**6. Types of surface coating**

#### **6.1 Physical vapor deposition (PVD) coating**

PVD process is well-known for offering corrosion, wear resistance, and thin protective films on the surface of the materials that are exposed to corrosive media, and its applications range from decorative objects to industrial parts. The benefit of this technique is that the mechanical, corrosion, and esthetic properties of the coating layers could be adjusted on demand. Generally, PVD is a method that occurs in a high vacuum, and the solid/liquid materials transfer to a vapor phase followed by a metal vapor condensation, which creates a solid and thick film. The common types of PVD methods are sputtering and evaporation. Since the coating layers created by PVD are thin in nature, there is always a need for multilayered coatings while the materials selection should be considered carefully.

#### **6.2 Chemical vapor deposition (CVD) coating**

Another type of vapor deposition is called CVD. This process undergoes a high vacuum and is widely used in the semiconductors industry providing a solid, high quality, and a high resistance coating layer on any substrate. CVD can be used for mechanical parts in continuous interaction, which need protection for corrosion and wear. In this method, the substrate, known as a wafer, would be exposed to a set of volatile material precursors where a chemical reaction creates a deposition layer on the surface of the material. However, some by products of these chemical reactions, which are removed by continuous airflow of the vacuum pump, can stay in the chamber.

#### **6.3 Micro-arc oxidation (MAO) coating**

MAO method is known as a flexible method of coating concerning the composition of coatings. In general, MAO utilizes a high voltage difference between anode and cathode to generate micro-arcs as plasma channels. When these arcs hit the substrate, they melt a portion of the surface, depending on the intensity of the microarcs. Simultaneously, plasma networks discharge their pressure, which supports the deposition of coating materials in the working electrolyte on the substrate surface. The existing oxygen in the electrolyte causes a chemical reaction resulting oxidation and the oxides gets deposited on the surface of the substrate. The adaptability of this process lies in the flexibility of combining preferred elements and compounds as a solute in the working electrolyte. With MAO, the most common substrate materials are Al, Mg, Ti, and their alloys.

#### **6.4 Electro deposition coating**

Electro deposition of materials is considered a type of protection utilizing the deposition of metallic ions on a coating substrate. In this method, a difference in potential between anode and cathode poles causes an ion transfer in the unit cell. After a while, a coating layer forms on the submerged sample by getting ions from the other electrode. The common group of metals that have been intensively studied includes, but is not limited to, Ni-P, Ag/Pd, Cu/Ag, Cu/Ni, and Co/Pt.

#### **6.5 Sol-gel coating**

Sol–gel synthesis is used to obtain coatings that can modify the surfaces of metals to avoid corrosion or to enhance the biocompatibility and bioactivity of metals and their alloys that are of biomedical interest. Anticorrosion coatings composed of smart coatings and self-healing coatings will be described.

#### **6.6 Thermal spray coating**

Thermal spray coating is a general term for a series of processes that utilize a plasma, electric, or chemical combustion heat source to melt a set of designed materials and spray the melt on the surface in order to produce a protective layer. These are reliable types of corrosion- and wear-resistant coatings. In this process, a heat source or plasma, heats up the coating materials to a fully molten or semi-molten phase and sprays them on the substrate material with a high velocity jet.

Thermal spraying dates back to the early 1900s when Dr. Schoop [1] first carried out experiments in which molten metal were atomized by a stream of high-pressure gas and propelled on to a surface. The Schoop process consisted of a crucible filled with molten metal while the propellant, hot compressed air, provided enough pressure to break up the molten metal, creating a spray jet. This system was quite rudimentary and inefficient. Following Schoop's work some improvements to the process were introduced. But the disadvantages of the process is that, it was only useful for low-melting-temperature metals, that the molten metal caused severe corrosion and that it was not possible to establish a continuous process, were enough to stop further progress.

Schoop then focused his efforts in another direction and in 1912 the first device for spraying metal wires was produced. The principle of this process is simple; a wire was fed into a combustion flame which melted the tip of the wire and then compressed air surrounding the flame atomized the molten metal and drove the tiny droplets on to a substrate to form a coating. In addition to improvements to nozzle and gun design along with the wire feed drive rolls, the basic principle of the process is the same today. This procedure is called flame spraying (FS) and covers an enormous group of thermal spray techniques which use powder, wires or rods.

A completely new concept in thermal spraying was introduced by Schoop in 1914 when he used electricity to melt the feedstock material. The most advanced equipment made by Schoop was quite similar to current electric arc spraying. This method is based on producing an electric arc among two wires of conducting

**293**

**Figure 17.**

*Classification of thermal spray coating process.*

*Wear: A Serious Problem in Industry*

particle velocity.

this coating process.

researchers (**Figure 17**).

**7.1 Powder flame spraying**

**7. Types of thermal spray processes**

*DOI: http://dx.doi.org/10.5772/intechopen.94211*

materials, which are fed together inside the gun. This arc is created at the tip of the

The concept of powder FS was introduced by F. Schori in the early 1930s, when a metallic powder was fed into a flame by the Venturi effect. The coating powder was heated in the nozzle and the exhaust gases (oxygen and acetylene) propelled the droplets. Improvements to the process incorporated in modern guns include an inert compressed gas that pressurizes the combustion chamber and results in rise in

The main problem associated with these early techniques was feedstock material.

