Journal of International Scientific Publications: Materials, Methods & Technologies, Volume 5, Part 1 ISSN 1313-2539, Published at: http://www.science-journals.eu

SUPERHARD MATERIALS BASED ON THE DENSE FORMS OF CUBIC (cBN). PROPERTIES AND APPLICATION AREAS Alexander A. Antanovich, Vitalii P. Modenov, Georgii N. Stepanov and Vladimir P. Filonenko Russian Academy of Sciences, Institute for High Pressure Physics, 142190 Troitsk, Russia

Abstract A method for the production o f samples of cBN-bctsed superhard materials by a directional infiltration o f a cBN micropowder mixture with aluminum at high pressures and temperatures has been proposed. Some physical-mechanical properties and applications o f obtained materials are described. Key words:superhard materials, cubic , high-pressure device, Bridgman anvils, metal­ working

1. INTRODUCTION Cubic boron nitride (cBN)-based polycrystalline superhard materials, having strength properties comparable to those of composites, are superior to diamond composites in temperature and chemical stability. Over time since the synthesis of cBN micropowders became commercialized, a fairly wide range of cBN micropowder-based superhard materials has been developed. It should be stressed that when the cBN micropowders, high pressures are necessarily used to prevent a reverse phase transition of the micropowders from a diamond-like to soft graphite-like structure. A successful sintering of cBN powders without activating additives can be performed at pressures of no less than 7.0 FHa. Therefore the sizes of the compacts produced in the above way cannot be large due to limited dimensions of the working volumes of high pressure apparatus capable of generating such high pressures. A maximum diameter of compacts in this case does not exceed 6-7 mm. A thermobaric sintering with the activating additives allows using lower pressures of 5.0-6.0 GPa. These pressures are used by world leading companies (Element six, SECO and others) for an activated sintering of cBN micropowders in the shape of plates on a hard substrate. These plates are of a fairly large size but their working layer thickness amounts to 0.5-0.7 mm. To reduce the thermobaric parameters of the production of composites based on dense modifications of boron nitride, one can quite efficiently use various metals (titanium, zircon, chromium, cobalt, copper) or their alloys. The low melting temperature of aluminum (660 °C), as well as high physical and mechanical properties of , resulting from a chemical interaction between aluminum and boron nitride (AIN, A1B2 h A1B12) and ensuring strong binding of cBN grains, make aluminum a technologically advantageous activating additive. Thus, for example, superhard material Kiborit [1], fabricated as plates of 7.0 mm in diameter and 3,18 mm in height, is obtained by reaction sintering of a cBN powder (up to 98%) with aluminum. In this case, as well as in many others, the A1 and cBN powders are initially premixed to prepare a reaction mixture which is then subjected to reaction sintering under high pressures and temperatures. Heterogeneity of reagent mixture in this case may cause inhomogeneity of the structure and deterioration of the physical and mechanical properties of a sintered composite. That is why in our work we use a method of directional infiltration of the cBN powder with aluminum with subsequent reaction sintering.

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2. APPARATUS AND SINTERING TECHNIQUE For the sintering of large-size samples of superhard composites, we used a high pressure apparatrus of a “chechevitsa” type [2], developed in the 1960s in the Institute for High Pressure Physics of the Russian Academy of Sciences. A photo of the half of this apparatus of 32 mm in the tapered cavity diameter with a fdled container and a sectional drawing of a fdled container are presented in Fig. 1.

r r r ^ r T T

Fig. 1 (a) High pressure apparatus “chechevitsa'’, (b) Sectional drawing of a filled container: (1) lithographic stone container, (2) end heater pressed from the mixture of hexagonal boron nitride (hBN) and graphite powders, (3) graphite busing-heater, (4) aluminum washer, (5) cBN powder or mixture of cBN powders, (6) cap pressed from the mixture of hBN and graphite powders

