Thin Films

Linköping Studies in Science and Technology Dissertation No. 1497

Growth and Heat Treatment Studies of Al-Cr-O and Al-Cr-O-N

Thin Films

Ali Khatibi

Thin Film Physics Division

Department of Physics, Chemistry, and Biology (IFM) Linköping University, SE-58183 Linköping, Sweden

Linköping, 2013

Cover:

X-ray diffraction patterns of Al-Cr-O films at different annealing temperatures and times, showing

the cubic to corundum phase transformation.

© Ali Khatibi, 2013

ISBN: 978-91-7519-710-4 ISSN: 0345-7524

Printed by LiU-Tryck, Linköping, Sweden

i

A BSTRACT

Aluminum oxide based thin films are applied on cutting tool inserts as a top layer to protect the underlying nitride or carbide functional layer from the harsh working environment in terms of abrasive and chemical wear under thermal and pressure load. This Thesis explores the synthesis and characterization of the next generations of multifunctional wear-resistant thin film coatings in the form of Al-Cr- O and Al-Cr-O-N compounds. The experiments include the deposition of oxide films by reactive magnetron sputtering and cathodic arc evaporation as well as investigation of structural and mechanical properties in as-deposited and annealed states. Ternary (Al

Cr

)

O

films were deposited on Si(001) and WC-Co substrates kept at 400-575 °C from elemental Al and Cr or alloyed Al/Cr cathodes in Ar/O

, O

/N

, and pure O

atmospheres. Also, quaternary (Al

Cr

)

(O

N

)

films were deposited at substrate temperature of ~400 °C on WC-Co substrates in O

/N

atmosphere.

X-ray diffraction and analytical electron microscopy combined with ab initio calculations showed the existence of a new face centered cubic (Al,Cr)

O

phase with 33% vacancies on the metallic Al/Cr sites. Increasing the temperature during annealing of these metastable cubic films resulted in phase transformation to corundum solid solution in the temperature range of 900-1100 °C. The apparent activation energy of this phase transformation process was calculated as 380-480 kJ/mol by using the Johnson-Mehl-Avrami model. The mechanical properties of the cubic and corundum oxide films were measured in terms of nanoindentation hardness and metal cutting performance. The cubic and corundum films showed hardness values of 26-28 GPa and 28-30 GPa, respectively. The oxynitride solid solution films showed to be predominantly cubic Al-Cr-N and cubic- (Al,Cr)

O

and secondary corundum-(Al,Cr)

O

with a hardness of

~30 GPa, slightly higher than Al-rich ternary oxides. Metal cutting

performance tests showed that the good wear properties are mainly

correlated to the oxygen-rich coatings, regardless of the cubic or

corundum fractions.

ii

iii

P OPULÄRVETENSKAPLIG S AMMANFATTNING

Vi lever i en materiell värld. Allt består av material i olika former, t.ex. metaller som finns i naturen eller konstgjorda plaster.

Materialvetenskap använder naturens lagar för att göra nya material med syftet att skapa nya egenskaper. Materialvetenskap visar oss hur vi kan blanda ett metalliskt grundämne som aluminium med en icke- metall som syre för att göra en förening: aluminiumoxid, en keram som har mycket högre smältpunkt, ca 3000 °C, än både aluminium och syre med smältpunkter på ca 660 °C respektive -218 °C.

Dessutom kan nya egenskaper styras genom nedskalning av materialen från bulk till mikro- (tusendels millimeter) eller nano- (miljondels millimeter) skalor. En ny vetenskapsgren kallas nanovetenskap. Nanovetenskap ger oss möjligheten att göra tunna filmer, skikt av material med tjocklekar från enstaka atomlager till mikrometer. De kan användas på ytan av ett annat material som kallas substrat. Tunna filmer har ofta helt olika egenskaper jämfört med samma material i bulk. Tunna filmer används i nästan varje enskild produkt som vi använder i vårt dagliga liv. Exempel är optiska skikt på ytor av glasögon och fönster, eller dekorativa skikt på våra klockor och dörrhandtag. En viktig tillämpning av tunna filmer i industri är i produktion av skärverktyg. Dessa verktyg är metaller, och beläggning av hårda skikt kan höja verktygs livslängd med 50 gånger eller mer.

I metallbearbetningsindustrin tillverkas skikt bildas oftast av en serie

av lager ovanpå varandra. Denna struktur kallas multilager i vilka

aluminiumoxid ofta används som det sista lagret. Anledningen är att

aluminiumoxid är en bra värmeisolator som skyddar de underliggande

hårda tunna filmerna, vilket resulterar i ökad produktivitet. Den här

avhandlingen handlar om produktion av nya typer av oxid- och

oxynitrid-baserade tunna filmer som är gjorda av aluminium, krom,

syre och kväve.

iv

Tillverkningen av tunna filmer i den här avhandlingen gjordes med en beläggningsmetod som kallas sputtring. Beläggningsprocessen sker i en kammare som först pumpas ner för att uppnå ultrahögt vakuum. I denna kammare placerar man beläggningskällorna (som kallas targets) som i mitt fall är små runda plattor av aluminium och krom. I det nästa steg släpper man in reaktiva gaser av syre och/eller kväve samt ädelgasen argon in vakuumkammaren. Gaserna joniseras därefter av en pålagd spänningsskillnad mellan källorna och kammarväggarna och ett plasma bildas. Plasma är det fjärde aggregationstillståndet av material som är en blandning av neutrala gasatomer, positivta gasjoner och negativta elektroner. Om man lägger en negativ spänning på källorna, så kommer plasmajoner att bombardera källornas ytor med hög hastighet och därför kommer atomer att slås ut från källan till plasmat. Slutligen bildas genom kondensation ett tunt kristallint skikt av atomer ovanpå substratytan, d.v.s. verktyget blir belagt. En stor del av mina skikt gjordes i industriella beläggningssystem. I industrin är det oftast viktigast att ha en hög beläggningshastighet och därför kan andra metoder än sputtring också användas. En vanlig metod kallas katodförångning i vilket en elektrisk urladdning används för att smälta och förånga material från källorna (kallas katoder). Dessa material kommer också att kondensera på substratytan och bilda den växande filmen.

