HOW TIN COATINGS IMPROVE THE PERFORMANCE OF HIGH-SPEED STEEL CUTTING TOOLS

TiN coatings deposited onto high speed steel (HSS) by physical vapour deposition have successfully been used in metal cutting applications for a number of years. Many papers have been presented which all show that TiN coatings often yield decreased wear rates and, usually, reduced friction coefficients. Although different theories are proposed in the literature (hardness theory, diffusion barrier theory, thermal barrier theory, reduced friction theory etc.), most papers have not dealt with the question of how and why TiN coatings modify the performance of HSS cutting tools. The present paper contributes to the answer of this question by (1) organizing published information about significant mechanical and chemical properties of TiN and (2) closely discussing different proposed theories and making comparisons with results from the broad spectrum of experiments (tool wear simulation testing, scratch testing in a scanning electron microscope, particle erosion testing, chip formation studies, metal cutting etc.) performed by the authors. A unique combination of sufficient adhesion to the substrate, high hot hardness, high wear resistance and an ability to improve the contact conditions at the cutting edge is concluded to be the answer. The complex interrelationships encompassed by this explanation are displayed in a flowchart. The thermal barrier function and the relatively low sliding friction coefficient of TiN are believed to be of minor importance. © 1990.