Microstructural and tribological characterisation of a nitriding/TiAlN PVD coating duplex treatment applied to M2 High Speed Steel tools

Abstract

We describe a duplex nitriding and TiAlN coating process performed with no break in vacuum in an industrial scale deposition chamber. Plasma nitriding at 480°C resulted in a fracture tough diffusion zone with no evidence of either a compound layer or grain boundary precipitation. Untreated, plasma-nitrided, coating-only and duplex-coated High Speed Steel (HSS) M2 coupons were microstructurally and mechanically characterised whilst similarly treated M2 ¼-inch jobber drills were subjected to accelerated drill testing (using D2 tool steel). In these tests, the duplex treated drills exhibited significantly improved wear performance and tool lives when compared with the other drills. After sectioning and electron microscopy, this extended tool life was attributed to increased toughness of the nitrided cutting edges coupled with improved adhesion at the substrate–TiAlN interface.