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indade Escola Superior de Tecnologia, Instituto Polite´cnico de Viseu, 3500 Viseu, Portugal
L.F. Menezes and A. Cavaleiro Departamento de Engenharia Mecaˆnica, Faculdade de Cieˆncias e Technologia da Universidade de Coimbra, Centro de Engenharia Mecânica da Universidade de Coimbra, Pinhal de Marrocos—Polo II, 3030 Coimbra, Portugal (Received 21 December 1999; accepted 23 May 2000)
Depth-sensing indentation tests were used to determine the hardness of amorphous W–C–Co coatings deposited on different steel and copper substrates. The hardness of the film, Hf, was chosen to be always greater than the hardness of the substrate Hs and within the range Hf/Hs ⳱ 2 to 18.5. The influence of the ratio Hf/Hs on the ratio (t/hD)C between the film thickness t and the critical value of the indentation depth (hD)C , for which the substrate starts to deform plastically, was studied. Two independent methods were used to determine (hD)C values. One utilized the differential analysis of the loading part of the indentation curve, and the other was based on the plot of (Hc − Hs)/(Hf − Hs) versus t/(hD), Hc being the measured hardness of the film/substrate composite at a given indentation depth (hD). A good correlation between both methods was found. I. INTRODUCTION
The mechanical behavior of thin coatings is usually characterized through indentation hardness tests, particularly using depth-sensing indentation equipment. One of the main problems associated with this test is the determination of the individual contributions of the substrate and the thin film to the measured hardness.1–20 If the ratio of the indentation depth to the thickness of the film is sufficiently small, the behavior observed is due only to the properties of the coating. For very thin films, the maximum indentation depth, which ensures no contribution from the substrate to the measured hardness, can be quite small. In this case, the hardness equipment may not allow us to perform accurate hardness tests down to such a low load, as is the case of ultra-microindentation equipment, which is widely used in industry. In industrial applications, the evaluation of the individual contribution of the substrate and the thin film to the composite hardness is a matter of crucial importance. Most of the research work on the behavior of coated samples shows how the mechanical properties of the film strongly influence the hardness behavior of the film/ substrate composite (see Refs. 14 and 20, for example). This paper focuses on the influence of the properties of the substrate on the composite behavior for the particular case where the film is harder than the substrate. Ultra1766
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 15 Mar 2015
microhardness measurements were carried out on different substrates coated with the same type of film but with different thicknesses. W–C–Co films were sputter deposited on six different substrate materials, and the behavior of the coated samples under different indentation loads was analyzed.
II. EXPERIMENTA
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