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Bismuth-based layered compounds have been considered during the last years as interesting materials for high temperature piezoelectric applications, due to their stability and wide thermal range of the ferroelectric activity. The high electrical conductivity present at high temperatures has been a disadvantage for the potential applications of these compounds. On Bi4Ti3O12 (BIT), different doping, such as Nb5+ and W6+ were used to increase resistivity. It has been shown that Nb5+ doping modifies piezoelectric response, passing from relatively high hysteretic to linear and almost completely non-hysteretic. In the present work, the effect of Nb5+ doping related to the conduction response is investigated. Studies of the I-V curves as a function of the temperature allows us, for the first time, to measure the conductivity of these compounds at room temperature (RT) and to discuss which are the microstructural elements that control the conductivity of the material using a serial electrical model. It is shown that at temperatures between RT and 125ºC the grain boundary conductivity limits the total conductivity. At higher temperatures, the material conducts mainly through the bulk of grains. Conduction type, conductivity thermal regimes and chemical composition may explain the change in the piezoelectric behaviour.Compuestos laminares de bismuto han sido considerados en los últimos años materiales interesantes como piezoeléctricos en aplicaciones de alta temperatura, debido a su estabilidad y amplio rango térmico de existencia de actividad ferroeléctrica. Sin embargo, la elevada conductividad presente a altas temperaturas ha sido una desventaja para las potenciales aplicaciones de estos compuestos. En el caso de Bi4Ti3O12 (BIT), diferentes dopantes como Nb5+ o W6+ han sido empleados para aumentar la resistencia eléctrica del material. Es conocido que el dopado con Nb5+ modifica la respuesta piezoeléctrica del material pasando de presentar una elevada histéresis a prácticamente lineal y no histerética. En este trabajo se estudia el efecto de este tipo de dopado sobre la respuesta de la conductividad. Estudios eléctricos mediante la medida de curvas I-V en función de la temperatura nos permiten, por primera vez en la literatura, medir la conductividad de estos compuestos a temperaturas entorno a la ambiente (TA) y discutir que elemento mesoestructural controla la conductividad total del material, usando un model eléctrico en serie. Como resultado, se ha visto que a temperaturas entre TA y 125ºC es la conductividad del borde de grano la que limita la conductividad de todo el compuesto. A temperaturas superiores, el material conduce principalmente a través del interior de grano. Combinaremos los resultados de los estudios sobre tipo de conducción, regímenes térmicos y la composición química para interpretar los cambios en la respuesta general introducidos por el dopado.

Coatings from diamond-like carbon (DLC) have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the total joint replacement field, layer performance, stability and adhesion in realistic physiological setups are quintessential and these aspects have not been consistently researched. In our team's efforts to develop long-term stable DLC implant coatings, test results gained from a simplified linear spinal simulator setup are presented. It is shown that metal-on-metal (MoM) pairs perform well up to 7 million loading cycles, after which they start to generate wear volumes in excess of 20 times those of DLC-coated implants. This is attributed to the roughening observed on unprotected metal surfaces. Furthermore, we illustrate that in contrast to DLC-on-DLC, MoM tribopairs require protein-containing media to establish low-friction conditions. Finally, results of defect monitoring during testing are presented, showing catastrophic failure of layers whose interfaces are too weak with respect to the stress-corrosion-cracking mechanism encountered in vivo., (Copyright 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

A quantitative method using Rockwell C indentation was developed to study the adhesion of diamond-like carbon (DLC) protective coatings to the CoCrMo biomedical implant alloy when immersed in phosphate-buffered saline (PBS) solution at 37 degrees C. Two kinds of coatings with thicknesses ranging from 0.5 up to 16 microns were investigated, namely DLC and DLC/Si-DLC, where Si-DLC denotes a 90 nm thick DLC interlayer containing Si. The time-dependent delamination of the coating around the indentation was quantified by means of optical investigations of the advancing crack front and calculations of the induced stress using the finite element method (FEM). The cause of delamination for both types of coatings was revealed to be stress-corrosion cracking (SCC) of the interface material. For the DLC coating a typical SCC behavior was observed, including a threshold region (60J m(-2)) and a "stage 1" crack propagation with a crack-growth exponent of 3.0, comparable to that found for ductile metals. The DLC/Si-DLC coating exhibits an SCC process with a crack-growth exponent of 3.3 and a threshold region at 470 Jm(-2), indicating an adhesion in PBS at 37 degrees C that is about eight times better than that of the DLC coating. The SCC curves were fitted to the reaction controlled model typically used to explain the crack propagation in bulk soda lime glass. As this model falls short of accurately describing all the SCC curves, limitations of its application to the interface between a brittle coating and a ductile substrate are discussed.