The quality inspection of micron diamond powder is a long debated matter, on which few agree and which is often misunderstood. The root of the problem obviously lies in the very small size of the product, which causes quality inspection to become very difficult and highly operator and instrument dependant. Micron diamond powders therefore fall into the category of products which users are usually unable to inspect themselves. Micron powders need to be very precisely graded and consistent, but there is no agreed common measurement technique and all attempts to agree or standardise such methods have failed (refer to the Product Standards page).
In addition to this technical problem, there is a human factor: in view of the size and nature of the product, it requires a lot of training for a single quality inspector using a well defined inspection technique and a single instrument to produce consistent inspection results, even in the relatively simple case of a coarse micron powder size. At the fine end of the size range, or when more than one operator and more than one location are involved, the problem becomes even worse.
The measurement unit is the micron or micrometre (µ or µm = 0.001mm). The common convention is that the particle size reflects its greatest length, at least in the commonly accepted size descriptions. However, the measurement methods do not always conform to this definition, unless specifically referred to.
Micron diamond powders are usually described by double-digit denominations, such as for instance 4-8µ, which means that the majority of particles are deemed to be located between these two sizes. Some manufacturers use single digit descriptions. In this case they would refer to a 6µ micron size, which represents the theoretical median size of the product.
Particle Size Distributions
Micron diamond powder is an abrasive product. Each carat, the weight unit in which it is traded (1ct=0.2g), contains a very large number of particles. Herewith some examples of the approximate number of particles contained in one carat:
|Micron Size||Particles p/carat|
The size measurement is therefore performed on a very small sample of the product, to establish its statistical particle size distribution, which may be expressed as a weight distribution or, more commonly, as a numbers distribution (or population distribution).
The particle size distribution needs to lie within certain limits, usually defined by the manufacturer. The main figure is the median size (d50) of the distribution. This is the mid-point of the distribution, 50% of all particles being located either size of it. This must lie within certain limits defined as the median size range. Sometimes, the mean size of the distribution is referred to, i.e. the arithmetic average size of all measured particles. The width of the distribution also needs to be defined. Most commonly, an upper size limit such as d99 and a lower size limit are set.
Example: Max. d99 means that the measured d99 of the distribution may not exceed this size limit and min. d10 means that the measured d10 may not lie under this size.In some cases, the definition of a maximum standard deviation is used, which is an easier alternative, depending on the instrument used. There is a downside to it: the standard deviation may be influenced by the instrument type, or the micron size step, or channel width used.
Calibration and Sampling Errors
Instrument calibration is a major problem, specially between different instrument types and locations. Between different suppliers, the calibration variances may easily exceed one full micron powder size. Measurements made in different locations may therefore not be compared. A valid comparison may only be made on the basis of measurements made in a single location.
In every case, whatever the instrument used, it is most important to ensure that the measured sample is totally representative of the product. This means that the product has to be homogeneous throughout, even before it is dried at the end of the grading process and that the sample is taken and prepared very carefully, in such a way as not to affect its size distribution or the inspection procedure.
No measurement is possible without a clearly defined and documented concept, which may obviously vary from one location to another. This concept is mostly a matter of internal convention and is usually defined to suit a company's own requirements, economy or convenience. The measurement results may vary significantly, depending on which measurement method and instrument is used.
Greatest Particle Length
This direct particle measurement obviously matches the very definition of a micron diamond powder denomination. It is used on optical instruments, such as projection microscope and image analysis. It can be used directly to compile the particle size distribution. However, it requires good and consistent particle shape to be meaningful. The circumscribed circle measurement is a variation of this method, used on projection microscopes.
The projection microscope is the most basic and inexpensive instrument, one that theoretically requires no calibration, providing it is adequately set up and equipped. It is limited to powders containing no particles under 0.5 micron, due to diffraction and the wavelength of light. The problem lies in the fact that measurements are very time-consuming, are limited to a small number of particles and require excellent and permanent operator training.
Image analysis is the preferred particle length measurement instrument, which automatically measures particles located in a microscopic slide. The difficulty here lies in the correct choice of the equipment, its correct calibration and the availability of an adequate measurement concept. Image analysis provides a wealth of other useful data, including particle shape measurement, which is collected simultaneously. Repeatability is excellent. Refer to the Inspection Methods section of this document for further details.
Particle Volume Measurement
This 'indirect' particle measurement method is now frequently used in many locations world-wide, mainly for reasons of convenience and ease of operation. Unfortunately, it requires the spherical radius measurement data to be adapted to a particle length 'equivalent', either through calibration or calculation. Particle shape is ignored and the distributions obtained are usually narrower than those measured optically.
