Self-lubrication mechanism of chlorine implanted TiN coatings

Wear 254 (2003) 668–679

Self-lubrication mechanism of chlorine implanted TiN coatings Thananan Akhadejdamrong a , Tatsuhiko Aizawa a,∗ , Michiko Yoshitake b , Atsushi Mitsuo c , Takahisa Yamamoto d , Yuichi Ikuhara e a

Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan b National Institute for Materials Science, Tsukuba 305-0003, Japan c Tokyo Metropolitan Industrial Technology Research Institute, Tokyo 115-8586, Japan d Department of Advanced Materials Science, The University of Tokyo, Tokyo 113-8656, Japan e Engineering Research Institute, The University of Tokyo, Tokyo 113-8656, Japan Received 25 May 2002; received in revised form 10 March 2003; accepted 10 March 2003

Abstract Different from the conventional physical modifications, significant reduction of wear and friction in severe dry conditions can be accommodated to titanium nitride (TiN) coating via the chlorine ion implantation. High friction coefficient with µ = 0.8–1.2 for the as-deposited TiN is reduced to be less than 0.2 at room temperature. Titanium mono-oxide (TiO) and oxides with oxygen deficiency or Magnèli phase with Tin O2n−1 , were formed inside the wear track of Cl-implanted TiN coating. Due to the shear deformability of titanium mono-oxide and crystallographic shearing planes in this Magnèli phase, vicinity of the Cl-implanted TiN surface can be elasto-plastically deformed, resulting in reduction of shear stress, wear and friction. Micro-X-ray photoelectron spectroscopy (XPS) measurement as well as high-resolution transparent electron microscopy (HRTEM), were an effective tool to describe local surface reaction taking place inside and outside of the wear track. Oxidation process of TiN during wear is drastically changed at the presence of Cl-atoms on the surface. Cl-atoms diffuse from the inside of TiN to the surface to accelerate the formation of titanium oxides, and to escape out of the system together with oxide debris. Both wear volume and friction coefficient, are preserved to be as low as or lower than diamond like carbon (DLC) coatings. This preferable tribological property comes from self-lubrication mechanism of the Cl-implanted TiN due to significant change of surface reaction by the effect of Cl-atoms. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Self-lubrication; Ion implantation; Chemical modification; Chlorine; Shear deformation

1. Introduction Ion implantation has been grown up as one of the most promising techniques for surface modification. As summarized in the textbook [1], ion doses and ion depth profiles can be widely controlled in the adaptive manner to computer simulation. No change occurs in the original dimensions, and, no limitation exists in the solid solubility for the implanted atoms. Hence, both the chemical composition and the microstructure can be selectively modified throughout the penetration depth without change of host material properties. Most of the studies positively evaluated a role of ion implantation on improvement of machinability, high strength and wear resistance [2–4]. Against its practical usefulness in the above, there still remain many unknowns or open issues with respect to the induced mechanism to improve the wear resistance via the ion implantation. ∗ Corresponding author. Tel.: +81-3-5452-5086; fax: +81-3-5452-5116. E-mail address: [email protected] (T. Aizawa).

Most of explanations for post-implantation properties stand on the physical modification effects. As pointed out in [5–7], the host metallic alloys and ceramics are work-hardened by the ion implantation. Recent works for ion implantation into titanium nitride (TiN) coating have supported this type of physical modification effects [8–11]. The common sense to these descriptions originates from the fact that both point defects and dislocations should be generated in high density via ion implantation. Increase of yield stress can be explained by this dislocation-induced strengthening. Increase of surface hardness, solid solution and dispersion strengthening processes as well as grain boundary strengthening mechanism, were all indebted to intense beam–solid interactions. Residual stresses in the starting PVD-coated TiN had influence on the wear behavior of physically modified TiN [12]. Through these studies, the implantation effects on the improved wear behavior have been rationally understood from the physical points of view. If every post-implantation wearing behavior were explained in the above manner, the implanted zone in the

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00249-7

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titanium nitride coating should be working as a hard or stiff buffer layer to protect TiN coating from adhesion or severe wear in dry. This tribological surface design, however, is completely against the modern self-lubricating tribo-films by diamond like carbon (DLC) or MoS2 coating [13,14]. In situ synthesized polymer layers or MoS2 work as a soft buffer to directly reduce the friction stress by significant plastic deformation. As had been partially discussed in [15], the chemical modification route plays a role on improvement of wear resistance and oxidation toughness by boron-, carbon- or aluminum implantation. Increase of the population in C–C bonding with increasing the carbon doses leads to enhancement of wear resistance [16]. Owing to the metallic aluminum or in situ synthesized cubic AlN in the aluminum implanted TiN, oxidation toughness can be promoted by in situ formation of protective Al2 O3 layer on the TiN surface [17]. These findings showed that ion-implanted TiN should have different mechanical performance on the chemical modification route. In the present paper, post-implantation wear resistance is reconsidered on the chemical point of view. As had been reported in [18–21], the halogen ion implantation into TiN has unique potential to accommodate the self-lubrication mechanism, where the in situ synthesized buffer layer with intermediate titanium oxides should deform elasto-plastically to relax the applied normal pressure and tangential stress. This self-lubrication mechanism is taken to reconsider the key-items in the chemical modification effects on the reduction of wear and friction. Precise description of tribo-reaction in the wear track, requires direct microscopic observation of wear track and wear debris. X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), are used to describe the wear mechanism concerning the role of implanted chlorine on the tribo-oxidation behavior. Comparison of the reacted compounds and their yield on the worn surface between as-deposited and Cl-implanted TiN coating, leads to a key to understand the effect of chlorine on the formation of lubricious titanium oxides.

