Friction and wear of rare earth metals in air

Friction and wear of rare earth metals in air

145 Journal of the Less-Cozen Met&, 30 (1973) 145-151 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands FRICTION AND WEAR OF RARE EART...

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145

Journal of the Less-Cozen Met&, 30 (1973) 145-151 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

FRICTION

AND WEAR OF RARE EARTH

E. RABINOWICZ

METALS

IN AIR

and P. A. MARCH

Department of Mechanical Engineering, Massachusetts institute

ofTechnology, Cambridge, Mass. 02139

(U.S.A.) (Received June 24,1972)

SUMMARY

Friction and wear rates have been measured for combinations of the rareearth metals cerium, lanthanum, dysprosium, holmium and yttrium, the alloys misch metal and didimium, and the metals zinc, aluminum, copper, iron, titanium and zirconium, sliding in air, unlubricated. It was found that the rare-earth metals show rather lower friction coe~cients and rather higher wear rates than do other metals, because they oxidize so readily. The cubic structured metals show higher friction than do the hexagonal structured metals ; of the hexagonal metals, especially low friction is associated with a high c/a ratio of the crystal lattice. It is concluded that, owing to their high oxidation rates, the rare-earth metals sliding in air do not show the great degree of promise that they display in a vacuum environment.

INTRODUCTION

Although the rare earths constitute about 20% of all naturally occurring metallic elements, they have received little consideration as materials for use in practical engineering applications. In particular, their friction and wear properties have been studied but little, even though the work which has been carried out suggests that the rare-earth elements have unusual and potentially valuable tribological properties. The first systematic study of a tribological effect, involving normal adhesion between rare earths which had been slid together in air, was carried out by Sikorski and Courtney-Pratt”‘. The metal yttrium (Y) and the rare earths lanthanum (La), cerium (Ce), samarium (Sm), gadolinium (Gd), dysprosium (Dy) holmium (Ho), erbium (Er), and ytterbium (Yb) were used, and it was found that these metals showed surprisingly small adhesion against themselves, except ytterbium and cerium. The anomalous behaviour of the latter metals was attributed to the fact that they are the only ones which have a cubic crystal structure. The second comprehensive study was that of Buckley and Johnson3*4. They carried out sliding tests in vacuum using yttrium and the rare-earth metals La, Ce, praseodimium (Pr), neodymium (Nd), Sm, Gd, Dy, Ho, and Er, sliding against stainless steel. They too found that crystal structure influenced friction greatly. The cubicstructured curium gave the highest friction, while the other metals, which have a hexagonal structure, gave lower friction. Lanthanum, which is hexagonal at room

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P. A. MARCH

temperature but transforms to a cubic structure at about 250°C shows a marked increase in friction at a temperature corresponding to the phase change. For the hexagonal metals, there was a strong correlation between friction and interbasal planar spacing, or c/a ratio, such that high c/a ratio corresponds to low friction. Buckley and Johnson showed that other hexagonal-structured metals obey the same relationship and that, in general, hexagonal metals with high c/a ratios (1.60 to 1.63) give unusually low friction when compared to other metals. More recently Nosovskii, Isaev and Kostetskii5 have confirmed the fact that, when sliding in vacuum, the friction of lanthanum does indeed increase when the temperature is raised above 250°C. It will be noted that these previous studies have involved such rather esoteric properties as adhesion or such unusual environments as a vacuum. It was decided to undertake a study of the friction and wear properties of the rare-earth metals when sliding in air, to see if the metals had potential as bearing materials in normal applications. MATERIALS

