WHAT IS THERMAL SPRAY?

What it is Thermal Spray? 

thermal spraying (noun): a group of coating processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semi-molten condition to form a coating. The coating material may be in the form of powder, ceramic-rod, wire, or molten materials. (Frank J. Hermanek, Thermal Spray Terminology and Company Origins, 2001, ASM International, Materials Park, Ohio).

Brief History of Thermal Spray

In the early 1900’s, Dr. M. U. Schoop and his associates developed equipment and techniques for producing coatings using molten and powder metals. Several years later, about 1912, their efforts produced the first instrument for the spraying of solid metal in wire form. This simple device was based on the principle that if a wire rod were fed into an intense, concentrated flame, (the burning of a fuel gas with oxygen), it would melt and, if the flame were surrounded by a stream of compressed gas, the molten metal would become atomized and readily propelled onto a surface to create a coating. This process was initially referred to as metallizing. Currently the technique is known as oxy-fuel or flame spraying. Oxy-fuel methods include wire, powder (metallic and ceramic), molten metal, ceramic-rod, detonation and high velocity oxy-fuel (HVOF).

In addition to using chemical means to plasticize the input consumables, electrical currents are also used. Typically, electrical energy is used to create a heat source into which powder, and more recently wires, are fed, melted/plasticized, and conveyed onto the surface to be coated. Major commercially employed electrical methods used to construct coatings include non-transferred arc plasma, RF plasma, and wire arc.

It has long been recognized that fluids may be broken up into very fine particles by a stream of high velocity gas emanating from a nozzle. Early experiments using this atomizing approach appear to have been directed at producing metallic powders rather than coatings. It was left to Schoop to appreciate the possibility that a stream of metallic particles, formed from a molten source, could produce a coating. Myth has it that Schoop developed the concept when playing “soldiers” with his son and observing the deformation of lead pellets being fired from a toy cannon against a brick wall. Whatever the rationale, it can be stated that the pioneer work of Schoop resulted in the discovery and development of metal spraying and subsequently the “Thermal Spray Process”.

The first spray technique developed by Schoop was the outcome of experiments in which molten metal was poured into a stream of high velocity gases. Schoop’s apparatus consisted of a compressor supplying air to a heated helical tube. The heated air was used to pressurize a crucible filled with molten metal and eject it out as a fine spray that would adhere to a suitable surface. This system was bulky, primitive and inefficient; however, the concept did lead to the development of portable and user friendly equipment.

Read M. U. Schoop's patents: 

https://patents.google.com/patent/US1133507A/en?oq=1133507+

https://patents.google.com/patent/US1128059A/en

As early as 1914, Schoop in collaboration with Bauerlin, an electrical engineer, experimented with electrical heating for spraying. Initial attempts were unsuccessful as they attempted to tailor their spray apparatus on the lines of molten metal equipment rather than wire. One pole was a graphite crucible, loaded with the consumable, the other a carbon rod. An arc was struck between the crucible and the rod causing the metallic consumable to melt and flow through an orifice. On exiting, the molten metal was atomized by jets of compressed gas. Eventually, a device was built utilizing two wires, insulated from each other, made to advance and intersect at some point. Generally, the wires were given a difference of electrical potential of about 89 V that caused the wires to melt and; in the presence of a gas stream, spraying was produced. Later guns, developed by Schoop, do not radically differ from those used today.

The gun is relatively simple. Two guides direct the wires to an arcing point. Behind this point a nozzle directs a stream of high-pressure gas or air onto the arcing point where it atomizes the molten metal and carries it to the workpiece as in the graphic above. Typically, power settings of about 450 A can spray over 50 kg/hr (110 lb/hr). Electric arc spray systems are offered that feed wire by either an air or electrical motor. Some units push the wire to the gun while others pull the wire into the arc. Controls include volt and ampere meters and air regulators.

