3.2. Friction Coefficient
- (1)
Friction coefficient at different movement speeds
Under dry sliding, the friction coefficient of the matrix material before laser hardening is shown in
Figure 3.
As shown in
Figure 3, the friction coefficient of the matrix material first increased significantly, and then the curve flattened with some fluctuation within a certain range. As the sliding speed increased, the value of the friction coefficient decreased.
Figure 4 shows the change of the typical friction coefficient between GCr15 steel and 40CrNiMo steel with laser scanning speeds of 700 and 800 mm/min. We found that, in the experimental range, when the load remained unchanged, with the increase in the sliding speed, the friction coefficient showed a decreasing trend; however, the increasing trend gradually became smaller. Compared with the friction coefficient of the matrix under the same sliding speed under dry sliding, the friction coefficient under oil lubrication decreased significantly.
Yang et al. [
24] studied the grinding situation between laser-hardened steel with a surface hardness of 640.3–706.08HV and GCr13 steel. The test load was 15 N, the rotating speed of the turntable was 50 r/min, and the wear time was 20 min. The dry friction coefficient was between 0.4 and 0.6. The surface hardness of the material was slightly lower than that of the material after laser hardening in this experiment, and the experimental conditions were also different; however, the friction coefficients were far from the friction coefficient under oil lubrication conditions. Thus, it can be inferred that the dry friction coefficient was significantly greater than that under oil lubrication at room temperature.
In fact, the contact area of the friction pair surface was not the nominal contact area of the two contact surfaces but should be the sum of the micro-area of the contact spots formed by the contact of some surface profile peaks. When the contact area was plastically deformed by pressure, adhesion occurred on the contact surface, and the node was formed. When relative sliding occurred between the contact surfaces, these nodes were sheared apart. The more nodes, the greater the shear force required and the greater the coefficient of friction.
Under the oil lubrication, the overall surface clearance of the friction pair was very small, there were lubricating oil films between some contact surfaces, and there was mutual contact of the microconvex peaks of the remaining contact surface. As a result, the boundary film with low shear strength formed can effectively reduce the contact area of micro-convex peaks on the friction surface under oil lubrication, resulting in fewer nodes. Compared to dry sliding, the friction coefficient can be greatly reduced under oil lubrication. In addition, during the whole process, due to the flattening effect of the microscopic elastohydrodynamic local pressure on the roughness peak, the actual roughness was greatly reduced, which was an important reason for the decrease in the friction coefficient under the oil lubrication.
From the fluctuation of the friction coefficient, the fluctuation range of the friction coefficient was small under the oil lubrication, which was mainly because the lubricating oil film generated between the contact surfaces of the friction pair during the friction process. In addition, the lubricating oil can quickly take the heat generated in the friction process away from the contact area, so that the friction surface was always kept at a relatively low temperature, and there was no serious plastic deformation and melting of the wear surface similar to the dry sliding friction process; therefore, the friction coefficient fluctuated less.
- (2)
Friction coefficient at different temperatures
Figure 5 shows the typical friction coefficient of 40CrNiMo steel Harden after laser scanning speeds of 700 and 800 mm/min. The sliding speed was 500 mm/min, and the temperatures were 20, 110 and 200 °C.
As shown in
Figure 6, with the increase in temperature, the friction coefficient increased significantly. This was because at high temperature the volume of the lubricating oil increased, the distance between the molecules increased, and the attraction between the molecules decreased, and thus the lubricating oil became thinner, the viscosity decreased, and the lubricating effect of the lubricating oil reduced. In addition, the hardness and strength of the material decreased, the plasticity increased, the deformation resistance would decrease. However, when the temperature rose, the stable sliding period but not the stable wear period still existed.
There was a larger fluctuation in the friction coefficient with increasing temperature. When the temperature reached 200 °C, the friction coefficient no longer fluctuated at a lower value, the fluctuation range was very obvious, and even the friction coefficient exceeded the maximum value of the running-in period. This was because the temperature rose, on the one hand, thermal expansion increased the microscopic free volume of the object. The thermal motion capability of each motion unit was improved, and the activity space of each motion unit was increased. On the other hand, the lubricating effect of lubricating oil was significantly reduced.