2. Chemical modification route for self-lubrication via chlorine ion implantation In the tribo-chemical reaction at the presence of lubricating agents or oils, metallic or inorganic constituents both in work/tool materials are reacted with those agents to form the solid lubricants or the lubricating soft buffer between tool and work materials [22]. In case of dry wear for TiN coated tools or dies, since no lubricants are supplied into system, a helper is necessary to accommodate the self-lubrication process. At the absence of lubricants, the wear mechanism of TiN coating is governed by the surface oxidation. Hence, a key for tribo-chemical modification lies in how to control the oxidation process in TiN during wearing. In general, there

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are two parameters governing this process: oxygen partial pressure and active use of oxidizer. Assuming the contact surface might be self-lubricated, the flash temperature on the contact surface is thought to become relatively low and the oxygen partial pressure in the wear track is often almost uniquely determined in air. Hence, a helper to control the tribo-oxidation process must be added via ion implantation. Among various kinds of lubricant oils, many metal forming experiments [23] reported that chlorine-bearing lubricant is recommended in use for reduction of wear and friction since a protective lubricant might be strictly fixed on the tool surface with aid of chlorine. Even in the electrical chemical treatment, the role of chlorine in the high temperature oxidation was high-lighted with respect to formation of unique oxide layer [24]. In those cases, the chlorine works as an oxidizing helper. The first success of self-lubrication accommodation via Cl-implantation to TiN [18] revealed that the implanted chlorine is found to control the post-treatment wearing behavior. As discussed in [25], fluorine has also the same potential of self-lubrication accommodation. Hence, exploration of chemical modification route via halogen base ion implantation can afford to provide a new application of ion implantation in practice. This type of chemical modification utilizes different implantation conditions from normal physical modification. Since the self-lubrication effect can be accommodated by lower energy implantation, the induced density of dislocations might well be relatively low. In the case of relatively excessive implantation condition with the energy of 100 keV and the dose of 1.0 × 1017 ions/cm2 , the scalar dislocation density on {1 1 1} plane of Cl-implanted TiN was measured by using the Fast-Fourier Transport method to pick up only the strained region from HRTEM image. It becomes 1.6 × 1013 , 7.0 × 1013 and 1.7 × 1013 cm−2 , in the vicinity near the surface, in the implanted zone and in the deeper zone with absence of chlorine atoms, respectively. Under this condition, the chlorine implantation also has sufficient momentum to induce a large amount of dislocations with near-zero micro-strains. Hence, argon implantation must be utilized as a reference to discriminate the chemical modification potential of Cl-implantation from the physical modification one, since both species have nearly the same ion implantation energy density as a physical modification capacity. Recent works [26,27] reveal that self-lubrication effects via Cl-implantation can be obtained by reduction of the energy by 1/3, the dose by 1/5 to 1/2, and, by using the single-charged beam with low current density. This reduction of implantation conditions is favored for practical use of this processing. Different from the carbon, boron or aluminum implanted into TiN, the chlorine has little distinct boding state in TiN to be discussed by XPS analysis. Different from the argon, no desorption of chlorine was detected unless the bulk temperature was increased over 773 K. This difference reflects on the self-lubrication process during wearing. Due to its weak bonding state with TiN lattice structure, the implanted

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chlorine might have sufficient mobility to diffuse in the columnar TiN structure. Although the chlorine atoms might be ejected together with wear debris in wear, most of chlorine is thought to be working as an oxidizing helper in TiN. Through precise analysis of chemical bonding state in TiN, the chemical modification process in TiN at the presence of chlorine is described.

Table 1 Summary of wear results obtained from friction sliding test of Cl-implanted TiN against AISI 304 stainless steel ball with reference to as-deposited and Ar-implanted TiN coatings

Friction coefficient Wear coefficient Wear volume Wear mode

As-deposited TiN

Ar-implanted TiN

Cl-implanted TiN

0.8–1.2 ∼10−5 to 10−4 ∼10−11 m3 Adhesive

∼1.0 ∼10−4 ∼10−11 m3 Adhesive with sticking mode

∼0.2 ∼10−7 to 10−6 ∼10−13 m3 Abrasive

3. Experimental method Substrate samples with the size of 15 mm×15 mm×2 mm were cut from high-speed tool steel AISI M 36. They were subsequently ground, polished, hardened, and cleaned by Ar-bombardment before coating. TiN films were deposited on all the surfaces of these substrates by a commercial-type hollow cathode discharge ion plating (HCD-IP) at temperatures about 723–773 K. The thickness of TiN coating was about 1 ␮m. Implantation was performed by an ion implanter which is equipped with a mass separation and a magnetic beam scanning. The samples were mounted on a target manipulator and irradiated by a scanning beam of Cl-ions at 100 kV. Ion beam was generated from AlCl3 in a Freeman type ion source with a vaporizer and mass-selected to yield single, positive-charged chlorine beam. Cl-ion doses were varied from 1.0 × 1016 to 1.0 × 1017 ions/cm2 . The incident angle was fixed in normal to the sample surface and the vacuum was controlled to be less than 2 × 10−5 Pa during implantation. To suppress the heating by ion beam itself, the beam current density was limited to 0.03–0.05 A/m2 throughout the process. Ball-on-disc testing was employed for evaluation of wear and friction behavior, and performed under dry conditions at room temperature with the relative humidity of 20–30%. Its experimental situation was depicted in Fig. 1. A stainless steel AISI 304 ball with the diameter of 6 mm (rb = 3 mm) was used as a counter material; width for sliding track became 12 mm. Normal load (W) was varied for 2 and 5 N, and sliding velocity (V), for 0.005–0.15 m/s. Total sliding distance (L) was fixed to 50 m in all the tests. From the measured tangential force (F), circular radius (a) and diameter of wear flat (d), the following five tribological parameters were calculated: the friction coefficient (µ), the archard sliding wear

