SURFACE ENGINEERING

A.S.Khanna

Corrosion Science & Engineering

IIT Bombay

Surface Engineering- Scope

Failure of an engineering

component occurs when its surface

cannot adequately withstand the

external forces or environment to

which it is subjected.

External forces can be

thermal, optical,

magnetic and electrical wear, or

corrosion.

Sometimes technological progress and

manufacturing efficiency

requires surface modifications.

The economic benefits -

According to a report the the UK coating market is

approximately £21.3 b.

(RSIC Report, 2005)

Surface Characterization is perhaps the most powerful function

which helps us the modify existing

surfaces, create new coatings, formulations,

understand mechanism of

surface degradation and its improvement

Definition To make changes to the surface of a material.

Purpose To gain or improve upon the desired surface properties of a material. To improve a components; performance, service lifetime, aesthetics or economics.

Surface Engineering Processes There are many processes for modifying surface properties. These can be grouped into three categories

Surface Engineering Processes

Surface modification without changing the material chemically Changes made by thermal or mechanical means, altering metallurgy or surface texture.

Surface modification by altering surface chemistry These processes involve diffusion of new elements into the surface of the material. The original substrate material constituents play an active part in the modified surface.

Surface modification by adding new material onto the surface (coating). These processes essentially add new material to the surface as a coating and do not involve the substrate material constituents at the surface.

Surface Modification without Changing

the Material Chemically • Thermal Processes

Surface Heat treatment, particularly those that undergo phase transformations like the martensitic reaction hardening of carbon steels, low alloy steels and cast irons - Laser, Flame, induction

• Mechanical Processes Cold working - surface by peening, shot blasting, explosive hardening or

other specialised machining processes induce compressive stresses, increasing hardness and fatigue resistance.

• Changing surface texture using machining and blasting. • Other Processes Modification of surfaces by chemical/electro-etching, laser engraving,

various chemical, solvent and ultrasonic cleaning processes could also be included here.

Surface Modification by Changing

Surface Chemistry • Thermochemical Diffusion Processes

– Carburising (carburizing) – carbonitriding – nitriding nitrocarburising – boronising

• In all these processes new element goes into interstitial position. – aluminising (aluminizing, calorising, alonising) – chromising (chromizing) – siliconising (siliconizing) – In this process new element goes into Substitutional SS

• Electrochemical Processes – Anodising (anodizing) of aluminium, titanium

• Chemical Conversion Coatings – Phosphating chemical blacking chromating

• Ion Implantation Processes

Surface Modification by Adding New Material onto the Surface (Coating)

Welding Type Processes

Thermal Spray Process

Electroplating Electroless plating

Galvanising, molten bath -tin aluminium (Al not aluminide) babbit

PVD Physical Vapour Deposition

CVD Chemical Vapour Deposition

Painting Spin Coating

Powder Coating Lubricants Tiling

Cladding

Material Properties

Bulk Surface

•Strength •Density •Ductility

•Hardness •Friction •Wear •Corrosion •Oxidation

Why Surface Treatment ?

To achieve desired surface properties which cannot be achieved by conventional alloying process.

What are the surface Properties?

Corrosion

Oxidation

Wear

Erosion/abrasion

Hardness

Conductivity

What are the various modes by which surface properties can be enhanced ?

• Coatings • Cladding • Surface Alloying

Creating a Barrier

• Homogenization • Hardening • Reflectivity change

Surface treatments

Different Techniques to achieve Surface Modifications

• Case Hardening • Slurry • Hot Dip • Quenching • Hardfacing • Electrodeposition

Conventional Methods

• PVD • CVD • Thermal Spray • Laser Based

Advanced Methods

Selection of Surface Treatment Method

Based upon Change in function property

Substrate characteristics

Thickness of the modified surface

Throughput of the process ( slow, fast)

Requirement of vacuum

Geometry of the component

Economics

Thickness as Selection Criteria for Coating Technique

Surface Engineering

Gaseous State Solution Molten/semi molten

•PVD •Ion Implantation •Ion Beam Assisted •CVD

Sol-Gel Electroplating

Laser Thermal Spray Welding

Classification of Coatings Types Of Coating

Processes

CVD PVD Thermal spray

Mechanism of Coatings Mechanism of Coatings

Mechanical Bonding Types Of Coating

Diffusion Of Coating

A

B

Important parameters to be optimized to control the properties at substrate coating and its interface

Substrate

Interface

Coating

Coating Surface •Roughness •Erosion •Corrosion/oxidation •Friction •Porosity •Electrical properties

Coating Bulk Cohesion Stress Adhesion Cracks/defects Graded composition Multilayers

Interface Adhesion Interdiffusion Cleanliness Roughness Expansion mismatch

Surface Property Improvement Triangle

Surface modification

technique

Examples of Surface Modification in Industry S.No.

