Preparation And Characterization Of Hafnium Nitride And Its Composite

Hf-N compounds have had more and more attention from the field of material research for the abroad application of TiN and ZrN films. The hafnium nitride phase HfN is a hard, high-melting, yellow compound and hence has attracted interest for use as a protective layer in technical and ornamental applications. The transition metal nitrides Hf-N ave received considerable attention as wear-resistant coatings for cutting tools because of their high mechanical hardness, wear resistance, chemical inertness, and ability to withstand high-temperature processing.In addition to the conducting mononitride HfN, an insulating and transparent higher nitride Hf3N4 has also been synthesized . In the past years, many hafnium nitrides were usually produced by gas-solids reaction between metal and gases containing nitrogen atoms, such as, N2 and NH3, etc., but little by solid-solid reaction between zirconium and solids containing nitrogen atoms. It is well known that hexagonal boron nitride (h-BN) is a kind of non-magnetic material with high heat conductivity and is an ideal matrix material of nanocomposites. It is expected to be a new method to produce zirconium nitride by solid state reaction between zirconium and BN on use of mechanical milling and high pressure. Especially, new zirconium nitrides are expected to be prepared by solid state reaction between zirconium and BN under high pressure. In addition, the synthesis of HfN/HfB2 composites in high pressure and high temperature are studied.In the present experiment, metal Hf and h-BN are chosen as precursor. Hf-N alloys were prepared by means of mechanical milling technologies. Their structures are examined by X-ray diffractory (XRD) and X-ray photoelectron spectroscopy (XPS).Hf/BN nanocomposites were prepared by mechanical milling, the thermodynamic and kinetic mechanisms of formation were studied .HfNx powders with rock salt structure were obtained by mechanically milling of a mixture of Hf and hexagonal boron nitride (h-BN) powders under argon atmosphere and isothermal annealing of the mixture after milling under vacuum of approximately Pa. The mole ratio of Hf to h-BN is 1:10. An analysis of the reaction kinetics under different milling time and isothermal annealing time show that the formation of an interstitial solid solution Hf(N), prior to its transformation to cubic HfNx. Rietveld refinements of the X-ray diffraction data were done in order to analyze the progress of the structural change. In the milling process, an amorphous Hf–N alloy was formed firstly by a diffusion reaction between Hf and BN, and then the amorphous Hf–N alloy transformed into HfNx driven by mechanical milling. However, in the annealing process, a Hf(N) solid solution was formed firstly by incorporation of N into Hf, and the N content in the Hf(N) increased with increasing annealing temperature. When the N content exceeded the solubility limit of the Hf(N) at some annealing temperature, the Hf(N) decomposed into HfNx. No self-sustaining reaction occurs in the present work. No HfB2 is observed to form in the two processes. The thermodynamic and kinetic mechanisms of formation of the HfNx are discussed. 5×10?3The Gibbs free energies of formation and the entropy of formation for the Hf–N and Hf–B alloys were calculated , indicating that the Gibbs energy of the Hf–N alloy is much more negative than that of Hf–B the alloy, which agree with the present experiment result .In terms of the kinetics of chemical reactions, a potential barrier has to be overcome when two elements react to form a compound. It is related to size, electronegativity, and chemical potential of the two elements. The higher the potential barrier, the more difficult it is for the reaction to occur. So, enough energy is required in the reaction to overcome the potential barrier. Obviously, a compound or alloy of two elements is produced easier by formation of their solid solution firstly, followed by a phase transition from the solid solution to the compound or alloy than by direct solid-state reaction of the two elements. This is because the potential barriers for formation of a solid solution and phase transition are usually smaller than the potential barrier involved in a solid-state reaction of two different elements. In the present experiment, the Hf(N) solid solution was formed, then the Hf(N) solid solution crystallized to Hf–N compound or alloy.But the solid solution of Hf(B) have not been reported. So the potential barrier of the former processes may be smaller than the potential barrier of the reaction between B and Hf. It is the reason why there is no Hf-B compound been found.Hf-B-N composites which have the face-centre cubic structure were synthesized from Hf and BN powders with Hf/BN molar ratio of 1:10 under high pressure(5Gpa) at temperatures above 1473K. The structure were analyzed by the XPS. The molar ratio of Hf-B-N almost is 1:7:2. The HfB2 compounds were synthesized from the mixtures of Hf and BN with Hf/BN molar ratio of 1:5 are milled for 20h under high pressure and temperatures as 5Gpa and 1573K, respective.In a vacuum conditions of the same sample, such as annealing temperature, and high-pressure conditions contrast, we found there only is a small amount of Hf-B-N, and a large number of HfN and HfO2, we can see that in the high-pressure conditions, the material will change the thermodynamic state, it is propitious to the synthesize of Hf-B-N, and the high pressure can inhibit the oxidation of Hf. Hf and the amorphous BN are experimented in 5GPa, 1300℃, isothermal processing 10-20 min. No Hf-B-N new phase, it means that the high pressure and high temperature synthetic samples structure rest with the precursor of the structure.The same sample of 1:5 molar ratio is experimented in high temperature and high pressure (5 Gpa, 1573K), the structure of the h-HfB2 is found. We find that it is propitious to the synthesize of h-HfB2 by the calculation and analysis of its structure characterization. Experiments show that in the high-pressure conditions, the existence of HfN stable, in contrast HfN in the annealing process decomposition, indicating the high pressure can inhibit the decomposition of HfN.