equation (δW), the wear volume (Wv ), the wear rate (Ws ) and the archard wear coefficient (Q). They are defined as follows: µ = F /W , δW = P /(π a 2 ), Wv = (π d 4 )/(64rb ), Ws = Wv /(WL) and Q = KW/H . Here K is the wear volume per unit sliding distance and H the hardness. Atomic force microprobe was utilized to measure the surface morphology both for as-deposited TiN and Cl-implanted TiN. Roughness was evaluated as the average value over 10 ␮m in length: Ra = 3.5 nm, Ry = 53 nm and Rz = 47 nm for as-deposited TiN, and Ra = 2.4 nm, Ry = 46 nm and Rz = 21 nm for Cl-implanted TiN. Since no significant difference was detected between two, the tribological data to be obtained might well be free from the initial roughness effects. XPS with monochromatic Al K␣ radiation was used to make analysis of local chemical composition and chemical state at the wear track and flat area on the worn surface of counter material. Survey spectra were taken in the energy range from 0 to 1400 eV at 1 eV per step prior to measurement of high-resolution spectra. Elemental spectra were obtained by intermittent surface sputtering with 2.0 kV Ar+ ions at the same rate (approximately 5 nm/min) as for a thermally oxidized SiO2 film. For chemical interpretation of relative intensity for XPS profiles, procedure for analysis with use of reference peaks is indispensable. In particular, since the secondary peaks of Ti and N overlap in TiN, the reference standard is necessary to convert the peak intensity to atomic concentration. In this measurement, Ti 2p, Cl 2p, N 1s and O 1s peaks were corrected with respect to C 1s peak, which was measured for the adventitious hydrocarbon at the energy of 285.3 eV. After sliding test, wear debris were directly observed by HRTEM without using any sample thinning techniques. energy dispersive X-ray spectroscopy (EDS) was also used for chemical analysis.

4. Experimental results

Fig. 1. Illustration of friction sliding test using the ball-on-disc method.

Effect of Cl-implantation on the friction behavior of the TiN coating against a stainless steel ball in the sliding test under dry condition, was first summarized in Table 1 after previous work [20]. History of friction coefficient with sliding distance was measured for as-deposited, Ar-implanted, and Cl-implanted TiN coating samples, respectively. The friction

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coefficient at the normal load of 2 N, ranged in 0.8–1.2 and 1.0 for the as-deposited and the Ar-implanted TiN, respectively. The friction coefficient was significantly reduced from 1.0 to be less than 0.2 by ion implantation with increasing Cl-dose up to 1.0×1017 ions/cm2 . This value is nearly equal to the friction coefficient for typical diamond like carbon coating. Furthermore, even with increasing the sliding distance, this friction coefficient for the Cl-implanted TiN was constant or slightly decreased. To be noted is that no improvement was seen in the friction coefficient and wear volume for Ar-implanted samples. As before mentioned, both the Ar- and Cl-implantation are thought to have the similar physical modification capacity against the wearing. Hence, hardening and strengthening via physical modification is never responsible for this reduction of wear and friction against the early success in improvement of wear resistance via Ar-implantation [28]. Although various factors are effective to explain this difference, severe dry wearing mode under high nominal pressure of 850 MPa in the present wear testing, might be never improved only by surface hardening effect. The self-lubrication mechanism should only work on the Cl-implanted TiN through the chemically modified surface reaction during dry wearing at the presence of chlorine. The wear volume of stainless steel ball after wear testing was also reduced in orders of magnitude with increasing Cl-doses. Fig. 2 showed the comparison of wear volume for counter material between as-deposited and Cl-implanted TiN samples. Wear volume proportionally increases with the sliding velocity for both samples, but the value is much smaller for Cl-implanted samples. The relationship of the wear rate in the function of normal load and sliding speed in

dry condition, told that the wear mode for all Cl-implanted TiN conditions was thought to be located in the mild oxidation regime (10−6 to 10−7 mm3 /(N m)) even for the highest sliding speed (0.15 m/s) in this study. Actual wear and friction took place in the wear track between two contact matters. Precise investigation on chemical reactions inside and beside the wear track or on the worn area of counter material, provides us important information about a change of wear and friction behavior for Cl-implanted TiN. XPS scanning analysis was carried out for the selected surfaces on the as-deposited, the Cl-implanted TiN coating and the counter material. Monochromatic Al X-ray beam diameter was fixed with 50 ␮m to discriminate the chemical composition and chemical state on the wear track and beside it. The analyzed spectrum patterns inside and beside the wear track were illustrated in Fig. 3. The surface spectrum beside the wear track showed the surface chemical composition before the wear test. Through comparison of spectra in Fig. 3, the effect of Cl-atoms on the surface reaction can be described before and after wear test. The binding energies of Ti 2p, O 1s, Fe 2p, Cr 2p, N 1s, C 1s, and Cl 2p were summarized in Table 2. Peak intensity was converted to atomic concentration by using the relative sensitivity factor. Chemical compositions

Fig. 2. Comparison of wear volume measured at W = 2 N and V = 0.005–0.15 m/s between the as-deposited TiN and the 0.5 × 1017 ions/cm2 Cl-implanted TiN coatings.

