Surface Engineering

 Definition

 "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

• Mechanical treatment (e.g. peening, shot blasting)
• Surface transformation (e.g. induction hardening, laser treatment)
• Surface composition changes (e.g. thermochemical treatment, ion implantation)
• Chemical treatment (e.g. conversion coatings)
• Coating (e.g. painting, spraying, plating, vapour deposition)
• Surface activation (etching, plasma treatment of polymers)

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

• Wear resistance
• Hardness/strength
• Corrosion resistance
• Impact resistance
• Thickness required

Processing factors

• Adhesion
• Porosity
• Residual stress
• Structure/property relationships

 Process factors

• Component shape
• Component size
• Deposition rate
• Substrate temperature during deposition
• Effect of surface treatment on fatigue strength of substrate
• Cost

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.

Adhesion

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

 Adhesion depends on:-

• Interfacial chemical bonding (covalent or ionic bonds stronger than van der Waals bonding)
• Surface cleanliness (contamination, method of preparation)
• Sharpness of interface (best adhesion from graded interfaces)
• Residual stress (high compressive residual stress promotes spallation)
• Deposition temperature (higher temperatures allow chemical bond formation and development of graded interfaces but also promote thermal expansion mismatch stress)
• Process factors (e.g. sputter cleaning in vapour deposition, particle velocity in spraying)

 How to improve adhesion:

• Remove weak, friable processed layers from components (e.g. grit blasting)
• Roughen surface (for sprayed coatings)
• Carefully clean and degrease
• Chemically etch or activate surface
• Sputter etch in vacuum processes
• Coat at as high a temperature as is practical for other considerations

 Porosity

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

 Effect of porosity:

• Reduced strength
• Allows penetration by gases and liquids (corrosion)
• Can act as an oil reservoir

 Control of porosity by:

• Process parameters (e.g. increase spray velocity or deposition temperature in PVD)
• Coating thickness (penetration of gases to interface reduced as coating thickness increases
• Use a post-coating sealant

 Component shape and size

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

• electrodeposits cannot be applied to components with sharp edges or blind cavities
• Spraying processes are best carried out with the spray stream normal to the surface
• Access to surface areas needed for weld deposits
• Vacuum coating is essentially line-of sight
• CVD and electroless nickel do not suffer from these problems

 Component size is also an issue:

• in PVD or CVD component size is limited by chamber size
• in high temperature processes (e.g. carburising) component size is limited by the amount of distortion which can be tolerated

 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

Coating	Hardness (HV)	Maximum service temperature (°C)	Corrosion resistance	Application method
Metals hardened by dispersed oxides Mo	390	250	moderate	Flame spraying
13% Cr steel	330	600	good	Flame spraying
Intermetallics NiAl	250-350	850	poor	Flame or plasma spraying
Co-Cr-Mo	1100	1000	very good	Plasma spraying, welding
Hardened steels 12% Mn	150-400 (work hardened)	150-200	poor	Welding
Martensitic	300-850	150-200	poor/ moderate	Welding, thermal and thermo-chemical treatment
Nitrided	800-1200	500	good/ moderate	Thermo- chemical treatment
Cast Irons Martensitic	400-600	200-250	poor/ moderate	Welding
High chromium	400-600	1000	good	Welding
NiCoCr alloys Cobalt alloys	300-700	850	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
Cermets WC/Co	1300-1600	550	good	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
Oxides alumina	2100	>1000	very good	D-gun, plasma spraying, CVD, PVD
chromia	2400	>1000	very good	D-gun, plasma spraying
Carbides TiC	3200	500	very good	CVD, PVD
VC	2600	500	very good	Thermo-chemical
Diamond	10000	650	very good	CVD, tiling
Diamond-like carbon (DLC)	1200	250	very good	CVD, PVD
Nitrides TiN	2400	500	very good	CVD, PVD
TiAlN	2300	850	very good	PVD
CrN	2000	850	very good	PVD
Carbonitrides TiCN	4500	450	very good	PVD, CVD, Thermo-chemical
Borides FeB	1650	200	moderate	Thermo-chemical
CrB	3500-4000	800	good	Fusion of powder coating

Case Studies: Applications of Surface Treatments and Coatings

Gears

 Requirements

• ability to sustain contact stresses on tooth flanks
• ability to sustain bending stresses at the base of the teeth
• resistance to surface fatigue (pitting-resistance)
• resistance to scuffing

 Contact stresses

• Hertzian stresses give a maximum shear stress below the surface
• The depth of this maximum increases with load and increasing tooth flank radius
• Hertzian shear stresses may reach 1.5 to 2GPa
• A thick, high hardness surface layer is required

 Possible surface treatments

• thermal hardening
• thermochemical treatments
  • carburising
  • nitriding

 Variation Of Maximum Shear Stress With Depth

Consider variation of maximum shear along the z axis

For circular contacts

 t_max=0.31P₀ at x=0, z=0.48a

 For line contacts

 t_max=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

• Electrodeposition (press tools)
• Electroless deposition (precision forming tools)

• Vapour deposition (cutting and forming tools)
  • Chemical vapour deposition
  • Physical vapour deposition

• Plasma nitriding (press and forming tools)
• Ion implantation (moulding tools)

SURFACE ENGINEERING 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.

