Search

An ablation resistant coating with self-transpiration cooling effect was prepared on the surface of aluminum alloy by dilution effect of high speed laser cladding. The coatings mainly contain ablation resistant phases ZrB 2 and SiC, and low-melting phases Al and Al 3.21 Si 0.47. The ablation property of coatings was investigated at different heat fluxes (35.39, 70.77, 106.16, 141.54) MW⸱m−2. The mass ablation rate increased gradually, and the morphology of ablation region changed significantly with the increased heat flux. The self-transpiration cooling effect of coatings was studied by phase, morphology, and element distribution of ablation region. The self-transpiration cooling effect weakened the oxidation behavior of ZrB 2 and SiC at heat fluxes of 35.39 and 70.77 MW⸱m−2 and protected the coating from being exhausted when the heat fluxes were 106.16 and 141.54 MW⸱m−2. [ABSTRACT FROM AUTHOR]

Herein, the effects of Gd2O3 on the laser cladding NiCoCrAlYTa coatings in terms of microstructure, phase structures, high temperature oxidation behaviors were investigated. The results showed that the addition of Gd2O3 changed the composition of coating phases, promoted homogeneous phase distribution, reduced porosity and improved coating density. During oxidation at 1100 °C for 150 h, the increase in the content of added Gd2O3 (at mass fractions of 0, 0.5, 1, 2, 5 and 10%) exhibited an initial inhibitory and then increased effect on the weight-gain rate of the coatings. Overall, 1% Gd2O3 addition resulted in the greatest improvement in the oxidation resistance of the coatings, with 49.58% decreases in oxidation kinetic constants, respectively, compared with those of unmodified coatings. However, at a Gd2O3 content of 10%, the thermally grown oxide on the coating surface exhibited large flaking areas. The high-temperature oxidation kinetic parameters of the coatings exhibited an inverted parabolic form with increasing Gd2O3 content. The higher Gd2O3 content led to the formation of excessive oxygen diffusion channels along grain boundaries, which is the reason for the decrease in the anti-oxidant performance of the coating. [ABSTRACT FROM AUTHOR]

There is a growing interest in enhancing the bioactivity of TC4-based metallic biomaterials, which are known for their excellent biocompatibility. Bioactive glass (BG) has been recognized for its high potential in promoting bioactivity, particularly in osteo tissue engineering. This study focuses on investigating the influence of BG addition on the microstructure and electrochemical properties of TC4 coatings. The TC4/BG composite coatings were fabricated through laser cladding, and their microstructure was characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical properties of the coatings were assessed through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in three different solutions. The results revealed that the incorporation of BG had a significant impact on the microstructure of the TC4 coatings, leading to the formation of a well-defined interface between the TC4 matrix and the BG aggregates. The distribution of BG aggregates within the TC4 matrix coating was found to be random and unrelated to the specific regions of the coating. The metallographic microstructure variations were attributed to different heat dissipation conditions during the laser cladding process. Furthermore, the electrochemical corrosion behavior of TC4/BG composite coatings reveals that they exhibit stability similar to that of passive films and good resistance against media corrosion compared to TC4, while also showing enhanced corrosion resistance in 3.5 wt% NaCl and Dulbecco's modified Eagle medium (DMEM) solutions, indicating their potential for biomedical applications; however, the corrosion resistance decreases gradually in all solutions, potentially due to the elevated Cl− concentration. Further research can explore bioactivity enhancement of TC4/BG composite coatings and investigate the long-term stability and biological response of these coatings in diverse physiological environments. [ABSTRACT FROM AUTHOR]

The bottleneck challenge of laser‐cladded coating preparation of magnesium alloys is the low melting point of alloys. This greatly restricts the selection of coating material. In this article, the composition of Ni–Ti–Cu coating is designed by cluster line criterion, and the thermal properties of the coating are simulated by JMatPro software. Through the analysis of simulation results, the Ni51.6Ti34.4Cu14 coating is selected and successfully prepared on AZ91D magnesium alloy via laser cladding technology. Then the microstructure, wear resistance, and corrosion resistance of the coating are studied in detail. The results of X‐Ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and transmission electron microscope (TEM) demonstrated that there is a good metallurgical bond between the coating and the substrate, and the coating is mainly composed of Ni3Ti, TiNi0.8Cu0.2, and (Cu, Ni) solid solution phases. Also, there are no obvious pores and crack defects in the coating. Remarkably, AZ91D alloy showed significant improvement in corrosion resistance and wear resistance. Among them, the wear volume of the coating is decreased by 71.9% than that of AZ91D alloy. In particular, the corrosion current density of the coating is three orders of magnitude lower than that of AZ91D alloy. [ABSTRACT FROM AUTHOR]

A novel Al 27 Cr 25 Ti 18 Nb 18 Zr 12 eutectic high entropy alloy (EHEA) coating with BCC/B2 and Laves phase has been developed by laser cladding on Ti–6Al–4V alloys. The average microhardness of EHEA coating was ∼697.6HV 0.3 , which significantly surpasses the values reported for most eutectic high-entropy alloy coatings. The strengthening effect and additional strain hardening sources generated by the intersecting nanometer-spaced stacking faults and Lomer-Cotrell locks which found in BCC/B2 phase are important reasons for the superior hardness. And the adhesive problem of Ti–6Al–4V alloy was ameliorated by the EHEA coating with lower friction coefficient (83.05 % of the Ti–6Al–4V alloy) and volumetric loss (27.75 % of the Ti–6Al–4V alloy). • A novel eutectic high entropy alloy (EHEA) coating consisting of BCC/B2 and Laves phases was firstly reported. • A large number of intersecting nanometer-spaced lattice imperfections were found inside the BCC/B2 phase. • Due to the rigid eutectic phases and lattice imperfections, microhardness in this work exceeds the majority of EHEAs. • The adhesive problem of Ti–6Al–4V alloy was ameliorated by this coating and the effect mechanism was investigated. [ABSTRACT FROM AUTHOR]

Two kinds of layered double hydroxides inhibition materials were synthesized on graphene oxide, and applied to strengthen the Ni-based coating on Mg-Li alloy. The composition and release properties of corrosion inhibition materials were studied. The formation of passivated film and the corrosion resistance mechanism of coating were summarized. The results show that two kinds of corrosion inhibition materials had good release properties. The addition of corrosion inhibition materials was helpful to form Ni-based composite coatings with fine and dense cell clusters, which strengthened the corrosion resistance of the coating, especially for a long time. • Two kinds of graphene-based corrosion inhibition materials were synthesized. • Corrosion resistance of Ni-based coatings in different soaking time was studied. • Inhibition materials enhanced the corrosion resistance of coatings on Mg-Li alloy. • Passivation film formation and corrosion resistance mechanism were studied. [ABSTRACT FROM AUTHOR]

Wear is one of the primary challenges for high-temperature working components. In this paper, FeCoCrNi 2 MoSi eutectic high-entropy alloy coatings were successfully prepared by introducing MoNiSi ternary silicides with Laves structure into FeCoCrNi high-entropy alloys using laser cladding technology. The alloy achieves a balance of strength and toughness through the nanoscale eutectic structure with alternating FCC and Laves phases. Tribological experiments conducted over a wide temperature range from 25 °C to 800 °C show that the synergistic effect of the nano-lamellar structure and the alternating soft and hard structure makes the coating have excellent wear resistance at room temperature. The improvement of wear resistance at high temperatures (800 °C) is mainly due to the formation of the oxide film on the coatings. • Eutectic high-entropy alloys with nano-lamellar structure prepared by laser cladding technique. • The formation of a flake structure of the oxide film has a protective effect on the wear surface. • The synergistic effect of the lamellar structure and the nano-scale layer results in excellent wear resistance. [ABSTRACT FROM AUTHOR]

The directional array TiN-reinforced AlCoCrCuNiTi high-entropy alloy (HEA) coating was designed and the correlative coating was successfully synthesized by magnetic-field-assisted laser cladding on Ti-6Al-4V (TC4) alloy. The effect of the directional array TiN on the phase structure, microstructure, microhardness and wear resistance of HEA coating were investigated methodically. As a result, the HEA coatings are mainly composed of body-centered-cubic solid solution phase, (Ni, Co)Ti2 inter metallic compound and TiN ceramic phase. With 1 T magnetic induction intensity assisted laser cladding, the microstructure of in-situ TiN displays a "cross" shape of the directional array. And the coating shows excellent wear resistance with an average microhardness of 760 HV. The worn surface morphologies of HEA coatings show that the wear mechanism is adhesive wear regardless of whether a magnetic field is added. This work can provide a novel and effective method to synthesize directional array microstructures to improve the performance of HEA coating, which will promote the further application of directional array microstructures in advanced manufacturing fields. [ABSTRACT FROM AUTHOR]

