The influence of surface crack on the wear behaviour of polyamide 66 under dry sliding condition

Contents Introduction Experimental Results Discussion Conclusions

1. Introduction Polymeric materials have been used in numerous mechanical and medical applications due to its excellent mechanical and tribological properties. The wear characteristics in polymers are considered a very complex phenomenon and may vary largely for different polymers. Surface fatigue wear in polymers probably results from repeated stress cycles applied to material associated with asperity interactions [2]. Microscopic investigation of polyamide 66 worn surface [4] showed a number of transverse vertical cracks, which suggested that the surface cracks play an important role in surface fatigue wear.

1. Introduction The initiation of the fatigue cracks is assisted by the defects, which are responsible for stress concentration. These are the scratches, dents, marks and pits on the surface, and impurities, voids, cavities in subsurface region. Fatigue wear is already occurred under static loads and is accounted for the majority of the wear of polymer surfaces. In cyclic loading, loading-unloading cycles generate highly subsurface stresses regions which increase the tendency to initiate surface and subsurface cracks. The purpose of the study is to investigate the influence of controlled imperfections in the Polyamide 66 surface upon its wear behavior. The polymer tested against steel counterface using reciprocating test rig in dry sliding condition under constant and cyclic load conditions. The imposed imperfections consisted of vertical deep crack perpendicular or parallel to the direction of sliding.

1. Introduction: Failure Criteria of artificial Joints

2. Expremental: 2.1 Tribometer The tribometer consisted of dual six-stations wear tracks, the wear tests under constant and fluctuating load can be performed at the same time. It was designed to provide a reciprocating motion to the matting counterface at a linear constant speed. The amplitudes, means, and frequencies of the cyclic load, as well as the magnitude of static load were easily controllable.

2. Expremental: 2.1 Tribometer Friction force Fr= µ.F Sliding direction Fmean Fmax Fmin F= Fmean sin ωt 6 1 3 5 4 2 Cyclic load system: (1) eccentric cam; (2) compression spring (3) pin holder; (4) polymer specimen; (5) steel counterface; (6) imposed perpendicular crack.

Tribometer

2. Experimental: 2.2 MATERIALS & METHODS 4 2 6 5 8 3 1 7 Closed side view of the polyamide 66 specimen with imposed vertical crack. Crack Formation Device 5 X 2.5 mm Crack Rig: (1) frame, (2) polymer pin, (3) pin holder, (4) vertical guide, (5) razor, (6) horizontal sliding beam, (7) power screw, (8) hand wheel.

crack orientation (θ) with respect to sliding direction 2. Experimental: 2.4 Tests Two groups of tests were conducted to investigate the influence of perpendicular and parallel imposed surface cracks on the wear rate (WR) of Polyamide 66. Constant load tests: F=90N and 135N (b) Cyclic load tests (f = 1.5 Hz): Fmean= 90N at three load ratios (R=Fmin / Fmxa) Crack Formation Device Polymer wear surface crack orientation (θ) with respect to sliding direction Sliding direction Parallel θ=00 Perpendicular θ=900

3. results: The wear rates (WR) were calculated according to the relationship: WR= V / X mm3/ m (1) Where; V Volume loss (mm3) X Sliding distance (m) Crack Formation Device

3. results: 3.1 constant load Tests: F N nc Crack Parameters Running-in Wear Steady state Wear θ deg. Ends after km Xrun WRrun 10-4mm3/m X WR 1 90 - 49 22.3 110 13.3 2  39 15.2 100 13.2 3 20 24.9 80 14.8 4 135 44 38.1 18.1 5 52 26.2 20.5 6 30 40 45.5 30.7 7 34.9 25.2 Crack Formation Device  the crack did not totally finish at the end of the test nc number of surface cracks X total sliding distance, km Xrun duration of running-in period, km

3. results: 3.1 constant load Tests: Crack Formation Device Variation of Polyamide 66 volume loss (V) with sliding distance (X), test (6) Variation of Polyamide 66 wear rates (WR) with number of transverse cracks (nc), F= 90N.

3. results: 3.2 cyclic load Tests: Crack parameters Running-in wear Steady state wear Fmax N R nc θ deg Ends after km Xrun WRrun 10-4 mm3/m X WR 8 170 0.06 - 50 22.4 100 15.8 9 1 45 40 27.3 80 22.1 10 90 27 22 46.6 30.1 11 135 0.33 30 23.2 16 12 70 24.7 14.7 13 28 33.7 14 110 0.64 34 25 18.6 15 65 19.9 24.4 20.4 Crack Formation Device

3. results: 3.2 cyclic load Tests: In order to get deep understanding to these results, Relative Change in Wear rate (RCW) due to existing of surface crack was calculated using Equation (2) and summed in Table. RCW=WR cracked specimen/WR uncracked specimen (2) Crack angle (θ) Relative Change in Wear rate (RCW) Constant Load Cyclic Load R=0.06 R=0.33 R=0.64 00, Parallel 1 1.22 0.92 1.07 900, Perpendicular 1.11 2.08 1.40 1.10 Crack Formation Device Perpendicular surface cracks have a considerable effect on wear rates of polymer, Parallel cracks showed a minor effect. Higher wear rates found at R= 0.06

