Assessment of the toughness of thin coatings using nanoindentation under displacement control J. Chen, S.J. Bull * Herschel Building, Department of Chemical Engineering and Advanced Materials, University of Newcastle Upon Tyne, NE1 7RU, UK Available online 10 October 2005 Abstract Various indentation models have been developed to measure the toughness of bulk materials and coatings. Most of them are based on the formation of well-developed cracks. However, in order to eliminate the influence of elastic– plastic deformation in the substrate, it is preferable to perform small indentations in thin coatings and thus the cracks may not be well-developed compared to the indentation size. Relatively little work has been done to investigate this kind of small cracks. The ultra small cracks (<500 nm in length) in thin coatings (¨ 500 nm in thickness) confined to indentation zone are investigated here. A new method to assess the toughness of the main components of solar control coatings such as SnO 2 , TiO x N y and ITO deposited on soda – lime glass is proposed here. This method is able to separate the energy contributions from other deformation mechanisms from that dissipated in the fracture event. The energy release rate of these ceramics coating are in the range 15 – 45 J/m 2 by this method. D 2005 Elsevier B.V. All rights reserved. 1. Introduction Multilayer coated systems (e.g. solar control coatings consisting of a selective absorbing layer, antireflection layers and barrier layers on glass) are widely used in architectural applications in order to save energy. Whatever the application, the coating lifetime, which is controlled by the mechanical properties of the coated system, is a key issue that needs to be addressed. For thin hard coatings, failure is often due to fracture during handling or cleaning, it is therefore important to understand the fracture mechanisms during contact so that we can understand how the coating performs in service. What is more, if current mechanical performance models (e.g. [1–3]) used to predict the hardness and Young’s modulus of the complex coated systems are to be improved, it is necessary to understand the fracture behaviour and include the effect of cracks in the model. Conventional indentation toughness methods were initially developed for monolithic bulk materials tested by microinden- tation when well-developed radial cracks form (i.e. Marshall and Lawn [4], Anstis et al. [5]). The toughness K IC is related to the applied load P and the crack dimension c . K IC ¼ v E H   1=2 P c 3=2 ð1Þ where E and H are the Young’s Modulus and hardness of the material. For Berkovich and Vickers indenters, v = 0.016. This method has been extended to coated systems where radial cracks are well-developed by some authors (e.g. [6–8]), generally for a coating much thicker than 1 Am. The values of toughness obtained by this method will depend on the residual stress in the coating since Eq. (1) is strictly only valid in the absence of internal stresses. For hard coatings on harder and stiffer substrates, it may be reasonable to assume the residual stress only modifies the crack shape. However, it is necessary to point out that this traditional method will be invalid when the cracks are confined to the indentation impression and coated systems consisting of a harder coating on softer substrate are usually an example of this. Chen and Bull [9] have shown that the radial cracks which run along the indenter edges in hard coatings on softer substrates are generally confined within the indentation impression provided 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.176 * Corresponding author. Tel.: +44 191 222 7913; fax: +44 191 222 8563. E-mail address: s.j.bull@ncl.ac.uk (S.J. Bull). Thin Solid Films 494 (2006) 1 – 7 www.elsevier.com/locate/tsf