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Abstract: TiC–TiB2 composite coatings were successfully synthesized using the technique of reactive plasma spraying (RPS) on a magnesium alloy. Phase composition, microstructure and wear resistance of the coatings were characterized by using X-ray diffraction, scanning electron microscopy and pin-on-disk wear test, respectively. The results showed that the resultant product in the RPS coatings was composed of TiC and TiB2. Depending on the ignition of self-propagating high-temperature synthesis reaction in the agglomerate particles, the RPS coatings displayed porous and dense microstructures. The porosity of the RPS coatings, to some extent, decreased when the feed powders were plasma sprayed with Ni powders. The RPS coatings provided good wear resistance for the substrate under various loads. For high loads (e.g., ≥15N), the wear resistance could be significantly improved by the proper addition of Ni into the RPS coatings. [Copyright &y& Elsevier]

Abstract: Al2O3 coatings with different thicknesses (160, 320, 480 and 640μm) were deposited on stainless steel substrate by plasma spraying. The variation in microstructural characteristics and properties of coatings with various thicknesses was investigated. Powders morphology and the microstructure of as-sprayed coatings were characterized by scanning electron microscopy and optical microscopy. The microhardness was measured using a Vickers'' indentor. The corrosion behaviour of plasma-sprayed Al2O3 coatings in 1N H2SO4 solution at a temperature of 25°C was evaluated by electrochemistry method. Experimental results indicated that surface roughness showed no obvious dependence on the coating thickness. However, the porosity of Al2O3 coating was increased with increased thickness. The enhanced coating thickness also resulted in decreasing microhardness and reduced corrosion resistance. In this study, the Al2O3 coating with thickness of 160μm possesses the lowest porosity, the highest hardness and superior corrosion resistance. [Copyright &y& Elsevier]

Abstract: Al2O3 and Cr2O3 coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al2O3 coating was superior to that of the Cr2O3 coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al2O3 coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al2O3 coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr2O3 coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation. [Copyright &y& Elsevier]

Abstract: Al/Al2O3 composite coating was prepared by plasma spraying and characterized by XRD and SEM. Some thermal–mechanical properties of the composite coating including thermal diffusivity, microhardness, fracture toughness and sliding wear rate were measured. The results showed that the Al/Al2O3 composite coating, compared with Al2O3 coating, exhibits denser structure and developed splat interface. The coexistence of Al metal phase and Al2O3 ceramic phase effectively increased the fracture toughness and thermal diffusivity of composite coating, in spite of the slight decrease in microhardness. Furthermore, the wear resistance of Al/Al2O3 composite coating is superior to that of Al2O3 coating. [Copyright &y& Elsevier]

Abstract: This paper aims to elaborate the particle characteristics during their flight into the spray and its significance in determining the microstructure and mechanical properties of resultant coatings. Study of plasma spraying of Al2O3 using three different grain fractions of feedstock was done by an on-line monitoring system (SprayWatch 2i). The as-sprayed coatings were characterized in terms of the microstructure and composition, microhardness, adhesion strength and porosity. In addition, a statistic study of Weibull analysis was employed to further examine the microstructural homogeneity of coatings. It was found that particle in-flight behaviors are strongly influenced by feedstock particle size. Improved particle velocity and temperature as well as melting extent can be obtained for the finer feedstock. This contributes to produce increasingly bonded splats resulting in coating with compact microstructure, where low porosity and increased microhardness and adhesion strength were realized. [Copyright &y& Elsevier]

Abstract: Al2O3 and Al2O3–Al composite coatings were prepared by plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction (XRD), while optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of impacted droplets, polished and fractured surface, and the element distribution in terms of wavelength-dispersive spectrometer (WDS). Mechanical properties including microhardness, adhesion and bending strength, fracture toughness and sliding wear rate were evaluated. The results indicated that the addition of Al into Al2O3 was beneficial to decrease the splashing of impinging droplets and to increase the deposition efficiency. The Al2O3–Al composite coating exhibited homogeneously dispersed pores and the co-sprayed Al particles were considered to be distributed in the splat boundary. Compared with Al2O3 coating, the composite coating showed slightly lower hardness, whereas the coexistence of metal Al phase and Al2O3 ceramic phase effectively improved the toughness, strength and wear resistance of coatings. [Copyright &y& Elsevier]

Abstract: Al2O3 and Al/Al2O3 composite coatings were prepared by atmospheric plasma spraying. The wear and friction properties of both coatings were evaluated under different degrees of severity using a block-on-ring arrangement. It was found that the wear resistance of the Al/Al2O3 composite coating was superior to that of the Al2O3 coating. The improvement in wear resistance of the Al/Al2O3 composite coating was attributed to its higher fracture toughness and adhesion strength as well as better thermal diffusivity. As for the Al/Al2O3 composite coating, the mechanism was mainly adhesion with micro-abrasion and fatigued-induced brittle fracture within splats, and the delamination along splat boundaries only occurred at high load. However, the failure of the Al2O3 coating was predominantly detachment of transferred film and brittle fracture within the splats and delamination along splat boundaries, which were enhanced with the increasing load. [Copyright &y& Elsevier]

In this paper, hydroxyapatite (HA) coatings having the crystallinities of 56% and 98% were deposited by the plasma spraying and vapor-flame treatment process. The phase composition and crystallinity of the coatings were investigated by X-ray diffraction and infrared spectra. The dissolution behavior of the coatings in tris-buffer solutions was examined. The results obtained indicated that the coating having the high crystallinity showed the lower dissolution as compared to the low crystallinity coating. The bone bonding ability of HA coatings were observed in vivo by implanted in dog''s femur. After 3 months implantation, the high crystallinity coating showed the higher shear strengths and remained integrated, whereas the separation of the coating fragments was clearly observed in the coating having low crystallinity. [Copyright &y& Elsevier]

