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Improvement in friction and wear of hard chromium layers by ion implantation
Materials Science and Engineering, A l l 6 (1989) 177-181
177
Improvement in Friction and Wear of Hard Chromium Layers by Ion Implantation* W. LOHMANN
Akzo Research Laboratories Obernburg, Corporate Research--New Materials, PO Box, D-8753 Obernburg (F.R.G.) J. G. P. VAN VALKENHOEF
Akzo Centrefor Materials and Corrosion Engineering, PO Box 25, NL- 7550 GC Hengelo Ov (The Netherhmds) (Received September 16, 1988)
Abstract Hard chromium layers are widely used for coating fibre-guiding elements in the man-made fibre industry. A low wear rate and a time-independent, constant friction coefficient (not necessarily minimized) are essential to this application. A screening implantation programme with singleormultiple implants has been carried out on hard chromium layers which had been electroplated onto steel cylinders. The experimental procedure, equipment and post-implantation analysis methods are described. The friction and wear evaluation shows a clear improvement after most ion beam treatments. Its relative degree is dependent on the wear conditions and the ion species. For the wear couple of TiO2-pigmented polyester yarn against chromium electroplate under abrasive-corrosive conditions, very high gain factors (up to 54) are found. It is also demonstrated that a desired combination between a time-dependent change of friction coefficient and wear rate can be adjusted by proper selection of the ion beam process. 1. Introduction Electroplated hard chromium layers are widely used in the man-made fibre industry as coatings for tools for thread guiding, drawing, heating and winding. Desirable surface properties for such applications are a low yarn-induced wear rate and a time-independent, constant friction coefficient. Because a defined slippage between the running thread and the contact area is required, e.g. for drawing purposes, an adjustable friction coefficient is often rated higher than a minimized one. *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
Although hard chromium plate is known to be effective against wear and for reduced corrosion of metallic materials [1], its wear under manufacturing conditions of synthetic yarns is still a concern [2]. The observed wear process consists of an abrasive and a corrosive mode. Abrasion is caused by hard particles incorporated into the soft fibre, and corrosion can be induced by spin finish fluids, especially in view of the tendency for cracks in hard chromium plate. Because ion implantation has been shown I3] to be useful for both wear and corrosion protection without affecting the dimensional stability of treated components, a screening programme has been carried out to determine the friction and wear improvement obtainable for ion-implanted hard chromium layers. 2. Experimental details
2.1. Specimens The specimens used for this study were C35 steel cylinders (height 30 ram, wall thickness 1.5 ram, inner diameter 30 ram). A dull, hard chromium layer 50/zm thick was electrodeposited onto the outer cylinder surface under usual industrial chromium plating conditions. The initial surface roughness R~ was about 0.7 ktm. 2.2. Ion implantation A modified Varian-Extrion medium-current ion implanter (model DF 3000) was used for a screening programme with single or multiple implants. The specimens were electrically ~nsulated for beam current measurement, and no intentional active cooling was provided. The essential implantation conditions are given in Table 1. © Elsevier Sequoia/Printed in The Netherlands
178 TABLE 1
lmplantationconditions
< 10 "mbar mainly 150 keV, in case of multiple implants various energies down to 32 keV to match the range of the initial one < 0.6 W cm 2 for Timp~< 300 °C (otherwise hardness loss [1 ]) generally 4 × 1017 cm 2, for multiple implants between 0.6 and 4 × I(P 7 cm-2 to meet the stoichiometry of the desired hard dispersed particles AI, Ar, B, C, Hf, Kr, N, Nb, O, P, Pb, Si, Sn, Ti, V
Pressure during implantation Ion energy
Beam power density Ion dose Ion species
Specimen rotation inside the beam and electrostatic beam scanning for uniform implantation
! i @
/ 5
=-~
TABLE 2
~ ~sca --- 4
Test conditions
Yarn speed
300mmin
Yarn type
TiO2-pigmented polyester (DIOLEN TM)in 50 dtex f 18, 100 dtex f 36 and 167 dtex f30 conditions
Yarn Friction Tester
~ 7 [1 Bobbin
]
; 2 Yarn tension device '3 Yarn tension head
j I
4 Specimen ' 5 Thermocouple
I
transport device [ 7 Air jet j i
6 Yarn
Fig. 1. Experimental set-up for friction coefficient measurement.
