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Tool wear and solid state reactions during machining
165
Wear, 53 (1979) 165 - 187 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
TOOL WEAR AND SOLID STATE
A. G. THORNTON*
REACTIONS
DURING
MACHINING
and J. WILKS
Clarendon Laboratory,
Oxford (Gt. Britain)
(Received September 8, 1978)
Summary Measurements have been made of the wear of diamond tools turning mild steel under a range of carefully specified conditions. High rates of wear are observed owing to graphitization of the diamond. Under certain conditions, this graphitization may proceed at a considerable rate even if there is no appreciable temperature rise at the diamond-steel interface. This result implies that the clean surfaces generated by the machining process have chemical activities different from those of the bulk material. Examples of unusual chemical activity have also been observed when using diamond tools to turn other metals, when using tungsten tools to turn graphite, and copper and nickel tools to turn sulphur. The experiments show that machining, under suitable conditions, offers a precise method of measuring rates of reaction between clean solids and of obtaining information on the relative bond strengths of the surface atoms.
1. Introduction Discussions of the wear of cutting tools used in machining operations often refer to chemical and diffusive modes of wear. Although these effects may play an important part in tool wear, there is generally little detailed information on the reactions involved. The study of chemical wear is also complicated by the fact that, besides producing wear directly, chemical reactions may create conditions more favourable to abrasive wear. The present experiments, arising out of studies of the use of diamond for grinding and precision machining, demonstrate some hitherto unobserved forms of chemical wear with rather unusual characteristics. Diamond is so much harder than brass that the rate of wear of a diamond tool turning brass is extremely small. In contrast, if the brass cylinder is replaced and the same tool is used to turn mild steel of comparable hardness, the rate of wear is extremely high: 10’ times greater than on brass and lo2 times greater than on the very abrasive aluminiumsilicon alloy LM13; see for example Wilks and Wilks [l] . The high rate of *Present address: A.M.T.E., Gt. Britain.
Experimental
Diving Unit, HMS Vernon, Portsmouth,
166
wear of diamond on steel is generally believed to arise from the chemical affinity between iron and carbon, but the exact details of the interaction are still a matter of discussion. Loladze and Bokuchava [2] proposed that the wear was due to carbon atoms diffusing into the steel at the high local flash temperatures generated by the machining process. However, if the carbon atoms at the surface of the diamond become sufficiently mobile to diffuse into the steel, they should also be sufficiently free to take up the more stable graphite configuration. Hence, we expect that the predominant effect of local flash temperatures will be to transform the diamond to graphite, which may then dissolve in the iron, or being mechanically weak be removed by abrasion. There are now several discussions in the literature which conclude that the wear process is controlled primarily by the rate of graphitization of the diamond [ 3 - 71. The reaction will be catalysed by the presence of the steel, e.g. the rate of graphitization of a diamond in a furnace is much enhanced when thin films of iron, nickel or titanium are deposited on the surface [ 81. However, although several workers have observed graphitization in static experiments, there is little information on the conditions at the interface between the steel and the diamond during machining. The present experiments were made to study the reactions that occur when diamond tools turn mild steel under controlled conditions. One of the principal results of the investigation is that graphitization of the diamond may occur even at virtually ambient temperatures [9], because of the enhanced chemical activity of the clean surfaces of steel which are produced by the machining process and rub across the flank of the diamond. The enhanced activity appears to arise not from particular characteristics of diamond and steel, but from experimental conditions which probably prevail when machining other materials. This report first describes experiments on turning mild steel with diamond tools and then some experiments involving other materials which give further examples of chemical wear at interacting surfaces.
2. Experimental 2.1. Measurement of wear rates Round nosed diamond turning tools with a nose radius of about 450 pm and a flank clearance angle of 10” were either purchased from commercial suppliers or fabricated by us from drillstones. They were mounted on a Bryant Symons diamond lathe in a specially designed tool holder which permitted the rake face of the tool to be set accurately horizontal by optical means. The tools were adjusted so that the rake face was on the centre line of the rod being turned, and 50 pm cuts were made on standard billets of mild steel. The lathe had a variable speed adjustment which permitted angular velocities of 3 - 50 rev s-l ; thus the use of billets with diameters ranging from 12 to 75 mm permitted a range of linear cutting speeds from about 0.12 m s- ’ to 12 m s- ‘. After it became apparent that turning at very
167
low speeds presented points of particular interest, an auxiliary gear drive was introduced to turn the lathe more slowly and to permit cutting speeds down to 0.02 m s-l. Although some wear occurs on the rake face of the tool, most occurs on the flank face in contact with the rotating billet, where the steel quickly wears a vertical flat as shown in Fig. l(a). This wear flat, viewed at a relatively low magnification, has a smooth appearance on which is imposed a pattern of fluting with a spacing equal to the feed rate of the lathe (Fig. 2). The geometry of the wear is also shown in Fig. 3 which gives a plan view of the rake face of a similar tool which has suffered a large amount of wear. The line on the micrograph has been drawn parallel to the edge of the turned billet and indicates that the wear has taken place on the leading (left hand) edge of the tool as we would expect. The fluting appears to be a secondary effect common to all the wear patterns but not relevant to our present measurements. (It probably arises because any small variation in the profile of the edge of the tool will lead to a corresponding variation in the thickness of the chip and hence to a change in the cutting conditions and the wear rate. If the resulting change in wear rate accentuates the effect of the variation in the edge of the tool, then the situation is unstable and a periodic variation of tool wear is likely to result.) MEASUREMENT
OF WORN AREAS tool tip
=+=_, flat
I i-
(a)
(b)
Fig. 1. Schematic diagram of a wear flat on a toof.
To obtain a quantitative measure of the rate of wear, the size of the wear flat is characterized by the dimensions 3cand y shown in Fig. l(b) and the product xy taken as a measure of the area. Then a plot of xy against the total area of metal traversed during the cuts is found to give an almost straight line, as shown for one particular tool in Fig. 4. This linear relation appears to be generally valid provided the product xy does not exceed a value of about 30 X low3 mm’. Hence by working in this range the rate of wear under particular cutting conditions is specified uniquely by the gradient of the line which we quote in units of 10s6 (mm2 mmB2). Subsidiary experiments showed that the rate of wear varied by about 30% as the depth of cut increased from 25 to 100 pm. As the depth in each experiment could be set to within 3 pm, any variations in the wear rates due to inaccuracies in this setting should be negligible. The feed rate of 25 pm rev- ’ was determined precisely by the gearing of the lathe. As the steel rods
168
Fig. 2. Optical micrograph of a typical wear flat. Fig. 3. Optical micrograph of the rake face of a heavily worn tool. The line is parallel to the axis of the lathe.
WORN AREA x’ 30 / mm2 x 10-3
0
I
C
2000
I
mm*
l
1
4000
WORK
Fig. 4. Area of wear flat us. metal turned in air at 10 m s-l.
decreased in diameter during the wear runs, the speed of the lathe spindle was adjusted to maintain the cutting speed. As a result of these adjustments the speed was not held exactly constant but was kept within about +lO% of its nominal value. Although the wear rates changes substantially with large changes in the velocity of cutting, any differences due to the above changes in the size of the rods are so small as to be negligible. The rate of wear of the diamond depends on the constitution of the steel. Rods made from a standard ENlA mild steel, from a free cutting ENlA mild steel containing lead and from EN34 a nickel-molybdenum steel gave wear rates differing by a factor of the order of 2. Similar observations on the wear of diamond grit grinding different steels have been reported by Tanaka and Ikawa [3] and by Graham and Nee [lo]. Therefore, all our experiments at speeds up to 11 m s-l were made on one type of steel, a standard ENlA mild steel. Tools were compared by turning either the same
169
rod of mild steel or rods made from the same billet of the steel. The wear rate was usually so high that it was sufficient to make one pass over a 10 mm length of a 75 mm diameter rod to produce measurable wear. In the experiments mentioned above, Tanaka and Ikawa observed that the rate of wear of a diamond grit machining different samples of steel depended on the crystallographic orientation of the diamond. The magnitude of the effect, which depended on the type of steel, was such that the wear rates varied with orientation by factors from about 1.3 to 2.1. During the present experiments we have observed the wear of turning tools made from specifically oriented diamonds and found similar results. Therefore, in order to obtain exact comparison of wear rates under different conditions it is essential to use similar tools of the same orientation. 2.2. Turning in different atmospheres Figure 5 shows a sketch of a chamber which could be fitted to the lathe in order to permit reductions of the air pressure and the introduction of other gases around the tool and workpiece. The specimen S is held on the end of the steel rod R which passes out of the chamber through the gland A with O-ring seals which contain the gas in the chamber and permit the steel rod to rotate freely and the chamber to move in a direction parallel to the rod. The tool is supported on a tool holder which passes into the chamber through a second gland B; during turning, the chamber remains fixed with respect to the tool and moves along with it. A tube T passing through a third gland connects the chamber with a pumping system which could maintain pressures down to about 0.1 Torr. Changes in the air pressure often produced marked effects on the rates of wear, but for the moment we refer only to one important feature of this behaviour. Figure 6 shows the wear rates obtained in an experiment in which all other parameters were held constant and the pressure of air in the chamber varied. The wear rate changes abruptly over a narrow range of pressure, but at other pressures is independent of the pressure, i.e. the wear rate CONTROLLED
l
::
Wring rotating
ATMOSPHERE
CHAMBER
sds parts
Fig, 5. Schematic diagram of controlled atmosphere chamber.
170
WEAR RATE
x
x
5, X
x
0
x
I I
10
x I 100
torr
x
AIR PRESSURE
x I’ 1000
Fig. 6. Wear rate of a tool (T58) turning at 0.13 m s-l as a function of the surrounding air pressure.
appears to take one of two values corresponding to low and high pressures. This pattern of behaviour appears to be fairly general, e.g. Fig. 7 shows the effect of pressure in another experiment with different machining conditions; as before there are two regions in each of which the wear is approximately independent of the pressure. It is important to note that the wear with which we are concerned occurs on the flank face of the tool which is pressed into contact with the steel billet (Fig. 8). Hence the wear flat on the tool is sealed off from the surrounding gaseous atmosphere by the presence of the freshly cut metal. The fact that changing the pressure of the surrounding air affects the rate of wear shows that changing the pressure affects the number of air molecules reaching the interface. Moreover, the fact that the wear above and below the A WEAR RATE
X X
10, xx
X
X
5_
X 0
X 0.01
I 0.1
I 1
I 10
I loo torr
X I ‘Ooo
+ AIR PRESSURE
Fig. 7. Wear rate of a tool (DS97R) turning at 0.013 m s-l as a function of the surrounding air pressure.
171
diiction
of rotation
steel c
region
billet
of flank
wear
Fig. 8. Schematic diagram showing the position of the wear flat with respect to the turned metal.
transition is largely independent of pressure suggests that so far as the wear reactions are concerned there are essentially two conditions, in which air molecules are either reaching or not reaching the interface in significant numbers. The pressure at which the transition occurs will depend on several factors which affect the access of air molecules to the interface, such as the cutting speed, the shape of the tool, the quality of the polish on the tool and the presence of any cracks near the tool edge. However, the magnitudes of the pressures and speeds at which transitions occur in the present experiments are of the same general order as those reported in other machining experiments [ 11,121.
