Hydrothermal Treatment of Si₃N₄ for the Improvement of Oxidation Resistance at 1400°C

Abstract

The surface of Si3N4 ceramics was hydrothermally treated with HCl or H2SO4 using an autoclave. The thickness of the oxide layers formed on the Si3N4 samples decreased to one‐fourth after oxidation at 1400°C by the treatment. The oxide layer of the treated samples was dense, and flaw formation in and beneath the layer did not occur at 1400°C. The avoidance of low melting Y‐silicates by leaching Y2O3 is the reason for the improved oxidation resistance of the hydrothermally treated Si3N4, despite an increase in surface porosity through a 70 μm layer.

Degradation of Silicon Nitride Glow Plugs in Various Environments Part 3: NGDI Engine

“…The estimated 1200 h life in service on the engine is not acceptable for this application (minimum 5000 h and ideally 20,000 h are desired), so means to protect the surface of the ceramic heater operating continuously at 1300°C in NGDI engine must be implemented. A variety of techniques to modify the Si 3 N 4 surface by ion implantation, coatings, hydrothermal surface treatment, have been studied in to improve the oxidation and corrosion resistance . Deposition of a mullite‐mullite coating, followed by in‐situ calcination by powering the glow plug has been evaluated in our labs (Wong et al [2010], unpublished data) and it looks very promising, as tested in air.…”

“…A variety of techniques to modify the Si 3 N 4 surface by ion implantation, coatings, hydrothermal surface treatment, have been studied in to improve the oxidation and corrosion resistance. [9][10][11][12][13][14] Deposition of a mullitemullite coating, followed by in-situ calcination by powering the glow plug has been evaluated in our labs (Wong et al [2010], unpublished data) and it looks very promising, as tested in air. After 100 h at 1300°C, the layer of Yb 2 Si 2 O 7 formed below the coating was about one-fourth of that developed on an uncoated glow plug.…”