Surface Modification

Surface modification is the alteration of surface composition or structure by the use of energy or particle beams. Elements may be added to influence the surface characteristics of the substrate by the formation of alloys, metastable alloys or phases, or amorphous layers. Surface-modified layers are distinguished from conversion or coating layers by their greater similarity to metallurgical alloying versus chemically reacted, adhered, or physically bonded layers.

Ion implantation is one of a number of surface modification processes that is emerging as an economical and viable process for improving the near-surface tribological properties of engineering components. In many engineering situations, material selection is often based on a compromise of bulk mechanical properties and near-surface tribological properties, with neither at their optimum values. As a result, there is considerable interest in fabrication processes, such as ion implantation, that make it possible to retain the bulk properties of a given compound yet still achieve desirable tribological properties in near-surface regions.

Ion implantation is a process by which virtually any element in the periodic chart can be injected into near-surface regions of any solid using a beam of high-velocity ions with energies typically ranging from 10 keV up to several MeV. As the ions slow down in the material, they are distributed at depths ranging from a few nanometers to several micrometers, depending on the particle energy, angle of incidence, and substrate composition. Depending on the ion type, mass, energy, dose, deposition temperature, and substrate composition, the chemical, electrical, thermal, microstructural, and crystallographic properties of near-surface regions can be significantly altered to improve the friction and wear performance of the component.

Examples of end-use applications of ion implanted stainless steels are bearing rings and ball bearings made from S44004 (type 440C) that are used in the Space Shuttle (Ref 10). The corrosive wear performance of the bearing rings was improved by implanting chromium plus nitrogen (CrN) ions. Significantly lower friction and wear rates were achieved in the ball bearings via ion implantation with titanium plus carbon (TiC) ions and titanium ions alone.

Laser surface processing includes laser transformation hardening, laser melting, laser alloying, laser cladding, and laser melt/particle injection. Of these five method, laser alloying and laser melt/particle injection have been carried out the most on stainless steels. A review of laser surface processing can be found in Ref 11.

Laser Alloying. A technique of localized alloy formation is laser surface melting with the simultaneous, controlled addition of alloying elements in powder form. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface; the degree of microstructural refinement will depend on the solidification rate.

Laser surface alloying was performed to incorporate molybdenum in type 304 stainless steel (Ref 12). The 304-3Mo material was similar in pitting resistance to type 316 stainless steel. The 304-9Mo material was superior to type 316 stainless steel and showed no pitting up to oxygen evolution potentials. Table 12 shows some of the results for the 304-Mo materials.

Table 12 Effect of laser surface alloying with molybdenum on pitting potentials of austenitic stainless steels in 0.1 M NaCl

Sample

(£pit), (V vs SCE)

Cr

Ni

Mo

Type 304

18-20

8-10

0

0.300

Type 316

16-18

10-14

2-3

0.550

304-3Mo

18.9

9.1

3.7

0.500

304-9Mo

19.2

11.7

9.6

Did not pit

Laser melt/particle injection produces an in situ, metal-matrix/particulate composite surface layer by mixing, but not melting, the second phase with the substrate. The particulate material is injected with sufficient velocity as a spray into the melt pool formed by the laser beam. If the second phase is hard, such as a carbide, the injected layer can be made to resist wear. Reference 13 describes the processing and properties of a type 304 stainless steel workpiece that was injected with titanium carbide.

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