Re-sharpenable thick CVD diamond-coated Si3N4 tools for hardmetal turning

Sharpening and re-sharpening ability of CVD diamond-coated ceramic tools for turning operations is addressed in this work. Thick CVD diamond films (150 mu m) were deposited by MPCVD on pressure-less sintered silicon nitride ceramics. These disc-shaped tools could be sharpened to the desired geometry using grinding wheels without film detachment. The tools were tested in the dry turning of three types of hardmetal differing in WC grain size (2-6 mu m) and Co content (18-27 wt.%). The lower Co content hardmetal grade (GD40) is more aggressive than GD50 or GD60 to the cutting edge, always resulting in faster wear and larger cutting forces for the same machining parameters. The depth of cut was fixed at 0.2 mm for the three hardmetal grades while the speed and feed were reduced from 30 m min(-1) and 0.15 mm rev(-1) (GD50 and GD60) to 20 m min(-1) and 0. 1 mm rev(-1) for GD40. Wear changes the cutting edge geometry and increases the cutting force to values above which the tool fails (F-d similar to 700 N). Wear of the tool occurs by microchipping of the CVD diamond, resulting in abrasion of the rake and flank faces by diamond debris. Up to 2000 m dry machining length per tool could be achieved before the re-sharpening operation has to be performed. When a cutting fluid is used, increased tool life is easily achieved due to reduction of Co adhesion and enhancement of diamond debris removal from the cutting edge. (c) 2006 Elsevier B.V. All rights reserved.