Coating Materials

The nonmetallic, inorganic materials used as ceramic coatings have several characteristics in common. Among these are relatively good chemical stability at elevated temperatures, hardness, brittle behavior under load, and mechanical continuity in thin cross section.

Silicate Glasses. Coatings prepared from glass powders, with or without additions of refractory compounds, have the greatest industrial usage of all ceramic coatings. Glass is typically manufactured by mixing specific proportions of minerals, heating the mixture to a molten or liquid state, and rapidly quenching. Quenching is normally accomplished by discharging the melt into cold water or by pouring it through water-cooled steel rolls. The former case results in small friable pieces of glass that can be ground into a powder with relative ease using a ball mill or other standard crushing equipment. These friable pieces are called frit. The word refers to the physical condition of the particle, not its composition or properties. Quenching through water-cooled steel rolls results in flake-like particles, somewhat less friable than those produced by water quenching but with less environmental nuisance.

Glass coatings are used for such long-duration elevated-temperature applications as aircraft combustion chambers, turbines and exhaust manifolds, and heat exchangers. Variations in composition of the glass are virtually unlimited. They range from alkali-alumina borosilicate glasses, which are relatively soft, low melting, and highly fluxed, to barium crown glasses.

Crystallized glass coatings have been developed. In these coatings, crystallization of the glass is controlled by formulation and heat treatment and by the presence of nucleating agents added to the glass during melting.

Several different refractory materials may be combined with glass to produce satisfactory coatings for elevated-temperature service. The addition of a refractory material depends on service requirements and on the compatibility of the refractory material with the glass, other mill-added materials, and the substrate metal. Raw batch compositions of glasses, their melted compositions, and slips (mixtures of frit and additives ground to a smooth consistency) for high-temperature service coatings are indicated in Tables 1, 2, and 3, respectively.

Table 1 Compositions of unmelted frit batches for high-temperature service silicate-based coatings

Constituent

Parts by weight for specific frits(a)

UI-32

UI-285

UI-346

UI-418

NBS-11

NBS-331

NBS-332

Quartz

29.3

21.2

18.3

31.2

18.0

38.0

37.5

Feldspar

42.0

30.2

47.4

31.0

Hydrated borax

28.9

21.0

17.9

37.1

Sodium carbonate

7.7

5.3

6.1

5.9

Sodium nitrate

5.0

4.0

4.4

3.8

Fluorspar

4.5

3.2

2.8

3.0

Tricobalt tetroxide

0.6

0.4

0.5

Nickel oxide

0.6

0.4

0.6

Manganese dioxide

1.8

1.1

1.1

Barium carbonate

26.3

56.6

56.6

Zinc oxide

4.2

5.0

5.0

Whiting

7.5

7.1

6.3

Vanadium pentoxide

1.3

Aluminum hydrate

15.1

1.5

Boric acid

12.0

11.5

11.5

Cerium oxide

4.2

Titania

4.2

Bismuth nitrate

4.2

Bismuth oxide

6.2

Beryllia

2.5

Zirconia

2.5

(a) UI numbers designate frit compositions developed at the University of Illinois; NBS numbers, frits developed at the National Bureau of Standards (now the National Institute of Standards and Technology, NIST)

Table 2 Compositions of melted silicate frits for high-temperature service ceramic coatings

Constituent

Percentage for specific frits(a)

UI-32

UI-285

UI-346

UI-418

NBS-11

NBS-331

NBS-332

Silicon dioxide (SiO2)

56.5

51.1

57.0

37.0

42.0

38.5

37.5

Aluminum oxide (Al2O3)

8.4

19.8

10.7

5.6

1.0

Boron oxide (B2O3)

10.6

9.5

8.0

8.0

25.8

6.5

6.5

Sodium monoxide (Na2O)

12.0

10.9

10.8

17.9

Potassium monoxide (K2O)