Jag har tillverkat och använt skikten för att studera hur beläggningsparametrar kan påverka strukturella och mekaniska egenskaper av skärverktygen. Röntgenmätningar användes för att studera bildning av olika faser. Mikrostrukturer undersöktes med transmissionselektronmikroskopi med vilket jag kunde visa strukturen på atomnivå. Hårdheten och förslitningsmekanismerna av filmer på vändskär studerades med nanoindentation och metallbearbetningstest.

Jag ville också se hur dessa egenskaper påverkas av höga

temperaturer och därför gjorde jag värmebehandling och

undersökningar i temperaturområdet 500-1100 °C.

v

Mina resultat visar att skikt av aluminium-krom-oxid eller oxynitrid kan bildas i två olika faser, kubisk som en sockerbit och korundum som en vaxkaka. För en bättre förståelse av dessa strukturer använde jag avancerade kvantmekaniska beräkningar. De visade hur atomer ska placeras i en kubisk eller korundum struktur i mina skikt.

Mätningarna visade att kubiska filmer är lika hårda som filmer med korundumstruktur. Slutligen bekräftade bearbetningstest att kristallstrukturen inte påverkar skärresultaten nämnvärt.

Oxynitridskikten gav det bästa resultatet i bearbetningstest, men hade

en lägre hårdhet.

vi

vii

P REFACE

This thesis is the result of my PhD studies performed from 2008 to 2013 in the Thin Film Physics Division at the Department of Physics, Chemistry, and Biology (IFM), Linköping University. The work is a continuation to my licentiate thesis presented in 2010 (Linköping Studies in Science and Technology, Licentiate Thesis No. 1474;

Deposition and Phase Transformation of Ternary Al-Cr-O Thin

Films). The work has been done in close cooperation with Sandvik

Coromant AB, Materials and Processes R&D in Stockholm and Seco

Tools AB in Fagersta. My work is financially supported by The

Swedish Research Council (VR) and the Swedish Foundation for

Strategic Research (SSF) program on Materials Science for

Nanoscale Surface Engineering (MS

E).

viii

ix

P APERS

Paper 1.

Face-centered Cubic (Al

Cr

)

O

A. Khatibi, J. Palisaitis, C. Höglund, A. Eriksson, P.O.Å. Persson, J. Jensen, J. Birch, P. Eklund, L. Hultman

Thin Solid Films 519 (2011) 2426–2429.

Paper 2.

Phase Transformations in Face Centered Cubic (Al

Cr

)

O

Thin Films

A. Khatibi, J. Lu, J. Jensen, P. Eklund, L. Hultman Surface & Coatings Technology 206 (2012) 3216–3222.

Paper 3.

Structural and Mechanical Properties of Cr-Al-O-N Thin Films Grown by Cathodic Arc Deposition

A. Khatibi, J. Sjölen, G. Greczynski, J. Jensen, P. Eklund, L. Hultman Acta Materialia 60 (2012) 6494–6507.

Paper 4.

Structural and Mechanical Properties of (Al

Cr

)

O

Coatings Grown by Reactive Cathodic Arc Evaporation in As-deposited and Annealed States

A. Khatibi, A. Genvad, E. Göthelid, J. Jensen, P. Eklund, L. Hultman Submitted for publication (2013)

Paper 5.

Theoretical Investigation of Cubic B1-like and Corundum (Cr

Al

)

O

Solid Solutions

B. Alling, A. Khatibi, S. I. Simak, P. Eklund, L. Hultman

In manuscript

x

xi

M Y C ONTRIBUTION TO THE P APERS

Paper 1.

I was involved in the planning of the project, did the depositions and all the characterizations except TEM and ERDA. I wrote the paper.

Paper 2.

I conceived the study together with my supervisors. I planned the project together with the co-authors, performed all the depositions, annealing experiments and characterizations except the TEM, where I participated, and ERDA. I wrote the paper.

Paper 3.

I conceived the study together with my supervisors. I was involved in the planning of the project, performed all the depositions and characterizations except the turning test, where I participated, ERDA, and XPS. I wrote the paper.

Paper 4.

I was involved in the planning of the project, performed all the depositions, annealing experiments and characterizations except the turning test, and ERDA. I wrote the paper.

Paper 5.

I conceived the study together with the co-authors. I was involved in

the planning of the project, discussion of the results, and writing of

the paper.

xii

xiii

A CKNOWLEDGEMENTS

I would like to thank everyone who helped me during my PhD studies at Linköping University. Particularly:

My first supervisor Lars Hultman. Thank you for giving me the chance to pursue my career in a group with such highly scientific spirit, also for your valuable scientific and strategic supervision. I have always liked your fast action in reading my writings and manuscripts.

My second supervisor Per Eklund. Thanks for the scientific and experimental supervision I received from you during all these years.

Jacob Sjölén at Seco Tools AB in Fagersta. Thanks for helping me in getting familiar with the industrial world and its realities and difficulties.

Axel Genvad and Emmanuelle Göthelid at Sandvik Coromant AB, Materials and Processes R&D in Stockholm. Thanks for your patience and kind attitudes.

Björn Alling and Sergey Simak. Thank you for the theoretical calculations and modeling of our amazing cubic structure.

The co-authors in my articles; Carina Höglund, Justinas Palisaitis, Anders Eriksson, Jens Jensen, Jun Lu, Jens Birch, Per Persson, and Grzegorz Greczynski. It was impossible to make all this work without your helps and support.

Hans Högberg and Johanna Rosén. Thanks for your valuable comments and ideas in discussion of my results.

Kalle Brolin, Thomas Lingefelt, and Harri Savimäki for your endless

helps with my broken instruments in the labs.

xiv

All my friends and colleagues in Thin Film Physics, Nanostructured Materials, and Plasma and Coating Physics divisions. Thanks to every single one of you for sharing your time at some point with me in the labs, conference trips, courses and even during the fika times. I enjoyed your company a lot. Also those who helped me to improve my little knowledge of Swedish language and culture. I will always remember you.

All my family members who have always been there for me in several stages of my life. Thank you all. No part of this progress was ever possible without your help.