Electric arc spraying has the advantage of not requiring the use of oxygen and/or a combustible gas; it has demonstrated the ability to process metals at high spray rates; and is, in many cases, less expensive to operate than either plasma and/or wire flame spraying. “Pseudo” alloy coatings, or those constructed by simultaneously feeding two different materials, are readily fabricated. An example would be copper-tin coatings constructed by feeding pure copper and tin wires into the arc to produce a heterogeneous mixture of each in the coating. Also, the introduction of cored wires has enabled the deposition of complex alloys (such as MCrAlYs) as well as carbide-containing metal alloys that were only attainable using powdered materials as feedstock. Some materials produce “self-bonding” coatings that are sprayed in a “superheated” condition. The overheated, hot particles tend to weld to many surfaces thereby increasing the coatings’ adhesive strength.

There are no further accounts of molten metal spraying by Schoop, it appears that his efforts were directed at developing and improving powder and wire flame spraying. However, work by others continued as a 1924 Dutch patent, describing equipment for spraying low melting point metals, was granted to Jung and Versteeg. Mellowes Ltd commercialized the process in the UK. Their system consisted of a gun, a furnace, an air compressor and a fuel supply. The gun had many air and gas valves, a heating chamber (burner), nozzle, handle and a melting pot. The pot was bulky having the ability to store 1.8 kg (4 lb) of molten lead. The pot sat atop the heating chamber, which was similar in construction to a Bunsen burner. Compressed air, fed to the burner, intensified the flame. The handle jutted out and downward from the pot; it was insulated using wood and asbestos. Metal exited the pot through a front orifice where it was directed into a nozzle. Compressed air surrounded the nozzle, atomizing the molten metal and propelling it to the surface to be coated.

Read Jung and Versteeg's patents:

https://patents.google.com/patent/US1705214A

https://patents.google.com/patent/US1758878A

The concept of powder flame spray was developed by Fritz Schori in the early 1930’s. However, the amount of powder that can be supported by a gas stream depends on many factors including powder characteristics. If air is not used then the density of the supporting gas influences the feed rate and, for any particular powder there is an optimum amount that can be carried in a gaseous stream. It depends upon the velocity and volume of the gases used. The usefulness and criticality of flowmeters and pressure gauges are governing factors. 

The molten metal process has advantages and disadvantages. Advantages include: cheap raw materials; use of inexpensive gases; and, gun design is very basic. Noteworthy disadvantages are: gun is cumbersome to use in the manual mode, can only be held in a horizontal plane; high maintenance due to high temperature oxidation and molten metal corrosion; and, useful only with low melting temperature metals.

Uses for the molten metal thermal spray process include the fabrication of molds, masks and forms for the plastics industry, using low melting point bismuth based alloys (the Cerro family of alloys); the deposition of solder alloys to joints that would be coalesced using torches or ovens; and, the production of metal powders.

Thermal Spray Methods

Flame Spray (powder, wire, rod)

Electric Arc Spray (wire)

High Velocity Oxy-Fuel (HVOF)/ Oxy-Liquid-Fuel 

Hybrid-Low Velocity Oxy Fuel (CERAJET)

Plasma Spray/ Vacuum Plasma Spray

Suspension plasma spray 

Solution Precursor Plasma Spray

Detonation Flame Spraying 

Cold Spray


Raw Materials and Applications

Basic thermal spray coating applications and their functions may be listed as follows:

Build-up and reclamation, Wear resistance, abrasive, adhesive, fretting, erosion, cavitation, Clearance control, abradable, abrasive, Thermal barrier, Environmental, high temperature oxidation and corrosion resistance, atmospheric corrosion control, Electrical conductivity and resistivity, Biomedical

Build-Up and Reclamation

The earliest commercial applications for thermal sprayed coatings, performed over seventy-five (75) years ago, were for repair and maintenance. Components worn or corroded were coated, machined and returned to service thereby saving the costs of replacement. Coatings for dimensional restoration are selected for their similarity and compatibility to the base metal rather than their ability to improve wear resistance. Selection is based on likeness in chemistry, color and performance. Galvanic corrosion is avoided by matching base metal chemistry especially with copper, aluminum and magnesium alloy parts. Self-bonding underlayments for surface preparation are seldom used on either aluminum or magnesium parts and never on copper, but are frequently used on iron, steels and superalloys.

