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Two commercial grade silicon nitride ceramics (Honeywell GS44 and Kyocera SN235) were exposed to an oil ash to evaluate the long-term corrosion/oxidation resistance in a simulated diesel engine environment. The exposure condition was at 850 °C for 1000 h in air. Subsequently, the exposed specimens were tested in flexure for strength degradation at room temperature and 850 °C at stressing rates of 30 MPa/s and 0.003 MPa/s in air, respectively. A similar set of specimens not exposed to the oil ash was also tested in flexure for purpose of comparison. Little change in strength was measured after 1000 h exposure in the oil ash environment. Also, the values of Weibull modulus obtained for all of the exposed silicon nitride materials were similar to those with the unexposed specimens whose strength were obtained under the same conditions. However, both exposed and unexposed GS44 specimens exhibited a low fatigue exponent, suggesting a susceptibility to slow crack growth at test temperature. In addition, detailed SEM/EDAX analyses indicated that no oil ash elements (e.g., Zn, Ca, P, Na, and S) were detected beneath a thin layer in the surface; thus, there were no changes in the chemistry of the secondary phase(s) within the bulk. These elements were detected only in a region about 1–3 μm below the exposed surface, but no apparent changes in microstructure observed. Results of mechanical properties and microstructural characterizations indicated that these candidate silicon nitride materials exhibited excellent corrosion/oxidation resistance in the simulated diesel engine environment and, based on their excellent mechanical strengths, would be ideal candidates for diesel engine exhaust valve applications. [ABSTRACT FROM AUTHOR]

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As/IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300 K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed. [ABSTRACT FROM AUTHOR]

Densification of silicon carbide without any sintering aids by hot-pressing and rapid hot pressing was investigated. Full density (>99% t.d.) has been reached at 1850 °C, a temperature of at least 150–200 °C lower compared to the up to now known solid state sintered silicon carbide powders. Silicon carbide was freeze granulated and heat treated prior the densification. Evolution of microstructure, mechanical properties and creep behavior were evaluated and compared to reference ceramics from as received silicon carbide powder as well as those of commercial one. Novel method results in dense ceramics with Vickers hardness and indentation fracture toughness of 29.0 GPa and 5.25 MPa m 1/2 , respectively. Moreover, the creep rate of 3.8 × 10 −9 s −1 at 1450 °C and the load of 100 MPa is comparable to the commercial α-SiC solid state sintered at 2150 °C. [ABSTRACT FROM AUTHOR]