Particle counts, particle shape and ferrography: reading wear debris
A particle count tells you how much is in the oil. Particle shape tells you how it was made. Together they separate normal wear from a fault in progress.
Counting particles
A particle count gives the number of particles in size bands per millilitre. In hydraulic and lube systems it is reported as an ISO cleanliness code. Counters that measure shadow or light blocking have limits: dark or dirty oil and high-viscosity oil need dilution, particles that overlap are counted as one larger one, and the signal saturates when the count is very high. Air bubbles and free water can be counted as particles unless the method can tell them apart.
Looking at the shape
A direct-imaging analyser photographs each particle in the oil and classes the larger ones by shape. This gives a count and a first idea of how the wear is happening, in minutes, without a skilled analyst. Ferrography does the same job on a prepared slide under a microscope. It takes longer and depends on the analyst, but it can see colour, texture and surface detail, which a silhouette cannot.
Wear particle classes
| Class | Look | Usual cause |
|---|---|---|
| Cutting wear | Curls and slivers, like machine shavings | Hard particles or a misaligned edge cutting a softer surface |
| Severe sliding wear | Large flat particles with scratches | Overload, loss of oil film, metal contact |
| Fatigue wear | Flakes and chunks, thicker, with a flat face | Rolling contact fatigue in bearings and gear teeth |
| Non-metallic | Crystals, sand, fibres, seal fragments | Dirt ingress, filter fibres, seal wear |
| Water droplets and bubbles | Round shapes | Water leak or aeration |
Interpreting a trend
- During break-in, a high count of cutting and sliding particles is expected and falls away. A repeat rise later is not normal.
- A rising count of fatigue particles, in several size bands, can appear well before a gear or bearing fails. In a test where a gearbox was overloaded to failure, the fatigue particle count rose long before teeth broke, while cutting and sliding particles did not rise.
- A rise in particles above about 20 micrometres is more urgent than a rise in fine particles.
- Take the sample point, dilution from top-ups and filter changes into account.
Know the limits of shape classes
A shape class from a silhouette is only a hint. Friction modifier flakes, carbon and seal fragments can be classed as wear particles because they block the light as solid shapes. If the abnormal classes rise over time, confirm with a microscope examination (ferrography or a filter patch) before you make a repair decision.
Colour clues under a two-light microscope
- Metal particles reflect light and look bright red under combined top and bottom light.
- Non-metallic particles transmit light and look green.
- Dark metal oxides point to heat or starved lubrication. Red oxides point to water.
- Copper and white metal chunks together with oxides point to bearing damage.