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Wear mechanisms with coated abrasives
Wear, 28 ( 1974) 33 l-343 (cl Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
WEAR ~EC~A~IS~S
J. BILLIN~HAM*
331
WITH COATED ABRASIVES
and J. LAURIDSEN
F~lnter Research Institute, Stoke Pages, Bucks. (Gt. Britain) J. F. BRYON English Abrasives Limiied, Tottenham, London (Gt. Brituin~ (Received January 14, 1974)
SUMMARY
A metallographic study is described of the grit failure mechanisms operating with coated abrasives when grinding a wide variety of engineering materials. Scanning electron microscopy has been used to both characterise and monitor the incidence of the various grit failure mechanisms for a standard aluminium oxide resin-bonded belt and a similar belt containing a grinding aid additive. The two predominant mechanisms are capping, where swarf becomes firmly attached to the grit surface preventing any further grinding action and dulling, which is a combination of attrition by chemical degradation or plastic flow and small-scaie grit fragmentation, which leads to the formation of flats on the grit surfaces. These mechanisms are discussed and the reasons for changes in both the incidence and rate of development of these failure modes are examined for belts containing coating additives which are responsible for improved grinding performance.
1. INTRODUCTION
In recent years there has been a marked increase in the use of coated abrasives in the metal grinding industry particularly in off-hand grinding practice’. Performance has been further improved with the help of certain chemical additives (grinding aids) incorporated in/on the grinding belt surfaces. Such additives usually consist of commercially protected formulations24 but some of the compounds cited in the patent literature include organic compounds that release HCl, HBr or H,S on heating, thiourea and other sulphur containing compounds. This type of coated abrasive is particularly effective when grinding materials which are normally difficult to machine such as nickel and cobalt base superalloys and stainless steels. This paper attempts to investigate the grit failure mechanisms operating during such grinding practices and forms part of a wider programme aimed at elucidating the reasons for the improved behaviour of these special grinding belts. The effect * Present address: Cranfield Institute of Technology, Bedford (ct. Britain).
332
.I. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
of grinding on the structural and hence mechanical properties of the ground component has been studied concurrently. Although there have been many investigations of the factors influencing the wear rate of meta@’ much less attention has been directed to the degradation of grinding belts or papers. Johnson’ however has studied the deterioration of silicon carbide grinding papers by utilising the technique of scanning electron microscopy and this technique, combined with the associated microanalysis facility, was widely used in the present study to follow the grit wear processes and swarf build-up on the belt surface. A wide range of conventional engineering materials have been ground and metallic wear rates and grit degradation mechanisms studied using standard aluminium oxide resin-bonded belts and similar belts that had been treated with a grinding aid chemical (hereafter referred to respectively as resinbond (RB) and anti-glaze (AG) belts). The relative importance of the possible grit wear mechanisms was assessed by measurements of the incidence of each type of mechanism as determined in the S.E.M. by examination of worn grits in a number of worn tracks. Following Johnson’ three major wear mechanisms were distinguished for the grits, capping where swarf is retained on the grit surface preventing its further participation in the abrasion process, pull-out where the grit is separated completely from the cementing resin base, and dulling where the grit is worn either by fragmentation or attrition wear. In addition the extent of wear debris entrapment in the grit interstices on the belt surface is also important. The two most important breakdown mechanisms in the case of the abrasive belt are capping and dulling. Capping in particular is much more serious in the case of coated abrasives than with conventional grinding wheels since it cannot be overcome by a redressing procedure. Dulling results in the formation of flat plateaux on the cutting points which reduces metal cutting and increases sideways ploughing effects causing heating and possible structural damage to the workpiece component. It can occur either by fragmentation of the cutting grit or by attrition wear which can be defined as the submicroscopic removal of material by chemical degradation, melting or plastic Row. Dyer’ has suggested that a proper balance between fragmentation and attrition is necessary for the effective use of an abrasive belt. Pull-out can usually be controlled by modifications to the resin bond chemistry and wear debris entrapment is not usually a serious problem in metal grinding. Although no direct studies have previously been reported of the relative importance of these wear mechanisms with such chemically treated belts, there have been a number of investigations of the effect of reactive gaseous environments in cutting and abrading processes”,“. The reactive gases” were found to reduce the frictional force if dull grits were used but with sharp grits deeper cuts resulted and the transverse force then increased. This correlates with similar effects which can be obtained in the presence of cutting oils. The gases were only effective under damp conditions, the oxides presumably protecting the metal from attack under dry conditions. Oxygen behaved in a similar manner to these reactive gases (HCl, HBr and H,S) but nitrogen and the inert gases gave increased friction and seizure between the Al,O, grit and the metal. Friction is also markedly reducedl’ by allowing hydrogen sufphide gas to react with a molybdenum surface and similar results have been reported for iodine on titanium and chlorine on
333
WEAR MECHANISMS WITH COATED ABRASIVES
chromium and tungsten. This was attributed to the ease of shearing the layer-lattice structure of the compounds formed on the metal. The effects produced by these gases are similar to those produced by the AG belts in practice, however it is possible that the reactive compounds could effect the abrasion process in a number of other ways including: (i) physical support for the grits when the grinding aids are apphed in the form of an adherent layer on the belt surface, (ii) chemical attack of the grits thereby modifying grit fragmentation mechanisms, (iii) chemical attack of the metal surface thereby modifying the properties of surface layers, (iv) chemical attack which modifies the interaction between the grit/metal interface Le. adhesion and frictianal effects. A major aim of this present study was to assess the relative importance of these effects in coated abrasive grinding with chemically treated belt surfaces. 2. EXPERIMENTAL The grinding belts used in the study were standard grade 60 aluminium oxide resin-bonded belts of both the standard (RB) and the treated anti-glaze type (AG) with an additional adherent surface layer. Grinding tests were carried out on a conventional backstand machine using a grinding belt speed of 6000 sf/min. A constant thrust of 4 lb wt, was used with a solid $ in, diam. metal rod sample TABLE I ALLOY COMPOSITIONS _~._ Ml&?&l Classification
-~lll_l----Cump0sitif3n
.Properties Mt. Pt. (“C)
WV at R.T.