And last but definitely not least my lovely wife, Maryam. Thank you

for everything!

xv

S YMBOLS AND A BBREVIATIONS

a Lattice parameter in cubic and hexagonal lattices c Lattice parameter in hexagonal lattice

CBED Convergent beam electron diffraction CSM Continuous stiffness method

CVD Chemical vapor deposition

DCMS Direct current magnetron sputtering DFT Density functional theory

E Young’s modulus

E

Reduced elastic modulus E

Exchange-correlation energy

EDX Energy dispersive x-ray spectroscopy ERDA Elastic recoil detection analysis GGA Generalized gradient approximation GIXRD Grazing incidence X-ray diffraction h

Recovered depth

h

Maximum indentation depth hkl Miller index

H Hardness

HAADF High-angle annular dark field

HRTEM High resolution transmission electron microscopy k

Boltzmann constant

K Kinematic factor

K

Incoming reciprocal lattice vector LDA Local density approximation NRA Nuclear reaction analysis

P

Maximum load

PIXE Particle induced x-ray emission PVD Physical vapor deposition Q Scattering vector

RBS Rutherford backscattering spectroscopy RFMS Radio frequency magnetron sputtering

S Structure factor

SAED Selected area electron diffraction

SEM Scanning electron microscopy

SIMS Secondary ion mass spectroscopy

xvi

STEM Scanning transmission electron microscopy TEM Transmission electron microscopy

ToF-E Time of flight-energy UHV Ultra high vacuum V

Floating potential

XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

α Incoming beam angle 2θ Scattering angle

ϕ Azimuth angle

ψ Tilt angle

ν Poisson’s ratio

xvii

T ABLE OF C ONTENTS

1. INTRODUCTION ... 1

1.1 Background to the Field ... 2

1.2 Scope and Objectives ... 3

1.3 Outline of the Thesis ... 4

Bibliography ... 5

2. CHOICE OF MATERIAL SYSTEMS ... 9

2.1 Al

O

and Cr

O

... 9

2.2 Ternary Al-Cr-O System ... 11

2.2.1 The Pseudo-Binary Phase Diagram of Al

O

-Cr

O

... 11

2.2.2 Mixing of Cr in Al

O

... 13

2.3 Quaternary Al-Cr-O-N System ... 15

Bibliography ... 17

3. DEPOSITION AND GROWTH OF THIN FILMS ... 21

3.1 Physical Vapor Deposition ... 21

3.2 Thin Film Deposition by Reactive Magnetron Sputtering ... 22

3.2.1 Sputtering Process ... 22

3.2.2 Sputtering Yield ... 24

3.2.3 Origin of the Energetic Particles ... 24

3.2.4 Plasma ... 26

3.2.5 Magnetron Sputtering ... 27

3.2.6 Reactive Sputtering ... 29

3.2.7 Radio-Frequency Magnetron Sputtering ... 31

3.2.8 Reactive RF Magnetron Sputtering ... 32

3.3 Thin Film Deposition by Cathodic Arc Deposition ... 34

3.3.1 What is an Arc? ... 34

3.3.2 Cathodic Arc Deposition ... 34

3.3.3 Microdroplets and Microparticles ... 36

3.3.4 Cathodic Arc Deposition of Al-Cr-O and Al-Cr-O-N Films ... 38

3.4 Thin Film Growth ... 39

3.4.1 Nucleation ... 40

3.4.2 Crystal Growth ... 42

Bibliography ... 44

4. CHARACTERIZATION OF THIN FILMS ... 47

4.1 X-Ray Diffraction ... 47

4.1.1 The

/2

xviii

4.1.2 The Grazing Incidence XRD Technique ... 50

4.1.3 Texture Analysis by Pole Figures ... 51

4.2 Electron Microscopy ... 52

4.2.1 Scanning Electron Microscopy ... 53

4.2.2 Transmission Electron Microscopy ... 54

4.2.2.1 High Resolution Transmission Electron Microscopy ... 54

4.2.2.2 Electron Diffraction ... 55

4.2.2.3 Scanning Transmission Electron Microscopy ... 57

4.2.2.4 Energy Dispersive X-Ray Spectroscopy in TEM ... 58

4.3 Compositional Analysis ... 59

4.3.1 Basics of Ion Beam Analysis ... 60

4.3.2 Elastic Recoil Detection Analysis... 61

4.4 Mechanical Characterization ... 63

4.4.1 Nanoindentation ... 63

4.4.2 Metal Cutting Performance Test ... 65

Bibliography ... 68

5. THEORETICAL MODELING ... 69

5.1 Density Functional Theory... 69

Bibliography ... 73

6. SUMMARY AND CONTRIBUTION TO THE FIELD ... 75

6.1 The Al-Cr-O System ... 75

6.2 The Al-Cr-O-N System ... 78

THE PAPERS ... 81

Paper 1……….83

Paper 2……….………...……….……….………….…………...89

Paper 3.………..………...….…………...99

Paper 4.……….………...….…………...115

Paper 5.……….………...……….…..……...151

1 INTRODUCTION

1

1. I NTRODUCTION

Many products benefit from thin film technology. Thin films, which

have thickness of only one atomic layer to several micrometers

functionalize the materials in a desirable way. Due to their low

dimensions, thin films can have remarkably different properties

compared to bulk of the same material system. Therefore,

applications can benefit from the combined properties arising from

the bulk and thin film. In industry, thin films are widely used in

microelectronics and semiconductor-related trades. They are also

used in decorative coatings on, e.g., home appliances, heat-filtering

coatings on a window glass, wear- and heat-resistive coatings on jet

turbine blades, and bio-compatible coatings applied on body joints or

surgical instruments. In a relevant application to the scope of this

thesis, hard and wear-resistant coatings can increase the cutting tool

life by a factor of 50-60; thin films add remarkable durability to the

products [1].

2

1.1

B

F

Al

O

is a widely used material in industry in both the bulk and thin film forms. Al

O

coatings are associated with high hardness as well as resistance to corrosion and wear and therefore find applications in, e.g., microelectronics [1], thermal and diffusion barriers [2,3], and tooling industry [4]. Cr

O

is the hardest among the common oxides with a value of 29.5 GPa [2]. It also shows very good mechanical, optical, and chemical inertness properties and therefore has been extensively used for applications in corrosion protection [3], wear protection [4], optics [5], and electronics [6]. Consequently, Cr

O

thin films have also been well-investigated regarding the deposition and properties [7-11].

