Burnishing (metal)

Burnishing may occur on any sliding surface if the contact stress locally exceeds the yield strength of the material.

The phenomenon can occur both unintentionally as a failure mode, and intentionally as part of a metalworking or manufacturing process.

If the ball is pushed directly into the plate, stresses develop in both objects around the area where they contact.

A bowl-shaped indentation will be left behind, surrounded by a ring of raised material that was displaced by the ball.

If the normal force is low, the ball will rub against the plate but not permanently alter its surface.

However, as the normal force increases, eventually the stresses in the plate's surface will exceed its yield strength.

Burnishing is normally undesirable in mechanical components for a variety of reasons, sometimes simply because its effects are unpredictable.

The plastic deformation associated with burnishing will harden the surface and generate compressive residual stresses.

Although these properties are usually advantageous, excessive burnishing leads to sub-surface cracks which cause spalling, a phenomenon where the upper layer of a surface flakes off of the bulk material.

The combination of higher friction and degraded form often leads to a runaway situation that continually worsens until the component fails.

To prevent destructive burnishing, sliding must be avoided, and in rolling situations, loads must be beneath the spalling threshold.

The benefits of burnishing often include combatting fatigue failure, preventing corrosion and stress corrosion, texturing surfaces to eliminate visual defects, closing porosity, creating surface compressive residual stress.

Ball burnishing, or ballizing, is a replacement for other bore finishing operations such as grinding, honing, or polishing.

[1] Ball burnishing tools of another type are sometimes used in CNC milling centres to follow a ball-nosed milling operation: the hardened ball is applied along a zig-zag toolpath in a holder similar to a ball-point pen, except that the 'ink' is pressurised, recycled lubricant.

Roller burnishing, or surface rolling, is used on cylindrical, conical, or disk shaped workpieces.

Typical applications for roller burnishing include hydraulic system components, shaft fillets, and sealing surfaces.