Fig. 3. XPS spectra detected from four surfaced regions of: (1) beside and (2) inside wear track for unimplanted TiN, (3) beside and (4) inside wear track for 1.0 × 1017 Cl/cm2 implanted TiN.

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Table 2 Binding energies of elemental spectra from worn and unworn surface of as-deposited and Cl-implanted TiN, after calibration with respect to C 1s peak

TiN Beside wear track Wear track TiN + Cl+ Beside wear track Wear track Ball White area Dark area

Ti 2p

O 1s

Fe 2p

Cr 2p

N 1s

C 1s

396.2 397.3

285.4

455.4 458.7

530.2 532.6

455.4 458.4

530.1 531.5

455.2 459.0

530.5 532.7

397.0

285.5 289.2

455.4 458.4

530.5 532.2

396.2 397.3

285.2 289.0

459.3

530.8 532.4

707.2 710.6

574.4 577.0

400.3

285.3 288.7

198.9 200.4

530.5 532.2

707.1 709.4 711

574.2 577.2

400.4

285.3 289.3

199.0 200.5

711.3

Cl 2p

285.3 288.8 199.1

Data in italic represent binding energies of shouldered peaks.

both for the as-deposited and Cl-implanted TiN can be estimated from the spectra (1) and (3) in Fig. 3, respectively. Nitrogen concentration was decreased from 19.1 to 7.7 at.% while oxygen was increased from 25.7 to 36.3 at.%, and titanium, nearly constant 19.3–21.9 at.%. It implies that a part of TiN at the surface had been oxidized to yield significant amount of titanium oxides at the presence of Cl-atoms before the wear test. The substance in the wear track of as-deposited TiN is only composed of iron oxide Fe2 O3 or FeOOH at 711.3 eV, which were adherent on the worn surface with about 40 nm in thickness. Intensity of Ti 2p peak was very small and N 1s peak became invisible. This suggests that the surface of nitride coating was mainly covered with iron oxides, which were formed by oxidation of adherent iron from stainless steel ball during the wear test. The binding energy of O 1s revealed the formation of metal oxides (529.5–531 eV) except for C 1s as a contaminant with C=O bond (∼532 eV). In case of the Cl-implanted TiN, Cl 2p peak vanished, and N 1s peak intensity in the wear track was enhanced again. Cl-atoms and titanium oxides might be removed from the wear track as wear debris. Two different Ti 2p spectra for as-deposited and Cl-implanted TiN after frictional sliding test, were compared in Fig. 4. Two peaks of Ti 2p mainly came from TiN (455.8 eV) and TiO2 (458.8 eV). Only main two Ti 2p peaks to TiO2 and TiN were seen in Fig. 4a, while additional peaks overlapped on them so that peaks looked broader in Fig. 4b. Between two distinct peaks, there exit many peaks having the binding energies of titanium oxides with mono-, di- or trivalent, which were difficult to separate into each chemical state by using the conventional Gaussian

Fig. 4. Comparison of XPS spectrum for Ti 2p on the wear track after the frictional sliding test at W = 2 N, V = 0.01 m/s and L = 50 m: (1) as-deposited TiN coating, and (2) Cl-implanted TiN coating with the dose of 1.0 × 1017 Cl/cm.

curve-fitting. This range of binding energies from 455.8 to 458.8 eV, may correspond to the intermediate titanium oxides and the oxygen deficiency or the Magnèli phase with Tin O2n−1 , which formed during wear. XPS analysis of the counter material or the stainless steel ball was performed after sliding test with Cl-implanted TiN. The flattened worn surface was composed of two areas. One is a flat worn surface covered with white wear debris and the other is beside the worn area or bare stainless steel surface. In the following, the former is called a white area and the latter is a black area. Fig. 5 showed XPS spectra for white and dark area, respectively. Main composition of this white area was titanium oxide. It was adherent on the worn surface of counter material in several nanometers in depth. In addition, N 1s and Cl 2p peaks can be detected on both white and dark area. The binding energies for Cl 2p and N 1s depicted in Fig. 6, were shifted from the regular binding energies of 199.1 eV for metal chloride (possibly TiClx ) and 397.3 eV for TiN to 200.5 and 400.4 eV, respectively. In this case, the binding energy of C 1s does not correspond to carbide state but saturated hydrocarbon (285.4 eV) or C=O bond (289 eV). This implies that a part of Cl-atoms should escape out of the implanted sample and might be trapped into carbon compound after tribo-chemical reaction. The peak shift for N 1s binding energy, explains that nitrogen atoms in TiN should be released by its oxidation and trapped either onto the stainless steel surface or into carbon as an organic compound.

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Fig. 7. Bright field micrograph of agglomerated wear debris collected from wear track of Cl-implanted TiN after friction test at normal load 2 N, sliding speed 0.01 m/s and total distance 50 m.

Fig. 5. Comparison of XPS spectra with wide scanning between two portions on the counter AISI 304 ball: (1) beside the worn area of counter ball, and (2) white debris particle on the counter ball.

Microstructure of wear debris was investigated by means of TEM, EDS, and selected area electron diffraction (SAED) pattern. Fig. 7 showed a typical bright field image of debris particles ejected from the wear track of Cl-implanted TiN sample. Each particle in this agglomerated debris had round shape with the size of 30–80 nm. This morphology of debris is associated with the actual wear mode. The round shape might be typical to mild abrasion mode. Various SAED patterns were obtained at the selected locations in the same debris particle as illustrated in Fig. 8. These debris particles were fixed on the perforated carbon copper grid and the SAED patterns were carefully detected from the place, which was not overlapped with carbon. Lattice distances in these patterns corresponded to TiO1.04 , Ti4 O7 , Ti5 O9 , Ti7 O13 , and TiO2 as well as some amorphous phase.

