Unlubricated sliding friction and wear of various Si3N4 pairs between 22° and 1000°C

Unlubricated sliding friction and wear of various Si3N4 pairs between 22° and 1000°C

Unlubricated sliding friction and w e a r of various SiaN4 pairs between 22 ° and 1000°0 A. Skopp, M. Woydt and K.-H. Habig* The tribological behaviou...

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Unlubricated sliding friction and w e a r of various SiaN4 pairs between 22 ° and 1000°0 A. Skopp, M. Woydt and K.-H. Habig* The tribological behaviour of three commercial self-mated Si3N4/Si3N4sliding pairs in pin-on-disc configuration was studied for sliding velocities between 0.03 m s ~ and 3 m s ~, constant load of 10 N and ambient temperatures between 22°C and 1000°C. The wear rate of Si3N4 is dependent on the overlap ratio, the ambient temperature and the sliding velocity. An influence of the phase composition was for the three tested commercial Si3N4 materials not observed. The coefficient of friction lies for solid state friction under steady-state conditions between 0.5 and 1. The tribological behaviour for temperatures greater than or equal to 400°C is characterized by a high wear/low wear transition with increasing velocities. Wear mechanisms were studied with scanning electron microscopy, infra-red spectroscopy and X-ray diffraction.

Keywords: silicon nitride, unlubricated bearings, ceramics, tribooxidation, friction, wear

Introduction Subjected to thermal conduction and high ambient temperatures typical for many technical applications, materials for uncooled engines, aeroengines, passenger car engines and turbo-chargers in continuous service have to withstand high thermal stresses especially for unlubricated tribosystems. For metallic bearings the maximum working temperature is restricted to 350-400°C, since their hardness decreases with increasing temperature, and their dimensional stability begins to reduce, with the wear rate and clearance increasing. It seems to be impossible to lubricate such tribosystems at higher ambient temperature with a liquid. Therefore one looks for tribological pairs, which possess unlubricated friction and wear behaviour similar to lubricated systems. Some of these requirements could be realized in rolling contacts with SigN42,-~. This has a high flexural strength at high temperatures and good thermal shock resistance, high hot-hardness and high erosion and corrosion resistance. Centrifugal forces can be reduced by up to 50% in comparison with metals owing to its low density, so that wear is diminished. Si.~N4 ceramics seem to be promising for high speed bearings (eg bearings for UHV pumps, turbines, spindle bearings and machine tools a,6-s) and also for bearings working in corrosive environments. Si3N4 balls mostly fail by pitting 5 as with M50 steel balls. The pitting depth, which can be smaller for HPSi3N4 than for M50 steel, is an important criterion for the use of Si,~N45. Balls made of SiC, ZrO2 or A1203 are not used in higher loaded bearings, because their *Federal Institute /or MateriaLs' Research and Testing (BAM), Division 5.2, Unter den Eichen 87, D-IO00, Berlin 45, FRG.

TRIBOLOGY INTERNATIONAL

failure is of catastrophic nature '~. Recent investigations show that under the conditions of E H D lubrication high overrolling cycles can also be achieved with SiC and ZrO2 m. For bearing applications the friction and wear behaviour of material pairs has not only to be characterized under rolling conditions, but also under sliding conditions, because the rolling movement has a sliding component. For material manufacturers and for users as well, less comparative tribological results are available for selfmated Si3N4/Si3N 4 pairs under unlubricated sliding conditions. This paper deals with the friction and wear behaviour of three commercial ceramics at ambient temperatures up to 1000°C and for a sliding speed range from 0 . 0 3 m s i t o 3 m s i. A general comparison of results found in this paper with those from the literature was omitted because about 50% of the published papers investigated metal/ Si3N4 sliding pairs, 25% looked at the rolling behaviour of SigN4 and metal/SigN4 pairs, and the final 25% worked at sliding speeds less than 0.02 m s -1 . Additionally, in many publications not all the material properties and tribological operating conditions necessary for a proper comparison were listed.

Experiments The friction and wear tester used was a pin-on-disc assembly which is constructed in such a way that a shaft presses the pin at an angle of 90 ° against the rotating disc. The load is applied by a dead weight (10 N). The overlap ratio for the pin is 100%, and that for the rotating disc is less than 2%. The pin and disc are installed with the shafts in a wire-heated furnace. The method of conducting the tests is

0301-679X/90/030189-11 © 1990 Butterworth-Heinemann Ltd

189

A. Skopp et aI--Unlubricated sliding friction and wear of various Si3N4 pairs

published elsewhere ~I.L'. The materials were tested as self-mated pairs.

