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UBC Theses and Dissertations

Yttrium disilicate as environmental barrier coating for silicon nitride-based glow plug Lin, Xin

Abstract

Silicon-based ceramics undergo severe degradation at temperatures above ~1000 ℃ in the presence of water vapor, which is inevitable in combustion environments. Therefore, Environmental Barrier Coatings (EBCs) are necessary for the protection of Si₃N₄-based ceramic components in the harsh combustion environments. Rare earth silicates, which have relatively low thermal expansion coefficients, good chemical stability at high temperatures and low recession rates in the presence of water vapor, are promising candidate materials for such EBC application. This study was related to the application of Si₃N₄ as part of Hot Surface Ignition Systems (“Glow-Plugs”, GP) in High-Pressure Natural Gas Direct Injection engines, currently under development by Vancouver company Westport Fuel Systems Inc. For certain kinds of commercially available Si₃N₄-based GPs, the use of Y₂O₃ as sintering additive results in the in-service formation of yttrium silicates on their ceramic pins. Therefore, taking the chemical compatibility into consideration, yttrium disilicate coating was chosen to provide corrosion protection for such GPs. A sol-gel dip-coating route, which is simple, cost effective and industrially applicable, has been developed to apply multi-layer Y₂Si₂O₇ EBCs on the GPs. Selective processing parameters, including the sol aging conditions and the withdrawal speed of the GP substrate during dip coating procedure, were investigated in detail. The thickness and microstructures of the coatings were controlled through the adjustment of these parameters during sol preparation and dip coating processes. To simultaneously achieve sufficient thickness and avoid the formation of cracks, thin layers of Y₂Si₂O₇ coating, each with the thickness of ~1 µm, were successively applied and processed. The 6-layer crack-free coating was able to achieve an average thickness of ~5.5 µm. The microstructures of the coatings were evaluated and their performance was tested at ~1200 ℃ in high concentration water vapor atmosphere and on a natural gas burner rig. Improved corrosion resistance of such EBC-protected glow plugs was observed in these tests.

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