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JTEKT Engineering Journal English Edition No. 1006E (2009)

Study of Tribology over the Past Twenty Years

−Fundamentals and Applications−

Professor Koji KATODepartment of Mechanical Engineering,

College of Engineering, Nihon University

1. IntroductionTribology, a fi eld whose name derives from the Greek

tribos, concerns the study of friction. Among specific areas included in this fi eld are friction heat, friction noise, wear, lubrication, friction electrification, friction gas generation, and friction electron emissions.

Friction is deeply related to numerous phenomena in the natural world and to contact-surface technology in the industrial world. Accordingly, understanding the fundamentals of friction and methods of best harnessing it for practical use is important in developing industrial products, in improving their performance and securing their reliability, and in raising productivity.

With this in mind, the author has endeavored together with young researchers and students over the last 20 years to carry out research in the fi eld of tribology and herein presents representative examples of the results of this research.

2. Micro-Mechanisms of Friction and Wear

2. 1 Initiation and Propagation of Micro-Slip in Contact Interface

In Fig. 1 (a), elastic deformation under load W results in formation of a circular contact surface. Friction force F is steadily increased on the lower spherical sample,

and accompanying the increase in F are increases in the sample's elastic deformation amount d and friction coeffi cient l = F/W.

Along with the increase in l, inside the circular contact surface the micro-slip indicated by the green area propagates from the circumferential region toward the center. Macro-slip occurs at the point this micro-slip has propagated throughout the contact surface area, and the l value (ls = 0.85) at that time becomes the static friction coeffi cient.

In Fig. 1 (b), two point-defects exist in the circular contact surface's center area. In this case the micro-slip propagates from around these defects toward the circumference. At the same time, micro-slip also propagates from the circumferential region toward the center. Once these two propagations meet, macro-slip occurs, and the static friction coefficient becomes ls = 0.69.

Figure 2 shows micro-slip occurrence and propagation in four different types of contact surface confi gurations. Micro-slip covers the entire contact surface most quickly in the case of the triangle shape, displaying the lowest of the four values at ls = 0.32.

The representative research results over the past 20 years at my laboratory are introduced as follows,(1) Static friction coeffi cient changes by the shape of contact area.(2) Different wear modes are classifi ed in one diagram as Wear Map by introducing indexes of severity of contact.(3) Repeated sliding friction at self-mated SiC and Si3N4 in water generates friction coeffi cient below 0.01.

Micro texture on contact surfaces generates friction coeffi cient in the order of 0.0001.(4) Tribo-coating of indium on the surface of SUS440C disk generates friction coeffi cient below 0.05 against Si3N4

pin in high vacuum.(5) N2 gas supply at the contacts of Si3N4/CNx and CNx/CNx in air generates friction coeffi cient in the order of

0.001.(6) Tribolayer formed by tribo-chemical reaction changes friction and wear drastically.

Key Words: micro-slip, wear map, water lubrication, tribo-coating, N2-gas lubrication, tribolayer

−Study of Tribology over the Past Twenty Years −Fundamentals and Applications−−

3JTEKT Engineering Journal English Edition No. 1006E (2009)

2. 2 Wear MapMetals are subject to various types of wear such as

abrasive wear, adhesive wear, fl uid wear, fatigue wear and corrosive wear, and even in regard to the same wear type, they are subject to differing wear modes.

Figure 3 shows a wear map depicting, by the two parameters of penetration depth Dp and contact surface dimensionless shear strength f, the wear occurrence ranges for the cutting mode, wedge mode and ploughing mode, three modes of abrasive wear.

W=7.85 N

0.6

0.5

0.4

0.3

0.2

0.1

00 0.1 0.2 0.3 0.50.4

F

Contact areaNon-slip region

W

F

FW

Fric

tion

coef

ficie

nt, l

(=

/ )

ls=0.85( )

Deformation amount of specimen , mm

Annular ringF

HemisphereF

Square pillarF

Triangle pole

Static frictioncoefficient

ls=0.49( )ls=0.48( )

ls=0.32( )