Dicalcium silicate coatings on titanium alloys substrates were prepared by plasma spraying and immersed in simulated body fluids for a period of time to investigate the nucleation and growth of apatite on the surface of the coatings. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of dicalcium silicate coatings before and after immersion in simulated body fluid. The plasma sprayed dicalcium silicate coating was bonding tightly to the substrate. The coating was mainly composed of β-Ca2SiO4 and glassy phase. A dense carbonate-containing hydroxyapatite (CHA) layer was formed on the surface of the plasma sprayed dicalcium silicate coating soaked in SBF solution for 2 days. In addition, a silica-rich layer was also observed between CHA layer and coatings. With an increase in the immersion time, the CHA layer gradually became thicker. The results obtained indicated that the plasma sprayed dicalcium silicate coating possesses excellent bioactivity. [Copyright &y& Elsevier]

In this study, Al 2 O 3 /Al 2 O 3 -Ca 2 SiO 4 powders was sprayed on stainless steel substrate which contains a nickel-chromium connecting layer. At the same time, Al 2 O 3 coatings were produced by air-plasma-spraying technology as comparison. Anti-corrosion tests in acidic corrosive atmosphere, potentiodynamic polarization measurements and thermal shock resistance tests were carried out to evaluate the protection performance of coatings. The effects of inorganic sealing agent on the microstructure and properties of the coatings were studied. The results indicated that Al 2 O 3 /Al 2 O 3 -Ca 2 SiO 4 double ceramic coating showed a better corrosion resistance, lower corrosion rate and a better thermal shock resistance than Al 2 O 3 coating. The sealing treatment can improve the corrosion resistance of coatings while decreasing the thermal shock resistance dramatically. [ABSTRACT FROM AUTHOR]

The Al 2 O 3 –TiB 2 –TiC/Al metal matrix composite coatings were deposited onto MB26 magnesium alloy substrates by atmospheric plasma spraying (APS) using the powders produced by self-propagation high-temperature synthesis (SHS). Phase composition and microstructure of the SHS powders and APS coatings were characterized by XRD and SEM. Microhardness, bond strength and wear resistance of the composite coatings were investigated by using a Vickers hardness tester, pull-off tester and pin-on-desk tribometer. The APS composite coatings mainly consisted of TiB 2 , TiC, excess Al and two types of alumina. The coatings bonded well to the substrate. Microhardness and wear resistance of the coatings increased significantly in comparison with that of the MB26 substrate. The applied load and the fraction of Al content were found to be the most effective factors on the wear rate of the coatings. [ABSTRACT FROM AUTHOR]

Al2O3‐YAG (Al5Y3O12) amorphous ceramic coatings exhibit excellent crack propagation resistance under harsh wear services due to the amorphous phase contributing to the plastic deformation performance of the coating. However, the formation mechanism of the amorphous phase is ambiguous. This study mainly investigated the formation mechanism of Al2O3‐YAG amorphous coating prepared by atmospheric plasma spraying from the perspective of crystallization chemistry. Nano and microsized powders with low eutectic point ratio were selected as feedstock for comparison. X‐ray diffraction, scanning electron microscope, and electron backscattered diffraction were used to analyze the phase composition, morphologies, phase distribution, and structure of the coating. It is concluded that the significant thermodynamically stable structure of polycompound with high coordination numbers of cations prioritized crystallizing in the Al2O3‐YAG melt, but it needed more time to crystallize and hardly crystallized in the limited time during plasma spraying. Therefore, the selection of as‐sprayable powder should also be considered the critical factor for preparing amorphous coatings. The nanoscale or submicro scale powder distributed uniformly with low eutectic point ratio was chosen as the feedstock to ensure the powder droplets diffuse sufficiently during deposition. [ABSTRACT FROM AUTHOR]

Al2O3-YAG amorphous ceramic coating is expected to apply to harsh wear service due to its excellent crack propagation resistance, but the crystallization of this coating may affect its service stability. Therefore, the crystallization behavior of the Al2O3-YAG coating prepared by atmospheric plasma spray (APS) was investigated in this study. The results showed that this coating had excellent microstructure stability over a broad temperature range and had a higher activation energy during non-isothermal crystallization process. The isothermal DSC curves showed that the crystallization process of this coating was controlled by the change in external thermodynamic conditions. The grain growth mechanism for α-Al2O3 and YAG was from three-dimensional direction bulk growth to one-dimensional direction flake thickening. The calculated fragility index showed that the amorphous ceramic coating had better stability and higher plastic deformation capacity than other typical brittle materials at its critical temperature range. [ABSTRACT FROM AUTHOR]

CoMoCrSi coatings with different thickness were prepared by high velocity oxy-fuel spraying (HVOF) and atmospheric plasma spraying (APS), respectively. Variation of microstructure and phase constitution the coatings were investigated by SEM and XRD, respectively. Thermal shock performance of the coatings was evaluated by heating (500 °C and 900 °C) and water quenching method. Compared with APS coatings, the as-sprayed HVOF coatings exhibited denser microstructure and higher hardness. The phase and microstructure of the coatings did not change significantly after 50 thermal cycles at 500 °C. On the contrary, spallation was observed in APS coatings with thickness of ∼350 μm during 900 °C thermal shock cycles. Obvious oxidation appeared in all of the coatings after 900 °C thermal shock. Meanwhile, segmentation cracks perpendicular to the coating surface were found in the edge of 900 °C thermal shocked HVOF coatings (both ∼350 μm and ∼130 μm) and APS coatings (∼350 μm). Element interdiffusion was not detected between the substrate and coating after 50 thermal shocks at 900 °C or 500 °C. After the thermal shock at 500 °C, the microhardness of the ∼130 μm coating did not change greatly while the microhardness of the ∼350 μm coating was greatly improved. After the thermal shock at 900 °C, the microhardness of the coating increased by about 40 % due to the reduction of defects in the coating and the generation of hard phases. At the same time, the migration of elements and the oxidation and recrystallization of some phases may increase the hardness of the coating. • CoMoCrSi coatings with different thickness were prepared by HVOF and APS. • HVOF coating exhibit better thermal shock resistance than APS coating. • No obvious changes in coating's phase and microstructure after 50 thermal cycles at 500 °C. • Oxidation, vertical crack and HV0.3 increase appeared in the coatings after 900 °C thermal shock. [ABSTRACT FROM AUTHOR]

