WO2015079035A1

WO2015079035A1 - A method of making a powder composition for production of a cubic boron nitride composite material

Info

Publication number: WO2015079035A1
Application number: PCT/EP2014/075993
Authority: WO
Inventors: Nelson Yee; Rui SHAO; Jacob S. PALMER; Torbjörn SELINDER; Malin MÅRTENSSON; Annika Kauppi; Gerold Weinl
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2013-11-29
Filing date: 2014-11-28
Publication date: 2015-06-04

Abstract

The present invention relates to a method of making a powder composition, which is suitable for production of a cBN composite material comprising cBN grains, a metallic binder phase and a ceramic binder phase. The method comprises a step of providing at least one ceramic binder phase forming powder in a milling device. Thereafter the at least one ceramic binder phase forming powder is subjected to a milling operation in the milling device to form a milled powder with 0.1 μm < d50 <0.6 μm, whereby the at least one ceramic binder phase forming powder is/are the only powder component(s) subjected to the milling operation. The milled powder is then mixed with at least cBN powder and aluminium powder to form the powder composition, whereby 0.10 μm < d₅₀≤ 1.40 μm of the aluminium powder. The invention also relates to a method of making a cBN composite material.

Description

A METHOD OF MAKING A POWDER COMPOSITION FOR PRODUCTION OF A CUBIC BORON NITRIDE COMPOSITE MATERIAL

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of making a powder composition, which is suitable for production of a cubic boron nitride (cBN) composite material, and to a powder composition obtainable by the method of making a powder composition. In addition, the present invention relates to a method of making a cBN composite material and to a cBN composite material obtainable by the method of making a cBN composite material.

BACKGROUND

Cubic boron nitride (cBN) composite materials are known as superhard abrasive materials and are commonly utilized in cutting tools for metal machining in order to provide a superhard cutting edge. In particular, cBN composite materials are utilized in cutting tools for hard part machining, e.g. machining of hardened steel and cast iron.

Usually, a cBN composite material for metal machining comprises cBN grains and a metallic binder phase comprising, for example, one or more aluminum compounds. The cBN composite material may also comprise a ceramic binder phase, which may comprise, for example, a nitride, carbide or carbonitride of a Group 4, 5 or 6 transition metal or mixtures thereof. The transition metal may, for example, be titanium.

By varying the components and the relative amounts of the components, cBN composite materials can be designed for optimum performance in different applications, e.g. continuous or interrupted cutting, and in machining of different metals.

Known methods for manufacturing a cBN composite material for metal machining are based on conventional powder metallurgical techniques, which include mixing and milling the raw materials to a powder mixture, forming the powder mixture to a green body and subjecting the green body to a sintering operation at high pressure and high temperature (HPHT sintering) to form a sintered body of a cBN composite material. The sintered body of a cBN composite material can either be formed on a support material of, for example, cemented carbide, or be formed without a support material. The sintered body of a cBN composite material may be cut into a tip which is then brazed to a corner of, for example, a cemented carbide substrate. Alternatively, the sintered body of a cBN composite material can be cut into the shape of an insert and utilized as a solid cBN cutting tool. Still alternatively, in case the cBN composite material is formed without a support material, it may be directly formed into the shape of an insert by the forming and sintering processes.

The inclusion of a ceramic binder phase in a cBN composite material greatly improves the chemical stability, but decreases the toughness. Thus, cBN composite materials comprising a ceramic binder phase suffer from more or less brittle behavior and may exhibit undesirable random breakages in toughness demanding operations such as intermittent hard part turning or milling. The decreased toughness and increased brittleness entail partly as a result of the ceramic binder phase being ceramic, but is also due to imperfect dispersion of the phases, i.e. the cBN grains, the metallic binder phase and the ceramic binder phase, in the material. Imperfect dispersion of the phases, and in particular of the cBN grains, leads to mechanically weak parts, or defects, in the material.

EP 1 831 130 discloses a method of making a powdered composition suitable for the manufacture of a cBN compact (i.e. a body of a cBN composite material). The method comprises attrition milling a powdered secondary hard phase and powdered binder phase to produce a fine mixture of the components. Furthermore, the method comprises adding cBN particles to the fine mixture and attrition milling this mixture to produce a powdered composition. The secondary hard phase consists preferably of a compound containing nitride, carbonitride or carbide of a Group 4, 5 or 6 transition metal, which preferably is titanium. The binder phase consists preferably of aluminum and optionally one or more other elements. The method of EP 1 831 130 is said to result in a highly homogenous distribution of cBN grains and the other phases in a cBN compact. However, there is still room for improvement concerning the dispersion of cBN grains and the other phases in a cBN composite material.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of making a powder composition, which is suitable for production of a cBN composite material, which results in an improved dispersion of all components in the powder composition and which results in an improved dispersion of all phases, and thus an improved toughness and reduced brittleness, of a cBN composite material produced from the powder composition.

This object is achieved with a method according to claim 1.

Preferred embodiments are listed in the dependent claims.

Another object of the present invention is to provide a method of making a cBN composite material which results in an improved dispersion of all phases in the cBN composite material and, thus, an improved toughness and reduced brittleness of the cBN composite material.

This object is achieved with a method according to claim 14.

A further object of the present invention is to provide a powder composition, which comprises cBN powder, aluminium powder and at least one ceramic binder phase forming powder, which is suitable for production of a cBN composite material, which has an improved dispersion of all components in the powder composition and which results in an improved dispersion of all phases, and thus an improved toughness and reduced brittleness, of a cBN composite material produced from the powder composition.

This object is achieved with a powder composition according to claim 15. Still another object of the present invention is to provide a cBN composite material, which comprises cBN grains, a metallic binder phase and a ceramic binder phase and which has an improved dispersion of all phases of the cBN composite material and, thus, an improved toughness and reduced brittleness. This object is achieved with a cBN composite material according to claim 16.

Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

In the drawings:

Figures la and lb show a Scanning Electron Microscope (SEM) image at Ι,ΟΟΟχ magnification of the microstructure of the cBN composite material produced in Example 3A and 3B, respectively.

Figures lc and Id show a SEM image at 20,000x magnification of the microstructure of the cBN composite material produced in Example 3A and 3B, respectively.

DEFINITIONS

The abbreviation "cBN" is herein intended to denote cubic boron nitride, i.e. one of the crystalline forms of boron nitride.

The term "cBN composite material" is herein intended to denote a material comprising cBN grains, a ceramic binder phase and a metallic binder phase. The term "ceramic binder phase forming powder" is herein intended to denote a powder utilized for forming a component of the ceramic binder phase in a cBN composite material.

The term "ceramic binder phase" is herein intended to denote a phase comprising one or more ceramic binder components.

The term "milled powder" is herein intended to denote the powder obtained as a result of the milling step of the method. The milled powder may comprise one or more milled ceramic binder phase forming powders.

The term "powder composition" is herein intended to denote the powder composition obtained as a result of the method of making a powder composition.

The phrase "the total dry powder weight of the powder composition" is herein intended to denote the total dry powder weight of all powder components of the powder composition.

The term "stoichiometric" is herein intended to denote that the atomic ratio between the nonmetallic and the metallic elements in a ceramic binder phase forming compound is close to 1, i.e. between 0.9 and 1.1. For example, this means that when the ceramic binder phase forming compound is stoichiometric TiN, the atomic ratio of N/Ti is > 0.9, but < 1.1, and when the ceramic binder phase forming compound is stoichiometric Ti(C,N), the atomic ratio of (C+N)/Ti is > 0.9, but < 1.1.

The term "substoichiometric" is herein intended to denote that the atomic ratio between the nonmetallic and the metallic elements in a ceramic binder phase forming compound is less than 0.9. For example, this means that when the ceramic binder phase forming compound is substoichiometric TiN, the atomic ratio of N/Ti is < 0.9, and when the ceramic binder phase forming compound is substoichiometric Ti(C,N), the atomic ratio of (C+N)/Ti is < 0.9.

