High-temperature oxidation of titanium diboride of different purity

Oxidation in air of TiB₂ powders of two different types in the temperature range from 100 to 900°C as well as of compact samples produced from them (at 600–1200°C) was studied by DTA, thermogravimetry, X-ray analysis, AES, EPMA, and SEM methods. The admixture elements in powders were found to prevent oxygen from enriching the diboride surface. In compact TiB₂ samples impurities form diffusion barriers during oxidation at the matrix/scale and scale/gas interfaces as well as facilitate sintering of the scale being formed.     

Oxidation mechanism of titanium nitride in oxygen

Despite the apparent complexity of the linear oxidation of titanium or titanium nitride resulting from the formation of a layered structure of the rutile scale formed, the uniqueness of the mechanism is proven. The limiting step appeared to be the predominant short-circuit diffusion of oxygen (E 44 kcal · mole^–1) through a rutile lamella of variable thickness growing at the nitride-oxide boundary and fracturing periodically to form a detached porous layer through which molecular oxygen can penetrate. The pressure dependence of the diffusion process in the case of the nitride was associated with the outward migration of nitrogen, while the undulations of the kinetics under certain conditions were caused by the growth of a sintered, recrystallized outer zone of oxide. The periodic exfoliation of the oxide was related to its poor adherence to the substrate, certainly due to the presence at the nitride-oxide interface of a thin gray film probably composed of intermediate phases between TiN_x (or Ti) and TiO₂.     

High-temperature oxidation of pure titanium in CO2 and Ar-10%CO2 atmospheres

The oxidation behavior of pure titanium has been investigated in the temperature range of 1000 K to 1300 K in CO₂ or Ar-10%CO₂. Optical microscopy, electron probe microanalyses, and X-ray measurements on the oxide scales formed during oxidation indicate that their structures are nearly independent of temperature and the corrosion atmosphere. The scales consisted of two layers, an external one and an internal one, having a rutile (TiO₂) structure. The parabolic rate law was confirmed for growth of the external scale and the permeation depth of oxygen in titanium with apparent activation energies of 266 and 226 kJ/mol, respectively. The rate-determining diffusion species in the oxidation processes are discussed.     

Oxidation of titanium nitride in oxygen: Behavior of TiN0.83 and TiN0.79 plates

The oxidation of 0.13 mm thick TiN_0.83 plates between 850 and 1005°C and 0.38 mm thick TiN_0.79 plates between 890 and 1220°C was studied at oxygen pressures in the range 15–600 Torr. The oxidation behavior of the first compound, TiN_0.83, was found to be very close to that recently described for the nitride TiN_0.91. There was formation of a rutile scale with a layered structure and the kinetics were quasilinear for conversion degrees 0.25<<0.9. By applying a pseudolinear oxidation model involving a diffusion regime through a scale of variable thickness, it was found that the activation energy of the limiting process, i.e., the diffusion of the reactants through the inner growing layer, was 43 kcal/mole. The second compound, TiN_0.79, showed a slightly better oxidation resistance. The irregular kinetic curves obtained (mainly above 1000°C) were associated with sintering and grain growth in the oxide starting at the outer surface of the scale and gave indirect information on the pressure dependence of the linear oxidation of nitrides TiN_0.83 and TiN_0.91.     

High-temperature oxidation of titanium-hydride powders

The oxidation kinetics of titanium-hydride powders were studied in the temperature range of 298–1378 K in air at atmospheric pressure. DTA, DSC, X-ray analysis, and scanning electron microscopy were used. Oxidation was found to take place by TiH_xO_y oxyhydride phases formation. The oxidation reaction rate at temperatures above 870 K was limited by diffusion of oxygen atoms through a rutile scale formed on the surface. The activation energy and preexponential values of the Arrhenius equation for different interaction stages as well as transformation enthalpies were calculated.     

High-temperature oxidation of titanium silicide coatings on titanium

Coatings of Ti ₅Si₃ on titanium have been prepared by means of decomposition of silane SiH₄ on heated titanium ribbons. Oxidation of the coated titanium specimens was much slower than that of the noncoated ones. Gravimetric and morphological experiments allowed to propose a mechanism describing the oxidation process.     

