Tribological Properties of Si3N4/SiC Nano–Nano Composite Ceramics

Jung-Hoo Shin, B. Venkata Manoj Kumar, Jae-Hee Kim, and Seong-Hyeon Hong†,*

Department of Materials Science and Engineering and Research Institute of Advanced Materials,Seoul National University, Seoul, 151-744, Korea

Fabrication of Si2N2O phase free, fully densified Si3N4/SiC

nano–nano composite ceramics was done by combining a car-

bothermal reduction treatment (CRT) and a spark plasma sin-

tering (SPS) using commercially available Si3N4 and SiCnano-powders, and their mechanical and tribological properties

were investigated. CRT was conducted at 1450°C for 10 h with

a phenolic resin and SPS was carried out at 1550°C for 5 min,

which yielded the fully dense (>98% of theoretical density)Si3N4/SiC nano–nano ceramics with ~100 nm grain size. The

elastic modulus, hardness, and fracture toughness increased

with increasing SiC content up to 20 wt%. The wear rate of

Si3N4/SiC nano–nano ceramics decreased from 2.0310�5

to6.76310�6 mm3/Nm with 20 wt% SiC addition, resulting from

the increased hardness and fracture toughness.

I. Introduction

S ILICON nitride-based ceramics are one of the most impor-tant engineering materials because of their superior

mechanical properties including good oxidation resistance,low thermal expansion, high hardness, and outstanding wearproperties, which allow them to be used in tribological appli-cations such as turbocharger, valves, ball bearings, cuttingtools, etc.1,2 The mechanical properties of Si3N4-basedceramics have been tailored by incorporating the secondphase (SiC, TiC, TiN, etc.) in the form of plates, whiskers,or particles, and a significant improvement has been achievedin hardness, strength, fracture toughness, and creep resis-tance.2 There were several attempts to improve the tribologi-cal performance by introducing the second phase ormicrostructure control, but the reported results were oftencontradictory or a clear relationship could not be obtained.3–8

Recently, the wear resistance of monolithic Si3N4 ceramicswas significantly improved by reducing the grain size tonano-meter range.7,9,10 Further improvement was achievedby developing the nano/nano composite such as Si3N4/TiN,which was achieved by mechano-chemical grinding andpulsed electric current sintering.11 Si3N4/SiC nano–nanocomposites have been fabricated by high pressure sinteringor electric field-assisted sintering of an amorphous Si–C–Npowder derived from the pyrolysis of a polymer precursorand showed the superior creep resistance at 1400°C undercompression,12–14 but other mechanical properties such aswear behavior have not been reported in these composites.

In the present study, Si2N2O phase free Si3N4/SiC nano–nano composite ceramics were fabricated using commerciallyavailable Si3N4 and SiC nano-powders and their mechanical

and tribological properties were investigated. For this, acombined processing method was employed, i.e., a carbother-mal reduction treatment to remove the excess oxygen and toavoid the Si2N2O phase formation and a spark plasma sinter-ing (SPS) to achieve a rapid consolidation with a limitedgrain growth.

II. Experimental Procedures

Commercially available Si3N4 nano powder containing 6 wt% Y2O3 and 3 wt% Al2O3 (~30 nm; PCT6Y3A, Plasma andCeramic Technology, Salaspils, Latvia) and SiC nano powder(<100 nm; Sigma Aldrich, St. Louis, MO) were used as thestarting materials. The SiC content in the composite was var-ied from 0 to 30 wt%. For carbothermal reduction, Si3N4

and SiC powders were mixed with 10 wt% phenolic resin(~5 wt% C; Kangnam Chemical Co., Seoul, Korea) by ballmilling in acetone using Si3N4 ball and polythene jar. Afterdrying in an oven, the powder was uniaxially pressed intopellets of 10 mm in diameter and then cold isostaticallypressed at 100 MPa. The powder compact was heat treatedat 1450°C for 10 h in N2 flowing atmosphere. For SPS, thecarbothermally reduced compact was placed into a 10 mmgraphite die and an electric current of ~1500 A was appliedunder a pressure of ~30 MPa in N2 flowing atmosphere. Theheating rate was 100°C/min and the sintering was conductedat 1550°C for 5 min.

