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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.

In this research, the effect of different amounts of Cr2O3 (2.5, 5, 7.5, and 10 wt.%) and sintering temperature (1850, 1900, and 1950 °C) on the sinterability and mechanical properties of liquid-phase sintered SiC-matrix composites was studied. First, raw materials were ground for 3 h using a planetary mill whose rotational speed was 180 rpm. The process of pressing the samples was completed using uniaxial pressing with the applied pressure of 90 MPa. Finally, the samples were sintered under an argon atmosphere at various temperatures for 1.5 h. In the end, the best sintered sample was annealed at 2000°C for 2 h. The phases, microstructure, and chemical composition of the samples were studied using X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (FESEM), respectively. The results suggested that the composite that contained 5 wt.% Cr2O3 and that was sintered at 1900 °C exhibited the best properties. The relative density, microhardness, elastic modulus, flexural strength, indentation fracture resistance, and brittleness index were 97.45%, 27.50 GPa, 395 GPa, 549 MPa, 6.2 MPa m1/2, and 282.58 ×10−6 m−1 respectively. At 1850 °C and 1950 °C, the best mechanical properties were acquired for the samples containing 5 wt.% Cr2O3. According to microscopic images, the formation of elongated grains and the activation of crack deflection as well as crack bridging mechanisms were the most significant mechanisms enhancing the toughness of these composites. The results showed that the sintered sample at 1900 °C, containing 5% additive and annealed at 2000 °C for 2 h, reached the highest fracture toughness (6.92 MPa m1/2). The microscopic images showed that the matrix grains of the as-annealed samples were elongated. There were signs of transgranular fracture at the cross-section of the samples.

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 research, the effect of different parameters on the behavior of suspension rheology, as well as the physical and mechanical properties of green and sintered SiC bodies, were investigated. Viscosity values ​​for slurries containing 0.1, 0.3, 0.5, 0.7, 0.9 and 1.1 wt% of dispersant (Tetramethyl ammonium hydroxide ((CH3)4NOH, TMAH) were investigated, and the minimum viscosity was obtained for the slurry containing 0.7 wt% TMAH. The results showed that the highest amount of SiC powder that could be used without causing any serious problem was 37 vol.%. The monomers used in the research were metacrylamide (MAM) and N,Ń- methylenbisacrylamide (MBAM). Afterward, the effect of monomers content (MAM and MBAM) and ceramic load on the shrinkage and warpage of green bodies were investigated. The results demonstrated that increasing the amount of monomers decreases warpage rate (WP) and shrinkage rate (SK). Other than that, with increasing monomers ratio, both of these parameters increase uniformly. Subsequently, the samples were sintered for 1 h at 2150 °C. The results proved that the sample containing 37 vol.% of SiC with a monomers ratio of 3:1 and a total monomers content of 20 wt.% had the highest hardness, fracture toughness, and relative density, equal to 27.51 ± 1.25 GPa, 6.2 MPa.m1/2, and 96.89%, respectively.

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...

This work discusses the pressureless sintering of a boron carbide-titanium diboride (B4C– TiB2) nanocomposite via in-situ reaction of the boron carbide/titanium dioxide/carbon system. Attempting to sinter pure boron carbide leads to poor mechanical properties. In this work, the effect of adding TiO2 to B4C on mechanical properties of the boron carbide was investigated. Thermodynamic simulations were performed with HSC chemistry software to determine the phases which were most likely to form during the sintering process. The reaction thermodynamics suggested that during the sintering process, formation of TiB2 occurs preferentially over formation of TiC. For examination of the microstructural evolution of the samples, Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized. The density, porosity, Young's modulus, microhardness and fracture toughness of the specimens were compared. Optimum properties were achieved by adding 10 wt% TiO2. In the sample possessing 10 wt% TiO2, the relative density, Young's modulus, hardness and fracture toughness were 94.26%, 428 GPa, 23.04 GPa and 5.19 MPa m0.5, respectively, and the porosity was decreased to 5.73%. Furthermore, phase analysis via XRD confirmed that the final product was free of unreacted TiO2 or carbon.

