Search

In this study, the microstructure and thermal properties of the plasma-sprayed free-standing Lu 2 Si 2 O 7 coating before and after thermal aging at 1350 °C were investigated. The tri-layer Yb 2 SiO 5 /Lu 2 Si 2 O 7 /Si EBCs were designed and prepared onto SiC f /SiC substrates, and its thermal shocking behaviors were also explored. Results showed that the as-sprayed coating was mainly composed of Lu 2 Si 2 O 7 and amorphous phase, and significant microstructural evolution, such as grain growth and defects reduction, was observed after thermal aging. Plastic deformation and well damage tolerance including twinning and dislocation were confirmed by TEM analysis. The CTE of the coating before and after heat treatment were similar, while the thermal conductivity increased after thermal aging. After 100 thermal cycles, penetrating microcracks in the Yb 2 SiO 5 top layer were mostly stopped at the Yb 2 SiO 5 -Lu 2 Si 2 O 7 interface, implying the excellent crack propagation resistance of the tri-layer EBCs. The thermal shock behaviors were clarified based on microstructure combined with thermal stresses analysis. • Microstructure and thermal property of Lu 2 Si 2 O 7 coating were studied. • Lu 2 Si 2 O 7 exhibited microstructure and thermal property evolution after aging. • The Yb 2 SiO 5 /Lu 2 Si 2 O 7 /Si EBCs possessed excellent thermal shock resistance. • Lu 2 Si 2 O 7 could be a promising interlayer material for tri-layer EBCs. [ABSTRACT FROM AUTHOR]

Rare-earth monosilicate (RE 2 SiO 5) have been considered to be a promising material for the environmental barrier coating because of their superior thermal and mechanical properties. However, the water vapor corrosion resistance of single-component RE-silicate materials, such as Y 2 SiO 5 , should be further improved. The high-entropy design is one of the most suitable methods to enhance the corrosion resistance for single-component RE-silicate materials. In this work, the multicomponent RE-silicate ((Lu 0.25 Yb 0.25 Er 0.25 Y 0.25) 2 SiO 5 , (4HES)) and single-component RE-silicate (Y 2 SiO 5) coatings were investigated with regard to its water vapor corrosion behaviors at 1350 °C for 300 h. A thinner and denser corrosion layer was generated in the 4HES coating, indicating that the 4HES coating possessed better corrosion resistance than the Y 2 SiO 5 coating. The improved corrosion resistance is attributed to the better hydrophobic property as well as the more stable crystal structure of the rare-earth oxide and 4HES phase which was resulted from the high-entropy design. [ABSTRACT FROM AUTHOR]

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb 2 O 3 have been introduced to the silicon bond layer, and three kinds of tri-layer Yb 2 SiO 5 /Yb 2 Si 2 O 7 /(Si- x Yb 2 O 3) EBCs with modified Si bond layer by different contents of Yb 2 O 3 (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb 2 O 3 (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb 2 Si 2 O 7 during long-term oxidation. The excessive addition of Yb 2 O 3 increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb 2 O 3 deserves further consideration and improvement. [ABSTRACT FROM AUTHOR]

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb 2 O 3 could react with SiO 2 TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb 2 O 3 were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb 2 Si 2 O 7 , could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb 2 O 3 -doped Si coatings were analyzed based on microstructure and phase composition observations. [ABSTRACT FROM AUTHOR]

Herein, a biomimetic coating of hydroxyapatite (HA)–Al2O3 and HA–ZrO2 was deposited on Ti–6Al–4V-alloy using vacuum plasma spray (VPS) technique. The bond-coat of ZrO2 has been introduced between the substrate and reinforced HA coatings to study the effect of bond-coat on structural, mechanical properties and electrochemical corrosion performance of the developed coatings. In addition, the impact of thermal treatment of coating was investigated on these properties too. Coating characteristics, such as morphology, porosity, surface roughness, and crystallinity were investigated. The corrosion performance of coatings was tested in Hank’s-based salt solution (HBSS). Significant enhancement in crystallinity and surface-hardness has been witnessed after heat treatment; nevertheless, porosity reduced. The electrochemical corrosion study revealed that the corrosion resistance of heat-treated samples was better than the as-sprayed coatings samples. The intensity of XRD peaks of all coatings increased after 24h immersion in HBSS for the electrochemical test in comparison to the intensity of peaks before the corrosion test. [ABSTRACT FROM AUTHOR]

The metallic glass powder of newly designed Fe46.8-Mo30.6-Cr16.6-C4.3-B1.7 composition was employed to fabricate Fe-based metallic glass coating layers using representative thermal spray processes, viz. atmospheric plasma (plasma), HVOF and vacuum plasma spray (VPS). The microstructure and wear properties of the thermal sprayed Fe based metallic glass coating layers were investigated. XRD analysis results showed that all the three coating layers had broad halo peaks. Alloying element depletion was observed in the coating layer in the vicinity of the oxides. However, the VPS coating layer showed no depletion area. Wear test results showed that the VPS coating layers had the best wear resistance among all the three coating layers, whereas the plasma material had the poorest wear resistance property. Worn surface and cross-section observation revealed that the plasma material had particle boundary delamination because of its lower bonding force between the particles. Directly below the worn-out surface of the HVOF coating layers, microcracks were generated along the particle boundaries. By contrast, no defects were found at all directly under the worn-out surface of the VPS materials. The wear behavior of Fe based metallic glass coating layers fabricated in diverse thermal spray processes was discussed with respect to the microstructure.

