of elements between coating and substrate particularly an inward diffusion of iron this is likely to produce a small change in the hardness of near surface regions sufficient to produce the degree of change noticeable in fig4 and 5 however the degree of change is not likely to be such as to cause problems of embrittlement cylindrical test pieces of imi829 alloy were coated using the hot rod target sputtering process mention previously to a depth of approximately 5 pm using targets consisting of a commercial alloy having the composition by weight of 15 5cr 4 8 al 0 3y 0 3si 0 03c balance fe no significant variation from this was identified when the composition of the coating deposited was checked by electron microprobe analysis specimens thus coated have been subjected to cyclic oxidation testing of the type described previously some at 650 u00b0 c furnace temperature others at 700 u00b0 c furnace temperature for some hundreds of hours at completion of the tests there was no coating damage by separation spallation or cracking evident by optical examination at sectioned specimens sectioned specimens were also subjected to microhardness measurement on a traverse through the surface layers the microhardness traverse for a specimen subjected to 400 hours oxidation testing.