Polymer-Derived Advanced Engineering Ceramics

2.1 Polymeric precursor synthesis

The phase composition and microstructure of the polymer-derived ceramic products are closely related to the molecular type and structure of the polymeric precursors used. Therefore, the design of polymer synthesis plays a crucial role in the field of PDCs. Several organosilicon polymers have proven suitable as precursors for Si-based ceramics. As presented in Figure 1, the backbone X defines the class of the polymer, such as polysilanes with X = Si, polysiloxanes with X = O, polycarbosilanes with X = CH2, polysilazanes with X = NH, and polysilylcarbodiimides with X = [N=C=N]. The functional groups R1 and R2 attached at the silicon atoms are usually hydrogen, aliphatic, or aromatic groups, which provide opportunities to design polymer precursors at the nanoscale level [7]. For example, transition metals or other organic substituents can be introduced onto the side groups R1 and R2, allowing precise control over the ceramic yield and the carbon content in the final ceramics [25, 26]. Moreover, the solubility and rheological properties of polymers, which are crucial features for subsequent processing, can also be finely controlled by the side groups [27].

The chlorosilanes (RxSiCl4−x, x = 0–3, R = organic group), generally formed as byproducts in the silicone industry, are the most frequently used raw materials for the synthesis of Si-based precursors because of their high reactivity, commercial availability, and low cost [2]. As shown in Figure 1, the synthetic route for Si-based precursors typically involves catalytic dechlorination of organic chlorosilanes, assisted by catalysts such as alkali metals (like lithium, sodium, potassium), ammonia, amines, or water [24].

2.2 Shaping and crosslinking

Due to the polymeric nature of preceramic precursors, they can be readily shaped and fabricated into various forms, including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) ceramic products [2, 7]. It should be noted that the polymeric precursor can exist in various forms: as a cross-linkable liquid, a meltable and curable solid, or an unmeltable but soluble solid [28]. This inherent flexibility in processing allows for a wide range of forming techniques, including extrusion, molding, and casting, making it possible to produce complex ceramic components. One of the advantages is that pre-ceramization machining prevents issues such as tool wear and brittle fracture during component finishing, ensuring a higher-quality final product with improved structural integrity and dimensional accuracy [29].

Importantly, to maintain its shape, the precursor must undergo crosslinking to transform into a thermoset after shaping, which helps to minimize weight loss during ceramization [30]. Thermal cross-linking is the predominant mechanism, typically occurring within the temperature range of 200 to 400°C [31]. During this process, cross-linking reactions take place between functional groups, including Si–H, Si–OH, N–H and vinyl/allyl substituents, leading to the formation of a three-dimensional network structure [32]. Other techniques like oxidative processes, γ-radiation, electron-beam irradiation, and reactive gases or plasma can also be used to initiate cross-linking reactions [21, 33]. In particular, UV-crosslinking can occur when there are photo-sensitive functional groups present on the polymeric precursors [34]. Besides, selective laser curing is often used in 3D printing processes, enabling the fabrication of complex and intricate 3D structures [35]. One can select the appropriate method for cross-linking based on the molecular structure of the polymeric precursor and the desired properties of the final ceramic derived therefrom. Noteworthy, careful control of the extent of cross-linking is crucial because it significantly influences the rheological behavior of preceramic polymers, such as their spinnability for fiber production and uniformity in coating fabrication [36, 37].

