Cavitation resistance of epoxy-based multilayer coatings: Surface damage and crack growth kinetics during the incubation stage G.L. García a , V. López-Ríos b , A. Espinosa a , J. Abenojar c , F. Velasco c , A. Toro d a Computational Mechanical Design Group, National University of Colombia, Medellín, Colombia b Statistical Research Group, National University of Colombia, Medellín, Colombia c Materials Science and Engineering Department, Universidad Carlos III, Madrid, Spain d Tribology and Surfaces Group, National University of Colombia, Medellín, Colombia article info Article history: Received 5 September 2013 Received in revised form 8 April 2014 Accepted 9 April 2014 Available online 28 April 2014 Keywords: Cavitation Epoxy-based coatings Instrumented microindentation H/E ratio abstract Four epoxy-based multilayer coating systems, with thicknesses of 380 720 μm, 650 710 μm, 720 730 μm and 920 720 μm, were applied manually onto stainless steel samples and subjected to vibratory cavitation tests according to ASTM G32-09 standard. In order to correlate the cavitation resistance of the coating systems with some of their mechanical properties, instrumented micro indentation tests were performed to determine hardness, resilience, total plastic work, among others, as a function of the thickness of the coatings. Examination of the surfaces by Scanning Electron Microscopy (SEM) revealed that the surface damage in all the coatings was caused by incubation and growth of cracks. Statistical analysis of crack growth data allowed determining a behavior law characteristic for each coating system, which was adjusted with proper parameters related to the mechanical properties measured by micro indentation. In particular, a good correlation was obtained among cavitation resistance, coating thickness and hardness- to-Young modulus ratio H/E. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Cavitation is a complex process that includes the steps of nucleation, growth, coalescence, collapse and successive rebound of bubbles and/or clusters of vapor and/or gas in a liquid when varying its thermodynamic and hydrodynamic conditions during short periods of time. Although this phenomenon has been studied for over a century it is not yet completely understood since it involves multiple variables and parameters related to the fluid, the surface and the testing technique [1–4]. When cavitation occurs close to the surface of a solid, it causes localized damage due to the high impact pressures that exceed the yield strength of the material and/or as a consequence of the fluctuating stresses that promote surface fatigue [4,5]. The surface damage mechanisms that take place during cavitation are divided into two main types, namely microjet impact and shock wave formation [6–8]. A complete agreement among researchers regarding the relative importance of each of these two mechanisms has not been achieved, although it is commonly accepted that when the bubble and/or cluster of bubbles collapse on or close to the solid surface both mechanisms are present, causing a synergistic effect [7–9]. Several authors have found that, when acting separately, the damaging effect of microjets and shock waves can be reasonably predicted, but due to surface's anisotropy and inevitable variations of pressure and velocity within the fluid the size and shape of the affected areas vary significantly, so comprehensive models for cavitation damage are still to be developed [8,10,11]. Similarly, it has not been possible to establish a general model to predict cavitation damage that can be applied to different families of materials, due to the fact that each material exhibits a characteristic behavior and the pressure distribution on the surface varies with position and time. Besides the experimental setups available in flow systems and vibratory equipment for cavitation accelerated testing, other methods and techniques to create appropriate conditions for cavitation were also used in the laboratory as well as in the field, but unfortunately the results of the different tests are not fully comparable to each other [12–14]. Experimental correlations between mechanical properties and cavitation resistance measured in terms of mass and/or volume loss when the material has completed its incubation period have been previously proposed by several researchers [5,12,15]. A number of mechanical properties such as hardness, yield strength and ultimate tensile strength have been measured, in most cases, by techniques traditionally developed for bulk material testing [15–17] and eventually through surface characterization techni- ques such as instrumented indentation, being a number of metals [18,19] and some hard coatings [20,21] the most common objects of study. However, the correlations between mechanical properties Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2014.04.007 0043-1648/& 2014 Elsevier B.V. All rights reserved. Wear 316 (2014) 124–132