Friction-induced intermittent motion affected by surface roughness of brake friction materials

• The normal contact stiffness of the friction material was significantly affected by the surface roughness and proportional to the tangential contact stiffness.
• The normal contact stiffness affects the shape of the stick-slip profiles.
• The friction force oscillation goes through transitions from stick-slip to sinusoidal oscillation and to steady sliding as the sliding velocity is increased.

The effect of the surface roughness of the brake friction material on friction-induced instability was studied. Commercial friction materials with different surface roughness levels were tested using a scale brake dynamometer to find a correlation between the surface roughness and friction oscillation. The results showed that the normal contact stiffness was strongly affected by surface roughness and played a significant role in determining the friction oscillation pattern. The friction force oscillation changed from stick-slip to sinusoidal oscillation and steady sliding as the sliding velocity increased, and the velocity ranges for different oscillatory patterns were changed by the surface roughness. A smooth surface with glazed patches produced friction oscillation with larger amplitudes, and the oscillation was maintained over a wider range of sliding velocities due to the high contact stiffness. The correlation between the contact stiffness and surface roughness suggested that friction-induced instability might be avoided by designing the friction material to have low surface stiffness.