The term "ά₁₀" is herein utilized in description of the particle size distribution of a powder material and is intended to denote a particle diameter. 10% by volume of the particles of the powder material has a particle diameter below the di₀ value. Unless otherwise stated, the dio values mentioned herein refer to the values obtained when measured by Microtrac-S3000 in the range 0.021-1408 μιτι. Distribution mode: volume, Number of channels: 64, Run time: 60 s, Absorption mode, Fluid: water, Flow rate: 60, Ultrasonic powder: 40 watts and Ultrasonic time: 90 s. The term "d₅₀" is herein utilized in description of the particle size distribution of a powder material and is intended to denote a particle diameter. 50% by volume of the particles of the powder material has a particle diameter below the d₅₀ value. Unless otherwise stated, the d₅₀ values mentioned herein refer to the values obtained when measured by Microtrac-S3000 in the range 0.021-1408 μιτι. Distribution mode: volume, Number of channels: 64, Run time: 60 s, Absorption mode, Fluid: water, Flow rate: 60, Ultrasonic powder: 40 watts and Ultrasonic time: 90 s.

The term "d₉₀" is herein utilized in description of the particle size distribution of a powder material and is intended to denote a particle diameter. 90% by volume of the particles of the powder material has a particle diameter below the d₉₀ value. Unless otherwise stated, the d₉₀ values mentioned herein refer to the values obtained when measured by Microtrac-S3000 in the range 0.021-1408 μιτι. Distribution mode: volume, Number of channels: 64, Run time: 60 s, Absorption mode, Fluid: water, Flow rate: 60, Ultrasonic powder: 40 watts and Ultrasonic time: 90 s.

The term "d₉₇" is herein utilized in description of the particle size distribution of a powder material and is intended to denote a particle diameter. 97% by volume of the particles of the powder material has a particle diameter below the d₉₇ value. Unless otherwise stated, the d₉₇ values mentioned herein refer to the values obtained when measured by Microtrac-S3000 in the range 0.021-1408 μιτι. Distribution mode: volume, Number of channels: 64, Run time: 60 s, Absorption mode, Fluid: water, Flow rate: 60, Ultrasonic powder: 40 watts and Ultrasonic time: 90 s. The term "d₉₉" is herein utilized in description of the particle size distribution of a powder material and is intended to denote a particle diameter. 99% by volume of the particles of the powder material has a particle diameter below the d₉₉ value. Unless otherwise stated, the d₉₉ values mentioned herein refer to the values obtained when measured by Microtrac-S3000 in the range 0.021-1408 μιτι. Distribution mode: volume, Number of channels: 64, Run time: 60 s, Absorption mode, Fluid: water, Flow rate: 60, Ultrasonic powder: 40 watts and Ultrasonic time: 90 s.

The term "specific surface area" is herein intended to denote the specific surface area of powder particles. Unless otherwise state, the specific surface area values mentioned herein refer to values obtained by using BET specific surface area measurement by Monosorb™, Quantachrome Instruments, in liquid nitrogen. The term "slurry" is herein intended to denote a fluid mixture comprising one or more powder components, milling liquid and optionally a dispersion agent, a pH-adjuster and an organic binder. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of making a powder composition, which is suitable for production of a cBN composite material comprising cBN grains, a metallic binder phase and a ceramic binder phase, the method comprising the steps of:

providing at least one ceramic binder phase forming powder in a milling device;

subjecting said at least one ceramic binder phase forming powder to a milling operation in said milling device to form a milled powder with 0.1 μιτι < d₅₀ < 0.6 μιτι, said at least one ceramic binder phase forming powder being the only powder component(s) subjected to said milling operation, and

- mixing said milled powder with at least cBN powder and a metallic binder phase forming powder to form said powder composition, whereby said metallic binder phase forming powder is aluminum powder and whereby 0.10 μιτι < d₅₀ < 1.40 μιτι, preferably 0.10 μιτι < d₅₀ < 1.20 μιτι, most preferably 0.10 μιτι < d₅₀ < 0.90 μιτι, of said aluminum powder (i.e. of the provided raw material aluminum powder).

Thus, 0.1 μιτι < d₅₀ < 0.6 μιτι of the milled powder, i.e. the at least one ceramic binder phase forming powder is milled such that 0.1 μιτι < d₅₀ < 0.6 μιτι of the resulting milled powder. Preferably, 0.2 μιτι < d₅₀ < 0.4 μιτι of the milled powder.

In one embodiment, the at least one ceramic binder phase forming powder is milled such that 0.6 μιτι < d₉₀ < 1.6 μιτι of the resulting milled powder. Preferably, 0.7 μιτι < d₉₀ < 1.4 μιτι of the resulting milled powder. In one embodiment, the at least one ceramic binder phase forming powder is milled such that 0.7 μιτι < d₉₅ < 1.9 μιτι of the resulting milled powder. Preferably, 0.9 μιτι < d₉₅ < 1.7 μιτι of the resulting milled powder.

For the aluminum powder (i.e. the provided raw material aluminum powder), preferably 0.5μιτι < d₉₀ < 5μιτι, most preferably Ο.δμιτι < d₉₀ < 3μιτι, even more preferably Ιμιτι < d₉₀ < 2μιτι. Thus, the method of making a powder composition according to the present invention comprises a milling step in which only one or more ceramic binder phase forming powders are milled in a milling operation to form a milled powder, i.e. no further powder components are milled together with the one or more ceramic binder phase forming powders in the milling step.

Furthermore, the method of making a powder composition according to the present invention comprises a mixing step, which is subsequent to the milling step and in which the milled powder is mixed with at least cBN powder and aluminum powder. The aluminum powder constitutes a metallic binder phase forming powder.

The aim of the milling step is to substantially reduce the particle size of the milled component(s), whereas the aim of the mixing step is not to mill the components but to mix the components and obtain a powder mixture (mixed powder composition).

In one embodiment of the present invention, the milling in the milling step is performed such that d₅₀ of the dry milled powder obtained after the milling operation is reduced >50%, preferably >70%, more preferably >75%, even more preferably >80%, still more preferably >90%, compared to d₅₀ of a representative powder sample of the ceramic binder phase forming powder(s) intended to be milled before start of the milling operation.

As mentioned above, in the milling step according to the method of the present invention at least one ceramic binder phase forming powder is subjected to a milling operation to form a milled powder with 0.1 μιτι < d₅₀ < 0.6 μιτι. Ceramic binder phase forming powders with 0.1 μιτι < d₅₀ < 0.6 μιτι are unstable and oxidized, i.e. an oxide layer is formed on the surface of the powder particles, upon storage. In addition, ceramic binder phase forming powders with 0.1 μιτι < d₅₀ < 0.6 μιτι reagglomerate upon storage. Further disadvantages connected to storage of ceramic binder phase forming powders with 0.1 μιτι < d₅₀ < 0.6 μιτι are water uptake and risk of autoignition. Accordingly, ceramic binder phase forming powders with 0.1 μιτι < d₅₀ < 0.6 μιτι are unsuitable to store.

Thus, by the inclusion of the milling step in the method of the present invention, the above mentioned disadvantages connected to the storage of ceramic binder phase forming powders with 0.1 μιτι < d₅₀ < 0.6 μιτι are avoided. The time period between the end of the milling step and the start of the mixing step in the method according to the invention should be minimized and should be kept as short as possible to avoid reagglomeration. Preferably, the time period between the end of the milling step and the start of the mixing step in the method according to the invention should not be longer than one hour. According to the invention the mixing step is performed such that no milling of the components is obtained in the mixing step or is performed such that a substantially reduced milling effect of the components is obtained in the mixing step compared to the milling effect obtained in the milling step.

As will be further described below in different embodiments, the mixing step of the method according to the invention may comprise one step of mixing the milled powder (i.e. the milled ceramic binder phase forming powder(s)) with at least cBN powder and aluminum powder. Alternatively, the mixing step may comprise a first sub-mixing step of mixing the milled powder with cBN powder and any optional further component(s) to form an intermediate powder composition and a second sub-mixing step of mixing the intermediate powder composition with aluminum powder and any optional further component(s).

