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The structure of MgO abrasive wear debris
255
Wear, 71 (1981) 255 - 258 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short Communication
The structure of MgO abrasive wear debris I. A. CUTTER* Department (Australia)
and R. MCPHERSON
of Materials
Engineering,
Monash
University,
Clayton,
3168
Victoria
(Received May 12, 1981)
An X-ray line-broadening study of MgC abrasive wear debris suggests that the finest particles have a structure comparable with that of a heavily cold-worked metal and that they are produced by deformational processes whereas the larger particles are not homogeneous in structure and are probably produced by fracture. 1. Introduction A previous study [l] of MgO abrasive wear debris, using X-ray linebroadening techniques, suggested that considerable plastic deformation occurred during abrasion and that the structure of the debris resembled that of a heavily cold-worked metal. Annealing of debris at temperatures up to 1200 “C produced little change in the contribution of the crystallite size to the line breadth but the strain contribution decreased to zero over the temperature range 500 - 900 “C. Analysis of line-broadening data involves an averaging over the whole sample and the results may therefore be misleading if the sample is not homogeneous in structure. However, the debris produced by abrasive wear would be expected to consist of a range of particle sizes having different degrees of distortion Within individual particles and from particle to particle. To investigate this effect, the structure of MgO abrasive wear debris as a function of particle size was examined by X-ray linebroadening methods. 2. Experimental results A hot-pressed polycrystalline MgO specimen was abraded on a motordriven 320 grade diamond-impregnated metal lap under a flow of ethanol. The resulting suspension of particles was dispersed ultrasonically and separated into three size fractions by sedimentation. The estimated
*Resent address: Pilkington AC1 Operations Pty. Ltd., Greens Road, Dandenong, 3175 Victoria, Australia.
256
equivalent sphere particle diameters calculated from Stokes’s law are given in Table 1. TABLE 1 Integral breadth of (222) reflections for three size fractions of MgO abrasive wear debris Sample number
Proportion of debris (wt.%)
Equivalent Mm)
1 2 3
11 76 13
.7.2 0.4 - 7.2 <0.4
diameter
(222) integral breadth @adI 6.2 x 1O-3 8.0 x 10-S 14.6 x 1O-3
Profiles of the (111) and (222) X-ray reflections from each of the size fractions were recorded with a Philips diffractometer at a scanning speed over the 20 range of f” mine1 using Cu Kcuradiation. The doublet area was measured with a plarumeter and the (Y~peak found by the method of Dumond and Kirkpatrick [2]. The true integral breadth of the lines (Table 1) was then determined by the method of Anatharaman and Christian [ 31 using the instrumental profile from an annealed polycrystalline specimen as the reference. The increase in the integral breadth with decreasing particle size demonstrates the inhomogeneity of the lattice distortion within the debris. The integral breadth from sample 2 was also determined after partial dissolution of the powder by successive treatments with hydrochloric acid. The results (Table 2) show a reduction in breadth after each treatment; this is consistent with the preferential solution of more severely distorted finer particles or more distorted regions of larger particles. TABLE 2 Effect of acid treatment on the integral breadth of (222) reflection from sample 2 Condition
Fraction dissolved
(222) integml WI
As prepared After fist treatment After second treatment
0 0.36 0.66
8 x 10-3 7.7 x10-a 6.5 x 1O-3
breadth
The decrease in the breadth after treatment is relatively small; this suggests that the structure of the intermediate size fraction consi8ts not of a mixture of undistorted fracture fragments and distorted material but of material which has been deformed to a greater or lesser extent. Attempts to analyse, by separation into strain and crystallite size contributions, the line breadth of debris containing a range of lattice distor-
257
tions will clearly give misleading results. There is evidence from electron microscopy studies that, on an extremely fine scale, material can be removed from a brittle solid by a chip-cutting process involving very large plastic strains [4]. Because of the large distortion and small particle size of sample 3 the line broadening was analysed on the assumption that its structure was homogeneous. The parabolic relationship of Halder and Wagner [ 51 was used to determine the strain e2 and crystallite size D contributions to the line breadth in the as-prepared condition and after annealing for 1 h at successively higher temperatures up to a maximum temperature of 1500 “C. The results, together with previously reported data [l] , are shown in Fig. 1.
