Wear 274–275 (2012) 60–67
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Tribological properties of nitrogen ion implanted steel N. Kumar ∗ , S. Kataria, S. Dash, S.K. Srivastava, C.R. Das, P. Chandramohan, A.K. Tyagi, K.G.M. Nair, Baldev Raj Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 23 September 2010 Received in revised form 26 July 2011 Accepted 8 August 2011 Available online 16 August 2011 Keywords: N+ ion implantation Amorphous carbon Nano-hardness Tribological properties
a b s t r a c t Nitrogen ions (N+ ) with three different doses were implanted on the AISI 304 LN steel samples under high vacuum at room temperature. Dose dependent morphological and structural changes were observed in the specimen. Structural changes were triggered by formation of nitrides; irradiation induced surface segregation of carbon and deposition of amorphous carbon (a-C) by the cracking of hydrocarbons during implantation from oil diffusion pump. Morphological and structural changes were found to inﬂuence nano-mechanical and tribological properties of ion implanted surfaces. The nano-indentation hardness was found to increase to 10.26 GPa with highest N+ ion dose due to formation of surface nitrides and amorphous carbon. Frictional force was found to decrease with increase in N+ ion dose and a minimum value of 0.078 N was obtained at higher dose presumably due to the formation of amorphous graphite like phase. In addition, amorphous diamond like carbon on the implanted surface can be contributing facts for high hardness. At higher dose, both deformation induced damage and wear rate (2.4 × 10−7 mm3 /Nm) were found to be minimum. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ion implantation is one of the techniques, which has been widely studied and applied to several areas of engineering and technology. This method induces modiﬁcations in the physical and chemical properties at surface layers. Ion implantation easily creates a progressive interface between the implanted surface layer and the volume of unaffected material without modifying the original dimensions of the material . The ion beam induced modiﬁcation of steel surface has established a method to improve surface mechanical properties in one hand and chemical inertness on the other hand . Due to above advantages, this technique has been extensively used for improving mechanical and tribological properties of metal surfaces . The hardening can be related either to the formation of hard phase like carbides and nitrides or to radiation-induced dislocation effects . Tribological and microstructural effects of high current density implantation of nitrogen into high speed steel have been studied which also shows signiﬁcant improvement in frictional response of the implanted surfaces [4,5]. Hard inclusions near the surface, solid solution formation due to implantation, generation of compressive residual stresses in the surface layers and surface chemistry variation improve the frictional properties of the implanted surface [2,6]. The compressive residual stress on the surface is important for
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(N. Kumar). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.08.017
preventing fatigue failure . Ion nitriding of implanted surface can improve surface hardness but leads to a decrease in toughness. However, the wear-resistance behavior of steel surfaces was found to be inﬂuenced not only by nitriding or nitride formation but also by the presence of C–N phases, where the carbon segregates from the atoms of host lattice [2,7]. In spite of pronounced inﬂuence of ion implantation on the tribological properties, its mechanism to signiﬁcantly reduce the friction, is still a subject matter of research. The present study focuses on the investigation of effect of nitrogen ion implantation on the nano-mechanical and tribological properties of implanted AISI 304 LN steel. The N+ ion implantation induced microstructural properties of the surfaces and deformation behavior of wear scars have been correlated with friction and wear resistance properties of the material. Structural changes and phase composition of implanted surface were studied for quantitative evaluation of tribological properties. 2. Experimental techniques AISI 304 LN steel samples with size of 10 mm in diameter and 5 mm thickness were prepared by mechanical polishing of surface to a level of 20 nm. All these samples were implanted with N+ at 140 keV energy and incident ﬂuences (doses) of 5 × 1016 N+ /cm2 , 1 × 1017 N+ /cm2 and 5 × 1017 ions N+ /cm2 . Implantation current density was kept constant at 3 A/cm2 . Sample temperatures were maintained below 27 ◦ C during implantation to prevent effects of heating. X-ray diffraction (XRD) analysis was used to detect the phases present in the ion implanted specimen. To probe the
