**3. Results and Discussion**

### *3.1. Structural Analysis of Non-Uniform Models of Bionic Unit of Grey Cast Iron*

According to the discussion on the formation process of the unit [29,30] in the literature, the ambient temperature around the unit formation is relatively low. After remelting, a grain boundary structure with higher dislocation density is formed. This process of remelting and rapid cooling can be compared to a self-quenching process. As the entire process is continuous, the processing of the unit will increase the temperature of the surrounding material environment. For a unit that has been processed, it is equivalent to the process of heat preservation and tempering after the self-quenching process. These two processes occur one after another, which means that the unit created by laser melting and solidification has increased dislocation density and hardness and enhanced ductility and toughness. However, the distribution mode of the cell changes the interaction degree of the adjacent cell forming process. Figure 7 depicts cross-sectional morphology photos of four different biomimetic cell combinations, and Figure 8 shows thee SEM photos of the cell microstructure of different cell combinations.

**Figure 7.** Photographs of cross section shapes of four different biomimetic unit combinations. The section of the bionic unit body is composed of the melting zone, transition zone, and base metal.

**Figure 8.** Electron micrograph of microstructure in melting and transition zone of biomimetic unit.

It can be seen from Figure 7 that there were some differences in cell size among the four different combinations. The maximum depth of P4 was 1.089 mm, and the minimum depth of P1 was 0.878 mm. The matrix structure of the base metal was pearlite with a small amount of ferrite. During laser treatment, the heat is transferred from the surface to the interior of the base metal. The lower the ambient temperature of the base metal, the more quickly the heat input from the laser is transferred to the surrounding area, and the more heat is transferred out. The input heat can quickly diffuse to the surrounding area, and the degree of heat accumulation in the molten pool becomes weak. Subsequently, the superheated temperature of the liquid metal in the molten pool decreases, so that the laser processing area on the sample surface reaches the melting point temperature of the gray cast iron, and the melting area becomes smaller. The heat transfer in each direction of the heating area of the four distribution modes is different. The heat transfer on the surface occurs through radiation and natural convection. As more units are processed continuously, the unit melts more slowly until it eventually solidifies. The distribution mode of the unit changes the surrounding environment of the base metal, which has an impact on the shape, size, and other characteristics of the unit body. The increase in the number of unit body combinations increases the depth of the unit body embedded in the base metal, and the thickness of the unit body increases.

The change in the unit distribution had no effect on the structure of the unit, which was composed of deformed ledeburite and a small amount of residual austenite. The deformed ledeburite had the structure of martensite distributed on the cementite substrate. Normally, the pearlite is distributed on the cementite substrate, but the unit body was formed under the condition of rapid cooling. At high temperatures, the carbon dissolve in the ferrite too late to precipitate, and the supersaturated solid solution becomes martensite in ferrite. According to the X-ray (D/Max, 2500PC, Tokyo, Japan) diffraction analysis and SEM photos of the units with different distribution modes shown in Figure 9, it can be seen that the higher the ambient temperature of the base metal during the laser processing, the lower the initial austenite volume, and the higher the amount of eutectic carbide in the unit body; meanwhile, the dendrite spacing of the carbide becomes denser and finer. As shown in Figure 7, the dendrite spacing of P4 was relatively large, while that of P1 was the densest and finest.

**Figure 9.** X-ray diffraction analysis of elements with different distribution.

The transverse and longitudinal microhardness distribution of the unit is shown in Figure 10b. The points are taken from the surface to the depth along the horizontal line at 25 um under the matrix and symmetrical center line of the unit, respectively. The measurement results are shown in Figure 10a,c. The microhardness of the surface of the two kinds of units was greater than that of the matrix. Due to the uniform structure on the same horizontal line, the hardness of a single unit showed little change, but there was an obvious hardness difference between the units arranged in different ways. The microhardness range of P1 was 765–820 HV, P2 was 675–725 HV, P3 was 590–640 HV, and P4 was 570–620 HV. The main reason for this hardness difference was the uneven heat transfer in the different arrangements, which resulted in different cooling rates across the entire molten pool. The top of the molten pool can radiate heat, and its cooling speed is faster. More carbide, martensite, and a small amount of austenite were formed during cooling. The temperature at the bottom of the molten pool was close to the melting point, and there was a large temperature gradient between the molten pool and the substrate. The cooling speed was slow. The austenite dendrite formed first, and then the austenite dendrite transformed into martensite. Therefore, in the depth direction, there were some differences in the microhardness of the unit, and the microhardness decreased gradually along the depth direction.

