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Wear 267 (2009) 669–678

Contents lists available at ScienceDirect

Wear

journa l homepage: www.e lsev ier .com/ locate /wear

he wear and friction of polyamide 46 and polyamide 46/aramid-fibreomposites in sliding–rolling contact

.H. Gordona,b, S.N. Kukurekaa,∗

School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UKDutch Polymer Institute (DPI), PO Box 902, 5600 AX Eindhoven, The Netherlands

r t i c l e i n f o

rticle history:eceived 22 October 2008eceived in revised form5 November 2008ccepted 25 November 2008

eywords:olyamide 4,6ramid fibre compositesear

riction

a b s t r a c t

Polyamide 4,6 (PA46) and its aramid fibre composites (6 wt.%, 12 wt.% and 15 wt.%) were tested as can-didate materials for tribological applications using a twin-disc wear test rig, built in our laboratories, tomeasure their wear and frictional properties under variations of number of cycles (103 to 106), appliedload (300–600 N) and applied velocity (500, 1000 and 1500 rpm), all at a slip ratio of 2%. The rig usespolymer-on-polymer non-conformal contacts with varying proportions of rolling and sliding and haspreviously been shown to be a more controllable model experiment than utilising the complex geometryof gears.

Over the range of tests, the average coefficient of friction results showed that the PA46 + 15% aramidfibres generally had the lowest values compared to the other types of samples; however they generallyhad the highest steady wear rates, especially at higher loads and velocities. Using a thermal imager to

record the temperatures created during the tests it was found that there was always an initial rapidrise in temperature followed by a slow decrease or a plateau, which seemed to follow the friction testresults, with temperatures of up to 200 ◦C being reached. Optical microscopy of the wear surfaces of thePA46 samples showed cracks occurring for low loads and velocities, while at higher loads and velocitiesmelting occurred. The PA46 + aramid fibre samples failed by pitting and large cracks/fractures occurringat the relatively high loads and velocities used. However, they may be suitable for tribological applications

ions.

under appropriate condit

. Introduction

Injection-moulded, fibre-reinforced polyamides have beenidely used for gear and bearing applications but the mechanisms

f wear and fracture, in such components, are not fully understood.mong the potential tribological damage mechanisms occurring,

ailure processes resulting from crack initiation and propagationnder the action of surface contact stresses have emerged as

mportant factors affecting the tribological performance. The objec-ive of the project is to investigate such contact fatigue processesn polyamides using an approach based upon model tribologicalxperiments. This will involve the analysis of surface cracking pro-esses within polymer-on-polymer, non-conformal contacts. Theechnique involves a twin-disc wear test rig with varying propor-

ions of rolling and sliding and has previously been shown to be aetter model experiment than utilising the complex geometry ofears (in which materials and mechanics effects cannot easily beeparated) [1–9]. Surface cracking phenomena in polyamides have

∗ Corresponding author.E-mail address: s.n.kukureka@bham.ac.uk (S.N. Kukureka).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2008.11.026

© 2009 Elsevier B.V. All rights reserved.

previously been observed with this technique but have not beenexplained. The approach was developed in the context of fibre-reinforced polyamides in relation to microstructural parametersand processing conditions. The following parameters were and willbe considered: fibre distribution and orientation close to the slidinginterface; matrix nature (polyamide type and grade); morphology;fibre type (glass, carbon or aramid) and the fibre/matrix morphol-ogy. Similar conditions in terms of load/pressure will be used as ourproject colleagues from ESPCI, Paris, France [10], so that the resultscould be comparable.

Most of the published work on the tribology of non-conformalpolymer pairs relate to the performance of gears. For a pair of gearsthe dominant operating parameters such as slip ratio, sliding veloc-ity and load and the geometric parameters such as module andcurvature of the contacting surfaces vary with the contact positionof the tooth profile. It is, therefore, very difficult to interpret thewear mechanisms of polymer gears [11]. For example it was found

that the surface topography of worn polyoxymethylene (POM) gearteeth depended on the position on the tooth surface with differentwear modes at the pitch line and on the addenda and dedenda [12].There is clearly a relationship between the mechanism of wear andthe sliding speed but this is obscured by the interaction between

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70 D.H. Gordon, S.N. Kukure

ear and load distribution. Wear in the addenda and dedendaeduces the contact load at the tooth extremities and increases itear the pitch line. In order to control the parameters it was decidedo find an alternative method to examine the wear characteristicnder conditions where the load, rolling and sliding speed coulde accurately defined.

