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Measurements of the friction and wear behaviour of ceramic guides during the production of nylon
Wear, 138 (1990)
209 - 224
209
MEASUREMENTS OF THE FRICTION AND WEAR BEHAVIOUR CERAMIC GUIDES DURING THE PRODUCTION OF NYLON* P. M. RAMSEY
OF
and T. F. PAGE+
Department of Metallurgy and Engineering Newcastle upon Tyne NE1 7RU (U.K.)
Materials, University of Newcastle upon Tyne,
Summary Man-made fibres, such as nylon and terylene, pass over the surface of up to 30 ceramic guides during their manufacture by the melt-spinning process. The rapid speed of modern threadlines (up to 100 m s-i) causes wear and a very high rate of frictional heating at the small contact spots and, after a few hours, smooth grooves can be observed in commercial aluminabased guide materials. The wear rate is extremely low, typically of the order of 10e9 g of ceramic removed by a kilometre of fibre. However, the friction between the guide and textile fibres is greatly increased (by a factor of about 2 - 3) by the wearing of these smooth grooves and this causes a significant increase in threadline tension and an unacceptable deterioration in fibre properties. This paper describes experiments whereby, for the first time, an instrumented guide has been used to continuously monitor friction behaviour as grooves are worn in a guide surface on an industrial spinning line. In addition to monitoring friction, the data from this device can be interpreted to provide insight into the events occurring as filament bundles pass over the guide. Frictional force measurements also appear sensitive to the very earliest stages of wear, and thus the instrumented guide is seen as a valuable means of monitoring threadline performance, friction and guide surface degradation.
1. Introduction Nylon and other man-made textile fibres are commonly manufactured by melt spinning. This process involves extruding molten polymer through the holes of a spinneret followed by cooling, drawing and winding-up onto bobbins. The individual filaments solidify in free-fall and can then be wound *Paper presented at the International Conference on Wear of Materials, Denver, CO, U.S.A., April 8 - 14, 1989. TFormerly at Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. 0043-1648/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
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up at high speeds, typically of the order of 50 m s- ‘. Further processes such as dyeing, texturizing and weaving may be required during the fabrication of clothes, carpets, toothbrushes etc. During each of these stages, the fibre, which usually consists of a bundle of individual filaments, is steered by passing it over several guide surfaces chosen to cause as little damage as possible to the fibre over extensive periods of time. The most common guide materials are ceramics (glass-bonded aluminas and titanias) and are manufactured in many different shapes, sizes and colours, a common feature being a rounded surface over which the fibre travels. Water-based lubricants are often used to lower fibre-guide friction, to minimize frictional heating and to reduce the build up of static charges. The majority of guide surfaces are intended merely to locate the fibre in the correct position at each stage of a process, and hence the lowest possible friction between guide and fibre is desirable. Over the last two decades, the speed at which man-made fibres are spun has risen by about an order of magnitude to around 100 m s-‘, As production speeds have risen, guide wear has become increasingly significant and can lead to large increases in friction by the formation of smooth grooves on the guide surfaces. High friction produces high fibre tensions further down the spinning line which, in extreme cases, can result in uneven drawing, filament surface damage, poor dyeing properties, difficulties in winding the bobbin or even fibre failure. The problem of high friction is likely to become worse as yarn speeds are increased still further. Hence, there is considerable interest in new guide materials and surface finishes designed to keep the fibre tension within reasonable limits over prolonged periods of spinning. This paper discusses the characteristics of low friction guide surfaces, the measurement of friction and wear on an industrial spinning line and the outline requirements for improved fibre guide materials. The guide material used in the study is a commercially available glass-bonded titania ceramic, known as “Teraglide”, supplied by Morgan Matroc, Textile Guide Division, Stourport, U.K. Some properties of this titania guide material are given in Table 1, while the microstructure of such materials has been described elsewhere [ 1 - 31.
TABLE 1 Teraglide properties TiOz content Grain size Hardness (1 kg) Measured density Theoretical density Thermal conductivity
93 wt.% 2-10pm 1100 VHN 4170 kg mP3 4250 kg me3 5 W mK-*
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2. Guide surfaces For a given fibre, the friction properties of a guide are known to depend critically on the detailed nature of its surface [l]. In particular, for the elastic contact of a textile fibre on a guide, microscopically undulating surfaces are known to produce the lowest friction (see, for example, ref. 4). In the present work, two techniques were used to study the topography of the titania guides. Scanning electron microscopy (SEM) micrographs (Fig. 1) provide detailed information about the size and distribution of the various surface features. However, this is essentially a two-dimensional image lacking depth, information, unless accurate stereo pairs are taken. Therefore, a useful complementary technique is profilometry which provides a quantitative measure of the surface’s height distribution.
Fig. 1. A secondary electron SEM image of an unworn Teraglide surface showing discrete grains generally 2 - 10 pm in size. The microstructure also contains small sub-micron pores between the grains and some larger pores due to poor powder mixing or handling damage.
Figure 1 shows an SEM secondary electron image of the titania guide surface. Discrete grains, typically 2 - 3 pm wide and often with a high aspect ratio (up to 5), can be seen. There appear to be small submicrometre pores between the grains and some larger depressions up to 10 I.crn across. The small pores are consistent with the lower-than-theoretical density measurement (Table l), suggesting that the ceramic has not been fully sintered. Therefore, considerable porosity can be expected principally at the triple points between grains. The larger pores are probably a result of poor powder mixing during the initial preparation of the guide material. The following topographic parameters were chosen to characterize the titania guide surface. R, Rt 0, PC
The arithmetic mean of the profile departure from the mean line. The height range (the highest peak to the lowest valley). The mean profile slope. The peak count. This is the number of peaks per unit length which project through a band of width R/40 about the mean line (where R is the range of measurement as set by the operator, see Table 2).
212 TABLE 2 Roughness parameters and values Traverse
Iength (mm) 4 4 4
Range(Pm)
Ra (Pm)
Rt Mm)
0, (deg.)
P, (cm-‘)
25 25 25
0.42 0.42 0.42
3.4 7.2 5.0
3.0 3.5 3.7
32 41 55
0.42
5.2
3.4
43
average values a
t
lsmL
50/lm
+ Fig. 2. Profilometry traces of the Teraglide surface (the traces (a) and (b) are both 750 (urn long): (a) is plotted with a high vertical magnification to show the scale of the surface roughness, however when the surface is plotted with equal vertical and horizontal magnifications (b) it is actually very flat.
