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.     

Ceramic Tool Concepts for the Semi‐Solid Processing of Steel Alloys

Two different ceramic tool concepts for the semi‐solid processing (Thixoforming) of steel alloys are presented. Materials selection is adapted to forming technology (Thixoforging, Thixoextrusion), preset die temperature, and resulting process conditions. Gas‐pressure sintered silicon nitride (Si₃N₄) is chosen as die material in low tool temperature (300...400 °C) thixoforging experiments due to its high strength and outstanding thermal shock resistance. High purity dense alumina (Al₂O₃) is applied as die material for high temperature (1200 °C) thixoextrusion tests. Thixoforging results using Si₃N₄ dies pre‐heated to 300 °C show sufficient thermal shock and corrosion resistance of Si₃N₄ and confirm the applicability of this tool concept. The high temperature tool concept developed at the Institute of Mineral Engineering (GHI) effectively reduced thermal shock impacts on extrusion dies. As expected, corrosion resistance of Al₂O₃ proved to be excellent. Further research will be carried out concerning long‐term behaviour of Si₃N₄ thixoforging dies as well as on the influence of extrusion speed and tool temperature on the quality of products extruded through Al₂O₃ dies at high temperature.     

Separation–permeation performance of porous Si3N4 ceramics composed of columnar β-Si3N4 grains as membrane filters for microfiltration

The separation–permeation performance of porous silicon nitride (Si₃N₄) ceramics (consisting of columnar grains connected at random in three dimensions) as membrane filters was evaluated, and compared with commercial Al₂O₃ membranes having a three-layer structure. Si₃N₄ membranes separate particles with diameters much less than their pore diameters. The permeability of Si₃N₄ membranes with separability values the same as those of the Al₂O₃ membranes was about 1.3–2.4 times as large as the Al₂O₃ membranes. Dead-end filtration examination, using Al₂O₃ particles with a particle size distribution, indicated that the Si₃N₄ membrane filtration mechanism obeyed the cake filtration mechanism although the particle size was smaller than the pore size of the Si₃N₄ membranes.     

Phase transformation of high aluminium fly ash in carbothermal reduction–nitridation at high temperatures

Abstract

Sialons (Si₅AlON₇, Si₄Al₂O₂N₆, Si₃Al₃O₃N₅ and SiAl₄O₂N₄) were synthesised from high aluminium fly ash by using a carbothermal reduction–nitridation process. The effects of temperature and the addition of carbon black on the phase compositions and morphologies have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermodynamic analysis has been used to study phase transformations. For the sample with the stoichiometric carbon black content, the product formed at 1500°C was single phase short prismatic Si₃Al₃O₃N₅. The products at 1600°C were Ca–α-Sialon accompanied by AlN, whereas at 1700°C, Si₄Al₂O₂N₆ and SiAl₄O₂N₄ formed. Increasing the carbon black at 1500°C, Si₃Al₃O₃N₅, β-SiC, 15R and Fe_xSi were produced as reaction products.     

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.     

Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Crystallisation of M-SiAlON glasses to I w -phase glass-ceramics: preparation and characterisation

The crystallisation of M-SiAlON glasses (M = Y, Ln) is particularly sensitive to small variations in composition and heat treatment temperature. The formation of I_w-phase in the YSiAlON system has been studied but little information concerning its nucleation, thermal stability, mechanical properties and extension into the lanthanide sialon series is available. In order to better understand the synthesis and to characterise more fully the I_w-glass-ceramic materials, a series of glass compositions have been prepared and characterised. These include M₃Si₃Al₂O_12.15N_0.90 (M = Y, Er, Ce, Yb); M_3.45Si₃Al₂O_12.76N_0.95 (M = Y, Er); Y₄Si₃Al₂O_13.50N and Y₄Si_3.37Al_1.50O_13.50N. Heat treatments were performed on these glasses and the resulting crystalline products have been studied by X-ray diffraction (XRD). Differential Thermal Analysis (DTA) in combination with XRD analysis of Y_3.45Si₃Al₂O_12.76N_0.95 and Ce₃Si₂Al₂O_12.15N_0.90 parent glass-compositions was used to investigate crystal phase formation. Young's modulus (E), hardness (H _v and fracture toughness (K _IC) of the glass-ceramics were measured. Glass-ceramics containing I_w-phase as the only detectable crystalline phase have been produced from M₃Si₃Al₂O_12.15N_0.90 and M_3.45Si₃Al₂O_12.76N_0.95 (M = Y and Er) compositions. No equivalent phase was found in the Ce- and Yb-sialon compositions.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these 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.     

