In situ analysis of the fragmentation of polystyrene films within sliding contacts

Abstract

Fracture processes of thin (10–100 μm) polystyrene films on PMMA substrates have been investigated within macroscopic single-asperity sliding contacts with rigid spheres. Using the resources of in situ contact visualization, the development of cracks has been analyzed under both elastic and plastic conditions for various values of the ratio of the contact radius to the film thickness. Under elastic contact conditions, damage mechanisms were dominated by the formation of a network of regularly spaced cracks at the leading edge of the contact. These processes were analyzed in the light of a fragmentation model based on contact mechanics simulations of the stress field induced within the cracked films. It emerged from this contact mechanics analysis that the mean spacing between adjacent cracks can be correlated to the strength of the polymer coating.

Introduction

Polymeric coatings are largely used to improve contact mechanical and tribological performance of engineering materials and optical components. However, the development and selection of such films is a very complex and costly task. The number of parameters is huge, spanning from material, physical, mechanical and surface properties to the behaviour of complex films/substrate systems. In addition, there is a lack of data and understanding regarding the actual deformation and fracture mechanisms involved in such coatings.

Within this context, abrasion resistance remains one of the key issues regarding the lifetime of organic coatings. During abrasive wear damage, the initial stage is usually considered to be the process of contact and scratch between the polymer surface and a sharp asperity. The accumulation of the associated microscopic failure events eventually generates wear particles and gives rise to weight loss. Investigating such processes within macroscopic contacts between rough surfaces is, however, a difficult task due to the multiple interactions between individual sliding micro-asperities. In order to overcome these limitations, model experiments are often considered which attempt to simulate the damage induced by a single asperity contact [1]. Although the wear rate itself is not monitored, such experiments provide the opportunity of getting a more detailed insight into the deformation and fracture mechanisms involved in asperity engagements. In such experiments, the selection of different indentor geometries and loading conditions offers the possibility of exploring the viscoelastic/viscoplastic response and brittle failure mechanisms over a wide range of strains and strain rates. For bulk polymers, the observed damage evolves through a range of severity as the contact strain is increased: visco-elastic smoothing or ‘ironing’, plastic or viscoplastic grooving, extensive plastic flow and tearing, pronounced fracture or tearing and finally cutting or chip formation can be identified [2], [3], [4], [5], [6], [7], [8]. These approaches have been popularized for a variety of amorphous and glassy and semi-crystalline polymers by Briscoe and co-workers [9], [10] who put together in the form of ‘deformation maps’ the different deformation regimes.

Similar regimes can be identified in the case of polymeric coatings. Within the fracture domain, regular crack patterns are often observed at the leading edge of the contact under the action of predominantly tensile stresses [7], [8], [11], [12]. A critical normal force is often ascribed to the occurrence of such cracking processes, but the way it relates to known polymer failure properties such as fracture toughness is still a matter of debate [6]. In addition, the contribution of substrate deformation to the development of contact cracks within thin polymer films remains largely unknown. As a first approach, the magnitude of these effects may be assumed to depend largely on the ratio of the contact area, a, to the film thickness, h. In many contact situations, this a/h ratio can vary by orders of magnitude depending on whether the macroscopic or micro-asperity contact lengths are considered. There is therefore a need for a better understanding of coating fracture processes as a function of this characteristic a/h ratio.

Within the frame of this investigation, fracture mechanisms of thin (10–100 μm) polystyrene (PS) films on polymethacrylate (PMMA) substrates have been investigated within macroscopic sliding contacts with smooth spherical asperities. Using the resources of in situ contact visualization, the various stages of the development of crack networks within the PS film have been observed for a range of contact conditions which were characterized by different ratios of the contact radius, a, to the film thickness, h. The resulting crack patterns have been analyzed in the light of a fragmentation model which considers that failure is driven by the evolving tensile stress field induced within the cracked PS film at the leading edge of the contact. For that purpose, a contact mechanics analysis of the cracked coated systems has been developed which is able to simulate the film unloading/reloading processes associated with the propagation of successive cracks during sliding.