5.1

4.6

6.7

3.4

Calcium fluoride (CaF2)

4.5

4.1

3.4

3.0

Cobalt oxide (CoO)

0.5

0.45

0.5

Nickel monoxide (NiO)

0.6

0.45

0.6

Manganese dioxide (MnO2)

1.8

1.3

1.2

Vanadium pentoxide (V2O5)

1.2

Bismuth dioxide(BiO2)

10.0

Calcium oxide (CaO)

5.0

3.5

3.5

Barium oxide (BaO)

25.0

44.0

44.0

Zinc oxide (ZnO)

5.0

5.0

5.0

Cerium dioxide (CeO2)

5.0

Titanium dioxide (TiO2)

5.0

Beryllium oxide (BeO)

2.5

Zirconium oxide (ZrO2)

2.5

(a) UI numbers designate frit compositions developed at the University of Illinois; NBS numbers, frits developed at the National Bureau of Standards (now the National Institute of Standards and Technology, NIST)

(a) UI numbers designate frit compositions developed at the University of Illinois; NBS numbers, frits developed at the National Bureau of Standards (now the National Institute of Standards and Technology, NIST)

Table 3 Compositions of slips for high-temperature service silicate-based ceramic coatings

Constituent

Parts by weight for specific coatings(a)

UI-32-22

UI-32-53

UI-285-1

UI-346-2

UI-346-4

UI-418-1

UI-418-4

A-19H

A-418

A-417

A-520

Frit(b)

88(c)

100(c)

100(d)

100(e)

100(e)

100(f)

100(f)

100(g)

70(h)

70(i)

90(i)

Diaspore

12

15

Clay

7

10

7

7

10

7

6

10

5

5

6

Hydrated borax

0.75

0.5

0.75

0.75

0.5

0.75

Water

50

50

50

55

50

50

50

50

48

48

45

Sodium pyrophosphate

0.05

0.05

Calcined aluminum oxide

25

25

25

Tricobalt tetroxide

1

Citric acid

0.05

Chromic oxide

25

30

30

Copper oxide

10

Sodium nitrite

0.025

(a) UI numbers designate coatings developed at the University of Illinois; A numbers, coatings developed at the National Bureau of Standards (now the National Institute of Standards and Technology).

(a) UI numbers designate coatings developed at the University of Illinois; A numbers, coatings developed at the National Bureau of Standards (now the National Institute of Standards and Technology).

(b) See Tables 1 and 2 for batch and melted compositions of frits.

Glass coatings can be applied by spraying or air brushing (for which the material is atomized and carried by compressed air), dipping and draining (which may be followed by spraying), slushing and draining, filling and draining, and flow coating. Under certain conditions, electrostatic spraying also can be used.

Spraying is the most commonly used method, except when the configuration of the part prevents complete coverage or when production requirements are great enough so that a saving in material costs would be realized by the use of dipping or slushing. Dipping and draining is the most economical procedure for coating small parts with simple shapes. For larger parts with restricted areas, filling and draining may be the best coating method. Rotation or shaking of parts is often necessary when dipping or filling is used to distribute the coating and obtain uniform draining.

Glass coatings are brittle, but when applied at the usual thickness of 25 to 50 /'m (1 to 2 mils), they will withstand considerable abuse, even at edges and unavoidable sharp corners. Mechanical roughening of the metal surface before applying the coating provides a greater surface area, affects the rate of oxidation, and frequently improves adherence. After application, coatings are dried at slightly elevated temperatures and are subsequently fired at higher temperatures to provide them with the desired performance and appearance characteristics.

Another type of coating for high-temperature service is a combination of glass and metal called cermet. A high proportion of metal powder (e.g., aluminum) is added after the glass has been made into a slip (see above). These coatings can be exposed to temperatures above the original firing temperature. Small steel sheets coated with this type of coating can be quenched red hot in water without chipping or blistering of the surface. Appropriate applications are heat exchangers, exhaust systems for internal combustion engines, marine service, structural members and panels for vehicular tunnels, and fire boxes of central heating boilers. Additional information is available in the section "Cermets" in this article.

