A study of the influence of humidity on the friction and squeal generationof automotive brake pads

In order to clarify the influence of humidity on the coefficient of frictio n and brake squeal generation, a series of brake rig tests has been perform ed. The influence of both air humidity and pad humidity was evaluated. The results show that, between 20 and 80 per cent relative humidity. air humidi ty has a limited influence on the coefficient of friction. Nevertheless, in a humid atmosphere the friction coefficient was considerably lower than in a wet environment for two of the pads and higher for one pad. This is beli eved to be an effect of different mechanisms of tribofilm formation on the pad surfaces. The tribofilm observed on the pads mainly consists of iron ox ide originating from the disc. The film is more easily formed in dry enviro nments and preferentially by the pads with low metal content. The friction increase often seen during each individual stop is slightly lower at high a ir humidity. This effect is more pronounced for the pads with high metal co ntent. Generally, the pads with smooth surfaces were less sensitive to chan ges in humidity. Only a limited correlation was found between relative humi dity and brake squeal generation. One of the pads generated considerably mo re squeal in low humidity. None of the pads showed the opposite behaviour.