Effect of porosity and mullite on the thermal fracture of plasma-sprayed ceramic thermal barrier coatings
Abstract
Thermal barrier coatings (TBCs) are used in many applications in order to protect metallic components from high temperatures. When subjected to a high heat flux, these coatings experience fracture due to the thermally activated time-dependent behavior of the coating. This objective of this research was to study the effect of porosity and mullite on the thermal shock performance of plasma-sprayed TBCs made of yttria-stabilized zirconia (YSZ). Mean field micromechanics models were used to estimate the effects of the variation of porosity and mullite volume fraction of the coating on its thermomechanical properties. An increase in porosity was found to lead to a decrease in elastic modulus and thermal conductivity of the coating and an increase in the time-dependent behavior; the increase in mullite volume fraction had the opposite effects on these thermomechanical properties. Computational simulations showed that an increase in both porosity and mullite content led to lower stresses in the coating as well as lower driving forces for crack growth. Laser thermal shock experiments were performed on five coating configurations. Three of the coatings contained no mullite and had nominal porosities of 14%, 21% and 25%. The other two coatings contained 17% mullite with 16% porosity and 22% mullite with 19% porosity. Among the porous YSZ coatings, the coatings with 14% porosity developed the longest surface and horizontal cracks during testing while there was no statistical difference in the crack lengths between the 21% and 25% porosity coatings. The two YSZ-mullite coatings developed surface cracks of similar lengths while the 22% mullite coatings developed shorter horizontal cracks. Furthermore, the YSZ-mullite coatings with 17% mullite and 16% porosity developed shorter cracks than the 14% porosity YSZ coatings while the YSZ-mullite coatings with 22% mullite and 19% porosity developed cracks of similar lengths to the 21% porosity YSZ coatings. The fracture toughness values of the coatings were estimated using a previously developed procedure. These estimated toughness values were used in conjunction with the results of the thermal shock tests as well as the results of the computational simulations in order to characterize the effects of porosity and mullite on the thermal fracture of the coatings. Finally, the potential for the development of porous YSZ-mullite coatings with improved resistance to thermal fracture was investigated.
Degree
Ph.D.
Advisors
Kokini, Purdue University.
Subject Area
Mechanical engineering
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