Crack healing in liquid-phase-pressureless-sintered silicon carbide–aluminum nitride composites

Crack healing in liquid-phase-pressureless-sintered SiC–AlN composites was investigated by introducing cracks into specimens and subsequently heat-treating the specimens. It was observed that cracks were healed and the strength was recovered. Cracks were filled with silica or mullite produced by the oxidation of the composites. It was shown that the healing temperature could be fixed in the range 1100–1300 °C and that large cracks up to about 300 μm could be healed completely. Our results imply that a simple oxidation heat-treatment can improve the reliability of silicon carbide–aluminum nitride components.     

Effect of Oxidation on the Room-Temperature Flexural Strength of Reaction-Bonded Silicon Carbides

The oxidation behavior of reaction-bonded silicon carbide (RBSC) and the effect of oxidation on the room-temperature flexural strength of RBSC were investigated. Four different types of RBSC, each having various SiC particle-size distributions and free silicon contents, were exposed to air at 1300°C for up to 200 h. Parabolic weight gains, with respect to oxidation time, were observed in all the specimens. The strength of the RBSC increased after oxidation for up to 50 h, because of the blunting of cracks by the silica layer that was formed on the surface. However, with further oxidation, the beneficial role of the oxide layer was negated by the cracks that were newly generated on the surface because of the thermal mismatch between the substrate and the silica layer. The amount of free silicon had a negligible effect on the strength retention of the specimens after the oxidation processes.     

Effect of Microstructure on Material-Removal Mechanisms and Damage Tolerance in Abrasive Machining of Silicon Carbide

Effects of microstructural heterogeneity on material-removal mechanisms and damage-formation processes in the abrasive machining of silicon carbide are investigated. It is shown that the process of material removal in a conventional silicon carbide material with equiaxed-grain micro-structure and strong grain boundaries consists of the formation and propagation of transgranular cracks which results in macroscopic chipping. However, in a silicon carbide material, containing 20 vol% yttrium aluminum garnet (YAG) second phase, with elongated-grain micro-structure and weak grain boundaries, intergranular micro-cracks are formed at the interphase boundaries, leading to dislodgment of individual grains. These different mechanisms of material-removal affect the nature of machining-induced damage. While in the conventional silicon carbide material the machining damage consists of transgranular median/radial cracks, in the heterogeneous silicon carbide material, abrasive machining produces interfacial micro-cracks distributed within a thin surface layer. These two distinct types of machining damage result in a different strength response in the two forms of silicon carbide materials. In the case of the conventional silicon carbide, grinding damage results in a dramatic decrease in strength relative to the as-polished specimens. In contrast, the ground heterogeneous silicon carbide specimens show no strength loss at all.     

高温氧化对再结晶碳化硅陶瓷断裂强度的影响

研究了在1500℃空气中的高温氧化对再结晶碳化硅(recrystallized silicon carbide,RSiC)陶瓷断裂强度的影响。用X射线衍射仪、扫描电镜分析了RSiC样品氧化后表面的物相组成及显微结构。结果表明:在1500℃下RSiC陶瓷的氧化质量增加遵循抛物线规律,氧化速率常数为3.77×10-5g2/(cm4·h1),其线性相关系数r2为0.998。RSiC陶瓷的室温抗弯强度随氧化时间增加呈现出先升后降的变化趋势,当氧化21h时,材料的室温抗弯强度最高,达到87MPa。氧化初期由于样品表面生成致密的非晶态SiO2,对样品表面的缺陷起到了钝化作用,导致材料室温断裂强度升高;氧化后期由于非晶态SiO2膜的析晶而产生裂纹以及循环氧化导致裂纹扩展,从而使材料的断裂强度降低。     

Effects of Sodium Silicate Exposure at High Temperature on Sintered α-Silicon Carbide and Siliconized Silicon Carbide

