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Introduction To Thermal Spraying (aka Metal Spraying)

What is a Thermal Sprayed Coating?

Thermal spraying (aka metal spraying) is a surface engineering/coating process that sprays metals, ceramics and polymers onto the surface of another material.

Molten spherical particles of metal, ceramic, carbide or blends of these
materials are sprayed onto a previously prepared surface where the
particles flatten out and instantaneously cool.
The material being sprayed is either in wire or powder form and can be
melted and sprayed in one of four different types of heat source:-

Flamespray (wire and powder)
Arcspray (wire only)
Plasma (powder only)
High Velocity Oxygen Fuel - HVOF (powder only)

Despite the heat source temperatures varying from 3000 to 15000 degrees
centigrade and the melting temperatures of the materials being sprayed,
Thermal Spraying can be defined as a cold working process. The
component being sprayed will normally never reach 150°C except when
applying fused coatings when fusing temperatures in excess of 700°C are experienced.
In general, coatings are harder and contain varying percentages of porosity
and oxides in comparison with similar wrought materials.

It’s widely used to provide corrosion protection to ferrous metals, as well as to change the item’s surface properties, for instance, to improve the wear resistance or thermal conductivity.
Corrosion and wear are a major problem for a long list of industries, so many use a metal spraying process, including:

• Offshore
• Oil and gas
• Marine
• Tube, pipe, and general fabrication
• Petrochemical
• Construction
• Water supply
• Sewerage
• Ship building
• Aerospace
• Airside support

Thermal spraying protects and extends the life of structures, equipment and vessels in many hostile environments and situations, where protective surface coatings are vital for longevity. Many will not have to have their first maintenance of the coating before 20 years’ service, even in harsh environments, leading to significant maintenance cost reductions.

There are four main methods of thermal spraying, all of which project small molten or softened particles onto a surface to adhere and form a continuous coating. The temperature increase of the coated part is minimal, meaning heat distortion is rare – a major advantage over hot-dipped galvanising or welding.

Thermal spraying for corrosion control is usually carried out by Flame and Arc spray – they are the least costly and quickest to implement, so are suitable for corrosion protection of larger structures. Plasma and HVOF sprays are used to apply engineering coatings and are of higher quality, density, and bond strength.

What is meant by Porosity?

Inherent in coatings applied by thermal spraying techniques are small
pores or voids termed porosity. Their size and distribution within a coating
will depend upon the material being sprayed and the process used.
Generally, for Flamespray wire and arc sprayed coatings porosity
represents approximately 10% to 15% of the coating volume. In
comparison using a very fine type powder via the plasma process, porosity levels can be reduced to less than 1%.

The porosity within the coating can be an advantage in certain applications
where lubrication is essential but a problem in corrosive environments or
where coatings are used where a seal is required. However various sealing
techniques may be employed to eliminate this
phenomenon.

What are Oxides?

During the spraying process, the passing of molten particles from the gun to the substrate will oxidise to varying degrees depending upon the heat source, spray distance and particle velocity. Oxidised particles cement together to improve coating integrity and will give an increase in hardness and hence wear resistance.

Special techniques involving spraying under inert atmospheres have been
employed to reduce oxide content in specialised applications.

What is meant by Bond/Adhesion strength?

The bond/adhesion of sprayed coatings to substrates is a matter of great concern to Engineers. Coatings which become detached during machining must be re-applied whereas those which fail during service will not only cause failure of the coated part but may seriously damage other components and could lead to injuries. However, provided that the basic rules for using sprayed metal coatings are applied and that materials are correctly sprayed on to properly prepared surfaces, bond failures are rare.

Many techniques have been used to assess the adhesion of coatings. The most commonly employed involve pulling in tension a known area of coating from a suitably prepared substrate. In order to do so, it is necessary to attach a pulling device to the coating with a suitable adhesive. This method has the advantage of giving a load failure and, knowing the area under test, a failure of bond strength can be calculated. Unfortunately, the test is generally restricted to test pieces which bear little resemblance to engineering components. Although the test is simple, it is subject to many variables: the strength and curing state of the adhesive; the degree of penetration into porous sprayed deposit (depending on porosity, coating thickness, adhesive viscosity, etc.,) axiality may not be achieved during testing (which may give rise to sheer and peel stresses as well as tensile stresses at the interface).

