Field of the Invention The present invention relates to the treatment of metal or metal alloys to increase their wear resistance. In particular, the present invention relates to the production of metal nitrides on corresponding metal substrates.

Background of the Invention

Light metals and light metal alloys, such as titanium and titanium alloys, are attractive materials because of their superior combination of good mechanical properties and excellent corrosion resistance. However, their poor tribological properties tend to restrict their usage. Attempts have therefore been made to improve the wear resistance of these materials, for example by surface treatment.

A commonly used surface treatment technique to enhance wear resistance of certain metals is nitriding. The primary aim of this technique is to produce a hard surface layer of metal nitride with this layer providing improved wear and corrosion resistance. Due to these effects, nitriding is of great industrial interest.

Surface nitriding was firstly applied to ferrous alloys and steels, as these are the most widely used structural materials and massive efforts have been devoted to increasing their surface hardness and corrosion resistance over the past century. Thus, many technical routes have been well established for industrial production and nitriding still is a traditional and practical route for surface treatment of ferrous alloys and steels.

However, given the drive to reduce weight of components light metals have been emerging as preferred structural materials over ferrous alloys and steels. Amongst them, titanium and titanium alloys have increasing applications in the automotive, aerospace and mechanical sectors due to their excellent mechanical properties and lightness. Hence, nitriding of these light metals to provide enhanced (surface) properties has also become of more interest to materials scientists and engineers.

Due to its high chemical reactivity at elevated temperature, nitriding of titanium and its alloys is relatively difficult to carry out using conventional gas nitriding methods developed for ferrous alloys and steels. Hence, various alternative nitriding techniques have been applied to titanium and titanium alloys to reduce the temperature at which nitriding takes place including the use of high-energy beam sources. Currently, there are three major nitriding processes assisted by high-energy sources available for titanium alloys, including plasma ^' nitriding, laser nitriding and ion-beam nitriding. These techniques have successfully decreased the nitriding temperature, and achieved significant increase in the surface hardness of titanium and titanium alloys. Plasma nitriding in particular has already reached an industrial application stage in the biomedical field. Nonetheless, the massive investment in equipment and on-going energy consumption, and dimensional limitations have restricted the industrial applicability of these techniques.

Therefore, conventional gas nitriding is still considered to be the most practical technique for generally hard nitride surfaces at low cost. The nitrogen sources used in this technique include pure nitrogen, nitrogen/argon gas mixtures, nitrogen/hydrogen gas mixtures and ammonia. However, in order to avoid hydrogen embrittlement and reduce environmental pollution the use of pure nitrogen is preferred. However, this then requires an ultra pure nitrogen environment (purity of at least 99.99%). This is very expensive and brings with it strict process control to avoid the purity of the nitrogen being compromised. Lower purity grades of nitrogen, such as industrial grade nitrogen are cheaper but include oxygen at levels that present a problem with respect to oxidation of titanium/titanium alloy.

Against this background it would be desirable to provide an alternative approach to surface nitriding of metals and metal alloys, such as titanium and titanium alloys, that does not suffer the disadvantages described above.. Summary of the Invention

Accordingly, the present invention provides a method of forming a metal nitride or metal alloy nitride on a corresponding metal or a metal alloy substrate, which method comprises: covering the substrate with an oxygen-reacting composition comprising (a) _, particles of a metal that oxidizes in preference to the metal or metal alloy substrate and that is unreactive with respect to the metal or metal alloy substrate and (b) particles of an inert material; and exposing the covered substrate to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce the metal nitride or metal alloy nitride on the substrate.

The present invention also provides a metal or metal alloy substrate that has been subjected to surface nitriding in accordance with the method of the present invention.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Brief description of the drawings

Embodiments of the present invention will be described, by way of example only, with reference to the accompanying figures. Figure 1 shows a schematic arrangement of a gas nitriding assembly that may be used to carry out the method of the present invention.