These parameters give quantitative information about the general surface roughness (0, and R,), any large departures from the mean surface line (I&) and the spacing of surface features (PC). Table 2 lists these values for three traverses of 4 mm. Plots of two shorter runs are shown in Fig. 2. The high vertical magnification of Fig. 2(a) is required to appreciate the microscopic topography of the surface. However, it should always be remembered that such surfaces appear flat on a macroscopic scale as can be seen from Fig. 2(b), where the vertical and horizontal magnifications are equal. The average values, which can be used to characterize the surface and compare it with other materials, are also given in Table 2. These three traverses cover a length of 12 mm, of the order of 1000 grains, enough to be statistically significant. 3. Friction
measurement
Previous investigations into fibre-guide friction have used simulations on slow-speed laboratory equipment with pre-spun yarn. These studies have
213
produced interesting and important results (see, for example, ref. 4). However, the conditions in these experiments are very different from those experienced on an industrial threadline, particularly in terms of speed and, hence, frictional heating. From a wear and friction point of view, the most critical guide is the first one encountered by the hot fibre. This is because, in most processes, no lubrication can be used at this stage since it affects the crystallization of the yarn, The fibre properties at this time (about 0.05 s after the yearn has left the spinneret) are very different from those on the final wound-up bobbin. The fibre is at a temperature of about 80 “C, has a draw ratio of only about a half of the final value and is essentially noncrystalline. The fibre is continuously drawn during spinning and so, at the fist guide, it is not travelling as fast as its wind-up speed and its diameter is also significantly greater than that of the final product. Therefore, it is clearly impossible to accurately reproduce these fibre properties in a laboratory test using cold, fully-processed fibre. Therefore, an on-line friction measuring device has been designed and developed to investigate fibreguide contact behaviour in situ on the production line. A simple cantilever beam arrangement was chosen for easy incorporation into the moving threadline without undue disruption of the process and to be robust enough to run for periods of several hours in an industrial plant. The friction was continuously but indirectly measured by monitoring the force applied to the guide by the fibre. This was measured by strain gauges mounted on the beam which detected its deflection from the vertical position (see Fig. 3). The design had to compromise between sufficiently high
Fig. 3. The instrumented guide apparatus. The test guide which has strain gauges attached to measure the force runs over the guide at a fixed contact angle determined the top and bottom of the perspex box improve the fibre
is mounted on the vertical beam exerted on the guide. The fibre by the beam position. Guides at stability.
214
beam stiffness, to avoid disruption of the fibre path and have enough compliance to provide the required sensitivity of measurement. With this contact geometry (Fig. 4) the well-known Capstan formula (eqn. (1)) can be used to calculate the friction coefficient (p) in terms of the fibre tension before (7’i) and after (To) passing through a contact angle of 8, on the guide
However, the derivation of this equation assumes that the frictional force is proportional to the normal load which may not be so for elastic contacts of various geometries. Thus a number of workers have fitted low-speed friction data to a revised version of the Capstan formula derived by Howell between frictional force (F) and normal [ 51 from an empirical relationship load (N), i.e. F=kN”
(2)
where k and n are constants. of ideal For plastic contacts n = 1, while for various geometries Hertzian single elastic contacts n = 0.67. For multiple elastic contacts, models have been proposed which have predicted values for n in the range 0.67 - 1 [6] and several workers have reported values around 0.8 for fibreguide contact (see, for example, refs. 7 and 8). In this present work, however, the simple single-valued friction coefficient of eqn. (1) has been used because the change of friction, due to the wearing of grooves, is easier to visualise in terms of p than the empirical parameters k and n. Two main forces act on any small element of fibre (Fig. 4) a normal force d/V due to the change in direction of the fibre, and a frictional force dF given by eqns. (3) and (4) respectively. dN=
Tde
Hence, assuming the frictional coefficient of friction, i.e.
(3) force and normal load are related through
the
dF=/..ldN then dF=pTdB
(4)
Therefore, the horizontal component of force acting on the guide d.Fh due to this fibre element can be expressed in terms of the angle of the element to the vertical 6 (eqn. (5)) dF~=T(sine-~cose)
(5)
215
e angle of element from input de, clement angle dN normal load dF: frictional forw dl elrment length H,, guide rad,ur
Fig. 4. Diagrams defining the symbols in the equations in the text: (a) defines the expressions for the contact angles and (b) shows the forces acting on a small element of fibre.
The Capstan formula (eqn. (1)) gives the tension of the fibre at any angle. Hence, substituting this into eqn. (5) and integrating around the contact length yields Fh = Ti sin Q!- To sin /3
(6)
which can be rewritten as Lln/&($
p = (a+/3)
-sinar)
1
(7)
A simplifying and reasonable assumption is that the ingoing tension Ti is not affected by an increase in fibre tension further down the spinning line. Therefore monitoring the variation of Fh leads directly to the change in friction coefficient via eqn. (7). A more refined version of the apparatus involving two perpendicular beam and gauge assemblies has also been tested. This device measures both
216
the horizontal and vertical components of the force exerted on the guide and, hence, the assumption of a constant initial fibre tension is not necessary. This adaptation of the apparatus appears promising, but is more difficult to stabilize.
4. Wear measurement The measurement of guide wear rates is made difficult by the extremely small amount of debris material involved at each contact area (typically less than 1 ,ug h-r). Therefore, the simple technique of monitoring the change in mass of the guide is not feasible and it was decided to take measurements from SEM micrographs of wear scars. Accurate SEM stereo pairs were taken and measured. However, this is time consuming and a simpler method has been found to be adequate and faster. The width of the groove is measured from a single SEM image and the wear volume calculated using a sensible assumption about the cross-sectional shape of the grooves. Based on elasticity calculations, the fibre is believed not to deform significantly on contact with the guide and, hence, a reasonable assumption is that the groove shape is the same as that of the filament which produced it. The wear volume V, guide-fibre contact area A, contact width w, groove depth d and a number of other quantities can then be calculated from simple geometric relations (see Fig. 5 and eqns. (8) - (ll), below).
Fig. 5. The calculation of the wear volume. This illustrates the quantities calculated from SEM micrographs of the wear scars. The distance between adjacent scars is a measure of the filament diameter Df. The scar length 2 and width w can also be measured from a micrograph. The scar volume V, contact area with the fibre A, depth d and contact width w can then be calculated using the equations given in the text.
217
y2P2}
d = f {Df - (Diz fl/Df2sin-‘(&)
V=
(8)
-Y(Dr’-_Y2)“2/
(9) (10)
A = wl
(11)
5. Results The instrumented beam was used to detect the change in friction during the wear of a titania guide. The conditions of the test were as follows. fibre: three filaments of clear nylon 6-6 expected filament diameter: about 40 pm guide: 5 mm radius titania pin fibre velocity: 50 m s-l at wind-up contact angle: c~= 0 = 30” lubrication: none The chart paper output from the strain gauge amplifiers showed a general gradual increase in force during the run with two discontinuous drops in force after 1 and 15 min. Measurements were taken from the chart paper every 10 s and the force at each point calculated from the calibration. These force values were then plotted against time (Fig. 6) which is, in effect, a smoothed version of the original trace. The gradual force increase coincides with surface smoothing and groove wear, while the discontinuous drops in 4
Guideforce(gf) and
p
0 . 0
-
guide force (gf)
----
P
C /
I
I
I
I
200
400
600
600
I :I 1000
1200 Time
(SCC)
Fig. 6. The smoothed force readings. Force measurements were made from the original output every ten seconds. The significant points are labelled A to D and a description of the events taking place at these times is given in the text. The variation of the friction coefficient is also plotted.