Oxides. Coatings based on oxide materials provide underlying metals, except refractory metals, with protection against oxidation at elevated temperatures and with a high degree of thermal insulation. Flame-sprayed oxide coatings do not provide refractory metals with the necessary protection against oxygen because of their inherent porosity. Oxide coatings can be readily applied in thicknesses up to 6.4 mm (0.25 in.), but their resistance to thermal shock decreases with increasing thickness.

Alumina (Al2O3) and zirconia (ZrO2) are the oxides most commonly used as coatings. Alumina coatings are hard and have excellent resistance to abrasion and good resistance to corrosion. Zirconia is widely used as a thermal barrier because of its low thermal conductivity.

Table 4 lists the principal oxides used for coatings and gives their melting points. Basic oxide is the major constituent of an oxide coating, usually being present in excess of 95 wt%. Other materials, such as calcium oxide (CaO), chromium oxide (Cr2O3), and magnesium oxide (MgO), are added in small percentages for stabilization, increase of as-sprayed density, modification of surface-emittance characteristics, and improvement of resistance to thermal shock. Physical properties for alumina and zirconia coatings flame sprayed from rod are given in Table 5.

Table 4 Melting points of principal oxides used in ceramic coatings

Oxide

Melting point

°C

°F

Aluminum oxide (AI2O3)

2070±28

3760±50

Aluminum titanate (Al2O3TiO2)

1860

3380

Beryllium oxide (BeO)

2570±84

4660±150

Calcium zircanate (CaZrO3)

2345

4250

Cerium oxide (CeO2)

Over 2600

Over 4710

Chromium oxide (Cr2O3)

2265±110

4110±200

Cobalt oxide (CoO)

1805±56

3280±100

Forsterite (2MgOSiO2)

1885

3425

Hafnium oxide (HfO2)

2900±110

5250±200

Magnesium oxide (MgO)

2S50±2S

5165±50

Mullite (3Al2O3 SiO2)

1S10

3290

Nickel oxide (NiO)

19S0±110

3600±200

Silicon dioxide (SiO2)

1720±14

3130±25

Spinel (MgOAl2O3)

2135

3S75

Thorium oxide (ThO2)

3220±56

5S30±100

Titanium oxide (TiO2)

1S70±2S

3400±50

Uranium oxide (UO2)

2S75±56

5210±100

Yttrium oxide (Y2O3)

2455±56

4455±100

Zircon (ZrO2SiO2)

1775±11

3225±20(a)

Zirconium oxide (ZrO2)

2710±110

4910±200

(a) Decomposes

Table 5 Physical properties of alumina and zirconia flame sprayed from rod

Coating

Bulk density

Porosity, %

Color

Typical compressive strength

Thermal expansion(a)

Thermal conductivity(b)

g/cm3

lb/in.3

MPa

ksi

^m/m^K

^in./in. • °F

W/mK

Btuin./ft2h°F

Alumina

3.3

0.12

8-12

White

255

37

7.4

4.1

33

19

Oxide coatings are usually applied by the flame or plasma-arc spraying methods. Before spraying by either method, the substrate surface should be clean and rough; abrasive blasting provides a satisfactory surface condition. Sprayed coatings usually range in thickness from 25 to 2500 pm (1 to 100 mils).

Flame spraying, using either oxyhydrogen or oxyacetylene systems, can deposit any refractory oxide whose melting point is below 2760 °C (5000 °F). However, certain refractory oxides, particularly silicon dioxide, do not spray well even though their melting points are considerably below 2760 °C (5000 °F).

An oxidation-resistant nickel chromium alloy often is applied to the substrate before an oxide coating is deposited by flame spraying. Without such a base coat, the adhesion of the oxide may be inadequate. Coating rates during flame spraying are slow, usually in the range of 16 to 410 cm3/h (1 to 25 in.3/h).