 Advantages

• Cheap
• Selective treatment possible
• Variable depth of treatment (1-10mm)

 Disadvantages

• Limited range of steels
• Possibility of distortion

 Types of Thermal Hardening

• Induction hardening
• High frequency resistance hardening
• Flame hardening
• Tungsten inert gas (TIG) hardening
• Laser transformation hardening
• Electron beam hardening

Types of Thermal Hardening

 Induction Hardening

• Alternating current flowing in an in an inductor induces eddy currents in the workpiece
• result in Joule heating
• High frequency (500kHz) for shallow case (0.5mm)
• Low frequency (1kHz) for deep case (5mm)
• Single shot or scanning
• Rapid repeatable and clean

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

• Water cooled proximity conductor is connected to the workpiece and a power source
• Heating occurs between the contacts when power is applied (400kHz)
• Selective hardening
• Cheap

Flame hardening

• Localised heating with gas flames (acetylene, propane or natural gas)
• Depth of hardening depends on heat transfer from the fuel-oxygen mixture
• Often integrated with quenching spray
• Can treat complex shapes
• Can be costly and difficult to control

 Tungsten inert gas (TIG) hardening

• Akin to TIG welding
• TIG used to harden cast iron by surface melting
• Large castings so that conduction in the bulk rapidly cools melted layer
• Cheap

 Laser transformation hardening

• High power laser beam impinges on surface to give rapid heating
• Localised treatment
• Operates in air
• Quenching by heat conduction to bulk
• Case depths around 1mm at high power density, deeper at increased laser defocus
• Can melt substrate surface or fuse applied powder layers
• High capital cost
• High throughput

Electron beam transformation hardening

• Vacuum process, prevents excessive component oxidation
• High energy electron beam impinges on component causing heating
• Depth of treated layer from 0.2 to 2.5mm
• Quenching by heat conduction to bulk
• High capital cost

 Thermochemical Treatments

• Carburising
• Carbonitriding
• Nitriding
• Nitrocarburising
• Boriding
• Metalliding

Carburising

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.

Advantages

• Applicable to a wider range of steel compositions
• Wider range of surface and core properties possible

Disadvantages

• Distortion occurs in processing which may require grinding
• Quench cracking can occur

Carburising Processes

Pack Carburising

• Components packed in a box with carburising agent (e.g. coke) and an energiser (e.g. barium carbonate)
• Heated to 850-950°C for diffusion of carbon
• Limited control of carbon profile by amounts of carburising agent and energiser
• Quenching from carburising temperature not possible
• Energy inefficient
• Good for "one-off" components

 Salt bath carburising

• Components immersed in a molten salt bath containing mixtures of sodium cyanide and an alkaline earth salt (e.g. barium chloride)
• Cyanide is oxidised to cyanate which dissociates at the steel surface to form CO

• 4NaCNO® 2NaCN+Na₂CO₃+CO+2N_Fe

  2CO® CO₂+C_Fe

• Both carbon and nitrogen diffuse into components

Gas Carburising

• Atmospheres with low carbon potential generated outside the furnace as carrier gas
• Gas is enriched with hydrocarbons on injection into the furnace to increase the carbon potential to the required level
• Carbon potential is the surface carbon concentration of the steel in the atmosphere under equilibrium conditions - measured directly by modern probes

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+CO_2® 2CO+2H₂

CH₄+H₂O® CO+3H₂

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

CO+H_2® C_Fe+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

• Gas blown through a bed of particles at sufficient flow rate will cause the particles to separate and "fluidise"
• High heat transfer rates and fuel efficiency
• fast process
• Carbon potential of fluidising gas can be controlled but it is difficult to achieve accurate carbon potentials
• Particles are generally inert

 Vacuum carburising

• Components heated to carburising temperature under low pressure (0.1Torr)
• Furnace then backfilled with a carburising gas to a fixed partial pressure
• Treatment rate depends on partial pressure
• Problems with soot formation

Plasma carburising

• Components raised to the carburising temperature by resistance heating in a low pressure inert gas environment
• The components are insulated from the chamber which acts as an anode in plasma generation
• A hydrocarbon/hydrogen gas mixture is then injected and a plasma produced by putting several hundred volts onto the components (cathode)
• Plasma activates the carburising atmosphere leading to a rapid increase in carburising rate
• Good process control is achieved by controlling the gas atmosphere and the electrical characteristics of the plasma
• Clean, efficient process
• Gas quench needed

