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Assessing the durability of electrodeposited metal films
Wear, 28 (1974) 271-275 ‘7; Elsevier Sequoia S.A., Lausanne
27‘1 - Printed
in The Netherlands
SHORT COMMUNICATION
Assessing the durability of electrodeposited metal films
B. J. BRISCOE* Physics and Chemistry of Solids, Cuvendish Laboratory,
Cambridge
(ct.
~rjfa~~~
T. S. EYRE Department
of Metallurgy,
Brunei
University, llxbridge
(Gt. Britain)
V. F. GOLOGAN Department
of Metal Cutting and Machine
Tools, Kishinev
Polytechnical
Institute, Kishinev
(U.S.S.R.)
(ReceivedNovember26, 1973)
The paper describes a simple test for predicting the durability of electrodeposited iron and chromium films in machine parts. These films are also shown to behave, to a limited extent, as brittle materials while undergoing plastic deformation, introduction Electrodeposited metal films are extensively used in machine parts. An almost infinite number of plating conditions may be chosen and it is the technologist’s task to select those conditions which provide the’best wear characteristics for a particular range of operating conditions. Direct measurement of wear rates is time consuming and the plating chemist requires an alternative which is both sensitive and simple to execute. Generally, normal micro-hardness testing does not provide a unique criterion for specifying the durability of these films. This paper describes what appears to be a more useful test. The static load is measured, for a given indenter radius, at which extensive microcracking of the film is observed. As an example data are given for iron films electrodeposited on hardened steel substrates. In order to quantify what is essentially an empirical test the study was extended to include a dynamic measurement of this “failure’* load during sliding and treatment of the data in the fashion described by several previous authors for brittle materials was attempted. Materials Iron films were deposited on steel substrates from 3M aqueous solutions of ferrous chloride (as FeC1,4H,O) at 40°C using a pH of 0.8. The current density was varied from 5 to 40 A dm- 2. This has the effect of significantly changing both the mechanical and chemical properties of the film. The films were approximately 0.5 mm in thickness. * To whom correspondence
should
be addressed.
273
SlIOKI ~‘oMhll:hlC’ATIoNS
The wear experiments were carried out on an Amsler-type wear testing machine where a cast iron shoe (face area I cm’) was held against a rotating steel shaft which was plated with the iron film. The normal load ranged from 1 to 5 kg and the operating temperature was about 40 C. Both lubricated and unlubricated tests werecarriedout, the former with straight mineral oil at a sliding speed of 100 msK’. and the latter at 2 rns~~I. Both the shoe and the film wore but most of the wear. over X0”,,, occurred in the film. The wear was rnonitor~~l h! measuring the change in the thickness of the shoe and tpe film. This work is described in greater detail elsewhere’ ‘. The static loads and the loads in a friction experiment required to induce micro-cracking were measured using a modified version of a friction measuring machine described by Eldredge and Tabor”. In the static tests spheres of diameters between 1 mm and 6 mm of either fired quartz or solvent cleaned steel were loaded against the films for 15 s and the load was progressively increased in steps untii a continuous circumferential crack was observed around the region of plastic deformation. Figure 1 shows such a condition. The dynamic measurements
Fig. 1. Illustration of microcracking complete ~ircurnfer~n~~~l microcrack.
under
a static
loading
condition
just
sufficient
to cause
;I
were made using the same indenters as above at a sliding speed of approximately 20 mm s-r. The metal films were degreased in petroleum vapour. The critical load to cause micro~r~cking was taken as the point where extensive mi~ro~ra~king was observed within the plastically deformed track. These cracks were mainly observed normal to the direction of the track and they were generally linear cracks extending across about SOo//,of the plastically deformed zone. No cracking was noted on the edge of the deformed zone. The observed coefficients of friction, p, ranged from 0.2 to 0.37. The microhardness measurements were made with a standard Leitz microhardness tester using a load of 100 g. Reszilts and discussion The critical loads under static and under dynamic, i.e. sliding conditions, to cause microcracking, W, and W;, were found to be proportional to the radii of the indenters. A similar relationship has been observed in the beha~iour of brittle solids5.6. However, the static and dynamic failure pressures, P; and P& that is the normal load divided by the observed area of plastic deformation, were each within experimental error independent of the curvature of the slider for all the films studied. Figures 2 and 3 show these quantities pfotted against
90
lDtA
dm-*)
I,(A dmw2) Fig. 2. Static pressure, normal load divided by the area of plast~caily deformed zone, to cause continuous circumferential microcracks against the current density used during electrodeposition.
