Search

The effect of lamellar orientation on the deformation behavior of eutectic high entropy alloy at the micrometer scale, and the roles of two rarely explored laminate orientations (i.e., the lamellar orientation at ∼ 0° and 45° angles with the loading direction) in regulating size-dependent plasticity were investigated using in-situ micropillar compression tests. The alloy, CoCrFeNiTa0.395, consists of alternating layers of Laves and FCC phases. It was found that the yield stress of the 0° pillars scaled inversely with the pillar diameters, in which the underlying deformation mode was observed to transform from pillar kinking or buckling to shear banding as the diameter decreased. In the case of the 45° pillars with diameters ranging from 0.4 to 3 μm, there exists a ‘weakest’ diameter of ∼ 1 μm, at which both constraint effect and dislocation starvation are ineffective. Irrespective of the lamellar orientations, the strain hardening rate decreased with decreasing pillar diameter due to the diminishing dislocation accumulation that originated from the softening nature of large shear bands in the 0° pillars, and the enhanced probability of dislocation annihilation at the increased free surfaces in the 45° pillars. The findings expand and deepen the understanding of the mechanical size effect in small-scale crystalline materials and, in so doing, provide a critical dimension for the development of high-performing materials used for nano- or microelectromechanical systems.

TiC nanoparticle-reinforced Mg−4Zn−0.5Ca matrix nanocomposites were processed by combining multidirectional forging (MDF) and extrusion (EX). The grain size of the nanocomposite after MDF+EX multi-step deformation was significantly decreased compared with that processed only by MDF. The average size of the recrystallized grains gradually increased after EX with increasing the number of MDF passes at 270 °C. However, the grain size significantly decreased by MDF processing at 310 °C. Both fine and coarse MgZn2 phases appeared in the (MDF+EX)-processed nanocomposites, and their volume fractions gradually increased with increasing the number of MDF passes before EX. Ultrahigh tensile properties (yield strength of ~404 MPa, ultimate tensile strength of ~450.3 MPa and elongation of ~5.2 %) were obtained in the nanocomposite after three MDF passes at 310 °C followed by EX. This was attributed to the refinement of the recrystallized grains, together with the improved Orowan strengthening provided by the precipitated MgZn2 particles that were generated by MDF+EX multi-step deformation.

The mechanical size effect of nanostructured, dual-phase CrCoNi medium-entropy alloy (MEA) was investigated by combining in-situ micro-compression testing with post-mortem electron microscopy analysis. The alloy possesses a superior yield strength up to ~4 GPa, primarily due to its hierarchical microstructure including column nanograins, preferred orientation, a high density of planar defects and the presence of the hexagonal close packed (HCP) phase. While the yield strength of the alloy has shown size-independency, the deformation behaviour was strongly dependent on the sample size. Specifically, with decreasing the pillar diameters, the dominant deformation mode changed from highly localized and catastrophic shear banding to apparently homogeneous deformation with appreciable plasticity. This transition is believed to be governed by the size-dependent critical stress required for a shear band traversing the pillar and mediated by the competition between shear-induced softening and subsequent hardening mechanisms. In addition, an unexpected phase transformation from HCP to face-centered cubic (FCC) was observed in the highly localized deformation zones, leading to strain softening that contributed to accommodating plasticity. These findings provide insights into the criticality of sample dimensions in influencing mechanical behaviors of nanostructured metallic materials used for nanoelectromechanical systems.

A new Mg-2.2 wt% Zn alloy containing 1.8 wt% Ca and 0.5 wt% Mn has been developed and subjected to extrusion under different extrusion parameters. The finest (~ 0.48 μm) recrystallized grain structures, containing both nano-sized MgZn2 precipitates and α-Mn nanoparticles, were obtained in the alloy extruded at 270 °C/0.01 mm s−1. In this alloy, the deformed coarse-grain region possessed a much stronger texture intensity (~ 32.49 mud) relative to the recrystallized fine-grain region (~ 13.99 mud). A positive work hardening rate in the third stage of work hardening curve was also evident in the alloy extruded at 270 °C, which was related to the sharp basal texture and which provided insufficient active slip systems. The high work hardening rate in the fourth stage contributed to the high ductility extruded at 270 °C/1 mm s−1. This alloy exhibited a weak texture, and the examination of fracture surface revealed highly dimpled surfaces. The optimum tensile strength was achieved in the alloy extruded at 270 °C/0.01 mm s−1, and the yield strength, ultimate tensile strength and elongation to failure were ~ 364.1 MPa, ~ 394.5 MPa and ~ 7.2%, respectively. Fine grain strengthening from the recrystallized fine-grain region played the greatest role in the strength increment of this alloy compared with Orowan strengthening and dislocation strengthening in the deformed coarse-grain regions.

FeMnCoCrC high entropy alloy coatings were deposited onto high speed steel substrates using magnetron sputtering under a range of substrate bias voltages. These coatings exhibited a novel α-Mn crystal structure and highly refined columnar grains. Higher bias voltages promoted strong texture development through favouring (332)-oriented grains along the coating growth direction, as well as the formation of well-defined twin boundaries. The -80 V coating possessed a high hardness (~12.5 GPa), more than triple that of its fcc-structured bulk counterpart. This is attributed to the cumulative effects of the α-Mn structure, strong texture, grain refinement and the presence of twin boundaries.

To meet the needs of corrosion resistance and electrically conductivity for metallic bipolar plates that are employed in proton exchange membrane fuel cells (PEMFCs), a TiSiN nanocomposite coating was fabricated on to a Ti–6Al–4V substrate using reactive sputter-deposition through the double cathode glow discharge plasma technique. The microstructure of the TiSiN coating comprised nanocrystallite TiN grains embedded in an amorphous Si3N4 matrix. Electrochemical measurements were employed to investigate the corrosion behavior of the TiSiN coating in the simulated operating environments of a PEMFC, specifically 0.5 M H2SO4 solution containing different HF concentrations (namely 2, 4 and 6 ppm) at 70 °C pumped with H2 at the anode and air at the cathode. With increasing HF concentration, a higher corrosion current density and lower corrosion potential were observed from both the coating and the uncoated substrate, indicating that the addition of HF accelerated their corrosion rates under these conditions. Compared to the uncoated substrate, the TiSiN coating showed a markedly higher corrosion resistance at all HF concentrations. The passive film that formed on the TiSiN coating, with a resistance of the order of magnitude of ~107 Ω cm2, displayed good electrochemical stability and was less affected by changes in HF concentration. For the TiSiN coating, the values of interfacial contact resistance (ICR) were 14.7 mΩ cm−2 and 18.3 mΩ cm−2, respectively, before and after 2.5 h potentiostatic polarization with 6 ppm HF under cathodic conditions under a compaction pressure of 140 N cm−2. Both values are much lower than those for the bare substrate. Moreover, the TiSiN coating was shown to improve the hydrophobicity of Ti–6Al–4V that would help facilitate water management in the PEMFC operating environment. This coating, which exhibited excellent corrosion resistance, electro-conductivity and hydrophobicity, is therefore a promising material for protecting metallic bipolar plates from corrosive attack.