(a)
(b) Fig. 7. Secondary electron SEM micrographs of the wear scar: (a) shows an area near to the entry point of the fibre on the guide and (b) the exit region. Six discrete parallel grooves can be seen in two sets of three. On the left hand side the grooves are far more pronounced (the “deep” grooves) than those on the right (“shallow” grooves). Also, the scars are more pronounced as the fibre travels around the guide (see the text). The filament forming the middle shallow groove has cut through a particularly large protrusion on the surface (X). Such features increase the friction markedly and should be removed by the surface finishing techniques during guide manufacture.
force unworn
occur
when
material
the
filaments
elsewhere
jump
out
of their
grooves
and
back
onto
on the guide surface.
An SEM micrograph of the wear scar (Fig. 7) shows six discrete grooves, which appear much smoother than the original guide surface. The grooves are in two sets of three and, within a set, the spacing is consistently about 43 pm. As a result of this regular series of parallel grooves it is reasonable to assume that the filaments run side-by-side over the guide. Therefore, the groove spacing gives a direct measurement of the filament diameter (see Fig. 5) and calculations suggest that this is hardly affected by the small (less than 1 pm) flattened zone along the contact line [ 91. Within the accuracy of measurement, the two sets are exactly twice this distance apart. The grooves are not continuous but show short worn areas, known as “groove segments”, separated by unworn ceramic. Also, the grooves become more pronounced in the direction of fibre travel and one set, the “deep grooves”, is distinctly more prominent than the other set, the “shallow” grooves. It is reasonable to assume that the deep grooves have been formed
219
(d)
(e) Fig. 8. The steps involved during the formation of the wear scars. (a) The original filament positions (point A in Fig. 6). (b) After 80 s the filaments have moved across and wear the deep grooves (B to C in Fig. 6). (c) After 917 s the filaments probably move across the guide by a twisting mechanism. (d) The deep grooves have been formed and the shallow grooves are worn (C to D on Fig. 6). (e) At the end of the run (after 1055 s) the fibre breaks and six grooves are visible in the SEM (see Fig. 7).
when the force reaches its highest value (point C) and that the shallow grooves are formed during the build-up of force to point D, before the yarn broke. Apart from providing a measure of fibre-guide friction, force traces of the type shown in Fig. 6 provide considerable insight into the sequence of events occurring as the fibre passes over the guide. The force trace of Fig. 6 and the wear scars of Fig. 7 suggest the following sequence of events (see Fig. 8).
220
The initial force (at point A) is low as the contact between the smooth filaments and microscopically rough guide surface is confined to small areas of the high asperity peaks. A-B: This increase in force is probably due to a very small amount of wear which is not yet visible as a groove but is sufficient to significantly affect the friction (see Fig. 8(a)). B: This discontinuous drop in force is due to the filaments moving across the guide surface. Since the force does not return to the original value we can infer that the filaments have only jumped across by one or two filament diameters (see Figs. 8(a) and 8(b)) so that one or two of them are running over the already slightly worn surface with the rest moving over unworn ceramic. The friction immediately increases again and at approximately the same rate as at point A. B-C: The large force increase is due to the wearing of the deep grooves which take 837 s to form. The time can be quoted this accurately because of the rapid response of the beam to changes in force. The small variations in the trace along B-C probably arise from shortterm variations in fibre tension. A discontinuous drop in force back to the original value. At this c: point all three filaments have moved out of the deep grooves onto fresh unworn material. Since the two sets of grooves are separated by a distance exactly twice the groove spacing, so a likely mechanism for this shift is that the filaments have passed over each other to reach their new positions (Fig. 8(c)). This probably arises from momentary instabilities in the fibre line. The shallow grooves were worn during this period of 138 s (Fig. C-D: 8(d)). The force rises more rapidly than for the section from points B to C. The reason for this is not clear although the friction is expected to be very sensitive to the rapid wear of particularly high asperities on the ceramic guide during the early stages of contact. One such area of the shallow wear scar is marked as point X on Fig. 7. D: The fibre broke at this point and the test was discontinued leaving the two sets of wear scars (Fig. 8(e)). At fast fibre speeds, the high interfacial temperature will have a significant effect on the fibre-guide friction and wear processes. The measurement of the interface temperature distribution has proved prohibitively difficult (see, for example, refs. 2, 9, 10). However, the steam evolved when drops of a water-based lubricant came into contact with the guide surface indicates local temperatures in excess of 100 “C. Theoretical calculations reported elsewhere [ 91 suggest that the steady state surface temperature distribution is formed very quickly and therefore the friction change due to the initial heating of the guide will not be detected by the beam apparatus. A:
221 TABLE 3 The force and friction values at points A to D (see Fig. 7)
Point A B C D
Time (s) 0
80 917 1055
Beam force (gf/fil) 0.46 0.88 (dropped to 0.67) 1.32 (dropped to 0.5) 1.07
0.94 1.72 (1.41) 2.16 (1.05) 1.93
Using a proprietary Rothschild fibre tensometer temporarily placed in the threadline, the ingoing fibre tension Z’i was measured to be 0.75 gf for the three-filament yarn. The friction coefficient can be calculated for any given initial fibre tension using eqn. (7). Table 3 lists the force and friction coefficient at the points already described in the run. The output tensions are also given since this is the quantity of greatest concern to the fibre manufacturer. The values of friction coefficient for the fibre travelling on the worn guide are very large (greater than 1.5). However, this is to be expected because of the very large area of contact between the two smooth surfaces. Measurements of the length and width of individual groove segments were taken from an SEM montage of the entire length of the worn area and wear volumes and contact areas were computed. An example of the computer output is given in Table 4 with the scar depth, volume and contact area calculated using eqns. (8) - (11). In this way detailed information can be obtained concerning the formation of wear scars in the titania guide, the movement of filaments from one part of the guide to another and the correlation between wear and friction. Table 5 gives a list of direct quantitative comparisons which can be made between the two sets of scars.