Slipping region

dx

dx

Fig. 2 Effect of shape of contact area on micro-slip and static

friction coefficient1)

l=0.29l=0.29l=0.30l=0.29

2mm2mm 2mm 2mm

Direction of friction force, FF

W

=l

Fric

tion

coef

ficie

nt, l

Deformation amount of specimen xd

W

F

Slipping region

Non-slip region

Contact area

W =7.85 N

Static friction coefficientls= 0.85

(a) Local slip in contact area

F

W(b) Effect of defect in contact area

FFriction force, WLoad,

Friction coefficient prior to generation of macro-slip

Friction coefficient

Static friction coefficientls= 0.69

Fig. 1 Micro-slip in contact area corresponding to the

increase in friction force and the static friction

coefficient1)

l=0.22 l=0.85l=0.70l=0.502mm2mm2mm2mm

l=0.30l=0.172mm2mm

l=0.69l=0.482mm2mm

(b) Abrasive wear map of metal

0.6

0.5

0.4

0.3

0.2

0.1

01.00.80.60.40.20

Dimensionless shear strength of contact surface, f

Cutting

Wedge

Ploughing

l( ) =Shear strength of contact surfaceShear strength of bulk material

(a) SEM (scanning electron microscope) images of three wear modes

Cutting Wedge Ploughing

: LoadW: Friction forceF: HardnessH: Projection radiusR: contact circle radiusa: Engagement depthh

DP = =R H2W

12−

Rp p 2H2W －1

f

Border line by so

lution of mo

dified slip-

line theo

ry

Deg

ree

of p

roje

ctio

n en

gage

men

t, Dp

DP = =R H2W

12−

Rp p 2H2W －1

Fig. 3 Abrasive wear map of metals2)

ha

ha


4 JTEKT Engineering Journal English Edition No. 1006E (2009)

Figure 4 shows a wear map depicting, by mechanical contact severity Scm and thermal contact severity Sct, the occurrence ranges of mild wear (a smooth wear surface is formed) and severe wear (a coarse wear surface is formed) observed on identical ceramic types.

Figure 5 shows a wear map depicting, by the coating and base material hardness ratio and the coating thickness and contact radius ratio, the occurrence range of three coating forms of delamination (delamination within the coating, delamination in the coating and base material interface, delamination within the base) occurring when a hard coating surface is subjected to repeated wear.

2. 3 Simulation of Flow WearFigure 6 shows thin filmy flow wear particles of

steel in oil. Figure 7 shows the results of continuous SEM observations of fl ow wear process on steel surface generated by friction cycles on a single surface area. By 1.6 × 104 passes of repeated friction, it is understood that a single flow wear particle has been formed. Figure 8 shows a Vickers indentation mark formed on a steel sliding surface that is covered by repeated fl ow of surface layer and gradually becomes smaller. When the change DDx of the mark size is measured, Fig. 9 is obtained. From Fig. 9, the relation between surface flow rate R and contact pressure as shown in Fig. 10 is obtained. In other words, it is understood that flow rate is sensitive to pressure and that fl ow wear occurs from surface fl ow around 1Å/pass.

Mild wear Severe wear

Potential pre-crack and distribution of contact pressure

Sev

erity

of m

echa

nica

l con

tact

Severity of thermal contact

l : Friction coefficientPmax: Hertz-contact surface pressure

d : Crack lengthKIc : Fracture toughness

c : Thermal distribution ratio

v : Slip velocityW : LoadHv : Hardness

DTs

DTs

: Thermal shock resistancek : Thermal conductivity

q : Densityc : Specific heat

Introduction of the concept of Sc (Severity of Contact)

vHWVkqc

Sc,t = (　　　　　　 )cl

KIC

Sc,

m=

(　　　　　　　　

)(1

+10l

)Pm

ax　

d

20

15

10

5

010−3 10−2 10−1 100

Fig. 4 Wear map of ceramics describing mild and severe

wear3)

(a3)

Es

Hs

Ys

(b)

(c) Coating delaminatednear interface

4

7

6

5

4

3

2

1

7

6

5

4

3

2

1

Har

dnes

s ra

tio, H

f/Hs

Yie

ld s

tres

s ra

tio, Y

f/Ys

3210

(d) Coating delaminated inside the layer

(a2)

Ef /Es=2l=0.25

Thickness of coating/half-breadthof contact, t/a

Fig. 5 Wear map of delamination of hard coatings4), 5)

