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Prototype fretting-wear testing machine and some experimental results
WEAR Wear 214 (1998) 221-230
Prototype fretting-wear testing machine and some experimental results Etsuo Marui *, Hiraki Endo, Norihiko Hasegawa, Hidetoshi Mizuno Department of Mechanical and Systems Engineering. Facility of Engineering. Gifll Unh-ersity. 1-1 Yanagido. Gifll-shi 501-11. Japan
Received 8 July 1997; accepted 8 October 1997
Abstract A new prototype fretting-wear testing machine is developed, and its performance is evaluated. In this testing machine, a spherical specimen is oscillated by a lead-zirconate-titanate semiconductor (PZT) actuator, driven by means of computer control. Thus, an arbitrary input waveform, such as sinusoidal wave, triangular wave and so on, can be easily operated to a spherical specimen. This is the special feature of our testing machine. As an example, a fretting experiment is carried out, using a steel flat specimen and a bearing steel sphere specimen. A sinusoidal input wave is added by a PZT actuator. It is clarified that there is a boundary fretting amplitude, beyond which fretting wear mass or volume increases abruptly. © 1998 Elsevier Science S.A. Ke.nmrds: Fretting wear; Fretting-wear testing machine; Sphere-an-flat type; PZT actuator \
1. Introduction Fretting occurs wherever small-amplitude reciprocating sliding between contacting surfaces is sustained for a large number of cycles. The sliding amplitude or fretting amplitude is smaller than the contact area size and wear debris is likely to build up within the contact zone, so the surface conditions are apt to affect more remarkably the fretting wear phenomena than the usual wear phenomena [I] .The main focus of previous research on fretting wear has been to basically understand its characteristics by clarifying the slip within the contact zone [2,3] and the stress distribution there [4]. Many experiments have been carried out to clarify the fretting wear characteristics of various engineering materials for practical use. The tested materials include mild steel [S7], eutectoid steel [8], bearing steel [9,10], stainless steel [ II ], Cr-Mo steel [12], aluminum alloy [ 13,14], titanium and nickel alloys [IS,16], and reinforced composite materials [17,18]. The effects of surface oxidation [6,7,9,IS], lubrication [S,19] and surface integrity by plasma [20] or laser treatment [10] have also been investigated. The following trend has been revealed by surveying the specimen configuration and the way in which specimens are oscillated. Specimen configurations used to date are: sphereon-flat surface type [9-11,13-16,19,21], cylinder-on-flat surface type [S,17,20] and flat surface on flat surface type [6,7,12,18]. Spheres or cylinders are oscillated in some
* Corresponding author. Fax: + 81-058-230-1892. 0043-1648/98/$ 19.00 © 1998 Elsevier Science S.A. All rights reserved Pll S0043-1648 (97) 00251-2
cases, and a flat surface in others, on a sphere-on-flat surface type or cylinder-on-flat surface type testing machine. Means to oscillate specimens are: mechanical devices such as eccentric cam mechanisms [8,9,14,21], hydraulic actuator mechanisms [6,12,13] and electro-magnetic devices [S,II,IS17,19]. A testing machine to observe the effect of environmental parameters such as temperature has also been constructed [7,16,21]. Basing on the previous research, we designed and constructed a new prototype testing machine for fretting wear in this research program. Our testing device is inexpensive, and the testing conditions can be easily altered. Some experimental results of steel sphere-an-steel flat surface are reported here.
2. Prototype fretting-wear testing machine 2. J. Construction of machine Fretting wear is apt to occur when the reciprocating sliding amplitude between two surfaces ranges from several to tenodd J.lm [I]. Fretting wear is influenced by reciprocating sliding amplitUde, normal force, frequency of reciprocating movement, fretting cycle number, environment and physical property of materials. Thus, a fretting-wear testing machine, in which these influencing parameters can be altered easily and over a wide range, is favorable in practical use.
E. Marui et al.llVear 214 (/998) 221-230
222
In our prototype testing machine, a lead-zirconate-titanate semiconductor (PZT) actuator, which can generate a small reciprocating displacement by a piezoelectric phenomenon [22], is used as a driving element for reciprocating sliding specimens. The PZT actuator is very advantageous because various waveforms of reciprocating sliding movements can be easily realized by computer-controlled voltage to the PZT actuator. Some of the electrical and mechanical properties of PZT actuator are given in Table 1. A combination of conventional spherical and flat specimens (sphere-on-flat type) are adopted in our machine. An outline of the fretting-wear testing machine used is given in Fig. I. The machine must be as rigid as possible to minimize the absorption of the small reciprocating sliding, which is induced by the PZT actuator, by an elastic deformation of each machine component. To realize this require- • ment, the spherical specimen is directly attached to the drive section of the PZT actuator. For this reason, a spherical specimen 20 mm in diameter made of bearing steel is used. However, a spherical specimen of a different size can be readily used in this testing machine, without any modification of the testing device. Table 1 Some properties of PZT actuator Properties of PZT Relative dielectric constant Loss tangent Young's modulus Curie point Maker
1800 0.02 44GPa 230°C Nippondenso
Size
iOmm
Diam'eter Length
A spherical specimen was arranged so as to be sandwiched between two identical flat specimens. By introducing this structure, a moment established owing to the friction force I acting on the spherical specimen can be eliminated, and the spherical specimen can be very rigidly held in place. The. driving clement and the sphere specimen are arranged on a ; vertical straight line, so the testing machine takes up little , space. Moving elements arc suspended by two sets of parallel leaf springs as seen in Fig. 2, so that the moving direction of the PZT actuator parallels the contacting surface between specimens. Normal load is applied by the two dead weights shown in Fig. I, and the load can be thus kept constant during the experiment. The magnitude of the displacement generated by the PZT actuator is controlled through a personal computer. Control voltage of 0-5 V, generated by a personal computer, is magnified 120-fold through an amplifier. Then, the driving voltage of the PZT actuator is between 0-600 V. Computer control of the fretting-wear testing machine enables the spherical specimen to be driven by an arbitrary waveform. Two flat specimens 35 mm-square are attached to each stage, which can move horizontally owing to linear guides and rails. Wires are stretched between stages via pulleys, and the tensile force of dead weights is exerted on the stages. A spherical specimen is suspended between two identical flat specimens, and a constant normal force is added between specimens. Two sensors of leaf spring construction are attached on stages to measure the reciprocating sliding movement of specimens. Their size and configuration are given in Fig. 3.
I:
Spherical specimen
120mm
Specification Maximum displacement Maximum driving force Maximum operating frequency Allowable temperature limit Control voltage
100j.lm 2kN 100Hz 0-50°C 0-5 V
PZT actuator
Fig. 2. Driving mechanism of specimens.
Fig. I. Prototype fretting-wear testing machine.