As shown in Fig. 9a, it was found through chemical analysis by EDS that no Fe, Cr, and Ni from counter material were seen in this debris but only Ti and Cl were detected. Therefore, most of fine debris was nanocrystalline titanium oxide. Fig. 10 illustrated two HRTEM micrographs of debris obtained from Cl-implanted TiN sample. This atomic structure was investigated without using any sample thinning techniques. No defects were produced by artificial effect before observation. Each of agglomerated debris is composed of many tiny debris particles. Its actual size was about 2 nm. Among these nanocrystalline debris particles, both deformed and non-deformed structures were shown in Fig. 10. From measurement of its lattice distance, this structure was found to be a titanium mono-oxide (TiO) with TiO1.04 . Fig. 10b depicted a distinct chevron pattern in atomic structure. This arises from the periodic twining bands in the shear-deformed titanium oxide during sliding test. Fig. 11 showed the HRTEM image of deformed TiOx (x = 1.04) in the debris particle with higher magnification. This wavy

Fig. 6. Binding energies for Cl 2p and N 1s were shifted from 199.1 eV for metal chloride (possibly TiClx ) and 397.3 eV for Cl-implanted TiN to 200.5 and 400.4 eV after friction sliding test, respectively.

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twinning pattern must be a proof of planar defects left in the plastically deformed titanium oxide crystal. In other words, the titanium oxide has elasto-plastic deformability to relax the applied shear stress during wear.

5. Discussion 5.1. Oxidation of TiN coating Oxidation behavior of titanium nitride to titanium oxide has been studied intensively [29,30]. Typical reaction process can be summarized by: mTiN + 21 nO2 = Tim On + 21 mN2 .

Fig. 8. Wear debris on different observed locations generated a variety of SAED patterns in which lattice distances corresponded to mixed titanium oxides: (a) TiO1.04 , Ti7 O13 , TiO2 ; (b) TiO1.04 , Ti4 O7 ; (c) TiO1.04 and (d) TiO1.04 , Ti4 O7 , Ti5 O9 , TiO2 .

(1)

Both nitrogen and oxygen partial pressure, have influence on this oxidation reaction. Lu and Chen [29] investigated the oxidation chemistry of TiN in the controlled oxygen partial pressure from 0.21 to 10−30 atm for the temperature ranging from 673 to 973 K by XPS analysis. Various intermediate titanium oxides from TiO through Ti2 O3 to Tin O2n−1 , form at lower temperature and rapidly diminish with increasing the ambient temperature. The relative intensity of TiN (Ti 2p = 455.0 eV) decreases with increasing temperature while that

Fig. 9. EDS analysis revealed that debris in (a) was titanium oxides and in (b) mainly iron oxides from counter material, respectively. Cu spectrum was generated from holy carbon copper grids.

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each step in this sequence of reactions. The first stage with TiN → TiO has the maximum activation energy barrier, about 439 kJ/mol. In the other steps, the activation energies become less than that for the first step, 251–334 kJ/mol. Hence, higher oxygen partial pressure or lower ambient temperature is difficult to sustain this sequence of reactions. Hence, in the wear track, the chained reaction process might be terminated at the intermediate stage and various intermediate oxides can be synthesized on the stationary manner. 5.2. Process conditions inside the wear track The abrasive wearing behavior is induced by oxidation process of TiN and counter material. With increase of the severity in this oxidation, the abrasive wearing mode changes to the adhesive mode, finally resulting in the seizure of two contact materials. Local temperature inside of wear track or flash temperature (Tf ) becomes an essential parameter to consider the oxidation process during dry sliding condition. Assuming the Hertzian contact between the flat circular disc and the ball in motion, the maximum flash temperature can be calculated [31]. Here, no chemical reactions are also assumed to take place on the contact surfaces, and physical properties, to remain constant during wear contact at this ambient temperature. The maximum flash temperature is given by Tf = γ1 µ Fig. 10. HRTEM micrographs of nanocrystalline debris on Cl-implanted TiN against AISI 304 stainless steel ball show: (a) non-deformed and (b) deformed titanium oxide structure, respectively. The inset of nanodiffraction pattern correspond to TiO1.04 .

of TiO2 (Ti 2p = 458.6–459.1 eV) increases with temperature. Ti 2p intensity of intermediate oxide ranging between 455.0 and 458.6 eV decreases with temperature. At 773 K, the intensities of Ti 2p for both TiN and intermediate titanium oxides also decrease with increasing oxygen partial pressure from 10−29 to 10−5 atm. Polyyakova and Hübert [30] demonstrated that the oxidation of titanium nitride film advances in the many-staged processes depending on the heating rate. Oxidation of TiN to titanium mono-oxide must the first step in the tribo-chemical reaction, and, it is followed by the successive reactions through several intermediate oxides to the final dioxide (TiO2 ): TiN + O2 → TiNy Ox → TiOx → Tin O2n−1 → TiO2 . (2) The first stage of oxidation, TiN → TiNy Ox → TiOx , is accompanied with a small weight change. Smaller mass gain by oxidation with the faster heating rate reveals that the oxidation process from TiN to TiO2 in series should be terminated when the oxygen partial pressure is insufficient to sustain further oxidation process leading to TiO2 in rutile structure. Different activation energies were reported for

