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Low amplitude reciprocating wear of sintered iron
Wenr , 176 (1994) 121-130
121
Low amplitud e reciprocatin g wear of sintere d iron* E.R . Leheup , D. Zhang , J.R . Moon Depanbzent of Materials Engineering and Materials Design Universi@ of Nottingham, Nottingham (UK)
(Received Decembe r 21, 1993; accepte d Februar y 3, 1994)
Abstract Th e unlubricate d wear , self-on-self, of sintere d iron du e to reciprocatin g motion ha s been investigated . Wrought, low-carbo n steel hav e also been studie d unde r identica l condition s for comparison. Th e genera l characteristic s of th e wear proces s for th e sintere d materia l ar e similar to thos e observe d for wrough t material . Wea r is mor e sever e when th e load is increase d and when th e amplitud e is increase d in the rang e up to abou t 250 pm . Furthe r increase s in amplitud e brough t abou t less marke d increase s in wear , until, ultimately , wear becam e independen t of amplitude. At high sliding speeds , sintere d iron loses mor e weight tha n does mild steel. At slow speeds , sintere d iron loses weight less rapidl y tha n does mild steel. Th e observation s ar e interprete d in th e light of oxide formatio n on th e bearin g surface s and of observations of wear debri s accumulate d in surfac e breakin g pores.
1. Introduction
The industria l proces s of stea m treatmen t of ferrous part s deliberatel y form s films of magnetit e on their surfaces . These ar e forme d mainl y to enhanc e appearanc e an d corrosio n resistance , bu t ar e claime d also to improv e wear resistance . Unfortunately , thi s commonly accepte d idea is neithe r well documente d nor well prove n in th e context of powde r metallurg y [ll]. Wha t is not clear from thes e previou s studie s is the role playe d by pore s in th e contactin g surfaces , The micromechanism s of wear of such surface s hav e not been addresse d in any detail , aithou ~ ther e is one observatio n tha t por e edges can brea k off, creating debris , whilst th e pore s themselve s can act as sinks int o which debri s can be packe d [12]. The wor k reporte d her e is intende d to be th e first par t of a coheren t stud y of th e wear of PM steels. To avoid unnecessar y complicatio n at thi s stage , it focuses on th e low amplitud e reciprocatin g wear (i.e. fretting) of pur e iro n at a constan t density.
Powde r metallurg y (PM ) part s ar e often used in circumstance s wher e thei r surface s ar e in slidin g contact with others . Nevertheless , th e phenomen a of wear of powde r metallurg y material s hav e attracte d few studies. Of these , most hav e examine d unidirectiona l sliding, typicall y usin g pin on disc experiment s [l-3]. They suggest tha t muc h of wha t is know n abou t wear in chemicall y equivalen t wrough t materia l is applicabl e to PM materials . For example , high load s an d slow sliding promot e metal-to-meta l contac t an d adhesiv e wear . At lower ioad s an d highe r speeds , th e wear behaviou r is controlle d primaril y by th e behaviou r of oxide layers forme d on th e contactin g surfaces . Adherent , lubricating films give low coefficients of frictio n an d low wear rates [3]. Possibl e fractur e an d delaminatio n of th e oxide layers , with consequen t increas e in frictio n an d wear, is promote d by high contac t pressure s an d by weak metalli c substrates , i.e. by fSSDI 0043-1648(94)06438-N actor s likely to caus e the layer s to ben d too much. 2. E~~rnen~l pqyamme In reciprocatin g wear of wrough t steels, generation of continuou s or semi-continuou s glaze bed s of oxide is a common observatio n [4-91. Their appearanc e is The wear behaviou r of presse d an d sintere d samples though t to accoun t for th e transitio n from severe to of pur e iro n at a relativ e densit y of p,=O.8 9 was mild wear as eithe r th e slidin g distanc e or th e test investigate d as a functio n of th e mechanica l variables temperatur e is increase d [lo]. of slidin g amplitude , reciprocatin g frequenc y an d normal load . The mai n programm e was carrie d out usin g a fixed tota l slidin g distanc e of 100 m. A smal l subsidiary *Thi s pape r is dedicate d to th e memor y of Dr . E.R . Leheup who died unexpectedly whilst this paper was under preparation. programm e of interrupte d testin g was used to examine
~3-1~8~4/$07.~ 0 1994 Etsevie r Science S.A. All rights reserved SSDI 0043-1648(94)06438-N
E.R. Leheup et al. / Reciprocatingwear of sintered iron
122
the a~umulat~on of wear after the first 10 m of relative motion. Annealed test-pieces of wrought mild steel were tested similarly for comparison. All tests were conducted in normal laboratory air. 2.1. Preparation of test pieces Test pieces were produced to final shape by a pressing and sintering route. Test piece geometry is shown in Fig. 1. Hogan& ABC 100.30 atomised iron powder was compacted by single-ended pressing. The surfaces to be worn were on the raised area, 10 mmX25 mm, which was in contact with the moving punch. Compaction pressure was 600 MPa. No lubricant was used. Compacts were sintered for 1 h at 1150 “C in an atmosphere of 95%N,-5%H,. Sintered densities were 7000 kg rne3, a relative density of 0.89. Test pieces produced in this way would be expected to contain variations in density with depth below the surfaces to be tested. Surface density was assessed by quantitative measurement of the area fraction of surface breaking pores. This gave pr = 0.8 + 0.05. An alternative assessment was made by repeatedly grinding away surface layers and weighing. These results are given in
Fig. 1. Form and size of test pieces (dimensions
TABLE 1. Quantitative test
assessments
in millimetres).
of density of surfaces under
(a) Area fraction of surface breaking Relative density, 0.8 f 0.05 Density, 6.3 f 0.4 mg rmC3
pores, 0.2~bO.05
(b) Results
experiments
of grinding
and weighing
Table 1 and suggest that the top 100 pm has an average density of 4.4kO.4 mg mmR3, i.e. p,=0.56f0.05. This discrepancy between these two measurements might be explained if some of the grinding debris is trapped in surface breaking pores, so that the loss of weight of a ground-off layer is less than it should be. Mild steel test pieces were machined from bright drawn bar, annealed for 1 h at 850 “C, furnace cooled, and finished to 240 grit on the wearing surfaces. 2.2. Wear testing Test pieces were tested, self on self, as crossed fiats. The experimental arrangement is shown schematically in Fig. 2. Both normal load and the reciprocating motion were applied hydraulically using an ESH servohydraulic machine. All experiments were done in normal laboratory air at 18 “C with no lubrication. Each test used two pairs of specimens, which were mounted symmetrically about the central activator. This was to avoid introducing bending moments to the actuator system and thereby to maintain planar contacts. Each specimen pair was brought into contact with their surfaces crossed at right-angles to give a nominal contact area of 10 mm X 10 mm. Amplitudes of relative motion were varied from 50 to 500 pm, frequencies from 1 to 100 Hz and normal loads from 200 to 1000 N. Amplitudes were monitored and controlled using LVDTs. Table 2 gives the schedule for the constant distance (100 m) tests. Section 4 details the wear accumulation programme. 2.3. Quantitative assessments of wear Net material loss was assessed from weight changes and by measuring volumes of wear scars. Weight loss was measured using a balance capable of resolving +O.l mg. Volume loss was measured using a three-dimensional profilometer. This uses a contacting stylus of 5 pm tip radius which is drawn across the surface in the manner of a Talysurf roughness tester. A series of parallel lines are drawn, the surface heights being data logged and processed to assess the total volume of the wear scar. Moving direction of the ram
Depth range below surface (pm)
Density of layer (mg mme3)
Relative density of layer h
o-loo 100-200 200-400 400-800 800-1600
4.4 f 0.4 5.6~tO.6 6.5 f 0.7 6.1*0.6 5.8f0.6
0.56 ic 0.05 0.71 f 0.08 0.83 f 0.09 0.78 f 0.08 0.74 f 0.08
Fig. 2. Arrangement
of test pieces in test-rig.
