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International audience; A piezoelectric glass-ceramic containing Sr2TiSi2O8 crystals is synthesized and used to develop a SAW device tested up to 950 degrees C. This device consists of an input IDT that generates the wave and an output IDT that converts it into a measurable electrical signal. The evolution of amplitude and frequency of the output signal is recorded. This study highlights that the glass-ceramic remains able to generate and propagate SAW up to 950 degrees C. The variations of the signal between room temperature and 950 degrees C are discussed with respect to the thermomechanical properties of the glass-ceramics. The softening of the residual glass above its Tg increases the signal's amplitude thanks to the relaxation of the stresses. At higher temperatures, the low residual glass viscosity causes the damping of the SAW. Below Tg, the variations of the signal are explained by the thermal expansion mismatch between the crystals and the glass, inducing tensile stresses and damages.

International audience; In this work, barium titanate thick films are produced at low-temperature using a modified sol–gel/ceramic powder technology named the composite route. In this process, a commercial BaTiO3 powder, doped or not by Li2O as sintering aid, is mixed with a stabilized titanium (IV) isopropoxide/barium acetate sol to obtain a composite ink. Two sols, using different solvents and chelating agents, are tested. Characterization of the evolution of theses sols with an increase in temperature is realized by thermal analyses (TGA and DTA) and X-ray diffraction. The four type of inks obtained are screen-printed on metallized alumina substrates and sintered. Microstructures and electrical properties of the sintered composite films are compared to those processed from a conventional ink obtained by dispersing the barium titanate powder in an organic vehicle. Measurements of dielectric/piezoelectric properties highlight higher charge coefficients d33 for composite thick films (69 vs. 45 pC/N for conventional thick films at close thickness values) despite a lower relative permittivity (340 vs. 658) certainly due to the presence of a single domain BaTiO3 nano-crystallites.

This paper aims at studying the influence of the reinforcement particle size and morphology on the wear resistance properties of NiCrBSi-based composites. Numerous papers have already been written on this subject but almost all of them studied the “conventional” particle size for laser cladding (i.e. between 20 and 200 μm). The objective of this study is to see the influence of coarser reinforcement particles, up to 1.2 mm, and the influence of the morphology (spherical and random shaped) on the coatings erosive and sliding wear resistance. Laser clad coatings were deposited on low carbon steel substrate S235JR with various amounts of WC/W 2 C particles up to 50 vol.%. The coatings were processed by using a 3.8 kW High Power Diode Laser (HPDL). Spherical tungsten carbide particles from 40 μm up to 1200 μm were used in this study as well as random shaped particles from 40 μm to 400 μm. To assess the influence of the reinforcement particle properties on wear properties, wheel tests and pin-on-disk tests were performed on each composition. From this study, it can be concluded that there is an obvious advantage in using larger particles (750–1200 μm) in harsh conditions while smaller particles (40–160 μm) improve the resistance in sliding conditions. The effect of morphology has not been proved significant.

Crystallization of strontium fresnoite Sr2TiSi2O8 piezoelectric crystals in Sr&ndash, Ti&ndash, Si&ndash, K&ndash, Al&ndash, O parent glasses is investigated with the aim of showing the influence of composition and crystallization conditions on the microstructure and piezoelectric properties of the resulting glass-ceramic. All the investigated conditions lead to a surface crystallization mechanism that induces a preferential orientation of crystal growth in the glasses. Near the surface, all the glass-ceramics obtained exhibit (002) planes preferentially oriented parallel to their faces. Deeper in the specimens, this preferential orientation is either kept or tilted to (201) after a depth of about 300 &micro, m. The measurement of the charge coefficient d33 of the glass-ceramic highlights that surface crystallization induces mirror symmetry in the polarization. It reaches 11 to 12 pC/N and is not significantly influenced by the preferential orientation (002) or (201). High temperature XRD shows the stability of the fresnoite phase in the glass-ceramics up to 1000 &deg, C. Mechanical characterization of the glass-ceramics by impulse excitation technique (IET) highlights that the softening of the residual glass leads to a progressive decrease of Young&rsquo, s modulus in the temperature range 600&ndash, 800 &deg, C. Damping associated to the viscoplastic transition become severe only over 800 &deg, C.

