MECHANICAL RESPONSE AND TRANSIT SCRATCH FORMATION IN THIN-COATED SYSTEMS ON GLASS

 

Krishna. J. Belde.

School of Chemical Engineering and Advanced Materials

University of Newcastle upon Tyne.

 

Many coating systems and surface treatments have been developed to enhance the mechanical performance of the surfaces of components for tribological applications, including protecting functional coatings (e.g. anti-reflective coatings, solar control coatings etc.) from contact damage. Such systems may now involve thin (~100nm) multilayer stacks and many different coating architectures have been proposed (i.e. different layer materials, layer thicknesses and stacking orders). Failure and contact damage- and thus service integrity and lifetime - are primarily controlled by the mechanical properties of the overall system and thus analysing and understanding the mechanical response of these coated systems at the challenging spatial scales involved remain key issues in enabling further coating developments.

 

This project has primarily concerned studies of the mechanical properties of thin solar control coating systems on glass, at loads ranging from macro- to microNewton levels. The project has involved both the measurement of the mechanical response of typical layers used in the solar control architecture, but also the development of laboratory test protocols to simulate the formation of transit scratches which are a major problem in the coated glass industry. Hardness and Scratch testers have been used along with light microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) to reveal the deformation structures around indentations and scratch tracks. A range of coated systems were studied, thin (<1µm) multilayer optical coatings (comprised of, TiO2, SnO2, ITO, ZnO, and Ag) on float glass and a commercial multilayer coated system (Pilkington Optithem, total coating thickness ~100nm) have been analysed.

 

The first part of the study was to investigate the effects of blunt and sharp indenters on the various coatings. The sharp indenter produces plastic behaviour at lower loads than the blunt indenter thus allowing better hardness values to be assessed at shallower depths. It also produces a lot of chipping along with radial cracks. This type of cracking is not seen in transit scratches and it demonstrates that the damage occurs because of a blunt contact rather than sharp contact (e.g. from glass debris). In contrast the blunt indentations produce hertz cracks with much less chipping, and also the damage is closer to that happening in real situations where through-thickness fracture is observed similar to Hertzian ring cracks [1]. Scratch tests were also performed on various thin coatings to check the deformation behaviour and confirm that blunt contact is responsible for transit scratches. The effect of coatings of fracture initiation and propagation was quantified.

 

The next area investigated wash the adhesion aspects of the coatings to the substrate as well as the adhesion between the coatings within the multilayer stack – most relevant to the problems occurring in service where damage was caused in these coatings during transportation. The weakest Ag/ZnO interface was found to fail giving real transit scratches their characteristic depth [1]. A simulation test in the laboratory was developed which gave the same failure. This was due to the stresses on the interface caused by the sliding of polymer interlevant balls between the glass plates during delivery. Reduction of loading on these balls and reduction of the ball/glass friction coefficient can be used to reduce damage.

 

Glass is exposed to the environment during delivery. The environmental effects especially water, can have a very destructive effect on these coatings; such responses are termed “chemomechanical effects” and include physical effects such as, reduction in hardness or softening of the top layer of the surface or chemical effects where in the layers present within the multilayer can react with atmosphere and produce a chemical residue [2]. Strong chemomechanical effects have been observed in ZnO and a smaller effect in TiO2. Although SnO2 shows no chemomechnical softening, it does suffer interfacial damage when exposed to water

 

The final stage of this study investigated the friction coefficients of the various coatings against the polymer balls and the effect that these would have on failure stress. These are very important as these can control the damage rate of the coatings. The friction coefficients of various coatings were measured and analysed and the combination of scratch induced stress with the residual stress in the coating was found to be responsible for local interfacial detachment by buckling which is the first stage in the formation of a transit scratch.

 

[1] Intentional Polymer Particle Contamination and the Simulation of Adhesion Failure in Transit Scratches in Ultra-Thin Solar Control Coatings on Glass, Krishna J. Belde and S.J. Bull, Proc. 10th Int. Symposium on Particles on Surfaces: Detection, Adhesion and Removal, Ed. R. Lacombe, June 19-21, 2006, Toronto, Canada.

[2] Chemomechanical effects in optical coating systems, K.J. Belde and S.J. Bull, Proc. Int. Conf. Metallurgical Coatings and Thin Films, San Diego, May 1-5, 2006, Thin Solid Films., 515 (2006) 859-865.