Wear, 34 (1975) 275 - 290 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
INSTABILITIES IN THE FRICTIONAL AND GRAPHITES*
J. K. LANCASTER Ministry of Defence (Procurement Executive), Materials Department, Establishment, Farnborough, Hants. (Ct. Britain)
(Received May 9, 1975)
Summary Both graphitic and non-graphitic carbons are widely used in a variety of tribological applications. A recent development is the use of carbons for aircraft brake discs and this has prompted an experimental investigation to determine the most important factors influencing the magnitude and stability of the coefficient of friction and rate of wear. Graphitic carbons sliding against themselves, or against metals, sometimes exhibit large and sudden transitions in friction and wear at critical conditions of sliding. These conditions are defined and the mechanisms involved are described. Carbons of low or negligible graphiticity are less prone to transitions than graphitic materials, but their friction and wear properties depend markedly on the way in which the conditions of sliding influence the structure and topography of the surface layers developed during sliding. Examples are given of typical variations of friction and wear with distance of sliding, speed and temperature. Discontinuities in friction and wear can also arise from locahsed surface misalignments induced either mechanically or thermally. The stability of the coefficient of friction and the rate of wear for both graphitic and non-graphitic carbons can be greatly improved by the incorporation of additives or by the presence of organic vapours in the environment. Examples are described.
Introduction Large fluctuations in the friction and wear behaviour of materials arising from minor changes in the conditions of sliding can be a serious limitation to the reliability of machine components. In some circumstances, the coefficient of friction and/or the rate of wear may increase suddenly to very high values, and examples of such transitions have been widely *Paper presented at the 3rd International Tribology Conference, “Tribology for the Eighties”, Paisley, 22 - 25 September, 1975.
reported in both dry and lubricated conditions for most types of materials -metals [ 11, polymers [Z] , ceramics  and carbons [4, 51. The latter appear to be particularly susceptible to transitions in friction and wear and it is with these materials that the present paper is concerned. The generic term “carbons” embraces a large group of manufactured products with widely differing properties. At one extreme are materials with a high degree of graphitic order (natural and synthetic graphite, pyrolytic graphite) whilst at the other are those with little or no such order (pitch-bonded cokes, glassy carbons). Between these extremes are compositions of varying degrees of graphiticity produced either by an appropriate choice of starting materials and subsequent heat-treatment temperatures, or by simple mixing of graphitic and non-graphitic constituents. The present paper discusses the friction and wear behaviour of carbons near to each end of the spectrum. Carbons are used in a wide variety of tribological applications of which the most important are electrical brushes, seals and bearings for hostile environments. In all these applications it is customary for the carbon to slide against a metallic or ceramic counterface which is intended to wear at a negligible rate compared to that of the carbon itself. A recent innovation, however, has been the introduction of carbon fibre-reinforced carbon composites for use as aircraft brake discs  and in this application the maximum benefits of weight-saving can be achieved only if carbon slides against itself. The tribology of the carbon-carbon system has received little attention in the past and most of the work to be described here is derived from a general research programme currently in progress in this area. Experimental A pin and ring apparatus has been used in which a small carbon sample is loaded against the curved surface of a larger, rotating ring. Several geometrical arrangements were involved at various stages of the work including crossed-cylinders, pins with truncated conical ends, and planeended cylindrical or square-section pins. In all configurations, the load was applied by weights attached to the arm supporting the pin. Friction was measured with a torque transducer interposed between the driving motor and the rotating ring, and wear was determined from microscopic measurements of the wear scar developed on the pin. The speed of rotation of the ring was variable between 1 and 10,000 r.p.m., and within these limits was continuously variable over a 9: 1 range. An infrared heater positioned beneath the ring enabled its temperature to be increased to about 300 “C!. All the materials examined were commercial products and are listed in Table 1 with an identifying code. To facilitate cross-reference, the code numbers are the same as those used in an earlier publication .
