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FINITE ELEMENT SIMULATION OF INDENTATION BEHAVIOR OF THIN FILMS

D.T. Madsen, R.J. Giovinazzo, and J.E. Ritter, Department of Mechanical Engineering, T.J. Lardner, Department of Civil Engineering, University of Massachusetts, Amherst, MA 01003

ABSTRACT Nanoindentation experiments are now widely used to study the elastic and plastic properties of thin films. Simulation of these experiments has been performed using finite element analysis. Results show the large influence that pile-up or sink-in behavior have on hardness calculations. Results also show that a compliant substrate significantly affects the measured hardness of a stiffer coating. The measured hardness of a compliant coating is less effected by a stiffer substrate.

INTRODUCTION Thin coatings are widely used to protect substrates from damage due to contact stresses, particle impact, and corrosion. In recent years, interest has grown in the use of depth-sensing, nanoindentation instruments to determine the elastic and plastic properties of these coatings. In particular, techniques have been developed to calculate the hardness and elastic modulus of a material from the indentation load-depth data. In this research, the indentation behavior of thin films have been simulated using finite element analysis. These finite element results are then used to determine hardness as a function of indentation depth.

FINITE ELEMENT MODEL Simulations of elastic/plastic indentations were conducted using the ABAQUS finite element code [1). Four-noded axisymmetric elements were used to model the coating and substrate continuum while the conical indenter with either a 70.3 or 68 half-angle was modelled as a perfectly rigid, frictionless surface. The indentation procedure was simulated by displacing the indenter downward into the coating. The indenter load was determined during the indentation process by summing the axial reaction forces along the bottom of the substrate. The indenter contact radius was determined directly from the finite element output. The overall dimensions of the model were large enough to be considered remote, i.e. any further increase in the dimensions made no significant change in the load-depth behavior. If the indentation simulation resulted in severe element distortion under the indenter, the remesh/restart feature of the ABAQUS program was implemented. This feature allows regions of elements experiencing extreme distortion due to the indentation process to be replaced by new, undistorted elements without significantly altering the stress distributions in the model. Both the coating and substrate were modelled as non-linear elastic/perfectly-plastic materials with Young's modulus, E, yield strength, Y, and Poisson's ratio, v.

Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society

432

HARDNESS CALCULATION Hardness, or a material's resistance to penetration by an indenter, is generally defined as the indenter load divided by the projected contact area. Historically, this area is determined by visually measuring the residual impress