123
E.R. Leheup et al. / Reciprocating wear of sintered iron TABLE 2. Test schedule for constant distance, 100 m) Frequency (Hz)
Relative sliding speeds amplitudes” 0.05 mm
1
0.1 mm
distance tests (total sliding
(mm s-l)
0.2 mm
for the following
0.25 mm
-
2 2.5 5 10 20 25 50 100
10
“All combinations of amplitude and frequency using loads of 200, 600 and 1000 W.
24
8
22
+MS.lkN.O.Olm/s
noted are tested
Weight loss values f 2% 24 Amplitude values_* 1% 22
1 kN,O.Olm/s
-20
2om1 kN,O.OOim/s 10
016 E
F -0
-16
600N,O.Olm/s
-16
*
-14
2 12 + 610 ‘5 * ‘6 j 14
0.50 mm
-2’ 0
I 0.1
I 0.2
-12 -10 -0 -6 -4 -2 I 0.3
1 0.4
I 0.5
-0 I 0.i2
sliding speeds. Increasing load, amplitude and speed all give greater weight loss. Figure 4 shows weight loss as a function of normal load. The slopes of the curves clearly increase with amplitude. A feature of both Fig. 3 and Fig. 4 is the small weight gain observed when either amplitude or load is low. It seems that low power inputs to the wearing surfaces result in the formation of mechanically stable surfaces. Presumably the weight gains are due to oxidation. It is worth noting that all low amplitude tests show this characteristic and that these were the only tests where the amplitude was less than the spacing of surface breaking pores. Summarising, both total weight losses and weight loss rates increase with increasing mechanical power input, once a small threshold has been exceeded. 3.2. Volume changes Examples of profilometer traces of worn surfaces are shown in Fig. 5. In each figure the wear scar is in the horizontal centre with unworn regions, used as a datum, to left and right. Note that the volume lost at high load extends deeper than one particle depth and that the scar is rather rougher than the original surface. Under a low load, the general effect of wear is to smooth the surface. The sample corresponding to Fig. 5(c) lost volume but gained weight. It is thought that debris is packed into pores and that some oxidation has taken place. Figure 6 shows the relationship observed between volume change and weight change. The straight line
Amplitude/mm
Fig. 3. The effect of sliding amplitude on weight sliding distance, 100 m. MS =mild steel.
loss. Total
Typically, this can resolve volumes to f lop3 mm3 as deviations from solid, flat surfaces. The radius of the stylus tip is of the same order as that of surface breaking pores. This introduces additional errors into defining the positions of original surfaces and of worn surfaces. The maximum error arising from this phenomenon is estimated to be about 10% of the measured wear volume. All four worn surfaces from one experiment were assessed in both ways.
24 22 2018018 E -;j14-
+ 0.25mm,20Hz * 0.5mm,l Hz
z12!+
*O.lmmSOHz 86-
22
- 0.5mm,lOHz
* 0.25mm,2Hz
P
24
Weight loss values * 2% Normal load values f 0.1%
+O.O5mm,lOOHz
42-
3. 100 m tests: effects of mechanical
variables
3.1. Weight changes Figure 3 summarises the influences of sliding amplitude on weight losses at three normal loads and two
-2’ 0
1 200
I 400
I 600
Normal Fig. 4. The effect of normal distance, 100 m.
I 800
I 1,000
1,2&
load/N
load on weight loss. Total sliding
124
E.R. Leheup et RI. I Reciprocating wear 7
of sintered iron 7
+ 0.05mm,lkN.100Hz l
6 _+ O.O5,kN,lOHz
-6
x 0.5mm,ZOON,lOHz -5
5 1 0.5mm,600N,1Hz v
x 0.25mm,fjOON,20Hz “E E 4- 0 O.zSMM,l kN,lHz Ti? .z 3_ . 0.35MM,800N,14.29Hz x 0.2MM.l
E jj 2 * >
0.5MM,l
-4 X
.
kN,2& kN,lOHz
(a Weight loss values & 2% Volume loss values f 2%
-1 0
-,I -2
’
0
’
2
’
4
’
6
’
8
Weight
’
10
’
12
’
14
I
16
’
18
’
20
2;’
loss/mg
Fig. 6. Relationship between weight and volume changes. Total sliding distance, 100 m. Straight line corresponds to test piece average density, i.e. 0.89X7.86=7.0 mg mmm3.
in the samples tested, the contacting surfaces are of lower densities than the main bulk of the samples. However, this cannot provide a complete explanation. The experimental data lie about a slope representing a density of 3.8 mg mmw3, i.e. a relative density of 0.48. This is much lower than the observed density of the surface layers. We are therefore left with the proposition that either wear debris is being retained in pores, or oxidation is occurring, or both.
Fig. 5. Examples of profilometer traces from wear scars. Total sliding distance in all cases is 100 m. (a) 1 kN, 0.5 mm, 1 Hz, 0.001 m s-l. @) 1 kN, 0.5 mm, 10 Hz, 0.01 m s-l. (c) 200 N, 0.5 mm, 1 Hz, 0.001 m s-‘. (d) 1 kN, 0.05 mm, 10 Hz, 0.001 m s-‘.
represents theoretical relationships for iron of relative density p,=O.89 Clearly, weight losses are significantly less than would be expected from the volume loss. In part, this observation might arise from density variations
3.3. Obse~a~o~ of worn surfaces All worn surfaces showed macroscopically flat scars, containing an obvious brown oxide. Interfacial conformity of the contacting surfaces was generally good. Scar areas ranged from about 10% of the nominal contact area at low mechanical inputs to 100% at high loads and amplitudes. Figure 7 shows scanning electron micrographs of scars formed under a load of 200 N, the direction of relative sliding is horizontal. Figure 7(a) is from a sample worn with a stroke of Q.5 mm. It shows a clearly defined scar boundary. At higher ma~ification (Fig. 7(b)), a discontinuous layer of oxide glaze is seen to cover part of the scar. The presence of oxide is indicated by atomic number contrast. A typical patch of glaze is several hundred microns across, well in excess of the pore spacing. The wear debris appears to have filled the smaller pores and to have been compacted or deformed into the flat patches observed. Presumably, these carry most if not all of the load and have low coefficients of friction. Such a view would be consistent with the evidence from Figs. 3 and 4 that the surfaces have developed into a state of no wear equilibrium.
