Mechanical and Tribological Properties of a Nano-Si3N4/Nano-BN Composite

Mohammad F. Wani*

Mechanical Engineering Department, National Institute of Technology Srinagar, Srinagar 190 006,Kashmir, India

The mechanical and tribological properties of a nano-Si3N4/nano-BN composite were studied. The composite was pre-pared via high-energy mechanical milling and subsequent spark plasma sintering. Y2O3 and Al2O3 were used as sinteringadditives. After sintering, the average crystalline size of Si3N4 and BN was 50 nm. Hardness (Vicker and Knoop) was evaluatedunder a high load of 0.05–2.0 kg for the nano/nano- and the micro/micro-Si3N4/BN composite with the same composition.The indentation fracture toughness values of both composites were also evaluated. Tribological studies were conducted tostudy the friction and wear behavior of both composites. A friction coefficient of 0.4–0.7 was obtained for the nano-S3N4/nano-BN composite under a normal load of 20–22 N, whereas, a friction coefficient of 0.37 was obtained for the micro-Si3N4/micro-BN composite. Specific wear coefficients of 0.418� 10�4 and 0.625� 10�4 mm3/N/m were obtained for nano-sizedand micro-sized Si3N4/BN composites, respectively. Higher hardness, higher fracture toughness, and lower wear were observedin the nano-sized composite, as compared with the micro-sized composite.

Introduction

Engineering ceramics are known for their low den-sity, high strength, superior thermal shock resistance,high wear resistance, fracture toughness, and chemicalinertness.1 These excellent mechanical and tribologicalproperties make them suitable candidate materials for a

wide range of mechanical and tribological applications,under harsh conditions of load, temperature, and environ-ment.2–7 In addition, these materials are preferred overnonlubricated and marginally lubricated applications.Compared with other engineering ceramics silicon nitridehas superior mechanical and tribological properties.5,6 Sil-icon nitride has long been used for the design and devel-opment of hybrid bearings under lubricated conditions.2,7

However, under dry conditions, the sliding contact of aself-mated tribopair (Si3N4/Si3N4) yields a high friction

Int. J. Appl. Ceram. Technol., 7 [4] 512–517 (2010)DOI:10.1111/j.1744-7402.2009.02362.x

Ceramic Product Development and Commercialization

*mfwnfw2002@yahoo.com

r 2009 The American Ceramic Society

coefficient (m) and a high wear rate.8–10 This problem isfurther exacerbated by the intrinsic brittle nature of a sil-icon nitride ceramic. The high friction and brittle nature ofsilicon nitride material results in poor reliability of siliconnitride bearings at the operational stage. Development ofself-lubricating composites of Si3N4 with carbon, boronnitride, and molybdenum has been proposed, to reducefriction and wear at sliding contact.6–14 Recently, nano-silicon nitride composites of BN have been developed toincrease superplasticity.15 The nano-sized silicon nitrideceramic shows high hardness, which is expected toincrease wear resistance.15 An increase in ductility andself-lubricating properties has been observed in nano-silicon nitride/C and nano-silicon nitride/BN compos-ites.7,9–12,15 The tribological behavior of a nano-siliconnitride/C composite has been studied recently.4,5,14 How-ever, the mechanical and tribological behavior of a nano-Si3N4/nano-BN composite under unlubricated conditionshas not been studied so far.

Mechanical and tribological tests of a nano-Si3N4/nano-BN composite have been conducted in this research.Hardness and indentation fracture toughness were evalu-ated using a hardness tester. Unidirectional sliding exper-imental studies were carried out on a nano-Si3N415 wt%nano-BN/Si3N4 tribopair at room temperature to evaluatethe friction and wear of the composite. Mechanical andtribological experimental studies were also conducted onmicro-sized Si3N415 wt% of BN to draw a comparison.This research work is valuable for engineers and scientists,working in the field of design and development of Si3N4

ceramics for various applications, in particular, for the de-sign and development of self-lubricating hybrid bearings,cutting tools, and turbine blades.

