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The influence of atmospheric composition on the abrasive wear of titanium and Ti-6Al-4V
Wear, 124 (1988)
165
165 - 176
THE INFLUENCE OF ATMOSPHERIC COMPOSITION ABRASIVE WEAR OF TITANIUM AND Ti-6Al-4V* A. P. MERCER?
ON THE
and I. M. HUTCHINGS
University of Cambridge, Department Street, Cambridge CB2 3Q.Z (U.K.)
of Materials Science and Metallurgy, Pembroke
Summary A pin-on-disc apparatus has been used to investigate the wear and friction (sliding force) behaviour of metals on bonded silicon carbide and alumina papers under conditions of controlled atmospheric composition. The wear rates of both commercial-purity titanium and the alloy Ti-G%Al4%V tested in air were found to remain constant with time, in contrast with the behaviour of other metals tested under similar conditions, which exhibited a progressive decrease in wear rate with increasing number of passes along the same track. In an atmosphere of dry inert gas (argon or nitrogen), the wear record for titanium and Ti-6Al-4V divided into two linear sections. In the first period, the wear rate was slightly higher in nitrogen and slightly lower in argon than that observed in dry air, while in the second it was substantially lower in both inert gases than in air. The influence of argon in reducing the wear rate was significantly greater than that of nitrogen. In an atmosphere of pure oxygen, an increase of about 30% was observed over the wear rate in dry air. It is proposed that the concentration of interstitial nitrogen and oxygen in the worn metal surface, which largely determines its mechanical properties, strongly influences both the ductility of the abraded material and the force of adhesion between the metal and the abrasive particles. Parallels are drawn between abrasive wear and machining to illustrate the importance of oxygen at the interface between workpiece and tool surfaces.
1. Introduction It is now recognized that abrasive wear depends in both mechanism and severity on many factors and that early models oversimplify the problem *Paper presented at the International Conference on Wear of Materials, Houston, TX, U.S.A., April 5 - 9,1987. TPresent address: Shell Thornton Research Centre, P.O. Box 1, Chester CH13SH, U.K. 0043-1648/88/$3&O
0 Elsevier Sequoia/Printed
in The Netherlands
166
by assuming that mechanical parameters such as indentation hardness and applied load alone determine material removal rates. Little work has been reported on the effects of atmospheric composition on abrasive wear behaviour, although enough literature exists to indicate the need to explore this area in more detail. The abrasive wear of metals and non-metallic materials by silicon carbide particles under both two-body and three-body conditions has been shown to depend on relative atmospheric humidity [l - 41, levels of which can cover almost the entire range from 0% to 100% in practical applications. The level of oxygen in the atmosphere, however, remains effectively constant at about 20% by volume, and this fact may account in part for the very limited information available on the influence of atmospheric oxygen on abrasive wear. Nevertheless, Duwell and McDonald [ 51 observed dramatic changes in frictional force and depth of cut when an inert gas atmosphere (such as nitrogen) was exchanged for one containing a reactive gas (such as oxygen or chlorine) during the grinding of mild steel with aluminium oxide coated abrasives. They concluded that chlorine and oxygen act as cutting lubricants by forming low shear strength compounds at the abrasiveworkpiece interface, the lack of which causes seizure. More recent work [6 - 91 has revealed that the machining of ferrous metals is strongly affected by the operating environment, and that only a small partial pressure of oxygen is required to restore the tool forces, rate of cut and wear of cutting tips observed in uacuo to those found under normal atmospheric cutting conditions. These results are particularly relevant to the two-body abrasive wear regime in which a micromachining mechanism may be expected to account for a high proportion of the wear debris produced from a metallic surface. The present work forms part of a wider-ranging study of the influence of atmospheric composition on the abrasion of metallic materials under twobody conditions [4]. Its aim was to determine the effect of variations in atmospheric oxygen content on the wear of titanium and the alloy Ti-GAl4V caused by coated abrasives and to characterize the mechanisms responsible.
2. Experimental
method
An instrumented pin-on-disc wear tester was used, as illustrated in Fig. 1. The pin specimen was loaded by gravity against the surface of a horizontal disc, coated with bonded abrasive paper and rotating about a vertical axis. In this design the pin specimen was mounted on the end of a horizontal radial arm, supported by a vertical shaft which was coaxial with the horizontal disc, and free to move downwards along its axis through a linear ball bushing. The torque on the shaft resulting from the sliding force on the specimen was measured from the deflection of a flexible radial reaction beam also mounted on the shaft and instrumented with strain
167
linear displacement transducer
1’50mm
150mm
1SOmm3OOmm
Fig. 1. Schematic diagram of the pin-on-disc apparatus.
gauges. The vertical movement of the shaft due to wear of the specimen was measured by a linear displacement transducer. In this way simultaneous continuous measurements were made of the sliding force on the specimen and of material removal. The two quantities were displayed on a two-channel chart recorder. In order to provide a confirmatory independent measurement of the overall wear, the mass lost by the specimen during the experiment was determined by weighing before and after wear. Figure 2 shows the correspondence between measurements of material loss taken from chart recordings of specimen length and those calculated from the mass loss and the density of the pin material for a number of pure metals and alloys. Agreement is generally within lo%, which demonstrates that the wear measured from the chart record is due to material loss from the specimen pin and that the contribution of abrasive deterioration to this measurement is negligible. The pin and abrasive-covered disc were enclosed in a chamber to provide atmosphere control. Atmospheres other than air were obtained by flushing out the chamber with gas from a cylinder of dry compressed gas (Air Products; argon and nitrogen, 99.999% purity; oxygen, 99.6% purity) at a flow rate of 7.5 dm3 min- ‘. A slight positive pressure was maintained in the chamber during all tests with gases other than air. The metal specimens were cylindrical pins 3 mm in diameter. The experiments reported here were all made under a normal load of 1.96 N
168
-I
0
10
2.0
3.0
4.0
50
wear measured by specimen displacement (mm)
Fig. 2. Comparison between wear measured by specimen displacement in the pin-ondisc apparatus and wear calculated from measurement of mass loss. Lines are shown which represent deviation by 5% and 10% from direct correspondence.