EG-A EG-I CG-A HC-D
Electrographitised coke and carbon black - brush grade Electrographitised pitch-bonded coke (HC-D) - bearing grade. 50% hard carbon/50% natural graphite mixture, pitch-bonded. Pitch-bonded coke, heat-treated 1000 “C
Results Transitions Previous work [ 51 has shown that most carbon-carbon combinations exhibit transitions to a regime of high friction and wear in normal, atmospheric conditions when a critical speed of sliding is reached. Figure 1 shows typical examples for an electrograph&, EC-A and a hard, largely nongraphitic carbon, HC-D. In these experiments the total distance of sliding was restricted to 1500 rev. of the carbon ring at each speed in order to minimize the general rise in.mean temperature of the components. With the graphitic carbon, the phenomena associated with the transition were very similar to those observed during “dusting” of carbon brushes on slip rings or commutators at high altitudes [ 71, in vacuum [ 41 or in dried inert gases [ 81. Copious quantities of finely-divided wear debris are generated in the high wear regime, and the surfaces are very severely damaged. With the nongraphitic carbon, the increase in wear appears to be only marginal, but the lower wear regime is ill-defined in the sense that the rate of wear varies with time in a characteristic manner; this aspect is discussed in more detail later. A more rapid and convenient way of determining the magnitude of the transition speed is to increase the sliding speed continuously from a relatively low value. Figure 2 shows typical results obtained in this way for an electrographite EG-A and hard carbon HC-D, and it may be noted that the latter exhibits two transient peaks before the final rise to a higher and approximately constant level of friction. The positions, with respect to speed, of these earlier peaks were somewhat irreproducible, but the final rise was always reproducible to about + 1 m/s. Earlier results with graphitic carbons  have shown that the transition to high friction and wear is characterised by a constant parameter W” V (Fig. 3A, a), where V is the speed and W is the applied load. Figure 3A, b, now shows that the same parameter is involved with a hard, relatively nongraphitic carbon. Flash temperature theory  leads to the relationship 0 r = 0 (I + Cp( W” V)% /(u where 0 r is the total temperature at the points of real contact, 19~is the mean surface temperature, C is a constant involving the thermal properties of the carbons and (Yis a parameter characterising the number and mode of deformation of the contact areas. A constant value of W” V thus implies that the transition occurs at a constant total temperature at the asperity contacts. Values of this temperature were derived earlier for
Fig. 1. Variation of friction and wear with speed for, a) electrographite load = 12N; b) hard carbon on itself, load = 32N.
on hard carbon,
graphitic carbons by determining the transition speed V at various values of the ambient temperature, Ba, Fig. 3B, a, thus enabling the unknown value of (Yto be eliminated. The resulting temperatures, for a range of different graphitic carbons, were within the range 150” - 180 “C. The results of similar experiments with the hard carbon HC-D are shown in Fig. 3B, b, and the calculated critical temperature is 560 “C. Similar high values have been obtained for other types of low-graphitic carbons [lo] . At this point it is relevant to mention that transitions in the friction and wear of graphitic carbons are not confined solely to sliding against carbon counterfaces but are also frequently observed against metals. The results in Fig. 4 show that the increases in wear rate of electrographite EG-I sliding against stainless steel and tool steel are similar in magnitude to those obtained against a carbon-graphite ring CG-A, but the increases in friction are smaller. The higher transition speeds on the metals are attributable to their greater thermal conductivities, and the lower coefficients of friction occur because hard metal surfaces are less severely damaged than a carbon counterface, thus reducing the deformation component of friction. Similar transitions in the wear of graphitic carbons on metals in air have been reported by Plutalova [ 111 at heavy loads and by Levens [ 121 at elevated temperatures.
Fig. 2. Continuous variation of friction with speed, load = 33N; B) hard carbon on itself, load = 32N.
on hard carbon,
Solid lubricant films of graphite on metals are also known to fail suddenly at temperatures in the range 50” - 150 “C [13, 141. After prolonged sliding against metals in air, the formation of oxide films may influence the incidence and type of transitions in friction and wear. There is some evidence to suggest that the presence of a coherent oxide film on copper inhibits transfer of graphite to the metal [ 151. Changes in the state of oxidation produced by changes in load, speed, or temperature, may thus alter the nature of the contacting surfaces and, in turn, the rate of wear. A discontinuity in the wear rate of an electrographite, EG-A, on copper arising from this cause is shown in Fig. 5A; the oxide film is disrupted at a critical load leading to transfer of graphite and a sudden increase in wear rate. This type of transition is quite distinct from those initiated by a critical temperature, as described earlier. However, the latter may also be affected by oxide film formation, as shown in Fig. 5B. The wear rate of electrographite EG-A on gold at relatively low speeds exhibits a transition at a critical ambient temperature, but on more reactive metals no transitions can be identified, and the wear rate varies continuously with temperature. Metal oxide additions to graphite are also known to prevent the disruption of solid-lubricant films on metals at temperatures of the order of 100 “C The passage of current
across a graphite-metal
100 200 Temperaturo,°C
Fig. 3. Variation of transition speed with, A) load, B) ambient graphite on carbon-graphite; b) hard carbon on itself.