3. Temperatures
generated
by machining
Very considerable temperature rises are generated at and near the face of the tool or abrasive grit when carrying out machining and grinding operations under normal industrial conditions and speeds. In most turning operations the heat is generated principally by the plastic deformation of the chip and by the friction of the chip moving over the rake face of the tool. During grinding operations, heat is produced by the plastic deformation as the grit grooves the workpiece and by friction between the chip and the grit. Many studies on the machining of a wide range of tool and workpiece materials show that at cutting speeds of about 10 m s-l temperatures on the tool face may rise to the order of 1000 K; see for example refs. 13 - 15. Loladze and Bokuchava [2] measured the temperature of the rake face of a diamond tool turning steel using an optical pyrometer, and recorded values ranging from 600 K to 1300 K at cutting speeds between 2.5 m s-l and 10 m s-l. Temperatures of this order have also been observed by Sagarda and Khimach [ 161 and by Panhorst and Triemel [17] when grinding steel with diamond grit. The work mentioned in the previous paragraph on diamond is not directly applicable to our present experiments, where the wear occurs on the
172
flank of the tool which is relatively remote from the plastic deformation in the chip and from the friction of the chip on the rake face. Therefore we need to know the temperatures on the interface between the wear flat on the diamond and the steel billet bearing against it. This temperature will be determined partly by an overall temperature rise associated with the plastic deformation of the chip, and partly by the generation of localized flash temperatures due to the rubbing of one surface over the other. Relative velocities of the order of 10 m s-1 readily give rise to excess flash temperatures of several hundred degrees; see for example refs. 18 and 19. Hence, it is quite likely that the temperatures on the diamond-steel interface may rise to the order of 1000 K. It is difficult to obtain precise values of the magnitude of the local excess temperatures concurrently with the wear measurements. However, we have studied the temperatures generated at the interface between a diamond tool and a metal workpiece during machining by turning metal with a semiconducting type IIb diamond and observing the e.m.f. generated. Some results obtained for a diamond tool turning copper are shown in Fig. 9 which gives the measured e.m.f. as a function of the linear speed at which the copper is being cut. Further experiments are needed to relate the e.m.f.s to the magnitude of the flash temperatures, but it is clear that the e.m.f.s decrease steadily as the cutting speed decreases. Hence, the excess temperatures are greatly reduced at the lowest turning speeds, a result we use in Section 4.4.
4. Wear of diamond
on steel
4.1. Wear at 30 m s-l Most of our measurements were made at a cutting speed of about 11 m s-l, which is typical of industrial machining processes, or at lower speeds. However, it is convenient to begin by describing some experiments made at higher speeds. To obtain these higher speeds it was necessary to use a specimen of larger diameter made from a sample of “good commercial quality” mild steel, different from that used in 2he other experiments. A subsidiary experiment showed that the form of the wear at 11 m s-l on this material was similar to that on the usual ENlA billets, with a wear rate about 30% smaller. At cutting speeds of about 11 m s-’ the wear rate has a value of about 10 mm2 mm-’ and depends only weakly on the speed. However, on increasing the speed the wear suddenly rises so rapidly that at 30 m s-l it is about 50 times greater and very short passes were necessary to avoid wearing out the tool during the measurements. A characteristic feature of this high rate of wear, not observed at lower speeds, was a shower of white sparks, presumably hot debris from the turning process. The vacuum chamber was then modified to take the extra large billet used to obtain the high cutting speeds and the experiment was repeated in a nominal vacuum of about
173
403
x. . x
E.M.F. l
(W
X.
A
l
.
75_ WORN AREA
0
X
X 80
20-
50-
rKx
X
mm2
I -0
x IO-3
0
x 25_
X’
x
x
X 0
Oc 0
x
0
SURFACE
SPEED
5
’
m s-l
0’
O WORK
I
100
InIn2
CI 200
Fig. 9. The thermal e.m.f. generated at the interface between a billet of copper and a turning tool (DS 90) fabricated from a type IIb semiconducting diamond: X , run 1; 0, run 2; 0, run 3. Fig. 10. Area of the wear flat on a tool (T39R3) turning at 29 m s-l vs. area of metal turned. The points were obtained at two different air pressures as indicated: X , 760 TOW 0, 0.2 Torr.
0.2 Torr. There was now no sign of hot debris but the wear rates were the same as in a full atmosphere of air. For example, Fig. 10 shows the worn area plotted against the area of metal turned in an experiment in which the pressure of air was varied between 760 and 0.2 Torr; the worn area steadily increases at virtually the same rate under both conditions. If the speed of cutting is continually increased, the surface temperature of the diamond must eventually rise to at least the melting point of the steel and become sufficient to produce graphitization. For example, Vishnevskii et at. [4] report graphitization when diamond is heated in contact with steel at a temperature above 800 “C. Bearing in mind (i) that the very high wear rates are the same in 0.1 Torr and 760 Ton of air, (ii) that the high cutting speeds generate temperatures sufficient to ignite the debris in air and (iii) that graphitization must eventually occur with increasing speed, we identify the extremely fast wear processes observed at speeds of about 30 m s-l as arising from ~aphitization of the diamond in the presence of iron which acts as a catalyst. Confirmation of graphitization is given in Fig. 11, which shows an optical micrograph taken with the Nomarski technique of the wear flat on a diamond tool which had worn on steel at a rate of approximately 400. Because the magnification is high and the wear band not absolutely flat, only the central part of each micrograph is fully in focus. The micrograph shows evidence of vertical grooving caused by the downward flow of metal over the tool, and also a structure built up of almost horizontal features lying almost perpendicular to the velocity of the moving metal. These horizontal features are inconsistent with abrasive wear but are not unlike those associated with
Fig. 11. Nomarski technique micrograph of the wear flat on a tool worn after turning at high speed.
the graphitization of diamond, which tend to develop along preferred crystallographic directions. As the wear flat on the tool of Fig. 11 has no simple crystallographic orientation, the precise orientation of these features is of no great significance and in any case the geometry of graphitization depends in a complex way both on the orientation of the face concerned and on the particular conditions and chemical environment. Not a great deal of information is available on the geometry of graphitization but the general form of the features we observe is not dissimilar to that observed in other studies [20, 211. Hence all the results obtained at 30 m s-l are consistent with the wear arising from graphitization. (Very high rates of wear have also been reported in diamond grinding experiments when the speed of the grit reaches the order of 30 m s-l [22] .) 4.2. Wear at 11 m s-l The wear rate when turning in an atmosphere of air at 11 m s-l has a value of approximately 10 (10-s mm2 mmP2) compared with a rate of about 10T3 for brass of comparable hardness. Reducing the air pressure to 0.1 Torr generally reduces the wear rate by the order of 50%; hence a component of the wear is dependent on the presence of air. Figure 12 shows a micrograph of the wear flat of a tool, T25R, turning at 11 m s-l in 760 Torr of air. Besides the vertical grooving observed in Fig. 11, there are linear features inclined at an angle of about 30” to the vertical similar to the horizontal lines in Fig. 11. The mode of wear appears to be similar in both cases, although the surface features are more marked in Fig. 11, presumably because the much higher rate of reaction produces a deeper etch pattern.
175
Fig. 12. Micrograph of the wear flat on tool T25R after turning at 11 m s-l in 760 Torr of air.
The different orientations of the linear features in Figs. 11 and 12 are due to different crystallographic orientations of the diamonds in the tools. An inspection of tool T25R in the cathodoluminescent mode of the scanning electron microscope revealed the position of {111) growth layers and suggested that the line features are approximately parallel to these layers. The inference from the above results is that the wear at 11 m s-l (Fig. 12) occurs by a rather similar mechanism to that at 30 m s-l (Fig. ll), i.e. by some form of graphitization. Diamond alone does not begin to graphitize until the temperature is raised to at least 1800 K [20, 211 but in the presence of iron graphitization is possible at the order of 1000 K [8] . It is also known that even a small pressure of oxygen begins to etch a diamond surface at temperatures above 1100 K [20,21]. Hence we expect that the graphitization of diamond in the presence of iron may be encouraged by oxygen and therefore occur at lower temperatures and lower cutting speeds. At least three factors may control the rate of the oxygen induced graphitization at 11 m s-l : (i) the rate of conversion of diamond to graphite; (ii) the rate of removal of the graphite; (iii) the availability of oxygen from the air. An experiment on the wear in air at speeds of lo,14 and 22 m s-l, in the region below the very high rate of graphitization, showed that the wear rates were appreciably less at 14 and 22 m s-l than at 10 m s-l, although the temperatures in the surface must be greater at the higher speeds. In another experiment a tool turned a steel rod which could be heated above ambient temperature by an induction heater and the wear rate was actually less when the rod was heated to 225 “C. We conclude that the higher temperatures produced by faster cutting do not increase the wear rate, so the limiting process can hardly be the rate of conversion of diamond to graphite which will certainly rise with temperature. The graphite formed in the conversion process is relatively weak mechanically and should be easily removed; therefore the removal process is unlikely to be the rate controlling one. There remains the third possibility, namely the ease with which air can penetrate to the interface. The results of
176 TABLE 1 Wear rates of a diamond tool (T20R) at different pressures
turning mild steel at different cutting speeds in air
---
9.8 m s-l 13.8 m s-l 19.9 m s-l
760 Torr
0.1 Torr
76 Torr
7 4 4
4.5 4 4
4.5
an experiment on this point are summarized in Table 1. At a speed of 9.8 m s-l, reducing the air pressure by a factor of only 10 reduced the wear rate to the value under 0.1 Ton. Hence, even under full atmospheric pressure, the air was only just reaching the interface. As the air must penetrate under the moving chip (Fig. 8), we expect access to become progressively more difficult as the cutting speed increases. In fact Table 1 shows that the wear rates in 760 Torr of air at 13.8 m s-l and 19.9 m s-l are no more than the value in 0.1 Ton. We conclude that while air at atmospheric pressure usually reaches the interface of tools cutting at 11 m s-l, there is the possibility that its rate of entry may be affected by relatively small changes in pressure and speed. The rate of wear is also reduced in an atmosphere of argon or by the introduction of water and lubricants which obviously restrict the entry of air. 4.3. Wear at 0.16 M S-I The wear rate when turning in air is usually reduced when the cutting speed is reduced. The wear rate at a nominal speed of 0.16 m s-l is generally considerably less than that at 11 m s-l. These lower wear rates are to be expected, as the surface temperatures will be much less at 0.16 m s-l and so will any thermally activated graphitization. We still have to account for an appreciable residual rate of wear even at these very low speeds, but first we consider some further observations. The next experiment at 0.16 m s-‘l brought a rather surprising result. When the air pressure was reduced to 0.1 Torr, the wear rates increased considerably, to values actually greater than those observed when turning in , i.e. in direct contrast to the behaviour at 11 m 760Torrofairatllms-1 the presence of 760 Torr of air resulted in a lower rate of wear, as shown S-’ for three typical tools in Table 2. The table also shows that an atmosphere of 760 Ton: of argon reduced the wear almost, but not entirely, to the value observed in 760 Torr of air. Another feature of these experiments at 0.16 m s-l is that the appearance of both the worn diamond and the machined metal after turning in 0.1 Ton and in 760 Torr of air is quite similar to the appearance after turning in ‘. Figure 13 is a Nomarski optical micrograph of the same tool airatllmsshown in Fig. 12, after turning at 0.16 m s-l in 0.1 Torr. The wear features are similar to those in the run at 11 m s-l in 760 Torr; note particularly that
177 TABLE 2 Wear rates of diamond tools turning mild steel at a cutting speed of 0.16 m 6-l in three different atmospheres
..-DSllR DS13R DS14R .-____
Air 0.1 Torr
Air 760 Torr
Argon 760 Torr
14 12 8
0 3 0
2 5 0
Fig. 13. Micrograph of the wear flat on tool T25R after turning at 0.16 m s-l in 0.1 Torr of air.