2.3 Pyrolysis and crystallization

Generally, the thermal treatment (pyrolysis) occurs after crosslinking to realize the polymer-to-ceramic transition, which is typically completed at temperatures between 400 and 1100°C [7, 15]. During the pyrolysis, the rearrangement and radical reactions occurring above 400°C lead to the cleavage of chemical bonds and the release of hydrogen, low-weight oligomers, and hydrocarbons, consequently forming amorphous ceramic materials. A flow of inert (Ar, N2) or reactive (NH3, H2, CO2, Ar + H2O) gas is often used. On the one hand, the inert gas can protect the material from oxidation or other possible reactions. On the other hand, the pyrolysis atmosphere plays an important role in adjusting the phase composition of resultant ceramics [38]. For instance, reactive gas NH3 is commonly used in the Si–N, Si–B–N, and Si–M–N systems, as it can effectively remove residual carbon introduced by the solvent and increase the nitrogen content in the final product [39]. The thermal decomposition accompanied by gas release results in a weight loss typically of 10–30%. It is noteworthy that high weight loss can cause substantial shrinkage in the final ceramic components. To enhance ceramic yield, efforts can focus on optimizing the elemental composition and thermal stability of the preceramic polymer, adjusting the structural arrangement (branched, ring, or linear) within the polymer backbone, improving the effectiveness of crosslinking, and incorporating suitable fillers [40]. Recently, ceramic yields as high as 90% have been reported in the literature [41, 42].

Figure 2 schematically summarized the temperature range for the transformation of amorphous to crystalline in PDC technology [15]. At temperatures exceeding 1100°C, the amorphous phase undergoes crystallization and decomposition, leading to the formation of multiphase crystalline structures [43]. The stability of the amorphous phase is primarily influenced by the addition of other chemical elements such as carbon, boron, aluminum, and transition metals (e.g., Hf, Zr, Ta) [44, 45]. These added elements form a percolation network within the amorphous matrix, acting as diffusion barriers that prevent local crystallization and maintain the size of nuclei below the critical radius. The nucleation and growth of the crystalline grains from the amorphous matrix occur at higher temperatures (above 1300°C) due to the increase in atomic mobility and the lower energy state of the crystalline phase compared to the amorphous phase. In addition, carbothermal reduction of SiO2 and Si3N4 takes place at elevated temperatures due to the existence of free carbon in SiOC and SiCN ceramics, which leads to the formation of SiC and a further weight loss [46].

3.1 Polymer-derived SiOC

SiOC is characterized as an amorphous material, with a significant amount of carbon incorporated into its silica glass structure [47]. The silicon atoms are tetrahedrally coordinated by oxygen and carbon atoms, forming SiO4C4−x units. The substitution of divalent oxygen atoms with tetravalent carbon atoms increases the bond densities in the system, which explains the enhanced physical properties such as viscosity, elastic modulus, and hardness when compared to vitreous silica counterparts [48]. At pyrolysis temperatures approximately 1100°C, polymer-derived SiOC ceramics remain largely amorphous. However, when advanced, high-resolution analytical techniques are applied, polymer-derived SiOC is found to exhibit nanoscale heterogeneity, due to its typical non-stoichiometric nature and inherent presence of free carbon phases within its structure. Nanosized phases or nanodomains that can exist within the amorphous SiOC matrix include amorphous silica (a-SiO2), free carbon (Cfree), amorphous or crystalline silicon carbide (SiC), and interfaces of SiO4C4−x [49].

It is noteworthy that the carbon content in polymer-derived SiOC can be easily adjusted by designing the molecular structure of the polymer precursor. For instance, varying the ratio of triethoxysilane (TH) and methyldiethoxysilane (DH) monomers during polymer synthesis allows for precise control over the carbon content [47]. Differences in microstructure can also be observed when varying the carbon content, as displayed in Figure 3a [50]. The microstructure exhibits graphitic carbon nanodomains represented by dark-shaded circles, surrounded by C-rich SiO4C4−x structural units depicted in light gray regions. The oxygen-rich SiO4C4−x structural units are shown as the gray patterned matrix. It is noted that a high content of carbon in the pre-ceramic polymer leads to the percolation of the graphitic carbon regions and the C-rich structural units within the ceramics. Moreover, by controlling the carbon content to low levels (i.e., ≤ 8 wt%), it is possible to achieve nearly stoichiometric SiOC, which facilitates the investigation of microstructural evolution of polymer-derived SiOC [51]. It is reported that the phase separation starts when the annealing temperature is up to 1300°C, due to the redistribution of the Si–O and Si–C bonds. Upon further annealing at higher temperatures (1400–1600°C), nanosized β-SiC phases form and grow due to carbothermal reactions, leading to a decrease in the free carbon content [52].

