Harald Nyberg

Tribochemically Active Ti–C–S Nanocomposite Coatings

We demonstrate a new concept of self-adaptive materials, where sulphur is incorporated into TiC/a-C coatings and may be released in, for example, a tribological contact. By reactive sputtering with H2S, sulphur goes into the carbide to form a TiC x S y phase in an amorphous carbon matrix. The addition of sulphur lowers the friction against steel. Significantly lower friction is obtained against a tungsten counter-surface, as WS2 is generated in the contact. Annealing experiments and formation energy calculations confirm that sulphur can be released from TiC x S y . Ti–C–S coatings are thus chemically active in tribological contacts, creating possibilities of new low-friction systems.

Calculated Trajectories of Curling Stones Sliding Under Asymmetrical Friction: Validation of Published Models

In the sport of curling, stones are slid across an ice sheet, aimed towards a target area. A sliding stone does not move in a straight line, but follows a curled trajectory, deviating in a direction determined by its rotation. As yet, no satisfactory explanation for this motion has been presented, although many attempts have been made. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, typically such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In this paper, the motion of a rotating curling stone sliding over ice is calculated, for different assumed distributions of the coefficient of friction in the contact between stone and ice, using a numerical method. It is shown that no redistribution of the friction, no matter how extreme, can explain the observed motion of a real curling stone.