This alternative method to studying wear is to apply non-onformal contact conditions experienced by gears, but applied to auch simpler geometry. An example is the use of two circular discs

oaded against one another in edge-to-edge contact and rotating atifferent speeds [1–9]. By varying the relative speeds of the discsnd the applied load, the normal load, slip ratio and sliding speedonditions experienced by gear teeth in contact may be approx-mated. This method has the advantages of applying uniform,teady-state wear to a pair of contacting surfaces without the effectsf accelerations in sliding speed and zero-sliding regions such as theitch point in gear contact and it also removes such uncertaintiesresent in gear testing as changes brought about by tooth flexibilityffects. A twin-disc experiment therefore allows clear comparisonf the tribological performance of different materials, which cane interpreted in terms of the wear surface topography and theicrostructure of the material.In previous work using the twin-disc wear test rig a wide vari-

ty of polymers and their composites, namely polyoxymethylenePOM) [2–4], polyamide 66 (PA66) [4,6–9], glass-fibre reinforcedA66 [3–5,7,8], PA66 and POM filled with 20 wt.% of polytetraflu-roethylene (PTFE) [6,8] and short fibre – aramid and carbon –einforced PA66 [4,7] were studied. The materials were tested underwide variety of conditions of a range of rolling speeds, a range

f loads, a range of slip ratios and a range of rolling cycles, totudy their wear and frictional properties and their potential tri-ological damage mechanisms. For the tests using POM, it wasound that at low temperatures, low loads, rolling velocities andlip ratios, the wear rate was low (5 × 10−7 �m/cycle) with sur-ace pitting and flaking on the wear surfaces. At high loads, rollingelocities and slip ratios the wear rate increased between 10−6 and0−4 �m/cycle and the wear became more severe leading to theetachment of large areas of the wear surfaces. It was found thatA66 had a high wear rate at low temperatures of less than 100 ◦Cf 2 × 10−6 �m/cycle. With increasing heat input it was found that aoft surface layer was produced which behaved as a self-lubricationayer that reduced the friction and wear. It was discovered thathe slip ratio had a significant effect on the wear rate and thathere was a critical slip ratio at a fixed load and running speedhere the PA66 showed poor wear resistance between 10−6 and

0−7 �m/cycle and a high friction coefficient from 0.42 to 0.72.t was also observed that the wear surfaces were covered in lat-ral cracks perpendicular to the direction of the frictional wearnd they appeared at temperatures above 80 ◦C, penetrating about50–450 �m into surface, but they did not lead to surface failure.he glass-reinforced PA66 had 30 wt.% or 40 wt.% of glass fibres. Athe start of the tests, where the wear rate was low (10−6 �m/cycle)nd the coefficient of friction was low (� < 0.1), a thin interfacialayer of PA66 formed on the surface which depended on the struc-ure, strength and fibre concentration and on the loads and slipatios, and masked the abrasive glass fibres. When this layer wasorn away, the surface became saturated with fibres which inter-

cted and the wear rate increased between 10−4 and 10−3 �m/cyclend the friction coefficient � between 0.28 and 0.35. However, thelass-fibre reinforced PA66 composites prevented transverse cracksrom the initiation and propagation that would have been found

n the unreinforced PA66 wear surfaces. In spite of the abrasiveature of the filler, polyamide glass-fibre composites can be useds gears if the crystallinity of the matrix material is low and max-mum surface temperature (150 ◦C) is limited. It was found thathe POM filled with 20 wt.% of PTFE reduced the coefficient of fric-

ear 267 (2009) 669–678

tion from around 0.3 to 0.13 and that PA66 filled with 20 wt.%of PTFE reduced the coefficient of friction from around 0.45 to0.11 and that there was a formation of thin film (probably PTFE)on the contacting surfaces. With the addition of the PTFE thewear increases with temperature, but only relatively slowly. Sur-face cracks on the wear surfaces that would normally occur onPOM and PA66 were eliminated with the inclusion of the PTFE. Thefibre-reinforced PA66 was reinforced with 30 wt.% carbon-fibre or20% aramid-fibre. It was found that the reinforcements increasedthe wear rate by a factor of 10 over unreinforced PA66. Usingcarbon-fibres significantly reduced the coefficient of friction, whichenabled the contact to carry higher loads and slips before the melt-ing point of the matrix was reached. All the reinforced materialsproduced large flake-like wear debris suggesting a thin surface layerformed over the stiffer underlying material during running, partlyprotecting it.

Recently there has been growing interest in wear and fric-tional properties of PA46 (also known as Nylon 46) and its fibrecomposites [13–20]. Kurokawa et al. [14] compared a number ofcarbon-reinforced polyamides for gear applications. They foundthat PA66 and PA46 composites failed by tooth wear in contrastto PA12 and PA6 composites which failed by fatigue with low wear.They attributed the higher wear of PA46 composites to its high crys-tallinity. Xiang et al. [15] studied PA46/HDPE blends. They foundthat PA46 had a high friction coefficient of up to 0.7 and wear fail-ures in a ring-on-block test characterised by surface melting andmaterial flow. PA46 with glass fibre reinforcement was investigatedby Unal et al. [16] who again found high friction coefficients anda high wear rate in comparison with PA66 composites and POMthough not as high as for PPS. They reported similar behaviour inwear test against unsaturated polyester composites [16] and againststeel [17].

