Figure 1. Comparison of mohs hardness and microhardness scales for several materials. Source: Gem Institute of AmericaWhat properties make diamond a super material for diamond tools, superabrasive and wear applications? Superhardness for one - diamond has the highest hardness or resistance to penetration of any material known and will even abrade the hardest ceramic materials.

Superhard Materials – Measuring Superhardness

What qualifies a material as superhard? Materials with Vickers hardness value over 40 GPa are considered superhard. There are several different ways to measure hardness. The scratch test is one of the oldest and simplest ways to determine hardness. A harder material will make a scratch in a softer material. Scratch testing is the basis of the Mohs scale, which classifies hardness on a scale of 0 to 10. The mohs scale is a very crude test. If someone gives you a diamond and it does not scratch a piece of glass, then the diamond gem is a fake gemstone of paste, glass or plastic.

IndeFigure 2. Hardness comparison of diamond, cubic boron nitride (CBN), and several common materials. Sources: Compiled from Ceramic Hardness (Plenum Press) and DeGarmo's Materials and Processes in Manufacturing (John Wiley & Sons)ntation tests using Brinell, Rockwell, Knoop (H_k) or Vickers (H_v) indenters with ASTM methods are now the most acceptable methods for testing material hardness. Knoop or Vickers tests are typically the most appropriate means for testing ceramics and superhard materials. The length of the cracks radiating out from the indentation can also be used to measure fracture toughness.

Nanoindentation and nanoscratch testing methods have been developed more recently for the measurement of the hardness and bulk, shear and elastic moduli mechanical properties, which are the key factors in the superhard classification process. Typically the Knoop or Vickers indenters for metal and ceramic measurement have a diamond tip because diamond is the hardest material.

Diamond Tools

Figure 3. Polycrystalline diamond (PCD) die materials. Source: SumitomoDiamond powders can also be consolidated to form polycrystalline diamond (PCD) tools. PCD tool materials are formed in a high temperature-high pressure (HT-HP) press usually as a diamond wafer on a backing of carbide. PCD is also formed as a "vein" of diamond within a carbide wafer or rod for certain applications. The wafers or rods are then cut using electrical discharge machining (EDM) to form inserts or segments for brazing onto a tool. Most wafers are polished to a mirror finish, then cut with an electrical discharge machining (EDM) tool into smaller, workable segments that are then brazed onto the sawblades, reamers, drill bits or other tools.

Figure 4. Polycrystalline diamond (PCD) cutting tool inserts. Source NTK Cutting ToolsPCD cutting tools can machine certain non-ferrous, plastics, composites, glass, ceramics, wood (especially laminate flooring) and very abrasive material with extended tool life compared to conventional tooling. PCD tools improve cycle times and productivity (parts per shift) through higher material removal rates. PCD tools can operate at increased speeds and feeds when compared to traditional cutting tool materials. Tight dimensional control and excellent surface finishes are produced with PCD tooling resulting in scrap reduction and superior workpiece quality. While PCD cutters are common in the machining or machine tool industry, the oil & gas industry also employs PCD rock bits cutters to drill wells deep into the earth.

Figure 5. Single point diamond tool dresser for truing and sharpening conventional grinding wheels. Source: Norton AbrasivesCertain tool applications can use single crystal diamond materials. Optical lenses, mirrors, and super flat surfaces are often machined using single point diamond tooling (SPDT), which use a single flawless diamond as the cutting edge. The single point is sharpened to the desired dimensions by mechanical grinding and polishing. The cutting edge of most diamond tools is sharpened to tens of nanometers, making it very effective for cutting non-ferrous materials with high resolution. SPDT machining provides extremely smooth and dimensionally accurate optical components, which do not need additional lapping or polishing to meet surface finish and flatness specifications. Dressers or dressing tools often use a single diamond grit particle or several grits to open up and shape aluminum oxide or silicon carbide grinding wheels. Rotary diamond dressers can have a metal bonded single layer of diamond for complex profile dressing.

Diamond Superabrasive Products

Figure 6. PARADIGM diamond superabrasive grinding wheels for rotary carbide and insert cutting tool manufacturing. Source: Norton AbrasivesDiamond particles (grits or grains) are used to make abrasive products called superabrasives. Superabrasives have outstanding performance on very hard materials or abrasive materials like stone, ceramics, glass, composites and certain hard non-ferrous alloys. Superabrasives are available in almost all of the forms used for conventional abrasives such as polishing compounds and slurries for lapping hard ceramic or carbide surfaces, cutoff wheels, grinding wheels, hones, abrasive discs and abrasive files. The IEEE GlobalSpec SpecSearch® form for Superabrasives and Diamond Tools allows engineers to parametrically search for superabrasive products based on abrasive type, bond type, grit size, applications, materials abraded, abrasive speed (rotary or surface), mounting, and special features.

The Saint-Gobain Norton Abrasives group has a long history of technological innovation to further enhance diamond’s productivity and performance effectiveness. For example, the Norton Abrasive group developed an infiltrated high density (i-HD™) technology for manufacturing high-performance diamond blades with Norton iHD™ laser welded high-density diamond segments that provides faster cutting, longer life and improved user safety. In addition, the i-HD™ technology products are more environmentally friendly because less energy is consumed during their manufacture. Another example of innovation in diamond superabrasives is Norton Abrasive’s PARIDIGM® bond technology for advanced, high performance grinding wheels. Norton Paradigm grinding wheels use a hybrid bond that combines the porosity of vitrified bonds with the grit retention of metal bonds. The new diamond bond technology reduces dressing and cycle times, improves ground part quality and enhances wheel life.

