COMPRISING SAME

FIELD OF INVENTION

This disclosure relates generally cubic boron nitride grains, methods of making them and methods of using them.

BACKGROUND

International patent application publication number WO/2012/130869 discloses cubic boron nitride (cBN) crystals containing a chloride salt compound including an alkali metal or an alkali earth metal. For example, the chloride salt compound may be selected from potassium chloride, magnesium chloride, lithium chloride, calcium chloride or sodium chloride. The crystal or crystals may have a relatively rough surface texture. Methods for making the cBN crystals are also disclosed.

United States patent number 6,984,448 discloses a method of growing cubic boron nitride clusters, including growing clustered ('polycrystal') cBN grains on seed grains each comprising a bonded mass of small cBN grains. United States patent application publication number 2014/0000177 discloses super- abrasive material comprising a core having a single crystal structure, and an outgrown region extending outwards from the core, wherein the outgrown region has a lower toughness index than that of the core. A method for making the material may comprise steps of providing a plurality of hexagonal boron nitride (hBN) grains; providing a catalyst; subjecting the plurality of hBN grains and the catalyst to a first high pressure for a first time period sufficient to form a core having a single crystal structure; and subjecting the plurality of hBN grains and the catalyst to a second high pressure for a second time period sufficient to form an outgrown region extending outwards from the core. In another embodiment, a super-abrasive material may comprise a single crystal having a tough core and an outgrown region, wherein the outgrown region has rough, friable and blocky structure.

There is a need for cBN grains and methods for making them, particularly but not exclusively for use in grinding tools. SUMMARY

Viewed from a first aspect there is provided a method for producing a synthetic cubic boron nitride (cBN) grain, including providing a seed grain comprising or consisting of plastically deformed cBN; the mass of the seed grain being at most one tenth the mass of the synthetic cBN grain; forming a synthesis compact comprising the seed grain, hexagonal boron nitride (hBN) material and catalyst material for transforming the hBN into cBN material at a pressure and a temperature at which the cBN allotrope of crystalline boron nitride is thermodynamically more stable than the hBN allotrope, the compact configured such that the hBN will be transformed into cBN material precipitated on the seed grain when subjected to the pressure and the temperature; the mass of the hBN being at least that of the synthetic cBN grain; subjecting the compact to the pressure and the temperature for a period sufficiently long to transform the hBN into the synthetic cBN grain. Variations of the method for making example cBN grains, as well as of cBN grains and tools comprising cBN grains are envisaged, non-limiting and non-exhaustive examples of which are provided below.

In some examples, the seed grain may comprise cBN plastically deformed such that its coefficient of plastic deformation (which may also be referred to as the coefficient of strain) may be at least about 0.01 , as determined using X-ray diffraction (XRD) measurement and use of the Williamson-Hall equation. In some examples, the coefficient of plastic deformation of the cBN seed grain may be at most about 0.7, 0.5 or 0.2.

In some examples, the seed grain size may be at least 1 micron; and / or the seed grain size may be at most 15 microns or at most 10 microns. In some examples, the synthetic cBN grain size may be at least about 20 microns, at least about 40 microns or at least 80 microns; and / or the synthetic cBN grain size may be at most about 500 microns, at most about 200 microns or at most about 150 microns.

In some examples, the pressure at which the synthetic cBN grain is synthesised may be at least about 2 gigapascals (GPa) or at least about 4 GPa; and / or the pressure may be at most about 8 GPa or at most 6 GPa. In some examples, the temperature at which the synthetic cBN grain is synthesised may be at least about 1 ,000 degrees Celsius; and / or the temperature may be at most 2,000 degrees Celsius. In some examples, the method may include providing the seed grains by mechanical treatment of an aggregation comprising a plurality of cBN gains, each having a mean mass of at most one tenth the mass targeted for the synthetic cBN grain, the mechanical treatment including subjecting the aggregation to a pressure of at least about 1 GPa and a temperature of at least about 1 ,000 degrees Celsius to stress the crystal lattice of the cBN grains beyond their elastic limit. The aggregation may comprise or consist of cBN grains contacting each other. The method may include milling the resulting aggregation of plastically deformed cBN grains in order to separate the grains from each other, and selecting grains having a size within a desired size range, such as by sieving means.

