A review of producing hard coatings by means of duplex treatments using an electroplated coating-thermochemical treatment combination

Héctor Cifuentes Aya¹, Jhon Jairo Olaya Flórez²

¹ Mechanical Engineering, Universidad Nacional de Colombia. Master of Education-University , Universidad Pedagógica Nacional. PhD Candidate inEngi-neering Science and Materials Technology, Universidad Nacional de Colombia. Assistant Professor, Universidad Nacional de Colombia. hcifuentesa@unal.edu.co

² Metallurgical Engineer, Master in Materials and Processes, Universidad Nacional de Colombia. Doctor of Engineering, Universidad Nacional Autónoma de México. Associate Professor, Universidad Nacional de Colombia. jjolayaf@unal.edu.co

ABSTRACT

Duplex treatments have been developed to overcome the disadvantages presented by simple treatments to surfaces of different materials and have, in a combined and complementary way, the properties that each of these methods supplies individually. The difference between thermal expansion coefficients for Fe and Cr in hard chrome plating leads to crack formation in the deposited coat, through which corrosive agents migrate and reduce the system's integrity.

Direct deposition by physical vapour deposition (PVD), used for obtaining chromium nitride films on steel substrates, is limited by high production costs, the low thickness obtained and low resistance to corrosion due to the presence of micro pores. Some studies have combined an electroplated chromium with thermochemical treatments made in a controlled atmosphere or vacuum furnaces or by plasma. This kind of duplex treatment allows compounds such as Cr_xN, Cr_xC_yN and Cr_xC_y to be obtained from chemical and micro structural transformation of chromium with nitrogen and/or carbon, the sealing of cracks in the coating and increasing the magnitude of properties like hardness and density, improving wear and abrasion and corrosion resistance.

Keywords: Duplex treatment, electroplated coating, thermochemical treatment, nitriding, carburising, nitrocarburising, plasma

Received: January 19th 2011 Accepted: November 15th 2011

Introduction

The constituent elements of a machine, instrument or equipment are subject to different kinds of physical and/or chemical phenomena (corrosive environments, high temperatures, contact loads, fatigue) that can lead to the gradual loss of its functional integrity and the performance of the engineering system to which they belong, by degradation that usually starts on its surface (Batchelor et al., 2002; ASM, IOM Communications, 2005). Different surface treatments have been developed aimed at extending their useful life, availability and reliability. These treatments transform the surface by the intentional growth or addition of a new layer or by surface or subsurface modifications without intentional growth or increase in the size of the piece (ASM Handbook Vol. 4, 1991; Celis et al., 1999). As a result, such region having few molecular diameters or microstructure surface having different chemical composition and behaviour regarding substrate materials can work in different conditions concerning its use (Kaufmann, 2002). However, each individually-applied treatment can present a number of disadvantages affecting a layer's final performance. In the case of electrolytic hard chromium coatings, development and relief of residual efforts in the substrate/coating system (Nakhimov et al., 1968; Pina et al., 1997; Torres-Gonzalez and Benaben, 2003) during deposition produces microcracks in the deposited layer, having adverse effects on mechanical properties, friction coefficient with reduction in corrosion and wear resistance (Karpov, 2001; Kim et al., 2003; Nam et al., 2004; Wang et al., 2005; Sommers et al., 2005; Ajikumar et al., 2006), especially at temperatures higher than 623°K (Menthe and Rie, 1999).

On the other hand, the production of chromium nitride films (Cr_xN) on steel substrates by treatments like direct deposition (e.g. by PVD) is constrained by different aspects such as high production cost, low resistance to corrosion due to micropore presence (Creus et al., 1998), low thickness (about 5µm) and high residual compression stresses produced by difference regarding thermal expansion coefficients between Cr_xN compounds and steel substrates (1:6 ratio), especially in high temperature applications (Ajikumar et al., 2006; Buijnsters et al., 2003). Different types of combinations of surface treatments have been developed in recent years whose design must be characterised both by their economy and technical viability in making up a duplex treatment integrating the advantages of simple processes to produce surfaces having multicomponent layers and compound layers having higher physical-chemical features not made available by other processes (Matthews and Leyland, 1995; Kessler et al., 1998; Celis et al., 1999; Wierzchon, 2004).