(200 gf); the mean contact pressure on the specimens was therefore 0.28 MPa. Commercial waterproof resin-bonded abrasive papers were used from single batches conforming to ASTM mesh sizes 1000, 600, 320 and 280 for silicon carbide (Tufbak Durite, Norton Abrasive Co.) and 800,600,400 and 240 for aluminium oxide (Tufbak Adalox, Norton Abrasive Co.). Fresh paper was used for each experiment. The rotational speed of the disc was 107 rev min-‘, and the specimen ran at a mean radius of 56 mm on a single track, giving a sliding speed of 0.63 m s-l and a total sliding distance of 2259 m over the standard test duration of 60 min. The metal specimen materials were commercial-purity titanium and the alloy Ti-6Al-4V, of Vickers’ pyramid hardness 200 kgf mm-* and 320 kgf mm-’ respectively. Both were tested in the hot-worked condition. In some tests performed in air the bulk temperature of the specimen was measured by inserting a thermocouple into a hole of 0.5 mm diameter drilled axially through the pin from its upper face to within approximately 1 mm of the wearing surface. The temperature was measured directly by a digital thermometer attached to the thermocouple, and was also displayed on a chart recorder to provide a continuous record throughout the test period.
3. Results In air, both commercial purity titanium and the alloy Ti-6Al-4V exhibited linear wear (i.e. a constant wear rate) throughout the test period
169
on both silicon carbide and aluminium oxide abrasive papers. The coefficient of friction rose rapidly at first to reach a steady value of about 0.6 in all cases. A typical record of wear (i.e. change in specimen length) under these conditions is shown in Fig. 3. The linear wear behaviour appears to be peculiar to titanium and its alloys since a number of other pure metals and alloys have been found to exhibit a steady decrease in wear rate under similar test conditions [4]. When the test atmosphere was changed from dry air to a dry inert gas (argon or nitrogen), the wear record divided into two linear sections after about 10 min of the test time had elapsed; this behaviour is illustrated for nitrogen in Fig, 3. An initial period, during which the wear rate was slightly higher or much lower than that measured in dry air, according to the inert gas used, was followed by a lower wear rate for the remainder of the test, so that the mean wear rate (measured over the entire test) was appreciably reduced in all cases. For example, nitrogen led to a significant decrease in the rate of wear of Ti-6Al-4V by 600 grit aluminium oxide abrasive paper while argon led to a further substantial decrease. The effect of replacing atmospheric oxygen by argon on the abrasive wear of the alloy Ti-6Al-4V is further illustrated by the wear record shown in Fig. 4. In this experiment the environmental chamber was alternately flooded with dry argon and opened to air (at 50% relative humidity) at the intervals indicated in the figure. Because it took time for argon to displace the air from the chamber, the wear rate is seen to fall gradually over a period of about 3 min before stabilizing at the lower value appropriate to a pure argon atmosphere. When air was readmitted to the chamber the wear rate increased abruptly and the friction increased briefly with the wear rate
0
05 i
k
s 10
0
10
20
110
30 Time
50
60
lmin)
Fig. 3. Wear records for typical tests in which the titanium alloy was worn by 600 grit alumina paper ln air and nitrogen. In each case the coefficient of friction remained roughly constant over the whole period of the test.
170
09 08 507 5 t 06 E $ 05 E
04
g
03
%_A02 01
0
10
20
30
40
50
60
Time IminI
Fig. 4. Wear record for an interrupted test on Ti-6Al-4V worn by 600 grit alumina. Dry argon was admitted to the air in the environmental chamber at the start of the test, gradually replacing it; the wear rate and friction force consequently decreased with the reduction in oxygen concentration Both wear rate and friction increased abruptly when the chamber was opened to air (after 12 - 13 min), and the sequence was then repeated reproducibly at intervals.
before returning to a steady value. This result was reproducible several times within the standard test time of 60 min and demonstrates clearly the influence of oxygen on the abrasion of the titanium alloy. Subsidiary experiments in which the relative humidity of the air was varied showed that humidity itself had an insignificant effect on the wear rate and friction force, in comparison with the difference between air and argon atmospheres. The pyrophoric nature of titanium particles (particularly in fine powder form such as abrasive wear debris) makes abrasion testing in pure oxygen somewhat hazardous. One such test was conducted, and sufficient time elapsed before the debris ignited for a measure of the wear rate to be taken from the chart recorder trace. This experiment was not repeated. The results of experiments in pure oxygen, air, nitrogen and argon are summarized in Fig. 5 for the wear of Ti-6Al-4V by 600 grit aluminium oxide abrasive papers and are representative of the results for all the combinations of alloy and abrasives studied in this work. X-ray diffractometry revealed no significant variation in the composition of the debris formed by the wear of the alloy by either silicon carbide or aluminium oxide abrasive papers under a variety of controlled atmospheres. All debris samples were predominantly metallic cu-Ti, with small traces of abrasive material. In no case was the bulk specimen temperature measured during tests under ambient conditions found to be greater than 10 K above room temperature. Scanning electron micrographs of Ti-6Al-4V surfaces after abrasion in air, dry argon and dry nitrogen are presented in Fig. 6. Unlike the uniform parallel scratches produced in air (Fig. 6(a)), the main features of the surface
171
600 qnt AI+&
I
50
7s d 40 I i?! 30 e L 20 E 3 10 n ”
02 Fig. The first with
Air
N2
Ar
5. Wear rates for Ti-6Al-4V worn against 600 grit alumina papers in different gases. initial wear rate, where different from the steady state wear rate, persisted for the 5 - 10 min of the test (see, for example, Fig. 3). Very similar results were obtained different grit sizes, with silicon carbide abrasive, and with gases at different relative
Fig. 6. Scanning electron micrographs of the surfaces of Ti-6Al-4V specimens worn against 600 grit alumina: (a) in air at 50% relative humidity, (b) in dry argon, (c) in dry nitrogen.