Consumables, based upon base metal composition and service requirements, often used to repair machine element components include:

· Pure aluminum

· Aluminum-silicon alloy

· Aluminum-iron-chrome-nickel composite

· Aluminum bronze

· Pure copper

· Copper-nickel alloy (Monel)

· Iron-chrome-aluminum-molybdenum composite

· Iron-aluminum-molybdenum-carbon-boron composite

· Iron-nickel-aluminum composite

· Iron-nickel-aluminum-molybdenum composite

· 304 stainless steel

· 316 stainless steel

· 410 stainless steel

· 420 stainless steel

· 431 stainless steel

· 17-4 PH

· Incoloy 800

· Incoloy 909

· Low carbon steel

· Pure molybdenum

· Nickel-aluminum alloys and composites

· Nickel-chrome-aluminum alloys and composites

· Nickel-chrome-aluminum-molybdenum-iron composites

· Nickel-chrome-aluminum-molybdenum-silicon-boron-iron-titania composites

· Nickel-chrome-iron alloy

· Various Stellites

· Inconel 625

· Inconel 718

· René 41

· René 80

· René 95

 Clearance Control Coatings

Clearance control systems or gas path seal coatings are those used in selective areas of gas turbine engines to maintain tight tolerances between rotating and static parts. This is best accomplished when the rotating member cuts a path into the static component. Typically, the static member is coated with an abradable material while the rotating part is coated with a hard, abrasive material. The rotating member functions like a grinding wheel.

The abradable component will exhibit good adhesion and erosivity; be easily rubbed with the rub surface being smooth; and, lastly, debris should not be detrimental to the engine’s overall performance.

The deposition of abradable coatings is particularly suited to thermal spraying based upon current knowledge of parameter interactions. This awareness permits the deposition of coatings with predetermined density levels vital for them to be highly abradable without causing damage to the incurring member.

Coatings for engine cold sections (<1200°F [649°C]), low (fan) and high-pressure compressor (HPC), are generally applied over a nickel-aluminum bond coat. Abradable products include:

· Commercially pure aluminum

· Aluminum-silicon alloys

· Aluminum quasicrystal alloy

· Nickel graphite composites

· Nickel-aluminum alloys and composites

· Silicon-aluminum graphite composites

· Aluminum-bronze graphite composites

· Silicon-aluminum+polyester blends

· Silicon-aluminum+polyimide blends

· Aluminum-bronze+polyimide blends

· Nickel-chrome+polyester blends

· Nickel-chrome+polyurethane blends

· Nickel-chrome+bentonite blends

· Nickel-chrome-aluminum/bentonite blends

· Nickel-chrome+boron nitride blends

· Nickel-chrome+hollow spheres blends

· Nickel-chrome-iron+boron nitride blends

At the rear of the engine, in the high pressure and low-pressure turbine (HPT and LPT) sections, temperatures are very hot, much greater than 1200°F (649°C), necessitating the use of high materials which are easily rubbed. High temperature abradables include:

· MCrAlY type alloys

· Exothermic MCrAlY’s

· Nickel-chrome-aluminum/bentonite blends

· Yttria-zirconia + polyester blends

As with front-end abradables the abrasives deposited onto hot section components are generally applied over a nickel-aluminum composite bond coat. However, unlike the abradables the abrasives are dense. Abrasive materials include:

· Nickel clad alumina

· Nickel-chrome clad alumina

· Nickel-chrome+clad alumina blends

· Nickel-chrome-aluminum+alumina blends

 Electrical Conductivity and Resistivity

Electrical conductivity

Materials for electrical conductivity include:

· Aluminum

· Copper

· Silver

Electrical resistivity

This electrical application is much more positive, from a marketing stand point, that those addressing conductivity. Coatings designed to be nonconducting exhibit high density with good inter particle cohesion. The best material for this application is high purity alumina.

 Environmentally Protective Coatings

Environments that thermal spray coatings may experience vary in temperature extremes as well and their corrosive nature. The former may be below freezing to red hot; the latter from mildly/highly caustic to mildly/highly acidic.

Environmentall protective coatings can therefore be separated into two (2) major categories:

· High temperature oxidation and hot corrosion resistance

· Atmospheric corrosion control

High temperature and hot oxidation resistance

All thermal sprayed claddings should exhibit some degree of corrosion resistance. However, there are many applications where the coating is solely intended to offer high temperature oxidation/corrosion protection. Materials are selected based upon their ability to function as a barrier between the corrosive environment and the substrate. Corrosion occurs slowly so the coatings are sacrificed to protect the substrate. It is important that their densities be high so that sealers are not required. In some instances, coating density may be intensified by Hot Isostatic Pressing (HIPping).

