Thermal Spraying

What is Thermal spraying ?

Thermal Spraying metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties. Thermal Spraying covers a wide range of techniques in which material is heated rapidly an a hot gaseous medium and simultaneously projected at high velocity onto a surface, to produce a coating. In other words, this method in its most basic form is the propelling of a powdered metal alloy via a heat source onto a component to form a metallurgical bond with the components base material. The range of alloys that can be sprayed is immense based mainly on Nickel, Cobalt, Aluminum and Iron but containing elements as diverse as boron, molybdenum and tungsten in fact it would be true to say that an alloy can be provided for almost any application.Sprayed metal coatings have been used for a number of years and exposure tests have proved them to be superior to conventional paint coatings.

Classfication

The different processes for thermal spraying can be grouped into two categories:

• There are lower energy processes often referred as metallising, or “cold” processes in which the alloy is sprayed onto the base material either directly or onto a pre-sprayed bondcoat of pure nickel. An exothermic reaction takes place bonding the alloy to the surface. In this category arc spraying and flame spraying processes are included. These are frequently used for spraying metals for corrosion resistance, such as zinc and aluminium.

• The second category covers higher energy processes sush as plasma spraying, detonation gun and high velocity combustion spraying. A technique recently attracting considerable interest is the High Velocity Oxy-Fuel (HVOF) process.The processes of the second category are basically the same as the first but when the alloy has been deposited it is then fused either in a vacuum furnace or with the aid of a gas pre-heating torch to form a dense homogenous coating metallurgical bonded to the base material, this second process is used where a point loading on the component is expected. Unfortunately a great deal of heat is generated with this second process, therefore it is not recommended for finished or slender components that could be susceptible to distortion. In all cases the types of structures differ considerably from those produced by either gaseous or solution-state processes.

How is the alloy sprayed ?

The alloy is sprayed via a torch that is connected to a gas source such as oxygen and acetylene.The delivery system of the torch is dependant on its manufacturer, however two main methods are employed:

• micro-pulverised alloy powder purchased in containers that fits onto the torch

• alloy in wire form that is either ground by the torch and fed into the flame or simply atomised in the flame itself

Page 1 of 4 Created by Javier Garcia

THERMAL SPRAYING

What materials can be coated ?

Almost all metals can be coated sometimes with a thickness' of as much as 25mm { ½ " ] although cost would prohibit thick coatings on large components. Here is a list of some of the materials that can be coated.

•Carbon Steels including tool steels such as D3,D2,O1 etc. •Stainless Steels •Titanium •Cast Irons•Cast Steels •Certain Bronzes and Brasses •Certain Magnesium Alloys

What alloys can be sprayed ?

As mentioned above the range of alloys that can be sprayed is immense, alloy selection is based upon the application to which it is being put, for example it is possible to spray phosphor bronze onto an aluminium component to produce a bearing surface, Tungsten Carbide can be sprayed onto a mild steel base to give superb abrasion resistance, Stainless Steel can be spayed onto medium carbon steel for resistance to rust and a nickel chrome iron material may be used to repair a worn shaft, putting it back into service quickly and almost certainly making it last longer than the original.

•Nickel Alloys - for corrosion resistance to acids and alkalis - provide good cutting edges on blades etc •Cobalt Alloys - good resistance to wear, abrasion and corrosion at elevated temperatures •Aluminum Alloys – for corrosion resistance•Iron based Alloys - Cutting edges, good self lubricating properties •Copper - gaskets, conductance etc •Phosphor Bronze - Excellent bearing material •Ceramics - Resistance to thermal shock and wear - High thermal and electrical resistance•Plastics – Resistance to corrosion from water and mild acid and alkali's

Applications

• Enhance wear and/or corrosion resistance• Provide specific frictional characteristics to the surface• Use for dimensional restoration• Uses as thermal barrier, thermal conductor, electrical conductor or resistor• Uses as electromagnetical shielding, enhance or retard of radiation

Advantages

• Extremely wide variety of materials that can be used to make a coating• Ability of most of the thermal spray processes to apply a coating substrate without significantly heating• Ability to strip or recoat worn or damaged coatings without changing the properties or dimensions of the part• Very cost effective especially on large components with small wear areas • Ideal in breakdown situations where spare parts are unavailable • Excellent for reclaiming obsolete parts - Classic cars/bikes etc • Inexpensive base materials can be used and coated in certain areas for a particular resistance• Specialised alloys for a specialised component - corrosion -RF shielding - thermal barriers etc • Coating where making a component from the alloy is unfeasible - Valve parts, offshore applications• Making moulds by spraying onto a component coated with release agent


Disadvantages

• Line-of-sight nature of these processes. They can only coat what the torch or gun can “see”• Size limitations prohibiting the coating of small, deep cavities into which a torch or gun will not fit

Processes

Arc Spraying

In this process, the raw material in the form of a pair of metallic wires (with electrically opposed charge), comprising the spray material, are fed together in such a manner that a controlled arc occurs at the intersection heating and melting them. This molten material is atomised by a cone of compressed air and propelled towards the workpiece. The molten spray solidifies on the component surface to form a dense, strongly adherent coating. This process differs from the other thermal spray processes in that there is no external heat source such as a gas fleme or electrically induced plasma.Major advantages of the Arc Spraying process are that the coatings are available for almost instant use with no drying or curing times and there is no risk of damaging the component.In addition, the deposits possess a higher degree of bond strength than most other thermally sprayed deposits and the use of compressed air and electricity alone mean more economic coatings. Another advantages are:

