In situ measurement of bulk modulus and yield response of glassy thin films via confined layer compression
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In situ measurement of bulk modulus and yield response of glassy thin films via confined layer compression Owen Brazil1 , Johann P. de Silva1, Mithun Chowdhury1,b), Heedong Yoon2, Gregory B. McKenna2, Warren C. Oliver3, Jason Kilpatrick4, John B. Pethica1, Graham L.W. Cross5,a) 1
School of Physics, CRANN & AMBER, Trinity College, Dublin 2, Ireland Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409-3121, USA 3 Nanomechanics Inc., KLA-Tencor, Oak Ridge, Tennessee 37830, USA 4 Adama Innovations Ltd., CRANN, Trinity College, Dublin 2, Ireland 5 School of Physics, CRANN & AMBER, Trinity College, Dublin 2, Ireland; and Adama Innovations Ltd., CRANN, Trinity College, Dublin 2, Ireland a) Address all correspondence to this author. e-mail: [email protected] b) Present address: Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India. 2
Received: 10 December 2019; accepted: 4 February 2020
The measurement of thin film mechanical properties free from substrate influence remains one of the outstanding challenges in nanomechanics. Here, a technique based on indentation of a supported film with a flat punch whose diameter is many times the initial film thickness is introduced. This geometry generates a state of confined uniaxial strain for material beneath the punch, allowing direct access to intrinsic stress versus strain response. For simple elastic–plastic materials, this enables material parameters such as elastic modulus, bulk modulus, Poisson’s ratio, and yield stress to be simultaneously determined from a single loading curve. This confined plastic yield phenomenon has not been previously observed in thin films or homogeneous materials. The phenomenon of confined plastic yield has not been previously observed in thin films or homogeneous materials, which we demonstrate here for 170 -470 nm thick polystyrene (PS), polymethyl-methacrylate (PMMA) and amorphous Selenium films on silicon. As well as performing full elastic -plastic parameter extraction for these materials at room temperature, we used the technique to study the variation of yield stress in PS to temperatures above the nominal glass transition of 100 °C.
Supported thin films and bonded coatings are critical to both fabrication and function in many technologies, as well as allowing for fundamental scientific study of size and boundary effects brought about by reduced dimensionality in materials [1, 2]. However, efforts to measure their mechanical constitutive character remain challenging despite decades of theoretical and experimental efforts. Nanoindentation with sharp, selfsimilar tip geometries constitutes the primary mechanical characterization technique at submicron length scales [3, 4] but suffers from two significant drawbacks in regard to thin film testing; as a consequence of the contact geometry, access to intrinsic stress–strain response is extremely limited and there exists significant and complex convolution of the film response with that of the substrate [5, 6]. Efforts to alleviate
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