Abstract: Rare-earth zirconates with a pyrochlore structure have been developed for potential application in thermal barrier coating systems to further improve the performance and durability of gas turbines. The Sm2Zr2O7 (abbreviated as SZ) powder was synthesized by solid state reaction and then deposited by air plasma spraying. The phase stability, microstructure and thermal conductivity of SZ and 8wt% Y2O3 stabilized zirconia (8YSZ) coatings were investigated. The X-ray diffraction results indicated that the crystal structure of the as-sprayed SZ coatings was defect-fluorite, and after heat treating at 1200°C for 50h, it started to transform to pyrochlore, and the content of pyrochlore increased with increase in temperature of the heat treatment. The thermal conductivities of SZ coatings were significantly lower than those of 8YSZ coatings before and after heat treatments, which increased considerably after heat treatments compared to the as-sprayed states for both coatings due to sintering effects. [Copyright &y& Elsevier]

Abstract: Nano-scaled lanthanum zirconate powder prepared by co-precipitation–calcination method was plasma-sprayed into a thick coating on an alloy substrate. We investigated the thermophysical properties of the free-standing coating, including thermal conductivity and thermal expansion coefficients (TECs). Minimum value of the thermal conductivity (at 900°C) of the coating was about 0.73Wm−1 K−1, and the average TEC in the measurement range was about 9.45×10−6 K−1. Although the TEC value was similar to that of the bulk material, the change tendency versus temperature was different. After annealing at 1300 or 1400°C for 50h, we found that the heat insulation performance of the coating decreased with the heat treatment temperature, while the hardness and fracture toughness increased. A peak of sudden decrease in TEC value can be observed in the curve, and with increasing temperature, the peak shifted to high-temperature direction. [Copyright &y& Elsevier]

Abstract: Vacuum plasma sprayed tungsten coating on graphite and copper substrates has been prepared. VPS-W coated graphite has multilayered silicon and tungsten interface pre-deposited by physical vapor deposition (PVD) and VPS-W coated copper has graded transition interlayer. VPS-W coating was characterized, and then the high heat flux properties of the coating were examined. Experimental results indicated that both VPS-W coated graphite and VPS-W coated copper could endure 1000 cycles without visible failure under a heat flux of approximately 5MW/m2 absorbed power density and 5s pulse duration. A comparison between the present VPS-W coated graphite and VPS-W coated carbon fiber composite (CX-2002U) with Re interface made by Plansee Aktiengesllshaft was carried out. Results show that both Re and Si are suitable as intermediate layer for tungsten coating on carbon substrates. [Copyright &y& Elsevier]

As a novel processing technology, plasma spray-physical vapor deposition (PS-PVD) enables coatings to be deposited mainly from vapor phase. To explore the potential advantages of such a plasma spray-based processes, the deposition mechanisms and their dependency on process conditions should be better investigated. In this work, yttria partially stabilized zirconia thermal barrier coatings with quasi-columnar structure were fabricated by PS-PVD. The morphologies of the coatings for each set of deposition durations (from 30 ms to 100 s) and spray distances range from 800 to 1000 mm were studied. Besides, a design-of-experiment was used to investigate the non-line-of-sight capability of PS-PVD coatings, which were deposited on flat superalloy substrates that mounted on a static cylindrical graphite holder approximately 40 mm in diameter at various deposition angle (from 0° to 270°). Moreover, the residual stresses in the topcoat and thermally grown oxide scale were measured non-destructively using Raman spectroscopy and photoluminescence piezospectroscopy, respectively. The stage growth process of PS-PVD coatings was obtained by electron backscattered diffraction characterization of the crystal orientations and distributions of crystal size. The various microstructures of the coatings help to improve the understanding of the deposition mechanisms of PS-PVD coatings. [ABSTRACT FROM AUTHOR]

NiCr–Al 2 O 3 -aluminum bronze (NAC) and NiCr–Ca 2 SiO 4 -aluminum bronze (NDC) coatings were prepared by atmospheric plasma spraying. Corrosion experiments were performed on coatings including immersion in simulated seawater and dry-wet composite salt spray test. Phase composition and microstructure of the coatings were investigated by X-ray diffraction, energy dispersive spectrometry and scanning electron microscopy, before and after the corrosion tests. The adhesion strength and hardness of coatings were also evaluated. As observed, Ca 2 SiO 4 had much lower hardness than Al 2 O 3 layer. Due to the rougher surface and higher thermal expansion of Ca 2 SiO 4 layer, NDC coatings possessed better adhesion. Further, NDC coatings exhibited relatively superior corrosion resistance. According to the microstructure and phase analysis, this finding was attributed to the hydration reaction of Ca 2 SiO 4 during the tests. [Display omitted] • Plasma sprayed ceramic (Al 2 O 3 , Ca 2 SiO 4)-aluminum bronze coatings were prepared. • Coatings with Ca 2 SiO 4 showed superior corrosion resistance and adhesion strength. • Hydration reaction of Ca 2 SiO 4 occurred during the corrosion tests. • Corrosion mechanism of ceramic-aluminum bronze coating was discussed. [ABSTRACT FROM AUTHOR]