Added preparation
1%-7.5% H20
Relative humidity Test temperature
60% 20 °C
Pretension Angle of wrap
0.5-1.67 N (1 cN dtex-') 90 °
Test duration
up to 10 h
2.3. Measurement offriction and wear
I
A schematic representation of the expenmental set-up (after ref. 2) is shown in Fig. 1. The yarn is drawn from a bobbin and passes through a tensioning device to give the desired pretension. Two tension heads are located in front of and behind the specimen; this arrangement allows direct calculation [4] of the friction coefficient/~ from
p=lln(F~)
(1)
when a is the angle of wrap, and F 1 and F 2 are the forces in front of and behind the specimen respectively. A thermocouple attached close to the specimen is used for temperature measurement. After the second tension head, there is a transport device to keep the yarn at the required speed. Finally, it is blown by an air jet into a reception box. The test conditions for reference and ion-implanted specimens are given in Table 2. Following this test step, which only gives friction coefficient data, the wear of the specimens was determined by means of another device
| t~3 _ ~ . .,...=...... ,9 ...r-:v=zT-.. _ o o o 12 ..... • _..43 - u u - °
1, B ~ ( /////////////
~
l
1-~
t6
Fig. 2. Schematicdiagramof wear measurementapparatus.
(Fig. 2). The yarn-induced incision is measured with the aid of a computerized profilometer (Talysurf-5, Rank Taylor Hobson Ltd.). It enables direct plotting of the surface roughness profile on a chart recorder as well as further processing of the various roughness parameters, e.g. generation of an enlarged picture of the cross-section of the worn surface (see position 6 in Fig. 2) and its integration. As described in ref. 2, this leads to a
179 ~jp~cies ploot~~
simple derivation of the specific wear rate k',
.
.
.
.
.
--~
22 ~°it,....... n
k'-
V
(2)
where V is the worn volume, F is the normal force and S is the running distance, k' is mainly used in case of smooth (R a < 0.1/~m) cylindrical model specimens. In practice, however, hard chromium plated guiding elements often have rougher, "orange peel"-like surface textures consisting of regularly textured spherical cusps. This complicates the computation and measurement of k'. A mode of computation was found by taking the expression
k'-(N2LBV')
(3)
in which
if= =ARtm (3R-AR,m) __
,~ ~
"I~~ ~
and L is the length of wear scar, B is the width of wear scar, and ARtm = Rtm (unworn)- Rtm (worn). V' is the average volume of a single segment. The other quantities in eqns. (3) and (4) are as follows: Rtm the average peak-valley distance of the roughness profile; R ( = 1/Rp~) the average radius of curvature of the roughness profile; N( = 1/Sm) the average number of roughness cusps per unit length, Hence, the determination of k' is only based on parameters from usual roughness measurement and on the dimensions of the wear scar. 3. Experimental results
3.1. Friction behaviour A series of friction coefficient measurements were carried through on three types of pigmented polyester yarn under identical pretension per iliament( 1 cN dtex- ~), but with different water additions according to the production conditions. The test results are presented in Fig. 3. Since usually an initial increase of the friction coefficient kt is observed, followed by a levelling-off after a run time of about 1-2 h, the typical values of/~ after 2 h are given, partly supplemented by data from longer test times. The initial friction coefficient/~0 shows some fluctuations, but no systematic reduction by ion
~
i
I D,OLEN,~ ,oof361 '
°'3
[+
~ water
~z
~n,t~o, .....
((~)
0°ira.......
IIIIII
o,
Ion~ecies Unlmplonted A~
J
0,
Frict]orlCoe,fJcient
T oI ~/P ~/~/~
]
. . . . ,too
, .... ,oog.aft.,
] ~Si 2 " ~/Aly,/,2~2 0 , - ~ - - ~ 0'2
/ B ~
after 2 h run time
DIOI D/OLEN tm 167f30]
+ 3.5=~ote~ J °.2
o,
Frictlo. Coefficient
~ ~
()
b
[[~ initialvalue m after 2 h run time I ~11
Ai]~c~/~i
(4)
o...2.
