Recent Vistas in Engineering Surface Modification Techniques

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 2, May-August (2012), IAEME

92

RECENT VISTAS IN ENGINEERING SURFACE

MODIFICATION TECHNIQUES

Mohammed Yunus1, Dr. J. Fazlur Rahman

2and Dr.A. Ramesh

3

1.

Research scholar, Anna University of Technology Coimbatore,Professor, Department of Mechanical Engineering H.K.B.K.C.E.,Bangalore,

India, [email protected]

2. Supervisor, Anna University of Technology Coimbatore,

Professor Emeritus, Department of Mechanical Engineering

H.K.B.K.C.E., Bangalore, India.

[email protected]

3. Supervisor, Anna University of Technology Coimbatore,

Professor and Head, Department of Mechanical Engineering,

Srikrishna College of Engineering and Technology, Coimbatore, India.

[email protected]

ABSTRACT

The Thermal sprayed coatings are commonly used on many advanced industrial

applications for their functional requirements like high strength at elevated

temperatures, resistance to chemical degradation, wear resistance and environmental

corrosion protection in Engineering components. The product design (design of a

surface) is concerned with design of enveloping surface which is achieved with some

suitable surface modifications. This paper highlights the several surface modification

techniques used for producing high quality coatings which involve the requirements

of one or more of mechanical and tribological properties.

Keywords: Engineering Surface; Surface Modification; Mechanical and Tribological

Properties; Thermal treatments; Thermo-chemical treatment; Plating and coating;

Implantation.

.

INTERNATIONAL JOURNAL OF MECHANICAL

ENGINEERING AND TECHNOLOGY (IJMET)

ISSN 0976 6340 (Print)ISSN 0976 6359 (Online)

Volume 3, Issue 2, May-August (2012), pp. 92-107

IAEME: www.iaeme.com/ijmet.htmlJournal Impact Factor (2011):1.2083 (Calculated by GISI)

www.jifactor.com

IJMET

I A E M E


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1. INTRODUCTION

The selection of technology to engineer the surface is an integral part of an

engineering component design. The first step in surface modification technique to

determine the surface and substrate engineering requirements which involves one or

more of the properties like wear resistance, corrosion and erosion resistance andthermal resistance, fatigue, creep strength, pitting resistance etc. [1, 2 & 3].

The various surface treatments generally used in engineering practice and

presented as under.

2. SURFACE MODIFICATION METHODS/ TECHNIQUES

A simplified classification of various groupings of non-mechanical surface

treatments could be reduced as [9, 10, 12, 13 & 14]

1. Thermal treatments 2.Thermo-chemical treatment 3.Plating and coating 4.

Implantation.

The figure illustrates different types of surface treatments and typical thickness of

engineered surface materials produced by them. The effectiveness depends on

particular surface and modification technique.

Fig.1. Typical thickness of engineered surface layers

1.PVD process 2.CVD process 3.Electoless Nickel 4.Composite 5.Thermal spraying

6.Surface welding 7.Ion Implantation 8.Anodising 9.Boronizing 10.Nitriding

11.Carbonitriding 12.Carborising 13.Nitrocarburising 14.Surface alloying 15.

Thermal hardening.


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There are two categories of vapor deposition processes: physical vapor deposition

(PVD) and chemical vapor deposition (CVD). In PVD processes, [4 ] the work piece

is subjected to plasma bombardment. In CVD processes, thermal [8] energy heats the

gases in the coating chamber and drives the deposition reaction.

2. 1. Physical Vapour Deposition (PVD)

In this process, the work piece or substrate is subjected to high [ ]films by the

condensation of a vaporized form of the material onto substrate surfaces. This process

contains the three major techniques; evaporation, sputtering and ion plating. It

produces a dense, hard coating. The primary PVD methods are.ion plating, ion

implantation, sputtering and laser surface alloying.

Fig. 2. PVD process using Plasma Fig. 3. PVD process using arc sputtering

evaporation


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Fig. 4. schematic diagram of CVD process.

