- Email: [email protected]
The wear resistance of an Ni-Cu-P brush plating layer on different substrates
63
Wear, 165 (1993) 6348
The wear resistance of an Ni-Cu-P substrates Yan-Sheng
brush plating layer on different
Ma, Jia-Jun Liu, Bao-Liang Zhu, Guang-Jie Zhai and Lin-Qing Zheng
Tribology Research Institute, Tsinghua University,100084 Beijing (China)
(Received May 14, 1992; accepted November 5, 1992)
Abstract In order to increase the bonding strength between plating layer and substrate and promote the formation of a transfer film, a new type of Ni-Cu-P brush plating layer with excellent wear resistance was developed on the basis of an Ni-P amorphous plating layer, although it showed different behaviour when the substrate was changed. This paper systematically investigated its tribological behaviour on a “ball-on-disc” testing machine under lubrication with paraffin oil, and analyzed in depth the effect of substrates (~nventionally heat treated 1045 steel and nitrided 1045 steel) on its microstructure, hardness, wear resistance and wear mechanism. The results showed that the load bearing capacity of an Ni-Cu-P brush piating layer on a nitrided 1045 steel substrate can be more than ten times that of a 1045 steel without a plating layer, and more than twice that of a 1045 steel substrate without a nitriding layer. The composite coating process can usually offer an immeasurable effect through the interaction between composite layers.
1. Intr~uction
mechanism, with the aim of demonstrating tages of this composite surface technique.
In order to increase the bonding strength between plating layer and substrate and make use of the selective transfer phenomenon of the Cu element, a new type of Ni-Cu-P brush plating layer was developed on the basis of the Ni-P amorphous plating layer [I, 21, but it showed an unusual difference in tribological behaviour between different substrates. (The brush plating technique, differing from the conventional vat plating, is an electrochemical process conducted with an electrolyte applied to the substrate by a so-called brush (or tampon pad) to form the adherent deposit.) The load bearing capacity of the Ni-Cu-P brush plating layer on a nitride 1045 steel substrate could be more than twice that of 1045 steel without a nitriding layer. The friction coefficient and wear rate of the former were also improved significantly compared with those of the latter. Therefore, the duplex process of nitriding plus brush plating of Ni-Cu-P on 1045 steel became a unique and effective surface technique for improving its wear resistance. This technique showed not only great potential and application prospect, but also an interesting approach to the tribological mechanism. This paper systematically investigated the tribological behaviour of an Ni-Cu-P brush plating layer on different substrate and analysed in depth the effect of different substrates on its microstructure, hardness and wear
2. Experimental method
the advan-
The friction and wear tests were conducted on a “ball-on-disc” testing machine. The upper ball specimen made from 52100 bearing steel was stationary, with radius R=6.35 mm, surface roughness Ra =O.Ol pm and hardness 770 IIV. The lower disc specimen made from 1045 steel was rotating, with dimensions 056 x 5 mm and surface roughness Ru =0.65 pm. All disc specimens were prepared in different ways: by conventional heat treatment, brush plating of the Ni-Cu-P layer, ion nitriding and the duplex process of ion nitriding plus brush plating of the Ni-Cu-P layer. The technolo~ of preparation and surface hardness of disc specimens is shown in Table 1. The microhardness of coatings was measured on their cross-section under a load of 5 g to avoid the effect of the substrate. In order to obtain a substrate with even lower hardness, specimen 3 was tempered at 590 “C instead of 550 “C for specimens 4 and 5. The surface roughness of the treated disc specimens was improved after different treatments: for nitriding, Ra = 0.25 pm; for nitriding plus brush plating of the Ni-Cu-P layer, Ru =0.265 pm; for a conventionally heat-treated disc plus Ni-Cu-P plating layer, Ru = 0.34 pm. 0 1993 - Elsevier Sequoia. All rights reserved