Figure 2 shows optical micrographs of cross-sections of pure Ti after nitriding at 950°C for 24 hours in (a) nitrogen flow, i.e. without oxygen-reacting composition and (b) using an oxygen-reacting composition of particulate Al ₂ 0 ₃ +5 wt.% particulate Mg.

Figure 3 shows SEM and optical images of cross-sections of pure Ti after gas nitriding in a powder mixture of A1 ₂ 0 ₃ + 5 wi% Mg at temperatures from 650°C to 850 °C for 24 hours.

Figure 4 reports X-ray diffraction spectra of pure Ti samples after gas nitriding at 650°C and 800°C for 24 hours.

Figure 5 shows bright field TEM images of TiN/Ti ₂ N and Ti ₂ N/Ti interface and corresponding diffraction patterns of TiN, Ti ₂ N and Ti on pure Ti after nitriding at 850°C for 24 hours in a powder mixture of Al ₂ 0 ₃ +5 wt.% Mg.

Figure 6 shows optical micrographs of cross-sections of pure Ti after gas nitriding in a powder mixture of Al ₂ 0 ₃ +5 wt.% Mg at 950°C for (a) 12 hours and (b) 24 hours.

Figure 7 shows optical micrographs on cross-sections on pure titanium after gas nitriding in a powder mixture of Al ₂ O ₃ +60 wt.% Cu at different temperature from 850°C to 1050°C for 24 h. Figure 8 shows optical micrographs of the cross-section on pure titanium after gas nitriding in a powder mixture of Al ₂ O ₃ +60 wt.% Cu at 950 °C for (a) 12 hours and (b) 24 hours.

Figure 9 shows microhardness on surface and cross-sectional profiles of pure Ti after gas nitriding in a powder mixture of Al ₂ 0 ₃ +5 wt.% Mg at temperatures from 650°C to 850°C for 24 hours. Figure 10 shows wear resistance, indicated by the wear depth and volume loss respectively, of the nitride layer on pure Ti and Ti-6A1-4V substrate measured at different loads for 30 minutes and compared to Ti-6A1-4V alloy. Figure 11 shows SEM micrographs showing the morphology on worn surfaces nitrided at 850 °C for 24 h after wear testing at different loads for 30 min: (a) Ti6A14V, (b) TiN on pure Ti and (c) TiN on Ti6A14V.

Figure 12 shows average coefficient of friction (COF) of nitrided layer on pure Ti and Ti- 6A1-4V substrate measured at different loads for 30 minutes and compared toTi-6Al-4V alloy.

Figure 13 shows images of nitrided pure Ti and Ti-6A1-4V exposure to salt spray for different time compared to as-sprayed Ti, Ti-6A1-4V and 316SS.

Figure 14 shows average weight gain/loss of nitrided pure Ti and Ti-6A1-4V samples exposure to salt spray for different time compared to as-received pure Ti, Ti-6A1-4V and 316SS. Figure 15 shows linear polarization behaviour of TiN on pure Ti and Ti6Al4V as compared with as-received 316SS, pure Ti and Ti-6A1-4V.

Detailed description of the Invention The invention involves on the formation of a metal nitride layer on the surface of metal or metal alloy substrates. This nitride layer provides improved wear resistance. The metal nitride layer forms by reactive growth when the surface of the substrate comes into contact and reacts with nitrogen at high temperature. The present invention uses industrial grade nitrogen (also known as industry purity nitrogen) as the source of nitrogen for the nitriding reaction. The use of industrial grade nitrogen would normally present problems in this context since it contains an amount of oxygen sufficient to induce formation of undesired metal oxides on the substrate surface. In accordance with the present invention the presence of oxygen is mitigated by the use of an . oxygen-reacting composition with the result that oxidation of the substrate surface can be avoided. The present invention may be used to coat the entire exterior surface of a substrate or only a part of the exterior surface. Surfaces not to be coated may be masked or otherwise prevented from being in contact with the composition that is used in the method.

