Mechanical interlocking mechanically roughened substrates

Surface Roughening - In adhesive bonding, a commonly used surface preparation technique in which the substrate surface is mechanically abraded. The roughened surface increases bondability by dramatically increasing the number of sites available for mechanical interlocking. 

Implant materials for coating. Prosthetic materials coated with HAp include titanium, Ti-6A1-4V, stainless steel, Co-Cr-Mo, and alumina (Jiang and Shi 1998). These materials are roughened by grit blasting for a mechanical interlock between the melted component of the particle and the substrate. The Ti-6A1-4V and Cr-Co-Mo alloys are the most common. Ideally, the elastic modulus and co-efficient of thermal expansion of the substrate and the coating material will be matched to minimize any residual stresses at the interface. Hydroxylapatite (E = 100 GPa and a = 12 x 10 °C (Perdok et al. 1987)) is... 

Mechanical interlocking occurs when the adhesive penetrates pores and openings in the substrate, producing a "lock and key" effect. This requires the adhesive to "wet" the substrate. Its effect can be enhanced by roughening the surface, thus creating pores and increasing the surface area. 

Coatings can be sprayed from rod or wire stock or from powdered materials. The material (e.g., wire) is fed into a flame, where it is melted. The molten stock is then stripped from the end of the wire and atomized by a high-velocity stream of compressed air or other gas, which propels the material onto a prepared substrate or workpiece. Depending on the substrate, bonding occurs either due to mechanical interlock with a roughened surface, due to localized diffusion and alloying, and/or by means of Van der Waals forces (i.e., mutual attraction and cohesion between two surfaces). 

However, the attainment of good adhesion between smooth surfaces exposes the mechanical interlocking theory as not being of wide applicability. For example, the elegant work of Tabor et al. [1,2] who studied the adhesion between two perfectly smooth mica surfaces and Johnson et aL [3] who examined the adhesion to optically smooth rubber surfaces, clearly demonstrates that adhesion may be attained with smooth surfaces. Also, detailed examination of surfaces roughened by typical industrial pretreatment methods, for example, grit-blasted metallic substrates (Fig. 3.1), usually reveals little... 

If the substrate is veiy smoodi, die adhesive bond strength can be increased by roughening its surface. This not only increases die area of the adhesive/substrate joint, but can improve the bond dirough mechanical interlocking. 

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