In this work, a novel lightweight AlCrTiNbMo refractory high-entropy alloy (RHEA) coating has been developed to overcome the surface damage defects in Ti-6Al-4V alloys under wear and erosion conditions. A crack-free in-situ nitride (AlCrTiNbMo)N x RHEA coating were successfully prepared by the gas assisted laser nitriding. According to XRD, SEM and TEM analysis, AlCrTiNbMo coating was composed of single BCC phase, while (AlCrTiNbMo)N x coating was constituted by BCC and TiN phases. The average microhardness of AlCrTiNbMo and (AlCrTiNbMo)N x RHEA coatings were 591.2 HV 0.3 and 1070.1 HV 0.3 respectively. (AlCrTiNbMo)N x RHEA coating has lower friction coefficient and wear rate than AlCrTiNbMo RHEA coating and the substrate from room temperature to 600 °C. At 600 °C, (AlCrTiNbMo)N x RHEA coating showed a rare self-lubricating behavior, the friction coefficient decreased to 0.275 (61.9 % of the substrate), and the wear rate was 0.06 mm3/N m× 10−4 (1.24 % of the substrate). The erosion rates of (AlCrTiNbMo)N x RHEA coating at erosion angle of 30°, 60° and 90° were 11.0 %, 22.3 %, and 36.8 % of those of the substrate, respectively. The homogeneous and hard TiN array-BCC structure generated by in-situ self-reaction was the main factor for the excellent wear and erosion resistance of (AlCrTiNbMo)N x RHEA coating. Thus, (AlCrTiNbMo)N x RHEA coating and corresponding laser nitride technique were expected to be applied in the severe working environment of high temperature, wear, and erosion. • In-situ nitride (AlCrTiNbMo)N x refractory high-entropy alloy coating by gas assisted laser nitriding was first proposed. • The wear and erosion performance of Ti-6Al-4V alloy was improved significantly and the effect mechanism was investigated. • The homogeneous and hard TiN array-BCC structure generated was the main factor for the excellent performance. [ABSTRACT FROM AUTHOR]

This study proposes the utilization of rare earth combined with ultrasonic-assisted laser cladding technology to address the dilution problem encountered during the laser cladding coating of Ti alloy. The aim is to improve the performance of the coating. Using Ti alloy surface as a base, a novel Ti-Al-Si-Cr-SiC multi-principal element alloy coating is fabricated. To characterize the coating, various techniques such as transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM) are employed to examine the microstructure and phase composition. The interface between the coating and the substrate is analyzed using focused ion beam and TEM. The coatings are also evaluated for corrosion and wear resistance through potentiodynamic polarization, electrochemical impedance spectroscopy, hardness, and friction tests. The results reveal that the rare earth combined with ultrasonic-assisted Ti-Al-Si-Cr-SiC coating consists of Ti 4 Cr phase, TiC phase, and eutectic phase (Ti 3 Al and Ti 5 Si 3). The combined effect of ultrasonic energy and rare earth reduces the dilution rate of the coating, resulting in a refined coating structure that improves the wear resistance and corrosion resistance. • A new preparation method of ultrasonic composite rare earth assisted laser cladding was proposed. • The dilution rate of the coating prepared by rare earth-ultrasonic assisted laser cladding is significantly reduced. • Microstructure refinement promotes the formation of stable passivation film and improves the corrosion resistance. • The coating prepared by rare earth combined with ultrasonic assisted laser cladding has good wear and corrosion resistance. [ABSTRACT FROM AUTHOR]

The excellent strength and hardness obtained from the customized phase compositions provide vast spaces for surface protection application of refractory high entropy alloy (RHEA) coatings in titanium alloys. However, the introduction of high-strength brittle phase in RHEA coatings commonly increases the cracking tendency and affects their service life. In this work, we propose a design strategy to enhance the mechanical properties of RHEA coatings by introducing an extremely stable high strength boride combined with a ductile phase. Experimentally, LaB 6 /NbNiTaTi RHEA coatings mainly consist of BCC phase, in-situ TaB phase and NiTi/IM eutectic phase. TaB phase with high-density (110) nanotwins inside presents needle-like morphology with the long side axis parallel to the [001] direction. Excessive needle-like TaB phase is fully fused and fractured due to its preferential growth along a specific direction. The fracture surface analysis shows that BCC phase bears the main deformation, NiTi phase can relieve stress concentration at the boundary while bearing a small amount of deformation. By contrast, the low-density dislocation distribution and the low energy extended dislocation structure in TaB phase exhibit the phase stability. In addition, the amorphous islands are formed near the high-deformation regions when the defect concentration reaches the critical value. [Display omitted] • TaB phase with high-density stacking faults was formed in situ. • The RHEA coatings exhibited superior compressive strength and ductility. • Different phases played different roles in the process of bearing deformation. • Grain boundary strengthening and dislocation strengthening were the main factors of strengthening mechanism. [ABSTRACT FROM AUTHOR]

This work is aimed to explore ways to further improve corrosion properties of single-phase RHEA coating by introducing in-situ TiC phase. Results shown that C/Al 0.1 CrNbSi 0.1 TaTiV RHEA coatings exhibited better comprehensive resistance to pitting and general corrosion compared to most of reported HEAs and traditional alloys. The passive film was dominated by a muti-oxide crystalline structure with a thickness of ∼ 11.52 nm. The small-size and evenly distributed TiC phase with most thermodynamically stable could be obtained via doping C and retard the nearby pitting process, improving the pitting behavior and the dense of passive film in 1.0 wt% C coating. [Display omitted] • A small amount of in-situ TiC phase enhances the corrosion resistance of RHEA coatings. • The passive films show biphasic semiconductor behavior. • The passive film is a muti-oxide crystalline structure with a thickness of ∼ 11.52 nm. • Al, Si and Ta elements are the main component of passive films. [ABSTRACT FROM AUTHOR]

Multi-principal element alloy coatings (MPEACs) have great application potential for high-temperature diffusion barrier coatings. The sluggish diffusion and lattice distortion of this new coating material due to its own atomic size effect give the coating excellent resistance to high temperature oxidation and grain coarsening. In this study, the high-temperature oxidation properties, and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings (whose lattice distortion magnitude is linearly increased) at 1100 °C were studied. The evolution of grain orientation, size, and grain boundaries of alloy coatings after high temperature exposure were analyzed by EBSD. The results showed that from quaternary to denary alloy coatings, with the rise in lattice distortion magnitude of the alloy system, the sluggish diffusion ability of the coating at high temperatures was more obvious. Specifically, the oxidative activation energy of quaternary to denary alloy coatings is 355.41, 480.96, 478.76, and 654.03 kJ/mol, respectively. A high distortion magnitude is beneficial to increase the activation energy of the alloy coating, as a result, grain growth of alloy coatings with high activation energy is significantly inhibited, and under the combined action of storage strain energy and solute dragging, the grain boundary migration rate of the coatings also declines. Finally, the denary alloy coating shows good resistance to high temperature softening. This study provides a positive reference for promoting the application of multi-principal alloys in diffusion barrier coatings. • High-temperature oxidation and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings at 1100 °C were studied. • The oxidative activation energy increases linearly from quaternary to denary alloy coatings. • The grain growth of the coating gradually weakened from quaternary to denary alloy coatings. • Grain boundary migration rates are affected by the combined effects of storage strain energy and solute dragging. • The denary alloy coating shows good resistance to high-temperature softening. [ABSTRACT FROM AUTHOR]

The poor surface properties of Mg Li alloys and the excellent comprehensive performance of high-entropy alloy (HEA) coatings are highly complementary. This study uses laser cladding technology to deposit an AlTiVNiCu/Cu–Al gradient functional coating with favourable wear and corrosion properties on Mg Li alloy substrates. Through the design of the gradient coating structure, laser cladding is used to coat HEA layers on low-melting-point Mg Li alloys, which effectively reduces further dilution of Mg and Li in the substrate. Notably, the Mg content in the prepared AlTiVNiCu protective layer is only 2.34 %. Furthermore, the microstructure, phase structure, microhardness, tribological and corrosion properties of the gradient coating are systematically studied. The protective layer consists of V-rich BCC 1 , (Ni, Ti)-rich BCC 2 , Cu-rich FCC, Mg-rich BCC 3 and nanoscale β -Ti phases, with a predominance of body-centered cubic (BCC) structural phases. The average microhardness is 547.46 HV0.3. Under a load of 3 N, the average friction coefficient of the protective layer is 0.54, with a wear volume of only 2.87 % compared with that of the substrate, which is satisfactory wear resistance. In a 3.5 wt% NaCl solution, the protective layer exhibits the highest corrosion potential (−0.49 V SCE , where SEC denotes saturated calomel electrode), and the lowest corrosion current density (1.14 × 10−6 A·cm−2), representing a remarkable enhancement compared with those of the substrate (−1.57 V SCE and 5.41 × 10−4 A·cm−2). [Display omitted] • Laser-cladded AlTiVNiCu HEA coatings were designed and prepared on Mg Li alloy. • LC-Cu 6 Al layer helps to balance the thermal mismatch between the HEA coating and Mg Li. • HEA coatings consist of V-rich, (Ni, Ti)-rich, Cu-rich, Mg-rich and β-Ti phases. • HEA coatings show good wear resistance with a wear volume of 2.87 % of the substrate. • HEA coatings have higher E corr and lower i corr values for good corrosion resistance. [ABSTRACT FROM AUTHOR]