Ferrous Alloys are Diamond’s “Kryptonite”

Figure 7. Diamond and CBN hardness changes with temperature. Source: DiametalDiamond does have a drawback in machining and abrading certain metal alloys. For example carbon, and therefore diamond, dissolves in iron at the high temperatures at the workpiece to cutting-edge interface, which makes diamond ineffective in cutting ferrous alloys (steel, stainless steel, cast iron, superalloys, etc.). Diamond also reacts with cobalt, nickel, chromium, and vanadium under the high temperatures generated in the grinding processes. Diamond’s sister superhard or superabrasive material, cubic boron nitride (CBN) is used to grind iron, cobalt and nickel-based alloys. CBN can also withstand higher temperatures before degrading. Norton Abrasives' Diamond and cBN Superabrasives catalog contains additional information on the differences between and selection of these two superhard abrasive materials.

Diamond Wear Surfaces

Figure 8. Wire dies made from (a) single crystal natural diamond and (b) polycrystalline synthetic diamond. Source: Fort Wayne Wire DieDiamond’s superhardness provides superior wear resistance for other manufacturing application such as dies, guides and punch faces. Diamond die applications include wire dies, compacting dies and shaving dies, and extrusion dies. The die blanks typically have a polycrystalline (PCD) and single crystal diamond (SCD) insert nib or core in the center of a tungsten carbide outer support layer. The die geometry is cut using EDM or laser machining processes. PCD dies have a matrix of interlocking diamond grains that are sintered using in diamond presses. The interlocking grains provide extremely high abrasion resistance. Characteristics of PCD dies include superior toughness and more uniform wear over single-crystal diamond due to randomly oriented grains. Diamond also has the highest thermal conductivity of any material, so frictional heating during drawing or extrusion can be extracted and dissipated faster.

Diamond materials, diamond coatings and diamond-like carbon (DLC) coatings are used to enhance the wear resistance of parts such as jewel bearings, watch crystals, cutting tools, valve components, pump impellers and wear parts, molding machine ejectors, sliding parts, extruder screws and capillaries. A new company, Akhan Semiconductor, is also commercializing "Miraj diamond glass" sheets for smartphone and VR display applications requiring extreme scratch resistance, Miraj diamond glass should be six times stronger and ten times harder than chemically hardened aluminosilicate glass (e.g. Corning Gorilla Glass or Schott BK-7, fused silica, and sapphire.

Types of Diamonds and Diamond Manufacturing

Figure 9. A mixture of bort and gem diamonds (larger inclusions) from the Crater of Diamonds State Park. Source: WikipediaBoth natural and synthetic diamonds are used to manufacture diamond tools and superabrasives. The synthetic diamonds are made by high pressure – high temperature (HPHT) pressing and chemical vapor deposition (CVD) processes. General Electric (GE) produced the first synthetic diamonds in 1954 in Schenectady, New York. The GE researcher Tracy Hall made his breakthrough with a “belt" press, which was capable of producing pressures above 10 GPa (1,500,000 psi) and temperatures above 2,000 °C (3,630 °F). Synthetic diamonds for cutting tools, grinding wheels, dies and other wear components tend to have a yellow or brown color. Natural diamond classified industrially as "bort," "borat" or "boort" tends to have high opacity with a dark or even black appearance due to a high level or internal flaws or twins. Bort diamonds have no value as gemstones and are used for diamond tools and abrasives.

General Electric produced the first synthetic gem-quality diamond crystals in 1970, but they were yellow or brown in color. Eventually, white gem quality diamonds were made by removing all nitrogen from the process by adding aluminum or titanium. Today, HPHT and CVD processes produce a variety of gem quality laboratory-created or artificial diamonds in a variety of colors – blue, pink, green, orange, yellow and brown using irradiation or by doping with chemical impurities. Both natural and synthetic diamonds continue to hold an intrinsic value for their beauty and superior properties.

Future Diamond and Superhard Materials

Researchers have been searching for new superhard materials with higher thermal and chemical stability than pure diamond. In fact, an entire scientific journal is dedicated to the subject, Journal of Superhard Materials. Not every superhard material makes a good superabrasive or tool material. Boron suboxide (B₆O) has hardness values, which would classify B₆O has a superhard material. B₆O has never been successfully used asFigure 10. Electron micrograph of nanodiamonds (10,000X). Source: Wikipedia superabrasive. Scientists have speculated that rhenium carbide or osmium diboride might have hardnesses exceeding diamond, but superhard forms of these materials have not yet been synthesized.

Diamond-like coatings (DLC), CVD diamond growth, and nanosuperhard materials are also promising research areas. Researchers have been trying to make coat cutting tools with CVD diamond or DLC layers to replace PCD insert, but the new tools don’t have the required performance yet. Argonne National Laboratory patented a process to deposit nanodiamond films at lower temperatures (400 C), which will open up the development and commercialization of diamond microelectronics.

We have only scratched the surface of some of the unique properties of diamonds. Diamond has additional properties such as the highest thermal conductivity of any material, high electron mobility and an extremely wide band gap, which are some of the answers to “Why is Diamond an Electrical Engineer's Best Friend?” – Diamond Materials Part 2.

More diamond materials articles:

Why is Diamond an Electrical Engineer’s Best Friend? Diamond Materials, Part 2

What is the Best Material for Electronics Thermal Management? Diamond Materials, Part 3

What is the Ultimate Dielectric Material? Diamond Materials, Part 4

Additional diamond articles coming soon!