Influence of SiC, Si3N4 and Al2O3 particles on the structure and properties of electrodeposited Ni

The structure and properties of electrodeposited nickel composites reinforced with inert particles like SiC, Si₃N₄ and Al₂O₃ were compared. A comparison was made with respect to structure, morphology, microhardness and tribological behaviour. The coatings were characterized with optical microscopy, Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) technique. The cross-sectional microscopy studies revealed that the particles were uniformly distributed in all the composites. However, a difference in the surface morphology was revealed from SEM studies. The microhardness studies revealed that Si₃N₄ reinforced composite showed higher hardness compared to SiC and Al₂O₃ composite. This was attributed to the reduced crystallite size of Ni — 12 nm compared to 16 nm (SiC) and 23 nm (Al₂O₃) in the composite coating. The tribological performance of these coatings studied using a Pin-on-disk wear tester, revealed that Si₃N₄ reinforced composite exhibited better wear resistance compared to SiC and Al₂O₃ composites. However, no significant variation in the coefficient of friction was observed for all the three composites.     

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.     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Preparation of Β-sialon from thermal-plant fly ash

Carbon reduction of fly ash from the Reftinskaya thermal plant was studied at temperatures from 1100 to 1600‡C in a nitrogen atmosphere. The ash was analyzed chemically and by x-ray diffraction. Β-Sialon phases with compositions Si₃Al₃O₃N₅ and Si₂Al₃O₇N were obtained by ash reduction. Conditions for the preparation of Si₃Al₃O₃N₅ were optimized.     

Thickness dependence of high-k materials on the characteristics of MAHONOS structured charge trap flash memory

The electrical characteristics of tunnel barrier engineered charge trap flash (TBE-CTF) memory of MAHONOS (Metal/Al₂O₃/HfO₂/SiO₂/Si₃N₄/SiO₂/Si) structure were investigated. The stack of SiO₂/Si₃N₄/SiO₂ films were used as engineered tunnel barrier, HfO₂ and Al₂O₃ films were used as charge trap layer and blocking oxide layer, respectively. For comparison, the electrical characteristics of MONOS (Metal/SiO₂/Si₃N₄/SiO₂/Si), MONONOS (Metal/SiO₂/Si₃N₄/SiO₂/Si₃N₄/SiO₂/Si), and MAHOS (Metal/Al₂O₃/HfO₂/SiO₂/Si) were also evaluated. The energy band diagram was designed by using the quantum-mechanical tunnel model (QM) and then the CTF memory devices were fabricated. As a result, the optimized thickness combination of MAHONOS structure was confirmed. The tunnel barrier engineered MAHONOS CTF memory showed a considerable enhancement of program/erase (P/E) speeds, retention time and endurance characteristics.     

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.     

Si3N4 ceramics formed by HIP using different oxide additions — relation between microstructure and properties

Si₃N₄-based ceramic materials formed by glass encapsulation and hot isostatic pressing (HIP) using different additions of Al₂O₃, Y₂O₃ and ZrO₂ have been characterized by analytical electron microscopy and X-ray diffractometry. The microstructures have been related to formation process and to room temperature hardness and fracture toughness of the ceramics. A high volume fraction of retained -Si₃N₄ after processing at 1550 °C gave the Si₃N₄ ceramics high hardness. The equi-axed grain morphology of the Si₃N₄ matrices in these materials, which contained only small amounts of residual glass, resulted in comparatively low fracture toughness values. Processing at 1750 °C reduced the amount of retained -Si₃N₄ substantially. When Y₂O₃ was added, the microstructure contained a comparatively large volume fraction of residual glass, and the Si₃N₄ was present mainly as high aspect ratio -Si₃N₄ grains. This type of microstructure gave an Si₃N₄ ceramic material with high fracture toughness combined with a lower hardness. Additions of ZrO₂ and/or Al₂O₃ resulted also at 1750 °C in an extremely small volume fraction of residual glass, and a major part of the Si₃N₄ was present as equi-axed grains. These ceramics exhibited medium hardness and toughness values, however, larger additions of ZrO₂ appeared to slightly increase toughness.     