The purpose of the present work was to test polyamide 4,6(PA46) and its aramid fibre composites (6 wt.%, 12 wt.% and 15 wt.%)using the twin-disc wear test rig and to ascertain their wear andfrictional properties by testing under a wide range of loads, veloc-ities and cycles as well as assessing potential tribological damagemechanisms.

2. Experimental method

2.1. Sample preparation

The PA46 was supplied by DSM Co. Ltd., The Netherlands andthe aramid fibres were supplied by Teijin-Twaron, The Nether-lands. The fibres were in lengths of 0.25 mm and included in thePA46 in amounts of 6%, 12% and 15% by wt. The original sam-ples were injection moulded and then machined so that they haddimensions of 30 mm diameter and 10 mm face width as shown inFig. 1.

All of the samples were conditioned before testing as polyamidesabsorb water from the atmosphere. The samples were initially runin on the twin-disc wear test rig for 1 h at a slip ratio of 4%, at a 100 Nload and at 1000 rpm. This ensured that the wear measured was notthe initial breaking-in wear rate but the equilibrium wear rate. Thesamples were then dried in a vacuum oven at 120 ◦C for 72 h andthen weighed using an electronic balance. The samples were thenreconditioned back to equilibrium conditions with normal atmo-sphere. This process was speeded up by immersing them in a water

the disc samples were put on the twin-disc wear test rig they wereweighed to see what the final water content was before testing andit was found to be between 3% and 6%. The longer a sample wasleft after being taken out of the water bath before testing, the lowerwas the water content.

D.H. Gordon, S.N. Kukureka / W

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Fig. 1. Geometry of specimen (all dimensions in mm).

.2. Instrumentation

A simple twin-disc wear rig used for unlubricated rolling–slidingests of the polymer and polymer composites is shown schemati-ally in Fig. 2. Two cylindrical disc samples are mounted on spindlesontained in a friction block and a pivoted loading block. An electricotor drives the discs at speeds which can vary from 0 to 2000 rpm.

wo toothed belts and a pair of change gears are incorporated in therive line so that the two discs are driven at controlled speeds whileniversal joints are used to eliminate any loads on the blocks. Byhanging the gears in the drive chain, the relative slip ratio at theontact can be adjusted and the system can simulate the contact ofwide range of components such as gears, cams, etc. Slip ratio isefined here as the ratio of sliding to rolling velocities and variesrom 0% for pure rolling to 100% for simple sliding with one disctationary [4]. If the tangential surface velocities are v and v then,

1 2he sliding velocity is (v1 − v2) and the rolling velocity is (v1 + v2)/2nd the slip ratio is the ratio of these two values. Loads were appliedy a dead weight to the upper, pivoted loading block to provide aormal force between the two discs of up to 1 kN. The lower block

ig. 2. Schematic view of the twin-disc wear test rig: 1, drive motor; 2, toothed belt;, universal joint; 4, friction measuring block; 5, strain gauges to measure horizontalriction; 6, loading and wear measuring block; 7, pivot; 8, speed change gears; 9,oading bar; 10, wear linear variable displacement transducer (LVDT); 11, top testisc; 12, bottom test disc.

ear 267 (2009) 669–678 671

is mounted on vertical leaf springs and strain gauges on these wereused to determine the sliding frictional force by noting the horizon-tal force on the lower disc. Wear of the samples was measured bydetecting the displacement of the upper block using a spring loadedlinear variable displacement transducer (LVDT) mounted above itto record the displacement of the disc centres. The sensors werecarefully calibrated and the friction and wear signals were con-verted into digital data and acquired by a computer logging systemto provide a continuous record of friction and wear during the tests.

2.3. Test procedure

The samples were cleaned with methanol before testing toremove any grease from the contact surface. The disc samples werethen remounted on the shafts in the twin-disc wear test rig inthe same positions as before and tested. Tests were run for rollingspeeds of 500, 1000 and 1500 rpm, for a range of loads between300 and 600 N, at a variety of rolling cycles between 2.5 × 103 and2.16 × 106, all at a slip ratio of 2%. The tests were all carried out ata slip ratio of 2%, since initial tests at a slip ratio of 4% appearedto create too high temperatures as melting occurred on the PA46samples, which was felt was unrealistic. During the tests the wearprocess was closely observed and the friction and wear data werecollected by an on-line data acquisition system, stored for laterretrieval and also displayed on a computer screen. After the teststhe samples were then re-dried in a vacuum oven at 120 ◦C for 72 hand re-weighed so that the mass loss could be recorded. A largenumber of material, speed and loads combinations were studied togive a wide ranging set of results and typical representative sam-ples are presented here to illustrate the appropriate mechanisms.Generally only one set of two disc samples of each type of materialwere tested for any particular rolling speed and load value, to givea wide ranging set of results because of lack of enough material tobe tested. However, previous work has shown that these tests arebroadly reproducible [1–9].

