Influence of residual stresses in hard tool coatings on the cutting performance

During the hard machining of powder metallurgical high-speed steel, finely dispersed carbides in the steel expose the tools to both thermal and mechanical load. This can influence the cutting performance and cause damage to the tools such as premature abrasive and adhesive wear. Thin hard coatings like TiAlN deposited by physical vapor deposition are widely used in order to improve the tool performance. High power pulsed magnetron sputtering (HPPMS) results in technical benefits, such as a more homogeneous coating thickness distribution on the tools compared to direct current magnetron sputtering (dcMS). The advantages of HPPMS can be combined with the high deposition rates of dcMS leading to a higher economic efficiency conducting a dcMS/HPPMS hybrid process. Adding silicon to the coating system TiAlCrN results in TiAlCrSiN leading to a nanocomposite coating architecture with improved mechanical properties. The influence of the residual stresses on the mechanical properties and on the roughing performance of nanocomposite coatings is of high interest and was therefore investigated in the present study. Four different TiAlCrSiN hybrid coatings deposited with four different substrate bias potentials were examined for this purpose. The residual stresses and the mechanical properties including the resistance against crack formation as well as the compound properties of the coatings on cemented carbide were investigated. Finally, the roughing performance of the coated cemented carbide tools were tested by milling the powder metallurgical high-speed steel HS6-5-3C. For the coatings investigated, it can be concluded that a compressive residual stress state of approx. -2 GPa ≤ σ ≤ −3 GPa leads to the highest resistance against crack formation, and thus to the best cutting performance.