6. Discussion and conclusions The friction between a high-speed nylon fibre and a static ceramic guide has been measured on an industrial spinning line. This is the first time that this type of measurement has been made under real industrial conditions and at real process speeds. Due to the continuous and significant guide wear and the different properties of the hot, partially drawn fibres, these experiments have produced appreciably higher friction values from those observed by previous workers using low-speed simulations with fully processed fibres. The wear rates were very low (typically 4 X 10-l’ g s-i) but were accompanied by surprisingly large increases in friction over very short periods of time (for example /J quadruples in the first minute of spinning at a speed of 50 m s-l) and correspondingly large rises in fibre tension. The reason for this behaviour is that the key factor controlling
222 TABLE 4 Wear scar parameter@
1(Pm) 49
88 95 23 148 85 88 113 176 120 21 67 21 25 78 42 127 39 64 131 21 35 120
Y
(Pm)
12.4 6.4 9.9 7.8 8.8 7.8 10.6 14.1 7.4 10.6 7.1 7.8 8.5 8.8 10.6 8.5 10.2 10.2 10.9 10.2 7.8 8.8 11.3
w (Pm)
d Wm)
V Wm3)
A
(m2)
12.57 6.42 9.99 7.84 8.86 7.84 10.71 14.35 7.44 10.71 7.13 7.84 8.55 8.86 10.71 8.55 10.29 10.29 11.01 10.29 7.84 8.86 11.43
0.89
0.23 0.56 0.35 0.44 0.35 0.65 1.16 0.31 0.65 0.29 0.35 0.41 0.44 0.65 0.41 0.60 0.60 0.69 0.60 0.35 0.44 0.74
363 88 355 42 387 154 404 1239 272 551 29 122 49 65 358 99 519 159 320 535 38 91 669
616 565 949 180 1311 667 942 1622 1309 1285 150 525 180 221 835 359 1307 401 705 1348 165 310 1371
23 scars Total length of scar = 1776 pm Total contact area = 17 324 pm2 Total wear volume = 6909 pm3 Vomputer output of values calculated for one of the deep grooves (the right hand deep groove in Fig. 8) using eqns. (8) - (11). The filament diameter was measured to be 43 pm and the individual scar lengths and widths were measured from an SEM montage of the whole worn area (about 2.5 mm in length).
the friction in this case is the fibre-guide contact area which is increased greatly even by the initial asperity smoothing effect of the wear process. The ceramic material is removed from the highest asperity tips which become plateaus (e.g. point X in Fig. 7(b)) increasing the contact area significantly. It should be noted that not all wear mechanisms would produce this surface smoothing. For example, abrasive wear by stray hard particles attached to the fibre could result in surface roughening from either plastic ploughing or surface fracture mechanisms (see, for example, refs. 10 and 11). Another factor which contributes to the large increase in friction is that, having formed a smooth groove, the filaments remain in them for long periods of time instead of traversing back and forth across the guide
223 TABLE 5 Quantitative comparisons between the two wear scars
Number of groove sections Apparent total contact length (mm) Total groove length (mm) Groove length as fraction of total length Total wear volume (pm3) Total contact area (pm’) Average contact width (pm) Time to form groove (s) Wear rate (pm3 s-l) (g s-l)
Shallow
Deep
22 2.64 0.94 0.36 1560 6620 2.5 138
23 2.64 1.78 0.67 7080 17330 6.6 837 11.3 (48 x 10-12)
:3”6 x 10-12)
surface. The latter would result in a more even distribution of wear rather than the formation of discrete grooves. Since wear leads to increased friction, some comment is necessary on likely wear mechanisms which may include plastic deformation, fracture, chemical wear or fatigue. No evidence of fracture has been observed on any of the worn guide surfaces examined in this or previous projects [2, lo]. Alumina-based guides, which have considerably higher hardness than titania (typically 1800 kg mm-* compared with 1100 kg mm-* for a 1 kg Vickers indentation at room temperature [ 111) suffer far less wear. However, hardness is not the only selection criterion since Sic guides have been shown to wear very quickly. Gibbs [ 111 and Czernuszka [2] attributed this behaviour to oxidative wear by the continual removal of Si02 from the surface of the guide. A similar chemical wear mechanism appears to operate for alumina and titania but, for these materials, the removal of thin hydrated layers from the surface seems responsible. For example, Czemuska and Page [3] reported the presence of hydrated &A1203 on worn alumina guide surfaces. Also, at high fibre speeds the frictional heating of the contact spots will be significant. Numerous papers (see, for example, refs. 12 and 13) have discussed the marked decrease in hardness of various ceramic materials with increasing temperature. Thus, the relevant hardness value for the description of the various surface plastic deformation response will be considerably lower than the room temperature value (see, for example, ref. 12). In conclusion, while the exact mechanisms controlling the low wear rates experienced in this study are not yet fully established, contributions from fine-scale plastic deformation and the removal of thin adsorbed and reacted layers (e.g. soft, hydrated, &Al,O,) are almost certainly responsible. The relative importance of these two processes is likely to vary with the material. Obviously, any change in either wear rate or materials wear behaviour which can maintain the undulating surface to longer periods without smoothing, will be beneficial in improving guide friction performance.
224
Acknowledgments The authors would like to thank Professor D. Hull for providing laboratory facilities in Cambridge and Professor R. N. Parkins for facilities in Newcastle. The research is supported by Morgan Matroc Limited (Stourporton-Sever-n) and ICI Fibres (Gloucester) under the SERC CASE scheme and Mr P. Love (Morgan Matroc Limited) and Mr F. Smith (ICI Fibres) are thanked for their interest and involvement. The authors are grateful for the use of a high-speed threadline at ICI Fibres (Gloucester) and to Ms. G. Hepple who kindly typed the manuscript.
References 1 P. E. Gallant and R. J. Merigold, The characterisation of the surfaces of technical ceramics used in the synthetic fibres industry by microscopy, J. Microsc., 124 (1981) 275. 2 J. T. Czernuszka, PhD Thesis, Cambridge University, 1985. 3 J. T. Czernuszka and T. F. Page, The importance of microscopy in studying the wear behaviour of ceramic surfaces, J. Microsc., 140 (1985) 159. 4 M. E. Baird and K. W. Mieszkis, Friction properties of nylon yarn and their relation to the function of textile guides, J. TextiZe Inst., 46 (1955) 101. 5 H. G. Howell, The general case of friction of a string round a cylinder, J. Textile Inst., 44 (1953). 6 J. F. Archard, Elastic deformation and the laws of friction, Proc. R. Sot., London, Ser. A, 243 (1957) 191. 7 C. Rubenstein, The friction and lubrication of yarns, J. Textile Inst., 49 (1958) 13. 8 D. G. Lyne, The dynamic friction between eelfulose acetate yarn and a cylindrical metai surface, J. Textile Inst., 46 (1955) 112. 9 P, M. Ramsey, PhD Thesis, Cambridge University, 1988. 10 N. K. Gibbs, PhD Thesis, Cambridge University, 1982. 11 J. T. Czernusxka and T. F. Page, Characterising the surface contact behaviour of ceramics. Part I: hardness response of glass bonded alumina and titania, J. Muter. Sci., 22 (1987) 3907. 12 P. M. Ramsey and T. F. Page, A new approach to predicting the wear behaviour of ceramic surfaces, Br. Ceram. Sot. Trans. J., 87 (1988) 74. 13 M. G. S. Naylor and T. F. Page, Microstructural studies of the temperature-dependence of deformation structures around hardness indentations in ceramics, J. Microse., 130 (1983) 345.
