WO1992017618A1

WO1992017618A1 - Abrasive compact composed mainly of cubic boron nitride and method of making same

Info

Publication number: WO1992017618A1
Application number: PCT/AU1992/000127
Authority: WO
Inventors: Alfred Edward Ringwood
Original assignee: The Australian National University
Priority date: 1991-03-26
Filing date: 1992-03-26
Publication date: 1992-10-15

Abstract

An abrasive compact comprising 40 to 90 volume percent of cubic boron nitride (CBN) crystals bonded by 60 to 10 volume percent of a bonding matrix comprised mainly of an intimate mixture of an aluminium nitride component and a metallic diboride component. The metallic diboride component comprises one or more of the metallic diborides MB2, wherein M represents metal atoms chosen from the group of titanium, zirconium, hafnium, niobium and tantalum atoms occurring as atoms of one or more elements in said group, or a solid solution of aluminium diboride in one or more of those diborides. The compact may be produced by a method which comprises intimately mixing particulate CBN crystals with a bonding agent in the proportion 95 to 60 volume percent of CBN and 5 to 40 volume percent of bonding agent, the bonding agent containing the metal atoms in the atomic proportions Al50M50 to Al90M10, subjecting the mixture of CBN crystals and bonding agent to temperatures in the range 900 °C to 1800 °C and pressures in the range 5 to 70 kilobars, and maintaining the temperature and pressure conditions for a period of at least about 30 seconds, sufficient to cause most of the bonding agent to form the bonding matrix.

Description

ABRASIVE COMPACT COMPOSED MAINLY OP CUBIC BORON NITRIDE AND

METHOD OF MAKING SAME TECHNICAL FIELD

This invention relates to an improved abrasive compact composed mainly of cubic boron nitride (CBN) and to a process for producing this compact.

Many different kinds of CBN compacts have been described in the literature and their properties vary widely. Generally speaking, a CBN compact is understood to mean a polycrystalline body possessing substantial abrasiveness and low or negligible porosity, comprised of more than about 50 percent by volume of CBN crystals in which a large proportion of contacts occur between individual CBN crystals. The classes of CBN which are most widely used in industry are produced in the thermodyna ic stability field of cubic boron nitride at high pressures and temperatures (e.g. -55 kilobars and ~1500°C) . US Pat. No. 3,743,489 ( entorf et al.) describes a cubic boron nitride compact comprising a thin layer ( < 1.5 mm) layer of CBN bonded to a cemented carbide support. The abrasive layer comprises an intergrowth of CBN crystals and a minor amount of one or more phases containing aluminium and at least one element selected from the group consisting of nickel, cobalt, manganese, iron, vanadium and chromium. It is believed that the product of this patent corresponds to the commercial material "BZN" manufactured by the General Electric Company.

Another type of compact is described in US Pat. No. 4,666,466 (Wilson). This consists of a monolithic piece of CBN not attached to a carbide substrate and comprises at least 80 percent by weight of CBN crystals which are directly bonded to each other and intergrown with a minor amount of bonding matrix comprised of aluminium nitride and/or aluminium diboride. This compact is also produced in the thermodynamic stability field of CBN, preferably at 50-65 kilobars and 1400-1600°C. The product of this invention is believed to correspond to the commercial material "AMBORITE" produced by the De Beers Company .

US Pat. No. 4,343,651 (Yazu and Hara) describes a compact comprising more than 80 percent of CBN consolidated by a bonding agent comprised mainly of a carbide, nitride, or boronitride of Ti, Zr, Hf, V, Nb, Ta, together with a lesser amount of one or more aluminium compounds. The compact is produced at high pressures and temperatures, preferably in the thermodynamic stability field of CBN. Compacts according to this invention could be produced either as monoliths or bonded to carbide substrates.

A second class of CBN compacts have been produced at substantially lower pressures, in the thermodynamic stability field of hexagonal boron nitride. US Pat. No. 4,353,714 (Lee et al.) described a compact in this latter class comprising 65-85 volume percent of CBN bonded mainly by elemental silicon which was infiltrated into a mass of metal-coated CBN crystals at a temperature of about 1500°C and a pressure of about 1 kilobar. Compacts produced by this method are much weaker and less abrasive than those produced according to the first class (made at very high pressures) but can be produced in larger individual sizes. US Pat. No. 4,220,455 (St. Pierre et al.) describes a compact comprised mainly of CBN crystals mixed with elemental carbon which is infiltrated by molten silicon in a vacuum at a temperature of about 1400°C. The silicon partially reacts with the carbon to produce silicon carbide, which becomes firmly bonded to CBN crystals. The compact thus comprises a mass of CBN crystals bonded by a mixture of silicon carbide and silicon. This compact is not nearly as strong and abrasive as those of the first class (made at very high pressures) but can be produced in larger individual sizes.

It will be appreciated by those skilled in the art that the qualities displayed by CBN compacts proposed in the literature vary over an extremely broad range, according to their compositions, structures, nature of their bonds and their pressures, temperatures and times of fabrication. Many composites, particularly those produced in the thermodynamic stability field of hexagonal boron nitride at pressures generally below 40 kilobars, may display substantial abrasiveness, but are relatively deficient in toughness and compressive strength and cannot be practically utilized in applications where a combination of high hardness with high toughness is required, for example, in turning, drilling and grinding hard metallic alloys. CBN compacts used for these purposes should possess high compressive strengths of at least 10 kilobars, combined with high fracture toughness and should be substantially superior to cemented tungsten carbide in turning hard metallic alloys. This invention relates to a compact in this latter category.

