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TRIBOLOGICAL TESTING OF Ca-Ba INFILTRATED ZIRCONIA

Rolf WÄSCHE, Dieter KLAFFKE, Gabriele STEINBORN

BAM – Federal Institute of Materials Research and Testing, Germany rolf.waesche@bam.de

ABSTRACT

Specific ceramic materials were prepared by sintering and a subsequent infil-tration process. Zirconia was first sintered by a special procedure in order to obtain an open porous body with porosity of 20 to 30 %. The porous zirconia body was then infiltrated with a Ca – Ba fluoride melt. The friction and wear behaviour of these materials was studied with oscillating sliding motion in a ball-on-disk configuration against different ball materials at room temperature. Special attention was paid to the effect of relative humidity (R.H.) of the surrounding air.

The friction depends on counter body material and on relative humidity. For different test conditions coefficients of friction are found <0.2 but also >0.7. The lowest friction and the smallest effect of R.H. are found in tests against Al2O3. The highest friction and the strongest effect of R.H. are found against Si3N4. KEYWORDS: friction, wear, Ca-Ba fluoride, infiltration, porous zirconia.

1. INTRODUCTION

Ceramic materials are of increasing interest for

tribological systems, owing to high hardness, good to high strength and high corrosion resistance. However, in many cases friction and wear are often too high to run ceramic tribosystems unlubricated. Improvement of friction and wear response of ceramics might be possible by use of solid lubricants. One of the well investigated materials group are the fluorides, espe-cially with the aspect of high temperature usage. These materials provide an extremely good thermo-dynamic stability making them useful for metals as well as ceramics. The application method can be manifold, sintering, transfer film application but also coatings are reported [1-7]. Infiltrations into porous metals have been reported, but to a lesser extend con-cerning melt infiltration into porous ceramics. Since friction and wear are not materials properties but always properties of a system, the behaviour of a selected material may differ considerably for different counter body materials [8-10].

2. OBJECTIVES

The goal of this study is the evaluation of a po-

tential improvement of friction and wear behaviour of ceramics by infiltration with lubricious Ca – Ba fluo-rides. The friction and wear behaviour is characterised for room temperature applications as a first step. Since the friction and wear behaviour of a material is often affected significantly by the choice of the mate-rial in contact, four different ball materials were used in order to optimise the tribological behaviour of the

tribo system. In contrast to former investigations with infiltration of porous alumina [11], the aim of this paper is the use of porous zirconia.

3. EXPERIMENTAL

3.1. Tribo Test Rig

Tests were performed with oscillating sliding motion with small amplitudes because this test method requires only small specimens. A test rig was used that is described in more detail in [12]. A ball (10 mm diameter) is fixed at the top of a lever with an integrated load cell and pressed (by a dead weight) against the disk. The disk is fixed on a horizontal table which can be activated to oscillations with a selectable frequency.

A blade spring with strain gauges touches the ball holder and measures the total linear wear Wl. The tribometer works in a chamber of acrylic glass inside which the relative humidity can be varied between 2 % R.H. and 100 % R.H., and kept constant during each test. The quantities friction force, linear wear, temperature and relative humidity are each stored with 1000 data per test by means of a PC system.

3.2. Materials The balls were commercially available, made of

bearing steel (100Cr6), alumina (Al2O3), silicon ni-tride (Si3N4) and zirconia (ZrO2), respectively.

The infiltrated disk materials were made by melt infiltration of a CaBa fluoride mixture. As a prerequi-

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site a porous ceramic is necessary having a good me-chanical stability. To achieve this, a nanoporous zir-conia ceramic was made from ZrO2 nano powder by a conventional ceramic manufacturing process. The disks were then infiltrated by the fluoride melt at a temperature range of 1200 to 1250°C in a gas pressure sintering device, by embedding the ceramic in the fluoride powder mixture and subsequent heating to the melting temperature range in vacuum. The melt infiltration process took place in a carbon crucible. After the powder mixture was melted a slight Ar pres-sure of 1 bar was applied to enhance infiltration. The coin like specimens were ground and polished prior to tribo testing.

