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Friction and wear of some engineering materials against hard chromium plating
Wear, 28 ( 1974) 49-57 c> Elsevier Sequoia S.A., Lausanne
49 - Printed
FRICTION AND WEAR OF SOME HARD CHROMIUM PLATING
V. GOLOGAN*
in The Netherlands
ENGINEERING
MATERIALS
AGAINST
and T. S. EYRE
Brunei University, Uxbridge, Mddx. UB8 3PH (Gt. Britain) (Received
October
26, 1973)
SUMMARY
Using a simple pin on disc machine the friction and wear characteristics of grey cast iron, aluminium bronze and phosphor bronze rubbing against channel type and conventionally ground surfaces of chromium plating were studied. The results are discussed and microscopical investigations of tested specimens used to illustrate the conclusions derived.
INTRODUCTION
Chromium is the most widely used electrodeposited coating for wear resistance, however, it has no engineering application in its ,pure form in either the cast or worked condition. In the electrodeposited condition the structure is much finer than in the cast or wrought form and the crystal structure contains many faults and built in oxide or hydroxide complexes. Because of this complex structure and built in stress pattern a high hardness may be developed (800 HV) which is coupled with good corrosion resistance. A low coefficient of friction is partly responsible for producing a high resistance to mechanical wear. Corrosion resistance was mainly responsible for the wide introduction of chromium plated cylinder bores for large marine diesel engines running on crude oil. The deposit thickness lies between 0.05-5 mm compared with up to 0.01 mm for coatings required solely for decorative purposes. Worn components which are subsequently stripped may be replated before being placed back in service. Chromium may be successfully plated onto a wide range .of non-ferrous and ferrous metals. Because of the poor wetability of chromium in contact with lubricating oils the development of the porous chrome technique by Dr. H. Van der Host in 1935 marked an important step forward. In this technique the electrodeposited coating is broken up by an etching process to produce surface porosity’. Currently two types of porous finish, referred to as intermediate and channel type are in use as well as the plain ground finish. This paper is primarily concerned with the more widely used channel type structure shown in Fig. l(a). The * Present
address:
Kishinev
Polytechnical
Institute,
Kishinev,
U.S.S.R.
50
V. GOLOGAN,
T. S. EYRE
Fig. 1. Appearance of unworn chromium plated surfaces. (a) Ground surface channel cracks masked by the grinding process. ( x 280) (b). As plated finish--channel cracked structure easily visible at low magnification. ( x 8.5)
partially
inter-connecting channel structure allows and promotes the free flow of lubricant through the surface by a capillary action. Mechanical keying of lubricant to the chromium surface is also said to be improved. Poor wear behaviour of the channel type chromium surface has been reported when rubbed against a sintered Fe~graphite material, this is rather surprising in the light of the low hardness of this material (5040 HV). As operating conditions in diesel engines have become more severe particularly to effect reduction in oil consumption, it has become apparent that the life expectancy of chromium plated piston ring surfaces has been called into question2. It was generally concluded that hard chromium is unsatisfactory when the lubricant condition becomes marginal and some metal to metal contact occurs. Diesel engine experience has shown that chromium wears by the removal of small granular particles which become distributed in the oil film. Some of these particles also become embedded in the rubbing surfaces of the piston rings producing an abrasive cap. The experiments reported here were carried out under unlubricated conditions so as to form a basis for both dry and marginally lubricated wear situations. The authors were particularly concerned to see if the surface structure of the chromium played an important part in the wear process and also ro see if wear varied with the nature of the mating material. EXPERIMENTAL
DETAILS
A pin on disc machine was used in which friction force and linear wear of the pin was measured continuously throughout the test. Wear pins were machined from phosphor bronze, aluminium bronze and grey cast iron, the composition and mechanical properties of which are given in Table I. Wear pins were cylindrical, approximately 5.0 cm long with flat ends of 30 mm2 area.
