4.1 Microhardness
The cross section microhardness of the gradient coating along the depth of the coatings is shown in Fig. 8. Across the depth of the cladding layer, the microhardness presents a decreasing trend. It is indicated that the coating can slow down the abrupt change of mechanical properties. The average hardness of the third layer, the second layer and the first layer stepwise decreased in corresponding order. The surface microhardness is the highest, and the microhardness value reaches 1342 HV0.2. The microhardness of the interface between the substrate and the cladding layer is the lowest due to the larger grains being produced over a long period of thermal interaction. The microhardness of WC particles in the cladding layer is 2685 HV0.2, and the mechanical properties of the cladding layer are enhanced by the dispersion of WC, diamond and other strengthening phases in the cladding layer.
4.2 Wear resistance
The comparison diagram of the friction coefficient and wear loss between the HSS substrate and composite gradient cladding layer is shown in Fig. 9. Before friction and wear testing, the frictional surfaces of each specimen were prepared with roughness values ranging from 0.13 μm to 0.15 μm. Variations on the friction coefficients within 30 minutes are presented in Fig. 9(a). One can conclude that the measured curves of the friction coefficients are characterized by the value of the oscillation of amplitude. As for the cladding layer, the friction curve fluctuates extremely in the initial stage. The mean value of the friction coefficient (ECof) is approximately 0.27. It can be also seen that the friction curve of the cladding layer fluctuates extremely at the initial stage due to protruded diamond particles leading to the fluctuation of the tangential force during the sliding test. As a feature of distinguishing the sliding stage, the curve of friction coefficient tending to be stable, as well as the appearance of smaller oscillations indicates that the transition from the initial stage to the stable stage begins to occur [29]. The sliding curves can be divided into different stages, as shown in Fig. 9(a) marked with the dash lines. Comparing with sliding the substrate, a longer wear initial stage was developed on sliding the cladding layer. It can be explained that the running-in time was prolonged owing to the superhard abrasive being distributed close to the top third layer. After stepping to the abrasion wear stage, the grinding ball began to contact with WC particles in addition to the diamond grains. The friction coefficient remains smooth and steady relatively due to the increase in the contact area between the ball and the cladding layer. In the breakage stage, part of the diamond grains were fractured and the broken particles were pushed into the contact area between the grinding ball and the worn surface of the layer along the sliding direction. The friction coefficient rose steeply. Subsequently, the sliding process steps into a stable stage with a slow increase of the friction coefficient. Three-body wear has been evoked due to the broken particles participating in the sliding process, which resulted in the fluctuation of the friction coefficient being risen.
One of the most glaring differences between the two curves is that the fluctuation of the friction coefficient in each stage(Fig. 9(a)). In the initial stage, the friction coefficient fluctuates widely owing to a poor surface quality similarity between grinding ball and sample. The higher surface quality similarity of the contact pairs will be produced as the sliding process develops. The friction coefficient tends to be stable. For the gradient cladding layer, there are significant variations in the material properties between layers. During the sliding process, not only would be the new texture of the coating ground, but the surface with the changed material arose at the contact area. And then it is difficult to form the high surface quality similarity between the ball and the ground surface. Therefore, a considerable oscillation of the friction coefficient is performed in four wear stages. The friction coefficients in sliding the two specimens become steady with the surface quality similarity of the pair increasing. A similar conclusion can be noted in the publication [30]. The larger ECof and wear loss were tested in the ball sliding the HSS substrate (Fig. 9(b)). ECof of the cladding layer is 0.33, and ECof of the substrate is 0.56. ECof of the cladding layer is decreased by twice as much as that of sliding the substrate. The wear loss of the cladding layer is 1.1 mg, and that of the substrate is 3.5 mg. The wear loss of the cladding layer is 1/3 of that of HSS substrate. Compared with the HSS substrate, the newly prepared gradient cladding layer significantly improves the wear resistance of HSS.
In-depth understanding of wear resistance of the gradient cladding layer, the micro mechanism of improved wear resistance, the grinding mark on the friction and wear surface, as well as the interface between the abrasive and adhesive phase, was observed by the means of scanning electron microscopy (SEM), as shown in Fig. 10. The wear mechanism can be divided into adhesive wear, abrasive wear and fatigue wear according to the damage and failure modes of the friction surface. These various wear modes often occur simultaneously in one sliding test.
The distribution of hard particles, such as WC, diamond, and Co-based solid solution are dispersive and homogeneous in the high preparing wear-resistant coating layer. These hard particles are usually combined with the hard toughness adhesive phase in preparing the cladding layer. The hard particles protruding out from the surface can bear the load applied in the sliding test. The adhesive phase contributes to forming the compactness structure for the hard particles in the cladding layer, which is positive to absorb the energy and buffer the loads as the grinding ball slides over the surface of the composites. It is a reasonable origin for improving the comprehensive performance of the cladding layer [31, 32].
