Mechanisms governing the high temperature erosion of thermal barrier coatings

The impact of thermal barrier coatings by hard projectiles at high temperature has been analyzed. Three different domains have been explored: each differentiated by particle size, velocity, temperature and TBC composition. Domain I applies when the projectile creates deeply penetrating plastic/densification zones. In this case, short time elastic effects are relatively unimportant. The response is dominated by stresses that arise after about 1 ms, at particle rebound. Deformation incompatibilities nucleate delaminations: which thereafter, extend in the TBC just above the interface with the TGO. An index governing material removal by delamination has been derived as Xi(1) = Gamma(TBC)E(TBC)(2) /(sigma(Y)(TBC))(3), where Gamma(TBC) is the toughness of the TBC, E-TBC its Young's modulus and sigma(Y)(TBC) its yield strength. In Domain II the plastic wave intensity is below the delarnination threshold: whereupon a thin densified zone is formed, without severe cracking. Subsequent impacts induce elastic bending, of the neighboring columns. The bending develops at short times (10-50 ns), causing large, transient stresses at the intersection between the dense and underlying columnar layers. These stresses can be large enough to form cracks that remove the dense layer. Analysis of this effect identifies an erosion index: Xi(2) equivalent to Gamma(TBC)/E(TBC)(3/5)dsigma(Y)(TBC), where d is the column diameter. Large values of Xi(2) reduce the erosion rate. Domain III arises for impact conditions that elicit an entirely elastic response in the TBC. The domain applies at low temperature and when the impacting particles are small. Again, bending effects at the tops of the columns arise at short times. Another erosion index arises, Xi(3) equivalent to Gamma(TBC)/E(TBC)(3/5)d, differing from that in Domain II because plasticity is not involved. (C) 2003 Elsevier B.V. All rights reserved.