FRICTION TABLE
AND WEAR
OF MATERIALS
AGAINST
51
Cr PLATING
I
COMPOSITION, Material
PERCENT Hardness
OF WEAR
MATERIALS
INVESTIGATED
Sn
P
CU
0.02Q4
Remainder
Al
Fe
Si
Mn
Ni
8.5-l 1.0
4.06.0
~
0.5
4.060
2.1
0.65
~
C
(HV). P bronze BS 36911963 PB 102
180/195
4.5-6.0
Al bronze BS 2874 CA 104
2101230
-
Grey C.I. BS 1452
190/210
-
Remainder
0.102
~
_
Discs were machined from steel to BS 970, EN24, hardened and tempered to 350 HV. The discs were electroplated with hard chromium (865 HV) to a depth of 0.25 mm. Half of the discs were ground to a surface finish of 20 pin. and the remainder were processed to prdduce a standard channel type structure. The discs were approximately 12.5.cm diam. and 1 cm thick and by varying the wear track radius three tests could be carried out on each side of the disc. All the experiments were carried out at a constant sliding speed of 300 cm/s under dry conditions with a relative humidity of approximately 5065% and a temperature in the range 60”-70°F. RESULTS
Equilibrium wear rates were obtained after all running-in was completed and were plotted against the load at which the test was carried out (Fig. 2). It will be observed that Al bronze is slightly superior to both P bronze and grey cast iron at loads below 6 kg. Al bronze, however, undergoes a marked mild to severe wear transition between 6 and 8 kg load. P bronze does not exhibit a wear transition, but the wear rate rapidly increased linearly with load. At loads above 20 kg there is little difference in wear rate between the Al bronze and P bronze. Grey cast iron does not exhibit a mild to severe wear transition load and the increase in wear rate with load is very small, wear only increasing by a factor of 2 whilst the load increases by an order of magnitude. At loads above 6 kg the cast iron is markedly superior to the other metals tested. For Al bronze the change from a low friction to a high friction condition is very marked (Fig. 3) and coincides with the wear rate change. It was observed that in the oxidative wear region, wear of the chromium surface was quite marked, which indicates the abrasive nature of the aluminium oxide. The friction changes are less well marked for both the P bronze and grey cast iron, but the values for both Al bronze and P bronze above 6 kg become approximately the same. Grey cast iron exhibits a much lower coefficient of friction and this is consistent with the lower wear rates for this material. Examination of the wear surfaces of the pin and disc indicated considerable
3.02
v. GOLOGAN,
I-. s. EYRE
0.8 Al I bronze
0.7 0.6.. c 9 .;0.5& 50.4F .E 0.3 -
““0
i
2*
4 L
6 I
8 I
IO I
12 ,I
14
16 I
18 II
Load (kgs)
20
22 ,
24
0’
x
I 2
t 4
b 6
+ 8
’ t ’ 10 12 14 Load ( kgs)
c
16
c
18
E 20
Fig. 2. Wear rate us. load. Fig. 3. Coefficient
of friction
I’S, load.
Fig. 4. Surfaces of cast iron pin and chromium plated disc worn at 5 kg load. {a) Cast iron pin showing evidence of abrasion and localised plastic flow. f x 75) (b) Chromium plated disc covered with patches of smooth oxide. No evidence
of channel
structure.
( x 150)
differences in wear behaviour which are consistent with the previous observations. At all loads the grey cast iron pin quickly built up an oxide layer and became relatively smooth (Fig. 4a). A smooth oxide was also produced on the channel structure chromium and completely masked the underlying structure of the chromium (Fig. 4b). There was no evidence of metallic adhesion and this is consistent with the low friction and wear rates. Wear of the pin proceeded by the removal of plates of oxide.
:
FRICTION
AND WEAR
OF MATERIALS
AGAINST
53
Cr PLATING
Fig. 5. Worn surface of P bronze and Al bronze I,ins- -load 15 kg. (a) P bronze-wear by an abrasive process. ( x 28;0) (b) Al bronze-wear by plastic Now; greater dam age t hart to P bronze.
