3.2.3. Phase Analysis

Figure 8a–e shows a comparison of the XRD between the matrix (A1–A5) and the biomimetic unit (LR-A1–LR-A5), and Figure 8f shows an XRD comparison diagram of the biomimetic unit (LR-A1–LR-A5). The results show that the main phase of the matrix is ferrite, and the phase of the laser biomimetic unit is martensite.

**Figure 8.** X-ray diffraction profiles of the biomimetic units and matrixes: (**a**) A1, (**b**) A2, (**c**) A3, (**d**) -A4, (**e**) A5; (**f**) comparative diffraction curves of LR-A1-LR-A5.

At the same time, Jade was used to analyze the half-height and width of the diffractive peak (FWHM) of the matrix and unit with different carbon content. The results show that the FWHM of the matrix materials are A1: 0.140◦, A2: 0.219◦, A3: 0.239◦, A4: 0.249◦, and A5: 0.300◦; the laser biomimetic

samples' FWHM is 0.239◦, 0.243◦, 0.282◦, 0.286◦, and 0.519◦. The FWHM of the bionic unit of the same material is larger than that of the matrix material. Here we use Scherrer's formula [27]:

$$D = \frac{K\lambda}{\beta \cos \theta'} \tag{1}$$

where D is the grain size, K (K value is 0.89) is the Scherrer constant, λ is the wavelength of the X-ray, β is the full width at half maximum, and θ is the Bragg diffraction angle. It can be seen that under the same testing conditions, the grain size of the unit has been refined to different degrees, and the order of the grain size of the unit is LR-A5(67 nm) < LR-A4(121 nm) < LR-A3(123 nm) < LR-A2(143 nm) < LR-A1(145 nm).

### *3.3. Mechanical Properties of Varying the Carbon Content Matrix and Bionic Samples*

### 3.3.1. Microhardness of Matrix and Unit

The microhardness of the base materials A1, A2, A3, A4, and A5 are 126 HV, 176 HV, 250 HV, 270 HV, and 320 HV, respectively. Figure 9a is the microhardness curve of the bionic unit along the X-axis direction, and Figure 9b is the microhardness curve of the bionic unit along the Y-axis direction. Figure 9a,b show that the hardness of the remelting zone and the HAZ of the biomimetic unit are higher than those of the matrix, and the hardness of the bionic unit is gradually increased as the carbon content increases. This also echoes the microstructure changes in Figure 7. After calculation, the average hardness of the bionic unit after laser fusion treatment is 321, 343, 466, 493, and 659 HV. The degree of hardness improvement is 155%, 89%, 86%, 83%, and 106%, relative to the respective matrix.

**Figure 9.** The hardness of the biomimetic unit: (**a**) section profile along the *X*-axis and (**b**) section profile along the *Y*-axis.
