Microstructure of direct current and pulse magnetron sputtered Cr-B coatings

Chromium diboride thin films possess desirable combinations of properties (such as high hardness, wear resistance, chemical inertness, high thermal and electrical conductivity), which are attractive for a wide range of potential industrial applications. However, these properties depend strongly on the deposition process and parameters. Investigation of the resultant coating structures could explain certain differences between them, giving important information about the characteristics of the deposition process (which in this particular case is a recently developed method involving magnetron sputtering of loosely packed blended powder targets) and pointing out directions for improvement. In this paper, Cr-B coatings deposited by direct current (DC) and DC-pulse magnetron sputtering of loosely packed blended powder targets are characterised by transmission electron microscopy (TEM) techniques (electron diffraction and bright-field/dark-field imaging). The structures of the coatings deposited with different parameters are investigated and compared, and the effect of oxygen contamination on the structure is discussed. Coatings with an extremely fine, nanocolumnar structure were observed. DC sputter deposited (and generally non-stoichiometric) Cr-B coatings exhibit a short range ordered 'zone T' microstructure, while DC-pulse deposited stoichiometric CrB₂ coatings are dense and defect-free, crystalline and show strong preferred orientation. A small amount of contamination by oxygen of the interfacial sub-layers (due to the target material being a powder) of the DC-pulse magnetron sputter deposited stoichiometric CrB₂ (and near-stoichiometric CrB) coatings was found to affect the structure by suppressing nanocolumnar growth and promoting equiaxed, nanometer-sized grains, close to the coating/substrate interface. The majority of the coating however remained nanocolumnar.