On the limits of the interfacial yield model for fragmentation testing of brittle films on polymer substrates

Abstract

Fragmentation testing is frequently used to probe film fracture strain and the interfacial properties of thin brittle films on compliant substrates. A model based upon complete yield of the film/substrate interface is frequently used to analyse data after cracking has saturated. Additionally, the film is either assumed to have a single-valued failure stress or a distribution of strengths described by Weibull statistics. Recent work by the authors showed that consideration of film thickness variations and the application of neighbour ratio analysis brought 96% of the data for an Al x O y /Cu film/substrate system into compliance with the predictions for a film with a single-valued failure stress. In the present work Cr/PI (polyimide) and Cr/PET (polyethylene teraphthalate) systems are analysed according to the same methodology. The Cr films on polymer substrates crack such that the neighbour ratios considerably exceed the predicted limit of 2. The influence of the relative thickness of the film and substrate and the strain rate of the test is investigated. A deviation from the idealised mechanical model due to the large difference in elastic moduli of film and substrate is put forward as a possible cause of the observed behaviour. The importance of these results to the application of the interfacial yield model is discussed.

Thickness effect on the fracture and delamination of titanium films

“…Recent work by the authors [6,11] has demonstrated that caution must be exercised in the application of shear lag type models to metallic and ceramic coatings on polymer substrates. This caution is required because the large difference in elastic moduli between film and substrate, sometimes two orders of magnitude difference, can cause bending of the film and a rather more complicated stress state at the interface than that assumed by shear lag.…”

“…This technique strains the film-substrate system in uniaxial tension to induce film cracking (fracture) and at higher strains (N8%) film delamination can occur between the crack fragments [4][5][6]. When fragmentation testing is performed in-situ, with optical light microscopy, scanning electron microscopy (SEM), or 4-point-probe (4PP) resistance measurements [6][7][8][9], the initial fracture strain, ε f , can be ascertained. The minimum crack spacing at saturation, λ min , and the initial fracture strain can be used with the shear lag model [4,10] to calculate the interfacial shear stress, τ IFSS (Eq.…”

“…A method that can be used to determine the mechanical and interfacial properties of metal films on polymer substrates is fragmentation testing [3][4][5][6]. This technique strains the film-substrate system in uniaxial tension to induce film cracking (fracture) and at higher strains (N8%) film delamination can occur between the crack fragments [4][5][6].…”

Electro-Mechanical Testing of Conductive Materials Used in Flexible Electronics

“…This occurs for ceramic films on metal substrates (Agrawal and Raj, 1989) but is not always true for metal and ceramic films on polymer substrates. Instead, the film fragments bend or curve between the through thickness crack ( Figure 6D) and has thus far only been observed with in situ AFM fragmentation (Douville et al, 2011;Taylor et al, 2012) and finite element modeling (Toth et al, 2013). This phenomena have not been experimentally studied much due to the complicated and time consuming in situ AFM testing method.…”

“…The fracture stress is often determined with Hooke's Law (σf = Eϵf) with E the elastic modulus of the film and ϵf is the fracture strain or crack onset strain (COS) when cracks are first observed during an in situ experiment (Agrawal and Raj, 1989;Taylor et al, 2011Taylor et al, , 2012. Equation 1 is based on the shear lag theory where the shear stress transfer along the film-substrate interface is limited by τIFSS (Cox, 1952;Kelly and Tyson, 1965).…”