Ti3SiC2 bulks were synthesized through reactive melt infiltration (RMI) method. To investigate the metallurgical and thermodynamical parameters during RMI method, the powders with different chemical compositions were pelleted by cold pressing. Then the melt infiltration process was carried out at 1500 °C for 1 h under 10−4 torr. The phase analysis, morphological observations and elemental analysis of different samples were performed by XRD, FE-SEM and EDS, respectively. The density, hardness and Young's modulus of the samples with optimum purity in each step were also measured. Thermodynamical analysis of all possible reactions was performed during RMI process. It was found that the preform composition of 3TiC/0.3Si was optimum. Among the various amounts of Si ingot (used during melt infiltration process), the amount of Si with 3 times of the stoichiometric value had the best results. Besides, addition of small amount of Si to the preform composition had a positive effect on the purity of Ti3SiC2. Finally, the sample with 93% Ti3SiC2 MAX phase was synthesized through RMI method.

This present work investigated the mechanical properties and microstructure of h-BN based ceramic composites reinforced with CNTs and GNPs. Accordingly, two different batches of pure h-BN, h-BN/0.1 wt%CNTs and h-BN/0.1 wt% GNPs were prepared through a high energy mixer mill to gain a uniform dispersion of reinforcement with the initial stable CNTs or GNPs solution in ethanol. After drying the mixtures, the pure h-BN and also, two different composite components were directly inserted into the graphite mold and the sintering process was performed with the initial and final pressure of 10 and 50 MPa, respectively, at 1900 °C, under the vacuum condition of 15–35 Pa. The relative density of the samples was calculated based on the Archimedes principle. The densification behavior of the samples showed the maximum amount of 98.31% for the theoretical density of the h-BN/GNPs composite. On the other hand, the minimum relative density of 96.41% was obtained for the h-BN/CNTs composite. The microstructure studies of the prepared sample showed the uniform distribution of GNPs in the h-BN layers; however, when the CNTs were added, some agglomerated area was found. Moreover, the fracture surface of all samples showed a laminar fracture as a result of the layer-by-layer structure of h-BN. The investigation of the mechanical properties of the prepared specimens also revealed the highest bending strength, fracture toughness and Vickers hardness of 199 MPa, 1.26 GPa and 3.62 MPa m−1/2, respectively, which belonged to the h-BN/GNPs composite. In the case of CNTs, this trend exhibited lower amounts, probably due to the agglomeration of CNTs.

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.

The mechanical alloying process was employed to produce C103 alloy with Nb-10% Hf-1% Ti (wt.%) composition using Nb, Hf and Ti powders. The mechanical alloying process was performed in an argon atmosphere in the chamber and bullets of tungsten carbide with a ball-to-powder weight ratio (BPR) of 20:1 at rotation speed of 200, 300 and 400 rpm for 2, 5 and 8 h. At rotation speeds of 200 and 300 rpm particle size decreased and became more spherical during MA. While increasing milling time at 400 rpm caused agglomeration of particles. XRD results showed that increasing milling time at a constant rotation speed has no considerable effect on reduction of crystallite size, but the lattice strain is strongly affected by it and increased obviously with further rotation speed. The results showed that the optimum milling time and rotation speed to attain Nb-10Hf-1Ti alloy powders with the least amount of contamination and appropriate morphology are 5 h and 300 rpm, respectively.

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.

Ca-α-SiAlON powder was prepared by carbothermal reduction nitridation process at nitrogen atmosphere without using high pressure and long holding time. The precursor was prepared by a novel colloidal sol-gel process consisting of silica, alumina, calcium oxide and carbon. The obtained powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The XRD and SEM analysis results revealed that by heating at 1200 °C for 2 h, spherical particles consisting of amorphous calcium aluminosilicate liquid phase were formed. With increasing temperature to 1360 °C and held for 2 h, the spherical particles with the dominant phase of O–SiAlON were observed. Finally, as the temperature climbed to 1500 °C and held for 2 h, spherical particles disappeared and Ca-α-SiAlON was formed. The XRD analysis of the products showed that with increasing temperature from 1360 °C to 1500 °C, O–SiAlON, β-SiAlON and Ca-α-SiAlON were formed, respectively, and colloidal sol-gel method, due to the high homogeneity of the reactants, had a significant role in increasing the purity of Ca-α-SiAlON.

