An introductory section outlining the main themes of the chapter and what each section will contain



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  • an introductory section outlining the main themes of the chapter and what each section will contain (approx. 250 words)

  • a short commentary on likely future trends (approx. 500 words)

  • a short descriptive section of sources of further information and advice (approx. 300 words). This might provide a brief commentary on key books to consult (cross-referenced to full details given in the references), major trade/professional bodies, research and interest groups, web sites etc.

  • references



Chapter 25. Duplex and hybrid surface treatments of light alloys
Dr. Y. Fu, Heriot Watt University, Edinburgh, UK
Abstract


    1. Introduction

The rapid development in engineering demands the requirement of materials with btter mechanical properties, resistance to frictional wear, resistance to corrosion and erosion etc. These demands can be satisfied by applying various surface engineering techniques which permit modifying the microstructure, phase and chemical composition of the surface layers of the treated parts. In real applications, TiN-, CrN- or DLC-coated aluminum alloys using various PVD methods often exhibit limited tribological performance due to the elastic and plastic deformation of the substrate under mechanical loading, which can result in eventual coating failure, since the coatings are usually too thin to support the heavy load and protect the substrate in the contact conditions. Deposition of thick (e.g. >10 μm) PVD coatings usually results in high compressive stresses, and thus low adhesion [2]. The problem is even more pronounced if the components are made of light metals, such as aluminum and its alloys due to their poor surface hardness and relatively low yield strength compared to steels and other materials, which usually exhibit poor wear resistance in sliding and rolling contact situations [3] R. Gadow and D. Scherer, Composite coatings with dry lubrication ability on light metal substrates, Surf. Coat. Technol. 151–152 (2002), p. 471477. Consequently, these issues reduce lifetime and limit the applications where a heavy surface load bearing is required. These layers improve essentially the performance properties and service life of the treated parts and widen significantly the application range of the materials, including titanium and its alloys which are now being increasingly used in industry [1].


Many engineering components will operate under severe conditions, such as large loads, high speeds and harsh environments, in order to achieve high productivity, high power efficiency and low energy consumption. There is requirement for properties of such as wear resistance, load bearing capacity, and fatigue performance) are required. These new challenges can be met only through realising the potential of duplex surface engineering [2]. Thin coatings such as PVD TiN can provide a surface with dramatically improved tribological properties in terms of low friction and high resistance to wear, but catastrophical premature failure will occur if the substrate plastically deforms under a high applied load; on the other hand, deep hardened layers produced by such surface modification techniques as energy beam surface alloying can sustain high contact stresses but still exhibit higher friction and wear rates when compared with most ceramic coatings. It is the combination of such surface engineering technologies that constitutes duplex surface engineering []. Need to correct this part.
Titanium and aluminium alloys show relatively high strength to density ratio and good corrosion resistance. Their main disadvantage in many applications is the low hardness and hence low resistance to abrasive wear. The wear resistance of titanium alloys can be enhanced by nitriding, but the compound layer thickness is limited. The requirement of thicker compound layers could be fulfilled by a combination process. The duplex process of nitriding plus moderate temperature chemical vapour deposition (MTCVD) TiCN coating of titanium alloys appears promising. Either pressure nitriding or gas nitriding can be applied to Ti-6Al-4V With pressure nitriding, homogeneous compound layers can produced on complex shaped components; however, the combined process must be carried out discontinuously in two reactors, due to the different process parameters. When using gas nitriding, the combined process can be performed continuously in the CVD equipment. With the combined nitriding + MTCVD coating route, thick compound layers could be produced in relatively short process times. The surface microstructures consist of. a nitrogen diffusion layer, an intermediate layer, a compound layer, and a TiCN coating. The Ti-6Al-4V surface was characterised by high hardness and good layer adhesion [3].
There is ever increasing interest in the applications of titanium and aluminum alloys in such sectors as automotive, medical, power generation and general engineering, in which tribological behaviour is often a major concern. Over the past 10 years significant progress has been made to overcome the inherent tribological problems of titanium alloys by means of surface engineering techniques (Fig. 5) including PVD and plasma nitriding, and also electron beam surface alloying techniques[17, 18, 19, 20]. Thin coatings such as TiN generated by plasma nitriding or by PVD deposition can provide a titanium alloy surface with greatly improved tribological properties in terms of low friction and high resistance to wear. However, premature failure will occur if the substrate deforms plastically under a high load, on the other hand, deep hardened layers produced by electron beam surface alloying can withstand a high contact stress, but do not endure sliding contact. Consequently, these techniques are technologically only suitable for components used under moderate loads or with a low or moderate sliding ratio. These include combining deep hardening with low friction high wear resistant thin coatings, which can provide titanium surfaces with good tribological behaviour to meet different requirements arising from diverse application conditions, especially pure sliding or high contact loads or a combination of both.
Therefore, various surface modification technologies have been developed to improve the wear behavior of Ti–6Al–4V alloys. Among them, hard ceramic coatings such as titanium nitride (TiN) are suggested to be promising materials for this purpose [3 and 4]. Up to now, there are many technologies for producing TiN coatings on Ti–6Al–4V alloy substrate, the most commonly used methods are conventional nitriding [4], conventional ion implantation [5] and its modified process, e.g. plasma source nitrogen ion implantation [6], Physical vapor deposition [9] and plasma-enhanced chemical vapor deposition [10, 11 and 12] are the most preferable methods used for the synthesis of hard TiN coatings on Ti–6Al–4V alloy substrates. However, the effective adhesion of the TiN coatings is not good enough because of the lack of load support provided by the relatively soft substrate beneath the coating. The processes investigated have included plasma nitriding, physical vapour deposition, laser surface alloying and electron beam surface alloying and currently ion implantation and plasma immersion ion implantation [4].
One of the techniques which appears to be most attractive, is producing surface layers under glow discharge conditions. This treatment is widely used for nitriding and oxynitriding of titanium alloys [13], [14] and [15]. The glow discharge conditions promote diffusion and nanocrystalization in the near-surface zone of the layers [16]. It also removes oxides and impurities from the surface, which ensures that the layers are of good quality.

