Study Of Preparation And Oxidation Resistance Of ZrB₂ Based Ceramic Coatings For C/C Composites

C/C composite is an important high temperature thermal structure material,which has the characteristics of low density,low thermal expansion coefficient,high specific strength and excellent thermal shock resistance.It can be used in aerospace,medical and other fields,is one of the most promising high temperature materials.However,C/C composites will be rapidly oxidized in an aerobic environment with temperature higher than 450°C,resulting in a significant decrease in mechanical properties,which limits their use as high-temperature structural materials.Coating modification is the most effective technologies to improve the oxidation resistance of C/C composites.Ultra-high temperature ceramics(UHTCs)represented by ZrB₂ have high melting point,good chemical stability,thermal shock resistance and oxidation resistance,are widely used in the oxidation-resistant ablation coatings of C/C composites.The pack cementation and atmospheric plasma spraying methods can prepare coating samples with good adhesion and uniform composition,are mostly used in the preparation of ceramic coatings.In this paper,C/C composites are used as matrix materials,and SiC bonding layer is mainly used to alleviate the thermal mismatch between ZrB₂-based ceramic coating and C/C composites.ZrB₂-SiC coating was prepared on C/C substrate by pack cementation.The effects of different embedding processes(composition and temperature)on the phase,structure,morphology and properties of the coating were studied.ZrB₂-based ceramic coatings(ZrB₂-SiC,ZrB₂-Ti B₂)modified by different components were prepared by atmospheric plasma spraying.The effects of different components on the morphology and structure of ZrB₂-based ceramic coatings and their oxidation failure behavior at 1200°C were studied.The results show that the surface morphology of the coatings prepared at 1800°C with different Si content(5 wt.%,10 wt.%,15 wt.%)is similar,and gray-SiC particles and white cluster network ZrB₂ particles are formed.The increase of Si content promotes the formation of the coating.When the Si content is 10 wt.%,the oxidation resistance of the coating is the best,and the weight loss rate is 5.43 wt.%after isothermal oxidation at 1200°C for 1 h.The morphology of ZrB₂-SiC coatings with Si content of 10 wt.%prepared at 1600°C and 1700°C changed,forming gray agglomerated?-SiC phase and dispersed white ZrB₂ phase.The coating prepared at 1700°C was uniform and dense with the best oxidation resistance,and the weight loss rate was 1.43 wt.%after oxidation for 1 h.In general,the preparation process of ZrB₂-SiC coating by pack cementation is simple,but the prepared coating has uneven composition,poor compactness,and thin.It is difficult to effectively cover the substrate surface,and it is difficult to play the antioxidant capacity of ZrB₂-SiC coating.The ZrB₂-based ceramic coating modified by different components prepared by atmospheric plasma spraying method,after isothermal oxidation at 1200°C for 5 h,the weight gain of ZrB₂-SiC coating is still 2.62 wt.%,and the oxidation resistance of the coating is greatly improved.The addition of SiC reduces the thermal expansion coefficient of the coating,alleviates thermal mismatch and improves the bonding strength of the coating.The dense borosilicate glass surface was formed after oxidation,which filled the cracks and holes of the coating,and provided more excellent antioxidant protection for the substrates.ZrB₂-Ti B₂ coating showed poor oxidation resistance.After oxidation for 5 h,the weight loss of ZrB₂-Ti B₂ coating reached 18.76 wt.%.The poor oxidation resistance of ZrB₂-Ti B₂ coating was due to the introduction of Ti B₂.The morphology of the coating was changed,and the porous structure was formed after oxidation,which could not effectively isolate oxygen.