The effect of blind holes diameter and depth on the surface properties uniformity in the DC-pulsed

C. CALAHONRA, P. CORENGIA, H. SVOBODA, E. DE LAS HERAS, M. BRIZUELA, A. GARCÍA-LUIS, D.

Garmisch-Partenkirchen, Alemania

Congreso; Tenth International Conference on Plasma Surface Engineering; 2006

Plasma nitriding is a plasma-activated thermochemical method widely used to increase the fatigue strength, hardness and wear resistance of low-alloy steels, tool steels and stainless steels. However, ion nitriding of materials with blind holes or different shapes may produce non-uniform surface characteristics (i.e. nitrided case thickness, microhardness profile and compound layer microstructure). These non-uniformity effects are related with the electric field distortion near edges, the variation of the thickness of the cathode fall and the hollow cathode effect produced in samples with complex geometries. These heterogeneities limit the plasma nitriding effectiveness for industrial use, where the control and reproducibility of the surface modifications are of high interest. Therefore, it is necessary to study systematically the surface properties influence on workpieces with different geometries. The aim of this present investigation is to study the influence of the DC-pulsed plasma nitriding (DCPPN) on the surface and subsurface characteristics of AISI 4140 steel containing blind holes with different diameters and depths. In this sense, cylindrical samples with diameter of 30 mm and 15 mm height were drilled to make holes with diameters of 2, 4, 6, 10 and 12 mm and depths of 3, 8 and 15 mm. The samples were nitrided in a DCPPN industrial equipment using a gas mixture consisting of 25% N2 + 75% H2 for 15h at 723 K. To characterize the DCPPN samples have been used optical and scanning electron microscopy and microhardness profiles. The results show that the blind holes dimensions have a strong influence on the uniformity and continuity of the layers, and also on the microhardness profiles. A simple model has been developed to predict the influences of the blind holes (diameter and depth) on the nitrided case thickness and the microhardness profile.