Low stress TiB2 coatings with improved tribological properties

Industrial use of physical vapour deposition TiB2 coatings has so far been strongly limited due to the very high compressive stresses normally possessed by these coatings. In this investigation it is shown that the residual stresses can be significantly reduced if the TiB2 coatings are deposited by magnetron sputtering utilising electrons to enhance adatom mobility on the growing film surface. This was done by switching the polarity of substrate bias from negative to positive. When the bias voltage changes from negative to positive the substrate bombardment by positive ions ceases. Instead, the necessary mobility of adatoms during a growth is obtained through an increased thermal activity resulting from both resistive heating due to a high electron current through the substrate, and the energy released through electron bombardment. These effects increase the substrate temperature from 350 to approximately 500 degreesC at the end of the deposition sequence. Even though the positively biased coatings are almost stress free their high hardness, Young's modulus and superior abrasive wear resistance are maintained compared to those of negatively biased TiB2 coatings. In addition, their intrinsic strength as measured by scratch testing, is significantly improved. (C) 2001 Elsevier Science B.V. All rights reserved.