A. P. M. Baptista

Tribological behaviour of food grade polymers against stainless steel in dry sliding and with sugar

Various food grade polymers 1 were tested to determine their tribological performance in dry sliding against AISI 316 stainless steel. Tests were made with a block on ring tribometer in order to measure the friction coefficient and the wear rate, under three different dry friction testing conditions. It was possible to conclude that, under the conditions used the polymer ultra high molecular weight polyethylene (UHMW-PE) green presents very low friction coefficient and wear rate values. The contact surface of the stainless steel rings and polymers was observed by optical microscopy and scanning electron microscopy to evaluate the involved wear mechanisms. Stainless steel and polymer wear debris were detected, which leads to the conclusion of the presence of wear mechanisms such as: adhesion, abrasion and thermo-oxidative wear. Another aim of this work was prompted by a real application in a sugar conveyor, in order to find which of the tested polymers would better resist to the wear caused by the insertion of sugar at the interface. Using the same block on ring tribometer, but modified to insert sugar at the contact zone, new tests have been performed under the same conditions. It was verified that the polytetrafluoroethylene (PTFE) wear debris form a barrier to the sugar inlet, while polyethylene terephthalate (PETP), although not preventing the sugar entrance into the interface, showed the lowest wear rate among all the polymers tested. The ideal polymer could be a composite of PETP and PTFE in such a percentage that the friction coefficient was reduced and it was able to form an efficient barrier to sugar entrance at the interface.

Tribological behaviour of CVD diamond films on steel substrates

Diamond films have been prepared by microwave plasma chemical vapour deposition (MPCVD) on high-speed steel substrates pre-coated with an intermediate metal multi-layer system. Wear tests were performed in a micro-abrasion apparatus by a rotating ball, using a slurry of diamond abrasive particles in water (sphere-on-flat geometry). The tests were carried out on the bare steel substrate, on the substrate provided with the multi-layered system and, finally, on the diamond-coated samples. The worn surfaces were characterised by scanning electron microscopy (SEM) and micro-Raman spectroscopy, in order to identify the wear mechanisms involved. Wear data are compared and discussed. CVD diamond films have revealed an excellent performance for medium film thickness and moderate normal loads.