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Title Super-hard coatings with materials based on ternary AlMgB
matrices
Author(s) Tsang, Mei Ka (曾美嘉)
Citation
Tsang, M. K. (2012). Super-hard coatings with materials based on ternary AlMgB matrices (Outstanding Academic Papers by Students (OAPS)). Retrieved from City University of Hong Kong, CityU Institutional Repository.
Issue Date 2012
URL http://hdl.handle.net/2031/6843
Rights This work is protected by copyright. Reproduction or distribution of the work in any format is prohibited without written permission of the copyright owner. Access is unrestricted.
CITY UNIVERSITY OF HONG KONG
DEPARTMENT OF
PHYSICS AND MATERIALS SCIENCE
BACHELOR OF ENGINEERING (HONS) IN MATERIALS ENGINEERING 2011-
2012
DISSERTATION
Super-hard coatings with materials based on ternary AlMgB matrices
by
Tsang Mei Ka
March 2012
Super-hard coatings with materials based on ternary AlMgB matrices
By
Tsang Mei Ka
Submitted in partial fulfilment of the
requirements for the degree of
BACHELOR OF ENGINEERING (HONS)
IN
MATERIALS ENGINEERING
from
City University of Hong Kong
March 2012
Project Supervisor : Prof. Igor Bello
i
Acknowledgements
To start with, I would like to express my deepest gratitude to my project supervisor,
Prof. Igor Bello, for his continuous and kindly support, encouragement and guidance
throughout the whole project. I am greatly touched by his heart of teaching. Prof. Bello is
always there to talk with when needed. He provides a lot of valuable advices in this
dissertation and shares his personal experiences to persuade positive thinking when facing
difficulties. I am really glad to carry out the project with and learn from my dearest Igor.
Secondly, I would like to thank Prof. Lawrence Wu for his further instructions. His
questions inspire me to think about my project in more details. His professional and precious
suggestions contributed to the clarity of this project.
Last but not least, I would like to thank to the research students, Mr. Yan Ce and Miss
Qian Jin Cheng, who have given me hands in experimental work. They have shared their
research experience with me. Mr. Yan Ce has helped and guided me in characterization of
thin films. He has shared his knowledge related to the topic of the project and given
suggestions in writing the thesis. Miss Qian Jin Cheng has demonstrated operation of a
sputtering system and also assisted me in XPS characterization.
ii
Table of Contents
Acknowledgements i
Table of Contents ii
List of Figures v
List of Tables vii
Abstract viii
Page
1 Introduction and Objectives 1
2 Literature Review 4
2.1 Super-hard material and their Interrelated Properties 4
2.2 Ternary AlMgB Materials and their Properties 7
2.3 Synthesis of AlMgB Films on Silicon Substrates 10
2.3.1 Sputtering system/ Sputter deposition 10
2.3.1.1 Advantages and disadvantages of sputter
deposition
13
2.4 Characterization Techniques used in Analysis of AlMgB Films 13
2.5 Applications of the AlMgB films 16
3 Experimental Methodology 17
3.1 Deposition of Ternary AlMgB Composite Films 17
3.1.1 Cleaning the Substrates 17
3.1.2 Synthesis of Ternary AlMgB Composite Films on
Silicon Substrates
17
3.2 Investigated Properties of AlMgB Films 20
4 Results and Discussion 22
4.1 Effect of Power Density Applied to AlMg Target on the
Composition of AlMgB films
22
4.2 Morphology of Deposited Ternary AlMgB Composite Films 23
4.2.1 Morphological Studies of AlMgB Films by Scanning
Electron Microscopy
23
iii
4.2.2 Studying Morphology and Roughness of the Deposited AlMgB
films
25
4.3 Compositional Analysis of AlMgB Thin Films 28
4.4 Surface Chemical Analysis of AlMgB films 29
4.5 Structural Analysis of the Deposited AlMgB films 32
4.6 Additional Structural Analysis of Boron Rich Al0.17Mg0.1B Thin Films
34
4.7 Study of Mechanical Properties of AlMgB films 35
5 Conclusion 38
6 Reference 39
iv
List of Figures
Page
Figure 2-1 The diamond structure. 6
Figure 2-2 Structures of boron nitride. 6
Figure 2-3 Structure of AlMgB in which Blue atom is aluminium, green
atom is magnesium and red atom is boron.
7
Figure 2-4 A BSE SEM micrograph of (a) baseline AlMgB14 material and
(b) AlMgB14 + 30%TiB2 material. In the image, the region with
high concentration of TiB2 was labelled as ‘A’ whereas the dark
region was referred to the composition of AlMgB14.
8
Figure 2-5 A system of sputter deposition. 11
Figure 2-6 Anti-reflective camera coatings. 11
Figure 3-1 The sputter system equipped with four magnetrons used in
deposition of AlMgB films.
18
Figure 3-2 (a) A configuration and (b) the assembly of a rectangular
planar magnetron with a wear track.
19
Figure 3-3 A configuration of sputtering system with four rectangular
planar magnetrons and planetary motion in a closed magnetic
field configuration.
19
Figure 3-4 A demonstration of the circular magnetron in operation. 19
Figure 4-1 Cross-sectional SEM micrograph of a boron rich Al0.17Mg0.1B
film.
23
Figure 4-2 Cross-sectional SEM micrograph of a metal rich Al0.55Mg0.21B
film. The experimental SEM image from Ref. [18] is also
shown for comparison.
24
Figure 4-3 Plane-view AFM images of the surface morphology of the
AlMgB thin films prepared at different power densities: a) 0.2
W/cm2; b) 0.5 W/cm2 and c) 1.0 W/cm2.
25
Figure 4-4 Variation of root mean square roughness of the AlMgB
composite films as a function of the power density applied to
the AlMg target.
27
Figure 4-5 EDX spectrum collected from Al0.17Mg0.1B thin film 28
v
fabricated at sputtering densities of 0.2 W/cm2.
Figure 4-6 (a) B 1s, (c) Al 2p and (e) Mg 2p XPS spectrum collected
before sputtering; (b) B 1s, (d) Al 2p and (f) Mg 2p XPS
spectrum collected after sputtering.
29
Figure 4-7 The X-ray diffraction patterns accumulated from composite
(a) metal rich Al1.38Mg0.64B and (b) born rich Al0.17Mg0.1B
thin film.
33
Figure 4-8 The IR transmittance spectrum of the boron rich Al0.17Mg0.1B
thin film Correlate.
34
Figure 4-9 The variation in hardness of AlxMgyB thin films as a function
of boron content in the films.
35
Figure 4-10 The loading and unloading curves of boron rich (a)
Al0.17Mg0.1B and metal rich (b) Al1.38Mg0.64B thin films as a
function of displacement.
36
vi
List of Tables
Page
Table 1 Hardness and fracture toughness of additive materials and
some reference materials
2
Table 2 Variation in selected mechanical properties of AlMgB14
composites with different proportions of AlN, TiC and TiB2
2
Table 3 Density, Hardness, bulk and Shear Moduli of Several Hard
Materials
6
Table 4 Density and hardness of materials 8
Table 5 The super hard thin films with materials based on ternary
AlxMgyB with different compositions synthesized by two
boron targets
23
vii
Abstract
Mechanical engineering, automotive, aeronautic and mining industries, as well as
military and space industry require hard and wear resistance materials. However military and
space industry often also needs light-weight materials in combination with the above wearable
and hardness properties.
