Journal article

A new method to evaluate the fracture toughness of thin films

A method based on microindentation is developed to determine the fracture toughness K-1c of a thin film bonded to a brittle substrate. Using an easily fabricated sample having a partially coated substrate, indentation on the uncoated portion of the substrate is used to generate a radial crack that propagates into and away from the coated region. Comparison of the lengths of the surface traces of the indentation crack into and away from the coated region is used to measure the toughening imparted by the coating. This test method avoids the complicating effects of delamination that often occur when the coating is indented directly. To extract quantitative results, a 3D finite element model of the system geometry is generated and a cohesive zone model is used to predict the complex equilibrium crack front. The model predicts the length of the crack penetration into the coating vs. the length of the crack growth away from the coating as a function of the elastic and toughness properties of the coating and the substrate, and the residual thermal stresses, which play an important contribution to the detailed crack growth. An approximate analytic model using energy balance ideas is developed to permit easy determination of the coating toughness from experimental data, and the model agrees well with full numerical results over a wide range of coating and substrate property values. The overall method is applied here to determine the toughness of a thin CVD diamond coating on a thick silicon single crystal substrate, and a coating toughness of K-1c = 8.4 MPa m(1/2) is obtained using a previously measured biaxial tensile coating residual stress of 1 GPa. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


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