Orfeo Sbaizero

Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings

The residual stresses in thick hydroxyapatite coatings, deposited by plasma spraying, have been determined experimentally using Raman piezo-spectroscopy. The stress dependence of the centre position of the 980 cm 1 Raman band, owing to the symmetric stretching of the phosphate ion, PO3(4), has been established and found to be 2.47 cm 3 GPa 1. Using this calibration, the residual stresses in hydroxyapatite coatings deposited onto Ti-6A1-4V substrates in air have been found to be 100 MPa (tensile), whereas those deposited in a vacuum have been found to be 60 MPa (compressive). Although desirable from a mechanical point of view, it is shown that coating under residual compression are thermodynamically more stable and, hence, the dissolution of the ionic species, necessary in the exchange between bone and hydroxyapatite coating, can be impeded. It is calculated that for the coating under examination the stresses have an effect comparable with almost an order of magnitude change of the [OH] concentration. The analysis explains the dissolution behaviour of hydroxyapatite coatings subject to cyclic stress reported previously.

Deposition of calcium ions on rutile (110) : A first-principles investigation

Abstract The deposition of calcium ions is the first and most crucial step of apatite nucleation on ceramic supports from ionic solution. This process is believed to initiate the growth of bone-like material on the surface of biocompatible implants. We have investigated the adsorption of Ca2+ from water solution on the rutile TiO2 (110) surface by means of first principles techniques. The preferential binding site of the calcium ion on the hydrated oxide surface was determined through a series of static calculations. Molecular dynamics simulations were then performed to elucidate the deposition pathway. The driving force for adsorption is identified in the electrostatic interaction between the Ca2+ complexes and negatively charged deprotonated sites present on the hydrated TiO2 (110) surface.

Influence of residual stresses on the mechanical properties of a layered ceramic composite

Abstract A layered Al2O3-ZrO2 ceramic composite has been fabricated using a colloidal processing method. The technique uses sequential centrifuging of slurries containing suspended ceramic powders to form a three-layered structure. The outer layers have a high alumina content while the central layer contains mainly stabilized zirconia. These laminae are subjected to residual stresses due to their different thermal expansion coefficients. These stresses depend on the configuration of the system as well as on the amount of the zirconia in the two layers. If the residual stresses exceed the strength of the inner layer, periodic parallel cracks are produced. Such cracks of course adversely influence the structural performance of the composite and should be avoided. A model for this problem is presented. Vickers indentations were also placed into the tensile layer with the intent to explore the crack propagation in such system. Cracks were split after they reached the compressive layer. The effects of the layer thickness on the depth of the cracks beneath the interface were systematically explored.

Electrical conductivity of an insulator matrix (alumina) and conductor particle (molybdenum) composites

Different concentrations of molybdenum (from 5 to 35 vol.%) were added to alumina matrices and the resulting mixture hotpressed to produce dense metal toughened-ceramic composites. Their resulting electrical properties were measured using two-probe impedance spectroscopy, in the range 25–1000 � C. Experimental and theoretical studies were done to examine how the volume fractions and the morphology of the embedded metal particles affect the electrical behaviour of these composite materials. Due to the percolation effect, a sharp increase in the electrical conductivity of these composites was observed for compositions with molybdenum contents higher than 20 vol.%. The experimental data were fitted into the GEM equation. It was shown that the conductivity of these metal-toughened-ceramic composites is strongly affected by the amount, and less by the size of the metal particles. For low concentrations of molybdenum only one semicircle was readily identified with impedance spectroscopy analysis. This indicates that the matrix is still the main conductor phase. For higher concentrations of molybdenum, two readily resolvable arcs were present. For concentrations of metal higher than 30 vol.% the second semicircle might be an overlapping of two semicircles and therefore the impedance spectra for these composites can be resolved with three semicircles. The presence of this third semicircle can be explained in terms of molybdenum clusters. # 2002 Elsevier Science Ltd. All rights reserved.

Mechanical properties in the ceria-zirconia system

Abstract The room temperature mechanical properties of samples ranging through the whole zirconia-ceria system have been determined and related to the phases present in order to establish the influence of crystallographic parameters on the material properties. Hardness, toughness and strength as a function of the amount of transformable tetragonal, non-transformable tetragonal or cubic zirconia are discussed.