Y.-S. Ma ct al. i Wear resistance of NiXu-P
64
TABLE
1. Technology
of preparation
and hardness
brush plating
of specimens --__
No. of specimen
Heat treatment
Hardness
1
860 “C water quench and 200 “C tempered
2
860 “C water quench 200 “C tempered
(HV)
Surface treatment
Hardness
627
_
627
and
627
Brush plating Ni-Cu-P layer Composition: Ni-64%, Cu-34%, P-2% Thickness: 10 pm
961
3
860 “C water quench and 590 “C tempered
243
Brush plating Ni-Cu-P
904
4
860 “C water quench 550 “C tempered
and
487
Ion nitriding Voltage: 370 V. Current: 7.6 A Temperature: 540 - 560 “C Holding time: 13 h
478
5
860 “C water quench and 550 “C tempered
487
ion nitriding plus brush plating Ni-Cu-P layer
502
A non-cyclic drop-feed lubrication mode was adopted. A paraffinic oil base stock without additives (~nematic viscosity: 14 N 16 X 10e6 m2 s-l at 40 “C) was used as the lubricant. The procedure for testing load bearing capacity (P-V curve) was as follows. After first setting a sliding speed, a 2 min running-in period under 120 N was adopted for every new pair of specimens. Afterwards, the stepwise loading was started with a rate of 120 N per 30 s. When the friction coe~cient increased steeply at a certain load, it was recorded as the scufhng load. The P-Y curve was obtained by linking up all data points of scuffing loads at different sliding speeds. The determination of wear rate was conducted under conditions of 1 m s-’ speed, 600 N load and 30 min duration by measuring the diameter of the wear track for ball specimens and the cross-sectional area of the wear track using a protilometer for the disc specimens. The boo-mo~holo~ of the wear track and wear debris were observed under a GM-950 scanning electron microscope. The analyses of the microstructure of the plating layer were completed using an H-800 transmission electron microscope and a D/MAX-RB X-ray diffractometer.
3. Results and discussion 3.1. Tribologkal behaviour of Ni-Cu-P
brush plating
layer
Figure 1 shows the load bearing capacity curves (P-V curves) of different disc specimens. It can be seen that specimen 5 with an Ni-Cu-P plating layer on nitrided 1045 steel exhibits the highest load bearing capacity, which is about two or three times that of specimens 2 and 3 which had a plating layer on conventionally
layer
(HV) -
_I_cI: OS
1.0
1.5
2.0
2.5
3.0
I m/s
Sliding speed
Fig. 1. P-V curves of different
disc specimens.
heat treated 1045 steel. If compared with specimen 1 without any surface treatment, the load bearing capacity of specimen 5 is increased by more than about ten times. Specimen 4 shows almost the same level of capacity as specimen 3. The hump appearing on the P-V curves of specimens 2 and 3 which was usually encountered in the same test for steel/steel rubbing pairs without coatings 131, is still hardly explained. Probably it is related to the frictional heat and behaviour of the oxide fdm on the surface [4]. The comparison of the friction coefficient and wear rate of five specimens is illustrated in Fig. 2. It is also obvious that specimen 5 of the duplex treatment shows the lowest friction coefficient, which is about one-half and one-third that of specimens 1 and 4 respectively. The wear rate of the ball rubbed with specimen 5 is decreased about 20 times compared with that of specimen 4 although the wear rate of the disc specimen
Y.-S. Ma et al. / Wear rwistance of Ni-Cu-P
brush plating
65
Friction coefficient
wear rate (lo-I’m’/ N.4
Fig. 4. The micro-morphology of scuffed surface of specimen 4. SEM original magnification X200 (V=l m s-r, P=1200 N.) Fig. 2. The comparison of friction coefficient and wear rate of different specimens (V=l m s-l, P=600 N, t=30 min.)
Fig. 5. The characteristic of wear debris of specimen 1. SEM original magnification ~700 (V=O.7 m s-‘, P= 360 N.) Fig. 3. The micro-morphology of scuffed surface of specimen 1. SEM original magnification X100. (V=O.7 m s-‘, P=360 N.)