1390 1420 1350
160 150 $90
Ni-2OC-lTi-4.5Al
1290
340
Co-25Cr-tONi-O.SC
f 260
290
Al-I2Si
565
65
A1-0.5Cu-2.7Mg-5.5Zn
600
1x5
-
ENIA EN58J J30
Free cutting mild steel Stainless steel High speed tool steel Tl-type Nimonic 108 High temperature nickel base alloy X40 Nigh temperature cobalt base alloy LM6 AIuminium silicon alloy High strength DTD 5w4 aluminium atioy
Fe-lMn-O.25S Fe-lKK3Ni Fe-O.?C-18W-Kr-1
______---
V
-
~__I_
grinding against a backing wheel of hard natural rubber. To prevent excessive heating the sample was rotated at an angle of 55” to the belt and the grinding interface cooled with an air blast (60 lbs/in2 pressure). The sample was water cooled at 30 s intervals and the weight change recorded at the end of each 60 s. The compositions of the alloys ground are given in Table I.
334
J. BILLINGHAM, J. LAURIDSEN, J. F. BRYON
Sections of worn track were removed from the grinding belt, coated with a gold palladium alloy and examined in a Cambridge Instruments Mark 2A scanning electron microscope. Additional examination was made with an optical stereomicroscope. The swarf distribution on the grit/grinding belt surface was examined using the microprobe analysis attachment to the S.E.M. or a Quantimet 720 Image Analysing Computer. Conventional metallographic sections of worn belts were also prepared by mounting transverse sections of belt in araldite and polishing with standard techniques. 3. GRINDING
EXPERIMENTS
AND ASSESSMENT OF FAILURE MECHANISMS
The amount of metal stock removed by the two types of belts during the grinding experiments varied from metal to metal as indicated by the histograms in Fig. 1 which compare the volumes of metal removed in 3 min grinding. A similar pattern was apparent after 10 min grinding. A further most important parameter
NIA
~
J 30
i
.M 6
ITD
5044
b-dt
0
R.B.
a
A.G. bell
F f
30
\ EN
58J
NM
108
X40
Fig. 1. Showing volume of stock removed in three minutes grinding.
WEAR MECHANISMS WITH COATED ABRASIVES
335
Fig. 2. Showing times for grinding to f cut condition.
is the rate of deterioration of cutting power particularly as this can be responsible in practice for causing structural damage to the ground component by promoting a glazing instead of a cutting action. This property was assessed somewhat arbitrarily in the present tests (Fig. 2) by measuring the elapsed time before the cutting rate was reduced to $ of its initial value, assessed from the results for the first minute of grinding. It can be seen that generally the treated belts cut faster and for a much longer time especially on the superalloy type materials. A comparison of the grinding behaviour of both types of belt on a Nimonic alloy, which is a typical example of materials which are usually difficult to machine, is shown in Fig. 3. The incidence of the various grit failure mechanisms is listed in Table II for belts that have ground the various alloys to a similar condition (i.e. until f cut condition). Additional information recorded includes the percentage of grits that have taken part in the grinding process and the number suffering from a particular form of attrition wear designated pitting. Figure 4 shows a typical capped grit formed when grinding a high strength alloy such as Nimonic. The surface of
336
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
IO-
6-
5-
LAcceptable
3-
2-
I-
I
O
I lUsefUl , belt Lfe
Fig. 3. Showing TABLE
ENlA
grinding
behaviour
I
30
20
(MMulES)
of standard
(RB) and treated
(AG) belt on Nimonic
108
II
MICROSCOPICAL Metul
I
I
‘O TIME
EXAMINATION
Grinding conditions
1 minute grinding to 4 condition EN58J 1 minute to i condition Nimonic 108 I minute to 5 condition x40 1 minute to i condition 530 1 minute to 3 condition LM6 1 minute to + condition DTD 5099 1 minute to -J-condition
OF WORN
1,:) Capped grits RB AB
0 2 0 30 0 20 13 45 2 2 10 10 5 9
BELTS
7” Pulled out grits RB AB
0
0
2
1
5
1
0 10 0 7 1 30
1
2 5 9 3 0 0 1
8 6 10 5 10
3 25 3 5
2 2 3 2 1 4 2 3 3 10 2 1
I
“/; Grits showirlg ‘, Pitted attrition wear grits RB AB RB AB
YOGrits that have taken part in the grinding process RB AB
12 3.5 20 35 30 30 20 50 20 30 16 45 13 65
12 40 20 40 30 30 20 50 20 30 20 70 16 70
50 65 45 70 35 45 40 75 45 75 48 65 37 60
0
0
30
20
20 13 46 7 2 0 0 0 0
3 4 60 14 65 0 0 0 0
50 65 45 70 40 45 40 80 50 75 54 75 40 60
Fig. 4. Capped grit produced can be clearly seen.
