Analysis of features and cutting properties of PSTM. Superhard materials are considered to be those with a Vickers hardness at room temperature above 35 GPa.
Natural diamond is the hardest material on Earth and has long been used as a cutting tool. The fundamental difference between monocrystalline natural diamond and all other tool materials with a polycrystalline structure, from the point of view of the toolmaker, is the possibility of obtaining an almost perfectly sharp and straight cutting edge. Therefore, at the end of the 20th century, with the development of electronics, precision engineering and instrument making, the use of natural diamond cutters for microturning of mirror-clean surfaces of optical parts, memory disks, copying equipment drums, etc. increases. However, due to their high cost and fragility, natural diamonds are not used in general mechanical engineering, where the requirements for processing parts are not so high.
The need for superhard materials led to the fact that in 1953 - 1957 in the USA and in 1959 in the USSR, small particles of cubic phases of synthetic diamond were obtained from the hexagonal phases of graphite (C) and boron nitride (BN) by catalytic synthesis at high static pressures and boron nitride. Large polycrystals intended for blade tools were produced industrially in the early 70s.
The state diagram of carbon and boron nitride is shown in Fig. 11.9.
The technology for manufacturing polycrystals with a diameter of 4-40 mm is based on two various process: phase transition of a substance from one state to another (synthesis itself) or sintering of small particles of pre-synthesized PSTM powder. In our country, the first method is to obtain polycrystalline cubic boron nitride (PCBN) of the composite 01 (el-boron RM) and composite 02 (belbor) grades, as well as polycrystalline diamond (PDA) of the ASPC (carbonado) and ACE (ballas) grades. Abroad, there are three manufacturers of PSTM using sintering technology: largest companies General Electric (USA), De Beers (South Africa) and Sumitomo Electric (Japan). Polycrystalline cutting tools from these three suppliers are produced by hundreds of companies around the world.
PSTM are fundamentally new tool materials, both in terms of manufacturing technology and operating conditions. They can be processed
cut products at cutting speeds an order of magnitude higher than the speeds allowed when using carbide tools. In addition, PCD tools have tens of times higher speeds than carbide tools.
* Thermal shock resistance coefficient R = ,
** Empirical wear resistance characteristic I/4 E ' N:
Polycrystalline superhard materials (PSHM) are systematized according to such defining features as the composition of the polycrystal base, production methods, and characteristics of the starting material. The entire range of polycrystals is divided into five main groups: diamond-based PSTM (DPA),
PSTM based on dense modifications of boron nitride (SPNB), composite superhard materials (KSTM), two-layer superhard composite materials (DSCM).
Diamond-based PSTM. Polycrystals based on synthetic diamond can be divided into four types:
1. Polycrystals obtained by sintering fine diamond powders in pure form or after special pre-treatment to activate the sintering process. Polycrystals produced according to this scheme are, as a rule, a single-phase product. Examples include megadiamond and carbonite.
2. Polycrystals of diamond type SV. They are a heterogeneous composite consisting of diamond particles held together by a binder - the second phase, which is located in the form of thin layers between diamond crystals.
3. Synthetic carbonates of the ASPK type. They are produced by exposing a carbon-containing substance with a significant amount of catalyst to both high pressure and high temperature. The density of such polycrystals varies widely, and the impurity content ranges from 2 to 20% by weight. Therefore, polycrystals of the ASPC type have lower hardness and strength than polycrystals of the first two varieties.
4. Diamond polycrystals obtained by impregnating diamond powder with a metal binder at high pressures and temperatures. Nickel, cobalt, iron, and chromium are used as binders. Diamond polycrystals obtained by this method have high mechanical properties.
The physical and mechanical properties of diamond-based PSTM are presented in Table. 11.20.
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Table 11.20 Physico-mechanical properties of diamond-based PSTM
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The microhardness of polycrystalline diamonds is on average the same as that of natural single crystals, but the range of its variation in synthetic diamonds is wider. The ratio of the maximum to minimum value for various types of polycrystals is in the range of 1.2 -2.28.
The microhardness at the periphery is 1.25 times greater than at the center of the sample in areas adjacent to the catalyst.
The density of synthetic ballas and carbonado is higher than the density of natural diamond single crystals, which is explained by the presence of a certain amount of metallic inclusions. As the concentration of the metal phase increases, the density also increases almost proportionally.
The thermal conductivity of diamond polycrystals exceeds the thermal conductivity of copper and silver, and in some cases reaches the thermal conductivity of diamond single crystals. The thermal conductivity of polycrystals depends on temperature. Moreover, for some materials, with an increase in temperature to 450°C, thermal conductivity increases, reaching a maximum, and then decreases. For others, such as ASB and SCM, it monotonically decreases to 900°C.
PSTM based on cubic boron nitride. There are several types of PSTM based on boron nitride.