(a1)

Wear track

20 µm

4 µm

2 µm

4

3

2

1

0

Z/a

0.00 1.601.200.800.40

w=rvm /Pmax

l=0.70l=0.50l=0.25

t/a=0.5X/a=0.0Ef/Es=2.0



S10C (0.1 wt.% C steel)Alkylnaphtalene (28～35 cst at 38℃)

Rmax=0.4 µmPm=9.8 MPaV=0.52 m/s

(a) (b)

Fig. 6 SEM images of flow wear particles of steel generated

in oil6)

S10C (0.1 wt.% C steel)Rmax=4 µmPm=9.8 MPaV=0.52 m/sAlkylnaphtalene(28～35 at cst 38℃)

Flow wear

(a) (b)

(c) (d)

Frictional Direction

1.6×103 Passes 8×103 Passes

1.6×104 Passes 4.8×104 Passes

Fig. 7 Generation process of a flow wear particle of steel by

repeated friction6)

10

0

20

2 4 6 8 (×104)Passage Number of Ball

(Passes)

Am

ount

of P

last

ic F

low

, DD

x (µ

m)

Fig. 9 Relation between surface flow, number of friction

passes and contact pressure7)

6.0 Mpa26.5 Mpa45.1 Mpa63.5 Mpa

20 µmIndentation

0.52 m/s

0.25 m/s; 26.5 MPa

3.2×104 Passes 0 Pass 4.8×104 PassesFrictional Direction

10 µm(a) (b) (c)

Fig. 8 Simulation of flow wear process by the change of

Vickers indentation mark in repeated friction7)

2.5

0

5.0

20 40 60 80 100

×10−4

Contact pressure (MPa)

Flo

w R

ate,

R (

µm/P

ass)

20 µmIndentation

Rough Surface(4 µm Rmax)

RxRy

R

0.52 m/s

Fig. 10 Relationship between surface flow rate and contact

pressure7)

3. Water Lubrication of Ceramics

3. 1 Generation of Low Friction by Running in Water

As shown in Fig. 11, when friction is generated repeatedly in water between SiC samples or Si3N4 samples, the wear surfaces become smooth on a nanometer order. A 10 nm-order water fi lm is formed by hydrodynamic effect, and the friction coeffi cient becomes less than 0.01. Figure 12 shows wear surfaces so smooth that the sliding direction of pin and disk is not clear. It is considered that formation of these smooth surfaces is due to the occurrence on the contact surfaces of "tribo-chemical reactions" such as the following:

Si3N4 + 6H2O → 3SiO2 + 4NH3SiC + 2H2O → SiO2 + CH4SiO2 + 2H2O → Si(OH)4

Even after eliminating water, wear surfaces such as these displayed low friction of l = 0.15 over approximately 104 cycles.

14121086420

1.2

1.0

0.8

0.6

0.4

0.2

0

Sliding contact in waterLoad: 5 NSliding velocity:120 mm/s

Fric

tion

coef

ficie

nt, l

l< 0.01

Water

Sliding cycles N, ×104 cycles

SiC/SiC

Si3N4/Si3N4

SiC/SiCSi3N4/Si3N4

Fig. 11 Reduction in friction coefficient of SiC/SiC and

Si3N4/Si3N4 by repeated friction in water8)



Si3N4 Pin/Si3N4 DiskSliding in water

Load W: 5 NSliding Velocity v: 120 mm/s

Sliding cycles N: 87 500 cycles

Fig. 12 Smooth wear surfaces on Si3N4 pin and Si3N4 disk

formed by repeated friction in water9)

3. 2 Effect of Surface Texture on Lubrication Figure 13 shows micro-pits formed on an SiC disk

surface. If the size of these pits and the ratio of overall area they occupy is changed, the critical load value Wc obtained through seizure testing in water changes significantly, as shown in Fig. 14. Figure 15 shows the influence of pit depth/diameter h/d and area ratio r (%) on Wc. Wco is the Wc value in the case of no pits and is constant. From this fi gure, it is known that the Wc/Wco > 2.0 to 2.5 value exists in the range h/d = 0.01 to 0.02, r = 5%.

