231 research outputs found
Mechanical behavior study of plasma sprayed hydroxyapatite coatings onto Ti6Al4V substrates using scratch test
Mechanical behavior and fracture mechanisms of plasma sprayed hydroxyapatite coatings on Ti-6Al-4V substrate were assessed taking into consideration two variables: the coating thickness and the substrate roughness. The results show that the specimens having a substrate arithmetic average roughness parameter Ra = 2.29 ÎŒm is favorable with respect to Ra = 1.23 ÎŒm. For coating thickness above 105 ÎŒm, cracks can be observed in the coating/substrate interface and the higher critical load Pc2 (used generally in comparative evaluation of adherence) decreases. A 90 ÎŒm coating thickness sprayed on a substrate having an arithmetic average roughness parameter Ra equal to 2.29 ÎŒm seems to be the best compromise between microstructure, mechanical resistance (high critical loads and fairly good contact quality) and long term stability in the physiological medium (low dissolution rate) for an orthopedic application
Bioinspired microstructures of chitosan hydrogel provide enhanced wear protection
We describe the fabrication of physical chitosan hydrogels exhibiting a layered structure. This
bilayered structure, as shown by SEM and confocal microscopy, is composed of a thin dense
superficial zone (SZ), covering a deeper zone (DZ) containing microchannels orientated
perpendicularly to the SZ. We show that such structure favors diffusion of macromolecules
within the hydrogel matrix up to a critical pressure, Ïc, above which channels were
constricted. Moreover, we found that the SZ provided a higher wear resistance than the DZ
which was severely damaged at a pressure equal to the elastic modulus of the gel. The
coefficient of friction (CoF) of the SZ remained independent of the applied load with ÎŒSZ =
0.38 ± 0.02, while CoF measured at DZ exhibited two regimes: an initial CoF close to the
value found on the SZ, and a CoF that decreased to ΌDZ = 0.18 ± 0.01 at pressures higher than
the critical pressure Ïc. Overall, our results show that internal structuring is a promising
avenue in controlling and improving the wear resistance of soft materials such as hydrogels
Mechanical characterization of elastoplastic materials - use of axisymetric indenters
Dans cet article, nous présentons une expression de la courbe d'indentation instrumentée d'un matériau élastoplastique pouvant s'écrouir. Cette expression nécessite d'expliciter la pression moyenne de contact entre l'indenteur et le matériau, ainsi que de tenir compte de la forme que prend la surface du matériau sous l'indenteur. Le cas d'un indenteur conique est d'abord étudié ; puis l'expression est généralisée aux indenteurs d'autres formes (supposés axisymétriques). Les résultats sont comparés aux modÚles déjà existants.In this article, an expression of the depth-sensing indentation curve for strain-hardening materials is introduced. This expression requires to formulate the mean contact pressure between the indenter and the material and to account for the shape of the material surface under the indenter. The case of a conical indenter is first studied, and extended to other axisymetric indenters. Results are compared to previous models
Mechanical characterization of elastoplastic materials - use of axisymetric indenters
Dans cet article, nous présentons une expression de la courbe d'indentation instrumentée d'un matériau élastoplastique pouvant s'écrouir. Cette expression nécessite d'expliciter la pression moyenne de contact entre l'indenteur et le matériau, ainsi que de tenir compte de la forme que prend la surface du matériau sous l'indenteur. Le cas d'un indenteur conique est d'abord étudié ; puis l'expression est généralisée aux indenteurs d'autres formes (supposés axisymétriques). Les résultats sont comparés aux modÚles déjà existants.In this article, an expression of the depth-sensing indentation curve for strain-hardening materials is introduced. This expression requires to formulate the mean contact pressure between the indenter and the material and to account for the shape of the material surface under the indenter. The case of a conical indenter is first studied, and extended to other axisymetric indenters. Results are compared to previous models
Mechanical behavior study of plasma sprayed hydroxyapatite coatings onto Ti6Al4V substrates using scratch test
