A Novel TiO2–TiC–TiC0.3N0.7–C–SiCN Multiphase Ceramic Nanocomposite from Preceramic Polymer Pyrolysis

The current investigation describes the synthesis of SiTiCNO ceramics derived from the mixture of a preceramic polymer (polyvinylsilazane) and tetrabutyl orthotitanate precursors by crosslinking at 300 °C and pyrolysis in the temperature range of 900–1400 °C in flowing nitrogen atmosphere. Crosslinked precursor was studied by thermogravimetry to estimate ceramization temperature as well as ceramic yield. Further, the evolution of phase and nanostructure with temperature in the composite SiTiCNO ceramics was analyzed by the help of different characterization techniques, such as XRD, Raman, and electron microscopy. The Ti-doped SiCN ceramic system appeared as single-phase SiTiCNO amorphous up to 1100 °C. The phase separation of SiTiCNO ceramics started at 1200 °C and exhibited TiO2 nanocrystals distributed in the amorphous SiCN matrix. Ti-doping was found to accelerate the separation of the free carbon phase from the SiCN matrix, and the said carbon had better graphitic order in the Ti-doped SiCN samples as compared to the undoped SiCN of equivalent thermal history. At 1400 °C, high temperature stable phases such as TiC and TiC0.3N0.7 were formed along with predominant rutile-TiO2 phase within the Si–Ti–O–C–N composite. A uniform distribution of these nanocrystals in the SiCN matrix at 1400 °C was observed by high resolution transmission electron microscopy. The current work exhibits the formation of a unique multiphase composite with the co-existence of nanocrystalline phases uniformly distributed within a polymer derived ceramic matrix.


Introduction
SiC or C-based fiber-reinforced ceramic matrix composites (CMCs) are preferred materials in the nose area or leading edges in hypersonic reusable launch vehicles, and modern gas turbine engine components [1]. These CMCs possess low weight, great mechanical property and can sustain ultra-high temperature (UHT) [2]. However, these CMCs face cataclysmic failure due to the disintegration reaction with combustion products (e.g., oxygen, steam, or salts), when exposed in combustion chamber environments, and require an environmental barrier coatings (EBC) [1,3,4]. Present work is aimed towards development of such a silica forming UHT ceramics through polymer derived ceramics (PDC) route, which could be applied on CMCs as an EBC material. Silicon-based polymer derived ceramic (PDC) nanocomposites have recently gained a lot of attention due to their low processing temperature, excellent thermal stability and thermomechanical properties, and excellent resistance towards oxidation and corrosion [5][6][7][8][9][10][11]. Such exceptional properties, along with the ability of being shaped by various fabrication techniques, have enabled its application in numerous sectors, including environmental systems, biomedical components, anode material in lithium batteries, aerospace and defense [12]. Previous investigations have evidenced that the nanocomposites of PDC derived SiCN ceramics and a filler such as glass [10,13], ZrO 2 [13], YSZ [14], SiC [15], BN [16], or TiB 2 [17] exhibit improved high temperature properties than the pure SiCN ceramics. These fillers can make considerable improvements in the different properties (thermal, mechanical, electrical, optical, etc.) of SiCN with improved homogeneity of the components within the amorphous matrix at the nanoscale [18][19][20][21], since the properties of the nanocomposites mostly depend on their phase assemblage and microstructure [7,22,23]. Therefore, initial precursors and high temperature processing conditions can have a profound influence on the final properties of polymer-derived ceramic 1 3 (PDC) nanocomposites [24,25]. Synthesis of such nanocomposites has been performed through the pyrolytic conversion of single source polymeric precursors. For instance, different metal precursors of Al [26,27], Y [28], Ti [29,30], Zr [31][32][33], and Hf [34] were used to transform the molecular structure of the preceramic precursors that exhibited altered microstructures of SiCN ceramics upon pyrolysis. These metals nucleate and crystallize in-situ as metal oxides, nitrides, carbides, or silicates within the SiCN matrix during the thermal treatment at temperatures above 1000 °C, and form uniform nanostructured composites [35]. PDC modified with the transition metal elements (Zr, Hf, Ta, and Ti) show few C free with completely different molecular structure [36,37]. For instance, Zr doping to SiCN leads to the precipitation of t-ZrO 2 phases, which reinforces the nanocomposites, thus enhancing thermal stability towards crystallization, oxidation, and corrosion even at temperatures above 1300 °C [32,33,38] B-doping into SiCN ceramic resulted in the high temperature stability towards crystallization up to temperature as high as 2200 °C [19,39]. Further, incorporation of B into SiZrCN ceramics diminishes the crystallization of ZrC x N y and suppress the reaction of SiN x with C, resulting in improved thermal stability of SiZrBCN as compared to SiZrCN ceramics [40]. Hf-doping to the SiCN matrix improved its thermal stability as well as oxidation and corrosion resistance [9,18,41]. A new class of SiC/ Hf y Ta 1−y C x N 1−x /C nanocomposite with improved oxidation resistance was synthesized at 1900 °C, and the composition as well as thickness of the HfC x N 1−x and TaC x N 1−x core shell structures could be tuned with varying the Hf:Ta atomic ratio [36]. Further, Ti-doping through Ti-isopropoxide precursor within the silazane precursor resulted into anatase precipitation within the Si-C-N matrix at 1000 °C, which eventually transformed into rutile above 1200 °C [42]. The current work is aimed towards development of such a Si/Ti-based nanocomposite ceramics by altering the polyvinylsilazane precursor with a molecular precursor of Ti through PDC route. The transformation of the precursor polymer to ceramic was studied through thermogravimetry (TG) and differential scanning calorimetry (DSC). The phase and nanostructural features of the pyrolyzed SiTiCNO ceramics were investigated through the Raman spectroscopy, X-ray diffraction (XRD), and high resolution transmission electron microscopy (HRTEM). The formation of a unique multiphase nanostructured composite has been demonstrated from the pyrolysis of a Ti-doped polyvinylsilazane precursor polymer.

Materials and Methods
A c o m m e r c i a l ly ava i l a b l e p o ly v i nyl s i l a z a n e (Durazane®1800, Merk Chemicals) precursor was doped by Ti using tetrabutyl orthotitanate (Ti(OC 4 H 9 ) 4 , Alfa Aesar, USA) as a metal precursor. The molecular structure of polyvinylsilazane (PVS) used comprises 20% methyl/ vinyl and 80% of methyl/hydride silazane based monomers ( Fig. 1). Tetrabutyl orthotitanate was dissolved in ethanol and instantly mixed with the PVS solution in a ratio of Ti:Si as 0.2 by mol%. Also, 1wt% of dicumyl peroxide (Sigma-Aldrich®, India) was added for easy crosslinking of the mixed solution. The crosslinking reaction was carried out at 300 °C for 2 h in a controlled box furnace having a vertical glass column attached with the flowing N 2 source. The key crosslinking reaction involves combination of Ti with Si via O bond, hydrosilylation between vinyl and Si-H groups and polymerization of the vinyl groups [42,43]. The crosslinking process resulted a thermoset (pale yellow solid mass), which was milled in fines (Fig. 2a) using a High-energy mechanical milling (SPEX 8000 M SamplePrep, USA) by means of zirconia pot and ball.
The resultant powder (termed PVS-Ti(b)-300) was heat treated in a tube furnace at high temperatures ranging 900 °C to 1400 °C for 2 h, with a uniform heating rate of 2 °C min −1 and nitrogen flow rate of 80 ml min −1 . The resultant samples were dark colored powders, termed as PVS-Ti(b)-900   Figure 2b) and 1400 °C (cf. Figure 2c), respectively.