The oxygen-reacting composition comprises a mixture of reactive metal particle and inert particles. The reactive metal has two requirements in terms of reactivity. Firstly, the reactive metal must be one that, at the temperatures employed in the method of the invention, can more readily react with oxygen to form a metal oxide than the metal or metal alloy of the substrate. Considering that the measure of a material to oxidize (or lose electrons) is known as its oxidation potential, the reactive metal will have a higher oxidation potential than the metal or ^' the metal alloy of the substrate.

Secondly, the reactive metal particles do not react with the metal or metal alloy substrate under the operating conditions employed. In other words, the reactive metal does not form any intermetallic species when contacting the substrate at elevated temperature. The oxygen-reacting composition also comprises inert particles. These are particles of a material that are entirely unreactive during the method of the invention. Thus, the inert particles do not react with any components of the industrial grade nitrogen, with the metal or metal alloy substrate or the reactive metal particles. The function of the inert particles is as a filler and to prevent sintering of the reactive metal particles under the elevated temperatures employed in the present invention.

In the method of the invention the oxygen-reacting composition covers the substrate and the nitrogen-containing gas being used must pass/permeate through the composition before reaching the substrate surface. An amount of reactive metal particles will also contact the surface of the substrate. Without wishing to be bound by theory, it is believed that the reactive metal particles can play two roles in the method of the invention. On one hand. they are believed to react with oxygen in the industrial grade nitrogen thereby reducing the oxygen concentration. At the very least this will significantly limit the concentration of oxygen available for reaction at the substrate surface. At best no oxygen will be available for reaction at the substrate surface. On the other hand, the reactive metal particles are believed to reduce metal oxide existing, or formed during the method of the invention, on the metal or metal alloy substrate. This would increase the surface area of metal or metal alloy available for reaction with nitrogen and/or mitigate any undesired oxidation of the substrate surface during application of the method of the invention.

In the oxygen-reacting composition the relative amounts and particles sizes of the reactive metal particles inert particles should be selected based on a number of practical considerations as follows:

• It must be possible to form a homogeneous mixture of the particles. The mixture must also remain essentially homogeneous when used in the method of the invention.

• The overall specific surface area of the reactive metal particles must be sufficient to achieve the necessary extent of reaction with oxygen present in the nitrogen source used. If too little reactive metal is present, the ability of the reactive metal to "mop up" oxygen may not be adequate.

• The composition should be suitably permeable to the nitrogen source. For example, a low weight percentage mixture of reactive metal particles that are too small compared to the inert particles may be unfavourable, as voids between the various particles may be large enough to allow for oxygen to flow through the composition with minimum interaction with the reactive metal particles.

In practice, the characteristics of the composition in terms of weight proportions and particles sizes of components can be assessed and optimised by experiment. As a general guide however, the reactive metal particles and inert particles each have a mean particle size of about 5 μιη to about 100 μηι. The weight percent of reactive metal particles can vary greatly, but is usually no more than about 65% by weight based on the total weight of the composition. The present invention is believed to have particular utility in relation to titanium and titanium alloys substrates. Various grades of pure titanium may be used, such as Grade 2. Examples of titanium alloys that may be treated in accordance with the invention include Ti6Al4V, TilOV2Fe3Al, Ti6Cr5 o5V4Al, Til 100 and IMI834. This list is not exhaustive however and the usefulness of the present invention in relation to other grades of titanium and other titanium alloys may be assessed by experiment.

When the invention is applied in relation to titanium and titanium alloys, it has been useful to employ as reactive metal particles magnesium (Mg) particles, copper (Cu) particles or mixtures of Mg and Cu particles. In this context Mg particles suitable for use in the present invention would have a mean particle size of from about 5 μηι to about 100 μπι. The use of atomized Mg powder with a mean particle size of 45 μιη may be preferred. A typical composition of oxygen reactive powder can be obtained mixing Mg particles with inert particles so that the Mg particle concentration would be in the range of from about 2% to about 25% by weight with respect to the total weight of the composition.

