Juan J. de Damborenea

Sol-Gel Coatings on 316L Steel for Clinical Applications

SiO2 and SiO2-CaO-P2O5 coatings have been prepared by dipping electropolished stainless steel 316L samples and microscope glass slides in three different sol-gel solutions. Multilayered dense SiO2 coatings, and thick silica films obtained from equimolar contents of TEOS and MTES were used. The latter were able to strongly reduce both the corrosion attack on the steel and the iron diffusion to the sample surface. SiO2-CaO-P2O5 coatings were also obtained and applied onto the silica films, in order to provide a bioactive external surface for contact with living tissue. In-vitro evaluation of these coatings and films is discussed.

Surface modification of metals by high power lasers

Abstract Lasers are a powewful tool for the surface modification of metals in improving their corrosion and tribological properties. Due to their intrinsic characteristics, lasers can be focused onto metallic surfaces producing a broad range of treatments from heating to melting depending on the input energy. So, phase transformation on the surface of a base metal can be carried out to increase the surface hardness or cladding or alloying to obtain new materials. Other applications such as laser physical deposition (LPVD) or laser chemical deposition (LCVD) are also currently being carried out. In the present paper, a review of laser surface alloying and cladding on different base materials is given, focussing on the modification of the corrosion properties of metals and alloys for applications at low temperatures (electrochemical corrosion) and high temperatures (oxidation process).

In vitro assessment of Staphylococcus epidermidis and Staphylococcus aureus adhesion on TiO2 nanotubes on Ti–6Al–4V alloy

The aim of this study was to evaluate Staphylococcus sp. adhesion to modified surfaces of titanium alloy (Ti-6Al-4V). Specimens of Ti-6Al-4V alloy 6-4 ELI-grade 23 that meets the requirements of ASTM F136 2002A (AMS 2631B class A1) were anodized in a mixture of sulfuric/hydrofluoric acid at 20 V for 5 and 60 min to form nanoporous (NP) and nanotubular (NT) oxide layers with pore diameter of 20 and 100 nm, respectively. The amount of fluorine incorporated in the oxide films from the electrolyte was 6 and 4 wt %, respectively. Bacterial adherence was studied using laboratory strains and six clinical strains each of Staphylococcus aureus and Staphylococcus epidermidis. Lower adherence of laboratory strains was demonstrated on fluoride nanostructured surfaces in comparison with the fluoride-free surfaces. Significant differences between clinical strains and laboratory strains were also found (p < 0.0001, Kruskal-Wallis test) when NP and NT specimens were compared with chemically polished (CP) surfaces. The results of the tests using multiple clinical strains confirmed a decrease in bacterial adherence on F-containing titanium oxide surfaces, suggesting a potential applicability of this surface, with a confirmed added value of decreasing clinical staphylococci adherence, for medical prosthetic devices.

Doped TiO2 anodic layers of enhanced antibacterial properties

Ti-6Al-4V joint replacement implants foster uncemented fixation in orthopaedic surgery. However, bacterial colonization competes with host cells and ultimately may produce implant-related difficult-to-treat infections, justifying the efforts to obtain infection-resistant materials. In a previous work, the authors demonstrated the antibacterial properties of anodic fluoride-TiO2 nanostructured layers on Ti-6Al-4V alloy. In this work, the anodizing bath has been modified in order to grow fluoride-TiO2 barrier layers (FBL). A bacterial adherence protocol, run with reference and six different clinical strains of Staphylococcus aureus and Staphylococcus epidermidis, showed a statistically significant decrease in the percentage of covered surface (p<0.0001, Kruskal-Wallis test) for FBL specimens when compared with non fluoride-containing specimens, i.e. chemically polished Ti-6Al-4V and F-free TiO2 barrier layers. The results obtained on the F-barrier layers allowed discrimination between the effects of the presence of fluoride in the layer and the layer nanostructure on bacterial adhesion.

Influence of the nanostructure of F-doped TiO2 films on osteoblast growth and function

The aim of this study was to evaluate the proliferation and mineralization ability of mouse osteoblastic MC3T3-E1 cells on F-containing TiO2 films with different morphology and nanostructure that previously confirmed antibacterial properties. F-containing TiO2 films were fabricated by anodizing Ti-6Al-4V alloy ELI -grade 23. By using a mixture of H2SO4/HF acid at 20 V for 5 and 60 min, a TiO2 film grows with nanoporous (NP) and nanotubular (NT) features, characterized with a pore diameter of 20 and 100 nm, respectively. Fluoride-TiO2 barrier films (FBL) were produced in 1M NH4H2PO4/0.15M NH4F solution at constant voltage controlled at 20 V for 120 min. The amount of F incorporated in the nanostructured oxide films was 6 at % and of 4 at %, for the NP and NT, respectively, while for the FBL film was 12 at %. MC3T3-E1 cells exhibited different behavior when seeded and grown onto these surfaces. Thus, F-doped TiO2 films with NP structures increased proliferation as well as osteogenic gene expression and the mineralization capacity of these osteoblastic cells. These results confirm that anodizing process is suitable to fabricate multifunctional surfaces on Ti-6Al-4V alloy with improved not only antibacterial but also osteogenic properties useful for bone fixation of prosthetic devices