abraded in dry argon (Fig. 6(b)) are the dark regions elongated in the direction of sliding. Close examination reveals these to be sheets or rafts of metallic material smeared over and parallel to underlying abrasion
172
grooves. The alloy surface abraded in dry nitrogen (Fig. 6(c)) exhibits rafts of smeared metal similar to those formed in argon but individually smaller and more widely spaced. So extensive is this raft formation after abrasion in dry argon that the underlying scratches are totally obscured in many places, and successive layers of rafts are observed, all of which exhibit long scars parallel to the direction of sliding. While some of these scars are clearly abrasive grooves, many appear to be due to simple sliding between metallic contacts. Examination of the corresponding abrasive paper surface reveals that this is indeed a possible cause of such marks, since frequent examples are found of abrasive particles which have suffered severe capping: blunting of the cutting tip by adherent metallic debris which is analogous to the “built-up edge” often formed on the tool during machining. In many cases the extent of the metallic caps is not revealed in the scanning electron microscope by secondary electron imaging but is shown clearly by the atomic number contrast provided by backscattered or primary electron images (Fig. 7). Many fine debris particles are also visible in Fig. 7, comprising both abrasive and workpiece material. Similar debris is seen on the alloy surfaces abraded in dry argon, and further examination of the used abrasive paper reveals large areas which have been almost denuded of abrasive particles, from which some fragments of abrasive material may have been transferred to the worn metal surface.
(a)
(b)
Fig. 7. Scanning electron micrographs of grit particles and debris on 600 grit alumina paper after abrasion of Ti-6Al-4V for 60 min in dry argon: (a) secondary electron image showing severe capping of the abrasive particle; (b) corresponding backscattered image, showing the extent of the metallic cap (lighter contrast).
4. Discussion The abrasion of titanium and the alloy Ti-6Al-4V responds to changes in atmospheric composition in a manner which is readily categorized. A change in the composition of the test atmosphere from 80% nitrogen-20% oxygen in air to 100% nitrogen reduces the steady state wear rate by about 25% (Fig. 5). An argon atmosphere reduces the steady state wear rate
173
further, by over 70%, while pure oxygen increases the rate over that obtained in dry air by nearly 30%. The fact that these results are qualitatively similar for both types of abrasive suggests that the mechanism responsible varies only in degree and not in nature. Further evidence to support this proposal is given by the marked change in slope which divides the wear record into two linear sections under nitrogen and argon atmospheres. The point at which the change in slope occurs corresponds to the removal of material from the pin surface to a depth of between 150 and 350 pm depending on the abrasive particle size; for 600 grit aluminium oxide and silicon carbide, 150 - 250 pm of material is removed prior to the change in slope. The wear of the alloy by 600 grit aluminium oxide abrasive paper is taken as a representative abrasive-workpiece combination and the mechanism responsible for this behaviour will be discussed in detail. Abrasion of the titanium alloy by 600 grit aluminium oxide paper in dry argon begins with the formation of typical abrasion scratches or grooves identical to those produced during abrasion in air until approximately 200 pm has been removed from the metal surface. At this point metallic debris, instead of leaving the worn surface as discrete machining chips or fine particles, accumulates on the surface in severely deformed sheets or rafts and the wear rate drops markedly. The appearance of these rafts (Fig. 6(b)) suggests that they form by an extension of the capping process, in which large numbers of otherwise discrete debris particles amalgamate under intense shear deformation to produce a mass of material. The nature of the subsequent metal-to-metal contact is not obvious, although McQuillan and McQuillan [lo] have shown that titanium surfaces will weld to each other (or to other metal surfaces) whether they are prepared and tested in air or in an inert gas. Furthermore, they found the static coefficient of friction of titanium on itself to be 0.65 - 0.68; the dynamic coefficient of friction was not quoted but is likely to be marginally lower than this. A value of approximately 0.6 was observed for the sliding of all combinations of titanium alloy and abrasive in the present work. This suggests that metal-to-metal contact occurs during abrasion of the alloy in air and in inert gas, albeit over a larger fraction of the nominal abrasiveworkpiece contact area in the latter, as is shown by the higher frequency of capping observed by scanning electron microscopy. The complex oxide film which forms on titanium surfaces in air [ll, 121, although chemically protective, is unlikely to play a direct protective role in the abrasion process. This is principally because it is only 2 - 5 nm thick [13] and is thus some three to four orders of magnitude smaller than the typical abrasive scratch penetration observed in this work. However, it is clear that the formation of oxide is important indirectly, since it facilitates the removal of metallic debris from the abrasion process by reducing capping and debris adhesion. In this respect the abrasive wear rate is reduced by the removal of atmospheric oxygen. The increase in wear rate observed in a pure oxygen atmosphere is due at least in part to the more widespread oxidation of debris and the subsequent reduction in abrasive capping. Similar conclusions were
