Preparation And Properties Of High-temperature Zirconium Carbide Coating

The increasingly severe performance environment of hypersonic aircraft vehicles has proposed higher requirements for thermal components.Protective coating has been proven to be effective for improving the anti-oxidation and anti-ablation properties of thermal materials,so it has already been highlighted throughout the world.In this dissertation,ZrC coatings were successfully prepared by chemical vapor deposition(CVD)with the Zr-Hr2-C3H6-H2-Ar system.The preparation process,structure and properties of ZrC coatings were systematically studied.A novel deposition system was designed and fabricated to solve the technical problem of zirconium halide transport.A steady-state ZrBr4 gaseous precursor was synthesized controllably by in situ halogenation of solid Zr-metal in the low temperature zone,and then ZrC coating was deposited by CVD in the high temperature zone.This process provides technical support for the study of CVD ZrC coating,and is conducive to engineering applications.The process parameters of the ZrBr4 gaseous precursor using the bromide process were studied and optimized.Thermodynamic analysis and kinetic study for Zr-Br2 reaction were carried out,revealing that the optimum bromination temperature,carrier Ar flow rate and pressure were 600?,10?60 ml/min and 5?101.13 kPa,respectively.Thermogravimetry-differential scanning calorimetry showed that zirconium halide mainly comprised ZrBr4.The effects of process parameters on the characteristics of ZrC coatings were studied.Based on the principle of crystal nucleation and growth as well as kinetic theory,the factors influencing the microstructure of ZrC were comprehensively analyzed.With rising deposition temperature,the deposition rate of coating increased first and then decreased,and the preferential orientation changed from(111),(200)to(220).At 1000?,the deposition was controlled by the surface reaction mechanism,forming a nanocrystalline ZrC coating with spherical and nodular morphology.Between 1100? and 1300?,the deposition was controlled by the mass transport mechanism,and the coatings were composed of well-faceted,pyramidal-shaped crystals.Meanwhile,the cross-section showed a structure of dense columnar crystals.At above 1400?,the deposition was dominated by the thermodynamic mechanism.The coatings were composed of many convex,lenticular-and needle-like ZrC crystals.The coating deposited at 1200? was uniform and dense,with a near stoichiometric composition.As the input C/Zr ratio increased from 0.5 to 1.5,the deposition rate increased evidently,and the growth mechanism changed from crystal growth to nucleation growth.However,when the input C/Zr ratio exceeded 2,ZrC crystals exhibited a lamellar structure,and the coating was porous and loose.When the input C/Zr ratio was 0.75?1,ZrC coating with near-stoichiometry was synthesized.As the H2 concentration increased from 10%to 70%,the deposition rate rose effectively and the free carbon in ZrC coating reduced.The(200)preferential orientation as well as the density of ZrC coating increased too.However,when the H2 concentration surpassed 90%,(220)became the preferential orientation,giving ZrC coating with a complex structure.When the H2 concentration was 50?70%,the structural stability and comprehensive properties of ZrC coating were optimal.With increasing deposition pressure,the deposition rate first increased rapidly and then leveled off.A near stoichiometric ZrC coating was obtained at 20 kPa.At blow 20 kPa,the deposition was dominated by crystal growth,so the coatings were composed of well-faceted crystals growing along the<001>direction.At above 40 kPa,the growth of ZrC coatings was controlled by the nucleation mechanism,so they were cluster-like crystals rapidly growing along the<110>direction.The reaction mechanisms,deposition processes and growth behaviors of CVD ZrC coatings were studied.Thermodynamic calculations showed that the CVD ZrC reactions mainly consisted of the decomposition reactions of C3H6 and ZrBr4 in gaseous diffusion,and the chemical reactions of C-and Zr-containing intermediate active groups.The simplified surface reaction involved seven important substances:H2,HBr,ZrBr4,ZrBr3,ZrBr2,C2H4 and C2H2.The deposition process of ZrC coating was simulated.The process mainly comprised the thermal decomposition of zirconium source and carbon source,diffusion and surface reaction,and the nucleation and growth of crystal.The growth behaviors of typical ZrC coatings were studied by high-resolution transmission electron microscopy.The equiaxed-crystal ZrC coating grew following the lamellar island mechanism,the columnar-crystal ZrC coating grew following the step flow mechanism,and the growths of nodular-and needle-crystal ZrC coatings were controlled by the twin nucleation mechanism and the vapor-solid mechanism respectively.The mechanical properties of CVD ZrC coating prepared at different parameters were investigated and discussed.Density,microdefect,grain size and preferential orientation mainly affected the mechanical properties of CVD ZrC coatings.In short,the optimum technical parameters were as follows:deposition temperature of 1100?1300?,input C/Zr ratio of 0.75-1,H2 concentration of 50?70%,and total pressure of 5-20 kPa.Finally,ZrC coating with complete crystal structure,smooth surface,near-stoichiometry and excellent mechanical properties was prepared.The microhardness,elastic modulus and bonding strength were 19.53 GPa,230.74 GPa and 5.86 MPa,respectively.The ablation properties were investigated by the oxyacetylene torch test.The ablation behavior,model and mechanism of CVD ZrC coating were studied and discussed.The columnar-crystal ZrC with(200)preferential orientation had high ablation stability,with the mass and linear ablation rates of-1.2×10-3 mg cm-2·s-1 and-4×10-3 ?m·s-1 respectively.There were more oxidation sites on(220)ZrC coating than on(200)ZrC coating,and catastrophic failure(e.g.cracking and bending)occurred during a long ablation process.During ablation,ZrC coating evolved from ZrC,ZrCxOy to ZrO2.The thickness direction of the ablated coating could be divided into a surface molten ZrO2 layer,an inner porous ZrO2 layer,an intermediate dense ZrCxOy layer and an inner non-oxidized ZrC layer.The ablation process of ZrC coating depended on the dissolution and diffusion rates of oxygen through the molten ZrO2 layer.In the long ablation process,ZrC coating was ablated following the mechanism gradually dominated by mechanical erosion.