Kinetics of high-temperature oxidation of boron carbide in oxygen

Thermogravimetry and gas-adsorption chromatography were used to study the kinetics of formation of solid and gaseous products during the hightemperature oxidation of compact boron carbide in oxygen at 740 Torr. Oxidation resistance was observed at temperatures up to 1200°C. The main oxidation products were B₂O₃ and CO₂. Oxidation was paralinear; the carbon consumption exceeded the consumption of boron as compared to the ratio of these elements in the compound B₄C. This difference resulted in carbon depletion of the carbide layer in the substrate near the scale>.     

High-temperature oxidation of SiC in a glow-discharge oxygen plasma

The oxidation kinetics and structure of the oxide scales formed on high-density SiC were studied in molecular oxygen at 740 Torr and in a glow-discharge oxygen plasma at 0.1 Torr at temperatures of 1000, 1100, and 1200°C. The monatomic oxygen formed by the glow discharge markedly increased the reaction rate and the vaporization of some of the oxidation products. The marked differences in kinetics suggest that the rate-controlling step during oxidation in molecular oxygen is the dissociation of adsorbed diatomic oxygen to the monatomic species. Films formed in molecular oxygen were mostly amorphous SiO₂ with small inclusions of SiC and graphite, whereas films formed in dissociated oxygen were primarily amorphous SiO₂ containing SiO, S₂O₃, and the coesite form of SiO₂.     

High-temperature oxidation of Fe-Cr alloys in wet oxygen

Fe-Cr binary alloys have been oxidized in a stream of oxygen containing different amounts of water vapor at 900–1000°C to study the effects of water vapor. The Fe-Cr alloys exhibit an initial protective behavior due to formation of a Cr-rich scale and followed by a nonprotective breakaway oxidation due to formation of iron-rich scale. The appearance of the breakaway oxidation was very sensitive to the water vapor content in the atmosphere. The higher the water vapor content, the earlier the breakaway oxidation takes place. Increasing the oxidizing temperature or decreasing the Cr content in the alloys facilitate an earlier breakaway oxidation. The breakaway oxidation was inhibited effectively by surface-applied CeO₂ particles before oxidation. The oxide scales were examined and analyzed by optical metallography, X-ray diffraction, SEM, and EPMA. A mechanism of the effects of water vapor has been proposed.     

A kinetic and thermoanalytical study on oxidation of powder and single-crystal samples of niobium carbide

The oxidation of two kinds of NbC, powder and single-crystal samples, was carried out isothermally at temperatures of 390 to 610°C at an oxygen partial pressure of 8 kPa using an electromicrobalance. The kinetics results suggested that the oxidation of both powder and crystal samples proceeds essentially by a phase-boundary-controlled reaction. It was found that cracking of grains or particles in the powder samples, which occurs in the later stage, accelerates oxidation below 485°C, and extremely rapid reaction proceeds above that temperature. Simultaneous TG-DTA-MS measurement showed that the reaction above 485°C is due to a large heat evolution resulting from rapid oxidation of bare NbC surfaces produced by cracking of grains or particles, which was partly caused by immediate release of CO₂ accumulated in them. Scanning electron microscopy of the crystals showed that columnar, porous grains with the major axis normal to the surface formed, which were responsible for the phase-boundary-controlled reaction of the oxidation of both powder and crystal samples.     

High-temperature oxidation of copper-manganese alloys

The oxidation behavior of copper-manganese alloys (2–35 wt. % Mn) in pure oxygen at 760 Torr was investigated at 100° intervals between 550 and 850°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The scales formed on Cu-2Mn, Cu-5Mn, Cu-10Mn are always composed of three different layers; in any case manganese is present only in the inner layer. The scales formed on Cu-20Mn and Cu-35Mn are composed of two layers, both containing manganese, with a more Cu-rich outer layer. In all the samples internal oxidation in combination with external scale formation is observed.     

