A. Durán

Structural considerations about SiO2 glasses prepared by sol-gel

Abstract Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. In the present work the transformation with temperature of these gels has been studied using IR spectroscopy together with measurements of density and specific surface area, thermal analysis and SEM. The IR spectra of these gels, measured as a function of temperature and H2O/TEOS ratio, confirm that the relative concentration of OH groups is temperature dependent. The total OH content, also depends on the H2O/TEOS ratio in the gel. The combined results of IR, density and specific surface area measurements allow some structural considerations to be advanced about the final SiO2 glasses obtained.

Protective glass coatings on metallic substrates

Abstract Sol-gel glass coatings of SiO 2 were prepared on different metallic substrates (stainless steels AISI 304, 310 and 316) using the dip-coating technique. The characteristics of the solutions and the films produced were evaluated for each type of substrate and certain properties measured. Coatings were given standard tests relating to chemical and mechanical resistance: dry and acid corrosion and microhardness. Sol-gel SiO 2 coatings act as a protective barrier against oxidation and acid corrosion and enhances the scratch resistance of these substrates.

Oxidation Protection Coatings for C/SiC based on Yttrium Silicate

Abstract The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young’s modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.

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.

Nanocrystallisation in oxyfluoride systems: mechanisms of crystallisation and photonic properties

Abstract Rare earth (RE) doped oxyfluoride glass ceramics possess interesting optical properties with applications in telecommunications and optoelectronics, such as solid state lasers, optical amplifiers, etc. These materials combine the transparency and mechanical and chemical resistance of aluminosilicate glasses with the low phonon energy and facile incorporation of RE ions in the fluoride crystals. The incorporation of RE ions in the crystalline phases enhances the laser emission intensity, a major property of these materials. Transparency is achieved when crystal size is in the nanometric scale, usually below 40 nm, which avoids light scattering. A strict control of the nucleation and crystal growth processes is therefore necessary which requires a deep knowledge of the crystallisation mechanisms. The great activity and publications in this field in the last decades merit a review providing a comparative study of the different nanoglass ceramic systems, their structural and optical characterisation and their main properties and applications. This is the objective of this review paper which includes 227 references. A general discussion on glass nucleation and crystallisation theories and more relevant crystallisation parameters and characterisation techniques are put forward in the first section of the review, focused on nanocrystallisation processes in oxyfluoride systems. In the second section, the principal RE doped glass ceramics are presented. After a general introduction about the luminescence processes, including up- and down-conversion, the behaviour of RE elements in glasses and crystals are discussed. Glass ceramic compositions have been divided as follows: glass ceramics with a glass composition following Wang and Ohwaki’s oxyfluoride glass ceramic, and glass ceramics with different matrix compositions, arranged by crystalline phases. Relevant properties, mainly optical and laser, are described in each system along with the most relevant applications of these materials.

Optical properties of glass coatings containing Fe and Co

Abstract Glass films of SiO 2 M x O y (M = Fe, Co, FeCo) have been prepared on soda-lime-silica flat glass by sol-gel employing the dip-coating technique. Different parameters affecting solution preparation and application and coating processing (H 2 O/TEOS and R-OH/TEOS ratios, type of catalyst, time and temperature of thermal treatment) were studied, determining the limits in the R-OH, H 2 O, TEOS diagram within which good coatings can be produced. Film thickness varies from 0.2 to 0.9 μm and its dependence on viscosity and oxide concentration in the solution and on the withdrawal rate has been studied. Furthermore, it has been determined the relation between film thickness and microcracks appearance using ROM. Optical properties of coatings have been investigated by spectrophotometric techniques, optical parameters obtained being compared with the corresponding parameters of soda-lime-silica glasses containing the same oxides. From these measurements, the coloration mechanism in each case has been determined.