3.2 Polymer-derived SiCN

In the SiCN system, a completely different microstructure is observed depending on the backbone structure of the polymeric precursor used: polysilazanes (X = N) or polysilylcarbodiimides (X = [N=C=N]), as presented in Figure 3b [53]. Polysilazanes produce a single-phase amorphous SiCxNy structure characterized by mixed Si–C–N bonding and localized areas of free carbon [54]. In this structure, silicon maintains its tetrafunctional coordination, allowing for mixed bonding with nitrogen and carbon atoms to form SiC4N4−x configurations. Despite some variations in microstructural evolution caused by the presence of linear or cyclic units in polysilazanes, the microstructure after pyrolysis at 1000°C can generally be depicted by the left picture in Figure 3b. However, SiCN ceramics derived from polysilylcarbodiimides form a multiphase amorphous system primarily consisting of amorphous Si3N4 and carbon clusters, with little to no presence of SiC4N4−x [55]. The formation of the amorphous SiC phase requires a sufficiently high carbon content. Compared to polysilazane-derived SiCN, the development of particular amorphous structures inhibits the processes of crystallization and phase separation. In addition, the backbone structure of polysilylcarbodiimides (whether branched or linear) influences the structural evolution during pyrolysis. For example, a branched poly(phenylsilsequicarbodiimide) forms a nanostructure with regions containing free carbon and amorphous Si3N4 (a-Si3N4), alongside an interfacial region characterized by C–N bonds. Instead, a linear poly (vinylsilylcarbodiimide) produces a nanostructure with dispersed a-Si3N4 surrounded by interconnected free carbon phase [54].

Similarly, after pyrolysis at 1100°C, polymer-derived SiCN ceramics remain mainly amorphous with some nanodomain structures. The onset of crystallization occurs within the temperature range of 1300–1500°C, varying depending on the C/Si ratio in the system. It has been reported that the thermal stability against crystallization can be enhanced with an increase in the C/Si ratio [56]. In this stage, the coarsening of nanodomains and the transformation of amorphous Si3N4 into α-Si3N4 can also be observed. The presence of the free carbon phase has been demonstrated to restrict the formation of the α-Si3N4 phase by acting as a diffusion barrier [57]. Upon thermal treatment at higher temperatures (1600–2000°C), many of the Si3N4 phase can undergo transformation into SiC phase due to carbothermal reaction, accompanied by the release of N2 gas [58]. Above 1700°C, all Si-based phases become fully crystalline, typically with a carbon matrix embedded with α- and β-SiC phases. Besides, many researchers have recently been interested in introducing boron or transition metals (M = Hf, Zr, Ta) into polysilazanes or polysilylcarbodiimides to form quaternary SiBCN or SiMCN systems, in order to increase the nitrogen content in the final ceramic products and improve high-temperature properties. Results show that the formed intergranular nanodomains BCxNy and MCxNy contribute a lot to the thermal stability, oxidation resistance, and high-temperature mechanical properties [59].

4.1 Dense ceramic bulks

Polymer-derived ceramic monoliths based on SiC and Si3N4 exhibit significant potential for applications in high-temperature structural components, as well as in grinding and cutting operations [60]. Compared with conventional sintering using binders to promote densification of green pellets, which requires burning out during processing and inevitably introduces impurities, preceramic polymer binders offer several advantages [61]. They enhance green density significantly, particularly with powders that have low packing density. Upon pyrolysis, these binders transform into ceramics with extremely small particle sizes, which facilitate sintering without the need of external additional aids. Besides, incorporating inert or reactive fillers into the polymer matrix is effective in minimizing pyrolysis shrinkage. This approach allows for achieving linear dimensional changes of less than 0.1% through precise control of time and temperature during partial pyrolysis of complex components. This process results in ceramic products, especially with complex shapes, exhibiting enhanced density and thus improved mechanical properties [2, 20].

