We have deposited superconducting YBCO (YBa2Cu3O7) thin films on polycrystalline diamond using a composite Si/YSZ (yttria stabilized zirconia) diffusion barrier. 10 - 20 micron thick, free standing diamond films were coated with a micron thick layer of amorphous Si by plasma CVD. This was vacuum annealed at 700 degree(s)C to crystallize Si prior to the in-situ deposition of YSZ and YBCO by laser ablation. A primary layer of Si was found essential to avoid the peeling of the multilayer structure during the cooling of the substrates. A Tc-zero equals 50 K was observed for the best films. A bolometer fabricated from this film exhibited a 18 V/W responsivity and NEP equals 6.3 X 10-9 W/(root)Hz at 60 K. Both the buffer layers and YBCO were polycrystalline. We believe single crystal Si backing CVD grown diamond can be used instead of a polycrystalline Si layer to further improve the quality of the YBCO deposited on polycrystalline diamond.