3.1. Morphology and Microstructure Observation
Figure 4 shows the micro-morphology of the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA powder. It can be seen from
Figure 4a that the powder presents a regular spherical shape. The surface of the powder is uneven, presenting an obvious dendritic structure, as shown in
Figure 4b.
Figure 5 shows the macroscopic cross-sectional morphology of PC, HLC, and DLC, respectively.
Figure 5a shows that the bottom of PC is relatively flat, with a small amount of melting of the base material. The coating thickness is 2.05–3.3 mm (obtained using a LAB-1 ZEISS Optical Microscope). The diameter of the plasma beam is about 10 mm. The powder melts quickly after flying out of the nozzle. The droplets accumulate and solidify quickly on the surface of the substrate. The bottom of HLC is extremely flat (
Figure 5b). The coating thickness is 0.9–1.05 mm. The laser and powder are focused above the substrate. The molten droplets are sprayed quickly onto the surface of the substrate and form a flat and uniform coating. The substrate material undergoes a short laser radiation time and barely melts due to the high-speed movement of the cladding robot. The bottom of the DLC is wavy (
Figure 5c). The coating thickness is 1–1.7 mm. The DLC cladding speed is slower than that of HLC. The surface of the substrate melts quickly. The molten droplets enter the molten pool and produce a metallurgical bond with the substrate.
Figure 6 shows the microstructure of different regions of PC, HLC, and DLC. Planar crystals were observed near the fusion line in
Figure 6a. PC is composed of cellular crystals, dendritic crystals, and equiaxed crystals, respectively, from the fusion line upward (
Figure 6b,c). The microstructure of HLC is similar to that of PC. But the grain size of HLC is smaller (
Figure 6d,e). DLC can be divided into a planar crystal region (
Figure 6f), a columnar crystal region (
Figure 6g), a recrystallization region (
Figure 6h), and a dendritic crystal region (
Figure 6i), based on the grain morphology and thermal field distribution during the cladding process. DLC has a finer grain boundary and larger grain size compared to HLC. A large number of irregular grains are distributed in the cladding feed direction in DLC (
Figure 6h). Because in the overlapping region, dendrites were secondary crystallized at high temperatures during cladding. Crystal boundaries solidify before they are fully formed due to the short time of laser radiation.
The grain morphology is related to the G (temperature gradient) and V (solidification velocity) at the solid–liquid interface [
26]. Due to the rapid heat dissipation of the substrate, the G at the fusion line is extremely large, while the V is extremely small, and there is almost no constitutional supercooling at the solid–liquid interface. The solid–liquid interface advances towards the interior of the coating at an extremely fast speed to form planar crystals. Subsequently, the G decreases and the V increases, resulting in constitutional supercooling at the front of the solid–liquid interface. Then, columnar and dendritic crystals grow on the planar crystals. The V is larger at the top of the coating. Nucleating particles are randomly generated in the liquid phase, resulting in uniformly distributed equiaxed crystals. The grain size depends on the cooling rate. High-speed laser cladding has the lowest overall heat input and the fastest cooling rate of the coating, so the grain size of HLC is slightly smaller than that of DLC and PC.
3.2. Crystal Structure and Composition Analysis
Figure 7 shows the XRD patterns of the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA powder, PC, HLC, and DLC. The crystal structure of the powder, PC, and HLC is typical single face-centered cubic (FCC), while that of DLC is body-centered cubic (BCC). The ICDD reference code for the powder, PC, and HLC is #47-1417, and that for DLC is #37-0474. The diffraction peak height of HLC is consistent with that of the powder. The (111) and (200) diffraction peak heights of PC exhibit different behaviors. The reason is that the (200) crystal direction has a lower surface energy, and the grains tend to align in the (200) direction during high-energy cladding [
27]. The complete change in the crystal structure of DLC may be related to the high dilution ratio of the coating. Therefore, it is necessary to further explore the microstructure of the three coatings using an EDS.
Figure 8 shows the line scanning results at the interfaces of PC, HLC, and DLC, respectively. Element diffusion occurs at the interfaces of the three coatings. The element diffusion region widths of PC, HLC, and DLC are 28 μm, 9 μm, and 31 μm, respectively. Therefore, the less the total heat input, the more difficult the element diffusion is. The element content of PC and HLC is the same as that of the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA powder. The content of Fe in DLC is extremely high, around 70%. The element of the substrate material entered the coating, greatly diluting the element content of the coating. Meanwhile, the DLC cladding process has a large heat input, resulting in the slow cooling rate of the coating. With extremely high Fe content and a slow cooling rate, DLC tends to form an α-Fe phase. Therefore, DLC has a BCC crystal structure, which is confirmed by the results of the XRD.
