Thermal Spray Coatings

Thermal spray coatings are surface coatings engineered to provide original equipment with resistance to wear, erosion, abrasion, corrosion, and oxidation. Thermal spraying is also used to repair and upgrade in-service equipment. In general, thermal spray coatings can be applied to a range of substrate materials, including metals, ceramics, plastics, and polymer composites. Such coatings often are used instead of paint because of their predictable service life, increased effectiveness, and lower life-cycle costs.

Thermal spray processes deposit finely divided metallic or nonmetallic feedstock surfacing material in a molten or semimolten condition onto a properly prepared, grit-blasted substrate to form a coating. The thermal spray feedstock material (wire, cored wire, ceramic rod, or powder) is heated to its plastic or molten state by combustion (flame, highvelocity oxygen fuel, or detonation) or by electric (arc or plasma) processes. The material is then accelerated toward the substrate. The particles or droplets strike the surface, flatten, and form thin platelets (splats) that conform, adhere, and interlock with roughened surface irregularities and with each other. As the sprayed particles impinge on the substrate, they cool and build up, particle by particle, into a lamellar-structured coating. Figure 7 in the preceding article "Surface Engineering of Cast Irons" in this Volume shows the lamellar structure of particle splats, oxide inclusions, and unmelted particles in a cross section of a typical thermal spray coating. In general, the substrate temperature can be kept at 200 °C (390 °F) or below to prevent metallurgical changes in the substrate material. Details of the thermal spray process can be found in Ref 53 and the article "Thermal Spray Coatings" in this Volume.

The properties of a thermal spray coating depend on such factors as its porosity, the cohesion between particles, adhesion to the substrate (including interface integrity), and the chemistry of the coating material. The particles bond to the substrate mechanically (the primary mechanism), metallurgically, or chemically. Particle impact velocity, particle size, substrate roughness, particle temperature, and substrate temperature influence bond strength.

Aluminum and zinc thermal spray coatings have a long history of corrosion protection in structural steel work, including:

• Radio and TV antenna masts

• Steel gantry structures

• High-power search radar aerials

• Overhead walkways

• Railroad overhead line support columns

• Electrification masts

• Traffic island posts

• Street and bridge railings

Corrosion Protection by Thermal Spraying. Thermal spray coatings are used extensively for the corrosion protection of steel and iron in a wide range of environments. Their long-term effectiveness (20 years or more) in rural, industrial, and marine environments is well documented (Ref 54, 55, 56, 57).

The selection of a thermal spray coating depends on the service environment, desired service life, operating duty cycle, and available maintenance and repair support. Tables 33 and 34 give current service-life information for thermal spray coatings in various service environments, and Fig. 22 and 23 plot the corresponding required thickness specifications. The service-life estimates for 85Zn-15Al alloy and 90Al-10MMC (metal-matrix composite) coatings--introduced in the late 1970s and 1980s, respectively—are based on accelerated laboratory tests and service applications through 1992. In a marine environment, powder spray coatings with higher aluminum contents exhibit improved corrosion resistance (Ref 55). Where resistance to wear and/or abrasion is required in addition to corrosion protection, 90Al-10MMC coatings should be considered. The 90Al-10MMC wire is composed of 90 vol% Al and 10 vol% Al2O3.

Table 33 Service life estimates for 85Zn-15Al thermal spray coatings in selected corrosive environments

Type of exposure

Coating thickness required for indicated service life

5-10 years

10-20 years

20-40 years

>40 years

in.

^m

in.

^m

in.

^m

in.

Rural atmosphere

75-125

0.003-0.005

125-175

0.005-0.007

250-300

0.010-0.012

Industrial atmosphere

150-200

0.006-0.008

300-375

0.012-0.015

350-400

0.014-0.016

Marine atmosphere

250-300

0.010-0.012

300-375

0.012-0.015

350-400

0.014-0.016

Freshwater immersion

150-200

0.006-0.008

250-350

0.010-0.014

300-375

0.012-0.015

Saltwater immersion

250-300

0.010-0.012

350-400

0.014-0.016

Table 34 Service-life estimates for aluminum and 90Al-10MMC (vol%) thermal spray coatings

5-10 years

10-20 years

20-40 years

>40 years

in.

^m

in.

^m

in.

^m

in..

Rural atmosphere

150200

0.0060.008

Industrial atmosphere

150200

0.0060.008

250300

0.0100.012

250375

0.0100.015

Marine atmosphere

150200

0.0060.008

200250

0.0080.010

250300

0.0100.012

250375

0.0100.015

Freshwater immersion

150200

0.0060.008

200250

0.0080.010

250300

0.0100.012

Saltwater immersion

200250

0.0080.010

250300

0.0100.012

300350

0.0120.014

High-temperature (100-540 °C, or 210-1000 °F)

150200

0.0060.008

200250

0.0080.010

250300

0.0100.012

Wear, abrasion, erosion, and impact (90/10 MMC preferred)

150200

0.0060.008

250300

0.0100.012

(a) With exception of wear abrasion, erosion, and impact properties, data for aluminum and 90Al-10MMC thermal spray coatings are identical.

(a) With exception of wear abrasion, erosion, and impact properties, data for aluminum and 90Al-10MMC thermal spray coatings are identical.

