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Surface & Coatings Technology 200 (2005) 2192 ­ 2207 www.elsevier.com/locate/surfcoat

Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods

Ani Zhechevaa, Wei Shaa,*, Savko Malinovb, Adrian Longa

b a School of Civil Engineering, The Queen's University of Belfast, Belfast BT7 1NN, UK School of Mechanical and Manufacturing Engineering, The Queen's University of Belfast, Belfast BT7 1NN, UK

Received 26 April 2004; accepted in revised form 29 July 2004 Available online 6 November 2004

Abstract Over the last 40 years, the commercial production of titanium and its alloys has increased steadily. Whilst these materials have some very attractive properties, enabling applications in many industries, they are seldom used in mechanical engineering applications because of their poor tribological properties. This paper starts with an introduction to the titanium material and a review of the different types of surface treatment. The processes of nitriding, oxidation and carburizing are among the most popular thermochemical treatments aiming at improving the surface properties of Ti-alloys. Different kinds of nitriding are investigated like plasma nitriding, ion nitriding, and laser and gas nitriding. The kinetics of nitriding and the conditions for the formation of nitrided layers are studied. The influence of the main processing parameters such as temperature, time on the microstructure and the formation of new phases during the processes of nitriding is discussed. Also based on investigations presented in the literature, the effects of nitriding on the microhardness and the corrosion resistance of titanium and titanium alloys are analyzed. The improved mechanical properties, which arise from these thermochemical treatments, are discussed in relation to the potential for applying these alloys to different industries. D 2004 Elsevier B.V. All rights reserved.

Keywords: Titanium alloys; Surface engineering; Nitriding; Microstructure; Hardness; Corrosion resistance; Application

1. Introduction and historical background Titanium is widely distributed in the universe. It is abundant on Earth and has been detected in meteorites, the moon, the sun and other stars. Its concentration within the earth's crust of about 0.6% makes it the fourth most abundant of the metals after aluminium, iron and magnesium [1,2]. There are 20 times more in quantity than chromium, 30 times more than nickel, 60 times more than copper, 100 times more than tungsten and 600 times more than molybdenum [1]. Some of the physical properties of titanium are listed in Table 1 [3].

* Corresponding author. Tel.: +44 28 90974017. E-mail address: [email protected] (W. Sha). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.115

The Titanium Metals Company of America (TMCA) produced the first commercial products around 1950 [1,4]. Since then, the production of the metal has grown at an average annual rate of about 8%, and over the past 50 years, titanium and its alloys have proven to be technically superior and cost-effective materials for structures. More specifically, a wide variety of aerospace, marine and other industrial applications is now commercially viable due to their superb combined properties, when compared with other metallic materials. Some of the developments in titanium alloys over this period are summarised in Table 2 [5,6]. As can be seen, the most common titanium alloy used today, Ti­6Al­4V, was introduced in the mid-1950s with a maximum working temperature of 300 8C. The highest temperature that a titanium alloy can be used today lies just below 600 8C. By implementing adequate cooling design, coating processing or thermochemical treatment, the max-

A. Zhecheva et al. / Surface & Coatings Technology 200 (2005) 2192­2207 Table 1 Physical properties of titanium [3] Name of element Titanium Atomic symbol Ti Atomic number 22 Atomic weight 47.90 Density (g/cm3) 4.5 Boiling point (8C) 3130

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Melting point (8C) 1812

imum working temperature of titanium alloys can be increased [6].

2. Characteristics and properties of titanium and titanium alloys 2.1. Types and microstructural characteristics of titanium alloys Titanium exists in two crystallographic forms. At room temperature, pure titanium has a hexagonal close-packed (hcp) crystal structure referred to as a-phase. At 883 8C, this transforms to a body-centered cubic (bcc) structure known as h-phase. The manipulation of these crystallographic variations through alloying additions and thermochemical processing is the basis for the development of a wide range of alloys and properties [1,4,7­12]. The alloying elements have different influences on the properties of titanium. For example, aluminium, vanadium, iron, chromium, tin and silicon increase the value of the tensile yield strength and ultimate tensile strength. At the same time, they decrease the ductility and the toughness. Aluminium, zirconium and molybdenum increase the heat resistance, while the corrosion resistance can be increased by molybdenum, zirconium, niobium, tantalum and palladium [7,8]. Some of the most common alloying elements and their stabilizing effect are shown in Table 3 [6,13]. On the other hand, elements such as nitrogen, carbon and especially oxygen have a strong a-stabilizing effect and thereby raise the aYh

Table 2 Introduction year and maximum working temperature for some titanium alloys [5,6] Alloy Composition (wt.%) Introduction year 1954 1956 1961 1961 1966 1967 1969 1972 1973 1974 1976 1984 Max working temperature (8C) 300 425 400 450 450 450 520 540 350 520 580 590

Ti-64 IMI-550 Ti-811 IMI-679 Ti-6246 Ti-6242 IMI-685 Ti-11 Ti-17 Ti-6242S IMI-829 IMI-834

