Correlation between Adhesion Strength and Thin film/Substrate Mechanical Properties using the Nano-Scratch Technique
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Correlation between Adhesion Strength and Thin film/Substrate Mechanical Properties using the Nano-Scratch Technique Bo Zhou1, Nicholas Randall1, Barton Prorok2 1 2
CSM Instruments Inc., Needham 02494, MA, U.S.A Materials Engineering, Auburn University, Auburn 36849, AL, U.S.A
ABSTRACT Scratch testing, as a mature technique for coating adhesion quantification, has been widely adopted by both industrial and academic fields in recent years. Following the urgent needs of very small materials characterization, nano-scratch testing has gradually replaced the traditional pull-off test for the study of ultra-thin film properties. In this research, the relationship between the adhesion strength and film/substrate mechanical properties was investigated to provide fundamental but crucial knowledge of the scratch mechanism. Scratch tests were performed on different film/substrate combinations using a Nano Scratch Tester with a spheroconical diamond indenter. A progressive load mode was employed to cause coating failure during scratch on the film surface. The critical values of different failure mechanisms, such as cracking and delamination were accurately determined according to the scratch panorama image, penetration and residual depth data. In addition, the hardness (H) and modulus (E) values of the thin films and substrates were measured with an Ultra Nanoindentation Tester. The scratch critical failure loads were then plotted versus film/substrate H and E ratios. A unique relationship was found between these parameters that could help understand the true mechanism behind scratch adhesion and leverage this methodology to a new theoretical level. INTRODUCTION Quantification of very thin film adhesion properties is crucial to many applications in the academic and industrial coating fields. In recent years, with the coating thickness dwindling down to the nanometer scale, the traditional pull-off tensile test can no longer provide accurate and consistent measurement of the adhesion strength. Nano scratch testing has emerged recently as a methodology specifically designed for ultra-thin film characterization with high resolution and accuracy. It can be employed to test a large variety of films, spanning from a few nanometers thick diamond-like carbon coating to soft polymer protective coatings [1-5]. Compared to the pull-off test, the nano scratch technique provides a more comprehensive picture of different film failure mechanisms such as cracking, chipping, and delamination. Other information such as residual depth determination is extremely useful for the study of elastic/viscoelastic recovery. In the scratch testing field, the influence of film/substrate mechanical properties on the determination of critical film failure is always a concern to researchers. In this study, three thin films were deposited using sputtering onto different polished substrate materials which span from silicon to sapphire. Nano scratch tests were performed on these specimens, from which the critical failure loads were determined and correlated wit
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