In this work, ZrC-SiC-Yb 2 O 3 ternary composite coatings with different contents of Yb 2 O 3 (0, 5, 15 and 30 wt.%) were fabricated by vacuum plasma spray and the anti-ablation property above 2000 °C was evaluated. The results showed that the microstructures of the oxide scales were changed with different Yb 2 O 3 contents and then greatly modified the ablation resistant property of the composite coatings. The composite coating with 15 wt.% Yb 2 O 3 exhibited the best ablation resistance. The disappearance of SiC-depleted layer and formation of ZrC x O y were observed. The mechanism of Yb 2 O 3 on the oxidation products and ablation behaviors of ZrC-SiC system was analyzed. [ABSTRACT FROM AUTHOR]

ZrC-TiC coatings were fabricated by vacuum plasma spray and their ablation resistance were evaluated and compared with ZrC-SiC coating by a plasma flame with a heat flux of 4.02 MW/m2. The microstructure and phase compositions of the as-sprayed and ablated coatings were characterized and the function of TiC addition on the ablation resistance was investigated. The results showed that the ablation resistance of the ZrC-TiC coating was much better than that of the ZrC-SiC coating under the present ablation conditions. The decrease of surface temperatures with the increasing of TiC content were observed. (Zr, Ti)O 2 eutectic phase in liquid state was observed. The low vapor and decomposition pressures of TiO 2 , combined with the formation of (Zr, Ti)O 2 liquid contributed to the excellent ablation resistance of the ZrC-TiC coating. This work affirmed that TiC could be an ideal addition to improve the ablation resistance of the ZrC coating in harsh environment above 2000 °C. [ABSTRACT FROM AUTHOR]

As one of the important Cu alloys, brass has been implanted in many fields of architecture or fabrication industry. Muntz (Cu60Zn40) brass alloy was used to manufacture the brass coatings in this study due to showing the best corrosion resistance. Two different spray methods i.e. vacuum plasma spray (VPS) and cold gas dynamic spray (CGDS), were chosen to perform the synthesis of brass coatings aiming to avoid oxidation. Upon utilization of VPS and CGDS techniques, spray process is crucial to the microstructures, phase transformation, mechanical property, and wear and corrosion properties of Muntz alloy coatings. Here results show that no oxidation was characterized in both two processes. Nevertheless, a vaporization of 19.32 wt% Zn occurred during VPS, which would lead to the significant differences in grain size, misorientation distribution, texture and phase composition of VPSed coating from those of CGDSed coating. The CGDSed coating exhibited excellent performances due to a preservation of chemical composition of original feedstock. For example, it yielded a hardness value of 211.8 ± 9.8 HV 0.3 , which was nearly two times higher than that of VPSed coating (119.3 ± 11.6 HV 0.3). It was also shown that a significant wear rate reduction ((1.3 ± 0.018) × 10−5 v.s. (2.36 ± 0.39) × 10−4 mm3/mN) and a corrosion resistance enhancement. • Muntz coatings were firstly produced by vacuum plasma spray and cold gas dynamic spray. • Vaporization of Zn element occurring during VPS was discussed. • Grain size, misorientation distribution and phase composition were explored by EBSD. • Microstructures, mechanical properties, wear and corrosion resistance were compared. [ABSTRACT FROM AUTHOR]

• ZrB 2 -SiC-WB coating prepared by vacuum plasma spray was firstly reported and its oxidation behavior was studied. • Appropriate amount of WB greatly improved the oxidation resistance of the coatings. • WB was more stable and a better boron source compared with ZrB 2 for promoting self-healing ability. • The oxidation product of WB could increase liquid viscosity, limit O 2 diffusion, stabilize B 2 O 3 and form WO 3 -ZrO 2 eutectic. In this work, different contents (5, 10, 15 mol.%) of WB were introduced into ZrB 2 -SiC coatings which were fabricated by vacuum plasma spray technique. The oxidation resistant property of the ZrB 2 -SiC-WB ternary composite coatings was evaluated in static air at 1500 °C. The results showed that appropriate amount of WB addition significantly improved the oxidation resistance of ZrB 2 -SiC coating, as proved by the reduced weight gain and increased thickness of the liquid layer. The influence of WB and its oxidation product WO 3 were analyzed based on the microstructure changes and thermodynamic calculations in detail. [ABSTRACT FROM AUTHOR]

