Oxidation kinetics of hot-pressed silicon carbide

The oxidation of silicon carbide, hot-pressed with 4 wt % Al₂O₃, in 1 atm dry oxygen follows classical parabolic behaviour with an activation energy of 481 kJ mol^–1 in the temperature range 1200 to 1400° C. The oxide film consists predominantly of cristobalite and a glassy phase in which additive (Al) and various impurity elements (Fe, Na, K, etc) concentrate. The desorption of CO(g) from the SiC/SiO₂ interface appears to be oxidation rate controlling.     

Time-resolved penetration into pre-damaged hot-pressed silicon carbide

We have conducted a series of experiments to examine projectile penetration of cylindrical hot-pressed silicon carbide (SiC) ceramic targets that are pre-damaged to varying degrees under controlled laboratory conditions prior to ballistic testing. SiC was thermally shocked to introduce non-contiguous cracks. Another set of targets was thermally shocked and then additional damage was induced by load–unload cycling in an MTS machine while the ceramic specimen was confined in a 7075-T6 aluminum sleeve. Finally, targets were made by compacting SiC powder into a 7075-T6 aluminum sleeve. For each of these target types, long gold rod penetration was measured as a function of impact velocity v_p over the approximate range of 1–3 km/s, with most data between 1.5 and 3 km/s. Penetration as a function of time was measured using multiple independently timed flash X-rays. Results are compared with previous results for non-damaged (intact) SiC targets. Key results from these experiments include the following: (1) penetration is nominally steady state for v_p>1.5 km/s; (2) for all target types, the penetration velocity u is a linear function of v_p (except for the lowest impact velocities); and (3) it is found that u_intact<u_pre-damaged<u_{in-situ comminuted}<u_powder<u_hydrodynamic.     

Solid-particle erosion of hot-pressed silicon carbide and SiC-TiB2 composite

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of polycarbosilane addition on mechanical properties of hot-pressed silicon carbide

Silicon carbide ceramics containing 20 wt% polycarbosilane was fabricated by hot-pressing with various additives (B, Al, AlN). The addition of polycarbosilane resulted in a considerable increase in flexural strength up to 1050 MPa for the 1 wt% AlN and 0.5 wt% B-doped specimens and fracture toughness up to 4.0 MPa m^1/2 for the 1 wt% Al-doped specimen. The improved fracture strength was a result of liquid-phase sintering and the improved toughness was a result of crack deflection along the grain boundaries. Crack branching was also observed in 1 wt% AlN and 0.5 wt% B-doped specimens.     

Characterization of creep cavity shape in hot-pressed silicon carbide using small-angle neutron scattering

Fine-grained silicon carbide with a continuous second-phase grain-boundary film was crept under compressive loading at 1600° C. The shape of the resultant grain-boundary cavities was characterized using small-angle neutron scattering. During the early stages of creep the cavities grew more rapidly in the plane of the grain boundary, as evidenced by an elongation of the isointensity contours and an increase in the radius of gyration,R _D, along the direction of the applied compressive stress. During the latter stages of creep the cavities grew more rapidly perpendicular to the grain-boundary plane, as evidenced by a gradual reduction in the scattering anisotropy and by an increase inR _D perpendicular to the compressive stress axis relative toR _D parallel to the compressive stress axis. Cavity aspect ratios calculated from the ratios of theR _D values parallel to and perpendicular to the compressive stress axis are shown to support a recent model of cavity growth in a viscous grain-boundary film.     

Effect of a new additive on mechanical properties of hot-pressed silicon carbide ceramics

The mechanical properties and microstructure of SiC ceramics, hot pressed by simultaneously adding nano-SiC and oxides (MgO+Al₂O₃+Y₂O₃) or nitrate salts (Mg(NO₃)₂+Al(NO₃)₃+Y(NO₃)₃) as additives, were evaluated. The oxide additives system slightly influenced the mechanical properties of the ceramics, while the addition of nano-SiC lead to finer microstructure, and 5 vol.% nano-SiC changed the fracture mode from intergranular type to transgranular type. The ceramics with nitrate salts had fine, equiaxed grains with an average grain size larger than that of the system added oxides, thus inducing lower Viker’s hardness and flexural strength, while the presence of crystalline YAG phase improved the fracture toughness by 54.7%. Also, an observed increase in grain growth—with decreasing weight fraction of liquid and the grounded grain morphology in this system—confirmed a diffusion-controlled growth mechanism. Although the sample with the least amount of additives has the lowest relative density and largest grain size, its flexural strength did not drastically decrease. The influence of nano-SiC on the fracture toughness in the nitrate additive system was negligible.     

Proof-testing of hot-pressed silicon nitride

Proof-testing was investigated as a method for insuring the reliability of hot-pressed silicon nitride in high temperature structural applications. The objective of the study was to determine if the strength distribution of a population of test specimens could be truncated by proof-testing. To achieve this objective the strength of silicon nitride was measured at 25° C and 1200° C, both with and without proof-testing. At 25° C, however, the strength distribution was effectively truncated by proof-test ing. At 1200° C, however, the effectiveness of proof-testing as a means of truncating the strength distribution was determined by the resistance of the silicon nitride to oxidation. Although oxidation removes machining flaws that limit the strength of silicon nitride, long-term exposure to high temperature oxidizing conditions resulted in the formation of surface pits that severely degraded the strength. Provided the effects of high temperature exposure are taken into account, proof-testing is shown to be useful for truncating the strength distribution of hot-pressed silicon nitride at elevated temperatures.     

Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol^?1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.     

Phenomenology of fracture in hot-pressed silicon nitride

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation 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 inherent processing flaws of the order of 12 to 24 m are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, , at 20° C for hot-pressed Si₃ N₄ is about 22 000 erg cm^–2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate.     

Production and properties of hot-pressed materials based on silicon carbide with additions of boron and titanium carbides

Special features of pressure sintering materials based on silicon carbide with additives of boron and titanium carbides have been investigated. The kinetic parameters of the densification have been established. Special features of the structure and physico-mechanical properties of hot-pressed materials of the SiC(8–20 wt %)–(B₄C–TiC) system have been studied. Dense materials have been produced based on si-licon carbide with increased (3.8 MPa?m^1/2) fracture toughness and low (0.07 Ω?m) electrical resistance.     

Origin of strength anisotropy in hot-pressed silicon nitride

Three- and four-point room-temperature bend tests were carried out on specimens of a hot-pressed silicon nitride oriented parallel and perpendicular to the hot-pressing direction. Scanning electron microscopy was used to identify and measure the fracture-initating flaws, thus enabling the fracture toughness to be calculated. The strength anisotropy exhibited by this material was attributed to variation in the severity of the inherent flaws in the material with orientation, with the fracture toughness showing no significant anisotropy in this case.     

Defects in silicon carbide

Defects in various forms of SiC, both single crystal and polycrystalline, have been examined using transmission electron microscopy. Dislocations were not as common as stacking faults, which were observed in all materials examined. The mechanism of formation of stacking faults is discussed and two types of both intrinsic and extrinsic faults are shown possible. The stacking-fault energy of SiC was measured to be 1.9 ergs/cm² by the extended node method.