Ceramic Turbine Tip Rub Damage Assessment

In achieving the dual-use turbo-shaft engine performance improvements specified in the HPTET initiative the use of advanced ceramic materials in the hot gas flowpath play a crucial role. Increased turbine inlet temperatures, and the attendant increase in un-cooled turbine blade material temperatures, enable the dramatic increases in specific power and thermal efficiency. In addition to increased turbine inlet temperature, the designer must provide minimal (near zero) turbine blade tip clearances to minimize tip leakage and the resulting losses in stage efficiency. Many variables control the hot operating tip clearances within a turbine engine and they difficult to analyze with consistent precision without model calibration from empirical engine data. Often the best data is not sufficiently accurate to prevent a "rub" from occurring between the turbine blade and the stationary shroud. In the case of a ceramic turbine blade severe damage may occur when a "rub" occurs but no data exists in the public domain to verify this conclusion. This program proposes to empirically determine if damage occurs to a candidate monolithic ceramic turbine blade material when subjected to "rub" incursions with a candidate high temperature abradable shroud coating material. The program consists of eight test with four variables organized in a Taguchi L(8) orthogonal array. The tests will be performed to conditions recommended by both Allied Signal Engines and Allison Gas Turbine Division and based upon analytically predicted conditions within their ATTAP engine test beds. Testing will occur at the Solar Turbines, Inc. test facilities on an existing hot turbine rub rig. Ceramic test specimens, designed with similarity to MIL-STD 1942-(B) flexure bars, will be flexure tested at Allied Signal Engines to determine retained strength at room temperature following exposure to the recommended "rub" testing conditions.