1. Polycrystals synthesized from hexagonal boron nitride (HNB) in the presence of the solvent VMgVMsf (composite 01 is a typical representative);
2. Polycrystals obtained as a result of the direct transition of the hexagonal modification into cubic BNrBN (composite 02);
3. Polycrystals obtained as a result of the transformation of a wurcytopo-like modification into cubic BNg VMdf. Since the completeness of the transition is regulated by sintering parameters, this group includes materials with noticeably different properties (composite 10, composite 09);
4. Polycrystals obtained by sintering cubic boron nitride (CBN) powders with activating additives (composite 05-IT, cyborite, etc.).
The main physical and mechanical characteristics of various grades of PSTM based on dense modifications of boron nitride are given in Table. 11.21.
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Table 11.21 Basic physical and mechanical characteristics of PSTM based on dense modifications of boron nitride |
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End of table. 11.21
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PSTM based on dense modifications of boron nitride, slightly inferior to diamond in hardness, are characterized by high heat resistance, resistance to cyclic exposure to high temperatures and, most importantly, weaker chemical interaction with iron, which is the main component of most materials currently subjected to cutting.
Polycrystals of the composite 01 type have a fine-grained structure, the dominant phase of which is small grains of CBN, intergrown and mutually intergrown to form a durable aggregate. Impurities are evenly distributed throughout the volume of the sample. Along with the main cubic modification, they may partially contain unreacted hexagonal boron nitride.
The sizes of grains and inclusions of accompanying phases are approximately 30 microns, the porosity is uniform, 10%.
Composite superhard materials (KSTM). Volume-homogeneous CSTMs are produced by sintering a mixture of synthetic diamond powders and cubic or wurtzite boron nitride. This includes materials such as PKNB - AS, SVAB (CIS), compact (Japan). These materials should be considered promising.
Of the materials of this class, materials SV-1 and SV-40 have the highest microhardness, and SV-14 and SVAB have the lowest. The unrecovered microhardness varies from 47.0 to 66.0 GPa, and the elastic modulus varies from 640 to 810 GPa.
Diamond-containing materials based on hard alloys also belong to the composite class. Among the materials of this group that have proven themselves well in operation, we should note “Slavutich” (made from natural diamonds) and tvesals (made from synthetic diamonds).
Two-layer composite polycrystalline materials (DSCM). The fundamental feature of DSCM is that the sintering of powders of superhard materials is carried out at high temperatures and pressures on a substrate made of hard alloys based on tungsten, titanium, tantalum carbides, resulting in the formation of a PSTM layer 0.5-1 mm thick, firmly bonded to the substrate material . The diamond-bearing layer may contain support components.
Two-layer materials have some advantages over STMs that are homogeneous in volume. The technology of fastening the cutting tool in the holder body by soldering to the carbide substrate is simplified. The presence of a substrate firmly connected to the working layer of STM gives the materials increased impact strength, and the use of a thin STM layer (0.5-2 mm) makes them more economical, since when sharpening and regrinding the tool, the irretrievable losses of expensive superhard materials are significantly reduced.
The most famous domestic two-layer superhard composite materials made of cubic boron nitride include composite 05-IT-2S, composite 10D, VPK, diamond-based - DAP, Diamet, AMK-25, AMK-27, BPA, ATP. Abroad, two-layer polycrystalline superhard materials based on diamond are produced by De Beers (South Africa) under the trademark syndite RKD010 and RKD 025. Syndite RKD025 is recommended mainly for rough processing, and finer-grained syndite grade RKD010 is recommended for final processing.
Areas of application of PSTM tools. The main area of effective use of blade cutting tools made from PSTM is automated production based on CNC machines, multi-purpose machines, automatic lines, and special high-speed machines.
In table 11.22 shows cutting speeds recommended for processing various materials with PSTM tools.
The choice of a specific cutting speed is determined by the amount of allowance removed, equipment capabilities, feed, the presence of shock loads during the cutting process and many other factors.
A wide range of tools made from PSTM has been developed and produced. These are turning pass-through, scoring, boring, grooving, thread cutters, including those of a stepped design for removing increased allowances from parts such as rolling rolls, end-face and shell cutters, including adjustable and adjustable ones, which can be equipped with plates made of various tool materials with optimal geometry for everyone, a range of brazed boring and prefabricated cutters, countersinks, boring heads, etc. Saws equipped with PSTM have been created for processing particle boards on automatic lines. The tools can be equipped with both brazed cutting elements (cylindrical and rectangular inserts, carbide polyhedral inserts with PSTM soldered at one of the vertices), and replaceable round and polyhedral inserts of a solid or two-layer design.
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Table 11.22 Cutting speeds with PSTM tools
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Note that for impact turning and milling of hardened high-speed steels and steels with a high chromium content (type X12), tools made of PSTM are not recommended.
