Wet clutches are required to transmit torque and also prevent motion in automatic transmissions. Their performance is critically dependent on a friction material which comprises one of the contacting surfaces. Friction materials are usually a composite of fibres, naturally occurring minerals and particles of silicon and graphite, which are all bonded together with a resin. The material formed has very rough surfaces with much steeper slopes than normally-finished steel surfaces. When the friction material is loaded against a relatively flat counterface the real area of contact is only a small percentage of the nominal area and consists of many small, independent “contact units”. It is important to know the conditions present in the contact units (spatial dimensions and pressure) in order to understand and model wet clutch lubrication.
In this study, the contact units formed between a paper based friction material and a glass counterface have been investigated under different pressures and during rubbing. A contact visualisation technique is used to directly view and capture images of the contact. The real area of contact and the number of individual units is subsequently determined by image analysis. It is found that the real area of contact increases approximately linearly with applied load, and increases rapidly with rubbing, due to wear. As the load is increased, the number of individual contact units increases up to a critical pressure, suggesting more parts of the material support the load. Above the critical pressure the contact units may be deforming elastically and/or plastically to form larger units. After rubbing, large contact units are formed by flat areas on the tops of the contacting fibres, which are formed during wear. The topography of individual fibres is studied before and after the wearing process using atomic force microscopy, and the results support the truncating wear mechanism.
Keywords: Clutches, Topography, Contact area, Running-in