The performance of molybdenum toughened alumina cutting tools in turning a particulate metal matrix composite

In this paper, a study of the tool wear mechanism in turning aluminium alloy reinforced with alumina using molybdenum-toughened alumina tools is presented. Alumina tools with three different amounts (15, 20 and 25 vol.%) of molybdenum were prepared and tested. The wear type was identified and its evolution with cutting time was measured. The results show that the main mechanism of tool wear is abrasion and not heat. The best overall performance was achieved, as far as flank wear is concerned, using the tool with 20 vol.% of molybdenum added. This has been explained using some of the composites intrinsic properties. SEM examination revealed that molybdenum particles are easily torn from the matrix by flowing chips. Under these conditions the molybdenum particles are not able to carry out their toughening action and they are responsible for the flank wear. Some ideas have been proposed as to engineer the alumina/molybdenum interface in order to increase their adhesion and the composite toughness.

R-curve behavior of alumina toughened with molybdenum and zirconia particles

Abstract Alumina matrix was added with metal molybdenum and different amounts of zirconias particles. R-curves were measured during stable crack propagation and a piezo-spectroscopic technique has been used to assess the bridging stresses developed along the crack wake at the critical condition for crack propagation. The theoretical R-curves calculated from the average (microscopic) bridging stress distribution obtained by in situ Raman spectroscopy were in good agreement with the experimental data. If the molybdenum content is kept constant but the ZrO2 amount is increased, the toughening due to t-m transformation increases, but at the same time the metal/ceramic interface is weakened by the presence of zirconia and the toughening due to metal particles bridging decreases. As a result of that, the presence of zirconia has only a limited beneficial influence on the overall toughness. The overall composite toughness is due to transformation and bridging toughening mechanisms, however, the two mechanisms do not have synergistic effects and crack bridging by molybdenum particles appears to be the dominant toughening mechanism.

Alumina/zirconia multilayer composites obtained by centrifugal consolidation

Abstract An attractive method for the fabrication of Al 2 O 3 -ZrO 2 laminar composites is described in this paper. The process consists in the centrifugation of colloidal suspensions containing alumina and zirconia particles. With this procedure, it is possible to grow thin layers with different composition and smooth interfaces. The effects of the slurry compositions, as well as aging times and centrifuging conditions on the morphology of the laminae are discussed.

Influence of residual and bridging stresses on the R-curve behavior of Mo- and FeAl-toughened alumina

Abstract Alumina matrix was toughened using either metal molybdenum or intermetallic FeAl particles. Mo and FeAl dispersoids were chosen because they have different thermomechanical properties (i.e. Youngs modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual stresses in the matrix. The R-curve behavior of these composites was first studied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different in the two composites: 25 and 15 vol% addition led to maximum toughness in the Mo- and FeAl added composite, respectively. This difference was ascribed to residual stresses. Microscopic observation of the crack path revealed, in both composites, the systematic presence of dispersoids acting as bridging sites in the crack wake, but only a few of them were plastically stretched. Residual stresses in the Al 2 O 3 matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed using microprobe fluorescence spectroscopy. Bridging microstresses were assessed in situ by a linear map along the crack profile, at the critical condition for fracture propagation. Experimentally collected residual stresses and bridging stresses were discussed to explain the different fracture behavior of the composites.

The Cardiomyopathy Lamin A/C D192G Mutation Disrupts Whole-Cell Biomechanics in Cardiomyocytes as Measured by Atomic Force Microscopy Loading-Unloading Curve Analysis.

Atomic force microscopy (AFM) cell loading/unloading curves were used to provide comprehensive insights into biomechanical behavior of cardiomyocytes carrying the lamin A/C (LMNA) D192G mutation known to cause defective nuclear wall, myopathy and severe cardiomyopathy. Our results suggested that the LMNA D192G mutation increased maximum nuclear deformation load, nuclear stiffness and fragility as compared to controls. Furthermore, there seems to be a connection between this lamin nuclear mutation and cell adhesion behavior since LMNA D192G cardiomyocytes displayed loss of AFM probe-to-cell membrane adhesion. We believe that this loss of adhesion involves the cytoskeletal architecture since our microscopic analyses highlighted that mutant LMNA may also lead to a morphological alteration in the cytoskeleton. Furthermore, chemical disruption of the actin cytoskeleton by cytochalasin D in control cardiomyocytes mirrored the alterations in the mechanical properties seen in mutant cells, suggesting a defect in the connection between the nucleoskeleton, cytoskeleton and cell adhesion molecules in cells expressing the mutant protein. These data add to our understanding of potential mechanisms responsible for this fatal cardiomyopathy, and show that the biomechanical effects of mutant lamin extend beyond nuclear mechanics to include interference of whole-cell biomechanical properties.