/~CNC~/'~N ]~I~NNN2
~ _ _ ±.... ~ ~_~L-~ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Friction Coefficient
* : terminated after 10 ml..... ime DIOLEN tm 50f181 + 7.5= wQter J (C)
Fig. 3. Friction coefficientsof reference and ion-implanted specimens tested with three yarn types: (a) DIOLEN T M 100f36+1% H20 , (b) DIOLEN T M 167f30+3.5% H20 and (c) DIOLEN T M 50f18 + 7.5% H20.
implantation. The average /~0 values are 0.18 (100f36), 0.13 (167f30) and 0.19 (50f18). However, distinct effects appear after some hours of testing. Many of the implants lead to a considerable reduction of/~ and, at the same time, to a more stable, time-independent behaviour. Taking /.~Ref/~Uimplafter 2 h as a figure of merit, one obtains gain factors relative to the unimplanted condition between 1.0 (A1) and 1.9 (Pb) for 100f36 + 1% H20, 1.1 (A1) and 2.3 (O, Ti/P, Ti/C/C) for 167f30 + 3.5% H20, and 1.2 (Ti/C/Nb) and 2.7 (C) for 5 0 f 1 8 + 7 . 5 % H 2 0 . The implanted species are given in brackets. The relative change of the friction coefficient after a run time of 2 h, A/~ = (/~ -/~o)//%, is a measure of the time stability of the friction coefficient and should be minimal for production. Further evaluation of Fig. 3 yields the following range of gain factors with respect to the untreated reference, i.e. A[tRef/AJ~irnpl: between 0.8 (A1, no gain) and 19.8 (Ti/C/C) for 100f36 + 1% H 2 0 (Pb:6.1), 1.2 (Al) and 7.7 (Ti/C/C) for 167f30+3.5% H 2 0 (0:4.5, Ti/P:5.9), and 1.2 (Ti) and 8.0 (Ti/A1/N/N) for 50f18 + 7.5% H 2 0
180 (C:6.8). As above, the implanted ions are given in brackets. It can be seen that the ions which are the most effective ones for reducing the friction coefficient are also beneficial in improving its time constancy, but to a lesser degree than implantation processes optimized for that purpose. However, the overall improvement of both properties remains acceptable for a variety of non-optimized implants.
3.2. Wear behaviour Wear analysis was not performed on all implanted materials, but only on specimens which had shown a large reduction of the friction coefficient (Section 3.1) after an ion beam treatment as simple (and economical) as possible. This led to closer consideration of the argon, carbon, nitrogen, oxygen and T i - N 2 implants. In view of the obvious tendency for increasing friction reduction with increasing water addition, which indicates improved corrosive wear behaviour, these tests were carried out on specimens which had been worn in the presence of only 1% H20 in order to emphasize the abrasive part of the wear process. The results can be seen in Fig. 4. All implants reduce the amount of wear with yarndependent gain factors up to 54. The smaller k' value of the 167f30 reference is due to a 40% larger filament diameter that decreases the Heftzian contact pressure by 50%. The wear rate after argon implantation should be proportionally smaller, but this is not observed,
4. Discussion Assuming a low and constant friction coefficient as an indication of low wear rate, the data from Fig. 3, which show a clear correlation between friction reduction and water addition, suggest an especially beneficial implantation effect on the corrosive wear mode that becomes more dominant at higher water concentration, Closing of microcracks present in hard chromium electroplate is one possible explanation. This phenomenon has been observed [5, 6] as a result of Cr2N layer formation after nitrogen implantation and seems to be responsible for the improved corrosion resistance of nitrogenimplanted chromium in HeSO 4 solution as reported by Terashima et al. [7]. It could also apply to our findings since many of the implants were aimed at generating hard dispersed particles
= E © E E " ~, ~ ~ ~° ~
~0
:
25
:
DIOLEN tm 50f18
DIOLEN tm 100f36 I
+ ,= ,~o
, ,= H2o
20 15
,,
10 ~
4
Ref.
C
•
II
~6 0 -
DIOLEN tm 187f30
.. + 1=,20
~ Ar Ti/N2 Ref.
N
I
1 Ar
i6 Pb
Ref.