In some examples, the synthesis compact may be provided by a process including blending the plastically deformed cBN seed grains with hBN powder, 5 to 15 weight per cent lithium boron nitride and 0.5 to 5 weight per cent ammonium chloride to provide a raw material blend, and compacting the raw material blend to provide the synthesis compact.

Viewed from a second aspect, a cBN grain comprising or consisting of a major cBN volume crystallographically joined to a minor cBN volume at most 10 per cent that of the major volume, the minor volume comprising or consisting of plastically deformed cBN. When the synthetic cBN grain has been manufactured by means of a disclosed example method, the minor volume will correspond to the seed grain and the major volume to the synthesised cBN grain.

In some examples, the minor volume may be cBN plastically deformed such that its coefficient of plastic deformation is at least about 0.01 , as determined using X-ray diffraction (XRD) measurement and the Williamson-Hall equation. In some examples, the coefficient of plastic deformation of the minor volume may be at most about 0.7, 0.5 or 0.2. In some examples, the major volume may be substantially free of plastic deformation or may have a mean coefficient of plastic deformation of less than 0.1 .

In some examples, the minor volume may be at least 1 micron; and / or the minor volume may be at most 3,375 cubic microns or at most 1 ,000 cubic microns. In some examples, the cBN grain size may be at least about 8,000 cubic microns, at least about 64,000 cubic microns or at least about 512,000 cubic microns; and / or the synthetic cBN grain size may be at most about 25,000,000 cubic microns, at most about 8,000,000 cubic microns or at most about 3,375,000 cubic microns (the cBN grain comprising or consisting of the minor and major volumes). In some examples, the major volume may comprise or consist of crystallographically twinned cBN parts (in other words, the synthesised cBN grain may comprise or consist of two or more crystalline parts that are reversed on each other and related in a definite geometrical manner, in which the twinned form may be described as though it had resulted from a geometrical symmetry operation on part of a cBN crystal to produce the twinned parts). The major volume may comprise more than one crystallographically twinned part.

In some examples, the cBN grain may have a friability index of 30 to 50 per cent. Viewed from a third aspect, there is provided a grinding tool comprising disclosed example cBN grains. The grinding tool may comprise the cBN grains dispersed in a vitreous bond material.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples will be described with reference to the accompanying drawings, of which

Fig. 1 shows schematic drawing a part of an example capsule assembly for an ultrahigh pressure, high temperature press;

Fig. 2A shows a schematic drawing of an example pre-compact assembly, and Fig. 2B shows a schematic plan view of an example salt nest ;

Fig. 3 shows a schematic diagram of an ultra-high pressure, high temperature (HPHT) cycle;

Fig. 4 shows a scanning electron microscope image of example plastically deformed cBN seed grains;

Fig. 5 shows a graph of parameters obtained from an XRD measurement of a plurality of example cBN crystals in powder form, the slope of the relationship between the parameters being the strain coefficient, or plastic deformation coefficient of the cBN crystals;

Fig. 6 shows a schematic cross section view through part of a capsule for synthesising cBN grains; Fig. 7 shows the number size distribution of example synthesised cBN grains;

Fig. 8A and 8B show optical micrographs of example synthesised cBN grains in the size band 120 to 140 US mesh (about 105 to 125 microns);

Fig. 9A to 9G show SEM micrographs of example synthesised cBN grains in the size band 120 to 140 US mesh (about 105 to 125 microns);

Fig. 10A shows an example of contact twinning within an example synthesised cBN crystal, having a planar composition surface; and Fig. 10B shows an example of penetration twinning within an example synthesised cBN crystal, having an irregular composition surface separating two or three individual crystal portions; and

Fig. 1 1 is a flow diagram showing exemplary steps.

