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Superhard materials for defined edge tooling may be defined as materials with a hardness exceeding 3,000 HV.
Polycrystalline diamond or “PCD” is produced by sintering micron diamond powders under ultrahigh pressure (>5 GPa) in the presence of a metal “catalyst” such as cobalt. These materials are commercially available in a range of grain sizes and typically are manufactured as disks (50–75 mm in diameter) with a thin (0.5 mm) layer of PCD bonded to a cemented carbide substrate. A “residue” of metal in the microstructure renders the materials electrically conductive, thereby greatly facilitating cutting tool production.
Polycrystalline thick-film diamond is produced by chemical vapor deposition. These materials are free-standing diamond layers of between 0.2 and 1 mm in thickness and comprise columnar grains of pure diamond.
Single crystal diamond is produced by ultrahigh pressure synthesis from carbon using a metal catalyst. Commercial production methods produce cubo-octahedral crystals of several mm to 10 mm in dimension, which are sliced into regular geometric plates for the production of ultraprecision cutting tools.
Single crystal diamond is produced by chemical vapor deposition method. Regular geometrical plates of such materials are commercially available with edge lengths up to 10 mm.
Thin-film diamond and diamond-like coatings are deposited on finish-ground cemented carbide tools such as end-mills and drills. The main benefit of CVD diamond coatings is that there is no subsequent grinding or finishing operation on the tool necessary.
In addition to the above five categories of synthetic diamond, natural single crystal diamond is still used to some extent in ultraprecision machining applications. All single crystal applications require the diamond to be precisely orientated so as to take advantage of the extreme anisotropy in the diamond lattice. Synthetic diamonds offer the advantage of having their crystal orientation aligned to the edges of cuboids/plates.
Boron nitride is an entirely synthetic material (with no natural occurrence) and the cubic crystal structure (cBN) is synthesized from the softer hexagonal (HBN) allotrope using ultrahigh pressures as in the case of diamond. CBN crystals exhibit less than half the hardness of diamond, but there are no industrial applications for such materials. Instead, all industrially relevant materials are polycrystalline in nature, produced by sintering micron cBN powder together with a variety of ceramic phases. Thus, polycrystalline cBN (PcBN) tooling materials may be classified most generally by their cBN content, with most practitioners distinguishing between high-content (>80%) and low-content PcBN. This distinction is useful also in terms of the relevant application areas. All commercial PcBN materials are produced in the form of disks (50–95 mm in diameter) with or without a hard metal substrate. Some of the most notable properties of polycrystalline PcBN tooling materials, in comparison with other cutting materials, are the high hot hardness, up to 1,200 °C, and the high chemical resistance. These properties are not only useful for withstanding the relatively hot chip forming temperatures in the cutting zone, but also make PcBN suitable for the machining of hardened steels. The tendency of the toughness of this material to increase as the hardness decreases does not occur with the same intensity as it does in other materials, such as cemented carbides or ceramics. PcBN is therefore an effective tool material for dry machining of high-performance materials (König and Hiding 1994; Clark and Sen 1998; Reuter 2001).
Indicative properties of superhard materials for defined edge tooling applications
Hardness vickers (N/mm^2)
Transverse bending strength (MPa)
Thermal conductivity (W/m.K)
Single crystal materials
Thick-film CVD Diamond
Although superhard materials other than diamond and CBN have been synthesized in recent years – including boron carbon nitride, cubic silicon nitride, and boron sub-oxide, these are presently not of industrial importance as their synthesis conditions are too extreme or their properties, other than hardness, insufficient for application as cutting tools.