8 6 0
Ar
Ion Species
Fig. 4. Wear parameter k' as a function of ion beam treatment for three different yarn types at 1% HzO addition. The gain factors from ion implantationare given above each bar.
like borides, carbides, carbonitrides, nitrides, oxides or phosphides to be formed either as a compound with the chromium matrix or together with additional injection of reactive species like boron, silicon, titanium or vanadium. On the other hand, implantation of inert gases like argon was found [7] to be detrimental to the corrosion of chromium in H2SO 4 solution, in contrast to our results (Figs. 3 and 4) which also show a positive effect of argon or krypton implantation under aqueous corrosion conditions. Other effects like tribodiffusion or surface amorphization may also be relevant to our case. A corresponding microstructural evaluation of our specimens by Auger electron spectroscopy and secondary ion mass spectrometry in order to determine the responsible protection mechanism is presently in progress. The abrasive wear contribution from the TiO2 particles in the fibre, which dominates the wear process at low water addition, was taken into account by two basic implantation processes: (a) surface hardening via lattice straining by injection of grossly oversized atoms (e.g. inert gases, lead, tin) which increase the internal stress; (b) dispersion hardening of the surface by formation of hard particles as mentioned above. Surface nitriding by nitrogen implantation has been reported to reduce wear of hard chromium layers in a Taber abrasion test up to a factor of 8.4 [7], and a gain factor of at least 20 was found in ball-on-disk tests at loads of 5.2 and 10.2 N [6]. Our results (Fig. 4) under mixed wear conditions (abrasive-corrosive) show an average improvement also in that range. Lead implantation, though, leads to a much higher wear reduction, possibly caused by both a sputter-induced surface
181
~.2 F-1F =o
, 0.6 :1 U "~
~
0.8
o
Ref. *
~/N2
" zx ,~ 0,4 D Pb ~ 0.2
,e,. o ,o, zx
//
/
/ /
/
"
zx so f~8 o 100f 36 :,k
, ' / qmplantation
167 f 30
o,woys
1 7o H20
~Test
:~ 0 A c
~.~;, 0 5
t~me:
10 h ON
110
Wear Parameter
~ J ' ~ - ~ - -
15
20
25
with running, TiO2-pigmented polyester yarn under water addition has been studied. T h e essential results are as follows.
50
k' [ l O - 9 m m 3 / ( N * m ) ]
Fig. 5. Correlation between relative change A# of friction coefficient and wear parameter k' for three different yarn types at 1% H20 after 10 h test time.
smoothing from the high-dose implant and a microlubricating effect during the wear process, A correlated plot (Fig. 5) of relative change A/~ in friction coefficient and the wear parameter k', both after 10 h test time, reveals some interesting features. T h e data points of all implants are located in the lower left area which represents the favourable regime of low wear and low friction change. Within that regime, p r o p e r selection of the implantation process allows a specific adjustment of desired, production-relevant combinations of both parameters. For 100f36, argon implantation minimizes A# at an already small k' value, and a lead implant leads to a minimized wear rate, but at the expense of a higher A/~. 5. C o n c l u s i o n s
T h e friction and wear behaviour of ionimplanted hard chromium electroplate in contact
(a) T h e majority of the investigated implants reduce both the friction coefficient and its timedependent change. (b) A beneficial implantation effect on the corrosive part of the abrasive-corrosive wear process is concluded from the stronger reductions of the friction coefficients at higher water concentrations. (C) Friction coefficient reduction is accompanied by wear reduction of implanted material with i m p r o v e m e n t factors u p to 54.
(d) A desired combination between low wear rate and small time-dependent change of friction coefficient can be adjusted by p r o p e r selection of the implantation process.
References 1 H. Simon and M. Thoma, Angewandte Oberfldchentechnik fiir metallische Werkstoffe, Carl Hanser, Munich, 1985, p. 28andpp. 51-55.
2 B.J. Tabor, Wear, 88(1983)93-101.
3 F. A. Schmidt, Nucl. lnstrum. Methods" B, IO/ll (1985) 532-538. 4 w. Beitz and K. H. Kiittner (eds.), Taschenbuch fiir den Maschinenbau/Dubbel, 15th edn., Springer, Berlin, 1983, p. 130. 5 R. Hutchings, Mater. Sci. Eng., 69(1985) 129-138. 6 w.C. Oliver, R. Hutchings and J. B. Pethica, Metall. Trans. A, 15(1984) 2221-2229. 7 K. Terashima, T. Minegishi, M. Iwaki and K. Kawashima, Mater. Sci. Eng., 90(1987)229-236.