The context of the present work is a large research project aiming at manufacturing functionalised surfaces by selected laser treatment with a focused beam [1]. More specifically, this paper aims at showing the possibility of obtaining a dense glass ceramic layer, with controlled microstructure, from a glass powder coating. In the present state of the research, laser scans are performed on glass powder compacts in order to investigate the operating conditions (power, speed and vectorisation) required to achieve the objective. The selected glass belongs to the Sr–Ti–Si–Al–K–O system, its crystalisation leads to a glass–ceramics containing piezoelectric Sr–fresnoite crystals (Sr2TiSi2O8) Maury et al. (2011). The selective laser treatment is carried out by mean of a focused Nd–YVO4 laser beam. Characterisation of the thermal state of the surface during the treatment is performed thank to an IR camera. The surface microstructure is evaluated by X-ray diffraction and scanning electron microscopy. The results presented shows that Sr–fresnoite glass ceramics layers can be processed by applying a two stage process: first the melting of a glass powder to form a dense amorphous layer; then the crystallisation of this layer. However, the microstructures resulting of a crystallisation step performed by mean of a one-scan laser treatment are heterogeneous. IR camera analysis highlights that this heterogeneity is due to thermal profile strongly depending on the position on the scanned surface. Moreover, the high energy scanning conditions required to promote crystallisation make that this later takes place on cooling after surface has been re-melted. Processing by a multi-scan mode (numerous successive rapid scans) seem more suitable. In that case, IR camera analysis shows a homogeneous thermal state of the surface. Additional investigations have to be performed in order to be able to master the relation between the thermal profile and the laser scan parameters.

International audience; In this work, foams were obtained by direct foaming of aqueous alumina suspensions containing butyric acid. Butyric acid is incorporated in the suspensions in order to hydrophobize alumina particles, leading to their adsorption at the air/water interface. After setting and drying, wet foams were sintered at 1585 °C for 2 h. The resulting porous samples were characterized in terms of microstructure, mechanical properties and thermal conductivity. Microstructures of sintered alumina foams reveal closed pores surrounded by a thin alumina layer. The microstructure of these macroporous ceramics is related to (i) the composition of the initial suspension (alumina and butyric acid contents) and (ii) the stirring velocity during the foaming process. Macroporous ceramics with pore sizes (d50) ranging from 20 µm to 140 µm and porosities between 25% and 89% were obtained. Three-point bending tests were performed on foams with porosities between 65% and 89%. The results of mechanical tests were analyzed with Weibull statistic. The bending strength values are between 5 MPa and 42 MPa. The Young’s modulus, obtained from 3-point bending tests, decreases with the porosity level according to Gibson–Ashby model. Thermal diffusivity measurements were made with the laser flash technique in order to determine the effective thermal conductivity. Experimental values are significantly higher than the predictions with Landauer’s relation and almost close to Maxwell/Hashin–Shtrikman upper bound.

This paper presents a new method for manufacturing macroporous ceramics by partial sintering of controlled ceramic powder agglomerates packing. Using this route, the final ceramic material is characterized by a hierarchical porous network that can contain up to three levels of interconnected pores: (i) the voids existing between the agglomerates (typically ≥10 µm in size), (ii) the porosity remaining inside the agglomerates after partial sintering (typically ≈100–1000 nm in size), (iii) the pores that may exist within the initial ceramic particles (typically

Spark Plasma Sintering presents many advantages in comparison with more conventional sintering techniques. Nevertheless, one of its main drawbacks is the heterogeneity of temperature field during the dwell of the temperature cycle, giving heterogeneous final microstructure. In this article, we propose to optimize the temperature field in the compact in order to obtain an almost homogeneous temperature only by modifying the tools dimensions. This optimization is based on the use of a coupled thermoelectric finite element model of the process. Finally, we present the sintering conditions influencing the optimization parameter.

The research work reported in this paper is an investigation of the behavior of barium titanate powders under selective laser irradiation. Our goal is to determine suitable conditions to sinter the powders and form dense layers usable in some electronic components. On that purpose, compacts of micro/nano BaTiO 3 powder mixes are used for a parametric investigation of the laser scans parameters (power, speed, etc.) with a Nd-YVO 4 laser (23 W). The microstructures obtained after laser treatments are evaluated by XRD, SEM and EDS and compared to a reference specimen manufactured in a conventional way. From this work it can be concluded that a high laser beam power is required to obtain a consolidation of the powder grains and the use of a high scan speed avoids the melting. The scanning speed also influences the final crystallographic state of BaTiO 3 . Optimal parameters were founded in order to form a dense and homogeneous tetragonal BaTiO 3 surface.