Polymer to ceramic transformation and mass loss of crosslinked PVS-Ti(b)-300 sample during ceramization was determined by thermogravimetric analysis (Netzsch, Germany, STA/TG-DSC) under flowing nitrogen. The calorimetric aspects of the crosslinked sample during ceramic conversion reactions, phase formation and crystallization phenomena were also studied by differential scanning calorimeter (DSC) attached with TG analyzer. The dissolution of Ti into ceramic matrix of pyrolyzed Ti doped PVS sample were confirmed through energy dispersive spectroscope (EDS) data obtained through scanning electron microscopy (SEM, FEI, Nova NanoSEM, Eindhoven, NL) by applying 15 kV accelerating voltage to the emission electron gun. Further, all the ceramic samples were scanned in the 1000 cm −1 to 2000 cm −1 spectral range through PL micro Raman spectrometer (XMB3000, WITec GmbH, Germany) having an Ar laser wavelength of 532 nm. The acquired Raman spectra were analyzed to understand the structural evolution of the free carbon (C free ) phase within the ceramic matrix at different pyrolysis temperatures. Also, the pyrolyzed samples were evaluated by an X-ray diffractometer (Rigaku Ultima IV, Japan) using Cu-Kα radiation in the 2θ range of 10° to 80° at a scan rate of 20° min −1 to understand the phase evolution in the ceramic. The evolved nanostructure within the SiTiCNO ceramic matrix was observed by bright field and high resolution imaging using a transmission electron microscope (TEM, FEI Tecnai G 2 80-300) at an accelerating voltage of 200 kV. The sample for TEM observation were prepared by dropcasting a uniformly dispersed solution of the ceramic powders in isopropanol on a carbon coated copper grid (300 mesh, Ted Pella, USA). The HRTEM micrographs were analyzed by a Java-based image processor "ImageJ" to identify the nanostructured phases. The lattice fringes were obtained by applying fast Fourier-transform (FFT) algorithm on a selected area over the HRTEM micrograph and the fringe width were calculated from the inverse of the FFT (IFFT) image.

Results and Discussion
The thermal behavior of the crosslinked Ti-modified polyvinylsilazane precursor was studied up to 1500 °C and shown in TGA curve (Fig. 3). The PVS-Ti(b)-300 sample shows no loss up to 300 °C and a continuous mass loss of 20.65% up to 750 °C. The cessation of mass loss after 750 °C indicated that the polymer to ceramic transformation was complete at this temperature, with the mass loss over the 300-750 °C temperature range attributed to the evaporation of H 2 , carbon in the form of CH 4 , and other hydrocarbons (C n H m ) during the decomposition of the polymer precursor [44].
A further small mass loss of ~ 1% in the temperature range of 1100-1500 °C was seen, which may be due to the loss of hydrogen or excess oxygen attached to the Ti precursor [20,38]. The total ceramic yield of the sample was found to be 78.2 wt%, which is in good agreement with the theoretical yield (71 0.1 wt%, where yield of pure PVS was considered as 83 wt%) of the selected composition for Ti-doped PVS in this work. The DSC thermogram of the sample (Fig. 3) shows a slow and constant rise of the curve up to 745 °C, which is due to the continuous transformation of the sample from polymeric to glassy state. Additionally, an exothermic peak at around 1328 °C as well as an endothermic peak at 1445 °C appeared, which may be due to the development of new phases and their crystallization within the SiCN matrix. Figure 4a shows SEM micrograph illustrating the morphology of the ceramics obtained from pyrolysis of the crosslinked powders at 1400 °C for 2 h. Though the SEM micrographs do not show any nanostructural features within the ceramics, the superior thermal stability of the SiTiCNO ceramics can be observed with glassy morphological appearance even at 1400 °C. Further, the EDS map images shown in Fig. 4b-f of the PVS-Ti(b)-1400 sample indicate the presence of Si, Ti, C, N, and O elements in the ceramic structure. The pyrolyzed SiTiCNO ceramics consists predominantly of Si, O, C, N, and the Ti distribution appears uniform throughout the ceramic microstructure.
The different phases evolved during different stages of pyrolysis were identified by analyzing XRD patterns of PVS-Ti samples of different thermal history (powders pyrolyzed in the temperature range of 900 °C to 1400 °C, cf. Figure 5). The PVS-Ti(b)-900, PVS-Ti(b)-1000, and PVS-Ti(b)-1100 samples were found to be completely amorphous and monophasic. The broad hump ranging from 2Ɵ = 17° to 28° shows Si-O based bonds present in the SiCN structure.
