Effect of impurities on the properties of silicon nitride materials

Conclusions The presence of oxides in silicon nitride leads to the formation of an intergranular vitreous phase which has an adverse effect on the high-temperature physicomechanical characteristics of constructional silicon nitride ceramics. This phenomenon is particularly pronounced with silicon nitride contaminated with calcium, silicon, and alkaline metal oxides. Future development work on oxidation-resistant constructional materials based on silicon nitride should go in the directions of removal of impurities and use of additions forming solid solutions or refractory compounds with Si₃N₄ and SiO₂. Additions which can be employed for this purpose include oxides of AI, Mg, Y, and some rare-earth elements.Translated from Poroshkovaya Metallurgiya, No. 1(193), pp. 75–80, January, 1979.     

Wear resistance of silicon carbide and nitride based ceramic materials

Various factors that affect the nature of wear in SiC and Si₃N4₂ based ceramics have been analyzed. It is shown that adhesion, mechanochemical and diffusion interactions in the contact zone and wear due to fatigue, thermal stresses and abrasion are the predominant factors. Ceramics based on SiC and Si₃N₄ are shown to have excellent wear resistance. Poreless silicon nitride materials that have good chemical stability, heat and crack resistance appear promising as ceramic—metal friction couples and for metal machining. Silicon carbide based poreless materials are efficient ceramic—ceramic friction couples and for service under severe hydro and gas abrasive media attack.Translated from Poroshkovaya Metallurgiya, No. 5, pp. 3–8, May, 1993.     

Reaction of a molten mixture of titanium,aluminum, and silicon oxides with hot-pressed silicon nitride

The formation temperature of a liquid phase and the solidification temperature of a molten mixture of Al₂O₃-TiO₂-SiO₂ oxides on a silicon nitride substrate are determined. Data are obtained for the change in kinetics. It is established that the intensity of interaction of molten Al₂O₃-TiO₂-SiO₂ with silicon nitride depends on the oxide mixture composition. With heating there are two possibilities: improvement and worsening of Si₃N₄ crystallite wetting with a liquid phase as well as solidification of the melt. The temperature range where a liquid phase exists for actual materials is about 15°C, which markedly worsens the process of structure formation with Si₃N₄ during sintering.Translated from Poroshkovaya M etallurgiya, No. 5, pp. 39–44, May, 1993.     

Recrystallization of silicon nitride powders during hot pressing

Conclusions The hot pressing of comminuted silicon nitride powders enables a uniform fine-grained structure of one and the same mean grain size to be obtained irrespective of the type of starting Si₃N₄, powder. At milling times of more than 100 h no significant decrease in particle size is observed. The recrystallization of milled silicon nitride powders during hot pressing takes place chiefly in the fine fractions appearing during milling. The degree of recrystallization attained is higher with ultrafine active silicon nitride powders (PCS) than with comminuted powders.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 43–47, October, 1982.     

Strength and thermophysical properties of porous silicon nitride materials

Conclusions A slurry casting process has been developed for the production of porous silicon nitride specimens of 40–60%. It has been established that the strength and thermal conductivity of such specimens sharply decrease with increasing porosity. However, porous Si₃N₄ possesses fairly high resistance to oxidation in air, which suggests the possibility of employing it as a heat insulating material.Translated from Poroshkovaya Metallurgiya, No. 11(263), pp. 67–70, November, 1984.     

Features of the wetting of materials based on aluminum nitride by nickel alloys

Wetting kinetics for aluminum and silicon nitrides as well as for materials based on them by nickel alloys (Ni-Al, Ni-Cr, Ni-Cr-Al) have been studied. Microprobe analysis is used to study the structure and phase composition of the interaction zone. On the basis of phase interaction data in the systems AlN(Si₃N₄)-Ni-alloys recommendations are made for creating composite ceramic materials with a metal binder.     

THE NITRIDING OF SILICON POWDER COMPACTS

Abstract

The reaction-bonding process to prepare silicon nitride by nitriding silicon compacts was studied, and an examination made of the influence of raw material and process variables on the properties of the resulting silicon nitride. The silicon powder grain size and the impurities content were considered as powder variables, and the green density and thermal cycles as process parameters. The examination of green-density effects indicates that, under the experimental conditions, the gas permeation of nitrogen through the silicon compacts was the rate-determining step of the reaction-bonding process. Regarding the effect of nitriding temperature, the final conversion, Si to Si₃N₄, is an increasing function of the temperature in the range 1300–1400°C. As to the composition of silicon nitride obtained, α-phase formation is favoured when oxygen is present as an impurity in silicon powder. Finally, physical, chemical, and thermomechanical tests showed that reaction-bonded silicon nitride has good bending strength (21 kgf/mm²) and can be used in very severe conditions up to 1200°C.     

Development of nonoxide ceramics based on silicon carbide and nitride

Research on nonoxide ceramics based on silicon carbide and nitride is reviewed along with related technological developments. The role of I. N. Frantsevich in initiating the development of such materials is shown. Three main stages in the development of the ceramics are distinguished. The relationship between the physical properties and applications of ceramics based on SiC and Si₃N₄.Institute of Materials Sciences, Ukranian Academy of Sciences, Kiev. Translated from Poroshkovaya Matallurgiya, Nos. 7/8(380), pp. 24–32, July–August, 1995.     

