CA2012820A1 - Process for preparing polycrystalline cbn ceramic masses and resulting product - Google Patents

Abstract

PROCESS FOR PREPARING POLYCRYSTALLINE CBN CERAMIC
MASSES AND RESULTING PRODUCT

ABSTRACT OF THE DISCLOSURE
Disclosed is a process for preparing sintered polycrystalline CBN/ceramic conjoint masses in an HP/HT process which comprises subjecting a mixture of GBN and one or more of a ceramic material or metal which reacts with BN to form a ceramic material, to HP/HT
conditions for inducing conversion of said GBN to CBN, inducing conversion of any of said metal to its corresponding metal ceramic, and forming said CBN/ceramic conjoint mass.

Description

20~2820 PROCESS FOR PREPARING POLYCRYSTALLINE CBN CERA~IIC
MASSES AND RESULTING PRODUCT

Cross Reference to Related Applications This application is a continuation-in-part of United States application Serial No. 07/269,965, filed November 10, 1988, entitled "Process for Preparing Polycrystalline Ceramic Masses and Resulting Product."

5 Back~round of the Invention The present invention relates to a high pressure/high temperature (HP/HT) process for making cubic boron nitride (CBN) ceramic masses and more particularly to a process which utilizes a single HP/lIT processing operation in the manufacture of CBN ceramic masses.
The manufacture of CBN by the HP/HT process is known in the art and is typified by the process described in U.S. Pat. No. 2,947,617, a basic monocrystalline CBN case. U.S. Pat. No. 4,188,194 describes a process for making sintered polycrystalline CBN compacts which utilizes pyrolytic hexagonal boron nitride (PBN) in the absence of any catalyst. An 15 improvement on such direct conversion process is disclosed in U S. Pat. No.
4,289,503 wherein boric oxide is removed from the surface of the HBN
powder before the conversion process.
A compact is a mass of abrasive particles bonded together in a self-bonded relationship (see U.S. Pats. Nos. 3,852,078 and 3,876,751); by means 20 of a bonding medium (see US. Pats. Nos. 3,136,615, 3,233,988, 3,743,489, 3,767,371, and 3,918,931); or by means of combinations thereof. A
composite compact is a compact bonded to a substrate material, such as cemented metal carbide. U.S. Pat. No. 3,918,219 teaches the catalytic conversion of hexagonal boron nitride (HBN) to CBN in contact with a 25 carbide mass to form a composite CBN compact. Compacts or composite compacts may be used in blanks for cutting tools, drill bits, dressing tools, and wear parts (see U S. Pats. Nos. 3,136,615 and 3,233,988).
Sintered CBN/cermet compacts prepared by HP/HT pressing of mixtures of CBN, various cermets, and aluminum compounds (with or 30 without metal additions) are disclosed in U.S. Pats. Nos. 4,334,928 and 4,389,465. U.S. Pat. No. 4,394,170 proposes a composite sintered compact 2~12820 6 OSDO o 4 5 9 containing high density BN (CBN and Wurtzite boron nitride, WBN), ceramic flnd various metal bases obtained by HP/HT processing of mixtures of CBN, WBN, and various ceramic and metal mixtures. A supported version of such CBN/cermet compact is disclosed in U.S. Pat. ~o. 4,403,015. These prior 5 CBN cermet compact disclosures, however, must initially make the CBN in one HP/HT step and the CBN cermet product in a second HP/HT step.
Extra wear of the HP/HT presses, reduced throughput, and increased manufacturing costs result from the use of two different HP/HT processing operations.
Broad Statement of the Invention The present invention is directed to a HP/HT process for making sintered polycrystalline CBN/ceramic conjoint masses utilizing a single HP/HT processing operation. Such method comprises subjecting, in the 15 absence of bulk catalytically-active material, a mixture of graphitic boron nitride (GBN) and one or more of a ceramic material, a metal which reacts with boron nitride (BN) to form a ceramic material, or mixture thereof (hereinafter, sometimes referred to as "second phase" or "second phase component(s)"), to HP/HT conditions for inducing conversion of GBN to 20 C8N, inducing conversion of any of said metal to a corresponding metal ceramic, and forming said CBN/ceramic mass. One technique for forming the initial reaction mixture for subjecting to the HP/HT process of the present invention involves the mixing of GBN powder and second phase powder. As the particle size of these powders decreases, however, surface 25 contamination may interfere with boron nitride conversion and/or sintering, thus special processing techniques often need to be employed (as described more fully below).
Conversion of "pristine" GBN/second phase mixtures to form CBN
conjoint masses would be advantageous with respect to obtaining high purity 30 composites via the elimination of the difficult mixing and cleaning steps needed in preparing CBN-based composites by conventional HP/HT
conversionlsintering methods utilizing individually-prepared mixed powders.
Accordingly, another aspect of the present invention involves the production of the feed GBN with simultaneous concomitant formation and/or deposition 35 of the second phase component(s) in a form suitable for subjecting to HP/HT
processing for forming the novel CBN/ceramic conjoint masses of the present invention. Such reaction mixture is termed "pristine" for present purposes due to the substantial absence of impurities in the GBN/second 2012~2~

