Preparation And Properties Of Field-based Antioxidant Repair Coating For C/C Brake Materials

Carbon/carbon composite(C/C)brake discs have inherent advantages such as light weight,high specific strength,low coefficient of thermal expansion(CTE),excellent friction and wear resistance.They have gradually replaced metal brake discs in aircraft brake systems.Under normal landing conditions,the temperature of the aircraft brake disc is between 600?800?.When the aircraft is landing under overload or suspension of the take-off state,sometimes it even reaches 1100?.However,the initial oxidation temperature of C/C composites in the atmosphere is 400?,and the oxidation rate increases rapidly with the increase of temperature.This rapid oxidation at high temperatures will cause its mechanical properties and friction properties to decline,seriously affecting the reliability and stability of aircraft braking.Therefore,C/C must be protected against oxidation.At present,phosphate coatings are commonly adopted domestically and abroad to treat the non-friction surface of C/C aircraft brake discs.However,bumps and collisions will inevitably occur during the process of transportation,equipment,disassembly and maintenance of C/C aircraft brake discs.As a result,the coating is damaged and its oxidation fails,which affects the safety of service and restricts the normal use of C/C aircraft brake discs.Re-preparation of the coating requires brushing and high temperature heat treatment(700?900?),which is not only long but also costly.Especially in the out-field of use and maintenance,there is no special high-temperature heat treatment equipment,so the development of field-based coating repair technology is very important.The field-based repair coating can directly repair the damaged parts of the coating instantly,and no special equipment is needed.After the coating material is painted,it only needs to be dried and does not need high-temperature heat treatment,which is convenient for field implementation.In this paper,the brush coating method is used to prepare a double-layer system repair coating,by studying the anti-oxidation performance of glass systems with different softening points.Field-based repair coatings with different service temperature ranges are prepared from borosilicate glass with lead glass,bismuth glass and B₄C added as coating materials.The temperature range and mechanism of each component in the coating for protection are analyzed,and the structure evolution model of borosilicate glass is established.The main research contents are as follows:(1)Designed the coating structure of C/C aircraft brake material field-based repair coating.The first layer is a ceramic precursor transition layer,the purpose of which is to improve the wettability between C/C and borosilicate glass.The second layer is borosilicate glass-B₄C layer.(2)The oxidation resistance of transition layers of different ceramic precursors was studied,and transition layers of different ceramic precursors were prepared.The PSN has the best antioxidant performance.When the thickness of the transition layer is?2?m,after curing,the PSN ceramic precursor can form a thin film on the C/C surface.(3)The effect of B₄C content on the oxidation resistance of the repair coating at 700?900?was studied,and a dynamic structural change model was established at 700?.The optimal value of B₄C content is 25 wt.%.After 10 hours of oxidation,the weight loss at 700? was-3.1%,and the weight loss at 900? was-2.9%.As the content of B₄C increases from 5 wt.%to 25 wt.%,the oxidation resistance of the coating improves.With the increase of oxidation temperature,the B₄C content in the repair coating can be quickly oxidized to form B₂O₃ glass within 700?900?.(4)The temperature range of each component of PSN/borosilicate glass-B₄C repair coating at different temperatures was studied.At 500?,PSN has short-term effective oxidation resistance.When the temperature rise reaches 600?,B₄C is oxidized to form B₂O₃ glass phase to repair the internal crack defects of the coating in situ.At 700?900?,the borosilicate glass begins to soften,forming a dense coating.(5)The effect of glass components on the oxidation resistance of borosilicate glass repair coating was studied,by adding bismuth glass or lead glass to adjust the oxidation resistance of the coating.Adding the repair coating of bismuth glass,a large amount of phase separation will occur in the glass system,which cannot form an effective anti-oxidation coating.The weight loss after oxidation at 500? for 10 h was 12.01%.However,the coating added with lead glass showed excellent oxidation resistance,and the oxidation weight loss was-3.41%when oxidized at 500? for 10 h.Under the same oxidation time,the weight loss of pure C/C was 23.32%.The best coating composition determined was the borosilicate glass-B₄C repair coating added to?20 wt.%lead glass content.