CA1181329A - Surface characteristics of boron rich cubic boron nitride - Google Patents

Abstract

IMPROVEMENT IN THE SURFACE CHARACTERISTICS OF

BORON RICH CUBIC BORON NITRIDE

Abstract of the Disclosure Boron rich cubic boron nitride suitable for use in electroplating processes (e.g., nickel plating) has been made by an acid leaching process. Boron rich aggregated cubic boron nitride grinding grits are leached in a mixture of nitric and sulfuric acids, the following conditions being typical:
(a) volume ratio of 67% nitric acid to 98% sulfuric acid of 10:20;
(b) temperature of 250°C; and (c) for a time of 1 hour.

The process removes sufficient free surface boron to reduce the conductivity at the surface, resulting in an abrasive which does not overplate.

Description

32~

De~ ticn 0~ A ~

Technic~l Field -lrhis invention is re1~ ted to a proc2ss for improv~ the plating characteristics of cubic boron nitride ~CBN~. More ~?arff~larly, the pracess is directed toward reduc~ the ~endency of boron rich csN grit to ~v~plate and to form nodules of the plating met~l (e.g. nickeV during ~e manu~ac~e of electroplated implement~, such as gr~ding wheelc.
10 ~E~:~
~ . S. Patent 2,947,61'? describes cubic boron nitride and its preparation by a catalytic, high pressure/high temperature (ElPt~T) technique. Ot~er ~.
c, Patents on the subject o~ ~:he preparation o~ C13N are: 3,15~,g29;
3,192,015; 3,~01,826; 3,918,931; ~,742,6S4; and 3,9592443. Sosne patents 15 directed sp~iIic~ny toward the conversion of hexagonRl boron nitride (HBN) to cubic boron nitride withou~ a cat~lyst present are: tJ. S. Patents 3,212,852 and 4,150,098 and Brit~sh Patents 1,317?716 arld 1,513,990. Also, Canadian App~cation Ser~ No. 352,170 filed May 16, 1980, disc:Loses an improved process for converting hexagonal boron nitrid~ to cubic boron nitride and also discloses agg~gated grinding grits manufactured by that i~lproved~process.
One of the methods for mak~g the aggregated c~bic boron nitr~de abrasive desc~bed above in Serial No. 352,110 com~r~ses the ~bllowinq-steps:
(a) a vacuum ~lring pre-treatment step ~n which hexagonal boron nitride powder is held at a temperature of About 1400 -1900 C for a time OI
about ~ minutes to 4 hours and at an ini~ial pressure of about 10 3 to 10 10 mm. Hg. (which would incre~se to greater l:han 10 3 mm.Hg during heating ~ue to nitrogen gas evolution);
(b) miadng the resulting powder with single-crystal CBN particles havu~g a maacimum dimension ranging from 5 to 150 microns and in a concentration of 5-30 volume percent single~ryst~l C8N;
(c) prepressing the mL~ture from part (b) in a hand pre~; at about 20,000 psi ' 1 3 ~ 9

