Structure of Al-targets Used for PVD Coating in Jewellery

Abstract

Aluminium, chromium, silver and other metal targets are often used for glass crystal coating in jewellery. The structure of targets strongly influences the quality of coating which leads to differences in their optical properties. The targets from two manufacturers were examined using scanning electron microscopy combined with EBSD with a goal to identify possible metallographic cause of defects arising on glass jewels.

Full text

Structure of Al-targets used for PVD coating in jewellery
Pavel KEJZLAR1, Zuzana ANDRSOVA1, Martin ŠVEC1
1Laboratory of Analytical Methods, Department of the Preparation and Analysis of Nanostructures, Institute for Nano-
materials, Advanced Technologies and Innovation, Technical Univertsity of Liberec, Studenstka 1402/2, 461 17 Liberec,
Czech Republic. E-mail: pavel.kejzlar@tul.cz, zuzana.andrsova@email.cz, martin.svec@tul.cz
Aluminium, chromium, silver and other metal targets are often used for glass crystal coating in jewellery. The
structure of targets strongly influences the quality of coating which leads to differences in their optical properties.
The targets from two manufacturers were examined using scanning electron microscopy combined with EBSD
with a goal to identify possible metallographic cause of defects arising on glass jewels.
Keywords: Aluminium, Structure, PVD, Thin layers, EBSD
1 Introduction
Plasma deposited metal coating has a wide variety of use, e.g. hard coating for machining tools, as oxidation and
corrosion protection, to decrease of coefficient of friction, in medicine, electronics, optics etc. [1]
An optical coating is one or more thin layers of material deposited on an optical component such as lenses, mirrors or
jewels, which alters the way in which the optic reflects and transmits light. The quality and performance of optical
components, and particularly their surfaces, are of crucial importance for the success of optical technologies in general.
In this field, plasma process represents an important tool for layer and surface property refinement. Plasma assisted
deposition of dense low-loss oxide coatings is being commercially introduced to the markets today. Optical layers can be
deposited by reactive or non-reactive magnetron sputtering of metal or ceramic targets or by evaporation of oxides or
pure metals in conjunction with an oxygen plasma generated by plasma source. [2-6]
High-reflection (HR) coating is usually based on the periodic layer system composed from two materials, one with a
high index, such as titanium dioxide (n=2.4) and second a low index material, such as silicon dioxide (n=1.49). This
periodic system significantly enhances the reflectivity of the surface in the certain wavelength range called band-stop,
whose width is determined by the ratio of the two used indices only, while the maximum reflectivity is increasing with a
number of layers in the stack. The thicknesses of the layers are generally quarter-wave, this time designed such that
reflected beams constructively interfere with one another to maximize reflection and minimize transmission. By
manipulating the exact thickness and composition of the layers, the reflection characteristics can be tuned to a particular
application, and may incorporate both high-reflective and anti-reflective wavelength regions. Common HR coatings can
achieve 99.9% reflectivity over a broad wavelength range. Naturally, HR coatings are affected by the incidence angle of
the light. When used away from normal incidence, the reflective range shifts to shorter wavelengths. [2-6]
In the jewellery these plasma deposited layers are used to achieve specific colours of different shaped glass beads.
Three samples are shown in Fig. 1. Their different colours were achieved by the variation of thicknesses and compositions
of TiOx and SiOx layers and a metal layer which serves here as the second surface metallic mirror. [7] An example of
TiOx/SiOx multi-layering in the crystal called Vitral Light is shown in Fig. 2. There is used multilayer SiOx/TiOx as HR
coating and Al-metallic mirror. The top layer is Cr, which serves as the metallic mirror’s mechanical and oxidation pro-
tection.
Fig. 1 Examples of decorative crystal. From left to right Lumin green; Bermuda blue and Vitral light

In the jewellery, the crucial point of the plasma coating process is a quality of metallic or ceramic targets, because
all imperfections and defects can reduce the reflection efficiency or can disperse light, causing different wavelengths
to refract at different angles.
The stimulus for this work was occurrence of optical defects on glass jewels. The goal given by our customer was
an identification of the possible reason through the metallographic evaluation of two aluminium targets supplied by
different manufacturers.
Fig. 2 HR SEM image of plasma deposited layers (the Vitral light crystal) taken in compositional contrast. On the
glass substrate (lead crystal glassware) there are deposited following layers: TiOx – SiOx – TiOx – SiOx – TiOx– Al –
Cr.
2 Results
In Fig. 3 there are two samples supplied by the customer. Under the No.1 there is an “old” target; the No. 2 marks the
second target from the new supplier. Samples for metallographic examination were taken from both materials using a
precision saw. Both samples were manually grinded on sandpaper GRIT #1200 and subsequently electrolytically polished.
Electrolytic polishing enables to achieve precise prepared sample without any surface deformation or contamination. The
structure was investigated using scanning electron microscope (SEM) and electron backscatter diffraction analysis
(EBSD).
Fig. 3 Aluminium targets from two suppliers. No.1 is previously used material; No. 2 comes from the new manufac-
turer.

SEM images taken in orientation contrast (Fig. 4) enable to see the fine (sub)grain structure. [8,9] Sample No.1
shows equiaxial grains with diameter of approx. 2 μm. Sample No.2 exhibits very fine inhomogeneous sub-grain structure.
In the case of sample No.2 there were also observed fine pores with diameter of approximately 1 μm (Fig. 5). A cubic
crystal structure is clearly visible in these pores.
Fig. 4 BSE SEM images taken in orientation contrast. On the left side there is sample No.1; right is No.2.
Fig. 5 Detailed SEM images of pores occurring in sample No.2.
Fig. 6 Band Contrast EBSD maps. Green colours marks pixels without solution (the SW was not able to index EBSD
pattern). Left sample No.1; right No.2.

Fig. 7 EBSD Euler maps and (sub-)grain boundaries. Blue borders correspond to crystal orientation difference from
2 to 5°; thin black from 5 to 10°; thick black lines show grain borders where difference in the crystal lattice orienta-
tion is > 10°. Left sample No.1; right No.2.
Fig. 8 Local missorientation EBSD maps showing local variations in crystal orientation. Red colour corresponds to
highly strained areas (plastic or elastic deformation; dislocations…). Green are non-indexed pixels. Left sample
No.1; right No.2.
Non-filtered Band Contrast EBSD images (Fig. 6) are mapping the quality of obtained EBSD patterns. Green pixels
indicate points where the solution for the EBSD pattern was not found (correspond to crystal lattice defects as
e.g. (sub-)grain boundaries, vacancies or dislocation arrays). This indicates significantly higher occurrence of defect in
the material No.2. Euler orientation maps and (sub-)grain boundaries are obvious from Fig. 7. In Fig. 8 there is used a
mapping of a local crystal lattice missorientation to reveal highly strained areas in the material [10]. All results obtained
from EBSD mapping clearly indicates presence of distinctively higher amount of crystal lattice defects and internal
residual stress in the sample No.2.
3 Conclusion
Based on metallographic observation it is possible to assume, that the observed optical defects of jewel’s coating are
related to the Al-target material provided by the new supplier. Compared to the previously used material (No.1), it shows
inhomogeneous sub-grain structure, occurrence of microporosity and considerably higher level of internal lattice defects
as dislocations and inner stress. It can be supposed, that this can negatively affect the PVD process stability leading to
appearance of optical defects.
Acknowledgement
The results of this project LO1201 were obtained with co-funding from the Ministry of Education, Youth and Sports
as part of targeted support from the "Národní program udržitelnosti I" programme.

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