3.1. Coating Cross-Sectional Morphology
The cross-sectional morphology of six typical samples is shown in
Figure 4. Within the coating of sample T0, numerous micro-cracks and micro-pores can be observed, likely caused by thermal mismatch between the coating and the substrate [
20]. Due to the significant difference in thermal expansion coefficients between the coating and the substrate, strains differ during the deposition and cooling processes, forming micro-cracks and micro-pores. In contrast, the coating adheres well to the substrate for textured samples, with only a few micro-pores observed and almost no micro-cracks. In the micro-dimpled surface processed by the picosecond laser, there are many micro-protrusions. Due to the small gaps between some of these micro-protrusions, the coating cannot nucleate and grow in these gaps, resulting in micro-pores. However, these micro-protrusions may simultaneously act as micro-anchors for the coating, and micro-dimples’ presence facilitates stress relief [
20], buffering substrate deformation. Therefore, it reduces the strain difference between the coating and the substrate during coating deposition and cooling, thereby reducing micro-cracks’ formation.
For different textured coating samples, it can be observed that samples D17 and R60 exhibit thinning of the coating at the edges of the micro-dimples. This is because the micro-dimples in the D17 and R60 samples have a larger depth-to-diameter ratio, resulting in a greater slope at their edges. This greater slope leads to stress concentration at the edges, causing a significant difference in deformation between the coating and the substrate in these areas. As a result, cracks are more likely to form, and even coating delamination can occur, causing the coating to thin. In contrast, the D3 and R140 samples exhibit gentler slopes at their edges, which reduces the stress concentration and results in a more uniform coating thickness in these regions, with no uncovered areas. Thus, as the depth-to-diameter ratio increases, the slope at the edges of the micro-dimples becomes steeper, leading to greater stress concentration, more cracks, thinner coatings, and potentially uncovered areas at the edges. This directly impacts the adhesion strength between the coating and the substrate at the edges.
3.2. Adhesion Strength of Coatings under Dynamic Contact Conditions
The critical load Lc at which continuous coating delamination begins was determined by combining changes in the coefficient of friction (COF) curve with two-dimensional optical micrographs and depth maps of the scratches. Typical COF curve and scratch morphology images are shown in
Figure 5. From the grayscale image of the scratch; it can be observed that the silver-white substrate becomes visible when the coating begins to delaminate continuously at the critical load. It is essential to note in the figure that due to minor elastic deformation of the loading rod caused by significant horizontal force during horizontal movement in the experiment, upon lifting the loading rod after the experiment, accumulated elastic potential energy is released, causing the loading rod to continue sliding along the scratch for a distance, leaving a ‘tail’ at the end of the scratch. The deepest point of the scratch, indicating the actual endpoint of the scratch in the experiment, can be determined from the scratch’s two-dimensional depth map.
The critical load Lc for coating failure in the scratching tests of each sample is shown in
Figure 6. It can be observed that the crucial load for all textured samples is higher than that for the non-textured sample, indicating stronger adhesion between the coating and the textured substrate surface. Additionally,
Figure 6a shows that for textured samples with a constant micro-dimple diameter, the critical load initially increases and then decreases with increasing micro-dimple depth. The maximum critical load is 59.8 N for a depth of 10 μm (sample AD), whereas it reduces to a minimum of 53.1 N for a depth of 17 μm (sample D17).
Figure 6b shows that with a constant micro-dimple depth, the critical load also initially increases and then decreases with increasing micro-dimple diameter. The maximum crucial load of 59.8 N is observed for a diameter of 100 μm (sample AD), while the minimum critical load of 53.1 N is observed for a diameter of 140 μm (sample R140). Furthermore, when the micro-dimple diameter is below 100 μm, the critical loads are relatively close. In contrast, they decrease more rapidly when the diameter exceeds 100 μm, indicating that 100 μm is a crucial threshold for maintaining good adhesion strength.
The reasons for the enhanced adhesion strength of the coating under dynamic contact conditions with micro-dimple textures may be attributed to two factors: (1) Micro-dimple textures increase the actual contact area of the substrate surface. The larger actual contact area provides more attachment space for the coating, allowing the micro-dimples to create a mechanical interlocking effect. Additionally, the micro-protrusions inside the dimples can also produce a micro-interlocking effect. When the coating experiences horizontal shear forces, the micro-interlocking effect of the micro-dimples can counteract some of the shear forces, reducing the shear strain on the coating. This enables the coating to withstand higher loads before failure, enhancing adhesion strength. (2) Textured surfaces reduce internal defects in the coating. During coating deposition and cooling, micro-dimples help to release some stress, thereby reducing the deformation difference between the substrate and the coating. This reduction helps minimize the formation of internal micro-cracks and micro-pores in the coating.