2.4. Contact temperature

Since the properties of polymers, both mechanical and tribologi-cal are far more sensitive to temperature than those of metals, it wasconsidered essential, to measure the maximum contact tempera-tures so that their effects on the wear behaviour could be examined.An Agema 900 Thermal Imager was obtained so that we were ableto take temperature measurements at particular periods of timewhile the experiments were being run on the twin-disc wear testrig and so the maximum contact temperature for that particulartime between the two disc samples could be measured. The Agema900 Thermal Imager is a long wave-band system (8–12 �m) which iscapable of real time scanning and analysis or the capture of imagesat up to 25 frames/s for subsequent analysis. It is cryogenicallycooled using liquid nitrogen and has a temperature range of −30to +1500 ◦C and a sensitivity of 0.08 at +30 ◦C. The Thermal Imagerhas a 20◦ × 10◦ lens which has a geometric resolution of 1.5 mradand a minimum focus of 0.6 m, which is the distance that the ther-mal imager was placed from the disc samples on the twin-disc weartest rig to give the ideal spatial resolution for the thermal images.Thermal images were taken at different time intervals and recordedonto a computer. ThermaCAM researcher software on the computerindicated which areas of the image were different temperatures

by prescribing each temperature with a different colour rangingfrom blue signifying the coolest regions up to white for the hottestregions. Next to the image was a temperature scale which signifiedwhat temperatures were being seen according to their colours, andthe highest temperature seen at that particular time period wasnoted.

672 D.H. Gordon, S.N. Kukureka / Wear 267 (2009) 669–678

Fr

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ig. 3. (a) PA46 (1000 rpm; 2% slip ratio; 400 N load). (b) PA46 (1000 rpm; 2% slipatio; 400 N load).

.5. Damage mechanisms

A Leica optical microscope was used to examine the potentialribological damage mechanisms on the wear surfaces of all theifferent samples tested and optical micrographs were obtained.

. Results and discussion

.1. Measured wear and friction

The samples on the twin-disc wear test rig were run for a setumber of cycles or to failure whichever occurred first. Wear rate

s defined here as the average depth of material removed from eachisc per rolling cycle. The wear and friction behaviour was given byhe on-line data acquisition system and typical examples are shownn Figs. 3–6 for the unreinforced PA46 and its three composites mea-ured against the number of cycles. All the wear curves show annitial apparent negative wear at the start of the tests produced byhermal expansion of the discs and the blocks. The coefficient ofinear thermal expansion, ˛, for PA46 (grade TW341) from +23 to80 ◦C is 1 × 10−4 K−1 and from +80 to +180 ◦C is 1.2 × 10−4 K−1. Toalculate the size of the thermal expansion the following equations used:

L = ˛L�T (1)

where �L is the change in length, L is the original length andT is the temperature change. From the measured contact temper-

ture results (see Section 3.2), temperatures of up to 140 ◦C were

Fig. 4. (a) PA46 + 6% aramid fibres (1000 rpm; 2% slip ratio; 500 N load). (b) PA46 + 6%aramid fibres (1000 rpm; 2% ratio; 500 N load).

being reached within the first few thousand cycles, therefore it iscalculated that the samples expanded out radially distances up to0.10 mm and up to 0.14 mm when temperatures of 180 ◦C were laterreached.

However, this section of the wear curves has to be disre-garded and the steady wear rate was measured from the endof this period. The steady wear rate results for the PA46 sam-ples ranged from 4.07 × 10−8 mm/cycle to 7.55 × 10−5 mm/cycle.The steady wear rate results for the PA46 + 6% aramid fibre sam-ples ranged from 7.17 × 10−8 mm/cycle to 2.68 × 10−4 mm/cycle;the steady wear rate results for the PA46 + 12% aramid fibre sam-ples ranged from 2.60 × 10−8 mm/cycle to 6.66 × 10−5 mm/cycleand the steady wear rates for the PA46 + 15% aramid fibre sam-ples ranged from 1.67 × 10−7 mm/cycle to 2.35 × 10−4 mm/cycle.The results show that the PA46 + 15% aramid fibres generally hadthe highest steady wear rates. It was noticed that the wear curvesfor the PA46 disc samples bottomed out at a higher number of cyclesthan the PA46 + aramid fibre samples whatever the load and veloc-ity. This may be due to the aramid fibres restricting the thermalexpansion of the PA46 matrix.