AND APPARATUS

The materials used in the study were those rare-earth metals which could be readily obtained. Thus, riders ofY, La, Ce, Dy and Ho were used, as well as two alloys, namely didymium (86% Nd, 14% Pr) and misch metal (Ce 53, Nd 16, other rare earths 3 l), these being referred to hereafter as di and mm, respectively. In addition, to provide a basis of comparison, a zinc rider was also used. The flat specimens were La, Ce, di, and mm, as well as aluminum, copper, iron (1010 steel), titanium and zirconium. In choosing the other materials used in these tests, it was decided to operate within a restricted range ofhardness values, in order to restrict the number ofvariables. The softest metal was cerium (40 kg/mm’), whilst the hardest was zirconium (250 kg/mm’). The friction apparatus was of the pin-on-disk type, as described in earlier papers6y7 and shown in Fig. 1. A load of 0.5 kg was used to load the rider specimen (a rod of diameter about 6 mm and with a hemispherical end of diameter about 12 mm), against a flat specimen rotating at 130 rpm. The diameter of the track was about 2.5 cm, giving a sliding speed of about 16 cm/s. The friction force was monitored continuously during the test and the amount of wear was measured by before and after weighing. The typical test lasted 30 min. From the test results the non-dimensional friction coefficient,f, was computed according to the relationship _f=

Average friction force (after first two minutes of sliding) Normal force

and the non-dimensional

wear coefficient, k*, according to the relationship

k = 3 x hardness of softer metal x total wear volume (both metals) Normal load x total distance slid In order to compute the wear coefficient values, it is necessary to know the penetration hardness of the sliding surfaces. Vickers microhardness values of the metals used in this study were measured and are shown in Table 1.

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AND WEAR OF RARE-EARTH

LEAF

METALS IN AIR

147

SPRINGY

TRAIN

GAGES

Fig. 1. Schematic illustration of the pm-on-disk friction apparatus. The friction is monitored continuously, while the wear is determined by before-and-after weighing. TABLE I MICRO~RDNESS Metal

VALUES OF THE METALS TESTED -.Vickers microhardness (kslmm?

Cerium Zinc Misch metal Lanthanum Aluminium Dysprosium Holmium Copper Didymium Y ttrmm Iron (1010 steel) Titanium Zirconium

40 43 58 59 80 80 93 105 110 130 200 250 250

RESULTS

The friction data obtained with the pin on disk apparatus are shown in Table II, while Table III shows the corresponding wear coeffzcient values. It will be seen that the friction coefficient values are in the range 0.25-0.62, while the wear coefficients arein therange0.22 x 10-3-10.7 x 10e3. Thesearenot untypicalvaluesfor unlubricated metals sliding in air, except that the friction values are rather lower than normal (values between 0.4 and 0.8 being more customary) while the wear values are rather higher than normal, (values between 0.1 x lo- 3 and 5 x lo- 3 being typical). However, it is clear that when sliding in air the rare-earth metals do not show the very unusual tribological properties described by earlier workers. Since the differences between various of the combinations are rather slight,

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TABLE II FRICTION ~.

COEFFICIENT

VALUES

Flats :

Ce

mm

La

Al

Cu

di

Fe( 1010)

Ti

Zr

Average

0.50 0.51 0.37 0.32 0.54 0.54 0.54 0.52 0.48

0.28 0.30 0.30 0.25 0.29 0.34 0.25 0.30 0.29

0.32 0.34 0.30 0.29 0.30 0.36 0.32 0.32 0.32

0.51 0.38 0.62 0.52 0.39 0.51 0.41 0.54 0.48

0.49 0.47 0.41 0.59 0.59 0.56 0.40 0.59 0.51

0.37 0.27 0.37 0.28 0.31 0.27 0.32 0.32 0.31

0.56 0.58 0.55 0.44 0.51 0.53 0.41 0.53 0.51

0.58 0.44 0.41 0.41 0.44 0.43 0.44 0.42 0.45

0.54 0.44 0.39 0.38 0.39 0.47 0.36 0.43 0.42

0.46 0.41 0.41 0.39 0.42 0.45 0.38 0.44

Riders Ce Zn Mfn La DY HO di Y Average

TABLE

III

WEAR

COEFFICIENT

Flats :

VALUES,

ALL

x 1O-3

Ce

mm

La

At

6.21

di

Fe(lOl0)