3.3. Test Programme

Tribological tests were performed with 4 ball materials with constant test parameters, compiled in Table 1.

Table 1. Test conditions for fretting tests. Ball material 100Cr6 - Al2O3 - Si3N4 - ZrO2

Stroke ∆x = 0.2 mm Frequency ν = 20 Hz Normal force Fn = 10 N Number of cycles n = 100 000 Temperature T = 24 °C Environment Lab. air Rel. humidity 3 % - 50 % - 100 % Lubricant none

Fig. 1. Specimen with wear scars and enlarged view. Because the wear scars in fretting tests are al-

ways rather small, figure 1, all tests can be preformed on one single specimen. This is a big advantage for achieving increased comparability of the testing re-sults, due to a drastically reduced effort of sample preparation and the increased probability to achieve

homogeneity in a single specimen rather than for a group of specimens.

3.4. Tribological Quantities

3.4.1. Friction The frictional behaviour is described by the co-

efficient of friction (COF) f, which is defined as the ratio of friction force and normal force. Since in many cases a certain running-in phase occurs the average value of f is selected for the sake of comparison.

3.4.2. Wear

The wear behaviour is described in terms of volumetric wear, calculated from the dimensions of the wear scar and an additional profile of the wear scar on the disk, as is shown exemplarily in figure 2. The circle line indicates the contour of the ball. In the case of the profile on the left side the distance be-tween profile and circle line indicates a rather high wear at the ball, while in the profile on the right side the distance of the two lines is small, indicating only small wear at the ball.

5 µm

500 µmcontourof ball

Wq = 2 030 µm²

Wq

5 µm

500 µm

disk wear

ball wear

Fig. 2. Profiles of two wear scars, indicating ball wear and disk wear.

The volumetric wear at both specimens was cal-

culated with the equations compiled in [13]. For com-parison of wear quantities the usually used -"coefficient of wear, k" was calculated that is defined as the ratio of volumetric wear and the product of sliding distance and normal force.

4. RESULTS

4.1. Friction

The evolution of COF is shown for tests against

different ball materials in figure 3 to figure 6. The steady state COF, determined from the sec-

ond half of each test, is plotted vs. R.H. in figure 7. Low COF is found against 100Cr6 in dry and normal air, against Al2O3 on all levels of R.H., against Si3N4 only in dry air and against ZrO2 in dry and in moist air.

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0.0

0.2

0.4

0.6

0.8

1.0

0 50 100Cycles n [103]

Coe

ffic

ient

of f

rictio

n f

Ball: 100Cr63 % R.H.

0 50 100Cycles n [103]

Ball: 100Cr649 % R.H.

0 50 100Cycles n [103]

Ball: 100Cr6100 % R.H.

Fig. 3. Evolution of COF in tests with infiltrated ZrO2

against 100Cr6 in dry, normal and moist air • Against 100Cr6 a more or less increase of COF

is found during the test. The friction at the end of each test is the highest in moist air (f = 0.55), moderate in normal air (f = 0.35, increasing trend), and the smallest in dry air (f = 0.25).

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100Cycles n [103]

Coe

ffic

ient

of f

rictio

n f

Ball: Al2O3

R.H. = 2 %

0 50 100Cycles n [103]

Ball: Al2O3

R.H. = 50 %

0 50 100Cycles n [103]

Ball: Al2O3

R.H. = 100 %

Fig. 4. Evolution of COF in tests with infiltrated ZrO2

against Al2O3 ball in dry, normal and moist air. • Against Al2O3, figure 4, the COF is smaller and

nearly constant during the tests on all levels of R.H. The effect of R.H. is rather small, the end value of COF is about 0.2 on all levels of R.H..

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100Cycles n [103]

Coe

ffic

ient

of f

rictio

n f

Ball: Si3N4

R.H. = 2 %

0 50 100Cycles n [103]

Ball: Si3N4

R.H. = 55 %

0 50 100Cycles n [103]

Ball: Si3N4

R.H. = 100 %

Fig. 5. Evolution of COF in tests with infiltrated ZrO2

against Si3N4 ball in dry, normal and moist air • Against Si3N4, figure 5, the COF is rather low

(0.3) in dry air and distinctly higher in normal air (0.8) and in moist air (0.7).

