Ceramic materials and phosphors based on silicon nitride and sialon

This review covers some of the main trends in the field of synthesis and the applications of materials based on silicon nitride presented in publications over the past 10–15 years. Attention is paid to the technique for synthesizing nitride and oxynitride compounds and for the production of ceramic construction materials and phosphors in silicon nitride systems.     

New heat treatment methods for glass removal from silicon nitride and sialon ceramics

Sialon ceramics were discovered simultaneously (but independently) in late 1971 at Newcastle University and also at the Toyota Research Laboratories in Japan. During the 30 years since their original discovery, the Newcastle laboratory has made a significant contribution to current understanding of the science and technology of these materials. Sialons are of interest as engineering materials for high temperature (>1000°C) applications because they can be pressureless-sintered to high density and be designed to retain good mechanical properties even up to 1350°C, whereas competing metallic materials are weaker and prone to corrosion. A characteristic disadvantage of all nitrogen ceramics is that an oxide additive is always included in the starting mix to promote densification, and this remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since the glass melts at 1000°C, the high temperature properties of the final ceramic are in fact determined by the properties of the grain-boundary glass. The most common method of improving high-temperature performance is to heat-treat the material at temperatures of 1100–1350°C in order to devitrify the glass into a mixture of crystalline phases. More specifically it is desirable to convert the glass into a sialon phase plus only one other crystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max 1400°C) in contact with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compounds which satisfy these requirements. The present paper gives an overall review of this subject area and then summarises recent work at Newcastle aimed at total removal of residual grain boundary glass. This has been achieved by: (1) a post-preparative vacuum heat treatment process to remove the grain boundary glass from silicon nitride based ceramics in gaseous form, (2) above-eutectic heat-treatment (AET) of sialon-based ceramics to crystallize grain-boundary liquid into five-component crystalline sialon phases.     

Effect of oxide additive in silicon nitride on interfacial structure and strength of silicon nitride joints brazed with aluminium

Silicon nitride ceramics with Y₂O₃ and Al₂O₃ as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.     

Interference coatings based on synthesized silicon nitride

Silicon nitrides are synthesized by ion-assisted deposition with only one coating material and a nitrogen-ion-beam source. All the SiN(x) films are amorphous and mechanically strong. A wide range of refractive indices from 3.43 to 1.72 at a wavelength of 1550 nm is obtained. Near-IR antireflection coating and a bandpass filter based on the multilayers of SiN(x) and Si are demonstrated.     

Creep of silicon nitride

General features of silicon nitride based ceramics, which may well influence their creep behavior are presented. Then, the most commonly invoked models for the microscopic mechanisms assumed to take place during creep (viscous flow, solution-precipitation, cavitation and shear thickening) are analyzed. Finally, the very numerous macroscopic and microscopic experimental findings about the plastic deformation of silicon nitride based ceramics at high temperatures, such as the fundamental role played by the secondary phases, the essential compressive-tensile asymmetry, and the microstructural evolution accompanying creep are summarized and discussed in terms of those models.     

Bond strength and interfacial structure of silicon nitride joints brazed with aluminium-silicon and aluminium-magnesium alloys

From the results of the bending strength and Weibull modulus of the joints of silicon nitride ceramics brazed using aluminium-silicon and aluminium-magnesium alloy filler metals at a temperature of 1073 K for 0.9 ksec in a vacuum of 1.3 × 10^–3 Pa, silicon, especially, present in a small amount in the filler metals, was found to be effective in improving the bond strength, while magnesium in the filler metals was harmful to the joining. This can result in the formation of a thick stable alumina layer on the surfaces of the filler metals containing magnesium during brazing which prevented contact of the filler metals with the silicon nitride ceramics.     

The interaction between silicon nitride and several iron,nickel and molybdenum-based alloys

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile.Trade Mark of the United Kingdom Atomic Energy Authority.Trade Mark of Henry Wiggin and Co. Ltd.Trade Mark of Union Carbide Corporation.     

The microstructures of silicon nitride/alumina ceramics

The microstructures of materials formed by sintering or hot-pressing mixtures of silicon nitride and alumina have been studied by transmission electron microscopy. The probable mechanism of transformation of the reactants to form β′-silicon aluminium oxynitride (β′-sialon) via a liquid phase sintering process, which is analogous to a similar transformation in hot-pressed silicon nitride containing a magnesia additive, is proposed. The origin and crystal symmetry of an unknown second phase is discussed. The residual quantity of this phase, known as the X-phase, is controlled mainly by the silica impurity content of the initial silicon nitride powder.     