?hase reaction mixture. The resulting CBN/ceramic conjoint mass forms another aspect of the present invention.
Advantages of the present invention include the ability to prepare CBN/ceramic masses directly from the low pressure form of BN. Another advantage is the ability to prepare such masses in a single HP/HT processing operation. Yet another advantage is the concomitsnt extension of the useful life of the HP/HT presses and increased throughput occasioned by the one-step HP/HT processing operation disclosed herein. These and other advantages will become readily apparent to those skilled in the art based upon the disclosure contained herein.

Brief Description of the Drawing The drawing illustrates, in cross-section, a configuration of reaction zone assembly or cell which was utilized in the examples, such cell being further described in U.S. Pats. Nos. 2,947,611, 2,941,241, and 2,941,248.
The drawing will be described in further detail in connection the the examples contained herein.

Detailed Description of the Invention The sintered polycrystalline CBN/ceramic masses of the present invention can be prepared in situ by HP/HT pressing of mixtures of low pressure form BN powders (GBN) and one or more of a ceramic material (including cermet) or metal. The temperature and pressure conditions utilized are sufficient to convert the low pressure phase BN to the cubic phase, and any metal powder to a ceramic material, for example, by reaction of the metal with the BN phase of the reaction mixture subjected eO the process. The proportion of GBN in the starting mixture can range from about 10 to 95 volume-% with the balance being composed of the ceramic material and/or metal. The GBN reactant can range in size on up to 1.5 mm, though typical size ranges of GBN in commercial operations ranges from about 0.1 to 10 microns in particle size. The ceramic/metal phase of the reaction mixture broadly ranges from about 0.1 to 100 microns in particle size. It is important that no impurities which inhibit the transformation of GBN to CBN be included in the reaction mixture.
Depending on the ceramic material in the starting mixture (e.g. TiN), conventional handling and pre-processing of the GBN phase may be desirable, such as, for example, removal of boric oxide from the surface of 2012~20 HBN powder as taught in U.S. Pat. No. 4,289,503. With other ceramic materials (e.g. AlB12, NiA13, ~IgB2), however, such vacuum firing pre-processing is not required as the ceramic material evidently suppresses oxygen inhibition in the CBN conversion process.
Although new process development may result in alternative techniques for implementation of the starting material modifications of the present invention, chemical vapor deposition (CVD), solgel and polymer pyrolysis techniques, and combustion synthesis processes should be particularly suitable techniques for producing "pristine" GBN/second phase mixtures for subjecting to the HP/HT process disclosed herein.
CVD processes currently are used in a variety of fields as those in the art will appreciate. Commercially, CVD is used to produce pyrolytic boron nitride, for example. In this process, boron trichloride and ammonia gases are allowed to flow through a high temperature furnace where reaction occurs and boron nitride is deposited on suitable substrates yielding large macroscopic pieces. This process can be adapted suitably to the production of the various type of modified starting materials disclosed herein.
Introduction of suitable gases during the pyrolytic BN deposition process allows for codeposition of various second phases during the PBN deposition, e.g. introduction of TiCl4 gas to obtain BN/TiN composites. By alternating the gas flow through the furnace, alternate layer type structures also could be obtained. Since the resulting products are obtained as large pieces, internal purity is maintained prior to HP/HT conversion of the GBN
composites to yield CBN based composites. These same processing techniques should be available for deposition of a variety of ceramic and ceramic-forming metals. Further information on CVD co-deposition may be found in the following citation: '~oore, et al., "Properties and Characterization of Co-deposited Boron Nitride and Carbon ~laterials", to be published in J. Appl. Phys., June, 1989.
It should be possible to disperse secondary phases in low pressure boron nitride during preparation by solgel techniques to form BN/secondary phase mixes for subsequent use as a starting material for the HP/HT preparation of CBN conjoint masses. The dispersion would comprise composite-forming materials (i.e. TiN, TiC, or the like) for preparing CBN based composites.
Since the solgel method for preparing BN starts with a liguid, incorporation of a second phase during solgel processing will allow obtaining extremely good homogeneity of fine powder mixtures with BN which is difficult to 201282~