• high deposition rates• low substrate heating• less expensive to operate

Flame Spraying

In this process, the raw material is melted in an oxygen-fuel gas flame. This molten material is atomised by a cone of compressed air and propelled towards the workpiece. Flame spray guns are available ton spray materials in either single wire, rod or powder form.Most flame spray guns can be adapted to use several combination of gases to balance operating cost and coating poperties. Aetylene, propane, methyl-acetylene-propadiene (MAPP) gas, anh hydrogen, along with oxygen, are commonly used flame spray gases. In general, changing the nozzle and/or air cap is required to adapt the gun to different alloys, wire sizes, or gases. Flame spray process is characterized by:

• low capital investment• high deposition rates and efficiencies• relative ease of operation and equipment maintenance

In general, as deposited its coatings exhibit:

• lower bond strenght,• higher porosity,• and higher heat transmittal

to the substrate than most other thermal spray processes.

Plasma Spraying

Plasma is the term used to describe gas which has been raised to such a high temperature that it ionises and becomes electrically conductive. In the case of Plasma spraying, the plasma is created by an electric arc burning within the nozzle of a plasma gun and the arc gas is formed into a plasma jet as it emerges from the nozzle. Powder particles are injected into this jet where they melt and then strike the surface at high velocity to produce a strongly adherent coating. Almost any material can be sprayed


including metals, ceramics and plastics. The workpiece remains cool because the plasma is localised at the gun.Advantages:

• Superior Bond Strength - As much as twice the strength of conventional metal spray processes.• Extreme Hardness - Plasma coatings equivalent to Rc70 can be applied, providing exceptional wear resistance even in extremely abrasive applications. In many cases, plasma-sprayed surfaces are suitable as a replacement for chrome plating.• Extreme Density - Over 150 materials can be plasma sprayed, including tungsten carbides, chromium carbides and oxide ceramics. Materials that are plasma-applied result in coatings that are very low in porosity, thus extremely dense. This enables Atlas to achieve exceptional finishing results.Finishing - Plasma coatings of all types can be finished ground and polished.

Surface Preparation

To ensure adequate bonding of thermal spray coatings, it is critical that a substrate be properly prepared. Surfaces must be clean, and usually substrates must be roughened after cleaning by grit blasting or some other means.

Finishing Treatment

Sealing

Thermal spray coatings usually have a structure with inherent porosity that ranges from less than 2 to more than 15 vol%, depending on the process and the material sprayed. These can contribute to premature failure of the coating. For this reason, many applications require the coated to be sealed before finishing, if necessary. Sealant materials such as waxes, epoxies, phenolics and inorganics are used.

Surface Finishing

Although thermal spray coatings are used with their surfaces in the as-deposited condition for some applications, these surfaces are too rough for some severe conditions. Therefore they are usually finished by methods such as grinding, lapping, polishing, machining, abrasive brushing or vibratory finishing.

Figure 1: Arc Spraying

Figure 2: Flame Spraying


Figure 2


From our papers:

Has the advantage of also reducing exfoliation and weld toe cracking.

MIL-M-6874: Metal Spraying, Process for (Cancelled, 1993)MIL-STD-1687A: Thermal Spray Process for Naval Ship Machinery Application

TAFA Inc.146 Pembroke Rd. Concord, NH 03301tel 603/224-9585fax 603/225-4342

MIL-M-6874: Metal Spraying, Process for (Cancelled, 1993)MIL-STD-1687A: Thermal Spray Process for Naval Ship Machinery Application

TAFA Inc.146 Pembroke Rd. Concord, NH 03301tel 603/224-9585fax 603/225-4342

METALLİSATİON’DAN

PROTECTION AGAINST CORROSION - THE BENEFITS OFSPRAYED METALLIC LAYERS1. Metallisation aluminium and zinc spraying wires are of consistent qualityand purity. Properly stored, shelf life is indefinite; there are no settlementproblems as may be experienced with powder spraying materials and nomixing as required with paints.2. The materials are simple to apply using Metallisation combustion flame orelectric arc spraying equipment. Operators can be trained in a relatively


