Effects of blade surface treatments in tip – shroud abradable contacts

In aero engines a thermally sprayed abradable lining is applied to the inside of the casing in order to provide a seal around the tips of the compressor blades. As engines become more efficient the materials used must be able to endure higher temperatures and moving parts must be made lighter. This combination is not currently possible in late stages of the compressor as hard abradable materials can cause titanium blades to wear excessively. One solution to this is to add a surface treatment to the blade tips.

Two surface treatments have been investigated, firstly cubic boron nitride (cBN) grits are bonded to the tip of the blade by electroplating. Secondly blades are coated in Cr(Al)N by cathodic arc deposition. The performance of these surface treatments is investigated on a test rig capable of monitoring the blade length, rub forces and abradable temperature during the test. Additional tests are performed against stepped coatings giving insight into the condition of the blade and abradable during the test. S.E.M., E.D.S., X.R.F., profilometry and wear debris have been used to characterise the wear mechanisms produced during the tests.

Grit (cBN) tipped blades load with abradable material after a period of efficient cutting causing grit pull out and fracture. Flat Cr(Al)N coated blades failed due to thermal damage to the coating caused by adhesion of abradable material onto the Cr(Al)N coating, while chamfered blades cut efficiently at low incursion rates.