Spallation of thin films driven by pockets of energy concentration

Abstract

A hypothesis is made that delamination can be driven by pockets of energy concentration (PECs) in the form of pockets of tensile stress and shear stress on and around the interface between a thin film and a thick substrate, where PECs can be caused by thermal, chemical or other processes. Based on this hypothesis, three analytical mechanical models are developed to predict several aspects of the spallation failure of elastic brittle thin films including nucleation, stable and unstable growth, size of spallation and final kinking off. Both straightedged and circular-edged spallations are considered. The three mechanical models are established using partition theories for mixed-mode fracture based on classical plate theory, first-order shear-deformable plate theory and full 2D elasticity. Experimental results show that all three of the models predict the initiation of unstable growth and the size of spallation very well; however, only the 2D elasticity-based model predicts final kinking off well. The energy for the nucleation and stable growth of a separation bubble comes solely from the PEC energy on and around the interface, which is 'consumed' by the bubble as it nucleates and grows. Unstable growth, however, is driven both by PEC energy and by buckling of the separation bubble. Final kinking off is controlled by the fracture toughness of the interface and the film and the maximum energy stored in the separation bubble. This work will be particularly useful for the study of spallation failure in thermal barrier coating material systems.

Determination of residual stress and interface adhesion toughness of thin films by blisters

“…Experimental observations, however, show no such preexisting delamination. Wang et al have recently proposed that spontaneous formation of blisters is driven by pockets of energy concentration (PECs) . Predictions of blister morphology from these PEC‐driven models are in excellent agreement with independent experimental results.…”

“…Wang et al have recently proposed that spontaneous formation of blisters is driven by pockets of energy concentration (PECs) . Predictions of blister morphology from these PEC‐driven models are in excellent agreement with independent experimental results. In this work, the measured morphology parameters are used to determine the compressive residual stress in thin films and the adhesion toughness at the interface.…”

“…Yuan et al have developed a completely analytical model to determine the four morphology parameters based on the PECs theory . The theoretical predictions are again in excellent agreement with extensive independent experimental data.…”

“…The results are recorded in Table , in which each group of results is for a different film thickness. It is worth noting that TCBs can form at different values of Ω > 1.5, which depends on the film thickness h , the residual stress σ 0 , and the adhesion toughness G c for a given material . It is observed that all Ω values in Table are larger than 1.5, which satisfies the formation condition of TCBs, that is, Ω > 1.5 .…”

“…According to the PECs theory in Wang et al and Harvey et al the spontaneous formation of a blister is determined by the parameter Ω, which represents the ratio between the plane‐strain residual strain energy density in the thin film and the adhesion toughness G c at the interface, that is, where σ 0 is the biaxial compressive residual stress. The mechanical condition for the spallation of a circular blister is Ω > 3/2 . It is worth noting that the value of Ω depends on the film thickness h , the residual stress σ 0 , and the adhesion toughness G c for a given material.…”