nanomaterials

Article Microstructure and Fracture Mechanism Investigation of Porous –Zirconia–Graphene Composite Using Multi-Scale and In-Situ Microscopy

Zhongquan Liao 1,* , Yvonne Standke 1, Jürgen Gluch 1, Katalin Balázsi 2 , Onkar Pathak 1, Sören Höhn 3, Mathias Herrmann 3 , Stephan Werner 4,Ján Dusza 5, Csaba Balázsi 2 and Ehrenfried Zschech 1

1 Fraunhofer Institute for Technologies and Systems IKTS, Maria-Reiche-Straße 2, 01109 Dresden, Germany; [email protected] (Y.S.); [email protected] (J.G.); [email protected] (O.P.); [email protected] (E.Z.) 2 Centre for Energy Research, Konkoly-Thege Str. 29-33, 1121 Budapest, Hungary; [email protected] (K.B.); [email protected] (C.B.) 3 Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Winterbergstraße 28, 01277 Dresden, Germany; [email protected] (S.H.); [email protected] (M.H.) 4 Helmholtz Zentrum Berlin, Albert-Einstein-Straße 15, 12489 Berlin, Germany; [email protected] 5 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia; [email protected] * Correspondence: [email protected]; Tel.: +49-(0)351-888-15598   Abstract: Silicon nitride–zirconia–graphene composites with high graphene content (5 wt.% and

Citation: Liao, Z.; Standke, Y.; Gluch, 30 wt.%) were sintered by gas pressure sintering (GPS). The effect of the multilayer graphene J.; Balázsi, K.; Pathak, O.; Höhn, S.; (MLG) content on microstructure and fracture mechanism is investigated by multi-scale and in-situ Herrmann, M.; Werner, S.; Dusza, J.; microscopy. Multi-scale microscopy confirms that the phases disperse evenly in the microstructure Balázsi, C.; et al. Microstructure and without obvious agglomeration. The MLG flakes well dispersed between ceramic matrix grains slow Fracture Mechanism Investigation of down the phase transformation from α to β-Si3N4, subsequent needle-like growth of β-Si3N4 rods Porous Silicon Nitride–Zirconia– and the densification due to the reduction in sintering additives particularly in the case with 30 wt.% Graphene Composite Using MLG. The size distribution of Si3N4 phase shifts towards a larger size range with the increase in Multi-Scale and In-Situ Microscopy. graphene content from 5 to 30 wt.%, while a higher graphene content (30 wt.%) hinders the growth of Nanomaterials 2021, 11, 285. https:// the ZrO phase. The composite with 30 wt.% MLG has a porosity of 47%, the one with 5 wt.% exhibits doi.org/10.3390/nano11020285 2 a porosity of approximately 30%. Both Si3N4/MLG composites show potential resistance to contact

Academic Editor: Babak Anasori or indentation damage. Crack initiation and propagation, densification of the porous microstructure, Received: 8 December 2020 and shift of ceramic phases are observed using in-situ transmission electron microscopy. The crack Accepted: 20 January 2021 propagates through the ceramic/MLG interface and through both the ceramic and the non-ceramic Published: 22 January 2021 components in the composite with low graphene content. However, the crack prefers to bypass ceramic phases in the composite with 30 wt.% MLG. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: porous ceramic composite; high graphene content; GPS; multi-scale microscopy; in-situ published maps and institutional affil- microscopy; contact-damage resistance iations.

1. Introduction Copyright: © 2021 by the authors. Materials for high temperature applications usually require tailored mechanical prop- Licensee MDPI, Basel, Switzerland. erties (e.g., fracture toughness, bending strength), good resistance to thermal shock, creep This article is an open access article resistance, high thermal conductivity, as well as good tribological and wear properties [1–4]. distributed under the terms and Ceramic materials have been extensively investigated in the previous decades due to their conditions of the Creative Commons high temperature performance in general. Silicon nitride (Si3N4) have the poten- Attribution (CC BY) license (https:// tial to meet the requirements mentioned above (e.g., low coefficient of thermal expansion creativecommons.org/licenses/by/ (CTE), good thermal conductivity and high strength, resulting in a higher thermal shock 4.0/).

Nanomaterials 2021, 11, 285. https://doi.org/10.3390/nano11020285 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 285 2 of 13