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near-surface region of specimen, the grazing incidence angle was kept below 1◦ . Cu K␣ radiation was used with a low intensity beam to minimize the ﬂuorescence from the iron. Elemental depth proﬁle analysis of the N+ ion implanted 304 LN stainless steel samples were carried out using dynamic SIMS (Cameca, IMS 7f). All the measurements were carried out in positive SIMS mode using Cs as primary ion source. The primary ion source (Cs+ ) of 5 keV impact energy and 25 nA beam current was rastered over an area of 150 m × 150 m in order to get uniform bombardment on the surface and the secondary ion species CCs+ , OCs+ , CrCs+ , NCs+ and H+ were collected with respect to time over a circular are of 60 m diameter. The process is repeated over three different places on the surface and the data presented is representative of the whole surface. The pressure inside the chamber during measurements was around 10−9 Pa. A nano-hardness tester (CSM Instruments, Switzerland) was used for extracting surface hardness of unimplanted and implanted samples at a constant load of 2 mN with a berkovich diamond indenter. Load–displacement curves were generated and resulting curves were analyzed using Oliver and Pharr method to obtain nano-hardness of the samples . Five indentations were performed on each sample. Friction and wear behavior of the implanted materials were determined by tribological tests using a ball-on-disk tribotester (CSM Instruments, Switzerland) in linear reciprocating mode. A steel ball (100Cr6 steel) with 6 mm diameter was used as a sliding probe. The normal load and sliding speed were kept constant at 1 N and 3 cm/s, respectively, with a stroke length of 3 mm during each experiment. The tests were performed at ambient (dry and unlubricated) conditions with relative humidity of 61%. Normalized wear rates of the coatings were evaluated from cross sectional proﬁles taken across the wear tracks after tribo testing by stylus proﬁlometry. Scanning electron microscope (SEM) technique was used to observe the morphology of implanted surface and the accompanying deformation response of wear tracks. Micro-Raman measurements were performed using a spectrometer equipped with a microscope to record the phase evolution on coatings and in wear surface. Laser with a wavelength of 514.5 nm was used as excitation source at a power of 5 mW. 3. Results 3.1. Nano-indentation studies Nanoindentation technique is a useful tool to characterize the surface and subsurface mechanical properties of materials . In this technique, load–displacement curves are recorded which are generally termed as mechanical ﬁngerprints of materials. The curves obtained for unimplanted and implanted samples are shown in Fig. 1. Shift of load–displacement curves towards lower penetration depths in implanted samples is observed with increase in ions dose. The shift is depicted by the values of maximum penetration depth (hm ) and residual or plastic depth (hp ) as shown in Table 1. A decrease in maximum penetration depth, at same indentation load of 2 mN, points towards an increase in penetration resistance of steel sample after ion implantation. This can be attributed to the presence of compressive residual stresses and formation of harder phases on the implanted sample. Also, the area underneath the Table 1 Parameters obtained from load–displacement curves. No.
(a) (b) (c) (d)
122.08 97.18 90.39 83.59
± ± ± ±
hp (nm) 5.30 2.12 1.12 1.61
111.3 77.34 65.77 45.66
± ± ± ±
5.13 2.60 3.06 1.16
6.37 18.34 25.65 44.43
± ± ± ±
0.18 0.75 3.58 2.48
Fig. 1. Nano-indentation hardness of (a) unimplanted steel, (b) implanted with dose 5 × 1016 N+ /cm2 , (c) 1 × 1017 N+ /cm2 and (d) 5 × 1017 N+ /cm2 . In the inset hardness is plotted.
