**3.1 Phase formation**

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, the γc and CrC phases are only detected.

### **3.2 Surface roughness**

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

### **3.3 Wear test and friction coefficient**

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

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 343

Fig. 3. Relative surface roughness for 304 ASS samples treated at different C2H2/N2 gas

Fig. 4. Relative coefficient of friction as a function of gas composition at different number of

ressure ratios.

cycles.

coefficient for 20000 numbers of tracks has nearly the same values. This reveals the homogeneity and the mechanical stability of the microstructure of the treated layer within the examined range in the near surface region.

The sliding wear behaviour of the untreated and treated samples was assessed using oscillating ball-on-disk type tribometer. The depth of the wear tracks of examined samples as a function of wear path at a load of 3 N is shown in Fig. 5. Generally, the wear resistance of the untreated samples in comparison to the treated is extremely poor. For all treated samples, examined up to 320 m wear path, maximum one micrometer wear depth has been observed and the wear depth slowly increases with increasing wear path. Otherwise the wear rates have been accounted as total volume loss in mm3 divided by the total sliding distance in meters. The wear rates for the untreated material were accounted in order to know the improvement in the wear rates for treated one. The wear rate for the untreated 304 austenitic stainless steel was 2.4 x 10-4 mm3/m at 20000 numbers of tracks (80 m wear path).

Fig. 2. X-ray diffraction pattern obtained at 2° grazing incidence from 304 stainless steel of untreated samples and samples treated at different C2H2/N2 gas pressure ratios.

coefficient for 20000 numbers of tracks has nearly the same values. This reveals the homogeneity and the mechanical stability of the microstructure of the treated layer within

The sliding wear behaviour of the untreated and treated samples was assessed using oscillating ball-on-disk type tribometer. The depth of the wear tracks of examined samples as a function of wear path at a load of 3 N is shown in Fig. 5. Generally, the wear resistance of the untreated samples in comparison to the treated is extremely poor. For all treated samples, examined up to 320 m wear path, maximum one micrometer wear depth has been observed and the wear depth slowly increases with increasing wear path. Otherwise the wear rates have been accounted as total volume loss in mm3 divided by the total sliding distance in meters. The wear rates for the untreated material were accounted in order to know the improvement in the wear rates for treated one. The wear rate for the untreated 304 austenitic stainless steel was 2.4 x 10-4 mm3/m at 20000 numbers of tracks (80 m wear path).

(200) (220)

 CrN Fe2 N

 C,N Fe3 N Fe4 N FeC Fe3 C

+75% C2 H2

+50% C2 H2

+30% C2 H2

+10% C2 H2

100% C2 H2

25% N2

50% N2

70% N2

90% N2

100% N2

304 ASS

30 40 50 60 70 80 90 100 110

2 (degrees) Fig. 2. X-ray diffraction pattern obtained at 2° grazing incidence from 304 stainless steel of

untreated samples and samples treated at different C2H2/N2 gas pressure ratios.

the examined range in the near surface region.

Intensity (a.u.)

(111)

Fig. 3. Relative surface roughness for 304 ASS samples treated at different C2H2/N2 gas ressure ratios.

Fig. 4. Relative coefficient of friction as a function of gas composition at different number of cycles.

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 345

Fig. 6. Wear rate of treated samples as a function of gas composition for a load of 3 N.

Fig. 7. Wear rate of treated samples as a function of gas composition at different loads and

fixed numbers of tracks (200000).

Fig. 5. Wear depth of treated samples at different gas composition compared to an untreated sample.

Fig. 6 presents the variation in the wear rate at different number of tracks as a function of gas composition ratios for a load of 3 N. The wear rate after treatment was reduced by more than two orders in comparison with the untreated material. At 20000 wear tracks, a decrease of the wear rate was observed as much as the C2H2 gas ratio increases. However, this improvement was continued up to 75 % C2H2 and 50 % C2H2 at 40000 and 80000 numbers of tracks, respectively. The decrease in the wear rate by increasing the C2H2 content is related to some improvement in the friction coefficient of treated layers due to fine carbon precipitations which work as solid lubricant on the first few hundred nanometers. However, for samples treated at high carbon content, a small increase in the wear rate can be seen with increasing the sliding distance.

Fig. 7 shows the resistance of treated samples toward physical wearing by accounting the wear rate at different loads of 3 N, 5 N and 8 N. The wear rate of untreated sample is increased by one order from 2.7 x 10-4 mm3/m to 2.7 x 10-3 mm3/m at 5 N and 8 N, respectively. Generally, for all treated samples the wear rate increases in the same order with the load. The wear rate decreases significantly with the increase of the C2H2/N2 gas ratio and for relatively high load (8 N) it reaches a minimum at 50 %.

