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.     

Effects of conditions of preparation on the superconducting properties of niobium nitride

Conclusions Niobium nitride produced by nitriding niobium wire is brittle, which makes it unsuitable for many practical applications. By comparison, nitride specimens produced by nitriding niobium powder compacts of 20% porosity (treatment under a nitrogen pressure of 5 atm for 3 h at 1400°C) seem to hold considerable promise. The superconducting characteristics attainable with such specimens are T_c=15.4°K and T= 0.1°K.Translated from Poroshkovaya Metallurgiya, No. 12 (144), pp. 76–78, December, 1974.     

A Thermodynamic Model to Estimate the Formation of Complex Nitrides of Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript>(1–<Emphasis Type="Italic">x</Emphasis>)</Subscript>N in Silicon Steel

A complex nitride of Al _x Mg_(1?x)N was observed in silicon steels. A thermodynamic model was developed to predict the ferrite/nitride equilibrium in the Fe-Al-Mg-N alloy system, using published binary solubility products for stoichiometric phases. The model was used to estimate the solubility product of nitride compound, equilibrium ferrite, and nitride compositions, and the amounts of each phase, as a function of steel composition and temperature. In the current model, the molar ratio Al/(Al?+?Mg) in the complex nitride was great due to the low dissolved magnesium in steel. For a steel containing 0.52 wt pct Al_s, 10 ppm T.Mg., and 20 ppm T.N. at 1100 K (827 °C), the complex nitride was expressed by Al_0.99496Mg_0.00504N and the solubility product of this complex nitride was 2.95?×?10^?7. In addition, the solution temperature of the complex nitride increased with increasing the nitrogen and aluminum in steel. The good agreement between the prediction and the detected precipitate compositions validated the current model.     

Kinetics of hafnium nitride formation in nitrogen and ammonia streams

Conclusions A study was made of the kinetics of hafnium nitride formation in nitrogen and ammonia streams. It was established that the kinetics of the reaction of hafnium powder of 30- particle size with nitrogen and ammonia is described by a pseudo-topokinetic equation. Calculations have shown that, owing to the formation and subsequent decomposition of intermediate products (nitrohydrides), the rate of nitride formation is less in ammonia than in nitrogen. It was found that the range of homogeneity of hafnium nitride is represented by the compositions HfN_0.80-HfN_1.0. No signs of formation of a lower hafnium nitride (of the Me₂N type) were detected in the temperature range 600–1200°C.Translated from Poroshkovay Metallurgiya, No. 9 (141), pp. 6–10, September, 1974.     

The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy

The mechanism and the kinetics of growth of the nitrided zone of ternary Fe-2 at pct Cr-2 at pct Ti alloy was investigated by performing gaseous nitriding experiments at temperatures of 833 K and 853 K (560 °C and 580 °C) and at nitriding potentials r _N = 0.004 atm^−1/2 and 0.054 atm^−1/2. The microstructure of the nitrided zone was investigated by transmission electron microscopy and the elemental compositional variation with depth was determined by employing electron probe microanalysis. Fine platelet-type mixed Cr_{1 – x} Ti _x N nitride precipitates developed in the nitrided zone. To describe the evolution of the nitrogen concentration depth profile, a numerical model was developed with the following parameters: the surface nitrogen content, the solubility product(s) of the alloying elements and dissolved nitrogen in the ferrite matrix, and a parameter defining the composition of the inner nitride precipitates. These parameters were determined by fitting model-calculated nitrogen depth profiles to the corresponding experimental data. The results obtained demonstrate that the type of nitride formation (i.e., whether Cr and Ti precipitate separately, as CrN and TiN, or jointly, as mixed Cr_{1 – x} Ti _x N) as well as the amounts of mobile and immobile excess nitrogen taken up by the specimen considerably influence the shape and extent of the nitrogen concentration profiles.     

