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Wear mechanisms of coated tools

Since the late 70:ies when the TiN-coating was introduced on HSS metal cutting tools, PVD
coating has become standard in tool wear protection, and today coating centres offer a
considerable number of thin ceramic coatings for HSS tools [9]. A thin (1 – 10 μm) PVD
coating will primarily protect the cutting edge in two ways:
• Acting as a shield againstabrasive and mild adhesive wear.
• Reducing the tool temperature by reducing the friction between tool and work material,
especially between chip and rake face.
The coatings combine a superior hardness (abrasive wear resistance) with relatively low
chemical reactivity with metallic materials (low solubility), the latter giving protection against
the welding mechanism that is the prerequisite foradhesive wear. Consequently, most of the
common PVD coatings of today rather fail by fatigue and discrete delamination/detachment
than removal by slow gradual wear [9]. Once the coating is removed, the wear mechanisms of
coated tools are the same as those of uncoated, although more severe because more severe
cutting parameters are normally used for coated tools.

7.1. Coating removal due topoor substrate preparation

Eliminación de capas, debido a la preparación del sustrato pobre

There are primarily two ways by which failure in HSS substrate preparation can occur.
• The surface temperature during grinding/polishing reaches above the austenitisation
temperature resulting in a brittle interlayer of untempered martensite, see Fig. 10.
• The resulting substrate surface is toorough, see Figs. 11 and 12.

[pic][pic]
a) b)

Fig. 10. Metallographic cross-sections through surface finished HSS materials.
a) Superficial layer of untempered martensite due to excessive heat generation during finishing.
b) Properly surface finished HSS.

Used as substrate for PVD coating, theuntempered martensite in Fig. 11a would constitute a
brittle interlayer inferior to coating adhesion.

PVD coatings on HSS tools possess internal compressive stresses of the order of 1-5 GPa.
Typically, TiN deposited on HSS has a lateral compressive residual stress of around 4 GPa.
This stress acts positively for the coating cohesion, but negatively on its adhesion to the
substrate. In combinationwith a rough substrate, excessively high compressive stresses may
cause spontaneous detachment without any external loads [10, 12]. The reason is that lateral
compressive stresses in the coating combined with a rough substrate will generate tensile
stresses across the coating/substrate interface as illustrated in Fig. 11a [12]. If such a system is
externally loaded, coating detachment isfacilitated along regions of maximum tensile stress,
i.e. along the coarse ridges on the tool of Fig. 11b. These ridges are the result of a too rough
grinding process / incorrect grinding parameters.
Another example of topographically induced coating failure is shown in Fig. 12 where it also
is indicated that cracks nucleated in the coating may spread to the underlying HSS material.
Throughfatigue, they may later cause edge chippings and large-scale edge fracture.

[pic] [pic]

a) b)
Fig 11. a) The lateral compressive stresses state σ present in most PVD coatings will generate interfacial
stresses S. At the top of e.g. grinding ridges this stress is a tensile “lift off” stress that mayreach the
same order of magnitude as the residual stress σ [12]. Such ridges can result from rough grinding.
b) TiN coating detachment along grinding ridges of a HSS cutting tool.

[pic] [pic]

Coating removal due to thermal softening of the substrate

Once the HSS substrate material reaches a temperature level of excessive softening, it fails to
resist the contact pressure, and the...