short time and with a small amount of practice are capable of producingconsistently sound and even coatings on properly grit blasted surfaces.3. There is almost no limitation on the size of component or structure whichcan be treated.4. Where large areas or large numbers of components are to be sprayed, theMetallisation wire fed spraying equipment is easily mechanised or fullyautomated. Both the combustion gas and electric arc spraying systemshave efficient stop/start devices for production economy.5. The process itself is simple involving only two or three stages; spraying ispreceded by grit blasting and may be followed by sealing of the deposit.This simplicity makes quality control relatively easy and offers fewer stagesfor errors to occur.6. Sprayed metal coatings may be handled immediately after treatment.There are no protracted drying times and factory floor space can be moreefficiently utilised.7. Properly applied sprayed metal coatings are more robust than paintsystems and are consequently able to withstand rougher usage.8. Even if a sprayed deposit is locally damaged, the sacrificial actionparticularly of zinc prevents corrosion from edges and discontinuities. Itmay also delay the onset of rusting of structures which have beenneglected.9. Although bare metal sprayed coatings give long lives they may be sealedto extend the life time or the enhance visual appearance.10. Sprayed aluminium or zinc coatings give long lives in most naturallyoccurring environments. (See Application Data Sheet AC-AC-002 for moredetail). Ten years to first maintenance is common and over twenty yearsmay be readily achieved with the appropriate system.11. There is no distinct limit to the thickness of sprayed coatings. Zinc may besprayed to over 3mm and unlike galvanising, thickness may be varied fromplace to place to provide protection in critical areas.12. The surface being sprayed remains cool. Consequently there is no risk ofheat distortion or metallurgical degradation of load bearing steel structures.Sealed tubular or hollow sections can be coated externally without danger.13. The sprayed metal surface maintains the efficiency of friction grip areasand ensures their effectiveness throughout the life of the structure14. Thick anti-corrosive paint systems are generally unnecessary but thetexture of "bare" sprayed coatings provides an excellent "key" forsubsequent paint treatment. In most cases where the sprayed metal isproperly sealed, these need only be applied for decorative purposes.15. The nature of the equipment makes it ideal for either factory or siteapplication and coatings can be deposited in ambient conditions totallyunsuited to other methods of protective treatment.

THE RANGE OF SPRAYED METAL COATINGS FOR THEPROTECTION OF IRON AND STEEL AGAINST CORROSIONThe selection of a coating system is dependent on the environment in which it is tooperate. These environments are detailed below. This is followed by the range ofsystems available and a chart to indicate the typical time to first maintenance. Thetreatments recommended for longer lives will always protect for shorter periods andare frequently also economical for these shorter lives. The information in thisbulletin is abstracted from BS 5493.The environments are:-CHARTREFERENCECATEGORY DESCRIPTION1.2.3.4.EXTERIOR EXPOSED


Non-polluted inlandPolluted inlandNon-polluted coastalPolluted coastalRAIN WASHED SURFACESAreas with low levels of acid,alkali salt or sulphur dioxideAreas with sulphur dioxide orother airborne pollutionAreas as 1. with saltdetectable but without saltsprayAreas as 2. with saltdetectable but without saltspray5. EXTERIOR SHELTERED As above, but not rain washed,normally badly ventilated andsubject to condensation.

CHARTREFERENCECATEGORY DESCRIPTION6.7.INTERIORNormally dryFrequently damp and wetINSIDE BUILDINGS (heated orunheated)Some condensationSubstantial condensation8.9.10.Non saline waterSEA WATERImmersedSplash zonePotable and non-potable waterSEA AND OTHER SALINEWATERSPermanent immersionWind and water exposed areas offloating and tidal structures.Areas subject to frequent saltsprayOTHER ENVIRONMENTS(a) Mines - Warm Humid Conditions (Water present - sometimes saline)Specialist advice should be sought as conditions in different mines varyconsiderably. Zinc coatings (not aluminium or paint in coal mines) shouldbe considered provided that the water pH is greater than 5. A sealedcoating is preferred. Guidance can be sought from Environments 8, 9 and10 but time to first maintenance may vary widely, depending on particularconditions.(b) Soil - Earth, sand, rock etc.Specialist advice is advised as the performance of the coatings will varyaccordingly to the nature of the soil. Coating lives may be shortened bysoluble sulphates and unburnt coke contained in clinker and ashes.Coatings are preferably sealed. Aluminium coatings are not recommendedfor direct contact with alkaline clays.


(c) Encasement in Concrete - Alkaline concrete away from atmosphereAluminium is unsuitable for direct contact with concrete due to its alkalinityand an inert barrier should be provided. This barrier is not required withzinc. Zinc coatings are beneficial in areas where carbonation of theconcrete may occur.(d) Refrigerated Surfaces - Subject to ice formation and condensationSealed or unsealed coatings are generally suitable (e.g. SC1A, SC1Z,SC5A or SC5Z). For temperatures below - 30°C advice should be sought.

(e) ChemicalsSealed 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 notspecifically attack the coating. The effect of the coating and sealer on thechemicals should be considered as well as the protection of the steel.(f) 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.For details of High Temperature Environments, See Application Data Sheet LE-AC-002The Systems are:-ReferenceNo.as BS 5493Metal NominalThicknessinch (microns)RemarksBare Metal CoatingsSC 1 ASC 1 ZSC 2 ASC 2 ZSC 3 ASC 3 ZSC 4 ZAluminiumZincAluminiumZincAluminiumZincZinc0.004 (100)0.004 (100)0.006 (150)0.006 (150)0.010 (250)0.010 (250)0.014 (350)SC 2 A may be used up to550°C.There is rarely any ad-vantage in applying alum-inium to thicknessesgreater than 0.006 in. (150microns)Sealed Metal CoatingsSC 5 ASC 5 ZSC 6 ASC 6 Z


