Material needs for turbine sealing at high temperature

There is considerable interest in reducing the volume of gas escaping around the blade tips in a gas turbine by fixing an abrasive, such as c-BN, onto the blade tip with an alloy, such as MCrAlY. Abrasion testing at 1100 degrees C showed that the MCrAlY deformed and the abrasive particles oxidised. The shear stress in the MCrAlY was estimated by finite element methods to be in the range of 5-30 MPa, while measurements of the frictional heating suggest the local temperature is between 1100 and 1200 degrees C. Creep testing showed that conventional MCrAlYs were too weak under these conditions. However, higher strengths were obtained using a continuous reinforcing phase together with NiAl. An Al2O3/ZrO2 abrasive was investigated as a more oxidation resistant abrasive. In this case, reaction with the MCrAlY caused Ni to diffuse into the abrasive forming layers of NiO, weakening the particles and causing premature failure.