# Studies on Nanocrystalline TiN Coatings Prepared by Reactive Plasma Spraying

Authors: Dong Yanchun; Yan Dianran; He Jining; Zhang Jianxin; Xiao Lisong; Li Xiangzhi
Journal: Journal of Nanomaterials
 (2008-01-30)

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## Abstract

Titanium nitride (TiN) coatings with nanostructure were prepared on the surface of 45 steel (Fe-0.45%C) via reactive plasma spraying (denoted as RPS) Ti powders using spraying gun with self-made reactive chamber. The microstructural characterization, phases constitute, grain size, microhardness, and wear resistance of TiN coatings were systematically investigated. The grain size was obtained through calculation using the Scherrer formula and observed by TEM. The results of X-ray diffraction and electron diffraction indicated that the TiN is main phase of the TiN coating. The forming mechanism of the nano-TiN was characterized by analyzing the SEM morphologies of surface of TiN coating and TiN drops sprayed on the surface of glass, and observing the temperature and velocity of plasma jet using Spray Watch. The tribological properties of the coating under nonlubricated condition were tested and compared with those of the AISI M2 high-speed steel andAl2O3 coating. The results have shown that the RPS TiN coating presents better wear resistance than the M2 high-speed steel and Al2O3 coating under nonlubricated condition. The microhardness of the cross-section and longitudinal section of the TiN coating was tested. The highest hardness of the cross-section of TiN coating is 1735.43HV100 g.


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## Body

## 1. Introduction

Titanium
nitride (TiN) coatings are extensively applied in machinery industry due to their high
hardness, low friction coefficient, beautiful color, excellent chemical
stability, and wear resistance [1–6]. TiN has
been produced by several techniques, which include direct nitridation of
titanium metal, reductive nitridation of TiCl4, plasma synthesis,
and laser synthesis. Direct nitridation of titanium metal powder by nitrogen
has been well studied [5, 6]. The formation of TiN is highly
exothermic and the nitridation can be sustained to completion even at
relatively low pressures of nitrogen. Chemical vapor deposition and plasma
synthesis of titanium nitride involve the use of TiCl using ammonia as the
nitriding agent. A vapor-phase chemical route using titanium tetrachloride,
magnesium or sodium, and nitrogen in the temperature range 750–1050°C has
been used in many studies [7–10]. Plasma processing in RF plasma torches has also been used to prepare titanium
nitride [11–13]. The
process involves the use of titanium halide or titanium metal powder with
ammonia or nitrogen as the reactive gas. These coatings have deadly
disadvantage, too. Namely, deposited efficiency is low (about 2~10 μm/h), and producing complicated structural
part is very difficult, and wear resistance in the high loading weight is not
acceptable, therefore the application of TiN is restricted.As known, bulk TiN
has such an excellent corrosion resistance that most acidic and alkaline
solutions cannot corrode it [14]. However, thin TiN coatings are not
corrosion resistant to aqueous or gaseous media due to some pinholes existing
in the coating [10, 11, 14, 15]. Rickerby
and Burnett found that the wear resistance of the coating
improved as the thickness of the coating increased [16].The disadvantage of these
coatings can be overcome when TiN
coating is prepared by plasma spraying. Because the deposited efficiency of
plasma spraying is higher than that of other ways, and the thickness of
coatings prepared by plasma spraying is bigger than that of other ways. The reactive plasma spraying
(RPS) technology has been introduced in recent years as a promising way to
develop dense composite coatings with a metallic or an intermetallic matrix and
finely dispersed ceramic phases [17–19]. The wear resistance of plasma sprayed coatings can be enhanced by means
of RPS techniques. Titanium nitride coatings developed via RPS are
characterized by a considerable hardness, over 1500 HV, without the
characteristic brittleness of TiN coatings obtained by physical vapor
deposition (PVD), or chemical vapor deposition (CVD) [20, 21]. In
this paper, nanocrystalline TiN coating is prepared by spraying
Ti powder with size of 30~40 μm using plasma spray gun with self-made
reactive chamber which is filled with N2. The microstructure and
property of nano-TiN coating are investigated in this paper.