Sintered α-silicon carbide and siliconized silicon carbide were exposed to combustion off-gas containing sodium silicate vapors and particulates in a combustion test facility for 24 to 373 h at 900° to 1050°C. Degradation was evaluated by measuring dimensional changes, by measuring loss in strength due to changes in flaw population, and by evaluating surface corrosion morphology. It is suggested that passive oxidation and dissolution of the silica oxidation scale play an important role in the corrosion process. These mechanisms were enhanced by the continuous removal and replenishment of corrosive material by the high-velocity gas. These degradation phenomena caused surface pitting and an approximately 50% reduction in strength for both materials after long-term exposure (>100 h). Morphological evaluation suggested that the grain boundaries in the α-silicon carbide were oxidized more rapidly than the grains, while for the case of the siliconized silicon carbide the silicon phase was oxidized rapidly along with preferential oxidation of the silicon carbide grains parallel to the {0001} plains.     

硅石—碳化硅窑具砖

针对硅砖使用过程中易断裂的弊病,在其生产工艺的基础上,以碳化硅代替部分硅石克服之。碳化硅的加入量为15％。初见成效,30％效果明显,1100℃的水冷次数高达13次。硅石—碳化硅砖集硅砖和碳化硅砖之优点而兼具荷重变形温度高,热震稳定好的特点。作为耐火材料烧成用的高温窑具,使用寿命比硅砖提高五倍以上。     

Oxidation behaviors and shear properties of z-pinned joint made of two-dimensional carbon fiber reinforced silicon carbide composite

Chemical vapor infiltration has been introduced for preparing z-pinned joint, which is made of two-dimensional carbon fiber reinforced silicon carbide composite. The effects of oxidation on the shear properties of the joint were investigated. The results showed that the joint strength increases with the increase of oxidation temperature, which is consistent with the oxidation consumption of the carbon phases. An exponential relationship is presented between the weight loss and the joint strength. In contrast, linear relationships are presented between the weight loss and the mechanical properties of the composite. The exponential relationship results from the coupled shear and bending stress states of the pin, according to the failure mechanisms of the joint. It is observed that in-plane and intra-layer cracks are formed under the shear stress. And these cracks are bridged by the fibers under the bending stress. Accordingly, the fiber bridging mechanism contributes to the joint strength before and after oxidation. For the conditions of this study, the joint strength can be roughly estimated as the plus of the in-plane shear strength and the tensile matrix cracking stress.     

Oxidation of Submicroscopic Fibrous Silicon Carbide

The oxidation rate of silicon carbide fibers of submicroscopic dimensions in static air was investigated by a gravimetric technique at 800°, 900°, and 1000°C. The fibers can be held near 800°C for several hours without significant oxidation, but they rapidly oxidize at 1000°C. A theoretical model for diffusion-controlled oxidation of the fibers, taking into account a changing reaction interfacial area, was obeyed to more than 60% conversion of the silicon carbide to silica. For the diffusion-controlled oxidation an enthalpy of activation of 55.8 or 39.8 kcal/mole was calculated depending on whether an amorphous silica sheath was initially present.     

Growth of Alumina/Metal Composites into Porous Ceramics by the Oxidation of Aluminum

Alumina/metal composites were grown into the pores of porous alumina, porous aluminosilicate, and porous silicon carbide substrates through the oxidation of Al–Si (5 wt%) powder compacts coated with magnesia powder (11 mg/ cm²). The thickness of the resulting composite increased with oxidation time and temperature, and was proportional to (pore size)^0.5 on using porous alumina. The composite thickness was more than 2 times larger in the silicon carbide and about 4 times larger in the aluminosilicate than in the alumina at 1523 K for 1 h. The products using these three types of substrates consisted of alumina, aluminum, and silicon, except that a silicon carbide phase occurred when using the silicon carbide substrate. Silica and mullite in the aluminosilicate substrate changed to silicon and alumina, and silica in the silicon carbide substrate changed to silicon because of the reduction by aluminum.     

