Effects of nitriding temperature on microstructures and ...
Transcript of Effects of nitriding temperature on microstructures and ...
�������� ����� ��
Effects of nitriding temperature on microstructures and vacuum tribologicalproperties of plasma-nitrided titanium
Dingshun She, Wen Yue, Zhiqiang Fu, Chengbiao Wang, XingkuanYang, Jiajun Liu
PII: S0257-8972(15)00063-8DOI: doi: 10.1016/j.surfcoat.2015.01.029Reference: SCT 20044
To appear in: Surface & Coatings Technology
Received date: 12 May 2014Accepted date: 12 January 2015
Please cite this article as: Dingshun She, Wen Yue, Zhiqiang Fu, Chengbiao Wang,Xingkuan Yang, Jiajun Liu, Effects of nitriding temperature on microstructures andvacuum tribological properties of plasma-nitrided titanium, Surface & Coatings Technology(2015), doi: 10.1016/j.surfcoat.2015.01.029
This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Type of contribution: Research paper, Full length article
Date of preparation: May 10, 2014
Number of text pages: 23
Number of tables & figures: 2 tables and 15 figures
Title: Effects of nitriding temperature on microstructures and vacuum tribological properties
of plasma-nitrided titanium
Authors: Dingshun She 1, 2
, Wen Yue 1, 2*
, Zhiqiang Fu 1
, Chengbiao Wang 1, Xingkuan Yang
3,
Jiajun Liu 4
1. School of Engineering and Technology, China University of Geosciences (Beijing), Beijing
100083, PR China;
2. Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources,
China University of Geosciences (Beijing), Beijing 100083, PR China;
3. Metals & Chemistry Research Institute, China Academy of Railway Sciences, Beijing
100081, PR China;
4. Mechanical Engineering Department, Tsinghua University, Beijing 100084, PR China
*Corresponding author. Tel: +086 10 82320255, Fax: +086 10 82322624, E-mail:
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Effects of nitriding temperature on microstructures and vacuum tribological properties
of plasma-nitrided titanium
Dingshun She 1, 2
, Wen Yue 1, 2*
, Zhiqiang Fu 1
, Chengbiao Wang 1
, Xingkuan Yang 3
,
Jiajun Liu 4
(1. School of Engineering and Technology, China University of Geosciences (Beijing),
Beijing 100083, PR China;
2. Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources,
China University of Geosciences (Beijing), Beijing 100083, PR China;
3. Metals & Chemistry Research Institute, China Academy of Railway Sciences, Beijing
100081, PR China;
4. Mechanical Engineering Department, Tsinghua University, Beijing 100084, PR China)
*Corresponding author. Tel.: +086 10 82320255; Fax: +086 10 82322624;
E-mail address: [email protected], [email protected]
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
ABSTRACT: Facing the outer space application of titanium and its alloys in drilling pipes
and drilling robots, it is necessary to find an optimized surface engineering method to
improve their vacuum tribological properties. In order to find an optimized nitriding
temperature, a series of plasma nitriding experiments were carried out for commercial
titanium (UNS R50400/Gr. 2/TA2/CP3) at various temperatures ranging from 650 to 950 ℃ for
8 h to investigate the effects of nitriding temperature on microstructures and mechanical
properties. Vacuum tribological properties of the nitrided and untreated samples were
examined using a ball-on-disc vacuum tribo-meter. The results show that the wear resistance
of titanium tested under vacuum condition is effectively enhanced, due to the formation of the
hard nitrided layer. The surface roughness, thickness and hardness of the nitrided layer
increase with the increase of nitriding temperature. Nevertheless, the load bearing capacities
of the samples nitrided at 900 and 950 ℃ are much lower than those of the samples nitrided at
800 and 850 ℃. Therefore, the wear volume keeps dropping at the nitriding temperature from
650 to 850 ℃, whereas, it converts to rise up when the nitriding temperature is above 850 ℃. In
general, it is an optimized process to be nitrided at the temperature of 850 ℃ for 8 h to improve
the vacuum tribological properties of titanium.
Keywords: titanium; plasma nitriding; vacuum tribology; wear
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
1. Introduction
Due to the excellent mechanical properties such as excellent strength-to-weight,
exceptional corrosion resistance and inherent flexibility, Titanium (Ti) and its alloys have
been found to provide operational benefits in geological drilling systems, such as drilling
pipes and drilling robots in space [1-3]. Nevertheless, Ti and Ti-based alloys exhibit low
surface hardness, poor wear resistance and higher friction coefficient under vacuum condition
[4-8]. Y. Liu et al. [8, 9] have demonstrated that plastic deformation and high flash
temperature can melt and soften the metal, and then cause severe adhesion under vacuum
condition. Accordingly, it is essential to find an optimized surface engineering method to
improve the vacuum tribological properties of titanium and its alloys.
Plasma nitriding (PN) has been proven to be one of the most effective methods improved
the tribological properties of titanium and its alloy [10-13]. Plasma nitriding can produce a
compound layer of TiN on the top of the matrix and Ti2N beneath, with a hardness of 3000
and 1500 HV, respectively [14, 15]. Generally, the composition, microstructure, thickness,
and mechanical properties of the nitrided layer strongly depend on the nitriding temperature
which usually can be chosen at a wide temperature ranging from 400 to 950 ℃ [14-22]. The
hardness and thickness of the nitrided layer increase with the increase of nitriding temperature
in references [16-20]. A. Molinari et al. [15] have reported that plasma nitriding temperature
is a basic and critical process parameter, and the tribological behaviours of the
plasma-nitrided Ti6Al4V alloy are deeply influenced by the nitriding temperature.
