Oxidation Behavior of a Fully Dense Polymer-Derived Amorphous Silicon Carbonitride Ceramic

The oxidation behavior of a polymer-derived amorphous silicon carbonitride (SiCN) ceramic was studied at temperature range of 900°–1200°C using fully dense samples, which were obtained using a novel pressure-assisted pyrolysis technique. The oxidation kinetics was investigated by measuring the thickness of oxide layers. The data were found to fit a typical parabolic kinetics. The measured oxidation rate constant and activation energy of the SiCN are close to those of CVD and single-crystal SiC. The results suggest that the oxidation mechanism of the SiCN is the same as that of SiC: oxygen diffusion through a silica layer.     

Oxidation Kinetics of an Amorphous Silicon Carbonitride Ceramic

The oxidation kinetics of amorphous silicon carbonitride (SiCN) was measured at 1350°C in ambient air. Two types of specimens were studied: one in the form of thin disks, the other as a powder. Both specimens contained open nanoscale porosity. The disk specimens exhibited weight gain that saturated exponentially with time, analogous to the oxidation behavior of reaction-bonded Si₃N₄. The saturation value of the weight gain increased linearly with specimen volume, suggesting the nanoscale pore surfaces oxidized uniformly throughout the specimen. This interpretation was confirmed by high-resolution electron microscopy and secondary ion mass spectroscopy. Experiments with the powders (having a particle size much larger than the scale of the nanopores) were also consistent with measurements of the disks. However, the powder specimens, having a high surface-to-volume ratio, continued to show measurable weight gain due to oxidation of the exterior surface. The wide range of values for the surface-to-volume ratio, which included all specimens, permitted a separation of the rate of oxidation of the free surface and the oxidation of the internal surfaces of the nanopores. Surface oxidation data were used to obtain the rate constant for parabolic growth of the oxidation scale. The values for the rate constant obtained for SiCN lay at the lower end of the spectrum of oxidation rates reported in the literature for several Si₃N₄ and SiC materials. Convergence in the behavior of SiCN and CVD-SiC is ascribed to the purity of both materials. Conversely, it is proposed that the high rates of oxidation of sintered polycrystalline silicon carbides and nitrides, as well as the high degree of variability of these rates, might be related to the impurities introduced by the sintering aids.     

Amorphous Silicon Carbonitride Fibers Drawn from Alkoxide Modified Ceraset™

We show that the rheology of a silazane-based precursor for a silicon carbonitride ceramic can be radically altered by reacting it with an alkoxide precursor. This modified precursor can be drawn into fibers by a simple process. The fiber-friendly rheology of the modified precursor is ascribed to the creation of linear polymer chains by a reaction between silazanes and alkoxides. The ceramic fibers made by this process have excellent mechanical properties, reaching tensile strengths of 2.8 GPa. Their structure is amorphous.     

Crystallization of Polymer-Derived Silicon Carbonitride at 1873 K under Nitrogen Overpressure

The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si₃N₄·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at p _N2 < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ∼10 bar, and is crystalline above ∼10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si₃N₄ are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ∼10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si₃N₄ crystals is calculated to be 30 kJ/mol.     

Crystallization Behavior of Amorphous Silicon Carbonitride Ceramics Derived from Organometallic Precursors

The crystallization behavior of organometallic-precursor-derived amorphous Si-C-N ceramics was investigated under N₂ atmosphere using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si-C-N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane-polysilazane blends, single-source polysilazanes, and single-source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si₃N₄ increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon-free amorphous Si-N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si-C-N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si-C-N network.     

Stability of Silicon Carbonitride Phases

Important hard phases are included in the quaternary compositional system Si-N-C-B. This paper reviews ternary amorphous and crystalline phases in the system Si-N-C and deliberates on the issue of stability of the binary C₃N₄, a hypothetical phase harder than diamond, and instability of nitrides in general. There is a tendency for nitrogen atoms to agglomerate and be released as nitrogen molecules. Stabilization of CN radicals can be achieved through ternary phases: carbonitrides metal-C-N. Ternary Si-N-C phases have been synthesized by pyrolysis of polyorganosilazanes, physical vapor deposition, and chemical vapor deposition. The crystalline α-Si₃N₄:C phase can incorporate about 6 at.% C and yields enhancement of hardness and wear resistance. Other crystalline phases contain more carbon, for example, Si₂CN₄.     

