Application of PEEK Coatings to C/PEEK Substrates by Plasma-Spray Process

Abstract

This article describes the development of a method for applying PEEK coatings to C/PEEK substrates by plasma spraying. Uniform, well-bonded PEEK coatings have been achieved using this technology. The process temperature is a critical parameter. "Hotter" coatings exhibit less porosity and better adhesion than "colder" coatings. No significant degradation of the PEEK has been observed as a result of the plasma-spray process as verified by DSC and TGA. Use of a plasma postheat cycle improves the uniformity and surface finish of the coating by reducing the number of unmelted PEEK particles on the surface of the coating. Use of a plasma preheat cycle provides a 52% increase in tensile bond strength of the coating. Plasma and reactive gas surface treatments do not appear to have any significant effect on bond strength when used in conjunction with a plasma preheat cycle. The optimized plasma-spray parameters developed in this study produce PEEK coatings 0.254 to 0.381 mm thick which are black in color and exhibit a moderate degree of porosity which is evenly distributed. Individual pores are isolated rather than interconnected and do not vary in size or distribution with substrate thickness, postheating, or annealing. No interface is visible between the PEEK coating and the C/PEEK substrate at magnifications up to 500 x. Tensile bond strengths of 13.37 ± 0.83 MPa have been measured on 3.175 mm thick C/PEEK substrates and similar tests on 12.7 mm thick C/PEEK substrates have yielded values of 24.28 + 0.65 MPa. The as-sprayed PEEK coatings are essentially amorphous and require an annealing treatment in order to ensure a reasonable degree of crystallinity. This treatment does not affect the bond strength of the coating.