For space application, it is required to investigate the tribological behaviours of
plasma-nitrided titanium under vacuum condition, and to optimize nitriding temperature to
enhance the vacuum tribological properties of titanium. T. Spalvin [23, 24] investigated the
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
effects of the constitution of plasma-nitrided steel on friction coefficient under vacuum
condition and revealed that the γ-Fe4N phase performed a lower friction coefficient than that
of ε-Fe3N, and the friction coefficient of the plasma nitrided steel displayed a significant
reduction. Previous studies [25, 26] have showed the predominant wear mechanism for
plasma-nitrided surface under vacuum condition is abrasive, in complete contrast with
oxidation wear and delamination under ambient condition. Unfortunately, few researches on
the tribological properties of plasma-nitrided titanium under vacuum condition have been
reported.
In this study, UNS R50400 titanium was plasma-nitrided under the temperatures of 650,
700, 750, 800, 850, 900 and 950 ℃ for 8 h. The effects of nitriding temperature on the surface
morphology, compound layer thickness, constitution, hardness, load bearing capacity and
vacuum tribological property have been investigated. It aims to optimize the nitriding
temperature to improve the wear resistant performance of titanium under vacuum conditions.
2. Experimental details
2.1 Materials
Commercial titanium (UNS R50400/Gr. 2/TA2/CP3) sheets with a thickness of 5 mm
and a hardness of 140 HV were used. Its chemical composition was shown in TableⅠ. Samples
were cut into discs with a diameter of 65 mm by a CNC wire-cut electric discharge machine.
The samples were then ground and polished to a mirror surface.
2.2 Plasma nitriding treatment
The discs were placed into a LDM 1-100 plasma nitriding furnace after cleaning with
acetone in an ultrasonic cleaner. The discs were connected to the cathode, and the furnace
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
wall acted as the anode. The vacuum chamber was evacuated to ultimate vacuum (below 10
Pa) and the leakage rate of the equipment was less than or equal to 2 Pa per 15 min prior to
the nitriding treatment. Before the nitriding process, the plasma chamber was back-filled by
argon and hydrogen (3:1 ratio) for sputtering at 900 Pa gas pressure. During the process of
sputtering, the maximum temperature was 200 ℃. After 45 min of sputtering, the ammonia
gas was introduced for plasma nitriding. The nitriding was carried out at 700 Pa in an NH3
containing atmosphere. Nitriding was performed at 650, 700, 750, 800, 850, 900 and 950 ℃ ,
and those nitrided discs were marked as 650PN, 700PN, 750PN, 800PN, 850PN, 900PN and
950PN, respectively. During the process of plasma nitriding, the glow discharge was operated
with a potential voltage 700~850 V to obtain the prescribed nitriding temperature. Finally, the
discs samples were cooled to room temperature in NH3 atmosphere with a pressure of 600 Pa.
2.3 Characterizations
Surface roughness and 3D topographies were characterized by 3D profiler
(Nano-Map-D). For the roughness measurements, the tests were repeated ten times and the
average value was calculated. JSM-7001 F model Scanning electron microscope (SEM) was
adopted to investigate surface morphologies of the discs.
Axio Imager M2m model optical Microscope (OM) was employed to observe the
microstructure of cross-section of the untreated and nitrided discs. The cross-sectional
samples mounted in the dental base acrylic resin was polished, and etched in Kroll’s reagent
(2% HF and 4% HNO3).
The constitution of the nitrided layers were identified by a D/max X-ray diffract-meter
using a Cu-Kα radiation source (wavelength of 1.5406 Å), 2θ range of 30-80° and an
increment of 0.04°/step with a time of 1.5 s per step.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Micro-hardness was used to characterize the hardness of the nitrided layer as a function
of depth. A MH-6 model Vickers micro-hardness tester was employed to measure the surface
hardness and hardness profile along the depth at a load of 50 gf. According to the ASTM
standard E384-11e1, the indentation was corrected before the hardness test. Each
measurement was repeated for five times and the average value was chosen. The thickness of
nitrided layer was defined as the depth where the hardness was 10% HV above the core
hardness [27, 28].
2.4 Scratch tests
Scratch test is generally accepted as one of the simple means in assessing the wear and
crack behavior of the nitride layer [29]. During the process of the scratch test, a diamond tip is
driven over a nitride layer surface to produce a scratch, and the load on the diamond tip is
increased linearly to induce a shear force that is proportional to the applied load and
transmitted through the bulk of the nitrided samples [29, 30]. As the plasma nitrided samples
possess a gradient nitrided layer, the mechanical properties of the nitrided samples are
different along the scratching depth [10-22, 29]. Therefore, there is a discontinuity in the
shear stress at the interface which, when sufficiently high, induces adhesive failure at a
critical load. Generally, for hard nitrided layer, microcracks appear during scratching before
the final adhesion failure [30, 31]. The minimum load at which the first crack occurs is termed
as the critical load (Lc). Some researchers directly used this critical load to indicate cracking
resistance of the hard coating (nitrided layer is also one of hard coatings) because the higher
Lc, the more difficult it is to initiate a crack in the nitrided layer [29-35]. Therefore, what the
lower critical load represents is a load bearing capacity: the minimum critical load of crack
initiation [29, 30].
