17MEDIA MESIN, Vol. 15, No. 1, Januari 2014, 17 - 25ISSN 1411-4348

APPLICATION OF SILICON NITRIDE (Si3N4) CERAMICSIN BALL BEARINGS

WijiantoMechanical Engineering Dept. Muhammadiyah University of Surakarta

Jl. A. Yani Tromol Pos I Pabelan Kartasura SukoharjoE-mail : wijianto@ums.ac.id

ABSTRACT

This paper is discus about silicon nitride ceramics in application to ball bearing.Silicon nitride (Si3N4) have advance properties such as consistent at temperatureoperation up to 1000°C, greater thermal shock resistance, lower density and lowthermal expansion. This properties gives some benefit for ball bearing materialsuch as higher running speed, reduce vibration of the shaft, will improve the lifetime and maintenance cost, lower heat generated, less energy consumption, lowerwear rate, reducing noise level and reduce of using lubricant.

The sintering methods are used to produce ball bearing from silicon nitride.Some techniques can be applied to increase ceramics strength which are reduceporosity, reduce grain size, reduce surface flaw and proof stressing.

The surface finishing of the ceramic bearing is very important because siliconnitride as a brittle material, its strength is limited to the flaw sizes especially theflaw at the surface.Ana

lysis can btain that the highest SPL is 104.7 dB and the lowest isKey words: Silicon nitride (Si3N4), Ball Bearing, Sintering

INTRODUCTION

In the machinery system, bearing is veryimportant to support of motion. A bearing is anengineering component that allows relative motionbetween two parts. Bearing also uses to supportloads from the component and secures the com-ponent position. Bearings are classified in twomajor types which are radial bearings and axialbearings. A radial bearing allows the motion aboutthe center for example: a bearing that permitsthe relative radial motion between a shaft andhousing. However, an axial bearing allows themotion in the axial direction. Bearing also can beclassified base on their relative motion and themain component. They are ball bearings, rollerbearings, ball thrust bearings, roller thrust bearingsand tapered roller bearings.

In addition, bearing materials can be clas-sified into two groups which are through-harde-ned materials and case-hardened materials. Thethrough-hardened materials usually contain allo-ying elements such as molybdenum, tungsten,chromium, vanadium, aluminum and silicon (http://www.machinedesign.com). They are used toproduce hardness at high temperature. The com-mon grades in this group are SEA52100 steels,M series steels, 440C stainless steel and WB49.SAE52100 steels contain high carbon chromiumand small amounts of manganese and silicon.However, SAE 52100 steels are operated onlyat temperature up to 177C. On the other hand,M series steels can be operated at the range tem-perature from 315°C to 538°C. The M seriesincludes M50, M10, M1 and M2.

18 Application of Silicon Nitride (Si3N4) Ceramics in Ball BearingsolehWijianto

The next bearing material group is case-hardened materials. These sort of steels are cha-racterized a hardened surface and a soft core.The common grades for this group include M50-NIL, AISI4320, AISI931, CBS600 and CBS1000. They have a surface hardness range fromRc 58 to 63 and operation limit temperature frombelow 177°C to 315°C (Zaretsky 1990, p. 7).

From both groups, the materials that arecommonly used for ball bearings are SAE52100steel, M50 steel, M50-NIL steel, 440C stainlesssteel and silicon nitride (http://www.mrcbearingservices.com). Among these materials, only sili-con nitride (Si3N4) is a non-metallic material ora ceramic material.

Silicon Nitride Ceramic Ball BearingsSilicon nitride (Si3N4) has some properties

that make this ceramic becomes a candidate ofbearing material. The advance properties include:the strength is consistent at temperature operationup to 1000°C, greater thermal shock resistance,lower density and low thermal expansion(Budinski 1989, p. 200). The structure of siliconnitride (Si3N4) is made of sintered crystal of he-xagonal alpha and beta forms.

Figure 1 Crystal structure of siliconnitride (http://www.iatia.com.au)

Figure 1 shows the crystal structure of sili-con nitride, where silicon atoms (red) and nitro-gen (green) form hexagonal crystal structures.