209 - 224
209
MEASUREMENTS OF THE FRICTION AND WEAR BEHAVIOUR CERAMIC GUIDES DURING THE PRODUCTION OF NYLON* P. M. RAMSEY
OF
and T. F. PAGE+
Department of Metallurgy and Engineering Newcastle upon Tyne NE1 7RU (U.K.)
Materials, University of Newcastle upon Tyne,
Summary Man-made fibres, such as nylon and terylene, pass over the surface of up to 30 ceramic guides during their manufacture by the melt-spinning process. The rapid speed of modern threadlines (up to 100 m s-i) causes wear and a very high rate of frictional heating at the small contact spots and, after a few hours, smooth grooves can be observed in commercial aluminabased guide materials. The wear rate is extremely low, typically of the order of 10e9 g of ceramic removed by a kilometre of fibre. However, the friction between the guide and textile fibres is greatly increased (by a factor of about 2 - 3) by the wearing of these smooth grooves and this causes a significant increase in threadline tension and an unacceptable deterioration in fibre properties. This paper describes experiments whereby, for the first time, an instrumented guide has been used to continuously monitor friction behaviour as grooves are worn in a guide surface on an industrial spinning line. In addition to monitoring friction, the data from this device can be interpreted to provide insight into the events occurring as filament bundles pass over the guide. Frictional force measurements also appear sensitive to the very earliest stages of wear, and thus the instrumented guide is seen as a valuable means of monitoring threadline performance, friction and guide surface degradation.
1. Introduction Nylon and other man-made textile fibres are commonly manufactured by melt spinning. This process involves extruding molten polymer through the holes of a spinneret followed by cooling, drawing and winding-up onto bobbins. The individual filaments solidify in free-fall and can then be wound *Paper presented at the International Conference on Wear of Materials, Denver, CO, U.S.A., April 8 - 14, 1989. TFormerly at Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. 0043-1648/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
210
up at high speeds, typically of the order of 50 m s- ‘. Further processes such as dyeing, texturizing and weaving may be required during the fabrication of clothes, carpets, toothbrushes etc. During each of these stages, the fibre, which usually consists of a bundle of individual filaments, is steered by passing it over several guide surfaces chosen to cause as little damage as possible to the fibre over extensive periods of time. The most common guide materials are ceramics (glass-bonded aluminas and titanias) and are manufactured in many different shapes, sizes and colours, a common feature being a rounded surface over which the fibre travels. Water-based lubricants are often used to lower fibre-guide friction, to minimize frictional heating and to reduce the build up of static charges. The majority of guide surfaces are intended merely to locate the fibre in the correct position at each stage of a process, and hence the lowest possible friction between guide and fibre is desirable. Over the last two decades, the speed at which man-made fibres are spun has risen by about an order of magnitude to around 100 m s-‘, As production speeds have risen, guide wear has become increasingly significant and can lead to large increases in friction by the formation of smooth grooves on the guide surfaces. High friction produces high fibre tensions further down the spinning line which, in extreme cases, can result in uneven drawing, filament surface damage, poor dyeing properties, difficulties in winding the bobbin or even fibre failure. The problem of high friction is likely to become worse as yarn speeds are increased still further. Hence, there is considerable interest in new guide materials and surface finishes designed to keep the fibre tension within reasonable limits over prolonged periods of spinning. This paper discusses the characteristics of low friction guide surfaces, the measurement of friction and wear on an industrial spinning line and the outline requirements for improved fibre guide materials. The guide material used in the study is a commercially available glass-bonded titania ceramic, known as “Teraglide”, supplied by Morgan Matroc, Textile Guide Division, Stourport, U.K. Some properties of this titania guide material are given in Table 1, while the microstructure of such materials has been described elsewhere [ 1 - 31.
TABLE 1 Teraglide properties TiOz content Grain size Hardness (1 kg) Measured density Theoretical density Thermal conductivity
93 wt.% 2-10pm 1100 VHN 4170 kg mP3 4250 kg me3 5 W mK-*
211
2. Guide surfaces For a given fibre, the friction properties of a guide are known to depend critically on the detailed nature of its surface [l]. In particular, for the elastic contact of a textile fibre on a guide, microscopically undulating surfaces are known to produce the lowest friction (see, for example, ref. 4). In the present work, two techniques were used to study the topography of the titania guides. Scanning electron microscopy (SEM) micrographs (Fig. 1) provide detailed information about the size and distribution of the various surface features. However, this is essentially a two-dimensional image lacking depth, information, unless accurate stereo pairs are taken. Therefore, a useful complementary technique is profilometry which provides a quantitative measure of the surface’s height distribution.
Fig. 1. A secondary electron SEM image of an unworn Teraglide surface showing discrete grains generally 2 - 10 pm in size. The microstructure also contains small sub-micron pores between the grains and some larger pores due to poor powder mixing or handling damage.
Figure 1 shows an SEM secondary electron image of the titania guide surface. Discrete grains, typically 2 - 3 pm wide and often with a high aspect ratio (up to 5), can be seen. There appear to be small submicrometre pores between the grains and some larger depressions up to 10 I.crn across. The small pores are consistent with the lower-than-theoretical density measurement (Table l), suggesting that the ceramic has not been fully sintered. Therefore, considerable porosity can be expected principally at the triple points between grains. The larger pores are probably a result of poor powder mixing during the initial preparation of the guide material. The following topographic parameters were chosen to characterize the titania guide surface. R, Rt 0, PC
The arithmetic mean of the profile departure from the mean line. The height range (the highest peak to the lowest valley). The mean profile slope. The peak count. This is the number of peaks per unit length which project through a band of width R/40 about the mean line (where R is the range of measurement as set by the operator, see Table 2).
212 TABLE 2 Roughness parameters and values Traverse
Iength (mm) 4 4 4
Range(Pm)
Ra (Pm)
Rt Mm)
0, (deg.)
P, (cm-‘)
25 25 25
0.42 0.42 0.42
3.4 7.2 5.0
3.0 3.5 3.7
32 41 55
0.42
5.2
3.4
43
average values a
t
lsmL
50/lm
+ Fig. 2. Profilometry traces of the Teraglide surface (the traces (a) and (b) are both 750 (urn long): (a) is plotted with a high vertical magnification to show the scale of the surface roughness, however when the surface is plotted with equal vertical and horizontal magnifications (b) it is actually very flat.
