Walter Lengauer

Properties of bulk δ-TiN1-x prepared by nitrogen diffusion into titanium metal

Abstract Compact titanium nitride (δ-TiN1-x) within the compositional range TiN0.53-TiN1.00 was prepared by a diffusion process and several of its properties were investigated as a function of composition. The microstructures showed an equiaxial polygonal grain boundary arrangement with some individual pores in the centre, particularly for the substoichiometric samples, probably as a result of the diffusion process. The samples exhibited good homogeneity. The microhardnesses HV0.1 (at 0.98 N) and HV0.3 (at 2.94 N) both show maxima for TiN0.67 of 24.4 and 23.8 GPa respectively, whereas TiN1.00 has values of 17.2 and 17.0 GPa respectively. The crack formation resistance measured by the Palmqvist method is of the order of 45 kJ m−2 and was found to be independent of the nitrogen content. The nitrogen sublattice occupancy was determined by density measurements. Within the accuracy of the results the titanium sublattice is fully occupied throughout the homogeneity range of TiN1-x whereas the nitrogen sublattice is occupied according to the stoichiometry TiN1-x. The electrical conductivity at room temperature increases with increasing nitrogen content from 0.48 × 10 4 Ω −1 cm−1 for TiN0.53 to 3.74 × 10 4 Ω −1 cm−1 for TiN1.00. The rate of oxidation increases with increasing nitrogen content at both 800 and 1000 °C for samples of composition TiN>0.67. Samples of composition TiN

Investigations in the scandium-nitrogen system

Single-phase samples of delta-ScN/sub 1-x/ and two-phase Sc/Sc N alloys were prepared by direct nitridation of Sc metal and by arc-melting of ScN + Sc, respectively. The lattice parameters of delta-ScN/sub 1-x/ prepared at 1370-1770 K were measured. The lattice parameter a = 0.45018(5) on the nitrogen-rich phase boundary and a = 0.45055(9) nm for delta-ScN/sub 1-x/ are in equilibrium with Sc metal. Chemical analysis of nitrogen and oxygen in single-phase delta-ScN/sub 1-x/ prepared at 25 kPa nitrogen pressure and 1770 K yielded a stoichiometry of ScN/sub 0.98 /plus minus/0.005/O/sub 0.02/plus minus/0.01/. Microprobe Sc profiles across diffusion layers of delta-ScN/sub 1-x/ on Sc metal prepared at 1380 and 1770 K indicate that in this temperature range ScN/sub 1-x/ has a homogeneity range of ScN/sub 0.87/-ScN/sub 1.00/. In the diffusion layers the dark blue color of ScN on the nitrogen-rich surface takes on a clearly visible violet tinge near the Sc/ScN boundary. Nitridation of solid Sc metal results in a porous or hollow nitride sample. This is probably due to the preferential diffusion of Sc/sup 3 +/ ions through the originally formed ScN layer.

Physical and mechanical properties of cubic δ-VN1−x

Abstract Compact samples of cubic δ-VN1-x were prepared by the reaction of vanadium metal with pure nitrogen in the pressure range 10 kPa ⩽ p(N2) ⩽ 4 MPa and at temperatures T from 1353 to 1923 K. The samples had compositions between VN0.765 and VN0.996 and lattice parameters between 0.4097(4) and 0.4135(3) nm. In this range the lattice parameter is a linear function of the nitrogen-to-metal ratio. The pressure isotherm at 1723 K as a function of composition was established. Near-stoichiometric VN1−x can only be prepared under elevated nitrogen pressures. The microhardness HV0.1 of VN1−x was measured. Hardness decreases significantly with increasing nitrogen content. Within the composition range investigated the vanadium sublattice is fully occupied at substoichiometric compositions, whereas the occupancy decreases just perceptibly as stoichiometry is approached. The occupancy of the nitrogen sites is a linear function of the [N] [V] ratio.

Formation of molybdenum nitrides by ammonia nitridation of MoCl5

Abstract The formation of molybdenum nitride from MoCl 5 and flowing NH 3 was investigated at temperatures of 770–1060 K. Up to 910 K the reaction yields single-phase WC-type δ-MoN with lattice parameters of a = 0.2851(2) nm and c = 0.2782(3) nm. At T ≥ 950 K, a reaction product consisting of the two phases δ-MoN + γ-MoN 1− x was obtained, where the amount of γ-phase increased with increasing reaction temperature. The γ-MoN 1− x lattice parameter is dependent on the reaction temperature, whereas the γ-MoN lattice parameters remain unchanged. At reaction temperatures of 1060 K, a reaction product with a conspicuosly different, coarse-grained facetted habit was formed - grown probably by a transport mechanism via the gas phase - with sharp diffraction lines and lattice parameters of a = 0.5740(2) nm and c = 0.5624(3) nm, which are significantly larger than the doubled cell parameters of the WC-type MoN. Debye-Schrrer photographs of this nitride suggest that it is isostructural with the recently observed MoN resulting from high-pressure high-temperature treatment of WC-type δ-MoN.