resistance than most other ceramic materials), therefore, it is commonly used in a variety of structural applications such as cutting tools, pump seal parts, balls, engine parts or heat exchangers [5–7]. To further extend its application field, forming multi-component materials or composites is under development to tailor both the me- chanical and functional properties of Si3N4 ceramics. Typical metal oxides (e.g., MgO, Al2O3,Y2O3, or ZrO2) in Si3N4 promote a liquid phase formation, which facilitates the consolidation [4]. The use of tetragonal zirconia as an energy-dissipation component can effectively result in an improvement of fracture toughness of Si3N4 ceramics [8–10]. Energy dissipation and consequently high fracture toughness of zirconia-containing ceramics can be related to the tetragonal-to-monoclinic phase transformation [9,10]. In another aspect, composites with nanofillers (e.g., carbon nanotube, graphene and hexagonal boron ni- tride (h-BN) show the potential to improve the properties of Si3N4-based ceramic matrix composites [11–13]. Due to the unique combination of electrical, thermal and mechanical properties [14], graphene and graphene oxide (GO) have been considered as components in ceramic matrix composites for the last decade. Several studies were reported on graphene reinforced Si3N4 matrix composites [15–22]. However, the problem of these composites is that graphene or graphite reacts with the sintering additives. This effect strongly reduces the densification of the material. Dense materials were observed only by hot pressing (HP) or spark plasma sintering (SPS), due to the fast densification caused by the high uniaxial pressure. Adding 1 wt.% graphene platelet (GPL) into Si3N4 with hot isostatic pressing (HIP) and gas pressure sintering (GPS) improved the fracture toughness, based on toughening mechanisms such as crack deflection, crack branching and crack bridg- ing [15,17,18]. Seiner et al. [19] investigated the elastic constants of silicon composites with variable content (3–18 wt.%) of graphene fillers (nanoplatelets and reduced GO sheets) by resonant ultrasound spectroscopy. Young’s modulus (E) and shear modulus (G) monotoni- cally decrease with the filler concentration for both types of fillers. Hvizdos et al. [20] and Balko et al. [21] observed no decrease in the coefficient of friction (COF) at room tempera- ture for Si3N4 matrix nanocomposites with 3 wt.% graphene. The authors mentioned that the graphene addition resulted in an insufficient densification and pores in the composite. The presence of porosity could be a reason for no change in the COF. Rutkowski et al. [22] investigated the thermal stability and conductivity of hot-pressed Si3N4 composite with a graphene content up to 10 wt.%. Anisotropic behavior was observed with excellent thermal properties in the major directions of graphene. Although using either ZrO2 or graphene nanofillers to improve the properties of Si3N4 has been reported, research on the combined use of these materials is rather rare [23,24]. On the other hand, it seems that the study on the effect of further increase in graphene content up to more than 20 wt.% into Si3N4 composite remains a gap to be filled. It is also important to explore the possibility of including very high graphene content to further develop novel Si3N4 matrix composites, for instance, porous ceramic composites. Porous carbon/silicon nitride composites showed tunable and weakly negative permittivity, which is necessary for the applications in solar energy harvesting, sensor and antennas [25]. Wave-transparent porous silicon nitride was produced using gel-casting and pressureless sintering [26]. Several studies demonstrated the potential of porous ceramics/ceramic composites to sustain mechanical damage and absorb energy, which benefits their applications in energy production, filtration, water treatment, absorption and catalysis as support or coating materials [27–32]. In this study, a porous ceramic composite with high graphene content (5 wt.% and 30 wt.%) was sintered by GPS from attrition milled Si3N4 ceramics and multilayer graphene (MLG), with in-situ incorporated ZrO2 particles. The studied porous silicon nitride– zirconia–graphene composites show potential resistance to contact or indentation damage. Combined use of ZiO2 and high content of graphene results in a complex ceramic system with high porosity, which is challenging to investigate using a conventional approach in ceramic studies. Therefore, the effect of the MLG content on the microstructure and the fracture mechanism of Si3N4-ZrO2/graphene composites is investigated using multi-scale and in-situ microscopy. Multi-scale and in-situ microscopy as a combined methodol- Nanomaterials 2021, 11, 285 3 of 13

ogy reported in this study also provides a potential unique approach to understand the microstructure and mechanical behavior correlation for other complex ceramic systems.

2. Materials and Methods 2.1. Silicon Nitride–Zirconia–Graphene Composite Preparation A commercial alpha silicon nitride powder (UBE Corp., Ube, Japan, particle size: 0.6 µm, specific surface area: 4.8 m2/g) was used as matrix material. The base powder consisted of 90 wt.% α-Si3N4, 4 wt.% Al2O3 (Alcoa, A16, Pittsburgh, PA, USA) and 6 wt.% Y2O3 (H.C. Starck, grade C, Goslar, Germany). It was mixed by attrition milling (Union Process, type 01-HD/HDDM, Akron, OH, USA) equipped with zirconia agitator discs and 3 ZrO2 grinding media (3 vol% Y2O3 stabilized, diameter of 1 mm) in a 750 cm zirconia tank. The milling process was performed at a high rotation speed of 3000 min−1 for 5 h in ethanol [33]. ZrO2 particles were incorporated into the Si3N4 during the milling, from the abrasion of zirconia balls under controlled conditions. The contribution of ZrO2 was adjusted between 30 and 42 wt.%. Commercial graphite powder (Aldrich, St. Louis, MO, USA, grain size: 1 µm) was milled intensively in ethanol for 10 h using the same attrition milling system, and subsequently added into powder mixture. The final powder mixture (with 5 wt.% and 30 wt.% MLG) was dried and sieved with a filter with a mesh size of 150 µm. Polyethylene glycol (PEG, 10 wt.%) surfactant and deionized water were added to the powder mixture before sintering. Samples with the dimension 5 mm × 5 mm × 50 mm were pressed by dry pressing at 220 MPa. The GPS was applied to form the final composites in atmosphere at 1700 ◦C and 20 MPa for 3 h.