( x 280)
Fig. 6. Surface of worn Al bronze pin. (a) Transverse shear cracks in the wear track. ( x 260) (b) Plastic flow and fracture to form wear debris. (xl 300)
Both Al bronze and P bronze exhibited wear by a mixed abrasive wear/ plastic deformation mechanism (Fig. S), particles of metal in the form of plates gradually becoming detached from the surface to form wear debris, some of which adhered to the steel disc. The Al bronze appeared to be more ductile and the scale of the damage was rather greater than on the P bronze. The generation of transverse shear cracks can be clearly seen in Fig. 6 from which the metallic wear debris was formed. A striking feature of the wear of Al bronze against the channel type chromium plating was the mechanism of adhesion of bronze wear particles to the chromium surface (Fig. 7a). Bronze particles became trapped in the channel cracks, particularly those running predominantly transverse to the rubbing direction.
V. GOLOGAN.
T. S. EYRE
Fig. 7. Chromium plated surface rubbed against Al bronze. ( x 320) (a) Transfer of Al bronze to the channel cracks. (b) Layers of Al bronze wear debris attached to the chromium surface.
Fig. 8. Chromium plated surface rubbed against cast iron, load (a) Upper part worn-lower part unworn. ( x 230) (b) Pitted area of the wear track. ( x 560)
1 kg.
This process became more obvious at the mild/severe transition for Al bronze and very quickly a transferred layer of bronze was formed on the chromium surface. The wear situation changed to Al bronze rubbing on Al bronze with a high coefficient of friction and a high wear rate (Fig. i’b). Experiments were repeated on both channel type and conventionally ground chromium surfaces and the mild/severe transition load was approximately 2 kg for Al bronze against channel chrome and 6 to 8 kg against as ground chromium. No significant differences were observed with grey cast iron and the differences with P bronze only became significant at higher loads. A further feature of the wear of channel chromium is shown in Fig. 8(a). After mild wear of grey cast iron with the production of a smooth oxide which
FRICTION
AND WEAR
OF MATERIALS
AGAINST
Cr PLATING
Fig. 9. Chromium plated surfaces rubbed against cast iron. (a) No evidence of cracking caused by wear. ( x 2140) (b) Cracks transverse to the rubbing direction ca used by wear. ( x 2801 (c) Well developed crack pattern. ( x 270)
55
56
V. GOLOGAN,
T. S. EYRE
masked the channel type chromium surface, the oxide was removed by swabbing with dilute hydrochloric acid. In the top half of Fig. 8(a), which coincides with the actual wear track fragmentation of the chromium has taken place. Higher magnification revealed that some of these particles have become detached leaving pits in the chromium surface. A further series of experiments was carried out to examine the stress dependency of the crack formation in chromium plated surfaces when rubbed against cast iron. Figure 9 shows that the density of cracks increases as the applied load increases and these cracks are formed predominantly at right angles to the rubbing direction. DISCUSSION
Wear of Al bronze and P bronze takes place predominantly by a mixed abrasive adhesive wear mechanism. Metallic wear debris attached itself to the channel type chromium plating by penetration into the open channels which has the effect of increasing the strength of adhesion of these particles to the disc surface. Wear then proceeded by further build up of transferred particles which become attached to the primary transferred layer which in turn gradually increased in the surface area covering the chromium. This process was much more marked with Al bronze than with P bronze. Aluminium oxide generated during oxidative wear, had the effect of rapidly wearing the chromium surface. There is a mild/severe transition with Al bronze on chromium plating and the transition load is lower with channel type chromium than with non channel type chromium. P bronze does not exhibit a marked mild/severe transition load but the severity of metallic adhesive wear increased gradually as the load increases. Both the friction force and wear rates became approximately the same for Al bronze and P bronze at loads greater than 18 kg. Grey cast iron Wears satisfactorily against channel type chromium plating with the build up of an oxide layer on both the pin and disc wear surfaces. There was no evidence of adhesive wear and oxidative wear appears to be the predominant wear mechanism. The wear rate increased extremely slowly as the load increased and the coefficient of friction was lower than that for Al bronze and P bronze. At the loads used in this investigation mild wear was always obtained which indicated that the mild/severe transition load was not exceeded and is, therefore greater than 24 kg. Rubbing of grey cast iron against channel type chromium had the effect of breaking up the chromium matrix between the channel cracks into smaller areas and some of these particles then broke away leaving a pitted surface. This appearance is similar to that obtained in industrial practice with diesel engines. The tendency to crack formation in the chromium plating increased as the load increased and is more predominant in the immediate vicinity of the channel cracks. This evidence indicates that a fatigue mechanism is responsible for this type of chromium failure.