Fig. 3. Pressure to cause microcracking under sliding conditions against current density.
the current density, 1, used to prepare the films. The curves have the same form, the maximum failure pressures occurring for films plated at approximately 20 A dme2, The dynamic values are, however, significantly less than the static values. Figure 4 shows the relative wear resistance of the films in the Amster testing machine under the conditions described earlier. There appears to be a correlation between static and dynamic “failure” conditions and the wear rate. Finally in Fig. 5 Vickers microh~rdness is shown against current density used during the deposition of the film; hardness does not provide a unique criterion for predicting the film’s durability.
274
SHORT C’OMMUNKYATIONS
I,( A dm- 2, &,(A dm- ‘) Fig. 4. Relative wear resistance (the reciprocal of the wear rates normalised to the value for the films prepared at 40 A dm ‘) against current density. Fig. 5. Vickers pyramidal hardness against current density.
Although extensive plastic deformation of the film both in static and dynamic tests was observed, the films show a characteristic exhibited by brittle materials, that is microcracking. This cracking appears at a critical load whose value is directly related to the radius of the indenter. Further, the onset of microcracking occurs at much lower loads for sliding experiments than static experiments. This is also a feature noted with brittle materials. In the sliding case the tangential stresses at the interface cause the onset of cracking at normal loads less than those observed for static tests. Gilroy and Hirst” have studied microcracking in glass and observed that
where A is a constant containing Poisson’s ratio for the film. For iron A may be put as 8.00. First, IV.,C/IV,C is independent of the radius of curvature of the slider and experimentally it appears to be so for the present data. Secondly, the calculated ratio may be compared with that given by experiment. The calculated value of W,C/IV; is approximately 0.02 while experimentally it was about 0.1. When experiments are carried out on truly brittle materials, the agreement is much closer. For example, experiments carried out by Powell and Tabor’ in air on titanium carbide gave a mean experimental ratio of approximately 0.2 compared with a calculated value about 0.1. The agreement in the present experiments is fair but cannot be considered highly significant as extensive plastic deformation is also observed in these experiments. Finally, a limited number of experiments has been carried out on chromium lilms( deposited on steel) and a similar type of behaviour was observed. Although this correlation between wear resistance and cracking behaviour suggests that wear is dominated by cracking of the film, optical and electron microscopic examination of the worn film does not show unequivocally that wear is a fracture dominated process.
(a) There
appears
to be a correlation
between
the wear rate under
SHORT COMMUNICATIONS
275
certain conditions and the static normal pressure required to cause microcracking in electrodeposited iron and chromium films. (b) Although the main mode of deformation is plastic, electrodeposited metal films behave in some regards like brittle materials.
The authors are particularly grateful to Professor David Tabor, F.R.S., for his help and encouragement and also for many helpful discussions during the preparation of this paper. REFERENCES I Y. N. Petrov, The effect of electrolysis conditions on the wear resistance of porous iron coatings, Trans. Kishinev SeLKhoz. Inst.. 40 (1966) 172-78, l&90. Y. N.-Petrov, Increasing the wear resistance of metat surfaces by iron and chromium platings, Electronnaya Obrabotka Metallot., Akad. Nauk Uddavsk SSR, 4 (1966) 21-28.
V. F. Gologan, The wear of electrolytic iron coatings during dry friction, Trans. Kishineu SelKhoz. Inst., 54 (1968) 7-77.
K. R. Eldredge and D. Tabor, Mechanism of rolling friction, the plastic range, Proc. Roy. Sot. A229 (1955) 181-198. D. R. Gilroy and W. Hirst, Brittle fracture of glass under normal and sliding loads, J. Phys. D, 2 (1969) 1784-1787. B. D. Powell and D. Tabor, The fracture of titanium carbide under static and sliding contact, J. Phys. D, 3 (1970) 783-788. (Lmdonj,
27‘1 - Printed
in The Netherlands
SHORT COMMUNICATION
Assessing the durability of electrodeposited metal films
B. J. BRISCOE* Physics and Chemistry of Solids, Cuvendish Laboratory,
Cambridge
(ct.
