Producing surface coatings on silicon nitride by pack cementation

Pack cementation processing was used to chemically alter the surface of dense silicon nitride materials and form a coating layer. Test samples were examined by XRD, SEM, and optical microscopy to characterize the chemical and microstructural changes in the materials after processing. The coating that was formed depended on the composition of the reactive powder pack used in the process and on the processing conditions of temperature and furnace atmosphere. Some of the coating compounds identified by XRD, such as Yb ₂Si ₂O ₇, SrAl ₂Si ₂O ₈, and Yb ₃Al ₅O ₁₂, are promising candidate materials for protective environmental barrier coatings. Preliminary exposure testing results in water vapor at high temperature indicate that a Yb ₂Si ₂O ₇ coating on NT154 Si ₃N ₄ reduces the rate of attack compared to the uncoated material.