further complications. Resistive heating enhances the frictionally-induced temperature rise, but at the same time the rate of oxidation increases and to maintain current transfer, the oxide film on the metal must be disrupted. Some recent results in this area are shown in Fig. 5C . A sudden increase in wear occurs for electrographite on copper in oxygen and nitrogen, but not on gold. It is postulated that the increase in wear occurs when the temperature raises the amount of oxidation to the point at which current transfer can only take place via sparking; the copper surface is then roughened, leading to increased localised stresses and higher wear. Friction- time variations Graphitic carbons sliding against themselves, or metals, normally exhibit relatively low and stable coefficients of friction in conditions insufficiently severe to induce transitions.. Low, or non-graphitic carbons, however, behave very differently, and Fig. 6 shows some results for the low-
6 8 Speed, m/e
Fig. 4. Variation of friction and wear with speed for electrographite graphite; b) 18/8 ctainless steel; c) 18 W tool steel, load = 42N.
on, a) carbon-
graphitic carbon HC-D sliding against itself. In the early stages of sliding between freshly-prepared surfaces, the friction decreases to a very low value for a finite time and then begins to increase again, Fig. 6A. This low value is associated with the formation of an uniform film of consolidated wear debris which appears to function as a classical thin-film solid lubricant - a film of low shear strength on a harder substrate [ 17 1. After the formation of the debris film, the wear rate also becomes very low. Insufficient debris is now produced to maintain the film and as it is gradually removed, the friction increases to a higher, constant value, Fig. 6B. The latter effect has also been reported by Longley et al. [ 18 ] and they attribute the increased friction to an increase in the real area of contact. Two other features of the results in Fig. 6 may also be noted. At high speeds, the initial period of high friction persists for a much longer time than at low speeds. Additional experiments have shown that the critical speed at which the difference begins - between 8 and 12 m/s in Fig. 6A decreases with increasing ambient temperature [lo] . Calculations from flash-temperature theory lead to the conclusion that the discontinuity occurs
Fig. 5. A) Variation of wear rate with load, electrographite on Cu, 18 m/s [ 181. B) Variation of wear rate with temperature, electrographite on metals, 0.1 m/s, 10N. C) Variation of wear rate with current, 0.75 m/s, 0.45N [ 161.
at critical contact temperatures of the order of 500 ‘C, in agreement with the value deduced earlier from Fig. 3B, b. Direct confirmation that temperatures of this order are, in fact, produced at high speeds comes from the observation that incandescent spots or streaks of “red-heat” are often present at the trailing edge of the contact area (Fig. 6D). The second feature of interest is the introduction of marked discontinuities in friction when sliding is interrupted for a wear measurement, Fig. 6B. Sudden decreases in friction sometimes also appear to occur spontaneously, as shown in Fig. 6C. These transients are attributable to small, transverse misalignments between the sliding surfaces induced either mechanically (e.g. by separating and replacing the surfaces or by vibration) or thermally (e.g. by differential thermal expansion between various parts of the assemblies supporting the surfaces). Stabilization
The most obvious solution to prevent or delay the transitions to high friction and wear in graphitic carbons is to incorporate additives into the structure. A variety of additives is already known to inhibit the “dusting” of carbon brushes intended for use at high altitudes [19,20]. Figure 7 shows
2 VOI mm’
Fig. 6. Variation of friction with time for hard carbon on itself, load = 22N, A) initial variation at different speeds, 23 “C. Long-term variations at 200 “C: B) 0.33 m/s; C) 2 m/s; D) 12 m/s. Friction - solid lines, wear volume - hatched lines.