accomplish by mixing of BN in secondary phase powders, for example. The solgel prepared mixtures would yield more uniformly dispersed and finer particle mixtures as starting materials for CBN production processes in accordance with the present invention and allow preparation of improved 5 ;,omogeneity of fine grain CBN composites. Further on solgel techniques can be found in Narula, "Precursors to Boron-2~itrogen ~lacromolecules and Ceramics", Mat. Res. Soc. Symp. Proc., vol. 73, 383, Materials Research Society (1986) and Research and Development, Dec. 1987, p. 49.
Combustion synthesis is a relatively new process for producing 10 refractory compounds. BN and various nitride composite ,naterials have been reported to be prepared by this process. The products can be obtained in solid, compacted form which would allow maintenance of purity prior to HP/HT processing. Because combustion synthesis involves high temperatures with rapid heating and cooling rates, the possibility of 15 producing non-equilibrium materials is enhanced greatly (cubic TaN was first reported by combustion synthesis before it was synthesized by high pressure techniques). Further information on combustion synthesis can be found in the following: "Combustion Synthesis: A New Area of Research in Materials Science", Los Alamos National Laboratory report pp. 11-29 (circa 20 1 98 5).
With respect to the ceramic (which includes cermet for present purposes) phase in the reaction mixture, such ceramics are taught in the art .o be a carbide, nitride, carbonitride, boride, or silicide, of a variety of metals including, for example, aluminum, titanium, silicon, chromium, 25 hafnium, zirconium, tantalum, molybdenum, niobium, vanadium, and tungsten. If a cermet is used in the reaction mixture, the metal for bonding the ceramic can include metals such as nickel, cobalt, chromium, manganese, iron, molybdenum, tungsten, vanadium, aluminum, magnesium, silicon, titanium, zirconium, hafnium, and the like and mixtures thereof. In 30 this regard, reference is made to the references in the Background section which utilize alternative processing techniques for forming polycrystalline CBN/ceramic masses. Such lists of ceramics and cermets are appropriate for present purposes.
Intact polycrystalline masses may be cleaned to remove any adhering 35 carbon, titanium, or other material. After cleaning, the samples can be milled to sizes which may be used in making resin bonded grinding wheels, metal bonded grinding wheels, metal bonded saw elements, and like 2012~20 conventional cutting and abrading tools; or the disk and cylinder shaped pieces may be shaped for cutting tool applications. In this regard, the masses may be used in unsupported or in conventional supported form utilizing a tungsten carbide or other appropriate support material. Joining 5 to a support may be achieved via brazing or other technique after transformation of the reaction mixture to the novel polycrystalline CBN/ceramic compact.
HP/HT cells or apparatus are known in the art, such as described in U.S. Pat. No. 4,289,503. With respect to the drawing which depicts the 10 reaction cell assembly utilized in making the samples in the Examples which follow, pyrophyllite cylinder bushing 10 which alternatively may be made of glass, soft ceramic, talc, alkali halide, steatite, or forms of soapstones has positioned concentrically within graphite resistance heater tube 12 about which metal foil wrap or shield metal 18 is placed. Shield metal 18 is a 15 refractory metal which may be selected from the group consisting of zirconium, titanium, tantalum, tungsten, and molybdenum.
Disposed within graphite resistance heater tube 12 are upper and lower graphite plugs 20 and 22 adjacent which graphite disks 24 and 26 are placed.
Within graphite disks 24 and 26 resides reaction sample 28 which is 20 comprised of a mixture of GBN and one or more of a ceramic material or metal which reacts with BN to form a ceramic material. It will be observed that reaction mixture 28 may be densified prior to its being placed in the reaction cell. Such densification results by the high pressure pressing of the reaction mixture without application of high temperature heating. Such 25 densification or consolidation of reaction mixtures in the super atmospheric pressure art is a known pre-processing step which is practiced in conventional fashion for present purposes. Temperatures ranging from about 1500 to 2300C with pressures ranging from about 45 to 80 Kbars, then, would be expected to be appropriate for forming the novel 30 C BN/ceramic masses.
The following examples show how the invention has been practiced but should not be construed as limiting. In this application, all citations are incorporated expressly herein by reference.