When the invention is applied in relation to titanium and titanium alloys, Cu particles would typically have a mean particle size of from about 1 μιη to about 50 μηι. The use of Cu 101 Metal Flakes, 99.9% pure, with mean particle size of 1-5 μηι may be given by way of specific example. A typical composition of oxygen reactive powder can be obtained mixing Cu particles with inert particles so that the Cu particle concentration would be in the range from about 2% to about 60% in weight with respect to the total weight of the oxygen-reacting composition.

Inert particles for the purposes of the present invention may be particles made of fused alumina (A1 ₂ 0 ₃ ), SiC or Zr0 ₂ . Typically, the average particle size is from about 5 μτη to about 100 μηι, preferably of about 45 μπι. The oxygen-reacting composition is prepared by simple mixing of the component particles. The homogeneity of the composition may be assessed to determine what constitutes adequate mixing.

In the method of the invention the oxygen-reacting composition should be in intimate contact with the substrate surface and the composition may be loosely packed onto and around the substrate to facilitate this. It is preferred that there minimal or no voids/air gaps at the interface between the composition and substrate surface to ensure good contact between the substrate and the composition. The composition should be provided as a relatively thick layer covering the surface of the substrate, for example at least about 5 mm thick, preferably at least about 10 mm thick, with the maximum thickness usually being about 20 mm. The composition is packed around the substrate in a suitable container. The composition should be used in dry form.

Prior to providing the composition on the substrate the relevant surface(s) of the substrate are preferably be cleaned (e.g. de-greased). It may also be desirable to prepare the surface(s) by machining or grinding as this may enhance the relevant reactions.

Figure 1 is a non-limiting representation of a possible set-up that can be used to carry out the method of present invention. The bottom of a container is pre-filled with particles of an inert material (these may be the same or different material from the inert particles in the oxygen-reacting composition) in order to facilitate sample removal at the end of the nitriding treatment. Over these inert particles is provided a layer of oxygen-reacting composition and this will be in contact with the bottom surface of the substrate. A substrate is then position on top of the composition and the substrate then covered with oxygen-reacting composition sufficient to completely cover the substrate. At all stages of filling the composition can be loosely packed by gentle tamping. In a preferred embodiment the top of the container is then topped up with flakes or particles of the reactive metal used in the composition (possibly collected from machining of reactive metal ingots). These flakes/particles have been found to prevent oxidation of the oxygen- reacting composition during the subsequent heating stage. Subsequently, the substrate and oxygen-reacting composition are exposed to an atmosphere of industrial grade nitrogen. Industrial grade nitrogen is intended as any gas mixture containing approximately 99% by volume of nitrogen with the balance being oxygen and traces of inert gas. The nitrogen mixture is produced by common industrial production techniques, i.e. cryogenic air distillation or membrane separation. Preferably the industrial grade nitrogen contains about 99.5% of nitrogen by volume with the balance being oxygen and traces of inert gas. The amount of oxygen actually present in the mixture will depend upon the specific mixture specifications. Preferably, the container (and surrounding heating chamber) are evacuated and backfilled with nitrogen. The evacuation- backfill process can be repeated to minimize the presence of atmospheric air. Industrial grade nitrogen is pumped into the top of the chamber via holes provided ^' for that purpose and exits the container through other holes. Thus, the container is continuously replenished with respect to nitrogen as reactant.

Desirably, the container (and furnace) are evacuated and backfilled before heating takes place. The prevailing temperature should be sufficiently high to allow nitrogen to chemically react with the metal or metal alloy substrate to form of metal-nitride or metal alloy nitride. This reaction is known per se and is believed to proceed in conventional manner. Generally, the temperature will be between about 650°C and 1050°C. Heating is generally carried out for a number of hours, for example at least about 12 hours, such as up to 24 hours. It has been found that the nitride layer generally develops more rapidly at higher temperatures than at lower temperatures. However, the thickness of the nitride layer that is formed at one particular temperature can also be increased using a longer treatment time.