When the mixing step comprises one step of mixing the milled powder with at least cBN powder and aluminum powder, it is by "mixing" in the mixing step herein meant that d₅₀ of the mixed dry powder composition obtained after the mixing operation is not reduced or is reduced <15%, preferably <10%, more preferably <8%, even more preferably <5%, still more preferably <2%, compared to d₅₀ of a representative powder sample of the powders intended to be mixed before start of the mixing operation (i.e. before start of the mixing action of the utilized mixing device) and that the specific surface area of the mixed dry powder composition obtained after the mixing operation is not increased or is increased <30%, preferably <20%, more preferably <16%, even more preferably <10%, still more preferably <4%, compared to the specific surface area of a representative powder sample of the powders intended to be mixed before start of the mixing operation (i.e. before start of the mixing action of the utilized mixing device).

When the mixing step comprises a first sub-mixing step and a second sub-mixing step in accordance with the above, it is by "mixing" in the first sub-mixing step herein meant that d₅₀ of the mixed dry intermediate powder composition obtained after the mixing operation of the first sub-mixing step is not reduced or is reduced <15%, preferably <10%, more preferably <8%, even more preferably <5%, still more preferably <2%, compared to d₅₀ of a representative powder sample of the powders intended to be mixed before start of the mixing operation in the first sub-mixing step (i.e. before start of the mixing action of the utilized mixing device) and that the specific surface area of the mixed dry intermediate powder composition obtained after the mixing operation of the first sub-mixing step is not increased or is increased <30%, preferably <20%, more preferably <16%, even more preferably <10%, still more preferably <4%, compared to the specific surface area of a representative powder sample of the powders intended to be mixed before start of the first sub-mixing operation (i.e. before start of the mixing action of the utilized mixing device). Likewise, it is by "mixing" in the second sub-mixing step herein meant that d₅₀ of the mixed dry powder composition obtained after the mixing operation of the second sub- mixing step is not reduced or is reduced <15%, preferably <10%, more preferably <8%, even more preferably <5%, still more preferably <2%, compared to d₅₀ of a representative powder sample of the powders intended to be mixed before start of the mixing operation in the second sub-mixing step (i.e. before start of the mixing action of the utilized mixing device) and that the specific surface area of the mixed dry powder composition obtained after the mixing operation of the second sub-mixing step is not increased or is increased <30%, preferably <20%, more preferably <16%, even more preferably <10%, still more preferably <4%, compared to the specific surface area of a representative powder sample of the powders intended to be mixed before start of the second sub-mixing operation (i.e. before start of the mixing action of the utilized mixing device).

It was surprisingly found that an improved distribution/dispersion of the components of a powder composition and the phases in a cBN composite material produced from the powder composition may be obtained by a combination of the features that only the one or more ceramic binder phase forming powders are milled to a milled powder in the milling step, that further components including at least cBN powder and aluminum powder are mixed with the milled powder in a subsequent mixing step without using milling bodies or with a substantially reduced milling effect compared to the milling effect of the milling step, and that 0.10 μιτι < d₅₀ < 1.40 μιτι of the aluminum powder. It was realized that aluminum is difficult to mill down since it is soft and ductile and that aluminum often undergoes cold rolling and smearing of the surfaces of the milling bodies. Furthermore, it was realized that aluminum might coagulate resulting in the formation of lumps. By milling only the one or more ceramic binder phase forming powders in the milling step, i.e. by not milling the ceramic binder phase forming powder(s) together with aluminum, it is avoided that aluminum sticks to the surface of the milling bodies and that lumps of aluminum are formed during the milling step. Thus, by milling only the one or more ceramic binder phase forming powders in a milling step to a milled powder and subsequently mixing the milled powder with cBN powder and aluminum powder without using milling bodies or with a substantially reduced milling effect, a powder composition with a substantial reduction of the amount of lumps of aluminum and aluminum stuck to the surface of the milling bodies may be obtained. Thereby an improved distribution of the aluminum in the powder composition is obtained and accordingly an improved distribution of the other components in the powder composition. Consequently, an improved distribution of all phases in a cBN composite material formed from the powder composition may also be obtained.

Furthermore, the fact that 0.10 μιτι < d₅₀ < 1.40 μιτι of the aluminum powder further contributes to the improvement of the distribution of the aluminum in the powder composition. Aluminum powder with 0.10 μιτι < d₅₀ < 1.40 μιτι comprises small spheres with a hard native coating of aluminum oxide, which are much less prone to stick together and form lumps than aluminum powder having particles with a greater particle size. In addition, by milling only the one or more ceramic binder phase forming powders in the milling step a more efficient milling (i.e. size reduction) of the ceramic binder phase forming powder(s) is achieved. It was realized that aluminum powder inhibits the milling effect on a ceramic binder phase forming powder if milled together with the ceramic binder phase forming powder. The inhibition of the milling effect is due to the formation of lumps of aluminum and sticking of aluminum to the milling bodies.

Furthermore, by not performing any milling of the aluminum powder in the milling step it is avoided that free metal surfaces are created such that aluminum oxidizes and/or a hydrogen gas evolution is obtained in the milling device during milling.

In one embodiment of the present invention, each ceramic binder phase forming powder is a powder of a compound selected from the group consisting of nitrides, carbides, carbonitrides, oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 (according to the new lUPAC format) transition metal. In another embodiment of the present invention, each ceramic binder phase forming powder is a powder of a compound selected from the group consisting of nitrides, carbides, and carbonitrides of a Group 4, 5 or 6 (according to the new lUPAC format) transition metal. In a further embodiment of the present invention, each ceramic binder phase forming powder is a powder of a compound selected from the group consisting of oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 (according to the new lUPAC format) transition metal.

The nitrides, carbides and carbonitrides of a Group 4, 5 or 6 transition metal may be stoichiometric or substoichiometric. The oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 transition metal may be stoichiometric.

The transition metal is preferably titanium.

In one embodiment of the present invention, at least one ceramic binder phase forming powder is a powder of a compound selected from the group consisting of TiN, TiC and Ti(C,N), which are stoichiometric or substoichiometric, and Ti(C_xN_yO_z)_a, which is stoichiometric. In another embodiment of the present invention, at least one ceramic binder phase forming powder is a powder of a compound selected from the group consisting of TiN, TiC and Ti(C,N), which are stoichiometric or substoichiometric. In a further embodiment of the present invention, at least one ceramic binder phase forming powder is a powder of a compound according to the formula Ti(C_xN_yO_z)_a, which is stoichiometric.

The composition of the Ti(C_xN_yO_z)_a powder is suitably so that 0≤x<0.95, preferably 0.01≤x<0.95, more preferably 0.3≤x<0.95, most preferably 0.5≤x<0.95, suitably 0≤y<0.95, preferably 0.01≤y<0.95, more preferably 0.01≤y<0.5, most preferably 0.01≤y<0.3 and suitably 0.05≤z<0.4, preferably 0.05≤z<0.3, most preferably 0.1≤z<0.3. For the Ti(C_xN_yO_z)_a powder 0.9<a≤l.l, preferably 0.95≤a<1.05. The at least one ceramic binder phase forming powder is preferably provided such that the content of the ceramic binder phase forming powder(s) in the powder composition is between 10 and 80 wt% of the total dry powder weight of the powder composition.

In one embodiment of the present invention, substoichiometric TiN or substoichiometric Ti(C,N) or mixtures thereof are utilized as ceramic binder phase forming powder(s).

In one embodiment of the present invention, substoichiometric TiN or Ti(C,N) and stoichiometric TiN or Ti(C,N) are utilized as ceramic binder phase forming powders. In one embodiment of the present invention, a Ti(C_xN_yO_z)_a powder having a high carbon content, i.e. 0.7≤x<0.95, and a stoichiometric TiN powder are utilized as ceramic binder phase forming powders.