All oxides that can be flame sprayed and those with higher melting points can be applied by plasma spraying. In general, plasma spraying produces coatings of greater density (porosity of sprayed oxide coatings ranges from 5 to 15%, depending on method of application), greater hardness, and smoother finish than those obtained by flame spraying. Also, the temperature of the substrate remains lower, because deposition is faster. Because of the inert gases used during plasma spraying, oxidation of the substrate is minimized.

In addition to spraying, any of the oxides may be applied by troweling. Troweled coatings usually are thicker than sprayed coatings and are designed to provide maximum thermal protection to the underlying metal. A bonding medium, such as sodium silicate, calcium aluminate, phosphoric acid, or glass, is used for coatings applied by troweling. In addition, the use of expanded-metal reinforcements greatly improves troweled coatings.

Carbides as ceramic coatings are principally used for wear and seal applications, in which the high hardness of carbides is an advantage. These applications include jet engine seals, rubber-skiving knives, paper machine knives, and plug gages. Carbide coatings for wear resistance are applied by flame spraying or detonation-gun techniques. Table 6 gives the melting points of ten carbides.

Table 6 Melting points of carbides

Carbide

Melting point

°C

°F

Boron carbide (B4C)

2470

4480

Chromium carbide (Cr3C2)

1900

3440

Niobium carbide (NbC)

3480

6295

Hafnium carbide (HfC)

3890

7030

Molybdenum carbide (Mo2C)

2410

4375

Silicon carbide (SiC)

2540

4605

Tantalum carbide (TaC)

3980

7200

Titanium carbide (TiC)

2940

5325

Tungsten carbide (WC)

2790

5050

Zirconium carbide (ZrC)

3400

6150

Silicides are the most important coating materials for protecting refractory metals against oxidation. Silicide-based coatings protect by means of a thin coating of silica that forms on the coating surface when heated in an oxygen-containing atmosphere. To improve the self-healing, emittance, chemical stability, or adherence of this thin silica coating, other elements, such as chromium, niobium, boron, or aluminum, are added to the coating formula.

Table 7 lists and describes several silicide coatings. These materials are usually applied to a substrate by some variation of the vapor-deposition process. Deposition, diffusion, and reaction of silicon (and any other elements added in small quantities) with the substrate metal at a high temperature produce the silicide-based coating.

Table 7 Silicide coatings for protection of refractory metals against oxidation

Constituents of as-applied coating

Suitable substrate metal

Oxidation protection(a)

Application

Temperature

Life, h

Method

Thickness

Silicide

Additives

°C

°F

mils

Molybdenum silicide (MoSi2)

None

Mo-0.5 Ti

1480

2700

10

Fluidized bed

25-50

1-2

Nb

Mo-0.5 Ti

1540

2800

12

Pack cementation

75

3

Cr, Al

Mo-0.5 Ti

1540

2800

8

Pack cementation1^"1

60

2

Cr

Mo-0.5 Ti

1480

2700

36(c)

Pack cementation

60

2

Cr, Al, B, Nb, Mn

Mo

1540

2800

19-45

Pack cementation

60-100

2-4

Niobium silicide (NbSi2)

None

Nb-33 Ta-0.8 Zr

1480

2700

3

Fluidized bed

25-50

1-2

Nb-10 Ti-10-10 Mo

1425

2600

15-25

Pack cementation

50

2

Cr, Ti

Nb-10 Ti-10 Mo

1370

2500

Over 100

Vacuum pack(b)(d)

100

4

Cr, B

Nb-10 Ti-10 Mo

1370

2500(e)

Over 15(e)

Pack cementation(b)

50

2

Niobium silicide (NbSi2),

Nb

1370

2500

396

Pack cementation

Over

Over

(NbAl3)(f)

150

6

Tantalum silicide (TaSi2), plus others®

Ta

1370

2500

275

Pack cementation

Over 150

Over 6

(a) Representative data only; can vary depending on test conditions.

