J.C. Labbe

Modifications structurales du nitrure de silicium en fonction de la temperature

Abstract The structural evolution of α and β - Si 3 N 4 has been studied up to 900°C by means of the time of flight neutron diffraction method. It has been shown that the relative evolution of lattice parameters is the same for both phases; the modifications versus temperature are isotropic along the cristallographic axas a and c. However, the atomic positions, bond angles and bond lengths at different temperatures, as well as the volume evolution of SiN4 tetrahedra compared to that of the unit cell volume, suggest a less stability of α Si 3 N 4 with regard to the β phase which explains the α → β transformation at higher temperatures.

Aluminium nitride-molybdenum ceramic matrix composites. Influence of molybdenum addition on electrical, mechanical and thermal properties

Abstract Aluminium nitride-molybdenum ceramic matrix composites are produced by hot pressing a mixture of two powders without any sintering aids. Sintered samples show homogeneous and dense structure. Electrical resistivity measurements conducted over a series of samples show a rapid decrease in its value once the molybdenum volume fraction in AlN matrix is increased from 0.2 to 0.22. The interface between AlN matrix and Mo particles is found to be devoid of any secondary phases. Mechanical thermal and thermomechanical properties of these composites are then studied in a way to investigate the influence of molybdenum addition on the behaviour of AlN. An important increase in the fracture toughness, bend strength and thermal shock resistance of the composites is observed as a function of Mo concentration. Thermal conductivity measured at room temperature and thermal diffusity measured at temperatures between 400 and 1000 °C also show an improvement when molybdenum concentration in the AlN matrix is increased from 0 to 40%. This improvement in mechanical thermal and thermomechanical behaviour of composites is attributed to a clean adherent AlN-Mo interface deprived of low melting and low thermal conductivity secondary phases as well as better mechanical properties and good thermal conductivity of Mo. Molybdenwn has a thermal expansion coefficient near the AlN one which allows to minimise the residual and thermal stresses at the AlN-Mo interface.

Wettability and corrosion of TiN, TiN–BN and TiN-AlN by liquid steel

The wettability and corrosion behaviour of titanium nitride by liquid steel has been studied. Due to its thermodynamic stability and low solubility in liquid iron, TiN has a good resistance to corrosion by liquid steel. However, its good wettability promotes adhesion to steel. Also, the effects of BN and AlN addition to TiN on the wetting parameters and the mechanisms of the ceramic corrosion by liquid steel have been investigated.

In-flight oxidation of stainless steel particles in plasma spraying

Air engulfment by the plasma jet in air plasma spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classic diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on the convective oxidation. Two different types of austenitic stainless steel particles were air plasma sprayed using a direct current plasma gun and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. The mass transfer from surface to interior of particle occurred, forming oxide nodules within particles. The mass transfer is governed by convective movements inside liquid particles within the plasma jet core due to the plasma-particle kinematic viscosity ratio greater than 50 and particle Reynolds number (Re) higher than 20. The nodules were composed of metastable phases consisting of mixed oxide of Fe and Cr. Convective movements within particles ceased roughly outside of the plasma jet core, and classic surface oxidation was found to be the dominant phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core may become entrained toward the tail of the particle if plasma conditions promote a higher particle Re number. The major oxide phase in collected particles was FeCr2O4, in a nonstoichiometric form of Fe3−xCrxO4.

Aluminium nitride—molybdenum ceramic matrix composites: Characterization of ceramic—Metal interface

Pure aluminium nitride can be hot pressed with an addition of molybdenum powder. With this technique we obtain a ceramic matrix composite having a dispersed metallic phase. Composites produced in this manner present a homogeneous structure with very little open porosity. Electrical resistivity measurements done over a series of composites show a sharp decrease in electrical resistivity when the molybdenum volume fraction in the material increases from 0.2 to 0.22. This value corresponds to the percolation threshold of metallic phase in the ceramic matrix. The interface between molybdenum particles and aluminium nitride grains is established through a fine oxide or oxynitride layer present at the surface of AlN grains.

Study of the behaviour of aluminium nitride in the iron and steel industry

Abstract Reactivity studies of different steels with nitride ceramics, and in particular with aluminium nitride, have shown the presence of complex chemical reactions at the solid-liquid and liquid-vapour interfaces. Reaction at the AlN-liquid steel interface shows a decomposition of AlN followed by the oxidation of aluminium, and an AlON phase attack at grain boundaries.

Study of the interaction between liquid aluminum and silicon nitride

Abstract The sessile drop method was used to determine the evolution as a function of time and temperature, under vacuum, of the contact angle θ of molten aluminum on two kinds of silicon nitride (SRBSN and CVD-SN). These results are discussed in terms of thermodynamic calculations (stability of a superficial metal oxide layer) coupled with morphological observations and characterization of the ceramic/metal interface. The limited interfacial corrosion and the formation of a protective dense aluminum nitride layer lead to the conclusion that silicon nitride is a good candidate as a corrosion resistant material for the handling or the melting of liquid aluminum.

Influence of boron nitride and carbon additives on the behaviour of sintered AlN in a steel-making environment

In an earlier work we showed that the corrosion of pure aluminium nitride by steels takes place via an attack on the grain boundary Al/O/N phases and through AlN decomposition, with oxidation of aluminium by the dissolved oxygen. To reduce this type of corrosion, we tried to remove alumina and Al/O/N phases from the grain boundary by adding to the initial powders a small quantity of BN or carbon. The quantity of added BN must be in strict accordance with the quantity necessary to clean the grain boundaries. Best results were obtained with 10 wt% of BN. Carbon addition, which helps to improve the thermal shock resistance as well, produces the same result, but through a different mechanism. Best results were obtained with quantities between 5 and 10 wt% of carbon.

Fast neutron and heavy ion bombardment of Si2N2O

Abstract A non-crystalline Si2N2O is formed by bombardment by ∼ 1016 Argon (3MeV) ions/cm2 or by ∼ 3. 2 × 1020 fast neutron (E>1MeV)/cm2. The factor of ∼ 3 × 104 lower fluence for heavy ions is compared to other reported values for heavy ion and fast neutron effects. The damage cross-section for Ar(3MeV) ions is 0. 03 ± 0. 01nm2 and for fast neutrons approximately 10−6nm2. The non-crystalline Si2N2O has many characteristics of a glass.

L'oxynitrure de silicium: Si2N2O: II. Evolution structurale et endommagement par irradiation aux neutrons rapides

Abstract Infrared spectroscopy and time of flight neutron diffraction methods have been used in order to study and to explain the structural modifications and damage effects in fast neutron irradiated silicon oxinitride. Atomic positions, bond lengths and bond angles are given for different irradiation doses, up to 3 10 20 n.cm −2 . An anisotropic behavior of the lattice parameters and a decrease of the volume of the unit cell have been set forth. This can be explained by a greater rigidity of the Si-1bN bonds compared to the Si-1bO bonds, and by the flexibility of the Si-1bO-1bSi bridge. During irradiation SiN 3 O tetrahedra undergo a rotation around the oxygen atom in association with a conter rotation of the opposite tetrahedron linked to the same oxygen. The destruction of the structure is a gradual process. The mechanism of amorphisation results from a more rapid breaking, in some cells, of the Si-1bO-1bSi bridge, in comparison with Si-1bN-1bSi, while the contiguous cells remain nearly undamaged.