Fig. 5. The appearance
when grinding
of a capped
a Nimonic
grit when grinding
alloy. The pitted appearance
a soft metal such as aluminium.
of the grit SWrface
338
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
the grits often contains micro-undulations (termed pitting in the table) which at high magnification can be seen to be smooth sided but not faceted. It can be seen that, the capped area covers only part of the grit surface and this was usually observed, although this was not the case when abrading soft metals such as aluminium, Fig. 5. Grit failure was usually by attrition rather than by large-scale fragmentation, although a few examples of obvious cleavage failures did occur, such as that shown in Fig. 6 for a belt grinding an aluminium alloy. The swarf distribution on the belt surface can easily be detected by using the characteristic X-ray emission technique, and whereas for example with a free cutting mild steel, fine swarf particles are distributed over all the belt surface, with most other metals and only swarf retained on the belts was in the form of metal caps. An alternative means of quantifying the amount of capping was by examination of the ground belt surface in the Quantimet 720 utilising the differences in reflectance between the metal swarf and the resinoid belt surface to supply the necessary contrast. The transverse metallographic sections of worn belt clearly showed the variation of cutting angle presented to the workpiece by the grits and also allowed the build-up of worn flats to be monitored (Fig. 7). Inclusions within the grits can clearly be seen in polished microsections, the identification and importance of which have formed part of a separate study.
Fig. 6. Showing
grit fragmentation
with large-scale
cleavage
fractures.
WEAR MECHANISMS WITH COATED ABRASIVES
339
Fig. 7. Optical micrograph of transverse belt section showing worn tlat on grit and area of capping.
Fig. 8. Swarf collected after abrasive grinding.
340
J. BILLINGHAM, J. LAURIDSEN, J. F. BRYON
Fig. 9. Metal surface after abrasive grinding. Note metal deformation at edges of cutting tracks.
Swarf collected from the grinding experiments was similar to that produced in conventionai machining with a tool (Fig. 8). Lengths of up to 2 mm were examined and because of the extreme ease of fracture it is likely that longer lengths were produced during the grinding. Most of the particles were approximately 50 pm in diameter (related to the grit size of the belts used) and up to 5 pm thick. When grinding a stainless steel EN58J alloy, the swarf from the RB belt showed more colouration (blueing) than the swarf from the AG belt, indicating a higher grinding temperature or thinner swarf section. The swarf size appeared to be of similar section but no detailed comparative measurements of swarf thickness were made. However, the surface roughness of metal ground with AG belts is somewhat coarser than for RB ground material with the same grit size, indicating a deeper cutting action. The metal surfaces showed typical well defined grit cutting paths (Fig. 9) with ploughed edges, indicating severe plastic deformation even with very strong high temperature materials such as Nimonic and tool steels. All this evidence points to high temperatures at the grit-metal abrading interface, a factor borne out by microanalysis of the interface regions”. 4. DJSCUSSION
Evidence was found for all the grit failure mechanisms previously discussed
WEAR MECHANISMS
WITH COATED
ABRASIVES
341
although pull-out was only really important when grinding the soft aluminium alloy and its incidence did not seem to be affected by the presence of the grinding aid. Dulling was the most common wear mode but little evidence was seen for large fragmentation indicating that attrition wear was most important. Capping was wide-spread on most alloys except for the free cutting mild steel. Two types of caps could be distinguished: a large build up of material usually covering the entire grit surface (Fig. 5) which seems merely to be physically attached to a smooth grit interface and much smaller amounts of material, often several to one grit surface and almost always associated with the pitted surface type of attrition attack (Fig. 4). The first type was present on aluminium alloys and the second on the superalloys and high strength steels i.e. on those materials where the treated grits gave improved grinding performance. The pitted appearance indicates that some chemical or deformation process has occurred rather than simple cleavage and this was confirmed by analysis of the interface region between an aluminium oxide grit and a piece of Nimonic swarf. Careful microprobe analysis work on the topside and underside of swarf caps, following their removal from the belt surface indicated the presence of aluminium on the swarf underside. These surfaces were very carefully examined in the stereoscan to ensure the complete absence of any particles of alumina. The inference from this preliminary studyi is that a chemical reaction does occur between the A1203 and the metal or possibly between the Al,O, based grit (i.e. plus impurities Si02, TiO, etc. at total levels of 5%) and the metal/metal oxide surface. Measurements of the shift in aluminium electron binding energy on these samples using photoelectron spectroscopy indicated that a spine1 compound was probably formed (NiO.Al,O,). Estimates of the temperature required for the formation of such a compound during the grinding tests indicates that the interface temperature was greater than 1100°C. The pitted nature of the grit surface seemed to be a feature of the material ground rather than whether or not the belt had been chemically treated i.e. no grit pitting was seen with aluminium or mild steel. This is further proof that it is a chemical rather than purely fractographic feature of the wear process. It is significant that the pitting only occurs with those metals on which the (AG) belt gives improved grinding performance and is likely to be