Microstructural observation of a composite film of c-BN and h-BN phases
- PDF / 154,442 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 52 Downloads / 187 Views
Jae Hyoung Choi and Jeong Yong Lee Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, Korea
Jun-Hee Hahn Materials Evaluation Center, Korea Research Institute of Standards and Science (KRISS), P.O. Box 102, Yusong, Daejon 305-600, Korea (Received 12 November 1999; accepted 21 August 2000)
BN films consisting of c-BN and h-BN phases were synthesized using an ion-beam-assisted deposition process. In contrast to conventional observations, the c-BN and h-BN phases did not form separate layers, but were distributed in the form of nano-sized grains throughout the film thickness. No distinctly aligned h-BN layer was observed before the c-BN phase. Such a mixed character of the film was attributed to a localized ion bombardment effect instead of the macro-stress. Possibly because of the presence of scattered h-BN phases, the thin film described here possessed a low hardness of about 20 GPa and a low stress of about 5 GPa, compared with other reported c-BN-containing films.
Over the past decade, c-BN thin films have attracted much attention from the hard coating industry due to their superior hardness combined with chemical stability.1 They have been successfully synthesized by various methods,2–9 and the microstructural evolution on the substrate has been well elucidated.3,9–13 An amorphous phase is formed at an early stage of deposition, and aligned h-BN and c-BN phases follow it sequentially. On the basis of their experimental data, Kester and Messier3 suggested a threshold value of momentum transfer per atom of 200 (eV amu)1/2 for the c-BN formation. They reported that once the growth of the cubic phase was initiated, the layer had no h-BN phase, and no grain boundary region was observed. Such a threshold value of momentum transfer for c-BN formation has been considered necessary to create the minimum compressive stress for the c-BN formation, as suggested by Mckenzie et al.6 They proposed that ion bombardment might introduce macro-stress into a film and the stable phase could be determined according to the stress level.6 In this case, different stress levels at the same depth of a film are hardly expected, and a homogeneous phase is favorably formed at a certain depth. Contrary to this view of the c-BN formation mechanism, Zhou et al.10,14 showed that the h-BN phase could a)
Address all correspondence to this author.
2292
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 27 Mar 2015
exist with the c-BN phase in the c-BN favorable region. They observed clear boundaries of c-BN grains and found that thin layers, 1–2 nm, of the h-BN phase often existed.10,14 However, while a layer-by-layer sequence of amorphous/h-BN/c-BN structure was maintained, they observed the h-BN phase mostly near the film surface as grain boundaries. The stress generated by the ion bombardment cannot be efficiently accumulated near the surface. Thus, even from the point of the macro-stress model for c-BN formation, it can be explained that h-B
Data Loading...
