The present invention relates to a coated cutting tool for metal machining, such as an insert for use in holding systems for turning, milling or drilling, or a solid tool, for instance, a drill or an endmill, having a body of cemented carbide or cermet, and a refractory coating, which entails excellent wear resistance of the tool, as well as toughness and resistance against edge chipping.
In the metal cutting industry there is a constant striving to increase productivity with demands on higher cutting speeds and feeds combined with increased tool life. To meet these demands, tool material development is largely focused on improving tool toughness and wear resistance.
By depositing a wear resistant coating by means of Physical Vapor Deposition, PVD, or Chemical Vapor Deposition, CVD, techniques the wear resistance can be dramatically improved compared to an uncoated tool.
Important improvements to coated cutting tools have been achieved by applying coatings of optimized thicker homogeneous PVD layers. However, with an increasing layer thickness, the positive effect on wear resistance is outbalanced by an increasing negative effect in the form of an increased risk of coating delamination and reduced toughness, making the cutting tool less reliable.
The tool generally has at least one cutting edge where the chip is separated from the work piece. The cutting edge separates two faces of the tool; a rake face, and a clearance face. This cutting edge is often subjected to chipping problems or fracture. This may happen as the result of a crack in the coating which may be present in the as coated tool or introduced as a result of, e.g., an intermittent cutting process.
The resistance to fracture or chipping is what is experienced as tool toughness, and this may be affected by the selection of coating and controlling its microstructure.
Examples of controlled microstructures are laminar superlattice coatings of multicomponent composition, for instance periodic TiN+VN, TiN+NbN or TiN+AlN thin layered structures, as disclosed, e.g., in EP 0 701 982 and EP 0 592 986.
EP 0 983 393 discloses aperiodic multilayer coatings of alternating metal nitrides or carbide layers where the multilayered microstructure provides for increased coating toughness compared to homogeneous coatings.
Albeit enhanced coating toughness, one of the most common failure modes of coated tools is related to fracture and chipping of the cutting edge.