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100Cycles n [103]

Coe

ffic

ient

of f

rictio

n f

Ball: ZrO2

R.H. = 1 %

0 50 100Cycles n [103]

Ball: ZrO2

R.H. = 50 %

0 50 100Cycles n [103]

Ball: ZrO2

R.H. = 100 %

Fig. 6. Evolution COF in tests with infiltrated ZrO2 against ZrO2 ball in dry, normal and moist air.

• Against ZrO2ball, figure 6, a quite different

influence of R. H. is found: the COF is rather low in dry and in moist air (0.34 and 0.24) but distinctly higher (0.6) in normal air.

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of f

rictio

n f

≅x, ≅

FnBall:

100Cr6

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of f

rictio

n f

≅x, ≅

Fn Ball: Al2O3

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of f

rictio

n f

≅x, ≅

Fn Ball: Si3N4

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of f

rictio

n f

x,

FnBall: ZrO2

Fig. 7. Coefficient of friction vs. R.H. from tests with infiltrated ZrO2 disk against different ball materials; mean

value of the second half of each test.

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4.2. Wear

In tests against steel ball wear occurs at both bodies of the tribo contact. Figure 8 shows the profiles of the wear scars from tests in dry, normal and in moist air. The wear at the disk is small and nearly equal in dry and in normal air. The very small dis-tance from the profile line to the circle line indicates nearly no wear at the ball. In moist air there is some material transfer to the disk, the width of the scar is greater than in dry and normal air. In this case (moist air) considerable wear occurs at the ball but no wear is detectable at the disk.

Ball: 100Cr6

3 % R.H.

5 µm 500 µm

Ball: 100Cr6

50 % R.H.

5 µm500 µm

Ball: 100Cr6

100 % R.H.

5 µm500 µm

Wq = 1 920µm² 2 005 µm² 20 µm²

Fig. 8. Profiles of wear scars on the infiltrated ZrO2 disk from tests against 100Cr6 ball in dry, normal and

moist air.

Ball: Al 2 O 3

3 % R.H.

5 µm 500 µm

Ball: Al 2 O 3

50 % R.H.

5 µm500 µm

Ball: Al 2 O 3

100 % R.H.

5 µm

500 µm

Wq = 680 µm² 2 175 µm² 800 µm²

Fig. 9. Profiles of wear scars on the infiltrated ZrO2 disk from tests against Al2O3 ball in dry, normal and

moist air.

Ball: Si 3 N 4

2 % R.H.

5 µm 500 µm

Ball: Si 3 N 4

50 % R.H.

5 µm500 µm

Ball: Si 3 N 4

100 % R.H.

5 µm500 µm

Wq = 1 455 µm² 13 445 µm² 8 100 µm²

Fig. 10. Profiles of wear scars on the infiltrated ZrO2 disk from tests against Si3N4 ball in dry, normal and

moist air.

The wear scars from tests against Al2O3, figure 9, show rather small wear at the disk in dry and in moist air and significantly more wear at the disk in normal air. In all cases, the wear at the ball is negligi-ble.

The profiles from tests against Si3N4, figure 10, show rather small wear in dry air, but extremely high wear in normal and in moist air. In the latter cases the wear at the ball is also more pronounced than in dry air.

The profiles from tests against ZrO2, figure 11, show high wear in dry and in moist air, but rather small wear in normal air. Ball wear is small on all levels of R.H.

Ball: ZrO 2

3 % R.H.

5 µm500 µm

Ball: ZrO 2

50 % R.H.

5 µm

500 µm

Ball: ZrO 2

100 % R.H.

5 µm 500 µm

Wq = 7 590 µm² 1 750 µm² 4 240 µm² Fig. 11. Profiles of wear scars on the infiltrated ZrO2 disk from tests against ZrO2 ball in dry, normal and

moist air.

The volumetric wear at ball and disk is summa-rized in figure 12: • Against 100Cr6 the wear at the ball is smaller

than that at the disk in dry and in normal air, but much higher in moist air.

• Against Al2O3 the wear at the ball is much smaller than that at the disk on all levels of R.H.