et al. I Reciprocating wear of sintered iron
125
(b)
id ) (d) Fig. 7. Scanning electron micrographs of wear scars formed under a normal load of 200 N, direction of relative motion is horizontal; totaf sliding distance, 100 m. (a), (b) 0.5 mm, 1 Hz, 0.001 m s-l. (c), (d) 0.05 mm, 10 Hz, 0.001 m s-l_
Decreasing the amplitude to 50 pm, still at the low load of 200 N, gives the scars shown in Figs. 7(c) and 7(d). Again, the worn area is smooth and contains patches of oxide, although these patches are smaller than those found at larger amplitude. Surface breaking
Fig. 8. Scanning electron micrographs of wear scars formed under a normal load of 1 Mu; direction of relative motion is horizontal; total sliding distance, 100 m. (a), (b) 0.5 mm, 10 Hz, 0.01 m s-r. (c), (d) 0.05 mm, 10 Hz, 0.001 m s-r.
pores can be seen in the scar area; the area fraction of large pores (>50 pm) appears to be the same as in the unworn areas. Fine surface channels linking the pores which can be seen in the unworn surfaces have disappeared in the worn area. Once again, filling of these channels by debris and oxide is most likely to
126
E.R. Leheup er al. I Reciprocating wear of sinfered iron
be responsible. Another possibili~, that the channels are simply ground away by the wear process, can be discounted. In these samples there is hardly any loss of volume at the scar. Figure 8 shows examples of wear scars formed under a high normal load (1 kN). At the larger amplitude (0.5 mm), all evidence of the porous substrate is obliterated (Figs. 8(a) and 8(b)). The debris bed is much thicker than those formed at lower loads. It appears to consist of overlapping regions of glaze, some of which are clearly spalling off. Figure 8(a) shows that some of the glaze is charging in the scanning electron microscope, suggesting that there is poor contact between the glaze and the metallic substrate. The sample illustrated in Figs. 8(a) and 8(b) had one of the greatest losses of weight. The micrographs suggest that the weight is lost by the spalling of glaze bed rather than by the immediate loss of debris as it isgenerated. It seems that the wear process goes through two stages. Fine debris is retained and consolidated into large beds of glaze. These beds appear to be unable to maintain their stability as they grow in area. Ultimately, they fail, producing flake-like debris which is rejected from the scar area. Figures 8(c) and 8(d) show the scar formed at the lower amplitude of 50 pm. It occupies only about 15% of the nominal contact area; this does not necessarily indicate that the initial conformity of the contact areas was poor. Figure 3 showed that this sample gained weight. The debris is retained in fields which are smaller and less coherent than those formed at higher amplitude. Perhaps this is to be expected, since it seems unlikely that continuous fields of glaze can be consolidated over regions of much more than the amplitude of relative motion.
4. Accumulation
of wear damage
Weight changes measured in two series of interrupted tests, using mild steel as well as sintered test pieces, are shown in Figs. 9(a) and 9(b). The wear conditions were a load of 1 kN, an amplitude of 0.5 mm and a frequency of 1 Hz, giving a sliding speed of 0.001 m S -‘. Note that the data are plotted both as weight changes of indi~dual samples and as the average for a sample pair. Throughout the whole of the range observed, sintered samples lost less weight than did mild steel, the difference becoming more and more apparent with an increasing number of cycles. In the early stages, up to about 5000 cycles (5 m sliding distance), there is a distinct difference between the two materials. Mild steel goes through a regime
8
0.5mm.l kN,lHr 7 _ Weight change
f
-2’ 0
' 1
/ 2
8
(0.001 ms-‘)
values * 2%
I, 3 4
I 5
I 6
I 7
I 8
I I;:
I 9
Totalcyclesx lo3
(a)
180
80, 0.5mm,lkN,lHz (0.001 m s-l) Weight change values f 2%
1
SS1
-60
*.S2
* (Sl +S2)/2
-50
*MS1
-40
* (MS1 +MS2)/2
- 30
: 5 20
- 20
_c 2 10
-10
.o, z
-lOi-_L-_i 2.5 3 (bt
-0
O 3.5
, 4
t 4.5
I 5
/ 5.5
‘-10 6
Log total cycles
Fig. 9. Accumulation of weight loss with increasing number of cycles (or sliding distance): (a) up to 9000 cycles; (b) total cycles, 3 X 16 (sliding distance, 300 m). MS-mild steel.
where one sample loses weight but its partner gains weight. Such behaviour indicates that an adhesive wear mechanism is operating; the corresponding wear scar (Fig. 10(a)) appears to confirm this inference. By contrast, both halves of a sintered pair behave similarly, gaining a small amount of weight. At this stage, the scars retain evidence of surface breaking pores (Figs. 10(b) and 10(c)), although there appears to be some charging up of the sample in the scanning electron microscope beam at the sites of the pores. Most likely, this would be caused by the presence in the pores of loosely packed debris of a nonconductive or weakly conductive nature.
E.R. Leheup et al. I ~eci~~~t~ng
wear of sintered iron
127
(a)
(b)
Fig. 10. Scanning electron micrographs of wear scars after 1700 cycles, 1.7 m sliding distance; direction of relative motion is horizontal; 1 kN, 0.5 mm, 1 Hz, 0.001 m s-‘. (a) Mild steel; (b), (c) Sintered iron.
After prolonged wear, the wear scars differ according to the frequency (or sliding speed) of the test. Figure 11 shows wear grooves formed at low speed (0.001 m s-‘) and the surface oxide is brown. At high speed (0.01 m s-l), a compact mat of black oxide is formed and wear grooves are not apparent (Fig. 8).
fd
5. Discussion
Fig. 11. Scanning electron micrographs of wear scars after 300 000 cycles, 300 m sliding distance; direction of relative motion is horizontal; 1 kN, 0.5 mm, 1 Hz, 0.001 m s-l. (a), (b) Mild steel. (c), (d) Sintered iron.
In general, sintered iron exhibits wear characteristics which mirror those found for wrought steels. Under reciprocating conditions, the mechanisms involve the generation of glazed beds of oxide in the contact zone which act as a lubricant as well as preventing metalto-metal contact.
Increasing loads and amplitudes both bring about more severe wear, although both effects are non-linear. Mechanical heat generated in the contact zone causes the generation of oxide beds covering, in whole or in part, the original surfaces; the greater the load and
)
128
E.R. Leheup et al. / Reciprocating wear of sintered iron
the faster the sliding speed, the more oxide would be expected. The next question concerns the stability of these oxide films. In wrought materials, large amplitudes of relative motion help to break the films and to sweep debris out of the contact zone. Much the same seems to be happening for sintered iron, although the process is modified by the ability of surface breaking pores to act as sinks for debris. Similarly, small loads would be expected to be less disruptive than large ones, in agreement with Fig. 3. An interesting feature of these experiments is the, perhaps surprising, observation that sintered iron loses weight more slowly than does mild steel at low sliding speed. To examine this further, we have to look at the relationships between volume and weight loss shown in Fig. 12. Relationships between weight loss and volume loss during the wear of solid metal surfaces have been modelled previously [ 131. The simple relationship, mass loss =volume loss X density, has to be modified to allow for weight gain due to oxidation. The formation of oxide beds of differing thicknesses give differing ratios of weight loss to volume loss. The most obvious difference between sintered iron and mild steel is the presence of surface breaking pores in the sintered material. These pores are going to modify the behaviour in a number of ways. (i) The pores provide channels through which air (oxygen) can be brought in relatively plentiful supply to the surfaces in contact. Kinetics of oxide formation will be affected. (ii) The presence of 10% pores (by volume) in the substrate material means that its specific heat is reduced by 10% and its thermal conductivity is reduced by about 20%. Consequently, equal inputs of frictional energy would give rise to interface temperatures that are higher for the sintered material than for mild steel. Again, more rapid oxidation of the sintered material would be expected.
