Deformation and failure of a film/substrate system subjected to spherical indentation: Part I. Experimental validation o
- PDF / 258,635 Bytes
- 9 Pages / 585 x 783 pts Page_size
- 96 Downloads / 180 Views
V. Jayaram Department of Metallurgy, Indian Institute of Science, Bangalore–560012, India
S.K. Biswasa) Department of Mechanical Engineering, Indian Institute of Science, Bangalore–560012, India (Received 27 June 2005; accepted 19 December 2005)
Our concern here is to rationalize experimental observations of failure modes brought about by indentation of hard thin ceramic films deposited on metallic substrates. By undertaking this exercise, we would like to evolve an analytical framework that can be used for designs of coatings. In Part I of the paper we develop an algorithm and test it for a model system. Using this analytical framework we address the issue of failure of columnar TiN films in Part II [J. Mater. Res. 21, 783 (2006)] of the paper. In this part, we used a previously derived Hankel transform procedure to derive stress and strain in a birefringent polymer film glued to a strong substrate and subjected to spherical indentation. We measure surface radial strains using strain gauges and bulk film stresses using photo elastic technique (stress freezing). For a boundary condition based on Hertzian traction with no film interface constraint and assuming the substrate constraint to be a function of the imposed strain, the theory describes the stress distributions well. The variation in peak stresses also demonstrates the usefulness of depositing even a soft film to protect an underlying substrate.
I. INTRODUCTION
It is known that the response of hard coatings to contact loading is a sensitive function of the coating thickness, substrate plasticity, and certain shear/fracture properties of the film which may, in turn, depend on the method of deposition and the residual stress.1,2 Depending on the combination of the parameters, films may display a mechanism of strain accommodation that is non-catastrophic and insensitive to load, or, alternatively, crack at very low loads in a manner that is likely to lead to spalling.3 Because these different domains of behavior have important implications for coating design, it is of interest to see whether such transitions in behavior can be predicted by an analysis of the driving forces for different accommodation mechanisms, One of the key driving forces for failure turns out to be the strain induced in a predominantly elastic film by a plastically deforming
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0094 774
J. Mater. Res., Vol. 21, No. 3, Mar 2006 http://journals.cambridge.org Downloaded: 16 Aug 2015
substrate. While analytical treatments exist for the solution of contact loading of a layered half-space, a full elastic-plastic solution is not available in closed form. The present two-part paper attempts to generate some insight into this field by explicitly writing down the solutions for stresses in an elastic bi-layer subject to Hertzian indentation and validating the estimated stresses and strains in a model system using photo elastic technique and surface strain measurement. In Part II [J. Mater. Res. 2
Data Loading...