. E. Marui et al. / lVear 214 (1998) 221-230
Each sensor consists of a leaf spring and a base. The base of the sensor is fixed to the stage. The tip of the leaf spring is arranged to be right above the spherical specimen. The deformation of the PZT actuator or the relative reciprocal sliding bet\veen spherical and flat specimen is measured by strain gauges pasted on both sides of the leaf spring. To minimize noise, all lead wires are sheathed. The calibration test result of these two sensors is shown in Fig. 4. The relation between output voltage from strain gauges and relative reciprocal sliding is shown here. Good linearity is recognized between them. Output noise is about 0.01 mY, which corresponds to the reciprocal sliding of 0.5 fLm. Accordingly, the relative reciprocal sliding between specimens can be measured precisely by these sensors. 2.2. Performance ofprototype fretting-wear testing machine
Performance of our prototype fretting-wear testing machine is checked through various calibration tests. Firstly, Fig. 5 gives a change with the passage of time of fretting amplitude. In this experiment, the input voltage waveform to the PZT actuator is sinusoidal, with an amplitUde and frequency of360 V and 10 Hz, respectively. In the figure, the result for the case of a bearing steel spherical specimen and a medium carbon steel flat specimen is shown at four levels
E E~
60~--~~--~~---+----~
8
40 ~---li--<"1"---l-----+-----I
is
20 I-.....,.,.--t---t----''---t--------I
'" ~
2
3
4
223
of normal force. This combination of materials is used in the under-mentioned fretting-wear experiment. The fretting amplitude decreases rapidly, during the initial 2000 fretting cycles. This decrement of the fretting amplitude is more remarkable as the normal force becomes larger. Surfaces of both specimens are fully degreased by washing in methyl acetate before the fretting-wear test. Rapid decreasing of the fretting amplitude in the initial stage may be induced by the change of surface oxide film or the breakdown of slightly remained lubricant film. After that, the variation in fretting amplitude is recorded till the fretting cycle number of 105 • It is confirmed that the fretting amplitude is almost constant in relation to the magnitude of the normal force. The relation between the mean fretting amplitUde and the normal force from 3000 to 104 fretting cycles is shown in Fig. 6. The fretting amplitude decreases almost linearly with the increase of the normal force, and the relation between them is expressed by Eq. (I). (I)
where, A is the fretting amplitude (fLm), andAo is the fretting amplitude (fLm) when the normal force is zero. Fn indicates the normal force (N). The fretting amplitude decreases when the normal force increases, because\the frictional force acting between specimens grows as the normal force increases. Therefore, the frictional force can be estimated by the size of the fretting amplitude. Next, the relation between the mean fretting amplitude and the input voltage amplitUde is given in Fig. 7, for constant input voltage frequency (to Hz). In the figure, the result is given for two levels of normal force, 0 Nand 66 N. The mean fretting amplitude changes linearly with input voltage amplitude, and the lines indicating the relation between them are parallel to each other. It is ascertained from this that the decrease in mean fretting amplitude owing to the frictional force effect is not affected by the input voltage amplitude. From Fig. 7, the mean fretting amplitudeA o when the normal force is 0 N, is represented by Eq. (2).
Vo
(2)
Output voltage (V) Fig. 4. Calibration result for leaf spring sensor.
50 50r--------------------,
40
E
40
(!)
30
•
"0
.~
C. ~
0 0
0
0
0
0
0
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.-o-
0000000000 0
~. _______
0
0
0
Fn=I6N
20 \ _ .... - _ _ 10
. _- .... -
20
FI1=66N
"'"
10
""
~
~
~
Fn=lI6N
~~-------------l
00
30 ~ --v
2000
4000 6000
00
20
40
60
80
100 120
8000 10000
Fretting cycle number Fig. 5. Variation of fretting amplitude owing to fretting cycle number at input voltage amplitude 360 V and frequency 10 Hz.
Normal load (N) Fig. 6. Influence of normal force on fretting amplitude at input voltage amplitude 360 V and frequency 10 Hz.
224
E. Marui el al. IlVear 214 (1998) 221-230
the normal force is zero and the fretting amplitude A when the normal force is applied. The action of the frictional force is shown in Fig. 9, when the sphere specimen is sliding in the direction M as seen in the figure. The downward frictional force 2Q acts on the spherical specimen and the upward,. frictional force Q acts on each flat specimen and stage. The " rigidity coefficient of the driving element in this direction is .' set at Rd and that of the stage at Rs. The variation or the I decrease ilX of the fretting amplitude due to frictional force' is written as follows:
80 ,.,.......
S
~ (!)
60
• Fn=ON = 66 N
0 Fn
'"0
.-C.Z S
40
ce
I=:
ce
(!)
20
::?:
2Q Q -+-=ilX Rd Rs
O~----~~-----L------~
o
200
400
600
Input voltage amplitude (V) Fig. 7. Influence of input voltage magnitude to PZT actuator on fretting amplitude at input voltage frequency 10 Hz.
where, Va is the input voltage amplitude (V) to the PZT actuator. From Eqs. (1) and (2), the fretting amplitude can be written as follows: A=0.13Va -0.24Fn -IS
(3)
Lastly, the effect of the input voltage frequency is investigated. Fig. 8 shows the result for the case of the normal force 66 N and the input voltage amplitude to the PZTactuator 600 V. The mean fretting amplitude is almost constant tiII the frequency of20 Hz. Above this frequency, the fretting amplitude decreases with the input voltage frequency. From this result, it is ascertained that the limiting input voltage frequency of this fretting-wear testing machine is about 20 Hz. 2.3. Estimation offrictional force This fretting-wear testing machine is not equipped with a sensor to measure frictional force. The magnitude of the frictional force can be approximately estimated as follows. As mentioned in Fig. 6, the frictional force is calculated using the difference between the fretting amplitude Ao when
Estimation of the rigidity coefficients is carried out as follows: The part of the spherical specimen attaching to the driving element is pulled by a spring balance, and the displacement of the part attaching the spherical specimen is read by a dial gauge as seen in Fig. 10. The rigidity coefficient of the driving element Rd is then calculated by the pulling force and the displacement there. The rigidity coefficient of the stage Rs is obtained by the same method. As a result, the rigidity coefficient of the driving element may be stated Rd= 16.54 N/v.m, and that of the stage by Rs=8.7 N/v.m. Then from Eq. (4), the frictional force Q is obtained as follows:
Q=4.24ilX
(5)
The frictional coefficient is also estimated by dividing this frictional force by the normal force Fn. The frictional coefficient variation obtained by this process is given in Fig. II. In this estimation, the input voltage frequency is 10Hz, its amplitude is 480 V and the applied normal force is 66 N. The stable frictional coefficient is estimated as seen in the figure, except the initial stage of the fretting cycle number.
50 ,-...
40
u
E :::t '-"
30
---
~(
~
(lJ
-0 ::l
.-::: 0.
~
Fig. 9. Frictional force acting on specimens.
"
20 10
o0
10
20
30
40
50
60
Frequency (Hz) Fig. 8. Influence of input voltage frequency to PZT actuator on fretting amplitude at normal force 66 N and input voltage amplitude 600 V.
Fig. 10. Evaluation of driving element rigidity.
E. Mami el al. IlVear 214 (/998) 221-230
1.2 I=:
.9 1.0 t> ;.s O. 8
- I-
---
"""0
....I=: C!}
'u
ij3 C!}
~
0.6 0.4
0
U
O. 2 2 4 6 8 Fretting cycle number Fig. II. Example of frictional coefficient evaluation at normal force 66 N. input voltage amplitude 480 V and frequency 10Hz.
225
number 105 • Fig. 12a shows the wear scar of the flat specimen and Fig. 12b shows the wear scar of the spherical specimen. The wear vestiges on the flat specimen have a conical shape. Its radius is about 600 fJ.m and the depth is about 14 fJ.m. There is a small plateau at the center of the worn portion, whose radius and depth account for about one-third of the wear scar. Around this area, there is an annular hump along the edge about 2.5 fJ.m high and around 100 fJ.m wide. On the other hand, the wear scar on the sphere specimen is a little smaller than the scar on the flat specimen. Its radius is about 570 fJ.m. There is a plateau at its center, similar to the flat specimen. The surface of the wear scar is almost smooth, except for the central plateau.The above features are similar to those of the fretting-wear scar already reported by many resf;!archers. Thus, the wear phenomena, which are observ,ed in o,ur fretting-wear testing machine, are considered to be induced by fretting wear. The fretting-wear volume is obtained [10] from the mean wear scar radius r and the scar depth d, which are easily measured by a surface roughness tester. Assuming the shape may be regarded as a sphere crown, the wear volume Vr is written as follows:
Vr= :d(3r 2 +d 2)
(6)
(a) Rat specimen
It is extremely difficult to measure the wear scar depth d on the sphere specimen from a surface roughness curve. So, the radius of curvature of the wear scar surface is approximately regarded as the radius of the spherical specimen and the wear scar depth is calculated by Eq. (7). The approximate wear volume of the sphe~ical specimen is obtained by substituting Eq. (7) into Eq. (6). (b) Spherical specimen
d=R-fR 2 -r 2
'Fig. 12. Measurement of fretting wear scar at normal force 66 N. fretting frequency 20 Hz, fretting amplitude 35 ILm and fretting cycle number 105.
(7)
where, Rand r represent radii of the sphere specimen and the mean wear scar radius, respectively.
