Gelson B. de Souza

Nanomechanical properties of rough surfaces

The nanoindentation technique allows the determination of mechanical properties at nanometric scale. Hardness (H) and elastic modulus (E) profiles are usually determined by using the Oliver-Pharr method from the load/unload curves. This approach is valid only for flat surfaces, or at least, when a very low degree of asperity is present (lower than 30 nm). The basic statement is the determination of the zero tip-surface contact point. If a rough surface is present, errors can occur in determining this contact point and, as a consequence, the surface hardness and elastic modulus profiles are drastically altered resulting in under evaluated values. Surfaces with different roughness were produced by controlled nitrogen glow discharge process on titanium. The changed nitriding parameters were different N2/H2 atmospheres and temperatures (600 °C-900 °C). The most correct H and E profiles were obtained by using the contact stiffness analysis method, proposed here, that overcomes the surface roughness. The obtained results were compared with available literature data.

Nanomechanical and nanotribological properties of bioactive titanium surfaces prepared by alkali treatment.

Alkali-heat treatment (AHT) is a simple and practical method to make titanium surfaces bioactive. Hydroxyapatite nucleates on Ti when in contact with body fluids due to the presence of a thin sodium titanate film produced by the AHT. This method was proposed more than a decade ago and it has been widely investigated at varied scopes. However, there is still little information about the mechanical properties of this film. In this work, the tribo-mechanical behavior of films produced by alkali treatment (AT) and AHT on Ti is investigated using instrumented indentation technique. The films were also characterized by TF-XRD, SEM, EDS and in vitro bioactivity tests. Analytical methods were employed to obtain the mechanical properties of the film from instrumented indentation data. The heat treatment subsequent to the alkaline processing increased the film elastic modulus from 1.7 GPa to 2.8 GPa, the hardness from 12 MPa to 20 MPa and the critical load for scratch test from 1.5 mN to 5.5 mN. Despite the overall improvement in the film bioactivity and tribo-mechanical behavior, the AHT elastic modulus is only 2% of the pristine Ti whereas hardness is less than 1%. This information must be considered for implant design purposes.

Effects of Ag addition on some physical properties of granular YBa2Cu3O7-δ superconductor

The effects of Ag addition on some physical properties of YBa2Cu3O7-δ superconductor has been studied. The samples were produced by different routes and characterized by scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, X-ray diffraction and nanoindentation. The superconducting properties were studied by dc magnetization and electrical resistivity measurements. The X-ray patterns revealed that all samples were polycrystalline and corresponded to the orthorhombic YBa2Cu3O7-δ phase. However, for some samples, silver appears to be present within the grains. The superconducting properties were not significantly influenced by Ag addition. Hardness and elastic modulus were also obtained by instrumented indentation. Ag addition was found to be effective in improving the mechanical properties, probably by filling the pores in the grain boundaries. Hardness profiles indicated values between 3.1 and 3.4 GPa at deep tip penetration depths. The highest elastic modulus of 146 GPa was attained for samples prepared using Y2O3, BaCO3, CuO and Ag2O as precursors, while the lowest modulus (125 GPa) was obtained for the pure YBa2Cu3O7-δ sample.

Mechanical properties of anodic titanium films containing ions of Ca and P submitted to heat and hydrothermal treatment

Anodic oxidation is a technique widely used to improve the bioactivity of Ti surface. In this study, micro-arc oxidation (MAO) was used to obtain an anodic film incorporating Ca and P ions to evaluate the effect of heat and hydrothermal treatment on the mechanical and in vitro bioactivity properties of these new layers. The MAO process was carried out using (CH3COO)2Ca·H2O and NaH2PO4·2H2O electrolytes under galvanostatic mode (150mA/cm(2)). The thermal treatments were made at 400°C and 600°C in air atmosphere while hydrothermal treatment was made in an alkaline water solution at 130°C. These surfaces presented desired mechanical properties for biomedical applications owing to the rutile and anatase phases in the anodic film that are more crystalline after thermal treatments; which provided an increase in hardness values and lower elastic modulus. The dry sliding wear resistance increased by performing thermal treatments on the surfaces with one condition still maintaining the film after the test. Bioactivity was investigated by immersion in simulated body fluid during 21 days and hydroxyapatite was formed on all samples. Finally, lower values of contact angle were obtained for heat treated samples.