Initial development of preceramic polymer formulations for additive manufacturing

Three preceramic polymer formulations for potential use in additive manufacturing technologies were investigated. The polymeric precursors include an allyl hydrido polycarbosilane (SMP-10), a mixture of SMP-10 with a reactive ester (1,6-hexanediol diacrylate, HDDA), and a polydimethylsiloxane (4690A/B). The SMP-10/HDDA proved to have outstanding photo-curing properties, high-resolution printing, and the ability to easily transform into the silicon carbide phase. The same polymeric mixture showed the lowest viscosity value which is preferred in vat additive manufacturing. Thermogravimetric analysis showed that, after pyrolysis to 1350 o C, the polydimethylsiloxane polymer showed the highest onset decomposition temperature and the lowest retained weight (52 wt%) while the allyl hydrido polycarbosilane showed the lowest onset decomposition temperature and highest retained weight (71.7 wt%). In terms of crystallography, X-ray diffraction and microstructural results showed that the ceramic matrix composites contained both silicon carbide and silicon oxycarbide. Overall, the results are very promising for the fabrication of ceramic materials using additive manufacturing technologies.


INTRODUCTION
Common industrial ceramic fabrication processes such as slip and tape casting [1], injection molding [2], and powder pressing [3][4] have many limitations. Such processes rely on mold usage which restricts the form of the fabricated ceramic part to relatively simple geometries. To produce a more complex product using these methods, further machining or shaping of the part is necessary. This proves challenging when working with inherently brittle ceramics. In the last few decades, additive manufacturing (AM) technologies have grown in popularity among researchers as they enable the manufacturing of more complex final products [5]. To successfully print parts with high precision in polymer-based AM, the polymeric precursors must have specific rheological properties. In particular, viscosity is crucial to successful printing. For example, in stereolithography or digital light processing, a polymer with low viscosity, below 5 Pas is preferred to facilitate layers coating and reduce the printing time [6].
To produce ceramic parts containing silicon carbide and silicon oxycarbide, the 3D printed polymeric precursors must have silicon in their main polymeric chain which converts to ceramic material with subsequent pyrolysis in inert gas [7]. This is a sophisticated process that includes many chemical and 2 physical changes in the polymer to the ceramic route. Adequate knowledge in many fields like inorganic/organic chemistry, mineralogy, materials science, and computer-assisted modeling is necessary to study polymer derived ceramics [8]. Among ceramics fabricated from preceramic polymers, silicon carbide has special importance due to its extraordinary environmental stability, mechanical properties, and resistance to oxidation (until 1400 o C) [9].
Eckel et al. [10] reported a process for the fabrication of a honeycomb structure by using preceramic polymers. The polymers were ultraviolet curable siloxanes prepared by mixing methylsiloxane with vinyl methoxy siloxane. After pyrolyzing at 1000 o C in inert gas, around 42% mass loss and 30% linear shrinkage were reported. The resulting ceramic products showed very limited defects and were fully dense.
Min et al [11] fabricated silicon nitride ceramics using digital light processing (DLP) additive manufacturing and subsequent pyrolysis. To optimize the heat treatment of the green body after printing, three distinct pyrolysis temperatures (1200 o C, 1400 o C, and 1600 o C) were investigated. The linear shrinkage, ceramic yield, and relative density at each temperature were calculated. It was found that the optimal pyrolysis temperature was near 1400 o C as it converted the preceramic polymer to a dense ceramic product with improved structural and mechanical properties. Wang et al [12] compared the ceramic yield of three different silicon-based polymers after pyrolysis. They applied thiol-ene free-radical addition which works with polymer derived ceramics containing carbon-carbon double bonds. For this study, three distinct polymers were chosen: methylvinylhydrogen polycarbosilazane, liquid methylvinylhydrogenpolysiloxane, and allylhydrydopolycarbosilane. The photoinitiator, phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, was added to the preceramic polymers to make them photosensitive. After stereolithography, the parts were pyrolyzed at 1100 o C in nitrogen at 40 o C/h. The authors observed a nearly fully dense product free of macroscopic voids and defects after pyrolysis.
In this work, three preceramic formulations were introduced for potential implementation in AM.
The photocuribility were investigated initially using a high power UV lamp to have a better understanding of polymer-UV interaction. Later, the polymeric formulations were tested for the actual 3D process. The rheological properties of the polymeric precursors, including the viscosity within a wide shear rate range, were compared to determine the suitability for AM technologies. The structural properties of the polymeric solutions and their photocureability were investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Moreover, thermogravimetric analysis was utilized to study the polymeric to the ceramic conversion of these three versatile precursors.

