POLYCRYSTALLINE CUBIC BORON NITRIDE MATERIAL

Abstract

This disclosure relates to a polycrystalline cubic boron nitride, PCBN, material that includes a binder matrix material containing nitride compounds. The nitride compounds are selected from HfN, VN, and/or NbN.

Claims

1. A polycrystalline cubic boron nitride, PCBN, material comprising: between 40 and 95 vol.% cubic boron nitride, cBN, particles, a binder matrix material in which the cBN particles are dispersed, the content of the binder matrix material being between 5 vol.% and 60 vol.% of the PCBN material, the binder matrix material comprising aluminium or a compound thereof, and/or titanium or a compound thereof, and the binder matrix material further comprising oxide compounds, nitride compounds and/or oxynitride compounds, wherein the nitride compounds are selected from: HfN, VN, NbN.

2. The PCBN material as claimed in claim 1, wherein said oxynitride compound is present in an amount of between 5 vol.% and 35 vol.% of the PCBN material.

3. The PCBN material as claimed in claim 2, wherein said oxynitride compound is present in an amount of between 10 vol.% and 25 vol.% of the PCBN material.

4. The PCBN material as claimed in claim 1, wherein said oxynitride compound comprises AlON.

5. The PCBN material as claimed in claim 1, wherein said oxide compound comprises Al.sub.2O.sub.3.

6. The PCBN material as claimed in claim 5, wherein the Al.sub.2O.sub.3 is present in an amount of 10 vol.% or 25 vol.% of the PCBN material.

7. The PCBN material as claimed in claim 1, wherein said HfN is present in an amount of 10 vol.% or 25 vol.% of the PCBN material.

8. The PCBN material as claimed in claim 7, the binder matrix material further comprising HfB.sub.2 and/or BN.

9. The PCBN material as claimed in claim 1, wherein said VN is present in an amount of 10 vol.% or 25 vol.% of the PCBN material.

10. The PCBN material as claimed in claim 9, the binder matrix material further comprising AIN and/or BN.

11. The PCBN material as claimed in claim 1, wherein said NbN is present in an amount of 10 vol.% or 25 vol.% of the PCBN material.

12. The PCBN material as claimed in claim 1, wherein said aluminium, Al, or a compound thereof, is present in amount of between 2 and 15 vol. % of the PCBN material.

13. The PCBN material as claimed in claim 1, comprising 50 to 70 vol.% cubic boron nitride, cBN.

14. The PCBN material as claimed in claim 1, comprising 60 vol.% cubic boron nitride, cBN.

15. A method of making a polycrystalline cubic boron nitride, PCBN, material, the method comprising: milling together precursor powders of: cubic boron nitride, cBN, powder, oxide-containing powder, nitride-containing powder, wherein the nitride-containing powders are selected from: HfN, VN, and/or NbN, aluminium-containing powder and/or titanium-containing powder, compacting the milled precursor powders to form a green body; sintering the green body at a temperature between 1250° C. and 2200° C. at a pressure of between 4.0 GPa and 8.5 GPa to form the sintered PCBN material of claim 1.

16. The method as claimed in claim 15, wherein the oxide-containing powders comprise Al.sub.2O.sub.3.

17. The method as claimed in claim 15, wherein the temperature is between 1250° C. and 1450° C.

18. The method as claimed in claim 17, wherein the temperature is 1350° C.

19. The method as claimed in claim 17, wherein the pressure is around 6.5 GPa.

20. The method as claimed in claim 15, wherein the temperature is between 1800° C. and 2100° C.

21. The method as claimed in claim 20, wherein the pressure is around 8 GPa.

22. A method of using the PCBN material as claimed in claim 1, the method comprising machining Heat Resistant Superalloys (HRSAs) using the PCBN material.

Description

BRIEF DESCIPTION OF THE DRAWINGS

[0050] Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which:

[0051] FIG. 1 is a flow diagram showing a known exemplary method of making a sintered PCBN material;

[0052] FIG. 2 is a flow diagram showing an embodiment of a process used to make a PcBN material in accordance with the invention;

[0053] FIG. 3 is an X-ray Powder Diffraction (XRD) pattern of sintered Example 1 produced using Powder 1, which contains HfN and Al.sub.2O.sub.3, sintered at 6.5 GPa;

[0054] FIG. 4 indicates a Scanning Electron Microscopy (SEM) micrograph of Example 1, at magnification X2000;

[0055] FIG. 5 are Energy Dispersive X-ray Spectroscopy (EDS) images of Example 1;

[0056] FIG. 6 is an XRD pattern of sintered Example 2 produced using Powder 2, which contains VN and Al.sub.2O.sub.3, sintered under 6.5 GPa conditions;

[0057] FIG. 7 is an SEM micrograph of Example 2, at magnification X2000;

[0058] FIG. 8 are EDS images of Example 2;

[0059] FIG. 9 is an XRD pattern of sintered Example 3 produced using Powder 2, which contains VN, under 8.4 GPa condition;

