稀有金属(英文版) 2017,36(08),651-658

Deposition of TiN/TiAlN multilayers by plasma-activated EB-PVD:tailored microstructure by jumping beam technology

Guo-Yuan Yang Hui Peng Hong-Bo Guo Sheng-Kai Gong

School of Materials Science and Engineering,Beihang University

Beijing Key Laboratory for Advanced Functional Material and Thin Film Technology,Beihang University

Key Laboratory of Aerospace Materials and Performance (Ministry of Education),Beihang University

收稿日期：12 November 2015

基金：financially supported by the National Natural Science Foundations of China (Nos.51201005 and 51231001);

Deposition of TiN/TiAlN multilayers by plasma-activated EB-PVD:tailored microstructure by jumping beam technology

Guo-Yuan Yang Hui Peng Hong-Bo Guo Sheng-Kai Gong

School of Materials Science and Engineering,Beihang University

Beijing Key Laboratory for Advanced Functional Material and Thin Film Technology,Beihang University

Key Laboratory of Aerospace Materials and Performance (Ministry of Education),Beihang University

Abstract：

Plasma-activated electron beam-physical vapor deposition(EB-PVD)was used for depositing nitride multilayer coatings in this work.Different from the conventional coating methods,the multilayers were obtained by manipulating electron beam(EB)to jump between two different evaporation sources alternately with variable frequencies(jumping beam technology).The plasma activation was generated by a hollow cathode plasma unit.The deposition process was demonstrated by means of tailoring TiN/TiAlN multilayers with different modulation periods(M1:26.5 nm,M2:80.0 nm,M3:6.0 nm,M4:4.0 nm).The microstructure and hardness of the multilayer coatings were comparatively studied with TiN and TiAlN singlelayer coatings.The columnar structure of the coatings(TiN,TiAlN,M1,M2)is replaced by a glassy-like microstructure when the modulation period decreases to less than 10 nm(M3,M4).Simultaneously,superlattice growth occurs.With the decrease of modulation period,both the hardness and the plastic deformation resistance(H³/E²,H-hardness and E-elastic modulus)increase.M4coating exhibits the maximum hardness of(49.6±2.7)GPa and the maximum plastic deformation resistance of⁰.74 GPa.

Keyword：

Nano-multilayer coatings; Superlattice; Plasma activation; TiN/TiAlN; EB-PVD; Hardness;

Author： Hui Peng,e-mail:penghui@buaa.edu.cn;

Received： 12 November 2015

1 Introduction

Physical vapor deposited (PVD) nitride coatings are commonly employed for cutting,anti-abrasion and anti-corrosion applications due to their excellent mechanical and chemical properties [ 1, 2, 3, 4] .In order to obtain better performance,nitride coatings with repeating alternate layers were developed to replace single-layer coatings in some cases.In particular,when the bilayer thickness is controlled under 10 nm,superlattice coatings can be obtained,depending on the crystal structure relationship of adjacent layers [ 5] .The superlattice coatings usually exhibit extremely high hardness,which is comparable to that of c-BN and is only second to diamond [ 6] .Up to now,various types of multilayer coatings,including superlattice coatings,e.g.,TiN/CrN,TiN/TiAlN,TiNVN,TiAlN/CrAlN and Cr/CrN,have been investigated from different aspects,such as deposition parameters,microstructure,hardening mechanism,thermal stability,wear and oxidation resistance [ 7, 8, 9, 10] .

Currently,arc ion plating (AIP) and reactive magnetron sputtering (MS) are the most commonly used methods for fabricating multilayer nitride coatings.The multilayer structure is usually generated by substrate rotation [ 11, 12, 13] or by alternate shutters [ 14, 15] .However,macroparticle contamination within AIP coatings which is inherent to the arc evaporation process can usually be detrimental to the quality of the coating [ 16] .In the case of MS coatings,the relatively low deposition rate and low ionization efficiency make it not a cost-effective way for industrial applications [ 17] .It is worth mentioning that new developments of AIP and MS are emerging,including filtered cathodic arc [ 18] ,pulse-enhanced electron emission (P3e^TM) [ 19] and highpower impulse magnetron sputter (HIPIMS) [ 20] ,aiming to improve both the coating rate and quality.