Ti

zr

Average

6.3 6.5 2.6 3.1 8.0 6.7 6.1 5.2 5.2

7.0 7.6 8.5 10.7 6.3 7.8 6.1 6.8 7.5

3.8 3.4 6.5 7.0 3.9 5.0 5.3 5.3 4.9

6.1 3.1 12.4 5.2 5.2 1.7 2.3 8.0 4.6

3.5 0.39 2.7 2.7 0.22 1.7 0.90 0.63 1.1

1.2 1.2 1.6 1.2 2.3 2.7 3.4 4.5 2.0

8.9 0.52 4.8 4.0 3.7 3.1 7.4 5.6 4.0

7.4 0.38 3.3 3.2 1.5 2.5 3.9 2.2 2.4

7.2 1.0 3.6 3.8 1.6 2.7 3.7 2.4 2.8

5.0 1.5 4.4 3.9 2.6 3.3 3.8 3.7

Riders Cl? Zn mm La DY Ho di Y Average

and in order to see more clearly the behavior pattern of the various metals, it was decided to compute average friction and wear values attributable to each metal when acting as rider or as flat in the array of Tables II and III. It was hoped in this way to obtain systematic information of the basis frictional properties of each metal, while at the same time reducing the random “noise” represented by slight errors in the individual friction values. The friction averages are the arithmetic means while the wear averages are geometric means. The results of the averaging process are shown in the last row and last column of Tables II and III. The average friction coefficient values, for the metals used as riders or flats or both, are listed in diminishing order, in Table IV. It will be seen that the cubic metals have higher friction coefficients than do the hexagonal ones. To test whether there is any relationship between friction and c/a ratio, the average friction of the hexagonal rare-earth metals is plotted against the c/a ratio in Fig. 2 (the didimium data are plotted as if they were obtained using pure neodimium at a c/a value of 1.613, since both its constituents have this c/a value). It will be seen, in

FRICTION

AND WEAR OF RARE-EARTH

METALS IN AIR

149

TABLE IV AVERAGE FRICTION

COEFFICIENT

VALUES

Metul

Averagefriction c~e~e~ent

Crystal strurture

Copper Aluminum Iron Cerium Titanium Holmmm Yttrium Zirconium ~isprosium mlsch metal Lanthanum didimmm

0.59 0.54 0.53 0.41 0.45 0.45 0.44 0.425 0.42 0.355 0.355 0.345

cubic cubic cubic cubic hexagonal hexagonal hexagonal hexagonal hexagonal mixed hexagonal hexagonal

I

I

I.57

I.58 4

I.59

I.60

/

I%I

j

1.61

RATIO

Fig. 2. Plot of the average friction coefficient of the hexagonal metals tested as a function of the c/a ratio of their crystal lattice spacing. Low friction is associated with a c/a ratio approaching 1.63.

agreement with the findings of Buckley and Johnson, that the friction drops systematically as the c/a ratio increases. The wear data for these metals do not show any such simple relationship. The only general comment that can be made is that all the rare-earth metals show a higher wear coefficient than all the non-rare-earths. DISCUSSION

According to the present theories of tribology, the friction and wear behavior of metalsare influenced by three pro~rties,namely oxide or other surfa~e~oatings formed by interaction with the environment, compatibility effects between the contacting metal atoms, and structural eflectsg. Previous workers with rare metals have worked mainly with like metal pairs (thus ensuring consistent perfect compatibility) or in