Effects of silicon powder structure of nitriding

A study has been made on how the structural state of the silicon affects the nitriding at 1200–1400C. As the initial powder becomes more defective, the nitriding accelerates, and there is an increase in the importance of reactions in the gas phase, with an increase in the proportion of the a modification of silicon nitride, and a tendency for elongated crystals to form.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 1–7, October, 1992.     

Thermodynamics of nitrogen in CaO-SiO2-AI2O3 slags and its reaction with Fe-Csat. melts

The solubility of nitrogen as the nitride ion in CaO-SiO₂-Al₂O₃ slags in equilibrium with N₂-CO gas mixtures and carbon was measured at 1823 K. The nitride capacity (C _N3-) was calculated to compare the nitrogen contents measured under different nitrogen and oxygen potentials.C _N3- decreased with increasing basicity and by replacing SiO₂ with A1₂O₃. The nitrogen partition ratio between carbon saturated iron and the slag was measured in CO gas at one atmosphere at 1823 K. By comparing the partition ratios with the corresponding nitride capacities measured by the gas-slag experiments, it was concluded that the oxygen partial pressure at the slag-metal interface was controlled by the Fe-FeO reaction. A new definition of nitride capacity was proposed based on the reaction between nitrogen and the network former,i.e., SiO₂ or A1₂O₃. This capacity could consistently explain the experimental results. Empirical equations were derived to estimate the activity coefficients of silicon and aluminum nitrides in the slags. On leave of absence from the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan.     

Thermodynamics of nitrogen in CaO-SiO2-AI2O3 slags and its reaction with Fe-Csat. melts

The solubility of nitrogen as the nitride ion in CaO-SiO₂-Al₂O₃ slags in equilibrium with N₂-CO gas mixtures and carbon was measured at 1823 K. The nitride capacity (C _N3-) was calculated to compare the nitrogen contents measured under different nitrogen and oxygen potentials.C _N3- decreased with increasing basicity and by replacing SiO₂ with A1₂O₃. The nitrogen partition ratio between carbon saturated iron and the slag was measured in CO gas at one atmosphere at 1823 K. By comparing the partition ratios with the corresponding nitride capacities measured by the gas-slag experiments, it was concluded that the oxygen partial pressure at the slag-metal interface was controlled by the Fe-FeO reaction. A new definition of nitride capacity was proposed based on the reaction between nitrogen and the network former,i.e., SiO₂ or A1₂O₃. This capacity could consistently explain the experimental results. Empirical equations were derived to estimate the activity coefficients of silicon and aluminum nitrides in the slags. On leave of absence from the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan.     

Physicochemical processes occurring in nitride and oxide-nitride composites with phosphate binders during heating

Conclusions The hardening of compositions based on aluminum and silicon nitrides is a result of the chemical reaction of their phosphate binder with the nitrides, while additions of, e.g.,-corundum act as inert fillers up to a temperature of 570K. The reaction products are amorphous acid mono- and disubstituted aluminum and silicon phosphates, which on heating at first condense, forming poly- and metaphosphates, and then, at temperatures above 870K, decompose, forming (depending on their composition) aluminum orthophosphate or silicon pyrophosphate.Aluminum nitride initially, after being combined with the phosphate binder, vigorously reacts with it, which may lead to clotting or rapid swelling of the composition. However, gradual mixing under conditions of cooling (or without it) enables masses of any consistency to be obtained.In the mechanism of hardening of the compositions investigated a dominant role is played by the hydrogen bond. With rise in temperature the OH groups evaporate and the role of the hydrogen bonds diminishes, but a new, more powerful mechanism becomes operative, linked with the polymerization of the phosphates. Above 970K the polymer structure, which attains its maximum development at 870–970K, gradually disintegrates, but at the same time a new process-sintering-commences. Accordingly, as has been demonstrated by our investigation into the temperature dependence of the strength of the compositions, all the materials investigated pass through stages of hardening (470K), strengthening (470–970K), partial strength loss (by 30–40% in the range 970–1270K), and high-temperature sintering.The end products of the thermal decomposition of the polyphosphates up to 1520K are (depending on the nature of the nitride filler) aluminum orthophosphate or silicon pyrophosphate, present in amounts of 5–10%.At 1270–1370K a liquid phase (aluminum metaphosphate or silicon polyphosphate) forms in the compositions and then gradually disappears as a result of its further reaction with the fillers. Under these conditions an additional amount of AlPO₄ or SiP₂O₇ is formed.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 50–54, May, 1982.     