2.2. Multi-Scale and In-Situ Microscopy High surface quality of composites was achieved by grinding, standard polishing, and polishing before performing scanning electron microscopy (SEM) studies. SEM imaging was performed at an operating voltage of 3 kV using a Carl Zeiss NVision 40 tool (Oberkochen, Germany), applying an Energy selective Backscattered (EsB) detector. The sample for X-ray microscopy (XRM) was firstly grinded with a relative flat surface, then a square pillar was prepared with a length of about 3 µm using focused ion beam (FIB) milling. An XRM study was carried out at the U41-XM beamline of the electron storage ring BESSY II, Helmholtz–Zentrum Berlin (Berlin, Germany). The used photon energy was 800 eV. The sample was tilted from −65◦ to +65◦ with 1◦ steps. The tomography was reconstructed by Tomo3D [34] (Almeria and Madrid, Spain), and rendered and sliced using the Tomviz software [35] (Ithaca, NY, USA). Standard lift-out lamellae with a thickness of about 200 nm for transmission electron microscopy (TEM) study were prepared using FIB milling, after local carbon and Pt deposition. TEM (Carl Zeiss Libra 200 Cs, Oberkochen, Germany, with an acceleration voltage of 200 kV) was used to study of the microstructure of the sintered composites. Energy-dispersive X-ray spectroscopy (EDX) was performed on the samples using a detector of Oxford Instruments attached to the TEM. A quantitative analysis of the microstructure from SEM and TEM images was performed in Fiji [36]. SEM images with magnification of 5000× and 10,000×, and TEM images with magnification of 20,000× and 30,000× were used. Si3N4 was treated as rod shape with round cross section, and ZrO2 was treated as globular shape in the analysis. Si3N4 phases with high aspect ratio larger than 3 were used to calculate the length and diameter of the cross section, while the rest was only used to calculate the diameter of the cross-section. The porosity was measured both by water intrusion porosimetry [21] and mercury intrusion porosimetry [37]. The was measured applying the Archimedes method. Vickers hardness measurement (hardness tester LECO 700AT, St. Joseph, MI, USA) was performed at loads from 9.81 to 150 N, the dwelling time was 10 s in all cases. The sample for in-situ TEM experiment was H-bar sample with a thickness of about 800 nm prepared by FIB milling. A wedge indenter with a piezo control in a TEM holder was used to perform the in-situ test. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13

The sample for in-situ TEM experiment was H-bar sample with a thickness of about 800

Nanomaterials 2021, 11, 285 nm prepared by FIB milling. A wedge indenter with a piezo control in a TEM holder4 ofwas 13 used to perform the in-situ test.

3. Results and Discussion 3. ResultsA square and pillar Discussion from the synthesized composite with 30 wt.% MLG was prepared for 3D microstructureA square pillar fromstudies the using synthesized XRM. compositeX-ray computed with 30 tomography wt.% MLG was (XCT) prepared data forare 3Dshown microstructure in Video S1 studies (supplementary using XRM. materials), X-ray computed two extracted tomography slices are (XCT) shown data in are Figure shown 1. inThe Video ZrO2 S1 phases (supplementary were easily differentiated materials), two by extracted the contrast, slices while are shownonly partial in Figure Si3N41 phases. The ZrOwere2 phasesdistinguished were easily from differentiated the mixed Si by3N the4 phase contrast, and whileMLG onlyflakes. partial The size Si3N of4 phases major wereZrO2 distinguishedphases is less than from 1 the µm, mixed the size Si3 Nof4 Siphase3N4 phases and MLG is less flakes. than 0.5 The µm. size The of major size of ZrO MLG2 phases flakes iscould less thannot be 1 µ unambiguouslym, the size of Si3 deN4terminedphases is by less XRM. than 0.5Emptyµm. space The size (open of MLG pores) flakes is clearly could notobserved be unambiguously as well from determined the 3D tomography by XRM. Emptydata as space shown (open in Video pores) S1 is clearly(Supplementary observed asMaterials). well from the 3D tomography data as shown in Video S1 (Supplementary Materials).

FigureFigure 1. ((aa,,bb)) Extracted Extracted slices slices from from a avolumetric volumetric recons reconstructiontruction of ofthe the composite composite with with 30 wt.% 30 wt.% multilayer graphene (MLG) by synchrotron-based nano-X-ray computed tomography (XCT). ZrO2 multilayer graphene (MLG) by synchrotron-based nano-X-ray computed tomography (XCT). ZrO2 phases are indicated by blue arrows, Si3N4 phases are indicated by red arrows. phases are indicated by blue arrows, Si3N4 phases are indicated by red arrows.

TheThe SEMSEM imagesimages onon thethe grindedgrinded andand ion-polishedion-polished samplessamples detecteddetected usingusing anan EsBEsB detectordetector showshow clear clear compositional compositional contrast, contrast, Si3 NSi4,3N ZrO4, ZrO2 and2 and MLG MLG flakes flakes are easily are distin-easily guisheddistinguished (indicated (indicated by arrows by arrows in Figure in2 ).Figure After 2). the After GPS process,the GPS spheroidprocess, ZrOspheroid2 particles ZrO2 (mostlyparticles c-ZrO (mostly2, as c-ZrO indicated2, as indicated by the XRD by datathe XRD in Figures data in S1 Figures and S2) S1 and and thin S2) MLGand thin platelets MLG wereplatelets successfully were successfully incorporated incorporated into the Si into3N4 thematrix Si3N for4 matrix bothcomposites. for both composites. MLG platelets MLG wereplatelets embedded were andembedded entangled and among entangled Si3N4 andamong ZrO 2Siphases.3N4 and Slight ZrO agglomeration2 phases. Slight of ceramicagglomeration particles of was ceramic also particles observed. was Hexagonal also observed.β-Si3N Hexagonal4 phases (rod-like) β-Si3N4 phases were commonly (rod-like) observedwere commonly in the sintered observed composite in the with sintered 5 wt.% composite MLG, while with approximately 5 wt.% MLG, 2.5 wt.% while of αapproximately-Si3N4 phases apart2.5 wt.% from of the α major-Si3N4β -Siphases3N4 phasesapart stillfrom remained the major in theβ-Si sintered3N4 phases compos- still iteremained with 30 in wt.% the sintered MLG (XRD composite data in with Figures 30 wt S1.% and MLG S2). (XRD Rod-like dataβ in-Si Figures3N4 phases S1 and with S2). a highRod-like aspect β-Si ratio3N4represent phases thewith majority a high in aspect the microstructure ratio represent in the the sample majority with 5in wt.% the MLGmicrostructure addition, in while the sampleβ-Si3N 4withphases 5 wt.% with MLG a low addition, aspect while ratio areβ-Si more3N4 phases common with in a low the sampleaspect ratio with are 30 wt.%more MLG common addition in the (Figures sample2 withand3 ).30 Due wt.% to MLG the high addition porosity, (Figures gas phase 2 and reactions3). Due to will the alsohigh influence porosity, the gas grain phase growth. reactions In the willα -alsoβ phase influence transformation, the grain growth. the liquid In phasethe α- isβ crucialphase intransformation, the whole process the ofliquid the dissolutionphase is crucial of the finein theα-phase whole starting process powder of the anddissolution subsequent of the precipitation fine α-phase of starting the β-phase powder [2]. and High subsequent content addition precipitation of MLG of couldthe β- react with the liquid phase, in particular by the reduction in sintering additives [38–40]. Therefore, it slowed down both the densification and the phase transformation from α β to -Si3N4. The high content of MLG plates slowed down the subsequent needle-like growth of β-Si3N4 rod as well (Figure2) by serving as barrier layer. The Si 3N4 and ZrO2 phases were quantitatively analyzed using the SEM images, the results are summarized in Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 13

