Preparation Of Ni, Ag, Cr Films And Study Of Their Microstructure And Mechanical Properties

Ni-film, Ag-film and Cr-film where deposited on different substrates by means of direct current electrodeposition, and the microstructures, grain sizes and microhardness of electrodeposited metallic films were investigated, and then the optimum processes were obtained. At the same time, the mean stresses in Ni film on Fe substrates and Ag film on Cu substrates and Ni film and Cu film on Ag substrates were evaluated by a modified cantilever beam method. Theoretical internal stresses were calculated using Thomas–Feimi–Dirac–Cheng (TFDC) electron theory model, which were compared to experimental results. The characteristics and origin of internal stresses in the films were analysed, and the correctness of generation mechanism model of intrinsic stress in film was proved.The results indicated that additives had obvious influences on microstructure, texture, grain size and hardness of films. The pinhole phenomenon in bright nickel plating disappeared, and the preferred orientation of the crystal plane (200) was enhanced due to the addition of C₁₂H₂₅SO₄Na. The grain size was refined significantly by adding saccharin, and then the microhardness of Ni-film increased. The coating brightness was improved by adding GL–100, and the (111) texture was generated by compound addition of saccharin and GL–100. Surface morphology and hardness of the coating achieved the best condition with adding all of the three kinds of additives. Under the same solution composition, the coating grain size decreased with increasing current density, meanwhile the hardness of the films increased. The current density range of 0.5–1.0 A·dm-2 was suited to nickelage in this experiment.When AgNO₃ was used as the main salt in cyanide–free solution, the adhesion strength of the coating is good. The current efficiency of the negative pole increased first and then decreased; however, grain size and microhardness for each crystallographic plane didn't increase obviously and the intensity of (111) texture was decreasing, and the preferred orientation of (222) plane was increasing. The optimum current density of electro-deposition silver was 0.25 A·dm-2.The morphology, grain size and microhardness of Cr-film were influenced by concentration of chromic anhydride, electroplating temperature and current density. The coating surface quality was the best at 10 A·dm-2. The grain size increased with the increasing current density, and the microhardness was decreasing. The optimal process parameters were determined as follows: the concentration of chromic anhydride is 250 g·L-1, the temperature is 46 oC, and the current density is 10 A·dm-2.The mean and distributed stresses in Ni film on Fe substrates and Ag film on Cu substrates and Ni film and Cu film on Ag substrates decreased with the increase in film thickness and then reached stable values. The internal stresses in the four kinds of films result from interfacial electron transfer, and the growth stresses are small. The measured values for the mean stresses and the calculated values based on TFDC theoretical model for those have the same variation trends and characters.