Materials

The Si3N4 materials used here are of commercial grade with different chemical compositions and fabrication processes. The tested materials were one sintered silicon nitride (SSi3N4), one hot-isostatic pressed silicon nitride (HIP-Si3N4) and one hot-isostatic pressed reaction-bonded silicon nitride (HIP-RBSi3N4). The mechanical properties and chemical compositions are listed in Table 1. In all Si3N4 materials the [3-Si3N4 phase could be detected by X-ray diffraction (XRD).

Hardness

The Vickers hardness was measured according to DIN 50133. The results are shown in Table 2 for normal loads in the range 2-50 N. At high loads the HIP materials showed intergranular radial cracks, whereas around the indentation of the sintered material additional lateral cracks occurred (Fig 1).

Microstructure

The polished samples were etched for 15 s in boiling hydrofluoric acid (Fig 2). • SSi3N4 (ND200): The microstructure is homogeneous and well-linked, with evidence of voids 10 ~m in diameter. The grain length/grain diameter ratio is 6 : 1. • HIP-Si3N4 (FX-950): The morphology is dense with good linking. The grain length/grain diameter ratio is6:l. • HIP-RBSi3N4 (HIP-RBSi3N4 411): The microstructure is dense with a grain length/grain diameter ratio of about 4 : 1. In the microstructure, impurities, 30 ~tm to 100 ~m in diameter as seen with the optical microscope, with higher iron content, are dispersed. The A1, Na, Ca and Mg content of these regions is also different from the normal composition. This was confirmed by small-spot ESCA analysis. Friction

The friction coefficients were measured on line at constant sliding velocities and constant ambient tem-

Table 1. Mechanical properties at 22°C and phase compositions of the Si3N4 materials E Material

KtC

X

103Nmm 2 MPam 12 W m

SSiaN4 (ND 200)

C 280

6.0

30

HIP-Si3N4 (FX-950)

C 300

7.0

20

HIP-RBSiaN4 (HIP-RBSN 411 )

C 290-315

5.6

46

SiO2 wt%

Si3N4 wt%

cp

1K 1 jg

(RT-1000°C)

1K 1

Ra (disc) #m

Rz ( d i s c ) txm

3.4

0.036 +0.012

0.335 -+0.177

3.6

0.028 _+0.004

0.210 -+0.041

0.085 +0.017

0.785 _+0.165

10 6K

0.8

0.7

3.9 (20-1200°C)

AI203 wt%

Y203 wt%

MgO wt%

SSi3N4 (ND 200)

A 78.99 C

6.825

3.97

10.16

0.008

HIP Si3N4 ( FX- 950 )

B 82.34 A 83.76

5.2 5.9

4.24* 3.80*

8 6.48

0.02 0.003

HIP-RBSiaN4 (HIP-RBSN 411)

A 90.53

2.33

2.27

4.83

0.008

Fe203 wt%

CaO wt%

p gcm 3

0.047

3.27 3.25

0.06

3.30 3.32

0.035

3.26

0.2

*AIN Si3N4 corresponds to 100% 13-Si3N4 A - Laboratory analysis; B - National Physical Laboratory, Prof. Dr M. Gee; C - Prospect data

Table 2. Hardness measurement data

SSi3N4 ( N D 2 0 0 ) a HIP-Si3N4 (FX-950) b HIP-RBSiaN4 (411 )c

Hv0.2

Hv0.5

Hv5

1474-+95 1725-+81 1467-+116

1408-+95 1677-+57 1356-+86

1311+-65 1605-+34 1500-+76

aFeldmdhle AG, Plochingen bToshiba, Japan cHoechst CeramTec AG, Selb 190

J u n e 90 Vol 23 No 3

A. Skopp et aI--Unlubricated sliding friction and wear of various SigN4 pairs

Fig 1 Hv5-Vickers indentations in several SigN4 materials. Top, SSi3N4 (ND200); middle, HIP-Si3N4 (FX-950); bottom, HIP-RBSi3N4 (H1P-RBSi3N4411) peratures. The friction coefficients after a sliding distance of 1000 m are plotted in Figs 3 and 4. Independent of the operating conditions the coefficients of friction after 1000 m sliding distance for all types of materials lie between 0.5 and 1.0. It seems that at 1 m s - ~ there is a minimum in the coefficient of friction which is shifted with increasing ambient temperature for SSi3N 4 (Fig 3) and HIP-RBSi3N 4 (Fig 4) to smaller sliding velocities. The coefficient of friction is especially increased at higher sliding velocities by increasing ambient TRIBOLOGY INTERNATIONAL