6Al, 4V 4Al, 2Sn, 4Mo, 0.5Si 8Al, 1Mo, 1V 2Al, 11Sn, 5Zr, 1Mo, 0.2Si 6Al, 2Sn, 4Zr, 6Mo 6Al, 2Sn, 4Zr, 2Mo 6Al, 5Zr, 0.5Mo, 0.25Si 6Al, 2Sn, 1.5Zr, 1Mo, 0.1Si, 0.3Bi 5Al, 2Sn, 2Zr, 4Mo, 4Cr 6Al, 2Sn, 4Zr, 2Mo, 0.1Si 5.5Al, 3.5Sn, 3Zr, 0.3Mo, 1Nb, 0.3Si 5.5Al, 4Sn, 4Zr, 0.3Mo, 1Nb, 0.5Si, 0.06C

transition temperature (h-transus), whereas hydrogen, which has a h-stabilizing effect, lowers the transus temperature. Increasing the amount of interstitial elements leads to a drastic increase in strength (Fig. 1), but at the same time, leads to a sharp drop in ductility and with an increased risk of embrittlement [13,14]. Based on the phases present, titanium alloys can be classified as a alloys, h alloys or a+h alloys [1,4,7­12]. Within the last category are the subclasses near-a and near-h, referring to alloys with compositions which place them near to a/(a+h) or (a+h)/h-phase boundaries, respectively. Alpha alloys contain elements such as aluminium and tin. These a-stabilizing elements increase the phase transformation temperature [10]. They are characterized by satisfactory strength, toughness and weldability but poorer forgeability than h alloys [15]. The absence of a ductile­ brittle transformation, a property of the bcc structure, makes a alloys suitable for cryogenic applications [16]. Alpha+beta alloys have a composition that supports a mixture of a and h phases. Although many binary hstabilized alloys in thermodynamic equilibrium are two phase, in practice, the a+h alloys usually contain mixtures of both a and h stabilizers. The properties of these alloys can be controlled through heat treatment, which is used to adjust the amounts and types of phases present [17]. Alpha+beta alloys generally exhibit good fabricability as well as high room temperature strength and moderate elevated-temperature strength. The most commonly used a+h alloy is Ti­6Al­4V. Beta alloys contain elements such as vanadium, molybdenum, iron and chromium, which decrease the temperature of the a to h phase transition. There are several commercial h alloys such as Ti­10V­2Fe­3Al, Ti­15V­ 3Cr­3Al­2Sn and Ti­3Al­8V­6Cr­4Mo­4Zr. Beta alloys, according to Ref. [15], are extremely formable. They are also prone to ductile­brittle transformation, and along with other bcc-phase alloys, are unsuitable for low-temperature application.

Table 3 Common alloying elements and their stabilizing effect [6,13] Alloying element Aluminium Tin Vanadium Molybdenum Chromium Copper Zirconium Silicon Range (wt.%) 2 to 7 2 to 6 2 to 20 2 to 20 2 to 12 2 to 6 2 to 8 0.2 to 1 Effect on structure a-stabilizer a-stabilizer h-stabilizer h-stabilizer h-stabilizer h-stabilizer a and h strengthening Improves creep resistance

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Fig. 1. Effect of interstitial alloying elements on strength and reduction in area of titanium [14]. The shaded area is for reduction in area values, when nitrogen, carbon or oxygen is present.

and implant and accelerate the process of healing and growth of the bone [35,36]. Titanium golf shafts, tennis racquet frames, pool cue shafts and bicycle frames are currently being fabricated using alloys such as Ti­3Al­2.5V. They have the properties needed for sports applications like low modulus of elasticity, damage tolerance, good strength-to-weight ratios and good corrosion resistance. The tensile properties of several commercial titanium alloys are listed in Table 4 [37]. At the same time, titanium and titanium alloys have found little use in mechanical engineering applications because of their poor tribological properties such as poor abrasive wear resistance, poor fretting behaviour and high coefficient of friction. The poor fretting behaviour can be improved by applying different surface treatments and coatings [38]. According to Ref. [38], there are four main mechanisms that can be used for such improvement: (i) to induce a compressive residual stress; (ii) to decrease the coefficient of friction; (iii) to increase the hardness; and (iv) to increase the surface roughness. The friction problem is related to the crystal structure and reactivity of titanium and can be largely overcome by changing the nature of the surface using such surface engineering technologies as different thermochemical treatments, so that the surface is no longer titanium, but a hard compound of titanium [39]. In order to improve such mechanical properties, surface thermochemical treatments can be applied.

The most commonly used titanium alloys are presented in phase-diagram format, Fig. 2 [18], to show the interactions between the alloy's a- and h-stabilizing components. 2.2. Properties and applications of titanium and titanium alloys Titanium and titanium alloys have some very attractive properties enabling them to be used in many industries. Some of their advantages are: excellent corrosion and erosion resistance; low densities, which give high specific strength-to-weight ratios allowing lighter and stronger structures; high-temperature capability and in some cases, cryogenic properties. They are widely used in aerospace applications [19], marine applications [20], many corrosive environments, sports equipment and medicine [21­29]. Titanium is completely neutral to the human body and is frequently used in the medical field to replace heart valves, joints and bones. Titanium replacements for hips and other joints are well established and have been in use for over 30 years. The biocompatibility and strength of titanium make it an ideal material for dental posts and other oral prosthetics. Bone naturally adheres to the surface oxide of titanium without additional coatings [30­34]. Hydroxyapatite coatings on titanium implants are becoming very popular because of enhancing the biocompatibility and the life of the implants. They also allow direct bonding between bone