2- 60SD-146 (137.9 kPa);
(d) subjecting the prepressed mi2cture to the HP/EIT process: (i) at apressure of55-80 kilob~rs (preferably 65-75 kbar)7 (ii) at a temperature of from 1600C to the reconversion temperature of cubic boron nit~ide S (prefer~bly 2000-2300C), (iii) for a time su~fieient to convert the he~agonal boron nitride to cubic boron nitride and sinter the cllbic boron nitride (about 8 minutes)j and (iY.) in the absence o~ catalyst and impurities; an~ -(e) recoveru~ the CBN.
~ recovering the CBN from the high pressure apparatus, most o~ the hi~h pressure reaction cell rnaterial is physically removed, leaYislg rel~ti~rely large pieces of the product specimen with c~r~on~ possibly !ihield metal and other cell materials present. The~e pieces are trefited with a mi~ture of sulfuric and r~itric acids to remove residual carbon and metal impurities. The und~ssolved solids are washed in water and then mi~ed with a mixture of nitric and hydrofluoric acids to dissol~re any remaining shield m~tal and gasket m~teriP1s from the high pressure celL This step is followed by a final water wash of the CBN pieces. Large lumps of the CBN may be impa~t milled to powder, size separ&ted, and ul~asonically cleaned to yield the 2û desired ag~regated grit.
~econversion temperab~re is defined as that tempe~ture at whi~h boron nitride reconYerts from the cubic crystal structure to the hexsgonaL
This tempe~ature is found along the ~uilibrium line separating the hexagonal boron nitride stable region from the cllbic boron nitride stab]Le region in the phase diagram ~or boron nitride ~see 1}. S. Patent 3,~1~,852; Fig 6. and Column 8, line 66 - Column 9, line 42).
The purpose of the vacuum firing Is two-fold, first, to remove boric oxide from the surfac~ of the hexagonal boron nib~de powder, and secondly, to generate a coating oi~ boron on the surfaces o~ the powder p~rticles. In order to accomplish thLs second purpo~e, it is necessary to carry out the vacuum firing in the boron nitride the~m~l decomposition rang~ The relative amount of the free boron developed can b~ infe~Ted visually from the discoloration of the vacuum fired powder. At the lower firinEr temp~rab~res 1 :~8~3~~
_ 3 _ 60SD-146 (1500-1650C) where only a slight amount of surface boron is generated, vacuum fired powder has a light reddish brown tint. The depth of color increPses with increasing firing temperature or time until at the higher firing temperatures (1800-1900C) the particles are covered with a b~ack boron 5 surface coating.
It is not absolutely necessary that the singie crystal CBN inclusions of step (b) be used. The above des~ribed process can be used with no such inclusions or with other types of inclusions, such as refractory metal powder, so Iong as the inclusion material does not interfere with the high pressure 10 conversion of he~cagonal boron nitride to CBN.
The vacuum fired hexagon~l boron nitride converts in the HP/HT
process to a polycrystalline material which may be du~led by attritious wear.
The alterrlative embodiments in which the single crystal cubic boron nitride (or other inclusions3 are mixed with the vacuum fired powder prior to high 15 pressure/high temperature processing are preferred because they seem to result in an aggregate particle (i.e. polycrystal containing single crystal or other inclusions) having breakdown character~stics which makes it adYantageous for use in some gr~ding applications.
~J. S. Patent 3,852,078 discloses bonded CBN bodies in ~vhich he~agaonal 2n boron nitride is mLYed with CBN before high pressure/high temperature processing, but no pre-treatment of hexagunal boron nitride LS required~
The hexagonal boron nitride utilized in the above-described process is ideal he2~agonal or graphitic boron nitride (GBN). Two forms of hexagonal boron nitride have been identified, blrbostratic and graphitic. The 25 turbostratic structure i~s characteristic of pyrolytic boron nitride ~nd is acontinuous structure characterized by t;wo-dimensional layers of he~agonal rings staclced at irregular intervaLs and randomly oriented. GBN generally ha a more ordered crystal structure than turbostratic or prylotic boron nitride. The boron and nitrogen atoms are believed to form more or less 30 para~lel stacks of fused BN layers in the hexagonal lattice, with the stacking being fairly ordered in translation para~lel to the layers and also in rotation about the normal to the layers. In other words, there are fewer imperfectians and distortions within the GBN structure. GE3N has a density ~ ~ ~3 1 3~9 of about 2.28 g/cm3 ~d an interlayer spacing of about 3.33 angstroms. The structure in any mass of GBN is continuous in any given direction, as opposed to being sep rated by crystal boundaries. The material is generally soft, ~alcy and light in color.
S Further details on the two forms of hexagonal boron nitride may be found in Thomas, J. et al, "Turbostratic Boron Nitrid, Thermal Transformations to Ordered-layer-lattice Boron Nitride", J. A. C. S., voL
84, (J~n. 25, 1963) p.~619; and Economy, J. and Anderson, R., "Boron Nitride Fibers~', ~, No. 19, (1967) p. 283;
- 10 ~ the HP/HT process step (d) the pressure and then the temperature are increased and held at the desired conditions for the desired time. The sintered sample is Pllowed to cool under pressure for a short period of time, and then the pressure is decreased to atmosphei ic. The mass of poLycrystalline cubic boron nitride is then recovered. Care must be exercised in the design of the high pressure cell to ensure against impurity penetration from exterior cell parts into the sample.
Beeause of the boron coating generated during the pre-treQtment step (a) the cubic boron nitride resulting from the above-i~escribed process (usually in the form of grinding grit) is itself boron rich. By following the 20 teachings of British Patent 1,513,990 (e.g.~ inco~porating boron powder, aluminum boride, or mixtureq of aluminum and boron into HBN in an HP/HT
process3 one also obtains boron rich CBN. In order to test the performance of the boron rich abrasives in plated tool`s (e.g. nickel plated grinding ~Ivheels) an atternpt was made to fabricate a nickei plated wheel. SeYere overplating of 25 the abrasive grains occurred which prohibited testing OI the~ wheel and whichwould prohibit use of the boron rich abrasives in plated tool applications. The invention disclosed herein presents a solution to the severP overplating problem. A discussion of pl1ting problems and solutions for diamond tools may be found in Pope~ B.J. and Stark, P., '~ynthetic Diamond for Plated 30 Products", in Proceed~ngs. "Diamond in the 80's" A Technical Symposium of Industrial Diamond Assn. of America, Chicago, ~, Oct. :L3-15, 1~80, pp.
113 - 126.

