Microstructural studies on silicon nitride

Thin specimens of reaction sintered and hot pressed silicon nitride have been prepared by ion beam thinning and examined in the Harwell million volt microscope. It has been found that reaction sintered material consists of large grains, which are mostly-Si₃N₄, in a fine grained matrix of-Si₃N₄. Fibres are frequently observed within the pores, the type of fibre depending on the size of the pore. The hot pressed material consists largely of two types of grain, small angular grains of-Si₃N₄ and larger irregular grains. There is also some non-crystalline material between the angular grains and there are numerous small unidentified inclusions.The grains of-Si₃N₄ generally contain dislocations and examination of these shows that most have a 0001 Burgers vector. The remaining dislocations appear to be more complex, frequently occurring as multiple images, and have not been unambiguously identified. An analysis of dislocations in-Si₃N₄ shows that 0001 dislocations are the most stable and are also likely to be most mobile with {10¯10} as the primary slip plane.     

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.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

Densification of silicon nitride ceramics under sinter-HIP conditions

The influence of several sinter-HIP variables on the densification behaviour of silicon nitride-based ceramics has been investigated. The processing conditions were studied for Si₃N₄ powder mixtures containing controlled amounts of Y₂O₃+Al₂O₃ or Y₂O₃+MgO. The specimens were subjected to sinter-HIP cycles under argon or nitrogen atmospheres at various temperatures and pressures. The final density of the powder compacts exhibited a strong dependence not only on the applied pressure, the composition and the processing temperature, but also on the pressurization rate and the initial pressurization time. The microstructural changes induced by the application of high pressure were followed by transmission electron microscopy. On examination by TEM, large concentrations of dislocations, generated inside some-Si₃N₄ grains, were observed. Characterization of these dislocations showed thatb=0001 is their most frequently found Burgers vector. Also, two relaxation mechanisms, tending to release the stored energy of deformation in the-Si₃N₄ grains, namely grain penetration (a form of strain-induced boundary migration) and grain fragmentation (the formation of subgrains due to rearrangement of dislocations into low energy configurations), have been identified. The intergranular phases formed were characterized by energy dispersive X-ray spectroscopic analysis, X-ray diffraction and electron diffraction. The influence of different sinter-HIP cycles on the transformation of silicon nitride without additives was also investigated by X-ray diffractometry.     

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si₃N₄ which was investigated involves the nitridation of Si:Si₃N₄ powder compacts containing additions of sintering aids (e.g. Y₂O₃ and Al₂O₃) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si₃N₄ forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si₃N₄ phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si₃N₄ grains. The grains are surrounded by a thin intergranular amorphous film.     

Effect of CeO2, MgO and Y2O3 additions on the sinterability of a milled Si3N3 with 14.5 wt% SiO2

Specimens of milled -Si₃N₄ with 0 to 5.07 equivalent per cent of CeO₂, MgO or Y₂O₃ additions were pressureless sintered at 1650 to 1820° C for 4 h in static nitrogen at 34.5 kPa (5 psi) gauge pressure and while covered with a mixture of Si₃N₄+SiO₂ powders. The density — per cent addition — temperature plots showed maxima which, for all three additives, occurred between 1.2 and 2.5 equivalent per cent. Maximum densities resulted on sintering in the 1765 to 1820° C range and were 99.6 per cent of theoretical with 2.5 equivalent per cent CeO₂, 98.5 per cent of theoretical with 1.24 to 1.87 equivalent per cent MgO, and 99.2 per cent of theoretical with 2.5 equivalent per cent Y₂O₃. Also, densities 94 per cent of theoretical were obtained with as little as 0.62 equivalent per cent additive (1.0 MgO, 2.11 CeO₂ or 1.85 Y₂O₃, in wt%). X-ray diffraction showed that the materials were predominantly -Si₃N₄ with some or no Si₂N₂O. Scanning electron photomicrographs showed microstructures of elongated grains with aspect ratios of about 5, with all additives.     

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.     

Gas pressure sintering of β-silicon nitride

The sintering behaviour of -Si₃N₄ powder was investigated in 980 kPa (10 atm) nitrogen at 1800–2000 °C. It is shown that -Si₃N₄ has a higher sinterability than the finer -Si₃N₄. The solution of small grains and reprecipitation on large grains occurred during sintering at >1600 °C. The rate-determining step in the liquid-phase sintering is believed to be the diffusion of material through the liquid phase at grain boundaries. There was no abnormal grain growth during gas pressure sintering of -Si₃N₄. The microstructures of gas pressure sintered materials from -Si₃N₄ were more uniform than those from -Si₃N₄. The densification mechanism of -Si₃N₄ is discussed in relation to that of -Si₃N₄.     