177
Improvement in Friction and Wear of Hard Chromium Layers by Ion Implantation* W. LOHMANN
Akzo Research Laboratories Obernburg, Corporate Research--New Materials, PO Box, D-8753 Obernburg (F.R.G.) J. G. P. VAN VALKENHOEF
Akzo Centrefor Materials and Corrosion Engineering, PO Box 25, NL- 7550 GC Hengelo Ov (The Netherhmds) (Received September 16, 1988)
Abstract Hard chromium layers are widely used for coating fibre-guiding elements in the man-made fibre industry. A low wear rate and a time-independent, constant friction coefficient (not necessarily minimized) are essential to this application. A screening implantation programme with singleormultiple implants has been carried out on hard chromium layers which had been electroplated onto steel cylinders. The experimental procedure, equipment and post-implantation analysis methods are described. The friction and wear evaluation shows a clear improvement after most ion beam treatments. Its relative degree is dependent on the wear conditions and the ion species. For the wear couple of TiO2-pigmented polyester yarn against chromium electroplate under abrasive-corrosive conditions, very high gain factors (up to 54) are found. It is also demonstrated that a desired combination between a time-dependent change of friction coefficient and wear rate can be adjusted by proper selection of the ion beam process. 1. Introduction Electroplated hard chromium layers are widely used in the man-made fibre industry as coatings for tools for thread guiding, drawing, heating and winding. Desirable surface properties for such applications are a low yarn-induced wear rate and a time-independent, constant friction coefficient. Because a defined slippage between the running thread and the contact area is required, e.g. for drawing purposes, an adjustable friction coefficient is often rated higher than a minimized one. *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
Although hard chromium plate is known to be effective against wear and for reduced corrosion of metallic materials [1], its wear under manufacturing conditions of synthetic yarns is still a concern [2]. The observed wear process consists of an abrasive and a corrosive mode. Abrasion is caused by hard particles incorporated into the soft fibre, and corrosion can be induced by spin finish fluids, especially in view of the tendency for cracks in hard chromium plate. Because ion implantation has been shown I3] to be useful for both wear and corrosion protection without affecting the dimensional stability of treated components, a screening programme has been carried out to determine the friction and wear improvement obtainable for ion-implanted hard chromium layers. 2. Experimental details
2.1. Specimens The specimens used for this study were C35 steel cylinders (height 30 ram, wall thickness 1.5 ram, inner diameter 30 ram). A dull, hard chromium layer 50/zm thick was electrodeposited onto the outer cylinder surface under usual industrial chromium plating conditions. The initial surface roughness R~ was about 0.7 ktm. 2.2. Ion implantation A modified Varian-Extrion medium-current ion implanter (model DF 3000) was used for a screening programme with single or multiple implants. The specimens were electrically ~nsulated for beam current measurement, and no intentional active cooling was provided. The essential implantation conditions are given in Table 1. © Elsevier Sequoia/Printed in The Netherlands
178 TABLE 1
lmplantationconditions
< 10 "mbar mainly 150 keV, in case of multiple implants various energies down to 32 keV to match the range of the initial one < 0.6 W cm 2 for Timp~< 300 °C (otherwise hardness loss [1 ]) generally 4 × 1017 cm 2, for multiple implants between 0.6 and 4 × I(P 7 cm-2 to meet the stoichiometry of the desired hard dispersed particles AI, Ar, B, C, Hf, Kr, N, Nb, O, P, Pb, Si, Sn, Ti, V
Pressure during implantation Ion energy
Beam power density Ion dose Ion species
Specimen rotation inside the beam and electrostatic beam scanning for uniform implantation
! i @
/ 5
=-~
TABLE 2
~ ~sca --- 4
Test conditions
Yarn speed
300mmin
Yarn type
TiO2-pigmented polyester (DIOLEN TM)in 50 dtex f 18, 100 dtex f 36 and 167 dtex f30 conditions
Yarn Friction Tester
~ 7 [1 Bobbin
]
; 2 Yarn tension device '3 Yarn tension head
j I
4 Specimen ' 5 Thermocouple
I
transport device [ 7 Air jet j i
6 Yarn
Fig. 1. Experimental set-up for friction coefficient measurement.