Correlation between the oxidation mechanism of titanium under a pure oxygen atmosphere,morphology of the oxide scale,and diffusional phenomena

The oxidation behavior of titanium was studied over the temperature range of 600 to 800°C in a pure oxygen atmosphere. A parabolic kinetics period is made up of a succession of short parts whose weight gain is nearly constant and whose beginning is characterized by an alteration of the oxidation rate. The kinetics curve is closely correlated with the morphology of the oxide scale which contains several layers separated by short cracks. These cracks decrease the diffusional flows of oxygen and titanium and alter the oxidation rate. A parabolic-linear kinetics transition is attributed to the formation of a continuous crack between the oxide scale and the matrix. The oxide scale then forms a porous barrier of constant thickness which induces a linear rate law due to the steady-state oxygen diffusion. Titanium cannot diffuse across the continuous crack. During the linear kinetics period several layers of constant thickness, separated by a continuous crack, spread out.     

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.     

Electrochemical corrosion behavior and MG-63 osteoblast-like cell response of surface-treated titanium

Commercially pure titanium is used as a clinical implant material for many orthopedic and dental implant devices owing to its excellent corrosion resistance and good biocompatibility. However, there remains concern over the release of metal ions from prostheses and unresolved questions about its behavior in a biological environment. Our research investigated the influence of surface oxide thickness and phase on the corrosion resistance in 0.9% NaCl solution by potentiostat and XRD. Also, the MG-63 osteoblast like cell morphology and proliferation were studied to evaluate the biocompatibility in terms of surface treatment. It is demonstrated that a substantial decrease in the current density may be attained due to surface oxide thickening and phase transformation by thermal oxidation. The osteoblast adhesion morphology and proliferation data indicated that the osteoblast cell response is not conspicuously influenced by the thermal oxidation and nitric acid passivation treatments but by surface roughness and porosity of 3^rd networking.     

High-temperature oxidation of Ti3Al−Nb alloys

Titanium aluminide (Ti₃Al–Nb) has potential for high-temperature applications because of its low density and high-temperature strength. This research is aimed at improving the high-temperature oxidation resistance of a Ti₃Al–Nb alloy by modification of its composition. The oxidation rates of Ti₃Al–Nb alloys were measured from 600 to 900°C in air. The oxide layer was examined by X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. The experimental results reveal that alloys with added Nb tend to form denser oxide layers and that oxidation rate can be reduced by increasing Nb content (up to 11 at.% in this study), which is in good agreement with other investigators. The only exception is a Ti₆₅Al₂₅Nb₁₀ alloy, which shows better oxidation resistance than the commercial Ti₆₅Al₂₄Nb₁₁ alloy. The oxidation resistance of Ti₆₅Al₂₅Nb₁₀ alloy can also be improved slightly by the addition of small amounts of Si or Cr. An increase in the oxidation resistance of Ti₆₅Al₂₅Nb₁₀ alloy containing Y was observed at 900°C but not at 800°C or below. The parabolic oxidation rate equation is adequate to describe the high-temperature oxidation reaction of the Ti₃Al–Nb alloys in the atmosphere.     

Synthesis of titanium carbide by induction plasma reactive spray

A novel method capable of sufficient mixing of titanium powder and methane of carbon source was developed in the synthesis of titanium carbide by induction plasma reactive spray. X-ray diffraction analysis, optical microscopy, scanning electron microscopy, and microhardness test were used to characterize the spray-formed deposit.The experimental results show that both primary carburization of the titanium particles inside the plasma flame and secondary carburization of the growing deposit on high temperature substrate contribute to the forming of titanium carbide. The transitional phase of TiC1-x has the same crystal structure as TiC, but has a slightly low lattice constant. The deposit consists of fine grain structure and large grain structure. The fine grain structure, harder than large grain structure, shows grain boundary fracture.     

Corrosion of titanium in supercritical water oxidation environments

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High-temperature coating for titanium aluminides using the pack-cementation technique

The practical application of titanium aluminide metal-matrix composites (MMCs) at high temperatures requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, the titanium aluminide alloys suffered from rapid oxidation attack at elevated temperatures, particularly under thermal cyclic conditions. The pack-cementation coating process was utilized to aluminize the surface region of a Ti₃Al-base alloy to TiAl₃, the most oxidation-resistant phase. With the existence of an adherent conversion coating, a thin protective alumina scale formed on the outer surface, and a significant improvement in the corrosion resistance was observed. Excellent coating efficiency and geometric flexibility were demonstrated in this study by the pack-cementation technique. Further development of the cementation process will focus on the elimination of surface cracking in the coating.