Abstract: A thick, adherent and coherent polycrystalline diamond (PCD) coated substrate product is disclosed which comprises either a metallic or ceramic substrate and a plurality of separately deposited PCD layers of substantially uniform microstructure and having high electrical resistivity. The method for depositing multi-layers of PCD film onto the substrate comprises chemically depositing at least two separate polycrystalline diamond layers onto the substrate deposition conditions which are substantially different between cycles. The method enables one to deposit PCD films having a thickness of at least 12 microns for applications on flat as well as curved substrates having wide use in the electronics industry. Thick PCT films of this invention have been found to be ideal for dissipating heat from radio frequency (RF) and microwave (MW) devices.