Effects during plasma nitriding of shaped materials of different sizes

Plasma nitriding of shaped materials and materials of different sizes may yield non-uniform nitrided layers and/or reproducibility problems limiting its industrial-scale use to specific cases. The energy balance at the substrate surface and several non-uniformity effects related to geometry, size and sample position in the chamber during plasma nitriding are discussed. To investigate several of these effects, rods of SAE 1020 steel, of 8 mm in diameter, with different heights (1, 3, 5, 8 and 10 mm) were plasma nitrided in a mixture of N-2-H-2 (20%) at different temperatures (673, 773 and 843 K) and pressures (200 and 500 Pa). The samples were characterized by optical and electron microscopy, X-ray diffraction and microhardness testing. Hardness increases from the center to the edge of the samples. In the case of samples nitrided at 500 Pa, a macroscopically visible ring is obtained with low hardness inside the ring. The diameter of the ring increases for taller samples. The various formed phases, the thickness of nitrided layer and the microhardness of the samples are directly related to the height of the rod. This is associated with surface energy balance. The results show that the top surface temperature increases with height. (C) 2002 Elsevier Science B.V. All rights reserved.