Pierre Lefort

Solid state reaction of zirconia with carbon

Abstract The solid–solid reaction between zirconia and carbon under flowing argon produces zirconium carbide via the intermediate formation of an oxycarbide ZrC0.84O0.06. In the temperature range 1623–1823 K, its final transformation into carbide is slow. The reaction producing the oxycarbide obeys the kinetic law between the degree of conversion α and time: F(α)=1−(1−α) 1/3 =K e −E/RI T , associated with an activation energy of 208±15 kJ·mol−1. This reaction occurs via two solid–gas reactions: ZrO 2 +0.84 CO=ZrC 0.84 O 0.06 +1.39 O 2 and 2.78 C+1.39 O 2 =2.78 CO . The oxycarbide appears on the surface of the oxide whose grain shape is not modified.

Role of some technological parameters during carburizing titanium dioxide

Abstract The influence of technological parameters is followed during the carbothermal synthesis of titanium carbide from the dioxide. The carbon grain size, the homogeneity of the carbon/oxide mixtures and the ventilation of the powders beds are the most important conditions for a rapid reaction. The oxide grain size, the mixing method and the compactness of the mixture has no influence, or very little. These results are explained by the carburizing mechanism where the Boudouards reaction: C+CO2→2CO plays a central role. This reaction is more rapid when fine carbon is used and when the carbon monoxide is eliminated as soon as formed. These conditions are those required for a complete synthesis of the carbide by: TiO2+3C→TiC+2CO for mixtures having the stoichiometric composition.

Formation of aluminium carbide by carbothermal reduction of alumina: Role of the gaseous aluminium phase

Abstract Formation of aluminium carbide by carbothermal reduction of alumina under an atmospheric pressure of argon involves the following reactions occurring from 1400°C: 1. (i) The decomposition of alumina: 2Al 2 O 3 ⇌ 4Al + 3O 2 and the correlative formation of carbon monoxide: 3O 2 + 6C ⇌ 6CO 2. (ii) The intermediate formation of the oxycarbide Al2OC for partial pressures of carbon monoxide of about 5 × 10−3 Pa: 4Al + 2CO ⇌ 2Al 2 OC 3. (iii) The final production of Al4C3, when the partial pressure of carbon monoxide becomes lower, accompanying the decomposition of the oxycarbide, may be by: 4Al + 3C ⇌ Al 4 C 3 Gaseous aluminium can be detected experimentally by an emission spectrometer from the lowest temperatures (1400°C) and all along the different steps.

Approche cinétique de la réduction carbothermique du dioxyde de titane

Kinetics of the titanium carbide synthesis by titania carbothermic reduction between 1470 and 1670 K, points out successively three reactions: (i) the fast reduction of the dioxide into the suboxide Ti3O5 up to a progress coefficient α = 0.17; (ii) the oxycarbide Ti2OC formation with c.f.c. structure (a 4.295 A) for progress coefficient α like 0.17<α<0.75, associated with an energy of activation E=380 ± 20 kJ mol−1, involving the following steps: (iii) the carbide formation resulting from the substitution of oxygen by carbon, according to a slow diffusional process within the oxycarbide grains.

Influence of the grain size on the reactivity of TiO2/C mixtures

Abstract Mixtures of TiO 2 and C of different grain sizes do not show the same reactivity during the synthesis of titanium carbide TiC. The finest powders (∅≈0.2 μm) lead to the serial reaction TiO 2 →Ti 3 O 5 →Ti 2 OC→TiC, while the coarsest powder leads to the intermediate formation of Ti 2 O 3 . For temperatures between 1503 and 1873 K, the diffusional oxygen loss leading to the suboxides is very rapid. It is followed by a complex carburization forming Ti 2 OC via a rate limiting process governed by the consumption of carbon via C+CO 2 →2CO. CO 2 comes from an equilibrium associated with the suboxide grains, such as 2Ti 3 O 5 +13CO=3Ti 2 OC+10CO 2 , this explains the coupling observed between temperature and partial pressure of carbon monoxide. The last stage of the reaction, associated with the diffusion of carbon inside the grains of the oxycarbide, is slowed down probably because of the inhomogeneity of the mixtures at the end of the reaction.

