Surface Engineering


 "Changing the properties of the surface of a material to give performance which cannot be achieved by the surface layer or bulk alone"

 Surface Engineering Processes

Only some of these processes will be useful to improve tribological performance, but all will affect surface properties in some way which can influence performance.

Factors Influencing Treatment Selection

 Performance factors

Processing factors

 Process factors

Hardness and Surface Engineering

 Many wear resistant coatings show high hardness, but hardness alone is not a good guide to wear resistance except in cases where abrasive wear dominates.

 The work of Kruschov indicates an almost linear relationship between wear resistance and hardness for metallic elements but the behaviour breaks down for multiphase materials. Hardness is a convenient monitoring tool for assessment of coatings.

However, as the coating thickness increases or its hardness increases the hardness is no longer directly related to plasticity but is a systems parameter depending on both coating and substrate.

 Impact Resistance

 The impact resistance of most wear-resistant coatings is low by engineering standards. In general relatively ductile materials show good impact resistance and high strength (high hardness) materials do not. There is almost an inverse relationship with hardness.

 In the vast majority of wear situations some impact cannot be avoided so the choice of coating will be a compromise between hardness and toughness.

Maximum Service Temperature

 The high temperature service of a coating is limited by microstructural changes leading to softening or oxidation.

 For instance martensitic steels soften appreciably above 200°C, whereas tungsten-carbide cobalt composite coatings oxidise above 550°C. High chromium, nickel or cobalt alloys are required for higher service temperatures and ceramics must be used at temperatures greater than 1000°C.Creep performance and oxidation/corrosion resistance dictate choice of material.


Of prime importance for coatings, but less so for thermochemical treatments where there is no interface between coating and substrate.

 Adhesion depends on:-

 How to improve adhesion:


 The size, shape and location of porosity is very important.

 Effect of porosity:

 Control of porosity by:

 Component shape and size

 Some coating methods are unsuitable for complex-shaped components:-

 Component size is also an issue:

 Coating rate and thickness

 In most coating processes there is a maximum thickness of coating which may be deposited, dictates by factors such as residual stress and adhesion.

However, a more important practical consideration is coating rate which dictates the economics of the coating process. For instance, though it is possible to deposit PVD wear-resistant coatings to 150mm thickness, layers of only 4mm are usually used because these can be deposited in a single shift.

 Fatigue Performance of Substrate

 Thermal and thermochemical processes such as carburising introduce surface compressive stresses which have a beneficial effect on substrate fatigue

Electroplating of chromium induces tensile residual stresses which have a detrimental effect

 Sprayed coatings can improved or reduce fatigue performance depending on spray parameters as it is possible to create both tensile and compressive stresses.

Vapour deposited coatings tend to reduce fatigue life due to the fact that columnar grain boundaries can act as a nucleation site for the fatigue crack.

Wear-Resistant Metal Coatings

Hardness (HV) Maximum service temperature (°C) Corrosion resistance Application method
Metals hardened by dispersed oxides









Flame spraying

13% Cr steel 330 600 good  Flame spraying





Flame or plasma spraying

Co-Cr-Mo 1100 1000 very good Plasma spraying, welding
Hardened steels

12% Mn


150-400 (work hardened)




Martensitic 300-850 150-200 poor/ moderate Welding, thermal and thermo-chemical treatment
Nitrided 800-1200 500 good/ moderate Thermo- chemical treatment
Cast Irons






poor/ moderate



High chromium 400-600 1000 good Welding
NiCoCr alloys

Cobalt alloys



very good


Welding, plasma spraying

Nickel alloys 350-700 850 very good Flame spraying plus fusion, welding
NiP, NiB 850-950 300 very good Electroless plating
Chromium 850-1000 350 very good Electroplating

Wear Resistant Ceramic Coatings

Coating Hardness (HV) Maximum service temperature (°C) Corrosion resistance Application method









D-gun, HVOF and plasma spraying

Cr3C2-Ni-Cr 1100 820 very good D-gun and plasma spraying
Cr3C2-Co 450-500 800 good Electro-deposition
WC/steel 500 300 poor/ moderate Welding







very good


D-gun, plasma spraying, CVD, PVD

chromia 2400 >1000 very good D-gun, plasma spraying







very good



VC 2600 500 very good Thermo-chemical
Diamond 10000 650 very good CVD, tiling

carbon (DLC)

