Functionalised polycrystalline boron nitride materials via laser surface engineering

Polycrystalline boron nitride cutting tools are widely used for hard-part steel turning due to their high wear resistance and long durability, however microstructurally different boron nitride composites are suited for different machining applications. For example, low cBN content grades (cBN<50%) are used in continuous and lightly interrupted cutting of automotive steels and in the machining of valve seat alloys; while high cBN content grades (cBN>85%) are designed to provide excellent abrasion and chip resistance and therefore are suitable for interrupted machining of grey and hard cast irons, hardened steel milling and in machining valve seat alloys where a longer tool life is required. In this paper, using a nanosecond fibre laser (1064-nm wavelength), surface engineering of polycrystalline of two microstructurally different boron nitride materials (50% and 90% cBN content) is proposed to functionalise the mechanical properties and enable to extend the applicability of a specific grade to a variety of machining applications. The materials’ response to different fluences, feed speeds and pulse durations was investigated and characterised through a combination of 3D white light interferometry, scanning electron microscopy (SEM) and microhardness measurements. 3D metrological data (Sa, Spk, Sku and Sq) of the samples were measured prior and post-process to provide an indication of the change in wear properties. Fractures between hard grains and binder and reduced surface integrity were highlighted by SEM analyses due to thermal stress caused by a difference in thermodynamic properties between grains and binder. Increased frequency and energy density caused local heating corroborated by larger Sa, Spk and Sq values but lower Sku values. Micro-hardness measurements revealed up to 20% increased hardness and 10% decreased root mean square height (Sq) with reduced feed speed and energy density per pulse. Improvements to the hardness and reductions in the surface roughness of both PCBN grades were observed, suggesting that the use of laser surface engineering could lead to improvements in performance in application in the automotive industry beyond the conventional application for the specific grade.