... 935938. 9. Palkar, VR, Thapa, D., Multani, MS, and Mal-ghan, SG, Densification of Nanostruct... more ... 935938. 9. Palkar, VR, Thapa, D., Multani, MS, and Mal-ghan, SG, Densification of Nanostructured Alumina Assisted by Rapid Nucleation of α-Alumina, Mater. Lett., 1998, vol. ... 17. Palmero, P., Simone, A., Stella, C., and Montanaro, L., Inventor's IT Patent TO 2004 A 000 453. ...
Nanoindentation experiments inside a transmission electron microscope are of much interest to cha... more Nanoindentation experiments inside a transmission electron microscope are of much interest to characterize specific phenomena occuring in materials, like for instance dislocation movements or phase transformations. The key points of these experiments are (i) the sample preparation and the optimization of its geometry to obtain reliable results and (ii) the choice of the transmission electron microscope observation mode, which will condition the type of information which can be deduced from the experiment. In this paper, we will focus on these two key points in the case of nanoindentation of zirconia, which is a ceramic material well known to be sensitive to stress because it can undergo a phase transformation. In this case, the information sought is the stress localization at the nanometre scale and in real time. As far as the sample preparation is concerned, one major drawback of nanoindentation inside a transmission electron microscope is indeed a possible bending of the sample occurring during compression, which is detrimental to the experiment interpretation (the stress is not uniaxial anymore). In this paper, several sample preparation techniques have been used and compared to optimize the geometry of the sample to avoid bending. The results obtained on sample preparation can be useful for the preparation of ceramics samples but can also give interesting clues and experimental approaches to optimize the preparation of other kinds of materials. The second part of this paper is devoted to the second key point, which is the determination of the stress localization associated to the deformation phenomena observed by nanoindentation experiments. In this paper, the use of convergent beam electron diffraction has been investigated and this technique could have been successfully coupled to nanoindentation experiments. Coupled nanoindentation experiments and convergent beam electron diffraction analyses have finally been applied to characterize the phase transformation of zirconia.
Hydrogen Snoek-Koster relaxation is generally attributed to the interaction of dislocations with ... more Hydrogen Snoek-Koster relaxation is generally attributed to the interaction of dislocations with hydrogen. In the case of iron, two components appear (SK1 around 120 K and SK2 around 160 K at 1 Hz). There is some disagreement about the interpretation of the two components of the SK(H) relaxation and the validity of the proposed models. In an attempt to resolve this, the authors make a comparison of the experimental features of SK(H) in iron with those predicted by the proposed models. A new analysis of the SK(H) relaxation is then proposed. This analysis is based on the kink pair formation on non-screw dislocations controlled by the hydrogen diffusion of atoms or clusters. It allows the authors to explain the complementarity between the SK(H) relaxation and the alpha -peak, the dependence of the SK(H) relaxation strength on the hydrogen concentration and the degree of deformation, and the activation parameters of the relaxation.
International Journal of Refractory Metals and Hard Materials, 2005
Powders used for this study were synthesized by the self-propagating high temperature synthesis (... more Powders used for this study were synthesized by the self-propagating high temperature synthesis (SHS) process. This method allows producing titanium nitride powders in an economic and easy way. Densification of these original powders is carried out by the hot pressing process. Sintering is achieved, under a 50 MPa pressure, at different temperatures (1400°C, 1600°C and 1800°C). At 1600°C properties are optimised and bulk specimen density is superior to 98%. In addition, microhardness reaches the average value of 1790 ± 225 H V and Young modulus measured is 430 ± 10 GPa. A tribological characterization is then carried out with an adapted four balls machine tribometer. Titanium nitride tribological behavior is compared with the one of other hard materials such as 100C6 steel, titanium carbide and two cemented carbides of different compositions. Parameters such as toughness, hardness and wear volume are correlated in order to understand wear mechanisms origins.
ABSTRACT The transformation of zirconia from its tetragonal to its monoclinic phase is an importa... more ABSTRACT The transformation of zirconia from its tetragonal to its monoclinic phase is an important feature of the zirconia system. First found to be an advantage due to its important toughening effect, it can also be very detrimental when it occurs in the framework of low-temperature degradation, particularly in the case of biomaterial applications. One way to avoid or to control this phase transformation is to understand how it initiates and more particularly the stress states that can trigger it. A new technique available inside a transmission electron microscope seems to be particularly well suited for that type of study: convergent beam electron diffraction, a well-known technique to reveal stresses, was coupled to in situ transmission electron microscopy mechanical nanoindentation. The experiments reveal the presence of sheared nanoregions at grain boundaries. These could act as embryos for tetragonal-to-monoclinic phase transformations. This is an important first step in the understanding of the earliest stage of zirconia phase transformation.
This paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapat... more This paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapatites Ca10(PO4)6-y(SiO4)y(OH)2-y(VOH)y (SiHAs) and details the characterization of powders with varying Si content up to y=1.25molmolSiHA(-1). X-ray diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to accurately characterize samples calcined at 400°C for 2h and 1000°C for 15h. This method allows the synthesis of monophasic SiHAs with controlled stoichiometry. The theoretical maximum limit of incorporation of Si into the hexagonal apatitic structure is y<1.5. This limit depends on the OH content in the channel, which is a function of the Si content, temperature and atmosphere of calcination. These results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.