Thermal expansion and adhesion of ceramic to metal sealings: Case of porcelain-kovar junctions

Abstract Glass to metal sealings necessitate a pre-oxidation of the metal, producing a thin superficial oxide layer. In the case of porcelain-kovar junctions a glassy interphase is necessary, which reacts and dissolves the oxide layer during the bonding thermal treatment. In the interfacial zone this leads to a good fitting of the thermal expansion coefficients of the phases present: in the alloy the superficial part is impoverished in iron and its thermal expansion coefficient is close to that of the FeO-rich glass which has dissolved the oxide layer. The resulting buffer zone limits the effect of strains appearing during cooling at the end of the bonding thermal treatment. Moreover, the FeO-rich glass penetrates the open porosity of the superficial alloy layer (due to its pre-oxidation) providing a good physical adherence of glass to kovar. At the porcelain-glass interface there is no problem because of the similarity of dilatometric behaviour of the two materials.

Ceramic to metal sealings: Interfacial reactions mechanism in a porcelain-kovar junction

Abstract Several processes occur during the sealing of preoxidized kovar with porcelain through a glassy interphase. In the kovar, the strongly perturbed superficial zone tends to homogenize in composition as well as in the porosity distribution. The oxide scale is rapidly dissolved by the glass but the mechanism is complex with two elementary steps: (1) the diffusion of iron into the glass; (2) the devitrification of the glass enriched in iron (and may be in cobalt) to form a fayalite-like phase (type Fe 2 SiO 4 ); however, crystallization is slower than iron diffusion so that, for well-chosen times and temperatures, the oxide scale is entirely dissolved into the glass before any devitrification. The glassy phase then penetrates into the open porosity of the alloy and a solid and helium-tight junction is achieved in this way. The iron-rich glass forms a “buffer zone” with a thermal expansion coefficient near that of the underlying alloy, but if the dissolved iron oxide layer is not thick enough, then the dilatometric behaviours of glass and alloy remain unadapted and the bonding breaks. The main knowhow of this kind of sealing consists in the good suitability of preoxidation of kovar with the characteristics of the thermal treatment for sealing.

Direct gaseous nitridation of the Ti–6Al–4V alloy by nitrogen

The gaseous nitridation of the Ti–6Al–4V alloy in a furnace with nitrogen improves strongly the superficial n microhardness of the alloy from 405 to 2133 HV0.3. For temperatures between 1175 and 1300°C, it leads to a very complex structure composed (in cross-section, from the surface to the heart) of several layers: (i) stoichiometric TiN, (ii) sub-stoichiometric δ-TiN0.7 (without vanadium and aluminium), (iii) a thin film of alloy enriched in vanadium and aluminium, (iv) an α-(Ti, N) phase, containing aluminium (from TiAl0.12N0.25 to TiAl0.12N0.12) and (v) the Ti–6Al–4V alloy with needles of the α-(Ti, N) phase together with aluminium and traces of nitrogen (less than TiAl0.12N0.05). The reaction, which is not influenced by the pressure of nitrogen, involves a gaseous diffusion through the superficial layers and is governed by the volumic diffusion of nitrogen in the α-phase, giving parabolic kinetics with the rate law: n nwhere Δm/S is the gain of mass per surface unit (g cm−2), R the gas constant and T the temperature (K).

Plasma-jet coating of preoxidized XC38 steel: influence of the nature of the oxide layer

Preoxidation of alloy substrates before spraying an oxide powder improves the adhesion of the coating. In the case of a low carbon steel (XC38) coated by plasma sprayed alumina, three preoxidation treatments in a classical furnace have been undertaken, in order to form respectively FeO, Fe3O4 and Fe2O3 layers. The best adhesion of alumina coating is obtained with FeO interlayers, and the worst with Fe2O3. The preliminary thermal treatment giving FeO implies a controlled atmosphere of carbon dioxide, but it points to some interesting prospects for coating complex parts. The origin of the improvement of adhesion may be a diffusion bonding at the FeO/Al2O3 interface or more simply it might be due to the increase of the substrate roughness with the time of oxidation. Besides, the epitaxial growth of FeO on steel probably explains the good mechanical resistance of the steel/FeO ninterface .

Compatibility between molybdenum and aluminium nitride

Abstract Molybdenum and aluminium nitride sintered discs are strongly bonded after hot pressing at 1850°C under 20 MPa in flowing argon. If aluminium nitride discs are pre-sintered with lime or yttria additive, the bonding is only achieved when the dopant content is weak (1 to 3%). By comparison with the behaviour of tungsten under the same conditions, the bonding is supposed to be due to the reaction of the secondary oxide phase of molybdenum with its analogue in aluminium nitride, providing a complex interfacial phase, able to exist at the grain boundaries of both the materials near the interface, and making the bonding strong after cooling. If the interfacial phase is too great in volume and too liquid (when AlN is doped with important amounts of lime or yttria) it is expelled from the interfacial zone and no bonding is observed.