Fabrication, microstructure and infrared optical properties of polycrystalline diamonds through surface seeding optimization

This study systematically investigated the impact of surface seeding density (5.5 × 10⁸ – 4.6 × 10⁹ cm⁻²) on the growth behavior, microstructure, and optical performance of polycrystalline diamond films synthesized via microwave plasma chemical vapor deposition (MPCVD). By modulating the composition of diamond suspensions, four samples (S1–S4) with controlled seeding densities were fabricated. Results demonstrated that higher seeding densities could reduce the grain sizes from ∼84 μm to ∼30 μm, lower the surface roughness (Sa: 3.81 μm to 1.53 μm), and enhance crystallinity, as were confirmed by TEM, EPR, and Raman spectroscopy. Under a seeding density of 2.7 × 10⁹ cm⁻², diamonds exhibited optimal performance with minimal lattice defects, low residual stress (0.07 GPa), and the highest infrared (IR) transmittance (∼70.9 % at 12 μm). Subsequent deposition of Y₂O₃ anti-reflective coatings on diamond substrates achieved an average transmittance of ∼93.3 % (peak: ∼96.1 %) in the 8–12 μm range, with only 4.4 % loss after thermal cycling at 800 °C. The work has established a quantitative relationship between seeding density, structural integrity, and optical properties, offering a scalable strategy for producing high-quality polycrystalline diamond materials with exceptional thermal stability and IR transparency. These advancements highlighted the material’s potential for demanding applications, such as IR windows in high-speed aircraft and high-temperature optical systems.