Abstract
Introduction
The tribological performance of grooved surfaces has been thoroughly analyzed, and such surfaces are thought to have great potential for hard-on-hard joint prosthesis. In related research, femtosecond laser-induced periodic surface structures (FLIPSSs) have been well developed to achieve grooved structures with submicron spacing (700nm) and amplitude (200nm). In this study, submicron-scale periodic grooved structures were made on SUS440C using a femtosecond laser, and its tribological performance was evaluated by both a pin-on-plate reciprocating sliding test and a ring-on-disk test.
Method
The pin-on-plate reciprocating test was performed using PAO6 (30.51cP at 37°C) as the lubricant. The pin and plate specimens were made of SUS440C and were polished to a surface roughness of 0.02μm Ra. The pin specimens were columnar in shape, and radial periodic grooved structures (700nm spacing x 200nm amplitude) were formed on the pin's outer periphery (from 4mm to 5mm in diameter).
The ring-on-disk test was performed using lubricants with different viscosity: PAO6 and PAO2 (4.60cP at 37°C). The ring-on-disk specimens were made of SUS440C and were polished to a surface roughness of 0.03μm Ra. Along the surface of the ring specimens, material was removed to create 4 elevated sections at 0°, 90°, 180° and 270°. These 4 sections were then polished and concentric grooved structures (700nm spacing x 200nm amplitude) were created along a 1.4mm circumferential path within each of these areas.
Result
Fig. 1 shows the relationship between sliding speed and friction coefficient for the pin-on-plate test, where the friction coefficient for the periodic grooved specimen was lower than that for the polished specimen under all sliding speed conditions. Fig. 2 shows the relationship between resting time and friction coefficient for the pin-on-plate test, where the friction coefficient for the periodic grooved specimen was lower than that for the polished specimen under all resting time conditions. Fig. 3 and Fig. 4 show the relationship between sliding speed and friction coefficient for the ring-on-disk test using PAO6 and PAO2, respectively. The friction coefficient for the periodic grooved specimen was also lower than that for the polished specimen under all sliding speed conditions regardless of the lubricant. In the case of PAO6, the lubrication mode for the periodic grooved specimen changed from mixed lubrication to fluid lubrication at speeds of 20mm/s or more. However, in the case of PAO2, the friction coefficient for the periodic grooved specimen was relatively high at lower sliding speed conditions.
Discussion
As human joint movement has a relatively long resting time, relatively low startup-friction performance is essential. Results shown in Fig. 1 and Fig. 2 suggest that the periodic grooved structure has the potential to improve lubrication and fluid film recovery during startup sliding. Despite results from the ring-on-disk test revealing higher friction coefficients in low-viscosity lubricant at low sliding speeds, grooved specimens still achieved a lower friction coefficient. Due to the fact that joint fluid viscosity changes with shearing speed and protein adsorption on the joint surfaces influences joint friction, we are now preparing a more clinically relevant testing system using bovine serum as a lubricant.