The role of mechanically activated area on tribocorrosion of CoCrMo

Co-Cr-Mo alloys are among the most used alloys for orthopedic implants because of their excellent corrosion resistance, mechanical properties, and biocompatibility. Although there is extensive literature on corrosion properties of Co-Cr-Mo alloys, fewer articles are focused on the synergistic effect of corrosion and wear in a simulated physiological solution. It is generally assumed that the current density measured during wear conditions for passive materials comes from the active area. However, there are no clear data supporting this statement. The current article correlates electrochemical measurements with the active area generated during sliding wear tests. Open circuit potential and current measurements, potentiodynamic scans, and electrochemical impedance spectroscopy were carried out on samples under static and sliding wear conditions. These measurements showed the importance of the active area, where the current coming from the surface not being abraded is negligible. Finally, by combining the sliding wear and electrochemical tests, the synergistic effect of wear and corrosion was characterized for this alloy, documenting the metal carbides detachment from the cobalt alloy matrix, which leads to a significant increase of total wear volume.

High-temperature oxidation behavior of laser-surface-alloyed Incoloy-800H with Al

The high-temperature oxidation resistance of laser-surface-alloyed Incoloy 800H with Al has been investigated. Several samples have been exposed to 1000°C in air and subsequently analyzed by optical microscopy, SEM, and electron microprobe X-ray fluorescence spectroscopy in order to determine their microstructure and composition. The results show that the laser-treated materials have a considerably higher oxidation resistance than the nontreated materials due to the formation of an Al-rich layer close to the surface, which acts as a barrier against oxygen diffusion into the bulk.

Influence of the base preheating on cracking of the laser-cladded coatings

Coatings of stellite SF6 are prepared on the chromium steel base by means of a direct laser cladding. A 1.2 kW CO2 laser is applied and the original material is delivered into the processing zone coaxially with the laser beam. The samples are produced with and without a controlled preheating of the substrate and are investigated by means of metallographic techniques. It is found that the observed micro-cracking susceptibility decreases markedly with increase of the base preheating temperature up to 750 K and the crack-free coatings are produced for preheating around 950 K. The substrate-coating interface reveals a metallic bond and the microstructure is characterized by a fine-grained, dendritic structure. The nearly constant concentration dependence of Fe, Co, Cr and Ni on the distance from interface indicates on homogeneous chemical composition of the produced coatings.

Effects of Laser Surface Modification of Nimonic with Aluminum on Oxidation Behavior

Using a high-power CO2 laser, aluminum-surface alloying was carried out on a nickel-base superalloy (Nimonic 80A) with the aim of improving its oxidation resistance. After the treatment, scanning-electron microscopy (SEM) studies show the alloy area to have a two-phase structure of Ni solid solution and Ni3Al intermetallic. These layers were subsequently subjected to a laser-remelting treatment with different beam-scanning speeds in order to homogenize their structure. Metallographic studies indicate the formation of a single dendritic phase rather than the two-phase structure present in the unmelted alloyed tracks and a decrease in the aluminum content throughout the laser track. To establish their oxidation behavior at high temperatures, the alloyed layers and remelted alloyed layers were oxidized at 1273 K for varying times, between 24 and 300 hr, comparing their behavior with that of untreated specimens. The results indicate the formation of a protective alumina layer on the alloyed specimens. The oxidation behavior differs, depending on the scanning rate (of the laser beam over the specimen surface) during remelting. Oxidation of the remelted specimens at the maximum rates studied (500 mm/min) leads to the formation of protective oxides on the superalloy. However, when remelting takes place at lower rates (100 and 300 mm/min), the amount of aluminum present is insufficient to develop a continuous protective-oxide layer.

Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels

In this work correlation between the 1000 MPa dual phase (DP) steel microstructure and the strain gained after roll-forming process have been studied by both microstructural and micromechanical analysis. The scanning electron microscope (SEM) inspection in the bent area reveals changes in the ferrite-martensitic microstructure. The plastic deformation of DP steels originates defects at the edges of bent sheet make them partly responsible for the damage caused. In addition, electron backscatter diffraction (EBSD) measurements have been carried out for an in-depth characterization after roll-forming. A high density of misorientation of the crystal lattice within the ferrite strained grains is observed, mainly concentrated in the ferrite/martensite grain boundaries. Furthermore, the ultramicrohardness tests exhibit little dependence between mechanical parameters and the material properties.