Gas turbine engine development continues to accelerate, creating more demanding requirements for abradable seal coatings. These coatings are necessary to provide very small clearances between the rotating and stationary parts in order to minimize gap losses and so Increase efficiency. The relatively few abradable coating materials developed over the last 20 years still perform well in many blade tip seal and labyrinth seal applications. However, rising operating temperatures, corrosion and other environmental changes, longer overhaul times and even better tip clearances are dictating the design of new coating materials which requires a strong scientific approach. For example, ways are being Investigated to replace Nickel-Graphite and other flame sprayed coatings being used between 450 and 700°C respectively because of steady state/corrosion/oxidation/erosion and wear problems respectively.
New plasma and HVOF sprayed coatings have been developed using a systematic approach based on material response to operating conditions, minimizing trial and error. The major steps in the programme were:
1. Selection of constituent materials able to withstand service temperatures up to 325 (AISI-Polyester or Polyimide), 450 (AISI base), 700 (MCrAlY base) and 1100°C (ceramic base) respectively.
2. Powder particle manufacture and coating deposition to guarantee highly reproducible coatings.
3. Coating optimization based on wear tests carried out using a fully instrumented abradability test rig and wear mechanism analysis.
4. An investigation of blade tipping systems for high temperature applications.
This paper discusses the results of plasma sprayed coatings developed for use at 450 and 700°C.