Theory and Application
Single Crystal Diamond for Ultraprecision Machining Applications
Single crystal diamond is the only practical tool material for ultraprecision machining applications. The extreme hardness of diamond provides for optimum wear resistance, but also enables the generation of near atomically sharp cutting edges (a characteristic which also draws upon the absence of grain boundaries). Diamond exhibits pronounced anisotropy, which is exploited in the production of ultraprecision tools – often, crystals are orientated so the (110) plane is presented to the polishing scaife so as maximize the rate of polishing. In addition to the crystal plane, the direction of polishing is also critical and each of the (100), (110), and (111) planes exhibits difference numbers of “soft” directions.
In addition to the material’s extreme hardness, monocrystalline diamond exhibits extremely low coefficients of friction on most metals and nonmetals (of the order of 0.05), resulting in minimal chip adhesion to the cutting edge. A thermal conductivity of up to 2,000 W/m.K and a thermal expansion coefficient of less than 2 ppm/K ensure minimum thermal deflections and deviations.
Such is the relative expense of single crystal diamond tools, they tend to be reserved for applications requiring below 10 nm surface finish and submicron form accuracies. The generation of optical components such as mirrors, lenses, and lens molds from materials such as aluminum, OFHC, silicon, electroless nickel, and germanium is a relatively common application of monocrystalline diamond tools. Certain large volume manufacturing processes use MCD tools, namely, precision finishing of laminate flooring and finishing of acrylic aircraft windows.
Polycrystalline Diamond Tooling Materials
PCD tooling materials are usually employed in very large volume manufacturing processes such as automotive engine and gearbox manufacturing, wheel turning, and wood machining. In such applications, tool lives may be 10–1,000 h and it is generally found that PCD will exceed cemented carbide tool lives by at least two orders of magnitude. PCD is also employed in the machining of fiber-reinforced composites, cemented carbides, and cast irons (precision finishing) (Kalpakjian and Schmid 2001).
CVD diamond coated tools are implemented in the machining of lightweight materials in the automotive and aeronautical sectors, for example, in the machining of aluminum silicon and aluminum lithium alloys. These alloys challenge the cutting material due to the hard particles in the soft aluminum matrix and depending on the percentage of hard particles, the tools exhibit adhesive or abrasive wear as well as coating delamination.
Although diamond is one of the most inert materials, it will rapidly graphitize in contact with ferrous metals at temperatures above 1,100 K, and as such, diamond is only by exception used in the machining of ferrous metals (and typically with relatively low cutting speeds). PcBN tools, however, have a greater inertness in the presence of ferrous metals at high temperature.
Polycrystalline Cubic Boron Nitride (PcBN)
Single crystal cBN is not of industrial importance for cutting applications as it offers few benefits over monocrystalline diamond and is intrinsically more difficult to synthesize. As such, all industrial cBN tools are composite materials prepared using powder metallurgical techniques, but sintered under similar conditions to those used for the synthesis of diamond and cBN from their softer allotropes (graphite in the case of diamond).
The majority of PCBN tooling materials contain 40–70% cBN with the remainder being primarily TiC, TiCN, or TiN ceramic. These tooling materials exhibit excellent abrasion resistance and chemical wear resistance and are mostly used in the machining of hardened steels. Grades with lesser amounts of cBN tend to be used for continuous turning operations, while grades for interrupted turning and milling more commonly contain 60–70% cBN. Despite being somewhat counterintuitive, thin PVD ceramic coatings applied to PcBN tools can increase tool lives or enable operation at higher cutting speeds. Approximately half of all PcBN tools in use today are coated.
High-content PcBN grades (containing >80% cBN) are used for machining of cast irons, heavy turning, and milling of hardened steels and for finish-machining of powder metal components. These materials tend to have better thermal shock resistance and strength but exhibit more rapid crater wear in comparison to low-cBN grades.
Due to the fact that PcBN composites exhibit only moderate strength values, yet are used to machine many of the highest strength metallic work materials, virtually all tools are employed with negative cutting geometries, and cutting edge chamfers of 20–30° are commonly employed, as are edge hones (cutting edge radii). This is in contrast to diamond tools, which invariably are used in a neutral or positive cutting geometry.
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