The effect of fillers on the wear resistance of thermoplastic polymeric coatings

Thermoplastic polymer matrix composites are used as coating materials for the bore of downhole tubulars used as water injectors in the oil industry. These coatings are primarily employed for corrosion resistance but must also resist mechanical damage from the inspection tools lowered at speed down the tubing. This mechanical damage is produced by the wearing action of the supporting wire against the coating (wireline wear) and by direct impact of the tool against the coating. Filler materials are added to these polymeric coatings and these additions are known to affect the wear resistance of the coating. In this study, three types of thermoplastic polymeric coatings were subjected to wear tests. Both abrasive wear tests — using silicon carbide papers as the abrasive, and wireline wear tests, utilising a true tribocouple consisting of the coating and a length of “slickline” wire on a modified pin on disc apparatus — were carried out to study the wear resistance of these three coatings. Detailed scanning electron microscopy was performed on the wear tracks produced to elucidate the wear mechanism and in particular the role of fillers. In abrasive wear a polymer with a brittle filler has a higher wear rate than an unfilled polymer due to the fact that the brittle fillers can be easily fractured and detached from the polymer matrix. In general, the weak bond between the filler and a thermoplastic polymer matrix leads to the filler particles detaching from the matrix causing enhanced wear. In wireline wear the presence of voids and unmelted particles is particularly deleterious.