 Carbonitriding

 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

 Advantages

• Reduced distortion
• Less cracking

Disadvantages

• Slower process
• Shallower case depths produced

Carbonitriding Processes

Salt bath carbonitriding

• Process is similar to salt bath carburising except that more oxygen is supplied to the bath
• This increases the amount of sodium cyanate in the bath and hence a greater production of nitrogen
• Process control is not easy due to complex salt bath chemistry
• Cheaper than salt bath carburising due to lower operating temperature

Gas carbonitriding

• Ammonia gas is introduced into a gas carburising atmosphere (typically around 5%)
• Control of the ammonia concentration is crucial as high levels lead to retained austenite and void formation at boundaries

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.

 Advantages

• Low temperature treatment, no distortion
• Hardness developed is temper resistant up to 500° C

 Disadvantages

• Long treatment times
• Thin case depth (<0.7mm)

 Nitriding Processes

 Gas nitriding

• Anhydrous ammonia gas dissociates catalytically on component surfaces (490-530° C)

2NH_3® 2N_Fe+3H₂

• Nickel alloy retorts necessary to prevent corrosion
• Case depth 0.5mm after 45h at 515° C
• White layer (~20mm) consisting of fcc g’ phase (Fe₄N) and a hcp e phase with a higher nitrogen content formed at surface, this is brittle and is often removed by grinding
• Amount of white layer can be controlled by minimising the nitriding potential

• Achieved by diluting the ammonia with hydrogen
• Diffusion layer with alloy nitrides formed below (900-1000HV)

 Plasma Nitriding

• Good control of compound layer properties and thickness
• Components are placed in the reactor vessel and made the cathode of a plasma by applying a voltage in the range 400-1500V to them. Chamber walls form the anode.
• Pressure reduced to 0.1-10torr of reaction gases hydrogen and nitrogen to strike discharge (1torr =1mm mercury)
• Gas is ionised by electrons streaming out from the components
• Nearly all the potential drop occurs in the few mm above the component surface, the "dark space" - ions are accelerated across this space and impinge on the components
• Ion bombardment provides the heat for the process.
• Adjusting the pressure changes the size of the dark space and allows complex features (blind holes, tubes) to be treated.
• Nitriding potential controlled by electric parameters - plasma nitriding can be carried out at lower temperatures that gas nitriding
• High degree of process control and reliability

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.

 Advantages

• Carbonitride layer has good resistance to adhesive wear
• Can be oxidised during quenching and sealed to give excellent corrosion resistance

 Disadvantages

• Typical treated layers are very thin
• Nitrocarburising atmospheres/salts are difficult to handle safely

 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.

 Advantages

• High surface hardness
• Good adhesion due to interdigitation of borides

 Disadvantages

• High process temperature (distortion)
• Brittle surface layer
• Poor fatigue and corrosion resistance

 Boronising Processes

 Pack Boriding

• Components packed into boron carbide powder with an activator (BaF₂, NH₄Cl or KBF₄) and an inert diluent (SiC or alumina)
• As B₄C content increases, the amount of FeB at the surface of the components increases
• Most widely-used process due to its simplicity

 Paste Boriding

• Boron-containing pastes can be painted on to components and dried
• Several coats may be needed
• Component then heated for boronising to take place

 Salt bath processes

• Molten salt baths used
• Thermochemical or electrolytic processes

 Gas boriding

• Nitrogen/hydrogen carrier gas with BCl₃ additions

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.

 Advantages

• High surface hardness
• Good abrasive wear resistance

 Disadvantages

• High process temperature (distortion)
• Thin treated layer

Coating Processes

• Electrodeposition
• Chemical coating
• Conversion coating
• Vapour deposition
  • Chemical vapour deposition
  • Physical vapour deposition

• Spraying
• Welding
• Cladding with solid tiles

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:-

• high hardness (1000HV)
• good corrosion resistance
• low coefficient of friction against steel
• thickness limit of 0.5mm due to residual stresses

 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).

 Advantages

• Low temperature treatment
• High hardness
• Low friction
• Applicable to a wide range of metal substrates
• Thick layers possible

 Disadvantages

• Poor thickness uniformity on complex components
• Hydrogen embrittlement
• Not applicable to insulating substrates
• Possible environmental concerns with plating baths

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).

 Advantages

• Low temperature treatment
• More corrosion resistant than electrodeposited chromium
• Can coat complex shapes uniformly
• Hard particles can be incorporated to increase hardness
• PTFE can be incorporated to reduce friction
• Can coat most metals and some insulators

 Disadvantages

• More expensive than electroplated chromium

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.

 Advantages

• Cheap and simple to perform
• Low temperature treatment

 Disadvantages

• Restricted range of materials can be treated
• Thin treated layer
• Poor treatment durability
• Difficult to control treatment quality on heterogeneous materials

 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.

