Papers

ICMCTF2004 Session D1-1: Carbon Nitride, Boron Nitride and Other Group-III Nitride Materials

Monday, April 19, 2004 10:30 AM in Room Royal Palm 4-6

Monday Morning

Start	Invited?	Item
10:30 AM		D1-1-1 Deposition and Characterization of Thick cBN Films Prepared by CVD Methods W. Zhang, C.Y. Chan, I. Bello, S.T. Lee (City University of Hong Kong) Cubic BN (cBN) is the most attractive BN allotrope due to its unique set of properties second and sometimes superior to diamond. Contrary to the successful chemical vapor deposition (CVD) of diamond, cBN deposition is based widely on ion assisted physical vapor deposition (PVD) methods by which energetic species of tens to hundreds eV produce cBN films but meanwhile induce a significant compressive stress leading to the delamination of the films and limiting the achievable thickness to ~100nm. The films deposited by PVD are characterized by a small crystalline size (<20nm), and are highly defective and highly stressed. The nature of such cBN films is similar to nanodiamond films obtained by the bias enhanced nucleation and growth process and very different from CVD diamond, which is single or polycrystalline. Recently, we found that application of fluorine chemistry enabled deposition of 20 μm thick cBN films with a crystalline size of several microns and a much reduced defect density. Fluorine works in many aspects as hydrogen in CVD diamond synthesis (etches the sp² constituents resulting in faceted crystalline films and also plays a role in the termination of sp³ dangling bonds). The cBN films deposited show a columnar structure, and each column was demonstrated to be a cBN single crystal. Introducing fluorine into the gas phase reduced the substrate bias voltage dramatically. As a result of the lower bombarding ion energy, the compressive stress in the films was reduced to about 1-2 GPa, and the films show good adhesion with substrates and long-term stability. Based on the experimental observations, a new growth mechanism of cBN films via chemical pathways can be suggested. The present work will briefly summarize the recent progresses of cBN deposition, emphasizing the interfacial engineering and CVD processes.
11:10 AM		D1-1-3 Complexity of the Growth Mechanisms of Ion Asisted Cubic Boron Nitride Films S.F. Wong, G.H.K. Pang, C.W. Ong (The Hong Kong Polytechnic University, Hong Kong) A multilayered cBN-rich film was fabricated under increasing assist ion energy (E_ion) from 200 to 280, to 360 and then to 450 eV. Transmission electron (TEM) microscopy analysis showed that the film had a complicated laminated structure. The 200-eV layer contained a 3.8-nm amorphous layer, and a 150-nm sp² layer having graphitic planes perpendicular to the substrate. At this low E_ion, the depositing species were too immobile to form large sp² planes. Low ion-induced compressive stress was established, which was merely enough to drive the sp² planes to align along the growth direction. The 280- and 360-eV layers (338 nm) consisted of curl and randomly orientated sp² planes, because the use of these higher E_ion ²s introduced more severe knock-on effect, which continuously altered the growth direction of the sp² planes during deposition. Moreover, some nano-sized sp³ clusters were observed in the 360-eV layer, supporting the quenching theory for the cBN growth. For the 450-eV layer, the first few tens nm showed sp² [0002] in-plane texture. It was in contact with a 643-nm cBN-rich layer (87 vol.% cBN) on top. The transition was so sharp that the theory of stress-induced sp²-to-sp³ transition was strongly supported [1]. The structural data and stress measurements attained in this study reflected that the increase in E_ion would enhance successively the severity of many ion-induced processes, including the increase in compressive stress, knock-on events, thermal spike generation and stress-induced phase transition etc.. One of these mechanisms, or a certain combinations of them may dominate the growth process under some conditions, which resulted in the observed features of a specific layer in the multilayered film structure. [1] W.J. Yu, W.M. Lau, S.P. Chan, Z.F. Liu, Q.Q. Zheng, Phys. Rev. B 67, Art. No. 014108 (2003).
11:30 AM		D1-1-4 Growth and Characterization of Thick c-BN Coatings on Silicon and Tool Substrates K Bewilogua, M. Keunecke, K. Weigel (Fraunhofer IST, Germany); E. Wiemann (Technical University Berlin, Germany) In the last years some few research groups in the world achieved essential progress in deposition of c-BN coatings with a thickness of 2 µm and more, sufficient for cutting tool applications. In our laboratory such thick c-BN coatings were sputter deposited on silicon substrates using a boron carbide target. Following a boron carbide interlayer (few 100 nm thick), a gradient layer with continuously increasing nitrogen content was prepared. After the c-BN nucleation the process parameters were modified for the c-BN film growth to a thickness of more than 2 µm. However, the transfer of this technology to technically relevant substrates like cemented carbide cutting inserts required some further process modifications. At first, a titanium interlayer had to be deposited followed by a more than 1 µm thick boron carbide layer. The next steps were identical to those on silicon substrates. The total coating thickness was in the range of 3 µm with a 0.5 to 1 µm thick c-BN top layer. In spite of the enormous intrinsic stress, both the coatings on silicon and on cemented carbide exhibited a good adhesion and a long-time stability in humid air. Oxidation experiments revealed a stability of the coating system on cemented carbide up to 800°C. Coated cutting inserts were tested in turning operations with different metallic workpiece materials. The test results will be compared to those of well established cutting materials like polycrystalline cubic boron nitride (PCBN) and oxide ceramics.