Effects of Sintering Aides on the Hydrothermal Oxidation of Silicon Nitride Spherical Rolling Elements
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.
Bateman, Allyssa; Queale, Abby J.; Butt, Darryl P.; and Jaques, Brian J.. (2019). "Effects of Sintering Aides on the Hydrothermal Oxidation of Silicon Nitride Spherical Rolling Elements". Corrosion Engineering, Science and Technology, 54(1), 22-27. http://dx.doi.org/10.1080/1478422X.2018.1523290