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
In order to characterize the load bearing capacity, a Rockwell-C diamond tip was adapted
to produce a scratch on the surface of nitrided samples. A MFT-4000 scratch tester was used
to carry out the scratch tests. The machine was equipped with acoustic emission and frictional
force measurement. The loading rate was 100 N/min. The nominal maximum load was 50 N.
The tests were carried out under ambient laboratory temperature and humidity conditions
which were 22 ℃ and 40-50% relative humidity. Five scratch tests per coating were performed
to obtain mean and standard deviation values.
2.5 Tribo-tests
Tribo-tests were performed on a MSTS-1 model ball-on-disc vacuum tribo-meter. The
schematic diagram of MSTS-1 vacuum tribo-meter was shown in Ref. [36]. AISI 52100 steel
ball with a diameter of 9.525 mm, hardness of 770 HV and surface roughness of Ra 32 nm
was chosen as the counter face materials of the untreated or nitrided discs. During the process
of tribo-tests, the upper ball was fixed, and the lower disc was rotated with a rotating speed of
100 rpm and a wear scar diameter of 26 mm (the sliding velocity is about 0.136 m/s).
Tribo-tests were carried out under a high vacuum (6.67×10-4 Pa) condition with a temperature
of 25 ℃. The test duration was 1200 s, and the applied load was 3 N (corresponding to the
mean Hertzian contact stress of 0.53 GPa). Wear volume of the discs was measured by a 3D
profiler (Nano-Map-D). To reveal the wear mechanism of the untreated and nitrided Ti, the
morphologies of worn surface were investigated by JSM-7001 F model Scanning electron
microscope (SEM) equipped with Oxford EDX-450 model energy dispersion spectrum
(EDS).
3. Results
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
3.1 Morphologies and microstructures
The SEM surface micrographs of the discs are shown in Fig. 1. It can be seen that the
surface of untreated discs is quite smooth. Nevertheless, some micro-pits and granular
micro-particles can be found on the surface of 750PN, 850PN and 950PN samples.
Additionally, it can be found that nitride particles are easy to aggregate larger-size particles at
a high nitriding temperature. The growing size of the nitrided micro-particles is accompanied
with the increase of surface roughness (Ra) of the nitrided discs. As shown in 3D profiler
images of the untreated and nitrided samples (Fig. 2), the 900 PN and 950PN samples exhibit
much rougher surfaces with many sharp peaks and valleys. Average surface roughness of the
untreated and nitrided samples is shown in Fig. 3. Obviously, the surface roughness of 900 PN
and 950PN samples is much higher than those of the other samples. Average surface
roughness of the nitrided discs keeps rising with the increase of nitriding temperature. S.R.
Hosseini et al. [21] have reported that variation of surface roughness versus process
temperature is similar to an exponential curve.
Fig.4 shows optical microscopy images of the cross-section of untreated and nitrided
discs. It can be seen that there is a uniform and continuous white compound layer on the
surface of the 750PN, 850PN and 950PN samples, and lamellar structure can be found in the
substrate of the 900PN and 950PN samples. The formation of lamellar structure is attributed
to the nitriding temperature is higher than the transition temperature (882 ℃) of α-Ti to β-Ti
phase [22]. In fact, it cannot be neglected that the formation of the lamellar structure in bulk is
deleterious to the mechanical properties of Ti.
In addition, the thickness of the white compound layer on the nitrided samples is listed in
Fig. 5. Obviously, there is no white compound layer formed on the surface of the 650PN
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
samples. As the nitriding temperature is above 700 ℃, the compound layer starts to grow and
its thickness gradually increases with the increase of nitriding temperature.
3.2 XRD analysis
XRD patterns of the untreated and nitrided discs are shown in Fig. 6. It can be seen that
the peaks of α-Ti keep dropping, nevertheless, the peaks of ε-Ti2N and δ-TiN keep rising with
the increase of nitriding temperature. For the 650PN and 700PN samples, a new phase ε-Ti2N
with a tetragonal crystal can be indentified compared with the untreated samples. Furthermore,
the conspicuous peaks of δ-TiN phase with NaCl-type crystal structure can be found on the
750PN samples. Accordingly, the results of XRD patterns demonstrate that a nitride layer was
formed on the titanium discs and the increasing nitriding temperature results in the increasing
growth of both ε-Ti2N and δ-TiN phase.