~rjfa~~~
T. S. EYRE Department
of Metallurgy,
Brunei
University, llxbridge
(Gt. Britain)
V. F. GOLOGAN Department
of Metal Cutting and Machine
Tools, Kishinev
Polytechnical
Institute, Kishinev
(U.S.S.R.)
(ReceivedNovember26, 1973)
The paper describes a simple test for predicting the durability of electrodeposited iron and chromium films in machine parts. These films are also shown to behave, to a limited extent, as brittle materials while undergoing plastic deformation, introduction Electrodeposited metal films are extensively used in machine parts. An almost infinite number of plating conditions may be chosen and it is the technologist’s task to select those conditions which provide the’best wear characteristics for a particular range of operating conditions. Direct measurement of wear rates is time consuming and the plating chemist requires an alternative which is both sensitive and simple to execute. Generally, normal micro-hardness testing does not provide a unique criterion for specifying the durability of these films. This paper describes what appears to be a more useful test. The static load is measured, for a given indenter radius, at which extensive microcracking of the film is observed. As an example data are given for iron films electrodeposited on hardened steel substrates. In order to quantify what is essentially an empirical test the study was extended to include a dynamic measurement of this “failure’* load during sliding and treatment of the data in the fashion described by several previous authors for brittle materials was attempted. Materials Iron films were deposited on steel substrates from 3M aqueous solutions of ferrous chloride (as FeC1,4H,O) at 40°C using a pH of 0.8. The current density was varied from 5 to 40 A dm- 2. This has the effect of significantly changing both the mechanical and chemical properties of the film. The films were approximately 0.5 mm in thickness. * To whom correspondence
should
be addressed.
273
SlIOKI ~‘oMhll:hlC’ATIoNS
The wear experiments were carried out on an Amsler-type wear testing machine where a cast iron shoe (face area I cm’) was held against a rotating steel shaft which was plated with the iron film. The normal load ranged from 1 to 5 kg and the operating temperature was about 40 C. Both lubricated and unlubricated tests werecarriedout, the former with straight mineral oil at a sliding speed of 100 msK’. and the latter at 2 rns~~I. Both the shoe and the film wore but most of the wear. over X0”,,, occurred in the film. The wear was rnonitor~~l h! measuring the change in the thickness of the shoe and tpe film. This work is described in greater detail elsewhere’ ‘. The static loads and the loads in a friction experiment required to induce micro-cracking were measured using a modified version of a friction measuring machine described by Eldredge and Tabor”. In the static tests spheres of diameters between 1 mm and 6 mm of either fired quartz or solvent cleaned steel were loaded against the films for 15 s and the load was progressively increased in steps untii a continuous circumferential crack was observed around the region of plastic deformation. Figure 1 shows such a condition. The dynamic measurements
Fig. 1. Illustration of microcracking complete ~ircurnfer~n~~~l microcrack.
under
a static
loading
condition
just
sufficient
to cause
;I
were made using the same indenters as above at a sliding speed of approximately 20 mm s-r. The metal films were degreased in petroleum vapour. The critical load to cause micro~r~cking was taken as the point where extensive mi~ro~ra~king was observed within the plastically deformed track. These cracks were mainly observed normal to the direction of the track and they were generally linear cracks extending across about SOo//,of the plastically deformed zone. No cracking was noted on the edge of the deformed zone. The observed coefficients of friction, p, ranged from 0.2 to 0.37. The microhardness measurements were made with a standard Leitz microhardness tester using a load of 100 g. Reszilts and discussion The critical loads under static and under dynamic, i.e. sliding conditions, to cause microcracking, W, and W;, were found to be proportional to the radii of the indenters. A similar relationship has been observed in the beha~iour of brittle solids5.6. However, the static and dynamic failure pressures, P; and P& that is the normal load divided by the observed area of plastic deformation, were each within experimental error independent of the curvature of the slider for all the films studied. Figures 2 and 3 show these quantities pfotted against
90
lDtA
dm-*)
I,(A dmw2) Fig. 2. Static pressure, normal load divided by the area of plast~caily deformed zone, to cause continuous circumferential microcracks against the current density used during electrodeposition.
