Improved Blade Tips for Abradable Coatings used in Gas Turbines

Gas turbine efficiency is of paramount importance in the modern carbon conscious global economy and the industry is always looking for ways to improve the efficiency of gas turbine engines.  Gas bypass between the rotating turbine blade tip and the engine casing affects both the efficiency and the power output of an engine.  An increase in this clearance of 125µm can result in an increase of 0.5% in the specific fuel consumption.  Abradable coatings have been designed to allow the turbine blade abrasive tip to cut a path into the shroud abradable coating to improve the seal between the blade tip and the casing.

A key impediment to improving the abradable coatings has been limited life of CBN blade tips.  Current blade tips lack the ability to cut abradable coatings for longer periods as the matrix oxidizes at high temperature losing its ability to hold as well as protect the CBN particles.  Improvement in blade tips – both in the cutting particles as well as the matrix which hold these particles – will therefore improve the performance of the abradable systems, as well as allow the use of denser, more erosion resistant abradable materials.

The current paper represents efforts to improve the oxidation resistance of the matrix materials which hold the CBN particles.  The current matrix is a low-aluminum containing NiCrAlY which is further aluminized to improve the oxidation resistance.  New coatings that have been tested are enhanced aluminide coatings, platinum aluminized coatings and platinum aluminized chromized coatings.  The results will be discussed in terms of matrix composition and microstructure as deposited and after static oxidation.  The effect of matrix and its impact on the blade tip performance will also be reviewed