Abrasive particles can cause massive problems, especially in friction systems. Therefore, it is often of particular importance to avoid and verify an entry. Some information on abrasive particles is summarized below.

Properties of abrasive particles

In most technical systems, mechanical abrasion of some components occurs over time. Often, the introduction of hard particles leads to accelerated wear. The abrasive wear leads to abrasion of the friction partners due to scoring, micro-chipping and grooving. Particles of high hardness and sharp-edged morphology are particularly problematic. These can be found, for example, in almost any ambient dust in the form of silicon oxide particles (SiO2, quartz). The mineralogical, ten-step hardness scale according to Mohs can be used as a benchmark. Minerals with a Mohs hardness of more than 7 are harder than most known materials and are therefore considered abrasive. These include most carbides as well as some naturally occurring minerals. The following table lists some materials, indicating the degree of hardness.

Material/Workpiece	Chem. Compound/Elements	Mohs Hardness
Minerals and Carbides
Diamond	C	10
Silicium carbide/-nitrid	SiC/ Si3N4	9,5
Bor carbide	B4C	9,5
Tungsten carbide	WC	9,5
Vanadium carbide	VC	9 – 9,5
Aluminium oxide, e.g. corundum	Al2O3	9 – 9,5
Boron nitride	BN	9
Titanium carbide	TiC	8 – 9
Silicium oxide e.g. silica quartz	SixOy	7
Metals
Carbonised Steel	acc. to alloy	8
Chromium (hard, electrolytic)	Cr	8
Tungsten	W	7 – 7,5
Manganese	Mn	7
Cobalt	Co	5
Nickel	Ni	3,5 – 7
Iron	Fe	3,5 – 4,5
Copper	Cu	3
Aluminium	Al	2 – 3
Tin	Sn	2
Lead	Pb	1,5

Examination for abrasive particles

A strongly accelerated wear is often due to the entry of abrasive particles. The corresponding examination of the components and lubricants can provide valuable information. GWP primarily uses scanning electron microscopy with X-ray micro-range analysis (SEM-EDX) for this purpose.

Various methods can be used. For example, particles can be extracted from the lubricants by filtration and analysed. Element distribution images are used to search for the corresponding abrasive components. The important elements are recorded (Si, C, W, V, Ti, Zr, etc.) and conspicuous particles are examined in detail for their elemental composition and morphology.

Another examination possibility is the observation of the component surface. Due to the influence of abrasive particles, abrasion and erosion (when influenced by liquids) occur, which are characterised by typical impressions of the corresponding particles. Often the particles are still imprinted in the surface.

Case study: Wear of plain bearings due to silicon oxide

Slide bearings made of bronze showed sudden wear. There was a clear clouding of the oil used. The particles in the oil were extracted via a gold core pore filter in order to analyse them using SEM-EDX. In the process, a strong filter clogging was already noticeable.

The heavy wear became visible in the SEM. The filter was almost completely covered by chips - a typical sign of wear mechanisms. The element analysis revealed copper and tin. This element combination corresponds to bronze. According to this, it was abrasion of the bearing, as expected. To find out the reason for the high wear, distribution images with the corresponding elements were created on several image fields. These included carbon (e.g. for diamond and carbides), silicon (for silicon oxides), aluminium (for aluminium oxides) and titanium (for titanium carbides). These are common materials introduced into systems that can lead to bearing failure. With the help of the analysis, accumulations of silicon could be found (blue in the picture).

These particles were analysed in detail. The EDX spectra showed the main elements to be oxygen and silicon. This composition therefore corresponds to silicon oxide. In the SEM image, the particles appear sharp-edged and of glassy structure - the typical morphology for mineral silicon particles, such as quartz.

Since silicon oxide particles have a high hardness (cf. hardness table) as well as a very sharp-edged morphology of small size (usually < 5 µm), they can exhibit highly abrasive properties. Especially in lubricated systems, the particles can act between the friction partners and lead to microspans due to the plastic deformation of the surface.

