Effects of sintering aids on the microstructure and mechanical properties of laminated Si3N4/BN ceramics

Laminated Si₃N₄/BN ceramics with two types of sintering aids, MgO–Y₂O₃–Al₂O₃ (MYA) and La₂O₃–Y₂O₃–Al₂O₃ (LYA), were fabricated through roll compaction and hot-pressing. Sintering aids influence evidently the microstructure and mechanical properties of laminated Si₃N₄/BN ceramics. In comparison with La₂O₃–Y₂O₃–Al₂O₃, MgO–Y₂O₃–Al₂O₃ sintering aid is easier to form a glassy phase with lower viscosity and lower eutectic temperature, which is much easier to migrate into BN interlayers. This results in the denser interlayer microstructure and good bending strength of laminated Si₃N₄/BN ceramics at room temperature, but poor work of fracture (WOF) at room temperature, low strength and work of fracture at elevated temperature. In addition, the LYA sintering aid is good for forming elongated and interlocked β-Si₃N₄ grains and beneficial to the mechanical properties of the laminated Si₃N₄/BN ceramics.     

Oxidation of sintered silicon nitride

The influence of oxidation at 1200 °C in air for up to 1000 h on the mechanical properties of two Si₃N₄-Y₂O₃-Al₂O₃ materials with different Y₂O₃/Al₂O₃ ratios, Material A (Si₃N₄-13.9 wt% Y₂O₃-4.5 wt% Al₂O₃) and Material B (Si₃N₄-6.0 wt% Y₂O₃-12.4 wt% Al₂O₃), was investigated. The oxidation significantly improves the high-temperature strength and fracture toughness of both materials, but more for Material A. After oxidation, Material A at 1300 °C retains 93% of its room-temperature strength and 87% higher than that before the oxidation. The oxidation has a different effect on the room-temperature K _IC for the two materials. The room-temperature Weibull modulus of Material A decreased by more than half while the 1200 °C Weibull modulus decreased slightly after oxidation. The annealing treatment prior to oxidation had no effect on the high-temperature strengths of the materials after oxidation. The effect of oxidation on mechanical properties is discussed in terms of the microstructure change of the materials.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

High-strength zirconium diboride-based ceramic composites consolidated by low-temperature hot pressing

Abstract

Two compositions of ZrB₂-based ceramic composites containing Si₃N₄, Al₂O₃ and Y₂O₃ have been hot-pressed at different temperatures between 1673 and 1773 K for 60 min in vacuum. The densification behavior of the composites was examined during the sintering process. The microstructures of the composites were characterized by scanning electron microscopy, and the crystalline phases were identified by x-ray diffraction. The effects of Al₂O₃ and Y₂O₃ additives on the densification behavior and flexural strength were assessed. A relative density of ～95% was obtained after sintering at 1723 K or higher temperatures. The microstructures of the composites consisted of (Zr,Y)B₂, α-Si₃N₄ and Y₃(Al,Si)₅O₁₂ phases. The room-temperature flexural strength increased with the amount of additives and approached 1 GPa.     

High-strength zirconium diboride-based ceramic composites consolidated by low-temperature hot pressing

Two compositions of ZrB₂-based ceramic composites containing Si₃N₄, Al₂O₃ and Y₂O₃ have been hot-pressed at different temperatures between 1673 and 1773 K for 60 min in vacuum. The densification behavior of the composites was examined during the sintering process. The microstructures of the composites were characterized by scanning electron microscopy, and the crystalline phases were identified by x-ray diffraction. The effects of Al₂O₃ and Y₂O₃ additives on the densification behavior and flexural strength were assessed. A relative density of ∼95% was obtained after sintering at 1723 K or higher temperatures. The microstructures of the composites consisted of (Zr,Y)B₂, α-Si₃N₄ and Y₃(Al,Si)₅O₁₂ phases. The room-temperature flexural strength increased with the amount of additives and approached 1 GPa.     

Pressureless sintering of Si3N4 with Y2O2 and Al2O3

Pressureless sintering of Si₃N₄ with Y₂O₃ and Al₂O₃ as additives was carried out at 1750°C in N₂ atmosphere. Si₃N₄ materials which had more than 92% relative density were obtained with 20wt% addition of additives. The flexural strength of as-sintered materials containing 5 to 8.6wt% Al₂O₃ and 15 to 11.4wt% Y₂O₃ was in the range of 480 to 560 MPa at room temperature. The glassy grain-boundary phase of as-sintered materials crystallized to 3Y₂O₃ · 5Al₂O₃ (YAG), Y₂O₃ · SiO₂ (YS), Y₂O₃ · 2SiO₂ (Y2S) and 10Y₂O₃ · 9SiO₂ sd Si₃N₄ (NA) by heat-treatment at 1250° C for 3 days. A specimen containing 15wt% Y₂O₃ and 5wt% Al₂O₃ sintered at 1750° C for 4 h was heat-treated at 1250° C for 3 days to precipitate YAG and YS. The nitrogen concentration of the grain-boundary glassy phase of the specimen was found to be very high, and therefore the flexural strength of the crystallized specimen scarcely decreased at elevated temperatures (the flexural strength of this specimen is 390 MPa at room temperature and 360 MPa at 1300° C). Resistance to oxidation at 1200° C of the specimen was good as well as the flexural strength, compared with that of as-sintered materials.     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.     