3 ~ ~.3i~g Di~closure of l~vention The invention is summarized as a process for improving the plating ch~racteristi~ of boron ri~h c~ic boron nitride which comprises leaching the cllbic boron nitride with an acid mixture selected from the group c~nsisting 5 of nitric/sulfilric aeid mixtures and phosphoric/sulfuric acid mixtures for a time sufficient to yield a cu~ic ~aron nitride having a substanl:ial absence of any surface electric~lly eonducting phase which would interfere with electropl~ting.
The pro~ess has been developed using mixtures of 67 weight percent 10 nitric and 98-99 weight per~ent sul~uric acids. ~ this description, whenever volume ratios are mentioned, they refer to volllmes of acids having these con~entraeions~ The pro~ess may be performed using an initial volume ratio of nitric ~id to sulfuric acid of from 2-98 to 75:25 (whi~h corresponds to a mole ratio of HNO3 to H2S04 of from 0.017 to 2,43), at a temperature range of 100 to 3~0C, and for a time of from 10 minute to 12 hours. ~ the e~perimental work through which this invention was developed, the acid mixture was initially boiling. With the passage of time, the boiling would cease after the components of higher volatilil~ had boiled off. ThLs would leave an acid mixture higher in concentratior~ of the less volaffle acid (usually 20 s.alfuric) and at a higher temperature. The ~cperiments were performed by simpl~ placing the baron rich cubic boron nib~ide grit into a beaker of acid on a hot plate.
AIter t~he aeid treatment, the resisti~ity OI the particles LS su~icient to allow ~abrication of elec~roplated tools. rt is believed t:hat this process 2S removes the boron~ontainirlg conductiYe phases ~rom the surface of the partiele~. Althollgh the mixture of hot nitri~/sulfuric acids hQs been found effective, similar treatment with hot nitric ~cid alane, with a nitric/hydrofluori~ acid mixtur~ or with aqua regia was not effective.
This process substantially eliminates abrasive particle ov~rplating, thus 30 allowing plated wheel.q and tools to be made with the boron rich CBN
abra~ives. Speci~icaLly, the ~bMsive~ developed may be succes~fully nickel plated; whereas7 the urltre~ted ~brasive grits may not.