SC 7 ASC 7 ZSC 8 ZAluminiumZincAluminiumZincAluminiumZincZinc0.004 (100)0.004 (100)0.006 (150)0.006 (150)0.010 (250)0.010 (250)0.016 (400)Pre-treatment after metalspraying with Sealer Type'B' is optional. Sealingwith the appropriate sealertype Range (See TB 251)should be applied immed-iately after spraying andcontinue until absorption iscomplete.SC 6 AH Aluminium 0.007 (175) For use up to 550°CPainted Metal CoatingsSC 9 ASC 9 ZSC 10 ASC 10 ZAluminium}Zinc }Aluminium}Zinc }0.004 (100)Metal +0.001 - 0.004(30 - 100) PaintInert paint coatings arepreferred. Painting ofsprayed metal coatings isonly done when: (seeoverpage)

Painting of sprayed metal coatings is only done when:-(i) The environment pH value is outside the range 5-12 for zinc or 4.9 foraluminium(ii) The metal is subject to direct chemical attack(iii) The desired finish can only be obtained by paint(iv) Additional abrasion resistance is required. Generally one or two coats ofpaint are sufficient, except in abnormally aggressive environments.(Sealed metal spray is normally preferable) Systems SC 9 A and SC 9 Zinclude one pre-treatment coat. Systems SC 10 A and SC 10 Z include twocoats.

SAFETY IN METAL SPRAYING


Metal Spraying is not a dangerous process, if equipment is treated with care andcorrect spraying practices are followed. However, as with any industrial process,there are a number of hazards of which the operator should be aware and againstwhich specific precautions should be taken.Ideally, equipment should be operated automatically in enclosures speciallydesigned to extract fumes, reduce noise levels and present direct viewing of thespraying head. Such techniques will also produce more consistent deposits.However, there are occasions when the type of components being treated, or lowproduction levels require manual operation. Under these conditions a number ofhazards peculiar to thermal spraying are experienced in addition to thosecommonly encountered in production or processing industries.NOISEMetal spraying equipment uses compressed gases which create noise. Soundlevels vary with the type of spraying equipment, the material being sprayed and theoperating parameters. Typical sound pressure levels taken 1 metre behind the arcspray or flame spray nozzle are 102-104 db(A). Specially designed enclosuresshould be used to attenuate these levels. Where this is not possible, operators andpassers-by should wear good quality ear defenders.LIGHTCombustion spraying equipment produces an intense flame which may have apeak temperature in excess of 3,100°C and is very bright. Electric arc sprayingproduces ultra-violet light which may damage delicate body tissues. Spray boothsand enclosures should be fitted with ultra-violet absorbent dark glass. Where this isimpracticable operators and others in the vicinity should wear protective gogglescontaining BS grade 6 green glass. Opaque screens should be placed aroundspraying areas. The nozzle of an arc pistol should never be viewed directly unlessit is certain that no power is available to the equipment.DUST AND FUMESThe atomisation of molten materials produces a certain amount of dust and fumes.Proper extraction facilities are vital, not only for personal safety, but to minimiseentrapment of re-frozen particles in the sprayed coatings. The use of breathingmasks fitted with suitable filters is strongly recommended where equipment cannotbe isolated.Certain materials offer specific known hazards.All finely divided metal particles are potentially pyrophorric and none should beallowed to accumulate.Certain materials e.g. aluminium, zinc and other base metals may react withwater to evolve hydrogen. This is potentially explosive and special precautionsare necessary in fume extraction equipment.Fumes of certain materials, notably zinc and copper alloys are unpleasant tosmell, and, in certain individuals, may cause a fever-type reaction. This mayoccur some time after spraying and usually subsides rapidly. If it does not,medical advice must be sought.HEATCombustion spraying pistols use oxygen and fuel gases. The fuel gases arepotentially explosive. In particular, acetylene may only be used under conditionsapproved by the Health and Safety Authorities. Oxygen, while not explosive, willsustain combustion and many materials will spontaneously ignite if excessiveoxygen levels are present. Care must be taken to avoid leakage and to isolateoxygen and fuel gas supplies when not in use.ELECTRICITYElectric arc pistols operate at low voltages (below 45 dc) but are relatively highcurrents. They may be safely hand held. The power supply units are connected to440 volts AC sources and must be treated with the normal caution afforded to suchequipment.COMPRESSED AIRThe air supply to spraying pistols is at high pressure. It should not be directedtowards people. The motor air supply is lubricated and on no account should it befitted to breathing apparatus. Any breathing equipment used with the thermalspraying process must be supplied with air of breathing quality.


Further information on safety aspects may be gained from the Metallisation Healthand Safety Brochure, the Area Sales Managers or from Metallisation Limited.

COMPARISON OF METAL SPRAYING WITH PAINTINGPainting is a widely used method of protecting steelwork (and other materials) fromcorrosion. Most paints are organic (polymer) bases with added metal particles,corrosion inhibiting compounds or inert filler materials. They may be applied bydipping, brushing or spraying to suitably prepared (grit blasted) surfaces.Protective paint systems are multi-layer comprising a priming coat, two or threeprimary protective coats and a decorative top coat. Extensive practical long termevaluation has shown that paint systems have shorter effective lives than sprayedzinc or aluminium coatings.METAL SPRAYING OFFERS THE FOLLOWING ADVANTAGES OVERPAINTING:Materials are of consistent quality and purity. No mixing is required beforeapplication.Materials have an infinite shelf life if properly storedFewer process steps are required. This allows simpler quality control and feweropportunities for errorSprayed articles require no protracted curing or drying times giving superiorutilisation to floor spaceMetals may be sprayed in a wider range of climatic conditions (temperatureand humidity) than paintsSprayed zinc and aluminium give effective corrosion protection immediatelySprayed metals are more robust than paints and can withstand rougher usageEven if the sprayed layer is damaged the sacrificial action, particularly of zinc,prevents corrosion from edges and discontinuitiesSprayed metal coatings maintain the efficiency of friction grip areas throughoutthe life of the structureAdhesion to steelwork is better. Sprayed zinc or aluminium are often specifiedas base layers for paint systems for this reason. However, experience showsthat properly sprayed metal coatings are adequate if sealed and that the paintoverlay offers no further advantage.