To be useful, the nitride layer developed in accordance with the present invention should have a thickness of at least 5 μηι, for example from 5 μιη to 200 μπι. The thickness depends upon the coating parameters used, primarily on the temperature and time of heating, and these may be manipulated and optimized as necessary. The optimum coating thickness for a given application may also be determined. After heating the substrate is allowed to cool before removal from excess coating composition. The coating on the substrate may then be subjected to surface finishing as may be required, for example to maintain dimensional tolerances. Grinding, machining and/or polishing may be applied to the coated substrate. Coated substrates may be assessed for coating quality and analysed using a variety of techniques, such as optical microscopy, electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), micro hardness testing and wear resistance testing, when appropriate.

The invention may also be applied to provide wear resistant coatings on light metal or light metal alloys of machine components. Conventionally, surface hardening of such components is undertaken by using gas nitriding, chemical/physical vapour deposition, thermal spraying or laser gas alloying in cases where components are subject to sliding forces or friction. According to this embodiment there is provided a method of providing a wear resistant coating on a surface of a light metal or light metal alloy machine component, which method comprises forming a metal nitride or metal alloy nitride on the surface by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the light metal or light metal alloy and that is unreactive with respect to the light metal or light metal alloy and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface.

The present invention may also find use in providing wear resistant coatings on a metal or metal alloy surface of a medical implants, such as joint replacement, where excellent biocompatibility combined with high wear resistance are required. According to this embodiment the method comprises forming a metal nitride or metal alloy nitride on the surface by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the metal or metal alloy of the surface and that is unreactive with respect to the metal or metal alloy of the surface and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface.

Other application substrates include metal or metal alloy surfaces of gas turbine components, such as discs and blades, which can be nitrided to reduce fretting wear. In addition, unlike the conventional nitriding process, which is normally limited to small scale components, the invention may be applied to large scale parts. According to this embodiment the method comprises forming a metal nitride or metal alloy nitride on a metal or metal alloy surface of a gas turbine component by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the metal or metal alloy of the surface and that is unreactive with respect to the metal or metal alloy of the surface and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface,

Embodiments of the present invention are illustrated with reference to the following non- limiting examples.

Examples

Materials used in the examples

The titanium and titanium alloys used in this study are commercially pure Ti (Grade 2) and Ti-6Al-4V alloy. The composition of the commercially pure Ti and Ti6A14V is shown in Table 1. Small blocks with the dimensions of 10 x 10 x10 mm ³ were cut from commercial plates of pure Ti and Ti-6A1-4V alloys, followed by mechanical grinding up to grade 4000 on silicon carbide paper. Table 1 Compositions of commercial pure Ti (Grade 2) and Ti-6A1-4V alloy in wt.%.

Ti alloys Al V Fe N 0 C H Ti

Ti 0.035 0.005 0.105 0.010 0.001 balance

Ti-6A1-4V 5.5-6.75 3.5-4.5 0.40 0.05 0.20 0.08 0.015 balance

Where used oxygen-reacting compositions consist of metal particles with a balance of fused A1 ₂ 0 ₃ (AWA, ~45 μπι). The metal particles used were commercial magnesium powder (Atomized Mg Powder, ~45 μπι) or commercial copper powder (Cu 101 , Copper Metal Flake, 99.9%, 1 -5 μηι). The composition is prepared by thorough mixing for 30 minutes in a mechanical power-mixer. Gas nitriding procedure used in the examples

Heating takes place in a small stainless steel container placed in a GSL 1600X tube furnace, which can work continuously at a maximum temperature of 1500°C. The furnace is sealed, evacuated to a pressure of -0.1 MPa and backfilled with nitrogen. The evacuation-backfill process is repeated three times to minimize atmospheric air in the furnace chamber. Pure nitrogen with industrial purity (min 99.5%) is used instead. The furnace is then heated using a heating rate of 5 °C/min to different target temperatures, usually between 650°C and 1050°C. Once the target temperature is reached, the furnace chamber is kept under constant pressure (0.04 MPa) using a nitrogen flow rate (industrial grade) of 0.1-0.3 ml/min, and the temperature maintained for up to 24 hours. After gas nitriding, the samples were cooled to room temperature under the same nitrogen atmosphere before removal from the furnace. Microstructure examinations

After gas nitriding, the samples were cut into two small blocks with dimensions of 10 x 10 x 5 mm. Cross-sections of these blocks, perpendicular to the surface, were mechanically ground using silicon carbide paper up to grade 4000. Then, a final polish was performed on a polishing cloth using a liquid suspension of 0.05 μηι alumina, followed by ultrasonic cleaning and drying.