Calculations showed that a necessary condition The effectiveness of introducing tools from PSTM on CNC machines and machining centers instead of carbide cutters and milling cutters is to increase the intensity of stock removal (volume of metal per unit time) by 1.5-2.5 times. However, the practice of introducing high-speed cutting indicates the possibility of increasing processing productivity by 3-6 times or more. Thus, when creating the automated plant “Krasny Proletary” for finishing machining of cast iron body parts with a surface roughness Ra of 1.25 microns on multi-purpose machines of the IR 500 type, it was proposed to use cassette face mills d = 125 mm of a new design with axial and radial adjustment of the position of the radius cutting cutters edges (with an accuracy of no worse than 0.005 mm) of square PCBN plates. Cutting mode n = 3000 rpm; v = 1177 m/min; SM = 2000 mm/min; t = 0.3-0.4 mm. When using high-speed machines with n = 6000 rpm, the cutting speed increases to 2350 m/min, feed to 4000 mm/min, and the productivity of the cutting process will be 10 times higher compared to the existing level.
Process development trends machining cutting suggest that in the coming years, high-speed cutting with the widespread use of new tool materials will become a completely ordinary phenomenon in enterprises equipped with advanced automated equipment.
The most effective use of diamond tools is in finishing and finishing operations when processing parts made of non-ferrous metals and their alloys, as well as non-metallic and composite materials. Diamond, as a tool material, has two significant drawbacks - relatively low heat resistance and diffusion dissolution in iron at high temperatures, which practically excludes the use of diamond tools when processing steels and alloys capable of forming carbides. At the same time, thanks to the very high thermal conductivity, the cutting edge of the blade is intensively cooled, making diamond tools suitable for working at high cutting speeds.
The types of diamond-based STMs existing in world practice are presented in Fig. 6.23.
Rice. 6.23 Ultra-hard materials for diamond-based blade tools
Monocrystalline diamond blade tools are used for processing radio ceramics, semiconductor materials, and high-precision processing of non-ferrous alloys. Monocrystalline diamond tools are characterized by record wear resistance and a minimum radius of rounding of the cutting edge, which ensures high quality treated surface. It should be taken into account that the cost of a single-crystal diamond blade tool is several times higher than the cost of a polycrystalline diamond tool. The advantages of instrumental polycrystalline diamonds (PCD, abroad PCD), in comparison with single-crystal diamonds, are associated with the arbitrary orientation of crystals in the working layer of cutting inserts, which ensures high uniformity in hardness and abrasion resistance in all directions with high strength values. From polycrystalline diamonds obtained on the basis of a phase transition, ASPC grades, which are obtained from graphite during synthesis in the presence of metal solvents, have become widespread for blade tools. ASPC grades are produced in the form of cylinders with a diameter of 2, 3 and 4 mm, and a length of up to 4 mm.
Of all types of PCD, the most common are diamond tools obtained by sintering diamond powders (size 1...30 microns) in the presence of a cobalt catalyst. An example would be the fine-grained CMX850 or the universal brand CTM302 from ElementSix, inserts various shapes VNIIALMAZ, JSC "MPO VAI". Significant advantages in terms of the strength of the plates and the convenience of their fastening by soldering in the tool body are provided by two-layer plates with a diamond layer on a carbide substrate, also called ATP - diamond-carbide plates. For example, such plates of various sizes are produced abroad by Diamond Innovations under the brand name Compax. Element Six produces Sindite inserts with diamond layer thicknesses from 0.3 to 2.5 mm and various diamond grain sizes. Domestic-made two-layer SVBN is soldered at the top of a carbide plate standard sizes. The composite class includes diamond-containing materials based on hard alloys, as well as compositions based on polycrystalline diamonds and hexagonal boron nitride. Of the diamond-hard alloy composites that have proven themselves in operation, it should be noted “Slavutich” (from natural diamonds) and “Tvesal” (from synthetic diamonds).
Polycrystals of diamond obtained by chemical vapor deposition (CVD-diamond) represent fundamentally new type STM based on diamonds. Compared to other types of polycrystalline diamonds, they are characterized by high purity, hardness and thermal conductivity, but lower strength. They represent thick films, and in fact - plates with a thickness of 0.3...2.0 mm (the most typical thickness is 0.5 mm), which, after growing, are peeled off from the substrate, cut with a laser and soldered to carbide inserts. When processing highly abrasive and hard materials, they have durability that is several times higher than other PCDs. According to ElementSix, which produces such PCDs under the general name CVDite, they are recommended for continuous turning of ceramics, hard alloys, and metal matrix compositions. Not used for processing steel. In recent years, publications have appeared about industrial cultivation single crystal diamonds using CVD technology. Thus, we should expect this type of single crystal diamond tools to appear on the market in the near future.
