Fabrication And Research On BCN Films

BCN is a new type of synthetic material with characteristic shorter bond length and higher proportion of covalent bonding. Theoretical research has demonstrated that BCN has a hexagonal and cubic structure similar to BN structure. It is noteworthy that the property of cubic BCN (c-BCN) should be in between diamond and cubic BN (c-BN):it has the high degree of hardness and wear of diamond and the thermal stability and chemical inertness of c-BN. As such, c-BCN is a new, ideal type of material for wear-resistant protective coating that potentially has wide applications. Researchers have attempted to use various methods to synthesize this new material. Most studies used single crystal Si or other crystals as substrate to make BCN film. Although progress has been made in the technique of film deposition and the analyses of its structure and performance, there is still a considerable gap between research and application. To diminish this gap, this work pioneered in China to use YG8hardmetal as substrate material to make BCN film and examine its properties.Using high purity graphite and h-BN as target material, this study succeeded in making BCN film on YG8hardmetal substrate by using DC and RF magnetron sputtering. Scratch test and ball-on-disc tribological test were used to systematically examine the effects of various conventional process parameters and treatment of the substrate on the adhesion performance and tribological properties of the film. FTIR, XRD and XPS methods were used to analyze the composition and structure of the film as well as factors that affect them. In addition, first-principles method was used to computationally simulate the adsorption characteristics of B、C and N atoms on WC and WC-Co surfaces. These studies provided the theoretical basis for studying the deposition and growth of BCN film.This research showed that the adhesion and tribological properties of BCN film are influenced by conventional process parameters including nitrogen gas partial pressure ratio, working atmospheric pressure, RF power and substrate bias, but not by the time of deposition. The optimal deposition parameters are:nitrogen gas partial pressure ratio25%, working atmospheric pressure1.OPa, RF power200W, and substrate bias-50V. In general, BCN film made with the conventional process does not have obvious antifriction effect and wear resistance, with the maximum adhesive force being only 31.7N, and the lowest friction coefficient against silicon nitride being0.32, and the lowest wear rate being2.94×10-6mm3/Nm.This study also demonstrated that the pretreatment of the substrate surface and the deposition of interlayer have relatively large effects on the adhesion performance and tribological properties of BCN film. The optimal treatment of the substrate is a two-step pretreatment of the substrate surface (grinding by diamond paste followed by acid etching) and deposition of a single-layer interlayer of TiN. The corresponding BCN films’ adhesive forces are37.1and48.7N, respectively, and the friction coefficients against silicon nitride are0.07and0.14, respectively; and the wear rate are0.88×10-6and1.81×10-6mm3/Nm, respectively. Therefore, the BCN film has a clear antifriction effect and wear-resistance, and serves as a good foundation for further studies on the application of BCN film.FTIR analysis revealed that the various elements in BCN film all form bonds with each other, resulting in the film being mainly a B, C, and N three-component compound, not the simple mixture of graphite and h-BN. The grinding by diamond paste followed by acid etching pretreatment and boronizing pretreatment of the hardmetal substrate is helpful for the formation of bonds between B, C and N atoms.XRD analysis found that BCN films are mainly in a non-crystal state. A minority of films form certain crystal structures, but the quantity is too low to measure. All the methods of treatment of the substrate help the crystallization of BCN film. Moreover, boronizing pretreatment also induces B element to form a compound and corresponding crystal phase with the Co and W elements in hardmetal surface layer.XPS analysis indicated that the B, C and N atoms do not exist as free atoms in BCN film, but instead form C-N, B-N and B-C chemical bonds. However, the relative content of N and B elements, especially B element is clearly low, resulting in low N/C, which does not meet the requirement for the formation of BC2N. The film should be a multi-crystal or non-crystal material. Pretreatment of substrate surface with grinding by diamond paste followed by acid etching helps to increase the amount of C=N bonds in BCN film.Computational analysis revealed that B, C and N atoms can form chemical adsorption at the high symmetry adsorption sites of both WC(001) and WC(100) surfaces. On WC(001) surface, B and N atoms tend to move to hollow site HC to for form a stable chemical adsorption, while the C atom tends to move to hollow site HO to form a stable chemical adsorption. On the other hand, on WC(100) surface, B, C and N atoms all tend to move to hollow site HC to for form stable chemical adsorptions. On both surfaces, B, C and N atoms mainly interact with the surface W atoms to form partially ionic covalent bonds. At the same adsorption site on both surfaces, the adsorption of N atom is the strongest, and that of B atom the weakest. WC(100) is generally more suitable than WC(001) for the adsorption of B, C and N atoms.Computational analysis also revealed that WC-Co surface formed by adding Co element to WC(100) has a reduced symmetry which increases potential adsorption sites. B, C and N atoms can still form chemical adsorption on the adsorptions sites on WC-Co surface, and they still tend to move to hollow site HC site to for stable chemical adsorption. B, C and N atoms on WC-Co surface mainly interact with the surface W and Co atoms to form partially ionic covalent bonds. At the same adsorption site on WC-Co surface, the adsorption of N atom is the strongest, and that of B atom the weakest. From a microcosmic perspective, this partially explains why the content of B element is relatively low in BCN film deposited on WC-Co surface. It was also shown that addition of Co element is harmful to the adsorption of C and N atoms on hardmetal surface, which from a microcosmic perspective, partially explains why Co element is not helpful to the deposition of related film on hardmetal surface, and provides guidance for future research.