the line features inclined to the vertical are well developed in both pictures. Figure 14 gives micrographs of the surface of a steel rod turned by the same tool at 11 m s-l in 760 Ton of air and at 0.16 m s-l in 0.1 Torr. There is no great difference between the two surfaces, though if anything the finish under the lower pressure is somewhat smoother. Figure 15 shows scanning electron micrographs of the corresponding chips; again the appearance under both conditions is similar. 4.4. Graphitization at ambient temperature Before discussing the results of the previous section, it is worth stressing that the patterns of wear on the tools, with line features inclined to the vertical, cannot be due to abrasive wear. In fact, the wear rates when turning brass of comparable hardness to mild steel, either at 11 m s-l in 760 Torr of
@I
(a) Fig. 14. Micrographs of the surface (b) 0.16 m s-l in 0.1 Torr.
of metal turned
(a) Fig. 15. Micrographs of the swarf produced air and (b) 0.16 m s-l in 0.1 Torr.
at (a) 11 m s-l
in 760 Torr of air and
0) when turning
at (a) 11 m s-l
in 760 Torr of
air or at 0.16 m s-l in 0.1 Torr of air, were approximately 10e3 compared with 10 for steel. We conclude that in all the present experiments on steel the contribution to the wear by abrasive processes is negligible. (The wear on brass is of the same order of magnitude as that observed in low speed rubbing experiments by Crompton et al. [23] who attributed the wear to a form of mechanical abrasion and attrition.) The inference from the above experiments is that the wear mechanism at 0.16 m s-l is not dissimilar from that in air at 11 m s-l, which we have already identified as graphitization. The high rate of wear observed under 0.1 Torr is at first sight surprising, as graphitization of diamond only usually occurs at temperatures above 800 “C. At 0.16 m s-l the cutting speed is down by a factor 70 on that at 11 m s-l, so the temperatures at the interface will be greatly reduced. It is not possible to calculate the change in the excess temperatures exactly but a rough conservative estimate is obtained by
179
assuming that the excess temperature varies as the square root of the velocity [ 181. Hence, assuming that the excess temperatures at 11 m s-l are of the order of 800 OC,it follows that the excess temperatures at 0.16 m s-l will certainly be less than 100 “C. The experiment with a semiconducting diamond turning copper described in Section 3 also implies a similar reduction of the excess temperatures. Two further experiments were made to study how the reaction was influenced by the temperature at the interface. First, the wear was measured under 0.1 Ton of air at still lower speeds down to 0.02 m s-l. The flash temperature in the surface was thus further reduced, but the wear rate was actually greater, as shown in Fig. 16. Next, the wear was measured with the mild steel rod heated to 220 “C by an induction heater. The area of wear of the diamond increased at the same rate whether the steel was heated or not, i.e. an excess temperature of 200 “C produced no increase in the rate of wear. Both experiments thus confirm the conclusion of the previous paragraph, namely that the wear reaction is not thermally activated.
WORN AREA
mm* x10-3
0.20
I
0
I’ 0
I 1000
I
I mm*
Fig. 16. Area of the wear flat on a tool (DS102R2) 0.20 m s-l and 0.02 m s-l in 0.1 Torr of air.
2000
b WORK
us. area of metal turned at speeds of
The high rates of wear which are observed at virtually ambient temperatures can only be explained by the fact that the turning process is continually producing large areas of fresh surface, and such surfaces have an enhanced chemical activity. A particularly clear example of such activity has been given by Grunberg [24] who reported the generation of hydrogen peroxide when zinc, aluminium, magnesium and nickel are machined under water, the volume of gas produced corresponding to each atom on the new surface being chemically active. In the case of diamond and steel the activity is clearly sufficient to produce surface graphitization. The presence of air contaminates the clean surfaces by a covering layer and so reduces the
180
activity and the wear. An atmosphere of argon at low speeds produces a rather similar effect as an atmosphere of air but may not reduce the wear so much as it is not absorbed so strongly; see for example ref. 25. Three modes of graphitization may contribute to the wear of the diamond tool: (i) thermally activated and catalysed by steel; (ii) thermally activated and catalysed by steel and oxygen; (iii) a clean surface reaction between diamond and steel. The first process gives an extremely high rate of wear but only at the highest speeds, of the order of 25 - 30 m s-l. Process (ii) is responsible for that component of the wear at 11 m s-l which depends on the presence of air and process (iii) is the clean surface reaction responsible for the wear at 0.16 m s- ’ at 0.1 Torr. The processes are also sufficient to account for the wear rates observed at 0.16 m s-r in 760 Torr of air and at 11 m s-l in 0.1 Torr of air, which though appreciably less than those discussed above are still of the same order of magnitude. The tool already shown in Figs. 12 and 13 was also used to turn at 0.16 m s-l in 760 Torr of air. The structure of the wear flat with similar features characteristic of graphitization is again visible (Fig. 17). The appearance of the turned metal was also similar to that observed under the other conditions. An inspection of a tool removed from the lathe generally shows fragments of steel still in contact with the face, so the air is probably only partly covering the interface. Hence we conclude that some wear must occur owing to the clean surface reaction. We note, for confirmation, that in this case an increase in cutting speed or a decrease in pressure should reduce the inflow of air, and hence the wear rate. Successive measurements on the same tool gave the wear rates shown in Table 3. We see that each change of pressure and speed affected the wear rate in the way to be expected. (As the tool
Fig. 17. Micrograph of the wear flat on tool T25R after turning at 0.16 m s-l in 760 Torr of air.
181 TABLE 3 Wear rates of a diamond tool turning mild steel as a function of cutting speed and air pressure Speed (m s-l) -__----
Air pressure (Torr)
Wear rate ---
0.10 0.02 0.20 0.02 0.02
760 760 80 760 80
4 1 15 3.5 16
was wearing progressively throughout the experiment the difference in the two wear rates observed at 0.02 m s-l and 760 Torr is probably not significant.) Finally, the wear flats produced by turning at 11 m s-l in 0.1 Torr (Fig. 18) appear quite similar to the flats produced under the other conditions discussed above. As clean surfaces react at 0.16 m s-l and low pressure, a similar reaction is to be expected at 11 m s-l and low pressure, perhaps assisted by the higher temperatures. We therefore identify this wear also as graphitization induced by clean surfaces.
Fig. 18. Micrograph of the wear flat on tool T25R after turning at 11 m s-l in 0.1 Ton of air.
4.5. Speed dependence of the wear at low pressure A characteristic feature of the wear rates observed in air at low pressure (0.1 Ton) calls for further comment. Figure 19 gives the results of five experiments in which the wear in 0.1 Torr was measured as a function of cutting speed, the results being normalized to the same value at low speed,
182 A
0.1 torr
WEAR RATE
0
’ 0.1
I 1
l ;. nl s-l
SURFACE SPEED
Fig. 19. The wear rate of five tools cutting in 0.1 Torr of air as a function of cutting speed. The rates are normalized to the same value at the lowest speed.
and we see that the wear decreases steadily with increasing cutting speed. It is not uncommon for a wear rate to decrease with increasing speed, owing, for example, to softening of the metal (e.g. ref. 26) or to a thermally induced transition in the metal (e.g. ref. 27). However, in the present experiments the main wear process is neither abrasive nor adhesive in the usual sense, so such explanations are not applicable. When turning in air at 760 Torr a change of speed may affect the entry of air but at speeds around 0.2 m s-l at 0.1 Torr the position is much simpler. Temperature effects must be negligible. Changes in the access of air to the wear flat cannot be responsible for the observed dependence of wear on speed, for an increased speed would reduce the entry of air, thus giving a cleaner surface and more wear. It might be that an increase of cutting speed makes a significant reduction in the time available for a reaction to occur. However, at a speed of 10 m s-l a steel and a carbon atom will be in close proximity for the order of lo-’ s, which should be ample time to allow the electron rearrangement involved in the reaction and to dissipate any excess energy released. (The thermal relaxation time involved is of the order of a period of the atomic vibrations in the lattice, i.e. lo-l2 s.) The electrons in a freshly machined surface will take a finite time to reach their new equilibrium configuration but this is the dielectric relaxation time which is of the order of 10-l’ s. In contrast, the height of the wear flat is of the order of 0.1 mm and so at a speed of 0.1 m s-l the freshly cut metal does not arrive at the lower part of the worn surface till after 1 ms. Hence, the wear cannot be due to a nonequilibrium arrangement of the metallic bonding. The position is still not clear but the most likely explanation for the decrease of wear with speed is as follows. The surface of a polished diamond
183
has an unusual topography with a rough and jagged microstructure of the order of 5 nm high and with a linear scale of about the same order [Z&29]. The surface is not flat but is delineated, at least approximately, by (111) cleavage planes steeply inclined to the surface, and the rate of the reaction will depend on the real area of contact between the steel and the diamond. As a diamond surface is loaded against a steel surface under static conditions, plastic flow occurs until a sufficient area of steel is in contact with the asperities on the diamond to support the load without exceeding the yield stress of the metal. This will take a small but finite time, so the effective area of contact during machining may fall with rising speed. It is difficult to estimate the likely magnitude of this effect, particularly for a metal such as steel where delayed yielding makes the position still more complicated. However, even at a speed as low as 0.1 m s-l the metal remains in contact with one particular asperity for the order of only 5 ns, while the delays for yielding are commonly much longer, see for example refs. 30 and 31. Recent experiments by Maan and Broese van Groenou [32] underline the importance of time effects during plastic flow. These authors observed the scratching of mild steel by a diamond stylus under a constant load driven at various constant low speeds. As the scratching speed was raised from 0.3 vrn s-l to 0.3 mm s-l the area of the cross section of the groove decreased by a factor of about 3, despite the very low magnitude of all the speeds involved.
