Electrodeposition of metal matrix composites (MMCs) is an effective method to enhance physical and chemical properties of materials by allowing dispersion of nanoparticles into the metal matrix. Various kinds of metals including Au, Cu and Ni have been reported as the metal-matrix depending on their target application. In co-electrodeposition with nanoparticles, uneven distribution of the nanoparticles in the metal matrix caused by aggregation of nanoparticles in the electrolyte would lead to an unevenly local enhancement of the desired property. Therefore, in co-electrodeposition of MMCs, controlling the dispersity, such as the size and space distribution, of the particles in the metal matrix is always a research topic of interest. Supercritical carbon dioxide (SC-CO₂) is a state of CO₂ exists when the temperature and pressure are above its critical point. SC-CO₂ has low surface tension and low viscosity, which are advantageous in co-electrodeposition of metal matrix composite. Dispersed phases in the SC-CO₂ emulsified electrolyte would improve transfer of materials in the electrolyte and expect to influence incorporation amount and distribution of suspension particles in the electrolyte then eventually affect the dispersity in the electrodeposited metal matrix. In this study, Ni-TiO2 composite coatings are electrodeposited with SC-CO₂ emulsified electrolyte. The effects of SC-CO₂ emulsion on the amount and distribution of TiO₂ particles incorporated in the Ni matrix, and the mechanical property of the Ni-TiO₂ composite coatings are evaluated for applications in miniaturized electronics.

Ni-TiO₂ composite coatings are deposited on cold-worked Cu plates with 3.5 cm² in surface area through a two-electrode deposition cell. Ni plates are used as counter electrode. Ni Watts bath-based electrolyte, which composed of NiSO₄‧6H₂O, NiCl₂‧6H₂O, and H₃BO₃, with and without addition of 30 g/L TiO₂ nanoparticles are used for the electrodeposition process. Since SC-CO₂ molecules are non-polar, 0.2 g/L of sodium dodecyl sulfate (SDS) is added to emulsify SC-CO₂ with the Ni electrolyte. Pure Ni and Ni-TiO₂ composite coatings fabricated from conventional electrodeposition method without addition of SC-CO₂ are prepared as comparisons. Pure Ni coatings are named as Ni-S-# (# represents the concentration of SDS in electrolyte) and the Ni-TiO₂ composite coatings prepared by conventional method are named as CV-S-#. For the composite coatings prepared from the SC-CO₂ emulsified electrolyte, the samples are designated as SC-S-#. For all electrodeposition processes, the temperature is maintained at 50 °C, the deposition time is 40 min, and the applied current density is 30 mA/cm². Conventional electrodeposition of pure Ni and Ni-TiO₂ composite are conducted under ambient pressure, and 15 MPa is used for the electrodeposition with SC-CO₂. Volume fraction of SC-CO₂ in the reaction cell is 20 vol.%. Surface morphologies of the as-deposited films are observed with a scanning electron microscope. The element composition are characterized by energy dispersive X-ray spectrometry (EDS). The crystalline structure annd grain size are measured by X-ray diffraction spectrometer (XRD). Local density of TiO₂ particles in Ni matrix are quantified by a python-based convolution process from Ti EDS mapping datasets. The Ti local density data is revisualized as heatmap and its coefficient of variation is calculated to evaluate the dispersity of TiO₂ in composite coatings. Hardness of the films is evaluated by Vickers hardness measurement.

TiO₂ content and Vickers hardness of Ni-TiO₂ composite coatings fabricated with representative electrodeposition conditions are shown in Fig. 1. Benefit from the accelerated mass transfer in SC-CO₂ emulsified electrolyte, a significant increase of TiO₂ incorporation amount could be found between SC-S-02 and CV-S-02. As a result of grain refinement and high incorporation amount of TiO₂ in Ni-TiO₂ composite coatings, the Vickers hardness of SC-S-02 is found at about 1150HV, which is threefold of the hardness of CV-S-02. Rest parts of the data, including the influence of SC-CO₂ on surface morphology of Ni-TiO₂ composite coatings and grain refinement of Ni matrix, and the effect of SC-CO₂ on the improvement of dispersity of TiO₂ particles incorporated in Ni matrix, will be presented in the ECS meeting.