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.     

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.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

Phase Relations and Stability Studies in the Si3N4-SiO2-Y2O3 Pseudoternary System

Composite powders were hot-pressed to determine the phase relations in the Si₃N₄-SiO₂-Y₂O₃ pseudoternary system. Four quaternary compounds, Si₃Y₂O₃N₄, YSiO₂N, Y₁₀Si₇O₂₃N₄, and Y₄Si₂O₇N₂, were identified. Studies of polyphase and single-phase materials in this system showed that these 4 compounds are unstable under oxidizing conditions. Materials within the Si₃N₄-Si₂N₂O-Y₂Si₂O₇ compatibility triangle precluded the unstable compounds, and are extremely resistant to oxidation.     

Hydrothermal Treatment of Si3N4 for the Improvement of Oxidation Resistance at 1400°C

The surface of Si₃N₄ ceramics was hydrothermally treated with HCl or H₂SO₄ using an autoclave. The thickness of the oxide layers formed on the Si₃N₄ samples decreased to one-fourth after oxidation at 1400°C by the treatment. The oxide layer of the treated samples was dense, and flaw formation in and beneath the layer did not occur at 1400°C. The avoidance of low melting Y-silicates by leaching Y₂O₃ is the reason for the improved oxidation resistance of the hydrothermally treated Si₃N₄, despite an increase in surface porosity through a 70 μm layer.     

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.     

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.     

Solid-Liquid Reaction in the Si3N4-AlN-Y2O3 System under 1 MPa of Nitrogen

The melting behaviors of selected compositions in the Si₃N₄-AlN-Y₂O₃ system were determined under 1 MPa of nitrogen. The phase diagrams of the ternary and their binary systems are presented. The lowest melting composition of the ternary system contains 15 mol % Si₃N₄, 25 mol % AIN, and 60 mol % Y₂O₃ and has a melting temperature of 1650°C. The binary eutectic compositions and temperatures are 15 mol % Si₃N₄ and 85 mol % Y₂O₃ at 1720°C, and 20 mol % AIN and 80 mol% Y₂O₃ at 1730°C.     

Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics

Si₃N₄/BN fibrous monoliths were prepared with 4 wt% Y₂O₃ added as a sintering aid to the Si₃N₄. Residual carbon, present in the billet before hot-pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hot-pressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot-pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y₂O₃ demonstrated linear-elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6-wt%-Y₂O₃/2-wt%-Al₂O₃ sintering aids.     

Oxidation Behavior of Silicon Yttrium Oxynitride

The oxidation behavior of the silicon yttrium oxynitride Y₁₀Si₇O₂₃N₄, so-called H-phase, in the temperature range 700–1400°C has been investigated. A nitrogen retention phenomenon in the oxidation product Y_4.67(SiO₄)₃O (O-apatite) is discussed. The H-phase is one of the four quaternary compounds identified in hot-pressed Si₃N₄ materials fabricated within the Si₃N₄–SiO₂–Y₂O₃ pseudoternary system.     

Strength and Creep Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics

The flexural strength and creep behavior of RE₂Si₂O₇–Si₃N₄ materials were examined. The retention in room-temperature strengths displayed by these ceramics at 1300°C was 80–91%, with no evidence of inelastic deformation preceding failure. The steady-state creep rates, at 1400°C in flexural mode, displayed by the most refractory materials are among the lowest reported for sintered Si₃N₄. The creep behavior was found to be strongly dependent on residual amorphous phase viscosity as well as on the oxidation behavior of these materials. All of the rare-earth oxide sintered materials, with the exception of Sm₂Si₂O₇–Si₃N₄, had lower creep strains than the Y₂Si₂O₇–Si₃N₄ material.     

Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4-SiC with Yb2O3 as a Sintering Aid

The oxidation behaviors of monolithic Si₃N₄ and nanocomposite Si₃N₄-SiC with Yb₂O₃ as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb₂Si₂O₇ and SiO₂ (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si₃N₄-SiC compared to the monolithic Si₃N₄. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb³⁺ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.     

Hot Isostatic Processing of Shocked Si3N4 Powder

To enhance the sinter ability of Si₃N₄, powders mixed with 0, 2, and 5 wt% Y₂O₃ were explosively shock-treated. Compacts of these powders were encapsulated in 96% silica glass containers and isostatically hot-pressed. The shocked Si₃N₄ with 5 wt% Y₂O₃ was pressed to a density of 3.09 g/cm³ (95.4% of theoretical) at 1400°C under 430 MPa for 3 h, whereas the unshocked material attained only 82.4% of theoretical density under the same hot isostatic pressing conditions.     

Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty-nine compatibility tetrahedra were established in the region Si₃N₄─AIN─Al₂O₃─Y₂O₃. The subsolidus phase relationships in the region Si₃N₄─AIN─YN─Y₂O₃ have also been studied. Only one compound, 2YN:Si₃N₄, was confirmed in the binary system Si₃N₄─YN. The solubility limits of the α'─SiAION on the Si₃N₄─YN:3AIN join were determined to range from m = 1.3 to m = 2.4 in the formula Y _{m /3}Si_{12- m} Al _m N₁₆. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si₃N₄─AIN─YN─Y₂O₃.     

Formation of N-phase and Phase Relationships in MgO–Si2N2O–Al2O3 System

Formation of N-phase in the system Mg,Si,Al/N,O was studied. Its composition was confirmed to be MgAl₂Si₄O₆N₄ (2Si₂N₂OMgAl₂O₄). Subsolidus phase relationships in the MgO–Si₂N₂O-Al₂O₃ system were determined. The results are discussed by comparing with two similar systems, CaO-and Y₂O₃–Si₂N₂O–Al₂O₃.     

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.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Phase Equilibria in the System Si3N4-SiO2-BeO-Be3N2

The 1780°C isothermal section of the reciprocal quasiternary system Si₃N₄-SiO₂-BeO-Be₃N₂ was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be₆Si³N₈, Be₁₁Si₅N₁₄, Be₅Si²N₆, Be₉Si₃N₁₀, Be₈SiO₄N₄, Be₆O₃N₂, Be₈O₅N₂, and Be₉O₆N₂. Large solid solubility occurs in β-Si₃N₄, BeSiN₂, Be₉Si₃N₁₀, Be₄SiN₄, and β-Be₃N₂. Solid solubility in β-Si₃N₄ extends toward Be₂SiO₄ and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be₂SiO₄ at 1880°C. A 4-phase isotherm, liquid +β-Si₃N₄ ( ss )Si₂ON₂+ BeO, exists at 1770°C.     

High/Low Modulus Si3N4-BN Composite for Improved Electrical and Thermal Shock Behavior

High-density Si₃N₄+6% CeO₂ composites with 5 to 50% BN were fabricated by hot-pressing. BN remained as a discrete phase. Dielectric constants were 4 to 8 and loss tangents were 0.0008 to 0.06 for the room temperature to 1100°C range for compositions with 10 to 50% BN. Thermal-expansion values perpendicular to the hot-pressing direction were somewhat less than those of hot-pressed Si₃N₄+6% CeO₂. Flexure strengths at room temperature were considerably lower than those of hot-pressed Si₃N₄+6% CeO₂ but values at 1000°, 1250°, and 1400°C in air were only slightly lower. Young's modulus values were found to decrease with increasing BN content at all temperatures. Better thermal shock resistance was found than for commercial hot-pressed Si₃N₄.