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- Influence of femtosecond laser surface nanotexturing on the friction behavior of silicon sliding against PTFEPublication . Alves-Lopes, Isabel; Almeida, Amélia; Oliveira, Vítor; Vilar, RuiThe aim of the present work was to investigate the influence of laser-induced periodic surface structures (LIPSS) produced by femtosecond laser on the friction behavior of silicon sliding on polytetrafluoroethylene (PTFE) in unlubricated conditions. Tribological tests were performed on polished and textured samples in air using a ball-on-flat nanotribometer, in order to evaluate the friction coefficient of polished and textured silicon samples, parallel and perpendicularly to the LIPSS orientation. In the polished specimens, the friction coefficient decreases with testing time at 5 mN, while it increases slightly at 25 mN. It also decreases with increasing applied load. For the textured specimens, the friction coefficient tends to decrease with testing time in both sliding directions studied. In the parallel sliding direction, the friction coefficient decreases with increasing load, attaining values similar to those measured for the polished specimen, while it is independent of the applied load in the perpendicular sliding direction, exhibiting values lower than in the two other cases. These results can be explained by variations in the main contributions to friction and in the wear mechanisms. The influence of the temperature increase at the interface and the consequent changes in the crystalline phases of PTFE are also considered.
- Influence of laser surface nanotexturing on the friction behaviour of the silicon/sapphire systemPublication . Alves-Lopes, I.; Almeida, Amelia; Oliveira, Vítor; Vilar, R.The aim of the present work was to investigate the influence of textures consisting of Laser-Induced Periodic Surface Structures (LIPSS) on the friction coefficient of silicon under unlubricated conditions, using nanoscale friction tests. The tests were performed on <1 1 1> single crystal wafers of p-doped silicon. Surface texturing was performed by a direct writing technique, using 560 fs pulses of 1030 nm wavelength radiation and parameters allowing large areas with a uniform LIPSS texture to be obtained. The tribological tests were performed in air using a ball-on-flat nanotribometer with 3 mm diameter sapphire balls as counterbodies. The wear scars were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The friction coefficient of polished Si remains approximately constant during the tests and is independent of the applied load, with values in the range 0.12-0.14. The morphology of the wear scars indicated that the predominant material removal mechanism is mild oxidative wear, independently of the testing conditions used, which explains the low values of the friction coefficient measured. The LIPSS texture leads to an increase of the friction coefficient, in particular in the perpendicular sliding direction. The friction coefficient increases with the applied load and is larger for the perpendicular sliding direction than for the parallel sliding direction, the difference increasing with increasing load. The morphology of the wear scars showed that the predominant wear regime remains oxidative wear, but significant contributions of plastic deformation and ploughing appear. This plastic deformation in Si, which is brittle at room temperature, shows that significant surface heating is being caused by the interaction of the asperities in motion and under load. This temperature increase also accelerates surface oxidation and increases the wear rate. Surface temperature calculations showed that the temperature increase is larger for the perpendicular sliding direction than for the parallel one, which explains the difference of friction coefficient observed in the two directions.