Surface engineering

Surface engineering refers to a wide range of technologies that aim to design and modify

the surface properties of components. There are two main categories of surface

engineering methods that can be used to optimize the surface properties and the bulk

materials. These are surface coatings and surface modification.

Surface coating processes involve depositing a layer of molten, semi-molten or chemical

material onto a substrate. One of the main functions of surface coating is to modify and

reinforce the surface functions instead of reforming the composition of the bulk material.

Some examples of surface coating processes include (1) Physical Vapour Deposition (PVD),

(2) Chemical Vapour Deposition (CVD), (3) plasma and thermal spraying, (4) cladding and

(5)electroplating.

Surface modification processes can be classified as hardening by flame, induction, laser or

electron beam, high energy treatments, e.g. ion implantation; and diffusion treatments,

e.g. carburizing and nitriding. Surface modification processes are applicable to control

friction, improve surface wear and corrosion resistance, and change the physical or

mechanical properties of the component. Surface modification treatments also can be

combined with surface coating processes, for instance laser cladding. This combination

enhances the advantages of surface coatings and surface modification, thus achieving

specific requirements and fitness for purpose.

Methods of Surface Coating

1: Electroplating

Electroplating is the application of electrolytic cells in which a thin layer of metal is deposited onto an electrically conductive surface. Or

it is the process of coating a metal with a thin layer of another metal by electrolysis to improve the metal's corrosion resistance.

The metals most commonly used in plating are:

Copper, Nickel, Gold, Silver, Chrome, Zinc and Tin

Electroplating is also known as electrodeposition and electroplated coating.

A cell consists of two electrodes (conductors), usually made of metal, which are held apart from one another. The electrodes are immersed in an electrolyte (a solution). When an electric current is turned on, positive ions in the electrolyte move to the negatively charged electrode (called the cathode). Positive ions are atoms with one electron too few. When they reach the cathode, they combine with electrons and lose their positive charge. At the same time, negatively charged ions move to the positive electrode (called the anode). Negatively charged ions are atoms with one electron too many). When they reach the positive anode they transfer their electrons to it and lose their negative charge.

In one form of electroplating, the metal to be plated is located at the anode of the circuit, with the item to be plated located at the cathode. Both the anode and the cathode are immersed in a solution which contains a dissolved metal salt (e.g., an ion of the metal being plated) and other ions which act to permit the flow of electricity through the circuit. Direct current is supplied to the anode, oxidizing its metal atoms and dissolving them in the electrolyte solution. The dissolved metal ions are reduced at the cathode, plating the metal onto the item. The current through the circuit is such that the rate at which the anode is dissolved is equal to the rate at which the cathode is plated.

There are several reasons why you might want to coat a conductive surface with a metal. Silver plating and gold plating of jewelry or silverware typically are done to improve the appearance and value of the items. Chromium plating improves the appearance of objects and also improves its wear. Zinc or tin coatings may be applied to confer corrosion resistance. Sometimes electroplating is done simply to increase the thickness of an item.

Electroplating Example

A simple example of the electroplating process is the electroplating of copper in which the metal to be plated (copper) is used as the anode and the electrolyte solution contains the ion of the metal to be plated (Cu2+ in this example). Copper goes into solution at the anode as it is plated at the cathode. A constant concentration of Cu2+ is maintained in the electrolyte solution surrounding the electrodes:

Anode: Cu(s) → Cu2+(aq) + 2 e -

cathode: Cu2+(aq) + 2 e - → Cu(s)

Common Electroplating Processes

Metal Anode Electrolyte Application

Cu Cu 20% CuSO4, 3% H2SO4 electrotype

Ag Ag 4% AgCN, 4% KCN, 4% K2CO3 jewelry, tableware

Au Au, C, NiCr 3% AuCN, 19% KCN, 4% Na3PO4 buffer jewelry

The main purpose of electroplating is to improve:

Appearance

Protection against corrosion

Special surface properties

Engineering or mechanical properties

In the process of electroplating the anode is connected to the positive terminal, and the cathode (metal to be plated) is connected to the negative terminal. Both are immersed in a solution that contains an electrolyte and then connected to an external supply of direct current. When DC power is applied, the anode is oxidized—its metal atoms dissolve in the electrolyte solution. These dissolved metal ions are reduced at the cathode and form a coating. The current through the circuit is adjusted so that the rate at which the anode is dissolved equals the rate at which the cathode is plated. Different metals can be coated using the electroplating process. Formulating the right electrolyte is important for the quality of plating. Electrolytes used in this process include:

Acids

Bases

Metal salts

Molten salts

Properties of the electrolyte that must be considered when making a selection are:

Corrosivity

Resistance

Brightness or reflectivity

Hardness

Mechanical strength

Ductility

Wear resistance

Chrome Plating

What does Chrome Plating mean

Chrome plating is a metal coating method used to create a thin layer chromium on the surface of a material. Chrome plating uses a technology known as electroplating to create chromium layers that can be less than 0.001 inch (0.025 mm).Chrome plating can be used to make different chromium alloy coatings with a variety of deposition thicknesses for corrosion and wear resistance.

There are two main types of chrome plating:

Decorative chrome plating. This type employs a layer of nickel and a layer of chromium. The nickel gives the surface of the object its shine and its polished look. Once the nickel layer has been deposited, a chromium layer is added on top of it. The chromium layer helps increase the corrosion resistance of the material and also improve the resistance to scratching and wear. Decorative chrome plating usually has a total thickness under 0.001 inch.

Hard chrome plating. This type is typically used in industrial settings where aesthetic appeal is not the primary concern. Hard chrome plating, while it can improve the corrosion resistance of the material to which it is applied, is primarily used to increase the wear resistance of certain components. Hard chrome plating is commonly applied to various types of steel and is almost always thicker than decorative chrome plating.

Abrasion-Resistant Coating

Abrasion-resistant coatings protect substrates against harsh environments. They can be applied on various structures and areas like dumpsters, industrial plants or factories, commercial establishment walls and floors and more.

Polymers are commonly used as abrasion-resistant coatings. This type of coating can be found in various applications, such as:

Chutes

Pumps and their casings

Slurry lines

Piping

Valves

Screens

Apart from these polymers, there are other types of protective paints available on the market to meet specific industrial needs such as:

Air dry epoxy - A cost-effective coating that promotes corrosion resistance

Phosphate - Coating for ferrous metal that provides protection from galling and minor corrosion

Ceramic epoxy - Offers protection by having ceramic particles bonded to resin system

There are numerous high-quality coatings on the market that can deliver according to industry requirements. Such coatings offer not only abrasion resistance, but the best protection to almost all structures and equipment through various means like reducing friction and restoration of equipment.

2: Galvanizing Process

Hot-dip galvanizing is the process of immersing iron or steel in a bath of molten zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. While the steel is immersed in the zinc, a metallurgical reaction occurs between the iron in the steel and the molten zinc. This reaction is a diffusion process, so the coating forms perpendicular to all surfaces creating a uniform thickness throughout the part.

Figure 1: Model of the Hot-Dip Galvanizing Process

Why Galvanize؟

Quite simply, galvanizing a metal gives it anti-corrosion properties. Without the protective zinc coating, the metal would remain exposed to the elements and potentially oxidize and corrode much faster. Galvanized Steel is a cost effective alternative to using materials such as austenitic stainless steel or aluminum in order to prevent corrosion.

How Does It Work?

Galvanizing can protect metal is a number of ways. Firstly, it creates a protective coating that shields the metal from the surrounding environment. The layer of zinc prevents water and moisture and other elements in the air from corroding the steel underneath. Should the zinc coating be scratched deep enough, the metal would become exposed and susceptible to corrosion.

Galvanizing can also protect metal through a process called “galvanic corrosion”. Galvanic corrosion occurs when two metals of a different electrochemical make up are placed into contact with one another with an electrolyte present, such as salty water. Depending on the atomic structure of the two metals, one metal is the anode and the other is the cathode. The anode corrodes more rapidly than it would by itself and the

cathode corrodes at a slower pace than it would by itself. The reason zinc is used for galvanizing is because it has an affinity towards being the anode when in contact with many different types of metals. Since the zinc coating in contact with the base metal is usually the anode, it slows the corrosion of the base metal, or the cathode.

Different Methods of Galvanizing

There are several different processes for galvanizing metal:

Hot-Dip Galvanizing

As the name implies, this method involves dipping the base metal into a molten pool of zinc. First, the base metal must be cleaned either mechanically, chemically, or both to assure a quality bond can be made between the base metal and the zinc coating. Once cleaned, the base metal is then fluxed to rid it of any residual oxides that might remain after the cleaning process. The base metal is then dipped into a liquid bath of heated zinc and a metallurgical bond is formed.