These parameters give quantitative information about the general surface roughness (0, and R,), any large departures from the mean surface line (I&) and the spacing of surface features (PC). Table 2 lists these values for three traverses of 4 mm. Plots of two shorter runs are shown in Fig. 2. The high vertical magnification of Fig. 2(a) is required to appreciate the microscopic topography of the surface. However, it should always be remembered that such surfaces appear flat on a macroscopic scale as can be seen from Fig. 2(b), where the vertical and horizontal magnifications are equal. The average values, which can be used to characterize the surface and compare it with other materials, are also given in Table 2. These three traverses cover a length of 12 mm, of the order of 1000 grains, enough to be statistically significant. 3. Friction
measurement
Previous investigations into fibre-guide friction have used simulations on slow-speed laboratory equipment with pre-spun yarn. These studies have
213
produced interesting and important results (see, for example, ref. 4). However, the conditions in these experiments are very different from those experienced on an industrial threadline, particularly in terms of speed and, hence, frictional heating. From a wear and friction point of view, the most critical guide is the first one encountered by the hot fibre. This is because, in most processes, no lubrication can be used at this stage since it affects the crystallization of the yarn, The fibre properties at this time (about 0.05 s after the yearn has left the spinneret) are very different from those on the final wound-up bobbin. The fibre is at a temperature of about 80 “C, has a draw ratio of only about a half of the final value and is essentially noncrystalline. The fibre is continuously drawn during spinning and so, at the fist guide, it is not travelling as fast as its wind-up speed and its diameter is also significantly greater than that of the final product. Therefore, it is clearly impossible to accurately reproduce these fibre properties in a laboratory test using cold, fully-processed fibre. Therefore, an on-line friction measuring device has been designed and developed to investigate fibreguide contact behaviour in situ on the production line. A simple cantilever beam arrangement was chosen for easy incorporation into the moving threadline without undue disruption of the process and to be robust enough to run for periods of several hours in an industrial plant. The friction was continuously but indirectly measured by monitoring the force applied to the guide by the fibre. This was measured by strain gauges mounted on the beam which detected its deflection from the vertical position (see Fig. 3). The design had to compromise between sufficiently high
Fig. 3. The instrumented guide apparatus. The test guide which has strain gauges attached to measure the force runs over the guide at a fixed contact angle determined the top and bottom of the perspex box improve the fibre
is mounted on the vertical beam exerted on the guide. The fibre by the beam position. Guides at stability.
214
beam stiffness, to avoid disruption of the fibre path and have enough compliance to provide the required sensitivity of measurement. With this contact geometry (Fig. 4) the well-known Capstan formula (eqn. (1)) can be used to calculate the friction coefficient (p) in terms of the fibre tension before (7’i) and after (To) passing through a contact angle of 8, on the guide
However, the derivation of this equation assumes that the frictional force is proportional to the normal load which may not be so for elastic contacts of various geometries. Thus a number of workers have fitted low-speed friction data to a revised version of the Capstan formula derived by Howell between frictional force (F) and normal [ 51 from an empirical relationship load (N), i.e. F=kN”
(2)
where k and n are constants. of ideal For plastic contacts n = 1, while for various geometries Hertzian single elastic contacts n = 0.67. For multiple elastic contacts, models have been proposed which have predicted values for n in the range 0.67 - 1 [6] and several workers have reported values around 0.8 for fibreguide contact (see, for example, refs. 7 and 8). In this present work, however, the simple single-valued friction coefficient of eqn. (1) has been used because the change of friction, due to the wearing of grooves, is easier to visualise in terms of p than the empirical parameters k and n. Two main forces act on any small element of fibre (Fig. 4) a normal force d/V due to the change in direction of the fibre, and a frictional force dF given by eqns. (3) and (4) respectively. dN=
Tde
Hence, assuming the frictional coefficient of friction, i.e.
(3) force and normal load are related through
the
dF=/..ldN then dF=pTdB
(4)
Therefore, the horizontal component of force acting on the guide d.Fh due to this fibre element can be expressed in terms of the angle of the element to the vertical 6 (eqn. (5)) dF~=T(sine-~cose)
(5)
215
e angle of element from input de, clement angle dN normal load dF: frictional forw dl elrment length H,, guide rad,ur
Fig. 4. Diagrams defining the symbols in the equations in the text: (a) defines the expressions for the contact angles and (b) shows the forces acting on a small element of fibre.
The Capstan formula (eqn. (1)) gives the tension of the fibre at any angle. Hence, substituting this into eqn. (5) and integrating around the contact length yields Fh = Ti sin Q!- To sin /3
(6)
which can be rewritten as Lln/&($
p = (a+/3)
-sinar)
1
(7)
A simplifying and reasonable assumption is that the ingoing tension Ti is not affected by an increase in fibre tension further down the spinning line. Therefore monitoring the variation of Fh leads directly to the change in friction coefficient via eqn. (7). A more refined version of the apparatus involving two perpendicular beam and gauge assemblies has also been tested. This device measures both
216
the horizontal and vertical components of the force exerted on the guide and, hence, the assumption of a constant initial fibre tension is not necessary. This adaptation of the apparatus appears promising, but is more difficult to stabilize.
4. Wear measurement The measurement of guide wear rates is made difficult by the extremely small amount of debris material involved at each contact area (typically less than 1 ,ug h-r). Therefore, the simple technique of monitoring the change in mass of the guide is not feasible and it was decided to take measurements from SEM micrographs of wear scars. Accurate SEM stereo pairs were taken and measured. However, this is time consuming and a simpler method has been found to be adequate and faster. The width of the groove is measured from a single SEM image and the wear volume calculated using a sensible assumption about the cross-sectional shape of the grooves. Based on elasticity calculations, the fibre is believed not to deform significantly on contact with the guide and, hence, a reasonable assumption is that the groove shape is the same as that of the filament which produced it. The wear volume V, guide-fibre contact area A, contact width w, groove depth d and a number of other quantities can then be calculated from simple geometric relations (see Fig. 5 and eqns. (8) - (ll), below).
Fig. 5. The calculation of the wear volume. This illustrates the quantities calculated from SEM micrographs of the wear scars. The distance between adjacent scars is a measure of the filament diameter Df. The scar length 2 and width w can also be measured from a micrograph. The scar volume V, contact area with the fibre A, depth d and contact width w can then be calculated using the equations given in the text.
217
y2P2}
d = f {Df - (Diz fl/Df2sin-‘(&)
V=
(8)
-Y(Dr’-_Y2)“2/
(9) (10)
A = wl
(11)
5. Results The instrumented beam was used to detect the change in friction during the wear of a titania guide. The conditions of the test were as follows. fibre: three filaments of clear nylon 6-6 expected filament diameter: about 40 pm guide: 5 mm radius titania pin fibre velocity: 50 m s-l at wind-up contact angle: c~= 0 = 30” lubrication: none The chart paper output from the strain gauge amplifiers showed a general gradual increase in force during the run with two discontinuous drops in force after 1 and 15 min. Measurements were taken from the chart paper every 10 s and the force at each point calculated from the calibration. These force values were then plotted against time (Fig. 6) which is, in effect, a smoothed version of the original trace. The gradual force increase coincides with surface smoothing and groove wear, while the discontinuous drops in 4
Guideforce(gf) and
p
0 . 0
-
guide force (gf)
----
P
C /
I
I
I
I
200
400
600
600
I :I 1000
1200 Time
(SCC)
Fig. 6. The smoothed force readings. Force measurements were made from the original output every ten seconds. The significant points are labelled A to D and a description of the events taking place at these times is given in the text. The variation of the friction coefficient is also plotted.