FRICTION AND WEAR OF MATERIALS AGAINST Cr PLATING
57
CONCLUSIONS
(1) Al bronze and chromium plated steel exhibited a classic mild/severe transition behaviour. (2) For both Al bronze and P bronze when the coefficient of friction is approximately 0.7 an adhesive form of wear takes place. (3) The oxide formed on Al bronze was sufficiently abrasive to wear hard chromium at a significant rate. (4) Under the conditions used in this investigation grey cast iron did not exhibit a mild/severe transition and there was no evidence of adhesive wear. (5) Two disadvantages directly related to the channel structure of the chromium have been shown in these ex~riments. (a) With Al bronze and to a considerabIy lesser degree with P bronze, mechanica keying of wear debris was facilitated by the presence of the channel structure in the surface of the chromium plating. (b) With grey cast iron, fragmentation and fracture of the chromium plating take place with the removal of some chromium particles to leave a pitted surface. ACKNOWLEDGEMENTS
One of the authors (V.C.) acknowledges the assistance of the British Council during his stay in the United Kingdom. Both authors thank Professor C. Bodsworth, Head of ~etaIlurgy Department, Brunel University for research facilities. REFERENCES 1 P. A. D. Fenton and A. Oolbekkink, Electra-plating applied to diesel engine components, Diesel Engineers and Users Association, preprint 356, 1973. 2 W. H. Charlesworth and W. L. Brown, Wear of chromium piston rings in modern automotive engines, Sot. Auto. Engrs., Auto. Eng. Congress, Detroit, 1967, 670042.
49 - Printed
FRICTION AND WEAR OF SOME HARD CHROMIUM PLATING
V. GOLOGAN*
in The Netherlands
ENGINEERING
MATERIALS
AGAINST
and T. S. EYRE
Brunei University, Uxbridge, Mddx. UB8 3PH (Gt. Britain) (Received
October
26, 1973)
SUMMARY
Using a simple pin on disc machine the friction and wear characteristics of grey cast iron, aluminium bronze and phosphor bronze rubbing against channel type and conventionally ground surfaces of chromium plating were studied. The results are discussed and microscopical investigations of tested specimens used to illustrate the conclusions derived.
INTRODUCTION
Chromium is the most widely used electrodeposited coating for wear resistance, however, it has no engineering application in its ,pure form in either the cast or worked condition. In the electrodeposited condition the structure is much finer than in the cast or wrought form and the crystal structure contains many faults and built in oxide or hydroxide complexes. Because of this complex structure and built in stress pattern a high hardness may be developed (800 HV) which is coupled with good corrosion resistance. A low coefficient of friction is partly responsible for producing a high resistance to mechanical wear. Corrosion resistance was mainly responsible for the wide introduction of chromium plated cylinder bores for large marine diesel engines running on crude oil. The deposit thickness lies between 0.05-5 mm compared with up to 0.01 mm for coatings required solely for decorative purposes. Worn components which are subsequently stripped may be replated before being placed back in service. Chromium may be successfully plated onto a wide range .of non-ferrous and ferrous metals. Because of the poor wetability of chromium in contact with lubricating oils the development of the porous chrome technique by Dr. H. Van der Host in 1935 marked an important step forward. In this technique the electrodeposited coating is broken up by an etching process to produce surface porosity’. Currently two types of porous finish, referred to as intermediate and channel type are in use as well as the plain ground finish. This paper is primarily concerned with the more widely used channel type structure shown in Fig. l(a). The * Present
address:
Kishinev
Polytechnical
Institute,
Kishinev,
U.S.S.R.