High-Resolution Transmission Electron Microscopy Study of the Microstructural Development of a Silicon Carbonitride Nanocomposite

Nanocrystalline ceramics are expected to possess enhanced superplasticity over their microcrystalline counterparts. In this effort of producing nanocomposites of silicon nitride and silicon carbide, amorphous Si-C-N derived from pyrolysis of a polysilazane precursor was sintered with yttria as an additive. High-pressure sintering at different temperatures resulted in sintered materials ranging from amorphous to nanocrystalline. High-resolution transmission electron microscopy was conducted to characterize the development and grain-boundary features of the nanocrystalline microstructure. The results provide a preliminary understanding of the process of the formation of the nanocrystalline structure from an amorphous matrix, under the condition of high pressure and relatively low temperature. The wide variation in the thickness of grain-boundary phases observed in this material suggests a nonequilibrium state of the grain boundary, which might be related to the processing conditions.     

Nanoscale Densification Creep in Polymer-Derived Silicon Carbonitrides at 1350°C

The measurement of axial and radial strains during uniaxial compression creep of SiCN shows the deformation to be entirely volumetric (as opposed to shear). Phenomenologically, the densification strain rate shows a good fit to an exponential stress dependence. This result is explained by the large volume of the diffusing molecular units in the oligomeric amorphous structure of SiCN, which causes the driving force to become nonlinear in stress. The size of the diffusing unit is estimated to be 1.2 nm.     

Electron Transport in Polymer-Derived Amorphous Silicon Oxycarbonitride Ceramics

The electron transport behavior of polymer-derived amorphous silicon oxycarbonitride ceramics is studied by measuring their temperature-dependent electrical conductivities. The experimental results are analyzed using theoretical models. The results reveal that the materials exhibit three conduction mechanisms: conduction in extended states, conduction in band tails, and conduction in localized states. Particularly, it is found that in a low-temperature regime, the conduction of the materials follows a band tail hopping mechanism, rather than the previously assumed variable range hopping mechanism. The results also reveal that energy gaps such as E _C− E _F and E _C− E _A decrease with increasing pyrolysis temperature.     

Thickness Alteration of Grain-Boundary Amorphous Films during Creep of a Multiphase Silicon Nitride Ceramic

Experimental observations of the creep response of a commercial sintered silicon nitride ceramic are presented. The stable microstructure of this material at high temperature contains secondary crystalline phases which result from partial devitrification of the intergranular phase. The widths of amorphous films along grain boundaries (between silicon nitride grains) and phase boundaries (between silicon nitride and secondary phase grains) are characterized by transmission electron microscopy. The thickness distributions of grain-boundary films before and after creep are analyzed by a statistical method. While the film widths are highly uniform before creep, a bimodal distribution is observed after creep. The results suggest that viscous flow of the boundary amorphous films occurs during creep deformation.     

先驱体转化法制备含硼SiC纤维

通过二甲基胺硼烷（dimethylamine borane,DMAB）与低分子量聚碳硅烷（low-molecular mass polycarbosilane,LPCS）反应合成硼溶胶,将硼溶胶与高分子量聚碳硅烷（high-molecular mass polycarbosilane,HPCS）共混制备含硼聚碳硅烷先驱...     

Pyrolysis Kinetics for the Conversion of a Polymer into an Amorphous Silicon Oxycarbide Ceramic

We present experimental and analytical results for the pyrolysis reactions underlying the conversion of a cross-linked polymer into an amorphous ceramic material. The activation energies, obtained from thermogravimetric data, and chemical analysis of the volatiles by mass spectroscopy are used to identify the reaction pathways. The reaction is determined to be first-order, which is consistent with its solid-state nature. The magnitude of the weight loss is analyzed to calculate the number of molecular sites in the polymer that participate in the reaction. The experiments were conducted on a polymer made from silsesquioxanes that convert into silicon oxycarbide ceramics on pyrolysis. The results show that <2.5% of the silicon atoms in the polymer are removed as volatile silanes, and less than one-half of the carbon atoms are lost as methane. These results are a first step in understanding the molecular basis for the ceramic yield, as well as the evolution of the nanostructure as the material changes from an organic into a ceramic state by reactions that can occur at <850°C.     

Newtonian Viscosity of Amorphous Silicon Carbonitride at High Temperature

The creep viscosity of chemical-precursor-derived silicon carbonitride (SiCN), which is known to remain predominantly amorphous at temperatures below 1400°C, was measured in the temperature range 1090-1280°C. Experiments were done in uniaxial compression at constant loads in pure nitrogen atmosphere. The creep behavior exhibited three stages. In stage I the strain rate decreased rapidly with time and deformation was accompanied by densification. In stage II the samples exhibited a steady-state creep rate. In stage III, which commenced after long-term deformation, creep gradually declined to rates that were below the sensitivity of our apparatus. The relative density of the specimens during stage II and stage III remained constant at ≅2.3 g/cm³. The shear viscosity in stage II was nearly Newtonian and was measured to be 1.3 × 10¹³-5.0 10¹³ Pa·s at 1280°C, which is approximately 10³ times the value for fused silica. The creep-hardened as well as uncrept specimens contained silicon nitride crystallites. The volume fraction of these crystals was variable but always less than 5%. Such a small volume fraction of crystals does not explain the dramatic creep-hardening behavior in stage III, even if it is assumed that the crystals formed during creep deformation in stage II.     

