1. Surface Topography and Composition
Surface Roughness
On the surface, NiCrBSi coatings exhibited a remarkably uniform appearance. As depicted in
Figure 1, three prominent peaks, representing adherent powder particles, adorned the surface, resulting in heightened surface roughness (Sa value of 12–15 μm). These resilient particles, primarily composed of partially melted and un-melted constituents, provided a foundation for attached particles [
46].
Moreover, an interesting revelation was the transformation in the average surface roughness (Ra), which increased to 1.5–2.5 μm, diverging from the base metal’s more modest Ra value of 1–2 μm. This transformation yielded tangible benefits in tribology.
In the laser metal deposition process Table 2, the laser power (P) influences the heat input and material melting, while the scanning speed (S) controls the rate of material deposition. Adjusting the powder feed rate (F), laser diameter (d), overlap, and energy density (E) allows precise control over the resulting material properties and geometry.
2.2. Microstructures post laser metal deposition
The microstructure examination, as presented in
Figure 2,
Figure 3 and
Figure 4, reveals the outstanding quality of the interface between the substrate and overlay. The absence of cracks or delamination defects underscores the success of the deposition process in achieving a strong bond between the two materials [
26,
27,
29].
The meticulous distribution of WC particles within the Ni matrix is a testament to the precision and effectiveness of the surface modification technique, further contributing to the enhancement of material properties. The presence of minuscule pores, particularly in the interface region, is a point of consideration. While their impact on the overall performance should be assessed, their presence does not diminish the overall achievement in creating a robust and cohesive interface.
Upon subjecting the Ni matrix to chemical etching using a mixture of 5% HNO3 and 95% Methanol, a distinct dendritic structure emerges. This dendritic structure, in line with the cooling direction, is characterized by a composition of 74% Ni, 6% Cr, and 10% W near the steel substrate. The interface zone, deeper in color, exhibits a composition rich in Fe and Ni. These distinct domains result from complex chemical reactions during the hard facing operation.
[47]
The hard facing operation initiates with the liquefaction of the Ni matrix, facilitating the dissolution of WC particles into the molten matrix. During this process, WC particles undergo a transformation, with a discernible reduction in size near the interfaces. This is well-established, as smaller particles tend to exhibit a heightened propensity for dissolution. The liberated carbon (C) from the dissolution of WC diffuses within the liquid matrix, leading to the formation of intermetallic phases with Cr, such as Cr23C6, and eutectic phases involving W2C in the vicinity of WC particle peripheries. These transformations explain the observed increase in microhardness within the hard-faced layers compared to the matrix. [
26,
27,
29,
46]
The presence of additional phases within the Ni matrix, including W4Ni
[31], can be attributed to the high melting point of W and its relatively generous solubility threshold in Ni, as per the W-Ni binary phase diagram. As the temperature recedes, dendritic phases with a high Ni content but low W content form
[32].
The entrainment of W within the matrix, fragmentation of diminutive WC particles, and alteration of WC particle morphology collectively poses challenges to the wear resistance of the WC particle-reinforced composite. The formation of intermetallic phases within the Ni matrix results in a reduction in matrix ductility, which can adversely affect wear resistance [
26,
32].
To mitigate this effect, careful control over the chemical composition and structural attributes of the WC particles, as well as judicious manipulation of matrix alloy parameters, becomes imperative. The size and configuration of WC particles also need to be considered to ensure their equitable dispersion within the matrix. The selection of an appropriate matrix alloy plays a critical role in shaping the wear and abrasive characteristics of the layer, striking a balance among mechanical properties like hardness, strength, and ductility.
The gradual transformation of Ni’s microstructure from a dendritic morphology near the substrate to an equiaxial configuration surrounding larger WC particles can be attributed to shifting nucleation mechanisms [
30,
32]. WC particles serve as heat sources during the cooling process, fostering the formation of equiaxial-shaped phases near the WC particles. The presence of oxides and pores at this interface is influenced by factors such as the interaction of materials with ambient air at elevated temperatures during the thermal spray process and the impact velocity of interface particles.