50
V. GOLOGAN,
T. S. EYRE
Fig. 1. Appearance of unworn chromium plated surfaces. (a) Ground surface channel cracks masked by the grinding process. ( x 280) (b). As plated finish--channel cracked structure easily visible at low magnification. ( x 8.5)
partially
inter-connecting channel structure allows and promotes the free flow of lubricant through the surface by a capillary action. Mechanical keying of lubricant to the chromium surface is also said to be improved. Poor wear behaviour of the channel type chromium surface has been reported when rubbed against a sintered Fe~graphite material, this is rather surprising in the light of the low hardness of this material (5040 HV). As operating conditions in diesel engines have become more severe particularly to effect reduction in oil consumption, it has become apparent that the life expectancy of chromium plated piston ring surfaces has been called into question2. It was generally concluded that hard chromium is unsatisfactory when the lubricant condition becomes marginal and some metal to metal contact occurs. Diesel engine experience has shown that chromium wears by the removal of small granular particles which become distributed in the oil film. Some of these particles also become embedded in the rubbing surfaces of the piston rings producing an abrasive cap. The experiments reported here were carried out under unlubricated conditions so as to form a basis for both dry and marginally lubricated wear situations. The authors were particularly concerned to see if the surface structure of the chromium played an important part in the wear process and also ro see if wear varied with the nature of the mating material. EXPERIMENTAL
DETAILS
A pin on disc machine was used in which friction force and linear wear of the pin was measured continuously throughout the test. Wear pins were machined from phosphor bronze, aluminium bronze and grey cast iron, the composition and mechanical properties of which are given in Table I. Wear pins were cylindrical, approximately 5.0 cm long with flat ends of 30 mm2 area.
FRICTION TABLE
AND WEAR
OF MATERIALS
AGAINST
51
Cr PLATING
I
COMPOSITION, Material
PERCENT Hardness
OF WEAR
MATERIALS
INVESTIGATED
Sn
P
CU
0.02Q4
Remainder
Al
Fe
Si
Mn
Ni
8.5-l 1.0
4.06.0
~
0.5
4.060
2.1
0.65
~
C
(HV). P bronze BS 36911963 PB 102
180/195
4.5-6.0
Al bronze BS 2874 CA 104
2101230
-
Grey C.I. BS 1452
190/210
-
Remainder
0.102
~
_
Discs were machined from steel to BS 970, EN24, hardened and tempered to 350 HV. The discs were electroplated with hard chromium (865 HV) to a depth of 0.25 mm. Half of the discs were ground to a surface finish of 20 pin. and the remainder were processed to prdduce a standard channel type structure. The discs were approximately 12.5.cm diam. and 1 cm thick and by varying the wear track radius three tests could be carried out on each side of the disc. All the experiments were carried out at a constant sliding speed of 300 cm/s under dry conditions with a relative humidity of approximately 5065% and a temperature in the range 60”-70°F. RESULTS
Equilibrium wear rates were obtained after all running-in was completed and were plotted against the load at which the test was carried out (Fig. 2). It will be observed that Al bronze is slightly superior to both P bronze and grey cast iron at loads below 6 kg. Al bronze, however, undergoes a marked mild to severe wear transition between 6 and 8 kg load. P bronze does not exhibit a wear transition, but the wear rate rapidly increased linearly with load. At loads above 20 kg there is little difference in wear rate between the Al bronze and P bronze. Grey cast iron does not exhibit a mild to severe wear transition load and the increase in wear rate with load is very small, wear only increasing by a factor of 2 whilst the load increases by an order of magnitude. At loads above 6 kg the cast iron is markedly superior to the other metals tested. For Al bronze the change from a low friction to a high friction condition is very marked (Fig. 3) and coincides with the wear rate change. It was observed that in the oxidative wear region, wear of the chromium surface was quite marked, which indicates the abrasive nature of the aluminium oxide. The friction changes are less well marked for both the P bronze and grey cast iron, but the values for both Al bronze and P bronze above 6 kg become approximately the same. Grey cast iron exhibits a much lower coefficient of friction and this is consistent with the lower wear rates for this material. Examination of the wear surfaces of the pin and disc indicated considerable
3.02
v. GOLOGAN,
I-. s. EYRE
0.8 Al I bronze
0.7 0.6.. c 9 .;0.5& 50.4F .E 0.3 -
““0
i
2*
4 L
6 I
8 I
IO I
12 ,I
14
16 I
18 II
Load (kgs)
20
22 ,
24
0’
x
I 2
t 4
b 6
+ 8
’ t ’ 10 12 14 Load ( kgs)
c
16
c
18
E 20
Fig. 2. Wear rate us. load. Fig. 3. Coefficient
of friction
I’S, load.
