Proceedings of SerbiaTrib '13

Figure 1. Transverse and longitudinal cross-section ofhardened specimenThe specimen test section had a cylindrical formof dimension 25 × 10 mm (diameter × height).Specimens with porosity of about 19% to 50%,were prepared by laser technique, followed byhardening at T ∈ [1000, 1400] °C and v ∈ [2, 5]mm/s. First, we changed two parameters of therobot laser cell: speed v ∈ [2, 5] mm/s with steps of1 mm/s and temperature T ∈ [1000, 1400] °C insteps of 100 °C (Fig. 2). Secondly, we repeated theprocess (Fig. 3). In addition, we hardened thespecimens again with equal parameters of the robotlaser cell. The microstructure of the specimens wasobserved with a field emission scanning electronmicroscope (JSM-7600F, JEOL Ltd.). An irregularsurface texture was observed with a few breaks,which are represented by black islands (Fig. 4). Fig.5 presents the boundary between the hardened andnon-hardened material.Figure 2. Robot laser hardening with differenttemperature and speedFigure 3. Repeated process of robot laser hardeningFigure 4. SEM picture of robot laser re-hardenedspecimenFigure 5. The boundary between work-hardened andnon-hardened materialWe used the method of determining the porosityfrom SEM images of the microstructure. It isknown that in a homogenously porous material thearea of pores is equal to the volume of pores inspecimens. The SEM pictures were converted tobinary images (Fig. 6), from which we calculatedthe area of pores of all pictures using the ImageJprogram (ImageJ is a public domain, Java-basedimage processing program developed at theNational Institutes of Health). The area of pores oneach picture of the material was calculated and thenthe arithmetic mean and standard deviation ofporosity were determined. To analyze he possibilityof the application of fractal analysis to the heattreatedsurface, we examined the relation betweenthe surface porosity and fractal dimensionsdepending on various parameters of the robot lasercell. In fractal geometry, the key parameter is thefractal dimension D. The relationship between thefractal dimension D, volume V and length L, can beindicated as follows:V~L D (1)356 13 th International Conference on Tribology – Serbiatrib’13