A Study On Design, Preparation Of Refractory Ceramic Coating Materials Containing Nano-Nickel Particles, Properties Of The Coating, Interface Reaction And Elements Diffusion Between Substrate And The Coating

Refractory ceramic coatings have been widely used on high-temperature structural parts due to their excellent properties such as resistance to thermal conduction, withstanding elevated temperatures, anti-oxidation and thermal insulation. However, many problems, for example, coating breaking off from substrate for low bond strength, or coatings oxidized at high temperatures, must be solved before they are used on the key parts of liquid oxygen/kerosene spacecraft rocket engine, which work at severe environment where oxygen-enriched high-velocity gases pass.In this work, under the support of the war industry subject of "investigation on ceramic-coating materials of the key parts of liquid oxygen/kerosene high-pressure reheat engines", a novel superfine refractory ceramic containing nano-nickel particles was developed successfully for the first time based on a comprehensive review of the progress of the high-temperature ceramic coatings. First of all, the binary phase diagrams and ternary liquidus projections of CaO-Al₂O₃-SiO₂ and MgO-Al₂O₃-SiO₂ systems have been evaluated and calculated by Thermo-Calc software. The glass forming area and a novel composition of the refractory ceramic coating have been confirmed subsequently in terms of the calculated results and the lowest eutectic principle. Furthermore, a new high-temperature ceramic coating has been successfully fabricated for the first time by the coatings-flow-coat process on the nickel-based superalloy, and it is applied on the inner surface of the turbine spheric shell in industry, which is the key part of the rocket engine. Finally, by means of optical microscopy (OM),scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron dispersive spectrometer (EDS), X-ray diffraction (XRD), tensile test and frictional wear test analyses, the dispersibility of the superfine ceramic coatings containing nano-nickel particles, the thermal shock resistance and anti-oxidation at high temperature of the coating as well as the element diffusion behaviour between the coating and the substrate have been systematically studied in theoretical and practice for the first time.1. The binary phase diagrams and ternary liquidus projections of CaO-Al2O3-SiO2 and MgO-Al2O3-SiO2 systems have been calculated and optimized by Thermo-Calc software. It shows that the lowest eutectic temperature of the CaO-Al2O3-SiO2 system is 1170°C and corresponding eutectic composition is CaO 23.3at.%, A12O3 14.7 at.% and SiO2 62 at.%. Accordingly, in the MgO-Al2O3-SiO2 system, the lowest eutectic temperature is 1345°C and its lowest eutectic composition is MgO 20.5at.%, A12O3 17.5at.% and SiO2 62 at.%. The ceramic glass phase forming area was obtained according to the lowest eutectic principle and glass forming ability, and a novel composition of our ceramic glass phase was designed as follows: CaO 17wt.%, A12O3 9.3 wt.%, SiO2 52.6 wt.% and MgO 1.8 wt.%.2. Nono-nickel particles were added to the superfine ceramic coating for the first time in order to improve the coating properties. The experimental and theoretical study on the dispersibility of the coating with nano-nickel particles shows that adding 1 .Owt.% sodium citrate and 0.1 wt.% sodium carboxymethyl cellulose (CMC) into the ceramic coating with nano-nickel particles exhibits enhanced dispersion and stability compared with the coating without dispersants during the sample standing for 72 hours. The critical additive concentration of sodium citrate, which is 21.7mol/m3, has been calculated from the DLVO theory, which was fitted very well to the experimental data 23.26mol/m3. The ^-potentials of the samples in the presence of two kinds of dispersants retain higher absolute value (|^|>35mv) in wide pH range than that in the absence of dispersant. The experimental resultsshow that the coating including 1 .Owt.% sodium citrate exhibits a higher negative ^-potential around neutral or in the weak alkaline range than the coating with O.lwt.% CMC. To ensure that the coating has not only good stability and dispersibility but also the practical applied conditions, adding 1.Owt.% sodium citrate into the superfine ceramic coatings and maintaining pH=7⁸ has been determined as the practical approach and an dispersive and stable superfine ceramic coating with nano-nickel particles was obtained.3. The flow coat parameters of the superfine ceramic coating containing nano-nickel powders have been investigated, and results indicate that the best rheologic parameters are as follows: the yield value is from 5Pa to lOPa, the thixotropic ratio is from 12% to 17% and the efflux cup viscosity is from 13 seconds to 16 seconds. The wetting ability of the ceramic paints to the alloy substrate has been improved because the surface tension between the coating and the substrate are decreased by adding the surface active agent and the superfine ceramic coatings flow and level on the alloy substrate. Furthermore, a new flow coating approach has been gained and applied to the inner surface coating of the turbine spheric shell by error and trail, which is the key part of the liquid oxygen/kerosene spacecraft rocket engine.4. The mechanical behavior of the interface between the high-temperature ceramic coating and alloy substrate has been studied. The theoretical and experimental analysis indicates that the stress intensity factor of the propagation of the cracks is directly proportional to the difference of thermal expansion coefficient between the ceramic coating and the alloy substrate. Therefore the process which can decrease the difference of thermal expansion coefficient all can improve the bonding strength of it. After adding nano-nickels to the coating, the thermal expansion coefficient of the ceramic coating changes from 4.37xlO'6 to 5.25xlO"6. The surface of the coating on Ni-based superalloy substrate was smooth without undulation and gaffing after 50 cycles at 900 °C in air. The interfacial bonding strength of ceramic coating and Ni-based alloy substrate at room temperature, which isgreater than 60MPa, was firstly measured by tensile test.The alloy substrate without coating cannot form continuous protective oxide film, the oxide layer contains Cr2O3, intergranular oxide A12O3, NiCr2O4 and TiO2. While coated alloy substrate obviously had less oxidation at 900 °C in air after 100 hours with oxidation rate of 0.12906mg/cm2 compared to the uncoated alloy sample, of which oxide rate is 1.13569 mg/cm2. It is noticeable that the coated sample with 10% nano-nickel powder only had 0.06892 mg/cm2 oxidation rate during oxidation at 900°C in air after 100 hours and it is indicated that adding nano-nickel powder can further improve the oxidation-resistance property of the ceramic coating.5. After various time annealed at 900 °C in vacuum, high-temperature ceramic coating had been turned from amorphous glass state into complex oxide ceramic inside with mica as primary crystal phase. Major elements in the ceramic coating and the alloy substrate have b een t he d iffused e ach o ther where N i, C r i n t he a Hoy substrate diffused into the coating and O, N, Si diffused from the coating to the alloy substrate. AI2O3 oxide layer and Ti, N compound exist respectively in the alloy substrate close to interface.Finally, the diffusion process of Al, Cr and Si in the coating and alloy substrate which annealed at 900 °C in vacuum has been firstly simulated through computation in the thermodynamic and dynamic aspects by Thermo-Calc and DICTRA commercial software. The calculated results were fitted very well to experimental datum.