Bernhard Wielage

Thermogravimetric and differential scanning calorimetric analysis of natural fibres and polypropylene

The thermal behaviour of variably separated flax and hemp fibres was characterised by means of thermogravimetric and differential scanning calorimetric analyses. Reference measurements of cellulose served to describe the degradation behaviour. Furthermore, thermoanalytic investigations on polypropylene were carried out to evaluate the thermal stability and the respective activation energy of the materials. The results prove the thermal stability necessary for the consolidation process of composite materials.

Processing of natural-fibre reinforced polymers and the resulting dynamic–mechanical properties

Abstract By means of dynamic–mechanical analysis (DMA), selected application-specific properties of flax- and hemp-fibre reinforced polypropylenes (PPs) have been determined for material characterisation. The compound samples were manufactured both by consolidation of hybrid non-wovens and compounding and injection moulding with the addition of natural fibres. The conditioning (long and short fibres), the manufacturing process and the processing parameters are the most important influencing factors on the mechanical properties of the final product. The results also reveal that the elastic properties (stiffness, storage modulus) of the composite material are dependent on the type of coupling agent. Other influencing parameters are the specific surface and the content of added fibre. The parameters mentioned can be varied by fibre separation or post-treatment procedures. The recycling behaviour of natural-fibre reinforced PP shows that multiple processing has only an insignificant influence on the fibre lengths and the mechanical properties. In addition, it is possible to very quickly draw conclusions about the quality of the composite material, such as fibre–matrix adhesion and damping behaviour. Fractographic evaluations in the scanning electron microscope (SEM) confirm the quantitative characterisation obtained from DMA.

Interface behaviour in nickel composite coatings with nano-particles of oxidic ceramic

Advances in micro-technology demand that new functional materials be developed so that the technical properties of micro-devices can be improved at reasonable cost. The co-deposition of nanoscaled particles during an electroplating process has been shown to bring such an improvement. This work focuses on particles of oxidic ceramics, in this case those of Al2O3 and TiO2. The diameters of the primary particles ranges from 10 to 30 nm, electrodeposited by means of a conventional Watts nickel electrolyte. A series of nickel nano-ceramic composites were produced, with co-deposition of particles as a single primary particle in the nanometre range at one end of the scale and as agglomerates up to a size of a micrometer at the other. The influence of the presence of particles on crystallisation behaviour, residual stress and texture of the deposited nickel coatings was examined by X-ray diffraction (XRD). There is a report on the interfaces between the nickel grains and the oxidic ceramic particles, investigated using transmission electron microscopy (TEM). A decreasing corrosion stability indicates an attack along the interface nickel/particles.

Interface phenomena and bonding mechanism of thermally-sprayed metal and ceramic composites

Abstract Metal and ceramic materials were sprayed on polished steel and iron substrates using atmospheric arc (nickel, molybdenum) and plasma (Al 2 O 3 , ZrO 2 -7Y 2 O 3 ) spraying. The bonding quality at the substrate-coating interface depends on the size and depth of the contact zones. The chemical-metallurgical interactions (diffusion, reactions) between the sprayed particles and the metal substrate as a function of the properties of the sprayed materials, contact temperature and solidification time were analysed. Higher contact temperature and longer interaction time for spraying of metals result in better bonding. Moreover, oxidation of the metal substrate to a limited degree by spraying of oxides allows one to improve the bonding quality. It is possible to correlate the results of microscopical investigations of the interface phenomena with the results of quantitative measurement of the bonding tensile strength.

DLC for tools protection in warm massive forming

Abstract In order to find an alternative to solid lubricants in bulk metal forming with contact temperatures between work piece and tool up to 600 °C, low wear friction coatings like hydrogenated amorphous diamond-like carbon (DLC), silicon doped diamond-like carbon (Si-DLC), sputtered molybdenum disulfide and a multilayer system of TiC and TiN (TiCN) were tested in a modified forced-in test at contact temperatures between room temperature and 500 °C. Caused by the high normal pressure between work piece and tool, this test models heavy sliding friction which can lead to adhesive wear. The tested solid lubricants showed, for all tested temperatures, lower coefficients of friction than the four coatings, but their values were very low too. Only the bolt coated with Si-DLC failed the test at room temperature and the TiCN coating did not work at all temperatures. The lowest press force of the coatings at 400 °C was needed for the Si-DLC, while for the other temperatures MoS 2 was best. The surfaces of the sleeves showed no visible failures. In endurance tests with the same bolt in different sleeves, surfaces formed with a Si-DLC coated bolt had no mistakes after 16 repetitions at 500 °C, while tests with the MoS 2 coating failed after nine times and with DLC after two times. At 400 °C the Si-DLC coating showed the best results too, while MoS 2 was best at room temperature.

High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders

Abstract Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybdenum disilicide which has a great potential as protective coating e.g. in aircraft engines and gas turbines in the temperature range between 1400 and 1800°C due to its high melting point and its low brittle-ductile transition temperature of approximately 800–1100°C. Four types of coatings were produced by high velocity oxyfuel spraying (HVOF): unreinforced MoSi 2 with low porosity, unreinforced MoSi 2 with high porosity, with silicon carbide reinforced MoSi 2 and with alumina reinforced MoSi 2 . The coatings as sprayed were characterized by XRD, SEM and EDX. Microhardness and porosity were measured. The oxidation behavior of the coatings was determined at 500, 1000 and 1500°C. The influence of the heating rate was investigated during oxidation tests at 1000°C. The tests at 500°C showed that the pesting depends on the porosity of the coating. SiC as reinforcing phase seems to accelerate pesting, while alumina reduces this reaction. Unreinforced MoSi 2 coatings form a protective SiO 2 layer on the surface with a thickness below 10 μm during oxidation at 1500°C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline two-phase system of α-MoSi 2 and hexagonal Mo 5 Si 3 .

