Torsional wear behaviours of TiO₂/PMMA film against Ti₆Al₄V alloy and alumina femoral head ball in dry condition

Abstract

The torsional fretting wear tests of Ti₆Al₄V alloy flat against alumina femoral ball in dry condition were carried out on a new high-precision torsional fretting-wear tester. The TiO₂/P-methoxymethamphetamine (PMMA)/Ti₆Al₄V pair showed higher contact stiffness, friction torque, wear damage and material transfer than that of the presentation of TiO₂/PMMA/Al₂O₃ pairs. The mechanisms of torsional wear under different counterbodies were discussed in detail. The mechanisms of torsional fretting wear of aluminium alloy are mainly oxidative wear, abrasive wear and delamination in the three fretting regimes. The torsional fretting running regimes transferred from partial slip regime (PSR) to SR were analysed; however, the mixed fretting regime was never appeared in this test. In PSR (lower angular displacement amplitudes), only a dint was found after the tests. The kinetics behaviours and damage mechanism of Ti₆Al₄V alloy under different angular displacement amplitudes with friction torque and dissipation energy, which are strongly dependent upon the imposed angular displacement amplitudes and presented in three stages, were discussed in detail.

Keywords:

• TiO₂/PMMA
• MFR
• wear

1. Introduction

The use of artificial joints for the treatment of degenerative diseases of the hip and knee is becoming widespread. Nowadays, there are several bioimplant materials available for total joint replacements (TJR). Early studies of clinical results provide evidence that loosening of one or both components of the TJR occurs in at least 10% to as high as 70% in patients 10 years after the surgery.[1] As such, three different sliding modes (tangential, radial, and circumferential) have been defined in the literature.[2,3] Torsional wear is one of the main motion modes of joints, and torsional fretting can be defined as the relative motion induced by reciprocating torsion under an oscillatory vibratory environment. Several solutions have been proposed to understand torsional contact and the mechanics of its behaviour.[4,5] Briscoe et al., for example, found that the damage brought about by torsional contact was more detrimental than those reduced by the rolling or rotating contacts.[6,7] However, the torsional wear behaviour of TiO₂/P-Methoxymethamphetamine(PMMA)/Ti₆Al₄V alloy and TiO₂/PMMA/Al₂O₃ has not been clearly discussed. In this research, a new testing method is used to reveal the damage characteristics of TiO₂/PMMA/Ti₆Al₄V alloy and TiO₂/PMMA/Al₂O₃ caused by torsional fretting wear. There have been many tribological research focused on rolling wear, reciprocating tangential sliding and pin-on-disc sliding,[8–11] and advance hip joint simulators were used to investigate the relative performance of artificial joint. These test method revealed the wear resistance of some artificial replacement materials. As a basic motion fashion, torsional motion occurs at different joints in human body such as hip joint, shoulder joint, temporomandibular joint and so on. However, relatively few studies focused on the torsion motion, and the results from previous reports cannot adequately reflect the damage originated from torsion wear.

PMMA is an amphetamine-related designer drug available in the illicit drug market and can be found by itself in ecstasy-like tablets with logos such as ‘Mitsubishi’, ‘E’, ‘Superman’ and ‘Elephant’ (EMCDDA. 2003). However, it is mostly found in combination with 3,4-methylendioxymethamphetamine (ecstasy, MDMA) or with other substances such as para-methoxyamphetamine, amphetamine, ephedrine, etc., causing serious health and social impairments.[12] In the torsional contact configuration, the resulting contact zone kinematics was found to have a very pronounced influence on the accumulation, the compaction and the displacement of the debris particles from the contact. In this research, a new testing method would be applied to reveal the damage characters of TiO₂/PMMA/Ti₆Al₄V alloy and TiO₂/PMMA/Al₂O₃ caused by torsional wear.

2. Experimental details

Torsional fretting tests of the TiO₂/PMMA in contact configuration of ball-on-flat were carried out on a new torsional fretting tester (Figure 1). Flat TiO₂/PMMA specimens (20 mm × 10 mm × 10 mm) were polished to an average roughness of R_a = 0.2 μm. Medical Al₂O₃ ceramic femoral head ball and Ti₆Al₄V titanium alloy ball (both with a diameter of 28 mm) were selected as the against counterbodies. The plate specimen TiO₂/PMMA was fixed on the upper holder to link a six-axis torques/forces sensor through a spring suspension. The three components of the torque and force (tangential, normal and transverse- x-, y- and z-direction) were measured directly at the contact interface. A ball specimen was mounted on the lower holder which was fixed on the low-speed reciprocating rotary motor system. Angular displacement of the contact pair was measured and controlled by a sensor in the motor system and was acted as the control signal fed back to the control unit of the tester. In this study, torsional fretting tests were performed under the normal load of 100 N at a constant rotary speed of 0.2º/s. The torsional angular displacement amplitudes were set as 0.1º, 0.2°, 0.5º, 2.5º, 15º and 45º, and the number of cycles varied from 1 to 1000. After testing, the fretting scars were measured by a profilometer (AMBIOS XP-2), and the morphologies of wear scar were examined by scanning electron microscope (SEM, Quanta200) and X-ray photoelectron spectroscopy (Axis ultra).

