Crystallization And High Temperature Oxidation Mechanisms Of Amorphous Si₂BC₃N Ceramics By High Pressure Sintering

The development of novel high-temperature materials for service under harsh conditions including high-temperature oxidation,thermal shock and ablation has become one of the most pressing needs in the aerospace industry.Silicoboron carbonitride?Si-B-C-N?ceramic and its matrix composites have great potential applications in the fields of high-temperature structural and multifunctional heat-resistant ceramics because of their special structures and outstanding high-temperature properties.Precursor pyrolysis route limits greatly the preparation of dense Si-B-C-N monoliths.Hot pressing combined with mechanical alloying?MA?technology using inorganic powders as the raw materials?referred to as inorganic processing route?is very simple and effective in the preparation of materials with uniform microstructures and excellent properties.It has been used to obtain dense Si-B-C-N monoliths and structural components resistant to high temperatures providing further impetues for industrial applications of Si-B-C-N materials.However,high sintering temperatures of 1800-2000?C cause crystallization of amorphous Si-B-C-N powders offering eventually crystalline ceramics with some residual amorphous strucutres sometimes.Additionally,hot pressing?2000?C/80 MPa/30 min?can not produce fully dense,pure Si-B-C-N monoliths.In such case,high pressure sintering was used here to densify amorphous Si-B-C-N powders synthesized using mechanical alloying technology.The evolution of phases,morphologies,microstructures and mechanical properties of Si-B-C-N monoliths during high pressure sintering were investigated using various characterization equipment including XRD,SEM,TEM,XPS and nano-hardness indenter.The feasibility of the preparation of dense,amorphous Si-B-C-N monoliths using MA-high pressure sintering method was discussed.Oxidation behaviors and damage mechanisms of amorphous Si-B-C-N monoliths at high temperatures were also studied in detail.Amorphous Si₂BC₃N powders were synthesized using mechanical alloying of c-Si,h-BN and graphite powders.Using high pressure technology?1000-1600?C/5GPa/30 min?,Si₂BC₃N monoliths can remain amorphous at?1100?C/5 GPa or 1200?C/3 GPa,while spark plasma sintering can not produce amorphous,dense Si₂BC₃N monoliths at 1000-1900?C/50 MPa/5 min.With the increasing temperatures at 5 GPa,Si₂BC₃N monoliths undergo partial phase segregation?1100-1200?C?,followed by initiation of nucleation?1200-1300?C?,and then nucleation and growth??1300?C?.Amorphous BN?C?and SiC tend to concentrate in the partical bridging areas and particle interiors,respectively,then BN?C?nucleats preceding SiC.At 1400?C,?-SiC and turbostratic BN?C?grains in amorphous Si-B-C-N ceramic matrices grow up to ca.10 nm in diam.and 3-5 nm in width,respectively.At 1600?C,grains grow up to 10-30 nm,and there are still residual amorphous structures in Si₂BC₃N monoliths.At 5 GPa the increasing sintering temperatures from 1000?C to 1600?C lead to monotonic increases in bulk densities,while mechanical properties?nano-hardness and elastic modulus?increase first,then decrease??1100?C?.Amorphous Si₂BC₃N monoliths at 1100?C offer densities of 2.75 g?cm^-3,6%higher than densities(2.60g?cm^-3)of hot-presed?1900?C/80 MPa?counterparts,and 2.85 g?cm^-3?9%higher?for amorphous/nanocrystalline Si₂BC₃N comoposite ceramics at 1600?C.Amorphous Si₂BC₃N monoliths show higher hardness and elastic modulus due to chemical bond-derived strong,amorphous 3-D network structures.These amorphous structures are lost with crystallization of?-SiC and�soft'BN?C?reducing the contributions from C-B,C-N?sp³?and C-B-N bonds thereby decreasing mechanical properties.Si₂BC₃N monoliths at 1600?C show lower hardness and elastic modulus.High-temperature oxidation of amorphous Si₂BC₃N monoliths leads to gas release?CO,CO₂ and SiO?and B₂O₃ volatilization causing mass loss,and coincident formation of silicon oxide scales causing mass gain.This makes,as a result,the mass of Si₂BC₃N change irregularly.Loose,porous oxide scales form on ceramic surfaces at 1500-1600?C in flowing air and flake off easily indicating poor adhesion.At 1700-1800?C,dense,smooth oxide scales with a few cracks form and adhere firmly to ceramic surfaces.The rate-controlling step at 1500-1600?C is the diffusion of O₂molecules through growing oxide scales,and the oxide scales show parabolic growth with parabolic rates of 32.5?m²/h and 86.1?m²/h for 1500?C and 1600?C,respectively.In contrast,at 1700?C the oxidation behavior is co-controlled by rates of O₂ diffusion through oxide scales and the reaction on scale/bulk ceramic interface,which causes complex growth of oxide scales.The calculated Arrhenius activation energy is ca.116 kJ�mol^-1for 1500-1600?C/16 h oxidation.The cross section of amorphous Si₂BC₃N monoliths after oxidization at 1700?C for 8 h can be devided into three zones from outside to inside:The outermost layer of N-containing amorphous SiO₂;the loose interlayer;the innermost unoxidized amorphous Si-B-C-N with some nano-sized precipitates of SiC and BN?C?.No obvious interfaces form between them indicative of strong adhesion.The loose interlayer mainly includes cristobalite,and unoxidized SiC and BN?C?precipitates.Oxygen isotope tracer oxidation at 1400?C indicates that oxide scales grow inward mainly by oxygen lattice diffusion inside scales during oxidation of amorphous Si-B-C-N monoliths.Amorphous Si₂BC₃N monoliths exhibit rivaling oxidation resistance at 1500?C with amorphous/nanocrystalline SiC,but manifest poor resistance compared to SiC at 1600?C.At 1700?C,Si₂BC₃N monoliths offer good resistance to oxidation.They can even resist flowing air up to 1800?C/?0.5 h.The oxidative damage of amorphous Si₂BC₃N monoliths is caused mainly by the interfacial reaction of SiO₂ scales/ceramic and rapid decomposition-evaporation of SiO₂ at high temperatures??1700?C?.These results noted above confirm,for the first time,the feasibility of preparation of dense,amorphous Si-B-C-N monoliths using MA-high pressure sintering method.These also reveal the evolution of pahses,morphologies,microstructures and mechanical properties of Si₂BC₃N monoliths during sintering and elucidate the mechanisms of crystallization and oxidative damage of amorphous Si₂BC₃N monoliths showing first their good resistance to oxidative damage at high temperaures up to 1800?C.This work enriches the experimental and theoretical knowledge about Si-B-C-N system ceramics,and also provides theoretical insight and experimental data for the further development of Si-B-C-N and related materials.