(a) Representative data only; can vary depending on test conditions.

(b) Multiple-cycle processing.

(d) Variation of pack-cementation process; pack is elevated to remove residual air before heating.

(e) Life of coating system is at least 10 h at 1425 °C (2600 °F).

(f) Proprietary

Vapor-deposited and diffused silicide coatings are characterized by their superior adhesion to the substrate. Fairly precise control of coating thickness is obtained through this process. Uniform silicide coatings with a thickness of a few tenths of a mil to several mils are produced on both simple and complex shapes by either the pack-cementation or the fluidized-bed technique.

The slurry fusion process is the most commonly used method for the deposition of silicide coatings on refractory metals. A slurry of fine silicon powder with desired additives (iron, chromium, hafnium, or titanium) in an organic liquid is applied to the part by dipping, spraying, or brushing. The coated part is heated in a vacuum or inert atmosphere at 1300 to 1400 °C (2370 to 2550 °F) for 30 to 60 min. An excellent coating-to-substrate bond is developed.

Because they are more brittle than the substrate metals, silicide coatings are highly susceptible to crack formation, which can act as a stress raiser on the substrate. In general, silicide coatings have an adverse effect on all room-temperature mechanical properties of the substrate; the thicker the coating, the greater the effect. Silicide coatings generally embrittle the metals to which they are applied, but they do not necessarily impair the usefulness of the coated metals for structural applications.

Phosphate-Bonded Coatings. Phosphates for metal protective coating systems are formed by the chemical reaction of phosphoric acid and a metal oxide such as aluminum oxide, chromium oxide, hafnium oxide, zinc oxide, and zirconium oxide. The phosphate-bonded materials are used to protect metals against heat and to act as a binder in thin ceramic paint films. Thicker composites are troweled, rammed, or sprayed to the desired thickness. Phosphate-bonded coatings have low density, low thermal conductivity, and high refractoriness after curing in place at temperatures ranging from 21 to 425 °C (70 to 800 °F), and they can be applied in greater thicknesses than other ceramic coatings. Thus, a thick refractory composite can be used to protect lower-temperature-resistant metal systems. Phosphate-bonded composites, depending on composition, withstand temperatures up to 2425 °C (4400 °F) and have been applied in thicknesses up to 50 mm (2 in.).

Reinforcements, bonded or welded to the metal substrate, usually are used within phosphate-bonded coatings to facilitate bonding to the substrate and to provide resistance to vibration and impact. Reinforcements are corrugated metal screen, expanded metal, open metal strips, and metal and nonmetallic honeycomb.

When phosphate-bonded composites are prepared, one of the strongest bonds between the metal oxide particles is obtained with 85% orthophosphoric acid (H3PO4). However, composites bonded with orthophosphoric acid have lower maximum service temperatures than composites formed by the reaction of metal oxide and fluorophosphoric acid (H2PO3F). The use of fluorophosphoric acid also permits the use of lower curing temperatures.

After preparation, the composites are aged for 24 h or more to permit reaction between the acid and the metal oxide. The aged composite is troweled either directly onto the substrate or over another protective coating. The coating is then cured with close control of time and temperature. Oxides bonded with orthophosphoric acid are cured for 1 h at each of the following temperatures successively: 93, 120, 150, 215, 315, and 425 °C (200, 250, 300, 420, 600, and 800 °F). Oxides bonded with fluorophosphoric acid are cured for 3 h at room temperature and then for 1 h at 120, 150, and 205 °C (250, 300, and 400 °F). Table 8 identifies several common phosphate-bonded coatings and gives their densities and maximum service temperatures.