is slightly increased. As for specimen 1 both the ball and the disc are already scuffed under the same experimental conditions. The friction coefficient of specimens 2 and 3 is moderate and the wear rate of balls rubbed with them is also significantly lower than that for specimen 4. However, their own wear rate is increased greatly due to the weaker load bearing effect of the substrate. 3.2. Wear mechanism of plating layer From the micro-morphology of the scuffed surface of specimens 1 and 4, adhesive wear is found to be present in both specimens as shown in Figs. 3 and 4, i.e. the material of the softer ball specimen has been smeared (adhered) onto the surface of the disc. The wear particles produced are block-like with a certain thickness (Figs. 5 and 6). However, specimen 5 shows the characteristic of delamination caused by strain fatigue (Fig. 7), and its wear particles are typically plate-like. (Fig. 8) [S]. Figures 9 and 10 are illustrations
Fig. 6. The characteristic of wear debris of specimen 4. SEM original magnification x700. (V= 1 m s-‘, P= 1200 N.)
of the cross-section of the wear track of specimens 2 and 5. In Fig. 9, a more severe plastic deformation can be found in the surface layer of specimens, which indicates that the material has been considerably softened, and a large crack has occurred between plating layer and substrate, which can result in the peeling off
66
Y.-S. Ma et al. I Wear resistance of Ni-Cu-P
brush plating
Fig. 7. The micro-morphology of scuffed surface of specimc en 5. SEM original magnification X 1000 (V= 1 m s-‘, P= 3840 N.)
Fig. 10. The section view of wear track of specimen 5. SEM original magnification X 1000. (V= 1 m SC’, P=3840 N.)
Fig. 8. The characteristic of wear debris of specimen 5. SEM original magnification x 1000 (V= 1 m s-r, P= 3480 N.)
Fig. 11. The surface morphology of specimen 2 in the early stages of wear. SEM original magnification x 500. (V= 1 m s-‘, P= 1200 N.)
Fig. 9. The section view of wear track of specimen 2. SEM original magnification ~500. (V=l m s-r, P=1200 N.)
of the whole plating layer. Figure 11 shows that many penetrating cracks have been formed in the early stage of wear on the plating layer of specimen 2. All these phenomena demonstrate that although the plating layer is quite hard, its wear resistance is difficult to develop
owing to the decrease of load bearing capacity of the substrate. Conversely, the plastic flow of the substrate beneath the plating layer is not visible for specimen 5 in Fig. 10 even though its surface hardness is only 502 HV, and a good bonding is also present, and no cracks can be seen at the interface. Figure 12 shows melted spherical wear debris produced from specimen 5. From EDAX it is found to be composed mainly from Fe. This indicates that the friction temperature has reached a considerably high level, which aids the increase of strain fatigue resistance. Meanwhile, the nitriding layer exhibits a better hardness (its hardness does not decrease until 500 “C), which can offer a larger resistance to plastic deformation at higher friction temperatures. 3.3. Microstructure of Ni-Cu-P different substrates
plating layer on
The microstructure of Ni-Cu-P plating layers on specimens 2 and 5 has been analysed by X-ray diffraction. Figures 13 and 14 show the analysis results. From Fig.
Y-S. Ma et al. / Wear resistance of Ni-Cu-P
67
brush plating
Ni-Cu
(222),
Ni-Cu Ni-Cu
(311)-222~ (220) -‘---,
Cu,P (223) -\ Cu,P (311) I--, Ni-Cu (200) --
Fig. 12. The spherical wear debris of specimen 5. SEM original magnification -X 2500. (V= 1 m s-‘, P= 3600 N.)
Fig. 15. The electron
--h__
XI--
I 38
nil
result of specimen
I
L I’
I
08
Fig. 13. The X-ray diffraction
“W
2.
1
n*r
Fig. 14. The X-ray diffraction
i//1
_.
mw
result of specimen
7:
pattern
of specimen
5.
It can be seen from Table 1 that specimens 2, 3 and 5 all have the same Ni-Cu-P plating layer, but their hardnesses are quite different; the hardness of specimen 5 is much lower than that of specimens 2 and 3. This phenomenon cannot be explained by the lower load bearing capacity of the substrate, because the substrate of specimen 3 is even softer than that of specimen 5, while the hardness of the Ni-Cu-P plating on the former is much higher than that on the latter. Obviously, the only cause is the difference in microstructure of the plating layers, i.e. the N&P and Cu,P phases are present in the plating layer of specimen 5 on the nitrided substrate. The mechanism explaining why the nitrided substrate can promote the formation of N&P and Cu,P phases is still a new phenomenon and topic of materials science. However, it can be considered that just by the formation of a plating layer with lower hardness, the internal stress can be decreased and resistance to crack propagation may be enhanced. This benefits its tribological behaviour.
