Abstract text (incl. figure legends and references)

Polymer-derived ceramics (PDCs) are promising candidates used as protective coatings on hard metal substrates for applications, requiring operating temperatures <1300°C. The polymer-to-ceramic synthesis route gives access to prepare ceramic coatings, such as environmental (EBC) or thermal barrier coatings (TBC) via several inexpensive liquid-phase deposition methods. Typically, PDCs consist of a highly refractory phase, such as early transition metal nitrides, carbides or borides, which are finely dispersed in a silica-former matrix phase (e.g., silicon carbide, silicon nitride, silicon(boro)carbonitride). As the microstructure and phase composition of a material strongly influence its high-temperature properties, a profound understanding of the microstructural development during its different processing steps is indispensable. During the synthesis of a PDC the starting preceramic polymers are firstly crosslinked at temperatures ranging from 200-1000°C and subsequently annealed at temperatures <1500°C, which induces phase separation and crystallization processes. To obtain dense monolithic materials, as-pyrolyzed powder samples are consolidated via spark plasma sintering. Dense monolithic ceramics are highly needed to gain information on mechanical properties and oxidation behaviour. In this context a series of novel PDCs were synthesized according to the polymer-to-ceramic route. As-pyrolyzed and subsequently annealed powder samples as well as spark plasma sintered (SPS) monolithic bulk materials in the Si(M)(BC)N system (M=Hf,Ta) were compared regarding their microstructural development upon different heat-treatments. Pyrolysis of Si(M)N powder samples was carried out at 1000°C under NH3 gas followed by an annealing at 1600°C in N2 atmosphere. As-pyrolyzed Si(M)(BC)N powders were sintered via SPS at 1950°C in vacuum employing different sintering parameters (different pressure & dwell times). The synthesized materials have been investigated via electron microscopy concerning their temperature-dependent microstructural evolution upon pyrolysis and subsequent heat treatments. Si(M)N powder samples as well as dense Si(M)(BC)N bulk materials were analysed via scanning (SEM) and transmission electron microscopy (TEM) with respect to their chemical composition, crystallization behaviour as well as their phase composition and distribution. The annealed Si(M)N powder samples revealed a microstructure consisting of two regions named bulk and surface regions, which differentiate in their intrinsic microstructure and phase composition. Bulk regions contain a silicon nitride-based matrix phase with finely dispersed nano-sized transition metal carbides (TMCs). In comparison, surface regions underwent phase separation, decomposition reactions and diffusion-controlled coarsening phenomena, showing a completely different microstructure, consisting of silicon carbide and grain coarsened TMCs. Similar microstructures were also found in the studied SPS dense monolithic materials. Former powder particles are consolidated via sintering, showing exactly the same above-mentioned two regions within the internal microstructure: surface and bulk. Furthermore, it is discussed, whether the microstructural evolution in powder samples compared to dense monolithic materials consolidated via SPS, show an almost identical microstructural development. A detailed microstructural evolution model for both, powder samples and dense monolithic materials is introduced.