Si3N4 Phase Research Articles

Microstructure Effect on the High-Temperature Oxidation Resistance of Ti–Si–N Coating Layers

The oxidation behaviors of Ti–Si–N coating layers were investigated in terms of Si content and their characteristic microstructures. The Ti–Si–N films were characterized as composites of fine TiN crystallites and amorphous Si3N4 phase. The continuity of amorphous Si3N4 phase, i.e., the degree of encapsulation of TiN crystallites by amorphous Si3N4 phase, was an important key factor governing the oxidation behavior of Ti–Si–N coating layers. In the case of Ti–Si–N coating layers containing 4 at.% Si, TiN crystallites were not fully surrounded by the amorphous Si3N4 phase due to the insufficient amount of Si3N4 and the relatively large TiN crystallites size. Hence, the Ti–Si–N coating layers contained 4 at.% Si fast oxidized through TiN crystallites above 600°C. However, Ti–Si–N coating layers containing the Si content above 10 at.% showed much improved oxidation resistance even above 800°C because it had the finer TiN crystallites, and these TiN crystallites were fully surrounded by amorphous Si3N4 phase. The oxidation rate dependence on the Si content, and in particular the microstructure of Ti–Si–N film as derived from Si content, was systematically studied in this work.

Diffusion bonding of Si3N4-TiN composite with nickel-based interlayers

The diffusion bonding of a Si3N4-TiN composite with Ni, INVAR (Fe-Ni alloy), and IN600 (Ni-Cr-Fe alloy) interlayers has been investigated between 1100 °C and 1350 °C, under argon or nitrogen atmosphere. For the chosen bonding conditions, the Si3N4 phase of the composite reacts with the interlayer phase, leading to the release of silicon and nitrogen, whereas the TiN phase remains stable. The bonding mechanisms with nickel and INVAR (Ni-Fe alloy) interlayers are rather similar. Released silicon diffuses into the reaction layer and into the interlayer, forming a solid solution, whereas the released nitrogen remains gaseous. The bonding rate depends then on the elimination rate of nitrogen from the reaction interface. The thermal stability of these joints is very high up to 1100 °C. However, the interfacial porosity and the internal stresses created by the high nitrogen pressure are pernicious for the mechanical strength. The bonding mechanism with IN600 (Ni-Fe-Cr alloy) interlayer is rather different. The released nitrogen can form nitrides with interlayer elements (Cr, Al). Released silicon diffuses into the reaction layer and forms silicides. The joint porosity is less significant for the IN600 interlayer, which suggests a good mechanical strength. However, the formation of silicide is pernicious, because of the brittleness of these phases.

Influence of Heat Treatment on the Structure of W-Si-N Sputtered Films

W-Si-N sputtered coatings with different chemical compositions were deposited by magnetron sputtering and annealed in N2(H2) and Ar(H2) atmospheres up to 1400oC. The results show that the structure of the as-deposited films depends on their chemical composition. Films with (W+Si)/N ratios close to 1 and low silicon contents are crystalline and formed by either a �-WN or a �-W2N structure. High silicon contents induce amorphicity of these films. This type of structure was also observed in the as-deposited films with low nitrogen contents. The W46N54 and W36Si15N49 crystalline films maintain their initial structure up to temperatures of 1200oC. The amorphous W27Si20N53 coating crystallise into �-W + �-W2N at 1050oC. The �-W phase was detected in all the coatings with (W+Si)/N ratio close to 1 annealed at temperatures equal or higher than 1050oC. Concerning the coatings with lower nitrogen contents (W69Si23N8 and W64Si9N27) annealed in an argon atmosphere the results showed that they crystallize at relatively low temperatures (� 700oC). Simultaneously, the films lose nitrogen. The final Si/N atomic ratio of these annealed films corresponds to the stoichiometry of the Si3N4 phase. The higher is the silicon content, the lower is the nitrogen loss during annealing.

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si3N4 phase and blunting crack front. Formation of Si3N4 whisker was also observed. The strength of the mixture composition was maximum.

Structure and bonding in a cubic phase of SiAlON derived from the cubic spinel phase of Si3N4

The structure and electronic bonding in the spinel SiAlON (Si6−zAlzOzN8−z, z=1) derived from the cubic c-Si3N4 are studied by a first-principles density functional method. Al prefers the octahedral site of the spinel lattice. The small energy difference between the four possible structural configurations indicates that the real SiAlON may be a random solid solution. The lowest energy configuration of c-Si5AlON7 is a semiconductor with a direct LDA band gap of 2.29 eV.