Electron beam-physical vapor deposition (EB-PVD) is a superior vacuum coating technology that can offer many desirable characteristics such as high deposition rate,free of macroparticle contamination and high thermal efficiency [ 21, 22] .Moreover,compound coatings can also be easily deposited by reactive deposition when being assisted with plasma activation [ 23] .In order to meet industrial requirements,several plasma activation processes,including spotless arc deposition (SAD) and hollow cathode-activated deposition (HAD),have been developed [ 24, 25] .The plasma densities produced by these ionization methods can be adapted to high rate electron beam (EB) evaporation.It has been demonstrated that SAD is particularly suitable for the deposition of a group of refractory metals,e.g.,Ti,Zr,W and Mo,but not for the application of other metals and alloys with low melting point.An expansion of SAD to some alloy materials can be realized by increasing the temperature and thermal electron emissivity of the molten pool by introducing Nb molten pool [ 26] .In the case of HAD,it can be well suited for most materials.One embodiment is the deposition of Al₂O₃ layer onto polymer substrates for packaging application [ 27] .

In this work,plasma-activated EB-PVD (HAD process)was used to produce nitride multilayer coatings.Different from the formation mechanisms in conventional deposition methods (MS and AIP),the multilayer structure was yielded by manipulating an EB to jump between different evaporation sources alternately,namely the jumping beamtechnology [ 28, 29] .By controlling the jumping frequency and dwelling time on each source,the modulation period and modulation ratio can be adjusted.In order to demonstrate the capability of the deposition process,four types of TiN/TiAlN multilayer coatings with different modulation periods were deposited.The microstructure and hardness of the coatings were investigated.

2 Experimental

2.1 Materials

High speed steel (HSS) was used as substrates,which were machined to rectangular-shaped specimens (20 mm×10 mm×3 mm).Prior to coating deposition,all the substrate surfaces were mirror polished and then were ultrasonically cleaned in acetone medium for 20 min and thoroughly dried.Pure Ti (99.9%) and TiAl (atomic ratio of 50:50) ingots produced by arc melting were used as targets for EB evaporation.

2.2 Coating preparation

Figure 1 shows schematic illustration of plasma-activated EB-PVD process.A vacuum system equipped with two bowl-shaped crucibles was used for experiments.One crucible served for evaporation of pure Ti,and the other one served for evaporation of TiAl.The diameter of each crucible was 55 mm.The plasma activation was gained by means of a hollow cathode plasma unit (HAD process),which is capable of generating a low-voltage,high-current EB.For optimal ionization efficiency,the two crucibles were closely arranged along the moving path of the hollow cathode EB.More detailed description of HAD process can be found in Ref. [ 24] ,as reported by which a high plasma density in the order of 1×10¹² cm^-3 can be expected.A pierce EB gun (accelerating voltage up to 17 kV,beam current up to 1.8 A) was utilized to evaporate the targets in the two separate crucibles by jumping beam technology.The jumping and scanning of EB were controlled by a computer system.As reported previously,multilayer coatings with different modulation periods can be produced by this technology.However,when the jumping frequency is higher than 10 Hz,a homogeneous mixing structure rather than multilayer structure will be formed [ 29] .

For coating deposition,the base pressure in vacuum chamber was about 1×10^-3 Pa.The substrates were mounted on the sample holder 300 mm above the evaporation source,with rotation speed adjustable.The substrate temperature was heated up to about 400℃by radiation.Prior to coating,a hollow cathode EB of 180 A was ignited with the argon feeding rate of 200 ml·min^-1.The chamber pressure was maintained at about 5 Pa with the substrates biased at-800 V for argon ion etching for 20 min.Then the argon feeding rate was decreased to 20 ml·min^-1 to get a moderate pressure of 0.1 Pa to allow the hollow cathode and pierce gun to work together.Ti plasma was generated once the Ti target was evaporated.The substrates were then bombarded with Ti plasma for 10 min with the same bias voltage.Following Ti ion cleaning,the bias was decreased to-100 V for depositing a thin layer of pure Ti as bond coat.For synthesis of TiN/TiAlN periodic structure,Ti and TiAl targets were evaporated alternatively by jumping beam technology.At the same time,nitrogen with a flow rate of 150 ml-min^-1 was admitted into the space between the crucibles and substrates,maintaining the vacuum pressure at about 0.2 Pa.The EB power for the targets evaporation was 6 kW,and the deposition was finished within 60 min.