## 2. Experiments

### 2.1. Materials

The
spraying equipment is LP-50B type, which is made in Jiujiang, China,
and its standard power is 50 Kw. The spraying gun is assembled using a BT-G3
type plasma spraying gun and a reactive chamber which is self-designed and
prepared [22]. The sketch of reactive plasma spraying gun is shown
in Figure 1. The pure titanium powder used in the present work is commercially
available and produced by Beijing General Research Institute of Mining and
Metallurgy, China. The average particle size distribution of the titanium
powder is about 30~40 μm. The substrate material is 45  (Fe-0.45wt.%C) steel, which is machined into samples of 30 mm × 25 mm × 10 mm and ground to rough surfaces. Prior to spraying TiN
coating, Ni-10 wt.%Al
self-melting alloy bond layer with a thickness of about 100 μm is sprayed
onto surfaces of samples, for increasing the adhesive strength between the TiN
coating and substrate.Figure 1
Sketch of the reactive plasma spraying gun.


### 2.2. Fabrication and Characterization of the TiN Coating

During
spraying, the titanium powders, the micrograph of which was shown in Figure2,
were carried by nitrogen gas into the reactive chamber of the RPS gun, where
pure nitrogen gas was also introduced. Ti and N2 reacted in the
reactive chamber, the product, which was TiN, deposited on the substrate. Thus,
a coating with a thickness of at least 400 μm was fabricated within a few
minutes. The morphology of the as-sprayed TiN coating was observed by means of
a PHILIPS XL30/TMP scanning electron microscope (SEM) and PHILIPS TECNAI F20 transmission
electron microscope (TEM). The JEOL Rigaku X-ray diffractometer 2500/PC
diffraction instrument with Cu target was adopted to analyze the phase
composition of the coating's exposed surface and cross-section. The grain size
of the TiN coating was calculated according to Scherrer formula [23]:
(1)Dhkl=kλBcosθ,B=B12−B22,
where
“Dhkl” is the average diameter in nm, “k” is the shape
factor (0.9), “B” is the broadening of the diffraction line measured
half of its maximum intensity in “radians,” “B1”
is the
half peak breadth of the diffraction line of test sample, “B2”
is half
peak breadth of the diffraction line of micron crystal TiN specimen prepared by
other way, “λ” is the wave length of X-ray, and “θ” is the Bragg
diffraction angle.Figure 2
SEM micrograph of the original Ti powders.Spray Watch
used during spraying is observation system for thermal spraying, made by Oseir Co., Ltd.,
Finland.


### 2.3. Hardness Test

Microhardness
of the cross and longitudinal sectional TiN coating was measured using the
digital Vickers microhardness tester, which is made in Shanghai Taiming Optical
Instrument Co., Ltd., China. Loads ranging from 25 to 1000 g and a dwell time
of 15 seconds were selected. Approximately, 10 indentations were made for each load. The
distance between every two indentations was controlled three times longer than
the indentation diagonal, so the stress field effect near the indentation can
be eliminated. The average microhardness is selected for discussion in this
paper.


### 2.4. Wear Tests

Friction and wear tests of the TiN coated specimens were performed using a
block-on-ring sliding apparatus, under nonlubricated condition.
The MM-200 wear test machine, made by Xuanhua Material Test Machine Co., Ltd.,
China, was operated with the sliding speed of 0.4 ms−1.
The test specimens were fixed by a sample holder; the wear rings were placed
under the specimens; loads ranging from 100 to 1470 N were
vertically applied at the top of specimens. For each load, three specimens were
tested, and the average wear volume loss was taken. The wear ring used was AISI
E52100 steel (0.95–1.05C, 0.2–0.4 Mn, 0.15–0.35 Si, 1.3–1.65 Cr, 0.3 Ni,
0.25 Cu, and bal Fe) (in wt.%). The ring is 38 mm in external diameter and 10 mm in thickness, which was heat-treated to an
average hardness of about 60 HRC. Each wear test was conducted for 60 minutes.
Wear loss of TiN coating, Al2O3 coating, and the AISI M2
steel specimens were obtained by measuring the volumetric loss after each test
according to (2). The friction coefficient
(FC) of the specimens was given by (3):
(2)ΔV=B×[r2×Sin−1(b2r)−b2(r2−b24)1/2],(3)μ=Tr⋅P×α+sinα⋅cosα2sinα,
where ΔV is the volumetric loss of specimens; B and r are, respectively, the thickness and radius of the ring; b is
the width of worn scars; T is the wear moment; P is the load
employed; α is the contacted
angle, according to α=sin−1(b/2r), and μ is friction coefficient.


