Microstructure of Coatings

The microstructure of a coating is critically dependent on the deposition technology used to produce it (Ref 1). In some technologies, such as physical vapor deposition (PVD), the structure of the coating can be controlled to a certain extent by the choice of deposition parameters. In other technologies, such as weld surfacing or diffusion treatment, it is the thermal history of the component surface that dictates microstructure. Good process control is needed to ensure the production of a layer with the correct microstructure and the required properties for the application. A number of microstructural factors are important in dictating the properties of the coating:

• Phase composition

• Size and shape of grains

• Size and distribution of porosity

• Defects (vacancies, dislocations, etc.)

• Presence of cracks and pin holes

• Anisotropy

• Stress and strain

In most cases, the first three factors are the most important. However, the others can be critically important for certain applications. Stresses and strains occur when the microstructural units of the coating (e.g., columnar grains in PVD coatings or splats in plasma-sprayed materials) are moved with respect to their equilibrium positions by externally applied forces, which can be due to mechanical or thermal loading or the mismatch of properties with the substrate. Stresses can lead to bending of the coating, distortion of the microstructural units, and generation of defects within microstructural units where they contact (e.g., dislocation loops are created by compressive stresses forcing columnar grains together in PVD films). Strains usually manifest themselves as changes to the lattice parameter of the material. There are two types: macrostrains (changes that affect the whole coating, produced by thermal expansion mismatch) and microstrains (highly localized distortions of the structure around defects). Described below is the evolution of microstructure in several important deposition technologies.

In the thermal spraying process, the powder form of a coating material is injected into a flame, where it melts and is ejected against the component surface to build up the coating. Thermal spraying is actually a generic term for a number of processes that all produce coatings with similar microstructures, ranging from flame spraying and high-velocity combustion processes to plasma spraying. The structure and properties of the coatings depend on the stability of the particles within the flame and the choice of process parameters. Ideally, the powder particles are completely melted, but not vaporized, in transit between the injection point and the substrate surface. The molten particles then strike the substrate surface, where they flatten and freeze. Succeeding particles acquire the same lenticular shape over material that has already been deposited, so that the coatings develop an anisotropic lamellar structure parallel to the interface (Fig. 1). The extent of flattening depends on factors such as degree of melting, viscosity of the liquid, and wetting of the surface. Coatings may contain voids that are due to outgassing, shrinkage, or topographical effects (e.g., shadowing). Metal coatings processed in air will also probably contain oxide inclusions.

Fig. 1 Scanning electron micrographs of fracture cross sections of air plasma-sprayed tungsten coatings. (a)

Lamellar structure. (b) Presence of columnar structure within the splats

Cooling rates can be very high (as much as 106 K/s) for plasma-sprayed particles. This produces a very fine microstructure within the lenticular splats (Ref 2), as shown in Fig. 1(b). The rapid solidification can form amorphous deposits from some ferrous alloys (Ref 3), whereas metastable or nonstoichiometric phases can be observed in some ceramics. For instance, alumina coatings contain an increasing proportion of the X phase as the energy of the spraying process increases or the particle size decreases (Ref 4).

Vapor Deposition Processes. The structure of coatings deposited from the vapor phase is controlled as much by the nucleation of the coatings as by the way in which they subsequently grow. One feature common to vapor-deposited coatings is that solid material is distributed in an array of fairly closely packed columns that run perpendicular to the substrate. It is this anisotropy in structure that controls many of the properties of the films.

A number of factors can influence the nucleation of vapor deposited coatings. The surface structure of the substrate is critically important, including grain size, defect density, texture, roughness, and surface composition, and it is often necessary to pretreat materials prior to coating to enhance nucleation. Surface contaminants introduced in the coating process can promote or inhibit nucleation, as can ion bombardment in PVD processes. In many cases a chemical or physical activation step is required to get the best surface structure for coating (e.g., a chemical etch is required to remove surface magnesium from aluminum alloys). This needs to be followed by careful control of the early stages of coating to ensure that reliable structures are produced.

Structure Zone Models. An essential feature of the structure of these thin films is that they are formed from a flux of atoms that approaches the substrate from a limited range of directions. This generates the columnar microstructure, but can lead to problems, because there are many boundaries running perpendicular to the interface that can act as planes of weakness (e.g., as short-circuit diffusion paths).

Movchan and Demchishin (Ref 5) were the first to classify thin-film microstructures. They identified three distinct structure zones as a function of the homologous temperature T/Tm, where T represents the substrate surface temperature and Tm represents the coating melting point. A low-temperature zone 1 structure (T/Tm < 0.25-0.3) corresponds to low adatom mobility and consists of tapered columns with domed tops. In a zone 2 structure (T/Tm = 0.3-0.5), surface diffusion becomes increasingly important, and the structure consists of straight columns with a smooth surface topography. With increasing temperature (T/Tm > 0.5), bulk diffusion becomes a dominant process. The zone 3 microstructure therefore consists of equiaxed grains, as are observed in recrystallized metals. For sputtered coatings, Thornton (Ref 6) later suggested that the presence of a sputtering gas could modify the model, and a further region was identified between zone 1 and zone 2. This region consists of poorly defined fibrous grains and is named zone T.

At the temperatures used in the chemical vapor deposition (CVD) process, surface diffusion of adatoms is actuated and structures of the zone 2 type are produced, in the case of most coating materials. However, the structure of the coating is often controlled by the nucleation density. In the early stages of coating, a very high density of small nuclei is established, which becomes constant after a time, that is, no new nuclei are formed, but newly deposited adatoms move to the existing nuclei and are trapped there. For longer deposition times, the nuclei grow until they eventually coalesce to form the film. In the case of some CVD coatings (e.g., diamond on silicon), the nuclei grow in both height and width, and the coalescence process may result in pores at the interface. Because porosity at the interface decreases adhesion, it is necessary to maximize the density of nuclei formed in order to obtain optimal adhesion. Porosity also remains at the triple points created as the nuclei grow together. These nuclei provide the seeds for the columnar units that compose the coating. Maximizing the nucleation density also reduces the size and extent of this through-thickness porosity.

