Scratch and wear characteristics of polyamide nanocomposites

1. Introduction Polymer nanocomposites have found wide application replacing metals in aerospace components, sporting goods and in the automotive industry; the advantages of polymers compared with metals are low cost, low density, vibration damping and ease of processing. Other research applications of nanocomposites include improving ballistic performance, high temperature applications, electronics, fire retardant coatings, reduction in flammability on coating systems and biodegradable polymers. The objective of this study was to analyze the effect of nano and micro particles on the tribological and mechanical performance of several types of composites prepared by extrusion mixing of the same polyamide PA66 matrix with varying quantities of nanoclay (NC, 1.75 and 3.50 wt%), polyhedral sislesquioxanes (POSS, 0.5 and 5.0 wt%), fullerene (0.7 wt%) and also micro-sized glass spheres, both hollow (GB7, 7wt%) and solid (GB20, 20wt%). 2. Results and discussion The inclusion of fillers did not change the matrix’s degree of crystallinity, as proved by differential scanning calorimetry. The particle dispersion was investigated by means of scanning electron microscopy and found to be quite uniform in all samples. Tensile and compressive mechanical properties were determined. A moderate increase of the elastic modulus and yield stress was reported for the composites with respect to the unfilled matrix. Unfortunately, this reinforcing effect was counterbalanced by a severe loss of ductility for the inorganic fillers (i.e. all but fullerene). Scratch tests were performed at constant load and sliding speed on a microscratch tester. Scratch resistance, was evaluated using a method already adopted on unfilled polymers. In this previous work a strong correlation between scratch hardness and the material’s compressive yield stress had been identified. In the present one, the same trend was observed. The friction and wear characteristics (against steel) of the investigated materials were measured on a thrust washer machine, applying constant load and rotating speed. While the friction coefficient was almost unaffected by filler addition, different wear phenomenology and different wear rates were displayed by different filler types: the wear rate (normalized by pressure and speed to give the wear factor) decreased substantially in the case of NC and POSS. 3. Conclusions The results confirm that, for these kinds of particles, the scratch hardness is directly linked to the effect their presence has on bulk mechanical properties (in particular on the compressive yield stress). On the other hand, fillers influence wear phenomena in a complex way and specific interactions need to be taken into account in order to have an accurate description of the wear process.