Fig 2 Etched microstructure of Si3Nz. Top, SSi3N4 (ND200); middle, HIP-Si3N4 (FX-950); bottom, HIPRBSi3N4 (HIP-RBSi3N4411) temperature. The rise of friction coefficients at high sliding velocities is shifted with increasing ambient temperatures to lower sliding velocities. Figs 3 and 4 show a maximum for temperatures of 800°C and 1000°C. The highest coefficient of friction was measured for SSi3N4 at 0.94 at operating conditions of 1000°C and 0.3 m s - ' and for HIP-RBSN at 0.97 at 800°C and 2 m s '.

Wear The sum of the volumetric wear coefficients of pin and disc (s -- 0-1000 m) is plotted for the three materials 191

A. Skopp et a I - - U n l u b r i c a t e d sliding friction and wear of various Si3N4 pairs I.O • O........

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- -

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.

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- e l Relative humidity 1 8 - 5 0 % I0 ff 0.1 I 0.1 I /

.

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--

: ....... "I

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i§ 05

• •

.~

d.x"

•••. % -

i0 °

i0 ~

Sliding velocity v , m s -~

Fig 5 Sum of volumetric wear coefficients" of self-mated Si3N4 sliding pairs under unlubricated sliding conditions

Fig 3 Coefficient of friction after 1000 m of SSi~N4/ SSi3N4 under unlubricated sliding conditions I.O

o

E 0 0

....:

o"..

o_ / x ~ x

x/

x

0_ X,E 'z

50 f 45 40

~

25

~

x X 7"= 22°C A - . - Z ~ T= 400°C on-or= 800oC {3..... [3 Y = IO00°C I

O.C I0 -2

~ [~

~j

3.28

3.56

55

I

10

~

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I I I II I

I i0 -I

I

I I ) I II

I i0 0

I

Oil

033

0.38

0.38

1.03

1.03

Sliding velocity, m s -t

I I I I II

200C

I0'

Sliding velocity v , m s -I

Fig 4 Coefficient of friction after 1000 m of HIPRBSi3N4/HIP-RBSi3N 4 under unlubricated sliding conditions

E z

At room temperature the volumetric wear coefficient for all materials reaches a minimum, which is not correlated to vibrations of the test rig. The magnitude of this wear minimum increases from HIP-RBSi3N4 through HIP-Si3N4 to SSi3N4.

The wear increases from a low-wear to a high-wear region in the speed range between 0.39 m s ] and 0.75 m s - ~. In the wear minimum at 22°C the formation of a solid wear debris layer was observed only on the pin. The transition point from the high-wear to lowwear region for ambient temperatures /> 400°C is shifted to lower sliding velocities with increasing ambient temperature (Fig 5). In Figs 6Ca) to 6(c) the wear rates (s = 0-1000 m) of the pin and disc are shown separately. Results are only plotted which were classed by the sliding velocity; this is not a plot as a function of sliding velocity.

160C

• []

Pin Disc

HIP-RBSi3 N4/HIP-RBSi3N 4 (411) Temperature : 400°C Normal force: I0 N

7,

~

120C

~

80C

"A

400

as a function of the sliding velocity in Fig 5. The volumetric wear coefficient can be calculated by dividing the wear volume by the normal force of 10 N and the sliding distance of 1 0 0 0 m .