In the present case, it was later found that the running surfaces of the bearing had previously been treated with a polishing paste containing silicon oxide. Due to the suspicion that the particles had been introduced by the paste, it was also examined using SEM-EDX. There were similarities in the elemental composition as well as in the morphology and size of the particles contained. It can be assumed that the particles were introduced into the lubricant due to insufficient cleaning after polishing.

See figure 1:
Search for abrasive particles; element distribution image of silicon (blue).

These particles were analysed in detail. The EDX spectra showed the main elements to be oxygen and silicon. This composition therefore corresponds to silicon oxide. In the SEM image, the particles appear sharp-edged and of glassy structure - the typical morphology for mineral silicon particles, such as quartz

See figure 2:
Typical morphology of abrasive mineral particles.

See figure 3:
EDX spectrum of a SiO particle.

Since silicon oxide particles have a high hardness (cf. hardness table) as well as a very sharp-edged morphology of small size (usually < 5 µm), they can exhibit highly abrasive properties. Especially in lubricated systems, the particles can act between the friction partners and lead to microspans due to the plastic deformation of the surface.

In the present case, it was later found that the running surfaces of the bearing had previously been treated with a polishing paste containing silicon oxide. Due to the suspicion that the particles had been introduced by the paste, it was also examined using SEM-EDX. There were similarities in the elemental composition as well as in the morphology and size of the particles contained. It can be assumed that the particles were introduced into the lubricant due to insufficient cleaning after polishing.

For the material identification of particles, the elements shown in the spectrum of the energy dispersive X-ray micro range analysis are used (see "Electron microscopy"). In this chapter you will find further information on material classification and interpretation of EDX spectra.

The EDX analysis
The examination by means of energy dispersive X-ray micro range analysis (EDX) helps to assign a particle to materials and materials. The characteristic X-ray radiation of each element is displayed in a spectrum. The combination of the elements provides information about a material. By means of EDX, only inorganic materials can be assigned in more detail.

The element combinations
The corresponding elements must be assigned to the peaks in the EDX spectra. Certain element combinations are typical for certain materials. The following table shows some typical examples of material assignment. In machine oils, for example, particles of mineral origin as well as various steel and copper alloys occur most frequently. Depending on the system from which the sample originates, there are many possibilities for material classification. This often depends on a specific question. With the appropriate background information on the system, the source of the particles can often be found on the basis of the material classification. The following table provides an overview of technically relevant element compounds with examples of material classification for download .

Preparation and examination of different sample types

A large number of different samples can be examined in the GWP. The decisive factor here is the preparation. The following types of samples are examined for inorganic constituents by means of scanning electron microscopy with energy-dispersive X-ray micro-range analysis (SEM-EDX) and for organic constituents by means of Fourier transform infrared spectroscopy (FT-IR) within the scope of ParticleCheck:

• Dusts and powders (SEM-EDX and FT-IR possible)
• Coatings and deposits (SEM-EDX and FT-IR possible)
• Particles and residues on filters and membranes (mostly only REM-EDX possible)
• Oils and lubricants (particles mostly only possible with SEM-EDX, oil itself with FT-IR)
• Water and various liquids (such as oils)
• Chips and single particles (SEM-EDX and FT-IR possible)

The sample preparation for each sample type is explained below.

Dusts and powders
Dust is produced in almost all technical areas. This can be abrasion from machines, "normal" ambient dust or other impurities. Within the scope of ParticleCheck, the material of unknown samples can be analysed and, with the appropriate background knowledge, assigned. Samples of this type require little preparation. For SEM-EDX analysis, the samples must be conductive. Carbon pads are available for this purpose. They are adhesive on both sides and can thus be attached to a sample plate. The powder or dust can now be sprinkled or stamped onto the other side. The sample is now ready for analysis, no further preparation is necessary.

Customers have the option of ordering a sampling set. This way, stamp samples of dusts or powders can be taken on site and the prepared samples can be sent to GWP. Please do not hesitate to contact us.

