Bonding strength and thermal shock resistance of glass coatings on anisotropic mullite fibrous ceramics

Abstract

Mullite fibrous ceramics with a two-dimensional planar random structure was prepared via a pressure infiltration method. The in-plane and out-of-plane mechanical properties, thermal expansion coefficients and thermal conductivities were compared. MoSi2-borosilicate glass coatings were applied on two adjacent surfaces of the as-prepared mullite fibrous ceramics via an in-situ reaction method. The bonding strength and thermal shock resistance of the two coatings were compared. Results show that higher crosslinking degree and more fiber orientating in the planes resulted in the fact that the in-plane compressive and tensile strength of the ceramics were 4.6 times and 2.1 times higher than the out-of-planes values, respectively. The in-plane CTE (Coefficient of Thermal Expansion), thermal conductivity and permeability were also higher than the out-of-plane ones. The interface bonding strength of Coating B (coated perpendicular to the planes) was higher than that of Coating A (coated parallel to the planes), because of the better mechanical locking at the interface and the higher tensile strength of the substrate. Besides, Coating B presented a better thermal shock resistance because the CTE of the substrate parallel to Coating B was closer to the CTE of the coating, and the substrate was more complaint parallel to Coating B so that could absorb some of the thermal stress by deformation.