Reducing CTE Mismatch Between Coatings and Si-Based Ceramics

Two techniques have been proposed to reduce thermal-expansion mismatches between (a) substrates made of silicon, silicon-based ceramics, and silicon-based-ceramic composite materials and (b) surface coats that protect the substrates against chemical attack in oxidizing and/or corrosive environments. Typical substrate materials include SiC/ Si composites. A typical coating material is mullite (Al6Si2O13), which can protect silicon-based substrates against water-free oxidizing and corrosive environments. Mullite can also be applied as intermediate coating layers to relax stresses and enhance the adhesion of overlying protective layers of zirconia (ZrO2) or nonstoichiometric anorthite (stoichiometric composition CaAl2Si2O8). The coefficients of thermal expansion (CTEs) of mullite and of some other typical oxide coating materials are greater than the CTEs of silicon-based substrates and, as a result, the coatings tend to crack through their thicknesses. The cracks become pathways for the entry of the chemical species from which one seeks to protect the substrates.