PREPARATION FOR SPRAYINGSprayed coatings adhere to surfaces mainly by mechanical and physical means; ina few instances, metallurgical or chemical bonding may occur to a small degree.Whatever the mechanism of adhesion, it is vital that the surface to be sprayed isclean and adequately roughened. Over 80% of coating failures are due to poor orincorrect surface preparation.INITIAL INSPECTIONSurfaces to be sprayed should be examined to ensure that previous coatings havebeen removed, that welds are properly dressed and that no cracks or other defectsexist.METHODS OF PREPARATIONDegreasing: Oil and grease must be removed before preparation begins. Withoutthis, grit and tools will become contaminated and the oil will spread over thesurface. Vapour degreasing is preferable, where this is not practicable, care mustbe taken to ensure that solvents do not simply re-distribute the contaminant thinlyover the entire surface Porous materials such as castings may require baking toensure removal of oils.Gritblasting: This is the most commonly used method of preparation. Sharpabrasive grit is projected towards the surface, either mechanically or bycompressed air. Blasting cleans the surface, increases the surface area andprovides a profile into which the surface will key. It is important that the grit is ofthe correct type and size, is not contaminated and does not contaminate thesurface. (For further information, see Technical Bulletin 5.2.2).Rough Machining: This method is commonly applied to surfaces which are


required to bear a thick deposit. It increases the surface area and provides aprofile which will resist shearing between coating and substrate. (For furtherinformation, see Technical Bulletin 5.2.4).Bond Coating: Originally developed to provide a rough keying profile on groundsteel surfaces which were too hard to grit blast, bond coats are now extensivelyused to enhance adhesion on mechanical and gritblasted surfaces. Laboratorytests indicate that bond coats not only increase bond strength, but will also givemore consistent adhesion. (For further information see Technical Bulletin 5.2.4).Combined Techniques: The above methods may be combined to give superioradhesion.Preheating: Preheating is rarely needed, but is essential for certain substrates, e.g.glass, to prevent thermal shock - usually no further preparation is needed in thesecases. Preheating is advisable when spraying bores or internal diameters with highshrink materials or thick deposits. It is also recommended when environmentalconditions are such that water (from burning gases or the atmosphere) maycondense onto the workpiece during spraying. Care must be taken to avoidexcessive temperatures (175°C maximum). Surfaces should be re-gritblastedimmediately after heating to remove the thin oxide film which will form.CARE OF THE PREPARED SURFACEPrepared surfaces are chemically and physically very active. They must not beallowed to deteriorate or become contaminated. They must be handled with careand not touched with naked hands, ropes or slings. Clean, lint-free cotton gloves orsheets should be used to protect prepared surfaces during handling.Spraying must begin as soon as possible after preparation. The allowable timeinterval depends on the material and on ambient conditions. It should not exceedfour hours: in hot or humid conditions the maximum allowable delay may be verymuch less. If longer delays occur, the surface must be re-prepared unless specialstorage facilities are available.

SURFACE PREPARATION BY GRIT BLASTINGGrit blasting is the most commonly used method of preparing surfaces for metalspraying. It removes rust, millscale and other surface contaminants and producesa suitably roughened surface by projecting a highly concentrated stream ofrelatively small abrasive particles at high velocity against the surface to becleaned. It has also been shown to be effective in reducing the loss of fatiguestrength.EQUIPMENT"Suction or Syphon Blasting": - Here the particles of abrasive are projected bysuction or by a venturi type nozzle into an air blast. It is mainly employed in thepreparation of small components in hand cabinets."High Pressure Blasting": - The particles of abrasive are directly fed from apressurised container into a high pressure air stream. This is the most widely usedform of blasting, either in hand cabinets, blast rooms or in portable form, on sitework."Centrifugal Blasting": - Involves the abrasive being centrifugally propelled fromrapidly rotating impellers. It is much more specialised equipment and is highlyefficient for low cost blasting of large volume repetitive production.TYPES OF ABRASIVESChilled Iron GritThis is by far the most widely used abrasive (ref: BS 2451) for metal spraying. It isan excellent general purpose abrasive, due to (a) its relatively high density, whichgives high particle energies, (b) its slow rate of breakdown and (c) the retention ofsharp cutting edges on the particles.Crushed Slags (Expendable Abrasive)An alternative to chilled iron grit when reclamation is not possible, as is the case onmany site jobs. While quite effective for "once only" use, they are not suitable forreclamation and re-use, due to their rapid breakdown to dust.Ceramic Grits (Aluminium Oxide and Silicon CarbidesUsed 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 normalblasting pressures and are effective when "Syphon Blasting". They are thereforewell suited to the preparation of thin metal surfaces which may distort if blasted withchilled iron at conventional pressures. Non-metallic grits must not be used toprepare surfaces for coatings which are to be fused.