Table 8 Characteristics of phosphate-bonded ceramic coatings

Type of phosphate

Constituents

Density

Maximum service temperature

kg/m3

lb/ft3

°C

°F

Aluminum

85% H3PO4 + Al2O3

3040-3600

190-225

1925

3500

H2PO3F + Al2O3

3040-3600

190-225

1980

3600

Hafnium

85% H3PO4 + HfO2

4490-4810

280-300

1925

3500

H2PO3F + HfO2

4490-4810

280-300

2205

4000

Zinc

85% H3PO4 + ZnO

1650

3000

Zirconium

85% H3PO4 + ZrO2

3200-4650

200-290

1925

3500

A reaction between the acidic coating and the substrate may cause bloating or blistering upon deposition or after initial curing as the result of the release of hydrogen from the acid. The volatilization of phosphorus pentoxide (P2O5), a decomposition product of the acid, also can cause blistering.

Various compounds, such as chromic oxide, ammonia compounds, or ferric phosphate, are added to the coating materials to prevent phosphorus pentoxide from corroding the substrate. These additives increase the pH of the coating without affecting the bonding action. Chromic acid may also be added to improve heat emission of the coating. Coatings are usually thixotropic and appear to have a greater viscosity than is actual because slight agitation causes the material to flow.

Coatings are formulated to possess optimum physical and thermal properties. Particle size and filler-to-binder ratio have a great influence on the final properties, including shrinkage, resistance to thermal shock, bond strength, porosity, and thermal conductivity. The common range of particle size for phosphate-bonded coatings is -14 to -325 mesh.

Phosphate-bonded coatings are used primarily to prevent deterioration of the substrate metal during high-temperature service. Applications include combustion-chamber linings, re-entry leading edges, hot gas ducts, and high-temperature insulation repairs.

Cermets. Table 9 lists the constituents of electrodeposited coatings based on cermets and indicates thicknesses, service life at elevated temperatures, and suitable substrates for these materials. Electrodeposited cermet coatings currently have only a few commercial applications.

Table 9 Cermet electrodeposited coatings for high-temperature oxidation protection

Constituents of coating as applied

Suitable substrate metal

Service temperature

Service life, min

Thickness

°C

°F

fini

mils

Cr + ZrB2

Mo-0.5Ti

2130

3S65

20

75-150

3-6

Tantalum

2130

3S65

20

75-150

3-6

Tungsten

2205

4000

10

75

3

Pt-Rh + ZrB2

Tungsten

2S70

5200

1

510-760

20-30

Cermet coatings, consisting of a mixture of metal and ceramic oxides, protect metallic substrates against oxidation and erosion. The electrodeposition process used for applying these coatings is a combination of electroplating (for metals) and electrophoresis (for ceramics). The amount of ceramic that can be deposited depends on particle size, density, and composition. Ceramic particles ranging in size from less than 1 pm to 44 pm (40 to 1730 pin.) can be plated. These particles are suspended in any common electroplating bath by agitation. With ordinary procedures, coatings containing about 20 wt% ceramic can be obtained in a deposit; with special procedures, this can be increased to 50 to 60 wt%. Because most cermet coatings are for erosion-resistance applications such as rocket nozzles, coatings are relatively thick (>75 /'m or >3 mils). Thinner coatings can be obtained, however, and thickness can be controlled to 25 pm (1 mil).

Cermets applied by plasma spraying or detonation gun processes are the basis for increasing the wear resistance of metals and superalloys. The most important cermets are metal-bonded carbides and borides, especially tungsten carbide with 8 to 15% Co. At the lower cobalt content, high hardness and wear resistance are produced. Increasing the cobalt content increases the toughness necessary for wear plus impact service. Tungsten carbides wear well to about 590 °C (1000 °F) in air. At higher temperatures, chromium carbide and certain nickel-chromium alloys are used because of self-lubricating qualities. Coatings based on aluminum oxide, refractory carbides, and an oxidation-resistant metallic binder are in use at temperatures above 870 °C (1600 °F).

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