Squeak and Rattle Behavior of Elastomers and Plastics:
Effect of Normal Load, Sliding Velocity, and Environment 2003-01-1521
The use of plastics and elastomers, for interior and exterior automotive components, presents a risk of frictionally incompatible materials contacting each other, resulting in squeaks, ticks, chirps… Ford's NVH S&R Department, and MB Dynamics have developed a tester (Figures 1 and 2) that can measure friction, and sound, as a function of sliding velocity, normal load, surface roughness, and environmental factors that allows us to provide up-stream engineering information to Forward Model Design Engineers.
When material pairs undergo sliding contact, friction forces can cause elastic deformation adjacent to the contacting surfaces. The elastic deformation is a mechanism for storing energy and sound is produced when the energy is released. The sound that we hear may be a squeak or squeal (multiple stick-slip) or a tick (single stick-slip). However if the sliding material composition (e.g. coatings, low friction additives…) and the structure (surface roughness or stiffness/compliance) of the sliding components is properly selected, we can avoid or minimize noise by minimizing elastic deformation and therefore the release of stored elastic energy.
The release of the stored elastic energy can occur when the kinetic friction is less than the static friction. This difference can be duplicated during single excursion events or when undergoing sine or random vibration. In the case of thermoplastics, cold temperatures can have a major influence on acoustic properties by reducing chain mobility (Tg) at the sliding contact surface leading to an increase in the surface contact stiffness and therefore changing the elastic deformation properties.