Effects of microstructure on the R-curve behaviour of sintered silicon nitride

The R-curve behaviour of sintered silicon nitride was investigated by using short bar specimens. The samples were fabricated by sintering at 1700° C for 2 or 8 h in a nitrogen atmosphere, varying the initial contents of the silicon nitride powder. The R-curve was evaluated with a loading and unloading technique to calculateK _R during stable fracture. Steep R-curves were observed in the specimens made from the high initial content powder. Thus, the grainbridging effect behind the crack seems to contribute to the R-curve, because a steep R-curve corresponds to fibrous texture development. ApparentK _c values with an assumption of linear elastic fracture mechanics for the specimens undergoing 8h sintering, are greater than those undergoing 2 h sintering. These results can be attributed to microcracking.     

Effects of proton irradiation on the microstructure and mechanical properties of Amosic-3 silicon carbide minicomposites

One dimensional Amosic-3 silicon carbide fiber reinforced silicon carbide matrix composites (SiC_f/SiC minicomposites) prepared by chemical vapor infiltration were irradiated with 2.8 MeV proton ions. The ion fluences were 1.0 × 10¹⁷ and 1.5 × 10¹⁷ cm⁻² at room temperature and 300 °C, respectively. The microstructure and mechanical properties were investigated before and after proton irradiation. Raman spectra showed no evident change in Amosic-3 fibers regardless of irradiation temperature, which is confirmed by high resolution transmission electron microscopy observation. Pyrolytic carbon interphase showed slightly expansion after 300 °C irradiation, however, no microstructure changes were observed in SiC matrix. Moreover, it can be deduced that no irradiation induced changes in mechanical properties were observed after present proton irradiation.     

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

TaC and TaC-1 wt.% B₄C powders were consolidated using spark plasma sintering (SPS) at 1850 °C and varying pressure of 100, 255 and 363 MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B₄C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255 MPa and higher at 1850 °C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B₄C leads to an increase in the density of 100 MPa sample from 89% to 97%. B₄C nano-powder resists grain growth even at high pressure of 363 MPa. The formation of TaB₂/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B₄C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B₄C addition.     

Microwave densification of electrophoretically infiltrated silicon carbide composite

A new method to fabricate SiC composites by microwave heating SiC preforms is described. Preforms were produced by electrophoretically infiltrating SiC fibre (Nicalon) preforms with SiC powder. Samples were heated to 1600°C in a matter of minutes and held at temperature for 5 min to minimize fibre degradation. To achieve densification, heated preforms required the application of a uniform load. Bulk densities increased from ∼ 0.8 gcm-3 for the as-infiltrated preforms to over 1.9 gcm-3 for microwave-heated samples with a small applied load of ∼13 kPa. Microstructural analysis revealed the presence of some pores and cracks in the matrix; however, large areas of dense SiC matrix material are apparent. This revised version was published online in November 2006 with corrections to the Cover Date.     

纳米晶钨粉对液相烧结93W合金组织性能影响

采用高能机械球磨方法制备了超细钨粉,经冷等静压和1465℃分解氨气氛中液相烧结制得高密度钨合金.研究了纳米晶亚微米颗粒钨粉对烧结态93W-4.9Ni-2.1Fe高密度钨合金微观组织及性能的影响.研究表明:采用超细钨粉与低温液相烧结技术,获得了高相对密度(大于99.7%)的烧结态高密度钨合金,且细钨颗粒组织均匀分布于粘结相中;与采用亚微米颗粒钨粉的烧结态钨合金相比较,不仅微观组织弥散分布,而且具有较高的力学性能;液相烧结态钨合金的力学性能主要与原始钨粉粒度及烧结温度有关.     

Mechanical characteristics of microwave sintered silicon carbide

The present work deals with the sintering of SiC with a low melting additive by microwave technique. The mechanical characteristics of the products were compared with that of conventionally sintered products. The failure stress of the microwave sintered products, in biaxial flexure, was superior to that of the products made by conventional sintering route in ambient condition. In firing of products by conventionally sintered process, SiC grain gets oxidized producing SiO₂ (∼ 32 wt%) and deteriorates the quality of the product substantially. Partially sintered silicon carbide by such a method is a useful material for a varieties of applications ranging from kiln furniture to membrane material.     

