Development of 1232°C (2250°F) erosion and impact tests for thermal barrier coatings

High temperature erosion and impact tests were developed for evaluation of thermal barrier coating (TBC) systems that are being generated for the High-Speed Civil Transport supersonic aircraft under NASA's Enabling Propulsion Materials program. The TBC-coated test specimen is a 6.1-mm (0.24-inch) diameter cylindrical pin. Twelve pins are mounted on a carousel with a pitch diameter of 46 mm (1.813 inch). This carousel is rotated at 500 rpm, 50 mm (two inches) from the combustor exhaust of a burner rig, while the TBC temperature is controlled at 1232 degrees C (2250 degrees F) using a pyrometer. The particulate is injected radially into the combustor in two horizontally opposed positions. Two separate types of particulate are used in the tests: 50-micron and 560-micron crushed alumina. The effects of the 50-micron erosion and 560-micron impact tests are quite different, but in each case excellent similarity is seen between damage to test specimens and that of commercial engine service hardware. The temperature of the impinging particulate is above 1450 degrees C (2642 degrees F), and the particulate velocities were determined to be 174 and 9 meter/second (570 and 30 feet/second), respectively, for the 50-micron and 560-micron materials. A mathematical expression was developed for erosion wear as a function of particle size, velocity, hardness and density. The ranges over which these parameters were varied are particle size 10 to 180 micron, velocity 107 to 332 meter per second, hardness 820 to 2100 kg/mm(2) Knoop, density 2.2 to 5.7 gram/cc. Erosion wear is linear with the accumulated mass of eroding particulate used. Impact wear caused by 560-micron alumina is distinctly nonlinear.