Polysilazane derived micro/nano Si3N4/SiC composites

The pyrolised polysilazanes poly(hydridomethyl)silazane NCP 200 and poly(urea)silazane CERASET derived Si–C–N amorphous powders were used for preparation of micro/nano Si3N4/SiC composites by hot pressing. Y2O3–Al2O3 and Y2O3–Yb2O3 were used, as sintering aids. The resulting ceramic composites of all compositions were dense and polycrystalline with fine microstructure of average grain size <1 μm of both Si3N4 and SiC phases. The fine SiC nano-inclusions were identified within the Si3N4 micrograins. Phase composition of both composites consist of α, β modifications of Si3N4 and SiC. High weight loss was observed during the hot pressing cycle, 12 and 19 wt.% for NCP 200 and CERASET precursors, respectively. The fracture toughness of both nanocomposites (NCP 2000 and CERASET derived) was not different. Indentation method measured values are from 5 to 6 MPa m1/2, with respect to the sintering additive system. Fracture toughness is slightly sensitive to the SiC content of the nanocomposite. Hardness increases with the content of SiC in the nanocomposite. The highest hardness was achieved for pyrolysed CERASET precursor with 2 wt.% Y2O3 and 6 wt.% Yb2O3, HV ≅23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives.

Spinel sialons.

Excellent mechanical properties make γ-Si2AlON3, one of the first spinel sialons and 4-3-spinels, a promising material for structural and abrasive applications. The hardness of this phase synthesized at 13 GPa/1800°C surpasses that of the low-pressure α- and β-sialons, which are used for metal-cutting and engineering applications. The picture shows a γ-Si2AlON3 surface that was used for Vickers hardness testing and indicates the structural relation between β- and γ-sialons and the corresponding phases of Si3N4.

Synthesis and Characterization of Ultra-Fine Si3N4 by Electric Arc Plasma

Under conditions of electric-arc low-temperature plasma (LTP), ultra-finely dispersed Si3N4 particles have been synthesized by using silicon powder and nitrogen as raw materials. The prepared samples are characterized by x-ray diffraction (XRD), scanning electron spectroscopy (SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The result indicates that the basic phase in Si3N4 produced is α- and β-Si3N4. The particle size of Si3N4 sample is in the range of 30-500 nm.

High‐Temperature Oxidation Behavior of High‐Purity α‐, β‐, and Mixed Silicon Nitride Ceramics

High‐temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive‐free α‐, β‐, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in α‐phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si3N4 phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO2 crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the α‐polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen‐rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. β‐Si3N4 is observed to form oxygen‐rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen‐rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse‐grained oxidation scales with thick intergranular glass film.

Nitride-bonded silicon nitride from slip-cast Si + Si3N4 compacts

The dispersability of Si and Si3N4 powders in aqueous media was monitored by particle-size distribution, sedimentation behavior, viscosity/rheological studies, and electrokinetic behavior [zeta potential (ZP) analysis] as a function of pH of their slips. The pH values of 4 and 8 for Si and 10 for Si3N4 resulted in optimum dispersion, characterized by minimum in sedimentation height, minimum in viscosity, and maximum in ZP. The optimum slips of Si + Si3N4 mixtures conditioned in the pH range 8 to 10 were slip cast to obtain green compacts having a density in the range of 59% to 66% theoretical value. When nitrided, these compacts yielded nitride-bonded Si3N4 products having a density of 2.06 to 2.28 g cm−3, Si3N4 bonding phase of 20–60%, and 3-point flexural strength values in the range of 50 to 150 MPa. The microstructure consisted of very fine particles as well as fibrous or whiskerlike α–Si3N4 binding phase enveloping the matrix Si3N4, in total consisting of 90% α–Si3N4 and the rest being β–Si3N4 phase.

Theoretical Prediction of Post‐Spinel Phases of Silicon Nitride

New phases of Si3N4 that may be stable at higher pressure than spinel have been searched using a first‐principles plane‐wave pseudopotential method. The CaTi2O4‐type phase is found to be the prime candidate for the post‐spinel phase among six phases selected on the analogy to high‐pressure oxides. The phase transformation from the spinel is predicted to occur at 210 GPa. All silicon atoms of the new phase are coordinated by six anions, similar to the case of the high‐pressure forms of SiO2 and SiC. Because of its high energy at zero pressure, this new phase may be difficult to quench. The bandgap increases with an increase of pressure when compared in the same polymorph. However, the bandgap and the net charge decrease in the order of β, spinel, and CaTi2O4‐type phases at zero pressure. The theoretical bulk modulus of the CaTi2O4‐type phase is comparable with that of spinel.

