Failure Mechanisms of Mg-Al Spinel Abradable Coatings Under Thermal Gradient Cycling Conditions

Abradable coatings are used to minimize the clearance distance between rotating and stationary gas turbine components with the aim of increased turbine performance and efficiency. Such coatings are applied by plasma spraying technology, which allows the deposition of coatings of approx. 1 mm with defined coating microstructure and properties.

In the present work ceramic coatings of non-stoichiometric magnesia alumina spinel are under investigation. Especially thermal gradient cycling tests were performed to evaluate the coating lifetime and failure mode. Additional testing with respect to e.g. sintering, hardness, and special coating features like unmolten particle amount were carried out. The main focus was to explain and understand the typical failure mode seen for such type of abradables during cycling, i.e. formation of a dense top layer and exfoliation from the coating surface. An empirical model is given which describes the failure mechanisms.