ANNEALING
TEMPERATURE
CC)
Fig. 1. Effect of annealing on the strain and crystallite size components of X-ray line broadening for the finest particle size fraction (less than 0.4 Mm) of MgO abrasive wear debris: -, present work; --- , Cutter and &fcPhereon [ 11.
The strain component is considerably higher for the fine fraction than for a sample’of debris taken as a whole; this component decreases to zero on annealing in the temperature range 300 - 1100 ‘%I.The crystallite size component increases slightly with mcreasing temperature until the strain is removed, after which it increases sharply. This behaviour is similar to that observed on annealing heavily cold-worked metals and may be interpreted as a reduction in microstrain caused by annihilation and rearrangement of dislocations followed by a rapid increase in the crystallite size as recrystallization commences. A dislocation density of 6 X 1011 cm- 2, with some evidence of increased strain energy owing to dislocation interaction, was obtained for the as-prepared fine debris fraction using the approach of Williamson and Smallman [6] to estimate dislocation densities from X-ray line broadening. This structure is similar to that deduced by Williamson and Smallman for
258
iron, tungsten filing.
and molybdenum
which had been heavily
cold worked
by
3. Discussion These results demonstrate that debris produced by the abrasive wear of MgO on 320 grade diamond is not structurally homogeneous but consists of a mixture of particles ranging from larger partly deformed fragments produced by fracture to submicrometre-sized particles which have a structure comparable with that observed in the most severely cold-worked metals. The observation of severe plastic deformation associated with the abrasive wear of MgO is consistent with the work of King and Tabor [ 71 who showed that, although some cracking and surface fragmentation was associated with sliding tracks in rock salt and other brittle materials, the behaviour of these materials was dominated by plastic deformation at the surface. They suggested that brittle fracture was suppressed under the high hydrostatic pressure at points of contact and that plastic flow occurred at a shear stress much greater than the static yield stress. The debris examined in the present study were prepared by grinding; this situation is much more complex than that in single-cut experiments. Not only would the abrasive tips vary in size and geometry but the abrasive action would be on a surface damaged by previous abrasion. This would be expected to favour the detachment of material cracked previously during the plastic formation of grooves so that although cracking may be a consequence of plastic flow the major contribution to the wear rate would be expected to be from fracture processes and not directly from deformation. 4. Conclusion The major proportion of the debris produced by the abrasive wear of MgO on 320 grade diamond is inhomogeneously distorted and is probably produced by fracture from a surface previously deformed during the formation of grooves by the abrasive particles. However, the finest particle size debris has a structure comparable with that of a heavily cold-worked metal; this is consistent with the formation of this debris by a plastic cutting process. 1 I. A. Cutter and R. McPherson, Examination of abraded MgO by X-ray diffraction line broadening, Philos. Meg., I9 (1969) 795 - 807. 2 J. W. M. Dumond and H. A. Kirlcpatrlck, Experimental evidence for electron velocities as the cause of Compton line breadth with the multicrystal spectrograph, Phys. Rev., 37 (1931) 136 - 159. 3 J. R. Anantharaman and J. W. Christian, Measurement of growth and deformation faulting in hexagonal cobalt, Acta Crystallogr., 9 (1956) 479 - 486. 4 R. L. Aghan and R. McPherson, Mechanism of material removal during abrasion of rutile, J. Am. Cemm. Sot., 56 (i973) 46 - 47. 5 N. C. Halder and C. N. J. Wagner, Separation of particle size and lattice strain in integral breadth measurements, Acta Crystallogr., 20 (1966) 312 - 313. 6 G. K. Wllllamson and R. E. Smallman, Dielocation densities in some annealed and coldworked metals from measurements on the X-ray Debye-Scherrer spectrum, Philos. Mag., 1 (1956) 34 - 36. 7 R. F. King and D. Tabor, The strength properties and frictional behaviour of brittle solids, Proc. R. Sot. London, Ser. A, 223 (1954) 225 - 238.
