Non-Stick and Scratch Resistant Sol-Gel Coating for Aluminum
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Non-Stick and Scratch Resistant Sol-Gel Coating for Aluminum Linda Y.L. Wu1 and Sandor Nemeth Surface Technology Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075 ABSTRACT PTFE-based coatings have been widely used, for example, on aluminum molds for molding of polyolefin packaging materials for its non-stick property. However, these coatings do not meet the users’ scratch resistance and durability requirements. This paper describes a preliminary study on the synthesis and characterization of a PTFE/sol-gel composite coating material which combines the required non-stick and low friction properties of PTFE filler with the high scratch resistance and durability of a silica-based sol-gel matrix. The non-stick and low friction properties were achieved by using both the PTFE filler and the lubricious compound resulting from the reaction of a solvent with siloxane. The high scratch resistance was attributed to the enhanced adhesion to the electro-chemically pre-treated surface and the optimized silica and alumina filler contents in the sol-gel material. FE-SEM/EDX, FTIR, contact angle goniometry, scratch testing and a pin-on-disc tribometry were used to evaluate the coating properties. INTRODUCTION Polytetrafluoroethylene (PTFE) has been widely used to impart non-stick, low friction properties to coatings for aluminum surfaces, due to its intrinsic low surface energy. Typical examples include the coatings applied on cooking utensils and mold sets for polyolefin packaging materials. The latter application requires non-stick, low friction against polyolefin foam, and high wear resistance and durability in a wet hot working environment A number of patents have been granted for the use of surface treatments [1,2], hard anodizing [3,4] and thermal spraying [3,5] technologies to build hard underlayers for the PTFE-based coating to improve the scratch and wear properties. Researchers [6,7,8] have demonstrated improved wear resistance by electron irradiation of PTFE/FEP (Fluoroethylene co-polymer) coatings. Great efforts have been given to the PTFE/hard fillers composite materials [9-14] for a combination of non-stick, low friction with high scratch resistance and durability. All of these materials used PTFE as the topcoat or at least in the matrix of the topcoat. Briscoe et al. [15] reported that the scratch resistance of a polymer surface is related to the deformation mechanisms. The small contact area between a scratching pin and the PTFE matrix, even under a small load, would cause local plastic deformation, which results in a visible scratch. Therefore, a hard and tough matrix is required. Good adhesion and sufficient mechanical support from the substrate are essential for a thin hard coating to function tribologically. As a first step in this work, an electro-chemical surface pre-treatment process was developed to replace the conventional grit blasting process. The pretreatment resulted in a porous layer into which the coating material penetrated to enhance the mechani
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