phase [2]. High content addition of MLG could react with the liquid phase, in particular by the reduction in sintering additives [38–40]. Therefore, it slowed down both the densification and the phase transformation from α to β-Si3N4. The high content of MLG Nanomaterials 2021, 11, 285 5 of 13 plates slowed down the subsequent needle-like growth of β-Si3N4 rod as well (Figure 2) by serving as barrier layer. The Si3N4 and ZrO2 phases were quantitatively analyzed using the SEM images, the results are summarized in Figure 4a,b. Compared with the sample Figurewith 54 wt.%a,b . ComparedMLG addition, with the the size sample distribution with 5 wt.% of Si MLG3N4 phase addition, shifts the towards size distribution a larger size of range for the sample with 30 wt.% MLG addition. The size (diameter) mainly ranges from Si3N4 phase shifts towards a larger size range for the sample with 30 wt.% MLG addition. The100 sizeto 300 (diameter) nm with mainly 5 wt.% ranges MLG from addition 100 to and 300 from nm with 100 5to wt.% 350 MLGnm with addition 30 wt.% and fromMLG 100addition, to 350 respectively. nm with 30 wt.%On the MLG contrary, addition, a higher respectively. MLG content On the (30 contrary, wt.% MLG) a higher hinders MLG the growth of ZrO2 phase (Figure 4b), in which the high content of graphene acts as barrier content (30 wt.% MLG) hinders the growth of ZrO2 phase (Figure4b), in which the high contentlayer. For of graphene5 wt.% MLG acts asaddition, barrier layer.the aver Forage 5 wt.% diameter MLG and addition, length the of average hexagonal diameter Si3N4 phases were 221 ± 9 nm and 1496 ± 39 nm, and the average size of spheroid ZrO2 particles and length of hexagonal Si3N4 phases were 221 ± 9 nm and 1496 ± 39 nm, and the average sizewas of867 spheroid ± 27 nm. ZrO For2 theparticles sample was with 867 30 ±wt.%27 nm.MLG For addition, the sample the average with 30 diameter wt.% MLG and addition,length of hexagonal the average Si3 diameterN4 phases and was length 249 ± 9 of nm hexagonal and 1363 Si ±3 31N4 nm,phases respectively, was 249 ±and9nm the andaverage1363 size± 31 of nmthe, spheroid respectively, ZrO2 and phases the was average 601 ± size19 nm. of theThe spheroidaverage size ZrO of2 Siphases3N4 phases was 601observed± 19 nm.in this The study average is smaller size of Sithan3N4 phasesin typical observed monolithic in this Si3 studyN4 ceramics is smaller sintered than inat typicalsimilar monolithicconditions Si [17]3N4 becauseceramics of sintered the additi at similaron of MLG conditions resulting [17] becausein the reduction of the addition in the ofliquid MLG phase resulting and a in high the residual reduction porosity. in the liquid The volume phase andratio a of high Si3N residual4 phase porosity.to ZrO2 phase The volumeobserved ratio by ofimage Si3N analysis4 phase towas ZrO about2 phase 2.65:1 observed in the bysample image with analysis 5 wt.% was MLG, about and 2.65:1 about in the2:1 samplein the sample with 5 with wt.% 30 MLG, wt.% and MLG. about Although 2:1 in the the sample addition with of ZrO 30 wt.%2 into MLG. the initial Although Si3N4 thepowder addition can of noticeably ZrO2 into thefacilitate initial Sithe3N dens4 powderification can process noticeably and facilitate decrease the densificationthe sintering processtemperature and decrease [10], open the pores sintering were temperature apparently observed [10], open in pores both were composites. apparently The observed interface inbetween both composites. ZrO2 and Si The3N4 interface was continuous between without ZrO2 and any Si 3apparentN4 was continuous cracks. Since without the open any apparentpores are cracks. closely Since associated the open with pores graphene are closely plat associatedelets, it is with expected graphene that platelets,the porosity it is expectedincreasesthat with the the porosity increase increases in the graphene with the increasecontent. inThe the porosity graphene data, content. measured The porosity by both data,water measured and mercury by both intrusion water porosimetry, and mercury intrusionare given porosimetry,in Table 1. A are porosity given inclose Table to1 50%.A porositywas observed close tofor 50% the wascomposite observed with for 30 the wt.% composite MLG, withwhich 30 proves wt.% MLG, that MLG which fillers proves in thatSi3N MLG4-ZrO fillers2 ceramics in Si 3makesN4-ZrO the2 ceramics densification makes of the the densification composite extremely of the composite difficult. extremely Even an difficult.addition Evenof 5 wt.% an addition MLG can of 5 generate wt.% MLG a porous can generate microstructure a porous microstructurewith about 30% with porosity. about 3 3 30%Correspondingly, porosity. Correspondingly, the are the 2.71 densities g/cm3 are and 2.71 1.84 g/cm g/cm3and. 1.84 g/cm .