Specifically, to produce dense, crack-free, and stoichiometric SiC bulks using the PDC route, four main steps are typically followed in PDCs: obtaining amorphous powder through pyrolysis; shaping using warm-pressing techniques; ceramization of the shaped green bodies through heat treatment; and liquid precursor infiltration and pyrolysis (PIP). Recently, dense SiC-based monoliths have also been prepared using hot-pressing (HP), spark plasma sintering (SPS), and 3D printing techniques [62, 63, 64]. Wen et al. [64] sintered boron-containing HfC/SiC-based ceramics using SPS. Incorporating boron results in the formation of low-viscous borosilicate, which facilitates the rapid formation of a continuous, dense scale upon oxidation, thereby providing excellent oxidation resistance over a wide temperature range (from 20 to 1500°C). Li et al. [41] prepared additive-free amorphous bulk polymer-derived SiHfN ceramics by using warm-pressing followed by ammonolysis and annealing. The critical issues related to gas evolution and crystallization, which can lead to bloating and cracking, were addressed through controlled thermolysis and pressure management. The SiHfN ceramic achieved a hardness of up to 19.7 GPa through warm-pressing at 120 MPa, followed by annealing the green sample at 1300°C. Xiong et al. [62] successfully prepared crack-free dense monolith and lattice skeleton structural polymer-derived SiOC ceramics by vat photopolymerization additive manufacturing. As displayed in Figure 4, the pyrolyzed samples exhibited relatively uniform shrinkage and crack-free structures because the introduction of phenolic resin (PR) enabled a smooth gas-release process by establishing ordered channels within the sample bodies during pyrolysis. The introduction of PR can enhance not only high-temperature stability but also mechanical properties.

4.2 Ceramic fiber and fiber-reinforced ceramic matrix composites (CMCs)

Ceramic fibers, particularly those based on SiC, have stood out as the most successful commercial application of PDCs since the last century. Ceramic fibers have paved the way for the development of fiber-reinforced ceramic matrix composites (CMCs), which have attracted considerable attention for their applications in high-temperature environments where the metallic materials suffer serious corrosion and mechanical property deterioration [23]. Therefore, ceramic fibers must possess superior mechanical and chemical properties to withstand the demanding operational environments of turbine engines, rocket nozzles, and furnaces. The polymer-derived ceramic fiber process has enabled the successful production of advanced ceramic fibers spanning from nanometers to micrometers in diameter [36].

To produce fibers, typically, a spinning step, either melting spinning or electrospinning, needs to be incorporated. Therefore, suitable rheology of the polymer precursor, both in its molten state and in solution, is essential for processes. Maintaining green fibers under tension during the polymer-to-ceramic conversion is crucial to prevent crimping of the fibers due to thermal shrinkage, and all stages of the process are carried out in a controlled atmosphere (either inert or reactive). For example, SiC-based ceramic fibers are commercially manufactured by melt spinning and widely employed as heat-resistant materials, as well as for reinforcing ceramic matrix composites (CMCs). SiC-based ceramic fibers are derived from polycarbosilane or polymetallic ocarbosilane precursors and have undergone the stages of development according to their chemical composition, oxygen content, and C/Si atomic ratio [65]. Amorphous SiBCN fibers have also been successfully prepared using polyborosilazane precursors [66]. As-prepared amorphous SiBCN fibers with a dense and smooth texture show high thermal stability in a N2 atmosphere of up to 1600°C. Electrospinning is also extensively studied for producing fibers with precise control over size and morphology, such as novel BN fibers and core-shell nanofibers [67]. This method offers a versatile and controllable approach to creating nanoscale ceramic fibers, facilitating the integration of functional materials with fibers and enabling new applications to be explored.