Fig. 22 Plot of service life versus coating thickness as a function of environment for an 85Zn-15Al thermal spray coating
Fig. 23 Plot of service life versus coating thickness as a function of environment for a 90Al-10MMC (vol%) thermal spray coating

Coatings of aluminum, zinc, and their alloys and composites provide broad atmospheric protection. Aluminum and zinc are anodic to steel and protect the ferrous substrate in electrolytic solutions. When applied sufficiently thick to prevent through-porosity, they provide both barrier and cathodic protection. When applied too thinly or when cut through to expose the underlying steel, these coatings provide galvanic protection. Aluminum corrodes less rapidly than zinc in highly acidic conditions, while zinc performs better in alkaline conditions. Aluminum thermal spray coatings immediately oxidize to form a loosely adherent Al2O3 protective film that prevents further oxidation. Thus, there is no advantage to applying aluminum coatings in a thickness greater than that which prevents through-porosity (150 to 200 pm, or 0.006 to 0.008 in.) unless wear or abrasion resistance is required in addition to corrosion resistance. Aluminum thermal spray coatings have greater wear, abrasion, and erosion resistance than zinc coatings.

Zinc alloyed with aluminum forms an effective corrosion-resistant coating, combining the attributes of both elements. The greater electrochemical activity of zinc provides greater galvanic protection than aluminum. Aluminum, with its lower electrochemical activity and a loosely adherent aluminum oxide film, provides long-term protection even when the coating is porous and gives better wear, abrasion, and erosion resistance than zinc.

Aluminum and aluminum composite thermal spray coatings can be used where the temperature is greater than 200 °C (390 °F). Aluminum composite coatings are used when resistance to wear, abrasion, and erosion is required beyond that provided by aluminum and zinc.

Wear coatings applied by thermal spraying are used to resist abrasion, erosion, cavitation, and fretting, and to reduce friction. These coatings consist of a wide range of metals and their alloys, ceramics, cermets, carbides, and even low-friction plastics. Typical coating hardness ranges from 20 to 70 HRC. Table 35 lists friction and wear (hardfacing) applications for various thermal spray materials.

Table 35 Thermal spray coatings used for hardfacing applications

Type of wear

process

Applications

Adhesive wear

Aluminum bronze

OFW, EAW, OFP, PA, HVOF

Babbitt bearings, hydraulic press sleeves, thrust bearing shoes, piston guides, compressor crosshead slippers

Soft bearing coatings:

Tobin bronze

OFW, EAW

Babbitt

OFW, EAW, OFP

Tin

OFW,EAW, OFP,

Hard bearing coatings:

Mo/Ni-Cr-B-Si blend

PA

Bumper crankshafts for punch press, sugar cane grinding roll journals, antigalling sleeves, rudder bearings, impeller shafts, pinion gear journals, piston ring (internal combustion); fuel pump rotors

Molybdenum

OFW,EAW, PA

High-carbon steel

OFW, EAW

Alumina/titania

OFP, PA

Tungsten carbide

OFP, PA, HVOF

Co-Mo-Cr-Si

PA, HVOF

Fe-Mo-C

PA

Abrasive wear

Aluminum oxide

PA

Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry); concrete mixer screw conveyors; grinding hammers

Chromium oxide

PA

(tobacco industry); core mandrels (dry-cell batteries); buffing and polishing fixtures; fuel-rod mandrels

Tungsten carbide

PA, HVOF

Chromium carbide

PA, HVOF

Ni-Cr-B-SiC/WC (fused)

OFP, HVOF

(fused)

OFP, HVOF

Ni-Cr-B-SiC (unfused)

HVOF

Surface fatigue wear

Fretting: Intended motion applications

Molybdenum

OFW, PA

Servomotor shafts, lathe and grinder dead centers, cam followers, rocker arms, piston rings (internal combustion), cylinder liners

Mo/Ni-Cr-B-SiC

PA

Co-Mo-Cr-Si

PA, HVOF

Fretting: Small amplitude oscillatory displacement applications:

Low temperature (<540 °C, or 1000 °F)

Aluminum bronze

OFW, EAW, PA, HVOF

Aircraft flap tracks (air-frame component); expansion joints and mid-span supports (jet engine components)

Cu-Ni-In

PA, HVOF

Cu-Ni

PA, HVOF

High temperature (>540 °C or 1000 °F)

Co-Cr-Ni-W

PA, HVOF

Compressor air seals, compressor stators, fan duct segments and stiffeners (all jet engine components)

Chromium carbide

PA, HVOF

Erosion

Chromium carbide

PA,HVOF

Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats

Tungsten carbide

PA, HVOF

WC/Ni-Cr-B-Si-C(fused)

OFP, HVOF

WC/Ni-Cr-B-SiC (unfused)

OFP, HVOF

Chromium oxide

PA

Cavitation

Ni-Cr-B-SiC-Al-Mo

PA

Wear rings (hydraulic turbines), water turbine buckets, water turbine nozzles, diesel engine cylinder liners, pumps

Ni-Al/Ni-Cr-B-SiC

PA

Type 316 stainless steel

PA

(fused)

OFP, HVOF

Ni-Cr-B-SiC (unfused)

HVOF

Aluminum bronze

PA, HVOF

Cu-Ni

PA, HVOF

(a) OFW, oxyfuel wire spray; EAW, electric arc wire spray;OFP, oxyfuel powder spray; PA, plasma arc spray; HVOF, high-velocity oxyfuel powder spray

(a) OFW, oxyfuel wire spray; EAW, electric arc wire spray;OFP, oxyfuel powder spray; PA, plasma arc spray; HVOF, high-velocity oxyfuel powder spray

Oxidation Protection. Thermal spray coatings are extensively used by industry to protect steel components and structures from heat oxidation at surface temperatures to 1095 °C (2000 °F). By ensuring long-term protection, thermal spray coatings show real economic advantages during the service lives of such items. Coatings such as pure aluminum, aluminum-iron, nickel-chromium, and MCrAlY are particularly effective in protecting low-alloy and carbon steels.

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