CeO2-x coatings were deposited under variable oxygen flow ratios (%fO2) onto Si substrates by reactive magnetron sputtering. Nanoindentation testing revealed an increase in the hardness, elastic modulus, H/E and H3/E2 ratio with increasing oxygen flow ratio, which in turn increased the adhesion and tribological performance of the coatings. Scratch testing yielded the highest critical load (LC2 = 28.8 N) and CPRS = 103 for the coating deposited with the highest oxygen flow ratio (57 %fO2). Cracking events during scratch testing were initiated by tensile forces behind the scratch stylus, which led to the formation of semi-circular ring cracks. As the normal load increased, transverse cracks emerged extending outwards from the scratch track towards the edge causing the exposure of substrate. Beyond LC2, severe spallation of the CeO2-x coatings led to coating failure. Furthermore, the specific wear rates of the CeO2-x coatings were determined to be within the ~10−15 m3/Nm range influenced by three-body abrasive wear. In-depth analyses from scratch and wear data indicates that these coatings possess good adhesion and durability.

The microstructure, texture and mechanical properties of a series of Mg–Zn–Ca–Mn alloys with three Zn/Ca ratios (2.63, 1.22 and 0.53, by weight ratio) were investigated. The dominant second phase changed from MgZn to Ca2Mg6Zn3 as the Zn/Ca ratio decreased from 2.63 to 1.22. With decreasing Zn/Ca ratio, the grain size of the as-cast alloys was significantly decreased, accompanied by an increase in the volume fraction of second phase. For as-extruded Mg–1.4Zn–2.6Ca–0.5Mn, the finest (~ 0.36 μm) recrystallized grain structures, containing both fine MgZn2 precipitates and α-Mn particles, were obtained at an extrusion speed of 0.01 mm/s. The texture of the deformed structure was more intense (~ 30.39 mud) relative to the recrystallized region (~ 8.33 mud). As the Zn/Ca ratio decreased, basal plane texture was weakened deriving from grain refinement following recrystallization. Superior mechanical properties with a yield strength of ~ 387.8 MPa and ultimate tensile strength of ~ 409.2 MPa were achieved in the Mg–1.4Zn–2.6Ca–0.5Mn alloy extruded at 270 °C at an extrusion speed of 0.01 mm/s. A number of factors were determined to contribute to strengthening including Hall–Petch effects from the fine recrystallized grains (contribution ~ 58.7%), dislocation strengthening of the deformed region (contribution ~ 29.3%) and Orowan strengthening (contribution ~ 12%).

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature.

CrAlNiN coatings were deposited on M2 tool steels using closed-field unbalanced magnetron sputtering. The coating microstructure transformed from coarse columnar grains to fine equiaxed grains with increasing Ni content, which was accompanied by a general decrease in both hardness (H) and elastic modulus (Er) values. The scratch tests showed both scratch toughness and adhesion strength were improved for coatings with higher ratios of H/Er and H3/Er2, whereby higher critical loads (LC1 and LC2) are required to initiate cohesive and adhesive failure. For the coatings with coarser columnar grains, the improved scratch toughness may be ascribed to tortuous crack deflection patterns. Improved wear resistance was found in the columnar grained coatings, which also exhibited the higher H/Er and H3/Er2 values. The inferior wear resistance in the equiaxed grained coatings was suggested to arise from these coatings being more susceptible to grain pull-out processes during abrasive sliding. The wear debris composed of tribo-oxides serving as additional capacity for load bearing via re-distribution of the applied stress. This was believed to boost the wear resistance of the coatings.

In this study, a β-Ta2O5 coating was deposited onto a Ti–6Al–4V substrate using a double-cathode glow discharge plasma technique. The hardness and elastic modulus values of the β-Ta2O5 coating were...

The development of hard, multi-layer coatings is an effective strategy to enhance the wear resistance of cutting tools and so extend their service life. In the present study, a sandwich structured TiN/g-TiSiN/TiSiN film (where a graded (g-) TiSiN layer with an increasing Si content from 0 to 10 at% was inserted as a transitional layer between the TiN layer and the TiSiN layer with a fixed silicon content of 10 at%) was prepared on to a M42 tool steel substrate. Its mechanical properties were compared to both a dual-layered TiN/g-TiSiN film and a monolithic TiN film. Nanoindentation testing, assisted by focused-ion-beam (FIB) microscopy, was employed to evaluate contact-induced deformation and the mode of fracture of these films. Indented regions created on samples by a 5 μm radius indenter were examined by transmission electron microscopy (TEM). Finite element analysis was used to model the stress distributions within these films and predict the regions where crack initiation and growth may occur. The deformation of the monolithic TiN film was found to be predominantly accommodated by shear sliding along columnar grain boundaries, leading to a lower resistance to deformation. For the bilayer TiN/g-TiSiN film, the g-TiSiN layer hindered the propagation of columnar cracks, however, this bilayer film exhibited a stress concentration together with radial cracks at the bottom of the film. Compared with the former two films, the sandwich-structured film that contained the graded TiSiN interlayer exhibited the highest resistance to contact damage. This is because the graded TiSiN interlayer altered the stress distribution in the film and lowered the overall stress concentration level. Keywords: TiSiN graded interlayer, Deformation mechanism, Nanoindentation, Finite element method

Ni incorporation is known to impose an intriguing influence on the composition, microstructure and mechanical properties of nitride-based coatings prepared by physical vapour deposition. This work examined the effect of varying Ni concentrations, controlled by NiCr alloy target current (INiCr), on the microstructures and properties of CrAlNiN coatings deposited on to M2 tool steel substrates. These coatings were deposited by closed field unbalanced magnetron sputtering (CFUMS). The composition and structure of the as-deposited coatings were investigated by XPS, XRD, FIB, and TEM. Residual stresses were determined using the XRD-sin2ψ method. Nanoindentation tests were performed to assess mechanical properties of the CrAlNiN outer layer. At relatively low INiCr values, below 3 A, the grains within CrAlNiN layer underwent refinement, yet maintained a columnar structure. At higher INiCr values, from 3 to 5 A, the columnar grains transitioned into more equiaxed grains. Hardness values of 25–28 GPa at low INiCr values were associated with the high residual compressive stress, solid solution hardening and grain refinement.

In this study, the authors have applied the “Oblique Cross Section” or OCS method, for the determination of the hardness changes in ion irradiated 316 stainless steel. The steel sample was irradiated with 1, 2 and 3 MeV He2+ ions. The samples were then polished at an angle of ∼15° from the ion irradiated surface, and the hardness on the cross-section was tested using nanoindentation in non-continuous stiffness measurement mode (non-CSM) at various distances from the ion irradiated surface. A profile of the hardness versus depth from the irradiated surface was thus obtained. It is shown that this method is quite suitable for single energy ion irradiation, when the energy is relatively high (≥1 MeV He in stainless steel), as it gives a better match between the damage peak and hardness peak locations than the “Top-down” method. The hardness profiles can be divided into two broad regions, viz. the pre-damage-peak plateau, and the peak hardness region. The results are contrasted with finite element (FE) models. The FE modelling shows formation of a secondary plastic zone in the unirradiated material beyond the irradiated layer, which was verified by cross-sectional transmission electron microscopy (TEM). Other aspects of the deformation below the nanoindents were also studied and illuminated by cross-sectional TEM. Thus, both experimental techniques and theoretical methods are employed here to elucidate the deformation processes and hardness measurement results, thereby establishing a more profound understanding of this relatively uncommon, but potentially powerful method for testing the mechanical property changes in ion irradiated materials, and of thin-layered materials in general.