Complete detachment of the deposit rarely occurs and the fracture is a mixture of bond and cohesive failure. These factors combine to produce considerable scatter and test results and quoted bond/adhesion strengths should be treated with considerable caution. If adhesion is critical, it is strongly recommended that a practical evaluation of a sprayed component be made before specifying a particular sprayed deposit. The adhesion test may then be used as a quality control tool rather than a design aid.

The bond to the base material should be considered to be primarily mechanical and occasionally metallurgical, depending upon the material being sprayed, the process used and the base material. For instance, coatings applied by the Powder Flamespray Process which are subsequently fused at red heat temperatures have a full metallurgical bond.
In contrast low melting point alloys such as zinc or aluminium applied to plastics bond only mechanically. Special bond coat materials containing Nickel Aluminium undergo an
exothermic reaction when sprayed creating a metallurgical bond (primarily
on ferrous substrates). At a local level, the temperature rises to the melting
point of the substrate and cause a diffusion bond.

In general the higher the temperature and velocity of the sprayed material, the
better the bond.

What are the typical comparisons of each Thermal Sprayed Process?
 

Typical Thermal Energy

Typical Kinetic Energy

PROCESS

Temperature °C

Particle Velocity M/Sec

Flamespray (wire)

3000

50-100

Flamespray (Powder)

3000

50-100

Arcspray

4000

20-50

Plasma

15000

50-150

High Velocity Oxygen Fuel - HVOF

2500

300-700

What are the coating comparisons of each Thermal Sprayed Process?

(a very approximate guide depending on many factors- such as surface preparation, bond coats, spraying procedure and materials used)

PROCESS

POROSITY

 OXIDES

 

 (%)

 (%)

Flamespray (wire)

10-15

5-15

Flamespray (Powder)

5-10

5-15

Flamespray (Powder-Fused Coatings)

0

1-5

Arcspray

10-15

5-15

Plasma

<1-5

<1-5

High Velocity Oxygen Fuel - HVOF

<1

<1-2

 

 

 

Advantages/Benefits Of Thermal Spraying

Why applying a Thermal Sprayed (Metal Sprayed) coating increases the life to first maintenance of a component in a given environment?

The selection of a coating system is dependent on the environment in which it is to operate. These environments are detailed below. This is followed by the range of systems available and a chart to indicate the typical time to first maintenance. The treatments recommended for longer lives will always protect for shorter periods and are frequently also economical for these shorter lives.

Environments

Enviroments Table For Faq

 

Other Environmental Considerations

Mines
Warm Humid Conditions (water present – sometimes saline).

Specialist advice should be sought as conditions in different mines vary considerably. Zinc coatings (not aluminium or paint in coal mines) should be considered provided that the water pH is greater than 5. A sealed coating is preferred. Guidance can be sought from Environment 8 but time to first maintenance may vary widely, depending on particular conditions.

Soil
Earth, sand, rock etc.

Specialist advice is advised as the performance of the coatings will vary accordingly to the nature of the soil. Coating lives may be shortened by soluble sulphates and un-burnt coke contained in clinker and ashes. Coatings are preferably sealed. Aluminium coatings are not recommended for direct contact with alkaline clays.

Alkaline concrete away from the atmosphere

Aluminium is unsuitable for direct contact with concrete due to its alkalinity and an inert barrier should be provided. This barrier is not required with zinc. Zinc coatings are beneficial in areas where carbonation of the concrete may occur.

Refrigerated Surfaces

Subject to ice formation and condensation.

Sealed or unsealed coatings are generally suitable. For temperatures below – 30°C advice should be sought.

Chemicals

Sealed metallic zinc is generally suitable for chemicals in the pH range 5 -12, sealed aluminium in the pH range 4 – 9, provided the chemical does not specifically attack the coating. The effect of the coating and sealer on the chemicals should be considered, as well as the protection of the steel.

