Coating Structures and Properties

Coating Microstructures. Thermal spray coatings consist of many layers of thin, overlapping, essentially lamellar particles, frequently called splats. Cross sections of several typical coatings are shown in Fig. 2, 11, 12, 13, and 14. Generally, the higher-particle-velocity coating processes produce the densest and better bonded coatings, both cohesively (splat-to-splat) and adhesively (coating-to-substrate). Metallographically estimated porosities for detonation gun coatings and some HVOF coatings are less than 2%, whereas most plasma sprayed coating porosities are in the range of 5 to 15%. The porosities of flame sprayed coatings may exceed 15%.

Fig. 11 Microstructure of plasma-sprayed chromium oxide. As-polished

Fig. 11 Microstructure of plasma-sprayed chromium oxide. As-polished

Fig. 12 Microstructure of detonation gun deposited alumina and titania. As-polished

Fig. 13 Microstructure of a detonation gun deposited tungsten carbide/cobalt cermet coating. (a) As-polished. (b) Etched

Fig. 13 Microstructure of a detonation gun deposited tungsten carbide/cobalt cermet coating. (a) As-polished. (b) Etched

Fig. 14 Microstructure of a mechanically mixed chromium carbide/nickel chromium cermet coating. (a) As-polished.(b) Etched

HO w

Fig. 14 Microstructure of a mechanically mixed chromium carbide/nickel chromium cermet coating. (a) As-polished.(b) Etched

The extent of oxidation that occurs during the deposition process is a function of the material being deposited, the method of deposition, and the specific deposition process. Oxidation may occur because of the oxidizing potential of the fuel-gas mixture in flame spraying, HVOF, or detonation gun deposition or because of air inspirated into the gas stream in plasma spraying or any of the other methods. Recall that the latter cause can be ameliorated by using inert-gas shrouds or low-pressure chambers with plasma spraying. Using carbon-rich gas mixtures with oxyfuel processes can cause carburization rather than oxidation with some metallic coatings. Metallic coatings are probably most susceptible to oxidation, but carbide coatings may suffer a substantial loss of carbon that is not particularly obvious in metallographic examination. Oxidation during deposition can lead to higher porosity and generally weaker coatings, and it is usually considered to be undesirable.

Most of the thermal spray processes lead to very rapid quenching of the particles on impact. Quench rates have been estimated to be 104 to 106 °C/s for ceramics and 106 to 108 °C/s for metallics. As a result, the materials deposited may be in thermodynamically metastable states, and the grains within the splats may be submicron-size or even amorphous. The metastable phases present may not have the expected characteristics, particularly corrosion characteristics, of the material, and this factor should be kept in mind in the selection of coating compositions.

The mechanical properties of thermal spray coatings are not well documented with the exception of their hardness and bond strength. These are discussed in the section "Quality Assurance" in this article. The sensitivity of the properties of the coatings to specific deposition parameters makes universal cataloging of properties by simple chemical composition and general process (e.g., WC-12Co by plasma spray) virtually meaningless. The situation is even more complex because the properties of coatings on test specimens may differ somewhat from those on parts because of differences in geometry and thermal conditions. Nonetheless, coatings made by competent suppliers using adequate quality control will be quite reproducible, and therefore the measurement of various mechanical properties of these standardized coatings may be very useful in the selection of coatings for specific applications. Properties that may be of value include the modulus of rupture, modulus of elasticity, and strain-to-fracture in addition to hardness. Examples of some of these are given in Table 9.

Table 9 Mechanical pro perties of representative plasma, detonation, and high-velocity combustion coatings

Table 9 Mechanical pro perties of representative plasma, detonation, and high-velocity combustion coatings

Parameter

Type of coating Tungsten-carbide-cobalt

Alumina

Nominal composition, wt%

W-7Co-4C

W-9Co-5C

W-11Co-4C

W-14Co-4C

Al2Os

Al2Os

Thermal spray process

Detonation gun

High-velocity combustion

Plasma

Detonation gun

Detonation gun

Plasma

Rupture modulus, 103 psi(a)

72

30

120

22

17

Elastic modulus, 106 psi(a)

23

11

25

14

7.9

Hardness, kg/mm2, HV300

1300

1125

850

1075

>1000

>700

Bond strength, 103 psi(c)

>10,000(b)

>10,000(b)

>6500

>10,000

>10,000(b)

>6500

Source: Publication 1G191, National Association of Corrosion Engineers

Source: Publication 1G191, National Association of Corrosion Engineers

(a) Compression of freestanding rings of coatings.

(b) Epoxy failure.

(c) ASTM C 633-89, "Standard Test Method for Adhesion or Cohesive Strength of Flame-Sprayed Coatings," ASTM, 1989.

Any measurement or use of mechanical properties must take into account the anisotropic nature of the coating microstructure and hence its properties (i.e., the coating properties are different parallel to the surface than perpendicular to the surface because of the lamellar nature of the microstructure). Most mechanical properties are measured parallel to the surface, in part because it is easier to produce test specimens in this plane because the coatings are typically thin. Unfortunately, the major load in service is usually perpendicular to the surface. This does not, however, make measurements in the plane of the coating useless. It is frequently important to know, for example, how much strain can be imposed on a coating due to extension or deflection of the part without cracking the coating. Cracks in a coating may not only affect its performance, but also initiate cracks and fatigue failures in the part.

0 0

Post a comment