To investigate the effects of Al alloying on the erosion-corrosion resistance of β-Ta5Si3, both a β-Ta5Si3 coating and an Al-alloyed β-Ta5(Si0.83Al0.17)3 coating were synthesized on a 316 substrate by the double cathode glow discharge technique. The phase constitution, composition and microstructure of the two coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The two coatings were composed of nearly rounded D8m-β-Ta5Si3 grains with an average size of ∼4 nm, and after the addition of Al, the preferred growth orientation for the β-Ta5Si3 coating changed from (4 0 0) to (0 0 2). The hardness, elastic modulus and contact damage resistance of the coatings were measured using a nanoindentation tester. The results showed that Al alloying improved the contact damage resistance of β-Ta5Si3 with only a slight decrease in hardness. The erosion-corrosion behavior of the two coatings was performed in a 3.5 wt% NaCl solution containing a 12 wt% concentration of silica sand under two phase slurry flow condition and in a 3.5 wt% NaCl solution under ultrasonic cavitation erosion conditions. This revealed that the Al alloyed β-Ta5Si3 has a higher resistance to both erosion-corrosion and ultrasonic cavitation erosion as compared to the binary β-Ta5Si3 coating.

A better understanding of structural coarsening mechanisms is of significance to effectively tackle structural instabilities in nanostructured materials. In this work, the microstructural evolution of nanostructured metallic powder particles during thermomechanical consolidation has been investigated to gain new insights into the evolution of reinforcing particles and matrix grains of a mechanically alloyed nanostructured Cu-5 vol.% Al2O3 composite powder extruded at either 300 or 900 °C, respectively. Unusual coarsening of Al2O3 particles and rapid growth of Cu grains occurred during extrusion at a low temperature of 300 °C; meanwhile the formation of elongated micron Cu grains was observed at a high extrusion temperature of 900 °C. Both the applied shear stress/strain and the sizes and distribution of second-phase nanoparticles were found to determine the grain structure of the Cu matrix after extrusion. Specifically, the shear stress-induced dynamic and particle-stimulated recrystallization and the movement of the Al2O3 particles are respectively responsible for the low-temperature rapid growth of the Cu grains and the coarsening of the Al2O3 particles. The development of the elongated micron Cu grains during extrusion at 900 °C is associated with anisotropic grain boundary migration and particle re-distribution.

To alleviate cavitation erosion damage in hydraulic machines operating, a TiCN coating was prepared on to a TC11 substrate using a double-cathode glow discharge technique. The TiCN coating consist of equiaxed nanocrystals with the grain size of about 10 nm. Under cavitation condition, the TiCN coated sample showed higher corrosion resistant, lower local corrosion tendency and less weight loss than the substrate. The excellent cavitation erosion resistance of the TiCN coating was ascribed to its nanoscale equiaxed grain structure and the good self-healing capability of the passive film, which was confirmed through both subsurface cross-sectional observation and electrochemical measurements.

Incorporation of Ni can have intriguing effects on the phase composition, microstructure and mechanical characteristics of titanium nitride coatings synthesised by physical vapour deposition. In this investigation, a series of TiNiN coatings, with varying Ni contents, were deposited onto Si substrates using the filtered vacuum cathodic arc method by changing the composition of the TiNi alloy target. The aim of this study is to investigate the effect of Ni concentration (as controlled by composition of the target material) on the phase composition, microstructure and mechanical behaviour of these TiNiN coatings. A transition from a fine columnar structure, at low Ni contents (∼2 at.%), to a much finer equiaxed structure at higher Ni concentrations (≥4 at.%) was noted. A maximum hardness value of ∼ 32.4 GPa was attained in the coating containing a Ni concentration of ∼ 4 at.%, which was ascribed to the synergistic effects of solid solution hardening, grain refinement and the presence of a high compressive residual stress. In addition, the density of macroparticles generated in these TiNiN coatings during arc deposition was shown to be inversely related to the melting temperature of the target material.

Medium entropy alloys (MEAs), such as CoCrNi, have been demonstrated to combine high hardness and excellent ductility, thereby outperforming many high entropy alloys reported to date. In this study, a multilayered CoCrNi/Ti coating was deposited onto a M2 steel substrate using a DC magnetron sputtering system. Columnar grains can be observed in both the CoCrNi and Ti layers. A high density of periodic twin boundaries, aligned in a direction normal to the growth direction, was also observed within the columnar CoCrNi grains. Moreover, different crystal structures were identified for different CoCrNi layers. The outermost CoCrNi layer exhibited a FCC structure, whilst in contrast, both the middle and bottom CoCrNi layers exhibited a BCC structure. It was assumed that Shockley partial dislocations were responsible for the FCC to BCC transition occurring in both the bottom and middle CoCrNi layers. A high hardness of ~7.6 GPa and elastic modulus of ~233 GPa were determined for this multilayered coating by nanoindentation testing. Further, extraordinary damage tolerance was found in the multilayered CoCrNi/Ti coating under indentation loading. The steady shear banding behaviour during deformation may benefit energy dissipation and promote structural plasticity.

An in-depth investigation of the structure, properties, scratch adhesion characteristics of graded Cr-CrN-Cr(1-x)AlxN coatings synthesized onto M42 steel substrates using closed – field unbalanced magnetron sputtering (CFUBMS) was carried out. Advanced microscopy (scanning and transmission electron microscopy), focused ion beam (FIB) imaging, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and micro–scratch tests was used to investigate the microstructure, mechanical properties and scratch performance as a function of Al content. FIB and TEM investigations revealed that the coatings exhibited a distinct structure; i.e., an adhesive Cr layer, a CrN transition layer and a graded CrAlN top layer with a face centered cubic (FCC) B1 structure. A columnar morphology was exhibited by the coatings and the dimensions of the columnar grains decreased with increasing Al content. Residual stress measurements, obtained from the XRD – sin2ψ method, revealed increasing compressive stresses with increasing Al content. Furthermore, nanoindentation tests showed an increase in mechanical properties, fracture toughness index (H/E) and plastic deformation resistance (H3/E2) as the Al content increased, accompanied by a decrease in the critical load, LC, during scratch testing implying a decrease in scratch toughness.

In this work, the structural stability, mechanical properties and electronic structure of β-Ta5Si3 with different Al additions were first predicted by first-principles density functional calculations. The binary and ternary β-Ta5Si3 are thermodynamically and mechanically stable, but their thermodynamic stability slightly decreases with Al additions. The elastic constants, bulk modulus (B), shear modulus (G) and Young's modulus (E) of Al-alloyed β-Ta5Si3 were calculated. According to the criteria involving the bulk/shear modulus (B/G ratio) and Poisson's ratio, the D8m-structured ternary Ta5Si3, where two Si4a atoms are replaced by two Al atoms, shows improved ductility. Based on the theoretical analysis, the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating was deposited onto polished Ti-6Al-4V alloy substrates by double cathode glow discharge plasma method. The phase constitution, microstructure and composition of the coating were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The as-deposited coating was composed of nearly rounded D8m-β-Ta5Si3 grains with an average size of ∼4 nm and exhibited strong (002) and (400) preferred orientation. After Al alloying, the hardness and the compressive residual stress of the β-Ta5Si3 decreased, and the contact damage resistance and adhesion strength increased. The electrochemical corrosion properties of the β-Ta5(Si0.83Al0.17)3 nanocrystalline coating were compared to the binary β-Ta5Si3 coating and Ti-6Al-4V in 3.5 wt% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. It was shown that the corrosion resistance of the β-Ta5Si3 was improved through the substitution of Al for Si.