Abrasion and Impact

Additional consideration in some applications.

The resistance to abrasion, rough handling or impact by sprayed metals (sealed or unsealed) is acceptable. The coating polishes by friction. Where abrasion is critical, specialist advice should be sought.

Table of typical coatings and life to first maintenance

Typical Coatings Table For Faq

Note a: Low corrosion category C1 has no specified coatings but will typically be 5-10 times longer than C2.

Note b: Sealed coatings will typically perform better than unsealed coatings and life to first maintenance will exceed that of an unsealed coating. Sealers should generally be applied until absorption is complete. Sealants and paints should be chosen for the specific environmental conditions.

Painting of sprayed metal coatings

Painting of sprayed metal coatings is normally only required when:

  1. The environment pH value is outside the range 5-12 for zinc or 4.9 for aluminium
  2. The metal is subject to direct chemical attack
  3. The desired finish can only be obtained by paint
  4. Additional abrasion resistance is required. Generally one or two coats of paint are sufficient, except in abnormally aggressive environments. (Sealed metal spray is normally preferable).

Metal spraying provides a very good bond surface for painted coatings and can increase the life of painted coatings.

The information in this bulletin is abstracted from BS EN ISO 14713 and BS EN ISO 2063.

How does thermal spraying (metal spraying) compare to electro plating?

The most common method is deposition from aqueous solutions, many of which are toxic. During electro plating, water decomposes and hydrogen is deposited at the surface being plated. This will cause embrittlement of high strength steels unless they are heat treated immediately after plating. This is only partially effective in reversing the damage.

Electro deposition is usually limited to the deposition of relatively thin, dense layers of pure metals or simple alloys. A limited range of composites may be plated. Electrically conductive substrates are essential but complex geometries are readily plated.

Metal Spraying Offers the Following Advantages Over Electro-Plating

  • Adhesion is usually better
  • Non-metallic substrates can be treated
  • Engineering alloys can be applied as coatings
  • Ceramic coatings can be applied
  • Thicker deposits can be applied
  • There is no risk of hydrogen embrittlement
  • Deposition rates are higher
  • There is no component size limitation
  • Coatings can be applied on site
  • There is no effluent disposal problem
  • Jigs and fixtures do not corrode
  • Complex chemical control techniques are not required
How does thermal spraying (metal spraying) compare to Galvanising?

Galvanising is a well established process for applying a corrosion protection layer of zinc to steel. Treated steelwork is immersed briefly in molten zinc and the coating builds up very quickly. Galvanised layers are usually of even thickness, dense and metallurgically bonded to the steel.

Thickness is typically 0.002 – 0.006 in (50 – 150μ). Coating compositions are sometimes varied to control deposit thickness and give better protection in certain specific environments.

Metal Spraying Offers the Following Advantages Over Galvanising 

  • Low heat input during spraying eliminates the risk of thermal distortion
  • Low heat input eliminates the risk of thermal metallurgical degradation
  • Sealed hollow fabrications may be treated without risk of explosion
  • The process is not limited to zinc. The coating material may be selected specifically for the environment
  • Coating thickness may be varied from place to place to provide extra protection in critical areas
  • There is no limit to the size of article which can be treated
  • Articles can be treated on site
  • There is no effluent disposal problem
  • Reduced stocks of zinc are required. Working capital is not tied up in a molten zinc bath
  • Fuel is not needed to keep zinc molten when the process is not working
  • Metal spraying is used to restore corrosion protection on damaged areas of welded galvanised steel.
How does thermal spraying (metal spraying) compare to Welding?

Welding is a method of joining materials or components to each other, either by direct fusion or through the use of an intermediate filter material. It is also used to apply surface coatings for reclamation or to confer improved wear resistance on OEM articles.

Weld overlay materials are typically steels, bronzes or nickel based alloys. The coatings are metallurgically bonded to their substrates and porosity levels are very low.