Cr

O

and α-Al

O

are isostructural; both have corundum crystal structure (crystallographic space group R3c) [18]. Moreover, they have similar lattice parameters with mismatch of ~4% [19-23].

Therefore, formation of a ternary Al-Cr-O solid solution is probable at higher temperatures of > 1200 °C [24,25]. However, deposition of thin films takes place at non-equilibrium conditions while the extraction of phase diagrams are based on the equilibrium conditions.

Much effort has been invested in the recent years to synthesis a ternary Al-Cr-O solid solution by using physical vapor deposition methods at substrate temperatures of 400-600 °C [26-32].

Besides the Al-Cr-O solid solutions, ternary nitride thin films of Al-

Cr-N solid solutions have also received attention [33-36]. This is due

to their high oxidation temperature (~900 °C), which makes them

beneficial compared to Ti-based alloys in tooling industry. Similar to

formation of ternaries from two binary systems, a quaternary Al-Cr-

O-N solid solution can also form by a combinatorial approach from

two ternary Al-Cr-O and Al-Cr-N systems. This is a new approach in

research and development of new hard coatings and has gained

interest in industry and academia [37-44]. The simultaneous presence

1 INTRODUCTION

3 of two non-metal elements in the Al-Cr-O-N thin films can likely increase the toughness of the coating by introduction of N-related covalent bonds to the pure ionic structure of the oxide.

Prior to my research work, it had become clear that Al-Cr-O solid solutions of corundum structure can form at substrate temperatures of 500-600 °C. There was also a patent on industrial synthesis by cathodic are deposition of (Al

Cr

)

O

films, without a determined crystal structure [45]. This Thesis presents the cubic structure of such, Al-Cr-O solid solutions. Also, original studies of the detailed process- structure-property relations in thin films of the ternary Al-Cr-O and quaternary Al-Cr-O-N materials systems are presented.

1.2

S

O

This thesis work has been inspired by scientific challenges on the

understanding of thin film synthesis and structure of complex oxides

and oxynitrides. The objectives are to understand the formation of Al-

Cr-O solid solutions at substrate temperatures as low as 450 °C and

below, incorporation of high level (> 75 at. %) of Al in the deposited

films, and thermal stability of the synthesized coatings when used in

real cutting tool applications. The technological drive is to use oxide-

based coatings to push the limits of tool life and productivity by

offering better chemical inertness for high-temperature applications,

compared to present nitride-based coatings. My research strategy is to

develop physical vapor deposition methods for ternary Al-Cr-O and

quaternary Al-Cr-O-N solid solution thin films. These are reactive

radio frequency magnetron sputtering and cathodic arc evaporation

techniques. The microstructure and mechanical properties of the films

have been characterized using different techniques of x-ray

diffraction, electron microscopy and hardness and cutting

performance tests in the as-deposited state and after thermal

annealing. The structural variations of the films occurred during the

4

depositions and annealing experiments are calculated using modeling techniques in density functional theory.

1.3

O

T

The thesis begins with chapters that present the research problem, materials system, deposition methods, and characterization methods.

The major findings and my overall contribution to the field are then discussed. The results are presented in the form of scientific papers submitted for publication in peer-reviewed international journals.

In Paper 1, the discovery of a cubic structure rather than the well- established corundum one is reported for the Al-Cr-O solid solutions.

Heat treatment studies of the magnetron sputtered cubic Al-Cr-O

films together with the calculation of activation energy for the phase

transformation process are presented in Paper 2. Paper 3 describes

the results of a comprehensive study on cathodic arc deposition and

application of Al-Cr-O-N thin films. In Paper 4 the experimental

conditions resulting in industrial cathodic arc deposition of cubic and

corundum Al-Cr-O thin films are investigated. Finally, the theoretical

calculations of the structure of the newly reported cubic phase of Al-

Cr-O thin films are presented in Paper 5.

1 INTRODUCTION

5 B

[1] Björn Ljungberg, Sandvik Coromant, private communication.

[2] G. V. Samsonov, The Oxide Handbook, 1^st ed., New York, Plenum Publishing Corporation, 1973, p. 254.

[3] U. Rothhaar et al, Temperature induced dissolution of Cr2O3 into tantalum, Thin Solid Film 302 (1997) 266.

[4] M. D. Bijker et al, The development of a thin Cr2O3 wear protective coating for the advanced digital recording system, Tribol. Int. 36 (2003) 227.

[5] F. D. Lai et al, Ultra-thin Cr2O3 well-crystallized films for high transmittance APSM in ArF line, Microelectron. Eng. 67-68 (2003) 17.

[6] E. Sourty et al, Chromium oxide coatings applied to magnetic tape heads for improved wear resistance, Tribol. Int. 36 (2003) 389.

[7] P. Hones et al, Characterization of sputter-deposited chromium oxide thin films, Surf. Coat. Technol. 120–121 (1999) 277.

[8] F. Luo et al, Role of deposition parameters on microstructure and mechanical properties of chromium oxide coatings, Surf. Coat. Technol.

202 (2007) 58.

[9] B. Bhushan et al, Characterization of R.F.-sputter-deposited chromium oxide films, Thin Solid Films 73 (1980) 255.

[10] A. Khanna et al, Growth and characterization of chromium oxide thin films prepared by reactive AC magnetron sputtering, J. Vac. Sci. Technol. A 24 (2006) 1870.

[11] X. Pang et al, Annealing effects on microstructure and mechanical properties of chromium oxide coatings, Thin Solid Films 516 (2008) 4685.

[12] W. H. Gitzen, Alumina as a ceramic material, 1^st ed., Westerville, American Ceramic Society, 1970, p. 3.

[13] J. Müller et al, Efficiency of α-alumina as diffusion barrier between bond coat and bulk material of gas turbine blades, Vacuum 71(2003) 247.

[14] R. Cremer et al, Thermal stability of Al–O–N PVD diffusion barriers, Surf.

Coat. Technol. 108–109 (1998) 48.

[15] N. I. Mukhurov et al, Anodic alumina: a promising constructional material for vacuum and semiconductor microelectronics, IEEE Cat. No.

97TH8257 (1997) 17.

[16] M. Åstrand et al, PVD-Al2O3-coated cemented carbide cutting tools, Surf.

Coat. Technol. 188-189 (2004) 186.

[17] B. M. Kramer et al, Computational design of wear coatings, J. Vac. Sci.