The sluggish diffusion effect of complex concentrated alloys (CCAs) affords them considerable application potential in the field of high temperatures. In this study, the role of refractory elements in the high temperature oxidation process of denary complex concentrated alloy coatings (CCACs, namely NiCoCrFeAl 0.8 Si 0.1 (MoNbTaW) x , where x is 0.025, 0.05, 0.1 and 0.15) prepared via laser cladding was investigated. Four groups of the CCACs with different refractory contents were subjected to 100 h of high-temperature oxidation experiments, and the oxidation products and microscopic characteristics of the samples were observed via XRD and SEM, respectively. Atomic-scale characterisation and analysis of the oxidised samples were performed using TEM and XPS technology to explore the diffusion behaviour of elements in the high-temperature process. Results indicate that the solid solution limit of the CCACs is obtained by regulating the topological instability of the lattice matrix via the addition of refractory elements. Consequently, when 0.1 > x > 0.025, the coating is a body-centred cubic (BCC) + face-centred cubic (FCC); when x = 0.1, the coating maintains a single-phase FCC structure; and when x > 0.1, the coating changes to an FCC + BCC duplex structure. The x 0.1 group exhibits the smallest relative strain among all coatings, which is conducive to maintaining the structural stability of the face-centred cubic at high temperatures and delaying atomic migration within the lattice. Furthermore, the oxidation kinetics curve of the CCACs with a single- phase FCC exhibits low K p values. In a single- FCC lattice, the atom diffusion rate through volume slows down and grain boundary diffusion decreases. Considering the Weibull modulus for the hardness statistical value after coating oxidation, the modulus of single-FCC phase CCACs is also higher than others. This study has the potential to contribute to the application of high-temperature thermal protection for CCACs. • The denary complex concentrated alloy coatings (CCACs) were prepared by laser cladding. • The solid solutioning limit of the CCACs can be adjusted by the topological instability of the lattice. • The diffusion rate of atoms in matrix is the combined action of the volume and the boundary diffusion. • The single-FCC CCACs with higher solid solubility have better oxidation resistance. [ABSTRACT FROM AUTHOR]

For the application of lightweight alloys, the corrosion and wear problems of magnesium-lithium (Mg-Li) alloys have always been important problems that cannot be ignored. The coupling protection of multiple technologies has gradually become a new trend. In this study, layered double hydroxides (LDH) nanosheet arrays were in-situ grown on Ni-P coating by the electroless plating technology combined with hydrothermal synthesis technology. The optimum deposition conditions (temperature-time-pH) were selected by orthogonal test. The surface morphology, structure, corrosion resistance and wear resistance of LDH/Ni composite coatings under different conditions were systematically studied by SEM, XRD, FT-IR, DSC, electrochemical and tribological tests. The growth mechanism of LDH suitable for Ni-P coating surface was proposed by analyzing the growth process of LDH nanosheet arrays. The results showed that the condition of 125 °C-12 h-13 was conducive to the growth of LDH nanosheet arrays. Compared with the Ni-P coating and the composite coating under other conditions, the LDH 125°C-12h-13 /Ni composite coating has the lowest self-etching current density (7.50 × 10−7 A·cm−2) and the average friction coefficient (0.35). The defect sealing effect and Cl− barrier property of LDH effectively improved the corrosion resistance of the coating. The frictional properties were improved by lubrication and heat absorption properties of LDH. In addition, it was found that the growth of LDH on the Ni-P coating surface followed the mechanism of "deposition nucleation-diffusion growth". • Nanosheet arrays in-situ coupled Ni-based coatings on Mg-Li alloy were prepared. • Optimum growth conditions of nanosheet arrays were selected by orthogonal test. • The composite coating had better corrosion and wear resistance for Mg-Li alloy. • Wear mechanism and growth mechanism of LDH nanosheet arrays were studied. [ABSTRACT FROM AUTHOR]

• A metallurgical AlCuTiVCr HEA coating was prepared on Mg-Li alloy. • Cu-Al layer can balance the thermal matching disparity among HEA layer and Mg-Li. • HEA coating consists of (V, Cr)-rich BCC 1 , Mg-rich BCC 2 , AlCu 2 Ti and β-Ti phases. • The corrosion resistance of Mg-Li alloy was significantly improved. AlCuTiVCr/Cu-Al lightweight high-entropy alloy gradient coating was successfully prepared by laser cladding to improve the corrosion resistance of Mg-Li alloy. The boundaries between AlCuTiVCr protective layer, Cu-Al transition layer and Mg-Li alloy substrate are distinct, forming an excellent metallurgical bonding. The gradient coating design has effectively balanced the thermal matching difference between HEA coating. The prepared AlCuTiVCr protective layer comprises (V, Cr)-rich BCC 1 , Mg-rich BCC 2 , AlCu 2 Ti and β-Ti phases. The Cu-Al transition layer is composed of β-Li, Cu 2 Mg and Al 2 Cu 3 phases. The corrosion potential of AlCuTiVCr protective layer (−0.46 V SEC) is 1.09 V SEC higher than that of Mg-Li alloy (−1.55 V SEC), while the corrosion current density of the protective layer (9.41 × 10−7 A cm−2) is about three orders of magnitude lower than that of the substrate (5.97 × 10−4 A cm−2). [ABSTRACT FROM AUTHOR]

In recent years, nickel–titanium (NiTi) coatings have shown excellent tribological performance due to their unique phase transition characteristics. However, in the laser cladding of NiTi coatings in a semi-open environment, there are still issues with nitrogen and oxygen impurities arising in the coatings due to their exposure to air. In this study, three rare earth (RE) compounds were used to modify NiTi coatings in a semi-open environment to obtain coatings without any impurities. The effect of different RE compounds (Y 2 O 3 , YF 3 and YCl 3) on phase composition, surface oxidation and mechanical properties were studied. NiTi, NiTi 2 and the impurity phase titanium nitride (TiN) were observed in the original and Y 2 O 3 modified coating. In addition, incompletely melted Y 2 O 3 exists in the Y 2 O 3 -modified coating. The coatings with YCl 3 and YF 3 were composed of NiTi and NiTi 2 phase and the TiN impurity phase was not observed in either coating. In addition, the oxidation phenomenon at the top of the coating gradually weakened in the original, Y 2 O 3 , YCl 3 and YF 3 -modified coatings. Furthermore, the R-phase was characterised in the YF 3 -modified coatings. The average microhardness values of the original, Y 2 O 3 , YCl 3 and YF 3 -modified coatings were 669.68 HV 0.3 , 755.1 HV 0.3 , 579.1 HV 0.3 and 390.47 HV 0.3 , respectively, of which the microhardness of the YF 3 -modified coating was the lowest. Although RE compounds did not significantly change the tribological properties, the Y 2 O 3 -modified coating exhibited the best tribological properties (∼0.886 mm, 2.6 mg). The results show that YF 3 can isolate pollutants from the air in the coating and protect the coating in a semi-open environment. • NiTi alloy coating was prepared by laser cladding in a semi-open environment. • The TiN impurity phase disappeared inside the coating after the addition of YF 3. • The coating with YF 3 addition has the lowest microhardness. • The YF 3 prevents the Ti element in coatings from being contaminated by N 2 in the air. [ABSTRACT FROM AUTHOR]

• The hypereutectic-eutectic-hypoeutectic gradient HEA coating is prepared. • The eutectic structure in the coating is nanoscale lamellar. • Hardness of the gradient HEA coating exhibits an outside-to-inside decrease trend. • Compared with FeCoCrNi HEA coating, the surface hardness is increased by 340%. • Compared with FeCoCrNi HEA coating, the wear rate is decreased by 94.1%. In this study, a hypereutectic-eutectic-hypoeutectic gradient high-entropy alloy (HEA) coating was successfully prepared by in-situ interfacial reaction between Nb and FeCoCrNi. Its microstructure is composed of Fe-rich FCC phase and Nb-rich Laves phase, and the eutectic structure is nano lamellar. The outside-to-inside hardness of the coating gradually decreases from 797.4 Hv 0.5 to 425.5 Hv 0.5. Compared with those of the FeCoCrNi HEA coating, the surface hardness, COF, and wear rate of the hypereutectic-eutectic-hypoeutectic gradient HEA coating are improved by 340%, 41.3%, and 94.1%, respectively. The design and preparation method applied in this study can provide new ideas for the development of new HEA coatings. [ABSTRACT FROM AUTHOR]

Ti-Al-Si-SiC-Cr x (X = 0, 5, 10, 15) coatings were prepared on TC4 alloy by laser cladding, and their microstructure and tribological properties were investigated. The results reveal that the coatings consist of Ti 3 Al, TiC, Ti 5 Si 3 , and Ti 4 Cr phases. In room temperature friction, with the addition of Cr, Ti 4 Cr soft phase was formed in the coating, which reduced the three-body wear of the coating. However, an excess of Ti 4 Cr soft phase induces a reduction in the wear resistance of the coating. Under high temperature friction conditions, due to the addition of Cr, a super-hard amorphous oxide protective layer is formed on the wear surface to improve the wear resistance of the coating. Therefore, Ti-Al-Si-SiC-Cr 10 coating has the best tribological properties. [ABSTRACT FROM AUTHOR]

