Adhesive strength of rubber bonded to Al₂O₃ surfaces: The role of chemistry and morphology

Tensile pull tests were performed on polydimethylsiloxane (PDMS) rubber bonded to single-crystal and ceramic Al₂O₃ substrates prepared using several surface treatments. An accurate and reliable mechanical test method was developed using a frustro-conical geometry to eliminate the influence of tri-axial stress effects at the periphery. Properties of the sample surface were quantified prior to encapsulation using surface science techniques. Chemical cleaning of atomically-smooth sapphire resulted in a significantly larger joint strength (1 MPa) than attained from as-received substrates (0.448 MPa). The introduction of a significant amount of contamination (∼ 25 nm thick) from isopropyl alcohol (IPA) residue resulted in even weaker adhesion (0.241 MPa). Abrasive cleaning of sapphire using fine (∼ 100 micron) Al₂O₃ beads created more 0.5-1.0 micron topographic features and significantly higher joint strengths (1.103 MPa) than for similarly-prepared surfaces using coarse (∼ 350 micron) Al₂O₃ beads (0.621 MPa). The application of a titanate-modified silicone primer resulted in the greatest joint strength encountered (> 1.207 MPa), even when significant levels of surface contamination were present prior to priming. Adhesion to ceramic Al₂O₃ substrates was systematically stronger than to sapphire for as-received samples (0.965 MPa), abrasively-cleaned surfaces (1.034 MPa), and surfaces containing IPA residue (0.827 MPa) as a result of the rougher topography.