Frictional properties of pyrolitic boron nitride and graphite

IVE:‘,

FRICTIONAL PYROLITIC

BORON

R

PROPEKTIES NITRIDE

AND

OF GKAPHITE

There are a number of problems which must be solved in order to explain the frictional properties of layer lattice materials. One of these concerns the effect of crystal orientation (i.e. what part a well-oriented film of a layer lattice material plays in ensuring effective lubricating action) ; another concerns the poor lubricating action of boron nitride. To throw some light on these questions, we have run a short series of tests using stainless steel riders on samples of pyrolytic boron nitride and pyrolytic graphite. In these samples, the low-shear plane of the various crystallites corresponded with the plane on which sliding took place. For comparison, tests were also carried out on compressed blocks of graphite and boron nitride, and for these the crystallitcs were randomly oriented. Most of the tests were carried out in an air environment (humidity 504b). A few tests were run in argon and these gave results very similar to those obtained in air. The results of room temperature friction tests using a hemispherically-ended rider on a flat surface at a load of 5ooorooo g and at speeds of 0.3~~1.0 cmisec, are shown in the table.

‘l-A1SI.E I I’yvolytir graphite

ITnlubricated Lubricated with cetane Lubricated with palmitic acid in cetane

It will be seen that the pyrolytic and non-pyrolytic samples behaved very similar-l>,, except that the unlubricated, compacted boron nitride gave rather less friction than did its pyrolytic counterpart. The wear rates at room temperature for the pyrolytic and compacted BN arc’ compared in Fig. I. The rather slight difference in wear rates is probably not significant. The wear rates of the two types of graphite samples were not measured since the> were lower than those of the boron nitride samples by about two orders of magnitude. Friction-temperature plots for the various samples were determined by heating them continuously in an air environment while the friction was monitored. These

FRICTION OF Bru’ AND

lo- 303

Grade

stainless

unlubricated

299

GRAPHITE

steel on

@N

Lood: 5009 8_ Speed: 30cm/sec

Shdirg distance (cm x low4 1

Fig. I. results are shown in Fig. 2. The graphite results of Fig. z are the averages of a number of separate runs, and an indication is provided of the standard error in the plotted curves as calculated from the separate runs. Only three boron nitride runs were carried out in all.

Ordinary

BN (2 runs

600 Temperature (“C)

800

Fig. -1. It will be seen that the two sets of boron nitride results are essentially identical, (and rather similar to those obtained by RowEI with compacted boron nitride), and that the two sets of graphite data are essentially identical over most of the temperature range. However, there is evidence that pyrolytic graphite gives lower friction than randomly-oriented graphite in the temperature range 5oo°C--Soo’C. The conclusion we draw from these data is that whatever mechanism is required to produce an oriented surface layer of graphite operates sufficiently with randomlyoriented graphite at room temperature, so that there is no benefit in using the initallyWeav, 7 (1964)

298-300

E. KAHINOWICZ, >‘I. IMAI

300

oriented pyrolytic grade. Conversely, whatever causes boron nitride to be a poor solid-film lubricant is not cured merely by using a sample having good crystal orientation. ACKNOWLEDGEMENTS

The authors

are indebted

to the Aeronautical

Systems

Division,

Wright-Patterson

Air Force Base, for financial support of this investigation, and to High Temperature Materials Inc. for providing samples of pyrolytic materials.

1 G. W. ROWE, Some observations Wear, 3 (1gGo) 274-285.

on the frictional behavior of boron nitride and of graphite,

Wear, 7 (1964) 29% p”