The high pressure apparatus was preliminary pressure calibrated (by fixing the phase transitions of bismuth) and temperature calibrated (by using a chromel-alumel thermocouple inserted into the central part of the container). Then the apparatus was placed into a hydraulic press of up to 500 ton press- force, which enabled creating a 2.5 - 3,0 GPa pressure in the working volume of the container. To attain a maximum density of a superhard material, two compositions of mixtures of cBN powders of different graininess were designed: (1) 14/10 pm (80%), 3/2 pm (15%), 3/0 pm (5%) and (2) 5/3 pm (80%), 3/0 pm (20%). It was experimentally found that if filling the container as above and using the above compositions of a cBN powder mixture, the complete infiltration of the powders with aluminum occurs at temperatures of 1100-1200 °C. In actual practice, the sintering process goes like this. Using a hydraulic press, we create the working pressure in the container. Then, using a transformer, we set the experimentally predetermined working voltage to achieve a required sintering temperature. Then the current is turned on for a certain period of time; the current is then switched off. The infiltration of the cBN powders with aluminum lasts about 5 seconds, which is followed by the reaction sintering resulting in the formation of aluminum nitride AIN and aluminum diboride A1B2. By varying the hold time of the sintering process at maximum power, one can obtain both conducting and non-conducting

12 I Publishing by Info Invest, Bulgaria, www.sciencebg.net Journal of International Scientific Publications: Materials, Methods & Technologies, Volume 5, Part 1 ISSN 1313-2539, Published at: http://www.science-journals.eu ceramics, which significantly expands the application area of said ceramics. Fig.2 shows diffraction patterns of two samples of composite ceramics sintered with different hold times.

1 - AIN 0 - AIB

_0 2 6 oI— o X mx o X 2a CD\- X

20 25 30 35 40 45 50 55 60 65 7 0 7 5 80

26

Fig.2 Diffraction patterns of two samples of composite ceramics (1) Hold time of 30 seconds at maximum power ((la) from the infiltration side, (16) the opposite side), (2) Hold time of 10 seconds at maximum power ((2a) from the infiltration side, (26) the opposite side)

The main phase, cubic boron nitride, fills 82-88% of the volume of ceramics. Aluminum nitride (AIN) and aluminum diboride (A1B2), formed from the reaction between A1 and cBN, are a binder phase in this composition. The binder phase is considerably softer than the main phase, which imparts some plasticity to superhard ceramics. The of AIN and A1B2 is 12.5 GPa and 10 GPa, respectively; the hardness of cBN is 60-80 GPa. A rough quantitative composition of the phases of a sintered superhard material (%), estimated from the peak intensity, is given in Table 1. Table 1 Phase Sam]pie 1 Sampie 2 Bottom Top Bottom Top cBN 82 86 86 88 AIN 15 6 6 3 AlBo 3 3 4

A1 _ 5 4 7

gBN - - - 2

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As is seen from the Table, Sample 1 is not conductive because there is no metal phase on its bottom surface, while Sample 2 is a conductor; its resistivity measured by the van der Pauw technique amounts to 10"4 ohm-cm.

3. SOME PROPERTIES OF THE MATERIAL The high pressure apparatus described above enables sintering large-sized samples of a superhard material of 15-20 mm in diameter (with the respective heater diameter) and 6-8 mm in height. Fig.3 presents an external view of some of the samples and items made from these samples.

0,1 o 1 2 3 4

Fig.3

Measurements of physical and mechanical properties were conducted on many of the sintered samples. The following properties were measured. Density (p) of the samples was determined by the hydrostatic weighing technique [3] based on Archimedes' law. Elastic moduli (the Young modulus E, G and K) were calculated using the known formulae [4] from the experimental data on the propagation velocity of ultrasound waves in the samples and from the density of the material. To measure the ultrasound velocity, the end surfaces of the samples were grinded and the deviation from the plane parallelism did not exceed 5 pm. Strength characteristics were measured using the Instron setup with the recording of the force- deformation diagram, which permitted high accuracy registration of the load at the moment of failure. To carry out the measurements, samples with a rectangular cross-section were cut from a sintered half-finished product, which was followed by the grinding of the side and end surfaces of the cut samples. The samples for compression strength testing with the sides of 2-3 mm cross-section had the ratio of the height to the side of the cross-section of the order of 2. The samples for bending

14 I Publishing by Info Invest, Bulgaria, www.sciencebg.net Journal of International Scientific Publications: Materials, Methods & Technologies, Volume 5, Part 1 ISSN 1313-2539, Published at: http://www.science-journals.eu strength testing with the length of 14 mm and cross-section ~ 2x3 mm were tested in a device having a 10 mm distance between the supports with the load applied in the center of the sample. Hardness of a composite was determined with a microhardness tester PTM-3M with the Vickers diamond pyramid as an indenter. The indentation load was 4.9 H. The length of the impression diagonals was measured with an optical microscope (JENAVERT) at the magnification 800 x. The Vickers hardness was determined by a formula recommended by the Operating Instructions for the microhardness tester PMT-3M.