The coefficients of friction were calculated by the tangential slid-ing force measured by the strain gauges mounted on the lower blockdivided by the normal load. As can be seen from Figs. 3(b)–6(b) thegraphs can be divided into two regions: an initial rapid increase toa high value and then a slow decay to lower values of coefficients of

friction. The friction curves shown in Figs. 4(b)–6(b) finally reduceto zero as these samples failed. The initial rapid increase regionsappear to follow the initial apparent negative wear regions at thestart of the tests produced by the thermal expansion of the discs and

D.H. Gordon, S.N. Kukureka / Wear 267 (2009) 669–678 673

FP

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imum contact temperature between 120 and 180 ◦C. The initialrapid increase seems to mirror the initial rapid increase of the aver-age coefficients of friction results. Within experimental error thechanges in maximum contact temperature are similar for the dif-

ig. 5. (a) PA46 + 12% aramid fibres (1000 rpm; 2% slip ratio; 400 N load). (b)A46 + 12% aramid fibres (1000 rpm; 2% slip ratio; 400 N load).

he blocks. Average coefficients of friction were calculated acrosshe whole of the friction results including both of the two differentegions. The average coefficients of friction results of the PA46 sam-les ranged from values of 0.08 to 0.38. For the PA46 + 6% aramidbre samples the average coefficients of friction ranged from 0.13o 0.36; for the PA46 + 12% aramid fibre samples from 0.04 to 0.36nd for the PA46 + 15% aramid fibre samples from 0.05 to 0.36. Theesults show that all the different types of samples have similaralues of coefficients of friction.

.2. Measured contact temperature

The thermal imager was used to measure the maximum con-act temperature for the different types of samples tested underhe different ranges of the different speeds, loads and number ofycles. It was generally assumed that the maximum temperatureeasured on the scale next to the images taken at that particular

ime period was the contact temperature of the two discs which wasndicated as the colour white on the scale. The white coloured areas,ndicating the maximum temperature, seen between the two PA46isc samples initially covered an area of approximately 19 mm2, ris-

ng up to approximately 30 mm2 with increasing time. For the twoA46 aramid composite disc samples this area was initially approxi-

ately 12 mm2, rising up to approximately 40 mm2 with increasing

ime.Figs. 7 and 8 show the change in the maximum contact temper-

ture of the PA46 and its aramid fibre composite samples duringhe tests at a speed of 1000 rpm and at two different loads of 400

Fig. 6. (a) PA46 + 15% aramid fibres (1500 rpm; 2% slip ratio; 400 N load). (b)PA46 + 15% aramid fibres (1500 rpm; 2% slip ratio; 400 N load).

and 600 N. In a similar way to the coefficient of friction results,the graphs can be divided into two different sections: an initialrapid increase to values of maximum contact temperature between80 and 100 ◦C and then a slow increase to higher values of max-

Fig. 7. Temperature for PA46 aramid fibre composites (1000 rpm with a 2% slip ratio;400 N load).

674 D.H. Gordon, S.N. Kukureka / W

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at the highest loads and velocities the steady wear rates values

ig. 8. Temperature for PA46 aramid fibre composites (1000 rpm with a 2% slip ratio;00 N load).

erent types of sample tested. The one main exception to the resultss the PA46 + 15% aramid fibre samples which at increasing loads andelocities created much higher temperatures of over 200 ◦C whichenerally led to failure of the samples. It is thought that was due tohe increased percentage of aramid fibres in the samples possiblyontacting which could have increased the friction and thereforeaused higher temperatures. The initial temperatures, as indicatedy the different colours on the thermal images, seen on the areasf the disc samples not in contact with each other were initiallypproximately between 30 and 50 ◦C, increasing up to between0 and 180 ◦C with increasing time, for all the different types ofamples tested.

It should be noted that examination of the thermal images takenf the different types of samples over time, showed that besideshite coloured areas showing where the two discs were in contactith each other and therefore the maximum contact tempera-

ures, there were also patches of white at different sections onhe wear surfaces on the top disc samples, ranging in area ini-ially from approximately 10 mm2 up to approximately 50 mm2,or all the different types of samples. It is assumed that this washowing the formation of tribological damage mechanisms such asracks/fractures occurring on the wear surfaces or small layers ofhe polymer sliding, due to the high temperatures being created athe top samples rolling at a velocity 2% slower than the bottom discsnd so increasing the friction and certainly the longer the tests wereun the more noticeable and frequent these white patches became,eading ultimately to failure of many of the samples, especially theomposite samples.

.3. Tribological damage mechanisms

The wear surfaces of the different types of samples tested underhe range of conditions were examined by optical microscopy. Fig. 9hows typical tribological damage mechanisms of the differentypes of samples. Fig. 9(a) and (b) shows typical wear surfaces ofwo PA46 samples that were tested at different loads and veloc-ties. As can be seen transverse cracks have appeared across the

ear surfaces as seen in previous work on POM and PA66 [2–4,6–9].ig. 9(c)–(h) shows typical wear surfaces of the PA46 + aramid fibreomposites that were tested at different loads and velocities. Asan be seen the transverse cracks across the wear surfaces are not

s prevalent as on the unreinforced PA46 samples and the mainource of tribological damage appears to be pitting on the wearurfaces, which appears on both the top and the bottom samplesested. This may be due to the fact that the aramid fibres are sep-

ear 267 (2009) 669–678

arating away from the polymer matrix, although there is no directexperimental evidence to support this. Under high loads and veloc-ities the aramid fibre composites, especially the 15 wt.% samples,had a tendency to fracture completely through the samples frominitial cracks being created and this led to failure. Typical exam-ples of these cracks/fractures are seen in Fig. 9(i) and (j) whichshows micrographs of the sides of two of the disc samples showingcracks/fractures initiating both on the wear surfaces and also fromthe key-hole cut in the samples to secure them on to the wear test-rig, which would have caused a stress concentration (see arrow inFig. 9(j)).