Experimental Procedure

Materials

The starting powders were 90.73 wt% submicrom-eter b-silicon nitride powder (NP500 Grade Denki

Kagaku Kogyo, Tokyo, Japan) with an average particlesize of 0.5mm, 7.85 wt% Y2O3 (99.9% pure, Shin-estuChemicals, Tokyo, Japan), and 1.42 wt% Al2O3 (99.9%pure, Sumitomo Chemicals, Tokyo, Japan). The amountof BN (Wako Pure Chemicals Indus, Osaka, Japan) was5 wt% based on the weight of the other starting powders.As proposed previously,6,7,15 the starting powders weremixed in ethanol using silicon nitride balls for 4 h. Afterdrying, the as-received powder mixture was high-energyball milled using Si3N4 balls of 5 mm diameter and Si3N4

pots of 359 mL volume. The ball-to-powder weight ratiowas 20:1, the milling speed was 475 rpm, and the millingtime was 6 h. The powder mixture was compacted in acarbon die (15 mm in inner diameter and 30 mm in outerdiameter) and sintered using Spark plasma sintering (Sum-ito Coal Mining, Tokyo, Japan) under a compressive stressof 30 MPa. The temperature was measured using an op-tical pyrometer through a 5.5-mm-depth hole in the outersurface of the graphite die. Heating and cooling were car-ried out at 300 and 6001C/min, respectively. The linearshrinkage of the specimen was obtained directly by mea-suring the movement of the crosshead. All processing stepswere conducted in a N2 atmosphere, to avoid oxidation.The micro-sized Si3N4/BN was obtained by SPS withouthigh-energy ball milling of the starting powders.

The composition of the Si3N4/BN composite andtheir source is shown in Table I.

Characterization

X-ray diffraction (XRD) studies were carried onCompact X-ray Diffraction System (M/s Philips,Eindhoven, The Netherlands). XRD patterns and anal-ysis of nano-Si3N415 wt% BN are shown in Figs. 1 and2. The average grain size of Si3N4 in the nano-Si3N415 wt% nano-BN disk sample is 56.5 nm. TheXRD pattern (Fig. 2) indicates that the major phasepresent in the nanocomposite is b-silicon nitride.

Table I. Composition of Materials of Si3N4/BN Composite and Si3N4

Si3 N4 Y2O3 Al2O3 BN

Materialcomposition(wt%)

NP 500 grade, 9.73 wt%, b-Si3N4, averageparticle size 0.5 mm

7.85 wt%,(99.9% pure)

1.42 wt%,(99.9% pure)

5 wt% of other powders(99.0% pure)

Source Denki Kagaku, Kogoyo Co. Shin-estuChemicals

SumitomoChemicals

Wako Pure ChemicalIndus

www.ceramics.org/ACT Mechanical and Tribological Properties of Nano-Si3N4/Nano-BN 513

Hardness, Friction, and Wear Tests

Hardness tests were performed on polished surfaces(mirror-like finish, 1 mm diamond polish) using Vickerand Knoop diamonds. Vicker hardness (VH) andKnoop hardness (HK) tests were performed on a Uni-versal high-load hardness tester (Model UHL VMHTMoT, Walter UHi, Asslar, Germany). The indentationwas observed under � 10–� 50 magnification. VH andHK were measured to study the effect of indentationload and time on the hardness values. The indentationload was varied from 2.943 N (0.3 kg) to 19.62 N(2.0 kg) and indentation time was changed from 6 to12 s. Each indentation test was repeated three to fivetimes for better repeatability.

Friction and wear tests were carried out on an in-digenous pin-on-disk (POD) machine, developed forcarrying out tribological tests under unidirectional slid-

ing conditions, as shown in Fig. 3. This POD has twomain features: (i) a cantilever-type load cell for the mea-surement of frictional force with a higher accuracy and(ii) a special arrangement for holding ball on disk, underunidirectional sliding. Wear was measured by theweight loss method. Initially, all the samples of disk(Si3N4 composites) and ball (monolithic Si3N4) werecleaned and degreased in an ultrasonic bath with ben-zene for 20 min. Then the samples were dried in an au-tomatic oven at room temperature. The disk sampleswere weighed repeatedly on a balance accurate to100 mg. After the test, the process was repeated to mea-sure the weight loss of ceramic disk and ball. Wear ofdisk and ball was calculated in terms of the specific wearcoefficient (Kw), which is obtained as

