Effects of Graphite Flake Diameter on Mechanical Properties and Thermal Shock

7/29/2019 Effects of Graphite Flake Diameter on Mechanical Properties and Thermal Shock

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Effects of graphite flake diameter on mechanical properties and thermal shock

behavior of ZrB2-nano SiC-graphite ceramics

Yang Hou, Ping Hu, Xinghong Zhang, Kaixuan Gui

PII: S0263-4368(13)00058-9

DOI: doi: 10.1016/j.ijrmhm.2013.02.015

Reference: RMHM 3566

To appear in: International Journal of Refractory Metals and Hard Materials

Received date: 29 November 2012

Accepted date: 21 February 2013

Please cite this article as: Hou Yang, Hu Ping, Zhang Xinghong, Gui Kaixuan, Effectsof graphite flake diameter on mechanical properties and thermal shock behavior of ZrB2-nano SiC-graphite ceramics, International Journal of Refractory Metals and Hard Materials(2013), doi: 10.1016/j.ijrmhm.2013.02.015

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http://dx.doi.org/10.1016/j.ijrmhm.2013.02.015http://dx.doi.org/10.1016/j.ijrmhm.2013.02.015http://dx.doi.org/10.1016/j.ijrmhm.2013.02.015http://dx.doi.org/10.1016/j.ijrmhm.2013.02.015


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Effects of graphite flake diameter on mechanical properties and

thermal shock behavior of ZrB2-nano SiC-graphite ceramics

Yang Hou*, PingHu, Xinghong Zhang, Kaixuan Gui

Science and Technology on Advanced Composites in Special Environment Laboratory,

Harbin Institute of Technology, Harbin, 150001, P. R. China

Abstract

ZrB2-20 vol.% nano SiC-20 vol.% graphite (ZSnpG) ceramics that contained

graphite flakes with different diameters were investigated to determine the effects of

graphite flake diameter on the microstructure and mechanical properties, as well as

thermal shock resistance. The results showed that the flexural strength and relative

density of ZSnpG ceramics firstly increased and then decreased with increasing the

diameter of graphite, whereas the fracture toughness did not change significantly. The

improvement in the flexural strength of ZSnpG ceramics was strongly dependent on

the controlling of graphite distribution, the reduction of the length of the microcrack

that resulted from thermal residual stress and the presence of intragranular nano-sized

SiC particles. The calculated thermal shock parameters revealed that the graphite with

finer starting diameter had a beneficial effect on the blocking of the crack initiation

and the graphite with larger diameter restrained the crack propagation of ZS npG

ceramics.

Keywords: Hot pressing; ZrB2-nanoSiC-graphite ceramics; Mechanical properties;

Thermal shock resistance;

* Corresponding author: Yang Hou

Tel/ Fax: +86 451 86402382, E-mail addresses: [email protected]


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1. Introduction

Borides, carbides, and nitrides of the transition metals comprise a family of

materials known as ultra-high temperature ceramics (UHTCs). Zirconium diboride

(ZrB2) based UHTCs has a desirable combination of high melting point (>2500C),

low density, resistance to chemical attack and other physical properties that make it

attractive for applications such as thermal protection materials on hypersonic

aerospace vehicles, and reusable atmospheric re-entry vehicles and rocket propulsion

[1-9].

The mechanical properties and oxidation resistance of ZrB2-based ceramics can be

significantly improved by introducing nanosized SiC particles (SiCnp) as the result of

the formation of intragranular nanostructures [10-13]. However, this approach cannot

overcome their intrinsic brittleness and thermal shock resistance, which limit their use

in extreme environmental applications [14]. One possible route that incorporating the

phase additions with a higher aspect ratio into the ZrB2-based ceramics was used to

overcome these deficiencies [15]. Natural graphite flake has been widely used in

aerospace fields and the reliability of brittle materials can be improved by the

introduction of graphite for the reason that graphite can deflect the crack propagation

and release the residual stress [16,17]. Previous studies have shown that a pronounced

increment in thermal shock resistance can be achieved with the controlled

introduction of graphite flake into ZrB2-SiC (ZS) ceramics while preserving

acceptable levels of oxidation resistance as reported in literature [18,19]. Actually, the

flexural strength of ZS ceramics will be obviously reduced with the addition of

graphite for the reason that the graphite flake acted as flaws in the ZrB2-SiC-graphite

