Hot Corrosion of SiC/BN/SiC Composites

SiC fiber-reinforced SiC matrix composites with a BN interphase (SiC/BN/SiC) are proposed for hot-section turbine engine applications.  NaCl ingested into the engine in marine environments can react with sulfur impurities present in fuel forming molten sodium sulfate deposits resulting in hot corrosion.  Hot corrosion of bulk SiC has been investigated, however, the effects of BN on SiC corrosion, as well as an understanding of hot corrosion resistance of SiC fibers and SiC matrix materials produced by different processing routes has not been elucidated.

 In this study, Na₂SO₄-induced hot corrosion of model materials (i.e. bulk CVD or Hexoloy SiC with thin BN surface films) and SiC/BN/SiC composites was characterized in furnace studies at temperatures between 900 and 1150°C, for times up to 96 hours, and in O₂ with 2.5 or 1000 ppm SO₂.  Na₂SO₄ was applied to the surface of coupons prior to furnace exposure.  After exposure, the amount of corrosion was determined from weight change measurements.  Corrosion products were characterized by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, and dissolution followed by Inductively Coupled Plasma Optical Emission Spectroscopy.   Thin film BN appeared to increase the hot corrosion attack of the model materials.  The composites exhibited more severe hot corrosion than the model materials, especially in matrix areas with initial porosity.  Sylramic fibers underwent more attack than Hi-Nicalon-S fibers.

 Ceramic matrix composites were also exposed in a Mach 0.3 Burner Rig at 950°C for times of 24 and 45 hours.  Simulated seawater (NaCl in deionized H₂O) was injected into the burner rig, reacted with sulfur in the fuel to form Na₂SO₄ (8ppm) to induce hot corrosion.  Severe hot corrosion was observed under these conditions.  Microstructural and compositional characterization of the corrosion products is underway.

 Finally, solubility of relevant oxides in sodium sulfate will be determined as a function of Na₂O activity at 900°C to elucidate the SO₂ and SO₃ partial pressure dependence of hot corrosion of silica- and boria- forming materials.