In-situ high temperature investigation of the residual stress evolution of thin Cr2AlC PVD-coatings on IN718 and WC-Co
Cr2AlC as a MAX-Phase is a promising coating material for high temperature environments and wear applications. One of the most important influence for coating lifetime are residual stresses, which are usually complex and the result of the whole production chain and application. For high temperature applications the coefficient of thermal expansion (CET) of the coating and substrate determines the arising residual stresses during heating and cooling cycles. If coating has a higher CET tensile stresses occur during cooling, which may result in coating failure. However moderate compressive stresses improve coating resistance to mechanical
load. Without plastic deformation or cracking thermal induced residual stresses are reversible. The main parameters for high temperature applications are the deposition and treatment temperature, the deposition residual stresses as well as the stability of them.
In order to gain insight on residual stress evolution in thin Cr2AlC coatings during heating and cooling cycles, in-situ experiments at the synchrotron microfocus station at DESY in Hamburg were performed. The focus of the experiments was laid to the stability of deposition residual stresses in order to gain information whether those stresses may influence coating lifetime over a longer time period. Additionally the influence of deposition parameters, heat treatment temperature and substrate on arising stresses was investigated. Cr2AlC coatings – deposited with High Power Pulsed Magnetron Sputtering and parameter variation – on IN718 and WC-Co were heat treated between 100 °C and 700 °C resp. 900 °C for three cycles. Furthermore an elastic FEM approach was conducted in order to find critical failure relevant steps for deposition and heat treatment.
The results revealed deposition residual stress relaxation during first 900 °C heating cycle thus the maximum of thermal induced residual stresses was determined at the lowest temperature (100 °C). However, experiments at 700 °C showed residual stresses over the whole time at peak temperature, which indicates that relaxation processes like plastic deformation and diffusion are impeded. As expected, the two different substrates showed high impact on thermal induced residual stresses. The residual stresses in the Cr2AlC coating on WC-Co led to cracking during first cooling cycle.