Figure 1. Schematic diagram of the torsional fretting tester under a ball-on-flat contact.

3. Results and discussion

3.1. T–θ curves analysis

Ft–D curve is used in tangential fretting and radial fretting to analyse kinetics behaviours, T–θ curve is applied in torsional fretting mode correspondingly. Figure 2 shows evolution of T–θ curves under a normal load of 90 N and angular displacement of 0.95° (Figure 2(a)–(c)) and 85° (Figure 2(d)–(f)). In both situations, the curves generally display quasi-parallelogram shape. When the angular displacement is 0.95°, the curves are much slimmer at the first 50 cycles. After 100 cycles, no considerable changes in the shape of curves are observed. And the friction torque is slightly increased with the number of cycles. When the angular displacement is 85°, the friction torques are much higher than those of 0.95°.

Figure 2. T–θ curves of TiO₂/PMMA under various torsional angular amplitudes and cycles, F_n = 100 N. (a), (c), (e) and (g) TiO₂/PMMA/Al₂O₃; (b), (d), (f) and (h) TiO₂/PMMA/Ti₆Al₄V.

In general, the slope of friction–displacement represents stiffness which is affected by system stiffness and contact stiffness. The system stiffness can be considered as a constant during the whole testing process. Therefore, the change of slope reflected the change of contact stiffness. The area enclosed by the T–θ curves represented friction energy that has been dissipated during the cycle. The evolution of contact stiffness and dissipated energy per cycle is illustrated in Figure 2(h). It can be seen that the value of contact stiffness rises rapidly at the first 200 cycles, and then it gradually reached a steady state. It is observed that the graphical representation of the energy dissipated per cycle as a function of time is similar to that of the contact stiffness. It may be explained that work hardening happened during torsional fretting wear.

In addition, it was obvious that the contact stiffness monotonically increased as a function of the number of cycles. From the slope of T–θ curves, the contact stiffness was extracted directly. Usually, the contact stiffness was found to be dependent on the loading conditions and sliding surface roughness. Increased contact stiffness may bring two effects on the torsional fretting processing. First, the increased stiffness of the tribo-pairs reduces the effect of vibrations on the slider interface. Second, plastic deformation and damage accumulation below the worn surfaces are enhanced.

3.2. Friction torque analysis

As the basic and important information in rotary motion, just like the significance of friction force in tangential wear, friction torque of torsional fretting was necessary to discuss in detail. Figure 3 displays the friction torque variation under various angular displacements and different normal loads. Generally, there were three typical stages occurred under all test conditions. In stage I (initial stage); the friction torques values were very low owing to the protection of the adsorbed and polluted surface films. After 10 or several decade cycles, the curves entered into the stage II (climbing stage), and the friction torques increased quickly. And then, in stage III (steady stage), the curves accessed a steady state, corresponded to the slight fluctuation within a narrow range.

Figure 3. Friction torque under various angular displacements and different normal loads.

Figure 3(a) shows the evolution of friction torques of TiO₂/PMMAcarried out at 100 N with various angular displacements. When θ = 0.1°, fretting run in PSR, and the friction torques kept at a lower level with approximate 4 N mm at about initial 100 cycles. Then the torque slowly rose as the increasing cycles and the value reached the steady stage with a final value of 6.2 N mm. There was a similar evolution for the torque under other angular displacements. Obviously, the process of stages I and II was shorten and torque reached the steady stage early. Compared with the curves of TiO₂/PMMA/Ti₆Al₄V alloy and TiO₂/PMMA/Al₂O₃ under 100 N (Figure 4(b) and 4(a)), the latter usually presented a larger value.

Figure 4. SEM morphologies of the alumina femoral head and Ti₆Al₄V alloy ball under lower angular displacements, F_n = 100 N, θ = 0.5°.

3.3. Wear damage analysis

Wear morphologies and profiles in the PSR are exhibited in Figure 4. The centre region is a well-known sticking zone and no relative slip occurs at the contact interfaces. This means that the low tangential force and relative high normal load were insufficient to drive relative motion at the contact centre. The wear, shaped like a narrow annulus, occurred on the contact edge. Under torsional fretting conditions, wear debris cannot easily be discharged from the contact interfaces, especially when angular amplitude is very low. Generally, the presence of a little TiO₂ does not give rise to any differences in the appearance of the worn surfaces. Therefore, the emphasis is on discussing the difference in wear behaviour for different counterfaces. It seems like the first wear step is micro-cracking of the resin between the fibres. The worn morphologies under different tribo-pairs were shown in Figures 4 and 5.

