374 Tribology of Metal Cutting
• Thermal diffusion (TD) is applied in a molten borax bath, with the addition of
vanadium, at approximately 1000
◦
C. The resultant vanadium carbide coating has
very good results in numerous applications.
• Dynamic compound deposition (DCD) coating process (developed by Richter
Precision Co.) is a proprietary low-temperature coating process that synthesized
dry-film lubricant and wear-resistant coating components.
CVD coatings have been commercially available for about 30 years, and the fact that
more than half of the inserts sold are CVD coated testifies to the effectiveness of these
coatings. CVD coatings usually are deposited in multi-layer composition. A TiC–TiN
multi-layer, for instance, provides the lubricity of TiN and the abrasion resistance of
TiC. Coating thickness is in the range of 5–10 µm.
However, the high temperatures (about 1000
◦
C) involved in the CVD process create
an embrittlement called “eta phase” at the coating–substrate interface. Depending on its
extent, the embrittlement can affect the operational performance involving interruptions
of cut and inconsistency of workpiece microstructure such as the one found in some
nodular irons. Recently developed medium temperature CVD (MTCVD) coatings have
shown a reduced tendency to the formation of eta phase. MTCVD-coated tools offer
increased resistance to thermal shock and edge chipping compared to the conventional
CVD-coated tools. The result is greater tool life as well as increased toughness compared
to high-temperature CVD coatings [70].
PVD coatings also offer advantages over CVD coatings in certain operations and/or work-
piece materials. Commercialized in the mid-1980s, the PVD coating process involves
relatively low deposition temperatures (approximately 500
◦
C), and permits coating of
sharp insert edges (CVD-coated insert edges are usually honed before coating to mini-
mize the effect of eta phase.). Sharp and strong insert edges are essential in operations
such as broaching, gear shaving, milling, drilling, threading and cutoff and for effective
cutting of the so-called “long-chip” materials such as low-carbon steels. In fact, a wide
range of “problem” materials – such as titanium, nickel-based high alloys and nonfer-
rous materials – can be productively machined with PVD-coated tools. From a workpiece
structure point of view, sharp edges reduce cutting forces, so PVD-coated tools can offer
a true advantage when machining thin-wall components or when the machining residual
stresses are the issue.
The first PVD coatings were titanium nitride (TiN), but more recently developed
PVD technologies include titanium carbonitride (TiCN) and titanium aluminum nitride
(TiAlN), which offer higher hardness, increased toughness and improved wear resis-
tance. TiAlN tools in particular, through their higher chemical stability, offer increased
resistance to chemical wear and thereby increased capability for higher speeds. Recent
developments in PVD coatings include “soft” coatings such as molybdenum disulfide
(MoS
2
) for dry drilling applications. Soft–hard coating combination such as MoS
2
over
a PVD TiN or TiAlN, also demonstrated great potential, as the hard (TiN or TiAlN)
coating provides wear resistance while the softer, more lubricious outer layer expedites
chip flow [70–72]. The basic PVD coatings are listed in Table 6.8 and their properties
are shown in Table 6.9. Effectiveness of various coatings on cermet cutting tools is
discussed in [73,74].