Refractory materials of high thermal fatigue resistance for arc welding guide bushings

Conclusions A study was made of the production of materials of the A1₂O₃-Si₃N₄ system involving pressing and subsequent sintering in a nitrogen atmosphere. It was established that the addition of as little as 2.8 mole % alumina approximately trebles the mechanical strength of silicon nitride. The sharp increase in strength is attributable to enhanced cohesion of the silicon nitride particles brought about by the formation of the mullitelike compound xAl₂O₃ · ySiO₂ during the reaction of the Al₂O₃ with SiO₂ surface films on the Si₃N₄ particles. Al₂O₃-Si₃N₄ electrode holders for the electric arc welding of metals were subjected to tests under industrial conditions, in which their useful life was found to be 25 times longer than that of similar electrode holders made of porcelain.Translated from Poroshkovaya Metallurgiya, No. 12(192), pp. 92–95, December, 1978.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

Nitrogen Ceramics: Liquid Phase Sintering

Abstract

The role of a minor silicate eutectic liquid phase as a transport medium in sintering hot–pressed silicon nitride (β Si₃N₄) ceramics was identified in the 1970s. A similar mechanism is applicable to hot–pressed Si–Al–O–N ceramic alloys which offer an advantage in control of the final liquid volume and hence in superior high temperature mechanical properties. By increasing the liquid volume it is possible to densify ceramic alloys without application of pressure at the sintering temperature and hence to fabricate components of complex shape. The Lucas Syalon ceramics typify the new range of pressureless–sintered ceramics based on the β Si₃N₄ structure. They are fabricated from the ultrafine compound powders α Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and a polytypoid phase (a substitute for A1N). The ceramics consist of submicrometre solid solution crystals of general composition Si_3?xAl_xO_xN_4?x(x < 1) within a minor matrix phase which may be either a glassy Y–Si–Al oxynitride or be crystallized to form yttrogarnet. Analysis of matrix glass compositions shows them to be residues of liquids near to a ternary eutectic in the Y₂O₃–SiO₂–Al₂O₃ system which is well below the sintering temperature of ～ 1800°C. Sintering models, based on particle rearrangement due to dissolution of the major α Si₃N₄ component in the eutectic liquid and its reprecipitation as a β Si₃N₄ solid solution, are discussed. Properties and current applications of Syalon ceramics are surveyed briefly. PM/0266     

Reactions in the systems MoSi3N4 and NiSi3N4

The reaction products, formed during annealing of porous powder mixtures of Si₃N₄ with non-nitride forming metals like Ni or Mo, will depend on the partial pressure of N₂ in the atmosphere. In a diffusion couple, however, nitrogen has to be released at the Si₃N₄-interface during the formation of a metal silicide. It cannot escape easily and builds up a higher pressure of nitrogen at this interface. Therefore, the reaction products are different from those in porous pellets. This has been verified for NiSi₃N₄ and MoSi₃N₄ couples. The role of traces of oxygen on these reactions will be discussed.     

Solid-state diffusion bonding of silicon nitride using titanium foils

This article presents an effective way to control the interfacial reaction during solid-state diffusion bonding of silicon nitride (Si₃N₄) using titanium foils. The interfacial structure and its growth kinetics were analyzed in detail with scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). The actual phase sequence of the joint interfaces bonded at temperatures between 1473 and 1673 K is concluded to be Si₃N₄/Ti₅Si₃(N)/α-Ti(N)+Ti₅Si₃(N), which is different from the phase sequence observed at room temperature after bonding. The joints are very weak due to the formation of a brittle Ti₅Si₃(N) layer at the interface. To suppress the growth of the Ti₅Si₃ layer, a nitrogen-solution treatment of titanium foils prior to each bonding experiment is implemented. Although a perfect prevention of the Ti₅Si₃(N) layer formation is not achieved with this treatment, it is shown that the growth of the layer is effectively suppressed enough to improve the joint strength to a level 3 times higher than the case in which pure titanium is employed.     

Chemical stability of silicon nitride powders in biochemical media

We have used chemical and x-ray analysis to study the stability and phase composition of silicon nitride powders with different panicle size ranges and morphology in biochemical media (physiological solution, gastric juice, blood serum) at 37°C. We have established the high solubility of Si₃N₄ in physiological solution and blood serum. The solubility increases with the dispersity of the powder in all the media. We explain this behavior of silicon nitride in biochemical media using modern theories of bioinorganic catalysis.     

Production of silicon nitride by the acid enrichment of products of combustion of ferrosilicon in nitrogen

Production of silicon nitride by acid enrichment of products of interaction between ferrosilicon and gaseous nitrogen under conditions of self-propagating high-temperature synthesis (SHS) is studied. The effect of the nature of acid, its concentration, agitation of solution, and process temperature is determined. The reaction of the Si₃N₄ + Fe composite powder and the hydrochloric acid solution is found to have a stage behavior. The apparent activation energy of iron passing into the solution is determined. The purity of the produced Si₃N₄ powder is demonstrated to depend on a degree of nitration of SHS products. The chemical and phase composition of the powder and its specific surface are determined.     

