Effects of silane- and MDP-based primers application orders on zirconia–resin adhesion—A ToF-SIMS study

Objective To evaluate the 3-methacryloyloxypropyltrimethoxysilane (MPS)- and 10-methacryloyloxydecyl-dihydrogen-phosphate (MDP)-base primers, in their single or sequential applications, with regard to modifying zirconia surfaces and improving resin–zirconia adhesion. Methods Zirconia disks received different treatments: without primer (Zr), MPS-base primer (S), MDP-base primer (M), MPS/MDP mixture (SMmix), MPS followed by MDP (SM), and MDP followed by MPS (MS). The compositions and chemical interactions of the coatings to zirconia were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and reconstructed 3D ion images. Surface wettability of these coatings to water and resin adhesive was assessed. The shear bond strength (SBS) between resin and the treated zirconia was also examined before and after thermocycling. Results Groups S and MS presented substantial OH⁻ ions in the coatings and zirconia substrate. PO₂⁻ and PO₃⁻ fragments existed in all MDP-treatment groups with various proportions and distributions, while groups M and SM showed higher proportions of PO₃⁻ and the zirconium phosphate related ions. In 3D ion images, PO₃⁻ in groups M and SM was denser and segregated to the interface, but was dispersed or overlaid above PO₂⁻ in SMmix and MS. All the primers increased the surface wettability to water and resin, with M and SM presenting superhydrophilic surfaces. All MDP-treatment groups showed improved SBS before thermocycling, while M and SM retained higher SBS after this. Significance The MDP-base primer shows a relevant function in facilitating P[sbnd]O[sbnd]Zr bonding and enhancing resin–zirconia bonding. The co-treated MPS impairs the chemical activity of MDP, especially if it is the final coat.