3.4. Pressure × velocity results

Because there is a wide range of tribological experimental tech-niques available to determine the wear and friction behaviours ofdifferent materials, one way of comparing the different techniquesis to plot the results as pressure × velocity (PV) graphs. The resultsof this for the present work are seen in Figs. 10–12. Fig. 10 showsthe steady wear rates of all the different types of samples testedplotted as a PV graph. As can be seen at low loads/pressures andvelocities the majority of the samples have wear rates averagingaround ∼1 × 10−6 mm/cycle. However as would be expected withincreasing loads/pressures and velocities the wear rates increase upto values of ∼1 × 10−4 mm/cycle. This is especially noticeable withthe PA46 + 15% aramid fibre samples.

Fig. 11 shows the average coefficients of friction of all the dif-ferent samples tested plotted as a PV graph. As can be seen atlow loads/pressures the average coefficients of friction values varybetween ∼0.15 and ∼0.4. However it is noticeable that at highloads/pressures and velocities the values drop down to valuesbetween ∼0.05 and ∼0.1, especially with the PA46 + 15% aramidfibre samples.

Fig. 12 shows the mass loss of all the different types samples(both top and bottom) tested plotted as a PV graph. The majorityof the results fall within values of ∼1 × 10−7 and ∼1 × 10−5 g/cyclewith no clear distinction between the different types of samplestested and only at some of the higher loads/pressures and velocitiesare there a few mass loss values above the majority values.

Fig. 13 shows the steady wear rate of all the different types ofsamples tested plotted against their mass loss. As can be seen thereis generally a linear relationship between the two sets of data withagain no clear distinction between the different types of samplestested.

3.5. General discussion

PA46 is a relatively new engineering polymer which is expectedto be superior to other nylons used in engineering parts and com-ponents such as gears because of its higher modulus and strengthas a result of being highly crystalline and the purpose of this exper-imental study was to investigate its tribological properties to see ifthis was the case.

From Fig. 10 it is clear that the steady wear rate increasesreasonably linearly with increasing load and velocity. This typeof behaviour has been seen with PA46 + 30% glass fibres under-taking sliding behaviour carried out on a pin-on-disc machinesliding against a 15% long fibre reinforced unsaturated polyesterdisc [16,17]. It is also clear that there is no real distinction betweenthe different types of samples tested and their steady wear ratesover the range of loads and velocities used. As would be expected

increase markedly up to ∼1 × 10−4 mm/cycle especially for thePA46 + 15% aramid fibre samples. Severe wear corresponds to thegeneration of excessive surface temperatures on the disc surfacesdue to increased frictional heating and it seems that the transition

D.H. Gordon, S.N. Kukureka / Wear 267 (2009) 669–678 675

Fig. 9. (a–j) Typical damage mechanisms on wear surfaces of discs, both top and bottom samples, at a slip ratio of 2% (direction of friction from left to right; bar on micrographsrepresents 1 mm).

676 D.H. Gordon, S.N. Kukureka / Wear 267 (2009) 669–678

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Fig. 12. Mass loss vs. pressure × velocity for all samples with a 2% slip ratio.

ig. 10. Steady-state wear rate vs. pressure × velocity for all samples with a 2% slipatio.

oint is associated with the softening temperature of the disc mate-ial. Provided the temperature is below this level, mild wear occursith very low rates of material loss; above this level severe wear is

ound. The increases in load also produce higher wear rates due tohe higher Hertzian contact stresses exceeding the yield points ofhe samples. Hertzian stress analysis for two discs in line contactives the maximum contact pressure using the following expres-ion:

o =(

PE∗

�R

)0.5(2)

here P is the load per unit length, E∗ =(1 − n2

1/E1) + (1 − n22/E2))

−1where E is the elastic modulus

nd � is Poissons ratio and R = (R−11 + R−1

2 )−1

where R1 and R2 arehe radii of the disc samples. Using the equation the calculated

aximum contact pressures for the different types of samples areiven in Table 1.

The high contact pressures as shown in Table 1 experienced byhe PA46 + 15% aramid fibre disc samples due to their high elastic

odulus value, might also explain their severe wear.From Fig. 11 it is clear that the average coefficient of friction

ecreases reasonably linearly with increasing load and velocitynd again this type of behaviour has been seen with PA46 + 30%lass fibres undertaking sliding behaviour as mentioned above16,17]. The heat generated due to friction at the wear surfaces

ig. 11. Average coefficient of friction vs. pressure × velocity for all samples with a% slip ratio.