HV = (0,189xP/D2) 104 GPa, (1) where P is the indentation load, N; D is the arithmetic mean of the diagonal lengths of the diamond pyramid impression, pm. A typical Vickers pyramid impression is shown in Fig.4.

/ X f

Fig. 4.

We estimated the average value of the square of the impression diagonal as 200 pm2, which gives the microhardness value of around 46 GPa for the given sample of a composite ceramics.

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Wear resistance (Am) of the samples was estimated by a special procedure, determining an averaged loss in the mass of the composites from the results of a series of three measurements after 5-minute wheel dressing with the end surfaces of a composite. Some physical and mechanical properties of a superhard material, obtained on the best samples, are presented in Table 2.

Table 2

Properties P- E, G, K, HV, oCH,

4. APPLICATION AREAS 4.1. Edge cutting tools The obtained cBN-based superhard materials were tested as materials for making edge tools for machining high hardened , high-strength cast , and other hard-to-machine materials. The tests gave fairly good results and showed that the cBN-based composites with 14/10 pm grain-size should preferably be used for semi-finishing, while those with 5/3 pm grain-size - for finishing operations. For example, when machining abrasion-resistant ball bearing IIIX15 with HRC 60- 62, the tool durability is no less than 50 min for the following cutting regime parameters: the cutting speed V = 80-90 m/min, feed s = 0,07-0,1 mm/rev, depth of cut t = 0,1-0,2 mm. 4.2. Material for high pressure devices High values of elastic moduli, high hardness, and a sufficient strength of the synthesized ceramics testify that this material is suitable for use in the components of high pressure devices. We verified this by using the Bridgman anvils as an example [5], A drawing of the Bridgman anvils made of natural diamond is shown in Fig. 5.

y / / / / / / A

Fig.5 (1,5) supports for the anvils, (2,4) diamond anvils, (3) gasket

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A cBN-based superhard material was used to make the Bridgman anvils. The Bridgman anvils arrangement and dimensions of one of the Bridgman anvil cell is given in Fig.6.

0 0,53

10°

0 14

Fig.6 To determine the pressure values that can be reached if using such anvils, a standard procedure of measuring the Ri-line shift of ruby luminescence was employed. For the performance of the respective optical measurements, a transparent natural diamond anvil with a roughly similar size of the working surface was used as the second anvil. Both anvils were placed in a standard high pressure cell whose diagram is shown in Fig.7. A compressive force exerted on the anvils by a hydraulic press was measured with a dynamometer and fixed with a special captive nut. Then the cell was installed in an optical spectrometer to determine the pressure value from the ruby pressure scale.

rzzzz, zzz />, "V K\ X X X X X- X X\ \ £♦;■ * * IZ <

$ e K - - - § N

Fig.7. Diamond anvil cell (1) captive nut, (2) piston, (3) half-sphere, (4, 5) anvils, (6) support, (7) cylinder

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A diagram for the dependence of the attained pressure value on the force applied to the anvils is presented in Fig.8. The experiment was terminated at a pressure of 36.2 GPa because of the natural diamond anvil failure, while the anvil made of superhard material remained intact.

40

35 /■

100 200 300 400 500 000 700 load (kg)

Fig. 8 5. CONCLUSION It is shown that the superhard material obtained by a directional infdtration of cubic boron nitride powders with aluminum at high pressures and temperatures can successfully be used in both edge cutting tools and high pressure devices.

REFERENCES 1. Novikov N.V., Shulzhenko A.A., Bezhenar N.P. et al. Kiborit: production, structure, properties, application. Superhard materials (in Russian), 2001; 2: 40-51. 2. Vereshchagin L.F. et al. Apparatus for Achieving High Pressure and High Temperature. Synopsis of patents, 1970: 3746484 (USA), 7100495 (France), 760788 (Belgium), 928465 (Canada), 551214 (Switzerland), 1360281 (Great Britain), 931644 (Italy), 71060453 (Sweden), 321113 (Austria). 3. Kivilis S.S. Procedure for measuring the density of liquids and solids (in Russian), 1959, Chapter. 4. 4. Landau L.D., Lifshitz E.M. Course of Theoretical Physics. Theory of Elasticity, 2003, v.7. 5. Bridgman P.W. Proc. Amer. Acad. Arts Sci., 1952, 81: 165

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