Ceramic composite cutting tools: are they tough enough?

As the demand for more fuel-efficient gas turbine engines continues to escalate, traditional heat-resistant superalloy (HRSA) materials are being replaced with new ultratough, high-temperature and wear-resistant alloys. The physical demands on these materials are high—as are the demands on the cutting tools applied to machine them.

Manufacturers tasked with cutting these alloys quickly learn that following traditional machining guidelines alone will not keep them competitive. Cutting tool manufacturers are familiar with these difficulties and are poised and ready to offer a productive and cost-effective alternative: ceramic composite cutting tools.

CCCTs are specifically engineered to combine extreme temperature resistance with high strength. This combination provides a highly productive and predictable cutting tool material capable of withstanding the extreme conditions encountered when machining HRSAs and other challenging materials.

Reinforcements to the Rescue

One of the most widely known CCCT materials for cutting HRSAs is whisker-reinforced ceramic. When introduced more than 30 years ago, whisker-reinforced ceramic tools took the market by storm by providing up to 10 times the productivity of carbide. Without the whisker reinforcement, ceramic cutting tools of the time lacked the predictability to excel at machining HRSAs. Whisker-reinforced ceramics continue to excel at machining these difficult-to-machine materials.

In addition, coated, whisker-reinforced ceramics introduced in the last decade provide additional time and cost savings compared to their uncoated counterparts. The addition of high-performance coatings increase an insert’s temperature and wear resistance, much like coatings on carbide cutting tools, and further boost productivity and tool life.

On average, by upgrading from an uncoated to a coated, whisker-reinforced ceramic tool, users can expect a 20 to 40 percent increase in cutting speed. And depending on the grade of the coated insert, up to a 20 percent increase in feed and up to a 50 to 100 percent increase in tool life can also be realized. In many applications, a coated grade imparts a finer surface finish and allows a longer length of cut than an uncoated grade.

Implementing CCCTs is not as simple as placing an insert in a pocket and hitting the green button. Users must understand the basic science behind why these tools are capable of such performance gains and how to accurately gauge and adjust for tool wear.

Recipe for Success

Whisker-reinforced ceramic inserts are made from fine-grain alumina oxide, combined with silicon-carbide crystals, commonly called whiskers. Al2O3 is a high temperature- and wear-resistant material but lacks strength and, therefore, predictability. By adding silicon-carbide whiskers, Al2O3 is reinforced and becomes predictable and strong, much like rebar in cement or fiber filaments in fiberglass. This reinforcement allows the ceramic tool material to operate at much higher cutting speeds than traditional carbide tools.

High cutting speeds are crucial for plasticization, or softening, of HRSAs, so material can be efficiently removed layer by layer. The plasticization process, however, generates a high level of heat in the cutting zone, which needs to be effectively managed and evacuated so the microstructure of the parent material is not compromised. Because of this, it is essential to properly balance the cutting speed with the feed rate and maintain the correct average chip thickness (ACT) to ensure the bulk of the heat is carried away with the chip.

The feed rate is expressed as ACT instead of feed per revolution or feed per tooth because the chip thickness can vary in relationship to the feed rate, depending on many factors. When applying a round insert, for example, the ACT depends on the DOC, because the effective lead angle changes as the DOC changes, varying the thinning or thickening effect on the chip.

When using a straight-edge insert, the ACT depends on the orientation of the tool to the part, otherwise known as the tool’s lead angle. While machining with a lead-angle tool can be beneficial for many reasons, such as reducing ceramic notching, it is important to adjust the programmed feed rate to account for these chip-thinning
phenomena and ensure a predictable tool wear pattern.

A rule of thumb for whisker-reinforced ceramics is to have an ACT of 0.002″ to 0.004″ (0.05mm to 0.10mm). This will ensure the chip maintains the proper cross section to carry the extreme heat developed by high cutting speeds from the tool/workpiece interface.