reached by Duwell et al. [14], who found that abrasive-covered belts used for grinding titanium showed rather more adhesion of metallic debris when oxygen was displaced by argon gas. If the presence or absence of oxygen were the only important factor in the formation of the debris rafts described above, identical results would be expected during abrasion of the titanium alloy under both argon and nitrogen atmospheres. However, Fig. 5 reveals that the steady state wear rate in nitrogen is three times that observed in dry argon; oxygen alone cannot be responsible for the difference in wear behaviour observed between abrasion in air and in the inert gases. Examination of Fig. 6 reveals that the difference in mean wear rates obtained in argon and in nitrogen is associated with the different extents to which raft formation occurs. In view of the fact that titanium has a high affinity for relatively small atoms such as carbon, oxygen and nitrogen, which form interstitial solid solutions in small quantities and carbides, oxides and nitrides in larger concentrations [lo], it is possible that the surface composition of the alloy is modified during abrasion in an inert atmosphere. This may occur by diffusion of interstitial species out of the high concentration in the metal to the lower concentration in the atmosphere, or by diffusion of interstitials into the subsurface metal driven by elevated temperatures at the abrasive-workpiece interface. Indeed, a number of such processes may occur simultaneously, although the net result appears from comparison of the alloy surfaces abraded under argon and under nitrogen to be a marked increase in the ductility of the metallic debris, which is more pronounced in argon than in nitrogen, and in its ability to form strongly adherent bonds. Both an increase in ductility and a lack of oxide are consistent with the removal of interstitial oxygen from the metal surface, and the less evident ductility produced by abrasion in nitrogen may be because a significant concentration of that element remains in solution in the metal. Support for this contention is given by elemental analyses of the abraded alloy surfaces by laser-microprobe mass spectrometry, which revealed constant concentrations of the interstitial elements carbon (0.05% - 0.5%), oxygen (0.1% - 1%) and nitrogen (0.025% 0.25%) down to a depth beneath the surface of approximately 10 pm after abrasion in air. While failure to detect these elements at greater depth may be due to the limitations of the analytical technique and not necessarily to the absence of any appreciable concentration, some estimate of concentrations down to approximately 10 pm can be made. In contrast to the results obtained after abrasion in air, the concentration profile of these interstitial elements below surfaces abraded in dry argon showed a minimum within a few micrometres of the surface. This is consistent with the diffusion of oxygen and nitrogen from atmospheric air back into the specimen surface, after testing, from which they diffused during abrasion. The removal of approximately 200 pm from the alloy surface appears to be necessary before the surface properties change sufficiently for raft formation to begin. This amount of material is sufficient to cover the abrasive track to a depth of ca. 1.3 I.tm. It is possible that the transition
175
occurs when a critical amount of debris has been transferred to the abrasive paper and is due to partial clogging of the abrasive. This suggestion is supported by the observation that the depth of material removed from the surface at the transition is greater for coarser abrasive particles [ 41. 5. Conclusions The following conclusions may be drawn from experiments conducted on the two-body wear of commercial-purity titanium and the alloy Ti-GAl4V by silicon carbide and aluminium oxide abrasive papers with nominal particle sizes of less than 50 pm. (1) Commercial purity titanium and the alloy Ti-6Al-4V exhibit linear wear behaviour during repeated sliding over the same track during two-body abrasive wear in air. (2) During abrasion in an inert gas atmosphere, the wear record divides into two linear sections. In the initial period, the wear rate of the alloy Ti-GAl-4V, compared with that obtained in dry air, is slightly higher in dry nitrogen and slightly lower in dry argon; in the steady state it is substantially lower in both cases. The mean rate of wear of the alloy in an inert gas atmosphere is therefore lower (by up to 70%) than that obtained in air. Abrasion in pure oxygen leads to eventual ignition of the fine metallic debris, with an increase in wear rate of about 30% over that measured in dry air. (3) Abrasion under an inert gas atmosphere possibly leads to modification of the surface alloy composition by removal of interstitial elements (primarily oxygen and nitrogen) and thereby increases surface ductility. Metallic debris adheres to the alloy surface in the absence of a protective oxide film and accumulates in extensive, heavily deformed sheets which reduce the wear rate. Qualitatively similar effects are observed in all abrasive systems, although the magnitude of the reduction in wear rate is greater in argon than in nitrogen. (4) A coefficient of friction of about 0.6 is observed during the abrasion of the alloy Ti-6Al-4V under all gases tested. This value is characteristic of the sliding of titanium on itself and confirms observations made by scanning electron microscopy of used abrasive papers that much of the normal load is borne by abrasive particles capped by metallic debris. In inert gas atmospheres, the nature of such contacts is essentially the same as that in air but the frequency is greater.
References 1 J. F. Carroll and R. C. Gotham, The measurement of abrasiveness of magnetic tape, IEEE Tmns. Magn., 2 (1966) 6 - 13. 2 J. Larsen-Basse, Effect of atmospheric humidity on abrasive wear, Proc. ht. Conf. on Mechanical Behauiour of Materials, Kyoto, 1971, Vol. 3, 1972,353 - 362.
176 3 J. Larsen-Basse and S. S. Sokoloski, Influence of atmospheric humidity on abrasive wear. II. Two-body abrasion. Wear, 32 (1975) 9 - 14. 4 A. P. Mercer, Ph. D. Dissertation, University of Cambridge, (1985). 5 E. J. Duwell and W. J. McDonald, The effect of reactive gases on the dry grinding of steel with aluminium oxide coated abrasive, Wear, 4 (1961) 384 - 386. 6 J. A. Williams and W. M. Stobbs, Changes in mode of chip formation as function of presence of oxygen, Met. Technol., 6 (1979) 424 - 432. 7 P. K. Wright, J. G. Horne and D. Tabor, Boundary conditions at the chip-tool interface in machining, Wear, 54 (1979) 371. 8 P. K. Wright, Frictional interactions in machining, Met. TechnoE., 8 (1981) 150. 9 T. H. C. Childs and A. B. Smith, Effects of atmosphere on flank wear of HSS tools used to turn medium carbon steels in BUE conditions, Met. Technol., 9 (1982) 292. 10 A. D. McQuillan and M. K. McQuillan, Titanium, Butterworths, London, 1956. 11 H. P. Godard, W. B. Jepson, M. R. Bothwell and R. L. Kane, The Corrosion ofLight Met&, Wiley, New York, 1967. 12 A. E. Jenkins, The oxidation of titanium at high temperatures in an atmosphere of pure oxygen, J. Inst. Met., 82 (1953 - 1954) 213 - 221. 13 V. V. Andreeva, Corrosion, 20 (1964) 35. 14 E. J. Duwell, I. S. Hong and W. J. McDonald, The effect of oxygen and water on the dynamics of chip formation during grinding, ASLE Trans., 12 (1969) 86 - 93.
