Microstructure and Property Characterization of Sintered Si3N4, SiC, and SiAlON

Commercially produced pressureless sintered Si₃N₄, SiC, and SiAlON were characterized with respect to density, phases present, bend strength, and oxidation resistance. The room-temperature bend strengths of sintered Si₃N₄, SiC, and SiAlON are comparable. However, the room-temperature strengths are much lower (=40 to 50%) than the room-temperature strength of hot–pressed Si₃N₄ (NC-132). The strength loss in Si₃N₄ and SiAlON materials at high temperature was attributed to a viscous grain-boundary phase retained during cooling from the sintering temperature. The oxidation resistance of sintered a-SiC was the best of any materials tested.     

Characteristics of Si3N4-SiO2-Ce2O3 Compositions Sintered in High-pressure Nitrogen

Full-density Si₃N₄-SiO₂-Ce₂O₃ compositions were prepared by sintering with 2.5 MPa nitrogen pressure at temperatures of 1900° and 2090°C. Room-temperature flexural strengths near 700 MPa for sintered material compared favorably with the strength of hot-pressed material. At 1370°C, where flexural strengths as high as 363 MPa were obtained, it was observed that the coarsest structure was the strongest and the finest structure was the weakest. One of the compositions tested, Si₃N₄-8.7 wt% SiO₂-8.3 wt%-Ce₂O₃, was found to have excellent 200-h oxidation resistance at 700°, 1000°, and 1370°C, without incidence of 700° to 1000°C phase instability and cracking.     

Effects of Oxidation and Oxidation Under Load on Strength Distributions of Si3N4

The room-temperature strength distributions of a sintered and a hot-pressed Si₃N₄ were examined in the as-machined condition, after oxidation at 1370°C and after oxidation under load at 1370°C. The strength-controlling flaw populations were highly transient in nature. Both the duration of oxidation and the magnitude of the applied load were observed to effect changes in strength. This dynamic situation is related to both strengthening and weakening processes, which at times may occur simultaneously in the same strength distribution.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO₂-dispersed Si₃N₄ composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO₂ (2.5 mol% Y₂O₃) in Si₃N₄ was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m^1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO₂. The dependence of fracture toughness of Si₃N₄–2.5YZrO₂ on ZrO₂ content can be related to the formation of zirconium oxynitride because of the reaction between ZrO₂ and Si₃N₄ matrix in hot isostatic pressing.     

Strength Retention in Hot-Pressed Si3N4 Ceramics with Lu2O3 Additives after Oxidation Exposure in Air at 1500°C

The effect of oxidation exposure on room-temperature flexural strength was examined in 3.33- and 12.51-wt%-Lu₂O₃-containing hot-pressed Si₃N₄ ceramics exposed to air at 1500°C for up to 1000 h. After oxidation exposure, the room-temperature strength of the ceramics was degraded, and strength retention decreased with time at temperature, dependent on the amount of additive. The retention in room-temperature strength displayed by the two compositions after 1000 h of oxidation exposure was 75%–80%. The degradation in strength was attributed to the formation of new defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk during oxidation exposure.     

Hot-Pressed Silicon Nitride with Lu2O3 Additives: Oxidation and Its Effect on Strength

The oxidation behavior and effect of oxidation on room-temperature flexural strength were investigated for hot-pressed Si₃N₄ ceramics, with 3.33 and 12.51 wt% Lu₂O₃ additives, exposed to air at 1400° and 1500°C for up to 200 h. Parabolic oxidation behavior was observed for both compositions. The oxidation products consisted of Lu₂Si₂O₇ and SiO₂. The Lu₂Si₂O₇ grew out of the surface silicate in preferred orientations. The morphology of oxidized surfaces was dependent on the amount of additive; Lu₂Si₂O₇ grains in the 3.33 wt% composition appeared partially in a needlelike type, compared with a more equiaxed type exhibited in the 12.51 wt% case. The high resistance to oxidation shown for both compositions was attributed to the extensive amounts of crystalline, refractory secondary phases formed during the sintering process. Moreover, after 200 h of oxidation at 1400° and 1500°C, the strength retention displayed by the two compositions was 93%–95% and 85%–87%, respectively. The strength decrease was associated with the formation of new defects at the interface between the oxide layer and the Si₃N₄ bulk.     

