Nanocoatings and Ultra-Thin Films

Technologies and Applications

Abdel Salam Hamdy Makhlouf, I Tiginyanu

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9781845698126
• eBook ISBN: 9780857094902
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Coatings are used for a wide range of applications, from anti-fogging coatings for glass through to corrosion control in the aerospace and automotive industries. Nanocoatings and ultra-thin films provides an up-to-date review of the fundamentals, processes of deposition, characterisation and applications of nanocoatings.Part one covers technologies used in the creation and analysis of thin films, including chapters on current and advanced coating technologies in industry, nanostructured thin films from amphiphilic molecules, chemical and physical vapour deposition methods and methods for analysing nanocoatings and ultra-thin films. Part two focuses on the applications of nanocoatings and ultra-thin films, with chapters covering topics such as nanocoatings for architectural glass, packaging applications, conventional and smart nanocoatings for corrosion protection in aerospace engineering and ultra-thin membranes for sensor applications.With its distinguished editors and international team of contributors, Nanocoatings and ultra-thin films is an essential reference for professional engineers in the glazing, consctruction, electronics and transport industries, as well as all those with an academic research interest in the field.- Provides an up-to-date review of the fundamentals, processes of deposition, characterisation and applications of nanocoatings- Focuses on the applications of nanocoatings and ultra-thin films, covering topics such as nanocoatings for architectural glass, packaging applications and ultra-thin membranes for sensor applications- Includes chapters on current and advanced coating technologies in industry, nanostructured thin films from amphiphilic molecules, chemical and physical vapour deposition methods and methods for analysing nanocoatings and ultra-thin films

Part I

Technologies

1

Current and advanced coating technologies for industrial applications

A.S.H. Makhlouf, 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 findings resulting from basic and applied research in the fields 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 industrial 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 fingertips 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 protection of various surfaces, other important applications for this type of coating should also be mentioned. Ancient Egyptian scientists developed a very fine coating technology that showed similarities with nanotechnology. Several theories therefore treat nanotechnology as a re-innovated technology, with 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 fulfill the demands of the manufacturing industries or of society.

The main driving forces behind the sharp increase in research and development 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 modification of the surface properties of the materials to be coated is one of the most desirable methods of improving corrosion and wear resistance, 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 solution 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, finding 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 final coating depends on the materials being plated. As a result, different chemical surface treatment processes must be developed for each material to be coated. Uneven distribution 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 significantly 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 resistance, hardness or abrasion.^4,6−9

1.3 Conversion coatings

Conversion coatings are produced by a chemical or electrochemical reaction 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.

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 efficiency: cost ratio. These advantages have made them a standard method of corrosion protection. Moreover, they provide the greatest level of under-film corrosion resistance and facilitate the application of further finishing 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 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 films are used to improve corrosion resistance and paint adhesion to the substrate.²³

The anodizing process includes the following stages: (i) mechanical treatment; (ii) degreasing, cleaning and pickling; (iii) electropolishing; (iv) anodizing using AC or DC current; (v) dyeing or post-treatment; and (vi) sealing.²⁴ The anodized films 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 film.²³

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 flaws, porosity and inclusions from mechanical treatment can also result in uneven deposition which, in turn, can enhance corrosion.²⁵

Another ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Introduction
• Part I: Technologies
• Part II: Applications
• Index