Industry Part/machine Problem for which coating is required

1 Aircraft , Gas Turbine

Turbine Disc, Lybrinth seals Turbine Blades Rotor Shafts

Fretting wear Friction / Hot Corrosion Hot Corrosion and erosion Fretting wear

2 Automobiles Exhaust Nozzles Cylinder jackets and Liners Piston Heads and crowns Camshafts/Crank shafts

High Temperature Corrosion Rubbing wear Thermal Fatigue and Corrosive wear Fatigue/wear

3 Textile Machinery

Yankey Dryers Grooved rolls Package drive rolls Twisting rolls Yarn guides Tension gates & drives

Abrasion and wear Abrasion and wear Abrasion and wear Abrasion and wear Abrasion and wear Abrasion and wear

4 Glass Work Diamond Work

Scoops and Moulds Dies Bushing Plates Diamond Polishing Pads

Hot Corrosion/abrasion Hot Corrosion/abrasion Hot Corrosion/abrasion Abrasive Wear

5 Paper & Pulp Printing Industry

Gripper Bars/pads Grip Rolls Transport Rolls Guide Plates Folder Rolls

Abrasive wear Abrasive wear Abrasion Rubbing wear Surface wear

6 Chemical, Petrochemical

Oil exploration Shafts Well Casing Tanks & Vessels Impellers Plug valves

Abrasive wear Corrosive wear Corrosive wear Fatigue & Corr. Wear Chemical corrosion

7 Power Plants Boilers

Gas Turbine Blades Furnace Pipes Superheater tubes Boiler Tubes

Corrosion and abrasion Heat Corrosion Oxidation/sulfidation/erosion/Hot Corrosion Heat & sulfidation attack

Coat Chuts Abrasive wear

Plasma & Ion-based Surface Engineering (PISE) techniques

• Large surfaces are easily treatable • PISE is based on dry technology, avoiding the use of

harmful solutions • unlike traditional techniques, the processes are virtually

pollution free • such processes can be easily automated • properties such as corrosion and wear resistance,

fatigue strength and biocompatibility, as well as the combination of these properties, are achievable and controllable

Electron Beam PVD

MCrAlY-coated blades produced by NTI's EBPVD process. EBPVD MCrAlY TBC Bond Coats

HYDROPHOBIC COATINGS

Plasma Assisted CVD Uses scrap teflon Used for corrosion resistance Also used in textiles as dust

repellant. Can be used for water collection

in moist climates

PLASMA ETCHING

Plasma etching improves adhesion properties of substrates. Cleans & activates surfaces. Reduces or eliminates requirements of primers &

toxic chemicals Wide applications on materials ranging from metals

to polymers. Few of typical examples are:

Plasma etching of autofacia (improved paintability & service life) Plasma etching of TPO profiles

PLASMA ETCHING – for cars

- One of the futuristic applications of Plasma Etching is ‘Plasma Car Wash’

PLASMA NITRIDING

UHV PN System for Space Quality Plasma Nitriding

Industrial Scale Plasma Nitriding Facility For Large size Industrial Jobs

Mould locking plates

Crankshafts

Connecting rods Cam chain sprocket

Basic Principal of Thermal Spray Systems ---- Heat Energy ---- Kinetic Energy

Flame Arc

HVOF Plasma

Comparison of various Thermal Spray Processes Attributes Flame SPRAY HVOF Detonation Wire Arc Air Plasma Vacuum

Plasma Cold spray

Jet Temp0C

3,200 5,200 5,000 >25,000 15,000 12,000 0-500

Jet Velocities M/S

50-100

500-1200 3000 50-100 300-1000 200-600

Gas Type O2,Acetylene CH4,C2H2,H2,O2 O2,Acetylene Air,N2,Ar Ar,He,H2,N2 Ar,He,H2 He, N2, Air

Gas Flow slpm 100-200 400-1100 N/A 500-300 100-200 150-250 Power Input,KW, Eqv.

20 150-300 N/A 2-5 40-200 40-120 5-25

Particle Temp0C Max.

2500 330 N/A >3800 >3800 >3800

Particle Velocities M/S,

50-100

200-1000

N/A 50-100

200-800

20-50

300-1200

Material Feed Rate g/min

30-50 15-50 N/A 150-2000 50-150 25-150

DEPOSIT/ COATING Density Range (%)

85-90 >95 >95 80-95 90-95 90-99 90-99

Bond Strength Mpa, (Ksi)

7-18 (1-3) 68 (10) 82 (12) 10-40 (1.5-6) <68 (<10) <68 (<10) 26-62

Oxides High Moderate to dispersed

Small Moderate to high

Moderate to coarse

None small

Principle of laser Cladding

• Single Step laser Cladding

Powder Feeder Laser Beam Cladding

Substrate Substrate

Two Step laser Alloying /Glazing

Laser Beam

Already existed Coating

Coating after Laser Glazing

Performance of laser clad heat exchanger tube with Ni-25Cr alloy exposed in coal

fired power plant for a year

Optical micrographs showing the fire cracks on the heat exchanger tube exposed in a

coal-fired power plant

Without Cladding

After Cladding with Ni-25Cr-alloy