3.3 Micro-hardness
Micro-hardness of the untreated and nitrided samples is listed in Fig. 7. Obviously, the
hardness of the nitrided samples is significantly enhanced by plasma nitriding. The higher
nitriding temperature, the harder surface layer formed. The hardness of 950PN samples is
approaching to 1560 HV, and that of the untreated samples is only about 140 HV. Fig. 8 shows
the micro-hardness variation versus the cross-sectional depth of the samples. For the nitrided
samples, the micro-hardness keeps dropping form the top surface to the substrate, and there is
no sharp and abrupt change of hardness between surface layer and the substrate. The higher
nitriding temperature, the harder and thicker nitrided layer is formed, which is in according
with the thickness of the compound layer shown in Fig. 4 and 5. In addition, the core hardness
of the 650PN, 700PN, 750PN, 800PN and 850PN is the same as the untreated samples,
whereas, the core hardness of the 900PN and 950PN samples increases up to about 200 HV.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
The formation of the hard β phase lamellar structure (as shown in Fig. 4 (d)) is one of the
reasons for higher hardness of the bulk materials. Many literatures [16, 22, 23] have showed
that the increase of surface hardness and the thickness of the nitride layer is attributed to the
formation of hard phases (ε-Ti2N and δ-TiN), and an increased diffusion of nitrogen and a
rapid reaction rate promoted the formation of hard phases (ε-Ti2N and δ-TiN) at a high
processed temperature.
3.4 Load bearing (cracking resistance) capacity
Fig. 9 shows morphologies of the typical scratches on the untreated and nitrided samples.
Visibly, the nitrided samples treated at the lower nitriding temperature (650, 700 and 750 ℃)
shows a minimum critical load of crack initiation (Lc) at 10~20 N. Such a low minimum
critical load of crack initiation results from a thin hard nitrided layer composed of α-Ti and
some brittle ceramic phases such as ε-Ti2N and δ-TiN phase on a soft substrate with a poor
mechanical support [29-31]. A suitable gradient of hardness (as shown in Fig. 8) can provide a
better mechanical support to the compound layer, as a comparative thick nitrided layer forms
on the 800PN and 850PN samples. As a consequence, the 800PN and 850PN samples exhibit
a better load bearing capacity with a higher Lc of 30~40 N, comparing with the 650PN,
700PN and 750PN samples. Nevertheless, for the 900PN and 950PN samples, the spalling on
the edge of scratches is severe, thus the Lc of is only about 6~8 N. The severe spalling and
low Lc of the 900PN and 950PN samples can be attributed to the following two reasons. The
one is that almost no α-Ti could be indentified on the surface of 900PN and 950PN samples,
as shown in Fig. 6. The compound layer is consisted of ε-Ti2N and δ-TiN phases (brittle
ceramic phase), and the other one is that the formation of the β phase lamellar structure, as
shown in Fig.4d, is easy to cause a crack. Accordingly, despite a higher hardness and a thicker
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
nitrided layer are formed on the 900PN and 950PN samples, the load bearing capacity and
cracking resistance of the 900PN and 950PN samples are much lower than those of the
800PN and 850PN samples.
3.5 Friction coefficient
Friction coefficient of the untreated and nitrided samples is shown in Fig. 10. Friction
coefficient of the nitrided samples is deeply influenced by nitriding temperature. The average
friction coefficients of the 650PN, 700PN, 750PN, 900PN and 950PN samples are around
0.65. Comparatively, the 800PN and 850PN samples exhibit the lowest friction coefficient
about 0.54, and the untreated samples perform the highest average friction coefficient about
0.73. The typical friction coefficient curves versus tribo-test time are shown in inner figures of
Fig. 10. During the process of tribo-tests, the friction coefficient is fluctuant under vacuum
condition. The 850PN samples perform the mildest friction coefficient fluctuation ranging
from 0.25 to 0.95, and the untreated samples exhibits the most severe fluctuation ranging
from 0.15 to 1.35.
3.6 Wear behaviours
3D profiler images of the worn surface the untreated and nitrided samples are shown in
Fig.11. Evidently, the untreated and 650PN samples suffer serious wear, and there are some
plastic flows on the worn surfaces. Comparing with the 950PN samples, the 850PN samples
exhibit a milder wear.
Wear volumes of the untreated and nitrided discs are shown in Fig. 12. It can be seen that
the wear resistance of the titanium discs tested under vacuum condition is effectively
improved by plasma nitriding. According to Archard theory, the wear volume is inversely
proportion to the hardness. Therefore, the wear volume of the nitrided sample decreases with
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
the increase of nitriding temperature when the nitriding temperature is lower than 850 ℃.
Nonetheless, the wear volume of the nitride samples tends to increase when the temperature
exceeds 850 ℃. Accordingly, the 850PN samples present the lowest wear volume.
As listed in Fig. 13, the variation of wear scar diameter of counter balls is in accordance
with the variation of wear volume. Wear scar diameter is initially decreased, and then
increased with the increase of nitriding temperature. Wear scar diameter of the ball against the
untreated discs is about 1.38 mm, which is higher than those of other counter balls. Counter
ball against the 850PN samples shows the smallest wear scar diameter (about 0.42 mm).
4. Discussion
The characteristics of the wear surface reveal quite different wear mechanisms between
the untreated samples and the samples nitrided at various temperatures. As shown in Fig. 14a,
typical characteristics of tongue-shaped wedges are clearly observed on the untreated samples,
which indicates that severe adhesion and plastic deformation occurred on the untreated and
650PN samples. Moreover, some scale-like debris and plowing along the sliding direction can
be found on the worn surface of 700PN and 750PN samples. Wear damage of the samples
nitrided at low temperature is a comprehensive wear model of abrasive wear and plastic
deformation, adhesive wear. For the 800PN and 850PN samples, no plastic deformation is
observed on the worn surface, whereas, some plowing along sliding direction can be found on
the worn surface. Accordingly, the dominant wear mechanism is abrasive wear. The 900PN
and 950PN samples show a feature of abrasive wear (plowing along direction) and spalling as
shown in Fig 14d.