5. Wear of other materials After completing the work described above, further experiments were made to see whether other examples of high rates of chemical wear could be observed when machining other materials. Tools and workpieces were prepared from materials with a probable affinity for each other, bearing in mind the need for the tool to be considerably harder than the workpiece to avoid abrasive wear. These experiments are still in a preliminary stage and are not as detailed as those discussed above but already they have produced some unexpected results. Rates of wear comparable with that of diamond on mild steel have been observed when diamond tools turn vanadium, titanium, zirconium and cobalt in air at speeds of about 5 m s-l. The wear of the diamond on all these metals is relatively smooth but the behaviour with molybdenum is quite different. After a very short time relatively large pieces of the diamond are tom out of the edge and the tool becomes useless. In addition the surface finish on the turned molybdenum billet is extremely poor and rough and it appears that the metal is being tom rather than cut. At low speeds the interaction of a diamond tool with nickel appears similar to that with molybdenum but at higher speeds the wear is smoother and more similar to that on steel. Another set of experiments was made turning billets of graphite with tools of tungsten. Tungsten is appreciably harder than graphite, so any wear
184
of the tool due to abrasion should be relatively small. This was confirmed by the fact that, when cutting at speeds in the range 0.1 - 10 m s--i, either in 0.1 Torr of air or in 760 Torr of argon, the wear was much less than 1 on our usual scale. However, on increasing the air pressure from 0.1 to 760 Torr, the wear rate increased to a value of about 40. Thus a component of the air is responsible for some chemical reaction at the interface but there is no obvious explanation of the very considerable wear of the tungsten tool in terms of the chemistry of bulk materials. For example, if oxygen is the critical constituent of the air, it might be supposed that it would react with the graphite rather than with the tungsten, yet it is the tungsten which wears. Nor are there any obvious reactions involving other constituents of the air. A third set of measurements was made with billets of sulphur which were turned with copper and nickel tools in air at cutting speeds of about 0.1 m s-l. Solid sulphur exhibits a range of crystalline structure and a billet cast from molten sulphur only takes up its equilibrium constitution after a period of at least 2 d. A billet turned immediately after casting gives a good continuous chip which will impede the flow of air to the freshly cut surface, whereas after ageing the chips are small and fragmented. Therefore the experiments were made on billets shortly after casting and due precautions were taken when comparing tools to use either the same billet or similar billets of similar age. Nickel and copper tools were chosen, as both metals are known to have considerable affinity for sulphur in conventional reactions. They have also a similar hardness appreciably greater than that of sulphur, so one would expect abrasive wear to be relatively negligible. The wear rates of the nickel tools were in fact quite low, appreciably less than 1. In contrast, the wear rates of the copper tools were very high -- of the order of 500!
6. Conclusion The rates of wear of diamond tools machining mild steel are very high over a wide range of machining conditions. The wear is due primarily to graphitization of the diamond brought about by three factors: (i) high local flash temperatures at the interface, (ii) the enhanced activity of the clean surface generated during the machining process and (iii) the catalytic action of the steel and the atmosphere. Although the local flash temperatures play an important role at normal cutting speeds, the experiments at speeds of 0.16 m s-l demonstrate that graphitization can occur even when the temperature rises are negligibly small. Hence, the activity of the clean freshly machined surfaces is sufficient to induce graphitization even at ambient temperatures. We stress that the conditions obtained during machining are different from those encountered in most experiments on surfaces. The present experiments avoid the usual complications which arise when the materials involved are covered with surface films of often unknown constitution different from
185
that of the bulk material, because an absolutely clean surface of machined metal is created by the turning process. The wear flat on the tool is wearing quickly, so a new clean surface is steadily being generated on the tools. These clean surfaces will only be contaminated, if at all, by the surrounding atmosphere, which may be monitored and controlled. The experiments described in Section 2.2 suggest that by reducing the pressure of the gaseous atmosphere surrounding the tool it is possible to obtain almost complete exclusion of the atmosphere from the interacting surfaces. The principal point of interest in the present experiments is that the forms of chemical wear observed during machining are different from what might be expected from a consideration of the chemical and thermodynamic properties of the materials in bulk. Besides the graphitization of diamond at ambient temperature described in detail earlier, three other examples have been given in Section 5. (i) The strong interaction between diamond and molybdenum is markedly different from that between diamond and steel. (ii) Both copper and nickel have a considerable affinity for sulphur, but the rates of wear of the nickel tools turning sulphur are two orders of magnitude less than those of the copper tools. (iii) Tungsten tools turning graphite at low speed in argon or in a low pressure of air show relatively little wear but the presence of an atmosphere of air raises the wear to a high value. As in the case of the diamond-steel reaction, each example illustrates behaviour not readily explicable in conventional chemical terms. Chemical reactions are usually discussed in terms of the thermodynamic functions of the two phases, which are determined by the bond strengths in the bulk solids. However, the wear processes in the present experiments, particularly when the temperature remains virtually ambient, must be essentially surface phenomena. We may begin to analyse this situation by considering the diamond-steel reaction. The diamond is certainly wearing away, so carbon atoms are being removed because of a linkage with the iron (or other) atoms in the steel. At least five types of bonds are involved in the wear process, as shown schematically: 11
2
3455
-Fe-Fe-Fe-Fe-C-C-C-CBonds 1 and 5 are characteristic of the bulk material, 3 is the bond formed across the interface, and bonds 2 and 4 are those linking the uppermost iron and carbon atoms to the next lower layer under the conditions at this particular interface. Any chemical reaction at the interface is controlled by the form and strength of the bonds 2,3 and 4, and these are not fully determined by the behaviour of the bulk material. The wear of diamond turning molybdenum gives an interesting contrast to the wear on steel. With steel the diamond wears steadily and relatively smoothly; this implies that in the schematic arrangement of bonds shown above bond 4 is appreciably weaker than the others. With molybdenum there is again a strong interaction with the diamond but instead of smooth wear fracture occurs in the bulk of the diamond and also tearing in the bulk of the
186
metal as described more fully by Casey and Wilks [33]. bonds schematically as previously : 1
1
2
Setting out the
3455
-MO-MO-MO-MO-C-C-C-Cwe see that the experimental results imply that a.ll five types of bond are stronger than the inherent mechanical strengths of the metal and of the diamond, which of course are determined by structural defects rather than bond strengths. The examples just discussed show that machining experiments under suitably controlled conditions give precise information on the relative strengths of bonds and thus provide a link between observations of chemical wear in working conditions and observations of atomic environments from more conventional surface studies, particularly X-ray photoelectron spectroscopy and allied techniques. We began this work because of our interest in the wear of diamonds. The experiments with other materials were made in the first place to confirm that high chemical activities due to clean surfaces occurred in other machining processes but they also indicate the possibility of further experiments for the better understanding of surface reactions in general, and of the diamond-steel reaction in particular. Both diamond and steel have complex constitutions which are difficult to analyse and control, diamond particularly so. (It is also difficult to identify any transfer of material on an atomic scale from a diamond tool to a steel workpiece ! ) In contrast, the materials used in the other experiments are more readily obtained in a known condition. Experiments with these and similar materials should permit correlations between rates of wear and atomic bonding strengths, e.g. a correlation between the different wear rates of copper and nickel on sulphur and the values of the bond strengths between copper and nickel atoms on sulphur obtained by conventional surface techniques. Experiments are being continued on these lines.
Acknowledgments We wish to thank the Science Research Council and De Beers Industrial Diamond Division for their support of this work and Mr. J. W. Naisby for his assistance with the experiments. We are also grateful for helpful discussions with Dr. T. H. Childs, Professor W. Hirst and Professor M. W. Roberts.
References 1 E. M. Wilks and J. Wilks, in J. E. Field (ed.), The Properties of Diamond, Academic Press, New York, 1979. 2 T. N. Loladze and G. V. Bokuchava, The Wear of Diamonds and Diamond Wheels, Mashinostroeniye, Moscow, 1967. 3 T. Tanaka and N. Ikawa, Ann. CIRP, 19 (1971) 153; Bull. Jpn Sot. Precis. Eng., 7 (4) (1973) 97.
187 4 A. S. Vishnevskii, V. G. Delevi et al., Sint. Almazy, 1 (1973). 5 A. S. Vishnevskii and A. V. Lysenko et al., Sint. Almazy, 4 (1974). 6 A. S. Vishnevskii, A. V. Lysenko, T. D. Ositinskaya and V. G. Delevi, IZV. Akad. Nauk SSR, Neorg. Mater., 11 (9) (1975) 1589. 7 R. Komanduri and M. C. Shaw, Nature (London), 255 (1975) 211. 8 A. E. Gorodetskii, L. L. Builov, V. M. Lukyanovich, R. I. Nazarova and Z. E. Sheshenina, Fiz. Khim. Probl. Krist., 2 (1971) 62. A. E. Gorodetskii, V. M. Lukyanovich, D. V. Fedoseev and L. L. Builov, Smachivayemost i Poverkhnostniye Svoistva Rasplavov i Tverdykh Tel, (1972) 125. 9 A. G. Thornton and J. Wilks, Nature (London), 274 (1978) 792. 10 W. Graham and A. Y. C. Nee, Prod. Eng. (London), 53 (1974) 186. 11 J. A. Williams and D. Tabor, Wear, 43 (1977) 275. 12 G. W. Rowe and E. F. Smart, Proc. Inst. Mech. Eng. (London), Part 30,181 (1966) 48. 13 M. C. Shaw, J. D. Pigott and L. P. Richardson, Trans. ASME, 73 (1951) 45. 14 V. A. Markitanova, Russ. Eng. J., 3 (1965) 71. 15 V. A. Zhilin, Mach. Tool. (USSR), 42 (12) (1971) 41. 16 A. A. Sagarda and 0. V. Khimach, Russ. Eng. J., 53 (6) (1973) 73. 17 J. Panhorst and J. Triemel, Ind. Diam. Rev., (1976) 320. 18 F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, 1st edn., Oxford Univ. Press: Clarendon Press, Oxford, 1950. 19 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. 20 T. Evans and D. H. Sauter, Philos. Mag., 6 (1961) 429. 21 G. Davies and T. Evans, Proc. R. Sot. London, Ser. A, 328 (1972) 413. 22 J. F. Prins, Ind. Diam. Rev., (1971) 364. 23 D. Crompton, W. Hirst and M. G. W. Howse, Proc. R. Sot. London, Ser. A, 333 (1973) 435. 24 L. Grunberg, Proc. Phys. Sot., 66 (1953) 153. 25 P. A. Redhead, J. P. Hobson and E. V. Kornelsen, The Physical Basis of Ultrahigh Vacuum, Chapman and Hall, London, 1968, Chap. 2. 26 W. Hirst and J. K. Lancaster, Proc. R. Sot. London, Ser. A, 259 (1960) 228. 27 J. F. Archard, Wear, 2 (1958) 438. 28 J. Wilks, Nature (London), 243 (1973) 15. 29 A. G. Thornton and J. Wilks, J. Phys. D, 9 (1976) 27. 30 N. P. Suh, Int. J. Mech. Sci., 9 (1967) 415. 31 J. Harding, Acta Metall., 19 (1971) 1177. 32 N. Maan and A. Broese van Groenou, Wear, 42 (1977) 365. 33 M. Casey and J. Wilks, in Koenigsberger and Tobias (eds.), Proc. 16th Int. Conf. on Machine Tool Design and Research, 1975, MacMillan, London, 1976, p. 553.
