Measurement of fracture toughness and interfacial shear strength of hard and brittle Cr coating on ductile steel substrate

Abstract

The fracture toughness and interfacial adhesion properties of a coating on its substrate are considered to be crucial intrinsic parameters determining performance and reliability of coatingsubstrate system. In this work, the fracture toughness and interfacial shear strength of a hard and brittle Cr coating on a normal medium carbon steel substrate were investigated by means of a tensile test. The normal medium carbon steel substrate electroplated with a hard and brittle Cr coating was quasi-statically stretched to induce an array of parallel cracks in the coating. An optical microscope was used to observe the cracking of the coating and the interfacial decohesion between the coating and the substrate during the loading. It was found that the cracking of the coating initiated at critical strain, and then the number of the cracks of the coating per unit axial distance increased with the increase in the tensile strain. At another critical strain, the number of the cracks of the coating became saturated, i.e. the number of cracks per unit axial distance became a constant after this critical strain. Based on the experiment result, the fracture toughness of the brittle coating can be determined using a mechanical model. Interestingly, even when the whole specimen fractured completely under an extreme strain of the substrate, the interfacial decohesion or buckling of the coating on its substrate was completely absent. The test result is different from that appeared in the literature though the identical test method and the brittle coating/ductile metal substrate system are taken. It was found that this difference can be attributed to an important mechanism that the Cr coating on the steel substrate has a good adhesion, and the ultimate interfacial shear strength between the Cr coating and the steel substrate has exceeded the maximum shear flow strength level of the steel substrate. This result also indicates that the maximum shear flow strength level of the ductile steel substrate can be only taken as a lower bound estimate on the ultimate shear strength of the interface. This estimation of the ultimate interfacial shear strength is consistent with the theoretical analysis and prediction presented in the literature.

Acoustic emission analysis on tensile failure of air plasma-sprayed thermal barrier coatings

“…9 then deflect into interface cracking or delamination with the increase of tensile stress [30,31] . Therefore, the characteristic frequency spectrums and corresponding dominant bands of an AE signal can be obtained after a series of the FFT analysis.…”

FEM Modeling of In-Plane Stress Distribution in Thick Brittle Coatings/Films on Ductile Substrates Subjected to Tensile Stress to Determine Interfacial Strength

“…Agrawal and Raj (A-R) [ 29 , 30 ] developed an experimental technique and analysis to evaluate the ultimate shear strength of a ceramic-metal interface. Due to its simplicity in experimental setup and its acceptable reliability in property prediction, the A-R model has been widely used in many reported works [ 31 , 32 , 33 , 34 , 35 , 36 , 37 ]. Furthermore, Chen et al [ 21 ] reported their modification of the sinusoidal profile of interfacial shear stress in the original A-R model and test results on a TiN (coating)—304 steel (substrate) system.…”

“…Furthermore, Chen et al [ 21 ] reported their modification of the sinusoidal profile of interfacial shear stress in the original A-R model and test results on a TiN (coating)—304 steel (substrate) system. It is noteworthy to point out that, besides films below or around 1 µm thick [ 19 , 21 , 33 , 36 , 37 , 38 ], coatings of tens or even hundreds of micron thick have also been reported to be tested using the original A-R model [ 31 , 32 , 39 ]. In these experimental papers, defects and residual stress are primarily mentioned and discussed as factors that are leading to the error in interfacial property prediction, nevertheless, thickness and in-plane stress distribution have been largely overlooked as a potential source of error.…”

Research on the Micro-Crack Density of a Chromium Coating/Steel Substrate with and without Laser Pre-Quenching Treatment

“…the number of cracks per unit axial distance became a constant after this critical strain [12][13][14][15][16]. Although the number of the cracks remained constant after the critical strain, the crack length or (and) width would increase with the increase of the tensile strain in order to accommodate the applied strain [12][13][14][15][16]. So, a more suitable parameter to characterize the crack density should be presented.…”

“…The crack density is often defined as the number of the cracks per unit distance [12][13][14][15][16]. It was found that the cracking of the coating initiated at critical strain, and then the number of the cracks of the coating per unit axial distance increased with the increase of the tensile strain [12][13][14][15][16]. At another critical strain, the crack density (the number of the cracks) of the coating became saturated, i.e.…”