A further EDS analysis has been carried out to discuss the wear mechanisms. As shown
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
in Table Ⅱ, Fe and Cr can be identified on the worn surface of the 900PN and 950PN samples,
which indicates that material transfers from the counter balls to the discs. The materials
transfer is the main reason for the big wear scar on counter balls against 900PN and 950PN
samples. Almost no N was detected on the worn surface of the samples nitrided at 650 ℃,
which also demonstrates that the nitrided layer has already been worn out.
There is no air convection for cooling under vacuum condition, meanwhile it is different
to regenerate the surface films (absorbed film and oxide film) with friction reduction property,
and thus the fresh surface of ball and disc contact each other directly [5-9, 37,38]. In addition,
the surface can be plowed and cut by hard particles easily, leaving behind deep grooves as
shown in Fig. 11. Plastic deformation and high flash temperature can melt and soften the
metal, which is able to cause severe adhesion [29, 30]. The above effects result in the
breakage of friction stationary and the aggravation of adhesion. Under the shearing action of
friction, the adhesion junctions with high strength are easy to be cut up and slide with each
other. Finally, sliding wear transforms to the alternating process of “stick-slip behaviour”.
Accordingly, the untreated samples show the most severe friction coefficient fluctuation and
severe adhesion wear.
For the 650PN, 700PN and 750 PN samples, the hardness has been improved.
Nevertheless, the thin hard nitrided layer without an enough thick diffusion layer cannot
provide a better mechanical support to the applied normal load. The load bearing capacity is
not higher enough, and then the compound layers on the surface of the discs tend to be
fractured and produced wear debris. These wear debris can cause slight scuffing and abrasive
wear. Meanwhile, these wear debris also can transform into scale-like debris adhered on the
worn surface after plastic deformation as shown in Fig. 11b. Furthermore, the wear debris is
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
able to prevent fresh surface of the counter balls and discs contact each other directly. As a
consequence, adhesive wear tends to be milder, and the “stick-slip behaviour” is restrained
during the process of the tribo-tests. Therefore, the milder fluctuation of friction coefficient
can be observed on the 650PN, 700PN and 750 PN samples, and relatively lower friction
coefficients comparing with the untreated sample. In addition, the hard nitrided layer can
provide excellent abrasive wear resistance [23-26]. Therefore, the wear volumes of the 650PN,
700PN and 750PN samples are lower than that of the untreated samples.
The surface hardness of the 800PN and 850PN samples has been effectively enhanced by
plasma nitriding. Additionally, a thicker nitrided layer with a hardness gradient can provide
better mechanical support to the applied normal load. Therefore, the load bearing capacities of
the 800PN and 850PN samples is perfect. The wear debris spalled from the discs acting as
abrasive particles is hard to deform. As a result, no “adhesion-cut up-adhesion” and plastic
deformation characteristics can be observed on the worn surface of the 800PN and 850PN
samples, such as tongue-shaped wedges and scale-like debris. In another words, the nitride
layer on the 800PN and 850PN samples acts as a barrier to protect titanium surfaces from
adhesion wear and plastic deformation under vacuum condition. The wear mechanism of the
800PN and 850PN samples is abrasive wear. M.M. Yazdanian et al. [39] have reported the
similar results that thermally oxidized layer on the titanium alloy can effectively protect
titanium surfaces from adhesion wear and plastic deformation under vacuum condition.
Accordingly, the fluctuation of friction coefficient is mildest, and the average friction
coefficient of the 800PN and 850PN samples also decreased due to the decrease of adhesion.
The high surface hardness requires the diffusion layer to provide better mechanical
support for the compound layer. Otherwise, any cracking on the compound layer resulting
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
from substrate deformation is likely to lead to higher localized spalling which results in severe
wear damage [13]. For the 900PN and 950PN samples, the hardness is enhanced to about
1500~1600 HV. As shown in Fig.9, the load capacities of the 900PN and 950PN samples were
poor. Meanwhile, obvious micro-cracks and spalling can be found on the scratches of the
900PN and 950PN samples. Therefore, a mixed failure mode of obvious micro-cracks and
spalling is harmful to keep the wear resistance of the compound layer. Actually, as shown in
Fig.15, the results of wear tests also demonstrate that micro-cracks and spalling can be
observed on the worn surface of the 900PN and 950PN samples. Those wear debris spalled
from the discs acting as hard abrasive particles plows and shears the worn surface of the
900PN and 950PN samples and their counter ball. Consequently, the 900PN and 950PN
samples and its counter balls suffer severe abrasive wear (as shown in Fig. 13). Comparing
with the 850 PN samples, there are more and harder abrasive particles on the worn surface of
the 900PN and 950PN samples during the wear tests. Therefore, the abrasive wear on the
900PN and 950PN samples is more serious than that on the 850 samples. D. Nolan et al. [13]
have demonstrated that the more gradual transition of the hardness and elastic modulus across
the nitrided layer/substrated interface, the more excellent load bearing capacity and sliding
wear resistance. Additionally, M. Rahman et al. [22] has demonstrated that the rougher
surface formed at a high nitriding temperature leads to more severe wear damage and high
average friction coefficient. As a consequence, the wear volume and average friction
coefficient of the 800PN and 850PN samples are lower than that of the 900PN and 950PN
samples. Furthermore, previous studies [40-42] have shown that the size and shape of wear
debris play basic role on the evolution of friction coefficient in dry sliding conditions. As a
result, the 900PN and 950PN samples perform a wilder fluctuation of friction coefficient.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
5. Conclusions
The following conclusions can be drawn from this work:
(1) Plasma nitriding can effectively enhance the wear resistance, and reduce the friction
coefficient of UNS R50400 titanium (Ti) under vacuum condition. In case of the nitriding
temperature ranges from 650 to 950 ℃ for 8 h, UNS R50400 Ti nitrided at the temperature of
850 ℃ exhibits the most excellent wear resistance and the lowest friction coefficient under
vacuum condition.