(iii) The pores provide edges and extra surface area capable of being oxidised. (iv) The pores provide sinks into which fine debris can be swept and packed. (v) Pores provide keying sites for oxide layers which would help them to resist defoliation. (vi) Porous surfaces are both elastically and plastically more compliant than solid ones. The first five of these observations point to a more ready oxidation and retention of oxide by the sintered iron. The question is, do these ideas make sense quantitatively? Figure 12 represents a wear scar which incorporates the following features. Metal is worn away to a depth X. Oxide beds cover an area fraction& of the surface to a depth y. Pores in the sub-surface are packed to a depth z with debris. The debris might be any mixture of metal, Fe,O, and Fe,O,, and will not be fully compacted in the pores; a density of p,, is assumed. Of necessity, these assumptions are simplistic, but they allow some deductions to be made. Detailed analysis in Appendix 1 shows the following. (1) In the absence of either oxide bed formation or pore packing, AM/AV is given simply by the density of the metal, as expected. (2) If there is no pore packing, AMIAV cannot be less than the metal density, no matter how much oxide is formed into beds. Therefore, the experimental data in Fig. 6 cannot be explained in this way. (3) If there is no oxide bed formation, only pore packing, then a mechanism exists for reducing AiWAV, as required by the experimental data. Table Al gives calculated values for x and z, assuming the surface is of the relative density measured metallographically and that the relative density of debris captured and compacted into pores is -0.75. The iron powder particles used to make the test-pieces averaged = 75 pm in size, and so the depths of debris penetration are, with one exception, of the order of one powder particle or less. The exception could be massaged to fit this pattern by making alternative assumptions about densities! The important thing is that reasonable assumptions give results which for the most part make sense.
6. Conclusions Fig. 12. Schematic representation of wear scar on sintered iron. x, depth of metal worn away; y, depth of oxide bed formed; z, depth to which pores are packed with debris; AfO, area fraction of scar surface covered by oxide beds; pr, relative density of metal substrate; pm, density of fully dense metal (for Fe, 7.86 mg mm-“); pm, density of fully dense oxide (for FeO, Fe,O,, 5.18 mg mme3; for Fe,O,, 5.24 mg mmT3); pd. density of debris packed in pores.
1. The reciprocating wear of sintered iron at low amplitudes exhibits phenomena which mirror the behaviour of wrought steels. The mechanism involves the generation of oxide glaze films in the gap between the two metal surfaces. These prevent metal-to-metal contact and act as a lubricant.
E.R. Leheup et al. I Reciprocating wear of sintered iron TABLE Al. Caluclated
129
values of x and z from Fig. 6
AM &PO (*O.l mg)
AV (mm’) (f0.2 mm3)
15.7 14.5 14.0 11.6 1.8 1.6
4.4 3.5 2.7 0.35 0.65
Ahf
X
= (mg mm-‘)
(pm)
3.6 f 0.2 4.1*0.3 5.2*0.4 4.6 f 0.4 5.1 f3.2 2.5 f 1.0
44*2 35+2 27*2 25rt2 3.5 f 2 6.5+2
2. Increasing loads and amplitudes bring about more severe wear. 3. Surface breaking pores in sintered iron act as sinks for wear debris. 4. At high sliding speed, sintered iron loses more weight than does mild steel. At low sliding speed, sintered iron loses weight less rapidly than mild steel. 5. The relationship between mass loss and volume loss can be explained adequately using a model based on accumulation of debris in surface-breaking pores.
Z/X
Z
(pm)
3.4+0.2 2.8f0.2 1.4*0.1 2.1*0.2 1.5 f 1.0 4.8il.8
150*20 95*15 35*5 55*10 5*6 30*20
Appendix A: Analysis of mass and volume changes in wear scars Al.
Volume changes per unit area of scar suface AV
metal -
A
= --x
oxide bed $
= +A,y
pore packing F
= + (1 - p,).z
7. Acknowledgments A2. Mass changes per unit area of scar sugace During the preparation of this paper, Dr Leheup died unexpectedly and prematurely. The other two authors wish to acknowledge their debts to their friend. Financial support was provided by the Science and Engineering Research Council.
oxide bed y
= +Afyp,,
pore packing y
= + (1 -p&p,
References A3. Measured volume change per unit area of scar 1 K. Gopinath, Wear, 71 (1981) 161. 2 T.S. Eyre and R.K. Walker, Powder Metall. I (1976) 22. 3 K. Gopinath, G.V.N. Rayudu and R.G. Narayanamurthi, Wear, 42 (1977) 245. 4 T. Sasada, Proc. JSLE Znt. Tribology Conj Tokyo, Ju& S-10, 1985, Japan Society for Lubrication Engineering, Tokyo, 1985, p. 623. 5 M. Godet, Wear, ZOO (1984) 437. 6 T. Kayaba and A. Iwabuchi, Wear, 66 (1981) 27. 7 H. Lyons and J.A. Collins, J. Mech. Des., 104 (1982) 619. 8 M. Kuno and R.B. Waterhouse, Proc. 5th. Znt. Congr. on Tribology, Helsinki, June 14, 1989, Vol. 3, The Finish Society for Tribology, Espoo, Finland, p. 30. 9 Ch. Colombie, Y. Berthier, A. Floguet, L. Vincent and M. Godet, J. Tribol., 106 (1984) 194. 10 P.L. Hurricks, Wear, 19 (1972) 207. 11 K. Razavizadeh and B.L. Davies, Wear, 69 (1981) 355. 12 S.C. Lim and J.H. Brunton, Wear, 113 (1986) 371. 13 E.R. Leheup and R.E. Pendlebury, Wear, I42 (1991) 351.
surjace AV = -x+Afy A
A4. Limiting case 1: oxide bed formation only, no pore packing, i.e. z=O: AM _ P~Pln--P0xAt&~~)
AV
1-&W)
using p,=O.8, so prp,=6.3
mg mmP2
130
E.R. Leheup
AM
6.3 -5.2&&/x)
AV
1 -A&Y/X)
-=
et al. I Reciprocating
AM
-
AV
of sintered iron
Note that AMIAV is greater than that for metal alone. However much oxide is formed as beds, AM/AV will always be greater than that for metal alone.
-3
mg mm
Range of possible values for A,Cy/x) (a) When there is no oxide bed, either A,=0 y=O, and
wear
or
=metal loss only
(b) If the scar is filled completely with oxide, A, = 1 andy/x = 1, giving AM/AV= CO,i.e. there is no volume loss, but weight is lost through the conversion of metal to oxide. (c) A reasonable value for A*, is = 0.5, for y= 2 pm and for x=20 pm. So, AMlAV=6.36 mg mrn3.