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E. RABINOWICZ,

P. A. MARCH

vacuum (thus avoiding oxide films). Our work has been mainly with unlike metals sliding in air, and under these conditions compatibility and oxide effects come more strongly into the picture. An extensive analysis of the data shown in Tables II and III shows that compatibility effects with rare metals seem to be relatively slight”. This conclusion may arise from the use of primitive compatibility data, because there has yet been insufficient work done on the binary phase diagrams between rare-earth metals and other common metallic elements, and such diagrams currently form the primary basis for an assessment of compatibility relationships’ ‘. But for the present, the conclusion seems to be that for rare-earth metals the correlation between friction or wear and compatibility is small. In regard to crystal structure a similar assessment can be made. It is true that according to Table IV the cubic structured copper, iron, aluminum and cerium give the highest friction, while Fig. 2 shows that of the hexagonal-structured metals, lanthanum and didimium with c/a ratios closest to the optimum value of 1.63 give very low friction. However, the effect is not numerically a very large one, and, more significantly, wear values do not obey these relationships. It seems that in our tests the effects ofoxidation have a predominating influence, especially on the wear results which are shown in Table III. It is well known that the rare earths oxidize readily. This apparently does not too significantly affect the friction coefficient values, since oxide films are fairly regularly penetrated during sliding and metal-metal contact is made. But it is clear that the oxide debris will be regularly swept away from the surfaces during sliding, and will contribute in very sizeable amounts to the reported wear rates. At the same time the oxide films will reduce the friction somewhat by hindering to some extent metal to metal contact. In consequence, the rare earths show relatively large wear rates combined with relatively low friction coefficients. In general, this conclusion bears out a point first propounded by Coffin’z that sliding tests in air show some of the same effects as tests in vacuum, but with the oxide layers as perturbing elements which reduce the correlation between theoretical prediction and experimental findings. The argument that our wear rates are too high because of excessive oxidation is reinforced by considering the limited wear data on rare earths in the literature, all of it obtained with rare-earth metals sliding in vacuum. From this data we have computed k values and these are listed in Table V. It will be seen that all these k values obtained with rare earths are lower than the values in Table III by some orders of magnitude.

TABLE WEAR

V DATA

OBTAINED

IN VACUUM

Investigators

Combination

Wear coefficient(k)

Buckley

La on 440C steel Sm on 440C steel 440C on 44OC steel La on La

0.02 x lo- 3 0.01 x 1o-3 1.0 x 1o-3 0.05 x 1o-3

Nosovskii

and Johnson’

et al.’

FRICTION

AND WEAR OF RARE-EARTH

METALS IN AIR

151

It is concluded that the very promising tribological behavior of the rare earth metals, as exemplified by testing in vacuum, is not borne out by testing in air. It is true that the friction is low, but the wear is excessively high. To what extent these findings would apply in the technologically important case of surfaces lubricated by a petroleum-based liquid (which tends to be oxidation-inhibiting) is not clear. ACKNOWLEDGEMENTS

Our thanks are due to the E. I. duPont de Nemours Company for supporting this work via a grant to the Mechanical Engineering Department at M.I.T., and to Dr. Donald H. Buckley of the NASA Lewis Research Center for kindly loaning us specimens of rare-earth metals. REFERENCES 1 M. E. Sikorski and J. S. Courtney-Pratt, Adhesion of rare earth metals, ASLE Trans., 7 (1964) 73-78. 2 M. E. Sikorski, The adhesion of metals and the factors that influence it, Wenr, 7 (1964) 144-162. 3 D H. Buckley and R. L. Johnson, Influence of crystal structure on friction characteristics of rare-earth and related metals in vacuum, ASLE Trans., 8 (1965) 123-132. 4 D. H. Buckley and R. L. Johnson, Friction and wear of hexagonal metals and alloys as related to crystal structure and lattice parameters in vacuum, ASLE Trans., 9 (1966) 121-135. 5 I. G. Nosovskii, E. V. Isaev and B. I. Kostetskii, The role of crystalline structure in friction and adhesion of metals, Soviet Physics, &&lady, 16 (1971) 393-395. 6 E. Rabmowtcz, Frictional properties of titanium and its alloys, Metal Progress, 65 (1954) 107-l 10 7 E. Rabinowicz, B. G. Rightnnre, C. E. Tedholm and R. E. Williams, The statistical nature of friction, 7‘rutis. ASME, 77 (1955) 981-984. 8 E. Rabmowicx, Friction and Wear ojkfaterials, Wiley, New York, 1965, p. 138. 9 E. Rabninowicz, Compatibility criteria for sliding metals, Fraction and ~u~~ica~i~n in ~e~or~~rz~~ Processmg. ASME, New York, 1966, pp. 9&101. 10 P. A. March, An experimental investigation of the friction and wear properties of the rare earth metals, B.S. thesis in Mechanical Engineering, M.I.T., 1971. 11 E. Rabmowicz, The determination of the compatibility of metals through static friction tests, ASLE Trans., 14 (1971) 19&205. 12 L. F. ColTin. A study of the sliding of metals, with special reference to atmosphere, Lub. Eng., 12 (1956) 50-58.