An objective of the present invention is to produce a CBN compact at relatively low pressures, preferably between 10-35 kilobars, said compact possessing properties as least as advantageous as those produced according to US Pat. No. 3,743,489 and No. 4,666,466 under pressures preferably in the range 50-60 kilobars. A further objective of the present invention is to conveniently produce monolithic CBN compacts which are larger than can be readily produced according to the above US patents. The larger size permits said compacts to be effectively employed in essentially new and industrially important applications such as grinding wheels and wire-drawing dies. Yet another objective of the present invention is to produce monolithic CBN compacts which possess electrical conductivities high enough to permit them to be shaped and fabricated readily by electrical discharge machining methods. This technique is not readily applicable to the monolithic CBN compact "AMBORITE" produced by the De Beers Company, which possesses a relatively low electrical conductivity. DISCLOSURE OF THE INVENTION

According to the invention there is provided a method of producing a CBN compact which comprises: intimately mixing a mass of particulate CBN crystals with a bonding agent in the proportions 95 to 60 volume percent of CBN and 5 to 40 volume percent of bonding agent, said bonding agent containing aluminium and other metal atoms chosen from the group of titanium, zirconium, hafnium, niobium and tantalum atoms in the atomic proportions A1₅₀M₅₀ to Al^^,, where M represents said metal atoms occurring as atoms of one or more of the elements in said group; subjecting the mixture of CBN crystals and bonding agent to temperatures in the range 900 to 1800°C and pressures in the range 5 to 70 kilobars; and maintaining the temperature and pressure conditions on the mixture for a period of at least about 3 seconds, sufficient to cause most of the bonding agent to form a bonding matrix composed of an intimate mixture of a aluminium nitride component and a metallic diboride component comprised of one or more of the metallic diborides MB₂ or a solid solution of aluminium diboride in one or more of those diborides, the aluminium nitride and metallic diboride components each comprising at least 25 volume percent of the bonding matrix.

The bonding agent may contain said aluminium atoms and metal atoms in the atomic proportions Kl₍^Λ._m to

^A185^M15-

Said bonding agent may comprise: (a) a mixture of elemental aluminium and metal (M) powders; (b) an alloy or intermetallic compound between aluminium and said metal atoms;

(c) a mixture of alloys or intermediate compounds between aluminium and said metal atoms and/or;

(d) a mixture of elemental aluminium with alloys or intermediate compounds between aluminium and said metal atoms.

Preferably, said temperatures and pressures are applied in a sequence and over a sufficient time to cause substantial plastic deformation of the CBN crystals. A preferred bonding agent is an alloy of aluminium and titanium in the atomic proportions Al^Ti^ -

^A-l-80^T--20*

The invention also provides an abrasive compact comprising 40 to 90 volume percent of CBN crystals bonded by 60 to 10 volume percent of a bonding matrix composed mainly of an intimate mixture of an aluminium nitride component and a metallic diboride component comprised of one or more of the metallic diborides MB₂, wherein M represents metal atoms chosen from the group of titanium, zirconium, hafnium, niobium and tantalum atoms occurring a atoms of one or more of the elements in said group, or a solid solution of aluminium diboride in one or more of those diborides, the aluminium nitride and metallic diboride components each comprising at least 25 volume percent of the bonding matrix, said bonding matrix producing a strong and coherent bond between CBN crystals. Preferably, said compact possesses a compressive strength of at least 10 kilobars. Preferably further said compact contains an electrically conductive metallic diboride phase whereby th compact possesses an electrical conductivity high enough t permit it to be shaped by electrical discharge machining. Preferably too, the CBN crystals display substantial plastic deformation at the boundaries between adjacent crystals and the bonding matrix penetrates betwee adjacent crystals to produce an interconnected, electrically conductive network. BEST MODES OF CARRYING OUT THE INVENTION Pressures in the range 15-35 kilobars are employed in producing compacts according to the preferred embodiment of the present invention. The mechanical strength of the compacts falls significantly as the pressure is reduced below 15 kilobars, although compacts possessing useful properties can be produced at 10 kilobars. CBN compacts according to the present invention can also be produced at pressures as high as 70 kilobars. However, in most cases, the quality of compacts so produce is not substantially superior to those produced at 35 kilobars. An important advantage of performing the invention at pressures below 40 kilobars is that it permits the use of relatively simple apparatus possessing much larger working volumes than the apparatus which is necessary if much higher pressures in the region of 50 kilobars are to be attained, as in US Patents Nos. 3,743,489 and 4,666,466. In the latter case, it is necessary to use apparatus such as that described in US Patent No. 2,941,248 (Hall) in which the pressure vessel and pistons are constructed of tungsten carbide and possess a complex geometry which severely restricts the size of the working volume in which the compact is fabricated. On the other hand, if the pressures necessary to produce good quality compacts are less than 40 kilobars, the apparatus used can possess a very simple geometry such as a straight piston which compresses the pressure medium axially within a straight cylinder. This kind of apparatus can readily be scaled up to yield a large working volume and thus can be used to fabricate correspondingly larger compacts. Because of these factors, CBN compacts can be produced below 40 kilobars at lower costs than in the more complex apparatus necessary for fabrication at higher pressures.

In a second embodiment of this invention, high pressures in the range of 40-70 kilobars, in the thermodynamic stability field of CBN, may be employed.

These higher pressures are found to be advantageous in the particular cases where the mean particle size of the cubic boron nitride employed is smaller than 5 microns.

A wide range of temperatures can be employed in the practice of this invention. The temperature should be high enough to permit the reaction of the metallic bonding agent with some of the CBN to produce a bonding matrix consisting of an intergrowth of aluminium nitride and one or more metallic diborides. Preferably this reaction should approach completion. The preferred temperature interval for achieving this objective lies between 1200 and 1600°C and still more preferably between 1300 and 1500°C. Strong compacts have been made at 1000°C but their strengths were significantly below those made at 1450°C. The strength of the compact also appears to decrease as temperature exceeds 1550°C although compacts possessing useful strengths can be prepared at higher temperatures. The practical range of temperatures for the performance of this invention is 900-l800°C. In producing CBN compacts according to a preferred embodiment of the invention, it is advantageous to apply pressure and temperature in a sequence which leads to maximum plastic deformation of CBN crystals. This causes the formation of contacts between adjacent grains of CBN in two dimensions, along faces, rather than at points and edges. Thin films of bonding matrix may form between such closely adjacent crystals. This structure provides greater compressive strength and rigidity in the resulting compact. A further advantage is that plastically deformed CBN crystals are harder than undeformed CBN crystals. In order to maximize plastic deformation of CBN crystals, a relatively low pressure, e.g. 2-10 kilobars is first applied to stabilize the pressure cell. Temperature is then increased to 1000-1200°C, after which pressure is slowly raised to its preferred level (e.g. 25 kilobars) over a period of 3-15 minutes. The gradual application of pressure whilst the CBN crystals are hot leads to extensive plastic deformation with the advantageous results noted above.