It was published that zirconate coatings containing Gd/Yb and yttria-stabilized zirconia (YSZ) coatings with high Y content had good calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance. However, there are few reports on CMAS resistance of zirconia-based thermal barrier coatings (TBCs) doped with Y, Gd and Yb. This article intends to explore the role of Y, Gd and Yb in resisting CMAS corrosion of zirconia-based TBCs. The purpose is to optimize the rare earth modification of zirconia-based TBCs. 8YSZ and ZrO2-9.5Y2O3-5.6Yb2O3-5.2Gd2O3 free-standing TBCs were produced by air plasma spraying (APS) with commercial powders. The CMAS corrosion behavior and failure mechanism of these coatings were studied by heat treatment at 1350 °C. The microstructure and composition were characterized. The results showed the CMAS deposited on the surface of 8YSZ/YSZ-Yb-Gd coatings gradually penetrated into coatings. ZrSiO4 and m-ZrO2 were observed in the shallow layer of coatings. The diffusion rate of Y3+ in coatings into CMAS was greater than that of Gd3+ and Yb3+. Compared with YSZ coatings, CMAS penetrated deeper in YSZ-Yb-Gd coatings after the same corrosion time. The spallation of YSZ-Yb-Gd coatings occurred earlier than 8YSZ. In general, adding a small amount of Yb and Gd did not improve the CMAS resistance of YSZ TBCs. [ABSTRACT FROM AUTHOR]

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y 2 O 3 on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y 2 O 3 , 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO 2 in TBCs because of the higher diffusion rate and solubility of Y3+ in CMAS than Zr4+. At the same time, 38YSZ/8YSZ + Y 2 O 3 reacts with CAMS to form Ca 4 Y 6 (SiO 4) 6 O/Y 4·67 (SiO 4) 3 O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y 2 O 3 layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y 2 O 3 dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties. [ABSTRACT FROM AUTHOR]

Ca2SiO4 coatings with and without metallic bonding layer (NiCr) have been prepared by atmospheric plasma spraying. Microstructure and corrosion behavior of the coatings with various exposure periods in simulated seawater were investigated by means of scanning electron microscopy, energy dispersive spectrometry, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that the after immersion, a layer of urchin-like aragonite (CaCO3) was formed on the surface and a dense silica-rich layer was found between CaCO3 and Ca2SiO4 layer. Besides, self-sealing behavior was observed in the Ca2SiO4 layers. Compared with the immersed coatings, as-sprayed coatings exhibited the highest corrosion current densities, the lowest polarization resistance and charge transfer resistance. The presence of the metallic bonding layer slightly decreased the corrosion resistance of Ca2SiO4 coatings. The plasma-sprayed Ca2SiO4 coatings have a potential use in marine corrosive environments. [ABSTRACT FROM AUTHOR]

The reliability and lifetime of mechanical systems often depend on their capacity to resist to friction and wear under high PV values (P contact pressure; V friction velocity). This study deals with plasma-sprayed amorphous Al2O3-YAG coatings that could be used under such conditions. Their phase transformation, microstructure stability and high PV wear behavior were investigated. The sprayable Al2O3/Y2O3 feedstock was obtained by spray-drying nano-sized Al2O3 and Y2O3 powders. In order to elucidate the effect of the original powder particle size on phase composition and anti-wear property of the coatings, micro-sized Al2O3 and Y2O3 powders were also utilized to produce Al2O3-Y2O3 coating. Simultaneously, Al2O3 coating was also deposited for performance comparison. The combination of coating microstructure analysis, solidification theory of deep eutectic system and practical plasma spraying conditions clarified the formation mechanism of amorphous Al2O3-YAG coating. Furthermore, amorphous Al2O3-YAG coating showed better wear-resistant performance than crystalline Al2O3 and Al2O3-Y2O3 coatings under severe conditions with high PV values, which indicate excellent engineering application potential of the former coating. The relationships between microstructure characteristics and high PV wear behavior of amorphous Al2O3-YAG coating were established, which contribute to deeply understand corresponding friction and wear mechanisms. The rapid solidification temperature (Te) in plasma spraying process is rather lower than the glass transition temperature (Tg) due to the cooling effects in the coating deposition. Amorphous phase is dominant in as-sprayed Al2O3-YAG coating. The amorphous Al2O3-YAG coating has better plastic deformation ability than Al2O3 and Al2O3-Y2O3 coatings. The amorphous Al2O3-YAG coating possesses greater crack propagation resistance. [ABSTRACT FROM AUTHOR]

Plasma spray-physical vapor deposition (PS-PVD) is an advanced technique to fabricate quasi-columnar structured thermal barrier coatings (TBCs) with excellent thermal cyclic lifetime. In this study, PS-PVD TBCs were investigated via burner rig test. The residual stresses in both of the topcoat layer and the thermally grown oxide (TGO) scale were measured non-destructively using Raman spectroscopy and Cr3+ photoluminescence piezo-spectroscopy, respectively. Evolution of the microstructures and distribution of residual stresses in such kind structured TBCs before and after thermal cycling test were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly and then essentially maintained at a relatively stable state with further testing. Moreover, the pre-heating of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat. [ABSTRACT FROM AUTHOR]

Abstract: Crystallization kinetics of the newly developed Al2O3‐Y3Al5O12 (YAG) amorphous ceramic coating fabricated by atmospheric plasma spraying (APS) were investigated via differential scanning calorimetry (DSC) under non‐isothermal conditions. The phase compositions and microstructure of the as‐sprayed coating were characterized by X‐ray diffraction (XRD) and Scanning electron microscopy (SEM). The glass transition temperature Tg, the onset temperature of crystallization Tc and the peak temperature of crystallization Tp presented heating rate dependence. The related kinetic parameters of activation energies (Eg, Ec, Ep) and Avrami exponents (n) were quantified using various methods including Kissinger, Augis–Bennett, Ozawa and Matusita–Sakka, etc., to understand the phase transition mechanism and crystallization process in depth. A series of parameters including devitrification interval ΔT, thermal stability (Tc, Ec), nucleation resistance Ec/RTg and fragility index F were quantified in order to evaluate the nucleation mechanism, crystallization behavior and thermal stability of Al2O3‐YAG amorphous ceramic coating. Excellent thermal stability was witnessed in the studied coating. Furthermore, the YAG crystalline phases can be reasonably controlled and independently precipitated from the amorphous matrix via proper annealing. [ABSTRACT FROM AUTHOR]

