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A Comparative Study of Nano-Fillers to Improve Toughness and Modulus of Polymer-Derived Ceramics

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A Comparative Study of Nano-Fillers to Improve Toughness and Modulus of Polymer-Derived Ceramics www.nature.com/scientificreports OPEN A comparative study of nano‑fllers to improve toughness and modulus of polymer‑derived ceramics Mohammad Mirkhalaf1,4*, Hamidreza Yazdani Sarvestani1, Qi Yang2, Michael B. Jakubinek3 & Behnam Ashraf1* Brittleness is a major limitation of polymer‑derived ceramics (PDCs). Diferent concentrations of three nanofllers (carbon nanotubes, Si3N4 and Al2O3 nanoparticles) were evaluated to improve both toughness and modulus of a commercial polysilazane (PSZ) PDC. The PSZs were thermally cross‑ linked and pyrolyzed under isostatic pressure in nitrogen. A combination of mechanical, chemical, density, and microscopy characterizations was used to determine the efects of these fllers. Si3N4 and Al2O3 nanoparticles (that were found to be active fllers) were more efective than nanotubes and improved the elastic modulus, hardness, and fracture toughness (JIC) of the PDC by ~ 1.5 ×, ~ 3 ×, and ~ 2.5 ×, respectively. Nanotubes were also efective in maintaining the integrity of the samples during pyrolysis. The modulus and hardness of PDCs correlated positively with their apparent density; this can provide a fast way to assess future PDCs. The improvement in fracture toughness was attributed to crack defection and bridging observed in the micro‑indentation cracks in the modifed PDCs. The specifc toughness of the modifed PDCs was 4 × higher than that of high‑purity alumina, and its specifc modulus reached that of commercially available technical ceramics. These PDCs can also easily take diferent shapes and therefore are of interest in protective armor, propulsion, thermal protection, device packaging and biomaterial systems. Ceramics are technologically important materials due to their high strength, low density, excellent thermal sta- bility, and high oxidation/corrosion resistance. As such, they are important in structural applications involving high service temperatures; e.g., in engine components, high temperature furnaces and exhausts 1–4. However, in comparison to high performance alloys, ceramics ofer limited shape fexibility in manufacturing and sufer from low fracture toughness and susceptibility to thermal shocks 5. In traditional ceramic manufacturing processes 6,7, porosity usually increases with the freeform capability of the manufacturing process. Polymer-derived ceramics (PDCs) can address this issue. PDCs are produced in a two-stage process where, in the frst stage, the polymer is covalently cross-linked (cured) to a green state. In the second stage (pyrolysis), the temperature is increased (usually to more than 800 °C) so that the organic moieties (e.g., methyl groups attached to the Si atoms in a polysilazane, PSZ) are eliminated and a ceramic residue called a PDC is obtained 8–14. Since the ceramic is derived from a polymer, the processing method has several advantages over more traditional methods. Tese advantages include ease of fabrication, near-net shape manufacturing of complex shapes (e.g., for advanced cooling systems in gas turbine engines), and being relatively environmental friendly 8,15,16. However, the main drawback of PDCs is the signifcant gas release and shrinkage during the transformation of the cross-linked polymer to the ceramic17. Tis gas release induces porosity in the resulting material9,18. To decrease the porosity of PDCs, two main approaches have been employed 4,5: (1) addition of nanometer sized fllers 9,10,12,19–21 and (2) using pressure during pyrolysis11. Passive fllers dilute the preceramic polymer and therefore decrease the amount of gas generated and the associated volume shrinkage. Tis can reduce the possibility of forming cracks/voids during pyrolysis12,20. On the other hand, active fllers can react with the decomposition gases generated during pyrolysis to form a secondary phase (e.g., silicon oxynitride)10,11,22,23, which reduces the amount of shrinkage and void formation10,24. Pressurizing the polymer during pyrolysis can also decrease the void content due to pressure-driven fow of the material into the cavities resulting from the gas release. Here we present a comparative study of diferent fllers 1Aerospace Manufacturing Technology Center, National Research Council Canada, 5145 Decelles Avenue, Montreal, QC H3T 2B2, Canada. 2Aerospace Manufacturing Technology Center, National Research Council Canada, 1200 Montreal Rd., Ottawa, ON K1A 0R6, Canada. 3Division of Emerging Technologies, National Research Council Canada, 100 Sussex Dr., Ottawa, ON K1A 0R6, Canada. 