Fig. 4. Surfaces of cast iron pin and chromium plated disc worn at 5 kg load. {a) Cast iron pin showing evidence of abrasion and localised plastic flow. f x 75) (b) Chromium plated disc covered with patches of smooth oxide. No evidence
of channel
structure.
( x 150)
differences in wear behaviour which are consistent with the previous observations. At all loads the grey cast iron pin quickly built up an oxide layer and became relatively smooth (Fig. 4a). A smooth oxide was also produced on the channel structure chromium and completely masked the underlying structure of the chromium (Fig. 4b). There was no evidence of metallic adhesion and this is consistent with the low friction and wear rates. Wear of the pin proceeded by the removal of plates of oxide.
:
FRICTION
AND WEAR
OF MATERIALS
AGAINST
53
Cr PLATING
Fig. 5. Worn surface of P bronze and Al bronze I,ins- -load 15 kg. (a) P bronze-wear by an abrasive process. ( x 28;0) (b) Al bronze-wear by plastic Now; greater dam age t hart to P bronze.
( x 280)
Fig. 6. Surface of worn Al bronze pin. (a) Transverse shear cracks in the wear track. ( x 260) (b) Plastic flow and fracture to form wear debris. (xl 300)
Both Al bronze and P bronze exhibited wear by a mixed abrasive wear/ plastic deformation mechanism (Fig. S), particles of metal in the form of plates gradually becoming detached from the surface to form wear debris, some of which adhered to the steel disc. The Al bronze appeared to be more ductile and the scale of the damage was rather greater than on the P bronze. The generation of transverse shear cracks can be clearly seen in Fig. 6 from which the metallic wear debris was formed. A striking feature of the wear of Al bronze against the channel type chromium plating was the mechanism of adhesion of bronze wear particles to the chromium surface (Fig. 7a). Bronze particles became trapped in the channel cracks, particularly those running predominantly transverse to the rubbing direction.
V. GOLOGAN.
T. S. EYRE
Fig. 7. Chromium plated surface rubbed against Al bronze. ( x 320) (a) Transfer of Al bronze to the channel cracks. (b) Layers of Al bronze wear debris attached to the chromium surface.
Fig. 8. Chromium plated surface rubbed against cast iron, load (a) Upper part worn-lower part unworn. ( x 230) (b) Pitted area of the wear track. ( x 560)
1 kg.