2. 3. Electroless Nickel Plating

Electroless nickel (EN) plating is a chemical reduction process that depends upon

the catalytic reduction process of nickel ions in solution containing a chemical

reducing agent and water and the subsequent deposition of nickel metal without the

use of electrical energy[15,16,18 & 20 ]. Thus in the EN plating process, the driving

force for the reduction of nickel metal ions and their deposition is supplied by a

chemical reducing agent in solution. This driving potential is essentially constant at

all points of the surface of the component, provided the agitation is sufficient to

ensure a uniform concentration of metal ions and reducing agents[15]. The electroless

deposits are therefore very uniform in thickness all over the parts shape and size. The

process is advantageous when plating complex shape devices, holes, recesses, internalsurfaces, valves, threaded parts etc. Electroless (autocatalytic) nickel coating provides

a hard, uniform, corrosion, abrasion, and wear-resistant surface to protect machine

components in many industrial environments. EN is chemically deposited, making the

coating exceptionally uniform in thickness. If carefully process is controlled good

surface finish can be produced which eliminates costly machining after plating. In a

true electroless plating process, reduction of metal ions occurs only on the surface of a

catalytic substrate in contact with the plating solution. Once the catalytic substrate is

covered by the deposited metal, the plating continues because the deposited metal is

also catalytic [18 ].

Deposits have unique magnetic properties. EN deposits containing more than 8%

P are generally considered to be essentially nonmagnetic in the as-plated condition. Asecond generation of EN plating has been developed by code positing micrometer-

sized particles of silicon carbide with the nickel, thereby creating an extremely wear-

and corrosion-resistant coating. The nickel alloy matrix provides corrosion resistance,

and the silicon carbide particles, which are actually the contacting surface, add wear

resistance.


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Fig. 5. Electroless nickel plating process

2. 4. Composite

A composite material is a macroscopic, physical combination of two or more

materials in which one material usually provides reinforcement [27]. Composites have

been developed where no single, quasi-continuous material will provide the required

properties. In most composites one phase (material) is continuous and is termed the

matrix, while the second, usually discontinuous phase, is termed the reinforcement, in

some cases filler is applied when the reinforcement is not a quasi-continuous fibre.

Matrix-filler nomenclature is one method of categorization. This yields the categories

metal matrix (MMC), polymer (plastic) matrix (PMC), and ceramic matrix (CMC)

composites the major subdivisions of this section. Other categories are given theshape and configuration of the reinforcing phase. The reinforcement is usually a

ceramic and/or glass. If it is similar in all dimensions, it is a particulate reinforced

composite; if needle-shaped single crystals, it is whisker-reinforced; if cut continuous

filament, chopped fibre-reinforced; and if continuous fibre, fibre composite. For fibre

composites configuration gives a further category. If fibres are aligned in one

direction, it is a uni-axial fibre composite; if arranged in layers, it is a laminar

composite; if a three-dimensional arrangement, it is a 3D weave composite.

Laminates and 3D weaves can be further divided by the weave used for the fibre.

2. 5. Thermal spraying

Energy surface treatment involves adding energy into the surface of the work

piece for adhesion to take place [17]. Conventional surface finishing methods involveheating an entire part. The methods described in this section usually add energy and

material into the surface, keeping the bulk of the object relatively cool and

unchanged. This allows surface properties to be modified with minimal effect on the

structure and properties of the underlying material. The different thermal spray

technologies, based on the process of heat source, type of coating material and spray

conditions etc., can be classified as follows:-


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1. Detonation gun spaying

2. Flame spraying

3. Electric arc spraying

4.

Wire explosion spraying5. Liquid metal spraying and

6. Plasma spraying

Plasmas are used to reduce process temperatures by adding energy to the surface

in the form of kinetic energy of ions rather than thermal energy. Advanced surface

treatments often require the use of vacuum chambers to ensure proper cleanliness and

control [1, 2 & 3]. Vacuum processes are generally more expensive and difficult to

use than liquid or air processes. Facilities can expect to see less-complicated vacuum

systems appearing on the market in the future. In general, use of the advanced surface

treatments is more appropriate for treating small components (e.g., ion beam

implantation, thermal spray) because the treatment time for these processes is

proportional to the surface areas being covered. Facilities will also have to address thefollowing issues when considering the new techniques[ ]. The following methods are

widely used in engineering applications.

2. 5.1. High-velocity oxy-fuel spraying (HVOF) process

Fig.6. HVOF process Fig.7. HVOF process setup

In general, the high velocity oxy fuel spraying (HVOF) process can be used for

the deposition of the bond coat materials, over which oxide coatings are sprayed for

good adherence. This process is based on the combustion of the fuel gas with oxygen

at high pressures within the combustion chamber. The exit jet velocity is generally

more than 1000 m/s and at this speed, the oxide powder which is axially injected is

moderately heated but highly accelerated through the expansion nozzle to large

particle velocities beyond 800 m/s. The stream of hot gas and powder is directed

towards the surface to be coated. The powder partially melts in the stream, and

deposits upon the substrate as shown in figure1. The process has been most successful

for depositing corrosion-resistant alloys (stainless steels, nickel-based alloys,

aluminium, etc). HVOF coatings are effectively used in the fields of general

manufacturing industry, gas turbine industry, petroleum industry, chemical process


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industry, pulp industry and automotive industry[ 1, 2, 3].

