Investigating of the electroless bath deposition temperature effect on the microstructural properties of Ni-P/Ni-B-CeO2 coatings

The current research aims to evaluate the effect of electroless bath deposition temperature on the microstructural properties of Ni-P/Ni-B-CeO2 nanocomposite coatings grown on 2024 aluminum substrate. Electroless plating is a coating method without applying an external current, and the required electrons are provided by chemical reactions in the bath, so choosing the right temperature for the coating process can provide the necessary energy for this reaction. Several samples of 2024 aluminum was cut in dimensions of 100mm × 100mm ×1mm. Effect of surface preparation on the coating adhesion was checked. The choice of 2024 aluminum substrate is directly linked to its wide range of applications in the aviation industry. Based on previous research, the reported properties of duplex coating of Ni-P and Ni-B prepared by electroless method such as microhardness, wear resistance and corrosion. In this type of double-layer coatings, the use of Ni-P coating as the top layer has higher corrosion resistance, and on the other hand, Ni-B coating results in higher hardness and wear resistance. In addition, due to its high chemical stability and the ability to modify the structure, ceria nanoparticles can play a significant role in improving the corrosion resistance, hardness and wear resistance of these types of coatings as reinforcing particles in the matrix of electroless coating. Therefore, we were used ceria nanoparticles to improve properties of coatings. For this purpose, electroless baths of high phosphorous (9-10wt%) were used. The pH of this bath was 4.6 with a coating temperature of 80°C. The second layer (Ni-B) was directly deposited on the sample without any surface preparation or activation. The nickel- boron coating was formed through an alkaline bath using a sodium borohydride reducing agent. After 30 minutes CeO2 nanoparticles were added with certain rate to electroless bath. The size of nanoparticles was 10-30 nm. Due to the strong tendency of nanoparticles to agglomerate, sodium dodecyl sulfate (SDS) has been used as a surfactant. To investigate and characterize the microstructure of the coatings, field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) was utilized. The results demonstrated that selecting an optimal temperature of T_O=85±3°C during the deposition process can lead to the formation of a suitable cauliflower-like morphology, appropriate deposition rate, and uniform distribution of nanoparticles in the coating. Increasing the deposition temperature (〖T T〗_O) resulted in unstable bath and the formation of non-uniform coatings with uneven distribution of nanoparticles.