192

SSi3N4/SSi3N 4 (ND200) Temperature: 22°C Normal force: I0 N

Pin Disc

;.,c

0.5

g

• []

0

0.045

b

E "7 450C z E

0.115 0149 0.364 0 3 7 1.03 103 Sliding velocity, ms -I



Pin

[]

Disc

193

315

333

H I P - R B S i 3 N 4 / H I P - R B S i 3 N 4 (411) Temperature 800% Normal force: I0 N

500C v,1

u 150C

YA VA w

0

C

v~, y,

Htl

~ 1 ~ 1 ~ 1 ~ 1 ~ 1 ~ 0 0 4 3 0 0 4 4 0.1090.1420.345 0.35 0.97 0 9 7 1.52 L99 2.98 3.1 Sliding velocity, ms-~

Fig 6 Volumetric wear coefficient of pin and disc at different sliding speeds under unlubricated conditions. Ca) 22°C,• (b) 400°C; (c) 800°C J u n e 90 Vol 23 No 3

A. Skopp et aI--Unlubricated sliding friction and wear of various SigN4 pairs The strong decrease in the volumetric wear coefficient of pin and disc (Fig 5) with increasing sliding velocity especially for ambient temperatures between 400°C and 1000°C is mainly caused by a strong decrease in the disc wear volume (Figs 6(a) to 6(c)). This means that the pin and disc wear volume of self-mated Si3N4 pairs is influenced by the overlap ratio, as the pin has an overlap ratio of 100% and the rotating disc of 2%. For sliding velocities < 1 m s-] the pin has a several orders of magnitude smaller wear volume than the disc 13-]5, if the temperature is increased. The differences observed for experiments under the same conditions are related to sharp transitions between the low and high wear regions and possibly caused by small changes in material composition. For sliding velocities smaller than 1 m s-~ and ambient temperatures -> 400°C the pin is protected by wear debris layers. For sliding velocities -> 1 m s ~ and high ambient temperatures soft SiO2 layers are indicated by IR measurements. The pin wear maximum and high disc wear are caused by fatigue mechanisms between p-Si~N4/interphase phases. The pin wear volume increases linearly with increasing sliding distance (Fig 7), so it can be concluded that the pin wear volume is independent of the surface pressure 12.

,~

0.3

o

~0.25

o 3.56 ms -i

o

"

1.08 0.36 '~ 0.12 x 0.05 *

~

O.2

o o

~: 0.15

o

-~ 0.1

o 0

O

SSi3N4/SSi3N 4 ( N D 2 0 0 )

FN=IO

N: T : 2 2 : C xxxxX Rehhurnidity 1 8 - 3 0 % xXX x x x xXX x

250

500

750

I000

I

I

1250

1500

Sliding distance

[

E 0.8

HIP- RBSi3N4/HIP-Si3N4~ (411)

F/V=

m s -I rn s -I rn s-I m s-I

s ,m

o 3.33 m s -I A 1.04 m s-i " 0 3 6 rn s-I vO.15 m s -I x 0 . 0 5 m s -I

.

I0 N: r = 4 0 0 ° C

*

Zh

0.6

,

=E

o

D, 0.4

(3-

X

**

X

~7

O

V

*

8

~S%

250

o o o o

500

~7

x

X

X X A~** xxXX O~ 0 ~' A.

~2

X

X

x

*

A

3 0.2 .E

*

0

I

I

750

I000

0

0 I

i

1250

1500

Sliding distance s , m

Additional investigations SEM investigations The surface of the pin is protected by a layer of wear debris (Fig 8). Only in the low-wear region is the surface of the disc also covered by such a protective wear debris layer (Fig 9). From Figs 10 and 11 it would seem that this protective layer has a higher strength than the base material, because it is detached by transgranular fracture of the base material.

H I P - R B S i 3 N 4 / H I P - R B S i 3 N 4 (411) E

FN=

ION: T = 8 0 0 ° c z~ z~ z.,

Besides the 6 w% to 18 w% additives in the three materials, only the ]3-Si3N4 phase could be detected (on the unworn surfaces) ~6. Weak peaks of c~-SiO2, Y2SiOs, Y2Si303N4 and Y-N-apatite phases are registered for the SSi3N 4 and HIP-Si3N 4 materials. In wear debris only the [~-Si3N 4 phase could be detected as well as an amorphous peak at small diffraction angles (Fig 12). The diffraction peaks for [3-Si3N4 at 400°C and 800°C are very weak or suppressed for wear debris originating in the low wear region; the diffraction curves seem to be similar to that of a gel ~v. By using the Debye-Scherrer diffraction technique these results could be confirmed. In the high wear region the [3Si3N4 diffraction peaks are generally stronger, whereas peaks or rings from the low wear region are weak (or diffuse).

