The evolution of hydriding and nitriding reactions during ball milling of titanium in ammonia

Abstract

Recently it has been demonstrated that a range of metal nitrides can be easily produced by ball milling elemental metals at room temperature in an environment of nitrogen or ammonia [1, 2]. However, details of the reaction processes which take place during milling and the possible phase transformations which occur during subsequent annealing have not been established. In many cases, milling in ammonia is found to enhance the nitriding process compared with milling in nitrogen gas [3, 4]. This raises the question of the role of ammonia in determining the reaction sequence leading to the final nitride phase. In a previous study [5], we reported on the formation of Ti2A1N by ball milling of Ti-A1 elemental mixtures in an ammonia atmosphere. The reaction process was studied by monitoring changes in gas pressure during milling. It was found that the adsorption of ammonia gas on newly exposed particle surfaces created by ball impact or shearing resulted in an initial decrease of the milling chamber pressure. Further milling appeared to induce the decomposition of adsorbed ammonia and, ultimately, the evolution of hydrogen as indicated by an increase in the milling chamber pressure. In this paper, we have studied the reaction process of a simple gas-solid system NH3-Ti in some detail by monitoring gas pressure changes and correlating these with compositional and phase analysis of milled powders. Our results provide considerable insight into the mechanochemical process occurring during milling. In this experiment, we used high purity elemental titanium powders (99.9%, 100 mesh) and anhydrous ammonia as starting materials. The milling process was performed in a vertical planetary ball mill. A stainless steel cell was loaded with 4 g of Ti powder and several hardened steel balls (diameter = 12 mm), and sealed with a Viton o-ring. In order to avoid oxygen contamination, the mill was purged with N H 3 several times and a pressure of 200 kPa was maintained prior to milling. The pressure of the milling chamber was monitored with a pressure gauge, over the pressure range from -100 kPa to 300 kPa. The structural development of powders at various stages of milling was investigated by X-ray diffraction (XRD) analysis using Co radiation (Z= 0.1789 nm). The annealing behaviour of milled powders was studied using a Shimadzu differential thermal analyser (DTA) at a heating rate of 20 °C/min in a dried argon flow. The H and N contents of as-milled powders were determined using combustion elemental analysis (Carlo Erba 1106). Fig. 1 shows the observed pressure variations as a function of milling time. The pressure initially decreased rapidly, dropping to a minimum pressure of -95 + 5 kPa (partial vacuum) at the end of 51 h. During further milling the pressure increased and attained the maximum measurement limit of the gauge (300 kPa) after 120 h milling. This pressure was maintained during continued milling up to 329 h. This final pressure is much higher than the starting ammonia pressure (200 kPa). Such a pressure variation is similar to that observed in the TiAI-NH3 system [5]. In order to establish the relation between the powder composition, structural changes and observed pressure variations, the milling process was halted at different milling times to give samples corresponding to different pressure changes. These samples were firstly examined for composition and the analysis results are illustrated in Fig. 2. It is clear that the N content increases with increased milling time, but the H content decreases for milling at times beyond the minimum in the mill chamber pressure. The evolution of microstructure during milling is illustrated by the typical XRD patterns of the milled samples in Fig. 3. A sample milled for 18 h, where the pressure had dropped to 50 kPa, has a complex structure. An fcc TiN phase appears to have been formed as indicated by peaks represented by the solid symbols. Appreciable unreacted Ti (crosses) was also found. A third phase (indicated by the open