Ultratough, high-temperature alloys often call for Ceramic Composite Cutting Tools (CCCTs) for their ability to handle extreme heat with high strength. They have been in circulation for decades, but developments in materials processing, coatings and most recently, cutting paths have unlocked new efficiencies that make them a better option than ever. That’s a big part of the reason they are used in more and more applications from aerospace to energy and automobiles.
We’ll cover some of the old innovations that first put ceramic on the map decades ago and discuss more recent phase-toughened ceramics like XSYTIN – 1. Then we’ll discuss how the unique properties of these materials is just now being understood and exploited by CAM software. Even if you use CCCTs regularly, there should be some new news here even for you.
Whisker-Reinforced Ceramic Upped Productivity by a Factor of 10
The secret of the Universe unfolded for machinists who worked with hardened AISI 4340. It was the 1980s, so crazy hair was already on the scene when Silicon Carbide (SiC) whiskers were added to Alumina Oxide (Al2O3), producing an insanely tough material with reported 2,100 MPa compressive strength and a Young’s modulus of 840 GPa. Cutting tools made with whisker-reinforced SiC literally shortened production times by a factor of 10. One article published in 1991 by Adams et al. reported the turning time for an aircraft landing gear made of hardened steel (~57 HRC) reduced from 12 and a half hours to 1 and a half by using SiCw tools (David W. Richerson, in Comprehensive Composite Materials, 2000).
How do Silicon Carbide whiskers x10 hardness?!
Alumina Oxide was already capable of withstanding heat, but the hardness and consistency achieved through whisker reinforcement made the material hard enough to cut steel.
Whiskers increase the strength of the material through crack deflection, crack bridging and whisker pullout. Because the whiskers are stronger than the alumina oxide matrix, cracks begin in the matrix first, so the whisker extends across the crack and holds it closed. This reduces the stress at the tip of the crack. And as the crack continues, it must pull the whisker against the matrix, causing an insane amount of friction that channels the force and heat toward the center of the tool, further stabilizing against the force of the opening crack.
The best cutting paths play to these strengths by inducing force toward the center of the tool, but more on tool paths later. We still need to talk about two more advancements that are widely used on CCCTs today: phase toughening and coatings.
Phase toughening with Zirconium, or coating?
The next advancement was less magnificent but still significant. When the alumina matrix is combined with flecks of zirconia, the resulting material becomes even harder. When temperature reduces to under 1150 degrees, zirconia increases volume by 3%.
Now let’s consider what can happen under cyclical stress conditions. Already with whiskers we have microcracks increasing cutting tool volume. This places intense pressure on the main crack to keep it closed. But with zirconia, disengaging from the cut allows the tool to cool below 1150 degrees. The zirconia expands and works to push those cracks closed. That’s why you need a completely different cutting path for phase toughened CCCTs. You want them to cool and heat cyclically.
Phase-toughened ceramics are two-to-four times as hard as your typical whiskered alumina with Compressive strength of 2,500 MPa and flexural strength of 675 MPa (compared to ~350 for Al2O3).
Greenleaf makes XSYTIN-1 inserts that are phase toughened, but the technology is still rarely applied today. Maybe it’s because working with zirconia is completely different than other ceramics.
Coatings are far more common, perhaps because they don’t require a path reboot like zirconia does.
Without zirconia you want to retain constant contact (more reasons on that below), so this slight difference in material makes a huge difference in path and capability.
A path for every ceramic treatment: 4 tips for CCCTs
Phase toughened ceramics are a different monster, so we’ve ‘zipped’ two lists of best practices together: “with zirconia” and “without zirconia.”
One thing that makes ceramic tools different is their penchant for heat, which means speed. But phase-toughened is like kerosene on a family BBQ. Compare these ideal speeds for hard turning Inconel 718. Coatings don’t make a difference in path, but they do add tool life:
- Whisker-reinforced ceramic insert: cutting speed of 800 sfm, feed of 0.008 ipr and DOC of 0.080”
- Phase-toughened: 700 to 750 sfm, 0.014 to 0.016 ipr and 0.080″ to 0.120″ (2.03mm to 3.05mm) DOC
Here’s another area of difference between phase-toughened and ‘traditional’ ceramics. Traditionally, interrupted cuts are bad because they allow the tool surface to cool. But phase-toughened ceramics actually prefer cyclical stress because it allows the zirconia to expand and repair itself.
As you can see in the feeds mentioned above, you want to feed fast.
Beware Sharp Edges and Corners
Despite whiskers, aluminum oxide is still relatively brittle. This necessitates caution when entering, exiting, and navigating around edges. If you can, round off areas of entry and exit before machining to avoid the risk of a chip or break. Reduce feed rate by as much as one-half around corners, as a greater portion of the insert engages the surface.
A typical path for a right angle corner is actually to disengage completely and then enter the far wall head on. Even though typical ceramics prefer constant contact, the risk of breakage is too great. If your project requires a lot of corners, then consider a phase-toughened tool that thrives under this kind of challenge.
Pile on the experience
We’ve covered just the very basics of ceramic tooling in this article, so if you have an experience to add, please pile it on! Follow MachiningCloud on Facebook for updates on the latest developments in CNC machining to share about your own odysseys into alumina oxide.