DETAILED DESCRIPTION

United States patent number 6,860,914 discloses a method of making a plurality of plastically deformed diamond or cubic boron nitride (cBN) grains by crushing an abrasive product comprising a polycrystalline mass of self-bonded abrasive grains of irregular shape, the product being substantially free of second phase and containing substantial deformation of the abrasive grains. The abrasive product is a polycrystalline mass of self-bonded abrasive grains and is substantially free of a second phase or additional component. Further, the abrasive grains made using the method are of irregular shape, potentially as a result of crushing or milling action. In the self-bonded product, there is evidence of asperities, sharp points or edges of one crystal bearing upon a substantially flat area of an adjacent crystal, resulting in plastic deformation at the contact points between crystals. An example method of manufacturing a plurality of plastically deformed cBN grains includes subjecting a loose (non-bonded) aggregation of cBN grains having a mean size of about 4 to 8 microns (and the consistency of powder) to mechanical treatment at an ultra-high pressure, high temperature (HPHT). The size distribution of the cBN grains was measured and found to have the following statistical characteristics: D(0.1 ) = 4.3 microns, D(0.5) = 6.2 microns, meaning that 50 per cent of the grains were greater and smaller than this value, and D(0.9) = 9.1 microns, meaning that 90 per cent of the grains were smaller than this value. About 3 g of the cBN powder was placed within a metal canister arrangement to provide a pre-compact assembly. With reference to Fig. 1 , the pre-compact assembly 10 comprised a cemented tungsten carbide substrate 14 located within a cup 13 consisting of niobium (Nb), the cBN powder aggregation 12 positioned adjacent an outer end surface of the Nb cup 13, thus separated from the substrate 14 by a barrier layer 13 of Nb in order to prevent cobalt (Co) binder material from the substrate 14 from infiltrating into the cBN powder 12. The cBN powder aggregation 12 was held in place by means of a jacketing arrangement comprising a pair of opposing titanium (Ti) cups 1 1 A, 1 1 B and the pre-compact assembly was loaded into a capsule for an HPHT apparatus. With reference to Fig. 2A and Fig. 2B, the capsule 20 comprised a nest 22 for accommodating the pre-compact assemblies 10 within chambers 24 provided within a housing 23 comprising salt material. The capsule 20 further comprised a pair of pads 21 A, 21 B consisting of salt material disposed on opposite sides of the nest 22, and an outer jacket 26 consisting of refractory ceramic material. The capsule was loaded into an ultra-high pressure, high temperature (HPHT) press apparatus and subjected to a pressure of about 5 to 6 gigapascals (GPa) and a temperature of about 1 ,100 to about 1 ,400 degrees Celsius, applying the pressure and heat according to the respective time cycles shown in Fig. 3, in which the maximum pressure and temperature were maintained for about 30 minutes. After the pressure and temperature had been reduced, the capsule was recovered from the HPHT press apparatus and the encapsulation material was removed by acid treatment. The resultant product was a solid and coherent compact, comprising the cBN grains in a plastically deformed condition. The compact was crushed by means of a cemented tungsten carbide (WC) ball mill device and the resulting cBN grains were sieved to provide a powder having a mean grain size of at most 20 microns. Grains that exceeded 20 microns were further broken down and screened again. The resulting fine cBN powder was treated in acid to remove any WC deposits from the ball mill that may have adhered to the grains. The size distribution of the cBN powder was measured and found to have a mean size of about 7.5 microns, and can be characterised by the following parameters: D(0.1 ) = 4.38 microns, D(0.5) = 7.08 microns and D(0.9) = 1 1.36 microns. A scanning electron microscope image of these grains is shown in Fig. 4. The plastic deformation of the processed cBN grains was calculated from the differences between the full-widths at half-maximum (FWHM) of XRD peaks before and after the mechanical treatment, taking into account the size distribution of the cBN grains and other line-broadening effects by applying a mathematical method known as de-convolution to the XRD peaks. The instrumental contribution to the XRD peak width was measured by means of an XRD scan of lanthanum hexa-boride (LaB ₆ ). Table 1 summarises the values of the FWHM of various Bragg peaks in the XRD spectra of cBN grains before and after mechanical treatment, and differences between these values and the FWHM of the calibration standard peaks of LaB ₆ . The strain is slope of the straight line relationship between the [(fi ² ₀ bs-fi ² instr) ^½ cosd] and [4(s/ ^' n0)] terms of the Williamson-Hall equation. In this example, the plastic deformation coefficient is about 0.15.

