Research On Preparation And Properties Of Borosilicate Compounds Multilayer Coating For C/C Composites

Carbon/carbon(C/C) composites are promising candidate materials for applications in aerospace industry due to their exceptional properties such as low density and highmodulus as well as excellent mechanical properties. Especially in high temperature environment, the higher the temperature is, the better its strength is. Because of these good performances, C/C composites have been widely used in many fileds, such as biological medicine, civil products and other aspects, of which the most important application area is the filed of aeronautics and astronautics. Unfortunately, the oxidation of C/C composites above 723 K in oxidizing atmosphere greatly limits their applications as high temperature structural materials. How to solve this problem has become the key to C/C composite application. To solve this problem, this paper mainly focused on the preparation of effective oxidation resistance coating for C/C composites. The borosilicate glass coating and Si B6-Mo Si2 coating were prepared on the sample of C/C composite material with Si C coating successfully by pulsed arc discharge(PADD). The phase composition, microstructure of the surface and cross-section of the as-prepared coatings were characterized by XRD, SEM and EDS. The oxidation resistance and the oxidation mechanism of the coating coated sample were also investigated. The results are as follows:The borosilicate glass coating was prepared by PADD. The borosilicate glass/Si C multilayer coatings can effectively protect C/C composites from oxidation in air at 1823 K for 156 h with a weight loss rate of 1.29×10-4g·cm-2·h-1.The failure of the double-layer coatings is attributed to the generation of oxidation holes and microcracks, which may be due to the volatilization of the borosilicate glass layer and the escape of CO and CO2. The influence of deposition voltage on the microstructure and oxidation resistance of the glass/Si C-C/C was investigated. Good oxidation resistance and compactness of the borosilicate glass coating can be obtained when the deposition voltage is 400 V(pulse frequency is 2000 Hz, pulse duty ratio is 70%, hydrothermal temperature is 373 K, deposition time is 30 minutes, iodine content is 3.0 g/L, concentration of the suspension is 20 g/L).The Si B6-Mo Si2 coatings were prepared on the Si C-C/C sample surface by PADD, and the main factors of the preparation process were studied. The optimal technological parameters are: pulse voltage of 450 V, pulse duty ratio of 50%, hydrothermal temperature of 373 K, iodine content of 3.0 g/L, crystal composition phase ratio of 1:4. The Si B6-Mo Si2 coating prepared by PADD can effectively protect C/C composites from oxidation in air at 1773 K for 168 h with a weight loss of only 1.79×10-3g·cm-2 and a corresponding weight loss rate of 1.01×10-4g·cm-2·h- 1. The failure of the Si B6-Mo Si2 coating is attributed to the generation of microcracks and oxidation holes, which possibly due to the oxidation of Si B6 and Mo Si2, release of B2O3 and Mo O3(gas phase), and the escape of CO and CO2.