Widok Tom 66 Nr 4 (2014)

www.ptcer.pl/mccm

P

R

, L

S

, G

G

, M

Z

, M

U

AGH University of Science and Technology, Faculty of Material Science and Ceramics, al. A. Mickiewicza 30, 30-059 Kraków, Poland

*e-mail: pawel.rutkowski@agh.edu.pl

1. Introduction

Graphene is a new monolayer powerful material, which shows high applicative potentials in electronics, high-ther-mal conductivity parts of different devices and in ceramic matrix composites because of its very good thermal, electri-cal and mechanielectri-cal 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 mechani-cal 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

Fracture toughness of hot-pressed Si

₃

N

₄

-graphene

composites

Abstract

The work concerns the infl uence of microstructure on fracture toughness of hot-pressed silicon nitride-graphene composites. The pha-se analysis was carried out to identify constituent phapha-ses of the matrix. The prepha-sence 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 gra-phene 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 zidentyfi kowania 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. Interpre-tację wyników pomiaru KIc podparto obserwacjami mikroskopowymi (SEM) propagacji pęknięć.

Słowa kluczowe: Si3N4, grafen, kompozyty, odporność na pękanie, propagacja pęknięcia

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 pha-se, 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

-gra-phen system, concerning the thermal, mechanical and tri-bological 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 coeffi cient were me-asured for the composites with 3% of graphene additive.

Kvetková at al. reported GPSed and HIPed 1 wt% gra-phene – Si3N4 composites [14], focusing the work on

reve-aling cracking mechanisms. Improved fracture toughness was observed after the addition of graphene when compared to a reference sample of polycrystalline pure silicon nitride.

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 mechani-cal 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

com-mercial powders: submicron silicon nitride Grade M11 of H.C. Starck, and nanometric graphene Grade AO-2 of Gra-phene 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 conta-ining 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

were calculated. The graphene-Si3N4 composite samples

were polished in directions perpendicular and parallel to a pressing axis during the HP sintering process. The mate-rials prepared in such a way were subjected to XRD phase analysis, and then amounts of -Si3N4 and -Si3N4 phases

were determined. Identifi cation 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 per-formed using the EDS method.

The fl exural 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 ob-served 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 densifi ed, and the

rela-tive densities are above 98% of theoretical density.

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

Table 1. Densifi cation of hot-pressed Si3N4-graphene composites.

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

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 -Si3N4,

-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

acti-vators. The incorporation of graphene leads to a decrease in the content of beta silicon nitride. Graphene can impede ac-cess of a liquid phase (melted YAM) to -Si3N4 grains, block

their dissolution, and thus transformation to the -phase. Raman spectroscopy allowed us to confi rm 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 fl akes 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 infl uence of the direction of the pressing force applied during the sintering on the arrangement of graphene fl akes. The orientation effect is stronger in the case of mi-crostructure perpendicular to the pressing direction, which should have a signifi cant positive infl uence on thermal con-ductivity of the graphene-Si3N4 composites.

The composite materials were mechanically examined. The results of bending strength and fracture toughness me-asurements 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 com-posites with good values of bending strength and fracture

toughness of above 600 MPa and above 7.5 MPa·m1/2_,

re-spectively. Higher concentrations of the graphene phase in the Si3N4 matrix decrease strongly these values

Crack propagation paths in the Si3N4-graphene

composi-tes were investigated in order to explain the infl uence of gra-phene 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 compo-sites 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 defl ected between the Si3N4 grains as is also well visible in Fig. 8c, showing the

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

func-tion of graphene concentrafunc-tion. Fig. 3. Raman spectra of graphene phase in Si3N4 matrix composites.

a) b)

Fig. 4. Identifi cation of graphene phase in 10 wt% graphene – Si3N4 composite by point EDS analysis: a) EDS spectra from selected points,

tures of the composites, and the resultant crack propagation paths are shown in Fig. 10.

The crack defl ection among the silicon nitride grains is detected in case of the 1 wt% graphene – Si3N4 composite

(Fig. 10a), confi rming that the lowest additions of graphene have no signifi cant infl uence 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 fractu-res of the 4 wt% graphene – Si3N4 composites indicate that

the cracks propagate along graphene layers in the fl akes (Fig. 10b) or between the graphene fl ake 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.

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 gra-phene grains. Such a way of crack propagation is responsi-ble 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 fl akes as a result of the uniaxial pressing applied during the sintering process, and reveal the graphene fl akes that extend above the fracture surface, suggesting some exfoliation of the graphene fl akes during the crack propaga-tion; this creates a replacement of the bridging toughening mechanism, but the contribution of this toughening mecha-nism to overall increase in fracture toughness is negligible as indicated by the results shown in Fig. 7.

The observation of crack propagation in fractured surfa-ces despite of the polished ones give some additional light to a way in which cracks propagate when the graphene fl akes exist. For that purpose, Vickers indents were made in

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. Fig. 7. Critical stress intensity factor of graphene-Si3N4 composites

a) b)

c) d)

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

4. Conclusions

The hot-pressing process allows manufacturing very well densifi ed Si3N4-graphene composites.

After the hot-pressing process the graphene phase exists in the material, as confi rmed by Raman spectra analysis.

The uniaxially applied pressure during the sintering leads to the texture of graphene fl akes in the Si3N4 matrix.

The fracture toughness and bending strength values of the silicon nitride matrix composites with up to 4 wt% gra-phene remain practically unchanged. In this case the crack propagates mostly across the YAM phase, and there is de-fl ection among Si3N4 grains (weak interfacial and

intergra-nular 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.

The crack propagation both along weakly bonded gra-phene layers of the fl akes and through weak gragra-phene/Si3N4

interfaces is responsible for a decrease in mechanical pro-perties.

The cracking process generates some defoliation of the graphene particles.

The alpha/beta silicon nitride ratio can noticeably infl uen-ce the mechanical properties of materials with up to 4 wt% of the graphene phase. For higher concentration of graphe-ne, the graphene particles distributed in the silicon nitride matrix start to play a signifi cant role in controlling fracture toughness.

Acknowledgement

The study constitutes a part of the project “Ceramic composites with graphene content as cutting tools and 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 fl akes, c) fracture of 4 wt% graphene – Si3N4

composite with cracks propagating between a graphene fl ake and a matrix phase, d) microcracks propagating through the matrix clo se to

device parts with unique properties no. GRAF-TECH/ NCBR/03/05/2012.

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Received 21 July 2014, accepted 29 October 2014.