Tribological performance of porous silicon hydrophobic and hydrophilic surfaces

Many studies have focused on surface texturing to control friction and wear in the past few decades. The underlying reasons for surface texturing effects under various contact situations remain a source of debate. The connection between surface roughness and friction between two sliding surfaces is more complicated than it looks at first glance. In this paper, samples of a highly resistive p-type silicon were anodized to get various pore sizes ranging from 0.01 up to 3.2 microns using two electrolyte solutions and varying the electric current. The wetting properties were determined via contact angle analysis, while pore size and shape measurements were performed by scanning electron microscopy. The tribological tests were performed on a tribometer (UMT, Bruker) in a reciprocating mode for 15 min with a 3N load, 5 Hz, and a 10 mm diameter steel ball. Wear surfaces were analyzed by optical microscopy and non-contact 3D profilometry. The results showed that low electric currents generate mesoporous silicon surfaces with low friction. Higher currents generate macroporous silicon surfaces, modifying the surface roughness and wetting properties from hydrophilic to hydrophobic and increasing friction. The pore size is directly related to the wear volume and scar diameter measured from optical micrographs and profilometry 3D. The larger the pore size and nanostructured peaks, the greater the contact angle, friction, and wear.