some examples of results obtained with various additives to EG-I sliding against an unimpregnated carbon-graphite ring CG-A. Five per cent of CdIs greatly increases the transition speed, and 7% of (NH,), HPOl appears to eliminate the transition entirely within the speed range available. Impregnation by fluids also eliminates the transition, but only at the expense of some increase to the rate of wear. The results shown in Fig. 7, curves d, are for a chlorinated silicone hydraulic fluid and show that this fluid greatly increases the coefficient of friction. The increased friction is attributable to the catalytic action of the carbon on polymerisation of the chlorinated silicone in the wear track . This particular effect appears to be a special feature of chlorinated silicone fluid and increased friction is not observed with other fluid impregnants, including hydrocarbons, diesters and dimethyl siloxanes. Hydrocarbon vapours in the environment also increase the transition speed, and curve b in Fig. 7 shows results for air saturated with benzene vapour. Experiments aimed at elucidating the mechanisms by which these, and other additives, influence the transitions are still in progress and will be reported in due course. The major tribological problem with low- or non-graphitic carbons is to prevent the large increase in friction with time of sliding. Figure 8 shows the effects of some additives on the friction-time relationships for an impregnated pin of HC-D sliding against a clean ring of the same carbon. The
Fig. 7. Effect of additives to electrographite on the transition speed during sliding against carbon-graphite, load = 22N. a) No additive; b) benzene vapour; c) CdI,; d) epoxy resin; e) chlorinated silicone fluid; f) (NH& HP04.
figures adjacent to each curve are the mean rates of wear over the final 60 mins of sliding, in units of low6 mm3/Nm. (NH4)2 HPOI additions (Fig. 8B) are clearly very effective in reducing both the coefficient of friction and the rate of wear, but Ca3 (PO,), additions are less so (Fig. 8C). If the high friction attained after long periods of sliding is attributable to an increased real area of contact, it might be supposed that the incorporation of small amounts of abrasive material could prevent the formation of very smooth surfaces and so reduce friction. Additions were therefore made of finely divided Al303 (1 pm particle size) and of compounds containing carbide-forming elements which might react to form abrasive material Na2 W04 and MoS2. Figure 8, curves D, E and F show that all these . additions prevent the build-up of a very high friction, and that Al203 and Na2W04 also significantly reduce wear. MO& additions, however, increase the rate of wear.
A’,“, (0 7)
II p?, 0
Fig. 8. Variation df friction with time for hard-carbon on itself. Pins impregnated with additives. Load = 22N, speed = 2 m/s, temperature = 200 “C. (Figures adjacent to each curve are wear rates - 10v6 mm3/Nm.)
Discussion The most obvious explanation for the transition in friction and wear of graphitic carbons at a critical contact temperature is that the quantity of water vapour available from the environment no longer suffices for adsorption on the freshly exposed surfaces generated by wear. The amount of physically adsorbed water vapour is controlled by its vapour pressure, P, relative to saturation pressure, PO,and the ratio P/P, decreases by more than two orders of magnitude over the temperature range 20” - 170 ‘?I!. A critical temperature of 170 “C corresponds to P/P, 2 0.0015,and this value is about 40 times less than that required to prevent “dusting” of carbon brushes in vacuum [ 51. The difference appears to be attributable to the part played in the present experiments by adsorption of other atmospheric gases, notably 0s and COz . The attainment of the critical contact temperature is governed by the interactions of two groups of factors. The first comprises those parameters which determine the rate of heat generation and dissipation - load, speed, ambient temperature, coefficient of friction and the thermal properties of the materials - and all of these are known, or easily measurable. The second group, however, involves parameters which influence the number, size and distribution of the real areas of contact - surface roughness generated during
sliding, degree of transfer or film-formation, mode of deformation and the apparent area of contact -- and the influence of most of these is less readily quantifiable. For example, it has been shown  that an increase in the apparent area of contact can proportionately increase the transition speed for graphitic carbons (at constant load and ambient temperature). The transition speed also increases with time of running-in of graphitic carbon surfaces before increasing the speed to its critical value. Both of these observations can be qualitatively explained in terms of a change in the number of real contact areas, but it is not easy to envisage how to quantify this change in number. Because of this uncertainty, the ways in which additives in graphitic carbons influence the transitions to high friction and wear cannot yet be established with certainty. Two possibilities, however, are worth mention. (a) The additives could increase the critical contact temperature by providing a substitute for environmental water vapour - organic vapours, fluids or resins (after thermal degradation). (b) Additives might influence the nature and topography of the surface layers generated during the early