Search

In this study, individual Al2O3 and Cr2O3 coatings and Cr2O3-25, 50, 75 wt% Al2O3 composite coatings were applied on carbon steel by atmospheric plasma spraying method. Corrosion experiments were performed on as-sprayed and epoxy resin sealed coatings including potentiodynamic polarization, electrochemical impedance spectroscopy and long-term immersion in 3.5 wt% NaCl solution. Phase composition and microstructure of the coatings were investigated by x-ray diffraction, optical microscopy and scanning electron microscopy, before and after the corrosion experiment. The results showed that the Cr2O3 coating exhibited the best corrosion resistance, due to the densest microstructure and highest adhesion strength. The Cr2O3-25 wt% Al2O3 coating had the highest interconnected porosities and thus had the least corrosion resistance compared to other coatings. In general, the as-sprayed coatings induced a maximum increase of 3.93 times the polarization resistance (Rp) in the polarization experiment and a 3.5 times increase in the charge transfer resistance (Rct) in the EIS experiment, which was not significant. Stresses caused by increased volume of corrosion products in the coating-substrate interface resulted in the spallation of Cr2O3-25, 50 wt% Al2O3 coatings from the substrate over long-term of immersion. The adhesion strength of the coatings was a determining criterion for the long-term durability of the coatings. The sealing treatment resulted in a significant increase in Rp and Rct.

In this research, the effect of the ZrB2 middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB2–SiC layers containing 10, 20 and 30 wt% SiC and, ZrB2 as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB2–SiC layer between SiC and ZrB2 coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, −0.072 × 10−4 g.cm−2.s−1, 0.0166 μm s−1 respectively had the best erosion resistance under oxy propane flame. In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB2-20% wt. SiC/ZrB2 coating had the lowest erosion rate.

This paper aimed to design and optimize the structure of a thick thermal barrier coating by adding graded layers to achieve a balance between high thermal insulation capacity and durability. To this end, conventional TBC, conventional TTBC, and functionally graded TTBCs were deposited on the superalloy substrate by air plasma spraying. To determine the quality of the bond strength of the coatings, the bonding strength was measured. The durability of coatings was evaluated by isothermal oxidation and thermal shock tests. Then, at a temperature of 1000 °C, the thermal insulation capacity of the coatings was carried out. The microstructure of the coatings was characterized by a scanning electron microscope. The results showed that the thickness of the TGO layer formed on the bond coat in the conventional TBC and TTBC under the oxidation test at 1000 °C after 150 h was 2.79 and 2.11 μm, respectively, whereas, in the functionally graded TTBC samples, no continuous TGO layer was observed as a result of internal oxidation. The functionally graded TTBC presented higher durability than conventional TTBC due to improved bonding strength, thermal shock resistance, and the lack of a TGO layer at the bond/top coat interface. Also, the thermal insulation capacity of the functionally graded TTBC (with 1000 μm thickness of YSZ coating) was better than TTBC.

In the present work, nanostructured La2(Zr0·7Ce0.3)2O7/YSZ functionally graded coatings were prepared by applying the atmospheric plasma spraying process. The La2(Zr0·7Ce0.3)2O7 compound was synthesized via the calcination process to get a nanostructured precursor precipitate at 600 °C. Two types of nanostructured functionally graded coatings were applied on the Hastelloy X substrates coated with NiCrAlY and 8YSZ. The agglomerates of La2(Zr0·7Ce0.3)2O7 were then used to apply the type I coating. Type II coating was then applied by the plasma spraying of the heat-treated agglomerates of La2(Zr0·7Ce0.3)2O7. The results of the phase and microstructural analysis showed that a nano-composite microstructure comprising the main phases of lanthanum zirconate with the partial solubility of cerium cation (LZ) and the solid solution of cerium lanthanum zirconate (LC) was formed in both types of coatings. Type II coating was relatively denser; so, it could be considered as a dense nanostructured coating. The results of the oxidation tests also showed that only the type II coating had higher oxidation resistance, as compared to that of the conventional YSZ coating. Parabolic oxidation rate constant of this coating was equal to 0.114 μm2/h. On the other hand, the oxidation rate constant for the type I coating and conventional YSZ coating was measured to be 0.141 μm2/h and 0.147 μm2/h, respectively. In addition, the results of the thermal insulation capability test indicated that the thermal barrier capability of the type I coating was 59% higher than that of the conventional YSZ coating. However, under similar conditions, this percentage was 29% for the type II coating.

In this study, for the first time, ZrC coating was deposited on SiC coated graphite by optimized atmospheric plasma spray (i.e. solid shielding shrouded plasma spray (SSPS)) method. Then SSPS-ZrC c...

In this research, the high-duration ablation resistance of SiC-gradient coating was investigated. SiC coating was prepared on graphite by reactive melt infiltration method at 1873 K under the argon...

In this study, La2(Zr0.7Ce0.3)2O7 nanostructured coatings were applied by plasma spraying of the nanostructured agglomerates. Initial nanostructured precursor, precursors calcined at 400°C, 600°C a...