Diamond is the hardest material known but some properties of the second hardest
material, cubic boron nitride (cBN), surpass those of diamond. Although diamond is superior
to cBN in hardness, its chemical stability is lower than that of cBN. Diamond is dissolved in
molten steel and ferrous materials, whereas cBN is inert. The two parameters as well as
friction coefficients are the reasons for using diamond and cBN as materials of choice in
commercial applications. Since diamond is unsuitable for machining steels and ferrous
materials and cBN coatings have not been mastered on commercial levels, new superhard
coatings are needed. The single elements and binary composite films have been investigated
thoroughly, but less effort has been given to ternary and quaternary composites that might
reach the hardness of superhard materials. In consistence with the properties for advanced
technological applications novel superhard materials in forms of films have been synthesized
and studied.
In this project, new materials based on aluminum, magnesium and boron were prepared
and their mechanical properties were investigated in correlation with their structural
composition. It is noted that composite films mentioned are related to the nano-scale since
there are nanocrystallites inside the film with amorphous structure, so it means
nanocomposite films. Composite films of aluminum magnesium boride (AlMgB) with ultra-
hard properties could be suitable alternatives to existing lower performance materials that are
currently used in mechanical applications such as cutting tools, or even they could be used as
more advanced materials in military and space applications. With suitable structures and
chemical compositions, these novel materials based on ternary compounds of Al, Mg and B
are remarkable light and counted in the group of superhard materials. Possibly by adding
optimum amount and some additives (TiB2) into the AlMgB composite, the mechanical
properties of the composite films are enhanced. The hardness can be effectively increased to
45 - 50 GPa, approaching the hardness of the cBN. However, the addition of AlN and TiC
would reduce film hardness. In addition, the AlMgB films are highly resistive to abrasion and
viii
have low coefficients of sliding friction. Therefore, the AlMgB materials approach the
mechanical properties of cBN and diamond.
The ternary AlMgB thin films were coated on silicon substrates by a sputter deposition
technique using three unbalanced rectangular magnetrons (one AlMg and two boron targets).
The magnetrons were installed on a deposition chamber in a closed magnetic field
configuration. The deposition parameters and power supplied to individual targets are
adjusted to optimize the structure and chemical composition of the film and achieve the
extreme level of mechanical properties of the AlMgB films.
The surface morphology and roughness were studied by atomic force microscopy and
scanning electron microscopy, structural properties were analysed using x-ray diffraction
(XRD) and fourier transform infrared (FTIR) spectroscopy, while compositional analysis and
chemical states were investigated by energy dispersive x-ray (EDX) spectroscopy and x-ray
photoelectron spectroscopy (XPS), respectively. Hardness and elastic modulus were evaluated
using a nanoindentation technique.
Analysis shows that the root mean square (rms) roughness varies between 1.0 and 18 nm
when the AlMg target at power density alters from 0.2 to 1.0 W/cm2 and the boron target is
maintained at 2 W/cm2. The chemical compositional analysis presents that aluminum,
magnesium and boron are the dominant elements in the film. The coating also comprises
some trace elements of oxygen, carbon and argon.
The chemical composition is changed when different power densities to the AlMg targets
are applied. By changing the power densities, the metal rich film Al1.38Mg0.64B with only
about 33 at.% boron content was prepared. The films demonstrate the hardness value of about
30 GPa. The obtained hardness is already impressive when it is compared to the hard
hydrogenated diamond-like carbon (DLC) films synthesized by plasma enhanced chemical
vapor deposition (CVD) showing hardness of 24 – 28 GPa. On the other hand, boron rich
films exhibit similar hardness (~30 GPa). The XRD pattern suggests that the prepared AlMgB
films confine nanocrystals in amorphous matrices. The nanocrystals are identified to be B12
icosahedra forming network in amorphous AlMgB matrices that considerably contributes to
the hardness enhancement of the films.
1
1 Introduction and Objectives
Hard and superhard materials are used in diverse industrial fields penetrating
mechanical engineering, automotive and aeronautic industry, as well as mining, military, and
space industry and others. The global market in superhard materials is forecasted to reach
US$16.5 billions by the 2015.1 This demand for hard and superhard materials will continue to
grow because of not only materials hardness but also other extreme properties that these
materials possess. Among them, high thermal conductivity or electronic properties including
resistance against the hard radiation and properties permitting operation of electronic devices
in harsh environments should be pointed out. Accordingly research and development in hard
and ultra-hard materials is very attractive topic in both short- and long-term vision.
The superhard materials being mostly used are diamond and cBN. The superhard
materials are bulk matters with hardness greater than 40 GPa. They are used in many fields of
production such as building industry, mechanical engineering, well-boring, medicine,
electronics and food industry.2 While diamond and cBN have extreme properties, they are not
absolutely universal. For example, the best material for machining steels is cBN, but cBN
deposited in the form of films suffers from extremely large internal stress, adhesion and
delamination problems. The best substitute for cBN seems to be the cost effective AlMgB
composite. Interestingly, coatings with single element materials and binary materials were rather
well investigated, but a little effort has been devoted to the development and research of
materials based on multi-element structures such as composite ternary borides of aluminum
and magnesium. These intermetallic composites are hard and very light. They are materials
which have high potential to develop to coatings with properties like those of cBN. Taking
into account drawbacks of highly stressed cBN films with delamination problems, AlMgB
coatings with some additives of titanium or silicon might be one of the best alternatives or
substitutes for cBN films, used whenever the coatings are in contact with steel and ferrous
materials.
For first time the intermetallic compound AlMgB was studied 30 years before,3 while
the unit cell AlMgB14 was discovered much later, and properties of AlMgB composites started
to be investigated by a group of the Amen University only recently. They investigated
mechanical, optical, electronic, and mechanical properties of these composite materials. Cook
2
et al.4 reported that the processing methods to prepare AlMgB14 composites with optimized
properties in which mechanical alloying (MA) and followed by a total of twenty seven
vacuum hot pressings were employed to prepare and consolidate the materials. It was
revealed that varying the content of additives such as AlN, TiC and TiB2 in AlMgB14 matrices
can considerably affect the mechanical properties of AlMgB14. For example, the hardness is
significantly increased to 45-50 GPa with 30 wt.% of TiB2 additive, 4 despite the fact that the
hardness of TiB2 additive is 28-35 GPa as listed in Table 1 by Tian et al..5 Ahmed et al.3
reported that the maximum hardness and toughness are attained at ~60 wt.% of TiB2 by
referring to Table 2. The sample materials were prepared by using second phase additions.
However the vast research work is referred to mechanical alloying to acquire powders that are
then sintered to bulk materials. Only few initial research works are related to AlMgB
composite films, which are prepared by pulse laser deposition (PLD) technique using
composite alloy targets.
Table 1: Hardness and fracture toughness of additive materials and some reference materials.
Table 2: Variation in selected mechanical properties of AlMgB14 composites with different
proportions of AlN, TiC and TiB2
3
Based on the current status of thin AlMgB film development and potential industrial
needs, the principal objective is synthesis of hard and adherent AlMgB thin films with properties
suitable for mechanical applications. The objective is planned to achieve via following research
tasks:
1) Synthesis of hard and adherent AlMgB composite films on silicon substrates using a
magnetron sputter-deposition method.
2) Development of a flexible technique to vary the composition of the deposited films
and their properties. For flexibility, approach of deposition from individual sputtering targets is
proposed to undertake.
3) Investigation of mechanical properties of AlMgB films in correlation with structural
properties.
4) Formation of base of knowledge needed in the next step of incorporation additives
into AlMgB matrices.
The individual tasks are achieved by synthesis of AlMgB films using multiple sputtering
targets that provide flexibility in variation of film composition by altering power applied to
individual sputtering targets. The deposition is preceded with substrate preparation for coating to
provide high adherence of the films to the substrates.