The advantages of this method are that it is economical; it can be performed quickly and to complex shapes. However, the final coating can be inconsistent relative to other galvanizing processes.

Pre-galvanizing

This method is very similar to hot-dip galvanizing but is performed at the steel mill, usually on materials that already have a specific shape. Pre-galvanizing involves rolling metal sheet through a similar cleaning process to that of the hot-dip galvanizing process. The metal is then passed through a pool of hot, liquid zinc and then recoiled.

An advantage of this method is that large coils of steel sheet can be rapidly galvanized with a more uniform coating compared to hot-dip galvanizing. A disadvantage is that once fabrication of the pre-galvanized metal begins, exposed, uncoated areas will become present. This means that when a long coil of sheet is cut into smaller sizes, the edges where the metal is cut are left exposed.

Electrogalvanizing

Unlike the previous processes, electrogralvanizing does not use a molten bath of zinc. Instead, this process utilizes an electrical current in an electrolyte solution to transfer zinc ions onto the base metal. This involves electrically reducing positively charged zinc ions to zinc metal which are then deposited on the positively charged material. Grain refiners can also be added which helps to ensure a smooth zinc coating on the steel. Similar to the pre-galvanizing process, electrogalvanizing is typically applied continuously to a roll of sheet metal.

Some advantages of this process are a uniform coating and precise coating thickness. However, the coating is typically thinner than the coating of zinc achieved by the hot-dip galvanizing method which can result in reduced corrosion protection.

The hot-dip galvanizing process (Figure 1) has been used since 1742, providing long-lasting, maintenance-free corrosion protection at a reasonable cost for decades. Although hot-dip galvanizing has been utilized to protect steel for generations, the galvanizing process continues to evolve with new technologies and creative chemistries. The three main steps in the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment, each of which will be discussed in detail. The process is inherently simple, which is a distinct advantage over other corrosion protection methods.

Surface Preparation: For high quality hot-dip galvanizing, steel must be properly prepared prior to being immersed in a bath of molten zinc. During the surface preparation stage, material going through degreasing/caustic cleaning, pickling, and fluxing.

Degreasing/Caustic Cleaning: A hot alkali solution, mild acidic bath, or biological cleaning bath removes contaminants from the steel such as dirt, grease and oil.

Pickling: To remove mill scale and iron oxides, the steel goes through a diluted solution of heated sulfuric acid or ambient hydrochloric acid.

Fluxing: Through the final surface preparation step, any remaining oxides are removed in a zinc ammonium chloride solution and a protective layer is deposited on the steel to prevent any further oxides from forming prior to galvanizing.

Figure : Hanging of Steel Products

Galvanizing: Following surface preparation, the steel will be immersed in a bath of molten zinc. The zinc kettle contains at least 98% pure zinc and is maintained at a temperature between 815º-850º F (435º-455º C). While steel is immersed in the kettle, the zinc reacts with the iron in the steel to form a series of metallurgically

bonded zinc-iron alloy layers with the final top layer 100% zinc

.

Figure : Photomicrograph of the galvanized coating

Corrosion Inhibitors

Corrosion inhibitors offer corrosion protection that is similar to coatings as they act as a barrier between the oxidizing agents and the metal surface. The inhibitor adsorbs onto the steel surface and slows down or eliminates one or more of the electrochemical reactions by blocking the reaction site. The degree of protection that a corrosion inhibiter provides is heavily dependent on the properties of the inhibitor, the properties of the steel and the fraction of the steel surface that is blocked by the inhibitor.

Liquid phase inhibitors (LPIs): LPIs are in wide scale use to combat CO2 corrosion in pipelines. The inhibitor is that is injected into the pipeline is transported in low concentrations and adsorbs onto the surface of the steel. Because the inhibitor is in the liquid phase only the interior of the pipeline needs to be in contact with the liquid either continuously or semi-continuously. One of the main benefits in using inhibitors to protect from CO2 corrosion is that it can continuously be added to the flow to provide continuous protection.

