Characterization and Tribological Performance of Titanium Nitrides in Situ Grown on Ti6Al4V Alloy by Glow Discharge Plasma Nitriding

KONG Weicheng, YU Zhou, HU Jun

(College of Mechanical Engineering, Donghua University, Shanghai 201620, China)

Abstract: Titanium (Ti) nitrides were in situ grown on Ti6Al4V alloy (TA) using a glow discharge plasma nitriding (GDPN). The morphology, chemical composition, phase and mechanical property of the obtained nitrided TA were analyzed using a scanning electron microscope (SEM), energy dispersive spectroscope (EDS), X-ray diffraction (XRD), and nanoindentation tester, respectively. The tribological performances of un-nitrided and nitrided TAs were evaluated using a ball-on-plate wear tester, and the wear mechanism was also discussed in detail. The results show that the nitrided layer with the compound and diffusion layers is formed on the nitrided TA, which is composed of δ-TiN and α-Ti phases. The nanohardness and elastic modulus of nitrided TA are 6.05 and 143.13 GPa, respectively, higher than those of un-nitrided TA.The friction reduction and anti-wear performances of nitrided TA are better than those of un-nitrided TA, and the wear mechanism is primary abrasive wear, accompanying with adhesive wear, which is attributed to the formation of Ti nitrides with the high nanohardness and elastic modulus.

Key words: glow discharge plasma nitriding (GDPN); Ti6Al4V alloy (TA); coefficient of friction (COF);wear mechanism

1 Introduction

Ti6Al4V alloy (TA) has been widely used in aerospace field owing to its low density, high strength and corrosion resistance[1], however, poor wear resistance prevents it from further applications.Researchers try to overcome the fatal weakness, and pay attentions towards the enhancements of hardness and wear resistance to prolong its service life[2,3].Although many physical and chemical methods are used to improve the wear performance of TA by depositing hard coatings, their applications in the surface field are still limited due to low bonding strength[4]. As an advanced surface modification technique, glow discharge plasma nitriding (GDPN)canin situgrow a layer of titanium (Ti) nitrides on TA[5], which hardens its surface and subsurface by introducing active nitrogen[6]. Previous studies also show that GDPN is recognized as one of the most promising technologies due to its green raw materials and process methods[2].

GDPN is a diffusion process to form a hard layer of TiN and Ti2N[7], and a lot of works have been reported that the nitriding processing is a very effective way for the wear resistance of TA[8,9]. The investigations on the plasma nitriding have confirmed that the surface color of TA is changed from the metallic gray to shiny golden, which is a formation proof of Ti nitrides[10],and the mechanical and tribological performance are improved due to the ceramic phases ofδ-TiN andε-Ti2N on TA[11,12].

There are many researches on the plasma nitriding of TA. Szymkiewiczet al[10]analyzed the effects of active screen plasma nitrided TA and Ti6Al7Nb alloy on the phases and chemical compositions of compounds and other layers, and found that the switching sample bias from the cathode to the floating potential resulted in covering it with the compact TiN layer backed by the α′′-Ti(N) marten-site and Ti3Al layers. Batoryet al[13]processed the TA by plasma nitriding with lowtemperature radio frequency, and revealed that the water droplet erosion resistance of nitrided TA significantly improved compared with the un-nitrided TA. Samantaet al[14]carried out a plasma nitriding on TA in a hot wall vacuum chamber, and confirmed that the nanohardness,Young’s modulus and wear resistance of nitrided TA were much higher than the un-nitrided TA due to the formation of nitrided metallic alloy layer. Liet al[15]performed a vacuum electromagnetic induction nitriding on TA, showing that the dense TiN layer was formed on the nitrided TA surface, and its corrosion resistance in F-corrosion solution was increased owing to the presence of TiN phase. And Shenet al[16]used ammonia as working gas to perform the plasma nitriding test, but the decomposition of ammonia caused the problems of environmental pollution. Although Dasireddyet al[17]reduced the decomposition temperature of ammonia from 800 to 550 ℃, the mixed gases of ammonia and hydrogen in the reaction furnace was a kind of waste gas to pollute the environment[18], which needed to be treated with a lot of subsequent costs. In view of current ammonia pollution problems[19], the implementations of sustainable clean production techniques[20]have become a hotspot in the plasma nitriding process, which may solve the above pollution problems. There is only limited literature on the plasma nitriding with clean nitrogen, especially the effects of plasma nitriding on the structural, mechanical and tribological performance of TA have rarely been reported[21], which hinders the development of GDPN in the surface modifications of TA.

In this work, clean nitrogen was used as working gas to nitride TA by glow discharge. The aim was to investigate the morphology, nanomechanical property and tribological performance of nitrided TA, and the wear mechanism was also discussed in detail, which provided a clean production method of plasma nitriding for the surface modification treatment of TA.

