Crystallization Kinetics and High-temperature Oxidation Behavior of SiBCN Amorphous Ceramics

Author:Li Da Zuo

Supervisor:jia de chang yang zhi hua


Degree Year:2018





The pursuit of strong and thermalstable materials is an active area as such materials are promising candidates for a variety of aerospace and aviation industries applications,typical in-service environments of high-temperature/pressure air erosion,corrosion,oxidation,thermal shock and ablation.Therefore,the development for high-temperature structural materials or structure-function integration materials with light weight,high strength,high toughness,desirable high-temperature performance and high reliability is an urgent need.SiBCN-type materials were publicly reported in the mid-1990s,which have become a field of growing interest due to their unique properties,such as the lowest oxidation coefficient in non-oxide ceramics,good mechanical properties at high-temperatures,low thermal expansion coefficient and large high-temperature viscosity.Precursor derived ceramics route is the main way to prepare SiBCN ceramcis with high chemical purity and homogeneous structure,but it is impossible to prepare highly dense bulk components with large dimensions.Based on mechanical alloying,inorganic method combined with sintering techniques can produce amorphous and/or nanocrystalline SiBCN monoliths with uniform structure and satisfying properties.However,there are still some problems remained to be solved.For example,the crystallization kinetics of SiBCN amorphous powders and monoliths are still in its fantasy;besides,the effects of crystallization behavior on microstructural evolution,mechanical properties and high-temperature oxidation resistance of SiBCN monoliths have not been clarified.Furthermore,the chemical composition-dependent phase composition,microstructures and mechanical properties of SiBCN monoliths were not excavated clearly.Using graphite,cubic silicon,boron and hexagonal boron nitride powders as raw materials,amorphous SiBCN powders with various carbon and boron contents were prepared by inorganic method based on mechanical alloying route.Mechanical alloying can mix the element atoms at atomic scale.The degree of solid-state amorphization increases with the increase of carbon content and decreases with the increase of boron content.The increase of boron content can reduce the thermal stability of amorphous powders at high-temperatures;however,the increase in carbon content can improve the thermal stability of the resulting amorphous powders.Based on JMAK theory,the crystallization transformation mechanisms of SiC wthinin amorphous SiBCN powders is governed by continuous nucleation plus three-dimension diffusion growth.The apparent crystallization activation energy for various carbon content containing amorphous Si2BC1-4N powders are 199.99 kJ/mol,229.10 kJ/mol,200.87 kJ/mol and 179.10 kJ/mol,respectively.The apparent crystallization activation energy for amorphous Si2B1-4C2N powders decreases with the increase of boron content,reaching to 192.94 kJ/mol,190.85 kJ/mol,149.29kJ/mol and 153.16 kJ/mol,respectively.High-pressure changes the equilibrium condition of the precipitated phases within amorphous ceramic matrix at high-temperatures,reversing the nucleation order,relative quantity and crystallization mechanisms.For various carbon content containing amorphous Si2BC2-4N monoliths,the sintering pressure mainly affects the nucleation stage of BN(C),greatly reducing the activation energy of nucleation.For various boron content containing amorphous Si2B1-4C2N monoliths,the sintering pressure mainly impacts the growing process of BN(C),enhancing the activation energy of growth.The crystallization activation energy decreases gradually with the increase of carbon and boron content,respectively.The crystallization process for amorphous SiBCN monoliths is mainly controlled by surface nucleation(ceramic particle bridging interfaces)and one-dimension growth.The crystallization temperature does not change monotonously with the increase of sintering pressure.In addition to the sintering pressure-dependent crystallization activation energy,the crystallization temperature is strongly impacted by the average chemical composition,chemical short-range ordering and short-range diffusion.The densification of amorphous SiBCN monoliths is strongly promoted with the increase of sintering pressure;however,the increase of boron and carbon contents hinders the densification process.The nano hardness and Young’s modulus for amorphous Si2B2C2N monoliths are satisfying.Nanocrystalline SiBCN monoliths were consolidated by hot-pressing sintering for comparison.Results show that the phase composition,microstructures and mechanical properties for the nano SiBCN monoliths are closely related to the average chemical composition.Nanocrystalline Si2BC0.1-2N monoliths are composed of substantial Si,BN and a small amount of SiC,while Si2BC2-4N monoliths are those of SiC and BN(C).The bulk density and mechanical properties of nano Si2B1.5C2N monoliths are desirable.The microstructural/morphological evolution of BN(C)is strongly impacted by the average chemical composition of the monoliths.The insertion of carbon into BN(0002)basal sheets can effectively increase the defect concentration,leading to splitting,twisting,rotating,expanding or even collapsing of the basal planes.Highly ordering of BN(C)along(0002)basal sheets are overlapped together for nano Si2BC0.1-2N and Si2BC3-4N monoliths;however,partial BN(C)within Si2BC2-3N nano monoliths displays turbostratic structures.The addition of boron powder contributes to the formation of Bx C phase,thereby reduces the defects concentration within BN(0002)basal planes leading to the ordering of BN(C).The oxidation products for amorphous or nano SiBCN monoliths oxidized at1500 oC are those of amorphous SiO2 and quartzite.The oxide surfaces for amorphous SiBCN monoliths are dense and smooth,without any pores,micro cracks and bubbling.The morphology of oxide scale for nano SiBCN monoliths strongly depends on the average chemical composition.Nano SiBCN nanoliths become porous with increasing carbon content;however,the oxide scale is contineous and seamless for various boron content containing nano monoliths.The oxidation behavior for amorphous Si2BC2-4N and Si2B1-4C2N monoliths at 1500 oC is controlled by the diffusion rate of oxygen within the oxide scale,showing a parabolic rate law.For nano Si2BC3.5N and Si2BC4N monoliths,1 h<t<6 h,the oxidation behavior is governed by the diffusion rate of oxygen within the oxide layer,6 h<t<15 h,the oxidation process is controlled by the interfacial reaction rate.The kinetic curves for nano Si2BC3.5N and Si2BC4N monoliths oxidizing at 1500 oC can be expressed as a parabolic-linear equation.The high-temperature oxidation behavior of nano Si2BC1-3N monoliths can be described approximately by a parabolic equation.Under the same oxidation conditions,the high-temperature oxidation resistance of amorphous SiBCN monoliths performs better than those of nanocrystalline ones with the same chemical composition.