CVD technology produces not only the diamond blade tools described above, but also diamond coatings on carbide and some ceramic tool materials. Since the process temperature is 600...1000 0 C, such coatings cannot be applied to steel tools. The thickness of coatings on tools, including complex-profile ones (drills, milling cutters, SMP), is 1...40 microns. Regions rational use diamond coatings are similar to CVD-diamond tools.
Diamond coatings should be distinguished from diamond-like coatings. Diamond-LikeCoating (DLC) amorphous coatings consist of carbon atoms with both diamond and graphite-like bonds. Diamond-like coatings applied by physical vapor deposition (PVD) and plasma activated chemical vapor deposition (PACVD) have a thickness of 1...30 microns (usually about 5 microns) and are characterized by high hardness and a record low coefficient of friction. Since the process of applying such coatings is carried out at temperatures no higher than 300 0 C, they are also used to increase the durability of high-speed tools. The greatest effect from diamond-like coatings is achieved when processing copper, aluminum, titanium alloys, non-metallic materials and highly abrasive materials.
Superhard composites based on boron nitride. STM based on polycrystalline cubic boron nitride (PCBN in Russia and PCBN abroad), slightly inferior to diamond in hardness, are characterized by high heat resistance, resistance to cyclic exposure to high temperatures and, most importantly, weaker chemical interaction with iron, therefore the greatest efficiency of use BN-based tools occur when machining cast irons and steels, including high-hard ones.
Abroad, according to ISO 513, the division of PCBN grades is carried out according to the content of cubic boron nitride in the material: with a high (70...95%) BN content (index "H") and a relatively small amount of binder, and with a low (40...70 %) BN content (index "L"). For low content PCBN grades, TiCN ceramic bond is used. Grades with a high BN content are recommended for high-speed machining of all types of cast iron, including hardened and bleached, as well as turning of heat-resistant nickel alloys. Low BN content PCBNs have greater strength and are used primarily for machining hardened steels, including interrupted machining. Sumitomo Electric also produces ceramic-coated PCBN inserts (BNC type), which have increased resistance to high-speed machining of steels and provide high quality surface finishes.
In addition to homogeneous in structure, PCBN are produced in the form of two-layer plates with a carbide base (similar to PKA). Composite PCBN is produced by sintering a mixture of synthetic diamond powders and cubic or wurtzite boron nitride. IN foreign countries materials based on wurtzite boron nitride are not widely used.
Purpose of STM based on cubic boron nitride:
Composite 01 (Elbor R), Composite 02 (Belbor R) - fine and fine turning without impact and face milling of hardened steels and cast irons of any hardness, hard alloys with a binder content of more than 15%.
Composite 03 (Ismit) - finishing and semi-fine processing of hardened steels and cast irons of any hardness.
Composite 05, composite 05IT, composite KP3 - preliminary and final turning without impact of hardened steels up to 55HRC and gray cast iron with hardness 160...600HB, cutting depth up to 0.2...2 mm, face milling of cast iron.
Composite 06 - fine turning of hardened steels up to 63HRC.
Composite 10 (Hexanit R), composite KP3 - preliminary and final turning with and without impact, face milling of steels and cast irons of any hardness, hard alloys with a binder content of more than 15%, intermittent turning, processing of deposited parts. Cutting depth 0.05...0.7 mm.
Tomal 10, Composite 10D - rough, semi-rough and finishing turning and milling of cast iron of any hardness, turning and boring of steels and copper-based alloys, cutting on casting crust.
Composite 11 (Kiborit) - preliminary and final turning, including impact turning, of hardened steels and cast irons of any hardness, wear-resistant plasma surfacing, face milling of hardened steels and cast irons.
Abroad, blade tools based on PCBN are produced by ElementSix, Diamond Innovations, Sumitomo Electric Industries, Toshiba Tungalloy, Kyocera, NTK Cutting Tools, Ceram Tec, Kennametal, Seco Tools, Mitsubishi Carbide, Sandvik Coromant, ISM (Ukraine), Widia, Ssangyong Materials Corporation, etc.
The main area of effective use of blade cutting tools made from STM is automated production based on CNC machines, multi-purpose machines, automatic lines, and special high-speed machines. Due to the increased sensitivity of STM tools to vibrations and shock loads, increased demands are placed on machines in terms of accuracy, vibration resistance and rigidity technological system. Different kinds CBN (cubic boron nitride composites) is used to process hardened steels and cast iron, which have high hardness and strength. Composites show excellent performance characteristics during processing and provide good surface quality due to its chemical composition and modern sintering technology (Fig. 6.24).
Figure 6.24 – Typical images of the microstructure of a CBN-based composite
The use of STM tools makes it possible to increase processing productivity several times compared to carbide tools, while improving the quality of machined surfaces and eliminating the need for further processing. abrasive processing. The choice of optimal cutting speed is determined by the amount of allowance removed, equipment capabilities, feed, the presence of shock loads during the cutting process and many other factors (Fig. 6.25, 6.26).
