Effect of diamond seeds size on the adhesion of CVD diamond coatings on WC-Co instrument

In this study, we investigated the effect of the size of diamond seeds on the adhesion of multilayered polycrystalline diamond (PCD) films, grown by microwave plasma-assisted chemical vapor deposition (MPCVD). For that, identical WC-Co substrates were separately seeded by a set of diamond powders with various average particle sizes from water-based suspensions using similar seeding procedures. This investigation included powders with a difference in particle sizes of nearly 3 orders of magnitude: from 5 nm up to 2-4 μm. Seeded substrates were used to grow 8-10 μm thick multilayered PCD films using MPCVD with time-limited cycling injections of N2 gas. The Raman spectra and scanning electron microscopy (SEM) studies showed the similarity of microstructure and phase composition of all grown films, which confirmed that all films were grown in similar conditions. The performed scratch tests revealed sufficient differences in the adhesion of the films seeded with different diamond particles. The PCD film grown on 250-500 nm particles delaminated even before any mechanical investigations. The substrates seeded with 50 nm particles allowed the formation of the stable PCD film, but it started flaking under a load as small as 15 N. The 2-4 μm powder allowed the formation of PCD film with decent adhesion, which had local flaking under scratch test, which can be explained by the inhomogeneity of seeds distribution. Detonation nanodiamond (DND) powders allowed the formation of continuous diamond films with decent adhesion, however, powders with positive zeta potential were superior due to a much lower agglomeration of separate particles.