Martine Desmaison-Brut

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.

Solid State Reaction between Tantalum (Ta) and Tantalum Carbide (TaC) Powders during Hiping

At high temperature and pressure, diffusion of carbon occurred between tantalum and tantalum carbide powders. Depending on the initial size of the tantalum grains, various ceramic grades were obtained by sintering Ta and TaC powders. Either an equilibrium composition was reached between the fine metal grains and the tantalum carbide grains, leading to the formation of monolithic but unstoichiometric carbides. On the other hand, under the same experimental conditions, ceramic-metal composites were prepared by using a coarse Ta grains powder. In this last case, tantalum inclusions were dispersed inside the ceramic matrix due to incomplete diffusion process and a carbon concentration gradient was detected at the Ta-TaC interface.

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.

Formation of Oxide Films on the Surface of Electroconductive Si3N4-TiN and Si3N4-TiB2 Materials under Electrochemical Polarization in a 3% NaCl Solution

It has been established that during electrochemical polarization, in a 3 % NaCl solution at 20 o C, at a rate of 5 mV/s, a very thin (~ 5 nm) protective oxide film is formed on the surface of a Si3N4-TiN ceramic. This film is constituted of three layers containing TiN0.7O0.3 in the inner part, TiO2 in the outer part and a TiO phase in the intermediate one. In the case of a Si3N4-TiB2 ceramic, treated in the same conditions, a 7-8 nm more friable oxide film was formed, consisting of rutile TiO2, oxynitride Si2N2O and of an oxygen-silicon nitride solid solution.

Fabrication and Fracture Behavior of TiN-TiB2/Cu/TiN-TiB2 Sandwich Composites

In order to improve the work of fracture of TiN-TiB2 ceramic composites and suppress catastrophic failure, an architecture has been design consisting of a copper interlayer inserted between two outer ceramic sheets. The TiN-TiB2 ceramic composite was densified to 99.7 % of theoretical density by hot-isostatic pressing of an equimolar mixture of TiN and TiB2 at 1850°C under a pressure of 190 MPa. The assembling of the three layered sandwich was performed by uniaxial pressing at 1000°C under 40 MPa. The very high malleability of copper results in an excellent intrusion into the surface roughness of the ceramic sheets ensuring a good mechanical interlocking. This mechanical join is supplemented by a chemical inter-diffusion. The stress-strain curves in 3-point bending of SENB samples exhibit a residual strength of 110 N once the ceramic sheets have broken. So, the rupture is not catastrophic. The deflection at rupture is five times greater than that of the monolithic material and the work of rupture is as high as 5450 J/m 2 . Introduction Though the toughness of TiN-TiB2 ceramic composite is rather high (6.6 MPa.m 1/2 ), it fails in a catastrophic manner. To produce materials that do not break catastrophically, one may design specific architectures which have the ability to absorb large amounts of energy such as ceramic laminates with weak interfaces or interphases [1,2]. Another possibility is to design metal/ceramic laminates [3,4]. In this case, the crack tip is blunted and energy dissipated by the formation of a plastic zone. It is this latter approach that was chosen to improve the work of fracture of an equimolar TiN-TiB2 ceramic composite: a sandwich arrangement was made with a thin layer of copper inserted between two thick layers of the ceramic composite. Materials and Sandwich Fabrication TiN-TiB2 composite. The starting powders (H. C. Stark) for TiN and TiB2 had a mean diameter of 3.5 and 5.8 μm and a specific surface area (BET) of 1.80Γ0.03 and 1.79Γ0.04 m 2 /g respectively. The equimolar mixture was cold pressed (200 MPa, 2 min) to 58 % of the theoretical density. The green compacts were covered with a BN coating acting as a diffusion barrier and introduced inside a silica capsule. The container was sealed off under vacuum and set in the HIP furnace. A density of 99.7 % of theoretical density was reached after HIPing for 1 hour under 190 MPa at 1850°C. No reaction occurred between TiN and TiB2 during the treatment and only these two phases were identified by X-ray diffraction. Grain growth was negligible (d = 3.5 to 4 μm) and the two phases were homogeneously distributed (Fig. 1). Sandwich fabrication. Copper was selected as the metal of the inner layer for three reasons: its high malleability is suitable to prevent from interfacial debonding due to difference in thermal expansion coefficients; its melting temperature (1083°C) allows an easy joining without any degradation of the properties of the ceramic layers that starts to be noticeable above 1000°C [5]. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 845-848 doi:10.4028/www.scientific.net/KEM.264-268.845

Mechanical Properties of Electroconductive Ceramic Composites Produced by Hot Isostatic Pressing

Electroconductive ceramic matrix composites of Si3N4 containing TiN or TiB2 as a secondary phase have been produced by hot isostatic pressing (HIP). The observed materials show a full or nearly full densification and a good dispersion of the secondary phase. Some improvements of the mechanical properties were clearly shown. The boride could be the best alternative even if it is difficult to totally avoid chemical reactions between Si3N4 and TiB2 under the used sintering conditions.

Creep and Oxidation Behaviour of a TiN-TiB2 Composite Material

This paper describes the compressive creep behaviour of an equimolar titanium nitride (TiN) -titanium diboride (TiB 2 ) particulate composite material in the temperature range 900-1200°C and the stress range 100 to 200 MPa. Attention was devoted to the specific role of the atmosphere in influencing the creep mechanism: air and impure argon were tested. In both atmospheres, the creep tests conducted during 24 hours exhibited primary and dominant secondary steady-state stages but did not show a tertiary stage. The stress exponent in air is 1.3 and the apparent activation energies are 140 and 160 kJ/mol, respectively, in air and argon. To understand the part played by oxidation, thermogravimetric analyses have been conducted in static air between 1000 and 1200°C. A reaction process is observed in this temperature range (Ea = 170 kJ/mol).