Ca-α-SiAlON powder was synthesized by carbothermal reduction nitridation process at a low temperature (i.e., 1400 °C, 2 h). The precursor was prepared by a novel sol-gel process. The effect of carbon content and seeding on the microstructure and purity of the Ca-α-SiAlON powder was investigated by X-ray diffraction and scanning electron microscopy. It was found that carbon, in addition to reduction the raw materials, preventing agglomeration of the final product via its steric hindrance. There was an optimal amount of carbon in the precursor and an excessive increase in that caused impurities such as AlN would increase again in the final product, which could be due to an increase in the diffusion distance of the reagents. The results showed that in some cases Ca-α-SiAlON with a distribution of X-values is formed by different reaction path. Seeding has an effective role in the purity and morphology of the products, and single phase Ca-α-SiAlON powder was prepared with the addition of 2 wt% Ca-α-SiAlON seed to the precursor at 1530 °C for 2 h.

One limitation of using silicon carbide (SiC) is the materials’ intrinsic brittleness. The mechanical properties of these ceramics can be improved through the use of additives during the sintering process. This research investigated the addition of boron carbide (B4C) on the Liquid Phase Sintered (LPS) SiC-Al2O3-Y2O3 system. The aim was to determine the effects on both the mechanical properties and microstructure of the LPS-SiC composite with varying levels of B4C as an additive. The Silicon Carbide preparation was composed of a 10 wt% mixture of Alumina (Al2O3) and Yttria (Y2O3) at a 3:2 M ratio, with different concentrations of boron carbide between 0 - 6 wt%. In addition, 1 wt% of carbon powder was added to the precursors to prevent oxidation during sintering. All precursors were milled for 2 h at 200 rpm and then samples were sintered by the pressureless technique for 1 h at 1900 °C in an argon atmosphere. A relationship was observed between the measured mechanical properties, the porosity and various phases formed in within the structure of the ceramic composite. The grain size and distribution of the B4C reinforcement phase provides a mechanism for the observed strengthening and increase in fracture toughness. Increasing the B4C content up to 5 wt% enhanced the strength and fracture toughness of the composite samples, however increasing beyond 5 wt% led to a decrease in these properties. Furthermore, a microstructural explanation of the grain-effects is proposed by considering the crack-growth micro-patterns. This work clarifies the link between the mechanical properties and microstructural characteristics of the SiC-Al2O3-Y2O3 composite with B4C additive. The strengthening mechanism of B4C additives was also explored in the microscopic scale.

To reveal the efficacy of Si amount on the high-purity Ti3SiC2 synthesis, starting powders of Ti:Si:TiC:Al = 1:(1 + x):2:0.2 with various Si amounts (x = 0.1, 0.2 and 0.3) were sintered through SPS. The sintering process was conducted at 1100–1250 °C for 8 min under pressure of 40 MPa. Undesirable TiC impurity initiated to form at temperatures above 1200 °C and left the primary phase near 1250 °C from the Ti:Si:TiC:Al = 1:1.3:2:0.2. It was found that Si content had a key role in the improvement of Ti3SiC2 purity. The outcomes demonstrated that high-purity Ti3SiC2 could be acquired though the Ti:Si:TiC:Al = 1:1.2:2:0.2 at 1150 °C. The Young's modulus of Ti3SiC2 sample was 346 GPa and decreased with increasing impurity phases.

In this research, acrylate gel maker monomers Mathacrylamide (MAM) and Methylenebisacrylamide (MBAM) were used through a gel casting process to fabricate SiC bodies. Investigation of rheological behavior of suspensions showed that optimum amount of surfactant (Tetramethylammonium hydroxide) was equaled with 0.7 wt% of ceramic powders and ceramic powders could be added up to 37 vol% of solvent without any problems in casting. The effects of monomers content, monomers ratio and also solid loading on gelation time, bending strength of samples were investigated. Results showed that increasing the monomers content (MAM + MBAM) and ceramic solid loading led to the reduction of gelation time. Also alteration of these parameters resulted in variation of samples bending strength from ~6 MPa to 19 MPa, the best strength was obtained in the sample containing 35 wt% monomers with the MAM/MBAM ratio of 3 and 35 vol% SiC. Finally the samples containing different amounts of SiC were casted according to the pre-determined optimum parameters and after being dried they were sintered at 2150 °C for 1 h. According to the results, the highest density, Young's modulus, hardness and indentation fracture resistance were obtained in sample containing 37 vol% SiC and were equaled with 96.89%, 499.24 GPa, 27.51 GPa and 6.3 MPa·m1/2, Respectively. Microstructural studies showed that cleavage fracture planes, crack deflection and branching were the most important mechanisms on toughness enhancement.

