A. Caron

Synthesis and properties of hydroxyapatite-containing porous titania coating on ultrafine-grained titanium by micro-arc oxidation

Equal channel angular pressing results in ultrafine-grained (approximately 200-500 nm) Ti with superior mechanical properties without harmful alloying elements, which benefits medical implants. To further improve the bioactivity of Ti surfaces, Ca/P-containing porous titania coatings were prepared on ultrafine-grained and coarse-grained Ti by micro-arc oxidation (MAO). The phase identification, composition, morphology and microstructure of the coatings and the thermal stability of ultrafine-grained Ti during MAO were investigated subsequently. The amounts of Ca, P and the Ca/P ratio of the coatings formed on ultrafine-grained Ti were greater than those on coarse-grained Ti. Nanocrystalline hydroxyapatite and alpha-Ca(3)(PO(4))(2) phases appeared in the MAO coating formed on ultrafine-grained Ti for 20 min (E20). Incubated in a simulated body fluid, bone-like apatite was completely formed on the surface of E20 after 2 days, thus evidencing preferable bioactivity. Compared with initial ultrafine-grained Ti, the microhardness of the E20 substrate was reduced by 8% to 2.9 GPa, which is considerably more than that of coarse-grained Ti (approximately 1.5 GPa).

Atomic Scale Mechanisms of Friction Reduction and Wear Protection by Graphene

We study nanoindentation and scratching of graphene-covered Pt(111) surfaces in computer simulations and experiments. We find elastic response at low load, plastic deformation of Pt below the graphene at intermediate load, and eventual rupture of the graphene at high load. Friction remains low in the first two regimes, but jumps to values also found for bare Pt(111) surfaces upon graphene rupture. While graphene substantially enhances the load carrying capacity of the Pt substrate, the substrates intrinsic hardness and friction are recovered upon graphene rupture.

Young's modulus, fracture strength, and Poisson's ratio of nanocrystalline diamond films

Youngs modulus, fracture stress, and Poissons ratio are important mechanical characteristics for micromechanical devices. The Poissons ratio of a material is a good measure to elucidate its mechanical behavior and generally is the negative ratio of transverse to axial strain. A nanocrystalline (NCD) and an ultrananocrystalline (UNCD) diamond sample with grain boundaries of different chemical and structural constitutions have been investigated by an ultrasonic resonance method. For both samples, the elastic moduli are considerably reduced, compared with the elastic modulus of single crystal diamond (sc-diamond). Depending on the chemical and structural constitution of grain boundaries in nano- and ultrananocrystalline diamond different values for Poissons ratio and for the fracture strength are observed. We found a Poissons ratio of 0.201 ± 0.041 for the ultrananocrystalline sample and 0.034 ± 0.017 for the nanocrystalline sample. We discuss these results on the basis of a model for granular media. Higher disorder in the grain boundary leads to lower shear stiffness between the single grains and ultimately results in a decrease of Youngs and shear modulus and possibly of the fracture strength of the material.

Structure vs Chemistry: Friction and Wear of Pt-Based Metallic Surfaces

In comparison of a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with a Pt(111) single crystal we find that wearless friction is determined by chemistry through bond formation alloying, while wear is determined by structure through plasticity mechanisms. In the wearless regime, friction is affected by the chemical composition of the counter body and involves the formation of a liquid-like neck and interfacial alloying. The wear behavior of Pt-based metallic surfaces is determined by their structural properties and corresponding mechanisms for plastic deformation. In the case of Pt(111) wear occurs by dislocation-mediated homogeneous plastic deformation. In contrast the wear of Pt57.5Cu14.7Ni5.3P22.5 metallic glass occurs through localized plastic deformation in shear bands that merge together in a single shear zone above a critical load and corresponds to the shear softening of metallic glasses. These results open a new route in the control of friction and wear of metals and are relevant for the development of self-lubricated and wear-resistant mechanical devices.

Lower nanometer-scale size limit for the deformation of a metallic glass by shear transformations revealed by quantitative AFM indentation

Summary We combine non-contact atomic force microscopy (AFM) imaging and AFM indentation in ultra-high vacuum to quantitatively and reproducibly determine the hardness and deformation mechanisms of Pt(111) and a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with unprecedented spatial resolution. Our results on plastic deformation mechanisms of crystalline Pt(111) are consistent with the discrete mechanisms established for larger scales: Plasticity is mediated by dislocation gliding and no rate dependence is observed. For the metallic glass we have discovered that plastic deformation at the nanometer scale is not discrete but continuous and localized around the indenter, and does not exhibit rate dependence. This contrasts with the observation of serrated, rate-dependent flow of metallic glasses at larger scales. Our results reveal a lower size limit for metallic glasses below which shear transformation mechanisms are not activated by indentation. In the case of metallic glass, we conclude that the energy stored in the stressed volume during nanometer-scale indentation is insufficient to account for the interfacial energy of a shear band in the glassy matrix.

On the glass transition temperature and the elastic properties in Zr-based bulk metallic glasses

The temperature dependence of the elastic moduli was estimated from ultrasound time of flight measurements performed on bulk metallic glasses of composition Zr63− x Cu24Al x Ni10Co3. Using the corresponding values at the glass transition temperature, the local atomic strain was determined. The obtained values for the critical atomic strain calculated for 8 at% < x < 15 at% are close to the predicted universal criterion derived from a topological model, but may also reflect the difference in the chemical interaction that are not accounted by a topological approach.