Si3N4 Matrix Grains Research Articles

Influence of Conductive Nano‐TiC on Microstructural Evolution of Si3N4‐Based Nanocomposites in Spark Plasma Sintering

A conductive TiC nanopowder was incorporated into nanosized β‐Si3N4‐based powder and consolidated by a spark plasma sintering (SPS) technique with a rapid heating process. The influence of the conductive phase on the microstructure development of the Si3N4matrix was demonstrated. After sintering, the conductive phase transformed into titanium oxycarbonitride. The Si3N4‐based composite containing 5 wt% nano‐TiCxOyNzshows a larger average grain size and aspect ratio than the monolithic Si3N4‐based ceramic. This is possibly because a leakage current hops across the conductive titanium oxycarbonitride grains and causes Joule heating during sintering. The transmission electron microscopy analysis confirmed that dissolution–reprecipitation and coalescence occur. In addition, although the increasing amount of incorporated nano‐TiCxOyNz(10 and 20 wt%) decreases the electrical resistivity of the composites, the pinning effect of the titanium‐based phase significantly suppresses the grain growth of Si3N4matrix grains. β‐Si3N4‐based nanocomposites containing nanosized titanium oxycarbonitride were thus obtained in the present study.

The Influence of Corrosion in an Aqueous Solution of NaCl on Fracture and Strength of Various Structural Ceramics

The present work studies the corrosion of three most widely used types of structural ceramics – silicon nitride, solid state sintered alumina and liquid phase sintered alumina – in 3 % aqueous solutions of sodium chloride at temperatures up to 290 °C and pressures up to 7 MPa. The corrosion of silicon nitride was controlled by attack of Si3N4 matrix grains, while yttrium oxynitride amorphous grain boundary phase was corrosion resistant. Corrosion of Si3N4 in reference media -distilled water - at 290 °C was characteristic by formation of passivation layer, which hindered further dissolution of silicon nitride matrix. The presence of sodium chloride resulted in formation of discontinuous layer of corrosion products, resulting in more severe corrosion than in distilled water. The corrosion of liquid phase sintered alumina was mainly attributed to congruent dissolution of SiO2 and CaO from grain-boundary amorphous film, which was accelerated at higher temperature, and accompanied by precipitation of siliceous phases from oversaturated solution at 200 °C. Pure polycrystalline alumina corroded by loss of alumina grains, which did not dissolve in the corrosion media. The corrosion impaired significantly the fracture strength of silicon nitride, creating new, corrosion related defects at the surface, while the influence of corrosion on fracture strength of polycrystalline aluminas was negligible.

Low Cost Si<sub>3</sub>N<sub>4</sub>/SiC Nanocomposites, Processing, RT and HT Properties

Silicon nitride - silicon carbide nanocomposite has been prepared by an in-situ method that utilizes formation of SiC nanograins by C+ SiO2 carbothermal reduction during the sintering process. The developed C/SiO2 derived nanocomposite consists of a silicon nitride matrix with an average Si3N4 matrix grain diameter of approximately 200 nm with inter- and intra- granular SiC inclusions with sizes of approximately 150 nm and 40 nm, respectively. The mean value of room temperature 4-point bending strength is 670 MPa with the Weibull modulus of 7.5 and indentation fracture toughness of 7.4 MPa.m1/2. The creep behaviour was investigated in bending at temperatures from 1200°C to 1450°C, under stresses ranking from 50 to 150 MPa in air. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix. The inserts machined from this composite have three times longer life time compared to those available on the market.

Ab initiostructural energetics ofβ−Si3N4surfaces

Motivated by recent electron microscopy studies on the Si3N4/rare-earth oxide interfaces, the atomic and electronic structures of bare beta-Si3N4 surfaces are investigated from first principles. The equilibrium shape of a Si3N4 crystal is found to have a hexagonal cross section and a faceted dome-like base in agreement with experimental observations. The large atomic relaxations on the prismatic planes are driven by the tendency of Si to saturate its dangling bonds, which gives rise to resonant-bond configurations or planar sp^2-type bonding. We predict three bare surfaces with lower energies than the open-ring (10-10) surface observed at the interface, which indicate that non-stoichiometry and the presence of the rare-earth oxide play crucial roles in determining the termination of the Si3N4 matrix grains.