An Olympus AX70 optical microscope was used to observe the microstructure on the cross-section and measure the thickness of nitride layers produced at higher temperature. The cross-sectional microstructure and thickness of the nitriding layer formed at low- temperature was examined in a JEOL6460LA SEM using backscattered electron images. The microstructure of nitrided layers was also determined using a JEOL-2100 Transmission Electron Microscope (TEM) operated at 200 kV. TEM specimens were prepared using focused ion beam (FIB) technique on a FEI Helios NanoLabDualBeam FIB apparatus.

X-ray diffraction (XRD) was used to characterize the nitride layers formed on pure Ti and Ti-6AI-4V substrates. The XRD analysis was carried out using a Bruker D8 diffractometer operated at 40 kV with A).

Mechanical properties evaluations

The micro-hardness of the surface and cross-sectional depth variation from the nitrided surface to the substrate core was determined using a Struers DURAMIN 20 hardness testing machine. The micro-hardness test was carried out using a load of 100 g on the surface, and 50 g on the sample cross-section. The load was applied for 12 s.

Dry sliding wear tests on pure Ti and Ti-6A1-4V surface after gas nitriding were performed on an Optimol SRVIII oscillating friction and wear tester at room temperature (25°C) in air with a relative humidity of 40-50%. The machine was equipped with a ball-on-disc contact configuration, which uses tungsten carbide - cobalt balls with a hardness of HV- 1750 as the counter friction pair. Prior to testing, all specimens were cut into 10 10 x 3 mm plates, and then the cutting plane was mechanically polished up to grit 1200 parallel to the nitride surface, followed by ultrasonic cleaning and air drying. Detailed testing parameters are as follows: 2 mm oscillating stroke with a frequency of 5 Hz, and a load ranging from 10 to 50 N were applied for 30 minutes; wear volume loss were determined by the formula proposed by Qu et al. For comparison, the tribo logical behaviour of commercial Ti-6A1-4V samples were measured under the same conditions. Corrosion resistance tests

Corrosion resistance of pure Ti and Ti-6A1-4V after gas nitriding was assessed visually using salt spray testing, and electrochemically using linear potentio-dynamie polarization, both in a neutral 5 wt.% NaCl solution. For comparison, as-received samples of pure Ti, Ti-6A1-4V and 316 stainless steel were also used in both tests.

The salt spray tests were conducted as per the ASTM B l 17 standard. During the tests, all samples were placed in a chamber at a temperature of 35°C under an aqueous spray (pH between 6.5 and 7.2). These samples were photographed at regular intervals by a digital camera, and then were cleaned and grit blasted with low air pressure to clear the surface corrosion product. The weight gain/loss was measured by an electronic balance to 0.0001 g as a function of spray duration.

For the polarization tests, only the nitrided surface was exposed to electrolyte; the other substrate surfaces were masked using an epoxy resin. The electrical contact was made via a sheathed copper wire drilled into the substrate and sealed with epoxy. The samples were cathodically polarized between -5V and 5V. Only the branch from -2.5 V to 2.5 V is shown in the Figure plots, and the scan rate used was 0.166 mV /s. Example 1

In order to explore the effect of metal particles on the formation of a nitride layer using industrial grade N ₂ , a powder mixture of 5 wt.% Mg with a balance of A1 ₂ 0 ₃ was used to protect pure Ti samples in the nitriding process. Figure 2 shows cross-section optical micrographs of a pure Ti sample after nitriding at 950°C for 24 h in nitrogen flow, in the absence (a) and in the presence of (b) a powder mixture of A1 ₂ 0 + 5 wt.% Mg. After direct nitriding in nitrogen flow (a), a 70 μη -thick nitrided layer was obtained on the surface of the pure Ti sample, but the layer appears porous and severely oxidated, as shown in Figure 2(a). However, a 40 μπι compact nitride layer showing no oxidation was achieved using a A1 ₂ 0 ₃ + 5 wt.% Mg powder mixture surrounding the sample, as shown in Figure 2(b).

