Cutting Tool Materials Part 3 pps

Moreover, ceramic tools at this juncture, were only really employed for turning operations and in particular, in ‘stable machining’ , where interrupted/intermittent cutting operations did not occur. With the recent advances in powerful and very rigid CNC machine tools, this has opened-up the pos- sibility of utilising ceramic tooling, either in a purely sintered monolithic tooling insert, or more recently as a multi-coated variant – more will be said on this topic shortly. Returning to the monolithic ceramic cutting tool materials, they have normally been available in three distinct grades, which will now be mentioned. ese cutting inserts consist of: • ‘Pure’ ceramic – this is the traditional tooling insert material, consisting of aluminium oxide. e alumina is white in colour and is produced by cold pressing powder in the desired insert geometry dies 8 , with subsequent sintering, the fused alumina particulates are sintered together, thereby signi- cantly decreasing porosity. ese ceramics, have been known in the past as ‘pure oxide’ , or ‘cold- pressed’ ceramics. e major disadvantage of such ceramics is their low thermal conductivity, making them highly susceptible to thermal shock (i.e. the hot and cold thermal cycles that can occur when interrupted cutting takes place). ese thermal shock 8 ese consolidated cutting inserts produced in compound, or ‘oating’ die sets from the admixed powders, are termed ‘green compacts’ and are friable, that is having very limited mechanical strength and must be gently handled, prior to sintering – thereaer the desired mechanical strength occurs. Figure 11. A typical ‘super-glide’ coating of molybdenum disulde (MoS 2 ) applied to a hard-coating on a tool’s substrate – weak bonds between crystal layers allow easy movement of the planes. [Courtesy of Guhring] . Cutting Tool Materials  problems are exacerbated by short machining cycle times, variable depths of cut and higher machining speeds. ‘Pure’ alumina inserts can be improved by additions of zirconia (Zr) to greatly increase the toughness somewhat, but such cutting tool material, has been widely superseded today, by ‘mixed grade’ ceramics, or cermets – to shortly be discussed, • Black, or mixed ceramics – tend to minimise the eects of thermal shock on the cutting insert, by having additions of titanium carbide added to the alumina, this causes the insert to turn black. A problem with these earlier ‘black ceramics’ was that they did not sinter as readily as the former ‘pure’ ceramic inserts. erefore they usually had an additional ‘hot pressing’ operation to achieve the desired densities, which tended to limit the geomet- ric shapes for such inserts. A later development of these cutting tool materials was termed ‘mixed ceramics’ , these had additions of titanium nitride, which improved thermal shock still further, with the sintered inserts tending to be brown, or choco- late in colour – the term ‘black’ for these later inserts, became irrelevant. ese ‘mixed ceramics’ had good hot hardness, enabling them to machine harder steel components, or chilled cast irons and at greater temperatures, where the combinations of higher cutting forces and greater chip/tool interface temperatures would have induced cutting insert plastic deformation in their previous counterparts. • Cermets – the original cermet was developed by Lucas under the trade name ‘Sialon’ which was a silicon nitride based material, having a very low co- ecient of thermal expansion. is low expansion rate when in-cut, tends to reduce the stresses between the hotter and cooler isothermal zones of the insert, giving very high thermal shock resistance. Originally, it was dicult to sinter these inserts to full density, although by substituting some of the silicon and nitrogen with aluminium and oxygen, the new material ‘Sialon’ 9 it had the added benets of: ease of pressing and sintering, with equally as good thermal shock resistance. A notable later el- emental addition was that of yttria (Y 2 O 3 ), which aided sintering performance and during sintering. e silica (SiO 2 ) on the surface of the silicon nitride 9 Sialon, this name was coined for the insert, as it represented the chemical symbols for the constituent elements: Si, Al, O and N. particles will react with the yttria forming a liquid. is chemical reaction forms a ‘glass’ on cooling, so depending upon the relative proportions of the reactants, the resultant ‘Sialon’ formed may have either of the following atomic arrangements: beta silicon nitride, or alpha silicon nitride. It is possible to produce a very complex cutting insert material, having both ‘beta-’ and ‘alpha-Sialons’ in atten- dance. A typical ‘beta-Sialon’ might be composed of: Si 6.Z Al Z O Z N 8.Z Where: ‘Z’ represents the degree of substitution of silicon and nitrogen by aluminium and oxygen. Conversely, an ‘alpha-Sialon’ can consist of: Mx(Si, Al) 12 (O,N) 16 Where: ‘M’ is the metal atom, such as yttrium. All this sounds quite confusing, but basically the ‘Si- alon’ microstructure consists of a crystalline nitride phase, held in a glassy, or partially crystallised matrix. ese crystalline grains can be either ‘beta-Sialon’ , or a mixture of ‘alpha’ and ‘beta’ , but generally it can be said that as the ‘alpha’ phase increases, the hardness of the ‘Sialon’ becomes greater. ese chemical and mechanical changes, result in a higher ‘hot-hardness’ for the cutting insert when in-cut. An additional and probably greater benet is gained by the signicant improvement in insert toughness, which can rival that of cemented carbide of equal hardness. One limitation in the past to such cermets, was that they could not satisfactorily machine steels, owing to their poor performance in resisting solution wear. However, these earlier cermets when machining nickel-based alloys, or cast irons they performed very well, but even the ‘mixed’ ceramics based on alumina, having 25% additions of carbide (i.e. ‘whisker-reinforcement’) within the insert’s substrate are a direct competitor to such cermets.  Chapter  Today, with the more complex material technology cermet 10 insert grades (Fig. 10b), they can easily machine ferrous-based workpieces at high cutting speeds, tool lives and excellent surface nishes. Complex powder particulates are utilised for the current turning inserts, such powders may have a large core of TiCN, surrounded by TiN – for superior hardness, adjacent particulates having a small core of niobium (Nb), surrounded by tungsten (W) and titanium (Ti) – for superior toughness. e sintered cutting insert product has a very complex substrate, which is further enhanced by subsequent multi-coating. Typical turning data for a high-performance steel product that can be rough-to-nish turned using the same insert on a 34CrMo4 grade workpiece has been shown to be: Cutting data: cutting speed (V c ) 140 m min –1 , feed (f) 0.2 mm rev –1 , depth of cut (D OC ) 1.0 mm and with ood coolant. In interrupted cutting trials with the cutting data mentioned on this workpiece material (i.e. having 4 equally-spaced splines around its periphery), the cermet insert’s edge withstood over 7,000 impacts per edge. is can be considered as a ‘true’ testament to the hardness, shock resistance and life of the latest such cermet tooling materials. .. Cermets – Coated To enable a wider range of machining applications while improving still further the original cermet grades available, tool coatings were introduced and with sophisticated high-technology cutting insert geometries (see Fig. 10b and c). e latest multi-coatings for indexable cutting inserts have individual ‘nano- coatings’ 11 and are extremely hard, approaching 4000 10 Cermet is derived from the two words ceramic and metallic and, the clear distinction between this and other cutting tool materials, such as cemented carbide and ceramic tooling has become somewhat ‘blurred’ , with one tooling manufacturer claiming it was developed in 1929, which is ‘at odds’ with the patented ‘Sialon’ product developed by the Lucas company – previously discussed. 11 One nanometre is equal to 10 –9 m, or one millionth of a mm. HV 12 and the surfaces of such coatings tend to be very smooth and having a total thickness of less than 3 µm thick – allowing around 2,000 durable layers. One of the key factors in successfully applying these complex metallurgy multiple coatings, has been the development of ‘super-lattice technologies’ at medium temperatures, which do not compromise the thermal properties of the substrate. e unit cost of the cermet substrate tends to be lower than its equivalent cemented carbide grade, this accounts for the fact that at present, in turning operations in Japan 35% of all the inserts utilised for a range machining steel grades tend to be cermets, whereas, in Europe less than 5% of cermets are employed. Cermets are considerably more wear and heat resistant than tungsten carbide-based cutting materials. By way of illustration for the reason for edge failure of tungsten carbide inserts, is the heat generated at the tool/chip interface – at high cutting speeds. For example, if one considers the pre-sintering temperature for a typical tungsten carbide material it is in the region of 1,150°C and, if turning a: 0.48% C, 0.8% Mn medium carbon steel workpiece at 200 m min –1 , this equates to the highest isothermal edge temperature of 1,000°C – cre- ating the potential for