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 BreakersChip 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.
Synthesizing the CoatingFigure 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.
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 SteelWhen 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.
A Complementary SolutionCubic 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.
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