Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Phase transformation and microstructural changes of Si3N4 during sintering

Changes of density, the - phase transformation, and composition of grains and grain boundaries during sintering of Si₃N₄ with various sintering conditions using additives of Y₂O₃ and Al₂O₃ were investigated. The phase determination of individual Si₃N₄ grains was performed by convergent beam electron diffraction. The relations between densification and transformation were divided into two groups, depending on the additive compositions. Aluminium dissolution into Si₃N₄ grains occurred mostly during - transformation process. The concentrations of aluminium and oxygen in the grain boundaries decreased as the - transformation progressed.     

Corrosion of S3N4 and sialons in V2O5 melts

Si₃N₄ based ceramics such as hot isostatically pressed Si₃N₄, hot pressed Si₃N₄, hot pressed sialons containing 0, 30, 60 and 100% of phase were corroded by V₂O₅ melts at 700 to 1000C. These Si₃N₄-based ceramics were oxidized to SiO₂ and dissolved into V₂O₅ melts. The surface chemical reaction controlled shrinking core model adequately described the relationship between the weight loss of the specimen and time for the corrosion reactions in V₂O₅ melts, and the apparent activation energies were 69 to 112kJ mor^–1. phase Si₃N₄ and sialon showed higher corrosion resistance than phase sialons, but no clear relationship between the corrosion rate and the content of additives was found. The specimens corroded by V₂O₅ melts showed no significant degradation in the fracture strength up to 30wt% of weight loss.     

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Processing and microstructural development of reaction-bonded sintering of a Si3N4/intermetallic composite

This paper focuses on the reaction-bonded sintering of a Si₃N₄/intermetallic composite. The preparation process and the microstructural characterization of this composite with different initial contents of Ni₃Al have been investigated. The effects of Ni₃Al addition on the nitridation of silicon and the phase transformation of Si₃N₄ formed, as well as the densification of the composite, also have been discussed. The addition of Ni₃Al particles accelerates the nitriding rate of silicon and results in the formation of -Si₃N₄ phase at low temperature due to the reaction occurring between silicon and Ni₃Al. Consequently, a completely nitrided and denser composite, compared with reaction-bonded sintered Si₃N₄, was obtained at low temperature. For comparison, pure silicon simultaneously processed was also investigated.     

Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Microstructure development of Si3N4-TiN composite prepared byin situ compositing

Si₃N₄-TiN nano-composite has been prepared by anin situ compositing method with silicon nitride powder, aluminium iso-propoxide and titanium butoxide as the raw materials. The densification mechanism during hot-pressing and the microstructure of the densified materials were characterized from the point of view of phase composition, TiN inclusion distribution and the interface properties. The dense materials were mainly composed of -sialons, residual -Si₃N₄, silicon oxynitride, and TiN. The materials were featured by the very fine size of the precursor-derived particulate TiN which was homogeneously distributed in the matrix.     

Phase relationships in Si3N4-AIN-MxOy systems and their implications for sialon fabrication

Phase relationships in Si₃N₄-AIN-M_xO_y systems involving -sialon, where M represents lithium, magnesium, calcium, yttrium, neodymium, samarium, gadolinium, dysprosium, erbium and ytterbium are outlined. Their implications for the formation and fabrication of single-phase -sialon and two-phase : sialon ceramics are discussed.     

In Situ synthesis of TiN-reinforced Si3N4 matrix composites using Si and sponge Ti powders

Dense Si₃N₄-TiN composites from Si and sponge Ti (0–40 wt %) powders were produced by in situ reaction-bonding and post-sintering under N₂ atomosphere. The fracture strength and toughness of Si₃N₄-17% TiN composite were found to be 537.9 MPa and 10.4 MPam^1/2, respectively. In the reaction-bonded bodies, Si₃N₄ grains were constructed with - and -Si₃N₄ structure as well as TiN and some amounts of residual Si phase. After post-sintering, the residual Si and -Si₃N₄ grains were transformed to -Si₃N₄ grains with rod-like shape. No intermetallic compounds (e.g., TiSi₂, Ti₅Si₃ and Ti₅Si₄) were formed at the interfaces between Si₃N₄ and TiN grains. The main toughening mechanisms were crack deflection and crack bridging caused by rod-like Si₃N₄ grains which were randomly dispersed in sintered body. Microcracking due to the dispersion of in situ formed TiN particles also contributed to the toughening of the sintered body.     

Compressive creep of Si3N4/MgO alloys

The compressive creep behaviour of four compositions within the Si₃N₄-Mg₂SiO₄-Si₂N₂O compatibility triangle were studied in air at 1400° C. Strain rate ( ) versus stress () was analysed to determine the stress exponent, n ( ). Cavitation during creep was determined by precise (sink-float) density measurements. Compositions close to the Si₃N₄-Si₂N₂O tie line exhibited no cavitation and had n1, whereas compositions close to the Si₃N₄-Mg₂SiO₂ tie line exhibited extensive cavitation and had n2. Test results are interpreted in terms of the volume fraction of the viscous phase present.     