The task referring to the investigation of films properties is tackled by a variety of
analytical techniques evaluating morphological, structural and chemical properties of the
synthesized films. At these analyses Scanning Electron Microscopy (SEM), Atomic Force
Microscopy (AFM), Energy Dispersive Spectroscopy (EDX), X-ray Photoelectron Spectroscopy
(XPS), X-ray diffraction (XRD), and Fourier transform infrared Spectroscopy (FTIR) are
exploited, while mechanical properties are investigated with a nanoindentation technique.
4
2 Literature review
2.1 Superhard Material and their Interrelated Properties
A material is classified as superhard when its hardness is greater than 40GPa. Most
superhard materials comprise simple and crystal structures with high-symmetry. They are solids
usually characteristic with high atomic density and bonding being highly covalent, high electron
density, small compressibility and often with high thermal conductivity. The superhard materials
can be used in cutting tools for machining of other materials, removing rock sediments and
mining.3,6 They can also be employed for abrasion and polishing metals or other surfaces as well
as protection and reliable operation of devices. For instance, ultrathin films of tetrahedral
amorphous carbon chemically and mechanically protect data stored in magnetic layers of hard
discs, or these films preserve mechanical and chemical integrity of reading and writing heads in
computers. These coatings can also be used for heat dissipation in power electronic devices,
semiconductor lasers or microprocessors with high density of active elements in a single chip.
Hardness refers to the incompressibility, elasticity and the resistance to deformation of a
material. Furthermore, hardness is a function of both the strength of the interatomic bonding and
of the rigidity of the lattice framework.4 A superhard material has high bulk modulus, high shear
modulus and does not deform plastically. The bulk modulus is directly related to the
incompressibility of the material. Higher bulk modulus can be obtained with possessing higher
electron density as the repulsive forces within the structure are greater. It implies that the
material with high modulus are likely less compressible. Moreover, the shear modulus is the
ability to resist the change in shape while keeping constant volume. A material with highly
directional bonds is said to have higher shear modulus. Furthermore, a material with shorter
covalent bonds, plastic deformation is less easy to occur than the materials with delocalized and
long bonds. If a material is covalently bonded, it gives higher bond-bending force constants.
Therefore, it can be considered as the material containing superhard structures.6
There are two groups of superhard materials, namely intrinsic and extrinsic compound.
The conventional superhard materials including diamond, cubic boron nitride (cBN) and ternary
compounds are classified as intrinsic compounds. The crystal structure of the conventional
superhard materials is based on the highly directional sp3 bonds. For those compounds like
5
nanocrystalline diamond with the high hardness and other properties being affected more by the
microstructure than the composition are classified as extrinsic. 6
Conventionally, there are two basic, well-known and ultra-hard materials. These
materials are diamond and cBN. Diamond is a bulk material and contains single element of
carbon atoms only. Diamond is one of many carbon allotropes. It exists in nature and it can be
prepared by different methods of synthesis. In diamond, carbon atoms are arranged in a face-
centered cubic lattice structure with strong covalent sp3 tetrahedral configuration7, as shown in
Figure 2-1. Due to the bonding structure and atomic density as well as strong covalent bonds,
diamond is far superior in hardness when compared with other materials. Depending on the
crystallographic orientation its hardness can be as high as 70 GPa. The diamond thermal
conductivity is also far superior property with respect to any other material. The thermal
conductivity of diamond is 20 Wcm-1K-1 which is five-times greater than that of copper.
However despite high thermal conductivity, diamond has very high electric resistivity and the
highest breakdown voltage of all materials (107 V/cm).
Alike carbon, boron nitride (BN) has several allotropes8 (Fig.2-2) that are similar to
carbon structures. For example, graphite is very similar to hexagonal boron nitride (hBN), and
therefore it is called also white graphite. Difference between these two materials is in the fact
that graphite is electrically conductive, but hBN is electrically insulating. At BN bonding a
valence electron from a boron atom is transferred to nitrogen which introduces iconicity that
is responsible for high electric resistivity of hBN.
Cubic BN is also an allotrope of BN and it is similar to diamond4 in its structure as
well as in its physical properties. Although cBN has not been found in nature, it can be
prepared synthetically by high pressure and high temperature methods as well as low pressure
methods assisted with energetic ions. However the synthesis of BN is much more challenging
than that of diamond. Cubic BN is the second hardest material (~50GPa) and materials with
the second highest thermal conductivity (13Wcm-1s-1) next to diamond. However the chemical
stability and graphitization temperatures of cBN are greater than those of diamond. Owing to
high chemical stability and preservation of mechanical and other properties at high
temperatures, cBN is suitable for tools and devices used in harsh environment. Table 3 shows
some mechanical properties of several hard materials as reported by Cook et al.4
Boron nitride has been prepared also as one dimensional structure of nanotubes which
are again structurally very similar to carbon nanotube.4
6
Figure 2-1 The diamond structure.
Figure 2-2 Crystalline structures of boron nitride (note -BN is regarded as cBN).
Table 3: Density, Hardness, bulk and shear moduli of several hard materials
7
One of the methods to measure the hardness of materials is a nanoindentation technique
which employs a diamond indenter. The bulk modulus test uses an indenter tool too to form a
permanent deformation in a material. The size of the deformation depends on the material
resistance to the volume compression made by the tool.
2.2 Ternary AlMgB Materials and their Properties
Intermetallic compound AlMgB materials composed of some nano-sized AlMgB14
crystals confining atoms in a body centered orthorhombic lattice structure. The structural
arrangement indicates that such an AlMgB matter is expected to be classified in the category of
superhard materials. This material has a complex and low-symmetry crystal structures. In the
unit cell of AlMgB14, it has 64 atoms in which four icosahedra B12 whereas the others eight
boron atoms are bonded to the inter-icosahedra Mg and Al atoms. Figure 2-3 shows the crystal
structure of aluminium magnesium boride along the crystal axis. 9 The icosahedra B12 are
arranged in the layers with distortion and packed closely.5
Figure 2-3 Structure of AlMgB in which blue atom is aluminium, green atom is magnesium
and red atom is boron.
The complex ternary borides of magnesium and aluminium show good mechanical
properties. They are ultra-hard and light and in terms of mechanical properties they belong to the
same group of materials as diamond and cBN. With suitable amount of additives, the hardness
8
can further be increased to the value as high as 46 GPa, as illustrated in Table 4. Cook et al4
report AlMgB materials with 30%TiB2 additives in a multi-phase microstructure yield the
highest hardness from all the AlMgB composites. Figure 2-4 shows the SEM backscattered
electron images of (a) baseline AlMgB14 material and (b) AlMgB14 with 30% TiB2 additive in an
AlMgB matrix indicating many phases in the structure.4
Table 4: Density and hardness of selected materials5
Density
(g/cm3)
Hardness
(GPa)
C (diamond) 3.52 70
BN (cubic) 3.48 45-50
AlMgB14 2.66 32-35
AlMgB14 + Si 2.67 35-40
AlMgB14 + TiB2 2.70 40-46
(a)
9
(b)
Figure 2-4 A BSE SEM micrograph of (a) baseline AlMgB14 material and (b) AlMgB14 +
30%TiB2 material. In the image, the light contrast region with high concentration of TiB2 is
labelled as ‘A’ whereas the dark contrast region is referred to the composition of AlMgB14.
The described compound above has high melting point, above 2000 oC, high chemical
stability, but it is relatively brittle. The production cost of the AlMgB composite is 5-10 times
lower than the cost of cBN and diamond.5 The composite is bonded by covalent, intraicosahedral
B-B bonds. Its hardness can be comparable with the conventional superhard material though B-B
bonds lack electrons. However, the presence of the metallic elements, aluminium and
magnesium donate boding electrons in the AlMgB orthorhombic borides which results in
stronger atomic bonding in the lattice structure. In amorphous structure, this effect is further
strengthened.