Microstructure and Short-Term Oxidation of Hot-Pressed Si3N4/ZrO2(+Y2O3) Ceramics

Composite ceramic materials based on Si₃N₄ and ZrO₂ stabilized by 3 mol% Y₂O₃ have been formed using aluminum isopropoxide as a precursor for the Al₂O₃ sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si₃N₄ matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO₂ grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO₂ incorporated some N during densification.     

Effect of Grain Size on the Thermal Conductivity of Si3N4

Polycrystalline Si₃N₄ samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si₃N₄ resulted in the formation of a large amount of ß-Si₃N₄ grains ∼10 µm in diameter and ∼50 µm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si₃N₄ materials were 80 and 102 Wm⁻¹K⁻¹, respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 Wm⁻¹K⁻¹). The calculated phonon mean free path of sintered Si₃N₄ was ∼20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si₃N₄ at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.     

The System Si3N4-SiO2-Y2O3

Subsolidus phase relations were established in the system Si₃N₄-SiO₂-Y₂O₃. Four ternary compounds were confirmed, with compositions of Y₄Si₂O₇N₂, Y₂Si₃O₃N₄, YSiO₂N, and Y₁₀(SiO₄)₆N₂. The eutectic in the triangle Si₃N₄-Y₂Si₂O₇-Y₁₀(SiO₄)₆N₂ melts at 1500°C and that in the triangle Si₂N₂O-SiO₂-Y₂Si₂O₇ at 1550°C. The eutectic temperature of the Si₃N₄-Y₂Si₂O₇ join was ∼ 1520°C.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Role of Si2N2O in the Passive Oxidation of Chemically-Vapor-Deposited Si3N4

The results of two-step oxidation experiments on chemically-vapor-deposited Si₃N₄ and SiC at 1350°C show that a correlation exists between the presence of a Si₂N₂O interphase and the strong oxidation resistance of Si₃N₄. During normal oxidation, k _p for SiC was 15 times higher than that for Si₃N₄, and the oxide scale on Si₃N₄ was found by SEM and TEM to contain a prominent Si₂N₂O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidation k _p increases over 55-fold for Si₃N₄, and 3.5-fold for SiC; the Si₃N₄ and SiC oxidize with nearly equal k _p's; and, most significant, the oxide scale on Si₃N₄ is found to be lacking an inner Si₂N₂O layer. The implications of this correlation for the competing models of Si₃N₄ oxidation are discussed.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Fracture Energies of Tape-Cast Silicon Nitride with β-Si3N4 Seed Addition

The fracture energies of the tape-cast silicon nitride with and without 3 wt% rod-like β-Si₃N₄ seed addition were investigated by a chevron-notched-beam technique. The material was doped with Lu₂O₃–SiO₂ as sintering additives for giving rigid grain boundaries and good heat resistance. The seeded and tape-cast silicon nitride has anisotropic microstructure, where the fibrous grains grown from seeds were preferentially aligned parallel to the casting direction. When a stress was applied parallel to the fibrous grain alignment direction, the strength measured at 1500°C was 738 MPa, which was almost the same as room temperature strength 739 MPa. The fracture energy of the tape-cast Si₃N₄ without seed addition was 109 and 454 J/m² at room temperature and 1500°C, respectively. On the contrary, the fracture energy of the seeded and tape-cast Si₃N₄ was 301 and 781 J/m² at room temperature and 1500°C, respectively, when a stress was applied parallel to the fibrous gain alignment. The large fracture energies were attributable primarily to the unidirectional alignment fibrous Si₃N₄ grains.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.