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.     

Nanoindentation of a Polymer-Derived Amorphous Silicon Carbonitride Ceramic

Nanoindentation has revealed significant scatter of the microscale hardness and elastic modulus of an amorphous SiCN ceramic, because of structural inhomogeneities (nanopores and clusters of free carbon within the material). As a consequence of the common feature, an amorphous nature, SiCN, in regard to its mechanical properties, resembles SiO₂ glass more than SiC or Si₃N₄. However, because of the stronger Si—C and Si—N covalent bonding, SiCN is harder and stiffer than SiO₂. The mean hardness—13 ± 2 GPa, measured at a load of 250 mN for SiCN—is approximately half that of polycrystalline Si₃N₄ (24.9 ± 0.6 GPa) but higher than that of SiO₂ glass (8.9 ± 0.04 GPa). The elastic modulus of the SiCN, measured at a load of 250 mN, is 121 ± 10 GPa.     

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 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.     

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.     

Oxidation of Chemically-Vapor-Deposited Silicon Nitride and Slnglem-Crystal Silicon

Oxidation of {111} single-crystal silicon and dense, chemically-vapor-deposited silicon nitride was done in clean silica tubes at temperatures of 1000° to woo°C. The oxidation rates of silicon nitride under various atmospheres (dry O₂, wet O₂, wet inert gas, and steam) were several orders of magnitude slower than those of silicon under the identical conditions. The activation energy for the oxidation of silicon nitride decreased from 330 to 259 kJ/mol in going from dry O₂ to steam while that for Si decreased from 120 to 94 kJ/mol. The parabolic rate constant for Si increased linearly as the water vapor pressure increased. However, the parabolic rate constant for silicon nitride showed nonlinear dependency on the water vapor pressure in the presence of oxygen. The oxidation kinetics of silicon nitride is explained by the formation of nitrogen compounds (NO and NH₃) at the reaction interface and the counterpermeation of these reaction products.     

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.     

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.     

Oxidation of Silicon Carbide in Environments Containing Potassium Salt Vapor

At high temperatures in clean oxidizing environments, SiC forms a very protective SiO₂ film, but, in environments containing low levels of gaseous alkali salt contaminants or where condensed salts may deposit on the surface, the resistance of the film is significantly reduced. Oxidation kinetics of SiC were measured by continuous thermogravimetric analysis in a controlled environment containing CO₂, H₂O, and O₂ plus low levels of potassium-containing salts. Potassium was found to be incorporated into the SiO₂ scale and to significantly change its transport properties and its morphology. The rate of scale formation was found to increase directly in proportion to K in the scale. A change in mechanism was observed when water vapor was added to the reacting gas stream.     

High-Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide

The oxidation behavior of chemically vapor deposited (CVD) SiC at high temperature was investigated using a thermogravimetric technique in the temperatures range of 1823 to 1948 K. The specimens were prepared by chemical vapor deposition using SiCl₄, C₃H₈, and H₂ as source gases. The oxidation behavior of the CVD-SiC indicated "passive" oxidation and a two-step parabolic oxidation kinetics over the entire temperature range. The crystallization of the SiO₂ film formed may have caused this two-step parabolic behavior. The parabolic oxidation rate constant ( K _p) varied with the square root of the oxygen partial pressure ( P ^1/2_O2). The activation energy for the oxidation was determined to be 345 and 387 kJ · mol⁻¹. These values suggest that the diffusion process of the oxygen ion which passes through the SiO₂ film is rate-controlling.     

Oxidation of Silicon and Silicon Carbide in Ozone-Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor-deposited SiC, and single-crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone-containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone-gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO₂ film apparently was the rate-controlling process.     

Improved Oxidation Resistance of Silicon Nitride by Aluminum Implantation: I, Kinetics and Oxide Characteristics

