The coating and modification of surfaces

9.3.1 The coating and modification of surfaces

Surface heat treatments such as carburizing and nitriding are long-established industrial processes. They are still used in many industries, and rely on temperature and diffusion. These processes are

summarized in Figure 9.13. The action of the new methods for coating or modifying material surfaces, such as vapor deposition and beam bombardment, can be highly specific and energy efficient. They allow great flexibility in controlling the chemical composition and physical structure of surfaces, and many materials which resisted conventional treatments can now be processed. Grain size and the degree of crystalline perfection can be varied over a wide range and beneficial changes in properties produced. The new techniques often eliminate the need for the random diffusion of atoms so that temperatures can be relatively low and processing times short. Scientifically, they are intriguing because their nature makes it possible to bypass thermodynamic restrictions on alloying and to form unorthodox solid solutions and new types of metastable phase.

Oxidation, corrosion and surface treatment 501 Table 9.5 Methods of coating and modifying surfaces (after R. F. Bunshah, 1984; by permission of

Marcel Dekker). Atomistic deposition

Surface modification Electrolytic environment

Particulate deposition Bulk coatings

Chemical conversion Electroplating

Thermal spraying

Wetting processes

Electrolytic Electroless plating

Plasma spraying

Painting

Anodizing (oxide) Fused salt electrolysis

Detonation gun

Dip coating

Electrostatic spraying Fused salts Chemical displacement

Flame spraying

Chemical – liquid Vacuum environment

Fusion coatings

Printing

Chemical – vapor Vacuum evaporation

Thick film ink

Spin coating

Thermal Ion beam deposition

Enameling

Cladding

Plasma Molecular beam epitaxy

Electrophoretic

Explosive

Leaching Plasma environment

Impact plating

Roll bonding

Mechanical Sputter deposition

Overlaying

Shot-peening Activated reactive evaporation

Weld coating

Liquid-phase epitaxy Thermal Plasma polymerization

Surface enrichment Ion plating

Diffusion from bulk Chemical vapor environment

Sputtering Chemical vapor deposition

Ion implantation Reduction

Laser processing Decomposition Plasma enhanced Spray pyrolysis

The number and diversity of methods for coating or modifying surfaces makes general classification difficult. For instance, the energies required by the various processes extend over some five orders of magnitude. Illustrating this point, sputtered atoms have a low thermal energy (<1 eV), whereas the energy of an ion beam can be >100 keV. A useful introductory classification of methods for coating and modifying material surfaces appears in Table 9.5, which takes some account of the different forms of mass transfer. The first column refers to coatings formed from atoms and ions (e.g. vapor deposition). The second column refers to coatings formed from liquid droplets or small particles.

A third category refers to the direct application of coating material in quantity (e.g. paint). Finally, there are methods for the near-surface modification of materials by chemical, mechanical and thermal means and by bombardment (e.g. ion implantation, laser processing).

Some of the methods that utilize deposition from a vapor phase or direct bombardment with particles, ions or radiation will be outlined: it will be apparent that each of the processes discussed has three stages: (1) a source provides the coating or modifying species, (2) this species is transported from source to substrate, and (3) the species penetrates and modifies the substrate or forms an overlay. Each stage is, to a great extent, independent of the other two stages, tending to give each process an individual versatility.