Erosion Resistant Coatings in Turbine EnginesTony Brozan - Rolls-Royce Corp.
As our involvement in the Middle East has mushroomed in the past decade, many new challenges have been presented in the use of turbine engines in aggressive erosive environments. The use of unimproved runways and the dry and dusty environment in the Middle East has caused a vast increase in cost of repair and a reduction in the time between overhaul for turbine engines. These problems have led to a aggressive search to reduce cost and life of engines. One method that is currently in use and further development is the use of coatings on compressor section components to limit erosive damage from the particulate found in the area of operation.
The Mid-east wars have created very high wear rates for blades and vanes from sand and dust impingement, particularly in the compressor sections of aircraft engines. Helicopters are better designed to avoid such damage, but conventional aircraft landing on sandy or dusty airstrips suffer very heavy damage. Impingement on the pressure side of blades and vanes causes severely eroded edges and very significant reduction in cord length. Coatings capable of combating this problem were developed several decades ago, but now are receiving serious attention due to high cost of engine rebuilds. Additionally worn compressor parts lead to inefficiency in engine operation and much greater fuel consumption. On a GE T-64 engine use of erosion resistant coatings demonstrated a 3:1 increase in life between tear downs.
Tests have been run using alternating coated and uncoated blades which dramatically demonstrate the improvement of using coatings by much less loss of cord length.
Hard coating materials are excellent for reducing wear at low angles of impingement, but these materials are brittle and can actually cause increased wear at high angles. Variations on TiN (Titanium Nitride) are the most successful. One approach to solving this is to put down multi-layer coatings of very hard layers alternated with ductile layers. This tends to reduce the rate of erosion by brittle failure at high impingement angles by stopping crack propagation as it reaches ductile layers.
Recently the Navy has been concerned with corrosion resistance of the erosion resistant coatings so many tests in salt fog and salt fog with Sulfur dioxide have been run on TiN with various additives. The results are highly varied.
Finally there has been some tailored application of coating to induce compressive stresses and thereby resist crack propagation as well.