Fiber-reinforced CMCs are fabricated by encapsulating carbon fibers (Cf) or ceramic fibers within a dense ceramic matrix. The matrix phase can consist of a single or multiple phases with compositional variations. The dense matrix is typically formed using three primary techniques: chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP), and reactive melt infiltration (RMI) or liquid silicon infiltration (LSI) [23]. Among them, the PIP method offers several advantages, including simplified and cost-effective equipment requirements, lower processing temperatures that maintain the strength of ceramic fibers, shorter cycle times, enhanced infiltration depth, improved uniformity, and the capability to produce large and intricate components. The well-known fiber-reinforced CMCs, for example, C/SiC, SiC/SiC, and C/C-SiC, are extensively used as hot structures for aerospace applications, like aero-engine combustor liners, ducts, nozzle flaps, acoustic liners, turbine vanes, turbine blades, and aircraft brakes. For example, using C/SiC composites as materials for combustion chambers and nozzles can elevate the maximum combustion temperature to above 1650°C [68]. Fiber-reinforced SiOC-based composites are also explored due to its low cost. The matrix in C/SiOC composites comprises nano-crystallites of SiC embedded within an amorphous SiOC matrix, which exhibits good oxidation resistance [69]. However, at elevated temperatures, the SiOC matrix suffers from poor creep resistance, primarily due to the presence of silica nanodomains. Introducing boron to the SiOC matrix can improve the oxidation resistance, as the SiBOC matrix forms stable borosilicate glass and inhibits crystallization during oxidization [70].

4.3 Ceramic membranes, coatings, and adhesives

Ceramic membranes and coatings can be prepared using polymer precursors through liquid phase deposition (e.g., dip coating, spray coating, spin coating) followed by pyrolysis [46]. This method is easier to handle and more cost-effective compared to physical vapor deposition (PVD) and chemical vapor deposition (CVD). Generally, thin layers of membranes are deposited by coating them onto tubular asymmetric porous ceramic substrates. Polymer-derived SiOC membranes demonstrate high permeability to hydrogen over carbon dioxide, showcasing molecular sieving capabilities for separating H2/CO2 [71]. Polymer-derived SiBCN membranes offer significant advantages over commercial alumina membranes, including ease of manufacturing, high thermal stability, and excellent oxidation resistance, making them suitable for high-temperature separation applications [72].

Polymer-derived ceramic coatings are extensively utilized as protective layers on metals or CMCs to prevent oxidation when used as hot structural components in the aerospace industry [23]. For example, ceramic-like gradient Si(C)N coatings can form on stainless steel sheets, which effectively shield it from oxidation up to 1000°C [73]. The gradient Si(N)CrO layers act as a diffusion barrier against oxygen, resulting in a significant reduction of both weight gain and the oxidation rate by up to two orders of magnitude. However, during the pyrolysis of precursors into ceramics, porosity and residual stress may occur due to shrinkage and decomposition. To tackle these issues, passive fillers such as BN, ZrO2, and Al2O3, as well as active fillers like ZrSi2 and TiSi2, can be incorporated to minimize shrinkage during the pyrolysis of ceramic precursors. This approach enables the creation of high-performance environmental barrier coatings capable of achieving substantial thicknesses, up to 100 μm [74]. Alternatively, another effective solution is the design of a double-layer coating system with a PDC bond coat and a glass/ceramic filled PDC topcoat [75]. The PDC bond coat plays a crucial role by shielding the metallic surface, enhancing adhesion between the surface and the topcoat, and reducing the coefficient of thermal expansion (CTE) mismatch between the metal and the topcoat. Apart from metallic components, it is also important to develop protective coatings for CMC components [76]. For example, C/C composites undergo degradation above 500°C in an oxidizing environment. Despite C/SiC, C/C-SiC, and SiC/SiC composites exhibiting good oxidation resistance, they still necessitate protective coatings to prolong their service life. The SiC-coated C/C composite experiences approximately one-fourth the mass loss compared to the uncoated C/C composite, after 50 minutes of exposure to an oxidizing environment [77].