1200 250 very good CVD, PVD







very good



TiAlN 2300 850 very good PVD
CrN 2000 850 very good PVD







very good


PVD, CVD, Thermo-chemical











CrB 3500-4000 800 good Fusion of powder coating

Important Factors in Treatment Selection
Wear Process Primary Surface Requirements Principal Surface Treatments Operating conditions affecting treatment selection

General environmental considerations

Fretting Metals of dissimilar composition, non metals, hard materials Sprayed copper alloys, anodising, nitrocarburising, plasma sprayed and electrodeposited cermets Degree of lubrication (solid lubricants), amplitude of vibration (Cu at low amplitudes, hard coatings at high), applied stress (thickness of treatment high for high stress)  
Contact fatigue High yield strength, adequate toughness, good adhesion Thermal and thermochemical treatments, weld deposits, spray and spray/fused coatings Applied stress (thickness of treatment must be high for high stress)  
Adhesive wear Metals of dissimilar composition to contacting surface, hard materials, ceramics Sprayed Cu, Mo, thermochemical treatment, phosphating, sprayed cermets, electroless nickel, TiC (CVD) or TiN (PVD) Degree of lubrication (harder coatings needed as lubrication is reduced), surface roughness (smoother surfaces reduce wear)

Elevated temperatures, corrosive environments will reduce range of treatments possible

Impact wear High yield strength, adequate toughness, good adhesion Weld deposits, thermal and thermochemical treatments, sprayed Co alloys, sprayed and electrodeposited cermets Impact stress (as stress increases the toughness of the treated layer becomes increasingly important  
Low stress abrasion High hardness All hard coatings Abrasive hardness, applied stress, impact loading  
Machining wear High hardness All hard coatings Abrasive hardness, applied stress, impact loads, chemical reactions with chips  
High stress abrasion High hardness, toughness and adhesion, thick coating Weld deposits, thermal and thermochemical treatments Size and hardness of abrasive particles, applied stress, impact  
Erosion - high angle impact High hardness, adequate toughness, good adhesion Weld deposits, plasma sprayed cermets and metals, vapour-deposited coatings Velocity of impact, size of erodent, hardness and toughness of erodent  
Erosion - low angle impact High hardness All hard coatings Properties of erodent (hardness, toughness)  

Case Studies: Applications of Surface Treatments and Coatings



 Contact stresses

 Possible surface treatments

 Variation Of Maximum Shear Stress With Depth

Consider variation of maximum shear along the z axis

For circular contacts

 tmax=0.31P0 at x=0, z=0.48a

 For line contacts

 tmax=0.3P0 at x=0, z=0.78a

 The maximum contact pressure (Hertz pressure), P0, and the semicontact width, a, can be calculated from Hertz equations (HO5)

 Tooling Applications

 Three main types

  1. Metal cutting tools (e.g. inserts, drills etc.)
  2. Metal forming tools (e.g. press tools, shears, etc.)
  3. Plastic moulding tools (injection moulding, etc.)

 Coated tools are used to improve tool life, throughput and surface finish.

 Tool Coating/Treatment Processes


Thermal Hardening

 A hard layer is produced on plain carbon and low alloy steels of medium carbon content (0.3-0.6%) by rapid heating of the surface followed by water or oil quenching to form martensite.



 Types of Thermal Hardening

Types of Thermal Hardening

 Induction Hardening

Induction Hardening conditions
 Depth of hardening (mm) Frequency (kHz) Typical Power Input (W/mm2)
0.5-1.1 450 15-19
1.1-2.3 450 8-12
1.5-2.3 10 15-25
2.3-3.0 10 15-23
3.0-4.0 10 15-22
2.3-3.0 3 23-26
3.0-4.0 3 22-25
4.0-5.0 3 15-22

Precise power input conditions will depend on component size and geometry

High frequency resistance hardening

Flame hardening

 Tungsten inert gas (TIG) hardening

 Laser transformation hardening

Electron beam transformation hardening

 Thermochemical Treatments


A process in which carbon (up to 0.8%) is diffused into the surface of a steel which is subsequently hardened by quenching and then tempered. It is generally carried out at 850-950°C to achieve reasonable carbon diffusion rates.