A ternary-phase SiC/ZrB2-MoSi2-SiC multilayer coating was prepared on graphite by two-step reactive melt infiltration (RMI) method. The formation mechanism of the coating was studied by HSC chemistry software 6.0. The erosion resistance of the coating was investigated by supersonic flame erosion test at 90° angle, temperature of 2173 K and speed of 1400 m/s (Mach 4) for 120 s. Erosion test results revealed that the SiC/ZrB2-MoSi2-SiC multilayer coating had very good erosion resistance. Weight change percentage, mass erosion rate and linear erosion rate of the coating were −0.18 %, −0.027 × 10−3g cm−2 s−1 and 0.33 μm s−1, respectively. Microstructural characterization demonstrated that interesting structures such as rod-like, flake-like, spherical, worm-like and fibrous structures were formed during erosion test. The erosion mechanism of ZrB2-MoSi2-SiC coatings is controlled both chemically and mechanically. The reduction of chemical degradation can be attributed to the presence of MoSi2 particles and the reduction of mechanical degradation can be related to the presence of ZrB2 particles.

In this study, a SiC/ZrB2 multilayer coating with a functional gradient structure was prepared on graphite to increase the ablation resistance. The SiC coating as the inner layer was applied by the reactive melt infiltration method. Then, a ZrB2 outer layer coating was applied by argon gas shrouded plasma spray. The effect of the shielding gas flow rate on the amount of oxides and quality of the ZrB2 coating was studied. To evaluate the ablation resistance of the coatings, the specimens were exposed perpendicular to an oxy-propane flame for 60 second with a temperature of 2473 K and heat flux of 3000 kW/m2. The single-layer SiC coating showed 24.24 pct mass loss. The results indicated that applying the ZrB2 coating significantly improved the ablation resistance and reduced the mass ablation rate. In the plasma spray process, by applying the argon shrouding with a flow rate of 150 L/min, the oxidation of ZrB2 phase decreased from 41.6 to 4.8 pct. In addition, the weight loss and mass ablation rate decreased from 12.79 and 1.857 × 10−3 g/cm2/s to 0.12 pct and 0.0393 × 10−3 g/cm2/s, respectively. The improvement of the ablation resistance of the coating prepared by argon gas shrouded plasma spray could be attributed to the lower amount of oxides and pores in the as-sprayed coating and better cohesion of splats.

In this research, the tribological and corrosion protection properties of atmospheric plasma sprayed Cr 2 O 3 -20YSZ (CZ) and Cr 2 O 3 -20YSZ-10SiC (CZS) composite coatings on 304L stainless steel substrates were evaluated and compared to pure Cr 2 O 3 (C) coating. Microstructural evaluations were carried out using an optical microscopy, X-ray diffraction (XRD) analysis, and a Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). The ball-on-disk wear test was performed using an alumina counterpart at room temperature. The protective efficiency of the coatings with/without sealing treatment was evaluated by electrochemical potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl solution. Evaluation of wear tracks showed that enhancement of the wear resistance of the composite coatings was ascribed to the zirconia phase transformation toughening and the presence of more ductile tribofilms participated in pore filling during the wear. The corrosion measurements revealed that the protection properties of the coatings were strongly dependent on the porosity content so that the as-sprayed coatings due to their porous nature could not protect the substrate. However, filling the porosity using an epoxy sealant prevented the penetration of the corrosive environment into the substrate and therefore increased the protective efficiency of the coatings. Moreover, the density of the unfilled defects such as micro-cracks determines the protection properties of the sealed coatings, so that the CZ coating due to its highest toughness had the lowest micro-crack density and thus the highest protective efficiency.

A bond coating of NiCrAlY followed by top coating of Al2O3/YSZ has been applied on an Inconel-718 using an air plasma spraying method, and radiation of a Nd-YAG laser was used to glaze the coated samples. The hot corrosion response of the coatings was measured in 55%V2O5 + 45% Na2SO4 at a temperature of 1000 °C for 3 and 30 h. Leaching of Y2O3 from YSZ, due to the reaction of V2O5 and Na2SO4, was found to be the major reason for the corrosion. Eventually, this led to the formation of YVO4 and tetragonal (t) zirconia to monoclinic (m)-ZrO2 phase transformation. In parallel, the volume fraction of m-ZrO2 in the sprayed coating decreased from 62 to 46%. Reduction of the m-ZrO2 in the glazed coating indicated an improvement of hot corrosion resistance. The results demonstrate that the existence of a dense Al2O3 layer on the upper surface of the sprayed coating led to increased resistance against corrosion. In addition, the laser operation reduced the surface roughness leading to the reduction of the reactive specific surface and the effective contact area between the corrosive materials and the glazed layer.

In order to provide a short-term oxidation protection system for graphite at high temperature, an aluminum metaphosphate coating was prepared on the surface of graphite by cathodic electrochemical treatment, and its oxidation protective ability was investigated using an isothermal oxidation test. The microstructure, chemical composition, and phase components of the aluminum metaphosphate coating were respectively analyzed through scanning electron microscope, dispersive X-ray analysis, and X-ray diffraction. The results showed that aluminum metaphosphate coating with a thickness of about 24 μm provides good oxidation resistance for graphite at 1250 °C, and it reduces the oxidation rate of graphite in a 5-h period by a factor of two. The good oxidation protective ability of the as-prepared coating at 1250 °C is related to the formation of a layer consisting of aluminum metaphosphate and aluminum orthophosphate phases.

