Overview

• Remove the damage from sectioning, the depth of damage depends upon the cutting process and the experience of the operator
• This damage can be mechanical damage from the cutting process or grinding burn produced by excessive heat or insufficient cooling
• If the damage from sectioning is excessive it is advised that another sample is cut
• The process uses ever finer grades of grinding paper to remove the damage produced at the previous stage
• At the end of the grinding phase the only damage to the surface of the sample should be that from the final stage 
• Produce a sample surface that is ready for polishing

Terms that are frequently used in grinding

• Planar grinding uses up to P220 – both belts and rotating wheels are used
• Plane grinding that is the initial stage that ensures that all specimens in a holder are the same level. This is accomplished using a coarse abrasive that removes material rapidly.
  • SiC papers used for softer materials
  • Al2O3 used for most ferrous materials
  • Diamond used for hard metals and ceramics
• Fine grinding that prepares the surface from the sectioning or plane grinding for final polishing. The grinding is carried out using a finer abrasive.
  • This process removes the deformation from the plane grinding
  • The traditional method is to use continuously finer SiC papers

Grit size

• The grit size of the abrasive determines how quickly the abrasive cuts the surface
  • The higher the grit number the finer the abrasive 
  • Fine abrasives cut more slowly than coarse abrasives
  • Fine abrasives leave shallower scratches on the surface
• The grit size used at the final grinding stage depends upon the hardness of the material – the harder the sample material the lower the final grit size (rougher surface) of the grinding media prior to polishing
• Low carbon steel will have been ground to P1200grit
• High carbon P600grit
• Aluminium between P1200 and P4000grit.

The grinding process - The Theory

• The grinding media can be seen as a number of single point tools with random orientation
• The stress at the point of each grain of abrasive results in the tensile strength of the material being exceeded at that point and is removed
• If the stress is too low the material will not be removed
• If the stress is too high the sample may be deformed
• If the abrasive becomes clogged or blunt the cutting will not be efficient resulting in deformation and smearing of the sample.
• When the abrasive is blunt the sample will plough rather than cut that results in plastic deformation and an unrepresentative surface.
• The grinding is carried out wet to prevent heat damage
• Wet grinding also carries away the wear particles preventing clogging of the abrasive extending its life. This also reduces the possibility for embedding of the abrasive in the specimen surface reducing potential for misinterpretation.

Abrasives

• Silicon Carbide (SiC) is the main type of abrasive due to its extreme hardness of 9.5 on the Mohs scale
• Other types of abrasive include
  • Alumina (Al2O3)
  • Emery (mainly Al203 but softer due to presence of other minerals)
• The softer abrasives are used on softer materials to reduce the possibility fro embedment of abrasive into the specimen surface.
• The abrasive takes the form of a number of forms
  • Sheets
  • Belts
  • Strips
  • Discs
  • Loose abrasive

Grades of Abrasive

• Abrasives are available in a number of grades
• The depth of the scratch is dependent upon the grit size
• Coarse abrasives leave deep scratches but remove a lot of material
  • Hence they are used at the start of grinding
• Finer abrasives leave shallower scratches but do not remove as much material
  • Hence they are used at the end of the grinding process
• The “standard” grades used in metallography are P120 -P240-P320-P400-P600 mesh although for poorly sectioned samples P60 grit may also be employed.

Lubricants

• The lubricant greatly affects the surface finish and rate of material removal
• The rate of metal removal is generally slow hence the heat generated is minimal.
• The simplest “lubricant” is water that removes heat from the system
• Water also prevents loading of the grit
• Water based lubricants containing soluble oils will produce a far superior surface to pure water alone.
• Alcohol based lubricants are thinner than water or emulsions hence will allow the fastest rate of material removal.
• The lubricant may also carry the abrasive (see platen grinding below)

Historical practice

• Carried out manually gently rubbing the sample on the abrasive.
• The technique takes time and patience to learn to produce the optimum results
  • Bevelling of the sample that results in areas of different gradient on the sample surface.
    • If a sample is bevelled the only way to produce a flat surface is to regrind and in extreme cases remount the sample.
  • It is very easy to apply too much pressure
    • This can lead to rocking of the sample
    • In turn leads to bevelling
  • Not enough time spent on previous stages hence scratches difficult to remove
  • Use of worn abrasives
  • Worn away fingers
• The loose abrasive on a sheet of glass
  • Abrasive (generally SiC powder) applied to the glass
  • Water added and sample rubbed against the abrasive generally in a figure of eight motion
  • This process could be classed as lapping as the abrasive is rolling rather than fixed
  • When all evidence of the previous stage has been removed the abrasive is removed, the glass washed and a finer abrasive applied and the process repeated
  • This is continued until the 600 (or finer) grit stage has been completed

• Abrasive strips
  • In this case the abrasive, generally SiC is bonded to strips of paper
  • The paper is stored in rolls.
  • In the very basic systems a length of abrasive paper is torn from the roll and held onto a sheet of glass with bulldog clips
  • The strips are then wetted and the sample rubbed lengthwise along the strip
  • When all of the evidence of the previous stage has been removed best practice is to turn the sample 90° and repeat until all of the previous scratches are removed.
  • The sample is then rubbed against a finer grade of abrasive again turning the sample 90° and continued until all evidence of the previous stage has been removed.
  • This is continued until the 600 grit stage has been completed
•  

Rotary wheels

• Abrasive discs are attached to discs using a number of methods
  • Pressure sensitive adhesive
  • Edge clip
  • Spray on adhesive
  • Gritfix
• The wheels are rotated with the coolant solution (water) sprayed onto the surface via nozzles.
• The samples can be processed individually by hand or by the use of a pendulum
• The discs are mainly SiC based
• Continuously finer grades of disc are used until the desired finish obtained

Consumables used on rotary equipment

Silicon Carbide Discs

Main range P60 to P1200 grit

Size range 200, 250, 300 and 350mm diameter

Relatively inexpensive but not as long lasting as diamond

Alumina

Main range P60 to P1200 grit

Size range 200, 250, 300 and 350mm diameter

Relatively inexpensive but not as long lasting as diamond

Lower cut rates and higher potential for surface damage than SiC abrasives

Fixed diamond

• Diamond Grinding Discs
  • Patterned surfaces containing diamond
  • Coarse grinding 250 to 15μm
    • P80 grit to P1200 grit
  • Fine grinding 15 to 3μm
• The discs can take 2 forms
  • Flexible polyester backed discs
  • Diamond bonded into nickel pellets attached to a flexible polyester disc
• Diamond abrasives are more expensive than alternatives but last far longer

Problems encountered during grinding

1. Deep scratch formed by
   1. Poor interstage cleaning resulting in carry over of coarse abrasives
   2. Breaking of brittle samples
2. Flowed layer caused by use of blunted abrasive

Deep scratch that prevailed until final polish