2.4. Estimatioll of wear volume
Fig. 12 shows the fretting-wear scar in the case of the bearing steel sphere specimen and the medium carbon steel flat specimen. This is done by combining the surface roughness profiles measured by a surface roughness tester. Here, the applied normal force is 66 N, the input voltage frequency is 20 Hz, the fretting amplitude 35 fJ.m and the fretting cycle
3. Some experimental data on pair of stcel specimcns A fretting-wear test is carried out using the bearing steel sphere specimen and the flat specimen of medium carbon steel for machine construction, and the validity of our testing machine is ascertained. The chemical composition and Vick-
Table 2 Chemical composition and Vickers hardness of specimens Vickers hardness
Elements (%)
Medium carbon steel for machine construction use High carbon chromium bearing steel
C
Si
Mn
Cu
Ni
Cr
p
0.55
0.23
0.75
0.05
0.15
0.12
0.02
0.02
235
1.05
0.15
0.20
0.12
0.15
1.48
0.01
0.01
850
S
Hv
\
I.,.
E. Mam; et al. / lVear 214 (1998) 221-230
226
"
ers hardness ofthe specimens are listed in Table 2. The radius of the spherical specimen is 20 mm. The flat specimen is a square plate 10 mm thick and 40 mm long. The fretting-wear test is carried out while varying some parameters affecting the fretting-wear characteristics.
°• Fn=I6N Fn=66N
Flat specimen
A Fn=1l6N
3.1. Influence offretting cycle number The influence of the fretting cycle number is examined, while varying the fretting cycle number between 103 and 106 for the constant normal force 66 N, the fretting frequency 20 Hz and the fretting amplitude 35 p.m. A sample experimental result is shown in Fig. 13. The fretting-wear rate of the sphere and the flat specimens, which is converted from the wear volume obtained by the abovementioned process, is shown here. The fretting-wear rate is defined as the wear volume corresponding to the unit sliding distance and unit normal force. The concept of the wear rate is important to compare the wear quantitatively under different fretting-wear conditions. The wear rates for the spherical and flat specimens are similar in magnitude. As is clear in the figure, the wear rate becomes small with the increase of the fretting cycle number. There is a linear relationship between the wear rate and the fretting cycle number on log-log paper. This relationship is written as follows:
01k-----~m_----_,~-----7,
Fretting amplitude (/1 m) Fig. 14. Influence of fretting amplitude and normal force on wear scar radius • at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
30
Fn=66N °• Fn=I6N Fn=I16 N A
I
r:-
-.g
I
§011"«:
3.2. Influence of normal force andfretting amplilllde In this section, the influence of the norrrial force and the fretting amplitude on fretting wear is examined. The fretting frequency (20 Hz) and the fretting cycle number (5 X 104 ) are kept constant in this experiment. Fretting-wear characteristicsare clarified for various fretting cycle numbers and normal forces. Figs. 14 and 15 show the relations between the wear scar radius and its depth on the flat specimen and the fretting 10-12
J •
~ 1°
3
Flat spec. Spherical spec.
r--
~ i~
IN
~
'13
5
104 lOS Fretting cycle number Fig. 13. Influence of fretting cycle number on fretting wear rate at normal force 66 N, fretting frequency 20 Hz and fretting amplitude 35 f.l.m.
A
"~S"
~I <:$1
\
(8)
../';.
"'~" -~ 11 1 "" :'::1'"
A
.Fl.-
oo
I"
0 ..........
,"
-
••
""""""0" "
I I I
1520 40 Fretting amplitude ( /1 m)
60
Fig. 15. Influence offretting amplitude and normal force on wear scar depth at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
amplitude. The parameter shown in the figures is the normal force. It is seen in Fig. 14 that the wear scar radius increases linearly with the increase in fretting amplitude, although a little scatter is recognized among the measured data. A wear scar of larger radius results as the"normal force is larger. The following tendencies are recognized from Fig. 15 regarding the wear scar depth. The wear scar depth is quite small and the influence of the normal force is not remarkable, for fretting amplitudes smaller than 15 p.m. On the contrary, for a fretting amplitude larger than 15 p.m, the wear scar depth increases abruptly. After this increase, the wear scar depth does not vary much and is almost constant. The fretting amplitude, where the abrupt increase in the fretting amplitude occurs, is called hereinafter the boundary amplitude. It is quite natural that the wear scar depth is large for a large normal force. Figs. 16 and 17 show the wear rate, which is converted from the fretting-wear volume obtained using the above-mentioned wear scar radius and the wear scar depth. As seen in Fig. 16, the wear rate characteristic of the flat specimen is different at the boundary amplitude. For a fretting
\
\
E. Marui el al. / \Year 214 (/998) 22/-230
6
Flat specimen
{ll "I
~E
o Fn=16N • Fn=66N fl. Fn=1l6 N _
:::1 ~I <::Sl
i:-I
"'I 4
-~I
0
§I
.-<
.,gl
~ .......,
:A= It"
~ ....
AO
.... 2
l:;.
l:;.
....
-(I
I-
I I I I I I
~
~
0
o riJ,JI. 15 o
20 40 Fretting amplitude ( Jl m)
60
Fig. 16. Influence of fretting amplitude and normal force on fretting wear rate in flat specimen at fretting frequency 20 Hz and fretting cycle number 5X 10'.
6
..,.
'I
o Fn=16N • Fn=66N l:;. Fn=116N
4
c-
0
.-<
O
~ .......,
0
~
.... .... 2 Ofl.
~
~
• • o • o
l:;.fl. ~
rI
l:;.
•• l:;.
•
15 20 40 Fretting amplitude ( Jl m)
regimes. This transition between fretting-wear regimes is again investigated in the under-mentioned fretting-wear surface observation by SEM. Fig. 17 shows the wear rate of the spherical specimen, for reference. A tendency that fretting wear becomes violent with the increase of the fretting amplitude is recognized in Fig. 17 showing the result for spherical specimen. But, the boundary fretting amplitude is not recognized, differing from the result for flat specimen (Fig. 16). However, the boundary amplitude feature found with the flat specimen is not observed. As mentioned previously, it is difficult to determine the wear scar depth of the sphere specimen exactly, and the wear volume of the spherical specimen is roughly calculated by Eq. (7), a convenient way to obtain such a result. So, the simple and accurate method to obtain the fretting-wear scar configuration of the spherical specimen must be established for further investigation of the spherical specimen.
4. Observation of wear scar by SEM
Spherical specimen
~E
227
It is clarified in the previous discussion that there is a boundary amplitude in the flat specimen. Then, by observing the wear scar surface by an electron microscope (SEM), the mechanism of the fretting wear is considered basing the stress on the relation with th~ fretting amplitude .
4.1. Wear scar for fretting amplitude larger thall the boundary amplitude
60
Fig. 17. Influence of fretting amplitude and normal force on fretting wear rate in spherical specimen at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
amplitude larger than this boundary amplitude, the wear rate is independent of the fretting amplitude and remains constant. Furthermore, the magnitude of the normal force does not influence the wear rate. This fact means that the fretting-wear volume is proportional to the normal force. For the fretting amplitude smaller than the boundary amplitude, the wear rate is extremely small. No influence of the fretting amplitude and the normal force can be recognized. Vingsbo and Soderberg [23] and Vingsbo et al. [24] summarized fretting-wear characteristics as the fretting map, by arranging the effect of various influencing factors. Referring to their result, fretting wear is divided into three regimes, that is, stick regime, mixed stick-slip regime and gross slip regime, owing to the difference of slip state at contact area. Intimate adhesion occurs almost on the whole contact area in the first stick regime. On the contrary, gross slip occurs on the whole contact area in the third gross slip regime. In mixed stick-slip regime, partial adhesion remains at the central area of contact. The abrupt increase of fretting-wear rate shown in Fig. 16 corresponds to the transition between fretting-wear
Fig. 18 is a micrograph of the flat specimen and Fig. 19 of the spherical specimen. Here, the normal force is set at 66 N and the fretting amplitude at 40 /Lm, while the fretting cycle number is 5 X 104 • From Fig. 18a and Fig. 19a of the whole image, the fretting-wear scar can be divided into two parts, a central area and a surrounding annular area. We first consider Fig. 18. Fig. 18b is an enlarged photograph of the central area where there are remarkable irregularities. The material is peeled off at the depressed area on the surface. And the peeled-off material on the opposite specimen surface migrates to adhere to the opposite surface. Small amounts of striped marks on these irregularities in the fretting direction are recognized. This indicates that adhesive and abrasive wear are mixed at this central area. On the contrary, only striped marks extending in the fretting direction are recognized in the surrounding annular area. Fig. 18c is an enlarged photograph of this area. Here, abrasion owing to scratching by peeled wear debris or asperities on the opposite specimen is a dominant wear pattern. Fig. 19 shows a wear scar of the sphere specimen corresponding to the flat specimen of Fig. 18. The appearance of the wear scar of the sphere specimen resembles that of the flat specimen. From these results of Figs. 18 and 19, it is clear that the fretting wear of 40 /Lm fretting amplitude belongs to the gross slip regime [23]. The appearance of the observed wear scars does not change with the magnitude of the normal force or fretting amplitude.
