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The nature of the `third-body layer' formed at the interface between the two elements of a friction couple in an automotive braking system has been the subject of much speculation. The development of the imaging x-ray photoelectron spectrometer is now allowing detailed studies of this layer. This investigation has examined the influence of temperature on the composition of transfer films (third-body layers) formed on grey cast iron after rubbing against asbestos-free friction materials in a friction assessment screening test (FAST). Evidence will be presented to show the strong dependence between transfer film composition and the coefficient of friction over a range of temperatures. The information obtained in this investigation may provide a partial solution to the problem of `early morning sharpness' and also poor cold friction coefficient, the latter is often referred to in the industry as `low cold mu'. These slight changes in friction coefficient are occasionally encountered by some drivers whose vehicles have a particular cast iron-friction material combination.

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