Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

A mechanism for the nitridation of Fe-contaminated silicon

The influence of iron impurity on both the oxidation and nitridation of high purity silicon has been investigated. It is shown that iron is effective in rapidly removing the protective silica film which normally covers silicon. Experimental evidence suggests that the removal is achieved by iron-induced devitrification and disruption of the silica, thus allowing the SiO (g) generated by the Si/SiO₂ interface reaction to escape. During the nitridation of iron-contaminated silicon powder compacts it is found that iron significantly enhances the extent of reaction for contamination levels of <1000 p.p.m. Fe (by weight). Above this level there is a decrease in the rate of formation of extra nitride. At all levels of contamination the percentage of silicon converted to -Si₃N₄ was observed to be directly proportional to the iron concentration, and it is shown that this -growth occurs within an FeSi_x liquid phase. The possible implications of the findings for the optimization of strength of reaction-bonded silicon nitride are briefly discussed.     

Optimization of time-temperature schedule for nitridation of silicon compact on the basis of silicon and nitrogen reaction kinetics

A time-temperature schedule for formation of silicon-nitride by direct nitridation of silicon compact was optimized by kinetic study of the reaction, 3Si + 2N₂ = Si₃N₄ at four different temperatures (1250°C, 1300°C, 1350°C and 1400°C). From kinetic study, three different temperature schedules were selected each of duration 20 h in the temperature range 1250°-1450°C, for complete nitridation. Theoretically full nitridation (100% i.e. 66.7% weight gain) was not achieved in the product having no unreacted silicon in the matrix, because impurities in Si powder and loss of material during nitridation would result in 5–10% reduction of weight gain. Green compact of density < 66% was fully nitrided by any one of the three schedules. For compact of density > 66%, the nitridation schedule was maneuvered for complete nitridation. Iron promotes nitridation reaction. Higher weight loss during nitridation of iron doped compact is the main cause of lower nitridation gain compared to undoped compact in the same firing schedule. Iron also enhances the amount of Β-Si₃N₄ phase by formation of low melting FeSi_x phase.     

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si₃N₄ which was investigated involves the nitridation of Si:Si₃N₄ powder compacts containing additions of sintering aids (e.g. Y₂O₃ and Al₂O₃) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si₃N₄ forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si₃N₄ phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si₃N₄ grains. The grains are surrounded by a thin intergranular amorphous film.     

Effect of Al2O3 and Fe3Al on the phase formation and mechanical properties of β-sialon

The paper evaluated the mechanical properties of β-sialon composites prepared by hot-pressing sintering at 1600 °C in N₂ atmosphere using α-Si₃N₄, Al₂O₃, Y₂O₃ and Fe₃Al as raw materials. The influence of Al₂O₃ and Fe₃Al content on flexure strength, fracture toughness, hardness, and relative density was investigated. And phase formation and morphology of the composites were characterized by X-ray diffraction and electron microscopy. The experimental results indicate that the raw material Fe₃Al reacts with α-Si₃N₄ to form silicides at elevated temperature, and supplies more liquid phase to assist densification. Besides, the variation of flexure strength, fracture toughness and hardness is mainly consistent, and also in good agreement with the relative density measurements. The values all increase firstly, and then decrease when the Al₂O₃ content increases. Scanning electron microscopy illustrates that the metal particles act to inhibit the crack propagation.     

Synthesis of β-Si3N4 whiskers by SHS

This paper presents the results of SHS of β-Si₃N₄ whiskers from silicon powders with α-Si₃N₄ as diluents under high nitrogen pressure. The effect of the addition of different amounts of Y₂O₃ on β-Si₃N₄ whisker synthesis has been investigated in detail. The results revealed that Y₂O₃ additive had a marked effect on the growth of β-Si₃N₄ whiskers compared that without additives. The optimum amount of Y₂O₃ addition is from 2.4 to 4.8 wt.%.     

