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|Advances in the Development of Ultrahard Cutting Tools – A Review|
|Bob Flynn, Materials Ireland Research Centre, University of Limerick, Ireland|
Several developments in the properties and processing of ultrahard cutting tools have occurred in recent years. Current innovation is tending towards more coating related technologies such as diamond and « diamond-like » coatings as well as the development of new ultrahard materials such as C4N4 with reported hardness values greater than natural diamond.
The Ideal Cutting Tool
In order to successfully produce ultrahard tooling materials several mechanical and thermal properties are required: wear resistance, hot hardness, high toughness and high transverse rupture strength. Figure 7 shows the relationship between these properties and today’s range of tooling materials as well as the direction needed to take in order to produce the ideal tooling material.
Tooling materials used today can be divided into two main types: tooling for ferrous metals:
and tooling for non-ferrous metals:
Diamond is the hardest of all known materials and has been available in natural form since ancient times. Natural diamond tools are used for special applications where no other tool has satisfactory performance. High quality single crystal industrial diamonds are the only option for applications such as finish turning of gold and copper. On the other hand, low quality industrial diamonds are used for high speed machining of non-ferrous metals, ceramics and plastics. However the anisotropy of such a « single stone » is well known and the unreliability of the tool due to easy cleavage has been well recognised. Moreover, limited supply, increasing demand and high cost have resulted in an intensive search for an alternative dependable source of diamond. This led to the ultra-high pressure and temperature synthesis of diamond from graphite and the subsequent development of polycrystalline diamond (PCD) tools in the late 1960’s.
Cubic Boron Nitride (cBN)
After diamond, cubic boron nitride is the hardest material in common use and is often used as a tooling material where the use of diamond is unsuitable, such as the machining of ferrous materials or where high temperature machining is required (>700°C).
Polycrystalline Diamond (PCD)
PCD tools consist of a thin layer (0.5 to 1.5 mm) of fine grain size, randomly orientated particles sintered with a binder phase (usually cobalt) and metallurgically bonded to a cemented carbide substrate. The diamond crystals and the blanks are synthesised and sintered at high pressure high temperature (HP-HT) conditions (~ 50 kbar and 500°C). Various shapes and formats are currently available from a range of manufacturers. Free standing PCD, where the substrate has been chemically dissolved away to leave just the polycrystalline diamond, is also available.
Diamond Coatings by Low Pressure Chemical Vapour Deposition (CVD)
Currently more than 30 modified methods have been developed for the manufacture of diamond coatings. However, all are variants of CVD methods, endeavouring to achieve a better bond, higher production volumes, and lower cost. In most processes hydrogen gas is broken down via a heating element or plasma stream into elemental hydrogen and dissolved in a heated hydrocarbon gas as methane at temperatures around 1300°C. When the mixture hits the relatively cooler (700-900°C) metal to be coated, carbon precipitates in pre-crystalline form and grain size can be controlled to between 1-50 µm. Such diamond, produced from the gas phase, is called CVD diamond. Diamond deposition can be summarised by the following conditions:
Many carbonaceous gases (hydrocarbons, alcohols, ketones, carbon oxides, carbon halides…) are suitable as long as hydrogen atoms are contained in the reactant gas. Recent reports suggest that diamond can be produced at temperatures as low as 150°C but only in laboratory conditions. CVD diamond has several advantages over polycrystalline products, including:
Diamond-Like Coatings (DLC)
DLCs have very similar properties to diamond films. However, DLC coatings differ from diamond in that they are mainly amorphous carbon but containing a proportion of carbon atoms bonded in a diamond structure and they may also contain up to 20% hydrogen. The coating process can be carried out by R.F., D.C. plasma-assisted CVD, sputtering, vacuum arc and ion beam deposition at temperatures from RT to 900°C. DLCs have many properties that make them very attractive from a tooling perspective: relatively high hardness (2000-5000 knoop), chemical inertness and highly smooth wear resistant coatings with an extremely low coefficient of friction (0.1). Furthermore, they are also less expensive than diamond coatings.
In recent years a range of new materials have been produced using ultrahigh pressures and temperatures. Materials such as B6O (hardness 8500 knoop) were produced at temperatures and pressures of 1700°C and 40 kbar and have been suggested as possible replacements for WC. C4N4 has been produced as a coating using pressures up to 1400 kbar with a reported hardness of between 10000 and 11000 knoop.
The development of other ultrahard materials based on a AxBy stoichiometry have been predicted using a neural network developed by Imagination Engines, Inc. These materials include: FrBe6+, CsB6, RbB6, CsB6, FrB6, Au4Zr5, Pt4Zr6+, Al6+Cs, Au5Mn6+, Au5V6+, Pt5V6+ and Au4Y6.
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