Plasma spraying in controlled atmosphere as very-low-pressure plasma spraying (VLPPS) allows to work with different states of matter, particularly vapor. As a result, the coating microstructure is unique (lower-scale elements, pore architecture) and the properties are improved. Another benefit of VLPPS process is the mold filling. Unlike other thermal spray processes, the vapor mode can follow the gas flow and reach the mold walls to fill. The vapor mode deposition is very soft and protects the surface state of the mold/substrate. The coating microstructure is homogeneous on all the geometry. The choice of a soluble salt mold permits to easily get back the coating and use it as a functional part. The objective is to demonstrate that VLPPS process can be used as an additive manufacturing device to create well-defined objects/pieces. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • ZrC-MoSi 2 /ZrC-SiC double-layered coating with excellent ablation resistance is firstly reported. • SiC-depleted zone and bubble burst of MoO 3 were found to be the fatal defects for ZrC-SiC and ZrC-MoSi 2 coatings, respectively. • Diffusion phenomenon of Si from ZM layer into ZS layer was observed and active oxidation of SiC was limited. • Introduction of Si from decomposition of MoSi 2 into ZrC-SiC layer is a promising way to solve the problem of SiC-depleted zone. Abstract A double-layered composite coating was designed for optimizing the ablation resistance of ZrC coating, which contained external ZrC-SiC and inner ZrC-MoSi 2 layers. The ablation resistance of the coating was evaluated and compared with the single-layered ZrC-SiC and ZrC-MoSi 2 coatings. SiC-depleted zone and bubble burst of MoO 3 were found to be the fatal defects for the ZrC-SiC and ZrC-MoSi 2 coatings, respectively. For the double-layered coating, the external layer acted as oxygen diffusion barrier and the inner layer offered extra Si source for the external one. The double-layered design effectively reduced the destructive effect of SiC-depleted zone and bubble burst. [ABSTRACT FROM AUTHOR]

In the present study, reinforced hydroxyapatite (HA) coatings (HA + 10 wt% Al 2 O 3 and HA + 10 wt% ZrO 2 ) were deposited on SS-316 L substrate with an intermediate layer (bond coat) of zirconia by vacuum plasma spray technique. The so-formed reinforced HA coatings were heat treated at 700 °C for 1 h. The influence of post coating heat treatment on phase composition, microstructure, mechanical and electrochemical corrosion properties were investigated. As-sprayed and heat treated coatings were characterized by x-ray diffraction, scanning electron microscope, surface roughness, porosity and crystallinity. Results showed that after post coating heat treatment, the structural integrity of HA has been completely re-established. Moreover, significant drop in porosity has been observed due to the sintering effect produced by heat treatment. Considerable improvement in nanohardness and shear strength was witnessed; however, the nanohardness of top layer was decreased after annealing due to the weak bonding of partially melted and un-melted particles with fully melted splats caused by diffusion process. As-sprayed coatings exhibited higher wear resistance compared to heat treated coatings. Nevertheless, post coating heat treatment effectively enhanced the corrosion resistance of coatings, since, the heat treatment lead to densification of coatings microstructure which further reduces the active sites for dissolution. [ABSTRACT FROM AUTHOR]

As structural materials for future fusion power plants, reduced activation ferritic martensitic steels as EUROFER97 can be used. Unfortunately, the interaction of the plasma with the steel would result in a limited lifetime, so protective layers are investigated. An excellent protective material is tungsten, as it shows unique properties with respect to low sputtering, high melting points and low activation. However, the mismatch of thermo-physical properties between tungsten and EUROFER97 can lead to large stress levels and even failure. A possible way to overcome this problem is the use of functionally graded material (FGM). The paper will describe the manufacture of these FGMs by vacuum plasma spraying and their characterization. First of all, two different feeding lines have been used to produce the coatings. A major problem lies in different melting points of tungsten and steel. So the particle size distribution has to be adjusted to achieve sufficient melting of both materials during the spray process. In a second step, the feeding rates were optimized to obtain the wanted amount of tungsten and steel phases in the graded structures. In a thermal spray process, the gradient cannot be made continuously, however it has to be applied in a step-wise manner. In this investigation, samples with 3 and 5 different concentrations (excluding the pure steel and tungsten part) have been produced. The microstructures of these layers have been investigated. In addition, hardness was measured and the residual stress state was determined by the hole drilling method. [ABSTRACT FROM AUTHOR]