WV2 , π ak1

(3)

where k1 is the thermal conductivity, γ 1 the heat partition factor, and V2 the velocity. The suffix number 1 and 2 denote stainless steel AISI 304 and TiN, respectively. Tf ∼ 680 K for the as-deposited TiN since µ = 1.2, W = 2 N, V2 = 0.05 m/s and 2a = 10 ␮m. Tf exceeds the oxidation temperature both for TiN and steel while Tf ∼ 340 K in the Cl-implanted TiN since µ = 0.2. This temperature is too low for TiN to oxidize to titanium oxides since TiN film is thermally stable in air, or, when the oxygen partial pressure is 0.21 atm, up to 673 K. In fact, Ti 2p spectra indicate the formation of intermediate oxide at 573 K and a phase transition of TiO2 from amorphous to anatase-phase occurs at the temperature above 673 K [29]. 5.3. Oxidation in the wear track of Cl-implanted TiN Both XPS analysis in wear track and TEM observation of the wear debris particle, reveal that titanium mono-oxide (TiO1.04 ) is first synthesized via Cl-implantation and followed by a series of reaction in the wear track to yield oxygen deficient titanium oxides (Ti4 O7 , Ti5 O9 , Ti7 O13 ) or Magnèli phase, Tin O2n−1 finally to dioxide (TiO2 ). In fact, no Ti–O binding was observed in the as-deposited TiN, but Ti–O binding was distinctly detected in the Cl-implanted TiN. Vicinity of Cl-implanted TiN surface might well be first oxidized to TiOx (x = 1.04).

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Fig. 11. A distinctive chevron appearance of Fig. 10b due to twinning planes resulting from applied shear stress caused remarkable change in friction coefficient for Cl-implanted TiN. Two arrows indicated shear stress direction and a line was drawn to present wavy sequence.

In the typical Ti–O phase diagram in [32], n varies from 3 to 10. This narrow zone is sandwiched by Ti2 O3 and TiO2 (rutile) phases. Although the oxygen partial pressure could not be directly measured, the population of titanium oxides analyzed by SAED in the same wear debris particle, indicated that the major phases were titanium mono-oxide and Magnèli phase but that titanium dioxide was a minor product. It implies that the tribo-oxidation, enhanced by Cl-atoms, may terminate at the intermediate stage in the wear track. Furthermore, flash temperature may be too low for the tribo-oxidation process to sustain the whole reaction series. No presence of hot spots inside the wear track and on the counter material is a proof that demonstrates that this low temperature oxidation takes place in the wear track.

5.4. Role of Magnèli phase As had been keenly discussed in [33–35], Magnèli phase can be easily shear-deformed. This might be because successive local collapse of Ti-sublattices in the Tin O2n−1 is induced by many oxygen vacancies along the crystallographic shearing planes. Hence, in situ formation of this Magnèli phase oxide on the interface between two materials is thought to change the interfacial mechanical behavior. Gardos et al. reported the effective role of Magnèli phase to improve the wear resistance in practice [36]. According to the theoretical consideration, titanium oxides with oxygen deficiency should have lower friction than titanium dioxide. In addition, Woydt et al. stressed that tribologically induced formation of Magnèli phases is effective to reduce both wear and friction coefficient through systematic wear tests on

SiC/TiC or Si3 N4 /TiN. In their study, Magnèli phase was detected by local spot TEM observation [37]. This is because TiC or TiN should be partially reacted into Tin O2n−1 during wearing and both wear volume and friction coefficient were reduced due to formation of lubricious oxides of Magnèli phase. As shown in Fig. 4, the Magnèli phase coexists with TiO2 in the worn surface of Cl-implanted TiN. In case of as-deposited TiN, little or no yield of Magnèli phase was detected that no improvement was observed in tribological properties. Therefore, a series of tribo-reactions from TiN to TiO2 rapidly advances at relatively high flash temperature so that every TiN is fully reacted to TiO2 during wear at the absence of chlorine in TiN. At the presence of chlorine, a part of TiN at the surface is oxidized into TiOx (x = 1.04) even before wearing. Owing to low flash temperature in the wear track, a series of reactions starting from TiOx (x = 1.04) runs too slowly to fully sustain this tribo-reaction to TiO2 . Various kinds of Magnèli phase oxides with Tin O2n−1 are synthesized at the intermediate steps of tribo-reaction. Serratoes and Bronson [38] first discussed the stability of Magnèli phase thermodynamically in the Ti–O system. Relatively low oxygen partial pressure is needed to stabilize any Magnèli phases, e.g. Tin O2n−1 can coexist with TiC in Ti–O–C system when 4.7 × 10−12 to 8.1 × 10−14 atm at 1673 K. This process condition is far away from the flash temperature and oxygen partial pressure in the wear track for formation of Magnèli phase. The conventional thermodynamic calculation cannot explain the stability of synthesized Magnèli phase at low ambient temperature in the wear track, as suggested by [29,30]. Owing to this stable Magnèli phase oxide or intermediate titanium mono-oxide, the contact interface can be

