A robust method for extracting the mechanical properties of thin films with rough surfaces by nanoindentation
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Teng Fei Zhang Department of Applied Hybrid Materials, School of Convergence Science, Pusan National University, Busan 609-735, Korea; and Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan 609-735, Korea
Byoung Hun Lee School of Materials Science and Engineering, Center for Emerging Electronic Devices and Systems, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea
Kwang Ho Kima) Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan 609-735, Korea; and School of Materials Science and Engineering, Pusan National University, Busan 609-735, Korea (Received 15 June 2016; accepted 27 October 2016)
Surface roughness and finite sample thickness are major sources of error in the nanoindentation measurements of thin films as the former makes it difficult to determine the effective contact point between the indenter and sample while the latter limits the usable depth range to be no more than ;10% of the film thickness. Combining a closed-form model of a film/substrate system with the ability of nanoindentation to monitor the contact depth, the present method defines the two-dimensional shape profile of the indenter contacting the composite system with one unknown constant associated with the model and another unknown constant associated with the effective contact point. On the basis that the obtained shape profile of the rigid indenter is identical to the pre-determined indenter shape profile function, the method extrapolates the two constants simultaneously so as to determine the effective contact point. The method was demonstrated for amorphous diamond-like carbon (DLC) coatings.
I. INTRODUCTION
Surface roughness and finite sample thickness are major sources of error in the nanoindentation measurements of thin films. The former makes it difficult to determine the effective contact point between the indenter and sample surface. Such difficulty may be avoided by using depths of indentation at least ;20 times the roughness depth to negate the surface roughness effect. However, the latter limits the usable depth range of indentation to be no more than ;10% of the film thickness. For a film/substrate system with a large elastic modulus mismatch, the upper limit of the depth range can decrease further. There is thus no window of usable indentation depth for many thin films with rough surfaces used for practical applications. Although a polishing technique can reduce the surface roughnesses of some materials, it is difficult to apply it to a material in thinfilm form as it can easily cause delamination or material Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.428
loss. Not to mention, the use of a polishing technique that alters the mechanical properties or/and residual stress state of the material (e.g., mechanical polishing for metals and alloys) must be avoided. To address this longstanding problem of nanoindentation, a robust method is presented for determinin
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