Development of a Strain Tolerant Ceramic Coating for the Protection of Rotating Blade Tips

Control of the clearance between the tip of rotating blades and the static components is important for the efficient operation of a gas turbine. Decreasing the gap to minimise gas leakage leads to higher efficiency and greater power, but also increases the risk of contact between the two components. Where this occurs, frictional forces cause localised heating which often results in damage to the components. In extreme cases this has resulted in fire and engine damage, particularly where titanium based components were used. The use of sealing systems, where the blade deliberately cuts a path into a softer or porous surrounding material, rely on a good cutting action in order to maintain efficiency and minimise component wear and damage. It is possible to protect blade tips by applying a wear resistant or abrasive coating which can minimise metal to metal contact and provide an enhanced cutting action. Design considerations have lead to the development of a strain tolerant ceramic coating (STCC) which consists of a two layered, thermally sprayed yttria stabilised zirconia coating, applied over a bond coat. The bond coat not only providing good adhesion for the ceramic top layer but by using an HVOF applied MCrAlY, also provides oxidation protection at higher temperatures. This system is suitable for use on both compressor and turbine blades. It is designed to survive between overhauls when it can be easily removed and replaced. The coating was rigorously tested including severe thermal shock loading and long term oxidation. An examination of the microstructure showed that as-sprayed, the ceramic top coat had a dense structure. After testing it was found that the coating had survived by microcracking to relieve the applied stresses. Results from engine operation confirm that this coating system provides the designed protection for rotating blade tips and enables them to operate effectively against a MCrAlY seal.