Mechanical behavior and fracture mechanisms of plasma sprayed hydroxyapatite coatings on Ti-6Al-4V substrate were assessed taking into consideration two variables: the coating thickness and the substrate roughness. The results show that the specimens having a substrate arithmetic average roughness parameter Ra = 2.29 ÎŒm is favorable with respect to Ra = 1.23 ÎŒm. For coating thickness above 105 ÎŒm, cracks can be observed in the coating/substrate interface and the higher critical load Pc2 (used generally in comparative evaluation of adherence) decreases. A 90 ÎŒm coating thickness sprayed on a substrate having an arithmetic average roughness parameter Ra equal to 2.29 ÎŒm seems to be the best compromise between microstructure, mechanical resistance (high critical loads and fairly good contact quality) and long term stability in the physiological medium (low dissolution rate) for an orthopedic application
Lubrication and wear protection of micro-structured hydrogels using bioinspired fluids
We report the fabrication and the use of a bioinspired synovial fluid acting as a lubricant fluid
and anti-wear agent at soft and porous chitosan hydrogel tribopairs. This synthetic synovial
fluid is composed of sodium hyaluronate (HA) and a bottle-brush polymer (BB) having a
polycationic attachment group and polyzwitterionic pendant chains. 2.5 %w/w chitosan
hydrogel plugs are organized in a bilayered structure exposing a thin and dense superficial
zone (SZ), covering a porous deep zone (DZ) and exhibiting microchannels perpendicularly
aligned to the SZ. Using a low-load tribometer, the addition of HA lubricating solution at the
hydrogel-hydrogel rubbing contact drastically decreased the coefficient of friction (CoF) from
Ό = 0.20 ± 0.01 to Ό = 0.04 ± 0.01 on the DZ configuration and from Ό = 0.31 ± 0.01 to Ό =
0.08 ± 0.01 on the SZ surface when increasing HA concentration from 0 to 1000 Όg/mL and
its molecular mass from 10 to 1500 kDa, similar to what was found when using BB polymer
alone. When combining the BB polymer and the 1500 kDa HA, the CoF remained stable at Ό
= 0.04 ± 0.01 for both studied contact configurations, highlighting the synergistic interaction
of the two macromolecules. Hydrogel wear was characterized by assessing the final gel
surface roughness by the means of an interferometer. Increasing HA concentration and
molecular weight plus the addition of BB polymer lead to a dramatic surface wear protection
with a final gel surface roughness of the hydrogels similar to the untested gels. In brief, BB
polymer in combination with high molecular weight HA is a potential lubricating fluid as well
as a wear resistant agent for soft materials lubrication and wear protection
Wear protection without surface modification using a synergistic mixture of molecular brushes and linear polymers
We describe the design of lubricating and wear protecting fluids based on mixtures of bottlebrushes (BB) and linear polymers solutions. To illustrate this concept we used hyaluronic acid (HA) - a naturally occurring linear polyelectrolyte, and a water soluble synthetic BB polymer. Individually, these two polymers exhibit poor wear protecting capabilities compared to saline solutions. Mixture of the two polymers in pure water or in saline allows to drastically increase wear protection of surfaces over a wide range of shearing conditions. We demonstrate that this synergy between the BB and HA polymers emerges from a strong cohesion between the two components forming the boundary film due to entanglements between both polymers. We show that this concept can be applied to other types of linear polymers and surfaces and is independent of the chemical and mechanical properties of the surfaces
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
Measurement of the CP-Violating Asymmetry Amplitude sin2
We present results on time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurements use a data sample of about 88 million Y(4S) --> B Bbar decays collected between 1999 and 2002 with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We study events in which one neutral B meson is fully reconstructed in a final state containing a charmonium meson and the other B meson is determined to be either a B0 or B0bar from its decay products. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2beta, is derived from the decay-time distributions in such events. We measure sin2beta = 0.741 +/- 0.067 (stat) +/- 0.033 (syst) and |lambda| = 0.948 +/- 0.051 (stat) +/- 0.017 (syst). The magnitude of lambda is consistent with unity, in agreement with the Standard Model expectation of no direct CP violation in these modes
A search for the decay
We search for the rare flavor-changing neutral-current decay in a data sample of 82 fb collected with the {\sl BABAR}
detector at the PEP-II B-factory. Signal events are selected by examining the
properties of the system recoiling against either a reconstructed hadronic or
semileptonic charged-B decay. Using these two independent samples we obtain a
combined limit of
at the 90% confidence level. In addition, by selecting for pions rather than
kaons, we obtain a limit of using only the hadronic B reconstruction method.Comment: 7 pages, 8 postscript figures, submitted to Phys. Rev. Let
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