The crystal structure of a new phase in the titanium-nitrogen system

Abstract During a systematic reinvestigation of the Ti-N system, a new high temperature phase, ζ-Ti4N3 − x, was observed. The crystal structure was determined from X-ray powder diffraction patterns. It is trigonal, space group D 3d 5 − R 3 m (No. 166), with a rhombohedral unit cell with ar = 0.98070 nm, and α = 17.47 °, or in the hexagonal setting, ah = 0.29795 nm, ch = 2.8965 nm and c h a h = 9.7214 . The titanium atoms occupy the 6(c) sites with z = 0.1255 and z = 0.2910. The structure consists of a close-packed titanium atom arrangement with 12 titanium layers per unit cell in the stacking sequence (hhcc)3. The nitrogen atoms are most probably randomly distributed among the 6(c) (z = 0.4150), 3(a) and 3(b) sites. ζ-Ti4N3 − x is considerably deficient in nitrogen and is isomorphous with ζ-Hf4N3, ζ-V4C3, ζ-Nb4C3 and ζ-Ta4C3.

Preparation and properties of compact cubic δ-NbN1−x

Compact δ-NbN1−x was prepared by heating niobium wire for several days in nitrogen at 4 MPa pressure and temperatures of 1 723 to 1 923 K. The samples obtained had compositions between NbN0.924 and NbN0.975±0.002 and were coarse-grained. The lattice parameter increases with the nitrogen content froma=0.43884 nm for NbN0.924 toa=0.43913 nm for NbN0.975. From the determination of the lattice parameters up to 1 073 K the coefficient of linear thermal expansion as a function of temperature was evaluated. The microhardness HV0.1 decreases from 1 300±80·107Nm−2 for NbN0.924 to 1080±60·107 Nm−2 for NbN0.975. The occupancies of both the niobium and the nitrogen sublattices were calculated using experimental density data. The occupancy of the niobium sublattice decreases linearly with increasing nitrogen content. An extrapolation gives 2.9±0.4% vacancies in both sublattices for stoichiometric δ-NbN.ZusammenfassungKompaktes δ-NbN1−x wurde durch mehrtägiges Erhitzen von Niobdraht in Stickstoff bei einem Druck von 4 MPa und Temperaturen von 1 273 bis 1 923 K hergestellt. Die dabei erhaltenen Proben hatten Zusammensetzungen von NbN0.924 bis NbN0.975±0.002 und zeigten ein grobkörniges Gefüge. Der Gitterparameter steigt mit dem Stickstoffgehalt vona=0.43884 nm für NbN0.924 bisa=0.43913 nm für NbN0.975 an. Von einer Bestimmung der Gitterparameter bis 1 073 K wurde der lineare thermische Ausdehnungskoeffizient erhalten. Die Mikrohärte HV0.1 sinkt von 1 300±80·107 Nm−2 für NbN0.924 auf 1 080±60·107 Nm−2 für NbN0.975 ab. Die Besetzung sowohl des Niob- als auch des Stickstoffteilgitters wurde unter Verwendung von experimentell gemessenen Dichten bestimmt. Die Besetzung des Niobteilgitters fällt mit zunehmendem Stickstoffanteil linear ab. Eine Extrapolation dieser Werte ergibt für stöchiometrisches δ-NbN einen Leerstellenanteil von 2.9±0.4% auf beiden Teilgittern.

The crystal structure of η-Ti3N2−x: an additional new phase in the Ti-N system

Abstract Phase equilibria studies in the Ti-N system have revealed the existence of an additional new phase which it is proposed be denoted by η-Ti3N2−x. This phase was obtained by quenching Ti-N alloys with about 30 at.% N from 1333–1363 K to room temperature. X-ray powder patterns of η-Ti3N2−x could be indexed on the basis of a rhombohedral unit cell, space group D 5 3d -R 3 m (No. 166), with the rhombohedral unit cell parameters (standard deviations in parentheses): ar = 0.74236(29) nm and α = 23.16(1) °, or, in the hexagonal setting: ah = 0.29809(4) nm and ch = 2.16642(85) nm with c h a h = 7.268(4) . In the hexagonal setting the titanium atoms occupy the sites 6(c) with z = 0.2222 and 3(a) whereas the nitrogen atoms are apparently randomly distributed among the sites 6(c) with z = 0.3889 and 3(b). η-Ti3N2−x is isostructural with ϵ-Hf3N2−x and Ta2VC2−x and contains considerably less nitrogen than Ti3N2.

Lattice parameters and thermal expansion of δ-VN1−x from 298–1000 K

AbstractThe thermal expansion of VN1−x was determined from measurements of the lattice parameters in the temperature range of 298–1000 K and in the composition range of VN0.707–VN0.996. Within the accuracy of the results the expansion of the lattice parameter with temperature is not dependent on the composition. The lattice parameter as a function of composition ([N]/[V]=0.707−0.996) and temperature (298–1000 K) is given by

WDS-EPMA nitrogen profile determination in TiN/Ti diffusion couples using homotypic standard materials

\begin{gathered} a([N]/[V],T) = 0.38872 + 0.02488([N]/[V]) - \hfill \\ - (1.083 \pm 0.021) \cdot 10^{ - 4} T^{1/2} + (6.2 \pm 0.1) \cdot 10^{ - 6} T. \hfill \\ \end{gathered}