Fig. 12.Towards titanium designer surfaces with desired combinations of properties through a novel duplex surface engineering system (Following T. Bell’s paper)


The processes investigated have included plasma nitriding, physical vapour deposition, laser surface alloying and electron beam surface alloying and currently ion implantation and plasma immersion ion implantation [5].. Some results from plasma nitriding and energy beam surface alloying processing and evaluation achieved to date are included which have culminated in the use of a duplex surface engineering process. This process has allowed a Ti6Al4V alloy (where the compositions is in approximate weight percent) to be tested at 1300 MPa maximum hertzian contact pressure and 50% slip. This represents an improvement of 1200% in contact pressure and 500% in slip ratio survival over untreated Ti6Al4V. This significant increase in wear resistance of the material after duplex treatment is caused by the presence of the Al2O3 on the surface. The presence of the intermetallic phases from the Ti­Al system is also very important, because they ensure continuous changes in the mechanical properties when passing from the surface (Al2O3) to the substrate (Ti6Al4V). The step change in the properties may cause a decrease of the wear resistance due to the decreased adhesion of the layer and the higher stresses at the interface between the layer and the substrate.

25.2. Duplex surface treatment and its advantages


Duplex or hybrid surface engineering involves the sequential application of two (or more) established surface technologies to produce a surface composite with combined properties which are unobtainable through any individual surface technology. Duplex surface engineering may invoves two different processes. Some examples of duplex surface engineering technologies are listed in Table 1.

Table 1.


Typical duplex surface engineering technologies



No

Complementary technology

Supplementary technology

1

PVD coatingof pre-nitrided




2

Plasma nitriding




3







4







5







6







7






Although the possible combinations of surface technologies are virtually unlimited and the list of duplex surface technologies could be endless, to date only a limited number of duplex treatments have been developed, and few of them have yet found real applications[6, 7]. In most coating systems plastic deformation initiates in the substrate near the coating–substrate interface when subject to relatively high intensity loading, and plastic deformation does not initiate in the coating until a large plastic zone has been developed in the substrate. The load bearing capacities of coating–substrate systems thus depends upon the substrate properties. Clearly, deep case hardening can significantly enhance the load bearing capacity of a coating-substrate system, for example, nitriding and oxidation, etc. Low friction coatings such as nitrides and oxides used as the top coating layer for duplex systems not only increase wear resistance but also diminish interfacial shear stress and strain, and thus reduce the tendency for debonding of top coatings. In this respect, DLC or diamond coating are more effective since they possess the lowest friction against most engineering surfaces.


The reasons for the combination of plasma nitriding and CNX films on Ti-6Al-4V substrate to improve the tribological properties can be listed as follows [Error: Reference source not found,6]:


  1. To synthesis carbon nitride films, it is important to provide a structural template to seed the growth of crystalline carbon nitride. An ideal structural template for this purpose is one with at least one low free-energy plane lattice matched to some low free-energy plane of C3N4. TiN is a reasonable substrate to realize this purpose [7]. By depositing carbon nitride films on plasma nitrided Ti-6Al-4V (from which TiN can be produced), it is easy to obtain an optimum chemical (both are nitrides) and structure transitions between plasma nitrided layer and carbon nitride film.




  1. Plasma nitriding of Ti-6Al-4V produces a graded hardened case which serves as an excellent supporting layer for the hard CNX film. Plasma nitriding also increases the hardening depth, provides a compressive stress and imparts better stability of carbon nitride film on Ti-6Al-4V substrate.




  1. CNX film deposited at low temperature can produce a wear resistant and low-friction surface without impairing the beneficial effects of plasma nitriding treatment. Smooth and low friction CNX film could effectively reduce both the tangential stress and the interfacial stresses thus providing a good tribological behavior [8];




  1. For CNX film deposited directly on Ti-6Al-4V substrate, the load bearing capacity is relatively poor thus affecting its tribological performance. With the application of CNX film on the pre-nitrided Ti-6Al-4V substrate, the load bearing capacity and adhesion strength increase dramatically, and therefore, the tribological properties can be improved significantly [Error: Reference source not found,9];

(5) For CNX films, if small amount of debris is generated, it might play an important role during wear processes depending on whether it is soft (graphitic) or hard (diamond-like) and how easily it is ejected from the surface [10]. From the low coefficient of friction and the presence of the transfer layer on the wear track, it can be concluded that the spalled CNX films act as a solid lubricant during sliding, i.e., as a third lubricating body between the two surfaces. Mechanical-chemical interactions between the sliding interfaces and the environment can lead to micro-graphitisation of the interlayer at the microcontacts as can be deduced from the Raman analysis on the worn surface of wear track shown in Figure 15. Compared with that of as-deposited films shown in Figure 5, the D band (1335 cm-1) and G band (1595 cm-1) of the worn coatings becomes sharp in the spectrum as a result of the graphitisation [11,12]. This phenomenon clearly shows that the graphitisation and degradation of CNX films occurs between the contact surfaces and it has a significant lubricating effect on the wear and friction properties of CNX films during sliding.


