Search

In this research, a SiC/ZrB2 coating was produced on graphite by reactive melt infiltration and plasma spraying method. The coating characterization was performed using XRD analysis, electron microscopy equipped with energy dispersive spectrometer (EDS), and supersonic flame ablation test at 2073 K. The results indicated that the dense C/SiC coating with good ablation resistance can be obtained at 1873 K. The coating thickness decreased with increasing infiltration temperature. The results of ablation test showed that by increasing the infiltration temperature and holding time, weight loss and mass ablation rate decreased from 22.63% to 9.83% and 3.63 × 10−3 g cm−2 s−1 to 1.34 × 10−3 g cm−2 s−1, respectively. The results showed that by using the ZrB2 as outer coating the ablation resistance improved remarkably. The weight loss and mass ablation rates for the SiC/ZrB2 coating were 12.79% and 1.857 × 10−3 g cm−2 s−1, respectively. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, a functionally graded SiC layer was prepared on a graphite substrate by a pack cementation method with Si, C, and AlO powders. Then a SiC-ZrB coating was developed by an in situ reaction method through a novel pack cementation technique at 1873 K with Zr, Si, and BC 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-ZrB coating was formed on the first coating. The SiC-ZrB 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-ZrB coating could be ascribed to the high thermally stable ZrSiO. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Alloyed powder of Nb-10Hf-1Ti was produced by mechanical alloying from elemental powders. • The optimum milling time and rotation speed were found to be 5 h and 300 rpm. • Variation of lattice parameter, strain and crystallite size were investigated. • The results are confirmed using XRD, SEM, and PSA. 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. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Sintering decreases porosities and cracks and consequently changing properties of thermal barrier coating during service. • Fine microstructure with tiny porosities resulting increasing of tendency to sintering • Sintering results increasing of the hardness and the porosity percent in the first 10 h. • In this study, important properties such as thermal shock and phase composition and hardness are evaluated. [ABSTRACT FROM AUTHOR]

Abstract: Aluminum matrix composite is one of the most conventional types of metal matrix composites. This paper deals with the effect of production parameters on wear resistance of Al–Al2O3 composites. Alumina powder with a particle size of 12, 3 and 48μ and pure aluminum powder with particle size of 30μ were used. The amount of added alumina powder was up to 20%. Ball milling was utilized to blend the powders. The range of sintering temperature and time were 500, 550 and 600°C and 30, 45, 60 and 90min respectively. It was found that increasing sintering temperature results in increasing density, hardness and wear resistance and homogenization of the microstructure. However at certain sintering temperatures and time, considerable grain growth and reduction of hardness value occurred, leading to the degradation of wear resistance. The results showed that at high alumina content, relative density of the composite increases. However, after raising the particle size of alumina, relative density initially increases and then drops to lower values. Increasing weight percent of alumina powder leads to higher hardness and consequently improves the wear resistance of Al–Al2O3 composite. The use of fine alumina particles has a similar effect on hardness and the wear resistance. Finally, a finer grain size was observed, at high amount and low size of the reinforcement particle. [Copyright &y& Elsevier]

Abstract: Al matrix composite is well known, in which Al2O3 is the most widely used reinforcement. The aim of this study is to investigate the effect of alumina particle size and its amount on the relative density, hardness, microstructure, wear resistance, yield and compressive strength and elongation in Al–Al2O3 composites. To this end, the amount of 0–20wt.% alumina with average particle sizes 48, 12 and 3μm was used along with pure aluminum of average particle size of 30μm. Powder metallurgy is a method used in the fabrication of this composite in which the powders were mixed using a planetary ball mill. Consolidation was conducted by axial pressing at 440MPa. Sintering procedure was done at 550°C for 45min. The results indicated that as the alumina particle size is reduced, density raises at first, then, declines. Moreover, as the alumina particle size decreases, hardness, yield strength, compressive strength and elongation increase and factors such as wear resistance, microstructure grain size and distribution homogeneity in matrix decreases. For instance, as the alumina particle size gets smaller from 48 to 3μm at 10wt.% alumina, hardness rises from 50 to 70BHN, compressive strength improves from 168 to 307MPa and wear rate rises from 0.0289 to 0.0341mm3/m. On the other hand, as the amount of alumina increases, hardness and wear resistance increase and relative density and elongation is decreased. However, compressive and yield strength rise at first, then drop. For example, if the amount of alumina with 12μm particle size increases from 5 to 10wt.%, hardness increases from 47 to 62BHN and compressive strength rises from 190 to 273MPa. Nevertheless, erosion rate after 300m decreases from 0.0447 to 0.0311mm3/m. [Copyright &y& Elsevier]

Abstract: The effect of Al addition on pressureless sintering of B4C ceramic was analyzed in the present work. Different amounts of Al, from 0 to 5wt.% were added to the base material and pressureless sintering was conducted at 2050 and 2150°C under argon atmosphere. Microstructure, crystal phases and density evolution were studied and correlated to Al additions and firing temperature. Density and grain size of sintered samples, increased significantly with Al load while less evidence is the effect of sintering temperature; 94% dense material was obtained by adding 4wt.% Al regardless of the maximum firing temperature. [Copyright &y& Elsevier]