Synthesis of Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;-TiN-SiC Composites by Combustion Reaction under High Nitrogen Pressures

Si3N4±TiN±SiC composites were synthesized from TiSi2 and SiC mixtures via the combustion reaction under high nitrogen pressure. The nitridation mechanism of TiSi2 was analyzed. The results show that the nitridation of TiSi2 produced TiN and Si ®rstly, and Si3N4 phase was formed by the further nitriding of Si. The molten eutectic phase and its agglomeration between Si and TiSi2 formed one core-shell structure and a€ected the nitridation process. Under higher nitrogen pressure, the nitridation reaction was complete and the relatively dense Si3N4±TiN±SiC composites obtained. TEM observation revealed inhomogeneous Si3N4 grain size, amorphous phase, cavities, microcracks and dislocations, and graphite from the nitridation of SiC in the microstructure. # 2000 Published by Elsevier Science Ltd. All rights reserved.

Ion irradiation induced spontaneous hypersonic long-range stimulation of silicon nitride synthesis in silicon

We studied the nature of the effect of medium-energy ion implantation on the defect system of a crystal target over distances exceeding by three to four orders of magnitude the average projected range of ions in the target material. Recently, we discovered an especially strong manifestation of this long-range effect in crystal targets: argon ion bombardment stimulated the formation of a Si3N4 phase in nitrogen-saturated layers of a silicon wafer, the effect being observed at a distance of up to 600 μm away from the ion stopping zone. An analysis of changes in the electrical and optical properties of the nitrogen-saturated layer depending on the argon ion dose, in comparison to the morphology development on the ion-irradiated silicon surface, suggests that sufficiently effective pulsed sources of hypersonic (in the initial propagation stage) shock waves appear in the Ar+ ion stopping zone. These shock waves arise as a result of the jumplike formation and evolution of a network of dislocation loops and argon blisters, accompanied by explosions of the blisters. These processes probably proceed in a self-synchronized or spontaneous manner. Argon in the blisters occurs at T = 773 K in a solid state under a pressure of 4.5×109 Pa, the blister energy reaching up to 5×108 eV. Estimates show that the synchronized explosions of blisters in the region of a nitrogen-saturated layer at the rear side of a 600-μm-thick silicon wafer may produce a peak pressure at the wave front exceeding 108 Pa, which is sufficient to cause the experimentally observed changes.

Characterization of Silicon Nitride Thin Films on Si and Overlayer Growth of Si and Ge

Crystalline silicon nitride (SiNx) thin films on Si(111) and amorphous SiNx films on Si(001) have been obtained after NH3 or NO exposure at T≈1175 K. The crystallinity of the film on Si(111) has been verified with high-resolution cross-sectional transmission electron microscopy (TEM) and scanning tunneling microscopy. The thickness of the SiNx film is 3–6 atomic layers. When compared with the known phases of Si3N4, our SiNx film is relatively close to β-Si3N4, but it could be a new phase of silicon nitride. Si or Ge forms 3D islands initially when deposited on both crystalline and amorphous SiNx films, and most of the islands are not aligned with the Si substrates. However, on SiNx/Si(111), the islands aligned with the Si substrate grow faster than other islands, so that the overlayer gradually grows into a (111)-oriented columnar film. On SiNx/Si(001), the overlayer films remain polycrystalline in later stages of growth.

MICROWAVE PERMITTIVITY OF NANO Si/C/N OMPOSITE POWDERS

The microwave permittivity of nano Si/C/N composite powders suspended in paraffin wax has been studied at the frequency range of 8.2—18GHz. The nano Si/C/N composite powders were synthesized from hexamethyldisilazane ((Me3Si)2NH) (Me:CH3) by a laser induced gas phase reaction. The dissipation factors of the nano Si/C/N composite powders are high at the microwave frequencies. The microwave permittivity of the mixture of nano Si/C/N composite powders and paraffin wax (or other dielectric materials) can be tailored by the content of the composite powders. And ε′, ε″ and tan δ increase with the volume filling factor ( v ) of nano Si/C/N composite powders. The ε′ and ε″ can be effectively modeled using second order polynomials ( ε′, ε″=Av2+Bv+C ). The ε′ and ε″ of the nano Si/C/N composite powders decrease with frequency at the frequency range of 8.2—18GHz. The difference being the microwave resonance is not sharply peaked but rather smeared out over a large frequency range. The promising features of nano Si/C/N composite powders would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si3N4 phase. The SiC microcrystallines in the nano Si/C/N composite powders dissolve a great deal of nitrogen. The local structure around Si atoms changes by introducing N into SiC. Carbon atoms around Si are substituted by N atoms. So there exist a large number of charged defects and dangling bonds in the nano Si/C/N composite powders. Thus charged defects and quasi free electrons move in response to the electric field, diffusion or polarization current resulting from the field propagation. The high ε″ and dissipation factor tan δ(ε″/ε′) of Si/C/N composite powders are due to the dielectric relaxation.