25.3. Duplex treatment of Ti alloys
Many surface engineering techniques have been investigated and applied to titanium alloys, such as laser surface treatments [1], PVD [2], CVD, plasma nitriding [3], plasma spraying [4], plasma immersion ion implantation [5] and plasma oxidising [6], [7] and [8] etc. Among them, carbon based coatings, especially DLC, have distinguished tribological performance due to their high hardness, low coefficient of friction and low wear rate [9]. Hard carbon coatings deposited on ‘hard’ substrates (ceramics and hardened steels) have displayed excellent tribological performance. However, when deposited on ‘soft’ substrates such as commercial pure titanium and Ti6Al4V alloy, the softer substrate deformed plastically when the applied load was high [10]. The deformation was considerable and repeated deflection of the coating caused fractures or fatigue cracks that eventually destroyed the film. Stress field analysis on sliding contacts under loading has established that when the coefficient of friction is reduced, significant shear stresses are developed and the location of the maximum shear stress moves gradually into the substrate away from the substrate/coating interface. There is initial evidence suggesting that under these conditions, soft substrate materials, i.e., Ti6Al4V alloy, may not be able to provide adequate support for the hard carbon films, adversely affecting their tribological performance and durability.

Titanium and its alloys have many attractive properties, including high specific strength and modulus, excellent corrosion resistance, and, in some cases, good cryogenic properties [13,14]. They are widely used in aerospace applications and many corrosive environments [15]. Ti–6Al–4V alloy has also been frequently used for orthopedic devices and other engineering components due to its beneficial properties, such as low density, low modulus of elasticity, excellent corrosion resistance and biocompatibility [1]. However, they are notorious for the poor tribological properties, such as poor abrasive and adhesive wear resistance; being prone to fretting wear and fretting fatigue; a tendency to galling and seizure; etc. [16]. Surface engineering methods are widely used to improve the wear resistance of titanium alloys [17,18,19,20]. The deposition of adherent diamond, diamond-like carbon (DLC) has been realised as a promising way to solve the above problem [21,22,23,]. Nowadays, carbon nitride films have also been prepared on Ti alloys for improving their tribological property [24]. A soft substrate such as Ti6Al4V alloy might not be able to provide adequate support for a hard carbon coating.