Metal Spraying Offers the Following Advantages

  • Low pre-heat or no pre-heating is required
  • No heat treatment is necessary after coating
  • Little heat is transferred to the work piece during coating
  • There is therefore no risk of thermal distortion
  • There is little risk of metallurgical degradation of the substrate
  • Almost any substrate can be coated
  • There is no dilution of the coating by the substrate material
  • A wider range of coatings can be applied
  • Operator skill requirements are lower
  • Spraying is usually (but not always) faster
  • There is better control over deposit thickness
  • Machining allowances are reduced, thus saving material
  • Machining times will also be reduced.

Surface Preperation Prior To Thermal Spraying Required

Why is preparation for thermal spraying required?

If anyone stage in the “building up” process can be described as the most important, it is without doubt preparation. In common with all types of deposition and surface coating work, whether it be welding, electroplating, vitreous enamelling or painting, unsatisfactory preparation can be disastrous. 90% of coating failures investigated have been a direct result of poor or inadequate surface preparation.

Is Preliminary Inspection necessary prior to thermal spraying?

All components must be inspected to ensure that no surface condition
exists which could interfere with any subsequent operation.

(a) Worn or mis-machined components should be examined for cracks,
previous deposits and a hardness check performed.

(b) In production engineering where the metal spraying operation is part
of the flow line process, dimensional inspection will naturally be carried
out before spraying.

Why Degreasing is necessary prior to thermal spraying?

If the surface of the component shows any trace of grease this should be
removed.

Options:

  1. Flood with cold solvent
  2. Vapour degrease
  3. Special cleaning may be required if the component has been crack
    detected to remove the dye penetrate.
    E.G. Soak in white spirit
  4. On porous castings that have been absorbed oil in service, methods 1, 2
    and 3 may not be satisfactory.
    Pre-heating the component to between 2600°C and 3700°C and maintaining
    this temperature until oil ceases to come to the surface or until all
    smoking stops will satisfactorily clean the component.
    After degreasing, it is important that the area to be sprayed is not handled or
    allowed to become contaminated. Castings which are contaminated with oil or grease should be heated to approximately 3000°C by suitable torch or in a furnace.
When is Preliminary Machining for preparation of thermal spraying required?

In some cases no preliminary machining will be required and preparation
of the surface in the chosen manner may be commenced immediately.
This is particularly the case in the restoration of mis-machined surfaces
where the surface to be reclaimed is a seating, housing or location
surface. However, in the case of a worn surface the worn area must be
pre-machined to ensure a uniform concentric deposit for both internal
and external diameters.

Applicable to machine element work such as shafts:
In general, the component is prepared by undercutting the area to be sprayed
using conventional machining techniques. EG. Turning or Grinding. The
amount of base material removed will be dependant upon the material to be
applied and the final dimension of the component.
The geometry for such undercuts and other types of machine preparation are
shown in the following diagrams;
Undercuts should have a straight shoulder or preferably a 450° chamfer. Sharp
corners should be replaced with radii of approximately 0.5mm (.020”) see fig 1:

Preliminary Machining For Preparation Of Thermal Spraying For Faq

The type of undercutting shown in fig 2 should NOT be used as this will cause

a poor bond and a porous area at the shoulder.

Preliminary Machining For Preparation Of Thermal Spraying for Faq

If possible, a shoulder should be left at the end of the shaft.
However if this is not possible, then the coating should be wrapped around
the end of the shaft. With a typical undercut of 0.5mm x 0.5mm (.020” x .020”)
see fig 3.

Preliminary Machining For Preparation Of Thermal Spraying Fig 3

For the reclamation of badly worn parts and to avoid deep shoulders a stepped undercut may be used fig 4.

Preliminary Machining For Preparation Of Thermal Spraying Fig 4

Using this method avoids any undue stresses that may cause cracking and
also reduces the amount of material sprayed.
Cutting fluids should not be used on machine preparation as contamination to
the previously degreased part may result. Surface finish is not critical, ideally the rougher the finish the better the bond. If any hardening process such as “Nitriding” have been applied to the surface
to be sprayed, then this will require removal.

Degree of Machining:

The degree of pre-machining determines the thickness of the finished machined deposit. The optimum thickness of the sprayed deposit is determined by the type of component and its duty and the contraction rate of the chosen metal. The component type and service conditions are the major
factors in determining deposit thickness.