Figure 1: VCI Molecules Adsorbing to a Metal

Figure 1: VCI Molecules Adsorbing to a Metal Surface

Surface Volatile corrosion inhibitors (VCIs): VCIs are similar to liquid phase inhibitors in that they are present in low concentrations in the pipeline. Because they are a volatile compound they can easily enter the vapour phase if one is present. VCIs can adsorb onto the steel directly from the vapour phase and can penetrate into complex shapes and imperfections better than LPIs. Any condensate that may form on the inside of the pipe may contain water and CO2 which will contribute to corrosion in the pipe. If VCIs are present in the vapour phase they will also condense and provide a protective film.

The volatility of the inhibitor is dependent on the vapour pressure of the compound. For the prevention of CO2 corrosion on steel, amines are used as the VCI. The vapour pressure of amines is ideal so that the VCI will be present in both phases and provide ideal coverage for the pipeline.

Cathodic Protection

Sacrificial Anode: A sacrificial anode is used to protect metal structures,

Fig.2: Iron protected with sacrificial anode

Figure 2: Iron Protected With Sacraficial Anode mainly iron, from corrosion. A sacrificial anode is a substance that is easier to oxidize then the structure being protected, typically it is a relatively small and easy to replace piece of metal. From the standard electric potentials below:

Fe(s) <-> Fe2+ + 2e- + 0.44

Zn(s) <-> Zn2+ + 2e- + 0.76

Mg(s) <-> Mg2+ + 2e- + 2.37

Both magnesium (Mg) and zinc (Zn) have higher potentials so they will be oxidized before iron if they are connected in a system. When either magnesium or zinc anodes are connected to an iron structure electrons will move from the sacrificial anode through the iron structure and to the reduction reaction site. This causes the iron structure to retain its electrons and not corrode until the anode has been used up.

Sacrificial anodes are used when the pipeline is temporary or where there is not access to a power supply.

Figure 3: Iron Protected With ICCP

Impressed Current Cathodic Protection (ICCP): This form of corrosion protection is applied by connecting the negative terminal of a DC power source to the iron structure and the positive terminal to an anode. Because the driving voltage is provided from a DC source and not electric potentials the anode does not need to oxidize and an inert anode can be used. There are 3 main advantages to using ICCP,

Thermal barrier coating

Thermal barrier coatings (TBCs) perform the important function of insulating components, such as gas turbine and aeroengine parts, operating at elevated temperature. Typical examples are turbine blades, combustor cans, ducting and

nozzle guide vanes. TBCs have made possible the increase in operating temperature of gas turbines.

TBCs are characterised by their very low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow. The most commonly applied TBC material is yttria stabilized zirconia (YSZ) which exhibits resistance to thermal shock and thermal fatigue up to 1150°C. YSZ is generally deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes. It can also be deposited by HVOF spraying for applications such as blade tip wear prevention, where the wear resistant properties of this material can also be used.

It is common practice to aluminise and pre-coat the substrate material (generally a nickel or cobalt superalloy) with an MCrAlY bond-coat. The bond-coat is necessary to accommodate residual stresses that might otherwise develop in the coating system, caused by the metallic substrate and the ceramic TBC having different coefficients of thermal

Requrements

The general requirements for an effective TBC can be summarize as needing: 1) a high melting point. 2) no phase transformation between room temperature and operating temperature. 3) low thermal conductivity. 4) chemical inertness. 5) similar thermal expansion match with the metallic substrate. 6) good adherence to the substrate. 7) low sintering rate for a porous microstructure. These requirements severely limit the number of materials that can be used, with ceramic materials usually being able to satisfy the required properties.[3]

Thermal barrier coatings typically consist of four layers: the metal substrate, metallic bond coat, thermally-grown oxide (TGO), and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ) which is desirable for having very low conductivity while remaining stable at nominal operating temperatures typically seen in applications. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface. However, above 1200 °C, YSZ suffers from unfavorable phase transformations, going from t'-tetragonal to tetragonal to cubic to monoclinic. Such phase transformations lead to crack formation within the top coating. Recent advancements in finding an alternative for YSZ ceramic topcoat identified many novel ceramics (rare earth zirconates) having superior performance at temperatures above 1200 °C, however with inferior fracture toughness compared to that of YSZ. In addition, such zirconates may have a high concentration of oxygen ion vacancies, which may facilitate oxygen transport and exacerbate the formation of the TGO. With a large enough TGO, spalling of the coating may occur, which is a catastrophic mode of failure for TBCs. The use of such coatings would require addition coatings that are more oxidation resistant, such as alumina or mullite.[4]

The bond-coat is an oxidation-resistant metallic layer which is deposited directly on top of the metal substrate. It is typically 75-150 μm thick and made of a NiCrAlY or NiCoCrAlY alloy, though other bond coats made of Ni and Pt aluminides also exist. The primary purpose of the bond-coat is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements that pass through the porous ceramic top-coat.