FigureFigure 2.2. SEMSEM imagesimages ofof thethe sinteredsintered silicon silicon nitride–zirconia–graphene nitride–zirconia–graphene composite. composite. (a (,ab,)b With) With 5 wt.%5 MLG;wt.% MLG; (c,d) with (c,d) 30 with wt.% 30 MLG.wt.% MLG.

Table 1. Density and porosity of synthesized silicon nitride–zirconia–graphene composite.

Porosity/Water Porosity/Mercury MLG Content Density (g/cm3) Intrusion (%) Intrusion (%) 5 wt.% MLG 2.71 28 33.4 30 wt.% MLG 1.84 47 47.5

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Table 1. Density and porosity of synthesized silicon nitride–zirconia–graphene composite.

MLG Density Porosity/Water Intrusion Porosity/Mercury Intrusion Content (g/cm3) (%) (%) 5 wt.% MLG 2.71 28 33.4 30 wt.% MLG 1.84 47 47.5

The morphology of the composites and the elemental distribution were investigated by TEM (Figure 3). MLG flakes distributed and embedded in the Si3N4-based matrix were clearly identified based on scanning TEM images for both composites. A certain amount of ZrO2 is located between the Si3N4 grains. The cross-section study of multilayered graphene flakes reveals their main presence between the rod-like Si3N4 particles. Compared with the composite with 30 wt.% MLG, more hexagonal β-Si3N4 grains were observed in the composite with 5 wt.% MLG. ZrO2 grains show an average size of less than 1 µm in both composites, while the size of silicon nitride rods is about 300 nm in diameter and 800 to 1200 nm in length. As shown in Figure 3, Si, N, C, Zr, Y, Al and O are the major elements detected by EDX in the TEM. Corresponding elemental mappings clearly indicate Si3N4, ZrO2 and MLG flakes in both 5 wt.% (Figure 3a) and 30 wt.% (Figure 3b) composites. Y distributes homogenously inside ZrO2, indicating the high degree of stabilization, which results in the formation of the cubic phase as proven by XRD. No transformation of ZrO2 is observed indicating that ZrO2 has no toughening effect. Al distributes around both ZrO2 and Si3N4; this is probably the remainder of the precipitated liquid phase. The elemental analysis reveals a low content of oxygen (2 to 4 at%) in the MLG flakes. There is no size difference observed in the TEM study for both composites. The thickness ranges mainly between 5 and 30 nm (thickness of about 20 nm (~ 65 carbon Nanomaterials 2021, 11, 285 layers) is also confirmed by XRD, Figures S1 and S2), while the length ranges mainly6 of 13 between 100 and 300 nm. The quantitative size distribution of MLG flakes is summarized in Figure 4c,d. The data reveal that the carbon is mostly in the form of thin graphite.

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FigureFigure 3. 3. TEMTEM image image and and corresponding corresponding elemental elemental maps maps of of the the sintered sintered composites. composites. ( (aa)) With With 5 5 wt.%, wt.%, ( (b)) with with 30 30 wt.%. wt.%.

3 4 2 FigureFigure 4.4. Histograms of of size size distribution distribution for for Si SiN3N (4a),(a ZrO), ZrO (b2),( band), and MLG MLG (c,d) ( cin,d the) in sintered the sintered compositescomposites with with different different graphene graphene content, content, analyzed analyzed from from SEM SEM and and TEM TEM images. images.

TheFigure morphology 5a shows ofa representative the composites optical and the microscopy elemental distributionimage of a Vickers were investigated indentation by TEM (Figure3). MLG flakes distributed and embedded in the Si 3N4-based matrix were site in dense Si3N4 without MLG. Classical radial cracking (extended cracks as indicated clearly identified based on scanning TEM images for both composites. A certain amount of by red arrows) is clearly observed in the micrograph. Figure 5b,c show representative ZrO2 is located between the Si3N4 grains. The cross-section study of multilayered graphene optical microscopy images of the Vickers indentation sites in Si3N4/MLG composites. No flakes reveals their main presence between the rod-like Si3N4 particles. Compared with classical radial cracks occur in the porous composites, indicating that the Si3N4/MLG the composite with 30 wt.% MLG, more hexagonal β-Si N grains were observed in the composites have potential resistance to contact or indentation3 4 damage [41]. The high composite with 5 wt.% MLG. ZrO grains show an average size of less than 1 µm in both porosity and shear-weak second2 phases (MLG) could play important roles in composites, while the size of silicon nitride rods is about 300 nm in diameter and 800 to redistributing under confined shear in indentation (contact loading), resulting in 1200 nm in length. As shown in Figure3, Si, N, C, Zr, Y, Al and O are the major elements the suppression of macroscopic (long) cracks. Since high porosity could deteriorate the detected by EDX in the TEM. Corresponding elemental mappings clearly indicate Si N , mechanical properties of the sintered composites, potential approaches (e.g., optimizing3 4 ZrO2 and MLG flakes in both 5 wt.% (Figure3a) and 30 wt.% (Figure3b) composites. Y MLG content, achieving high density, forming sandwich structure with alternating low and high MLG content layers [42,43]) can be applied to compensate the deteriorated mechanical properties.

Figure 5. Optical microscopy images of indents after Vickers hardness measurements. (a) With 0 wt.% MLG, (b) with 5 wt.% MLG, and (c) with 30 wt.% MLG.

