Research On Preparation And Properties Of Ytterbium Silicate Coating For C/C Composites

Carbon/carbon?C/C?composites are widely used in aerospace and military field for their excellent properties such as low density,high strength rate,high modulus rate,good thermal shock resistance and low thermal expansion coefficient.However,the oxidation of C/C composites at 400 °C in oxidizing atmosphere limits their applications.Anti-oxidation coatings are considered an effective way to solve this problem.The anti-oxidation coatings and high temperature structure materials are mainly composed of silicon-based ceramics.SiO₂ dense film will formed at high temperature and dry atmosphere,and it can increase the oxidation resistance.But under the condition of high temperature steam flushing,SiO₂ can react with water vapor and generate loosened Si?OH?₄.This material is easy to peel off,so that C/C matrix will direct contact with the environment and cause material failure.The thermal barrier/environment barrier coatings?TBC/EBC?could weaken the oxidation and corrosion of material.The previous environmental barrier coating systems,such as mullite,YSZ?yttria-stabilized zirconia?coating and BSAS coating have some unavoidable problems so that they cannot solve this difficulty effectively.By contrast,rare earth silicates,such as Yb₂Si₂O₇,Y₂Si₂O₇ and Lu₂Si₂O₇ can be used as another excellent EBC,due to their low thermal expansion coefficient,superior high-temperature stability and chemical stability,outstanding corrosion resistance in water vapor.In addition,Yb₂Si₂O₇ has the highest water corrosion resistance,it can be used as a promising EBC coating.In this work,we prepared pure phase ?-Yb₂Si₂O₇ powders by the molten salt synthesis.The thermal history of xerogel and the mixture of xerogel and Na2 Mo O4 were characterized by TG-DSC.The infrared spectra and valence states of ytterbium silicate prepared by molten salt synthesis were analyzed by FI-IR and XPS,and compared the thermal conductivity of Yb₂Si₂O₇ prepared by sol-gel and molten salt synthesis.Further explore the different kinds of molten salt,heat treatment temperature,the reaction ratio of raw materials for the preparation of ytterbium silicate,the phase composition and microstructure were analyzed by XRD,SEM,TEM.The Yb₂Si₂O₇ coating was prepared by hot dipping,the crystal structure and microstructure of the coating were analyzed by SEM and EDS,the oxidation resistance were also tested.Ytterbium silicate was prepared by different methods.Single phase was hard to prepare by hydrothermal method.The single phase Yb₂Si₂O₇ powder was prepared by sol-gel at 1200 °C,the particle size was about 50-500 nm.The single phase Yb₂Si₂O₇ powder could obtained by molten salt synthesis at 700 °C for 3 h,and particle size was 50-200 nm.Compared the thermal conductivity of Yb₂Si₂O₇ prepared by sol-gel and molten salt synthesis,the Yb₂Si₂O₇ prepared by molten salt synthesis has a lower thermal conductivity,the measured thermal conductivity at 1773 K was 2.16 W/?m·K?-1.The influence of phase composition and microstructure of the powder were investigated by different molten salts,heat treatment temperature and molar ratio of raw materials.Different kinds of molten salts have a significant effect on the phase composition of the products.When used Na Cl-KCl and Na₂SO₄ as molten salt,single phase Yb₂Si₂O₇ powder could not obtained at 900 °C for 3 h.Na₂MoO₄ was used as molten salt,the single phase Yb₂Si₂O₇ can be prepared at different heat treatment temperature,but different heat treatment temperatures could affect the Yb₂Si₂O₇ morphology.Yb₂Si₂O₇ whisker can be prepared with a diameter of 50-200 nm,length-diameter ratio of 10-30 under 800 °C.With the increase of heat treatment temperature,Yb₂Si₂O₇ whisker gradually became Yb₂Si₂O₇ particle.Controlling the molar ratio of raw material can change the phase of products.Using Na2 Mo O4 as molten salt,the heat treatment temperature is 800 C,the heat treatment time is about 3h and raw material mole ratio is 1:1,and the single phase Yb₂Si₂O₇ whisker can be synthesized.When the raw material mole ratio is 1:0.8,the single phase Yb2 Si O5 particle can be synthesized.The ?-Yb₂Si₂O₇ coating was prepared on the surface of Si C-C/C sample by hot dipping,and the influence of hot dipping temperature,concentration of silica sol on the coating structure and properties were studied.The optimum process conditions are as follows: hot dipping temperature is 200 °C,the volume ratio of silica sol and water is 1:4.At this time the thickness of coating is about 230 ?m,the coating surface is uniform with fewer cracks,and the inner structure of the coating was density.After the oxidation test,it was found that the ?-Yb₂Si₂O₇ coating prepared by hot dipping could effectively protect the Si C-C/C sample from oxidation at 1600 °C for 168 h with a weight loss of 2.43 %.