This article reviews advances regarding surface treatments combining using electrolytic hard chromium coatings as a method of layer addition from a liquid phase on ferrous substrates (e.g. steels) with thermochemical treatment of nitriding, nitrocarburising and carburising producing chemical and microstructural changes in coatings from a gaseous phase or plasma. The duplex process was selected for its high industrial impact associated with increased chemical and mechanical property values in the surface layers of treated parts, thereby obtaining a combination of properties that allow complex loads (e.g. an increase in resistance because of fatigue-corrosion synergy) (Celis et al., 1999; Kessler et al., 1998). In particular, this article emphasises vacuum gaseous treatments due to the economic benefits obtained with hard layers because of the lower cost of these treatments compared to other techniques (e.g. PVD) (ASM - IOM Communications, 2005) and its impact on potential reduction in losses related to corrosion which can reach 5% of GDP in many countries (Groysman and Brodsky, 2006), i.e. $276 billion, in the U.S., equivalent to 3.1% of GDP for 2001 (ASM Handbook, Vol. 13, 2003).

Electrolytic hard chrome coatings combined with thermochemical treatments

Such duplex treatments start with the deposition of an electrolytic hard chrome coating on a ferrous metal substrate. The characteristics of the coating are selected according to hardness, wear resistance and the piece's required thickness, shape and size, substrate and dimensional specifications. The coated substrate is then subjected to chemical-mechanical surface cleaning (Groover, 2007) and then placed in a plasma chamber, in a low vacuum furnace or a controlled atmosphere furnace. This chamber may have been previously evacuated and purged with an inert gas like argon to minimise O2 partial pressure to prevent its corrosive effects. The thermochemical treatment selected (nitriding, nitrocarburising, carburising) is then carried out in a gaseous phase or by interaction with plasma. The pressure in the chamber has to be reduced to around 10-2 Pa to make the treatment, at a temperature whose magnitude is a function, regarding the type of precursor gas used. For example, the formation of composite type Cr_xN has been reported when T ≥ 873 °K for gaseous nitriding with NH3 (Ajikumar et al., 2006; Buijnsters et al., 2003; Basu et al., 2007). To avoid corrosion of the system so obtained at the end of the treatment, cooling takes place in the chamber or vacuum furnace in the presence of the precursor gases used. Thermochemical treatments allow atom species (N and/or C) to be obtained that subsequently diffuse on the surface of the hard chromium coating as a result of chemical kinetics (King et al., 2005; Pierson, 1999). The chemical adsorption of reactants by active sites on the surface of metallic substrate in heterogeneous processes (Whitten and Yang, 1996) leads to the start of dissociation of carrier gases present in the atmosphere and chemical potential and activities required for the treatment developing (Mittemeijer and Slycke, 1996) which subsequently, as a result of reactive diffusion, lead to the development of the desired phase (Arkharov and Konev, 1960), see Figure 1.

Electrolytic hard chromium coatings combined with nitriding thermochemical treatment

This treatment combines the deposition of a hard chromium electroplated coating on a ferrous substrate (as a method of adding layers from a liquid phase) with a nitriding thermochemical treatment as surface transformation method either by plasma nitriding or using controlled atmosphere or low vacuum furnaces (gaseous nitriding). Surface and subsurface layers of Cr_xN type compounds that seal the cracks of the electrolytic chromium and generate a barrier that prevents different corrosive agents reaching the surface of the substrate are obtained with this treatment.

