High-performance ceramic composite-coated chip breakers can produce tool life equivalent to the industrial benchmark while offering greater flexibility in geometry and cost.
Cubic boron nitride (cBN), the second-hardest known ceramic material next to diamond, possesses the high hardness, thermal stability and chemical inertness desirable for ferrous material machining. For the majority of machining applications, polycrystalline cubic boron nitride (PCBN) ceramics are provided in bulk shape or tipped form, and they are fabricated using high-temperature and high-pressure (HT-HP) processes.
In spite of its high cost, PCBN is one of the main choices for ferrous material machining. In practical applications, especially the turning of hardened steel, the cutting edges of PCBN tools are often chipped at the micron scale. To circumvent the edge-chipping issue, a sizable T-land or K-land is typically placed on the cutting edge. This change of edge profile usually induces high contact stress and thus generates excessive heat on the workpiece, which often degrades its surface quality by forming a white layer and produces entangled chips (commonly referred as a "bird nest") that scratch the machined surfaces.
Tailoring Chip Breakers
Chip breakers deliver favorable performance benefits to both the machining process and end user applications. These features include an improved surface finish by properly controlling chip flow and breakage, as well as a reduced chance of white layer formation by lessening the contact stress at the tool-workpiece interface. However, due to its structural rigidity, it is difficult to produce PCBN compacts in complex geometries such as chip breakers, despite some encouraging progress made by creating simple grooves as alternatives to chip breakers on PCBN compacts using laser processing and electrical discharge machining.
To date, applying cBN as a coating or thin film is the only practical option to tailor chip breakers of different geometries. Various physical and chemical vapor deposition (PVD and CVD, respectively) techniques have been explored for synthesizing cBN film. However, no significant progress has been made or reported in terms of achieving thick cBN films. Driven by the need for cBN in complex shapes and viewing a thick cBN coating as a cost-effective alternative to PCBN tools for machining applications, major cutting tool companies worldwide have invested in unremitting development efforts.
A major advancement in synthesizing thick cBN ceramic composite coatings (5-20 µm) has resulted from the application of an innovative synthesis and deposition process.* This process combines the electrostatic spray coating of cBN particles as a coating preform, followed by the chemical vapor infiltration (CVI) of a ceramic binder, such as titanium nitride (TiN), titanium carbonitride (TiCN) or titanium carbide (TiC), to provide the desired application-specific coating chemistry, thickness, geometry and properties.
Figure 1. Examples of cBN composite coating-coated carbide inserts with chip breakers.
Synthesizing the Coating
Figure 1 shows examples of carbide inserts with different chip breakers coated with a cBN composite coating of about 14 µm. The carbide inserts feature percentages of cobalt binder ranging from 2.5 to 12.0%. The coating clearly shows the features of chip breakers and is conformal to the surface profile of the carbide substrates.
The composite coating is realized in sequential steps by starting with the deposition of a porous cBN particle coating preform (with a particle size of less than 2 µm) using electrostatic spray coating (ESC), followed by the CVI of TiN as a binder. ESC deposition involves the physical spraying of micro- and nano-sized particles. The particles are charged at the exit of the spray gun and exposed to an electric field generated by a pointed electrode.
Figure 2. A typical cross-section of the cBN composite coating.
Since powder particles are generally electrically insulating in nature and can carry their static charge over a certain distance, the charged particles follow the electrostatic field lines toward the grounded substrate and form a uniform coating. For deposition on cutting inserts, graphite trays that are typically used in CVD can be readily integrated to facilitate the insert transfer from the deposition unit to the subsequent CVI reactor. The CVI process uses standard CVD equipment with slight modifications for the necessary processing parameters.
With its combination of hardness and toughness, the composite coating adheres well to the carbide substrate and does not show particle pull-out or coating delamination at a critical loading of 10 kg in scratch testing. A typical cross-section profile of a cBN composite coating is illustrated in Figure 2. It is uniform with approximately 50% cBN particles.
The cost of the coating is comparable to conventional tool coatings. As a complement to PCBN inserts, the cBN composite coating on carbide inserts with different geometries widens the opportunities for cutting tools while being less expensive than PCBN compact or tipped inserts.
Turning AISI 4340 Steel
When turning AISI 4340 hardened steel (50-52 HRC) at the machining conditions recommended for PCBN-tipped inserts listed in Table 1, the cBN composite coating-coated chip breakers produced more than 40 minutes of tool life without reaching the flank wear limit of 0.20 mm (0.008 in.) specified by ISO 3685. PCBN-tipped inserts were also tested at identical conditions.
Figure 3. Tool wear progression of cBN composite coating-coated chip breakers and PCBN-tipped inserts.
A comparison of tool wear progression between the cBN-coated chip breakers and PCBN-tipped flat inserts is shown in Figure 3. The wear rate of the cBN-coated chip breakers is evidently lower than PCBN in the test period. PCBN-tipped inserts had a tool life of about 34 minutes before they reached the tool failure criterion of 0.2 mm (0.008 in.). Examination of the tested chip breakers under an optical microscope indicated a retained edge profile with coating still on the edge (see Figure 4). Such a wear progression provides much better predictable tool life.
Figure 4. Flank surface (A) and crater surface (B) of cBN composite coating-coated chip breakers after machining test.
Importantly, the cBN composite coating-coated chip breakers produced short chip segments that were effectively shed away from the workpiece without rubbing the machined surface (see Figure 5A). This observation is in significant contrast to the long and entangled chips from the PCBN tool. The controlled chip flow and reduced tool wear contributed to a surface roughness (Ra) of less than 0.8 µm in the whole testing process, which was an improvement over the 1.8 µm achieved by the PCBN-tipped inserts.
Figure 5. Segment chips (A) and machined surface (B) produced by cBN composite coating-coated chip breakers.
Examination of the chip segments did not show discoloration, indicating smooth cutting without excessive heat generated in the machining process. It is consistent with the outcome from inspecting the machined surface (see Figure 5B). Metallographic analysis of the sample pieces from the machined surface did not reveal any white layer formation, in spite of some plastic flow observed near the surface. The plastic flow was induced due to the interfacial stress from shearing.
A Complementary Solution
Cubic boron nitride particle composite coatings can be readily applied to cutting inserts with chip breakers through the use of patented coating technology. In straight turning of AISI 4340 hardened steel, the coated chip breakers produced a tool life equivalent to PCBN-tipped flat inserts under identical conditions.
The machined surface with chip breakers has a surface finish approaching 0.8 µm without the formation of a white layer. Thus, the ceramic coating-coated inserts can be an important complement to PCBN ceramics, offering extended tool life, improved machined surfaces and better economics.
*Developed at NanoMech in partnership with the University of Arkansas. For additional information, contact NanoMech at 2447 Technology Way, Springdale, AR 72764; call (479) 756-9999; e-mail email@example.com; or visit www.nanomech.biz.