,
I
diffraction
j.
II
5.
13 it can be seen that the Ni-Cu-P plating layer on specimen 2 consists of Ni crystals, Ni-Cu solid solution and amorphous phase (as shown by the hump at the right of the X-ray diffraction pattern). Figure 14 shows an obvious difference from Fig. 13. Besides the Ni-Cu solid solution and amorphous phase, the N&P and Cu,P phases are present. Figure 15 is the electron diffraction ring of the Ni-Cu-P plating layer on specimen 5, which shows full agreement with the X-ray analysis.
3.4. The role of the Ni-Cu-P transfer film In addition to the reasons mentioned above, the formation of an Ni-Cu-P transfer film on the surface of the ball is also very important to the tribological properties of specimen 5. Figure 16 shows the morphology of the transfer film on the surface of the ball rubbed with disc specimen 5. The result of its analysis by EDAX is shown in Fig. 17. The composition of this transfer film is similar to that of the original plating layer. It can be considered that, in fact, the friction and wear process is conducted between the plating layer and the transfer film on its counterpart, and the improvement of wear resistance and friction coefficient of specimen 5 is partly due to the presence of the
68
Y.-S. Ma et al. I Wear resistance of Ni-Cu-P
brush plating
investigations on this interesting topic of the transfer film of an Ni-Cu-P plating layer. 4. Conclusions
Fig. 16. The surface morphology disc specimen 5.
: 1
:
:
of a ball specimen
:
I
I
/
rubbed with
: : i
/
:
: ;
: :
: i
i
:
:
!
:
i
e.e0a
:
:
(1) The Ni-Cu-P brush plating layer on nitrided 1045 steel shows an excellent wear resistance and friction coefficient. (2) The hardness of an Ni-Cu-P plating layer on nitrided 1045 steel is decreased significantly owing to the presence of Ni,P and Cu,P phases, this improving its tribological behaviour, compared with that of the plating layer made directly on 1045 steel. (3) The hardness of the nitriding layer is also beneficial in maintaining a better load bearing role of the substrate on the plating layer. (4) A transfer film is easily formed from the Ni-Cu-P plating layer onto the surface of its counterpart, which can effectively separate the direct contact of ball and disc and diminish their friction and wear.
:
;;i
i
:
:
References
i
j
:
:
ws : 8152 x.24
Fig. 17. The EDAX analysis result of a ball surface rubbed with disc specimen 5.
transfer film which can play the. role of separating direct contact of ball and disc and diminishing friction and wear. The phenomenon is similar to the selective transfer [6, 71, but no obvious enrichment of the Cu element was detected. It is worth carrying out further
X.-H. Zhang, The study of brush plating layers for reducing friction and increasing wear-resistance, Ph.D. Dissertation, Tsinghua University, 1990. H. Gao, H. Gu and H. Zhou, Sliding wear and fretting fatigue resistance of amorphous Ni-P coatings, Wear, 14.2 (1991) 291-301. J.-J. Liu, Y.-W. Zhao and L.-Q. Zheng, The effect of hardness on the P-V diagram for steel/steel rubbing pair under lubrication, Proc. of International Symposium of Tribochemis~, Lanzhou, China, 1989, pp. 158-165. Y.W. Zhao, J.-J. Liu and L.-Q. Zheng, The nature of the friction transition as a function of load and speed for the steel/steel system, STLE Tribol. Trans., 33 (1990) 642-652. J.-J. Liu, Z.-Q. Lu and Y.-Q. Cheng, The study of scuffing and pitting failure of cam-tappet rubbing pair, Wear, 240 (1990) 135-147. D. N. Garkunov, I. V. Kragelsky and A. A. Polyakov, Selective Transfer at Friction Points, Transport, Moscow, 1969 (in Russian). Y. Zhao, An approach to the effect of friction-reduction and wear-resistance through selective transfer, Friction and Wear, 3 (1982) l-10 (in Chinese).