3 ~ 9 60SD 1~6 The acids specified ~i.e. nitric and phosporic) are solYents for boron, and it is believed that this treatment removes elemental boron from the surface of the grits. For purposes of this description, the term boron rich means having boron in a proportion which is substantially greater than the normal stoichiometric amount, which in the case of CBN is a 1:1 ratio of boron atoms to nitrogen atoms.
The invention resulted from an investigation ow how to make the boron rich yrits of Canadian Application S.N. 352,170 usable in plated tools. The essence of the invention lies in the discovery of the particular acids and operating condil,ions which accomplish the desired end. It also lies in the investigation of the differences between the physical properties (i.e., electrical resistance and surface composition) of the treated and untreated cubic boron nitride, Best Mode For Carrying Out the Invention _ It is preferred that nitric and sulfuric acids be used in the leaching process in an initial volu~le ratio of nitric acid to sulfuric acid of 4:96 to 10:20 (corresponding to a mole ratio of HNO3 to H2S04 of 0.034 to 0.41~, and the preferred temperature range is 150C or a~ove.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary. In these examples, after the acid leaching treatment, the CBN yrits were rinsed with water and acetone. Boron rich CBN containing single crystal CBN inclusions, 13~
7- 60SD-1~6 as described in the background section, are designated ~-7. The designa~ion ~;-7D3 is gi~en to those grits containing single crystal CBN
inlusions having a mesh slze of 270 to 325 mesh (54-45 microns), at a concentration of 30 volume percent single crystal CBN.
H~rd niclcel plating tests were ~qm on various samples. The process of making nickel plated tools containing grits such as diamond or cubic boron nitride is we~l known in the art. Typically a Watts bath is used. A
Watts-~pe solution utilized for several of the tests described here~nafter c~mpri~d the foilowing ingredients:
Nickel sulphate-hexahydrate - 100 grams/liter (gO
(NiS04.6H2~3 Ammcnium ~ulphate - 21 (g/I.) ~NH4)2S04) 50dium Chloride - 8 (gll) (~aCl) Ammonium Hydroxide - Added as needed for pH
(NH40H) adjustment SulfuPic Acid - Added as needed for pH
adjustment 2~ A ~ace amount of wetting agent was also used.
Th~ general p]Ati31g procedure was as follow3: First, the p~I of the wetting solution was adjusted to about 5.3 with the ammonil~lm hydroxide and then bac~ to about 4.9 with sulfuric acid. Second, the anode lePd w~s connected to a nickel anode in an anade bag, and the cathode lead was co71nected to the sample holder. Third, a we~l cleaned brass cathode disc was mounted i~ the sample holder or plating fi~ture, a quantity of the cubic baron nitride grit was placed on top of the cathode, and the pl~t~g fixture was 3 ~

immersed in the plating solution or elec~lyte. Fourth, the plating cUrreJlt was increased to the appropriat2 current density (e.g. 3.2 amperes per squ~re decimeter) and p~ating was conduc~ed for a sufficient time ts tack down the abrasive and pl~te over the spaces on the cathode between the abrasive S grairLs. It is recommended that the sample holder be tapped at 5 minute intervals to dislodge hydrogen bubbl~s. The term "tac~ down" refew to the build up of ni~kel on the cathode arolmd th~ CBN gri ts which is the ~Inechanism whereby the grits are held to the cathode plate or substrate.
Taclc down time v~ri2s with the size o~ the grits, incre~sing with increasing grit size. The overplate time (i.e.7 the ~dditional time needed to plate over the remaining sp~ces on the ~sthode) ~Isa increases with increasing grit size.
Typica1 total plating times w~uld range between 20 minutes and 3 hours.
Some references on nickel plated abrasiYe tools and electroplatin~ in general are: Grenier, J. W. ~nd Falovchik, S. T., nElectroplated Tool3 Fabricstion and Per~ormance"9 presented at Diamond, - P~rtner in Productivity a Technic2l Symposium Presented by ~ndustrial Diamond Association o~ A merica, Inc., November 11 - 12, 19~4, Washington, D.C.;
O ~ rd~ E-A-~ l~9E9~YY ~Ys!_~LCY9t9E~Y~9 y, R~bert Drap0r Limited, Teding~on~
England, 196g. ~ ~
Metals and PLastics Pub~cations, Inc., H~ckensack, New Jersey; Graham, K.
A., ~y~, 3rd ed.~ Van Nostrand Reinhold Co., NoY~ 19~11;
Brimi, M. A. and Luck, J. R., ~, American E3sevier Pub. Co., Inc., N.Y. 1965; Lowenheim, F. A., ~, McC~raw Hill Book Co., lg78.
In the manufacture of plated tools there is ~ minimum metal coating thiclmess (as a percent of ~verage grit diarneter or micron size) needed to re'cain the grit. The acid leached grits of this invention are rough Md have an irre~r shape ~llowing them to be s~ongly held in the metal bond. It is be~iel/ed that the minimum nickel coating thickness 3s therefore less than 30 that required ~or presently ~qvailable single crystal CBN grit. This permits a greater portion of the acid leached grits to protrude and do the work of grinding. On the other hand, at normal nickel thickne~ses, the grits would be held ~or a longer time, giving better tool life.

1 ~8:~3~9 .
_ 9 _ 60SD-146 Certain terminology has been used to report the plating results. As used in the following examples, the term overplate measls the-encapsulation of abrasive g ains in nic~el. A nodule is a lump of nickel coating. Percent nodules equals the tot 1 number of nod~lles observed on CBN grit~ or grains 5 (in 3 microsc~pi~ fields~ divided by the tot~l number of CBN grains observed in those 3 microscopic ~ields, that quotient being multiplied by 100 to obtain percent.