At room temperature deposition method, single B12 icosahedron is not fully formed; at
573 K, well-developed B12 icosahedron is present in the AlMgB composite film and it results in
a denser structure. Therefore, the AlMgB films deposited at 573 K shows higher hardness due to
greater atom migration and development of crystallites at higher deposition temperature. The
material has frictional coefficient of about 0.4 to 0.5, and friction is characteristic with self-
lubricant effect.10
10
The composite AlMgB materials can be compared to other materials in different
application. Only couple examples can be chose. For instance diamond-like carbon (DLC) films
cannot protect the micro-devices at temperature higher than 723 K and they tend to delaminate
when they are thicker than 100 nm due to their large compressive residual stress and low thermal
stability. On the contrary, low surface energy polymeric coatings, despite their low frictional
coefficient, have remarkable low hardness and low thermal stability at mild temperature causing
rapid wear when applied to e.g., micro-devices.10
In comparison to cubic boron nitride films, it can be said that cBN has not been mastered
on the level of industrial applications. Cubic BN films exhibit extreme compressive stress (up to
10GPa) that leads to the film delamination. However the fabricated AlMgB films have a
comparably lower stress, better adhesion to the substrates and thus great mechanical stability.11
2.3 Synthesis of AlMgB Films on Silicon Substrates
2.3.1 Sputtering system/ Sputter deposition
Sputter deposition is the technique that belongs to the group of techniques denoted as
physical vapor deposition (PVD). It is also known as sputtering. Sputter deposition is technique
that is chosen for preparation of thin AlMgB films in this project. In sputtering process, material
in close surface regions is removed via momentum transfer from energetic ions to surface atoms
of a solid target. The practical energies of ions (usually inert such as argon) impinging to a
negatively charged target surface12 are from several hundreds to few thousands of electron-volts
(eV). At their ion impact, surface atoms are compressed and at subsequent relaxation process
atoms overshoot their original equilibrium positions and break their bonds with the solid to be
emitted to vacuum with energies of 5 to 10 eV at the target surface. Since sputtering is carried
out in inert argon gas, the sputtered tend to equalize their energies in collision process
(thermalization process) and are deposited to a suitable positioned substrate, like shown in Figure
2-5, with lower energies (1 – 2 eV). The deposition can be carried out in an inert argon
atmosphere at pressure ranging from 100 to 0.1 Pa. Sometimes other gas constituents can be
supplied into the processing gas. The activated gases react with sputtered material and form
composite films on substrate. This process is then called reactive sputter deposition.
11
Figure 2-5 Illustration of a sputter deposition8 in two parallel-electrode configuration.
The geometrical configuration of a sputter deposition may confine a disc target electrode
and a parallel disc anode with a substrate. The interelectrode spacing can be several inches. The
application voltage across the electrode at suitable pressure of argon leads to electric discharge
and sputtering process above.13
Since during the deposition the inert process gas is present between the electrodes, and
atom or molecules removed from target lose their energy at their collisions with the gas
molecules, the important deposition parameter is interelectrode distance. The distance has to be
adjusted to optimize the film properties.13 Alike the interelectrode distance, gas pressure is also
closely associated with final particle energy before their depositions. Hence the distance and the
gas pressure affect the evolution of the microstructure and morphology of growing films which
is further interrelated with the film stress and generally physical properties of the films.
Nevertheless, the vital parameter is power density applied to the sputtered target, which is in fact
the product of applied voltage and electric current density. The applied voltage is related to the
energy of impinging ions and that is a function of sputtering yield. The sputtering yield depends
not only on the energy of impinging ions, but also on chemical nature of the sputtered materials.
Because sputtering yield of chemically pure elements differs, the composition of deposited films
could be expected to may deviate from that of the alloy of the target.
Figure 2-6 Anti-reflective coatings on lenses for camera objectives.
12
However, unlike in thermal evaporation the composition of the films deposited on the
substrates using sputtering should correspond to that of target because there is no compositional
bias in the ejection of the atoms from the source to the substrate. In sputtering there is self-
regulative mechanism.14 The sputtered target surface is enriched by the atomic component with
lower sputtering yield which finally compensates difference in sputtering yield.14 However for
flexibility, it is preferable to deposit alloys from individual pure element targets at substrate
rotation using confocal or other more sophisticated configuration with planetary rotation. Such
techniques enable us to prepare films with considerable flexibility in their composition.
Sputter deposition gives another parameter, bias voltage of substrate, to control the films
properties. The bias voltage applied to the substrate can be facilitated in either a three electrode
configuration using planar electrodes or magnetron configuration.
Since the deposition rate is fairly high and argon gas used in deposition has high purity
films prepared by sputtering can be regarded as pure when background pressure is on the order
of 10-4 Pa and deposition is carried at 100 – 0.1 Pa.
There are six types of primary sputtering deposition methods including DC diode,
magnetron, radio-frequency, reactive gas, ion beam and pulse DC/AC sputtering. In this project,
the AlMgB composite films on silicon are fabricated by the magnetron sputter-deposition. So,
the working principle of the magnetron system is focused. Magnetron sputter-deposition uses the
magnetic field to trap electrons, which are secondary electrons, close to the magnetron surface,
target. As electrons are trapped on long helical paths, they make many collisions with argon
atoms along their trajectories and induce high density of electrons and ions near the target
surface. The ions are accelerated against the negative target across plasma sheath and cause the
target puttering. Hence, intense plasma can sustain at a lower pressure (about 0.1 Pa) while high
sputtering rate is maintained at these conditions. Since the trapped electrons are concentrated
above the target, ionizations of gas occur in that region leading to non-uniform ion bombardment
and wear of the target. Uniform wear of target and its effective use could be provided with
mowing the magnets13 which is not simple in many commercial systems.
Sputtering process is commonly used in a variety of coating materials. For instance
sputtering process is used in optical applications such as IR-reflective architectural glass, anti-
reflective camera lens coatings, 15 shown in Figure 2-6, and double-pane window assemblies.
The technique is also used in processing integrated circuits, thin-film transistors and many
13
electronic devices. Even in daily used disks, an aluminium layer is also coated by sputter
deposition.14
2.3.1.1 Advantages and disadvantages of sputter deposition
The quality of films, possibility to prepare a variety of films, possibility to scale the
system and control the film properties are counted for advantages of a considered deposition
technique. Sputter deposition is a versatile technique that permits us to prepare a variety of films
in pure and composite form. The films with high melting points can be deposited. The film
structures and properties can be engineered by tuning the deposition parameters that include the
total operational pressure, partial pressure of reactive gases at reactive sputter deposition,
temperature of substrate, the spacing between the target, power applied to the target and bias
voltage applied to the substrate. Accordingly films with high uniformity in composition and
thickness as well as desired microstructures can be obtained. By adjusting the partial pressure of
the inert gas, the functionally gradient can be obtained. Using different deposition techniques,
like reactive gas sputter-deposition, coatings with multi-layers can be obtained. The sputtered
films have fairly good adhesion to the substrates since particle energy is larger at least by factor
of tens when compared to evaporated films. In addition, there is a certain flexibility to control the
film stress. 13
Disadvantage of sputter deposition is that target and substrate has to be on line of sight,
which means the substrate cannot be deposited from back side and at deposition of complex
shapes some difficulties in film uniformity can appear. However these problems can be solved
with planetary rotation of the substrates in large extent. Since the energy of particles from which
films growth is higher the stress built in film can be greater which might affect some film
properties, particularly mechanical stability of the thick films. Controlling the deposition
parameters allows us to control the film stress in some extent too.