IN THE E~AMPLES

A 6.8 g sample of a mixture comprising 40.3 wt-% TiN and 59.7 wt-%
PBN powder was pre-pressed at about 50 kpsi in a hardened steel mold at room temperature and inserted directly from the mold into the HP/HT cell as set forth in the drawing. The TiN powder used in the reaction mixture was -325 mesh size (United States Sieves Series, average particle size of about 5 microns). The PBN powder (less than 14 mesh sverage particle size) was vacuum fired at 1700C for approximately 4 hours prior to its incorporation into the reaction mixture for prepressing. This vacuum heat treatment resulted in a weight loss of about 6 wt-%, primarily due to partial thermal decomposition with the evolution of nitrogen (see U.S. Pat. No.
4,289,503) and resultant boron surface enrichment.
The sample in the reaction cell was pressed at a force of about 65 kbar and a temperature of about 1800C for four minutes. Thereafter, the sample was recovered from the cell in the form of a large, compacted mass primarily disk-shaped with the individual CBN particles obtained on conversion of the PBN particles clearly visible in the titanium nitride matrix. Additional samples totaling 600 g were pressed in the same manner described herein. The resulting composite masses were treated with a 10%
nitric/sulfuric acid mixture to remove adhering carbon and then milled to mesh size particles (-40 mesh) suitable for grinding applications.

The procedure described in Example 1 was implemented to press a total of 585 g of a reaction mixture comprising 48.8 wt-% TiC (1.5 micrometers average particle size) and 51.2 wt-% PBN powder (-14 mesh, vacuum fired as described in Example 1). Each reaction mixture in the cell comprised 7.2 g total weight. The samples were pressed at a force of about 65 kbar at a temperature of about 1800C for four minutes pressing time.
During pressing, the PBN particles were converted to CBN, yielding large, primarily disk-shaped conjoint masses.

A series of press runs were made on additional samples of vacuum-fired HBN (5 wt-% weight loss) which were mixed with TiN and ALB12 having a composition as set forth below.

20~282~

Composition (wt-%) Sample No. HBN TiN AlB12 TN7 39.06 60.04 ---TN8 44.86 42.00 13.14 TN 10 52.57 41.01 6.42 TN 11 62.13 34.62 3.25 TN12 64.50 28.75 6.75 TN 13 77.72 15.16 7.12 Conversions of the HBN to CBN with sintering into CBN/cermet conjoint masses were obtained at pressing leveis ranging from about 50 to 65 kbar and temperatures ranging from about 1500 to 2000C. Press times of about 4 to 20 minutes were used.
15 Sample TN 13 was ground to size for use as a cutting tool insert (1.11 cm diameter by 0.32 cm thick, 7/16 inch diameter by 1/8 inch thick).
Comparison cutting tests were conducted with cutting tool insert TN 13 and a commercially available CBN-based cutting tool. The workpiece subjected to the cutting test comprises hardened D2 steel (Rockwell hardness, Rc 20 scale, 59-61). The test conditions and results recorded are set forth below.

Cutting Speed: 200 SFPM
Infeed: 0.010 IPR
Depth of Cut: 0.050 in.