アモルファス・ナノ結晶制御と高機能材料 イミド法によるＳｉ‐Ｃ‐Ｎ系非晶質粉体の合成と結晶化挙動

The imide powders of Si-C-N-H system were formed by liquid phase reaction in SiCl4-CH3SiC13-NH3 or SiCl4-(C2H5)2NH system in n-hexane and decomposed at 900°C in vacuum to amorphous powder of Si-C-N system. The powders were heat-treated in N2 at 1400-1800°C. Si3N4 was crystallized above 1500°C and SiC was formed at higher temperatures. The formation temperature of SiC lowered with an increase of the carbon content in the synthesis system. The SEM-EDX indicated that the detected carbon content was small, when Si3N4 phase was predominant, and increased according to SiC formation at high temperatures. This means that carbon may be included inside Si3N4 particles, reacting with Si3N4 to form SiC at higher temperatures. This results suggests that Si3N4-SiC composite can be fabricated from the Si-C-N amorphous powder by optimizing starting materials and heat treatment condition.

Solid-State NMR Study of Crystallizing Process of Silicon Nitride Powder Synthesized by the Thermal Decomposition Method of Imide.

Our recent investigation established a method for quantifying the amount of amorphous silicon nitride by 29Si MAS NMR technique, which involved the full relaxation of the 29Si nuclear spin system between pulses in order to make the signals proportional to the number of nuclei in each phase. In this work, solid-state NMR study of the crystallizing behavior of silicon nitride powder, synthesized by the thermal decomposition method of imide, has been attempted. Because the crystalline and amorphous phases of Si3N4 have different thermal expansion and oxidation susceptibilities, this difference strongly affects sinterability and the mechanical properties of the final sintered body. In view of the effects of crystalline vs. non-crystalline phases on sinterability and mechanical properties, it is crucial to establish a reliable method for measuring the quantities of α-, β-, and amorphous Si3N4 in the batches of Si3N4 powder before carrying out the sintering process. Moreover, because remains of hydrogenous products generally exist on the surface of the silicon nitride powder, this affects the characteristic of slurry and sinterability of the Si3N4 powder. Thus, the quantification of the amount of hydrogenous products in the silicon nitride powder is also required. According to 29Si MAS NMR spectra of silicon diimide (Si(NH)2) during thermal decomposition at a series of temperatures, the crystalline phase increased as calcination temperatures increased. The integrated intensities of 1H MAS NMR spectra varied directly as specific surface areas. The bands near 1630 and 3300cm-1 arising from adsorbed molecular water and the unknown band near 1400cm-1 were observed in IR spectra. This result can be interpreted as indicating that there exists a proportional amount of compound consisting of hydrogen to specific surface area on the surface of silicon nitride powder, which was measured by 1H MAS NMR.

Si3N4/SiC nanocomposite powder from a preceramic polymeric network based on poly(methylsilane) as the SiC precursor

Si3N4/SiC nanocomposite powders were obtained from a preceramic polymeric network based on poly(methylsilane) as the in situ quasi-stoichiometric SiC source. These powders were constituted of nanosized SiC particles homogeneously distributed in the Si3N4 particulate matrix. beta-SiC whiskers were grown at 1400 °C in the pores of the matrix. At 1600 °C, the alpha -> beta Si3N4 phase transition took place, but no elemental silicon from Si3N4 decomposition was detected, evidencing the protective effect of the SiC phase.

A long-range influence of the argon-ion irradiation on the silicon nitride layers formed by the ion implantation

The effect of stimulating synthesis reactions for the Si3N4 phase in nitrogen-enriched silicon layers under the influence of argon-ion implantation into the rear side of silicon wafers was investigated. Dependences of variations in the IR absorption and the resistivity of the synthesized layers on argon-implantation dose were obtained. The morphology of a wafer surface after argon implantation with various doses was investigated by atomic-force microscopy. The effect is attributed to the action of shock waves arising owing to microbursts of argon blisters.

Role of Sintering Aids in Phase Transformation, Microstructure and Mechanical Properties of Si3N4/SiC Nanocomposites.

Details of α→β Si3N4 phase transformation, microstructural development and mechanical properties of SiC reinforced-Si3N4 matrix nanocomposites were investigated. Two sintering aid systems were used: 7.5mass% MgAl2O4+7.5mass%ZrO2 (MA-Z) and 10mass%Y2O3+4mass%Al2O3 (Y-A). The MA-Z system enhanced α→β phase transformation at low temperatures (1400-1550°C), which was interpreted to arise from nucleation of β-Si3N4 upon the SiC addition. This phenomenon was not observed in composites added with the Y-A system. Due to a large number of β-Si3N4 nuclei, microstructure of Si3N4/SiC nanocomposites added with the MA-Z system exhibited a reduced grain growth and high flexural strength.