3. Surface engineering of titanium and titanium alloys Surface engineering techniques that may be applied with varying degree of success to titanium alloys to combat wear and galling can be divided into different categories. For example, in Ref. [2], the authors divide the surface engineering of titanium and titanium alloys into three broad categories--surface coatings, surface modifications and duplex surface treatments. In terms of the different methods for the surface engineering of titanium and titanium alloys in general, these can be divided into three main groups: heat treatment, coatings and thermochemical treatment. 3.1. Heat treatment Heat treatment of materials is a fundamental metallurgical process. Titanium and titanium alloys are heat treated to achieve different properties, for example, to optimize special properties such as fracture toughness, fatigue strength, to increase strength, to produce an optimum combination of ductility, machinability and structural stability [40]. The heat treatment can be divided into two kinds: surface and bulk heat treatment. Different technologies for surface treatment aiming at improving the tribological properties of titanium alloys are being attempted. Some of the recent works include plasma-assisted surface treatments [41­49], laser-assisted surface treatments

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Fig. 2. Compositions of U.S. technical alloys mapped onto a pseudobinary h-isomorphous phase diagram [18].

[50­55] and ion implantation [47,56­58]. The main aim of some of these treatments is to change the microstructure of the surface layers of the materials (without changing the chemistry) via extraordinary high heating and cooling rates or the effect of the melting of the surface by laser techniques [59]. Most of these technologies are thermochemical surface treatments and they will be discussed in the following sections of the paper. Various types of bulk heat treatments, such as single, duplex, beta and recrystallization annealing, solution treating and aging treatments, are used to achieve selected mechanical properties [60]. The response of

titanium and titanium alloys to heat treatment depends on the composition of the metal. As the bulk heat treatment discussed in this paragraph do not significantly improve the surface properties, the other alternatives need to be considered. 3.2. Coatings Different types of coatings can be applied to titanium and titanium alloys. They can be classified as chemical conversion coatings, plating, physical vapour deposition

Table 4 Tensile properties of several commercial titanium alloys: typical room temperature values [37] Alloy composition Ti­8Al­1Mo­1V Ti­6Al­4V Ti­6Al­2Sn­4Zr­2Mo Ti­10V­2Fe­3Al Ti­15V­3Cr­3Sn­3Al Condition Annealed Annealed aged Annealed Annealed Annealed (8 h/788 8C) (2 h/700­870 8C) (2 h/700­840 8C) (1 h/760 8C) aged (0.25 h/790 8C) aged Ultimate strength 10 N/m 10.0 9.6 10.0 9.7 7.9

8 2

Yield strength 10 N/m 9.3 9.0 9.3 9.0 7.7

8 2

Elongation (%) 12 17 15 9 20­25

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(PVD), sprayed coatings, etc. [61,62]. Chemical conversion coatings are used on titanium to improve lubricity by acting as a base for the retention of lubricants. Other types of coatings are different kinds of plating such as copper plating, platinum plating, coatings for emissivity and coatings for wear resistance. Copper-plated titanium wire has an outstanding property, which is the lubricity of its copperplated surface. On the other hand, the application of a thin film of platinum to titanium results in a material with excellent electrochemical properties. Different types of coatings for emissivity like electrodeposits and sprayed coatings of gold on titanium are used to provide a heatreflecting surface that reduces the temperature of the base metal. Plating of hard chromium and electroless nickel are the most widely used for wear resistance. Physical vapour deposition (PVD) is a term that covers several processes (including evaporation, ion plating and various forms of sputtering) to deposit metals, alloys, compounds, or metastable materials on a wide variety of substrates [61,63]. The most successful engineering application is the coating of tool material substrates by reactive ion plating or reactive sputtering of TiN and related materials. Sprayed coatings may be applied to titanium alloy substrates as to many other materials. Sprayed coatings are mainly deposited using plasma spraying, but detonation gun, highvelocity oxyfuel (HVOF) and vacuum plasma spraying have also been used. There is a vast range of materials that can be sprayed onto titanium substrates and their main function is to reduce wear [62]. 3.3. Thermochemical treatment--classification In general, all the established surface engineering technologies may be applicable to titanium alloys [2], but the following considerations are important. First, commercially available titanium alloys are not amenable to significant case hardening by any established surface engineering technologies without changing the composition of the surface. That is why in most of the cases the thermal hardening processes are ineffective [64]. Second, titanium and titanium alloys are chemically active and easily react with most interstitial elements and especially oxygen; thus, all surface treatments are carried out in vacuum or protecting inert gas. Third, titanium alloys react at various temperatures with all but the most stable elements, giving the possibility of a wide range of diffusion-based surface treatments [2]. In order to improve the mechanical properties of titanium and titanium alloys, different types of thermochemical treatments can be applied. Nowadays, there is an increasing interest in the methods of thermochemical treatment, connected with diffusion saturation of the surface of the material with different elements. As discussed in Ref. [65], such treatments can increase the wear and corrosion resistance, decrease the coefficient of friction and easily harden the surface of the material. The processes of oxidation, carburizing and

