Wettability of PVD compound materials by lubricants

The wetting properties of coolants are important for tribological systems. In order to optimize the wettability on steel alloys coolants have been modified for decades. Using PVD-coatings for wear and corrosion resistance the thermodynamical interactions between coolant and surface have been changed. The characteristic factor, which affects interfacial interactions such as adsorption, wetting and adhesion, is the surface free energy. Test results indicate that all PVD hard film systems have much lower surface energies than uncoated low-alloyed steels. The surface free energy of solids and liquids can be examined by using a drop shape analysis where, due to spreading, the contact angle of coolants on metals is not measurable. In this work a theoretical solution is presented. The wetting behaviour can be calculated and is given as the spreading coefficient S. Results of these theoretical equations have been verified by experimental tests.