WEAR MECHANISMS OCCURRING IN PLASTIC GEARS B. GOFFIN, R. LEGRAS and D. DEBIER CERTECH (CEntre de Ressources TEchnologiques en Chimie) - UCL, Zone Industrielle C, B-7180 Seneffe, BELGIUM; e-mail: benedicte.goffin@certech.be SUMMARY The surface and bulk of injection moulded spur gears made glass fibres reinforced polyamide internally lubricated with polytetrafluoroethylene PTFE have been characterised before and after wear testing on a gear test rig. The use of irradiated PTFE proved to be essential to obtain the best wear behaviour. The evolution of surface morphology across testing time allowed to visualise the formation and breaking of a film layer between the two running gears. This film, formed through the melting of the polymers at the surface, plays an important role in polymer tribology in reducing the dry wear. In long lasting tests, the film is not only observed around the pitch line, but everywhere at the worn tooth surface. Generated wear debris could be parts of the film, broken through high shear and abrasive effect of fibre fragments, as confirmed by thermogravimetric and elemental analysis. The conclusions could be extended to various polyamide based composite gears. Running dissimilar materials against each other showed that the film is formed from the bulk of the gear. Therefore the film formation does not involve material transfer from one surface to its counterpart.

Keywords: polymer gear, wear mechanism, transfer film, dry wear behaviour, polymer composites

1 INTRODUCTION Plastic gears provide unique advantages over gears made of metal. Injection mouldable thermoplastic composites are being used increasingly in gear and bearing applications [1 - 3]. Dry wear resistance is one of the main reason for choosing polymer composites. Indeed in addition to a polymer matrix and a reinforcement, composites intended for tribological applications contain an internal lubricating component which allows to eliminate the need for external lubrication [4].

A huge amount of polymer tribological data is available. However, this data is mostly based on plastic running against metal in a pure sliding configuration [5 - 7]. For plastic-on-plastic gear pairs, the wear behaviour is difficult to predict [8]. Relatively little research has been carried out to study the wear mechanisms of polymer composites operating against themselves in non-conformal rolling-sliding contact [9]. This work aims to understand the key morphological parameters involved in the wear behaviour of thermoplastic composites gears.

2 EXPERIMENTAL Dry wear tests were conducted on a rig running spur gears against each other at a speed of 1000 rpm and a torque of 10 Nm. Indeed, it is important in a wear test to ensure representative service conditions [10 - 12]. These testing conditions corresponds to a maximum sliding speed of 1 m/s and a contact stress of 46 Mpa.

The morphological characterisation of the worn surface of composite gears has been done by environmental scanning electron microscopy (ESEM) using a detector for backscattered electrons (BSE) and X-ray dispersive elemental analysis (EDX). The microscope is a Philips XL30 ESEM equipped with a field electron gun (FEG). For the observations, no sample preparation is required.

Differential scanning calorimetry was performed on a DSC 821e from Mettler Toledo.

Thermogravimetric analysis (TGA) was performed on a TGA 850 supplied by Mettler Toledo.

Since polyamide (PA) and polyacetal (POM) account for 85 % of gear materials used [13], gears based on polyamide 6.6 matrix (PA 6.6) were chosen as model compound. The PA matrix was reinforced with 30 % glass fibre (GF). Polytetrafluoroethylene (PTFE) was used as internal lubricant (15 %). 3 RESULTS

3.1 Effect of the PTFE type

The type of internal lubricant was examined. Figure 1 shows the observed morphology of two compounds having the same composition (PA 6.6 with 15% PTFE and 30 % GF) made from two different types of PTFE.

The PA matrix appears in black, while the PTFE corresponds to grey regions as confirmed by EDX analysis. Pellets were cut transversally. Therefore the fibres appear as bright spots perpendicular to the plane of the picture.

In the case of non-treated PTFE, the dispersion is heterogeneous and large chunks of PTFE are observed (Figure 1a). Using irradiated PTFE leads to a fine and homogeneous dispersion of PTFE in the PA matrix (Figure 1b). The type of PTFE appears thus as an important parameter. The use of irradiated PTFE proved to be essential to obtain the best wear behaviour.

a) non-treated PTFE

b) irradiated PTFE

Figure 1: Morphology of PA 6.6 pellets containing 30% GF and 15% PTFE (250x)

3.2 Influence of the test duration

We characterised gear surface and bulk before and after running in order to determine the major wear mechanisms. Gears made of the model compound (with irradiated PTFE) were tested against themselves during 24, 48 and 72 hours. Different worn areas were examined, from the tip of the tooth, to the pitch line and the root. Figure 2 shows the morphology of a worn tooth surface after 24 hours of testing.

After 24 hours, a film covers an important surface of the tooth around the pitch line. At the tip and near the root, this film seems less developed and is oriented due to wear as indicated by the white arrow on Figure 2.

