Method of forming a coating resistant to deposits and coating formed thereby

A method of forming a thermal barrier coating system on a surface of a gas turbine engine component, the thermal barrier coating system comprising a thermal barrier coating that defines an external surface of the component that is resistant to infiltration by CMAS, the method comprising the steps of:

• placing the component in a coating chamber of an electron beam physical vapor deposition apparatus;
• evaporating at least a first source material with an electron beam to deposit a bond coat on the component;
• while the component remains in the coating chamber, evaporating a first ceramic source material with an electron beam to deposit an inner portion of the thermal barrier coating on the bond coat, the inner portion having a columnar grain structure and being formed of a first ceramic composition corresponding to the first ceramic source material;
• while the component remains in the coating chamber, evaporating the first ceramic source material and a second ceramic source material with an electron beam to deposit an outermost portion of the thermal barrier coating on the inner portion, the outermost portion having a columnar grain structure that is a continuation of the columnar grain structure of the inner portion so that the thermal barrier coating is continuous and the inner and outermost portions are not discrete layers, the outermost portion having a thickness of at least 10 micrometers and containing a mixture of at least two weight percent of the second ceramic composition and a balance essentially of the first ceramic composition, the second ceramic composition being reactive with molten CMAS to form a compound with a melting temperature higher than CMAS so as to inhibit infiltration of the outermost portion by molten CMAS.