In electrozone type instruments, such as the 'Coulter Multisizer', particles are dispersed in an electrically conductive liquid (saline water), which is forced to flow through a small orifice. Each particle produces an electrical resistivity signal as it passes through the orifice, which is approximately proportional to its volume. Each orifice is capable of measuring particles ranging between say 5% and 60% of the orifice size. The resulting numbers distribution of particle volume related values then needs to be converted into an adequate size distribution, usually through calibration against a known calibration powder, or against spherical latex beads, with the addition of a suitable 'shape' multiplication factor. Measurement consistency is very good, but limited to powders containing no particles under say 1 micron.
Laser diffraction or light scattering instruments have been gaining popularity due to their ease of operation. However, they are some steps further away from the desired target, since they measure volume related distributions of particle volumes. A laser beam is sent through a suspension of particles, which is diffracted at various angles proportional to the particle volumes, or scattered backwards on a target. It is difficult to find a useful correlation between such measurements and even the simplest optical measurements.
Oversize Particles and Particle Shape
To perform its duty, i.e. obtain an acceptable surface roughness whilst maintaining the required abrasive performance, a micron powder should not contain unduly large particles. An optical oversize scan may be performed on the inspected sample, but this is not statistically significant, due to the insufficient number of particles concerned. To increase the relevance of this inspection, it may be carried out on a concentrate of the largest particles, which is obtained by the sedimentation of the sample. The main guarantee for good oversize control however remains in a manufacturing process that is perfectly controlled in every aspect, from grading to handling, from the absence of pollution to the cleanliness of containers, etc.
Another important aspect of micron diamond powder quality is its particle shape. For good performance, the powder should contain no plates (flats or shales) and no needles (splinters), as these may affect performance and surface roughness. Also, they may directly affect the measurement results obtained in quality inspection. A good particle shape is a blocky shape, exempt of plates and needles, but still showing sharp cutting edges.
Example: A micron diamond powder made of badly shaped, splinty material may come out as a perfect distribution when measured with certain instruments, where measurement is initially based on particle volume. The distribution may however be perfectly meaningless if particle length is measured. Furthermore, its abrasive performance may not relate to the measured size. This is where the greatest particle length measurement offers an additional quality guarantee
Submicron Powder Measurement
Laser diffraction instruments are often used to measure submicron powders, when particle sizes allow no optical or electrozone measurements. However, the Disk Centrifuge is to be preferred for its consistency and performance.
This instrument includes a clear, hollow disk with a central opening on the front side and which rotates between say 600 and 24'000 RPM. The disk chamber is partly filled with a liquid which holds to the periphery of the chamber by centrifugal force and in which a density gradient has been built, with a higher density at the periphery and gradually reducing towards the centre. Some diluted particle suspension is introduced at the centre, which then sediments towards the periphery. A detector beam measures the density of particles near the outside of the liquid ring as a function of time.
The density versus time data is then converted into a volume distribution of particle volumes, which again needs to be converted to a meaningful distribution through an adequate, proprietary calibration method. Once properly set up and calibrated, which is not an easy task, the disk centrifuge provides very sensitive and consistent submicron powder measurements, down to approx. 0.01 micron.
Scanning Electron Microscope
Submicron powders cannot be 'seen' by other instruments to measure or evaluate their particle shape. SEM photographs may therefore be used to occasionally check submicron particle shape or produce pictures, of which some are shown elsewhere in this website. However, SEM cannot be used for routine inspection measurement, due to the prohibitive cost of producing a sufficient, statistically significant number of particle images.
Matching Product and Application
Micron powder selection
The interpretation of inspection data and the choice of products to match exact application requirements are a matter of thorough staff training. The graphs and figures shown on the quality record form the basic quality information. We then need to match the product with its application environment.
Some applications require narrow distributions, where most particles are similar in size. This is particularly true for bonded applications, or ‘short cycle’ lapping and polishing jobs, in which the expected surface roughness is directly related to the powder size: the lapping cycle is considerably shorter than the abrasive life, or the the abrasive is dispensed in a continuous pattern.
In some cases, however, a wider distribution may be required, to fulfil the needs of a ‘long cycle’ application: The initial powder size provides material removal, whilst the surface roughness is related to the final size of the worn abrasive. A typical application in this category would be the final polishing of diamond drawing dies, or the lapping and polishing of some large components.
Some applications are very sensitive to the presence of large particles, such as precision grinding wheels or glass smoothing pellets. In this case, powders are specially selected for their properties at the coarse end of the distribution.
Most importantly, once the right product has been found for an application, it has to remain absolutely identical from one delivery to the next. This is one of the great strengths of the Van Moppes Quality system. It is designed to eliminate the ‘diamond powder variable’ from the workplace and thus provide better production quality and consistency.
Micron Powder and Mesh Size Overlap Area
By definition, Micron Diamond Powders include all fine diamond abrasives below the D46 (325/400) mesh size, which is the finest commercially produced standard sieve size. The micron diamond powder size range however extends from 0 to say 120 microns and therefore overlaps with sieve sizes up to the D76 (200/230) mesh size.