Fig. 5. Wear depth of treated samples at different gas composition compared to an untreated

Fig. 6 presents the variation in the wear rate at different number of tracks as a function of gas composition ratios for a load of 3 N. The wear rate after treatment was reduced by more than two orders in comparison with the untreated material. At 20000 wear tracks, a decrease of the wear rate was observed as much as the C2H2 gas ratio increases. However, this improvement was continued up to 75 % C2H2 and 50 % C2H2 at 40000 and 80000 numbers of tracks, respectively. The decrease in the wear rate by increasing the C2H2 content is related to some improvement in the friction coefficient of treated layers due to fine carbon precipitations which work as solid lubricant on the first few hundred nanometers. However, for samples treated at high carbon content, a small increase in the wear rate can be seen with

Fig. 7 shows the resistance of treated samples toward physical wearing by accounting the wear rate at different loads of 3 N, 5 N and 8 N. The wear rate of untreated sample is increased by one order from 2.7 x 10-4 mm3/m to 2.7 x 10-3 mm3/m at 5 N and 8 N, respectively. Generally, for all treated samples the wear rate increases in the same order with the load. The wear rate decreases significantly with the increase of the C2H2/N2 gas

ratio and for relatively high load (8 N) it reaches a minimum at 50 %.

sample.

increasing the sliding distance.

Fig. 6. Wear rate of treated samples as a function of gas composition for a load of 3 N.

Fig. 7. Wear rate of treated samples as a function of gas composition at different loads and fixed numbers of tracks (200000).

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 347

**20 μm** 

**100%N2 after Corrosion** 

**20 μm**

**70%N2, 30%C2H2 after Corrosion** 

**100%N2**

**70%N2, 30%C2H2**

**20 μm** 

**20 μm** 

**Ass** 

**20 μm** 

#### **3.4 Corrosion test and surface morphology**

Fig. 8 shows the anodic polarization curves obtained from treated and untreated 304-AISI samples. These results was published elsewhere and represented here to make a correlation to the study of the surface morphology before and after corrosion effect [12]. The increase in the corrosion current and decrease in corrosion potential indicate a degradation of the corrosion resistance for the treated samples. The highest degradation in comparison to the untreated sample is observed for the sample treated in pure nitrogen and carbon plasma. The lowest degradation in the corrosion resistance is observed for the sample treated at the gas composition of 70 % N2 and 30 % C2H2.

The SEM pictures, given in Fig. 9, show the surface morphology of the untreated in comparison to treated samples. Moreover the treated surfaces have been scanned after corrosion test. The original material (304-AISI) may be characterized by a non-uniform shapes, thin grain boundaries and very weak links between the grains. In general the treated samples have wider grain boundaries and smaller grain size. Nitrocarburized samples show also a tendency of grain coalescence. At preparation with 100 % C2H2, the grain boundaries are not clearly visible, which is caused by the higher carbon precipitations at the surface. However, it seems that the grain size is larger than that obtained in the nitrocarburized samples.

Fig. 8. Anodic polarization curves for untreated and treated samples at different C2H2/N2 gas pressure ratios obtained in 1 wt. % NaCl solution.

Fig. 8 shows the anodic polarization curves obtained from treated and untreated 304-AISI samples. These results was published elsewhere and represented here to make a correlation to the study of the surface morphology before and after corrosion effect [12]. The increase in the corrosion current and decrease in corrosion potential indicate a degradation of the corrosion resistance for the treated samples. The highest degradation in comparison to the untreated sample is observed for the sample treated in pure nitrogen and carbon plasma. The lowest degradation in the corrosion resistance is observed for the sample treated at the

The SEM pictures, given in Fig. 9, show the surface morphology of the untreated in comparison to treated samples. Moreover the treated surfaces have been scanned after corrosion test. The original material (304-AISI) may be characterized by a non-uniform shapes, thin grain boundaries and very weak links between the grains. In general the treated samples have wider grain boundaries and smaller grain size. Nitrocarburized samples show also a tendency of grain coalescence. At preparation with 100 % C2H2, the grain boundaries are not clearly visible, which is caused by the higher carbon precipitations at the surface. However, it seems that the grain size is larger than that obtained in the nitrocarburized


Fig. 8. Anodic polarization curves for untreated and treated samples at different C2H2/N2

100 % N2

100 % C2H2

Potential (V)

50 % N2 70 % N2 Ref

**3.4 Corrosion test and surface morphology** 

gas composition of 70 % N2 and 30 % C2H2.

samples.

1e-9

gas pressure ratios obtained in 1 wt. % NaCl solution.

