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)

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

Materials for electrical conductivity include:

· Aluminum
· Copper
· Silver

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.

Environmental protecting 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.
Environmental protective coating 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

Putting aside PTA (Plasma Transferred Arc) and Laser cladding there are only two (2) commercially viable thermal spray processes. These are: flame spraying and plasma (non-transfer) arc. Historically, knowing which coatings and their in-put raw materials, perform best in specific applications, enables the researcher to categorized the materials. Based upon known industrial applications/functions the materials can be further identified according to chemical composition. This effort is an overview of coating applications and the suitability of sprayed coatings to the functions.

Basic thermal spray coating applications and their functions may be listed as follows:
Underlayments and bond coats
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
Metal and 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: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

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