Infra-red spectroscopy By means of Fourier-transformation IR spectroscopy in the wavenumber range from 440 cm -~ up to 4000 cm-~ the ~3-Si3N4 lines at 400 cm 1 and 575 cm TRIBOLOGY

INTERNATIONAL

m s -I m s-r m s -I m s -i m s -I

~F X

x

~

**

._c Q_ ~ l ~ ' ~

250

X-ray diffraction

o 2.98 z~ 0 9 8 z~ * 0 . 3 5 '~014 x 0.05

0

0 LO

500

0

01 0

750

0

0I 0

I

I000

1250

I

1500

Sliding distance s ,m

Fig 7 Wear volume of the pin as a function of sliding distance under unlubricated conditions

of wear debris pressed in KBr from SSi3N4 and HIPRBSi3N 4 sliding pairs which were tribologically stressed at temperatures of 400°C and 800°C were found (Figs 13(a) and (b)). For wear debris arising in the low wear region of SSi3N 4 and HIP-RBSi3N4 sliding pairs (eg 800°C, 1.52 m s -~) additional amorphous SiO2 lines 18 at 480 cm -I, 1040 cm i and 1200 cm 1 were obtained (Fig 13(b)). The transmittance of the wear debris powder, which was formed in the high wear region, decreases in the wavenumber range from 2000 cm ~ to 4000 cm ~ (Fig 13) by a factor of 2 to 3 more than for the powder obtained in the low wear region at 400°C and 800°C. In all IR spectra no lines corresponding to the Si2N20 phase 19 could be detected. Fig 14 shows the IR spectra of the wear powder from the low wear and high wear regions at 1000°C. Phases corresponding to absorption bands at 500, 520,600 and 610 cm-~ together with the 193

A. Skopp et aI--Unlubricated sliding friction and wear of various Si3N4 pairs

Fig 8 Wear track (left) and wear scar (right) o f sliding pairs showing that the pin is protected by a wear debris layer. T o p - T = 22°C, v = 0 . 3 8 m s 1; b o t t o m - T = 400°C, v - 0.37 i n s

strong 660 cm-~ band have not yet been found. The line at 660 cm ~ was only detected in wear powder from the low wear region at high temperatures.

Oxygen and nitrogen analyses of wear debris from high wear regions With the oxygen and nitrogen analyses of wear debris performed by a L E C O tester the influence of tribooxidation as a wear increasing mechanism was investigated 2-~. The results for SSi3N 4 and HIP-RBSi3N4 wear debris are given in Table 3. Unfortunately, only the wear debris volume from the high wear region was sufficient for analysis. It can be seen from the results, that at 400°C, 800°C and 1000°C at small sliding velocities the tribo-oxidation is an effective mechanism, but not the wear-determining one.

Discussion This paper has shown that for tribological testing a wide range of operating conditions is important ~ 12 194

Table 3. Wear debris T (°C)

v (m s -1 )

Oxygen (Wt%)

Nitrogen (Wt%)

SSi3N4 (ND200) Bulk material 400 400 400 800 800 1000

6.49 0.053 0.60 0.72 0.04 0.1 O.1

18.2 8.6 9.8 14.2 14.3 14.3

32.3 21.8 29.8 29.4 25.7 25.0 25.7

HIP-RBSi3N4 (411) Bulk material 400 800

3.15

35.8

0.045 0.345

19.3 8.8

23.9 30.9

1000

0.1

15.1

24.7

June 90 Vol 23 No 3

A. Skopp et aI--Unlubricated sliding friction and wear of various Si3N4 pairs

Fig 9 Wear track (righO attd wear scar (left) o f an SSi3N4/SSi~N4 (ND200) sliding pair at 400°C and 3 m s - l after 1122 m

By using two different selected sliding speeds (Fig 5) the wear ranking of self-mated Si3N4 sliding pairs is changed. On the other hand one has to consider the sample costs and test times, when tests are performed over a wide range of operating conditions.