A5., Limiting case 2: pore packing only no oxide bed retention, i.e. y =0
AM TV
= PrPm -
(1 - Pr)Pdw>
sing pr = 0.8, i.e. prpm= 6.3 mg mmP3, and assuming the pores to be packed to a relative density of ~0.75 with oxide debris gives pd = 4 mg mme3, AM cv = 6.3 -O&lx
mg mme3
121
Low amplitud e reciprocatin g wear of sintere d iron* E.R . Leheup , D. Zhang , J.R . Moon Depanbzent of Materials Engineering and Materials Design Universi@ of Nottingham, Nottingham (UK)
(Received Decembe r 21, 1993; accepte d Februar y 3, 1994)
Abstract Th e unlubricate d wear , self-on-self, of sintere d iron du e to reciprocatin g motion ha s been investigated . Wrought, low-carbo n steel hav e also been studie d unde r identica l condition s for comparison. Th e genera l characteristic s of th e wear proces s for th e sintere d materia l ar e similar to thos e observe d for wrough t material . Wea r is mor e sever e when th e load is increase d and when th e amplitud e is increase d in the rang e up to abou t 250 pm . Furthe r increase s in amplitud e brough t abou t less marke d increase s in wear , until, ultimately , wear becam e independen t of amplitude. At high sliding speeds , sintere d iron loses mor e weight tha n does mild steel. At slow speeds , sintere d iron loses weight less rapidl y tha n does mild steel. Th e observation s ar e interprete d in th e light of oxide formatio n on th e bearin g surface s and of observations of wear debri s accumulate d in surfac e breakin g pores.
1. Introduction
The industria l proces s of stea m treatmen t of ferrous part s deliberatel y form s films of magnetit e on their surfaces . These ar e forme d mainl y to enhanc e appearanc e an d corrosio n resistance , bu t ar e claime d also to improv e wear resistance . Unfortunately , thi s commonly accepte d idea is neithe r well documente d nor well prove n in th e context of powde r metallurg y [ll]. Wha t is not clear from thes e previou s studie s is the role playe d by pore s in th e contactin g surfaces , The micromechanism s of wear of such surface s hav e not been addresse d in any detail , aithou ~ ther e is one observatio n tha t por e edges can brea k off, creating debris , whilst th e pore s themselve s can act as sinks int o which debri s can be packe d [12]. The wor k reporte d her e is intende d to be th e first par t of a coheren t stud y of th e wear of PM steels. To avoid unnecessar y complicatio n at thi s stage , it focuses on th e low amplitud e reciprocatin g wear (i.e. fretting) of pur e iro n at a constan t density.
Powde r metallurg y (PM ) part s ar e often used in circumstance s wher e thei r surface s ar e in slidin g contact with others . Nevertheless , th e phenomen a of wear of powde r metallurg y material s hav e attracte d few studies. Of these , most hav e examine d unidirectiona l sliding, typicall y usin g pin on disc experiment s [l-3]. They suggest tha t muc h of wha t is know n abou t wear in chemicall y equivalen t wrough t materia l is applicabl e to PM materials . For example , high load s an d slow sliding promot e metal-to-meta l contac t an d adhesiv e wear . At lower ioad s an d highe r speeds , th e wear behaviou r is controlle d primaril y by th e behaviou r of oxide layers forme d on th e contactin g surfaces . Adherent , lubricating films give low coefficients of frictio n an d low wear rates [3]. Possibl e fractur e an d delaminatio n of th e oxide layers , with consequen t increas e in frictio n an d wear, is promote d by high contac t pressure s an d by weak metalli c substrates , i.e. by fSSDI 0043-1648(94)06438-N actor s likely to caus e the layer s to ben d too much. 2. E~~rnen~l pqyamme In reciprocatin g wear of wrough t steels, generation of continuou s or semi-continuou s glaze bed s of oxide is a common observatio n [4-91. Their appearanc e is The wear behaviou r of presse d an d sintere d samples though t to accoun t for th e transitio n from severe to of pur e iro n at a relativ e densit y of p,=O.8 9 was mild wear as eithe r th e slidin g distanc e or th e test investigate d as a functio n of th e mechanica l variables temperatur e is increase d [lo]. of slidin g amplitude , reciprocatin g frequenc y an d normal load . The mai n programm e was carrie d out usin g a fixed tota l slidin g distanc e of 100 m. A smal l subsidiary *Thi s pape r is dedicate d to th e memor y of Dr . E.R . Leheup who died unexpectedly whilst this paper was under preparation. programm e of interrupte d testin g was used to examine
~3-1~8~4/$07.~ 0 1994 Etsevie r Science S.A. All rights reserved SSDI 0043-1648(94)06438-N
E.R. Leheup et al. / Reciprocatingwear of sintered iron
122
the a~umulat~on of wear after the first 10 m of relative motion. Annealed test-pieces of wrought mild steel were tested similarly for comparison. All tests were conducted in normal laboratory air. 2.1. Preparation of test pieces Test pieces were produced to final shape by a pressing and sintering route. Test piece geometry is shown in Fig. 1. Hogan& ABC 100.30 atomised iron powder was compacted by single-ended pressing. The surfaces to be worn were on the raised area, 10 mmX25 mm, which was in contact with the moving punch. Compaction pressure was 600 MPa. No lubricant was used. Compacts were sintered for 1 h at 1150 “C in an atmosphere of 95%N,-5%H,. Sintered densities were 7000 kg rne3, a relative density of 0.89. Test pieces produced in this way would be expected to contain variations in density with depth below the surfaces to be tested. Surface density was assessed by quantitative measurement of the area fraction of surface breaking pores. This gave pr = 0.8 + 0.05. An alternative assessment was made by repeatedly grinding away surface layers and weighing. These results are given in
Fig. 1. Form and size of test pieces (dimensions
TABLE 1. Quantitative test
assessments
in millimetres).
of density of surfaces under
(a) Area fraction of surface breaking Relative density, 0.8 f 0.05 Density, 6.3 f 0.4 mg rmC3
pores, 0.2~bO.05
(b) Results
experiments
of grinding
and weighing
Table 1 and suggest that the top 100 pm has an average density of 4.4kO.4 mg mmR3, i.e. p,=0.56f0.05. This discrepancy between these two measurements might be explained if some of the grinding debris is trapped in surface breaking pores, so that the loss of weight of a ground-off layer is less than it should be. Mild steel test pieces were machined from bright drawn bar, annealed for 1 h at 850 “C, furnace cooled, and finished to 240 grit on the wearing surfaces. 2.2. Wear testing Test pieces were tested, self on self, as crossed fiats. The experimental arrangement is shown schematically in Fig. 2. Both normal load and the reciprocating motion were applied hydraulically using an ESH servohydraulic machine. All experiments were done in normal laboratory air at 18 “C with no lubrication. Each test used two pairs of specimens, which were mounted symmetrically about the central activator. This was to avoid introducing bending moments to the actuator system and thereby to maintain planar contacts. Each specimen pair was brought into contact with their surfaces crossed at right-angles to give a nominal contact area of 10 mm X 10 mm. Amplitudes of relative motion were varied from 50 to 500 pm, frequencies from 1 to 100 Hz and normal loads from 200 to 1000 N. Amplitudes were monitored and controlled using LVDTs. Table 2 gives the schedule for the constant distance (100 m) tests. Section 4 details the wear accumulation programme. 2.3. Quantitative assessments of wear Net material loss was assessed from weight changes and by measuring volumes of wear scars. Weight loss was measured using a balance capable of resolving +O.l mg. Volume loss was measured using a three-dimensional profilometer. This uses a contacting stylus of 5 pm tip radius which is drawn across the surface in the manner of a Talysurf roughness tester. A series of parallel lines are drawn, the surface heights being data logged and processed to assess the total volume of the wear scar. Moving direction of the ram
Depth range below surface (pm)
Density of layer (mg mme3)
Relative density of layer h
o-loo 100-200 200-400 400-800 800-1600
4.4 f 0.4 5.6~tO.6 6.5 f 0.7 6.1*0.6 5.8f0.6
0.56 ic 0.05 0.71 f 0.08 0.83 f 0.09 0.78 f 0.08 0.74 f 0.08
Fig. 2. Arrangement
of test pieces in test-rig.