Correlation between fracture toughness and microstructure of seeded silicon nitride ceramics

Microstructure development and fracture toughness of Si₃N₄ composites were studied in the presence of seeds and Al₂O₃ + Y₂O₃ as sintering aids. The elongated β-Si₃N₄ seeds were introduced into two different α-Si₃N₄ matrix powders; one was the ultra fine powder matrix and the other was the coarse powder matrix. The amount of seeds varied from 0 to 6 wt%. The grain growth inhibition and the mechanism of toughening were discussed and correlated with microstructure. The maximum fracture toughness of 9.0 MPa m^1/2 was obtained for ultra fine powder with 5 wt% seeds hot pressed at 1,700 °C for 6 h.     

Some factors influencing the formation of reaction-bonded silicon nitride

Several salient factors influencing the formation of reaction-bonded silicon nitride (RBSN) compacts have been studied. These include the effects of mullite and alumina furnace tubes typically employed during high-purity nitridation studies, pre-sintering of green silicon compacts, free powder versus compact nitridation, and the influence of metal/metal oxide additions. The latter studies have provided experimental evidence for enhancement due to dissociated nitrogen, and suggest that (1) -Si₃N₄ formation does not necessarily require a liquid phase, (2) atomic nitrogen stimulates -phase formation, and (3) the liquid phase provides an efficient source for volatile silicon, promoting -Si₃N₄. These conclusions are consistent with accepted mechanisms for the formation of the two phases.     

Spark plasma sintering of silicon nitride using nanocomposite particles

The spark plasma sintering (SPS) of silicon nitride (Si₃N₄) was investigated using nanocomposite particles composed of submicron-size α-Si₃N₄ and nano-size sintering aids of 5 wt% Y₂O₃ and 2 wt% MgO prepared through a mechanical treatment. As a result of the SPS, Si₃N₄ ceramics with a higher density were obtained using the nanocomposite particles compared with a powder mixture prepared using conventional wet ball-milling. The shrinkage curve of the powder compact prepared using the mechanical treatment was also different from that prepared using the ball-milling, because the formation of the secondary phase identified by the X-ray diffraction (XRD) method and liquid phase was influenced by the presence of the sintering aids in the powder compact. Scanning electron microscopy (SEM) observations showed that elongated grain structure in the Si₃N₄ ceramics with the nanocomposite particles was more developed than that using the powder mixture and ball-milling because of the enhancement of the densification and α-β phase transformation. The fracture toughness was improved by the development of the microstructure using the nanocomposite particles as the raw material. Consequently, it was shown that the powder design of the Si₃N₄ and sintering aids is important to fabricate denser Si₃N₄ ceramics with better mechanical properties using SPS.     

Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si₃N₄) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si₃N₄ powders was studied, and the results were compared with a commercial powder. The α-Si₃N₄ powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm^?3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si₃N₄ powders produced by CRN from silica initially containing 5 wt.% Y₂O₃. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.     

Stabilization of α-SiAlONs using a rare-earth mixed oxide (RE2O3) as sintering additive

α-SiAlONs are commonly produced by liquid phase sintering of Si₃N₄ with AlN and Y₂O₃ as additives. The formation of the α-SiAlONs using a mixed oxide (RE₂O₃), containing yttria and rare-earth oxides, as an alternative additive was investigated. Dense α-SiAlONs were obtained by gas-pressure sintering, starting from α-Si₃N₄ and AlN-Y₂O₃ or AlN-RE₂O₃ as additives. The mixed oxide powder RE₂O₃ was characterized by means of high-resolution synchrotron X-ray diffraction and compared to Y₂O₃. The X-ray diffraction analysis of the mixed oxide shows a pattern indicating a true solid solution formation. The Rietveld refinement of the crystal structure of the sintered α-SiAlON using AlN-RE₂O₃ as additive revealed a similar crystal structure to the α-SiAlON using AlN-RE₂O₃ as additive. The comparison of the microstructures of the both α-SiAlONs produced using pure Y₂O₃ or RE₂O₃, revealed similar grain sizes of about 4.5 μm with aspect ratios of about 5. Both materials show also similar mechanical properties, with hardness of 18.5 GPa and fracture toughness of 5 MPa m^1/2. It could be, thus, demonstrated that pure Y₂O₃ can be substituted by the rare-earth solid solution, RE₂O₃, in the formation of α-SiAlONs, presenting similar microstructural and mechanical properties.     