Research into plasma nitriding (Menthe and Rie, 1999; Lunarska et al., 2001; Kuppusami et al., 2002; Wang et al., 2003; Poporska, 2005; Wang et al., 2007; Dasgupta et al., 2007; Han et al., 2009; Keshavarz et al., 2009) and gaseous nitriding on chromium substrates or electrolytically-applied chromium (Buijnsters et al., 2003; Ajikumar, 2004; Ajikumar et al, 2006; Nam and Lee, 2007) have studied the microstructure of the metallurgical system so obtained, the effect of production parameters (time and temperature) on corrosion resistance, wear resistance and hardness and their potential application in developments applicable, for example, to fuel cells. Nitriding transforms the chromium coating's chemical and microstructure as a result of the reaction between atomic nitrogen obtained by precursor gases dissociation (N₂, NH₃), either in plasma glow discharge or by heterogeneous reaction between the carrier gas and the surface of the metallic chromium coating in which these atoms diffuse.

The working temperature in gaseous nitriding can be lower than 1,273°K when NH₃/Ar gas mixtures are used in different proportions (Buijnsters et al., 2003; Ajikumar et al, 2006, Basu et al., 2007; Nam and Lee, 2007). When T> 1,273°K, N₂ can be used as atomic nitrogen precursor gas because this gas dissociates at T> 1,273°K (Grafen and Edenhofer, 2005). A multi-layer structure (Figure 2) can usually be obtained as a product of this reaction, consisting of the following.

1. An outer surface layer composed by chromium nitride Cr_xN, followed by a mixture of CrN/Cr₂N and below these regions Cr₂N/Cr (Ajikumar et al, 2006). It has been suggested (Matthews and Leyland, 1995) that the Cr₂N sub-stoichiometric phase is the first to be formed and then the CrN stoichiometric phase. On the other hand, the presence of different phases in the compound layer is also a function of process temperature (between 873 and 973°K is obtained for Cr₂N/Cr, between 973 and 1,323 °K for CrN/Cr₂N and, if T> 1,323°K, Cr₂N surface occurs by thermodynamic instability of the CrN phase) (Kuppusami et al., 2002);

2. An unmodified region consisting of electrolytic hard chromium coating;

3. An inner layer of Cr_xC_y chromium carbides. Because of the treatment temperature, ferrous substrate decarburisation is produced, followed by carbon atom diffusion into the chromium coating in the coating-substrate interface. The formation of such compounds (Cr, Fe)₇C₃ (Poporska, 2005) is related to the free energy associated with the formation of chromium and iron carbides, whose change for 973°K is around 380 kJ/mol for Cr₂₃C₆, while this is - 5 kJ/mol for Fe₃C formation. Hence the higher affinity of C for Cr than Fe and the chromium carbide stability so developed. On the other hand, Kim et al., (2003) have reported the formation of chromium carbides in the coating-substrate interface by transformation of the initial layer of amorphous chromium obtained in the electrolytic bath modified with organic compounds (formamide); and

4. An unmodified substrate.

Phases and their crystal orientations were determined by X-ray diffraction (XRD) using a X-PertPro Panalytical system in Bragg-Brentano mode, with Cuα monochromatic radiation of 1.540998 Å wavelength. Figure 3 presents the XRD results and the cross-section obtained by nitriding of a ferrous substrate (H13) electrolytically coated with hard chromium in low vacuum gaseous atmosphere, using N₂ as nitrogen precursor gas (T: 1,323°K, t: 10 hours). Layers preferably consisting of Cr₂N chromium nitrides and hardness having values up to 2,200 HV_0,5 have been obtained.