Technol. A 3 (1985) 2439.

[18] C. Giacovazzo, Symmetry in crystals, In: C. Giacovazzo ed., Fundamentals of Crystallography, 2^nd ed., New York, Oxford University Press, 2002, pp.

24-26.

6

[19] P. Eklund et al, α-Cr2O3 template-texture effect on α-Al2O3 thin-film growth, Thin Solid Films 516 (2008) 7447.

[20] P. Jin et al, Low temperature deposition of α-Al2O3 thin films by sputtering using a Cr2O3 template, J. Vac. Sci. Technol. A 20 (2002) 2134.

[21] J. M. Andersson et al, Microstructure of α-alumina thin films deposited at low temperatures on chromia template layers, J. Vac. Sci. Technol. A 22 (2004) 117.

[22] M. Ristić et al, Structural properties of the system Al2O3 -Cr2O3, Mater.

Lett. 16 (1993) 309.

[23] P. Jin et al, Localized epitaxial growth of α-Al2O3 thin films on Cr2O3

template by sputter deposition at low substrate temperature, Appl. Phys.

Lett. 82 (2003) 1024.

[24] T. M. Besmann et al, Thermochemical Analysis and Modeling of the Al2O3–Cr2O3, Cr2O3–SiO2, and Al2O3–Cr2O3–SiO2 Systems Relevant to Refractories, J. Am. Ceram. Soc., 89 (2006) 638.

[25] S. S. Kim et al, Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3 – Cr2O3 and V2O3– Cr2O3 Systems, J. Am. Ceram. Soc., 84 (2001) 1881.

[26] D. Diechle et al, Combinatorial approach to the growth of α-(Al1−x,Crx)2O3

solid solution strengthened thin films by reactive r.f. magnetron sputtering, Surf. Coat. Technol. 204 (2010) 3258.

[27] K. Pedersen et al, Texture and microstructure of Cr2O3 and (Cr,Al)2O3

thin films deposited by reactive inductively coupled plasma magnetron sputtering, Thin Solid Films 518 (2010) 4294.

[28] J. Ramm et al, Pulse enhanced electron emission (P3e™) arc evaporation and the synthesis of wear resistant Al–Cr–O coatings in corundum structure, Surf. Coat. Technol. 202 (2007) 876.

[29] J. Ramm et al, Thermal Stability of Thin Film Corundum-Type Solid Solutions of (Al1–xCrx)2O3 Synthesized Under Low-Temperature Non- Equilibrium Conditions, Adv. Eng. Mater. 9(2007) 604.

[30] L. A. Vieira et al, Approaches to influence the microstructure and the properties of Al–Cr–O layers synthesized by cathodic arc evaporation, Surf. Coat. Technol. 204 (2010) 1722.

[31] M. Pohler et al, Cathodic arc deposition of (Al,Cr)2O3: Macroparticles and cathode surface modifications, Surf. Coat. Technol. 206 (2011) 1454.

[32] J. Ramm et al, Correlation between target surface and layer nucleation in the synthesis of Al–Cr–O coatings deposited by reactive cathodic arc evaporation, Surf. Coat. Technol. 205 (2010) 1356.

[33] S. R. Pulugurtha et al, A study of AC reactive magnetron sputtering technique for the deposition of compositionally graded coating in the Cr–

Al–N system, Surf. Coat. Technol. 201 (2006) 4411.

[34] J. Vetter et al, (Cr:AI)N coatings deposited by the cathodic vacuum arc evaporation, Surf. Coat. Technol. 98 (1998) 1233.

1 INTRODUCTION

7

[35] X. Z. Ding et al, Cr1−xAlxN coatings deposited by lateral rotating cathode arc for high speed machining applications, Thin Solid Films 516 (2008) 1710.

[36] R. J. Smith et al, Using CrAlN Multilayer Coatings to Improve Oxidation Resistance of Steel Interconnects for Solid Oxide Fuel Cell Stacks, J. Mater.

Eng. Perform. 13 (2004) 295.

[37] D. Diechle et al, Combinatorial approach to the growth of α- (Al1−x,Crx)2+δ(O1−y,Ny)3 solid solution strengthened thin films by reactive r.f.

magnetron sputtering, Surf. Coat. Technol. 206 (2011) 1545.

[38] P. E. Gannon et al, Simulated SOFC Interconnect Performance of Crofer 22 APU with and without Filtered Arc CrAlON Coatings, Electrochem.

Solid-State Lett. 11 (2008) B 54.

[39] H. Najafi et al, Correlation between anionic substitution and structural properties in AlCr(OxN1−x) coatings deposited by lateral rotating cathode arc PVD, Thin Solid Films 520 (2011) 1597.

[40] M. Stueber et al, Magnetron sputtering of hard Cr–Al–N–O thin films, Thin Solid Films 519 (2011) 4025.

[41] Q. M. Wang et al, Ion-plated Al–O–N and Cr–O–N films on Ni-base superalloys as diffusion barriers, Surf. Coat. Technol. 197 (2005) 68.

[42] M. Hirai et al, Mechanism of hardening in Cr–Al–N–O thin films prepared by pulsed laser deposition, J. Vac. Sci. Technol. A 21 (2003) 947.

[43] M. Hirai et al, Oxidation behavior of Cr-Al-N-O thin films prepared by pulsed laser deposition, Thin Solid Films 407 (2002) 122.

[44] A. Kayani et al, Oxidation studies of CrAlON nanolayered coatings on steel plates, Surf. Coat. Technol. 201 (2006) 1865.

[45] D. Kurapov, International Patent No. WO 2010/040494 A1, 2010.

8

2 CHOICE OF MATERIAL SYSTEM

9

2. C HOICE OF M ATERIAL S YSTEMS

This chapter presents the different oxides and oxynitrides that are the subject of my research.

2.1 Al

O

Cr

O

Aluminum oxide (alumina; Al

O

) exists in several polymorphs;

different crystal structures with an identical chemical formula of

Al

O

. Besides the thermodynamically stable α-phase (called

corundum) at room temperature and atmospheric pressure, there are

thermodynamically metastable phases of γ and η in cubic symmetry,

δ and κ in Tetragonal-Orthorhombic symmetry, and θ and λ in

monoclinic symmetry. Some of these polymorphs in metastable

alumina (e.g., γ-alumina) have found applications in industry as

catalysts and adsorbents due to their high surface area and catalytic

activities. α-Al

O

is a suitable choice of material for the applications

carried out in harsh and corrosive environments as well as at high-

temperatures. Therefore, it has been desirable to deposit the Al

O

10

films in this thermodynamically stable α-phase rather than those metastable polymorphs [1-5].