Al2O3, Cr2O3 and Cr2O3-25, 50 and 75 wt% Al2O3 composite coatings were deposited by atmospheric plasma spraying and their wear performance against tungsten carbide were evaluated using a ball-on-disk configuration under mild conditions. It was found that at the short sliding distances, alumina and rich-alumina coatings show better anti-wear performance that is related to flattening behavior and higher toughness. However for longer sliding distances, friction coefficients increase and lead to progressive weight losses. On the other hand, chromia and rich-chromia coatings show stable friction coefficients and correspondingly lower wear rates due to effective formation of transferred tribofilm from counterpart on these coatings. The Cr2O3-25 wt%Al2O3 showed the best wear performance and selected for investigating effect of some post-treatment e.g. grinding, and lubricating. Correlations between microstructure, mechanical properties and wear performance as well as wear mechanisms were discussed in detail.

The aim of the present research was to investigate the effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings. In this regard, specimens of Inconel 718 were sprayed with Ni–22Cr–10Al–Y bond coat and Al2O3/YSZ top coat using an atmospheric plasma spraying approach. Then, top surfaces of specimens were under exposed to Nd-YAG laser radiation and thereby were glazed. Thermal stabilities of both coatings were evaluated by rising the environment temperature up to 1000 °C, keeping at this temperature for 15 min, and also rapid cooling of specimens in cold water. Using FESEM with EDS and XRD, the microstructure and morphology observations, elemental investigations and also phase identification were carried out. Findings displayed that both sprayed and glazed coatings were damaged as a spallation of top coats. Furthermore, by checking the life-time of coatings, the number of thermal cycles led to 20% destruction of top surfaces of coatings, increased from 232 cycles in sprayed coating to 325 cycles in laser glazed Al2O3/YSZ multilayer coating which displayed 40% improvement of thermal shock resistance after laser glazing process. This enhancement in resistance to thermal shock was due to strain adaptability improvement via a network of segmented cracks on the top surface produced by laser glazing process.

In this study, the microstructure and mechanical properties of the atmospheric plasma-sprayed Cr2O3 (C), Cr2O3-20YSZ (CZ), and Cr2O3-20YSZ-10SiC (CZS) coatings were evaluated and also compared with each other, so as to explain the coatings wear behavior. Microstructural evaluations included X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and porosity measurements. Mechanical tests including bonding strength, fracture toughness, and micro-hardness tests were used to advance our understanding of the correlation between the coatings properties and their wear behavior. The sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of multimodal YSZ and subsequent SiC reinforcements to the Cr2O3 matrix resulted in an increase in the fracture toughness and Vickers micro-hardness, respectively. It was found that the composite coatings had comparable coefficients of friction with pure Cr2O3 coatings. When compared with the C coating, the CZ and CZS composite coatings with higher fracture toughness exhibited superior wear resistance. Observation of the wear tracks of the coatings indicated that the lower wear rates of the CZ and CZS coatings were due to the higher plastic deformation of the detached materials. In fact, improvement in the wear resistance of the composite coatings was attributed to a phase transformation toughening mechanism associated with tetragonal zirconia which created more ductile tribofilms during the wear test participated in filling the pores of coatings.

The evaluation of hot corrosion behavior of sprayed and laser glazed YSZ + Al2O3 composite coatings was conducted in the presence of 55% V2O5 + 45% Na2SO4 molten salt was conducted. Inconel 718 coupons were sprayed with a Ni–22Cr–10Al–1Y bond-coat and then with an YSZ–Al2O3 top-coat as composite coatings. The sprayed coatings were glazed using a Nd:YAG laser. The results displayed that both coatings illustrated similar mechanisms of hot corrosion. However, the reaction between corrosive molten salt with Y2O3 and production of YVO4, leaching Y2O3 from YSZ, led to a destructive phase transformation of tetragonal-ZrO2 phase (t-ZrO2) to monoclinic-ZrO2 phase (m-ZrO2) and production of YVO4, accompanied by 3–5% volume expansion during cooling which could lead to cracking and degradation of TBCs. Findings revealed that by comparing the destabilization fractions of coatings, the content of m-ZrO2 fraction decreased from 80% in sprayed coating to 47% in laser glazed YSZ + Al2O3 coating, which displayed double improvement of hot corrosion resistance in laser glazing process. Reduction of the reaction between molten salt and zirconia stabilizer was the result of a surface roughness decrease and a specific reactive surface gained by laser glazing process.

