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.