(ZSG) ceramics. Besides, the reduction of flexural strength also resulted from the

weaker bonding within the ZSG ceramics and the lower load transfer ability of ZSG


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ceramics since the lower mechanical properties of the graphite flake [18]. Therefore,

the improvement of the flexural strength of graphite-containing ZrB2-based ceramics

and the effects of graphite diameter on the mechanical properties and thermal shock

behavior need to be investigated.

In the present work, ZrB2-20 vol.% nano SiC-20 vol.% graphite ceramics

containing the graphite flakes with various diameters were investigated and the

predicted thermal shock parameters (R, RIV) were compared with the experimental

thermal shock behavior to determine the effects of graphite diameter on the

microstructure, the mechanical properties and thermal shock resistance of the ZSnpG

ceramics. The aim of this paper is to find a method to obtain high comprehensive

performance of ZrB2-based ceramics and lay the foundation for the study of UHTCs.

2. Material and methods

2.1. Materials

Commercially available ZrB2 with a mean size of about 2 m, SiC with a mean size

of about 50 nm were purchased from Alfa Aesar Co., Ltd., USA. Three kinds of

graphite with different mean diameters have been used (mean diameter: 5 m (G1), 10

m (G2), 20 m (G3); thickness: about 1.5 m; Kaier Nanometer Energy &

Technology Co. Ltd., Hefei, China). PEI (10000) was used as dispersant (aladdin Co.

Ltd., Shanghai, China). Absolute ethyl alcohol with a water level around 0.1% was

used as a solvent for all suspensions.

2.2. Suspension Preparation and Sintering

The suspensions used for all measurements were prepared by adding the

as-received powder to ethanol at selected powder mass percentages. The dosage of

PEI was expressed as a dry weight percentage of powder basis (dwb%). The powder

mixtures of ZrB2 plus 20 vol.% nano SiC and 20 vol.% graphite with different


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diameters (labeled as ZSnpG1, ZSnpG2, and ZSnpG3) were added to the premixed

solution with the desired dispersant content and ultrasonically deflocculated

(JYD-801, JIATUANDA Technology Co. Ltd, Shenzhen, China) at an output power

of 160 W in an ice bath, then ball milled after ultrasonication, with SiC balls

(diameter: 2-3 mm) as the ball milling media at 220 rpm for 8 h (in order to reduce the

mean graphite diameter


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selected temperature in an electric resistance furnace and then dropped into a water

bath of 20 C. Retained flexural strength was tested after thermal shock for all the

specimens.

3. Results and discussion

3.1 Microstructure characteristics and mechanical properties

Figure 1 shows typical SEM micrographs of the fracture surfaces of the ZSnpG

ceramics. The results of EDS analysis demonstrated (not shown here) that the small

darker phase was SiC. High-magnification SEM images shown in Fig. 1b revealed

that nano SiC particle was observed within ZrB2 grains (intragranular) as indicated by

arrow. The as-prepared ZSnpG ceramics showed a preferred orientation of the graphite

flake with its basal planes perpendicular to the hot pressing direction and the size of

graphite flake presented in the fracture surfaces was analogous to the raw graphite

flake.

The ZSnpG ceramics achieved near full density when the graphite diameter

increased from 5 to 10 m, and further increasing the graphite diameter to 20 m

resulted in a reduction of the relative density (98.7%) as listed in table 1. The

observed high relative densities of ZSnpG ceramics were due to the fact that the

oxygen impurities on the borides surface were removed by reactions with graphite

while sintering at high temperatures [20]. Furthermore, fine ZrB2, ZrC and SiC

particles formed in situ on the particle surface had a high sinterability, which also

provide a higher sintering driving force, since densification is driven by the

minimization of surface free energy [14]. The reason for reduction of the relative

density of ZSnpG3 will be discussed in the subsequent research.