2 Experimental

2.1 Nitriding treatment

The substrate was commercial TA with the chemical composition (wt%) of Al 5.7, V 4.1, Fe 0.05, C 0.08, O 0.13, N 0.03 and balanced Ti, which followed by grinding in turn with SiC emery paper of 800-2000 mesh and polishing with diamond spray, then cleaning in acetone to remove oil and impurities, and drying for the nitriding treatment. The plasma nitriding test was conducted in an LnMC-150AQK type glow discharge plasma nitriding (GDPN) furnace, in which the nitrogen and hydrogen were used as working gas and catalysis, respectively, as shown in Fig.1. Process parameters: pressure of 300 Pa; frequency conversion of 20 Hz; working voltage of 780 V; substrate bias of-250 V; duty ratio of 88%; nitrogen and hydrogen ratio of 1:2; elevating temperature rate of 24 ℃/10 min; temperature of 950 ℃; and holding time of 12 h.When the nitriding process was completed, the nitrided TA was cooled down to room temperature.

2.2 Characterization methods

The morphologies and chemical composition of nitrided TA were analyzed using a JSM-6360LA type scanning electron microscope (SEM) and its configured energy dispersive spectroscope (EDS), respectively.The roughness was measured using a CSPM5500 type atomic force microscope (AFM) with the Imager-4.60 software, and the phases was analyzed using a D/max-2500PC type X-ray diffraction (XRD). The valence states of chemical elements for the nitrided TA were analyzed using a Thermo ESCALAB 250 type X-ray photoelectron spectroscope (XPS), and test parameters:X-ray excitation source of monochromatic with the Al target and Ka radiation (HV=1 486.6 eV); power of 150 W; spot size of 500 μm; and pass energy of 30 eV. The nanohardness and elastic modulus were measured on a U9820A type nanoindenter, and test parameters: load of 50-85 mN; displacement of 700 nm; and indenter tip radius of 60 nm. The nanoindentation tests were repeated for three times, and then took the average values as the experimental results.

2.3 Friction and wear test

The wear test was performed on a CFT-I type friction tester with the sliding method, in which the COF was online recorded by a computer. Test parameters: tribo-pair of Si3N4ball with the diameter of 4 mm; normal load of 2 N; reciprocating speed of 500 times/min; reciprocating length of 4 mm; and operation time of 30 min. Under the same conditions,the friction-wear test was repeated three times for each sample, and the average COFs in the three-time values were presented as the experimental COFs. After the wear tests were ended, the profiles of worn tracks were analyzed using a VHX-700FC type super depth of field microscope (SDFM), and the morphologies and image mappings of worn tracks were analyzed using a SEM and EDS, respectively.

In this case, the wear rate was[22]

whereVwwas the wear volume;Fwas the normal load;andDwas the total length of worn track.

The wear volume in Eq.(1) was

wherenwas the reciprocating speed(times•min-1);Lwas the length for a single reciprocating(mm); andSwas the profile area of worn track(mm2), which was measured by SDFM.

3 Results and discussion

3.1 Microstructure of surface and crosssection

The metallic grey color of TA was turned into golden after the plasma nitriding process[23], and the TiN particles werein situgrown on the nitrided TA,suggesting that the compounds of Ti nitrides were formed[5], as shown in Fig.2(a). The EDS analysis of nitrided TA is shown in Fig.2(b). The Ti, Al and V were detected on the nitrided TA surface, which were the constituent elements of TA; while the N was the action result of plasma nitriding.

Line scan analysis was used to understand the distributions of chemical elements on the cross-section of nitrided TA. The Ti and N contents were enriched on the near surface zones of nitrided TA, which decreased with the increase of nitriding depth, as shown in Fig.3(a) and 3(b); while the Al and V contents were very low on the near surface zones of nitrided TA,which increased with the increase of nitriding depth,and tended to be stable, as shown in Fig.3(c) and 3(d).It was indicated that the Ti content on the nitrided TA increased than the TA, and the Al and V contents were the opposite. This was because the Ti was migrated to the surface of nitrided TA, while the Al and V were migrated into the nitrided TA[24]. The nitrided layer contained a compound layer with the high N content and a diffusion layer with the low N content, and theβ-Ti phase in the diffusion layer was obviously reduced due to the solid solution of N[11], which was conducive to improve the bearing capability of TA.

