Research On Oxidation And Tribological Properties Of C/C-SiC Composites Modified By Aluminum Phosphate

At the air environment,the oxidation sensitivity of carbon phase（carbon fiber and carbon matrix）in C/C-SiC composites limits its long-term reliability application in brake discs of high-speed trains.In order to improve the oxidation resistance and tribological properties of C/C-SiC composites,at medium-low temperature（400～1000℃）aluminum phosphate with excellent oxidation resistance is applied treat the C/C-SiC composites.In this study,C/C-SiC composites are prepared by chemical vapor infiltration（CVI）combined with liquid silicon infiltration（LSI）.Firstly,the oxidation behavior of the full-scale C/C-SiC brake disc without modification was studied through a 1:1 bench experiment.C/C-SiC composites are treated by aluminum phosphate coating and matrix modification.The oxidation behavior of C/C-SiC composites modified by aluminum phosphate and the tribological properties of pairing C/C-SiC composites are analyzed.The main research contents and results are as follows:（1）The oxidation behavior of full-scale C/C-SiC brake disc is studied on 1:1 bench,and its oxidation mechanism is also revealed.The oxidation behavior is highly depended on the friction surface area of the C/C-SiC brake disc due to the distribution of microcracks,the formation of the friction film,the temperature difference,and the contact content with O₂.Specifically,the oxidation depths of the friction layers on the inner circumferential surface,the middle friction surface and the outer circumferential surface are 278.3μm,252.1μm,and 359.9μm,respectively.Furthermore,the oxidation reaction preferentially occurred in the active areas of carbon fibers and pyrolytic carbon（Py C）.（2）The preparation,microstructure,isothermal oxidation mechanism and tribological properties of C/C-SiC composites modified by aluminum phosphate coating were studied.The isothermal oxidation was carried out at 400°C,600°C,800°C and 1000°C for 10 h.After oxidation at600°C for 10 h the mass loss rate of C/C-SiC composites modified by the aluminum phosphate coating in which borosilicate powder accounts for10 wt.%of the total mass was 2.65%.Compared with unmodified C/C-SiC composites,the mass loss rate was reduced by 36.48%.The failure of the oxidation protection for the coating is attributed to the mismatch between the thermal expansion coefficients of the coating and the substrate.Under the same braking conditions,compared with the unmodified C/C-SiC composites,the friction coefficient of C/C-SiC composites modified by aluminum phosphate coating is increased by12%.（3）The preparation,microstructure,oxidation properties and tribological properties of aluminum phosphate matrix modified C/C-SiC composites were studied.Compared with the unmodified C/C-SiC composites,the weight loss rate decreased by 8.92%,35.04%and 28.27%after oxidation at 600°C,800°C and 1000°C for 5 h,respectively.The friction experiment shows that at rotating rates of 1500 r/min,3000 r/min,6000 r/min and 7500 r/min the friction coefficient is affected by the braking times,and the braking curve is more stable than that of unmodified C/C-SiC composites.It illustrates that the C/C-SiC composite modified by aluminum phosphate can improve the braking stability and give rise to different degrees of abrasive wear and oxidation wear.