In this work, the phase composition and microstructure evolution of vacuum plasma-sprayed MoSi2 coating between room temperature and 1200 °C in air was evaluated and characterized. The results showed that hexagonal MoSi2 (h-MoSi2) became the main phase in the deposited coating, which remained even after 50 h oxidation at 500 °C, exhibiting excellent thermal stability. MoO3 bundles and SiO2 clusters were generated by consuming tetragonal MoSi2 (t-MoSi2) after 1 h, and white powders formed on the coating’s surface after 10-h exposure to air at 500 °C. Most h-MoSi2 transformed to t-MoSi2 at 800 °C; moreover, a protective silica layer formed on the coating surface. Similar phenomenon was observed for the coating exposed to 1000 °C where grain growth also occurred. Vacuum heat treatment at 900 °C effectively improved the thermal stability of the MoSi2 coating. The formation of silica layer alleviated negative effects of structural defects and helped the MoSi2 coating serve as a protective coating for varied substrates. [ABSTRACT FROM AUTHOR]

To improve the ablation resistance of ZrC coating, MoSi 2 incorporated ZrC composite coatings were fabricated by vacuum plasma spray. The ablation resistance of the composite coatings was evaluated using a plasma jet with a heat flux of 1.94 MW/m 2 . The phase compositions and microstructures of the coatings before and after ablation were investigated, and the ablation mechanisms and effect of MoSi 2 were analyzed based on thermal dynamics and microstructure changes. Results showed that MoSi 2 addition could improve the ablation resistance of ZrC coating by means of decreasing the surface temperature and changing the microstructure of the oxidation layer. Si derived from the decomposition of MoSi 2 , which occurred within coating, was beneficial to maintain the thickness and integrity of the SiO 2 layer and reduce the oxygen pressure beneath. The thickness of the SiO 2 layer was related to the formation rate (V f ) and the consumption rate (V c ) of SiO 2 . The diffusion of Si was in favor of increasing the value of V f . MoSi 2 could be one choice to improve the ablation resistance of the ZrC coating. [ABSTRACT FROM AUTHOR]

Hydroxyapatite powder reinforced individually with 10 wt percentage (wt%) of Al 2 O 3 and ZrO 2 (HA + 10 wt% Al 2 O 3 and HA + 10 wt% ZrO 2 ) was thermally sprayed onto SS-316L substrate with a bond coat of Zirconia by vacuum plasma spray (VPS) technique. The resulted coatings were heat treated at 700 °C for 1 h to study its effects on microstructural and mechanical properties of the deposited coatings. The characterization of the coatings was carried out using scanning electron microscope, x-ray diffraction, porosity, surface roughness and crystallinity using Rietveld analysis. The results indicated that after post coating heat treatment substantial decrease in porosity was witnessed along with significant improvement in crystallinity. Besides, the hardness across the cross-section of the coatings and bond strength was considerably improved; however the hardness of top coat was reduced owing to the loosening of un-melted and partially melted particles by diffusion process which takes place during heat treatment. [ABSTRACT FROM AUTHOR]

Two reinforced hydroxyapatite (HA) coatings with an intermediate layer of zirconia were deposited on Ti-6Al-4V by vacuum plasma spray (VPS) technique. In first coating, HA was reinforced with 10 wt % AlO whereas in second coating, HA was reinforced with 10 wt % ZrO. The objective of this study was to investigate the microstructure, phase formation and mechanical properties like hardness and bond strength of as-sprayed coatings and the coatings after post coating heat treatment at 700 °C for 1 h. The characterization of the coatings was performed by using SEM/EDAX, XRD, porosity, crystallinity and roughness measurement. The coatings were also evaluated for mechanical properties like hardness and tensile bond strength. It was observed that after post coating heat treatment, crystallinity increased and porosity decreased which indicated recrystallization of amorphous phases of as-sprayed coatings. Heat treatment resulted into improvement in cross-sectional hardness, however sharp decrease in bond strength was observed. [ABSTRACT FROM AUTHOR]

The aim of this present work was to deposit reinforced hydroxyapatite coatings with a bond coat of zirconia on Ti-6Al-4V substrate using the vacuum plasma spray technique in order to improve the surface properties. To achieve that, HA + 10 wt% Al2O3 and HA + 10 wt% ZrO2 coatings with an intermediate layer of zirconia were deposited on Ti-6Al-4V implant substrate. Further, these coatings were subjected to post-coating heat treatment at 700 °C for 1 h. The morphology and phase analysis of bio-ceramic powders, as-sprayed and heat-treated coatings, have been characterized using a scanning electron microscope and an X-ray diffractrometer. Crystallinity, surface roughness, porosity, hardness and shear strength were also investigated. The wear behaviour of the coatings was evaluated using a Taber abrasion tester as per ASTM F1978 standard. The results indicate that heat-treated coatings exhibit lower wear resistance than as-sprayed coatings and show lower porosity and higher crystallinity and hardness after heat treatment. Slight improvement in shear strength was observed after heat treatment. [ABSTRACT FROM AUTHOR]