Sol–gel coatings for protection and bioactivation of metals used in orthopaedic devices

The aim of this work is the production and characterisation of sol–gel coatings for protection and bioactivation of metals used as standard surgical implant materials, such as stainless steel 316 L (ASTM F138), Co based alloys (ASTM F75) and titanium alloy Ti-6Al-4V (ASTM F67). These films should both prevent degradation of the substrates by wear or corrosion, and bioactivate the material for inducing the formation of a hydroxyapatite (HA) rich layer onto the material surface, thereby permitting a natural bonding to living tissues. Formation of HA layers can be observed on performing in vitro tests by soaking the material in simulated body solutions. The work describes the development of coatings containing bioactive glass and glass-ceramic particles in hybrid methyl-triethoxysilane (MTES) and tetraethylorthosilicate (TEOS) acidic sol, applied by dip-coating to surgical alloys, AISI 316 L, ASTM F75 and ASTM 67, with the aim of accomplishing both high corrosion resistance of the metal in the body environment and adhesion of the implant to the surrounding tissue. The performance of the coated metal was evaluated in vitro by electrochemical techniques including potentiodynamic polarisation curves and electrochemical impedance spectroscopy, to follow the formation of hydroxyapatite on the surface, as well as the in vitro release of ions by plasma atomic emission spectroscopy (ICP-MS) after up to one year of immersion. In vivo behaviour was evaluated by subcutaneous tests and endomedullar implantation in Hokaido rats to study possible rejection reactions and natural bonding to living tissue.

Bioactive and Protective Sol-Gel Coatings on Metals for Orthopaedic Prostheses

The aim of this work has been the preparation and evaluation of sol-gel coatings for clinical applications. Research was focussed in the development of highly corrosion resistant and/or bioactive sol-gel coatings onto AISI 316L stainless steel. Hybrid SiO2 sol-gel coatings inhibited corrosion and Fe diffusion, although no signal of bioactivity was detected. The inclusion of Ca- and P-alcoxides in the sol composition did not promote bioactivity. Bioactive coatings were obtained from suspensions prepared by adding glass (CaO·SiO2·P2O5) particles to an hybrid organic-inorganic SiO2 sol. The dissolution of glass particles promoted in vitro induction of apatite along with a slight reduction in the corrosion resistance of coated pieces. By combining an inner SiO2 hybrid film acting as barrier against corrosion with an outer coating containing bioactive glass particles, a significant improvement in the electrochemical behaviour was observed. This double-layered coating showed in vitro signals of bioactivity, and preliminary in vivo tests gave promising results.

Protection and surface modification of metals with sol-gel coatings

Abstract The preparation and properties of protective sol–gel coatings on metallic substrates are reviewed. Control of the coating microstructure is critical to obtain the desired properties and this requires optimisation of processing parameters that include sol composition, synthesis and processing conditions, deposition method and post-deposition sintering or curing. Current knowledge in these areas, e.g. structure–property–processing relationships, and the characterisation of the coatings produced are discussed. SiO2 layers have been found to confer good oxidation resistance, but for electrochemical corrosion resistance, mixed oxide layers or hybrid organic–inorganic layers are required. Incorporation of alkylalkoxides with polymerisable groups attached has allowed synthesis of hybrid nanostructured coatings consisting of interpenetrating, chemically bonded organic and inorganic networks. Doping of hybrid sol–gel coatings with environmentally friendly inhibitors is also a promising approach combining barrier properties with active inhibition of corrosion. The potential to control functional properties such as biocompatibility is also briefly considered. It is concluded that both fundamental work on new coating compositions and the development of improved industrial technologies are priorities for future research.

Electrochemical and in vitro behaviour of sol–gel coated 316L stainless steel

The corrosion resistance of AISI 316L stainless steel for biomedical applications, was significantly enhanced by means of inorganic and hybrid SiO2 sol–gel coatings deposited by dip-coating. Coatings of 0.5–1.1 μm with different hybrid character were obtained by varying the temperature of thermal treatment from 400 to 550 °C. The pitting potential is increased up to 1 V and the current intensities were reduced up to 10−8 A/cm2. After one month of immersion in simulated body fluids, all the coatings showed a high degree of corrosion resistance and integrity, being slightly better the behaviour of the hybrid coatings. Not any bioactivity signal was observed.