Measurement of Thin-film Coating Hardness in the Presence of Contamination and Roughness: Implications for Tribology
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VARIOUS hard thin-film coatings are increasingly being used for a variety of technological applications ranging from enhancing the durability of cutting tools to reducing friction and wear in automotive and aerospace components due to favorable properties such as oxidation, corrosion, and wear resistance.[1–6] As in all materials, there is a relationship between structure, properties, and performance of thin-film coatings. The quality of the coating and deposition process is usually evaluated based on a measure of some pertinent coating properties. For tribological applications, hardness and elastic modulus are often the most significant. In recent years, the development of instrumented indentation techniques ranging from micro- to nano-Newton load range has enabled convenient simultaneous measurement of the hardness and elastic modulus of hard thin-film coatings. Currently, nanoindentation is commonly used to measure the mechanical properties of coatings and thin films.[7–10] Because of the small indentation depths, some challenges have arisen with this technique, such as NICHOLAOS G. DEMAS, Assistant Materials Scientist, CINTA LORENZO-MARTIN, Principal Materials Scientist, OYELAYO O. AJAYI, Principal Materials Scientist, and ROBERT A. ERCK, Metallurgist, are with Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL, 60439. Contact e-mail: [email protected]. IQBAL SHAREEF, Professor, is with Bradley University, 1501 W Bradley Ave., Peoria, IL, 61625. Manuscript submitted June 10, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
surface roughness, typical to engineering samples, but also surface roughness resulting from the deposition of thin films, and the existence of surface contamination layers, which can vary in nature depending on the film composition.[11–15] The nature and morphology of the coating’s surface topmost layer can significantly affect nanomechanical measurements often leading to erroneous measurements of the real hardness of the bulk portion of a coating or thin film. When measuring the nanomechanical properties of thin films, there are several general guidelines developed over the years that are commonly followed. Based on empirical observations, it is commonly accepted that the indentation depth should not be more than 10 pct of the thin-film coating thickness in order to avoid substrate effects, also known as the ‘‘10 pct rule’’ of indentation depth, for the condition that the surface is perfectly smooth. It has been observed that repeatability of hardness and modulus is greatly affected by surface roughness. The presence of surface roughness often leads to large scatter in measured indentation hardness and modulus, often giving reduced values for these properties.[11,13] However, surface roughness may not be the only factor that can affect nanoindentation results. The properties of the deposited films can vary with the preparation conditions and methods; thus, the bulk and surface properties of the coating may be different. This paper presents a simple method to measure the nanomechanical properties o
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