predicting the permissible external loading that a diamond-coated cutting tool can withstand without premature de-bonding. 3.1.6. Wear mechanisms. The failure of CVD diamond-coated inserts during machining can be in the form of flaking (interfacial failure) or abrasive wear (gradual cohesive failure) [22]. Ideally, a test of superb adhesion is when the diamond coating fully deteriorates by wear rather than flaking. Flaking will occur primarily due to poor adhesion between the diamond coating and the carbide substrate [6]. Therefore, flaking is clearly undesirable because the benefit of using a diamond coating is lost, except for the chip breaking assistance of faceted diamond crystals at the rake surface [29, 75]. If the adhesion strength of the CVD diamond coating is sufficient to withstand the machining stresses, then the abrasive action between the workpiece material and the diamond coating becomes the primary failure mechanism. Unless the CVD diamond coating is polished, a two-step wear mechanism is ex­ pected to occur. The first step is caused by the initial high surface roughness of the CVD diamond coating in which crack initiation occurs at the surface. The mecha­ nism that describes such behavior was proposed by Gunnars and Alahelisten [56]. They described a three-zone wear model as shown in Fig. 6. In this model, the role of residual stresses becomes significant in controlling crack propagation from the surface to the interface that could lead to interface failure (flaking). As outlined earlier, the high total compressive residual stress present in CVD diamond coatings on carbide inserts was assumed to be biaxial and oriented parallel to the interface. Wear starts to occur at the surface, which, because of geometry, allows stress to relax. A crack is more likely to initiate at protruding grains in zone I and propa­ gate preferentially along the (111) easy cleavage planes of diamond. The geometry at deeper depths, however, prevents the compressive residual stress from relaxing. Therefore, as the crack propagates deeper in the coating, it encounters higher com­ pressive stresses that cause the cracks to redirect their paths deviating from cleavage planes to a direction parallel to the interface in region II. The high compressive stress now causes cracks to propagate fast parallel to the interface resulting in a smooth surface in region III. Due to the smoother surface, fewer asperities will be present and it becomes harder to nucleate cracks.