The surface damages due to the transient heat load and particle fluxes on the surface of the coated stainless-steel (SS) samples have been investigated. University of Saskatchewan compact torus injector has been used to inject the high heat load and the particle flux on the surface of stainless samples with three different types of coatings. The type 1 samples were made from a SS plate coated by tungsten. The other two samples had additional films of either nickel (type 2) or chromium (type 3) on the type 1 samples. The vacuum plasma spray technique has been applied for the coating of the samples. Surface cracking and melting have been observed on the surface after the samples were impacted by many high-speed and dense plasmoids. [ABSTRACT FROM PUBLISHER]

An efficient and effective method, vacuum plasma spray, has been utilized to fabricate tungsten disilicide coating in the present work. The phase composition and microstructure characteristics of the as-sprayed coating have been focused as well as its thermal stability and oxidation behavior from room temperature to 1500 °C. Results showed that the coating consisted of tetragonal WSi 2 and a little amount of tetragonal W 5 Si 3 , presenting a uniform and lamellar structure with porosity less than 6%. The WSi 2 coating was stable at temperatures below 500 °C, while rapid oxidization occurred between 800 and 1200 °C leaving loose and coarse surface consisting of mixed SiO 2 and WO 3 . Excellent oxidation resistance at temperature range of 1300–1500 °C could be observed with dense and integrated SiO 2 film formed on the coating surface. The oxidation mechanism was also discussed based on theoretical analysis, which could provide strong illustration of the oxidation processes of the WSi 2 coating at different temperatures. [ABSTRACT FROM AUTHOR]

Metal silicides are often utilized to enhance high temperature oxidation resistance for refractory ZrB 2 ceramics or coatings owing to the capability to form glassy silica layer. However, the role of metal elements in silicides on maintaining the glassy layer needs to be elucidated. In the present study, TaSi 2 and MoSi 2 incorporated ZrB 2 composite coatings were respectively prepared by vacuum plasma spray (VPS) technology and their oxidation behaviors were comparatively investigated. The oxidation tests carried out in ambient air at 1773 K confirmed the crucial function of glassy silica layer on high temperature oxidation resistance for both of the metal silicide incorporated ZrB 2 coatings. More importantly, the metal elements in silicide additives were revealed to have great influence on the integrity of glassy silica layer. The oxidation of element Ta in TaSi 2 generated plate-like compound TaZr 2.75 O 8 , which impaired the consistency and completeness of silica layer, creating opportunity for oxygen diffusion. However, MoB, the oxidation product of element Mo in MoSi 2 , could help supporting the integrity of silica layer. MoB played the role defending the wholeness of silica layer and reducing the evaporation of B 2 O 3 and MoO 3 at the same time. Better oxidation resistance of ZrB 2 –MoSi 2 composite coating was found as compared with ZrB 2 –TaSi 2 composite coating. [ABSTRACT FROM AUTHOR]

TaC-25 vol%MoSi 2 and TaC-25 vol%SiC composite coatings were prepared by vacuum plasma spray on SiC coated C/C composites, aiming to improve their ablation resistance. The phase composition and microstructure of the coatings were characterized by scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction, respectively. The ablation behavior of the coatings was evaluated by a plasma flame over 2000 °C for 60 s. TaC coating was also fabricated for comparison. The results showed that MoSi 2 and SiC uniformly distributed in the composite coatings. The ablation tests demonstrated that the TaC coating was totally oxidized during the ablation process with significant volume expansion. The formed Ta 2 O 5 was porous and blown away by the gas flow. The surface layer of the TaC–MoSi 2 coating was oxidized during ablation and the oxide layer was partially detached from the underneath layer due to the evaporation of MoO 3 . The TaC–SiC coating exhibited a low mass loss of 0.32%, 68% lower compared to the TaC coating, and exhibited excellent ablation resistance. It was thought that the comparatively dense Ta 2 O 5 –SiO 2 layer formed on the coating surface was beneficial for both the oxidation–corrosion and mechanical-erosion resistance. [ABSTRACT FROM AUTHOR]

This work was aimed at reporting a new method for fabrication of TaSi 2 coating. In this research, vacuum plasma spray (VPS) process was applied to fabricate TaSi 2 coating. The chemical composition and microstructure of the coating was characterized by SEM, XRD, TEM and EDS. Its thermal stability was evaluated by TG-DTA and oxidation tests at 500 °C–1000 °C. The results showed that the TaSi 2 coating was composed of hexagonal TaSi 2 phase and presented dense lamellar microstructure combining defects, such as pores, interfaces and oxygen impurity. There was a sharp weight increase in TG curve at range of 800 °C–1000 °C with an exothermic phenomenon. The coating exhibited excellent thermal stability at 500 °C up to 50 h in atmospheric environment. Obvious oxidation was observed at 800 °C. The TaSi 2 coating was completely oxidized and fragmented into pieces after oxidation at 900 °C for 1 h. The oxidation mechanism was also discussed. [ABSTRACT FROM AUTHOR]