The development of external chromium nitride (Cr_xN) layers and chromium carbide (Cr_xC) inner layers leads to increased ferrous substrate corrosion resistance (Han et al., 2009; Keshavarz et al., 2009). The formation of ceramic compounds and sealing by chromium nitrides and carbides in typical electrolytic coating cracks prevents corrosive agents penetrating the substrate and causing corrosiion (Menthe and Rie, 1999; Kim et al., 2003; Somers and Christiansen, 2005). Sub-surface Cr_xN type layers obtained from pre-coated steels with pure chromium and produced by PVD present better performance in corrosive environments because of their high-density columnar grain microstruc-ture and less intergranular porosity (Ahn et al., 2002). It also leads to an increase in wear and erosion resistance (ASM Handbook Vol 4, 1991; Celis et al., 1999; Somers and Christiansen, 2005) and maintains hardness values at high temperature (Pina et al., 1997; Ahn et al., 2002; Wang et al., 2005; Wang et al, 2007).

Electrolytic hard chromium coatings combined with thermochemical treatment or nitrocarburising

This treatment develops chemical and microstructural transformation of electrolytic chrome coatings by the diffusion of nitrogen and carbon atoms on its surface. The effect of plasma nitro-carburising on microstructure and electroplated chromium coatings properties involving mixtures of N₂/H₂/CH₄ and NH₃/CH₄ (Wang et al., 2005) as carbon and nitrogen precursor gases have been studied (Menthe and Rie, 1999; Wang et al., 2005; Hedaiat Mofidi et al., 2008). From a structural point of view, layers of Cr_xN type chromium nitrides are obtained from dissociation of ammonia (NH₃) and methane (CH₄) (Wang et al., 2005) or N₂ (Menthe and Rie, 1999). A carbon-enriched outer layer forms when treatment temperature is above 1,173°K, followed by Cr_xC_y type chromium carbides (Cr₃C₂ and Cr₇C₃) (Menthe and Rie, 1999). An inner layer of chromium carbides is also formed in the substrate-coating interface, as in the combined process with nitriding; this is associated with steel decar-burisation followed by chromium coating diffusion and Cr_xC_y type compound formation (Wang et al., 2005). The final microstructure depends on temperature and processing time (Menthe and Rie, 1999). Among the advantages associated with the application of this combined process is a great increase in hardness values, such as 1,450 HV (Nam et al., 2004) and 2,200 HK (Menthe and Rie, 1999; Rie, 1999), higher than those obtained with nitriding, as well as sealing microcracks typical of any electrolytic process, with a consequent increase in corrosion resistance (Menthe and Rie, 1999; Rie, 1999; Wang et al., 2005 ) due to the high values of layer thickness (Hedaiat Mofidi et al., 2008). It is also shows a decrease in wear rate associated with the presence of chromium carbides in the system as well as in nitriding.

Nam et al., (2004) have carried out studies aiming to improve electroplated Cr_(III) resistance by oxy-nitrocarburisation in atmospheres consisting of a mixture of NH₃/N₂/CO₂ gases. A Fe₃O₄/ Fe₂O₃/Fe₄N compounds system without chromium nitride formation has been obtained as a result of the treatment; the author related this to the process's low energy. The results have shown that typical cracks in hard chrome plating increase in magnitude, instead of becoming sealed. However, compounds such as Cr_xC_y are produced due to the addition of additives in the preparation of bath chromium. Similar results have also been developed by Kim (2003). Microstructure based on transition metal carbides produces increased system hardness and wear resistance. Although the coating cracks are not sealed, corrosion resistance is promoted by iron oxide Fe_xO_y and iron nitride Fe₄N formation protecting the system.

Electrolytic hard chromium coatings combined with ther-mochemical treatment or carburisation