In this work, a new dual-phase high-entropy alloy (HEA) coating is successfully synthesized by laser cladding. XRD, SEM, TEM, EBSD, and TG-DSC are employed to investigate the microstructures and high-temperature stabilities. Simultaneously, under high-temperature friction and universal mechanical tester, the wear properties at 25 °C, 400 °C, and 800 °C and the mechanical properties at the compression speeds of 1 %, 2 %, and 3 % are explored. As indicated by the results, the FeCoNiAlCu HEA coating is composed of FCC phase with small angle grain boundaries and small size grains and BCC phase with HAGBs and large size grains. In terms of mechanical properties, FeCoNiAlCu HEA coating has good strength and toughness. When considering the fracture mechanism, it transfers gradually from a ductile fracture to a brittle fracture with increased compression speed. Besides, FeCoNiAlCu HEA coating maintains good stability at a high temperature of 764.2 °C. Meanwhile, the wear resistance and friction reduction of HEA coating are improved obviously at 800 °C. With the rise of temperature, the friction coefficient decreases gradually, the wear rate reduces, and the antifriction and wear resistance are enhanced. • The FeCoNiAlCu HEA is composed of FCC phase (LAGBs and small size grains) and BCC phase (HAGBs and large size grains). • The yield strengths of FeCoNiAlCu HEA are 1516.2 MPa, 1189.1 MPa and 929.8 MPa at the compression speeds of 1%, 2% and 3%. • With the increase of compression speed, the fracture gradually changes from ductile fracture to brittle fracture. • The FeCoNiAlCu HEA has good high temperature stability from 25°C to 764.2°C. • The frictional performance of HEA are improved obviously at 800°C. [ABSTRACT FROM AUTHOR]

Refractory high entropy alloys (RHEAs) with single-phase structure hold far great potential in the field of surface protection application under extreme environment such as high temperature, abrasion and corrosion. In this work, the effects of Cr, Zr and W on the microstructure evolution and properties of a single-phase body-centered-cubic (BCC) NbTaTiV RHEA coating have been investigated. Experimentally, in addition to a small amount of Laves phase introduced by Cr element, NbTaTiV-(Cr, Zr, W) RHEA coatings basically maintain the single BCC structure. The addition of different elements affects segregation degrees in dendrite microstructure, and Zr shows nanoscale chemical heterogeneity. Notably, grain refinement, elevated geometrically necessary dislocation density and high-density low angle grain boundaries lead to a dramatically increase of micro-hardness due to elements addition. The wear rate of NbTaTiVCr and NbTaTiVW RHEA coatings reduce by 74.98 % and 70.79 % compared with NbTaTiV coating. The wide passive passivation zones close to 1000 mV obtained by potentiodynamic polarization exhibit excellent corrosion resistance. In addition, the formation mechanism of oxide layers is studied by thermodynamic parameters and oxidation behavior of simple oxides. The current work provides theoretical support for the application of strengthening elements in a single-phase BCC RHEA coating. [Display omitted] • Lattice distortion of BCC phase after adding elements is affected to various degrees. • There exists nanoscale chemical heterogeneity in NbTaTiVZr RHEA coating. • The interaction between different types of grain boundaries and dislocations enhances hardness of RHEA coatings. • The wide passive passivation zones close to 1000 mV are exhibited in potentiodynamic polarization curves. • The thermodynamic parameters and oxidation behavior of simple oxides determine the formation of oxide layers. [ABSTRACT FROM AUTHOR]

The metastable materials have attracted the attention of researchers due to their promising physicochemical properties, but the thermal stability limits their practical application. In this paper, Ni-B/La 2 O 3 amorphous/nanocrystalline composite coatings were prepared on AISI 1045 steel substrate by electro-brush plating technique. The surface morphology, component and the phase structure of coatings were observed by Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD), respectively. The coatings were further analyzed by Transmission Electron Microscope (TEM). Then the thermal stability of coatings with different content of La 2 O 3 was also analyzed under different annealing temperature. It was found that the coating had a unique microstructure consisting of amorphous and nanocrystalline structures. The addition of La 2 O 3 does improve the thermal stability of metastable Ni-B coatings when the annealing temperature is over 355 °C. In addition, the thermal stability of the amorphous/nanocrystalline coatings depends on the percentage of amorphous phase and grain sizes. [ABSTRACT FROM AUTHOR]

In the pursuit of high performance development of alloy materials, it is a long-standing problem to enhance strength and toughness simultaneously. In this study, by selecting heterogeneous high-entropy alloy (HEA) materials and conducting structural design and laser process optimization, an external-hard internal-tough HEA composite coating with an in-situ dual-phase interface layer was successfully prepared. For the microstructure, the phase structures at the top, middle, and bottom of the coating are single-phase body-centered cubic (BCC), dual-phase BCC/face-centered cubic (FCC), and single-phase FCC, respectively. Among them, the dual-phase interface layer gives priority to the FCC phase (79.4 %), supplemented by the BCC phase (16.7 %). In terms of mechanical properties, the hardness of the coating from the top to the bottom is 9.85 GPa, 5.53 GPa, and 4.86 GPa, respectively. As demonstrated by employing the three methods of K, H/E, and H3/E2, the HEA composite coating has favorable integrated characteristics of strength and toughness. Meanwhile, due to the in-situ generated dual-phase interface layer, the structural and performance differences between the high-hardness layer and the high-toughness layer are effectively alleviated, and the stress concentration is reduced, thus stabilizing the overall structure of the coating. Hence, the HEA composite coating designed and prepared in this study possesses better erosion resistance (mass erosion rate = 0.141 × 10−3 g.cm−2 s−1), tribological properties (wear rate = 0.30 × 10−6 mm3/(N · m)), and impact resistance (AKU = 103.5 ± 2.8 J). The study results can provide new methods and new theories for new HEA composite coatings with integrated strength and toughness, and can be used in guiding the development and design of similar HEA composite coating structures. [Display omitted] • The FCC/FCC + BCC/BCC HEA composite coating with gradient structure is successfully prepared. • The FeCoCrNiAl interface layer is formed in situ by FeCoCrNi and FeCoCrAl. • The interface layer alleviates the structural and performance differences between the surface and bottom layers. • The HEA composite coating possesses good strength and toughness. • The HEA composite coating has better erosion resistance, preferable tribological properties, and superior impact resistance. [ABSTRACT FROM AUTHOR]

Eutectic high-entropy alloys (EHEAs) possess in-situ composite structure, which provides a novel strategy for developing wear-resistant alloys or coatings. In this work, a new-type AlCoCrFeMn 0.5 Mo 0.1 Nb x (x = 0.4 hypoeutectic, 0.65 eutectic and 0.8 hypereutectic) EHEA coatings were designed and prepared by ultrasonic assisted laser cladding. The design strategy, microstructure and tribological behaviors of the EHEA coatings were studied in detail. The Nb 0.4 , Nb 0.65 and Nb 0.8 coatings were all composed of BCC solid solution and Laves phase. The Nb 0.65 coating showed typical nano-lamellar/nano-granular eutectic microstructure, while the Nb 0.4 and Nb 0.8 coatings had hypoeutectic and hypereutectic microstructures, respectively. The wear rates of the EHEA coatings (5.5 × 10−6 mm3/N·m for Nb 0.4 coating, 5.3 × 10−6 mm3/N·m for Nb 0.65 coating and 8 × 10−6 mm3/N·m for Nb 0.8 coating) were significantly lower than those of most metallic and metal-ceramic composite materials. The wear mechanisms of Nb 0.4 , Nb 0.65 and Nb 0.8 coatings were slight adhesive wear and oxidation wear. Numerous regular rolling structures formed on the wear scar of Nb 0.65 coating, which provides an anti-friction effect for the coating. The good carrying capacities and high hardness of the BCC, Laves and eutectic microstructures contributed to the excellent wear resistance of the EHEA coatings. [Display omitted] • A new type AlCoCrFeMn 0.5 Mo 0.1 Nb x eutectic HEA coating system were successfully designed and prepared. • Typical nano-lamellar and some nano-granular eutectic microstructures were obtained in the coatings. • The wear rates of the EHEA coatings were significantly lower than those of most metallic materials. • The high hardness, good elastic recovery and oxidation structure contribute to the excellent wear resistance. [ABSTRACT FROM AUTHOR]

Microcapsules with self-healing coatings for corrosion protection of magnesium substrates have attracted tremendous interest due to their capability of preventing the propagation of cracks in protective coatings by releasing inhibitors from the microcapsules. Gelatin-chitosan microcapsules containing La(NO 3 ) 3 were prepared by complex coacervation. The morphology of the cerium-based conversion coating shows a globular structure with microcapsules distributing in the tiny cracks of the coating. In 3.5 wt% sodium chloride solution environment, the coating is damaged and microcapsules migrate into the damaged area, the triggering release of inhibitors supplement to the conversion coating. The cerium conversion coating with microcapsules shows a much higher corrosive resistance than the coating without microcapsules, and demonstrates the achievement of a self-healing performance. [ABSTRACT FROM AUTHOR]

