Investigation of the effect of hafnium and chromium additions on the microstructures and short-term oxidation properties of DC magnetron sputtered β-nial bond coats deposited on Ni-based superalloys
Thermal barrier coatings play a major role in protecting turbine blades from extreme operating environments and extending service lifetimes. Reactive elements (e.g. Zr, Hf, Y, Si, etc.) have been shown to enhance the oxidation performance of such coating systems when added in appropriate amounts to overlay bond coats. This study investigated the influences of processing parameters along with Hf and Cr additions on the short-term oxidation performance of β-NiAl based coatings deposited via direct current magnetron sputtering onto CMSX-4 and René N5 substrates. Sputtering parameters were optimized to yield a zone T microstructure. The results indicate that the coatings are deposited as a solid solution and precipitation with the RE-doped coatings occur following annealing at 1000°C for times up to four hours. Precipitates form heterogeneously within the coatings with larger precipitates forming at grain boundaries and smaller ones forming within the grains at prior dislocation lines. Small incorporations of Cr into the NiAl-1Hf coating increased the average grain size and precipitate size. Transmission electron microscopy and atom probe tomography confirmed that the chemistry of the precipitates is mostly β'-Ni2AlHf accompanied by HfC and α-Cr. The results from the isothermal oxidation studies at 1050°C indicated that increasing the preoxidation annealing time from two to four hours decreased the mass gains observed with the specimens up to 100 hours. However, significant oxidation at the coating/substrate interface was discovered and the amount of oxidation increased with Hf content and preoxidation annealing time. This oxidation is thought to be caused by the large number of pinhole defects with the iii zone T microstructure and large grain boundary volume. Increased Hf concentrations were also found at the coating/substrate interface and this has been shown to lead to dramatic internal oxidation. The NiAlCrHf samples contain larger grain sizes and precipitates and a thinner TGO than the NiAl-Hf coatings. This combined with the lower mass gains during isothermal oxidation indicate that the NiAlCrHf coatings rapidly form a thin, protective α-Al2O3 layer that limits additional transport of oxygen to the bond coat. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl-Hf coatings.