3.1. Fabrication of Microstripe Textures
In order to solve the problem of abnormal wear on bearing linings at low speeds, heavy loads, and frequent start and stop conditions, and to improve the tribological performance and lubrication characteristics of the bearings, a regular microstripe texture was constructed on the surface of water-lubricated materials. The dimensions of the microtextures were the following: the spacing L between two adjacent stripes was 5 μm, and the depth H of the stripes was 0.5 μm. The direction of the stripe microtexture was aligned perpendicular to the direction of motion of the copper ring, as shown in
Figure 4. The design of the microtexture was inspired by the microstructure of the epidermal scales of a lizard called sandfish (
Scincus scincus). The dimensions of the microtexture (L and H) were based on the scale parameters of the lizard’s epidermal scale microstructure.
The microstripe texture was fabricated onto the surface of the water-lubricated material by a hot-embossing method. The water-lubricated blocks were placed into the mold container, and the mold was then placed into the precision hot-pressing machine. Specific temperatures and pressures were applied to the mold according to the thermoplastic characteristics of the Thordon and polyurethane. After a certain forming time, Thordon and polyurethane samples featuring imprinted microstripe textures or pristine surfaces were obtained. All the samples were finally cleaned with acetone to remove residual material, dust, or other contaminants in preparation for tribological testing.
Table 1 summarizes the forming temperatures, pressures, and times for embossing the microstripe textures on the Thordon and polyurethane materials. The parameters for each material varied depending on their different thermoforming properties. Due to the poor fluidity of the thermoplastic polyurethane, it was not possible for the flow to spread evenly over a short period of time and it also could not overflow in time, resulting in the microstripe texture not being effectively embossed onto the surface of the material. Therefore, an extended pressing time for the polyurethane, as opposed to the Thordon material, allowed the melt to be fully fluidized and spread out so that the microtexture of the mold was fully embossed onto the surface of the material. The pictures of the Thordon and polyurethane blocks after hot embossing are shown in
Figure 5a,d.
The topography of the Thordon and polyurethane samples was characterized by laser microscopy, and the results are shown in
Figure 5b,e. Comparing the surface topographies of the samples, regular microstripes were successfully fabricated onto the surfaces of the Thordon and polyurethane materials by the hot-embossing technique.
Figure 5c,f show the surface profile curves of the samples truncated by the red dashed lines in
Figure 5b,e, respectively. It can be seen that the profile curves of the pristine surfaces show irregular bumps or pits, representing random micro-convex bodies and micro-pits that occur on the sample surfaces, whereas the profile curves passing through the microtextured surfaces reveal regular jagged shapes, indicating regular microstripes. The spacing between two neighboring stripes was about 5 μm, and the depth of the stripe monomer was about 0.5 μm. The results demonstrate that the microstripe texture was successfully fabricated on the surfaces of the Thordon and polyurethane materials by the hot-embossing technique.
Table 2 summarised the surface roughness (Sa) of Thordon and PU with pristine and microtextured surfaces. By comparing the roughness of the material surface, we found an interesting phenomenon that the roughness of the microtextured surface was lower than that of the pristine surface. This suggests that fabricating the microtextures on the surface of water-lubricated materials can reduce the microconvex bodies such as burrs on the surface and make the surface more regular, which leads to a lower roughness on the surface of the material. This is due to the fact that the pristine surface has a higher adhesion force than the microtextured surface (discussed in the following
Section 3.3), which leads to part of the materials adhering to the mold during mold opening process without fully separating, resulting in some irregular burrs on the pristine surface.
3.3. Influence of Microstripe Texture on the Adhesion of Materials
To investigate the effect of the microstripe texture on the surface adhesion properties of the water-lubricated materials, the adhesion force between the samples and a silicon AFM tip was measured using atomic force microscopy (Dimension Edge, Bruker, Billerica, Germany). A sharp silicon tip of a commercial AFM cantilever was used to characterize the adhesion and friction properties of the sample surface in all the AFM experiment. A schematic image of the working principle of the typical AFM measurement is illustrated in
Figure 7a. Atomic force microscopy allows the measurement of adhesion by recording force–distance curves [
23]. Adhesion analysis of the sample surface was carried out in contact mode, i.e., the cantilever was fixed in the X and Y positions while bending in the Z direction. The force between the cantilever tip and the sample surface was calculated from the spring constant and the deflection sensitivity of the cantilever tip as it approached or detached from the sample surface.
Figure 7a illustrates the theoretical force–distance curve for the measurement of adhesion forces using AFM. The elastic constant of the cantilever was obtained from the manufacturer (0.4 N/m). All adhesion measurements for the experiment were carried out at a ramp frequency of 1 Hz with a velocity of 3.8 μm/s.
The silicon AFM tip moved towards the water-lubricated material sample and contacted the sample surface with a pre-defined load. Afterwards, the AFM cantilever was retracted until the silicon tips completely detached from the sample surface. Four typical force–distance curves recorded in this manner are shown in
Figure 7c–f. The blue lines in the figure indicate trace motion, while red lines indicate retrace motion. The minimum value of the retrace force defines the adhesion force required to lift the AFM tip from the sample surface. The shaded area between the zero and retrace lines is defined as the adhesion energy required to disengage the probe from the sample surface [
24]. Each specimen was tested at five arbitrary positions, and the adhesion force was measured five times at each point. The average of 25 adhesion tests was taken as the adhesion force of the sample.