In one embodiment of the present invention, a Ti(C_xN_yO_z)_a powder having a high nitrogen content, i.e. 0.7≤y<0.95, and a stoichiometric TiC powder are utilized as ceramic binder phase forming powders.

In one embodiment of the present invention, a Ti(C_xN_yO_z)_a powder having a nitrogen and carbon content of 0.3≤x<0.69 and 0.3≤y<0.69 and a substoichiometric Ti(C,N) powder and/or substoichiometric TiN powder are utilized as ceramic binder phase forming powders. The cBN powder is preferably provided such that the cBN content in the powder composition is between 20 and 79 wt% of the total dry powder weight of the powder composition. Preferably, 0.5 μιτι < d₅₀ < 10 μιτι of said cBN powder (i.e. of the provided raw material cBN powder). Furthermore, the cBN may have a unimodal or multimodal (e.g. bimodal) particle size distribution.

The aluminum powder is preferably provided such that the aluminum content in the powder composition is between 1 and 10 wt%, more preferably between 3 and 9 wt%, most preferably between 4 and 8 wt% of the total dry powder weight of the powder composition. Furthermore, the aluminum powder may have a unimodal or multimodal (e.g. bimodal) particle size distribution. The step of mixing the milled powder with at least cBN powder and aluminum powder may involve mixing of the milled powder with only cBN powder and aluminum powder (i.e. with no further powder components). Alternatively, this step may involve mixing the milled powder with cBN powder, aluminum powder and one or more further components common in the art of making cBN composite materials such as, for example, elements of group 4, 5 and/or 6, i.e. for example Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo. Preferably, the total content of the further components in the powder composition is less than 3 wt% of the total dry powder weight of the powder composition.

In one embodiment of the present invention, the cBN powder is provided such that the cBN content in the powder composition is between 20 and 40 wt% of the total dry powder weight of the powder composition, the at least one ceramic binder phase forming powder is provided such that the content of ceramic binder phase forming powder(s) in the powder composition is between 52 and 76 wt% of the total dry powder weight of the powder composition and the aluminum powder is provided such that the aluminum content in the powder composition is between 4 and 8 wt% of the total dry powder weight of the powder composition.

In one embodiment of the present invention, the cBN powder is provided such that the cBN content in the powder composition is between 41 and 60 wt% of the total dry powder weight of the powder composition, the at least one ceramic binder phase forming powder is provided such that the content of ceramic binder phase forming powder(s) in the powder composition is between 32 and 55 wt% of the total dry powder weight of the powder composition and the aluminum powder is provided such that the aluminum content in the powder composition is between 4 and 8 wt% of the total dry powder weight of the powder composition.

In one embodiment of the present invention, the cBN powder is provided such that the cBN content in the powder composition is between 61 and 79 wt% of the total dry powder weight of the powder composition, the at least one ceramic binder phase forming powder is provided such that the content of ceramic binder phase forming powder(s) in the powder composition is between 13 and 35 wt% of the total dry powder weight of the powder composition and the aluminum powder is provided such that the aluminum content in the powder composition is between 4 and 8 wt% of the total dry powder weight of the powder composition.

The step of providing at least one ceramic binder phase forming powder of the method according to the invention may comprise providing the at least one ceramic binder phase forming powder in a milling liquid so that a slurry is formed before the milling step. Then the milling step comprises milling the at least one ceramic binder phase forming powder in the milling liquid. Thereby, the milled powder, which is formed as a result of the milling step, is comprised in the slurry. Furthermore, the mixing step then comprises mixing the milled powder with at least the cBN powder and the aluminum powder in the slurry. Thereby, the powder composition, which is formed as a result of the mixing step, is formed in the slurry. In order to obtain a dry powder composition, the mixing step is then followed by a drying step.

The milling liquid is preferably water, an alcohol or an organic solvent, more preferably an alcohol mixture, most preferably ethanol or a mixture of ethanol and water. The properties of the slurry are dependent on, among other things, the amount of milling liquid added. Since drying of the slurry requires energy the amount of milling liquid should be minimized in order to keep costs down. However, depending on the utilized milling device, it might be necessary to add such an amount of milling liquid so that a pumpable slurry is obtained and so that clogging of the pump system of the milling device is avoided.

Also, other compounds commonly known in the art can be added to the slurry, e.g. dispersion agents, pH-adjusters, etc. Furthermore, an organic binder such as e.g. polyethylene glycol (PEG) or wax is preferably added to the slurry in order to facilitate formation of granules during the drying step and to act as a pressing agent during pressing of the powder composition to a green body.

In the drying step the slurry is dried according to any known drying technique in the art, e.g. spray drying or freeze drying. Preferably, spray drying is utilized. Then the slurry containing the powder composition, the milling liquid and possibly other components as mentioned above is atomized through an appropriate nozzle in a drying tower where the small drops are instantaneously dried by a stream of hot gas, for instance a stream of nitrogen, to form spherical powder granules with good flow properties. The granules may range from 2 to 150 μιτι, preferably 20 to 100 μιτι, in diameter.

The milling device may be any suitable milling device known in the art, such as e.g. an attritor mill having an agitator unit arranged to rotate around a vertical axis or a horizontal axis, a pearl mill, a ball mill, a roll mill, a planetary mill or a basket mill. Preferably, the milling device is an attritor mill having an agitator unit arranged to rotate around a vertical axis. Furthermore, the milling device may be provided with a pumping arrangement for circulation of the slurry.

The choice of the type of milling device may depend on the chemistry, morphology and final particle size of the milled powder to be achieved. Energy input, size of the milling device and chemistry of lining in the milling device will influence the chemical and physical properties of the milled powder. However, independently of the type of milling device utilized for milling in the step of milling at least one ceramic binder phase forming powder, the method according to the invention will result in an improved distribution of the components of the formed powder composition.

The utilized milling bodies may be composed of any suitable material such as e.g. a cemented carbide, a cermet, or ceramics and may have a size in the range of 1-10 mm, preferably 1-6 mm.

When milling bodies of cermets or other W and Co containing material are utilized, W and Co will be added to the ceramic binder phase forming powder(s) during milling and mixing (in case milling bodies are present during the mixing as will be further described below) as a residue from the milling bodies. Then the composition of the milling bodies, the milling time and the mixing time need to be designed to give the desired amount of W and Co addition. Preferably, the milling bodies, the milling time and mixing time are designed such that the resulting content of W plus Co in the powder composition is between 4 and 20 wt% of the total dry powder weight of the powder composition and such that the ratio between W and Co is 1.0 to 2.0 in the powder composition.

The mixing step (i.e. the step of mixing the milled powder with at least cBN powder and an aluminium powder to form the powder composition) may be performed in the milling device used for the milling step. Alternatively, the mixing step may be performed in a separate mixing device or in another milling device. Furthermore, the mixing step may include a first sub-mixing step and a second sub-mixing step. Then the first sub-mixing step comprises mixing the milled powder (i.e. the milled one or more ceramic binder phase forming powders) and at least the cBN powder so as to form an intermediate powder mixture. The second sub-mixing step comprises then mixing the intermediate powder mixture with at least the aluminum powder to form the powder composition. The first and second sub-mixing steps may both be performed in the milling device. Alternatively, the first sub-mixing step is performed in the milling device, whereas the second sub-mixing step is performed in a separate mixing device. In another alternative, the first and second sub-mixing steps may both be performed in a separate mixing device. In a further alternative, the first sub-mixing step is performed in one separate mixing device, whereas the second sub-mixing step is performed in another separate mixing device. The separate mixing device may be, for example, a roll mill or any other suitable mill, an ultrasonic device, a mechanical stirring device, e.g. a propeller, or a paint shaker.

In one embodiment of the present invention, the mixing step (i.e. the step of mixing the milled powder with at least cBN powder and an aluminium powder to form the powder composition) includes:

adding the cBN powder, the aluminum powder and optionally one or more further components to the milled powder in the milling device, and subjecting the milled powder, the cBN powder, the aluminum powder and any optional one or more further components to a mixing operation in the milling device so as to form the powder composition.

