Investigation of abradable seal coating performance using scratch testing

Since the real conditions at the blade tip and the casing in a gas-turbine engine present a complex (very high speed) deformation situation that is difficult and expensive to replicate, we have evaluated the use of a (low speed) standard scratch tester, as a means of assessing the performance of abradable coatings. Three proprietary plasma-sprayed coatings, Ni-graphite, Al–Si-graphite and Al–Si-polyester, were chosen for the tests. The scratch test behaviour was correlated with the mechanical properties of each coating (elastic modulus, microhardness and UTS (ultimate tensile strength)). Our results 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” (also called “specific grooving energy”), 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 “PAH" can change with scratch length due to coating densification ahead of the slider, which is not easily revealed by other tests (such as the pendulum method). We therefore believe that scratch testing is a useful means of evaluating the likely in-service performance of abradable coatings, prior to carrying out engine trials.