If the various pit patterns shown in Fig. 16 are formed, the changes in friction coefficient l and critical load Wc become even greater by each pattern, as shown in Figs. 17 and 18. The value l ≈ 0.0001 shown in Fig. 17 can contribute significantly to practical applications of water lubrication.

20mm

Fig. 13 Micro-pits formed on SiC disk surface10)

Pit diameter: 150 µmPit depth: 3 to 4 µm

WcWcWcoWc

0

0.01

0.02

0.03

0.04

Fric

tion

coef

ficie

nt l

SiC/SiCRotational speed n: 1 200 min−1

Lubricant: Purified waterSupply rate: 60 ml/min

0 500 1 000 1 500

Load W, N

2 000 2 500 3 000 3 500

Fig. 14 Effect of size and area ratio of micro-pits on critical

seizure load10)

Untexturedu150, 2.8%, Depth 3-4 µmu150, 22.5%, Depth 3-4 µmu350, 4.9%, Depth 3-4 µm

2.0～2.51.5～2.01.0～1.50.5～1.0

Wc/Wco

0

0.01

0.02

0.03

0.04

0 5 10 15 20 25

Dep

th-d

iam

eter

rat

io,

/ d

h

rPit area ratio , %

SiC cylinder/SiC diskPit diameter d: 50-650 µmPit depth h: 1.9～16.6 µmLubricant: Purified water

Fig. 15 Relationship among depth/diameter ratio h/d and area

ratio r (%) of pits on disk surface and the normalized

critical seizure load Wc/Wco10)

SiC (CIP)

35040RSPCLP

3.4-82.7-7.92.6-7

7.74.94.0

Lower specimenUpper specimen10mm

100 µm 500 µm

Material

Texture’s name

Pit diameter:d (µm)

Pit depth:h (µm)

Pit area ratio:r (%)

SiC (CIP) SiC (CIP)

RSP(Rectangular

Small Pit)

CLP(Circular Large Pit)

CLRSP(Circular Large

and Rectangular

Small Pit)

350 40

500 µm

Fig. 16 Three pit patterns formed on SiC disk surface11)



0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

CLRSPCLPRSPNo texture

Sta

ble

fric

tion

coef

ficie

nt, µ

s Lubricant: refined waterSupply rate: 60 ml/minTemperature: 14-20℃

*Under detection limit

Fig. 17 Effect of three pit patterns on friction coefficients of

SiC/SiC in water11)

4. Solid Film Lubrication in VacuumIon plating or plasma coating is used to form solid

film for lubrication in vacuum, and when usage has caused the fi lm to wear to the point of disappearing, the system using that bearing stops. By "tribo-coating" to form a vapor-deposition coating immediately before and after sliding as shown in Fig. 19, a lubricant film can be supplied repeatedly at intervals as shown in Fig. 20. Moreover, the indium film formed by this tribo-coating can maintain a low friction coeffi cient much lower than that of conventional coatings for more than 104 cycles, as shown in Fig. 21.

It is important to note that these superior lubrication characteristics are obtained through the combination of Si3N4 balls and SUS440C disks. Figure 22 shows a TEM image of an indium tribo-layer formed by tribo-coating on an SUS440C disk sliding surface. The layer thickness is approximately 400 nm. The EDX analysis of Fig. 23 shows a matrix comprising mainly the elements Si, Cr, and O. The black points distributed in the matrix are nano-particles whose main element is indium. The TEM image of Fig. 24 shows the indium nano-particles to be crystalline phase and the matrix to be amorphous phase.

0

1 000

2 000

3 000

4 000

5 000

CLRSPCLPRSPNo texture

Cri

tical

load

Wc,

N

Lubricant: refined waterSupply rate: 60 ml/minTemperature: 14-20℃

Fig. 18 Effect of three pit patterns on critical seizure load Wc

of SiC/SiC in water11)

Tribo-coating process Friction test

Load

(～1 min) (1～2 min) 104～106 (10 cycles)

Fig. 19 Friction assisted coating lubrication (Tribo-coating):

Vapor-deposition of soft metal In for a few minutes

just before and after initiation of sliding12)

0 21 30

0.2.

