3.1. Macroscopic Forming and Crack Sensitivity of the Coating
Figure 2a shows a diagram of the typical macroscopic morphology of the coating produced by laser cladding. The forming parameters of the coating mainly include melting height (
H), melting width (
W), and melting depth (
h). Dilution rate (
η) refers to the extent to which the substrate metal melts into the coating in the laser cladding process, resulting in a change in the material composition of the coating. Its calculation is performed using the following Formula (2):
Figure 2b,c display the macroscopic morphology and nondestructive testing results of the coatings with varying mass fractions of ZrW
2O
8. In coatings 1 to 5, the ZrW
2O
8 content is 0 wt%, 2 wt%, 4 wt%, 7 wt%, and 10 wt%, respectively. Nondestructive testing results indicate that as the ZrW
2O
8 content increases, the overall tendency for coating cracking gradually decreases. Notably, when the ZrW
2O
8 content reaches 7 wt% or 10 wt%, no cracks are observed in the cladding coating.
The cross-section morphology of the coatings is shown in
Figure 3. As ZrW
2O
8 content increased, WC particles in the coating gradually melted, with the number of pores rising initially and then falling, and the thickness of the composite coating gradually decreasing. In actual production, it is necessary to polish the surface layer of the coating to a certain thickness. The influence of near-surface pores on the coating can be ignored. Therefore, when the addition of ZrW
2O
8 reaches 7% or 10%, the obtained crack-free coatings partially meet the requirements for practical applications to some extent.
3.3. Phase Analysis of the Coating
Figure 5 shows the X-ray diffraction patterns of the coatings with different ZrW
2O
8 contents. The test results indicate that in the absence of ZrW
2O
8, the phases of the coating consist of γ (Ni,Fe), FeNi
3, Cr
4Ni
15W, Ni
4B
3, M
23C
6, CrB, Cr
5B
3, Fe
3Ni
2, WC, and W
2C, with the presence of W
2C indicating WC decomposition. When the ZrW
2O
8 mass fractions are 2%, 4%, and 7%, the coating phases include γ (Ni,Fe), FeNi
3, Cr
4Ni
15W, Ni
4B
3, M
23C
6, FeNi, M
3C
2, Ni
17W
3, Cr
5B
3, NiCr
2O
4, ZrO
2, WC, and W
2C. At a ZrW
2O
8 mass fraction of 10%, the phases are γ (Ni,Fe), FeNi, Ni
17W
3, Cr
4Ni
15W, Cr
2Ni
3, M
3C
2, Ni
4B
3, NiCr
2O
4, ZrO
2, WC, and W
2C. The XRD patterns reveal that as the ZrW
2O
8 content in the coating increases, WC is progressively eroded by the liquid metal matrix, leading to greater WC dissolution. This results in the recrystallization of free W and C elements with Cr, Fe, and Ni to form complex carbides, and an increase in W-containing compounds in the coating. Based on studies of the C-Cr-Fe quaternary alloy system [
22] and experimental results, it is inferred that complex phase transitions may occur in the coating, including:
3.4. Microstructure of the Coating
Figure 6 and
Figure 7 display the microstructure at the bottom, middle, and top of single-pass composite coatings with varying ZrW
2O
8 contents. Rapid solidification at the interface of the composite coatings forms a bright band of planar crystals, indicating atomic diffusion between the solution and the substrate, thus achieving metallurgical bonding. Independent fine-line regions within the coatings suggest that all samples have formed metallurgical bonds with the substrate.
The white particles in the images are unmelted WC particles; as the molten pool solidifies, the temperature around these particles drops, causing heat to flow toward them and resulting in micro-area directional cooling [
23]. Consequently, the edges of the WC particles are composed of radially oriented blocky or rod-like structures.Without ZrW
2O
8, the bottom of the cladding layer mainly consists of epitaxially grown dendrites and inter-dendritic eutectic structures, with blocky, rod-like, and dendritic hard phases dispersed between the dendrites. The upper and middle parts primarily comprise dendrites and a few equiaxed crystals. As the ZrW
2O
8 content increases, the bottom dendrites gradually disappear, replaced by fine columnar crystals. The proportion of dendritic and equiaxed crystal structures in the cladding layer increases, with the grain structure becoming progressively refined, more uniform, and denser. The hard phases become more finely and uniformly dispersed among the dendrites.