The flexural strength of the ZSnpG ceramics containing graphite with different

diameters is also summarized in Table 1. The results indicated that the flexural


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strength of ZSnpG ceramics increased when the diameter of graphite increased from 5

to 10 m and then decreased as the diameter of graphite increased from 10 to 20 m,

whereas the fracture toughness did not change significantly. The flexural strength of

the ZSnpG2 was as high as 523.06 MPa, which was much higher than as-reported ZSG

ceramic with the same volume fraction of SiC and graphite (367.77 MPa) [19]. It is

well-known that as a weakening factor, the introduction of graphite into ceramics will

reduce the flexural strength owing to the formation of weak interface [21]. Therefore,

the decrease of graphite diameter will increase the distribution capacity of graphite in

the matrix and greatly reduce the flexural strength, as ZSnpG1.

However, it cannot predicate that the larger the diameter of graphite, the higher the

flexural strength of ZSnpG ceramics. Due to the mismatching of the coefficients of

thermal expansion (CTE) among ZrB2, SiC and graphite, a thermal residual stress

develops in this material system and cause the formation of microcracks during the

cooling process [22]. Figure 2 shows the micrographs of the polished surfaces for

perpendicular specimens of ZSnpG containing graphite with different diameters. There

was no significant microcrack in Fig. 2a and b, but a long microcrack was found on

the boundary of ZrB2/graphite or SiC/graphite as indicated by a square in Fig. 2c.

These features exhibited that the smaller the graphite diameter, the fewer the

concentration of pre-existing connecting cracks and the lower the thermal residual

stress, which was consistent with the research on magnesia-spinel composites [23].

Finer particles resulted in shorter crack propagation distances from the particle, but

the larger particles were the origin of longer cracks. From this point of view, the

graphite with large diameter in ZSnpG3 will lead to the formation of microcracks,

which is the main reason for the reduction of relative density and flexural strength

[20].


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The pits on the polished surface for parallel specimens of ZSnpG ceramics showed

in Fig. 3 were graphite flakes, which revealed that significant pullout of graphite

occurred during the polishing process, indicating the weaker bonding within ZS npG

composite due to the presence of soft graphite [24]. So with the increase of graphite

diameter, the diameter of pits that resulted from the pull-out of graphite increased (Fig.

3). The large pits act as gaps, which lead to the stress concentration and the reduction

of the flexural strength during the test.Therefore, the combination of microcracks and

pits in ZSnpG3 may be the reason for the significant decreasing of flexural strength.

3.2 Thermal shock behavior

The flexural strength of the ZSnpG ceramics before and after thermal shock at

different temperature differences is shown in Fig. 4. As the thermal shock temperature

difference (T) increased up to 400 C, the flexural strength of the ZSnpG1 and ZSnpG2

ceramics after thermal shock were statistically equivalent to the original strength.

However, the flexural strength of the ZSnpG1 and ZSnpG2 that quenched from a

temperature difference of 400 C to 450 C was sharply reduced and kept steady with

increasing the temperature difference up to 600 C. The curve for ZSnpG3 showed a

characteristic drop of flexural strength afterT=350 C. This drop was followed by a

modest decrease as T further increased. The retention rate of the residual strength for

ZSnpG3after thermal shock at T=450C remained about 54% of the primary strength,

which was much higher than that for ZSnpG1 and ZSnpG2 (about 25% and 29%,

respectively). When the temperature difference was above 500 C, the retained

strength of ZSnpG3 maintain stable. The presence of these stages was a typical

characteristic of the Hansselmans model for thermal shock [25]. The critical

temperature difference is identified using a linear interpolation between points that

first reduced the average flexure strength of the quenched bars by more than 30% of


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the mean strength of the as-sintered material as described in ASTM C1525-04 [26].