Graded Cr-CrN-Cr (1−x) Al (x) N coatings were synthesized onto M42 HSS substrates used in advanced machining operations by closed-field unbalanced magnetron sputtering (CFUBMS). The tribological behavior of these graded coatings was explored in detail by advanced electron microscopy, confocal laser scanning microscopy, nanoindentation and dry sliding wear tests. The presence and magnitude of residual stresses in these coatings were determined by the XRD – sin 2 ψ method, which revealed increasing compressive stresses with increasing Al content. The coating surface morphology, mechanical properties were determined prior to dry sliding wear by atomic force microscopy (AFM) and nanoindentation methods, which yielded decreasing surface roughness (R a ) as well as enhancement of hardness and modulus along with increase in H/E and H 3 /E 2 ratios with increasing Al content. Tribological investigation was performed with a pin-on-disc arrangement by keeping the sliding velocity (0.2 ms −1 ) and normal axial load (10 N) constant and varying the sliding distance. Specific wear rates of the order ~ 10 –17 m 3 N −1 m −1 were encountered for all coatings with the wear rates increasing as the Al content increased implying a decrease of wear resistance of the coatings. Abrasive wear has been found to be the dominant wear mechanism during dry sliding wear. Increasing modulus mismatch between coating and substrate can be mainly attributed to a decrease in wear resistance of the coatings.

To improve the durability and bioactivity of Ti–6Al–4V alloy used for medical implants, the β-Ta2O5 nano-crystalline coatings were introduced using double cathode glow discharge technique. The coating microstructure was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The coating exhibits an assembly of near-equiaxed grains, locally aligned normal to the coating surface. The β-Ta2O5 coating exhibits strong adhesion to substrate and a strong resistance to deformation and cracking under applied loads. Cells culture tests showed that the coating is more beneficial to the adhesion and proliferation of NIH-3T3 cells as compared to the uncoated alloy. In-vitro bioactivity was evaluated by immersion of the coating in simulated body fluids (SBF) for different periods up to 14 days at 37 °C. The results indicated that bioactivity of Ti–6Al–4V was dramatically improved after the deposition of β-Ta2O5, since the coating has a higher apatite forming ability than the Ti–6Al–4V substrate. Finally, the electrochemical behavior of the β-Ta2O5 coating after soaking in SBF at 37 °C for 0, 3, 7, and 14 days was studied through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). EIS measurements also confirm that the presence of a hydroxyapatite layer on the coating becomes thicker and denser during soaking in SBF. Moreover, the coating exhibits better corrosion resistance than the bare alloy. Hence, the β-Ta2O5 coating is a promising candidate coating for protection of orthopedic implants with enhanced bioactivity and corrosion resistance.

Medium entropy alloys (MEA) are defined as alloys consisting of three equiatomic elements, such as CoCrNi. MEAs are reported to have superior mechanical properties and high thermodynamic stability, as well as excellent fracture toughness at cryogenic temperatures. Here, we investigate a series of equiatomic medium entropy alloy coatings, containing three elements, Co, Cr, Ni. These coatings were deposited onto M2 steel substrates with a range of coating thicknesses using a DC magnetron sputtering system using a CoCrNi alloy target (1:1:1 at.%). The microstructure and mechanical properties were examined by a number of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. XRD analysis showed that the coatings were dominated by a fcc CoCrNi phase, with a smaller amount of hcp Co. TEM analysis demonstrated that the elongated grains contained a high density of {111} nanotwins. In addition, the residual stresses in the coatings were analysed using X-ray diffraction by adopting the conventional sin2ψ method. A high hardness value, ~ 10 GPa, was determined by nanoindentation of these coatings. Exceptional damage-tolerance was also found in these coatings under contact loading. It is believed that the nanotwinned structure is responsible for the high hardness and damage tolerance observed in the new coatings.

Ultrafine-grained Cu–5 vol%Al2O3 nanocomposite rods were fabricated by a combination of high-energy mechanical milling of Cu and Al2O3 powders and powder compact extrusion at 300, 500, 700 and 900 °C. The extruded rods were investigated to evaluate microstructures, mechanical properties, fracture behavior and electrical resistivity. It was found that the extrusion temperature has a pronounced effect on Cu grain growth, Al2O3 particle coarsening and particle distribution. High-temperature extrusion leads to directional coarsening of certain grains. As such, a heterogeneous matrix structure of large elongated and equiaxed grains is created, and this unique matrix structure brings beneficial effects in tensile ductility and electrical resistivity. While Al2O3 dispersions in the matrix improve the overall performance of the nanocomposite, an incorrect selection of the extrusion temperature may have detrimental effects on yield strength and resistivity. Tensile fractography investigation shows that the presence of Al2O3 results in failures along grain boundaries. This study also provides a framework for modeling the mechanical and electrical properties of such complex matrix structures. Modeling tools/formulae can be used to predict mechanical/electrical properties via microscopic characteristics and hence can also be used to understand the effect of processing variables.

TiSiN coatings were synthesised onto AISI M42 tool steel substrates via closed field unbalanced magnetron sputtering ion plating, using bias voltages of −40 and −50 V. The aim of this study is to investigate the underlying deformation mechanisms of TiSiN coatings, prepared at two different substrate bias voltages, following scratching, wear and indentation tests. A graded columnar microstructure evolved in these coatings. A hardness value of ~30.2 GPa was determined in the coating prepared at the lower bias voltage (i.e., −40 V), which was correlated to the fine nanocomposite structure and the presence of a high compressive residual stress. Of note, for the coating deposited at −40 V enhanced scratch adhesion strength, i.e., higher critical loads (Lc1 and Lc2) against cohesive and adhesive failure were determined with the higher H/Er and H3/Er2 values. The improved scratch resistance was ascribed to the hierarchical structure that hindered crack propagation in the TiSiN coatings during progressive loading. An approximately 21% decrease in wear rate was obtained at the lower bias voltage, which was attributed to the slightly lower Si concentration (~8.3 at.%) and, in turn, higher hardness. Deformation behaviour under indentation loading was dominated by shear sliding along the columnar grain boundaries.

The indentation response, scratch behaviour and wear performance of binary TiN and ternary TiSiN coatings under a variety of loading conditions were comparatively studied. The coatings were fabricated onto M42 steel substrates via closed field unbalanced magnetron sputtering ion plating. A maximum hardness value of ~40 GPa was obtained for one of the TiSiN coatings as compared to ~28 GPa for TiN. This was attributed to the nanocomposite structure, grain refinement, solid solution hardening and the higher compressive residual stress in the ternary coatings. The damage resistance of both the TiN and TiSiN coatings under indentation loading was governed by the dampening effects of sliding or shearing of the columnar grains along the grain boundaries coupled with the coatings’ respective mechanical characteristics. Improved scratch adhesion properties (i.e., higher LC1, LC2 and CPR values) were also observed for the TiSiN coatings that were underlain by their superior mechanical properties along with the graded structure, promoting the capacity to resist crack formation and delamination. Lower wear rates for the TiSiN coatings during dry sliding were found to be consistent with their higher H/Er, H3/Er 2 and We values.