They all used a low-melting-point material, which leads to limited applications. Years passed, and the demand for high-temperature-resistant materials increased, until in the 1950s new systems that would boost the thermal spray market appeared. Firstly a modification of wire FS, the ceramic rod FS technique, which could use stabilized zirconias and aluminas appeared. However, it was the development, in about 1955, of the detonation gun (D-Gun) and atmospheric plasma spraying (APS) in about 1960 that proved to be the watershed as regards thermal spray applications. The thickness achieved in thermal spray coating techniques can range from 20 micron to several milli meters which are significantly higher than the thickness offered by electroplating, CVD, or PVD processes. In addition, the materials that can be used as feedstock of thermal spray coatings range from refractory metals and metallic alloys to ceramics, plastics, and composites and can easily cover a relatively high surface area of a substrate. Therefore the current chapter will mostly focus on

There are various types of thermal spray coating processes introduced by the

In powder flame spraying, the feedstock material is injected to the plume for heating and melted by the heating zone. After melting the molten particles are propagated towards the substrate surface. Then the molten particles are deposited on the substrate surface or pre-deposited splat to form a coating (**Figure 18**). The molten particles are ejected by the flame spray gun. The only difference between

powder flame spraying and wire flame spraying is the feedstock material.

wires and a jet of compressed air propels the molten metal to the substrate.

#### *Wear: A Serious Problem in Industry DOI: http://dx.doi.org/10.5772/intechopen.94211*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

materials are Al, Mg, Ti, and their alloys.

**6.4 Electro deposition coating**

**6.5 Sol-gel coating**

**6.6 Thermal spray coating**

enough to stop further progress.

deposition of coating materials in the working electrolyte on the substrate surface. The existing oxygen in the electrolyte causes a chemical reaction resulting oxidation and the oxides gets deposited on the surface of the substrate. The adaptability of this process lies in the flexibility of combining preferred elements and compounds as a solute in the working electrolyte. With MAO, the most common substrate

Electro deposition of materials is considered a type of protection utilizing the deposition of metallic ions on a coating substrate. In this method, a difference in potential between anode and cathode poles causes an ion transfer in the unit cell. After a while, a coating layer forms on the submerged sample by getting ions from the other electrode. The common group of metals that have been intensively studied

Sol–gel synthesis is used to obtain coatings that can modify the surfaces of metals to avoid corrosion or to enhance the biocompatibility and bioactivity of metals and their alloys that are of biomedical interest. Anticorrosion coatings composed of

Thermal spray coating is a general term for a series of processes that utilize a plasma, electric, or chemical combustion heat source to melt a set of designed materials and spray the melt on the surface in order to produce a protective layer. These are reliable types of corrosion- and wear-resistant coatings. In this process, a heat source or plasma, heats up the coating materials to a fully molten or semi-molten

Thermal spraying dates back to the early 1900s when Dr. Schoop [1] first carried out experiments in which molten metal were atomized by a stream of high-pressure gas and propelled on to a surface. The Schoop process consisted of a crucible filled with molten metal while the propellant, hot compressed air, provided enough pressure to break up the molten metal, creating a spray jet. This system was quite rudimentary and inefficient. Following Schoop's work some improvements to the process were introduced. But the disadvantages of the process is that, it was only useful for low-melting-temperature metals, that the molten metal caused severe corrosion and that it was not possible to establish a continuous process, were

Schoop then focused his efforts in another direction and in 1912 the first device for spraying metal wires was produced. The principle of this process is simple; a wire was fed into a combustion flame which melted the tip of the wire and then compressed air surrounding the flame atomized the molten metal and drove the tiny droplets on to a substrate to form a coating. In addition to improvements to nozzle and gun design along with the wire feed drive rolls, the basic principle of the process is the same today. This procedure is called flame spraying (FS) and covers an enormous group of thermal spray techniques which use powder, wires or rods. A completely new concept in thermal spraying was introduced by Schoop in 1914 when he used electricity to melt the feedstock material. The most advanced equipment made by Schoop was quite similar to current electric arc spraying. This method is based on producing an electric arc among two wires of conducting

phase and sprays them on the substrate material with a high velocity jet.

includes, but is not limited to, Ni-P, Ag/Pd, Cu/Ag, Cu/Ni, and Co/Pt.

smart coatings and self-healing coatings will be described.

**292**

materials, which are fed together inside the gun. This arc is created at the tip of the wires and a jet of compressed air propels the molten metal to the substrate.

The concept of powder FS was introduced by F. Schori in the early 1930s, when a metallic powder was fed into a flame by the Venturi effect. The coating powder was heated in the nozzle and the exhaust gases (oxygen and acetylene) propelled the droplets. Improvements to the process incorporated in modern guns include an inert compressed gas that pressurizes the combustion chamber and results in rise in particle velocity.

The main problem associated with these early techniques was feedstock material. They all used a low-melting-point material, which leads to limited applications. Years passed, and the demand for high-temperature-resistant materials increased, until in the 1950s new systems that would boost the thermal spray market appeared. Firstly a modification of wire FS, the ceramic rod FS technique, which could use stabilized zirconias and aluminas appeared. However, it was the development, in about 1955, of the detonation gun (D-Gun) and atmospheric plasma spraying (APS) in about 1960 that proved to be the watershed as regards thermal spray applications.

The thickness achieved in thermal spray coating techniques can range from 20 micron to several milli meters which are significantly higher than the thickness offered by electroplating, CVD, or PVD processes. In addition, the materials that can be used as feedstock of thermal spray coatings range from refractory metals and metallic alloys to ceramics, plastics, and composites and can easily cover a relatively high surface area of a substrate. Therefore the current chapter will mostly focus on this coating process.
