Fundamental Research Of Nitrides-oxides-carbon Composite Prepared By High-temperature Nitriding

Carbon containing refractories (MgO-C and Al2O3-C materials) have been widelyused in steelmaking due to their excellent thermal shock and slag resistance. However, nochemical bonding between oxides and graphite is formed. Nitrides-oxides-carbonrefractories are prepared in this study by a method of in-situ nitriding at elevatedtemperatures, which exhibiting excellent mechanical and thermal properties. And theformation of chemical bonding between oxides and nitrides, nitrides and graphite areachieved.In this paper, effects of preparation processes, types and particle sizes of rawmaterials on phase composition, microstructure and properties of Si3N4-MgO-C (SMC)and Sialon-Al2O3-C (SAC) materials are first discussed; the relations among phaseevolution, morphologies and sizes of nitrides, mechanical strength and thermal propertiesare also analyzed. Second, effects of bonding forms of resin and nitrides are comparedthrough physical properties, mechanical strength, thermal shock resistance and oxidationresistance; and effects of binders of phenolic resin and calcium lignosulphonate solution,which are used in nitrides-oxides-carbon refractories, are also compared. Then, themechanisms of nitridation and slag corrosion are investigated by using the methods ofthermodynamics and kinetics, and the relevant kinetics models are also built. The last, theinterface bonding situations between oxides and nitrides, nitrides and graphite followedtemperatures varying are first simulated and calculated by using the molecular dynamicssoftware, and then compared and analyzed against the practical interface bondingsituations and phase relations that observed by HRTEM, all of which provide theoreticalbasis for the design and development of nitrides-oxides-carbon refractories. Theconclusions can be drawn as follows:(1) The preparation technologies including nitriding temperatures, nitriding time,nitrogen flows and briquetting pressures are studied. The optimal nitriding temperature ofSMC material is1500°C, and when the temperature continues increasing, silicon basicallyreacts to form SiC that decrease the amount of Si3N4and deteriorate the properties ofSMC material. When the nitriding time is3h, nitrogen flow is60ml/min and briquettingpressure is400MPa, the optimal compression strength is generated due to the formation oflargest size of β-Si3N4crystals in SMC material. The optimal nitriding temperature ofSAC material is1450°C, and when the temperature continues increasing, a larger numberof gases are formed that deteriorate the properties of SAC materials. When the nitridingtime is4h, nitrogen flow is90ml/min and briquetting pressure is300MPa, the optimalcompression strength is generated due to the formation of proper amount and largest sizeof β-Sialon crystals in SAC material.(2) Effects of different kinds of alumina, sizes of alumina powder, silicon powder and graphite powder on the properties of SAC material are investigated. Adding reactivealumina powder and alumina powder in SAC material generate the optimal cold crushingstrength (CCS) and cold modulus of rupture (CMOR) due to the formation of densereticular structure and large size of O′-Sialon tabular whiskers in reducing atmosphere.When the graphite size is200mesh, SAC material possesses the highest CCS and CMORdue to the formation of tapered β-Sialon crystals and O′-Sialon whiskers, and also thehighest hot modulus of rupture (HMOR) due to the screw dislocation secondary growth ofcolumnar β-Sialon crystals. Effects of different sizes of silicon powder and graphitepowder on the properties of SMC material are investigated. Varing of sizes of siliconpowder do little effect on the properties of SMC material. When the graphite size is325mesh in SMC material, the better mechanical and thermal properties are generated due tothe formation of larger size of columnar β-Si3N4crystals and flake-like SiC crystals.(3) Effects of different kinds of bonding form and binder on the properties of SMCand SAC materials are investigated and compared; using nitrides bonding form andcalcium lignosulphonate solution as binder generate better properties.(4) Reaction mechanisms of preparing SAC and SMC materials are investigated byusing the methods of thermodynamics and kinetics. The chemical reactions among Si,graphite and oxides, SiO(g)and CO(g)in SMC and SAC materials affect the morphologiesof nitrides and SiC crystals. At the temperatures of1300°C~1350°C, the rate of nitridedprocess is controlled by chemical reaction and diffusion in SAC material, the relevantapparent activation energies are109and191kJ/mol; the rate of nitrided process iscontrolled by chemical reaction, mix of chemical reaction and diffusion, diffusion in SMCmaterial, the relevant apparent activation energies are129,235and134kJ/mol,respectively. At the temperatures of1420°C~1460°C, both the rates of nitrided processesin SAC and SMC materials are controlled by chemical reaction and diffusion, the relevantapparent activation energies are370and410kJ/mol in SAC material, and226and154kJ/mol in SMC material.(5) Mechanisms of slag corrosion in SAC and SMC materials are investigated by themethods of thermodynamics and kinetics. The dissolving into slag of SiO2, which isformed due to the oxidation of Sialon and Si3N4in SAC and SMC materials, caneffectively increase the viscosity of slag that prevent the further slag corrosion andpenetration. The rate of slag corrosion process is controlled by mix of chemical reactionand diffusion in SAC material, the relevant apparent activation energy is214kJ/mol. Therate of slag corrosion process is controlled by chemical reaction in SMC material, therelevant apparent activation energy is170kJ/mol; the rate of slag corrosion andpenetration process is controlled by diffusion in SMC material, the relevant apparentactivation energy is94kJ/mol.(6) Interface bonding situations between the phases in SAC and SMC materials areobserved and simulated by HRTEM and molecular dynamic software, respectively. The direct chemical bonding between β-Si3N4and graphite, β-Si3N4and MgO, β-Sialon andgraphite, β-Sialon and Al2O3interfaces can be formed in SAC and SMC materials by thepreparation technology of in-situ nitriding. These interfaces possess higher interfacialbonding energies according to the simulated results. The formation of chemical bondingbetween these interfaces achieve the designed mind of building bonding between oxides(MgO and Al2O3) and graphite by forming nitrides (Si3N4and Sialon).