Transfer layer formation of high performance polymers, polymer blends and polymer composites

Abstract

Polymers and polymer-based composites have previously been used for tribological applications in systems where lubricants are deemed unfit. However, one of the main challenges faced by polymer-based components over the years, is their narrow operating temperature range and their poor resistance to frictional heating. In this work, the performance of two high performance polymers: Polyetheretherketone (PEEK) and Polybenzimidazole (PBI) were evaluated under high temperature and various test conditions. The poor perfor- mance of PEEK composites at temperature above 260 ◦C and the poor processability of PBI are pushing the need for new composites. For this reason, PEEK-PBI blend (TU60) and PEEK-PBI-CF composite (TL60) were investigated for tribological applications. In this thesis, the frictional heating generated during rubbing for the various polymers was studied using a in-situ IR thermography. The validity of this technique was compared to a temperature prediction model. The temperature distribution inside the contact was compared to the tribological and material properties of the polymers. The effect of the transfer layer on the surface temperature was answered. In the present work, the formation of a transfer layer at the tribological interface is of particular interest. This layer is often associated to desirable tribological performance. The formation and degradation mechanism of the transfer layer was analysed using sev- eral surface characterisation techniques such as Scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared spec- troscopy (FTIR), Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to understand the formation mechanism and chemical composition of this transfer layer. The results show the excellent tribological performance of PBI and PBI-PEEK compos- ites under high operating temperatures. In all cases, the formation of a polymeric transfer layer on the steel countersurface was observed. It is demonstrated that frictional heating influences the formation of the transfer layer and that shear stress is the major factor affecting its chemical degradation. Although PEEK and PBI have previously been studied, this work has contributed to access the potential of PBI, PEEK-PBI blends and composites at high operating tempera- ture. This thesis shows encouraging results to further the research on PBI and PBI-based polymers for tribological applications.