Surface modification with high-energy beams

9.3.4 Surface modification with high-energy beams

9.3.4.1 Ion implantation

The chemical composition and physical structure at the surface of a material can be changed by bombarding 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. Currently, implantation requires several hours. The ions may be derived from any element in the Periodic Table: they may be light (most frequently nitrogen) or heavy, even radioactive. Ion

implantation 3 is a line-of-sight process: typically, a bombardment dose for each square centimeter of target surface is of the order of 10 17 –10 19 ions. These ions penetrate to a depth of 100–200 nm and their concentration profile in a plane normal to the surface is Gaussian. Beyond this modified region, the properties of the substrate are unaffected.

The beam usually has a sputtering effect which ejects atoms from the surface and skews the concentration profile. This effect is most marked when heavy ions or heavy doses are used. It is possible for a steady state to be achieved, with the rate of sputter erosion equal to the rate of implan- tation. Thus, depending upon the target, the type and energy of ion and the substrate material, sputter erosion is capable of limiting the amount of implantation possible. As a general guide, the maximum concentration of implanted ion is given, roughly, by the reciprocal of the sputtering yield (Y ). As one would expect, Y increases in value with increases in ion energy. However, Y -values for pure metals are broadly similar, being about 1 or 2 for typical argon ion energies and not differing from each

3 Pioneered by the UKAEA, Harwell, in the 1960s.

Oxidation, corrosion and surface treatment 507 other by more than an order of magnitude. Thus, because of sputter, the maximum concentration

of implanted ions possible is of the order of 40–50 at.%. In cases where it is difficult to attain this concentration, a thin layer of the material to be implanted is first deposited and then driven into the substrate by bombardment with inert gas ions (argon, krypton, xenon). This indirect method is called ‘ion beam mixing’.

During ion bombardment each atom in the near-surface region is displaced many times. Various forms of structural damage are produced by the cascades of collisions (e.g. displacement spikes, vacancy/interstitial (Frenkel) pairs, dislocation tangles and loops, etc.). Damage cascades are most concentrated when heavy ions bombard target atoms of high atomic number (Z). 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 process variables being beam energy, ion species, ion dose, temperature and substrate material.

Ion implantation is used in the doping of semiconductors, as discussed in Chapter 5, and to improve engineering properties such as resistance to wear, fatigue and corrosion. The process tem- perature is less than 150 ◦ C; accordingly, heat-treated alloy steels can be implanted without risk of tempering effects. Nitrogen implantation is applied to steel and tungsten carbide tools, and, in the plastics industry, has greatly improved the wear resistance of feed screws, extrusion dies, noz- zles, etc. The process has also been used to simulate neutron damage effects in low-swelling alloys being screened for use in atomic fission and fusion reactors. A few hours’ test exposure to an ion beam can represent a year in a reactor because the ions have a larger ‘cross-section’ of interac- tion with the atoms in the target material than neutrons. However, ions cannot simulate neutron behavior completely; unlike neutrons, ions are electrically charged and travel smaller distances (see Chapter 5).

9.3.4.2 Laser processing

Like ion implantation, the laser 4 process is under active development. A laser beam heats the target material locally to a very high temperature; its effects extend to a depth of 10–100 µm, which is about 1000 times greater than that for an ion beam. Depending on its energy, it can heat, melt, vaporize or form a plasma. The duration of the energy pulse can be 1 ns or less. Subsequent cooling may allow a metallic target zone to recrystallize, possibly with a refined substructure, or undergo an austenite/martensite transformation (e.g. automotive components). There is usually an epitaxial relation between the altered near-surface region and the substrate. Cooling may even be rapid enough to form a glassy structure (laser glazing). Surface alloying can be achieved by pre-depositing an alloy on the substrate, heating this deposit with a laser beam to form a miscible melt and allowing to cool. In this way, an integral layer of austenitic corrosion-resistant steel can be built on a ferritic steel substrate. In addition to its use in alloying and heat treatment, laser processing is used to enhance etching and electroplating (e.g. semiconductors).

The principal variables in laser processing are the energy input and the pulse duration (see Figure 9.19). For established techniques like cutting, drilling and welding metals, the rate of energy

4 Light Amplification by Stimulated Emission of Radiation (LASER) devices provide photons of electromagnetic radiation that are in-phase (coherent) and monochromatic (see Chapter 5).

508 Physical Metallurgy and Advanced Materials

10 10 2 ) 10 9 ⫺ Shock

8 10 hardening Cladding

10 7 and alloying

10 6 Laser 10 5 glazing

Power density (W cm 10 4 Transformation hardening

Interaction time (s)

Figure 9.19 Laser processing variables and applications. transfer per unit area (‘power density’) is of the order of 1 MW cm −2 and pulses are of relatively

long duration (say, 1 ms). For more specialized functions, such as metal hardening by shock wave generation, the corresponding values are approximately 100 MW cm −2 and 1 ns. Short pulses can produce rapid quenching effects and metastable phases.