Fracture Toughness and Adhesion Energy of Sol-gel Coatings on Glass
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Ju¨rgen Malzbender and Gijsbertus de With Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Ruud Balkenende Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands (Received 15 August 2001; accepted 31 October 2001)
The reliability of coatings that are used in industrial applications critically depends on their mechanical properties. Nanoindentation and scratch testing are well-established techniques to measure some of these properties, namely the elastic modulus and hardness of coatings. In this paper, we investigate the possibility of also assessing the coating fracture toughness and the energy of adhesion between the coating and the substrate using indentation and scratch testing. Various existing and new methods are discussed, and they are illustrated by measurements on particle-filled sol-gel coatings on glass. All methods are based on the occurrence of cracking, and they are therefore only applicable to coating systems that act like brittle materials and exhibit cracking during indentation and scratching. The methods for determining the fracture toughness give comparable results, but the values still differ to within about 50%. The values of the adhesion energy obtained from different measurements are consistent, but it remains uncertain to which extent the obtained values are quantitatively correct. The results show that the methods used are promising, but more research is needed to obtain reliable quantitative results.
I. INTRODUCTION
Sol-gel coatings are widely used in industrial applications. The coatings are often designed to improve the mechanical properties (such as the friction coefficient and scratch resistance) of the surface to which they are applied. In other applications, the coatings are used for different reasons, such as achieving favorable optical or electrical properties. Whatever the reason for applying the coating, it is evident that both its durability and functionality critically depend on its mechanical properties such as the elastic modulus, the hardness, the coating fracture toughness, and the adhesion between the coating and the underlying substrate. Nanoindentation and scratch testing are frequently used to assess the mechanical properties of coatings. Indentation experiments are a fairly well established technique for determining the elastic modulus and the hardness [see for example the review papers by Bhushan (1995),2 Fischer-Cripps (2000),6 and Malzbender et al. (2001)9]. As a way of estimating the fracture toughness a
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J. Mater. Res., Vol. 17, No. 1, Jan 2002 Downloaded: 12 Mar 2015
and the energy of adhesion between the coating and the substrate, however, this technique is still unsatisfactory. Only a few papers have been published on the measurement of the fracture toughness of coatings using indentation [Li et al. (1997)7 is one such example]. Although various models have been proposed to determine the adhesion energy from scratch test
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