Thermal fatigue behaviour of Al2O3-DBC substrates under high temperature cyclic loading

Abstract

Purpose The purpose of this paper is to study the phenomenology of Al2O3-DBC substrate thermal-cracking under different high temperature cyclic loadings. The extremely low cycle fatigue (ELCF) life prediction model for ductile materials was used to describe the thermal fatigue life of Al2O3-DBC substrates. Design/methodology/approach Four groups of thermal cycling tests using Al2O3-DBC substrates with 0.65 mm thick copper were conducted using different peak temperatures. The failure samples were observed by optical microscope. The thermal plastic strain distribution in the Al2O3-DBC substrates was analyzed using a finite element method with the Chaboche model for describing plastic deformation of copper. The ELCF life prediction model was used to predict the life of Al2O3-DBC substrates under high temperature cyclic loadings. Findings Interface cracking was observed to initiate at the short edge of the bonded copper and deviated into the ceramic layer when the crack grew beyond the critical length of 0.1-0.8 mm. The interface crack deviated into the ceramic layer at different thickness and grew parallel to the interface layer between the ceramic layer and copper layer. The crack propagation stopped after certain cycles. The copper layer with 10-20 μm thick alumina inside was not split away totally from the ceramic layer. The ELCF life prediction model could predict the life of Al2O3-DBC substrates well under high temperature cyclic loading. The material constants in the extremely low fatigue life prediction model were obtained using thermal fatigue tests results. Research limitations/implications The influence of copper layer thickness and ceramic layer thickness on thermal cracking characteristics of DBC substrate should be studied in the future. Failure models should also be further investigated. Originality/value The failure model of Al2O3-DBC substrates under high temperature cyclic loading was studied. A method for predicting the life of the substrate samples under high temperature cyclic loading was proposed.