Electro-deposition Processes, Microstructures And Properties Of Ni-P & (Ni-P)-SiC Coatings

Electro-deposition of Ni-P alloy has been given more and more attention because of the excellent performance of obtained coatings. In this work, the initial deposition behavior, microstructure, corrosion-resistance and abrasion resistance of Ni-P alloy coatings are studied systemically on the base of successful development of electro-deposition technology with industrial value. The role of rare earth elements on electro-deposition of Ni-P alloy is also studied A fast speed electro-deposition method in centrifugal force field is proposed. The technical parameters and the performance of (Ni-P) -SiC composite coating obtained in centrifugal field are also studiedThe cyclic voltammeter and the cathode polarization techniques have been used to study the effect of additives on Ni-P alloy coatings. On the base of experimental results, the compositions of bath, containing FM as a main complexing agent and 2g/L rare earth chlorides in it, as well as the technical conditions are determined. High quality coatings with low, middle and high phosphorus can be obtained by using this bath.Study on the initial electro-deposition behavior of Ni-P alloy shows that it is influenced by the microstructure of matrix material. On the surface of annealed 45 steel, as the pearlite in it contains cementite phase and has high crystal boundary density, the nucleation and growth of deposits prefer to occur at the pearlite at the beginning during depositioa The coating exists in form of nano-polycrystal layers composed by gathering of nano-sized crystals. With plating time going on, the nano-polycrystal gathering grows in two dimensions, and the new nano-polycrystal layer appears simultaneously on the formerly formed nano-polycrystal surface. Because the boundary and defect densities in quenched and tempered 45 steel is higher than those in annealed steel, the nano-polycrystal gatherings formed at its surface at the beginning of electro-deposition are more concentrated, and then their size is smaller.The microstructure of the Ni-P alloy coating has closed relation with its phosphorus content With the increase of phosphorus content, the microstructure of coating turns from nano-crystalline to the amorphous. X-ray and TEM are used to analyze the crystallization behavior and the phases formed during afterward heat treatment In the amorphous coating with 10.6%P, meta-stable phase NisP2, forms during crystallization; the crystallization completes, when the temperature reaches 400 癈; the crystallization process is as follows: Ni-10.6wt%P梟ickel solid solution + NisP2 meta-stable phase桸i solid solution + NisP. In the coating with Ni-3.4wt%P, the NisP phase precipitates during heat treatment and the nickel crystal grows up gradually with the raising of temperature.Hardness, corrosion resistance, and abrasion resistance of Ni-P alloy coating are investigated.The corrosion resistance of Ni-P alloy coating has close relationship with the content of phosphorus in it. The influence of phosphorus content on corrosion-resistance of Ni-P alloy shows different regulations in various corrosion mediums. The corrosion resistance of the coating drops down remarkably after heat treatment. Before heat treatment, the coating with low phosphorus content is harder than that with high phosphorus content and its abrasion-resistance is also better. After heat treatment, hardness of the coating is greatly improved because of precipitation of NiaP phase, while the difference of wear resistance between low-P coatings and high-P ones becomes little. The friction coefficient and the abrasion mechanism of the Ni-P alloy are related to the applied load. The abrasion form transforms from light abrasion to oxidation one, and then becomes adhere abrasion with the increase of load.The investigations about the effects of rare earth elements on Ni-P alloy coating show that the properties of bath for Ni-P alloy plating and obtained coating are improved by adding some rare earth chlorides in the bath. The corrosion resistance is the best when the content of rare ear...