Performance of CVD Mullite Coatings on Silicon Nitride under High Temperature High Load Conditions

Abstract

Previous studies have demonstrated that dense coatings of CVD mullite (3Al2O3×2SiO2) provide excellent oxidation protection for Si3N4 and SiC in a high pressure, steam environment. In this study the mechanical properties of CVD mullite coated silicon nitride materials from different vendors (AS800, NGKSN88, Kyocera SN281) were evaluated following ASTM test procedures. The dynamic fatigue tests werep erformed in ambient air at temperatures of 850 and 1200°C under fast (30 MPa/s) and slow (0.003 MPa/s) load rates. The static fatigue tests were carried out at a constant load of 350 MPa for 1000h at 1200°C. The cyclic fatiguetests at 850°C consisted of a loading ramp from 20 to 400 MPa in 30 seconds followed by unloading ramp from 400 to 20 MPa. A total of 10,000 cycles were applied to the fatigue test specimens before fast fracture tests were conducted at room temperature. The strength test results indicated that CVD mullite coatings showed excellent adhesion during dynamic fatigue tests and exhibited no creep behavior. Minor flexure strength reduction observed at low stressing rate and at high temperatures appeared to be related to Si3N4 properties such as SCG (slow crack growth) susceptibility. During cyclic and static fatigue tests, a glassy silica/aluminosilicate phase was formed due to oxidation. This resulted in localized coating separation and buckling. However, accumulation of this corrosion layer was not critical since the coated specimens showed a flexure strength increase of ~7-9.5%.