Wear, 165 (1993) 6348
The wear resistance of an Ni-Cu-P substrates Yan-Sheng
brush plating layer on different
Ma, Jia-Jun Liu, Bao-Liang Zhu, Guang-Jie Zhai and Lin-Qing Zheng
Tribology Research Institute, Tsinghua University,100084 Beijing (China)
(Received May 14, 1992; accepted November 5, 1992)
Abstract In order to increase the bonding strength between plating layer and substrate and promote the formation of a transfer film, a new type of Ni-Cu-P brush plating layer with excellent wear resistance was developed on the basis of an Ni-P amorphous plating layer, although it showed different behaviour when the substrate was changed. This paper systematically investigated its tribological behaviour on a “ball-on-disc” testing machine under lubrication with paraffin oil, and analyzed in depth the effect of substrates (~nventionally heat treated 1045 steel and nitrided 1045 steel) on its microstructure, hardness, wear resistance and wear mechanism. The results showed that the load bearing capacity of an Ni-Cu-P brush piating layer on a nitrided 1045 steel substrate can be more than ten times that of a 1045 steel without a plating layer, and more than twice that of a 1045 steel substrate without a nitriding layer. The composite coating process can usually offer an immeasurable effect through the interaction between composite layers.
1. Intr~uction
mechanism, with the aim of demonstrating tages of this composite surface technique.
In order to increase the bonding strength between plating layer and substrate and make use of the selective transfer phenomenon of the Cu element, a new type of Ni-Cu-P brush plating layer was developed on the basis of the Ni-P amorphous plating layer [I, 21, but it showed an unusual difference in tribological behaviour between different substrates. (The brush plating technique, differing from the conventional vat plating, is an electrochemical process conducted with an electrolyte applied to the substrate by a so-called brush (or tampon pad) to form the adherent deposit.) The load bearing capacity of the Ni-Cu-P brush plating layer on a nitride 1045 steel substrate could be more than twice that of 1045 steel without a nitriding layer. The friction coefficient and wear rate of the former were also improved significantly compared with those of the latter. Therefore, the duplex process of nitriding plus brush plating of Ni-Cu-P on 1045 steel became a unique and effective surface technique for improving its wear resistance. This technique showed not only great potential and application prospect, but also an interesting approach to the tribological mechanism. This paper systematically investigated the tribological behaviour of an Ni-Cu-P brush plating layer on different substrate and analysed in depth the effect of different substrates on its microstructure, hardness and wear
2. Experimental method
the advan-
The friction and wear tests were conducted on a “ball-on-disc” testing machine. The upper ball specimen made from 52100 bearing steel was stationary, with radius R=6.35 mm, surface roughness Ra =O.Ol pm and hardness 770 IIV. The lower disc specimen made from 1045 steel was rotating, with dimensions 056 x 5 mm and surface roughness Ru =0.65 pm. All disc specimens were prepared in different ways: by conventional heat treatment, brush plating of the Ni-Cu-P layer, ion nitriding and the duplex process of ion nitriding plus brush plating of the Ni-Cu-P layer. The technolo~ of preparation and surface hardness of disc specimens is shown in Table 1. The microhardness of coatings was measured on their cross-section under a load of 5 g to avoid the effect of the substrate. In order to obtain a substrate with even lower hardness, specimen 3 was tempered at 590 “C instead of 550 “C for specimens 4 and 5. The surface roughness of the treated disc specimens was improved after different treatments: for nitriding, Ra = 0.25 pm; for nitriding plus brush plating of the Ni-Cu-P layer, Ru =0.265 pm; for a conventionally heat-treated disc plus Ni-Cu-P plating layer, Ru = 0.34 pm. 0 1993 - Elsevier Sequoia. All rights reserved