 Advantages

• High coating hardness
• Good adhesion (if the coating is not too thick)
• Good throwing power (i.e. uniformity of coating)

 Disadvantages

• High temperature process (distortion)
• Sharp edge coating is difficult (thermal expansion mismatch stresses)
• Limited range of materials can be coated
• Environmental concerns about process gases

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:-

• Evaporation
• Sputtering

 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.

 Advantages

• Excellent process control
• Low deposition temperature
• Dense, adherent coatings
• Elemental, alloy and compound coatings possible

 Disadvantages

• Vacuum processes with high capital cost
• Limited component size treatable
• Relatively low coating rates
• Poor throwing power without manipulation of 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:-

• Resistive heating (e.g. aluminium evaporation from TiB₂ boat)
• Electron beam evaporation (e.g. metals such as tungsten)
• Cathodic arc evaporation (e.g. titanium evaporation for TiN coatings)

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:-

• DC diode sputtering (for conducting targets)
• RF sputtering (for insulating targets)

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:

• raise the substrate temperature
• allow some ion bombardment of the coating to occur as it grows (ion plating)

The substrate temperature needs to be very high (>0.5T_m 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:

• Passing the flux through a plasma (either as part of the sputtering process, or artificially generated for evaporation)
• Using a hot filament to generate a flux of electron into the system
• Using an arc evaporation process (~90% of the material ejected from the arc is ionised)
• Using an ion beam

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.

 Advantages

• Low temperature process
• Very versatile - every stable element in the periodic table can be implanted into any vacuum compatible target
• Highly controlled
• No distortion - can be applied to finished components
• Not a coating process

 Disadvantages

• Line of sight process
• Expensive vacuum equipment needed
• Very thin treated layer

 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.

Process	Typical Velocity (m/s)
Flame spraying	100
Atmospheric plasma spraying	300
Vacuum plasma spraying	600
High velocity oxy-fuel/ detonation gun spraying	800

 Advantages

• Most metals and ceramics and some polymers can be sprayed
• A wide range of metallic substrates can be coated.
• Localised treatments possible

 Disadvantages

• Most sprayed coatings contain some porosity
• The adhesion of sprayed coatings is generally poor compared to other processes
• The quality and reliability of the coatings depends on careful control of both powder source and spray parameters
• Difficult to produce high quality coatings on re-entrant surfaces

 Thermal Spray Processes

 Wire Spraying

• Consumable wire fed into gun where it is melted and atomised by a gas blast (generally compressed air)
• Melting by flame or arc
• Metal wires only
• Much oxide incorporated in deposit
• Used for building up worn or badly machined products
• Arc processes are faster but noisier and lead to metal fumes so not used so much

 Flame spraying

• Consumable powder is injected into a torch
• Simple and cheap
• Range of metal powders can be sprayed plus some oxides
• Coatings sprayed by simple torches have poor properties due to low particle speeds
• Fusible alloys such as Ni-Cr-B-Si have been developed to overcome this

 High velocity flame spraying

• Fuel/air mixtures undergo controlled combustion in the gun
• High velocity spray guns burn fuel/air mixtures at high pressure to achieve high gas velocities
• Shock waves created which can accelerate gas and powder feed to very high velocity
• High velocity oxyfuel processes developed for wear-resistant cermet coatings (WC/Co)

Detonation Gun Process

• D-gun detonation coatings have fuel/air mixture ignited by a spark plug
• During the flame burn period the flame front accelerates down the barrel, compressing the gas ahead of it
• At a critical temperature, self ignition produces a shock wave
• Detonation wave heats and carries powder to substrate at high velocity
• Flame temperature limited to 3500K
• Detonation between four and eight times a second
• Very noisy process
• Detonation gun cermet coatings well established for wear (e.g. WC/Co)

 Plasma Spraying

• A carrier gas is fed into a torch in which a high current arc is struck
• Gases passing through the arc are dissociated and ionised resulting in a plasma jet from the nozzle
• Powders are injected into the plasma jet where they are heated and ejected at the target - the injection position can be adjusted depending on their melting point
• Very high temperatures possible in the are (10000K) and heat transfer is very efficient, especially from diatomic gases
• Ceramic, metal and polymer coatings are possible
• The process can be carried out in vacuum to reduce atmospheric braking and oxidation of metallic coatings

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.

 Advantages

• Cheap
• Applicable to large components
• Localised coating possible
• Excellent adhesion

 Disadvantages

• Limited range of coating materials
• Minimum thickness limits

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

 Advantages

• Very thick layers possible
• No affect on substrate properties
• Can be replaced
• Different tiles used in different locations to optimise performance

 Disadvantages

• Limited substrate geometries
• Limited coating materials
• Labour intensive
• Problems at tile boundaries
• Expensive