Annealing effects on platinum coating morphology
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Date
2009
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Abstract
The importance of phase transformation for the physical, chemical and mechanical
properties of coated systems has been of enormous scientific interest for a long time.
Due to a huge application field, the metallic coatings and platinum coated systems in
particular, has been studied extensively. Metallic platinum is important for many
industrial applications and the demand has increased considerably in the past decade,
particularly in applications such as jewellery and catalytic converters (which together
account for over 80 % of platinum consumption). In addition, the applications of
platinum and its alloys in fuel cells, gas sensors and MEMS are also in increased
demand in the last decade. On the other hand, aluminium also plays an important role
in electronics, microelectronics and many others applications.
It is known that platinum and platinum alloys readily form intermetallics.
Intermetallic compounds based on platinum group metals have almost unique
properties. Many of them exhibit outstanding thermodynamic stability in aqueous and
elevated temperature applications. Therefore, the platinum group metals base
intemetallics are of particular interest as next generation high temperature materials
due to their high melting temperatures and better oxidation or corrosive resistance
than refractory metals. High melting temperatures of these compounds make them
promising candidates for high-temperature structural materials. However, they are
hard and brittle which could be a significant limitation when the surface hardness and
structural integrity are of importance.
Besides the applications as protective coatings and electronics, the intermetallic
phases of the Pt-Al binary system can play a significant role in jewellery industry
where they can contribute towards increased surface hardness without compromising
the purity of platinum.
However, the research on platinum/aluminium coated systems conducted in this study
was concentrated onto morphological issues and thus, the effect of coating thickness,
the temperature and annealing time on coating morphology was studied. Thin
platinum coatings deposited on thick aluminium substrates by electron beam
deposition process were used as a model system to study the effect of annealing on Pt
coating morphology. Samples were annealed in a vacuum furnace at different
temperatures and time. Several complementary techniques such as a scanning electron
microscope (SEM) equipped with an energy dispersive spectroscopy (EDS), Atomic
force microscope (AFM), X-ray diffraction (XRD), Rutherford backscattering
spectroscopy (RBS), and Particle induced X-ray emission (PIXE) were used to
characterise the coated systems in terms of surface morphology, identification of
intermetallics and elemental distribution.
The scanning electron microscope studies revealed that the morphology of thin
platinum coatings is affected by both annealing parameters, the temperature and time.
Our study showed that rumpling and increased surface roughness, flaking and
thickening are the main degradation mechanisms in annealed coated systems. The
investigation of phase formation by XRD and RBS revealed the formation of the
following intermetallic phases: Al2Pt, Al6Pt, Al21Pt8, and Al21Pt6 when annealed at
different temperatures and times.
The change in coating morphology has been attributed to the formation of the
platinum/aluminium intermetallic phases.
Description
Thesis presented in fulfillment of the requirements for the degree of Master of Science
at the University of Zululand, 2009.
Keywords
Metallic platinum, Platinum coated systems, Aluminium