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

The effect of pulsed magnetron sputtering on the structure and mechanical properties of CrB2 coatings

CrB2 thin films possess desirable combinations of properties (high hardness, wear resistance, chemical inertness, high thermal and electrical conductivity), which are attractive for a wide range of potential applications. Pulsed magnetron sputtering (PMS) of loosely-packed blended powder targets has allowed the deposition of stoichiometric chromium diboride coatings. The structure and properties of these coatings were found to be strongly dependent on the deposition process parameters; therefore investigation of the coating structures could explain certain differences between them and provide important information about the characteristics of the deposition process. In this study, characterization of the CrB2 films was performed by scanning and transmission electron microscopy (SEM and TEM) techniques. The microstructures and properties of coatings deposited with different parameters are compared and changes that resulted from the variation of these parameters (particularly the pulsing duty cycle and the substrate biasing conditions) are discussed. The results show that besides the pulsing frequency, the target pulsing duty cycle is an important parameter of the PMS process, which is able to affect such coating properties as hardness, thickness and stress. Coating thickness measurement results suggest more intense bombardment of a growing film by energetic ions at lower values of duty cycle. Structural TEM analysis revealed two extremely different types of coating microstructures, obtained at quite similar substrate biasing conditions, i.e. floating ( − 15 V) and negatively biased (− 30 V). It appears that the structures of the coatings deposited at the negatively biased substrate are significantly affected by high-energy ion bombardment, which is a peculiarity of PMS that can modify film growth conditions. These conditions are not present when the substrate is allowed to float