Fig. 13. Steady wear rate vs. mass loss with a 2% slip ratio.

could explain this behaviour, as polymers are viscoelastic materials,their mechanical and physical properties are very sensitive to hightemperatures. As the temperature increases during the friction pro-cess and the material softens, this affects the adhesion and transferbehaviour of the polymers and hence leads to a decrease in the shearstrength of the polymers and to the decrease in the coefficient offriction value [16]. Again it is also clear that there is no real distinc-tion between the different types of samples tested and their averagecoefficients of friction over the range of loads and velocities tested.With increasing load and velocity the average coefficients of frictionvalues fall from ∼0.35 to ∼0.05, especially for the PA46 + 15% aramidfibre samples. Yamada et al. [13] report a friction coefficient of pure

PA46 of 0.75 and 0.70 sliding on rough and smooth steel counter-faces and Cong et al. [15,19,20] report a friction coefficient of 0.77,also from sliding against steel. The average coefficient of frictionvalues reported here are much lower than these values. However,

Table 1Maximum contact pressures for the different types of samples.

Load (N)aramidfibresc

PA46a

(MPa)PA46 + 6%aramidfibresb

(MPa)

PA46 + 12%aramidfibresd

(MPa)

PA46 + 15%(MPa)

300 33.960 36.085 38.576 41.816400 39.214 41.667 44.544 48.285500 43.842 46.585 49.802 53.984600 48.027 51.032 54.555 59.136

Data obtained from DSM Co. Ltd., The Netherlands: aE = 3100 MPa, � = 0.38;bE = 3500 MPa, � = 0.38; cE = 4000 MPa, � = 0.38; dE = 4700 MPa, � = 0.38.

ka / W

twpfac

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toadsstcorosicwpapf

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D.H. Gordon, S.N. Kukure

he results presented here are from non-conformal polymer pairshere it is expected that the values would be high, since when aolymer is in tribological contact with a metal, any heat created byriction is conducted away much more efficiently so lowering theverage coefficient of friction values and yet references [13,15,19,20]ontradict this by reporting high values.

Typical maximum contact temperatures against time results arehown in Figs. 7 and 8 and seem to follow the average coefficient ofriction results which is as expected since friction should be approx-mately proportional to the maximum surface temperature, whichs the general rule for polymeric materials. The initial section of theraphs show that for all of the different types of samples tested,emperatures of around 70 ◦C were reached within 1000 cycles ofesting. The temperatures recorded then slowly increased up toalues of between 120 and 180 ◦C showing very little differenceetween the different types of samples tested with the excep-ion of the PA46 + 15% aramid fibre samples which did not have alow increase section but instead had a second rapid increase sec-ion where temperatures of over 200 ◦C were reached, especiallyt higher loads and velocities. The melting temperature of PA46s 295 ◦C and if temperatures were being reached near this valuehen the surface of the discs would soften and the wear rate wouldncrease substantially leading to catastrophic rapid failure whichas indeed the case for the PA46 + 15% aramid fibre samples. Itould also explain the low average coefficients of friction values

een by these samples at high loads and velocities due to the highemperatures created. It is also assumed the higher percentage oframid fibres in these samples would allow more of the stiff fibreso interact with each other, possibly increasing the friction and heat.

Transverse cracks, spaced unevenly, are typically seen acrosshe wear surfaces of the PA46 samples as well as the formationf polished surfaces and areas of melting especially at high loadsnd velocities. Fine powder debris was also generated by the PA46isc samples tested. The cracks form initially because of the ten-ile stresses produced by the high friction coefficient and the highurface temperatures created also play a major role in crack forma-ion. The Hertzian stress field controls the depth and spacing of theracks. Deep transverse cracks spaced evenly have been observedn unreinforced PA66 [4,7,8] after testing on the twin-disc wear testig with the cracks being about 0.5 mm apart, over a wide rangef slip ratios used except for 2%. As transverse cracks on the wearurfaces are observed on PA46 at the only slip ratio used in thisnvestigation of 2%, this poses the problem that the material is sus-eptible to cracking which is critical for components such as gearshere their surfaces are subject to a tensile bending stress. Theseropagate rapidly and all tests of polyamide (Nylon) gear pairsppear to produce failure by fracture, normally originating at theitch line but occasionally near the root which produces premature

ailure.Transverse cracks are also seen on the PA46 composites but

hey are not as prevalent as on the unreinforced PA46 samples buthey could lead to complete fracture of the disc samples leading toatastrophic failure. This behaviour could also be explained by therittleness of the PA46 matrix and the weak bonding between thebres and the matrix. In contrast to the unreinforced PA46, the PA46omposite samples generated thin flakes through the testing, buthese were not examined further. The main source of tribologicalamage on the PA46 composites appears to be pitting irrespective ofhe load or velocity used or the percentage of aramid fibres. Pittingas previously been observed with POM tested on the twin-discear test rig and was found to be repeated in a regular pattern

round the perimeter of each disc and would appear to arise fromrapped debris recirculating for many cycles [2,4]. However the pit-ing observed in the PA46 composite samples was unpredictablend it is assumed to be due from mechanical fatigue. It could alsoe due to the aramid fibres becoming detached from the polymer

ear 267 (2009) 669–678 677

matrix through an increase in load values and higher temperaturescreated.