This occurs due to the substitution reaction between Si-H site of PVS precursor and Ti-O of tetrabutyl orthotitanate, which forms Si-O-Ti bond. Also, the hump may be of Si-O based bonds occurred due to oxygen adsorbed from the atmosphere due to the high affinity of silicon for oxygen [45,46]. Therefore, the hump remains present in all pyrolyzed samples and indicates their amorphous nature.  Thus, the PVS-Ti(b)-1400 sample was essentially found to be consisting of multiple phases including, rutile-TiO 2 , TiC, TiC 0.3 N 0.7 , and the nanodomains of SiO 2 in the ceramic.
The XRD-patterns were fitted using Gaussian function after background subtraction, and peak widths (full width at half maximum, FWHM) were calculated. The crystallite size of different evolved phases in different samples was calculated by Debye-Scherrer equation (cf. Table 1). Size of the anatase-TiO 2 was found to be 6.1 nm for the PVS-Ti(b)-1200 sample, which eventually grew to 9.9 nm and 10.4 nm for the PVS-Ti(b)-1300 and PVS-Ti(b)-1400 samples, respectively. Rutile and TiC 0.3 N 0.7 precipitates of 2.2 nm and 2.7 nm in the PVS-Ti(b)-1300 composite coarsened to 13.6 nm and 3.2 nm in the PVS-Ti(b)-1400 sample, respectively. From these data one can assess that the TiC 0.3 N 0.7 phase was relatively coarsening resistant as compared to rutile. The TiC nanocrystals were of 4.6 nm in the SiTiCNO ceramics synthesized at 1400 °C. (1) The C free phase within the PDC influences their high temperature properties such as crystallization and oxidation resistance [21,49,50]. Thus it is important to understand the evolution of C free in the prepared SiTiCNO ceramics. Figure 6 shows Raman spectra of the SiTiCNO ceramics prepared at different temperatures, which clearly exhibits the presence of D and G bands of graphitic carbon. The D band appears due to the out of plane vibration caused by laser scattering from local defects present in C free , whereas the G band is a result of in-plane vibrations of sp 2 bonded carbon atoms caused by tangential stretching of the sp 2 bonded carbon in the graphitic structure [51,52]. The Raman spectra of few undoped samples (synthesized in the similar conditions) were also presented in Fig. 6 for comparison purpose. The D, G peak positions, their width, and the size of C free were calculated (cf. Table 2) after the baseline correction of spectra followed by the peak fittings using Lorentzian function.
The D and G peaks were observed in the SiTiCNO samples over the spectral range of 1326-1340 cm −1 and 1589-1614 cm −1 , respectively (Table 2). In the Raman spectra of pure SiCN samples (PVS-1000 and PVS-1400), the D and G peaks appeared at ~ 1330 and ~ 1620 cm −1 . Presence of these D and G Raman bands for all the pyrolyzed PVS-Ti(b) samples suggests that the formation of the C free phases started at earlier temperature (even at 900 °C) in the Ti-doped SiCN ceramics than the undoped SiCN ceramics. It could be concluded that Ti doping in the SiCN matrix accelerates formation of the free carbon phase (C free ).