(2) As nitriding temperature rises from 650 to 850 ℃, the wear resistance of nitrided UNS
R50400 titanium shows a upward trend, which results from the increase of the thickness,
hardness and load bearing (cracking resistance) capacity of the nitrided layer.
(3) For the case of the UNS R50400 titanium nitrided at 900 and 950 ℃, a brittle ceramic
top layer and a lamella structure in the substrate are formed, which results in the poor load
bearing (cracking resistance) capacity. The poor load bearing capacity accompanied with the
sharp rise of roughness leads to a higher wear rate.
Acknowledgments
The authors are grateful for the financial support by the National Natural Science
Foundation of China (51375466), the Beijing Higher Education Young Elite Teacher Project
(YETP0646), the Beijing Natural Science Foundation (3132023), the Fundamental Research
Funds for the Central Universities (2652013080) and the Tribology Science Fund of State Key
Laboratory of Tribology (SKLTKF13B10). The authors would like to thank Prof. Haidou
Wang and Dr. Guozheng Ma from National Key Lab for Remanufacturing, Academy of
Armored Forces Engineering, for their help with the use of the vacuum tribotester.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
References
[1] K. Zacny, Y. Bar-Cohen, K. Davis, P. Coste, G. Paulsen, S. Sherrit, J. George, B.
Derkowski, S. Gorevan, D. Boucher, J. Guerrero, T. Kubota, B.J. Thomson, S. Stanley, P.
Thomas, N. Lan, C. McKay, T.C. Onstot, C. Stoker, B. Glass, S. Wakabayashi, L. Whyte,
G. Visentin, E. Re, L. Richter, M. Badescu, X. Bao, R. Fincher, T. Hoshino, P. Magnani, C.
Menon, Extraterrestrial drilling and excavation, Y. Bar-Cohen, K. Zancy (Eds.), Drilling
in Extreme Environments, Wiley-VCH Verlag GmbH , New York, 2009, pp. 347–557
[2] P. Younse, A. Stroupe, T. Huntsberger, M. Garrett, J.L. Eigenbrode, L.G. Benning, M.
Fogel, A. Steele, Sample acquisition and caching using detachable scoops for mars
sample return, Aerospace conference, 10.1109/AERO.2009.4839312, 2009.
[3] M. Anttila, Concept evaluation of mars drilling and sampling instrument, Ph.D. Thesis,
Helsinki University of Technology.
[4] K. Zacny, G. Cooper, Considerations, constraints and strategies for drilling on Mars, Planet.
Space Sci. 54 (2006) 345-356.
[5] K. Miyoshi, Aerospace mechanisms and tribology technology-case study, Tribol. Int.
32(11) (1999) 605-616.
[6] K. Miyoshi, Considerations in vacuum tribology (adhesion, friction, wear, and solid
lubrication in vacuum), Tribol. Int. 32(11) (1999) 673-685.
[7] I.L. Lebedeva, G.N. Presnyakova, Adhesion wear mechanisms under dry friction of
titanium alloys in vacuum, Wear. 148(2) (1991) 203-210.
[8] Y. Liu, D.Z. Yang, S.Y. He, W.L. Wu, Microstructure developed in the surface layer of
Ti6Al4V alloy after sliding wear in vacuum, Mater. Charact. 50 (2003) 275-279.
[9] Y. Liu, D.Z. Yang, S.Y. He, W.L. Wu, Study on dry sliding wear of TC4 alloy in vacuum,
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Rare Metal. Mat. Eng. 34 (1) (2005) 128-131.
[10] D.G. Bansal, O.L. Eryilmaz, P.J. Blau, Surface engineering to improve the durability and
lubricity of Ti–6Al–4V alloy, Wear 271(9-10) (2011) 2006-2015.
[11] G. Cassar, J.A.B.S. Wilson, S. Banfield, J. Housden, A. Matthews, A. Leyland, A study of
the reciprocating-sliding wear performance of plasma surface treated titanium alloy, Wear
269(1-2) (2010) 60.
[12] A.F. Yetim, F. Yildiz, Y. Vangolu, A. Alsaran, A. Çelik, Several plasma diffusion
processes for improving wear properties of Ti6Al4V alloy, Wear 267(12) (2009)
2179-2185.