,-----.-- .---'---'----~---------~-~~~-~~------..... "'I:~ 228
E Marui eta{ /Weur214 (1998) 221-230
Fig. I g. SEM observation of fretting wear scar on f,,,( specimen at norm"l force 66)l, fretting number 5 X 10". (a) Whole image; (b) Central area; (c) Surrounding annular area.
Fig. 19. SEM observation of fretting wear scar on spherical specimen
ciI
'"e'CllIN"',"
20 Hz, frelling amplitude 40 !-L m and fretting cycle
norca; fcrce 66:\" frcIting ~'re(!uency 20 Hz. fretting amplitude 40 fLm and fretting
cycle number 5 X 10-l. (a) Whole area; (b) Central area; (c) SurrouraJing annular area.
E. Marui
el
ai. / Wear 214 (1998) 221-230
229
Fig. 20. SEM observation of fretting wear scar un specimen at Honnal force 66 N. f:'etting frequency 20 Hz. fretting anlpIituGc 5 /-LIn and fretting cycle number5X104 . (a) Whole image; (b) Central area; (c) Surrounding annular area.
Fig. 21. SE~1 observation of fretting \\'e~:- scar on ~pheric:ll specimen at notnml f'lJ[CC 66 ;..;. fretting frequency 20 HL, fretting a.mp!itudc 5 J.Lm and fretting cycle number 5 X 104
4.2. Wear scarfor fretting amplitude smaller than the boundary mnplitude Where the fretting amplitude is smaller than the boundary amplitude, the appearance of the wear scar is fairly different from that of the large fretting amplitude explained in Section 4.1. Figs. 20 and 21 are SEM micrographs of the flat specimen and the spherical specimen, respectively. In this experiment, the normal force is set at 66 N and the fretting amplitude at 5 fLm, while the fretting cycle number is 5 X The appearance of wear scar is classified roughly into two regions, central area and the surrounding annular area, in the
same manner as when the fretting amplitude is larger than the boundary amplitude. Fig. 20b is an en larged SEM micrograph of the central area. Striped marks extending parallel to the frctting direction are seen in the figure. These striped mar:(s have a rising pattern, dissimilar to the case in which the amplitude is larger than the boundary amplitude. Adherence between the spherical specimen and the flat specimen may be induced in this area. The striped marks are forn1ed by stretching corresponding to the fretting action. Some oxide rises and striped marks due to slight abrasion are seen in the annular area of Fig. 20c. The appearance afthe spherical specimen corresponds well to that of the flat specimen, as seen in Fig. 21. From these results of Figs. 20 and 21, it is clear that the fretting wear of 5 !-Lm fretting amplitude belongs to the mixed stick-slip regime [231. So, the boundary recognized in Fig. 16 corresponds to the transition of wear regime from mixed stick-slip to gross slip regime. In the fretting condition in which the fretting amplitude is smaller than the boundary amplitude, the tangential force is also small and perfect adherence is generated at the central area only. the surrounding annular area is fretted, and the fretting-wear rate becomes small.
5. Conclusion A prototype fretting-wear testing machine, in which a sphere specimen is oscillated by the PZT actuator, is con-
230
E. Marui et al. / Wear 214 (1998) 22/-230
structed and its perfonnance is evaluated in this paper. The most advantageous feature of this testing machine, in which a specimen's reciprocal sliding displacement is driven by the PZT actuator controlled by a personal computer, is that the specimen's reciprocal sliding displacement of sinusoidal, triangular wave fonn or others can easily be realized. The relation between the experimental conditions and the fretting amplitude is clarified in detail, considering the PZTactuator' s properties. Using this result, the frictional force or the frictional coefficient acting on the specimens is estimated. The maximum input voltage frequency for stable operation of this testing machine is 20 Hz. As an example, a fretting test of a bearing steel spherical specimen and a medium carbon steel flat specimen is carried out, while driving the spherical specimen by sinusoidal wavefonn. It is clarified that there is a boundary amp Ii tude at which the fretting-wear volume of the flat specimen increases abruptly. The fretting-wear regime changes at this boundary amplitude. This is ascertained by SEM observation of the fretting-wear scar. .In a research to follow, we plan to elucidate the frettingwear characteristics of various advanced engineering materials using our new fretting-wear testing machine.
References [I] G.W. Stachowiak, A.W. Batcheolor, Engineering Tribology (Tribology Series 24), Elsevier, Amsterdam, 1993,684 pp. [2) S. Fouvry, Ph. Kapsal, L. Vincent, Wear 185 (1995) 35-46. [3) S. Faanes, Int. J. Solids Struct. 33 (1996) 3477-3489. [4) LJ. Fellows, D. Nowell, D.A. Hills, Wear 185 (1995) 235-238. [5) A. Neyman, Wear 152 (1992) 171-181. [6) D.Aldham,J. Warburton, Wear 106 (1985) 177-201. [7) P.L. Hurricks, Wear 30 (1974) 189-212. [8) R.B. Waterhouse,I.R. McColl, SJ. Harris, M. Tsujikawa, Wear 175 (1994) 51-57. . [9) T. Kayaba, A. Iwabuchi, Wear 66 (1981) 27-41. [10) D. Yaub,X.Zhang,Q.Xue, Wear 171 (1994) 13-18. [II) M. Odfalk, O. Vingsbo, Tribol. Trans. 33 (1990) 604-610. [12) J.F. Carton, A.B. Vannes, L. Vincent, Wear 185 (1995) 47-57. [13) Z.R. Zhou, S.R. Gut, L. Vincent, Tribol. Int. 30 (1997) 1-7. [14) X.P. Niu, L. Froyen, L. Delacy, C. Peytour, Wear 193 (1996) 78-90. [15) R.C. Bill, NASA-81570, AVRADCOM-TR-80-C-15, 1980, pp. 238250. [16) A. Koenen, Ph. Virmoux, R. Gras, J. Blouet, J.M. Dewulf, J.M. De Monicault, Wear 197 (1996) 192-196. [17) O. Jacobs, K. Friedrich, Wear 135 (1990) 207-210. [18) I.R. McColl, S.J. Harris, GJ. Spurr, Wear 197 (1996) 179-191.
[19] Y. Qiu, BJ. Roylance, Lubricat. Eng. 48 (1992) 801-808. [20] P.W. Sandstrom, K. Sridharan, J.R. Conrad, Wear 166 (1993) 163168. [21] M. Kuno. K. Dinsdale, B.R. Pearson, B. Ottewell, Tribol. Int. 21 ( 1988) 929-931. [22] W. Wersing, K. Lubitz, J. Mohaupt, Ferroelectrics 68 (1986) 77-97. [23] O. Vingsbo, S. Soderberg, Wear 126 (1988) 131-147. [24] O. Vingsbo, M. Odfalk, N. Shen, Wear 138 (1990) 153-167.