Wear, 53 (1979) 165 - 187 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
TOOL WEAR AND SOLID STATE
A. G. THORNTON*
REACTIONS
DURING
MACHINING
and J. WILKS
Clarendon Laboratory,
Oxford (Gt. Britain)
(Received September 8, 1978)
Summary Measurements have been made of the wear of diamond tools turning mild steel under a range of carefully specified conditions. High rates of wear are observed owing to graphitization of the diamond. Under certain conditions, this graphitization may proceed at a considerable rate even if there is no appreciable temperature rise at the diamond-steel interface. This result implies that the clean surfaces generated by the machining process have chemical activities different from those of the bulk material. Examples of unusual chemical activity have also been observed when using diamond tools to turn other metals, when using tungsten tools to turn graphite, and copper and nickel tools to turn sulphur. The experiments show that machining, under suitable conditions, offers a precise method of measuring rates of reaction between clean solids and of obtaining information on the relative bond strengths of the surface atoms.
1. Introduction Discussions of the wear of cutting tools used in machining operations often refer to chemical and diffusive modes of wear. Although these effects may play an important part in tool wear, there is generally little detailed information on the reactions involved. The study of chemical wear is also complicated by the fact that, besides producing wear directly, chemical reactions may create conditions more favourable to abrasive wear. The present experiments, arising out of studies of the use of diamond for grinding and precision machining, demonstrate some hitherto unobserved forms of chemical wear with rather unusual characteristics. Diamond is so much harder than brass that the rate of wear of a diamond tool turning brass is extremely small. In contrast, if the brass cylinder is replaced and the same tool is used to turn mild steel of comparable hardness, the rate of wear is extremely high: 10’ times greater than on brass and lo2 times greater than on the very abrasive aluminiumsilicon alloy LM13; see for example Wilks and Wilks [l] . The high rate of *Present address: A.M.T.E., Gt. Britain.
Experimental
Diving Unit, HMS Vernon, Portsmouth,
166
wear of diamond on steel is generally believed to arise from the chemical affinity between iron and carbon, but the exact details of the interaction are still a matter of discussion. Loladze and Bokuchava [2] proposed that the wear was due to carbon atoms diffusing into the steel at the high local flash temperatures generated by the machining process. However, if the carbon atoms at the surface of the diamond become sufficiently mobile to diffuse into the steel, they should also be sufficiently free to take up the more stable graphite configuration. Hence, we expect that the predominant effect of local flash temperatures will be to transform the diamond to graphite, which may then dissolve in the iron, or being mechanically weak be removed by abrasion. There are now several discussions in the literature which conclude that the wear process is controlled primarily by the rate of graphitization of the diamond [ 3 - 71. The reaction will be catalysed by the presence of the steel, e.g. the rate of graphitization of a diamond in a furnace is much enhanced when thin films of iron, nickel or titanium are deposited on the surface [ 81. However, although several workers have observed graphitization in static experiments, there is little information on the conditions at the interface between the steel and the diamond during machining. The present experiments were made to study the reactions that occur when diamond tools turn mild steel under controlled conditions. One of the principal results of the investigation is that graphitization of the diamond may occur even at virtually ambient temperatures [9], because of the enhanced chemical activity of the clean surfaces of steel which are produced by the machining process and rub across the flank of the diamond. The enhanced activity appears to arise not from particular characteristics of diamond and steel, but from experimental conditions which probably prevail when machining other materials. This report first describes experiments on turning mild steel with diamond tools and then some experiments involving other materials which give further examples of chemical wear at interacting surfaces.
2. Experimental 2.1. Measurement of wear rates Round nosed diamond turning tools with a nose radius of about 450 pm and a flank clearance angle of 10” were either purchased from commercial suppliers or fabricated by us from drillstones. They were mounted on a Bryant Symons diamond lathe in a specially designed tool holder which permitted the rake face of the tool to be set accurately horizontal by optical means. The tools were adjusted so that the rake face was on the centre line of the rod being turned, and 50 pm cuts were made on standard billets of mild steel. The lathe had a variable speed adjustment which permitted angular velocities of 3 - 50 rev s-l ; thus the use of billets with diameters ranging from 12 to 75 mm permitted a range of linear cutting speeds from about 0.12 m s- ’ to 12 m s- ‘. After it became apparent that turning at very
167
low speeds presented points of particular interest, an auxiliary gear drive was introduced to turn the lathe more slowly and to permit cutting speeds down to 0.02 m s-l. Although some wear occurs on the rake face of the tool, most occurs on the flank face in contact with the rotating billet, where the steel quickly wears a vertical flat as shown in Fig. l(a). This wear flat, viewed at a relatively low magnification, has a smooth appearance on which is imposed a pattern of fluting with a spacing equal to the feed rate of the lathe (Fig. 2). The geometry of the wear is also shown in Fig. 3 which gives a plan view of the rake face of a similar tool which has suffered a large amount of wear. The line on the micrograph has been drawn parallel to the edge of the turned billet and indicates that the wear has taken place on the leading (left hand) edge of the tool as we would expect. The fluting appears to be a secondary effect common to all the wear patterns but not relevant to our present measurements. (It probably arises because any small variation in the profile of the edge of the tool will lead to a corresponding variation in the thickness of the chip and hence to a change in the cutting conditions and the wear rate. If the resulting change in wear rate accentuates the effect of the variation in the edge of the tool, then the situation is unstable and a periodic variation of tool wear is likely to result.) MEASUREMENT
OF WORN AREAS tool tip
=+=_, flat
I i-
(a)
(b)
Fig. 1. Schematic diagram of a wear flat on a toof.
To obtain a quantitative measure of the rate of wear, the size of the wear flat is characterized by the dimensions 3cand y shown in Fig. l(b) and the product xy taken as a measure of the area. Then a plot of xy against the total area of metal traversed during the cuts is found to give an almost straight line, as shown for one particular tool in Fig. 4. This linear relation appears to be generally valid provided the product xy does not exceed a value of about 30 X low3 mm’. Hence by working in this range the rate of wear under particular cutting conditions is specified uniquely by the gradient of the line which we quote in units of 10s6 (mm2 mmB2). Subsidiary experiments showed that the rate of wear varied by about 30% as the depth of cut increased from 25 to 100 pm. As the depth in each experiment could be set to within 3 pm, any variations in the wear rates due to inaccuracies in this setting should be negligible. The feed rate of 25 pm rev- ’ was determined precisely by the gearing of the lathe. As the steel rods
168
Fig. 2. Optical micrograph of a typical wear flat. Fig. 3. Optical micrograph of the rake face of a heavily worn tool. The line is parallel to the axis of the lathe.
WORN AREA x’ 30 / mm2 x 10-3
0
I
C
2000
I
mm*
l
1
4000
WORK
Fig. 4. Area of wear flat us. metal turned in air at 10 m s-l.
decreased in diameter during the wear runs, the speed of the lathe spindle was adjusted to maintain the cutting speed. As a result of these adjustments the speed was not held exactly constant but was kept within about +lO% of its nominal value. Although the wear rates changes substantially with large changes in the velocity of cutting, any differences due to the above changes in the size of the rods are so small as to be negligible. The rate of wear of the diamond depends on the constitution of the steel. Rods made from a standard ENlA mild steel, from a free cutting ENlA mild steel containing lead and from EN34 a nickel-molybdenum steel gave wear rates differing by a factor of the order of 2. Similar observations on the wear of diamond grit grinding different steels have been reported by Tanaka and Ikawa [3] and by Graham and Nee [lo]. Therefore, all our experiments at speeds up to 11 m s-l were made on one type of steel, a standard ENlA mild steel. Tools were compared by turning either the same
169
rod of mild steel or rods made from the same billet of the steel. The wear rate was usually so high that it was sufficient to make one pass over a 10 mm length of a 75 mm diameter rod to produce measurable wear. In the experiments mentioned above, Tanaka and Ikawa observed that the rate of wear of a diamond grit machining different samples of steel depended on the crystallographic orientation of the diamond. The magnitude of the effect, which depended on the type of steel, was such that the wear rates varied with orientation by factors from about 1.3 to 2.1. During the present experiments we have observed the wear of turning tools made from specifically oriented diamonds and found similar results. Therefore, in order to obtain exact comparison of wear rates under different conditions it is essential to use similar tools of the same orientation. 2.2. Turning in different atmospheres Figure 5 shows a sketch of a chamber which could be fitted to the lathe in order to permit reductions of the air pressure and the introduction of other gases around the tool and workpiece. The specimen S is held on the end of the steel rod R which passes out of the chamber through the gland A with O-ring seals which contain the gas in the chamber and permit the steel rod to rotate freely and the chamber to move in a direction parallel to the rod. The tool is supported on a tool holder which passes into the chamber through a second gland B; during turning, the chamber remains fixed with respect to the tool and moves along with it. A tube T passing through a third gland connects the chamber with a pumping system which could maintain pressures down to about 0.1 Torr. Changes in the air pressure often produced marked effects on the rates of wear, but for the moment we refer only to one important feature of this behaviour. Figure 6 shows the wear rates obtained in an experiment in which all other parameters were held constant and the pressure of air in the chamber varied. The wear rate changes abruptly over a narrow range of pressure, but at other pressures is independent of the pressure, i.e. the wear rate CONTROLLED
l
::
Wring rotating
ATMOSPHERE
CHAMBER
sds parts
Fig, 5. Schematic diagram of controlled atmosphere chamber.
170
WEAR RATE
x
x
5, X
x
0
x
I I
10
x I 100
torr
x
AIR PRESSURE
x I’ 1000
Fig. 6. Wear rate of a tool (T58) turning at 0.13 m s-l as a function of the surrounding air pressure.
appears to take one of two values corresponding to low and high pressures. This pattern of behaviour appears to be fairly general, e.g. Fig. 7 shows the effect of pressure in another experiment with different machining conditions; as before there are two regions in each of which the wear is approximately independent of the pressure. It is important to note that the wear with which we are concerned occurs on the flank face of the tool which is pressed into contact with the steel billet (Fig. 8). Hence the wear flat on the tool is sealed off from the surrounding gaseous atmosphere by the presence of the freshly cut metal. The fact that changing the pressure of the surrounding air affects the rate of wear shows that changing the pressure affects the number of air molecules reaching the interface. Moreover, the fact that the wear above and below the A WEAR RATE
X X
10, xx
X
X
5_
X 0
X 0.01
I 0.1
I 1
I 10
I loo torr
X I ‘Ooo
+ AIR PRESSURE
Fig. 7. Wear rate of a tool (DS97R) turning at 0.013 m s-l as a function of the surrounding air pressure.
171
diiction
of rotation
steel c
region
billet
of flank
wear
Fig. 8. Schematic diagram showing the position of the wear flat with respect to the turned metal.
transition is largely independent of pressure suggests that so far as the wear reactions are concerned there are essentially two conditions, in which air molecules are either reaching or not reaching the interface in significant numbers. The pressure at which the transition occurs will depend on several factors which affect the access of air molecules to the interface, such as the cutting speed, the shape of the tool, the quality of the polish on the tool and the presence of any cracks near the tool edge. However, the magnitudes of the pressures and speeds at which transitions occur in the present experiments are of the same general order as those reported in other machining experiments [ 11,121.
