Papers by Author: Gilbert Fantozzi

Abstract: Tantalum and hafnium carbides are classified as Ultra High Temperature Ceramics (UHTC) because of their extreme melting temperatures (above 3900°C). Therefore, these materials can safely operate in the range of temperature that any other materials could hardly exist. However, these applications can be strongly restricted due to (1) processing difficulties and (2) low fracture toughness. In this work, to address these two difficulties we have used two additives, which are multi-walled carbon nanotubes (CNTs) and molybdenum disilicide (MoSi₂). The CNTs were added aimed to improve the fracture toughness of the composites, and the MoSi₂ to facilitate sintering. Application of such a sintering aid, add to the novel SPS technique, allowing quick processing at relatively lower temperature, results in (1) fully densified specimens (> 99%) and (2) well-surviving CNTs after sintering. Moreover, microstructural analysis points out fair-enough dispersion of the CNTs within the ceramics particles, in both the green and sintered bodies. Also the specimens phase characterization shows inter dissolution of TaC and HfC and formation of binary carbides solid solution.

Abstract: Composites containing 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) reinforced with multiwalled carbon nanotubes (CNTs) with various amounts of CNTs (3Y-TZP / X wt% CNT, X= 0, 0.5, 1.5, 3 and 5) were processed by spark plasma sintering. Microscopic analysis proves that CNTs were well dispersed and embedded in grain boundaries of the sintered body. High temperature mechanical properties have been investigated using mechanical spectroscopy and low stress (6 MPa) creep. The isothermal spectrum (measured at 1600 K) consists of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at low frequency. The absence of a well-marked peak in monolithic 3Y-TZP is justified considering that restoring force decreases at low frequencies or high temperatures due to the elasticity of neighboring grains. Therefore, strain is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. However, with CNT additions the mechanical loss decreases and a better resolved peak was observed. In parallel, the results have shown that the creep rate drastically decreases with CNT additions. These results can be interpreted by the pinning effect of CNTs which can hinder grain boundary sliding at high temperatures, resulting in a creep resistance improvement.

Abstract: Despite systemic prophylaxis, infection rates after orthopedic surgery can reach more than 1%. A new HAP/TCP bone substitute loaded with 125 mg of gentamicin was designed for prophylactic use. Its aim was to enhance the efficacy of systemic prophylactic treatments by increasing the local antibiotic concentration. For prophylactic applications, release had to take place within 48 hours not to select antibiotic-resistant bacterial strains. The purpose of this study was to investigate the releasing mechanisms of gentamicin from the porous HAP/TCP matrix. The release rate of gentamicin trough the porosities of the bone substitute was investigated in vitro, in 0.9% sodium chloride solution. The rate appeared to be related to the bone substitute volume and fit classical diffusion laws. All the gentamicin was released in less than 48 hours: this rate corresponds to the recommendations for the prophylactic use of antibiotics.

Abstract: Non-oxide composites are interesting materials for long term applications at high temperature under oxidizing atmosphere. To improve their lifetime, self-healing [Si-B-C] matrix has been developed recently. On this new generation of composite, fatigue has been studied at high temperature (up to 1200°C) in air. For the SiCf/[Si-B-C] composite, lifetime at 600°C is higher than 1000 h for a tension/compression cyclic loading of -50/+170 MPa. Nevertheless, the mechanical behaviour during the cyclic fatigue test is different at 600°C from that at 1200°C. Contrarily to 1200°C, at 600°C no specific evolution of the classical parameters used to characterize the mechanical behaviour during fatigue allows us to estimate the approach to fracture,. This indicates that the fatigue phenomena involved at 600°C are different from those involved at 1200°C. At 600°C, lifetime is mainly controlled by slow-crack-growth in the fibres in presence of air, and at 1200°C lifetime is controlled by fibre creep and oxidation.