3.2 XRD and XPS analysis

XRD was used to analyze the phases of Ti nitrides on the nitrided TA, and the XRD patterns of un-nitrided and nitrided TAs are shown in Fig.4(a). The intensity of hexagonal close packed (HCP)α-Ti peak was much stronger than that of body-centered cubic (BCC)β-Ti peak, indicating that the un-nitrided TA contained a large amount of α-Ti and fewβ-Ti phases[25]. The formed nitrided layer on the nitrided TA contained the compact HCPα-Ti and face centered cubic (FCC)δ-TiN as the main phases[14], which depended on the nitriding temperature and time[11]. Besides, theα-Ti andβ-Ti peaks of un-nitrided TA with the low intensities were observed on the nitrided TA, which was because the X-ray penetrated the nitrided layer to detect the peaks of TA. As a result, theδ-TiN phase was the main crystalline phase on the nitrided TA. The crystal transformations of Ti lattice before and after the plasma nitriding test are shown in Fig.4 (b). The plasma nitriding process was divided into preheating,tempering and nitriding stages, especially the HCP α-Ti in the tempering stage was partially transformed to the BCCβ-Ti[26], which was beneficial to forming the FCCδ-TiN lattice.

Table 1 Crystal transformation data of Ti in plasma nitriding process

The crystal transformation data of Ti lattice in Fig.4 (b) are listed in Table 1. The lattice constant ofβ-Ti lattice was larger than that of α-Ti lattice, and the hardness of nitrided TA was improved by the formed TiN lattice.

XPS was used to further analyze the composition of chemical bonds for the nitrided TA. The Ti2p peak was decomposed into TiN (Ti2p3/2) bond at the binding energy (BE) of 455.2 eV, TiO2(Ti2p3/2) bond at the BE of 458.6 eV[27], TiN (Ti2p1/2) bond at the BE of 461.0 eV and TiO2(Ti2p1/2) at the BE of 464.5 eV, as shown in Fig.5 (a). The N1s (N-O) peak was divided into TiN bond at the BE of 397.3 eV and N1s bond at the BE of 399.5 eV, as shown in Fig.5 (b). It was concluded that the Ti compounds was formed on the nitrided TA, which consisted with the results of XRD in Fig.4 (a).

3.3 Mechanical property

Nanoindentation was useful in assessing the mechanical property of nitrided TA at the micro- and nano-scales[28,29], and the typical loading/un-loading curves of un-nitrided and nitrided TAs are shown in Fig.6(a). The stabilities of loading and un-loading periods for the nitrided TA were better than those of un-nitrided TA, which was related to the formation of Ti nitrides. The nanohardness and elastic modulus of nitrided TA were 6.05 and 143.13 GPa, respectively,increasing by 69.94% and 19.01% than the unnitrided TA, respectively, as shown in Fig.6(b) and 6(c). Combined with the XRD results, theδ-TiN phase improved the mechanical property of nitrided TA,which was beneficial to enhancing its wear resistance capability.

3.4 Friction-wear performance

3.4.1 Coefficients of friction and wear rates

Wall-on-plate sliding wear test was used to evaluate the COFs of un-nitrided and nitrided TAs.Fig.7(a) shows the curves of COFsvsoperation time. After the running-in period, the COFs of unnitrided and nitrided TAs increased with the increase of operation time. The average COFs of un-nitrided and nitrided TAs were 0.647 and 0.516, respectively,showing that the average COF of nitrided TA decreased by 20.25% than the un-nitrided TA, and the friction reduction of nitride TA was better than the un-nitrided TA.

The profiles of worn tracks on the un-nitrided and nitrided TAs were measured by SDFM, which was used to calculate their wear rates in Eq.(1), as shown in Fig.7(b). The maximal depths of un-nitrided and nitrided TAs were 16.8 and 2.3 μm, respectively, and the corresponding wear rates were 104.489 and 7.598 μm3•N-1•mm-1, respectively. It was indicated that the wear rate of nitrided TA decreased by 92.73% than the un-nitrided TA, showing that the nitrided TA exhibited the higher wear resistance than the un-nitrided TA.

3.4.2 Worn images of tribo-pairs

SDFM was used to observe the wear degree of tribo-pairs, which was helpful to analyze the wear mechanism of nitrided TA. Fig.8(a) shows the optical microscopic image of tribo-pairs against the un-nitrided TA. The debris of un-nitrided TA was attached on the dark regions of worn track, and the obvious adhesive wear occurred in the friction process, which was attributed to the material transfer to the worn track of tribo-pair. There was trace debris from the nitrided TA stuck on the tribo-pair surface, and the parallel scratch marks were observed on its worn track, indicating that the abrasive wear occurred between the nitrided TA and the tribo-pair, accompanied with adhesive wear, as shown in Fig.8(b). The wear areas of tribopairs against the un-nitrided and nitrided TAs were 6.23×102and 3.96×102mm2, respectively. The wear degree of tribo-pair against the un-nitrided TA was more severe than that of nitrided TA, which was the combined action result of abrasive wear and adhesive wear due to its low hardness. After the plasma nitriding process, the hardness of nitrided TA demonstrated great improvements to decrease the wear degree of tribopair, and the wear mechanism was changed from the adhesive wear to abrasive wear, which enhanced the wear resistance of TA.

