Observing Precipitated Wear Debris Particles Technological Advances For Particle Counting

V. F. Leavers, V4L Particles Ltd.
Tags: particle counting, wear debris analysis

Wear debris particle analysis is an established equipment health monitoring technique for oil-wetted components. Particle sizing and counting establishes a cleanliness level for an oil sample expressed as an ISO or NAS code. Most equipment manufacturers specify the optimum cleanliness level required for their components and will not honor their guarantee (warranty) if the machinery is operated with oil that is outside of the prescribed cleanliness limits. This is because dirty oil is potentially disastrous for hydraulic machinery and lubricated equipment and is the reason why it is also important to analyze the morphology of wear debris particles in order to identify possible incipient failure modes. Solid particle counts in oil can be determined using optical instruments. These have changed over time according to advances in technology. This article compares the various methods, their advantages and disadvantages and ends with the latest advances in technology in this area.

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Figure 1. Optical Instrument for the Direct Observation of Precipitated Wear Debris Particles

Microscope
In the first instance, the microscope was used to observe the wear debris directly. Particles are precipitated from oil samples, which are taken from the components' lubrication system, either by cleaning and deposition onto a substrate or by draining through a filter patch. These are then used to interactively count and size the particles. In this case, size is defined as the longest chord length. However, there are disadvantages to using a microscope. Most laboratory microscopes are stereo-zoom, binocular instruments, which require a fixed aperture size. High magnification leaves only a short working distance between the objective lens and the sample, which makes manipulation of the sample difficult. At the magnification required to resolve particles of 4 microns (100x), the effective field of view becomes narrow. This makes it inconvenient for use in particle sizing and counting tasks with as many as 10 randomly selected fields of view needing to be processed to give a reliable, repeatable result.1 Similarly, at this magnification the depth of focus is shallow and larger three-dimensional shapes will not appear sharply in focus. This means that particles in different size ranges need to be viewed at different magnifications.

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Figure 2. Percentage Differences Between Longest Chord Length and Projected Area Equivalent Diameter

The Birth of Automatic Particle Counters
Because of the microscope's practical difficulties involved and its labor-intensive nature, interactive particle sizing and counting using a microscope was superseded in the 1960s by automatic particle counters (APCs). First-generation APCs contain a laser light source and a detector, which are separated by an optical cell. The oil sample flows through the cell and when a particle passes through it, an area of light is obscured, causing a shadow to move across the light sensor. The detector senses the shadow and outputs a voltage. The voltage pulse generated increments the particle count and the height of the pulse is used to determine the size of the particle. First-generation APCs have the disadvantage that they cannot distinguish between multiple particles. If two or more particles enter the cell together, they are counted as one large particle. In addition, first- generation APCs are blind to the shape of the particle and are able to report size only in terms of a projected area equivalent diameter. That is, size is defined as the diameter of the disc with an area equivalent to the area of the particle's shadow.

A further disadvantage of using a first-generation APC is that it is not possible to check the results of testing by direct observation, and calibration of instruments is achieved only by using a medium that has been certified by visual inspection using a microscope. Until the introduction of ISO 11171 in December 1991, no comprehensive particle counter calibration procedure existed. Previous standards (ISO 4402-1991) left many important instrument parameters open for interpretation or referred to manufacturers' recommendations. The matter is further complicated by changes in the type of test dust used for calibration purposes.2 In 1993, a project was undertaken by the U.S. National Institute of Standards and Technology (NIST) to certify the particle size distribution of suspensions of ISO Medium Test Dust (MTD) as a reference material to be used for APC calibration. This material was chosen because it is optically similar to the contaminants typically found in filter testing and field contaminants. The particle size distribution is checked using an electron microscope. Particle sizes are defined using the projected area equivalent diameter, which is smaller than the longest chord dimension used in optical microscopy. The complexity of the new test procedures makes it difficult to implement the ISO 11171 requirements and many existing instruments cannot meet the requirements of the new standards. Whereas, manufacturers may certify that their instruments meet ISO 11171 requirements, they stop short of providing detailed documentation and in a recent case study of five different models of instrument, only one of the five passed all ISO 11171 performance criteria.3