### *3.2. Thermal Fatigue Resistance Analysis of Non-Uniform Model of Biomimetic Unit*

Under the action of cyclic thermal stress and thermal strain, the surface defects tend to produce stress concentration, thus becoming the location of crack nucleation. Due to the large amount of flake graphite in gray cast iron, each flake graphite can approximately form a micro notch in the matrix. In the process of the thermal cycle, because of thermal expansion and contraction, the internal stress is concentrated at the tip of the graphite, leading to crack initiation. As shown in Figure 11, after 20 thermal cycles, microcracks initiated at the tip of the two graphite samples and propagated along the interface, with poor bonding properties and grain boundaries. The contact area between the crack and the air was large, allowing for easy oxidation. The formation of loose oxide further promoted

the crack growth. Energy Dispersive Spectrometer analysis showed that the oxide from the iron distributed into the cracks. The results showed that the stress concentration effect and oxidation corrosion promoted crack initiation and propagation.

**Figure 11.** Crack initiation source of gray cast iron and EDS analysis of the elemental composition.

The biomimetic unit embedded in the base metal acted as a crack-arresting unit that blocked crack growth. The local structural transformation of the base metal and the change in the temperature difference between the laser irradiation zone and the base metal generate certain types of stress that are conducive to improving surface wear resistance and fatigue resistance. At the same time, the existence of residual stress can reduce the sensitivity of the crack tip to stress as well as offset part of the driving force of the crack. The residual stress values of different models were different when they were processed; consequently, different models were used to measure residual stress. As the penetration depth of x-ray to the metal was about 20 μm, the stress value measured by the test should be the plane stress value of 20 μm deep on the surface of the unit. During the measurement, three points were selected along the laser scanning direction along the laser scanning direction in the middle of the melting unit, which were marked as A1, A2 and A3, and three points were selected as B1, B2, and B3 in the phase transition region of the unit. The D ϕ-sin2 ϕ curve of each measuring point is shown in Figure 12. Residual stress and its average values were calculated by the stress measuring instrument's own software according to the slope of the regression line of D ϕ to sin2 ϕ. Residual tensile stress in the melting area of the biomimetic coupling unit in the laser scanning direction was approximately 202.8 MPa, and the residual compressive stress in the phase transformation area of the biomimetic coupling unit in the laser scanning direction was approximately 103.4 MPa. The molten metal in the molten pool shrank because of solidification through movement of the light beam. The melting layer and its surrounding transformation zone were combined metallurgically, which resulted in the contraction of the melting zone and the surrounding transformation zone, increasing the formation of residual tensile stress in the melting zone. Figure 13 shows the surface stress values of the unit in the four different distance distribution models. Surface stress values of the unit were lower as the number of biomimetic units in the model increased, which reflects the function of heat preservation and tempering in the processing of the unit.

**Figure 12.** The D ϕ—sin2 ϕ curve of each measuring point.

**Figure 13.** Surface stress values of elements in four models of different distance distribution.

When the microcrack growth is no longer dependent on the surface conditions of the material, the crack initiation phase ends, and the crack growth resistance depends on the overall properties of the material. We found that during the thermal cycle, due to the interaction between the surface layer and the thermal medium, the oxygen in the air reacts with the matrix to oxidize the surface layer. Additionally, a large amount of pearlite decomposes into ferrite and graphite, which reduces the hardness and strength of the material. As shown in Figure 14, due to the decomposition of cementite, the pearlite content was reduced, allowing the crack to expand into the matrix more easily. In the gray cast iron, the presence of graphite in the matrix was an excellent bridge for crack growth. Figure 15 shows the crack growth morphology in the macro state under the micro state. It can be seen from

the figure that rapid crack growth was achieved by bridging the graphite in the matrix. Therefore, different types of graphite lead to different growth levels of macro thermal fatigue cracking.

**Figure 14.** Microstructure of the cross sections of specimen after thermal 600 cycles. (**a**) the substrate; (**b**) the bionic unit.