## 2.1. Materials

The
spraying equipment is LP-50B type, which is made in Jiujiang, China,
and its standard power is 50 Kw. The spraying gun is assembled using a BT-G3
type plasma spraying gun and a reactive chamber which is self-designed and
prepared [22]. The sketch of reactive plasma spraying gun is shown
in Figure 1. The pure titanium powder used in the present work is commercially
available and produced by Beijing General Research Institute of Mining and
Metallurgy, China. The average particle size distribution of the titanium
powder is about 30~40 μm. The substrate material is 45  (Fe-0.45wt.%C) steel, which is machined into samples of 30 mm × 25 mm × 10 mm and ground to rough surfaces. Prior to spraying TiN
coating, Ni-10 wt.%Al
self-melting alloy bond layer with a thickness of about 100 μm is sprayed
onto surfaces of samples, for increasing the adhesive strength between the TiN
coating and substrate.Figure 1
Sketch of the reactive plasma spraying gun.


## 2.2. Fabrication and Characterization of the TiN Coating

During
spraying, the titanium powders, the micrograph of which was shown in Figure2,
were carried by nitrogen gas into the reactive chamber of the RPS gun, where
pure nitrogen gas was also introduced. Ti and N2 reacted in the
reactive chamber, the product, which was TiN, deposited on the substrate. Thus,
a coating with a thickness of at least 400 μm was fabricated within a few
minutes. The morphology of the as-sprayed TiN coating was observed by means of
a PHILIPS XL30/TMP scanning electron microscope (SEM) and PHILIPS TECNAI F20 transmission
electron microscope (TEM). The JEOL Rigaku X-ray diffractometer 2500/PC
diffraction instrument with Cu target was adopted to analyze the phase
composition of the coating's exposed surface and cross-section. The grain size
of the TiN coating was calculated according to Scherrer formula [23]:
(1)Dhkl=kλBcosθ,B=B12−B22,
where
“Dhkl” is the average diameter in nm, “k” is the shape
factor (0.9), “B” is the broadening of the diffraction line measured
half of its maximum intensity in “radians,” “B1”
is the
half peak breadth of the diffraction line of test sample, “B2”
is half
peak breadth of the diffraction line of micron crystal TiN specimen prepared by
other way, “λ” is the wave length of X-ray, and “θ” is the Bragg
diffraction angle.Figure 2
SEM micrograph of the original Ti powders.Spray Watch
used during spraying is observation system for thermal spraying, made by Oseir Co., Ltd.,
Finland.


## 2.3. Hardness Test

Microhardness
of the cross and longitudinal sectional TiN coating was measured using the
digital Vickers microhardness tester, which is made in Shanghai Taiming Optical
Instrument Co., Ltd., China. Loads ranging from 25 to 1000 g and a dwell time
of 15 seconds were selected. Approximately, 10 indentations were made for each load. The
distance between every two indentations was controlled three times longer than
the indentation diagonal, so the stress field effect near the indentation can
be eliminated. The average microhardness is selected for discussion in this
paper.


