Moradshahi et al [6] have practiced the plasma nitriding of Al alloys using a DC glow discharge. The experimental results of GDS show a slight decrease in the thickness of nitride layer y with increasing pressure. The low density of nitrogen atom is evident by EDX and RBS. The XRD results of nitride samples shows fcc AlN phase (lattice parameter α= 4.045 Å).
Meletis et al [7] attempted nitriding of aluminum by using intensified glow discharge plasma. They reported the feasibility of formation of aluminum nitride under enhanced plasma conditions at low pressure. AES, XPS, XRD, SEM and TEM characterizations were performed. AES shows a decrease in the thickness of the nitrided layer. An early presence of Al2O3 detected but for longer treatment time only AlN detected by XPS. The face centre cubic structure of AlN is observed in the XRD analysis of the nitride samples with a 200 preferred orientation. The SEM microstructure of nitrided-specimens posses smooth surface morphology of AlN surface. The electron diffraction pattern AlN by TEM reveals fine nuclei size AlN with face centre cubic structure of lattice parameter 4.38 A which is in accordance with as calculated from XRD. The bright field image of TEM shows the presence of dislocation substructure in Al layer near to the nitrided surface.
Shah et al [8] reported the plasma nitriding of Al-Mg2-Si performed for various duration of time at a gas pressure of 1 and 2 mbar. The crystallite size and the residual stresses were calculated by the XRD analysis. The thickness of layer increases linearly with the treatment time for both the pressures. An increase in Microhardness is observed with the time however the increase is more prominent in the case of when the pressure is 2 mbar.
Ebisawa et al [9] reported the formation of aluminum nitride by treating the negatively biased aluminum substrate in electron cyclotron resonance ECR nitrogen plasma. A thick layer of about 15 um has been formed resulted in to the increased vicker hardness i.e. 1400. The crystal structure of AlN synthesized by ECR nitrogen plasma is consisting of wurtzite crystal structure which is confirmed by the XRD and EPMA results. SEM results showed that nitride layer have two different morphologies one is the bottom layer which is consist of dense columnar structure whereas the surface layer have pores and a granular morphology.
Quast, et al [10] reported the nitriding of aluminum alloys using nitrogen along with argon hydrogen plasma in a furnace evacuated to a pressure less than 2 Pa. The sample nitride is a piston of Al-Mg-Si which is treated for 6 hour at a temperature of 480 C, pressure of 200 Pa, with 10:1 nitrogen to hydrogen ratio and a discharge voltage of 500 V. A thicknes of 1 um layer is achieved and the presence of chemically homogeneous AlN composition is evident from the XPS results however the concentration of the nitrogen reported on the surface of treated sample is less than 25% at. With increase in the time thickness of the nitrided layer increased to 2um resulting into a smooth wear resistant layer. On further increase in time or thickness, the surface roughening has been observed.
Budzynski et al [11] reported the implantation of 120 KeV nitrogen ion having fluencies ranging from 3xl017 to 1.1x1018 N/cm2 at a chamber pressure of <1.3x10-4 Pa with 1uA/cm2 ion current density at a temperature of 55-60oC. A deeper diffusion of nitrogen ions detected by RBS from their projected range claimed by the author for the above mentioned parameters. The diffusion coefficient for free nitrogen atoms is large enough to allow migration from the implanted layer towards the nitrogen with lower atomic N/Al ratio, which helps the nitrogen ions to diffuse further in to the substrate. The XRD spectra shows the presence of (0 0 2), (1 0 1) and (1 1 0) of aluminum nitride crystals with hcp structure when compared with the ASTM file. The observed increase in the micro hardness with ion fluencies is attributed to the defects produced by implantation and the formation of hcp AlN layer on the surface. In nitrogen implanted samples immersed in methanol the friction coefficient increases slowly from a relatively low value over the first 500 cycles and then at an even slower rate for subsequent cycle. So an overall improvement in tribological properties of the aluminum after implantation has been achieved.
Srivatsan et al [12] studied the N2+ ion implantation effect on the tensile deformation of 1018 carbon steel in air and dry nitrogen gas atmosphere. The implantation of nitrogen ions was performed under vacuum of 10-6 Torr. N2+ ions were accelerated to energy of 100 KeV in an ion beam accelerator. The ion beam current density is maintained at a fluence of 1.5 x 10-17 N2+ cm-2 which helps to control the implantation temperature. Srivatsan et al observed that the yield strength of the ion implanted specimens decreased 13% as compared to untreated specimens deformed in laboratory air atmosphere. This decrease in yield strength reduces to 4% percent for the specimens deformed in dry nitrogen gas atmosphere. The decrease in the yield strength is associated with an increase in the ductility which was measured upto 35% improved elongation for specimens deformed in laboratory air and 13% for the specimens deformed in dry nitrogen gas atmosphere. He reported the reason for the decrease in the yield strength was free dislocation produced on the surface of implanted specimens which assists the deformation of the specimens at low stresses. However along the decrease in the yield strength of material Srivatsan et al reported a marginal increase in the ultimate tensile stress of the implanted specimens. This marginal increase was attributed to the precipitates formed by the diffusion of nitrogen in to the specimens. Fractography analysis of both implanted and unimplanted specimens in both environments is moderately ductile.
Tokaji et al[13]. Studied the effect of gas nitriding on fatigue and bending tests of titanium and its alloys. Nitriding was carried out in an evacuated electric furnace using 4N nitrogen gas at a constant pressure of 0.13MPa. An increase in the depth and hardness of nitrided layer was observed with an increase in the nitriding temperature and duration. In case of the pure titanium the fatigue strength of nitrided specimen is enhanced as compared to that of untreated specimen, however in case of titanium alloy the resistance to fatigue deformation decreases for the nitrided specimens. The variation in the fatigue resistance was attributed to the initiation of cracks which are relatively less for nitrided pure titanium, and in case of nitrided alloys the decrease in the fatigue resistance is attributed to the premature initiation of cracks.
ZHU. et al [14]. Investigated the effects on low cycle fatigue resistance of ferrite alloy, by nitrogen ion implantation. Implantation of nitrogen ion was done by using nitrogen ions of energy 65keV at 2 x 2017 ions/cm2 fluence at a temperature of 300oC. They observed an inconsistent fatigue life enhancement and significant suppression of persistent slip bands on the surface caused by the formation nitride precipitates on the surface.
Qian et al [] investigated the effect of ion nitriding on the uniaxial fatigue deformation using polished samples of SAE 1045 HR steel. Increases in surface hardness along with residual stresses were observed in the ion nitrided specimens. Two type of cyclic fatigue test i.e. Low Cycle Fatigue LCF and High Cycle Fatigue HCF was conducted at room temperature on various specimens. It was observed that ion nitriding have unfavorable effects in case of low cycle fatigue whereas an increase in the fatigue life has been observed in case of high cycle fatigue. In low cycle fatigue test cracks are generated on the nitrided surface due to the difference in plastic flow stresses of hard nitrided surface and soft ductile bulk, which causes the failure of material at early stages. Residual stress were found not to have any kind of detrimental effect in case of high cycle fatigue test resulting in to improved fatigue life.