Wear, 71 (1981) 255 - 258 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short Communication
The structure of MgO abrasive wear debris I. A. CUTTER* Department (Australia)
and R. MCPHERSON
of Materials
Engineering,
Monash
University,
Clayton,
3168
Victoria
(Received May 12, 1981)
An X-ray line-broadening study of MgC abrasive wear debris suggests that the finest particles have a structure comparable with that of a heavily cold-worked metal and that they are produced by deformational processes whereas the larger particles are not homogeneous in structure and are probably produced by fracture. 1. Introduction A previous study [l] of MgO abrasive wear debris, using X-ray linebroadening techniques, suggested that considerable plastic deformation occurred during abrasion and that the structure of the debris resembled that of a heavily cold-worked metal. Annealing of debris at temperatures up to 1200 “C produced little change in the contribution of the crystallite size to the line breadth but the strain contribution decreased to zero over the temperature range 500 - 900 “C. Analysis of line-broadening data involves an averaging over the whole sample and the results may therefore be misleading if the sample is not homogeneous in structure. However, the debris produced by abrasive wear would be expected to consist of a range of particle sizes having different degrees of distortion Within individual particles and from particle to particle. To investigate this effect, the structure of MgO abrasive wear debris as a function of particle size was examined by X-ray linebroadening methods. 2. Experimental results A hot-pressed polycrystalline MgO specimen was abraded on a motordriven 320 grade diamond-impregnated metal lap under a flow of ethanol. The resulting suspension of particles was dispersed ultrasonically and separated into three size fractions by sedimentation. The estimated
*Resent address: Pilkington AC1 Operations Pty. Ltd., Greens Road, Dandenong, 3175 Victoria, Australia.
256
equivalent sphere particle diameters calculated from Stokes’s law are given in Table 1. TABLE 1 Integral breadth of (222) reflections for three size fractions of MgO abrasive wear debris Sample number
Proportion of debris (wt.%)
Equivalent Mm)
1 2 3
11 76 13
.7.2 0.4 - 7.2 <0.4
diameter
(222) integral breadth @adI 6.2 x 1O-3 8.0 x 10-S 14.6 x 1O-3
Profiles of the (111) and (222) X-ray reflections from each of the size fractions were recorded with a Philips diffractometer at a scanning speed over the 20 range of f” mine1 using Cu Kcuradiation. The doublet area was measured with a plarumeter and the (Y~peak found by the method of Dumond and Kirkpatrick [2]. The true integral breadth of the lines (Table 1) was then determined by the method of Anatharaman and Christian [ 31 using the instrumental profile from an annealed polycrystalline specimen as the reference. The increase in the integral breadth with decreasing particle size demonstrates the inhomogeneity of the lattice distortion within the debris. The integral breadth from sample 2 was also determined after partial dissolution of the powder by successive treatments with hydrochloric acid. The results (Table 2) show a reduction in breadth after each treatment; this is consistent with the preferential solution of more severely distorted finer particles or more distorted regions of larger particles. TABLE 2 Effect of acid treatment on the integral breadth of (222) reflection from sample 2 Condition
Fraction dissolved
(222) integml WI
As prepared After fist treatment After second treatment
0 0.36 0.66
8 x 10-3 7.7 x10-a 6.5 x 1O-3
breadth
The decrease in the breadth after treatment is relatively small; this suggests that the structure of the intermediate size fraction consi8ts not of a mixture of undistorted fracture fragments and distorted material but of material which has been deformed to a greater or lesser extent. Attempts to analyse, by separation into strain and crystallite size contributions, the line breadth of debris containing a range of lattice distor-
257
tions will clearly give misleading results. There is evidence from electron microscopy studies that, on an extremely fine scale, material can be removed from a brittle solid by a chip-cutting process involving very large plastic strains [4]. Because of the large distortion and small particle size of sample 3 the line broadening was analysed on the assumption that its structure was homogeneous. The parabolic relationship of Halder and Wagner [ 51 was used to determine the strain e2 and crystallite size D contributions to the line breadth in the as-prepared condition and after annealing for 1 h at successively higher temperatures up to a maximum temperature of 1500 “C. The results, together with previously reported data [l] , are shown in Fig. 1.
