A bimetallic bond layer (26, 28) for a thermal barrier coating or TBC (30) on a superalloy substrate (22) for a high temperature environment. An interlayer (26) is applied on the substrate. A bond coat (28) comprising a CoNiCrAlY or NiCoCrAlY alloy is applied on the interlayer. A ceramic TBC (30) such as 8YSZ is applied on the bond coat. The interlayer (26) is an alloy that is compatible with the substrate and the bond coat, and that blocks or delays diffusion of aluminum from the bond coat into the substrate at high operating temperatures. This preserves aluminum in the bond coat that maintains a beneficial alumina scale (29) between the bond coat and the TBC. This delays spalling of the TBC, and lengthens the coating and component life.