Polymer Brushes for Silicon Nitride-Steel Contacts: a Colloidal Force Microscopy Study

Introduction Polymer brushes have promising characteristics which could lend themselves to the lubrication of hybrid contacts [1]. In this paper the interaction between surface-initiated polymer bushes grown on silicon nitride and a steel counter contact is investigated. The importance of the hybrid contact relies on silicon nitride and its unique material properties which show clear reductions in friction in engineering applications [2]. Although there are many positives for hybrid bearings more can be done, namely that the most lubrication solutions are metal protection driven, and therefore the protection of this contact is usually based on protecting steel. These steel protection mechanisms, such as zinc dialkyldithiophosphates (ZDDPs) and molybdenum dithiocarbamate (MoDTC) contain environmentally unfriendly components such as S and P. The removal of these elements is required due to the effect they have on catalytic converters and increasingly rigorous emissions regulations. Although many polymer brushes have been applied on a variety of surfaces the hybrid contact has not been explored and neither have polymer brushes been applied to silicon nitride for tribological applications. The challenges of testing the hybrid contact in the nanoscale are insuring that a high quality polymer film is created, creating an appropriate steel counter surface, forming brushes in a way that avoids using S and P compounds and forming an appropriate polymer that synergises well with the chosen lubricant. Polymer brushes rely on good solvent to facilitate swelling and therefore better sliding performance. To form high quality brushes atom transfer radical polymerisation (ATRP) is utilised. ATRP is the most popular type of polymerisation due to the relative robustness of the technique. For example, unlike the other techniques, rigorously dry working conditions are not needed and reactions are very tolerant of a variety of monomers, ligands and catalysts [3]. However, in ATRP, a small amount of oxygen can result in a large drop in the rate of polymerisation. In order to overcome this problem and to facilitate the growth of polymer brushes, Activators Re-Generated by Electron Transfer (ARGET) is Introduced by incorporating a readily available source of a reducing agent [4]. Another benefit of ARGET is this technique can significantly reduce the concentration of metal catalysts up to 1000 times to ppm levels. Although polymer brushes repel interactions from sharp AFM probes, it is likely that the high contact pressures will be in excess of what the brush can withstand and cause interpenetration of the brush system resulting in high friction and damage to the brush system [5]. To be able to understand the tribological performance of the polymer brushes under oprtational conditions and without losing the force sensitivity or penetrating the polymer, spherical colloidal probes instead of sharp AFM tips are custom-made and used. One of the benefits of this procedure is that the load is distributed across the sample surface whilst accurately recording the forces therefore making this a ideal technique for the characterisation of the tribological performance of polymer brushes. One of the novelties of this research is nanotribological testing using a steel colliodal probe.