Because the typical deposition temperatures in PVD processes is very low, many coatings are deposited with a zone 1 microstructure. Instead of increasing the deposition temperature, the occurrence of this microstructure can be overcome by bombarding the growing films with particles having sufficient energy so that the resulting momentum transfer will cause the coating atoms to fill the voided boundaries. Messier (Ref 7) has suggested a modification to the structure zone models that accounts for the evolution of morphology with increasing film thickness, as well as the effect of both thermal and bombardment-induced mobility. The model draws attention to the fact that ion bombardment promotes a dense structure of the zone T type, but it also indicates that the atomic rearrangement can have either thermal or ion-bombardment-induced origins.

Increasing the energy (or the flux) of ion bombardment, by applying a substrate bias, for example, has a significant effect on the structure of PVD films (Ref 8, 9). The coating on an unbiased substrate shows an open columnar structure (zone 1, Fig. 2a), whereas the film on a biased substrate appears more dense, because the individual columns are less well defined (zone T, Fig. 2b). The two coatings will have different properties that are due primarily to changes in the packing density of the columns.

Fig. 2 Scanning electron micrographs of fracture cross sections of sputtered tungsten films on a tungsten substrate. (a) Unbiased (zone 1 structure). (b) -100V bias (zone T structure)

Fig. 2 Scanning electron micrographs of fracture cross sections of sputtered tungsten films on a tungsten substrate. (a) Unbiased (zone 1 structure). (b) -100V bias (zone T structure)

The deposition of predominantly covalent coatings, such as silicon carbide or diamond-like carbon, causes a further complication. Because of bonding directionality, it becomes very difficult to incorporate adatoms into their correct crystallographic location at low deposition temperatures. For this reason, amorphous coatings are produced with smooth, featureless, fracture cross sections. At a critical deposition temperature, some crystallization occurs, and structures of the zone T or zone 2 type can be produced. However, identifiable zone 1 structures are not usually observed.

Zone 1, zone T, and zone 2 microstructures are all associated with the development of texture in PVD films. For titanium nitride coatings, for example, a {111} orientation is commonly reported, although both {200} and {220} orientations have also been observed (Ref 10, 11, 12). The development of texture occurs in three stages:

• Nucleation, where crystallites are nucleated on the substrate from the vapor phase, the distribution and orientation of which depend on the substrate surface structure and deposition parameters

• Competitive growth, where certain favorably oriented nuclei will grow into the vapor phase faster than others, but which may not constitute the majority of the nuclei population

• Steady growth, which occurs once a preferred orientation has achieved dominance

The detailed deposition conditions in any PVD process can change any or all of the above stages and will also affect the morphology of the coating. At the interface region, a very fine grain size is established initially, but with increasing thickness, the columnar structure becomes established and the grain size increases. During the competitive growth phase, intercolumnar voids will open up because of shadowing processes. These voids will be increasingly closed up once the steady-state growth conditions are achieved. Clearly, if porous coatings are to be avoided, it is advisable to know in which thickness range these changes are taking place, as well as how to minimize their effects.

Electrodeposition was one of the earliest plating processes developed for depositing one metal onto another. The method is now widely used for both decorative and engineering purposes. The process involves the reduction of metallic ions at the surface of the substrate, which acts as the cathode of an electrolytic cell. The electrodeposit structure is controlled by the composition of the electrolyte, by the plating conditions, and, in particular, by the presence of growth-inhibiting substances and by the substrate itself (Ref 13). In solution, the metal ions (surrounded by their solvation sheath) migrate toward the deposit, where they lose their sheath and accept electrons to become atoms. The atoms are adsorbed onto the surface and migrate until they encounter a site where they can be incorporated into the existing structure. Impurities or bath additions (inhibitors) may block such sites and can thereby control the structure of the deposit. Under growth-inhibiting conditions (i.e., when the current density is high and the bath temperature is low, so that atoms cannot readily diffuse), the deposit is finely grained and there is essentially continuous nucleation. Under other conditions, extended three-dimensional crystallite networks can grow.

The growth rate perpendicular to the surface is not the same for all grains, because the adsorption of inhibiting substances is anisotropic. As the deposit thickness increases, the slower-growing grains can become covered, and a texture will develop in the film. Slight misorientations between the grains lead to the need for misfit dislocations at boundaries. This is the reason for the high dislocation densities associated with coatings deposited under inhibited conditions. The microstructure of hard electrodeposits used for tribological applications (such as hard chromium) is equiaxed, and extremely finely grained, and it contains some oxide inclusions and microcracks, which give the coating some porosity (Fig. 3). Tensile residual stresses can be produced in the coating during plating and can lead to a network of larger cracks on the material. Similar structures are observed for autocatalytic coatings, such as electroless nickel, where phosphorus-containing reducing agents are present in the plating solution and get incorporated in the coating. Both electrolytic and autocatalytic coatings can be modified to incorporate fine particles (typically, 0.5-5 pm, or 20-200 pin. in size) into the growing film. They can be hard particles, such as silicon carbide, as well as solid lubricants, such as polytetrafluoroethylene. A nonuniform distribution of codeposited substances can lead to crevices or a banded structure (Ref 13).

Fig. 3 Scanning electron micrographs of an electrodeposited chromium film. (a) Fracture cross section. (b) Plan view showing the presence of cracks within the coating
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