The microstructure and microhardness of the GO/UHMWPE composites were examined with a high-resolution scanning electron microscope (HR-SEM; JSM-6701) and hardness testing machine (MH-5-VM, Shanghai Hengyi Precision Instrument Co., Ltd.), respectively. At least 15 points were measured by a permanent load of 10 g to obtain accurate average values of the microhardness of these samples. According to the research of Kumar, Piconi, and coworkers,[34-37] zirconia ceramic, as a joint prosthesis implant material, may have some advantages, including a low friction coefficient, no harmful effects to the human body, and a high strength and toughness. Therefore, in the DW and NS lubrication conditions, the friction and wear properties of the composite samples were investigated in the joint simulator of an improved UMT-3MT tribometer apparatus (Universal Micro Tribometer, CETR; Figure 1) by sliding against ZrO2 balls (Moh's hardness = 7.0 Pa, roughness = 0.03 μm, and diameter = 4.0 mm).
The ball-on-disc UMT-3MT tribometer [Figure 1(a)] was used to evaluate the properties of the GO/UHMWPE composites.[37, 38] The zirconia ceramic ball was fixed on the load arm, and the sample plates were fixed and submerged at the bottom of the chamber that contains the lubricating fluid [Figure 1(b)]. The chamber and sample plates were then rotated by an electrical motor controlled by a frequency converter. At the start of each wear test, pure DW or a 0.9% NS solution was introduced as the lubricant. The composites and ZrO2 balls were ultrasonically cleaned with acetone before each test. A new ball or a new position of the ball was used for each friction test. Each wear test was repeated at least three times. To compare the samples, the applied normal load (N), the testing time, the sliding speed, the frequency, and the actual contact pressure between the balls and the composites were 5.0 N, 1 h, 10 mm/s, 3 Hz, and 6.56 MPa (calculated with the Hertzian contact model[4, 39]), respectively. In the experimental apparatus, the strain gauge of the sensor was used to measure the frictional force on time, and the COF was calculated as follows:[20, 34]
where μ is the specific coefficient of friction, F is the friction force (N), and N is the applied normal load (N). After the test, the WR was evaluated with a micro-XAM three-dimensional (3D) surface profiler and was calculated as follows:[13, 20, 34, 39, 40]
where K is the wear rate, ΔV is the wear volume (mm3), and L is the sliding distance (m). In addition, all of the data values were determined with SPSS (Version 16.0), and the changes were analyzed with one-way analysis of variance (ANOVA) test for each group comparison among five levels of different GO contents.[20, 41, 42] Then, a two-way ANOVA and Tukey multiple-comparisons post hoc analysis were used to examine the effects and interaction of the reinforcement GO level and fabrication method on the measured microhardness of the composites and the effects and interaction of the reinforcement GO level and lubricant on the measured WR and scratch depth of the composites in different lubrication conditions (Table 1). Significance level (p) values of less than 0.05 indicated that there were statistically significant differences in the group comparison. The correlation between the GO level and WR was studied with the correlation coefficient calculated by Origin 8.0, and the correlation simple linear regression equation was made as follows (Table 2):
where A is a constant, B is the slope, X is the value of the independent variable, and Y is the value of the dependent variable. B = 0 and B ≠ 0 indicate that the slope of the regression line is equal to zero and not equal to zero, respectively. If there is a significant linear relationship between X and Y, the slope will not equal zero. The p value was also chosen to be equal to 0.05. Finally, the morphologies of the worn surfaces of these composites and the balls were examined by a JSM-5600LV scanning electron microscope. The X-ray photoelectron (ESCALAB 210) spectrum was detected with Mg Kα as the excitation source with a radiation of 1253.6 eV to determine the element species on the ball surface.