165
165 - 176
THE INFLUENCE OF ATMOSPHERIC COMPOSITION ABRASIVE WEAR OF TITANIUM AND Ti-6Al-4V* A. P. MERCER?
ON THE
and I. M. HUTCHINGS
University of Cambridge, Department Street, Cambridge CB2 3Q.Z (U.K.)
of Materials Science and Metallurgy, Pembroke
Summary A pin-on-disc apparatus has been used to investigate the wear and friction (sliding force) behaviour of metals on bonded silicon carbide and alumina papers under conditions of controlled atmospheric composition. The wear rates of both commercial-purity titanium and the alloy Ti-G%Al4%V tested in air were found to remain constant with time, in contrast with the behaviour of other metals tested under similar conditions, which exhibited a progressive decrease in wear rate with increasing number of passes along the same track. In an atmosphere of dry inert gas (argon or nitrogen), the wear record for titanium and Ti-6Al-4V divided into two linear sections. In the first period, the wear rate was slightly higher in nitrogen and slightly lower in argon than that observed in dry air, while in the second it was substantially lower in both inert gases than in air. The influence of argon in reducing the wear rate was significantly greater than that of nitrogen. In an atmosphere of pure oxygen, an increase of about 30% was observed over the wear rate in dry air. It is proposed that the concentration of interstitial nitrogen and oxygen in the worn metal surface, which largely determines its mechanical properties, strongly influences both the ductility of the abraded material and the force of adhesion between the metal and the abrasive particles. Parallels are drawn between abrasive wear and machining to illustrate the importance of oxygen at the interface between workpiece and tool surfaces.
1. Introduction It is now recognized that abrasive wear depends in both mechanism and severity on many factors and that early models oversimplify the problem *Paper presented at the International Conference on Wear of Materials, Houston, TX, U.S.A., April 5 - 9,1987. TPresent address: Shell Thornton Research Centre, P.O. Box 1, Chester CH13SH, U.K. 0043-1648/88/$3&O
0 Elsevier Sequoia/Printed
in The Netherlands
166
by assuming that mechanical parameters such as indentation hardness and applied load alone determine material removal rates. Little work has been reported on the effects of atmospheric composition on abrasive wear behaviour, although enough literature exists to indicate the need to explore this area in more detail. The abrasive wear of metals and non-metallic materials by silicon carbide particles under both two-body and three-body conditions has been shown to depend on relative atmospheric humidity [l - 41, levels of which can cover almost the entire range from 0% to 100% in practical applications. The level of oxygen in the atmosphere, however, remains effectively constant at about 20% by volume, and this fact may account in part for the very limited information available on the influence of atmospheric oxygen on abrasive wear. Nevertheless, Duwell and McDonald [ 51 observed dramatic changes in frictional force and depth of cut when an inert gas atmosphere (such as nitrogen) was exchanged for one containing a reactive gas (such as oxygen or chlorine) during the grinding of mild steel with aluminium oxide coated abrasives. They concluded that chlorine and oxygen act as cutting lubricants by forming low shear strength compounds at the abrasiveworkpiece interface, the lack of which causes seizure. More recent work [6 - 91 has revealed that the machining of ferrous metals is strongly affected by the operating environment, and that only a small partial pressure of oxygen is required to restore the tool forces, rate of cut and wear of cutting tips observed in uacuo to those found under normal atmospheric cutting conditions. These results are particularly relevant to the two-body abrasive wear regime in which a micromachining mechanism may be expected to account for a high proportion of the wear debris produced from a metallic surface. The present work forms part of a wider-ranging study of the influence of atmospheric composition on the abrasion of metallic materials under twobody conditions [4]. Its aim was to determine the effect of variations in atmospheric oxygen content on the wear of titanium and the alloy Ti-GAl4V caused by coated abrasives and to characterize the mechanisms responsible.
2. Experimental
method
An instrumented pin-on-disc wear tester was used, as illustrated in Fig. 1. The pin specimen was loaded by gravity against the surface of a horizontal disc, coated with bonded abrasive paper and rotating about a vertical axis. In this design the pin specimen was mounted on the end of a horizontal radial arm, supported by a vertical shaft which was coaxial with the horizontal disc, and free to move downwards along its axis through a linear ball bushing. The torque on the shaft resulting from the sliding force on the specimen was measured from the deflection of a flexible radial reaction beam also mounted on the shaft and instrumented with strain
167
linear displacement transducer
1’50mm
150mm
1SOmm3OOmm
Fig. 1. Schematic diagram of the pin-on-disc apparatus.
gauges. The vertical movement of the shaft due to wear of the specimen was measured by a linear displacement transducer. In this way simultaneous continuous measurements were made of the sliding force on the specimen and of material removal. The two quantities were displayed on a two-channel chart recorder. In order to provide a confirmatory independent measurement of the overall wear, the mass lost by the specimen during the experiment was determined by weighing before and after wear. Figure 2 shows the correspondence between measurements of material loss taken from chart recordings of specimen length and those calculated from the mass loss and the density of the pin material for a number of pure metals and alloys. Agreement is generally within lo%, which demonstrates that the wear measured from the chart record is due to material loss from the specimen pin and that the contribution of abrasive deterioration to this measurement is negligible. The pin and abrasive-covered disc were enclosed in a chamber to provide atmosphere control. Atmospheres other than air were obtained by flushing out the chamber with gas from a cylinder of dry compressed gas (Air Products; argon and nitrogen, 99.999% purity; oxygen, 99.6% purity) at a flow rate of 7.5 dm3 min- ‘. A slight positive pressure was maintained in the chamber during all tests with gases other than air. The metal specimens were cylindrical pins 3 mm in diameter. The experiments reported here were all made under a normal load of 1.96 N
168
-I
0
10
2.0
3.0
4.0
50
wear measured by specimen displacement (mm)
Fig. 2. Comparison between wear measured by specimen displacement in the pin-ondisc apparatus and wear calculated from measurement of mass loss. Lines are shown which represent deviation by 5% and 10% from direct correspondence.
