PRELIMINARY STUDIES OF TIN PARTICULATE-REINFORCED SI3N4 MATRIX COMPOSITE (SYALON-501) FOLLOWING EXPOSURE IN OXIDIZING AND OXY-CHLORIDISING ENVIRONMENTS

Silicon nitride ceramics are used for engine components, turbochargers and recent modification has been devoted to particulate (mainly TiN) reinforcement of Si3N4-based composites for specific application as heaters, and structural and wear resistant components. The purpose of this study was to compare and contrast the corrosion behaviour (as determined by mass change, scale morphology, elemental composition and phase constitution) of TiN-Si3N4 (Syalon 501) composite at 1050, 1150, 1250 and 1350 degrees C under oxidising (pO(2) congruent to 0.2x10(5) Pa) and oxy-chloridising (pCl(2) congruent to 0.02x10(5) Pa, pO(2) congruent to 0.2x10(5) Pa) conditions for 5 to 100 hours. A discontinuous gravimetric method was used to evaluate corrosion performance of polished specimens of Syalon 501 composite. Oxidation kinetics at 1050, 1150, 1250 and 1350 degrees C indicated parabolic behaviour until 50 hours of exposure. The weight gain after 100h isothermal oxidation at 1050 degrees C was 2.987mg/cm(2): weight gains increased considerably with temperature, ie 43.838mg/cm(2) after 100h oxidation at 1350 degrees C. The surface of the oxidised samples was covered by a scale of rutile (TiO2) and various amounts of tridymite (SiO2), yttrium silicate (Y2Si2O7) and titanium yttrium oxide (YTiO3). The oxy-chloridation kinetic curves revealed a progressive increase in mass gain implying that the scale formed was less protective in the presence of 2%Cl-2 which was further substantiated by a highly degraded surface morphology. The mass gains after 100 hours oxy-chloridation were 5.603 and 48.171mg/cm(2) at 1050 degrees C and 1350 degrees C respectively. The post-exposure corrosion products were identified as rutile (TiO2), yttrium silicate (Y2Si2O7) and trace amounts of titanium yttrium oxide (YTiO3), and cristobalite (SiO2).