A CoCrFeNiTiMo high entropy alloy (HEA) coating was prepared onto a Ti-6Al-4 V substrate using the double cathode glow discharge method. This innovative HEA coating exhibited a duplex phase microstructure, composed of a BCC-structure solid solution phase and a tetragonal structured intermetallic phase, Co2Mo3, and possessed high hardness, high load bearing capacity and strong adhesion to the substrate. The HEA coating underwent much less severe cavitation erosion damage in a 3.5 % NaCl solution relative to Ti-6Al-4 V according to the results of electrochemical measurements and the cumulative weight loss, because of its unique combination of excellent mechanical properties and high corrosion resistance.

To combat cavitation erosion-corrosion damage inflicted upon mechanical components in marine applications, NbC nanocrystalline coatings were synthesized by a double cathode glow discharge assisted reactive sputtering method. The coating comprised equiaxed B1-NaCl-type NbC grains with an average grain size of ~10 nm and a strong NbC (200)-oriented texture by TEM observation. The electrochemical response of the coating exposed to a 3.5 wt% NaCl solution with and without the influence of cavitation was monitored by electrochemical noise (EN) measurements, in addition to open circuit potential and cyclic polarization. To differentiate the cavitation erosion-corrosion resistance between the NbC coating and a reference Ti-6Al-4V alloy substrate, the EN data were further evaluated, based on a series of analyses, including frequency domain, time-domain, shot noise and wavelet transform. For the Ti-6Al-4V alloy, with increasing output power, the noise resistance (Rn) and slope (k values) of potential and current power spectral density (PSD) decreased, and the average charge in each corrosion event (q) and the total energy of d series crystals increased markedly at the output power levels above 400 W, indicative of a high propensity of pitting corrosion for this alloy. Conversely, the Rn and k values for the NbC coating were found to be almost independent of the output power, and the charge of the corrosion event and the total energy of the d series crystals were much lower than that of the Ti-6Al-4V alloy. The findings garnered from analyzing the EN signals is in accord with what were uncovered from examining the eroded surface morphologies, suggesting that the EN method might offer a reliable and effective tool to probe the cavitation erosion-corrosion behavior in developing durable material.

Substrate bias is one of the many factors, influencing the microstructure and, thus, properties of physical vapor deposition coatings. The aim of this study is to investigate the effect of varying substrate bias on the microstructure and mechanical properties of TiAlSiN coatings deposited on M42 tool steel substrates at 500 °C by a pulsed-DC close-field unbalanced magnetron sputtering system (CFUBMS). The microstructure of the as-deposited coatings was characterized by a range of techniques, including transmission electron microscopy, glancing angle X-ray diffraction and X-ray photoelectron spectroscopy. In addition, nanoindentation measurements were conducted to evaluate the mechanical properties of these coatings. It was found that an increase in substrate bias imposed minimal effects on the composition of the as-deposited TiAlSiN coatings, but had a significant influence on both the phase composition and microstructure of the coatings. As the substrate bias voltage increased from − 40 to − 80 V, a transition from zone-2 type structure to zone-3 type structure was observed, together with a reduction of the width of columnar grain from ~ 180 to ~ 60 nm. There was also a structural transition from a mixed fcc TiN + fcc AlN phases into a single fcc TiAlN phase as the negative substrate bias was increased above 60 V. Hardness and modulus were observed to increase with increasing bias voltage. Solid solution hardening and Hall-Petch effects are believed to be responsible for the hardness improvement.

In this study, the effects of Ti substitutional additions on the mechanical and electrochemical corrosion behavior of β-Ta 5 Si 3 were investigated through both theoretical calculation and experiment. Initially, first-principles calculations, based on density functional theory, were used to guide compositional design, according to the calculation of mechanical parameters (for example, bulk modulus, shear modulus, Young's modulus, the shear modulus/bulk modulus ratio and Poisson's ratio). This analysis showed that optimum mechanical performance may be achieved through Ta atoms in the Ta 20 Si 12 unit cell being replaced by two Ti atoms. Subsequently, both the binary β-Ta 5 Si 3 coating and the optimized Ti-alloyed β-Ta 5 Si 3 coating (i.e., β-(Ta 0.902 Ti 0.098 ) 5 Si 3 ) were deposited onto Ti–6Al–4V substrates using a double cathode glow discharge plasma method. Both as-deposited coatings exhibited a tetragonal crystal structure with fine equiaxed grains ∼4 nm in diameter. The mechanical properties of the coatings were determined by both nanoindentation and Vickers indentation techniques, and their electrochemical corrosion behavior was examined by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis in naturally aerated 0.9 wt.% NaCl solution at 37 °C. These investigations showed that the addition of Ti significantly improved the damage resistance of β-Ta 5 Si 3 , with little negative impact on its corrosion resistance.

Ultrafine grained AA6063-SiCnp nanocomposites with 1, 5 and 10 vol.% SiCnp have been fabricated by a novel powder metallurgy process. This process combines high energy ball milling of a mixture of 6063 alloy granules made from machining chips and SiC nanoparticles and thermomechanical powder consolidation by spark plasma sintering and hot extrusion. The microstructure and tensile mechanical properties of the samples were investigated in detail. Increasing the SiC nanoparticle content from 1 to 10 vol.%, the yield strength and ultimate tensile strength increased from 296 and 343 MPa to 545 and 603 MPa respectively, and the elongation to fracture decreased from 10.0%, to 2.3%. As expected, a higher SiC nanoparticle content generates a stronger inhibiting effect to grain growth during the thermomechanical powder consolidation process. Analysis of the contributions of various strengthening mechanisms shows that a higher SiC nanoparticle content leads to a higher contribution from nanoparticle strengthening, but grain boundary strengthening still makes the largest contribution to the strength of the nanocomposite. When the SiC nanoparticle content increased to 10 vol.%, the failure of the nanocomposite was initiated at weakly-bonded interparticle boundaries (IPBs), indicating that with a high flow stress during tensile deformation, the failure of the material is more sensitive to the presence of weakly-bonded IPBs.

The scratch adhesion and tribological behavior of graded Cr/CrN/CrTiN coatings deposited on tool steels by closed-field unbalanced magnetron sputtering (CFUBMS) by varying the Ti sputtering current (I Ti ) was investigated in detail. The hardness and modulus of these coatings were determined prior to scratch and wear testing using nanoindentation methods, which yielded increasing hardness, modulus, and H/E and H 3 /E 2 ratios with increasing Ti content. Progressive loading scratch tests from 1 N to 150 N at a speed of 4 mm/min were carried out using a Rockwell diamond indenter at a loading rate of 100 N/min on a 6 mm linear track. The dry sliding wear tests with a pin-on-disc arrangement under various wear parameters (sliding velocity, sliding distance and normal load) were carried out to investigate the tribological response of these graded coatings. The scratch test yielded the highest critical loads, i.e. L C1 =~141 N and L C2 =~149 N and a high scratch crack propagation resistance parameter (CPR S )=~1074 for the coating with the lowest Ti content. The initial failure of the coatings was due to conformal cracking leading to significant plastic deformation before complete substrate exposure. The coatings also yielded wear rates of the order ~10 –16 m 3 /N m with abrasive wear as the dominant wear mechanism. Both the scratch toughness and wear resistance of the coatings decrease with increasing Ti content. The decrease in scratch toughness, wear resistance and damage tolerance of these coatings can be attributed to increasing modulus of these coatings as the Ti content increases, implying that a modulus mismatch exists between coating and substrate.