Biographies Etsuo Marui was born in Ichinomiya, Japan in 1942. He finished his postgraduate doctoral course in mechanical engineering at Nagoya University in 1969 and received his doctor's degree in 1969 from Nagoya University. He has worked at Gifu University, Gifu, Japan since 1975 and is at present • professor in the Mechanical and Systems Engineering Department, Gifu University. His research interests are tribological phenomena in contact surfaces and the boundary lubricant technology. Hiroki Endo was born in Kakamigahara, Japan in 1961. He finished his postgraduate master's course in precision engineering at Gifu University in 1986 and received his master's degree in 1986 from Gifu University. He worked at Okuma until 1994, and developed an ultra-precision machine tool. Then he moveCi to Gifu University, Gifu, Japan. He is at present assistant in the Mechanical and Systems Engineering Department, Gifu University. His research interests are rolling friction mechanisms and ultra-precision positiQning devices of nanometer order. Norihiko Hasegawa was born in Konan, Japan in 1945. He finished his postgraduate master's course in Mechanical engineering at Gifu University in 1971. He received his doctor's degree in 1984 from Tokyo Institute of Technology. He has worked at Gifu University since 1971. He is at present professor in the Mechanical and Systems Engineering Department of Gifu University. His research interests are fatigue strength of materials and statistic!ll properties of metals and nonmetals. Hidetoshi Mizuno was born in Tajimi, Japan, 1971. He finished his postgraduate master's course in mechanical engineering at Gifu University in 1997 and received his master's degree in 1997 from Gifu University. He is now working at Onda International Patent Office, Gifu, Japan.
Prototype fretting-wear testing machine and some experimental results Etsuo Marui *, Hiraki Endo, Norihiko Hasegawa, Hidetoshi Mizuno Department of Mechanical and Systems Engineering. Facility of Engineering. Gifll Unh-ersity. 1-1 Yanagido. Gifll-shi 501-11. Japan
Received 8 July 1997; accepted 8 October 1997
Abstract A new prototype fretting-wear testing machine is developed, and its performance is evaluated. In this testing machine, a spherical specimen is oscillated by a lead-zirconate-titanate semiconductor (PZT) actuator, driven by means of computer control. Thus, an arbitrary input waveform, such as sinusoidal wave, triangular wave and so on, can be easily operated to a spherical specimen. This is the special feature of our testing machine. As an example, a fretting experiment is carried out, using a steel flat specimen and a bearing steel sphere specimen. A sinusoidal input wave is added by a PZT actuator. It is clarified that there is a boundary fretting amplitude, beyond which fretting wear mass or volume increases abruptly. © 1998 Elsevier Science S.A. Ke.nmrds: Fretting wear; Fretting-wear testing machine; Sphere-an-flat type; PZT actuator \
1. Introduction Fretting occurs wherever small-amplitude reciprocating sliding between contacting surfaces is sustained for a large number of cycles. The sliding amplitude or fretting amplitude is smaller than the contact area size and wear debris is likely to build up within the contact zone, so the surface conditions are apt to affect more remarkably the fretting wear phenomena than the usual wear phenomena [I] .The main focus of previous research on fretting wear has been to basically understand its characteristics by clarifying the slip within the contact zone [2,3] and the stress distribution there [4]. Many experiments have been carried out to clarify the fretting wear characteristics of various engineering materials for practical use. The tested materials include mild steel [S7], eutectoid steel [8], bearing steel [9,10], stainless steel [ II ], Cr-Mo steel [12], aluminum alloy [ 13,14], titanium and nickel alloys [IS,16], and reinforced composite materials [17,18]. The effects of surface oxidation [6,7,9,IS], lubrication [S,19] and surface integrity by plasma [20] or laser treatment [10] have also been investigated. The following trend has been revealed by surveying the specimen configuration and the way in which specimens are oscillated. Specimen configurations used to date are: sphereon-flat surface type [9-11,13-16,19,21], cylinder-on-flat surface type [S,17,20] and flat surface on flat surface type [6,7,12,18]. Spheres or cylinders are oscillated in some
* Corresponding author. Fax: + 81-058-230-1892. 0043-1648/98/$ 19.00 © 1998 Elsevier Science S.A. All rights reserved Pll S0043-1648 (97) 00251-2
cases, and a flat surface in others, on a sphere-on-flat surface type or cylinder-on-flat surface type testing machine. Means to oscillate specimens are: mechanical devices such as eccentric cam mechanisms [8,9,14,21], hydraulic actuator mechanisms [6,12,13] and electro-magnetic devices [S,II,IS17,19]. A testing machine to observe the effect of environmental parameters such as temperature has also been constructed [7,16,21]. Basing on the previous research, we designed and constructed a new prototype testing machine for fretting wear in this research program. Our testing device is inexpensive, and the testing conditions can be easily altered. Some experimental results of steel sphere-an-steel flat surface are reported here.
2. Prototype fretting-wear testing machine 2. J. Construction of machine Fretting wear is apt to occur when the reciprocating sliding amplitude between two surfaces ranges from several to tenodd J.lm [I]. Fretting wear is influenced by reciprocating sliding amplitUde, normal force, frequency of reciprocating movement, fretting cycle number, environment and physical property of materials. Thus, a fretting-wear testing machine, in which these influencing parameters can be altered easily and over a wide range, is favorable in practical use.
E. Marui et al.llVear 214 (/998) 221-230
222
In our prototype testing machine, a lead-zirconate-titanate semiconductor (PZT) actuator, which can generate a small reciprocating displacement by a piezoelectric phenomenon [22], is used as a driving element for reciprocating sliding specimens. The PZT actuator is very advantageous because various waveforms of reciprocating sliding movements can be easily realized by computer-controlled voltage to the PZT actuator. Some of the electrical and mechanical properties of PZT actuator are given in Table 1. A combination of conventional spherical and flat specimens (sphere-on-flat type) are adopted in our machine. An outline of the fretting-wear testing machine used is given in Fig. I. The machine must be as rigid as possible to minimize the absorption of the small reciprocating sliding, which is induced by the PZT actuator, by an elastic deformation of each machine component. To realize this require- • ment, the spherical specimen is directly attached to the drive section of the PZT actuator. For this reason, a spherical specimen 20 mm in diameter made of bearing steel is used. However, a spherical specimen of a different size can be readily used in this testing machine, without any modification of the testing device. Table 1 Some properties of PZT actuator Properties of PZT Relative dielectric constant Loss tangent Young's modulus Curie point Maker
1800 0.02 44GPa 230°C Nippondenso
Size
iOmm
Diam'eter Length
A spherical specimen was arranged so as to be sandwiched between two identical flat specimens. By introducing this structure, a moment established owing to the friction force I acting on the spherical specimen can be eliminated, and the spherical specimen can be very rigidly held in place. The. driving clement and the sphere specimen are arranged on a ; vertical straight line, so the testing machine takes up little , space. Moving elements arc suspended by two sets of parallel leaf springs as seen in Fig. 2, so that the moving direction of the PZT actuator parallels the contacting surface between specimens. Normal load is applied by the two dead weights shown in Fig. I, and the load can be thus kept constant during the experiment. The magnitude of the displacement generated by the PZT actuator is controlled through a personal computer. Control voltage of 0-5 V, generated by a personal computer, is magnified 120-fold through an amplifier. Then, the driving voltage of the PZT actuator is between 0-600 V. Computer control of the fretting-wear testing machine enables the spherical specimen to be driven by an arbitrary waveform. Two flat specimens 35 mm-square are attached to each stage, which can move horizontally owing to linear guides and rails. Wires are stretched between stages via pulleys, and the tensile force of dead weights is exerted on the stages. A spherical specimen is suspended between two identical flat specimens, and a constant normal force is added between specimens. Two sensors of leaf spring construction are attached on stages to measure the reciprocating sliding movement of specimens. Their size and configuration are given in Fig. 3.
I:
Spherical specimen
120mm
Specification Maximum displacement Maximum driving force Maximum operating frequency Allowable temperature limit Control voltage
100j.lm 2kN 100Hz 0-50°C 0-5 V
PZT actuator
Fig. 2. Driving mechanism of specimens.
Fig. I. Prototype fretting-wear testing machine.