Added preparation
1%-7.5% H20
Relative humidity Test temperature
60% 20 °C
Pretension Angle of wrap
0.5-1.67 N (1 cN dtex-') 90 °
Test duration
up to 10 h
2.3. Measurement offriction and wear
I
A schematic representation of the expenmental set-up (after ref. 2) is shown in Fig. 1. The yarn is drawn from a bobbin and passes through a tensioning device to give the desired pretension. Two tension heads are located in front of and behind the specimen; this arrangement allows direct calculation [4] of the friction coefficient/~ from
p=lln(F~)
(1)
when a is the angle of wrap, and F 1 and F 2 are the forces in front of and behind the specimen respectively. A thermocouple attached close to the specimen is used for temperature measurement. After the second tension head, there is a transport device to keep the yarn at the required speed. Finally, it is blown by an air jet into a reception box. The test conditions for reference and ion-implanted specimens are given in Table 2. Following this test step, which only gives friction coefficient data, the wear of the specimens was determined by means of another device
| t~3 _ ~ . .,...=...... ,9 ...r-:v=zT-.. _ o o o 12 ..... • _..43 - u u - °
1, B ~ ( /////////////
~
l
1-~
t6
Fig. 2. Schematicdiagramof wear measurementapparatus.
(Fig. 2). The yarn-induced incision is measured with the aid of a computerized profilometer (Talysurf-5, Rank Taylor Hobson Ltd.). It enables direct plotting of the surface roughness profile on a chart recorder as well as further processing of the various roughness parameters, e.g. generation of an enlarged picture of the cross-section of the worn surface (see position 6 in Fig. 2) and its integration. As described in ref. 2, this leads to a
179 ~jp~cies ploot~~
simple derivation of the specific wear rate k',
.
.
.
.
.
--~
22 ~°it,....... n
k'-
V
(2)
where V is the worn volume, F is the normal force and S is the running distance, k' is mainly used in case of smooth (R a < 0.1/~m) cylindrical model specimens. In practice, however, hard chromium plated guiding elements often have rougher, "orange peel"-like surface textures consisting of regularly textured spherical cusps. This complicates the computation and measurement of k'. A mode of computation was found by taking the expression
k'-(N2LBV')
(3)
in which
if= =ARtm (3R-AR,m) __
,~ ~
"I~~ ~
and L is the length of wear scar, B is the width of wear scar, and ARtm = Rtm (unworn)- Rtm (worn). V' is the average volume of a single segment. The other quantities in eqns. (3) and (4) are as follows: Rtm the average peak-valley distance of the roughness profile; R ( = 1/Rp~) the average radius of curvature of the roughness profile; N( = 1/Sm) the average number of roughness cusps per unit length, Hence, the determination of k' is only based on parameters from usual roughness measurement and on the dimensions of the wear scar. 3. Experimental results
3.1. Friction behaviour A series of friction coefficient measurements were carried through on three types of pigmented polyester yarn under identical pretension per iliament( 1 cN dtex- ~), but with different water additions according to the production conditions. The test results are presented in Fig. 3. Since usually an initial increase of the friction coefficient kt is observed, followed by a levelling-off after a run time of about 1-2 h, the typical values of/~ after 2 h are given, partly supplemented by data from longer test times. The initial friction coefficient/~0 shows some fluctuations, but no systematic reduction by ion
~
i
I D,OLEN,~ ,oof361 '
°'3
[+
~ water
~z
~n,t~o, .....
((~)
0°ira.......
IIIIII
o,
Ion~ecies Unlmplonted A~
J
0,
Frict]orlCoe,fJcient
T oI ~/P ~/~/~
]
. . . . ,too
, .... ,oog.aft.,
] ~Si 2 " ~/Aly,/,2~2 0 , - ~ - - ~ 0'2
/ B ~
after 2 h run time
DIOI D/OLEN tm 167f30]
+ 3.5=~ote~ J °.2
o,
Frictlo. Coefficient
~ ~
()
b
[[~ initialvalue m after 2 h run time I ~11
Ai]~c~/~i
(4)
o...2.
