Tribological performance of high density polyethylene (HDPE) composites with low nanofiller loading

Abstract

High density polyethylene (HDPE) nanocomposites were prepared by melt mixing of the polymer with 0.5–1.0 wt-% hydrophobic surface treated fumed silica and titanium nitride (TiN) nanopowders, fiber-shaped halloysite nanotubes (HNT), and graphene oxide (GO) nanoplatelets in the presence of trace amount of organic peroxide. Surface treatments of the nanofillers with hydrophobic vinyltrimethoxy silane (VTMS) were accomplished in boiling toluene solvents and in the case of fumed silica, most feasibly in ethanol solvent. Sliding friction and wear of the nanocomposites were characterized by pin-on-disk (POD) test against polished bearing steel disc. Microstructures of the nanocomposites, and the wear surfaces were characterized by scanning electron microscopy (SEM). Microindentation experiments were carried out to measure the surface hardness, elasticity and the plastic and elastic work of indentation. VTMS surface coverage was highest on fumed silica and GO powders, which also correlated with the most homogeneous and fine filler dispersion in HDPE polymer, and showed the best performance of the nanocomposites in the POD test. In all cases, the addition of silane treated nanopowders reduced the sliding wear of the HDPE against polished steel counter surface. Moreover, deviation of the wear rate data between parallel measurements was greatly reduced by the nanofiller addition indicating homogenous structure. The coefficient of friction (COF) of the neat polymer was reduced only in the case of TiN filler. Inverse linear trend was found between the specific wear rate of the nanocomposites and a parameter derived from micro-indentation data (HIT*total work of indentation)−1, which is formally similar to the well-known Ratner-Lancaster correlation.