Characterization of Porous Alumino-Silicate Bonded SiC-Ceramics Prepared by Hand-Pressing and Extrusion Methods

Silicon carbide was mixed with alumina and kaolin to obtain porous alumino-silicate bonded SiC ceramics. Starch was added as sacrificial template. The mixtures were processed by hand-pressing and extrusion method. The effect of firing temperature (1450 ^°C) and the addition of starch on the composition and characteristics of fired specimens were studied. The changes in phase composition and microstructure of fired SiC specimens were followed through x-ray diffraction analysis and scanning electron microscopy, respectively. The sintering parameters and thermal oxidation in air of such specimens were determined. The results indicated that silicon carbide is oxidized during firing into silica which reacted with silica-alumina mixture forming alumino-silicate bonding. Meanwhile, starch burnt out leaving pores inside the specimens. Porous SiC specimens of 1.72 to 1.79 g.cm^?2 bulk density, 40 to 45% open porosity and 250 to 350 N.cm^?2 compressive strength could be obtained by using a mixture of 80 mass% SiC and 20 mass% alumina and kaolin as starting materials. The properties of porous SiC specimens depend on the type and amount of used starch. The extrusion method is favorable for preparing porous SiC articles of homogeneous microstructure and good properties.     

Residual thermal stress analysis of SiC joint by polysiloxane silicon resin YR3187

Reaction-bonded silicon carbide was joined using a polysiloxane silicon resin YR3187. Residual thermal stress distribution in joint structure was calculated by finite element analysis method. Factors influencing the distribution of residual thermal stress, including joining temperature, thickness of join layer, and presence of crack in join layer, were studied. The simulated results were compared with actual joint strength. It is showed that in determining the joining temperature, the pyrolysis procedure of silicon resin should be taken into consideration; in addition, the reasonable thickness of the join layer and the reduction of cracks in join layer are important to obtain higher joint strength.     

Stress-Oxidation Behavior of a Carbon/Silicon Carbide Composite in a High-Temperature Combustion Environment

A carbon/silicon carbide composite with a silicon carbide coating was prepared by chemical vapor infiltration. Stressed oxidation testing was performed on the composites in a self-built high-temperature combustion environment. The gas in this environment contained oxygen, steam, carbon dioxide, and some nitrogen. Test conditions were controlled at temperatures of 1300°, 1500°, and 1800°C, and the stress was sustained at 40, 80, 120, 160, and 200 MPa. The effect of combustion environment and applied load on stress-oxidation behavior was discussed by analyzing the residual strength and weight loss. The morphology of the fracture surface of the tested specimens was observed by scanning electron microscopy. The high-temperature combustion environment and the high sustained stress above 80 MPa enhanced the material failure and led to strength reduction by determining crack openings and thus oxidation of fibers. However, sustained stress below 80 MPa resulted in no strength degradation after exposure for 10 min at 1500°C.     

Improved Low-Temperature Environmental Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystals by Surface Encapsulation

An encapsulating layer was deposited on the surface of tetragonal zirconia polycrystals doped with 3 mol% of yttria (3Y-TZP), to prevent low-temperature environmental degradation (aging) of the material. The layer, which was composed of silica and zircon, was formed on the surface by exposing the specimens next to a bed of silicon carbide powder in a flowing hydrogen atmosphere that contained ∼0.1% water vapor at 1450°C. The layer was ∼0.5 µm thick and is expected to be under strong residual compressive stress. This encapsulation process remarkably improved the low-temperature degradation of the material. The strength of the specimens also was improved by this process.     

Oxidation Behavior of Silicon Carbide

The oxidation of purified green silicon carbide in controlled atmospheres was studied by weight-gain measurement and by observation of the surface reaction products, including optical measurement of the thickness of the oxide surface film. The rate of oxidation was much greater for silicon carbide in contact with fluid silicate glasses than for silicon carbide alone. In a vacuum of 0.1 mm., oxidation proceeded with loss of weight, because of the formation of volatile SiO₂, and at a greater rate than at atmospheric pressure. It is postulated that the rate-controlling process in the normal oxidation of silicon carbide is the formation of solid SiO₂ on the surface.     

