The interfacial indentation test to determine adhesion and residual stresses in NiCr VPS coatings G. Marot a,b, ⁎ , Ph. Démarécaux a , J. Lesage b , M. Hadad c , St. Siegmann c , M.H. Staia d a École des Hautes Études d’Ingénieur, Pole de Recherche en Structures et Matériaux, Lille, France b Université des Sciences et Technologies de Lille, Laboratoire de Mécanique de Lille, UMR CNRS 8107, Villeneuve d’Ascq, France c EMPA, Materials Science and Technology, Thun, Switzerland d Universidad Central de Venezuela, Centro CENMACOR, Caracas, Venezuela Available online 8 April 2008 Abstract The interfacial indentation test allows determining the interface toughness of a coating obtained by thermal spraying. During this test, a Vickers indentation is carried out on a cross-section of the sample. A crack is initiated and propagated along the interface. An analytical model allows defining an interface toughness representing the coating adhesion. The objective of this study is to compare this test with other tests (tensile adhesive test, shear test) and to specify its applicability. The residual stresses are also estimated by two different methods. Their influence on adherence is discussed in a third part. Those experiments were conducted on NiCr 80-20 VPS coatings with different thicknesses and roughnesses. In particular, it is shown that the interfacial indentation test is the most universal one and that compressive residual stresses improve coating adhesion. © 2008 Elsevier B.V. All rights reserved. Keywords: Thermal spray coatings; Interfacial indentation; Adhesion; Residual stress; NiCr 1. Introduction Adhesion of thermal sprayed coating is one of the most important parameters to consider. The development of this surface treatment technique depends on this point since more and more severe loadings are submitted to mechanical parts. The adhesion determination is then one of the main problems in scientific and technical laboratories. Since few decades, the interfacial indentation test is developed and started to be employed by different international teams [1–3]. The goal of this work is to confront this test with other standard tests which allows determining of adhesion and residual stresses in the coatings. Adhesion of NiCr 80-20 vacuum plasma sprayed coatings were determined with EN 582 standardized tensile adhesive test, EN 15340 standardized shear test and interfaciale indentation test. Residual stresses were evaluated with the interfacial indentation test results and the curvature method. 2. Experimental methods 2.1. Samples preparation NiCr 80-20 coatings were vacuum plasma sprayed in three different thicknesses (180, 280, 480 μm) on S235 construction steel with three different surface roughnesses (2.3, 4.5 and 6.0 μm Ra). The temperature at the beginning of the spraying process was 350 °C and 500 °C at the end. Particle size of the NiCr powder was - 53 + 20 μm. The nomenclature is presented Table 1. Mechanical properties of coating and substrate were determined by depth-sensing indentation. S235 steel has a hardness of 151 HV, a Poisson ratio of 0.30 and a Young modulus of 200 GPa. NiCr coating has a hardness of 244 HV, a Poisson ratio of 0.31 and a Young modulus of 218 GPa. 2.2. Tensile adhesive test The tensile adhesive test is one of the few standardized tests to determine interface strength. The European EN 582 and American ASTM C 633 norms are the most diffused and this test is largely used in industrial and scientific laboratories. To Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 4411 – 4416 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. École des Hautes Études d’Ingénieur, Pole de Recherche en Structures et Matériaux, Lille, France. Tel.: +33 3 28 38 48 58; fax: +33 3 28 38 48 04. E-mail address: guillaume.marot@hei.fr (G. Marot). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.018