Effects of additives on the nitridation process of foamed materials containing silicon and silicon carbide powders

Without using any additive, the nitridation process of silicon powder was slow and the main product was α-Si₃N₄ due to the cycling production of SiO species. The addition of Al₂O₃ and Y₂O₃ could facilitate the nitridation process resulting in a higher β-Si₃N₄ content presumably due to the liquid phase formed between Al₂O₃, Y₂O₃ and surface silica on silicon powder. When a small amount of 1.76% Fe₂O₃ was added, the accelerated nitridation process was attributed to the FeSi₂ liquid phase produced by reaction Fe element with surface silica at a lower temperature of 1212 °C, but the Al₂O₃ and Y₂O₃ additives could still be active for sustaining the nitridation process at higher temperature. At a higher Fe₂O₃ concentration of 3.46%, the nitridation process was mainly controlled by the formed FeSi₂ liquid phase. This study has demonstrated the active role of using Al₂O₃ and Y₂O₃ combination and Fe₂O₃ on the nitridation process, which could be helpful for further investigation on reaction bonding of SiC and Si₃N₄ ceramics.     

Composition and microstructure of silicon carbide whiskers

The bulk and surface chemistries of four sets of commercially available SiC whiskers made by three manufacturers were determined. The oxygen content varied significantly, ranging in the bulk from 1.9 to 0.6 at.% and on the surface from 35 to 15 at.%. Surface analysis as obtained by X-ray photoelectron spectroscopy also indicated that the oxygen species differed significantly with whisker supplier; each of three of the whisker sets contained a surface species that is very similar to that found in a Si-O-C glass, while one whisker surface appeared to have a silica-rich surface. Surface carbon concentrations varied significantly, while silicon concentrations did not. Scanning transmission electron micrographs indicate significant morphological variations (i.e., twinning, branching, kinks, surface roughness, etc.) occur in all of the whisker types.     

Phenomenology of fracture in sintered alpha silicon carbide

Crack propagation mechanisms in a sintered alpha silicon carbide were studied as a function of initial flaw size, temperature, loading rate and applied stress. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw (IIF) technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 40m were strength controlling. The flexural strength was found to be independent of temperature (20 to 1400° C) and the fracture faces did not show the presence of subcritical crack growth (SCG). Preliminary results from stress rate testing also failed to show the presence of SCG in tests made at 1200° C in air. In contrast, flexural stress rupture tests carried out at 1200 and 1300° C in air using precracked specimens indicated the materials susceptibility to time-dependent deformation and showed the presence of SCG. Fractographic evidence for transgranular crack propagation during fast fracture (catastrophic failure) and intergranular crack propagation during SCG is presented.     

R-curve behaviour and microstructure of sintered silicon nitride

R-curves for two in situ reinforced silicon nitrides A and B, of different microstructures, were characterized using indentation-crack growth measurements. Silicon nitride B, with its coarser microstructure and 8 MPa m^1/2 toughness, showed higher resistance to crack growth and more damage tolerance than silicon nitride A, with its finer microstructure and 7 MPa m^1/2 toughness. However, silicon nitride A showed a higher Weibull modulus than that of silicon nitride B due to the relatively narrow critical grain-size distribution. These results suggest that a coarse microstructure with narrow flaw-size distribution is beneficial to toughening, damage tolerance, and reliability.     

Study the formation mechanism of silicon carbide polytype by silicon carbide nanobelts sintered under high pressure

In this paper, in order to reveal the formation mechanism of SiC polytype, four SiC specimens sintered under high pressure has been investigated, after being prepared from SiC nanobelts as initial powders. The structure and morphology variation dependence of SiC specimens with temperature and pressure was studied based on experimental data obtained by XRD, SEM, and Raman. The results show that SiC lattice structure and the crystallite size are greatly affected by pressure between 2 and 4 GPa under different sintering temperatures of 800 and 1200 degrees C. At the largest applied pressure and temperature, 4 GPa and 1200 degrees C, 3C-SiC crystal structure can be changed into to R-SiC due to the stress resulted in dislocations instead of planar defects. Based on our results, the multiquantum-well structure based a single one-dimensional nanostructure can be achieved by applying high pressure at certain sintered temperature.     

Formation of silicon carbide whiskers and their microstructure

Thermodynamic and kinetic conditions for the formation of SiC whiskers are established. The mechanism of their nucleation and growth are studied and, on this basis, the magnitude of the thermally activated barrier is determined from the rate of reduction data. The microstructures of whiskers are analysed and the role of interfacial tension between the nuclei and impurities, and the metallic iron catalyst is studied in relation to the formation of SiC whiskers. A possible reason for polytypism in SiC whiskers is also proposed.     

碳化硅陶瓷的活化烧结与烧结助剂

综述了作为碳化硅陶瓷烧结助剂的热力学条件及液相烧结的有效条件,介绍了碳化硅陶瓷烧结助剂的研究进展.