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shear-deformed to allow relative displacement between tool and work materials, resulting in significant reduction of frictional stress. 5.5. Demonstration of shear-deformation of Magnèli phase Wear debris particle is employed as a tracer to describe the tribo-oxidation phenomena in the wear track, which is invisible in the conventional manner. Hence, precise observation and analysis by HRTEM and SAED provides us important information to build up the physical/chemical scenario for the self-lubrication mechanism. Close examination by the HRTEM image in Fig. 11 reveals the twin bands left in the debris particles. An imperfection of lattice corresponds to distortion of lattice structure. That is, plastic deformation occurs in the distorted titanium mono-oxide TiO1.04 during sliding test. Transient structure is easily shear-deformed along the slip system when subjected to severe straining. Titanium mono-oxide has an NaCl-structured defect. The smaller atom or Ti-atom, can be distorted or have defect variations in this kind of structure [39]. The twinning deformation is induced to make strain relaxation, i.e. the shear strain associated with an un-twinned band can be reduced by this twinning. The titanium oxides with oxygen deficiency or Magnèli phase might have another deformation mechanism, but other deformed traces were not found in the present HRTEM observation. SAED pattern analysis proves that Magnèli phases, such as Ti4 O7 , Ti5 O9 , Ti7 O13 are in situ synthesized in the wear track. Further investigation is necessary to describe the deformation mechanism of each Tin O2n−1 structured oxide both in theory and experiments. 5.6. Role of implanted chlorine atom Ti–Cl binding state coexists with TiN and TiO2 beside the wear track as shown in Fig. 6. This bond is easily dissociated under high normal pressure in wear. Physically induced micro-strains and defects by ion implantation as well as temperature gradient between top surface and inner region, attract this free chlorine to diffuse toward the surface. At the presence of chlorine, formation of intermediate oxides is enhanced at relative low temperature. Whenever the growth rate of this oxide is faster than the removal rate, this lubricious oxide layer can protect the original TiN coating from adhesion of counter material. Due to the concentration gradient of Cl-atoms between the oxidizing surface and inner region, Cl-atoms can also diffuse in backward direction to inner region of TiN. As partially observed in [18,20], the self-lubrication works well even in the deeper zone where the worn depth exceeds the initial Cl-implantation zone. Oxygen and titanium intensity information obtained by XPS, told significant difference in the oxide formation between beside wear track of as-deposited and Cl-implanted TiN. As shown in Fig. 3, higher oxygen and lower ni-

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trogen concentrations were detected at the surface after Cl-implantation. In the Ar-sputtering, oxide formation rate was very low, 0.66 nm/min. On the contrary, titanium oxide via Cl-implantation becomes thicker in several magnitudes than that for as-deposited TiN. This is just a proof of chemical modification via Cl-implantation. Owing to the oxidizing capacity of implanted chlorine in TiN, a part of TiN is easily oxidized to titanium mono-oxide just after implantation at room temperature. Cl-atom has been considered as a detrimental, hazardous agent on TiN. The presence of Cl-atoms on the worn surface increases the yield of intermediate titanium mono-oxide and Magnèli phase on the wear track. Effect of Cl-atoms on high reduction of wear rate and friction coefficient might be well explained by this chemical modification effect. 5.7. Comparison to other self-lubricating mechanisms Low friction coefficient is attained in Cl-implanted TiN and AISI 304 system without using any lubricants. This self-lubrication mechanism can be explained by the role of Cl-atoms and in situ formed intermediate titanium oxides that have shear deformability. This process is essentially different from other several mechanisms, which have been recently reported elsewhere [40–44]. In these applications, graphite, molybdenum disulphide (MoS2 ), or polytetrafluoroethylene (PTFE) are necessary to be housed as a lubricious solid. In each case, however, the lubrication is often suppressed by decomposition or oxidation of these lubricious agents in air. Since the titanium mono-oxide and Magnèli phase are only in situ formed by tribo-oxidation reaction, TiN coating remains its mechanical properties because of no change in the parent structure even after Cl-implantation. This self-lubrication can be sustained until the original TiN coating is worn out together with the chlorine atoms. Furthermore, since the frictional and wear condition is governed by shear-deformation of lubricious oxides, the macroscopically measured friction coefficient and wear parameters are indifferent to the normal loads and sliding velocities. This indifference to process conditions in wearing is a new aspect to discuss further characteristics in self-lubricating wear mechanism.

6. Conclusion Self-lubrication accommodated by Cl-implantation is a key to improve the original tribological behavior of TiN coating under dry condition without externally applying any lubricious material. The intrinsic lubricious film can be directly formed onto the parent TiN. Wear mode of TiN/AISI 304 system was successfully changed from adhesive to abrasive. Addition of Cl-atoms into TiN coating by ion implantation can reduce the wear rate and friction coefficient significantly because of formation of stable lubricious

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titanium oxide layer on the TiN. The chemical modification route explores a new way of ion implantation besides the conventional physical modification routes. At the absence of Cl-atoms, the surface of TiN was fully oxidized to TiO2 . In parallel, the counter material was further oxidized and imprinted on the wear track as an adhesive iron oxides. With Cl-atoms, tribo-oxidation process of the TiN during wear was drastically changed. The implanted chlorine atoms act as an oxidizing agent to enhance the formation of TiOx (x = 1.04) and Tin O2n−1 at low temperature except for TiO2 . A formation of titanium oxide mixture layer was stabilized in the wear track to protect TiN from direct contact with the constituent elements such as Fe, Cr and Ni in the counter material. Due to shear deformation of intermediate TiOx (x = 1.04) and titanium oxides with oxygen deficiency Tin O2n−1 , the wear rate as well as friction coefficient were then significantly reduced to solid-lubrication wearing mode. Although further study is necessary, the Magnèli phase must be stable in the wear track since it was detected on all the surfaces and the debris particles at room temperatures. This self-lubrication is favored for new tribological design aiming for dry forming and dry machining.

