Scratch analysis and control strategy of friction surface of SiCp/A356 brake disc/synthetic brake pad friction pair

SiCp/A356 brake disc and synthetic brake pad, as a light friction pair, have broad application prospects in intercity and city trains. However, a friction surface scratch is prone to occur during braking. This paper aimed to study the cause and mechanism of friction surface scratch through a combination of experiments and simulations, and considering noise control, proposed a corresponding scratch control strategy. Firstly, the thermal cycle tests of the samples made from a SiCp/A356 brake disc were carried out, and the degradation law of the SiCp/A356 brake disc material properties caused by thermal cycle load was clarified. Subsequently, the tests of the samples with different degradation degrees were carried out by using the MM-1000/Ⅳ friction testing machine. It was found that under heat and pressure, abrasive debris was compacted on the friction surface to form a third body layer (TBL). The TBL was then ploughed along the friction direction by locally welded hard points, resulting in friction surface scratches. The scratching phenomenon became more pronounced with material properties deteriorating and abrasive debris, which were identified as the source of these scratches. Furthermore, flow field simulation showed significant differences in debris accumulation within grooves at different positions. It was observed that accumulation at the entry side was more severe than at the exit side. Based on the findings, an optimization structure design for a synthetic brake pad was carried out using the response surface analysis method of orthogonal test and Box-Behnken test design. The optimized synthetic brake pad demonstrated an increase in mass flow rate by 28.22%, convective heat transfer coefficient by 1.34%, while reducing the coefficient of tendency to instability (TOI) by 49.53%. As a result, there was a significant reduction in the tendency for braking noise. In addition, it was suggested that by changing the groove of the friction surface of the brake pad, and the ability to effectively remove debris can be improved, thus significantly reducing the degree of scratches on the friction surface.