Abstract
Diamond-like carbon (DLC) coatings are becoming well established across many industrial sectors including aerospace, automotive, oil and gas, and cold-forming tools. While DLC coatings exhibit good mechanical properties and a low coefficient of friction, the coating–substrate systems may suffer from insufficient wear resistance. This paper describes the effect of mechanical and thermochemical tool steel substrate pre-treatment on DLC coating durability. We have investigated two tool steel substrates, Sverker 21 (AISI D2) and an advanced powder metallurgy alloyed steel Vanadis 8. Initially, the substrates were heat treated in a vacuum furnace and gas quenched resulting in hardness of 59 ± 1 and 64 ± 1 Hardness Rockwell C (HRC) respectively. Subsequently, the samples were subjected to mechanical turning and burnishing with 130 N and 160 N forces, using diamond composite tools with a ceramic bonding phase. Afterwards, a plasma-assisted vacuum nitriding process in a physical vapour deposition (PVD) coating chamber, as a pre-treatment for subsequent DLC coating deposition, was carried out. Coated samples were subjected to a series of ball-on-disc wear tests against Al 2 O 3 and Si 3 N 4 counterparts. X-ray diffraction, instrumented indentation and scanning electron microscopy were employed to examine the mechanical and chemical properties of the wear scars. Selected variable factors, including the type of steel, the burnishing force and the type of counterbody material, were analysed in order to correlate them with the durability of DLC coating deposited on a pre-treated steel substrate. The effect of sequential processes used as pre-treatment on DLC coating durability was demonstrated. The wear resistance was over 180 (Sverker 21 substrate) and 10 (Vanadis 8 substrate) times greater against the Al 2 O 3 counterbody for samples subjected to the following treatment: turning + burnishing with 160 N force + vacuum nitriding + DLC coating, comparing with the sample after grinding. The results are discussed in light of improving the cold-forming tools' tribological performance. • Surface preparation significantly affects the durability of the coating-substrate system • Selected pre-treatment increased wear resistance against Al 2 O 3 counterbody • Sequential surface treatment decreased friction comparing to grinding • H/E & H 3 /E 2 for DLC coating were highest after pre-treatment with burnishing of 160 N
Highlights
The global metal cutting tools market was valued at $22.2bn in 2018 and is projected to grow at a compound annual growth rate (CAGR) of 8.8% to reach $38.3bn by 2024 [1]
The wear resistance was over 180 (Sverker 21 substrate) and 10 (Vanadis 8 substrate) times greater against the Al2O3 counterbody for samples subjected to the following treatment: turning + burnishing with 160 N force + vacuum nitriding + Diamond-like carbon (DLC) coating, comparing with the sample after grinding
We have studied the effect of mechanical and thermochemical steel substrate pre-treatments on DLC coating dura bility and their correlations with the tribological properties of Sverker 21 (AISI D2) and Vanadis 8 tool steels
Summary
The global metal cutting tools market was valued at $22.2bn in 2018 and is projected to grow at a compound annual growth rate (CAGR) of 8.8% to reach $38.3bn by 2024 [1]. Machining is a subtractive manufacturing process using cutting tools to manufacture products through material removal to specified shapes and properties. Machining operations are being constantly optimized with new difficult-to-machine workpiece materials, strict environmental requirements, increased productivity demands and manufacturing costs. Increases in cutting speed lead to an increase in tool–workpiece interaction, along with increases in the temperature and wearing of the tool. To reduce wear and prevent fracture, the tool must be ductile and hard, which presents contradictory demands – a material is either hard and with poor ductility, or vice versa. A solution to this problem is to protect the ductile tool with hard coating. The substrate provides resistance to fracture, while hard coating protects the cutting edge against abrasive and mild adhesive wear. The coating can help to reduce tool temperature by reducing the friction between chip and rake face of a tool [2,3]
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