Fig.1 Schematic illustration of deposition system

Because plasma-activated EB-PVD is a nearly line-ofsight process,the modulation period of the multilayer coatings can be determined by both EB jumping parameters and substrate rotation speed.It can be simply derived as follows:

1.When EB jumping period (T) is integer time ofsubstrate rotation period (τ),T=ητ(n=2,3,4...),

where∧'is the nominal modulation period of the bilayers,d is the total thickness of the multilayer coating and t is the total deposition time.

2.Whenτ=nT (n=2,3,4...),

3.When T≠nT (n=2,3,4...),τ≠nT (n=2,3,4...),but T andτare close in value,a great cycle of the layered structure will be formed.The great cycle period (T) is the least common multiple of T andτ.A'can be calculated by following relationship:

Besides,interlayers exist in each modulation period,as discussed in Sect.3.2.

4.When T≠nT (n=2,3,4...),T≠nT (n=2,3,4...),and T》T,∧'can be calculated by Eq.(1).

5.When T≠nT (n=2,3,4...),T≠nT (n=2,3,4...),and T<<T,∧'can be calculated by Eq.(2).

According to above derived results,the microstructure of multilayer coatings can therefore be tailored.Four types of TiN/TiAlN multilayer coatings (denoted as Ml,M2,M3and M4) with different modulation periods were demonstrated in this work.Deposition parameters for the coatings are shown in Table 1.Single-layer TiN and TiAlN coatings were also produced by using the same parameters for comparative study.

2.3 Microstructure and hardness measurement

The phases of the coatings were investigated by X-ray diffractometer (XRD,Bruker D8 Advance) using Cu Kαradiation,which was operated at 40 kV and 40 mA.The scanning angle (2θ) was ranged from 20°to 90°with a scanning speed of 4 (°)·min^-1.Grazing incidence X-ray diffraction (GIXRD,in-plane mode) was also used for the measurement of thin coatings,with a fixed incidence angle of 1°.The microstructure of the coatings was characterized by scanning electron microscope (SEM,Zeiss Merlin) and transmission electron microscope (TEM,JEM-21 00F).Samples with preformed slits were cooled rapidly in liquid nitrogen and then were fractured to get brittle fractures.Samples for TEM observations were prepared by focused ion beam system (FIB,FEI Helios Nanolab 600).The coating composition was analyzed by electron probe micro-analyzer (EPMA,JEOL JXA-8100),with the measurements done on polished cross sections at different positions all over the sample to acquire an average composition.The roughness of the coatings was measured by means of atomic force microscope (AFM,TMX 2000Discoverer) in contact and constant force modes.

Nanoindentation tests (Nanoindenter XP equipped with Berkovich diamond tip,MTS Systems Corp.) were conducted to measure the hardness and elastic modulus of the coatings by continuous stiffness measurement (CSM)method.During test,the indenter was controlled to penetrate 10%-15%of the coating thickness to avoid substrate effects.Five measurements were taken at randomly selected points on each sample and subsequently averaged in order to get accurate results.

3 Results and discussion

3.1 Chemical and XRD analysis

Elemental analysis by EPMA reveals that nearly stoichiometric single-layer TiN and Ti_0.45 Al_0.55N coatings were prepared.The variation of Ti/Al atomic ratio in TiAlN coating compared with that of TiAl target is ascribed to the different degrees of ionization and deposition of Ti and Al vapor emitted from crucible.It can also be concluded that the nitrogen partial pressure during coating is high enough for the growth of stoichiometric nitride coatings.The measured composition for single-layer coatings provides a reference on the composition of TiN/TiAlN multilayers.