Novel duplex or hybrid systems combining nickel diffusion (ND) deep case hardening with low friction wear resistant TiN and diamond-like carbon (DLC) coatings have been designed and applied to high strength Timetal 550 titanium alloy [25]. The load bearing capacity of these low friction wear resistant coatings can be improved dramatically when deposited on ND-treated Timetal 550 substrate relative to material coated with TiN and DLC alone. Continuous duplex process consisting of low temperature ( 420 °C) plasma nitriding followed by in situ deposition of the DLC coating is reported [26]. The duplex treatment significantly increased the composite hardness and reduced the plastic deformation of the substrate. An improvement in the adhesion in the duplex treated sample compared to the non-duplex treated sample. In complex design situations two surface technologies can be combined -duplex surface engineering -to give very substantial improvements in wear and load bearing capacity. This duplex approach is illustrated through the industrially accepted TiN/plasma nitrided combination technology. Recent contact mechanics modelling of this duplex system is reviewed showing that thin ceramic coatings can act as “stress barrier” coatings on realistic industrial engineered surfaces. A new method of plasma nitriding and plasma-enhanced chemical vapor deposition (plasma-duplex process) was conducted on Ti–6Al–4V alloy in order to modify its wear performance [27]. The results indicate that the plasma nitriding processing temperature plays a dominant role in nitrided layer formation when compared to other nitriding parameters (e.g. treatment time, gas flow ratio of H2/N2 etc.). It is also shown that plasma nitriding and TiN coatings produced by plasma-duplex processing provide a marked improvement in the wear performance of the modified Ti–6Al–4V alloy surface. However, the friction coefficients of plasma-duplex treated samples increased significantly compared to the substrate. There is also a negative effect on the wear behavior if the TiN coating adhesion is poor.
25.3.1. Plasma nitriding based duplex treatment
Amorphous CNX films show attractive mechanical and tribological properties, characterised by high hardness, high elastic recovery, good adhesion to substrate and low friction coefficient [Error: Reference source not found], and it is considered as a potential material which will compete with DLC films [28, 29]. Amorphous CNX films have already been applied in magnetic recording industry rivalling the amorphous DLC films with their high hardness and elastic property, superior heat conductivity, and low friction coefficient [30]. Amorphous carbon nitride films have also been prepared on Ti alloys for improving the tribological property and biocompatibility of Ti alloys [31,32,33]. However, the soft Ti alloys may not be able to provide adequate support for the hard DLC or CNX films, thereby adversely affecting their load bearing capacity, tribological performance and durability. An approach to solve this problem is to design and develop duplex diffusion/coating treatments [34,35,36].
Duplex surface engineering involves the sequential application of two (or more) established surface technologies to produce a surface composite with combined properties which are unobtainable through any individual surface technology [37]. Current studies on duplex treatments concentrate on combining plasma nitriding with TiN, CrN or DLC coating on steel substrate [38,39,40]. The best tribological coating should combine high hardness, low coefficient of friction and high load bearing capacity (not only the high adhesion strength) [Error: Reference source not found,41]. Higher coating hardness and lower coefficient of friction are desirable for the good tribological performances of DLC or CNX films. However, high hardness usually corresponds with brittleness and high stress in the coating, which may deteriorate both the load bearing capacity and the tribological performance [42].
Load bearing capacity evaluated by indentation tests
Indentation tests are used to measure the static load bearing capacity of coating-substrate system [16,21]. Fig. 4 shows the SEM micrographs of the indentation impressions of CNX films deposited on Ti-6Al-4V and plasma nitrided Ti-6Al-4V. For the CNX films deposited directly on Ti-6Al-4V substrate, there is usually large-area cracking and spallation occurring after indentation (see Fig. 4(a) and (d)) due to the brittle nature of CNX film and their poor load bearing capacity. The poor load bearing capacity of CNX films on Ti-6Al-4V is probably attributed to: (1) the high hardness, brittleness and high internal stresses in thin film; (2) the significant differences in elastic modulus and hardness between CNX film and Ti alloy; (3) the high contact pressure during indentation which causes the severe plastic deformation of the soft substrate, thus inducing the cracking and spallation in thin and hard CNX film.
Under a small normal load of 60 kgf, for a 2 m CNX film deposited on plasma nitrided Ti-6Al-4V substrate, there are some small ring cracks within the impression and ejecting radial cracks formed at the circumference of the indentation indicating the brittle nature of the deposited CNX films as shown in Fig. 4(e). During indentation, the circular surface wave-like topographic deformation serves as the driving force for these lateral cracks. Under a high normal load of 150 Kgf, severe radial and ring cracks (but no spallation) can be observed indicating a high load bearing capacity of CNX films deposited on plasma nitrided substrates (see Fig. 4(f)).
