The Effect of TaN Interlayer on the Performance of Pt-Ir Protective
Coatings in Glass Molding Process
Hsi-Hsin Chien1,a, Kung-Jeng Ma1,b, Chien-Huang Kuo1,c, Cheng-Bang Huo2,d,
Choung-Lii Chao2,e, and Ying-Tung Chen3,f
1
Institute of Engineering Science, Chung Hua University, No. 707, Sec. 2, Wu Fu Rd., Hsin Chu,
Taiwan 30067.
2
Department of Mechanical and Electro-Mechanical Engineering, Tam Kang University, No.151
Ying-Chuan Road, Tamsui, Taipei Hsien, Taiwan 251.
3
Department of Mechatronic, Energy and Aerospace Engineering, National Defense University
a
hhchien@chu.edu.tw, bma600229@ms17.hinet.net, cpots0382@yahoo.com.tw
694341594@s94.tku.edu.tw, eclchao@mail.tku.edu.tw, fd92522026@ntu.edu.tw
d
Keywords: TaN, Pt-Ir protective coating, diffusion, glass molding
Abstract. Glass molding process provides a great potential for mass production of precise glass
optical components at low cost. The key issue for achieving a low production cost is to extend the
service life of the expensive mold inserts. The precious metal based alloy is one of the coating
materials for the molds which provide excellent glass anti-sticking results. However, the
inter-diffusion between the WC/Co mold materials and precious metal coatings will deteriorate the
coatings which need to be resolved. It is essentially to deposit an interlayer as the diffusion barrier to
improve inter-diffusion problem. A thin layer of TaN was deposited on the WC/Co substrate as the
diffusion barrier using magnetron sputtering system, and followed by the deposition of Pt-Ir layer as
the protective layer. Low Tg Glass gobs (L-BAL 42) were placed on the coated substrate to
investigate inter-diffusion between the substrate and coating at high temperature. The surface
interaction between the glass gobs and protective coatings was also examined. The obtained TaN and
Pt-Ir multilayer had dense nano-crystalline structure. High temperature wetting tests showed that the
TaN film could effectively resist the cobalt and tungsten diffusion into the precious metal protective
layer and, as a result, minimized the possibility of oxidation and interaction between glass and
protective coating. The coated substrates remained good surface finish and the glass gobs stayed fully
transparent after 6 hours reaction test at 700℃.
Introduction
Glass molding process provides a great potential for mass production of precise glass optical
components at low cost [1, 2]. The key issue for achieving a low production cost is to extend the
service life of the expensive mold inserts. To apply protective coating on the mold surface is the most
popular method to improve the service life of the mold insert. The materials have been selected as the
protective coatings for molding glass coatings can be divided into four groups including [3-10]: (1)
single layer carbides, nitrides, oxides and borides such as TiN, BN, TiAlN, NiAlN, TiBC, TiBCN,
NiCrSiB and Al2O3 (2) nitrides or carbides based gradient and multi-layers, (3) amorphous carbon or
diamond-like carbon and (4) precious metal based alloys. Among these coating materials, the
precious metal based alloys are the most promising protective coating materials because they can
provide the best high temperature oxidation resistance and glass anti-sticking results. However, the
inter-diffusion between the sintered WC/Co mold materials and precious metal coatings will
deteriorate the coatings which need to be resolved. It is essentially to deposit an interlayer as the
diffusion barrier to improve inter-diffusion problem.
Ta and TaN barrier layers have been extensively used in copper interconnection system due to
their good diffusion barrier properties and relatively stable structure [11]. In this work, a thin layer of
TaN was deposited on the WC/Co substrate as the diffusion barrier using magnetron sputtering
system, and followed by the deposition of Pt-Ir film as the protective layer. The effect of TaN layer on
the inter-diffusion behaviour between the glass gob, precious metal layer and substrate was
investigated.