Conventionally, nanostructured ceramics are susceptible to brittle fracture due to processing-induced flaws such as porosity. In this study, the microstructure and deformation behavior of two nanostructured Ta2N coatings, one with a fully-dense structure and the other with a nanoporous structure, both prepared by double cathode glow discharge technique, were investigated. It was found that the elastic modulus of the Ta2N coatings is more sensitive to the presence of nanoporosity than their hardnesses. Compared with the fully-dense Ta2N coating, the nanoporous Ta2N coating exhibits a marked increase in the damage tolerance, as well as a good potential for wear-resistance.

In this study, we observed a strong strain rate dependence of tensile ductility for an ultrafine grained Cu-5vol%Al2O3 nanocomposite prepared by powder compact extrusion. This dependency of tensile ductility of the material to strain rate, combined with detailed materials characterization, suggests that the strain at which nearly ideal plastic deformation occurs is associated with the magnitude of the flow stress. The measured apparent activation volume and observed alignment of Cu grains suggest that plastic deformation during this nearly perfect deformation stage is dominated by the co-operative grain boundary sliding.

A novel wear and corrosion resistant β-Ta5Si3 coating was deposited onto a Ti-6Al-4V substrate using a double glow discharge plasma method. The structure of the coating was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and its mechanical properties were determined by a combination of nanoindentation and scratch testing. Dry sliding wear behavior of the β-Ta5Si3 coating was evaluated using a ball-on-disc tribometer. Further, the electrochemical behavior of the β-Ta5Si3 coating was assessed through immersion in a 3.5 wt% NaCl solution. The as-deposited coating consisted of equiaxed tetragonal D8m-structured Ta5Si3 exhibiting a ∼4 nm grain size with a pronounced (400) preferred orientation. The specific wear rates of the β-Ta5Si3 coating were ∼10−6 mm3/(N m), which is two orders of magnitude better than that of the uncoated substrate under the same test conditions. This is because the coating exhibits higher values of H/E. Finally, the corrosion resistance of the coating was improved relative to bare alloy in this solution. Mott-Schottky analysis showed that the passive film formed on the coating during corrosion testing behaved like an n-type semiconductor. The carrier density was one order of magnitude less than that formed on bare substrate in the same solution. In light of this, the β-Ta5Si3 nanocrystalline coating is expected to provide protection to metallic components against wear and corrosion and as such extend their service life.

Graded Cr/CrN/CrTiN coatings were deposited onto M42 HSS tool steel substrates by close field unbalanced magnetron sputtering (CF-UBMS). The effects of varying Ti sputtering current, I Ti , on the chemical composition, crystal structure, morphology and mechanical properties of the graded Cr/CrN/CrTiN coatings were investigated via XPS, XRD, AFM, electron microscopy techniques and nanoindentation tests. XPS and XRD results indicate that the CrTiN layer had formed as a solid solution nitride with fcc B1 type phase. The layers in the coatings were distinct and all the coatings exhibited a columnar microstructure, which decreased in size as the Ti content increased due to the increase in the Ti sputtering current. The mechanical properties of these coatings were determined by nanoindentation tests, which revealed an increase in hardness and modulus as the Ti content increased. However, the damage resistance of the coatings decreased with increasing Ti concentration. Furthermore, deformation behaviour of the coatings at high loads revealed a good resistance to plastic deformation and good adhesion strength for all coatings. These improvements, and the increase in hardness, are attributed to a combination of a decrease in grain size and a formation of solid solution nitride.

In this study, two tantalum nitride-based coatings were synthesized onto Ti-6Al-4V substrates with two different Ar/N 2 flux ratios using a form of plasma assisted bias sputtering deposition termed the double cathode glow discharge deposition technique. Their microstructures and mechanical properties were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy and nanoindentation tests. At a low nitrogen partial pressure, the tantalum nitride coating consists of a hexagonal Ta 2 N phase, with a preferred (101) orientation, while at a high nitrogen partial pressure, the as-deposited coating is composed of a face-centered cubic (fcc) TaN phase with a strongly (200) oriented texture. The two as-deposited coatings exhibited striated nanostructured composed of equiaxed grains about ~ 10 nm in diameter, embedded with an array of homogenously distributed nanopores. The mechanical properties and damage resistance of the coatings were evaluated by nanoindentation techniques. The hardness and elastic modulus of the Ta 2 N coating was higher than those of the TaN coating, indicating that the Ta 2 N coating may offer better protection ability for the underlying metal substrate under load-bearing conditions. In addition, the presence of nanopores is beneficial to the contact damage resistance for both coatings.

NiCrN coatings, with varying Ni contents, were deposited on AISI M2 tool steel substrates using closed field unbalanced magnetron sputtering ion plating (CFUMSIP) where the NiCr target current (INiCr) was varied. An evolution of the graded columnar structures was found in these coatings, where increased incorporation of nickel induced grain refinement, as well as a reduction in values of both hardness (H), elastic modulus (Er) and elastic recovery (We). The influence of variations in the microstructure and mechanical properties on the adhesion behaviour and tribological performance of these NiCrN coatings was investigated. Scratch test results demonstrated that higher critical loads (Lc1 and Lc2) suppressed both cohesive and adhesive failure in coatings which exhibited higher values of H/Er, H3/Er2 and We. The failure mode under progressive scratch loading was controlled by the hierarchical design of the NiCrN coatings that enhanced adhesion and prevented crack propagation through the coating. For the coatings with lower Ni contents improved wear resistance was ascribed to not only higher H/Er, H3/Er2 and We values of these coatings, but also the graded interlayers that delocalised the stress distribution during dry sliding.

Ni feedstock particles were sprayed onto a grit-blasted stainless-steel substrate using the high-velocity oxy-fuel (HVOF) deposition process to analyse the flattening behaviour and bonding features of single splats on a roughly textured substrate surface. Both substrate surface features and splat morphologies were analysed through scanning electron microscopy (SEM), while cross-sectional features, including the nature of the splat-substrate interface, were characterised through focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The chemistry of both the splat and substrate were analysed by energy-dispersive X-ray spectroscopy (EDS) interfaced to both the SEM and TEM. It was observed that despite the surface roughness, fully melted splats demonstrated good flattening performance, as well as efficient bonding, with the substrate surface. Such bonding was revealed through elemental interdiffusion across the splat-substrate interface. The particle size and the location of impact on the rough surface played important roles in defining the melting state, flattening degree and, ultimately, adhesion of the particles. Fully melted splats derived from smaller feedstock particles demonstrated good flattening behaviour, comparable to that observed on polished surfaces. Such particles exhibited better bonding efficiency than the partially melted splats, which originated from relatively larger feedstock particles. The presence of residual alumina contaminants, retained after grit blasting, is also analysed and discussed.

