Jackson, Ryan Daniel (2010)
Ph.D. thesis, University of Birmingham.
A series of isothermal and cyclic oxidation tests were carried out on two thermal barrier coating systems consisting of a CMSX-4 nickel-based superalloy substrate, NiCoCrAlY overlay bond coat applied by two different techniques and a yttria stabilised zirconia topcoat applied by electron beam physical vapour deposition (EBPVD). The bond coats were applied by either high velocity oxy-fuel spraying (HVOF) or EBPVD. Isothermal oxidation tests were carried out at 950°C, 1050°C and 1150°C for both coating system for up to 3000 hours. Cyclic oxidation testing was conducted at 1150°C in one hour cycles to coating failure on both coatings. A detailed examination on the oxide thickness was conducted on all specimens, along with characterisation of the bond coat and TGO. This was coupled with examination of specimen cross-sections for cracking and signs of coating degradation. Isothermal oxidation showed sub parabolic oxide growth consistent with the literature. Detailed analysis of oxide thickness showed a normal distribution but with increasing standard deviation with increasing oxidation time. Both bond coats were dual phase, β + γ. The EBPVD applied bond coat only, was found to contain yttrium rich precipitates in the bond coat and TGO. Both coatings showed no increase in surface roughness after either isothermal or thermal cycle testing. Short sub-critical cracks were observed at the TGO/topcoat interface in the HVOF applied bond coat only associated with the flanks of asperities. Coating failure in both coatings occurred at the TGO/bond coat interface on cooling and was likely driven by the thermal expansion mismatch between the TGO, topcoat and substrate. The initial mechanism of crack formation was not determined conclusively but could be a wedge cracking type mechanism.
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