Abstract: Non-oxide ceramic-matrix composites (CMCs) are subjected to be used in aeronautic applications which require very long duration (up to 100 000h) of materials at high temperatures and under air. Recently a self-healing [Si-B-C] matrix has been developped to enhance strongly the lifetime of CMCs under air. The aim of this work is to study the mechanical behaviour of a SiCf/[Si-B-C] composite with a self-healing matrix under static fatigue, and to determine its lifetime. During the mechanical tests, acoustic emission is detected in order to characterize the damage of the composite in addition to the measurement of the longitudinal deformation of the composite. The analysis of acoustic emission follows a non-supervised procedure of classification. Each event of acoustic emission is described by a set of several parameters, and the total activity can be divided in four classes. The assignment of each class to a damage mechanism is required to follow the spread of damage during fatigue and to determine the mechanisms controlling the failure of the composite.

Abstract: High damping materials exhibiting a loss factor higher than 10-2 are generally considered as polymer or metallic materials. But, it will be interesting to consider ferroelectric or ferrimagnetic ceramics, in which internal friction can be due to the motion of ferroelectric or magnetic domains. High level of internal friction can be obtained in these ceramics in a given temperature range. In the case of ferroelectric ceramics, hard ferroelectrics, such as BaTiO3 or PZT, can show some relaxation peaks below the Curie temperature due the motion of domain walls and the interaction between the domain walls and the oxygen vacancies or cationic vacancies. In the case of ferrimagnetic ceramics, some anelastic manifestations due to the ferrimagnetic domain walls appear below the Curie Temperature TC. These peaks are linked to the interaction of domain walls with cation vacancies or cation interstitials or the lattice. Above the Curie temperature, a relaxation mechanism due to the exchange of cations Mn3+ and their vacancies on octahedral sites should occur.

Abstract: A compliance function is used to quantify the shielding capacity of grain bridging, the degradation of which is the main cyclic fatigue mechanism in alumina. Materials with different grain sizes were processed and the fatigue experiments were performed using the double torsion test. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.

Abstract: Two α-Al2O3/YAG composite powders have been prepared by reverse-strike precipitation, starting from chlorides aqueous solutions, the former containing 50 vol% of the two phases (labelled as AY50) and the latter made of 90 vol% of alumina and 10 vol% of YAG (AY90). The as-prepared powders were characterised by DTA/TG simultaneous analysis as well as by XRD analysis performed after calcination at different temperatures. A systematic TEM analysis was performed on AY50 powders pre-treated at different temperatures, in order to investigate the crystallites size evolution as a function of the temperature. After that, samples were compacted by uniaxial pressing and sintered at 1600°C for 3h. SEM observations revealed a homogeneous microstructure made of micronic alpha-alumina and YAG grains. For limiting grain growth through the decreasing of the maximum sintering temperature, an innovative activation procedure by coupling suitable thermal and mechanical treatments of the powders was performed. After that, high densification (>95% of the theoretical density) was easily achieved by performing a free sintering in the temperature range between 1320° and 1420°C, with different soaking times at the maximum temperature. The resulting sintered bodies showed an effective retention of the nano-size of the primary particles. By SEM, highly-homogeneous nanostructures, with an average grains size of about 200 and 300 nm for AY50 and AY90, respectively, were observed.

Abstract: The target of this work is to investigate the effect of small additions of SiO2 or CaO on the sintering behavior and the microstructure of an ultrapure α-alumina compound. The sintering behavior has been investigated through extensive dilatometric study. SiO2 additions lead to a significant decrease in shrinkage rate during the intermediate stage of sintering whereas CaO is beneficent to densification. It has been found that during this stage which corresponds to the maximum of densification rate, grain boundaries diffusion controls densification through oxygen vacancies. The study of the densification behavior under different atmospheres help us to explain the role of the additives in agreement with electroneutrality equations. S.E.M. investigations confirm the well know correlation between doping and heterogeneous microstructures. After doping with SiO2 or CaO, abnormal grain growth appears at temperatures corresponding to the lowest eutectics given by Al2O3-SiO2 or Al2O3-CaO phase diagrams. H.R.T.E.M. observations show that below the critical temperatures for abnormal grain growth, additives enrichment is observed near grain boundaries (GBs). Above these temperatures, glassy phase for SiO2-doping and calciumhexaluminate (CA6) for CaO-doping are present at grain boundaries.