Herstellung, Umwandlungsverhalten und keramischer Polysilazanschichten auf Stahlsubstraten Eigenschaften (Volume 2) (English shop)

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Surface and thin-film technology is one of the fastest-growing sectors in Germany and is applied in many industrial fields. Important branches of industry such as the automotive industry, environmental and energy technology, aerospace, microsystems and medical technology, or information and communication technology depend, in part very strongly, on surface technology. In 2002, the estimated annual turnover of companies in the surface-finishing sector amounted to approximately 10 billion euros. The most important processes were painting, electroplating, vacuum technology and thermal spraying. The functions of coatings are extremely diverse, ranging from specific sliding properties or high wear resistance through corrosion and oxidation protection to particular electrical, optical or decorative properties [KOE08, MOH05].

Against the background of sharply rising raw material prices, particularly in recent years, the prevention of corrosion, oxidation and wear of metallic components has gained enormously in importance. Economic estimates assume that tribologically induced damage accounts for a volume of approximately 1 % of gross domestic product (GDP). The economic damage caused by corrosion and oxidation is even put at about 4 % of GDP [BAC05].

Surface technologies for the protection of metals can be roughly divided into coating and heat treatment processes.

Paints or coatings are most frequently used for corrosion protection. These can be applied using simple painting processes such as dipping or spraying. However, they are limited in their chemical, mechanical and thermal stability.

Oxide as well as non-oxide ceramic coatings have great potential for protecting metals against oxidation, corrosion and wear. These coating systems, which are mostly applied by vacuum techniques (Physical Vapour Deposition (PVD) or Chemical Vapour Deposition (CVD)) or by thermal spray processes, possess good chemical resistance and high hardness values. Disadvantages of these processes are, however, the considerable and cost-intensive equipment requirements as well as the porosity of the coatings (thermal spraying) [BAC05].

In heat treatment processes such as nitriding, carburising or boriding, the surface region of the metals is enriched with certain elements by means of a diffusion-controlled reaction. This can be achieved by a thermal treatment in the corresponding donor media. In this way, coatings with good tribological properties and excellent adhesion can be produced. However, the often difficult transferability to different component geometries as well as the health- and environmentally hazardous material systems of many processes have a negative effect.

An alternative to the already established surface technologies for producing polymeric and ceramic coatings is the use of precursors. Since the 1970s, these preceramic precursors have been used for the production of ceramics [VER73, YAJ78]. A key advantage compared with powder-ceramic processes is their processability using polymer technology. The precursors are predominantly based on silicon-containing compounds, with polysiloxanes, polycarbosilanes and poly(carbo)silazanes being the most widespread. After a suitable thermal treatment (pyrolysis), these yield ceramics in the systems SiCO (silicon oxycarbide), SiC (silicon carbide), Si3N4 (silicon nitride) or SiCN (silicon carbonitride).

These ceramics are characterised by good thermal shock resistance, high creep resistance, high thermal stability as well as good oxidation and corrosion resistance [CHO00, JAC01, KOL04, NIC99, RIE06]. The formation and growth of a protective and passivating SiO2 layer, which has the lowest oxygen diffusion coefficient of all simple oxides, is the main reason for the high oxidation resistance of Si-based ceramics [AN04]. Polysilazane-based ceramics exhibit a somewhat higher oxidation resistance compared with polycarbosilane- and polysiloxane-based systems. This is due to the fact that the activation energy for the oxidation of a Si-N bond (Ea = 330-490 kJ/mol) is higher compared with Si-C bonds (Ea = 90-140 kJ/mol) [CHO00].

The production of precursor-based coatings can be roughly divided into the steps of polymer synthesis, coating and thermal treatment. The process can therefore be regarded as a combination of coating and heat treatment processes. The advantages of producing ceramic coatings via the precursor route are above all the applicability of the coatings by simple painting processes such as spraying or dipping, the coatability of components with complex geometries, the low ceramisation temperatures as well as the high thermal and chemical stability of the coatings.

At the Chair of Ceramic Materials Engineering at the University of Bayreuth, work has been carried out in the field of polysilazanes for several years. Specifically for the production of ceramic coatings and fibres, a suitable preceramic polymer (ABSE: ammonolysis product of bis-dichloromethylsilylethane) was developed, which has already been produced on a technical scale (approx. 50 kg). This organic polycarbosilazane possesses a high carbon content, so that after pyrolysis a SiCN ceramic with a free carbon fraction results [MOT00, MOT02a, TRA01]. Earlier investigations have shown that the commercially available polysilazane PHPS (perhydropolysilazane) is likewise very well suited for application as a coating material [BAU05, BRA07b, GUE04]. In contrast to ABSE, it is a purely inorganic polysilazane with an excess of Si [KRO00].