The above results indicate that with the application of plasma nitrided interlayer between the substrate and coating, the load bearing capacity has been improved significantly. Fig. 5(a) demonstrates a failure mode with only ring cracks and radial cracks around the indentation, which indicate that the coating is brittle but with a high load bearing capacity [43,44]. Fig. 5(b) shows that during indentation, many cracks which form in the coating reorient along the interface resulting in discrete debonding, but most of the films remain attached to the substrate [45]. This situation corresponds to a film with a relatively poor load bearing capacity. When there are high stresses existed in the film due to the sharp difference in elastic modulus or hardness between coating and substrates, the compressed films may buckle at the interface, and these buckles are susceptible to propagation by interface crack growth, followed by large-area spallation as shown in Fig. 5(c).
Scratch Tests
Scratch testing was used to evaluate the load bearing capacity of coatings under both normal and tangential force, and it is considered to assess the dynamic load bearing capacity of the coating-substrate system [16]. For the scratch tests on CNX films, the load-friction curves of coated samples often show a linear increase at the beginning period, with an abrupt increase at a critical load. The critical loads, i.e., the load bearing capacity of CNX films on untreated and plasma nitrided Ti-6Al-4V substrate are shown in Fig. 6. Compared with that of carbon nitride film deposited directly on Ti-6Al-4V substrate, the load bearing capacity is improved dramatically with the application of plasma nitrided layer between Ti-6Al-4V substrate and CNx film.
Fig. 7 (a) to (f) show the typical friction-load curves of scratch tests for Ti-6Al-4V substrate, plasma nitrided layer, CNX film deposited on Ti-6Al-4V substrate and pre-nitrided layer. Fig. 8 shows the corresponding curves of the coefficient of friction vs. normal load for different specimens.
For the stylus scratching on Ti-6Al-4V substrate, the coefficient of friction is extremely high (larger than 1.0, see Fig. 8). Examination on the scratch track shown in Fig. 9 reveals the typical severe abrasive wear (i.e., ploughing) and delamination. With the stylus scratching on the plasma nitrided Ti-6Al-4V substrate, the coefficient of friction is low and stable (around 0.15) as shown in Fig. 8. Examination on the wear track shows that there is no much wear and only slight scratching lines on the wear track indicating a good wear resistance (see Fig. 10). When the load is increased to about 55 N, there is a large variation in coefficient of friction which is probably caused by the collapse of nitrided layer under high normal load.
For the scratch test of CNX film deposited on Ti-6Al-4V substrate, during the beginning period, the coefficient of friction is very low (see Fig. 8) indicating a good lubricating effect of CNX films. However, after a short period, the coefficient of friction increases abruptly. SEM observation shown in Fig. 11 reveals that this sudden increase in friction force is attributed to the spallation of CNX film due to the poor adhesion strength and load bearing capacity on CNX film with Ti-6Al-4V substrate.
The duplex treated coating shows excellent load bearing capacity and tribological performance. For the 2 m CNX film deposited on pre-nitrided specimen, the long-term coefficient of friction is rather low and stable as shown in Fig. 8. The typical scratch morphology on duplex treated surface indicates a mild wear on CNX film. There is a sudden jump in friction force occurring at a critical load around 60 N as shown in Fig. 8. SEM examination reveals that the film can not endure the combination of the high normal load and frictional force, so it collapses and causes the large-area spallation of the film as shown in Fig. 12. The similar phenomenon can be observed for the 5 m CNX films deposited on plasma nitrided surface, and the only difference is that the long-term coefficient of friction is a little lower and the critical load is slightly smaller. The relatively low coefficient of friction under a higher normal load is probably related to the graphitization of CNX coating under high load and sliding conditions [46], but this needs further investigation.
Plasma nitriding of Ti-6Al-4V produces a graded hardened case which can serve as a supporting layer for the hard CNX films improving load bearing capacity, and it also provides a compressive stress which can assist in excellent fatigue resistance and impart better stability of carbon nitride films to the Ti substrate. Hard and low-friction CNX films could effectively reduce the coefficient of friction thus providing a good tribological behaviour. After plasma nitriding, usually the surface becomes rough and the high surface roughness gives rise a highly stressed layer. By depositing CNX films on plasma nitrided specimen, the surface roughness can be decreased, therefore, reinforcing plasma nitrided layer by deposition of CNX films can improve the wear resistance significantly [Error: Reference source not found,47].