3. results: 3.2 cyclic load Tests: Crack Formation Device Effect of surface crack angle (θ) on steady state wear rate (WR) for different cyclic loading ratios (R), Fmean= 90N and f = 1.5 Hz

Polyamide 66 wear pin surface after 80 km of sliding, test (1). 3. results: 3.3 Photography & optical microscopy: (a) Uncracked polymer: 50X 500 µm sliding direction wear grooves Polyamide 66 wear pin surface after 80 km of sliding, test (1). Crack Formation Device

3. results: 3.3 Photography & optical microscopy: (b) Cracked polymer: Polyamide 66 wear pin side view after 10 km of sliding, θ= 900, test (13). Sliding direction 500 µm 20X 60X 500 µm sliding direction wear grooves Polyamide 66 wear pin surface after 20 km of sliding showing crack face fracture, pitting, trapped wear debris and wear grooves, test ( 3). trapped wear debris crack face fracture pitting Crack Formation Device

3. results: 3.3 Photography & optical microscopy: 50X friction tracks 1 mm b 1 mm sliding direction a Photography (a) and optical microscopy (b) of pin surface of test (7) after 20 km sliding distance.

High density of wear grooves 3. results: 3.3 Photography & optical microscopy: 50X 500 µm Sliding direction Wear grooves sliding direction High density of wear grooves Test (1), Wear pin surface. Test (7), Wear pin surface. The arrangement of the surface grooves that found in perpendicular cracked pins is different to those in uncracked and parallel cracked polymers, it suggested that the polymer has transferred non-homogeneously to the counterface and not in a uniform manner as occurred with uncracked polymers. Consequently, relative change in wear rate due to perpendicular surface crack is expected.

3. results: 3.3 Photography & optical microscopy: 3X Sliding direction 1 cm transfer film 3X Sliding direction 1 cm transfer film Test (1), Wear track 1 cm 3X transfer film sliding direction Test (7), Wear track Surface crack may change the topography and the conformity of the polymer which, in turn, affect the characteristics of the formed transferred layer.

4. discussion: The probable form of surface fatigue wear may well explain the occurrence of high wear rates in polymer bearing applications under cyclic loading which are more than two orders of magnitude greater than wear rates under constant load on laboratory tribometers. Scratches and surface cracks upon the polymeric parts after service in such applications were detected and reported in many literatures [11, 13]. The present work has demonstrated that even a single vertical crack in the rubbing surface can increase the wear rate of the polymer significantly. This applies particularly to imposed cracks marked perpendicular (θ=900) to the sliding direction, whereas parallel cracks (θ=00) show a much smaller effect. Wear rates increase rapidly as the polymer subjected to dynamic load s while constant loads have little effect.

4. discussion: The imposed perpendicular crack is considered as a nucleation of further subcracks that nucleate from the original imposed crack and propagate long enough to link up with other subsurface cracks until eventually one crack large enough to break from the bulk polymer as wear debris. Under cyclic loading, loading-unloading cycles generate highly subsurface stresses regions which increase the tendency to initiate surface and subsurface cracks. Higher wear rates were found in load ratio (R=0.06) which corresponding to a maximum loading peak value (Fmax= 170N). The unloading phase has a minimum value and the ability of generated wear debris to escape from the worn surface increased. Thus fresh polymer surface will be exposed to the metallic counterface at every pass and the overall wear will be expected to increase specially for cracked pin.

4. discussion: The effect of crack was found significantly during the running-in wear. Results also indicated that steady state wear was affected by the presence of surface crack although, in some cases, it has been totally vanished at the end of the primary stage of wear. Test F (N) nc Relative Change in Wear (RCW) Running-in Steady state (1) (3) 90 - 1 1.11 (4) (6)* 135 1.19 1.69 (7)* 3 1.56 1.89 * crack was totally vanished after running-in period RCW due to perpendicular imposed crack, constant load test

4. discussion: Test R (Fmin/Fmax) nc Relative change in wear (RCW) Running-in Steady state (8) 0.06 - 2.08 1.90 (10)* 1 (11) 0.33 1.45 1.40 (13)* (14) 0.64 0.98 1.09 (16)* * crack was totally vanished after running-in period RCW due to perpendicular imposed crack, Cyclic load test

4. discussion: The arrangement of the surface grooves that found in perpendicular cracked pins is different to those detected in uncracked polymers, it would suggested that the polymer has transferred non-homogeneously to the counterface and not in a uniform manner as occurred with uncracked polymers. The effect of surface crack on the transfer film formation is not limited to the existence of surface crack during the initial stage of wear. It may extend to the steady state stage of wear even after the crack becomes totally vanished during the running-in period.

4. conclusions: The effect of surface crack on the polyamide66 was found related to the orientation of crack with respect to the sliding direction. Perpendicular cracks resulted in about 10 to 90% higher wear rates compared to parallel cracks. The effect of perpendicular surface crack was also found sensitive to the nature of applied load. Under cyclic loads, polymer showed a significant increase (up to 90%) in wear rates than those found under constant load. Wear mechanisms were related to progressive surface fatigue due to surface crack existence which affected by load ratio (R); higher effect produced at lower load ratio. The increase in wear rates of polymer due to the existence of perpendicular surface cracks was found directly related to the number of cracks on the rubbing surface.