stages of sliding by abrasion or by preferential transfer, thus affecting both the coefficient of friction and the number, and/or size of the real contact areas. The transitions to high friction and wear with carbons of low or negligible graphiticity appear to be more complex than those with graphitic carbons. In the early stages of sliding, films of consolidated wear debris form over the contacting surfaces, and previous work has shown that these films may exhibit a greater degree of graphitic order than the original carbon . The transient increases in friction occurring when the speed is increased after the formation of debris films (Fig. 2B) are associated with the disruption and removal of these debris films at contact temperatures of the order of 150 “C. The final transition, however, occurs at much higher contact temperatures and there is additional evidence to suggest that the magnitude of this transition temperature for different carbons is related to that at which oxidation begins to occur at a significant rate [lo] . If so, then the transitions in low-graphitic carbons might arise from the removal of chemisorbed oxygen complexes from the surfaces, leading to increased adhesion or, alternatively, from preferential oxidation of the binder-phase between grains, weakening their support and thus leading to easy detachment and an increase in wear. All the additives to low-graphitic carbon so far examined prevent the build-up of very high coefficients of friction. As already mentioned, AlsOs may achieve this by interrupting the development of very smooth surfaces and a high real area of contact. Microscopic examination of worn surfaces containing AlsOa showed, in fact, that numerous, fine longitudinal scratches were present, and a similar degree of damage was observed with the sample containing NasWO*. In the latter case it is not known whether the scratches are attributable directly to the abrasiveness of Na2W04 or indirectly to the formation of WC by reaction with the carbon. The surfaces
(a) X 600
(b) x 280
(d) x 5700
Fig. 9. Worn surfaces of hard carbon after sliding against itself. Load = 22N, speed = 2 m/s, temperature = 200 “C; a - d, f. Scanning electron micrographs: e. Optical micrograph.
generated on carbon containing ; MO&, however, were very severely scratched, and this, together with the iIncreased rate of wear, implies that chemical reaction and carbide formation from MO& is a distinct possibility. The
difference in behaviour of carbons impregnated with the two phosphates is very marked, and may be related to their different thermal stabilities. Gas (PO,), is stable to above 1500 ‘C, but (NH,), HP04 decomposes at around 150 “C [ 231. Phosphorus-containing compounds are well-known as oxidation inhibitors for graphite [24, 251 and are believed to function by selective adsorption at dislocation cores on basal planes, thus inhibiting the formation of etch-pits . In the present work with (NH,), HPO,, it was noted that a uniform film of wear debris remained on the carbon surfaces at all times. This observation, together with the fact that (NH,)zHPO, also inhibits the transition in graphitic carbons (Fig. 7) suggests that the action of this additive may well result from adsorption of phosphorus-containing decomposition products. One of the main obstacles to interpreting the effects of additives on the tribological behaviour of carbons is that, even in the absence of additives, the mechanisms of wear are still obscure. It is widely assumed that the mode of asperity deformation on worn carbon surfaces is elastic and, in consequence, it has been postulated that a form of fatigue wear occurs on an asperity scale [ 221. There is a considerable volume of indirect evidence supporting the idea of fatigue wear  but, unfortunately, little direct confirmation has so far emerged from electron microscope examinations of worn carbon surfaces. The latter, in fact, suggest that a variety of different wear mechanisms exist, each operating on a different scale of size. Some examples are shown in Fig. 9 for the low-graphitic carbon, HC-D, sliding against itself in the same conditions as in Fig. 8A. (a) Flaking of surface films in the low friction regime, shortly after the onset of sliding. (b) Preferential removal of weakly-bonded inter-granular material occurring as the coefficient of friction increases. (c) Formation and propagation of cracks within coke grains, ultimately leading to the removal of parts of grains. (d) Chipping and delamination of the edges of grains. (e) Sub-surface fracture of whole grains; in this optical micrograph, the smooth bright patch reflecting oblique illumination is inclined at about 15” to the general plane of the surface. (f) The very smooth surfaces generated on individual grains after prolonged sliding imply that material must have been removed on a scale of size of a few tens of nanometres or less. In terms of its direct contribution to the total wear, this process may be quantitatively negligible; its indirect contribution, however, by increasing the real area of contact and the coefficient of friction, could well be very significant and possibly be responsible for the sub-surface fracture of whole grains, as in Fig. 9e. Until the particular processes responsible for the majority of wear occurring in specific conditions of sliding can be identified unambiguously, the precise ways in which additives modify the wear of carbons must remain distinctly speculative.
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