Thus, in this embodiment both the milling step and the mixing step are performed in the same milling device, whereby the milling bodies are present in the milling device both during milling and mixing. The mixing may then be performed by reducing the rotational speed of the milling device in the mixing step compared to the rotational speed utilized in the milling step so as to obtain a mixing as defined previously herein. Alternatively, when d₅₀ of the milled powder is <30%, or <20%, or <10%, of d₅₀ of the cBN powder, the mixing may be performed by utilizing a similar or reduced rotational speed of the milling device in the mixing step, compared to the rotational speed utilized in the milling step, so as to obtain a mixing as defined previously herein. Since the particles of the milled powder then are much smaller than the cBN particles, the milling effect on the milled powder is minimal in the mixing step independent of the rotational speed of the milling device, i.e. the milling effect on the milled powder is minimal in the mixing step even though the same or a similar rotational speed of the milling device is utilized as in the milling step. In another embodiment of the present invention, the mixing step includes:

adding the cBN powder and optionally one or more further components to the milled powder in the milling device;

- subjecting said milled powder, the cBN powder and any optional one or more further components to a first sub-mixing operation in the milling device to form an intermediate powder mixture;

adding the aluminum powder and optionally one or more further components to the intermediate powder mixture in the milling device, and

- subjecting the aluminum powder, the intermediate powder mixture and any optional one or more further components to a second sub-mixing operation in the milling device so as to form the powder composition.

Thus, in this embodiment both the milling step and the mixing step are performed in the same milling device. However, the mixing step is divided into two sub-mixing steps, i.e. a first sub-mixing step and a second sub-mixing step. The mixing may then be performed by reducing the rotational speed of the milling device, compared to the rotational speed utilized in the milling step, in both the first and second sub-mixing steps so as to obtain a mixing as previously defined herein. Alternatively, when d₅₀ of the milled powder is <30%, or <20%, or <10%, of d₅₀ of the cBN powder, the mixing may be performed by utilizing a similar or reduced rotational speed of the milling device in the first sub-mixing step, compared to the rotational speed utilized in the milling step, so as to obtain a mixing as defined previously herein, and a reduced rotational speed of the milling device, compared to the rotational speed utilized in the milling step, in the second sub-mixing step so as to obtain a mixing as defined previously herein. Since the particles of the milled powder are much smaller than the cBN particles then, the milling effect on the milled powder is minimal in the first sub-mixing step independent of the rotational speed of the milling device, i.e. the milling effect on the milled powder is minimal in the first sub-mixing step even though the same or a similar rotational speed of the milling device is utilized in the first sub-mixing step as in the milling step. Furthermore, the first sub-mixing operation may be performed 80-95% of the total mixing time of the method and the second sub-mixing operation may be performed 5-20% of the total mixing time of the method. By only mixing aluminium powder with the other components during a small part of the total mixing time of the method, the risk of formation of lumps of aluminium is further reduced. In addition, the risk of aluminium oxidation and hydrogen gas evolution is reduced.

In still another embodiment of the present invention, the mixing step includes:

adding the cBN powder and optionally one or more further components to the milled powder in the milling device; subjecting the milled powder, the cBN powder and any optional one or more further components to a first sub-mixing operation in the milling device to form an intermediate powder mixture;

providing the intermediate powder mixture, the aluminum powder and optionally one or more further components in a separate mixing device; and subjecting the intermediate powder mixture, the aluminum powder and any optional one or more further components to a second sub-mixing operation in the mixing device so as to form the powder composition.

Thus, in this embodiment the mixing step is divided into two sub-mixing steps. The first sub-mixing step is performed in the milling device and the second sub-mixing step is performed in a separate mixing device. Accordingly, the intermediate powder mixture is transferred from the milling device to the separate mixing device after the first sub-mixing operation. When the milling is performed in a slurry, the slurry comprising the intermediate powder mixture is transferred to the separate mixing device. The milling bodies are not transferred to the separate mixing device. In this embodiment, the first sub- mixing operation performed in the milling device may be performed by reducing the rotational speed of the milling device compared to the rotational speed utilized in the milling step so as to obtain a mixing as defined previously herein. Alternatively, when d₅₀ of the milled powder is <30%, or <20%, or <10%, of d₅₀ of the cBN powder, the mixing may be performed by utilizing a similar or reduced rotational speed of the milling device in the first sub-mixing step, compared to the rotational speed utilized in the milling step, so as to obtain a mixing as defined previously herein. Since the particles of the milled powder are much smaller than the cBN particles then, the milling effect on the milled powder is minimal in the first sub-mixing step independent of the rotational speed of the milling device, i.e. the milling effect on the milled powder is minimal in the first sub-mixing step even though the same or a similar rotational speed of the milling device is utilized as in the milling step. In the second sub-mixing step performed in the separate mixing device there are no milling bodies present, i.e. there is no, or essentially no, milling effect in the second sub-mixing step. Furthermore, the first sub-mixing operation may be performed 80-95% of the total mixing time of the method and the second sub-mixing operation may be performed 5-20% of the total mixing time of the method. By only mixing aluminium powder with the other components during a small part of the total mixing time of the method, the risk of formation of lumps of aluminium is further reduced. In addition, the risk of aluminium oxidation and hydrogen gas evolution is reduced.

In a further embodiment of the present invention, the mixing step includes:

providing the milled powder, the cBN powder, the aluminum powder and optionally one or more further components in a separate mixing device; subjecting the milled powder, the cBN powder, the aluminum powder and any optional one or more further components to a mixing operation in the mixing device so as to form the powder composition.

Thus, in this embodiment the milling step is performed in the milling device and the mixing step is performed in a separate mixing device. Accordingly, the milled powder is transferred from the milling device to the separate mixing device after the milling operation. When the milling is performed in a slurry, the slurry comprising the milled powder is transferred to the separate mixing device. The milling bodies are not transferred to the separate mixing device.

In another embodiment of the present invention, the mixing step includes:

providing the milled powder, the cBN powder and optionally one or more further components in a separate mixing device;

subjecting the milled powder, the cBN powder and any optional one or more further components to a first sub-mixing operation in the mixing device to form an intermediate powder mixture;

adding the aluminum powder and optionally one or more further components to the intermediate powder mixture in the mixing device, and

subjecting the aluminum powder, the intermediate powder mixture and any optional one or more further components to a second sub-mixing operation in the mixing device so as to form the powder composition.

Thus, in this embodiment the milling step is performed in the milling device and the mixing step, which is divided into a first sub-mixing step and a second sub-mixing step, is performed in a separate mixing device. Accordingly, the milled powder is transferred from the milling device to the separate mixing device after the milling operation. When the milling is performed in a slurry, the slurry comprising the milled powder is transferred to the separate mixing device. The milling bodies are not transferred to the separate mixing device. Furthermore, the first sub-mixing operation may be performed 80-95% of the total mixing time of the method and the second sub-mixing operation may be performed 5-20% of the total mixing time of the method. By only mixing aluminium powder with the other components during a small part of the total mixing time of the method, the risk of formation of lumps of aluminium is further reduced.

The present invention relates also to a powder composition obtainable by the method of making a powder composition according to the invention.

Furthermore, the present invention relates to a method of making a cBN composite material comprising the following steps: making a powder composition according to the method of making a powder composition according to the invention;

forming the powder composition to a green body, and

subjecting the green body to a high pressure and high temperature operation to form said cBN composite material.

The forming step comprises forming a green body of the powder composition using conventional techniques such as cold tool pressing technology including MAP (multi axial pressing), extruding or MIM (metal injection moulding), cold isostatic pressing, tape casting and other methods known in the powder metallurgy art. Forming yields a green density and/or strength that permit easy handling and green machining.

In one embodiment of the present invention, the forming is done by a pressing operation. Preferably, the pressing is done by a uniaxial pressing operation at a force, for example, from 5 to 40 ton.

