improved adhesion of thermal barrier coating systems to nonmetallic substrates using a layer of silica on an underlying nonmetallic substrate that includes a silicon containing material is described . the improved adhesion occurs because the improved thermal barrier system improves the temperature capability of the system by providing a layer adjacent the substrate that has improved thermal performance and that is not subject to deterioration . the application of a silica layer of predetermined thickness adjacent the substrate also forms a diffusion barrier for oxygen while eliminating the need to transform an external layer of the substrate by a pre - oxidation step . this diffusion barrier prevents any oxidizable component of the substrate from decomposing . the thermal / environmental barrier coating system includes an intermediate layer containing ysz and bsas , mullite and / or alumina , which is preferably used in combination with a mullite layer that overlies the surface of the silica layer , a layer of bsas between the mullite layer and the intermediate layer , and a thermal barrier coating of ysz overlying the intermediate layer .