closely associated with the mechanism for their improved grinding performance. Examination of Table II indicates that less caps are present on the treated (AG) than on the untreated (RB) belts for these materials. This is an especially significant result in view of the way the grinding tests were carried out since the AG belts examined in this case had removed significantly more metallic material. The results indicate that the reduction in the amount of capping seen with the AG belts is a very important feature of their improved behaviour. The second most important result to be recognised from Table II is that the AG belts generally show a higher proportion of the grits taking part in the abrasive process at all stages of the grinding but particularly when only a small amount of material has been removed. Thus the grinding process is modified in some way so as to become a more-uniform process. The keying of swarf to grits is obviously important and the reason for the success of the AG belts could be that less keying occurs, or that caps once formed could be more easily removed in subsequent grinding. Examination of belts at
342
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
intermediate and more extensive stages of the grinding process and attempts to remove caps physically and re-use the belts (dressing) indicated that the first of these factors is most important. From examination of the physical and mechanical properties of the materials it seems certain that greater heat is generated when grinding the metals EN58J, Nimonic 108 and X40 (all have low thermal conductivity and reasonable strength at high temperature) and these are precisely the metals on which the AG belt performs best. Some possible mechanisms for the AG improvement are, reduced welding of Al,O, and metal, or effective cooler cutting due to chemical attack of either or both interfaces by volatile components in the grinding aid possibly resulting in the formation of a low friction compound. Some evidence exists to support this with blueing of, swarf from RB belt compared to AG indicating deeper cuts as discussed in the introduction for the vapour grinding experiments. The very irregular nature of the grits, which leads to the existence of wide variations in the distance from the backing cloth to the cutting points, results in certain critical cases (e.g. Nimonic 108) to the rejection of a belt when a very large proportion of the grits has not taken part in the grinding process. Thus both the capping and the gradual wear prevent large numbers of grits from taking part. If belts with much more uniform grit size (here we really mean backing cloth to cutting point distance) could be produced they might result in remarkable improvements in this particular case (Nimonic 108) which could offset the extra expense of their production. This is one possible reason for how the treated belts work because a much higher proportion of grits take part in the grinding due possibly to ease of grit fracture or to less capping. The two most important grit failure mechanisms are capping and dulling. Capping is substantially reduced in the presence of grinding aid chemicals on materials such as superalloys and stainless steels due to a modification in the interface reaction between the swarf and the alumina grit. Dulling occurs by some small-scale fragmentation ofgrits but the main wear mechanism appears to be a form of chemical attack probably related to high temperature reaction with metallic oxides which leads to a macroscopically flat surface which in fact consists of micro-undulations termed pitting. The reasons for the improved behaviour of the treated (AG) belts have not been clearly defined. However this study has shown that physical support of the grits is unimportant and indicates that the AG belts most probably work by reducing adhesion (or reaction) between the grits and the metal surface (or metal oxides). The fact that the behaviour varies from metal to metal would appear to rule out mechanisms based entirely on reaction between the grit and the grinding aid, whether these involve modified fracture behaviour or interfacial reaction. However this may be a slight over simplification of the problem since the metals where no improvement takes place are also those where the interface temperatures should be lower. Experiments involving modified grinding temperature may therefore be very important and although we carried out preliminary experiments with these aims they may not have been di~riminatory enough to detect possible effects. Further photoelectron spectroscopy is being carried out in an attempt to see how the AG additive modifies the grit-swarf interaction. It is hoped that this information will enable
WEAR
MECHANISMS
WITH
COATED
ABRASIVES
343
the present grinding aids to be improved either by improved performance per se or by extending the range of materials on which they give improved grinding behaviour. ACKNOWLEDGEMENTS
The authors are grateful to the Department of Trade and Industry to English Abrasives Ltd. for permission to publish this work.
and
REFERENCES 1 K. Lacy, Metalworking Production, 113 (21) (1969) 45. 2 J. P. Bryon and A. G. Rolfe, Coated Abrasive Articles, English Abrasives Ltd., B.P.1, 145, 082 (1969). 3 Supersize Film Forming Resin Coated Abrasives, Minnesota Mining and Manufacturing Co. Ltd., U.S.P. 3, 256, 076 (1966). 4 Anti-Weld Additioesfor Coated Abrasioe Bonds, Carborundum Co. Ltd., U.S.P. 3, 058, 819 (1962). 5 T. 0. Mulhearn and L. E. Samuels, Wear, 5 (1962) 478. 6 B. W. E. Avient, J. Goddard and H. Wilman, Proc. Roy. Sot. (London), A258 (1960) 159. 7 M. M. Khruschov, Proc. Con& Lubrication and Wear, London, 1957, p. 655. 8 R. W. Johnson, Wear, 12 (1968) 213, 9 H. N. Dyer, Ind. Eng. Chem., 47 (12) (1955) 2500. 10 E. J. Duwell and W. J. McDonald, Wear, 4 (1961) 384. 11 G. W. Rowe, Conf. Lubrication and Wear, Proc. Inst. Mech. Eng., 1957, p. 333. 12 M. Vowles, J. Billingham and J. P. Bryon, submitted for publication in Metallography.