After the forming step, the green body may be subjected to an elevated temperature for organic binder removal. Preferably, this is done in the same apparatus as the pre-sintering. The pre-sintering will be further described below. Suitable temperatures for organic binder removal are between 100 °C and 450 °C in hydrogen atmosphere. The temperature is dependent on the type of organic binder used.

The green body is thereafter pre-sintered at a temperature T, where T is between about 650 °C to about 950 °C, preferably between about 700 °C to about 950 °C, more preferably between about 850 °C to about 930 °C, depending on the composition.

In one embodiment of the present invention, the temperature may be increased at a rate of about l°C/min to about 10°C/min up to the desired pre-sintering temperature. The temperature may be maintained for about 1 to about 90 minutes until the entire charge of bodies in the sintering furnace has reached the desired temperature. The pre-sintering step may be conducted in vacuum, or in a reactive or non-reactive atmosphere e.g. N₂, Ar or a carbon containing gas.

After the pre-sintering, the green body is sintered by subjecting the green body to a high pressure and high temperature (HPHT) operation. For example, this may be performed at 25 to 75 kbar, preferably 35 to 50 kbar, at temperatures between from 1300 to 1600°C to form a body of a cBN composite material. The body of a cBN composite material can either be formed on a support material of, for example, cemented carbide (i.e. "a carbide back"), or be formed without a support material. In case the cBN composite material is formed on a support material the green body is pressed and sintered in the HPHT process together with the support material. The formed body of a cBN composite material will then be attached to the support material during the HPHT sintering. Elements in the support material may then diffuse into the cBN composite material. For example, Co, W, Cr and C may diffuse into the cBN composite material in case the support material is of cemented carbide. Furthermore, the sintered body of a cBN composite material may be cut into a tip which is then brazed to a corner of a substrate, e.g. an insert of cemented carbide. Alternatively, the sintered body of a cBN composite material can be cut into the shape of an insert and utilized as a solid cBN cutting tool. Still alternatively, in case the cBN composite material is formed without a support material, it may be directly formed into the shape of an insert by the forming and sintering processes.

The body of a cBN composite material may be coated with a wear resistant coating comprising single or multiple layers of at least one carbide, nitride, carbonitride, oxide or boride of at least one element selected from Si, Al and the groups 4, 5 and 6 of the periodic table by known CVD-, PVD- or MT-CVD-techniques.

The present invention relates also to a cBN composite material obtainable by the method of making a cBN composite material. The cBN composite material comprises cBN grains, a metallic binder phase comprising one or more aluminum compounds, such as Al₂0₃ and AIN, and a ceramic binder phase. The ceramic binder phase comprises one or more ceramic binder components depending on the use of one or more ceramic binder phase forming powders.

Each utilized ceramic binder phase forming powder being a powder of a compound of the group of nitrides, carbides, and carbonitrides of a Group 4, 5 or 6 (according to the new lUPAC format) transition metal form a binder phase component in the cBN composite material. For example, when substoichiometric TiC, TiN and/or TiCN are utilized as ceramic binder phase forming powder(s), the structure of the cBN composite material comprises cBN grains, a ceramic binder phase comprising TiC, TiN and/or TiCN and a metallic binder phase comprising aluminium-containing compounds such as e.g. Al₂0₃ and/ or AIN embedded in the phase comprising TiC, TiN and/or TiCN. The aluminium-containing compounds can be found as isolated islands in the phase comprising TiC, TiN and/or TiCN. Depending on the utilized milling bodies, W and/or Co in the form of WC-Co and/or W-Co islands (debris from milling bodies) might also be present. Furthermore, each utilized ceramic binder phase forming powder being a powder of a compound selected from the group consisting of oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 (according to the new lUPAC format) transition metal is transformed to a binder phase component during the pre-sintering/HPHT treatment. A reaction of Al to Al₂0₃ takes place then. For example, when one or more Ti(C_xN_yO_z)_a powder(s) is/are utilized as ceramic binder phase forming powder(s), the structure of the cBN composite material comprises cBN grains, a ceramic binder phase comprising TiC, TiN and/or TiCN and a metallic binder phase comprising aluminium-containing compounds such as e.g. Al₂0₃ and/ or AIN. The type of ceramic binder phase depends on the composition of the one or more Ti(C_xN_yO_z)_a powder(s) used. The islands of aluminium- containing compounds are very few. Instead the aluminium-containing compounds can be found in a phase that surrounds the TiC, TiN and/or TiCN grains. Depending on the utilized milling bodies, W and/or Co (milling debris) can also be present in the structure. However, the islands comprising W and/or Co are very few. Instead, W and/or Co can be found in a phase that surrounds the TiC, TiN and/or TiCN grains.

A body of a cBN composite material according to the present invention may be used as a cutting tool. By that is herein meant that the body of the cBN composite material either constitutes a whole cutting tool, e.g. an insert, or a smaller piece, e.g. a tip, fastened to a cutting tool insert, possibly of cemented carbide.

Cutting tools produced from a body of the cBN composite material according to the present invention have particular application in machining of e.g. hardened steel or cast iron, but may also be used in toughness demanding operations such as intermittent turning or milling.

EXAMPLES Example 1A

1 kg of substoichiometric TiN (ssTiN) was added to 0.75 dm³ of 91% ethanol in water. The ssTiN utilized was TiN₀.₇₂. The slurry was intensively pre-mixed in a commercial paint shaker for 60 s to break up soft agglomerates before milling in a Labstar LMZ pearl mill together with 4400 g of spherical milling bodies with a diameter of 1.7 mm of a 10% Co WC grade. After the intense pre-mixing and deagglomeration, TiN had an average particle size of 2.440 μιτι, a d₅₀ of 0.487 μιτι, a d₉₀ of 4.208 μιτι, a d₉₅ of 11.660 μιτι and a d₉₇ of 22.20 μιτι.

The rotational speed during milling was 1275 rpm and the pump speed was 170 l/min and the pressure of the milling balls was 0.2 bar. At start the dry-content was about 62 wt% but more ethanol was added during the milling to keep the viscosity and volume constant. During milling the temperature was increased from 40 to 43 °C.

When the milling was completed after 90 minutes the viscosity was considered higher than normal. Table la shows the average particle size, di₀, d₅₀, d₉₀, d₉₅ and d₉₇ versus milling time at 1275 rpm at constant volume measured using Microtrac S-3000 in the range 0.021- 1408 μιτι.

Table la

The pearl mill was very effective and after only 15 min milling the slurry was milled to the desired particle size d₉₇< 4 μιτι, but to be able to study the particle size distribution versus milling time the milling continued to 90 minutes and slurry samples were taken after 30, 60 and 90 minutes, respectively. The final over-milled and partly re-agglomerated slurry were poured into a plastic bottle that was sealed. Several days later the slurry properties was the same and cBN and Al could be added to it in order to make a cBN composite powder. Example IB

1 kg of substoichiometric TiN and 88 g of Al-powder having an average particle size of about 5.4 μιτι and a d₅₀ of 4.8 μιτι were added to 0.75 dm³ of 91% ethanol in water. The ssTiN utilized was TiN₀.₇₂ from the same batch as described in Example 1A. The slurry was intensively pre-mixed in a commercial paint shaker for 60 s to break up soft agglomerates before addition to the Labstar LMZ pearl mill together with 4400 g of spherical milling bodies with a diameter of 1.7 mm of a 10% Co WC grade. During milling the rotational speed was 1275 rpm and the pump speed was 44 l/min and the pressure of the milling balls was 0.2 bar. At start the dry-content was about 62 wt% but more ethanol was added during the milling to keep the viscosity and volume constant. During milling the temperature was increased from 30 °C to 46 °C.