Tungsten (W) is the primary candidate material as a plasma facing material in fusion devices, as for its high melting temperature, good thermal conductivity and low sputtering rate, and vacuum plasma spray (VPS) technique is preferred as it is applicable for large area without brittle interlayer, but the thermal conductivity of W layer is very poor, and easy to detach, mainly caused by its porous structure. W Friction stir processing (FSP) was applied on VPS-W to improve these poor properties, and it was suggested that FSP can contribute to significant improvement in both mechanical and thermal properties of the VPS-W coating. [ABSTRACT FROM AUTHOR]

In order to examine the impacts of friction stir processing (FSP) on irradiation effects in vacuum-plasma-spray (VPS) coated tungsten (W), nano indentation hardness was evaluated of three kinds of W materials after self-ion-irradiation to 5.0–5.4 dpa at 500 and 800 °C. The VPS-FSP clearly got grains refined and isotropic compared to bulk-W and the as-VPS-W. Nano indentation hardness remains unchanged for the as-VPS-W and VPS-FSP × 2-W irradiated to 5.4 dpa at 500 °C and it decreased from 1 dpa at 800 °C, while typical irradiation induced hardening was observed for the bulk-W irradiated at 500 °C. [ABSTRACT FROM AUTHOR]

A series of (HfN)1−x(TaN)x, ceramics with x representing the starting powder blend compositions of 0.0, 18.8, 28.1, and 46.7 at.%, have been fabricated by vacuum plasma spraying. During the plasma spraying, the mixture lost approximately 25 at.% nitrogen facilitating the precipitation of metallic and metal-rich nitride phases. These specimens underwent static air oxidation exposure up to 1700 °C. In general, it was found that the addition of tantalum nitrides to the hafnium nitrides resulted in poorer oxidation behavior. However, the 18.8 at.% specimen deviated from this trend and had the lowest observed mass change. This specimen formed a dark-colored oxide scale, indexed as Hf6Ta2O17, which acted as a passivation layer. Within the scale, hafnium oxynitride phases were observed. A transformation pathway in forming these rhombohedral oxynitride phases is proposed by the filling in of oxygen in the light element interstitial locations of the rhombohedral ε-Hf3N2 and ζ-Hf4N3 structures. [ABSTRACT FROM PUBLISHER]

Plasma spray is one of the suitable technologies to deposit carbide coatings with high melting point, such as ZrC. However, in the spray processes performed under atmosphere, oxidation of the carbide powder is inevitable. To investigate the influence of the oxidation behavior of feedstock on microstructure and ablation resistance of the deposited coating, ZrC coatings were prepared by atmospheric and vacuum plasma spray (APS and VPS) technologies, respectively. SiC-coated graphite was applied as the substrate. The obtained results showed that the oxidation of ZrC powder in APS process resulted in the formation of ZrO and ZrO phases. Pores and cracks were more likely to be formed in the as-sprayed APS-ZrC coating. The VPS-ZrC coating without oxides possessed denser microstructure, higher thermal diffusivity, and lower coefficients of thermal expansion as compared with the APS-ZrC coating. A dense ZrO layer would be formed on the surface of the VPS-ZrC-coated sample during the ablation process and the substrate can be protected sufficiently after being ablated in high temperature plasma jet. However, the ZrO layer, formed by oxidation of the APS-ZrC coating having loose structure, was easy to be washed away by the shearing action of the plasma jet. [ABSTRACT FROM AUTHOR]

TiAlCrY coating was fabricated on TiAl single crystals by vacuum plasma spray (VPS) technique to investigate the oxidation resistance and microstructure evolution via isothermal oxidation tests at 1100 and 1200 ℃, respectively. The results indicated that the oxidation resistance of TiAlCrY coating was excellent with the maximum spallation degree of oxide layer extremely below the critical value of coating failure. The oxide growth kinetic curves of the TiAlCrY coating complied well with parabolic law, and the oxidation rate constants were comparable to those of typical MCrAlY coatings on nickel superalloys. In particular, the strong α-Al 2 O 3 formation ability in the initial oxidation process and the great chemical compatibility at coating-substrate interface contributed to the durable oxidation resistance of TiAlCrY coating. This work highlights the potential application of VPS-TiAlCrY coating for TiAl single crystals at 1100–1200 ℃. • It is the first time to investigate the oxidation resistance of VPS-TiAlCrY coating on TiAl single crystals. • The oxidation resistance of TiAlCrY coating on TiAl single crystals was excellent. • The chemical compatibility at TiAlCrY-TiAl interface was better than that of the conventional MCrAlY coated TiAl systems. [ABSTRACT FROM AUTHOR]

