Tribology Testing Dovetails: Coatings & Lubricants

Fretting and wear have been a major failure mode of land-based turbines and aircraft engines. Tribology has been a major focus on improving the overall performance and life span for these applications.  The focuses for improvements are centered on the contact faces seen by turbine dovetails.  New coatings, lubricants, and surface processes are being investigated to be applied to these surfaces to deter wear and cracking during operation.  To test the value of adding these surface improvements, we needed to determine ways to simulate the motion of the dovetails through Tribology testing.   Dovetails experience high stresses and relatively small displacements through a wide range of elevated temperatures.   In the pursuit of locating a standard test rig to perform this testing, it was quickly determined that none could meet this criteria.

Innovative Test Solutions, Inc. (ITS) took on the challenge of developing a custom test rig that could accurately simulate stresses and displacements seen by turbine dovetails.  Numerous road blocks challenged the design for this custom rig.  First was to maintain adequate stiffness to achieve very small displacements without any compliance on the fixture side.  Second, we needed the ability to make fixturing adjustments while the test samples and fixturing were at elevated temperature. To accommodate customer requirements, the test rig also needed to be able to operate in two axes: one to simulate the displacement of the dovetail and one to simulate changing loads through the displacement cycle.  After several iterations of designs and testing, ITS has a fully computer controlled closed loop biaxial servo-hydraulic test rig.

Combined with the physical testing, ITS provides plenty of proof illustrating the work that was performed and the results of the testing.  While the test is running, the static and dynamic coefficients of friction (COF) as well as periodic wear rates can be calculated in real-time.  Pictures are taken with a high-powered digital microscope before and after testing to visually display the effects of the testing.  In addition to pictures, 3D surface topographical images are taken of the samples before and after testing to illustrate any material loss/gain during the test.

Improvements to the test rig and design are constantly being made as testing is ongoing. With seeing changing customer requirements, changes have been made so the rig can now perform fretting fatigue and high frequency fretting, making the rig more versatile.