In order to analyze the change of the friction coefficient better, two additional wear experiments were performed on the laser-hardened specimens, and the change trend of the friction coefficient obtained was basically the same. At room temperature, under oil lubrication, after reaching the stable period, the fluctuation of the friction coefficient was obviously small, and the wear was relatively stable. The friction coefficient during the wear period is shown in
Figure 6.
Table 5 shows the different friction coefficients under oil lubrication at room temperature.
From
Table 5, the surface hardness of the 40CrNiMo steel after laser hardening with a scanning speed of 800 mm/min was slightly larger than that of the material after laser hardening with a scanning speed of 700 mm/min laser, the friction coefficient was also different but not greatly.
The friction coefficient decreased with the increase in the sliding speed. This was mainly because at the lower sliding speed, the friction pair squeezed less lubricating oil into the friction surface, and the bearing capacity of the liquid extrusion film would be weaker, the formation ability of the liquid film was poor, and there was a large proportion of solid–solid contact between the two solid surfaces.
As wear progressed, the surface of the contacting object would appear to a certain extent, and the surface was uneven, it is difficult to form a complete and continuous oil film. Therefore, the lubricating effect was poor. At high sliding speed, the action of the liquid extrusion film and the depth of the furrow would be deeper, thus, forming a better lubricating oil flow path, and then the better oil lubricating film was formed.
For laser-hardened materials with different scanning speeds, the decrease in friction coefficient would increase with the increase in the sliding speed. This may be because more furrows would be formed with the increase in the sliding speed, the better lubricating oil flow path was formed, which can make the lubrication effect better. Thereby, the coefficient of friction was reduced even more [
25].
At a sliding speed of 500 mm/min, the friction coefficient values at different temperatures are shown in
Table 6.
From
Table 7, it can be seen that the material has been laser hardening with a scanning speed of 800 mm/min had a smaller friction coefficient and better resistance to fluctuations because, relatively speaking, the surface hardness of the material treated at this scanning speed was higher. With the increase in temperature, the increase in friction coefficient was larger; however, the increase would gradually decrease. This was mainly because the increase in thermal motion capacity would decrease to a certain extent within the experimental range.
3.4. Wear Morphology
- (1)
3D morphology analysis of friction at different speeds at room temperature
The typical wear morphologies s at sliding speeds of 300, 400, and 500 mm/min without lighting effects enabled are shown in
Figure 8.
From
Figure 9, it can be seen that, with the increase in the speed, the wear pattern of the 40CrNiMo steel specimen hardened by the scanning speed of 700 mm/min or 800 mm/min changed. When the sliding speed was 300 mm/min, the wear depth was shallow, and wear in different areas was not exactly the same, and there was no obvious furrow along the sliding direction on the surface, which has a certain polishing effect. When the sliding speed increased to 400 mm/min, the furrows were gradually obvious; however, the furrows were shallower.
When the sliding speed was increased to 500 mm/min, the number of furrows increased, the furrows became more obvious, and the maximum wear depth also increased significantly. It can be seen that under the condition of oil lubrication, due to the high hardness of the material, under the combined action of normal load and tangential force, local ploughing and micro-cutting would appear, which were typical abrasive wear. Under the influence of lubricating oil, the increase in the sliding speed would bring more particles into between the two contact surfaces and gather together, deeper furrows were prone to appear.
- (2)
3D morphology analysis of friction at different temperatures
Figure 9 showed the typical wear scars of the hardened layer without lighting effects enabled after laser hardening at a scanning speed of 700 and 800 mm/min, at 20 °C, 110 °C and 200 °C, at the sliding speed 500 mm/min.
As can be seen in
Figure 10, at the same sliding speed, with the increase in temperature, the wear scars of the hardened layer under different scanning speeds have obvious differences.