Materials for high temperature use include:

· CoCrAlY

· CoNiCrAlY

· FeCrAlY

· NiCrAlY

· NiCoCralY

· Exothermic MCrAlY’s

· Nickel-chromium alloys

· Inco 718

· IN-625

· René 41

Atmospheric corrosion control

Flame sprayed coatings of wire aluminum and zinc are the most common thermal spray answers to atmospheric and marine corrosion. Both are anodic to steel.

Zinc provides extends the service life of steel by twenty to thirty (20 to 30) times. Its corrosion products are friable and easily removed thereby presenting an unprotected open surface.

Aluminum protects somewhat differently. Its corrosion product is more tenacious and inert. Internal pores fill with oxide products to prevent the progression of rust. Either coating will provide a steel component with:

· Longer life

· Compatibility with many paints and sealers

· Resistance to mechanical damage

· Resistance to ultraviolet light

· Can usually be applied in-situ

· Do not sag or run

· Can be applied thicker than competitive coatings

Materials for atmospheric and marine protection include:

· Commercially pure aluminum

· Pure zinc

· Zinc-aluminum alloys

Aluminum protects somewhat differently. Its corrosion product is more tenacious and inert. Internal pores fill with oxide products to prevent the progression of rust. Either coating will provide a steel component with:

· Longer life

· Compatibility with many paints and sealers

· Resistance to mechanical damage

· Resistance to ultraviolet light

· Can usually be applied in-situ

· Do not sag or run

· Can be applied thicker than competitive coatings

Materials for atmospheric and marine protection include:

· Commercially pure aluminum

· Pure zinc

· Zinc-aluminum alloys

Metal/Ceramic Matrix Composites

Metal/ceramic matrix composites (MMC/CMC)

Reinforcing metals and/or ceramics with fibers offers potential for improving their overall mechanical properties when used without the fiber reinforcement.

MMC’s are constructed by applying a metallic coating over metal windings. It is a layering technique where the coating and winding filaments are dispersed throughout the structure. The freestanding structure is a composite with properties greater than the input materials.

CMC have been constructed using sol gel powder composites consisting of fibers blended with a refractory oxide.

Powders for either application are based upon research demands.

For more information, contact author Frank J. Hermanek, thermal spray engineering consultant, Indianapolis, Indiana USA, email: fhermanek@aol.com

 Thermal Barrier Coatings

Thermal Barrier Coatings (TBC) are thermally insulating coating systems protecting the substrate from the hotter temperatures of the surrounding environment. They are used in heavy diesel engines, some gasoline powered engines and in both aero and stationary gas turbines. When properly applied they can provide a 300°F (149°C) temperature difference between their outer surface and their base metal interface.

TBC’s are complex coating systems consisting of two (2) or more layers of sprayed material. The initial coating deposited onto the substrate is generally an MCrAlY metallic alloy, performing the function of a bond coat while also offering hot corrosion and oxidation protection. Selection is based on how its coefficient of thermal expansion matches that of the host metal. Subsequent layers may be wholly refractory oxides or blends of the MCrAlY with the ceramic component.

Typical bond coat materials for TBC’s include:

· CoCrAlY

· CoNiCrAlY

· FeCrAlY

· NiCrAlY

· NiCoCralY

· MCrAlY modifications with silicon, platinum, yttrium, tantalum

· Exothermic MCrAlY’s

· Nickel-chromium alloys

Metallic oxides used for the insulating layer are:

· 22% magnesia stabilized zirconia

· 6% yttria stabilized zirconia

· 12% yttria stabilized zirconia

· 20% yttria stabilized zirconia

 Underlayments and Bond Coats

Primer thermal sprayed underlayments, useful as surface preparation tools, perform an anchoring function for subsequent overlayments. They are typically, but not always, “self-bonding”, that is they are metallurgically bonded to the substrate in the as-sprayed condition. Self-bonding materials include:

