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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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