Workpiece ~olume Cutting Volume Flank Craterper mil Time Removed Wear WearFlank Wear SampleNo. (min) (in3) (in) (in)in3/.001 in -Comm ercial~ 7.07 7.8 0.0049 0.0013 1.59 35 Tool TN13 7.64 9.1 0.0039 0.0003 2.33 * BZN Compact which is a cemented WC supported polycrystalline CBN compact, General Electric Co., Worthington, Ohio.

The above-tabulated results show improved wear performance for inventive CBN/cermet mass TN13 compared to the commercially available CBN compact also tested. Significantly, the inventive masses are made by a simplified one-step process. Thus, the invention provides improved processing in the manufacture of the conjoint masses which have improved properties.

The reaction mixture comprised 35.7 wt-% titanium metal powder (-325 mesh particle size) and 64.3 wt-% vacuum-fired HBN powder. This reaction mixture was pressed at 65 kbar and 1800C for four minutes.
Confirmation of the reaction of titanium metal with a portion of the BN
phase in the reaction mixture to form TiN during the HP/HT processing was evidenced by the gold coloring of the resulting CBN-based mass. The sample was machined to the tool insert size set forth in Example 3 and tested by turning a hardened 4340 steel workpiece (Rockwell hardness, Rc scale, 48-52). The results recorded are set forth below:

Cutting Speed: 300 SFPM
Infeed: 0.010 IPR
Depth of Cut: 0.050 in.

Workpiece Volume Cutting Volume FlankCrater per mil TimeRemoved Wear WearFlank Wear SampleNo. (min)(in3) (in) (in)in3/.001 in 30 Tl 10.4818.1 0.0085 0.005 2.13 Commercial 12.8223.0 0.0158 0.0010 1.46 Tool*
35 *See Table 2 Again, the above-tabulated results clearly demonstrate improved cutting performance being exhibited by the novel polycrystalline CBN/ceramic masses compared to a commercial polycrystalline CBN
40 compact.

_g _ 201~

A series of press runs were made using mixtures of unfired HBN with additions of TiN and AlB12 using the same HBN powder source as in Example 3.

Composition (wt-%) Sample No. HBN TiN AlB12 -.

Conversion of the HBN to CBN with sintering into CBN/ceramic conjoint masses was obtained at pressures of from about 45 to 65 Icbars and temperatures ranging from about 1500 to 2000C with pressing times of 4 minutes. This example demonstrates that firing of the HBN feed to remove surface oxygen content was not required by virtue of the TiN/AlB12 addition.

A mixture of 65 wt-% vacuum-fired HBN (10.0% weight loss), 30 wt-%
TiN, and 5 wt-% NiA13 was pressed at a pressure of about 60 kbars and temperature of about 1800C for 4 minutes. The press run resulted in the conversion of the HBN feed to CBN and the production of 8 sintered intact CBN/cermet conjoint disc-shaped mass.

A number of samples containing 60 wt-% vacuum-fired HBN (6.0%
weight loss), 30 wt-% TiN, and 10 wt-% MgB2 were pressed at pressures ranging from about 55 to 60 kbars and temperatures ranging from 1600 to 1800C for times of four minutes. The samples were recovered ~rom the press as intact disc-shaped CBN/ceramic conjoint pieces.

Claims (25)

1. In an HP/HT process for making sintered polycrystalline CBN/ceramic masses, the improvement which comprises, subjecting a mixture of GBN and one or more of a ceramic material or metal which reacts with BN to form a ceramic material, to said HP/HT conditions for inducing conversion of said GBN to CBN, inducing conversion of any of said metal to its corresponding metal ceramic, and forming said CBN/ceramic mass.

2. The process of claim 1 wherein the proportion of GBN in said mixture ranges from between about 10 to 95 vol-%.

3. The process of claim 1 wherein said GBN in said mixture is in the form of particles having a size not above about 1.5 mm.

4. The process of claim 3 wherein the GBN particle size ranges from about 0.1 to 10 microns.

5. The process of claim I wherein said ceramic material or said metal in said mixture is in the form of particles having a particle size ranging from about 0.1 to 100 microns.