According to this definition, the measured critical temperature differences of ZSnpG1,

ZSnpG2 and ZSnpG3 were almost the same (as high as about 425C), which was higher

than that of the ZrB2-15 vol.% SiC ceramic (385C) [27]. Nevertheless, the same

critical temperature differences of these materials do not mean that they had the same

response to thermal shock. It can be seen from the presented results that the

decreasing trend of flexure strength of ZSnpG3 was different from those of ZSnpG1 and

ZSnpG2 during water quenching.

In order to further evaluate the effect of graphite diameter on the thermal shock

behavior of ZSnpG ceramics, two thermal shock resistance parameters were used to

predict the crack initiation and propagation behavior induced by the thermal shock

stress of ceramic matrix composites [26]. One was the thermal stress fracture

resistance parameterR, expressed by Eq. (1) in terms of the strength of the material

(), Youngs modulus (E), Poissons ratio () and the coefficient of thermal expansion

().

(1 )R

E

(1)

The thermal shock parameter R predicts the maximum change in temperature (the

critical thermal shock temperature difference) that can occur without the initiation of

cracks. It is clear that high resistance to fracture initiation can be achieved in materials

with high strength and thermal conductivity, and with low values of thermal

expansion and Youngs modulus [23].

The other parameter was the thermal stress damage resistance parameter RIV, as

expressed in Eq. (2).

2

2 1-

ICK

R

(2)


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where is the strength of material, KIC is the fracture toughness and is Poissons

ratio, respectively. RIV decides the resistance to catastrophic crack propagation of

ceramics under a critical temperature difference. In terms of parameter RIV, the

superior thermal shock resistance depends on the higher toughness and Poissons ratio,

and the lower strength of the materials [28].

Because the curves of retained strength versus temperature difference for ZSnpG1

and ZSnpG2 had the same trend, as shown in Fig. 4, only the thermal shock parameter

of ZSnpG2 was taken for the comparison with that of ZSnpG3. From Table 1, the

calculated crack initiation parameterR of ZSnpG2 was approximately 11% larger than

that of ZSnpG3. It means that the crack initiation was more difficult for ZSnpG2 than

ZSnpG3 ceramic, which resulted in the higher starting temperature of flexure strength

reduction (Fig. 4). For the simplicity of the calculation, the magnitudes of the

Poissons ratio and the thermal expansion coefficient for ZSnpG were supposed to be

identical, respectively, due to the same of compositions. So the high flexural strength

of ZSnpG2 was the main reason for the increasing ofR.

The calculated crack propagation parameterRIV of ZSnpG3 was much larger (more

than 33%) than that of the ZSnpG2, indicating that the crack propagation was more

restricted for ZSnpG3 ceramics. Hence, unstable crack propagation may occur

infrequently in ZSnpG3 ceramics during water quenching, which was proved by the

modest decreasing of flexure strength and lower terminating temperature of strength

reduction (Fig. 4). The reason for this was that the larger diameter of graphite might

induce higher thermal residual stress, which has been studied in the former discussion.

When a crack propagates through the thermal residual stress field, its propagation

energy is dissipated and the crack is deflected or pinned. The thermal residual stress

also brings on interfacial friction between graphite and other phases when the graphite


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flake is pulled out during crack propagation. Meanwhile, because the graphite flake

boundary is a weak interlayer and possesses, crack deflection occurs at the boundaries

of graphite flake, as well as significant delamination cracking and sliding which

consume more crack propagation energy [29].