Machining Allowance:
The sprayed deposit thickness is the final deposit thickness plus a machining allowance. This depends on the diameter of the component and whether it is to be ground or turned, but normally it will range from 0.125mm (0.005”) on
radius for a 25.4mm (1”) diameter ground finish, up to 0.5mm (0.020”) for a 254mm (10”) diameter and above turned finish.

It is important to appreciate that if a deposit fails to machine to final
dimensions through lack of metal, the whole deposit must be removed and the complete operation recommenced.

Keyways:
If the edges of a keyway are in reasonable condition, it should be masked off immediately before spraying with a dummy key as described in section 8 -
Spraying Procedure (Masking for Spraying).
If damage has occurred to such an extend that the refitting of a key would not be satisfactory, no reliance should be made on the sprayed deposit to rectify
the damage. In this event another keyway should be cut if this is permissible and the existing keyway filled in, or alternatively the keyway may be reclaimed
by welding.

Having determined the depth of pre-machining in accordance with the above recommendations and maintained the type of profile discussed, the machining may be commenced without the use of lubricant or cutting fluid

What is surface preparation by grit blasting?

Grit blasting is the most commonly used method of preparing surfaces for metal spraying.

Grit blasting is the most commonly used method of preparing surfaces for metal spraying. It removes rust, mill scale and other surface contaminants and produces a suitably roughened surface by projecting a highly concentrated stream of relatively small abrasive particles at high velocity against the surface to be cleaned. It has also been shown to be effective in reducing the loss of fatigue strength.

Equipment

Suction or Syphon Blasting
Here the particles of abrasive are projected by suction or by a venturi type nozzle into an air blast. It is mainly employed in the preparation of small components in hand cabinets.

High Pressure Blasting

The particles of abrasive are directly fed from a pressurised container into a high pressure air stream. This is the most widely used form of blasting, either in hand cabinets, blast rooms or in portable form, on site work.

Centrifugal Blasting

Involves the abrasive being centrifugally propelled from rapidly rotating impellers. It is much more specialised equipment and is highly efficient for low cost blasting of large volume repetitive production.

Prior to gritblasting, components should be degreased to prevent contamination
media and masked where areas not to be sprayed need to be protected.

Methods of Masking:
1. Grit blasting tape
2. Mechanical masks

What Gristblast media is available?

Chilled Iron Grit

This is by far the most widely used abrasive for metal spraying. It is an excellent general purpose abrasive, due to (a) its relatively high density, which gives high particle energies, (b) its slow rate of breakdown and (c) the retention of sharp cutting edges on the particles.

Crushed Slag’s – Expendable Abrasive

An alternative to chilled iron grit when reclamation is not possible, as is the case on many site jobs. While quite effective for “once only” use, they are not suitable for reclamation and re-use, due to their rapid breakdown to dust.

Ceramic Grits – Aluminium Oxide and Silicon Carbides

Used where the base material has a hardness greater than 360HV which cannot be effectively blasted by chilled iron grit. They can be used at lower than normal blasting pressures and are effective when “Syphon Blasting”. They are therefore well suited to the preparation of thin metal surfaces which may distort if blasted with chilled iron at conventional pressures. Non-metallic grits must not be used to prepare surfaces for coatings which are to be fused. Grit blasting standards for metal spraying should not be confused with blast cleaning as used to prepare surfaces for painting.

Of the various standards of surface finish “SA 3” are comparable in surface cleanness with grit blasting quality for metal spraying.

The blast profile, defined as “height from trough to adjacent peak”, should not exceed 0.004″-0.005” (100 – 125μ) experience has shown that chilled iron grit Grade G24 provides a surface of appropriate amplitude. Comparable surface amplitudes are similarly achieved with expendable non-metallic abrasives of around N° 16 mesh.

On Site Blasting Grit

There are two types of grit available for on site blasting, Garnet and Copper Slag. These are of the disposable type and are only used once.