localised thermal soening and edge failure. While an equivalent multi-coated Cermet, can readily turn alloy carbon steels at a depth of cut (D OC ) of up to 3 mm, with cutting speeds of between 200 to 300 m min –1 , with feedrates ranging from 0.1 to 0.3 mm rev –1 . Moreover, as less ank wear takes place, the dimensional size of subsequent components in a batch will not signicantly ‘statistically-dri’ , produc - ing much less tolerance variation (i.e. reliable size-for- size consistency) in the completed turned parts. is increased multi-coated cermet tool life, allows for an excellent surface nish and dimensional consistency, whether cut wet, or dry. In general, the multi-coated cermet cutting tool materials, can be consolidated (i.e. pressed) in compound die-sets with very complex tool geometries and have integrated chip-breakers present – as illustrated in Fig. 10c. Such inserts, have seen a slow take-up in Europe and oer considerable economical advantages when in particular, turning hardened steel parts. 12 A comparison of the hardness of dierent popular coatings may be applicable here, as TiCN coating has a hardness of around 2,700 HV and TiAlN coating has a hardness of approximately 3,200 HV. Cutting Tool Materials  Figure 12. Ultra-hard cutting tool materials – cubic boron nitride (CBN). [Courtesy of DeBeers – element 6].  Chapter  .. Cubic Boron Nitride (CBN) and Poly-crystalline Diamond (PCD) Cubic Boron Nitride (CBN)/Synthetic Diamond – Extraction and Sintering Cubic boron nitride (CBN) is one of the hardest materials available and for machining operations it can be considered as a ultra-hard cutting tool, it was rst synthesised in the late 1950’s. In many ways, CBN and natural diamond are very similar materials, as they both share the same atomic cubic crystallographic structure (see Fig. 12a and b). Both materials exhibit a high thermal conductivity, although they have pro- foundly dierent properties. For example, diamond is prone to graphitisation and will readily oxidise in air, reacting to ferrous workpieces at high temperatures, conversely, CBN is stable to higher temperatures and can eortlessly machine ferrous components. CBN can therefore machine ferrous materials, such as: tool steels, hard white irons, surface hardened steels, grey cast irons, (some) austempered ductile irons and hard- facing alloys. Normally, CBN tools should be used on workpiece materials with hardnesses greater than 48 HR C , because if workpieces are less hard than this, the cutting edge will result in excessive tool wear. In graphite, the carbon atoms are arranged in a hexagonal layered structure (Fig. 12ai) and, by the appli- cation of very high temperatures and pressures 13 , it can be transformed into the cubic structure of diamond (Fig. 12aii) – this transformation does not occur easily. As boron and nitrogen are two elements on either side of carbon in the Periodic Table, it is possible to form a compound of boron nitride, that exhibit’s a hexagonal boron nitride (HBN) as depicted in Fig. 12bi, having the characteristics of being both slippery and friable. HBN can be transformed in a similar fashion to that of CBN (Fig. 12 bii). In practice, to facilitate the rate of transformation in the reaction chamber, additions of solvents/catalysts are utilised for synthesis at more easily obtainable levels: pressures of approximately 60 GPa and temperatures 1,500°C. As this transforma - tion proceeds in the reaction volume of a high pres- sure system, the CBN/synthetic diamond grows, being embedded in a portion of reaction mass and extracted aerward from this special-purpose press. By dissolv- ing away the unwanted matrix, the CBN/synthetic diamond can be liberated and recovered for subsequent processing. Grain sizes vary from large dimensions of approximately 8 µm, down to sub-micron sizes, for ne-grain tooling. Once the synthesised CBN/diamond has been extracted, it is possible to sinter together these crystals of CBN, or diamond, with the aid of a ceramic binder, to produce polycrystalline masses. Commercially, in 13 To transform hexagonal graphite into the cubic diamond structure, requires exceedingly high temperatures > 2000°C and applied pressures > 60 GPa, to enable the conversion to take place. Table 1. Cutting tool materials – with some important physical properties Cutting tool material: Black ceramic (Al 2 O 3 + TiC) Cemented carbide (ISO K10 grade) CBN (DBC50) CBN (DBC80) Physical properties: Density [g cm –3 ] 4.28 14.7 4.28 3.52 Knoop hardness [GPa] 17 17 27.5 30 Young’s modulus [GPa] 390 593 587 649 Fracture toughness [MPam ½ ] 2.94 10.48 3.7 5.90 Thermal expansion [10 –6 K –1 ] 7.8 5.4 4.7 4.6 Thermal conductivity [Wm –1 K –1 ] 9.0 100 