2.5.2. Atmospheric Plasma spraying process

When a strong electric arc is struck between tungsten electrode (cathode) and a

nozzle (anode) in the presence of Argon and nitrogen / hydrogen mixture in the

chamber, the gas gets ionized producing high temperature plasma. Injected particlesof coating materials are heated inside the plasma jet and molten droplets sprayed on

the substrate with high velocities to form the coating [1&3]. APS ceramic coatings are

widely employed in the engineering applications which demand wear resistance,

corrosion resistance and high strength at elevated temperatures.

Figh.8. Atmospheric Plasma spraying Fig.9.Atmospheric Plasma Spray set up.

process.

2. 6. Ion Implantation

In the Ion plating (IP) process, the target material is initially melted while the

substrate is bombarded with ions before deposition to raise it to the required

temperature. The coating flux ion is attracted to the substrate by biasing the substrate

with a negative voltage. Thus sufficient ion energy [ 28 ] is available for good inter

mixing of coating and substrate at the interface. Ion implantation is the introduction of

ionized dopant atoms into a substrate with enough energy to penetrate beyond the

surface. The most common application is substrate doping. The use of 3 to 500 keV

energy for boron, phosphorus, or arsenic dopant ions is sufficient to implant the ions

from 100 to 10,000A below the silicon surface. The depth of implantation, which is

proportional to the ion energy, can be selected to meet a particular application.

Implantation offers a clear advantage over chemical deposition techniques. Themajor advantage of ion implantation technology is the capability of precisely

controlling the number of implanted dopant atoms. Furthermore, the dopants depth

distribution profile can be well-controlled.

Disadvantages of Ion Implantation are very deep and very shallow profiles are

difficult, not all the damage can be corrected by annealing [9 & 10], typically has

higher impurity content than does diffusion. Often uses extremely toxic gas sources

such as arsine (AsH3), and phosphine (PH3) and expensive


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They are generally used in Doping, SIMOX, H and He isolation in GaAs, and

Smart cut technologies.

Fig.10. Ion Implantation setup and doping process

2.7. Anodizing

Anodizing involves the electrolytic oxidation of a surface to produce a tightly

adherent oxide scale that is thicker than the naturally occurring film. Anodizing is an

electrochemical process during which aluminium is the anode. The electric current

passing through an electrolyte converts the metal surface to a durable aluminium

oxide. The difference between plating and anodizing is that the oxide coating is

integral with the metal substrate as opposed to being a metallic coating deposition.

The oxidized surface is hard and abrasion resistant, and it provides some degree of

corrosion resistance [28].

Anodic coatings can be formed in chromic, sulphuric, phosphoric, or oxalic acid

solutions. Chromic acid anodizing is widely used with 7000 series alloys to improve

corrosion resistance and paint adhesion, and unsealed coatings provide a good base

for structural adhesives. However these coatings are often discoloured and where

cosmetic appearance is important, sulphuric acid anodizing may be preferred.

Fig.11. Anodizing Process


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2. 8 Boronising

Boronising is also called as boriding. It is a thermo-chemical treatment involving

diffusion of boron into the surface of a component from the surrounding environment

which results in the formation of a distinct compound layer of a metal boride. The

reaction takes place between boron and component, therefore it can be generallylimited to steels, titanium-based alloys and cobalt-based hard metals. In steels,

boronising is carried out in the austenite regime (between 8101020 C) for several

hours, resulting in the formation of layers commonly between 60 and 165m thick.

The surface reaction layer thus formed consists of two separate phases, namely a layer

of Fe2B adjacent to the substrate and an outer layer of FeB. The proportions of the

two phases are dependent upon the composition of the boronising environment and

the alloy content of the steel (higher alloy content favours FeB formation). Care is

taken to reduce the proportion of FeB in the boride layer since this always exists in

tension; as such, high-alloy and stainless steels are unsuitable for boronising. The

hardness of the boronised layer is dependent upon the exact composition of the steel

but is commonly in the range 16002350 kgf/mm2 (as measured on the Vickers

scale). This is significantly higher than many commonly occurring abrasives and, assuch, boronising has been employed in situations requiring abrasive wear resistance.

Materials that can be processed for Ferrous materials such as irons, plain carbon,

alloy, stainless, and tool steels are all possible. This is because the boride compound

formed is an iron boride so we only need iron to be present in the material to do this

are Nickel-based alloys, Cobalt-based alloys, Molybdenum, Sintered carbides [5 & 6

].

