CHARACTERIZATION OF PLASMA NITRIDED LAYERS PRODUCED ...
CHARACTERIZATION OF PLASMA NITRIDED LAYERS PRODUCED ON SINTERED IRON
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M. A. Fontes1; R. G. Pereira2; F. A. P. Fernandes2; L. C. Casteletti2; P. A. P. Nascente1 Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, São Carlos, SP, Brasil. 2 Departamento de Engenharia de Materiais, Universidade de São Paulo, São Carlos, SP, Brasil.
Plasma nitriding is a thermo-physical-chemical treatment process, which promotes surface hardening, caused by interstitial diffusion of atomic nitrogen into metallic alloys. In this work, this process was employed in the surface modification of a sintered ferrous alloy. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses and wear and microhardness tests were performed on the samples submitted to ferrox treatment and plasma nitriding carried out under different conditions of time and temperature. The results showed that the nitride layer thickness is higher for all nitrided samples than ferrox treated samples, and this layer thickness increases with nitriding time and temperature, and temperature is the more significant variable. The XRD analysis showed that the nitrided layer, for all samples, near to the surface consists in a mixture of ’-Fe4N e -Fe3N phases. Both wear resistance and microhardness increase with nitriding time and temperature, and temperature influences the most both characteristics.
Keywords: plasma nitriding, powder sintered metal, scanning electron microscopy.
Introduction Ion nitriding is a thermal-physical-chemical treatment process that provokes surface hardening by interstitial diffusion of atomic nitrogen into both ferrous and non-ferrous metallic surfaces. This process causes the formation of a case layer, which may comprise of an oxide layer, a compound layer, and a diffusion zone [1]. In the case of ferrous alloys, the compound layer, also called white layer, is constituted of mainly ’Fe4N, -Fe3N, and possibly Fe2N. It is very important to control the nitriding parameters in order to obtain the best performance of nitrided components used in different engineering applications [2]. The composition and thickness of the nitrided layers are directly related to the treatment temperature, pressure, and time, as well as the composition of the base material [3]. In this work, iron sintered samples with and without ferrox treatment were plasma nitrided, and the modified layers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, and wear and microhardness tests. Experimental part Sintered iron samples with dimensions of 31.75 mm x 12.65 mm x 10 mm were nitrided in an atmosphere of plasma 80 vol.% H2 and 20 vol% N2 and pressure of 5 mbar, at temperatures of 500, 540, and 580°C and periods of 3 and 5 hours. The microstructural analysis were carried out using a Philips SL-30 field emission gun (FEG) SEM apparatus equipped with an energy-dispersive spectrometer (EDS). The images were acquired employing the backscattering electron (BSE) method. XRD analysis was performed by a Rigaku Geiger-Flex equipment using Cu K radiation ( = 1.54056 Å) and 2 step scan of 0.032 per second in the range of 5-90. The wear tests were carried out in a fixed ball device using a 52100 steel sphere of 25.4 mm in diameter, a rotation speed of 300 rpm, a load of 245 g (2.45 N), and testing
times of 5, 10, 20, and 30 minutes. The microhardness measurements were obtained using a Buehler hardness tester, in the Vickers scale (HV), with a load of 100 gf. For each specimen, eight measurements were done at as many different points on the surface. Results and discussion SEM analysis revealed that the greater diffusion of nitrogen was observed for the samples treated at both 540 and 580°C (Figure 1). Higher thickness values were presented by the samples treated for 5 hours for a given treatment temperature. Figure 2 displays the XRD diffractograms for the samples nitrided at 580C for 5 hours, revealing the presence of ’-Fe4N and, in a lesser scale, -Fe3N phases. Regarding to the wear analysis, the wear volumes (in mm3) as a function of testing times are plotted in figure 3. The lower wear rate values are obtained for the samples nitrided for 5 hours at 540 and 580C, indicating that higher nitriding temperatures and times had a more favorable impact on the wear resistance. Figure 4 depicts the Vickers microhardness test results for all samples. The higher values were obtained for the samples nitrided at 580C for 3 and 5 hours, while the lower values were obtained for nitriding temperature of 500C.
Fig.1 - SEM micrographs of the plasma nitrided samples, and sample submitted to the ferrox treatment.
Fig.3 - The wear volumes (in mm3) as a function of testing times.
Fig.2 - XRD diffractograms for plasma nitriding temperature of 580°C for 5 hours.
Fig.4 - Vickers microhardness test results.
Conclusions Plasma nitriding of sintered ferrous samples can promote modifications on their surface properties. Regarding to the nitrided layer thickness dependence, the temperature variable predominates over the nitriding time variable. According to the Fick's second law, the nitride layer thickness is less dependent on t - time (non-linearly) than on T temperature (exponentially).
The nitrided layer consists predominantly of ’-Fe4N phase, with a smaller amount of Fe3N phase. The amount of the ’-Fe4N phase increases with higher nitriding temperatures and longer times. Higher nitriding temperatures and times enhanced both the wear resistance and hardness of the samples. References [1] Gontijo LC, Machado R, Miola EJ, Casteletti, Nascente PAP. Characterization of plasma-nitrided iron by XRD, SEM and XPS. Surf Coat Technol 2004; 183: 10-7. [2] Inokuti Y, Nishida N, Ohashi N. Formation of Fe3N, Fe4N and Fe16N2 on surface of iron. Metall Trans A 1975; 6: 773-784. [3] Collins GA, Hutchings R, Short KT, Tendys J, Van Der Valk CH. Development of a plasma immersion ion implanter for the surface treatment of metal components. Surf Coat Technol 1996; 84: 537-543. Acknowledgement CNPq and FAPESP
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M. A. Fontes1; R. G. Pereira2; F. A. P. Fernandes2; L. C. Casteletti2; P. A. P. Nascente1 Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, São Carlos, SP, Brasil. 2 Departamento de Engenharia de Materiais, Universidade de São Paulo, São Carlos, SP, Brasil.