(200 gf); the mean contact pressure on the specimens was therefore 0.28 MPa. Commercial waterproof resin-bonded abrasive papers were used from single batches conforming to ASTM mesh sizes 1000, 600, 320 and 280 for silicon carbide (Tufbak Durite, Norton Abrasive Co.) and 800,600,400 and 240 for aluminium oxide (Tufbak Adalox, Norton Abrasive Co.). Fresh paper was used for each experiment. The rotational speed of the disc was 107 rev min-‘, and the specimen ran at a mean radius of 56 mm on a single track, giving a sliding speed of 0.63 m s-l and a total sliding distance of 2259 m over the standard test duration of 60 min. The metal specimen materials were commercial-purity titanium and the alloy Ti-6Al-4V, of Vickers’ pyramid hardness 200 kgf mm-* and 320 kgf mm-’ respectively. Both were tested in the hot-worked condition. In some tests performed in air the bulk temperature of the specimen was measured by inserting a thermocouple into a hole of 0.5 mm diameter drilled axially through the pin from its upper face to within approximately 1 mm of the wearing surface. The temperature was measured directly by a digital thermometer attached to the thermocouple, and was also displayed on a chart recorder to provide a continuous record throughout the test period.
3. Results In air, both commercial purity titanium and the alloy Ti-6Al-4V exhibited linear wear (i.e. a constant wear rate) throughout the test period
169
on both silicon carbide and aluminium oxide abrasive papers. The coefficient of friction rose rapidly at first to reach a steady value of about 0.6 in all cases. A typical record of wear (i.e. change in specimen length) under these conditions is shown in Fig. 3. The linear wear behaviour appears to be peculiar to titanium and its alloys since a number of other pure metals and alloys have been found to exhibit a steady decrease in wear rate under similar test conditions [4]. When the test atmosphere was changed from dry air to a dry inert gas (argon or nitrogen), the wear record divided into two linear sections after about 10 min of the test time had elapsed; this behaviour is illustrated for nitrogen in Fig, 3. An initial period, during which the wear rate was slightly higher or much lower than that measured in dry air, according to the inert gas used, was followed by a lower wear rate for the remainder of the test, so that the mean wear rate (measured over the entire test) was appreciably reduced in all cases. For example, nitrogen led to a significant decrease in the rate of wear of Ti-6Al-4V by 600 grit aluminium oxide abrasive paper while argon led to a further substantial decrease. The effect of replacing atmospheric oxygen by argon on the abrasive wear of the alloy Ti-6Al-4V is further illustrated by the wear record shown in Fig. 4. In this experiment the environmental chamber was alternately flooded with dry argon and opened to air (at 50% relative humidity) at the intervals indicated in the figure. Because it took time for argon to displace the air from the chamber, the wear rate is seen to fall gradually over a period of about 3 min before stabilizing at the lower value appropriate to a pure argon atmosphere. When air was readmitted to the chamber the wear rate increased abruptly and the friction increased briefly with the wear rate
0
05 i
k
s 10
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lmin)
Fig. 3. Wear records for typical tests in which the titanium alloy was worn by 600 grit alumina paper ln air and nitrogen. In each case the coefficient of friction remained roughly constant over the whole period of the test.
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09 08 507 5 t 06 E $ 05 E
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Time IminI
Fig. 4. Wear record for an interrupted test on Ti-6Al-4V worn by 600 grit alumina. Dry argon was admitted to the air in the environmental chamber at the start of the test, gradually replacing it; the wear rate and friction force consequently decreased with the reduction in oxygen concentration Both wear rate and friction increased abruptly when the chamber was opened to air (after 12 - 13 min), and the sequence was then repeated reproducibly at intervals.
before returning to a steady value. This result was reproducible several times within the standard test time of 60 min and demonstrates clearly the influence of oxygen on the abrasion of the titanium alloy. Subsidiary experiments in which the relative humidity of the air was varied showed that humidity itself had an insignificant effect on the wear rate and friction force, in comparison with the difference between air and argon atmospheres. The pyrophoric nature of titanium particles (particularly in fine powder form such as abrasive wear debris) makes abrasion testing in pure oxygen somewhat hazardous. One such test was conducted, and sufficient time elapsed before the debris ignited for a measure of the wear rate to be taken from the chart recorder trace. This experiment was not repeated. The results of experiments in pure oxygen, air, nitrogen and argon are summarized in Fig. 5 for the wear of Ti-6Al-4V by 600 grit aluminium oxide abrasive papers and are representative of the results for all the combinations of alloy and abrasives studied in this work. X-ray diffractometry revealed no significant variation in the composition of the debris formed by the wear of the alloy by either silicon carbide or aluminium oxide abrasive papers under a variety of controlled atmospheres. All debris samples were predominantly metallic cu-Ti, with small traces of abrasive material. In no case was the bulk specimen temperature measured during tests under ambient conditions found to be greater than 10 K above room temperature. Scanning electron micrographs of Ti-6Al-4V surfaces after abrasion in air, dry argon and dry nitrogen are presented in Fig. 6. Unlike the uniform parallel scratches produced in air (Fig. 6(a)), the main features of the surface
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600 qnt AI+&
I
50
7s d 40 I i?! 30 e L 20 E 3 10 n ”
02 Fig. The first with
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Ar
5. Wear rates for Ti-6Al-4V worn against 600 grit alumina papers in different gases. initial wear rate, where different from the steady state wear rate, persisted for the 5 - 10 min of the test (see, for example, Fig. 3). Very similar results were obtained different grit sizes, with silicon carbide abrasive, and with gases at different relative
Fig. 6. Scanning electron micrographs of the surfaces of Ti-6Al-4V specimens worn against 600 grit alumina: (a) in air at 50% relative humidity, (b) in dry argon, (c) in dry nitrogen.