What Blasting Techniques should be used?

i) Blasting pressures must not be excessive. If pressures are too high, grit breakdown will be rapid, grit may be embedded in the surface and mechanical distortion of the component may occur. Particular care is required with most non-ferrous alloys, plastics and fragile or highly stressed parts.

ii) Grit must be inspected regularly. Blunt particles, fines and contaminants are deleterious and should be removed.

iii) Blasting air must be free of water, oil and other contaminants. Accordingly, suitable after coolers, moisture traps, filters etc. should be fitted to the air lines.

iv) Excessive blasting should be avoided. It is expensive and can be detrimental to the metal spraying process.

v) Blasting debris must be removed from the surface before spraying. Vacuum cleaning or brushing is preferable; blowing with compressed air may not remove debris but move it from one place to another.

vi) Grit blasted surfaces must not be contaminated before spraying. If handling is unavoidable, clean cotton gloves should be used.

vii) Spraying must commence as soon as possible after surfaces have been blasted, certainly before any visible deterioration occurs. In temperate climates, deterioration (and impairment of adhesion) may occur in less than four hours. In hot, humid conditions, deterioration will be more rapid.

What precautions should be made when Gritblasting?

i) Except in open site work, where special precautions must be taken to protect personnel; blasting should always be done in a blast room or cabinet.

ii) Never start up a blasting unit until the hose is firmly held pointing in a safe direction.

iii) The blast hose should be of an approved anti-static type and must be inspected regularly for wear and security of fittings.

iv) Always wear full protective clothing; for example, helmet, hood, gloves, aprons and leggings, to ensure protection from flying abrasive.

v) The provision of an inspection window in a blast room is advised.

NOTE – Under The Blasting (Castings and other Articles) Special Regulations 1949 Part II, it is forbidden to use sand, or other substance containing free silica in any blasting apparatus.

How to use rough threading for preparation of surface prior to thermal spraying?

Rough cutting or Rough Thread preparation consists essentially of cutting a
thread form on the area to be sprayed.
This process increases the surface area and roughens it up, there by adding
to the mechanical bond of the Thermal Sprayed coating.
This type of preparation correctly carried out gives a very high bond strength,
and has the advantage that it may be done in the lathe immediately after the
pre-machining operation , without necessitating the removal of the
component and subsequent re-setting for spraying. Which is necessary when gritblast preparation is used. It must be emphasised that the cutting must be rough, as a conventional
smooth thread is virtually useless.
The tool should be ground with a 90° inclusive angle a slight radius at the top and a relief angle of 50°, see Fig 5.

The tool should be mounted in the conventional manner and set so that it
enters the surface between 0.5mm (1/64”) and 1mm (1/32”) below centreline
according to the diameter being treated (this produces a torn and jagged
cut). No cutting fluid or other lubricant is used and the cut should be taken
with one traverse of the tool which should be fed into full depth immediately.
Between 10 and 12 threads per cm (24 & 30 T.P.I.) should be used, the greater
numbers for small diameters, e.g. a 12mm (1/2”) diameter shaft would require
12 thread per cm (30 T.P.I.) The surface speed should be the lowest possible
for the traverse speed selected, the whole purpose being to produce a torn
and jagged thread form. The illustration in Fig 6 shows an example of this type of preparation.
In internal diameters a modification of the thread form is desirable and a buttress thread should be used with an included root angle of 60° see Fig 7.

This permits the spray to enter the thread fully without risk of bridging.

FIGURE 6

FIGURE 6

Fig 5

Fig 5

When are Bond Coats used?

Certain materials will bond to clean surfaces that have not subsequently been
prepared by either gritblasting or Rough threading.

These materials generally contain Nickel and Aluminium which when sprayed,
will create an exothermic reaction causing particles to become superheated
and will metallurgically bond with the substrate material.
These materials (T405-1 for Wire Flamespray, 75E for Arcspray and P636 for
the powder processes) are applied as bond coats (thickness approximately
0.125mm (.005”)) prior to spraying the desired top coat material.

A Metallisation 99E molybdenum wire coating Flame sprayed will exhibit self
bonding properties but will not bond to bronze, copper, chrome and nitrided
surfaces (unless the nitrided area is removed).

