Abrasive Machining of Brittle Materials and Engineering Ceramics

As the title suggests loose or embedded abrasive materials are employed to machine away small fragments of a material. In brittle materials such as engineering ceramics, hard abrasive grains are placed in contact with the surface to be machined and a micro-fracture at the surface of the brittle solid leads to the removal of small fragments of materials.

The diagram below provides and indication of sliding wear grooves and micro-fractures caused by an abrasive grain on a brittle solid.

Types of Abrasives used in Abrasive Machining

Some materials that are used as abrasives include:

• Diamond, most often used in ceramic grinding or final polishing due to high hardness and cost. Diamond is commonly used by Insaco for machining of technical ceramics and polishing of sapphire.
• Silicon carbide, generally used for non-ferrous metals
• Aluminium oxide or alumina, the most widely used abrasive, generally used for ferrous alloys, high tensile materials and wood.
• Cubic boron nitride (CBN)
• Zirconia/Alumina alloys, suited to carbon and stainless steels and welds
• Glass, usually used for pressure blasting operations.
• Colloidal silica, generally used for finishing operations as it is only available as a suspension of extremely fine particles.

Abrasive Sizing Conventions

Two scales that are used to classify particle sizes are the US Sieve Series and Tyler Equivalent, sometimes called Tyler Mesh Size or Tyler Standard Sieve Series.

Abrasive Machining Technology for Engineering Ceramics

Abrasive machining is divided into two distinct areas, loose abrasive machining and bonded abrasive machining.

Loose Abrasive Machining

In loose abrasive machining the abrasive is free to move within a carrier fluid. Carrier fluids can be oils, water or in the case of sand blasting, high pressure air. Although there are several loose abrasive techniques such as "tumbling" where components are placed in a vessel with abrasives and "tumbled" to produce a smoothing or polishing effect, in the machining of engineering ceramics, the most common loose abrasive process used to produce highly toleranced engineering components is Lapping for flat surfaces or Honing for cylindrical components.

Lapping

Lapping employs abrasive particles embedded in a lapping plate, often made from iron. The lapping abrasives and a lapping fluid are introduced between the two plates and in addition to free abrasives other abrasives become embedded in the lapping plate.

The surface to be machined is typically placed on a bottom plate, with both plates being rotated to produce a relatively random grinding path for the embedded abrasives - see diagram below.

Insaco routinely achieves TTV (Total Thickness Variation) of less than 0.0001" (2 micron) on 6" (150mm) diameter sapphire discs by lapping.

Honing

Honing employs the same fundamental principles as Lapping, but the abrasive is embedded or bonded in "stones" that are spring loaded within the bore of the cylindrical component being machined. The honing tool moves backwards and forwards inside the component being machined to produce bores with very tight dimensional tolerances.

Insaco excels at maintaining very tight concentricity and cylindricity of polished ceramic tubes and rods to meet piston/sleeve customer design requirements for pumping and metering without seals.

Bonded Abrasive Machining

This is the most common form of abrasive machining used in the machining of brittle materials, typically this involves the bonding of an ultrahard abrasive such as diamond particles in a supporting matrix.

The function of the bonding matrix is to withstand the stresses, strains, temperatures and thermal stresses imposed on it during the machining process, but also to be able to generate the required material removal rate and subsequent surface finish on the machined component.

Designation System for Bonded Abrasive Wheels

The standard designation system for abrasive wheels is shown in the diagram below;

Metal Bonded Diamond Grinding Wheels

The common manufacturing technique for metal bonded diamond grinding wheels is to produce a preformed metal wheel geometry with Diamond or CBN abrasive grains being held in place by an electroplated nickel coating.

Other forms of metal bonding matrices include sintered bronze, iron, nickel combinations and Tungsten Carbide.

Resin Bonded Diamond Grinding Wheels

In this case the abrasive grains are mixed with a combination of thermo-setting resins or rubbers and plasticisers, the mixture is then placed into a mold of the desired geometrical form before curing at temperatures up to 200°C.

Examples of precision ground engineering ceramic components machined by Insaco using a range of abrasive machining operations are shown below.

All of these processes required highly skilled operators and high precision equipment to achieve the highest levels of precision, surface finish and low levels of surface and sub-surface damage.

This information has been sourced, reviewed and adapted from materials provided by INSACO Inc.

For more information on this source, please visit INSACO Inc.