In the practice of this invention, the bonding agent is premixed with CBN crystals, rather than infiltrated from the outside as in US Pat. No. 3,743,489. Premixing provides support for the hard CBN particles during application of pressure and thereby minimizes the extensive fracturing of CBN crystals which is characteristic of infiltration processes. This procedure also enhances the degree of plastic deformation of CBN crystals during pressurization. The aluminium-titanium alloy which provides a preferred bonding agent in the present invention may be produced by pre-reacting a mixture of elemental aluminium and titanium to form one or more intermetallic compounds or alloys such as Al₃Ti, Al₂Ti or AITi or a mixture thereof. This bonding agent may be prepared by intimately mixing Al and Ti powders, sealing them in an evacuated silica tube, and then heating the tube at 1000-1200°C for 15-30 minutes. The product is then finely ground until it is mostly finer than 5 microns. Alternatively, the bonding agent can be prepared by mixing finely particulate aluminium powder with one or more of the above alloys and/or intermetallic compounds.

In the practice of the present invention, the finely ground bonding agent is intimately mixed with the mass of CBN crystals prior to being placed in the high pressure-high temperature apparatus. Intimate mixing of bonding agent and CBN particles can be conveniently performed in a commercially available vibratory ball mill. In order to ensure good mixing, the particle size of the bonding agent is preferably smaller than 20 microns and still more preferably smaller than 5 microns. However, the particle size of the bonding agent should preferably be not much smaller than 0.1 microns. Powders finer than this are covered by proportionally large oxide films or absorbed gases which may be deleterious to the quality of the compacts. The particle size of the CBN powders is preferably in the range 1-1000 microns and more preferably in the range 5-200 microns. Still more preferably, the CBN particles are in a range of sizes from 5 to 100 microns with the size distribution chosen so as to maximize the efficiency of packing.

In one embodiment of this invention, the mean particle size of CBN crystals is smaller than 5 microns. However, when such small particles are used, it is desirable to employ higher pressures, extending to within the thermodynamic stability fields of CBN (e.g. 55 kilobars at 1350°C) , in order to produce compacts possessing the optimum combination of abrasive properties. Because of their small particle-sizes, compacts prepared according to this embodiment of the invention produce very smooth surface finishes when used in machining operations.

Bonding agents employing other metals, zirconium, hafnium, niobium and tantalum, in place of or in combination with titanium are prepared using analogous procedures. Intermetallic compounds and alloys which may be prepared in this way include TiAl, TiAl₂, TiAl₃, ZrAl₂, ZrAl₃, HfAl₂, HfAl₃, Hf-_jALj, NbAl₃, NbAl^Nb_Al, TaAl₃ and TaAl₂. These compounds may be alloyed with additional Al or mixed with Al powder to produce bonding agents with compositions in the range M^A -^ to M₁₀A1₉₀, where M refers to Zr, Hf, Nb, Ta and Ti atoms, including mixtures of the atoms. An advantage of preparing bonding agents in this manner is that most of these intermetallic compounds are brittle and are readily ground to sizes smaller then 5 microns. Moreover, they provide intimate mixing between and M atoms at the atomic scale which is advantageous in achieving homogeneous, fine-grained microstructures in the resultant compacts.

The time over which maximum pressure and temperature are applied to the charge (defined henceforth as run time) is governed by the objective that an adequate degree of reaction occurs between the bonding agent and CB particles to produce the desired bonding matrix of silicon nitride plus titanium diboride. At temperatures around 1400°C and 25 kb, where the bonding agent consists of an Al_xTi alloy, run times between 3 and 30 minutes lead to extensive degrees of reaction and equilibration, accompanied by the production of mechanically strong and abrasive compacts. A run time of 1 minute also produced a compact which, whilst it was of adequate quality, was nevertheless significantly inferior in mechanical properties to specimens produced with run-times of 5 minutes. The minimum practical run time for application o maximum pressure and temperature is taken as 30 seconds. There does not seem to be much practical advantage in carrying out runs for longer than 60 minutes under the preferred-temperature conditions. In the preferred embodiment of this invention, a liquid phase is present during the reaction between bondin agent and CBN. Accordingly it is necessary to adjust the composition of the bonding agent and the temperature of th run to ensure the presence of at least a small proportion of liquid phase. This facilitates rapid and pervasive reaction between CBN and the bonding agent, leading to the production of a homogeneous and fine grained microstructur in which a metallic diboride phase forms an interconnected electrically conductive network extending throughout the compact, in this embodiment of the invention, compacts possessing optimum properties are produced at relatively low pressures, between 15-35 kilobars and the use of higher pressures is not necessary. Compacts of this type are preferably prepared using CBN with mean particle sizes greater than 5 microns.

