Measuring critical stress for shear failure of interfacial regions in coating/interlayer/substrate systems through a micro-pillar testing protocol (original) (raw)
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Materials
The ceramic-metal interface is present in various material structures and devices that are vulnerable to failures, like cracking, which are typically due to their incompatible properties, e.g., thermal expansion mismatch. In failure of these multilayer systems, interfacial shear strength is a good measure of the robustness of interfaces, especially for planar films. There is a widely-used shear lag model and method by Agrawal and Raj to analyse and measure the interfacial shear strength of thin brittle film on ductile substrates. The use of this classical model for a type of polymer derived ceramic coatings (thickness~18 µm) on steel substrate leads to high values of interfacial shear strength. Here, we present finite element simulations for such a coating system when it is subjected to in-plane tension. Results show that the in-plane stresses in the coating are non-uniform, i.e., varying across the thickness of the film. Therefore, they do not meet one of the basic assumptions of the classical model: uniform in-plane stress. Furthermore, effects of three significant parameters, film thickness, crack spacing, and Young's modulus, on the in-plane stress distribution have also been investigated. 'Thickness-averaged In-plane Stress' (TIS), a new failure criterion, is proposed for estimating the interfacial shear strength, which leads to a more realistic estimation of the tensile strength and interfacial shear strength of thick brittle films/coatings on ductile substrates.
Interlayer Shear Failure Evolution with Different Test Equipments
Procedia - Social and Behavioral Sciences, 2012
This research analyses the relationship between several configurations, failure mechanisms and states of stress imposed by testing machines, comparing the results of two devices. It focalizes on the evolution of numerous tests, performed on identical specimens, and reports the correspondent response curves obtained with two devices suitably designed to cover two kind of devices used in recent years and modified to ensure the comparison between the outcomes. The observation of a regular trend in the results suggests a strict relationship between them and the specimens' features and also warrants the statistic reliability of the testing machines.
Multiscale monitoring of interface failure of brittle coating/ductile substrate
In this paper, we proposed a non-destructive evaluation method combined digital image correlation with acoustic emissi on techniques. The method was used to in situ monitor interface failure and internal damage of brittle coating/ductile substrate systems with different size scales. The results show that there is a good relationship between digital image correlation and acoustic emission signals, which can be applied to judge cracking formation and coating delamination and to determine fracture toughness of a thermal barrier coating system subjected to bending.
1996
A Constant-Depth Scratch Test (CDST) technique to quantitatively determine the shear strength of interfaces between thin metallic or non-metallic films and metal or ceramic substrates is revealed. The test overcomes two problems associated with other types of scratch tests, namely the instrumental complexity required for real-time detection of interfacial failure, and the inability to quantify interfacial strength. These problems are circumvented by maintaining a constant depth during scratching through the coating and the substrate, monitoring the horizontal and vertical forces to sustain the constant depth scratch, and finally by using a model to analyze the test results to quantify the interfacial shear strength. Unlike other scratch tests, this test is capable of measuring interfacial shear strength as a function of position on the film-substrate sample.