Chromium oxide (chromia; Cr

O

) has only the thermodynamically stable form called eskolaite with the same crystal structure as α- Al

O

. Cr

O

is known as the hardest among the common oxides with a hardness of 29.5 GPa [6]. It has a wide range of applications in corrosion protection, wear resistance, electronics and optics [7-10].

The Cr

O

coatings have also been well-investigated in terms of their deposition, characterization, and properties [11-15].

Corundum crystal structure of α-Al

O

and Cr

O

has the

crystallographic space group R3c. In this structure, oxygen atoms

form a semi-hexagonal close packed (hcp) sublattice in which Al/Cr

atoms are placed in two-thirds of the octahedral sites [16]. The top

view of corundum structure (figure 2.1.b) shows the semi-hcp

structure of oxygen atoms. The side view of structure shows the

presence of 18 O and 12 Al/Cr atoms. It is important to note that the

primitive cell of α-alumina has a rhombohedral lattice with a basis

that consists of 6 O and 4 Al atoms, but it is more convenient to show

the structure as hexagonal shown in figure 2.1. α-Al

O

and Cr

O

are

close in their lattice parameters in a way that a and c values in their

corundum structure differs for 4% and 4.7%, respectively [17,18].

2 CHOICE OF MATERIAL SYSTEM

11

(a) (b) (c)

Figure 2.1 (a) perspective, (b) top, and (c) side views of schematic illustration of corundum crystal structure in which big and small circles represent oxygen and metal (Al in alumina and Cr in chromia), respectively.

2.2

T

Al-Cr-O S

2.2.1

T

P

-B

P

D

Al

O

- Cr

O

Figure 2.2.a shows the pseudo-binary phase diagram of Al

O

-Cr

O

at the thermodynamically equilibrium conditions of room temperature and atmospheric pressure [19-21]. According to this phase diagram, the solid solution of Al-Cr-O (marked by (Al,Cr)

O

(ss)) exist for the whole range of compositions if the temperature is above 1250 °C.

Below this temperature and for the compositions with molar fraction

of Cr

O

< 0.8, the solid solution area is limited by a dashed line (or

full line at higher pressures). This line is the boundary of miscibility

gap area and separates the solid solution from the phase separated Al-

rich and Cr-rich regions. A miscibility gap occurs if two elements or

phases form an endothermic solid solution, i.e., when the difference

in the enthalpy of system before and after the mixture, ∆H

, has a

positive value (i.e., ∆H

> 0). We know that the Gibbs free energy,

12

∆G, of a system is defined as ∆H - T∆S in which, ∆S is the difference in entropy. At high temperatures, T∆S

is a large term and therefore,

∆ G

becomes negative meaning that the solution is thermodynamically stable. At low temperatures, T∆S

is small and therefore, ∆G

becomes positive for a certain range of compositions; meaning that solid solution is not anymore thermodynamically stable. In a miscibility gap, there is a region in which the Gibbs free energy curve has a negative curvature, i.e.,

2

0

2

G dx <

d (Figure 2.2.b). This region is called chemical spinodal and upon cooling, one-phase alloys with a composition falling in this region may undergo a phase transformation named spinodal decomposition. Spinodal decomposition is an up-hill process in which the alloy components move toward the higher concentrations [22].

Spinodal decomposition in the Al

O

-Cr

O

system was observed by Schultz and Stubican for a powder-metallurgy prepared (55 mol%

Al

O

-45 mol% Cr

O

) solid solution of α-(Al

Cr

)

O

annealed at

temperature of 800 °C [23]. In this thesis, annealing of the Al-Cr-O

films is performed and reported in Papers 2 and 4. The results show

that the annealing of corundum Al-Cr-O solid solution films does not

result in a phase separation to the binary Al

O

and Cr

O

by spinodal

decomposition. This is likely to be due to the limitation in increasing

the annealing temperature to > 1100 °C and more. On the other hand,

it is predicted that the spinodal decomposition occurs if the annealing

temperature is increased to temperatures above the dashed line in

figure 2.2.a; i.e., 1250 °C and above where the alloy is in one-phase

solid solution region. This, however, was not practical in this thesis

due to the melting of Co in the substrate at temperatures > 1100 °C.

2 CHOICE OF MATERIAL SYSTEM

13

(a) (b)

Figure 2.2 (a) Pseudo-binary phase diagram of Al2O₃-Cr₂O₃; adopted from [19], and (b) Gibbs free energy of an alloy falling in a spinodal region; redrawn from [22]. The single phase region at 1 bar is marked by α-(Al1-xCr_x)₂O₃(ss) in (a). By increasing the pressure, the miscibility gap moves toward the lower temperatures.

2.2.2

M

Cr

Al

O

For several years, the deposition of α-Al

O

thin films has been

performed by using chemical vapor deposition (CVD) method at

temperatures of 1000

C and more [24]. This range of substrate

temperature is too high for a variety of substrates, e.g., polymeric-

based materials like plastics and high speed steels. Therefore, to

decrease the deposition temperature in CVD method, several attempts

have been performed; e.g., plasma-assisted CVD to deposit the α-

Al

O

coatings at temperature of ~600 °C [25,26]. In this technique,

the energetic particles present in the plasma bombard the growing

film. This provides the required energy for the growth of corundum

phase in a decreased process temperature [27].

14

Along with CVD, many efforts have been done to deposit the α-Al

O

films by using different techniques of PVD at lower substrate temperatures. Techniques like magnetron sputtering [28-32] and cathodic arc deposition [33-35] were employed to deposit the α-Al

O

films at substrate temperatures of ~500-700 °C. In majority of these works, a seed layer has been used as a facilitator for the lower- temperature deposition of corundum phase in the functional α-Al

O

film. The Cr

O

layer was either deliberately deposited from a source or formed as a result of the oxidation of the Cr-containing substrate.