load–displacement curve provides the magnitude of work done by the system. The ratio of elastic work to the total work, IT , characterizes the elastic fraction of the total work done during an indentation cycle . A larger value of this ratio indicates that the material is stiffer which in turn indicates the higher hardness of the material. This ratio for unimplanted and implanted samples is given in Table 1. A gradual increase in the value of IT from 6.37% for unimplanted sample to 44.43% for the sample implanted with highest dose was obtained. Correspondingly, a gradual increase in nanohardness from 3.63 GPa for unimplanted sample to 10.26 GPa for the sample implanted with highest dose was observed as shown in the inset of Fig. 1. Increase in hardness of ion implanted materials has been observed earlier [7,10]. Nearly three-fold increase in nano-hardness and seven-fold increase in IT indicates formation of harder and stiffer phases on the steel surface implanted with highest ion dose (5 × 1017 N+ /cm2 ). This improvement in surface hardness of steel samples after nitrogen ion implantation arises from the effects of solid solution hardening due to solute nitrogen ingress precipitation and formation of metastable iron nitride phases, like Fe2–3 N. The implantation induced formation of hard phases act as hampering sources for dislocation movement and lead to an enhancement of surface hardness [6,7]. On the other hand, the implantation induced dislocations can also cause the increase of the surface hardness [3,11]. Formation of diamond like carbon phase is also responsible to increase the hardness and elastic modulus and will be discussed below. 3.2. Tribological properties of ion implanted surfaces Tribological properties of implanted samples were also found to be signiﬁcantly inﬂuenced by the ion dose. Fig. 2 shows the evolution of frictional force during tribotests for all the samples. Frictional force is found to be minimum (0.078 N) and stable up to a wear distance of 40 m for the sample implanted with N+ ion dose of 5 × 1017 N+ /cm2 as seen in Fig. 2, curve (d). In case of lower implantation dose 5 × 1016 N+ /cm2 , frictional force is found to be higher and increases continuously with wear distance, as seen in curve (b) of Fig. 2. The trend of frictional force is similar for both the unimplanted and steel implanted with lowest dose 5 × 1016 N+ /cm2 . At the end of wear distance, frictional force is found to be 0.34 N and 0.28 N for unimplanted and lowest dose implanted steel, respectively, as seen in curves (a) and (b) of Fig. 2. The typical bar graph, in the inset of Fig. 2, represents coefﬁcient of friction measured at
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Fig. 2. Frictional force vs. wear distance for (a) unimplanted steel, (b) implanted with dose 5 × 1016 N+ /cm2 , (c) 1 × 1017 N+ /cm2 and (d) 5 × 1017 N+ /cm2 . In the inset the coefﬁcient of friction is plotted.
the end of wear distance. It is shown that the coefﬁcient of friction is higher for unimplanted steel and lower N+ dose implanted steel. At highest dose (5 × 1017 N+ /cm2 ), signiﬁcantly low value of coefﬁcient of friction (0.078) is obtained. Wear rates are also calculated and it is found to decrease from 3.8 × 10−5 mm3 /Nm at ion dose of 5 × 1016 N+ /cm2 to 2.4 × 10−7 mm3 /Nm at 5 × 1017 N+ /cm2 . Others have also observed a decrease in coefﬁcient of friction and wear rate after ion implantation [12,13]. Unimplanted sample is found to exhibit the highest wear rate of 6.4 × 10−5 mm3 /Nm. Enhancement in wear resistance in ion implanted specimen accrues from mitigation of fatigue induced adhesive failure in a oxidational surface wear where pristine surfaces are involved. Hardness, coefﬁcient of friction and wear rate are plotted against ion dose as presented in Fig. 3. The wear rate is found to be reduced for implanted samples because of formation of nitride and internal precipitates. 3.3. X-ray diffraction analysis of nitrogen implanted steel surface The diffraction patterns of unimplanted and implanted samples are shown in Fig. 4. ˛ (1 1 0), (1 1 1) and (2 0 0) diffraction peaks are observed [4,7,14]. ␣-phase lattice parameter was found to be 0.26 nm for unimplanted sample and which increased to 0.32 nm for the sample implanted with a dose of 1 × 1017 N+ /cm2 , as shown in Fig. 4(d). Interestingly, lattice parameter of ␣-phase decreases (0.25 nm) at highest dose can be explained by the
Fig. 4. XRD of (a) unimplanted and implanted steel with dose (b) 5 × 1016 N+ /cm2 , (c) 1 × 1017 N+ /cm2 and (d) 5 × 1017 N+ /cm2 .
