Authors: F. Arianpour, Hamid Reza Rezaie, F. Golestani Fard, Gilbert Fantozzi, Mehdi Mazaheri
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.
145
Authors: Mehdi Mazaheri, Daniele Mari, Robert Schaller, Gilbert Fantozzi
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.
265
Authors: Aurélien Bignon, F. Laurent, J. Goldnadel, Jérôme Chevalier, Gilbert Fantozzi, E. Viguier, T. Roger, Georges Boivin, D.J. Hartmann
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.
1203
Authors: Gilbert Fantozzi, Pascal Reynaud, Dominique Rouby
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.
1664
Authors: M. Moevus, Pascal Reynaud, M. R'Mili, N. Godin, Dominique Rouby, Gilbert Fantozzi
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.
141
Authors: Gilbert Fantozzi, E.M. Bourim, Sh. Kazemi
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.
157
Authors: Hassan El Attaoui, Malika Saâdaoui, Jérôme Chevalier, Gilbert Fantozzi
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.
449
Authors: Paola Palmero, Claude Esnouf, Laura Montanaro, Gilbert Fantozzi
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.
267
Authors: Nicolas Louet, Thierry Epicier, Gilbert Fantozzi
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.
1
Authors: S. Deville, Jérôme Chevalier, Gilbert Fantozzi, J.F. Bartolomé, J. Requena, J.S. Moya, Ramon Torrecillas, L.A. Díaz
2013