Cr

O

films can be deposited at substrate temperatures of 400 °C and lower, which is remarkably lower compared to α-Al

O

films. The low-temperature deposition of Cr

O

films is likely due to the absence of metastable phases like γ, κ, etc., which are present in the case of Al

O

(section 2.1). In this regard, Cr

O

layer is employed as a pre- deposited template to provide the nucleation site for continued growth of isostructural α-Al

O

through a coherent interface/boundary at lower substrate temperatures [17, 36-38].

In a different approach, several efforts have been made for low- temperature deposition of corundum phase through alloying of Cr

O

and Al

O

by forming a (Al

Cr

)

O

solid solution [39-44]. This is supported by the fact that Cr

O

and α-Al

O

have small lattice mismatch of about 4% and 4.7% in their a and c axes in corundum structure, respectively.

Here, solid solutions of Al-Cr-O thin films have been deposited in two different crystal structures of cubic (Papers 1, 2, and 4) and corundum (Paper 4). The cubic films deposited at substrate temperature of 400 °C by magnetron sputtering are most likely vacancy-stabilized solid solutions, which are also reported by Ref.

[45]. The corundum films are deposited by cathodic arc deposition at

600 °C.

2 CHOICE OF MATERIAL SYSTEM

15 2.3

Q

Al-Cr-O-N S

Thin films of quaternary Al-Cr-O-N solid solutions can be formed by alloying of Al-Cr-O and Al-Cr-N material systems. Al-Cr-N films have been recently used in several applications like fuel cells and tooling industry. In the latter case, they have shown to have superior properties compared to the Ti-based nitrides. This is mainly due to the formation of aluminum and chromium oxynitrides on the surface, which eventually prevents the further diffusion of the oxygen to the underlying layers of the tool [46,47]. Also, the solubility of CrN in AlN results in the formation of Cr-Al-N solid solutions with super hardness (values of ~ 40 GPa). The hardening mechanism in such alloys is explained as pinning of dislocations by the dissolved Al/Cr atoms in the lattice [48-50].

The ternary Al-Cr-O and Al-Cr-N thin films are only different in their non-metal elements; i.e., O and N, which are placed next to each other in the periodic table of elements and have different valency.

Therefore, in order to maintain the electroneutrality of the material during e.g. introduction of O atoms in Al-Cr-N, two nitrogen atoms have to be replaced by three oxygen atoms (2 3 ). An alternative to that is to reduce the negative charge by creation of

cation vacancies ( 1 ) [45].

The latter approach is likely the case in the arc deposited Al-Cr-O-N films in Paper 3, in which solid solution of cubic phase is observed.

Reviewing the nature of the bonds in ternary oxide and nitrides

encourages that quaternary Al-Cr-O-N solid solution can be

advantageous. This is in terms of having the simultaneous presence of

ionic and covalent bonds, which are respectively formed by oxygen

and nitrogen atoms. A material with such ionocovalant structure is

likely to be tougher compared to the majority of hard oxides with a

purely ionic structure, as previously observed for Al-Ti-O-N films

[51-53]. The incorporation of covalent bonds in Al-Ti-O-N films had

16

increased the wear resistant and hardness of the coatings. The effect of covalent bonds in this process is attributed to the fact that they are highly directional and can affect the deformation mechanisms.

Examples of hard materials with covalent bonds are diamond (purely

covalent) and c-BN (dominantly covalent) [54]. In this thesis, Al-Cr-

O-N thin films are deposited by cathodic arc deposition technique in

Paper 3.

2 CHOICE OF MATERIAL SYSTEM

17 B

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97TH8257 (1997) 17.

[2] J. Müller et al, Efficiency of a-alumina as diffusion barrier between bond coat and bulk material of gas turbine blades, Vacuum 71(2003) 247.

[3] R. Cremer et al, Thermal stability of Al–O–N PVD diffusion barriers, Surf.

Coat. Technol. 108–109 (1998) 48.

[4] M. Åstrand et al, PVD-Al2O3-coated cemented carbide cutting tools, Surf.

Coat. Technol. 188-189 (2004) 186.

[5] I. Levin et al, Metastable Alumina Polymorphs: Crystal Structure and Transition Sequence, J. Am. Ceram. Soc. 81 (1998) 1995.

[6] G. V. Samsonov, The Oxide Handbook, 1^st ed., New York, Plenum Publishing Corporation, 1973, p. 254.

[7] U. Rothhaar et al, Temperature induced dissolution of Cr2O3 into tantalum, Thin Solid Film 302 (1997) 266.

[8] M. D. Bijker et al, The development of a thin Cr2O3 wear protective coating for the advanced digital recording system, Tribol. Int. 36 (2003) 227.

[9] F. D. Lai et al, Ultra-thin Cr2O3 well-crystallized films for high transmittance APSM in ArF line, Microelectron. Eng. 67-68 (2003) 17.

[10] E. Sourty et al, Chromium oxide coatings applied to magnetic tape heads for improved wear resistance, Tribol. Int. 36 (2003) 389.

[11] P. Hones et al, Characterization of sputter-deposited chromium oxide thin films, Surf. Coat. Technol. 120–121 (1999) 277.

[12] F. Luo et al, Role of deposition parameters on microstructure and mechanical properties of chromium oxide coatings, Surf. Coat. Technol.

202 (2007) 58.

[13] B. Bhushan et al, Characterization of R.F.-sputter-deposited chromium oxide films, Thin Solid Films 73 (1980) 255.

[14] A. Khanna et al, Growth and characterization of chromium oxide thin films prepared by reactive AC magnetron sputtering, J. Vac. Sci. Technol. A 24 (2006) 1870.

[15] X. Pang et al, Annealing effects on microstructure and mechanical properties of chromium oxide coatings, Thin Solid Films 516 (2008) 4685.

[16] M. Barsoum, Fundamentals of Ceramics, 1^st ed., New York, McGraw-Hill (1997), p. 66.

[17] P. Jin et al, Low temperature deposition of α-Al2O3 thin films by sputtering using a Cr2O3 template, J. Vac. Sci. Technol. A 20 (2002) 2134.

[18] M. Ristić et al, Structural properties of the system Al2O3 -Cr2O3, Mater.

Lett. 16 (1993) 309.

