Sonia López-Esteban

Zirconia/stainless-steel continuous functionally graded material

Abstract A close to theoretical density zirconia/stainless steel continuous functionally graded material has been fabricated starting from coarse (≈20 μm) commercial metal powder by a wet processing method (pressure slip casting). The shape of the metal concentration profile has been characterized by image treatment. The dependence of the electrical properties of the material with the metal concentration presents a percolative behaviour with a metal-insulator transition, in addition to an increment of the capacity in the neighbourhood of a critical volume concentration, f c ≈0.285, approximately. Finally, the Vickers hardness vs metal concentration has been determined. Likewise, the results were compared with those obtained from composites with uniform metal concentration.

Sliding wear of ceramics and cermets against steel

Abstract The wear resistance of ceramics and ceramic/metal hybrid composites against steel was studied under dry sliding condition by the use of a pin-on-disc type wear test. The results were compared not only on the basis of the specific wear rates of the various ceramic based materials, but also on the basis of the total cumulative wear rates, which show accumulated wear losses of both sample material as a sliding body and its steel counterpart. From this point of view, it can be considered whether the tribo-materials are optimised with regard to the whole tribo-system or not. The specific wear rates of metal and ceramic/metal composites showed roughly 1.7–16 times higher values than the monolithic ceramics. But the total cumulative wear rate of the ceramic/metal composite, which contained larger sized metal particles, exhibited more than twice better total wear performance than the other systems. The mechanisms responsible for these behaviours were discussed by means of microscopical observations on the worn surfaces and the microstructures of the samples.

Silver-hydroxyapatite nanocomposites as bactericidal and fungicidal materials

Abstract Silver-hydroxyapatite nanocomposites containing 1 wt.% of metallic silver have been obtained by a colloidal chemical route and subsequent chemical reduction process. The silver nanoparticles are supported on the hydroxyapatite surface without a high degree of agglomeration. The bactericidal effect against common Gram-positive and Gram-negative bacteria, as well as antifungal activity against yeast have been investigated. The results indicated a high antimicrobial activity for E. coli, M. Luteus and I. Orientalis, so this material can be a promising antimicrobial biomaterial for implant and reconstructive surgery, among other applications.

Zirconia/nickel interfaces in micro- and nanocomposites

Abstract Experimental tests for zirconia/nickel composites determined lower toughness than expected. In this regard, the combination of ab initio density functional theory calculations and high-resolution transmission electron microscopy observations allowed to describe the nature of the zirconia/nickel interface and to justify the absence of any toughening mechanism by grain bridging of the metal particles. In the particular case of nanocomposites, enhanced values of hardness were found. Hardness increases have been justified by a novel model which takes into account the Hall — Petch effect and mean particle size according to the percolation theory. It has been found that this model is not exclusive to zirconia/nickel but can be extended to several other superhard composites.

Mechanical performance of a biocompatible biocide soda–lime glass-ceramic

A biocompatible soda-lime glass-ceramic in the SiO2-Na2O-Al2O3-CaO-B2O3 system containing combeite and nepheline as crystalline phases, has been obtained at 750°C by two different routes: (i) pressureless sintering and (ii) Spark Plasma Sintering. The SPS glass-ceramic showed a bending strength, Weibull modulus, and toughness similar values to the cortical human bone. This material had a fatigue limit slightly superior to cortical bone and at least two times higher than commercial dental glass-ceramics and dentine. The in vitro studies indicate that soda-lime glass-ceramic is fully biocompatible. The in vivo studies in beagle jaws showed that implanted SPS rods presented no inflammatory changes in soft tissues surrounding implants in any of the 10 different cases after four months implantation. The radiological analysis indicates no signs of osseointegration lack around implants. Moreover, the biocide activity of SPS glass-ceramic versus Escherichia coli, was found to be >4log indicating that it prevents implant infections. Because of this, the SPS new glass-ceramic is particularly promising for dental applications (inlay, crowns, etc).

Osteoblastic cell response to spark plasma-sintered zirconia/titanium cermets

Ceramic/metal composites, cermets, arise from the idea to combine the dissimilar properties in the pure materials. This work aims to study the biocompatibility of new micro-nanostructured 3 Y-TZP/Ti materials with 25, 50 and 75 vol.% Ti, which have been successfully obtained by spark slasma sintering technology, as well as to correlate their surface properties (roughness, wettability and chemical composition) with the osteoblastic cell response. All samples had isotropic and slightly waved microstructure, with sub-micrometric average roughness. Composites with 75 vol.% Ti had the highest surface hydrophilicity. Surface chemical composition of the cermets correlated well with the relative amounts used for their fabrication. A cell viability rate over 80% dismissed any cytotoxicity risk due to manufacturing. Cell adhesion and early differentiation were significantly enhanced on materials containing the nanostructured 3 Y-TZP phase. Proliferation and differentiation of SaOS-2 were significantly improved in their late-stage on the composite with 75 vol.% Ti that, from the osseointegration standpoint, is presented as an excellent biomaterial for bone replacement. Thus, spark plasma sintering is consolidated as a suitable technology for manufacturing nanostructured biomaterials with enhanced bioactivity.

