Nature of contact deformation of TiN films on steel
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Z-H. Xie and M. Hoffman School of Materials Science and Engineering, The University of New South Wales, Sydney NSW 2052, Australia
V. Jayarama) Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India
S.K. Biswas Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India (Received 19 October 2003; accepted 18 May 2004)
Nanoindentation experiments were carried out on a columnar ∼1.5-m-thick TiN film on steel using a conical indenter with a 5-m tip radius. Microstructural examination of the contact zone indicates that after initial elastic deformation, the deformation mechanism of the TiN is dominated by shear fracture at inter-columnar grain boundaries of the TiN film. A simple model is proposed whereby the applied load is partitioned between a deforming TiN annulus and a central expanding cavity in the steel substrate. It is possible to obtain a good fit to the experimental load–displacement curves with only one adjustable parameter, namely the inter-columnar shear fracture stress of the TiN film. The implication of results in the context of the performance of TiN films in service is also discussed.
I. INTRODUCTION
Among the many types of nanoindentation experiments that have been carried out on thin films on substrates, the deconvolution of film hardness has posed special problems. Applications have dictated the film– substrate combinations as to whether it is soft on hard or hard on soft. “Soft” and “hard” may be further qualified as to whether these are brittle or ductile. Models to describe and analyze thin film performance have therefore tended to be combination specific1–5 and may be classified into two generic categories; (i) where plastic flow is emphasized and (ii) where fracture is emphasized. The focus of our attention is a combination of a hard and brittle film on a ductile substrate; a common example of which are TiN coatings on steel where one typically encounters films of 1–5 m thickness of a columnar TiN with strong texture, often with the 〈111〉 crystal plane normal to the surface. In such cases, it is routine to extract elastic modulus by fitting depth-sensing indentation curves with low maximum loads. However, significant plastic deformation of the hard coating, particularly in
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0339 2616
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 9, Sep 2004 Downloaded: 14 Mar 2015
thin (1 m) coatings, is very likely preceded by irreversible deformation of the substrate even at a penetration depth (normalized by the coating thickness) of 0.05.6 The premise of the present work is that, to the best of our knowledge, there has been no attempt to link microstructural observations which reveal the nature of deformation beneath an indentation to a model that satisfactorily reproduces, not just the final contact size after unloading, but the full load–displacement response of the film– substrate combination in TiN films on a ductile subs
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