Fe Nitride Formation in Fe–Si Alloys: Crystallographic and Thermodynamic Aspects

A Fe–3wt pctSi alloy was gas nitrided to study the effect of Si on the Fe nitride formation. Both ε-Fe₃N_1+x and γ′-Fe₄N were observed at nitriding conditions only allowing to form single-phase γ′ layers in pure α-Fe. During short nitriding times, ε and γ′ simultaneously grow in contact with Si-supersaturated α-Fe(Si). Both nitrides almost invariably exhibit crystallographic orientation relationships with α-Fe, which are indicative of a partially displacive transformation of α-Fe being involved in the initial formation of ε and γ′. Due to Si constraining the Fe nitride growth, such transformation mechanism becomes highly important to the nitride layer formation, causing α-Fe-grain-dependent variations in the nitride layer morphology and thickness, as well as microstructure refinement within the nitride layer. After prolonged nitriding, α-Fe is depleted in Si due the pronounced precipitation of Si-rich nitride in α-Fe. The growth mode of the compound layer changes, now advancing by conventional planar-type growth. During nitriding times of 1 to 48 hours, ε exists in contact with the NH₃/H₂-containing nitriding atmosphere at a nitriding potential of 1 atm^−1/2 and 540 °C, only allowing for the formation of γ′ in pure Fe, indicating that Si affects the thermodynamic stability ranges of ε and γ′.

     

Use of (NH4)3AlF6 in the synthesis of aluminum nitride powder

Conclusions A study was made of the kinetics of formation of aluminum nitride in a charge consisting initially of aluminum mixed with ammonium hexafluoroaluminate. It was established that the addition of (NH₄)₃AlF₆ increases the rate of nitriding of aluminum and decreases the energy of activation of the process. Details are given of a new method for the preparation of aluminum nitride of improved quality.Translated from Poroshkovaya Metallurgiya, No. 3 (147), pp. 11–14, March, 1975.     

A Novel Bonding Method of Pure Aluminum and SUS304 Stainless Steel Using Barrel Nitriding

A great deal of research is being carried out on welding or bonding methods between iron and aluminum. However, it is not so easy to make Fe-Al bonding materials with both high strength and light weight. Recently, a new nitriding process has been proposed to produce aluminum nitride on an aluminum surface using a barrel. This study proposes a new concept in the production of a multilayer which has an AlN and Fe-Al intermetallic compound layer between the aluminum and steel using a barrel nitriding process. The bonding process was carried out from 893 K to 913 K (620 °C to 640 °C) for 18, 25.2, and 36 ks with Al₂O₃ powder and Al-Mg alloy powder. After the process, an aluminum nitride (AlN) layer and a Fe-Al intermetallic compound (Fe₂Al_5.4) layer were formed at the interface between the pure aluminum and SUS304 austenitic stainless steel. The thicknesses of the AlN layer and the intermetallic compound layer increased with increasing treatment temperature and time. The maximum hardnesses of the AlN layer and Fe₂Al_5.4 layers were found to be 377HV and 910HV, respectively, after barrel nitriding at 893 K (620 °C) for 18 ks.     