This process became more obvious at the mild/severe transition for Al bronze and very quickly a transferred layer of bronze was formed on the chromium surface. The wear situation changed to Al bronze rubbing on Al bronze with a high coefficient of friction and a high wear rate (Fig. i’b). Experiments were repeated on both channel type and conventionally ground chromium surfaces and the mild/severe transition load was approximately 2 kg for Al bronze against channel chrome and 6 to 8 kg against as ground chromium. No significant differences were observed with grey cast iron and the differences with P bronze only became significant at higher loads. A further feature of the wear of channel chromium is shown in Fig. 8(a). After mild wear of grey cast iron with the production of a smooth oxide which
FRICTION
AND WEAR
OF MATERIALS
AGAINST
Cr PLATING
Fig. 9. Chromium plated surfaces rubbed against cast iron. (a) No evidence of cracking caused by wear. ( x 2140) (b) Cracks transverse to the rubbing direction ca used by wear. ( x 2801 (c) Well developed crack pattern. ( x 270)
55
56
V. GOLOGAN,
T. S. EYRE
masked the channel type chromium surface, the oxide was removed by swabbing with dilute hydrochloric acid. In the top half of Fig. 8(a), which coincides with the actual wear track fragmentation of the chromium has taken place. Higher magnification revealed that some of these particles have become detached leaving pits in the chromium surface. A further series of experiments was carried out to examine the stress dependency of the crack formation in chromium plated surfaces when rubbed against cast iron. Figure 9 shows that the density of cracks increases as the applied load increases and these cracks are formed predominantly at right angles to the rubbing direction. DISCUSSION
Wear of Al bronze and P bronze takes place predominantly by a mixed abrasive adhesive wear mechanism. Metallic wear debris attached itself to the channel type chromium plating by penetration into the open channels which has the effect of increasing the strength of adhesion of these particles to the disc surface. Wear then proceeded by further build up of transferred particles which become attached to the primary transferred layer which in turn gradually increased in the surface area covering the chromium. This process was much more marked with Al bronze than with P bronze. Aluminium oxide generated during oxidative wear, had the effect of rapidly wearing the chromium surface. There is a mild/severe transition with Al bronze on chromium plating and the transition load is lower with channel type chromium than with non channel type chromium. P bronze does not exhibit a marked mild/severe transition load but the severity of metallic adhesive wear increased gradually as the load increases. Both the friction force and wear rates became approximately the same for Al bronze and P bronze at loads greater than 18 kg. Grey cast iron Wears satisfactorily against channel type chromium plating with the build up of an oxide layer on both the pin and disc wear surfaces. There was no evidence of adhesive wear and oxidative wear appears to be the predominant wear mechanism. The wear rate increased extremely slowly as the load increased and the coefficient of friction was lower than that for Al bronze and P bronze. At the loads used in this investigation mild wear was always obtained which indicated that the mild/severe transition load was not exceeded and is, therefore greater than 24 kg. Rubbing of grey cast iron against channel type chromium had the effect of breaking up the chromium matrix between the channel cracks into smaller areas and some of these particles then broke away leaving a pitted surface. This appearance is similar to that obtained in industrial practice with diesel engines. The tendency to crack formation in the chromium plating increased as the load increased and is more predominant in the immediate vicinity of the channel cracks. This evidence indicates that a fatigue mechanism is responsible for this type of chromium failure.
FRICTION AND WEAR OF MATERIALS AGAINST Cr PLATING
57
CONCLUSIONS
(1) Al bronze and chromium plated steel exhibited a classic mild/severe transition behaviour. (2) For both Al bronze and P bronze when the coefficient of friction is approximately 0.7 an adhesive form of wear takes place. (3) The oxide formed on Al bronze was sufficiently abrasive to wear hard chromium at a significant rate. (4) Under the conditions used in this investigation grey cast iron did not exhibit a mild/severe transition and there was no evidence of adhesive wear. (5) Two disadvantages directly related to the channel structure of the chromium have been shown in these ex~riments. (a) With Al bronze and to a considerabIy lesser degree with P bronze, mechanica keying of wear debris was facilitated by the presence of the channel structure in the surface of the chromium plating. (b) With grey cast iron, fragmentation and fracture of the chromium plating take place with the removal of some chromium particles to leave a pitted surface. ACKNOWLEDGEMENTS
One of the authors (V.C.) acknowledges the assistance of the British Council during his stay in the United Kingdom. Both authors thank Professor C. Bodsworth, Head of ~etaIlurgy Department, Brunel University for research facilities. REFERENCES 1 P. A. D. Fenton and A. Oolbekkink, Electra-plating applied to diesel engine components, Diesel Engineers and Users Association, preprint 356, 1973. 2 W. H. Charlesworth and W. L. Brown, Wear of chromium piston rings in modern automotive engines, Sot. Auto. Engrs., Auto. Eng. Congress, Detroit, 1967, 670042.