nitriding are among the most popular thermochemical treatments used for the improvement of the surface properties of titanium and titanium alloys. They are briefly introduced in the following sections, and then the paper is focused entirely on the processes of nitriding. 3.3.1. Oxidation Over the past 40 years, many studies have been carried out on the oxidation of titanium [66­69], but less attention has been paid to the oxidation as a tribological surface thermochemical treatment of titanium alloys [61]. Oxidation of titanium and titanium alloys can be used to improve their tribological properties. Oxygen in solution with a-Ti produces significant strengthening of the material. The usually excellent corrosion resistance of titanium under normal conditions is largely due to the formation of very stable, highly adherent and protective oxide films on the surface. During thermal oxidation, this oxide film becomes thicker and tougher thus giving additional protection against corrosion. When titanium and titanium alloys are heated in air at a temperature of 450­800 8C for 2­10 min, protective oxide films can be formed [66]. At the same time, these oxide layers are quite brittle so they can be easily damaged by mechanical impact, and according to Ref. [10], the process cannot significantly improve the wear resistance. Relatively few investigations have been carried out concerning the oxidation treatment of titanium for improving wear resistance [66]. It has been reported [67] that after heat treatment of Ti­6Al­4V in argon/oxygen atmosphere, rutile surface oxide (TiO2) is produced, giving optimum wear characteristics. The maximum depth of hardening was 125 Am at 850 8C and 250 Am at 900 8C. Plasma electrolytic oxidation is another technique that can produce thick (maximum thickness of about 500 Am) wear-resistant layers with excellent adhesion. Plasma thermochemical interactions in the multiple-surface discharges result in a coating growing in both directions from the substrate surface [70,71]. 3.3.2. Carburizing Carburizing of titanium and titanium alloys may be achieved in nonoxidizing, carburizing environments. The phase equilibrium system for Ti­C differs from those for Ti­ O and Ti­N, as there is a very low solid solubility for carbon in titanium. The TiC compound layer can be formed with a thickness of 1­10 Am, but there is no significant diffusion zone beneath the TiC [10,61]. The carburizing process can be performed at temperatures up to 1050 8C in a carbonaceous media. Carburizing creates wear-resistant surfaces for titanium and titanium alloys and could be used for surface modification of engine valves [61]. 3.3.3. Nitriding Nitriding of titanium and titanium alloys has been investigated for many years and is used effectively for protection against wear. Nitrogen has a high solubility in

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a-Ti so it strengthens the surface layer significantly. Nitriding processes can cause the formation of a compound layer of TiN on top and Ti2N beneath, with a hardness that can reach 3000 and 1500 HV, respectively [61]. Different values are given in Ref. [72], namely 1200 HV for TiN0.6 and 1900 HV for TiN0.97. Nitriding cannot be achieved in air because of the tendency for titanium to form TiO2 in preference to either of the nitrides.

4. Nitriding--classification The major types of nitriding, such as plasma, ion, laser and gas nitriding, will be reviewed in this paper. 4.1. Plasma nitriding Plasma nitriding is a method for thermochemical treatment that has many advantages such as control of the phase formation and the depth of the nitrided layer. It requires short periods of nitriding time and it avoids oxidation. Different experiments have been performed at low temperatures from 400 to 950 8C for various periods of time from 15 min to 32 h. Microhardness values from 600 to 2000 HV for Ti­6Al­4V and Ti­10V­2Fe­3Al and a compound layer with a thickness of about 50 Am have been obtained [26,27,29,73­79]. This type of treatment requires special equipment and high ionizing energy. Conventional plasma nitriding occupies the low-energy end-spectrum (practical energies ranging from thermal energy 1­2 to 200 eV) [80]. The two major plasma processes developed for titanium nitride synthesis are PVD (mentioned above as a type of coating technology) and ion nitriding [30]. One disadvantage of plasma nitriding is that it reduces the fatigue strength of titanium alloys; however, this problem can be overcome by reducing the processing temperature as reported in Ref. [27]. 4.2. Ion-beam nitriding Ion-beam nitriding, which occupies the high-energy endspectrum, is another method for hardening the surface of titanium and titanium alloys. The treated surface is exposed to the ion beam using N2 and Ar. Nitrogen bombards the specimen surface and mainly leads to desorption and sputtering of atoms of impurities. Data in the literature indicate that the treatment is performed at temperatures from 500 to 900 8C for 30 min to 20 h. Microhardness of 800­1200 HV and thickness of the compound layer of 5­8 Am for Ti­ 6Al­4V and Ti­8Al­1Mo­1V have been obtained [81­84]. 4.3. Laser nitriding

attractive because it provides potential for an excellent metallurgical bond between the hardened surface layer and the substrate. Nitrogen is fed through a nozzle into the melt pool and the angle between the nozzle and the substrate surface has to be at least 308. The main problem with laser nitriding of titanium alloys is the surface cracking. Microhardness between 900 and 1300 HV has been obtained for Ti­6Al­4V by adjusting the processing parameters such as laser pulse energy, scanning speed and nitrogen concentration [85­90]. The main disadvantages are that this method requires special equipment and is dependent on the geometry of the material. 4.4. Gas nitriding Gas nitriding is considered to be a promising method available for engineering applications because it can easily form a harder layer on the surface of the materials. The main advantage of gas nitriding is that it is independent of the geometry of the sample and does not require special equipment. A big disadvantage is that it requires high temperatures, 650­1000 8C, and a long time for nitriding, 1­ 100 h, according to the literature. It is also well known that gas nitriding reduces the fatigue limit of titanium alloys [27]. The microhardness varies between 450 and 1800 HV for Ti­6Al­4V and Ti­6Al­2Sn­4Zr­2Mo. Compound layers with a thickness of about 2­15 Am for Ti­6Al­4V have been obtained [91­99].