WEAR ~EC~A~IS~S
J. BILLIN~HAM*
331
WITH COATED ABRASIVES
and J. LAURIDSEN
F~lnter Research Institute, Stoke Pages, Bucks. (Gt. Britain) J. F. BRYON English Abrasives Limiied, Tottenham, London (Gt. Brituin~ (Received January 14, 1974)
SUMMARY
A metallographic study is described of the grit failure mechanisms operating with coated abrasives when grinding a wide variety of engineering materials. Scanning electron microscopy has been used to both characterise and monitor the incidence of the various grit failure mechanisms for a standard aluminium oxide resin-bonded belt and a similar belt containing a grinding aid additive. The two predominant mechanisms are capping, where swarf becomes firmly attached to the grit surface preventing any further grinding action and dulling, which is a combination of attrition by chemical degradation or plastic flow and small-scaie grit fragmentation, which leads to the formation of flats on the grit surfaces. These mechanisms are discussed and the reasons for changes in both the incidence and rate of development of these failure modes are examined for belts containing coating additives which are responsible for improved grinding performance.
1. INTRODUCTION
In recent years there has been a marked increase in the use of coated abrasives in the metal grinding industry particularly in off-hand grinding practice’. Performance has been further improved with the help of certain chemical additives (grinding aids) incorporated in/on the grinding belt surfaces. Such additives usually consist of commercially protected formulations24 but some of the compounds cited in the patent literature include organic compounds that release HCl, HBr or H,S on heating, thiourea and other sulphur containing compounds. This type of coated abrasive is particularly effective when grinding materials which are normally difficult to machine such as nickel and cobalt base superalloys and stainless steels. This paper attempts to investigate the grit failure mechanisms operating during such grinding practices and forms part of a wider programme aimed at elucidating the reasons for the improved behaviour of these special grinding belts. The effect * Present address: Cranfield Institute of Technology, Bedford (ct. Britain).
332
.I. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
of grinding on the structural and hence mechanical properties of the ground component has been studied concurrently. Although there have been many investigations of the factors influencing the wear rate of meta@’ much less attention has been directed to the degradation of grinding belts or papers. Johnson’ however has studied the deterioration of silicon carbide grinding papers by utilising the technique of scanning electron microscopy and this technique, combined with the associated microanalysis facility, was widely used in the present study to follow the grit wear processes and swarf build-up on the belt surface. A wide range of conventional engineering materials have been ground and metallic wear rates and grit degradation mechanisms studied using standard aluminium oxide resin-bonded belts and similar belts that had been treated with a grinding aid chemical (hereafter referred to respectively as resinbond (RB) and anti-glaze (AG) belts). The relative importance of the possible grit wear mechanisms was assessed by measurements of the incidence of each type of mechanism as determined in the S.E.M. by examination of worn grits in a number of worn tracks. Following Johnson’ three major wear mechanisms were distinguished for the grits, capping where swarf is retained on the grit surface preventing its further participation in the abrasion process, pull-out where the grit is separated completely from the cementing resin base, and dulling where the grit is worn either by fragmentation or attrition wear. In addition the extent of wear debris entrapment in the grit interstices on the belt surface is also important. The two most important breakdown mechanisms in the case of the abrasive belt are capping and dulling. Capping in particular is much more serious in the case of coated abrasives than with conventional grinding wheels since it cannot be overcome by a redressing procedure. Dulling results in the formation of flat plateaux on the cutting points which reduces metal cutting and increases sideways ploughing effects causing heating and possible structural damage to the workpiece component. It can occur either by fragmentation of the cutting grit or by attrition wear which can be defined as the submicroscopic removal of material by chemical degradation, melting or plastic Row. Dyer’ has suggested that a proper balance between fragmentation and attrition is necessary for the effective use of an abrasive belt. Pull-out can usually be controlled by modifications to the resin bond chemistry and wear debris entrapment is not usually a serious problem in metal grinding. Although no direct studies have previously been reported of the relative importance of these wear mechanisms with such chemically treated belts, there have been a number of investigations of the effect of reactive gaseous environments in cutting and abrading processes”,“. The reactive gases” were found to reduce the frictional force if dull grits were used but with sharp grits deeper cuts resulted and the transverse force then increased. This correlates with similar effects which can be obtained in the presence of cutting oils. The gases were only effective under damp conditions, the oxides presumably protecting the metal from attack under dry conditions. Oxygen behaved in a similar manner to these reactive gases (HCl, HBr and H,S) but nitrogen and the inert gases gave increased friction and seizure between the Al,O, grit and the metal. Friction is also markedly reducedl’ by allowing hydrogen sufphide gas to react with a molybdenum surface and similar results have been reported for iodine on titanium and chlorine on
333
WEAR MECHANISMS WITH COATED ABRASIVES
chromium and tungsten. This was attributed to the ease of shearing the layer-lattice structure of the compounds formed on the metal. The effects produced by these gases are similar to those produced by the AG belts in practice, however it is possible that the reactive compounds could effect the abrasion process in a number of other ways including: (i) physical support for the grits when the grinding aids are apphed in the form of an adherent layer on the belt surface, (ii) chemical attack of the grits thereby modifying grit fragmentation mechanisms, (iii) chemical attack of the metal surface thereby modifying the properties of surface layers, (iv) chemical attack which modifies the interaction between the grit/metal interface Le. adhesion and frictianal effects. A major aim of this present study was to assess the relative importance of these effects in coated abrasive grinding with chemically treated belt surfaces. 2. EXPERIMENTAL The grinding belts used in the study were standard grade 60 aluminium oxide resin-bonded belts of both the standard (RB) and the treated anti-glaze type (AG) with an additional adherent surface layer. Grinding tests were carried out on a conventional backstand machine using a grinding belt speed of 6000 sf/min. A constant thrust of 4 lb wt, was used with a solid $ in, diam. metal rod sample TABLE I ALLOY COMPOSITIONS _~._ Ml&?&l Classification
-~lll_l----Cump0sitif3n
.Properties Mt. Pt. (“C)
WV at R.T.
