Effects of Sintering Aides on the Hydrothermal Oxidation of Silicon Nitride Spherical Rolling Elements

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Commercially available silicon nitride (Si3N4) spherical bearing rolling elements containing TiO2, Y2O3, MgO and Al2O3 additives were evaluated for corrosion-resistance in high-temperature, high-pressure hydrothermal tests designed to simulate aero propulsion conditions. Spheres were exposed in an autoclave at 523–623 K and 5.2–16.5 MPa for 12–48 h and characterised using mass change and pH measurements, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy and inductively coupled plasma mass spectrometry. The oxidation resistance of the TiO2/Y2O3/Al2O3-sinter-aided Si3N4 ceramic closely matches the Y2O3/Al2O3-doped Si3N4 and outperforms the MgO-sinter-aided Si3N4. Additional studies on the TiO2/Y2O3/Al2O3 Si3N4 composition show pitting initiates around titanium-rich inclusions, due to a break in the protective hydroxide layer, accessible diffusion paths around inclusions and the catalytic nature of titanium. This study demonstrates that the addition of TiO2/Y2O3/Al2O3 to hot pressed Si3N4 reduces corrosion rates in high-temperature, high-pressure, hydrothermal environments.