Fracture toughness of hot-pressed Si3N4-graphene composites - PTCer

MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 66, 4, (2014), 463-469

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Fracture toughness of hot-pressed Si3N4-graphene composites Paweł Rutkowski*, Ludosław Stobierski, Gabriela Górny, Magdalena Ziąbka, Michał Urbanik AGH University of Science and Technology, Faculty of Material Science and Ceramics, al. A. Mickiewicza 30, 30-059 Kraków, Poland *e-mail: [email protected]

Abstract The work concerns the influence of microstructure on fracture toughness of hot-pressed silicon nitride-graphene composites. The phase analysis was carried out to identify constituent phases of the matrix. The presence of graphene was controlled by analysis of Raman spectra, and a graphene phase distribution in the composite microstructure was studied by SEM observations with EDS analysis. The microstructural observations of fractures and polished surfaces of the composites were made to determine the arrangement of the graphene phase. The fracture toughness was measured with the use of the three-point bending test on notched beams. The results of KIC measurements were supported in interpretation by the SEM observations of the crack propagation. Keywords: Si3N4, Graphene, Composites, Fracture toughness, Crack propagation

ODPORNOŚĆ NA PĘKANIE KOMPOZYTÓW GRAFEN-Si3N4 OTRZYMYWANYCH METODĄ PRASOWANIA NA GORĄCO Praca dotyczy badań nad wpływem mikrostruktury na odporność na pękanie kompozytów grafen – azotek krzemu prasowanych na gorąco. Analizę fazową przeprowadzono w celu zidentyfikowania faz składowych osnowy. Obecność grafenu kontrolowano za pomocą spektroskopii Ramana, zaś dystrybucję grafenu w mikrostrukturze kompozytu badano metodą skaningowej mikroskopii elektronowej z analizą EDS. Obserwacje mikroskopowe przełomów i powierzchni polerowanych próbek kompozytowych wykonano w celu określenia sposobu ułożenia fazy grafenowej. Odporność na pękanie zmierzono za pomocą trójpunktowego testu zginania belek z karbem. Interpretację wyników pomiaru KIc podparto obserwacjami mikroskopowymi (SEM) propagacji pęknięć. Słowa kluczowe: Si3N4, grafen, kompozyty, odporność na pękanie, propagacja pęknięcia

1. Introduction Graphene is a new monolayer powerful material, which shows high applicative potentials in electronics, high-thermal conductivity parts of different devices and in ceramic matrix composites because of its very good thermal, electrical and mechanical properties. As a single phase graphene presents very good thermal and electrical properties [1-4]. Such a perspective phase might be used as the dispersed part of ceramic matrix composite materials e.g. based on the silicon nitride matrix. Polycrystalline pure silicon nitride shows good mechanical properties such as bending strength, fracture toughness or abrasive wear, and also oxidation resistance at higher temperatures [5-7]. It is the reason why this material is used for cutting tools, bearing balls, and other parts of devices working under heavy conditions [8-10]. From this point of view, Si3N4 sintered bodies show a thermal conductivity of approx. 30 W/(m·K) [11] (which is not very high) and are very resistant to mechanical treatment [1]. So there is a possibility that the combination of silicon nitride and graphene phases can give a better composite material with higher thermal and electrical properties, and

similar values of mechanical characteristics in comparison to the reference polycrystalline pure Si3N4. There is also a hope that doping the silicon nitride materials by the graphene phase, which leads to an increase in electrical properties, can allow shaping the materials by spark erosion cutting. There is only a few published papers on the Si3N4-graphen system, concerning the thermal, mechanical and tribological properties [12-16]. They report the materials of silicon nitride matrix with low concentration of graphene, that were obtained by gas pressure sintering (GPS), hot isostatic pressing (HIP), and spark plasma sintering (SPS). The hot-pressed graphene-Si3N4 composites show a strong anisotropy of thermal conductivity [12]. Hvizdoš at al. obtained the Si3N4 composite materials containing up to 3 wt% graphene by the HIP method [13]. An additive of 1 wt% graphene improved fracture toughness. The lowest abrasive wear and friction coefficient were measured for the composites with 3% of graphene additive. Kvetková at al. reported GPSed and HIPed 1 wt% graphene – Si3N4 composites [14], focusing the work on revealing cracking mechanisms. Improved fracture toughness was observed after the addition of graphene when compared to a reference sample of polycrystalline pure silicon nitride.