Gd 2 O 3 and Sm 2 O 3 co-doped LZ ceramic ((La 0.5 Gd 0.25 Sm 0.25) 2 Zr 2 O 7 /YSZ) are designed and prepared by atmospheric plasma spraying (APS). It is the aim of this study to investigate the similarities and differences between co-doped sample and mono-doped samples, (La 0.5 Gd 0.5) 2 Zr 2 O 7 /YSZ and (La 0.5 Sm 0.5) 2 Zr 2 O 7 /YSZ on hot corrosion behaviors exposed to Na 2 SO 4 + V 2 O 5 molten salts. Thermodynamic calculations are used to predict the hot corrosion mechanism before experiments. Characterization and hot corrosion behavior are analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). Results indicate that the main hot corrosion products are composed of rare-earth vanadates ((La, Gd)VO 4 , (La, Sm)VO 4 and (La, Gd, Sm)VO 4 , respectively) and ZrO 2. After 10 h of corrosion, the reaction layer evolves into trilayered structure containing Zr-rich layer, V-rich layer and mixed-layer. The formation mechanism is discussed in detail. The co-doped sample exhibits excellent degradation resistance, which can be attributed to the higher entropy value. The formation of the particular trilayered structure occurs earlier in the co-doped sample, which also provides a role in reducing the degradation rate of ceramics. • A co-doped ceramic composition (La 0.5 Gd 0.25 Sm 0.25) 2 Zr 2 O 7 was designed. • Thermodynamic calculations were used to predict hot corrosion mechanisms. • The hot corrosion behaviors of mono-doped and co-doped coatings were compared. • The reaction layer of co-doped coating evolved into trilayered structure earlier. • The trilayered structure can prevent further penetration of molten salts. [ABSTRACT FROM AUTHOR]

In this work, the effect of the dual-phase microstructure (FCC and Laves phases) of Ni 1.5 CrCoFe 0.5 Mo 0.1 Nb x (x = 0.55 hypoeutectic, 0.68 eutectic and 0.8 hypereutectic) high-entropy alloy coatings on their corrosion and tribo-corrosion behaviors were studied. In a 3.5 wt.% NaCl solution, the FCC phase behaved as anodic sites. With the increase of Nb content, the passive regions of the coatings increased slightly. Mott–Schottky analysis revealed that the improvement in the performance of the passive film is mainly attributed to the reduced point-defect density. The increased Laves phase changed the wear mechanism of the coatings from adhesive to abrasive wear. [Display omitted] • The effect of the dual-phase microstructure on the tribo-corrosion behaviors of the eutectic HEA coatings was studied. • The interaction between FCC and Laves phases in the coatings was revealed by EBSD. • Nb 0.68 and Nb 0.8 coatings exhibited nano-scale pitting sizes. • With the increase of Laves phase, the protective capability of the passive films on the coatings was improved. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni-Ti-Cu/Cu-Al functionally graded coating was prepared on Mg-Li alloy by laser cladding. • The graded coating was constructed to solve the problem that the influence of evaporation and dilution about Mg and Li elements on the performance of coating. • Typical hypoeutectic structure composed of Li 3 Mg 7 and CuMg 2 was obtained in the interlayer (Cu-Al). • The corrosion and wear resistance of Mg-Li got huge improvement. The bottleneck challenge of preparing laser cladded coating on Mg-Li alloys is the evaporation and dilution of substrate, which seriously affect the performance of coatings. In order to solve these deficiencies, Ni-Ti-Cu/Cu-Al functionally graded coating was proposed in this work. The graded coating with excellent performance was successfully prepared on Mg-Li alloy. More remarkable, the top-layer was not affected by dilution and substrate evaporation. The top-layer was mainly composed of Ni 3 Ti and NiTi intermetallic compounds, as well as TiNi 0.8 Cu 0.2 phase that dispersed on the surface of Ni 3 Ti. Typical hypoeutectic structure was found in the interlayer, which was composed of Li 3 Mg 7 and CuMg 2. Furthermore, the corrosion and wear resistance of Mg-Li alloy were greatly improved. Specifically, the wear volume of the top-layer and interlayer is reduced by 98.04% and 82.41% than that of substrate, respectively. The corrosion potential of the top-layer (-242 mV) and the interlayer (-991 mV) are 1311 mV and 562 mV higher than that of the substrate (-1553 mV), while the corrosion current density of the top-layer (2.71 × 10-7 A·cm−2) and the interlayer (1.93 × 10-6 A·cm−2) are about three and two orders of magnitude lower than that of the substrate (2.53 × 10-4 A·cm−2). [ABSTRACT FROM AUTHOR]

NiTi, NiTi-4Si, and NiTi-4Si-2YF 3 coatings were prepared on Ti-6Al-4V substrate by laser cladding to obtain composite multilayer protective coating for composite structure. The effects of Si and YF 3 addition on the microstructure, phase composition, microhardness, and phase transformation of the coatings were systematically analyzed. The coatings are mainly composed of NiTi, NiTi 2, and TiN phases. The addition of YF 3 results in the decrease of the dendrite TiN phase in the coating. TEM results show that the core-shell structure is observed in NiTi-4Si coating. The β -Ti phase was surrounded by the shell NiTi 2 phase. After the addition of YF 3 , the core-shell structure was found to disappear. With the addition of the Si element, the eutectic structure was found in the coatings. The eutectic structure is mainly composed of NiTi (B2) and Ti 2 Si phases. According to the SEAD result, there are crystallographic orientation relationships (ORs) between NiTi (B2) phase and Ti 2 Si phase as follows: 1 ¯ 11 B 2 / / 1 ¯ 23 Ti 2 Si. The average microhardness of the NiTi, NiTi-4Si and NiTi-4Si-2YF 3 coatings were at 468.33 HV 0.5 , 499.86 HV 0.5 , and 433.65 HV 0.5 , respectively. At the same time, the uniformity of the NiTi phase in NiTi-4Si-2YF 3 coatings is improved, which is conducive to a more complete phase transition. [Display omitted] • With the addition of Si element, the eutectic structure was found in the coatings. • The addition of YF 3 results in the decrease of the dendrite TiN phase in coatings. • the core-shell structure (NiTi 2 @ β -Ti) is observed in NiTi-4Si coating. • The NiTi-4Si-2YF 3 coating exhibits the lowest microhardness. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, neodymium-based conversion coating is used as a substitute for toxic chromate conversion coating on AZ91D magnesium alloys. Its formation and growth were observed via SEM, EDS, XPS, electrochemical tests and weighting experiment. The influence of post-treatment on neodymium conversion coating was measured by FTIR and electrochemical experiments in terms of morphology, component, surface functional group and corrosion resistance. The dissolution of matrix and the deposition of neodymium/magnesium oxides compete with each other in initial time. Then the deposition of neodymium oxides dominates the process. Compact coating is obtained after 20min immersion and it is mainly made of neodymium oxides and a small amount of magnesium oxides/hydroxides. The coating post-treated is rich in OH− and PO4 3+. The post-treatment can improve the corrosion resistance of the neodymium conversion coating effectually examined by EIS. [Copyright &y& Elsevier]

Abstract: In this paper, a non-toxic Ce-based conversion coating was obtained on the surface of bio-medical AZ31 magnesium alloys. The micro-morphology of the coating prepared with optimal technical parameters and immersed in physiological solution (Hank''s solution) in different time was observed by scanning electron microscopy (SEM), composition of the cerium conversion coating and corrosion products in Hank''s solution were characterized by X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS), respectively. In addition, the corrosion property in Hank''s solution was studied by electrochemical experiment and immersion test. The results show that the dense Ce-based conversion coating is obtained on the surface of AZ31 magnesium alloys in optimal technical parameters and the conversion coating consists of a mass of trivalent and tetravalent cerium oxides. The cerium conversion coating can provide obvious protection of magnesium alloys and can effectively reduce the degradation speed in Hank''s solution. Also the degradation products have little influence on human body. [Copyright &y& Elsevier]