Figure 5. SEM micrographs of the worn scars under torsional fretting condition. F_n = 100 N, θ = 15°. (a), (b) and (c) TiO₂/PMMA/Al₂O₃; (d), (e) and (f) TiO₂/PMMA E/Ti₆Al₄V.

For the alumina femoral head, smooth worn surface without any trace could be observed at θ = 0.5° (Figure 4(a)), while slight ploughing could be found at the same condition with alumina femoral head and Ti₆Al₄V alloy ball contact (Figure 4(b)). At small displacements, the torsional fretting was running at partial slip regime (PSR) which means that the centre was sticking and micro-slip occurred at the contact edge.

With the increasing angular displacements, severe surface damage occurred at both tribo-pairs and different characteristics appeared. Figure 5 shows the worn photomicrography of TiO₂/PMMA under a high angular displacement of 15° and the debris morphology at the contact edge of the scar. Brown particles could be found at the contact edge of the contact. It was clear that debris is formed from the steel ball being oxidised by tribo-chemical action (Figure 5(a)). On the steel ball surface, some oxidative products cover at the wear zone and a large amount of fine oxidative debris with brown colour were found to scatter at the contact edge after 1000 cycles (Figure 5(a) and 5(b)). Similar results were mentioned in Geringer's research, which emphasised that, when linking with orthopaedic fields, these oxides could be assimilated by bone cells around a femoral stem where friction occurs between the stem and bone cement. When the test duration was extended to 10,000 cycles, a transfer film of TiO₂/PMMA/Ti₆Al₄V and TiO₂/PMMA/Al₂O₃ film was formed at the surface of the ball specimen (Figure 5(c) and 5(d)). Thus, TiO₂/PMMA adhesion also probably contributed to the large bulge observed.

The damage of TiO₂/PMMA under other test modes had been widely examined by many researchers. When a polymeric material was rubbed against metal surfaces, adhesion, abrasion and fatigue were the most recognised wear modes.

More ripples were found on the worn surface of TiO₂/PMMA /Al₂O₃ than that of and TiO₂/PMMA /Ti₆Al₄V alloy ball. Figure 6(a) showed the mean size of ripples below 5 μm × 2 μm. Delamination and cracking appeared at large angular displacement, which corresponded to obvious deformation (Figure 6(b) and 6(c)). The ripples perpendicular to the sliding direction on the scars have been discussed in early studies under the condition of sliding. In this study, perhaps there was the same mechanism due to the plastic flow of the polymer. Because the ripples on TiO₂/PMMA/Al₂O₃ were more easily broke off when they are subjected to the cross-directional shearing, more wear particles were removed and higher wear mass loss of TiO₂/PMMA would be induced. For TiO₂/PMMA/Ti₆Al₄V alloy ball contact, the characters of abrasive wear were presented adequately (Figures 6 and 7)

Figure 6. SEM micrographs of the ripple on worn scars, TiO₂/PMMA/Al₂O₃. (a) F_n = 100 N, θ = 5°; (b) F_n = 100 N, θ = 15°; (c) F_n = 200 N, θ = 45°.

Figure 7. SEM micrographs of the scratch on worn scars of TiO₂/PMMA/Ti₆Al₄V alloy ball. (a) F_n = 100 N, θ = 15°; (b) F_n = 200 N, θ = 15°; (c) F_n = 200 N, θ = 45°.

The surface morphologies of the corresponding balls were observed via optical microscope. Less material transferring on the worn surfaces of the ceramic ball specimens for the TiO₂/PMMA/Ti₆Al₄V contact was detected than that of the TiO₂/PMMA/Al₂O₃ alloy ball contact (Figure 8). It might be the reason why the severe adhesion occurred on the interfaces of the TiO₂/PMMA/Ti₆Al₄V contact. It could be seen clearly that TiO₂/PMMA had transferred to the titanium ball counterpart exhibiting morphology of concentric circles. Therefore, it could be concluded that a transfer film of TiO₂/PMMA was formed on the surface of the ball specimen. However, no titanium was found on the surface of TiO₂/PMMA/Ti₆Al₄V, quite different from the damage occurred at metal materials for the different deformation behaviours of polymer.

Figure 8. (Colour online) Optical micrographs of worn ball surface, θ = 45º, F_n = 100 N. (a) worn Al₂O₃ ball; (b) worn Ti₆Al₄V ball.