1390 1420 1350
160 150 $90
Ni-2OC-lTi-4.5Al
1290
340
Co-25Cr-tONi-O.SC
f 260
290
Al-I2Si
565
65
A1-0.5Cu-2.7Mg-5.5Zn
600
1x5
-
ENIA EN58J J30
Free cutting mild steel Stainless steel High speed tool steel Tl-type Nimonic 108 High temperature nickel base alloy X40 Nigh temperature cobalt base alloy LM6 AIuminium silicon alloy High strength DTD 5w4 aluminium atioy
Fe-lMn-O.25S Fe-lKK3Ni Fe-O.?C-18W-Kr-1
______---
V
-
~__I_
grinding against a backing wheel of hard natural rubber. To prevent excessive heating the sample was rotated at an angle of 55” to the belt and the grinding interface cooled with an air blast (60 lbs/in2 pressure). The sample was water cooled at 30 s intervals and the weight change recorded at the end of each 60 s. The compositions of the alloys ground are given in Table I.
334
J. BILLINGHAM, J. LAURIDSEN, J. F. BRYON
Sections of worn track were removed from the grinding belt, coated with a gold palladium alloy and examined in a Cambridge Instruments Mark 2A scanning electron microscope. Additional examination was made with an optical stereomicroscope. The swarf distribution on the grit/grinding belt surface was examined using the microprobe analysis attachment to the S.E.M. or a Quantimet 720 Image Analysing Computer. Conventional metallographic sections of worn belts were also prepared by mounting transverse sections of belt in araldite and polishing with standard techniques. 3. GRINDING
EXPERIMENTS
AND ASSESSMENT OF FAILURE MECHANISMS
The amount of metal stock removed by the two types of belts during the grinding experiments varied from metal to metal as indicated by the histograms in Fig. 1 which compare the volumes of metal removed in 3 min grinding. A similar pattern was apparent after 10 min grinding. A further most important parameter
NIA
~
J 30
i
.M 6
ITD
5044
b-dt
0
R.B.
a
A.G. bell
F f
30
\ EN
58J
NM
108
X40
Fig. 1. Showing volume of stock removed in three minutes grinding.
WEAR MECHANISMS WITH COATED ABRASIVES
335
Fig. 2. Showing times for grinding to f cut condition.
is the rate of deterioration of cutting power particularly as this can be responsible in practice for causing structural damage to the ground component by promoting a glazing instead of a cutting action. This property was assessed somewhat arbitrarily in the present tests (Fig. 2) by measuring the elapsed time before the cutting rate was reduced to $ of its initial value, assessed from the results for the first minute of grinding. It can be seen that generally the treated belts cut faster and for a much longer time especially on the superalloy type materials. A comparison of the grinding behaviour of both types of belt on a Nimonic alloy, which is a typical example of materials which are usually difficult to machine, is shown in Fig. 3. The incidence of the various grit failure mechanisms is listed in Table II for belts that have ground the various alloys to a similar condition (i.e. until f cut condition). Additional information recorded includes the percentage of grits that have taken part in the grinding process and the number suffering from a particular form of attrition wear designated pitting. Figure 4 shows a typical capped grit formed when grinding a high strength alloy such as Nimonic. The surface of
336
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
IO-
6-
5-
LAcceptable
3-
2-
I-
I
O
I lUsefUl , belt Lfe
Fig. 3. Showing TABLE
ENlA
grinding
behaviour
I
30
20
(MMulES)
of standard
(RB) and treated
(AG) belt on Nimonic
108
II
MICROSCOPICAL Metul
I
I
‘O TIME
EXAMINATION
Grinding conditions
1 minute grinding to 4 condition EN58J 1 minute to i condition Nimonic 108 I minute to 5 condition x40 1 minute to i condition 530 1 minute to 3 condition LM6 1 minute to + condition DTD 5099 1 minute to -J-condition
OF WORN
1,:) Capped grits RB AB
0 2 0 30 0 20 13 45 2 2 10 10 5 9
BELTS
7” Pulled out grits RB AB
0
0
2
1
5
1
0 10 0 7 1 30
1
2 5 9 3 0 0 1
8 6 10 5 10
3 25 3 5
2 2 3 2 1 4 2 3 3 10 2 1
I
“/; Grits showirlg ‘, Pitted attrition wear grits RB AB RB AB
YOGrits that have taken part in the grinding process RB AB
12 3.5 20 35 30 30 20 50 20 30 16 45 13 65
12 40 20 40 30 30 20 50 20 30 20 70 16 70
50 65 45 70 35 45 40 75 45 75 48 65 37 60
0
0
30
20
20 13 46 7 2 0 0 0 0
3 4 60 14 65 0 0 0 0
50 65 45 70 40 45 40 80 50 75 54 75 40 60
Fig. 4. Capped grit produced can be clearly seen.