Tribological properties
Figures 13 (a) to (d) show the comparison of coefficient of friction under different normal loads for four types of specimens. Under a normal load of 1 N and 2 N, it can be observed that both plasma nitrided specimen and duplex treated samples showed a relatively low value of coefficient of friction compared with that of Ti substrate. However, the coefficient of friction of plasma nitrided specimen increased gradually during the further sliding, whereas that of duplex treated specimen remained low and stable. CNX film deposited on Ti-6Al-4V substrate also showed a low value of coefficient of friction. However, due to poor adhesion and the occurrence of spallation, the coefficient of friction varied significantly during sliding. With the normal load increased to 5 N and 10 N, the coefficient of friction of duplex treated specimen remained a low value of about 0.25. However, that of the plasma nitrided specimen jumped to a high level of 0.6 to 0.7 due to the spallation of the compound layer. In brief, the duplex treated system was more effective in maintaining a favourable low and stable coefficient of friction than both individual plasma nitriding and individual CNX films.
Figures 14 (a) to (d) show the comparison of wear rates for three types of specimens. Both Plasma nitriding and duplex treated coating could improve the wear resistance significantly under low normal loads of 2 N and 5 N. However, with the normal load increased to 10 N, due to the collapse of the compound layer, wear of plasma nitrided layer became significant. Clearly, the duplex treated system was more effective in terms of improving wear resistance than both individual plasma nitriding and individual CNX film.
For untreated Ti-6Al-4V, the long-term coefficient of friction remained a constant at about 0.5-0.6 and was almost independent upon the applied normal load. With an increase in normal load, the wear volume increased significantly. The dominant wear mechanisms for untreated Ti-6Al-4V were abrasive wear, adhesive wear and delamination. Fig. 6 (a) and (b) show the worn surface morphology of wear track indicating the extensive ploughing and delamination on Ti-6Al-4V substrate. There were usually large quantities of wear debris on wear track which were the oxide particles according to EDX analysis results. Examination on the worn Al2O3 ball using EDX indicated the transfer of substrate materials on the balls. In brief, tribological behaviour of untreated Ti-6Al-4V was characterised by high coefficient of friction and severe wear of materials [48].
Fig. 7(a) shows the coefficient of friction of plasma nitrided samples under different normal loads. It revealed that there was a critical load at which the wear mechanism was probably changed. Under normal loads of 1 N and 2 N, the coefficient of friction remained a low value (about 0.2-0.3) during the long-term sliding. Wear of plasma nitrided sample was difficult to be detected. Fig. 7(b) shows the worn surface morphology of plasma nitrided sample under a normal load of 2 N. There are only some small scratch lines indicating the mild wear of the nitrided layer. However, with the normal load increased to 5 N and 10 N, the dominant wear mechanism changed to spallation and severe abrasive wear. The coefficient of friction of nitrided sample under 5 N increased significantly during the long-term sliding (see Fig. 7(a)). Under a normal load of 10 N, the coefficient of friction also increased abruptly to a high value of 0.6, then varied significantly. On the worn surface, the crushing or spallation of compound layer could be observed as shown in Figure 7 (c). EDX analysis on worn Al2O3 ball indicated the transfer of materials from the plasma nitrided layer.
Wear of CNX films on Ti-6Al-4V substrate was only performed under a normal load of 1 N with the consideration of poor load bearing capacity of CNX film. Fig. 8(a) shows the worn surface morphology indicating a large-area spallation of CNX films. Fig. 8(b) shows the coefficient of friction curves of CNX films on Ti-6Al-4V substrate. After a slight increase during the beginning period, the coefficient of friction fluctuated significantly during the further sliding. The sudden fluctuation in coefficient of friction was caused by the rapid spallation of CNX films. It is interesting to note that even though the CNX films were completely worn out during wear, the coefficient of friction still remained a relatively low value. On the worn coating surface, the coating fragments were entrapped within the wear track, comminuted into fine particles, and finally mixed up with the substrate materials to form a mechanically alloyed surface layer (see Fig. 8(a)). Fig. 4 shows the Raman analysis on these mechanically alloyed layer. Compared with that of as-deposited film, the D band (1335 cm-1) and G band (1595 cm-1) became sharp in the spectrum as a result of the graphitization [49,50]. The formation of these graphitized layer can act as a lubricating third layer, and probably this is the reason why after the coating spallation, the coefficient of friction fluctuated significantly but still remained a low value of 0.2 to 0.3 as shown in Figure 8 (b).
Wear tests with the normal loads of 2 N, 5 N and 10 N were conducted on CNX films deposited on plasma nitrided layer. Wear of duplex treated specimen was minimal under normal loads of 2 N and 5 N. Only with the normal load increased to 10 N could the wear of CNX films on nitrided layer be detected. Figure 9 (a) shows the surface morphology of the wear track under a normal load of 10 N. There was only some small spallation of the films occurring at the edge of the wear track, probably because of the original rough surface of plasma nitrided layer. The spalled debris were trapped in the wear track and promoted the maintaining of a low coefficient of friction. Figure 9 (b) shows the variation of the coefficient of friction of duplex treated specimen under different normal loads. The frictional response was characterised by an initial break-in period which was followed by an intermediate constant stage. This break-in period probably corresponded to the removal of the oxidised top layers and the build-up of a transfer film. Thereafter, the coefficient of friction decreased a little and reached a steady value. The long-term coefficient of friction of duplex treated coating remained roughly about 0.25 under normal loads of 2 N and 5 N. However, under a normal load of 10 N, the long term value of coefficient of friction was less than 0.2. The reason was not clear, and probably under a high normal load, there was more transfer material which further decreased the coefficient of friction.
Counterface materials on the tribological characteristics
The successful application of duplex coating system for tribological protection requires knowledge of not only the intrinsic properties of the coating composites, but also their dependence on the counterface materials and operating environment [51,52].