The worn profiles on TiO₂/PMMA surfaces under different angular displacements were given in Figure 8. The diameter (D) and maximal depth (H) of the worn scar were analysed in detail. The outer lines clearly demonstrated that the worn profile for the TiO₂/PMMA/Ti₆Al₄V was deeper or wider (Figure 8(a)) than that of TiO₂/PMMA/Al₂O₃ (D_A > D_T and H_A > H_T). When θ increased to a large value of 45°, the difference of the worn width (ΔD) and the worn depth (ΔH)) enlarged. In addition, some shakes occurred on the profile of TiO₂/PMMA/Ti₆Al₄V at 200 N, which proved the deep scratches.

As observed in the SEM micrographs of the untreated wear scars in Figure 8, abrasive wear was manifested by grooves, pits and scratches on the worn, flat specimen surfaces. The distinct plastic grooves and deformation, detached metallic particles and debris accumulation were clear under high magnification of the worn surfaces. The external third body particles (debris) were definitively eliminated from the contact zone and distributed in a crown flake around the fretting scar. As a result, the wear mechanisms of the Ti₆Al₇Nb alloy were abrasive wear and oxidative wear.

The differences can be explained by the following reasons. First, the Al₂O₃ ceramic ball had a higher hardness (Hv = 1920) and elastic modulus (E = 380 GPa) than that of Ti₆Al₄V alloy (Hv = 465, E = 310 GPa). According the classical Hertzian contact theory, larger elastic modulus would induce a wider and deeper contact scar. Second, the Al₂O₃ ceramic ball had a smooth surface (R_a = 0.02 μm) than that of Al₂O₃ ceramic ball (R = 0.2 μm). It has been now accepted that when the metallic surface roughness is higher than 0.05 μm, the abrasion mechanism becomes significant. Therefore, since the roughness of the titanium alloy ball is 0.2 μm, abrasion is at the origin of the transfer film. At a lower angular displacement of TiO₂/PMMA on a metal hard surface, lumps of polymer transferred to the counterbody. As for the lumpy transfer mechanism, the lumps of polymer were removed from asperity peaks and left adhering on the counterbody. The viscoelastic nature of the TiO₂/PMMA/Ti₆Al₄V induced the tearing and raised lips formation. The appearance of arc-like track in the ball surface shown in Figure 8(b) indicated the characteristics of this wear mechanism.

According to the results of the damage analyses, the damage law of flat TiO₂/PMMA under torsional fretting conditions can be schematically outlined in Figure 9. At lower angular displacement amplitudes (in the PSR), slight damage occurs. Since the damage mechanisms were almost same in the mixed fretting regime (MFR) and SR (no sticking zone in the SR), only physical models in the MFR were supplied.

Figure 9. The diameter and maximal height of the worn scar under various angular displacement and different normal loads F_n = 200 N, (D_A - diameter of the worn scar of U/A; D_T - diameter of the worn scar of U/T; H_A - maximal height of the worn scar of U/A; H_T - maximal height of the worn scar of U/T; ΔD = D_A − D_T; ΔH = H_A − H_T).

For the TiO₂/PMMA/Ti₆Al₄V contact pair, a similar evolution in friction torques occurred during the torsional fretting process (Figure 9(b)). An obvious difference, however, is that at angular displacement amplitude of 4.5° the friction torque is still present in stage I for all test cycles, and the fretting running state remains in the PSR. As such, the TiO₂/PMMA/Al₂O₃ contact pair enters the MFR earlier compared to the TiO₂/PMMA/Ti₆Al₄V contact pair. The distribution of the torsional fretting running regimes for the two TiO₂/PMMA/Ti₆Al₄V contact pairs under various angular displacement amplitudes F_n = 200 N.

4. Conclusion

A comparing analysis of torsional fretting behaviour of TiO₂/PMMA against different tribo-pairs was presented in this experiment. The running regime transfer from partial regime to gross SR as the increase of the angular amplitudes, and the mixed SR was not found in this research. The TiO₂/PMMA/Ti₆Al₄V contact presented higher contact stiffness, friction torque, wear damage and material transfer than that of the TiO₂/PMMA/Al₂O₃ contact. Damage occurred at lower angular displacement amplitudes was slighter than that of higher torsional angular displacements. The wear mechanisms of TiO₂/PMMA/Ti₆Al₄V were combined with abrasive wear and delamination. The generation of radial ripples and micro-cracking was the main damage modes for TiO₂/PMMA/Al₂O₃.

For both contact pairs, friction torques increased with the increase of angular displacement amplitude. Friction torques of the TiO₂/PMMA/Al₂O₃ contact were higher than those of the TiO₂/PMMA/Ti₆Al₄V contact under the same test conditions.