Ž . Thin Solid Films 381 2001 143154 Indentation-induced interface delamination of a strong film on a ductile substrate A. Abdul-Baqi, E. Van der Giessen  Delft Uni  ersity of Technology, Koiter Institute Delft, Mekelweg 2, 2628 CD Delft, The Netherlands Received 15 January 2000; received in revised form 12 July 2000; accepted 12 July 2000 Abstract The objective of this work is to study indentation-induced delamination of a strong film from a ductile substrate. To this end, spherical indentation of an elastic  perfectly plastic substrate coated by an elastic thin film is simulated, with the interface being modeled by means of a cohesive surface. The constitutive law of the cohesive surface includes a coupled description of normal and tangential failure. Cracking of the coating itself is not included and residual stresses are ignored. Delamination initiation and growth are analyzed for several interfacial strengths and properties of the substrate. It is found that delamination occurs in a tangential mode rather than a normal one and is initiated at two to three times the contact radius. It is also demonstrated that the higher the interfacial strength, the higher the initial speed of propagation of the delamination and the lower the steady state speed. Indentation load vs. depth curves are obtained where, for relatively strong interfaces, the delamination initiation is imprinted on this curve as a kink.  2001 Elsevier Science B.V. All rights reserved. Keywords: Adhesion; Coatings; Hardness; Interfaces 1. Introduction Indentation is one of the traditional methods to quantify the mechanical properties of materials and during the last decades it has also been advocated as a tool to characterize the properties of thin films or coatings. At the same time, for example for hard wear- resistant coatings, indentation can be viewed as an elementary step of concentrated loading. For these reasons, many experimental as well as theoretical stud- ies have been devoted to indentation of coated systems during recent years. Proceeding from a review by Page and Hainsworth  Corresponding author. Tel.: 31-15-278-65002150; fax: 31-15- 278-2150. Ž E-mail address: e.vandergiessen@wbmt.tudelft.nl E. Van der . Giessen .  1 on the ability of using indentation to determine the  properties of thin films, Swain and Mencik 2 have ˇ considered the possibility to extract the interfacial en- ergy from indentation tests. Assuming the use of a small spherical indenter, they identified five different classes of interfacial failure, depending on the relative Ž properties of film and substrate hardbrittle vs. duc- . tile , and the quality of the adhesion. Except for elastic complaint films, they envisioned that plastic deforma- tion plays an important role when indentation is con- tinued until interface failure. As emphasized further by  Bagchi and Evans 3 , this makes the deduction of the interface energy from global indentation load vs. depth curves a complex matter. Viable procedures to extract the interfacial energy will depend strongly on the precise mechanisms in- volved during indentation. In the case of ductile films on a hard substrate, coating delamination is coupled to 0040-609001$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 0 - 6 0 9 0 00 01344-4