The high-entropy design of rare-earth silicates has been expected to improve CMAS corrosion resistance, which is essential to the development of EBCs for aero-engines. In this work, CMAS corrosion behaviors of plasma-sprayed high-entropy monoslicates ((Lu 0.25 Yb 0.25 Er 0.25 Y 0.25) 2 SiO 5 and (Lu 0.2 Yb 0.2 Er 0.2 Ho 0.2 Y 0.2) 2 SiO 5) and Y 2 SiO 5 coatings at 1350 °C for 25–50 h were investigated and compared. The corrosion products included apatite and garnet based on the dissolution-reprecipitation mechanism. The high-entropy coatings possessed improved corrosion resistance compared with the Y 2 SiO 5 coating, which was mainly due to the doped RE elements with smaller ionic radius could improve the stability of crystal structure and form garnet phase more easily. • (4RE 0.25) 2 SiO 5 and (5RE 0.2) 2 SiO 5 coatings possess improved corrosion resistance. • The doped RE with smaller ionic radius are easier to form garnet during corrosion. • RE ionic radius affects the crystal structure stability and corrosion behaviors. [ABSTRACT FROM AUTHOR]

The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 °C/1050 °C surface/substrate exposure temperatures. [ABSTRACT FROM AUTHOR]

Because of the nature of the plasma spraying process, the physical and mechanical properties of vacuum-plasma-sprayed structures of Ti-6Al-4V alloy are completely different from those of conventionally manufactured alloys such as bulk materials from casting and forging. To obtain desired mechanical and physical properties, vacancy and internal defects must be reduced, splat boundaries must be eliminated, and optimal phase compositions should be obtained through postdeposition heat treatments. To determine appropriate heat treatment processes, one needs to study the kinetic behavior of the as-sprayed microstructure at elevated temperatures. In the current study, the kinetics of the solid transformations found in Ti-6Al-4V alloys produced during the vacuum plasma spraying process was studied based on the Johnson-Mehl-Avrami theory. For the kinetic behavior of this alloy, the nonconstant temperature dependence of the transformation rate constant exhibits an irregularity at 900 °C, marking a change in the transformation mechanism. For the lower-temperature (<900 °C) curves, the constant gradient indicates a lack of change in the transformation mechanism, including homogeneous nucleation, with growth of α phase. For higher temperatures (>900 °C), a gradient change indicates a change in the transformation mechanism. The first mechanism was the formation of α-phase grain boundary, and the second mechanism was α-plate nucleation and growth from grain boundaries. The value of the transformation rate constant in the kinetics study of as-sprayed Ti-6Al-4V alloy was much higher than for material produced by the casting method. Using the results obtained from the kinetics of the $$ \upbeta \to \upalpha + \upbeta $$ phase transformation at different constant temperatures, a time-temperature-transformation (TTT) diagram for as-sprayed Ti-6Al-4V alloy was developed. [ABSTRACT FROM AUTHOR]

Abstract: Five specimens of (TaC)1−x (HfC) x , where x is 0.0, 0.3, 3.0, 16.5, and 19.8at.%, were fabricated by vacuum plasma spraying. As HfC content increased, the grain size was reduced and the volume fraction of TaC, Ta2C and Ta4C3 changed, with the TaC phase being more dominate. Reduced grain sizes also lead to an increase in Knoop hardness values. The reduction of grain size with increasing HfC content has been explained by the system being driven further into a compositionally lower melting temperature phase field. This increase in liquid fraction caused greater under-cooling and the formation of more nucleation sites that lead to a finer grain size. The changing volume fraction of (TaC)1−x (HfC) x and sub-stoichiometric tantalum carbide phases has been contributed to the loss of carbon intrinsic to vacuum plasma spray processing. [Copyright &y& Elsevier]

In this study, molybdenum disilicide (MoSi) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi coatings was also explored. The results show that the MoSi coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi phases. A small amount of tetragonal MoSi phase is randomly distributed in the MoSi matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO film formed on their surface. A thick layer composed of MoSi is found to be present under the SiO film for the MoSi coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place. [ABSTRACT FROM AUTHOR]

This study determines the optimal processing parameters for vacuum plasma spraying boron carbide (BC), employing an integrated approach based on the Taguchi design method, a neural network, and a genetic algorithm (GA). The proposed method comprises two stages. In the first stage, the Taguchi design method is used to establish a preliminary solution for the optimal set of processing parameters. In the second stage, the experimental results acquired from the Taguchi trials are used to construct a neural network model of the spraying process. A GA is then used to establish the optimal combination of processing parameters. The experimental results show that the coating void content of the specimen prepared using the processing parameters identified by the GA is significantly lower than that of the specimen prepared using the processing parameters identified by the Taguchi method alone. [ABSTRACT FROM AUTHOR]