Columnar-structure is favourable to the durability of thermal barrier coatings (TBCs), and the efficacy of TBCs increases with the thickness of the TBCs. In this study, thick columnar YSZ coatings (∼1.5 mm) were deposited by multiple PS-PVD processes. The phase compositions, microstructures and mechanical properties were examined with various techniques including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nanoindentation. Results show that there are only tetragonal phase on the top surface while both monoclinic and tetragonal zirconia are present on the bottom surface. The first layer of the coatings is composed of randomly oriented grains. Immediately after the quasi-equiaxed zone, columns start to grow. The top coatings exhibit looser microstructure compared with the middle and bottom regions, and thus have lower values of hardness and Young’s modulus. [ABSTRACT FROM AUTHOR]

In recent years, numerous techniques have been developed to mimic nacre-like hierarchical architectures in order to improve the damage tolerance of materials. We present herein a simple strategy to fabricate such a hierarchical architectured Al2O3–Y2O3 composite coating via atmospheric plasma spraying. The evolution of the phase and microstructure of the Al2O3–Y2O3 composite coating were characterized under conditions of high-temperature exposure in air at 800-1350 °C. The hardness and porosity of several typical coatings were determined. In situ formation of dense hierarchical architectured Al2O3–YAG composite coating with improved hardness was achieved after heat treatment at 1350 °C. Compared with Al2O3 coating, elevated toughness was found for the hierarchical architectured Al2O3–YAG composite coating, which can be ascribed to the distribution of YAG phase that contributed to crack termination and deflection, and microbridging. After thermal aging treatment at 1350 °C, the hierarchical architectured Al2O3–YAG composite coating was quite stable after 100 h of thermal exposure. Furthermore, the Al2O3–Y2O3 composite coating exhibited superior sintering resistance compared with the Al2O3 coating. [ABSTRACT FROM AUTHOR]

The microstructure and wear resistant performance of plasma-sprayed Al 2 O 3 –Y 3 Al 5 O 12 (YAG) coating were investigated. As-sprayed Al 2 O 3 –YAG coating was chiefly composed of amorphous phase, which may reveal superplastic feature in supercooled liquid region (from 503.0 °C to 906.5 °C). The as-sprayed Al 2 O 3 –YAG coating exhibits fine plasticity and toughness under the following conditions: a load of 2000 N, a rotational speed of 500 rpm and worn surface temperature above 500 °C, which significantly contributes to relieve accumulated coating stress and maintain the coating integrity. Therefore, plasma-sprayed Al 2 O 3 –YAG coating possesses excellent wear resistance under severe conditions. [ABSTRACT FROM AUTHOR]

In the current work, by means of suspension plasma spraying (SPS), yttria stabilized zirconia splats and coatings were deposited onto mirror-polished and rough substrates, respectively. The splats were characterized by scanning and transmission electron microscopes (TEM). The influence of substrate temperature on splat morphology was analyzed via comparing the splats deposited onto substrates held at different temperatures, namely room temperature (RT), 300 °C, 500 °C, 700 °C and 900 °C. As the substrates were heated from RT to 300 °C, the feature of splats changed rapidly. However, when the substrate temperature further increased, the variation of morphology was found less conspicuous. Integrated splats were peeled off from the substrate using a “plating and etching” approach for TEM examination. The observation on morphological and crystallographic features of splats revealed that the cooling rate during impacting and spreading of SPS splats was lower than that of splats deposited by conventional atmospheric plasma spraying. Based on the characteristics of splats, a modified mechanism consisting changing of droplets trajectory and shadowing effect was proposed to explain the formation of the microstructure of as-sprayed SPS coating. In addition, the coatings were also examined by X-ray diffraction and nano-indenter to determine the phase composition and microhardness, respectively. [ABSTRACT FROM AUTHOR]

Yttria-stabilized zirconia coatings were deposited by plasma spraying and heat-treated at 1100 °C for 50, 100, 150, and 200 h in air, respectively. Mechanical properties including microhardness and Young's modulus were evaluated using the nanoindentation test. Residual stresses in the ceramic topcoat and the thermally grown oxide (TGO) layer were measured using Raman spectroscopy and photoluminescence piezo-spectroscopy (PLPS) techniques, respectively. The results showed that both the modulus and hardness increased with the thermal exposure time up to 100 h and then gradually decreased. The accumulated tensile stress in the as-sprayed topcoat changed to compressive stress after thermal exposure, and the compressive stress in the topcoat increased with an increase of thermal exposure time up to 150 h. The average compressive stresses in the TGO layer were higher than that of the cross-sectional topcoat, and the measured in-plane compressive stress increased firstly and then gradually decreased with increasing exposure time. The local interface geometry strongly affect the nature and evolution of hydrostatic stresses in the TGO. Finally, the crack initiation and propagation at the topcoat/TGO/bondcoat interface has been discussed with respect to the residual stresses in the plasma-sprayed TBC system. [ABSTRACT FROM AUTHOR]

In the current study, alumina and alumina-yttria composite coatings were prepared on stainless steel substrates via atmospheric plasma spraying. Their dry sliding wear performances against graphite under severe conditions were evaluated by using a ring-on-disk tribometer. The results showed that the addition of yttria has successfully decreased the friction coefficients of the alumina coatings. Further, the temperatures in the friction process rose sharply in the initial wear stage and then reached the state of thermal equilibrium. The temperatures of alumina-yttria coatings at thermal equilibrium stages were lower than those of alumina coatings, which may be due to their lower friction combined with the slightly higher thermal conductivity. High friction interface temperatures were considered to greatly aggravate the thermal stress, thereby intensifying the cracking of alumina coatings. Additionally, the addition of yttria improved the toughness of composite coatings and the related crack propagation resistance. [ABSTRACT FROM AUTHOR]