• Against Si3N4 the wear at ball and disk are nearly of the same magnitude on all levels of R.H.

• Against ZrO2 the wear at the ball is significantly smaller than that at the disk on all levels of R.H. The coefficient of total wear, k, is shown in

figure 13 vs. R.H. The coefficient of wear is calcu-lated from the total volumetric wear of the system, taking into account the wear at the ball and at the disk. It is evident, that the counter body material has a strong influence on the wear characteristic: • Against 100Cr6 k is small in dry air and in-

creases by more than one order of magnitude for increasing R.H.

• Against Al2O3 k is rather small on all levels of R.H., the wear is highest in normal air.

• Against Si3N4 k is rather small in dry air but very high in normal and in moist air,

• Against ZrO2 k is medium and only slightly affected by R.H., the wear is here smallest in normal air.

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10

100

1000

10000

100000

3 50 100

Rel. humidity R.H. [%]

Vol

umet

ric w

ear W

v [10

-6m

m3 /N

m] Ball: 100Cr6

Disk: ZrO2-inf

x,

Fn

3 50 100

Rel. humidity R.H. [%]

Ball: Al2O3Disk: ZrO2-inf

x,

Fn

2 50 100

Rel. humidity R.H. [%]

Ball: Si3N4Disk: ZrO2-inf

x,

Fn

3 50 100

Rel. humidity R.H. [%]

Ball: ZrO2Disk: ZrO2-inf

∆x, ν

Fn

Fig. 12. Volumetric wear at ball and disk vs. R.H. in tests with different counter body materials.

0.1

1

10

100

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of w

ear k

[10-6

mm

3 /Nm

]

∆x, ν

Fn

Ball: 100Cr6

0.1

1

10

100

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of w

ear k

[10-6

mm

3 /Nm

]

∆x, ν

Fn

Ball: Al2O3

0.1

1

10

100

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of w

ear k

[10-6

mm

3 /Nm

]

≅x, ≅

Fn

Ball: Si3N4

0.1

1

10

100

0 50 100

Rel. humidity R.H. [%]

Coe

ffic

ient

of w

ear k

[10-6

mm

3 /Nm

]

∆x, ν

Fn

Ball: ZrO2

Fig. 13. Coefficient of wear vs. R.H. in tests with infiltrated ZrO2 disk against different counter body materials.

4.3. Influence of Mating Material

The tribological characterisation of a material requires information about friction and wear quanti-ties. The results can be plotted in a k vs. f-plane. In many cases a low friction and low wear rates are beneficial and are consequently the goal of materials development. Figure 14 summarises the effect of counter body and humidity on friction and wear behaviour. Low friction and wear quantities are found against: • 100Cr6 in normal and moist air, • Al2O3 on all levels of R.H. • ZrO2 in normal air, and • Si3N4 in dry air.

The influence of R.H. is strongest for tests against Si3N4 and smallest for tests against Al2O3.

0.1

1

10

100

0.0 0.2 0.4 0.6 0.8 1.0

Coefficient of friction f

Coe

ff. o

f wea

r k [1

0-6m

m3 /N

m]

Al2O3

Ball: 100Cr6

Si3N4

∆x, ν

Fn

ZrO2

Fig. 14. Results of tribotest with different counter body materials in a k vs. f-plane.

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5. DISCUSSION

Chemical analysis inside the wear scar and on wear particles provide information concerning the dominating wear processes. Figure 15 shows a SEM micrograph of a wear scar on the zirconia disk after the test against a Si3N4 ball, including the wear debris as obtained after the experiment.

Figure 16 shows the EDX analysis. It can be seen, that the CaBa fluoride mixture is well present in the ceramic. From the element analysis several con-clusions can be drawn. Si is found mainly in the wear debris, whereas the fluorides as well as zirconia are found more in the disk material. This is in agreement with the expectation and secondly with the experi-mental results, too, shown in figure 12, namely that there is substantial wear of the silicon nitride ball relatively to the disk. In this regard most likely tribo-oxidation and formation of silicon oxide plays a major role.