Example 2

A pure Ti sample underwent a nitriding procedure at temperatures from 650°C to 850°C for 24 h in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% Mg. Back-scattered SEM images of Figures 3(a)-(c) show the nitride layer at temperature from 650°C to 750°C. At higher temperature the thickness of the nitride layer significantly increased. At 850°C, a 12-μπι thick layer was observed after nitriding for 24 h as shown in the optical microscope image of Figure 9(e). Figure 3(f) shows a plot of the nitride layer thickness observed after 24 hours at a temperature in the 650-950°C range. Nitriding temperature can promote the formation of nitride layer, especially above 800°C. At 950°C, the thickness of the nitride layer was 40 μηι after nitriding for 24 h.

As seen from Figure 3, two distinct sub-layers can be observed in the nitride layers. X-ray diffraction was performed to characterize the phase structure of these layers. Figure 4 shows the X-ray diffraction patterns of pure Ti after nitriding at 650°C (a) and 800°C (b) for 24 h in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% Mg. The XRD patterns indicate that the titanium nitride layers comprise TiN and Ti ₂ N. TEM investigations were performed to spatially identify the nitride layers. Figure 5 shows that the nitride layers contain two interfaces, which are recognisable in bright field mode. Diffraction patterns of the corresponding layers clearly clarified that the outer sub-layer is TiN and the inner one is Ti ₂ N. Example 3

Nitriding of pure titanium at 950°C for different annealing times has been carried out in a powder mixture of Al ₂ (½ + 5 wt.% Mg. Figure 6 shows cross-section optical micrographs of pure Ti samples after gas nitriding in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% Mg at 950°C for 12 h (a) and 24 h (b). As seen from Figure 6(a), the thickness of the nitride layers is about 24 μηι after nitriding for 12 h, whilst it almost doubled after nitriding for 24 h at the same temperature, as shown in Figure 6(b). This indicates that the nitride layer thickness can also be increased by prolonging the nitriding time, besides raising the nitriding temperature.

Example 4

Cu metal particles were used instead of Mg particles. A powder mixture of A1 ₂ 0 ₃ + 60 wt.% Cu was used to purify the industrial grade nitrogen during the nitriding of pure Ti samples at temperatures from 850°C to 1050°C, for up to 24 h.

Figure 7 shows the optical micrographs of the cross-section of the pure titanium samples after gas nitriding in a powder mixture of A1 ₂ 0 ₃ + 60 wt.% Cu at different temperature from 850°C to 1050°C for 24 h. It can be seen that Cu powder can also protect the substrate from oxygen, thus favouring the formation of a nitride layer on its surface, similarly to what is obtained using Mg particles. Upon increasing the temperature from 850°C to 1050°C, the thickness of nitride layer after nitriding for 24 h increases slowly below 900 °C, but significantly above 950 °C. A 150-μιτι thick nitride layer can be achieved at 1050 °C after 24 h (Figure 7(e)). Example 5

Figure 8 shows optical micrographs of the cross-section of pure Ti samples after gas nitriding in a powder mixture of Al ₂ 0 ₃ + 60 wt.% Cu at 950 °C for 12 h and 24 h. A 23- μηι thick nitride layer can be achieved on pure Ti at 950 °C for 12 h using a powder mixture of Al ₂ O ₃ +60 wt.% Cu, which is similar to the thickness obtained under the same conditions using an Al ₂ 0 ₃ +5 wt.% Mg powder mixture. However, as the nitriding time increased up to 24 h, the thickness of the nitride layer increased 5 times in the powder mixture of Al ₂ O ₃ +60 wt.% Cu, whereas it only doubled in the Al ₂ 0 ₃ +5 wt.% Mg powder mixture. Therefore, comparing these two powder mixtures, it was found that the powder mixture of Al ₂ O ₃ +60 wt.% Cu is more effective than the powder mixture of Al ₂ 0 ₃ +5 wt.% Mg above 950°C, opposite to what was observed below 950°C. As suggested by Malinov et al [S. Malinov, A. Zhecheva, and W. Sha, Metal Science and Heat Treatment, Vol. 46, Nos. 7 - 8, 2004, p. 286-293], gas nitriding of titanium alloys should be performed at a temperature below their β-transus temperature. Hence, the A1 ₂ 0 ₃ + 5 wt.% Mg powder mixture is more suitable for pure Ti other than the powder mixture of A1 ₂ 0 ₃ + 60 wt.% Mg.