The typical chemical composition of siliconnitride is shown in Table 1

Table 1. Chemical compositionof Si3N4 in wt %

(Jiang & Komanduri 1998, p. 270)

The use of silicon nitride ceramic for ballbearings can be divided into two types that areceramic ball bearings and ceramic hybrid ballbearings. The sort of bearing can be seen in figure 2.

Figure 2 (a) Ceramic ball bearings(http://www.lily-bearings.com)

Figure 2 (b) Ceramic hybrid ball bearing(http://www.ferrovac.ch)

Al C Ca Fe Mg O Si3N4

0.5%

0.88

0.04

0.17

0.6-1.0

2.3-3.3

97.1-94.1

Figure 2 (a) Ceramic ball bearings(http://www.lily-bearings.com)

Figure 2 (b) Ceramic hybrid ball bearing(http://www.ferrovac.ch)

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The main deference between both bearingsis that in full ceramic ball bearings all the bearingcomponents are made of silicon nitride ceramic.However, in ceramic hybrid ball bearing only ballsare made of silicon nitride ceramic and the innerrace and outer race are made of steel.

Over the past two decades, ceramic bea-rings have been used in a wide range of applica-tions such as machine tool spindles, dental drills,motor racing, aerospace, high speed air turbinebearing and biotechnology industry. Typically,silicon nitride ball bearings are used for high tem-

perature applications and for high running speedapplications (Ling & Yang 2005, p.31).

Comparison Between Candidate BearingMaterials

As mentioned before that ball bearingsgenerally uses 52100 steel, M50 steel, M50-NIL steel, 440C stainless steel and silicon nitride(Si3N4). Table 2 shows the mechanical propertiesof silicon nitride and two metallic materials forbearings which are M50 steels and 440C stainlesssteels.

Table 2. Properties of silicon nitride (Si3N4), M50 steels and 440C stainless steels

Properties unit Si3N4 M50 440C

Metallic/ non metallic Ceramic Tool steel Stainlesssteel

Density kg/m3 3190 7850 7800Bending strength MPa 867 2400 2400Tensile strength Mpa 414-1000 1720 1790Working teemperature C 800 538 300Weibull modulus 14Fracture toughness MPa m1/2 41-53Young’s modulus GPa 303 208 200Poisson ratio 0.28 0.30 0.30Hardness (Rc) 75-80 62 66Thermal shockresistance TK 500/700

Thermal conductivity W/mK 26 26 24.2Thermal expansion 10-6/K 2.7 12.5 10.2Specific heat kJ/kgK 0.76 0.48 0.46

Advantages And Disadvantage Of SiliconNitride Ceramic Ball Bearings.

The advantages of silicon nitride ball bea-rings if compared to steel ball bearings are comefrom some advanced properties of silicon nitride.By evaluation some features such as density, mo-dulus elasticity, coefficient of linear expansion,hardness, coefficient of friction, wear resistance

(Lube & Dusza 2006, p. 3), (Budinski 1989, p.189) & (Callister 2003, pp.737-770)

and thermal resistance the benefits of using siliconnitride for ball bearings can be known.

Density: Table 2 shows the density ofSi3N4 is about 3.2 g/cm3and 7.9 g/cm3for steels,the resulting of the lighter weight produces thesmaller centrifugal force then reduces slippingand the higher running speed can be achieved.

Modulus elasticity: the higher moduluselasticity of silicon nitride tells that silicon nitride

20 Application of Silicon Nitride (Si3N4) Ceramics in Ball BearingsolehWijianto

is more rigid that steel, this property will producethe higher ball accuracy and it will reduce vibrationof the shaft/ spindle and allowing higher speed.

Thermal expansion: the low thermal ex-pansion coefficient of silicon nitride allows theceramic balls experience smaller change of con-tact angle then it will improve the life time andmaintenance cost (http://www.bearingworks.com).

