Effect of intergranular phase of Si3N4 substrates on MPCVD diamond deposition

The role of the intergranular phase of Si3N4 ceramics in CVD diamond coating is investigated. Two compositions from the SiO2-Y2O3-Al2O3 sintering aid system were selected: a more refractory one (A) inside the Y4Si3O12-mullite-alumina compatibility triangle and another (B) belonging to the Y2Si2O7-mullite pseudo-binary system. Samples with 7, 14 and 28 wt.% of these additive phases were diamond coated on a MPCVD reactor using H2/CH4 mixtures. SEM observations after 10 min of CVD diamond deposition showed a fully covered surface by a nano-scale diamond crystallite (∼100nm; Nd ∼ 1010 cm−2) layer, regardless the intergranular phase nature. However, the emerging number of diamond micro-crystals from this primary layer decreased with the additive content. The 7 wt.% doped A substrate was almost covered by such large crystals (∼3 μm; Nd∼6×106 cm−2) after 30 min of deposition and a continuous film was observed after 3 h, while samples with 14 and 28 wt.% of additive amounts reveal non-homogeneous films. The emerging micro-crystals after 30 min were fewer and bigger on B based samples, continuous films never being attained even for larger depositions periods. The distinct influence of the chosen intergranular phases on diamond deposition is due to the intrinsic glass properties. Glass type B, possessing lower thermal conductivity than A, leads to higher surface temperatures, increasing the Si3N4 intergranular phase sink effect with respect to carbon enrichment of the surface and subsequent diamond formation.