Example 6 A micro-hardness test was performed on pure Ti samples after nitriding in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% at temperatures from 650°C to 850°C for 24 h. Figure 9 shows the micro-hardness profiles on the surface and the cross-section of pure Ti samples after gas nitriding in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% Mg at temperatures from 650°C to 850°C for 24 hours. A micro-hardness of HVo.i 1650 can be achieved on titanium surface after nitriding at 850°C for 24 h, as result of the thick nitride layer shown in Figure 3(e).

Therefore, in order to investigate the wear behaviours of nitride layer, both pure Ti and Ti- 6AI-4V were nitrided at 850°C for 24 h in a powder mixture of A1 ₂ 0 ₃ + 5 wt.% Mg. Figure 10 shows the wear resistance of nitrided layer on pure Ti and Ti-6A1-4V measured at different loads from 10 N to 50 N for 30 min and compared to as-received Ti-6A1-4V, which was indicated by the wear depth and volume loss respectively. SEM micrographs showing the moφhology on worn surface of these samples after wear testing were given in Figure 1 1.

It can be seen from Figure 1 1 that as-received Ti-6Al-4 V has larger worn depth and volume loss than two nitrided titanium alloys substrate at each load. This indicates that Ti-6A1-4V has the poorest wear resistance among these three samples, and could be evidenced by largest worn scar on the surface.

The nitrided titanium and nitrided titanium alloy samples have a similar wear resistance, indicated by worn depth and volume loss in Figure 1 1 , when using a load below 40 N. When the load exceeds 40 N, the nitrided pure Ti sample exhibits higher wear resistance than the nitrided Ti-6A1-4V sample. SEM observations show that the nitride layer on the Ti-6A1-4V sample was worn out at 40 N, which leads to a similar morphology of worn scar with as-received Ti-6A1-4V. The nitride layer on the pure Ti sample shows a typical morphology on worn scar compared to as-received Ti-6A1-4V under a 50 N load. This is attributed to a thicker nitride layer produced on pure Ti than that on Ti-6A1-4V.

Although the nitride layers on both pure Ti and Ti-6A1-4V exhibits an excellent wear resistance, they have the similar average coefficient of friction compared to as-received Ti-6Al-4V. This is probably due to the high surface roughness caused by gas nitriding.

Example 7

Corrosion behaviour - Salt spray testing

Figure 13 shows macrographs of nitrided pure Ti and Ti-6A1-4V samples surface exposed to neutral salt spray for 180 h and 326 h, compared with 316 SS, pure Ti and Ti-6A1-4V substrates. No visible corrosion attack is observed. The weight gain/loss recorded at regular intervals is shown in Figure 14. No obvious weight change is detected in all samples exposed to salt spray for 326 h. The fluctuation of curve is caused by the salt remaining on samples. Example 8

Electrochemical corrosion testing

The response of the nitride layer subject to linear polarization is shown in Figure 15, plotted with respect to the standard Ag/AgCl- reference electrode potential. It can be seen that the nitride layer has a significant effect on the polarization behaviour of the titanium and titanium alloy substrates. Although the corrosion current density remains the same for both pure Ti and Ti-6A1-4V before and after nitriding, the corrosion potential significantly increases after nitriding. This indicates that the nitriding can improve the corrosion resistance of titanium alloys as well.