Evaluating indent pile-up with metallic films on ceramic-like substrates
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This work focused on ascertaining the effect of pile-up during indentation of thin films on substrates. Conventional understanding has postulated that differences in contact area resulting from pile-up or sink-in significantly alter the extraction of material properties. In this work, the specific case of pile-up with compliant, plastically deforming films on stiff, nonplastically deforming substrates was studied. Several literature methods to assess pile-up were leveraged, and a new technique was developed and validated to quantify projected pile-up. Indentation testing was performed on gold films of multiple thicknesses on several ceramic-based substrates. The results indicated that the degree of pile-up was solely a function of indent depth into the film. Pile-up was not influenced by film thickness or substrate elastic modulus. In other words, the pile-up development was insensitive to the presence of the substrate and how it contributes to the composite’s elastic properties. In such case, if the elastic response of the film/substrate composite was independent of the degree of pile-up, then elastic data acquired from unloading did not require a contact area correction. The findings are confirmed using the Zhou–Prorok model for extracting film elastic properties for both gold and platinum films.
I. INTRODUCTION
In the past several decades, the mechanical properties of thin films have been an important aspect in applications, such as electronics, protective coatings, and microelectrical mechanical systems. Nanoindentation has become a popular tool to evaluate these properties of thin film materials because it is minimally destructive and there is no need to image the indent.1–9 Various models have been developed to account for substrate effects, including Doerner and Nix4 with modifications,5,6 Gao et al.,7 and more recently from Zhou and Prorok.10,11 In addition, there have been barriers to collecting accurate values due to pile-up and sink-in during nanoindentation.12 Generally in film/substrate systems, pile-up occurs when there is a compliant film on a stiff substrate, and sink-in when there is a stiff film on a compliant substrate,13 leading to a difference in the indent impression, see Fig. 1. A normal indent from a Berkovich indenter tip is presented in Fig. 1(a), followed by (b) pile-up, and (c) sink-in. These are represented by platinum films on (a) platinum, (b) silicon oxide, and (c) tin. Previous researchers have shown that pile-up and sink-in can affect the contact area of the indenter, which in turn can alter the extracted mechanical properties.14,15 An indent experiencing pile-up underestimates contact area because area due to the
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.167 2046
J. Mater. Res., Vol. 30, No. 13, Jul 14, 2015
additional pile-up is not included. Inversely, sink-in produces an overestimation of contact area.6,14–16 These phenomena can play a role in the calculations of extracting film properties when using the Ol
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