In a second embodiment of the invention, the reaction between the bonding agent and CBN is performed essentially in the solid state, in the absence of a liquid phase. Under these conditions, it may be desirable to employ finely particulate CBN and bonding agent, each with mean particle sizes smaller than 5 microns, in order to cause the reaction between these components to proceed to the desired degree of completion and to produce compacts with optimum textural homogeneity and mechanical properties. The use of higher pressures may be desirable when producing compacts under these conditions. Preferably, the pressures used are between 40-70 kilobars and the reaction is carried out within the thermodynamic stability field of CBN. The resultant product possesses a different microstructure to that produced in the presence of liquid. However, provided that sufficient bonding agent is employed, the resultant compact possess an electrical conductivity sufficiently high to permit shaping by EDM. Preferred bonding agents include alloys or mixtures of alloys between titanium and aluminium, e.g. TiAl, TiAl₂ TiAl₃ and mixtures of these with pure aluminium powder. A preferred bonding agent is TiAl₂ which reacts with CBN under the desired pressure and temperature conditions according to the following equation

2 BN + TiAl₂ = TiB-₂ + 2 AlN

This reaction is preferably carried out around 1350-1400°C at 25-35 kb in the presence of a liquid phase and yields compacts possessing excellent properties in which the bonding matrix comprises an intimate intergrowth of titanium diboride and aluminium nitride. If the proportion of titanium in the bonding agent is higher than in TiAl₂, as for example, if the compound TiAl is used, it is found that some titanium nitride, (TiN) is also produced in the bonding matrix together with aluminium nitride and titaniu diboride. The additional presence of TiN in the bonding matrix does not cause an improvement in the properties of the compact. If the bonding agent contains a higher proportion of aluminium, as in the compound TiAl₃, the temperature at which liquid phase is produced is lowered significantly and the reaction between bonding agent and CBN seems to be even more effective. Compacts made using TiAl₃ as the bonding agent possess properties which are at least as good and may be somewhat superior to those in which TiAl₂ is used. The bonding matrix in runs at 1330 - 1380°C and 25-35 kb is found to comprise a mixture of aluminium nitride and a diboride solid solution ranging in composition between (Ti₀₅Al₀₅)B₂ and (Ti₀₉Al₀₁)B₂. The diboride possessed a high electrical conductivity and was distributed throughout the compact as an interconnected network thereby making it possible to machine the compact using EDM techniques. The proportion of Al in the bonding agent can be increased still further, by mixing TiAl₃ with Al powder or by direct reactions between Ti and Al powders. For compositions between Ti₇₀Al₃₀ and Ti₁₀Al₉₀, the bonding matri is found to comprise an intimate intergrowth of aluminium nitride and two diboride phases. One of these is a

(Ti,Al)B₂ diboride solid solution in which (atomic) Ti > Al, and the second is essentially pure A1B₂. Compacts produced using these bonding agents possessed useful abrasive properties and were sufficiently conductive to be machined via EDM. However, the electrical conductivity of the compact made from the Ti₁₅Al₈₅ bonding agent possessed a lower conductivity than the others. Another compact using Ti_joAl^ bonding agent possessed an electrical conductivity which permitted it to be machined only with difficulty. Thus, the effective range of compositions using Ti-Al bonding agents which can be employed in the practice of this invention is from Ti₅₀Al₅₀ to TijoAlgo (atomic proportions) . The applications of Zr-Al, Hf-Al, Nb-Al and Ta-Al alloys as bonding agents are analogous to those employing Ti-Al bonding agents. Compositions between M^!^ and M₂₅A1₇₅ possess higher melting points than the corresponding Ti-Al alloys. The temperatures at which liquids form in the presence of CBN are lowered somewhat by solution of B and N in the melts. Nevertheless, in order to produce composites in the presence of liquid phase at 25-35 kb, temperatures between 1400 and 1600°C are preferably employed, where the bonding agents possess compositions between M^lrø and M^Al^ (M = Zr, Hf, Nb, Ta) . Compacts thereby produced consist of CBN particles bonded by matrices composed mainly of AIN and ZrB₂, AIN and HfB₂, AIN and NbB₂ and/or AIN and TaB₂. The diboride phases form an electrically conducting interconnected network within the compacts thereby permitting them to be machined readily by EDM. Obviously, bonding agents comprising mixtures or alloys of Zr, Hf, Nb and Ta aluminides can also be employed in the practice of this invention. Hot-pressing temperatures can be lowered significantly by utilizing alloys containing larger proportions of aluminium, for example, with compositions between M-_jjAl-^ and M₁₅A1₈₅, where M = (Zr, Hf, Nb, Ta) . Compacts prepared from bonding agents in this compositional range each contain AIN and A1B₂ as components of their bonding matrices, together with a second diboride solid solution, (Zr,Al)B₂, (Hf,Al)B₂, (Nb,Al)B₂, (Ta,Al)B₂ respectively. In these latter diboride solid solutions, the Zr, Hf, Nb and/or Ta atoms are present in excess of Al atoms. Compacts produced using bonding agents in this range of compositions possess good abrasive properties and can also be machined by EDM.

Alternatively, where bonding agents possessing compositions between ^l^ and M-yAl^ (M = Zr, Hf, Nb, Ta) are employed, reactions can be performed under subsolidus conditions in the absence of liquid, at temperatures between 1200-1500°C. In order to produce compacts possessing optimum properties under these conditions, particle sizes of CBN and bonding agent are preferably smaller than 5 microns and pressures in the range 40-70 kilobars are preferably employed. Compacts produced under these conditions possess good abrasive properties and produce smooth surface finishes on workpieces because of their fine crystal sizes. Compacts produced according to this embodiment of the invention can usually be machined using EDM techniques.

It is possible to utilize other forms of bonding agents possessing overall chemical compositions similar to those described previously. For example, bonding agents can be prepared by intimately mixing elemental aluminium and M powders (M = Ti, Zr, Hf, Nb, Ta) in previously defined ratios and further mixing this bonding agent with CBN particles. However, application of this procedure did not produce compacts possessing a high degree of strength and abrasiveness. An alternative procedure was tested in which the bonding agent was introduced as a mixture of finely particulate (minus 5 micron) aluminium nitride plus metallic diboride (MB₂) in the previously defined ranges of proportions. Although some of the compacts produced by these methods possessed useful abrasive properties, the resultant compacts were inferior to the best compacts produced using M_Al_y alloys and/or intermetallic compounds as essential components of the bonding agents.

The considerable difference in results obtained by mixing CBN with pre-prepared M^Al-. alloys, as compared to mixing CBN with the equivalent amounts of pure metallic powders was quite unexpected and is believed to be caused by the following factors:

(1) Most of the pure metallic powders (e.g. Ti, Zr, Hf, Al) are chemically highly reactive. During heating in the pressure cell, they may become poisoned by reacting with gaseous and other components including oxygen, carbon, and nitrogen, which are often present.

