Creator:RH Wintory
Date Created:
Place Created:
Keywords:cubic boron nitride,synthetic diamonds
Context:talk for APS conference
**************************************************
"The Uses of Synthetic rianond and Cubic Boron Nitride in Industry end Tochnolocy"
(Talk for American Physical Socioty, Karch 22, 1977)
As yon all knovr, It is nice vhen now and then some products of the scientific laboratcry fire actually used for practical purposes. The unique properties and relative scarcity of diamond made its price high enough to ease the task of finding economically acceptable methods for large-_cale production of synthesized diamond. An additional favorable factor was the discovery that the crystal morphology of the diamond affects its utility, and that this morphology could be pretty veil controlled during growin.
For example, a lot of diamond abrasive is ^sod to grind cemented tungsten carbides, a;; depicted in F3 gnv<* )-i • these carl idea arc used for a '.'id? variety of b1 3h-prcdu.M-.jvj ty tools In industry. The <<inc of diamond which dee? this Job the best has an appearance which a crystal-grovrer or solid-state physicist would regard as terrible: it's full of fissures and overgrowths and inclusions, as yai can see in Figure U-2. When such crystals are bonded into an abrasive wheel, usually by means of a high-melting resin or a low-molting ccrarric, -the ro-igh crystal surfaces help to hold the crystals in the wheel as they cut on the workpiece. Vhen the cutting edge of a crystal becomes dull, the forces on it increase. If the crystal were strong, it would be torn from the wheel and its useful life would be finished. Put if the crystal is not too strong and is firmly held, the dull edge portions will, chip off and newly-formed sharp edges will be formed to continue' cutting. Thus the usefulness of the 'diamond grain is increased. If is necessary to have a true cutting acttor here because the worVpi^ce material is ductile and strong in tension, and so it can't be cracl-ed out very easily. If the cutting edge beeones dull and cutting action is replaced by a sort of plowing action, excessive heating by friction occurs locally and the resulting higher temperatures favor chemical attack or graph!tization of the dirmond.	' i&fop
Another large use for diamond abrasive is saving or grinding stone, concrete, glass, ceramics, etc, as shown in Figure ^-3. You night think that the sane principle? about cutting action would apply to this use as they did for cemented tungsten carbide. In the beginning you would be right. Figure ^-"M shows some curly glass chips cut from the> surface of a microscope slide by the sharp edges of diamond grains. Put the cutting edges soon become dull and a kind of plowing actio:: then occurs in which the workpiece material fails in tension near the advancing abrasive grain. If the grain is strong, it can continue to pic./ for a long time with satisfactory results, basically because the workpiece material is weak in tension. Some of the tough, strong diamond crystals useful for cutting ceronics, glass, rock, etc. are shown in Figure
( , As Kerb Strong mentioned, the sizes and crystal morphology of
the synthesized diamonds can be controlled by the conditions of growth,
and so the manufactured diamonds are tailored to the end uses, with
several kinds of diamond available ir„ appropriate sir.es. In this way
the most effective and economical use of diamond is obtained, and
the average improvemat in the performance of diamond abrasive is
aprroximatoly a factor, of two from the time when the synthesized
diamonds first appeared on the market, about 20 years ago. The bulk
of abrasive diamond use is now met by synthesized diamond, hundreds
of kilograms rer year. In spite of the improvements in performance, the
price of diamond abrasive his remained about the same, £2.7? per
carat or £6000 per pound.
i
Not all Industrial diamond is used as abrasive, however. Respectable amounts aro used as single crystals, of gemstone size, in special cutting tools, rock drills, wire-drawing dies, and tools for " dressing grinding wheels. However, single cyrstal diamond is easily cleaved on planes parallel with the octahedral faces, and this characterise.tic of diamond generally acts to shorten the life of tools made from single crystals. Some natural polycrystallinc lumps of diamond known as carbonado can be found in Brazil and western Africa where the two continents were once joined. There lumps are
much tougher than single crystals but the supply is United and the quality is uneven. For many years a goal of diamond research was to sinter svy.o.11 diamonds top ether into strong pieces.
Some formidable natural obstacles stand in the way to this goal. First of all, glueing the diamonds together vith seme sort of a bonding agent is not a satisfactory solution, "'he strength, melting point, anr* thermal conductivity of diamond are so far aVove those of any potential glue that the composite would have properties more like the glue than like diamond. V.ony investigators will agree on this point, having worked over all the glues they could think of. The best glue is diamond itself, that is, the crystals must be held togethe by direct diamond-to-diamond bonds.