Sma~l quantitie~ of boron rich CBN particles were prepared and le~ched 10 with variou~ ~cids ~ listed in Table 1. After a~id tre~tment and r~rLsing with water and a~etone, hard rlickel pla ting tests were run on these v~rious samples. The results are given in Table 1.

Tabl~ 1 ~L~

Mesh Sam~le Size Pretreetment"~ Results X-7D5~ 80/100 HN03/H2S04 - 6 hrs. 3-5% Nodules (180/153 micron) X-7D3 45/50 None 99~6 Overplated (350/300 micron) X-7D3 60/80 H~03 - 5 hrs. 100% Overplated (250/183 mi~ron) X-7D3 80/100 None 1009~ Overplated (180/150 micron) X-71)3 30/100 E~N03/H2S04 -12 hrs. 296 Nadules X-7D5 Boron rich polycrystalline CBN containing 20 volume percent 230/2~0 Me~h (63/53 micron) catalyst grown CBN Gry~tal inclusions ~* ~Ieated on hot p~te for times indicated.

I ~ ~1 3;~.3 - 10 - 60SD-1'16 The above results indicate that nitric/sulfuric acid mixtures are effectiYe in le~ching the particles sufficiently to allow their use in plated tool applications. Surface leaching of the particles was also ~fesTed from the visual appearance of the p~ticles - the virgin and HN03 treated samples S (~1 o~lerplated~ h~d a smooth shiny appearance while the lsached HN03/H2~04 treated samples developed a dull matted appearan~e~

A number of experiments looking at the effects of acid eoneentr~tion~
temQe~ature and time on the leaching process as re~ated to the plateability of 10 the leached material have been run.
In these experiment~ the samples and desirec~ acid mixtures contained in gl~s~ be~cers were placed on a preheated hot pl~te for the desired leaching time. After leaching, ~e acid was decanted and the sampl~ rinsed with w~ter, cleaned in an ultrasonic water bath, rinsed with acetone and air dried.
15 In some OI the e2cperiments the samples we~e removed periodically from the hot pl~te a~ter a ~ven time inte~ral, the acid decanted ant replenished with fresh acid and rehePted. After leach~ng, the samples were subjected to hard nickel p~Qteabili~ testing.
The leaching conditions and plateability test results are gnren in Table 20 2. E~cept for e~periments 8 and 9, various mixtures of nitric and sulfuric acids were used as the leaching medium (prep~red from technical grade 67~6 nitric and g8-99~6 sulfuric a~ids). The ~amples used were 80/100 or 120/140 mesh size boron rich polycrystalline aggregates containing 2096, 30-g5 micron sized CBN cryst~ls prepared as d~3scribed Ln IJ.S. Serial No. 47,656 and the 25 E3ac~round section of this description.

1 3 ;G ~

~ 11 ~ 60SD-146 Table 2 Ini~i~
Vol~lme EIot Pereent Total Leaching Plate HNO3 in Leaehing ~te~s~l Temp- Plateability Test~MixblreTime-hrs hrs erature Test Results 33-1/3 1/4 - 357C Partial o~erplaffng 2 " 1 - '~ Partial ' . oYe~1~ting 3 ~ 2 1 n Few ~odules