(a)
(b) Fig. 7. Secondary electron SEM micrographs of the wear scar: (a) shows an area near to the entry point of the fibre on the guide and (b) the exit region. Six discrete parallel grooves can be seen in two sets of three. On the left hand side the grooves are far more pronounced (the “deep” grooves) than those on the right (“shallow” grooves). Also, the scars are more pronounced as the fibre travels around the guide (see the text). The filament forming the middle shallow groove has cut through a particularly large protrusion on the surface (X). Such features increase the friction markedly and should be removed by the surface finishing techniques during guide manufacture.
force unworn
occur
when
material
the
filaments
elsewhere
jump
out
of their
grooves
and
back
onto
on the guide surface.
An SEM micrograph of the wear scar (Fig. 7) shows six discrete grooves, which appear much smoother than the original guide surface. The grooves are in two sets of three and, within a set, the spacing is consistently about 43 pm. As a result of this regular series of parallel grooves it is reasonable to assume that the filaments run side-by-side over the guide. Therefore, the groove spacing gives a direct measurement of the filament diameter (see Fig. 5) and calculations suggest that this is hardly affected by the small (less than 1 pm) flattened zone along the contact line [ 91. Within the accuracy of measurement, the two sets are exactly twice this distance apart. The grooves are not continuous but show short worn areas, known as “groove segments”, separated by unworn ceramic. Also, the grooves become more pronounced in the direction of fibre travel and one set, the “deep grooves”, is distinctly more prominent than the other set, the “shallow” grooves. It is reasonable to assume that the deep grooves have been formed
219
(d)
(e) Fig. 8. The steps involved during the formation of the wear scars. (a) The original filament positions (point A in Fig. 6). (b) After 80 s the filaments have moved across and wear the deep grooves (B to C in Fig. 6). (c) After 917 s the filaments probably move across the guide by a twisting mechanism. (d) The deep grooves have been formed and the shallow grooves are worn (C to D on Fig. 6). (e) At the end of the run (after 1055 s) the fibre breaks and six grooves are visible in the SEM (see Fig. 7).
when the force reaches its highest value (point C) and that the shallow grooves are formed during the build-up of force to point D, before the yarn broke. Apart from providing a measure of fibre-guide friction, force traces of the type shown in Fig. 6 provide considerable insight into the sequence of events occurring as the fibre passes over the guide. The force trace of Fig. 6 and the wear scars of Fig. 7 suggest the following sequence of events (see Fig. 8).
220
The initial force (at point A) is low as the contact between the smooth filaments and microscopically rough guide surface is confined to small areas of the high asperity peaks. A-B: This increase in force is probably due to a very small amount of wear which is not yet visible as a groove but is sufficient to significantly affect the friction (see Fig. 8(a)). B: This discontinuous drop in force is due to the filaments moving across the guide surface. Since the force does not return to the original value we can infer that the filaments have only jumped across by one or two filament diameters (see Figs. 8(a) and 8(b)) so that one or two of them are running over the already slightly worn surface with the rest moving over unworn ceramic. The friction immediately increases again and at approximately the same rate as at point A. B-C: The large force increase is due to the wearing of the deep grooves which take 837 s to form. The time can be quoted this accurately because of the rapid response of the beam to changes in force. The small variations in the trace along B-C probably arise from shortterm variations in fibre tension. A discontinuous drop in force back to the original value. At this c: point all three filaments have moved out of the deep grooves onto fresh unworn material. Since the two sets of grooves are separated by a distance exactly twice the groove spacing, so a likely mechanism for this shift is that the filaments have passed over each other to reach their new positions (Fig. 8(c)). This probably arises from momentary instabilities in the fibre line. The shallow grooves were worn during this period of 138 s (Fig. C-D: 8(d)). The force rises more rapidly than for the section from points B to C. The reason for this is not clear although the friction is expected to be very sensitive to the rapid wear of particularly high asperities on the ceramic guide during the early stages of contact. One such area of the shallow wear scar is marked as point X on Fig. 7. D: The fibre broke at this point and the test was discontinued leaving the two sets of wear scars (Fig. 8(e)). At fast fibre speeds, the high interfacial temperature will have a significant effect on the fibre-guide friction and wear processes. The measurement of the interface temperature distribution has proved prohibitively difficult (see, for example, refs. 2, 9, 10). However, the steam evolved when drops of a water-based lubricant came into contact with the guide surface indicates local temperatures in excess of 100 “C. Theoretical calculations reported elsewhere [ 91 suggest that the steady state surface temperature distribution is formed very quickly and therefore the friction change due to the initial heating of the guide will not be detected by the beam apparatus. A:
221 TABLE 3 The force and friction values at points A to D (see Fig. 7)
Point A B C D
Time (s) 0
80 917 1055
Beam force (gf/fil) 0.46 0.88 (dropped to 0.67) 1.32 (dropped to 0.5) 1.07
0.94 1.72 (1.41) 2.16 (1.05) 1.93
Using a proprietary Rothschild fibre tensometer temporarily placed in the threadline, the ingoing fibre tension Z’i was measured to be 0.75 gf for the three-filament yarn. The friction coefficient can be calculated for any given initial fibre tension using eqn. (7). Table 3 lists the force and friction coefficient at the points already described in the run. The output tensions are also given since this is the quantity of greatest concern to the fibre manufacturer. The values of friction coefficient for the fibre travelling on the worn guide are very large (greater than 1.5). However, this is to be expected because of the very large area of contact between the two smooth surfaces. Measurements of the length and width of individual groove segments were taken from an SEM montage of the entire length of the worn area and wear volumes and contact areas were computed. An example of the computer output is given in Table 4 with the scar depth, volume and contact area calculated using eqns. (8) - (11). In this way detailed information can be obtained concerning the formation of wear scars in the titania guide, the movement of filaments from one part of the guide to another and the correlation between wear and friction. Table 5 gives a list of direct quantitative comparisons which can be made between the two sets of scars.
