A Stretch/Bend Method for In Situ Measurement of the Delamination Toughness of Coatings and Films Attached to Substrates

Abstract

A stretch/bend method for the in situ measurement of the delamination toughness of coatings attached to substrates is described. A beam theory analysis is presented that illustrates the main features of the test. The analysis is general and allows for the presence of residual stress. It reveals that the test produces stable extension of delaminations, rendering it suitable for multiple measurements in a single test. It also provides scaling relations and enables estimates of the loads needed to extend delaminations. Finite element calculations reveal that the beam theory solutions are accurate for slender beams, but overestimate the energy release rate for stubbier configurations and short delaminations. The substantial influence of residual stress on the energy release rate and phase angle is highly dependent on parameters such as the thickness and modulus ratio for the two layers. Its effect must be included to obtain viable measurements of toughness. In a companion paper, the method has been applied to a columnar thermal barrier coating deposited onto a Ni-based super-alloy.

A method for in situ measurement of the elastic behavior of a columnar thermal barrier coating

“…When loaded in this manner the TBC/bond coat bilayer contains a combination of bending and stretch (membrane) loads. Sufficiently long narrow beams and can be described with analytical solutions recently proposed by He et al [11], but the geometry of the microbeams used in the current study limit the utility of these solutions. The response of the bilayer bridge has instead been analyzed by the FE method using the commercial code ABAQUS Standard.…”

On modeling bodies with delaminating coatings taking into account the fields of prestresses

“…В работе [16] решена задача о потере устойчивости отслоившегося участка покрытия в случае жесткой подложки с прямолинейной поверхностью. Статья [19] посвящена исследованию отслоений, вызванных остаточными напряжениями. Метод исследования заключается в измерении ударной вязкости покрытий при последовательном растяжении и изгибе.…”

In Situ Measurement of the Toughness of the Interface Between a Thermal Barrier Coating and a Ni Alloy

“…It cannot be applied to either thick or curved substrates. This paper presents a new flexure test method that obviates this limitation and allows in situ measurement on films and coatings attached to substrates; the mechanics‐based design for which is described in a companion article 5 . In the current study, this method has been used to measure the toughness between an oxide coating and a relatively small diameter cylindrical substrate, the configuration for which is depicted in Fig.…”

“…The test design is guided by beam theory solutions for a planar four‐point bending configuration 5 . A three‐point bending configuration with asymmetric loading was used in this study but is difficult to solve analytically.…”

“…In the absence of residual stress, the energy release rate, G P , for this configuration can be expressed in the normalized form: where P is the load/width, a is the delamination length, L is the span, b is the separation of the inner load points in the four‐point bending setup, and h 1 , h 2 are the thickness of the coating and substrate, respectively, and E 1 , E 2 the corresponding values of the Youngzs modulus. The functional form for Π has been elaborated elsewhere 5 . One desirable feature of this test is the fact that the energy release rate diminishes with increase in crack length, so that the delamination progresses only upon increasing the load (stable behavior).…”