Fig. 5. The appearance
when grinding
of a capped
a Nimonic
grit when grinding
alloy. The pitted appearance
a soft metal such as aluminium.
of the grit SWrface
338
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
the grits often contains micro-undulations (termed pitting in the table) which at high magnification can be seen to be smooth sided but not faceted. It can be seen that, the capped area covers only part of the grit surface and this was usually observed, although this was not the case when abrading soft metals such as aluminium, Fig. 5. Grit failure was usually by attrition rather than by large-scale fragmentation, although a few examples of obvious cleavage failures did occur, such as that shown in Fig. 6 for a belt grinding an aluminium alloy. The swarf distribution on the belt surface can easily be detected by using the characteristic X-ray emission technique, and whereas for example with a free cutting mild steel, fine swarf particles are distributed over all the belt surface, with most other metals and only swarf retained on the belts was in the form of metal caps. An alternative means of quantifying the amount of capping was by examination of the ground belt surface in the Quantimet 720 utilising the differences in reflectance between the metal swarf and the resinoid belt surface to supply the necessary contrast. The transverse metallographic sections of worn belt clearly showed the variation of cutting angle presented to the workpiece by the grits and also allowed the build-up of worn flats to be monitored (Fig. 7). Inclusions within the grits can clearly be seen in polished microsections, the identification and importance of which have formed part of a separate study.
Fig. 6. Showing
grit fragmentation
with large-scale
cleavage
fractures.
WEAR MECHANISMS WITH COATED ABRASIVES
339
Fig. 7. Optical micrograph of transverse belt section showing worn tlat on grit and area of capping.
Fig. 8. Swarf collected after abrasive grinding.
340
J. BILLINGHAM, J. LAURIDSEN, J. F. BRYON
Fig. 9. Metal surface after abrasive grinding. Note metal deformation at edges of cutting tracks.
Swarf collected from the grinding experiments was similar to that produced in conventionai machining with a tool (Fig. 8). Lengths of up to 2 mm were examined and because of the extreme ease of fracture it is likely that longer lengths were produced during the grinding. Most of the particles were approximately 50 pm in diameter (related to the grit size of the belts used) and up to 5 pm thick. When grinding a stainless steel EN58J alloy, the swarf from the RB belt showed more colouration (blueing) than the swarf from the AG belt, indicating a higher grinding temperature or thinner swarf section. The swarf size appeared to be of similar section but no detailed comparative measurements of swarf thickness were made. However, the surface roughness of metal ground with AG belts is somewhat coarser than for RB ground material with the same grit size, indicating a deeper cutting action. The metal surfaces showed typical well defined grit cutting paths (Fig. 9) with ploughed edges, indicating severe plastic deformation even with very strong high temperature materials such as Nimonic and tool steels. All this evidence points to high temperatures at the grit-metal abrading interface, a factor borne out by microanalysis of the interface regions”. 4. DJSCUSSION
Evidence was found for all the grit failure mechanisms previously discussed
WEAR MECHANISMS
WITH COATED
ABRASIVES
341
although pull-out was only really important when grinding the soft aluminium alloy and its incidence did not seem to be affected by the presence of the grinding aid. Dulling was the most common wear mode but little evidence was seen for large fragmentation indicating that attrition wear was most important. Capping was wide-spread on most alloys except for the free cutting mild steel. Two types of caps could be distinguished: a large build up of material usually covering the entire grit surface (Fig. 5) which seems merely to be physically attached to a smooth grit interface and much smaller amounts of material, often several to one grit surface and almost always associated with the pitted surface type of attrition attack (Fig. 4). The first type was present on aluminium alloys and the second on the superalloys and high strength steels i.e. on those materials where the treated grits gave improved grinding performance. The pitted appearance indicates that some chemical or deformation process has occurred rather than simple cleavage and this was confirmed by analysis of the interface region between an aluminium oxide grit and a piece of Nimonic swarf. Careful microprobe analysis work on the topside and underside of swarf caps, following their removal from the belt surface indicated the presence of aluminium on the swarf underside. These surfaces were very carefully examined in the stereoscan to ensure the complete absence of any particles of alumina. The inference from this preliminary studyi is that a chemical reaction does occur between the A1203 and the metal or possibly between the Al,O, based grit (i.e. plus impurities Si02, TiO, etc. at total levels of 5%) and the metal/metal oxide surface. Measurements of the shift in aluminium electron binding energy on these samples using photoelectron spectroscopy indicated that a spine1 compound was probably formed (NiO.Al,O,). Estimates of the temperature required for the formation of such a compound during the grinding tests indicates that the interface temperature was greater than 1100°C. The pitted nature of the grit surface seemed to be a feature of the material ground rather than whether or not the belt had been chemically treated i.e. no grit pitting was seen with aluminium or mild steel. This is further proof that it is a chemical rather than purely fractographic feature of the wear process. It is significant that the pitting only occurs with those metals on which the (AG) belt gives improved grinding performance and is likely