Linear friction coefficient: the coefficientof friction for the oil lubricated Si3N4 on steelsare from 0.04 to 0.09, while for M50 steels onsteels are between 0.10 and 0.17 (Wang et al2003, p. 662). The lower coefficient of frictionof silicon nitride affects on the lower heat gene-rated, less energy consumption, lower wear rate,reducing noise level and reduce of using lubricant.

Working temperature: as can be seen inTable 2, the working temperature of silicon nitrideis higher than steels have. So silicon nitride ballbearings can be used in higher temperature en-vironment.

Wear resistance: the wear resistance ofsilicon nitride that was evaluated by Wang showsthe wear rate of Si3N4 at least two times lessthan M50 steels (Wang et al 2003, p. 667). Thisproperty leads the silicon nitride ball bearingshave a longer life time than steel ball bearings.

Corrosion resistance: silicon nitride ballbearings also show the greater corrosion resis-tance compared to steel ball bearing.

However, the main disadvantage of usingsilicon nitride for ball bearings is their price isrelatively high if compared to steel ball bearings.The reason of high cost of silicon nitride ball bea-rings is because of surface condition is one ofthe major factors of silicon nitride strength butthe surface finishing process in ceramic bearingmanufacturing is cost up to 50% (Wang 2000, p162). As a consequence, the silicon nitride ballbearings remain more expensive than steel ballbearings. In the market, finished silicon nitrideballs price is from 2000-4000 US$/kg (Callister2003, p.768).

Manufacture Of Silicon Nitride CeramicBall Bearings.

Silicon nitride ceramics like other mostceramics are produced by sintering process. Sin-tering process is a thermal treatment of a com-pacted ceramic powder at a temperature belowthe melting point of the main component so itwill bond together and its strength increases. Thesintering methods for silicon nitride ball bearinginclude reaction sintering, hot pressing and stan-dard pressure sintering or pressureless sintering.However, the hot isostatically pressed (HIP) isthe most commonly used for sintering processof the silicon nitride ball bearing (Thoma et al.2004, p. 461). The typical manufacturing processof silicon nitride is illustrated in Figure 3 (Davies2006).

As can be seen in Figure 3, Y2O3 andAl2O3are sintering aid components then they mixwith main component. After that the mixture isadded by binder additive such as a kind of poly-mer. The forming and sintering process are de-pended on the type of sintering. At hot pressing,the mixture pressing and heating at the same time.

Finishing process

Inspection

Si3N4 Y2O3 Al2O3

Mixing

Forming process

Sintering

Binder addition

Figure 3 Schematic diagram of siliconnitride production.

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In addition, to help the formation of a li-quid phase during sintering process in elevatedtemperature, the process needs sintering aidcomponents. Sintering aids that produce a liquidphase at high temperature allow mass transferby rearrangement and solution-reprecipitationprocesses then producing the phase transfor-mation and densification of Si3N4 (Vuckovic etal 2006, p. 303). Also, sintering aids is used toimprove mechanical properties of the ceramicsbecause they will help in reducing porosity. Sin-tering aid components or sintering additives mayin form of oxide additives such as yttria (Y2O3),alumina (Al2O3), magnesia (MgO) and Lithia(Li2O). For a low temperature sintering usessodium oxide (Na2O). Sintering aids such asMgO, Y2O3 during sintering process react withSiO2 on silicon nitride particle to form silicateliquid phase and then transform into glassy phaseafter cooling down at triple junction points andbetween grain boundaries (Ling & Yang 2005,p. 33). This glassy phase or second phase has agreat effect on the properties of the ceramics.

Finishing Process Of Silicon Ni-trideBalls.

In addition, the reducing of porosity andgrain size process are included in manufacturingprocess, however, the reducing surface flaw orsurface finishing of the ceramic bearing is finalprocess that determines the ceramic ball bearingquality.

The surface finishing of the ceramic bearingis very important process because silicon nitrideas a brittle material, its strength is limited to theflaw sizes especially the flaw at the surface orfollows the theorem of ‘weakest link hypothesis’.This theorem states that the strength is deter-mined by the largest flaw size or in other wordthat ceramic components will fail due to the lar-gest flaw size at the surface. If the surface flawsize can be reduced the strength will be increase.

