Fracture and solid particle erosion of micro-crystalline, nano-crystalline and boron-doped diamond films

Chemical vapor deposition (CVD) diamond film has been considered as a good candidate protective coating under hostile and abrasive conditions due to its outstanding erosion behavior. In the present study, the conventional micro-crystalline diamond (MCD) film, nano-crystalline diamond (NCD) film and the boron-doped diamond (BDD) film are respectively fabricated by the hot filament CVD (HFCVD) method. Solid particle erosion tests are conducted on diamond films in an air-sand erosion rig under different impact velocities (v) and angles (α) with angular SiC sands as the erodents, and WC-Co and SiC materials are adopted as comparisons. The field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, Rockwell hardness tester and surface profilometer are respectively used to characterize diamond films before and after tests, exhibiting that compared with MCD and NCD films, the BDD film performs relatively higher fracture strength and better adhesion. Typical solid impact erosion stages of the different diamond films are observed. The solid particle erosion behavior of diamond films has close relationships to their fracture strength and adhesion. As a result, when the impact angle α ≤ 30°, the steady-state erosion rate of the BDD film is lower than those of MCD and NCD ones. More importantly, under all the impact angles and velocities, much quicker film delamination and removal can be noticed on the MCD and NCD films, especially the NCD one. By contrast with WC-Co and SiC materials, diamond films present much better erosion resistance and higher velocity exponents, which should be attributed to different impact mechanisms caused by the different hardness of eroded samples.