The microstructures of silicon nitride ceramics during hot-pressing transformations

The microstructure of silicon nitride hot-pressed with a magnesium oxide additive has been studied by transmission electron microscopy. This includes material at various stages in a hot-pressing process: the initial (～ 90%α) silicon nitride powder; specimens partially densified and partially transformed from α-silicon “nitride” (Si_11.5N₁₅O_0.5) to β-silicon nitride (Si₃N₄); and almost fully dense and fully transformed β-Si₃N₄. The observations substantiate a solid/liquid/solid transformation mechanism, whereby Si and N are transported from α grains through a silicate liquid phase to nucleation sites for β at α/liquid interfaces or to β grains nucleated homogeneously in the liquid phase.     

Synthesis of submicron-sized spherical Y2O3 powder for transparent YAG ceramics

Spherical monodispersed, submicron-sized Y₂O₃ powder was prepared via a homogeneous precipitation method using nitrate and urea as raw materials. The structure, phase evolution and morphology of Y₂O₃ precursor and the calcined powder were studied by FTIR, TG/DTA, XRD and SEM methods. The sphere size of the precursor was about 250 nm and that of Y₂O₃ powder calcined at 800 °C for 2 h was about 200-210 nm. With the spherical Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder, high transparent YAG ceramics was fabricated by vacuum sintering at 1780 °C for 6 h through a solid-state reaction method. The in-line transmittances of the as-fabricated YAG ceramics at the wavelength of 1064 nm and 400 nm were 82.8% and 79.5%, respectively, which were much higher than that of the YAG ceramics with a commercial Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder directly. The superior properties are attributed to the good morphology, dispersibility and uniform grain size of the as-prepared spherical Y₂O₃ powder, which matches that of the commercial Al₂O₃ powder.     

Hot isostatic pressing of Si3N4 with Y2O3 additions

The effect of Y₂O₃ additive on the properties of hot isostatically pressed silicon nitride was studied. The influence of small additions of Y₂O₃ on the densification of silicon nitride was investigated. The density and elastic moduli of the product increase with increasing of the Y₂O₃ additions. The hot isostatically pressed pure silicon nitride consists of -Si₃N₄, -Si₃N₄and Si₂N₂O; phase content of the hot isostatically pressed silicon nitride with 10 wt % Y₂O₃addition consists of -Si₃N₄, yttrium silicate and Y₂Si₃O₃N₄. The effect of the outgassing of the specimens prior to hot isostatical pressing on the properties of the final material is discussed.     

Properties of reaction bonded silicon nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (>1·2 g/cm³) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si₃N₄, 7–22%β-Si₃N₄ and 7–23% Si₂N₂O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si₃N₄) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K _ic values are almost constant up to 1000°C, and thereafter increase sharply. Pore size distribution, existence of Si₂N₂O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.     

Liquid-phase bonding of silicon nitride ceramics using Y2O3–Al2O3–SiO2–TiO2 mixtures

Hot-pressure sintered β-Si₃N₄ ceramic was bonded to itself using Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures. Reactive behavior at interface between Si₃N₄ and Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures during silicon nitride ceramic joining was studied by means of scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray diffraction (XRD) and auger electron spectroscopy (AES). The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA, AES and XRD analyses show that the liquid glass solder reacts with silicon nitride at interface, forming the Si₃N₄/Y–Si–Al–Ti–O–N glass/TiN/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increase of bonding temperature and holding time, the joint strength increased reaching a peak, and then decreased. The maximum joint strength of 200 MPa measured by the four-point bending tests is obtained for silicon nitride bonded at 1823 K for 30 min.     