transformation of phases. Average grain size of ␥N -phase is around 18–22 m with lattice parameter 0.38 nm. Same behavior has been observed by others [6,7]. High N+ ion implantation dose results in an expanded austenite lattice, due to high concentrations of nitrogen in solution, as extensively studied by Vardjman and Singer . 3.4. SIMS analysis The elemental depth proﬁle analysis of N+ ion implanted steel 304 LN with various doses using SIMS are shown in Fig. 5. Mainly N, Cr, O, C and H depth proﬁle is analyzed. It is clearly seen from the ﬁgure that the concentration of carbon is low at low N+ ion implantation [Fig. 5(a)] compared to concentration of carbon observed at high doses of 1 × 1017 and 5 × 1017 N+ /cm2 as shown in Fig. 5(b) and (c), respectively. The peak proﬁle of N is sharp at low doses of 5 × 1016 and 1 × 1017 N+ /cm2 and proﬁle of Cr is more or less similar as a function of depth. However, at high dose of 5 × 1017 N+ /cm2 , the peak intensity of N is high and broad features are observed indicating high concentration with larger depth. However, at lower doses depth proﬁling of N is less. Sharp decrease of Cr with N indicates replacement of Cr. At low dose, the C diffusion is observed up to ∼40 nm with weak intensity count. At high dose of 1 × 1017 N+ /cm2 , carbon deposition with the thickness of ∼25 nm is observed on the implanted surface. The sharp decrease of C concentration indicates diffusion in subsurface. However, at high dose C deposition is comparatively thicker 130 nm and sharp decrease indicates diffusion of C and low concentration. In this case, weak intensity of diffused O peak is observed which may appear due to contamination. Concentration vs. depth proﬁles of N and C in N+ ion implanted steel is extensively studied by Singer and coworkers [16,17]. It is demonstrated that the mechanisms by which C can get to the subsurface is attributed ﬁrstly by diffusion from the bulk and secondly, by surface chemisorptions and ion beam mixing. 3.5. Morphology and microstructure of unimplanted and implanted surface
Fig. 3. Comparison of coefﬁcient of friction, wear rate and hardness vs. ion dose.
Micro-structure of the unimplanted and implanted surfaces is shown in Fig. 6. Morphological observation of unimplanted surface of steel reveals distinct grain boundaries as shown in Fig. 6(a). It can be seen that the well structured grain boundaries on the surface of material are disrupted by the ion implantation. Microscopic
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Fig. 5. SIMS analysis of N+ ion implanted steel (a) 5 × 1016 N+ /cm2 , (b) 1 × 1017 N+ /cm2 and (c) 5 × 1017 N+ /cm2 .
pits are formed on the surface as depicted in (b) and (c) of Fig. 6. This happens due to N+ ions beam induced sputtering . Higher dose of ions causes to generate blistering on the surface, as seen in Fig. 6(d). Sputtering yield calculated by SRIM code is around
0.5 atoms/ion. At high dose (5 × 1017 ions N+ /cm2 ) thickness reduction of steel due to sputtering during implantation is around 60 nm. The scale of sputtering is low enough to produce surface morphologies clearly seen in the SEM image. Blistered surface morphology
Fig. 6. Morphology of (a) unimplanted surface, (b) implanted with dose 5 × 1016 N+ /cm2 , (c) 1 × 1017 N+ /cm2 and (d) 5 × 1017 N+ /cm2 .