Figure 6.26 – Areas of application of some grades of composites
Figure 6.26 – Example of processing hardened steels with STM tools
7 PRINCIPLES OF CONSTRUCTION OF TECHNOLOGICAL PROCESSES WHEN PROCESSING MATERIALS BY CUTTING.
The hardest material on Earth, which has long been used as a cutting tool, is natural diamond. Diamond is a mineral, a type of native carbon. Opaque diamond is used as a tool material. The hardness of diamond (HV » 60–100 GPa) at room temperature is much higher than the hardness of carbides or oxides, and under conditions abrasive wear he is irreplaceable. Density
3500–3600 kg/m3. The thermal conductivity of diamond polycrystals exceeds the thermal conductivity of copper.
Natural diamond is a single crystal and allows you to obtain almost ideal sharp and straight cutting edges. With the development of electronics, precision engineering and instrumentation, the use of natural diamond cutters for turning mirror-clean surfaces of optical parts, memory disks, copying equipment drums, etc. is increasing.
Diamond can be effectively used for processing copper collectors - removing a small layer of copper at a fine feed and very high cutting speeds. This ensures low roughness and high precision of the machined surface. Diamond tools effectively finish machining pistons made of aluminum alloys with a high silicon content, while when machining such pistons with carbide cutters, large silicon crystals cause rapid tool wear. Diamond is good for processing ceramics and partially sintered carbides. Diamond can be used for dressing grinding wheels, etc.
Diamond wears out when interacting with iron at high temperatures, and therefore it is not recommended to use diamond tools for machining steels. The heat resistance of diamond is relatively low – 700–750 °C. Diamonds have insufficient impact strength; the sharp edges of diamond tools are easily chipped and destroyed. The high cost and scarcity of natural diamonds limits their use as a tool material.
The need for less expensive and scarce superhard materials led to the fact that in 1953–1957 in the USA and in 1959 in the USSR, small particles of cubic phases of synthetic diamond were obtained from hexagonal phases of graphite (C) by catalytic synthesis at high static pressures and temperatures . The color ranges from black to white; depending on the manufacturing technology, synthetic diamond can be translucent or opaque.
The crystal sizes usually range from a few tenths to 1–2 mm. Larger, dense, spherical polycrystalline synthetic diamond formations intended for blade tools were produced commercially in the early 1970s. Synthetic polycrystalline diamonds have a high elastic modulus E = 700–800 GPa, high compressive strength s – IN» 7–8 GPa, but low flexural strength s AND» 0.8–1.1 GPa.
Using a similar technology, a modification of boron nitride BN was obtained from boron and nitrogen, which in structure and properties resembles synthetic diamond. The crystal lattice is cubic, the hardness is slightly lower than that of diamond, but still very high: 40–45 GPa, i.e., more than twice as high as that of hard alloys, and almost twice as high as the hardness of cutting ceramics. Polycrystalline cubic boron nitride (PCBN) is sometimes called “borazon”, “cubanit”, “elbor”. Boron nitride elastic modulus
E = 700–800 GPa, compressive strength is approximately the same as that of hard alloys: s – IN» 2.5–5 GPa, and a lower bending strength than that of hard alloys and polycrystalline diamonds: s AND» 0.6–0.8 GPa.
The heat resistance of cubic boron nitride is significantly higher than that of synthetic and natural diamonds: about 1000–1100 °C. For this reason, and also due to its lower chemical affinity with carbon, cubic boron nitride is more effective than diamond and carbide in finishing cutting of steels, especially when cutting hardened steels of high hardness with small sections of the cut layer.
The technology for manufacturing polycrystals is based on two different processes: the phase transition of a substance from one state to another (synthesis itself) or the sintering of small particles of pre-synthesized PSTM powder. In our country, the first method is to obtain polycrystalline cubic boron nitride (PCBN) of the following grades: composite 01 (elbor RM) and composite 02 (belbor), as well as polycrystalline diamond (PDA) of the ASPC (carbonado) and ACE (ballas) grades.
Polycrystalline superhard materials (PSHM) are systematized according to such defining features as the composition of the polycrystal base, production methods, and characteristics of the starting material. The entire range of polycrystals is divided into five main groups: diamond-based PSTM (DBA), PSTM based on dense modifications of boron nitride (SPNB), composite superhard materials (KSTM), two-layer superhard composite materials (DSCM).
Polycrystals based on synthetic diamond can be divided into four types:
1) Polycrystals obtained by sintering fine diamond powders in pure form or after special pre-treatment to activate the sintering process. Polycrystals produced according to this scheme are, as a rule, a single-phase product. Examples include megadiamond and carbonite.
2) Polycrystals of diamond type SV. They are a heterogeneous composite consisting of diamond particles held together by a binder - the second phase, which is located in the form of thin layers between diamond crystals.