Biocompatibility assessment of spark plasma-sintered alumina-titanium cermets

Alumina-titanium materials (cermets) of enhanced mechanical properties have been lately developed. In this work, physical properties such as electrical conductivity and the crystalline phases in the bulk material are evaluated. As these new cermets manufactured by spark plasma sintering may have potential application for hard tissue replacements, their biocompatibility needs to be evaluated. Thus, this research aims to study the cytocompatibility of a novel alumina-titanium (25 vol. % Ti) cermet compared to its pure counterpart, the spark plasma sintered alumina. The influence of the particular surface properties (chemical composition, roughness and wettability) on the pre-osteoblastic cell response is also analyzed. The material electrical resistance revealed that this cermet may be machined to any shape by electroerosion. The investigated specimens had a slightly undulated topography, with a roughness pattern that had similar morphology in all orientations (isotropic roughness) and a sub-micrometric average roughness. Differences in skewness that implied valley-like structures in the cermet and predominance of peaks in alumina were found. The cermet presented a higher surface hydrophilicity than alumina. Any cytotoxicity risk associated with the new materials or with the innovative manufacturing methodology was rejected. Proliferation and early-differentiation stages of osteoblasts were statistically improved on the composite. Thus, our results suggest that this new multifunctional cermet could improve current alumina-based biomedical devices for applications such as hip joint replacements.

Micro-Structural Characterization and Stress-Corrosion Cracking Behavior of a FGM Glass-Based Coating on Ti6Al4V for Biomedical Applications

Microstructural and stress-corrosion cracking characterization of two glass-based coatings on Ti6Al4V with different SiO2 content (61% and 64% of SiO2) have been investigated in this work. These coatings belong to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system and were obtained using a simple enamelling technique. They will be used as the first layer of the bioactive FGM coating. Microstructural characterization performed in the coatings by SEM shows the separation of the sintered glass particles after acid etching. The XRD integration method shows that the percentage of the crystalline phase (2.4CaO•0.6Na2O•P2O5) due to the partial devitrification is between 3-16 % vol. Mechanical characterization was made using Vickers and Hertzian indentation. Both coatings were sensitive to Vickers indentation subcritical crack growth with longer crack lengths for the smaller SiO2 content. This coating was also more sensitive to stress-corrosion “ring” cracking by Hertzian indentation. These two results are related with the larger residual stresses due to the thermal expansion mismatch.

Wetting of metals and glasses on Mo

Abstract The wetting of low melting point metals and Si – Ca – Al – Ti – O glasses on molybdenum has been investigated. The selected metals (Au, Cu, Ag) form a simple eutectic with Mo. Metal spreading occurs under nonreactive conditions without interdiffusion or ridge formation. The metals exhibit low (non-zero) contact angles on Mo but this requires temperatures higher than 1100 °C in reducing atmospheres in order to eliminate a layer of adsorbed impurities on the molybdenum surface. By controlling the oxygen activity in the furnace, glass spreading can take place under reactive or nonreactive conditions. We have found that in the glass/Mo system the contact angle does not decrease under reactive conditions. In all cases, adsorption from the liquid seems to accelerate the diffusivity on the free molybdenum surface.

Influence of Firing Time on Micro-Mechanical Properties of Glass Based Coating on Ti6Al4V for Biomedical Applications

In this work, the microstructure and micro-mechanical behaviour for a system composed by a glass coating of the SiO 2-CaO-MgO-Na 2O-K2O-P2O5 system on a titanium alloy (Ti6Al4V) have been studied in terms of firing time. The coating was produced using a simple enamelling technique with three different firing times (15, 20 and 30 seconds). The chemical analysis by Electron Dispersive Spectroscopy shows the formation of a titanium rich reaction layer, most probably composed of nanocrystals of Ti 5Si 3. Hertz indentation on the coating surface, Vickers microindentation at the interface and scratch test indentation show that the best mechanical response is obtained with a firing time of 30 seconds, which corresponds to the conditions for which the titanium-rich layer is thicker.