6. The process of claim 1 wherein said GBN of said mixture has been vacuum-fired for removal of surface boric oxide.

7. The process of claim 1 wherein said GBN of said mixture has been vacuum-fired for its partial decomposition and resultant surface boron enrichment.

8. The process of claim 1 wherein said mixture is formed from the group consisting of chemical vapor deposition, solgel techniques, polymer pyrolysis techniques, and combustion synthesis.

9. The process of claim 1 wherein said ceramic material of said mixture is a carbide, nitride, carbonitride, boride, or silicide of a metal selected from the group consisting of aluminum, titanium, silicon, chromium, hafnium, zirconium, tantalum, molybdenum, niobium, vanadium, tungsten, and mixtures thereof.

10. The process of claim 1 wherein said ceramic material comprises a cermet formed from a ceramic which is a carbide, nitride, carbonitride, boride, or silicide of a metal selected from the group consisting of aluminum, titanium, silicon, chromium, hafnium, zirconium, tantalum, molybdenum, niobium, vanadium, tungsten, and mixtures thereof, wherein the metal for bonding said ceramic is selected from the group consisting of nickel, cobalt, chromium, manganese, iron, molybdenum, tungsten, vanadium, aluminum, magnesium, silicon, titanium, zirconium hafnium, and mixtures thereof.

11. The process of claim 1 wherein said mixture is subjected to a temperature ranging from between about 1000° to 2300° and a pressure ranging from between about 45 and 80 kbars for inducing said conversion.

12. The process of claim 1 wherein the formed CBN/ceramic mass is subjected to size attrition for forming particles thereof.

13. A CBN/ceramic conjoint mass wherein said CBN and said conjoint mass both were in situ formed in an HP/HT process from a mixture of GBN and one or more of a ceramic material or metal which reacts with BN to form a ceramic material.

14. The conjoint mass of claim 13 wherein the proportion of GBN in said mixture ranges from between about 10 to 95 vol-%.

15. The conjoint mass of claim 13 wherein said GBN in said mixture is in the form of particles having a size not above about 1.5 mm.

16. The conjoint mass of claim 15 wherein the GBN particle size in said mixture ranges from about 0.1 to 10 microns.

17. The conjoint mass of claim 13 wherein said ceramic material or said metal in said mixture is in the form of particles having a particle size ranging from about 0.1 to 100 microns.

18. The conjoint mass of claim 13 wherein said GBN of said mixture has been vacuum-fired for removal of surface boric oxide.

19. The conjoint mass of claim 13 wherein said GBN of said mixture has been vacuum-fired to its partial decomposition and resultant surface boron enrichment.

20. The conjoint mass of claim 13 wherein said ceramic material of said mixture is a carbide, nitride, carbonitride, boride, or silicide of a metalselected from the group consisting of aluminum, titanium, silicon, chromium, hafnium, zirconium, tantalum, molybdenum, niobium, vanadium, tungsten, and mixtures thereof.

21. The conjoint mass of claim 13 wherein said ceramic material comprises a cermet formed from a ceramic which is a carbide, nitride, carbonitride, boride, or silicide of a metal selected from the group consisting of aluminum, titanium, silicon, chromium, hafnium, zirconium, tantalum, molybdenum, niobium, vanadium, tungsten, and mixtures thereof, wherein the metal for bonding said ceramic is selected from the group consisting of nickel, cobalt, chromium, manganese, iron, molybdenum, tungsten, vanadium, aluminum, magnesium, silicon, titanium, zirconium hafnium, and mixtures.

22. The conjoint mass of claim 13 wherein said mixture was subjected to a temperature ranging from between about 1000° to 2300° and a pressure ranging from between about 45 and 80 kbars for inducing said conversion.

23. The conjoint mass of claim 13 wherein the formed CBN/ceramic mass is subjected to size attrition for forming particles thereof.

24. The conjoint mass of claim 12 wherein said mixture forming said conjoint mass was prepared from the group consisting of chemical vapor deposition, solgel techniques, polymer pyrolysis techniques, and combustion synthesis.

25. The invention as defined in any of the preceding claims including any further features of novelty disclosed.