Quantitative evaluation of adhesion of metallic coatings with an extended microbridge test
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xtended microbridge test (eMBT) was proposed to assess the adhesion of metallic coatings on metallic substrates. Through loading on the backside of narrow striped freestanding coatings, a two-dimensional stable interfacial delamination was introduced. A cross-sectional scanning electron microscope (SEM) was used to examine the interfacial fracture process. A large deflection solution for elastic deformation of the coating was derived, and an approximate model was established for the estimate of interfacial crack extension force G. The eMBT samples of electroplated Ni coatings on C45 carbon steel substrate were tested, and the measured interfacial fracture toughness was about 5.28 J/m2. Cross-sectional SEM examination showed that the interface crack extended along the interface plane, and therefore the interfacial fracture proceeded by the debonding of Ni/steel interface.
I. INTRODUCTION
Coatings deposited on metallic substrates have been widely used to protect metallic materials from oxidation, corrosion, abrasion, etc. Reliability of the coatings’ bond to the underlying substrate is crucial for the durability and functionality of the coatings system. While many methods have been utilized to assess the bonding strength and interfacial adhesion, techniques providing quantitative values of adhesion are rare.1 Indentation and scratching are the most widely used techniques for the assessment of the interfacial adhesion of hard coatings.2–9 However, for metallic coatings, because of their tendency to deform plastically in testing, it is difficult to induce delamination at the interface. In order to suppress the plastic deformation during the indentation and scratch test, a layer of hardness was often coated upon the top surface of metallic coatings, and the high stresses in superlayer can provide additional driving force for delamination.10–13 However, the major disadvantage limits the viability of the superlayer technique for strong adhesive evaluation, which is that the bonding of interface of superlayer and coating must be higher than that of coating and substrate. Blister testing is another successful technique for the adhesive evaluation of ductile coatings.14–18 If one side
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0309 J. Mater. Res., Vol. 22, No. 9, Sep 2007
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of the freestanding thin film is pressurized through a hole in the substrate, the film can be peeled from the substrate. Through measurement of the debonding radii, the interface fracture toughness can be obtained along with the critical load. However, it is difficult to measure the delaminated zone when the blister test is utilized to estimate the adhesion of opaque metallic coatings. The microbridge testing method has been proposed for mechanical evaluation of thin films.19,20 In the present study, taking the advantages of both the microbridge test and the blister test, we propose introducing an extended microbridge technique (eMBT) for measur
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