Elsevier

Wear

Volume 265, Issues 3–4, 31 July 2008, Pages 507-515
Wear

Mechanisms of blistering and chipping of a scratch-resistant coating

https://doi.org/10.1016/j.wear.2007.11.019Get rights and content

Abstract

In most cases, scratching of the surface of a polymeric glass elicits brittle behavior. Industrial solutions have been successfully used to improve the scratch resistance of polymeric glasses and a common way is to coat the substrate with a thin film. However, one of the limitations of this method is the risk of cracking and chipping. The origin of the success of the coating technique is still of great research interest and further work will be required to explain the improvement in scratch resistance and predict the cracking in anti-scratch coatings. The present study contributes to these aims.

Using a single-asperity scratching device allowing in situ observation of the scratch, the fracturing of a thin (3.5 μm) nano-composite coating deposited on a viscoelastic–viscoplastic substrate (polycarbonate) was investigated under different conditions of temperature and scratching speed. Four types of fracture mechanisms were observed, depending on these two variables. The processes involved in deformation of the system were: (i) delamination (blister formation) and fracture (chipping) of the coating and (ii) viscoelastic–viscoplastic deformation of the substrate. Image analyses were performed on video sequences of the different processes leading to damage of the film. The quantitative results are discussed in terms of the damage mechanisms involved.

Introduction

Most polymeric glasses are sensitive to scratching and wear and this usually constitutes their time-limiting factor. The mechanical and tribological performances upon contact can be improved by coating such materials with an elastic thin film, which increases the elastic component in the elastic–plastic behavior of the system (substrate/thin film). The most recent scratch-resistant coatings for optical components associate the large elastic domain of an elastomeric polymer with the hardness of nano-sized particles of silica in a single layered nano-composite coating deposited on the substrate. However, there is a need for a better understanding of the deformation processes in the film. In this context the single-asperity experiment, i.e. the scratching test, is commonly accepted as being a convenient simplification for tribological and contact mechanical problems.

In any case, the limiting factor for wear of the coating when attempting to improve the scratch resistance of an optical component is the occurrence of fracture of the coating. A scratch test performed on this kind of coated material can involve different deformation processes: (i) delamination (blister formation) and fracture (chipping) of the coating and (ii) viscoelastic–viscoplastic deformation of the substrate. The difficulties in understanding deformation and damage in complex film/substrate systems are numerous. Among these, the main problems are:

  • Since the mechanical properties of polymeric materials are time and temperature dependent, one cannot determine a single value of the critical load for damage to the film (normal load value at which damage occurs).

  • The adhesion of the coating, which governs the interfacial behavior, has not been very successfully correlated with regard to the test conditions.

  • The contribution of the substrate to the deformation within thin polymer films remains largely unknown.

  • The geometric scales are of prime importance since for example the thickness of the film relative to the contact area and/or the roughness of the scratching tip (which requires an exact knowledge of the tip shape) is critical for the mechanical behavior of the system.

Some answers and clues for a mechanical treatment of these processes and solution of some of the above difficulties may be drawn from the literature and these approaches will first be reported here for the three different situations.

Thouless et al. [1] considered a circular blister in a thin film. A mode dependence of the blister propagation was found in experiments: mode II predominated over mode I as the interfacial crack propagated, so that the interface toughness increased as the blister grew. Hutchinson and Suo [2] examined the propagation of a straight-sided blister (i.e. only in one direction) like those leading for example to telephone-cord buckles [3]. In a scratching process, Kriese et al. [4] described the establishment of a large delamination ahead of the indenter.

With or without prior delamination, fracture of the coating has been reported to involve various processes. Bull [5] and Bull and Berasetegui [6] found that, under some conditions, wedge cracks could form some distance in front of the indenter, due to the large displacement of the coating caused by compressive stresses ahead of the indenter. Malzbender et al. [7] described a fracture of the coating in the form of two radial cracks [8], [9], as was also reported with delamination by Kriese et al. [4] and by Venkataraman et al. [10] in a Pt thin film/NiO substrate system. Lastly, Bull [5] described the interfacial failure which results from the pile-up occurring in front of the moving indenter before the film buckles. More recently, Demirci et al. [11] observed that cracking of the coating may appear within the contact area, rather than at the rear edge, while the ratio of the contact radius to the radius of the grooving tip proved to be a pertinent parameter to predict the damage and did not depend on the scratching velocity or temperature.

Gauthier et al. [12] have previously shown that the substrate deformation can be studied through the shape of the contact area between the indenter and the material. The rear contact is due to the elastic recovery of the polymer and depends on the plastic contribution to the elasto-plastic strain in a characteristic spherical volume of matter under the contact area. Ideally, an inexistent rear contact area leads to a perfectly plastic contact, while identical rear and front contact areas lead to a fully elastic contact. Intermediate cases give rise to elasto-plastic contacts.

This paper provides insight into the scratching behavior of a film/substrate system with regard to the temperature and scratching speed. Using in situ contact visualization, the delamination and crack networks within a nano-composite film were observed over a wide range of test conditions. The experimental observations are discussed in terms of the adhesion, the substrate contribution, the appearance of cracking and the delamination growth. In this work, the geometric scales were kept constant, i.e. a single rigid spherical indenter with a well-known geometry ensured a viscoelastic–viscoplastic response of the substrate and was used to examine the delamination and brittle failure of a thick coating.

Section snippets

Experimental procedures

The material was provided by the company Essilor and consisted of an amorphous polycarbonate with a nano-composite coating (obtained by the dip technique) composed of a thermoset matrix filled to about 20% of its volume with nano-sized silica particles (about 10 nm in diameter). The samples received were circular in shape with a diameter of 80 mm, a total thickness of about 2 mm and a thin film thickness of 3.5 μm. The Young's modulus of the thin film was given by the manufacturer as 5.5 GPa. As it

Mechanisms of the scratching damage

A complete “morphology” of the delamination and cracking processes occurring during scratching of the test material was established for the chosen experimental conditions. Four different sets of mechanisms of delamination and/or fracture of the thin film were identified, while the “critical normal load” ranged from 1.2 to 0.6 N with increasing temperature. Representative images of these mechanisms are shown in Fig. 1 and may be described as follows:

  • (1)

    At −20 °C: two radial cracks occur at the rear

Conclusions

The contact mechanical and tribological performances of polymeric glasses can be improved by coating the material with an elastic thin film. However, contact damage limits the improvement of the scratch behavior. In this work, the fracturing of a thin nano-composite coating deposited on a viscoelastic–viscoplastic substrate (polycarbonate) was investigated using an experimental device which allows observation and recording of the true contact area during scratching.

Depending on the temperature

Acknowledgements

The authors are very grateful to Mrs. L. Scodellaro and Mrs. A. Roos of the company Essilor for providing the samples and for helpful discussions.

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