Specific wear coefficient (Kw) 5 wear volume (Wv)/Normal load (FN) � Sliding distance (Sd) (mm3/N/m).The operating parameters adopted for friction and wearmeasurements were as follows:

Normal load (FN) 5 18, 19, 21 N; sliding velocity(Vs) 5 0.3, 0.5/ms; sliding distance (Sd) 5 75, 90, 180,360, 450 m; temperature (T) 5 301C; and relative hu-midity (RH) 5 55%.

Results and Discussions

The hardness values (VH and HK) of Si3N4/BNcomposites against indentation load and indentationtime are shown in Figs. 4 and 5. The effect of inden-tation load (0.05–2.0 kg) on VH and HK is shown inFig. 4, and the effect of indentation time (6–12 s) onVH and HK is shown in Fig. 5. It is obvious from Figs.4 and 5 that hardness decreases with the increase in loadand remains unchanged with the increase in the inden-tation time. The hardness values obtained are given inTable II. Higher hardness was obtained in the case ofnano-Si3N415 wt% of nano-BN, as compared with thecomposite of micro-sized Si3N415 wt% of BN. Thehigher values of hardness in the case of nano-Si3N415 wt% of BN can be attributed to the fact thatthe hardness of fine-grained ceramics generally increaseswith decreasing grain size, for example, due to Hall–Petch-type effects on the associated plastic flow. Inden-tation images on the surface of nano-Si3N415 wt% ofnano-BN are shown in Fig. 6. The indentation imageon the nanocomposite at 0.5 kg is given in Fig. 6a,whereas indentation image at 2 kg is shown in Fig. 6b.The knoop indentation image of nanocomposite is

Fig. 1. X-ray diffraction of nano-Si3N415 wt% nano-BN.

Fig. 2. X-ray diffraction of nano-Si3N415 wt% nano-BN.

514 International Journal of Applied Ceramic Technology—Wani Vol. 7, No. 4, 2010

shown in Fig. 6c. It is evident from Fig. 6b that cracksare developed at the edges of the Vicker’s indenter at anindentation load of 2 kg. The fracture toughness ofceramic composites was obtained by measuring cracklength at the edges. Indentation fracture toughnessis calculated in terms of the toughness parameter(TP) relationship given in the reference:15

TP 5 10.282E0.4P0.6a�0.7 (c0/a)1.5 MPa m1/2 where Eis the elastic modulus (GPa), P is the indentation load(N), c0is the 1/2 total crack length (mm), and a is thediagonal length (mm). Fracture toughness values of 11.0and 8.17 MPa m1/2 were obtained for nano-sizedSi3N415 wt% BN and micro-sized Si3N415 wt%BN, respectively. The fracture toughness of the nano-sized Si3N4/BN composite is higher than that of micro-sized Si3N4/BN composite.

The friction coefficient (m) of nano-sized and mi-cro-sized Si3N4/BN composites against monolithicSi3N4 is shown in Fig. 7. m of 0.4 was obtained in thecase of nano-Si3N4/BN composite against monolithicSi3N4 under a normal load of 19 N. m of 0.370 wasobtained in the case of the micro-sized Si3N4/BN com-posite against monolithic Si3N4. m obtained in this re-search study is higher than the value of m (0.2–0.3)observed for Si3N4/C composite and the Si3N4 tribo-

pair.4 However, a higher m (0.8) was obtained for themonolithic Si3N4 and the Si3N4 tribopair.5 The frictioncoefficient increased from 0.40 to 0.70 when the normalload was increased from 19 to 22 N in the case of nano-Si3N415 wt% nano-BN and Si3N4 tribopair (Fig. 8).