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Fig. 7. Morphology of wear scars formed on (a) unimplanted surface, (b) implanted with dose 5 × 1016 N+ /cm2 , (c) 1 × 1017 N+ /cm2 and (d) 5 × 1017 N+ /cm2 .
is resulted due to N gas bubbles . Ion implantation leads to signiﬁcant transformation of the near-surface microstructure [16,20]. Deformed surface of wear tracks is shown in Fig. 7. Cracks and adhesive failure of surfaces are clearly seen in Fig. 7(a)–(c). Cracks are generated by the formation of persistent slip band during plastic deformation caused by generation of cyclic compressive stress during sliding. Typical persistent slip band is shown in the inset of Fig. 7(a). This band occurs near the edge of wear track on both sides. This is related to the formation of dislocation motion during plastic deformation. Adhesive failure of the thick oxide scales is dominant on the reference material, as shown in Fig. 7(a), and in samples with lower ion doses as seen from Fig. 7(b) and (c). Surface morphology of wear scars on N+ ion implanted samples with highest dose is shown in Fig. 7(d). Quantity of oxide scale is found to be less on the surface of wear scars. Cracks, adhesive failure, fatigue and surface grooving are not found in surfaces exposed to highest ion dose. 3.6. Raman spectroscopy of implanted samples and wear scars Micro-Raman measurements are performed to detect the various phases formed on the surface of implanted samples and in the tribo-induced wear scars. For reference, micro-Raman measurements are also performed on the unimplanted sample. Fig. 8 shows the Raman spectra collected from the worn and unworn implanted and unimplated surfaces. Fig. 8(a)–(d) depicts the Raman spectra obtained in wave number region from 100 to 1000 cm−1 whereas Fig. 8(a*)–(d*) shows the Raman spectra from 1000 to 2000 cm−1 . Unworn surfaces of implanted samples are found to exhibit two Raman bands at 366 cm−1 and 715 cm−1 at higher N+ ion dose. These bands have been assigned to ␣-Fe2 N phase. Sanchez et al. have also observed this phase after nitrogen ion implantation in steels . Two Raman bands at 1369 and 1589 cm−1 are seen in higher wave number region of worn implanted surfaces. These
bands are the characteristic D and G bands of amorphous carbon or generally termed as diamond like carbon (DLC) phase [22,23]. This points towards the presence of a superﬁcial or subsurface carbon layer. As Raman technique cannot distinguish between superﬁcial and subsurface carbon, secondary ion mass spectroscopy is used to analyse the chemical composition of the implanted surfaces and is presented in Section 3.4. However, it is observed that the presence of carbon on implanted surfaces is attributed to the crackdown of hydrocarbons present in the vacuum during implantation [24,25]. In contrast to Raman spectra of unworn surfaces, several features were observed in the acquired spectra of worn surfaces after wear testing. Mainly oxides phases corresponding to ␣-Fe2 O3 , Fe3 O4 and Cr2 O3 are observed in the wear tracks which have high intensity compared to the phases formed on the unworn implanted surfaces in the wave number range of 100–1000 cm−1 . These are shown in Fig. 8(a)–(d). The oxides phases of ␣-Fe2 O3 and Fe3 O4 correspond to the frequency bands at 207, 272, 275, 538, 541, 705, 709 cm−1 , respectively [26–31]. Oxide scale of Cr2 O3 is also observed at higher wave number (858, 868 and 871 cm−1 ) and this was found to be absent at the wear track of reference material, as shown in Fig. 8(a). Interestingly, ␣-Fe2 N phase is not observed in the wear tracks. Compared to wear tracks, the unworn implanted surfaces did not show noticeable presence of oxide phases. This could be due to the tribo induced oxidation of the implanted surfaces which can lead to the formation of oxide phases, as observed in the present study. Tribo-induced oxide phase evolution is a dominant phenomenon in wear tracks . Such process gets accelerated due to stress gradients in one hand and continuous renewal of exposed surface occurring due to repeated sliding. Relatively softer phase of oxides like ␣-Fe2 O3 can be formed at initial sliding stages when the formation of tribo-induced temperature is low [28,30]. Much higher oxygen potential is needed to convert Fe2 N to oxides like ␣Fe2 O3 and Fe3 O4 [26,28]. With increase in sliding distance, where
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Fig. 8. Raman spectroscopy of surface and wear scars formed in (a) unimplanted material (b), (c) and (d) implanted with dose 5 × 1016 N+ /cm2 , 1 × 1017 N+ /cm2 and 5 × 1017 N+ /cm2 , respectively, in wave number between 100 and 1000 cm−1 and (b*), (c*) and (d*) implanted with dose 5 × 1016 N+ /cm2 , 1 × 1017 N+ /cm2 and 5 × 1017 N+ /cm2 , respectively, in wave number between 1000 and 2000 cm−1 .