44 85 . Cutting Tool Materials  order to speed-up the rate of sintering, additions of a solvent/catalyst are utilised (i.e. normally metals, or metal nitrides), but during sintering the whole mass must be held in the ‘cubic region’ of the respective pres- sure/phase diagram – to prevent these hard crystals reverting back to their original so hexagonal form. By sintering these hard particles together, it is possible to form a conglomerate of CBN/diamond, in which randomly orientated crystals are combined to produce a large isotropic 14 mass. A very wide range of polycrystalline products can be produced, utilising either CBN, or synthetic diamond as a base. For example, by changing the: grain size (see Figs. 12 c and d), solvent/ catalyst employed, degree of sintering and particle size distribution and, the presence/absence of inert llers, this will have a profound eect on the mechanical and physical properties of the nal product – Table 1 lists 14 Isotropic materials can be considered to have the same properties in dierent directions. the physical properties of various comparable cutting tool materials. In order to produce the required tool geometries, both the polycrystalline layer of CBN and polycrystalline diamond (PCD) are bonded to a thick tungsten carbide backing layer, then cutting inserts are wire- cut out of this large blank – obtaining the maximum number of insert shapes per blank (see Fig. 13a). ese CBN/PCD inserts are either full-size, or smaller tips that are then brazed onto suitably-shaped blanks, to t the desired tool holder (as illustrated in Fig. 13b). Both CBN and PCD cutting tools can successfully machine: super-alloys (ie with low iron content), grey cast iron and non-ferrous metals, but show distinct dierences when other workpiece materials are to be productively machined – as depicted in Fig. 14. Polycrystalline diamond cutting tools are not utilised for machining ferrous workpieces, this is because when machining under the high temperatures and sustained pressures that occur during cutting, the diamond has a tendency to revert back to graphite, aer only a few seconds in-cut. is reversion, does Figure 13. Cutting tool materials: Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD) . [Courtesy of DeBeers – element 6] .  Chapter  Figure 14. A diagram illustrating how Cubin Boron Nitride (CBN) and Polycrystalline Diamond (PCD) applications are grouped, by workpiece materials. Their eectiveness when ei- . ther machining highly abrasive components, or high temperatures in the cutting vicinity Cutting Tool Materials  Figure 15. Turning operations with Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD). [Courtesy of DeBeers – element 6] .  Chapter  not take place when machining many non- ferrous and non-metallic workpiece materials. Although CBN is synthesised in a similar fashion to that of PCD cutting tool products, it is not as hard as PCD and is therefore less reactive with ferrous metals, as long as the cutting temperature is less than 1,000°C, it will not revert to its soer hexagonal form and oxidise in air. is means that CBN can machine many ferrous parts and cast iron grades. e complementary nature of both CBN and PCD is clearly depicted in Fig. 14, where the ‘cross-over’ between these ultra-hard cutting tool materials is shown. In both CBN and PCD machining applications, an excellent machined surface nish can be obtained (see Figs. 15a and b). In the case of many PCD operations, the cutting tool must not only machine widely diering materials that are situated adjacent to one another in many passes over such a diverse material workpiece, but produce an excellent machined surface nish, which really ‘challenges’ the tool. Tool life can be extended greatly by utilising either CBN, or PCD tooling, oen tool lives can be increased by 50 to 200 times that of the previous cemented carbide alterna- tives. is boost in output, makes their additional purchase price irrelevant, when considered against the massive productive gains that are to be made by their adoption. Today, both CBN and PCD can oen be found as either thin-coated layers on tooling (see Fig. 3 for their relative tool insert hardnesses/toughnesses), or as a ‘sandwich’ between metallic backing layers. ese ‘sandwiched’ tool edges, permit brazing on both sides of the hardened product, which are then accurately positioned and held onto a tungsten carbide shank, making them an ideal alternative for many micro- drilling operations. Such compound drilling edge technology, gives considerably improved edge reten- tion and resistance to any abrasive particles present in the workpiece and its severely work-hardened swarf, typically found with the latest metal matrix