Experimental
Sintered WC-Co (8%) was selected as the substrate materials. A thin layer of TaN (~ 50 nm) was
deposited on the WC/Co substrate as the diffusion barrier using magnetron sputtering system, and
followed by the deposition of Pt-Ir film (~ 110 nm) as the protective layer. The Low Tg (502℃) Glass
gobs (L-BAL 42) were placed on the coated substrate to investigate inter-diffusion between the
substrate and coating at high temperature (600~700℃). The chemical composition of glass is as
follows: SiO2 (40~50％), BaO (20~30%), B2O3 (2~10％), Al2O3 (2~10％), ZnO (2~10％), Sb2O3
(0~2％). High temperature inter-diffusion experiments were performed in a quartz tube furnace with
IR heating system, in which N2 can be purged into the chamber.
A high resolution field emission SEM and EDX were used to examine microstructure and interface
reaction between coatings and glass materials. X-ray diffraction analysis (XRD) was carried out to
evaluate the interaction between layers. XRD measurements were performed with the grazing
incident angle (2.0°) attachment in a RIGAKU RINT-2000 diffractometer, using Cu K_ X-ray at 50
kV and 20 mA from 10° to 80° with a 0.05° step and 1°/min scanning rate. The elemental distribution
were analyzed by XPS using focused 400 μm in diameter monochromatic Al-Kαradiation at a pass
energy of 20 eV. The surface of the sample was eroded by Ar+ (1 keV) ion etching under a pressure of
10−7 Torr in order to determine the depth profiles of the Ta, O, Si Pt, Ir, W, Co and C.
The performance of TaN/Pt-Ir multilayer coated die was also investigated by hot pressing test at
580℃ by Toshiba molding machine. The surface integrities of molded lenses were examined by
optical microscope and high resolution SEM.
Results and discussions
The cross-sectional SEM photomicrographs showed that both Pt-Ir and TaN/Pt-Ir multilayer coatings
exhibited fully dense microstructure (Fig. 1). X-ray diffraction (XRD) spectrums show the TaN and
Pt-Ir layer with (110) and (111) preferred orientation respectively. Both Pt-Ir and TaN/Pt-Ir multilayer
coated substrates remained good surface finish and the glass gobs stayed fully transparent after 6 hours
reaction test at 600 and 650oC. Further increasing the temperature to 700 oC, leads to an obvious
difference in surface morphologies between Pt-Ir and TaN/Pt-Ir coated substrate. The Pt-Ir layer
coated substrate lost surface finish and the glass gob looked semi-transparent (Fig. 2(a)). It can be
observed that the surface became very rough and the W and Co oxides were detected on the surface
from SEM/EDX results (Fig. 2(b) and (c)). Fig. 3(a) showing a few glass sticking products appears on
the glass contacted surface. The reaction products containing elements of O, Al, and Zn were
identified from EDX results (Fig. 3(b)). It is suggested that the precious metal films, such as Pt-Ir alloy
is unable to completely inhibit W and Co out diffusion and to be oxidized at 700 °C. The formation of
W and Co oxides on the surface of Pt-Ir layer may trigger the glass sticking. The TaN/Pt-Ir multilayer
coated substrates remained much better surface finish and glass gob looked fully-transparent after 700
o
C/6hrs reaction test (Fig. 4(a)). A few small blisters can be observed on the glass contacted area, as
shown in Fig. 4(b). The blisters may be caused by the aggregated N2 gas which comes from the
decomposed TaN due to the oxidation reaction. However, there is no obvious W/ Co rich oxides to be
detected on the surface of Pt-Ir layer (Fig. 4(c)).
(a)
(b)
Fig. 1 Cross-sectional SEM images of (a) Pt-Ir and (b) TaN/Pt-Ir multilayer coated substrate
(a)
(b)
(c)
Fig. 2 (a) Appearance of glass gob (b) surface morphology of Pt-Ir coated substrates and (c) EDX
results of surface precipitates after 700℃/6hrs annealing test
(a)
(b)
Fig. 3 (a) Glass sticking products appears on the glass contacted surface and (b) EDX results
(a)
(b)
(c)
Fig. 4 (a) Appearance of glass gob (b) surface morphology of TaN/Pt-Ir coated substrate and (c) EDX
results after 700℃/6hrs annealing test
XRD spectra of the annealed samples are shown in Fig. 4. After 650℃/6hrs heat treatment, the
phase structure has no significant change compared to the as deposited samples. Increasing the
annealing temperature to 700℃ resulted in significant difference in phase structure between Pt-Ir and
TaN/Pt-Ir coated samples. In the case of Pt-Ir coated samples, the peak of TaN and Pt-Ir phases
become very sharp due to the grain growth effects. The obvious Co and WO3 phases were observed
which are in agreement with SEM/EDX results. However, there are no obvious Co or W oxides to be
detected in the TaN/Pt-Ir coated samples after 700℃/6hrs treatment. It shows that the TaN barrier is
able to against Co or W out diffusion and to avoid the formation of unwanted Co and W oxides on the
surface. Instead, Ta2O5 phases were observed from the X-ray diffraction pattern. Fortunately, these
phases were confined inside the Pt-Ir protective layer rather than immerging out to the surface, which
was proved from XPS results.
