Investigations on friction-fade and friction-recovery performance of phenolic composites based on fly ash–graphite combinations for braking applications

Friction composite materials based on combinatorial variations of fly ash and graphite were fabricated and evaluated on Krauss friction tester under inertia-braking mode following the ECE R-90 regulation. The frictional responses, such as friction fade at elevated temperatures and friction recovery on cooling, of the composites have demonstrated three regimes of friction evolution irrespective of the composition and the test runs. The composite with the highest fly ash content (i.e. 65 wt.%) and without graphite, shows the highest frictional performance, comparable frictional fluctuation, permissible fading but the highest wear with the highest rise in disc temperature. The composite with 60 wt.% fly ash in combination with 5 wt.% graphite showed higher frictional performance, permissible fading, lower wear volume and relatively lesser rise in disc temperature. However, the other composites having <60 wt.% fly ash and >5 wt.% graphite contents show unacceptable frictional level and friction-fade characteristics. Recovery performances of all composites remained above 100%. The study comprehensively established that composite material with ≥60 wt.% fly ash content in combination with ≤5 wt.% graphite content is functionally ideal to meet the desired performance requirements. Analysis of frictional data revealed the predominance of fade coefficient, whereas material coefficient remained minimally influential. Wear data analysis indicates that the inherent material factor predominates the wear performance followed by temperature-induced effects. Among the fade and recovery influences, the recovery coefficient has been observed to be predominant over the fade coefficient in the determination of the wear behaviour. Worn surface morphology revealed the role of topographical attributes in controlling the friction and wear performances.