2.4 Characterization Techniques used in Analysis of AlMgB Films
In analysis of AlMgB several analytical techniques which is available in AP laboratories
can be considered. To understand reason behind using these analytical techniques, their
properties are described very briefly.
14
Materials Scanning electron microscopy (SEM)16 is the very first analytical technique
that should be used in materials analysis. It allows us to visualize and study surface morphology,
grain size, precipitation in alloys, surface defects and other properties. These characteristic
features can be obtained by collection of secondary electrons that are emitted upon interaction of
very fine focused electron beam with a solid surface and scanning the electron beam over the
area of interest. On the other hand the collection of inelastically backscattered electrons instead
of secondary electrons enables us to observe atomic contrast, and thus resolve the surface areas
with a low atomic number from those with a high atomic number. The advantage of SEM over
optical microscope is in higher magnification and resolution as small as 1.5 nm. Very important
characteristic is also viewing three dimensional images due to very high view of the field.
Obviously the film thickness and surface topography can be investigated through the SEM
analysis.
An atomic force microscope (AFM)17 is another analytical technique that can be used
for evaluation of topographic features of material surfaces on nanoscales. The important feature
of the AFM microscopes is evaluation of surface roughness that can be averaged over scanned
areas of micrometers. The surface is probed with a stylus that is broad to vicinity of surface of
several angstroms and scanned over a small sample area. At such distances interactive forces on
the order of 10-8 N are exerted between the stylus and surface atoms. The forces deflect a
cantilever on which the stylus is installed. The deflection of cantilever and thus the force can be
determined by using a laser beam that is reflected from the cantilever and measured by a four-
quadrant detector. In conventional mode operation, often tapping mode, an image of the surface
morphology and also the root mean square roughness of the AlMgB composite films can be
revealed.
X-ray photoelectron spectroscopy (XPS)18 can be used for compositional and chemical
analysis involving chemical bonding, particularly composite films as investigated in this work.
The principle of the technique is based on photoelectron emission. Core level electrons are
emitted upon interaction of soft X-rays with material surfaces. The x-ray are induced by electron
impact on aluminium or magnesium anode. The emitted x-rays are achromatic and for more
precise analysis they can be monochromatized by using crystal diffraction. For example quartz
crystal is used for monochromatization of Al K alpha with photon energy of 1486.6 eV. This
implies if photons with energy of hv interact with the sample the binding energy, B.E., of
electrons in atoms can be determined using a simple equation
15
analEKhvEB ....
where K.E. stands for kinetic energy of emitted photoelectrons that is measured in an
energy analyzer and anal is the work function. The B.E. is measured with respect to the Fermi
level and is unique to the chemical element where photoelectron originates from. However the
typical depth of analysis is about 5.0 nm, which suggest that some precaution have to be
undertaken since adsorbed atoms may considerable affect the compositional analysis.
X-ray diffraction (XRD)18,19 is non-destructive analytical methods that can be applied to
the materials which confines materials crystallites. The materials are probed by x-rays which can
be constructively diffracted from crystal planes. The constructive diffraction occurs for an
interplanar spacing in crystallites at a specific impact angle of x-rays. The relationship between
the diffraction angle and interplanar spacing d depends on x-ray wavelength as given by
Bragg’s law, which mathematical formulation is follows
sin2dn
Since interplanar spacing is unique to in crystalline materials the chemical nature and
phases in can be determined is solids. Therefore this technique was used to search for crystallites
in AlMgB films with different compositions.
Fourier transform infrared (FTIR) spectroscopy18 is the technique used for acquisition of
infrared absorption/transmission spectra. The sample is irradiated by a broad range of
wavelength in infrared radiation region and the absorption data are obtained very fast by
acquisition spectra over whole analyzed region of wavelengths. Then data obtained are
mathematically treated using Fourier transform methods to obtain the dependence upon
wavelength. However in principle the absorption or transmission depends on the inducing
phonon vibration states that are characteristic with bonding length and masses of atoms or
chemical groups that are enforced to the motion by absorption of infrared energy. Again the
vibration states are unique and they can be used to identify structural characteristics and phases
in solids. The technique is also applicable to analysis of gases and liquids. Therefore these
techniques might be also used to explore some structural properties of the deposited AlMgB
composite films. Thus the FTIR analysis can be used for identification of presence crystallites
such as icosahedrons in amorphous matrices of hard AlMgB films.
Since the project is also devoted to hardness of the prepared thin films in some extent, it
is useful to describe the technique used in these measurements. Very often used technique is
16
nanoindentation test which is applied especially for measurement hardness of very thin films.
Nanoindentation18 is the technique that confines indentation and deformation to a very small
volume. For indentation is usually used a tip of diamond which properties are well known. Then
hardness is determined from the indented area and load applied to the indenting diamond tip.
Loading and loading characteristics may indicate behaviour of the films at indentation. In this
project, it is used to assess the hardness of the AlMgB films. In the test, when the diamond tip
indents the sample with a constant strain rate to an indentation depth of 100nm, the hardness is
measured continuously.
2.5 Applications of the AlMgB films
The ternary AlMgB composite can be applied in hard coatings, for example, protective
coatings on LIGA micro-devices and Si-based MEMS components. They can be used for coating
of machining tools and substitute expensive bulk cBN tools. Since they are ultra-light and
superhard, they can be process to bulk materials using laminate film structures that can be
heavily exploited in military, armour and space applications.18
17
3 Experimental Methodology
3.1 Deposition of Ternary AlMgB Composite Films
3.1.1 Cleaning the Substrates
Before carrying out the sputter deposition, the silicon substrates were ultrasonically
cleaned in baths of methanol and distilled water. The clean substrates were then etched in a
5% hydrofluoric solution in order to remove the native oxide (SiO2) which was formed on the
substrates’ surface. After etching with diluted hydrofluoric (HF) acid, the substrates were
washed in deionized water and subsequently dried in nitrogen gas flow.
3.1.2 Synthesis of Ternary AlMgB Composite Films on Silicon Substrates
Sputter deposition is the method used to deposit AlMgB films on the device quality Si
(100) substrates. The films were prepared in Teer’s magnetron deposition system as shown in
Figure 3-1.
A large cylindrical deposition chamber (60 cm in diameter and 60 cm in height) and a
pumping unit composed of a turbomolecular pump backed up with a rotary pump and
provided with a polycold trap were comprised in the Teer’s magnetron deposition system. The
chamber was equipped with four rectangular unbalances magnetrons with target sizes of 30
cm by 15 cm, as shown in Figure 3-2 (a) and (b). The magnetrons were installed on the
chamber with rotation angles of 90o. Apart from the magnetrons, the system was also
equipped with a rotary holder rack providing uniform deposition in film thickness and
composition when deposited from multiple targets distributed around the axis of the
cylindrical chamber.
The vacuum chamber was pumped down to about 10-5 Pa background pressure. This
pressure was obtained with assistance of a polycold trap cooled to temperature about – 120 oC.
The residual water vapor was reduced by the polycold trap inside the vacuum deposition
chamber. Prior to pumping the vacuum chamber, the substrates were placed on a rotatable
holder rack at a distance of 13 cm from the targets. The substrate temperature was set up to be
200 oC. Then pure argon was supplied into the evacuated chamber to reach pressure of 0.4 Pa.
At this pressure, samples were sputter-cleaned using a bias voltage of – 500 V applied to the
18
substrate. After sputter cleaning for ~30 minutes the substrate bias was reduced to –60 V to
carry out film deposition. The deposition was performed at substrate rotation of 15 rpm to
achieve uniform deposition and prevent the formation of a laminated structure. This way
heterogeneous ternary compound was produced at deposition from different targets.