A variety of methods are employed to produce the boron-rich environment for the

boronising process such as pack boronising, paste boronising, salt bath boronising and

gas boronising[ ]. In pack boronising (the most commonly employed method), the

source of boron is B4C which is mixed with an activator and an inert diluent to make

up the pack powder.

Fig. l2. Boronising Process

2. 9. Nitriding

Steels containing nitride-forming elements such as chromium, molybdenum,

aluminum, and vanadium can be treated to produce hard surface layers, providing


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improved wear resistance. Many of the processes employed are proprietary, but

typically they involve exposure of cleaned surfaces to anhydrous ammonia at elevated

temperatures. The nitrides formed are not only hard but also more voluminous than

the original steel[ ], and therefore they produce compressive residual surface stresses.

Therefore, nitrided steels usually exhibit improved fatigue and corrosion fatigue

resistance. Similar benefits can be achieved by shot-peening [28].

Fig. 13. Nitriding process setup. Fig. 14. Nitriding Process

2.10. Carburizing

Carburizing is a heat treatment process in which iron or steel is heated in the

presence of carbon material (in the range of 900 to 950 C). Depending on the amount

of time and temperature, the affected area can vary in carbon content. Longer

carburizing times and higher temperatures lead to greater carbon diffusion into thepart as well as increased depth of carbon diffusion. When the iron or steel is cooled

rapidly by quenching, the higher carbon content on the outer surface becomes hard via

the transformation from austenite to martensite, while the core remains soft and tough

as a ferritic and/or pearlite microstructure

Generally it is used for low-carbon work-piece to increase their toughness and

ductility; and it produces case hardness depths of up to 6.4 mm.

.


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Fig.15. Carburising Process

Plasma carburization is increasingly used in major industrial regimes toimprove the surface characteristics (such as wear and corrosion resistance, hardness

and load-bearing capacity, in addition to quality-based variables) of various metals,

notably stainless steels [28]. The process is used as it is environmentally friendly (in

comparison to gaseous or solid carburizing). It also provides an even treatment of

components with complex geometry (the plasma can penetrate into holes and tight

gaps), making it very flexible in terms of component treatment.

The process of carburization works via the implantation of carbon atoms in to the

surface layers of a metal.

A main goal when producing carbonized work pieces is to insure maximum

contact between the work piece surface and the carbon-rich elements. In gas and

liquid carburizing, the work pieces are often supported in mesh baskets or suspendedby wire. In pack carburizing, the work piece and carbon are enclosed in a container to

ensure that contact is maintained over as much surface area as possible. It is possible

to carburize only a portion of a part, either by protecting the rest by a process such as

copper plating, or by applying a carburizing medium to only a section of the part.

2. 11.Carbo-nitriding

Carbo-nitriding [28] is similar to cyaniding except a gaseous atmosphere of

ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be

quenched then the part is heated to 775885 C if not then the part is heated to 649

788 C.


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Fig. 16. Nitro- carburizing process

2.12. Ferritic nitro-carburizing

Ferritic nitro-carburizing diffuses mostly nitrogen and some carbon into the caseof a work-piece below the critical temperature, approximately 650 C. Under the

critical temperature the work pieces microstructure does not convert to an austenitic

phase, but stays in the ferritic phase, which is why it is called ferritic nitro

carburization[28].

It is used in Parts that are subject to high pressures and sharp impacts are

commonly case hardened, e.g. firing pins and rifle bolt faces, or engine camshafts.

2.13 Short Peening, Water-Jet Peening and Laser PeeningIn short peening the surface of the work piece[28] is hit repeatedly with large

number of cast-steel, glass or ceramic shot (size of 0.125mm to 5mm diameter),

making overlapping indentation on the surface; this action causes plastic deformation

of the surfaces[ ]. Thus improving the fatigue life of the component. Extensively usedon shafts, gears, springs, oil-well drilling equipment, and jet engine parts.

In water-jet peening, a water jet at pressure as high as 400 MPa impinges on the

surface of the work piece, inducing compressive residual stresses. This have been

successfully used on steels and aluminum alloys[28].

In laser peening, the surface is subjected to laser shocks from high powered laser up

to 1KW and at energy levels of 100 J/pulse. This method has been used on jet engine

fan blades with compressive residual stresses deeper than 1mm [28 ].