STANDARD OF PREPARATION (See British Standard 2569 Parts 1 and 2, BS5493, BS 4232 and Svensk Standard SIS 05 59 00)Grit blasting standards for metal spraying should not be confused with blastcleaning as used to prepare surfaces for painting.Of the various standards of surface finish, only BS 4232 "White Metal Finish" orSIS 05 59 00 "SA 3" are comparable in surface cleanness with grit blasting qualityfor metal spraying.The blast profile (defined as "height from trough to adjacent peak") should notexceed 0.004"-0.005" (100 - 125) experience has shown that chilled iron grit to BS2451 Grade G24 provides a surface of appropriate amplitude. Comparable surfaceamplitudes are similarly achieved with expendable non-metallic abrasives ofaround N16 mesh.BLASTING TECHNIQUESi) Blasting pressures must not be excessive. If pressures are too high, gritbreakdown will be rapid, grit may be embedded in the surface andmechanical distortion of the component may occur. Particular care isrequired with most non-ferrous alloys, plastics and fragile or highly stressedparts.ii) Grit must be inspected regularly. Blunt particles, fines and contaminantsare 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 airlines.iv) Excessive blasting should be avoided. It is expensive and can bedetrimental 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 airmay not remove debris but move it from one place to another.vi) Grit blasted surfaces must not be contaminated before spraying. Ifhandling is unavoidable, clean cotton gloves should be used.vii) Spraying must commence as soon as possible after surfaces have beenblasted, certainly before any visible deterioration occurs. In temperateclimates, deterioration (and impairment of adhesion) may occur in less thanfour hours. In hot, humid conditions, deterioration will be more rapid.

PRECAUTIONS RELATING TO GRIT BLASTINGi) Except in open site work, where special precautions must be taken toprotect personnel; blasting should always be done in a blast room orcabinet.ii) Never start up a blasting unit until the hose is firmly held pointing in a safedirection.iii) The blast hose should be of an approved anti-static type and must beinspected 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.NOTEUnder 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 blastingapparatus.

SEALED SPRAYED COATINGS


Porosity inherent in 'as-sprayed' coatings will allow the ingress of fluids which maygive rise to:-Corrosion of the coating or the base materialContamination of the coating or of products handled by the coated articlesPressure loss in hydraulic systems due to seepage beneath sealsPremature coating failure due to sudden or fluctuating pressure changesThese problems may be overcome by applying a suitable sealant deforming thecoating to seal porosity (only for some materials), preventing interconnecting porenetworks with thicker coatings (in some cases only). Choice of method andmaterial depends on the type of coating, its thickness, the coating function and theoperator environment.PROPERTIES REQUIRED OF SEALANTSSealability - The sealant must fill the pores sufficiently to prevent fluid penetration.Chemical Stability - To be effective the sealant must not react chemically with theenvironment unless the reaction promotes sealing without harming the coating orany products.Thermal Stability - Many organic materials deteriorate rapidly at moderately hightemperatures (>120°C). The sealant must be effective at the expected operatingtemperatures of the coating.Mechanical Stability - Sealants must be strong enough to resist abrasive orcavitation erosion and hydraulic washing by the environment.Viscosity - Sealants must be sufficiently fluid to penetrate the deposit yetsufficiently viscous to prevent drainage from the coating.

SEALANT TYPE ADVANTAGES LIMITATIONSNON-CURING(Oils, grease, wax)Easy to apply.Good penetrationMay be flushed out bysolvents or hydraulic action offluidsCURING-TYPES(Vinyls, polyesters,epoxies, anaerobics)Usually stronger thannon-curing types.Usually relatively inertwhen curedPenetration often poor withhigh viscosity materials. Ifthinned, lower solidsconcentration may not alwaysfill pores.SELF SEALING METALS(Tin, copper, aluminium,18/8 stainless steel)Higher temperaturecapability. Loweradded costCoating may be damagedduring compression. Depthof sealing usually shallow andmay be removed by wear.METHODS OF SEALINGSealants may be applied by painting (brushing or spraying), dipping or vacuumimpregnation depending on which method is appropriate to the sealant and thecomponent. For instance, vacuum impregnation may not be appropriate for fastcuring anaerobics or very large components.'Self Sealing' coatings may be sealed by machining using high pressures and blunt


tools, by rolling or by peening. These methods may be used to control surfacefinish and, in some cases, dimensional tolerances. However, care must be takensince excessive tool pressures may detach the coating from its substrate.For further details of sealing materials, see Technical Bulletin Numbers 2.5.1 and2.5.2.1.TYPICAL USES OF SEALINGAtmospheric CorrosionProtectionZn or Al sealed with vinyls, epoxies, alkyds,polyesters (Metallisation ProtectometRange)High TemperatureProtectionZn or NiCr sealed with silicones or silicates.(Metallisation Protectomet Range)Hydraulic Rams 60E Sealed with Metallisation Sprayseal MBearings 10E &15ESealed with Metallisation Sprayseal MJournals 30E &60ESealed with Metallisation Sprayseal MPrinting Cylinders Cu, 30E Sealed with Metallisation Sprayseal MFood ProcessingMachineryAl Sealed with Vinyl (MetallisationProtectomet Range)Chemical Vessels Al, 80E Self sealed

