Morphology,Structure And Properties Of Si-Mo-Zr Antioxidant Coatings On C/C Composites

Applications in the field of aero-engine hot-end components require C/C composites with high temperature and long-time oxidation resistance.In this paper,single SiC coatings were prepared by pack cementation after pre-oxidation treatment of C/C composites,and the effects of Si powder content in the raw material,sintering temperature and sintering atmosphere on the coating structure and antioxidant properties were investigated.Moreover,SiC-Mo Si₂,SiC-ZrSi₂ and SiC-Mo Si₂-ZrSi₂monolayer coatings were prepared by pack cementation to investigate the effects of Mo Si₂ and ZrSi₂ on the antioxidant properties of SiC coatings and the mechanism of these effects.Then,SiC-Mo Si₂-ZrSi₂/SiC bilayer coating was prepared by two-step pack cementation on SiC coating.Isothermal static oxidation experiments at 1500℃were used to examine the oxidation resistance of each coating.The coatings before and after oxidation were characterized by SEM,EDS and XRD to investigate the antioxidation mechanism of the coatings.The main findings are as follows:（1）The study of SiC coating preparation parameters showed:with the increase of Si powder content in the raw material,the SiC grains on the coating surface become larger,the surface porosity decreases,and the residual Si increases,The coating prepared with Si powder content of 60wt%in the raw material had the best antioxidant performance;As the sintering temperature was increased from 1700°C to 1900°C,the denseness of the coating improved,the oxidation resistance of the SiC coating increased with the increase of denseness of the coating;The thickness of the SiC coating prepared under vacuum atmosphere is thinner than that of the SiC coating prepared under Ar atmosphere,and the SiC coating prepared under Ar atmosphere has better oxidation resistance due to the long oxygen diffusion path.The weight loss of the SiC coating prepared using the best process parameters was 10 mg/cm² after oxidation at 1500℃for 250 h.（2）The study of the morphology,structure and antioxidant properties of three monolayer coatings of the Si-Mo-Zr system showed:the distribution pattern of Mo Si₂,ZrSi₂ in the SiC coating is similar to residual Si,and the width of cracks in the coating increases compared to the single SiC coating due to the increase in the coefficient of thermal expansion;The excellent oxidation resistance of Mo Si₂ led to a significant decrease in the oxidation rate of the coating,and ZrSi₂ contributes to the faster formation of a complete protective layer of glass film and the sample oxidation weight gain period lasts longer;The antioxidant property of SiC-Mo Si₂-ZrSi₂ coating is more stable than SiC,SiC-Mo Si₂ and SiC-ZrSi₂ coating with lower oxidation rate,its stable antioxidant duration is 120 h,and the sample weight gain is 0.1 mg/cm² after 200 h of oxidation.（3）The study of SiC-Mo Si₂-ZrSi₂/SiC bilayer coating shows that No obvious delamination between inner and outer coating,and the coating can provide stable antioxidant protection for C/C composites for up to 150 h and oxidation for nearly 300 h with a sample weight gain of 3 mg/cm³,which is better than the SiC-Mo Si₂-ZrSi₂ monolayer coating.The thermal shock resistance of the SiC-Mo Si₂-ZrSi₂/SiC coating was significantly improved compared to the SiC-Mo Si₂-ZrSi₂ monolayer coating,with the weight gain of 0.07 mg/cm² after 24 thermal cycles.In contrast,the weight loss of SiC-Mo Si₂-ZrSi₂ coating reached 12.9 mg/cm² after 5 thermal cycles.The oxidation mechanism analysis shows that the oxidation rate of SiC-Mo Si₂-ZrSi₂/SiC coating decreases with the thickening of the oxide film in the first stage of oxidation,the thickness of the oxide film is stable and dense in the second stage and the oxidation rate is low,and the coating cannot effectively prevent oxygen diffusion in the third stage and the coating fails.