An approach to the microscopic study of wear mechanisms during hard turning with coated ceramics

• We relate forces and wear mechanisms to the effective parameters of chip area.
• Sudden damage of tool edge as a consequence of cutting force and temperature.
• Coated mixed oxide ceramics reduces parasitic part of the total cutting force.
• Column-like wear of uncoated ceramics due to temperature and parasitic force.
• Iron in wear particles associated with wear of coated ceramics.

The influence of tool edge microgeometry on the wear of tool inserts made from mixed oxide ceramics is investigated. The microgeometry of ceramic inserts is described using quantifiable data, including, tool edge radius (rn), roughness of the rake face (Ra) and tool edge roughness (sometimes called ‘tool edge sharpness’) Rt. Applied coatings affect these quantifiable data. Total force and mean temperature were measured to identify the safe operating region in which the tool edge is not chipped or damaged. The effect of tool microgeometry on wear progress for two types of mixed oxide ceramics-TiN-coated and uncoated, were compared using both macroscopic and microscopic tool wear data. A geometrical approach was used to determine the effective chip area at the chamfered tool edge. This involved mathematical modelling where the effective chip area, effective tool edge length and maximum distance between two subsequent transient surfaces were determined. The total forces were divided into cutting parts and parasitic parts, using both effective chip area and effective tool edge length. Total force division during hard machining operations with uncoated and TiN-coated ceramic inserts, enabled us to compare the effects of tool edge microgeometry on the main mechanisms of wear. Secondary wear areas at the chamfered tool edge were identified for both ceramic types. Hard machining produced abrasive wear pattern and smearing particles due to thermal load for uncoated ceramics; while it produced particles with iron content in the secondary wear area of TiN-coated ceramics.