Determination of relative in vivo osteoconductivity of modified potassium fluorrichterite glass–ceramics compared with 45S5 bioglass

Potassium fluorrichterite (KNaCaMg₅Si₈O₂₂F₂) glass–ceramics were modified by either increasing the concentration of calcium (GC5) or by the addition of P₂O₅ (GP2). Rods (2?×?4?mm) of stoichiometric fluorrichterite (GST), modified compositions (GC5 and GP2) and 45S5 bioglass, which was used as the reference material, were prepared using a conventional lost-wax technique. Osteoconductivity was investigated by implantation into healing defects in the midshaft of rabbit femora. Specimens were harvested at 4 and 12?weeks following implantation and tissue response was investigated using computed microtomography (μCT) and histological analyses. The results showed greatest bone to implant contact in the 45S5 bioglass reference material at 4 and 12?weeks following implantation, however, GST, GC5 and GP2 all showed direct bone tissue contact with evidence of new bone formation and cell proliferation along the implant surface into the medullary space. There was no evidence of bone necrosis or fibrous tissue encapsulation around the test specimens. Of the modified potassium fluorrichterite compositions, GP2 showed the greatest promise as a bone substitute material due to its osteoconductive potential and superior mechanical properties.     

Fabrication and characterization of porous unidirectional Si2N2O-Si3N4 composite

Porous unidirectional Si₂N₂O-Si₃N₄ composite was fabricated by in-situ nitriding of a porous unidirectional Si substrate. The porous unidirectional Si substrate having a diameter of 450 μm, was prepared by forming ethanol bubbles in a slurry which contained Si, Y₂O₃, Al₂O₃ and methylcellulose powder. After nitridation at 1400 °C, the Si substrate was transformed into Si₂N₂O-Si₃N₄ composite and the pore surface of the unidirectional Si₂N₂O-Si₃N₄ composite was covered throughout with Si₂N₂O fibers, which had a diameter of about 55 nm. The Si₂N₂O fibers were orthorhombic single-crystals with an amorphous layer having a thickness of about 1 nm. The compressive strength of the in-situ synthesized Si₂N₂O-Si₃N₄ composite was about 30 MPa.     

Effect of oxide addition on the sintering and high-temperature strength of Si3N4 containing Y2O3

The effect of oxide addition on the sintering behaviour and high-temperature strength of Si₃N₄ containing Y₂O₃ was studied at 0.1 to 30 MPa N₂ at 1600 to 2000° C. The addition of oxide, i.e. MgO, Al₂O₃, La₂O₃, or Nd₂O₃, was found to lower the densification temperature and increase the densification rate. The addition of Al₂O₃ or MgO reduced the strength of sintered materials at >1350° C. The addition of La₂O₃ or Nd₂O₃, on the other hand, did not affect high-temperature strength which remained equivalent to that of the material containing only Y₂O₃. These results indicate that the glassy phases in these systems are as refractory as that in the Si₃N₄-Y₂O₃.     

Effect of crosshead speed on the fracture toughness of Soda-lime glass, Al2O3 and Si3N4 ceramics determined by the surface crack in flexure (SCF) method

The fracture toughness of soda-lime glass, Al₂O₃ and Si₃N₄ specimens was measured by the surface crack in flexure method. For the soda-lime glass specimens, the fracture toughness was calculated from the initial crack size and flexure strength, and the value increased with increasing crosshead speed. This trend seems to be related to the difficulty in determining the critical crack size at fracture, since slow crack growth occurs during bending test. For the Al₂O₃ specimens, a halo region (stable crack growth region) was formed around the initial precrack during bending test. The halo size increased and the resultant flexure strength decreased with decreasing in the crosshead speed. The halo region, however, could not be observed in the Si₃N₄ specimens. Despite of the difference in the appearance of halo region, the fracture toughness of the Al₂O₃ and Si₃N₄ specimens was constant irrespective of the crosshead speed when the values were calculated with the critical crack sizes at fracture (halo incorporated crack sizes). The constant fracture toughness with the crosshead speed could be explained by the relation between the changes of halo size (thus critical crack size at fracture) and resultant flexure strength.