Plasma Surface Engineering of Metals
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frequently employed for PDT. (See the article by Lieberman et al. in this issue.) A potential difference of 300-1,000 V is applied between the workpiece, which forms the cathode of the discharge, and the chamber walls. The discharge covers the workpiece with a conformal "glow seam," allowing uniform treatment of geometrically complex three-dimensional components. Because of the high current density required, care must be taken to ensure that the glow does not
transform into an arc with consequent melting and damage to the component. For a long time, this limited the use of PDT but the limitations have been overcome with the better design of modern switched-mode power supplies. Many PDT units pulse the dc bias applied to the components, which helps suppress the occurrence of arcs. By regulating the pulse duty cycle, some control over the relative population of active species in the plasma is also possible by allowing ionized species to recombine or excited species to decay to a lower state.2 Plasma Nitriding and Nitrocarburizing Mechanical properties and corrosion resistance can be improved considerably by plasma nitriding and nitrocarburizing processes carried out in ferrous materials below 600°C. Treatments are usually conducted in N 2 /H 2 mixtures at pressures between 50 and 500 Pa with CH4 or CO2 providing a source of carbon if required. A nitrogen-rich compound layer (between 2- and 10-/xm thick) is formed on the surface and is responsible for the improvement of the wear and corrosion resistance. Below this a diffusion layer (up to several hundred microns thick) influences the fatigue and loadbearing characteristics.3 A cross section
0.1 mm
Compound Zone Diffusion Zone Nitriding Depth Figure 1. Cross section of a plasma-nitrided steel.
MRS BULLETIN/AUGUST 1996
Plasma Surface Engineering of Metals
of a piasma-nitrided steel is shown in Figure 1. In iron, carbon steel, and low-alloy steels, the compound layer consists of y'-Fe4N and e-Fe2_3N. The relative abundance of these two phases depends on the nitrogen content of the N2/H2 gas mixture and the treatment temperature. The growth rate of the compound layer and the microstructure of both the compound layer and the diffusion zone are influenced by the carbon content of the substrate material and of the gas mixture. Small, acicular precipitates of y'-Fe4N and finely dispersed a"-FewN2 plates are formed in the diffusion layer of pure iron and plain carbon steel (Figure 1). In low-alloy steels, chromium nitride and chromium carbonitride are precipitated. Figure 2 shows the improvement, obtained by plasma nitriding, on the fatigue properties of a low-alloy steel. The improvement in the corrosive fatigue properties are indicated by tests carried out in water. Although nitriding leads to dramatic improvements in the wear resistance and surface hardness of austenitic stainless steels (used extensively in the chemical and food-processing industries), these improvements are usually accompanied by a significant reduction in corrosion resistance. By plasma nitriding
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