## 2.4. Wear Tests

Friction and wear tests of the TiN coated specimens were performed using a
block-on-ring sliding apparatus, under nonlubricated condition.
The MM-200 wear test machine, made by Xuanhua Material Test Machine Co., Ltd.,
China, was operated with the sliding speed of 0.4 ms−1.
The test specimens were fixed by a sample holder; the wear rings were placed
under the specimens; loads ranging from 100 to 1470 N were
vertically applied at the top of specimens. For each load, three specimens were
tested, and the average wear volume loss was taken. The wear ring used was AISI
E52100 steel (0.95–1.05C, 0.2–0.4 Mn, 0.15–0.35 Si, 1.3–1.65 Cr, 0.3 Ni,
0.25 Cu, and bal Fe) (in wt.%). The ring is 38 mm in external diameter and 10 mm in thickness, which was heat-treated to an
average hardness of about 60 HRC. Each wear test was conducted for 60 minutes.
Wear loss of TiN coating, Al2O3 coating, and the AISI M2
steel specimens were obtained by measuring the volumetric loss after each test
according to (2). The friction coefficient
(FC) of the specimens was given by (3):
(2)ΔV=B×[r2×Sin−1(b2r)−b2(r2−b24)1/2],(3)μ=Tr⋅P×α+sinα⋅cosα2sinα,
where ΔV is the volumetric loss of specimens; B and r are, respectively, the thickness and radius of the ring; b is
the width of worn scars; T is the wear moment; P is the load
employed; α is the contacted
angle, according to α=sin−1(b/2r), and μ is friction coefficient.


## 3. Results

### 3.1. Phases Constitute of the TiN Coating

Figure3 is XRD pattern of the TiN coating sprayed on the surface of
substrate and pure Ti powders. The coating is mainly composed of two phases,
TiN and small quantities of Ti3O, without pure Ti. Observing the
spectrum, five sharp TiN diffraction peaks appeared, and the diffraction planes
are (111), (200), (220), (311), and (222). Intensities of Ti3O are
very low, the diffraction planes of the Ti3O phase are (103), (113),
(116), and (223). According to the Scherrer formula, the grain size of
TiN coating was estimated, which was shown in Table 1. The grain size of
the TiN coating ranged 78~97 nm, which is consistent
with what is observed by TEM (see Figure 5). Pure Ti diffraction does not
appear in the coating, which proves that the Ti and N2 react fully
in RPS process. This is consistent with literature [9]. Ti3O is
metastable solid solution, which comes from Ti unsufficient oxidation at
temperature of 650~750°C and decomposes when temperature exceeds 750°C
[24]. The Ti3O in the coating is product of the
oxidation of TiN and exists in the coating for shock cooling under plasma
spraying condition.Table 1
Scherrer crystallite sizes of the TiN coatings.
Diffraction plane (hkl)(111)(200)(220)Dhkl (nm)92~9783~8878~83Surface XRD pattern of original Ti powders and the reactive plasma sprayed TiN coating.
(a)(b)


### 3.2. Microstructural Characterization of the TiN Coating

The SEM photograph of the TiN coating cross-section is shown in Figure4. The entire cross-sectional morphology of the
RPS TiN coating in Figure 4(a) indicates that its thickness is 420 μm, which is
about 100 times than that of TiN films prepared by CVD or PVD. The coating presents layer structure, which is tightly piled. The structure
with few pores should be attributed to gas which exists between the TiN liquid  drops and has no time to be released during coating forming. Small quantities of cracks appear in the multilayered structures of the coating (see Figure
4(b)). Further
study is needed to reduce the pores and cracks, and improve the structure of
the coating.The cross-sectional morphologies of the
RPS TiN coating, (b) is partial
enlarged drawing of (a).
(a)(b)TEM morphology of the
reactive plasma sprayed TiN coating: (a)
nanocrystallines, (b) the selected area
electron diffraction pattern.
(a)(b)TEM is an
indispensable analytical tool in the study of the microstructure of coatings. Figure5 indicates the TEM
morphology and the selected area electron diffraction (SAED) pattern of the
reactive plasma sprayed TiN coating. It can be seen from Figure 5(a) that the
most of grain size of the coating is smaller than 100 nm. The SAED pattern of the
coating is given in Figure 5(b). The grain size of 82 ± 10 nm was measured by linear
intercepting, which is approximate to the result obtained by calculation, and smaller
than that of the original Ti powders, which is 30~40 μm. The
diffraction rings of the SAED pattern in Figure 5(b) are continuous, dense, and
broader, which indicates the (111), (200), and (220) orientation. (311)
orientation is not clear. The diffraction rings show that the orientation of
TiN crystal grains is random. Weak diffraction spots distribute in diffraction
rings because the size of small quantity of crystal grains is bigger than
100 nm. XRD
and SAED examinations revealed that the TiN coating has the cubic structure of
NaCl type (a = 0.42 nm).