There are other Metallisation materials which have these self bonding
properties plus additional coating qualities. EG Selfbonding Stainless, Bronze
etc. These materials are commonly referred to as one steps. (Metallisation
83E for Wire Flamespray, 79E for Arcspray and the P600 series when powder
spraying).

Bond strengths will be improved if prior to spraying a bond coat or one step
material the surface is either gritblasted or thread cut.
It is recommended that in the engineering environment a Bond Coat should
always be used.

Bond Coats are used in the following circumstances:-

To improve adhesion when basis materials have been prepared mechanically.
Where the substrate material is too hard for mechanical preparation.
Where mechanical preparation would damage the substrate material.

Fig 7

Fig 7

Where mechanical preparation is uneconomic.
Where the chosen coating will not adhere to the substrate material.
As a buffer where brittle coatings may be subjected to shock loading or excessive mechanical strains.

As a buffer between coatings and basis materials where elevated temperatures may lead to excessive strains due to mis-match of thermal expansion co-efficient.

Selection of Material:

Choice of bonding material depends on basis material and spraying process and may be influenced by corrosion considerations. In somecases, it may be necessary to experiment with different materials to obtain optimum selection.

99E Molybdenum
This is applied by flame spraying as a bond for steels and aluminium materials. It does not bond to copper and its alloys. It should not be used above 400°C.

T405/1 Ni alloy
This is applied by flame spraying as a bond for steels. It may improve coating adhesion on aluminium alloys but it will not bond to copper and its alloys.

85E Ni chrome
This is used as a high temperature bonding material (up to 1000°C) and as a buffer layer beneath ceramic coatings. It adheres well to steels but does not bond to copper and its alloys. It may be arc or flame sprayed.

10E Aluminium Bronze
This is used as a bond coat for steel substrates and may be used on copper alloy substrates. Arc

sprayed aluminium bronze gives better results than when flame sprayed.

05E Copper
This is sometimes used as a bond when spraying on to copper and copper alloy substrates. Mechanical preparation is also necessary.

02E Zinc
This is frequently used as a bond coating on to plastics, paper and wooden substrates. Sometimes, it is effective as a bond onto ceramics and glasses. It may be arc or flame sprayed.

75E Ni alloy

Gives the best adhesion on steels and good adhesion to aluminium alloys. It is not suitable for copper and bronzes. It should be applied only by arc spraying.

01E Aluminium

Bonds well to steel and may be used for certain ceramic and glass substrates. It should not be used for plastics.

Application of Bond Coats

Bond coats should be applied thinly to produce a roughened surface to which the subsequent coat can adhere. For most purposes 0.002-0.004″ (50-100μm) is adequate; thicker coatings are not only more expensive but often less effective. Where the bond coat is applied as a buffer beneath ceramics which may be subjected to excessive mechanical or thermal strains, thicker deposits are necessary. Depending on application, the buffer layer may be between 0.005″-0.15″ (125 – 375μm).

The bond coat must be applied to a clean, and preferably grit blasted surface as soon as possible after the surface has been prepared and before there is any visible contamination. Subsequent coatings should be applied immediately after bond coating.

Summary of Surface Preparation Methods

Method - Processes

  1. GRITBLAST
  2. PRE-MACHINING PLUS BOND COAT
  3. GRITBLAST PLUS BOND COAT
  4. ROUGH THREAD PLUS GRITBLAST
  5. ROUGH THREAD PLUS BONDCOAT
  6. ROUGH THREAD PLUS GRITBLAST PLUS BONDCOAT

Applications:

All these surface preparation methods have their place, e.g Grit Blast for an anti-corrosion coating on a steel structure. However, the recommend method
where possible would be, rough thread plus grit blast plus bondcoat for most
applications.
Method 1: used for thin coatings less than 1.25mm (.050”) (however this will
depend on grit size used) on irregular shaped components where
bond coats are not applicable.

