Wear resistant solid lubricating coatings via compression molding and thermal spraying technologies

Abstract

This work combines two industrially friendly processing methods in order to create wear resistant and solid-lubricating composite coatings potentially suitable for high load applications. Layered composite coatings were fabricated over wrought stainless steel 444 (SS444) by compression molding a mixture of solid lubricant polymer, polytetrafluoroethylene (PTFE, 80 wt%), and wear resistant polymer, polyimide (PI, 20 wt%), onto iron aluminide (Fe3Al) thermal spray coatings without the need of either primers or adhesives. The fabrication process consisted of three main steps: deposition of the Fe3Al thermal spray coating onto a SS444 substrate and transfer into a metal mold; transfer, compress, and sinter mixed polymeric powder onto the thermal spray coating; and finally, sample cooling to room temperature. This method takes advantage of the high surface roughness of thermal spray coatings, which increases mechanical adhesion of slippery PTFE to the underlying metallic material. Coatings were produced with and without a small amount of graphite (5 wt%) to analyze its impact on sliding and wear properties. Unlike current coating technologies, the thickness of the coatings presented herein can be easily and quickly tailored by varying the amount of polymer powder added to the mold prior to compression or by grinding after fabrication. We produced and analyzed coatings ~1.3 mm in total thickness that portray coefficient of frictions ~0.1, similar to pure PTFE. The calculated wear rates for both coatings with and without graphite are an order of magnitude lower than what has been previously reported for coatings of similar composition. The influence of graphite on wear properties was found to be minimal due to the high content of self-lubricating PTFE yet can act as a way to lower material costs and increase the coatings load capacity.