0.4

0.6

0.8

1

Number of cycles, ×104 cycles

Pin/Disk: Si3N4 / SUS440cLubricant: InH=2 nm/min, th=1-2 minP=1.3 GPa, v=24 mm/sVR=10−6 Pa

Evaporation

Fric

tion

coef

ficie

nt, l

Fig. 20 Effect of interval tribo-coating12)

0

0.1

0.2

0.3

1050 15

Fric

tion

coef

ficie

nt, l


Range of friction coefficientobtained with soft metal films

Pin/Disk: Si3N4/SUS440CP=1 GPa, v=22 mm/sec,VR=10−6 Pa In (Tribo-coating)

In (Vapor deposition)Ag (Ion-plating)Pb (Ion-plating)

Fig. 21 Lower friction and longer life of tribo-coating film in

comparison with films by vapor-deposition and ion

plating13)

Carbon film (overcoat)

Tribo-layer

Si3N4 pin

(a) (b)

Fig. 22 TEM image of tribo-layer on disk surface formed by

tribo-coating of In14)

100 nm 10 nm



(a)

(b)

Count，a.u.

Count，a.u.

In

In

Cr

Cr

Cr

Cr

CuFe

Fe

Si

Si

O

O

0 1 2 3 4 5 6 7 8 9 10X-ray Energy，keV

0 1 2 3 4 5 6 7 8 9 10X-ray Energy，keV

Fig. 23 Composition in tribo-layer on disk surface analyzed

by EDX14)

(b) Black points in Fig. 22(b)

(a) Matrix in Fig. 22(a)

(b) Matrix(a) Nano-particle

Fig. 24 TEM image of tribo-layer14)

2.5 nm

5. N2 Gas LubricationFriction between a Si3N4 ball and CNx coating varies

dramatically depending on the gas environment. Of the seven gas environments shown in Figs. 25 and 26, only N2 reduced the friction coefficient to less than 0.01. Figure 27 shows that by blowing N2 gas on the contact surface in an air environment, a friction coefficient of around 0.7 is reduced very quickly to a level of 0.02.

The effectiveness of blowing gas on the contact surface in an air environment is observed as well in the case of blowing dry air or O2. Moreover, blowing N2 on the surface after fi rst blowing dry air or O2 for 50 to 100 cycles significantly reduces the friction coefficient and wear ratio Ws. Figure 28 shows that a value of l ≈ 0.005 is obtained by blowing N2 on a CNx/CNx surface in air after fi rst blowing O2 on the surface for 50 cycles. Figure 29 shows the l and Ws values after blowing various gas types on the contact surface. The observed values l ≤ 0.05, l ≤ 5 × 10−8 mm3/N・m are very useful for practical applications.

0

0.1

0.2

0.3

0.4

0.5

0.16

0.36

0.030.0090.05

VacuumAir N2 CO2 O2

Pin: Si3N4 ball (r=4.0mm)Disk: 100 nm CNx/SiNormal load: 100 mNMaximum contact pressure: 200 MPaSliding speed: 4 mm/s

Si3N4

Si3N4CNx

(1x105 Pa) (2x10−4 Pa) (7.4x104 Pa) (7.4x104 Pa) (7.4x104 Pa)

Fric

tion

coef

ficie

nt, l

Fig. 25 Effect of gas environment on friction of Si3N4/CNx15)

0

0.1

0.2

0.3

0.4

0.5

N2VacuumAir ArHe

(4x10−3 Pa) (1.0x105 Pa)

0.35

0.14

0.01

0.20

0.31

Fric

tion

coef

ficie

nt, l

Disk: 100 nm CNx/SiPin: Si3N4 ball (r=4.0mm)Normal load: 750 mNPmax=400 MPaV=0.26 m/s

(1.0x105 Pa)

Fig. 26 Effect of gas environment on friction of Si3N4/CNx16)

0.0

0.2

0.4

0.6

0.8

1.0

14121086


420

0 L/min 1.2 L/min 4.8 L/minN2 N2Air

Disk: CNx (100 nm)/Si3N4Pin: Si3N4 ball (r=4mm)Normal load: 200 mNRotary speed: 250 min−1 (0.4 m/s)

Fric

tion

coef

ficie

nt, l

Air

Fig. 27 Effect of blowing N2 gas to the contact on friction of

Si3N4/CNX in air16)