### 3.3. Microhardness of the TiN Coating

It is well known that the
apparent microhardness of solid materials depends on the applied indentation test
load. This phenomenon is known as the indentation size effect (ISE). Figure6
shows the dependence of the microhardness for the cross-sectional and longitudinal-sectional
RPS TiN coating on the indentation loads. As the applied load ranges from 100
to 1000 g, the Vickers
microhardness drops from 1735.43 to 1125.27 HV and 1267.78 to 962.26 HV, respectively,
which is an evident phenomenon of ISE. When the test load is standard load of 100 g, the microhardness is 1189.36 HV.Figure 6
The microhardness of TiN coating longitudinal section and cross-section.


### 3.4. Volumetric Wear Loss of TiN Coating Specimens

In
order to study the wear resistance of the TiN coating, the test load changes
from 100~1470 N. For Al2O3 ceramic coating cannot resist
wear of high load, when the test load is higher than 500 N, the friction
coefficient increases notably, and friction noise is high. M2 high-speed steel
is selected as the comparison samples when test load is above 500 N. Figure 7(a)
shows the volumetric wear loss of the Al2O3 coating and
TiN coating at loads between 100 and 500 N and at 0.4 ms−1; Figure 7(b)
shows the volumetric wear loss of TiN coating and M2 high-speed steel at loads
between 500 and 1470 N. The volumetric wear loss of the Al2O3 coating, the TiN coating, and M2 steel increases with the loads increased. Under
low loads (100~500 N), the volumetric wear loss of the Al2O3 coating is bigger than that of the TiN coating. When the load changes from
490~980 N, the TiN coating and M2 high-speed steel are close in wear loss. With
the continuous increase of the loads, the TiN coating remained a mild increase
in wear loss, and reached 6.86 mm3 when the load is up to 1470 N, while the curve of the M2 steel's volumetric
wear loss versus loads presents distinctly moving up tend. The volumetric wear
loss of the TiN coating is approximately half under 1225 N and two-fifths of
that of the M2 high-speed steel under 1470 N, respectively. Figure 7 indicated
that the wear resistance of the TiN coating is better than Al2O3 coating and M2 high-speed steel.Wear volume loss of the TiN
coating, Al2O3 coating, and M2 steel plotted as a function of loads: (a)
load is smaller than 500 N, (b) load is bigger than 500 N.
(a)(b)


### 3.5. Friction Coefficient of TiN Coating and M2 High-Speed Steel

Figure8 presents the dependence of friction
coefficient on loads of both the TiN coating and the M2 steel under dry sliding
wear. It can be seen from the curves that with the increase of loads, the FC of
the TiN coating and AISI M2 steel generally changes in a small range of about 0.372~0.412
and 0.331~0.375, respectively. The two are close in friction coefficient when
load is 1470 N. Therefore, the antifriction property of TiN coating is close to
that of the AISI M2 steel at dry sliding wear under heavy loads.Figure 8
Friction coefficients of the TiN coating and M2 steel plotted as a function of loads.