6. Discussion and conclusions The friction between a high-speed nylon fibre and a static ceramic guide has been measured on an industrial spinning line. This is the first time that this type of measurement has been made under real industrial conditions and at real process speeds. Due to the continuous and significant guide wear and the different properties of the hot, partially drawn fibres, these experiments have produced appreciably higher friction values from those observed by previous workers using low-speed simulations with fully processed fibres. The wear rates were very low (typically 4 X 10-l’ g s-i) but were accompanied by surprisingly large increases in friction over very short periods of time (for example /J quadruples in the first minute of spinning at a speed of 50 m s-l) and correspondingly large rises in fibre tension. The reason for this behaviour is that the key factor controlling
222 TABLE 4 Wear scar parameter@
1(Pm) 49
88 95 23 148 85 88 113 176 120 21 67 21 25 78 42 127 39 64 131 21 35 120
Y
(Pm)
12.4 6.4 9.9 7.8 8.8 7.8 10.6 14.1 7.4 10.6 7.1 7.8 8.5 8.8 10.6 8.5 10.2 10.2 10.9 10.2 7.8 8.8 11.3
w (Pm)
d Wm)
V Wm3)
A
(m2)
12.57 6.42 9.99 7.84 8.86 7.84 10.71 14.35 7.44 10.71 7.13 7.84 8.55 8.86 10.71 8.55 10.29 10.29 11.01 10.29 7.84 8.86 11.43
0.89
0.23 0.56 0.35 0.44 0.35 0.65 1.16 0.31 0.65 0.29 0.35 0.41 0.44 0.65 0.41 0.60 0.60 0.69 0.60 0.35 0.44 0.74
363 88 355 42 387 154 404 1239 272 551 29 122 49 65 358 99 519 159 320 535 38 91 669
616 565 949 180 1311 667 942 1622 1309 1285 150 525 180 221 835 359 1307 401 705 1348 165 310 1371
23 scars Total length of scar = 1776 pm Total contact area = 17 324 pm2 Total wear volume = 6909 pm3 Vomputer output of values calculated for one of the deep grooves (the right hand deep groove in Fig. 8) using eqns. (8) - (11). The filament diameter was measured to be 43 pm and the individual scar lengths and widths were measured from an SEM montage of the whole worn area (about 2.5 mm in length).
the friction in this case is the fibre-guide contact area which is increased greatly even by the initial asperity smoothing effect of the wear process. The ceramic material is removed from the highest asperity tips which become plateaus (e.g. point X in Fig. 7(b)) increasing the contact area significantly. It should be noted that not all wear mechanisms would produce this surface smoothing. For example, abrasive wear by stray hard particles attached to the fibre could result in surface roughening from either plastic ploughing or surface fracture mechanisms (see, for example, refs. 10 and 11). Another factor which contributes to the large increase in friction is that, having formed a smooth groove, the filaments remain in them for long periods of time instead of traversing back and forth across the guide
223 TABLE 5 Quantitative comparisons between the two wear scars
Number of groove sections Apparent total contact length (mm) Total groove length (mm) Groove length as fraction of total length Total wear volume (pm3) Total contact area (pm’) Average contact width (pm) Time to form groove (s) Wear rate (pm3 s-l) (g s-l)
Shallow
Deep
22 2.64 0.94 0.36 1560 6620 2.5 138
23 2.64 1.78 0.67 7080 17330 6.6 837 11.3 (48 x 10-12)
:3”6 x 10-12)
surface. The latter would result in a more even distribution of wear rather than the formation of discrete grooves. Since wear leads to increased friction, some comment is necessary on likely wear mechanisms which may include plastic deformation, fracture, chemical wear or fatigue. No evidence of fracture has been observed on any of the worn guide surfaces examined in this or previous projects [2, lo]. Alumina-based guides, which have considerably higher hardness than titania (typically 1800 kg mm-* compared with 1100 kg mm-* for a 1 kg Vickers indentation at room temperature [ 111) suffer far less wear. However, hardness is not the only selection criterion since Sic guides have been shown to wear very quickly. Gibbs [ 111 and Czernuszka [2] attributed this behaviour to oxidative wear by the continual removal of Si02 from the surface of the guide. A similar chemical wear mechanism appears to operate for alumina and titania but, for these materials, the removal of thin hydrated layers from the surface seems responsible. For example, Czemuska and Page [3] reported the presence of hydrated &A1203 on worn alumina guide surfaces. Also, at high fibre speeds the frictional heating of the contact spots will be significant. Numerous papers (see, for example, refs. 12 and 13) have discussed the marked decrease in hardness of various ceramic materials with increasing temperature. Thus, the relevant hardness value for the description of the various surface plastic deformation response will be considerably lower than the room temperature value (see, for example, ref. 12). In conclusion, while the exact mechanisms controlling the low wear rates experienced in this study are not yet fully established, contributions from fine-scale plastic deformation and the removal of thin adsorbed and reacted layers (e.g. soft, hydrated, &Al,O,) are almost certainly responsible. The relative importance of these two processes is likely to vary with the material. Obviously, any change in either wear rate or materials wear behaviour which can maintain the undulating surface to longer periods without smoothing, will be beneficial in improving guide friction performance.
224
Acknowledgments The authors would like to thank Professor D. Hull for providing laboratory facilities in Cambridge and Professor R. N. Parkins for facilities in Newcastle. The research is supported by Morgan Matroc Limited (Stourporton-Sever-n) and ICI Fibres (Gloucester) under the SERC CASE scheme and Mr P. Love (Morgan Matroc Limited) and Mr F. Smith (ICI Fibres) are thanked for their interest and involvement. The authors are grateful for the use of a high-speed threadline at ICI Fibres (Gloucester) and to Ms. G. Hepple who kindly typed the manuscript.
References 1 P. E. Gallant and R. J. Merigold, The characterisation of the surfaces of technical ceramics used in the synthetic fibres industry by microscopy, J. Microsc., 124 (1981) 275. 2 J. T. Czernuszka, PhD Thesis, Cambridge University, 1985. 3 J. T. Czernuszka and T. F. Page, The importance of microscopy in studying the wear behaviour of ceramic surfaces, J. Microsc., 140 (1985) 159. 4 M. E. Baird and K. W. Mieszkis, Friction properties of nylon yarn and their relation to the function of textile guides, J. TextiZe Inst., 46 (1955) 101. 5 H. G. Howell, The general case of friction of a string round a cylinder, J. Textile Inst., 44 (1953). 6 J. F. Archard, Elastic deformation and the laws of friction, Proc. R. Sot., London, Ser. A, 243 (1957) 191. 7 C. Rubenstein, The friction and lubrication of yarns, J. Textile Inst., 49 (1958) 13. 8 D. G. Lyne, The dynamic friction between eelfulose acetate yarn and a cylindrical metai surface, J. Textile Inst., 46 (1955) 112. 9 P, M. Ramsey, PhD Thesis, Cambridge University, 1988. 10 N. K. Gibbs, PhD Thesis, Cambridge University, 1982. 11 J. T. Czernusxka and T. F. Page, Characterising the surface contact behaviour of ceramics. Part I: hardness response of glass bonded alumina and titania, J. Muter. Sci., 22 (1987) 3907. 12 P. M. Ramsey and T. F. Page, A new approach to predicting the wear behaviour of ceramic surfaces, Br. Ceram. Sot. Trans. J., 87 (1988) 74. 13 M. G. S. Naylor and T. F. Page, Microstructural studies of the temperature-dependence of deformation structures around hardness indentations in ceramics, J. Microse., 130 (1983) 345.