PROPERTIES OF SPRAYED MATERIAL

ADHESIONThe adhesion of sprayed coatings to substrates is a matter of great concern toEngineers. Coatings which become detached during machining must be re-appliedwhereas those which fail during service will not only cause failure of the coated partbut may seriously damage other components and could lead to injuries. However,provided that the basic rules for using sprayed metal coatings are applied and thatmaterials are correctly sprayed on to properly prepared surfaces, bond failures arerare.Many techniques have been used to assess the adhesion of coatings. The mostcommonly employed involve pulling in tension a known area of coating from asuitably prepared substrate. In order to do so, it is necessary to attach a pullingdevice to the coating with a suitable adhesive. This method has the advantage ofgiving a load failure and, knowing the area under test, a failure of bond strengthcan be calculated. Unfortunately, the test is generally restricted to test pieceswhich bear little resemblance to engineering components. Although the test issimple, it is subject to many variables: the strength and curing state of theadhesive; the degree of penetration into porous sprayed deposit (depending onporosity, coating thickness, adhesive viscosity, etc.,) axiality may not be achievedduring testing (which may give rise to sheer and peel stresses as well as tensilestresses at the interface). Complete detachment of the deposit rarely occurs andthe fracture is a mixture of bond and cohesive failure.These factors combine to produce considerable scatter and test results and quotedbond strengths should be treated with considerable caution. If adhesion is critical, itis strongly recommended that a practical evaluation of a sprayed component bemade before specifying a particular sprayed deposit. The adhesion test may thenbe used as a quality control tool rather than a design aid.

HARDNESSHardness is of interest to engineers because it relates to the tensile strength of


wrought or cast metals, or because it gives an indication of resistance to abrasivewear. The tests, which measure the resistance to penetration of a hardened ball,cone or diamond under known loads are simple, rapid and require a minimum ofskill. With homogenous wrought or cast materials, hardness can be converted fromone scale to another with reasonable accuracy by using appropriate conversioncharts. Hardness of sprayed deposits cannot be converted into other scale or intotensile strength values.Sprayed coatings differ in that they consist of many individual particles, are un-homogenousand may contain appreciable levels of porosity and oxides.Hardnesses of sprayed deposits must be treated with considerable caution. Valuesmeasured on a single specimen may vary depending on whether the area under theindenter contains porosity, oxide or uncontaminated coating. In general for unfusedcoating, Brinell hardnesses made with a large diameter ball will give the mostconsistent results. However, such impressions may penetrate deeply into thecoating and particularly with thin deposits, may be influenced by the hardness ofthe basis material. Coating macrohardness tests are not recommended ondeposits less than 0.125 in (3mm) thick.The hardnesses given below are based on laboratory tests made at MetallisationLimited, certain Customers and Universities. They are for guidance purposes only,except under abrasive wear conditions, they should not be used for designpurposes and even where abrasive wear is experienced, they should be treatedwith caution.HARDNESSMATERIAL REF HB Rc Rb HV Knoop100Nickel Manganese 76E 200Monel 70E 110-120 80-84Aluminium Bronze 10E 118-128 144Phosphor Bronze 15E 95-105Copper 05E 58Molybdenum 99E 250-800Zinc 02E 12-15Aluminium 01E 25-30 5518/8 StainlessSteel80E 275 240-260ChromeManganese Steel65E 435 38-44Chrome Steel 60E/61E420-460 35-4018/5 StainlessSteel55E 340 220-240 360Low Carbon Steel 30E 210-230 336MATERIAL SURFACEPREPARATIONBONDMN/M²STRENGTHPSI75E Nickel Aluminium CleanedBlasted28.9633.094200480010E Aluminium Bronze CleanedBlasted


20.6824.133000350070E Monel Blasted 17.24 250015E Phosphor Bronze Blasted 10.00 145005E Copper Blasted 10.00 145002E Zinc Blasted 4.82 70001E Aluminium Blasted 13.78 200030E Low Carbon Steel Blasted 39.30 570055E 18/5 Stainless Steel Blasted 28.77 410060E Chrome Steel Blasted 20.68 300065E Chrome ManganeseSteelBlasted 21.37 310080E 18/8 Stainless Steel Blasted 28.34 410080/20 Tin Zinc Blasted 8.27 120099E Molybdenum Blasted 37.92 5500