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P. Rutkowski, L. Stobierski, G. Górny, M. Ziąbka, M. Urbanik

Walker at al. proved the possibility to sinter composites of 0-1.5 vol.% graphene and Si3N4 using the SPS method [15]. The composites showed a very fine microstructure, a very high hardness of circa 20 GPa and a fracture toughness between 2.8 MPa·m1/2 and 6.6 MPa·m1/2, increasing with the graphene content. Pfeifer at al. showed that the addition of graphene to HIPed silicon nitride could decreased the friction coefficient [16]. The presented work is focused on determination of bending strength and fracture toughness of hot-pressed Si3N4 matrix composites with dispersed graphene flakes. The influence of different weight fractions of the graphene particles on the value of these mechanical properties is studied. The tests show the impact of a crack propagation path on fracture toughness as a function of graphene content in the materials. The microstructural observations reveals cracking mechanisms in areas rich in the graphene phase, and are used for explanation of the measured values of mechanical properties. The work shows the original results of the research carried out in new graphene – Si3N4 composites, that have been described only in a few publications which concerned another manufacturing methods and sintering activators. The results refers to a graphene content up to 10 wt%, and all mechanical properties were measured by using large samples and the three-point bending method, what has not been shown in the literature yet.

2. Preparation and measurement route Si3N4-graphene composites were prepared from commercial powders: submicron silicon nitride Grade M11 of H.C. Starck, and nanometric graphene Grade AO-2 of Graphene Laboratories (Fig. 1). Aluminium nitride Grade C of H.C. Starck and yttria Grade C of H.C. Starck were used to activate the sintering. The activators were added to the Si3N4 powder in quantities of 2.5 wt% AlN and 4 wt% Y2O3. The component powders were blended to obtain mixtures containing 0, 0.5, 1, 2, 4, 6, 8 and 10 wt% of the graphene phase. The mixtures were homogenized using a rotary-vibratory mill. The homogenization step was performed in isopropyl

alcohol for 6 hours at the 80% chamber filling, using silicon nitride milling balls. Such a high filling rate of a milling chamber was aimed at the good dispersing and complete destroying agglomerates of the graphene flakes. After the drying and granulation processes, the mixtures were hotpressed by using a HP apparatus of Thermal Technology INC. The sintering under uniaxial pressing was carried out in a nitrogen flow under a pressure of 25 MPa with a 10 °C/min heating rate up to a temperature of 1750 °C and a holding time of 60 min at the maximum sintering temperature. The graphene-Si3N4 composite samples with a diameter of 50 mm were manufactured. Apparent densities of the sintered composites were measured by the hydrostatic method, and then relative densities were calculated. The graphene-Si3N4 composite samples were polished in directions perpendicular and parallel to a pressing axis during the HP sintering process. The materials prepared in such a way were subjected to XRD phase analysis, and then amounts of α-Si3N4 and β-Si3N4 phases were determined. Identification of the graphene phase was performed by Raman spectroscopy (a Horriba Yvon Jobin LabRAM HR micro-Raman spectrometer equipped with a CCD detector). Microstructural observations of polished surfaces and fractures of the graphene-Si3N4 composites were conducted by means of scanning electron microscopy (SEM). The chemical analysis of silica and carbon was performed using the EDS method. The flexural strength was determined by the three-point bending test of a beam of 3 mm × 5 mm × 25 mm in size at a support span of 20 mm and a feed rate of 1 mm/min. The critical stress intensity factors, KIc, were calculated based on the Evans’ formula using the results of three-point bending of notched beams of 3 mm × 5 mm × 25 mm in size with at a support span of 20 mm, and a feed rate of 0.1 mm/min. The notches were 0.15 mm and 1.5 mm in width and depth, respectively. Cracks were generated by Vickers indentation on both polished surfaces and fractures of the composite samples, and propagation paths of the cracks were observed by SEM.