Carburising Processes

Pack Carburising

 Salt bath carburising

Gas Carburising

Traditional Gas Atmosphere

Methane or propane are burnt in a controlled manner producing a gas of the following composition
N2 H2 CO CO2 CH4
35-40% 40-45% 15-25% 0.1-1.0% 0.5-1.5%

Controlled additions of methane or propane are then added to the gas to increase the carbon potential

CH4+CO2® 2CO+2H2

CH4+H2O® CO+3H2

At the carburising temperature several reactions take place of which the most important is

CO+H2® CFe+H2O

The balance of the atmosphere’s constituents is maintained by the water gas reaction

CO+H2O® CO2+H2

Nitrogen-based atmosphere

Mixture of nitrogen, hydrogen, oxygen and carbon-containing gases produced by a gas blender

 Fluidised bed carburising

 Vacuum carburising

Plasma carburising


 A variant on carburising in which a small amount of nitrogen (up to 0.5%) is diffused into the steel along with carbon. Nitrogen lowers the ferrite-austenite transformation temperature and increases hardenability, so carbonitriding is a lower temperature process. A lower quenching rate is also used



Carbonitriding Processes

Salt bath carbonitriding

Gas carbonitriding


Nitrogen is diffused into the steel surface by heating to 500-525° C in a nitrogen-containing atmosphere. To obtain high surface hardnesses (>750HV) nitride forming elements such as Al, Cr, Mo and V must be present in the steel. No quenching is required to develop hardened surface layer.



 Nitriding Processes

 Gas nitriding

2NH3® 2NFe+3H2

 Plasma Nitriding

Plasma Nitrided Gears

Plasma nitrided gears with a controlled white layer are used to resist scuffing. Scuffing is wear generated under high load/velocity conditions where frictional heating of the material is excessive, leading to softening. It often occurs in asperities during run-in.


A variation on nitriding in which carbon is diffused into the surface together with nitrogen at 570°C to give a carbonitride phase in the surface layer. Carried out in either a salt bath containing sodium cyanide and cyanate or in a gas mixture with ammonia and a carburising gas.




 Boron is diffused into the surface of plain carbon or low alloy steels at approximately 950°C to form a layer of iron borides about 100mm thick with a hardness in the range 1800-2100HV. Can also be applied to cobalt, nickel and titanium alloys.



 Boronising Processes

 Pack Boriding

 Paste Boriding

 Salt bath processes

 Gas boriding

Morphology of Boride Layers

 For iron substrates:-

(a) Exterior layer of orthorhombic FeB

(b) Internal layer of body-centred tetragonal Fe2B

 In alloy substrates, alloying elements inhibit boride formation and the amount of FeB increases with alloy content. Stainless steels are not suitable for boriding.


 Diffusion of metals into the surface to form compounds with substrate elements.

 The best known process is the Toyota Diffusion process in which vanadium and niobium are diffused into steel from a salt bath at 1000°C to form carbide layers. The carbide layers are typically 5-12mm thick and have a hardness of ~3000HV.




Coating Processes

Elecrochemical deposition

Coatings produced by electrolysis of an aqueous solution of a salt containing the coating material, the component to be coated being the cathode.

 For wear resistance chromium is the coating most widely used:-

 Thicker coatings of nickel can be produced but the deposit is relatively soft (250HV). Hard particles (e.g. oxides or carbides) can be incorporated into coating during deposition to increase its hardness (e.g. to ~600HV with SiC).



Chemical Coatings

Chemical coatings are produced by the immersion of the component in a solution of a salt of the coating metal with no impressed current. So-called "electroless" coatings of Nickel-boron or nickel-phosphorus are commonly used produced by the reduction of a nickel salt by sodium hypophosphite or sodium borohydride respectively.

 Electroless coatings have a reasonable as-deposited hardness but can be heat-treated to give a high hardness (~1000HV).



Heat treatment is needed to develop optimum properties

Conversion Coatings

Thin compound layers can be produced by reacting a metal surface with an acidic solution. e.g. Thin (10mm) coatings of metal phosphates are formed on steel substrates exposed to phosphoric acid. These provide low friction surfaces with some resistance to adhesive wear. Often used to help components run-in.



 Chemical Vapour Deposition (CVD)

 Gaseous compounds react to form a dense layer on a heated substrate. The most widely deposited wear-resistant coatings are TiC, TiN, chromium carbide and alumina. Deposition temperatures are generally in the range 800-1000°C which restricts the range of materials which can be coated and can lead to component distortion. Thicknesses are limited to about 10mm due to the thermal expansion mismatch stresses which develop on cooling which also restrict the coating of sharp edged components.



