Origin of frictional ageing by molecular dynamics simulation of a silicon tip sliding over a diamond substrate

In this paper, frictional ageing is investigated through simulating a silicon tip sliding over a diamond substrate with a molecular dynamics model. It is demonstrated that contact strengthening in ultra-high vacuum is mainly caused by surface dimerization. With the increase of temperature, tip-substrate contact evolves from incommensurate to commensurate due to the lattice transition from $\left(1\times 1\right)$ to $\left(2\times 1\right)$ lattice ordering. The combination effects of contact strengthening and thermal lubrication lead to a nonmonotonic variation of the mean friction force with temperature rise. However, the friction increase trend induced by the surface dimerization can be suppressed efficiently with the structural lubrication.     

Sputtered Si-containing low-friction carbon coatings for elevated temperatures

This work presents a tribological study on three sputtered amorphous carbon-based coatings containing Si and Cr (a-C, a-C:Cr and a-C:Si). Molecular dynamics simulations predict tetrahedral bonds between C and Si in the a-C matrix. Ball-on-disk-tests against Al₂O₃ carried out at room temperature revealed a coefficient of friction of 0.08–0.1 for all films. Between 250 and 325 °C, Si decreases the COF and wear rate to $<0.05$ and $<5\times {10}^{-17}{\mathrm{m}}^3/\mathrm{N}\times \mathrm{laps}$, respectively. The a-C reference shows a COF of 0.15±0.05 and a wear rate of $1\times {10}^{-16}{\mathrm{m}}^3/\mathrm{N}\times \mathrm{laps}$, whereas the a-C:Cr film failed. The improved tribological performance of a-C:Si expands its application temperature to 450 °C and is most probably related to formation of Si-compounds on the film surface, as evidenced by X-ray photoelectron spectroscopy.     

Elastic–plastic contact analysis of an ellipsoid and a rigid flat

The work presents a finite element model (FEM) of the equivalent von-Mises stress and displacements that are formed for the different ellipticity contact of an ellipsoid with a rigid flat. The material is modeled as elastic perfectly plastic and follows the von-Mises yield criterion. The smaller the ellipticity of the ellipsoid is, the larger the depth of the first yield point from the ellipsoid tip happens. The FEM produces contours for the normalized normal and radial displacement as functions of the different interference depths. The evolution of plastic region in the asperity tip for a sphere (k_e=1) and an ellipsoid with different ellipticities $(k_e=\frac{1}{2}\mathrm{and}\frac{1}{5})$ is shown with increasing interferences. It is interesting to note the behavior of the evolution of the plastic region in the ellipsoid tip for different ellipticities, k_e, is different. The developments of the plastic region on the contact surface are shown in more details in Fig. 7. When the dimensionless contact pressure is up to 2.5, the uniform contact pressure distribution is almost prevailing in the entire contact area. It can be observed clearly that the normalized contact pressure ascends slowly from the center to the edge of the contact area for a sphere (k_e=1), almost has uniform distribution prevailing the entire contact area for an ellipsoid $(k_e=\frac{1}{2})$, and descends slowly from the center to the edge of the contact area for an ellipsoid $(k_e=\frac{1}{5})$.     

Effect of the reinforcement (carbon or glass fibres) on friction and wear behaviour of the PEEK against steel surface at long dry sliding

PEEK (poly-ether-ether-ketone) is a high performance engineering semicrystalline thermoplastic. PEEK has excellent tribological behaviour, which is optimised in the specially formulated tribological composite grade.This paper presents a comparative study of wear and friction on PEEK, PEEK-CF30 (wt%) and PEEK-GF30 (wt%) against steel, at long dry sliding. A plan of experiments was performed on a pin-on-disc machine, under the following conditions $pv=2\text{MPa}\text{m/s}$ (p = 8 MPa and $v=0.25\text{m/s}$; p = 2.68 MPa and $v=0.75\text{m}\text{/}\text{s}$) at the ambient temperature for a sliding distance of 15 km.PEEK-CF30 presented the lesser friction coefficient followed by PEEK. PEEK-GF30 presented the higher friction coefficient throughout all sliding distance. Both PEEK-CF30 and PEEK-GF30 have presented an excellent wear resistance relatively to PEEK while PEEK-CF30 presented the best tribological behaviour.