The influence of heat treatment on the microstructure, surface topography and selected properties of PEEK coatings electrophoretically deposited on the Ti-6Al-4V alloy

Abstract

The aim of this work was to fabricate high temperature PEEK 708 coatings on the Ti-6Al-4V alloy substrate using electrophoretic deposition and post-deposition heat treatment, as well as to determine the influence of the heat treatment process on the structure, microstructure, surface topography and selected properties of the coated alloy. The design of the EPD parameters (voltage and time) enabled the deposition of homogeneous coatings. The heat treatment of the as-deposited coatings, carried out above the polymer melting point, changed the polymer morphology from a granular coating into a dense and continuous one. Depending on the cooling rate, the coating exhibited an amorphous or semi-crystalline structure. The semi-crystalline coating consisted of numerous spherulites and had a well-developed surface topography. In contrast, the amorphous coatings were smooth, with lines consisting of individual crystallites. The polymer structure and microstructure significantly influenced the micromechanical properties and scratch resistance of the coatings. Semi-crystalline coatings had higher hardness and Young's modulus as well as better scratch resistance. Tribological properties were investigated using ball-on-disc tests in dry sliding contact with an alumina ball at room temperature and at elevated temperatures, i.e. 150 °C and 260 °C. It was found that the semi-crystalline coatings showed a significantly higher wear resistance and lower coefficient of friction at all the investigated temperatures compared to the amorphous coatings. The resistance to motion and the wear of the coatings increased with the ambient temperature of the friction pair. Electrochemical studies showed that both coatings improved the Ti-6Al-4V alloy's corrosion resistance in a NaCl solution. The improvement was more significant in the case of amorphous coatings.