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Figure 3. Effect of Shape on Size Shifting Using Projected Area Equivalent Diameter

Fine Tuning
As stated above, first-generation APCs are calibrated using a certified oil sample. That is, the instrument is effectively tuned according to the known particle counts and size distributions in the calibration fluid. However, the process of tuning an APC with respect to a certified oil sample assumes a uniform shift in sizes and a stable size distribution profile. This is not the case, the estimated projected area equivalent diameter is a function of the shape of the particle and this introduces significant uncertainty in estimating the size of a particle. Figure 2 shows the projected area equivalent diameter of three particle silhouettes. It is clear that the size is increasingly underestimated as the particle shape becomes more elongated. The percentage differences between the longest chord length and the projected area equivalent diameter used in estimating the size of the fatigue, severe sliding, and cutting wear particles shown are 4 percent, 49 percent and 76 percent, respectively.

This means that long thin particles will be systematically undersized to the point where they may slip into a size range smaller than their actual size indicates or even disappear from the count all together. Figure 3 shows the effect of shape on size shifting when using the projected area equivalent diameter. The particle silhouettes have been scaled to 15 units to illustrate the way in which long thin particles will be systematically undersized to the point where they may slip into a size range smaller than their actual size indicates or even disappear from the count all together. This is significant because cleanliness codes, such as the ISO 4406 and NAS 1638, divide particle sizes into ranges meaning that size distribution, and not just particle count, is an important source of information concerning the cleanliness of the oil samples tested. Preferential size shifting with respect to shape will produce uncertainties both in the total number of particles counted and in their size distribution. This source of error cannot be mitigated for by simply tuning the APC results according to a calibrated oil sample containing MTD as this medium is not truly representative of the variety of shapes present in metal wear debris particles.6 Thus, testing an oil sample containing a high percentage of elongated particles such as severe sliding or cutting particles will be of limited value. This problem does not occur when using a microscope where longest chord measurements can be calculated. The effect of size shifting with respect to shape may also be a reason why first-generation APCs produce particle counts with significantly more small particles and significantly fewer large particles than if the same sample is sized and counted using a microscope.

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Figure 4. Effect of Particle Rotation on the Projected Image

Second Generation
More recently, a second generation of APCs has emerged. These use the same basic design as first-generation APCs but operate using microsecond duration-pulsed lasers. This has the effect of freezing the image of the particles present in the optical cell. In addition, the optical cell has a larger aperture (1.6 x 1.2 mm2) and the detector is replaced by a charge-coupled device (CCD) array. A CCD is an image sensor consisting of an integrated circuit which contains an array of linked, or coupled light-sensitive capacitors. This device is also known as a color- capture device.8

In this way, second-generation APCs are able to collect the silhouette images of multiple particles. Image processing is then used to count and size the particles. Second-generation APCs are able to use the same definition of size as the microscope, that of the longest chord length. Users of second-generation APCs claim that using neural net technology, it is possible to classify the wear debris mode of the imaged particles from their silhouette images. However, determination of wear debris mode using shape has limited value when the particles are shredded or torn (as is generally the case) and their characteristic shapes destroyed. Moreover, because only the outlines of the particles are imaged, no use can be made of surface appearance, which is a key factor in determining wear debris mode.

An additional source of uncertainty in estimating size when using both first- and second-generation APCs is that particles pass in suspension through a cell which has a volume that is relatively large compared to the size of the particles. For example: in a second-generation APC this is 1,600 x 1,200 x 100 cubic microns. In this case, particles are free to rotate in three dimensions and the projected silhouette will not always be that of the mechanically stable orientation seen when particles are precipitated onto slides or filter patches. Figure 4 shows the effect of rotating particles about the vertical axis and then coupling this with a rotation about the horizontal axis. In the third column of Figure 4, all of the rotated particles appear smaller, and the shapes of the fatigue and severe sliding particles are quite different from those in the first column, which show the stable orientation of precipitated particles. The NIST certification procedure recognizes this source of uncertainty and because the uncertainty increases with size, APCs calibrated in this way cannot claim to reliably size any particles greater than 30 microns.5