Overlap ratio

Fig 10 Back view o f wear debris broken by transgranular fracture in the SigN4 surface ( T = IO00°C, v = 1 m s l) TRIBOLOGY INTERNATIONAL

The results showed that the specimens with a small overlap ratio have higher wear than the specimens with a high overlap ratio. The ratio of wear of specimens with a small overlap ratio to that for specimens with a high overlap ratio increases with increasing temperature and decreasing humidity. This was confirmed by Ishigaki et al. 2°, Tomizawa and Fischer 2~ and Yust and Carignan 22 as well as by Gee et al. 14. The reason for the higher wear could be a cyclic mechanical fatigue mechanism on the rotating disc caused by high tangential stresses. Plastic deformation and/or surface layers were not observed in dry atmospheres 20-2~. 195

A. Skopp et aI--Unlubricated sliding friction and wear of various SisN 4 pairs

Fig 11 Fracture in the wear track of HIP-RBSi3N4 (T = 22°C, v = 3 m s ~)

9C 8C 7C 6C 5C

~.,~,.,~"4r~'r"'"=u''~=~"~°°'3'"~'°°'.3'"4~'~ucuL'J~~., AJ T=800°C v = 0 5 5 ms -I,

4C

low wear region C u - K a , Uo602/Uol02

3C

a

55

5'0

4'5

4'0

:5t5 50

2'5

2'0

I'5

2c

I0

7"= 4 0 0 ° C v - - , m s-' High wear region

/

IO

a

/ / z

k

~ z

05zi I

0 4000

I

52100

I

I

2400

~

I

I

I

1800 1400 Wavenurnbers

I ~J~

I000

L..

I

600

t E

~

j low wear region C u - K a , Uo632/Uo115b

b

I

I

55

50

45

40

55

I

i

:50

25

20

15

90

"7-

I0

80

4

-r

Fig 12 X R D profilogram of SSi3N4 wear powder from." (a) high wear region; (b) low wear region

0

,

70

0

0 0

= 60 o !--

g 5C c o

~ 4c

Mechanical stress/grain boundary fatigue

3(

The ratio of properties for the [B-Si3N4 and glassy phase are as follows:



HIP- RBSi3N4/HIP- RSSi3N 4 (411) 7":800°C v =1.52 m s -t Low wear region

IC ESi3N4 EC~,.... = 2 - 3 ;

O/-Si3N4

-

Or'Glass

~Si3N 4

=2-3;

~-Glass

> 15

where E is Y o u n g ' s modulus, o~, linear thermal expansion, and k, thermal conductivity. The hardness and the fracture toughness of the glassy phase are not 196

I

4000 b

I

3200

I

I

2400

I

I

[

z, z

o,

I

1800 1400 Wovenumbers

)ZZ ._ .& .& u3 "^ Vl~tJ3 I

Iooo

I

I

600

Fig 13 IR transmittance sheet of HIP-RBSi~N4 wear debris." (a) high wear region," (b) low wear region J u n e 90 V o l 23 N o 3

A. Skopp et aI--Unlubricated sliding friction and wear of various SigN4 pairs considered here, because they are strongly temperature-dependent. Si3N4 materials have an inhomogeneous microstructure. Commonly they are composed of hexagonal [3-Si3N4 crystals, which are surrounded by an oxygen- or nitrogen-containing multicomponent glassy phase. The different temperature-dependent mechanical and physical properties of both [3-SigN4 and the glassy phase lead under unlubricated sliding conditions to rapid and local changing thermal and mechanical stresses not only in the phases, but more severely at the boundary of the phases. These cyclic stresses cause quick fatigue of the 13-Si3N4/glass boundary and the formation of wear particles. This coincides with tension-compression fatigue and bending fatigue tests 25'2~ which revealed that at the [3-Si3N4/glass interface intergranular fatigue failure occurred.

9°f 80

70

o

v

F= 60 2 ~u~ 50

-r-

~

cD

13

~. 4o 04 50

SSi~N4/SSi3N 4 (ND200) T =IO00°C v = 0.92 ms-t L o w wear r e g i o n

z~

2o i0 i.0

i

I

4000

I

~200

i

I

2400

i

i

1800

a

z,

i

I

i

1400

I~)

[

6OO

1000

Wavenumbers

9c

Tribo-oxidation Tribo-oxidation is the enhanced formation of a reaction layer resulting from a reaction of the surface with the ambient gas during tribological operation. With increasing sliding speed not only does the frictional power increase, but also the contact-free time of the rotating specimen decreases, so that during the break in contact the surface can cool down only slightly. The wear of the rotating specimen decreases at 400°C, 800°C and 1000°C by around 1 m s ~ It can be concluded from IR spectroscopy, and XRD and chemical analyses of the wear debris that above approximately 1 m s ~ the wear of the rotating specimen is reduced by the formation of a dense, amorphous Si-O layer (Fig 14). The Si-O layer not only protects Si3N4 against catastrophic oxidation, but also reduces the tribological stresses of the surfaces by distributing the load over a greater contact area caused by the softer Si-O layer.