From start the slurry was very in-homogenous and contained mm sized lumps which still were present after 15 minutes of milling, se Table lb. The lumps disappeared and d₉₇ and d₉₀ decreased as the milling continued, but after 45 minutes the viscosity started to increase rapidly and the milling was stopped after 60 minutes despite that d₉₇ still was above the target (d₉₇ <4 μιτι). After milling the slurry was put into a sealed plastic bottle. One day later the slurry had become a bubbling lump and an over pressure had been created within the plastic bottle probably due to the reaction of Al(s) to AI(OH)₃(s) under formation of H₂(g). During this reaction the viscosity in the slurry had increased dramatically. The "lump" could not be used for making a cBN composite powder and was scrapped.

The particle size distribution in the slurry was investigated after 15, 30, 45 and 60 minutes milling using a Microtrac S-3000 in the range 0.021-1408 μιτι. Table lb shows the average particle size, d₅₀, d₉₀ and d₉₇ vs milling time at 1275 rpm at constant volume measured using Microtrac S-3000.

Table lb

The aim of Example 1A and IB was to by a short and intensive milling be able to significantly reduce the size of the largest ssTiN particles to d₉₇ < 4 microns, measured by Microtrac S-3000, as well as de-agglomerate the fine TiN particles to make the final cBN- composite more homogenous and the ssTiN more evenly distributed between the cBN grains. In a conventional ball mill process (2.4 I mill using cylindrical milling bodies with OD around 1 cm and height around 2 cm and with the weight relation 1:20 for ssTiN rmilling bodies and at approx. same dry content) the pre-milling time is 7-10 h to achieve a d₉₇< 4 microns.

Examples 1A-B are not examples of the method according to the invention, but are initial tests performed in order to compare milling of only TiN with milling of TiN together with Al.

Example 1C

The slurry from Example 1A showed a high viscosity even when the dry content was decreased to 55 wt% and according to the Microtrac measurements in Table la the ssTiN started to re-agglomerate when the part of fine grained increased already between 15 and 30 minutes of milling. To disperse the fine particles properly and to reduce the viscosity at a constant dry content a polymeric dispersing agent Solsperse41000 from Lubrizol was added to the slurry obtained after milling during 90 min in Example 1A.

When 0.5 wt% Solsperse41000 was added viscosity decreased significantly. The particle size distribution was measured by Microtrac S-3000 in the range 0.021-1408 μιτι. Table lc shows the average particle size, d₁₀, d₅₀, d₉₀ and d₉₇ before and after addition of the dispersing agent Solsperse41000 measured by Microtrac S-3000 in the range 0.021-1408 μιτι.

Table lc

0.5, 1 and 2 wt% calculated on the dry content of the ssTiN was added to the slurry obtained after 90 min milling in Example 1A respectively and the viscosity was measured at shear rates between 0 and 1000 1/s by a Physica US200/32 V2.50 universal dynamic spectrometer from Paar Physica using a Z3 DIN (25 mm) measuring system. As may be seen in Table Id the addition of Solsperse41000 decreased the viscosity significant and the lowest viscosity was obtained when 2 wt% Solsperse41000 was added, calculated on the dry mass of ssTiN.

Table Id

Added Dry Shear Viscosity Shear Viscosity Shear Viscosity amount content

Rate (Pas) Rate (Pas) Rate (Pas)

Solsperse

(wt%)

41000

(1/s) (1/s) (1/s)

(wt%)

0.5 53 26.4 0.259 336 0.0285 1000 0.0161

1.0 53 26.4 0.0301 336 0.00905 1000 0.0112

2.0 53 26.4 0.0149 336 0.00724 1000 0.0106 Example 2A

695 g substoichiometric TiN with d₅₀ of 3.4 μιτι, d₉₀ of 7.9 μιτι and d₉₅ of 10.6 μm was pre- milled in dry ethanol in an attritor mill of 5.7 liters during 16 hours at 80 rpm milling speed (as measured on the axis). The ssTiN utilized was TiN 15 kg milling bodies of a cermet material having a composition (weight%) of W 17.36, Co 17.47, Ti 50.65, N 4.84, and C 9.83 were used. After pre-milling, a slurry sample was taken from the attritor mill. The slurry sample was dried and d₅₀, d₉₀ and d₉₅ of the dried powder sample were measured. After pre-milling d₅₀ was 0.34 μιτι, d₉₀ was 1.1 μιτι and d₉₅ was 1.4 μιτι of the dried milled powder. Furthermore, immediately after finished pre-milling 574 g cBN with d₅₀ of 1.3 μιτι and 74 g Al with d₅₀ of 0.18 μιτι were added to the slurry in the attritor mill and mixed together during 9 hours, still at 80 rpm to produce a powder composition in the slurry. Thereafter the slurry was mixed with polyethylene glycol (PEG)-water solution (organic binder) and spray dried into spherical granules using nitrogen. The granules were pre-compacted into soft green discs of 60 mm in diameter and 2 mm in height using a 50 tons hydraulic press and subsequently fired at 400°C in hydrogen gas for organic binder removal and then pre- sintered at 900 °C in vacuum. The hard green discs were then high pressure-high temperature (HPHT) sintered at a temperature of about 1380°C and a pressure of about 55 kbar to produce a cBN composite material. Example 2B

695 g substoichiometric TiN with d₅₀ of 3.4 μιτι, d₉₀ of 7.9 μιτι and d₉₅ of 10.6 μιτι was pre- milled in dry ethanol in an attritor mill of 5.7 liters during 16 hours at 80 rpm milling speed (as measured on the axis). The ssTiN utilized was TiN 15 kg milling bodies of a cermet material having a composition (weight%) of W 17.36, Co 17.47, Ti 50.65, N 4.84, and C 9.83 were used. After pre-milling, a slurry sample was taken from the attritor mill. The slurry sample was dried and d₅₀, d₉₀ and d₉₅ of the dried powder sample were measured. After pre-milling d₅₀ was 0.34 μιτι, d₉₀ was 1.1 μιτι and d₉₅ was 1.4 μιτι of the dried milled powder. Furthermore, immediately after finished pre-milling 574 g cBN with d₅₀ of 1.3 μιτι was added and mixed together with the ssTiN during 7.5 hours, still at 80 rpm in a first sub- mixing step. It should be noted that due to the high hardness of cBN, its particle size is not reduced in the first sub-mixing step. After the first sub-mixing step 74 g Al with d₅₀ of 0.18 μιτι was added and the mill speed reduced to the lowest possible speed which is 60 rpm in order to mix the milled TiN and the cBN with Al in a second sub-mixing step and produce a powder composition in the slurry. The second sub-mixing step was conducted for 30 minutes. After the mixing steps the slurry was mixed with polyethylene glycol (PEG)-water solution (organic binder) and spray dried into spherical granules using nitrogen. The granules were pre-compacted into soft green discs of 60 mm in diameter and 2 mm in height using a 50 tons hydraulic press and subsequently fired at 400°C in hydrogen gas for organic binder removal and then pre-sintered in vacuum at 900 °C. The hard green discs were then high pressure-high temperature (HPHT) sintered at a temperature of about 1380°C and a pressure of about 55 kbar to produce a cBN composite material.

In Examples 2A and 2B the particle size distribution values were measured by Microtrac S3500.

A chemical analysis of the pre-sintered green discs obtained in Examples 2A and 2B was performed. The results in wt% are shown in Table 2. The elements Al, Co, W, Ti, Cr, Fe, Ni and Si were analyzed using X-ray fluorescence (XRF). The XRF instrument was a Philips PW2404 instrument used in the semi-quantitative mode IQ+. The non-metallic elements N and O were analyzed using LECO.

Table 2

Example 2C

From the cBN composite materials of Example 2A and 2B, respectively, inserts in style CNGA120408S01030 were manufactured. The inserts were tested in two wear resistance tests (Test 1 and Test 2), facing a 8620 case hardened steel disc, work piece hardness 62 HRC. Speed 200m/min, feed 0.2mm/rev, depth of cut (DOC) 0.15 mm, length of cut: 113 mm, cutting time per pass: 1.2 mm, cutting fluid: none (dry). Tool life criterion was 0.2 mm flank wear or edge fracture.