composites (MMC’s). Such ultra-hard tooling, can be readily used on high-silicon aluminium alloys used in the automo- tive industries, while not discounting the wide range of workpiece composites employed by the aerospace industries and the resin-based components utilised in the furniture industry. .. Natural Diamond Monolithic, or single-crystal diamond (SCD), is the hardest material available today. If such natural diamond is used correctly in a very rigid machine-tool- workpiece setup for materials that require the best possible surface nish, then there is simply no alternative. By way of illustration of this fact, if production turning high-silicon content aluminium pistons with polycrystalline diamond (PCD) tooling, the best surface nish that can be obtained will be in the region of 0.4 µm, conversely using an SCD tool this will give a surface nish of better than 0.15 µm. If one really wants the ultimate surface nish currently obtainable by machining – in the ‘nano-range’ , then a monolithic diamond tool, mounted in a special-purpose diamond turning lathe is the only manner in achieving such su- perb ‘mirror-nish’ surfaces. SCD tool edges are produced as either razor sharp edges, or are made with a perfect radius being chip-free, imparting machined ‘mirror-nishes’ of just a few angströms (i.e. 10 –10 m). e optical industries in particular nd that the latest blemish-free ultra-sharp cutting edges of SCD, means that diamond (paste) polishing aer machining has been virtually, if not completely eliminated, this fact in particular being a very big production cost for the - nal manufacture of large monolithic astronomical mir- rors. A cautionary note, is that to use SCD tooling for anything other than as a nishing cut is totally uneco- nomic, as these precision components to be machined, should have been roughly congured to the desired shape, prior to diamond machining. erefore, SCD tools should be employed for exceedingly light nish cuts of no deeper than 0.0008 m. Natural diamond is a truly remarkable material, that exhibit’s a diverse range of mechanical and physical properties. For example diamond has the highest known: bulk hardness, thermal conductivity, while having a very low coecient of friction and will not corrode, these properties make it an ideal tool material for the highest precision and accuracy machined components. Of these properties, hardness is probably the most important characteristic in machining operations and, when measured by the Knoop indentor 15 . By way of comparison of ultra-hard cutting tool materials, the following two examples may prove informative: 15 Knoop indentors produce a wedge-shaped indentation in the form of a parallelogram, with one diagonal seven times lon- ger than the adjacent one. e Knoop test method is generally considered the optimum technique, for crystalline solids – having crystallographic directionality (i.e. anisotropy). Cutting Tool Materials  • Natural diamond – has a hardness of 9,000 kg mm –2 (ie diamond orientation and test conditions): Dia- mond (111) surface, <110> direction, 500g load, • Cubic boron nitride (CBN) – has a hardness of 4,500 kg mm –2 , (111) surface, <110> direction, 500g load. One of the main limitations of natural diamond is that it has distinct cleavage planes (111) 16 . is consistent cleavage plane makes it ideal for jewellery-makers to cleave the beautiful facets demanded of diamond jewellery, but this means that monolithic diamonds must be mounted in their respective tool holders in exactly the correct orientation/plane, so avoiding any potential cleavage in-cut. SCD tool cost is a draw-back, because these tools cost in the region of four times more than the equivalent PCD tool. However, despite this very high cost dierence, SCD can reduce the overall operating costs and signicantly improve productivity, when applied to the correct machining process. Expensive tooling such as SCD, must be handled with care, because although it is the hardest material known, it is also very brittle and subject to thermal shock, the problem being exacerbated with its very sharp tool edges. erefore, it is essential that sudden impacts to the tool’s edge must be avoided, through either inappropriate cutting applications, or by rough handling. References Journal and Conference Papers Boller, R. Crystal Clear – DLCoatings. Cutting Tool Engg., 36–40, May 2002. Craig, P. Behind the Carbide Curtain. Cutting Tool Engg., 26–41, Aug., 1997. Dzierwa, R. Slippery when Blue – Coatings. Cutting Tool Engg., 36–41, Jan., 2003. Eastman, M. Inserts Show their True Colors. Cutting Tool Engg., 30–36, April 1999. Feir, M. Post-treatment of PM Parts. 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