[0060] FIG. 10 is an image of an example indentation in a PCBN material indicating the measurements used in calculating the hardness;

[0061] FIG. 11 is a line chart showing the performance in profile operation of aged Inconel™ 718 (HRC 44 - 46) ofHPHT sintered samples with different binder chemistries;

[0062] FIG. 12 is a bar chart showing the performance in longitudinal machining of aged Inconel™ 718 (HRC 44 - 46) ofHPHT and LPLT sintered samples with VN and Al.sub.2O.sub.3 binder chemistry;

[0063] FIG. 13 is an optical image showing the wear scar of reference PCBN material with TiC binder from FIG. 12;

[0064] FIG. 14 is an optical image showing the wear scar of the PCBN material with Al.sub.2O.sub.3-VN binder sintered at HPHT conditions from FIG. 12; and

[0065] FIG. 15 is an optical image showing the wear scar of the PCBN material with Al.sub.2O.sub.3-VN binder sintered at LPLT conditions from FIG. 12.

DETAILED DESCRIPTION

[0066] FIG. 2 is a flow diagram showing exemplary steps, in which the following numbering corresponds to that of FIG. 2.

[0067] S1. Precursor powders are milled together to form an intimate mixture and obtain a desired particle size. The precursor powders comprise oxide-containing powder, nitride-containing powder, aluminium powder and cBN powders. The precursor powder mixing was carried out in organic solvent using ball-milling techniques and drying with a rotary evaporator.

[0068] S2. The milled precursor powders are dry pressed together to form a green body in metal encapsulation before putting it into a HPHT capsule. In the case of HPHT sintering, Specifically, after drying, the powder is filled into a soft mould, then compressed using a Cold Isostatic Press to compact the powder and form the green body with high green density in order to have less dimensional change after sintering.

[0069] The green body is then cut into different heights to fit into a HPHT capsule. S3. The dry pressed green body is then subjected to high temperature vacuum heat treatment and subsequently sintered in a capsule.

[0070] Materials generated thus far were sintered under two conditions: [0071] a pressure of around 6.5 GPa and at a temperature between 1250° C. and 1450° C., and typically at 1350° C.; and [0072] a pressure of around 8 GPa and at a temperature between 1800° C. and 2100° C.

[0073] The sintering temperature was calibrated up to 1800° C. using S-type thermocouples.

[0074] S4. After sintering, the resultant sintered articles cool to room temperature. The cooling rate is uncontrolled.

EXAMPLES

[0075] Table 1 lists all the PcBN compositions that were included in this work, together with a TiC and a TiCN reference sample. In this section, LPLT stands for Lower Pressure and Lower Temperatures, and HPHT stands for Higher Pressure and Higher Temperatures.

TABLE-US-00001 Powder 1 Al.sub.2O.sub.3-HfN binder Sintering Conditions cBN (vol %) Al.sub.2O.sub.3 (vol %) HfN (vol %) Al (vol %) LPHT (Example 1) 60 10 25 5 Powder 2 Al.sub.2O.sub.3-VN binder Sintering Conditions cBN (vol %) Al.sub.2O.sub.3 (vol %) VN (vol %) Al (vol %) LPLT (Example 2) & HPHT (Example 3) 60 25 10 5 Powder 3 Al.sub.2O.sub.3-NbN binder Sintering Conditions cBN (vol %) Al.sub.2O.sub.3 (vol %) NbN (vol %) Al (vol %) HPHT 60 25 10 5 Reference 1 TiC binder Sintering Conditions cBN (vol %) TiC (vol %) Al (vol %) HPHT 60 35 5 Reference 2 TiCN binder Sintering Conditions cBN (vol %) TiCN (vol %) Al (vol %) HPHT 60 35 5

[0076] Examples 1, 2 and 3 are described in more detail below. Other samples provided in Table 1, both inventive and reference, were prepared, characterised and subsequently tested in a similar way to Examples 1, 2 and 3.

Example 1

[0077] S1. Precursor powders comprising Al.sub.2O.sub.3 and HfN were mixed together with cBN powders and Al powder, in the proportions provided in Table 1, as per the description above.

[0078] S2. The precursor powders were then compacted to form a green body inside metal encapsulation.

[0079] S3. The green body was placed inside a capsule, and then sintered.

[0080] S4. The sintered article, PCBN material, was cooled to room temperature, ready for subsequent characterisation and application testing.

[0081] The XRD trace is provided in FIG. 3 and indicates the presence ofHfN, HfB.sub.2, Al.sub.2O.sub.3 and BN in the sintered article. FIG. 4 conveys the resulting microstructure and the EDS images in FIG. 5 provide a breakdown of the composition of the microstructure in select areas of the sample.

Example 2

[0082] S1. Precursor powders comprising Al.sub.2O.sub.3 and VN were mixed together with cBN powders, in the proportions provided in Table 1, as per the description above.

[0083] S2. The precursor powders were then compacted to form a green body inside metal encapsulation.