Wear particle adhesion Figure 15 shows wear particle islands, the plastic deformation of these islands and their breaking off by repeated stress. The plastic deformation of wear particles and the build up of wear particle islands is a typical phenomenon for unlubricated ceramic/ceramic sliding pairs ~,j2, leading independently of test conditions to high friction coefficients between 0.5 and 1 for nearly all ceramics. Fig 15 shows schematically that between two wear particle islands in the contact surface a geometrical misalignment or hooking during sliding can occur, which increases the friction force. The friction coefficient of Si3Na/Si~N4 sliding pairs after removing the wear particles from the wear tracks and scars lies at approximately 0.2 at 22°C. These low values caused by surface layers are reported for short sliding distances, low loads, low geometrical contact pressures and low sliding velocities ~.2°~24. The surface of the rotating disc with an overlap ratio < 2% is cyclically stressed. For a constant friction coefficient the friction-induced energy and contact temperature TRIBOLOGY

INTERNATIONAL



7C

T

O

6C

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Fig 14 IR transmittance sheet of SSi3N4 wear debris for high temperature in the." (a) low wear region," (b) high wear region

increase with increasing sliding speed. For a given wear track diameter the break in contact decreases simultaneously.

Conclusions • The coefficient of friction lies under steady state conditions between 0.5 and 1. • Wear rate increases with rising ambient temperature, especially at sliding speeds < 1 m s ~. • The influence of the overlap ratio on wear increases with increasing ambient temperature. A small overlap ratio is disadvantageous for Si3N4 sliding pairs. • Grain boundary fatigue and abrasion is significant at room temperature. • Surface-protecting layers formed by wear particles at 22°C and sliding velocities between 0.3 m s-~ and 0.75 m s-t reduce the wear rate. •

Tribo-oxidation is effective as a wear accelerating mechanism at 22°C and sliding speeds I> 1 m s J. 197

A. Skopp et aI--Unlubricated sliding friction and wear of various SisN4 pairs

a b c Fig 15(a) Transfer of deformed wear particles; (b) plastic deJ~)rmation of tran~ferred wear particle islands; (c) fracture

The breakdown of a tribochemical layer was observed at sliding velocities ~> 1 m s ~ leading to abrasion and higher wear rates. • Layers of wear particles protect the pin at sliding speeds ~< 0.3 m s J and temperatures /> 400°C. • There is a high wear/low wear transition at temperatures ~> 400°C due to protective tribo-oxidation.

schungsbericht Nr. 133, Bundesanstalt fiir Material-forschung und -pHifung, 2nd edition, 1988, D-IO00 Berlin 45 2. Steinmann D. Silicon nitride. Elektroschmelzwerk Kempten, 1983, Firmenschrift 3. Brusban B. and Sihley L.B. SigN 4 rolling bearings for extreme operating conditions. A S L E Trans. 25 (4), 1982, 417-428 4. Hosang G.W. Application of ceramic ball bearings to the Meradcom 10 kW-turbine. Research report SR-78-R-4440-22, US

Army Mobility

• The tribological behaviour in the high wear region at sliding speeds <~ 1 m s ~ and temperatures ~> 400°C is determined first by cyclical mechanical and second by friction-induced cyclical thermal stresses. A fatigue process at the interface ~-Si3N4/ glass is the reason for high wear rates. •

Tribo-oxidation determines the tribological behaviour of temperatures >7 400°C and sliding velocities ~> 1 m s ~. Soft oxide-layers reduce thermal and/or mechanical stresses, leading to lower wear rates.

Acknowledgements Financial support by the G e r m a n Ministry for Research and Technology ( B M F T ) is gratefully acknowledged. The specimens were furnished by Feldmiihle A G , Hoechst CeramTec A G and those for the V A M A S round robin tests by Toshiba Tungaloy Co., Ltd. This is also gratefully acknowledged. Thanks are due to Mrs S. Binkowski and R. Pahl for the ceramography. The X-ray diffraction analyses were conducted by Mr H. Walter. The authors would also like to thank Dr L. Vogel and Mrs B. StrauB for carrying out the IR and SEM analyses.