5. Testing and measurement techniques A number of widely known analytical techniques such as X-ray diffraction (XRD) [73,74,76,79], optical microscopy [21,27,33,100], scanning electron microscopy (SEM) [22,78,101,102], electron microprobe [22], atomic force microscopy (AFM) [31] have been used in studying the phase transformations and microstructure changes taking place during the process of nitriding. Similarly, hardness measurements, using Vickers and Knoop indentation geometries have been extensively used. Different techniques for measuring the corrosion resistance such as potentiodynamic method [21,33,74,75,78,90,100,103,104], potentiostatic method [104,105], weight-loss test [106] have been applied. Various techniques for measuring the wear resistance such as three rollers+taper method [21,33,78,100], ring-on-disk and ball-on-disk [22,101] have been used.

6. Microstructure of the nitrided layers of titanium alloys 6.1. Phase transformations during the process of nitriding

Laser nitriding works by melting the surface (1 to 1.5 Am deep), using a focused laser beam in a nitrogen gas environment to form a hard titanium nitride layer. It is

The formation of nitrided layers in titanium alloys is a complicated process and involves several reactions taking

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Fig. 3. A schematic presentation of the kinetics of formation and growth of surface layers during nitriding of titanium [110].

place simultaneously at the boundary between the gas and the metal and within the substrate. The kinetics of the diffusion process of nitriding have been studied by several research groups [97­99,107­109]. A simplified physical model for the formation and growth of nitrided layers during gas nitriding in titanium is suggested [110]. The model is based on reaction diffusion rules and is applicable for nitriding temperatures below the h-transus. If the titanium material is in an active nitrogen-containing environment at high temperature, a nitrogen mass transfer from the medium

to the solid occurs. The nitrogen absorbed at the surface diffuses into the titanium forming interstitial solution of nitrogen in the hcp a-titanium phase (Fig. 3 top). The surface layer formed is called the diffusion zone (a(N)). This process can continue as long as the a-titanium matrix can dissolve nitrogen at the nitrogen medium/solid interface (where the nitrogen concentration is the highest). If the concentration of nitrogen at the gas/metal interface becomes higher than the a phase is able to retain in interstitial solution, a reaction at the interface occurs leading to the formation of a new phase--Ti2N (Fig. 3 middle). There is a concentration jump of nitrogen at the sample surface, and as a result, the total nitrided layer consists of a compound layer (Ti2N) on the top and a diffusion zone underneath. Following the same rules when the concentration of nitrogen at the gas/metal interface becomes higher than the one acceptable in Ti2N, there is a phase transformation at the sample surface and the Ti2N transforms to TiN (Fig. 3 bottom). The sublayer with titanium nitrides only (TiN and Ti2N) forms the compound layer, while a(N) is the diffusion zone. The phase transitions of the sample surface during nitriding can be written as: aÀTi Z aðNÞÀTi Z Ti2 N Z TiN ð1Þ

The evolution of the surface layer during nitriding is presented schematically in Fig. 3. The physical model for formation of nitrided layers described above is developed assuming the diffusion of nitrogen in pure titanium, and it can be inferred by applying the rules of the reaction diffusion to the binary Ti­N phase diagram (Fig. 4)

Fig. 4. Ti­N phase diagram [111].

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6.2. Microstructure and layer growth of the nitrided layers The main phases observed on the surface after nitriding of different titanium alloys under various thermodynamic conditions are TiN and Ti2N [91], which generally form compound layers that cover the entire surface. X-ray diffraction patterns and SEM micrographs (Fig. 5) show the presence of TiN and Ti2N phases on the surface of plasma-nitrided Ti­6Al­4V [22]. A clear boundary between the compound and the diffusion layer can be seen in Fig. 5 and a typical micrograph of plasma nitrided Ti­6Al­4V is given in Fig. 6 [112]. Underneath the compound layer is the diffusion zone that consists of an interstitial solution of nitrogen in the hcp a-titanium phase. The microstructure of the diffusion zone depends on the chemical composition of the titanium alloy and the temperature of nitriding as is indicated in Fig. 7 for Ti­6Al­2Sn­4Zr­2Mo gas-nitrided at two different temperatures [113]. The thickness of the new surface layers formed during the diffusion process of nitriding depends mostly on the temperature, the time and the method of nitriding. Experimental results of nitrided layers in titanium and titanium alloys show that the thickness of the compound layer varies between 1 and 50 Am, while the diffusion zone is in the range of hundreds of micrometers [113]. It can be seen from Fig. 8 how the thickness of the nitrided layers of Grade 3 titanium after gas nitriding increases with the increase of the treatment temperature and time [32]. The growth of the nitrided layer can be described by the parabolic time law [101]; that is, the thickness of the layer increases linearly with the square root of the nitriding time t (Fig. 9) [113]. It can be seen that for Ti­6Al­2Sn­4Zr­2Mo alloy the rate of thickness growth is enhanced by the higher temperature of nitriding. This is due to the diffusion-controlled nature of the process [113]. Similar findings about the thickness of the surface nitrided layers after gas nitriding are reported in Refs. [31,91­ 97,102,114­121]. Fig. 10 shows the variation of the

Fig. 5. X-ray diffraction patterns of Ti­6Al­4V plasma nitrided for 6 h at (a) top 900 8C and (a) bottom 700 8C; (b) SEM micrograph of Ti­ 6Al­4V, plasma nitrided at 900 8C for 6 h with A-TiN and B-Ti2N [22].

