S. Steinhäuser

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.

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 .

Plasma-sprayed wear-resistant coatings with respect to ecological aspects

Abstract This work introduces some aspects of further improvement of the environmental friendly character of thermal spraying. After a short overview of the present state of the technical development, general relations between the spraying process and the environment will be discussed. Then, the possibilities to contribute to an improvement of the environmental friendliness will be shown. The use of spraying powders on the basis of Fe-TiC will be presented first of all. These powders are produced by means of SHS synthesis. They do not contain any elements — or only on a limited scale — that can be classified as being critical. The spraying behaviour of these powders and the wear resistance of the coatings that are sprayed with them are not yet optimal. The results that have been obtained, however, allow us to draw the conclusion that the way taken so far is the right way. Furthermore, possibilities will be presented to reduce the amount of sealings or to use alternative, more environmentally friendly materials. Finally, a possible improvement of the properties of thermally sprayed coatings, which is based on a laser shock treatment, will be indicated. This type of laser treatment is presently not yet practicable in a techno-economically reasonable way, but it offers innovative possibilities for future developments.

Fracture Behaviour of an Electron Beam Weld

This paper deals with cracks in an electron beam welded joint made of severely different materials. The motivation is to improve the design and reliability of an electron beam welded worm wheel, which may contain large cracks due to hot cracking. Material properties of the weld zones were determined by minitensile specimens. The fracture behaviour of the weld was investigated with CTSand SENB-specimen. So far, estimates of the threshold va lues for the fatigue crack growth can be presented. Further experimental and numerical investigations are pla nned.