/~CNC~/'~N ]~I~NNN2
~ _ _ ±.... ~ ~_~L-~ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Friction Coefficient
* : terminated after 10 ml..... ime DIOLEN tm 50f181 + 7.5= wQter J (C)
Fig. 3. Friction coefficientsof reference and ion-implanted specimens tested with three yarn types: (a) DIOLEN T M 100f36+1% H20 , (b) DIOLEN T M 167f30+3.5% H20 and (c) DIOLEN T M 50f18 + 7.5% H20.
implantation. The average /~0 values are 0.18 (100f36), 0.13 (167f30) and 0.19 (50f18). However, distinct effects appear after some hours of testing. Many of the implants lead to a considerable reduction of/~ and, at the same time, to a more stable, time-independent behaviour. Taking /.~Ref/~Uimplafter 2 h as a figure of merit, one obtains gain factors relative to the unimplanted condition between 1.0 (A1) and 1.9 (Pb) for 100f36 + 1% H20, 1.1 (A1) and 2.3 (O, Ti/P, Ti/C/C) for 167f30 + 3.5% H20, and 1.2 (Ti/C/Nb) and 2.7 (C) for 5 0 f 1 8 + 7 . 5 % H 2 0 . The implanted species are given in brackets. The relative change of the friction coefficient after a run time of 2 h, A/~ = (/~ -/~o)//%, is a measure of the time stability of the friction coefficient and should be minimal for production. Further evaluation of Fig. 3 yields the following range of gain factors with respect to the untreated reference, i.e. A[tRef/AJ~irnpl: between 0.8 (A1, no gain) and 19.8 (Ti/C/C) for 100f36 + 1% H 2 0 (Pb:6.1), 1.2 (Al) and 7.7 (Ti/C/C) for 167f30+3.5% H 2 0 (0:4.5, Ti/P:5.9), and 1.2 (Ti) and 8.0 (Ti/A1/N/N) for 50f18 + 7.5% H 2 0
180 (C:6.8). As above, the implanted ions are given in brackets. It can be seen that the ions which are the most effective ones for reducing the friction coefficient are also beneficial in improving its time constancy, but to a lesser degree than implantation processes optimized for that purpose. However, the overall improvement of both properties remains acceptable for a variety of non-optimized implants.
3.2. Wear behaviour Wear analysis was not performed on all implanted materials, but only on specimens which had shown a large reduction of the friction coefficient (Section 3.1) after an ion beam treatment as simple (and economical) as possible. This led to closer consideration of the argon, carbon, nitrogen, oxygen and T i - N 2 implants. In view of the obvious tendency for increasing friction reduction with increasing water addition, which indicates improved corrosive wear behaviour, these tests were carried out on specimens which had been worn in the presence of only 1% H20 in order to emphasize the abrasive part of the wear process. The results can be seen in Fig. 4. All implants reduce the amount of wear with yarndependent gain factors up to 54. The smaller k' value of the 167f30 reference is due to a 40% larger filament diameter that decreases the Heftzian contact pressure by 50%. The wear rate after argon implantation should be proportionally smaller, but this is not observed,
4. Discussion Assuming a low and constant friction coefficient as an indication of low wear rate, the data from Fig. 3, which show a clear correlation between friction reduction and water addition, suggest an especially beneficial implantation effect on the corrosive wear mode that becomes more dominant at higher water concentration, Closing of microcracks present in hard chromium electroplate is one possible explanation. This phenomenon has been observed [5, 6] as a result of Cr2N layer formation after nitrogen implantation and seems to be responsible for the improved corrosion resistance of nitrogenimplanted chromium in HeSO 4 solution as reported by Terashima et al. [7]. It could also apply to our findings since many of the implants were aimed at generating hard dispersed particles
= E © E E " ~, ~ ~ ~° ~
~0
:
25
:
DIOLEN tm 50f18
DIOLEN tm 100f36 I
+ ,= ,~o
, ,= H2o
20 15
,,
10 ~
4
Ref.
C
•
II
~6 0 -
DIOLEN tm 187f30
.. + 1=,20
~ Ar Ti/N2 Ref.
N
I
1 Ar
i6 Pb
Ref.