4 " 2 - n Partial overplating 15 5 n 4 2 n Few nodules 6 n 4 - ~ Partial overplating 7 " 8 2 n Ni free 8 0 2 - " lU0%
oYerplated n 100%
overplated 1 _ n Par1:ial overplating 11 5~ 1 ~ 24~C Partial overplati3lg 1~ 33-1/3 1/2 ~ 405C Pa~tial overplating 13 16-2/3 1/2 - " Eew Nodule~
14 8-2/3 1/2 - n Ni free 15 16-2/3 1 1/2 n Vesy few nodules 16 8~2/3 1 1/2 n Ni free 17 It 1/2 ~ 357C Ni free 18 n . 1 _ n ~i fre~
19 4 1/2 - n Partial overplating 4 1 _ n Ni free 21 4 l/2 - 405C Ni free 40 ~ Aqu~ Regia 1 lL~13~,9 - 12 - 60SD-1~16 The results in Table 2 show that various HN03/H2S04 mixtures can be used for leaching the boron rich abrasives to Ni ~ree state ~i e., no overplating or Ni nodules formed during hard nickel plating). The data so show that although ni~ic acid is necessary to the process, leaching effieiency S is best at the lower nitric acid concentrations. ~t the higher nitric acid ~on~entrations, periodic refreshening of ~he acid mixnlr~ is req7lired, compare results for test numbers 3, 4, S, 67 and 7.
At this point~ it should be mentioned that later experiments demonstrated that ~he actual temperature of the leaching solution was 10 subs~cantially lower than the temQeFsture of the hot plate on which the be~ker stood, thu~ th@ differences between the temeeratures in Table 2 and those appearing under th~ heading Disclosure of Invention previously.
It has been found that the friability of the cubic boron ni~ride grits incre~ses with increasing leaching time. This fact eould provide a met~od of 15 varying the friability of ~e CBN to match that needed for a gi~ren applieation.
.

Comparative slot grinding tests ha~e be~n made on the series of catalyst grown CBN and leached boron rich polycrystalline CBN nickei plated ~0 gr~nding wheels. These tests eon~isted o~ repe~itive grinding of slots in hardened ~-2 steel wor!cpieces with the relative perforrnance being determined by the number of slots ground over the lifetime of the gr~ding wheels. M 2 steel is a molybdenu~sl based high speed tool steel with a Rockwe~l C hardness of 60~62. The tests were in wet grinding using a water 25 soluble oil under the conditions shown in Table 3 and the results are tabul~ted in Table 4.
Table 3 __ Test Conditions __ Ma~hine: 3 HP
Wheel Si~e: 152 mm x 6.35 mm x 31.8 mm fu31 radius (1/8 inO wide x 6in. nominal dia.) Wheel Spe~d: 2~.~ x M/~e~ond DowT~eed: 1.2~ mm Table Speed: 0.30 M/min (12 in/min) Metal Removal Rate: 82.D2 cm3/hr Worlcpieceo M-2 Steel E~RC 60-62 102 mm ~1S2 mm x 51 mm ~lots/Workpiece: 15 (lS2 mm long) I L~13~

Table 4 Test Results Number of Abr~ re Slots Ground __ _ Control I 855 Coll~ol I~ 240 Control IlI 725 ~-7D3 1740 The control samples were variouS commercially available single 10 cryst 1 cubic bor~n n~trid~ grits.
.~!!
Slot grinding tests simila~ to those desr~ribed in Example I~l were done at 0.40 M/min (16 irllmin) table speed givirlg the results shown in Table

5.
Table 5 Number o~
Abrasive Slots Ground __ __ Control I 510 Control II 23~
2~ C~ntrol m 450 ~Bo~on rich polycr~stalL;ne CBN containing 20 volume percent 20 - 40 micron c~talyst grown CBN crystal incl~ions.

L~cts Electricsl re~tance me~surements were taken on untre~ted and leached 60/80 me~h e (250/180 micron~ samples of boron rich CBN
aggregates. The leached samples were ~eated with 33-1/3~6 H~O3/H2SO4 mixt~re for vario~Ls durations a~ a hot plate tempera~ure of 357C, the acid 30 being changed at one hour intervals~ Resistance values were obtained by pressing the same ~mount o~ sample between metallic conduetors at a given îorce and meRsuring the resistance across the pre~sed sarnple.