0.00.20.40.60.81.0

10 0001 000100

Number of cycles N, x103 cycles

101

N2 at 50th cycle

O2

Fric

tion

coe

ffici

ent, l

Load: 1 NSliding speed: 0.21-0.27 m/sTemperature: 20-24℃Humidity: RH 20-40%

100 µm 100 µm

10 cycles 10 000 cycles

l= 0.005

Fig. 28 Change in friction coefficient of CNx/CNx by

supplying N2 gas after initial 50 cycles in O2

gas17)

0

1

2

3

4

5

0

Air O2 N2 O2 N2Air N2From

1st cycleFrom

1st cycle

Load: 1 NSliding speed: 0.21-0.27 m/s (250 min−1)Temperature: 20-24℃, Humidity: RH 20-40%Gas flow rate: 2.0 L/min. (2.10 cc/mm2s)Inner radius of tube: 4.5mm

Ste

ady

stat

efr

ictio

n co

effic

ient

, lS

peci

fic w

ear

amou

ntW

s, x1

0−7

mm

3 /N・

m

0.1

0.2

0.3

100 cycles(400 mN)

50 cycles

Fig. 29 Values of friction coefficient l and wear ratio Ws (mm3/N・m) at CNx/CNx in dry air, O2, N2 and combination of gases17)

6. Tribo-Chemical Reactions and Tribo-LayersZDDP (zinc dialkyl dithio phosphate) was invented

in America and has been used for more than 60 years as an engine oil additive to improve wear resistance. When friction is repeated between a steel sheet on which an iron oxide film has been grown and a steel cylinder in PAO (Poly-Alpha Phosphate) oil containing ZDDP, the friction coeffi cient drops and a brownish friction surface is formed, as shown in Fig. 30. This surface is covered with a 200 nm tribo-layer as shown by the TEM image in Fig. 31. The main elements include Zn, Fe, P, C, S, O and Cr, which are distributed as shown by the EDX profi le in Fig. 31. The mechanisms by which this microstructure

with layer thickness of 200 nm is formed and by which friction and wear are reduced are not clear. Certainly these mechanisms belong to the world of "tribo-chemical reactions" caused by the operation of friction.

Steel Cylinder

TEM Analysis

LoadWear Track

Plate(10×8×2mm)

Lubricant: 1%ZDDPin PAO (80℃)

7mm stroke@7 Hz

Cylinder (u6×6mm,bearing steel)

Iron Oxide Plate

R-O S S O-R

S SP P

R-O O-R R:-CH-CH2-CH-CH3CH3

CH3

ZDDP: 4-methyl-2-pentanol

Zn

1.0mm

l

0 1 000 2 000 3 000

0.20

0.15

0.10

0.05

0.00

Time, sec.

Fig. 30 Outlook of surface layer on oxidized steel plate

formed by friction against steel cylinder in oil

(PAO) containing ZDDP18)

Upper Layer

Middle Layer

Bottom Layer

Tribofilm

R-O S S O-R

S SP P

R-O O-R R:-CH-CH2-CH-CH3CH3

CH3

ZDDP: 4-methyl-2-pentanol

Zn

40 nm

Iron Oxide Layer

TEM Cross Section PhotographNear the Surface

Carbon Layer(For protection)

Fig. 31 TEM image of tribo-layer (to 400 nm) on friction

surface of steel plate and depth profiles of elements

by EDX18)



7. Tribo-Applied TechnologyFigure 32 shows the shrink-fi tting mechanism between

lens and a tube adopted for laser printers in 1995. Here the shrink-fitter invented by Nitta and Kato was used, achieving a field of view 100 times larger than the conventional type. This is brought by contact micro-mechanisms as basic knowledge19).

Figure 33 shows a totally ceramic X-Y stage utilizing an ultrasonic motor for use in electron beam lithography in nonmagnetic vacuum environments. This stage was designed to have a drive pin and driven rail wear-resistant alumina, which are the key elements of ultrasonic motor, and was manufactured in 2000 by Kyocera. This is brought by wear mechanisms ceramics as basic knowledge20), 21).

