Diamond and Related Materials, 3 (1994) 791-798 791 Characterization of the strength and adhesion of diamond films on metallic substrates using a substrate plastic straining technique L. Chandra and T. W. Clyne Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3Q Z ( U K ) Abstract Diamond films have been deposited by microwave plasma-assisted CVD on two different metallic substrates (304 stainless steel and Ti 6A1 4V). The film thickness was also varied within the range of a few micrometres. The strength and adhesion of these coatings have been measured by a substrate plastic straining technique. The information about the distribution of the crack spacing has been utilized to estimate the Weibull moduli of the films. The data presented in this paper indicate that the films exhibit excellent adhesion to the substrates. The average strength of the films appears to be greater for lower thicknesses. The films on titanium substrates show greater strengths and a somewhat higher value of Weibull modulus. 1. Introduction stress can help drive the interfacial crack) and the initiation of through-thickness cracks. A high compres- Adhesion is one of the main factors governing the sive stress in the coating discourages through-thickness durability of coatings. The adhesive strength of diamond cracks but can still be deleterious, in view of its capacity film is expected to depend on the substrate material, i.e. to drive interfacial cracks (for example, to cause for similar diamond deposition conditions, differences in blistering). the substrate material may result in a variation in A usefulcharacterizationofinterfacialandfilmcharac- adhesion characteristics [1-3]. This difference can often teristics is possible even with very thin and strongly be related to the nature of the substrate-diamond inter- adhering films, using the simple procedure and analysis face [3], which determines the shape, faceting and outlined by Agrawal and Raj [16]. Here, we use this adhesive strength of diamond grown under metastable technique to explore the interfacial adhesion strength conditions. Therefore, characterization of interfacial between the diamond film and the substrate. This test strength is essential. Although a number of mechanical also provides a measure of the film fracture strength. tests have been employed to measure the adhesion of Furthermore, information about the distribution of the thin diamond films to their substrates [ 1, 4-10], none crack spacing of the coating has been utilized to estimate of these gives fundamental measures of the interfacial the Weibull modulus of the coating. Input data for this strength. They all effectively measure the ease with which analysis include the film stiffness and level of residual the film becomes debonded in response to an applied stress. An estimate of the film stiffness was made by an stress. Unfortunately, such methods are usually very in situ resonance method, but this is not an accurate sensitive to the initiation event and, hence, to the distri- technique for very thin films on relatively thick bution of flaws. They tend to be useful only as qualitative substrates. (ranking) tests. Ideally, the critical strain energy release rate of the interface, which is a fundamental measure of its tough- 2. Experimental procedures ness, should be evaluated. However, methods available to obtain this [11-14] are not readily applicable to very Diamond films were deposited on two different sub- thin, strongly-adhering films. For example, the four- strates (304 stainless steel and Ti-6A1-4V, referred to point bend delamination test [11, 14, 15] is not viable hereafter as steel and Ti respectively) using a standard if the strain energy stored in the bent coating is insuffi- microwave plasma-assisted chemical vapour deposition cient to drive the advance of the interfacial crack. In technique. Plate specimens of steel with film 1 and 2 ~tm addition, the level of residual stress in the coating should thick and of Ti with film 0.9 and 2 lain thick on one of be taken into account. This affects both the apparent the 30 mm x 10 mm faces were supplied. The plate speci- interfacial toughness (since the release of stored residual mens were cut into "dog-bone" specimens using a spark 0925-9635/94/$7.00 © 1994 -- Elsevier Sequoia. All rights reserved SSDI 0925-9635(93)05 116-T