(a)
(b)
Fig. 4 XRD results of (a) Pt-Ir and (b) TaN/Pt-Ir coated substrate after 700℃/6hrs annealing
treatment
The electron binding energies of oxygen and tantalum atoms of TaN/Pt-Ir coated sample after
700℃ reaction test were detected by the X-ray photoelectron spectroscopy (XPS), as shown in Fig. 5.
The Ta spectra are very similar to the spectra of the reference TaOx materials. The broadening of the
peaks and the changes in the intensity are indications of Ta being in more than one valence state,
which is agreement with other published spectra [12]. The Ta-O bond at surface is attributed to the
reaction of Ta atoms and the residual O2 molecules in the annealing ambient. It is worth to note that
the formation of Ta-O bonds was only confined in the TaN layer. The lack of O 1s orbit in the Pt-Ir
layer indicates that the outer layer of Pt-Ir alloy is oxygen free. The binding energies of other elements
of W, and Co are agreed well with the values of pure element state. This indicates that no oxidation
reaction occurs on WC/Co substrate.
(a)
(b)
Fig. 5 XPS binding energy spectrum of element (a)Ta and (b) O for TaN/Pt-Ir coated substrate after
700℃/6hrs annealing treatment
Fig. 6 shows the elements depth profile of TaN/Pt-Ir coated substrate after 700C/6hrs annealing
treatment. It can be observed that there are no O, Ta, Co and W to be detected in the Pt-Ir layer. With
increasing the etching time, oxygen accompanied with Ta gradually increases their concentration.
The Pt-Ir alloy layer is thermodynamic stable phase which can avoid oxidation at 700℃. However,
the oxygen from the ambient diffused through the Pt-r layer and reacted with nanocrystalline TaN to
form Ta2O5 complex compounds. Near the substrate, the sintered WC grains were decomposed and
lead to the diffusion of W and C into the TaN layer. Fortunately, the diffusion of these elements was
effectively hindered by TaN, or complex Ta2O5 layer. It is believed that Ta has very high affinity to
oxygen and reacts with it and forms Ta2O5, which may increase the effectiveness of the diffusion
barrier by decorating the extended defects such as grain boundaries, thereby blocking the active paths
for grain boundary diffusion.
Fig. 6 Elements depth profile of TaN/Pt-Ir coated substrate after 700℃/6hrs annealing treatment.
The coated molding dies can extend the service life over 2000 shots in glass molding test at
molding temperature of 580℃. The surface of both molding dies and molded lenses successfully met
the requirements of the desired specifications.
Conclusion
The single Pt-Ir protective layer coated on WC/Co substrate is thermally stable at 650℃. Further
increasing the temperature to 700 ℃ leads to the out diffusion of W and Co from the substrate and
forming oxides on the surface of Pt-Ir film, which triggers the glass sticking. Applying a thin layer of
TaN between WC/Co substrate and Pt-Ir film is able to against Co or W out diffusion and the
formation of unwanted Co and W oxides. This ensures the TaN/Pt-Ir multilayer coated substrates
remained better surface finish and glass gob keep fully-transparent after 700 ℃/6hrs reaction test.
Acknowledgement
This work was supported by the National Science Council of R.O.C. under contract
NSC-96-2211-E-216-030. Thin film deposition was done at Shining Optics Ltd, Taiwan, R.O.C..
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