123
E.R. Leheup et al. / Reciprocating wear of sintered iron TABLE 2. Test schedule for constant distance, 100 m) Frequency (Hz)
Relative sliding speeds amplitudes” 0.05 mm
1
0.1 mm
distance tests (total sliding
(mm s-l)
0.2 mm
for the following
0.25 mm
-
2 2.5 5 10 20 25 50 100
10
“All combinations of amplitude and frequency using loads of 200, 600 and 1000 W.
24
8
22
+MS.lkN.O.Olm/s
noted are tested
Weight loss values f 2% 24 Amplitude values_* 1% 22
1 kN,O.Olm/s
-20
2om1 kN,O.OOim/s 10
016 E
F -0
-16
600N,O.Olm/s
-16
*
-14
2 12 + 610 ‘5 * ‘6 j 14
0.50 mm
-2’ 0
I 0.1
I 0.2
-12 -10 -0 -6 -4 -2 I 0.3
1 0.4
I 0.5
-0 I 0.i2
sliding speeds. Increasing load, amplitude and speed all give greater weight loss. Figure 4 shows weight loss as a function of normal load. The slopes of the curves clearly increase with amplitude. A feature of both Fig. 3 and Fig. 4 is the small weight gain observed when either amplitude or load is low. It seems that low power inputs to the wearing surfaces result in the formation of mechanically stable surfaces. Presumably the weight gains are due to oxidation. It is worth noting that all low amplitude tests show this characteristic and that these were the only tests where the amplitude was less than the spacing of surface breaking pores. Summarising, both total weight losses and weight loss rates increase with increasing mechanical power input, once a small threshold has been exceeded. 3.2. Volume changes Examples of profilometer traces of worn surfaces are shown in Fig. 5. In each figure the wear scar is in the horizontal centre with unworn regions, used as a datum, to left and right. Note that the volume lost at high load extends deeper than one particle depth and that the scar is rather rougher than the original surface. Under a low load, the general effect of wear is to smooth the surface. The sample corresponding to Fig. 5(c) lost volume but gained weight. It is thought that debris is packed into pores and that some oxidation has taken place. Figure 6 shows the relationship observed between volume change and weight change. The straight line
Amplitude/mm
Fig. 3. The effect of sliding amplitude on weight sliding distance, 100 m. MS =mild steel.
loss. Total
Typically, this can resolve volumes to f lop3 mm3 as deviations from solid, flat surfaces. The radius of the stylus tip is of the same order as that of surface breaking pores. This introduces additional errors into defining the positions of original surfaces and of worn surfaces. The maximum error arising from this phenomenon is estimated to be about 10% of the measured wear volume. All four worn surfaces from one experiment were assessed in both ways.
24 22 2018018 E -;j14-
+ 0.25mm,20Hz * 0.5mm,l Hz
z12!+
*O.lmmSOHz 86-
22
- 0.5mm,lOHz
* 0.25mm,2Hz
P
24
Weight loss values * 2% Normal load values f 0.1%
+O.O5mm,lOOHz
42-
3. 100 m tests: effects of mechanical
variables
3.1. Weight changes Figure 3 summarises the influences of sliding amplitude on weight losses at three normal loads and two
-2’ 0
1 200
I 400
I 600
Normal Fig. 4. The effect of normal distance, 100 m.
I 800
I 1,000
1,2&
load/N
load on weight loss. Total sliding
124
E.R. Leheup et RI. I Reciprocating wear 7
of sintered iron 7
+ 0.05mm,lkN.100Hz l
6 _+ O.O5,kN,lOHz
-6
x 0.5mm,ZOON,lOHz -5
5 1 0.5mm,600N,1Hz v
x 0.25mm,fjOON,20Hz “E E 4- 0 O.zSMM,l kN,lHz Ti? .z 3_ . 0.35MM,800N,14.29Hz x 0.2MM.l
E jj 2 * >
0.5MM,l
-4 X
.
kN,2& kN,lOHz
(a Weight loss values & 2% Volume loss values f 2%
-1 0
-,I -2
’
0
’
2
’
4
’
6
’
8
Weight
’
10
’
12
’
14
I
16
’
18
’
20
2;’
loss/mg
Fig. 6. Relationship between weight and volume changes. Total sliding distance, 100 m. Straight line corresponds to test piece average density, i.e. 0.89X7.86=7.0 mg mmm3.
in the samples tested, the contacting surfaces are of lower densities than the main bulk of the samples. However, this cannot provide a complete explanation. The experimental data lie about a slope representing a density of 3.8 mg mmw3, i.e. a relative density of 0.48. This is much lower than the observed density of the surface layers. We are therefore left with the proposition that either wear debris is being retained in pores, or oxidation is occurring, or both.
Fig. 5. Examples of profilometer traces from wear scars. Total sliding distance in all cases is 100 m. (a) 1 kN, 0.5 mm, 1 Hz, 0.001 m s-l. @) 1 kN, 0.5 mm, 10 Hz, 0.01 m s-l. (c) 200 N, 0.5 mm, 1 Hz, 0.001 m s-‘. (d) 1 kN, 0.05 mm, 10 Hz, 0.001 m s-‘.
represents theoretical relationships for iron of relative density p,=O.89 Clearly, weight losses are significantly less than would be expected from the volume loss. In part, this observation might arise from density variations
3.3. Obse~a~o~ of worn surfaces All worn surfaces showed macroscopically flat scars, containing an obvious brown oxide. Interfacial conformity of the contacting surfaces was generally good. Scar areas ranged from about 10% of the nominal contact area at low mechanical inputs to 100% at high loads and amplitudes. Figure 7 shows scanning electron micrographs of scars formed under a load of 200 N, the direction of relative sliding is horizontal. Figure 7(a) is from a sample worn with a stroke of Q.5 mm. It shows a clearly defined scar boundary. At higher ma~ification (Fig. 7(b)), a discontinuous layer of oxide glaze is seen to cover part of the scar. The presence of oxide is indicated by atomic number contrast. A typical patch of glaze is several hundred microns across, well in excess of the pore spacing. The wear debris appears to have filled the smaller pores and to have been compacted or deformed into the flat patches observed. Presumably, these carry most if not all of the load and have low coefficients of friction. Such a view would be consistent with the evidence from Figs. 3 and 4 that the surfaces have developed into a state of no wear equilibrium.
