Development of Oxide‐Based EBC for Silicon Nitride

Abstract

In search of a candidate EBC material for silicon nitride ceramics, water vapor corrosion test was performed for a series of monolithic materials such as Al 2 O 3 , TiO 2 , ZrO 2 , HfO 2 , Al 2 TiO 5 , Al 6 Si 2 O 13 , Lu 4 Hf 3 O 12 , Ln 2 Si 2 O 7 (Ln = Yb and Er) and mullite/Lu 2 Si 2 O 7 eutectic bulk at above 1300°C. Based on the high‐temperature water vapor corrosion test results and the resultant microstructures, the mechanisms of water vapor corrosion of these materials were discussed in this paper. In this present study, ZrO 2 and HfO 2 showed excellent resistance to water vapor corrosion at high temperatures. Among low‐thermal‐expansion materials, specifically silicate‐based materials, Lu 2 Si 2 O 7 showed better results in the aspect of weight loss rate during the corrosion test, which also desirably has a coefficient of thermal expansion very close to that of silicon nitride. To verify the recession mechanism of Lu 2 Si 2 O 7 , a recession test was performed for this bulk in high‐velocity combustion gas flow at 1300‐1500°C. This material showed excellent recession resistance compared to Al 2 O 3 and Si 3 N 4 . Different coating techniques, such as sputtering, sol‐gel, and reaction sintering, were employed to coat a high‐density Lu 2 Si 2 O 7 EBC layer on a silicon nitride substrate. However, in polycrystalline Lu 2 Si 2 O 7 phase, the grain boundary silica was prone to water vapor corrosion. It was observed that the corrosion resistance of a Lu 2 Si 2 O 7 /mullite eutectic bulk, without boundary silica, was more excellent than that of polycrystalline Lu 2 Si 2 O 7 bulk.