. E. Marui et al. / lVear 214 (1998) 221-230
Each sensor consists of a leaf spring and a base. The base of the sensor is fixed to the stage. The tip of the leaf spring is arranged to be right above the spherical specimen. The deformation of the PZT actuator or the relative reciprocal sliding bet\veen spherical and flat specimen is measured by strain gauges pasted on both sides of the leaf spring. To minimize noise, all lead wires are sheathed. The calibration test result of these two sensors is shown in Fig. 4. The relation between output voltage from strain gauges and relative reciprocal sliding is shown here. Good linearity is recognized between them. Output noise is about 0.01 mY, which corresponds to the reciprocal sliding of 0.5 fLm. Accordingly, the relative reciprocal sliding between specimens can be measured precisely by these sensors. 2.2. Performance ofprototype fretting-wear testing machine
Performance of our prototype fretting-wear testing machine is checked through various calibration tests. Firstly, Fig. 5 gives a change with the passage of time of fretting amplitude. In this experiment, the input voltage waveform to the PZT actuator is sinusoidal, with an amplitUde and frequency of360 V and 10 Hz, respectively. In the figure, the result for the case of a bearing steel spherical specimen and a medium carbon steel flat specimen is shown at four levels
E E~
60~--~~--~~---+----~
8
40 ~---li--<"1"---l-----+-----I
is
20 I-.....,.,.--t---t----''---t--------I
'" ~
2
3
4
223
of normal force. This combination of materials is used in the under-mentioned fretting-wear experiment. The fretting amplitude decreases rapidly, during the initial 2000 fretting cycles. This decrement of the fretting amplitude is more remarkable as the normal force becomes larger. Surfaces of both specimens are fully degreased by washing in methyl acetate before the fretting-wear test. Rapid decreasing of the fretting amplitude in the initial stage may be induced by the change of surface oxide film or the breakdown of slightly remained lubricant film. After that, the variation in fretting amplitude is recorded till the fretting cycle number of 105 • It is confirmed that the fretting amplitude is almost constant in relation to the magnitude of the normal force. The relation between the mean fretting amplitUde and the normal force from 3000 to 104 fretting cycles is shown in Fig. 6. The fretting amplitude decreases almost linearly with the increase of the normal force, and the relation between them is expressed by Eq. (I). (I)
where, A is the fretting amplitude (fLm), andAo is the fretting amplitude (fLm) when the normal force is zero. Fn indicates the normal force (N). The fretting amplitude decreases when the normal force increases, because\the frictional force acting between specimens grows as the normal force increases. Therefore, the frictional force can be estimated by the size of the fretting amplitude. Next, the relation between the mean fretting amplitude and the input voltage amplitUde is given in Fig. 7, for constant input voltage frequency (to Hz). In the figure, the result is given for two levels of normal force, 0 Nand 66 N. The mean fretting amplitude changes linearly with input voltage amplitude, and the lines indicating the relation between them are parallel to each other. It is ascertained from this that the decrease in mean fretting amplitude owing to the frictional force effect is not affected by the input voltage amplitude. From Fig. 7, the mean fretting amplitudeA o when the normal force is 0 N, is represented by Eq. (2).
Vo
(2)
Output voltage (V) Fig. 4. Calibration result for leaf spring sensor.
50 50r--------------------,
40
E
40
(!)
30
•
"0
.~
C. ~
0 0
0
0
0
0
0
0
:.';.~-r:.
.-o-
0000000000 0
~. _______
0
0
0
Fn=I6N
20 \ _ .... - _ _ 10
. _- .... -
20
FI1=66N
"'"
10
""
~
~
~
Fn=lI6N
~~-------------l
00
30 ~ --v
2000
4000 6000
00
20
40
60
80
100 120
8000 10000
Fretting cycle number Fig. 5. Variation of fretting amplitude owing to fretting cycle number at input voltage amplitude 360 V and frequency 10 Hz.
Normal load (N) Fig. 6. Influence of normal force on fretting amplitude at input voltage amplitude 360 V and frequency 10 Hz.
224
E. Marui el al. IlVear 214 (1998) 221-230
the normal force is zero and the fretting amplitude A when the normal force is applied. The action of the frictional force is shown in Fig. 9, when the sphere specimen is sliding in the direction M as seen in the figure. The downward frictional force 2Q acts on the spherical specimen and the upward,. frictional force Q acts on each flat specimen and stage. The " rigidity coefficient of the driving element in this direction is .' set at Rd and that of the stage at Rs. The variation or the I decrease ilX of the fretting amplitude due to frictional force' is written as follows:
80 ,.,.......
S
~ (!)
60
• Fn=ON = 66 N
0 Fn
'"0
.-C.Z S
40
ce
I=:
ce
(!)
20
::?:
2Q Q -+-=ilX Rd Rs
O~----~~-----L------~
o
200
400
600
Input voltage amplitude (V) Fig. 7. Influence of input voltage magnitude to PZT actuator on fretting amplitude at input voltage frequency 10 Hz.
where, Va is the input voltage amplitude (V) to the PZT actuator. From Eqs. (1) and (2), the fretting amplitude can be written as follows: A=0.13Va -0.24Fn -IS
(3)
Lastly, the effect of the input voltage frequency is investigated. Fig. 8 shows the result for the case of the normal force 66 N and the input voltage amplitude to the PZTactuator 600 V. The mean fretting amplitude is almost constant tiII the frequency of20 Hz. Above this frequency, the fretting amplitude decreases with the input voltage frequency. From this result, it is ascertained that the limiting input voltage frequency of this fretting-wear testing machine is about 20 Hz. 2.3. Estimation offrictional force This fretting-wear testing machine is not equipped with a sensor to measure frictional force. The magnitude of the frictional force can be approximately estimated as follows. As mentioned in Fig. 6, the frictional force is calculated using the difference between the fretting amplitude Ao when
Estimation of the rigidity coefficients is carried out as follows: The part of the spherical specimen attaching to the driving element is pulled by a spring balance, and the displacement of the part attaching the spherical specimen is read by a dial gauge as seen in Fig. 10. The rigidity coefficient of the driving element Rd is then calculated by the pulling force and the displacement there. The rigidity coefficient of the stage Rs is obtained by the same method. As a result, the rigidity coefficient of the driving element may be stated Rd= 16.54 N/v.m, and that of the stage by Rs=8.7 N/v.m. Then from Eq. (4), the frictional force Q is obtained as follows:
Q=4.24ilX
(5)
The frictional coefficient is also estimated by dividing this frictional force by the normal force Fn. The frictional coefficient variation obtained by this process is given in Fig. II. In this estimation, the input voltage frequency is 10Hz, its amplitude is 480 V and the applied normal force is 66 N. The stable frictional coefficient is estimated as seen in the figure, except the initial stage of the fretting cycle number.
50 ,-...
40
u
E :::t '-"
30
---
~(
~
(lJ
-0 ::l
.-::: 0.
~
Fig. 9. Frictional force acting on specimens.
"
20 10
o0
10
20
30
40
50
60
Frequency (Hz) Fig. 8. Influence of input voltage frequency to PZT actuator on fretting amplitude at normal force 66 N and input voltage amplitude 600 V.
Fig. 10. Evaluation of driving element rigidity.
E. Mami el al. IlVear 214 (/998) 221-230
1.2 I=:
.9 1.0 t> ;.s O. 8
- I-
---
"""0
....I=: C!}
'u
ij3 C!}
~
0.6 0.4
0
U
O. 2 2 4 6 8 Fretting cycle number Fig. II. Example of frictional coefficient evaluation at normal force 66 N. input voltage amplitude 480 V and frequency 10Hz.