For the purpose of good product design, is is important to know the exact size correlation between the coarser micron powders and the overlapping mesh sizes. This may result in better delivery consistency and more reliable tools and wheels. To that effect, Van Moppes developed its own optical mesh size inspection system, designed to establish a proper size correlation in the overlap area and reinspect the size distributions of mesh sizes in the grinding size range. The optical inspection limits have been validated by numerous inspection measurements.
|ISO / FEPA Denominations||Normal||High|
Evaluation and Measurement of Surface Roughness
Micron Diamond Powders are most frequently used to achieve a particular geometry on a hard material, usually with a well defined surface texture or surface aspect. In some cases, the surface needs to fit an optical aspect requirement only, but in many other cases the surface texture needs to fall within some prescribed measurement limits. To that effect, a roughness profile is measured, using a dedicated roughness measurement instrument.
Surface texture measurement is fully described in the ISO 4287 Standard, which covers the subject in full detail. Here, we only refer to the Surface Roughness measurement, which is most commonly used to characterize a surface texture after lapping or polishing with micron diamond powders. The surface roughness is denoted by some R parameters, such as Ra, Rz or Rt. Note: W parameters relate to the waviness of a surface profile, whilst P parameters relate to the primary profile, before any geometrical adjustments. For full details, refer to ISO Standards 4287, 3274, 4288 and 11562.
|Profile peak||High par of profile, over mean line|
|Profile valley||Low part of profile, under mean line|
|Ordinate value||Total of peak + adjacent valley|
|Sampling length||Length of one measurement (Lr)|
|Evaluation length||Total of several sampling lengths|
|R parameters||Calculated from Roughness profile|
|W parameters||Calculated from waviness profile|
|P parameters||Calculated from Primary profile|
Surface Roughness Measurement
The table below shows the surface roughness measurements most commonly used in the evaluation of the surface finish obtained with a micron diamond powder. The size unit used is the micrometer or micron (μm), the nanometer (nm) or the Angstrom (Å), depending on powder size used and surface finish achieved (1 μm = 1’000 nm = 10’000 Å).
Description of the most common surface roughness measurements
These descriptions are the most common roughness measurements extracted from the ISO 4287 Standard. This obviously contains numerous other possible measurements, such as profile waviness, profile element width or spacing, relative contact surface ratio, or profile skewness, kurtosis, slope, etc. Some old, still used codes are shown in brackets.
|Type of Measurement||Summary Description of Measurement||Code|
|Roughness values measured over one sampling length||Largest peak height over mean line|
|Largest valley depth under mean line|
|Sum of the largest peak + deepest valley|
|Mean value of ordinates (peaks + adjacent valleys)|
|Average roughness values measured over one sampling length||Arithmetic mean of absolute ordinate values within one sampling length|
|Root mean square value (geometric) of ordinates within sampling length|
|Measured over total evaluation length (total of all sampling lengths)||Sum of the largest peak height and the largest valley depth over the total evaluation length|
Relationship between Surface Roughness and Micron Powder
This relationship can roughly be defined as follows: Ra is mainly guided by the median size (d50) of the micron powder, as well as particle shape. Rt (Rmax) is mainly guided by the top of the particle size distribution of the powder (such as the cumulative d95 or d99 of the size distribution), as well as particle shape.
Oversized particles or particle agglomerates tend to produce large single scratches. It is therefore essential to use the appropriate micron powder quality, which also needs to be totally consistent between deliveries. This is what Van Moppes guarantee to supply
Moreover, the achievement of the correct, targeted surface roughness lies in the adequate choice of powder sizes and the correct succession and timing of lapping and polishing sequences.
In a succession of short polishing cycles (with continuous powder supply, or in which the powder has no time to wear) the surface finish is most directly related to the powder size. In a long polishing cycle (with no intermediate powder feeding), the surface finish is essentially dependant on the residual size of the powder at the end of the cycle.
Resolving Surface Roughness Problems
In an established lapping or polishing operation, a sudden loss in surface finish can usually be traced back to a fairly straightforward cause. The following points should always be investigated first:
- Any change in the polishing cycles, timing, succession of powder sizes?
- Any changes in the component, in the powder or in the working parameters?
- Has the powder been adequately dispersed in its carrier liquid and has the mix been further dispersed by ultrasonics? Residual powder agglomerates are by far the most frequent source of problems.
- Has some abrasive slurry been supplied shortly before completion of the polishing cycle?
Note: Too slow a polishing cycle, or a very low cutting rate may also cause a loss of surface geometry.
If none of these points helps to resolve the problem, please contact us immediately. We are here to help.
Physical Properties of Diamond and CBN Powders
|Product Type||Mesh Sizes and
|Coating factor||Density||Specific volume||Gross weight of
100 carats of nett product weight
|Uncoated Diamond||SSX, SSO
SYP, SYG, SYS, SYT, SYM
SYV, SYR, RBM, PM
|Coated Diamond||SYR-..30Ni SYV-..30Ni
|Uncoated CBN||CBN-A, CBN-B