localized temperature continuously increases, hard oxides phases like Fe3 O4 and Cr2 O3 are observed. Tribo-induced oxidation of iron at low temperature results in the formation of ␣-Fe2 O3 in the outer layer and Fe3 O4 in the inner layer. During the initial stages, Fe3 O4 is formed which grows and thickens over the surface of tribo-surface. Nucleation and growth of ␣-Fe2 O3 phase occurs on the surface of Fe3 O4 . Raman spectra of worn surfaces acquired in higher wave number region from 1000 to 2000 cm−1 reveal the absence of D and G bands of amorphous carbon phase at lowest ion doses, as shown in Fig. 8(a*)–(d*). However, both the bands are observed at highest dose even after wear testing. The spectral proﬁle is best ﬁtted by two Gaussian proﬁles corresponding to the D and G band
of vibrational mode. It is seen that with increase in ion dose the G band intensity decreases and band width at the full width at half maximum (FWHM) increases as formed in Fig. 8(a). This broad feature is assigned to amorphous graphitic phase with the evolution of D band known as breathing mode in sp2 -atoms in rings. D band also corresponds to diamond like phase formation. Vibrational mode of D band appears at frequencies below 1500 cm−1 [23,31]. Detail of Raman spectral D and G bands are presented in Table 2. The shapes of spectra are found to be similar as seen in Fig. 8(b*), (c*) and (d*) and ID /IG values of 0.862, 0.876 and 0.882 were derived from the Gaussian ﬁts. It is seen that the ratio ID /IG also increases with implantation dose and it is signiﬁcantly higher (1.01) at the higher dose of 5 × 1017 N+ /cm2 . Higher implantation dose results
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Table 2 Summary of Raman spectral lines of unimplanted and implanted specimen. No.
Bands (D and G)
Peak position (cm−1 )
Band width ω1/2 (cm−1 )
Ratio of D and G bands frequency (ID /IG )
D G D G D G D G
1369 1589 1349 1538 1355 1535 1345 1530
67 80 184 91 152 98 181 114
(b*) (c*) (d*)
in larger ID /IG value which is indicative of disorder sp3 carbon like phase. At lower dose, the vibrational mode of D and G bands are found to be weak in wear tracks compared to bands formed on the surface as seen from Fig. 8(b*) and (c*). Peak frequencies of these bands are found to be centered at similar positions in the wear tracks as well as on the implanted surface. This is seen in Fig. 8(d*). 4. Discussion Microstructure of the implanted samples was found to change with the dose. With increase in nitrogen ion dose, hardness of SS 304 showed an increase along with a reduction in friction for the highest dose implanted surface. X-ray diffraction studies revealed the strained austenitic phase due to the high concentration of nitrogen in the solution. The volumetric size of lattice increase can be attributed to the irradiation induced defects generation and agglomeration of voids. The noticeable result is the formation of oversaturated nitrogen solid solution in austenite, which results in displacement of the austenite phase lines . Most of the microstructures, produced by ion implantation are metastable and characterized by enhanced level of stored energy. This excessive energy can be used for transformation of surface microstructure during friction test to optimize tribological properties of the surface . In this case, precipitates produce a high stress concentrated in the surface layers causing accumulation of residual compressive stress. This is one of the important mechanisms which can improve the wear resistance and hardness and considerably lowers the frictional force . In the surface region of N+ implanted steel, a ﬁne dispersion of nitrides and a supersaturated solid solution of interstitial nitrogen co-exist as described by Feller et al. . In analogy, low friction with superior wear resistance and hardening can be obtained by implantation induced nitride precipitate dispersion. These smaller precipitates have the ideal range of size to effectively pin dislocations by interacting elastically with them and consequently impeding their motion in the near surface