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

Tensile pull tests were performed on polydimethylsiloxane (PDMS) rubber bon ded to single-crystal and ceramic Al2O3 substrates prepared using several s urface treatments. An accurate and reliable mechanical test method was deve loped using a frustro-conical geometry to eliminate the influence of tri-ax ial stress effects at the periphery. Properties of the sample surface were quantified prior to encapsulation using surface science techniques. Chemica l cleaning of atomically-smooth sapphire resulted in a significantly larger joint strength (1 MPa) than attained from as-received substrates (0.448 MP a). The introduction of a significant amount of contamination (similar to 2 5 nm thick) from isopropyl alcohol (IPA) residue resulted in even weaker ad hesion (0.241 MPa). Abrasive cleaning of sapphire using fine (similar to 10 0 micron) Al2O3 beads created more 0.5-1.0 micron topographic features and significantly higher joint strengths (1.103 MPa) than for similarly-prepare d surfaces using coarse (similar to 350 micron) Al2O3 beads (0.621 MPa). Th e application of a titanate-modified silicone primer resulted in the greate st 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-recei ved samples (0.965 MPa), abrasively-cleaned surfaces (1.034 MPa), and surfa ces containing IPA residue (0.827 MPa) as a result of the rougher topograph y.