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 study, SiC-TiC composite was fabricated by the reaction between TiO2 and SiC and in addition to the effect of TiO2 additive, influence of sintering temperatures on the electrical resistivity, the relationship between electrical resistivity and microstructure, density, indentation fracture resistance and hardness were investigated. The main goal of this study was to improve electrical resistivity while preserving mechanical properties of SiC body. The results showed that with 10 wt% Al2O3-Y2O3, the electrical resistivity reached to 9 × 108 Ω·m. Increasing the amount of TiO2 particles from 2.5 to 10 wt% and changing the sintering temperature from 1850 °C to 1950°C made the electrical resistivity to be variable in the range of 2.2×105 Ω·m to 9 × 108 Ω·m. In this report, the highest density, hardness and indentation fracture resistance for the samples containing 5 wt% additive which were sintered at 1900 °C were 96.2%, 24.4 GPa and 5.8 MPa.m1/2, respectively. Microscopic images showed that if the grain boundary phase is located in the triple points or multiple points of grain boundary, the electrical resistivity will decrease and if it is located in full circumference of the SiC particles, due to failure of conducting pathways through SiC grains, the resistance of ceramics will increase instead.

In the current research, the effects of TiO2 additive on mechanical and physical properties of SiC bodies, sintered by liquid phase methods were investigated. Al2O3 and Y2O3 were used as sintering-aids (10 wt% in total) with an Al2O3/Y2O3 ratio of 43/57 to provide liquid phase during Sintering. TiO2 was also used as the oxide additive with an amount ranging from 0 to 10 wt%. After scaling and mixing the starting materials by a planetary mill, the obtained slurry was dried at 100 ℃ for four hours. The derived powders were finally pressed under a pressure of 90 MPa. The samples were then pyrolyzed and sintered at 600 ℃ and 1900 ℃, respectively under argon atmosphere for 1.5 h. Phase analysis showed no trace of TiO2 after the sintering process, demonstrating the complete TiO2 to TiC transformation. The results showed that an increase in TiO2 content up to 5 wt%, led an improvement in all the measured properties including the relative density, hardness, Young's modulus, bending strength, indentation fracture resistance and the brittleness factor, reaching to 96.2%, 24.4 GPa, 395.8 GPa, 521 MPa, 5.8 MPa m1/2 and 286.5 × 10−6 m−1, respectively. However more than 5 wt% additive resulted in a decrease in all the above-mentioned properties. Microstructural studies demonstrated that crack deflection and crack bridging were the major mechanisms responsible for an increase in the indentation fracture resistance.

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.

To reveal the effect of Al content on the purity and mechanical properties of Ti3SiC2, the powder mixture of 1Ti/1.2Si/2TiC/xAl (x = 0–0.4) were sintered to synthesize Ti3SiC2 through spark plasma sintering (SPS). The sintering process was conducted at 1150 °C for 8 min under a pressure of 40 MPa. To evaluate the phase constituents and morphology of the sintered samples, X-ray diffraction and field emission scanning electron microscopy were used. It was found that addition of Al from 0.2 to 0.4 mol in the starting powder mixture led to synthesis of high-purity Ti3SiC2. Besides, with increasing the Al content in the starting powder mixture, the densification behavior of sintered samples was promoted. The results showed that the Vickers microhardness of high-purity samples became almost same at higher loads and moreover the measured Young's modulus (E) of the high-purity Ti3SiC2 samples were near to each other and increased with promoting the densification.

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.

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.