[13] D. Nolan, S.W. Huang, V. Leskovsek, S. Braun, Sliding wear of titanium nitride thin
films deposited on Ti–6Al–4V alloy by PVD and plasma nitriding processes, Surf. Coat.
Technol. 200(20-21) (2006) 5698-5705.
[14] S.L. Ma, K.W. Xu, W.Q. Jie, Wear behavior of the surface of Ti–6Al–4V alloy modified
by treating with a pulsed d.c. plasma-duplex process, Surf. Coat. Technol. 185(2-3) (2004)
205-209.
[15] A. Molinari, G. Straffelini, B. Tesi, T. Bacci, G. Pradelli, Effects of load and sliding speed
on the tribological behaviour of Ti6Al4V plasma nitrided different temperatures, Wear
203-204 (1997) 447-454.
[16] F.Yilidiz, A.F. Yetim, A. Alsaran, A. Çelik, Plasma nitriding behavior of Ti6Al4V
orthopedic alloy, Surf. Coat. Technol. 202(11) (2008) 2471-2476.
[17] M. Rahman, M.S.J. Hashmi, Saddle field fast atom beam source: A new low pressure
plasma nitriding method for a alloy Ti–6Al–4V, Thin Solid Film 515(1) (2006) 129-134.
[18] S.Taktak, H. Akbulut, Diffusion kinetics of explosively treated and plasma nitrided
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Ti–6Al–4V alloy, Vacuum 75(3) (2004) 247-259.
[19] S.Taktak, H. Akbulut, Dry wear and friction behaviour of plasma nitrided Ti–6AL–4 V
alloy after explosive shock treatment, Tribol. Int. 40(3) (2007) 423-432.
[20] D.S. She, W. Yue, Z.Q. Fu, Y.H. Gu, C.B. Wang, J.J. Liu, The effect of nitriding
temperature on hardness and microstructure of die steel pre-treated by ultrasonic cold
forging technology, Mater. Des. 49 (2013) 392-399.
[21] S.R. Hosseini, A. Ahmadi, Evaluation of the effects of plasma nitriding temperature and
time on the characterisation of Ti 6Al 4V alloy, Vacuum 87 (2013) 30-39.
[22] M. Rahman, I. Reid, P. Duggan, D.P. Dowling, G. Hughes, M.S.J. Hashmi, Structural and
tribological properties of the plasma nitrided Ti-alloy biomaterials: Influence of the
treatment temperature, Surf. Coat. Technol. 201(9-11) (2007) 4865-4872.
[23] T. Spalvin, Tribological and microstructural characteristics of ion-nitrided steels, Thin
solid films 108(2) (1983) 157-163.
[24] T. Spalvin, Frictional and structural characterization of ion‐ nitrided low and high
chromium steels, J. Vac. Sci. Technol. A 3(6) (1985) 2329-2333.
[25] J.Q. Yang, Y. Liu, Z.Y. Ye, D.Z. Yang, S.Y. He, Microstructure and tribological
characteristics of nitrided layer on 2Cr13 steel in air and vacuum, Surf. Coat. Technol.
204(5) (2009) 705-712.
[26] J.Q. Yang, Y. Liu, Z.Y. Ye, D.Z. Yang, S.Y. He, A study of friction and wear behavior of
nitrided layer on 2cr13 steel in vacuum, Tribol. Lett. 40(3) (2010) 285-294.
[27] S.D. Oliveira, A.P. Tschiptschin, C.E. Pinedo, Simultaneous plasma nitriding and ageing
treatments of precipitation hardenable plastic mould steel, Mater. Des. 28(5) (2007)
1714-1718.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
[28] S.Y. Sirin, E. Kaluc, Structural surface characterization of ion nitrided AISI 4340 steel,
Mater. Des. 36 (2012) 741-747.
[29] H. Paschke, M. Weber, P. Kaestner, G. Braeuer, Influence of different plasma nitriding
treatments on the wear and crack behavior of forging tools evaluated by Rockwell
indentation and scratch tests, Surf. Coat. Technol. 205 (20010) 1465-1469.
[30] S. Zhang, D. Sun, Y.Q. Fu, H.J. Du, Toughness measurement of thin films: a critical
review, Surf. Coat. Technol. 198 (2005) 74-84.
[31] S. Zhang, X.M. Zhang, Toughness evaluation of hard coatings and thinfilms, Thin Solid
Films, 520 (2012) 2375-2389.
[32] A.A. Voevodin, C. Rebholz, J.M. Schneider, P. Stevenson, A. Matthews, Wear resistant
composite coatings deposited by electron enhanced closed field unbalanced magnetron
sputtering, Surf. Coat. Technol. 73 (1995) 185-197.
[33] E. Harrry, A. Rouzaud, P. Juliet, Y. Pauleau, M. Ignat, Failure and adhesion
characterization of tungsten–carbon single layers, multilayered and graded coatings, Surf.
Coat. Technol. 116–119 (1999) 172-175.
[34] A.A. Voevodin, J.S. Zabinski, Supertough wear-resistant coatings with ‘chameleon’
surface adaptation, Thin Solid Films 370 (2000) 223-231.
[35] A.A. Voevodin, J.S. Zabinski, Load-adaptive crystalline–amorphous nanocomposites, J.
Mater. Sci. 33 (1998) 319-327.