This in turn may inhibit their subsequent reactivity. On the other hand, the preferred M_A1 intermetallic compounds present in the bonding agent are much less reactive chemically with these components and can be heated to abov -ø

1500°C in the pressure cell without losing or reducing their ability to react with CBN to form the bonding matrix

(2) Al and M atoms are intimately mixed in the intermetallic aluminide compounds at the atomic level. Moreover, the intermetallic aluminides are very brittle an can readily be ground to particle sizes smaller than 5 microns. These factors enhance the homogeneity of mixing between CBN and bonding agent. On the other hand, it is difficult to prepare and handle several of the metallic powders (e.g. Ti, Zr, Hf, Al) in particle sizes smaller than 20 microns because they may be pyrophoric. Metallic powders with particle sizes as large as 20 microns cannot readily be intimately mixed with CBN particles with the degree of homogeneity which is necessary to produce compacts possessing optimum microstructure and other properties.

It is possible that the problem discussed in (1) above would be solved by very careful preparation of mixtures of M and Al powders with CBN particles under clean conditions and under vacuum, and that the charge could be sealed under vacuum into a metallic container prior to being subjected to high pressure and temperature. This would be inconvenient, however, and would not provide the homogeneity of mixing which are obtained by the use of M-Al_y alloys.

The difference in mechanical properties between compacts produced using M_Al_y alloys as bonding agents and those which use equivalent amounts of mixed AIN and MB₂ powders is also unexpected. It is believed that this difference arises from the fact that in one case, the M^l_y alloy chemically reacts with CBN to produce the desired AIN + MB₂ bonding matrix in situ, and that this reaction produces strong chemical bonding between the bonding matrix and the CBN crystals. Moreover, as described below, the bonding matrix possesses a unique microstructure. On the other hand, when pre-prepared AIN and TiB₂ powders are mechanically mixed with CBN particles and then hot pressed, the strength of the bonds between the bonding matrix and CBN particles seems to be weaker and a different microstructure is produced.

The beneficial results obtained from the use of M-Al_y alloys as bonding agents, rather than M and Al powders, or AIN and MB₂ powders are not obvious, and could not have been predicted. This discovery is an important aspect of the present invention.

In its preferred embodiment, the reaction of M-Al_y alloys with CBN according to the process of the present invention produces a bonding matrix composed mainly of AIN and MB₂ (which may contain additional Al atoms replacing up to half of the M atoms) , said bonding matrix being pervasively distributed along CBN grain boundaries producing a unique microstructure in which MB₂ (M - Ti, Zr, Hf, Nb, Ta) possessing high electrical conductivities form an interconnected network of fine plates and films. It is this microstructure which is responsible for the high electrical conductivity of the compact, thereby enabling the compact to be machined by EDM methods. This microstructure is different to that produced when either the individual elements M and Al, or the compounds MB₂ and AIN are mixed as powders with CBN and hot-pressed. It is believed that this microstructure may be partly responsible for the advantageous mechanical properties of the aforesaid compacts, in addition to their electrical conductivity. Compacts produced according to the present invention possess important practical and commercial advantages over CBN compacts currently in use. They can be produced with larger physical dimensions which is of special benefit in certain applications, particularly grinding. Moreover, they are cheaper to produce. The metallic diborides (MB₂ where M = Ti, Zr, Hf, Nb, Ta) of the compact of the present invention are much harder than A1B₂ which is a key bonding agent in the commercially produced CBN compact "AMBORITE" and is also believed to be present in BZN compacts produced by the General Electric Company. Thus, TiB₂ possesses a Vickers micro-hardness of 3250 kg/mm² as compared to 980 kg/mm² for A1B₂. The melting point of TiB₂ is 2980°C as compared to 1710°C for A1B₂. These characteristics combined with the unique microstructure of the product of the present invention, yield a product possessing great mechanical toughness, combined with a high compressive strength, greater than 10 kilobars, and high abrasiveness. Compacts produced according to the present invention readily turn, drill, grind and machine hardened metallic alloys such as tool steels, high-speed steels, cast iron, and nickel- and cobalt-based super-alloys. Their performance in machining these alloys is at least as good as that of current CBN compacts such as the product commercially known as AMBORITE, and in certain uses, is superior to those of existing CBN compacts. This is particularly the case in grinding applications. A further major advantage of the product of the present invention is its substantial electrical conductivity, which permits the product to be worked and shaped by electrical discharge machining (EDM) . Presently available monolithic commercial compacts containing more than 70 volume percent of CBN, such as AMBORITE , possess much lower electrical conductivity and cannot be machined by EDM methods.

The production of CBN compacts according to the present invention is described in the following examples and it is to be understood that these are not to be considered as limiting the scope of the invention in any way:

EXAMPLE 1

A bonding agent with a mean atomic composition TiAl₂ was prepared by intimately mixing 47 wt% titanium metal powder (minus 40 microns) with 53 wt% aluminium powder (minus 30 microns) . The mixture was placed in an evacuated silica tube and heated to 1100°C for 15 minutes. The mixture reacted to form an alloy of aluminium and titanium. The product was then ground under acetone to a particle size smaller than 5 microns. This alloy, possessing a mean composition equivalent to TiAl₂, constituted the bonding agent in the present example.

A mixture comprising 80 wt% of CBN (mean particle size: 30 microns) and 20 wt% CBN (mean particle size: 6 microns) was then prepared. An amount of 82.5 wt% of the CBN mixture was then intimately mixed with 17.5 wt% of the powdered TiAl₂ bonding agent. The mixture was placed in a cylindrical capsule of hexagonal boron nitride possessing internal dimensions 12 mm diameter x 22 mm long. The capsule was closed with a lid of hexagonal boron nitride and placed within a piston-cylinder high pressure-high temperature apparatus. The design of the apparatus and pressure cell was generally similar to the piston-cylinder apparatus described by F. Boyd and J. England (J. Geophys . Res . 65, 741, 1960). The internal diameter of the pressur vessel was 2.54 cm, its height was 8 cm, and the heater was a graphite tube which itself was surrounded by a ductile pressure medium.

