Protective high temperature coatings with intrinsic depletion sensor

Abstract

A novel approach for non-destructive residual life-time estimation of coatings for high temperature applications is presented. Coatings of ferromagnetic Cr-doped AlN are applied on a paramagnetic metallic substrate (Ni-base alloy) using a diffusion treatment. The Cr-content in AlN achieved by this procedure is about 2at.%. Applied on a metallic plant component (turbine blade, chemical reactor, boiler tubes etc.) these coatings serve simultaneously two functions. On the one hand they deliver protection against high temperature corrosion because they act as a reservoir phase for alumina and chromia scale formation, while on the other hand due to their magnetic properties they can be used as a depletion sensor for the reservoir of the protective elements. Knowledge about the oxidation kinetics combined with monitoring of the coating's magnetic moment before and after oxidation can allow prediction of its residual life-time. Magnetic investigations of the coating indicate soft ferromagnetic behaviour. The ferromagnetic domains are visualised with the help of magnetic force microscopy (MFM). During oxidation of the coating a change in the microstructure of the alloy subsurface zone takes place. This is characterised by a gradual decrease of the amount of the ferromagnetic Al(Cr)N-phase and by the formation of new ferromagnetic precipitates of Ni and Fe throughout the diffusion zone. Their influence on the net magnetic moment of the coating–alloy system is discussed.