*	m
Second, the sintering process has be carried out at some temperature where the carbon atoms are somewhat mobile, which implies a high pressure, otherwise the diamonds will change to graphite. Getting a high pressure all around a diamond crystal is not so easy in a mass of diamonds. If you put some diamonds in-a tox and sq^e^ze on the sides, you do not rove the diamonds together very much because they are se hard and strenr. I-'ariv voids remain among the grains, ir you squeeze harder, the diamonds dimply indent the walls of the box. It is like Iryinr to compress a mass of sand inside a box rade of modeling clay. Where the diamond grains touch each other, the pressures are qui^e high, and	they don't t'uch, the pressures
are quite low, a few atmospheres. If the rass is heated, graphite can form where the pressure -is 1ow. In order to change this graphite back into diamond, it has to he subjected to fairly high pressures, but the pressure won't be there unless the grains which are supporting th= compressive load deform. Sut these rrains are the strongest material known.- * .
From the foregoing remarks one might conclude that sintered diamond masses are almost impossible to make except at pressures and temperatures lil'e those used, by Bundy to transform graphite into dianond, say 130 Vbar and 3000°C. If natural carbonadoes formed in this way, one must regard them as very special visitors to the surface of the earth, because 130 kcars corresponds to a depth in the earth
of about ^00 km or miles.
Tracy Hall found that 1)0 Vhar was not needed to rake something useful out of diamond powder. He used 70 kbar or so and controlled the time and temperature so that only small amounts of graphite formed while the diamonds were hot enough to stick together where they touch each other. You might call this a pure;physics approach to the problem.
Meanwhile,In our lab at G.E. we thought we'd try a little chemistry, too, if you'll pardon the thought, and we found a way to make sintered diamond masses which are extremely thoroughly bonded by diam^nd-to-diamond bonds, with no graphite arid only a little metal in the mass. The main fault of such lumps appears to be that they are so strong and hard that they take almost forever to shape and polish, even on a diamond lap. They are now manufactured In several shapes, some of which are shown in Figure '<-6. Figure 7 shows the polished surface of such a diamond mass and reveals the extensive diamond-to-diamond bonding.
The wire-drawing dies made with these lumps have worked out quite well because they don't burst easily and they wear uniformly and slowly: 100,000 miles of copper wire is not unusual for such a die before it needs retouching. The tools are excellent for cutting hard abrasive, materials like ceramics, rock, fibcr-reinforced materials, and certain alloys like the silicon-aluminum used for automotive engine pistons. But don't try them on steel or nickel-based alloys. Here once more chemistry rears its beautiful head; these metals when hot have a devastating effect on diamond.
Luckily nature has provided us with some small atoms on either side of carbon, namely boron and nitrogen, and boron nitride, EN, bears many resemblances to carbon. Like graphite, it can exist as a soft, slippery solid. And at high pressures and temperatures like those used for synthsising diamond from graphite, the soft IN enn be transformed into a hard, diamond-liVe cubic form. The catalysts for this transformation are not iron, nickel, and the like; instead the best catalysts are fouv.d to be nitrides, particularly the salt-like nitrides such as those of lithium-or magnesium.	_
Cubic EN has almost as much bonding enerpy per cubic cm as diamond, and so it Is almost aLr hard. Figure shov/s the structure of Cubic EN, and Figure shows s^rre crystals of it. Cubic EN is more inert than than diamond toward attack by o::ygen or hot iron or nickel, ana therefore has found wide use as an abrasive for hare] steals and nic' el-hased alloys. A cubic boron nitride alrasive wheel wears very slowly and is extremely useful where close tolerances must be maintained on the wcrkpiece, as in tools with many teeth such as broaches or milling cutters, or where many parts must be made to exact sizes, such as hearings.
Ctibic EN gVrains may also 1 be bonded and sintered together into hard, strong masses which make excellent cutting tools for hard steels and nickel-1ased alloys, and chilled cast iron. Figure U-/9 shows such a tool, and Figure U-li) shows one peeling off a red-},ot chip from-a piece of jet-engine alloy, here the tool is so strong and refractory that it can be operated at temperatures above those at which the workpioce material softens.
So we see that diamond and cubic EN form a complementary pair, with one taking up where the other leaves off. As far as we can tell, they don't seem to te soluble in each other to any great extent. Neither does something harder than diarr'-.nd seem to be available from such a combination.
Someti-as people ask us why, with all this high-pressrure equipment we don't make something harder than dia-ond. !7e always reply that we would be glad to, but what should we make it out of? It is difficult to improve on carbon, with its small effective atomic radius and four strong chemical Vnds per atom. We really need some more elements in the first row of the periodic table, and as you all know, making new elements is not easy, even for a physicist.
bopp that you have enjoyed our talk on Just the three elements that have been so interesting to us: boron, nitrogen, and carbon. Thank you.
,.iKur„ i Autoiimtrd ( ut Olf Mnchine
COMPAX diomond tool blanks
Figure |7 Diamond compact, sinlcied lor (a) 5 mm, (h) (>0 mm (< -00).
•	CARBON
•	BORON
« NITROGEN
BORON NITRIDE "DIAMOND"
Machining Inconel 718 willi .1 CBN