'3 ~ ~

Tabl~ 6 ~e~
Besssts~ce (ohms) Control I~ 107 1o8 ~-7- ~nleached 2 x 104 X-7 - Leached 2 1~. 1.7 ~108 ~:-7 - Leached 8 hr3. 5.6 x 108 ~C)btained s,s BO~ZONDI (60/80 mesh3 CBN.
The betteP than four orders of magnitute ia~crease in re~istance 10 ob~ained on leaching correlates with the improvement Ln plateability obtained by leaching. The low resistance and ove~platIng experienced with the un1~e~ted material is attributed to a conductive phase in the boron rich CBN
p~rticl~s which is r~mov~d (at le~s~ to some depth on the surface) by the le~ching process.
T sts hav~ bee~ performed to investig~te the effec~ of long term storage of the ab~iYe in the nicl~el plating solution. ~ one such te~t, after 29 weeks in the p~ating soluffon, the abrasive showed no ill effee~ This is not sllrprisin~ since the leached samples do nut rely on a high resL~tance co~ting on the erystals which could be at:tac}ced by the acidic plating bath.
20 . ~ an eIIort to f~ther characteri . e ~he surface o~ the new cubic boron nitride formed by the proceæ of this invention, X~ay difIra~tion scan were made on a number of sampleg of varying boron conten~ Two fairly strong peaks in addition to the ~ormal CBN peaks were observed in ~11 samples, with the peak heights in~reasing~ with in~re~s~ boron contPnt~
25 Tllese two addil:ional peaks correspond to the two strongest pe~ks r~ported by Wentori ('lBoron: Another ~orm", Science, Jan. 1, 1965, Volume 147, No.
3653, pp 49-aO) Ior high pressure boron. These results would indicate that at least a por~ion (probably a major portion) of the boron (generated on the graphitic boron nitride i~ the pre-treatment step described under Background) 30 r~mains in the elemental state during high pressure/high temperature processing.
On comparison of X-ray diffraction patterns for a typical sample OL leaehed boron rich grit and unleaehed grit respectively, these can be seen to have in eo~mo~ two large peaks at about 43 and 50.5 whieh are characteristic 3.1~3~9 of cuhic boron nitride. In addition two smaller peaks appearing at ahout 35 and 39 can also he ohserved which are characteristic of the high pressure boron phase discovered by Wentorf. (These 2 ~ values correspond ~ith the DeBye-Scherrer d-values of 2.51-2.54 and 2.30-2.32 reported in the Wentorf article.l These la-tter two peaks in the leached grit are ahout 1/3 to 1/2 the size of the corresponding peaks in the unleached grit indicating a re~oval of the high pressure ~oron phase in the leaching operation. It is felt that the leaching process described herein may be used as a general one for re~oval of high pressure phase boron from solid ma-terials.
Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles de~cribed herein may be made by one skilled in the art without departing from the true scope and spiriti of the inveniton which is indicated by the following claims.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for improving the plating charac-teristics of boron rich cubic boron nitride which comprises leaching the cubic boron nitride with an acid mixture selected from the group consisting of nitric/sulfuric acid mixtures and phosphoric/sulfuric acid mixtures for a time sufficient to yield a cubic boron nitride having a substantial absence of any surface electrically conducting phase which would interfere with electroplating.

2. The process as recited in claim 1 wherein a mixture of nitric and sulfuric acids is used.

3. The process as recited in claim 2 wherein the leaching process takes place under the following conditions:
(a) initial mole ratio of nitric to sulfuric acids of from 0.017 to 2.43;
(b) temperature range of 100° to 300°C; and (c) a time of from 10 minutes to 12 hours.

4. The process as recited in claim 3 wherein the initial mole ratio of nitric to sulfuric acids is from 0.034 to 0.41, and the temperature is greater than or equal to 150°C.

5. An improved boron rich cubic boron nitride mass, wherein the improvement comprises having a substantial absence of any surface electrically conducting phase which would interfere with electroplating.

6. The improved boron rich cubic boron nitride as recited in claim 5 which specifically does not interfere with the electroplating of nickel in the manufacture of nickel plated objects containing cubic boron nitride.

7. The improved boron rich cubic boron nitride mass of claim 6 having a resistivity which is comparable to that of single crystal, catalyst grown CBN, and which is several orders of magnitude greater than the resistivity of boron rich CBN which has been made from graphitic boron nitride but which remains untreated for removal of any surface conducting phase.

8. The improved boron rich cubic boron nitride as recited in claim 6 which is characterized as an aggregated cubic boron nitride consisting essentially of single-crystal, catalyst-grown cubic boron nitride embedded in a matrix of boron-rich polycrystalline cubic boron nitride made from graphitic hexagonal boron nitride.

9. A metal plated grinding implement containing grit made from the improved cubic boron nitride of claim 8.

10. A process of removing high pressure phase boron from a solid material which comprises leaching the material with a mixture of nitric and sulfuric acids.

11. The process as recited in claim 10 wherein the initial mole ratio of nitric acid to sulfuric acid is from 0.017 to 2.43, and the leaching takes place at a temperature of from 100° to 300°C during a time of from 10 minutes to 12 hours.