[111]. However, for real cases of nitriding of titanium alloys, the alloying elements present can cause different deviations and modifications. The presence of alloying elements in the general case might result in: ! Simultaneous formation of two or more titanium nitrides; ! Formation of complex stable and/or metastable nitrides of the alloying elements; ! Change of the composition ranges of existence of the different phases. Despite these possible influences, the alloying additions in the classical titanium alloys are small in quantity and usually are dissolved in the hcp a-titanium matrix by forming a substitutional solid solution. Hence, dramatic changes to the kinetics of formation and the phase compositions of nitrided layers in titanium alloys are not very likely. This has been confirmed by experimental studies of the microstructure and the phase compositions of nitrided layers in different titanium alloys [73,74,76,78,79].

Fig. 6. Optical micrograph of the cross-section of Ti­6Al­4V plasma nitrided for 14 h at 900 8C in nitrogen [112].

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Fig. 9. Thickness of the whole nitrided layer vs. square root of time for Ti­ 6Al­2Sn­4Zr­2Mo after gas nitriding [113].

diffusion layer thickness of Ti­6Al­4V after plasma nitriding with temperature and time [122]. The thickness of the nitrided layers depends also on the voltage and the gas pressure when plasma and ion nitriding is performed. According to Ref. [26], the thickness of the compound layer increases with increasing temperature, increasing voltage and increasing gas pressure (Fig. 11). The thickness of the nitrided layers after plasma nitriding of titanium and titanium alloys has been studied by many research groups [21­23,27,33,74,76,78,101,105,112].

7. Influencing factors

Fig. 7. Microstructure of Ti­6Al­2Sn­4Zr­2Mo alloy after gas nitriding for 5 h at (a) 950 and (b) 1050 8C [113].

Various nitriding conditions influence the surface properties of titanium and titanium alloys. The main parameters are temperature, time, gas mixture, gas flow rate, heating and cooling rate, gas pressure, current, voltage and each can be varied depending on the type of nitriding. For example, for gas nitriding, the most important parameters that can be varied to obtain desirable surface properties of titanium and titanium alloys are temperature and time. For plasma and ion nitriding, they are temperature, time, gas mixtures, pressure and electrical parameters [122]. Laser nitriding has more specific process conditions. Parameters such as laser

Fig. 8. Thickness of the nitrided layer of Grade 3 titanium vs. treatment temperature and time [32].

Fig. 10. Variation with the temperature and the time of the diffusion layer thickness after plasma nitriding of Ti­6Al­4V [122].

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Fig. 12. Surface hardness values (HV) in relation to the treatment temperature and time [32].

Fig. 11. Compound layer thickness of plasma nitrided samples (a) vs. applied voltage and (b) vs. pressure: (o) Ti­6Al­4V: TiN; ( ) Ti­5Al­ 2.5Fe: TiN; (5) Ti­6Al­4V: Ti2N; (n) Ti­5Al­2.5Fe: Ti2N [26].

.

power, scan speed and gas mixture are among the most frequently used for property control [123].

8. Materials properties 8.1. Hardness The hardness is probably the surface property of nitrided titanium alloys that has been studied most extensively. The initial hardness of titanium and titanium alloys varies usually between 200 and 400 HV depending on the chemical composition of the material [102]. There is an increase of the hardness after nitriding that is caused by the increase of the nitrogen concentration and the formation of new phases like TiN and Ti2N on the surface of the material [102]. The hardness can be controlled by changing the different processing parameters depending on the type of nitriding. In gas nitriding, the most important processing parameters are temperature and time, and changing either influences the hardness of the material. The hardness increases with the increase of the temperature and the time and this tendency is shown in many references

[31,32,102,117­121]. The hardness varies between 450 and 1800 HV for Ti­6Al­4V and Ti­6Al­2Sn­4Zr­2Mo, and compound layers with a thickness of 2­15 Am for Ti­ 6Al­4V have been obtained [91­99]. An example of the surface hardness values of Grade 3 titanium, after gas nitriding, in relation to the treatment temperature and time, is given in Fig. 12 [32]. The gas nitriding was performed in the temperature range between 700 and 950 8C for 1 to 16 h. A maximum value of the surface hardness of about 1100 HV has been obtained at 950 8C after 16 h of treatment [32]. Similar investigations have been made for pure titanium in Ref. [94]. The nitrogenenriched layer becomes harder and the surface hardness, which is due to the hardness of the nitride TiN, reaches 1734 HV (17 GPa) at 950 8C, which is in contradiction with the result reported in Ref. [32]. With the increase of the temperature, the diffusion is highly activated and the nitride formation and gas saturation are accelerated, which results

Fig. 13. Microhardness profiles of Ti­6Al­2Sn­4Zr­2Mo after gas nitriding at 950 8C for 1 (o) and 5 h (5) [113].