At peak operating conditions found in gas-turbine engines with temperatures in excess of 700 °C, oxidation of the bond-coat leads to the formation of a thermally-grown oxide (TGO) layer. Formation of the TGO layer is inevitable for many high-temperature applications, so thermal barrier coatings are often designed so that the TGO layer grows slowly and uniformly. Such a TGO will have a structure that has a low diffusivity for oxygen, so that further growth is controlled by diffusion of metal from the bond-coat rather than the diffusion of oxygen from the top-coat.[5]

The TBC can also be locally modified at the interface between the bondcoat and the thermally grown oxide so that it acts as a thermographic phosphor, which allows for remote temperature measurement

A key feature of all TBC components is well matched thermal expansion coefficients between all layers. Thermal barrier coatings expand and contract at different rates upon heating and cooling of the environment, so materials when the different layers have poorly matched thermal expansion coefficients, a strain is introduced which can lead to cracking and ultimately failure of the coating.

Cracking at the thermally-grown oxide (TGO) layer between the top-coat and bond-coat is the most common failure mode for gas turbine blade coatings. TGO growth produces a stress associated with the volume expansion which persists at all temperatures. When the system is cooled, even more mismatch is introduced from the mismatch in thermal expansion coefficients. The result is very high (2-6GPa) stresses which occur at low temperature and can produce cracking and ultimately fracture of the barrier coating. TGO formation also results in depletion of Al in the bond-coat. This can lead to the formation of undesirable phases which contribute to

the mismatch stress. These processes are all accelerated by the thermal cycling which occurs in many thermal barrier coating applications.[6]

Types

YSZ : Yeteria stabilized Zirconia

YSZ is the most widely studied and used TBC because it provides excellent performance in applications such as diesel engines and gas turbines. Additionally, it was one of the few refractory oxides that could be deposited as thick films using the then-known technology of plasma spraying.[8] As for properties, it has low thermal conductivity, high thermal expansion coefficient, and low thermal shock resistance. However, it has a fairly low operating limit of 1200C due to phase instability, and can corrode due to its oxygen transparency.

Mullite

Mullite is a compound of alumina and silica, with the formula 3Al2O3-2SiO2. It has a low density, along with good mechanical properties, high thermal stability, low thermal conductivity, and is corrosion and oxidation resistant. However, it suffers from crystallization and volume contraction above 800C, which leads to cracking and delamination. Therefore, this material is suitable as a zirconia alternative for applications such as diesel engines, where surface temperatures are relatively low and temperature variations across the coating may be large.

Alumina

Only α-phase Al2O3 is stable among aluminum oxides. With a high hardness and chemical inertness, but high thermal conductivity and low thermal expansion coefficient, alumina is often used as an addition to an existing TBC coating. By incorporating alumina in YSZ TBC, oxidation and corrosion resistance can be improved, as well as hardness and bond strength without significant change in the elastic modulus or toughness. One challenge with alumina is applying the coating through plasma spraying, which tends to create a variety of unstable phases, such as γ-alumina. When these phases eventually transform into the stable α-phase through thermal cycling, a significant volume change of ~15% (γ to α) follows, which can lead to microcrack formation in the coating.

CeO2 + YSZ

CeO2 (Ceria) has a higher thermal expansion coefficient and lower thermal conductivity than YSZ. Adding ceria into a YSZ coating can significantly improve the TBC performance, especially in thermal shock resistance. This is most likely due to less bond coat stress due to better insulation and a better net thermal expansion coefficient. Some negative effects of the addition of ceria include the decrease of hardness and accelerated rate of sintering of the coating (less porous).

Rare-earth zirconates

La2Zr2O7, also referred to as LZ, is an example of a rare-earth zirconate that shows potential for use as a TBC. This material is phase stable up to its melting point and can largely tolerate vacancies on any of its sublattices. Along with the ability for site-substitution with other elements, this means that thermal properties could potentially be tailored. Although it also has very low thermal conductivity compared to YSZ, it also has a low thermal expansion coefficient and low toughness.