ANNEALING
TEMPERATURE
CC)
Fig. 1. Effect of annealing on the strain and crystallite size components of X-ray line broadening for the finest particle size fraction (less than 0.4 Mm) of MgO abrasive wear debris: -, present work; --- , Cutter and &fcPhereon [ 11.
The strain component is considerably higher for the fine fraction than for a sample’of debris taken as a whole; this component decreases to zero on annealing in the temperature range 300 - 1100 ‘%I.The crystallite size component increases slightly with mcreasing temperature until the strain is removed, after which it increases sharply. This behaviour is similar to that observed on annealing heavily cold-worked metals and may be interpreted as a reduction in microstrain caused by annihilation and rearrangement of dislocations followed by a rapid increase in the crystallite size as recrystallization commences. A dislocation density of 6 X 1011 cm- 2, with some evidence of increased strain energy owing to dislocation interaction, was obtained for the as-prepared fine debris fraction using the approach of Williamson and Smallman [6] to estimate dislocation densities from X-ray line broadening. This structure is similar to that deduced by Williamson and Smallman for
258
iron, tungsten filing.
and molybdenum
which had been heavily
cold worked
by
3. Discussion These results demonstrate that debris produced by the abrasive wear of MgO on 320 grade diamond is not structurally homogeneous but consists of a mixture of particles ranging from larger partly deformed fragments produced by fracture to submicrometre-sized particles which have a structure comparable with that observed in the most severely cold-worked metals. The observation of severe plastic deformation associated with the abrasive wear of MgO is consistent with the work of King and Tabor [ 71 who showed that, although some cracking and surface fragmentation was associated with sliding tracks in rock salt and other brittle materials, the behaviour of these materials was dominated by plastic deformation at the surface. They suggested that brittle fracture was suppressed under the high hydrostatic pressure at points of contact and that plastic flow occurred at a shear stress much greater than the static yield stress. The debris examined in the present study were prepared by grinding; this situation is much more complex than that in single-cut experiments. Not only would the abrasive tips vary in size and geometry but the abrasive action would be on a surface damaged by previous abrasion. This would be expected to favour the detachment of material cracked previously during the plastic formation of grooves so that although cracking may be a consequence of plastic flow the major contribution to the wear rate would be expected to be from fracture processes and not directly from deformation. 4. Conclusion The major proportion of the debris produced by the abrasive wear of MgO on 320 grade diamond is inhomogeneously distorted and is probably produced by fracture from a surface previously deformed during the formation of grooves by the abrasive particles. However, the finest particle size debris has a structure comparable with that of a heavily cold-worked metal; this is consistent with the formation of this debris by a plastic cutting process. 1 I. A. Cutter and R. McPherson, Examination of abraded MgO by X-ray diffraction line broadening, Philos. Meg., I9 (1969) 795 - 807. 2 J. W. M. Dumond and H. A. Kirlcpatrlck, Experimental evidence for electron velocities as the cause of Compton line breadth with the multicrystal spectrograph, Phys. Rev., 37 (1931) 136 - 159. 3 J. R. Anantharaman and J. W. Christian, Measurement of growth and deformation faulting in hexagonal cobalt, Acta Crystallogr., 9 (1956) 479 - 486. 4 R. L. Aghan and R. McPherson, Mechanism of material removal during abrasion of rutile, J. Am. Cemm. Sot., 56 (i973) 46 - 47. 5 N. C. Halder and C. N. J. Wagner, Separation of particle size and lattice strain in integral breadth measurements, Acta Crystallogr., 20 (1966) 312 - 313. 6 G. K. Wllllamson and R. E. Smallman, Dielocation densities in some annealed and coldworked metals from measurements on the X-ray Debye-Scherrer spectrum, Philos. Mag., 1 (1956) 34 - 36. 7 R. F. King and D. Tabor, The strength properties and frictional behaviour of brittle solids, Proc. R. Sot. London, Ser. A, 223 (1954) 225 - 238.