In this research, Cr2O3 ceramic nano-sized powder particles were prepared using ball milling and then were granulated to reach the proper size for spraying. Afterward, Cr2O3 nano-coatings were deposited by atmospheric plasma spraying (APS) process onto stainless steel substrates. To optimize APS parameters, spraying was carried out under six conditions with different parameters. Microstructures of the elemental/milled powder and coatings were characterized via a field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). In this research, Cr2O3 coatings were deposited under different spraying conditions to understand the effect of APS parameters on the splat formation, deposition efficiency, and porosities of the coatings. After parameter optimization, spraying was performed under a high deposition efficiency of 46.0±1.3%. The optimized Cr2O3 coatings showed porosity content, Knoop microhardness, and adhesive strengths of 8.7±2.2%, 823±27 HK0.2, and 49±4 MPa, respectively; making them a good candidate for industrial use.

Plasma spray coatings 8YSZ (Zirconia stabilized with 8 wt% Yttria) are widely used for the protection of gas turbine parts against heat. Operating temperature of turbine engine causes sintering of the coating and this alters the microstructure of the coating. Sintering rate is dependent on the primary microstructure of the coating. The objective of this study is to compare the effect of spraying parameters on the sintering rate of YSZ TBC. Therefore, the effect of Hydrogen flow rate, powder feed rate and spray angle on sintering was studied. Sintering in constant temperature of 1100 °C for 10, 20, 50, 100 and 150 h was conducted in the environment atmosphere furnace. Microstructure studying was conducted using OM and FESEM and phase studying was conducted with XRD. Porosity and hardness measurement were considered as a measure of sintering. The results showed that the initial conditions of the coatings (the size and quantity of pores and residual stress) have a tremendous impact on the sintering behavior, such that a higher percent of the pores and smaller porosities increase sintering rate.

In this research, the effect of the addition of multimodal yttria-stabilized zirconia (YSZ) and SiC reinforcements on the mechanical properties of Cr2O3 plasma-sprayed coatings was studied. For this purpose, the starting powders were ball milled for 5 h and then mixed and agglomerated, prior to spraying. Cr2O3, Cr2O3-20YSZ (CZ), and Cr2O3-20YSZ-10SiC (CZS) coatings were then deposited onto 304L steel substrates using the atmospheric plasma spray process. Microstructural evaluations of the initial/milled powders and the plasma-sprayed coatings were conducted through x-ray diffraction, field emission scanning electron microscopy (FESEM) equipped with energy-dispersive x-ray spectroscopy and porosity measurements. The microscopic images indicated that the multimodal milled powders resulted in nanostructured coatings. Mechanical tests including adhesive strength, fracture toughness, and micro-hardness were used to understand the dependence of the properties of coatings and their microstructure. Adding tough YSZ particles to the C coating considerably increased the toughness through the phase transformation-toughening mechanism of tetragonal zirconia while decreasing micro-hardness of the coating; therefore, intrinsically hard SiC particles were added to the CZ coating to deal with the reduced hardness. Moreover, when compared to pure C coating, CZ, and CZS Composite coatings showed comparable bonding strengths and higher porosities.

Electrochemical treatment is a new method for accelerating the phosphating processes at low temperatures that can be performed as cathodic and anodic treatments. In this study, aluminum phosphate coating was deposited on graphite by cathodic electrochemical treatment, and, then heat treatment was performed on it. Microstructure, chemical composition, phase structure and adhesion of the coating produced in current densities of 35, 45, and, 55 mA/cm2, temperatures of 55, 70, and, 85 °C, and, treatment types of a three-layer coating at 120 min and a single-layer coating at 360 min were characterized using a scanning electron microscope, energy dispersive X-ray analysis, X-ray diffraction, and, scribe-grid test. The results revealed that the three-layer aluminum electrophosphate coating deposited in the current density of 35 mA/cm2, temperature of 70 °C, and, treatment time of 120 min was favorable due to a dense microstructure, suitable thickness, and, good adhesion. Furthermore, the phase that was formed in this coating was characterized as Al(PO3)3.

It is tried in this study to find relation between erosion behavior at ultra-high temperature and W-W and W-Cu interfacial strength in infiltrated W-Cu composites. For this purpose, W-10wt.%Cu composites were fabricated by compacting tungsten powder at 180–200 MPa, sintering and infiltration rout. Erosion of composites carried out at 3000 °C under high velocity impact of alumina droplets. The obtained results show there is an indirect relation between erosion rate and hardness. Also composite with higher cold tensile strength exhibited higher erosion loss. Based on microstructural study, lower sintering temperatures led to lower copper strengthening effect after infiltration that finally, resulted into higher erosion resistance during erosion. These results suggested that instead of cold tensile strength, tensile strength difference between 25 and 700 °C had main role in determining erosion manner of composite. Based on relation between erosion behavior and fracture mode in composites, addition of sintering activators has a positive effect on erosion resistance. These elements improve W-W interfacial strength and diminish some factors affecting W-W interfacial weakness. The obtained results in this study provide a comprehensive understanding about correlation between erosion manner and microstructural features in infiltrated W-10wt.%Cu composite for aerospace applications.

Microstructure, thermal insulation behavior and bonding strength of nanostructured 5%scandia, 0.5%yttria codoped zirconia (5.5 ScYSZ) and 25%ceria, 2.5%yttria co stabilized zirconia (27.5%CYSZ) coatings were investigated. ScYSZ and CYSZ nanostructured granules (as a top coat) and commercial NiCrAlY powder (as a bond coat) were thermal sprayed on an Inconel 738 substrate. The results revealed ScYSZ coating to have higher bonding strength. Nevertheless, due to higher porosity and the presence of local strains, the thermal insulation of nanostructured CYSZ coating was higher than for ScYSZ coating.