Table 1

In an example method, the mechanically treated plastically deformed cBN grains were used to seed the synthesis of cBN grains. A quantity of 0.025 weight per cent of the seed crystals was blended into a powder mixture comprising hexagonal boron nitride hBN, lithium boron nitride (Li ₃ BN ₂ ) and ammonium chloride (NH ₄ CI). This mixture was blended for 30 minutes in a double-cone blender using alumina balls to provide blended raw material powder substantially consisting (besides minor amounts of unavoidable impurities that may be present) of 88.18 weight per cent hBN powder, 9.80 weight per cent lithium boron nitride, 2 weight per cent ammonium chloride and 0.025 weight per cent of the plastically deformed cBN seed grains described above.

About 78 g of the raw material powder was compacted by means of cold iso-static pressing (CIP) at a pressure of about 2,000 bar (200 MPa) to make it easier to handle. The compacted powder was then compacted by means of uniaxial pressing at a load of up to 100 tonnes in order to provide two cylindrical reaction bodies of mass 34 g each. With reference to Fig. 6, the reaction bodies 31 A and 31 B were placed against each other, within a molybdenum (Mo) sheet 32 wrapped around part of the reaction bodies 31 A and 31 B, and placed within an opposing pair of cups 34 consisting of refractory ceramic material to provide the capsule 30. The capsule 30 was loaded into an HPHT press apparatus and the reaction bodies 31 A, 31 B were subjected to an ultra-high pressure of about 5 to 6 GPa at a temperature of about 1 ,600 degrees Celsius for a period of about 25 minutes. Such conditions are known to be suitable for the synthesis of cBN grains. After HPHT synthesis process, the joined reaction bodies were crushed and about 55 carats (1 1 g) of synthetic cBN grains having a size distribution as shown in Fig. 7 was recovered.

Synthesised cBN grains falling within the range of 120 / 140 US Mesh (about 105 to about 125 microns) were examined by means of optical microscopy and scanning electron microscopy (SEM), and their strength was measured in terms of the known friability index (Fl). This grain size will likely be suitable for use in grinding applications. Optical micrographs of cBN grains within this size band are shown at different magnifications in Fig. 8A and Fig. 8B. SEM micrographs of the cBN grains are shown in Fig. 9A to 9H. It is apparent from the micrographs that the cBN grains are crystalline and that a relatively high percentage of them are crystallographically twinned and have very sharp edges. The seed grains shown in Fig. 9F and Fig. 9G evidently comprise a major volume of synthesised cBN crystal grown onto a respective minor volume of the cBN seed crystal according to the crystallographic planes of the seeds, some of which are exposed in these micrographs. It appears that the crystallographic shape of a synthetic cBN crystals depends to some extent on the degree of plastic deformation of each seed as well as damage caused to the seed grain during the crushing of the seeds. The friability indices (Fl) of the cBN grains at both ambient temperature (about 20 to 30 degrees Celsius) and after being heat treated at 1 ,100 degrees Celsius were measured. The comparison of the friability indices at these two temperatures is expected to provide an indication of the thermal stability of the cBN grains, since it provides an indication of the extent to which the strength of the grains is diminished by the heat treatment. The friability of the cBN grains prior to heat treatment was 37.25 per cent and that after heat treatment was 19.50 per cent, the thermal stability value therefore being 52.35 per cent.