3. Results and discussion The apparent and relative densities of graphene-Si3N4 composites as a function of graphene concentration are shown in Table 1. The results show that the hot-pressed graphene-Si3N4 composites are well densified, and the relative densities are above 98% of theoretical density. Table 1. Densification of hot-pressed Si3N4-graphene composites.

Fig. 1. Morphology of graphene powder Grade AO-2.

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MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 66, 4, (2014)

Graphene powder addition [wt%]

Apparent density [g/cm3]

Relative density [%]

0

3.21±0.01

99.9±0.1

0.5

3.18±0.01

99.4±0.1

1

3.23±0.01

99.9±0.1

2

3.23±0.01

99.9±0.1

4

3.09±0.01

97.8±0.1

6

3.09±0.01

98.8±0.1

8

3.06±0.01

98.5±0.1

10

3.03±0.01

98.4±0.1

Fracture toughness of hot-pressed Si3N4-graphene composites

Fig. 2. Phase composition of graphene-Si3N4 composites as a function of graphene concentration.

a)

Fig. 3. Raman spectra of graphene phase in Si3N4 matrix composites.

b)

Fig. 4. Identification of graphene phase in 10 wt% graphene – Si3N4 composite by point EDS analysis: a) EDS spectra from selected points, b) SEM images with marked points of EDS measurements. The graphene phase is found in the point 1.

The phase composition and Raman spectra of graphene-Si3N4 composites are shown in Figs. 2 and 3. The results of phase composition analysis indicate the presence of α-Si3N4, β-Si3N4 and also very small amounts of YAM (Y4Al2O9) in the studied composites. The YAM phase is a result of a chemical reaction between AlN and Y2O3 added as the sintering activators. The incorporation of graphene leads to a decrease in the content of beta silicon nitride. Graphene can impede access of a liquid phase (melted YAM) to α-Si3N4 grains, block their dissolution, and thus transformation to the β-phase. Raman spectroscopy allowed us to confirm the presence of graphene in the all prepared composites (Fig. 3), taking into consideration wavenumbers of pure graphene powder. The EDS analysis carried out in a fracture of 10 wt% graphene – Si3N4 composite in selected points is shown in Fig. 4. The silica and carbon contents indicated by EDS and the above discussed Raman spectroscopy analysis indicate the graphene phase in the form of thin plate-shaped particles in the composite’s microstructure. Aluminium detected by EDS comes from the phase activating the sintering. In order to show quality of graphene flakes distribution in the Si3N4 matrix, the microstructural SEM observation were performed. The images of the composite surface taken in both a parallel and a perpendicular direction to the pressing axis are shown in Figs. 5a and 5b, respectively. The results

illustrate the influence of the direction of the pressing force applied during the sintering on the arrangement of graphene flakes. The orientation effect is stronger in the case of microstructure perpendicular to the pressing direction, which should have a significant positive influence on thermal conductivity of the graphene-Si3N4 composites. The composite materials were mechanically examined. The results of bending strength and fracture toughness measurements are shown in Figs. 6 and 7, respectively. The determined values of mechanical properties show that the addition of graphene up to 4 wt% permits to obtain the composites with good values of bending strength and fracture toughness of above 600 MPa and above 7.5 MPa·m1/2, respectively. Higher concentrations of the graphene phase in the Si3N4 matrix decrease strongly these values Crack propagation paths in the Si3N4-graphene composites were investigated in order to explain the influence of graphene concentration on the values of mechanical properties measured. The results of observations of cracks generated by a Vickers indenter in the polished surface of the studied composites are shown in Fig. 8. The images of the composites with low fraction of the graphene phase, below 4 wt%, show the cracks propagating mostly across grain boundaries rich with the YAM phase, bonding the silicon nitride matrix grains (Figs. 8a and 8b). The cracks are deflected between the Si3N4 grains as is also well visible in Fig. 8c, showing the


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a)

b)

Fig. 5. SEM images of microstructure of 10 wt% graphene – Si3N4 composites: a) pressing direction, b) direction perpendicular to the pressing.

