Novel Metal Diboride Coatings in the System Zr-Hf-Ti-B by LPCVD

Recently, there has been growing interest in the synthesis of thin films made from metal diboride. Boron forms binary compounds with a wide variety of metals. These diborides are refractory, ultra-hard solids characterized by high melting points, exceptional thermal stability, and pronounced chemical inertness. This work describes the preparation of metal diboride coatings made of binary ZrHfB2, HfTiB2 and ZrTiB2 as well as ternary HfZrTiB2. In the low-pressure chemical vapor deposition (LPCVD) process used, MeCl4 (Me = Zr, Hf, Ti), BCl3, H2, and Ar were employed at deposition temperatures of 850 °C. The coatings were characterized with respect to phase composition, crystal structure, hardness, residual stress and wear behavior. A hardness of 38 GPa was achieved with a modulus of elasticity of around 700 GPa and a moderate tensile residual stress of approx. 400 MPa was obtained for the ternary alloys as well as 44 to 633 MPa for the binary alloys, respectively. The phase composition and structure of the deposited layers were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The investigations revealed dense, crack-free, well defined crystalline single-phase diboride layers with grain sizes of 0.1-1.5 µm. A TiN interlayer applied prior to diboride deposition significantly enhanced adhesion between the diboride coating and hard-metal inserts. Scratch test measurements revealed critical loads of approximately 90 N. In the wear test milling against TiAl6V4, the HfZrTiB2 coating (with ZrCl4:HfCl4:TiCl4 = 1:2:1) demonstrated the best tool life with ~15% improvement over the state-of-the-art CVD TiB2 reference coating using a single cutting condition. The tool life for the ZrTiB2 coating was 20% below the tool life of the reference coating.