Study On Electrodeposition Process And Properties Of SrSO₄ And Ni-SrSO₄ Coatings

Strontium sulfate (SrSO₄) shows excellent friction and wear properties as a solid lubricant. An electrochemical deposition process was used to deposit SrSO₄ coatings onto the anode surfaces of SS304 stainless steel and Inconel 718 superalloy, respectively, from the EDTA stabilized electrolytes dispersively distributed with nanoscale SrSO 4 particles. The crystal growth behavior of SrSO₄ during electrodeposition process was investigated by means of scanning electron microscope (SEM) and X-ray diffractometer (XRD). Ni-SrSO₄ composite coatings were deposited on the surface of Inconel 718 superalloy to evaluate the effect of nanoscale SrSO₄ particles on the microstructure and properties of composite coatings.The electrochemical reactions on the surfaces of different anodes of SS304 stainless steel and Inconel 718 superalloy depend mainly on the type and concentration of anions in the electrolyte. The morphology of SrSO₄ crystals deposited on SS304 stainless steel exhibit orthorhombic tetrahedron due to the limited variations of the pH value. Oriented crystal growth of SrSO₄ occurred along (210) crystal plane, however, and the crystal growth along (002) crystal plane was restrained. The morphology of SrSO₄ deposited on Inconel 718 superalloy exhibit a decahedron structure due to great variations on the pH value, and no orientation growth was found from the results of SEM and XRD. The crystal growth behavior of SrSO₄ during the electrodeposition process was controlled by both the pH value of the electrolyte and the current density.After an appropriate pretreatment before electrodeposition, highly adherent Ni-SrSO₄ composite coatings can be deposited on the surface of Inconel 718 superalloy. The SrSO₄ content in composite coatings and the deposition rate depend on different technological parameters such as current intensity, SrSO₄ powder concentration in the electrolyte etc. With the increase of both the SrSO₄ powder concentration in the electrolyte and the current intensity, the SrSO₄ content in composite coatings increased at first and then decreased. With the increase of temperature, the SrSO₄ content in deposits decreases. The optimum parameters for electroplating Ni-SrSO 4 composite coatings are given as 1.5A/dm 2 of the current (Dk), 60 g / L of the SrSO₄ powder concentration, T = 40°C of temperature, 30 min of electroplating duration, 4 of the pH value with an interval stirring mode (stirring time15s and intermittent time 75s for each cycle).The introduction of nanoscale SrSO 4 particles into the Ni-SrSO₄ composite coating improves greatly the surface roughness, microstructure and hardness of coatings on Incone718 superalloy due to the dispersion-strengthening mechanism of SrSO 4 in the deposits.The introduction of nanoscale SrSO 4 particles into the Ni-SrSO₄ composite coating improves the tribological properties of coatings. Low friction coefficients and small wear are obtained with the increase of SrSO 4 content in Ni-SrSO₄ composite coatings. This is mainly contributed to the dispersion-strengthening effect, the grain refinement and the self-lubrication effects of SrSO₄ particles in composite coatings.