burnishing

Burnishing is a metalworking surface finishing process in which a hard, smooth tool is pressed against a workpiece with sufficient force to plastically deform the surface. The deformation compresses and rearranges surface grains, reducing roughness and creating a shallow, hardened layer without removing material. Burnishing can produce a bright, uniform finish and can improve surface integrity for subsequent wear performance.

Common forms include ball burnishing, where a hard ball contacts the surface; roller burnishing, which uses hard rollers to impart deformation; and friction burnishing, which relies on sliding contact under load. The process is typically performed on lathes, turning centers, mill machines, or dedicated burnishing equipment, with the workpiece rotating or the tool traversing while applying controlled pressure and feed. Key variables include tool geometry, contact force, feed rate, speed, number of passes, and lubricant or cooling conditions.

Effects and outcomes of burnishing include a substantial reduction in surface roughness, the development of a compressive residual stress field beneath the surface, and work hardening that can slightly increase surface hardness. These changes can enhance fatigue strength, wear resistance, and dimensional stability of finished parts. The depth of the affected layer is typically a few micrometers to tens of micrometers, depending on material, tool, and process parameters.

Applications span many metals, including steels, stainless steels, cast iron, aluminum, and copper alloys, with use in automotive components (shafts, gears, hydraulic cylinders), bearings, structural and aerospace parts, and medical devices. Limitations include reduced suitability for brittle materials that may crack under plastic flow, limited effectiveness on deep internal geometries, and the need for proper access and fixturing to ensure uniform treatment. Quality is assessed by surface roughness, residual stress measurements, and microhardness profiling near the surface.