region . Frictional behavior of the dose dependent ion implantation and unimplanted surfaces could be discussed in terms of tribo-induced deformation to understand the relation between deformation, wear rate and friction. Sliding contact between the surfaces of solids is accompanied by plastic deformation, which may result in signiﬁcant transformation of the surface microstructure of wear scars . However, adhesive failure is caused by fatigue of oxide scales that attribute to weakened interfacial bonding between the oxides and metal during continuous sliding which causes fretting of surface. Weakening of the bonding between these interfaces can be accelerated by rise in localized tribo-induced temperature which can also produce severe fretting. Surface and subsurface nitride restricts oxide formation. Such phenomena prevent cracking and adhesive failure. However, prolonged sliding did cause severe damage to the implantation hardened surface. Subsurface plastic deformation as well as tensile stress accumulation causes microcracks to propagate at the interface, ultimately leading to adhesive failure [6,7]. Hydrostatic stress accumulation in contact zone causes residual
0.876 0.882 1.01
compressive stress to rise. This renders void nucleation and microcracks propagation. With implantation hardening the magnitude of plastic strain as well as strain rate accompanying wear deformation reduces [4,10]. Above both factors causes a tapered damage gradient across specimen depth. Formation of a-C in the wear track of unimplanted steel is recorded by Raman spectroscopy [Fig. 8(a*)]. It may be explained by the continuous tribo-induced shearing which frees the carbon atoms located at the interstitial lattice site due to a raise in stress induced energy of carbon atoms. These carbon atoms move to the surface when the ﬂux of deformation induced dislocation triggers vacancy motion. Tribo-induced continuous sliding produces dislocation induced defects on the surface of material. But some of these defects relax due to dissipation of elastic energy . This elastic energy can contribute to the movement of interstitial carbon atoms to the surface. This energy can also relax the residual compressive stress. At the surface, migrated carbon atoms from the sub-surface can interact with each other forming a-C like structure. Formation of a-C like phase is also observed on the implanted steel surfaces mainly by the cracking of hydrocarbons during implantation. This is also driven by the pining of nitrogen ions into the carbon sites which are present in metallic lattice. At excess energy, carbon atoms can become free from the lattice which can thus segregate to the implanted surface. During implantation, vacancies and interstitials move in opposite direction causing diffusion of ions. This mechanism represents that carbon atom from the subsurface region migrates to the surface with the aid of irradiation induced vacancies. Band width of D and G mode of a-C increases with increase in ions dose which could be related to irradiation induced amorphization. Ratio of ID /IG is nearly unity at highest dose while little variation is observed at lower ion doses. It is found to be 0.862 for the reference material. Characteristic D and G modes have different band widths of around 67 cm−1 and 80 cm−1 , respectively, for the reference material. This is related to the minimum defects produced by shear induced deformation during sliding of counter body. The amorphous carbon embedded in metallic matrix, can signiﬁcantly reduce the friction and improve the wear resistance. As seen in Fig. 8(d*), the intensity of Raman spectra of D and G bands is similar on the surface and in wear scars. The low friction characteristics arise due to the formation of graphite like structure of sp2 -bonded carbon network which act as a lubricant phase while the disorder diamond like D mode acts as a harder phase embedded in softer matrix. Ideal mixing of G mode and harder D mode in amorphous structure of carbon causes signiﬁcant reduction in friction and improves the wear resistance, as seen in curve (d) of Fig. 2. Formation of these phases is also observed in the wear tracks, shown in Fig. 8(b*) and (c*), where frictional force is found to be high, as depicted by curves (b) and (c) of Fig. 2. This relationship can be established by (a) wearing of surface due to tribo-induced sliding where the removal of carbon atoms is dominant (b) carbon segregation driven by ion ﬂuence towards the surface. Formation of various oxides, which are mentioned above, is also observed to be insigniﬁcant in wear track formed