A synthesis of mono-crystalline silicon nitride filaments

A catalytic process for synthesis of pure, mono-crystalline-Si₃N₄ filaments, with iron particles of some micrometres in diameter as catalyst, was investigated. Silicon subhydrides, producedin situ by reaction of silicon powder with hydrogen at 1300° C, were used as the silicon source. Experiments with molecular nitrogen as the nitrogen source failed, but the use of ammonia was successful. At 1300° C mono-crystalline-Si₃N₄ filaments up to some micrometres in diameter and some centimetres in length were successfully produced. These filaments exhibit tensile strength values between 30 and 50 GPa, and Young's modulus values between 550 and 750 GPa.     

Thermal conductivity of unidirectionally oriented Si3N4w/Si3N4 composites

-silicon nitride whiskers were aligned unidirectionally in silicon nitride sintered with 2 wt% Al₂O₃ and 6 wt% Y₂O₃. It was be densified by the Gas Pressure Sintering (GPS) method. Thermal conductivity of the sintered body with different amount of - silicon nitride whiskers was measured by the direct contact method from 298 K to 373 K. This unidirectionally oriented -silicon nitride whiskers grew into the large elongated grains, and improved also the thermal conductivity. The amount of -silicon nitride whiskers changed the microstrcuture, which changed the thermal conductivity.     

Mechanism of Si3N4 nucleation during carbothermal reduction of silica

The process of carbothermal reduction of SiO₂ in a nitrogen flow at temperatures of 1673–1723 K was investigated. It was established that mixtures obtained by the sol-gel technique are not microhomogeneous and consist of silica and carbon aggregates, inside which the processes of structure and phase ordering occur during heat treatment. The contact region of these aggregates is the place where SiO₂ reduction occurs. In this region the following transformation takes place: partial SiO₂ reduction, appearance of a film of melt of the composition SiO _x , where x<2, enveloping of the carbon particles by this melt, destruction-activation of the carbon particles, implantation of nitrogen in the highly oxygen-defective melt, formation of silicon oxynitride and, subsequently, silicon nitride. -Si₃N₄ forms in the presence of a large carbon nucleus, -Si₃N₄ forms in its absence (or on two-dimensional particles).     

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.     

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.     

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.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y₂O₃ as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol^–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined.     

HIP-sinteredβ- and mixedα-β sialons densified with Y2O3 and La2O3 additions

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y₂O₃ and/or La₂O₃ as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y₂O₃/La₂O₃:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreacted-Si₃N₄ was present in the samples in addition to-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favoured-sialon formation. The amount of intergranular phase increased by about 50% when Y₂O₃ was replaced by La₂O₃. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam^–1/2, but the Vicker's hardness is slightly lower, being 1400 kg mm^–2 at a 98 N load.     

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.     

Silicon nitride crystal structure and observations of lattice defects

In view of the considerable progress that has been made over the last 40 years on the microstructural design of silicon nitride and related materials of tailored properties for specific applications, a clear review of the current understanding of the crystal structure and crystal chemistry of silicon nitride is timely. The crystal structures, crystal chemistry, and lattice defect nature of silicon nitride are critically reviewed and discussed, with emphasis placed firstly on the structural nature of -silicon nitride (whether it is a pure silicon nitride, or should better be regarded as an oxynitride); and secondly on the space group of -silicon nitride (whether it is P6₃/m or P6₃). In conjunction with recent observations of vacancy clusters in -silicon nitride, a comprehensive view compatible with all the experimental facts with respect to the structural nature of -silicon nitride is tentatively presented.     

The nature of sialon joints between silicon nitride based bodies

Quite strong joints between silicon nitride based bodies have been made by incorporating a layer of aluminium and oxides between the bodies and heating in a nitriding atmosphere. The joints are resistant to thermal shock and maintain their strength at 1200° C. Microscopic, DTA and X-ray diffraction studies indicated that sialon phases are present in the joints, and that the bonding reaction involves the reduction of Si₃N₄ by aluminium and the subsequent renitriding of the resultant silicon, as well as the simultaneous nitriding of a portion of the aluminium. Transmission electron microscopy of a joint between hot pressed and reaction bonded silicon nitrides showed that 15R aluminium nitride polytype sialon was present on the reaction bonded side of the joint and ß-sialon on the hot pressed side.