Plasma nitriding is a thermo-physical-chemical treatment process, which promotes surface hardening, caused by interstitial diffusion of atomic nitrogen into metallic alloys. In this work, this process was employed in the surface modification of a sintered ferrous alloy. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses and wear and microhardness tests were performed on the samples submitted to ferrox treatment and plasma nitriding carried out under different conditions of time and temperature. The results showed that the nitride layer thickness is higher for all nitrided samples than ferrox treated samples, and this layer thickness increases with nitriding time and temperature, and temperature is the more significant variable. The XRD analysis showed that the nitrided layer, for all samples, near to the surface consists in a mixture of ’-Fe4N e -Fe3N phases. Both wear resistance and microhardness increase with nitriding time and temperature, and temperature influences the most both characteristics.
Keywords: plasma nitriding, powder sintered metal, scanning electron microscopy.
Introduction Ion nitriding is a thermal-physical-chemical treatment process that provokes surface hardening by interstitial diffusion of atomic nitrogen into both ferrous and non-ferrous metallic surfaces. This process causes the formation of a case layer, which may comprise of an oxide layer, a compound layer, and a diffusion zone [1]. In the case of ferrous alloys, the compound layer, also called white layer, is constituted of mainly ’Fe4N, -Fe3N, and possibly Fe2N. It is very important to control the nitriding parameters in order to obtain the best performance of nitrided components used in different engineering applications [2]. The composition and thickness of the nitrided layers are directly related to the treatment temperature, pressure, and time, as well as the composition of the base material [3]. In this work, iron sintered samples with and without ferrox treatment were plasma nitrided, and the modified layers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, and wear and microhardness tests. Experimental part Sintered iron samples with dimensions of 31.75 mm x 12.65 mm x 10 mm were nitrided in an atmosphere of plasma 80 vol.% H2 and 20 vol% N2 and pressure of 5 mbar, at temperatures of 500, 540, and 580°C and periods of 3 and 5 hours. The microstructural analysis were carried out using a Philips SL-30 field emission gun (FEG) SEM apparatus equipped with an energy-dispersive spectrometer (EDS). The images were acquired employing the backscattering electron (BSE) method. XRD analysis was performed by a Rigaku Geiger-Flex equipment using Cu K radiation ( = 1.54056 Å) and 2 step scan of 0.032 per second in the range of 5-90. The wear tests were carried out in a fixed ball device using a 52100 steel sphere of 25.4 mm in diameter, a rotation speed of 300 rpm, a load of 245 g (2.45 N), and testing
times of 5, 10, 20, and 30 minutes. The microhardness measurements were obtained using a Buehler hardness tester, in the Vickers scale (HV), with a load of 100 gf. For each specimen, eight measurements were done at as many different points on the surface. Results and discussion SEM analysis revealed that the greater diffusion of nitrogen was observed for the samples treated at both 540 and 580°C (Figure 1). Higher thickness values were presented by the samples treated for 5 hours for a given treatment temperature. Figure 2 displays the XRD diffractograms for the samples nitrided at 580C for 5 hours, revealing the presence of ’-Fe4N and, in a lesser scale, -Fe3N phases. Regarding to the wear analysis, the wear volumes (in mm3) as a function of testing times are plotted in figure 3. The lower wear rate values are obtained for the samples nitrided for 5 hours at 540 and 580C, indicating that higher nitriding temperatures and times had a more favorable impact on the wear resistance. Figure 4 depicts the Vickers microhardness test results for all samples. The higher values were obtained for the samples nitrided at 580C for 3 and 5 hours, while the lower values were obtained for nitriding temperature of 500C.
Fig.1 - SEM micrographs of the plasma nitrided samples, and sample submitted to the ferrox treatment.
Fig.3 - The wear volumes (in mm3) as a function of testing times.
Fig.2 - XRD diffractograms for plasma nitriding temperature of 580°C for 5 hours.
Fig.4 - Vickers microhardness test results.
Conclusions Plasma nitriding of sintered ferrous samples can promote modifications on their surface properties. Regarding to the nitrided layer thickness dependence, the temperature variable predominates over the nitriding time variable. According to the Fick's second law, the nitride layer thickness is less dependent on t - time (non-linearly) than on T temperature (exponentially).
The nitrided layer consists predominantly of ’-Fe4N phase, with a smaller amount of Fe3N phase. The amount of the ’-Fe4N phase increases with higher nitriding temperatures and longer times. Higher nitriding temperatures and times enhanced both the wear resistance and hardness of the samples. References [1] Gontijo LC, Machado R, Miola EJ, Casteletti, Nascente PAP. Characterization of plasma-nitrided iron by XRD, SEM and XPS. Surf Coat Technol 2004; 183: 10-7. [2] Inokuti Y, Nishida N, Ohashi N. Formation of Fe3N, Fe4N and Fe16N2 on surface of iron. Metall Trans A 1975; 6: 773-784. [3] Collins GA, Hutchings R, Short KT, Tendys J, Van Der Valk CH. Development of a plasma immersion ion implanter for the surface treatment of metal components. Surf Coat Technol 1996; 84: 537-543. Acknowledgement CNPq and FAPESP
