Research On Preparation Of Scanning Micro-arc Oxidation Coating On SiCp/A356 Composite And Its Properties

Lightweight SiCp/A356 composite brake discs are gradually replacing the traditional steel brake disc and are quickly becoming the first choice for the next generation of urban rail trains.However,one challenge is that under wet(rain and snow)conditions or poor matching of brake pads,SiCp/A356 brake discs have the risks of surface scratching and an unstable friction coefficient.In this paper,a scanning micro-arc oxidation ceramic film(SMAO film)was successfully prepared on the surface of a SiCp/A356 material sample.The thermal shock resistance of the ceramic film and its friction matching performance with a synthetic brake pad material were also studied.An automatic scanning bipolar micro-arc oxidation test platform was designed and built for SiCp/A356 SMAO treatment.Treatment process experimental research was carried out on this platform and an electrolyte formulation of aluminate-based electrolyte system was developed:8g/L NaAlO2 +3g/L Na2SiO3 +2 g/L NaOH +2 g/L NaF,high quality and high efficiency film forming process parameters was also established in this paper.The surface and cross-section morphology,element distribution,phase composition and SiC particle evolution of SMAO films were studied by means of X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).The results shows that,SiC particles decompose during the growth of SMAO films,the main phase composition of the ceramic film are ?-Al2O3,?-Al2O3 mullite and a small amout of SiO2 can also be detected.At the same time,the effects of electrode distance,scanning speed and scanning pass times on SMAO film formation efficiency and film properties were studied.An empirical formula relating SMAO film thickness and equivalent processing time was also obtained,and can achieve accurate control of the film thickness over the range of 0?m-80 ?m.The thermal shock performance of the applied SMAO film was using both thermal shock test and finite element simulation which took crack and spalling of ceramic film as the judgement standard.The results show that the SMAO film has good thermal shock resistance.Under thermal cycling conditions of 30?-300?,80 ?m thick SMAO films can bear 600 times circulation without macro-peeling and 400 times circulaition without micro-peeling while ?m thick SMAO films can bear 1000 times circulation without macro-peeling.The thinner the applied SMAO film is,the smaller the shear stress level at the film-substrate interface is during the thermal shock cycle and result in better coating thermal shock performance.The higher the thermal shock cycle temperature,the greater the shear stress level at the film-substrate interface and the worse the thermal shock performance of the ceramic film.Based on the constant-speed ring-disc friction test,the friction matching performance between SMAO film and synthetic brake pad material under dry friction,wet friction and different temperature conditions was also studied.The results show that,SMAO film can protect the matrix,ease the temperature rise of the matrix and stabilize the friction coefficient,it was found that the wet friction coefficient of the friction pair tends to be maintained above 0.3.The friction and wear mechanism of SMAO film was discussed according to the morphology of friction surface under different test conditions.The research results of this paper can provide scientific guidance regarding maintenance schedules and the service safety of SiCp/A356 composite brake disc and play an instumental role in promoting its industrial application.