Processing
Thin polymeric films were manually deposited onto glass slides to simulate 3D printing process which allowed for quicker evaluation of the multiple formulations and process conditions. The film thickness was controlled by using a reference adhesive tape. The three preceramic polymer formulations involve SMP-10, SMP-10/HDDA (1wt%:1wt%), and PDMS (1wt% KER 4690A, 1wt% KER 4690B).
After mixing for one hour, the formulations were spread onto a glass slide to produce a thin film which was followed by UV illumination for 10 seconds or 30 seconds, depending on the formulation. The cured film was subsequently peeled from the glass substrate, thermally and structurally characterized, and then transferred to the tube furnace for ceramitization under inert conditions. A tube furnace (Thermo Scientific, Linderg Blue M, USA) with 1500 o C maximum temperature was utilized to pyrolyze the cured samples.
The pyrolysis cycle involved heating from room temperature to 1350 o C under argon gas with a heating rate of 10 o C/min. Figure 2 illustrates the experimental workflow used to evaluate the suitability of the three preceramic polymer systems for use in AM.

Figure 2.
Thin film deposition process -polymeric formulation deposition on a glass slide; UV exposure, and polymer to ceramic conversion in a tube furnace.
To confirm the printability of the preceramic formulations, the DLP additive manufacturing technique using ANYCUBIC Photon S, Shenzhen, with a UV-LED light source (50 W, λ = 405 nm) was employed to 3D print parts with different geometries.

Examinations
The viscosity-shear rate relationship of the three preceramic polymers was measured using a rotational rheometer (MCR 302, Anton Paar, Graz, Austria) equipped with a 50 mm diameter cone-plate

Viscosity Test
Since the resin viscosity plays a vital role in determining the suitability for AM technology, the viscosities of the three preceramic polymer systems, namely SMP-10, SMP-10/HDDA, and PDMS, were investigated at a shear rate from 1 to 1000 s -1 . For stereolithography (SLA) and digital light processing (DLP) 3D printing technologies where each layer builds onto the previously cured layer, the polymer should display a low viscosity and near Newtonian behavior [13]. The photosensitive resins must be able to quickly flow to the print area and self-level to be effective using these technologies.
Overall, It can be seen from Figure 3

Crosslinking of Preceramic Polymers
Being a low vinyl-containing preceramic polymer, the polymerization rate and photo-curing depth of SMP-10 are limited. Thus, the polymerization requires a high energy density UV illumination [14].
Furthermore, unlike PDMS and SMP-10/HDDA, SMP-10 is translucent amber which has a slight contribution in hindering UV light penetration through the material thickness (Figure 4 a). In comparison, SMP-10/HDDA is more transparent than SMP-10 while PDMS is transparent colorless, making it even more responsive to UV illumination.  FTIR results of SMP-10 before and after exposure to UV light (Figure 6-a) show that the peak around 1630 cm -1 , which is attributed to the C=C stretch of the silicon-allyl group (Si-allyl) [12], was reduced upon UV exposure. The formation of the carbonyl (C=O) peak after curing SMP-10 may be attributed to either the carbonyl of the photoinitiator and/or oxidation of the silicon-hydrogen (Si-H) or silicon-methylene (Si-CH 2 -) groups, considered active sites for vinyl photopolymerization [14]. The addition of HDDA to SMP-10 contributed to an increase in the number of vinyl (C=C) groups ( Figure 6-b) in comparison to pure SMP-10 due to the acrylate of HDDA. As seen in Figure 6-b, the C=C bonds of the acrylate group, appearing as a doublet peak (1635 cm −1 and 1619 cm −1 ), effectively disappeared after UV 8 illumination for 10 sec which indicates the HDDA fully reacted and a successful co-photopolymerization took place. According to the manufacturer, KER 4690AB is cured by hydrosilylation under a photoactivated Pt catalyst, which arises between Si-H and vinyl groups within the resin (Figure 6-c).

Polymer to Ceramic Conversion
The polymer to the ceramic conversion of the successfully printed and pyrolyzed parts to 1350 o C, SMP-10/HDDA, showed uniform shrinkage without noticeable macroscopic defects. An estimated linear shrinkage of around 30% was recorded for such a sample (Figure 7). wt% is attributed to incomplete hydrosilylation and dehydrocoupling from UV crosslinking, which occurs at the first stage of pyrolysis. Thus, the evaporation of hydrogen gas and short oligomers are the major events at this temperature. Massive weight loss of around 20 wt% is associated with the second stage of pyrolysis due to the evolution of both hydrogen and methane gases [12]. In the third stage, there was a slight weight loss due to Si-C and Si-O-C bridging formations, which means that the polymer to ceramic conversion occurred beyond this temperature [17].
A dramatic weight loss (around 30 wt%) between 400 and 500 o C is connected with the HDDA addition to SMP-10. The mass loss is attributed to small moieties separation from the acrylate and preceramic polymer main chain at this temperature range.
On the other hand, PDMS showed the highest onset temperature and lowest ceramic yield with major depolymerization temperature between 400-750 o C. During this range, most PDMS converted to cyclic oligomers through two mechanisms. At lower temperature ranges around 400-500 o C, unzip degradation is the dominant mechanism generating cyclic siloxanes. At temperatures over 500 o C, rearrangement degradation proceeds by readjustment of Si-O-Si bonds in the siloxane backbone and heterolytic cleavage. The subsequent species from this degradation are cyclic siloxanes and short moieties [18].   outstanding photocurability, which makes it an excellent candidate for polymer-based AM technologies.