Hertzian indentation was used to assess the fracture toughness (KIc) and the surface residual stresses of tempered soda lime glass. Although such measurements are usually performed on chemically strengthened glasses, where the thickness of the layer under compression is limited to several micrometres, our study considers the most useful case of thermally quenched glass, widely used in industry and where the compressed layer is several hundred micrometres thick. Measurements were performed on annealed and thermally quenched samples and residual stresses were calculated using two different models of the literature. Results were compared to stress measurements performed by a conventional photoelasticimetry technique. It is shown that Hertzian indentation gives qualitatively correct results which are comparable for each of the literature models used, provided that interfacial friction between the spherical indenter and substrate is correctly taken into account in the calculations. It is shown that a scratch tester used to perform sliding indentation onto the surface of the samples is a suitable tool for assessing the friction coefficient value.

A commercial polysilazane is used as a silicon carbonitride matrix precursor for the manufacture of ceramic matrix composites using bi-directional SiC Nicalon fabrics as reinforcing material. The objective is to develop a simple and fast process leading to materials able to compete with SiC/C/SiC composites obtained by the Chemical Vapour Infiltration (CVI) route. Two processes are investigated: (1) a ‘conventional’ process using the densification of a SiC fibre preform by several cycles of impregnation of the preform with the polymer followed by pyrolysis and (2) a ‘modified’ process consisting in a powder filling of the fibre preform prior to the precursor impregnation and pyrolysis. This paper describes the different steps of both processes. The materials obtained are characterised in terms of their porosity, microstructure and mechanical properties. ©

This paper reports the ability to use a commercial polysilazane † as matrix precursor for the manufacture of ceramic matrix composites. By pyrolysis under nitrogen in the temperature range 1000–1400 °C this polysilazane leads to an amorphous silicon carbonitride with a ceramic yield around 65 wt%. The rheological behaviour and the thermal decomposition of the polysilazane have been studied. Parameters for the impregnation and pyrolysis cycles required by the manufacture of composites are also proposed.

A low temperature process for the manufacture of non-oxide monolithic ceramic bodies is presented. This process is based on the pyrolysis at temperatures between 1000 and 1400 °C of the polysilazane precursor. The different steps of the process are precisely described. The physical (porosity, density, crystalline phases) and mechanical properties (flexural strength, Young's modulus, toughness and hardness) of the bodies obtained are studied versus the maximum temperature of the pyrolysis. The high temperature oxidation resistance in air is also presented.

A new process route for ceramic composites, associating CVI of SiC and impregnation and pyrolysis cycles with a polysilazane precursor, has been developed. The interest of this process is, firstly, a shorter time of elaboration than for CVI alone and, secondly higher mechanical properties than those resulting from a conventional use of the precursor. Composites with mechanical properties comparable to those of composites entirely obtained by CVI of SiC have been realised. The flexual strength of these materials is about 400 MPa, whereas that of composites resulting from a conventional use of the same polysilazane does not exceed 200 MPa.

International audience; The aim of this work is to study the machinability of aluminium nitride green bodies obtained from dry pressing of spray-dried granules. The characterisation of the green machinability of ceramics is not easy. Indeed, to date, no conventional mechanical test able to represent the machining behaviour of green powder ceramic compacts. Therefore, the first target of this work is to determine possible correlations between mechanical properties, microstructure and machinability and to propose suitable tests. Three types of aluminium nitride granules are investigated. Two of theses types of granules are commercial grades using a thermoplastic binder: a conventional grade for pressing and a specific grade especially developed for green machining. The third grade is under development and uses a thermoset binder. From this work it appears that the compacts containing the thermoset binder exhibit simultaneously the highest mechanical properties and the best behaviour at machining. The good machinability has been correlated to a highwork of fracture together with a transgranular mode of fracture of the material.

The purpose of this work is the manufacture of dense thin mullite substrates by tape casting of the natural silicate mineral andalusite. The targeted application for such substrates is the manufacture of thin film solar cells. Indeed, in addition to a good resistance at high temperature (up to 1200 °C), this application requires a good correspondence between the thermal expansion coefficient of the substrate and silicon film in order to lower the stresses in the film and in the substrate after cooling. The work was performed in three successive stages. First, the study of the transformation during sintering of andalusite+alumina mixtures. Second, the optimisation of the slurries and of the parameters for tape casting. Finally, green tapes prepared from various powder compositions were characterised before and after sintering. The addition of alumina to andalusite reduces the final amount of vitreous phase. This limits the risk of impurity diffusion in the silicon film. However, the addition of alumina also slows down the sintering process leading to more porosity in the substrates. A good compromise is obtained with an initial mixture of 80 wt.% andalusite with 20 wt.% alumina. In that case, the thermal expansion behaviour is very close to pure mullite and the mechanical properties of the substrates can be considered as sufficient for the targeted application.