Typical color and shapes of synthetic diamonds
In today’s economic reality, as a result of the introduction of synthetic diamonds, it is apparent that increased communication between the field and the tool manufacturer is necessary in order to enhance the success of the drilling industry. Not only the traditional forms of communication need to be improved, but new forms must be encouraged. Hopefully, this paper will germinate ideas that will help improve the dialogue between the field and manufacturer. After all, both are users of synthetic diamonds.
In order to gain a better understanding of why synthetic diamonds work so well, it is useful to discuss diamond synthesis.
Diamond is an extremely hard dense phase of carbon. Commercially, it is formed at pressures approaching 1,000,000 lb. per sq. in. and temperatures equal to that of molten iron, approximately 2,600 degrees F.
To envision diamond synthesis, imagine a molten bath of iron, into which carbon is dissolved. The molten bath acts like a catalyst for the carbon, and while the system is kept at very high pressure, crystals begin to grow. The size and shape of the crystals depend upon the length of time the graphite is kept at diamond stable conditions, but in the order of minutes is generally enough to produce a commercial product of 20-mesh diamonds. Although 20-mesh is small, about 500 to the carat or 0.03 in. in diameter, several tons per year are produced worldwide.
Knowing how the diamonds are made, certain synthetic diamond characteristics are important from a user’s viewpoint: first, crystal morphology and hardness vectors; second, included impurities.
The crystal morphology, or shape exhibited by synthetics ranges from incomplete cubes and octahedrons to perfect cubes, octahedrons and cubic octahedrons. The shapes used in drill bits are preferably the cubic octahedrons, which if compared to natural diamonds, would be graded AAA+.
While synthetic diamonds are identically as hard as natural diamonds, the high strength obtained from synthetics is directly related to their resistance to cleave, or fracture under load. For example, whereas a gem cutter takes advantage of cleavage planes to form a gem from a rough diamond, the diamond tool producer attempts to ensure the diamonds on the tool face are oriented so that cleavage doesn’t occur. A cleaved diamond on the tool face is termed “polished” and when abundant will cause the bit to build pressure and stop cutting.
Even with random orientation, the synthetic cubic octahedrons have less than a 20% probability of being in an orientation conducive to easy cleavage. This is calculated using the (1,1,1) plane as the plane with the lowest cleavage energy on the fourteen-sided cubic octahedron.
When the diamond is grown in the metal bath, up to 10% by weight of the metal is incorporated into the body of the diamond.
Although information is scarce, examinations of reclaimed diamond and literature references indicate that the metal impurities are planar and are parallel to the preferred (1,1,1) cleavage plane. Since the incorporated metal has a far greater rate of thermal expansion than the diamond, bit manufacturing temperatures are kept below a certain threshold in order to prevent diamond splitting by the expanding included metal.
High speeds and low loads recommended for rig performance.
To obtain the maximum performance from an impregnated bit, the highest rotational speeds that ground and rig conditions will allow, coupled with low bit loads, are generally recommended. From field experience, bit loads of 1,000 lb. per sq. in. and rotational speeds of 1,400 ft. per min. are targets to aim for. To convert these numbers so that they will apply to a particular rig, one must take into account the bit diameter and size, pressure and type of hydraulic feed system used.
As the matrix wears, the diamonds are exposed to increasing shear moments and eventually cleave. The matrix is designed so that erosion is gauged to diamond wear. When cleavage flats appear and bit pressure builds, the matrix, ideally, will erode only until enough diamonds are again exposed so that the bit pressure drops and penetration rate increases. This “stripping” as it has been called, should occur continuously as a function of load and revolutions per min. and not as a deliberate action of the runner. This design eliminates potentially injurious field remedies such as shutting off the coolant, lowering revolutions per min. and increasing the bit load.
The wear characteristics discussed thus far relate only to sound, unfractured formations. With fracturing, the drilling characteristics become much less predictable. One reason for this is the lack of information flow between the field and manufacturer. But, as many ore deposits characteristically are highly fractured, it is clear that an improvement in fractured ground performance is needed.
The problem of quantifying rock properties has been studied by several people. An appropriate rock fracture intensity measurement used by North American geological engineering design firms is termed the Rock Quality Designation (RQD). The RQD is given as a percentage and is easily determined in the field. It is the ratio of the length of core recovered greater than 4 in. divided by the total length of the core run. Used statistically, the RQD can be shown to be proportional to formation fracturing and ultimately can be used as a measure in conjunction to rock abrasion and hardness.
What improvement will this measurement bring? It enables the field user once on a job to quickly refine his bit type to suit the ground; it can be included in bid documents so that prospective bidders have a better understanding of the formation; it enables quantitative communications between the manufacturer and the field; and it also renders field work more valuable to certain groups who contract out drilling programs. Most, if not all geotechnical work requires a knowledge of the RQD. Any contractor who is familiar with this system intrinsically offers extra value for his services over those who don’t.
Diaset Products is interested in establishing a protected data bank for any contractors who wish to improve their bit performance in fractured conditions. This data bank will statistically monitor through the use of the RQD the effect of various bit parameters versus performance.
To continue with the description of the synthetic diamond in the bit, we now come to the final stage of the diamond tool. Although most impregnated tools wear evenly to the end of the diamond bearing matrix, some do not, for a variety of reasons. It doesn’t make a lot of sense to discard the diamonds, and although the diamonds can be released from the matrix, it is a contentious issue as to what exactly remains.
Recalling the earlier comments regarding synthetic crystal shapes and inclusions, microscopic examinations of reclaimed synthetic diamonds reveal that the ultimate hardness of the reclaimed diamond remains unchanged, but the shapes do not. Many crystals, either due to thermal shock in manufacturing, or mechanical shock arising from vibration and use have cleaved so as essentially to reduce their size. It is possible, through sophisticated reclaiming methods to separate these very fine diamonds according to shape, but it is expensive and time-consuming to do so. (see footnote *)
Synthetic diamonds are here to stay
What will the future bring? One can be sure synthetic diamonds are here to stay. Soon, large solitary synthetic diamond crystals will be available, competing directly with natural stones in soft sedimentary formations. Polycrystalline aggregates, similar in design to carbonados having the inherent advantage of a lack of a continuous cleavage plane, are proving useful in high production sedimentary drilling programs.
From any viewpoint, improvements in diamond abrasives coupled with a commensurate improvement in toolmaking is welcome. It will inevitably lead to lower drilling costs and as a result, encourage exploration companies to do more drilling. This is, after all, what we all want.
This paper is based on a talk given to the Western Division of the Canadian Diamond Drilling Association, September 1984. A shorter version of this paper was published in the Northern Miner, October 4, 1984
*Nevertheless, this material should still be considered excellent abrasive material and therefore be returned to firms equipped to return this material back to industry in appropriate form. The question that may be forming in you minds is, “What are the diamonds reclaimed from bits worth?” The answer is, of course, “What did you pay for them?” It is apparent that only when diamonds are put into a tool do they assume any field value. Remember, a thousand dollars recovered from your scrap bin is probably equivalent to more than ten thousand dollars in sales.