Surface coating by vapor deposition

9.3.2 Surface coating by vapor deposition

9.3.2.1 Chemical vapor deposition

In the chemical vapor deposition (CVD) process a coating of metal, alloy or refractory compound is produced by chemical reaction between vapor and a carrier gas at or near the heated surface of a

502 Physical Metallurgy and Advanced Materials

Bell jar Enclosure

Inlet Substrate

Substrates Induction

Susceptor Seal

coil

Base plate

Rotary seal

Exhaust

Inlet

Heater leads

(b) Inductively heated pedestal Figure 9.14 Experimental CVD reactors ( from Bunshah, 1984; by permission of Marcel Dekker).

(a) Resistively heated plate

substrate (Figures 9.14a and b). CVD is not a ‘line-of-sight’ process and can coat complex shapes uniformly, having good ‘throwing power’. 1 Typical CVD reactions for depositing boron nitride and titanium carbide respectively are:

BCl 500 −1500 ◦ C 3(g) + NH 3(g) −−−−−−→ BN (s) + 3HCl (g)

800 −1000 ◦ C

TiCl 4(g) + CH 4(g) −−−−−−→ TiC (s) + 4HCl (g) . It will be noted that the substrate temperatures, which control the rate of deposition, are relatively

high. Accordingly, although CVD is suitable for coating a refractory compound, like cobalt-bonded tungsten carbide, it will soften a hardened and tempered high-speed tool steel, making it necessary to repeat the high-temperature heat treatment. In one variant of the process (PACVD) deposition is plasma assisted by a plate located above the substrate which is charged with a radio-frequency bias voltage. The resulting plasma zone influences the structure of the coating. PACVD is used to produce

C (minimum) is still too high for heat-treated alloy steels. The maximum coating thickness produced economically by CVD and PACVD is about 100 µm.

ceramic coatings (SiC, Si 3 N 4 ) but the substrate temperature of 650 ◦

9.3.2.2 Physical vapor deposition

Although there are numerous versions of the physical vapor deposition (PVD) process, their basic design is either evaporation or sputter dependent. In the former case, the source material is heated by a high-energy beam (electron, ion, laser), resistance, induction, etc. in a vacuum chamber (Figure 9.15a). The rate of evaporation depends upon the vapor pressure of the source and the chamber pressure. Metals vaporize at a reasonable rate if their vapor pressure exceeds 1 N m −2 and the cham- ber pressure is below 10 −3 Nm −2 . The evaporant atoms travel towards the substrate (component), essentially following lines of sight.

1 The term ‘throwing power’ conventionally refers to the ability of an electroplating solution to deposit metal uniformly on a cathode of irregular shape.

Oxidation, corrosion and surface treatment 503 PS ⫹

Substrate (s) ⫺

Flux profile at

Molten pool

Components Crucible

Electron

beam source

Ingot rod

Feed ⫺

PS ⫹

(a) (b) Figure 9.15 Evaporation-dependent (a) and sputter-dependent (b) PVD (from Barrell and

Rickerby, Aug 1989, pp. 468–473; by permission of the Institute of Materials, Minerals and Mining).

When sputtering is used in PVD (Figure 9.15b), a cathode source operates under an applied potential of up to 5 kV (direct current or radio-frequency) in an atmosphere of inert gas (Ar). The vacuum is ‘softer’, with a chamber pressure of 1–10 −2 Nm −2 . As positive argon ions bombard the target, momentum is transferred and the ejected target atoms form a coating on the substrate. The ‘throwing power’ of sputter-dependent PVD is good and coating thicknesses are uniform. The process benefits from the fact that the sputtering yield (Y ) values for metals are fairly similar. (Y is the average number of target atoms ejected from the surface per incident ion, as determined experimentally.) In contrast, with an evaporation source, for a given temperature, the rates of vaporization can differ by several orders of magnitude.

As in CVD, the temperature of the substrate is of special significance. In PVD, this temperature can

C, making it possible to apply the method to cutting and metal-forming tools of hardened steel. A titanium nitride (TiN) coating, <5 µm thick, can enhance tool life considerably (e.g. twist drills). TiN is extremely hard (2400 HV), has a low coefficient of friction and a very smooth surface texture. TiN coatings can also be applied to non-ferrous alloys and cobalt-bonded tungsten carbide. Experience with the design of a TiN-coated steel has demonstrated that the coating/substrate system must be considered as a working whole. A sound overlay of wear-resistant material on a tough material may fail prematurely if working stresses cause plastic deformation of the supporting substrate. For this reason, and in accordance with the newly emerging principles of surface engineering, it has

be as low as 200–400 ◦

504 Physical Metallurgy and Advanced Materials

Substrate

Substrate B B

PLASMA

E PLASMA

Target N

Target

SN

Permanent magnets

Convectional magnetron Unbalanced magnetron

(b) Figure 9.16 Comparison of plasma confinement in conventional and unbalanced magnetrons

(a)

(PVD) ( from Kelly, Arnell and Ahmed, March 1993, pp. 161–165; by permission of the Institute of Materials, Minerals and Mining).

been recommended that steel surfaces should be strengthened by nitriding before a TiN coating is applied by PVD.

Two important modifications of the PVD process are plasma-assisted physical vapor deposition (PAPVD) and magnetron sputtering. In PAPVD, also known as ‘ion plating’, deposition in a ‘soft’ vacuum is assisted by bombardment with ions. This effect is produced by applying a negative potential of 2–5 kV to the substrate. PAPVD is a hybrid of the evaporation-and sputter-dependent forms of PVD. Strong bonding of the PAPVD coating to the substrate requires the latter to be free from contamination.

Accordingly, in a critical preliminary stage, the substrate is cleansed by bombardment with positive ions. The source is then energized and metal vapor is allowed into the chamber.

In the basic magnetron-assisted version of sputter-dependent PVD, a magnetic field is used to form

a dense plasma close to the target. The magnetron, an array of permanent magnets or electromagnets, is attached to the rear of the target (water cooled) with its north and south poles arranged to produce

a magnetic field at right angles to the electric field between the target and substrate (Figure 9.16a). This magnetic field confines electrons close to the target surface, increases the rate of ionization and produces a much denser plasma. The improved ionization efficiency allows a lower chamber pressure to be used; sputtered target atoms then become less likely to be scattered by gas molecules. The net effect is to improve the rate of deposition at the substrate. Normally the region of dense plasma only extends up to about 6 cm from the target surface. Development of unbalanced magnetron systems (Figure 9.16b) has enabled the depth of the dense plasma zone to be extended so that the substrate itself is subjected to ion bombardment. These energetic ions modify the chemical and physical properties of the deposit. (In one of the various unbalanced magnetron configurations, a ring of strong rare-earth magnetic poles surrounds a weak central magnetic pole.) This larger plasma zone can accommodate large, complex workpieces and rapidly forms dense, non-columnar coatings of metals or alloys.

Target/substrate separation distances up to 20 cm have been achieved with unbalanced magnetron systems.

Oxidation, corrosion and surface treatment 505

Spark plug

Workpiece

Powder Nitrogen

gas

Oxygen gas

Barrel

Acetylene gas Figure 9.17 Coating by detonation gun ( from Weatherill and Gill, 1988; by permission of the

Institute of Materials, Minerals and Mining).