Y.-S. Ma ct al. i Wear resistance of NiXu-P
64
TABLE
1. Technology
of preparation
and hardness
brush plating
of specimens --__
No. of specimen
Heat treatment
Hardness
1
860 “C water quench and 200 “C tempered
2
860 “C water quench 200 “C tempered
(HV)
Surface treatment
Hardness
627
_
627
and
627
Brush plating Ni-Cu-P layer Composition: Ni-64%, Cu-34%, P-2% Thickness: 10 pm
961
3
860 “C water quench and 590 “C tempered
243
Brush plating Ni-Cu-P
904
4
860 “C water quench 550 “C tempered
and
487
Ion nitriding Voltage: 370 V. Current: 7.6 A Temperature: 540 - 560 “C Holding time: 13 h
478
5
860 “C water quench and 550 “C tempered
487
ion nitriding plus brush plating Ni-Cu-P layer
502
A non-cyclic drop-feed lubrication mode was adopted. A paraffinic oil base stock without additives (~nematic viscosity: 14 N 16 X 10e6 m2 s-l at 40 “C) was used as the lubricant. The procedure for testing load bearing capacity (P-V curve) was as follows. After first setting a sliding speed, a 2 min running-in period under 120 N was adopted for every new pair of specimens. Afterwards, the stepwise loading was started with a rate of 120 N per 30 s. When the friction coe~cient increased steeply at a certain load, it was recorded as the scufhng load. The P-Y curve was obtained by linking up all data points of scuffing loads at different sliding speeds. The determination of wear rate was conducted under conditions of 1 m s-’ speed, 600 N load and 30 min duration by measuring the diameter of the wear track for ball specimens and the cross-sectional area of the wear track using a protilometer for the disc specimens. The boo-mo~holo~ of the wear track and wear debris were observed under a GM-950 scanning electron microscope. The analyses of the microstructure of the plating layer were completed using an H-800 transmission electron microscope and a D/MAX-RB X-ray diffractometer.
3. Results and discussion 3.1. Tribologkal behaviour of Ni-Cu-P
brush plating
layer
Figure 1 shows the load bearing capacity curves (P-V curves) of different disc specimens. It can be seen that specimen 5 with an Ni-Cu-P plating layer on nitrided 1045 steel exhibits the highest load bearing capacity, which is about two or three times that of specimens 2 and 3 which had a plating layer on conventionally
layer
(HV) -
_I_cI: OS
1.0
1.5
2.0
2.5
3.0
I m/s
Sliding speed
Fig. 1. P-V curves of different
disc specimens.
heat treated 1045 steel. If compared with specimen 1 without any surface treatment, the load bearing capacity of specimen 5 is increased by more than about ten times. Specimen 4 shows almost the same level of capacity as specimen 3. The hump appearing on the P-V curves of specimens 2 and 3 which was usually encountered in the same test for steel/steel rubbing pairs without coatings 131, is still hardly explained. Probably it is related to the frictional heat and behaviour of the oxide fdm on the surface [4]. The comparison of the friction coefficient and wear rate of five specimens is illustrated in Fig. 2. It is also obvious that specimen 5 of the duplex treatment shows the lowest friction coefficient, which is about one-half and one-third that of specimens 1 and 4 respectively. The wear rate of the ball rubbed with specimen 5 is decreased about 20 times compared with that of specimen 4 although the wear rate of the disc specimen
Y.-S. Ma et al. / Wear rwistance of Ni-Cu-P
brush plating
65
Friction coefficient
wear rate (lo-I’m’/ N.4
Fig. 4. The micro-morphology of scuffed surface of specimen 4. SEM original magnification X200 (V=l m s-r, P=1200 N.) Fig. 2. The comparison of friction coefficient and wear rate of different specimens (V=l m s-l, P=600 N, t=30 min.)
Fig. 5. The characteristic of wear debris of specimen 1. SEM original magnification ~700 (V=O.7 m s-‘, P= 360 N.) Fig. 3. The micro-morphology of scuffed surface of specimen 1. SEM original magnification X100. (V=O.7 m s-‘, P=360 N.)