### 3.6. Analyses and Discussion

Figure9
shows the surface layer morphologies of a few TiN drops sprayed on the surface
of glass and the TiN coating under same spraying condition. Obviously, the
topography of TiN drops on the glass surface presents the character of
high-temperature molten drop spattering. It is indicated that the coating is deposited
with molten drops. The
forming process of the TiN coating is that the molten reactants remain in liquid state in
the plasma jet, spatter, spread, deform, and freeze on the sample surface. Owing to the fact that the temperature gradient from plasma jet to the sample surface is so big that the molten
drops nucleate quickly, but nucleates have no time to grow, therefore the grain
of the TiN is refined.Surface morphologies of the TiN sprayed on  the glass and the TiN coating:
(a) the TiN on the glass, (b) the TiN coating.
(a)(b)Solid
Ti particles are melted in high temperature plasma jet and acutely reacted with
N2 ion gas and N2 gas in reactive chamber, while a great
deal of reaction heat is given out. The reaction belongs to combustion
synthesis [25]. As a result, the temperature of the liquid particle increases.
When the spraying state is steady, the velocity and temperature of the
particles in plasma jet are measured by Spray Watch. Figure 10 shows the
temperature and the velocity of the particles flying in plasma jet. It can be
seen that the velocity of particle decreases with its flying distance to
reactive chamber exit increasing, while the temperature of the particles remains steady, which is
different to normal plasma spray. For normal plasma spray, the temperature of
particle decreases with the distance increasing. The reaction heat reinforces
the heat of the plasma jet and has particles remained liquid before it reaches to metal
substrate. The measured average velocity and temperature of particles arriving
to substrate are 105 m/s
and 3360 K, respectively (see Figure 10). The molten point of TiN is 3223 K,
which is lower than 3360 K. During spraying, the distance (/) from metal substrate to spray gun exit is 100 mm. The time (t) in which a particle arrives to
substrate from reactive chamber is
(4)t=lv=100×10−3m105m/s=0.95×10−3s.The velocity and temperature of the particles in plasma jet when the RPS state is
steady, which are measured by Spray Watch: (a) the velocity of the particle,
(b) the temperature of the particle.
(a)(b)In so
short time, the TiN drops quench from 3360 K to
about 800 K and quickly freeze; the cooling rate is higher than general plasma
spraying. The Huge cooling velocity and undercooling increase the nucleation
rate. The reason for forming nanocrystal TiN coating is that combustion synthesis between
Ti and N2 and plasma spraying condition coacts. Combustion synthesis
reaction in plasma jet fabricates liquid TiN with high temperature. TiN liquid
drops, shock-cooling under the plasma spraying condition, solidify and form
nanocrystal TiN coatings on substrates.


## 3.1. Phases Constitute of the TiN Coating

Figure3 is XRD pattern of the TiN coating sprayed on the surface of
substrate and pure Ti powders. The coating is mainly composed of two phases,
TiN and small quantities of Ti3O, without pure Ti. Observing the
spectrum, five sharp TiN diffraction peaks appeared, and the diffraction planes
are (111), (200), (220), (311), and (222). Intensities of Ti3O are
very low, the diffraction planes of the Ti3O phase are (103), (113),
(116), and (223). According to the Scherrer formula, the grain size of
TiN coating was estimated, which was shown in Table 1. The grain size of
the TiN coating ranged 78~97 nm, which is consistent
with what is observed by TEM (see Figure 5). Pure Ti diffraction does not
appear in the coating, which proves that the Ti and N2 react fully
in RPS process. This is consistent with literature [9]. Ti3O is
metastable solid solution, which comes from Ti unsufficient oxidation at
temperature of 650~750°C and decomposes when temperature exceeds 750°C
[24]. The Ti3O in the coating is product of the
oxidation of TiN and exists in the coating for shock cooling under plasma
spraying condition.Table 1
Scherrer crystallite sizes of the TiN coatings.
Diffraction plane (hkl)(111)(200)(220)Dhkl (nm)92~9783~8878~83Surface XRD pattern of original Ti powders and the reactive plasma sprayed TiN coating.
(a)(b)