Graphical abstract, Highlights • The NbN coating exhibits high contact damage resistance and good adhesion to Ti6Al4V substrate. • The intermediate product of NbOxNy is further oxidized to Nb2O5 with increasing cavitation erosion time. • The passive film formed on the coating has lower concentration and diffusivity of oxygen vacancies than Ti6Al4V. • The coating has lower formation energy of oxygen vacancies and the Cl− adsorption energies than Ti6Al4V., To alleviate the cavitation damage of metallic engineering components in hydrodynamic systems operating in marine environments, a NbN nanoceramic coating was synthesized on to a Ti-6Al-4V substrate via a double cathode glow discharge technique. The microstructure of the coating consisted of a ~13 μm thick deposition layer of a hexagonal δ′-NbN phase and a diffusion layer ~2 μm in thickness composed of face-centered cubic (fcc) B1-NaCl–structured (Ti,Nb)N. The NbN coating not only exhibited higher values of H/E and H2/E than those measured from NbN coatings deposited by other techniques, but also possessed good adhesion to the substrate. The cavitation erosion resistance of the NbN coating in a 3.5 wt% NaCl solution was investigated using an ultrasonic cavitation-induced apparatus combined with a range of electrochemical test methods. Potentiodynamic polarization measurements demonstrated that the NbN coated specimens demonstrated both a higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than the uncoated substrate. Mott-Schottky analysis, combined with the point defect model (PDM), revealed that, for a given cavitation time, the donor density (ND) of the passive film on the NbN coating was reduced by 1 ~ 2 orders of magnitude relative to the uncoated Ti-6Al-4V, and the diffusivity of the point defects (D0) in the passive film grown on the NbN coating was nearly one order of magnitude lower than that on the uncoated substrate. In order to better understand the experimental observations obtained from Mott-Schottky analysis and double-charge layer capacitance measurements, first-principles density-functional theory was employed to calculate the energy of vacancy formation and the adsorption energy for chloride ions for the passive films present on both the NbN coating and bare Ti-6Al-4V.

CrN coatings are often brittle and prone to abrupt failure. Incorporation of Ni is known to have significant effects on the chemical composition, microstructure and mechanical behaviour of chromium nitride coatings synthesised by PVD, and thus imparting toughness. In this study, a series of NiCrN coatings, with varying Ni concentrations, were deposited onto AISI M2 tool steel substrates through closed-field unbalanced magnetron sputtering ion plating (CFUMSIP). The phase compositions, microstructure, mechanical properties and deformation behaviour of the coatings were characterised by XRD, AFM, FIB, TEM/EDS and indentation testing. Residual stresses of these as-prepared coatings were measured by the XRD-sin2ψ method. A columnar structure was observed in all coatings, although grain refinement at higher NiCr target currents was noted. High hardness values of ~20 GPa were found in the coatings with the lower Ni (~5–14 at%) contents, associated with the effects of solid solution hardening, high compressive residual stress and grain refinement. Moreover, significant damage-tolerance, coupled with good hardness values (greater than ~12 GPa), was found in the NiCrN coatings deposited at INiCr ≥ 2 A. The presence of a metallic nickel-rich phase, together with nanoscale porosity, may contribute to stress dissipation and help maintain structural integrity.

Novel nanocomposites based on an AZ91 magnesium alloy matrix reinforced with a hybrid mixture of SiC and TiC nanoparticles with different nanoparticle contents (0.5% and 1%, by mass fraction) were fabricated by semi-solid stirring followed by ultrasonic vibration. Microstructural analysis indicated that the hybrid nanoparticles were uniformly distributed in the nanocomposite. The morphology of the Mg17Al12-based eutectic phase changed from plate-like, in the as-cast AZ91 alloy, to lamellar in the as-cast nanocomposites. The tensile strength and micro-hardness of the nanocomposites were improved by increasing the fraction of nanoparticles in the composite. Following extrusion, the matrix grains in the nanocomposite were significantly refined. The extent of the grain refinement increased at higher mass fractions of the nanoparticle. This refinement was not only due to dynamic recrystallization, but also the synergistic pinning effects arising from both the externally added nanoparticles as well as dynamically precipitated Mg17Al12 particles. The nanoparticle additions led to increases in yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (EL). The increase in YS was mostly attributed to the effects of grain refinement, with additional contributions from the influence of Orowan strengthening and thermal expansion effects. The values of both the work hardening rate (θ) and strain hardening exponent (n) increased with increasing fraction of nanoparticles. The high θ value in stage III was attributed to both grain refinement and weakening of the basal plane texture, while the high n value was mainly related to the increase in resistance to dislocation movement caused by pinning effects of the nanoparticles.

Low stacking fault energy alloys often exhibit low ductility. However, sometimes these alloys show unusual mechanical behavior following specific processing parameters. In this study, a low stacking fault energy α+β brass was processed by equal channel angular pressing (ECAP) at 350 °C in ‘route C’ through three passes. Room temperature tensile testing showed that ductility, surprisingly, reached ~80%, whilst good tensile strength was maintained. Investigation of alloy microstructure revealed a combination of deformation mechanisms, including slip and twinning, accompanied by grain boundary serration and grain fragmentation. It is suggested that these deformation modes triggered these unexpected mechanical properties in this intrinsically brittle alloy. The required energy for discontinuous recrystallization was supplied after only one pass in the α phase, and three passes in the β phase, which then prompted good ductility. In order to study texture evolution, the macro-texture was measured. A high fraction of recrystallized grains and change in strain path fostered the development of Goss, Rotated Goss and Rotated Cube components in the α phase. While in the β phase {011} and Goss components were dominant. Despite the high Schmid factor, Hall-Petch effects and work hardening led to increase of strength and hardness after the final pass.

In this work, the authors apply the “top-down” nanoindentation testing method to assess the mechanical property changes in ion-irradiated metallic alloys for three different ion energies in order to understand the relationship between ion energy, damage peak depth and hardness peak depth. The samples were irradiated with He +2 ions having 1, 2 and 3 MeV beam energies respectively. The curves for ΔH (radiation induced hardness) have been obtained by calculating the difference of the irradiated and unirradiated hardness curves after these were corrected for indentation size effect. Three-dimensional analytical and numerical models have been developed to obtain greater insight into the mechanisms involved in the nanoindentation processes, the nature of the plastic zone, and how these affect the hardness results, including the full hardness profiles with respect to depth. This is particularly valuable in situations where the damage profile is non-uniform, as in the present case, and provides the means to predict expected hardness peak positions and values for a given irradiation dose.

In this study, two different types of nanoceramic coatings; namely, β-Ta2O5 and TaON, were synthesized by double glow discharge plasma technique on Ti–6Al–4V alloy to improve its wear and corrosion resistance. The new coatings were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The two coatings were found to have a thickness of about ~ 25 μm and exhibit similar microstructural characteristics, i.e., composed of equiaxed grains with an average grain size of ~ 15 nm. They are tightly adhered to the Ti–6Al–4V substrate. Nanoindentation was used to determine the hardness and elastic modulus of the two coatings and the adhesion strength between the coatings and substrate was also evaluated by scratch testing. Dry sliding wear tests were performed using a ball-on-disk type tribometer, in which the two coatings were slid against Si3N4 ceramic balls under applied loads ranging from 2.3 N to 5.3 N at room temperature. Compared with the Ti–6Al–4V, the coated samples showed a decrease of two orders of magnitude in the specific wear rate, and the specific wear rates of the two coatings were also insensitive to the applied normal loads. The electrochemical behavior of the two coatings was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis in a naturally aerated 5 wt.% HCl solution. While both types of coatings were able to greatly improve the corrosion resistance of Ti–6Al–4V alloy, the TaON coating exhibit a higher corrosion resistance than the β-Ta2O5, due primarily to the TaON coating with a lower carrier density.