Results of Example 2C:

Test 1:

Tool life insert 1A made from Example 2A: 8.80 min (criterion edge fracture)

Tool life insert IB made from Example 2B: 32.11 min (criterion flank wear 0.2 mm) Test 2:

Tool life insert 2A made from Example 2A: 11.20 min (criterion edge fracture)

Tool life insert 2B made from Example 2B: 25.93 min (criterion flank wear 0.2 mm) Example 3A

Powders of aluminum (5 wt%) with d₅₀ of 1.1 μιτι, substoichiometric titanium carbon nitride (ssTi(C,N)) with d₅₀ of 2.3 μιτι, d₉₀ of 5.4 μm and d₉₅ of 7.1 μm (50 wt%), and cBN with d₅₀ of 2.8 μιτι (45 wt%) were milled in 100% ethanol in an attritor mill of 1.4 liters at 200 rpm for 5.5 hours with a weight ratio between milling liquid and powder of 0.86:1 and 10:1 between milling bodies and powders (dry weight). The ssTi(C,N) utilized was Ti(C,N)_{0 8}. The milling bodies had a composition (weight%) of W 17.36, Co 17.47, Ti 50.65, N 4.84, and C 9.83. After co-milling of aluminum, ssTi(C,N) and cBN, the slurry was mixed with polyethylene glycol (PEG)-water solution (organic binder) and spray dried into spherical granules using nitrogen. The granules were pre-compacted into soft green discs of 60 mm in diameter and 2 mm in height using a 50 tons hydraulic press and subsequently fired at 400°C in hydrogen gas for organic binder removal and then at 700 to 1000 °C in vacuum for pre-sintering. The hard green discs were then high pressure-high temperature (HPHT) sintered to produce a cBN composite material.

Example 3B

Powders of the same composition as that of Example 3A were milled in a different procedure. The ssTi(C,N) powder was pre-milled in the same attritor mill at 200 rpm for 12 hours. Then the cBN and aluminum powders were added in the mill and the slurry was mixed for another 5 hours at 200 rpm. After the 5 hour mixing, a polyethylene glycol (PEG)- water solution (organic binder) was mixed into the slurry. The spray dried granules, pre- compacted greens, and HPHT sintered discs were produced in the way as described in example 3A in order to produce a cBN composite material.

It should be noted that due to the high hardness of cBN, its particle size is not reduced during mixing in this example. In Example 3A, because the ssTi(C,N) particles are bigger than the hard cBN particles, the ssTi(C,N) particles are milled down, but the final particle size is restricted by the cBN particle size. While in Example 3B, the ssTi(C,N) particles are actually milled down in the pre-milling step. In the mixing step, because the ssTi(C,N) particles are much smaller than the cBN particles, the effect of milling on particle size reduction is minimal.

Table 3 shows the d₅₀, d₉₀, d₉₅, d₉₉ and the specific surface areas of the ssTi(C,N) before and after premilling, and the final blends. The values specified in Table 3 for premilled ssTi(C,N), and the final blends are measured on dry powder samples (i.e. samples were taken from the respective slurries, dried and analyzed). It can be seen that with premilling, the ssTi(C,N) particle size was significantly reduced, and the specific surface area was significantly increased, which can help improve the distribution of all the ingredients. In Examples 3A and 3B particle size distribution values were measured by Microtrac S3500 and the specific surface area was measured using the BET method.

Table 3

Figures la and lb show a SEM image at l,000x magnification of the microstructure of the cBN composite material produced in Example 3A and 3B, respectively. The black particles denoted with "A" in the SEM images are cBN and the light grey areas denoted with "B" are ssTi(C,N). As may be seen in the images, the large cBN particles are dispersed to the same level in the cBN composite materials of Figures la and lb. However, the small cBN are better distributed in the cBN composite material of Figure lb.

Figures lc and Id show a SEM image at 20,000x magnification of the microstructure of the cBN composite material produced in Example 3A and 3B, respectively. The black particles denoted with "A" in the SEM images are cBN, the light grey areas denoted with "B" are ssTi(C,N) and the dark grey areas denoted with "C" are aluminium-containing compounds. As may be seen in Figure lc, due to the large particle size of ssTi(C,N), only a few aluminium-containing particles can exist within the binder (non-cBN phase), and a lot of aluminium-containing particles are accumulated around the cBN particles in the cBN composite material of Example 3A. On the contrary, as may be seen in Figure Id, the aluminium-containing particles are much better distributed in the binder, which is more desired for sintering of low-cBN material, of the cBN composite material of Example 3B.

While the invention has been described in connection with various exemplary embodiments, it is to be understood that the invention is not to be limited to the disclosed exemplary embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the appended claims. Furthermore, it should be recognized that any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the appended claims appended hereto.

Claims

A method of making a powder composition, which is suitable for production of a cBN composite material comprising cBN grains, a metallic binder phase and a ceramic binder phase, the method comprising the steps of:

providing at least one ceramic binder phase forming powder in a milling device;

mixing said milled powder with at least cBN powder and a metallic binder phase forming powder to form said powder composition, whereby said metallic binder phase forming powder is aluminum powder and whereby 0.10 μιτι < d₅₀ < 1.40 μιτι of said aluminum powder.

A method according to claim 1 wherein each ceramic binder phase forming powder is a powder of a compound selected from the group consisting of nitrides, carbides, carbonitrides, oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 transition metal.

A method according to claim 2 wherein said nitrides, carbides and carbonitrides of a Group 4, 5 or 6 transition metal are stoichiometric or substoichiometric and wherein said oxycarbonitrides, carboxides and oxynitrides of a Group 4, 5 or 6 transition metal are stoichiometric.

4. A method according to claim 2 or 3 wherein said transition metal is titanium.

A method according to anyone of claims 1-4 wherein at least one ceramic binder phase forming powder is a powder of a compound selected from the group consisting of TiN, TiC, and Ti(C,N), which are stoichiometric or substoichiometric, and Ti(C_xN_yO_z)_a, which is stoichiometric.

A method according to anyone of the preceding claims wherein said cBN powder is provided such that the cBN content of said powder composition is between 20 and 79 wt% of the total dry powder weight of said powder composition.

A method according to anyone of the preceding claims wherein 0.5 μιτι < d₅₀< 10 μιτι of said cBN powder.

8. A method according to anyone of the preceding claims wherein said aluminum powder is provided such that the aluminum content of said powder composition is between 1 and 10 wt% of the total dry powder weight of said powder composition.

9. A method according to anyone of the preceding claims wherein said at least one ceramic binder phase forming powder is provided in a milling liquid so that a slurry is formed before said milling step and wherein said mixing step is followed by a drying step to dry said powder composition.

10. A method according to anyone of the preceding claims wherein said milling device is selected from the group consisting of an attritor mill, a ball mill, a pearl mill, a roll mill, a planetary mill or a basket mill.

A method according to anyone of the preceding claims wherein said mixing step is performed in said milling device or in a separate mixing device.

A method according to anyone of the preceding claims wherein said mixing step includes:

a first sub-mixing step which comprises mixing said milled powder and at least said cBN powder so as to form an intermediate powder mixture, and a second sub-mixing step which comprises mixing said intermediate powder mixture with at least said aluminum powder to form said powder composition,

whereby said first and second sub-mixing steps are both performed in said milling device or in a separate mixing device or whereby said first sub- mixing step is performed in said milling device and said second sub-mixing step is performed in a separate mixing device or whereby said first sub- mixing step is performed in a separate mixing device and said second sub- mixing step is performed in another separate mixing device.

A method according to claim 11 or 12 wherein said mixing device is selected from the group of: a roll mill, an ultrasonic device, a mechanical stirring device and a paint shaker.

14. A method of making a cBN composite material comprising making a powder

composition according to anyone of claims 1-13, forming the powder composition to a green body and subjecting said green body to a high pressure and high temperature sintering operation to form said cBN composite material.

15. A powder composition obtainable by the method of anyone of claims 1-13.

16. A cBN composite material obtainable by the method of claim 14.