Growth and Characterization of Ti-Si-N Hard Coatings

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Linköping Studies in Science and Technology Licentiate Thesis No. 1270

Growth and Characterization of

Ti-Si-N Hard Coatings

Axel Flink

LiU-TEK-LIC-2006:51 Thin Film Physics Division

Department of Physics, Chemistry, and Biology (IFM) Linköping University, 581 83 Linköping, Sweden

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ISBN: 91-85643-85-8 ISSN: 0280-7971

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Abstract

Metastable (Ti,Si)N alloy and TiN/SiNx multilayer thin solid films as well as SiNx/TiN

surfaces have been explored. Cubic Ti1-xSixN (0 x 0.14) films deposited onto cemented

carbide (WC-Co) substrates by arc evaporation exhibited a competitive columnar growth mode where the structure transforms to a feather-like nanostructure with increasing Si content as revealed by x-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy revealed the presence of Ti-N and Si-N bonding, but no amorphous Si3N4. Band structure calculations showed that phase separation of

NaCl-structure Ti1-xSixN solid solution into cubic SiN and TiN phases is energetically

favorable. The metastable microstructure, however, was maintained for the Ti0.86Si0.14N

film annealed at 900 °C, while recrystallization in the cubic state took place at 1100 °C annealing during 2h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x=0 and x=0.14, nanoindentation gave a hardness of 29.9±3.4 GPa and 44.7±1.9 GPa, respectively. The hardness was retained during annealing at 900 °C.

Nanostructured materials, e.g., nanocomposites and nanolaminates, are defined by internal interfaces, of which the nature is still under debate. In this work two-phase model systems were explored by depositing SiNx/TiN nanolaminate films, including

superlattices containing cubic SiNx, by dual target reactive magnetron sputtering. It is

demonstrated that the interfacial phase of SiNx onto TiN(001) and TiN(111) can be

crystalline, and even epitaxial with complex surface reconstructions. Using in situ structural analyses combined with ab initio calculations, it is found that SiNx layers grow epitaxially, giving rise to strong interfacial bonding, on both TiN(001) and TiN(111) surfaces. In addition, TiN overlayers grow epitaxially on SiNx/TiN(001) bilayers in

nanolaminate structures. These results provide insight into the development of design rules for novel nanostructured materials.

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Preface

This Licentiate Thesis is based on my research carried out with the Thin Film Physics Division at Linköping University in collaboration with SECO Tools AB in Fagersta, the Materials Science Department at University of Illinois at Urbana-Champaign, and the Department of Materials Chemistry at Uppsala University. The work is supported by SECO Tools AB, the Swedish Research Council (VR), and the Swedish Foundation for Strategic Research (SSF).

Included Papers

Paper I Influence of Si on the Microstructure of Arc Evaporated (Ti,Si)N Thin Films; Evidence for Cubic Solid Solutions and their Thermal Stability A. Flink, T. Larsson, J. Sjölén, L. Karlsson, L. Hultman,

Surf. Coat. Technol. 200 (2005) 1535-1542

Paper II Toward Understanding Interface Structure in Superhard TiN-SiN Nanolaminates and Nanocomposites

L. Hultman, J. Bareno, A. Flink, H. Söderberg, K. Larsson, V. Petrova, M. Odén, J. E. Greene, I. Petrov,

Submitted for publication

Other Papers by the Author

Paper III Deposition of Ti2AlN Thin Films by Reactive Magnetron Sputtering

T. Joelsson, A. Flink, J. Birch, L. Hultman, Manuscript in final preparation

Paper IV MAX-Phase Ti2AlN Coatings by Arc Deposition

A.Flink, J. Sjölén, L. Karlsson, L. Hultman, In manuscript

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Paper V Growth and characterization of single crystalline TiN/SiNx superlattice

films

H. Söderberg, A. Flink, J. Birch, L. Hultman, M. Odén, In manuscript

Paper VI Role of Carbon in Boron Suboxide Thin Films

D. Music, V. M. Kugler, Zs. Czigany, A. Flink, O. Werner, J. M. Schneider, L. Hultman, U. Helmersson,

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Acknowledgements

Many persons have contributed to my work and I would especially like to thank:

Lars Hultman, my supervisor. Thank you for giving me the opportunity to do both my diploma work and PhD in the Thin Film Group. I am very grateful for all support you have given me so far.

Lennart Karlsson, Jacob Sjölén, and Tommy Larsson at SECO Tools AB for fruitful collaboration. I always enjoy going to Fagersta and SECO for meetings and work. Javier Bareno, Vania Petrova, and Ivan Petrov from University of Illinois at Urbana-Champaign for taking care of me during my visits in Chambana.

Hans Söderberg and Magnus Odén from Luleå University of Technology. Hasse, for nice collaboration and discussions. Magnus, for your efforts to always answer my endless list of questions regarding nanoindentation.

Karin Larsson for introducing me and Javier to CASTEP.

Per Persson, for helping me learning TEM, both by practice and discussions.

Jens Birch for giving fast and accurate answers, and for increasing the already good spirit in the group.

Hans Högberg, for always taking time to answer my questions, and for encouragement. Karl-Olof Brolin, Inger Eriksson, and Thomas Lingefelt for your endless helping spirit. Anders Hörling, for sharing your thoughts, both about science and life in general. Per Eklund, my old neighbor in Ryd, for our interesting football discussions, and for recommending me to apply for a diploma work at the Thin Film Physics Division. Anders E, Johan, and Timo for our unforgettable golf/poker trips.

Erik, Fredrik, Martina, and Naureen for a fantastic week on Iceland!

All friends and colleagues, both past and present, in the Thin Film and Plasma groups. I really have a lot of fun both at work and beside work with you!

My family, of course, for all your support throughout these years (and the years before I started in graduate school).

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1 Introduction

Ceramics is an interesting class of material in the sense of heat resistance, high hardness, thermal shock resistance, and producabililty as thin films. Ceramics can be defined as …“solid compounds that are formed by the application of heat, and sometimes heat and pressure, comprising at least two elements provided one of them is a metal or a non-metallic solid. The other element(s) may be a metal(s) or another non-non-metallic solid(s).”1

The properties of the ceramics make them attractive for thin film applications within the metal cutting industry. Many industries are dependent of tools for metal cutting applications; this has created a strong interest for developing new methods and materials for making the tools more efficient and cheaper.

1.1 Hard Coatings for Cutting Tools

During the 1970’s replaceable cutting inserts together with hard coatings were introduced. One of the first wear-resistant coating materials in the modern cutting tool industry used was TiN.2_{It exhibits high hardness and stiffness which makes it suitable as}

a cutting tool coating. The main shortcoming is, however, limited stability due to oxidation at temperatures above 500 °C.3_{Today the work temperatures of tools are}

typically between 800-1200 °C. Therefore, there is a growing interest concerning coating materials with improved thermal stability. The present design concept has been to employ ternary compounds, e.g. (Ti,Al)N4,5_{. (Ti,Al)N offers improved oxidation resistance, better}

thermal stability, defect (compressive residual stress) hardening, as well as newly discovered age hardening6_{. Together, these factors improve the life time of the tool, and}

provide the possibility to work at higher cutting speed. (Ti,Al)N has been used as a protective coating for cutting tools since the early 1990’s and is still a work horse for hard coatings.7_{Today, (Ti,Al)N used in metal cutting industry are examples of}

metastable hard coatings that can be synthesized by arc evaporation at low temperature. Further on, the coating technology development and research on ternary compounds have expanded to cover a range of compounds based on the Ti-Al-N, Cr-Al-N, and most recently, Ti-Si-N systems (see Paper I). One example from the latter are the TiN/SiNx

nanocomposites8_{, which exhibit thermal stability and very promising mechanical}

properties including superhardness. These nanocomposites consist of TiN nanocrystallites embedded in what is assumed to be amorphous SiNx. However, there is

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TiN/SiNx multilayers, or nanolaminates, have been synthesized.10,11 Furthermore, recent

work points to the possibility of fabricating (Ti,Si)N solid solutions12,13,14_{(see Paper I) as}

well as epitaxially stabilized cubic-SiNx (see PaperII). The objective of this thesis is to

explore the synthesis, structure, and properties of the novel (Ti,Si)N alloy and SiNx/TiN

nanolaminate thin films.

1_{M. W. Barsoum, Fundamentals of Ceramics, McGraw-Hill (1997)} 2_{P. O. Snell, Jernkontorets Anm. 154 (1970) 413}

3_{H. Ichimura, A. Kawana, J. Mat. Res. 8 5 (1993) 1093}

4_{O. Knotek, W. Bosch, T. Leyendecker, Proc. 7th Int. Conf. Vacuum Metallurgy, Linz, Austria 1985} 5_{W. –D. Münz, J. Göbel, Proc. 7th Int Conf. Vacuum Metallurgy, Linz, Austria 1985}

6_{A. Hörling, PhD Thesis (Linköping Studies in Science and Technology, dissertation no. 922, Linköping}

University, Sweden 2005

7_{S. PalDey, S. C. Deevi, Mat. Sci. And Eng. A342 (2003) 58} 8_{S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64} 9_{S. Hao, B. Delley, C. Stampfl, Phys. Rev. B 74 (2006) 035402}

10_{H. Söderberg, M. Odén, J. M. Molina-Aldereguia, L. Hultman, J. Appl. Phys. 97 (2005) 114327} 11_{X. Hu, H. Zhang, J. Dai, G. Li, M. Gu, J. Vac. Sci. Technol. A23 (2005) 114}

12_{F. Vaz, L. Rebouta, B. Almeida, P. Goudeau, J. Pacaud, J. P. Riviere, J. Bessa Sousa, Surf. Coat.}

Technol. 120-121 (1999) 166

13_{J. L. He, C. K. Chen, M. H. Hon, Mater. Chem. Phys. 44 (1996) 9}

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2 The Ti-Si-N System

In Paper I cubic Ti1-xSixN metastable solid solution coatings deposited by arc evaporation

were studied. In Paper II TiN/SiNx multilayers were deposited by reactive DC sputtering

with different thicknesses of the SiNx layers.

2.1 Phase Diagram

Fig. 2.1 The ternary phase diagram for Ti-Si-N at 1000 °C.1

The phase diagram at 1000 °C in Fig. 2.1 shows that Si3N4 is the only stable Si-N

compound. TiN is stable over a wide stoichiometry range. Furthermore, there is no ternary phase present.

2.2 Titanium Nitride

TiN is a ceramic which is used in a wide field of thin film applications, from diffusion barriers to wear-resistant coatings to decorative coatings. TiN has a rocksalt structure (NaCl) with a unit cell consisting of 8 atoms; 4 Ti and 4 N. The lattice parameter is 4.24 Å.2_{TiN exhibits high hardness, 20 GPa}3_{as single crystal thin film, and 26 GPa}4_as

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Fig. 2.2 Image illustrating the rocksalt, or NaCl, structure.

2.3 Silicon Nitride

Silicon nitride as a thin film material is mostly used within electronics. It exists as Si3N4

in three different polytypes, two hexagonal, - and , and one amorphous, a.5_{There is}

also a high pressure, high temperature cubic phase.6_{In a-Si}

3N4 the average binding

distance is 1.74 Å.7

2.4 TiN/SiN

x

Nanocomposites

A nanocomposite can be defined as a composite structure whose characteristic dimensions are found at the nanoscale.8_{The superhard nanocomposites}9_{of nc-TiN/SiN}

x10

exhibit relatively good thermal and chemical stability. In the idealized case, this nanocomposite consists of crystalline TiN grains which are embedded in a tissue phase of amorphous Si3N4. A prerequisite to synthesize nanocomposites is a strong segregation

tendency between the constituents in order to get a strong interface between the nanocrystals and the Si3N4 phase. This is the case for TiN and Si3N4, which have

essentially no solid solubility; see the pseudo-binary phase diagram in Fig. 2.3. For the nanocomposites with the highest hardness, the grain sizes should be below 10 nm, and the tissue phase that separates the nanocrystallites on the order of 1-2 monolayers (ML) thick.11_{The hardness enhancement is then explained by small crystallite sizes of TiN,}

which gives grain boundary hardening, together with inhibited grain boundary sliding and crack propagation from the Si3N4 phase.

N

Ti

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2.5 TiN/SiN

x

Multilayers

A multilayer thin film consists of alternating layers of two or more materials. The sum of two consecutive layers in a bilayer system is called multilayer period ( ). A multilayer containing epitaxial layers is called superlattice. Also, multilayered structures with characteristic dimensions on the nanoscale are referred to as nanolaminates. Recently, publications regarding superhard TiN/SiNx multilayers have been published,2,12,13 where

metastable c-SiNx have been epitaxially strained between TiN layers in an artificial

superlattice structure. These hardness correlates to the thickness of the SiNx layer, and the

hardness is highest for the case of superlattice with SiNx layer thickness of 1-2 ML. The

high hardness is explained by hindering of dislocation motion.12

2.6 Ternary Solid Solutions

A solid solution can be defined as follows…“A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. Both of these types of solid solution affect the properties of the material by distorting the crystal lattice and disrupting the physical and electrical homogeneity of the solvent material.”14

It was stated in section 2.1 that there exist no thermodynamically stable ternary Ti-Si-N compounds. However, based on the use of our method for growth of metastable thin films in section 3.4, metastable (Ti,Si)N cubic solid solutions can in fact be realized. Figure 2.3 shows the pseudo-binary phase diagram for TiN and SiN and indicates a strong phase separation tendency from a solid solution into the binary phases. This implies, that for a metastable (Ti,Si)N solid solution, a phase separation into the binary phases may be expected during annealing. The region of ´+ ´´ is the miscibility gap, where the cubic binary phases are preferred.

The chemical spinodal is indicated by a dashed curve in Fig. 2.3. Within this curve the eventual decomposition is spinodal and outside of which towards the binodal (solid curve), phase separation by nucleation and growth would take place. Spinodal

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decomposition can briefly be described as ‘up-hill’ diffusion, in which atoms diffuse towards high-concentration regions.15

0,000 0,25 0,50 0,75 1,00 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 Te m pe ra tu re (K ) x, Ti1-xSixN Te m pe ra tu re ( oC) δδδδ' + δ + δ + δ + δ'' δδδδ_-Ti1-xSixN TiN SiN

Fig. 2.3 Pseudo-binary phase diagram for TiN-SiN together with the

chemical spinodal (dashed line), calculated down to 1727 °C (Liquid state not considered). From Paper I.

The pseudo-binary phase diagram was calculated in the following way. Gibbs free energy, G, of a system is defined by

G = H − TS Eq. 1

where H, T, and S are the system’s enthalpy, temperature, and entropy, respectively. The total energies were calculated by ab initio density functional theory (DFT) in Paper I for the Ti1-xSixN system at 0 K. This gives G = H in Eq. 1. To include the temperature and

entropy dependence of G an ideal solution16_{is assumed.}

1_{S. Sambasivan, W. T. Petuskey, J. Mater. Res. 9 (1994) 2362}

2_{Powder Diffraction Files, JCPDS International Center for Powder Diffraction Data, Swarthmore, 1989,}

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6_{A. Zerr, G. Miehe, G. Serghiou, M. Schwarz, E. Kroke, R. Riedel, H. Fuess, P. Kroll, R. Boehler, Nature}

400 (1999) 340.

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14_{http://en.wikipedia.org/wiki/Solid_solution}

15_{A. Hörling, PhD Thesis (Linköping Studies in Science and Technology, dissertation no. 922, Linköping}

University, Sweden 2005

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3 Thin Film Deposition

3.1 Physical Vapor Deposition

Thin solid films can be synthesized by physical vapor deposition (PVD) techniques. Generally, the coating material is vaporized in vacuum from a solid material, target. The vapor will eventually condense onto a substrate surface. Next follows descriptions of arc evaporation and DC magnetron sputtering, the PVD methods employed in this work.

3.2 Arc Evaporation

Arc evaporation has been widely used because of its promise of an efficient source of highly ionized material for producing dense, adherent coatings having a wide range of compositions.1

A cathodic arc can be described as a low voltage, high current plasma discharge between two electrodes. The evaporation process of target material is a consequence of the very high local surface temperature in an arc spot. This creates a molten pool from which evaporation of the cathode (or target) material and electron emission occurs. The electrons are then attracted by an electric field and will collide and ionize evaporated atoms; this is called the ionization zone, see Fig. 3.1. The ions are transported to the substrate surface where they condensate and react with a reactive gas (if present) from the surrounding. In Paper I, N2 was utilized as reactive gas. However, the molten pool also

emits macro particles.

To synthesize high quality thin films, the importance of plasma ionization2,3,4 _should

be emphasized. Arc evaporation, in contrast to sputtering, provides highly ionized plasmas and can therefore be manipulated with electric and magnetic fields5_{. Other ion}

induced effects are acceleration of the nucleation stage,6_{enhanced adhesion,}7

modification of crystal structure,3_{film stress,}8_{densification, and in the case of deposition}

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Fig. 3.1 Schematic illustration of particle flux at the arc spot.9

The changes in stoichiometry appear when arc evaporation is applied to a compound target where the different materials cause different degree of ionization. This gives the ions different acceleration towards a negatively biased substrate. Therefore, ions with higher degree of ionization will impinge on the surface with higher energy and thus penetrate deeper into the film compared to an element with lower degree. This will cause preferential resputtering of the surface near material and the film will contain a higher concentration of the material with higher degree of ionization.1_{This phenomenon was}

apparent in Paper I, where a slightly higher Ti:Si ratio was observed in the film compared to the target. The average charge state during arc evaporation for Ti and Si is typically +2.1 and +1.4, respectively.10_{However, during reactive arc evaporation, the average}

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Fig. 3.2 Image of an arc evaporation deposition chamber at SECO Tools AB.

3.3 DC Magnetron Sputtering

The process of sputtering starts by introducing a sputtering gas, preferably inert, into a vacuum chamber. A high voltage is applied to the target; this creates a visible glow discharge, often referred to as plasma, by ionization of the inert gas. The gas ions will be accelerated towards, and eventually collide, with the negatively charged target. If the kinetic energy of the incoming ions is higher than the binding energy of the target surface atoms, the atoms will be ejected, sputtered. The ejected target material will vaporize and travel through the plasma to the substrate. Depending on the kinetic energy of the incoming coating material and the temperature of the substrate, ad-atoms may or may not migrate on the surface until they occupy an energetically favorable position. As the ions collide with the target they will also cause emission of secondary electrons. Since the electrons are negatively charged they will be repelled from the target and instead collide with other atoms and ions to free electrons. This will create positively charged ions to maintain the process.

In Paper II reactive DC magnetron sputtering was used to deposit TiN/SiNx

multilayers from two separate targets, where the reactive N2 gas was mixed with the inert

Ar gas. In this process the vaporized target species in the plasma mix and react with N2

before they migrate on the substrate. In Fig. 3.3 a schematic view of the interior of the

Target, cathode

Plasma

Substrates Trigger

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with magnetic coupling induced by a coil around the substrate holder enhances the ionization near the growing film. The ions will then be accelerated towards the negatively biased substrate, in this case -50 V.

Fig. 3.3 Schematic interior view of the deposition chamber.

3.4 Growth of Metastable Solid Solution Films

In Paper I metastable (Fig. 3.4) ternary solid solutions was synthesized by arc evaporation. Two mechanisms are described below for depositing metastable solid solutions. E ne rg y Atomic arrangement Ea I II E ne rg y Atomic arrangement Ea I II

Fig. 3.4 Schematic illustration of metastable (I) and

thermodynamically stable (II) states. Energy Ea is needed

to activate transformation from state I − to − II. Substrate Coil Magnetrons Shutter Ar inlet N2 inlet

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3.4.1 Low-temperature Synthesis

Low temperature deposition is preferable from an industrial point of view, since it allows a wider range of substrate materials with respect to their thermal stability and cost. Utilizing lower temperatures also decrease the production time. Low temperatures can be at or below 500 °C; a typical process condition in arc evaporation. In addition, low-temperature film synthesis (being far from the thermodynamical equilibrium of the deposition material) induces kinetic limitations which, e.g., allow for synthesis of metastable phases.11_{This is the first mechanism for the metastable ternary solid solution}

synthesized in Paper I.

3.4.2 Ion-induced Recoil Implantation

When a negatively biased target (or substrate in the thin film deposition case) is bombarded by the energetic ion beam, atoms will be redistributed in the target.12_These

collisions also cause atomic recoils, which more precise are energetic ions traversing the target and goes on to generate a cascade of secondary atoms. The collisions induced by the ion bombardment cause recoil implantation, wherein target atoms are knocked downstream by collisions and ion-mixing. This process resembles thermal diffusion, but is driven by collisions rather than by thermal motions.13_{The ion-induced recoil mixing is}

proposed as a second mechanism to form the metastable ternary solid solutions in Paper I.

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4 Theoretical Modeling

To receive further understanding in why the materials of the present thesis behave as they do, theoretical modeling is used to investigate phase stability as well as structural and elastic properties. Modeling is an area that has expanded tremendously during the last decade due to increased computer power and more efficient program codes. However, in order to provide reliable results with physical meaning, a deep understanding and knowledge is necessary. In this thesis density functional theory was used to determine lattice parameters and total energy for Ti1-xSixN solid solutions (0 x 1) and for the

determination of surface reconstructions for SiNx onto TiN.

4.1 Density Functional Theory

Today the density functional theory (DFT) formalism is the most used ab initio method in computational material science and solid-state physics. The reason for this is the high computational efficiency combined with high accuracy. Ab initio is a Latin term that means first principles. This implies that the calculation relies on basic and established laws of nature without additional assumptions or special models.

In the 1960s Hohenberg and Kohn1_{presented and proved two theorems, which}

became the fundament for DFT. The first theorem states that the external potential in which the electrons move, is a unique functional of the ground state electron density. This means that the systems are fully determined by the electron density. Hence, the total energy of the system can be expressed as a functional of the density. The second theorem states that the ground state electron density minimizes the total electronic energy of the system.

The theory was then further developed by Kohn and Sham2 who used these theorems to derive the Kohn-Sham equations:

( )

_{( )}

_r

_{( ) ( )}

_r _r

_{( )}

_r r r r n n n xc ext v E v d n e m∇ + − + + ψ = ψ − r' ' ' 2 2 2 _{(Eq. 2)}

where the external potential, vext(n(r)), is determined from the electronic density, n(r),

instead of from the electron wave functions as for the general Schrödinger equation. (r)

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4.1.1 Approximations for Many-body Interactions

The expression for the energy contribution from many-body interactions, which are captured by the exchange-correlation energy vxc(r), is unknown. However, different

approximations have been developed; the generalized gradient approximation (GGA) and the local density approximation (LDA) are perhaps the most commonly used ones. In LDA the exchange-correlation energy is taken from known results of the many-body interactions in a uniform electron gas. This apparently easy approximation works surprisingly well for most applications3_{and requires relatively short computational time.}

However, for more rapid changes in the electron gas, LDA seems coarse and great effort has been made to find better approximations. To overcome this problem gradient correction were included to the exchange-correlation potential. This, together with constraints on the exchange-correlation functions led to the implement of GGA.

In a comparison between LDA and GGA, the latter tends to improve total energies4, atomization energies2,5,6_{, energy barriers, and structural energy differences}7,8_{. GGA also}

expands and softens bonds6, an effect that sometimes corrects9 and sometimes overcorrects10_{the LDA prediction.}

4.1.2 Pseudo Potentials and Plane Waves

Cambridge serial total energy package (CASTEP) is a powerful simulation package from Accelrys Inc.11_{In this package either GGA or LDA can be utilized.}

CASTEP uses pseudo potentials, which are approximations where the core electrons are treated as frozen. Since the computational time is heavily dependent on the number of electrons, this decreases the computational time dramatically. One type of pseudo potential method is the ultra soft pseudo potentials12, these were used in Paper II.

4.1.3 Linear Muffin-Tin Orbital

The full-potential linear muffin-tin orbital (FP-LMTO) method within LDA was used for the calculation in Paper I. Here, the unit cell is divided into non-overlapping muffin-tin spheres around the atoms. The potentials and charge densities in the crystal can have any, and not necessarily spherical, shape.

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10_{C. Filippi, D. J. Singh, C. Umrigar, Phys. Rev. B 50 (1994) 14947}

11_{M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, M. C. Payne, “First}

principles simulation: ideas, illustrations and the CASTEP code”, J. Phys. Cond. Matt. 14(11) (2002) 2717-2743

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5 Thin Film Characterization

To obtain necessary information about material properties, several characterization techniques are needed. In this work, investigation about composition, morphology, microstructure, thermal stability and mechanical properties using the techniques described below were performed.

5.1 X-ray Diffraction

In x-ray diffraction (XRD) an x-ray beam is scattered, or diffracted, by atoms in the investigated material. X-ray diffractograms are in principle created from the condition of constructive interference from Bragg’s law

2d·sin =n (Eq. 3)

which requires that the path difference between the traveled x-rays is equal to an integer number of wavelengths. In Eq. 3, d is the atomic plane spacing of the investigated crystal,

the wavelength of the x-rays and 2 is the angle of diffraction.

Fig. 5.1 Schematic of diffraction according to Bragg’s law.

XRD is a powerful analytical technique for microstructural investigations and can be used for characterization of crystal structure, phase transformations, residual stress, thickness measurement etc.

d

2

X-ray source Detector

d

2

X-ray source Detector

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5.2 Electron Microscopy

Transmission electron microscopy (TEM) is an invaluable analysis technique due to its ability of getting both a physical image and an electron diffraction pattern. In this thesis an FEI analytical TEM, Technai G2 UT FEG operating at 200 keV, equipped with scanning transmission electron microscope (STEM), energy loss electron spectroscopy (EELS), and energy dispersive x-ray spectroscopy (EDX) has been used for high resolution imaging and analytical analysis. A Philips EM 400T (120 keV) and a Philips CM 20 UT (200 keV) were used for the overview images and electron diffraction. A TEM is in many ways similar to a light optical microscope. Both types are built up with an illumination and an image part, where the first illuminates the sample and the second creates the image. However, instead of photons, electrons are irradiating the sample in TEM; the sample has to be viewed in vacuum in order to increase the mean free path of electrons. Also different lenses are used, instead of ordinary optical lenses, electromagnetic lenses are employed. Since electrons are accelerated with, in this thesis, 120 and 200 keV, respectively, their wavelength is on the 10-12_{m scale, which can be}

compared to wavelength of photons, 10-7_{m scale. The output is better resolution.}

In STEM the electron beam been focused to a small probe, i.e. a convergent beam, which scans over an area of the sample. Some imaging modes in STEM supply information that cannot be obtained in a conventional TEM, e.g. micrographs containing mainly z-contrast (described below). Due to the small probe size, chemical analysis can be performed in different fashions, e.g., EDX line-scan.

TEM and STEM requires careful sample preparation in order to obtain an electron transparent thin area (<100 nm thick). Depending on the purpose of the analysis the sample are studied in cross-section or plan-view. The cross-sectional samples in this thesis were prepared by gluing two small slices cut out from the sample face-to-face and mount them in a titanium grid. This was continued by mechanically grinding until a thickness of about 50-60 µm were achieved. Finally, the sample was etched by a 2-5 keV Ar+ ion beam at a 5° incident angle in a precision ion polish sputter (PIPS) until electron transparency was achieved.

TEM and STEM combined with the analytical techniques, EELS and EDX, give possibilities to measure chemical composition on the nanoscale with a point-to-point resolution in the subnanometer range.

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50nm

Figure 5.2 a) Cross-sectional scanning transmission electron micrograph, and b) transmission electron

micrograph, of Ti0.86Si0.14N deposited onto WC(Co) and annealed at 1100 °C. From Paper I.

The STEM image in Fig. 5.2 a) is obtained by collecting high-angle scattered electrons collected with a high angle annular dark field detector (HAADF)1_{at a low camera length.}

This configuration provides mainly z-contrast (thus, very little diffraction contrast) which gives bright contrast from heavy elements. In Fig. 5.2 a) the heavy elements at the grain boundaries correspond to W and Co according to a line-scan measurement with EDX/STEM. Compare also with the bright field TEM image in Fig. 5.2 b), were Z-contrast and diffraction Z-contrast are present. In TEM the heavy elements have dark atomic number contrast. Note that some grains appear black due to diffraction contrast.

5.3 Nanoindentation

In nanoindentation an indenter deforms a material on a very small scale. During the indent displacement and indent load are continuously recorded and by using the Oliver and Pharr2_{method, hardness – a material’s ability to resist deformation upon a load}3_–,

and Young’s modulus can be evaluated. To avoid influence from substrate the penetration depth should not exceed ~10%4_{of the film thickness.}

It is important to take into account that the acquired hardness data is affected by the material’s microstructure (grain size and boundaries, voids etc.), loading orientation (for non-isotropic materials), and environment, i.e. temperature, humidity etc. The hardness is

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thus a system response and property that should be evaluated using a statistical approach, i.e., each samples hardness should be evaluated from multiple indents.

5.4 Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) delivers images of a solid surface by moving a sharp conductive tip in a very precise and controlled manner across the sample surface and recording the electron tunneling current between the tip and sample as a function of position. The tip edge ideally consists of only one atom in order to provide atomic resolution.

Tunneling is a quantum mechanical effect in which electrons from one conductor penetrate through a classically impenetrable potential barrier (for STM, vacuum) into a second conductor. The phenomenon arises from the leaking of the respective wave functions into the vacuum and their overlap within classically forbidden regions. This overlap is significant only over atomic-scale distances and the tunnel current depends exponentially on the distance between the conductors.

Since an image of the surface is obtained in STM, the analysis has to be carefully performed, keeping in mind that there is always a risk for unwanted features, like for example double tip image, vibrations, and electrical noise. A double tip image forms when the tip picks up contamination from the sample surface or by other structural modification with features to which tunneling can take place. The image created from a double tip will contain doublets of the true surface features. Fig. 5.3 shows a micrograph created from a double tip.

The Material Research Laboratory at the University of Illinois at Urbana-Champaign has a deposition system, equipped with one sputtering target and one sublimation*_{target, together with variable temperature scanning tunneling microscope}

(VT-STM) and low-energy electron diffraction (LEED)†_{; all in the same ultra high}

vacuum (UHV) system. Hence, new possibilities of in situ surface studies where depositions of the order of sub-monolayer directly can be probed by means of STM and LEED. This system was used in Paper II.

*_{Sublimation is a PVD process where the target is heated to make the target material evaporate from the}

surface.

†_{LEED works in principle as ordinary electron diffraction, but is utilized at lower energies which makes it}

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Fig. 5.3 Scanning tunneling micrograph from SiNx deposited on

TiN scanned by a double tip. Note that all features appear twice in the image.

5.5 X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface sensitive technique, which can provide information about elemental composition and chemical bonding.

X-ray photons, typically Al or Mg K , travels incident on a surface and eject valence or core electrons from a depth of 0-10 nm. The kinetic energy, Ekin, from the

ejected photoelectrons are detected and the electron binding energy, EB, is calculated

from

Ekin = h −EB− (Eq. 4)

where h is the incident photon energy, and a work function that corresponds to the x-ray source and the sample. The binding energy of a core electron does not only depend on the element but also on the surrounding atoms. The change in binding energy is often referred to as chemical shift. This information is of great interest since it could provide information about the surrounding atoms, e.g. if a Si atom is located in an octahedral (like atoms in a NaCl lattice) or in a tetrahedral (like atoms in a ZnS lattice) position. The

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energy resolution of the XPS also plays a central role to resolve these shifts. In Paper I XPS was used to determine the Si bond character.

1_{E. M. James and N. D. Browning, Ultramicroscopy 78 (1999) 125} 2_{W. C. Oliver, G. M. Pharr, J. Mater. Res. 7 (1992) 1564} 3_{C. –M. Sung, M. Sung, Mater. Chem. Phys. 43 (1996) 1}

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6 Results

6.1 Ti

1-x

Si

x

N Alloy Films

In Paper I Ti1-xSixN (0 x 0.14) thin films were deposited onto cemented carbide

(WC-Co) substrates by arc evaporation at ~500 °C. Cross-sectional scanning electron microscopy showed that all films exhibited a columnar structure. A closer investigation using XTEM revealed that within each column for the x=0.14 film, a feather-like domain structure could be revealed. This is an effect originating from point defects and dislocations yielding low-angle grain boundaries. TEM together with XRD showed that the obtained films phases were of cubic NaCl type with a lattice parameter close to the reference value of -TiN at 4.24 Å. Calculations by FP LMTO gives that the lattice parameter of NaCl Ti1-xSixN (x=0, 0.25, 0.5, 0.75, 1) are very similar (within 1%) of each

other. This implies that it can be hard to distinguish SiNx from TiN in nanocomposites or

multilayers or even from a (Ti,Si)N solid solution using XRD. Furthermore, the calculations in Paper I suggests that a NaCl-lattice is more favorable than a ZnS-lattice for Si-content x<0.67. The synthesis of the (Ti,Si)N alloy films is made possible by the kinetic limitations for atom mobilities at the chosen low substrate temperature (~500 °C) combined with the employed high-flux low-energy metal ion bombardment, which induces recoil mixing. Effects of N content on the phase stability and properties of SiNx

polytypes, however, were not investigated.

The microstructure of as-deposited solid solution films with a Si content of x=0.14 was retained up to an annealing temperature of at least 900 °C/120 min (see Paper I). This absence of phase separation −despite the deep miscibility gap− is likely due to a limited driving force for the nucleation of a Si3N4 phase due to molar volume mismatch

(Si3N4 has a larger unit cell than the NaCl-structured solid solution). When annealing at

1100 °C/120 min, interdiffusion of W and Co occurs in the film grain boundaries which transforms the film into a nanocrystalline cellular microstructure. Mechanical properties of the films were investigated with nanoindentation. The measured hardness increased close to linearly with increasing Si content and is retained up to 900 °C. At 1100 °C the hardness decreased to below 30 GPa due to the Co and W diffusion, which weakens the grain boundaries.

Also, initial deposition experiments were performed for the Ti1-xSixN system in the

wider composition range (0 x 0.22) onto cemented carbide (WC-Co) substrates by arc evaporation. The as-deposited Ti0.78Si0.22N film exhibited a dense fine grained and

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columnar two-phase structure consisting of a defect-rich crystalline cubic phase and a nanocrystalline − to − amorphous structure as seen by cross-sectional and plan-view transmission electron micrographs, see Fig. 6.1. The crystalline structure is a saturated (Ti,Si)N solid solution similar to the Ti1-xSixN x 0.14 alloys. Due to the supersaturation

of N2 gas in the deposition, the amorphous phase is likely to by rich in nitrogen as for the

thermodynamically stable a-Si3N4. However, no z-contrast was obtained for the structure

using XSTEM at low camera length (90 mm) which indicates that the two phases have a similar composition, i.e., a-Si3N4:Ti and c-(Ti,Si)N, respectively. The maximum hardness

was reached for Si content of 0.135 x 0.175 and was of the same order as for the Ti0.86Si0.14N film in Paper I. Higher Si contents resulted in a leveling out or slight

reduction of the hardness.

a-Si

₃

N

₄

2 nm

200 nm

a)

b)

a-Si

₃

N

₄

2 nm

200 nm

a)

b)

Fig. 6.1 Transmission electron micrographs from a Ti0.78Si0.22N film deposited by arc-evaporation, a)

plan-view containing amorphous area of Si3N4 between defect-rich (Ti,Si)N crystallites, b) cross-sectional

bright-field overview image showing diffraction contrast indicating location of amorphous and crystalline phases, respectively. A. Flink, T. Larsson, J. Sjölén, L. Karlsson, L. Hultman, unpublished.

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6.2 TiN/SiN

x

Nanolaminate Films

The nc-TiN/a-Si3N4 nanocomposites can exhibit extraordinary mechanical properties and

thermal stability. The interface between TiN and SiNx is of great importance to

understand since the atoms involved occupy a relatively large volume fraction of the nanocomposite. However, there is little knowledge about the bonding at the interface and for any tissue phase forming or effects of any segregated contamination. In Paper II, an interface study was performed with three different approaches. 1) Investigation of the interface between TiN and SiNx by using in situ STM to probe sub atomic layer

coverages of SiNx deposited onto TiN(001)/MgO(001) and TiN(111)/MgO(111), 2)

multilayers of TiN/SiNx deposited onto SiOx and MgO(001) which reduces the interface

to a two dimensional, instead of a three dimensional, problem, and 3) surface calculations of different SiNx coverages placed onto TiN surfaces by using ab initio DFT methods.

In the in situ UHV STM and LEED study, epitaxial TiN(001)/MgO(001) was deposited by magnetron sputtering at 700 °C. These substrates were then inserted in the vacuum deposition system equipped with LEED and VT-STM. The deposition system contains one Ti magnetron sputtering source and one Si evaporation source and N2 and

Ar gas supplies. Here, a TiN(001) template layer containing atomistically flat terraces was sputtered. Then, different SiNx surface coverages, SiNx, was deposited onto the TiN

surface by sublimation of Si followed by annealing in N2 atmosphere during 12 h at

temperatures ranging from 600 to 800 °C. Several different crystalline reconstructions were found for different SiNx, containing rows in the <110> directions.

The multilayers in Paper II were deposited by reactive magnetron sputtering at 500 °C onto MgO(001) substrates. The TiN layers were kept at fixed thickness, 40 Å, while the SiNx layer thicknesses (lSiNx) were varied between 3 and 25 Å. For a SiNx layer

thickness up to 5 Å, both the TiN and SiNx layers were epitaxial, forming a superlattice.

Increasing the SiNx thickness to 13 Å yielded a polycrystalline TiN layer structure

alternating with SiNx layers that are initially crystalline to a thickness of 5-13 Å before

becoming amorphous. The epitaxial stabilization of c-SiNx is explained by the

minimization of surface area energy at the early stages of layer nucleation, i.e., instead of forming a high energy crystalline/amorphous interface a low-energy crystalline/crystalline interface is formed. However, when the interfacial strain energy, which depends on the thickness of the SiNx layer, is sufficiently large the epitaxial

growth breaks down, and an amorphous Si3N4 layer forms.

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several areas exhibit local epitaxy as in a superlattice. A HAADF detector was used with a camera length of 300 mm for the purpose of increased image intensity. Correspondingly, both diffraction and z-contrast are present in the images, which promote the TiN crystals to be more or less pronounced. The dark and bright contrast

Figure 6.2 High resolution scanning transmission electron micrographs of a TiN/SiNx multilayer with

lSiNx=13 Å. The inset image shows local epitaxy of c-SiNx and TiN. A. Flink, H. Söderberg, P. O. Å.

Persson, L. Hultman, M. Odén, unpublished.

correspond to SiNx and TiN, respectively. The higher-magnification inset shows that

cubic SiNx is stabilized between the TiN layers.

In Paper II also the hardness of the multilayers was determined by nanoindentation. Both multilayer series deposited on MgO(001) and SiO2 substrates had their maximum

hardness (34±2 GPa) at lSiNx close to the epitaxial breakdown limit in the respective

series. The SiNx/TiN superlattice films exhibit Koehler hardening1, in which dislocation

generation and glide across interfaces is hindered for phases with a large difference in shear moduli. However, the maximum hardening observed in the present multilayer samples is significantly larger than expected from Koehler hardening. Thus, additional hardening mechanism, including strengthening due to the coherency strain between the

10 Å

TiN

SiN

x

20 Å

TiN

SiN

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two epitaxial layers, (SiNx and TiN(001)) must be present. Effects of lattice strain can be

seen in Fig. 6.2 with the meandering trace of {200} crystallographic planes in the successive TiN and SiNx layers.

Finally, ab initio DFT calculations were used to calculate different surface formations. Three different coverages were tested; 0.4 ML with a Si to N ratio of 1, 0.9 ML with a Si to N ratio of 5/6, and 1 ML with a Si to N ratio of 1. The DFT calculations suggest that for increasing Si content a tetrahedral binding configuration as in hexagonal or amorphous Si3N4 is preferred over an octahedral as in NaCl-structure SiN.

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Influence of Si on the microstructure of arc evaporated (Ti,Si)N thin films;

evidence for cubic solid solutions and their thermal stability

A. Flinka,*, T. Larssonb

, J. Sjo¨le´nc, L. Karlssonc, L. Hultmana

a_{Thin Film Physics Division, Department of Physics, IFM, Linko¨ping University, SE-581 83 Linko¨ping, Sweden} b_{Ombenning by 14, SE-737 90 A¨ngelsberg, Sweden}

c_{SECO Tools AB, SE-737 82 Fagersta, Sweden}

Available online 19 September 2005

Abstract

Ti1 xSixN (0 x 0.14) thin solid films were deposited onto cemented carbide (WC-Co) substrates by arc evaporation. X-ray diffraction

and transmission electron microscopy showed that all films were of NaCl-structure type phase. The as-deposited films exhibited a competitive columnar growth mode where the structure transits to a feather-like nanostructure with increasing Si content. Films with 0 x 0.01 had a b111 crystallographic preferred orientation which changed to an exclusive b200 texture for 0.05 x 0.14. X-ray photoelectron spectroscopy revealed the presence of Si – N bonding, but no amorphous Si3N4. Band structure calculations performed using a

full potential linear muffin tin orbital method showed that for a given NaCl-structure Ti1 xSixN solid solution, a phase separation into cubic

SiN and TiN is energetically favorable. The microstructure was maintained for the Ti0.86Si0.14N film annealed at 900 -C, while

recrystallization in the cubic state took place at 1100-C annealing during 2 h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x = 0 and x = 0.14, nanoindentation gave a hardness of 31.3T 1.3 GPa and 44.7 T 1.9 GPa, respectively. The hardness was retained after annealing at 900 -C, while it decreased to below 30 GPa for 1100-C following recrystallization and W and Co interdiffusion.

Keywords: Nitrides; Arc evaporation; Transmission electron microscopy (TEM); Thin films; Solid solution; Microstructure

1. Introduction

Advanced surface engineering of transition metal nitride wear-resistant coatings by the introduction of alloying elements is a growing field of research. TiN has been widely used as hard coating on cutting tools, but the poor oxidation resistance at temperatures above 500-C[1,2]has created an interest in ternary compounds, e.g., on Ti – Al – N [3 – 5] and Cr – Al – N [6 – 8] and also the more complex quaternaries, e.g., Ti – Al – Si – N [9,10]. These coating materials show a much improved oxidation behavior at high temperatures and are now used by market leaders within metal cutting tools. Recently, Choi et al.[11]showed for Ti – Si (11 at.%) – N compounds that Si forms SiO2on

the surface which acts as an oxygen diffusion barrier up to

an annealing temperature of 800-C for a duration of 150 min. Also, improvement on mechanical properties has been realized with super hardness, H > 45 GPa, for specific Si contents[12,13]in TiN – Si3N4nanocomposites.

The equilibrium phase diagram for Ti – Si – N does not contain any stable ternary phases[14]. However, Vaz et al. found phases originating from a possible (Ti,Si)N solid solution [15]. The maximum Si content in metastable supersaturated cubic solution Ti1 xSixN is 10 – 15 at.%

[14]. Furthermore, Procha´zka et al. [16] conclude that a totally segregated amorphous Si3N4only can occur when

the nitrogen activity is larger than about 10 6. This suggests that PVD is a preferred technique to suppress nitrogen segregation by virtue of the lower substrate temperature employed. Thus, during deposition by reactive magnetron sputtering So¨derberg et al.[17], first, and Hu et al. [18], more recently, stabilized sub-nm thick layers of cubic SiN in

* Corresponding author.

Surface & Coatings Technology 200 (2005) 1535 – 1542

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In this work, we show that as-deposited Ti1 xSixN thin

films with a Si content up to x = 0.14 can be prepared by arc deposition. Kinetic limitations during film deposition com-bined with ion bombardment induced collisional mixing are proposed as conditions for the growth of metastable Ti1 xSixN films instead of the thermodynamically stable –

and much more studied – TiN/Si3N4 system. Interestingly,

film hardness was found to increase close to linearly with the Si content. The as-deposited solid solutions exhibited thermal stability above 900-C for 2 h. Residual stress recovery and recrystallization, however, resulted in transformation to a nanocrystalline structure after annealing at 1100-C. Band structure calculations performed using a full potential linear muffin tin orbital method showed that for a given NaCl-structure, a phase separation into cubic SiN and TiN phases is energetically more favorable than a Ti1 xSixN solid solution.

2. Experimental details

Cemented carbide WC-Co (6 wt.%) 12 12 4 mm3

plates were used as substrates. Before the deposition the substrates were ground and polished to a mirror-like finish, Raå 0.01 Am, and cleaned in an ultrasonic alkaline

degreasing agent.

The films were deposited by a commercial arc evapo-ration system. Three cathodes of composition Ti, Ti90Si10,

and Ti80Si20, respectively, located on top of each other in the

chamber were used to produce Ti1 xSixN films of varying

composition from one batch. Substrates with a bias of 50 V were kept at a temperature of 500 -C in an Ar/N2

atmosphere with N2-flow of 300 sccm.

Isothermal annealing of samples were performed in a Sintevac Furnace from GCA Vacuum Industries. The samples were annealed for 2 h at 900-C and 1100 -C, respectively. The annealing experiments were performed in an Ar flow at atmospheric pressure to prevent oxidation of the sample surfaces.

The microstructure of the coatings was studied with X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM), and scanning electron microscopy, (SEM). X-ray diffractometry was performed using a Philips PW 1820 powder diffractometer with a line-focused Cu Ka X-ray source. h – 2h scans were recorded in the 2h range of 5- to 90-. A Philips EM 400T microscope operating at 120 kV was used for the overview imaging and an FEI Technai G2 UT FEG microscope equipped with an electron energy loss spectrometer, EELS, and an energy-dispersive X-ray analysis spectrometer, EDX, operating at 200 kV was used for the high-resolution imaging and the EELS analysis.

Chemical analysis of the film compositions was per-formed using an Oxford Link ISIS EDX equipment, operating at 20 kV, in connection with a LEO 1550 SEM. Elemental mapping by EDX was measured for 30 min on a

photoelectron spectroscopy, XPS, using a VG Microlab 310F system. The XPS was equipped with a

non-mono-chromated Al Ka at 1486.6 eV X-ray source and a

hemispherical electron energy analyzer. To compensate for eventual sample charging, the peak position of the adventitious carbon was recorded before Ar-etching, thereby setting the peak position to 284.75 eV. The samples were then Ar-etched and survey scans of the binding energy 0 – 1100 eV was recorded with a step size of 1 eV for each sample. For accurate determination of the exact peak positions of the Si2p and C1s peaks, local region scans were recorded with step size of 0.1 eV. To suppress the background noise, each scan was recorded 10 times.

After mechanical polishing of the surface, nanoindenta-tion analysis of films was performed using a Nano Instru-ments NanoIndenter II with a Berkovich diamond tip. The maximum indent load was 25 mN. The indentation procedure is described in more detail by Ho¨rling et al.[5]. Ten indents in each sample were made to obtain statistically reliable results. Indents in a bulk fused silica reference sample were made with an indent load of 8 mN, yielding a similar penetration depth, < 200 nm, as in the investigated coatings. The average hardness and Young’s modulus with standard deviations was determined[19]. Poisson’s ratio, t, for TiN was set to t = 0.22, as used, e.g., by Sue[20]. The average hardness and Young’s modulus for the reference

sample, SiO2, was measured to 9.65T 0.4 GPa and

72.31T1.32 GPa, respectively.