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6Al­4V for 14 h at 900 8C [124]. The dependence of the surface properties of titanium on the gas composition has been studied in Ref. [27]. The authors observed an increased rate of nitriding when nitrogen­hydrogen gas mixtures were employed [27]. These observations are in agreement with the work in Ref. [125] and partly in contradiction with the work in Ref. [124]. The discrepancy may be caused by different processing parameters used. Results on the effect of the N2:Ar ratio on the nitriding process of pure titanium by intensified plasma ion nitriding are presented in Fig. 15a [83]. They show that the most effective nitriding could be achieved at an N2:Ar ratio of 3:1 [83]. In the same reference, the effect of the working pressure

Fig. 14. Microhardness profile of gas-nitrided Grade 3 titanium at 950 8C for 8 h [32].

in an increase of the hardness and growth of the diffusion layer [94]. Similar observations have been made for Ti­ 6Al­4V and Ti­15Mo­5Zr­3Al in Ref. [23]. The relation between the nitrogen interaction and the increase of the surface hardness after gas nitriding has been studied for different titanium alloys by many research groups [24,76,101,108,109]. The hardness values of the surface (compound) layer are usually very high due to the phase composition that consists of titanium nitrides and they decrease through the diffusion zone to approach the base microhardness of the matrix in the unsaturated core. Microhardness profiles are given in Figs. 13 [113] and 14 [32]. After plasma nitriding, microhardness from 600 to 2000 HV for Ti­6Al­4V, Ti­10V­2Fe­3Al and Ti­8Al­1Mo­ 1V, and compound layer with a thickness of about 50 Am, have been obtained [26,27,29,73­79]. For example, the plasma nitriding at temperatures between 700 and 1000 8C for 1.5­14 h increases the surface hardness of Ti­6Al­4V to 1700 HV [112]. For plasma and ion nitriding, over and above the influence of temperature and time, the gas composition plays an important role in determining the characteristics of the nitrided layers [124]. Table 5 shows the effect of the gas composition on the surface hardness and thickness of the compound layer after plasma nitriding of titanium and Ti­

Table 5 Effect of gas composition on surface hardness and thickness of compound layer after plasma nitriding for 14 h at 900 8C [124] Gas composition (vol.%) N 100 80 H ­ 20 Surface hardness (HV0.025) Titanium 945 1003 Ti­6Al­4V 808 830 Thickness of compound layer (Am) Titanium 7­8 7 Ti­6Al­4V 5 5-6 Fig. 15. Surface microhardness of pure titanium after intensified plasma ion nitriding as a function of (a) the gas composition and (b) the working pressure [83].

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on the ion nitriding process is given (Fig. 15b). The effect of the processing parameters of plasma and ion nitriding on the microhardness of different titanium alloys has been studied in the following Refs. [21,22,30,33,80,100,105,122,126]. The gas mixtures, the laser pulse energy and the scanning speed are the most important processing variables for laser nitriding. Microhardness of 900­1300 HV has been obtained for Ti­6Al­4V [85­90]. The effects of the N2/Ar ratio and scanning speed on the hardness profile of lasernitrided Ti­6Al­4V are given in Fig. 16 [127]. Near-surface hardness of about 800 HV was obtained for scanning speeds between 0.02 and 0.05 m/s [127] and further results for the surface hardness of titanium alloys after laser nitriding are given in Refs. [27,128­130]. 8.2. Corrosion resistance Titanium has excellent corrosion resistance, high reactivity at elevated temperatures and a relatively high melting point. All these characteristics make titanium difficult to refine [131]. Commercial Ti contains some oxygen, nitrogen and carbon. Almost all metallic elements are soluble in Ti

Fig. 17. General corrosion of different titanium alloys in naturally aerated HCl solutions [133].

and commercial alloys have been made containing Al, Fe, Cr, Mn, V, Zr, Mo and Sn. Certain major alloying elements influence the general and crevice corrosion behavior of various commercial Ti alloys [132]. Results indicate that V and especially Mo additions (z4% Mo) improve corrosion resistance, but the increase of Al content appears to be disadvantageous. Because Ti metal itself is highly reactive and has an extremely high affinity for oxygen, Ti owes its corrosion resistance to a protective oxide film [17,131,133]. This film resists attack by oxidizing solutions, in particular, those containing chloride ions. It has outstanding resistance to corrosion in marine environments and other chloride salt solutions. Areas in which Ti is being used extensively because of a good corrosion performance are in seawater, chemical brines, wet chlorine gas and chlorine dioxide [134]. Ti may corrode rapidly under conditions where the protective film breaks down [134]. General corrosion rates for Ti alloys can be determined from weight-loss data, dimensional changes and electrochemical methods [132,133,135]. Electrochemical anodic and cathodic polarization testing is often used to supplement weight-loss testing. The most common corrosive media for testing are nitric acids, hydrochloric acids and sulphuric acids. Corrosion rates in millimeters per year for Ti alloys can be calculated from weight-loss data as follows: À Á 8:76 Â 104 ðW Þ Corrosion rate ¼ ð2Þ ðd Þð AÞðt Þ where d is Ti alloy density (in grams per cubic centimeter), A is the sample surface area (in square centimeters), t is the exposure time (in hours) and W is the weight change (in grams). General corrosion of Ti alloys in HCl solutions can be seen in Fig. 17 [133]. When titanium and titanium alloys are thermochemically treated by different processes of nitriding to improve their

Fig. 16. Effect of (a) N2/Ar ratio and (b) scanning speed on the hardness profile of laser nitrided Ti­6Al­4V [127].

