Cubic Boron Nitride Micron

CBN Particle Toughness / Thermal Toughness Index and Size

Q: What does MICRON mean? 

A: MICRON refers to abrasive particle sizes typically 40 µm (1 µm = 40 microinches) or less.

Q: What is a SUPERABRASIVE?

A: A very hard abrasive, usually natural or synthetic diamond and cubic boron nitride (CBN).  Hardness is measured on the Knoop Scale in megapascals (1 Mpa = 145 lb / in2 ) and diamond is 7000 Mpa with CBN following at 5000.  Conventional abrasives such as silicon carbide and aluminum oxide have much lower Knoop values of 2500 and 2200 respectively.

Q: How are MICRON superabrasives measured and classified?

A:

• It depends on the size range:
• 50 nm - 3 mm, Laser Diffraction
• 0.5 nm - 10 µm, Dynamic Light Scattering
• 1 nm - 100 µm Electrophoretic Light Scattering
• 50 nm - 10 mm, Automated Imaging
• 10 nm - 100 µm, Sedimentation

For more information on fine particle measurement, visit www.malvern.com.  Download this white paper: "A Basic Guide to Particle Characterization" 

Q: What are the key properties of superabrasive particles?

A: Size, shape, color, surface (extrinsic) chemistry, internal (intrinsic) chemistry, mechanical properties -- Ti and TTi (toughness index / impact strength and thermal toughness index), zeta potential (surface charge) and microstructure.

Q: What is Particle Size Distribution (PSD)?

A: Most individual grains of a superabrasive powder batch are not the same size (or shape).  The powder consists of a distribution of particles of different sizes represented as a frequency distribution curve.

Q: In a Particle Size Distribution (PSD) curve, what do the terms Median, Mode and Mean represent?

A:

• Median: the size where 50% of the particles is below/above that size
• Mode: the size with the highest frequency (i.e. the greatest number of particles with the largest size)
• Mean: the average size of the entire particle population

Q: What is electroless nickel plating?

A: Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus on a solid workpiece, such as metal or plastic.  The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2·H2O) which reacts with the metal ions to deposit metal.  Alloys with different percentages of phosphorus, ranging from 2 - 5 (low phosphorus) to up to 11 - 14 (high phosphorus) are possible.  The metallurgical properties of alloys depend on the percentage of phosphorus.

Q: What is electroless copper plating?

A: Essentially the same process as electroless nickel, but copper is the coating element. 

Q: Why are superabrasives offered with nickel or copper coatings?

A: To improve the adhesion of the superabrasive in the bond matrix of the grinding wheel and to aid removal of heat from the grinding zone.

Q: What is Ti and TTi?

A: Ti and TTi stand for Toughness Index and Thermal Toughness Index.  This is a measurement of the impact strength of the superabrasive and its ability to resist mechanical breakdown (friability).

The Ti test is performed on a superabrasive at room temperature.  The TTi test is run at room temperature on a superabrasive previously raised to an elevated temperature and allowed to cool down – this mimics the performance of the superabrasive in a grinding wheel in “real world” conditions. 

The Ti and TTi test is performed to ANSI (American National Standards Institute).

Q: What is friability?

A: A measure of the ability of a superabrasive to be pulverized, crumbled or otherwise reduced to a powder.  Ti and TTi testing are employed to measure friability in superabrasives.

Q: Why is diamond so hard?

A: Diamond is an allotrope of carbon, where the atoms are arranged in a variation of the face-centered cubic crystal structure called an atomic diamond lattice. It is this atomic structure which defines diamond's hardness. Diamond is well-known as a material with excellent physical qualities, most of which depend on strong covalent bonds between its atoms.  Diamond has the highest hardness and thermal conductivity of any material and those properties make it the abrasive of choice in many industrial drilling, grinding, lapping and polishing applications.

The basic physical and chemical properties of synthetic diamond and CBN types can influence the performance of grinding wheels and superabrasive tools.  Several properties should be considered when choosing the optimum superabrasive for a specific application:

• Color
• Particle size and particle size distribution (PSD)
• Particle shape / morphology / aspect ratio / ellipticity
• Toughness and thermal toughness index (Ti & TTi)
• Extrinsic / surface chemical purity and cleanliness
• Intrinsic chemical purity
• Coating types and percentage by weight