Pin-on-disk wear tests with different types of counterfaces, i.e., 52100 steel balls, alumina balls and UHMWPE (ultrahigh molecular weight polyethylene) pins, were performed to evaluate the wear and friction characteristics of the duplex treated coatings. For the wear tests sliding with steel balls, the transferring of materials from steel ball was one of the main wear mechanisms. For the wear tests of duplex treated coating sliding with alumina ball, the generation, compaction and graphitization of coating debris layer (which acted as a lubricating third-body) improved the friction property and wear resistance of the coating. For the wear tests of titanium substrate sliding with UHMWPE pins, severe abrasion was found on both titanium alloy and UHMWPE pin surface. With the application of the duplex treated coating system on titanium substrate, both the coefficient of friction and wear volumes of two counterfaces were reduced significantly.
Plasma nitriding was carried out with a total power of 2 kW and a voltage of 1500 V. The deposition temperature was 800oC and the nitriding duration was 9 hours. The surface roughness of plasma nitrided sample was about 0.258  0.12 m. CNX films with a thickness of 2 m were deposited on both untreated and plasma nitrided Ti-6Al-4V plates by an unbalanced magnetron sputtering system under a base pressure of 510-5 Torr. A high purity (99.99%) graphite target was used in a pure (99.999%) nitrogen discharge at a gas pressure of 5 Pa and a constant gas flow rate of 40 sccm. The discharge current on the cathode was held at 1 A, the substrate temperature was below 200oC, and the negative substrate bias voltage was -300 V.
Fig. 1 (a) to (c) show the coefficient of friction data for untreated Ti-6Al-4V, plasma nitrided Ti-6Al-4V and duplex treated coatings under different normal loads sliding with 52100 steel balls. Fig. 2 shows the wear volumes of above specimens under different normal loads. Table 1 lists the wear volumes of steel balls under different normal loads.
For untreated Ti-6Al-4V, the long-term coefficient of friction remains a high value at about 0.5 to 0.6 and is almost independent upon the applied normal load (see Fig. 1(a)). The wear rates of Ti-6Al-4V remain a high value (see Fig. 2). The dominant wear mechanisms for untreated Ti-6Al-4V sliding with steel balls are adhesive and abrasive wear, delamination and transferring of materials from the counterfaces as shown in Fig. 3 [53]. EDX analysis can detect the transferred materials from the steel ball.
For plasma nitrided Ti-6Al-4V sliding with steel balls, the coefficient of friction is about 0.3 to 0.4 (see Fig. 1(b)). The lower value of coefficient of friction compared with untreated Ti-6Al-4V can be attributed to the surface hardening effect after plasma nitriding [54]. Examination on the worn nitrided layer reveals the large quantities of transfer materials from steel balls as shown in Fig. 4, probably due to the hard and rough nitrided surface. These transferred materials exist within the wear track and act as a third layer, affecting the friction and wear behavior of plasma nitrided layer and maintaining a constant values of coefficient of friction during the long-term sliding. Cross-section of wear scar reveals the transfer materials from the counterface. The wear of the steel balls sliding with plasma nitrided specimen is much higher than that of steel ball sliding with untreated Ti alloy (see Table 1). Fig. 5 shows the worn morphology of the steel balls indicating the abrasive wear.
The long-term coefficient of friction for the duplex treated coating sliding with steel balls is about 0.15 to 0.25 (see Fig. 1 (c)). Wear of the duplex treated coating sliding with steel ball is generally minimal, and the wear rate of CNX film deposited on plasma nitrided Ti-6Al-4V is quite low as shown in Fig. 2. Some discrete spallation can be observed on the wear track as shown in Fig. 6. EDX analysis on the wear track of duplex treated coatings reveals the transfer materials from steel balls. Detection on the steel ball using EDX analysis indicates the transfer of carbon nitride coating debris on steel balls, which can be used to explain the low coefficient of friction during sliding wear. The above results indicate the significant improvement in wear resistance and reduction of coefficient of friction with the application of duplex treatment on Ti-6Al-4V substrate. The wear of the steel ball can also be reduced significantly when sliding with the duplex treated coatings (see Fig. 2 (b)).
Fig. 7 and 8 show the comparison of coefficient of friction and wear rates for the above three types of specimens sliding with alumina balls under different normal loads. Table 1 lists the wear rates of alumina balls sliding with different specimens.
For untreated Ti-6Al-4V, the long-term coefficient of friction remains a high value of about 0.4 to 0.5 as shown in Fig. 7(a). Wear rate is slightly higher for untreated Ti-6Al-4V sliding with alumina balls compared with the specimen sliding with steel balls (see Fig. 8). Under dry sliding wear with alumina balls, the dominant wear mechanisms for untreated Ti-6Al-4V were adhesive wear, abrasive wear and delamination as shown in Fig. 9. Compared with the wear rate of the steel balls, the wear volumes of alumina balls are slightly higher (see Table 1). This phenomenon has been well explained by the different tribochemical reactions between these two counterfaces sliding with titanium alloy [55].
For plasma nitrided Ti-6Al-4V sliding with alumina balls, there is a critical load at which the wear mechanism is probably changed. Under a relatively low normal load of 5 N, both the coefficient of friction and wear rate remain low as shown in Fig. 7(b) and 8(a). With the normal load increased to 10 N and 20 N, the coefficient of friction increases abruptly to a high value of 0.4 to 0.5 and the wear rates also show high values (see Fig. 7(b) and 8). The dominant wear mechanism changes from mild wear to spallation (or crushing) of nitrided layer as shown in Fig. 10. The above results indicate that plasma nitriding is not effective in improving the friction property and wear resistance under high normal loads. The wear rates of alumina balls is much less than steel balls sliding with plasma nitrided Ti-6Al-4V probably due to its relatively high hardness (see Fig. 7(b)).
Fig. 7(c) shows the variation of coefficient of friction for duplex treated specimens under different normal loads sliding with Al2O3 ball. The long-term coefficient of friction for duplex treated coating remains low and stable (less than 0.2). The wear rates of duplex treated specimen (see Fig. 8) are much smaller than those of untreated Ti-6Al-4V and plasma nitrided samples sliding with alumina balls. Fig. 11 shows the surface morphology of wear track under a normal load of 10 N. There is only some small spallation of the films occurring on the wear track due to the fracture and fragmentation of the original coating asperities. Some of the coating fragments are entrapped within the wear track, comminuted into fine particles, and finally mixed up with the substrate materials to form a mechanically alloyed debris layer. Mechanical-chemical interactions between the sliding interfaces can lead to the micro-graphitization of this debris layer (see Fig. 12). Compared with that of the as-deposited CNX film, the D band (1335 cm-1) and G band (1595 cm-1) of Raman spectrum of worn coating surface becomes sharper as a result of the graphitization [56,57]. The graphitization of CNX films occurring in the contact zone has a self-lubricating effect and can be used to explain the good tribological behaviour of duplex treated system. Observation on the worn alumina ball reveals the transferring of lubricating CNX films debris (as shown in Fig. 13). The wear rates of alumina balls sliding with duplex treated specimen are quite low as listed in Table 1.