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surface properties, it is likely that some changes in their corrosion behavior will occur. According to Ref. [103], nitrogen-diffusion hardening performed in a nitrogen atmosphere for 8 h at 566 8C considerably changes the surface morphology of Ti­6Al­4V alloy and it does not significantly alter its corrosion properties in the neutral and acidic Hanks' solution comparing to the initial samples. There was no significant difference between the sample groups in the corrosion current density values. These results are in agreement with the corrosion data presented in Ref. [136]. An improvement of the wear and corrosion resistance after gas nitriding of Ti­6Al­4V alloy at 800 8C for 4 h is reported in Ref. [137]. Electrochemical measurements were performed in a lactic Ringer's solution [137]. In order to study the corrosion stability of the plasmanitrided samples of Ti­6Al­4V with time and the effect of the different treatment conditions, a weight-loss test was performed [106] at 90 8C in a 4 M HCl solution (Fig. 18). In general, the plasma nitriding improves not only the surface hardness but also the corrosion resistance when concentrated hydrochloric acid solutions are encountered. However, the samples with the smallest thickness of the compound layer, after a short immersion time, were heavily corroded with weight losses higher than the ones measured on the untreated alloy after the same immersion time. Plasma nitriding was also found to improve the corrosion resistance of OT4-0 alloy [78]. The corrosion test was conducted in 1.8 M H2SO4 solution using potentiodynamic method. The same method has been used in Ref. [75] for measuring the corrosion resistance in 0.5 M NaCl solution of Ti­6Al­3Mo­2Cr alloy after plasma nitriding at 850 8C for 6 h. It has been reported [75] that plasma nitriding and carbonitriding of Ti­6Al­3Mo­2Cr alloy produce surface layers with good wear and corrosion resistance, considerably better than the untreated titanium alloy. Some other

investigations show that plasma nitriding worsens the corrosion performance of titanium alloys [104]. More results for the corrosion behavior of nitrided titanium alloys are presented in Refs. [26,74,80,83,90,100,138].

9. Applications of titanium alloys after nitriding Because of the poor tribological properties of titanium and titanium alloys, surface thermochemical treatments such as different types of nitriding are recommended to expand the range of industrial and medical applications. Surface treatment is essential for example for engineering applications such as gears and bearings [29]. Different components including watch cases, racing car components, including engines and precision machine parts, can be gas nitrided [61], and it has also been applied to the surface hardening of the ball-and-socket joint of dental implants in order to obtain titanium nitrides [32]. Surface gas-nitrided titanium can be considered as a promising material for dental implants and moving parts of artificial joints [31]. The nitriding of titanium alloy surfaces using plasma processes has already reached the industrial application stage in the biomedical field. Orthopaedic companies in the USA and Europe are manufacturing nitrided or TiN-coated hip implants [30]. For example, plasma nitriding has provided new possibilities for producing a nontoxic, human fibroplast-compatible surface on parts made of titanium alloys with sophisticated shapes for in vitro tests [33]. It has been reported [34] that ion implantation is widely used for treatment of titanium components for medical implants and also small components that the chemical industry is beginning to use routinely. According to Ref. [139], highpressure nitriding can be successfully applied to fuel injection nozzles machined from Ti­6Al­4V alloy in pure nitrogen. The application of various types of titanium alloys has been discussed elsewhere by different research groups [30­34].

10. Summary Titanium and titanium alloys are very attractive materials because of their excellent combination of properties that give them the possibility to be used in many industries. However, they have some disadvantages that reduce the number of possible applications, especially those which require good tribological properties. These problems can be overcome using such surface engineering technologies as different thermal treatments, coatings and thermochemical treatments to obtain desirable properties. Thermochemical treatments such as oxidation, carburizing and nitriding are quite effective because they change the chemistry of the surface layers. Different types of nitriding such as plasma, ion, laser and gas nitriding are among the most popular methods for thermochemical treatment used for this

Fig. 18. Weight loss of differently nitrided samples of Ti­6Al­4V in 4 M HCl solution at 90 8C: N1-700 8C, 2 h (x); N2-900 8C, 2 h (.); N3-900 8C, 8 h (D); Ti­6Al­4V-untreated (x) [106].

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purpose. Gas nitriding is considered to be a promising method available for engineering applications because it can easily form a harder nitriding layer on the surface of the materials. It is comparatively cheap and has a small amount of processing parameter variables that allows modeling of the diffusion process and simulation of the properties and the characteristics of the nitrided layers. The surface properties of titanium and titanium alloys strongly depend on the chemical composition of the materials and the processing parameters of nitriding. To study the microstructural changes and properties of the materials, it is essential to understand the kinetics of the process of nitriding. Overall, the unique characteristics of nitrided titanium and titanium alloys can be summarized as below: ! Formation of TiN and Ti2N on the surface of the materials; ! Thick diffusion layers consisting mainly of a-Ti solid solution enriched with nitrogen; ! Microstructural changes depending on the chemical composition of the materials and the processing parameters; ! High values of surface hardness depending on the chemical composition of the materials and the processing parameters; and ! Very good corrosion resistance before and after nitriding. Several specific areas of nitriding of titanium and titanium alloys have been identified, which still require further research. They include studies of the process of gas nitriding at temperatures slightly below and above the htransus for different alloys, their surface roughness and the corrosion resistance after gas nitriding. Interest in other thermochemical processes such as plasma electrolytic oxidation and carburizing treatments, which have shown promising results, is also growing.

Acknowledgement Dr. Sha was sponsored by a Royal Academy of Engineering's Global Research Award whilst contributing to this research.

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