Process optimization for Si(Hf,Ta)(B)(C,N) polymer-derived ceramics via microstructural analysis

Abstract

Polymer-derived ceramics (PDCs) gained significant importance because of the unique combination of their commercial availability, molecular tailorability, reduced processing costs, high-temperature stability, chemical resistance and adherence. Thus, they can serve as potential thermal barrier coating (TBC) protecting an underlying alloy from high thermo-mechanical loads and degradation in extreme environments. The main focus in the development of novel PDCs lies in the modification of the polymeric precursors on a molecular level and in the subsequent treatment tailoring the nano- and microstructure to achieve attractive structural and functional properties. However, detailed information of the emerging microstructures upon different heat treatments is lacking. In this work, the microstructure development of novel PDC nanocomposites (PDC-NCs) upon different heat treatments is investigated, primarily using scanning and transmission electron microscopy (SEM, TEM). Polysilazane-based SiHfCN, Si(HfxTa1-x)(C)N and polysilane-based Si(B)(HfxTa1-x)C ultra-high temperature ceramic nanocomposites (UHTC-NCs) were analyzed in the as-pyrolyzed, annealed and sintered state. In a first step, particular emphasis was placed onto the microstructure development of novel Si(HfxTa1-x)(C)N UHTC-NCs upon pyrolysis and subsequent annealing. A microstructure evolution model shows the conversion from a single-source precursor into a mostly amorphous single-phase ceramic after pyrolysis. Annealing resulted in crystalline UHTC-NC, presenting two distinct microstructure regions, namely the bulk and surface of the powder particles with inherently different microstructures and phase compositions. Initiated by high-temperature annealing, the residual “free” carbon present in the system initiated the formation of SiC and thus, the thermal decomposition of Si3N4, along with the release of gaseous nitrogen in the outer region of the powder particles. A detailed study of the crystallization behaviour and phase composition found local chemical variations and a gradual average increase of transition metal carbide (TMC) grain sizes in the proximity to surface-near regions. Grain sizes effects correlate with an outward zoning of increased carbon- and oxygen compositions accompanied by depleted nitrogen compositions in the matrix. The experimental data clearly showed that polysilazane-based UHTC-NCs are prone to phase separation, accompanied by thermal decomposition and diffusion-controlled coarsening. This is disadvantageous when considering these materials as TBC materials, as the coarsening phenomena could induce thermal stresses and therefore lead to spallation. By changing the precursor from polysilazane to polysilane, a reduced coarsening of TMCs was achieved. However, the polysilane-based UHTC-NC showed very similar microstructures upon sintering. Increased TMC grain sizes were attributed to a combination of the pulsed direct current (DC), the accompanying Joule heating and the presence of oxygen impurities promoting particle coarsening during evaporation, condensation and diffusion processes. A further modification of the polysilane-based UHTC-NC with small amounts of boron resulted finally in a homogeneous distribution of the constituting phases, a reduction of TMC grain sizes and a reduced porosity. The findings demonstrate that SEM and TEM are useful for unravelling complex microstructures of multiphase PDC-NCs and guiding their development. The results show that the correlation of different parameters such as the precursor selection as well as the process parameters of the sintering procedure have an enormous impact on the microstructural development. Therefore, microstructure characterization and continuous feedback is essential when designing and synthesizing novel PDC NCs for potential TBC materials.