3. Temperatures
generated
by machining
Very considerable temperature rises are generated at and near the face of the tool or abrasive grit when carrying out machining and grinding operations under normal industrial conditions and speeds. In most turning operations the heat is generated principally by the plastic deformation of the chip and by the friction of the chip moving over the rake face of the tool. During grinding operations, heat is produced by the plastic deformation as the grit grooves the workpiece and by friction between the chip and the grit. Many studies on the machining of a wide range of tool and workpiece materials show that at cutting speeds of about 10 m s-l temperatures on the tool face may rise to the order of 1000 K; see for example refs. 13 - 15. Loladze and Bokuchava [2] measured the temperature of the rake face of a diamond tool turning steel using an optical pyrometer, and recorded values ranging from 600 K to 1300 K at cutting speeds between 2.5 m s-l and 10 m s-l. Temperatures of this order have also been observed by Sagarda and Khimach [ 161 and by Panhorst and Triemel [17] when grinding steel with diamond grit. The work mentioned in the previous paragraph on diamond is not directly applicable to our present experiments, where the wear occurs on the
172
flank of the tool which is relatively remote from the plastic deformation in the chip and from the friction of the chip on the rake face. Therefore we need to know the temperatures on the interface between the wear flat on the diamond and the steel billet bearing against it. This temperature will be determined partly by an overall temperature rise associated with the plastic deformation of the chip, and partly by the generation of localized flash temperatures due to the rubbing of one surface over the other. Relative velocities of the order of 10 m s-1 readily give rise to excess flash temperatures of several hundred degrees; see for example refs. 18 and 19. Hence, it is quite likely that the temperatures on the diamond-steel interface may rise to the order of 1000 K. It is difficult to obtain precise values of the magnitude of the local excess temperatures concurrently with the wear measurements. However, we have studied the temperatures generated at the interface between a diamond tool and a metal workpiece during machining by turning metal with a semiconducting type IIb diamond and observing the e.m.f. generated. Some results obtained for a diamond tool turning copper are shown in Fig. 9 which gives the measured e.m.f. as a function of the linear speed at which the copper is being cut. Further experiments are needed to relate the e.m.f.s to the magnitude of the flash temperatures, but it is clear that the e.m.f.s decrease steadily as the cutting speed decreases. Hence, the excess temperatures are greatly reduced at the lowest turning speeds, a result we use in Section 4.4.
4. Wear of diamond
on steel
4.1. Wear at 30 m s-l Most of our measurements were made at a cutting speed of about 11 m s-l, which is typical of industrial machining processes, or at lower speeds. However, it is convenient to begin by describing some experiments made at higher speeds. To obtain these higher speeds it was necessary to use a specimen of larger diameter made from a sample of “good commercial quality” mild steel, different from that used in 2he other experiments. A subsidiary experiment showed that the form of the wear at 11 m s-l on this material was similar to that on the usual ENlA billets, with a wear rate about 30% smaller. At cutting speeds of about 11 m s-’ the wear rate has a value of about 10 mm2 mm-’ and depends only weakly on the speed. However, on increasing the speed the wear suddenly rises so rapidly that at 30 m s-l it is about 50 times greater and very short passes were necessary to avoid wearing out the tool during the measurements. A characteristic feature of this high rate of wear, not observed at lower speeds, was a shower of white sparks, presumably hot debris from the turning process. The vacuum chamber was then modified to take the extra large billet used to obtain the high cutting speeds and the experiment was repeated in a nominal vacuum of about
173
403
x. . x
E.M.F. l
(W
X.
A
l
.
75_ WORN AREA
0
X
X 80
20-
50-
rKx
X
mm2
I -0
x IO-3
0
x 25_
X’
x
x
X 0
Oc 0
x
0
SURFACE
SPEED
5
’
m s-l
0’
O WORK
I
100
InIn2
CI 200
Fig. 9. The thermal e.m.f. generated at the interface between a billet of copper and a turning tool (DS 90) fabricated from a type IIb semiconducting diamond: X , run 1; 0, run 2; 0, run 3. Fig. 10. Area of the wear flat on a tool (T39R3) turning at 29 m s-l vs. area of metal turned. The points were obtained at two different air pressures as indicated: X , 760 TOW 0, 0.2 Torr.
0.2 Torr. There was now no sign of hot debris but the wear rates were the same as in a full atmosphere of air. For example, Fig. 10 shows the worn area plotted against the area of metal turned in an experiment in which the pressure of air was varied between 760 and 0.2 Torr; the worn area steadily increases at virtually the same rate under both conditions. If the speed of cutting is continually increased, the surface temperature of the diamond must eventually rise to at least the melting point of the steel and become sufficient to produce graphitization. For example, Vishnevskii et at. [4] report graphitization when diamond is heated in contact with steel at a temperature above 800 “C. Bearing in mind (i) that the very high wear rates are the same in 0.1 Torr and 760 Ton of air, (ii) that the high cutting speeds generate temperatures sufficient to ignite the debris in air and (iii) that graphitization must eventually occur with increasing speed, we identify the extremely fast wear processes observed at speeds of about 30 m s-l as arising from ~aphitization of the diamond in the presence of iron which acts as a catalyst. Confirmation of graphitization is given in Fig. 11, which shows an optical micrograph taken with the Nomarski technique of the wear flat on a diamond tool which had worn on steel at a rate of approximately 400. Because the magnification is high and the wear band not absolutely flat, only the central part of each micrograph is fully in focus. The micrograph shows evidence of vertical grooving caused by the downward flow of metal over the tool, and also a structure built up of almost horizontal features lying almost perpendicular to the velocity of the moving metal. These horizontal features are inconsistent with abrasive wear but are not unlike those associated with
Fig. 11. Nomarski technique micrograph of the wear flat on a tool worn after turning at high speed.
the graphitization of diamond, which tend to develop along preferred crystallographic directions. As the wear flat on the tool of Fig. 11 has no simple crystallographic orientation, the precise orientation of these features is of no great significance and in any case the geometry of graphitization depends in a complex way both on the orientation of the face concerned and on the particular conditions and chemical environment. Not a great deal of information is available on the geometry of graphitization but the general form of the features we observe is not dissimilar to that observed in other studies [20, 211. Hence all the results obtained at 30 m s-l are consistent with the wear arising from graphitization. (Very high rates of wear have also been reported in diamond grinding experiments when the speed of the grit reaches the order of 30 m s-l [22] .) 4.2. Wear at 11 m s-l The wear rate when turning in an atmosphere of air at 11 m s-l has a value of approximately 10 (10-s mm2 mmP2) compared with a rate of about 10T3 for brass of comparable hardness. Reducing the air pressure to 0.1 Torr generally reduces the wear rate by the order of 50%; hence a component of the wear is dependent on the presence of air. Figure 12 shows a micrograph of the wear flat of a tool, T25R, turning at 11 m s-l in 760 Torr of air. Besides the vertical grooving observed in Fig. 11, there are linear features inclined at an angle of about 30” to the vertical similar to the horizontal lines in Fig. 11. The mode of wear appears to be similar in both cases, although the surface features are more marked in Fig. 11, presumably because the much higher rate of reaction produces a deeper etch pattern.
175
Fig. 12. Micrograph of the wear flat on tool T25R after turning at 11 m s-l in 760 Torr of air.
The different orientations of the linear features in Figs. 11 and 12 are due to different crystallographic orientations of the diamonds in the tools. An inspection of tool T25R in the cathodoluminescent mode of the scanning electron microscope revealed the position of {111) growth layers and suggested that the line features are approximately parallel to these layers. The inference from the above results is that the wear at 11 m s-l (Fig. 12) occurs by a rather similar mechanism to that at 30 m s-l (Fig. ll), i.e. by some form of graphitization. Diamond alone does not begin to graphitize until the temperature is raised to at least 1800 K [20, 211 but in the presence of iron graphitization is possible at the order of 1000 K [8] . It is also known that even a small pressure of oxygen begins to etch a diamond surface at temperatures above 1100 K [20,21]. Hence we expect that the graphitization of diamond in the presence of iron may be encouraged by oxygen and therefore occur at lower temperatures and lower cutting speeds. At least three factors may control the rate of the oxygen induced graphitization at 11 m s-l : (i) the rate of conversion of diamond to graphite; (ii) the rate of removal of the graphite; (iii) the availability of oxygen from the air. An experiment on the wear in air at speeds of lo,14 and 22 m s-l, in the region below the very high rate of graphitization, showed that the wear rates were appreciably less at 14 and 22 m s-l than at 10 m s-l, although the temperatures in the surface must be greater at the higher speeds. In another experiment a tool turned a steel rod which could be heated above ambient temperature by an induction heater and the wear rate was actually less when the rod was heated to 225 “C. We conclude that the higher temperatures produced by faster cutting do not increase the wear rate, so the limiting process can hardly be the rate of conversion of diamond to graphite which will certainly rise with temperature. The graphite formed in the conversion process is relatively weak mechanically and should be easily removed; therefore the removal process is unlikely to be the rate controlling one. There remains the third possibility, namely the ease with which air can penetrate to the interface. The results of
176 TABLE 1 Wear rates of a diamond tool (T20R) at different pressures
turning mild steel at different cutting speeds in air
---
9.8 m s-l 13.8 m s-l 19.9 m s-l
760 Torr
0.1 Torr
76 Torr
7 4 4
4.5 4 4
4.5
an experiment on this point are summarized in Table 1. At a speed of 9.8 m s-l, reducing the air pressure by a factor of only 10 reduced the wear rate to the value under 0.1 Ton. Hence, even under full atmospheric pressure, the air was only just reaching the interface. As the air must penetrate under the moving chip (Fig. 8), we expect access to become progressively more difficult as the cutting speed increases. In fact Table 1 shows that the wear rates in 760 Torr of air at 13.8 m s-l and 19.9 m s-l are no more than the value in 0.1 Ton. We conclude that while air at atmospheric pressure usually reaches the interface of tools cutting at 11 m s-l, there is the possibility that its rate of entry may be affected by relatively small changes in pressure and speed. The rate of wear is also reduced in an atmosphere of argon or by the introduction of water and lubricants which obviously restrict the entry of air. 4.3. Wear at 0.16 M S-I The wear rate when turning in air is usually reduced when the cutting speed is reduced. The wear rate at a nominal speed of 0.16 m s-l is generally considerably less than that at 11 m s-l. These lower wear rates are to be expected, as the surface temperatures will be much less at 0.16 m s-l and so will any thermally activated graphitization. We still have to account for an appreciable residual rate of wear even at these very low speeds, but first we consider some further observations. The next experiment at 0.16 m s-‘l brought a rather surprising result. When the air pressure was reduced to 0.1 Torr, the wear rates increased considerably, to values actually greater than those observed when turning in , i.e. in direct contrast to the behaviour at 11 m 760Torrofairatllms-1 the presence of 760 Torr of air resulted in a lower rate of wear, as shown S-’ for three typical tools in Table 2. The table also shows that an atmosphere of 760 Ton: of argon reduced the wear almost, but not entirely, to the value observed in 760 Torr of air. Another feature of these experiments at 0.16 m s-l is that the appearance of both the worn diamond and the machined metal after turning in 0.1 Ton and in 760 Torr of air is quite similar to the appearance after turning in ‘. Figure 13 is a Nomarski optical micrograph of the same tool airatllmsshown in Fig. 12, after turning at 0.16 m s-l in 0.1 Torr. The wear features are similar to those in the run at 11 m s-l in 760 Torr; note particularly that
177 TABLE 2 Wear rates of diamond tools turning mild steel at a cutting speed of 0.16 m 6-l in three different atmospheres
..-DSllR DS13R DS14R .-____
Air 0.1 Torr
Air 760 Torr
Argon 760 Torr
14 12 8
0 3 0
2 5 0
Fig. 13. Micrograph of the wear flat on tool T25R after turning at 0.16 m s-l in 0.1 Torr of air.