Figure 9 shows the microstructure of the three coatings. An EDS was used to detect the elemental distribution of PC (
Figure 9a), HLC (
Figure 9b), and DLC (
Figure 9c). The EDS results are listed in
Table 4. In the three coatings, the content of Cr and Mo elements is higher at the solidification boundary (region A, C, and E) than that in the grain (region B, D, and F). The melting points of the Mo and Cr elements are higher than those of the Fe, Co, and Ni elements, so Mo and Cr solidify first in the molten pool. As the solid–liquid interface advances, Mo and Cr aggregate at the grain boundaries. Therefore, the content of the Mo and Cr elements at the grain boundary is relatively high [
22].
3.3. Microhardness and Friction Properties
Figure 10a shows the microhardness of Q235, PC, HLC, and DLC. The average microhardness of Q235, PC, HLC, and DLC is 195 HV, 333 HV, 407 HV, and 408 HV, respectively. Compared to the substrate material, the microhardness of the coating increased by about 109%. The average hardness of the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA is 170 HV higher than that of the CrMnFeCoNi HEA through plasma cladding [
28]. The standard deviations of microhardness are 9.44 HV, 7.79 HV, 79.26 HV, and 19.83 HV, respectively. The microhardness of PC and HLC is relatively uniform, while DLC exhibits a stepped distribution. The microhardness of HLC is higher than that of PC due to its lower heat input. The solidification speed of the droplets is extremely fast. There is enormous thermal stress inside the coating. At the same time, the SEM of the coatings shows that the grain size of HLC is significantly smaller than that of PC. Therefore, the resistance to dislocation movement during plastic deformation increases, and the microhardness of HLC also increases accordingly. The microhardness of DLC is extremely high at the fusion line. The reason may be that the cooling rate near the fusion line is very fast, resulting in high thermal stress, which increases the microhardness. From the fusion line to the top of DLC, the cooling speed of the molten pool decreases. Therefore, the hardness of the coating gradually decreases and ultimately presents a stepped distribution.
Figure 10b shows the friction coefficient curves of Q235, PC, HLC, and DLC. The four friction coefficient curves all stabilized after 6 min. The friction coefficient curves of the three coatings do not fluctuate sharply, indicating that the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA has extremely stable friction and wear resistance. The average friction coefficients for Q235, PC, HLC, and DLC are 0.7653, 0.5118, 0.4252, and 0.3735, respectively, within 6–16 min. The friction coefficient was reduced by more than half compared to Q235. According to the microhardness curve and XRD results, the friction coefficient of the coatings is related to the microhardness and crystal structure. The higher the microhardness of the material, the smaller the average friction coefficient.
The friction coefficient of the grinding ball sliding on the friction surface is
μ. It can be calculated by the following equation:
where l/2 and
r are the groove radius and abrasive particle radius, respectively. If the ratio of the groove radius to the abrasive particle radius (l/2
r) is replaced by
A, Equation (1) can be expressed as:
where
A is a characterization of the grinding ball penetration depth. The deeper the grinding ball is pressed, the higher the value of
A, and the larger the coefficient of friction. The grinding ball is pressed shallowly due to the high microhardness of DLC and HLC. Therefore, the average friction coefficients of DLC and HLC are relatively lower. In addition, the friction and wear resistance of the BCC crystal structure is superior to that of the FCC crystal structure [
29]. Therefore, the average friction coefficient of DLC is lower than that of HLC.
According to the width of the wear track, the depth of the wear track can be calculated from Equation (3):
The volume of material wear can be calculated from Equation (4) [
18]:
where
R is the wear track radius,
L is the wear track width, and
r is the grinding ball radius. The wear track depths and wear volumes are recorded in
Table 5. The wear volumes of Q235, PC, HLC, and DLC are 0.0682, 0.0418, 0.0268, and 0.0245 mm
3, respectively. The results show that DLC has the best friction and wear resistance.
Figure 11 shows the wear tracks of Q235, PC, HLC, and DLC. There is a great number of furrows on the wear tracks of Q235, PC, HLC, and DLC. This indicates that some hard phases fall off during friction, resulting in abrasive wear. The wear track surfaces of Q235, HLC, and DLC are uneven, resulting in adhesive wear. High temperature caused by high-speed friction leads to welding and tearing. Cracks were generated and expanded under the cyclic load. Large areas of black phases and cracks were observed on the wear tracks of PC, HLC, and DLC. The EDS results in
Table 6 show that the black phase on the wear track of Q235 is oxide of Fe. Therefore, oxidation wear occurred on Q235 and the three coatings during friction. Although the microhardness of HLC and DLC is similar, the average friction coefficient of DLC is lower than that of HLC. The wear track surface of DLC in
Figure 11d is smoother than that of HLC in
Figure 11c. The reason is that there are many cracks and stacks on the wear track surface of HLC, which increases the friction force on the grinding ball.