As shown in Figure 3-3, four magnetrons were closed magnetic field configuration to
spatially confine the plasma and enhance its density. Ternary AlMgB composite films were
then prepared from multiple targets. Only three targets of four were used since one of the
targets was metallic AlMg alloy (1:1), while two targets were made of pure boron. Two boron
targets were used because of a lower sputter yield of boron. The fourth magnetron was not
used at deposition of AlMgB composite. The magnetron with metallic AlMg target was
operated in a direct current (DC) mode, whereas the two magnetrons with boron targets were
operated pulse modes with the pulse frequency of 250 kHz. A demonstration of the circular
magnetron in operation is shown in Figure 3-4.
During the experiment, some parameters were changed continuously in order to obtain
films with different compositions. So, for the electric power, different electric powers were
supplied to the magnetron with the metal target were used. Hence, for each substrate, there
was a small part being masked to form a step in order to enabling the thickness measurement
using a surface profiler.
Figure 3-1 The sputter system equipped with four magnetrons used in deposition of AlMgB
films.
19
(a) (b)
Figure 3-2 (a) A configuration and (b) the assembly of a rectangular planar magnetron with a
wear track.
Figure 3-3 A configuration of sputtering system with four rectangular planar magnetrons and
planetary motion in a closed magnetic field configuration.
Figure 3-4 A demonstration of the circular magnetron in operation.
20
3.2 Investigated Properties of AlMgB Films
The morphological structures of the AlMgB films were studied using a Joel JSM-820
scanning electron microscope (SEM). The film thickness was determined from the cross
sectional images. The cross sectional structures was also analysed in SEM images.
Compositional analysis of the deposited AlMgB films was studied by an energy dispersive x-
ray (EDX) spectroscopy which is complementary techniques installed on a Joel JSM-820
SEM. The EDX spectra were obtained at electron energy of 5 keV. The background pressure
in the analysis chamber was on the order of 10-4 Pa.
The roughness of the deposited AlMgB films was studied by using a Nanoscope
Veeco atomic force microscope (AFM). In this experiment, the AFM instrument was operated
a tapping mode. The topography as well as the root mean square roughness in a view field of
5 μm was determined.
Compositional analysis and chemical nature of bonding in AlMgB films were
investigated by an X-ray photoelectron spectroscope (XPS) installed on a Vacuum Generator
ESCALAB 220i-XL facilities. Monochromatized x-ray Al Kα source with photon energy of
1486.6 eV was used at analysis.
Presence of crystalline phases and structural analysis of AlMgB films grown on silicon
substrates were studied by X-ray diffraction (XRD). The analysis was carried out by a Cu Kα
X-rays and using a small incident angle geometry. The 2θscan ranged from 30° to 80° with
angular step of 0.02° and dwell time of 5 seconds.
Additional structural characteristics of deposited ternary AlMgB films grown on silicon
substrates were acquired by studying FTIR spectra by a Perkin-Elmer PC 1600 Fourier
Transform Infrared Spectrometer. Some representative peaks specifying the structures of the
films are identified in representative infrared reflection spectra.
Mechanical characteristics of hard ternary AlMgB composite films, prepared by
sputtering, in particularly hardness and elastic modulus were investigated by a MTS
nanoindenter XP. Measurement of nanohardness of the AlMgB films with thickness of 500
nm was evaluated. In this work, a Berkovich diamond tip with the radius smaller than 20 nm
was continuously indented into the specimen to a depth of 100 nm at a constant rate. The
hardness was determined from the indentation size and load in reference to the standard of
21
fuse silica with known hardness (10 GPa). The fused silica was used systematically before
and after each measurement of AlMgB films.
22
4 Results and Discussion
4.1 Effect of Power Density Applied to AlMg Target on the Composition of AlMgB
films
AlMgB thin films with various compositions were synthesized on the Si substrates by
magnetron sputtering using three targets, one AlMg and two pure boron targets. The film
thickness of about 0.5μm was maintained at each deposition. Both the boron targets were
sputtered at the power density of 2 W/cm2 while the power density applied to the AlMg
magnetron target changed from 0.2 to 1 W/cm2. As a result of the change of power density
applied to the AlMg magnetron target, the sputtering yields of Al and Mg and the overall rate
of deposition were changed too.
As observed in the XRD patterns of the AlMgB films, there was no diffraction peak
found when the films were prepared at a power density applied to the AlMg target smaller
than 0.5 W/cm2. That meant the films were amorphous. However the analysis of the films
deposited at higher power densities revealed a boride nanocrystalline structure. The
compositional analysis of the AlMgB thin films and the total corresponding boron content
were determined by XPS as indicated in Table 5. Generally, it was expected that when the
power density and the sputtering rate of the metallic AlMg target were increased, the boron
content was decreased. Despite the fact that the boron concentration was dropped with
increasing power increment, a local maximum of boron concentration was shown at a power
density of 0.6 W/cm2. The local increase of boron concentration is related to the changes in
the incorporation probability of boron, which can be explained by the phase variation of
ternary AlMgB thin films.
23
Table 5: The hard thin films with materials based on ternary AlxMgyB with different
compositions synthesized using two boron targets sputtered at power density of 2 W/cm2 and
an AlMg target sputtered at variable power density.
Sample
number
Sample atomic
ratio
Boron
content
(at.%)
Al/Mg ratio AlMg target at
power density
(W/cm2)
1 Al0.17Mg0.1B 79 1.7 0.2
2 Al0.39Mg0.2B 63 1.9 0.3
3 Al0.80Mg0.45B 44 1.8 0.5
4 Al0.55Mg0.21B 57 2.6 0.6
5 Al1.38Mg0.64B 33 2.2 1.0
4.2 Morphology of Deposited Ternary AlMgB Composite Films
4.2.1 Morphological Studies of AlMgB Films by Scanning Electron Microscopy
Figure 4-1 Cross-sectional SEM micrograph of a boron rich Al0.17Mg0.1B film.
24
Figure 4-2 Cross-sectional SEM micrograph of a metal rich Al0.55Mg0.21B film. The
experimental SEM image from Ref. [18] is also shown for comparison.
Figure 4-1 shows the SEM image of sample 1, in cross-sectional view Al0.17Mg0.1B
prepared at power density of 0.2 W/cm2. This boron rich film shows isotropic characteristics
which indicate more likely amorphous AlMgB structures. However cleaved cross section
interface carries some features of columnar growth which might be signatures of some
structures with a very short range of crystallinity assembled to columns. The metal rich film
in Figure 4-2 reported by Yan et al. 18, shows higher degree of crystallinity, where the
evolution of polycrystallites are more evident.
Indeed, the electrical conductivities of metal rich and boron rich films differ. The films
with higher metal content, the metal rich films, being deposited at higher sputtering power
density (greater than 0.5 W/cm2) are characteristic with higher electrical conductivity. In other
words, due to lack of the contribution from the metal content, the films deposited at lower
power density yields lower electrical conductivity.
25
4.2.2 Studying Morphology and Roughness of the Deposited AlMgB films
Further morphological studies have been carried out by AFM. Figure 4-3 presents three
surface morphologies of the AlMgB composite films deposited at different target power
densities applied to the AlMg target. Sample 1, 3 and 5 are selected for presentation of the
AFM analysis.
26
Figure 4-3 Plane-view AFM images of the surface morphology of the AlMgB thin films
prepared at different power densities: a) 0.2 W/cm2; b) 0.5 W/cm2 and c) 1.0 W/cm2.