• The formation of solid solution phase is promoted by increasing Ta content. • The microstructure evolution mechanism with varied Ta content is investigated. • The RHEA coatings possess the good structure stability below 900 °C. • The stability of passive film is improved by increasing Ta content. [Display omitted] In this work, the novel NiTiCrNbTa x (x = 0.1, 0.3, 0.5, 1) refractory high-entropy alloy (RHEA) coatings were successfully synthesized on Ti6Al4V by laser cladding technology. The effect of Ta content on microstructure, thermal stability, micro-hardness and corrosion resistance of RHEA coatings was investigated in detail. The results showed that NiTiCrNbTa x RHEA coatings were composed of BCC, NiTi 2 -Laves and Cr 2 Nb-Laves phases. With Ta content increasing, the formation of BCC phase was promoted while the precipitate of Cr 2 Nb-Laves phase was affected. The average micro-hardness values of Ta 0.1 , Ta 0.3 , Ta 0.5 and Ta 1.0 RHEA coatings were calculated as ~922.8 HV 0.3 , ~985.1 HV 0.3 , ~851.3 HV 0.3 and ~825.4 HV 0.3 , respectively. The decrease of micro-hardness value might be ascribed to the reduction of hard Laves phase. After annealing at 900 °C for 3 h, the phase compositions of all RHEA coatings remained unchanged but the grains of Cr 2 Nb-Laves phase were refined obviously. Besides, RHEA coatings exhibited excellent corrosion resistance and the minimum corrosion current density was 1.08 × 10−7 A/cm2. Moreover, the secondary passivation phenomenon emerged in Ta 0.5 and Ta 1.0 coatings, and the passive film mainly consisted of the oxidation state of Ti, Cr, Nb, Ta and the sub-oxide state of Nb, Ta. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a new innoxious and pollution-free chemical protective coating for magnesium alloys, phytic acid conversion coating, was prepared. The conversion coatings are found to have high cover ratio and no cracks are found by atomic force microscopes (AFM) and scanning electron microscopy (SEM). The main elements of the conversion coatings are Mg, Al, O, P and C by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The chemical state of the elements in the coatings was also investigated by Fourier transform infrared spectroscopy (FTIR). AES depth profile analysis suggests that the thickness of the conversion coating is about 340nm. The corrosion resistance of the coatings was evaluated by polarization curves. The results indicate that the corrosion resistance for the conversion coated AZ91D magnesium alloys in 3.5% NaCl solution increases markedly. The mechanisms of corrosion resistance and coatings formation are also discussed. [Copyright &y& Elsevier]

Abstract: In this study, the phytic acid conversion coating, a new environmentally friendly chemical protective coating for magnesium alloys, was prepared. The influences of phytic acid concentration on the formation process, microstructure, chemical state and corrosion resistance of the conversion coatings on AZ91D magnesium alloy were investigated by means of weight gain measurement, field-emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, potentiodynamic polarization method and electrochemical impedance spectroscopy (EIS), respectively. And the depth profile of all elements in the optimal conversion coatings was analyzed by auger electron spectroscopy (AES). The results show that the growth, microstructure, chemical state and corrosion resistance of the conversion coatings are all obviously affected by the phytic acid concentration. The concentration of 5gl−1 corresponds to the maximum weight gain. The main elements of the coating are Mg, Al, O, P, and C, which are distributed gradually in depth. The functional groups of conversion coatings formed in higher concentration phytic acid solution are closer to the constituent of phytic acid than those formed in lower concentration phytic acid solution. The coatings formed in 1–5gl−1 are integrated and uniform. However, those formed in 20–50gl−1 have some micro-cracks on the α phase. The coating formed in 5gl−1 has the best corrosion resistance, whose open circuit current density decreases about six orders than that of the untreated sample, although the coatings deposited in 1–20gl−1 can all improve the corrosion resistance of AZ91D. [Copyright &y& Elsevier]

[Display omitted] • A multi-step ultrasonic assisted laser cladding for Mg-Li alloy was proposed. • A metallurgical Ni 2 MnCuSnAl 0.1 MPEA coating was prepared on Mg-Li alloy. • Typical gradient microstructure was obtained in Ni 2 MnCuSnAl 0.1 MPEA coating. • The corrosion-resistance and wear-resistance of Mg-Li alloy were improved. The evaporation and dilution of substrate are the key deficiencies that limit the performance of laser-cladded coatings on Mg-Li alloys (or Mg alloys). In order to overcome these drawbacks, in this work, a novel Ni 2 MnCuSnAl 0.1 multi-principal element alloy (MPEA) coating and a multi-step ultrasonic assisted laser cladding technique were proposed. Dense metallurgical Ni 2 MnCuSnAl 0.1 MPEA coatings were successfully synthesized on Mg-Li alloy. Typical gradient microstructure of low-diluted layer, diffusive layer and interfacial layer were obtained in the coating. The low-diluted layer was composed of BCC solid solution, Ni 2 MnAl and Ni 2 MnSn while the diffusive layer and interfacial layer showed an interactive microstructure of Mg-CuMg 2 eutectic, Ni 2 MnAl, Li 2 MgSn/SnMg 2 and Mg/Mg-Li substrate. The corrosion potential of Ni 2 MnCuSnAl 0.1 MPEA coating (-108 mV SHE) was 1283 mV SHE higher than that of Mg-Li alloy (-1391 mV SHE), while the corrosion current density of the coating (5.85 × 10−7 A·cm−2) was about three orders of magnitude lower than that of Mg-Li alloy (1.12 × 10−4 A·cm−2). The micro-hardness gradually increased from the substrate to the coating, and the average micro-hardness of the coating was ~ 541.2 HV 0.2 , which is about 10 times higher than that of Mg-Li alloy (~57.8 HV 0.2). The wear resistance of Mg-Li alloy was also improved approximately 10 times by cladding Ni 2 MnCuSnAl 0.1 MPEA coating. [ABSTRACT FROM AUTHOR]

NiTi 98-x Si x C 2 (x = 0, 4, 8 wt%) coatings were prepared on Ti-6Al-4V substrate by laser cladding to improve the surface mechanical properties of NiTi coatings. The effects of Si content addition on the microstructure, phase composition, microhardness, and tribological properties of the coatings were systematically analyzed. The results show that NiTi 98-x Si x C 2 (x = 0, 4, 8) coatings are mainly composed of NiTi, NiTi 2 , TiC, and Ti 5 Si 3 phases. The rod-like phase of Ti 5 Si 3 was found in NiTi 90 Si 8 C 2 coating, while the rod-like phases were found broken. The fragmentation of the rod-like phase is related to the growth of the NiTi phase around it. At the same time, TEM results show that the NiTi (B2) phase and Ti 2 Si second phase are exited in the NiTi 90 Si 8 C 2 coating. Ti 2 Si, which has not been decomposed into Ti 5 Si 3 , remains during the laser cladding process. In addition, the eutectic structure in NiTi 90 Si 8 C 2 coating was observed in some regions. The average microhardness of the NiTi 98-x Si x C 2 (x = 0, 4, 8) coating were at 534.08 HV 0.5 , 598.06 HV 0.5 , and 974.08 HV 0.5 , respectively. The wear resistance of NiTi 98-x Si x C 2 (x = 0, 4, 8) coatings increased with the increased Si content. The NiTi 90 Si 8 C 2 coating exhibits the highest microhardness (974.08 HV 0.5) and wear resistance (friction coefficients, wear trace, widths loss weight, and wear rate are 0.320, ~1.25 mm, 12.2 mg, and 5.78 × 10−6 g/Nm, respectively). In NiTi 90 Si 8 C 2 coating, Ti 5 Si 3 phase plays a significant role in the second phase strengthening to improve its tribological properties. The above analysis confirms that NiTi 90 Si 8 C 2 coating has optimal tribological properties. [Display omitted] • NiTi 90 Si 8 C 2 coatings are mainly composed of NiTi, NiTi 2 , Ti 5 Si 3 , and TiC phases. • The fragmentation of Ti 5 Si 3 phase is related to the growth of NiTi phase around it. • Ti 2 Si phase and the eutectic structure were found in NiTi 90 Si 8 C 2 coating. • The microhardness of NiTi 90 Si 8 C 2 coating is 1.8 times than NiTi 98 C 2 coating. [ABSTRACT FROM AUTHOR]

The new parameter ε we proposed in this study is to be more conducive to the hardness design of high-entropy alloy coatings. Here, the parameter ε is defined as the ratio of the atomic size mismatch to the atomic stacking mismatch. This parameter can be used as the criterion for measuring the magnitude of lattice distortion caused by the atomic size effect, which reveals the influence of the atomic size effect on the hardness of the alloy. We collected nearly 100 groups of on high-entropy alloy coatings in order to verify the feasibility of parameter ε, and to analyze the relevant data from available literatures by using the parameter ε. We found that the hardness value of the high-entropy alloy coatings showed monotonicity with the ε increased. When ε < 6.4, the phase structure of the coating is mainly fcc phase, the increase in hardness of the coating is smaller with the increments of ε. The opposite of bcc phase as the main phase for ε > 6.4, hardness value of the coating escalates with the increase of parameter ε. Based on this finding, it is suggested that the parameter ε can then be used to design a high-entropy alloy coatings with the desired hardness. • The influence of atomic size effect on the hardness of high-entropy alloy coating is briefly analyzed. • A dimensionless parameters is proposed and tested using available experimental data. • The new criterion is able to predict the hardness of high-entropy alloy metallic coatings. [ABSTRACT FROM AUTHOR]