The duplex coating system consisted of electroplated Re-Ni as the diffusion barrier layer and the thermally sprayed NiCoCrAlY coating deposited. The adhesion strength and thermal shock resistance of the NiCoCrAlY coatings with and without the diffusion barrier were evaluated. Thermal shock resistance was investigated by quenching the coated samples in cold water from the temperature of 1100 °C. Also, the capability of Re-Ni as the diffusion barrier layer was investigated by isothermal oxidation at 1100 °C. The results showed that application of the thin interlayer of Re-Ni as a diffusion barrier beneath the NiCoCrAlY coating could slightly decrease the thermal shock and the adhesion strength of the NiCoCrAlY coating. The thermal expansion coefficient mismatch between the Re-Ni diffusion barrier and the NiCoCrAlY coating and the reduction of the substrate surface roughness due to Re-Ni deposition were the two main mechanisms limiting the lifetime of the NiCoCrAlY coating. But with significant improvement in oxidation resistance, these limitations could be compensated.

Ablation behavior of SiC nanowhisker toughened-SiC/ZrB2-SiC ultra-high temperature multilayer coatings was evaluated under supersonic flame at ∼2173 K for 120s. SiC nanowhisker toughened-SiC/ZrB2-SiC coatings were prepared on graphite substrate by two-step reactive melt infiltration (RMI) method. The ZrB2–SiC outer coating was prepared on the SiC coated graphite surface by in-situ and ex-situ methods. In ex-situ processing, ZrB2 powder was directly introduced into the powder mixture as a raw material. In in-situ processing, ZrB2 phase was formed in the coating by in-situ reaction between ZrO2 and B2O3. The linear and mass ablation rates for the in-situ ZrB2-SiC coating after 120s ablation under supersonic flame were 1.75 μm s−1 and 0.083 × 10−3 g cm−2 s−1 respectively. For the ex-situ ZrB2-SiC coating the linear and mass ablation rates were 1.5 μm s−1 and 0.064 × 10−3 g cm−2 s−1. The results showed that, the ablation resistance of in-situ ZrB2-SiC coating is very close to ex-situ ZrB2-SiC coating which can provide stable protection for the carbon substrate. Moreover, The SiC nanowhisker network structure in both in-situ ans ex-situ ZrB2-SiC coatings not only improved the toughness of the protective film by crack bridging, crack deflection and crack impedance, but also enhanced the bonding ability of protective film by the pinning and framework effects.

W-Cu Composites were fabricated by three powder particles with average particle sizes of 2, 6 and 8 μm at compressing pressure between 350 and 550 MPa. Sintering and infiltration rout was employed as fabrication method. Erosion test was done by means of HVOF at 2400 °C at the velocity of 1200 m.s− 1. The obtained results were discussed based on microstructure evolution, conductivity and hardness. It was found that, there is an indirect relation between erosion resistance and electrical resistivity. Also, composites prepared by fine particles exhibit higher erosion resistance due to higher hardness. Studied composites show erosion loss ratio in the range of 65–95%.

The main goal of this study was to evaluate the microstructure and capability of the diffusion barrier Re-base layer between the NiCoCrAlY coating and a Ni-based superalloy. To this end, a duplex coating system consisting of the Re-base layer and NiCoCrAlY coating were deposited by the electroplating method and High Velocity Oxygen Fuel (HVOF) spraying, respectively. The mechanism of the deposition process and the effect of deposition time and surface preparation and cleaning on the Faradaic Efficiency (FE), surface morphology, thickness and quality of the Re-base diffusion barrier were studied. Also, the inter-diffusion behaviour of the substrate- NiCoCrAlY coating with and without the diffusion barrier was investigated by isothermal oxidation at 1000 °C. The microstructural features and the diffusion barrier behaviour of the Re-base layer were evaluated by a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS). Criteria related to the properties of diffusion barrier layer based on to increase the Faradaic efficiency and the rhenium content in the coating were evaluated. Isothermal oxidation test results showed that the diffusion barrier coating could inhibit the inter-diffusion of the elements of the superalloy substrate and significantly decrease the growth rate of the inner β-NiAl depletion zone.

In this work, the effect of vacuum annealing on the erosion resistance of infiltrated W-10wt%Cu composite against the impact of alumina droplets at 3000 °C, was studied. SEM images were employed to identify the microstructure evolution. The obtained results indicated that the post-sintering annealing reduced erosion rate from 19.7% to 11.2%.