A pressure of 2 kilobars was first applied to consolidate the components of the pressure cell. The temperature and pressure of the CBN-TiAl₂ mixture were then increased to 600°C and 10 kilobars over a period of 6 minutes. Temperature was then raised to 1100°C and held steady. Pressure was then increased to 25 kilobars over a period of 5 minutes. This step causes a desirable degree of densification of the charge accompanied by plastic deformation of the CBN crystals. Temperature was then increased to the run temperature of 1380°C over a period of 2 minutes. Pressure and temperature were then held constant for 10 minutes to allow the desired reaction between the bonding agent and the CBN crystals. After completion of the run, temperature was first reduced to 800°C whilst maintaining full pressure. Pressure was then slowly released over 30 minutes from 25 to 5 kilobars whilst holding temperature steady at 800°C. Temperature and pressure were then lowered in parallel to ambient conditions over a further 20 minutes. The sample capsule was then removed from the apparatus.

The powder mixture comprising the starting material was found to have formed a CBN compact which was recovered in the form of an intact cylinder. After sandblasting, the compact was found to possess a diameter of 11.5 mm, a height of 13 mm and a density of 3.50 g/cm³. The compact possessed a substantial electrical conductivity and could readily be shaped by electrical discharge machining (EDM) . Examination of a sample by X-ray diffraction showed that it consisted of CBN, TiB₂ and AIN. Hexagonal boron nitride was below the detection limit (< 0.5%). These identifications were confirmed by electron-probe microanalyses and optical studies. The sample was essentially fully dense and CBN crystals displayed extensive plastic deformation, so that crystals were in contact or adjacent to one another along shared complementary surfaces produced by plastic deformation. The X-ray and optical study showed that the TiAl₂ bonding agent had reacted essentially completely with CBN crystals to form a bonding matrix compromising a mixture or intergrowth of titanium diboride (TiB₂) and aluminium nitride (AIN) . Although the pressure and temperature conditions under which this reaction occurred were deeply in the thermodynamic stability field of hexagonal boron nitride, the amount of this latter phase, which could have been formed by retrogressive transformation of CBN, was negligible. The amount of bonding matrix (TiB₂ + AIN) present was estimated at about 25 volume percent, these two phases being present in approximately similar proportions. The microstructure of the compact showed that the bonding matrix had penetrated along grain boundaries in thin films and had infiltrated the mass of CBN crystals. Optical and electronprobe examination showed that the CBN crystals were to a large extent surrounded by films of AIN whilst the TiB₂ phase occurred as intergranular plates and films forming an interconnected network. This unique microstructure of the CBN compact produced according to the present invention is believed to be substantially responsible for. its excellent mechanical properties and for its high electrical conductivity.

A number of tools and other pieces were cut from the cylindrical CBN compact by EDM. The compact was found to possess high impact strength and outstanding hardness. When broken, fractures were found to extend through CBN crystals, showing the strength of the bond between CBN and the bonding matrix. Tools prepared from the product of the invention were used to turn a range of alloys including Bohler K720 cold work steel (Re 60) , Assab Werke 45 high speed steel (Re 65) , cast iron, stellite, and nimonic and inconel super-alloys. These alloys were turned readily with the tool with depth of cut up to 0.5 mm and varying surface feed rates. Tool wear was minimal. The turning performance of tools made according to the present invention was considerably superior to that of tungsten carbide tools. The performance of the present tools was compared with that of AMBORITE tools. (AMBORITE is a commercially-produced CBN compact manufactured by De Beers Ltd.). In most turning operations under similar conditions, the performance of the product of the present invention was superior to that of AMBORITE, in terms of the amount of tool wear in relation to the amount of stock removed from the test sample.

EXAMPLE 2

This Example was prepared similarly to Example 1 except that the CBN used was prepared as a mixture of 80% CBN crystals possessing a mean size of 80 microns and 20% of CBN crystals possessing a mean size of 30 microns. After preparation, and removal from the apparatus, the sample was found to possess a diameter of 11.3 mm, a length of 12.5 mm and density of 3.48 g/cm³.

The sample was then fabricated by EDM into a true cylinder OD 10 mm, length 10 mm, with an axial hole 5 mm in diameter. The cylinder was etched in molten potassium hydroxide for 2 minutes, thereby dissolving some TiB₂ and AIN from the bonding matrix at the surface and loosening the outermost layer of CBN crystals. The sample was then sandblasted to produce a rough surface of pristine CBN crystals. This sample was then mounted on a steel shaft to produce a CBN grinding wheel.

The grinding wheel was used for internal and external grinding of several hard alloys including Bohler K720 cold work steel (Re 60) , Assab Werke 45 high speed steel (Re 65) , cast iron, stellite, and nimonic and inconel super-alloys. Parallel tests on these materials were carried out using commercially available impregnated CBN grinding wheels. The performance of the product of the present invention was vastly superior to that of commercial impregnated CBN grinding wheels. Stock-removal rates displayed by the wheel of the present invention were 3-5 times higher and wheel wear was 5-10 times lower than for commercial CBN grinding wheels. The difference arises . because of the much higher packing-density of CBN particles in the wheel of the present invention, combined with high compressive strength and the use of a bonding matrix which is vastly stronger than the matrices (resin, metal, vitreous) of commercial impregnated CBN grinding wheels.

Commercial manufacturers of CBN compacts have not hitherto found it practical to market CBN compact grinding wheels. It is believed that this is because the production technology currently used to produce commercial compacts cannot readily produce them in monoliths large enough to permit practically useful grinding wheels to be fabricated. Moreover, there are practical difficulties in fabricating grinding wheels from existing monolithic commercial CBN compacts such as AMBORITE which possesses low electrical conductivity and cannot readily be shaped by EDM. EXAMPLE 3

A bonding agent with a mean atomic composition TiAl₃ was prepared by intimately mixing 37.2 wt% titanium metal powder (minus 40 microns) with 62.8 wt% aluminium powder (minus 30 microns) . The mixture was placed in an evacuated silica tube and heated to 1100°C for 15 minutes. After cooling, the alloy was shown by X-ray diffraction to consist of the intermetallic compound TiAl₃. The product was then ground under acetone to a particle size smaller than 5 microns. This alloy constituted the bonding agent in the present example.

