If you ask a production engineer in the ceramic industry
about the best choice of cutting tools for machining ceramic, it’s likely that
materials such as carbide, polycrystalline diamond (PCD) and diamond-like
coatings will make the list. But consider that production engineers in other
industries actually turn to ceramic for its use as a cutting tool material. In
fact, today’s ceramics are used to machine some of industry’s most
difficult-to-cut materials while still offering productivity gains.
Ceramics first became an attractive option for cutting tools
in the 1940s, when tungsten (used in the most common cutting tool materials,
high-speed steel and carbide) was in high demand. The extreme density of
tungsten made it suitable for military applications such as anti-tank weaponry.
Tungsten’s use in high-speed tool steel for machining purposes had already been
demonstrated, but limited supplies of the element created a need for an
alternative cutting tool material.
had been considered as a cutting tool material, and the short supply of
tungsten pushed them to the front lines of development. While these tools were
a viable alternative to tungsten-based cutting tools, they were still limited
in terms of their mechanical strength. As a result, the ceramics could only be
applied for machining cast irons and steels on machines with high rigidity.
Advances in pressing technology and further
experimentation with alloying materials gave rise to a new breed of ceramic
cutting tools in the 1960s and ’70s. Hot press and hot isostatic press
technology provided some improvements in the overall toughness of ceramic
tooling. The toughness and reliability of the product was further improved when
Japanese researchers began experimenting by adding titanium carbide (TiC) to
the alumina base material. The addition of TiC provided improvements in the
thermal conductivity and toughness of the ceramic, allowing it to become more
accepted throughout the cutting tool industry.
In the ever-changing world of indexable machining, Al2
ceramics are still the most widely used type of ceramic cutting tools. From
these humble beginnings, the family of ceramic cutting tools has grown to
include multiple classes: Al2
silicon nitride (Si3
whisker-reinforced, and cell-fiber composites. While there is some overlap in
terms of application, these different classes meet a variety of needs for the
aerospace, medical, steel, bearing and automotive industries.
Often referred to as cold
press ceramics, pure Al2
ceramics still find use in the area of cast iron
machining. Gray cast iron is often an ideal material to be machined by alumina.
It does not produce a true “chip” during machining, but instead essentially
breaks down into a powder. A traditional carbide insert with a chipbreaker is
therefore not truly necessary to machine the cast iron part.
ceramics can run much
faster than their carbide counterparts, offering huge gains in productivity.
The purity of this class of ceramic allows it to excel at the high-speed
finishing of cast iron, though it does suffer from low durability due to its
lack of toughness.
ceramics are suited for the high-speed finishing
of gray cast iron, the Si3
class is the first choice for rough turning and
milling of this material. Si3
-based ceramics give up some wear resistance to
cousins, but they provide huge gains in the area
of toughness. Extra durability allows these tools to take heavier depths of cut
and run more reliably with coolant.
Coolant is often used to help keep dust from building up during the machining
of cast iron. Ceramics are naturally exposed to heat during use and the
introduction of coolant can create rapid changes in temperature at the cutting
edge, making them susceptible to thermal cracking. While this vulnerability can
be especially true of the alumina-based ceramics, the silicon nitrides are more
forgiving of coolant.
The same toughness that allows Si3
run with coolant also provides the opportunity to run in applications that
include slight interruptions, such as bolt-holes or keyways. The combination of
toughness and wear resistance makes the Si3
of ceramics the best choice for the general-purpose machining of cast iron.
The steel and bearing
industries are the largest consumers of Al2
-TiC ceramics. This class of ceramic cutting tool
is normally called upon to machine steels with hardness values ranging from 45
to 64 Rockwell C. At this hardness range, cutting forces are generally 30-80%
greater than the forces generated during the conventional machining of softer
To account for this increase in cutting force, careful attention is paid to the
edge preparation of the ceramic insert itself. Unlike carbide inserts, which
are readily available in a variety of insert shapes and chipbreakers, tools
designed for “hard” machining are more focused in terms of their design.