Plasma spray-physical vapor deposition (PS-PVD) is a novel technique which can offer new opportunities to obtain advanced microstructures. In this study, dense yttria-stabilized zirconia (YSZ) coatings were deposited using PS-PVD with proper parameters. The dependencies of the microstructure of the as-sprayed coatings on axial spray distance and radial position were discussed. Four typical coatings were selected for the current work. The corresponding microstructures, phase compositions and mechanical properties were studied in detail. These as-sprayed coatings had dense lamellar or lamellar/columnar hybrid microstructures, the lowest porosity was 0.44%. Both monoclinic and tetragonal zirconia were detected in the as-sprayed coatings. The distribution of yttrium was not homogeneous, especially in the coatings deposited at shorter spray distance. Furthermore, oxygen got lost partly during the spraying process with decreasing spray distance. The hardness ( H ) and Young's modulus ( E ) of the coatings changed simultaneously with the different microstructures. The maximum values of H and E were 16.6±0.6 and 234.3±10.1 GPa respectively, which were found in the densest coatings. The dense PS-PVD ceramic coatings might be applicable in anti-corrosion fields (such as alkali corrosion, marine corrosion and hot corrosion). [ABSTRACT FROM AUTHOR]

Columnar-structured thermal barrier coatings, owing to their high strain tolerance, are expected for their potential possibilities to substantially extend turbine lives and improve engine efficiencies. In this paper, plasma spray-physical vapor deposition (PS-PVD) process was used to deposit yttria partially stabilized zirconia (YSZ) coatings with quasi-columnar structures. Thermal cyclic tests on burner rigs and thermal shock tests by heating and water-quenching method were involved to evaluate the thermal cycling and thermal shock behaviors of such kind of structured thermal barrier coatings (TBCs). Evolution of the microstructures, phase composition, residual stresses and failure behaviors of quasi-columnar YSZ coatings before and after the thermal tests was investigated. The quasi-columnar coating obtained had an average life of around 623 cycles when the spallation area reached about 10% of the total coating surface during burner rig tests with the coating surface temperature of ~1250 °C. Failure of the coating is mainly due to the break and pull-out of center columnar segments. [ABSTRACT FROM AUTHOR]

C f /SiC composites were plasma sprayed with Yb 2 Si 2 O 7 /LaMgAl 11 O 19 coatings with and without Si bond layer. Microstructure and oxidation protection of the coatings at 1673 K were investigated. The results showed that the coatings have the same phase constituent as that of the powders and uniformly distributed on the substrate. The bonding interface between the coating and substrate with Si bond layer is better than that of without Si bond layer. After 15 cycles of heating at 1673 K for 90 min, the weight losses of the coated samples with and without Si bond layer were 2.54% and 21.89%, respectively, however, for the uncoated sample, its weight loss was up to 36.47%. To certain extent, the Si bond layer efficiently healed the porosity on the surface of substrate and relieved the mismatch of coefficient of thermal expansion (CTE) between the coating and substrate, and therefore significantly improved the oxidation protection of the coating. The difference of CTEs between the coating and substrate, the chemical reactions between the LaMgAl 11 O 19 and Yb 2 Si 2 O 7 layers and the holes and cracks inside C f /SiC composites were disadvantageous to the oxidation resistance of the coatings. [ABSTRACT FROM AUTHOR]

The microstructure and property of plasma-sprayed Al 2 O 3 –Y 3 Al 5 O 12 (YAG) composite coatings were investigated. The sprayable feedstock was composed of Al 2 O 3 /Y 2 O 3 granules obtained from the spray drying. As-sprayed Al 2 O 3 –YAG composite coating showed obvious amorphous characteristics. Post-heat treatment facilitated the complete crystallization of the Al 2 O 3 –YAG composite coating. The heat-treated Al 2 O 3 –YAG ceramic coating possessed the deposition microstructure comprising three-dimensional interpenetrating network of α-Al 2 O 3 /YAG eutectic phases, which is very similar to the directional solidification microstructure of Al 2 O 3 /YAG eutectic bulk ceramics. This eutectic coating microstructure does not correspond to a traditional eutectic microstructure. Al 2 O 3 –YAG ceramic coating with eutectic composition is obtained from the crystallization of an amorphous solid ceramic instead of the solidification of the liquid. Furthermore, the plasma-sprayed Al 2 O 3 –YAG eutectic ceramic coating with the heat treatment exhibits excellent microstructure and performance stabilities under high temperature. [ABSTRACT FROM AUTHOR]

Perovskite-type La 1 − x Sr x CoO 3 − δ (0.2 ⩽ x ⩽ 0.8) ceramics were prepared by tape casting and solid state reaction method. The room temperature optical properties of these samples were studied in detail. The temperature dependent emittance measurements and high-temperature durability tests were carried out. Results showed that these materials exhibited low reflectance at the solar spectrum wavelengths and high reflectance in the infrared region, with an optimal spectral selectivity of 3.2. The investigated samples showed low thermal emittance at high temperatures. More importantly, the sample was highly stable in air at 800 °C, as the performance criterion value was lower than 0.05 after 10 cycles (90 h) heat treatment. Our results demonstrate that the La 1 − x Sr x CoO 3 − δ ceramics have the potential to emerge as high temperature (∼800 °C) solar absorbers for the future solar power systems. [ABSTRACT FROM AUTHOR]