Figure 7 clearly shows that the adhesion force of the Thordon with a microstripe texture on the surface was 80 nN, which is significantly lower than the adhesion force of 119 nN for the Thordon with a pristine surface, leading to a reduction of about 32.8%. In the case of polyurethane, the magnitude of adhesion force reduction was approximately 47.7%. For the adhesion energies, the adhesion work of the Thordon on the pristine surface was 166.8 fJ, and that of the Thordon with microstripe texture was 59.7 fJ, representing a reduction of about 64.2% in the adhesion work. The value for polyurethane was approximately 38.9%. The results show that embedding a microstripe texture on the surface of water-lubricated material helps to reduce the adhesion on the material surface, thereby attenuating the occurrence of adhesive wear.
3.4. Comparison of Tribological Properties between Pristine and Microtextured Surfaces
Microscopic measurements recording friction vs. normal force curves on Thordon and polyurethane with pristine and microstripe-textured surfaces were recorded by AFM. The results are shown in
Figure 8. The scanning size was set to 50 μm × 50 μm. For each measurement, a friction loop [
25] was recorded for the AFM tip scanning in the forward and backward directions with a varying normal force. The averaged friction forces for both directions were calculated. For each sample, three different positions were analyzed, and the results were averaged into each single data point in
Figure 8. Increasing the defined normal force
Fload and recording the frictional force
Ffric, the friction coefficient
μ can be calculated by linearly fitting the applied load and its corresponding friction data (
Ffric =
Fadh +
μ ×
Fload).
As shown in
Figure 8, the data show a nearly linear relationship between the normal loads and friction forces. The friction coefficient
μ was calculated as the slope of this linear fit (provided in the legend) and was compared between the results for the pristine and microstripe-textured surfaces. For polyurethane, the friction coefficient of the pristine surface decreased from 1.83 to 0.99 with the introduction of a microstripe-textured structure, and this value decreased from 2.36 to 1.25 for the Thordon. This suggests that the presence of microstripe textures can effectively reduce the friction on water-lubricated materials.
Figure 9 shows the friction (
Figure 9a), average coefficient of friction (COF), and specific wear rate (
Figure 9b) of the above samples for the 2 h friction test conducted at a 50 N load, 100 r/min rotary speed, and water lubrication in the ring-on-block friction/wear test. As shown in
Figure 9a, the friction of all Thordon samples was greater than that of the polyurethane samples. In addition, compared to Thordon, the friction trend of polyurethane was more stable over time, while the friction jitter amplitude of Thordon was larger, which was determined by the material’s properties. Combined with the analysis of the surface morphology after abrasion, this is due to the fact that the abrasion mark on Thordon is deeper than that on polyurethane, by about 8–9 times. In particular, it is interesting to note that for the same material, surfaces with microstripe textures always exhibited lower friction than the pristine surfaces. This suggests that the microstripe texture contributed to the reduction in friction on the water-lubricated materials.
Figure 9b shows that the samples with microstripe-textured surfaces demonstrated a lower coefficient of friction and wear rate relative to the samples with pristine surfaces. The COFs of Thordon and PU with microstripe-textured surfaces were reduced by about 17.7% and 19.8%, respectively, compared to the pristine surfaces. In terms of wear performance, the wear rates of Thordon and PU with microstripe-textured surfaces were reduced by 27.3% and 25.5%, respectively, compared to those of the samples with pristine surfaces. The excellent tribological properties of microstripe-textured surfaces can be attributed to several factors. Firstly, the microtexture reduces the actual contact area and adhesion between the friction pairs, resulting in lower friction. In addition, the microtexture, with a depth of 0.5 μm between the stripes, helps to trap abrasive particles and can reduce the occurrence of abrasive wear. Finally, the uniformly arranged microstripe texture contributes to the rapid formation of a stable lubricating water film, which in turn improves the lubrication state between the friction pairs, thus reducing the friction and wear properties of the material.
A summary of the photographs, 3D topographies, and 2D profiles (perpendicular to microstrips, marked in red lines) of the Thordon and PU samples after the friction experiments is shown in
Figure 10. The widths of the abrasion scar were about 8.5 mm and 6.4 mm, and the depths were about 234 μm and 165 μm for the Thordon samples with pristine and microstripe-textured surfaces, respectively. Microstripe-textured Thordon demonstrated a smaller abrasion width and depth than the pristine surface sample, where the width and depth of the wear scar were lower by 24.7% and 29.5%, respectively. The width and depth of the wear scar on the microstripe-textured PU reduced by approximately 5.7% and 17.4% compared to those of the pristine surface sample. The reason for this may be that the microstripe texture contributes to the formation of a stable lubricating water film, which provides a certain load-bearing capacity under dynamic pressure lubrication, reducing the load of the copper ring acting on the surface of the material. Thus, it can be concluded that embedding microstripe textures can improve the tribological properties of water-lubricated materials, such as optimizing the formation of a lubricating water film, reducing adhesion, friction, and wear, thereby leading to a superior anti-wear performance compared to pristine surfaces.