Fig. 14 and 15 show the comparison of coefficient of friction and wear volumes for the above three types of specimens sliding with UHMWPE pins under different normal loads. The wear volumes of UHMWPE pins sliding with different counterfaces are listed in Table 1.
When Ti-6Al-4V sliding with UHMWPE, due to the continuous rubbing against UHMWPE, the passive outer layer of the Ti-6Al-4V is broken. Hard titanium oxide particulates are produced that remain between the disc and the pin and start a severe abrasion process, resulting in a quick deterioration of both Ti-6Al-4V specimen and UHMWPE pins. Severe abrasion with deep scratches is present on the surface of UHMWPE when rubbing against the untreated Ti-6Al-4V alloy as shown in Fig. 16. Untreated Ti-6Al-4V also shows severe wear, with deep abrasive grooves plastic deformation and formation of titanium oxide debris (see Fig. 17).
For plasma nitrided samples sliding with UHMWPE pins, the coefficient of friction is lower than that of untreated titanium specimens (see Fig. 14 (b)). The wear rates of the UHMWPE are quite high, whereas those of plasma nitrided layer show much less wear (see Table 1 and Fig. 15). SEM observation on the worn surface of UHMWPE pins reveals the severe abrasion.
Duplex coating shows a coefficient of friction value between 0.1 to 0.15 against UHMWPE (see Fig. 14 (c)). The wear rates of both UHMWPE and duplex treated coatings are much less than those of UHMWPE pins sliding with untreated and plasma nitrided Ti-6Al-4V as shown in Table 1 and Fig. 15. There is only some mild abrasive wear on UHMWPE pin surface (see Fig. 18) and a uniform pattern of fine scratches are visible on duplex treated Ti-6Al-4V (see Fig. 19). There is no much evidence for the formation of extensive transfer film or wear debris on duplex treated coating surface. The good tribological performance of duplex treated coatings sliding with UHMWPE can be mainly attributed to the hardened, smooth and lubricating surface of CNX films on plasma nitrided surface. Further experimental work will be conducted in the friction and wear properties of duplex treated Ti-6Al-4V sliding against UHMWPE pins under the lubricated conditions with the simulated body fluids.
Erosion performance of duplex coating[58]
Aircraft, rockets and other aeronautical engines are often subjected to severe erosion situation from sands, rains or other solid particles in the space. Erosion by these solid particles or raindrop impingement can cause the rapid degradation of mechanical properties and even catastrophic failure. Examples of the failure components include the blades and discs of aircraft compressors, helicopter rotor blades, and valves, piping etc. [59]. Most of these components are made of titanium alloys, which are notorious for their poor erosion and wear resistance [60].
To design and use protective coatings or surface treatments that withstand solid particle erosion is thought as the best approach to solve this problem. For solid-particle erosion testing, a standard sandblasting apparatus was used to impact the target specimens (with a dimension of 10 mm  10 mm) with erodents at various velocities. The solid particles used in erosion tests were natural angular alumina sands with dimensions of 300-600 m. The impingement angle of the particles on the surface was 90  2o. The erodent was accelerated by compressed air with the impact velocities of the particles 100 m/s and 350 m/s. The nozzle-to-specimen distance was maintained at 50 cm. The erosion performance was measured by weighing specimens to an accuracy of  0.01 mg before and after exposure to the erodent for different test durations, so that the evolution of the mass loss with time could be determined.
Fig. 8 (a) and (b) show the eroded mass loss of the duplex treated system under different erosion conditions. The duplex treated coating can improve the erosion resistance of titanium substrate under a low impact velocity. It contains hard layers, being able to resist the particle flux at low velocity. However, under a high velocity, the improvement of erosion resistance is not so significant. Carbon nitride thin films exhibits a unique combination of properties: i.e., extreme hardness, chemical inertness and excellent tribological behavior, however, adhesion problems, high residual stress limit the thickness of the coating. For duplex treated coating, micro-cracks propagating through the interface between CNX film and plasma nitrided layers are observed. This crack deflection mechanism improves the impact resistance until the rupture of the coating happens. This mechanism is only effective under a low impact velocity of erodents, however, under a high impact velocity, the spallation of the CNX layer is more significant thus the erosion resistance is not good.
For carbon nitride coating deposited on plasma nitrided layer, when the erodent impacts on the coating surface, the cracks will form in the brittle carbon nitride coating. Then these cracks will grow parallel at the interface between carbon nitride coating and plasma nitrided layer. When cracks have developed thoughout the film, small parts are torn out by impinging particles as shown in Fig. 10 (a). The predominant features of the eroded surface were microchipping, some degree of plastic deformation from direct impacts and craters which resulted from the detachment of thin platelets [61] (see Fig. 10 (b) and (c)).
Corrosion properties of duplex treatment
TiN used as a hard coating is chemically inert, which in turn provides a good corrosion protection. When producing the TiN coatings on the substrate surface, some defects, such as pores or cracks may occur, which leads to a pitting and delamination of the surface [12, 13 and 14].
The corrosion properties of duplex-treated and nitrided Ti–6Al–4V have been investigated in 0.025, 0.25 and 2.5 M NaCl. The electrochemical results showed a three-fold increase in corrosion rate for every ten-fold increase in chloride concentration. [62].Electrochemical testing of a duplex treated Ti–6Al–4V alloy is considered [63]. The duplex treatment is carried out in two steps. In the first step, the alloy surface is laser nitrided while in the second step, the nitrided alloy surface is PVD TiN coated. Corrosion rates and electrochemical properties of the treated surfaces are investigated using polarization techniques. It is found that the duplex treated workpiece surface is more resistant to corrosion as compared to TiN coated and untreated surfaces. Among the three metals contained in the alloy, aluminum is found to selectively dissolve in large proportion in solution during oxidation.
Yilbas et al. [18] conducted a study on the corrosion properties of TiN coated and nitrided Ti–6Al–4V alloy. They showed that TiN coating improved the corrosion properties, but nitriding made worse the corrosion resistance of the substrate. A comparative study of the corrosion subjected to nitriding, coating and duplex treatment was carried out by Dingremont et al. [19]. They showed that duplex treatment improved the corrosion resistance of the construction and hot working steels when compared with nitriding or PVD coating.

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