· Pure molybdenum

· Pure tantalum

· Pure niobium

· Nickel-aluminum alloys and composites

· Nickel-chrome-aluminum alloys and composites

· Nickel-chrome-aluminum-cobalt-yttria composites

· Nickel-chrome-aluminum-molybdenum-silicon-boron-iron-titania composites

· Nickel-chrome-aluminum-molybdenum-iron composites

· Exothermic MCrAlY’s

Mechanical or non-metallurgical bond coats include:

· Nickel-chrome alloys

· Nickel-chrome-iron alloys

Biomedical coatings

Human bone and dental implants currently used are fabricated from Vitallium (a cobalt alloy), titanium Ti6-4 and some sintered ceramics. While these products excellent biocompatibility and high strength their surface finishes do not promote tissue adherence and/or growth. Rather an adherent, coarse, porous structure is required. Materials selected for attachment to mammalian implant should be:

· Biocompatible with the host, · Adherent to the implant, · Sensitive to application parameters to control adhesion and density, · Promote tissue adhesion and growth

Current used materials, meeting these criteria include:

· High purity titanium

· Titanium-6 aluminum-4 vanadium alloy

· Vitallium, cobalt base alloy (V-75)

· High purity alumina

· Hydroxyapatite (HA)

Specifications and Standards 

 

United States Published Standards

 

United States Standards In Preparation


British/International Standards

This listing of Thermal Spray related Standards has been provided courtesy of the Thermal Spraying and Surface Engineering Association, Ivor Huff, Secretary. Holding the secretariat of the BSI committee STI/40 the TSSEA continues to provide UK input to the European Committee, CEN/ TC 240. This committee is comprised of spokesman/delegates representing each of the member states in the community.

ISO are informed of CEN proposals and they may choose to participate in their development by making comments at each stage of the discussions. Such a dual document is finally subject to parallel voting. When finally approved, the standard will be identified in the UK as BS EN ISO. Others are designated BS EN followed by the number.

ISO standards are available from ASM's Standards Store; all others can be purchased through the TSSEA web site TSSEA members receive a 50% discount; add 10% handling charge, minimum £3.00)

BS EN 582: 1993 Determination of tensile strength.

BS EN 657: 1994 Terminology, classification. (Revision in progress)

BS EN 22063: 1994 Metal and other inorganic coatings - Thermal Spraying - Zinc, Alumium and their alloys. (Updated standard to be available in the near future)

BS EN 1395: 1996 Acceptance Inspection of Thermal Spraying Equipment

BS EN 1274: 1997 Powders - Composition - Technical supply conditions. (Revision in progress)

BS EN ISO 14920 Spraying and fusing of thermally sprayed coatings of self fluxing alloys.

BS EN ISO 14918: 1998 Approval Testing of Thermal Sprayers.

BS EN ISO 14922 - 1, 2, 3, 4: 1999 Quality requirements of thermally sprayed coatings Parts 1, 2, 3, 4

BS EN ISO 14919: 2001 Wires, Rods and Cords for Flame and Arc spraying

BS EN ISO 14921: 2001 Procedures for the application of thermally sprayed coatings for engineering components

BS EN 13214: 2001 Tasks and responsibilities

BS EN 13507: 2001 Pre Treatment of surfaces of metallic components for thermal spraying.

Quality requirements of thermally sprayed coatings Parts 1, 2, 3, 4 BS EN ISO 14919: 2001 Wires, Rods and Cords for Flame and Arc spraying BS EN ISO 14921: 2001 Procedures for the application of thermally sprayed coatings for engineering components BS EN 13214: 2001 Tasks and responsibilities BS EN 13507: 2001 Pre Treatment of surfaces of metallic components for thermal spraying. Quality requirements of thermally sprayed coatings Parts 1, 2, 3, 4 BS EN ISO 14919: 2001 Wires, Rods and Cords for Flame and Arc spraying

BS EN ISO 14921: 2001 Procedures for the application of thermally sprayed coatings for engineering components BS EN 13214: 2001 Tasks and responsibilities BS EN 13507: 2001 Pre Treatment of surfaces of metallic components for thermal spraying.

Note that existing standards are subject to review, normally after 5 years.

Other topics which are still in public enquiry or discussion stages include:


Thermal spray processes used by various industrial segments 

Thermal spray coating applications according to industry served 

Industrial use of gas metallic materials