This paper reports on a study that investigated the effect of sintering on the microstructure, thermal properties and phase composition of nanostructured and conventional 8YSZ thermal barrier coatings. For this purpose, conventional and nanostructured YSZ coatings were heated at 1100 °C in an atmospheric box furnace for 10, 20, 50, 100, and 150 h. Optical microscope and FE-SEM were used for microstructural studies and XRD was used for phase analysis. The percent of pores and micro hardness were used as measures of sintering. According to the findings, the percent of pores and cracks decreased because of the inter-diffusion of splats. Changes in properties, such as thermal conductivity and micro hardness, during the sintering process were less significant for nanostructured coatings than they were for the conventional ones, but phase changes were more noticeable. XRD investigation showed that picks shift for both samples were the same. The findings showed that using nanostructured coatings could be a way of reducing sintering effects.

It is tried in this paper to study microstructural features related to erosion of infiltrated W-10 wt%Cu at ultra-high temperature against liquid alumina droplets. Also the effect of post-sintering annealing on the fracture mode and erosion resistance of the composite has been examined. For this purpose, the composite material has been prepared by compressing, sintering (also vacuum annealing) and infiltration of initial tungsten powder. The erosion test carried out by static firing of the bi-propellant engine (using two types of propellant: typically a fuel and an oxidizer) and the microstructure evolutions were studied by SEM and EDS. It is found that the main erosion mechanism of infiltrated W-10 wt%Cu composite, includes inter-granular fracture of tungsten skeleton and chipping. Extra sintering of skeleton takes place in outer copper-free skin of eroded composite. Also, melting and evaporation of copper occurs due to ultra-high temperature of erosion test and leads to formation of evacuated porosities. On the other hand, alumina droplets can penetrate into evacuated porosities. Also, vacuum annealing of sintered skeleton resulted into about 33% higher erosion resistance of composite. The obtained results were discussed based on weak points in infiltrated composite, transpiration cooling and microstructural evolution during erosion. This study provides an insight into correlating microstructural evolution with the erosion mechanism of infiltrated W-10 wt%Cu composite.

A nanostructured SiC/SiC-ZrO 2 coating was prepared on a graphite substrate by a two-step technique to improve the oxidation protection ability of graphite. The first step was to prepare a functionally graded SiC layer by a pack cementation process and the second one was to obtain a SiC-ZrO 2 coating through a pack cementation technique at 1873 K. The phase compositions and microstructure of the coating were characterized by X-ray diffraction and scanning electron microscopy. The isothermal oxidation test of the coated samples was performed at 1773 K for 10 h. A gradient C–SiC transition layer can be observed at the graphite-coating interface and a SiC-ZrO 2 coating was formed. The SiC nanofibers with the diameter in the range of 30–160 nm were observed on the coating. The SiC-ZrO 2 coating could efficiently improve oxidation resistance of graphite with a weight loss of 11%, as compared with a 70% weight loss of the first step coating.

In this paper, the erosion mechanism of infiltrated W-10wt%Cu composites against liquid alumina particles as well as the effect of sintering activators was studied. The fabrication of samples consists of infiltration of copper into sintered porous tungsten skeleton in presence of activators such as Ni and Co. Ni and Co was used as solid state sintering activators. The erosion of prepared specimens observed after the static firing and explained in terms of microstructure, transpiration cooling and erosion mechanism. Chemical analysis of the material after erosion test was obtained using energy dispersive X-ray analysis (EDS). SEM images were employed to identify the microstructure evolution due to erosion. The obtained results show that Composite prepared by Ni additive show about 33% better erosion resistance compare to specimen without additive. Also, presence of sintering activator could enhance transpiration cooling mechanism during erosion. These results discussed based on hardness, contiguity and connectivity.

It has been a while since abradable coatings have been used in turbine engines to seal the gap between rotating and stationary components to protect these parts and increase the efficiency of engines. The properties of these coatings should satisfy the need of low wear resistance to seal the gap without any damage to the parts involved, and high erosion resistance against compressed air trying to pass through the gap. Many parameters are involved in order to tailor the coatings' properties to the needs; in this work, the effect of atmospheric plasma spraying (APS) process parameters such as hydrogen flow rate as the secondary plasma gas and spraying distance on wear behavior of nickel-graphite (Ni-G) abradable seal coatings at 400 °C have been investigated. The results showed these parameters have a direct impact on the hardness, adhesion strength, coefficient of friction and consequently wear resistance of the coatings.

This study investigated the thermal insulation and thermal shock resistance behavior of nanostructured versus conventional yttria-stabilized zirconia (YSZ) thermal barrier coatings. To evaluate their coating performance in service conditions, samples were furnace sintered at 1150 °C for 100 h in ambient atmosphere. The results show that the nanostructured coatings exhibited better heat transfer resistance and thermal shock resistance compared with the conventional coating. In addition, the larger size of the initial agglomerates in the nanostructured coatings increased the percentage area of nanozones and decreased the heat transfer resistance. The thermal insulation behavior of the conventional coating was improved after heat treatment because of horizontal cracking. Disappearance of cracks, bridging between grains, and their growth by connecting with each other were observed in the conventional coating. However, in the nanostructured coatings, the nanoareas and their related properties disappeared. Microstructural and phase investigations were carried out by optical microscopy, field-emission scanning electron microscopy (FE-SEM), and x-ray diffraction (XRD) analysis. The thermal behavior was investigated by thermal insulation capability testing.

