Microstructure And Component Characterization Research Of ZrB₂-SiC Gradient Coatings

Preparing ZrB2/SiC coatings is one of effective method to improve the ablation resistance of carbon/carbon composites in real operational environment. In this thesis, the microstructure, component distribution and ablation property of ZrB2/SiC gradient coating that deposited on C/C composite by high power plasma sprayed(HPPS) were characterized and researched. The Induction Plasma Spheroidization(IPS) was selected to sintering the ZrB2-xSiC(x is the volume fraction, vol%) agglomerated powders, Zr B2/SiC complex ceramic powders fit for the HPPS were successfully synthesized; the mechanism and microstructure characterization of sintering process for the ZrB2/SiC agglomerated powders treated by IPS were researched. The formation mechanism of ZrB2/SiC gradient coating microstructure were characterized and analyzed. Comparison ablation experiments were adopted for Zr B2/SiC coatings, the microstructure evolution of gradient structure and the failure mechanism in oxidation environment were characterized.The ZrB2-x SiC composite powders were synthesized by spray drying. The agglomerated powders made by this process had good sphericity and the particle sizes ranged from 15μm to 100μm. ZrB2-xSiC agglomerated powders were obtained preferable density and smooth surface after IPS treatment. The property results revealed that the powder fluidity were improved by 4.94%,10.5%,28% and 89% respectively, and the bulk density were increased by 22.6%,30%,67.2% and 93% respectively. Morphologies suggested that the microstructure were changed during the treatment of IPS, as a result of the eutectic reaction between ZrB2 and SiC occurred. The new microstructure include particle-melting & eutectic organization, as the the ZrB2/SiC mole ratio reaching 0.7, eutectic organization tend to be generated.ZrB2/SiC gradient coating was prepared on the C/C substrate by HPPS, which structure and composition distribution present the continuous change along the coating thickness direction. The unmatched thermal physical properties between C/C and Zr B2/SiC coatings were effectively relieved by the gradient structure. XRD results showed that ZrB2 and SiC was the mainly phase composition existed in coatings, and trace amount of ZrO2 was founded. The average porosity of ZrB2/SiC gradient coating is 15.82%.The property of anti-ablation of ZrB2/SiC coating was test by Oxy-acetylene flame(OAF).The result showed that the gradient structure attributed to the greater improvement of C/C anti-ablation property. After 600 s OAF test in 50 mm ablated distance, the mass ablative rate of Zr B2/Si C gradient coating is 3.89×10-4 g/s,while the mass ablative rate of single-configuration Z8S2&Z5S5 coating reached 2.54×10-3g/s and 1.7×10-3g/s respectively. During the OAF assessment, ZrB2 / SiC gradient coating formed two kinds of oxide layer structure, the outer layer is ZrO2 dense oxide layer, the inner is ZrO2 loose scale. This dual structure oxide layer effectively protected the C/C matrix was eroded by heat flux and alleviated the heat stress, which increased the C/C material lifetime in high temperature atmosphere.