Friction and wear performance of different carbon coatings for use in dry aluminium forming processes

Abstract

Abstract Lubricants are commonly used in metal forming processes to reduce the friction between the workpiece and the forming tool, to protect semi-finished products and goods against corrosion and to reduce the load on the tool. One aim of environmentally friendly production technologies is to achieve dry forming without the use of lubricants. An oscillating ball-on-plate tribometer (load: 10\u202fN, frequency: 2.5\u202fHz, ball diameter: 10\u202fmm, 99,900\u202fcycles) was used to compare the suitability of different carbon coatings for dry contact against the aluminium alloy EN AW 5083. As candidates a graded and multilayered hydrogenated amorphous carbon coating (a-C:H) system by physical vapour deposition (PVD) as well as a polycrystalline diamond coating by chemical vapour deposition (CVD) were studied. The CVD diamond coatings were tested in two surface conditions, a rough as-deposited (roughness Sa 1.19\u202fμm) and a subsequent polished (Sa 0.01\u202fμm) variant. An uncoated, hardened steel plate of ledeburitic cold working steel (X153CrMoV12-1) was used as reference material. Worn surfaces were examined using laser scanning and atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine their wear rates and to characterize the degree of adhesion, the change in nano-topography and the type and quantitative change of chemical bindings. The tests showed that polishing of CVD-diamond coatings is mandatory for the use in regards to dry forming. Sliding against the hydrogenated amorphous carbon (a-C:H) coating and polished polycrystalline CVD-diamond coating (pCVDD) resulted in the lowest sliding friction coefficients (COF) of 0.13 respectively 0.12. The polished polycrystalline diamond coating showed the lowest wear rates at the round-ended aluminium pin of 4\u202f·\u202f10−9\u202fmm3/Nm and at the coating of 1\u202f·\u202f10−8\u202fmm3/Nm. Cycle-dependent friction coefficient transitions were exhibited by the a-C:H-coating. For the pCVDD-coating as well as for the a-C:H-coating the formation of a transfer film was detected, which explains the small COFs. Due to the nearly negligible wear rate the pCVDD-coating is the most promising one of the tested carbon coatings to enable dry aluminium forming with a long lifetime of the coated tool.