Mullite-mullite environmental barrier coatings on silicon nitride by composite sol-gel

• Main Author: Wong, Frankie
• Language: English
• Published: University of British Columbia 2009
• Online Access: http://hdl.handle.net/2429/12612

Silicon nitride is susceptible to paralinear oxidation in applications exposed to combustion environments, such as glow plugs in direct injection natural gas engines. The oxide layer may become volatile in moist high-temperature environments. To protect Si₃N₄, environmental barrier coatings (EBC) maybe used. Mullite has high corrosion resistance, high temperature strength, and its thermal expansion (CTE=4.5-6 x 10⁻⁶ /°C), is relatively close to that of Si₃N₄ (CTE=3.3 x 10⁻⁶ /°C). However, the CTEs still differ by a factor of 1.4-2, thus it is difficult to produce crack free EBCs >1 µm thick. To address this, it is proposed that a mullite-mullite EBC be applied by composite sol gel. The addition of calcined powder should reduce the overall shrinkage of the sol, making deposition of thicker (1-5µm) coatings possible. Processing methods to produce mullite-mullite EBC by composite sol gel were studied. Sol gel (SG) mullite precursors were prepared by combining silica-precursor sol with AlOOH sol. Commercially available calcined mullite (CM) was dispersed in the sol, with a solid loading of 36%. Gelation of these sols was studied and it was found that using AlOOH sols at a pH of 2.5 gave the best balance of gelation rate and CM sedimentation rate. Mullite sols were prepared and fired at different temperatures and times. It was determined that mullitizatoin of SG samples required 20min at 1300°C, however, CM addition allowed a similar extent of mullitization at 1250°C. It is hypothesized that the CM acted as seeds, shortening the nucleation step during mullite formation. Calcined samples had a density of 3.01g/cm³, with a total porosity of 9.4%. Glow plugs were dip coated. Final dipping parameters were sol viscosity of 40cps and withdrawal speed of 0.1mm/s. Calcined coatings with CM were relatively crack free with thicknesses of ~3μm. A coated glow plug was tested on an electric rig for effectiveness against oxidation. After 100hrs at 1300°C, the corrosion products layer on the coated glow plug was about one fourth as compared to the as received glow plugs. In conclusion, the composite sol-gel mullite EBC appears to provide significant oxidation protection to the silicon nitride glow plugs.