A CBN compact was then produced using the same procedure as for Example 1 except that 17.5 wt% of the powdered TiAl₃ was used as the bonding agent. The compact possessed a substantial conductivity and could readily be shaped by electrical discharge machining (EDM) . Its density was 3.45 g/cm³.

Examination of a sample by X-ray diffraction and electronprobe microanalysis showed that it consisted of CBN, AIN and a diboride solid solution with composition varying between (Ti₀₅Al₀₅)B₂ and (Ti₀₉Al₀₁)B₂. Hexagonal boron nitride was below the detection limit (<0.5%). The microstructure of the sample was similar to that of the compact produced in Example 1. The mechanical and abrasiv properties of the compact were slightly but significantly superior to those of the compact produced in Example 1. EXAMPLE 4

In this and in the following Examples a pressure vessel with a diameter of 1.5 cm and a length of 5 cm was employed. The hexagonal boron nitride containment capsule possessed an ID of 5 mm and length of 6 mm. Except for these specific variations, the runs were carried out similarly to the procedures described in Example 1. The CBN compacts recovered after the runs were approximately 4.2 mm (dia) x 4 mm long.

Also in this and in the following examples, the CBN employed comprised a mixture of 80 percent of 30 micro (mean size) particles with 20 percent of 6 microns (mean size) particles.

An alloy possessing the bulk atomic composition ZrAl₂ was prepared by heating an intimate mixture of zirconium and aluminium powders under conditions similar t those employed in producing TiAl₂ in Example 1. The alloy was crushed to a particle size smaller than 6 microns.

24.9 wt% of the alloy was intimately mixed with 75.1 wt% of the CBN mixture. The resultant mixture was then hot-pressed at 25 kilobars, 1500°C for 20 minutes. The compact thereby produced comprised about 75 volume percent of CBN consolidated by about 20 volume percent of a bondin matrix composed mainly of an intergrowth of aluminium nitride and zirconium diboride. About 5 percent of unreacted zirconium aluminide(s) was also present. The strength, abrasive properties and electrical conductivity of the resultant compact were generally similar to those of the compact produced in Example 1. The compact could be shaped readily by EDM methods. EXAMPLE 5 An alloy possessing the bulk atomic composition

HfAl₂ was prepared by heating an intimate mixture of hafnium and aluminium powders under conditions similar to those employed in producing TiAl₂ in Example 1. The alloy was crushed to a particle size smaller than 6 microns. 39.8 wt% of the alloy was intimately mixed with 60.2 wt% of the CBN mixture. The resultant mixture was then hot-pressed at 25 kilobars, 1500°C for 20 minutes. The compact thereby produced comprised about 75 volume percent of CBN consolidated by about 20 volume percent of a bonding matrix composed mainly of an intergrowth of aluminium nitride and hafnium diboride. A few percent of unreacted hafnium silicide(s) was also present. The strength, abrasive properties and electrical conductivity of the resultant compact were generally similar to those of the compact produced in Example 1. The compact could be shaped readily by EDM methods. EXAMPLE 6

.An alloy possessing the bulk atomic composition NbAl₃ was prepared by heating an intimate mixture of niobium and aluminium powders under conditions similar to those employed in Example 1. The alloy was crushed to a particle size smaller the 6 microns and further mixed with aluminium powder, thereby producing a bonding agent with bulk composition NbAl₅. 19.4 wt% of this bonding agent was mixed with 80.6 wt% of the CBN mixture. The resultant mixture was then hot-pressed at 25 kilobars, 1380°C for 10 minutes. The compact thereby produced comprised about 75 volume percent of CBN consolidated and infiltrated by about 25 volume percent of a bonding matrix composed mainly of an intergrowth of aluminium nitride (AIN) , niobium diboride (NbB₂) and aluminium diboride (A1B₂) , which contained several percent of niobium in a solid solution. The compact displayed high strength and useful abrasive properties, but these were not quite as advantageous as the properties of the compact produced in Example 1. The electrical conductivity of the compact was smaller than that of the compact of Example 1, but it was still possible to machine it by EDM methods. EXAMPLE 7

An alloy possessing the bulk atomic composition TaAl₃ was prepared by heating an intimate mixture of tantalum and aluminium powders under conditions similar to those employed in Example 1. The alloy was crushed to a particle size smaller than 6 microns and further mixed with aluminium powder, thereby producing a bonding agent with bulk composition TaAl₅. 27.1 wt% of this bonding agent was mixed with 72.9 wt% of the CBN mixture. The resultant mixture was then hot-pressed at 25 kilobars, 1380°C for 10 minutes. The compact thereby produced comprised about 75 volume percent of CBN consolidated and infiltrated by about 25 volume percent of a bonding matrix composed mainly of an intergrowth of aluminium nitride, tantalum diboride and aluminium diboride containing a few percent of tantalum in solid solution. The properties of the compact were similar to those of Example 6.

EXAMPLE 8

A bonding agent possessing an overall atomic composition (Nb₀₇Zr₀₃)Al₅ was prepared by intimately mixing NbAl₃ ZrAl₃ and Al powders in the required proportions.

19.5 wt% of this bonding agent was mixed with 80.5 wt% of the CBN mixture. The resultant mixture was hot-pressed at 25 kilobars, 1380°C for 10 minutes. The compact thereby produced comprised about 75 volume percent of CBN consolidated and infiltrated by a bonding matrix consisting mainly of an intergrowth of aluminium nitride (AIN) , niobium diboride (NbB₂) containing about 3 wt% of Al replacing Nb, and a solid solution of aluminium and zirconium diboride possessing the composition (Al₀₈Zr₀₂)B₂. The properties of this compact were generally similar to those of the compact produced in Example 6.