3) Synthetic carbonates of the ASPC type, obtained by exposing a carbon-containing substance with a significant amount of catalyst to both high pressure and high temperature. ASPCs have lower hardness and strength than polycrystals of the first two varieties.
4) Diamond polycrystals obtained by impregnating diamond powder with a metal binder at high pressures and temperatures. Nickel, cobalt, iron, and chromium are used as binders.
There are several types of PSTM based on boron nitride:
1) polycrystals synthesized from hexagonal boron nitride (HNB) in the presence of the solvent VM g VM sf (composite 01 is a typical representative);
2) polycrystals obtained as a result of the direct transition of the hexagonal modification into cubic BNrBN (composite 02);
3) polycrystals obtained as a result of the transformation of the wurtzite-like modification into cubic BN g ® VM df. Since the completeness of the transition is regulated by sintering parameters, this group includes materials with noticeably different properties (composite 10, composite 09);
4) polycrystals obtained by sintering cubic boron nitride (CBN) powders with activating additives (composite 05-IT, cyborite
and etc.).
PSTM based on boron nitride, slightly inferior to diamond in hardness, they are distinguished by high heat resistance, resistance to cyclic exposure to high temperatures and, most importantly, weaker chemical interaction with iron, which is the main component of most materials currently subjected to cutting.
Homogeneous in volume composite superhard materials obtained by sintering a mixture of synthetic diamond and cubic boron nitride powders. This includes materials of the PKNB type - AS, SV, SVAB. Diamond-containing materials based on hard alloys also belong to the composite class. Among the materials of this group that have proven themselves well in operation, we should note “Slavutich” (from natural diamonds) and “Tvesal” (from synthetic diamonds).
Fundamental feature two-layer composite polycrystalline materials is that the sintering of powders of superhard materials is carried out at high temperatures and pressures on a substrate made of hard alloys based on tungsten, titanium, tantalum carbides, resulting in the formation of a PSTM layer 0.5–1 mm thick, firmly bonded to the substrate material. The diamond-bearing layer may contain support components.
According to the classification, all superhard blade materials based on dense modifications of boron nitride are composites. Depending on the production technology, physical and mechanical properties, and conditions of use, they are divided into certain groups. The most widely used in domestic metalworking are composite 01 (Elbor-R), composite 03 (Ismite), composite 05, composite 09 (PTNB), single-layer and two-layer composite 10 (hexanit-R).
Similar and similar tool materials based on modification of cubic boron nitride (CBN) have been created and are used in many industrial applications. developed countries, and their use is steadily expanding.
The listed tool materials are distinguished by high hardness, thermal stability and chemical inertness to ferrous metals, i.e. everything that makes these advanced tool materials very effective in the operations of turning, boring and face milling of both smooth and intermittent precision surfaces of parts for basic engineering purposes.
The high efficiency of using tools equipped with polycrystalline composites is due to their unique combination physical and chemical characteristics, including exceptionally high hardness, high heat resistance and thermal conductivity, close to the thermal conductivity of hard alloys and not decreasing with increasing temperature (Table 6.1). Polycrystalline cubic boron nitride has a wear resistance 50 times higher than cemented carbide and 10 to 25 times higher than oxide or nitride ceramics. These composites maintain their strength at high temperatures typical for processing hardened ferrous metals with relatively high intensity removal of material. These instrumental materials come into chemical reaction with ferrous metals in air and at high temperatures, which provides them with certain advantages over diamonds and other traditional tool materials.
The areas of application of various grades of composites are determined by the sizes of polycrystals and their physical and mechanical characteristics. Despite the variety of brands, composites do not create competition with each other, but successfully complement each other. Available regulations, catalogues, guidelines and reference literature, which fairly fully and broadly describe the basic properties of composites.
Thus, composites 01 and 02 are used for fine and fine turning, mainly without shock loads, of parts made of ferrous metals of any hardness; composite 03 - for preliminary and final turning of cast iron of any hardness; composite 05 - for finishing and semi-finish turning without shock loads of hardened steels and cast iron of any hardness, for face milling of cast iron; composite 10 - for preliminary and final turning (boring) with and without shock loads of steels and cast irons of any hardness, for face milling of hardened steels and cast irons.
Currently mastered industrial production boron nitride composites with different properties, with each type of tooling material offering advantages under certain processing conditions.
There are four main groups of materials that can be effectively processed by modifications of cubic boron nitride:
- bleached cast iron; white cast iron alloyed with nickel or chromium (50...65 HRC);
- hardened steels and parts with surface hardening(50... 65 HRC);
- some hardenable alloys (38 HRC);
- some brands of gray cast iron (200...220 HB).