Typical geometries for hard machining are round and square inserts with a
landed-edge preparation, which is basically a chamfering process designed to break
the sharp corner of the insert at a specific width and angle.
Edge preps for the general-purpose turning of hard materials are in the range
of 0.006-0.008 in. wide by 25-30°, while edge preps designed for turning steel
rolls are considerably larger and consist of both a primary and secondary
landed edge. The typical edge prep for a roll-turning insert consists of a
0.080 in. x 15° primary land, with a 0.006 in. x 25° secondary land. In
addition, a small hone (~ 0.0008 in.) is also applied to further strengthen the
As the hardness of the machined workpiece
increases, the size of the edge prep changes accordingly to account for the
increase in cutting forces. While cutting tool manufacturers maintain inventory
of many “standard” edge preps, it is widely understood that optimal tool life
can only be achieved when the edge prep of a ceramic insert is tailored to a
specific customer and application.
Nickel-based alloys are commonly used in the aerospace market
due to their high strength and heat resistance. This combination creates a
product that is difficult to machine with conventional tooling because the
material tends to work-harden during machining, which can lead to “notching” on
the cutting tool and, ultimately, insert failure.
Whisker-reinforced ceramics feature an
alumina base with the addition of silicon carbide (SiC) whiskers, which have
a high tensile strength that allows them to act like rebar in the alumina-based
matrix. This whisker reinforcement improves the notch resistance of the insert.
The end result is a ceramic insert that can run at speeds five to six times
that of a conventional carbide insert in nickel-based materials. As an added
benefit, the toughness of the SiC whiskers also makes this category of ceramic
available for machining harder materials with interruptions.
The benefits of whisker-reinforced ceramics involve certain limitations. The
SiC whiskers themselves have been identified as a carcinogen, and special precautions
must be taken in order to manufacture this type of ceramic safely.
The newest category of
modern-day ceramics is cell-fiber composites, which are designed to mimic the
strength found in nature. The substrate of one such composite* consists of
bundles of Al2
“cells” surrounded by Si3
“cell walls.” This approach is similar to the
design found in human muscle tissue, with individual muscle fibers being
surrounded by membranes. The toughness of the Si3
cell wall provides some strength and flexibility
to the insert, while the harder Al2
core offers the wear
resistance for the grade. The end result is a product that can offer notch
resistance similar to that of a whisker-reinforced ceramic without the need for
* Kyocera’s grade CF1
Figure 1. When a coating is applied to an advanced ceramic substrate, the productivity improvements can be substantial.
The technology involved in manufacturing ceramic cutting
tools has changed dramatically over the past 100 years. Like many other
industries, the indexable cutting tool industry has continued to experiment
with coatings to improve wear resistance and prolong the life of its products.
When properly applied, a coating can essentially make a ceramic insert both
tougher and more wear resistant than a similar insert without the coating.
When the coating is applied to an advanced ceramic substrate, the productivity
improvements can be substantial. One new material** combines an Al2
substrate with a proprietary coating. In field testing at a leading bearing
manufacturer, this coated ceramic provided the customer with a 20% improvement
in throughput while reducing the overall tooling costs compared to the existing
cubic boron nitride (CBN) tool (see Figure 1). The increased productivity and
cost savings were so substantial that the manufacturer has begun switching all
of its tooling to the coated ceramic.
does the future hold for ceramic cutting tools? With ongoing experimentation in
ceramic substrates and coatings, only time will tell. One thing is certain-the
next generation of ceramic inserts is sure to stretch the barriers of
toughness, wear resistance and, most importantly, user productivity.
For additional information regarding ceramic cutting tools, contact Kyocera
Industrial Ceramics Corp., Cutting Tool Division, 100 Industrial Park Rd.,
Mountain Home, NC 28758; (800) 823-7284; e-mail firstname.lastname@example.org; or visit www.kyocera.com/cuttingtools.