is slightly increased. As for specimen 1 both the ball and the disc are already scuffed under the same experimental conditions. The friction coefficient of specimens 2 and 3 is moderate and the wear rate of balls rubbed with them is also significantly lower than that for specimen 4. However, their own wear rate is increased greatly due to the weaker load bearing effect of the substrate. 3.2. Wear mechanism of plating layer From the micro-morphology of the scuffed surface of specimens 1 and 4, adhesive wear is found to be present in both specimens as shown in Figs. 3 and 4, i.e. the material of the softer ball specimen has been smeared (adhered) onto the surface of the disc. The wear particles produced are block-like with a certain thickness (Figs. 5 and 6). However, specimen 5 shows the characteristic of delamination caused by strain fatigue (Fig. 7), and its wear particles are typically plate-like. (Fig. 8) [S]. Figures 9 and 10 are illustrations
Fig. 6. The characteristic of wear debris of specimen 4. SEM original magnification x700. (V= 1 m s-‘, P= 1200 N.)
of the cross-section of the wear track of specimens 2 and 5. In Fig. 9, a more severe plastic deformation can be found in the surface layer of specimens, which indicates that the material has been considerably softened, and a large crack has occurred between plating layer and substrate, which can result in the peeling off
66
Y.-S. Ma et al. I Wear resistance of Ni-Cu-P
brush plating
Fig. 7. The micro-morphology of scuffed surface of specimc en 5. SEM original magnification X 1000 (V= 1 m s-‘, P= 3840 N.)
Fig. 10. The section view of wear track of specimen 5. SEM original magnification X 1000. (V= 1 m SC’, P=3840 N.)
Fig. 8. The characteristic of wear debris of specimen 5. SEM original magnification x 1000 (V= 1 m s-r, P= 3480 N.)
Fig. 11. The surface morphology of specimen 2 in the early stages of wear. SEM original magnification x 500. (V= 1 m s-‘, P= 1200 N.)
Fig. 9. The section view of wear track of specimen 2. SEM original magnification ~500. (V=l m s-r, P=1200 N.)
of the whole plating layer. Figure 11 shows that many penetrating cracks have been formed in the early stage of wear on the plating layer of specimen 2. All these phenomena demonstrate that although the plating layer is quite hard, its wear resistance is difficult to develop
owing to the decrease of load bearing capacity of the substrate. Conversely, the plastic flow of the substrate beneath the plating layer is not visible for specimen 5 in Fig. 10 even though its surface hardness is only 502 HV, and a good bonding is also present, and no cracks can be seen at the interface. Figure 12 shows melted spherical wear debris produced from specimen 5. From EDAX it is found to be composed mainly from Fe. This indicates that the friction temperature has reached a considerably high level, which aids the increase of strain fatigue resistance. Meanwhile, the nitriding layer exhibits a better hardness (its hardness does not decrease until 500 “C), which can offer a larger resistance to plastic deformation at higher friction temperatures. 3.3. Microstructure of Ni-Cu-P different substrates
plating layer on
The microstructure of Ni-Cu-P plating layers on specimens 2 and 5 has been analysed by X-ray diffraction. Figures 13 and 14 show the analysis results. From Fig.
Y-S. Ma et al. / Wear resistance of Ni-Cu-P
67
brush plating
Ni-Cu
(222),
Ni-Cu Ni-Cu
(311)-222~ (220) -‘---,
Cu,P (223) -\ Cu,P (311) I--, Ni-Cu (200) --
Fig. 12. The spherical wear debris of specimen 5. SEM original magnification -X 2500. (V= 1 m s-‘, P= 3600 N.)
Fig. 15. The electron
--h__
XI--
I 38
nil
result of specimen
I
L I’
I
08
Fig. 13. The X-ray diffraction
“W
2.
1
n*r
Fig. 14. The X-ray diffraction
i//1
_.
mw
result of specimen
7:
pattern
of specimen
5.
It can be seen from Table 1 that specimens 2, 3 and 5 all have the same Ni-Cu-P plating layer, but their hardnesses are quite different; the hardness of specimen 5 is much lower than that of specimens 2 and 3. This phenomenon cannot be explained by the lower load bearing capacity of the substrate, because the substrate of specimen 3 is even softer than that of specimen 5, while the hardness of the Ni-Cu-P plating on the former is much higher than that on the latter. Obviously, the only cause is the difference in microstructure of the plating layers, i.e. the N&P and Cu,P phases are present in the plating layer of specimen 5 on the nitrided substrate. The mechanism explaining why the nitrided substrate can promote the formation of N&P and Cu,P phases is still a new phenomenon and topic of materials science. However, it can be considered that just by the formation of a plating layer with lower hardness, the internal stress can be decreased and resistance to crack propagation may be enhanced. This benefits its tribological behaviour.