abraded in dry argon (Fig. 6(b)) are the dark regions elongated in the direction of sliding. Close examination reveals these to be sheets or rafts of metallic material smeared over and parallel to underlying abrasion
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grooves. The alloy surface abraded in dry nitrogen (Fig. 6(c)) exhibits rafts of smeared metal similar to those formed in argon but individually smaller and more widely spaced. So extensive is this raft formation after abrasion in dry argon that the underlying scratches are totally obscured in many places, and successive layers of rafts are observed, all of which exhibit long scars parallel to the direction of sliding. While some of these scars are clearly abrasive grooves, many appear to be due to simple sliding between metallic contacts. Examination of the corresponding abrasive paper surface reveals that this is indeed a possible cause of such marks, since frequent examples are found of abrasive particles which have suffered severe capping: blunting of the cutting tip by adherent metallic debris which is analogous to the “built-up edge” often formed on the tool during machining. In many cases the extent of the metallic caps is not revealed in the scanning electron microscope by secondary electron imaging but is shown clearly by the atomic number contrast provided by backscattered or primary electron images (Fig. 7). Many fine debris particles are also visible in Fig. 7, comprising both abrasive and workpiece material. Similar debris is seen on the alloy surfaces abraded in dry argon, and further examination of the used abrasive paper reveals large areas which have been almost denuded of abrasive particles, from which some fragments of abrasive material may have been transferred to the worn metal surface.
(a)
(b)
Fig. 7. Scanning electron micrographs of grit particles and debris on 600 grit alumina paper after abrasion of Ti-6Al-4V for 60 min in dry argon: (a) secondary electron image showing severe capping of the abrasive particle; (b) corresponding backscattered image, showing the extent of the metallic cap (lighter contrast).
4. Discussion The abrasion of titanium and the alloy Ti-6Al-4V responds to changes in atmospheric composition in a manner which is readily categorized. A change in the composition of the test atmosphere from 80% nitrogen-20% oxygen in air to 100% nitrogen reduces the steady state wear rate by about 25% (Fig. 5). An argon atmosphere reduces the steady state wear rate
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further, by over 70%, while pure oxygen increases the rate over that obtained in dry air by nearly 30%. The fact that these results are qualitatively similar for both types of abrasive suggests that the mechanism responsible varies only in degree and not in nature. Further evidence to support this proposal is given by the marked change in slope which divides the wear record into two linear sections under nitrogen and argon atmospheres. The point at which the change in slope occurs corresponds to the removal of material from the pin surface to a depth of between 150 and 350 pm depending on the abrasive particle size; for 600 grit aluminium oxide and silicon carbide, 150 - 250 pm of material is removed prior to the change in slope. The wear of the alloy by 600 grit aluminium oxide abrasive paper is taken as a representative abrasive-workpiece combination and the mechanism responsible for this behaviour will be discussed in detail. Abrasion of the titanium alloy by 600 grit aluminium oxide paper in dry argon begins with the formation of typical abrasion scratches or grooves identical to those produced during abrasion in air until approximately 200 pm has been removed from the metal surface. At this point metallic debris, instead of leaving the worn surface as discrete machining chips or fine particles, accumulates on the surface in severely deformed sheets or rafts and the wear rate drops markedly. The appearance of these rafts (Fig. 6(b)) suggests that they form by an extension of the capping process, in which large numbers of otherwise discrete debris particles amalgamate under intense shear deformation to produce a mass of material. The nature of the subsequent metal-to-metal contact is not obvious, although McQuillan and McQuillan [lo] have shown that titanium surfaces will weld to each other (or to other metal surfaces) whether they are prepared and tested in air or in an inert gas. Furthermore, they found the static coefficient of friction of titanium on itself to be 0.65 - 0.68; the dynamic coefficient of friction was not quoted but is likely to be marginally lower than this. A value of approximately 0.6 was observed for the sliding of all combinations of titanium alloy and abrasive in the present work. This suggests that metal-to-metal contact occurs during abrasion of the alloy in air and in inert gas, albeit over a larger fraction of the nominal abrasiveworkpiece contact area in the latter, as is shown by the higher frequency of capping observed by scanning electron microscopy. The complex oxide film which forms on titanium surfaces in air [ll, 121, although chemically protective, is unlikely to play a direct protective role in the abrasion process. This is principally because it is only 2 - 5 nm thick [13] and is thus some three to four orders of magnitude smaller than the typical abrasive scratch penetration observed in this work. However, it is clear that the formation of oxide is important indirectly, since it facilitates the removal of metallic debris from the abrasion process by reducing capping and debris adhesion. In this respect the abrasive wear rate is reduced by the removal of atmospheric oxygen. The increase in wear rate observed in a pure oxygen atmosphere is due at least in part to the more widespread oxidation of debris and the subsequent reduction in abrasive capping. Similar conclusions were