et al. I Reciprocating wear of sintered iron
125
(b)
id ) (d) Fig. 7. Scanning electron micrographs of wear scars formed under a normal load of 200 N, direction of relative motion is horizontal; totaf sliding distance, 100 m. (a), (b) 0.5 mm, 1 Hz, 0.001 m s-l. (c), (d) 0.05 mm, 10 Hz, 0.001 m s-l_
Decreasing the amplitude to 50 pm, still at the low load of 200 N, gives the scars shown in Figs. 7(c) and 7(d). Again, the worn area is smooth and contains patches of oxide, although these patches are smaller than those found at larger amplitude. Surface breaking
Fig. 8. Scanning electron micrographs of wear scars formed under a normal load of 1 Mu; direction of relative motion is horizontal; total sliding distance, 100 m. (a), (b) 0.5 mm, 10 Hz, 0.01 m s-r. (c), (d) 0.05 mm, 10 Hz, 0.001 m s-r.
pores can be seen in the scar area; the area fraction of large pores (>50 pm) appears to be the same as in the unworn areas. Fine surface channels linking the pores which can be seen in the unworn surfaces have disappeared in the worn area. Once again, filling of these channels by debris and oxide is most likely to
126
E.R. Leheup er al. I Reciprocating wear of sinfered iron
be responsible. Another possibili~, that the channels are simply ground away by the wear process, can be discounted. In these samples there is hardly any loss of volume at the scar. Figure 8 shows examples of wear scars formed under a high normal load (1 kN). At the larger amplitude (0.5 mm), all evidence of the porous substrate is obliterated (Figs. 8(a) and 8(b)). The debris bed is much thicker than those formed at lower loads. It appears to consist of overlapping regions of glaze, some of which are clearly spalling off. Figure 8(a) shows that some of the glaze is charging in the scanning electron microscope, suggesting that there is poor contact between the glaze and the metallic substrate. The sample illustrated in Figs. 8(a) and 8(b) had one of the greatest losses of weight. The micrographs suggest that the weight is lost by the spalling of glaze bed rather than by the immediate loss of debris as it isgenerated. It seems that the wear process goes through two stages. Fine debris is retained and consolidated into large beds of glaze. These beds appear to be unable to maintain their stability as they grow in area. Ultimately, they fail, producing flake-like debris which is rejected from the scar area. Figures 8(c) and 8(d) show the scar formed at the lower amplitude of 50 pm. It occupies only about 15% of the nominal contact area; this does not necessarily indicate that the initial conformity of the contact areas was poor. Figure 3 showed that this sample gained weight. The debris is retained in fields which are smaller and less coherent than those formed at higher amplitude. Perhaps this is to be expected, since it seems unlikely that continuous fields of glaze can be consolidated over regions of much more than the amplitude of relative motion.
4. Accumulation
of wear damage
Weight changes measured in two series of interrupted tests, using mild steel as well as sintered test pieces, are shown in Figs. 9(a) and 9(b). The wear conditions were a load of 1 kN, an amplitude of 0.5 mm and a frequency of 1 Hz, giving a sliding speed of 0.001 m S -‘. Note that the data are plotted both as weight changes of indi~dual samples and as the average for a sample pair. Throughout the whole of the range observed, sintered samples lost less weight than did mild steel, the difference becoming more and more apparent with an increasing number of cycles. In the early stages, up to about 5000 cycles (5 m sliding distance), there is a distinct difference between the two materials. Mild steel goes through a regime
8
0.5mm.l kN,lHr 7 _ Weight change
f
-2’ 0
' 1
/ 2
8
(0.001 ms-‘)
values * 2%
I, 3 4
I 5
I 6
I 7
I 8
I I;:
I 9
Totalcyclesx lo3
(a)
180
80, 0.5mm,lkN,lHz (0.001 m s-l) Weight change values f 2%
1
SS1
-60
*.S2
* (Sl +S2)/2
-50
*MS1
-40
* (MS1 +MS2)/2
- 30
: 5 20
- 20
_c 2 10
-10
.o, z
-lOi-_L-_i 2.5 3 (bt
-0
O 3.5
, 4
t 4.5
I 5
/ 5.5
‘-10 6
Log total cycles
Fig. 9. Accumulation of weight loss with increasing number of cycles (or sliding distance): (a) up to 9000 cycles; (b) total cycles, 3 X 16 (sliding distance, 300 m). MS-mild steel.
where one sample loses weight but its partner gains weight. Such behaviour indicates that an adhesive wear mechanism is operating; the corresponding wear scar (Fig. 10(a)) appears to confirm this inference. By contrast, both halves of a sintered pair behave similarly, gaining a small amount of weight. At this stage, the scars retain evidence of surface breaking pores (Figs. 10(b) and 10(c)), although there appears to be some charging up of the sample in the scanning electron microscope beam at the sites of the pores. Most likely, this would be caused by the presence in the pores of loosely packed debris of a nonconductive or weakly conductive nature.
E.R. Leheup et al. I ~eci~~~t~ng
wear of sintered iron
127
(a)
(b)
Fig. 10. Scanning electron micrographs of wear scars after 1700 cycles, 1.7 m sliding distance; direction of relative motion is horizontal; 1 kN, 0.5 mm, 1 Hz, 0.001 m s-‘. (a) Mild steel; (b), (c) Sintered iron.
After prolonged wear, the wear scars differ according to the frequency (or sliding speed) of the test. Figure 11 shows wear grooves formed at low speed (0.001 m s-‘) and the surface oxide is brown. At high speed (0.01 m s-l), a compact mat of black oxide is formed and wear grooves are not apparent (Fig. 8).
fd
5. Discussion
Fig. 11. Scanning electron micrographs of wear scars after 300 000 cycles, 300 m sliding distance; direction of relative motion is horizontal; 1 kN, 0.5 mm, 1 Hz, 0.001 m s-l. (a), (b) Mild steel. (c), (d) Sintered iron.
In general, sintered iron exhibits wear characteristics which mirror those found for wrought steels. Under reciprocating conditions, the mechanisms involve the generation of glazed beds of oxide in the contact zone which act as a lubricant as well as preventing metalto-metal contact.
Increasing loads and amplitudes both bring about more severe wear, although both effects are non-linear. Mechanical heat generated in the contact zone causes the generation of oxide beds covering, in whole or in part, the original surfaces; the greater the load and
)
128
E.R. Leheup et al. / Reciprocating wear of sintered iron
the faster the sliding speed, the more oxide would be expected. The next question concerns the stability of these oxide films. In wrought materials, large amplitudes of relative motion help to break the films and to sweep debris out of the contact zone. Much the same seems to be happening for sintered iron, although the process is modified by the ability of surface breaking pores to act as sinks for debris. Similarly, small loads would be expected to be less disruptive than large ones, in agreement with Fig. 3. An interesting feature of these experiments is the, perhaps surprising, observation that sintered iron loses weight more slowly than does mild steel at low sliding speed. To examine this further, we have to look at the relationships between volume and weight loss shown in Fig. 12. Relationships between weight loss and volume loss during the wear of solid metal surfaces have been modelled previously [ 131. The simple relationship, mass loss =volume loss X density, has to be modified to allow for weight gain due to oxidation. The formation of oxide beds of differing thicknesses give differing ratios of weight loss to volume loss. The most obvious difference between sintered iron and mild steel is the presence of surface breaking pores in the sintered material. These pores are going to modify the behaviour in a number of ways. (i) The pores provide channels through which air (oxygen) can be brought in relatively plentiful supply to the surfaces in contact. Kinetics of oxide formation will be affected. (ii) The presence of 10% pores (by volume) in the substrate material means that its specific heat is reduced by 10% and its thermal conductivity is reduced by about 20%. Consequently, equal inputs of frictional energy would give rise to interface temperatures that are higher for the sintered material than for mild steel. Again, more rapid oxidation of the sintered material would be expected.