[0084] S3. The green body was placed inside a capsule, and then LPLT sintered.

[0085] S4. The sintered article, PCBN material, was cooled to room temperature, ready for subsequent characterisation and application testing.

[0086] The XRD trace is provided in FIG. 6 and indicates the presence ofVN, AlN, Al.sub.2O.sub.3 and BN in the sintered article. FIG. 7 conveys the resulting microstructure and the EDS images in FIG. 8 provide a breakdown of the composition of the microstructure in select areas of the sample.

Example 3

[0087] S1. Precursor powders comprising Al.sub.2O.sub.3 and VN were mixed together with cBN powders, in the proportions provided in Table 1, as per the description above.

[0088] S2. The precursor powders were then compacted to form a green body.

[0089] S3. The green body was cut to size, placed inside a capsule, and then HPHT sintered.

[0090] S4. The sintered article, PCBN material, cooled to room temperature, ready for subsequent characterisation and application testing.

[0091] The XRD trace is provided in FIG. 9 and indicates the presence ofVN, AlN, Al.sub.2O.sub.3 and BN in the sintered article. SEM micrographs and EDS images of the sample were taken but are not included here.

Hardness

[0092] The samples were further characterised using the Vickers hardness test. The Vickers Hardness (HV) is calculated by measuring the diagonal lengths (e.g. see FIG. 10) of an indent in the sample material left by introducing a diamond pyramid indenter with a given load.

[0093] Table 2 indicates the hardness of samples sintered from powder 1 and 2 in different conditions.

TABLE-US-00002 HPHT condition LPLT condition Powder 1 (Al2O3-HfN binder) n/a 35.44 GPa Powder 2 (Al2O3-VN binder) 34.33 GPa 32.08 GPa

[0094] The results show that all samples have a relatively high hardness, but moreover that sintering at higher pressures and temperatures increases the hardness only slightly.

Applications Testing

[0095] The PCBN variants with different binder chemistries were then tested in profiling aged Inconel™ 718, which has a Rockwell Hardness of HRC 44 - 46. The results are shown in FIG. 11. FIG. 11 is a graph plotting surface cutting speed v.sub.c in m/min, with wear rate, in .Math.m/min. The wear rate was measured at three different surface cutting speeds, for most samples. These surface cutting speeds were 280 m/min, 350 m/min and 420 m/min.

[0096] The reference TiC binder is indicated generally at 10 and the TiCN binder at 12. Al.sub.2O.sub.3-VN (HPHT) has reference 14. Al.sub.2O.sub.3-VN (LPLT) has reference 16. Al.sub.2O.sub.3-NbN (HPHT) has reference 18. Al.sub.2O.sub.3-HfN (HPHT) has reference 20 and comprises a single data point.

[0097] From FIG. 11, it is clear that all samples from Table 1 perform better than the reference samples.

[0098] Also, referring to the samples with reference 14 and 16 (i.e. with binder chemistry Al.sub.2O.sub.3-VN) on the graph, there is marginal difference in wear rate when sintering under LPLT conditions compared to sintering under HPHT conditions.

[0099] Al.sub.2O.sub.3-VN (whether HPHT or LPLT) performs better than any of the samples. Al.sub.2O.sub.3-NbN performs second best, followed by Al.sub.2O.sub.3-HfN.

[0100] Turning now to FIG. 12, a second application test, similar to the first, was carried out. The second test focused on the performance of the Al.sub.2O.sub.3-VN binder chemistry in longitudinal machining of aged Inconel™ 718, with a Rockwell Hardness ofHRC 44 - 46. Both LPLT and HPHT variants were considered.

[0101] FIG. 12 is a bar chart plotting surface cutting speed v.sub.c, in m/min, with wear rate, in .Math.m/min. A single surface cutting speed was used, 350 m/min. The results showed that both LPLT and HPHT variants performed significantly better than the reference TiC binder chemistry. Furthermore, that there was minimal difference in wear rate performance between the LPLT and the HPHT variants.

[0102] FIGS. 13 to 15 indicate the resulting wear scars. The wear scars for the LPLT and HPHT Al.sub.2O.sub.3-VN binder chemistries are significantly smaller than for the TiC reference sample.

[0103] In summary, the inventors have successfully identified several materials which are suitable for use in extreme tooling applications and are viable alternatives to CRMs. In particular, the PCBN materials are especially suitable for machining Inconel™ 718 and offer many advantages over cemented carbide solutions.

Definitions

[0104] As used herein, “PCBN” material refers to a type of super hard material comprising grains of cBN dispersed within a matrix comprising metal or ceramic. PCBN is an example of a super hard material.

[0105] As used herein, a “binder matrix material” is understood to mean a matrix material that wholly or partially fills pores, interstices or interstitial regions within a polycrystalline structure.

[0106] The term “binder matrix precursor powders” is used to refer to the powders that, when subjected to a HPHT or LPLT sintering process, become the matrix material.

[0107] While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.