to be closely associated with the mechanism for their improved grinding performance. Examination of Table II indicates that less caps are present on the treated (AG) than on the untreated (RB) belts for these materials. This is an especially significant result in view of the way the grinding tests were carried out since the AG belts examined in this case had removed significantly more metallic material. The results indicate that the reduction in the amount of capping seen with the AG belts is a very important feature of their improved behaviour. The second most important result to be recognised from Table II is that the AG belts generally show a higher proportion of the grits taking part in the abrasive process at all stages of the grinding but particularly when only a small amount of material has been removed. Thus the grinding process is modified in some way so as to become a more-uniform process. The keying of swarf to grits is obviously important and the reason for the success of the AG belts could be that less keying occurs, or that caps once formed could be more easily removed in subsequent grinding. Examination of belts at
342
J. BILLINGHAM,
J. LAURIDSEN,
J. F. BRYON
intermediate and more extensive stages of the grinding process and attempts to remove caps physically and re-use the belts (dressing) indicated that the first of these factors is most important. From examination of the physical and mechanical properties of the materials it seems certain that greater heat is generated when grinding the metals EN58J, Nimonic 108 and X40 (all have low thermal conductivity and reasonable strength at high temperature) and these are precisely the metals on which the AG belt performs best. Some possible mechanisms for the AG improvement are, reduced welding of Al,O, and metal, or effective cooler cutting due to chemical attack of either or both interfaces by volatile components in the grinding aid possibly resulting in the formation of a low friction compound. Some evidence exists to support this with blueing of, swarf from RB belt compared to AG indicating deeper cuts as discussed in the introduction for the vapour grinding experiments. The very irregular nature of the grits, which leads to the existence of wide variations in the distance from the backing cloth to the cutting points, results in certain critical cases (e.g. Nimonic 108) to the rejection of a belt when a very large proportion of the grits has not taken part in the grinding process. Thus both the capping and the gradual wear prevent large numbers of grits from taking part. If belts with much more uniform grit size (here we really mean backing cloth to cutting point distance) could be produced they might result in remarkable improvements in this particular case (Nimonic 108) which could offset the extra expense of their production. This is one possible reason for how the treated belts work because a much higher proportion of grits take part in the grinding due possibly to ease of grit fracture or to less capping. The two most important grit failure mechanisms are capping and dulling. Capping is substantially reduced in the presence of grinding aid chemicals on materials such as superalloys and stainless steels due to a modification in the interface reaction between the swarf and the alumina grit. Dulling occurs by some small-scale fragmentation ofgrits but the main wear mechanism appears to be a form of chemical attack probably related to high temperature reaction with metallic oxides which leads to a macroscopically flat surface which in fact consists of micro-undulations termed pitting. The reasons for the improved behaviour of the treated (AG) belts have not been clearly defined. However this study has shown that physical support of the grits is unimportant and indicates that the AG belts most probably work by reducing adhesion (or reaction) between the grits and the metal surface (or metal oxides). The fact that the behaviour varies from metal to metal would appear to rule out mechanisms based entirely on reaction between the grit and the grinding aid, whether these involve modified fracture behaviour or interfacial reaction. However this may be a slight over simplification of the problem since the metals where no improvement takes place are also those where the interface temperatures should be lower. Experiments involving modified grinding temperature may therefore be very important and although we carried out preliminary experiments with these aims they may not have been di~riminatory enough to detect possible effects. Further photoelectron spectroscopy is being carried out in an attempt to see how the AG additive modifies the grit-swarf interaction. It is hoped that this information will enable
WEAR
MECHANISMS
WITH
COATED
ABRASIVES
343
the present grinding aids to be improved either by improved performance per se or by extending the range of materials on which they give improved grinding behaviour. ACKNOWLEDGEMENTS
The authors are grateful to the Department of Trade and Industry to English Abrasives Ltd. for permission to publish this work.
and
REFERENCES 1 K. Lacy, Metalworking Production, 113 (21) (1969) 45. 2 J. P. Bryon and A. G. Rolfe, Coated Abrasive Articles, English Abrasives Ltd., B.P.1, 145, 082 (1969). 3 Supersize Film Forming Resin Coated Abrasives, Minnesota Mining and Manufacturing Co. Ltd., U.S.P. 3, 256, 076 (1966). 4 Anti-Weld Additioesfor Coated Abrasioe Bonds, Carborundum Co. Ltd., U.S.P. 3, 058, 819 (1962). 5 T. 0. Mulhearn and L. E. Samuels, Wear, 5 (1962) 478. 6 B. W. E. Avient, J. Goddard and H. Wilman, Proc. Roy. Sot. (London), A258 (1960) 159. 7 M. M. Khruschov, Proc. Con& Lubrication and Wear, London, 1957, p. 655. 8 R. W. Johnson, Wear, 12 (1968) 213, 9 H. N. Dyer, Ind. Eng. Chem., 47 (12) (1955) 2500. 10 E. J. Duwell and W. J. McDonald, Wear, 4 (1961) 384. 11 G. W. Rowe, Conf. Lubrication and Wear, Proc. Inst. Mech. Eng., 1957, p. 333. 12 M. Vowles, J. Billingham and J. P. Bryon, submitted for publication in Metallography.