It should be noted that there was also a little difference between the thermal shock

resistance parameters of ZSnpG1 and ZSnpG2. Nevertheless, the thermal shock

resistance mode of ZSnpG1 was still the same as ZSnpG2. So it can be speculated that

there is a critical diameter of graphite. If the diameter of graphite that contained in

ZSnpG ceramics increases above the critical diameter, the thermal shock behavior

obviously changes, but if the diameter of graphite is below the critical diameter, the

thermal shock behavior of ZSnpG ceramics is similar. However, in order to understand

the underlying mechanism, the further analysis and theoretical study should be carried

out. Base on the above results, it can be confirmed that the graphite with finer starting

diameter had a beneficial effect on the blocking of crack initiation and the graphite

with larger diameter can restrain crack propagation of ZSnpG ceramics.

4. Conclusions

The mechanical properties and thermal shock behavior of ZSnpG ceramics

containing graphite flake with various diameters were investigated. The ZSnpG

ceramics achieved near full density when the graphite diameter increased from 5 to 10

m and the further increasing of graphite diameter to 20 m resulted in a reduction of

the relative density (98.7%). The flexural strength of ZSnpG ceramics firstly increased

and then decreased with increasing the diameter of graphite, whereas the fracture

toughness did not change significantly. The flexural strength of the ZSnpG2 was as

high as 523.06 MPa, which was much higher than ZSG ceramic with the same volume

fraction of SiC and graphite. The improvement of flexural strength was attributed to


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the controlling of the graphite distribution, the reduction of microcrack length, as well

as the presence of intragranular nano-sized SiC particles. The critical temperature

differences of ZSnpG ceramics containing graphite with different diameters were

almost the same, but the thermal shock behavior was different. The calculated thermal

shock parameters confirmed that the graphite with finer starting diameter had

beneficial effects on the blocking of crack initiation and the graphite with larger

diameter can restrain crack propagation of ZSnpG ceramics.

Acknowledgements

This work was supported by the National Science Foundation of China (51072042

and 51202048)

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Figure Captions

Fig. 1 SEM micrographs of fracture surface of the ZSnpG ceramics: (a) ZSnpG1, (b)

ZSnpG2, (c) ZSnpG3

Fig. 2 The SEM micrographs of polished surfaces for perpendicular specimens: (a)

ZSnpG1, (b) ZSnpG2, (c) ZSnpG3

Fig. 3 The SEM micrographs of polished surfaces for parallel specimens: (a) ZS npG1,

(b) ZSnpG2, (c) ZSnpG3

Fig. 4 Retained strength versus temperature difference for ZSnpG1, ZSnpG2 and ZSnpG3

ceramics

Table Captions

Table 1 The mechanical properties and relative densities of the ZSnpG ceramics

containing graphite with different diameters.


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Fig. 1 SEM micrographs of fracture surface of the ZSnpG ceramics: (a) ZSnpG1, (b)

ZSnpG2, (c) ZSnpG3


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Fig. 2 The SEM micrographs of polished surfaces for perpendicular specimens: (a)

ZSnpG1, (b) ZSnpG2, (c) ZSnpG3


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Fig. 3 The SEM micrographs of polished surfaces for parallel specimens: (a) ZS npG1,

(b) ZSnpG2, (c) ZSnpG3


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Fig. 4 Retained strength versus temperature difference for ZSnpG1, ZSnpG2 and ZSnpG3

ceramics


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Table 1 The mechanical properties and relative densities of the ZSnpG ceramics

containing graphite with different diameters.

MaterialsFlexural strength

(MPa)

Fracture toughness

(MPa.m1/2

)

Youngs modulus

(GPa)

Relative density

(%)ZSnpG1 451.9323.21 4.310.11 330.8919.43 99.4

ZSnpG2 523.0618.52 4.540.15 356.8224.64 100

ZSnpG3 405.5819.63 4.060.19 306.5227.96 98.7


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Highlights

With increase of graphite diameter, flexural strength increased and then decreased Relative density changed in the same trend as flexural strength Strength depend on graphite dispersion, microcrack length, intragranular particle The graphite with finer diameter can block crack initiation The graphite with bigger diameter can restrain crack propagation

Effects of Graphite Flake Diameter on Mechanical Properties and Thermal Shock

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