Boundary lubrication of stainless steel and CoCrMo in aqueous systems

Oil-based lubricants are widely used in many mechanical applications, but they cannot be used for applications with a high risk of polluting the environment or for applications that involve a bio-medical environment. Water-based lubricants can be used as alternative because they can potentially overcome these problems while maintaining the required tribological performance. A water based environment requires interacting surfaces that combine hardness, wear resistance and corrosion resistance. Typically, stainless steel and CoCrMo alloys are used for such tribological applications including food processing equipment as well as implants. In this thesis, a new concept of aqueous lubrication, i.e. lubrication by hydration of surface active polymers combined with graphene oxide from water or an oil-in-water (O/W) emulsion, is presented. Based on the literature review and the lubrication concept, this work started with the interaction of bearing steel, stainless steel and CoCrMo with several newly developed additives. Then the aqueous lubrication by using O/W emulsions incorporating the additives was examined. The friction profile of the O/W emulsion was quite different to that of the oil lubricant. It exhibited three stage frictional behaviour, including running-in, water dominated status and oil dominated status. Secondly, the CoCrMo alloy was coated with surface-active polymers PAA and PEG and graphene oxide layers. Enhanced friction reducing capability was found in water based fluids for the polymeric coatings in combination with a graphene oxide. Finally, an amphiphilic coating, PEG-lactide, was combined with graphene oxide and used with the O/W emulsions for CoCrMo. The tribological performance of PEG-lactide coating in O/W emulsion was enhanced further compared to the performance of the PEG coated surfaces: a clear indication of the advantage of using hydrophilic and lipophilic group containing surface-active polymers for emulsion lubrication. The overall maximum reduction in friction that was achieved for a sliding contact of coated engineering surfaces from CoCrMo at low sliding velocity and moderate contact pressure was of about 63 % compared to uncoated CoCrMo sliding in water at the same operational conditions.