In order to gain an in-depth understanding of the mechanical behavior and the fracture mechanism of sintered composites, in-situ wedge indentation tests were performed as shown in Figures 6 and 7. The corresponding videos from experiments are presented in Videos S2 and S3 (Supplementary Materials). Representative TEM images

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distributes homogenously inside ZrO2, indicating the high degree of stabilization, which results in the formation of the cubic phase as proven by XRD. No transformation of ZrO2 is observed indicating that ZrO2 has no toughening effect. Al distributes around both ZrO2 and Si3N4; this is probably the remainder of the precipitated liquid phase. The elemental analysis reveals a low content of oxygen (2 to 4 at%) in the MLG flakes. There is no size difference observed in the TEM study for both composites. The thickness ranges mainly

Figurebetween 4. Histograms 5 and 30 nm of size (thickness distribution of about for Si3 20N4 nm(a), ZrO (~652 ( carbonb), and MLG layers) (c,d is) alsoin the confirmed sintered by compositesXRD, Figures with S1 different and S2), graphene while thecontent, length analyzed ranges from mainly SEM betweenand TEM images. 100 and 300 nm. The quantitative size distribution of MLG flakes is summarized in Figure4c,d. The data reveal that theFigure carbon 5a shows is mostly a representative in the form of optical thin graphite. microscopy image of a Vickers indentation site inFigure dense5 aSi shows3N4 without a representative MLG. Classical optical radial microscopy cracking image (extended of a Vickers cracks indentationas indicated bysite red in densearrows) Si3 Nis 4clearlywithout observed MLG. Classical in the micrograph. radial cracking Figure (extended 5b,c show cracks representative as indicated opticalby red microscopy arrows) is clearly images observed of the Vickers in the indentation micrograph. sites Figure in Si53b,cN4/MLG show composites. representative No classicaloptical microscopy radial cracks images occur of thein the Vickers porous indentation composites, sites indicating in Si3N4/MLG that composites.the Si3N4/MLG No compositesclassical radial have cracks potential occur resistance in the porous to contact composites, or indentation indicating damage that the [41]. Si3 NThe4/MLG high porositycomposites and have shear-weak potential resistancesecond phases to contact (MLG) or indentation could play damage important [41]. Theroles high in redistributingporosity and shear-weak stress under second confined phases shear (MLG) in indentation could play important (contact loading), roles in redistributing resulting in thestress suppression under confined of macroscopic shear in indentation (long) cracks. (contact Since loading), high porosity resulting could in the deteriorate suppression the of mechanicalmacroscopic properties (long) cracks. of the Since sintered high porositycomposites, could potential deteriorate approaches the mechanical (e.g., optimizing properties MLGof the content, sintered achieving composites, high potential density, approaches forming sandwich (e.g., optimizing structure MLG with content, alternating achieving low andhigh high density, MLG forming content sandwich layers [42,43]) structure can with be applied alternating to compensate low and high the MLG deteriorated content mechanicallayers [42,43 properties.]) can be applied to compensate the deteriorated mechanical properties.

Figure 5. Optical microscopymicroscopy images images of of indents indents after after Vickers Vickers hardness hardness measurements. measurements. (a) With(a) With 0 wt.% 0 wt.% MLG, MLG, (b) with (b) 5with wt.% 5 MLG,wt.% MLG, and (c and) with (c) 30with wt.% 30 MLG.wt.% MLG.

InIn orderorder toto gain gain an an in-depth in-depth understanding understandi ofng the of mechanical the mechanical behavior behavior and the and fracture the fracturemechanism mechanism of sintered of composites,sintered composites in-situ wedge, in-situ indentation wedge testsindentation were performed tests were as performedshown in Figuresas shown6 and in 7Figures. The corresponding6 and 7. The corresponding videos from videos experiments from experiments are presented are presentedin Videos in S2 Videos and S3 S2 (Supplementary and S3 (Supplementa Materials).ry Materials). Representative Representative TEM images TEM selected images from an in-situ experiment for a composite with 5 wt.% MLG are shown in Figure6. Rod-like β-Si3N4 phases with high aspect ratio account for the majority of Si3N4 phase. The positions of major cracks observed in the experiment are highlighted by dashed lines (Figure6a). Cracks initiated in multiple locations, and propagated along weak interfaces (Figure6b ). Cracks turned towards different directions at the ceramic/MLG interface, within the ceramic phase and within the MLG platelets. Pulled out MLGs are visible in the partially fractured interface, fracture within ceramic phases is frequently observed as well (Figure6c ). The fracture surface typically looks sharp and straight in ceramic phases (Figure6c–f). As shown in Figure6f, a small part of the composite was completely delaminated and removed at the end of the experiment. Apart from the cracking process, shift of ceramic grains is also observed. Due to the high porosity, densification process occurs commonly in a relatively homogeneous pace. In the composite with 30 wt.% MLG, Nanomaterials 2021, 11, 285 8 of 13