Fig. 6. Bending strength of graphene-Si3N4 composites as a function of graphene concentration.

composite with 8 wt% graphene. The higher concentrations of the dispersed graphene phase than 4 wt% (Figs. 8c and 8d) lead to cracks that start to propagate straight through dark areas in the microstructure, being attributed to the graphene grains. Such a way of crack propagation is responsible for the decrease in both fracture toughness and bending strength of the studied composites. The fracture images shown in Fig. 9 prove the textured graphene flakes as a result of the uniaxial pressing applied during the sintering process, and reveal the graphene flakes that extend above the fracture surface, suggesting some exfoliation of the graphene flakes during the crack propagation; this creates a replacement of the bridging toughening mechanism, but the contribution of this toughening mechanism to overall increase in fracture toughness is negligible as indicated by the results shown in Fig. 7. The observation of crack propagation in fractured surfaces despite of the polished ones give some additional light to a way in which cracks propagate when the graphene flakes exist. For that purpose, Vickers indents were made in frac-

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Fig. 7. Critical stress intensity factor of graphene-Si3N4 composites as a function of graphene concentration.

tures of the composites, and the resultant crack propagation paths are shown in Fig. 10. The crack deflection among the silicon nitride grains is detected in case of the 1 wt% graphene – Si3N4 composite (Fig. 10a), confirming that the lowest additions of graphene have no significant influence on mechanical properties of the composites. Other features of cracking are observed, when there is more graphene phase in the manufactured materials. The SEM images taken from the indented fractures of the 4 wt% graphene – Si3N4 composites indicate that the cracks propagate along graphene layers in the flakes (Fig. 10b) or between the graphene flake and the matrix phase (Fig. 10c), where the interface is very weak. This two ways of crack propagation in the graphene reach areas give contribution to the decrease in fracture toughness and bending strength of the composites. The crack propagates also among the Si3N4 grains and across the YAM phase (Fig. 10d) similarly to the composites with low concentration of graphene.


a)

b)

d)

c)

Fig. 8. SEM images of surfaces of graphene–Si3N4 composites showing crack propagation paths as a function of graphene concentration: a) 1 wt%, b) 4 wt%, c) 8 wt%, and d) 10 wt%.

a) b) Fig. 9. SEM images of fracture of graphene-Si3N4 composites in direction perpendicular to the pressing: a) 6 wt% graphene, and b) 10 wt% graphene.


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a) b)

c) d)

Fig. 10. SEM images of crack propagation paths in fractures of graphene-Si3N4 composites: a) 1 wt% graphene – Si3N4 composite, b) fracture of 4 wt% graphene – Si3N4 composite with cracks propagating between the graphene flakes, c) fracture of 4 wt% graphene – Si3N4 composite with cracks propagating between a graphene flake and a matrix phase, d) microcracks propagating through the matrix close to a Vickers’ indent in 4 wt% graphene – Si3N4 composite.

4. Conclusions The hot-pressing process allows manufacturing very well densified Si3N4-graphene composites. After the hot-pressing process the graphene phase exists in the material, as confirmed by Raman spectra analysis. The uniaxially applied pressure during the sintering leads to the texture of graphene flakes in the Si3N4 matrix. The fracture toughness and bending strength values of the silicon nitride matrix composites with up to 4 wt% graphene remain practically unchanged. In this case the crack propagates mostly across the YAM phase, and there is deflection among Si3N4 grains (weak interfacial and intergranular boundaries). The addition of more than 4 wt% of graphene strongly decreases the bending strength and the fracture toughness of the silicon nitride matrix composites.

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The crack propagation both along weakly bonded graphene layers of the flakes and through weak graphene/Si3N4 interfaces is responsible for a decrease in mechanical properties. The cracking process generates some defoliation of the graphene particles. The alpha/beta silicon nitride ratio can noticeably influence the mechanical properties of materials with up to 4 wt% of the graphene phase. For higher concentration of graphene, the graphene particles distributed in the silicon nitride matrix start to play a significant role in controlling fracture toughness.

Acknowledgement The study constitutes a part of the project “Ceramic composites with graphene content as cutting tools and device parts with unique properties no. GRAF-TECH/ NCBR/03/05/2012.


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♦ Received 21 July 2014, accepted 29 October 2014.


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