A Modified Indentation Method And Characterization Of Thermal Barrier Coatings Under High-temperature By Vickers Indentation

Thermal barrier coatings（TBCs） have played a significant role in aerospace, thermal power generation and many other fields for their excellent performance in heat insulation, wear resistance and corrosion resistance. With a special multilayers structure and servicing a complex high-temperature environment, the practicability of thermal barrier coatings is restricted by the crack and spallation of coating seriously. As the critical parameters to characterize the properties of coatings, it is necessary to measure the fracture toughness and residual stress of coatings accurately and efficiently, which is of great significance to detect the quality, assess the performances and predict the service life of coatings. Therefore, a modified indentation method for fracture toughness and residual stress determination of air plasma-sprayed thermal barrier coatings was proposed in this article. A high-temperature Vickers indentation test system was designed and constructed to measure the properties of thermal barrier coatings under high temperature environment. The main research contents are summarized as follows:Firstly, a modified indentation method for fracture toughness and residual stress determination of thermal barrier coatings was proposed. For indentation test at the coating surface, considering the residual stress with gradient distribution characteristic, the ceramic is divided into many strips from the coating surface to the maximum depth of indentation crack along the direction of indentation load and the residual stress in each strip is uniform distribution but different from each other. Then, the results of fracture toughness and residual stress considering the coating with multi-layer structure are obtained. For indentation test at the coating interface, the volume ratio of indenter occupied in the coating and substrate regions during an indentation process is used to analyze this problem. The method that takes the logarithm of indentation load and the crack length was utilized to exclude singular points during the analysis of experimental data because the ceramic materials was a kind of anisotropic and uneven material. The results of fracture toughness and residual stress of thermal barrier coatings measured by the modified method are expected to be much more accurate.Secondly, the fracture toughness and residual stress of thermal barrier coatings were measured by Vickers indentation test. The results were compared with that measured by SENB and Raman spectroscopy, respectively. The relationship of fracture toughness and residual stress versus thermal cycles were obtained. The results show that the fracture toughness of air plasma-sprayed thermal barrier coatings ranges from 1.32±0.2 MPa·m^-1/2 to 4.08±0.3 MPa·m^-1/2. The residual stress at the coatings surface changes from-25.9±5 MPa to-190.8±10 MPa, The residual stress in the interior of ceramic coating varies from-53.29±10 MPa to-253.4±10 MPa. The fracture toughness at the interface ranges from 0.23±0.05 MPa·m^-1/2 to 0.65±0.05 MPa·m^-1/2 and the corresponding residual stress varies from-11.32±1 MPa to-42.17±2 MPa.Thirdly, the high-temperature Vickers indentation test system was designed and constructed to conduct the indentation tests under high temperature. A Vickers hardness tester was transformed into a high-temperature Vickers indentation test system by coupling with heating and cooling system, exhaust system and computer. The hardness of thermal barrier coatings with the temperature ranging from 700 to 1000 °C was obtained. The results show that the hardness of ceramic coating ranges from 4.85±0.4 GPa to 2.19±0.2 GPa and the hardness of bond coat varies from 2.25±0.2 GPa to 1.32±0.2 GPa.