Microstructural development in aluminide diffusion coatings on nickel-base superalloy single crystals

An investigation has been made of the microstructural stability of aluminide diffusion coatings during post-coating thermal exposure. This study has employed edge-on transmission electron microscopy to examine high-activity pack aluminised single crystals of a gamma prime strengthened nickel-base superalloy. The influence of exposure temperature, duration and atmosphere as well as the initial coating thickness has been assessed. Two major processes have been found to contribute to microstructural changes in the coating. These are, firstly, the transformation of the coating matrix (β-phase, nominally NiAl) to other Ni-Al based phases, especially γ' (nominally Ni3(Al, Ti)) and, secondly, the precipitation of chromium containing phases. The work has enabled the roles of three processes contributing to γ formation, namely: oxidation of the coating surface, interdiffusion with the substrate and ageing of the coating, to be understood. In addition, the factors leading to the formation of a sequence of chromium-containing phases have been identified.

Nanoscratch and nanowear testing of TiN coatings on M42 steel

Constant load, progressive load and multipass nanoscratch (nanowear) tests were carried out on 500 and 1500 nm TiN coatings on M42 steel chosen as model systems. The influences of film thickness, coating roughness, scratch direction relative to the grinding grooves on the critical load in the progressive load test and number of cycles to failure in the wear test have been determined. Progress towards the development of a suitable methodology for determining the scratch hardness from nanoscratch tests is discussed. © 2011 W. S. Maney & Son Ltd.