shear strength from (14) for elastic case. Obviously, from figure 3, the energy contributed from plastic deformation is very high. Figure 4 shows the results for the higher hardening thin film material case, N=0.3. The variations of the horizontal driving force with the material shear strength and the interface separation strength are similar to those of the lower hardening material. Clearly, the work dissipated in the plastic deformation for higher hardening material is lower than that for lower hardening material. Figure 5 shows the relation of the driving force (applied work) changing with normalized thin film thickness. The curves are for different direction angles of indenter surface, , , , and 165 . In the figure, the experimental results for the Pt/NiO film/substrate system and for the two kinds of techniques annealed at 500 o 90 β= o 120 o 135 o 150 o oC and 800oC from [4] are also shown. From figure 5, the horizontal driving force increases as thin film thickness increases and as the indenter angle increases. The indenter shape with the largest β corresponds to the high driving force. The simulation results are roughly consistent with the experiment results. CONCLUDING REMARKS By the detailed analyses in the present research, some important conclusions are obtained as follows: (1) Thin film plastic deformation has the important influence on the advance of delaminated film strap in the scratch test. (2) The interface separation strength and material shear strength have important influence on the failure of thin film/substrate system. (3) The horizontal driving force depends on the thin film or coating layer thickness. With the thin film thickness increase, the horizontal driving force increases and asymptotes to a stable value, which corresponds to the small scale yielding case. When either the interface separation strength or the material shear strength is large, a strong shielding effect from plastic deformation can be produced when the failure strengths are increased. In other words, with any cohesive strength increase, it is difficult or even impossible to make a film failure strap advance due to the strong plastic shielding. Such a prediction from using the conventional elastic-plastic theory seems somewhat contradictory. Actually, for the strong separation strength of interface or for the high shear strength, or for both, a strong plastic strain gradient effect could dominate the crack tip fields [10]. A reasonable simulation for this behavior might be obtained by using the strain gradient plasticity theory. A success application of the strain gradient plasticity to the crack growth problem has been shown in [11]. Acknowledgements The work is supported by National Science Foundations of China through Grants 19891180 and 19925211; and jointly supported by Fundamental Research Project from Chinese Academy of Sciences through Grant KJ951-1-201 and "Bai Ren" Project. The work is also supported by NSF Grants CMS-96-34632 in USA. References 1. Blees, M.H., Winkelman, G.B., Balkenende, A.R et al.(2000). Thin Solid Films, 359,1. 2. Pistor, C.and Friedrich, K. (1997). J. Appl. Polymer Science, 66, 1985. 3. Maekawa, H., Ikeda, T. and Horibe, H., et al.(1994). Quart. J. Japan Welding Society, 12, 262. 4. Venkataraman, S., Kohlstedt, D.L. and Gerberich. W.W. (1996). J. Mater. Res., 11, 3133. 5. Thouless, M.D. (1998). Eng. Frac. Mech., 61, 75. 6. Tvergaard, V. and Hutchinson, J.W. (1993). J. Mech. Phys. Solids, 41, 1119. 7. Wei, Y. and Hutchinson, J.W. (1997). J. Mech. Phys. Solids, 45, 1137. 8. Wei, Y. and Hutchinson, J.W. (1998). Int. J. Fracture, 93, 315. 9. Dean, R.H. and Hutchinson, J.W. (1980). In: Fracture Mechanics, ASTM STP700, p.383. 10. Fleck, N.A. and Hutchinson, J.W. (1997). Advances in Applied Mechanics, 33, 295. 11. Wei, Y. and Hutchinson, J.W. (1997). J. Mech. Phys. Solids, 45, 1253.
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