Influence of Sintering Conditions on the Microstructure and Mechanical Properties of Si3N4 Ceramics with WB Addition

Si3N4/W composites with high wear resistance were fabricated by reduction reaction of tungsten boride (WB) under hot-press sintering in a nitrogen atmosphere. Composites sintered above 1850°C consisted of β-Si3N4, W, W2B and W2C. The Si3N4 matrix grains in the composite were composed of an elongated and bimodal structure similar to conventional Si3N4. Mechanical properties, i.e. fracture toughness and strength, were almost the same as monolithic Si3N4 ceramics produced under the same sintering conditions and with the same additive system.

Mechanical and wear properties of Si3N4–W composites using tungsten boride powder

Silicon nitride–tungsten (Si3N4–W) composites were fabricated by reduction of tungsten boride under hot-press sintering in a nitrogen atmosphere. The fabricated composite consisted of mainly β–Si3N4 and W. The Si3N4 matrix grains were composed of an elongated and bimodal structure similar to conventional Si3N4. The mechanical properties of the composites in terms of fracture toughness and strength were almost the same as those of a monolithic Si3N4 produced under the same sintering conditions. The sliding wear properties of the composites were evaluated using a ball-on-disk machine under unlubricated sliding conditions against a commercial Si3N4 ceramic ball. The tungsten (W) content had a significant effect on the composite wear properties. In particular, for a composite disk with a W content of 8 vol% the specific wear rate of the opposing ball was decreased around ten times compared to the monolithic Si3N4. The composites had higher wear resistance compared with the conventional silicon nitride, which was attributed to the formation of debris consisting of W, Si, and O. The debris restricted the adhesion of the two surfaces.

Comparison between SEM and TEM Imaging Techniques to Determine Grain‐Boundary Wetting in Ceramic Polycrystals

Two different non‐oxide ceramics, Si3N4 and SiC, were characterized with respect to their grain‐boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain‐boundary wetting occurred. SEM imaging of plasma‐etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si3N4 as well as SiC, suggesting the presence of an intergranular glass film. High‐resolution TEM studies of the Si3N4 sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si3N4 matrix grains. However, when high‐resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma‐etching response. Although SEM imaging showed a similar interface contrast for both Si3N4 and SiC ceramics, HRTEM studies clearly revealed grain‐boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.

Si3N4–TiN–Y2O3 ceramics derived from chemically modified perhydropolysilazane

[Si–Y–Ti–O–C–N] multicomponent powders were synthesized by pyrolysis at 1000 °C, in NH3 flow, of chemically modified perhydropolysilazane using yttrium triisopropoxide and titanium tetrachloride. [Si–Y–Ti–O–C–N] powders yielded uniform and fine-grained Si3N4–TiN–Y2O3 ceramics by heat treatment at 1800 °C in N2. The fully densified Si3N4–TiN–Y2O3 ceramics were also synthesized by heat treatment at 1800 °C, followed by powder-vehicle hot pressing at 1800 °C in N2. The resulting ceramics revealed that TiN was dispersed as particles having a size range of about 60–600 nm and the fine particles less than 80 nm were dispersed within the β–Si3N4 matrix grains.

Microstructure and Fracture Toughness of Si3N4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry

Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary‐phase chemistry. Si3N4 grain growth was dominated by diffusion‐controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack‐propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary‐phase chemistry plays a dominant role with respect to the mechanical behavior of liquid‐phase‐sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary‐phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary‐phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.

Si3N4–SiC–Y2O3 ceramics derived from yttrium-modified block copolymer of perhydropolysilazane and hydroxy-polycarbosilane

A polymeric precursor for the Si3N4–SiC–Y2O3 ceramic system was synthesized by block copolymerization of perhydropolysilazane (PHPS) with hydroxy-polycarbosilane (PCS-OH), followed by chemical modification with yttrium methoxide. Fully dense Si3N4–SiC–Y2O3 ceramics were successfully synthesized by pyrolysis of the polymeric precursor at 1000 °C, followed by hot pressing at 1800 °C in N2. The resulting ceramics revealed that β–SiC particles were dispersed in a size range of about 10–600 nm, and a large amount of β–SiC submicron particles were segregated at the β–Si3N4 matrix grain boundaries. It was found that the yttrium-modified block copolymer of PHPS and PCS-OH yielded unique binary ceramics composed of β–SiC–Y2O3 and β–SiC nanoparticle-dispersed Si3N4–Y2O3.