Limitations of APCs
In addition to the problems of reliably estimating size and size distribution, it is widely recognized that both first- and second-generation APCs have significant limitations, which make some samples unsuitable for testing. If the sample is too heavily contaminated, then a first-generation APC cannot be used because the light emitted by the analyzer will not pass through the sample in the same way as the equipment is calibrated to receive. Moreover, first-generation APCs count water droplets and air bubbles as solid particles, giving spuriously high particle counts. In addition, various forms of contamination such as tribopolymers, varnishes and oil additives are not detected because their optical properties make them invisible to the system. Because direct observation does not have these disadvantages, attempts have been made to reintroduce the microscope as the optical system of choice. This is partly because of its ready availability and also the ease with which a digital camera and computer interface can be integrated into the viewing system.

However, this co-option of the microscope does not mean that it is best-suited to the task due to the practical difficulties already discussed. One problem is that to date, the camera has been seen as merely a replacement for the human eye. That is, simply a convenient way of digitizing the image seen through the eyepiece of the microscope. Thus, microscope/camera combinations rely on the microscope to produce the required magnification. Recent advances in technology have produced a novel imaging combination that splits the burden of magnification between the optical system and the camera. The result is a lower powered lens than would normally be the case when using a microscope coupled with an extremely high-resolution (21 mega pixel) camera (Figure 1). In this way, it is possible to achieve a relatively high magnification (200x) over a significantly larger field of view (12 mm2). Mounting the camera and lens on a motorized stand and adding images taken in increments of two microns such that the final image appears sharply focused resolves the lack of depth of focus issue.

Case Study
In a case study7 using the novel instrumentation, readings were taken over randomly selected areas of filter patches. For each filter patch, these varied by no more than one ISO code number in each category with the variation occurring most frequently in the 4-micron category. This variation is within the acceptable limit for particle sizing and counting5 and means that for most oil samples, it would be necessary to process only one area of the patch. The case study7 also found that the novel instrumentation and software have a range of measurement similar to that of an APC. The main difference is that the software does not give an indeterminate measurement when the oil sample is too dirty (as is the case with an APC), but rather warns the user that a dilution of the sample is appropriate. The novel instrumentation can also be used in conjunction with automatic particle classification software.

Gravimetric analysis using filter patches is already routinely carried out in many laboratories, even those which routinely use APCs. This is usually done for three reasons. First, to ascertain the weight of debris deposited on the filter patch. Secondly, to perform colorimetric tests on the resulting patch; and thirdly, to give the client an image that allows a qualitative impression of the cleanliness of the sample to be made. Where this procedure is already in place, it requires little extra effort or cost to introduce automatic particle sizing and counting using the novel instrumentation into the laboratory routine. The novel instrumentation has all of the benefits of using a conventional microscope/camera combination without the practical difficulties. Moreover, reduced cost, ease of sample mounting and portability, together with the ready availability of automatic particle sizing and counting software, make the instrument an excellent choice for on-site use.

References

  1. Millipore Particle Monitoring Guide. Lit. No. AD030, Rev. 1998.
  2. Leonard Bensch. "How the New ISO Particle Count Standard Will Affect You." Practicing Oil Analysis magazine, May 2000.
  3. Holger Sommer. "Implementing Particle Counter Calibration per ISO 11171-1999." Society of Automotive Engineers, 2002.
  4. Abbas Vijilee. "Standardized Test Dusts - ACFTD vs. ISO MTD." Machinery Lubrication magazine, November 2002.
  5. National Institute of Standards and Technology 2004. Certificate of Analysis, Standard Reference Material 2806. Medium Test Dust (MTD) in Hydraulic Fluid.
  6. R. Fletcher and D. Bright. Shape Factors of ISO 12103-A3 (Medium Test Dust). Filtration and Separation ISBN 0015-1882, 2000.
  7. V. Leavers and M. Hanlon. "Vision for the Future: Case Studies in Image Processing for Automatic Particle Sizing and Counting and Wear Debris Mode Analysis." Lubrication Excellence Conference, May 2007.
  8. Wikipedia encyclopedia. "Charge-coupled Device." http://en.wikipedia.org/wiki/Charge-coupled_device.