(iii) The pores provide edges and extra surface area capable of being oxidised. (iv) The pores provide sinks into which fine debris can be swept and packed. (v) Pores provide keying sites for oxide layers which would help them to resist defoliation. (vi) Porous surfaces are both elastically and plastically more compliant than solid ones. The first five of these observations point to a more ready oxidation and retention of oxide by the sintered iron. The question is, do these ideas make sense quantitatively? Figure 12 represents a wear scar which incorporates the following features. Metal is worn away to a depth X. Oxide beds cover an area fraction& of the surface to a depth y. Pores in the sub-surface are packed to a depth z with debris. The debris might be any mixture of metal, Fe,O, and Fe,O,, and will not be fully compacted in the pores; a density of p,, is assumed. Of necessity, these assumptions are simplistic, but they allow some deductions to be made. Detailed analysis in Appendix 1 shows the following. (1) In the absence of either oxide bed formation or pore packing, AM/AV is given simply by the density of the metal, as expected. (2) If there is no pore packing, AMIAV cannot be less than the metal density, no matter how much oxide is formed into beds. Therefore, the experimental data in Fig. 6 cannot be explained in this way. (3) If there is no oxide bed formation, only pore packing, then a mechanism exists for reducing AiWAV, as required by the experimental data. Table Al gives calculated values for x and z, assuming the surface is of the relative density measured metallographically and that the relative density of debris captured and compacted into pores is -0.75. The iron powder particles used to make the test-pieces averaged = 75 pm in size, and so the depths of debris penetration are, with one exception, of the order of one powder particle or less. The exception could be massaged to fit this pattern by making alternative assumptions about densities! The important thing is that reasonable assumptions give results which for the most part make sense.
6. Conclusions Fig. 12. Schematic representation of wear scar on sintered iron. x, depth of metal worn away; y, depth of oxide bed formed; z, depth to which pores are packed with debris; AfO, area fraction of scar surface covered by oxide beds; pr, relative density of metal substrate; pm, density of fully dense metal (for Fe, 7.86 mg mm-“); pm, density of fully dense oxide (for FeO, Fe,O,, 5.18 mg mme3; for Fe,O,, 5.24 mg mmT3); pd. density of debris packed in pores.
1. The reciprocating wear of sintered iron at low amplitudes exhibits phenomena which mirror the behaviour of wrought steels. The mechanism involves the generation of oxide glaze films in the gap between the two metal surfaces. These prevent metal-to-metal contact and act as a lubricant.
E.R. Leheup et al. I Reciprocating wear of sintered iron TABLE Al. Caluclated
129
values of x and z from Fig. 6
AM &PO (*O.l mg)
AV (mm’) (f0.2 mm3)
15.7 14.5 14.0 11.6 1.8 1.6
4.4 3.5 2.7 0.35 0.65
Ahf
X
= (mg mm-‘)
(pm)
3.6 f 0.2 4.1*0.3 5.2*0.4 4.6 f 0.4 5.1 f3.2 2.5 f 1.0
44*2 35+2 27*2 25rt2 3.5 f 2 6.5+2
2. Increasing loads and amplitudes bring about more severe wear. 3. Surface breaking pores in sintered iron act as sinks for wear debris. 4. At high sliding speed, sintered iron loses more weight than does mild steel. At low sliding speed, sintered iron loses weight less rapidly than mild steel. 5. The relationship between mass loss and volume loss can be explained adequately using a model based on accumulation of debris in surface-breaking pores.
Z/X
Z
(pm)
3.4+0.2 2.8f0.2 1.4*0.1 2.1*0.2 1.5 f 1.0 4.8il.8
150*20 95*15 35*5 55*10 5*6 30*20
Appendix A: Analysis of mass and volume changes in wear scars Al.
Volume changes per unit area of scar suface AV
metal -
A
= --x
oxide bed $
= +A,y
pore packing F
= + (1 - p,).z
7. Acknowledgments A2. Mass changes per unit area of scar sugace During the preparation of this paper, Dr Leheup died unexpectedly and prematurely. The other two authors wish to acknowledge their debts to their friend. Financial support was provided by the Science and Engineering Research Council.
oxide bed y
= +Afyp,,
pore packing y
= + (1 -p&p,
References A3. Measured volume change per unit area of scar 1 K. Gopinath, Wear, 71 (1981) 161. 2 T.S. Eyre and R.K. Walker, Powder Metall. I (1976) 22. 3 K. Gopinath, G.V.N. Rayudu and R.G. Narayanamurthi, Wear, 42 (1977) 245. 4 T. Sasada, Proc. JSLE Znt. Tribology Conj Tokyo, Ju& S-10, 1985, Japan Society for Lubrication Engineering, Tokyo, 1985, p. 623. 5 M. Godet, Wear, ZOO (1984) 437. 6 T. Kayaba and A. Iwabuchi, Wear, 66 (1981) 27. 7 H. Lyons and J.A. Collins, J. Mech. Des., 104 (1982) 619. 8 M. Kuno and R.B. Waterhouse, Proc. 5th. Znt. Congr. on Tribology, Helsinki, June 14, 1989, Vol. 3, The Finish Society for Tribology, Espoo, Finland, p. 30. 9 Ch. Colombie, Y. Berthier, A. Floguet, L. Vincent and M. Godet, J. Tribol., 106 (1984) 194. 10 P.L. Hurricks, Wear, 19 (1972) 207. 11 K. Razavizadeh and B.L. Davies, Wear, 69 (1981) 355. 12 S.C. Lim and J.H. Brunton, Wear, 113 (1986) 371. 13 E.R. Leheup and R.E. Pendlebury, Wear, I42 (1991) 351.
surjace AV = -x+Afy A
A4. Limiting case 1: oxide bed formation only, no pore packing, i.e. z=O: AM _ P~Pln--P0xAt&~~)
AV
1-&W)
using p,=O.8, so prp,=6.3
mg mmP2
130
E.R. Leheup
AM
6.3 -5.2&&/x)
AV
1 -A&Y/X)
-=
et al. I Reciprocating
AM
-
AV
of sintered iron
Note that AMIAV is greater than that for metal alone. However much oxide is formed as beds, AM/AV will always be greater than that for metal alone.
-3
mg mm
Range of possible values for A,Cy/x) (a) When there is no oxide bed, either A,=0 y=O, and
wear
or
=metal loss only
(b) If the scar is filled completely with oxide, A, = 1 andy/x = 1, giving AM/AV= CO,i.e. there is no volume loss, but weight is lost through the conversion of metal to oxide. (c) A reasonable value for A*, is = 0.5, for y= 2 pm and for x=20 pm. So, AMlAV=6.36 mg mrn3.
A5., Limiting case 2: pore packing only no oxide bed retention, i.e. y =0
AM TV
= PrPm -
(1 - Pr)Pdw>
sing pr = 0.8, i.e. prpm= 6.3 mg mmP3, and assuming the pores to be packed to a relative density of ~0.75 with oxide debris gives pd = 4 mg mme3, AM cv = 6.3 -O&lx
mg mme3