Finishing process of silicon nitride ballsconsist of two techniques which are the con-ventional technique and magnetic float polishingtechnique (Jiang & Komanduri 1998, p. 267).The conventional plate grinding technique com-

prises grinding and lapping process using dia-mond abrasive with low speeds and higher loads.This technique needs a long time to finish andgenerally more expensive. Defects such as flaws,scratches, micro-cracks and pits generally stillpresent on ball bearing surface in conventionalfinishing process.

The next finishing technique is magneticfloat polishing. Generally, the magnetic float po-lishing provides higher quality of silicon nitrideballs. Figure 4 (a) shows the conventional plategrinding and magnetic fluid float polishing (b).

Figure 4 Silicon nitride grindingtechniques (Wang et al 2000, p. 162).

The magnetic float polishing technique isbased on magneto hydrodynamic action that ableto float non-magnetic float and abrasive sus-pended by magnetic field. The main differencebetween these techniques on the force appliedbetween abrasive and balls. In the magnetic floatpolishing technique, the force applied by abrasiveto the ball (3-point contacts) is quite small andcontrollable. The force is come from magneticbuoyancy force when a magnetic field is applied.But, in conventional technique the force is comefrom spring force.

Porosity And Surface Flaws In SiliconNitride Balls.

Some techniques can be applied to increa-se ceramics strength which are reduce porosity,reduce grain size, reduce surface flaw and proofstressing.

In order to reduce surface flaw size, thefinishing process is done step by step from the

22 Application of Silicon Nitride (Si3N4) Ceramics in Ball BearingsolehWijianto

coarser grinding then the finer grinding and po-lishing but unfortunately the fine polishing can notremove the flaws that initiated by the coarse grin-ding steps. But it should be noted that for anyfinishing methods that applied, surface defects/flaws are still introduced by sharp contact of ab-rasive finishing process. The micro-cracks espe-cially subsurface cracks are still present in thesilicon nitride ceramics ball (Swab & Sweeney1995, p.188). The micro-cracks or flaws arereported lead to spalling formation in silicon nitrideballs and decrease the life of the bearing (Swab& Sweeney 1995, p.188). Hence, for these rea-sons it is recommended that the material removalrate during grinding process should be reducedand eliminates grinding with coarse grid wheels.

As mention earlier that sintering aids willimprove mechanical properties by decreasing theamount of porosity. The density of the ceramicsalso will increase as porosity decrease. One ofmethods to make the higher density of the siliconnitride was introduced by Vuckovic et al. Theirresearch was based on introducing the preparedb-Si3N4 seeds into a mixture of a-Si3N4and 10wt% sintering additive. With 0-3% b-Si3N4 seedsthe full densification can be achieved after gaspressure sintering at 1800°C for 4 hours(Vuckovic et al 2006, p. 303).

The pressureless sintering also can pro-duce silicon nitride with a low porosity. The com-position of a-Si3N4and 9% wt sintering aids (4wt% Y2O3 and 5wt % MgO) then sintered withstandard pressure at 1700°C for 60 min will pro-duce 99% of theoretical density (Ling & Yang2005, p.33).

Moreover, another technique on reducingporosity is recently done by Tatli and Thompson.Densification of silicon nitride at relatively lowtemperature was introduced by hot pressing ofSi3N4 with Na2O as the sintering aid. Powderwith the composition of a-Si3N4and 5wt.% Na2Owas hot press sintered at 1500°C for 20 minproduced silicon nitride with 97% of theoreticaldensity (Tatli & Thompson 2006, p. 4).

Fracture Mechanism Of Silicon NitrideCeramic Bearings.

