Critical Load Analysis for a Layered Half-Space under Sliding Indentation

Abstract

Analyses are presented for cracking or delaminating problems of thin coatings on dissimilar thick substrate materials under a normal and a tangential force. Based on the energy principle associated with the consideration of substrate deformation, an analytical expression has been proposed to predict the critical load of an adherent stiff thin coating on a compliant substrate. The critical load has been shown to depend on the hardness, the coating thickness, the surface energy of adhesion, the coefficient of friction, contact areas of a scratch track, and elastic moduli of coatings and substrates. Experimental observations of microscratch testing on a CrN coating deposited on a soft coating, labeled as SC-I, over a Cu/Zn alloy substrate have shown that buckling and cracking in a semicircular arc ahead of an indenter are the predominant failure modes, thereby confirming the assumption of the theoretical calculation. Using Beuth's solution, a simple fracture model describing the cracking of thin coatings has been developed over a range of practical elastic mismatches and applied to solve cracking problems of compliant coatings in this work. Microscratch results from another soft coating, labeled as SC-II, on a Cu/Zn alloy substrate have revealed that cracks first occur along the edge of the circular contact area in the rear of the indenter due to the tensile stress. The stress formulae of Hamilton and Bower and Fleck are therefore introduced into this model to compute the critical load required to initiate a crack in the coating and the stress intensity factor of the coating.

Review led by Dong Zhu

KEY WORDS:

• Coating
• Cracking
• Delaminating
• Microscratch
• Failure Analysis

ACKNOWLEDGEMENT

The financial support of this work by the National Science Foundation Industry/University Cooperative Research Center in Coatings at Eastern Michigan University is greatly appreciated. The authors are grateful to the MASCO Corporation R & D for preparation of CrN-SCI-Cu/Zn and SCII-Cu/Zn systems.

Notes

Review led by Dong Zhu