In the present study, Al 2 O 3 and Al 2 O 3 –Y 2 O 3 composite coatings were deposited by atmospheric plasma spraying. The microstructure, hardness, thermal conductivity and dry sliding wear behavior of the coatings were examined. The results showed that the mass ratio of α-Al 2 O 3 /γ–Al 2 O 3 in as-sprayed Al 2 O 3 coating was 0.08. However, the corresponding value in Al 2 O 3 –Y 2 O 3 composite coating was 0.53. This exhibits salient influence on the stabilization of α-Al 2 O 3 . The addition of Y 2 O 3 was beneficial to improve the thermal conductivity of the composite coating. In addition, Al 2 O 3 –Y 2 O 3 composite coating/graphite pairs possess more stable friction coefficients, which may be related to the formation of more effective and stable graphite transferred film on the surface of the coating subjected to abrasion. Further, the wear resistance of Al 2 O 3 –Y 2 O 3 composite coatings was superior to that of Al 2 O 3 coatings. [ABSTRACT FROM AUTHOR]

Two kinds of segmentation-crack structured YSZ thick thermal barrier coatings (TTBCs) were deposited by suspension plasma spraying (SPS) and atmospheric plasma spraying (APS) with nano-based suspension and agglomerated particles, respectively. The phase composition, microstructure evolution and failure behavior of both TBCs before and after thermal shock tests were systematically investigated. Microstructure of the APS coating exhibits typical segmentation-crack structure in the through-thickness direction, similar with the SPS coating. The densities of segmentation-crack in APS and SPS coatings were about 3 cracks mm −1 and 4 cracks mm −1 , respectively. The microstructure observation also showed that the columnar and equiaxed grains existed in the SPS coating. As for the thermal shock test, the spallation life of the APS TTBCs was 146 cycles, close to that of the SPS TTBCs (166 cycles). Failure of the APS coating is due to the spallation of fringe segments and splats. [ABSTRACT FROM AUTHOR]

This study reveals that the use of a Ca dopant can increase the solar absorption of GdBaCo 2 O 5+δ , with an optimal composition of Gd 0.8 Ca 0.2 BaCo 2 O 5 achieving 85% solar absorptance. Electrical conductivity measurements revealed that this composition allows for faster insulator-metal transition and higher conductivity, with the observed variation in conductivity with 0.8 W/cm 2 of solar illumination explained in terms of a first-order spin-state transition. Thermal radiation imaging confirmed that regions of different radiative temperature are related to a change in free carriers that promote scattering, and therefore, cause an increase in infrared reflection. This rapid electrical conductivity transition and the low infrared radiation properties at high temperatures strongly suggest that Gd 0.8 Ca 0.2 BaCo 2 O 5 could be used in a variety of potential fields, such as high-temperature thermosensitive or thermal storage materials. [ABSTRACT FROM AUTHOR]

A novel design of a solar absorber making use of different optical properties of materials for high temperature solar-thermal applications is proposed. For the two sides of the sample, the solar absorbing surface is designed to absorb sunlight and reduce radiative heat loss, while the high emission surface is aimed at enhancing the heat transfer. The composite samples were prepared by tape casting, alternately laminating and high temperature sintering. Room temperature optical properties and temperature-dependent radiative properties of the composite samples were studied. The results showed that the emittances of the absorbing surface and heat transfer surface were 0.32 and 0.70, respectively. Due to this difference of emittances, the two surfaces exhibited a radiation temperature (read from thermography camera) difference of 140 °C when the actual sample temperature is 600 °C. [ABSTRACT FROM AUTHOR]

In the present study, the phase composition and generation mechanism of the nanocrystals located in the microcracks of plasma-sprayed AlO coating were reevaluated. The AlO coatings were investigated using transmission electron microscopy and x-ray diffraction. We supply the detailed explanations to support the new viewpoint that in situ formation of α-AlO nanocrystals in the preexisting microcracks of the as-sprayed AlO coating may be due to the stress-induced phase transformation. Owing to the partially coherent relationship, the phase interfaces between the α-AlO nanocrystals with the preferred orientation and the γ-AlO matrix may possess better bonding strength. The α-AlO nanocrystals could repair the microcracks in the coating, which further strengthens grain boundaries. Grain boundary strengthening is beneficial to the coating fracture toughness enhancement. [ABSTRACT FROM AUTHOR]

In this paper, Al 2 O 3 –Cr 2 O 3 composite coating was fabricated by plasma spraying. It has better mechanical performances than Al 2 O 3 coating. Erosion–wear resistance of the coatings was evaluated by a new type of solid particle impact test (slurry jet). Slurry was mixed with compressed air in the nozzle and eventually injected on coating surface at high velocity. Injected slurry on coating surface resulted in a wear progression (wear rate) proportionately to the erosion strength of the coating material. Al 2 O 3 –Cr 2 O 3 composite coating possesses better erosion–wear resistance than pure Al 2 O 3 coating. [ABSTRACT FROM AUTHOR]

In this work, yttria-stabilized zirconia splats deposited onto different bond coatings were characterized. The influence of both substrate topography and temperature on splat morphologies was investigated. The substrate surface was described using a method characterized by multi-level roughness. On non-heated substrate, the peak-valley structure in scale of micrometer contributed to the formation of crater-like holes on splats. The amount of such holes was reduced with the increase of substrate temperature. It was also observed that on micro-smooth surface, island-like fragments in the center of splat would warp and their edges could be detached from the underlying substrate, especially when the substrate was heated. The obtained results reveal that the influence of substrate pre-heating on the splat formation is realized via thinning the air film near substrate surface. [ABSTRACT FROM AUTHOR]