In order to improve the wear and corrosion resistance of Ti-6Al-4V, a novel β-Ta 2 O 5 nanoceramic coating was synthesised using reactive sputter deposition enabled by double glow discharge plasma technique. The surface topography, chemical composition, and microstructure of the newly developed coating were characterised by a variety of surface analytical techniques. The coating microstructure was found to exhibit a compact striated pattern extending in a direction perpendicular to coating surface, which is composed of equiaxed β-Ta 2 O 5 grains with an average grain size of ∼20 nm, well adhered to the Ti-6A1-4V substrate. The hardness and the Young's modulus of the as-deposited coating were obtained by nanoindentation, and the adhesion strength between the coating and substrate was determined by a scratch tester. The dry sliding wear behaviours of the coating were investigated at room temperature against Si 3 N 4 ceramic balls at room temperature under applied loads ranging from 2.3 N to 5.3 N using a ball-on-disc tribometer. The specific wear rates of the coating exhibited only a slight increase with applied normal load, and were shown to be two orders of magnitude lower than that for Ti-6Al-4V under the same loading condition. Furthermore, the electrochemical behaviour of the coating immersed in 3.5 wt.% NaCl solution was systematically examined by using a range of complementary electrochemical techniques including potentiodynamic polarisation, electrochemical impedance spectroscopy (EIS), Mott–Schottky analysis as well as potential of zero charge (PZC). The results showed that the corrosion resistance of the β-Ta 2 O 5 nanoceramic coating was better than that of Ti-6Al-4V alloy in 3.5 wt.% NaCl solution. Hence, by possessing higher mechanical properties and good wear and corrosion resistance, the β-Ta 2 O 5 nanoceramic coating is considered to be a promising candidate for protection of engineering components operating under harsh conditions.

To improve the corrosion resistance, surface electrical conductivity and wettability of Ti-6Al-4V used in polymer electrolyte membrane fuel cell (PEMFC), a ZrCN nanocrystalline coating was deposited on Ti-6Al-4V substrate using double cathode glow discharge technique. The new coating exhibited a nanocomposite structure, consisting of amorphous C, CNx and nanocrystalline ZrCN. The effect of the HF concentrations on the corrosion behavior of the coating was investigated by potentiodynamic, potentiostatic polarizations and electrochemical impedance spectroscopy (EIS) in a simulated the operating conditions of a PEMFC. With increasing HF concentrations, the corrosion potential (E-corr) decreased and the corrosion current density (i(corr)) of the ZrCN coating increased, indicating that corrosion resistance decreased with the increase of HF concentrations. However, at any given concentration of HF, the corrosion resistance of the ZrCN coating was significantly higher than that of uncoated Ti-6Al-4V. The results of EIS measurements showed that with increasing the concentration of HF, the resistance of the passive film (R-b) formed on the ZrCN coating decreased slightly, being of the order of magnitude of similar to 10(7) Omega cm(2), which was an improvement by four orders of magnitude compared to uncoated Ti-6Al-4V. At a compaction force of 140 N cm(-2), no perceptible difference in the interfacial contact resistance (ICR) of ZrCN-coated Ti-6Al-4V was observed before and after potentiostatic polarization for 120 min, and its ICR values were reduced by one order of magnitude in comparison to that of uncoated Ti-6Al-4V. Moreover, ZrCN-coated Ti-6Al-4V exhibited a much low surface wettability than uncoated Ti-6Al-4V alloy, which was beneficial for both water management and improving corrosion resistance. (C) 2015 Elsevier B.V. All rights reserved.

To enhance the corrosion resistance, biocompatibility and mechanical durability of biomedical titanium alloys, a novel β-Ta2O5 nanoceramic coating was developed using a double glow discharge plasma technique. The surface morphology, phase composition and microstructure of the as-deposited coating were examined by atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating exhibits a striated structural pattern along the growth direction, which consists of equiaxed β-Ta2O5 grains, 15-20 nm in diameter in cross-section, showing a strong (001) preferred orientation. The mechanical properties and contact damage resistance of the β-Ta2O5 coating were evaluated by nanoindentation. Additionally, scratch tests were performed to evaluate the adhesion strength between the β-Ta2O5 coating and the Ti-6Al-4V substrate. The β-Ta2O5 coating shows high hardness combined with good resistance to both indentation and scratch damage, thus favoring it for long-term load-bearing application in the human body. Electrochemical behavior of the coating was analyzed in both a 0.9 wt % NaCl solution and Ringer's solution at 37 °C, by various electrochemical analytical techniques, including potentiodynamic polarization, electrochemical impedance spectroscopy, potential of zero charge and Mott-Schottky analysis. Compared with uncoated Ti-6Al-4V and commercially pure tantalum, the β-Ta2O5 coating possesses a more positive Ecorr and lower icorr in both aqueous solutions, which is attributed to the thicker and denser β-Ta2O5 coating that provides more effective protection against corrosive attack. In addition, the β-Ta2O5 coating shows stable impedance behavior over 5 days immersion under both simulated body solutions, corroborated by the capacitance and resistance values extracted from the EIS data. Mott-Schottky analysis reveals that the β-Ta2O5 coating shows n-type semiconductor behavior and its donor density is independent of immersion time in both aqueous solutions. Its donor density is of the order of 1 × 1019 cm-3, which is an order of magnitude less than that of the passive films formed on either commercially pure Ta or uncoated Ti-6Al-4V. Moreover, according to the differences between corrosion potential and potential of zero charge, the β-Ta2O5 coating exhibits a greater propensity to repulse chloride ions than both commercially pure Ta and uncoated Ti-6Al-4V. Therefore, the newly developed coating could be used to protect the surface of biomedical titanium alloys under harsh conditions.

Ultrafine structured Al-4wt. %Cu- (2.5-10) vol. % SiC nanocomposites were produced by high energy mechanical milling of a mixture of Al and Cu powders and SiC nano-powder to produce nanocomposites powders, followed by consolidation of the powders using powder compact extrusion (PCE). Scanning and transmission electron microscopy as well as tensile testing were used to characterize the extruded nanocomposite bars. Increasing the volume fraction of SiC nanoparticles from 2.5 to 5.0 causes the yield strength, ultimate tensile strength and microhardness of the nanocomposite to increase from 98 MPa, 168 MPa and 104 HV to 391 MPa, 400 MPa and 153 HV, showing the effectiveness of SiC nanoparticles and microstructural refinement in strengthening the material. However, the ductility decreases from 6.8% to 2%, possibly due to the existence of SiC nanoparticle agglomerates in the Al-4wt%Cu-5vol.%SiC nanocomposite. The ultrafine structured Al-4wt%Cu-(7.5 and 10)vol.%SiC nanocomposite bars fractured prematurely during tensile testing. The possible reason for this may be the existence of SiC nanoparticles agglomerates in their microstructure.