Granule spray process for fabrication of adherent,low thermal conductivity ceramic coatings

This study proposes an effective strategy to fabricate ceramic thick films with controllable porosity but strong mechanical adhesion. The technique uses pre-heat-treated granules for a dry spray coating process, granule spray in vacuum. For proof-of-concept, Y₂O₃ spherical granules were prepared and subsequently pre-heat-treated at various temperatures to control their strength. During film deposition, hard granules (i.e., pre-heated at 1400 °C) caused damage to the substrate and pre-deposited film, owing to a strong hammering effect, which lead to limited film growth (1–2 μm). Contrarily, soft granules (i.e., pre-heated below 800 °C) produced a powder compact with low adhesion, resulting in delamination of some parts. This is attributed to the reduction in kinetic energy caused by the elastic cushioning effect during deposition. Regarding the granules with appropriate strength (i.e., pre-heated at 1000 °C), relatively porous (relative density: 74%) but mechanically well-adhering (adhesion strength: 41 MPa) thick films (~60 μm) were successfully coated on Al substrates. These films exhibited four times higher adhesion strength than those prepared with Y₂O₃ coatings using atmospheric plasma spraying. The films showed low thermal conductivities (~0.76 W/m·K at R.T.), suggesting potential application of our approach in the field of thermal insulation.