Example cBN grains made using disclosed example methods may have the aspect that they have relatively sharp edges and a tendency to exhibit tetrahedral crystallographic habit. Example cBN grains may exhibit a strong tendency to be twinned, in which individual cBN grains may be twinned along one or more crystallographic planes (which may be referred to a 'multiple twinning'), and / or they may tend to be uniquely angular in the appearance of their shapes, and / or they may tend to have shapes exhibiting relatively high aspect ratios.

Example cBN crystals may comprise crystallographically twinned parts that may be generally symmetric about a twin plane. Examples of crystallographic twin arrangements include generally mirror-symmetric crystal portions on either side of a crystallographic twin plane, rotationally-symmetric portions about an axis (n-fold rotational symmetry about the axis) or inversion-symmetric portions through a point, or centre. It may be relatively easier to identify the mirror-symmetric planar twinning arrangement, which may be referred to as contact twinning, in which a planar 'composition surface' P separates two crystal volumes i, ii, as shown in Fig. 10A. A composition surface is a boundary along which the lattice points are shared by twinned crystals. Twinning arrangements defined by an axis or centre of inversion, which may be referred to as penetration twins, can be more complex in shape, with an irregular composition surface separating two or more individual crystal volumes i, ii, iii as shown in Fig. 10B, in which the centre of inversion appears to be located in the circled area A.

Example cBN grains may be suitable for use in tools for removing material from work- piece objects in industrial machining operations, such as in grinding tools or saws, particularly but not exclusively vitreous bond grinding wheels. The shapes, properties and other characteristics of example cBN grains may result in improvement of the efficiency of tools. In some examples, this may arise owing to at least some of the cBN grains exhibiting 'self-sharpening' behaviour, in which the cBN grains may fracture in use such that relatively sharp edges remain and thus allow the cBN grain to persist in the tool and continue to remove material efficiently from the work-piece. Twinned cBN grains may have the aspect of exhibiting more controllable break-down in use, and fracture of the grains may more likely occur at twin planes. The rate at which example cBN grains break down in use in some applications, particularly but not exclusively grinding, may be relatively more constant over at least a period of their working life. So-called 're-entrant' cBN crystal shapes, which have the appearance of generally 'V'shaped' depressions into the grain and may arise from twinning, may improve the retention of the cBN grains in a bond holding the grains within a tool.

Fig. 1 1 is a flow diagram showing exemplary steps. The following numbering corresponds to that of Fig. 1 1 :

S1 . A seed grain comprising plastically deformed cBN is provided. The mass of the seed grain is at most one tenth the mass of the synthetic cBN grain. S2. A synthesis compact is formed. The compact comprises the seed grain, hBN material and catalyst material for transforming the hBN into cBN material at a pressure and a temperature at which the cBN allotrope of crystalline boron nitride is thermodynamically more stable than the hBN allotrope. The compact is configured such that the hBN will be transformed into cBN material precipitated on the seed grain when subjected to the pressure and the temperature, and the mass of the hBN is at least that of the synthetic cBN grain to be grown on the seed grain.

S3. The compact is subjected to the pressure and temperature for a period sufficiently long to transform the hBN into the synthetic cBN grain.

Certain terms and concepts as used herein are briefly explained below.

As used herein, 'plastic deformation' means the persisting strain induced in crystalline or non-crystalline material as a result of the application of mechanical stress, the magnitude of which exceeds the elastic limit of the material, and is quantified by means of the Williamson-Hall equation as follows:

W ² obs - tf _imtr fcose = A /D _v + 4 e _str (sind) where, λ is the X-ray wavelength, D _v is the volume weighted volume size, and e _s t _r is a coefficient corresponding to the non-uniform strain, which can arise due to plastic deformation. The slope of the straight line obtained by plotting the [fi ² ₀ bs - 0inst ^A cosQ term versus the [4 (sinO)] will be the strain e _s t _r in the material being examined.

