Determination of the optimum TiN coating thickness on WC inserts for machining carbon steels

Developing coated cemented carbide inserts exhibiting enhanced wear performance has been of major interest for the cutting tool industry in recent years. This study evaluates the influence of titanium nitride (TiN) coating thickness deposited by the physical vapor deposition (PVD) technique on the machining performance of cemented carbide inserts. TiN coatings with thickness ranging from 1.75 to 7.5 μm were deposited onto CNMG 432 cemented carbide inserts (6% Co). Coating adhesion was assessed using the Rockwell indentation method, while coating thickness was determined using scanning electron microscopy (SEM) and optical profilometry. The glancing angle X-ray diffraction (XRD) technique using Cu Kα radiation was used to determine the influence of coating thickness on peak intensity. The intensity of the (1 1 1) peak was found to increase with increase in TiN coating thickness. The influence of TiN coating thickness on the machining performance of the WC inserts was evaluated by dry external cylindrical turning of AISI 1040 carbon steel. Based on these tests, the TiN coating with thickness of 3.5 μm was found to exhibit the best turning performance on the WC inserts. For these coatings a minimum of a 40-fold increase in tool life in comparison to the uncoated WC tool was recorded during dry machining using the specified turning parameters. The inferior performance of the thicker coatings is due to their higher levels of compressive stress resulting in coating failure due to chipping of the coating at the cutting edge exposing the WC substrate.