Thermal fracture of ceramic coatings under high heat flux with time-dependent behavior
Abstract
The objective of this research was to study the effect of time-dependent behavior on the thermal fracture of ceramic coatings under high heat flux laser heating. Continuously plasma sprayed zirconia coatings with thicknesses varying from 0.26 to 1.5 mm were studied. A CO2 laser was used to heat the coatings to maximum surface temperatures varying from 800 to 1700°C. Temperature differences from 700 to 1300°C across the ceramic coating were applied. An analytical model based on fracture mechanics was developed that enables the prediction of surface and interface crack initiation and propagation in ceramic coating systems subjected to high heat flux heating. High heat flux heating was found to cause changes in the material leading to a denser microstructure and relaxation of compressive stresses. These changes lead to the development of surface and interface cracks during ambient air cooling following laser heating. Under high heat flux heating followed by ambient air cooling, increasing the coating thickness was found to decrease the number of surface cracks developed and increase the crack spacing. Surface cracks were found to extend further into thin coatings than thick coatings. Increasing the maximum surface temperature was found to increase the surface crack length. Closely spaced multiple surface cracks were found to inhibit interface crack development between the ceramic and bond coat. In contrast, long widely spaced surface cracks were found to increase the possibility of interface crack development. The thinnest and thickest coatings did not readily develop interface cracks while intermediate thicknesses did. Under high heat flux thermal loads, increasing the ceramic coating elastic modulus and the thermal expansion coefficient was found to increase the probability of initiating surface and interface cracks, as well as increase the final surface and interface crack lengths.
Degree
Ph.D.
Advisors
Kokini, Purdue University.
Subject Area
Mechanical engineering
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