Salient features of thermal expansion of multilayer cermet composites

The linear thermal expansion coefficient (LTEC) of multilayer brazed cermet composites made of alternating plates of Si₃N₄ and Cu (ZrO₂ and Cu) has been studied. The coefficient was observed to be anisotropic parallel and perpendicular to the plane of brazing. In the first case the coefficient of the composite was practically equal to that of silicon nitride and in the second case reached or at times even exceed that of copper. Moreover the LTEC was considerably higher than the value calculated for the composite on the basis of the additivity principle, the assumption being that the copper and silicon nitride expand independently. The anisotropy observed in the LTEC of the composite can be attributed to the anisotropy of copper expansion in such a material. The linear dimension of the same sample, heated repeatedly, was observed to decrease in the lateral direction; this is due to plastic deformation of the copper and the resultant thinning of the sample.Institute of Problems in Material Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 43–46, July–August, 1996.     

Model Tests and Analysis of Corrosion Resistance of Ceramic Tool Dies for Steel Thixoforming

Contact corrosion and cyclic melt corrosion test results of silicon nitride (Si₃N₄) ceramics in contact with semi‐solid and molten steel alloys are analysed in view of steel thixoforming applications. Corrosion effects are identified and model tests are evaluated by comparison of corrosive attack on steel thixoforming dies in application‐relevant forming experiments. The performed corrosion experiments show that a reaction zone of up to 20 μm thickness forms on the ceramic in model tests and forming experiments, respectively. Si₃N₄ ceramics reveal sufficient corrosion resistance in small‐scale forming series. Results show good agreement of contact corrosion tests with thixoforming experiments. Comparability of cyclic melt corrosion tests with thixoforming applications is limited by the casting powder used to prevent oxidation of steel melt.     

Improvement in the Mechanical Properties of Silicon Nitride Ceramic in High-Temperature Deformation Processes

Studies have been made on the changes in structure and properties of sintered materials: Si₃N₄ - 5 mass% Y₂O₃ - 2 mass% Al₂O₃, Si₃N₄ - 5 mass% Y₂O₃ - 5 mass% Al₂O₃, and Si₃N₄ - 40 mass% TiN on deformation in a high-pressure chamber of toroid type (pressure 4–5 GPa, temperature 1000–1600 °C), and also by direct extrusion with degrees of reduction of 55 and 72% (temperature 1750–1850 °C, pressure on the plunger 20–30 MPa). After pressure-chamber treatment, the materials have elevated mechanical characteristics: HV₁₀ ≈ 16.7 GPa, K_Ic up to 8.4 MPa · m^1/2 for the system Si₃N₄ - Y₂O₃ - Al₂O₃; and HV₁₀ ≈ 16.9 GPa, K_Ic up to 9.4 MPa · m^1/2 for Si₃N₄ - TiN. A structure feature is the small size of the coherent-scattering regions: 51 nm for Si₃N₄ and 65 nm for TiN in the system Si₃N₄ - TiN, and 33 nm for specimens in the system Si₃N₄ - Y₂O₃ - Al₂O₃. High-temperature extrusion results in a structure with β-Si₃N₄ grains elongated along the deformation direction. The anisotropic structure has K_Ic values in directions perpendicular to and parallel to the direction of extrusion of 11.5–12.0 MPa · m^1/2 and 7.5–7.8 MPa · m^1/2, respectively. The hardness after extrusion becomes 16.0 GPa.     

Impurities influence on multicrystalline photovoltaic Silicon

The formation of the grain structure in multicrystalline photovoltaic Silicon relies on basic phenomena which are not well understood and mastered in industrial processes. Nucleation, growth modes (facetted or not), grain competition and kinetics, coarsening are at the origin of the mean grain size and of the morphology of the grain structure which impacts drastically on the photovoltaic properties. During solidification, solute rejection (especially carbon and nitrogen) at the growth interface leads to an increase of the impurity concentration in the liquid phase and then to the precipitation of silicon nitride (Si₃N₄) and silicon carbide (SiC). As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation set up has been developed in order to measure the undercooling versus impurity concentration. The impurity concentration in the levitated Si droplet is controlled by the partial pressure of nitrogen or hydrocarbon gas. The concentration of nitrogen and carbon dissolved will be compared with theoretical predictions (phase diagram). And a new analytical model is proposed to understand the precipitation of SiC during the solidification process of an ingot.     

Effect of physicochemical treatment of initial powders on the properties of sintered ceramics in the Si3N4-Al2O3-SiO2-Y2O3 system. II. The structure and properties of sintered ceramics

In the first report [1], we described the methods and results of examining the properties of initial powders, and also the results of modifying these powders by means of different physicochemical interaction effects. In the second report, we examine the dense ceramics based on modified powders, and also generalize the data, obtained these investigations, on the role of the composition and state of the surface of the initial silicon nitride powder and the formation of the final properties of the Si₃N₄-Al₂O₃-SiO₂-Y₂O₃ ceramics.