Figure 8 presents the SEM microstructure of coatings with ZrW
2O
8 contents of 0%, 2%, and 10%, with the corresponding EDS results detailed in
Table 4. Point 1 identifies a WC particle, and around its edges and the nearby matrix, many irregular needle-like, rod-like, petal-like, or blocky structures are observed at Points 2, 4, 6, and 8. These belong to carbide-hard phases such as M
23C
6 and M
3C
2. The dendritic structure at Point 7, dispersed throughout the cladding layer, is mainly composed of W, Ni, Cr, and Fe, forming hard phases like M
3C
2 and solid solution-strengthened products like Ni
17W
3 and Cr
4Ni
15W. Points 3, 5, 9, and 10 contain high concentrations of Ni and Fe, primarily constituting the γ (Ni,Fe) matrix phase. Notably, when the ZrW
2O
8 content is 10%, the upper layer of the cladding consists of finer WC particles and dendritic structures (Point 7). The middle layer is filled with fine, dense black particles (Point 9) and a small number of smaller rod-like structures (Point 10). The edges of the WC particles exhibit fine, blocky M
xC
y phases. The directionally solidified structure of the cladding layer with 10% ZrW
2O
8 is more uniform and denser than that of the layer without ZrW
2O
8. Grain refinement enhances the material’s plasticity and toughness, while the special directional structure inhibits crack propagation, grain boundary sliding, and dislocation movement [
23]. Therefore, adding an appropriate amount of ZrW
2O
8 can improve the microstructure of the cladding layer and reduce the crack sensitivity of the cladding coating.
Figure 9 and
Figure 10 show the EDS surface scanning results for Ni, Cr, Fe, W, C, Si, and B in samples without ZrW
2O
8 and with 10% ZrW
2O
8, respectively. Comparing
Figure 9 and
Figure 10, it is evident that local segregation occurs in the element distributions of the samples without ZrW
2O
8, whereas the sample with 10% ZrW
2O
8 shows a more uniform distribution of elements, particularly W.
These phenomena further indicate that the addition of a certain amount of ZrW
2O
8 can play a role in reducing the microstructure size of the coating and making the microstructure more uniform and finer, consistent with the results shown in
Figure 8.
Combining the XRD results with the microstructure of the cladding coating reveals that adding ZrW
2O
8 to the composite powder increases WC dissolution, enhances the solid solution strengthening effect of W, refines the microstructure, and generates ZrO
2 in situ. The addition of an appropriate amount of ZrW
2O
8 reduces the overall thermal expansion coefficient of the alloy powder, improving the coating’s resistance to residual stress. Additionally, ZrW
2O
8 decomposes into different ZrO
2 phase structures under varying solidification conditions. During the laser cladding process, the stress-induced phase transformation of ZrO
2 particles generates compressive stress on the main phase of the coating, halting crack propagation. Furthermore, the dispersed ZrO
2 particles pin the cracks, causing deflection, twisting, and branching, dissipating the driving force of crack propagation. The in situ generated ZrO
2 enhances the toughness of the coating and promotes crack self-healing [
24,
25]. Therefore, when an appropriate amount of ZrW
2O
8 is added, the phase transformation and dispersion of in situ generated ZrO
2 play a crucial role in crack self-healing.
3.6. Wear Behaviors of Coatings
Figure 12 displays the wear curves of various samples under identical conditions. Initially, the wear coefficients of most samples are unstable but stabilize over time. The minimum wear coefficient values are observed in the coating without ZrW
2O
8. As the ZrW
2O
8 content increases, the amplitude of the wear curves initially rises and then decreases. When the other samples reach the stable wear stage, the wear coefficient of the sample with 2% ZrW
2O
8 remains in the rising stage and is higher. Conversely, the entire wear process of the coatings with 7% and 10% ZrW
2O
8 is steady.