Salt Bath Nitriding of Fe<Subscript>3</Subscript>Al Based Intermetallic Compound

Iron aluminide Fe₃Al was produced in a vacuum arc melting furnace. The alloy was heat treated by salt bath nitriding at 580 °C for durations of 3, 6, and 9 h. The nitride layers formed on the surface were characterized with light optical microscopy (LOM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDXS), X-ray diffraction (XRD), and micro hardness measurements. The results showed that the nitride layer thickness increased with an increase in nitriding duration, while the layer hardness did not vary. The nitride layers were composed chiefly of iron nitride and aluminum nitride phases. The dry sliding friction and wear behaviors of nitrided iron aluminides were determined. The results revealed that the wear resistance decreased with increase in the length of nitriding.     

Structural state and mechanical properties of the material in the sialon-titanium nitride system after radiant heating

Conclusions Short-term radiant heating of the material of the Sialon-titanium nitride system in the temperature range 1230–2300°C is accompanied by oxidation of the titanium nitride with oxidation of rutile and titanium oxynitride (the lattice spacing of these compounds increases with increasing temperature) and also by the breakdown of '-Si₃N₄ to -Si₃N₄ and '-phase with a higher aluminum and oxygen content. The formation of rutile reduces the hardness of the material whereas formation of the oxynitride increases this parameter. A liquid phase consisting mainly of silicon and titanium disilicate appears on the surface of the specimens at temperatures above 1500°C. The material retains satisfactory strength in air at temperatures of up to 1200°C.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 60–66, May, 1988.The authors are grateful to V. V. Kovylyaev and V. V. Traskovskii for help in the experiments.     

Nitrogen solubility and aluminum nitride precipitation in liquid iron alloys containing nickel and aluminum

The solubility of nitrogen in liquid iron-base Fe-Ni-Al alloys has been measured up to the solubility limit for formation of aluminum nitride using the Sieverts’ method. Measurements were conducted over the temperature range from 1843 to 2023 K and aluminum concentration range from 1.5 to 3.0 wt pct Al. The effect of nickel additions was determined at 2, 5 and 10 wt pct Ni. The cross interaction parameter describing the effect of nickel and aluminum on the activity coefficient of nitrogen in iron was determined. The first and second order effects of nickel on the activity coefficient of aluminum also were determined. The solubility product of aluminum nitride increases with increasing aluminum content and increasing temperature. Addition of nickel decreases the solubility products of aluminum nitride in lower aluminum content alloys. However, the effect of the cross interaction terme _Al ^NiAl becomes significant with increasing aluminum content and compensates for the effects of the first and second order nickel-nitrogen and nickelaluminum interaction terms. Therefore the effect of nickel additions show little effect on the solubility products of aluminum nitride in higher aluminum alloys.     

Mechanical properties of materials obtained from an ultrafine aluminum nitride powder

Conclusions A study was made of the mechanical properties of materials produced from an ultrafine aluminum nitride powder. It is shown that use of such a fine powder enables virtually nonporous specimens to be produced having high strength characteristics: _c=1600 MPa, _tr= 320 MPa, microhardness of 16,800 MPa, microstrength of 3100 MPa, and microbrittleness of 1.9. On the basis of the results obtained it is possible to recommend aluminum nitride sintered from an ultrafine powder as a good-quality constructional ceramic material.Translated from Poroshkovaya Metallurgiya, No. 12(240), pp. 65–69, December, 1982.     

Solubility product of aluminum nitride in 3 percent silicon iron

The solubility product of aluminum nitride in 3 pct silicon iron was determined experimentally from 1273 to 1473 K with results described by the equation $$\begin{gathered} \log [pct \underline {Al} _{\alpha (3Si) } pct \underline N _{\alpha (3Si)} ] \hfill \\ = {\text{--11,900/}}T + 3.56 \hfill \\ \end{gathered} $$ whereT is in kelvins and concentrations are in weight percent. In the experiments the equilibrium distribution of nitrogen between purified gamma iron (fcc) and 3 pct silicon alpha iron (bcc) was determined between 1273 and 1523 K.