References 1. Woydt M. and Habig K.-H. Technisch-physikalische Grundlagen zum tribologischen Verhalten keramischer Werkstoffe. For-

198

5. Gugei E. and Loroseh H.-H. ASME, J. Engng Power, 102,

1980, 128 6. Hamburg G., Cowley P and Valori R. Operation of an allceramic mainshaft roller bearing in a J-402-gas-turbine engine. Lubr. Engng, 37 (7), 19811, 4117-415 7. Cundill R. Werkstuffe mit geringer Dichte for W~ilzk6rpcr wm Triebwerkslagern. SKF-Kugellagerzeitschr(lL 4, 216, 1984, 33 8. Weigand A. Kcramik-Werkstoff fiir W~ilzlager yon morgen?

Wiilzlagertechnik 1988, No. 1 9. Haas H and Fleischer M. Untersuchungen von Funktion und Lebensdauer an Drehdurchfiihrungen, Lagern und Keilmech-

anismen unter HTR-Bedingungen Reaktortagung, 4 7 April 1978, Hannover, Deutsches Atomforum, 737 10. Piispanen T. Rolling contact fatigue of ceramic materials.

Tribologia (Finn. J. Tribol.) 6 (41, 1987, 5-19 11. Gienau M. Woydt M and Hahig K.-H. Hochtemperaturtribometer ftir Reibungs- und Verschlei6-untcrsuchungen his 10110°C. Materialpn~fang, 7/8, 29, 1987, 197-199 12. Woydt M. and Habig K.-H. High temperature tribok)gy of ceramics. Tribology Intern. 22, 2, 1989, 75-88 13. Becket S. VAMAS-Wear Test Methods; interlaboratory-Exercises Conf. on Wear of Ceramics' - Test Methods and Mechanisms,

National Physical Laboratory, Institute of Metals, 5-6 December 1988 14. Gee M.G., Matharu C.S., Almond E.A. and Eyre T.S. The measurements of sliding friction and wear at high temperature.

Wear of Materials', 1, 1989, 387-397 15. lwasa M., Toibana Y., Yoshimura S. and Kobayoshi E. Wear of SigN4 ceramics measured with various testers. Yogyo-KyokaiShi 93, 2, 1985, 73-80

June 90 Vol 23 No 3

A. Skopp et aI--Unlubricated sliding friction and wear of various Si3N4 pairs 16. Mencik Z., Short M.A. and Peters C.R. Quantitative phase analysis of synthetic Si3N4 by XRD. Advances X-Ray Anal., 23, 1980, 375-379 17. Salmang H. and Scholze H. Keramik, TeE 2: Keramische

Werkstoffe, 6th edition, 1983 18. Hammel/Scholl Atlas der Kunststoffanalyse, Band 2, Zusatz-

stoffe + Verarbeitungshilfsmittel Hansa-Verlag, 1973 19. Baraton M.I., Labbe J.C. and Quintard P. L'oxynitride de silicium: Si2N20. I Attribution des absorption du spectre infrarouge. Mat. Res. Bull, 20, 1985, 1239-1250 20. lshigaki H., Kawaguchi I. and Iwasa M. Friction and wear of HP-SN and other ceramics. Wear of Materials, 1985, 13-21 21. Fischer T.E. and Tomizawa H. Interaction of tribochemistry

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and microfracture in the friction and wear of silicon nitride.

Wear of Materials, 1985, 22-32 22. Yust C.S. and Carignan F.J. Observations on the sliding wear of ceramics• A S L E Trans• 18, 2, 1985, 24.5-252 23. Klafl'ke D. Fretting wear of Si3N4. BAM, unpublished 24. Fischer T.E. Friction and wear of ceramics. 1. Int. Symposium

High-Tec Materials and Finishing, 12-14 March 1989, PraxisForum, FRG, D-IO00 Berlin 22, Ritterfelddamm 82 25. Kawakuho T. and Komeya K. Cyclic fatigue of SSi3N4 at room temperature. Zairyo. J. Soc. Mat. Sci. Jap. 34, 1985, 387,

1460-1465 26. Masuda M., Soma T., Matsui M. and Oda I. Cyclic fatigue of SSi3N4. Ceram. Engng. Sci. Proc. 9, 9-10. 1988, 1371-1382

199

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