Hot-isostatically-pressed, additive-free Si₃N₄ ceramics were implanted with aluminum at multi-energies and multidoses to achieve uniform implant concentrations at 1, 5, and 10 at.% to a depth of about 200 nm. The oxidation behavior of unimplanted and aluminum-implanted Si₃N₄ samples was investigated in 1 atm flowing oxygen entrained with 100 and 220 ppm NaNO₃ vapor at 900–1100°C. Unimplanted Si₃N₄ exhibits a rapid, linear oxidation rate with an apparent activation energy of about 70 kJ/mol, independent of the sodium content in the gas phase. Oxides formed on the unimplanted samples are rough and are populated with cracks and pores. In contrast, aluminum-implanted Si₃N₄ shows a significantly reduced, parabolic oxidation rate with apparent activation energies in the range of 90–140 kJ/mol, depending on the sodium content as well as the implant concentration. The oxides formed on the implanted samples are glassy and mostly free from surface flaws. The alteration of the oxidation kinetics and mechanism of Si₃N₄ in a sodium-containing environment by aluminum implantation is a consequence of the effective modification of the properties of the sodium silicates through aluminum incorporation.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in an Ar─O2 Atmosphere

Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O₂ atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures ( P _O2^t). The values of P _O2^t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates ( k _a) increased with increasing O₂ partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O₂ diffusion through the gaseous boundary layer.     

Oxidation Studies of Aluminum-Implanted NBD 200 Silicon Nitride

The effects of aluminum-ion-implantation on the oxidation behavior of NBD 200 Si₃N₄ were investigated over an implant concentration range of 0–30 at.%, at 800°–1100°C, in 1 atm dry O₂. Oxidation of both unimplanted and implanted samples follows a parabolic rate law. The parabolic rate constant decreases and the activation energy increases with aluminum concentration. Smooth and crack-free oxides are formed under the combination of high implant concentrations and low oxidation temperatures. Outward diffusion of Mg²⁺ from the bulk of NBD 200 to the oxide layer remains the rate-limiting step for aluminum-implanted samples. The enhancement of the oxidation resistance of NBD 200 by aluminum implantation is attributed to the retardation of the outward diffusion of Mg²⁺.     

Passive-Oxidation Kinetics of SiC Microparticles

We investigated the oxidation kinetics of SiC materials in the form of powders (average dimension 4 μm) in the temperature range 1100°–1500°C in dry air. The oxidation process was monitored through the relative mass gain in a thermobalance. As the specific surface area of the particles was measured, the recorded mass gain could be converted into the corresponding oxide thickness. The oxidation isotherms were fitted to a linear-parabolic equation, and the parabolic rate constant was evaluated. Up to 1400°C, temperature dependence can be described by a single activation energy of 179 kJ/mol, which increases in the 1400°–1500°C temperature range. These results are compared with the oxidation behavior of sintered polycrystalline and monocrystalline SiC materials.     

Precursor-Derived Si-B-C-N Ceramics: Oxidation Kinetics

The oxidation behavior of three precursor-derived ceramics—Si_4.46BC_7.32N_4.40 (AMF2p), Si_2.72BC_4.51N_2.69 (AMF3p), and Si_3.08BC_4.39N_2.28 (T2/1p)—was investigated at 1300° and 1500°C. Scale growth at 1500°C in air can be approximated by a parabolic rate law with rate constants of 0.0599 and 0.0593 μm²/h for AMF3p and T2/1p, respectively. The third material does not oxidize according to a parabolic rate law, but has a similar scale thickness after 100 h. The results show that at least within the experimental times these ceramics develop extremely thin scales, thinner than pure SiC or Si₃N₄.     

渗硅碳化硅材料的高温氧化

研究了全碳粉反应渗硅碳化硅(PCRBSC)材料,在1300℃静态空气中的高温氧化行为.研究结果表明:PCRBSC材料的氧化过程遵循直线-抛物线规律,其结构对高温氧化有很大的影响,特别是游离硅fsi和游离碳fc的含量对氧化影响更大,fsi含量高的PCRBSC材料单位面积氧化增重(Δm/s)明显,fc含量高的PCRBSC材料氧化后表现为先减重后增重,氧化层断口经扫描电镜观察有明显的气孔存在.     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen at 1823 to 1923 K

The oxidation of chemically vapor-deposited SiC in wet O₂ (water vapor partial pressure = 0.01 MPa, total pressure = 0.1 MPa) was examined using a thermogravimetric technique in the temperature range of 1823 to 1923 K. The oxidation kinetics follow a linear-parabolic relationship over the entire temperature range. The activation energies of linear and parabolic rate constants were 428 and 397 kJ · mol⁻¹, respectively. The results suggested that the rate-controlling step is a chemical reaction at an SiC/SiO₂ interface in the linear oxidation regime, and the rate-controlling step is an oxygen diffusion process through the oxide film (cristobalite) in the parabolic oxidation regime.