High velocity thermal spraying of PEEK onto stainless steel

Abstract

Thermal spray is a group of processes widely used in the coating industry. This thesis reports the relationship of the substrate chemistry, morphology and substrate temperature to the properties of Polyether ether ketone (PEEK) single splats and coatings on stainless steel 304 substrates deposited using high velocity thermal spray techniques. Single splats and coatings of PEEK were deposited onto stainless steel 304 substrates with 6 different surface treatments using HVAF, HVOF and AC-HVAF thermal spray techniques. Thermal spraying was done at substrate temperatures of room temperature, 163 °C, 230 °C and 275 °C. Analysis of PEEK splats showed that surface chemistry has a minimal effect on the number of splats adhering with surface roughness being the major determinant. Further to this result for similar roughness value substrates, the skewness parameter had an effect on the adherence of splats. Positive skewness value substrates lead to a reduction in adherence of splats compared to negative skewness value substrates. However positive skewness value parameter substrates had splats of higher circularity than substrates with negative skewness value. Coatings of PEEK in general showed minimal degradation during the thermal spray process. The degradation of the PEEK occurred at either the ether or ketone connecting bonds in the PEEK molecule. Substrate pre-heating had minimal effect on the degradation of the PEEK coating surface. Reducing post deposition flame impingement onto the PEEK coatings reduces the surface degradation. Scratch testing of coatings showed that parameters of 150 mm nozzle length and 300 mm spray distance produced the most cohesive and highest scratch resistant coating. High roughness substrates showed cohesive coating failure. For low roughness value substrate, positive skewness value substrates showed a combination of both adhesive and cohesive coating failure as opposed to complete adhesive failure for substrates with negative skewness value. Salt spray corrosion tests revealed that coatings are permeable to the salt solution. Corrosion products formed on the coating surface, however no de-bonding of the coating from the substrate was observed. Linear wear testing of PEEK coatings showed excellent wear resistance properties. Mechanism of wear was initial ploughing of coating surface resulting in a lubricious wear track after which minimal wear occurs.