A Silicon Carbonitride Ceramic with Anomalously High Piezoresistivity

The piezoresistive behavior of a silicon carbonitride ceramic derived from a polymer precursor is investigated under a uniaxial compressive loading condition. The electric conductivity has been measured as a function of the applied stress along both longitudinal and transverse directions. The gauge factor of the materials was then calculated from the data at different stress levels. The results show that the material exhibits an extremely high piezoresistive coefficient along both directions, ranging from 1000 to 4000, which are much higher than any existing ceramic material. The results also reveal that the gauge factor decreases significantly with increasing applied stress. A theoretical model based on the tunneling–percolation mechanism has been developed to explain the stress dependence of the gauge factor. The unique piezoresistive behavior is attributed to the unique self-assembled nanodomain structure of the material.     

High-Temperature Mechanical Properties of Si-B-C-N-Precursor-Derived Amorphous Ceramics and the Applicability of Deformation Models Developed for Metallic Glasses

The characterization of Si-B-C-N amorphous ceramics using isothermal compression creep testing in the temperature range of 1200°–1500°C is reported. The deformation rate contains a stress-dependent component that is proportional to the applied stress, which indicates that this portion of the deformation mechanism is based on viscous flow. An increase in the creep resistance is observed, following either preliminary annealing or hot isostatic pressing, which may be explained by a reduction of free volume in the amorphous material. The application of two deformation models that are used to predict similar deformation behavior in metallic glasses also is discussed. Although both models accurately predict the time dependence of the deformation rate of precursor-derived amorphous ceramics, the free-volume model fits the observed temperature dependence better than the two-step rearrangement model.     

Fracture Toughness of Amorphous Precursor-Derived Ceramics in the Silicon-Carbon-Nitrogen System

The toughness of amorphous precursor-derived ceramics in the Si-C-N system is investigated. Crack-growth data are obtained from DCB specimens, whereas the crack-tip toughness is determined from the crack-tip profile of indentation cracks. For amorphous Si-C-N ceramics that have been fabricated via the powder route, an R -curve effect is observed. The initial values of the rising R -curve are consistent with the estimated crack-tip toughness.     

Visualization of Knoop Indentation Damage of Silicon Nitride by Plasma Etching

A visualizing technique for indentation damage of ceramics was developed. Plasma etching was used to enhance the view of cracks and the subsurface microcracking crush zone following Knoop indentation of hot pressed Si₃N₄. The microcracking zone was readily identified from the surface view of the indented surface as a grain-falling-off region (GFOR), defined as a region in which grains were removed by preferential etching using CF₄ gas, followed by ultrasonic cleaning. A fissure-like opening corresponding to the indentation cracks was also observed. It is inferred that the formation of the GFOR region and the fissure-like opening were caused by the etching/cleaning treatment. Meanwhile, the etching on a section which included diagonals of the impression provided a section view of the microcracking zone.     

Effect of Ammonia Treatment on the Crystallization of Amorphous Silicon–Carbon–Nitrogen Ceramics Derived from Polymer Precursor Pyrolysis

An effort was made toward modifying the Si₃N₄-SiC phase ratio in bulk nanocomposites obtained from polymer precursors. While pyrolyzing the polymer, flowing ammonia was introduced, to facilitate a chemical exchange, resulting in a different C/N ratio in the ceramic pyrolysis product. A pre-pyrolysis/binding/pyrolysis approach was used for sample consolidation. Comparison was made between the crystallization behavior of the pyrolysis-derived ceramic powders and consolidated bulk samples. A profound enhancement in crystallization tendency was observed in the consolidated samples whose nitrogen content was increased by ammonia treatment. A mechanism based on the particle/binder interface energy was proposed to account for this observation.     

Indentation Crack Length Anisotropy in Silicon Nitride with Aligned Reinforcing Grains

Vickers indentation was performed on surfaces of silicon nitride with an aligned microstructure in order to study the interaction between cracks and the microstructure. Although there was not much evidence of crack bridging, the transverse radial cracks were very short, resulting in high fracture toughness values. The longitudinal radial cracks tended to propagate along the grain boundary of the reinforcements and were much longer than the transverse cracks. As the sintering temperature increased, the lateral cracks on the casting surface led to spalling and consumed more energy for the crack formation, making the longitudinal cracks shorter. On the surface normal to the alignment direction, there was no spalling and the indentation cracks became longer as the sintering temperature increased.