The apparent density of the sintered specimens was mea-sured using the Archimedes method in deionized water. Thephases were examined by X-ray diffraction (XRD;M18XHF-SRA, MAC Science Co., Ltd., Yokohama, Japan)and the fracture morphology was observed by a field emis-sion scanning electron microscope (FE-SEM; JSM7401F,JEOL, Tokyo, Japan). Microstructure and chemical elementanalysis were conducted by an analytical transmission elec-tron microscope (TEM; Tecnai F20, FEI, Hillsboro, OR).The elastic modulus (E) was determined by an ultrasonicpulse-echo testing (Tektronix TDS 220; Tektronix Inc., Bea-verton, OR). Vickers Hardness (Mituyoto, Kawasaki, Japan)was measured at 1 kg load for 15 s, while fracture toughnesswas estimated from the crack length measurements based onAnstis’s formula after indenting at 20 kg load for 15 s.15 Aunidirectional ball-on-disk tribometer was used to evaluatethe friction and wear characteristics of sintered specimens inambient conditions (20±5°C, 50 ± 10% RH). As a counter-body material, Si3N4 ball of 6.35 mm diameter was used andthe detailed description of wear test can be found in the pre-vious report.10

III. Results and Discussion

The XRD patterns of the sintered specimens with variousamount of SiC are shown in Fig. 1. Only b-Si3N4 and SiCwere observed in the diffraction patterns, and the peak inten-sity of SiC increased with SiC contents. No secondary phase

R. Scattergood—contributing editor

Manuscript No. 30038. Received July 21, 2011; approved August 11, 2011.*Member, American Ceramic Society.†Author to whom correspondence should be addressed. e-mail: shhong@snu.ac.kr.

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J. Am. Ceram. Soc., 94 [11] 3683–3685 (2011)

DOI: 10.1111/j.1551-2916.2011.04847.x

© 2011 The American Ceramic Society

Journal

such as Si2N2O was found within the detection limit. Thestarting Si3N4 nano powder contains a significant amount ofoxygen (~4.6 wt%),16 but the carbothermal reduction usingphenolic resin was very effective to remove the oxygen in thepowder and to fabricate the Si2N2O phase-free Si3N4/SiCcomposite ceramics.

A nearly full densification (� 98% of theoretical density)was successfully achieved by SPS up to 30 wt% SiC addition.However, a 40 wt% SiC added composite was not fully con-solidated at 1550°C (~88% of theoretical density) and itrequired a higher temperature to densify, which resulted in asignificant grain growth.

The fracture morphologies of fabricated Si3N4/SiC com-posite ceramics are shown in Fig. 2. A nano-grained mono-lithic Si3N4 ceramics was developed at 1550°C. Some ofgrains were elongated, but the aspect ratio was not greaterthan 3 [Fig. 2(a)]. With addition of SiC, the grains becameequi-axed, and the microstructure was composed of ~100 nmsized matrix and ~50 nm sized spherical, dispersed particles[Fig. 2 (b)]. The number of dispersed particles increased withSiC contents [Figs. 2(c) and (d)] and the dispersed particlesare believed to be SiC nanocrystallites. The grain size esti-mated from the polished and plasma etched surfaces by lin-ear intercept method was 120 and 100 nm for monolithicSi3N4 and Si3N4/SiC (20 wt%) ceramics, respectively. The100 nm sized grains were also confirmed by TEM [Fig. 3(a)].Thus, Si3N4/SiC nano–nano composite ceramics were suc-cessfully developed by combining carbothermal reductionand SPS.

