Finite-element Analysis Of The Residual Stress In ZrO₂Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are widely used in aerospace, chemical industry, power generation and other fields for its good oxidation resistance and low heat conductivity. In the process of service the thermal barrier coatings provide effective thermal insulation and heat protection for the components, which can make the components work in an environment of high temperature. A typical thermal barrier coating has multi-layer structures, the substrate, intermediate layer (bond-coat), the thermally growth oxide and the ceramic top-coat. It is believed that the residual stress produced during the service process was the main reason for the failure of the thermal barrier coatings. In order to research the failure mechanism of thermal barrier coatings, it is very important to study the residual stress. For the TBCs fabricated by electron beam physical vapor deposition (EB-PVD), there are columnar crystals in the top coating, the gaps of the columnar crystals provide channels for the oxygen to spread into the TBCs, which leads to the failure of the thermal barrier coatings. So we should isolate the outside oxygen in order to improve the properties of thermal barrier coatings, the high-current pulsed electron beams (HCPEB) is an effective surface modification technology developed in recent years which can achieve the surface sealing so as to isolate the outside oxygen.In this paper, two kinds of finite-element models were built using ANSYS11.0which incorporated the thermal physical parameters of the materials and which allowed the simulation of the distributions of residual stresses during thermal cycling and during the modification of the ZrO2ceramic coating by high-current pulsed electron beam. The results can be useful to the practical applications of the HCPEB surface sealing treatment. The main research results are as follows:1、For the simulation of residual stress in the ZrO2/NiCoCrAlY thermal barrier coatings during the thermal cycle, a high residual stress exists at the interface between bond and ceramic top-coat layers. The residual stress in curved interface is larger than that in planar interface, and the stress concentrates mainly at the troughs. The residual stress in the TBCs fabricated by electron beam physical vapor deposition was smaller than that of plasma spraying fabrication, and the service life was much longer. The thermally grown oxide layer plays an important role in the distributions and numerical value of residual stress. The residual stress increases obviously with the presence of thermally grown oxide layer in TBCs.2、For the simulation of thermal stress in ZrO2ceramic coating irradiated by high-current pulsed electron beams, we have found that during the irradiation there was a large compressive stress at the surface of the sample which caused the deformation of the sample. The radius tensile thermal stress at the subsurface can lead to the formation of microcracks while the axial stress and shear stress were the main cause for the spallation of the surface layer. HCPEB of the energy density between5～8J/cm2can make an excellent sealing and improve the antioxidant properties of TBCs. The thermal stress generated by HCPEB of a low energy can lead to microcracks in the sample, which would not influence the sealing. Large cracks could destroy the sample along with the increase of energy density.