1e-8

1e-7

1e-6

1e-5

Current (A/cm²)

1e-4

1e-3

1e-2

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 349

304 ASS 0 0 72.3 19.1 8 100 % N2 15.2 0 58.5 17.6 8 50 % N2 18.8 12.1 46.2 15.5 6.9 100 % C2H2 0 46 37.9 9.3 5.8

Table 1. Surface elemental composition of ASS and selected treated samples determined by

The SEM pictures of the treated layer after corrosion test show that the corrosion occurs predominantly by bitting, intergranular or by general attack, depending on the difference of the corrosion rate of the grain boundary zones or the grain faces. This difference in rate is determined not only by the metallurgical structure and the composition of the boundary, but also by the characteristics of the corroding solution. The corrosion attack decreases with increasing the carbon content up to 30 % C2H2. After that the attack increases again. At 100 % N2 the surface undergoes significant corrosion leading to visible intergranular stress corrosion cracking. The surface obtained by treatment with 100 % C2H2 exhibits dealloying by selective material dissolution over large surface regions. The substrate is anodic to carbon-bound region and corrodes, leaving behind a mass of carbon compound related areas. In both cases, with pure nitriding or pure carburizing, the loss of corrosion resistance is associated with the depletion of Cr in regions near the grain boundaries. That component,

These results are in accordance with those from potentiodynamic polarization curves, as shown in Fig. 8. Samples treated at 100 % N2 or 100 % C2H2 corrode significantly as evidenced by their more negative corrosion potentials and high corrosion currents. Moderate nitrogen content appears to degrade the corrosion resistance insignificantly,

The plasma efficiency may be increased due to creation of more plasma species such as H, CH, NH, HCN or CN by adding C2H2 to N2 gas during rf plasma carbonitriding. The microstructure of the modified layers depends on the pressure ratio between nitrogen and acetylene plasma gas. The nitride phases and their intensities increased by adding C2H2. The effect of adding acetylene has been also observed in [18] where 0.7 % of C2H2 was used in addition to the N2 gas. Even though Blawert et. al. [8, 19] has observed the nitrogen and carbon expanded austenite phases nearly at the same peak positions as in our case. However, we can not ignore that at high N-content by XRD the γn phase can not be easily distinguished from the Fe4N phase. Both nitride and carbide phases contribute to the improvement of the mechanical properties of the treated samples. Nitride phases (γn) are harder than the carbide phases (γc) [8]. The interplay between the temperature and gas composition might be caused by the effect of hydrogen from the acetylene gas. Compared to carbon and nitrogen the mass difference between plasma species and the ionization potential of atoms can play an important role in the resulting plasma temperature (electron

however, is necessary for regeneration of corrosion protective film.

especially for the gas ratio 70 % N2/30 % C2H2.

**4. Discussion** 

EDAX.

N (at.%) C (at.%) Fe (at.%) Cr (at.%) Ni (at.%)

Fig. 9. SEM pictures of the surface of untreated 304 AISI and treated samples, respectively, in comparison with pictures of treated samples after corrosion test. The last image is for treated sample at 100 % C2H2 scanned after removing few tens nanometers from the surface.

Two distinguishable regions are observed on the surface of the samples, beside the white points and the black base. For 100% N2, one observes a very fine white uniform deposition all over the surface. On the surface of sample treated with 100% C2H2 the white deposition is larger. For 50% N2, surface fractures and little particle deposition can be seen. EDAX analysis shows that the white precipitations contain more carbon than the black ones. The elemental composition of the untreated, treated samples and selected parts was analysed by EDAX and the results are listed in Table 1. For nitrocarburizing with 50 % N2 and 50 % C2H2, the nitrogen concentration is higher than that of carbon.


Table 1. Surface elemental composition of ASS and selected treated samples determined by EDAX.

The SEM pictures of the treated layer after corrosion test show that the corrosion occurs predominantly by bitting, intergranular or by general attack, depending on the difference of the corrosion rate of the grain boundary zones or the grain faces. This difference in rate is determined not only by the metallurgical structure and the composition of the boundary, but also by the characteristics of the corroding solution. The corrosion attack decreases with increasing the carbon content up to 30 % C2H2. After that the attack increases again. At 100 % N2 the surface undergoes significant corrosion leading to visible intergranular stress corrosion cracking. The surface obtained by treatment with 100 % C2H2 exhibits dealloying by selective material dissolution over large surface regions. The substrate is anodic to carbon-bound region and corrodes, leaving behind a mass of carbon compound related areas. In both cases, with pure nitriding or pure carburizing, the loss of corrosion resistance is associated with the depletion of Cr in regions near the grain boundaries. That component, however, is necessary for regeneration of corrosion protective film.

These results are in accordance with those from potentiodynamic polarization curves, as shown in Fig. 8. Samples treated at 100 % N2 or 100 % C2H2 corrode significantly as evidenced by their more negative corrosion potentials and high corrosion currents. Moderate nitrogen content appears to degrade the corrosion resistance insignificantly, especially for the gas ratio 70 % N2/30 % C2H2.