For textured samples, assuming the micro-dimple shape approximates an inverted cone with smooth inner walls, the adhesion area
for the textured substrate per unit projected area can be approximated as:
where
represents the texture area ratio, and
denotes the total inner wall area of micro-dimples within the unit projected area
Here,
is the projected area of a single micro-dimple, and
is the area of the inner wall of a single micro-dimple.
Here, is the diameter of the micro-dimple, and is the depth of the micro-dimple.
Substituting Equations (3)–(5) into Equation (2), we obtain:
From Equation (6), it can be seen that when the micro-texture area ratio is determined, the adhesion area of the textured surface is proportional to the depth-to-diameter ratio
. When the diameter of the micro-dimple remains constant, an increase in depth increases the depth-to-diameter ratio, thereby increasing the adhesion area of the textured surface. This enhances the mechanical interlocking effect on the substrate surface, improving the adhesion strength between the coating and the substrate. However, at the same time, due to the concentration of internal stresses in the layer at the edges of the micro-dimples [
19], as the depth-to-diameter ratio increases, the corners at the edges of the dimples become smaller, leading to increased stress concentration at the edges. This increases the likelihood of thinning or even delamination of the coating at the edges, thereby reducing the adhesion strength between the coating and the substrate. When the depth of the micro-dimple remains constant, an increase in diameter decreases the depth-to-diameter ratio, reducing the adhesion area and weakening the mechanical interlocking effect. However, lower edge stresses minimize the likelihood of edge thinning and delamination, strengthening the adhesion between the coating and the substrate. Therefore, an optimal depth-to-diameter ratio, which in this study is 0.1, corresponds to sample RD.
3.3. Coating Crack Propagation Behavior under Static Contact Conditions
After the indentation tests, SEM images of the six typical samples’ indentation sites are shown in
Figure 7. For the T0 sample, partial coating delamination appeared at the edge of the indentation, with larger coating delamination areas around the indentation. From the magnified images of the delamination areas, it can be seen that the shapes of the delaminated regions are mostly regular rectangles, with radial cracks (from now on referred to as “radial cracks”) and circumferential cracks (from now on referred to as “circumferential cracks”) distributed around the indentation (
Figure 7, T0, C). This indicates that these delaminations are caused by the network of cracks formed by radial and circumferential cracks. This is more pronounced in an area where the coating is about to delaminate (
Figure 7, T0, D). No coating delamination was observed around the indentation of the textured samples, with only a small amount of coating delamination observed around the micro-dimple areas of some samples, and the shapes were irregular. Among them, samples D17 and R60 had larger delamination areas, with larger cracks appearing around the delamination areas. No coating delamination was observed on sample RD. Around its micro-dimples, some cracks were observed extending along the edges of the micro-dimples, and a few cracks extended into the micro-dimples before stopping. On samples D3 and R140, some cracks extended into the interior of the micro-dimples, with a small amount of coating delamination appearing at the edges of the micro-dimples. Thus, it is evident that textured samples can effectively suppress crack formation, thereby reducing coating delamination and enhancing adhesion strength. As the texture diameter or depth increases, the ability of the texture to suppress cracks initially improves but then deteriorates. When the depth-to-diameter ratio is large, cracks tend to grow along the texture edges, while a smaller ratio causes cracks to propagate inward.
After static contact is applied to the surface, the pressure around the indentation center shows a radial distribution, causing the material to deform in a bulging manner. As the load linearly increases, the bulge expands outward, increasing in diameter, thereby inducing circumferential tensile stress in the material. Radial cracks form when this tensile stress exceeds the material’s tensile limit. Simultaneously, as the indentation depth increases, more material is squeezed out, increasing bulge height. Differences in bulge height between the inner and outer rings of the indentation result in normal shear stress on the material. When this shear stress exceeds the material’s tensile limit, circumferential cracks form. The network of cracks formed by radial and circumferential cracks ultimately leads to coating delamination [
23]. When the surface is textured, the extension of radial and circumferential cracks changes due to the obstruction of the texture, extending along the edges of the texture and increasing the energy required for crack propagation, thereby reducing the formation of crack networks and minimizing coating delamination.
Figure 8 illustrates the formation of radial and circumferential cracks and the inhibitory effect of micro-dimple textures on crack propagation. However, when the depth-to-diameter ratio is small (samples D3, R140), this inhibitory effect is reduced, allowing some radial cracks to extend into the interior of the micro-dimples, forming a network of cracks inside and even causing a small amount of coating delamination. Conversely, when the depth-to-diameter ratio is large (samples D17, R60), radial cracks are more likely to extend along the edges of the micro-dimples due to coating thinning at the edges of the micro-dimples, intersecting with circumferential cracks to form a network of cracks, resulting in more coating delamination.