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Table 1 Details of plasma-activated EB-PVD parameters

Fig.2 XRD patterns of TiN,TiAlN,Ml,M2,M3,M4 coatings and sub strate

Figure 2 shows XRD patterns of substrate and TiN,TiAlN,M1,M2 coatings and GIXRD patterns of thinner coatings,M3 and M4.It can be seen in Fig.2 that all the coatings present a cubic structure (PDF No.38-1420).The texture coefficient (T_c,Table 2) is calculated for (111) and(200) diffraction planes from XRD data using the following well-known formula [ 30] :

where I_(hkl) is the measured intensity of reflection from a given (hkl) plane,I_o(hkl) is the relative intensity of the reflection from the same plane as indicated in a standard sample (PDF No.38-1420),and n is the total number of reflections observed,which is 4 ((111),(200),(220),(222)reflections) in the present investigation.The highest value of T_c is 4 for a perfectly oriented coating,and its value is 1for a randomly oriented one.For TiN,TiAlN single-layer coatings and Ml,M2 coatings,the texture coefficients of(200) plane are between 2.4 and 2.9,implying preferential growth of (200) plane.The (200) plane has been reported to possess the lowest surface energy in Bl-NaCl type micro structure [ 31] .Since the diffraction planes of GIXRD performed with in-plane configuration are nearly perpendicular to sample surface [ 32] ,the texture coefficients of M3 and M4 coatings provide information of crystalline orientation parallel to the sample surface.For M3 and M4coatings,the coefficient is close to unity for both (111) and(200) planes,indicating randomly orientation parallel to sample surface.

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Table 2 Texture coefficient (T_c) of (111) and (200) diffraction planes of coatings

XRD peaks of TiAlN and multilayer coatings shift toward high angle side remarkably compared with that of TiN.This is because when partial Ti atoms of TiN lattice are replaced by A1 atoms,the lattice constant and the internal stress are changed.The shifting degree increases with the increase of jumping frequency (from Ml to M4).This might result from compressive stresses caused by multilayer effect when the modulation period is reduced,as reported in Ref. [ 33, 34] .Further investigations are still needed to determine the stress state.Another point should be mentioned here is that the peaks of TiN and TiAlN cannot be separated in XRD patterns.One possible reason for this is that the layered structure results in the broadening of the peaks.It is therefore hard to distinguish XRD peaks of TiN and TiAlN with the same crystal structure and similar lattice parameters.

3.2 Microstructural analysis

Figure 3 shows typical surface morphology of as-deposited coating (M4 as an example) prepared by plasma-activated EB-PVD.The coating exhibits a smooth and compact surface without macroparticles,voids or cracks.This is mainly attributed to the HAD process which allows for the deposition of dense and smooth layers at high rate.It is widely accepted that the above-mentioned macro-defects are extremely detrimental to the coating quality by reducing wear and anti-corrosion properties [ 35] .Minimizing coating defects is therefore an effective way to improve the performance of coatings.In this work,the average roughness of the coating measured by AFM is only about 5 nm,much lower than that of AIP coating and comparable with that MS coating [ 36, 37, 38] .

Fig.3 SEM image of surface microstructure of M4 coating

The fractured sections of the coatings are shown in Fig.4.All the coatings adhere well with the substrates without any visible delaminating or cracks.From Fig.4ad,it can be observed that TiN,TiAlN,M1 and M2 coatings exhibit typical columnar microstructure,with compact crystallites grown perpendicular to substrate surface.As forM3 and M4 coatings with higher jumping frequency,the coatings tend to possess glassy-like microstructure,as shown in Fig.4e,f.The microstructural changes lead to the variation of mechanical properties,which will be discussed in the following section.

The thickness of the coatings measured from SEMimages and corresponding deposition rate are listed in Table 3.For the multilayer coatings,the modulation period was also calculated according to the equations in Sect.2.2,as also seen in Table 3.The deposition rate for TiN coating is about 94 nm·min^-1.Actually,HAD process is capable of providing a coating rate up to hundreds of nm·s^- [ 24] .The deposition rates of nano-multilayer coatings are lower than those of single-layer coatings.That is because when EB dwells at one target,the molten pool formed on the other target will solidify within a few seconds after the EB moves away.Once solidified,the target needs to be reheated for some time and then melts again.This causes a decreased deposition rate accordingly.From the images at higher magnification (Fig.5),the layered structure with relatively sharp interfaces can be clearly recognized.The bilayer thickness is measured to be 26.5,80.0,6.0 and4.0 nm for coatings from Ml to M4,respectively.According to Sect.2.2,M1,M2,M3 and M4 coatings meet Eq.(4),(3),(1) and (2),respectively.The measured values tally with the calculated results well (Table 3).In addition,a careful observation of Fig.5b shows that several small interlayers exist in the periodic structure of M2 coating (as mentioned in Sect.2.2).This complex microstructure is resulted from the synergistic effect of EB jumping and substrate rotation,which may provide a route for microstructural design.However,further investigations are still needed to reveal the relationship between microstructure and mechanical properties.

