Advanced Ceramics from Preceramic Polymers Modified at the Nano-Scale: A Review

Materials 2014, 7, 1927-1956; doi:10.3390/ma7031927 OPEN ACCESS materials ISSN 1996-1944 www.mdpi.com/journal/materials Review Advanced Ceramics from Preceramic Polymers Modified at the Nano-Scale: A Review Enrico Bernardo 1,*, Laura Fiocco 1, Giulio Parcianello 2, Enrico Storti 3 and Paolo Colombo 1,4 1 2 3 4 Department of Industrial Engineering, University of Padova, Via Marzolo 9, Padova 35131, Italy; E-Mails: (L.F.); (P.C.) EMPA—Swiss Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland; E-Mail: Institut für Keramik, Glas- und Baustofftechnik, TU Bergakademie Freiberg, Agricolastraße 17, Freiberg 09596, Germany; E-Mail: Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16801, USA * Author to whom correspondence should be addressed; E-Mail: ; Tel.: +39-049-827-5510; Fax: +39-049-827-5505. Received: 13 December 2013; in revised form: 24 February 2014 / Accepted: 26 February 2014 / Published: 6 March 2014 Abstract: Preceramic polymers, i.e., polymers that are converted into ceramics upon heat treatment, have been successfully used for almost 40 years to give advanced ceramics, especially belonging to the ternary SiCO and SiCN systems or to the quaternary SiBCN system. One of their main advantages is the possibility of combining the shaping and synthesis of ceramics: components can be shaped at the precursor stage by conventional plastic-forming techniques, such as spinning, blowing, injection molding, warm pressing and resin transfer molding, and then converted into ceramics by treatments typically above 800 °C. The extension of the approach to a wider range of ceramic compositions and applications, both structural and thermo-structural (refractory components, thermal barrier coatings) or functional (bioactive ceramics, luminescent materials), mainly relies on modifications of the polymers at the nano-scale, i.e., on the introduction of nano-sized fillers and/or chemical additives, leading to nano-structured ceramic components upon thermal conversion. Fillers and additives may react with the main ceramic residue of the polymer, leading to ceramics of significant engineering interest (such as silicates and SiAlONs), or cause the formation of secondary phases, significantly affecting the functionalities of the polymer-derived matrix. Materials 2014, 7 1928 Keywords: precursors-organic; polymer-derived ceramics; nanocomposites; silicates; SiAlON 1. Introduction Preceramic polymers, especially in the form of organo-silicon compounds (e.g., polymers based on a backbone of Si atoms containing also C, O, N, B and H atoms), have been widely recognized for the last 40 years as an extremely powerful tool for the production of advanced ceramics. Their key advantage over conventional (powder) synthesis procedures is represented by the possibility of adopting plastic-forming techniques (e.g., fiber spinning, foaming, warm pressing, extrusion, injection molding or resin transfer molding) to generate shaped components, later transformed into the desired ceramic parts (usually defined as polymer-derived ceramics or PDCs) by thermal treatment (pyrolysis) above ~800 °C, typically in a non-oxidative atmosphere (nitrogen or argon) [1–3]. The ceramics obtained from polymeric precursors usually feature a chemical composition not achievable by other techniques: as an example, silicones (polymers with a -Si-O- backbone) yield, upon firing in a non-oxidative atmosphere, an amorphous SiCO (silicon oxycarbide) residue, which can be considered as a silica glass modified by the presence of nano-sized domains based on SiC (Si atoms surrounded by four bridging carbon atoms instead of two oxygen atoms), together with Si atoms bonded to a varying number of C atoms and C clusters (“free carbon”) [4,5]. Such nano-domains transform into distinct phases upon treatment above 1200 °C. A similar microstructural evolution is found for another well investigated class of preceramic polymers, i.e., polysilazanes, yielding a SiCN (silicon carbonitride) ceramic containing nano-sized N- and C-rich areas at low pyrolysis temperature and Si3N4 and SiC nano-sized regions after heating at high temperature. Several efforts have been devoted to the control and understanding of the phase separation occurring in the ceramic residue with increasing pyrolysis temperature, with consequent (partial) crystallization, as well as to precisely describe the microstructure of the PDCs at various stages during pyrolysis [1,2,6]. A distinctive drawback of PDC technology is the poor control of shrinkage and structural integrity of the products of the polymer-to-ceramic transformation. The transformation implies the elimination of the organic moieties typical of a polymer (e.g., methyl or phenyl groups attached to the Si atoms), with consequent significant gas release (in the form of methane, benzene and hydrogen) and shrinkage (the density goes from ~0.8–1.2 g/cm3, typical values for a polymer, to ~2.2 g/cm3, a standard value for amorphous Si-based ceramics) [1,7]. Gas release not only leads to the formation of unwanted/uncontrolled porosity, but also causes a substantial cracking of monolithic pieces [7,8]. Thin-walled components (e.g., fibers, microtubes or highly porous open-celled foams) represent a notable exception, due to the intrinsic short diffusion paths for the generated gases, leading to very limited internal pressure build-up [9]. Hot-pressing of pre-pyrolyzed material has been proposed as a solution for crack-free polymer-derived ceramics since the early applications of the technology [4], and recently, spark plasma sintering (SPS) has been successfully used to produce fully dense, nano-structured components [10–12]. Although successful, a (more or less sophisticated) hot pressing treatment needs to be employed on PDC Materials 2014, 7 1929 powders at least already partially pyrolyzed, as the generation of decomposition gases during pressing would create problems for the equipment. The pioneering work of Greil [7–9,13–16] provided a fundamental solution for obtaining crack-free, almost dense (porosity typically below 15 vol%), strong monoliths with a single ceramization step, based on a more or less pronounced modification of the chemistry of PDCs. He demonstrated, in particular, the impact of two types of solid additives, or fillers:  Inert, or passive, fillers, which are ceramic powders that do not react with the ceramic residue from the preceramic polymer, the decomposition gases or the heating atmosphere [17]. Such fillers simply dilute the preceramic polymer, therefore decreasing the amount of gas generated and the associated volume shrinkage, reducing the likelihood of forming macroscopic cracks during processing. The final ceramic has a modified chemistry in the sense that a polymer-derived matrix is accompanied by secondary phases;  Active fillers, i.e., metallic or intermetallic powders that react, dur (...truncated)