**Figure 15.** Micromorphology of macro thermal fatigue crack propagation.

Figure 16 shows the statistical curve of the thermal fatigue crack length and cycles of bionic units arranged in different ways from 0 to 600 times. It can be found that the fatigue resistance of the bionic model was better with the increase in the amount of bionic cell arrangement, which occurs because the bionic cell embedded in the substrate surface hinders the crack growth. With the increase in the number of thermal cycles and the continuous decarburization of the unit, the carbide and martensite phases in the unit were greatly reduced, the hardness of the unit was reduced, and the blocking effect was reduced. Therefore, in terms of the number of cracks, when the number of cycles reached 600, the number of cracks in P4 was 80, while that of P1 was 96. However, when P4 was 600 times, the maximum crack length was 2.8 mm, while that of P1 was 6.9 mm. This is due to the accumulation of energy at the crack tip because of the continuous thermal cycling, which allows the crack to break through the potential barrier of the unit. When breaking through the cell barrier, the crack deflects along the cell edge, which means that the crack propagation is interrupted. Therefore, with the increase in the number of elements in the arrangemen<sup>t</sup> mode, the maximum length of the crack was effectively shortened in the thermal cycle test of the sample, and the surface crack mode changed from a large, long crack to a fine, small crack (although the difference in the number of cracks was not particularly significant).

(**b**) 

**Figure 16.** Statistical curves of the length and number of thermal fatigue cracks of biomimetic units arranged in different ways from 0 to 600 times. (**a**) The quantity of cracks, (**b**)The length of cracks.

### *3.3. Thermal Fatigue Resistance Analysis of Non-Uniform Models of Biomimetic Units*

Figure 14 depicts the bionic unit structure after 600 fatigue tests. From Figure 14b, many dispersed point graphite points can be seen. This shows that in the process of thermal fatigue, the cyclic thermal process causes the martensite phase to decompose and the carbon element to precipitate. In the biomimetic unit, tiny point like and strip like cementite could also be seen, and were distributed in the basic phase composed of ferrite. Figure 17 shows the weight loss results of the bionic samples with different distribution combinations and the semi-metallic brake pads after 150, 300, 450, and 600 thermal cycles. It can be seen from the results that with the increase in fatigue time, the wear amount of the bionic samples also increased. Compared with the blank sample, the wear loss of the bionic sample was much less important. Thus, in the early stages of fatigue, the bionic unit in the bionic

sample effectively resists wear. With the increase in the fatigue time, the structure of the unit and the matrix change to varying degrees. The number of non-equilibrium phases with higher hardness in the unit decreases, which makes the hardness of the unit decrease.

**Figure 17.** Weight loss results of thee bionic samples in the wear test with semi-metallic brake pads after thermal cycling.

There are different numbers of microcracks on the surface of the fatigue specimen. During the wear process, the edge of these microcracks is easier to peel <sup>o</sup>ff, which increases the wear amount of the specimen. Therefore, the number of microcracks in the sample surface has a grea<sup>t</sup> influence on the wear resistance of the sample. According to the results of wear loss, the samples with different unit distributions have different wear surfaces. P1 had the largest wear loss weight, and it also had the largest wear loss compared to the worn brake pads. This is because during the process of wear, with the increase in fatigue times, the unit is gradually impacted by the parent crack, which causes it to crack, and a large number of microcracks are produced on the surface. Moreover, with the increase in wear on the surrounding parent metal, the unit embedded in the parent metal will gradually be exposed to the friction surface. In this case, the microcracks on the surface of the unit will cause the unit to wear off and increase the amount of wear. The wear of P4 was most similar to that of P1. This is because the principle of the same unit area was adopted in the design of the bionic surface, so the surface of the base metal with a relatively large area on the P4 surface was exposed. In the process of wear, the area loses the protection of the unit body, so it also experiences relatively significant wear loss. P3 had the lowest amount of wear of all of the samples due to many factors. When the unit had the P3 distribution combination, the heat preservation and tempering process of the unit produced during the processing process increased the toughness of the unit. At the same time, the combined distribution of the units can protect each other during the wear process, which effectively blocks the development of cracks, reduces the micro cracks on the surface of the unit, and prevents excessive wear of the brake pads on the unit.

Figure 18 shows the surface wear morphology of different unit combinations after 600 thermal fatigue cycles. With the increase in fatigue times, different degrees of wear appeared on the surface of the unit, caused by the micro cracks peeling off the surface of the unit. With the increase in the number of units in each combination of units, the degree of wear was lighter, because the combined units protect each other, resulting in the relatively uniform wear that occurred between the units. Due to the wear and loss of support and protection of the matrix structure around P1, the brake pads caused more damage to P1 during the wear process, and the surface peeling was more serious.

**Figure 18.** Surface wear morphology of different unit combinations after 600 thermal fatigue cycles.
