A comparative study of the wear resistance of thermoplastic and thermoset coatings

Thermoset and thermoplastic polymeric materials are used as coatings on the bore of downhole tubulars such as water injectors in the oil industry. Previous work has indicated that such coatings can fail by impact damage and by abrasion from the wire used to lower inspection tools down the tubular. The wire is commonly called “slickline” and the type of wear is called “wireline wear”. However, the different wear mechanisms of thermoset and thermoplastic polymeric coatings under wireline wear conditions have not been clarified. Filler materials such as calcium silicate, calcium fluoride and alumina are often added to polymeric coatings to enhance the mechanical properties of the matrix materials; nevertheless, fillers can improve the wear resistance or exacerbate the wear rate of polymeric coatings depending on the characteristics of the filler material such as its shape, concentration, and the boundary condition between the filler and the matrix material. In this study, two types of thermoset polymeric coatings, a modified novolac containing calcium silicate fillers and a modified epoxy containing alumina as the filler, and one type of thermoplastic coating, a fluoropolymer with 2% calcium fluoride filler, were selected for wear tests. The disc specimen in a pin-on-disc (POD) apparatus was modified to enable embedding a circular loop of wire into its surface. That arrangement was used to study the effect of normal force and sliding distance on wireline wear of the three polymeric coatings. Detailed scanning electron microscopy (SEM) was carried out on the wear tracks produced to investigate the wear mechanisms. Energy dispersive X-ray (EDX) spectroscopy was used to X-ray map the wear scars so as to quantify the amount and size distribution of filler present in the wear scar compared to that in the bulk material and thus elucidate the role of fillers in the wear mechanism.