[36] G.Z. Ma, B.S. Xu, H.D. Wang, S.Y. Chen, Z.G. Xing, Excellent vacuum tribological
properties of Pb/PbS film deposited by Rf magnetron sputtering and ion sulfurizing, Appl.
Mater. Interfaces 6(1) (2014) 532-538.
[37] S.Z. Wen, Tribology Principle, Tsinghua university press, Beijing, 2003. (in Chinese)
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
[38] H.D. Wang, G.Z. Ma, B.S. Xu, H.J. Shi, D.X. Yang, Microstructure and vacuum
tribological properties of 1Cr18Ni9Ti steel with combined surface treatments, Surf. Coat.
Technol. 205(11) (2011) 3546-3552.
[39] M.M. Yazdanian, A. Edrisy, A.T. Alpas, Vacuum sliding behaviour of thermally oxidized
Ti–6Al–4V alloy, Surf. Coat. Technol. 202(4-7) (2007) 1182-1188.
[40] A.I. Dmitriev, W. Österle, H. Kloß, G. Orts-Gil, A study of third body behaviour under
dry sliding conditions. Comparison of nanoscale modelling with experiment, Est. J. Eng.
18(3) (2012) 270-278.
[41] A. Zmitrowicz, Wear debris: a review of properties and constitutive models, J. Thero.
Appl. Mech-pol.43(1) (2005) 3-35.
[42] J. Denape, Paper XI (iii) Wear Debris Action in Sliding Friction of Ceramics, Tribol.
Series 21 (1992) 453-462.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Figure captions:
Fig. 1 Typical SEM micrographs of the (a) Untreated, (b) 700PN, (c) 750PN and (d) 950PN
samples.
Fig. 2 3D profiler images of the (a) untreated (b) 700PN (c) 800PN and (d) 900PN samples.
Fig. 3 Surface roughness of the untreated and nitrided samples.
Fig. 4 Optical microscopy images of the cross-section of the (a) Untreated, (b) 750PN, (c)
850PN and (d) 950PN samples.
Fig. 5 Thickness of the white compound layer forming on the nitrided samples.
Fig. 6 XRD patterns of the nitrided samples compared with the untreated samples.
Fig. 7 Surface hardness of the untreated and nitrided samples.
Fig. 8 Micro-hardness variation versus cross-sectional depth.
Fig. 9 Scrathes evaluated in scratch test showing the influence of the nitriding temperature on
the load bearing capacity of the nitrided layer.
Fig. 10 Friction coefficients of the untreated and nitrided samples.
Fig. 11 3D profiler images of the untreated and nitrided samples.
Fig.12 Wear volumes of the untreated and nitride samples.
Fig. 13 Wear scar diameter of the counter balls.
Fig. 14 SEM morphologies of worn surfaces of the (a) Untreated, (b) 750PN, (c) 850PN and
(d) 950PN samples.
Fig. 15 SEM morphology of worn surface of the 950PN sample.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Table captions:
Table Ⅰ Nominal composition of commercial titanium (wt.%).
Table Ⅱ EDS analysis results on the worn surfaces.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 1 Typical SEM micrographs of the (a) Untreated, (b) 700PN, (c) 750PN and (d) 950PN
samples.
Fig. 2 3D profiler images of the (a) untreated (b) 700PN (c) 800PN and (d) 900PN samples.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 3 Surface roughness of the untreated and nitrided samples.
Fig. 4 Optical microscopy images of the cross-section of the (a) Un-treated, (b) 750PN, (c)
850PN and (d) 950PN samples.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 5 Thickness of the white compound layer forming on the nitrided samples.
Fig. 6 XRD patterns of the nitrided samples comparing with the untreated samples.
Fig. 7 Surface hardness of the untreated and nitrided samples.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 8 Microhardness variation versus cross-sectional depth.
Fig. 9 Scrathes evaluated in scratch test showing the influence of the nitriding temperature
on the load bearing capacity of the nitrided layer.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 10 Friction coefficients of the un-treated and nitrided samples.
Fig. 11 3D profiler images of the (a) 650PN, (b) 750PN, (c) 850PN and (d) 950PN samples.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig. 12 Wear volumes of the untreated and nitride samples.
Fig. 13 Wear scar diameter of the counter balls.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Fig.14 SEM morphologies of worn surfaces of the (a) 650PN, (b) 750PN, (c) 850PN and (d)
950PN samples.
Fig.15 SEM morphology of worn surface of the 950PN sample.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Table 1 Nominal composition of commercial titanium (wt.%).
Element Ti Fe O C H N Si
Content Bal. ≤0.30 ≤0.35 ≤0.08 ≤0.015 ≤0.03 ≤0.15
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Table Ⅱ EDS analysis results on the worn surfaces.
Spectrum
number
Samples Element content (at.%)
Ti N Fe Cr
1 un-treated 99.96 ---- 0.04 ----
2 650PN 98.33 1.65 0.02 ----
3 700PN 86.73 13.61 0.18 ----
4 750PN 82.33 17.49 0.16 ----
5 800PN 78.78 19.99 1.23 ----
6 850PN 71.52 26.21 2.26 ----
7 900PN 63.37 28.76 7.28 0.59
8 950PN 57.52 30.27 11.95 1.26