Evaluation of abradable seal coating mechanical properties

Three proprietary plasma-sprayed coatings, based on Ni-graphite, Al-Si-graphite and Al-Si-polyester, were chosen for evaluation by the use of a (low speed) scratch tester, as a means of assessing the performance of abradable coatings. The scratch test behaviour was also correlated with the mechanical properties of the coatings (elastic modulus, microhardness and ultimate tensile strength). The results obtained were compared with those from industrial trials, to ascertain if the scratch test could be used as a relatively cheap and effective alternative to expensive engine trials. We have shown that the Progressive Abradability Hardness, abbreviated below as PAH, can be utilised as a measure of abradability in the scratch test, and can be related to the mechanical properties, in a manner consistent with engine test-bed findings. We have also found that the abradability and the PAH can change with scratch length due, we believe, to coating compression and densification ahead of the slider. In this work, the PAH has been related to coating hardness, ultimate tensile strength and elastic modulus.