Carbon nanotubes represent superior multifunctional nanofillers in polymer nanocomposites, as they enable excellent mechanical reinforcement, electrical conductivity and other similar properties. Using a nanoindenter, we studied the influence of the concentration of single-walled carbon nanotubes on mechanical strengthening of polymer nanocomposites made of low-density polyethylene and high-density polyethylene. The results showed that the geometric entanglement of carbon nanotubes leads to the additional mechanical reinforcement when the critical mass fraction of the nanofiller (0,7 % mass.) is exceeded. The best improvement was found at about 4 %wt. of nanofiller, where the modulus of elasticity, compared to the same material without any nanofillers increases by 274 % and the hardness by 246 % for low-density polyethylene. For high-density-polyethylene the modulus of elasticity, compared to the same material without any nanofillers increases by 70 % and the hardness by 73 %. At 5 %wt. of the added nanofiller, the agglomeration occurs, which is evident from the decrease of mechanical properties. The concentration of the nanofiller has no significant effect on the surface roughness. With the scratch test we obtained the surface roughness, the maximum load, the depth of the load, the cross-section.
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