Initially blue shift of the G bands was observed for the SiTiCNO samples pyrolyzed up to 1200 °C. The lack of meaningful signal intensity of the G peak in the PVS-1000 sample is attributable to fluorescence, which indicates substantial polymeric existence and considerable amount of hydrogen still being present in the pyrolyzed ceramic. Therefore, it could be concluded that amorphous carbon nanostructures dominate in the samples pyrolyzed up to 1200 °C. The ratio of integral intensity of D and G peaks increases for high temperature pyrolysis (cf. Table 2). The G peak from   Table 2) indicating graphitization of the C free . [53]. However, in the Raman spectrum of the undoped PVS-1400 sample, there were no major shifts observed in the D and G peak position. Nevertheless, the D and G peaks were sharper for the PVS-1400 samples than those of PVS-1000, which could be attributed to the graphitization of the C free present in SiCN ceramics at 1400 °C. The FWHM of G peak for the PVS-1400 spectrum is larger than the corresponding values of G peak widths for all the pyrolyzed Ti-doped PVS samples (cf. Table 2). Similar observations can be made for the I D I G values of PVS-Ti(b)-1400 and PVS-1400 sample. Therefore, it can be surmised that Ti doping in the SiCN matrix promotes ordering of the C free phase. There were no major changes in the D peak widths of the doped and undoped samples. Further, residual C free knot size ( L a ) in the SiTiCNO ceramics were calculated (cf. Table 2 for values) by using Tuinstra-Koenig relation (Eq. 3) [54] For the laser beam used in this study (laser wavelength, λ = 532 nm), C is ~ 4.95 nm [55,56]. No major change in the dimensions of the nanocrystals of residual carbon between the undoped and doped samples when the pyrolysis temperature was 1200 °C or higher. The sample PVS-1000 and PVS-Ti(b)-1000 showed a larger L a than the high temperature pyrolyzed samples. The main reason behind it is the pyrolysis temperature (1000-1100 °C), which is apparently low to eradicate remaining hydrogen attached with the carbon [21,42,57]. It is interesting to note that though the carbon segregates at earlier temperatures in the SiTiCNO matrix, the lateral size of carbon nanocrystals remain fine even at 1400 °C (~ 2.6 nm size range). Therefore, it can be concluded that Ti doping to SiCN does not affect crystallite size of the C free phase.
The different nanostructures developed within the SiT-iCNO ceramics during pyrolysis at 1400 °C as evidenced by XRD analysis, were observed through TEM/HRTEM, as presented in Figs. 7 and 8. Micrographs shown in Fig. 7a-c display a homogeneous distribution of the nanocrystals evolved within the SiCN matrix at different microscope resolution. In Fig. 7d, mixed precipitates of small (grayish dot structures) and relatively larger nanocrystals appear. The nanocrystals could be observed in spherical, elliptical, as well as cuboid shape, with sizes ranging from a few to 12 nm. These nanostructures were further identified by analyzing it through HRTEM as shown in Fig. 8. Figure 8a and b show different lattice fringes of the nanostructures evolved within the ceramic matrix. Also, the SiCN matrix clearly appeared amorphous in nature. The insets within the HRTEM images shows IFFT image of the selected areas for the calculation of the respective lattice fringe widths. The lattice fringe width computed as 0.32 nm   corresponds to the d-spacing of (110) plane of rutile-TiO 2 (JCPDS file No. 77-0441). The fringe widths corresponding to 0.35 nm, and 0.24 nm, belong to the d-spacing of (101) and (004) planes of anatase-TiO 2 (JCPDS #21-1272), respectively. Also, the fringe width measured as 0.25 nm corresponds to the d-spacing of (111) plane of TiC (JCPDS file no. 73-0472). These observations of the different phases in an amorphous SiCN matrix are in confirmation with the X-ray diffractograms (Fig. 6). Other than these, an unusual lattice fringe of 0.445 nm width was reported, which belongs to (200)  In many previous studies, efforts have been made to develop Ti doped SiCN/SiOC ceramics with a stable nanostructure at high temperatures. The dopant precursor sources influence the evolution of nanostructures within the PDC during pyrolysis, which consequently influence materials properties. For instance, TiN nanocrystals within SiOCN matrix were observed from the thermolysis of titanium tetrabutoxide modified polyhydridomethylsiloxane at 1000 °C in