In this study, a functionally graded SiC layer was prepared on a graphite substrate by a pack cementation method with Si, C, and Al2O3 powders. Then a SiC–ZrB2 coating was developed by an in situ reaction method through a novel pack cementation technique at 1873 K with Zr, Si, and B4C powders to improve the oxidation protection ability of SiC-coated graphite. The phase compositions, microstructure, and element distribution of the coating were identified by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, respectively. The isothermal oxidation test of the coated samples was accomplished at 1773 K in air for 10 h. According to the results, a 550-μm thick graded C–SiC layer was detected at the graphite–coating interface and a SiC–ZrB2 coating was formed on the first coating. The SiC–ZrB2 coating could efficiently enhance oxidation resistance of graphite with a mass gain of +1.1 %, as compared with a −1.2 % mass loss of the first step coating. The excellent protection ability of SiC–ZrB2 coating could be ascribed to the high thermally stable ZrSiO4.

A C/SiC-SiC nano functionally gradient coating was prepared on graphite by reactive melt infiltration (RMI) at 1873 K under argon atmosphere. The phase composition, microstructure, isothermal oxidation behavior and thermal shock resistance of the coatings were investigated. The results showed the presence of α-SiC and β-SiC phases as well as a little amount of Si in the coating. The addition of SiC nanoparticles resulted in surface continuity improvement and increase transition interface thickness. Isothermal oxidation test was performed at 1773 K for 10 h in air. Results showed with increasing the SiC nanoparticles content the weight loss of the samples dropped from 48.72% to 5.03%. Despite the results of the isothermal oxidation test, the samples without SiC nanoparticles exhibited the minimum weight loss after thermal cycling oxidation between 1773 K and room temperature for 14 times due to the formation of SiC whiskers.

In this paper, ablation behavior of C/SiC/ZrB2 ultra-high temperature ceramic coatings applied by solid shielding shrouded plasma spray method was studied for as nose tip, leading edges, solid-fueled rocket nozzles, propulsion systems, re-entry missile nozzles throat applications. For this purpose, SiC inner layer coating was applied by reactive melt infiltration (RMI) under argon atmosphere. Then, ZrB2 outer layer coating was deposited on SiC coated graphite by solid shielding shrouded plasma spraying (SSPS). To evaluate the performance of the coating, SiC and SiC/ZrB2 coatings were exposed to oxy-propane and oxy-acetylene flames at 90o angle. The results of ablation test showed that mass ablation rates of SiC/ZrB2 coating under oxy-propane flame after 300 s and under oxy-acetylene flame after 120 s is 0.426 × 10−4 g.cm−2.s−1 and 4.8 × 10−4 g.cm−2.s−1 respectively. It seems that ablation behavior of SiC/ZrB2 coatings are controlled by the formation of the ZrO2 dense layer.

This paper investigates the effect of hydrogen and argon shrouding gas flow rates in solid shielding shrouded plasma spray (SSPS) method on in-flight oxidation, formation and growth of the TGO layer and high-temperature oxidation behavior of NiCrAlY coating. With this in mind, NiCrAlY coating was applied on the Hastelloy X substrate by solid shielding shrouded plasma spray (SSPS) and compared with the conventional atmospheric plasma spray (APS). High-temperature oxidation was carried out at 1000 °C for 200 h. Microstructure of the coatings before and after oxidation was evaluated by scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). To study the effect of solid shielding shrouded plasma spray on the properties of metallic coatings, the variable parameters such as the shroud gas nature (Ar, H2), the gas injection method (internal, external or both) and their flow rates, were examined. The results indicated the better operation of the coatings deposited under the protection of internal argon shroud gas with a flow rate of 75SLPM, which could be due to better protection of the metallic particles against in-flight oxidation. This sample represented the lowest thickness of the TGO layer (2.25 μm). The formation and growth of the TGO layer were higher in APS coating (3.82 μm) than that of SSPS. Consequently, this can prove by the preservation of mono-layered TGO of SSPS coating even after 200 h oxidation without the formation of any CSN clusters.

The hot corrosion behavior of plasma-sprayed ceria–yttria co-stabilized zirconia (CYSZ) nanostructured coatings (with the chemical composition of ZrO2–2.5 wt%Y2O3–25 wt%CeO2) was studied in the presence of 45 wt% Na2SO4+55 wt%V2O5 as the corrosive molten salts, for 6, 12, 18, 30, 72, 156, and 300 h at 1000 °C. As the hot corrosion products, YVO4, CeVO4 and Ce0.75O2Zr0.25 crystals were formed by leaching Y2O3 and CeO2 from CYSZ due to the reaction between the corrosive salts and (Y2O3, CeO2) stabilizers of zirconia. The CYSZ nanostructured coating showed remarkable resistance to degradation throughout the hot corrosion test, irrespective of an additional reaction with the corrosive salts. A mechanism was suggested based on the prohibition of the further diffusion of molten salts into the deeper layers of the coating, via diffusional resistance in the nanoporous media.

