Study On The Ultra High Temperature Ceramic Borides Modified Si-based Coating Prepared By In-situ Reaction Method

The broad application of carbon/carbon(C/C) composites is limited because of its oxygen sensitivity, and the coating technology is considered to be an effective method to protect the C/C from oxidation at high temperature. In this paper, Si-based ceramic coatings modified by ultra high temperature ceramic borides, Zr B2,Ta B2,Hf B2,(Zr,Ta)B2 and(Ta,Hf)B2, were prepared by in-situ reaction method. The oxidation protective abilities of the coatings were evaluated by static isothermal oxidation test at 1773 K, TGA test from room temperature to 1773 K, severe high temperature 45 s oxyacetylene flame ablation test whose thermal flow is 2400KW/m2 and wind tunnel abrasive test at 1873 K. The crystalline structure, morphology and element distribution of the samples before and after the tests were analyzed with XRD, SEM and EDS. The oxidation protection mechanisms of the coatings were also investigated. The main contents and the results were summarized as follows:Mx Oy(Mx Oy=Zr O2,Ta2O5 and Hf O2) were chosen as raw materials of transition metal boride M in ultra high temperature cereamic borides MB2. B2O3 was used as the source of B, C was used as reducing agent and the source of C, Si was used as the source of Si. The Si-based ultra high temperature cereamic coatings MB2-Si C(M=Zr, Ta and Hf O2) were prepared by in-situ reaction method through high temperature heat-treatment. MB2 were obtained through the carbo-thermal reduction reaction between the Mx Oy, B2O3 and C powders. The Si C phase was obtained through the solid reaction between the Si and C powders. According to the synthesis theory of solid solution, the boride solid solution(Zr,Ta)B2 and(Ta,Hf)B2 modified Si-based ceramic coating were prepared by in-situ reaction method through high temperature heat-treatment. The double coexistence of Mx Oy was chosen as raw materials combining with B2O3, Si and C powders, through which Zr O2 and Ta2O5 were used to provide the source of Zr and Ta, while Hf O2 and Ta2O5 were used to provide the source of Hf and Ta.The weight loss of C/C composites reaches to 23.1% after oxidation at 1773 K in air for only 25 minutes, which indicates the large oxidation rate of C/C. But the oxidation protection performances of the samples are largely improved after the preparation of Si-based ultra high temperature ceramic coatings on the surface of C/C. With the protection of Zr B2-Si C/Si C coating, Ta B2-Si C/Si C coating and Hf B2-Si C/Si C coating, the samples could protect the C/C from oxidation at 1773 K in air for 550 h, 330 h and 265 h, respectively, whose weight loss are 0.22%, 1.1% and 1.6%, respectively. While with the protection of(Zr,Ta)B2-Si C/Si C and(Ta,Hf)B2-Si C/Si C coatings, the oxidation resistances of the samples are largely improved, which could protect the C/C from oxidation at 1773 K in air for 1412 h and 1480 h, respectively, whose weight loss are only 0.1% and 0.57%, respectively.After the modification of borides for the Si-based ultra high temperature ceramic coatings, the oxidation protection ability of the coatings in the dynamic TGA test from room temperature to 1773 K were effectively improved. After the TGA test, the weight loss of C/C coated with pure Si C coating reaches to 21.8%. While after the modification of Zr B2, Ta B2, Hf B2,(Zr,Ta)B2 and(Ta,Hf)B2, the weight loss of the coated C/C are 10.3%, 11.2%, 8.7%,-1.8% and 1.37%.The formation of compound glass layers on the surface of coating is responsible for the excellent oxidation protection effect of the Si-based ultra high temperature ceramic coatings modified with borides. In the oxidation environment, owing to the modification of Zr B2, a Zr-Si-O compound glass layer is covered on the surface of the coating, which is composed of Si O2, Zr O2 and Zr Si O4. Owing to the existence of Ta B2, a compound Ta-Si-O glass layer consisted of multiple oxides Tax Oy, B2O3 and Si O2 is covered on the surface of the coating. A Hf-Si-O compound glass layers is covered on the surface of Si-based coatings modified with Hf B2, which is composed of Hf O2, Si O2 and Hf Si O4. While for the modification of Zrx Ta1-x B2 or Tax Hf1-x B2 borides solid solutions, their existences in the Si-based ceramic coating make the generation of Zr-Ta-Si-O glass layer consisted of Si O2, Zr O2, multiple Tax Oy and Zr Si O4, and Hf-Ta-Si-O glass layer consisted of Hf O2, Si O2, multiple Tax Oy and Zr Si O4.Owing to the existence of high melting oxides Zr O2, Hf O2, Hf Si O4 and Zr Si O4 in silicate glass as incompatible phase, the Zr-Si-O and Hf-Si-O compound glass layers exhibit a kind of embedding structure. The incompatible phases exsit as “pinning phases”, which can force the formed microcracks in the glass layers to deflect or teminate, thus capable of restricting the spread of microcracks, reducing the penetration of oxygen into the C/C substrate and finally improve the oxidation protective ability of the coatingsDue to the fusion of Tax Oy with Si O2 in the Ta-Si-O compound glass, the compound glass layer actually present a double layer structure, which contains inner Si O2 glass and outer Ta-Si-O glass. In addition, the stability of the compound glass is improved, which shows stronger restriction for microcracks. The Ta-Si-O/Si O2 glass exhibits the mechanism of “microcrack reinforcement”, which can generate more microcracks in the inner Si O2 glass, thus reducing the amounts of cracks in the outer Ta-Si-O glass, hence further improving the high temperature stability of glass layer.Zr-Ta-Si-O and Hf-Ta-Si-O glass layer are actually the combination of Zr-Si-O and Hf-Si-O glass layer with Ta-Si-O glass, which possesses the excellent characteristics of the two kinds of glass layers, showing stable oxidation protective ability.With the modification of Zr B2, Ta B2, Hf B2,(Zr,Ta)B2 and(Ta,Hf)B2 as well as the synergistic effect of multi component oxides, after the severe 45 s oxyacetylene flame ablation tests whose thermal flow is 2400KW/m2, the mass ablation rates of the coated samples are.58×10-3g/cm2, 3.98 × 10-3 g/cm2, 3.87 × 10-3 g/cm2, 2.07 × 10-3 g/cm2 and 1.89 × 10-3 g/cm2,respectively, while the linear ablation rate are 4.32×10-3mm/s, 5.62 × 10-3 mm/s, 4.72 × 10-3 mm/s, 2.73 × 10-3 mm/s and 2.37 × 10-3 mm/s.The oxidation protective effects of the C/C coated with(Zr,Ta)B2 and(Ta,Hf)B2 modified Si-based coating in wind tunnel abrasive test at 1873 K were studied respectively. The(Zr,Ta)B2 phase modified Si-based coating could protect the C/C in wind tunnel abrasive test at 1873 K for 76.5 h, and the phenomenon of fracture finally occurred in the transition zone, while for the(Ta,Hf)B2 phase modified Si-based coating, it could protect the C/C in wind tunnel abrasive test at 1873 K for as long as 97 h without fracture as well as no obvious oxidation. The formation of penetrating cracks in the coatings during test is responsible for the ultimate failure of the samples.