ammonia, and subsequent annealing at 1200 °C in nitrogen [59]. In another work, titanium isopropoxide modified polymethylphenylsilsesquioxane precursor formed TiC-SiOC ceramics at 1000 °C [60]. The TiC within the SiOC improved the oxidation resistance of the SiTiOC nanocomposite. Ti doping using titanium isopropoxide to polyvinylsilazane resulted in the precipitation of anatase-TiO 2 within the SiCN ceramics at 1100 °C, and a small fraction of the anatase-TiO 2 phase converted into rutile-TiO 2 at 1200 °C [42]. Such nanocrystallites of anatase-TiO 2 within the SiCN ceramic matrix improved ceramic yield (during pyrolysis) as well as oxidation resistance of the ceramics up to 1400 °C. Further, the pyrolysis of titanium tetrabutoxide altered polyhydridomethylsiloxane precursor led to the formation of nanocrystallites of anatase-TiO 2 in SiOC matrix that remained stable up to 1200 °C [61]. Similarly, in the current investigation the rutile-TiO 2 was found predominantly at 1300 °C and 1400 °C in the amorphous SiCN matrix. TiC is a high temperature ceramic (melting point of 3067 °C) that shows excellent resistance towards high temperature creep, corrosion, and thermal shock [62]. Further, TiC exhibits high hardness (28)(29)(30)(31)(32)(33)(34)(35), high elastic modulus (450 GPa) and good thermal conductivity (22-35 W/ (m·K)) [63]. Formation of TiC in the SiTiCNO composite in the current investigation imparts beneficial high temperature properties. The formation of TiC 0.3 N 0.7 is very rare and it has got properties similar to that of TiC and TiN [63]. Moreover, Si 2 N 2 O has low diffusion coefficient and shows excellent properties, such as excellent oxidation resistance (up to 1600 °C), high thermodynamic stability (up to 1800 °C and more stable than Si 3 N 4 ), high flexural strength (up to 1400 °C), high thermal shock resistance, and high fracture toughness [64,65]. The carbon in the PDC matrix acts as barrier and improves their thermal stability towards crystallization and decomposition [21]. Therefore, assimilation of all the results indicate the formation of a multicomponent multiphase nanocomposite ceramic with the simple doping of Ti in a polysilazane based preceramic polymer. The formation of such a composite with nanostructured phases of TiO 2 , C, TiC, TiC 0.3 N 0.7 , and Si 2 N 2 O dispersed in an amorphous SiCN matrix, which is stable at 1400 °C, promises structural stability and better high temperature properties. These compounds merit exploration as functional materials, high temperature coatings, and engineered intergranular phase compounds in bulk ceramic components.

Summary
The present investigation describes synthesis, high-temperature stability, and microstructure evolution of SiT-iCNO ceramics. Polyvinylsilazane precursor was mixed with tetrabutyl orthotitanate and subsequently crosslinked at 300 °C and pyrolyzed in the range of 900-1400 °C in flowing nitrogen atmosphere to get SiTiCNO ceramics. The precursor to ceramic conversion completed at 750 °C and the Ti incorporated SiCN ceramics remained as a single phase SiTiCNO ceramic up to 1100 °C. SEM-EDX confirmed uniform doping of Ti in the SiCN ceramic matrix. Ti-doping in SiCN ceramics was found to promote separation of the carbon phase. The free carbon phase was found to possess better graphitic structural order in the Ti-doped SiCN as compared to the undoped ceramic of equivalent thermal history. However, doping did not affect the lateral size of the free carbon phase. The Ti-doped SiCN ceramics annealed at 1 3 1200 °C exhibited formation of nanocrystallites of anatase-TiO 2 (size ~ 6.1 nm) thus forming a TiO 2 -SiCN-C nanocomposite. At 1300 °C, these anatase-TiO 2 converted into rutile-TiO 2 and the carbothermal decomposition of SiCN matrix produced high temperature stable TiC 0.3 N 0.7 phase. Further annealing of the SiTiCNO materials at 1400 °C, produced novel multiphase and multicomponent ceramic nanocomposite system consisting of phases, such as TiO 2 , SiCN, C, TiC, TiC 0.3 N 0.7 , and Si 2 N 2 O. These constituent phases were of few nanometers to 13.6 nm in size and distributed throughout glassy SiCN matrix. The results presented in this work indicate the formation of a unique multiphase nanostructured composite with excellent thermal stability that merits further exploration of its potential for high temperature structural and functional applications.
Funding The authors have not disclosed any funding.