Previous tests on PA66-aramid fibre composites on the twin-discwear test rig have shown high friction values which means that thetemperature limit of 200 ◦C would be reached at much lower loadsand slip ratios [4,7]. This limits the maximum duty for which theymay be used as gear materials. This would also appear to be thecase with the present investigation of PA46 and PA46-aramid fibrecomposites. More tests using lower loads giving a wider range ofresults would need to be done on the PA46 and PA46-aramid fibrecomposites to give a more complete set of tribological properties ofthese materials to see if they could be used as materials for gears.

4. Conclusions

The tribological behaviour of pairs of disc samples of PA46 andits aramid fibre composites in unlubricated rolling–sliding non-conformal contacts were tested on a twin-disc wear test rig undera range of loads between 300 and 600 N (giving contact pres-sures between 33.960 and 59.136 MPa), a range of rolling speedsof 500, 1000 and 1500 rpm and at a variety of rolling cyclesbetween 2.5 × 103 and 2.16 × 106, all at a slip ratio of 2%, to sim-ulate the tribological behaviour of plastic gears. The wear resultsfrom the twin-disc wear test rig generally showed an initial neg-ative wear due to thermal expansion followed by a steady wearrate. The steady wear rate results for the PA46 samples rangedfrom 4.07 × 10−8 mm/cycle to 7.55 × 10−5 mm/cycle. The steadywear rate results for the PA46 + 6% aramid fibre samples rangedfrom 7.17 × 10−8 mm/cycle to 2.68 × 10−4 mm/cycle; the steadywear rate results for the PA46 + 12% aramid fibre samples rangedfrom 2.60 × 10−8 mm/cycle to 6.66 × 10−5 mm/cycle and the steadywear rates for the PA46 + 15% aramid fibre samples ranged from1.67 × 10−7 mm/cycle to 2.35 × 10−4 mm/cycle. The friction resultsshowed a rapid increase followed by a plateau which sometimesdecreased in value. The average coefficients of friction results of thePA46 samples ranged from values of 0.08–0.38. For the PA46 + 6%aramid fibre samples the average coefficients of friction rangedfrom 0.13 to 0.36; for the PA46 + 12% aramid fibre samples from0.04 to 0.36 and for the PA46 + 15% aramid fibre samples from 0.05to 0.36.

Using a thermal imager to measure the maximum contact tem-peratures between the two disc samples created during the tests itwas found that there was always an initial rapid rise in tempera-ture up to and between 80 and 100 ◦C followed by a slow increaseto temperatures between 120 and 180 ◦C, which seemed to followthe shape of the average coefficient of friction plots. It was alsofound that with increasing load and increasing velocity, higher tem-peratures were created of up to and over 200 ◦C especially for thePA46 + 15% aramid fibre samples. The damage mechanisms for thePA46 samples showed cracks on the wear surface for low loads andvelocities while higher loads and velocities caused high temper-atures to be created and therefore melting occurred. The damagemechanisms for the PA46 + aramid fibre samples showed pittingand large cracks/fractures occurring on the wear surfaces, irrespec-tive of what load or velocity was used.

The steady wear rate vs. pressure × velocity results showed thatthe PA46 + 15% aramid fibre samples generally had the highest wearrates compared to the other types of samples especially at higherloads and velocities. The average coefficient of friction vs. pres-sure × velocity results showed that the PA46 + 15% aramid fibres

generally had the lowest values compared to the other types ofsamples whose values increased with lower loads and velocities.The average mass loss vs. pressure × velocity results showed that allthe different samples tested had generally the same values. Com-paring the steady wear rate against the mass loss there was found

6 ka / W

tpfigl

A

PbatLF

R

[

78 D.H. Gordon, S.N. Kukure

o be a linear relationship for all of the different types of sam-les tested. Generally the results showed that PA46 and its aramidbre composites could be suitable materials to be used as a plasticears if they are used under appropriate conditions, i.e. low appliedoads/pressures and velocities.

cknowledgements

This research is part of the Research Programme of the Dutcholymer Institute (DPI), Eindhoven, The Netherlands, Project Num-er #465. We would also like to thank DSM Co. Ltd., The Netherlandsnd Teijin-Twaron, The Netherlands for the supply of the materialsested, The EPSRC Engineering Loan Pool, The Rutherford-Appletonaboratories, Didcot, UK, for the loan of the thermal imager and Mrrank Biddlestone for his technical assistance.

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