It is evident from these experimental studies that ahigher friction coefficient (m) is obtained in the case ofthe nano-Si3N4/BN composite and Si3N4 tribopair, ascompared with the micro-sized Si3N4/BN compositeand Si3N4 tribopair. The lowest value of m (0.2) wasobtained in the case of the nano-Si3N4/C composite andthe Si3N4 tribopair.4,14 The friction reduction in Si3N4

composites of C against Si3N4 can be attributed to thegraphite lubrication effect of carbon fibers under mutualtransfer of carbon from the disk to the counter surface ofthe ball.4,14 It has been observed in this research that BNhelps to reduce the friction between the Si3N4/BN andthe Si3N4 tribopair. However, the influence of BN ismore predominant in the case of the micro-sized Si3N4/BN and the Si3N4 tribopair. Boron nitride is also con-ferred as white graphite, as it has a layered structure andit acts as a lubricant between the disk (Si3N4/BN) andball (Si3N4) tribopair, through mutual transfer of BNfrom the disk to the ball. The higher influence of BN inreducing friction, in the case of the micro-sized Si3N4/BN and Si3N4 tribopair, as compared with the nano-Si3N4/BN and Si3N4 tribopair, is due to early mutual

Si3N4 ball holder

Cantilever type load cell

Si3N4 + BNrotating disc

Digitalmeter

Fixture

Arm

Si3N4 ball

Test conditions:Load 18N, 19 N, 21 N, Slidingvelocity 0.3ms , 0.5 ms , Slidingdistance 75m, 90m, 180m,360 m450m

Fig. 3. Schematic diagram of pin on disk with a cantilever load cell.

0

1000

2000

3000

4000

5000

6000

0.05 0.1 0.2 0.3

Load (Kg)

0.5 1 2

Vic

ke (

VH

) an

dK

noop

Har

dnes

s(H

k)

Fig. 4. Vicker hardness (VH) and Knoop hardness (HK) of nano-Si3N415 wt% BN (~, VH; & , HK) and Si3N415 wt% BN(& , VH; �, HK) versus time.

0500

1000150020002500

6 8 10 12Time ( secs)

Vic

ker

hard

ness

(H

v)an

d K

noop

har

dnes

s (H

K)

Fig. 5. Vicker hardness (VH) and Knoop hardness (HK) of nano-Si3N415 wt% BN(m, VH; �, HK)and Si3N415 wt% BN (~,VH; & , HK) versus time.

www.ceramics.org/ACT Mechanical and Tribological Properties of Nano-Si3N4/Nano-BN 515

transfer of BN from the Si3N4 disk to the ball in theformer case. That is, BN is first of all removed from thesurface of the disk due to higher wear observed, in thecase of disk (micro-sized Si3N4/BN) and ball (Si3N4)tribopair. The transferred BN material, from disk toball, is subsequently distributed uniformly on the sur-face of ball at the point of contact and acts as a lubri-cant, whereas in the case of the disk (nano-Si3N4/BN)and ball (Si3N4) tribopair, wear of disk is less, andtransfer of BN material is almost negligible from thedisk to the ball. Therefore, BN is less effective in re-ducing the friction in this case, although BN helps inreducing friction under sliding conditions of silicon nit-ride with other tribo-elements. However, it is imperativeto state here that addition of BN beyond 5 wt% has a

negative effect on the microstructure and mechanicalproperties of a silicon nitride ceramic.9,16

The specific wear coefficient (Kw) of nano-siliconnitride/BN and micro-sized Si3N4/BN is shown in Fig.7. The lowest wear coefficient of 0.418� 10�4 mm3/N/m was obtained in the case of nano-Si3N415 wt%nano-BN and a higher wear coefficient of7.162� 10�4 mm3/N/m was obtained for the Si3N4

ball in this case. A specific wear coefficient of0.625� 10�4 mm3/N/m was obtained in the case ofSi3N415 wt% BN and a wear coefficient of2.89� 10�4 mm3/N/m was obtained for the Si3N4

ball in this case. This means that the wear resistanceof nano-Si3N415 wt% BN is higher thanSi3N415 wt% BN. However, the wear resistance of

Table II. Vicker and Knoop Hardness Values of Si3N4 Composites

Ceramic material Density (g/cm3) Load (kg) 0.05 0.1 0.2 0.3 0.5 1.0 2.0

Nano-Si3N41 5 wt% nano-BN 3.22 HV 5432 2169 2192 2506 1864 1506 1737�

KH x 2005 1820 1733 1654 1639 1569Si3N41 5 wt% BN 3.20 HV x 1718 1450 1432 1362 1333 1556�

HK x 1536 1307 1296 1227 1139 x�Cracks observed at the edges. x = value not available.