In this study, the surface of AZ91 magnesium alloy plasma sprayed with WC–12 wt-%Co powder was melted by pulsed Nd:YAG laser. Microstructure, chemical composition and phase analyses of the treated layers were studied by utilising a scanning electron microscope equipped with energy dispersive spectroscopy, X-ray diffraction technique and a microhardness testing machine. Laser treated layers showed much finer structure compared to that of the original base material with suitable distribution of WC particles. Microhardness of melted zone achieved at optimum condition was ∼3.5 times higher than that of the base material. In addition, the presence of WC particle and refined structure reduced the wear rate to less than half in the laser treated layer.

In the present study, the feasibility of the formation of surface layers containing hard iron aluminides on AA6061-T6 aluminum via pre-plasma spraying with iron and subsequently double surface melting by pulsed Nd:YAG laser is studied. The effects of single and double laser surface melting on microstructure, phase formation, and hardness of the treated layers are examined. Single-step laser treatment resulted in the presence of undissolved iron particles surrounded by lump-like Al5Fe2 and needle-like Al3Fe intermetallic compounds. Double laser surface melting dissolved the retained undissolved irons and resulted in the formation of Al-Al3Fe eutectic structure. Microhardness profiles along cross section and top surface of the treated layers indicated that laser surface alloying with iron enhanced the hardness of the aluminum to more than twice of that of the base material.

Concurrent with the development of new generation of gas turbines, many attempts have been made to introduce advanced thermal barrier coatings with lower thermal conductivity and higher temperature stability. Most of the research to improve TBCs performance are based on two general approaches: structural modifications and chemical modifications. In most cases, the improvements in some properties are at the expense of loss of some other properties. Changing in the TBCs architecture and the application of multilayer coatings, consisting of layers with engineered properties based on the requirements, is a solution to achieve a combination of desired properties. In all of these development methods it is to be understood that the principle is reducing the possibility of formation of cracks, but, once formed, all such cracks can grow under and thermal cycles and eventually lead to coating delamination and spallation. Self-healing is the most precious phenomenon to overcome this problem.

Aramid fabrics are used in impact-resistant equipment because of their lightness and flexibility. However, the resistance of aramid fabrics to high-energy impacts is not sufficient. Applying ceramic coatings is one of the best methods to increase the stab resistance of the fabrics. In this rout, the type of metallic bond coat has an effective role. Therefore, the aim of this study was to investigate the effect of bond coat type on the stab resistance of aramid fabrics coated with Al2O3–13%TiO2 as a top coat. For this purpose, this coating was applied on two different bond coats (copper and aluminum) by atmospheric plasma spraying method. Tensile strength, adhesion strength and stab resistance of the samples were evaluated. The results showed that the use of copper bond coat increased the adhesion strength of the Al2O3–13%TiO2 coating by 12% compared with the aluminum bond coat (adhesion strength of samples with copper and aluminum bond coats was 13 and 11.5 MPa respectively). Furthermore, the use of copper bond coat enhanced the stab resistance by 10% compared with the aluminum bond coat. The results of tensile strength also indicate that the structure of the fabric has been preserved and no damage to its fibers was observed after the tensile test. The tensile strength of 172 and 168 MPa was obtained for Al2O3–13%TiO2 with copper and aluminum bond coats, respectively.

The spallation of ceramic coating from the bond coat is important problem for TBC systems. And the spallation is caused the oxidation and corrosion at the interface of ceramic layer and bond coat. In this research the stability of conventional YSZ coating was compared to the particulate nanostructured YSZ + alumina composite coating by means of oxidation and hot corrosion tests. The coatings have been deposited by plasma spray method. High temperature oxidation test at 1100 °C and hot corrosion test at 1050 °C using Na2SO4 and V2O5 molten salts were performed on the coatings. The experimental results showed nanostructured YSZ + Al2O3 composite coating had higher oxidation and hot corrosion resistance than those of the conventional YSZ coating. The microstructural analysis indicated that the growth of TGO was much less for this nanostructured composite coating while it reduced infiltration of both oxygen and aggressive molten salt.

In this study, chemical and mechanical erosion mechanism of SiC single-layer coatings were evaluated using supersonic flame erosion test, X-ray diffraction, and scanning electron microscopy. In the first step, the formation mechanism of SiC coating on graphite substrate was investigated by HSC Chemistry software version 6.0 and then the effects of infiltration temperature and SiC nanoparticles addition on erosion behavior of the coatings were studied. The results of erosion test under supersonic flame at a 90° angle after 120 s indicated that chemical and mechanical erosion occur simultaneously during erosion test but mechanical erosion is the dominant mechanism which causes to sample destruction. However, with SiC nanoparticles addition and increasing the infiltration temperature, mechanical erosion decreased. By increasing of infiltration temperature, mass erosion rate and linear erosion rate decreased from 2.52 × 10−3 g cm−2 s−1 and 32.7 μm s−1 to 1.09 × 10−3 g cm−2 s−1 and 3.33 μm s−1 respectively. Also, with addition of SiC nanoparticles, mass and linear erosion rate decreased from 2.52 × 10−3 g cm−2 s−1 and 32.7 μm s−1 to 1.04 × 10−3 g cm−2 s−1 and 2.17 μm s−1 respectively.