The wetting of steel, DLC coatings, ceramics and polymers with oils and water: The importance and correlations of surface energy, surface tension, contact angle and spreading

The importance of wetting is becoming increasingly obvious and its control is inevitable in many engineering applications, including tribology and interface nanotechnology. However, the relations between the key parameters affecting surface-liquid wetting behaviour under realistic conditions are not very well understood, especially for typical engineering materials and lubricants (oils), often leading to exceptions and contradictions, which impede their use in engineering models and theories, and so the possible optimisation of the interfaces of engineering systems. In this paper we present the correlations between the contact angle, the spreading, the surface tension and the surface energy of fourteen frequently used engineering materials belonging to four different classes of materials (steel, DLC coatings, ceramics, and polymers) wetted with four different liquids: three oils (a non-polar synthetic oil of two different viscosities and a polar natural-based oil) and water. The results represent systematically and consistently obtained data about the wetting-relevant parameters of the selected materials and lubricants and numerous correlations between them. However, the most striking result suggests that the spreading parameter correlates very linearly with the surface energy for all the materials and liquids studied, in both the adhesion-wetting and spreading-wetting regimes. The experimentally determined spreading vs. surface energy correlation functions that appear generally valid for a broad range of properties of the materials and oils can thus be applied as an engineering tool to tailor and design the required/desired wetting performance and nature of the solid-liquid interfaces. The spreading parameter SP - in contrast to the contact angle - was found to be a reliable and relevant parameter for describing the wetting of oils with selected engineering materials. (C) 2013 Elsevier B.V. All rights reserved.