Nitridation in the Processing and Preparation of Metals and Ceramics O. Addemir, A. Tekin and C.K. Gupta* Faculty of Chemistry and Metallurgy, Istanbul Technical University, 80286 Maslak, Istanbul, Turkey; *Materials Group, Bhabha Atomic Research Centre, Bombay 400 085, India ABSTRACT 1. INTRODUCTION The formation and separation of metal nitrides from Most of the elements in the periodic table combine a variety of materials including ferroalloys, oxides, with nitrogen to form nitrogen compounds or nitrides. natural materials and minerals is an important com- Based on their structure and bonding characteristics, ponent in current materials processing practice. they have been divided into four groups and among Ferroalloys like ferroniobium and ferrovanadium these the transition metal nitrides and the diamond-like undergo nitridation by ammonia yielding a mixture of nitrides exhibit more readily usable properties. Among iron nitrides and the refractory metal nitride. Acid the many properties of the nitrides, the high melting leaching effectively separates the iron nitride from the point, high hardness and resistance to corrosion are mixture. Niobium nitride thus obtained is used to very attractive. However, nitrides are also brittle, produce niobium metal by pyrovacuum treatment. readily oxidise at high temperatures and many of them Vanadium nitride converts to metal when pyrovacuum thermally decompose. While some of these limitations treatment is followed by electrorefining. Nitride have been partially overcome in novel compositions suitable for decomposition to metal can be obtained like sialon, the extensive use of the majority of nitrides from niobium pentoxide by reacting ammonia or by is restricted by these limitations. In recent times, the reacting a mixture of carbon and nitrogen. Rice husk is special combination of nitride properties has been a ready-made intimate mixture of silica and carbon. It exploited in devising some useful processes. These can be converted to silicon nitride or to a mixture of processes, which finally result in the production of silicon carbide and silicon nitride by caibonitrothermic metals, use nitrides as the intermediates. These involve reduction, β sialon can be directly prepared from preparation of nitrides, starting from ferroalloys, pure kaolinite by carbonitrothermic reduction. Ilmenite is compounds and also minerals by easily executable another mineral amenable to nitridation processing. It processes. Nitrides of silicon have a special status as yields after reaction with nitrogen in presence of end-product nitrides on account of their unique carbon, a separable titanium nitride-iron mixture. All combination of properties. Preparation of these nitrides these attractive features of nitridation have been from new types of raw materials, therefore, remains an discussed in this paper. The possibility of incorporating important area of research. Both these aspects, nitridation into the process flowsheets has been pertaining, respectively, to preparation and use of highlighted. nitrides as intermediates and to preparation of nitrides as end products have been discussed in this paper. 273 Vol. 15, No. 4, 1996 Nitridation in the Processing and Preparation of Metals and Ceramics 2. PREPARATION OF NITRIDES 2.1. J. Ferroniobium Iron can be nitrided both in nitrogen and in There are many general methods for the preparation ammonia. The nitridation takes place to a greater of nitrides. A convenient method is extent with ammonia at temperatures exceeding 400°C. Similarly, the nitridation of pure niobium has been investigated in both nitrogen and ammonia 161. M+—N 22 = MNn 2 However, no published information was available /4/ on the nitridation of ferroniobium for the formation of This method is based on the fact that nitride is thermo- nitrides. Some predictions can be made on the basis of dynamically stable at temperatures at which significant the phase diagrams of the Fe-Nb, Fe-N and Nb-N reaction rates between metal and nitrogen at about 1 systems 111. At room temperature, the stable phases are atm pressure are established. Many metals which do FeNb and Fe2Nb in the Fe-Nb system at 35% Fe; in the not undergo the above reaction or undergo that reaction Fe-N system they are Fe2N, Fe4N and Fe2Ni.x, and in only at high nitrogen pressures can be nitrided by the Nb-N system they are NbN and Nt^N. Nitrided reaction with ammonia ferroniobium would, therefore, contain a mixture of iron and niobium nitride phases. M + nNH3 = MNn+ |-nH2 It is known from the nitriding behaviour of pure metals that, for efficient reaction, the metal should be The nascent nitrogen produced during the dissociation used in the form of a fine powder. A similar considera- of ammonia is very reaction and nitrides the metal. tion applies to the nitridation of a ferroalloy. Thus Other important reactions useful for the formation of ferroniobium powder was used as the starting