4Present address: School of Biomedical Engineering, The University of Sydney, Sydney, NSW 2008, Australia. *email: [email protected]; [email protected] Scientifc Reports | (2021) 11:6951 | https://doi.org/10.1038/s41598-021-82365-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Nano-modifed preceramic polymers. (a) Optical images of samples with diferent concentrations of alumina nano-fllers before the pyrolysis, and (b,c) Afer the pyrolysis. (d–f) Optical microscopy images of PDCs with diferent concentrations of (d) Al2O3, (e) Si3N4, and (f) CNTs. Scale bars are (a–c) 15 mm, (d–f) 2 mm. (Al2O3 and Si3N4 nanoparticles as well as carbon nanotubes) added to a commercial PSZ resin on the properties of the resulting PDCs, which are characterized in terms of composition, density, harness, modulus, and fracture toughness to guide the selection of these fllers for future studies (e.g. on 3D–4D printing of PDCs). Results and discussion Samples with diferent concentrations of fllers were easily detached from the aluminum mold afer crosslink- ing to the green state (at 150 °C for 15 min) in the presence of a radical initiator (see “Materials and methods” section, Fig. 1a). Filler concentrations in the range of 0–15 wt% were explored for alumina and silicon nitride nanoparticles. Concentrations beyond 15 wt% resulted in a very viscous paste that could not be processed. For nanotubes, this threshold concentration was 3 wt%. Afer pyrolysis (at 1000 °C under isostatic pressure of 30 MPa in nitrogen), extensive macro-cracking and high volume of voids were observed within the material with no or low concentration (i.e., 2 wt%) of alumina (Fig. 1a,b). Tailoring the heating and cooling cycles did not prevent shattering of the samples or creation of voids. However, higher concentrations of alumina or silicon Scientifc Reports | (2021) 11:6951 | https://doi.org/10.1038/s41598-021-82365-3 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 2. Properties of the modifed PDCs obtained from nano/micro-indentation. (a) Representative nano- indentation force–displacement results for the samples containing alumina nano-fller. (b) Elastic Modulus, (c) hardness and (d) fracture toughness of the PDC modifed by diferent concentrations of alumina, silicon nitride or carbon nanotube nano-fllers. Colored regions show 2 standard deviations (number of samples N = 6). nitride eliminated the cracking and resulted in solid samples with relatively retained shape (Fig. 1c). At low fller concentrations, most voids were located halfway through the thickness of the samples (Fig. 1d), which provided a sign that they resulted from pyrolysis reaction gases that could not escape from the material. Although there were fewer voids with a higher concentration of alumina nano-fller (6 and 15 wt%, Fig. 1d), there were some macroscale-cracks in the material. Conversely, samples containing silicon nitride nano-fllers were almost free of defects in the optical microscope images (Fig. 1e). In contrast to the alumina and silicon nitride nano-particles, adding CNTs did not dramatically decrease the void content (Fig. 1f), although the integrity of the samples was retained during pyrolysis by adding CNTs, an efect that has been observed in previous studies 25. Figure 2 shows the mechanical test results obtained from nano-indentation and micro-indentation fracture with diferent concentrations of alumina (1, 3, 6, and 15 wt%) and silicon nitride (2, 6, and 15 wt%) nanoparti- cles as well as diferent concentrations of CNTs (1, 2, and 3 wt%). Indentation was selected over other common mechanical tests, such as 3-point bending (for measuring the modulus) and single edge notch test (for measuring the fracture toughness), because of: (a) the irregular shape of the ceramic samples, in particular the baseline PSZ- derived ceramic, which shattered into smaller pieces during pyrolysis, and (b) the high porosity of the samples with low fller content, which resulted in their breakage at an unintended position during the bending tests. Several studies show that the fracture toughness values obtained from indentation tests on ceramics agree with those obtained from 3-point bending notch or double cantilever beam fracture tests26,27. Also, indentation has been used in the literature to measure both elastic modulus and toughness of PDCs 28–30. However, the indentation fracture toughness values of PDCs might not be absolute due to possible densifcation of the samples under the indenter. Te fracture toughness values are therefore best-used as a comparison between qualitatively similar materials30, such as those in the present work. Nano-indentation experiments (maximum force: 300 mN) were used to characterize the hardness and elas- tic modulus of the material. Micro-indentation tests (max. force: 49 N) were used to create cracks within the ceramics and measure their fracture toughness 26,31 (see “Materials and methods” section for details); creating cracks was not possible using nano-indentation. All measurements were performed
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