18

[19] M. Fujita et al, Sintering of Al2O3-Cr2O3 powder prepared by sol-gel process, J. Soc. Mater. Sci. Japan 56 (2007) 526.

[20] T. M. Besmann et al, Thermochemical Analysis and Modeling of the Al2O3

–Cr2O3, Cr2O3–SiO2, and Al2O3–Cr2O3–SiO2 Systems Relevant to Refractories, J. Am. Ceram. Soc., 89 (2006) 638.

[21] S. S. Kim et al, Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3 –Cr2O3 and V2O3–Cr2O3 Systems, J. Am. Ceram. Soc.

84 (2001) 1881.

[22] D. A. Porter, and K. E. Easterling, Phase Transformation in Metals and Alloys, 3^rd ed., Boca Raton, CRC Press, 2009, pp. 308-314.

[23] A. H. Schultz et al, Separation of Phases by Spinodal Decomposition in the Systems Al2O3-Cr2O3 and Al2O3-Cr2O3-Fe2O3, J. Am. Ceram. Soc. 53 (1970) 613.

[24] B. Lux et al, Preparation of alumina coatings by chemical vapor depositon,Thin Solid Films 138 (1986) 49.

[25] K. Jiang et al, Low temperature synthesis of α-Al2O3 films by high-power plasma-assisted chemical vapour deposition, J. Phys. D 43 (2010) 325202.

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[28] J. M. Andersson et al, Phase control of Al2O3 thin films grown at low temperatures, Thin Solid Films 513 (2006) 57.

[29] O. Zywitzki et al, Effect of the substrate temperature on the structure and properties of Al2O3 layers reactively deposited by pulsed magnetron sputtering, Surf. Coat. Technol. 82 (1996) 169.

[30] E. Wallin et al, Influence of residual water on magnetron sputter deposited crystalline Al2O3 thin films, Thin Solid Films 516 ( 2008) 3877.

[31] T. Kahora et al, Deposition of α-Al2O3 hard coatings by reactive magnetron sputtering, Surf. Coat. Technol. 185( 2004) 166.

[32] O. Zywitzki et al, Correlation between structure and properties of reactively deposited Al2O3 coatings by pulsed magnetron sputtering, Surf.

Coat. Technol. 94–95 (1997) 303.

[33] J. Rosén et al, Effect of Ion Energy on Structure and Composition of Cathodic Arc Deposited Alumina Thin Films, Plasma Chem. Plasma Process. 25 (2005) 303.

[34] Y. Yamada-Takamura et al, Characterization of α-phase aluminum oxide films deposited by filtered vacuum arc, Surf. Coat. Technol. 142-144 (2001) 260.

[35] R. Brill, Crystal structure characterisation of filtered arc deposited alumina coatings: temperature and bias voltage, Surf. Coat. Technol. 174 –175 (2003) 606.

2 CHOICE OF MATERIAL SYSTEM

19

[36] P. Jin et al, Localized epitaxial growth of α-Al2O3 thin films on Cr2O3

template by sputter deposition at low substrate temperature, Appl. Phys.

Lett. 82 (2003) 1024.

[37] J. M. Andersson et al, Microstructure of α-alumina thin films deposited at low temperatures on chromia template layers, J. Vac. Sci. Technol. A 22 (2004) 117.

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[39] J. Ramm et al, Pulse enhanced electron emission (P3e™) arc evaporation and the synthesis of wear resistant Al–Cr–O coatings in corundum structure, Surf. Coat. Technol. 202 (2007) 876.

[40] J. Ramm et al, Thermal Stability of Thin Film Corundum-Type Solid Solutions of (Al1–xCrx)2O3 Synthesized Under Low-Temperature Non- Equilibrium Conditions, Adv. Eng. Mater. 9(2007) 604.

[41] L. A. Vieira et al, Approaches to influence the microstructure and the properties of Al–Cr–O layers synthesized by cathodic arc evaporation, Surf. Coat. Technol. 204 (2010) 1722.

[42] M. Pohler et al, Cathodic arc deposition of (Al,Cr)2O3: Macroparticles and cathode surface modifications, Surf. Coat. Technol. 206 (2011) 1454.

[43] D. Diechle et al, Combinatorial approach to the growth of α-(Al1−x,Crx)2O3

solid solution strengthened thin films by reactive r.f. magnetron sputtering, Surf. Coat. Technol. 204 (2010) 3258.

[44] K. Pedersen et al, Texture and microstructure of Cr2O3 and (Cr,Al)2O3

thin films deposited by reactive inductively coupled plasma magnetron sputtering, Thin Solid Films 518(2010) 4294.

[45] H. Najafi et al, Correlation between anionic substitution and structural properties in AlCr(OxN1−x) coatings deposited by lateral rotating cathode arc PVD, Thin Solid Films 520 (2011) 1597.

[46] R.J. Smith et al, Using CrAlN Multilayer Coatings to Improve Oxidation Resistance of Steel Interconnects for Solid Oxide Fuel Cell Stacks, J. Mater.

Eng. Perform.

[47] S. R. Pulugurtha et al, A study of AC reactive magnetron sputtering technique for the deposition of compositionally graded coating in the Cr–

Al–N system, Surf. Coat. Technol. 201 (2006) 4411.

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Al–O–N thin films, Surf. Coat. Technol. 201 (2007) 6392.

20

[52] F. Mei et al, Microstructure and mechanical properties of (Ti,Al)(O,N) films synthesized by reactive sputtering, Mater. Lett. 60 (2006) 375.

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3 DEPOSITION AND GROWTH OF THIN FILMS

21

3. D EPOSITION AND G ROWTH OF T HIN F ILMS

The deposition of thin films can be done by several techniques, which can be categorized in three major families of (1) physical, (2) chemical, and (3) hybrid methods. In this thesis, the focus is entirely on physical methods, although chemical methods are touched for comparison. Hybrid methods are combinations of physical and chemical methods; a plasma discharge is used for activation of the chemical deposition processes. This family is widely used in deposition of inorganic and polymer thin films. A more detailed list of these processes and methods can be found in Refs. [1-3].

3.1

P

V

D

Chemical vapor deposition is based on the formation of chemical

bonds between the source species in precursor and dangling bonds of

the atoms present at the substrate surface. In contrast, PVD methods

are generally purely physical processes (reactive PVD techniques are

exceptions).