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on the implanted surface. The weakly formed oxide phases have secondary effect on friction while amorphous carbon mixed with the metallic structure profoundly inﬂuence the frictional behavior. Oxide phases are observed to be stronger in the wear track formed on the unimplanted surface. Along with the oxide formation, amorphous carbon phase is also observed in the wear track formed on the reference material although friction force was high. Higher friction could be related to the evolution of various metallic oxides which also interact during tribo-induced sliding. Formation of both softer and harder mode of amorphous carbon (D and G bands) can reduce the friction. Oxides ␣-Fe2 O3 and Fe3 O4 increase resistance to sliding resulting in higher frictional energy dissipation, as shown in curve (a) of Fig. 2. These metallic oxides form the strong adhesive bonding between the interfaces which disrupt the sliding motion due to adhesion induced jamming. The most probable sites for ␣-Fe2 O3 and Fe3 O4 formation are vicinities of contact spots between the counter body and hard steel. 5. Conclusions Nitrogen ions with 140 keV energy at various doses were implanted on the steel surfaces in vacuum condition at room temperature. Dose dependent morphological and structural changes of implanted surface were observed. Deposition of amorphous sp3 and sp2 like carbon network was observed on the implanted surface due to cracking of hydrocarbons from the oil diffusion pump. The carbon network signiﬁcantly increased hardness and wear resistance properties especially at high N+ ions implantation dose. The signiﬁcant reduction of frictional force owes its genesis to the presence of thin layer of amorphous carbon on the implanted surface which contributed to remains in wear tracks despite tribo-induced sliding. This amorphous carbon layer together with hard nitride phase signiﬁcantly reduces the frictional force. Such a layer also arises at wear track of unimplated bulk specimen due to elastic energy induced segregation of carbon from host lattice. Wear rate was also found to be low with increase in ion dose 5 × 1017 N+ /cm2 where the frictional force were found to be 0.078 N. Relation between hardness, friction and wear rate was established. With increase in hardness, friction and wear rate was found to decrease. Acknowledgements Authors gratefully acknowledge the helpful comments and suggestions from the Reviewers to improve the quality of the manuscript. Authors would like to thanks Prof. B.S. Murty (IIT, Chennai) for providing the XRD facility. We thank Mr. Ashok Bahuguna, from SND/IGCAR for the measurements of wear dimensions using proﬁlometer. Authors especially thank to Dr. S. Rajagopalan and Mr. Ashok Bahuguna, SND/IGCAR for SIMS measurement. Authors are also grateful to Dr. C.S. Sundar, Director MSG/IGCAR and Dr. A.K. Arora, Head, CMPD/IGCAR for useful technical discussions. References  M.J. Marques, J. Pina, A.M. Dias, J.L. Lebrun, J. Feugeas, X-ray diffraction characterization of ion-implanted austenitic stainless steel, Surf. Coat. Technol. 195 (2005) 8–16.  C. Neelmeijer, R. Grötzschel, E. Hentschel, R. Klabes, A. Kolitsch, E. Richter, Ion beam analysis of steel surfaces modiﬁed by nitrogen ion implantation, Nucl. Instr. Meth. Phys. Res. B66 (1992) 242–249.  M. Li, E.J. Knystautas, M. Krishnadev, Enhanced microhardness of four modern steels following nitrogen ion implantation, Surf. Coat. Technol. 138 (2001) 220–228.
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