In this work, a novel Ni 1.5 CrCoFe 0.5 Mo 0.1 Nb 0.68 eutectic HEA coating with nano-lamellar microstructure was successfully designed by binary eutectic compositions strategy, thermodynamic calculation and simple experimental approach. Compared with the predicted eutectic compositions, the actual eutectic composition had a higher Nb content due to the increased solid solubility of Nb in the FCC phase. The Ni 1.5 CrCoFe 0.5 Mo 0.1 Nb x (x = 0.55 hypoeutectic, 0.68 eutectic, 0.8 hypereutectic) HEA coatings were prepared by laser cladding, which were composed of FCC phase with lattice parameter of a = ~3.59 Å and Laves phase with lattice parameter of a = b = ~4.82 Å and c = ~7.80 Å. The FCC phase was enriched in Ni, Cr, Co and Fe elements, while the Laves phase was enriched in Nb and Mo elements. Typical eutectic lamellar and hypoeutectic/hypereutectic microstructures were obtained at the central regions, while the columnar crystals formed at the interfacial regions. The average micro-hardness of Nb 0.55 , Nb 0.68 and Nb 0.8 coatings were calculated as ~573.5 HV, ~665.8 HV and ~715.6 HV respectively. From x = 0.55 to x = 0.8, the wear resistance of Ni 1.5 CrCoFe 0.5 Mo 0.1 Nb x HEA coatings are increased, and the dominant wear mechanism of the HEA coatings transforms from adhesive wear to abrasive wear. Unlabelled Image • A feasible design strategy for laser-cladded eutectic HEA coatings was proposed. • A novel Ni 1.5 CrCoFe 0.5 Mo 0.1 Nb 0.68 eutectic HEA coating with nano-lamellar microstructure was successfully prepared. • Typical eutectic microstructures were obtained at central regions, while columnar crystals formed at interfacial regions. • The Nb content of actual eutectic composition high than that of predicted composition. [ABSTRACT FROM AUTHOR]

• High entropy alloys (HEAs) was first introduced in underwater laser wetcladding process process. • The utilization rate of laser energy underwater can be enhanced. • The incorporation of Nb can enhance the corrosion resistance and prevent the expansion of pitting pits. Underwater wet laser cladding technology (UWLC) offers advantages such as short repair cycles, low costs, and independence from drainage equipment, making it highly practical. The exceptional design flexibility of high entropy alloys, combined with the convenience of UWLC technology, presents significant application potential. To assess the feasibility of utilizing high entropy alloys (HEAs) in underwater wet laser cladding, (NiCrCoFe) 100-x Nb x (x = 0, 5, 10) coatings were prepared. This study investigates the effects of Nb addition on the formation characteristics and metallurgical process of (NiCrCoFe) 100-x Nb x HEA coatings. The results show that the addition of Nb promotes the efficient utilization of laser energy underwater by altering the melting latent heat of UWLC materials and enhancing the fluidity of the molten pool. Furthermore, the introduction of Nb element improves the corrosion resistance of the coatings and inhibits the expansion of corrosion pits. This study confirms the feasibility of utilizing high entropy alloys for underwater wet laser cladding and offers new ideas and methods for the underwater online repair of marine equipment. [ABSTRACT FROM AUTHOR]

As reinforcements, carbonaceous nanomaterials (CNMs) especially multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO), have attracted increasing attention in the application of electrodeposition. In this work, Ni-based nanocomposite coatings with MWCNTs and GO were prepared by electro-brush plating technique (EBP). The results showed that adding MWCNTs and adding GO both significantly improved the compactness of Ni coatings, however, the latter contributed more uniform microstructure and morphology of nanocomposite coatings owing to the excellent dispersion of GO. In addition, MWCNTs and GO could not only provide numerous nucleation sites for Ni primary crystals contributing the grain refinement, but also co-deposited with Ni atoms to form a strong interfacial bonding with Ni matrix. The addition of MWCNTs and GO can significantly reduce the worn volume of coatings from 7.52 × 107 μm3 to 3.49 × 107 μm3 and 2.82 × 107 μm3, respectively. Moreover, Ni nanocomposite coatings adding GO exhibited better wear resistance and friction stability than that of adding MWCNTs. Microstructural characterizations revealed that during wear testing, the agglomerated MWCNTs in nanocomposite coatings were destroyed severely and transformed into the shorten MWCNTs or the unfolded sidewalls, thereby leading to the instability of the wear process. In contrast, the damage degree of rGO was relatively slight. The nanosheets of rGO uniformly distributed in nanocomposite coatings became smaller and some of them were stacked between each other, which promoted the stability of friction process. Image 1 • Comparison study of MWCNTs and GO reinforced Ni composite coatings was conducted. • Both MWCNTs and GO significantly improved the compactness of pure Ni coatings. • HRTEM characterization provided evidences for grain refinement of MWCNTs and GO. • Adding GO promoted the stability of friction process compared with MWCNTs. • Structural variations of MWCNTs and GO after wear testing were investigated. [ABSTRACT FROM AUTHOR]

Graphene (Gr) has been regarded as the promising reinforcement for metal matrix composite coatings. However, it is hard to mix nano-scale Gr with micron-scale metal powders effectively at present, restricting the extensive application of Gr in engineering field. Therefore, this work proposed a new two-step method to in-situ synthesize multilayer Gr on the surface of Ni60 powders by combining ultrasonic and wet milling. Under protective atmosphere, Ni60 + Gr coatings were fabricated using laser cladding technique. In-situ synthesized multilayer Gr were successfully incorporated into Ni60 coatings, but part of them were transformed into the fullerene-like structures. In addition, a new matrix phase of Ni-Cr-Fe with bcc structure was generated in Ni60 + Gr coatings owing to the super adsorption effect of Gr on Ni, Cr and Fe atoms. The second phases such as chromium carbides in Ni60 + Gr coatings were refined and distributed more uniformly compared with that of Ni60 coatings. Furthermore, Ni60 + Gr coatings showed excellent tribological performance than that of Ni60 coatings, which was attributed to the synergistic effect between the lubrication of Gr or fullerene-like structures and the superior dispersion strengthening of the second phases. • Multilayer Gr on the surface of Ni60 powders by combing ultrasonic and wet milling was in-situ synthesized. • Multilayer Gr were incorporated into Ni60 coatings, but part of them were transformed into the fullerene-like structures. • Ni-Cr-Fe with bcc structure was generated in Ni60+Gr coatings. • The second phases in Ni60+Gr coatings were refined and distributed more uniformly compared with that of Ni60 coatings. [ABSTRACT FROM AUTHOR]

Refractory high entropy alloy (RHEA) coatings containing W and Ta elements at the same time usually have excellent performance at high temperature, which are promising candidate materials for high temperature protective coating materials. In this work, the effects of W and Ta on the microstructure and performance evolution of TiMoCrW x Ta y RHEA coatings have been investigated. Experimentally, TiMoCrW x Ta y coatings are mainly composed of BCC phase, Laves phase and Ti-rich phase, with W and Ta atoms preferably dissolved in BCC phase. The W atoms promote the formation of dendrites and the growth of rod-like and equiaxed crystals and the Ta atoms promote the formation of cellular crystals and produce the effect of fine crystal strengthening. W and Ta atoms have significant influence on the tribological properties of TiMoCrW x Ta y coatings. The tribological properties of TiMoCrW x Ta y coatings at 800 °C are better than those at room temperature. TiMoCrW x Ta y RHEA coatings follow near-linear oxidation kinetics during the oxidation of 50 h at 800 °C.The oxidation resistance of the coatings first increases and then decreases with the increase of Ta content. Ta 2 O 5 , TiO 2 and other oxides may have a shielding effect on the oxidation of W. In particular, The W 0.3 Ta 0.5 shows the best tribological properties and oxidation resistance, and the mass gain of W 0.3 Ta 0.5 is only 13.4 mg/cm2 after the oxidation test for 50 h. It indicates that TiMoCrW x Ta y coatings can obtain good high temperature performance by optimizing the content of W and Ta in RHEA coatings. This work can provide guidance for the research and application of TiMoCrW x Ta y RHEA coatings. • The Ta atom is preferentially dissolved in the BCC phase and it significantly affects the BCC lattice. • The variable content of W and Ta elements significantly affects the crystal growth rate and precipitation sequence of phases. • The tribological properties of the coatings can be improved by controlling the content of W and Ta elements. • Appropriate addition of W and Ta elements can improve the oxidation resistance of the coatings. [ABSTRACT FROM AUTHOR]

Based on the two-way diffusion behavior of FeCoCrNi and W elements and the lattice distortion principle during alloy solidification, a new type of HEA wear-resistant coating with Laves phase and hardness gradient was prepared in this work. Analysis results show that the Laves phase has higher hardness and resistance to deformation, while the FCC phase exhibits higher plastic deformation ability. The tribological properties of the coating material under both room- and high-temperature wear conditions are improved by the synergistic effect of the hard Laves phase and the ductile FCC phase. Moreover, the transition effect of the intermediate eutectic layer alleviates the stress concentration during the friction process, and the oxide film shows a strong wear resistant under the room-temperature wear condition. [Display omitted] • The HEA wear-resistant coating with Laves phase and hardness gradient distribution was prepared in situ. • The FCC-Laves dual-phase structure with soft and hard cross has good strength and toughness. • The wear mechanism of W-CC at room temperature is three-body wear with slight oxidation. • The high hardness of Laves phase and the oxidation of FCC phase improve the high temperature anti-friction performance. • When the oxide film is worn at room temperature, it shows a strong wear protection effect. [ABSTRACT FROM AUTHOR]