Percolation study on electrical resistivity of SiC/Si3N4 composites with segregated distribution

Experimental and theoretical studies are made to account for the formation of segregated networks in SiC/Si3N4 particulate composite. It was observed in the present experimental work that fine SiC particles are uniformly dispersed along boundaries of Si3N4 matrix grains. The proposed theoretical model takes into account the volume fraction of dispersed particles necessary to form a percolating network as a function of the size ratio of SiC particle to Si3N4 grain. Threshold particle loading for percolation is estimated by applying a modified two-dimensional bond percolation model and then electrical resistivity of the composite by using percolation and general effective media equations. Both predicted values of threshold particle loading and electrical resistivity were found to agree closely with the present experimental values.

Fabrication of Si3N4/SiC Composite Ceramics by Reactive Hot-Pressing from Elemental Powders.

Using elemental powders, reactive hot-pressing was carried out to prepare Si3N4/SiC composite ceramics. Silicon and graphite powders with additives (Al2O3 and Y2O3) were reacted at 1873K in gaseous nitrogen (0. 15MPa) and hot-pressed at 2073K for 7.2ks with applied pressure of 70MPa, and finally formed a Si3N4(β-Si3N4)-SiC(β-SiC) composite with a small amount of glassy and crystalline phases from additives. The resulting composites had 12-34vol%SiC, and the SiC grains were granular with sizes of about 0.1-1μm, and homogeneously distributed and well bonded to the matrix Si3N4. On the other hand, the Si3N4 matrix grains were rod-like in shape with sizes of 0.3-3μm. It was found that the composite ceramics had a relatively higher bending strength at both ambient and elevated (at 1423K) temperatures; about 1000 and 700 MPa, respectively. Fracture toughness of 5.3 MPam1/2 was obtained by indentation fracture tests. It was evident that reactive hot-pressing was very effective process for in-situ synthesis of Si3N4/SiC composite ceramics.

Fabrication and Mechanical Properties of Si<sub>3</sub>N<sub>4</sub>/SiC Nanocomposite with Pressureless Sintering and Sinter-Post-HIPing

Si3N4/SiC composites containing 0-30 vol% SiC were fabricated by pressureless sintering and sinter-post-HIPing method using fine Si3N4 and nano-sized SiC powders. Specimens reached full density (>96% theoretical density (th.d.)) except the composites made by pressureless sintering which contained a small amount of sintering additives or a large amount of SiC. All composites with 10 mass% sintering additives reached full density (-99% th.d.) by sinter-post-HIPing. The addition of SiC particles greatly influenced the grain morphology of the Si3N4 matrix. The average grain thickness, length and aspect ratio decreased with increasing SiC content because of grain boundary pinning by SiC particles. Also the mechanical properties were markedly dependent upon the fraction of SiC particle reinforcement, as well as matrix grain size and porosity. When the materials were pressureless sintered, the strength of the composite increased with SiC additions below 20 vol%. Further increase in the SiC content resulted in a corresponding decrease in strength. Maximum strength of 1150MPa was attained at 20 vol% SiC addition. The increase in strength by addition of SiC is due to decrease of grain size of Si3N4 matrix grain size, and the decrease by further increase in SiC content is caused by porosity and SiC agglomerate. On the other hand, toughness decreased monotonically with SiC content. The strength of all samples was enhanced by sinter-post-HIPing without any change in fracture toughness. The strength of sintered samples with 10-15 vol% SiC increased by -20% to 1260MPa by post-HIPing. The increase in strength by post-HIPing was interpreted as being due to a decrease flaw size.

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si3N4/SiC) were studied by combined high‐resolution transmission electron microscopy and electron energy‐loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si3N4 matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si3N4, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si3N4 grain boundaries and at phase boundaries between Si3N4 and SiC. The Si3N4 grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω‐Si3N4 grains are discussed on the basis of these experimental results.

Influence of SiC Particle Size on Creep Properties of Si<sub>3</sub>N<sub>4</sub>/SiC Composite Ceramics

Si3N4/SiC composite ceramics containing 20mass% dispersed SiC particles were fabricated by hot-pressing at 1800°C under 35MPa for 2h. SiC powders with average particle sizes of 0.03, 0.27, 0.60 and 1.20μm were used, and the influence of the SiC particle size on the creep properties was investigated. Creep tests were performed by 4-point bending for 100h at 1250-1350°C under 250-450MPa in air. Grain growth in the Si3N4 matrix was prevented by the dispersed SiC particles. The Si3N4 matrix grain size decreased with decreasing SiC particle size. Steady state creep strain rate ε increased with decreasing Si3N4 matrix grain size. Accordingly, the creep properties were not improved by the dispersed SiC particles. Stress exponents, n were 1.5-2.7. It was assumed that the creep deformation was controlled by grain boundary sliding accompanied by cavitation. Moreover, weight gains of Si3N4/SiC composite ceramics were larger than that of monolithic Si3N4 after creep tests. However, the bending strength of Si3N4/SiC slightly increased.