reached by Duwell et al. [14], who found that abrasive-covered belts used for grinding titanium showed rather more adhesion of metallic debris when oxygen was displaced by argon gas. If the presence or absence of oxygen were the only important factor in the formation of the debris rafts described above, identical results would be expected during abrasion of the titanium alloy under both argon and nitrogen atmospheres. However, Fig. 5 reveals that the steady state wear rate in nitrogen is three times that observed in dry argon; oxygen alone cannot be responsible for the difference in wear behaviour observed between abrasion in air and in the inert gases. Examination of Fig. 6 reveals that the difference in mean wear rates obtained in argon and in nitrogen is associated with the different extents to which raft formation occurs. In view of the fact that titanium has a high affinity for relatively small atoms such as carbon, oxygen and nitrogen, which form interstitial solid solutions in small quantities and carbides, oxides and nitrides in larger concentrations [lo], it is possible that the surface composition of the alloy is modified during abrasion in an inert atmosphere. This may occur by diffusion of interstitial species out of the high concentration in the metal to the lower concentration in the atmosphere, or by diffusion of interstitials into the subsurface metal driven by elevated temperatures at the abrasive-workpiece interface. Indeed, a number of such processes may occur simultaneously, although the net result appears from comparison of the alloy surfaces abraded under argon and under nitrogen to be a marked increase in the ductility of the metallic debris, which is more pronounced in argon than in nitrogen, and in its ability to form strongly adherent bonds. Both an increase in ductility and a lack of oxide are consistent with the removal of interstitial oxygen from the metal surface, and the less evident ductility produced by abrasion in nitrogen may be because a significant concentration of that element remains in solution in the metal. Support for this contention is given by elemental analyses of the abraded alloy surfaces by laser-microprobe mass spectrometry, which revealed constant concentrations of the interstitial elements carbon (0.05% - 0.5%), oxygen (0.1% - 1%) and nitrogen (0.025% 0.25%) down to a depth beneath the surface of approximately 10 pm after abrasion in air. While failure to detect these elements at greater depth may be due to the limitations of the analytical technique and not necessarily to the absence of any appreciable concentration, some estimate of concentrations down to approximately 10 pm can be made. In contrast to the results obtained after abrasion in air, the concentration profile of these interstitial elements below surfaces abraded in dry argon showed a minimum within a few micrometres of the surface. This is consistent with the diffusion of oxygen and nitrogen from atmospheric air back into the specimen surface, after testing, from which they diffused during abrasion. The removal of approximately 200 pm from the alloy surface appears to be necessary before the surface properties change sufficiently for raft formation to begin. This amount of material is sufficient to cover the abrasive track to a depth of ca. 1.3 I.tm. It is possible that the transition
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occurs when a critical amount of debris has been transferred to the abrasive paper and is due to partial clogging of the abrasive. This suggestion is supported by the observation that the depth of material removed from the surface at the transition is greater for coarser abrasive particles [ 41. 5. Conclusions The following conclusions may be drawn from experiments conducted on the two-body wear of commercial-purity titanium and the alloy Ti-GAl4V by silicon carbide and aluminium oxide abrasive papers with nominal particle sizes of less than 50 pm. (1) Commercial purity titanium and the alloy Ti-6Al-4V exhibit linear wear behaviour during repeated sliding over the same track during two-body abrasive wear in air. (2) During abrasion in an inert gas atmosphere, the wear record divides into two linear sections. In the initial period, the wear rate of the alloy Ti-GAl-4V, compared with that obtained in dry air, is slightly higher in dry nitrogen and slightly lower in dry argon; in the steady state it is substantially lower in both cases. The mean rate of wear of the alloy in an inert gas atmosphere is therefore lower (by up to 70%) than that obtained in air. Abrasion in pure oxygen leads to eventual ignition of the fine metallic debris, with an increase in wear rate of about 30% over that measured in dry air. (3) Abrasion under an inert gas atmosphere possibly leads to modification of the surface alloy composition by removal of interstitial elements (primarily oxygen and nitrogen) and thereby increases surface ductility. Metallic debris adheres to the alloy surface in the absence of a protective oxide film and accumulates in extensive, heavily deformed sheets which reduce the wear rate. Qualitatively similar effects are observed in all abrasive systems, although the magnitude of the reduction in wear rate is greater in argon than in nitrogen. (4) A coefficient of friction of about 0.6 is observed during the abrasion of the alloy Ti-6Al-4V under all gases tested. This value is characteristic of the sliding of titanium on itself and confirms observations made by scanning electron microscopy of used abrasive papers that much of the normal load is borne by abrasive particles capped by metallic debris. In inert gas atmospheres, the nature of such contacts is essentially the same as that in air but the frequency is greater.
References 1 J. F. Carroll and R. C. Gotham, The measurement of abrasiveness of magnetic tape, IEEE Tmns. Magn., 2 (1966) 6 - 13. 2 J. Larsen-Basse, Effect of atmospheric humidity on abrasive wear, Proc. ht. Conf. on Mechanical Behauiour of Materials, Kyoto, 1971, Vol. 3, 1972,353 - 362.
176 3 J. Larsen-Basse and S. S. Sokoloski, Influence of atmospheric humidity on abrasive wear. II. Two-body abrasion. Wear, 32 (1975) 9 - 14. 4 A. P. Mercer, Ph. D. Dissertation, University of Cambridge, (1985). 5 E. J. Duwell and W. J. McDonald, The effect of reactive gases on the dry grinding of steel with aluminium oxide coated abrasive, Wear, 4 (1961) 384 - 386. 6 J. A. Williams and W. M. Stobbs, Changes in mode of chip formation as function of presence of oxygen, Met. Technol., 6 (1979) 424 - 432. 7 P. K. Wright, J. G. Horne and D. Tabor, Boundary conditions at the chip-tool interface in machining, Wear, 54 (1979) 371. 8 P. K. Wright, Frictional interactions in machining, Met. TechnoE., 8 (1981) 150. 9 T. H. C. Childs and A. B. Smith, Effects of atmosphere on flank wear of HSS tools used to turn medium carbon steels in BUE conditions, Met. Technol., 9 (1982) 292. 10 A. D. McQuillan and M. K. McQuillan, Titanium, Butterworths, London, 1956. 11 H. P. Godard, W. B. Jepson, M. R. Bothwell and R. L. Kane, The Corrosion ofLight Met&, Wiley, New York, 1967. 12 A. E. Jenkins, The oxidation of titanium at high temperatures in an atmosphere of pure oxygen, J. Inst. Met., 82 (1953 - 1954) 213 - 221. 13 V. V. Andreeva, Corrosion, 20 (1964) 35. 14 E. J. Duwell, I. S. Hong and W. J. McDonald, The effect of oxygen and water on the dynamics of chip formation during grinding, ASLE Trans., 12 (1969) 86 - 93.