Effect of TiCN and AlCrN Coating on Tribological Behaviour of Plasma-nitrided AISI 4140 Steel

Effect of TiCN and AlCrN Coating on Tribological Behaviour of Plasma-nitrided AISI 4140 Steel

Santosh Vitthal Bhaskar (Research Scholar, AVCOE, Sangamner and Associate Professor, Mechanical Engineering Department, Sanjivani College of Engineering, Savitribai Phule Pune University, Kopargaon, India) and Hari Narayan Kudal (SND College of Engineering and Research Center, Savitribai Phule Pune University, Yeola, India)
DOI: 10.4018/IJSEIMS.2017070101
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In the present article, samples made of AISI 4140 steel, pre-treated with plasma nitriding (PN), and coated with different coatings like Titanium Carbonitride (TiCN), Aluminium Chromium Nitride (AlCrN), using Physical Vapor Deposition (PVD) technique, were investigated in terms of their microhardness, surface roughness, and dry sliding wear behaviour. Wear tests were performed with a pin-on-disc machine. Coatings were deposited on plasma nitrided samples. The wear behaviour, and wear mechanisms of TiCN- and AlCrN-coated, PN treated AISI 4140 specimens were investigated using a field emission Scanning Electron Microscope (SEM), equipped with an Energy Dispersive X-ray (EDX) analyzer. An SEM was used to study the surface morphology of the worn surfaces. Also, adhesion tests were conducted to investigate the adhesion quality of the coated specimens. The results of the investigation showed improved wear properties. Furthermore, the compound layer formed during nitriding was found to act as an intermediate hard layer, leading to superior sliding wear properties.
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AISI 4140 steel constitutes a very important engineering material which is extensively used in the manufacture of many different parts and components, which encompass connecting rods, conveyor rolls, hydraulic machinery shafts, hollow shafts, axles, and forming dies, ejectors, crankshafts, trim dies, and guides, among others. Components of forming tool dies such as draw ring, ejector pin use AISI 4140 as material for their manufacturing. The integrity of the die cutting tools is essential to achieve adequate product quality. An improperly formed tool will result in inferior product shape. All these components could be subjected to conditions of sliding contact, with the material being formed, and, subjected to wear. However, as-received AISI 4140 steel exhibits poor tribological properties such as poor sliding wear resistance and unstable friction characteristics. It also possesses strong adhesion when it is sliding against itself and other metals (Devaraju, Perumal, Alphonsa, Kailas, & Venugopal, 2012). Such parts necessitate pretreatment on AISI 4140 before being put to service. Surface engineering, which may include surface treatment, coating and surface modification are generally employed to improve wear resistance of steel substrates by increasing surface hardness and minimizing adhesion (reduce friction) (Zeghni & Hashmi, 2004a). One of the approaches adopted by the industry to reduce wear and enhance life of the part is the use of physical vapor deposition (PVD) technique (Luo, Fridrici, & Kapsa, 2011). PVD hard coatings are well-known for providing engineering surfaces with high surface hardness and improved tribological properties. However, it is well-known fact that the application of PVD hard coatings to the substrate materials does not necessarily lead to optimal tribological performance if the substrate material does not undergo suitable pretreatment. It is due to plastic deformation of the substrate, which may result in coating failure (Zeghni and Hashmi, 2004b). The fundamental requirement to improve the wear properties of the substance is the proper and strong adhesion of the coat to the substrate. Presence of hard nitride layer on the lower surface helps achieve proper adhesion of the coat to the substrate (Zeghni and Hashmi, 2004a). Nitriding prior to coating deposition strongly affects the growth and properties of the hard coating (Yilbas & Nizam, 2000). Iron nitride formed in the hardened surface layer after plasma nitriding increases surface hardness substantially (Sharma & Swami, 2014). Moreover, nitriding could promote the development of compressive stresses in the surface of the substrate (Yildiz, Yetim, Alsaran, Celik, & Kaymaz, 2011); therefore, lessening the difference in stress environments in the coating and steel substrate (Yilbas & Nizam, 2000). This was supported by the fact that the best adhesion results were obtained in work pieces with a larger diffusion zone and a thicker compound layer (Devaraju et al., 2012). However, in order to increase the shear properties of the resulting coat, development of the interface layer between the coat and the substrate may be useful. Zeghni and Hashmi (2004a), and Staia et al. (2009) studied the beneficial effects of nitriding of tool steel prior to PVD coating and found improved hardness and wear resistance. The authors, Devaraju et al. (2012), and Yildiz et al. (2011) experimented with plasma nitrided AISI 316 LN stainless steel and found improved wear behaviour at elevated temperature and improved fatigue properties, respectively. The nature and the prevailing wear mechanisms of nitrided and coated tools had been the subject matter of interest in many investigations. To name a few; Nickel et al. (2000), and Yilbas and Nizam (2000) experimented with PN treated and TiN coated drills, and observed longer tool lives under all machining conditions. The improvement in machining performance of the pre-nitrided drills was attributed to the role of the plasma nitriding in improving coating adhesion, and, thus, increasing the wear resistance of the coating. Abrasive wear properties of steel reported to depend upon their microstructure, and chemical composition, as well (Modi, 2007).

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