Valence band XPS and FT-IR evaluation of thermal degradation of HVAF thermally sprayed PEEK coatings

Coatings of PEEK (poly-ether-ether-ketone) have been produced on stainless steel 304 using the High Velocity Air Fuel (HVAF) thermal spray technique. These coatings were produced using 50 and 100 mm nozzle lengths with 200, 300 and 400 mm gun to substrate distances. The thermal degradation of the PEEK during the production of the coatings was assessed with valence band X-ray Photoelectron Spectroscopy (XPS) and attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR FT-IR). Valence band XPS shows that in general there is minimal degradation of the PEEK during the HVAF thermal spraying process. The FT-IR results show that at the 200 mm gun to substrate (standoff) distance for both nozzle lengths there is more surface degradation of the PEEK coating than at the longer gun to substrate distances. Specifically absorption bands appeared at 2918 and 2850 cm⁻¹, which correspond to alkane -CH₂- asymmetric stretching modes. The reduction of the 1250 cm⁻¹ band as compared with the 1220 cm⁻¹ band, both representing the stretching modes of the ether bonds in PEEK suggests that the degradation occurs at only one of the ether bonds. The phenyl C-H vibration at 673 cm⁻¹ was split for coatings produced at gun-substrate distance of 200 and 300 mm. This indicates a structural change in the phenyl ring possibly indicating a change in the extent of crystallinity of the PEEK polymer.