The removal of surface silica and its effect on the nitridation of high-purity silicon

Compacts of high-purity silicon powder were pretreated in hydrogen or argon to remove the surface silica and then nitrided in gas at atmospheric pressure and at 1623 K. The kinetics of nitridation were monitored continuously, the fracture surfaces of the nitrided samples examined using scanning electron microscopy and the alpha/beta nitride contents determined by X-ray diffraction. The experiments confirm that high-purity silicon powder, usually regarded as unreactive, can be rapidly nitrided to nearly complete conversion following pretreatments designed to remove the silica layer. The results suggest that nitridation occurred by the reaction of silicon vapour with nitrogen gas resulting in the deposition of massive Si₃N₄.     

Effects of surface area, polymer char, oxidation, and NiO additive on nitridation kinetics of silicon powder compacts

The oxidative stability of attrition-milled silicon powder under reaction-bonding processing conditions has been determined. The investigation focused on the effects of surface area, polymer char, preoxidation, nitriding environment, and a transitional metal oxide additive (NiO) on the nitridation kinetics of attrition-milled silicon powder compacts tested at 1250 and 1350°C for 4 h. Silicon powder was wet-attrition-milled from 2 to 48 h to achieve surface areas (SAs) ranging from 1.3 to 63 m²/g. A silicon powder of high surface area (63 m²/g) was exposed for up to 1 month to ambient air or for up to 4 days to an aqueous-based solution with the pH maintained at 3, 7, or 9. Results indicated that the high-surface-area silicon powder showed no tendency to oxidize further, whether in ambient air for up to 1 month or in deionized water for up to 4 days. After a 1-day exposure to an acidic or basic solution, the same powder showed evidence of oxidation. As the surface area increased, so did the percentage nitridation after 4 h in N₂ at 1250 or 1350°C. Adding small amounts of NiO significantly improved the nitridation kinetics of high-surface-area powder compacts, but both preoxidation of the powder and residual polymer char delayed it. Conversely, the nitridation environment had no significant influence on the nitridation kinetics of a high-surface-area powder. Impurities present in the starting powder, and those accumulated during attrition milling, appeared to react with the silica layer on the surface of silicon particles to form a molten silicate layer, which provided a path for rapid diffusion of N₂ and enhanced the nitridation kinetics.     

Reaction sintering and the α-Si3N4/β′-sialon transformation for compositions in the system Si-Al-O-N

The reaction sintering of three powder compositions corresponding to points near theβ′-phase line in the region of z=0.75 has been studied using apparatus which allowed the continuous monitoring of the densification kinetics. The powder compositions were prepared from mixtures ofα-Si₃N₄, Al₂O₃ and AIN, and the weight changes in the compacts could be kept to less than ∼ 1% over a two hour period. The densification rate is sensitive to small changes in powder composition and decreases markedly as theβ∼'-sialon phase is approached from the oxygen-side. Theα-Si₃N₄ toβ′-sialon conversion rate, on the other hand, is almost independent of the powder composition. The sintering and transformation kinetic data, combined with surface area measurements and observations on the microstructure of the sintered compacts, indicate that the sintering behaviour is controlled by at least two processes, namely the vapour phase transport of material and a solution-diffusion-reprecipitation process involving a grain boundary liquid phase. Both processes result in the conversion ofα-Si₃N₄ toβ′-sialon and result in microstructural coarsening but only the second process leads to overall densification in the powder compact.     

Behavior of magnesium and silicon in the formation of MgO/AlN composite

MgO/AlN composites have been fabricated by directed metal nitridation of Al–Si alloy in flowing N₂ at 1473 K. A mixture of magnesia particles and chemically pure magnesium powder was placed on the surface of Al–Si alloy block as reinforcement materials. Mg powder initiates the infiltration and nitridation of Al alloy melt by eliminating protective Al₂O₃ film at the reaction frontier. New Mg vapor from the interface reaction between Al and MgO particles, keeps as continuous deoxidization agent as the added Mg powder. The spinel layer thickness due to the reaction of Al melt with MgO particles is controlled by Mg content. Si not only reduces the surface tension and viscosity of Al alloy melt, but also leads to increase in N₂ content.