Novel ZrB2 and HfB2 metaldiboride coatings by LPCVD

The synthesis of metal diboride thin films has recently attracted great interest. Boron forms binary compounds with most metals. In general, these materials are high-melting, extremely hard solids with high degrees of thermal stability and chemical inertness. In this work the preparation of metal diboride coatings of ZrB2 and HfB2 by CVD is described. A low-pressure CVD (LPCVD) process using MeCl4 (Me = Zr or Hf), BCl3, H-2 and Ar was applied. Diboride layers were prepared at deposition temperatures between 800 degrees C and 1000 degrees C. The coatings were characterized with respect to phase composition, crystal structure, hardness and wear behavior. The deposited diboride layers show well defined crystallites with a high hardness up to 3200 HV[0.01] for ZrB2 and 3900 HV[0.01] for HfB2. Depending on the substrate temperature and precursor ratio, layers with different textured crystalline structure were obtained at different deposition rates. Phase composition and structure of the deposited layers were examined using SEM and EDS-analysis. A TiN bonding layer prior the diboride deposition ensures a strong adhesion to the hard metal inserts. Scratch test measurements showed critical loads of about 90 N. In wear tests a high performance of the CVD diboride coatings were observed. HfB2 coated inserts exceeded the lifetime of state-of-the-art CVD-TiB2 coatings in face-milling TiAl6V4.