8 6 0
Ar
Ion Species
Fig. 4. Wear parameter k' as a function of ion beam treatment for three different yarn types at 1% HzO addition. The gain factors from ion implantationare given above each bar.
like borides, carbides, carbonitrides, nitrides, oxides or phosphides to be formed either as a compound with the chromium matrix or together with additional injection of reactive species like boron, silicon, titanium or vanadium. On the other hand, implantation of inert gases like argon was found [7] to be detrimental to the corrosion of chromium in H2SO 4 solution, in contrast to our results (Figs. 3 and 4) which also show a positive effect of argon or krypton implantation under aqueous corrosion conditions. Other effects like tribodiffusion or surface amorphization may also be relevant to our case. A corresponding microstructural evaluation of our specimens by Auger electron spectroscopy and secondary ion mass spectrometry in order to determine the responsible protection mechanism is presently in progress. The abrasive wear contribution from the TiO2 particles in the fibre, which dominates the wear process at low water addition, was taken into account by two basic implantation processes: (a) surface hardening via lattice straining by injection of grossly oversized atoms (e.g. inert gases, lead, tin) which increase the internal stress; (b) dispersion hardening of the surface by formation of hard particles as mentioned above. Surface nitriding by nitrogen implantation has been reported to reduce wear of hard chromium layers in a Taber abrasion test up to a factor of 8.4 [7], and a gain factor of at least 20 was found in ball-on-disk tests at loads of 5.2 and 10.2 N [6]. Our results (Fig. 4) under mixed wear conditions (abrasive-corrosive) show an average improvement also in that range. Lead implantation, though, leads to a much higher wear reduction, possibly caused by both a sputter-induced surface
181
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~
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o
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~/N2
" zx ,~ 0,4 D Pb ~ 0.2
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/ /
/
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, ' / qmplantation
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~.~;, 0 5
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110
Wear Parameter
~ J ' ~ - ~ - -
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20
25
with running, TiO2-pigmented polyester yarn under water addition has been studied. T h e essential results are as follows.
50
k' [ l O - 9 m m 3 / ( N * m ) ]
Fig. 5. Correlation between relative change A# of friction coefficient and wear parameter k' for three different yarn types at 1% H20 after 10 h test time.
smoothing from the high-dose implant and a microlubricating effect during the wear process, A correlated plot (Fig. 5) of relative change A/~ in friction coefficient and the wear parameter k', both after 10 h test time, reveals some interesting features. T h e data points of all implants are located in the lower left area which represents the favourable regime of low wear and low friction change. Within that regime, p r o p e r selection of the implantation process allows a specific adjustment of desired, production-relevant combinations of both parameters. For 100f36, argon implantation minimizes A# at an already small k' value, and a lead implant leads to a minimized wear rate, but at the expense of a higher A/~. 5. C o n c l u s i o n s
T h e friction and wear behaviour of ionimplanted hard chromium electroplate in contact
(a) T h e majority of the investigated implants reduce both the friction coefficient and its timedependent change. (b) A beneficial implantation effect on the corrosive part of the abrasive-corrosive wear process is concluded from the stronger reductions of the friction coefficients at higher water concentrations. (C) Friction coefficient reduction is accompanied by wear reduction of implanted material with i m p r o v e m e n t factors u p to 54.
(d) A desired combination between low wear rate and small time-dependent change of friction coefficient can be adjusted by p r o p e r selection of the implantation process.
References 1 H. Simon and M. Thoma, Angewandte Oberfldchentechnik fiir metallische Werkstoffe, Carl Hanser, Munich, 1985, p. 28andpp. 51-55.
2 B.J. Tabor, Wear, 88(1983)93-101.
3 F. A. Schmidt, Nucl. lnstrum. Methods" B, IO/ll (1985) 532-538. 4 w. Beitz and K. H. Kiittner (eds.), Taschenbuch fiir den Maschinenbau/Dubbel, 15th edn., Springer, Berlin, 1983, p. 130. 5 R. Hutchings, Mater. Sci. Eng., 69(1985) 129-138. 6 w.C. Oliver, R. Hutchings and J. B. Pethica, Metall. Trans. A, 15(1984) 2221-2229. 7 K. Terashima, T. Minegishi, M. Iwaki and K. Kawashima, Mater. Sci. Eng., 90(1987)229-236.