the line features inclined to the vertical are well developed in both pictures. Figure 14 gives micrographs of the surface of a steel rod turned by the same tool at 11 m s-l in 760 Ton of air and at 0.16 m s-l in 0.1 Torr. There is no great difference between the two surfaces, though if anything the finish under the lower pressure is somewhat smoother. Figure 15 shows scanning electron micrographs of the corresponding chips; again the appearance under both conditions is similar. 4.4. Graphitization at ambient temperature Before discussing the results of the previous section, it is worth stressing that the patterns of wear on the tools, with line features inclined to the vertical, cannot be due to abrasive wear. In fact, the wear rates when turning brass of comparable hardness to mild steel, either at 11 m s-l in 760 Torr of
@I
(a) Fig. 14. Micrographs of the surface (b) 0.16 m s-l in 0.1 Torr.
of metal turned
(a) Fig. 15. Micrographs of the swarf produced air and (b) 0.16 m s-l in 0.1 Torr.
at (a) 11 m s-l
in 760 Torr of air and
0) when turning
at (a) 11 m s-l
in 760 Torr of
air or at 0.16 m s-l in 0.1 Torr of air, were approximately 10e3 compared with 10 for steel. We conclude that in all the present experiments on steel the contribution to the wear by abrasive processes is negligible. (The wear on brass is of the same order of magnitude as that observed in low speed rubbing experiments by Crompton et al. [23] who attributed the wear to a form of mechanical abrasion and attrition.) The inference from the above experiments is that the wear mechanism at 0.16 m s-l is not dissimilar from that in air at 11 m s-l, which we have already identified as graphitization. The high rate of wear observed under 0.1 Torr is at first sight surprising, as graphitization of diamond only usually occurs at temperatures above 800 “C. At 0.16 m s-l the cutting speed is down by a factor 70 on that at 11 m s-l, so the temperatures at the interface will be greatly reduced. It is not possible to calculate the change in the excess temperatures exactly but a rough conservative estimate is obtained by
179
assuming that the excess temperature varies as the square root of the velocity [ 181. Hence, assuming that the excess temperatures at 11 m s-l are of the order of 800 OC,it follows that the excess temperatures at 0.16 m s-l will certainly be less than 100 “C. The experiment with a semiconducting diamond turning copper described in Section 3 also implies a similar reduction of the excess temperatures. Two further experiments were made to study how the reaction was influenced by the temperature at the interface. First, the wear was measured under 0.1 Ton of air at still lower speeds down to 0.02 m s-l. The flash temperature in the surface was thus further reduced, but the wear rate was actually greater, as shown in Fig. 16. Next, the wear was measured with the mild steel rod heated to 220 “C by an induction heater. The area of wear of the diamond increased at the same rate whether the steel was heated or not, i.e. an excess temperature of 200 “C produced no increase in the rate of wear. Both experiments thus confirm the conclusion of the previous paragraph, namely that the wear reaction is not thermally activated.
WORN AREA
mm* x10-3
0.20
I
0
I’ 0
I 1000
I
I mm*
Fig. 16. Area of the wear flat on a tool (DS102R2) 0.20 m s-l and 0.02 m s-l in 0.1 Torr of air.
2000
b WORK
us. area of metal turned at speeds of
The high rates of wear which are observed at virtually ambient temperatures can only be explained by the fact that the turning process is continually producing large areas of fresh surface, and such surfaces have an enhanced chemical activity. A particularly clear example of such activity has been given by Grunberg [24] who reported the generation of hydrogen peroxide when zinc, aluminium, magnesium and nickel are machined under water, the volume of gas produced corresponding to each atom on the new surface being chemically active. In the case of diamond and steel the activity is clearly sufficient to produce surface graphitization. The presence of air contaminates the clean surfaces by a covering layer and so reduces the
180
activity and the wear. An atmosphere of argon at low speeds produces a rather similar effect as an atmosphere of air but may not reduce the wear so much as it is not absorbed so strongly; see for example ref. 25. Three modes of graphitization may contribute to the wear of the diamond tool: (i) thermally activated and catalysed by steel; (ii) thermally activated and catalysed by steel and oxygen; (iii) a clean surface reaction between diamond and steel. The first process gives an extremely high rate of wear but only at the highest speeds, of the order of 25 - 30 m s-l. Process (ii) is responsible for that component of the wear at 11 m s-l which depends on the presence of air and process (iii) is the clean surface reaction responsible for the wear at 0.16 m s- ’ at 0.1 Torr. The processes are also sufficient to account for the wear rates observed at 0.16 m s-r in 760 Torr of air and at 11 m s-l in 0.1 Torr of air, which though appreciably less than those discussed above are still of the same order of magnitude. The tool already shown in Figs. 12 and 13 was also used to turn at 0.16 m s-l in 760 Torr of air. The structure of the wear flat with similar features characteristic of graphitization is again visible (Fig. 17). The appearance of the turned metal was also similar to that observed under the other conditions. An inspection of a tool removed from the lathe generally shows fragments of steel still in contact with the face, so the air is probably only partly covering the interface. Hence we conclude that some wear must occur owing to the clean surface reaction. We note, for confirmation, that in this case an increase in cutting speed or a decrease in pressure should reduce the inflow of air, and hence the wear rate. Successive measurements on the same tool gave the wear rates shown in Table 3. We see that each change of pressure and speed affected the wear rate in the way to be expected. (As the tool
Fig. 17. Micrograph of the wear flat on tool T25R after turning at 0.16 m s-l in 760 Torr of air.
181 TABLE 3 Wear rates of a diamond tool turning mild steel as a function of cutting speed and air pressure Speed (m s-l) -__----
Air pressure (Torr)
Wear rate ---
0.10 0.02 0.20 0.02 0.02
760 760 80 760 80
4 1 15 3.5 16
was wearing progressively throughout the experiment the difference in the two wear rates observed at 0.02 m s-l and 760 Torr is probably not significant.) Finally, the wear flats produced by turning at 11 m s-l in 0.1 Torr (Fig. 18) appear quite similar to the flats produced under the other conditions discussed above. As clean surfaces react at 0.16 m s-l and low pressure, a similar reaction is to be expected at 11 m s-l and low pressure, perhaps assisted by the higher temperatures. We therefore identify this wear also as graphitization induced by clean surfaces.
Fig. 18. Micrograph of the wear flat on tool T25R after turning at 11 m s-l in 0.1 Ton of air.
4.5. Speed dependence of the wear at low pressure A characteristic feature of the wear rates observed in air at low pressure (0.1 Ton) calls for further comment. Figure 19 gives the results of five experiments in which the wear in 0.1 Torr was measured as a function of cutting speed, the results being normalized to the same value at low speed,
182 A
0.1 torr
WEAR RATE
0
’ 0.1
I 1
l ;. nl s-l
SURFACE SPEED
Fig. 19. The wear rate of five tools cutting in 0.1 Torr of air as a function of cutting speed. The rates are normalized to the same value at the lowest speed.
and we see that the wear decreases steadily with increasing cutting speed. It is not uncommon for a wear rate to decrease with increasing speed, owing, for example, to softening of the metal (e.g. ref. 26) or to a thermally induced transition in the metal (e.g. ref. 27). However, in the present experiments the main wear process is neither abrasive nor adhesive in the usual sense, so such explanations are not applicable. When turning in air at 760 Torr a change of speed may affect the entry of air but at speeds around 0.2 m s-l at 0.1 Torr the position is much simpler. Temperature effects must be negligible. Changes in the access of air to the wear flat cannot be responsible for the observed dependence of wear on speed, for an increased speed would reduce the entry of air, thus giving a cleaner surface and more wear. It might be that an increase of cutting speed makes a significant reduction in the time available for a reaction to occur. However, at a speed of 10 m s-l a steel and a carbon atom will be in close proximity for the order of lo-’ s, which should be ample time to allow the electron rearrangement involved in the reaction and to dissipate any excess energy released. (The thermal relaxation time involved is of the order of a period of the atomic vibrations in the lattice, i.e. lo-l2 s.) The electrons in a freshly machined surface will take a finite time to reach their new equilibrium configuration but this is the dielectric relaxation time which is of the order of 10-l’ s. In contrast, the height of the wear flat is of the order of 0.1 mm and so at a speed of 0.1 m s-l the freshly cut metal does not arrive at the lower part of the worn surface till after 1 ms. Hence, the wear cannot be due to a nonequilibrium arrangement of the metallic bonding. The position is still not clear but the most likely explanation for the decrease of wear with speed is as follows. The surface of a polished diamond
183
has an unusual topography with a rough and jagged microstructure of the order of 5 nm high and with a linear scale of about the same order [Z&29]. The surface is not flat but is delineated, at least approximately, by (111) cleavage planes steeply inclined to the surface, and the rate of the reaction will depend on the real area of contact between the steel and the diamond. As a diamond surface is loaded against a steel surface under static conditions, plastic flow occurs until a sufficient area of steel is in contact with the asperities on the diamond to support the load without exceeding the yield stress of the metal. This will take a small but finite time, so the effective area of contact during machining may fall with rising speed. It is difficult to estimate the likely magnitude of this effect, particularly for a metal such as steel where delayed yielding makes the position still more complicated. However, even at a speed as low as 0.1 m s-l the metal remains in contact with one particular asperity for the order of only 5 ns, while the delays for yielding are commonly much longer, see for example refs. 30 and 31. Recent experiments by Maan and Broese van Groenou [32] underline the importance of time effects during plastic flow. These authors observed the scratching of mild steel by a diamond stylus under a constant load driven at various constant low speeds. As the scratching speed was raised from 0.3 vrn s-l to 0.3 mm s-l the area of the cross section of the groove decreased by a factor of about 3, despite the very low magnitude of all the speeds involved.
