A CoNiCrAlY-hBN/Polyester material has recently been developed for clearance control applications in gas turbine compressors that use titanium alloy blades. While engine tests serve as the final evaluation of the coating performance, quality assurance laboratories and production shops would rely upon the more readily available coating hardness values to predict performance. This paper will focus on the reproducibility of coating macrohardness with a plasma spray process. It is shown that plasma spray parameters affect the hardness of CoNiCrAlY-hBN/Polyester coatings by changing the level of polyesters retained in the coating and the volume percentage of metallic matrix. The correlation between hardness, retained polyester level and microstructure of these coatings is captured in a coating hardness map from which desired microstructure and polyester entrapment are determined. Based on the understanding of the correlation between coating hardness and microstructural features, the use of additional criteria other than hardness such as retained polyester level and non-metallic portion of the coating is recommended in order to assure the quality of the coating more effectively.

Very many gap-sealing products are now available for use in the compressor section of gas turbines. This paper attempts to give an overview of these and where they are best used. Data is presented for abradability, erosion resistance and application technique. By explaining how abradables function tribologically it is hoped that selection will be simpler for designers. New products are introduced as well as the way forward described. It is shown how with time the diversity of application techniques has decreased with thermal spraying becoming the preferred technique, and that abradables are now available to run against titanium blading up to 600°C.