TiC-based composite coating using Mo as an additive has been fabricated by vacuum plasma-spraying, and then the phase composition and microstructure of TiC-Mo composite coating were investigated. Wear resistance of the TiC-Mo composite coating was comparatively studied with pure TiC coating. The experimental results showed that the microstructure of the TiC-Mo composite coating was relatively homogeneous and compact, exhibiting typical lamellar structure of plasma-sprayed coating. Orientated columnar grains of TiC can be found in the composite coating, and a (Ti, Mo)C transition phase was also observed. Due to the formation of (Ti, Mo)C transition phase, strong interface between TiC and Mo splats was obtained, which positively influenced the wear performance of the composite coating. As compared with pure TiC coating, the TiC-Mo composite coating exhibited improved wear resistance both at low and high loads. Wear mechanisms for the TiC coatings have been changed by adding Mo element. [ABSTRACT FROM AUTHOR]

Abstract: In this study, tungsten coatings were fabricated onto copper alloy substrate without and with gradient Cu+W and Ti+W bond coatings via vacuum plasma spraying technology (denoted as W NON, W Cu+W and W Ti+W, respectively). The bonding strength and residual stresses of coatings were measured and thermal loading tests with high heat flux were performed. The obtained results showed that the W Cu+W coating had the highest bonding strength among the three kinds of coatings. The value of residual stresses in the W Cu+W coating was much lower than those in the W NON and W Ti+W coatings. The existence of hydrogenated titanium and high residual stresses resulted in the low bonding strength of the W Ti+W coatings. Microcracks were observed on the surface of the W Ti+W and W NON coatings after the thermal loading experiments. However, the W Cu+W coating kept intact and no microcracks were found on its surface. It indicated that the addition of the gradient layer of Cu+W improved the mechanical and thermal loading properties of tungsten coatings, and it had potential application used as bonding coatings for plasma sprayed tungsten coatings on copper alloy substrates in fusion reactors. [Copyright &y& Elsevier]

Abstract: This paper deals with the diagnostic yttria-stabilized zirconia (YSZ) in-flight particles in Vacuum Plasma Spray (VPS) process using an optical measurement device. Particle velocity, temperature and diameter were correlated to spray distance under a fixed chamber pressure of about 14kPa. Experiments were carried out with a two-color pyrometer. Results show that correlations can be satisfactory described with linear relationships. Particle velocity and temperature decrease when increasing spray distance whereas particle diameter exhibits a linear increase with the spray distance. [Copyright &y& Elsevier]

Abstract: The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr3C2–NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 °C and 800 °C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 °C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 °C. Potentiodynamic polarization showed that the annealed coating at 800 °C has a better corrosion resistance than the as-deposited coating. The corrosion current density (I corr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating–substrate interface, which ensures the presence of a metallurgical bond. [Copyright &y& Elsevier]

The effect of particle size distribution on the degradation behavior of plasma sprayed CoNi- and CoCrAlY coatings during isothermal oxidation was investigated, in terms of the oxygen content, porosity, surface roughness, and oxide scale formation. The results show that the degradation of both coatings was considerably influenced by the starting particle size distribution. It also shows that in the as-sprayed vacuum plasma spray (VPS) coatings the oxygen content on the coating surface increased significantly with decreased average particle size. But after thermal exposure, the difference of the oxygen contents between the coatings with different particle size was decreased. The powder with various particle size resulted in low porosity inside the coatings during the deposition process. The surface roughness of the coatings increased with increased particle size. The small particles produced a relatively smooth surface, and the oxide growth in the coating deposited by small particle was slower than that in the large particle coating. [ABSTRACT FROM AUTHOR]

Abstract: The performance of several overlay coatings on Cu–8at.%Cr4%Nb alloy (GRCop-84) thermally cycled in a high heat flux hydrogen–oxygen combustion flame are reported. Finite element analyses (FEA) were conducted to evaluate the expected performances of coatings with different thermal conductivities as well as to determine a suitable thickness for coating deposition. Top coats consisting of Cu–26%Cr, NiAl and NiCrAlY along with a Cu–8%Cr bond coat were vacuum plasma sprayed on GRCop-84 thin sheet specimens. It was observed that the Cu–26%Cr coating formed blisters whereas the NiAl and NiCrAlY coatings showed no significant differences between the exposed and unexposed specimens. Water-cooled rectangular specimens specially designed to simulate a rocket engine liner were coated with a NiCrAlY-top coat using either a Cu–8%Cr or a Cu–8%Cr–1%Al bond coat. Preliminary tests conducted on these specimens showed no visible evidence of coating spallation after 40 cycles. The measured temperatures and heat fluxes were used in FEA models to estimate the stresses, strains and temperatures across the coating interfaces in these specimens. [Copyright &y& Elsevier]