Property of Ni-Based Metallic Glass Coating
Produced by Gas Tunnel Type Plasma Spraying
Akira Kobayashi1, Toshio Kuroda1, Hisamichi Kimura2 and Akihisa Inoue2
1
Joining & Welding Research Institute, Osaka University, Osaka, Japan
2
Institute for Materials Research, Tohoku University, Sendai, Japan
Abstract: Metallic glass is one of excellent functional materials which have high strength and
corrosion resistance. Plasma spraying makes it possible to fabricate various metallic glass
composites. The gas tunnel type plasma spraying, which has high energy density and efficiency, is useful to obtain high quality metallic glass coatings. In this study, the Ni-based metallic glass coatings were produced by the gas tunnel type plasma spraying, and their microstructure and mechanical property was investigated. The Ni-based metallic glass coatings of
200 m in thickness were formed densely with Vickers hardness of about Hv=700 at plasma
current of I=300A.
Keywords: Ni-based metallic glass, Plasma sprayed coating, Gas tunnel type plasma spraying,
Microstructure, X- ray diffractometer (XRD), Vickers hardness.
1. Introduction
New functional materials act as the keystone for the
development of advanced technology in most of the industries. Metallic glass material is one of the most attractive advanced materials, as it has excellent physical and
chemical functions such as high strength and corrosion
resistance [1, 2]. Therefore, many researchers have
focused on it and conducted various developmental
research works. However, as the metallic glass material
is expensive, the application for small size parts has been
carried out only in some industrial fields. In order to widen the industrial application fields, a composite material
is preferred for the cost performance. In the coating
processes of metallic glass with the conventional deposition techniques such as plasma sputtering, it is difficult to
obtain thick coatings because of their low deposition rate.
Due to ease of operation thermal spraying is one of
advantageous methods to produce metallic glass
composites.
The gas tunnel plasma spraying is the most superior
technology for obtaining high quality ceramic coating [3,
4] and synthesizing new functional materials [5], because
the plasma jet has a high-speed and a high-energy density under controlled operating conditions [6]. The performance of gas tunnel type plasma jets has been described
in previous publications [7, 8]. Because of its superior
advantages against other conventional plasma jets [9],
this method has great potential for a range of applications
in thermal processing. Indeed, high performance materials, high quality ceramic coatings have been fabricated
by the gas tunnel type plasma spraying method [10-13].
By gas tunnel plasma spraying, the metallic glass
coatings can be formed on the stainless steel substrate by
using Fe-based or Zr based metallic glass powder
[14-16].
In this study, the Ni-based metallic glass coating was
formed by the gas tunnel plasma spraying using Ni-based
metallic glass powder. The microstructure and surface
morphology of the metallic glass coatings were examined using SEM and XRD and the Vickers hardness
was measured on the cross section of the coating.
2. Method
Ni-based metallic glass powder (Ni60Nb20Zr20) was
used in this study. This powder was atmospherically
plasma sprayed (APS) on a flat 304 stainless-steel substrate by a gas tunnel type plasma spraying torch as
shown in Fig.1. Ni-based metallic glass powder was externally fed into the plasma flame from the exit of the gas
divertor nozzle, in order to melt the metallic glass powder effectively. The powder is supplied from the outside of
the gas divertor nozzle of the torch in this study.
Experiments were carried out under selected spraying
conditions, which were selected from the basis of earlier
experimental results and its correlation with torch operating parameters. Meanwhile, the present experiments
was carried out in open atmosphere, hence the oxidation
of in-flight metallic glass powder was inevitable during
spraying because of the unique nature of plasma spraying.
However, it could be controlled by selecting the appropriate spraying conditions.
Fig.2 SEM micrographs of the Ni based metal
glass powder.
Fig. 1 Gas tunnel type plasma spraying used in this study
(L=spraying distance).
Table 1 shows the spraying conditions employed in
the present investigation. The plasma torch is operated at
a power level of 10-20kW and the arc current I of
150-350 A. The plasma jet is generated with the argon
gas flow rate of 200 l/min, at a spray distance of 40 mm.
The powder feed rate is about 12 g/min. The stainless
steel substrate is traversed 16 times during the spraying
time of 24 s. The substrate with dimension of 50 x50 x2
mm3 is grit-blasted with alumina grit on one side.
Microstructure of the Ni-based metallic glass coatings
was observed by using a scanning electron microscope
(SEM). The surface morphology of the feedstock powder
and the metallic glass coating cross-section was also
examined by a scanning electron microscope. Phase constituents of metallic glass coating were identified by using a X-ray diffractometer (XRD) system with Cu-K
radiation source at voltage of 40 kV and current of 40
mA. Vickers microhardness measurement was made on
the coating cross section by using a load of 100g. Indentation parameters were set as 20s loading time.
Table 1 Spraying conditions.