β-Si3N4 grains with low aspect ratio were mainly obtained after sintering process (Figure7 ). A small amount (about 2.5%) of α-Si3N4 phase remained. The cracks propagated in a much faster pace after initiation. The cracks penetrated easily through the composite mainly within the MLG phase. A long crack within the MLG was quickly observed (step 23, Figure7c), and subsequently a large fracture interface was formed two steps later (step 25, Figure7d ). Pulled-out MLG components are commonly visible at the fracture surface (Figure7d). Shift of ceramic grains and a densification process were observed, but with a much faster pace. It seems that the cracks prefer to bypass the ceramic phases in the composite with 30 wt.% MLG. Such a crack propagation behavior could be caused by a high content of the weak carbon phase which forms a three-dimensional network. The microstructure has a strong influence on the fracture behavior of the Si3N4/MLG composites. Contact loading (indentation) with highly concentrated loads act on a very small region, resulting in an intensely confined shear. Therefore, highly heterogeneous ceramic matrix composites with shear-weak graphite and high porosity here result in a considerable redistribution of stress in the region below the indentation. The behavior can be described by a distributed shear anelasticity in the form of microstructure-localized shear-sliding along numerous interfaces [41]. The high porosity (28% and 47% for 5 wt.% MLG content and 30 wt.% MLG content, respectively) improves the shear-deformability of the composites even more. This type of dispersed damage caused by significant redis- tributed stress consequently prevents the formation of long macro cracks (classical radial cracks), as observed in the homogenous Si3N4 ceramics (Figure5a). Considering the fine scale of ceramic grains (about 200 nm in diameter and about 1µm in length for Si3N4, less than 1 µm in diameter for ZrO2) and of the carbon-based MLG as reinforcing component (tens of nm in thickness and less than 1 µm in size), it is not surprising that no macro toughening is observed, which requires this kind of carbon-based reinforcing component with larger scale (e.g., carbon fiber with a length of hundreds of micrometers [41]). The fine scale of the reinforcing component (MLG) in the composites studied here results in a small toughening zone relative to the crack size. Although both Si3N4/MLG (5 wt.% and 30 wt.%) composites are characterized by a resistance against contact damage (Figure5), the content of MLG plays an important role in the micro fracture behavior. In the composite with 30 wt.% MLG, the high content of the weak carbon component forms a three-dimensional network. As summarized in Table S1, a higher volume ratio (about 2:1 for Si3N4:ZrO2 compared to 2.65:1) of spheroid shape of ZrO2 with 260 nm smaller in average diameter, lower aspect ratio (5.47:1 compared to 6.77:1) of Si3N4 phase, and high porosity of about 50% facilitate the crack propagation along the three-dimensional network of the weak carbon component. The typical crack paths are sketched in Figure8b. Since such a three-dimensional network of MLG is not formed in the composite with 5 wt.% MLG, crack deflection and crack-bridging occur locally (see Figure8a). The cracks propagate into Si 3N4 phase as well. The high porosity in the composites causes a redistribution of stress that shifts ceramic and carbon components, and consequently, leads to the direction change of cracks and new crack initiation. Nanomaterials 2021, 11, 285 9 of 13 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 13

Figure 6.6. Representative TEM images selected from an in-situ experiment for a composite with 5 wt.% MLG.MLG. ((aa)) BeforeBefore thethe in-situin-situ experiment,experiment, majormajor crackscracks observedobserved inin thethe followingfollowing experimentexperiment areare highlightedhighlighted byby dasheddashed lines;lines; ((b––ee)) selectedselected TEM images during the in-situ experiment, crack propagation and fracture interfaces are highlighted by green arrows; (f) TEM images during the in-situ experiment, crack propagation and fracture interfaces are highlighted by green arrows; last TEM image recorded before retracting the indenter, fracture interfaces and rotated ceramic phases are highlighted by (f) last TEM image recorded before retracting the indenter, fracture interfaces and rotated ceramic phases are highlighted by dashed ellipses. dashed ellipses.

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Figure 7. Representative TEM images selected selected from from an in-sit in-situu experiment for for a a compos compositeite with with 30 wt.% MLG. ( a)) Before Before thethe in-situ experiment, major major cracks cracks observed observed in in the the fo followingllowing experiment experiment are are high highlightedlighted by by dashed lines; ( (bb,,cc)) the initiationinitiation and and propagation propagation of of first first cr crack,ack, highlighted highlighted by by green green arrows; arrows; ( (dd)) delamination delamination caused caused by by the the first first long long crack; crack; ( (ee,,ff)) development of further cracks before retracting the indenter. development of further cracks before retracting the indenter.

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Figure 8. The crack propagation paths in Si 3NN44/MLG/MLG composites. composites. (a ()a With) With 5 5wt.%, wt.%, (b (b) )with with 30 30 wt.%. wt.%.