27
It is observed that when the target power density rises, the surface morphology of the
films have alters. The films deposited at low power density are nearly atomically smooth but
the films prepared at higher deposited power density become relatively rough. Figure 4-3
indicates that some surface constituents of the thin film are aggregated and even signatures of
faceted surfaces are observable which contributes to the increase of the surface roughness
increases. The deposition at higher power densities induces higher plasma density and thus
higher ion current at the grown surface which increases the surface temperature of the
growing films. The higher temperature then leads to the higher migration rate of deposited
surface constituents. The amorphization caused by the ion bombardment at the – 60 V bias is
less effective when the substrate temperature attains 200oC. Therefore the surface migration
rate is enhanced via increase of surface temperature which facilitates rougher surfaces.
Figure 4-4 Variation of root mean square roughness of the AlMgB composite
films as a function of the power density applied to the AlMg target.
As illustrated in Figure 4-4, the measured root mean square (rms) roughness of the
AlMgB composite films in the sample 1, 3 and 5 are plotted as a function of power density.
The root mean square roughness of the AlMgB films rises from a value of ~0.8 nm when it is
sputtered at low (0.2 W/cm2) power densities to a relatively rough surface for films deposited
(~13 nm) when it is sputtered at high (1.0 W/cm2) power densities.
28
4.3 Compositional Analysis of AlMgB Thin Films
Bulk compositional analysis was performed by EDX spectroscopy integrated in an
SEM as previously described.
Figure 4-5 EDX spectrum showing the composition of Al0.17Mg0.1B thin film
fabricated at target power density of 0.2 W/cm2.
The compositional analysis of EDX is indicated in Figure 4-5. The above EDX
spectrum is collected from the ternary composite Al0.17Mg0.1B thin film (Sample 1). Figure 4-
5 shows that the composite compound prepared at sputtering rate of 0.2 W/cm2 reveals the
Al/Mg atomic ratio is 3.1, as determined by EDX spectroscopy. However XPS compositional
analysis of the same sample shows that the Al/Mg atomic ratio is 1.7. This discrepancy is
primarily caused by analytical techniques which are different in nature. The EDX analysis
gives information on composition typically from the depth of 1 to 2 m, while XPS provides
information from the depth of about 5 nm, which is very surface sensitive. At XPS analysis
surface is sputter clean to remove adventitious surface impurities. However such surface
treatment may also alter the ratio of other surface constituents including aluminium and
magnesium surface content. EDX spectroscopic analysis also indicates incorporation of small
amount of oxygen, The oxygen content is however convolution of oxygen finding on the
29
surface from adventitious impurities as well as traces oxygen incorporated into the film bulk
from vacuum background during the film deposition by the sputtering process. The oxygen
impurities are estimated on the level of about 5%. The spectrum also reveals the most
abundant constituent of the film, boron. Boron is the chemical element with a low atomic
number, which can be identified by recording Kα x-ray appearing at energy of 192 eV. This
energy is fairly low to be recorded using a number of instruments, since low energy photon
may be absorbed in non-functional region of the detector. The EDX spectroscopic system
used herein enables us to measure the photon energy however sensitivity is reduced
significantly and accuracy of measurement may suffer from the same reasons.
4.4 Surface Chemical Analysis of AlMgB films
Surface chemical analysis was carried out by x-ray photoelectron spectroscopy (XPS).
The core level spectra were induced by a monochromatic Al K x-ray source with energy of
1486.6 eV.
(a)
(b)
30
(c)
(d)
(e)
(f)
Figure 4-6 (a) B 1s , (c) Al 2p and (e) Mg 2p XPS spectrum collected before sputtering; (b)
B 1s , (d) Al 2p and (f) Mg 2p XPS spectrum collected after sputtering.
Mg 2p, O1s, C 1s, Mg 1s, B 1s, Al 2p XPS spectra for all samples are collected before
and after sputtering with high resolution. Argon is also detected herein. Small amount of
argon is usually observed in sputtered films and in surfaces treated by sputtering. Since argon
is in a subsurface region, the intensity of photoelectrons originated in argon atoms traverse via
adsorbed layer (water, hydrocarbon and oxygen) and therefore the measured Ar 2p intensity
of photoelectrons is attenuated. Argon as chemically inert element is trapped in vacancies and
31
extended defects induced by sputtering processes. Since argon does not affect the films
chemically and exists only in trace amounts (~1.0 at.%), it is not a scope of this analysis.
Similarly no spectral data of O 1s and C 1s is given since the signals arise from adventitious
adsorption of oxygen/water and hydrocarbons, respectively. These surface impurities can be
removed or suppressed by in situ sputter cleaning.
Despite sputtering yields for B and Mg and Al differ by some factors, the sputtering at
lower energy (1 keV) and angle impact (45o) has removed surface oxides and adventitious
impurities, while the ratio Al:Mg:B before and after sputtering is changed only insignificantly
from 25 : 6 : 69 to 14 : 3 : 83. This finding entitles us to carry out deeper analysis. High
resolution core level XPS spectra reveal chemical states of metallic components present in the
film. The Al, Mg and B components are found in pure metallic forms as well as forms of
metal borides.
The high resolution XPS spectra acquired from the sample 1 (Al0.17Mg0.1B) deposited
at power density of 0.2 W/cm2 are shown in Figure 4-6. The B 1s spectra analysed before and
after sputtering are indicated in Figure 4-6 (a) and (b) respectively. The B-B bonding and
borides are present in the AlMgB film before and also after sputtering, since the deconvoluted
peaks emerge at binding energies of 187.96 eV and 189.49 eV that correspond to B-B
bonding and metallic borides. The shift of the peak position towards the higher binding
energy for aluminium borides is logical and consistent with electron affinities of the bonding
chemical elements. The core level peak at binding energy 192.89 eV corresponds to the B-O
bonds. However, this chemically shifted B 1s core level peak only appears at the film analysis
before the sputter cleaning which indicates that boron oxide is adventitious surface
constituents confined to a very surface region.
The high resolution Al 2p XPS spectra acquired before and after sputtering are shown
in Figure 4-6 (c) and (d), respectively. It should be noted that Al 2p3/2 and 2p1/2 are unresolved
since spin orbital splitting is too small. Thus single chemical state is presented by a single
peak. More complete peak fitting is performed only for data obtained after the sputter
cleaning
The deconvoluted peaks, in Fig. 4-6 (d), are located at the binding energy of 73.96 eV
and 75 eV and they are ascribed to Al-borides and Al-Al/Al-Mg metallic composites,
respectively. Thus the dominant peak at the lower binding energy is ascribed to Al borides
32
based on the same concept of logic of electron affinities. The Al-Al and Al-Mg bonding can
hardly be resolved because insignificant chemical shift between these two chemical states.
The concept of analysis of chemical states of magnesium has a very similar
characteristic as that of used aluminium analysis. The high resolution Mg 2p XPS spectra
obtained before and after sputtering are shown in Figure 4-6 (e) and (f), respectively. Again
only the core level spectrum after the sputter cleaning is deconvoluted. The deconvolution
shows two peaks. The dominant peaks at lower binding energy (50.87 eV) are ascribed to
magnesium boride, while that at binding energy of 51.85 eV is assigned to Mg-Mg and Mg-
Al bonding. Alike in the case of Al 2p, the metallic chemical states of Mg-Mg and Mg-Al in
Mg 2p cannot be resolved either.
The XPS chemical analysis shows that Al and Mg are preferentially involved in
forming borides, which is consistent with fundamental objective in synthesis of AlMgB
composite films.