The study of the thermochemical treatment of vacuum carburis-ing shows significant progress, as demonstrated by developments by Basu et al., 2007, Zhang et al., 2006, Gawronski, 2000, Weber, 1982 and Krishtal et al., 1980. This process allows working with "oxygen-free" atmospheres and transforming alloyed steels whose chemical composition has strong oxide-forming elements (Morral and Law, 1990). Processes combining low vacuum with high-temperatures (Graf and Edenhofer, 2005) with less working time, better mechanical properties because of preventing intergranular corrosion, greater control of process generating uniformity and repeatability as a better control of layer depth and less environmental impact according to the carbon precursor gas used (preferably acetylene) have all been described (Tsepov, 1979; Kristhal and Tsepov, 1980; Hitoshi, 2005). The importance of this treatment at industrial level is evident from its development in the treatment of moulds, dies and tools (Oleinik, 2004) or technological innovations such as vacuum carburising with acetylene which allows removing impurities such as soot and tar with increased carburising power, process repeatability and higher quality surface. However, regarding this type of duplex treatment, there has only been one report by Arkharov et al., (1974) in which an electrolytic chromium coating applied to structural steel became transformed by means of a thermo-chemical treatment in a controlled gaseous atmosphere (mixture of vaporized benzene as carbon precursor agent with argon or hydrogen, at T = 1,323°K), obtaining Cr_xC_y chromium carbides both on the surface of the electrolytic chromium coating and on the interface of this with the ferrous substrate by carbon effusion.

Table No. 1 gives a comparative summary of the results obtained with duplex treatments combining electrolytic hard chrome coatings on ferrous substrates with nitriding and nitrocarburising.

Conclusions

Several studies have shown that thermochemical treatment regarding electrolytic hard chromium coatings helps to improve their physicochemical, structural and microstructural properties. This improvement is due to surface and subsurface formation of nitride and / or chromium carbide phases. Major advances have been made with plasma nitriding, controlled atmosphere or low vacuum processes. While the development of Cr_xN phases starts at 600°C, its formation on chrome is best evidenced at T > 700° C. Cr₂N + Cr phase appears at T > 700°C, then with 700°C > T < 1,000°C the CrN and Cr₂N phases are formed and if T > 1,000°C, only one phase is formed, Cr₂N, because of CrN's ther-modynamic instability. Transformed layer thicknesses vary between 1 and 20 µm and hardness can reach values up to 22 GPa in both treatments (plasma and gaseous atmosphere). Formation of Cr_xC_y type compounds by carbon effusion to chromium at the interface with the substrate is presented.

These phases seal typical electroplating cracks and act as a barrier preventing corrosive agents' access to ferrous substrate surface and, because of their high intrinsic toughness, improve coating properties such as hardness (to 22 Gpa), wear resistance and abrasion. Nitrocarburising of electrolytic hard chrome coatings applied to ferrous substrates has been less studied. However, in addition to Cr_xN type phases obtained by nitriding, this duplex treatment allows obtaining additional chromium carbide (Cr_xC_y) phases combined with chromium nitrides which allows improving properties such as surface hardness to values up to 22 GPa and resistance to corrosion by sealing typical electrolytic chromium coating micro-cracks. Only one study of controlled gaseous atmosphere with vaporised benzene has been carried out regarding carburising electrolytic hard chromium coatings on steel substrates. It reports Cr_xC_y type carbide phase formation on both the surface and chromium-steel interface. This paper is referenced only for information because of its date of publication and the quality of the characterisation tests. The reviewed studies present evidence of dispersion in parameters such as electrolytic chromium coating thickness (from 2 to 200 mm) and treatment time (2 hours to 142 hours) without selection criteria. There is an opportunity to develop experimental duplex treatments combining electrolytic hard chromium coatings on ferrous substrates, with thermochemical treatment of gas carburising in gas vacuum atmosphere, especially with acetylene as precursor gas; as well as being an undeveloped treatment, it may lead to obtaining carbide-like compounds which are widely used in manufacturing. Regarding gas nitriding duplex treatments there is an opportunity to validate Cr_xN type compound development at industrial/manufacturing level since studies have been focused on evaluating properties by different characterisation techniques, without a projection into the afore mentioned field of work having been found. These duplex treatments should contribute towards increasing the lifetime of electrolytic chromium (Cr_(VI)) coatings, decreasing its requirement in various applications due to its negative impact on the environment and public health.

Acknowledges

The authors wish to acknowledge the Universidad Nacional de Colombia's School of Engineering's Research and Extension unit and the university's research department in Bogotá for financing this research.

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