Ergebnisse von Sol‐Gel‐Beschichtungen auf austenitischem CrNi‐Stahl

Results from sol‐gel coatings on austenitic CrNi steel Thin aliuminia films were applicated on austenitic CrNi steel by sol‐gel‐process and tested with several examinations. The phase transformation and the measuring of residual stresses in the coatings were investigated by X‐ray diffraction. This research was completed by wear tests.     

Cost‐efficient conversion coatings for corrosion protection prepared by the sol‐gel process

For several years, chromate compounds were used as effective and inexpensive corrosion inhibitors. Studies showed that these materials were toxic and carcinogenic. A subsequent ban on these materials has led to extensive research to develop effective alternative inhibitors. The sol‐gel method appeared as an effective technique forming protective silane layers from hydrolyzed tetraethylorthosilicate (TEOS) and glycidoxypropyltrimethoxy silane (GPTMS) in an aqueous sol–gel process. The final cross‐linking of the layer is realized by heat treatment to form thin, fully dense, protective films of silane on diverse metal surfaces. Most sol‐gel layers use a considerable amount of solvent VOC (v olatile o rganic c ompound) in order to achieve a dispersion of silane with water. However, in a wide range of industries, the high amount of VOC is the major hindrance for the application of silane layers. This article shows a coating concept without VOC, which is cost‐efficient, effective for corrosion protection and applicable for metal surfaces. The corrosion protection effect is given by means of comprehensive impedance spectroscopy measurements.     

Development and characterization of sol‐gel composite coatings on aluminum alloys for corrosion protection

For several years, Cr^IV compounds were used as effective and inexpensive corrosion inhibitors. Studies showed that these materials were toxic and carcinogenic. This has led to extensive research to develop alternative inhibitors. Organo‐silicate hybrid coatings appeared as an effective technique for forming protective layers on different metal alloys. A silane film was obtained by dip coating of the sample in sol solution prepared from the hydrolysis of 3‐glycidoxypropyltrimethoxysilane (GPTM), tetraethylorthosilicate (TEOS) with acid as catalyst and water as solvent instead of VOC (volatile organic compound). The sol solution was aged at ambient temperature to to enhance the effectiveness of the solution for the hydrolysis process. The sample was cured at 150 °C to ensure cross‐linking of the film. The experiments have shown that heat treatment leads to increased density and corrosion resistance of the films. Organic and inorganic inhibitors were added in different concentrations to improve the protection and self‐healing properties of the coating even after long‐time immersion in corrosive solution. The protection performance of the film was evaluated by electrochemical impedance spectroscopy (EIS) in 0.5 M NaCl solution (pH value 7). The surface morphologies of the treated samples were investigated using SEM.     

AFM characterisation of sol‐gel TiO2 films deposited on stainless steel

One‐layer, two‐layer and three‐layer titania films on AISI 304 stainless steel were deposited by sol‐gel process and dip‐coating method. Two sols were prepared by using titanium isopropoxide as a precursor, propanol as a solvent, nitric acid as a catalyst and acetylacetone for peptization. Both of the prepared sols contained the same amount of mentioned components, the only difference was in the addition of polyethylene glycol (PEG) as templating reagent to one of the sols. After calcination at 550°C, deposited films on the stainless steel substrate were characterized by atomic force microscope (AFM). Influence of layer number as well as addition of polyethylene glycol on morphology of titania films was analysed and discussed. Generally, by increasing the number of layers and by addition of polyethylene glycol, roughness parameters increase by changing surface topography. The surface topography analysis is very important when choosing the adequate industrial application of prepared layers.     

Eine innovativ im Sol‐Gel‐Prozeß hergestellte,hochporöse Siliziumoxidkeramik zum Knochenersatz – In‐vivo Langzeitergebnisse

A new high porous silica‐sol‐gel‐ceramics for bone grafting – in‐vivo long‐time investigations The new calcium phosphate ceramics was produced by a sol‐gel‐process at 200 °Celsius with silica (SiO₂) as adjuvant. The aim of this investigation was to test the osteoinductive effect of these bioceramics and to prove its biodegradation by means of animal experiments. One year old minipigs were used and divided into three groups (n=6). Critical size defects (>5cm³) in the mandible were filled by different materials (group1: 60 % hydroxilapatite [HA] + 40 % ß‐tricalciumphosphate, group 2: only HA; group 3: control, without ceramics). Eight months later clinical, histological, morphometrical and REM investigations concerning the state of former defected mandible were made. In groups 1 and 2 a complete reossification of the bone defects and a biodegradation rate of ceramics of more than 96 % were recognized. In conclusion silica‐calcium phosphate ceramics made by a sol gel method seems to be suitable for filling bone defects in men and is of interest for orthopedic surgery, traumatology, craniomaxillofacial surgery and dentistry. Recently a clinical study was started.     

C/C‐SiC Composites for Advanced Friction Systems

Ceramic Matrix Composites (CMC), based on reinforcements of carbon fibres and matrices of silicon carbide, show superior tribological properties in comparison to grey cast iron or carbon/carbon. In combination with their low density, high thermal shock resistance and good abrasive resistance, these Si‐infiltrated carbon/carbon materials, called C/SiC or C/C‐SiC composites, are promising candidates for advanced friction systems. Generally, the carbon fibres lead to an improved damage tolerance in comparison to monolithic SiC, whereas the silicon carbide matrix improves the wear resistance compared to carbon/carbon. In combination with new design approaches cost‐efficient manufacturing processes have been developed and have lead to successfully tested prototypes of brake pads and disks, especially for passenger cars and emergency brake systems.     

Structural and optical properties of nitrogen‐iron co‐doped titanium dioxide films prepared via sol‐gel dip‐coating: Effect of urea and iron nitrate concentration in the sol

In this study, nitrogen‐iron co‐doped titanium dioxide films were prepared via sol‐gel dip‐coating method using urea and iron nitrate as nitrogen and iron source, respectively. Nonmetal doping of TiO₂ have some disadvantages such as massive charge carrier recombination and losing the photo‐catalytic capability. Three different nitrogen‐iron co‐doped titanium dioxide sols with different urea and iron nitrate concentration were prepared. The resulting sols were homogeneous and transparent, and no precipitation was observed in any of them. It was observed that the film prepared with middle urea‐iron nitrate concentration sol got opaque in a short time after the dip‐coating process. All prepared films were characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, confocal microscopy and ultraviolet–visible (UV‐Vis) spectroscopy. It was found that the concentration of the urea and iron nitrate in the sol had an effect on the crystal structure, microstructure, surface morphology and optical properties of the resulting films. Samples with middle concentrations had amorphous structure and bigger particle size. It was seen that sample with higher iron amount has lower band‐gap. It is concluded that we can prepare transparent anatase, transparent amorphous and opaque amorphous titanium dioxide films by changing the urea and iron nitrate concentration in the sol.