Fracture toughness measurement of ultra-thin hard films deposited on a polymer interlayer Audrey Favache a, ⁎, Laure Libralesso b , Pascal J. Jacques a , Jean-Pierre Raskin c , Christian Bailly d , Bernard Nysten d , Thomas Pardoen a a Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Place Sainte Barbe 2 L5.02.02, 1348 Louvain-la-Neuve, Belgium b Advanced Coatings Coating Solutions — CRM Group, Boulevard de Colonster 57, 4000 Liège, Belgium c Information and Communications Technologies, Electronics and Applied Mathematics (ICTEAM), Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium d Bio and Soft Matter, Institute of Condensed Matter and Nanoscience, Université catholique de Louvain, Croix du Sud 1 L7.04.02, 1348 Louvain-la-Neuve, Belgium abstract article info Article history: Received 30 November 2012 Received in revised form 14 October 2013 Accepted 16 October 2013 Available online 24 October 2013 Keywords: Fracture toughness Thin film Channel cracking Nanoindentation The fracture toughness of 100 nm-thick chromium nitride films deposited on a soft interlayer is evaluated using two different test methods. Both methods take advantage of the enhanced crack driving force resulting from the large elastic stiffness mismatch between the film and the polymer interlayer. The first method is based on the presence of channel cracks developing during the deposition as a result of large internal stress. The second method relies on nanoindentation induced cracking. The presence of a soft interlayer is shown to be essential to promote cracking otherwise impossible on a hard substrate, hence to determine the fracture toughness of very thin films. The assumptions underlying the two methods are very different, which allows critical cross comparison and assessment. A fracture toughness value between 1.5 and 2 MPa·m 1/2 is obtained. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The mechanical properties of thin films play a crucial role in the reliability of a variety of technologies, ranging from microelectronics to functionalized nanostructured coatings [1,2]. One important aspect is the ability to withstand, without fracture, the mechanical loads originating from deposition, forming operations or mechanical contacts. The fracture toughness is the relevant material property to quantify the resistance to cracking, at least for sufficiently brittle materials. Indeed, for ductile films or sheets the use of fracture mechanics, even within an elastoplastic formalism, might not be relevant due to size effects associated to the small thickness, e.g. [3]. Although well-established techniques for measuring the fracture toughness of bulk materials are widely used [4], these techniques cannot be directly applied to the measurement of the fracture toughness of thin films, essentially due to their restricted size. Various techniques have been developed in recent years in order to tackle with this issue (see the recent review [5]). Techniques based on free- standing films, such as, for instance, micro-tensile testing or bulging, as well as techniques based on coatings bonded to a substrate, such as, for instance, nanoindentation, multi-strain flexure test or tensile tests [6] have been investigated. The microstructure and hence the film properties depend on the deposition conditions and on the roughness, crystallography, and composition of the underlying substrate [2]. Hence, it is crucial to measure the fracture toughness on systems which have been produced under conditions similar to the end use applications. This is often not possible when using techniques based on free-standing films. The focus of the paper is on the description and assessment of a methodology for the measurement of the fracture toughness of films deposited on a polymer layer. Flexible electronics and surface protection are two mainstream fields of application where hard films on soft substrate stacks are encountered. The small thickness (100 nm) of the films makes this analysis very challenging. The goal is to deliver methodological information and conclusions applicable to other hard coating based systems with similar or even smaller thicknesses. Two independent approaches are presented to evaluate the fracture toughness of thin brittle chromium nitride films deposited on a polymer interlayer. In the spirit of the approach developed by Stafford et al. [7] for measuring the elastic modulus of polymer thin films, the two techniques proposed here take advantage of the intrinsic “mechanical compliance” of layered systems where a stiff film lies on a soft thicker substrate. Indeed, crack propagation is highly facilitated in these systems by the elastic property mismatch between the film and the underneath substrate as predicted by interface fracture mechanic arguments [8,9]. The first technique is based on channel-cracking of thin films with the driving force originating from the internal stresses building up during deposition. Channel-cracking in thin hard films on soft substrates has been extensively studied in the literature, e.g. [6,8,9]. The second technique, which has already been repetitively Thin Solid Films 550 (2014) 464–471 ⁎ Corresponding author at: Université catholique de Louvain, Place Sainte Barbe 2 L5.02.02, 1348 Louvain-la-Neuve, Belgium. Tel.: +32 10 47 24 73; fax: +32 10 47 40 28. E-mail address: audrey.favache@uclouvain.be (A. Favache). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.10.052 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf