Nanocoating and ultra-thin films

Makhlouf, Max Planck Institute of Colloids and Interfaces, Germany self-assembling and self-cleaning coatings 13 1.10 New composite and powder coatings 161.11 Advanced polymers and fi ll

Nanocoatings and ultra-thin fi lms

Electroless copper and nickel-phosphorus plating

(ISBN 978-1-84569-808-9)

Compared with electroplating, electroless plating allows uniform deposits over ferent surfaces Electroless copper and nickel-phosphorus deposits provide protec- tive and functional coatings in industries as diverse as electronics, automotive, aerospace and chemical engineering Written by leading experts in the fi eld, this important book reviews the deposition process and the key properties of electroless copper and nickel-phosphorus deposits as well as their practical applications.

dif-Thermal barrier coatings

(ISBN 978-1-84569-658-0)

Thermal barrier coatings are used to counteract the effects of high temperature corrosion and degradation of materials exposed to environments with high operat- ing temperatures The book covers both ceramic and metallic thermal barrier coat- ings as well as the latest advances in physical vapour deposition and plasma spraying techniques Advances in nanostructured thermal barrier coatings are also discussed The book reviews potential failure mechanisms in thermal barrier coatings as well

as ways of testing performance and predicting service life A fi nal chapter reviews emerging materials, processes and technologies in the fi eld.

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Nanocoatings and ultra-thin fi lms

Technologies and applications

Edited by Abdel Salam Hamdy Makhlouf

and Ion Tiginyanu

Oxford Cambridge Philadelphia New Delhi

Published by Woodhead Publishing Limited,

80 High Street, Sawston, Cambridge CB22 3HJ, UK

First published 2011, Woodhead Publishing Limited

© Woodhead Publishing Limited, 2011

The authors have asserted their moral rights.

This book contains information obtained from authentic and highly regarded sources Reprinted material is quoted with permission, and sources are indicated Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or

indirectly caused or alleged to be caused by this book.

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Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identifi cation and explanation, without intent to infringe.

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library.

Library of Congress Control Number: 2011934932

ISBN 978-1-84569-812-6 (print)

ISBN 978-0-85709-490-2 (online)

The publisher’s policy is to use permanent paper from mills that operate a

sustainable forestry policy, and which has been manufactured from pulp

which is processed using acid-free and elemental chlorine-free practices.

Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards.

Typeset by Toppan Best-set Premedia Limited, Hong Kong

Printed by TJI Digital, Padstow, Cornwall, UK

A S H Makhlouf, Max Planck Institute of Colloids

and Interfaces, Germany

self-assembling and self-cleaning coatings 13

1.10 New composite and powder coatings 161.11 Advanced polymers and fi llers 171.12 Developments in coating processes 18

vi Contents

2.4 Metal/semiconductor nanoparticles 412.5 2-D arrays of colloidal spheres 44

I V Shishkovsky, P N Lebedev Physics Institute of the

Russian Academy of Sciences, Russia

3.1 Substrate preparation for ultra-thin fi lms and functional

3.2 Paradigm of functional graded layer-by-layer coating

fabrication 603.3 Nanocoating fabrication methods 613.4 Physical vapor deposition-based technologies 633.5 Chemical vapor deposition-based technologies 713.6 Conclusion and future trends 74

4 Surface-initiated polymerisation for nanocoatings 78

V Harabagiu, L Sacarescu, A Farcas,

M Pinteala and M Butnaru, ‘Petru Poni’ Institute

of Macromolecular Chemistry, Romania

D M Bastidas, M Criado and J.-M Bastidas,

National Centre for Metallurgical Research (CENIM),

CSIC, Spain

5.2 Electrochemical methods 1325.3 Surface-sensitive analytical methods for ultra-thin fi lm

coatings 1405.4 Spectroscopic, microscopic and acoustic techniques for

ultra-thin fi lm coatings 145

A S H Makhlouf, Max Planck Institute of Colloids

and Interfaces, Germany

6.2 Conventional coating technologies for the automotive

and aerospace industries 159

6.3 Advanced coating technologies for the automotive

and aerospace industries 162

6.4 Packaging applications 170

6.5 Coatings for the electronics and sensors industry 171

6.6 Paints and enamels industry 173

6.7 Biomedical implants industry 174

7 Nanocoatings for architectural glass 182

J Mohelnikova, Brno University of Technology,

Czech Republic

7.2 Spectrally selective glass 183

7.3 Dynamic smart glazings 188

7.4 Glass surface protections 194

8.6 Packaging as a drug carrier and for drug delivery 218

8.7 Nanotechnology solutions for the packaging waste

problem 219

M G S Ferreira, M L Zheludkevich and J Tedim,

University of Aveiro, Portugal

9.1 Introduction: corrosion in aeronautical structures 2359.2 Types of corrosion in aircraft 2369.3 Factors infl uencing corrosion 2419.4 Corrosion of aluminum and its alloys 2439.5 Corrosion of magnesium alloys 2449.6 Protective coatings in aerospace engineering 246

9.11 Self-healing coatings: nanostructured coatings with

triggered responses for corrosion protection 2619.12 Application of nanomaterials for protection of

I Tiginyanu, V Ursaki and V Popa, Academy of

Sciences of Moldova, Republic of Moldova

10.1 Lithography techniques and nanoimprint lithography

Contents ix

11 Ultra-thin membranes for sensor applications 330

I Tiginyanu, V Ursaki and V Popa, Academy of

Sciences of Moldova, Republic of Moldova

11.2 Graphene and related two-dimensional (2D) structures 331

11.3 Nanometer-thick membranes of layered semiconductor

12 Nanocoatings for tribological applications 355

S Achanta and D Drees, Falex Tribology NV, Belgium

and J.-P Celis, Katholieke Universiteit Leuven, Belgium

12.2 Use of nanostructured coatings in tribology 356

12.3 Review of nanostructured coatings for friction and

12.4 Advanced techniques for characterizing tribological

properties of nanostructured coatings 382

12.5 Conclusions and future trends 391

13 Self-cleaning smart nanocoatings 397

J O Carneiro, V Teixeira, P Carvalho, S Azevedo

and N Manninen, University of Minho, Portugal

13.1 Introduction: TiO2 photocatalysis 397

Contributor contact details

Department of Interfaces,

Am Mühlenberg 1

14476 Potsdam-GolmGermany

E-mail: abdelsalam.makhlouf@mpikg.mpg.de

Chapter 2

J Y Park and R C Advincula*Department of Chemistry and Department of Chemical and Biomolecular EngineeringUniversity of HoustonHouston

Texas 77204-5003USA

E-mail: radvincula@uh.edu

xii Contributor contact details

Chapter 3

I V Shishkovsky

P N Lebedev Physics Institute

Russian Academy of Sciences

National Centre for Metallurgical

Research (CENIM), CSIC

Avda Gregorio del Amo, 8

602 00 BrnoCzech RepublicE-mail: mohelnikova.j@fce.vutbr.cz

Chapter 8

A SorrentinoDepartment of Industrial Engineering

University of Salernovia Ponte Don MelilloI84084 Fisciano - SAItaly

CICECOUniversity of AveiroAveiro, 3810-193Portugal

E-mail: mgferreira@ua.pt

Contributor contact details xiii

E-mail: carneiro@fi sica.uminho.pt

Ultra-thin fi lms and nanocoatings play a major role in many areas such as micro- and nanoelectronics, machine building, car and aircraft manufactur-ing, robotics, etc Nanocoatings, in particular, represent the interface between the product and the environment and therefore determine not only aes-thetic aspects of goods, but also important specifi c properties such as, for example, anti-corrosion, self-cleaning, chemical and scratch resistance, etc The term ‘nanocoatings’ is usually used when the coating is nanostruc-tured or its thickness is in the nanometer scale Nanostructuring is usually applied because of its ability to increase hydrophobicity, radiation hardness, and corrosion resistance and because it makes materials much more

fl exible

Ultra-thin fi lms and nanocoatings represent two-dimensional (2D)systems, i.e free electrons in conductive systems can propagate only in the

x–y plane Confi nement in the z-direction may add many specifi c

charac-teristics, especially in the case of electronic materials Properly designed ultra-thin fi lms and nanocoatings are sometimes used to reduce stiction and light refl ection, for surface modifi cation in extreme conditions, and to enhance dirt release properties Nowadays there is increasing interest in nanophase thermal barrier coatings that exhibit extremely low thermal conductivity A great deal of attention is also paid to decorative nanocoat-ings based on special paints and inks

There are various methods of producing ultra-thin fi lms and ings: vacuum deposition, thermal spray, electrochemical deposition, etc Vacuum deposition can be based on evaporation, sputtering, thermal decomposition, etc Among thermal spray methods, one can mention plasma spray and arc spray as well as the high velocity oxygen fuel thermal spray process that provides high density coatings with unique performances in aggressive wear and corrosive environments The most widely used indus-trial coating processes are, in fact, based on electroplating and electroless plating These approaches are relatively simple and cost-effective and are applicable for a wide variety of coatings

nanocoat-xvi Introduction

Currently, nanotechnology enables the production of ultra-thin fi lms and nanocoatings consisting of just one monolayer or a few atomic layers Such ultra-thin fi lms may functionalize the surface to support desired chemical interactions, or, in contrast, passivate the surface to make it chemically inert Note that the formation of a few-atomic-layer thick native oxide on the surface of many semiconductor materials is an example of surface passivation

Nanotechnology revolutionizes the application of nanocoatings in many

fi elds, especially taking into account the potential to fabricate nanocoatings with specially designed nanoarchitecture, e.g., nanocomposite polymer-based coatings comprising networks of metal nanodots, aligned metal nano-rods, nanowires, or nanotubes, etc The occurrence of phenomena related

to surface plasmon-polariton excitation and negative refraction opens new opportunities for the development of novel focusing optical elements with super-resolution

The discovery of graphene (one-atom-thick sheet of carbon) can be sidered as an important breakthrough in the development of nanocoatings Graphene possesses excellent electrical conductivity and therefore is a unique material for anti-electrostatic applications Over the last years, researchers have succeeded in fabricating a few-atoms-thick membranes of

con-BN, MoS2, Bi2Te3, Bi2Se3 and GaN Besides obvious applications in micro- and nanoelectronics, nanomembranes of GaN seem to be promising for spintronic and biomedical applications

When choosing the type of nanocoatings and the technological approaches for their fabrication, it is very important to take into account their possible impact on the environment As a rule, increasing investment is made in technologies that are characterized by high effi ciency-to-cost ratio and at the same time are environmentally friendly There is no doubt that in the near future a new generation of multifunctional nanocoatings will be devel-oped with fl exible characteristics controlled, in particular, by the conditions

of the environment (temperature, pressure, intensity of illumination, etc.)

Nanocoatings and ultra-thin fi lms is both a reference and a tutorial for

understanding the most common thin-fi lms and coating techniques The book encompasses recent approaches and future trends in coating and thin-

fi lms technology, looking at essential innovations in the development of industrial nanocoatings and ultra-thin fi lms based on new fi ndings resulting from basic and applied research in the fi elds of both physics and chemistry

The goal of this book is to discuss the basics of ultra-thin fi lms and coatings and their synthesis techniques, surface characterization, and per-formance for possible industrial applications It addresses important questions frequently posed by end-user design engineers, coaters, and coat-ings suppliers in their quest for multifunctional and superior coating

Introduction xviiqualities for industrial applications Therefore, contributions in this book

emphasize thin fi lms, self-healing coatings, self-cleaning coatings, super-hard

nanocoatings, corrosion, tribological and nano-ceramic and nanocomposite

coatings with respect to their mechanical and physical properties

Chapter 1 addresses the most common coating techniques It includes

recent developments and future trends in coatings technology and

consid-ers the essential innovations in the development of industrial coatings The

chapter highlights future improvements in coating processes based mainly

on reduction of the number of coating layers; full automation of the coating

process; controlling the end product color through a module method and

automatic quality control

Chapter 2 discusses the nanostructuring of thin fi lms of amphiphilic

mac-romolecules and nanomaterials at the air–water interface The chapter

introduces several synthesized amphiphilic materials which have been

recently used in the Langmuir–Blodgett (LB) technique The surface

chem-istry and properties of the synthesized amphiphilic materials at the air–

water interface are also described Examples of thin fi lm applications using

LB fi lm are discussed

Chapter 3 provides a comprehensive analysis of vacuum deposition

methods for nanocoating and the production of functional graded (FG)

multilayers A general approach of FG layer-by-layer synthesis is based on

a paradigm of the type of connectivity of the internal structure The

objec-tive of the chapter is to demonstrate the particularities and versatility of

PVD, CVD, laser-, electron-, and ion-assisted technologies in the

engineer-ing of FG nanocoatengineer-ings with control microstructure The chapter also

pro-vides a description of the nanoperspectives of FG thin fi lms and surface

structures with nanoelectromechanical systems (NEMS) properties

Chapter 4 discusses surface-initiated polymerization for nanocoatings In

this chapter, thin polymer layer–surface conjugates are proposed as

appro-priate materials for studying surface/interface physicochemical properties

and material interactions with the environment, allowing performance

control over the entire system Recent advances in surface-attached polymer

layers are presented, and thermodynamic and kinetic aspects of polymer

physi- and chemisorption are discussed The chapter also summarizes the

preparation methods for polymer-grafted surfaces with the emphasis on

controlled processes able to achieve polymer surfaces meeting well-defi ned

criteria A comparison between the unique properties of polymer brushes

and the bulk characteristics or the physisorbed layers is highlighted

Chapter 5 reports the most common and advanced methods for

charac-terization and surface-sensitive analysis of nanocoatings and ultra-thin

fi lms A correlation between the linear potential sweep and impedance

measurements for copper specimens under different tarnishing treatments

is discussed The changes in the dielectric constant caused by water

xviii Introduction

absorption and the pigment/polymer proportions and porosity of the organic coatings are described with a reasonably good approximation using electrochemical methods These coatings are characterized by different ana-lytical techniques such as AFM, XPS, infrared, Raman and Mössbauer spectroscopies, x-ray diffraction, ion spectroscopy, glow discharge optical emission spectroscopy, electronic microscopy, scanning acoustic microscopy, and Kelvin probe force microscopy

Chapter 6 provides an overview of conventional and advanced coatings for industrial applications and describes the role of coating technologies in some important industrial applications The chapter also presents a critical review of recent research and development work on advanced coatings such

as smart coatings, ‘super’-hard coatings, and multifunctional coatings, etc The most important aspects of coating technologies for the automotive industry and for sensing, packaging, and biocompatible applications are discussed

Chapter 7 provides a general overview of the main types of nanocoatings for architectural window glass Glass plays an important role in building design because of its infl uence on thermal and visual comfort in buildings Highly transparent coatings are deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy for heating and air conditioning They offer environmental benefi t because they reduce heat loss and allow passive solar heat gain, reducing the energy consumption required to heat a building as well as energy-related CO2 emissions from buildings

Chapter 8 discusses the challenges of nanocoatings and ultra-thin fi lms for packaging applications Packaging technology is of strategic importance

as it can be a key to competitive advantage in the modern industry

An innovative pack design can open up new distribution channels, providing a better quality of presentation, enabling lower costs, increasing margins, enhancing brand differentiation product safety and integrity, and improving the logistics service Thus, there is a persistent challenge to provide cost-effective pack performance, with health and safety being of paramount importance At the same time, there is a continuous legislation and political pressure to reduce the amount of packaging used and packag-ing waste The chapter reports a variety of polymers currently used in packaging and the most widely used plastics in fl exible packaging It also reports different designs and processing techniques used to produce pack-aging products

Chapter 9 deals with conventional coating technologies and smart nanocoatings for corrosion protection in aerospace engineering The types and factors which infl uence corrosion are reviewed as well as the protective coatings that have been in use or which have shown potential for future applications Moreover, particular attention is given to functional

Introduction xixnanocoatings for sensing corrosion, nanostructured coatings which self-heal

when either corrosion starts or the corrosivity of the environment becomes

critical, and other coating properties important in reducing maintenance

costs The chapter concludes that fundamental and applied research in the

area of sensor-based, corrosion active and anti-icing/self-cleaning smart

coatings is expected to grow in the near future, contributing to the

genera-tion of high performance, added-value products

Chapter 10 discusses nanoimprint lithographic (NIL) techniques for

elec-tronics applications The potential of these techniques to surpass

photoli-thography in resolution, and, at the same time, to allow mass fabrication at

a lower cost is highlighted Current and potential uses of NIL are discussed

in such fi elds as data storage, optical components, image sensors, and phase

change random access memory devices Challenges faced by nanoimprint

lithography in becoming a standard fabrication technique are also

considered

Chapter 11 addresses some technological approaches for the fabrication

of ultra-thin membranes for sensor applications and fl exible, stretchable,

foldable electronics The discussion focuses on graphene and 2D sheets

of layered compounds The potential to build multifunctional

three-dimensional (3D) nanoarchitectures based on 2D graphene hybridized with

one-dimensional (1D) semiconductor nanostructures is highlighted The

chapter also reviews the fabrication of ultra-thin GaN membranes of

nano-meter scale thickness by using the concept of surface charge lithography

based on low energy ion treatment of the sample surface with subsequent

photoelectrochemical etching

Chapter 12 discusses the use of nanostructured coatings as tribological

surfaces for both friction and wear reduction with examples from

state-of-the-art research The chapter gives a general overview of common friction

and wear mechanisms encountered in engineering applications Moreover,

it provides a brief review of methods used to deposit nanostructured

coat-ings on substrates Different advanced techniques for friction and wear

characterization of nanostructured coatings and the scale dependence of

tribological properties are discussed The challenges encountered in

extra-polating laboratory experiments to fi eld applications are discussed

Chapter 13 looks at the concept of smart materials/coatings – terms

usually applied to materials able to change their properties in response to

an external stimulus such as light or temperature New insight is provided

into self-cleaning smart coatings and the chapter expands to cover the

major features of the photocatalytic materials developed to date The

chapter also gives a historical overview of TiO2 photocatalysis in order to

clarify the fundamental characteristics of the photocatalysis processes

which take place on TiO2 surfaces The electronic processes are also

dis-cussed, highlighting the main factors controlling the intensity of light

xx Introduction

absorption by the molecule or substrate The chapter discusses actual and potential applications of TiO2 photocatalysis in industry and in the develop-ment of self-cleaning glass materials, giving some practical examples of the application of TiO2 nanoparticles in environment protection

Abdel Salam Hamdy Makhlouf

Ion Tiginyanu

Current and advanced coating technologies

for industrial applications

A S H M A K H L O U F, Max Planck Institute of Colloids

and Interfaces, Germany

Abstract: This chapter addresses the most common coating techniques

currently in use Recent developments and future trends in coating technology are discussed, taking into account the essential innovations in the development of industrial coatings These are based on new fi ndings resulting from basic and applied research in the fi elds of both physics and chemistry.

Key words: nanocoatings, coating processes, coating techniques,

composite coatings, trends in coatings.

1.1 Introduction

Coatings have been used for centuries in numerous areas of society The main function of coatings lies in the protection and decoration of materials, and the extent of their use has broadened with increasing social and indus-trial development

Gooch1 provided a review of the history of paints and the development

of coatings He claimed that the earliest reported paints originated in Europe and Australia approximately 20 millennia ago During that period, paints based on iron oxide, chalk or charcoal were applied with the fi nger-tips or with brushes made by chewing on the tips of soft twigs In 9000 bc, the North American people used their primitive paints in the same manner

as their European and Australian counterparts to paint the rock walls of their living quarters with pictures of animals and people

More advanced coating technology based on polymeric coatings and paints was developed in ancient Egypt, and later in Greece, Rome and China Ancient Egyptians used natural resins and wax to form coatings, and artists employed lacquers based on dried oils to protect their paintings Although polymeric coatings were traditionally mainly used for the protec-tion of various surfaces, other important applications for this type of coating should also be mentioned Ancient Egyptian scientists developed a very fi ne coating technology that showed similarities with nanotechnology Several theories therefore treat nanotechnology as a re-innovated technology, with

4 Nanocoatings and ultra-thin fi lms

the initial attempts at developing nanoscale coatings carried out by Egyptian and, later, Chinese artists

Nowadays, there are probably a few thousand coating systems, ranging from simple systems based on one or two coating steps to sophisticated systems based on multilayers and complicated instruments However, most

of these have an adverse effect on the environment and, in many cases, do not wholly fulfi ll the demands of the manufacturing industries or of society.The main driving forces behind the sharp increase in research and devel-opment in coatings science and surface technology are:

• an increase in industry requirements for high performance coatings at relatively low cost;

• increasing regulatory pressure to reduce the hazardous waste (such as hexavalent chromate and volatile organic compounds (VOC)) produced

by coating processes, which results in air and water pollution

There are several techniques employed for the application of a coating onto a substrate Coatings may be applied as liquids, gases or solids The following section describes some of the most common coating technologies for metal and alloy substrates

1.2 Electro- and electroless chemical plating

The modifi cation of the surface properties of the materials to be coated is one of the most desirable methods of improving corrosion and wear resis-tance, electrical conductivity or decorative appearance Historically, the chemical processes of electroplating and electroless plating have always constituted the most common, cost-effective and simple techniques for applying a metallic coating to a substrate In both cases, a metal salt in solu-tion is reduced to its metallic form on the surface of the material to be coated

1.2.1 Electrochemical plating

In electrochemical plating, the electrons for reduction are supplied from an external source High reactivity materials such as magnesium alloys can quickly form an oxide layer when exposed to air; this oxide layer must be removed prior to plating Therefore, fi nding the appropriate chemical surface treatment to prevent oxide formation during the plating process is one of the major challenges involved in plating processing.2–5

Another potential issue is that the quality of the fi nal coating depends

on the materials being plated As a result, different chemical surface ment processes must be developed for each material to be coated Uneven

treat-Current and advanced coating technologies 5distribution of current density in the plating bath, resulting in non-uniform coatings, is a further problem with this technique Electroplating also uses

a large amount of electricity which can signifi cantly increase the cost of the plating process

1.2.2 Electroless chemical plating

In electroless chemical plating, the reducing electrons are supplied by a chemical reducing agent in solution or from the material itself This process does not suffer from the same disadvantages as those noted previously for electroplating and even allows complex shapes to be coated Another advantage of electroless plating is that second-phase particle such as alumina, carbides or diamonds can be co-deposited during the plating process in order to improve some desirable properties such as wear resis-tance, hardness or abrasion.4,6–9

Conversion coatings are produced by a chemical or electrochemical tion at a metal surface, which creates a layer of substrate metal oxides, vanadate, chromates, cerate, molybdate, phosphates or other compounds that are chemically bonded to the substrate surface Conversion coatings are widely used as low-cost coating processes which are able to protect the metal substrate from corrosion by acting as an insulating protective barrier between the metal surface and the environment

reac-1.3.1 Chromate conversion coating

Chromate conversion coating is the most common type of conversion coating applied to improve the corrosion protection performance of many metals and their alloys, including aluminum, zinc, copper and magnesium Major reasons for the widespread use of chromating are the self-healing nature of the coating, the ease of application, the high electric conductivity and the high effi ciency : cost ratio These advantages have made them a standard method of corrosion protection Moreover, they provide the great-est level of under-fi lm corrosion resistance and facilitate the application of further fi nishing treatment However, the Environment Protection Agency (EPA) ranks hexavalent chromate as one of the most toxic substances due

to its carcinogenic effect and because it is environmentally hazardous as a waste product As a result of current environmental legislation, along with increasing calls for a total ban on toxic hexavalent chromate in coating processes, many attempts have been made to develop less toxic or

6 Nanocoatings and ultra-thin fi lms

eco-friendly alternatives Trivalent chromate was proposed as a possible alternative but proved to be less effective than hexavalent chromate

1.3.2 Chrome-free conversion coatings

In the last few decades, chrome-free conversion coatings based on salts such

as cerate, stannate, vanadate, molybdate, silicate and zirconate have been developed These can provide covalent bonding for strong coating adhesion and can act as a barrier coating, limiting the transport of water to the surface

of the material.10–23

1.3.3 Anodizing

Anodizing is an electrolytic process which is used to produce a thick oxide layer on the surface of metals and alloys These fi lms are used to improve corrosion resistance and paint adhesion to the substrate.23

The anodizing process includes the following stages: (i) mechanical ment; (ii) degreasing, cleaning and pickling; (iii) electropolishing; (iv) anod-izing using AC or DC current; (v) dyeing or post-treatment; and (vi) sealing.24 The anodized fi lms formed consist of a thin barrier layer at the metal–coating interface and a relatively thick layer of a cellular structure Each cell contains a pore the size of which is determined by the type of electrolyte and the experimental conditions The pore size and density in turn determine the quality of the anodized fi lm.23

treat-Electrochemical inhomogeneity due to phase separation in the material

to be coated is one of the main challenges faced in the production of uniform anodic coatings The presence of fl aws, porosity and inclusions from mechanical treatment can also result in uneven deposition which, in turn, can enhance corrosion.25

Another disadvantage of anodizing is that the fatigue strength of the materials to be coated can be affected by localized heating at the surface during the treatment,25 especially in thicker fi lms Moreover, the anodized

fi lm formed is made of a brittle ceramic material that may not have the appropriate mechanical properties to fulfi ll the requirements of some industrial applications

1.4 Chemical and physical vapor deposition

(CVD and PVD)

1.4.1 Chemical vapor deposition

Chemical vapor deposition (CVD) is one of the most common processes used to coat almost any metallic or ceramic compound, including elements,

Current and advanced coating technologies 7metals and their alloys and intermetallic compounds The CVD process involves depositing a solid material from a gaseous phase; this is achieved

by means of a chemical reaction between volatile precursors and the surface

of the materials to be coated As the precursor gases pass over the surface

of the heated substrate, the resulting chemical reaction forms a solid phase which is deposited onto the substrate The substrate temperature is critical and can infl uence the occurrence of different reactions

There are several types of CVD process, including atmospheric pressure chemical vapor deposition, metal-organic chemical vapor deposition, low pressure chemical vapor deposition, laser chemical vapor deposition, pho-tochemical vapor deposition, chemical vapor infi ltration, chemical beam epitaxy, plasma-assisted chemical vapor deposition and plasma-enhanced chemical vapor deposition

1.4.2 Physical vapor deposition

Physical vapor deposition (PVD) is a vaporization coating technique, involving the transfer of material on an atomic level under vacuum condi-tions The process is in some respects similar to CVD, except that in PVD the precursors, i.e the material to be deposited, start out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in gaseous form

The process involves four steps: (i) evaporation of the material to be deposited by a high energy source such as an electron beam or ions–this evaporates atoms from the surface; (ii) transport of the vapor to the sub-strate to be coated; (iii) reaction between the metal atoms and the appropri-ate reactive gas (such as oxygen, nitrogen or methane) during the transport stage; (iv) deposition of the coating at the substrate surface

PVD has several advantages including: (i) coatings formed by PVD may have improved properties compared to the substrate material; (ii) all types

of inorganic materials and some types of organic materials can be used; (iii) the process is environmentally friendly compared to many other pro-cesses such as electroplating However, PVD has also some disadvantages including: (i) problems with coating complex shapes; (ii) high process cost and low output; (iii) complexity of the process

Spray coating is a process in which molten or softened particles are applied

by impact onto a substrate to produce a coating Spray coating techniques are widely used in industry for organic lacquers and for coating irregularly shaped glass and metals.26 Examples of some common spray coating tech-niques are on the following pages

8 Nanocoatings and ultra-thin fi lms

1.5.1 Thermal spraying

In the thermal spraying process, melted coating materials are sprayed onto the substrate to be coated Particles of 1–50 μm are partially melted and accelerated to high velocities by a fl ame or an arc The particles deposit onto a surface forming a coating, the quality of which is determined by the oxide content, porosity and adhesion to the substrate The coating materials are usually heated by electrical or chemical means, and the sprayed material can be metal, ceramic or polymer

One of the main advantages of the thermal spray technique is its ability

to provide coatings ranging from 15 μm to a few mm thick for substrates with large surface areas, at a high deposition rate compared with other conventional coating processes such as electro- and electroless deposition, CVD and PVD Another advantage is the possibility of feeding powders of different coating materials such as ceramics, plastics and composites, or pure metal, and spraying them over the substrate surface.27,28

1.5.2 High-velocity oxygen fuel spraying

High-velocity oxy-fuel spraying (HVOF) is a modifi ed version of the thermal spray technique, developed in 1980 In this technique, a mixture of liquid or gaseous fuel in addition to oxygen is fed into a combustion chamber, where these are ignited and react with each other There are several types of fuels used in HVOF Gaseous fuels such as hydrogen, natural gas, methane, propane, or liquid fuels such as kerosene are com-monly used

The resultant hot gas at high pressure (about 1 MPa) passes through a jet of very high velocity (∼1000 m/s) A powder of the coating materials is injected into the hot gas stream, which accelerates the powder up to 700–800 m/s The stream of hot gas and powder is directed towards the substrate to be coated The powder partially melts in the stream, and is deposited over the substrate.26 The resultant coating has a thickness of about 10 mm and is commonly used to improve corrosion and wear resistance

1.5.3 Plasma spraying

Plasma spraying is a coating process in which powders of the coating rials are fed into the plasma jet at around 10 000 K, at which the coating materials melt and are sprayed over the substrate to be coated Owing to the interaction between the plasma coating materials and the substrate to

mate-be coated, several factors affect the fi nal properties of the coating, such as the nature of the coating powders, composition of the plasma gas, gas fl ow

Current and advanced coating technologies 9rate, energy input, torch geometry, distance from the substrate and fi nal coating/substrate cooling parameters.29

1.5.4 Vacuum plasma spraying

Vacuum plasma spraying was developed on the basis of the plasma spray technique, and can operate at relatively low temperatures, ranging from 40–120 °C, thus avoiding thermal damage to some types of coating materials such as polymers, rubbers or plastics Moreover, this process can induce non-thermally-activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmo-spheric pressure

1.5.5 Cold spraying

The cold spraying coating technique is broadly based on the same ideas as the HVOF spraying technique, in that high velocity is used in order to enhance the interaction between the coating materials and the substrate to

be coated However, in the cold spraying technique, particles are ated to very high speeds by the carrier gas, which is forced through a nozzle Upon impact, solid particles deform plastically and bond mechanically to the substrate to form a coating

acceler-Selecting a high velocity range is an important issue in cold spraying, where the velocity must be suffi cient to create bonds between the coating materials and coating substrate The velocity depends on the properties of the material, powder size and temperature

The fi rst attempt to use the cold spraying technique was carried out in

1990 by a Russian research team who were testing the particle erosion of

a target exposed to a high velocity gas steam loaded with fi ne powder One major advantage of this technique is the possibility of using soft metals such

as copper or aluminum as well as elements with high melting points such

as tungsten, titanium and tungsten carbide and cobalt.26 Another advantage

is the possibility of using an inert carrier gas such as nitrogen or helium instead of oxygen The disadvantages of this technique are the low deposi-tion effi ciency, and the need to use a very fi ne powder in order to allow higher velocities, which is industrially unattractive.26

1.5.6 Warm spraying

The warm spray technique was recently introduced as a novel modifi cation

of HVOF spraying.26 In this technique, the temperature of the combustion gas is lowered by mixing it with nitrogen, which is similar to the principle behind the cold spraying technique

10 Nanocoatings and ultra-thin fi lms

The advantage of this technique is that the coating effi ciency is higher than when cold spraying is employed Moreover, the lower temperatures used for warm spraying result in reduced melting and fewer chemical reac-tions in the feed powder compared to HVOF These advantages are espe-cially important for coating materials such as titanium, plastics and metallic glasses, which rapidly oxidize or deteriorate at high temperatures

1.6.1 Sol–gel coatings

New developments in the chemical tailorability of mixed alkoxide sol–gel coatings have led to the creation of an environmentally friendly and long-lasting conversion coating for many ferrous and nonferrous alloys Two of the most common problems associated with applying the sol–gel technique

to protect metals against corrosion are: (i) the poor adhesion performance

of the coatings formed using sol–gel processing; and (ii) the absence of high-performance coating systems based on environmentally acceptable salts Recent research has shown that coatings based on silica, ceria, vanadia and molybdate can be adapted using sol–gel technology to produce a func-tionally gradient coating This coating is able to provide covalent bonding for strong coating adhesion and can act as a high performance surface treat-ment, limiting water transport-induced attack on the surface of the mate-rial.12,13,30–37 This technique was successfully applied with different aluminum alloys

1.6.2 Spin coating

Spin coating has been used for several decades for the application of thin

fi lms In this process, a small drop of the coating material is loaded onto the centre of a substrate, which is then spun at a controlled high speed In the spin coating process, the substrate spins around an axis which should be perpendicular to the coating area As a result, the coating material spreads towards, and eventually off, the edge of the substrate leaving a thin fi lm of coating on the surface Final fi lm thickness and other properties will depend

on the nature of the coating (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process such as the rotation speed

1.6.3 Gravure coating

The gravure coating process relies on an engraved roller running in a coating bath, which fi lls the engraved dots or lines of the roller with the

Current and advanced coating technologies 11coating material The excess coating on the roller is wiped off by the doctor blade and the coating is then deposited onto the substrate as it passes between the engraved roller and a pressure roller.38

1.6.4 Roll-to-roll coating

Roll-to-roll coating is the process of applying a coating to a fl at substrate

by passing it between two (or more) rollers In this technique, the coating material is applied by one or more auxiliary rolls onto an application roll after the gap between the upper roller and the second roller has been appropriately adjusted The coating is wiped off the application roller by the substrate as it passes around the support roller at the bottom After curing, the coated substrate is then shaped to the fi nal form; this has no effect on the properties of the coating

Roll-to-roll coating is made up of two different techniques: direct roll coating and reverse roll coating In the direct roll coating technique, the applicator roll rotates in the same direction as the substrate In the reverse roll coating technique, the applicator roll rotates in the opposite direction

to the substrate.38

1.6.5 Knife over roll coating

The knife over roll coating process is one of the most suitable coating techniques for high viscosity coatings and rubbers In this process, the coating material being applied to the substrate passes through a gap between the knife and the roller The excess coating is scraped off using the knife.38

1.6.6 Air/knife coating

The air/knife coating process is similar to the knife over roll coating process However, a powerful air jet is used instead of the knife This is an extremely simple process, in which the coating is applied to the substrate and the excess is ‘blown off’ by the air jet; however, the noise associated with the air jet makes the process industrially unattractive.38

1.6.7 Meyer rod coating

In the Meyer rod coating process, the coating is applied onto the substrate as

it passes over a roller partially immersed in the coating material The quantity (and sometimes the shape) of the coating to be applied on the substrate is controlled by a wire-wound metering rod (known as the Meyer rod), with the quantity determined specifi cally by the dimensions of the wire used.38

12 Nanocoatings and ultra-thin fi lms

1.6.8 Slot/die and slot/extrusion coating

In the slot/die process, the coating is squeezed out by gravity or by externally-applied pressure through a slot and onto the substrate surface

If the coating is composed entirely of solids, the process is known as sion’ The coating thickness can be controlled by adjusting the line speed compared with the speed of the extrusion.38

‘extru-1.6.9 Dip coating

In the dip coating process, the substrate is immersed into a bath of the coating material of determined viscosity, with withdrawal speed and immer-sion time carefully controlled The substrate is then removed from the bath and allowed to drain The coated substrate can then be dried by force-drying

or baking

Dip coating is extremely dependent on the viscosity of the coating The coating viscosity must remain constant during the coating process The main use of this process is the application of primers prior to fi nal coats

1.6.10 Curtain coating

In the curtain coating process, a bath with a slot of a determined dimension

in its base allows a continuous curtain of the coating to fall into the gap between two conveyors The coating substrate is passed along the conveyor

at a controlled speed so the coating material can be applied at the substrate surface.38

1.7 New lightweight materials

Interest in lightweight materials is increasing both in industry and in research circles Magnesium alloys are one example of these lightweight materials to replace heavy alloys in the automotive and aerospace indus-tries, resulting in savings in fuel consumption and a reduction in CO2 emis-sions Magnesium alloys have a variety of excellent properties, including a high strength-to-weight ratio, low density, dimensional stability and casta-bility However, despite their excellent mechanical properties, magnesium alloys remain very susceptible to corrosion

Several coating schemes have been proposed to improve the corrosion resistance of magnesium.10–22 However, the existing methods are frequently either expensive, such as PVD and nitrogen ion implantation, or unable to create the surface properties desired for many applications where magne-sium alloys would otherwise be highly competitive

On the other hand, an increased demand for new plastics and composite materials will require the development of both new coating solutions and

Current and advanced coating technologies 13new application processes Innovative changes in manufacturing processes will also require effi cient coatings to be developed, which meet the appro-priate surface quality standards.39,40

1.8 Trends in environmentally friendly coatings,

self-assembling and self-cleaning coatings

This section describes the future trends in environmentally friendly ings, self-assembling and self-cleaning coating technologies, reviewing the essential innovations in (i) the materials to be coated, (ii) the structure and chemistry of the coatings and (iii) coating techniques, based on new fi ndings from basic research in physics and chemistry

coat-1.8.1 Environmentally friendly coatings

The demand for more environmentally friendly coatings will increase as a result of stricter environmental legislation and tightening regulation on the use of hexavalent chromate and VOCs The industrial coatings of the future will always be chrome-free, and low in solvent or solvent-free, provided that the resulting properties, both in terms of protection and appearance, are still acceptable

The old days of trial and error formulating with standard binder systems will give way to new binder resins with tailor-made properties and well-defi ned molecular structures, especially in the paint industry where a broad molecular weight distribution increases the viscosity of the solution More attention will be paid to the statistical design of functionality in polymers

to ensure greater uniformity of crosslinking during curing, and to optimize

fi lm performance A number of monomers will cease to be used due to their toxic potential.39

1.8.2 Self-assembling molecules

Self-assembling molecules can arrange themselves regularly and closely on

a metal surface, and can then polymerize as a second step This very fl exible, strongly-anchored layer could improve coating adhesion, corrosion protec-tion and mechanical and chemical resistance

The use of natural materials such as oils and resins has declined in paint binders because their performance did not match the desired requirements for modern coatings, but biochemical gene modifi cations could make natural raw materials attractive as components for paint binders Natural polymers such as cellulose or chitin can be modifi ed to make them more acceptable in paint binders.39–41

14 Nanocoatings and ultra-thin fi lms

1.8.3 Self-cleaning coatings

Another interesting trend is the development of self-cleaning coatings which are resistant to dirt, water and oil when applied to a wide range of substrates Micro-patterns on surfaces are known to resist water and dirt penetration, and this so-called ‘lotus effect’ is already used in self-cleaning roof tiles and sanitary ware.39 Bright surfaces on car bodies, however, do not seem to be a realistic goal: artifi cial lotus surfaces are not self-renewing,

so their sensitivity to cleaning and other mechanical treatments presents a problem Other surface effects such as shark skin and fur-like fi lms might

fi nd outlets in the coating of plastics.39

1.9 Trends in nanocoatings

The evolution of nanotechnology is behind the recent dramatic changes in several areas of scientifi c research and technology In the area of surface coatings, new approaches that make use of nanoscale effects can be used

to create tailor-made coatings with signifi cantly enhanced properties The ultimate impact of nanotechnology in the area of coatings will depend on its ability to direct the assembly of hierarchical systems that include nano-structures Future approaches will focus on tailor-made coatings for specifi c functions, either by incorporating existing identifi able components into the desired coating or by the formation of new structures during the coating process.42

Interest in nanocoatings has increased because of the potential to thesize materials with unique physical, chemical and mechanical properties There are several types of design models for nanocoatings, such as nano-composite coatings, nanoscale multilayer coatings, super lattice coatings, nanograded coatings, etc.43

syn-In the last decade, research interest in nanostructured coatings has increased due to the potential for enhancing the coating functionality for specifi c applications such as environmentally friendly anti-corrosion coat-ings for the automotive and aerospace industries.13,15,37 The data showed a signifi cant improvement in the corrosion resistance of materials as com-pared to materials processed using conventional coating methods The fol-lowing section reviews the most common nano-based coating systems and their expected future impact

1.9.1 Micro- and nanocapsule-based coatings

Micro- and nanocapsules or containers are of great interest to both industry and the scientifi c research community, and have a wide range of applica-tions such as molecular biology, electronic materials, medical imaging and

Current and advanced coating technologies 15photonic crystals Moreover, they have also been increasingly used as fi llers, coatings, capsule agents, etc., because of their low density and optical prop-

erties These shells are created either by in situ hydrolysis of the

correspond-ing metallic salt in the presence of core materials,44–49 or by calcination of polymer particles coated with uniform inorganic shells.41, 50–56

Recently, several nanocontainers were synthesized using a two-step process In the fi rst step, charged polystyrene nanospheres were prepared using emulsion polymerization or polymerization in suspension In the second step, the polystyrene lattices were coated using the sol–gel method to form a layer of inorganic oxide(s) The composites were treated

in air to burn off the polystyrene latex Using this approach, different nanocontainers such as cerium/molybdenum oxide, cerium/titanium oxide, iron/titanium oxide, silicon/calcium oxide, polypyrrole and polyaniline were produced.55 However, further research still needs to be carried out in order to optimize the experimental coating conditions such as the coating thickness and temperature Moreover, a multistep process is industrially unattractive

To make self-repairing coatings, the researchers fi rst encapsulated a lyst into spheres less than 100 μm in diameter They also encapsulated an inhibitor or a healing agent into similarly sized microcapsules The micro-capsules are then dispersed within the desired coating material and applied

cata-to the substrate.39 When the coating is subjected to corrosion or scratching, some of the capsules break open, spilling their inhibitor contents onto the damaged region The healing agent reacts with the environment to form a protective oxide to repair the damage, depending upon environmental conditions

Another approach based on a dual-function tailor-made capsule ing a healing agent and a catalyst has also been proposed The healing agent (inhibitor) offers a self-healing property which protects against scratches and corrosion and the catalysis provides extra functions such as anti-microbial effects or other desired functions

16 Nanocoatings and ultra-thin fi lms

given to increasing the hardness of a material, insuffi cient attention is paid

to its toughness Accordingly, further research should be carried out which considers both hardness and roughness while taking into account corrosion behavior In designing nanocomposite coatings, several factors and applica-tion requirements must be considered Research areas in the fi eld should involve prevention of the formation of dislocations in the nanocrystalline phase; blocking of the grain-boundary sliding of nanograins; the effect

of changes made to the lattice parameter; the role of the crystal size; the nature of the grain boundaries; and fi nally the effects of impurities and intermediate phases.56

1.10 New composite and powder coatings

1.10.1 Composite coatings

Composite coating technology has been developed to fulfi ll the industrial demands for coatings whose specifi cations exceed the capabilities of con-ventional coating technologies, and that are capable of functioning in extreme environments and in the face of challenges posed by temperature, corrosion, abrasion, fatigue, friction and erosion.57

Tungsten carbide hard metals and their analogues are now a mature technology; however, recent research has focused on changing the design

of the microstructure with the aim of producing alternatives to the tional two-phase structure Signifi cant improvements in the performance of coatings have been achieved by changing the size, shape and distribution

conven-of the phases to produce ultra fi ne-grained materials.57 Some approaches involve the use of nanoscale powder to form nanocomposite coatings as discussed in Section 1.9.2

1.10.2 Powder coatings

Designing a high performance clear coating is a key target for the tive industry Powder coating offers a limitless choice of colors and fi nishes Moreover, powder coating produces a high specifi cation coating which is relatively hard, abrasion-resistant and tough The thickness of the coating applied can be varied considerably according to requirements

automo-The prospects for powder coatings will improve with the development of materials with lower curing temperatures, and the production of thinner

fi lms will become an achievable goal as powder particle morphology improves The use of UV curable powder coatings seems to have great potential Controlling the humidity content of the surface can also facilitate electrostatic coating The size and shape of the powder particle will be important factors in achieving much smoother fi lms Ultrasonic waves seem

Current and advanced coating technologies 17

to be a promising means of adjusting the shape of the powder particle to make it more spherical

1.11 Advanced polymers and fi llers

Developments in the structure of coatings using advanced polymers sent one noteworthy future trend in coating technology

repre-1.11.1 Hyperbranched polymers

Controlled radical polymerization will be used for the synthesis of branched polymers with low melt viscosities; this is ideal for coatings with

hyper-a high solid content hyper-and for powder cohyper-atings The hyper-addition polymerizhyper-ation

of acrylic monomers in hypercritical fl uids is one potential means of ing solvent-free binders with narrow melting ranges for use in powder coatings with low fi lm thickness and low temperature curing characteristics.39,40

produc-1.11.2 Organic–inorganic hybrid polymers

Hybrid polymers have proved to be of great interest in the development of future coating systems Combinations of organic polymers and silicates make it possible to improve the overall coating qualities, as the stability and scratch resistance of inorganic networks can be combined with the elasticity

of organic polymers The sol–gel method has been successfully used for the synthesis of hybrid polymers.39

Most of the current multiphase polymer systems do not produce lucent fi lms, a fact that has limited their wider acceptance as coatings A method for producing polymers with improved translucent qualities was reported by Brock.39 This method uses a construction of block and comb polymers with intramolecular incompatibility, followed by phase separation

trans-in the nanofi eld Future developments trans-in phase separation, along with a better understanding of the interface energies of polymer mixtures, will lead

to improved adhesion to the surface with no negative effect on the coating qualities

1.11.3 Conductive polymers

Newly-developed conductive polymers based on polyanilines and phens might offer improved corrosion resistance These could also permit electrostatic and electrochemical coating of non-metallic substrates,

polythio-as well polythio-as producing heat-conductive layers for electrically heatable surfaces

18 Nanocoatings and ultra-thin fi lms

Microporous coatings containing catalysts are currently the subject of extensive research in environmental science, due to their ability to remove toxic gases Coatings that contain a small amount of active components (such as photochromic coatings, which are translucent or opaque according

to light intensity) will be increasingly used as a means of identifying ship of stolen items.39,40

owner-1.11.4 Water-soluble paint

An increase in the use of water-soluble latex paints is one of the biggest future trends in architectural paint industries Water-soluble paints are less expensive, lower in odor than alkyd-based solvent-borne paints, and produce

no toxic waste In the coatings and paints industries, water-soluble paints have met with strong competition from recently-developed superior resins with unique characteristics

1.11.5 Fillers

Fillers are widely used in every coatings industry, mainly to reduce costs, although they can negatively affect the coating quality Future develop-ments in the chemistry and structure of the fi llers will lead to an increase

in their use in tailor-made coating systems; the fi llers will then have a cifi c function such as strengthening the mechanical properties of the coat-ings or improving the coating quality for specifi c applications and decorative effects Nanoscience and nanotechnology will have a signifi cant impact on the design of ultra-thin fi lms containing nanolayers of special fi llers or addi-tives embedded into the matrix of the polymer It is expected that such

spe-fi llers will improve the mechanical strength, coating transparency and rosion resistance

cor-Phyllo silicates with their unique leaf-like structure will increasingly be the subject of both industrial and basic research as a fi ller for tailor-made coatings Some studies used phyllo silicates interlocked into the polymer matrix in nano form to improve the barrier effect of the coating This approach will become more popular and will be used for thermal and elec-trical insulation, and especially for fi re protection.39,40

1.12 Developments in coating processes

The processing of nanocoatings will be of interest to researchers due to the superior hardness and strength that these coatings can offer However, developments in nanocoatings are determined by improvements in coating processing and the availability of nanopowders The use of nanocoatings is still in its infancy because the process requires large-scale control during

Current and advanced coating technologies 19the synthesis of nanoparticles Moreover, the technology requires sophisti-cated instruments and multistep processes.

Nanopowders are used as feedstock materials for thermal spray cesses (plasma spraying or HVOF spraying) Thermal spraying offers the unique advantage of moderate to high rate of throughput and the ability

pro-to coat target materials with complex shapes using nanostructured stock powders prepared from vapor, liquid and solid routes Thermal sprayed nanocoatings with moderate hardness showed better wear resis-tance than those fabricated by micro powders.58 HVOF will get the most research attention in the next generation of nanocoatings, due to its ability

feed-to deposit dense nanocrystalline ceramic coatings with wear properties superior to those produced by plasma spraying, thanks to the lower spraying temperature involved Moreover, HVOF allows the development of nano-coatings with low porosity, high strength and increased wear resistance.Some modifi cations will be made to the electro-dip processes in order to allow lower curing temperatures; new curing mechanisms using radiation curing will also extend their fi eld of application to low melting point materi-als such as plastics.39 Automation of the coating process in order to increase line speed, reduce labor dependency and save energy will be the main target for coating industries

Future improvements in coating processes will include reduction of the number of coating layers; full automation of the coating process; controlling the color of the end product using a module method; and automatic quality control The future trends in coating technology are generally based on:

1 Advanced lightweight materials such as magnesium alloys and ite materials Developments in magnesium alloys and composite materi-als will continue, focusing on rare-earth alloying elements such as cerium and yttrium with the aim of providing some desirable properties

compos-2 Tailor-made coating systems with unique chemistry and structures will become commonplace thanks to innovations in polymers, new binders and fi llers

3 Nanoscience and nanotechnology will play a distinct role in the next generation of coating technology More attention will be paid to self-healing coatings based on nanocontainers or nanocapsules that can

be fi lled with inhibitor to protect the substrate from corrosion upon damage or scratching in the coating layer However, more studies still need to be carried out to optimize the coating conditions since the technology currently involves multisteps (around 8–10 steps) and expensive raw materials, both of which are industrially unattractive

4 Many future efforts will be devoted to the design of biocompatible nanocoatings systems for medical implant applications and to the

20 Nanocoatings and ultra-thin fi lms

development of suitable techniques for preparing high performance hydroxyapatite coatings

5 Developments in the coating process will be an important topic for industrial and basic research New surface modifi cation techniques and faster and cheaper curing methods using UV and electrostatic applica-tion will be the focus of the next decade of research

1.13 Acknowledgements

Many thanks to all the authors of papers, books, and websites and to all published sources (listed below) that were used to prepare the materials for this chapter

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