There is also found a significant sign for the presence of carbon in the disk as observed by the carbon peak. For this result it may only be guessed that this carbon peak is due to carbon resulting form the manufacturing process, since the infiltration was done in a carbon crucible and in a reducing atmos-phere. How the carbon got into the ceramic is not clear yet.

Looking at figure 7 it is interesting to find that at low relative humidity friction is lowest, with the ex-ception of the zirconia ball. This is in ceramic sliding

couples rather unusual, since normally low R.H. val-ues cause higher friction and also higher wear. It seems that the effect of lubrication in these cases is better at low R.H. As was found in investigations with BN [14] as solid lubricant and also with SiC-TiC-TiB2 material sliding pairings [15], chemical reactions in the sliding interface is most probably the cause. However, to prove this hypothesis, more experiments and detailed analysis are necessary.

Fig. 15. Wear scar on infiltrated zirconia disk with wear debris obtained with Si3N4 ball at 4% R.H.

Fig. 16. EDX analysis of wear scar and wear debris.

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6. CONCLUSIONS Fluoride infiltration of zirconia can improve the

friction and wear behaviour of a tribo couple at room temperature, especially at low relative humidity. An improvement of friction and wear behaviour, how-ever, is not found for all different counter body mate-rials. The best behaviour is found for tests against Al2O3. The friction and wear behaviour is more or less affected by the relative humidity of surrounding air, the smallest sensibility is again found for tests against Al2O3.

ACKNOWLEDGEMENTS

Thanks are due to Mrs. Ch. Neumann for assis-

tance in tribo testing, and Mr. J. Schwenzien for pro-filometric measurements.

REFERENCES

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5. Sliney H. E., Strom T. N., Allen, G. P., 1965, Fluoride Solid Lubricants for extreme Temperatures and corrosive Environments, ASLE Trans. Vol. 8, pp. 307-322. 6. Sliney H. E., 1984, Status and new directions for solid lubricant coatings and composite materials. I: Tribology in the 80’s, NASA conference Publications 2300, Vol. II, pp. 665-689. 7. DellaCorte C., Edmonds B. J., Benoy P. A., 2001, Thermal Processing Effects on the Adhesive Strength of PS304 High Tem-perature Solid Lubricant Coatings. NASA/TM-210944. 8. Klaffke D., 2001, Influence of counter body material and relative humidity on the tribological behaviour of steel/ceramic couples. Proc. 2nd World Tribology Congress, ed. F. Franek, W. J. Bartz, A. Pauschitz, pp. 93-100 9. Klaffke D., Wäsche, R., Wötting G., 2004, Tribological Char-acterisation of a Si3N4 - TiN composite ceramic in fretting tests at room temperature. Proc. 14th Int. Coll. Tribology, (ed. W. J. Bartz), TAE, Esslingen, Vol. III, pp. 1413-1419. 10. Klaffke D., Wäsche R., Yarim R., 2003, Friction and Wear Behaviour of Ceramic SiC-TiC-TiB2 Composites under Unlubri-cated Sliding Conditions, Tribotest Journal, Vol. 9(3), pp. 185-195. 11. Wäsche R., Klaffke D., 1994, Schwingungsverschleiß von fluoridinfiltriertem Aluminiumoxid bei Temperaturen bis 600°C an Luft. Trib. Schmier.tech., Vol. 41, pp. 290-293. 12. Klaffke D., 1995, On the Influence of Test Parameters on Friction and Wear of Ceramics in Oscillating Sliding Contacts. Tribotest Journal, Vol. 1(4), pp. 311-320 13. Klaffke D., 1989, Fretting wear of ceramics, Tribol. Int., Vol. 22(2), pp. 89-101. 14. Wäsche R., Klaffke D., Skopp A., 1992, Tribological behav-iour of Silicon Nitride with Boron Nitride as a solid lubricant, Proc. 8th Int. Coll. Tribology, TAE, Esslingen, (ed. W.J. Bartz), pp. 10.2.1-10. 15. Wäsche R., Klaffke D., 1998 In situ formation of tribologically effective oxide interfaces in SiC-based ceramics during dry oscillat-ing sliding, Tribol. Lett., Vol. 5, pp. 173-190.