The micro-cutting furrow is obvious on the worn surface, as shown in Fig. 10(a). The micro-cutting furrow is consistent with the sliding direction of the counter-grinding ball. Due to the hardness of the ceramic ball being much higher than that of the cladding layer, the characteristic of the wrinkles generated on the surface of the adhesive phase along the wear scar indicates that the wear mode could be tagged with abrasive wear. Additionally, diamond particles are still embedded in the cladding surface despite a small amount being pulled out during the friction and wear process. A few microcracks could be discovered on the sliding track. Multi-layer laser cladding usually leads to geometric deformation in the superposition, while stress concentration is formed in the gradient layers. When the tensile stress close to the top layer exceeds the allowable stress, the microcrack propagation will be developed both in the cladding process and the sliding tests. Fig. 10(b) is an enlarged local view of the bottom of the wear marks. It can be noted that a little debris, as well as a few scabs, are sticking to the marks. It is indicated that adhesive wear was generated between the grinding ball and the cladding layer in the sliding test. On side of the wear mark (Fig. 10(c)), accumulation due to sliding plastic material can hardly be found on the side of the wear mark and a lot of plastic scratches were obviously observed in the adhesive alloy (Fig. 10(d)). It is implied that the hardness of the cladding material is much higher to cause little plastic formation.
The ingredient of debris close to the worn WC particle is marked in Fig. 10(e). It mainly includes the elements such as aluminum, oxygen, chromium, cobalt, and tungsten, as shown in Fig. 10(f). The element of aluminum contained in the debris indicates that the adhesive wear was generated in the sliding test. Also, a much higher content of oxygen detected on the wear surface implies that the oxidation wear mode occurs simultaneously. During the sliding test, the WC particles are difficult to be ground and the particles begin to emerge as the adhesive phase becomes worn out. The WC particles would bear the plow load coming from the grinding ball along the sliding direction and the micro material of the ball located in the contact area had been removed [33]. The removed material of the ball got buried in the worn surface and adhesive wear was produced. As well, a large amount of friction heat was induced during the sliding process. The brittle oxide film of the Co-Cr alloy would be produced on the worn surface under the condition of exposure to air. Due to the diamond particles affording much of the sliding load, the edge of the particle was gradually ground to be dull and a little material around the particles had hardly been removed even if a few of the diamond grits were damaged (Fig. 10(g)). Micro spalling pits (Fig. 10(h)) observed on the diamond surface were formed arising from the brittleness removal according to the material removal mechanism [34]. A few diamond particles would be slightly damaged to a low strength status and turned to be brittleness after the laser cladding. Therefore, a small number of thermal cracks inevitably existed in diamond particles [35]. When the ball slides over the defect particles, the cracks will be propagated and diamond particles will be crushed to form the breakage (Fig. 10(i)).
The main wear modes induced in sliding the gradient cladding layer include abrasive wear, slight adhesive wear, and brittle peeling wear. The wear resistance of the gradient cladding layer is significantly improved due to the hard particles of the diamond and WC being embedded in the composites.
4.3 Corrosion resistance
Two specimens of the HSS substrate and the substrate with cladding layer were prepared for detecting the corrosion resistance. The corrosion kinetics are characterized by potentiodynamic polarization in NaCl solution with a molarity of 3.5% at a temperature of 25℃ and the results are presented in Fig. 11. The corrosion potential of the gradient cladding layer is higher than that of the HSS substrate. Passivation can be monitored on both surfaces of two samples when the corrosion potential is ranged on a scale of -0.6 to -0.4 V. The electrochemical parameters were determined based on analyzing the Tafel curves and the results are listed in Table 5. The corrosion potential (Ecorr), corrosion current density (Icorr) and Tafel constant (βa, βc) of anode/cathode were extracted by Tafel fitting method. The polarization resistance (Rp) can be calculated with the Stern-Geary equation [36]:
![](https://myfiles.space/user_files/69515_16346c490bab499e/69515_custom_files/img1624600517.png)
According to the standpoint of corrosion kinetics, the corrosion rate is positively correlated with current density, while polarization resistance is inversely proportional to the current density. In general, the parameters of Ecorr, Icorr, and Rp are important for evaluating the corrosion characteristics of materials. Larger values of positive Ecorr and Rp together with the lower value of Icorr will reflect an excellent corrosion resistance [37, 38]. Comparing the electrochemical data of the two samples, poor corrosion resistance can be predicted for the HSS substrate.
Table 5 Electrochemical data of substrate and coating
Sample
|
Ecorr(V)
|
Icorr (A/cm2)
|
βa (mv)
|
βc (mv)
|
Rp (Ω·cm2)
|
Substrate
|
-0.884
|
9.50E-05
|
3.742
|
6.744
|
11003.5
|
Coating
|
-0.755
|
4.74E-06
|
4.023
|
7.205
|
236488.1
|