Rare earth oxides

The mixture of rare earth oxides is readily available, cheap, and may have promise as effective TBCs. The coatings of rare earth oxides (ex: La2O3, Nb2O5, Pr2O3, CeO2 as main phases) have lower thermal conductivity and higher thermal expansion coefficients when compared to YSZ. The main challenge to overcome is the polymorphic nature of most rare earth oxides at elevated temperatures, as phase instability tends to negatively impact thermal shock resistance.

A powder mixture of metal and normal glass can be plasma-sprayed in vacuum, with a suitable composition resulting in a TBC comparable to YSZ. Additionally, metal-glass composites have superior bond-coat adherence, higher thermal expansion coefficients, and no open porosity, which prevents oxidation of the bond-coat.

Uses

Automotive

Thermal barrier ceramic coatings are becoming more common in automotive applications. They are specifically designed to reduce heat loss from engine exhaust system components including exhaust manifolds, turbocharger casings, exhaust headers, downpipes and tailpipes. This process is also known as "exhaust heat management". When used under-bonnet, these have the positive effect of reducing engine bay temperatures, therefore reducing the intake air temperature.

Although most ceramic coatings are applied to metallic parts directly related to the engine exhaust system, technological advances now allow thermal barrier coatings to be applied via plasma spray onto composite materials. It is now commonplace to find ceramic-coated components in modern engines and on high-performance components in race series such as Formula 1. As well as providing thermal protection, these coatings are also used to prevent physical degradation of the composite material due to friction. This is possible because the ceramic material bonds with the composite (instead of merely sticking on the surface with paint), thereby forming a tough coating that doesn't chip or flake easily.

Although thermal barrier coatings have been applied to the insides of exhaust system components, problems have been encountered because of the difficulty in preparing the internal surface prior to coating.

Processing

In industry, thermal barrier coatings are produced in a number of ways:

Electron beam physical vapor deposition: EBPVD

Air Plasma Spray: APS

High velocity oxygen fuel: HVOF

Electrostatic spray-assisted vapour deposition: ESAVD

Direct vapor deposition

Additionally, the development of advanced coatings and processing methods is a field of active research. One such example is the Solution precursor plasma spray process which has been used to create TBCs with some of the lowest reported thermal conductivities while not sacrificing thermal cyclic durability.one being that the DC power sources can provide much higher driving voltages then sacrificial anodes (100 to 10,000 times higher) and can protect larger areas. The second main advantage is that the anodes that are used, such as graphite, are inert and don’t need to be continuously replaced. The final advantage is the degree of control over the electron transfer that ICCP has, the current can be increased or decreased as need be. ICCP is what is typically found on all pipelines as it does not have to continuously monitored like the sacrificial anode and there is more control over the electron flow.

Physical Vapor Deposition

Physical vapour deposition PVD) is an atomistic deposition process where target material is changed from a solid or liquid source (in the form of atoms or molecules) to vapour in a vacuum or low pressure gaseous (or plasma environment before it is changed back into a solid form and transferred to the substrate . PVD can produce a hard coating such as TiN, TiCN, TiAlN, and CrN. The advantages of this technique is that it can be used with low processing temperatures (<500 oC) and provide a wide range of coating thickness. The problems with high processing temperatures it that they have a detrimental effect on the physical and mechanical properties of the base material, which restricts the types of substrates that can be used, promote unexpected phase transitions, and create excessive residual stresses due to the difference in thermal expansion between the deposited material and substrate

The processing parameters used to create a PVD coating are important factors for determining the quality of the coating. The common processes and parameters used

to create a PVD coating are:

a) Generation of particles from the target materials

b) Transport and film growth

c) Particle energy, density, substrate temperature, and reactive gas properties.

Even PVD has very versatile and many advantages as compared with others surface modification method, it has limitation when it contact to Cl- ion which is appears in sea water and body fluids. Several studies have been conducted to solve this issue. Past reports highlighted that coated PVD layer consists of pores, pin holes and columnar growth which act as channels for the aggressive medium to attack the substrate. Duplex and multilayer coatings seem able to address this issue at certain extent but at the expense of manufacturing time and cost.