A multiphase coating was prepared on a graphite substrate by a two-step technique to enhance the oxidation protection ability of graphite. The first step was to obtain a functionally graded SiC layer and the second one was to develop a SiC-ZrB 2 coating by an in-situ reaction method through a novel pack cementation technique with Zr, Si and B 4 C powders. The phase compositions, microstructure and element distribution of the coating were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The isothermal oxidation and thermal shock tests of the coated samples were performed at 1773 K in air for 10 h. A 650 μm thick graded C SiC layer was observed at the graphite-coating interface and a SiC-ZrB 2 coating was formed on the first coating. The v coating could efficiently improve oxidation resistance of graphite with a mass gain of +1.7%, as compared with a −63.2% mass loss of the first step coating. The excellent protection ability SiC-ZrB 2 coating could be attributed to the formation of the thermally stable phase ZrSiO 4 .

Due to having a nanolayered microstructure and unique properties, titanium silicon carbide (Ti 3 SiC 2 ) has the advantages of both the ceramics and metals and has attracted a great deal of attention. Like ceramics, the Ti 3 SiC 2 MAX phase enjoys a low density, excellent high-temperature mechanical properties and a high melting point and thermal stability; and like relatively soft metals, it can be machined, is resistant to thermal shock and has a good thermal and electrical conductance. Numerous methods have been employed for synthesizing this material; and among them, the mechanical alloying technique has many advantages. In this article, the effects of different additives such as SiC, Al and TiO on the synthesis of Ti 3 SiC 2 MAX phase by mechanical alloying have been investigated.

The oxidation of Ti2AlC MAX phase has been studied in this paper. MAX phases are a class of materials with nano-layered structure. These materials have been formed from a transition metal (M), an element from the IIIA or IVA groups (A) and carbon or nitrogen (X). Ti2AlC MAX phase is a ternary carbide with layered structure and hexagonal crystalline lattice. Physical and chemical properties are the most significant characteristics of the Ti2AlC which have introduced this material as the most practical MAX phase known so far. This material is currently considered as the most used MAX phase at elevated temperatures, the application of which requires correct recognition of the oxidation mechanism. Many of researchers investigated on the oxidation of Ti2AlC. Therefore, it has been tried in this paper to introduce this MAX phase and also report all the investigations of its oxidation characteristics at elevated temperatures.

In this study, a multiphase coating was prepared on a graphite substrate by a two-step technique to improve oxidation resistance of graphite. The first step was to prepare a functionally graded SiC layer by a pack cementation process and the second step was to obtain a nano SiC and SiO 2 thin film coating by a slurry painting technique. The microstructure and the surface chemistry of the coating were characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The spectral deconvolution was conducted using CasaXPS software based on Gaussian-Lorentzian function. The isothermal oxidation test of the coated samples was performed at 1773 K for 10 h. A 500 μm thick graded C–SiC layer was observed at the graphite-coating interface. An amorphous SiO 2 thin film and SiC nanofibers with the diameter in the range of 30–75 nm were formed on the coatings. The SiC nanofibers can efficiently improve oxidation resistance of graphite.

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.

A graded SiC coating was prepared on a graphite substrate by a pack cementation technique at 1 873 K to improve the oxidation resistance. The HSC Chemistry software was used for thermodynamics calculations. The coating was characterized by means of X-ray diffraction and scanning electron microscopy. Isothermal oxidation testing of the samples was performed at 1 773 K in air. The modeling analysis indicated that aluminum oxide can be reduced and an SiC coating can be formed due to the reaction of silicon with CO and carbon with SiO. The growth of the SiC nanofibers was attributed to the gaseous reaction of SiO with CO. A 600 μm thick SiC layer can be observed at the graphite–coating interface. The simulation and experimental results were consistent with each other. The mass loss of the SiC coated sample was 1.1 % after oxidation for 10 h, whereas the graphite was fully oxidized after 1 h.

