Mario Roberto Rosenberger

Characterization of TiO2 Nanofilms Obtained by Sol-gel and Anodic Oxidation

The influence of sol-gel dip-coating and anodic oxidation process parameters in producing thin TiO2 films is studied. As the size of the films is in the order of nanometres (20–140 nm), to obtain a precise measurement of their thickness and analyse their crystalline structures, glancing incidence angle X-ray techniques (X-ray reflectometry and X-ray diffraction) using synchrotron radiation are used. A relationship between the colour and thickness of the films was found. This enables the film thickness to be estimated by the film colour. Within the range of the parameters studied, both techniques produce thin films with smooth surfaces which at most reproduce the roughness of the polished substrate. Independently of the technique, thermally-treated films thicker than 30 nm presented different crystalline structures with anatase and rutile phases.

Fabrication of TiO2 Crystalline Coatings by Combining Ti-6Al-4V Anodic Oxidation and Heat Treatments

The bio- and hemocompatibility of titanium alloys are due to the formation of a TiO2 layer. This natural oxide may have fissures which are detrimental to its properties. Anodic oxidation is used to obtain thicker films. By means of this technique, at low voltages oxidation, amorphous and low roughness coatings are obtained, while, above a certain voltage, crystalline and porous coatings are obtained. According to the literature, the crystalline phases of TiO2, anatase, and rutile would present greater biocompatibility than the amorphous phase. On the other hand, for hemocompatible applications, smooth and homogeneous surfaces are required. One way to obtain crystalline and homogeneous coatings is by heat treatments after anodic oxidation. The aim of this study is to evaluate the influence of heat treatments on the thickness, morphology, and crystalline structure of the TiO2 anodic coatings. The characterization was performed by optical and scanning electron microscopy, X-ray diffraction, and X-ray reflectometry. Coatings with different colors of interference were obtained. There were no significant changes in the surface morphology and roughness after heat treatment of 500°C. Heat treated coatings have different proportions of the crystalline phases, depending on the voltage of anodic oxidation and the temperature of the heat treatment.

Modeling the interaction of convex solidifying interfaces with spherical particles

The phenomenon of pushing during solidification is modeled for the case of particles producing a convex interface. The thermal and fluid fields generated by the particle–melt–solid system are calculated in a decoupled way determining in the first place the shape of the interface and then, the two main forces acting during pushing; the drag and repulsion forces. The thermal and fluid flow fields were calculated using finite element methods. Both, the drag and repulsion forces are integrated at each step and compared until both are equal and the steady state of pushing is reached. The repulsion force is integrated using the Casimir–Lifshitz–Van der Waals interaction. The model predicts the equilibrium distance in a steady state of pushing for spherical particles and a convex solidifying interface. It is shown that the equilibrium separation distance for a convex interface results in a larger solidification velocity for trapping with respect to an ideal planar interface. The model results were in good agreement with experimental results for the critical velocity reported in the literature.

Wear Resistance of Anodic Titanium Dioxide Films Produced on Ti-6Al-4V Alloy

Ti-6Al-4V alloy with TiO2 coating is the most commonly selected material to construct an aortic heart valve. Wear resistance is the main mechanical property to be evaluated for this purpose. In this paper, the wear resistance of TiO2 thin films obtained by anodic oxidation of Ti-6Al-4V is evaluated. Anodic oxidation was performed at 20 V to 70 V with a H2SO4 1 M electrolyte. The samples were thermally treated at 500°C for 1 h, and crystalline phases of TiO2 were obtained. The wear was performed in a ball-on-flat reciprocating machine with a range of loads from 1 gf to 4 gf and times between 60 s and 1200 s, using a diamond sphere as counterface. The counterface oscillates at 0.5 Hz and 4 mm in amplitude. The wear is measured using a profilometer and is calculated as the worn volume. The wear resistance of the coated samples is larger than the substrate, and increases with thickness and with crystalline coating.

Construction and calibration of a goniometer to measure contact angles and calculate the surface free energy in solids with uncertainty analysis

Abstract Here, we present the construction and calibration of a low-cost goniometer to measure contact angles by the sessile drop method. Besides, we propose a simple and fast method to calculate the uncertainty in the determination of the surface free energy (SFE) and its polar and dispersive components through the Owens-Wendt model and tested it by using two testing liquids. The goniometer performance and the SFE uncertainty were determined on two polymers: polytetrafluorethylene (PTFE) and polyoxymethylene (POM), by using water and methylene iodide. The values of contact angle measured were used to calculate the SFE and its components with their errors. The SFE values obtained for PTFE were 17.57–17.91 mJ/m2, with a relative error lower than 5.5%, whereas those for POM were 42.80–43.23 mJ/m2, with a relative error lower than 4.3%. Both the SFE values and the errors were in the range of those previously reported. Based on the mathematical analysis of the uncertainty propagation in the determination of SFE, we concluded that the uncertainty is minimized when the testing liquids are an apolar liquid and water.

Stability analysis of the solution of the one-dimensional Richards equation by the finite difference method

The solution by the Finite Difference Method of the Richards equation written as a function of the degree of saturation of the domain and the matrix potential is obtained and the convergence of the solutions is analyzed. The necessary time and spatial sizes for convergence are obtained and established.

Adhesion of Anodic Titanium Dioxide Coatings on Titanium Grades 5 Alloys

In the present research the adhesion of thin anodic coatings of TiO2 (40 nm to 140 nm) is evaluated. Coatings were produced by anodic oxidation of Ti-6Al-4V, using H2SO4 as electrolyte at different voltages (20 V to 70 V), employing heat treatments of 1 h at 500 ℃ and 600 ℃.

Analysis of Thermal and Structural Parameters and Microhardess Variations in Different Al‐Cu Alloys Directionally Solidified

The columnar — to — equiaxed transition (CET) was investigated in Al-Cu alloys (Al-lwt%Cu, Al-4.5wt%Cu, Al-15wt%Cu and Al-33.2wt%Cu) solidified directionally from a chill face in a vertical setup. The CET occurs when the temperature gradient in the melt ahead of the columnar front reach critical values. Also, we investigate correlations between structural parameters (grain size) with Vickers microhardness measurements in the directionally solidified samples. We observed that the Vickers microhardness is greater in the equiaxed zone than in the columnar or columnar to equiaxed transition (CET) zone, additionally, is greater on the edges of the samples than in the centre. Also, we determined that the grain size increases from the columnar to equiaxed structure.

Microhardness, Corrosion Behaviour and Microstructures of Directionally Solidified Al-Cu Alloys

The aim of this work is to analyze the effect of micro structural parameters (secondary dendritic arm spacings) on the microhardness and corrosion behavior of hypoeutectic Al-Cu alloys. Experimental results include HV microhardness values, corrosion and pitting potential and current density. It was found that high cooling rates during solidification provide finer dendritic spacings, which encourage better mechanical properties. The Vickers microhardness increases as the contents of copper in the alloy increases. The most influencing microstructural variable for corrosion resistance has been found to be the secondary dendrite arm spacing. It has been found that that corrosion resistance decreases with the increasing in secondary spacing until 10wt.% of Cu.