Phase Evolution in Silicon Carbide–Whisker‐Reinforced Mullite/Zirconia Composite during Long‐Term Oxidation at 1000° to 1350°C

Abstract

A composite consisting of 30 wt% SiC whiskers and a mullitebased matrix (mullite-32.4 wt% ZrO 2 -2.2 wt% MgO) was isothermally exposed in air at 1000°-1350°C, for up to 1000 h. Microstructural evolution in the oxidized samples was investigated using X-ray diffractometry and analytical transmission electron microscopy. Amorphous SiO 2 , formed through the oxidation of SiC whiskers, was devitrified into cristobalite at T > 1200°C and into quartz at 1000°C. At T > 1200°C, the reaction between ZrO 2 and SiO 2 resulted in zircon, and prismatic secondary mullite grains were formed via a solutionreprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also were found after long-term exposure at T > 1200°C.

Phase Evolution of SiO2–Al2O3–ZnO–CaO–ZrO2–TiO2‐Based Glass with Added Y‐PSZ Particles

“…It is suggested that zircon formation is caused by the inward migration of Si 4+ . [33][34][35][36][37] Due to the smaller ionic radius of Si 4+ (0.026 nm) than that of Zr 4+ (0.072 nm), Si shows higher diffusivity than Zr. 33,37,38 Furthermore, Zr 4+ is already arranged in ZrO 8 polyhedra which have only to separate in one direction to incorporate SiO 2 .…”

“…Once the YSZ particles were entirely clad by the newly formed zircon, the inward growth of zircon began and was controlled by a diffusion process. It is suggested that zircon formation is caused by the inward migration of Si 4+ . Due to the smaller ionic radius of Si 4+ (0.026 nm) than that of Zr 4+ (0.072 nm), Si shows higher diffusivity than Zr .…”

Oxygen diffusivities in mullite/zirconia composites measured by 18O/16O isotope exchange and secondary ion mass spectrometry

“…Several investigations on the oxidation behavior of mullite/PSZ/SiC composites have been carried out. [5][6][7][8][9][10] In a pioneering study, Lin et al 10 first phenomenologically proposed two distinct oxidation modes, designated as mode I and mode II, of mullite/PSZ/SiC composites at elevated temperatures: mode I was based on the assumption that oxygen diffusion in the silica layer was much faster than in the mullite/zirconia matrix, and mode II operated otherwise. However, oxygen diffusivities in various mullite/PSZ composites have not been available to date.…”

Wear mechanisms of silicon carbide-whisker-reinforced alumina (Al2O3–SiCw) cutting tools when high-speed machining aged Alloy 718

“…Hard water has a higher concentration of both elements (normally >100 ppm) when compared to 30 ppm found by Brandt et al [2] in Alloy 718. In either case expected are the reactions with Al 2 O 3 with generation of MgO-n·Al 2 O 3 spinel which forms at temperatures around 1200 ºC [27]. Figure 12 shows the FIB cross-section taken on the crater of worn tool at v c =250 m/min, f=0.19 mm/rev and a p =0.53 mm.…”