Hard Times Call for Harder Cutting Tools
May 1, 2009
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.
Historical PerspectiveCeramics 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.
Alumina (Al2O3)-based ceramics 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, Al2O3-TiC 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: Al2O3, silicon nitride (Si3N4), Al2O3-TiC, 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.
AluminaOften referred to as cold press ceramics, pure Al2O3 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.
The Al2O3 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.
Silicon NitrideWhile Al2O3 ceramics are suited for the high-speed finishing of gray cast iron, the Si3N4 class is the first choice for rough turning and milling of this material. Si3N4-based ceramics give up some wear resistance to their Al2O3 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 Si3N4 to 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 Si3N4 class of ceramics the best choice for the general-purpose machining of cast iron.
Alumina-Titanium CarbideThe steel and bearing industries are the largest consumers of Al2O3-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 materials.
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 secondary edge.
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.
Whisker-ReinforcedNickel-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.
Cell-Fiber CompositeThe 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 Al2O3-ZrO2 “cells” surrounded by Si3N4 “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 Si3N4 cell wall provides some strength and flexibility to the insert, while the harder Al2O3-ZrO2 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 SiC whiskers.
* Kyocera’s grade CF1
CoatingsThe 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 Al2O3-TiC 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.
Next GenerationWhat 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 email@example.com; or visit www.kyocera.com/cuttingtools.