Grain-size distribution of silicon carbide as a factor in the properties of electric heaters

Conclusions The variation of the proportion of the main fractions in the powders of green silicon carbide are a source of irregularity in the properties (porosity, pore structure, strength, and oxidation resistance) of silicon carbide electric heaters.The properties of the heaters can be improved by increasing and stabilizing the proportion of main fraction. It would be advisable to define the relevant norms in the standard for granular silicon carbide more precisely.Translated from Ogneupory, No. 4, pp. 46–51, April, 1977.     

Reaction sintering of alumina/mullite nanocomposites from nano-sized starting powders

Alumina/mullite ceramic nanocomposites were prepared by the mixtures of nano-sized starting powders of alumina with silica and alumina with silicon carbide. Silica from deliberate addition and as the product of silicon carbide oxidation reacted completely with alumina to form mullite. Silica from direct addition segregated at the grain boundary and intergranular mullite was formed whereas silica from oxidation was surrounded by alumina matrix and intragranular mullite was formed after reaction sintering. The most significant difference was fracture behaviour where intragranular mullite nanoparticles promoted transgranular fracture in alumina matrix due to thermal mismatch around nanoparticles and intergranular mullite nanoparticles gave rise to intergranular fracture similar to pure alumina. Wear resistance of the nanocomposites was better than that of alumina. Pull-out formation in the nanocomposites was less and pull-out size was also smaller. Fracture toughness of the nanocomposites was significantly higher than that of alumina.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Influence of silicon carbide as a mineralizer on mechanical and thermal properties of silica-based ceramic cores

Ceramic cores have been designed with compounds based on fused silica due to its excellent thermal stability and chemical inertness against molten metals. To endure the high temperatures present during investment casting, mineralizers have been widely used to enhance the flexural strength and shrinkage of ceramic cores. In this study, we demonstrated a silica-based ceramic core with silicon carbide as a mineralizer for improving the mechanical and thermal properties. The SiC in the silica-based ceramic cores can enhance the mechanical properties (i.e., flexural strength and linear shrinkage) by playing a role as a seed for the crystallization of fused silica to cristobalite. The SiC also improves the thermal conductivity due to its higher value compared with fused silica. The results suggest that using the optimal amount of silicon carbide in silica-based ceramic cores can provide excellent mechanical properties of flexural strength and linear shrinkage and improved thermal conductivity.     

Crack-healing in ytterbium silicate filled with silicon carbide particles

A study on the influence of the silicate composition on the self-healing ability of a promising environmental barrier coating (EBC) material is reported. The EBC material consists of silicon carbide particles dispersed in a ytterbium silicate matrix. The composition of the silicate was varied from pure monosilicate to pure disilicate including a mixture of 50 vol-% of both phases. Pre-cracked specimens were mechanically tested before and after thermal annealing in air at 1400 °C accompanied by microstructural investigations. It is shown, that the self-healing mechanism is based on the application of compressive surface stresses due to graded oxidation of the SiC particles. With increasing amount of disilicate strength and self-healing ability increase.     

Influence of the pH on the Morphology of Sol–Gel-Derived Nanostructured SiC

This investigation deals with the use of the carbothermal reduction reaction of carbonaceous silica xerogel for synthesizing nanostructured SiC. The effect of the pH of the early-prepared gel (EPG) on the properties of the finally obtained nanostructured silicon carbide was investigated. The produced silicon carbide samples were characterized by X-ray diffraction, fourier transform infrared spectroscopy, transmission electron microscopy, and N₂ porosimetry. The results revealed that the variation of EPG pH greatly affected the structural properties of the finally obtained nanostructured silicon carbide. The morphology of the produced nanostructured silicon carbide was changed from nanoparticles to nanorods and vice versa with changing the pH values of the gel. The nitrogen adsorption–desorption isotherms of the samples are of classical type IV, characteristic of mesoporous materials. The BET surface areas of the nanostructured silicon carbide are increased from 68 to 106 m²/g with increasing the pH of the EPG from 3 to 5, respectively.