In general, this method includes obtaining an XRD scan of powder, resulting in a series of diffraction (Bragg) peaks as a function of twice the incident angle 2Θ of the X-ray beam, and mathematically de-convoluting the contributions to the width of at least one of the XRD peaks arising from various phenomena, such as the random orientation of grains within the powder and strain present in the grains. As used herein, the width of an XRD peak means the full-width at half-maximum (FWHM, which may be referred to simply as the 'width' of the peak). The method includes assuming that the XRD peaks have the shape of a Gaussian function (also referred to as 'normal' distribution), the observed width (3 ₀ b _S ) of which arises from various factors such as systematic broadening due to the measurement (instrumental broadening, 3 _ins tr), broadening due to the size distribution of the grains (3 _S i _Ze ) and broadening due to strain in which (3 ² obs - 3 ² inst) = 3 ² size + 3 ² strain- Substantially strain-free lanthanum hexa-boride (LaB ₆ ) can be used as a standard material for calibrating the effects of grain size and instrument on the XRD peak widths, as described in Chantler et al. (C. T. Chantler, C. Q. Tran, D. J. Cookson, 2004, "Precise measurement of the lattice spacing of LaB ⁶ standard powder by the x-ray extended range technique using synchrotron radiation"; Phys. Rev. A 69 (4): 042101 ).

As used herein, comminution is the process in which grains are reduced in size, by crushing, grinding or other processes. As used herein, friability is an extrinsic property of solid material indicating the degree to which it can to be reduced to smaller pieces when energy is applied to it. A comminution device known as a friability tester can be used for providing an indication of the friability of super-hard grains. Broadly, the term "friability" refers to the degree to which grains of hard or super-hard material tend to fracture when subjected to impact with a body. A friability tester assembly comprises a cylindrical capsule which can be closed at its ends and a ball consisting of a hard, wear resistant material, which can be accommodated by the capsule and move freely within it. The test process includes introducing a plurality of super-hard grains having a certain known mass and size distribution into the capsule with the ball, closing the capsule and causing it to be shaken back and forth in the direction of its cylindrical axis, causing the ball and the super-hard grains to be violently agitated and the consequent fracturing the super-hard grains. The shaking action is maintained for a certain number of cycles, at a certain amplitude and a certain frequency, after which all the super-hard grains are recovered and the size distribution is measured (using the same method as used to measure the initial size distribution of the grains). Based on the differences between the initial and final size distributions, a friability index can be computed as the mass percentage of the unbroken grains in relation to the initial mass. The friability index had been used as an indicator of the likely performance of super- hard grains of various particular grades in various industrial applications, such as grinding. United States patent number 5,140,857 discloses an electronically controlled friability testing apparatus, in which the cycle frequency is disclosed as being 40 cycles per second. Further information can be found in international patent application publication number WO/2014/177476. The size range of abrasive grains may be expressed in terms of U.S. Mesh size, in which two mesh sizes are provided, the first being a mesh size through which the grains would pass and the second being a mesh size through which the grains would not pass. Mesh size may be expressed in terms of the number of openings per (linear) unit length of mesh.

As used herein, grain sizes less than about 20 microns are expressed in terms of equivalent circle diameter (ECD), in which each grain is regarded as though it were a sphere. The ECD distribution of a plurality of grains can be measured by means of laser diffraction, in which the grains are disposed randomly in the path of incident light and the diffraction pattern arising from the diffraction of the light by the grains is measured. The diffraction pattern may be interpreted mathematically as if it had been generated by a plurality of spherical grains, the diameter distribution of which being calculated and reported in terms of ECD. Aspects of a grain size distribution may be expressed in terms of various statistical properties using various terms and symbols. Particular examples of such terms include mean, median and mode. The size distribution can be thought of as a set of values Di corresponding to a series of respective size channels, in which each Di is the geometric mean ECD value corresponding to respective channel / ^' , being an integer in the range from 1 to the number n of channels used.