An estimation of the temperature generated near the surface was calculated using the low melting peak of PA 6.6 observed by differential scanning calorimetry (DSC). Indeed the melting behaviour of polyamide is closely linked to the thermal history experienced by the material sample [14]. The results indicate a temperature of about 220 °C. Nevertheless this temperature corresponds to a sample thickness of about 100 µm. It is not the temperature at the extreme surface which is usually called the flash temperature. Furthermore the DSC method is limited by the melting point of PA6.6. As a melted polymer film is observed on the worn surface, it is likely that the flash temperature is above the melting point of PA 6.6 (~260 °C).

a) around the tip

b) near the pitch line

c) near the root

Figure 2: Morphology of worn tooth surface after 24 hours of testing (200x)

The EDX analysis shows that the film is enriched in fluorine when compared to the unworn surface. However, it is not composed of PTFE alone. Therefore the PA matrix and the PTFE both contributes to the film formation. The film is thus probably formed by surface melting due to high temperature and high shear conditions encountered at the interface between the gears when the teeth are in contact.

Near the root, fibres appear completely broken already after 24 hours. This could be explained by the recipro-cating movement which occurs near the root of the tooth. This mechanism is more damaging than the roll/slide wear.

After 72 hours, the morphology is different from the one observed after only 24 hours of testing (Figure 3).

The wear orientation is still observed. The film is formed everywhere at the worn tooth surface (Figure 3, a, b and c), together with broken fibres.

a) close to the tip

b) around the pitch line

c) near the root

Figure 3: Morphology of worn tooth surface after 72 hours of testing (200x)

After 72 hours, a transition occurs on the wear curves and the wear starts to increase. The damage is more severe and wear debris are generated. The TGA thermogramm of the wear debris (Figure 4) shows that PA 6.6 is degraded (a broad weight loss occurs earlier). Furthermore, the weight loss due to PTFE is not observed any more indicating that the content of PTFE is low. It is likely that PTFE is degraded. Therefore the wear debris could be parts of the film, broken through high shear and abrasive effect of fibre fragments, as confirmed by elemental analysis.

~30% fibres

~54% PA

~15% PTFE

carbon black and degraded material

wear debris 72h

72 hours24 hours

Figure 4: Thermogravimetric analysis (TGA) of a worn tooth surface after 24 and 72 hours,

and the corresponding wear debris 3.3 Various polyamide based composite gears

The formation of a film at the surface through the melting of polymers was also observed using polyamide 4.6, polyamide 6.10 and polyphthalamide as matrix materials. As previously concluded with the model compound based on a PA 6.6 matrix, both polyamide and PTFE contribute to the formation of the film.

3.4 Dissimilar mating polymers

It is well known that the primary wear mechanism for thermoplastics is adhesive wear. Adhesive wear (or interfacial wear) occurs when the counterface is smooth and is characterised by the transfer of polymer to the harder counterface (e.g. steel) [10]. The tribological properties of polymers closely relate to this transfer film formation. In this work, we showed that a film layer is formed between the contacting polymer surfaces. In order to detect any polymer transfer, the wear behaviour of the model compound against unreinforced polyacetal (or polyoxymethylene POM) was studied.

As previously observed, a film is formed at the worn tooth surface through melting of polymers. However, the interesting result is that the composition of the film at the POM tooth surface is close to pure POM. No fluorine could be detected by EDX analysis at the POM surface. At the nylon composite tooth surface, the film is made of melted PA and PTFE. Therefore, the film is formed from the bulk, and its formation does not involve material transfer from one surface to its counterpart.

4 CONCLUSIONS A morphological approach was followed in order to understand wear mechanisms occurring in plastic gears. Complementary research (e.g. thermal and elemental analysis) was performed to link the morphological observations with the wear behaviour.

A film is created during nylon based composite gear run through the melting of PA and PTFE at the surface of the tooth. This film plays an important role in the tribology of polymers in reducing the dry wear. The evolution of surface morphology across testing time helped us to visualise the formation and breaking of the

film layer between the two running gears. Running dissimilar materials against each other (PA 6.6 composite against POM) showed that the film is formed from the bulk of the gear. Thus in the case of polymer gears running against each other, the wear mechanism does not imply transfer of material from the surface to its counterpart.

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[9] Chen, Y.K.; Nukureka, S.N.; Hooke, C.J.: The wear and friction of short glass-fibre-reinforced polymer composites in unlubricated rolling-sliding contact. J. Mater. Sci., 31 (1996) 5643 – 5649 [10] Hutchings, I.M.: Tribology: Friction and Wear of Engineering Materials. Edward Arnold 1992 [11] Matériaux et contacts: une approche tribologique. Eds. Zambelli, G.; Vincent, L.: Presses Polytechniques et Universitaires Romandes 1998 [12] Neale, M.J.; Gee, M.: Guide to Wear Problems and Testing for Industry. Professional Engineering Publishing 2000 [13] Tsukamoto, N.: Argument on Plastic Gears for Power Transmission. JSME Int. J. Series C, 38 (1995) 1 – 7 [14] Quintanilla, L.; Rodriguez-Cabello, J.C.; Pastor, J.M.: Structural analysis of injection-moulded semicrystalline polymers by Fourier-transform infra-red spectroscopy with photoacoustic detection and differ-ential scanning calorimetry: 2. Polyamide-6,6. Polymer, 35 (1994) 2321 – 2328 6 ACKNOWLEDGMENT The authors wish to express their thanks to the European Union for financial support through Brite-Euram funding (contract n° BRPR – CT98 – 0703). Furthermore, the help and assistance provided by the Davall Moulded Gear Company in supplying gears, and the School of Manufacturing and Mechanical Engineering (University of Birmingham) in testing gears are gratefully acknowledged.