... 935938. 9. Palkar, VR, Thapa, D., Multani, MS, and Mal-ghan, SG, Densification of Nanostruct... more ... 935938. 9. Palkar, VR, Thapa, D., Multani, MS, and Mal-ghan, SG, Densification of Nanostructured Alumina Assisted by Rapid Nucleation of α-Alumina, Mater. Lett., 1998, vol. ... 17. Palmero, P., Simone, A., Stella, C., and Montanaro, L., Inventor's IT Patent TO 2004 A 000 453. ...
Nanoindentation experiments inside a transmission electron microscope are of much interest to cha... more Nanoindentation experiments inside a transmission electron microscope are of much interest to characterize specific phenomena occuring in materials, like for instance dislocation movements or phase transformations. The key points of these experiments are (i) the sample preparation and the optimization of its geometry to obtain reliable results and (ii) the choice of the transmission electron microscope observation mode, which will condition the type of information which can be deduced from the experiment. In this paper, we will focus on these two key points in the case of nanoindentation of zirconia, which is a ceramic material well known to be sensitive to stress because it can undergo a phase transformation. In this case, the information sought is the stress localization at the nanometre scale and in real time. As far as the sample preparation is concerned, one major drawback of nanoindentation inside a transmission electron microscope is indeed a possible bending of the sample occurring during compression, which is detrimental to the experiment interpretation (the stress is not uniaxial anymore). In this paper, several sample preparation techniques have been used and compared to optimize the geometry of the sample to avoid bending. The results obtained on sample preparation can be useful for the preparation of ceramics samples but can also give interesting clues and experimental approaches to optimize the preparation of other kinds of materials. The second part of this paper is devoted to the second key point, which is the determination of the stress localization associated to the deformation phenomena observed by nanoindentation experiments. In this paper, the use of convergent beam electron diffraction has been investigated and this technique could have been successfully coupled to nanoindentation experiments. Coupled nanoindentation experiments and convergent beam electron diffraction analyses have finally been applied to characterize the phase transformation of zirconia.
Hydrogen Snoek-Koster relaxation is generally attributed to the interaction of dislocations with ... more Hydrogen Snoek-Koster relaxation is generally attributed to the interaction of dislocations with hydrogen. In the case of iron, two components appear (SK1 around 120 K and SK2 around 160 K at 1 Hz). There is some disagreement about the interpretation of the two components of the SK(H) relaxation and the validity of the proposed models. In an attempt to resolve this, the authors make a comparison of the experimental features of SK(H) in iron with those predicted by the proposed models. A new analysis of the SK(H) relaxation is then proposed. This analysis is based on the kink pair formation on non-screw dislocations controlled by the hydrogen diffusion of atoms or clusters. It allows the authors to explain the complementarity between the SK(H) relaxation and the alpha -peak, the dependence of the SK(H) relaxation strength on the hydrogen concentration and the degree of deformation, and the activation parameters of the relaxation.
International Journal of Refractory Metals and Hard Materials, 2005
Powders used for this study were synthesized by the self-propagating high temperature synthesis (... more Powders used for this study were synthesized by the self-propagating high temperature synthesis (SHS) process. This method allows producing titanium nitride powders in an economic and easy way. Densification of these original powders is carried out by the hot pressing process. Sintering is achieved, under a 50 MPa pressure, at different temperatures (1400°C, 1600°C and 1800°C). At 1600°C properties are optimised and bulk specimen density is superior to 98%. In addition, microhardness reaches the average value of 1790 ± 225 H V and Young modulus measured is 430 ± 10 GPa. A tribological characterization is then carried out with an adapted four balls machine tribometer. Titanium nitride tribological behavior is compared with the one of other hard materials such as 100C6 steel, titanium carbide and two cemented carbides of different compositions. Parameters such as toughness, hardness and wear volume are correlated in order to understand wear mechanisms origins.
ABSTRACT The transformation of zirconia from its tetragonal to its monoclinic phase is an importa... more ABSTRACT The transformation of zirconia from its tetragonal to its monoclinic phase is an important feature of the zirconia system. First found to be an advantage due to its important toughening effect, it can also be very detrimental when it occurs in the framework of low-temperature degradation, particularly in the case of biomaterial applications. One way to avoid or to control this phase transformation is to understand how it initiates and more particularly the stress states that can trigger it. A new technique available inside a transmission electron microscope seems to be particularly well suited for that type of study: convergent beam electron diffraction, a well-known technique to reveal stresses, was coupled to in situ transmission electron microscopy mechanical nanoindentation. The experiments reveal the presence of sheared nanoregions at grain boundaries. These could act as embryos for tetragonal-to-monoclinic phase transformations. This is an important first step in the understanding of the earliest stage of zirconia phase transformation.
This paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapat... more This paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapatites Ca10(PO4)6-y(SiO4)y(OH)2-y(VOH)y (SiHAs) and details the characterization of powders with varying Si content up to y=1.25molmolSiHA(-1). X-ray diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to accurately characterize samples calcined at 400°C for 2h and 1000°C for 15h. This method allows the synthesis of monophasic SiHAs with controlled stoichiometry. The theoretical maximum limit of incorporation of Si into the hexagonal apatitic structure is y<1.5. This limit depends on the OH content in the channel, which is a function of the Si content, temperature and atmosphere of calcination. These results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.
Uploads
Papers by C. Esnouf