5. Wear of other materials After completing the work described above, further experiments were made to see whether other examples of high rates of chemical wear could be observed when machining other materials. Tools and workpieces were prepared from materials with a probable affinity for each other, bearing in mind the need for the tool to be considerably harder than the workpiece to avoid abrasive wear. These experiments are still in a preliminary stage and are not as detailed as those discussed above but already they have produced some unexpected results. Rates of wear comparable with that of diamond on mild steel have been observed when diamond tools turn vanadium, titanium, zirconium and cobalt in air at speeds of about 5 m s-l. The wear of the diamond on all these metals is relatively smooth but the behaviour with molybdenum is quite different. After a very short time relatively large pieces of the diamond are tom out of the edge and the tool becomes useless. In addition the surface finish on the turned molybdenum billet is extremely poor and rough and it appears that the metal is being tom rather than cut. At low speeds the interaction of a diamond tool with nickel appears similar to that with molybdenum but at higher speeds the wear is smoother and more similar to that on steel. Another set of experiments was made turning billets of graphite with tools of tungsten. Tungsten is appreciably harder than graphite, so any wear
184
of the tool due to abrasion should be relatively small. This was confirmed by the fact that, when cutting at speeds in the range 0.1 - 10 m s--i, either in 0.1 Torr of air or in 760 Torr of argon, the wear was much less than 1 on our usual scale. However, on increasing the air pressure from 0.1 to 760 Torr, the wear rate increased to a value of about 40. Thus a component of the air is responsible for some chemical reaction at the interface but there is no obvious explanation of the very considerable wear of the tungsten tool in terms of the chemistry of bulk materials. For example, if oxygen is the critical constituent of the air, it might be supposed that it would react with the graphite rather than with the tungsten, yet it is the tungsten which wears. Nor are there any obvious reactions involving other constituents of the air. A third set of measurements was made with billets of sulphur which were turned with copper and nickel tools in air at cutting speeds of about 0.1 m s-l. Solid sulphur exhibits a range of crystalline structure and a billet cast from molten sulphur only takes up its equilibrium constitution after a period of at least 2 d. A billet turned immediately after casting gives a good continuous chip which will impede the flow of air to the freshly cut surface, whereas after ageing the chips are small and fragmented. Therefore the experiments were made on billets shortly after casting and due precautions were taken when comparing tools to use either the same billet or similar billets of similar age. Nickel and copper tools were chosen, as both metals are known to have considerable affinity for sulphur in conventional reactions. They have also a similar hardness appreciably greater than that of sulphur, so one would expect abrasive wear to be relatively negligible. The wear rates of the nickel tools were in fact quite low, appreciably less than 1. In contrast, the wear rates of the copper tools were very high -- of the order of 500!
6. Conclusion The rates of wear of diamond tools machining mild steel are very high over a wide range of machining conditions. The wear is due primarily to graphitization of the diamond brought about by three factors: (i) high local flash temperatures at the interface, (ii) the enhanced activity of the clean surface generated during the machining process and (iii) the catalytic action of the steel and the atmosphere. Although the local flash temperatures play an important role at normal cutting speeds, the experiments at speeds of 0.16 m s-l demonstrate that graphitization can occur even when the temperature rises are negligibly small. Hence, the activity of the clean freshly machined surfaces is sufficient to induce graphitization even at ambient temperatures. We stress that the conditions obtained during machining are different from those encountered in most experiments on surfaces. The present experiments avoid the usual complications which arise when the materials involved are covered with surface films of often unknown constitution different from
185
that of the bulk material, because an absolutely clean surface of machined metal is created by the turning process. The wear flat on the tool is wearing quickly, so a new clean surface is steadily being generated on the tools. These clean surfaces will only be contaminated, if at all, by the surrounding atmosphere, which may be monitored and controlled. The experiments described in Section 2.2 suggest that by reducing the pressure of the gaseous atmosphere surrounding the tool it is possible to obtain almost complete exclusion of the atmosphere from the interacting surfaces. The principal point of interest in the present experiments is that the forms of chemical wear observed during machining are different from what might be expected from a consideration of the chemical and thermodynamic properties of the materials in bulk. Besides the graphitization of diamond at ambient temperature described in detail earlier, three other examples have been given in Section 5. (i) The strong interaction between diamond and molybdenum is markedly different from that between diamond and steel. (ii) Both copper and nickel have a considerable affinity for sulphur, but the rates of wear of the nickel tools turning sulphur are two orders of magnitude less than those of the copper tools. (iii) Tungsten tools turning graphite at low speed in argon or in a low pressure of air show relatively little wear but the presence of an atmosphere of air raises the wear to a high value. As in the case of the diamond-steel reaction, each example illustrates behaviour not readily explicable in conventional chemical terms. Chemical reactions are usually discussed in terms of the thermodynamic functions of the two phases, which are determined by the bond strengths in the bulk solids. However, the wear processes in the present experiments, particularly when the temperature remains virtually ambient, must be essentially surface phenomena. We may begin to analyse this situation by considering the diamond-steel reaction. The diamond is certainly wearing away, so carbon atoms are being removed because of a linkage with the iron (or other) atoms in the steel. At least five types of bonds are involved in the wear process, as shown schematically: 11
2
3455
-Fe-Fe-Fe-Fe-C-C-C-CBonds 1 and 5 are characteristic of the bulk material, 3 is the bond formed across the interface, and bonds 2 and 4 are those linking the uppermost iron and carbon atoms to the next lower layer under the conditions at this particular interface. Any chemical reaction at the interface is controlled by the form and strength of the bonds 2,3 and 4, and these are not fully determined by the behaviour of the bulk material. The wear of diamond turning molybdenum gives an interesting contrast to the wear on steel. With steel the diamond wears steadily and relatively smoothly; this implies that in the schematic arrangement of bonds shown above bond 4 is appreciably weaker than the others. With molybdenum there is again a strong interaction with the diamond but instead of smooth wear fracture occurs in the bulk of the diamond and also tearing in the bulk of the
186
metal as described more fully by Casey and Wilks [33]. bonds schematically as previously : 1
1
2
Setting out the
3455
-MO-MO-MO-MO-C-C-C-Cwe see that the experimental results imply that a.ll five types of bond are stronger than the inherent mechanical strengths of the metal and of the diamond, which of course are determined by structural defects rather than bond strengths. The examples just discussed show that machining experiments under suitably controlled conditions give precise information on the relative strengths of bonds and thus provide a link between observations of chemical wear in working conditions and observations of atomic environments from more conventional surface studies, particularly X-ray photoelectron spectroscopy and allied techniques. We began this work because of our interest in the wear of diamonds. The experiments with other materials were made in the first place to confirm that high chemical activities due to clean surfaces occurred in other machining processes but they also indicate the possibility of further experiments for the better understanding of surface reactions in general, and of the diamond-steel reaction in particular. Both diamond and steel have complex constitutions which are difficult to analyse and control, diamond particularly so. (It is also difficult to identify any transfer of material on an atomic scale from a diamond tool to a steel workpiece ! ) In contrast, the materials used in the other experiments are more readily obtained in a known condition. Experiments with these and similar materials should permit correlations between rates of wear and atomic bonding strengths, e.g. a correlation between the different wear rates of copper and nickel on sulphur and the values of the bond strengths between copper and nickel atoms on sulphur obtained by conventional surface techniques. Experiments are being continued on these lines.
Acknowledgments We wish to thank the Science Research Council and De Beers Industrial Diamond Division for their support of this work and Mr. J. W. Naisby for his assistance with the experiments. We are also grateful for helpful discussions with Dr. T. H. Childs, Professor W. Hirst and Professor M. W. Roberts.
References 1 E. M. Wilks and J. Wilks, in J. E. Field (ed.), The Properties of Diamond, Academic Press, New York, 1979. 2 T. N. Loladze and G. V. Bokuchava, The Wear of Diamonds and Diamond Wheels, Mashinostroeniye, Moscow, 1967. 3 T. Tanaka and N. Ikawa, Ann. CIRP, 19 (1971) 153; Bull. Jpn Sot. Precis. Eng., 7 (4) (1973) 97.
187 4 A. S. Vishnevskii, V. G. Delevi et al., Sint. Almazy, 1 (1973). 5 A. S. Vishnevskii and A. V. Lysenko et al., Sint. Almazy, 4 (1974). 6 A. S. Vishnevskii, A. V. Lysenko, T. D. Ositinskaya and V. G. Delevi, IZV. Akad. Nauk SSR, Neorg. Mater., 11 (9) (1975) 1589. 7 R. Komanduri and M. C. Shaw, Nature (London), 255 (1975) 211. 8 A. E. Gorodetskii, L. L. Builov, V. M. Lukyanovich, R. I. Nazarova and Z. E. Sheshenina, Fiz. Khim. Probl. Krist., 2 (1971) 62. A. E. Gorodetskii, V. M. Lukyanovich, D. V. Fedoseev and L. L. Builov, Smachivayemost i Poverkhnostniye Svoistva Rasplavov i Tverdykh Tel, (1972) 125. 9 A. G. Thornton and J. Wilks, Nature (London), 274 (1978) 792. 10 W. Graham and A. Y. C. Nee, Prod. Eng. (London), 53 (1974) 186. 11 J. A. Williams and D. Tabor, Wear, 43 (1977) 275. 12 G. W. Rowe and E. F. Smart, Proc. Inst. Mech. Eng. (London), Part 30,181 (1966) 48. 13 M. C. Shaw, J. D. Pigott and L. P. Richardson, Trans. ASME, 73 (1951) 45. 14 V. A. Markitanova, Russ. Eng. J., 3 (1965) 71. 15 V. A. Zhilin, Mach. Tool. (USSR), 42 (12) (1971) 41. 16 A. A. Sagarda and 0. V. Khimach, Russ. Eng. J., 53 (6) (1973) 73. 17 J. Panhorst and J. Triemel, Ind. Diam. Rev., (1976) 320. 18 F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, 1st edn., Oxford Univ. Press: Clarendon Press, Oxford, 1950. 19 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. 20 T. Evans and D. H. Sauter, Philos. Mag., 6 (1961) 429. 21 G. Davies and T. Evans, Proc. R. Sot. London, Ser. A, 328 (1972) 413. 22 J. F. Prins, Ind. Diam. Rev., (1971) 364. 23 D. Crompton, W. Hirst and M. G. W. Howse, Proc. R. Sot. London, Ser. A, 333 (1973) 435. 24 L. Grunberg, Proc. Phys. Sot., 66 (1953) 153. 25 P. A. Redhead, J. P. Hobson and E. V. Kornelsen, The Physical Basis of Ultrahigh Vacuum, Chapman and Hall, London, 1968, Chap. 2. 26 W. Hirst and J. K. Lancaster, Proc. R. Sot. London, Ser. A, 259 (1960) 228. 27 J. F. Archard, Wear, 2 (1958) 438. 28 J. Wilks, Nature (London), 243 (1973) 15. 29 A. G. Thornton and J. Wilks, J. Phys. D, 9 (1976) 27. 30 N. P. Suh, Int. J. Mech. Sci., 9 (1967) 415. 31 J. Harding, Acta Metall., 19 (1971) 1177. 32 N. Maan and A. Broese van Groenou, Wear, 42 (1977) 365. 33 M. Casey and J. Wilks, in Koenigsberger and Tobias (eds.), Proc. 16th Int. Conf. on Machine Tool Design and Research, 1975, MacMillan, London, 1976, p. 553.