Structural Evolution And Mechanical Preperties Of Boron Carbon Nitride Thin Films Prepared By Pulsed Laser Deposition

Ternary boron-carbon-nitrogen, a promising superhard material, which combines the excellent properties of cubic boron nitride and diamond, has extensive and important applications in the protective coatings or cutting tools. The particular structure in the B-C-N films is essential for obtaining excellent mechanical properties. However, there are few studies referring to their structural evolution. In this paper, B-C-N films were therefore prepared by pulsed laser deposition(PLD), focusing on the structural evolution and mechanical properties as a function of deposition processes including nitrogen pressure(PN2), substrate temperature(Tsub) and laser energy density(DL). The relationship between bonding structure and mechanical properties was attempted to be established in order to optimize the mechanical properties of B-C-N films.Amorphous B-C-N thin films were obtained by adjusting the deposition processes of nitrogen pressure(PN2=2.5 ~ 5.0 Pa), substrate temperature(Tsub=RT ~ 600 oC), and laser energy density(DL=1.0 ~ 3.0 J/cm2). The results of FTIR and XPS indicated the existence of a ternary B-C-N compound with a variety of B-N, BC, C-N and C=N bonds.With increasing PN2 from 2.5 Pa to 5.0 Pa, the deposition rate of B-C-N films was increased constantly, while the surface roughness increased at first and then decreased. At the same time, the content of N was increased. The content of B was increased at first and then decreased; whereas the structural evolution changed from sp2 B-N and C=N bonds to B-C and sp3 C-N bonds with increasing PN2 from 2.5 to 3.5 Pa, and then changed contrarialy from B-C and C-N bonds to B-N and C=N bonds when further increasing PN2 to 5.0 Pa.With increasing Tsub from RT to 600 ?C, the deposition rate and surface roughness of B-C-N films were decreased constantly. The content of C and N was decreased constantly, while that of of B was increased. The structural evolution changed from sp2 B-N and C=N bonds to B-C and sp3 C-N bonds with increasing Tsub from RT to 400 ?C, and then changed contrarialy with further increasing Tsub to 600 ?C.With increasing DL from 1.0 to 3.0 J/cm2, the deposition rate of B-C-N films was increased constantly and the surface roughness was increased first, and then decreased. The content of B and N was gradually increased, while that of C was decreased. The structural evolution changed from sp2 B-N and C=N bonds to B-C and sp3 N-C bonds with increasing laser energy density.The hardness and elastic modulus of B-C-N films were changed in the range of 7.5 ~ 33.7 GPa and 128 ~ 256 GPa, respectively, depending on the deposition process, which corresponded to the evolution of bonding structure and bond content. The B-CN films with a high fraction of B-C and sp3 N-C bonds showed better mechanical propertie, since those bonds were thought as “hard” phases. On the contrary, the sp2 C=N and B-N bonds were “soft” phases. As a result, the B-C-N films with a high content of sp2 B-N and C=N bonds have a comparatively lower hardness.