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The reliability of silicon nitride radial turbine rotors for automotive gas turbine was evaluated with hot-gas spin test and reliability analysis. The turbine rotors were made of a newly developed silicon nitride, having a 4 point bending strength higher than 700 MPa at 1400°C. The turbine rotors withstood at a turbine tip speed of 500m/s at TIT 1400°C, the same level as the rated condition of turbine rotors for the two-shaft type automotive gas turbines. The reliability of turbine rotors is discussed in relation to the hot-gas spin test results and failure probability analysis using CARES program developed by NASA as well as three-dimensional flow analysis and finite element analysis.

Evaluations were made of the short-term (5 minute) durability at high temperatures of 85 mm outer diameter ceramic radial turbine rotors made of silicon nitride using an improved hot gas spin test rig. Under no load conditions, the rotors withstood a maximum turbine tip speed (Ut) of 550 m/s without bursting at a turbine inlet gas temperature (TIT) of 1240°C. Under partially loaded conditions, they withstood up to Ut of 500 m/s at TIT of 1280°C without bursting. The test results indicated that the knee point temperature in terms of TIT at which the failure tip speed decreased sharply was about 40°C higher under partially loaded condition than under no load condition. The temperature and stress distribution in the turbine in the hot gas spin test were calculated by the finite element analysis, which supported the dependence of the knee point temperature in terms of TIT at spin test on load condition.

The high-temperature durability of 85 mm tip diameter silicon nitride ceramic radial turbine rotors was evaluated with a hot gas spin test rig. The rotors withstood up to a turbine tip speed of 700 m/s at TIT of 1300°C under partially loaded conditions and 570 m/s at TIT of 1300°C under fully loaded conditions, respectively. The material of the rotors was a post-HIPed silicon nitride. The basic fatigue properties of the material were measured at high temperatures. In the hot gas spin test, the temperature and stress distributions at the turbine blade were calculated with a finite element method. The results of the hot-gas spin test are discussed by means of a failure prediction analysis on the basis of the Weibull statistics.