Arc current
Voltage
Spraying distance
Working gas flow rate (Ar)
Powder feed gas flow rate(Ar)
Powder feed rate
Traverse number
Spraying time
150-350A
40-50V
40 mm
200 l/min
10 l/min
12 g/min
16 times
24 s
Figure 2 shows the SEM micrographs of the Ni
based metal glass powder. The size of powder was
10-25m, and it was spherical in nature with average
diameter of 18m. The above kinds of spherical nature
feedstock powders have good flow quality and uniform
melting scenario during the in-flight in thermal plasma
jet.
3. Results and Discussion
3.1 Microstructure of the metallic glass coating
Figure 3 shows the SEM micrographs of the
cross-section of Ni-based metallic glass coatings sprayed
at different plasma spraying current of 100A and 300 A,
at L=40mm. The coating was sprayed by 16-times traversed substrate during the spraying time of 24 sec.
There are some large pores in the Ni-based metallic
glass coating at a low current of 100A in Fig.3(a). It is
estimated that the adhesion state is not good in this condition. The coating thickness increased with increase in
the plasma current and it was more than 200 m at 300A
which is shown in Fig.3(b). In this case, the coating was
much denser on the cross section of the surface side and
it was rather less porous than that at 100A as shown in
Fig.3(a). The bonding condition between the coating and
the substrate seemed to be good.
The XRD pattern from the surface of the metallic
glass coating at 300A is shown in Fig.4(a). For comparison, the XRD pattern of the Ni-based metallic glass
powder used in this study is shown in Fig.4(b). The
broad amorphous phase (Phase center is about 42 degree)
was observed in this pattern. But the XRD pattern has
some crystalline peaks corresponding to some compounds of Ni, Nb, Zr, etc. Ni crystalline peak was recognized as shown in this figure.
On the other hand, these crystalline peaks near 40 degree were suppressed and disappeared in the metallic
glass coating at 300A. The broad amorphous phase was
clearly observed in the pattern which is shown in
Fig.4(a). There were some new peaks related to Ni, Nb,
Zr, etc. at different diffraction angles in the XRD pattern
of the metallic glass coating. Two crystalline peaks near
30 and 50 degree were recognized as the peaks from zirconium (ZrO2) oxide phase.
When the metallic glass powder is injected into the
plasma jet, particles in the high temperature region are
heated and may be simultaneously decomposed. So,
there is large possibility of crystalline peaks, and peaks
of any other oxidized materials, but there were no peaks
from Ni, Nb, Zr detected in the XRD spectra.
Fig.3 SEM micrographs of the cross-section of the Ni
based metallic glass coating sprayed at 100A and 300A,
on the 16 times traversed substrate.
Fig.4 XRD patterns of the Ni-based metallic glass coating sprayed at 300 A plasma current on stainless-steel
substrate (a) and Ni-based metallic glass powder (b).
3.2 Vickers hardness of the metallic glass coating
Figure 5 shows the dependence of Vickers hardness
of the metallic glass coating on the plasma current. The
Vickers hardness on the cross section of the metallic
glass coating was increased from Hv100 = 350 to 850 with
increase in the plasma current from I = 100A to 350A.
The Vickers hardness was Hv100 =350-500 at low current
of 100-150A. This value is lower than that of bulk metallic glass (about Hv100= 650). This means that high porosity leads to lower hardness of the sprayed coating as
shown in Fig.3(a).On the contrary, the Vickers hardness
of metallic glass coating is more than Hv= 800 at higher
current of 350A. It is rather a higher value compared to
the coating at 300A, in which case, the Vickers hardness
of metallic glass coatings is around Hv100= 700. This
hardness value was similar to that of the bulk metallic
glass.
Fig.5 Dependence of Vickers hardness of Ni-based
metallic glass coating on the plasma current.
It is estimated that the sprayed particle was melted
and heated more than the melting point and some part of
the particle was decomposed or oxidized after spraying
at higher current of 350A. The hardness of oxides or intermetallic compounds is generally high.
Thus, Ni-based metallic glass coatings were formed
by the gas tunnel type plasma spraying. More amorphous
phases would be realized by choosing the appropriate
spraying conditions. The results indicate the possibility
to form high functional metallic glass coatings, which
will be useful for various industrial applications.
Conclusions
The Ni-based metallic glass coatings were produced
by gas tunnel type plasma spraying and the following
results were obtained.
(1) The Ni based metallic glass sprayed coating with
thickness of more than 200 m was obtained at 300A.
The coating thickness depends on the spraying parameters.
(2) XRD study confirms the presence of amorphous
phases in the sprayed coatings and Ni-based metallic
glass particles are decomposed and the oxidation occurs during deposition.
(3) The Vickers hardness of metallic glass coating at
300A was around Hv100= 700 in the cross section of
coating.
Acknowledgment
The authors would like to thank Mr. Takuya Ueda,
and other coworkers for their valuable and helpful
discussions.
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