Platinum-Aluminide Coatings

The improvement in environmental resistance that platiniun-aluminide coatings can impart to some of the nickel-base superalloys used by the gas turbine industry has resulted in much interest in combinations of these materials. Now a further paper by investigators at the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, and at Rolls-Royce, Bristol, England, reports on the effect of substrate composition on the oxidation behaviour of selected platinum-aluminised nickel-base superalloys, with particular emphasis on the thermal stability of the coating and on microstructural features of the surface scale (H. M. Tawancy, N. M. Abbas and T. N. Rhys-Jones, Surf. Coat. Technol., 1992, 54/55, (1–3), 1–7).

Platinum-aluminide coatings were applied to polycrystalline, directionally solidified and single-crystal superalloy rods before they were oxidised in still air at temperatures of 1000 and 1100°C. The microstructures were characterised by analytical electron microscopy, scanning electron microscopy and X-ray diffraction. Interdiffusion between the surface coatings and the substrates occurred at both temperatures. Variations in the protective nature of the coatings were believed to result from the outward diffusion of elements from the substrate. Refractory and reactive elements appeared to have the most significant effects. The beneficial and adverse effects of these elements are discussed. It is concluded that although the refractory elements tend to degrade the protective nature of the scale developed on the coating, the presence of the reactive elements can outweigh these adverse effects.