## 3.2. Microstructural Characterization of the TiN Coating

The SEM photograph of the TiN coating cross-section is shown in Figure4. The entire cross-sectional morphology of the
RPS TiN coating in Figure 4(a) indicates that its thickness is 420 μm, which is
about 100 times than that of TiN films prepared by CVD or PVD. The coating presents layer structure, which is tightly piled. The structure
with few pores should be attributed to gas which exists between the TiN liquid  drops and has no time to be released during coating forming. Small quantities of cracks appear in the multilayered structures of the coating (see Figure
4(b)). Further
study is needed to reduce the pores and cracks, and improve the structure of
the coating.The cross-sectional morphologies of the
RPS TiN coating, (b) is partial
enlarged drawing of (a).
(a)(b)TEM morphology of the
reactive plasma sprayed TiN coating: (a)
nanocrystallines, (b) the selected area
electron diffraction pattern.
(a)(b)TEM is an
indispensable analytical tool in the study of the microstructure of coatings. Figure5 indicates the TEM
morphology and the selected area electron diffraction (SAED) pattern of the
reactive plasma sprayed TiN coating. It can be seen from Figure 5(a) that the
most of grain size of the coating is smaller than 100 nm. The SAED pattern of the
coating is given in Figure 5(b). The grain size of 82 ± 10 nm was measured by linear
intercepting, which is approximate to the result obtained by calculation, and smaller
than that of the original Ti powders, which is 30~40 μm. The
diffraction rings of the SAED pattern in Figure 5(b) are continuous, dense, and
broader, which indicates the (111), (200), and (220) orientation. (311)
orientation is not clear. The diffraction rings show that the orientation of
TiN crystal grains is random. Weak diffraction spots distribute in diffraction
rings because the size of small quantity of crystal grains is bigger than
100 nm. XRD
and SAED examinations revealed that the TiN coating has the cubic structure of
NaCl type (a = 0.42 nm).


## 3.3. Microhardness of the TiN Coating

It is well known that the
apparent microhardness of solid materials depends on the applied indentation test
load. This phenomenon is known as the indentation size effect (ISE). Figure6
shows the dependence of the microhardness for the cross-sectional and longitudinal-sectional
RPS TiN coating on the indentation loads. As the applied load ranges from 100
to 1000 g, the Vickers
microhardness drops from 1735.43 to 1125.27 HV and 1267.78 to 962.26 HV, respectively,
which is an evident phenomenon of ISE. When the test load is standard load of 100 g, the microhardness is 1189.36 HV.Figure 6
The microhardness of TiN coating longitudinal section and cross-section.


## 3.4. Volumetric Wear Loss of TiN Coating Specimens

In
order to study the wear resistance of the TiN coating, the test load changes
from 100~1470 N. For Al2O3 ceramic coating cannot resist
wear of high load, when the test load is higher than 500 N, the friction
coefficient increases notably, and friction noise is high. M2 high-speed steel
is selected as the comparison samples when test load is above 500 N. Figure 7(a)
shows the volumetric wear loss of the Al2O3 coating and
TiN coating at loads between 100 and 500 N and at 0.4 ms−1; Figure 7(b)
shows the volumetric wear loss of TiN coating and M2 high-speed steel at loads
between 500 and 1470 N. The volumetric wear loss of the Al2O3 coating, the TiN coating, and M2 steel increases with the loads increased. Under
low loads (100~500 N), the volumetric wear loss of the Al2O3 coating is bigger than that of the TiN coating. When the load changes from
490~980 N, the TiN coating and M2 high-speed steel are close in wear loss. With
the continuous increase of the loads, the TiN coating remained a mild increase
in wear loss, and reached 6.86 mm3 when the load is up to 1470 N, while the curve of the M2 steel's volumetric
wear loss versus loads presents distinctly moving up tend. The volumetric wear
loss of the TiN coating is approximately half under 1225 N and two-fifths of
that of the M2 high-speed steel under 1470 N, respectively. Figure 7 indicated
that the wear resistance of the TiN coating is better than Al2O3 coating and M2 high-speed steel.Wear volume loss of the TiN
coating, Al2O3 coating, and M2 steel plotted as a function of loads: (a)
load is smaller than 500 N, (b) load is bigger than 500 N.
(a)(b)


## 3.5. Friction Coefficient of TiN Coating and M2 High-Speed Steel

Figure8 presents the dependence of friction
coefficient on loads of both the TiN coating and the M2 steel under dry sliding
wear. It can be seen from the curves that with the increase of loads, the FC of
the TiN coating and AISI M2 steel generally changes in a small range of about 0.372~0.412
and 0.331~0.375, respectively. The two are close in friction coefficient when
load is 1470 N. Therefore, the antifriction property of TiN coating is close to
that of the AISI M2 steel at dry sliding wear under heavy loads.Figure 8
Friction coefficients of the TiN coating and M2 steel plotted as a function of loads.


