**2. Experimental work**

The samples were treated at a fixed input plasma power of 450 W and for a processing time of 10 min. The gas pressure related to N2/C2H2 ratio was varied from 100% N2 to 100% C2H2. The pressure was increased from an atypical base pressure of 1.3x10-2 mbar to a total gas pressure

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 341

The anodic polarization curves were recorded with potential scan rate of 10 mV /sec. The potentiodynamic polarization curve is plotted using AutoLab PGSTAT 12 + GPES software. The surface morphology before and after corrosion tests and the chemical composition of selected parts of the as-prepared layers were examined by scanning electron microscopy

The microstructure of the untreated sample and samples treated at different gas compositions (N2/C2H2) obtained by GIXRD have been studied by us before [11] and it is shown in Fig. 2. It is briefly described here to correlate tribological and corrosion results to the microstructure of the modified surface layers. Only fcc austenitic stainless steel (γ) and bcc ferritic iron (α) were detected in the untreated sample. After treatment at 100 % N2, the Fe2N and CrN phases are observed. The formation of CrN phase is typical for such a high treatment temperature (475 oC). Residual signals from fcc γ-austenite and bcc ferritic iron are present. At high percentage of nitrogen (90 %) iron nitride phases of Fe2N, Fe3N and chromium nitride phase CrN are detected beside the main phase/phases, cubic Fe4N and/or nitrogen-expanded austenite (γn). Due to an overlapping of the strong reflections, the existence of both phases is possible. The intensities of the CrN are lower in comparison to the case of pure nitriding. This might be due to nitrogen atoms, which are dissipated in favor of the formation of the iron nitrides (Fe3N, Fe4N) and γn phases. In the sample treated at high carbon content (75 % C2H2 and 25 % N2), most of the peaks are correlated to Fe3C, carbon-expanded austenite (γc) beside the CrN phase. For the sample treated at 100 % C2H2,

Fig. 3 shows the relative surface roughness, determined by the ratio of the roughness of treated samples to untreated one, as a function of different C2H2/N2 gas pressure ratios. The value of the surface roughness of the untreated sample was 46.3 nm. Due to pure nitriding the surface roughness is increased only by a factor of 1.33. By addition of C2H2, the surface roughness increases abruptly up to a maximum value of 4.12, reached at 30 % C2H2. The value is nearly the same up to a gas content of 50 % C2H2 and decreases significantly for

The friction coefficient is a mechanical parameter, which depends on the surface material composition and the nature of the surface itself. Fig. 4 presents the relative friction coefficient for the samples treated at different gas composition. It relates the friction coefficient of the treated sample to the value of the untreated stainless steel (0.78). The measurement of the friction coefficient has been done for different number of tracks. For pure nitriding, after the first 2000 tracks, at which the wear depth is lower than 0.6 μm, in all examined treated samples, the friction coefficient is reduced to 59 %. While the C2H2/N2 gas ratio increases, the values of the friction coefficient decrease significantly and reaching approximately 14 % for pure carburizing. As a function of gas composition, the friction

samples treated at high carbon content (75 % C2H2) and at pure carburising.

(SEM) and energy dispersive X-ray analysis (EDAX), respectively.

**3. Results** 

**3.1 Phase formation** 

the γc and CrC phases are only detected.

**3.3 Wear test and friction coefficient** 

**3.2 Surface roughness** 

of 8.4x10-2 mbar. The sample was heated mainly by the rf field. The sample temperature was measured during the rf plasma process by a Chromel-Alumel thermocouple, attached to the sample holder. As shown in Fig. 1, the substrate temperature was influenced by the effect of gas compositions. It was found that the temperature gradually increases from 475 °C for pure nitriding up to 550 °C for carbonitriding (50% C2H2, 50% N2) and raises up to 600 °C for pure carburizing. Grazing incidence X-ray diffraction (GIXRD) with Cu Kα radiation was used to determine the phases, present in the treated layers. For the chosen incidence angle of 2° the (1/e)-penetration depth of the X-rays was approximately 700 nm. The recorded diffraction pattern shows therefore mainly the structure of the phases formed in this near-surface region. In this paper we concentrate on the study of corrosion resistance, surface morphology before and after corrosion, and tribological properties of the treated samples. The surface roughness was measured by use of the rough machine (Dektak 8000, Veeco Instrument GmbH). Wear and friction measurement were performed at room temperature in laboratory air with low humidity of 16 to 24 % using an oscillating ball-on-disk type tribometer wear tester without lubrication. The 3-mm ball of cobalt tungsten carbide was moved at mean sliding speed of 15 mm/sec with different normal loads of 3, 5 and 8 N. The corrosion properties were evaluated using the electrochemical testing technique. The corrosion tests were performed in a 1 wt. % NaCl solution by application of the potentiodynamic polarization method. A three-electrode electrochemical cell has been used, counter and reference electrode were related to Pt and saturated calomel electrode, respectively.

Fig. 1. Temperature variation as a function of the gas composition.

The anodic polarization curves were recorded with potential scan rate of 10 mV /sec. The potentiodynamic polarization curve is plotted using AutoLab PGSTAT 12 + GPES software. The surface morphology before and after corrosion tests and the chemical composition of selected parts of the as-prepared layers were examined by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX), respectively.