Control of the insulator–metal transition (IMT) is a constant focus of intensive study, owing to its importance to the exploration of novel effects and in Information Technology. Complex oxides with competitive mechanisms are suitable objects of study in this regard, because of their susceptibility to external stimuli. The double-perovskite cobaltate Gd 1− x Ca x BaCo 2 O 5+ δ (GCBC) is one such oxide. In this paper, the combined effects of the GCBC structure and Ca 2+ doping have been studied for 0≤ x ≤0.4, which offers the possibility of systemic modulation of the conductivity ( σ ) of the GCBC via microwave stimulation. The maximum value and rate of increase of σ increase with the microwave power, and it has been found that the conductivity of Gd 0.8 Ca 0.2 BaCo 2 O 5+ δ can increase from 158 to 1488 S/cm at a frequency of 2.45 GHz (1500 W) over 2 min. Thus, our experimental study confirms that microwaves are crucial for controlling the electrical properties of cobaltates. [ABSTRACT FROM AUTHOR]

In the present study, Cr 2 O 3 –Al 2 O 3 composite coatings were fabricated by atmospheric plasma spraying (APS) technique. Aluminum phosphate with small fraction of Al 2 O 3 nanoparticles was applied as high-temperature sealant to enhance the anti-corrosion performance of the coatings. The inorganic sealant significantly reduced the open pore of the as-sprayed coatings and showed low shrinkage after heat treatment (600 °C, 30 min). Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and salt spray tests were used to assess anti-corrosive characteristics of the coatings. The sealants resulted in an augment of corrosion resistance properties of the coatings both before and after heat treatment, indicating effective sealing and good high temperature applicability. The sealed Cr 2 O 3 –Al 2 O 3 composite coatings have a potential use in severe corrosion condition. [ABSTRACT FROM AUTHOR]

Carbon/carbon composites were plasma sprayed with a ZrB 2 –SiC-Si/Yb 2 SiO 5 /LaMgAl 11 O 19 coating to improve the oxidation resistance at high temperature. Oxidation protection behavior of the coating was investigated by thermal cycling test using a gas flame about 2273 K. Results showed that the weight loss for the completely coated sample was only 0.30% after 10 cycles of heating for 79 min, and it gradually increased to 4.8% until the failure of coating with 24 cycles. The presence of through-cracks and horizontal cracks in the coating due to the durative oxidation of ZrB 2 –SiC–Si layer progressively accelerated the oxidation of substrate and failure of coating. [ABSTRACT FROM AUTHOR]

Gadolinium zirconate (Gd 2 Zr 2 O 7 , GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3 mol% yttria partially-stabilized zirconia (YSZ) incorporation. The fracture toughness of the composite of 90 mol% GZ-10 mol% YSZ (GZ–YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ–YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock lifetime of GZ–YSZ composite coating was improved, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ–YSZ composite coatings were discussed. [ABSTRACT FROM AUTHOR]

7.5YSZ thermal barrier coatings (TBCs) were deposited onto the stainless steel substrates using axial suspension plasma spraying (ASPS). Free-standing coatings were isothermally aged in air from 1200 to 1600 °C for 24 h and at 1550 °C for 20 to 100 h, respectively. Thermal aging behavior such as phase composition, microstructure evolutions, grain growth, and mechanical properties for thermal-aged coatings were investigated. Results show that the as-sprayed metastable tetragonal (t′-ZrO) phase decomposes into equilibrium tetragonal (t-ZrO) and cubic (c-ZrO) phases during high-temperature exposures. Upon further cooling, the c-ZrO may be retained or transform into another metastable tetragonal (t″-ZrO) phase, and tetragonal → monoclinic phase transformation occurred after 1550 °C/40 h aging treatment. The coating exhibits a unique structure with segmentation cracks and micro/nano-size grains, and the grains grow gradually with increasing aging temperature and time. In addition, the hardness ( H) and Young's modulus ( E) significantly increased as a function of temperature due to healing of pores or cracks and grain growth of the coating. And a nonmonotonic variation is found in the coatings thermal aged at a constant temperature (1550 °C) with prolonged time, this is a synergetic effect of coating sintering and m-ZrO phase formation. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, the relationship between phase transformation and stress-inducing is established. The residual stress would decrease the phase transition temperature of the γ-Al2O3 to α-Al2O3. In order to further ensure the reliability of stress-induced phase transformation, the as-sprayed Al2O3 coating was heat treated at 600°C for 24h. The Al2O3 coatings were investigated using X-ray diffraction and transmission electron microscopy. The relative low temperature heat treatment facilitates the growth of γ-Al2O3 nanocrystals, amorphous-to-nanocrystalline transition and stress-induced phase transformation in the Al2O3 coating. The heat treated Al2O3 coating has greater Vickers hardness and bending strength than the as-sprayed Al2O3 coating. However, the fracture toughness descent might be attributed to the pore-toughening effect reduction due to phase transition (from γ-Al2O3 to α-Al2O3) accompanied by a certain extent volume shrinkage and the brittleness of new generated α-Al2O3. [Copyright &y& Elsevier]

Yttria (YO) and zirconia (ZrO) stabilized by 8 and 20 wt.%YO thermal barrier coatings (TBCs) subjected to calcium-magnesium-alumino-silicate (CMAS) have been investigated. Free-standing YO, 8 and 20 wt.%YSZ coatings covered with synthetic CMAS slurry were heated at 1300 °C in air for 24 h in order to assess the effect of YO on the corrosion resistance of the coatings subjected to CMAS. The microstructures and phase compositions of the coatings were characterized by SEM, EDS, XRD, RS, and TEM. TBCs with higher YO content exhibited better CMAS corrosion resistance. Phase transformation of ZrO from tetragonal (t) to monoclinic (m) occurred during the interaction of 8YSZ TBCs and CMAS, due to the depletion of YO in the coating. Some amounts of original c-ZrO still survived in 20YSZ TBCs along with a small amount of m-ZrO that appeared after reaction with CMAS. Furthermore, YO coating was found to be particularly highly effective in resisting the penetration of molten CMAS glass at high temperature (1300 °C). This may be ascribed to the formation of sealing layers composed of Y-apatite phase [based on CaY (SiO)O and Y(SiO)O] by the high-temperature chemical interactions of YO coating and CMAS glass. [ABSTRACT FROM AUTHOR]