Squealing occurrence of worn brake pads to foreign particles embedment into the friction layers

Abstract

The disc brake squeal is a very annoying sound and a source of considerable discomfort that leads to customer dissatisfaction. There are various possible mechanisms that could trigger brake squeal generation either from a structural dynamics or tribological point of view. Unlike drum brake design, the disc brake assembly, particularly the disc and the pads are exposed to any unwanted road particle, wear debris and water spray. Their presence into the disc and pad interfaces may create dynamic and physics phenomena induced by friction surface changes which lead to the brake noise and vibration issues. Thus, the objective of this research is to investigate a characterization of the worn surface of squealing brake friction material with the effect of different sizes of foreign particles using laboratory scale brake test rig. The correlation between squeal generation and tribological characteristics of the pad including surface topography, surface roughness, wear, element composition and friction coefficient are established by using squeal index and qualitative analysis. The foreign particles (silica sand and road sand particles) with the sizes of 100-150, 200-300 and 300-400 um are introduced into the brake disc and pad interfaces. The sensitivity of sand particles in producing squeal noise is also examined at different brake pressures, disc temperatures and speeds. The experimental results show that both sand particles have a significant effect on the brake squeal occurrences. The tribological properties reveal that squeal is more affected by smaller sand particle size. The micrometric particles act as punctual contact surfaces generating more wear debris which are accumulated and compacted inducing a reduction of the friction level. However, the biggest particle size damages the pad surface, reduces the real contact surface and decreases the friction coefficient, yet generates more wear lost. It was found that foreign particles play an important role in reducing the squeal level on the pad surface as well as increasing the value of the squeal index number.