Infoscience

Thesis

Self-Replenishing, Wear-Resistant and Anti-Sticking Surfaces Based on Liquid Impregnation of Microstructured Mesoporous α-AI₂O₃ [Alpha Alumina] Matrices

Fabrication of wear-resistant omniphobic surfaces is a persistent scientific challenge and essential for various applications such as in paint industries, fluid transport, antifouling, reduction of friction drag on ship hulls, and stain resistant textiles. However, there are very limited efforts to address this problem, especially because the conventional omniphobic surfaces are intrinsically wear-sensitive, due to the loss of either non polar coating materials or surface roughness elements during wear. Consequently, there is a need to employ novel approaches to fabricate such surfaces. The present research work mainly focuses on fabricating anti-sticking and wear-resistant materials using liquid impregnation of mesoporous alumina (MPA) matrices. Secondly, and equally important is to study their wetting properties post wear. To achieve the outlined objectives, different impregnating liquids such as water, Hexadecane, Dodecane, Fomblin® oil and 3M HFE 7200 were employed in conjunction with various alumina matrix densities ranging from 70 to 99.5 %. Fomblin® oil was found to be the most efficient impregnating liquid. Indeed, anti-sticking and pinning free sliding behaviors were observed for water, Hexadecane, Dodecane, water based and oil based paints atop Fomblin® oil impregnated/lubricated composites. Subsequently, the friction coefficient (FC) and the wear-resistance of Fomblin® impregnated MPA composites were evaluated by varying the normal load and alumina matrix density. The optimum alumina density of 90 %, leading to a low FC and high wear-resistance was determined. Ultra-low FC values of 0.025 and small wear coefficients of 10-8 mm3 N-1 m-1 were measured. The wear mechanism is mild abrasion majorly by intergranular fracture and/or third body abrasion. Finally, the wetting characterizations of Fomblin® impregnated MPA composites post wear were evaluated using Hexadecane and water. The worn composite surfaces lost anti¿sticking properties to water and Hexadecane immediately after wear, exhibiting superhydrophilicity and oleophilicity. Nevertheless, given a sufficient self-replenishing/healing time, they exhibited enhanced omniphobicity. The contributing factors towards self¿replenishment are the strong capillary forces of the impregnating liquid, coupled with its surface diffusivity on the polar alumina matrix. Further, the replenishing efficiency may be improved by providing surface microstructuring to MPA samples. Consequently, a fabrication process of large area surface microstructuring the bulk MPA samples was developed using replication and slip casting techniques. The microstructures are first replicated into polydimethylsiloxane (PDMS) membranes using excimer laser ablated polycarbonate sheets and are subsequently employed as molds in the alumina slip casting process. The developed process can produce microstructured areas up to 120 cm2, but can be further extended to 3 m2 and on samples as thick as 10 mm. The drying of the ceramic slurry occurs as a one dimensional process and a linear mass diffusion model developed predicts that the drying times are linearly proportional to the slurry height, PDMS membrane thickness and independent of the microstructured areas. The 3D geometries include tear cavities, micro bowls, conical micro pillars, bone pits and are not limiting. Such microstructured liquid impregnated MPA composite surfaces may also lead to anisotropic wetting and tribological properties if desired.

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