Influence of powder characteristics on gas pressure sintering of Si3N4

The sintering behaviours of four kinds of Si₃N₄ powders were investigated by dilatometry in 10 atm N₂ at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si₃N₄ grains and reprecipitation as elongated -Si₃N₄ grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Microstructure of Si3N4-TiN composites prepared by chemical-vapour deposition

The shapes and the distribution of TiN within Si₃N₄-TiN composites prepared by the chemical-vapour deposition of a SiCl₄-TiCl₄-NH₃-H₂ system have been examined using an electron microscope. The TiN dispersion in the amorphous Si₃N₄ matrix was granular and its maximum size was 3 nm. The TiN dispersions in- and-Si₃N₄ matrices were contained in their respective crystal grains; however, the shape of the TiN dispersions in the-Si₃N₄ matrix was markedly different from that in the-Si₃N₄ matrix. Granular TiN dispersions with an average size of 10 nm were observed in the-Si₃N₄ matrix. On the other hand, the TiN dispersions in the-Si₃N₄ matrix were columnar with a diameter of several nm having its axis extended to the direction parallel to thec-axis of the-Si₃N₄ crystal.     

Texture development in Si3N4/BN fibrous monolithic ceramics

Preferred orientation was measured in Si₃N₄/BN fibrous monolithic ceramics using x-ray diffraction. The materials were manufactured by co-extrusion of polymer binder/ceramic blends which were subsequently pyrolized and then hot-pressed to produced a fully dense ceramic composite. A very strong modified wire texture was present in the BN with the basal planes aligned parallel to the axis of extrusion due to shear-induced reorientation of the platelet-shaped BN particles during co-extrusion. Texture was also observed in the Si₃N₄ and was attributed to a combination of co-extrusion and hot-pressing. After hot pressing, the basal planes of the rod-shaped -Si₃N₄ were observed to be preferentially aligned perpendicular to the extrusion direction. Measurements prior to hot-pressing revealed that a small amount (5%) of -Si₃N₄ was present in the -Si₃N₄ starting powder. Although texturing of the predominant -Si₃N₄ did not occur during co-extrusion, significant texturing of the -Si₃N₄ was observed. During subsequent hot-pressing, the pre-existing textured -Si₃N₄ particles appeared to act as seeds for transformation and preferred growth of rod-shaped grains parallel to the axis of extrusion.     

Preparation and some properties of chemically vapour-deposited Si3N4-TiN composite

Chemically vapour-deposited (CVD) Si₃N₄-TiN composite (a plate with the maximum thickness of 1.9 mm) has been prepared on a graphite substrate using a mixture of SiCl₄, TiCl₄, NH₃ and H₂ gases. The CVD was carried out at deposition temperatures,T _dep, in the range of 1050 to 1450 ° C, total gas pressures,P _tot, from 1.33 to 10.7 kPa and gas flow rates of 136 (SiCl₄), 18 (TiCl₄), 120 (NH₃) and 2720 (H₂) cm³ min^–1. The deposits thus obtained appeared black. The Ti content in the composites ranged from 2.1 to 24.8 wt % and was found in the form of Tin. The structure of the Si₃N₄ matrices varied from amorphous (initially) to the- and-type, with increasingT _dep. Most of the- and-type deposits had a preferred orientation (001) parallel to the deposition surface. While the deposition surface of the amorphous deposits showed a pebble structure, the surfaces of the- and-type deposits were composed of various kinds of facets. The heat-treating experiment suggested that-Si₃N₄ obtained in the present work was formed directly via a vapour phase, and not from crystallization of amorphous Si₃N₄ or from transformation of-Si₃N₄.     

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Pressure sintering of Si3N4

The sintering of Si₃N₄ with 5% MgO was investigated at 1450 to 1900 C under a pressure of nitrogen. A maximum density of 95% of the theoretical value was obtained, which is greater than that obtained by pressureless sintering. The sintering process was inferred to be liquid-phase sintering and divided into two processes; rearrangement and solution-precipitation. The contribution of rearrangement to densification was about 10% in the present system, and the rest, up to 17% was due to solution-precipitation. Application of the present method of sintering Si₃N₄ with a high strength grain-boundary phase at high temperature is surveyed.     

Stability of Si3N4-Al2O3-ZrO2 composites in oxygen environments

Oxidation of dense Si₃N₄-Al₂O₃-ZrO₂ and Si₃N₄-Al₂O₃ compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<1623 K (H= 100kJ mol^–1)), whereas at T> 1623 K (H = 800kJ mol^–1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si₃N₄-Al₂O₃-ZrO₂ composites compared to other ZrO₂-Si₃N₄ materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.     

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the + phase field at 900 C. The microstructures obtained at room temperature after cooling from 900 C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the -phase, molybdenum and silicon to the -phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the -phase and the resultant microstructure. In quenched material containing transformed , substantial age hardening can be obtained in the range 350 to 600 C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.     

Microstructures and subcritical crack growth in oxidized hot-pressed Si3N4

The microstructure of the oxide scales, primarily the size, distribution, and density of the pits, was characterized in hot-pressed Si₃N₄ oxidized at different temperatures from 1300 to 1450 C. These microstructural features and the chemical changes in Si₃N₄ due to oxidation were related to the elevated-temperature subcritical crack-growth (SCG) behaviour. Oxidation at 1375 C for 240h resulted in a measurable improvement in SCG over that in as-hot-pressed and 1300 C-oxidized Si₃N₄.