Adhesion properties of MgO-ZrO₂ insulation coatings for 5 Tesla HTS coils

Self insulating substrate tapes (SIST) is the most promising insulation technique for high temperature MgO-ZrO₂ coatings on Ag and AgMg sheathed Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) superconducting tapes and wires in applications of HTS/LTS coils and magnets. We have already reported successful results as to the synthesis, characterizations and applications of the insulation coatings using the SIST. In order to provide no electrical short circuit in J_c measurements of HTS/LTS coils, the bonding of the coatings onto the substrate is a very important issue. In this present research, the adhesion properties of high temperature MgO-ZrO₂ coatings were scrutinized for different processing parameters. Lap joints were fabricated by laying fresh sol-gel coated silver tape samples over each other and then by heat-treating at temperature range of 500-800 °C for several times in air. These joint samples were pulled to failure by using a mini tensile tester. MgO-ZrO₂ was coated on Ag tapes by sol-gel process using Mg and Zr based precursors. The obtained results obviously pointed out that the best Mg precursor is Mg(C₅H₇O₂)·2H₂O to prepare solution and there is a strong relationship between film growth and adhesion properties. Also, MgO content in ZrO₂ increased its bonding strength. The optimum heat treatment conditions are 600 °C and 15 min for best bonding for these high temperature insulation coatings on HTS tape conductor. The failure mode of all samples was in the form of a mixed type interfacial/cohesive defects in MgO-ZrO₂ coating.