(a) (b) (c)

20µm20µm 20µm

Fig. 6. Vickers and Knoop Indentations on nano-Silicon nitride15 wt% nano-BN (a) 0.5 kg, (b) 2.0 kg, (c) Knoop.

0.10.20.30.40.50.6

20 100

175

300

400

Sliding distance (m)Fn =4 N, N=450 rpm

Coe

fiici

ent o

f fric

tion

(µ)

Fig. 7. Coefficient of friction versus sliding distance: ~, nano-Si3N415 wt% BN; & , Si3N415 wt% BN.

012345678

Coe

ffici

ent o

f wea

rm

m3

/Nxm

(10–4

)

1

2

3

4

Fig. 8. Coefficient of wear of silicon nitride composites and siliconnitride 1, nano-Si3N415 wt% BN; 2, Si3N415 wt% BN; 3,Si3N4 ball/Si3N415 wt% BN; 4, Si3N4 ball/nano-Si3N415 wt%BN.

516 International Journal of Applied Ceramic Technology—Wani Vol. 7, No. 4, 2010

the Si3N4 ball against nano-Si3N415 wt% BN is lowerthan the wear resistance of the Si3N4 ball againstSi3N415 wt% BN. The increase in wear resistance inthe case of the nano-Si3N4/nano-BN composite, ascompared with the micro-sized Si3N4/BN composite,is based on the fact that the wear in ceramics varies withthe mechanical severity factor.17,18 The mechanical se-verity factor depends on the mechanical properties,hardness, and fracture toughness.17,18 Higher values offracture toughness and hardness reduce the mechanicalseverity factor, which ultimately reduces the wear of ce-ramics. It has already been observed that hardness andfracture toughness values of the nano-Si3N4/nano-BNcomposite are higher, as compared with the Si3N4/BNcomposite. Therefore, less wear is observed in the case ofthe nano-Si3N4/nano-BN composite.

Conclusions

Nano-Si3N4/nano-BN was developed using high-energy mechanical milling and by subsequent sparkplasma sintering. Mechanical and tribological testswere conducted on nano-sized Si3N4/BN and micro-sized Si3N4/BN composites against monolithic Si3N4 toassess the mechanical and tribological properties of theseceramic composites. The following results were ob-tained:

1. The nano-Si3N4/nano-BN composite possesseshigher values of Vicker and Knoop hardness, as com-pared with the micro-sized Si3N4/BN composite.

2. Fracture toughness (11.0 MPa m1/2) of thenano-Si3N4/nano-BN composite is higher, as comparedwith that (8.17 MPa m1/2) of the micro-sized Si3N4/BNcomposite.

3. BN acts as a lubricant under dry sliding condi-tions between the Si3N4 and Si3N4 tribopair at roomtemperature..

4. A higher friction coefficient (0.4–0.7) was ob-tained for the nano-Si3N4/nano-BN and monolithicSi3N4 tribopair, as compared with the friction coeffi-cient (0.37) obtained for the micro-sized Si3N4/BN andmonolithic Si3N4 tribopair.

5. The nano-Si3N4/nano-BN composite possesseshigher wear resistance, as compared with the micro-sized Si3N4/BN composite.

References

1. J. E. Sergent, ‘‘‘‘Ceramics and Ceramic Composites’’, Chap. 7,’’ Hand Book ofMaterials for product design. ed., A. Harper C. McGraw Hill, New York, 7.1-7.60, 2003.

2. K. Kanimoto, K. Kajihara, and K. Yanai ‘‘Hybrid ceramic ball bearings forturbochargers,’’ SAE Technical paper series No, 950981, 2000.

3. K. Kitamura, H. Takebayashi, M. Ikeda, and H. M. Percoulis ‘‘Developmentof ceramic cam roller follower for engine applications,’’ SAE Technical paperseries, No. 972774, 1997.

