The coating and modification of surfaces

12.3.1 The coating and modification of surfaces

The action of the new methods for coating or modi- fying material surfaces, such as vapour deposition and beam bombardment, can be highly specific and energy- efficient. They allow great flexibility in controlling the chemical composition and physical structure of sur- faces 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 pro- duced. The new techniques often eliminate the need for the random diffusion of atoms so that tempera- tures 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.

388 Modern Physical Metallurgy and Materials Engineering Table 12.3 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

Fused salts Chemical displacement

Flame-spraying

Electrostatic spraying

Chemical-liquid Vacuum environment

Fusion coatings

Printing

Chemical-vapour Vacuum evaporation

Thick film ink

Spin coating

Thermal Ion beam deposition

Enamelling

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

Thermal Plasma polymerization

Liquid phase epitaxy

Surface enrichment Ion plating

Diffusion from bulk Chemical vapour environment

Sputtering Chemical vapour deposition

Ion implantation Reduction

Laser processing Decomposition Plasma enhanced Spray pyrolysis

The number and diversity of methods for coating or carrier gas at or near the heated surface of a substrate modifying surfaces makes general classification diffi-

(Figures 12.11a and 12.11b). CVD is not a ‘line-of- cult. For instance, the energies required by the various

sight’ process and can coat complex shapes uniformly, processes extend over some five orders of magni-

having good ‘throwing power’. 1 Typical CVD reac- tude. Illustrating this point, sputtered atoms have a

tions for depositing boron nitride and titanium carbide, low thermal energy (<1 eV) whereas the energy of

respectively, are:

an ion beam can be >100 keV. A useful introductory 500–1500 classification of methods for coating and modifying ° C

⊲ s⊳ C 3HCl ⊲ g⊳ material surfaces appears in Table 12.3, which takes

BCl 3⊲g⊳ C NH 3⊲g⊳

⊲ s⊳ C 4HCl ⊲ g⊳ atoms and ions (e.g. vapour deposition). The second

some account of the different forms of mass trans-

800–1000 ° C

TiCl 4⊲g⊳ fer. The first column refers to coatings formed from C CH 4⊲g⊳

It will be noted that the substrate temperatures, column refers to coatings formed from liquid droplets

which control the rate of deposition, are relatively high. or small particles. A third category refers to the direct

Accordingly, although CVD is suitable for coating application of coating material in quantity (e.g. paint).

a refractory compound, like cobalt-bonded tungsten Finally, there are methods for the near-surface modifi-

carbide, it will soften a hardened and tempered high- cation of materials by chemical, mechanical and ther-

speed tool steel, making it necessary to repeat the mal means and by bombardment (e.g. ion implantation,

high-temperature heat-treatment. In one variant of the laser processing).

process (PACVD) deposition is plasma-assisted by a Some of the methods that utilize deposition from

plate located above the substrate which is charged with

a radio-frequency bias voltage. The resulting plasma ions or radiation will be outlined: it will be apparent

a vapour phase or direct bombardment with particles,

zone influences the structure of the coating. PACVD that each of the processes discussed has three stages:

is used to produce ceramic coatings (SiC, Si 3 N 4 ) but (1) a source provides the coating or modifying specie,

C (minimum) is still (2) this specie is transported from source to substrate

the substrate temperature of 650 °

too high for heat-treated alloy steels. The maximum and (3) the specie penetrates and modifies the substrate

coating thickness produced economically by CVD and or forms an overlay. Each stage is, to a great extent,

PACVD is about 100 µ m.

independent of the other two stages, tending to give each process an individual versatility.

12.3.2.2 Physical vapour deposition

12.3.2 Surface coating by vapour deposition

Although there are numerous versions of the physical vapour deposition (PVD) process, their basic design is

12.3.2.1 Chemical vapour deposition In the chemical vapour deposition (CVD) process a

1 The term ‘throwing power’ conventionally refers to the coating of metal, alloy or refractory compound is

ability of an electroplating solution to deposit metal produced by chemical reaction between vapour and a

uniformly on a cathode of irregular shape.

Corrosion and surface engineering 389

Figure 12.11 Experimental CVD reactors (from Bunshah, 1984; by permission of Marcel Dekker) .

Figure 12.12 (a) Evaporation-dependent and (b) sputter-dependent PVD (from Barrell and Rickerby, Aug 1989, pp. 468–73; by permission of the Institute of Materials) .

either evaporation- or sputter-dependent. In the former m . The evaporant atoms travel towards the substrate case, the source material is heated by high-energy

(component), essentially following lines-of-sight. beam (electron, ion, laser), resistance, induction, etc.