The flaws are the important phenomenonin discussion of the fracture mechanism of theceramics. The flaws can be divided into intrinsicand extrinsic flaws (Kutz 2002, p. 789). Intrinsicflaws are produced during manufacturing pro-cess. They can be in form of pores, shrinkagecracks or area with low density. However, extrin-sic flaws are caused by surface treatment suchas machining and grinding. The flaws size has acorrelation with the fracture strength of the cera-mics. Its relation is formulated as: (Kutz 2002,p. 790)

aYK IC

c (1)

Where sc is the fracture strength, a is theflaw size, KIC is the fracture toughness and Y is aconstant that depends on the shape of the flaw.For example Y value for a semicircular surfaceflaw is 1.28 with the flaw size is 50mm and fromTable 1 the fracture toughness of silicon nitrideis 4.9 MPa m1/2 , the strength of silicon nitridecan be calculated as:

MPac 54100005.028.19.4

If the flaw size is 100mm, the silicon nitridestrength becomes:

MPac 3820001.028.1

9.4

The comparison of two calculations aboveshows that flow size has a significant effect tothe strength of silicon nitride, with two times offlaw size the fracture strength decreases about29%.

The intrinsic flaws such as pores also pre-sent in the silicon nitride balls because the sin-tering process may not produce fully density ce-ramics. The example, the pore that initiates a

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fracture on the silicon nitride is shown in Figure5 below (O’Brien et al 2003, p.464).

Figure 5 (a) shows the point C is the junc-tion of three cracks (D, F, G) and Figure 5 (b)shows the magnification of point C that indicatesthe fracture of silicon nitride ball that initiated bya 2mm pore.

In addition, the wear mechanism in siliconnitride (Si3N4) ball bearings can be explained bythe following stages. The research that was con-ducted by Chen, concluded that contact stressfatigue is the main cause of wear mechanism(Chen et al 1996, p. 205). As a consequence,the wear mechanism in silicon nitride balls alsoproduces Si3N4 debris which then deposit in thelubricant. The debris in the lubricant can createother scratches in silicon nitride balls. This stageshows the second wear mechanism is occurred.The process could be repeated with the largerdebris will produce more scratches and flaws asmultiple wear mechanisms. Then finally fracturesin silicon nitride balls such as spallings clearlyappear on the surface.

The location of fractures can be seen bythe fractographic analysis. The initial fracture islocated in the raceway of inner race (Swab &Sweeney 1995, p.186). The fracture at the innerrace is caused by the stress created by a ball.The compressive forces are perpendicular to theball-raceway contact area. But the contact areaof the inner race is smaller than contact area ofthe outer race, so the inner race experiences ahigher stress than the outer race does.

CONCLUSION

a. The advance properties of silicon nitride(Si3N4) that make this ceramic becomes acandidate of bearing material are the strengthis consistent at temperature operation up to1000C, greater thermal shock resistance,lower density and low thermal expansion.

b. The advance properties of silicon nitride givesome benefit for ball bearings include thehigher running speed, reduce vibration of theshaft, will improve the life time and main-tenance cost, lower heat generated, lessenergy consumption, lower wear rate,reducing noise level and reduce of usinglubricant.

c. The sintering methods for silicon nitride ballbearing include reaction sintering, hot pres-sing and standard pressure sintering or pres-sureless sintering. However, the hot isostati-cally pressed (HIP) is the most commonlyused for sintering process of the silicon nitrideball bearing.

d. Sintering aid components or sintering additi-ves that used in manufacturing of silicon nitridemay in form of oxide such as yttria (Y2O3),alumina (Al2O3), magnesia (MgO), Lithia(Li2O) and sodium oxide (Na2O).

e. Some techniques can be applied to increaseceramics strength which are reduce porosity,reduce grain size, reduce surface flaw andproof stressing.

f. The surface finishing of the ceramic bearingis very important because silicon nitride as abrittle material, its strength is limited to theflaw sizes especially the flaw at the surface.

g. In order to minimize the surface flaws thatcaused by coarse grinding process, it is sug-gested that the material removal rate duringgrinding process should be reduced and elimi-nates grinding with coarse grid wheels.

h. The sintering methods, sintering aids used,sintering temperature and soaking time insilicon nitride manufacturing are the factoron obtaining the desire porosity of silicon ni-tride.

i. The pores and surface flaws are the causeof the fracture initiation on the silicon nitrideball bearings.

24 Application of Silicon Nitride (Si3N4) Ceramics in Ball BearingsolehWijianto

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