4. H. Hyuga, M. Jones, K. Hirao, and Y. Yammauchi, ‘‘Friction and Propertiesof Si3N4/Carbon Fibre Composites With Aligned Microstructure,’’ J. Am.Ceram. Soc., 88 [5] 1239–1243 (2005).

5. M. Nakamura, K. Hirao, Y. Yammauchi, and S. Kanazaki, ‘‘TribologicalProperties of Unidirectionally Aligned Silicon Nitride,’’ J. Am. Ceram. Soc.,84 [11] 2579–2584 (2004).

6. X. Xu, T. Nishimura, N. Hirosaha, R.-J. Xie, and H. Tanaka, ‘‘New Strat-egies for Preparing Si3N4 Ceramics,’’ J. Am. Ceram. Soc., 88 [4] 934–937(2005).

7. X. Xu, T. Nishimura, N. Hirosaki, R.-J. Xie, Y. Zhu, Y. Yamamoto, and H.Tanaka, ‘‘Fabrication of Nano-Si3N4/nano-C Composite by HighEnergy Ball Milling and SPS,’’ J. Am. Ceram. Soc., 90 [4] 1058–1062(2007).

8. P. Andersson and K. Holmberg, ‘‘Limitations on the Use of Ceramics inUnlubricated Sliding Applications due to Transfer Layer Formations,’’ Wear,175 [1–2] 1–8 (1991).

9. M. F. Wani, B. Prakash, P. K. Das, S. S. Raza, and J. Mukerji, ‘‘Friction andWear of HPSN Bearing Materials,’’ J. Am. Ceram. Soc. Bull., 76 [8] 65–69(1997).

10. B. Dumont, P. J. Blau, and G. M. Crosbie, ‘‘Reciprocating Friction and Wearof Silicon Nitride-Based Ceramics Against Type 316 Stainless Steel,’’ Wear,238 93–109 (2000).

11. F. Gutierrez-Mora, A. Erdemir, K. Goretta, A. Dominguez-Rodriguez, and J.Routbort, ‘‘Dry and Oil-Lubricated Sliding Wear of Si3 N4 and Si3N4/BNFibrous Monoliths,’’ Tribol. Lett., 18 [2] 231–237 (2005).

12. J. M. Carrapichano, J. R. Gomes, and R. F. Silva, ‘‘Tribological Behavior ofSi3N4–BN Ceramic Materials for Dry Sliding Applications,’’ Wear, 2531070–1076 (2002).

13. A. H. Jones, R. S. Dobedoe, and M H. Lewis, ‘‘Mechanical Properties andTribology of Si3N4 _TiB2 Ceramic Composite Produces by Hot Pressing andHot Isostatic Pressing,’’ Eur. Ceram. Soc., 21 969–980 (2001).

14. H. Hyuga, K. Hirao, M. I. Jones, and Y. Yammauchi, ‘‘Processing andTribological Properties of Si3N4/C Short Fiber Composites,’’ J. Am. Ceram.Soc., 86 [7] 1081–1087 (2003).

15. X. Xu, T. Nishimura, N. Hirosaki, R.-J. Xie, Y. Zhu, Y. Yamamoto, and H.Tanaka, ‘‘Super Plastic Deformation of Nano-Sized Silicon Nitride Ceram-ics,’’ Acta Mater., 54 254–262 (2006).

16. Y. Sun, Q. Meng, D. Jia, and C. Guan, ‘‘Effect of h-BN on Microstructureand Mechanical Properties of Si3 N4 Ceramics,’’ J. Mater. Process. Technol.,182 134–138 (2007).

17. K. Adachi, K. Kato, and N. Chen, ‘‘Wear Map of Ceramics,’’ Wear, 204291–301 (1997).

18. M. Mursaleen ‘‘Wear modelling of self mated ceramics’’, Masters of Tech-nology Thesis, http://www.nitsri.ernet.in/faculty/mfwani.html

www.ceramics.org/ACT Mechanical and Tribological Properties of Nano-Si3N4/Nano-BN 517