4.5 Structural Analysis of the Deposited AlMgB films
(a)
33
(b)
Figure 4-7 The X-ray diffraction patterns accumulated from composite (a) metal rich
Al1.38Mg0.64B and (b) born rich Al0.17Mg0.1B thin film.
The structure of deposited AlMgB films was investigated by x-ray diffraction (XRD).
Figure 4-7(a) shows XRD pattern of metal rich film (sample 5) obtained at a glancing angle of
1o. A representative diffraction pattern collected from a deposited metal rich film is shown in
Figure 4-7(a). The metal rich film gives a broad but week diffraction peak at the θ-2θ scan
of around 34o, indicating that the film is prevalently amorphous but it confines some
nanocrystals. The metal rich films with boron content smaller than 55 at.% is rather
featureless with exception of a fairly board and weak diffraction peak occurring at 2θ of
approximately 34o. This indeed illustrates an amorphous film matrix confining some
nanocrystalites. The observed diffraction peak can be correlated with the diffraction from the
(211) crystallographic planes of the AlMgB14 orthorhombic crystal structure that also appears
at ~ 34o. The AlMgB14 crystal structure normally yields the diffraction of (211) but it also
shows the strongest peaks, which are (011) peak at 13.89o, (132) peak at 40.69o and (213)
peak at 42.96o. However the presented diffraction pattern in Figure 4-7(a) does not reveal any
other diffraction peak of the AlMgB14 orthorhombic crystal structure except that of (211) at
34o. Since the structure is prevalently amorphous and only the week peak matching the (211)
34
of AlMgB14 diffraction is present, the nanocrystallites may be assigned to the AlMgB14
structure.
Figure 4-7(b) shows an XRD pattern of a boron rich film (sample 1) acquired at a
glancing angle of 1o. The boron rich film yields again a very weak diffraction peak at around
42o, suggesting that the thin film has prevalently amorphous structure with embedded
nanocrystallites with B icosahedral structure (AlMgB14). This structure yields the film
hardness of approximately 30 GPa.20
4.6 Additional Structural Analysis of Boron Rich Al0.17Mg0.1B Thin Films
Figure 4-8 The IR transmittance spectrum of the boron rich Al0.17Mg0.1B thin film
Correlate.
Additional structural analysis was carried out by Fourier infrared (FTIR) spectroscopy in
a reflection mode. In particular boron rich AlMgB films were investigated. Figure 4-8 shows
a typical FTIR spectrum collected from the B-rich film (sample 1). The spectrum reveals a
peak in vicinity of the wavenumber of 1000 cm-1. This peak correlates with Si-O bonds and
therefore it can emerge only from the silicon substrate. More importantly, a strong absorption
peak is observed at ~ 1100 cm-1, which can be ascribed to the F1u vibration mode of a single
B12 icosahedra.21
35
4.7 Study of Mechanical Properties of AlMgB films
Figure 4-9 The variation in hardness of AlxMgyB thin films as a function of boron content
in the films.
Some mechanical properties of the AlMgB films were studied using a nanoindentation
method. It should be noted that the nanoindentation is dependent on the indentation depth
particularly in the case of thin films when substrate effect in measurement can be considerable.
Therefore it is recommended that the penetration depth should be less than 10-20% of the film
thickness in order to minimize the effect of the underlying silicon substrate on the intrinsic
properties of the film. Since the films prepared have thickness of 500 nm, the maximal
indentation depth should not be larger than 100 nm.
Figure 4-9 shows that the measured hardness of the deposited AlMgB composite films
plotted against the Boron content.22
The limited number of deposited AlMgB samples and evaluated hardness do not allow
making very conclusive statements. The trend of hardness variation indicates that at very low
boron concentration (metal rich structures) film hardness is practically 30 GPa and at very
high boron concentration hardness tends to reach value of 30 GPa, too. It seems that hardness
should be at a minimum when the boron content is in neighbourhood of 60%. Unfortunately
36
the minimum position has not been determined herein, because of fluctuating values with
considerable deviation and above mentioned number of sample prepared. The deviations in
hardness are more likely caused by presence or absence of nanocrystallites in the regions of
nanoindentation.
Figure 4-10 The loading and unloading curves of boron rich (a) Al0.17Mg0.1B and metal rich
(b) Al1.38Mg0.64B thin films as a function of displacement.
From the result of nanoindentation measurements with the indentation depth of 100nm,
the loading-unloading curves of boron rich film (sample 1) and metal rich film (sample 5) are
plotted in Figure 4-10, as a function of displacement. It is reminded that the elastic recovery is
calculated by the difference between the displacement at the maximum and residual load over
the minimum displacement. From the Figure 4-10, both of the films even in different
compositions achieve about 60% of elastic recovery. It is originated in the B icosahedral
structure of hardening mechanism. Due to the composition of the boron rich film very similar
to the stoichiometric AlMgB14, these films comprises the B icosahedron crystallites in
metastable amorphous structures. These films yield the measured hardness of ~ 30 GPa,
which is very close to that of the ternary super hard films of single and polycrystalline
AlMgB14.23,24
The mechanism of hardening of a composite material was examined by a group of
researchers. They found that the thin films of a few nanometers having nanocrystallites in an
(a)
(b)
37
amorphous matrix gives the highest hardness. The extraordinarily high hardness may be
explained as follows: (1) Dislocation cannot be found on the tiny and very hard nanocrystals
inside the films, (2) The effect of coherent strain can be governed by the clear boundary
between the hard nanocrystallites and the thin amorphous matrix inside the films, and (3) The
propagation of cracks can be suppressed by the flexible amorphous matrix of the films.25
Those deductions help suggesting the structure of the metal rich AlMgB films and
deriving the extreme hardness. The small and hard nanocrystals found in the analysis of XRD
and B icosahedra are embedded in the flexible and thin amorphous matrix of the metal rich
AlMgB films. A sharp boundary is also suggested between the small and hard nanocrystals or
B icosahedra and the thin amorphous matrix inside the AlMgB films. More importantly,
because the thickness of separating metal matrix is dissimilar to that of the boron icosahedra,
the icosahedra would be aggregated and propagation of cracks would be inhibited.
38
5 Conclusion
Superhard thin coatings with materials based on ternary AlMgB have been
synthesized on silicon substrates by sputter-deposition using unbalanced planar rectangular
magnetrons with closed magnetic field configuration. The deposited films with different
chemical compositions were prepared when the supplied power density at the metallic AlMg
alloy target of atomic composition 1:1 has been increased from 0.2 to 1 W/cm2; whereas the
supplied power density at the two pure boron targets has been kept at a constant value (2
W/cm2).
By the adjustment of supplied power density to smaller than 0.5 W/cm2, a group of
AlMgB films rich in boron with more than 60 at.% have been synthesized, while other group
of metal rich films with less than 55 at.% has been produced at higher target power density. A
superhard coating based on a ternary boride compound AlMgB14 has not been fabricated.
However, at low boron concentration the film hardness is practically equal to 30 GPa. In
medium range of boron content in AlMgB films the hardness is lower but at higher
concentration again it has tendency to reach 30 GPa. The boron rich films are found to
contain AlMgB14 nanocrystallites embedded in metastable structures. Impressively, their
hardness obtained in nanoindentation test is just a bit below the boundary of the hardness of
the superhard films. So, these materials are considered as matrices of a new class of superhard
and extraordinarily lightweight materials. Due to the investigation on the film morphology
and chemical composition and bonding, the difference between analysis results from two
types of AlMgB films implies a much clear classification between composite films with
different target power densities.
The presence of the B12 icosahedral framework in the films causes both boron rich and
metal rich films comprising excellent mechanical properties showing that they can be
outstanding candidates to compete with other superhard materials and meet requirements for
their commercialization.
39
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