Physical Vapor Deposition

Physical vapor deposition is a vacuum coating process that has a higher corrosion tolerance than electroplating and other forms of metal finishing. Because of the corrosion resistant properties, PVD coatings provide a brilliant and more durable product. Additionally, PVD coatings can be done in a wide range of colors. Physical vapor deposition (PVD) has a variety of benefits as a metal coating, including corrosion resistance. Corrosion resistance is an important factor when manufacturing metal products as the formation of rust can be destructive and having a devastating impact on a manufacturer’s bottom line. Rust and corrosion can spread quickly, which is why it’s important to protect metal products with physical vapor deposition coatings.

Because of its high corrosion and oxidation resistance, PVD has become a popular finishing for multitude of industries and applications. Physical vapor deposition is an environmentally sound process that can be applied directly on strong materials like stainless steel and titanium. Applying a corrosion resistant coating like PVD will enhance the chemical and wear resistance of products, as well as increase the durability and lifespan.

When corrosion and oxidation resistance matter, choose physical vapor deposition for metal coating and finishing. PVD coatings prevent unwanted rusting and increase the lifespan of metal products. Contact Bend Plating for more information about PVD and it’s corrosion resistant properties.

Chemical vapour deposition

Chemical vapour deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase and is similar in some respects to physical vapour deposition (PVD).

PVD differs in that the precursors are solid, with the material to be deposited being

vaporised from a solid target and deposited onto the substrate.

How Does CVD Work?

Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate. The substrate temperature is critical and can influence

what reactions will take place.

Coating Characteristics

CVD coatings are typically:

• Fine grained

• Impervious

• High purity

• Harder than similar materials produced using conventional ceramic fabrication

Processes CVD coatings are usually only a few microns thick and are generally deposited at fairly slow rates, usually of the order of a few hundred microns per hour.

CVD Apparatus

A CVD apparatus will consist of several basic components:

• Gas delivery system – For the supply of precursors to the reactor chamber

• Reactor chamber – Chamber within which deposition takes place

• Substrate loading mechanism – A system for introducing and removing substrates, mandrels etc

• Energy source – Provide the energy/heat that is required to get the precursors to react/decompose.

• Vacuum system – A system for removal of all other gaseous species other than those required for the reaction/deposition.

• Exhaust system – System for removal of volatile byproducts

from the reaction chamber.

• Exhaust treatment systems – In some instances, exhaust gases may not be suitable for release into the atmosphere and may require treatment or conversion to safe/harmless compounds.

• Process control equipment – Gauges, controls etc to monitor process parameters

such as pressure, temperature and time. Alarms and safety devices would also be

included in this category.

Energy Sources

There are several suitable sources of heat for CVD processes. These include:

• Resistive Heating e.g. tube furnaces

• Radiant Heating e.g. halogen lamps

• Radio Frequency Heating e.g. induction heating

• Lasers

Other energy sources may include UVvisible light or lasers as a source of photo energy.

Precursors

Materials are deposited from the gaseous state during CVD. Thus precursors for CVD

processes must be volatile, but at the same time stable enough to be able to be

delivered to the reactor.

Generally precursor compounds will only provide a single element to the deposited

material, with others being volatilised during the CVD process. However sometimes

precursors may provide more than one. Such materials simplify the delivery system, as they reduce the number of reactants required to produce a given compound.

Materials That Can be Produced by CVD Processes

CVD is an extremely versatile process that can be used to process almost any metallic or ceramic compound. Some of these include:

• Elements

• Metals and alloys

• Carbides

• Nitrides

• Borides

• Oxides

• Intermetallic compounds

CVD Gas Products

An often neglected byproduct of the CVD process are volatile gases. However, these gases may be toxic, flammable or corrosive so must be treated appropriately.

Analysis of the offgasesncan also lead to a better understanding of the CVD reaction mechanisms and the information used to refine the process.

Applications

CVD has applications across a wide range of industries such as:

• Coatings – Coatings for a variety of applications such as wear resistance, corrosion resistance, high temperature protection, erosion protection and combinations thereof.

• Semiconductors and related devices – Integrated circuits, sensors and opto electronic devices

• Dense structural parts – CVD can be used to produce components that are difficult or uneconomical to produce using conventional fabrication techniques. Dense parts

produced via CVD are generally thin walled and maybe deposited onto a mandrel or

former.