In situ and ex situ examination of plasma-assisted nitriding of aluminium alloys

Abstract Pre-sputtering and plasma-assisted nitriding of pure aluminium and 2024 aluminium alloy were studied in situ by plasma monitoring and ex situ by several surface analysis methods. The influence of the process parameters, time, temperature, pressure and gas composition on mass and energy distributions of ions, as well as on topography and chemical composition, was examined. The chronology of the sputtering process could thus be clarified. An oxide film which is relatively thick compared to the native oxide layer and enriched with magnesium is formed at the initial stage of the sputtering treatment, followed by the removal of this film with a subsequent roughening of the surface. The cleaning effect of the plasma can be enhanced by increasing the argon pressure or by an intermixture of hydrogen. Due to a supplementary chemical etching, hydrogen admixture reduces the surface roughness and effectively decreases the oxygen content of the surface. During the nitriding treatment, nanocrystalline hexagonal aluminium nitride AlN is formed. Growth rate and nitrogen content of the nitride layer were found to strongly depend on the gas composition during sputtering and nitriding. The positive effect of addition of argon and hydrogen, respectively, arises from an intensification of the nitrogen plasma, the production of nitrogen–hydrogen molecular ions and an increase of the ion energy. The nitrogen pressure affects the layer thickness due to higher nitrogen activity and ion energies. The chemical composition of the cathode material influences discharge current and, as a result, the ion energies.

High-resolution microstructural investigations of interfaces between light metal alloy substrates and cold gas-sprayed coatings

Interfaces between light metal alloys, aluminum AA7022, and magnesium AZ91, and optimized cold gassprayed zinc-based coatings are characterized. The analyses include scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). Investigations by SEM show a seam with intensive mixing of the substrate and coating material, which is indicated by different values of gray due to element contrast. In energy-dispersive spectroscopy analyses, increased zinc concentrations compared with the substrate material are detected in <1 μm thick vortexes inside the seam. The TEM investigations prove that these areas consist of a homogeneous solid solution and submicron-sized or nanosized intermetallic phases with different concentrations of aluminum, zinc, and magnesium. Because diffusion processes cannot result in the observed microstructure. local melting followed by precipitation of intermetallic phases is concluded as the consequence of the intensive mechanical interaction at the substrate-coating interface during particle impact during the cold gas spraying of zinc on magnesium or aluminum substrates.

A cost effective route for the densification of carbon-carbon composites

Abstract Carbon–carbon (C/C) composites are candidate materials for many high temperature structural applications, but the high costs of manufacture prohibit their application in many cases, where the properties are desired but economically not affordable. This study focuses on the catalytic effects of metal carbonyl additions on the cross-linking efficiency of pre-ceramic silicon polymers, which are used for a rapid and cost effective densification of porous C/C composites. Whereas, the open pores in the C/C matrix, obtained via the polymer pyrolysis, are effectively closed by a one shot infiltration/pyrolysis step applying a dicobaltoctacarbonyl [Co 2 (CO) 8 ] modified polysilane, a similar level of densification of porous C/C composites by a one time infiltration/pyrolysis of polycarbosilane could not be achieved, although catalytic additions of Co 2 (CO) 8 were used as well. Besides the successful densification of C/C composites using the modified polysilane, improvement in the oxidation behaviour of the composites at elevated temperatures was also recorded. The influence of certain manufacturing process variables, such as curing temperature of the carbon precursor before pyrolysis and heat-treatment temperatures of the manufactured C/C composites before densification, on the structures and properties of the obtained composite materials were investigated.

Improvement of the corrosion resistance of C/Al-composites by diamond-like carbon coatings

Abstract Because of the extreme sensitivity of carbon fiber reinforced aluminum (C/Al-compound) to electrochemical corrosion suitable protection methods are necessary. Improved corrosion resistance can be achieved by applying appropriate coatings. Diamond-like carbon (DLC) coatings provide properties, which make them interesting materials for external corrosion protection of MMCs (Metal Matrix Composites). The hydrogenated, amorphous carbon coatings not only reveal outstanding wear resistance and good adherence to the heterogeneous MMC-substrate, but also results in an improvement of corrosion resistance. The lightweight and dense DLC-coating covers up the underlying substrate and separates the MMC from the attacking electrolyte. The electrochemical corrosion behavior of uncoated and DLC-coated C/Al-composites is tested in 3.5 wt.% NaCl solution by using potentiodynamic polarization with low scanning rates tests. It has been found that the pitting potential is shifted significantly in the anodic direction and the corrosion current density is much lower due to the presence of the sealing DLC-coating. Additionally, tranmission electron microscope (TEM) and scanning electron microscopy (SEM) studies were performed in order to characterize the fiber-matrix interface and the corrosion mechanisms of uncoated and DLC-coated MMCs.