Experimental Study on Laser Surface Texturing for Friction Coefficient Enhancement

Laser surface texturing of alloy steel 42CrMo4+QT (AISI 4140) was studied in order to significantly increase the friction coefficient of tribological systems. Two different surface topologies were investigated to functionalize the contact surface, such as dimple-shaped micro structures and deep welding dots. The dependencies of the laser processing parameters on the characteristics of these micro-scaled surface structures were evaluated. The coefficient of friction mu, the static coefficient for dimple-shaped and kinetic for dot textures respectively, was determined in experimental torsion tests. Dimple-shaped micro structures were processed by using q-switched near-infrared laser pulses. The dimple diameter, wall height, and wall hardness were analyzed depending on both pulse energy and laser spot size. The maximum static coefficient of friction of dimple-shaped specimen was measured to be mu = 0.53, tested against fine grinded steel surfaces. This is almost twice the friction coefficient of the non-laser processed fine grinded reference contact surface which was determined of mu = 0.3. In a second approach, singular deep welding dots were fabricated by using high-power continuous wave laser radiation emitted by a single mode fiber laser. The geometric dimensions of these lenticular dots were studied depending on irradiated laser power and laser exposure time. Torsion testing of deep welding dot patterned surfaces indicated steadily increasing slipping curves with a maximum kinetic coefficient of friction of mu = 0.82 as obtained with the largest investigated torsion angle of 3 degrees.