The interface between the WC-NiCrBSi-Fe alloy hard facing and the steel substrate is seamless, resulting from the interplay of Ni, Cr, Fe, and W [
37,
39]. SEM line scan analyses reveal shown in the
Figure 6–8. the variation in chemical composition along the interfaces. The mutual solubility of these elements, particularly Ni and Fe, contributes to the inter-diffusion across the interface, strengthening the bonding and mitigating stress at the junction.
The SEM EDS analysis provides insight into the microstructural composition of the NiCrBSi coatings and reveals the presence of various phases, including Ni/Ni3B, Cr7C3 carbides, and CrB borides. The microstructure of the composite coatings is complex, with NiCrW solid solutions, WC, Cr7C3, and Cr23C6, along with eutectic phases and oxides. The composition varies depending on the proportion of WC and matrix alloy in the coatings, impacting hardness and other mechanical properties
[46].
The microstructure and composition play a crucial role in the wear resistance and performance of hard-facing materials. The transformation of WC particles, the formation of intermetallic phases, and the evolution of microstructure provide insights for optimizing material properties in demanding applications.
The microstructure analysis provides valuable insights into the formation of intermetallic phases, the impact on wear resistance, and the intricate interplay of elements within the material. It also underscores the importance of carefully controlling the composition and structure of WC particles and the choice of the matrix alloy.
The seamless interface between the hard facing layer and the steel substrate is vital in mitigating stress at the junction, contributing to the overall effectiveness of the coating.
Figure 9 presents a comprehensive overview of microhardness depth profiles of coatings fabricated under various processing parameters. Notably, an undulatory pattern is discernible in the hardness values as they traverse different depths in all specimens. This pattern is intricately linked to the stratified microstructure within the molten pool’s cross-section. The stratification is influenced by the morphology of structural constituents and the distribution of tungsten-rich carbide precipitates, which impact the concentration of the carbide phase in distinct cross-sectional regions.
The influence of processing parameters on hardness is a critical aspect of our study, as demonstrated in
Figure 9. This figure highlights the impact of varying the laser current while maintaining a constant rate, as well as the effect of altering the laser operating speed while keeping the current constant. Notably, our analysis reveals that the average hardness values within the remelted zones exhibit a relatively narrow range, fluctuating between approximately ~1050 HK0.5 and ~1150 HK0.5 across the different samples. This observation underscores the significance of fine-tuning processing parameters to achieve the desired hardness characteristics in laser-clad materials. [
5,
9,
19,
35]
Interestingly, there is a reasonable but small decrease in hardness when laser operating speed decreases and laser current increases. This trend correlates harmoniously with the size of the main tungsten-rich phase, as indicated in the
Figure 9. Notably, samples 2 and 3, remelted at maximum power density and heat input, exhibit the lowest hardness levels at the interface.
Within samples characterized by a penetration depth exceeding the coating thickness (sample 3), a significant iron concentration of approximately ~16 wt% is observed, nearly threefold higher than its counterparts. This elevated iron concentration results from the extensive intermingling between the coating and substrate materials during the remelting process, coupled with a slightly coarser structure. These factors collectively contribute to the comparatively lower hardness of the remelted layers.
The microhardness depth profiles obtained in our study shed light on the intriguing characteristics of spherical-cast carbides dispersed throughout the coating’s microstructure, yielding remarkable hardness levels ranging from 2500 HV to 3100 HV. As depicted in the figures, a distinctive undulatory pattern in hardness values is discernible, closely related to the stratified microstructure within the molten pool [
35]. This stratification is intricately woven by the morphology of structural constituents and the distribution of tungsten-rich carbide precipitates, significantly impacting the concentration of the carbide phase in distinct cross-sectional domains. Our findings underscore the intricate relationship between processing parameters and coating hardness, with a reasonable yet discernible decrease in hardness as laser operating speed decreases and laser current increases. Notably, this trend harmonizes with the size of the main tungsten-rich phase. Furthermore, our study elucidates the factors contributing to variations in hardness levels, particularly the extent of intermingling between coating and substrate materials during the remelting process. Understanding these dynamics is pivotal for optimizing the properties of hard-facing materials, where spherical-cast carbides play a pivotal role in achieving exceptional hardness levels ranging from 2500 HV to 3100 HV distributed throughout the layer.