- (3)
SEM analysis of wear scars of the hardened layer with a scanning speed of 700 mm/min
The surface morphology of the hardened layer with a scanning speed of 700 mm/min at a normal force of 20 N and sliding speed of 300 mm/min, 400 mm/min and 500 mm/min respectively under 20 °C are shown in
Figure 10.
As the equipment was different, the wear surface morphology was different from the one observed from the 3D Optical Microscope. Some surface features cannot be displayed when the 3D Optical Microscope companion software was used to analyze images without lighting effects enabled.
The shallow furrow and particle can be seen from
Figure 10a, and thus abrasive wear occurred. Some areas appeared smooth, not enough for furrows, confirming there were a certain polishing effect again. More small particles and deeper furrow can be seen from
Figure 10b, it can be inferred that abrasive wear has increased. Flake and deeper furrow can be seen from
Figure 10c, and it can be inferred that the abrasive wear further increased.
The surface morphology of the hardened layer with a scanning speed of 700 mm/min at a normal force of 20 N and sliding speed of 500 mm/min, respectively, under 110 °C and 200 °C are shown in
Figure 11.
From
Figure 11, it can be seen that the furrows at the bottom of the friction surface of the hardened layer were more obvious and the depth was further deepened as the temperature increased. At the same time, due to the increase in temperature, there were obvious wear debits, thereby, indicating that the adhesive wear was intensifying.
In addition, the increase in temperature caused the hardness of the material to decrease, and the oxidation reaction would also increase, resulting in oxidative wear, and the generated oxides adhere to the wear surface under the action of lubricating oil. Since oxides were less hard than laser-hardened steel, they also promote abrasive wear during friction, resulting in deeper furrows.
3.5. Wear Rate
After laser hardening at scanning speeds of 700 and 800 mm/min, the wear rates of the hardened layer under different conditions are shown in
Figure 12.
The wear rate values are shown in
Table 7. From
Table 7, it can be seen that with the increase in speed and temperature, the wear rate showed an increasing trend. The details as follows:
(1) After laser hardening with a scanning speed of 700 mm/min, under oil lubrication, the increments of the average wear rate of the hardened layer at the sliding speed of 500 mm/min relative to the sliding speed of 400 and the sliding speed of 400 mm/min relative to the sliding speed of 300 mm/min were 0.0167 and 0.0221 (10−3 mm/min), respectively. After laser hardening with a scanning speed of 800 mm/min, the average wear rate of the hardened layer at the sliding speed of 500 mm/min compared with the sliding speed of 400 mm/min and the sliding speed of 400 mm/min compared with the sliding speed of 300 mm/min.
The increments are 0.0162, 0.0284 (10−3 mm/min) respectively. At high sliding speed compared with low sliding speed, the sliding distance would increase, resulting in an increase in the amount of wear, and there would be more wear products in the lubricating oil and aggregated, resulting in deeper furrows.
It can be seen that the performance of the specimens hardened with a speed of 700 and 800 mm/min showed the same change law, that is, as the movement speed increased, the increase in the wear rate would change from more to less. This was because the friction was accompanied by the flow of lubricating oil. When the relative movement speed between the contact surfaces was slow, the flow of the lubricating oil was relatively slow, and thus that there may be more hard particles in the oil film between the contacting objects, which would promote a larger increase in wear. After reaching a certain the sliding speed, the speed continued to increase, and the oil flow accelerated to take away more wear products, which formed a relatively pure oil film between the friction objects, thereby the wear increment reduced.
(2) The increase in temperature had obvious effect on the increase in wear rate. Within the experimental range, the wear rates of the specimens hardened at 700 and 800 mm/min showed the same variation law. The wear rate at 110 °C was more than twice that at 20 °C, and the wear rate at 200 °C was about 140% of that at 110 °C. This was because the lubricating oil would become thinner and the viscosity would decrease after the temperature rose, which would change the characteristics of the lubricating oil film, the bearing capacity of the lubricating film would decrease, and the thickness of the oil film would be lower. Moreover, the increase in oil temperature would also promote oxidative wear and adhesive wear.
In particular, the research has certain limitations, and the contents of the article may be only applicable within the experimental parameters.