225
number 105 • Fig. 12a shows the wear scar of the flat specimen and Fig. 12b shows the wear scar of the spherical specimen. The wear vestiges on the flat specimen have a conical shape. Its radius is about 600 fJ.m and the depth is about 14 fJ.m. There is a small plateau at the center of the worn portion, whose radius and depth account for about one-third of the wear scar. Around this area, there is an annular hump along the edge about 2.5 fJ.m high and around 100 fJ.m wide. On the other hand, the wear scar on the sphere specimen is a little smaller than the scar on the flat specimen. Its radius is about 570 fJ.m. There is a plateau at its center, similar to the flat specimen. The surface of the wear scar is almost smooth, except for the central plateau.The above features are similar to those of the fretting-wear scar already reported by many resf;!archers. Thus, the wear phenomena, which are observ,ed in o,ur fretting-wear testing machine, are considered to be induced by fretting wear. The fretting-wear volume is obtained [10] from the mean wear scar radius r and the scar depth d, which are easily measured by a surface roughness tester. Assuming the shape may be regarded as a sphere crown, the wear volume Vr is written as follows:
Vr= :d(3r 2 +d 2)
(6)
(a) Rat specimen
It is extremely difficult to measure the wear scar depth d on the sphere specimen from a surface roughness curve. So, the radius of curvature of the wear scar surface is approximately regarded as the radius of the spherical specimen and the wear scar depth is calculated by Eq. (7). The approximate wear volume of the sphe~ical specimen is obtained by substituting Eq. (7) into Eq. (6). (b) Spherical specimen
d=R-fR 2 -r 2
'Fig. 12. Measurement of fretting wear scar at normal force 66 N. fretting frequency 20 Hz, fretting amplitude 35 ILm and fretting cycle number 105.
(7)
where, Rand r represent radii of the sphere specimen and the mean wear scar radius, respectively.
2.4. Estimatioll of wear volume
Fig. 12 shows the fretting-wear scar in the case of the bearing steel sphere specimen and the medium carbon steel flat specimen. This is done by combining the surface roughness profiles measured by a surface roughness tester. Here, the applied normal force is 66 N, the input voltage frequency is 20 Hz, the fretting amplitude 35 fJ.m and the fretting cycle
3. Some experimental data on pair of stcel specimcns A fretting-wear test is carried out using the bearing steel sphere specimen and the flat specimen of medium carbon steel for machine construction, and the validity of our testing machine is ascertained. The chemical composition and Vick-
Table 2 Chemical composition and Vickers hardness of specimens Vickers hardness
Elements (%)
Medium carbon steel for machine construction use High carbon chromium bearing steel
C
Si
Mn
Cu
Ni
Cr
p
0.55
0.23
0.75
0.05
0.15
0.12
0.02
0.02
235
1.05
0.15
0.20
0.12
0.15
1.48
0.01
0.01
850
S
Hv
\
I.,.
E. Mam; et al. / lVear 214 (1998) 221-230
226
"
ers hardness ofthe specimens are listed in Table 2. The radius of the spherical specimen is 20 mm. The flat specimen is a square plate 10 mm thick and 40 mm long. The fretting-wear test is carried out while varying some parameters affecting the fretting-wear characteristics.
°• Fn=I6N Fn=66N
Flat specimen
A Fn=1l6N
3.1. Influence offretting cycle number The influence of the fretting cycle number is examined, while varying the fretting cycle number between 103 and 106 for the constant normal force 66 N, the fretting frequency 20 Hz and the fretting amplitude 35 p.m. A sample experimental result is shown in Fig. 13. The fretting-wear rate of the sphere and the flat specimens, which is converted from the wear volume obtained by the abovementioned process, is shown here. The fretting-wear rate is defined as the wear volume corresponding to the unit sliding distance and unit normal force. The concept of the wear rate is important to compare the wear quantitatively under different fretting-wear conditions. The wear rates for the spherical and flat specimens are similar in magnitude. As is clear in the figure, the wear rate becomes small with the increase of the fretting cycle number. There is a linear relationship between the wear rate and the fretting cycle number on log-log paper. This relationship is written as follows:
01k-----~m_----_,~-----7,
Fretting amplitude (/1 m) Fig. 14. Influence of fretting amplitude and normal force on wear scar radius • at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
30
Fn=66N °• Fn=I6N Fn=I16 N A
I
r:-
-.g
I
§011"«:
3.2. Influence of normal force andfretting amplilllde In this section, the influence of the norrrial force and the fretting amplitude on fretting wear is examined. The fretting frequency (20 Hz) and the fretting cycle number (5 X 104 ) are kept constant in this experiment. Fretting-wear characteristicsare clarified for various fretting cycle numbers and normal forces. Figs. 14 and 15 show the relations between the wear scar radius and its depth on the flat specimen and the fretting 10-12
J •
~ 1°
3
Flat spec. Spherical spec.
r--
~ i~
IN
~
'13
5
104 lOS Fretting cycle number Fig. 13. Influence of fretting cycle number on fretting wear rate at normal force 66 N, fretting frequency 20 Hz and fretting amplitude 35 f.l.m.
A
"~S"
~I <:$1
\
(8)
../';.
"'~" -~ 11 1 "" :'::1'"
A
.Fl.-
oo
I"
0 ..........
,"
-
••
""""""0" "
I I I
1520 40 Fretting amplitude ( /1 m)
60
Fig. 15. Influence offretting amplitude and normal force on wear scar depth at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
amplitude. The parameter shown in the figures is the normal force. It is seen in Fig. 14 that the wear scar radius increases linearly with the increase in fretting amplitude, although a little scatter is recognized among the measured data. A wear scar of larger radius results as the"normal force is larger. The following tendencies are recognized from Fig. 15 regarding the wear scar depth. The wear scar depth is quite small and the influence of the normal force is not remarkable, for fretting amplitudes smaller than 15 p.m. On the contrary, for a fretting amplitude larger than 15 p.m, the wear scar depth increases abruptly. After this increase, the wear scar depth does not vary much and is almost constant. The fretting amplitude, where the abrupt increase in the fretting amplitude occurs, is called hereinafter the boundary amplitude. It is quite natural that the wear scar depth is large for a large normal force. Figs. 16 and 17 show the wear rate, which is converted from the fretting-wear volume obtained using the above-mentioned wear scar radius and the wear scar depth. As seen in Fig. 16, the wear rate characteristic of the flat specimen is different at the boundary amplitude. For a fretting
\
\
E. Marui el al. / \Year 214 (/998) 22/-230
6
Flat specimen
{ll "I
~E
o Fn=16N • Fn=66N fl. Fn=1l6 N _
:::1 ~I <::Sl
i:-I
"'I 4
-~I
0
§I
.-<
.,gl
~ .......,
:A= It"
~ ....
AO
.... 2
l:;.
l:;.
....
-(I
I-
I I I I I I
~
~
0
o riJ,JI. 15 o
20 40 Fretting amplitude ( Jl m)
60
Fig. 16. Influence of fretting amplitude and normal force on fretting wear rate in flat specimen at fretting frequency 20 Hz and fretting cycle number 5X 10'.
6
..,.
'I
o Fn=16N • Fn=66N l:;. Fn=116N
4
c-
0
.-<
O
~ .......,
0
~
.... .... 2 Ofl.
~
~
• • o • o
l:;.fl. ~
rI
l:;.
•• l:;.
•
15 20 40 Fretting amplitude ( Jl m)
regimes. This transition between fretting-wear regimes is again investigated in the under-mentioned fretting-wear surface observation by SEM. Fig. 17 shows the wear rate of the spherical specimen, for reference. A tendency that fretting wear becomes violent with the increase of the fretting amplitude is recognized in Fig. 17 showing the result for spherical specimen. But, the boundary fretting amplitude is not recognized, differing from the result for flat specimen (Fig. 16). However, the boundary amplitude feature found with the flat specimen is not observed. As mentioned previously, it is difficult to determine the wear scar depth of the sphere specimen exactly, and the wear volume of the spherical specimen is roughly calculated by Eq. (7), a convenient way to obtain such a result. So, the simple and accurate method to obtain the fretting-wear scar configuration of the spherical specimen must be established for further investigation of the spherical specimen.
4. Observation of wear scar by SEM
Spherical specimen
~E
227
It is clarified in the previous discussion that there is a boundary amplitude in the flat specimen. Then, by observing the wear scar surface by an electron microscope (SEM), the mechanism of the fretting wear is considered basing the stress on the relation with th~ fretting amplitude .