## 3.6. Analyses and Discussion

Figure9
shows the surface layer morphologies of a few TiN drops sprayed on the surface
of glass and the TiN coating under same spraying condition. Obviously, the
topography of TiN drops on the glass surface presents the character of
high-temperature molten drop spattering. It is indicated that the coating is deposited
with molten drops. The
forming process of the TiN coating is that the molten reactants remain in liquid state in
the plasma jet, spatter, spread, deform, and freeze on the sample surface. Owing to the fact that the temperature gradient from plasma jet to the sample surface is so big that the molten
drops nucleate quickly, but nucleates have no time to grow, therefore the grain
of the TiN is refined.Surface morphologies of the TiN sprayed on  the glass and the TiN coating:
(a) the TiN on the glass, (b) the TiN coating.
(a)(b)Solid
Ti particles are melted in high temperature plasma jet and acutely reacted with
N2 ion gas and N2 gas in reactive chamber, while a great
deal of reaction heat is given out. The reaction belongs to combustion
synthesis [25]. As a result, the temperature of the liquid particle increases.
When the spraying state is steady, the velocity and temperature of the
particles in plasma jet are measured by Spray Watch. Figure 10 shows the
temperature and the velocity of the particles flying in plasma jet. It can be
seen that the velocity of particle decreases with its flying distance to
reactive chamber exit increasing, while the temperature of the particles remains steady, which is
different to normal plasma spray. For normal plasma spray, the temperature of
particle decreases with the distance increasing. The reaction heat reinforces
the heat of the plasma jet and has particles remained liquid before it reaches to metal
substrate. The measured average velocity and temperature of particles arriving
to substrate are 105 m/s
and 3360 K, respectively (see Figure 10). The molten point of TiN is 3223 K,
which is lower than 3360 K. During spraying, the distance (/) from metal substrate to spray gun exit is 100 mm. The time (t) in which a particle arrives to
substrate from reactive chamber is
(4)t=lv=100×10−3m105m/s=0.95×10−3s.The velocity and temperature of the particles in plasma jet when the RPS state is
steady, which are measured by Spray Watch: (a) the velocity of the particle,
(b) the temperature of the particle.
(a)(b)In so
short time, the TiN drops quench from 3360 K to
about 800 K and quickly freeze; the cooling rate is higher than general plasma
spraying. The Huge cooling velocity and undercooling increase the nucleation
rate. The reason for forming nanocrystal TiN coating is that combustion synthesis between
Ti and N2 and plasma spraying condition coacts. Combustion synthesis
reaction in plasma jet fabricates liquid TiN with high temperature. TiN liquid
drops, shock-cooling under the plasma spraying condition, solidify and form
nanocrystal TiN coatings on substrates.


## 4. Conclusions

(1)
The TiN coating, prepared via RPS Ti powders using spraying gun with self-made reactive
chamber, is mainly composed of two phases, TiN and small
quantities of Ti3O. TiN coatings present typical layer structure. The
size of most of crystal grains in the TiN coating is smaller than 100 nm.(2)
The
forming mechanism of the nanostructure coating is that Ti powders are melted
and reacted with N2 in plasma jet and in the chamber. Heat given out by combustion synthesis reaction
increases the temperature of molten drops. The Huge cooling velocity and degree
of supercooling under plasma spraying condition have the drops quench, nucleate
quickly, and form nanostructure.(3)
The
highest hardness of the TiN coating is 1735.43 HV100 g; the wear
resistance of the coating is better than that of Al2O3 coating and M2 high-speed steel.



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*Source: 690951-2008-04-22.xml*