3. Computational details

Ab initio calculations on the TiN – SiN system were carried out using the full potential linear muffin tin orbital

method (FP-LMTO) [21,22] within the local density

approximation (LDA) of density functional theory (DFT). The exchange correlation function used is described by Hedin and Lundqvist[23]. Starting with the TiN structure, and by exchanging Si for Ti atoms in the NaCl and zincblende, ZnS, structure, respectively, a total of five different compositions, x = 0, 0.25, 0.5, 0.75, and 1, were investigated. The unit cell dimensions for all structures were varied uniformly and the theoretical size of the unit cells was obtained from the minimum in total energy. Energy convergence was reached for all compositions with respect to the number of k points used.

4. Results and discussion

EDX analysis of as-deposited films from the different positions within the deposition chamber showed a continuous composition range between 0 x 0.14 Si for samples positioned between Ti and Ti0.9Si0.1 or Ti0.8Si0.2 targets.

A. Flink et al. / Surface & Coatings Technology 200 (2005) 1535 – 1542 1536

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x = 0.14. However, the apparent loss of Si during arc evaporating can be explained by the stronger impact from Ti atoms than Si on the sample surface. Since the Ti atoms have a higher grade of ionization, they will retrieve higher acceleration towards the negatively biased substrate. This will distribute the Ti atoms deeper into the structure than for

Fig. 1a shows X-ray diffractograms from the as-deposited Ti1 xSixN films. The diffractograms for all compositions

revealed a NaCl-structure compound with a lattice parameter very close to TiN, 0.424 nm. The preferential film growth orientation changed from mixed b111 to exclusive b200 with increasing Si content. For x 0.07, however, the (200) peak broadening increased substantially.

Cross-sectional TEM micrographs from the as-deposited

samples are presented in Fig. 2a (TiN), Fig. 2b

(Ti0.92Si0.08N), and Fig. 3 (Ti0.86Si0.14N), respectively. The

films exhibited a dense columnar structure where the column width ranged from 100 to 400 nm. Interestingly, for all films, the top surface correlated directly with the substrate topography. This implies that 3D-island growth and eventual faceting was effectively suppressed during the deposition. FromFigs. 2 and 3, it is evident that the Si content also affected the structure and increased the defect density. The as-deposited Ti0.86Si0.14N, seeFig. 3, exhibited

within each column a feather-like structure. Higher magni-fication imaging revealed nm-structure of feathers (elon-gated crystalline grains) with large strain contrast and moire´ fringes from overlapping features. The high-resolution electron micrograph (HREM) inFig. 3b shows a typical appearance of three feather features of the cubic (Ti,Si)N phase with high defect density of dislocations. The observations in Fig. 3show an interesting growth mode with a rotating lattice by branching into subgrains over each column. Branching begins at the column boundaries and the subgrains merge at the apparent stem of the columns. This takes place to maintain the (002) growth surface. The selected area electron diffraction pattern (SAED) inFig. 3a confirms the texture seen in XRD. No volumes of any amorphous phase were found by the TEM analysis.

Fractured cross-sections from as-deposited Ti1 xSixN,

x = 0, x = 0.05, x = 0.08, x = 0.14, presented inFig. 4, were investigated by scanning electron microscopy. The micro-graphs showed dense columnar microstructure with macro particles incorporated to a similar density as for (Ti,Al)N coatings. The thickness of the (Ti,Si)N coating ranged between 1.6 and 2.0 Am. As-deposited Ti0.86Si0.14N a) b) c) 34 36 38 40 42 44 46 48 50 WC (101) TiN (200) TiN (111) WC (100)

Intensity (arb. units)

2 Theta (degrees)

Co

34 36 38 40 42 44 46 48 50

2 Theta (degrees) WC (100) TiN (111) TiN (200) WC (101) 0.14 0.13 0.11 0.08 0.07 0.06 0.05 0.03 0.01 0 Co x: 0.14 0.13 0.11 0.08 0.07 0.06 0.05 0.03 0.01 0 34 36 38 40 42 44 46 48 50

2 Theta (degrees) WC (100) TiN (111) TiN (200) WC (101) Co x: 0.14 0.13 0.11 0.08 0.07 0.06 0.05 0.03 0.01 0 x:

Fig. 1. X-ray diffractograms from Ti1 xSixN films in (a) as-deposited, (b)

annealed at 900-C, and (c) annealed at 1100 -C states. The Si content of each film is indicated.

a) b)

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behaved as a fine structure when fractured in agreement with the nanostructure seen by XTEM.

InFig. 5a, the total energy of a relaxed NaCl type unit cell of Ti1 xSixN is plotted as a function of Si content. As a

comparison the total energy of Ti1 xAlxN, Ti1 zZrzN, and

TiC1 xNxare included in the figure. The calculations show

that NaCl-structure (Ti,Si)N is metastable with respect to phase separation into NaCl-structure SiN and TiN. Compar-ison of the energy for ZnS and NaCl structure Ti1 xSixN,Fig.

5b, however, suggests that a NaCl-structure (Ti,Si)N solid solution is energetically more favorable than a cubic ZnS-structure only up to x¨ 0.67. The calculations of lattice

lattice parameter for TiN. Underestimation of the unit cell size is normal for the local density approximation (LDA) within density functional theory (DFT) as seen for TiN here. The relatively constant a values are consistent with our exper-imental results, c.f.Figs. 1a, 3, and 6). Also, So¨derberg et al. [17]was not able to distinguish the y-SiN and y-TiN phases by XRD due to the small difference in lattice parameters. The present findings, further, implies that the peak with a lattice parameter of 0.429 nm observed in an alleged (Ti,Si)N solid solution by Vaz et al.[15]is not from a NaCl-structure phase. Results from XRD of Ti1 xSixN films isothermally

annealed at 900-C and 1100 -C are presented inFig. 1b and c. The texture of the as-deposited samples was maintained. However, the peak width decreases as annealing temperature increases.

Fig. 6shows a cross-sectional TEM micrograph from the Ti0.86Si0.14N sample annealed at 1100-C. The film now

exhibits a cellular structure of elongated ¨ 10-nm-wide grains with a texture that is reminiscent from the columnar

d) a) b) c) a) b) Column boundaries Feather features

8 °

I

II

III

(002) (002)

Fig. 3. Cross-sectional TEM micrographs from an as-deposited Ti0.86Si0.14N thin film on WC-Co substrate in, (a) overview with selected

area electron diffraction pattern and (b) HREM image. The columnar microstructure with internal branching of subgrains of (002) crystallo-graphic orientation is indicated in (a). The trace of (200) and (002) planes in neighboring subgrains I – III with zone axes [010], [hk0], and [110], respectively, after mutual rotation around [001] is shown in (b).

A. Flink et al. / Surface & Coatings Technology 200 (2005) 1535 – 1542 1538

Growth and Characterization of Ti-Si-N Hard Coatings