Understanding of the adhesive strength enhancement mechanisms of bilayer diamond film at nanoscale

In virtue of its outstanding comprehensive performances, the development of multilayer diamond holds significant potential for expanding the range of applications for diamond. However, there still is a major challenge to synthesize multilayer diamond with high adhesive strength. In this work, a bilayer diamond film with enhanced adhesive strength was successfully prepared through depositing nanocrystalline diamond film on plasma-etched polycrystalline diamond substrate using chemical vapor deposition. The adhesion strength was evaluated by the critical load when film failed during scratch test. The nanocrystalline diamond film deposited on plasma-etched substrate exhibited a critical load of 121.6 N, which is higher by 57 % than that deposited on substrate without plasma etching. TEM observation demonstrated that the nanocrystalline diamond has a preference for nucleating at the bottom of pits on the substrate produced by plasma etching, constructing a unique interface with a mortise and tenon joint structure. Moreover, Raman results verified the removal of the graphite phase in substrate surface during plasma etching. Our findings reveal the mechanisms responsible for the enhanced adhesive strength of the developed bilayer diamond film at nanoscale. The improved adhesive strength can be attributed to the combination of the mechanical interlocking interface and the removal of the graphite phase. In addition, the method presented in this work can also be employed on chemical vapor deposition of other multilayer materials beyond diamond, offering the potential to enhance their adhesive strength.