In many cases, it is more cost-effective to construct complex shapes by assembling simple geometric forms. Ceramic adhesive bonding is the most cost-effective method of joining without significant energy wastage, and it does not cause severe damage to the substrates [78]. Therefore, high-temperature adhesives are crucial structural materials for aerospace and nuclear power applications, where they must offer strong bonding capabilities in elevated temperature conditions. NASA has developed an adhesive called Non-Oxide Adhesive eXperimental (NOAX), which blends a sophisticated formulation of preceramic sealant (hydridopolycarbosilane) with silicon carbide powder and additional additives. This adhesive meets NASA’s stringent requirements for space missions [79]. It is applied to fill cracks or gaps in protective coatings measuring up to 0.5 mm wide and 100 mm long. Following exposure to temperatures exceeding several hundred degrees, NOAX achieves a semi-rigid state and reaches complete hardening only upon encountering the extreme heat during reentry into Earth’s atmosphere.

4.4 Microelectromechanical systems (MEMSs) and sensor

While silicon-based MEMSs have been applied in sensors and actuators, their performance is frequently constrained by poor thermal stability above 250°C and mechanical stability above 600°C [80]. Certain PDCs, such as SiOC, SiCN, and SiCNO, also exhibit semiconducting properties, primarily attributed to the presence of graphitic carbon and other nanodomains within the amorphous matrix. Reportedly, both SiCN and SiOC ceramics exhibit remarkable piezoresistive properties, with exceptionally high coefficients observed along both longitudinal and transverse directions, even under elevated temperatures [81, 82]. According to Ryu et al., semiconductive properties of SiCNO PDCs were measured and confirmed up to 1300°C without dopants. The band-gap of SiCNO PDCs decreased from 2.2 to 0.1 eV as the annealing temperature was raised from 1100 to 1400°C [83].

Additionally, the semiconductive properties of PDCs can be readily tuned by controlling their microstructure. SiC, with its wide bandgap semiconductor properties, has been proposed as a solution to address temperature limitations in Si MEMSs for high-temperature sensing applications. Its superior resistance to corrosion, excellent thermal stability, chemical stability, and mechanical performance have sparked considerable interest in high-temperature semiconductor applications [84]. Figure 5 illustrates the shaping process and microscopic features of PDC MEMSs, demonstrating how the polymer route utilizes micropatterning to shape liquid precursors [80].

Due to their temperature-dependent electrical resistances, PDCs have shown promise for use as flux sensors or temperature sensors in harsh environments. As an example, Nagaiah et al. engineered a high-temperature heat flux sensor for gas turbine engines utilizing polymer-derived SiCN ceramics [85]. This advanced sensor shows a temperature coefficient of resistance of 4000 ppm/ºC and maintains extended functionality at 1400°C, surpassing conventional heat flux sensors. In addition, amorphous SiCN-based ceramics have demonstrated effectiveness in hydrogen (H2) sensing at temperatures as high as 500°C, attributed to their semiconducting nature. Thinner layers of SiCN ceramics exhibit heightened sensitivity in measuring resistivity upon exposure to adsorbed hydrogen molecules [86].

4.5 Porous ceramics

Porous ceramics, such as cellular ceramics/ceramic foams, ceramic sandwich structures, and ceramic aerogels, are ideal for lightweight thermal protection systems (TPS) [87]. Cellular ceramics are ceramic materials known for their substantial porosity, with hollow cells that can vary in repetition and shape. These cells can be regular in size and form, akin to a honeycomb structure, or irregular, resembling a foam structure. Using preceramic polymers to produce cellular ceramics offers several benefits, such as controlling the composition of ceramics, adapting to new processing techniques, and easily machining the precursor foam followed by post-curing. The fabrication of ceramic foams using preceramic polymers can be achieved through three different methods: (i) replica method, (ii) sacrificial template method, and (iii) direct foaming, as illustrated in Figure 6 [88]. For example, SiOC foams typically undergo a decrease in compressive strength at elevated temperatures, mainly due to carbothermal reaction [89]. In contrast, SiC foams can maintain their strength under similar conditions due to the absence of carbothermal reaction [90]. Derived from polycarbosilane, SiC foams demonstrate an enhanced load-bearing capability through increased strut density, attributable to a reduction in pore size. Besides that, ceramic aerogel, with its ultra-low density and low thermal conductivity, is highly sought after as a thermal protection system (TPS) material for space applications [91].