Metallographic Cutting

 

The first step in preparing a specimen for metallographic or micro structural analysis is to locate the area of interest. Sectioning or cutting is the most common technique for obtaining this area of interest. Proper sectioning guarantees minimal micro structural damage. Excessive subsurface damage and damage to secondary phases (e.g. graphite flakes, nodules or grain pull-out) should be avoided.

Sectioning can be categorized into two areas: Abrasive Cutting and Precision Wafer Cutting. Abrasive cutting is generally used for metal specimens and is accomplished with silicon carbide or alumina abrasives in resin or resin rubber bonds. Proper blade selection is required to minimize burning and heat generation during cutting which degrades both the specimen surface as well as the blade cutting efficiency. Precision wafer cutting is accomplished with thin diamond blades. Wafer cutting is especially useful for cutting ceramics and minerals as well as some metallic materials.

ABRASIVE CUTTING

 

Sample preparation starts with cutting and good cutting means a good start. Abrasive cutting is primarily used for ductile materials. Examples include metals, plastics, polymer matrix composites, metal matrix composites, etc.. Proper selection of abrasive blades requires an understanding of the relationship between the abrasive particle, abrasive bonding and the specimen properties.

Selecting the right cut-off wheel ensures freedom from burn and distortion and is the best way to save time and consumables. Correct cutting produce specimens which are in perfect condition for the next preparation steps. The most commonly used abrasives for the cutting of different materials are SiC and Al2O3.

Silicon carbide is suitable for non-ferrous metals whereas Aluminium oxide is preferred for ferrous metals. Hard wheels are used for cutting soft materials while soft wheels are recommended for cutting harder materials.

• Select the appropriate abrasive blade.
• Secure specimen – improper clamping may result in blade and specimen damage
• Check coolant level and replace when low or excessively dirty. Since abrasives
blades breakdown during cutting they produce a significant amount of debris.
• Allow blade to reach its operating speed before beginning the cut.
• A steady force or light pulsing action will produce the best cuts and minimize blade
wear characteristics as well as the maintain sample integrity (no burning).

Lubrication during abrasive cutting is required to minimize damage and to remove the cutting debris or swarf. It should have a relatively high flash point because of the sparks produced during abrasive sectioning.

 

PRECISION CUTTING

 

Precision wafer cutting is most commonly accomplished with diamond wafering blades, however for some materials the use of cubic boron nitride (CBN) is the more efficient wafering blade. In addition, optimal wafer cutting is accomplished with by maximizing the abrasive concentration and abrasive size, choosing the most appropriate cutting speed and load.

Sectioning with diamond wafering blades is most commonly used for delicate materials requiring a precise cut. The important parameters for sectioning with wafering blades are speed, load, diamond concentration and diamond size. The cutting speed and applied loads are a function of the material being cut. Most common diamond wafering is done between 50 rpm and 4000 rpm with loads varying from 10-1000 grams. Generally, harder specimens are cut at higher loads and speeds (e.g. ceramics and minerals) and more brittle specimens are cut at lower loads and speeds (e.g. electronic silicon substrates).

Several factors are important for choosing the appropriate wafering blade. These include: diamond concentration (low and high), diamond bond (metal plate), diamond size (fine or medium), blade diameter and blade thickness. The diamond concentration is important because it directly affects the load which is applied during cutting. For example, brittle materials such as ceramics require higher effective loads to section, whereas ductile materials such as metals require more cutting points. The result is that low concentration blades are recommended for sectioning hard brittle materials such as ceramics and high concentration blades are recommended for ductile materials containing a large fraction of metal or plastic.

Condition the wafering blade with the appropriate dressing stick to remove previous cutting swarf and smeared metal from the wafering blade. A properly conditioned blade will cut faster and last longer. It is recommended that the dressing stick be mechanically applied to avoid twisting and chipping of the blade.

• Clamp the specimen appropriately so that the sample does not shift during cutting.
• For brittle materials clamp the specimen with a rubber pad to absorb vibration from
the operation.
• Begin the cut with a lower load to set the blade.
• Orient the specimen so that the cut is through the smallest cross section.
• Use largest appropriate blade flanges to prevent the blade from becoming distorted.
• Reduce the load towards the end of the cut for brittle specimens (reduces fracturing at the end of the cut).

For diamond wafer cutting of metals an oil based cutting fluid is required to prevent the metal from coating the blade. Oil based cutting fluids therefore prolong the life and cutting efficiency of the diamond blade when cutting specimens requiring a high level of lubrication for ceramics, minerals, and composite type of samples a water based cutting fluid is recommended. Water based cutting fluids are easier to clean off the specimen, especially if the samples are porous.

application cutting