Influence of boron doping level on the basic mechanical properties and erosion behavior of boron-doped micro-crystalline diamond (BDMCD) film

Abstract

For the chemical vapor deposition (CVD) of diamond films, boron doping with appropriate boron doping levels can enhance the basic mechanical properties of the as-deposited films, especially the film-substrate adhesive strength. However, the boron doping level, which is thought to play a critical role in modifying the properties of the boron-doped diamond (BDD) film, must be further investigated. In the present investigation, the commonly used reaction-bonded silicon carbide (RB-SiC) material is selected as the substrate upon which boron-doped micro-crystalline (BDMCD) films with similar film thickness (24.8–26.3μm) are synthesized using mixed reactant gas with different B/C atomic ratios, i.e., different boron doping levels. Systematic characterization and solid particle erosion tests are conducted on all specimens to elucidate the influence of the boron doping level on the diamond films' basic mechanical properties and erosion behavior and to elaborate the impact velocity and impact angle dependence of the erosion behavior. The results demonstrate that moderate boron doping levels (5000 and 8000ppm) are able to maximize the growth rate, reduce the surface roughness, guarantee diamond quality and hardness, minimize the residual stress, and significantly enhance the film-substrate adhesive strength, thereby providing favorable erosion behavior. By contrast, very high boron doping levels (12,000 and 16,000ppm) deteriorate the erosion behavior of as-deposited BDMCD films, mainly because of the excessive reduction of the film quality and hardness, together with the deterioration of the film-substrate adhesive strength caused by the transformation of the residual stress from compressive to tensile. With increasing either impact velocity or impact angle, the stable erosion rates of all specimens increase, and the film lifetime of the coated specimens become shorter. Moreover, the impact velocity dependence of stable erosion rates for diamond-coated specimens is considerably stronger than that for the uncoated RB-SiC specimen, as indicated by a much higher velocity exponent.