EXAMPLE 9

An intimate mixture comprising 60.2 wt% of CBN with a mean particle size of 3 microns and 39.8 wt% of HfAl₂ with a similar particle size was prepared. It was contained in a boron nitride capsule 5 mm diameter and 5 mm deep which was placed in a "girdle" high pressure-high temperature apparatus. Pressure was raised to 55 kilobars and the temperature was then increased to 1350°C and held for 10 minutes. The compact thereby produced was removed from the apparatus. It was found to possess generally similar abrasive properties to the compact of Example 1, except that it produced a smoother finish when machining hard steel workpieces. This was probably caused by its finer particle-size. In contrast, when a pressure of 25 kb was used on similarly prepared 3 micron starting material, the product was significantly weaker. This example demonstrates that high operating pressures may be advantageous when the particle-size of CBN used in the performance of the invention is smaller than about 5 microns.

Claims

1. A method for producing a compact composed mainly of cubic boron nitride (CBN) which comprises: intimately mixing a mass of particulate CBN crystals with a bonding agent in the proportions 95 to 60 volume percent of CBN and 5 to 40 volume percent of bonding agent, said bonding agent containing aluminium and other metal atoms chosen from the group of titanium, zirconium, hafnium, niobium and tantalum atoms in the atomic proportions A1₅₀M₅₀ to A1₉₀M₁₀, where M represents said metal atoms occurring as atoms of one or more of the elements in said group; subjecting the mixture of CBN crystals and bonding agent to temperatures in the range 900 to 1800°C and pressures in the range 5 to 70 kilobars; and maintaining the temperature and pressure conditions on the mixture for a period of at least about 30 seconds, sufficient to cause most of the bonding agent to form a bonding matrix composed of an intimate mixture of an aluminium nitride component and a metallic diboride component comprised of one or more of the metallic diborides MB₂ or a solid solution of aluminium diboride in one or more of those diborides, the aluminium nitride and metallic diboride components each comprising at least 25 volume percent of the bonding matrix.

2. A method as claimed in claim 1, wherein the bonding agent contains said aluminium atoms and metal atoms in the atomic proportions Alg_fj ^ to Alg₅M₁₅.

3. A method as claimed in claim 1, wherein the bonding agent comprises an alloy of aluminium and titanium in the atomic proportion

to Al₈₀Ti₂₀.

4. A method as claimed in claim 1, wherein said bonding agent comprises an alloy of or a mixture of intermetallic compounds of aluminium with any one or more of the metals titanium, zirconium, hafnium, niobium and tantalum.

5. A method as claimed in claim 4, wherein the aluminium and metal atoms in the bonding agent are generally in the overall atomic proportions A1-₅₀M₄₀ to Al^M^.

6. A method as claimed in any one of the preceding claims, wherein said temperatures and pressures are applied in a sequence and over a sufficient time to cause substantial plastic deformation of the CBN crystals.

7. A method as claimed in claim 6, wherein the intimately mixed mass of CBN crystals and bonding agent are subjected initially to an elevated pressure below the maximum pressure to be applied thereto in the method, and subsequently to elevated temperatures of at least 800°C after which the applied pressure is increased to said maximum.

8. A method as claimed in claim 1 , wherein the applied pressure is increased to said maximum over a period of about 3 to 15 minutes.

9. A method as claimed in any one of the preceding claims, wherein the intimately mixed mass of CBN crystals and bonding agent are subjected to a maximum temperature in the range 1200 to 1800°C and a maximum pressure in the range 20 to 40 kilobars.

10. A method as claimed in claim 9, wherein a major part of the volume occupied by the CBN crystals is comprised of crystal particles of more than 5 microns particle size.

11. A method as claimed in any one of claims 1 to 8, wherein the intimately mixed mass of CBN crystals and bonding agent are subjected to a maximum temperature in the range 1200 to 1800°C and to a maximum pressure in the range 40 kilobars to 70 kilobars.

12. A method as claimed in claim 11, wherein the CBN crystals have a mean particle size smaller than 5 microns.

13. A method as claimed in any one of the preceding claims, wherein the metallic diboride component comprises an electrically conductive phase whereby the resulting compact possesses electrical conductivity high enough to permit it to be shaped by electrical discharge machining.

14. A method as claimed in any one of the preceding claims, wherein the proportion of bonding agent is in the range 10 to 25 volume percent.

15. An abrasive compact comprising 40 to 90 volume percent of CBN crystals bonded by 60 to 10 volume percent of a bonding matrix composed mainly of an intimate mixture of an aluminium nitride component and a metallic diboride component comprised of one or more of the metallic diborides MB₂, wherein M represents metal atoms chosen from the group of titanium, zirconium, hafnium, niobium and tantalum atoms occurring as atoms of one or more of the elements in said group, or a solid solution of aluminium diboride in one or more of those diborides, the aluminium nitride and metallic diboride components each comprising at least 25 volume percent of the bonding matrix, said bonding matrix producing a strong and coherent bond between CBN crystals.

16. An abrasive compact as claimed in claim 15, wherein the metallic diboride component comprises said solid solution and the aluminium atoms in that solution number up to 50% of the number of M atoms therein.

17. An abrasive compact as claimed in claim 15 or claim 16, wherein the proportion of bonding matrix is in the range 40 to 15 volume percent.

18. An abrasive compact as claimed in any one of claims 15 to 17 and possessing a compressive strength of at least 10 kilobars.

19. An abrasive compact as claimed in any one of claims 15 to 18, wherein said diboride comprises an electrically conductive phase whereby the compact possesses an electrical conductivity high enough to permit it to be shaped by electrical discharge machining.

20. An abrasive compact as claimed in claim 19, wherein the CBN crystals display substantial plastic deformation at the boundaries between adjacent crystals and the bonding matrix penetrates between adjacent crystals along said boundaries to produce an interconnected electrically conductive network.