Surface modification with high-energy

12.3.4 Surface modification with high-energy

beams

12.3.4.1 Ion implantation The chemical composition and physical structure at

the surface of a material can be changed by bombard- ing it, in vacuo, with a high-velocity stream of ions. The beam energy is typically about 100 keV; efforts are being made to increase the beam current above

5 mA so that process times can be shortened. Cur- rently, implantation requires several hours. The ions

392 Modern Physical Metallurgy and Materials Engineering The injection of atoms and the formation of vacancies

tend to increase the volume of the target material so that the restraint imposed by the substrate produces a state of residual compressive stress. Fatigue resistance is therefore likely to be enhanced.

As indicated previously, the ions penetrate to a depth of about 300–500 atoms. Penetration is greater in crystalline materials than in glasses, particularly when the ions ‘channel’ between low-index planes. The collision ‘cross-section’ of target atoms for light ions is relatively small and ions penetrate deeply. Ion implantation can be closely controlled, the main

Figure 12.15 Coating by plasma-spray torch (from process variables being beam energy, ion species, ion Weatherill and Gill, 1988; by permission of the Institute of

dose, temperature and substrate material. Materials) .

Ion implantation is used in the doping of semicon- ductors, as discussed in Chapter 6, and to improve engineering properties such as resistance to wear,

may be derived from any element in the Periodic fatigue and corrosion. The process temperature is Table: they may be light (most frequently nitrogen)

less than 150 ° C; accordingly, heat-treated alloy steels

can be implanted without risk of tempering effects. of-sight process; typically, a bombardment dose for

or heavy, even radioactive. Ion implantation 1 is a line-

Nitrogen-implantation is applied to steel and tungsten each square centimetre of target surface is in the order

carbide tools, and, in the plastics industry, has greatly improved the wear resistance of feed screws, extrusion

of 10 17 –10 19 ions. These ions penetrate to a depth dies, nozzles, etc. The process has also been used to

of 100–200 nm and their concentration profile in a plane normal to the surface is Gaussian. Beyond this

simulate neutron damage effects in low-swelling alloys modified region, the properties of the substrate are

being screened for use in atomic fission and fusion unaffected.

reactors. A few hours’ test exposure to an ion beam The beam usually has a sputtering effect which

can represent a year in a reactor because the ions have ejects atoms from the surface and skews the concen-

a larger ‘cross-section’ of interaction with the atoms in tration profile. This effect is most marked when heavy

the target material than neutrons. However, ions cannot ions or heavy doses are used. It is possible for a steady

simulate neutron behaviour completely; unlike neu- state to be achieved, with the rate of sputter erosion

trons, ions are electrically charged and travel smaller equal to the rate of implantation. Thus, depending

distances (see Chapter 6).

upon the target, the type and energy of ion and the substrate material, sputter erosion is capable of limit-

12.3.4.2 Laser processing ing the amount of implantation possible. As a general

Like ion implantation, the laser 2 process is under active guide, the maximum concentration of implanted ion is

development. A laser beam heats the target material given, roughly, by the reciprocal of the sputtering yield

locally to a very high temperature; its effects extend (Y). As one would expect, Y increases in value with

to a depth of 10–100 µ m, which is about a thousand increases in ion energy. However, Y values for pure

times greater than that for an ion beam. Depending on metals are broadly similar, being about 1 or 2 for typi-

its energy, it can heat, melt, vaporize or form a plasma. cal argon ion energies and not differing from each other

The duration of the energy pulse can be 1 ns or less. by more than an order of magnitude. Thus, because of

Subsequent cooling may allow a metallic target zone sputter, the maximum concentration of implanted ions

to recrystallize, possibly with a refined substructure, possible is in the order of 40–50 at.%. In cases where

or undergo an austenite/martensite transformation (e.g. it is difficult to attain this concentration, a thin layer of

automotive components). There is usually an epitaxial the material to be implanted is first deposited and then

relation between the altered near-surface region and driven into the substrate by bombardment with inert

the substrate. Cooling may even be rapid enough to gas ions (argon, krypton, xenon). This indirect method

form a glassy structure (laser glazing). Surface alloying is called ‘ion beam mixing’.

can be achieved by pre-depositing an alloy on the During ion bombardment each atom in the near-

substrate, heating this deposit with a laser beam to surface region is displaced many times. Various forms

form a miscible melt and allowing to cool. In this way, of structural damage are produced by the cascades of

an integral layer of austenitic corrosion-resistant steel collisions (e.g. displacement spikes, vacancy/interstitial

can be built on a ferritic steel substrate. In addition to (Frenkel) pairs, dislocation tangles and loops, etc.).

Damage cascades are most concentrated when heavy ions bombard target atoms of high atomic number (Z). 2 Light Amplification by Stimulated Emission of Radiation (LASER) devices provide photons of electromagnetic

radiation that are in-phase (coherent) and monochromatic 1 Pioneered by the UKAEA, Harwell, in the 1960s.

(see Chapter 6).

Corrosion and surface engineering 393 its use in alloying and heat-treatment, laser processing

Further reading

is used to enhance etching and electroplating. (e.g. semiconductors).

Bell, T. (1992). Surface engineering: its current and future The principal variables in laser processing are the

impact on tribology. J. Phys D.: Appl. Phys. 25, A297–306. energy input and the pulse duration. For established Bunshah, R. F. (1984). Overview of deposition technologies with emphasis on vapour deposition techniques. Indus-

techniques like cutting, drilling and welding metals, trial Materials Science and Engineering , Chapter 12 (L.E. the rate of energy transfer per unit area (‘power den-

Murr, (ed.)). Marcel Dekker, New York. sity’) is in the order of 1 MW cm

Picraux, S. T. (1984). Surface modification of materi- are of relatively long duration (say, 1 ms). For more

and pulses

als — ions, lasers and electron beams. Industrial Materials specialized functions, such as metal hardening by

Science and Engineering , Chapter 11 (L.E. Murr, (ed.)). shock wave generation, the corresponding values are

Marcel Dekker, New York.

approximately 100 MW cm and 1 ns. Short pulses Shreir, L. L. (1976). Corrosion, Vol. 1 and 2, 2nd edn. can produce rapid quenching effects and metastable Newnes-Butterworth, London. Trethewey, K. R. and Chamberlain, J. (1988). Corrosion for

phases. students of science and engineering . Longman, Harlow.