Fig.4 SEM images of fractured sections of coatings:a TiN,b TiAlN,c Ml,d M2,e M3 and f M4

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Table 3 Thickness,modulation period and deposition rate o coatings

Considering the bilayer thickness for M3 and M4 coatings is in the range of less than 10 nm,the formation of superlattice structures might occur.Figure 6 shows the high-resolution TEM (HRTEM) image of M4 coating.As the three grains (marked as A,B,C) with different orientations shown in Fig.6,well-defined poly crystalline grains with the size of 5-10 nm can be observed.The lattice fringes continuously go across adjacent TiN and TiAlN layers through their interface (Area D in Fig.6),suggesting the coherent growth between them.Meanwhile,lattice distortion and mismatch exist in some regions (marked by E in Fig.6).This superlattice with distortion results in internal stress which can strongly influence the mechanical properties of the coatings.

Fig.6 HRTEM image of M4 coating

Selected area electron diffraction (SAED) was performed for M4 coating as well (Fig.7).This sample exhibits clear polycrystalline rings in SAED pattern,which is of Bl-NaCl crystal structure.In addition,diffraction rings of some crystal planes can be distinguished in pairs(Areas A and B in Fig.7),corresponding to TiN and TiAlN phases.

Fig.5 High magnified images of multilayer coatings:a M1 (SEM),b M2 (SEM),c M3 (TEM),and d M4 (TEM)

Fig.7 SAED pattern of M4 coating

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Table 4 Nanoindentation results of coatings

3.3 Nanoindentation tests

Results of nanoindentation tests are listed in Table 4.The measured hardness values for TiN and TiAlN coatings are(27.3±1.2) and (32.2±1.2) GPa,respectively,which are comparable with that of the same type coatings produced by other popular methods.The highest hardness of(49.6±2.7) GPa is observed for M4 coating with the lowest bilayer thickness.The high hardness is only one parameter that ensures scratch and abrasion resistant.Protective overcoat must be highly resistant also to plastic deformation during contact events.It has been widely accepted that the ratio H³/E²(H-hardness and E-elastic modulus) is a good measure of the resistance to plastic deformation [ 5, 39] .A graphical comparison of the hardness and plastic deformation resistance (H³/E²) of each coating plotted versus bilayer thickness is illustrated in Fig.8.The plastic deformation resistance increases with bilayer thickness decreasing.Similar tendency is also observed for the hardness,except for the slight fluctuation for Ml and M2 coatings.One thing should be noticed is that the hardness and plastic deformation resistance increase dramatically when the bilayer is thinner than10 nm.The plastic deformation resistance value for M4 is as high as～0.74 GPa,which is more than 7 times that of TiN single-layer coating and 3.5 times that of TiAlN single-layer coating.By comparing the morphologies andmechanical properties of single layer with those of multilayer coatings in this work,it is obvious that the structural growth of multilayer plays an important role.Especially in regime where the bilayer thickness is of several nanometers,the grains grow finer and the number of grain boundary will increase,thus restricting the dislocation and grain boundary sliding.

Fig.8 Hardness and plastic deformation resistance (H³/E²) of coatings

4 Conclusion

Plasma-activated EB-PVD (HAD process) was used for depositing TiN/TiAlN multilayer coatings.It is verified in this work that TiN/TiAlN multilayer coatings with different modulation periods could be produced by jumping beam technology.The modulation period and the micro structure of the coatings are strongly affected by the processing parameters of EB jumping and substrate rotation.With the decrease of modulation period,the columnar growth of multilayer coatings is replaced by a glassy-like morphology,which reveals superlattice micros true ture.The superlattice coatings exhibit superior mechanical properties compared with contrast coatings.For the coating with the lowest modulation period (4 nm in this work),hardness value of (49.6±2.7) GPa and plastic deformation resistance (H³/E²) of～0.74 GPa are obtained.

Acknowledgements

This study was financially supported by the National Natural Science Foundations of China (Nos.51201005 and51231001).

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