Table 6.1
Properties of composites based on dense modifications of boron nitride (according to TU 2-035-982-85)
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composite |
strength tensile strength o„, MPa |
Ultimate compressive strength, MPa |
Hardness HV, MPa |
Heat resistance, “C” |
|
Composites 01 and 02 |
75 000... 80 000 |
|||
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Composites 05 and 06 |
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Composite 09 |
||||
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Composite 10 |
When processing ferrous metals with a hardness of over 45 HRC, rough and finishing turning with the help of composites is widely used instead of grinding. Due to the insufficiently high durability of traditional tool materials, such metals are ineffectively processed, for example, with hard alloys or cutting ceramic materials. The use of grinding with electrocorundum wheels for these purposes is a rather lengthy process characterized by low metal removal rate and rapid wear of the wheel, which limits productivity.
The development of the design of tools equipped with artificial superhard materials is proceeding in two main directions - the creation of tools with mechanical fastening of solid, multi-layer round and multi-faceted cutting inserts, as well as the use of grindable cutting inserts when a design with mechanical fastening of inserts is practically impossible.
The hardness of synthetic diamond is approximately 90... 100 GPa. It is used for the manufacture of diamond drills, cutters, drill cores and tools for drilling the hardest rocks, as well as for the manufacture of tips for instruments for measuring hardness and surface cleanliness.
The most favorable conditions for the synthesis of diamonds are pressure 4...6 GPa and temperature 1125...1325°C.
It is technically quite difficult to obtain large cutting inserts for composites, as this is associated with the very high temperature and pressure that are necessary for the synthesis of these materials. Therefore, tools with brazed plates made of composites, which can be sharpened several times, are more often used, which are simpler in production technology.
Further development of the design of cutting tools equipped with superhard blade materials is moving towards expanding the capabilities of composites and their use in automated production conditions. Cutters made of composites are widely used for processing parts of various configurations on lathes, turret lathes, boring machines, as well as on multi-purpose machines. Effective use of tools is achieved on rigid, high-precision machines with increased power. Such equipment must have sufficient rigidity, since when turning and boring with tools made of composites, relatively large cutting forces occur. Vibrations of the equipment are not allowed, as this not only worsens the roughness of the machined surface, but in some cases also causes chipping of the cutting edges. Tools made of composites have high durability and relatively low wear important when processing on automated lathes, since the number of tool changes is noticeably reduced, and the specified dimensions of the workpiece are maintained without frequent operator intervention. If loading and unloading of parts is carried out by a robot, then tools equipped with modifications of composites are quite suitable for processing in conditions of unmanned technology.
The processes of processing metals with blade tools obey the classical laws of the theory of metal cutting.
Throughout the development of metal cutting, the emergence of qualitatively new tool materials with increased hardness, heat resistance and wear resistance was accompanied by an increase in the intensity of the processing process.
Created in our country and abroad in the late fifties and early sixties of the last century and widely used, instruments equipped with artificial superhard materials based on cubic boron nitride (CBN) are characterized by great diversity.
According to information from domestic and foreign tool manufacturers, the use of CBN-based materials is currently significantly increasing.
In industrialized countries, the consumption of blade tools made of artificial superhard materials based on CBN continues to grow by an average of 15% per year.
According to the classification proposed by VNIIinstrument, all superhard materials based on dense modifications of boron nitride are given the name composites.
In the theory and practice of materials science, a composite is a material that is not found in nature, consisting of two or more components with different chemical compositions. The composite is characterized by the presence of distinct
boundaries separating its components. The composite consists of a filler and a matrix. The filler has the greatest influence on its properties, depending on which composites are divided into two groups: 1) with dispersed particles; 2) reinforced with continuous fibers and reinforced with fibers in several directions.
The thermodynamic features of boron nitride polymorphism have led to the emergence of a large number of materials based on its dense modifications and various technologies for its production.
Depending on the type of the main process occurring during synthesis and the determining properties of superhard materials, in modern technologies There are three main methods for obtaining instrumental materials from boron nitride:
- phase transformation of hexagonal boron nitride into cubic. Polycrystalline superhard materials obtained in this way differ from each other in the presence or absence of a catalyst, its type, structure, synthesis parameters, etc. The materials of this group include: composite 01 (elbor-R) and composite 02 (belbor). Materials from this group are not published abroad;
- partial or complete transformation of wurtzite boron nitride into cubic. Individual materials of this group differ in the composition of the initial charge. In our country, materials from this group are used to produce one- and two-layer composite 10 (hexanite-R) and various modifications of composite 09 (PTNB, etc.). Abroad, materials of this group are produced in Japan by the company Nippon Oil Fat under the brand name Wurtzip;
- sintering cubic boron nitride particles with additives. This group of materials is the most numerous, since it is possible various options binders and sintering technologies. Using this technology, composite 05, cyborite and niborite are produced in the domestic industry. The most famous foreign materials are boron zone, amborite and sumiboron.
Let's give short description the most famous superhard tool materials.
Composite 01(elbor-R) - created in the early 70s.
