Evolution of surface deposits on a high-pressure turbine blade - Part I: Physical characteristics

Turbine blade coupons with three different surface treatments were exposed to deposition conditions in an accelerated deposition facility. The facility simulates the flow conditions at the inlet to a first stage high-pressure turbine (T=1150 degrees C, M=0.31). The combustor exit flow is seeded with dust particulate that would typically be ingested by a large utility power plant. The three coupon surface treatments included: (1) bare polished metal; (2) polished thermal barrier coating with bondcoat; and (3) unpolished oxidation resistant bondcoat. Each coupon was subjected to four successive 2 h deposition tests. The particulate loading was scaled to simulate 0.02 parts per million weight (ppmw) of particulate over 3 months of continuous gas turbine operation for each 2 h laboratory simulation (for a cumulative 1 year of operation). Three-dimensional maps of the deposit-roughened surfaces were created between each test, representing a total of four measurements evenly spaced through the lifecycle of a turbine blade surface. From these measurements the surface topology and roughness statistics were determined. Despite the different surface treatments, all three surfaces exhibited similar nonmonotonic changes in roughness with repeated exposure. In each case, an initial buildup of isolated roughness peaks was followed by a period when valleys between peaks were filled with subsequent deposition. This trend is well documented using the average forward facing roughness angle in combination with the average roughness height as characteristic roughness metrics. Deposition-related mechanisms leading to spallation of the thermal barrier coated coupons are identified and documented.