Stéphane Valette

Preparation of boron nitride-based coatings on metallic substrates via infrared irradiation of dip-coated polyborazylene

BN-based coatings on metallic (titanium, aluminium and copper) substrates are prepared by infrared irradiation of a dip-coated polyborazylene. This innovative strategy creates an access towards the molecular design of hexagonal-boron nitride coatings as protective materials for low melting point metals.

The effects of femtosecond laser-textured Ti-6Al-4V on wettability and cell response

To study the biological activity effects of femtosecond laser-induced structures on cell behavior, TA6V samples were micro-textured with focused femtosecond laser pulses generating grooves of various dimensions on the micrometer scale (width: 25-75μm; depth: 1-10μm). LIPSS (Laser Induced Periodic Surface Structures) were also generated during the laser irradiation, providing a supplementary structure (sinusoidal form) of hundreds of nanometers at the bottom of the grooves oriented perpendicular (⊥ LIPPS) or parallel (// LIPPS) to the direction of these grooves. C3H10 T1/2 murine mesenchymal stem cells were cultivated on the textured biomaterials. To have a preliminary idea of the spreading of biological media on the substrate, prior to cell culture, contact angle measurement were performed. This showed that the post-irradiation hydrophilicity of the samples can decrease with time according to its storage environment. The multiscale structuration either induced a collaborative or a competitive influence of the LIPSS and grooves on the cells. It has been shown that cells individually and collectively were most sensitive to microscale grooves which were narrower than 25μm and deeper than 5μm with ⊥ LIPPS. In some cases, cells were individually sensitive to the LIPSS but the cell layer organization did not exhibit significant differences in comparison to a non-textured surface. These results showed that cells are more sensitive to the nanoscale structures (LIPSS), unless the microstructuress size is close to the cell size and deeper than 5μm. There, the cells are sensitive to the microscale structures and go on spreading following these structures.

Femtosecond laser ablation of polypropylene: A statistical approach of morphological data.

We have investigated femtosecond (fs) laser (130u2009fs, 800u2009nm, 5u2009kHz) ablation of polypropylene (PP). The following laser process conditions were varied: power density and number of pulses. The morphological parameters response (depth, ablation diameter, ablation volume) to the laser process conditions, measured by an optical profiler, was investigated by the statistical analysis technique to determine the relationship between them. For this, the simple linear regression and the multiple linear regressions are compared. The simple linear regression shows that the ablation volume follows a linear relationship with the product of the power and the number of pulse.

Influence of the polypropylene structure on the replication of nanostructures by injection molding

In this paper, an experimental study of replication by injection molding of sub-micrometer features is presented. Two polypropylenes with different melt flow rates (MFR) were used throughout this study. The used removable mold insert was textured with a femtosecond laser. Replication of these periodic structures, called ripples, is thus investigated. Despite different MFR, we show that the viscosities of the two polymers at the investigated temperatures and injection shear rates are similar. The reproducibility of the injected molded samples and the texture of the mold were analyzed. We propose a specific methodology to quantify the reproducibility quality replicas. The latter introduces morphological parameters such as anisotropy rate, power density, etc. A difference between the two replicas was noticeable. Based on rheological analysis, the viscosity was ruled out as the origin of this difference. Other properties were identified as the source such as the chain length and the stress relaxation time. Their impact on the replication quality was investigated and found interesting.

The effect of the orientation and the height of periodic sub-micrometric texturing on dropwise condensation

Controlling condensation conditions by surface topography is of prime interest. The aim of this work is to investigate the behavior of water droplets condensing on oriented sub-micrometric structures representing ripples with wavelengths around 800u202fnm. Droplet behavior was studied on different ripples heights and on untextured surfaces. It was specifically looked at how the presence of ripples creates geometrical confinement, and how that influences the deformation and the orientation of single droplets. Results show that the condensed droplets follow the orientation of textured features, especially with high structures height (150u202fnm). This is shown by the decreasing of droplet roundness with ripples height. The relative number of circular droplets (roundness near to 1) is around 0.6 for 70u202fnm high ripples and decrease to around 0.2 for 150u202fnm high ripples. The corresponding relative number on untextured surface is around 0.5. A mechanism, based on droplets pinning and hysteresis, is proposed to explain the influence of the ripples orientation in a vertical plane, onto the droplet deformation during coalescence step. Finally, the presence of ripples is shown to barely impact breath figure dynamics. Number of droplets and mean droplet radius for the textured and untextured surfaces present a comparable evolution.

Fem Calculations and Experimental Determination of Residual Stresses in Alumina/Nickel Alloy Joints. Optimization of Fabrication Parameters

The study relates to joints fabricated by solid state bonding between alumina and nickel alloy HAYNESTM214®, using an intermediate nickel metallic foil. Experimentally, damages and cracks often are observed close to the metal/ceramics interface. Consequently, the residual stresses distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and indentation techniques and predicted by Finite Element Analysis (FEA) calculations using an elastic-plastic-creep model. We demonstrate that a good correlation between FEA calculations and experimental results is obtained. Then, the effect of elaboration and geometrical parameters has been studied in order to minimize the residual stresses in alumina close to the metal-ceramics interface. However, the Al2O3/Ni/HAYNESTM214® system always leads to high residual stresses. To solve this problem, we show that the use of a multi-layer Cu/Ni/Cu joint, associated with the Direct Copper Bonding method (DCB), by pre-oxidation of copper, allows reducing significantly the tensile residual stresses in ceramics.

Modeling thermal effects produced by nanosecond and femtosecond laser pulses applied to metals

The widths of the Heat Affected Zone (HAZ) produced by nanosecond and femtosecond laser pulses applied to metals, measured experimentally in recent experiments, are quantified by numerical simulations. The calculations use the finite element technique in the case of a two-temperature model. In both experiments and simulations, the HAZ width is defined by physical metallurgy considerations. This allows to describe the lateral thermal effects induced by laser-metal interaction, and thus to explain the large difference in microdrilling sharpness between nanosecond and femtosecond pulse duration regimes.The numerical simulations give a HAZ width of 1.71u2005µm for Cu, 290u2005nm for Ni, and 220u2005nm for Al after femtosecond pulses. In the nanosecond case, the HAZ width becomes 8.34µm for Cu, 1.64µm for Ni, and 3.15µm for Al. This outlines the difference between the HAZ produced by these two pulse duration regimes (a 6-times to 23-times factor is obtained).Experimental results obtained on Al samples are finally compared with the numerical results. A reasonable agreement is obtained. This validates the use of a two-temperature model to represent the laser-metal interaction in ultra-short pulse duration regimes.The widths of the Heat Affected Zone (HAZ) produced by nanosecond and femtosecond laser pulses applied to metals, measured experimentally in recent experiments, are quantified by numerical simulations. The calculations use the finite element technique in the case of a two-temperature model. In both experiments and simulations, the HAZ width is defined by physical metallurgy considerations. This allows to describe the lateral thermal effects induced by laser-metal interaction, and thus to explain the large difference in microdrilling sharpness between nanosecond and femtosecond pulse duration regimes.The numerical simulations give a HAZ width of 1.71u2005µm for Cu, 290u2005nm for Ni, and 220u2005nm for Al after femtosecond pulses. In the nanosecond case, the HAZ width becomes 8.34µm for Cu, 1.64µm for Ni, and 3.15µm for Al. This outlines the difference between the HAZ produced by these two pulse duration regimes (a 6-times to 23-times factor is obtained).Experimental results obtained on Al samples are finally compared...