,
I
diffraction
j.
II
5.
13 it can be seen that the Ni-Cu-P plating layer on specimen 2 consists of Ni crystals, Ni-Cu solid solution and amorphous phase (as shown by the hump at the right of the X-ray diffraction pattern). Figure 14 shows an obvious difference from Fig. 13. Besides the Ni-Cu solid solution and amorphous phase, the N&P and Cu,P phases are present. Figure 15 is the electron diffraction ring of the Ni-Cu-P plating layer on specimen 5, which shows full agreement with the X-ray analysis.
3.4. The role of the Ni-Cu-P transfer film In addition to the reasons mentioned above, the formation of an Ni-Cu-P transfer film on the surface of the ball is also very important to the tribological properties of specimen 5. Figure 16 shows the morphology of the transfer film on the surface of the ball rubbed with disc specimen 5. The result of its analysis by EDAX is shown in Fig. 17. The composition of this transfer film is similar to that of the original plating layer. It can be considered that, in fact, the friction and wear process is conducted between the plating layer and the transfer film on its counterpart, and the improvement of wear resistance and friction coefficient of specimen 5 is partly due to the presence of the
68
Y.-S. Ma et al. I Wear resistance of Ni-Cu-P
brush plating
investigations on this interesting topic of the transfer film of an Ni-Cu-P plating layer. 4. Conclusions
Fig. 16. The surface morphology disc specimen 5.
: 1
:
:
of a ball specimen
:
I
I
/
rubbed with
: : i
/
:
: ;
: :
: i
i
:
:
!
:
i
e.e0a
:
:
(1) The Ni-Cu-P brush plating layer on nitrided 1045 steel shows an excellent wear resistance and friction coefficient. (2) The hardness of an Ni-Cu-P plating layer on nitrided 1045 steel is decreased significantly owing to the presence of Ni,P and Cu,P phases, this improving its tribological behaviour, compared with that of the plating layer made directly on 1045 steel. (3) The hardness of the nitriding layer is also beneficial in maintaining a better load bearing role of the substrate on the plating layer. (4) A transfer film is easily formed from the Ni-Cu-P plating layer onto the surface of its counterpart, which can effectively separate the direct contact of ball and disc and diminish their friction and wear.
:
;;i
i
:
:
References
i
j
:
:
ws : 8152 x.24
Fig. 17. The EDAX analysis result of a ball surface rubbed with disc specimen 5.
transfer film which can play the. role of separating direct contact of ball and disc and diminishing friction and wear. The phenomenon is similar to the selective transfer [6, 71, but no obvious enrichment of the Cu element was detected. It is worth carrying out further
X.-H. Zhang, The study of brush plating layers for reducing friction and increasing wear-resistance, Ph.D. Dissertation, Tsinghua University, 1990. H. Gao, H. Gu and H. Zhou, Sliding wear and fretting fatigue resistance of amorphous Ni-P coatings, Wear, 14.2 (1991) 291-301. J.-J. Liu, Y.-W. Zhao and L.-Q. Zheng, The effect of hardness on the P-V diagram for steel/steel rubbing pair under lubrication, Proc. of International Symposium of Tribochemis~, Lanzhou, China, 1989, pp. 158-165. Y.W. Zhao, J.-J. Liu and L.-Q. Zheng, The nature of the friction transition as a function of load and speed for the steel/steel system, STLE Tribol. Trans., 33 (1990) 642-652. J.-J. Liu, Z.-Q. Lu and Y.-Q. Cheng, The study of scuffing and pitting failure of cam-tappet rubbing pair, Wear, 240 (1990) 135-147. D. N. Garkunov, I. V. Kragelsky and A. A. Polyakov, Selective Transfer at Friction Points, Transport, Moscow, 1969 (in Russian). Y. Zhao, An approach to the effect of friction-reduction and wear-resistance through selective transfer, Friction and Wear, 3 (1982) l-10 (in Chinese).