3.4. Corrosion Resistance
Figure 12a shows the potentiodynamic polarization curves of Q235, PC, HLC, and DLC. It can be seen from
Figure 9a that the passivation region width of PC and HLC is significantly wider than that of DLC.
Table 7 lists the pitting potential of Q235, PC, HLC, and DLC. Compared with Q235, the E
pit of PC and HLC increased by 1.6068 V and 1.6710 V, respectively, while that of DLC only increased by 0.5735 V. This shows that the stability of the PC and HLC passivation films is superior to that of DLC. Due to the similar elemental composition, PC and HLC exhibit the same passivation ability. Research shows that the passivation film of the Fe
30Ni
20Co
20Cr
20Mo
3.5 HEA consists of Cr
2O
3 and MoO
3[
30]. The first reason is that the atomic gap of the BCC crystal structure is larger than that of FCC. The difficulty of Cl moving towards the interior of the coating is lower [
31]. Secondly, DLC contains a large amount of Fe. The iron oxide has a strong adsorption effect on anions and the passivation film is more easily damaged by Cl
−. Therefore, the passivation film of DLC is more easily damaged by Cl
− [
32]. Therefore, the passivation region of DLC is relatively narrow.
Table 7 lists the kinetic parameters of the polarization curves obtained by the Rp fitting method. E
corr, I
corr, and C
corr are the corrosion potential, corrosion current density, and annual corrosion rate, respectively. The E
corr of Q235, PC, HLC, and DLC is similar, while the corrosion current density and annual corrosion rate of PC, HLC, and DLC are significantly lower than Q235. The smaller the I
corr, and C
corr, the better the corrosion resistance of materials. Therefore, HLC and PC have the best corrosion resistance.
Figure 12b,c show the Nyquist plot and Bode plot obtained by fitting the electrochemical impedance spectroscopy (EIS) of Q235, PC, HLC, and DLC. The equivalent circuit of the coatings in 3.5 wt% NaCl solution is R
s (CPE-R
p), as shown in
Figure 12b, where R
s and R
p represent the solution resistance and polarization resistance. CPE is the constant phase element defined by two values, CPE-T and CPE-P. The fitting results of the components in the equivalent circuit are listed in
Table 8. The values of capacitive reactance arc radius and R
p, in descending order, are HLC, PC, DLC, Q235. This is consistent with the results of Rp fitting in
Table 7.
In the Bode plot (
Figure 12c), the value of the impedance modulus |Z| is Rp in the low-frequency region and parallel connection of Rp and CPE in the high-frequency region. The larger the |Z| in the low-frequency region, the better the corrosion resistance of the material. PC and HLC maintain extremely high phase angles over a wide frequency range, indicating that the passivation film stability of PC and HLC is better than that of DLC, which is confirmed by the results of the potentiodynamic polarization curves. Compared to PC and HLC, DLC exhibits completely different corrosion resistance, and more analyses are needed through observing the morphology of the corroded surface.
Figure 13 shows the corrosion morphology of PC, HLC, and DLC. There are many small corrosion pits on the corrosion surface of PC, with a diameter of approximately 3.5 μm (
Figure 13a). After amplification, there are many nano phases at the grain boundary (
Figure 13b). These nano phases may be crystals left after the precipitates are corroded. A large number of dendrite frames are distributed on the corrosion surface of HLC (
Figure 13d). The corrosion surface of HLC is uniform and no corrosion pits are found, indicating that the type of corrosion is general corrosion (
Figure 13c). The corrosion surface of DLC has larger corrosion pits with a diameter of approximately 70 μm (
Figure 13e). The corrosion type of DLC is pitting corrosion. Corrosion pits are distributed in the overlapping region. Dendrite frames can be observed in the corrosion pits (
Figure 13f).
The EDS results of the coatings are listed in
Table 9. The element atomic ratio of the phase on the surface of PC is the same as the original composition (Region B as shown in
Table 4), indicating that the corrosion mainly occurred between the dendrites. Therefore, based on the SEM pictures of corrosion morphology, the corrosion types of PC are pitting corrosion and intergranular corrosion. The elemental composition of the corrosion product of HLC is the same as the original composition at the grain boundary of HLC (Region C as shown in
Table 4). Therefore, the corrosion of HLC occurred within the grains. The content of Co and Ni in the corrosion product of DLC is extremely low. This indicates that the secondary heat input in the overlapping region caused changes in the microstructure and composition of DLC, which is different from the potential in the non-overlapping region. Therefore, the overlapping region is more susceptible to corrosion.