Method 2: used for coatings less than 1.25mm (.050”) on machinable bases where bond coats are applicable and grit blasting is not available.
Advantage - quicker than Method 1

Method 3:- used on irregular shaped components that cannot be prepared
by machining. Can also be used for machine element work but
will require resetting after gritblasting.

Method 4: used for thick coatings in excess of 1.25mm (.050”) on
machinable bases where bond coats are inapplicable
e.g.Copper substrates.

Method 5: is a quicker alternative to Method 4 where bond coats are applicable.

Method 6: the most expensive method with the least risk of subsequent coating failure through lack of adhesion.

Limitations Of Thermal Spraying

What are the limitations of a thermal sprayed coating on a component?

The term engineering applications covers the field of thermal spraying
where material is deposited on machine components to reclaim worn or mis-machined surfaces, and to impart desirable surface characteristics such as wear resistance and improved corrosion resistance to new components in primary manufacture.

The original use of thermal spraying in this connection was for the repair of worn parts, this was soon extended to mis-machined components. In recent years refinement of technique and modern equipment have permitted the salvage of highly stressed components in applications such as gas turbine aircraft engines. In the same manner these techniques and equipment have been used to extend the process into production engineering both in mechanical engineering e.g. spraying fork lift truck masts with bronze to prevent galling, automotive transmission components hardfaced with
molybdenum, and in chemical engineering, e.g. spraying mild steel drying rolls with stainless steel and pump shafts with Monel.

The process is also used in many specialised industries such as the
manufacture of printing machinery, paper machinery and glass making
machinery.

The suitability of the metal spraying process for any particular application
may be determined according to the following fundamental principals:-

No strength is imparted to the base material by the sprayed metal
deposit. It is essential that the component to be sprayed should in its prepared form be able to withstand any mechanical loading to which it will be subject in service, e.g. a crankshaft pin worn below its final regrind limit should not be undertaken, but a crankshaft worn to slightly above its final regrind limit may be restored to original dimensions.

If the area to be sprayed on a component or any section of the total area is subject to shear loading in service then it is not a suitable
subject, e.g. gear teeth, splines, threads etc., cannot be reclaimed by metal spraying.

A point loading with line contact on a metal sprayed deposit will
eventually spread the deposit causing detachment. If the deposit is on a moving component with such a loading the deposit failure will occur very rapidly, e.g. needle and roller bearing seating where the bearing elements are in direct contact with the sprayed deposit
cannot be treated.

d) If any hardening process such as “Nitriding” have been applied to the surface to be sprayed, then this will require removal.

Subject to the above almost any metal or alloy surface may be sprayed and it is convenient to divide applications into the following types:

i) Seatings
These are usually bearing seating, gear and pinion seating etc. The
bearings or other components may be press fits, interference fits or
sliding fits on the seating. Various forms of keyway may be present
but all can be metal sprayed satisfactorily using the techniques
described later.

ii) Housings
The technique used for treating seating is employed suitably modified for
internal diameters.

iii) Location Surfaces
Except in the case of internal or flat surfaces, no complication arises.

iv) Bearing Surfaces
The treatment of bearing surfaces is also quite straightforward. The
selection of the metal to be sprayed (see 5 - Coating Selection) will be governed by the type of bearing material in contact with the sprayed surface, e.g. a crank pin running in white metal bearings may be sprayed with many materials, but if it runs in lead/bronze, lead/indium or aluminium alloy shell bearings 60E steel or 103T Cored wire would
be used.

v) Bearings
The usual practice where ball or roller bearings are concerned is to
spray the seating or housing as described above. Bronze bearings,
however, are reclaimable by spraying, and babbit liners can be built up or restored completely in the same way.

vi) Gland Contact Areas, Oil Seal Contact Areas
Gland packing areas may be treated in the normal way but regard should be given to the type of packing and the environment to
determine the material used to build up the worn area. Oil seal
contact areas are treated in the same manner.

Basic Thermal Spraying Procedures

Simple thermal spraying procedures

The following is a guide of simple thermal spraying procedures. Metallisation offer various levels of training for all kinds of different applications/components in order to find and develop with our customers best practises and improve efficiencies. Please get in touch for further information.

Masking for Spraying