3.4.3 Morphologies of worn tracks

SEM was used to analyze the morphologies of worn tracks on the un-nitrided and nitrided TAs.Fig.9(a) shows the overall morphology of worn track on the un-nitrided TA. The un-nitrided TA surface was easily deformed due to its low hardness; the reciprocating motion yielded the debris from the unnitrided TA, and stuck on the tribo-pair surface, which formed new adhesive wear between the un-nitrided TA and the tribo-pair. The fractured debris was embedded to the worn track to form some furrows, and the marks of abrasive wear also appeared on the worn track.Therefore, the wear mechanism was primary adhesive wear, accompanied with abrasive wear in the friction process. Fig.9(b) shows the marks of adhesive wear.The worn track underwent high plastic deformation and adhesive wear due to its low hardness[30], and the deformed and smeared wear debris was adhered to the worn track, and the plate-like wear debris indicated adhesive wear. This was because the high ductility and chemical activity of un-nitrided TA led to adhesion wear, which resulted in abrasive wear on the worn track[31]. Meanwhile, the wear mechanism was also accompanied with abrasive wear. Fig.9(c) shows the marks of abrasive wear. The severe adhesive wear occurred on the worn track, and the deep grooves were also found on the rough worn track, suggesting that the friction process primarily undergone adhesive wear,accompanied with abrasive wear.

Fig.10(a) shows the overall morphology of worn track on the nitrided TA. The relatively smooth and shallow worn tracks were found on the worn track, showing its wear resistance was higher than the un-nitrided TA. The marks of abrasive wear and adhesive wear were observed on the worn track, in which the material removal was caused by adhesive wear. Fig.10(b) shows the marks of adhesive wear.The broken debris was observed on the worn track of nitrided TA, but the adhesive wear degree was less than the un-nitrided TA, which was attributed to its increase of hardness. Fig.10(c) shows the marks of abrasion wear. The smooth shallow scars appeared on the worn track; the grooves were observed due to hard debris from the compound layer. The Ti nitrides with the high hardness reduced the amount of plastic deformation,which increased its adhesive wear resistance. The wear mechanism was primary abrasive wear, companied with adhesive wear. The formation of Ti nitrides was the main factor of enhancing wear resistance, which caused the adhesive wear change to abrasive wear.As a result, the nitrided TA presented an outstanding wear resistance, which was contributed to its high nanohardness and elastic modulus.

3.4.4 Image mapping analysis of worn tracks

Image mapping was used to further understand the wear mechanisms of un-nitrided and nitrided TAs. The image mapped position of worn track on the un-nitrided TA is shown in Fig.11(a). The chemical composition of worn track was composed of Ti, Al and V, accompanied with a small amount of Si, as shown in Fig.11(b). The Ti, Al and V were the constituent elements of TA, and there were no atoms-poor zones, as shown in Fig.11(c)-(e). The Si originated from the debris from the tribopair stuck on the worn track, suggesting that the adhesive wear occurred due to the low hardness of unnitrided TA, as shown in Fig.11(f).

The image mapped position of worn track on the nitrided TA is shown in Fig.12(a). The chemical composition of worn track was composed of Ti, Al,V, N and Si, as shown in Fig.12(b). The Ti, V and N were uniformly distributed on the worn track; while the Al formed the atom-rich zone, which originated from the TA, as shown in Fig.12(c)-(f). The Si did not form the atom-rich zone due to its low content, indicating that a certain amount of adhesive wear occurred in the friction process, as shown in Fig.12(g).

4 Conclusions

a) The nitrided TA contains the high N compound layer and the low N diffusion layer, and its phases are composed ofδ-TiN andα-Ti peaks, which enhances the bearing capability of TA.

b) The nanohardness and elastic modulus of nitrided TA are 6.05 and 143.13 GPa, respectively,increasing by 69.94 and 19.01% than the un-nitrided TA, respectively. Theδ-TiN phase improves the nanomechanical property of nitrided TA, which is beneficial to enhancing its wear resistance.

c) The average COF of nitrided TA under the normal load of 2 N is 0.516, decreasing by 20.24%than the un-nitrided TA, showing the excellent friction reduction.

d) The nitrided TA exhibits the excellent anti-wear performance, and its wear rate is 7.598 μm3•N-1•mm-1,decreasing by 92.73% than the un-nitrided TA.The wear mechanism is primary abrasive wear,accompanying with adhesive wear, which is attributed to the formation of Ti nitrides with high nanohardness.