An electron energy loss spectroscopy (EELS) analysis wascarried out to determine the elemental distribution in thenano–nano composite (20 wt% SiC) and the resulting EELSmappings of Si, O, N, and C are shown in Fig. 3. The twophases, Si3N4 and SiC, were relatively well distributedthroughout the specimen. The oxygen was mainly located atthe grain boundary and triple junction forming the glassygrain boundary phase. Some of the carbons which weredetected at the grain boundary, originated from the residualphenolic resin.

The elastic modulus and hardness of the developed Si3N4/SiC composites linearly increased with increasing the SiCcontents (elastic modulus: 246, 307, 318, and 325 GPa andhardness: 16.5 ± 0.12, 16.6 ± 0.13, 18.1 ± 0.16, and 18.7 ±0.19 GPa for 0, 10, 20, and 30 wt% SiC added Si3N4, respec-tively). The fracture toughness increased from 2.58 to3.67 MPa·m1/2 up to 20 wt% SiC addition and then slightlydecreased with further addition (3.35 MPa·m1/2 at 30 wt%

SiC). Thus, both hardness and fracture toughness weresimultaneously improved by producing the nano–nano com-posite.17 It was reported that Si3N4/SiC nano–nano compos-ites had a hardness of 18.6 GPa and a fracture toughness of3.2 MPa·m1/2 (200 nm of Si3N4 and 200 nm of SiC).18

When subjected to unlubricated sliding against Si3N4 balls,all sintered Si3N4/SiC nano–nano composite ceramics exhib-

Fig. 1. XRD patterns of developed Si3N4/SiC nano–nanocomposite ceramics; (a) 0 wt% SiC, (b) 10 wt% SiC, (c) 20 wt%SiC, and (d) 30 wt% SiC.

(a) (b)

(c) (d)

Fig. 2. FESEM micrographs of fractured morphology for thedeveloped Si3N4/SiC nano–nano composite ceramics; (a) 0 wt% SiC,(b) 10 wt% SiC, (c) 20 wt% SiC, and (d) 30 wt% SiC.

(a) (b)

(c) (d)

(e)

Fig. 3. (a) TEM micrograph and EELS mapping of (b) Si, (c) O,(d) N, and (e) C of developed Si3N4/SiC (20 wt%) nano–nanocomposite ceramics.

3684 Rapid Communications of the American Ceramic Society Vol. 94, No. 11

ited an almost identical evolution of coefficient of friction(COF) with time regardless of the SiC content. The generalobservation was that COF increased to a peak value duringinitial 100 s (running-in-period) and thereafter maintaineda steady state. The average steady state COF decreasedfrom 0.52 to 0.43 with increasing the SiC content up to20 wt% and then slightly increased at 30 wt% SiC (Fig. 4).The experimentally determined wear rate of Si3N4/SiC com-posites was reduced from 2.0 9 10�5 to 7 9 10�6 mm3/Nmwhen the SiC content was varied from 0 to 20 wt%. Thereported COF and wear rate in Si3N4/SiC micro-nano com-posite were 0.64–0.71 and 0.89–1.91 9 10�5 mm3/Nm,respectively.8 Consequently, Si3N4/SiC nano–nano compositewith 20 wt% SiC showed the lowest COF and highest wearresistance resulting from the increased hardness and fracturetoughness.

IV. Conclusions

Thus, Si3N4/SiC nano–nano composite ceramics were suc-cessfully prepared by carbothermal reduction and sparkplasma sintering. The fabricated composites were fully dense(>98% of theoretical density) up to 30 wt% SiC additionand the grain size was ~100 nm. The elastic modulus, hard-ness, fracture toughness, and wear resistance increased withincreasing SiC content up to 20 wt%. Thus, nano–nano com-posites exhibited improved mechanical and tribological prop-erties.

Acknowledgments

This study was supported by a grant from the Fundamental R&D Programfor Core Technology of Materials funded by the Ministry of Knowledge Econ-omy, Republic of Korea.

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Fig. 4. Average coefficient of friction (COF) and wear rate of thedeveloped Si3N4/SiC nano–nano composite ceramics.

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