4.4. Conclusions Conclusions InIn summary,summary, porous porous silicon silicon nitride–zirconia–graphene nitride–zirconia–graphene composites composites with high with graphene high graphenecontent (5 content wt.% and (5 30wt.% wt.%) and were 30 wt.%) sintered were by GPS.sintered Multi-scale by GPS. microscopy Multi-scale confirmed microscopy that confirmedall components that all had components been dispersed hadevenly been di inspersed the composite evenly withoutin the composite obvious agglomera- without obvioustion. Graphene agglomeration. caused aGraphene reduction caused in the sintering a reduction additives, in the and sintering therefore additives, an increased and thereforeporosity. MLGan increased fillers in Siporosity.3N4-ZrO 2MLGceramics fillers hampered in Si3N the4-ZrO densification2 ceramics of hampered the composites, the densificationobservable by of porositythe composites, values of observable about 30% by (5 porosity wt.% MLG) values and of about about 50% 30% (30 (5 wt.% wt.% MLG) MLG). andA quantitative about 50% (30 analysis wt.% onMLG). SEM A and quantitative TEM images analysis revealed on SEM that and the sizeTEM distribution images revealed of the thatSi3N the4 phase size shiftsdistribution towards of a largerthe Si size3N4 rangephase with shifts increased towards graphene a larger content. size range The higherwith increasedporosity ingraphene the composite content. with The the higherhigher grapheneporosity contentin the (30composite wt.%) hinders with the the growthhigher grapheneof the ZrO content2 phase. (30 The wt.%) average hinders diameters the growth of Si 3ofN 4thegrains ZrO2 were phase. 221 The± 9average nm (5 wt.% diameters MLG) ofand Si3N 2494 grains± 9 nm were (30 221 wt.% ± 9 MLG), nm (5 andwt.% the MLG) average and sizes249 ± of 9 spheroidnm (30 wt.% ZrO 2MLG),particles and were the average867 ± 27 sizes nm of (5 spheroid wt.% MLG) ZrO and2 particles 601 ± 19were nm 867 (30 ± wt.% 27 nm MLG). (5 wt.% The MLG) volume and ratio 601 ± of 19 Si nm3N4 (30and wt.% ZrO 2MLG).phase wasThe aboutvolume 2.65:1 ratio in theof Si composite3N4 and withZrO2 5phase wt.% MLG,was about and about 2.65:1 2:1 in in the the compositecomposite with with 5 30wt.% wt.% MLG, MLG. and The about MLG 2:1 in flakes the composite well dispersed with 30 between wt.% MLG. ceramic The grainsMLG flakesslowed well down dispersed the phase between transformation ceramic grains from slowedα to β-Si down3N4 and the thephase subsequent transformation longitudinal from αgrowth to β-Si3 ofN4β and-Si3 theN4 subsequentrods due to longitudinal the interaction growth with of the β-Si sintering3N4 rods additives,due to the particularlyinteraction withvisible the for sintering the composite additives, with particularly 30 wt.% MLG. visible Both for Sithe3N composite4/MLG composites with 30 wt.% show MLG. a po- Bothtential Si3 resistanceN4/MLG composites against contact show or a indentationpotential resistance damage dueagainst to significant contact or redistributed indentation damagestress. Crack due initiationto significant and propagation, redistributed densification stress. Crack of the initiation porous materialand propagation, and shift of densificationceramic components of the porous were mate observedrial and in shift in-situ of ceramic experiments. components The cracks were prefer observed to bypass in in- situceramic experiments. components The cracks in the prefer composite to bypa withss ceramic 30 wt.% components MLG, which in is the mainly composite caused with by 30the wt.% formed MLG, three-dimensional which is mainly networkcaused by with the aformed high content three-dimensional of weak carbon network components. with a highWithout content such of a weak three-dimensional carbon components. network Without of MLG such in the a compositethree-dimensional with 5% network MLG, crack of MLGdeflection in the and composite crack-bridging with 5% occurMLG, locally. crack deflection The cracks and propagate crack-bridging into the occur Si3N locally.4 phase Theas well. cracks Redistribution propagate into of stressthe Si moves3N4 phase the ceramicas well. phasesRedistribution and the carbonof stress phase, moves which the ceramicleads to phases the change and the of crackcarbon direction phase, which and to leads new crackto the initiation.change of Multi-scalecrack direction and and in-situ to newmicroscopy crack initiation. as a combined Multi-scale methodology and in-situ reported microscopy in this studyas a combined also provides methodology a potential reporteduniqueapproach in this study to understand also provides the microstructure a potential unique and mechanical approach behaviorto understand correlation the microstructurefor complex ceramic and mechanical systems. behavior correlation for complex ceramic systems.

SupplementarySupplementary Materials: Materials: TheThe following following are are available available online online at at www.mdpi.com/xxx/s1, https://www.mdpi.com/2079-499 Figure S1: XRD1/11/2/285/s1 data from the, Figure sintered S1: XRDsilicon data nitride-zircon from the sinteredia-graphene silicon composite nitride-zirconia-graphene with 5 wt.% MLG, composite Figure S2:with XRD 5 wt.% data MLG, from Figure the sintered S2: XRD silicon datafrom nitride- thezirconia-graphene sintered silicon nitride-zirconia-graphene composite with 30 wt.% composite MLG, Tablewith 30S1: wt.% Microstructure MLG, Table S1:comparison Microstructure for comparisonsilicon-zirconia-graphene for silicon-zirconia-graphene composites, composites,Video S1: Tomography,Video S1: Tomography, Video S2: In-situ Video S2:TEM In-situ for the TEM composite for the composite with 5 wt.% with MLG, 5 wt.% Video MLG, S3: VideoIn-situ S3: TEM In-situ for theTEM composite for the composite with 30 wt.% with MLG. 30 wt.% MLG. Author Contributions: Conceptualization, Z.L. and E.Z.; methodology, Z.L.; investigation, Z.L., Y.S., J.G., K.B., O.P., S.H., M.H., and S.W.; formal analysis, Z.L. J.G. O.P. and M.H.; writing—

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Author Contributions: Conceptualization, Z.L. and E.Z.; methodology, Z.L.; investigation, Z.L., Y.S., J.G., K.B., O.P., S.H., M.H. and S.W.; formal analysis, Z.L., J.G., O.P. and M.H.; writing—original draft preparation, Z.L.; writing—review and editing, J.G., K.B., M.H., J.D., C.B. and E.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Hungarian National Research Development and Innova- tion Office (projects NK-FIH NN 127723 and NKFIH-NNE 129976), and DFG in Germany (project number 397380564). This research was funded also by the “Research Centre of Advanced Materials and Technologies for Recent and Future Applications PROMATECH”, ITMS 26220220186, supported by the Operational Program “Research and Development” financed through European Regional Development Fund. Data Availability Statement: The data presented in this study are available on request from the corresponding author. This data is not publicly available due to excessive size and complex format. Acknowledgments: The authors would like to thank B. Jost and A. Potthoff (Fraunhofer IKTS, Dresden), P. Guttmann and G. Schneider (BESSY II, Berlin), and R. Sedlák (Slovak Academy of Sciences, Košice) for scientific discussions and technical support. Conflicts of Interest: The authors declare no conflict of interest.

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