S. V. Shevtsov

Titanium-to-rutile oxidation kinetics in the direct-oxidation fabrication of thin-wall ceramics

The oxidation kinetics of titanium preforms for the direct-oxidation fabrication of thin-wall ceramics have been studied in the temperature range 750–1100°C using titanium plates up to 5 mm in thickness. Empirical relations for the kinetics of these processes have been obtained. The preform weight has been shown for the first time to influence the oxidation rate of titanium and the amount of forming rutile, which is inconsistent with the known patterns of metal oxide formation. One possible mechanism of rutile formation during the direct-oxidation fabrication of thin-wall ceramics is considered.

Structure and hardness of ceramics produced through high-temperature nitridation of zirconium foil

We have studied the structure and microstructure of ZrN prepared by high-temperature nitridation of zirconium foil and estimated its hardness. The results demonstrate the feasibility of producing a composite heterostructure with the composition ZrN-(α-solid solution of nitrogen in zirconium/ZrN)-ZrN in the conditions of the process under consideration by heating to a temperature above the peritectic reaction temperature.

Stability of the structure of compact zirconium nitride ceramics to irradiation with high-energy xenon ions

X-ray diffraction and transmission electron microscopy (TEM) data demonstrate that irradiation with high-energy +24Xe136 ions causes no changes in the grain microstructure of compact zirconium nitride ceramics. According to Raman spectroscopy and high-resolution TEM data, the irradiated ceramics may have local distortions of the zirconium nitride lattice within regions on the order of the lattice parameter in size.

Irradiation of titanium, zirconium, and hafnium nitrides with high-energy ions

We have identified structural and morphological changes produced by irradiation with 167-MeV +24Xe136 ions to a fluence of 5.3 × 1014 cm–2 in Ti, Zr, and Hf nitrides prepared using oxidation-assisted engineering. Irradiation of TiNx and HfNx leads to the formation of nano- and micropores in the surface layer of the samples. The surface layer of ZrNx samples contains nanopores both before and after irradiation. The presence of pores in the unirradiated ZrNx samples probably ensures the possibility of structural relaxation without further pore formation under irradiation. The irradiated ZrNx samples have local crystal structure distortions unrelated to dislocations and attributable to the impact of high-energy xenon ions.

High-temperature titanium nitridation kinetics

Titanium samples 60.0 mm in length and 3.0 × 0.3 mm in cross section were heated in a nitrogen gas atmosphere for 60 min at temperatures from 1300 to 2100°C. At temperatures below 2000°C, the titanium nitridation process comprised two stages. The lower temperature limit of exponential nitridation kinetics was determined to be ~1000°C. At temperatures above the melting point of the metal, the presence of liquid phase in the bulk of the material has no significant effect on the titanium nitridation process.

Creation of Ceramic Composites by High Temperature Oxidation of Iron and Nickel Alloys Using Oxidative Constructing Approach

High temperature oxidation of iron and nickel alloy blanks is characterized by the formation of bilayer ceramic and cermet structures, holding the original shape of the metal blank. The composition of the resulting ceramic depends on the temperature and synthesis time and the ratio of components with different sensitivity to oxygen in the initial alloy. Dwell at a low temperature leads to stabilization of the composition of the synthesized ceramic within the outer layer owing to the concentration leveling of the spinel phase. The outer and inner layers are characterized by different morphology of the chip surface and different porosity. The inner polycrystalline layer is porous; the outer layer of the sample is monolithic. The structure of interphase boundaries of the heterophasic sample ensures the integrity of the material and provides high adhesion properties of different phases to each other. The resulting oxidative constructed composites of ferrous alloys with nickel are promising for testing as inert anodes.

High-temperature oxidation of nickel using oxidative constructing approach

The kinetics and structural-phase behavior of the high-temperature oxidation of nickel are considered. It is found that the kinetics of high-temperature oxidation of nickel using the oxidative constructing approach is described by a parabolic law. The resulting compact bunsenite ceramic has a high adhesion to the metal surface. The presence of a gradient penetration of oxide inclusions in the porous structure of metal leads to a blurring of the phase boundary. It is shown that the limiting stage of the nickel oxidation process in the range of 1250–1400°С is nickel oxide dissociation with formation of free ions.

High-temperature iron oxidation within the oxidative development approach

High temperature iron oxidation within the oxidative development approach was carried out. It was found that, in the temperature range of 750–850°C, the kinetics of the iron oxidation with the use of the oxidative development approach was described by a parabolic law. The formed compact oxide ceramic has a uniform thickness which reaches 7 mm at 850°C within 14 days. It was found that, with the increase in volume of the initial metallic sample, the rate of the high temperature iron oxidation decreased. The ceramic obtained within 14 days at 850°C was characterized by a laminated structure.

Structure of ceramics produced through high-temperature nitridation of hafnium foil

The high-temperature hafnium nitridation process, with HfNx formation, comprises two stages. With increasing nitridation temperature and time, the lattice parameter of the nitride decreases. The observed discrete nucleation of nitride microcrystals on the surface is a consequence of metal diffusion through an amorphous carboxynitride. No structural defects were detected by high-resolution transmission electron microscopy in selected areas.

Structural and phase transformations and hardness of ceramics produced by high-temperature zirconium nitriding

Resistive heating of zirconium in a gaseous nitrogen atmosphere yields ceramics based on zirconium nitride with a heterophase structure. X-ray powder diffraction analysis determined the compositions of phases of the synthesized ceramics. The surface of the material consists of zirconium nitride close in composition to ZrN. In the bulk of the materials, in shallower layers, a nitrogen-deficient nitride phase forms, and in deeper layers, a phase of solid solution of nitrogen in zirconium does. The hardness of ceramics based on heterostructures of the type Zrsolid solution/ZrN1–x/ZrN was studied. Changes in the structure and phase composition during high-temperature nitriding of zirconium foil at 1500 and 2400°C were described.