3.3. Optimization of Composite Plating Process Parameters
In the composite plating process, there are many factors affecting the quality of the composite coating, among which plating time, plating temperature and nanoparticle additions have the greatest influence. The results of the L9 (3
4) orthogonal experiments and range analysis are shown in
Table 5.
According to the experimental results, the degree of influence of the three factors on electroless composite plating is ranked as follows: A > B > C.
The effect of plating time on the electroless composite plating. After a certain time of deposition of metal ions in the plating solution, the thickness of the coating thickened with the increase in plating time. If the plating time was too short, the thickness of the coating did not meet the standard and had poor surface continuity. If the plating time was too long, it led to a decrease in ionic concentration, and the reaction was weak or even did not react, which consumed a lot of energy. Therefore, it needed to be strictly controlled by the time of the plating in the plating process. Under the conditions of the experiments, the optimum plating time was 60 min.
The effect of the nanoparticle concentration on the electroless composite plating. In the experimental process, the plating rate increased as the concentration of nanoparticle increased. When the nanoparticle concentration reached a certain level, the plating rate decreased accordingly, as shown in
Figure 10. Regarding the mechanism, a certain amount of nanoparticles was added to the plating solution. They were deposited with Ni-P on the surface of the sample, thereby increasing the surface area of autocatalysis. When the nanoparticle concentration was saturated, the non-conductive nanoparticles were prone to agglomeration [
30], which masked some catalytic active points and they lost their effectiveness, and the plating rate slowed down, so the optimal amount of nanoparticle addition was 1.5 g/L.
The effect of temperature on electroless composite plating. Temperature can provide energy for the reaction, making more molecules more active so that the molecules attach to the surface of the sample faster and the rate of reaction speeds up. The optimal temperature obtained from the experiment was 85 °C.
In summary, the optimal combination of this experiment was A2B1C1, with the plating time of 60 min, the nanoparticle addition of 1.5 g/L and the plating temperature of 85 °C.
3.4. The Analysis of the Composite Coating
The surface morphology and constituent elements of the samples prepared under the optimal process were analyzed by SEM and EDS. As shown in
Figure 11, the coating surface contained four elements: Al, Ni, P and O. The elemental distribution map shows that Ni, Al and O were uniformly distributed on the surface of the samples. The scan data were combined and
Figure 12 shows that the Al
2O
3 nanoparticles uniformly dispersed on the surface of the sample along with the nickel grains, which corroborated the successful addition of the nanoparticles.
The cross-section of the coating prepared under the optimal process is shown in
Figure 13a. The coating was tightly bonded to the substrate with uniform thickness, good conformality and a thickness of 12 μm. To test the binding force of the coating, thermal shock experiments were conducted on the coating prepared under the optimal process conditions. The coating surface showed no cracking or peeling after the thermal shock experiments.
A cross-cut tester was used for scratch experiments. The sample was placed on a flat plate and the cross-cut tester was perpendicular to the sample plane. The instrument was then used to cut at a uniform pressure and stable cutting speed. Next, the sample was rotated by 90° and the above operation was repeated to form the lattice pattern, as shown in
Figure 13b. The figure shows that the coating coverage was completely covered, and no peeling of the coating was found at the cut and cross, indicating that the coating prepared was of good adhesion. According to the GB/T9286-2021 standard for the evaluation of the sample adhesion, the binding level of the sample was 0, while the coating prepared by traditional zinc dipping method showed a peeling phenomenon in the scratch test, as shown in
Figure 13c.
A digital microhardness tester was used to perform an indentation process on the sample surface and a light microscope was used to observe the measured indentation and measure the diagonal length of each indentation. Then, the surface hardness was obtained through Formula (3). The results are shown in
Table 6. Comparing testing results of the hardness, the microhardness of the substrate surface was 200.1 ± 0.5 HV
500gf and the surface hardness of the Ni/Al
2O
3 composite coating by electroless plating was increased to 688.5 ± 0.5 HV
500gf, while the surface hardness of the sample prepared by the traditional process was only 593.7 ± 0.5 HV
500gf. It shows that the surface hardness of the sample prepared by the new process was significantly improved compared with the traditional process. The main reason for the improvement of hardness was that the hard particle phase Al
2O
3 nanoparticles themselves had extremely high hardness. From the analysis of
Figure 12, it can be seen that Al
2O
3 nanoparticles deposited on the substrate surface to form a composite coating, and were embedded in the coating, which played the role of dispersion reinforcement. As one of the main components of the coating, the performance of Al
2O
3 nanoparticles reacted to the coating at the same time, so the hardness of the composite coating was improved.
Rtec Viewer (1.0.4.0) software was used to fit the friction and wear experimental data, and compare friction coefficients.
Figure 14a shows that the friction coefficient of the titanium alloy substrate surface was around 0.45 ± 0.05 in the friction and wear experiments. Under the optimal process conditions, the friction coefficient of the electroless nickel plating layer was around 0.35 ± 0.05, and the friction coefficient of the electroless Ni-P/Al
2O
3 composite plating layer was around 0.2 ± 0.05. This further demonstrates that electroless composite plating can enhance the surface wear resistance of the material, thereby enhancing the life span of the material in a frictional wear environment.
SEM was used to observe the wear surface of the sample. As shown in
Figure 14b, the abrasion marks on the wear surface were mainly typical parallel furrows, indicating that the main mechanism of sample wear was abrasive wear. The figure shows that the surface abrasion marks were shallow and did not damage the substrate. Some particles were scattered on the abraded surface, which should be the Al
2O
3 nanoparticles embedded on the surface of the coating during the abrasion process. The addition of nanoparticles increased the hardness of the coating, and the anti-abrasion performance was greatly improved. At the same time, the scattered nanoparticles also indicated that the nanoparticles reduced the friction by changing the friction form, i.e., sliding friction to composite friction of sliding and rolling, thus improving the wear resistance of the composite coating.
The roughness of the material surface also had some influence on the wear resistance of the substrate, as seen in
Figure 15. The roughness was kept between 11.52 μm and 12.29 μm, and the change in roughness was relatively small, which indicated that the coating prepared by this method was relatively smooth.