In this research, it was found that the C/SiC transition interface thickness increases without a significant decrease in toughness by modifying the reactive melt infiltration method through the addition of SiC nanoparticles. Also, the effect of the infiltration temperature on the transition interface thickness, isothermal oxidation behavior, and thermal shock resistance of the C/SiC graded coating were investigated. Coatings were characterized by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy with energy-dispersive spectroscopy. Microstructural observations showed that with an increase in the heat treatment temperature, a higher amount of β-SiC (as a product of the infiltration process) is produced, which results in lower surface continuity in the coatings produced at a lower temperature. Moreover, the transition interface thickness increased with a decreasing infiltration temperature. The addition of SiC nanoparticles increased the transition interface thickness and oxidation resistance. After isothermal oxidation at 1773 K (1500 °C) for 10 hours, samples containing 7 wt pct SiC nanoparticles heat treated at 1773 K and 1873 K (1500 °C and 1600 °C) showed 13.3 and 5.03 pct weight loss, respectively.

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.

In order to improve the oxidation resistance of graphite, a SiC/ZrO2-SiC nanofibers multiphase coating was developed on a graphite substrate using the pack cementation and slurry painting techniques. The microstructure of the coating was characterized by X-ray diffraction and scanning electron microscopy. The HSC chemistry software package was used for the thermodynamic calculations. The isothermal oxidation test of the coated samples was performed at 1773 K in air. A 500 µm thick gradient C–SiC transition layer was formed at the graphite-coating interface SiC nanofibers with the diameter in the range of 32–88 nm were observed on the coating whose growth was ascribed to the gas phase reaction of SiO with CO. The oxidation test results revealed that the SiC nanofibers and thermally stable phase ZrSiO4 were the mainly effective tools protecting graphite from oxidation.

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.

Due to its favorable physical, mechanical and chemical properties, boron carbide (B 4 C) is one of the most important ceramics used in various industries. After diamond and boron nitride with a cubical structure, this material is the third hardest substance known; and due to its low density, large cross-sectional surface area for neutron absorption, excellent chemical stability, and other desired properties, it is considered as a strategic material. The sintering of boron carbide ceramics is very difficult due to the existence of strong covalent bonds in pure boron carbide and its low self-diffusion, high resistance against grain boundary slipping, and low surface energy. For these reasons, many additives have been added to boron carbide, as sintering aids. These additives, in addition to facilitating the sintering of boron carbide, do not have an adverse effect on its properties, and they improve the characteristics of the resulting product. Iron is one of the additives that reduces the sintering temperature and improves the mechanical properties of boron carbide by producing a liquid phase and thus preventing the growth of B 4 C particles. In this paper, the role of iron additive in the wetting, sintering ability, and the physical and mechanical properties of boron carbide composites has been investigated.

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.

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.

The goal of this study was to produce hydroxyapatite (HAp), a bioactive biomaterial, in a decomposition-free form with fracture toughness comparable to bone by metal fibre-reinforcement. This goal was ultimately achieved. Glass encapsulation of FeCralloy®-reinforced HAp was an unsuccessful technique due to the excessive low-temperature volatilisation, which aerated the glass. Therefore a graphite/stainless steel encapsulation system was used in the present study. Hot isostatic pressing enabled the production of fully dense decomposition-free HAp with toughness improvements of 14 times (FeCralloy® fibres, optimally 15 vol%), comparable to cortical bone. Further, it was found that the HAp decomposition temperature was higher at 100 MPa (the HIPing pressure) than for pressureless sintering. Addition of the FeCralloy® fibre additive induced significant plastic deformation and ductile fracture of the hydroxyapatite.

Fracture Toughness Improvement of the Hydroxyapatite Matrix Composite, to a Level Comparable to that of Natural Bone for in Vivo Applications, Was the Aim of the Present Work. Hot Isostatic Press Using a Graphite/stainless Steel Encapsulation System Enabled the Production of Fully Dense Decomposition-Free Hap with Toughness Improvements of: 2.4 Times (Al2O3 Fibres, Optimally 20 Vol%). Glass Encapsulation of Fibre-Reinforced Hap Resulted in Aeration from Sample Volatilization. Further, it Was Found that the Hap Decomposition Temperature Was Higher at 100 Mpa (the Hiping Pressure) than for Pressureless Sintering. the Toughening Effect of the Al2o3 Fibre Additive Induced Plastic Deformation and Ductile Fracture.