Dusts or powders can also be analysed for organic components by FT-IR, provided that it can be assumed that the material is homogeneous or equivalent within. The sample can be applied directly to the diamond measuring cell (ATR) of the instrument. This has a radius of 1mm and should be covered as completely as possible by dust or powder. Therefore, a sufficient amount is required for the FT-IR, whereas single particles are sufficient for the SEM-EDX examination.

Coatings and deposits
The chemical composition or origin of coatings and deposits such as corrosion products is often of particular interest in cases of damage or damage prevention.  The preparation is comparable to that of dusts and powders. If they are already loose, the procedure is the same as for the sample type "powders and dusts". If they still adhere to surfaces, an attempt can be made to scrape off some particles with the help of a scalpel and transfer them to a C-pad. Another possibility is to put the sample into an ultrasonic bath with a solvent. The deposits dissolve in the liquid, which in turn can be filtered through a membrane or a core pore filter (see "The filtration" and "Water and various liquid").

Coatings and deposits can also be partially transferred to the ATR measuring cell of the FT-IR, provided enough material is available and can be scraped off. (see "Dusts and powders").

Particles and residues on filters and membranes
In many technical applications, particles are retained in filters. Often the question then arises as to what kind of materials they are and where they come from. Filters and membranes can be examined directly depending on their size and symmetry. For this purpose, a small piece can be taken out. To make it conductive, it is vapourised with gold or carbon and fixed on an aluminium sample plate. The sample is then ready for analysis using SEM-EDX.

Often, however, the particles are stuck between several membrane layers or the meshes or pores. Here again, it is possible to prepare the samples in an ultrasonic bath with a solvent. The liquid obtained in this way contains the particles fixed in the membranes and can then be filtered (see "Filtration").

Oils and lubricants
A particularly frequent question is the chemical composition of residues in oils and lubricants. The materials found can give an indication of the wear of certain components. However, oils and greases are very viscous and thick, so they cannot be filtered easily. Therefore, dilution is necessary.

Oils are pre-diluted with the help of light petrol until a suitable concentration and flow behaviour are achieved. Lubricants and greases are often more difficult to dissolve. In this case, dilution tests are carried out with various solvents, sometimes also acids or bases. In addition, centrifuges are used to separate poorly soluble from easily soluble components. A number of quite complex steps are repeated several times until the fat is diluted to such an extent that it can be filtered. This process is quite sample-specific.

Water and various liquids
In some cases, liquids similar to water are provided, such as washing liquids. If they have a suitable viscosity, they can be filtered without further preparation (see "The filtration"). If they are too viscous, a dissolution test must also be carried out until a suitable dilution occurs and the particles of the sample can be recovered.

Chips and single particles
Depending on the size and material, chips and single particles can first be cleaned (rinsing and dispersing with acetone or ethanol) and then prepared (see "Dusts and powders") on a conductive C-pad.

If the particles are large enough to be handled macroscopically, they can also be transferred to the diamond measuring cell of the FT-IR and analysed for possible organic components.

Filtration
Particles from liquids can be extracted via core pore filters or filter membranes. For this purpose, the samples must be as thin as possible. This can be achieved with the help of different solvents, depending on the matrix material. The liquids are then extracted via a filtration unit by means of pumping action and reaching a negative pressure. The filter is placed on a ceramic or metallic support filter, which prevents the ultra-thin material from being sucked in or destroyed. As standard, we use core pore filters made of polycarbonate that are already vapour-plated with gold. This eliminates the need for subsequent sputtering to make the samples conductive. The main advantage of this is that the gold adhering to the particles does not influence the element analysis and evaluation too much due to the Au peaks measured in the spectrum. The pore size is 0.8 µm, so that even the smallest particles remain measurable. Other pore sizes (up to 0.1 µm) or non-vapourised filters (e.g. for gravimetry) are also available for certain questions, as well as various membranes.

During filtration, attention must be paid to the particle concentration in the sample. In the case of heavily contaminated liquids, it can happen that a high number of particles clogs the pores and suction is no longer possible. Therefore, only a small proportion of the sample can be filtered at any one time. This depends on the particle concentration and empirical values.

After particle collection, the samples are dried and fixed onto a SEM sample plate for analysis using a self-adhesive C-pad.