material the nitride are carbothermic reduction of an oxide IM. The powder was contained in an alumina boat and under nitrogen or ammonia. charged into a tube reactor. The charge was heated to the experimental temperature in the range of 500 to 1050°C and then maintained at the reaction tempera- M02n + —nC + —N 22 =MN+—CO Y 3 2 5 ture for a predetermined duration. Ammonia flow was maintained in the reactor during heating, soaking and The temperature required for the occurrence of all these cooling. The product was characterised by measuring reactions is in the range of 800 to 1200°C. Metal weight changes and by X-ray diffraction. The results nitridation may be initially inhibited by a protective are summarised in Table 1. surface oxide film. To achieve a practically useful The results in the table indicate that the weight gain conversion rate, it is advantageous to work with increased with increase of temperature up to 950°C to powders of <10 μηι size. These general methods of 1000°C and then remained steady. Nitriding appears to nitridation are application to a variety of materials, be very sensitive to the particle size of the powder; very including ferroalloys, oxides and also minerals. poor nitriding was observed when powder coarser than 270 mesh was used. X-ray diffraction pattern showed the presence of discrete nitride phases of iron and 2.1. Nitriding of Ferroalloys niobium and also a small amount of FeNbN phase. The processing of ferroalloys by nitridation begins with nitriding the alloy to obtain the mixed-iron alloy constituent nitride. Kirby and Fray /1-3/ were the first 2.1.2. Ferrovanadium to investigate a process for upgrading ferrochromium Even though vanadium nitride could be obtained by by a route involving nitriding. Subsequently, Suri, heating vanadium metal powder at 1250°C under a Singh and Gupta /4,5/ investigated processes for nitrogen or ammonia atmosphere 161, there is no upgrading ferroniobium and ferrovanadium by a mention in the literature of the conditions for nitriding similar route. ferrovanadium to obtain the nitrides of the ferroalloy 274 Ο. AdJemir, Α. Tekin and C.K. Gupta. High Temperature Materials and Processes Table 1 Nitriding of ferroniobium by ammonia Particle size of ferroalloy Nitriding Weight gain, Nitrogen % powder, mesh size temperature, °C content, wt% -270 750 4 - -270 800 7 - -270 900 11.5 5.76 -270 950 10.5 7.20 -270 1000 11.5 7.34 -140 950 5 - -270 950 10.2 8.50 -270 1000 12.17 - -325 1000 12.8 8.60 Nitriding time : 4.5 to 5 h, Sample weight: 3 g, Ammonia flowrate : 200 ml/min. constituents. According to the phase diagrams of the The nitridation of ferrovanadium was carried out 151 Fe-N and V-N systems 111, iron and vanadium each by loading a known quantity (3, 20 or 80 g) of form a number of nitrides. The nitrides of iron were ferroalloy powder in an inconel boat in a silica tube mentioned earlier. Vanadium forms VN].X and V2Ni-y reactor. The charge was heated under ammonia flow to phases in addition to the vanadium-nitrogen solid temperatures ranging from 900 to 1000°C. Nitrided solution. Nitrided ferrovanadium can be expected to samples were cooled and weight gain was measured. yield a mixture of iron nitride and vanadium nitride The results are summarised in Table 2. phases. Table 2 Nitriding of ferrovanadium by ammonia Temperature, °C Duration, h Weight gain, % jj 900 5 8.5 900 6 9.6 950 5 10.3 950 6 11.3 950 7 11.2 950 8 13.0 1000 7 13.0 Sample weight: 3 g, Powder size : -270 mesh, Ammonia flowrate : 200 ml/min. 275 Vol. 15, No. 4, 1996 Nitridation in the Processing and Preparation of Metals and Ceramics The results in the table indicate that the weight to the nitrides was accomplished by Krishanamurthy, gain, which is indirectly a measure of uptake of Venkataramani and Garg /10/ by carbonitrothermic nitrogen by the alloy, depends on temperature and reduction. A compacted mixture of niobium pentoxide duration of the experiment. The maximum weight gain and carbon was heated to 1500°C under flowing of 12 to 13% is obtained at 950°C to 1000°C in 6 to 7 nitrogen. The oxide-carbon mixture converted h. X-ray diffraction pattern of the nitrided samples completely to the nitride in less than 2 h. Residual showed the presence of Fe2.5N, Fe3N and 5VNi.x carbon and oxygen contents in the nitrides thus phases. Chemical analysis of the nitride product gives obtained were 0.2 and 0.4%, respectively. The residual 36.4% Fe, 48.8% V and 11.49% N. impurity contents tend to be higher in the case of tantalum nitride. While the presence of significant quantities of residual carbon and oxygen impurities 2.2. Nitriding of Oxides may be considered as a disadvantage of the process as The conversion of oxide to nitride as a step in the far as nitride preparation per se is considered, it is not a production of pure metal from oxide was pioneered by serious limitation if the nitride formation is considered Guidotti, Atkinson and Kesterke /8,9/.