Figure 13 illustrates the two-dimensional morphology of wear tracks and the volume wear rate of coatings after wear at various ZrW
2O
8 contents. It is observed that as the ZrW
2O
8 content in the coatings increases, both the maximum depth and maximum width of the wear tracks initially increase before decreasing. Simultaneously, the volume wear rate of the coatings also exhibits a trend of an initial increase followed by a decrease. At a ZrW
2O
8 content of 10%, the wear rate is minimized, indicating the highest wear resistance.
Figure 14 and
Table 5 present SEM images and EDS analysis of the worn surfaces on the coatings. The coating without ZrW
2O
8 exhibits a few thin furrows and shallow adhesion pits. The combination of wear scar morphology and energy spectrum analysis reveals that the primary wear types are adhesive and oxidation wear.
Figure 14b,c show adhesive pits and peeling layers caused by crack propagation and tearing. Analysis of point D indicates significant adhesive and fatigue wear, with slight oxidation wear, in the coatings with 2% and 4% ZrW
2O
8. Consequently, these coatings have higher wear rates, as shown in
Figure 13b. When the ZrW
2O
8 content is between 7% and 10%, the main components of the spalling layers shift from γ (Ni,Fe) to boride and carbide, indicating strengthened γ (Ni,Fe) as the primary matrix. The worn surface of the coating with 7% ZrW
2O
8 is smooth, with minimal adhesive pits and peeling layers. In contrast, the coating with 10% ZrW
2O
8 shows only a few adhesive pits and no overall exfoliation.
When the ZrW2O8 content is low, numerous cracks form in the composite coating, compromising its microstructure and performance. This results in an unstable friction coefficient and a higher wear rate. However, when the ZrW2O8 content exceeds 7%, the crack sensitivity of the coating decreases, producing a fine, crack-free microstructure. The increased dissolution of WC-reinforced phases leads to the dispersion of small WC particles in the γ (Ni,Fe) matrix, enhancing its toughness. The dissolved WC particles further strengthen the γ (Ni,Fe) matrix through a new metallurgical reaction with Ni and Cr atoms, forming Ni17W3- and Cr4Ni15W-reinforced phases. These small, abundant reinforcement phases bear the main load during friction, reducing stress concentration and providing effective pinning reinforcement. Additionally, the unique, uniform directional structure formed by non-equilibrium solidification imparts high strength and toughness to the composite, improving its resistance to external scratches and preventing the penetration of abrasive particles. This uniform support mitigates crack formation and spalling during wear. In conclusion, the addition of ZrW2O8 not only avoids negative effects on the coating’s wear resistance but also ensures wear stability.
3.7. Electrochemical Measurements of Coatings
Figure 15 shows the potentiodynamic polarization curves of the coatings in a 3.5 wt% NaCl solution at room temperature.
Table 6 lists the corrosion potential (E
corr), corrosion current density (I
corr), and polarization resistance (R
p) of the investigated coatings. Lower I
corr, higher E
corr, and higher R
p indicate better corrosion resistance. As seen in
Figure 15 and
Table 6, the earlier onset of the passivation zone, increased corrosion potential and polarization resistance, and decreased corrosion current density demonstrate that the add ition of ZrW
2O
8 significantly enhances the corrosion resistance of the Ni-based coating. Notably, the coating with 10% ZrW
2O
8 exhibits the highest corrosion resistance, while the coating with 4% ZrW
2O
8 shows the lowest.
When the ZrW
2O
8 content is low, defects such as microcracks, pores, impurities, and structural segregation in the coating impede the formation of a stable passivation film, enhancing electrochemical non-uniformity on the composite coating’s surface. This makes the coating susceptible to pitting, crevice, and grain boundary corrosion, significantly reducing its corrosion resistance [
26]. When the ZrW
2O
8 content exceeds 7%, the coating exhibits fewer pores and suppressed cracks, diminishing pitting, crevice, and grain boundary corrosion. Moreover, the more evenly distributed elements, similar grain orientation, and fine microstructure enhance the nucleation sites for the passivation film, facilitating the formation of a dense passivation layer on the surface. Additionally, the increased solid solution of W in the coating’s dendrites improves its thermodynamic stability, reducing the number of galvanic cells. Consequently, the corrosion resistance of the coating with a ZrW
2O
8 content greater than 7% is significantly enhanced.