This material consists of randomly oriented cubic boron nitride crystals obtained by catalytic synthesis. As a result of high-temperature pressing under high pressure, the initial BN K crystals are crushed to sizes of 5...20 microns. The physical and mechanical properties of composite 01 depend on the composition of the initial charge and the thermodynamic parameters of the synthesis (pressure, temperature, time). The approximate mass content of the components of composite 01 is as follows: up to 92% BN K, up to 3% BN r, the rest is impurities of catalyst additives.
Modification of composite 01 (Elbor-RM), in contrast to Elbor-R, is obtained by direct synthesis BN r -> BN k, carried out at high pressures (4.0...7.5 GPa) and temperatures (1300...2000°C). The absence of a catalyst in the charge makes it possible to obtain stable performance properties.
Composite 02(belbor) - created at the Institute of Solid State and Semiconductor Physics of the Academy of Sciences of the BSSR.
It is obtained by direct transition from BN r in high-pressure apparatuses with static load application (pressure up to 9 GPa, temperature up to 2900 °C). The process is carried out without a catalyst, which ensures high physical and mechanical properties of composite 02. With a simplified manufacturing technology due to the introduction of certain alloying additives, it is possible to vary the physical and mechanical properties of polycrystals.
Belbor is comparable in hardness to diamond and significantly exceeds it in heat resistance. Unlike diamond, it is chemically inert to iron, and this allows it to be effectively used for processing cast iron and steel - the main engineering materials.
Composite 03(ISM) - first synthesized at the Institute of Materials and Mathematics of the Academy of Sciences of the Ukrainian SSR.
Three grades of material are produced: Ismit-1, Ismit-2, Ismit-3, differing in physical, mechanical and operational properties, which is a consequence of differences in the starting raw materials and synthesis parameters.
Niborite- received by the Institute of Physics and Physics of the USSR Academy of Sciences.
The high hardness, heat resistance and significant size of these polycrystals determine their high performance properties.
Cyborite- synthesized for the first time in the Institute of Materials and Mathematics of the Ukrainian SSR Academy of Sciences.
Polycrystals are produced by hot pressing of the charge (sintering) at high static pressures. The mixture contains cubic boron nitride powder and special activating additives. The composition and amount of additives, as well as sintering conditions, provide a structure in which intergrown BN K crystals form a continuous frame (matrix). Refractory solid ceramics is formed in the intergranular spaces of the frame.
Composite 05- the structure and production technology were developed at NPO VNIIASH.
The material basically contains crystals of cubic boron nitride (85...95%), sintered at high pressures with the addition of aluminum oxide, diamonds and other elements. According to their own physical and mechanical properties Composite 05 is inferior to many polycrystalline superhard materials.
A modification of composite 05 is composite 05IT. It is characterized by high thermal conductivity and heat resistance, which are obtained by introducing special additives into the charge.
Composite 09(PTNB) was developed at the Institute of Chemical Physics of the USSR Academy of Sciences.
Several grades are produced (PTNB-5MK, PTNB-IK-1, etc.), which differ in the composition of the initial charge (a mixture of BN B and BN K powders). The difference between composite 09 and other composite materials is that it is based on particles of cubic boron nitride measuring 3...5 microns, and the filler is wurtzite boron nitride.
Abroad release of materials of this class using the transformation of wurtzite boron nitride is carried out in Japan by Nippon Oil Fate in collaboration with Tokyo State University.
Composite 10(hexanite-R) was created in 1972 by the Institute of Materials Science Problems of the Academy of Sciences of the Ukrainian SSR together with the Poltava Plant of Artificial Diamonds and Diamond Tools.
This is a polycrystalline superhard material, the basis of which is the wurtzite modification of boron nitride. Technological process obtaining hexanite-R, like previous composites, consists of two operations:
- synthesis of BN B by the direct transition BN r -> BN B under impact on the starting material and
- sintering of BN B powder at high pressures and temperatures.
Composite 10 is characterized by a fine-grained structure, but the crystal sizes can vary within significant limits. The structural features also determine the special mechanical properties of composite 10 - it not only has high cutting properties, but can also work successfully under shock loads, which is less pronounced in other brands of composites.
Based on hexanite-R, an improved grade of composite 10 - hexanite-RL, reinforced with thread-like crystals - "sapphire whisker" fibers - was obtained at the Institute of Materials Science Problems of the Academy of Sciences of the Ukrainian SSR.
Composite 12 is obtained by sintering at high pressures a mixture of wurtzite boron nitride powder and polycrystalline particles based on Si 3 N 4 (silicon nitride). The grain size of the main phase of the composite does not exceed 0.5 microns.
The prospect for further development, creation and production of composites is associated with the use of thread-like or needle-shaped crystals (whiskers) as fillers, which can be obtained from materials such as B 4 C, SiC, Si 2 N 4. VeO et al.