Oxidation weight gain and strength degradation of Si3 N4 with various additives

Oxidation behaviour and strength degradation, due to long-term high temperature exposure in air, of Si₃N₄ with MgO, ZrO₂ or Y₂O₃, as densification aids have been studied. It was found that the weight change and strength degradation in the specimens depend largely on the kind of densification aid. Strength degradation, which generally occurred and was greatest at higher additive levels, was related to the generation of pits in the specimen surface. Possible mechanisms for pit formation are discussed.     

Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

High Temperature Construction Materials on the Basis of Nanodisperse Silicon Nitride

Si₃N₄‐Al₂O₃‐Y₂O₃, Si₃N₄‐TiN and Si₃N₄‐AIN‐Al₂O₃‐Y₂O₃ (β‐sialon) nanopowders with the specific surface area of 60–70 m²/g and average particle size of 30–50 nm have been prepared by plasmachemical synthesis. By means of the hot pressing method at 1850°C compact materials with fine‐grained structure were prepared from this powders as well as from mixture of Si₃N₄‐Al₂O₃‐Y₂O₃ with the second phase (10 wt.% SiC‐Si₃N₄, ZrO₂, TiN nanopowder). Addition of the second phase to silicon nitride improves material strength.     

Microstructural control of green bodies prepared from Si-based multi-component non-aqueous slurries and their effects on fabrication of Si3N4 ceramics through post-reaction sintering

A Si-based slurry containing Si particles covered with Y₂O₃ and MgO nanoparticles (NPs) has been successfully prepared and then applied to shape Si-based green compacts for the fabrication of silicon nitride (Si₃N₄) ceramics via post-reaction sintering. It was found that Y₂O₃ and MgO NPs modified with polyethyleneimine-oleic acid complex (PEI-OA) could be effectively attached to Si particles by simple mixing in dense toluene slurry. Field emission scanning electron microscopy observations confirmed the attachment of PEI-OA-modified sintering aids to Si particles without forming large NP agglomerates. The adsorption of the PEI-OA-modified sintering aids and PEI-OA on the surface of Si particles drastically improved the stability of the Si-based toluene slurry, which was subsequently molded through wet vacuum casting and dewaxed to fabricate a Si-based green body. The obtained green body was nitrided at 1375?°C for 4?h at a N₂ pressure of 0.15?MPa and further sintered at 1850?°C for 2?h at a N₂ pressure of 0.9?MPa. The adsorption of sintering aid particles on the Si surface reduced the number of contact points between Si particles in the green body, which effectively suppressed the Si melting process during nitriding and improved the characteristics of the produced nitride body such as the degree of nitriding and α/(α+β) ratio of Si₃N₄, leading to the successful fabrication of high-density Si₃N₄ ceramics during the subsequent densification step.     

Corrosion of silicon nitride ceramics in aqueous HF solutions

The leaching behaviours of hot-pressed Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as additives and hot isostatically pressed Si₃N₄ without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF₃. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol^–1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si₃N₄ ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions.     

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.     

Sintering and compensation effect of donor and acceptor codoped 3 mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350°C. The enhancement is accounted for by a liquid phase sintering mechanism. While the addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol% at 1300–1600°C results in the decrease of final density and in the destabilization of the tetragonal (t) phase of the 3 mol% Y₂O₃-t-ZrO₂ (TZP). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m) ZrO₂ phase and a second Ta₂Zr₆O₁₇ phase. The decrease is attributed to the increase of m-ZrO₂ content in these samples. Complete phase transformation from t-ZrO₂ to m-ZrO₂ observed in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor and acceptor codoping defect chemistry.     

Phase relationships in the system Si3N4-SiO2-La2O3

Phase relationships in the system Si₃N₄-SiO₂-La₂O₃ have been investigated after cooling from 1700° C. Two phases, 2Si₃N₄·La₂O₃ (monoclinic) and La₅ (SiO₄)₃N (hexagonal), were identified; the other two phases in the system, LaSiO₂N (monoclinic) and La₄Si₂O₇N₂ (monoclinic), were found to dissociate to La₅ (SiO₄)₃N and a glass after cooling from temperatures above 1650° C. The unit cells of 2Si₃N₄·La₂O₃, LaSiO₂N and La₄Si₂O₇N₂ have been determined and compared with those of preceding works. The results are discussed in relation to the intergranular phases observed when Si₃N₄ is sintered with La₂O₃ additions.     

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.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si₃N₄ ceramics containing Y₂O₃ and Al₂O₃ as sintering aids.The test environments for this study consisted of air with 0, 1.2,and 6.4 v/o H₂O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.     

Pressureless sintered SiAlON with low amounts of sintering aid

-SiAlONs of compositions Si_2.6Al_0.393Y_0.007O_0.4N_3.6 and Si_2.6Al_0.384Y_0.014O_0.4N_3.6 were pressureless sintered from mixtures of Y₂O₃ and separately milled -Si₃N₄, AlN, and SiO₂. On sintering, the carbon content of these SiAlONs was reduced to negligible levels and their oxygen content was also proportionately reduced, presumably due to reaction of carbon with SiO₂. These SiAlONs had densities in excess of 98% of theoretical, four-point bend strengths of 460 and 155 MN m^–2 at r.t. and 1400° C, respectively, and 1400° C oxidation rates lower than those reported in the literature for hot-pressed Si₃N₄ and for a similar but stronger SiAlON with 2.5 wt % Y₂O₃. These results indicate that increasing the Y₂O₃ content of SiAlONs increases their strength but decreases their oxidation resistance.