Jean Desmaison

Comparison of the oxidation behaviour of two dense hot isostatically pressed tantalum carbide (TaC and Ta2C) Materials

Isothermal oxidation of dense HIPed tantalum carbide materials TaC and Ta2C, has been performed in flowing oxygen between 750 and 850 °C. The behaviour of the two carbides: i.e. TaC (NaCl type structure) and Ta2C (hexagonal type), is characterized by the growth of a non-protective oxide scale which, on square section samples, forms a maltese cross. X-Ray diffraction analysis has only shown the formation of tantalum hemipentoxide βTa2O5. The oxidation of TaC proceeds by an interfacial reaction process. For Ta2C, the mechanism could be more complex due to the presence of an intermediate oxycarbide layer TaCxOy which has been detected at the Ta2C-Ta2O5 interface. Indeed, in this case, it is not possible to exclude a diffusion limiting process through this oxynitride sublayer of constant thickness with time.

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 TiNx (or Ti) and TiO2.

Microwave plasma enhanced CVD of aluminum oxide films: OES diagnostics and influence of the RF bias

Aluminum oxide films are obtained in a remote-microwave-plasma-enhanced chemical vapour deposition (RMPECVD) reactor. In this technique, only oxygen gas plasma is generated while trimethylaluminum (TMA) carried by argon is fed near the substrate holder which can be RF biased and deposition occurs at the surface of a silicon wafer. Optical emission spectroscopy (OES) is used in order to gain information on the plasma excitation of various species. The presence of an inert gas (argon) allows an evaluation of the relative densities of reactive species by the actinometry technique. A comparison has been done with and without RF biasing or TMA injection at different microwave powers in the same conditions of Ar and O2 flow rates. The RF biasing creates an additional plasma near the substrate holder but has a weak influence on the OES signal. The microwave plasma is the major mode in the present experimental set up. Less than 2% of TMA modifies the plasma chemistry by improving the atomic oxygen formation. In addition, the TMA decomposition involves a cooling effect of the electron energy. Although this dual mode configuration of the plasma production induces a slight decrease of the deposition rate, it limits the incorporation of impurities (O, H) in the layer by improving the ion bombardment and etching of the growing layer.

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.

Comportement du nitrure de niobium cubique δ en atmosphere d'oxygene

Abstract The oxidation of cubic δ NbN .87 plates and NbN .90 powder in oxygen at pressures of 25–730 Torr in the temperature range 425–600°C obeys pressure independent sigmoidal kinetics. There is formation of a low temperature form of Nb 2 O 5 . The reaction is governed by an inner interface process, the shape of the curves being associated to a chain fragmentation of the nitride substrate.

Remote microwave plasma-enhanced chemical vapour deposition of insulating coatings (SiO2) on metallic substrates: film properties

Abstract Silicon dioxide thin films were prepared on stainless steel substrates by remote microwave plasma-enhanced chemical vapour deposition (RMPECVD) using an oxygen plasma and a mixture of 5% silane in argon injected in the afterglow. The influence of the process parameters on the film properties was investigated by Fourier transform infrared (FTIR) spectroscopy and electron recoil detection analysis (ERDA). The content of hydrogen was deduced. The electrical insulation was analysed in terms of the dielectric constant, loss factor ( tan δ), resistivity and dielectric strength of the films. An increase in temperature or microwave power leads to a decrease in hydrogen incorporation and improves the insulation properties. Moreover, the use of a radiofrequency (r.f.) substrate bias voltage of 200 V aids the desorption of hydrogen.

Study of the adhesion between a-CH films and TA6V substrates by electron-induced X-ray emission spectroscopy (EXES)

Abstract The Ti 3d valence states have been analyzed by electron-induced soft X-ray emission spectroscopy (EXES) in the TA6V bulk alloy and at a-CH/TA6V and a-SiO x C y H/TA6V interfaces. These states stem from metallic titanium in both cases. In contrast, EXES analysis shows that aluminum is in an oxide environment. Both aluminium compounds have a different behaviour depending on the presence of a-SiO x C y H in contact with the alloy. Adherence of a-CH films on the TA6V alloy can be explained from physicochemical interactions.

Modelling of a microwave flowing oxygen discharge: application to remote plasma enhanced CVD of silica films

A kinetic model of low-pressure surface wave discharges in flowing oxygen has been used to study the remote microwave plasma enhanced CVD of silica. This work deals with the effect of pressure and microwave power on the computed concentrations of O2(X3?), O2(a1?), O2(b1?), O(3P) dominant species, present in a symmetrical discharge created in a quartz tube (diameter:?30?mm). The calculations are conducted at a position located 5?cm below?the centre of the wave guide. This length which corresponds to the end of the quartz tube defines the beginning of the afterglow. By correlation with the?coating characteristics, the model suggests that the oxygen O(3P) active species in the afterglow plays an important role in the deposition process.

Oxidation behaviour of vanadium nitride in carbon dioxide-carbon monoxide mixtures

Abstract The oxidation of vanadium nitride by CO 2 -CO mixtures, at a total pressure of 1 atm, for ratios 0.54 ⩽ P co 2 ,/ P co ⩽ 4 leads to the formation of a porous V 2 O 3 scale between 1050 and 1200°C. The kinetics are continuously decelerated. Their shape, intermediate between linear and parabolic, is well represented by a model based on the hypothesis of two parallel processes: a reaction regime slowed by the progressive closure of the open porosity of the scale and a diffusion process of the reactants through lattice defects or, more probably, low-resistance paths in V 2 O 3 . The effect of the nature of the substrate and atmosphere is discussed.

High-Temperature Oxidation Behaviour of a Hot Isostatically-Pressed Si3N4-HfB2 Ceramic Composite

A nearly fully-dense (>98%) electroconductive silicon nitride 27vol% hafnium diboride composite was prepared by Hot Isostatic Pressing (HIP). A sintering temperature of 1710°C is required to restrain the decomposition of the silicon nitride phase, but this temperature is too low to obtain a full densification. The necessary presence of sintering aids (2wt% Y2O3 + 1wt% Al2O3) induced reactions between the phases. The only noticeable secondary phase formed by reaction between the initial powders (and detected by XRD and TEM, after sintering), was the formation of a Y2Hf2O7 phase, (Y2O3*2HfO2). The oxidation tests were carried out under pure flowing oxygen (10L/h) between 900 and 1400°C for 24 hours. The composite material started to oxidize at 800°C but the weight gain was low up to 1400°C. From 900 to 1200°C, the rate of oxidation decreased with time. The oxides HfO2, B2O3 and a borosilicate glass were formed. At higher temperature (1200-1400°C), SiO2 and HfSiO4 appeared, and played an increasingly important role. At 1400°C, the whole oxidized sample was covered with a silicon oxide glassy phase while Yttrium and Hafnium were detected near the surface by EDS analysis, suggesting the presence of the mixed oxide Y2Hf2O7. The HfSiO4 phase occasionally formed on the surface large hollow spheres, with diameters greater than 50 μm while, in cross section, a very porous and titanium-depleted sublayer was observed. Since the silicon oxide glassy phase acted as a protective coating, this composite has promise for high-temperature applications.