2. 14.Thermal hardening

In case of steels, to achieve a full conversion of austenite into hard martensite,

cooling needs to be fast enough to avoid partial conversion into perlite or bainite

[26]. If the piece is thick, the interior may cool too slowly so that full martensiticconversion is not achieved. Thus, the martensitic content, and the hardness, will drop

from a high value at the surface to a lower value in the interior of the piece.

Hardenability is the ability of the material to be hardened by forming martensite.

The shape and size of the piece, together with the heat capacity and heat

conductivity are important in determining the cooling rate for different parts of the

metal piece. Heat capacity is the energy content of a heated mass, which needs to be

removed for cooling. Heat conductivity measures how fast this energy is transported


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to the colder regions of the piece.

2.15. Precipitation Hardening

Hardening can be enhanced by extremely small precipitates that hinder dislocation

motion. The precipitates form when the solubility limit is exceeded. Precipitation

hardening is also called age hardening because it involves the hardening of thematerial over a prolonged time. Precipitation hardening is achieved by solution heat

treatment where all the solute atoms are dissolved to form a single-phase solution or

by rapid cooling across the solvus line to exceed the solubility limit[ ]. This leads to a

supersaturated solid solution that remains stable (metastable) due to the low

temperatures, which prevent diffusion or precipitation heat treatment where the

supersaturated solution is heated to an intermediate temperature to induce

precipitation and kept there for some time (aging).

The requirements for precipitation hardening [28]are appreciable maximum

solubility, solubility curve that falls fast with temperature and composition of the

alloy that is less than the maximum solubility. Mechanism of Hardening involves the

formation of a large number of microscopic nuclei, called zones. It is accelerated athigh temperatures. Hardening occurs because the deformation of the lattice around

the precipitates hinder slip. Aging that occurs at room temperature is called natural

aging, to distinguish from the artificial aging caused by premeditated heating.

2. 16. Microwave Irradiation

Microwave heating is fundamentally different from conventional heating process.

In conventional thermal processing energy is transferred to the material through

conduction, convection and radiation of heat from the surface of the material [3]. On

the other hand microwave energy is delivered directly to materials through molecular

interaction with the electromagnetic field. As microwave radiation penetrates and

interacts with molecules, transfer of electromagnetic energy takes place throughout

the volume of material, leading to volumetric heating. Microwave material interactiondepends on dielectric property of materials. While conducting, metals reflect,

insulators are transparent to microwaves.

Once the ceramics composite material starts coupling with microwave at an

elevated temperature, temperature rises rapidly and induces phase transformation

associated with increase in volume. This results in micro structural changes likepartial filling of pores and voids, healing of micro cracks and consequent

densification. Typical enhancement in observed porosities across the microwave

glazed coating [3], typical improvement in hardness of glazed composite and a

significant improvement in Vickers hardness are observed, which is associated with

the densification of the coatings structures during post processing. Morphological

improvement during glazing is indicated by the observed improvement in surfacetexture.

2.17. Laser Surface Alloying (LSA)

The LSA is used to mix an additional material with the molten surface of the

substrate so that, upon solidification, an alloy surface with a different composition

from that of the substrate can be obtained[ 22, 23, 24, 24 25 & 26]. The properties of

the alloyed surface can be tailored to suit different requirements. Much work on LSA

has been done towards producing a hardfacing layer or an improved corrosion-

resistant layer [22]. The structure of the laser-alloyed layer often contains


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supersaturated solid solutions and sometimes intermetallic compounds. In addition to

the metallic components that can be alloyed to the substrate surface, ceramic

components can also be added so as to achieve a metal matrix composite surface with

significantly increased hardness on the metal substrate.

Application of a laser can improve the properties of thermally sprayed coatings.

These improvements having been studied for the following applications such as

biomedical coatings, thermal-barrier coatings, wear-resistant composite coatings, wet

and hot corrosion-resistant alloys.

3. CONCLUSION

In an engineering component design, the selection of surface technology to

engineer the surface involves selection of suitable surface modification technique to

determine the surface and substrate requirements which involve one or more

properties. The very important proprties are mechanical, tribological, corrosion

reisistance, erosion resistance, creep strength, thermal resistance, pitting resistance

etc.

The various surface treatments generally used in engineering practice arehighlighted in this technical paper. Details aspect of various surface modification

techniques have been dealt. Applications in engineering field has been mentioned.

Further, the most widely used ceramic material coating technique namely thermal

spraying technique ( e.g. HVOF and APS )is dealt in detail aspect of it. Besides, the

special post-processing methods of surface modification of ceramic coatings using

microwave glazing and laser treatment are also highlighted for the production of

better quality coating for any functional requirement of engineering surfaces.

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Recent Vistas in Engineering Surface Modification Techniques

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