When sputtering is used in PVD (Figure 12.12b), in a vacuum chamber (Figure 12.12a). The rate of

a cathode source operates under an applied poten- evaporation depends upon the vapour pressure of the

tial of up to 5 kV (direct-current or radio-frequency) source and the chamber pressure. Metals vaporize at

in an atmosphere of inert gas (Ar). The vacuum is

a reasonable rate if their vapour pressure exceeds ‘softer’, with a chamber pressure of 1–10 Nm . As 1Nm and the chamber pressure is below 10 N

positive argon ions bombard the target, momentum is

390 Modern Physical Metallurgy and Materials Engineering transferred and the ejected target atoms form a coating

and sputter-dependent forms of PVD. Strong bonding on the substrate. The ‘throwing power’ of sputter-

of the PAPVD coating to the substrate requires the dependent PVD is good and coating thicknesses are

latter to be free from contamination. Accordingly, in uniform. The process benefits from the fact that the

a critical preliminary stage, the substrate is cleansed sputtering yield (Y) values for metals are fairly simi-

by bombardment with positive ions. The source is lar. (Y is the average number of target atoms ejected

then energized and metal vapour is allowed into the from the surface per incident ion, as determined exper-

chamber.

imentally.) In contrast, with an evaporation source, for In the basic magnetron-assisted version of sputter-

a given temperature, the rates of vaporization can differ dependent PVD, a magnetic field is used to form by several orders of magnitude.

a dense plasma close to the target. The magnetron, As in CVD, the temperature of the substrate is of

an array of permanent magnets or electromagnets, is special significance. In PVD, this temperature can be

attached to the rear of the target (water-cooled) with its as low as 200–400 °

north and south poles arranged to produce a magnetic method to cutting and metal-forming tools of hard-

C, making it possible to apply the

field at right angles to the electric field between the tar- ened steel. A titanium nitride (TiN) coating, <5 µ m get and substrate (Figure 12.13a). This magnetic field

thick, can enhance tool life considerably (e.g. twist confines electrons close to the target surface, increases drills). TiN is extremely hard (2400 HV), has a low

coefficient of friction and a very smooth surface tex- the rate of ionization and produces a much denser ture. TiN coatings can also be applied to non-ferrous

plasma. The improved ionization efficiency allows a alloys and cobalt-bonded tungsten carbide. Experience

lower chamber pressure to be used; sputtered target with the design of a TiN-coated steel has demonstrated

atoms then become less likely to be scattered by gas that the coating/substrate system must be considered as

molecules. The net effect is to improve the rate of

a working whole. A sound overlay of wear-resistant deposition at the substrate. Normally the region of material on a tough material may fail prematurely

dense plasma only extends up to about 6 cm from the if working stresses cause plastic deformation of the

target surface. Development of unbalanced magnetron supporting substrate. For this reason, and in accor-

systems (Figure 12.13b) has enabled the depth of the dance with the newly-emerging principles of surface

dense plasma zone to be extended so that the substrate engineering, it has been recommended that steel sur-

itself is subjected to ion bombardment. These energetic faces should be strengthened by nitriding before a TiN

ions modify the chemical and physical properties of the coating is applied by PVD.

deposit. (In one of the various unbalanced magnetron Two important modifications of the PVD process are

configurations, a ring of strong rare-earth magnetic plasma-assisted physical vapour deposition (PAPVD)

poles surrounds a weak central magnetic pole.) This and magnetron sputtering. In PAPVD, also known as

larger plasma zone can accommodate large complex ‘ion plating’, deposition in a ‘soft’ vacuum is assisted

workpieces and rapidly forms dense, non-columnar by bombardment with ions. This effect is produced

coatings of metals or alloys. Target/substrate separa- by applying a negative potential of 2–5 kV to the

tion distances up to 20 cm have been achieved with substrate. PAPVD is a hybrid of the evaporation-

unbalanced magnetron systems.

Figure 12.13 Comparison of plasma confinement in conventional and unbalanced magnetrons (PVD) (from Kelly, Arnell and Ahmed, March 1993, pp. 161–5; by permission of the Institute of Materials) .

Corrosion and surface engineering 391

Figure 12.14 Coating by detonation-gun (from Weatherill and Gill, 1988; by permission of the Institute of Materials) .