Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 506

oxidation, and corrosion of metals are estimated at several billions $ per year. New
developments in the industry as well as political regulations demand coatings with
excellent chemical and thermal resistance for applications in power plants, in
electronics, in energy conversion or storage, in the chemical industry, or in the
automotive sector. Varnishes, paints, and enamel layers have traditionally been
used as protective layers. They are characterized by simple and well-established
application techniques. However, organic varnishes and paints are limited in tem-
perature stability as well as resistance to wear and solvents, whereas enamel layers
are very sensitive to mechanical deformations. Ceramic coating systems can pro-
vide high-temperature resistance, abrasion protection, oxidation protection, thermal
insulation, as well as functional properties. Non-oxide and oxide ceramic layers or
even amorphous and diamond-like carbon coatings are now applied mainly with
CVD or PVD processes to improve wear and hardness of the surface. Drawbacks of
these well-established techniques are the high cost due to specialized equipment
(e.g., plasma or high vacuum technology), the limited adhesion of the layers, as
well as the formation of cracks, especially under thermal stress. In addition, it is still
dif cult to coat complex-shaped components.

In technical applications, surface protection with hard coatings such as TiC, SiC,
or Si 3 N 4 can extend the lifetime of moving parts by minimizing abrasion and wear.
Thermal barrier coatings (TBC), for example, yttria-stabilized zirconia (YSZ),
TiB 2 , or WC, perform the important function of insulating components, such as gas
turbines and aeroengine parts, operating at elevated temperature (Clarke and Levi
2003 ). However, high costs, scale-up problems, and the limited geometry of the
substrates are crucial disadvantages of conventional techniques used to apply these
non-oxide or oxide ceramic layers, including physical or chemical vapor deposition
or plasma spraying.
As an alternative approach, preceramic polymers can be utilized for coating
processes and converted into cross-linked polymer coatings (at temperatures below
300

C) or into polymer-derived ceramic (PDC) coatings (at temperatures higher
than 500

°

C). They often possess a composite structure, due to more than one phase
formed during thermal conversion or after reaction with substrate of

°

filler material.
Figure 7 shows an overview of the polymer-derived ceramic process in coating
applications. The following advantages have been identi ed for polymer-derived

ceramic coatings over the last two decades of research:

Use of inexpensive coating techniques such as spray coating, spin coating, or
dip coating,

￿

Application as solid (dissolved in a liquid) or liquid polymeric precursors,

￿

￿ Chemical resistance and thermal stability up to 500 ° C in the polymer state,

￿ High ceramic yield of the precursors after thermal conversion (pyrolysis),

￿ Coating of complex-shaped parts or large-volume parts,

￿ Control of the polymer-to-ceramic conversion and resulting properties by

temperature, as well as

Easy incorporation of functional properties.

￿