Method of depositing a compositionally-graded coating system

A method of forming a thermal/environmental barrier coating system on a substrate formed of a silicon-containing material, the method comprising the steps of:

• depositing a silicon-containing bond coat on the substrate;
• depositing a mullite-containing first layer on the bond coat;
• depositing a second layer on the first layer, the second layer consisting essentially of BSAS;
• depositing a third layer on the second layer, the third layer consisting essentially of yttria-stabilized zirconia and either mullite or BSAS;
• and
• depositing a dense, strain-tolerant, vertically-cracked topcoat of yttria-stabilized zirconia on the third layer using a plasma spraying technique while maintaining the substrate at a temperature of not greater than 600° C. if the third layer consists essentially of YSZ and mullite, and a temperature of not greater than 500° C. if the third layer consists essentially of YSZ and BSAS.