Fig. 3. Pressure to cause microcracking under sliding conditions against current density.
the current density, 1, used to prepare the films. The curves have the same form, the maximum failure pressures occurring for films plated at approximately 20 A dme2, The dynamic values are, however, significantly less than the static values. Figure 4 shows the relative wear resistance of the films in the Amster testing machine under the conditions described earlier. There appears to be a correlation between static and dynamic “failure” conditions and the wear rate. Finally in Fig. 5 Vickers microh~rdness is shown against current density used during the deposition of the film; hardness does not provide a unique criterion for predicting the film’s durability.
274
SHORT C’OMMUNKYATIONS
I,( A dm- 2, &,(A dm- ‘) Fig. 4. Relative wear resistance (the reciprocal of the wear rates normalised to the value for the films prepared at 40 A dm ‘) against current density. Fig. 5. Vickers pyramidal hardness against current density.
Although extensive plastic deformation of the film both in static and dynamic tests was observed, the films show a characteristic exhibited by brittle materials, that is microcracking. This cracking appears at a critical load whose value is directly related to the radius of the indenter. Further, the onset of microcracking occurs at much lower loads for sliding experiments than static experiments. This is also a feature noted with brittle materials. In the sliding case the tangential stresses at the interface cause the onset of cracking at normal loads less than those observed for static tests. Gilroy and Hirst” have studied microcracking in glass and observed that
where A is a constant containing Poisson’s ratio for the film. For iron A may be put as 8.00. First, IV.,C/IV,C is independent of the radius of curvature of the slider and experimentally it appears to be so for the present data. Secondly, the calculated ratio may be compared with that given by experiment. The calculated value of W,C/IV; is approximately 0.02 while experimentally it was about 0.1. When experiments are carried out on truly brittle materials, the agreement is much closer. For example, experiments carried out by Powell and Tabor’ in air on titanium carbide gave a mean experimental ratio of approximately 0.2 compared with a calculated value about 0.1. The agreement in the present experiments is fair but cannot be considered highly significant as extensive plastic deformation is also observed in these experiments. Finally, a limited number of experiments has been carried out on chromium lilms( deposited on steel) and a similar type of behaviour was observed. Although this correlation between wear resistance and cracking behaviour suggests that wear is dominated by cracking of the film, optical and electron microscopic examination of the worn film does not show unequivocally that wear is a fracture dominated process.
(a) There
appears
to be a correlation
between
the wear rate under
SHORT COMMUNICATIONS
275
certain conditions and the static normal pressure required to cause microcracking in electrodeposited iron and chromium films. (b) Although the main mode of deformation is plastic, electrodeposited metal films behave in some regards like brittle materials.
The authors are particularly grateful to Professor David Tabor, F.R.S., for his help and encouragement and also for many helpful discussions during the preparation of this paper. REFERENCES I Y. N. Petrov, The effect of electrolysis conditions on the wear resistance of porous iron coatings, Trans. Kishinev SeLKhoz. Inst.. 40 (1966) 172-78, l&90. Y. N.-Petrov, Increasing the wear resistance of metat surfaces by iron and chromium platings, Electronnaya Obrabotka Metallot., Akad. Nauk Uddavsk SSR, 4 (1966) 21-28.
V. F. Gologan, The wear of electrolytic iron coatings during dry friction, Trans. Kishineu SelKhoz. Inst., 54 (1968) 7-77.
K. R. Eldredge and D. Tabor, Mechanism of rolling friction, the plastic range, Proc. Roy. Sot. A229 (1955) 181-198. D. R. Gilroy and W. Hirst, Brittle fracture of glass under normal and sliding loads, J. Phys. D, 2 (1969) 1784-1787. B. D. Powell and D. Tabor, The fracture of titanium carbide under static and sliding contact, J. Phys. D, 3 (1970) 783-788. (Lmdonj,