The wetting of silicon nitride by chromium-containing alloys

The wetting of ceramic materials by metallic melts is the most important characteristic of brazing alloys. The effect of chromium additions to copper-base alloys on the wetting of silicon nitride was investigated. Wetting experiments were carried out on Si₃N₄ using liquid Cu-Cr, Cu-Ni-Cr, Cu-Si-Cr and Cu-Ni-Si-Cr alloys. The addition of chromium to liquid copper up to its solubility limit promoted wetting on Si₃N₄. Improved wetting with a higher chromium content was achieved by the addition of nickel to the Cu-Cr alloys. The formation of an interfacial reaction layer, which is detrimental for brazing ceramics, was suppressed by the addition of silicon to the chromium-containing brazing alloys.     

The strength and oxidation of reaction-sintered silicon nitride

The structure of reaction-sintered silicon nitride is studied using scanning electron and optical microscopy at various stages during nitriding, for a range of nitriding and compacting conditions. The strength is then evaluated and interpreted in terms of the microstructure. It is found that fracture always occurs in a brittle manner by the extension of the largest pores. The effects of prolonged annealing in air above 1000† C on both the structure and strength are investigated. At 1400† C, cristobalite is formed. If the temperature is then maintained above 250† C, the strength is enhanced, but below this temperature the oxide layer cracks and reduces the strength.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

Reaction chemistry at joined interfaces between silicon nitride and aluminium

Joined interfaces of HIPed additive-free silicon nitride ceramics/aluminium braze bonded at a low temperature of 1073 K for 18 ks or at a high temperature of 1473 K for 1.8 ks in vacuum of 1.3 mPa and of β silicon nitride powders/aluminium powders bonded at the low temperature for 1.8 ks or 18 ks in the same vacuum are identified by analytical transmission electron microscopy and X-ray diffraction method. Mullite, some small crystals and β′-sialon are detected at the interface of the ceramics/aluminium braze bonded at the low temperature and 15R AIN-polytype sialon, β′-sialon, aluminium nitride, mullite and silica-alumina noncrystalline are detected at that bonded at the high temperature. At the interface of the two kinds of powders, aluminium nitride and silicon are also detected besides β′-sialon and silica-alumina noncrystalline even though the bonding was conducted at the low temperature. The interfacial reactions of the joints are influenced not only by bonding temperature but also by the oxide formed at the interface before bonded.     

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5?5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.     

Creep of reaction-bonded silicon nitride

The high temperature mechanical properties, mainly the creep behaviour of reaction-bonded silicon nitride (RBSN), a new engineering ceramic for the gas turbine, have been a point of considerable interest. During the recent development a remarkable increase of the creep resistance of RBSN has been reached and the latest data show creep rates of below 10^–6 h^–1 at 1300° C and 70 to 100 MM m^–2. Activation energies between 540 and 700 kJ mol^–1 and stress exponents of 1in vacuo. Methods to determine the amount of internal oxidation,namely X-ray diffraction analysis, electron microprobe analysis and Rutherford backscattering of -particles were used. The deleterious effects of the internal oxidation are explained in terms of the microstructure, mainly porosity and pore size distribution, and ways to avoid this effect are discussed.     

Analysis of coating interlayer between silicon nitride cutting tools and titanium carbide and titanium nitride coatings

Silicon nitride (Si₃N₄) cutting tools exhibit excellent thermal stability and wear resistance in the high-speed machining of cast irons, but show poor chemical wear resistance in the machining of steel. Conventional chemical vapour deposition (CVD) coating of Si₃N₄ tools has not been very successful because of thermal expansion mismatch between coatings and the substrate. This problem was overcome by developing a CVD process to tailor the interface for titanium carbide (TiC) and titanium nitride (TiN) coatings. Computer modelling of the CVD process was done to predict which phases would form at the interface, and the results compared with analyses of the interface. Three Si₃N₄ compositions were considered, including pure Si₃N₄, Si₃N₄ with a glass phase binder, and Si₃N₄ + TiC composite with a glass phase binder. Results of machining tests on coated tools show that the formation of an interlayer provides superior wear resistance and tool life in the machining of steel as compared to uncoated and conventionally coated Si₃N₄ tools.     

Proof-testing of hot-pressed silicon nitride

Proof-testing was investigated as a method for insuring the reliability of hot-pressed silicon nitride in high temperature structural applications. The objective of the study was to determine if the strength distribution of a population of test specimens could be truncated by proof-testing. To achieve this objective the strength of silicon nitride was measured at 25° C and 1200° C, both with and without proof-testing. At 25° C, however, the strength distribution was effectively truncated by proof-test ing. At 1200° C, however, the effectiveness of proof-testing as a means of truncating the strength distribution was determined by the resistance of the silicon nitride to oxidation. Although oxidation removes machining flaws that limit the strength of silicon nitride, long-term exposure to high temperature oxidizing conditions resulted in the formation of surface pits that severely degraded the strength. Provided the effects of high temperature exposure are taken into account, proof-testing is shown to be useful for truncating the strength distribution of hot-pressed silicon nitride at elevated temperatures.