4.1. Wear scar for fretting amplitude larger thall the boundary amplitude
60
Fig. 17. Influence of fretting amplitude and normal force on fretting wear rate in spherical specimen at fretting frequency 20 Hz and fretting cycle number 5 X 10'.
amplitude larger than this boundary amplitude, the wear rate is independent of the fretting amplitude and remains constant. Furthermore, the magnitude of the normal force does not influence the wear rate. This fact means that the fretting-wear volume is proportional to the normal force. For the fretting amplitude smaller than the boundary amplitude, the wear rate is extremely small. No influence of the fretting amplitude and the normal force can be recognized. Vingsbo and Soderberg [23] and Vingsbo et al. [24] summarized fretting-wear characteristics as the fretting map, by arranging the effect of various influencing factors. Referring to their result, fretting wear is divided into three regimes, that is, stick regime, mixed stick-slip regime and gross slip regime, owing to the difference of slip state at contact area. Intimate adhesion occurs almost on the whole contact area in the first stick regime. On the contrary, gross slip occurs on the whole contact area in the third gross slip regime. In mixed stick-slip regime, partial adhesion remains at the central area of contact. The abrupt increase of fretting-wear rate shown in Fig. 16 corresponds to the transition between fretting-wear
Fig. 18 is a micrograph of the flat specimen and Fig. 19 of the spherical specimen. Here, the normal force is set at 66 N and the fretting amplitude at 40 /Lm, while the fretting cycle number is 5 X 104 • From Fig. 18a and Fig. 19a of the whole image, the fretting-wear scar can be divided into two parts, a central area and a surrounding annular area. We first consider Fig. 18. Fig. 18b is an enlarged photograph of the central area where there are remarkable irregularities. The material is peeled off at the depressed area on the surface. And the peeled-off material on the opposite specimen surface migrates to adhere to the opposite surface. Small amounts of striped marks on these irregularities in the fretting direction are recognized. This indicates that adhesive and abrasive wear are mixed at this central area. On the contrary, only striped marks extending in the fretting direction are recognized in the surrounding annular area. Fig. 18c is an enlarged photograph of this area. Here, abrasion owing to scratching by peeled wear debris or asperities on the opposite specimen is a dominant wear pattern. Fig. 19 shows a wear scar of the sphere specimen corresponding to the flat specimen of Fig. 18. The appearance of the wear scar of the sphere specimen resembles that of the flat specimen. From these results of Figs. 18 and 19, it is clear that the fretting wear of 40 /Lm fretting amplitude belongs to the gross slip regime [23]. The appearance of the observed wear scars does not change with the magnitude of the normal force or fretting amplitude.
,-----.-- .---'---'----~---------~-~~~-~~------..... "'I:~ 228
E Marui eta{ /Weur214 (1998) 221-230
Fig. I g. SEM observation of fretting wear scar on f,,,( specimen at norm"l force 66)l, fretting number 5 X 10". (a) Whole image; (b) Central area; (c) Surrounding annular area.
Fig. 19. SEM observation of fretting wear scar on spherical specimen
ciI
'"e'CllIN"',"
20 Hz, frelling amplitude 40 !-L m and fretting cycle
norca; fcrce 66:\" frcIting ~'re(!uency 20 Hz. fretting amplitude 40 fLm and fretting
cycle number 5 X 10-l. (a) Whole area; (b) Central area; (c) SurrouraJing annular area.
E. Marui
el
ai. / Wear 214 (1998) 221-230
229
Fig. 20. SEM observation of fretting wear scar un specimen at Honnal force 66 N. f:'etting frequency 20 Hz. fretting anlpIituGc 5 /-LIn and fretting cycle number5X104 . (a) Whole image; (b) Central area; (c) Surrounding annular area.
Fig. 21. SE~1 observation of fretting \\'e~:- scar on ~pheric:ll specimen at notnml f'lJ[CC 66 ;..;. fretting frequency 20 HL, fretting a.mp!itudc 5 J.Lm and fretting cycle number 5 X 104
4.2. Wear scarfor fretting amplitude smaller than the boundary mnplitude Where the fretting amplitude is smaller than the boundary amplitude, the appearance of the wear scar is fairly different from that of the large fretting amplitude explained in Section 4.1. Figs. 20 and 21 are SEM micrographs of the flat specimen and the spherical specimen, respectively. In this experiment, the normal force is set at 66 N and the fretting amplitude at 5 fLm, while the fretting cycle number is 5 X The appearance of wear scar is classified roughly into two regions, central area and the surrounding annular area, in the
same manner as when the fretting amplitude is larger than the boundary amplitude. Fig. 20b is an en larged SEM micrograph of the central area. Striped marks extending parallel to the frctting direction are seen in the figure. These striped mar:(s have a rising pattern, dissimilar to the case in which the amplitude is larger than the boundary amplitude. Adherence between the spherical specimen and the flat specimen may be induced in this area. The striped marks are forn1ed by stretching corresponding to the fretting action. Some oxide rises and striped marks due to slight abrasion are seen in the annular area of Fig. 20c. The appearance afthe spherical specimen corresponds well to that of the flat specimen, as seen in Fig. 21. From these results of Figs. 20 and 21, it is clear that the fretting wear of 5 !-Lm fretting amplitude belongs to the mixed stick-slip regime [231. So, the boundary recognized in Fig. 16 corresponds to the transition of wear regime from mixed stick-slip to gross slip regime. In the fretting condition in which the fretting amplitude is smaller than the boundary amplitude, the tangential force is also small and perfect adherence is generated at the central area only. the surrounding annular area is fretted, and the fretting-wear rate becomes small.
5. Conclusion A prototype fretting-wear testing machine, in which a sphere specimen is oscillated by the PZT actuator, is con-
230
E. Marui et al. / Wear 214 (1998) 22/-230
structed and its perfonnance is evaluated in this paper. The most advantageous feature of this testing machine, in which a specimen's reciprocal sliding displacement is driven by the PZT actuator controlled by a personal computer, is that the specimen's reciprocal sliding displacement of sinusoidal, triangular wave fonn or others can easily be realized. The relation between the experimental conditions and the fretting amplitude is clarified in detail, considering the PZTactuator' s properties. Using this result, the frictional force or the frictional coefficient acting on the specimens is estimated. The maximum input voltage frequency for stable operation of this testing machine is 20 Hz. As an example, a fretting test of a bearing steel spherical specimen and a medium carbon steel flat specimen is carried out, while driving the spherical specimen by sinusoidal wavefonn. It is clarified that there is a boundary amp Ii tude at which the fretting-wear volume of the flat specimen increases abruptly. The fretting-wear regime changes at this boundary amplitude. This is ascertained by SEM observation of the fretting-wear scar. .In a research to follow, we plan to elucidate the frettingwear characteristics of various advanced engineering materials using our new fretting-wear testing machine.
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Biographies Etsuo Marui was born in Ichinomiya, Japan in 1942. He finished his postgraduate doctoral course in mechanical engineering at Nagoya University in 1969 and received his doctor's degree in 1969 from Nagoya University. He has worked at Gifu University, Gifu, Japan since 1975 and is at present • professor in the Mechanical and Systems Engineering Department, Gifu University. His research interests are tribological phenomena in contact surfaces and the boundary lubricant technology. Hiroki Endo was born in Kakamigahara, Japan in 1961. He finished his postgraduate master's course in precision engineering at Gifu University in 1986 and received his master's degree in 1986 from Gifu University. He worked at Okuma until 1994, and developed an ultra-precision machine tool. Then he moveCi to Gifu University, Gifu, Japan. He is at present assistant in the Mechanical and Systems Engineering Department, Gifu University. His research interests are rolling friction mechanisms and ultra-precision positiQning devices of nanometer order. Norihiko Hasegawa was born in Konan, Japan in 1945. He finished his postgraduate master's course in Mechanical engineering at Gifu University in 1971. He received his doctor's degree in 1984 from Tokyo Institute of Technology. He has worked at Gifu University since 1971. He is at present professor in the Mechanical and Systems Engineering Department of Gifu University. His research interests are fatigue strength of materials and statistic!ll properties of metals and nonmetals. Hidetoshi Mizuno was born in Tajimi, Japan, 1971. He finished his postgraduate master's course in mechanical engineering at Gifu University in 1997 and received his master's degree in 1997 from Gifu University. He is now working at Onda International Patent Office, Gifu, Japan.