 Physical Vapour Deposition (PVD)

A generic term for a range of low pressure coating processes in which the coating flux is produced by a physical process. There are two main types:-

 In both cases the source material is a solid (metal or ceramic). A reactive gas may be used in the deposition chamber to deposit compound coatings from an elemental source or maintain the stoichiometry of coatings from compound sources. Typical coating thicknesses range from 1-10mm for wear-resistant coatings, though thinner layers are used in microelectronics and thicker layers are used for high temperature corrosion protection of gas turbine components.



Evaporation Processes

 The vapour pressure of most materials increases with temperature and if it exceeds the ambient pressure the material will rapidly evaporate into the environment. In a coating chamber the pressure is reduced and the source material heated until a desired vapour flux is maintained which is controlled by the source material, the source temperature and the system pressure.

 Heating can be performed in several ways:-

The vapour pressures of different metals vary over several orders of magnitude so it is difficult to evaporate alloys and control composition.

As-deposited evaporated coatings are porous due to the limited mobility of coating atoms on component surfaces. This can be controlled by heating or ion plating (see later)

Spatter from localised boiling can lead to droplet formation which affects coating performance.

Sputtering Processes

When energetic ions strike a surface, material is ejected by the transfer of momentum from the ion to the target atoms (akin to billiard ball collisions at the atomic scale). This can be conveniently achieved in a low pressure glow discharge of an inert gas such as argon.

In such a process the target material is made the cathode and is raised to a potential of several hundred volts. Electrons leaving the cathode stream out into the gas phase where they can impact with argon atoms, ionising them. The positively charged argon is then accelerated to the cathode where it impacts and sputters away material.

The sputtering yields of different elements for given impact conditions do not vary very much so target alloy compositions can be maintained in the coating except in cases where there are large differences in the atomic weights of alloy constituents.

The coating rate scales with the electrical power used to sustain the discharge. The coating rate also depends on the plasma density, so techniques to increase this (e.g. by confining the electrons close to the target using magnets) will increase the coating rate. However, as much as 95% of the power is dissipated as heat in the target so good cooling is essential.

Main sputtering processes:-

Ion Plating

Coatings produced by vacuum evaporation or sputtering onto components at room temperature are rarely very dense. This is due to low adatom mobility and shadowing processes affecting where incoming atoms can be incorporated into the structure. There are two ways to increase coating density and performance:

The substrate temperature needs to be very high (>0.5Tm of the coating) to achieve a fully dense coating which is not generally practical to achieve (distortion, softening etc.). Ion plating is thus preferred.

 Ion plating is achieved by putting a small negative voltage onto the components during deposition and ensuring that a proportion of the coating flux is ionised. This can be achieved by:

The energy and flux of ion bombardment needs to be controlled to produce a dense coating without introducing excessive compressive residual stresses.

Ion Implantation

 A vacuum process in which a beam of ions is directed at the surface and injected into it. The ions lose energy in collisions with the target atoms and come to rest in the surface layer of the material with an approximately Gaussian distribution. The ion penetration depth depends on the ion species, ion energy and target material, bur is generally less than 1mm. For steels the main ion used is nitrogen, which hardens the surface by forming nitride precipitates and solid solutions. The damage introduced by the implantation process also introduced a compressive residual stress which improves fatigue performance.



 Ion implantation is routinely used for semiconductor doping and treatment of expensive plastics injection moulding tools where any wear is detrimental.

Thermal Spraying Processes

 A number of processes have been developed in which particles of the coating material are heated to a molten state and projected at the substrate which is relatively cold (<200°C). Coating density and strength of bonding to the substrate increase with projection velocity.

Typical Velocity (m/s)

Flame spraying


Atmospheric plasma spraying


Vacuum plasma spraying


High velocity oxy-fuel/ detonation gun spraying




 Thermal Spray Processes

 Wire Spraying

 Flame spraying

 High velocity flame spraying

Detonation Gun Process

 Plasma Spraying


Welding Processes

 The same methods which can be used for joining materials can be used to deposit wear resistant coatings (hardfacings). Coating materials range from low alloy steels to tungsten carbide composites.

 High deposition rates are possible and very thick coatings can be produced. It is impractical to produce layers less than 2-3mm thick.

 There can be problems with cracks in weld deposits.



Can affect properties of substrate

Solid Tiles

 Used in applications where a large amount of wear can be tolerated.

Tiles can be attached mechanically of by adhesives, cements or brazing.

 Materials used include fused basalt, alumina, 13% Mn steel, high chromium cast iron and cemented carbides