Refractory High Entropy Alloys (RHEAs) have great potential for ultra-high temperature environments and are one of the potential candidates for high temperature applications. In this work, a series of Al 0.75 NbNi 0.1 Mo 0.1 Cr 0.5 Si 0.1 W x (x = 1, 1.5, 2) refractory high-entropy alloy coatings is prepared by laser cladding technology. The microstructure and oxidation behavior at 1000 °C and effect of changing oxidation temperature on oxidation behavior of the coatings is investigated. The results show that Al 0.75 NbNi 0.1 Mo 0.1 Cr 0.5 Si 0.1 W x RHEA coatings are composed of BCC, W 5 Si 3 and AlNi 3 phases. The microstructure of RHEA coatings exhibits dendritic-cellular grain transition with increasing W content. The specific mass gains of coatings due to high temperature oxidation is 65.5 mg/cm2 for x = 1, 8.5 mg/cm2 for x = 1.5 and 14.5 mg/cm2 for x = 2 at 1000 °C for 70 h. After oxidation in 1000 °C for 70 h under air atmosphere, a protective oxide film (such as Al 2 O 3 and SiO 2) adhere around NiWO 4 on the alloy coatings, which impedes the inward oxygen transport. Moderate addition of W elements is found to be beneficial in enhancing the oxidation resistance of coatings. However, excessive addition of W elements prevents the formation of protective oxides (Al 2 O 3 and SiO 2) and reduces the high-temperature oxidation resistance of the coatings. With the increase of oxidation temperature, simple oxides are significantly reduced while compound oxides are increased on the surface of the coatings. The surface appears hill-shaped oxides with the increase of NiWO 4. • The increase in W element content promotes the formation of cellular crystal structure. • The combination of SiO 2 and Al 2 O 3 can form a dense oxide layer. • Al diffuses rapidly to the top of the oxide layer during the oxidation process. • Part of the elemental distribution shows a continuous horizontal band in the oxide film. [ABSTRACT FROM AUTHOR]

Highlights • Protective material assisted underwater wet laser cladding was proposed. • The promotion mechanism of protective material is studied in detail. • The 316L coatings without obvious defects were successfully prepared underwater. • The performance of 316L coatings was improved by using protective materials. Abstract In this paper, a new underwater processing technology was proposed, called underwater wet laser cladding (UWLC). Protective material assisted UWLC was provided to increase the processed water depth of UWLC and promote the quality of UWLC coatings without changing the laser power. For comparison, the direct UWLC and protective material assisted UWLC were performed under various water depths with uniform experimental parameters. The UWLC processes under water depth of 15 mm were studied in detail. The blue-purple plasma appears during the direct UWLC processes leading to the failure of UWLC. The mental plume appears during the protective material assisted UWLC processes. In addition, the protective material assisted UWLC is more likely to succeed by increasing the content of protective material. The 316L stainless steel coatings were successfully prepared by protective material assisted UWLC. The microstructure of the 316L coatings by protective material assisted UWLC is well in agreement with 316L coatings prepared by laser cladding in air, while the 316L coating by direct UWLC is mainly composed of columnar dendrites. Moreover, the corrosion resistance of 316L coating by direct UWLC in 3.5 wt.% NaCl solutions is improved markedly by applying protective material. The energy loss in the process of underwater wet laser processing is significantly reduced through the protective material assisted method, which provides meaningful theoretical support for underwater wet laser repair. [ABSTRACT FROM AUTHOR]

• Heat treatment led to the growth of the grain size of the NiTi phase in the coatings. • The spinodal decomposition occurred in the NiTi 2 phase by cryogenic treatment. • The post-treatments affected the phase transformation characteristics of the coatings. • The coating after cryogenic treatment has the highest microhardness. YF 3 modification NiTi-Si coatings were prepared on Ti-6Al-4 V substrate by laser cladding. On this foundation, heat and cryogenic treatments were conducted respectively to explore the effect of after-treatment procedures. The consequence of post-treatment methods on the phase constituent, microstructure, mechanical property, and phase transition characteristics of post-treatment coatings was studied. The phase constituent of the three coatings was roughly the same, composed of NiTi and NiTi 2. The results show that heat treatment led to the coarsening of NiTi phase grain size and increased Si content in the NiTi 2 phase. In addition, due to the rapid cooling rate provided by cryogenic treatment, the spinodal decomposition of the Si element occurred in the NiTi 2 phase. Furthermore, the post-treatments also affected the phase transformation characteristics of the coatings. As the temperature of martensitic transformation is sensitive to the content of Ni element in the NiTi phase, the M s and M f points of the heat treatment coating were slightly higher. In contrast to the heat treatment, the M s and M f points of the cryogenic treatment coating decreased. The microhardness of the three coatings is as follows: cryogenic treatment > original coatings > heat treatment. The cryogenic treatment coating has the highest microhardness due to grain refinement and spinodal decomposition. In addition, the cryogenic treatment coating showed the best wear morphology. [ABSTRACT FROM AUTHOR]

In this work, the CoNiAlBx coatings were successfully prepared on 38CrMoAl steel by laser cladding technology. The effects of B content addition on the microstructure, phase composition, microhardness, and tribological properties of the coatings were systematically analyzed. We found that CoNiAlB coatings were composed of β,γ,and FeB phase. TEM results shown that the orientation relationships between FeB phase and γ phase were [—112]FeB//[12]γ. With the addition of B, the microstructure changed from cellular morphology to dendritic morphology, then generated FeB phase, the transformation of microstructure led to the improvement of the coating: The microhardness improved from 380 Hv 0.3 to 790 Hv 0.3 , the friction coefficient reduced from 0.75 to 0.45, wear rate decreased from 3.59·10−14 m3 N−1 m−1 to 1.34·10−14 m3 N−1 m−1. The wear mechanism of CoNiAlB coatings was changed from only oxidation wear to only abrasive wear, exhibited excellent tribological property. [Display omitted] • FeB phase was appeared in CoNiAlB coatings due to the addition of B. • The crystallographic orientation relationships and d-spacing of two phases were analyzed. • The appearance of FeB phase improved the microhardness and friction property of the coating. • The microhardness was increased by ∼390 Hv0.3, wear rate decreased to 1/3 of the original. [ABSTRACT FROM AUTHOR]

[Display omitted] • The coherent interface of TiN/(Ni, Co)Ti 2 ensures higher interfacial bonding force. • In the interface zone, N-Co bond is a covalent bond with higher bonding energy. • Heterojunction with larger W ad confirms that failure site tends to occur at the interface near the coating side. TiN-reinforced high-entropy alloy (HEA) coating has been widely applied in the field of surface strengthening. The isotropy, high modulus, high hardness and excellent chemical stability of TiN combine with the ductility of HEA, so as to significantly optimize the wear resistance and service life of HEA coating. In this work, in-situ TiN-reinforced AlCoCrCuNiTi HEA coating was synthesized, and TiN phase had a coherent interface between (Ni, Co)Ti 2 phase. Besides, the relationship between reinforcing behavior of TiN and wear resistance of HEA coating was analyzed based on experimental observation and first-principle model. [ABSTRACT FROM AUTHOR]

In this study, graphite/CeO 2 reinforced Co-based coatings were prepared on the surface of HT250 gray cast iron by the plasma cladding technology. The influence of graphite and CeO 2 on microstructure and properties of Co-based coatings were studied in detail. The phase composition and microstructure of coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The micro-hardness, wear resistance and corrosion resistance of coatings were evaluated using micro-hardness tester, friction wear testing machine and electrochemical workstation. The results showed that the addition of graphite was beneficial to enhance micro-hardness and wear resistance of coatings, but there was still shortcoming in improving corrosion resistance. On this basis, CeO 2 was further introduced together with graphite. The microstructure of coatings was found to be more uniform under the synergy of graphite and CeO 2. The micro-hardness increased from 684.5 HV 0.3 (coatings adding graphite alone) to 767.4 HV 0.3 (coatings adding graphite and CeO 2), which was 29.7% and 115.1% higher than that of as-cladded coatings and substrate respectively. More importantly, the corrosion current density of coatings was an order of magnitude lower than that of coatings adding graphite alone. Therefore, adding CeO 2 together with graphite was beneficial to the synergistic improvement of micro-hardness, wear resistance and corrosion resistance of Co-based coatings. • Co-based coatings adding graphite/CeO 2 were prepared by plasma cladding. • The microstructure of coatings became uniform after adding graphite/CeO 2. • Wear resistance of coatings adding graphite together with CeO 2 was enhanced. • Corrosion resistance of coatings adding graphite together with CeO 2 was raised. [ABSTRACT FROM AUTHOR]