WEAR RESISTANCEThermally sprayed coating materials differ in many ways from their wrought or castcounterparts. During spraying, excessive currents and/or voltages may result in thepreferential burn-off of elements such as carbon and chromium during atomisation.Under normal conditions, the chemical change will not be deleterious. However,sprayed coatings are always porous and they usually contain appreciable levels ofoxides. In addition the sprayed particles quench at very high rates when theyimpact upon metallic basis components. As a result the mechanical propertiescannot be compared with other materials of similar composition and the chemicalproperties may differ.Hardness tests on sprayed coatings are unreliable since, on thick coatings theresults may be affected by oxides or porosity in the material beneath theindentation; on thin coatings, indenters may penetrate sufficiently for the result tobe influenced by the substrate material. Even when carefully measured thecoatings hardness often gives little indication of wear resistance (an application forwhich sprayed coatings are frequently specified). This is partly because the wearperformance will depend on the counterface material, the operating speed,pressure and temperature and on lubrication. Often the pores in coatings will actas lubricant reservoirs and provide fluid under potentially disastrous conditions.Frequently, the oxides present within sprayed coatings will give wear resistancesmuch higher than might be expected from hardness tests. The metallurgicalstructures of sprayed deposits are different from wrought or cast materials sincethe extremely rapid cooling on impact followed by short term re-heating ofsubsequent particles gives rise to unusual metallurgical phenomena. Several ofthe steels containing high chromium contents show work hardening propertieswhich, combined with the oxides present in the coating, give excellent wearproperties.For composite (or pseudo-alloy) coatings formed by arc spraying with dissimilarwires, it is important to recognise that alloying does not occur during spraying.Each wire produces discrete particles and the coating will be a true compositecontaining approximately equal volumes of material of each composition.Chemically, most coatings behave much as their wrought or cast counterparts.However, the porosity will allow fluid penetration in corrosive situations or inpneumatic or hydraulic equipment. In order to avoid problems coatings used insuch applications should be sealed. There have been occasional instances wheresprayed stainless steels have performed better than wrought materials in corrosivesituations. It can only be assumed that the oxide present within the coating hascontributed to the improvement by retarding corrosion of the particles.In view of the above, care should be taken when selecting materials for sprayedcoatings. In case of difficulties, advice may be sought from the TechnicalDepartment, Metallisation Limited, Pear Tree Lane, Dudley, West Midlands, DY2


OXH : Tel: 01384 252464.WEAR RATING i.e. depth of wear in micronsContact SurfaceWear Specimen (slider) 0.1% Steel Hardened Steel 60 RcGas sprayed molybdenum (hard) 20 20Arc sprayed 1.5% C steel composite 20 19Arc sprayed chrome steel 60E 25 76Gas sprayed chrome steel 60E 44 24Wroughthardened steel 60Rc 50 142Arc sprayed low carbon steel 30E 50 450Arc sprayed (+) 30E/copper mix 80 119Wrought hardened steel 40 Rc 100 133Arc sprayed 18/8/2 Mo stainless 115 128Arc sprayed 55E 18/5/8 Mn stainless 120 155Arc sprayed phosphor bronze 146 840Arc spray (+) copper/30E mix 148 98Arc spray 80/20 nickel chrome 205 85Wrought 18/8 stainless 235 790Gas sprayed 18/8 stainless 264 525Arc sprayed 18/8 stainless 355 362Gas sprayed 30E low carbon steel0.1%370 300Gas sprayed aluminium bronze 645 778Arc sprayed aluminium bronze 840 500Wrought mild steel 0.1% C 1040 975Gas sprayed phosphor bronze 1070 940Wrought phosphor bronze 1880 1295A maximum velocity of 1 Km/Hr was adopted as standard, giving a mean velocityof 0.6 Km/Hr. At this speed, the specimens suffered no measurable rise in bulktemperature. This loading required to produce a measurable depth of wear onwrought metals and sprayed coatings was proved by experiment to be 4Kg. Toobtain consistent results both slides and bars were lightly polished immediatelybefore testing using a very fine, dry abrasive paper. The tests reported are anincomplete selection of sprayed coatings, wrought metals and contact surfaces, butthe results provide a clear indication that, the spraying process confers propertiesthat are beneficial under conditions which result in metal-to-metal wear.

PROPERTIES OF SPRAYED COATINGS - EFFICIENCY ANDCOVERAGE1. Efficiency is used as a test to assist in establishing the optimum economicand technical deposition parameters. In general, conditions giving highdeposition efficiencies are close to those for optimum fuel utilisation. Theyare also close to those for maximum integrity.2. Factors Affecting EfficiencyEfficiency will be affected by:i) The shape and size of the componentii) The basis material and its preparationiii) The spraying parametersMeasured efficiency will be reduced when spraying onto small componentswhich are not completely within the spray stream. Even with largecomponents, overspray at edges will reduce efficiencies. Spraying atangles other than normal to the surface will reduce efficiency.Normally, efficiencies will be higher when spraying onto similar materialsand onto properly gritblasted surfaces.Deviation from the recommended spraying parameters will r educeefficiencies. This will be particularly noticeable if atomising pressures andspraying rates are increased, when the deposit volatilises easily or forms a


volatile oxide.IT IS IMPORTANT THAT DEPOSITION RATE AND FEED RATE ARENOT CONFUSED. By increasing the fuel consumption, it is possible tospray slightly faster with most materials. In combustion gas sprayingparticularly, the resultant reduction in an efficiency together with increasedfuel consumption renders the practice extremely uneconomic.3. Measurement of EfficiencyA known weight of material is sprayed under closely controlled conditionson to a suitably prepared flat plate, round bar or tube of such a size that nopart of the spray pattern will extend beyond the sample. The efficiency iscalculated as the weight gain of the sample per 100gm of material sprayed.The quoted results were measured using a flat mild steel plate 18" x 18"gritblasted to Swedish Standard SA3. The pistol was moved over thesurface in a pattern which prevented local hot spots whilst ensuring nooverspray. Efficiencies quoted are those obtained under the normallyrecommended spraying parameters.


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Thermal Spraying