Influence of SiC Particle Size on Microstructure and Mechanical Properties of Si<sub>3</sub>N<sub>4</sub>/SiC Composite Ceramics

Si3N4/SiC composite ceramics containing 20mass% dispersed SiC particles were fabricated by hot-pressing at 1800°C under 35MPa for 2h. SiC powders with average particle sizes of 0.03, 0.27, 0.60 and 1.20μm were used, and the influence of the SiC particle size on the microstructure and mechanical properties (3-point bending strength and fracture toughness values) was investigated. As a result, grain growth and α-β transformation in Si3N4 matrix were prevented by the dispersed SiC particles. The Si3N4 matrix grain size decreased with decreasing SiC particle size, and in Si3N4/SiC (0.03μm) and Si3N4/SiC (0.27μm), α-phase remained. The fracture toughness decreased with decreasing Si3N4 matrix grain size. Accordingly, the fracture toughness was not improved by the dispersed SiC particles. In Si3N4/SiC (0.03μm), the bending strength showed a maximum value of 1161MPa at room temperature and 950MPa at 1400°C. It is considered that the remarkable improvement in the strength of Si3N4/SiC (0.03μm) is attributed to fine grains of Si3N4 matrix. On the other hand, other composites showed almost the same strengths as monolithic Si3N4.

HREM and AEM studies of Yb2O3-fluxed silicon nitride ceramics with and without CaO addition

Abstract The grain-boundary microstructure and interface chemistry of Yb2O3/Al2O3-fluxed sintered Si3N4 materials with and without the addition of CaO has been investigated by means of high-resolution and analytical electron microscopy. Each Si3N4 grade examined, exhibited a thin amorphous intergranular layer at both homophase and heterophase boundaries. The presence of calcium within this amorphous layer is confirmed in the CaO-doped materials. Furthermore, no solid solution of CaO within the Si3N4-matrix grains could be detected. The observed variability in the grain-boundary film thickness appears to be closely related to changes in the glass composition, in particular to the Ca content. The addition of CaO resulted in a widening of the intergranular film of up to 0·4 nm. Moreover, postsintering heat-treatment, which induces crystallization of the secondary phase pockets, led to only small changes in the intergranular film thickness of 0·1 nm when compared to the as-sintered specimens, for both CaO-doped and undoped materials.

Elemental Analysis of Matrix Grain Boundaries in Sic Whisker Reinforced Si3N4 Based Composites

ABSTRACTThe structures of SiC/Si3N4 interfaces and Si3N4 matrix grain boundaries in Ceramic Matrix Composites (CMC) were investigated by high resolution electron microscopy. The light element chemistry of the interfaces was analyzed by high spatial resolution (∼3 nm) position resolved EELS in a field emission TEM and by high spatial resolution EDS in a dedicated scanning transmission electron microscope (STEM). High-angle annular dark-field (HAADF) imaging (resolution < 1 nm) technique was used to determine the distribution of yttrium atoms at matrix grain boundaries and at SiC/Si3N4 interfaces. HAADF images suggest that yttrium might diffuse into Si3N4 crystals bounding the interfacial and grain boundary regions.

Microstructures and mechanical properties of Si3N4-SiC composites prepared by combustion synthesis and HIP sintering.

The microstructures and mechanical properties of Si3N4-SiC composites containing 8 to 46 vol% Sic, which were prepared from combustion synthesis and HIP sintering without additives, were studied and compared with those of Si3N4 prepared from conventional methods. Transmission electron microscopic observation revealed the existence of some small SiC particles within the Si3N4 matrix grains. The fracture toughness of the Si3N4-SiC composite was independent of Si, C content, probably because the SiC grains were smaller than the critical size for effective toughening and the formation of elongated Si3N4 grains was not promoted by the Sic dispersions. But SiC additions modified the high-tempera-ture strength of the Si3N4-SiC composites and increased the microhardness at all temperatures.