Lens-barrelLens

Thermal expansion of the shrink- fitter during temperature rise

Polygon mirrorSlit Shrink-fitter

Shrink-fitter

Beam spot: small

Laser microscope

Shrink-fitter and lens

Spacer Image forming surface

Beam spot: large

Conventional lens joining method

Polygon mirror

Achieved approx. 100 times larger visual field than conventional ones by applying shrink-fitter for lens alignment

Shrink-fitter

Fig. 32 Laser microscope using shrink-fitter for ceramics

polymer/metal shrink fitting

Figure 34 shows the principle of magnetic fluid grinding, a method invented in 1990 enabling Si3N4 ceramic balls to be grinded 40 to 100 times faster than by the conventional method with the same degree of roundness. The cost of magnetic fluid has not dropped, making this technology commercially unfeasible in terms of equipment investment and running costs. Wear mechanisms of Si3N4 in water is the basic knowledge for the method22), 23).

Figure 35 shows a ball bearing currently being developed for space applications that has a built-in micro-heater and utilizes tribo-coating. The balls are Si3N4, and the races are SUS440C. As a result, knowledge concerning ceramic/metal optimal combinations and tribo-layers has become basic knowledge12), 13), 14).

1. Non-magnetic float acts large work force on balls and makes the ball motion stable.

2. Si3N4 balls are ground as a result of abrasive wear and tribo-chemical wear among SiC abrasive grain, Si3N4 drive shaft and water-base magnetic fluid.

Magnetic fluid grinding method of Si3N4 balls

Magnetic fluid and abrasive grain

Ball

Float

7.7mm 7.1mm1.0 µm 0.01 µm500 µm 0.14 µm

Ball diameter

Ball appearance

Average roughnessSphericity

Grinding time: 180 min.Over 40 times faster than

conventional method

Si3N4 ceramic ball

Si3N4 grinding ball

Fig. 34 Magnetic fluid grinding of ceramic balls

XY Stage driven by Ultrasonic Motorfor LSI of 0.15 µm line width

Electron BeamLithography System

X-Y stage

Fig. 33 Totally ceramic X-Y stage driven by ultrasonic wave

for electron beam lithography



In

Prototype of micro-heater

Pt heaterAu wire

Membrane area Siwafer

SiO2

5mm

Fig. 35 Ball bearing with micro-evaporators of in for space

application

8. ConclusionThe life of technology supporting products tends to

be short-lived. Put differently, such technology jumps forward dramatically every year. Attempting to survive by developing new products featuring little more than a changed appearance leads to weariness, a lack of uniqueness, little sense of responsibility for results, and frustration. Such an approach causes us to fall into an inescapable spiral and become little more than those humoring children with temporary novelty, as it were.

Development bringing dignity and true joy to those involved in "monozukuri" manufacturing is that which pioneers new technology and applications, improves performance (function and efficiency), reduces environmental load and cost, and calls to conscience of the mature consumer. It is the "high road" that gives one the satisfaction of having contributed to society and also leads to sure profitability. Pursuing profit by repeatedly developing new products intended mainly to excite consumers with superficial novelty while avoiding the rigor of advanced development is not the way of this road.

Many products of JTEKT can be considered to be on this high road. JTEKT has acquired much knowledge and basic experience related to tribology, and this certainly has contributed long-term to the signifi cant improvement of performance and reliability and reduction of environmental load and cost in regard to these products.

The 35 fi gures provided in this paper show a sampling of the results the author obtained working mainly with a group of around 20 or 25 young researchers and students over the last 20 years. The fact that this small group of young people working without significant funding has been able to achieve such results in this short period of time attests to how deep, expansive, and relatively new

the fi eld of tribology science and technology is.It is thought that technology in the 21st century will be

the key to determining the fate of our planet, and my hope is that JTEKT will play an important role in securing this technology. I would be delighted if this paper somehow contributed to that effort.

AcknowledgementIn connection with the results presented herein, I

wish to express my heartfelt thanks to Koyo Seiko and JTEKT for supporting our research efforts over the last 15 years related to Si3N4 and SiC water lubrication, Si3N4/SUS440C indium lubrication, and Si3N4/CNx N2 gas lubrication mainly by providing ceramic balls and bearings.

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study of tribology over the past twenty years fundamentals...

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