Freestanding abradable coating manufacture and tensile test development

Abradable coatings are used extensively within gas turbines. Abradable material is applied to the inside surface of the compressor and turbine shroud sections using thermal spray methods, coating the periphery of the blade rotation path. The function of an abradable seal is to wear preferentially when rotating blades come into contact with it, while minimising the over-tip clearance, and improving the overall efficiency of the engine. There is a distinct lack of established materials property data for all abradable materials, due to the difficulty of testing this very unique class of materials. Abradables understanding is historically limited, with the field often described as a ‘black art’, and component/material improvements habitually the result of ‘firefighting’ actions. This work is part of a wider programme in partnership with Rolls-Royce plc to gain a greater understanding of abradable materials, how they perform, and ultimately how to improve their performance in-service. The paper describes a novel method, devised in tandem with Rolls-Royce plc, of producing free-standing abradable tensile test specimens via thermal spray. The specimen mould is composed of a dissolvable polymer composite which maintains its integrity during spraying and cooling, and is then ‘washed’ away in an ultrasonic water bath. This results in near-net shape specimens, which are machined to a specific geometry. The paper details the iterative testing development that contributed to a final working design and testing methodology for a previously untested class of materials.