Patrick Verdier

Nitrides and oxynitrides: Preparation, crystal chemistry and properties

Abstract As compared to oxides, nitride-type compounds can have specific properties and applications. The outline of this review focuses on the crystal chemistry of nitrides and oxynitrides. Given the nitride-oxynitride association, this article mainly deals with compounds which are predominantly covalent in character and does not include compounds which are metallic in character and in which nitrogen can be considered as an interstitial element. The classification which is used thus highlights the two main coordination polyhedra: nitrided or oxynitrided tetrahedra or octahedra.

MSiAlON glasses (M = Mg, Ca, Ba, Mn, Nd), existence range and comparative study of some properties

Abstract Glasses have been prepared in the MSiAlON systems (M = Mg, Ca, Ba, Mn, Nd). The determination of their fields in the M x O y SiO 2 AlN sections allows a comparison with the corresponding oxide glasses. The density change versus the nitrogen content indicates a higher compaction resulting from stronger electrostatic forces between Si(O,N) 4 tetrahedra and cations. The variation of the number of gram-anions per unit volume with the cationic radius is a first approach of the structure of these glasses. Increasing the nitrogen content makes significant variations in some properties such as hardness, viscosity and Youngs modulus.

SiC particle reinforced oxynitride glass and glass-ceramic composites: crystallization and viscoplastic forming ranges

Abstract The viscoplastic forming and the crystallization ranges of SiC particle reinforced oxynitride glass composites were characterized. Crystallization starts beyond 1050°C, with MgAl2O4, MgSiO3, z - and δ-Y2Si2O7 as the main phases. A non-Newtonian flow behaviour was observed from 800 to 900°C, i.e. on both sides of Tg (Tg = 863°C), with n ≈ 0.6 (shear thickening). The viscosity versus temperature curves (drawn from creep tests), show that temperature must be higher than 950°C for the viscosity to be lower than 1010 Pa s. In the light of these results, the feasibility for viscoplastic forming of a 40 vol.% SiC composite was demonstrated by shaping a parabolic shell at 980°C within 30 min.

Cryomilling of Al/AlN powders

Abstract Elemental powders of 80 vol% 5000 Al-alloy (3 wt.% Mg) and 20 vol% AlN were milled in different mechanical conditions and at three different temperatures (in liquid nitrogen (cryomilling), low temperature (about −50°C) and room temperature). The effects of these parameters on the milling efficiency of Al and AlN and on the mechanical alloying were investigated. The cryomilling of both Al and AlN resulted in a rapid decrease in size of the coherently diffracting domains (

AlN dispersed reinforced aluminum composite

Abstract In this paper the first results are presented on nitriding of aluminium powder and the fabrication of Al-AlN composites after milling and hot-pressing. Nitriding appears to follow a complex process. High energetic milling of these powders is an important factor in obtaining homogeneous materials with AlN nanometric grains. TEM and EDX nanoanalyses have shown that Al grains are surrounded by AlN nanocrystals, with some A12O3 needles and AlON crystals. Physical properties — thermal expansion, thermal conductivity, electrical conductivity, hardness, Youngs modulus, fracture strength — of these composites change with the AlN content, and the values for 0 vol.% AlN (process powders) always correspond to higher or lower values than for pure Al (unprocessed powders), reflecting the fact that processing introduces impurities. A comparison of composites fabricated from composite powders and from a mixture of Al-AlN commercially available powders is interesting. Generally these new composites exhibit better properties than those for Al-SiC or Al-Al2O3 composites with an apparently similar reinforcement content.

Preparation and properties of gadolinium oxide and oxynitride glasses

The composition ranges in which oxide glasses in the Gd2O3SiO2 system and oxynitride glasses in the Gd2O3AlNSiO2 system are formed have been determined. The introduction of gadolinium gives these glasses a neutron-absorbing property. Physical properties such as the density, the Vickers hardness, the glass transition temperature, the thermal expansion coefficient and elastic moduli have been studied vs. nitrogen content.

SiC particle reinforced oxynitride glass: Processing and mechanical properties

Abstract After optimization of the processing route, the mechanical properties of the composites were evaluated with varying particle sizes and volume fractions of reinforcement. The best dispersion of the particles in the composite was obtained by using attrition milling followed by spray-drying; nevertheless, ball-milling led to satisfactory results for particle sizes higher than 3 μm. Elastic moduli, hardness and fracture toughness increase with the volume fraction of SiC. Fracture strength increases with both decreasing particle size and increasing volume fraction to reach 400 MPa for a glass matrix composite containing 47 vol% of SiC with 1 μm average panicle size. A further improvement is achieved by crystallizing the matrix.

Structural approach of sialon glasses: MSiAlON

Abstract Glass samples with the formular composition: M II 0.37 Si 0.53 A 0.10 O 1.58 − 3x 2 N x′ (M II = Mg, Ca or Ba) and M I 0.46 Si 0.46 Al 0.08 O 1.27 − 3x 2 N x (M I = Li) have been studied by infra-red absorption spectroscopy. The nitrogen-free alkaline-earth glass structure is built up with SiO 4 and AlO 4 linked units as in the metasilicate glasses. The M II cations are network modifiers, except Mg, which participate in the formative network inducing a certain part of the A 3+ cations in six-fold co-ordination which play the role of network modifiers. The lithium glass presents a silicate layer structure formed with (Si 2 O 5 ) − units and Al 3+ ions in four and/or five-fold co-ordination; the Li + cations are located between the layers. The presence of nitrogen leads to very high cross-linkage of aluminosilicate chain or layer. The M II cations are inserted into increasingly large sites (Mg- to Ba- glasses). The structural change could be correlated with the physical properties: the network rigidity increases from Ba- to Mg- glasses and with the amount of nitrogen, and the disorder was emphasized with an increasing amount of nitrogen, but especially from Mg- to Ba- glasses. These effects are less perceptible for the lithium glasses with a layer structure.

Li-Si-Al-O-N and Li-Si-O-N oxynitride glasses study and characterization

The extent of oxynitride glass regions has been determined within Li-Si-Al-O-N and Li-Si-O-N systems. Physical properties such as density, hardness, elastic moduli and characteristic temperatures have been studied in terms of nitrogen content in the vitreous network. A comparison between the two systems studied enables the role of aluminium in these alumino-silicated glasses to be precisely determined.

SiC particle reinforced oxynitride glass: Stress relaxation, creep, and strain-rate imposed experiments

The stress relaxation and creep behaviors of particulate composites consisting of SiC particles embedded in an oxynitride glass matrix were characterized for particle size and volume fraction from 3 to 150 {micro}m and 0. to 40%, respectively. The presence of rigid particles results in significant decreases in the relaxation kinetics and creep rates. Furthermore, the apparent viscosity of the composites exhibits a strain dependence (strain hardening), and a critical strain, depending on both particle size and volume fraction, at which flow is apparently blocked has been successfully introduced to interpret the data. A preliminary study of the behavior at constant strain rates in the 10{sup {minus}5}--10{sup {minus}1}/s range, shows a tension-compression flow asymmetry as well as a shear thinning effect for strain rates above 10{sup {minus}3}/s.