Acknowledgements This study was financially supported by the Grant-in-Aid from the Japanese Ministry of Education, Science, and Culture with the contract number of #12305047. References [1] G.R. Fenske, Ion Implantation, ASM Handbook, Friction, Lubrication, and Wear Technology, vol. 18, ASM International, Materials Park, OH, 1992, 850–860. [2] J.L. Walter, D.W. Skelly, W.P. Minnear, Ion implantation of cobolt–tungsten carbide tools for machining titanium, Wear 170 (1993) 79–92. [3] K.T. Kern, K.C. Walter, A.J. Griffin Jr., Y. Lu, M. Nastasi, W.K. Scarborough, J.R. Tesmer, Fayeulle, boron and nitrogen implantation of steels, Nucl. Instrum. Methods Phys. Res. B 127–128 (1997) 972–976. [4] X. Qiu, A. Hamdi, A. Elmoursi, G. Malaczynski, Development of bench test methods for the evaluation of ion-implanted materials: piston/bore application, Surf. Coat. Technol. 88 (1996) 190–196. [5] S. Lucas, J. Chevallier, Nanohardness and transmission electron microscopy study of nitrogen-implanted aluminum, Surf. Coat. Technol. 65 (1994) 128–132. [6] S.J. Bull, P.C. Rice-Evans, A. Saleh, A.J. Perry, J.R. Treglio, Slow positron annihilation studies of defects in metal implanted TiN coatings, Surf. Coat. Technol. 91 (1997) 7–12. [7] R.R. Manory, C.L. Li, C. Fountzoulas, J.D. Demaree, J.K. Hirvonen, R. Nowak, Effect of nitrogen ion-implantation on the tribological properties and hardness of TiN films, Mater. Sci. Eng. A 253 (1998) 319–327. [8] A.J. Perry, Microstructural changes in ion implanted titanium nitride, Mater. Sci. Eng. A 253 (1998) 310–318. [9] Y.P. Sharkeev, A.N. Didenko, E.V. Kozlov, High dislocation density structures and hardening produced by high fluency pulsed-ion-beam implantation, Surf. Coat. Technol. 654 (1994) 112–120.

[10] A.J. Perry, D.E. Geist, On the residual stress profile developed in titanium nitride by ion implantation, Nucl. Instrum. Methods Phys. Res. B 127/128 (1997) 967–971. [11] Y.P. Sharkeev, B.P. Gritsenko, S.V. Fortuna, A.J. Perry, Modification of metallic materials and hard coatings using metal ion implantation, Vacuum 52 (1999) 247–254. [12] R.R. Manory, A.J. Perry, R. Nowak, D. Rafaja, Some effects on ion beam treatments on TiN coatings of commercial quality, Surf. Coat. Technol. 114 (1999) 137–142. [13] K.Y. Eun, K.-R. Lee, E.-S. Yoon, H. Kong, Effect of polymetric debris on the tribological behavior of diamond-like carbon films, Surf. Coat. Technol. 87 (1996) 569–574. [14] N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Tear, J. Hampshire, Performance of MoS2 /metal composite coatings used for dry machining and other industrial applications, Surf. Coat. Technol. 123 (2000) 84–91. [15] T. Akhadejdamrong, Self-protection/self-lubrication of ceramic coating by light element ion implantation, Ph.D. Thesis, University of Tokyo, 2002. [16] A. Mitsuo, K. Kanda, T. Aizawa, Friction and wear properties of carbon-ion implanted titanium nitride films, Surf. Coat. Technol. 128–129 (2000) 440–445. [17] T. Akhadejdamrong, C. Iwamoto, Y. Ikuhara, A. Mitsuo, T. Aizawa, Formation of protection layer during oxidation of Al-implanted TiN coating, Mater. Trans. 43 (6) (2002) 1291–1297. [18] A. Mitsuo, T. Aizawa, Improvement of friction and wear performance of titanium nitride films by chlorine ion implantation, Mater. Trans. JIM 40 (12) (1999) 1361–1366. [19] T. Aizawa, A. Mitsuo, Improvement of tribological behavior in thin hard coatings by chlorine ion implantation, in: Proceedings of the 20th ASM Heat Treating Soc. Conference, St. Loius, 2000, pp. 366–372. [20] T. Aizawa, T. Akhadejdamrong, C. Iwamoto, Y. Ikuhara, A. Mitsuo, Self-lubrication of chlorine-implanted titanium nitride coating, J. Am. Ceram. Soc. 85 (1) (2002) 21–24. [21] A. Mitsuo, T. Akhadejdamrong, T. Aizawa, Self-lubrication coating and surface modification, in: Proceedings of the Powder Metallurgy World Congress, vol. 1, Orlando, 2002, pp. 23–36. [22] S. Mori, H. Nanao, T. Numata, Tribo-chemistry. Fundamentals and applications of micro-tribology, Materia 40 (10) (2001) 850–853. [23] S. Kataoka, Lubrication in the low-speed metal forming, Ph.D. Thesis, University of Tokyo, 1995. [24] Y. Sato, M. Hara, Chlorine induced corrosion of Ni and Ni–Cr alloys at elevated temperature, Jpn. J. High Temp. 28 (5) (2002) 219–225. [25] A. Mitsuo, T. Akhadejdamrong, T. Aizawa, Self-lubrication via halogen ion implantation to TiN coating, in: Proceedings of the 53rd Japan Joint Conference on Technol. Plast., Hamamatsu, November 2002, pp. 303–304. [26] T. Akhadejdaomrong, T. Aizawa, M. Yoshitake, A. Mitsuo, Feasibility study of self-lubrication by chlorine implantation, Nucl. Instrum. Methods Phys. Res. B, 2003, in press. [27] A. Mitsuo, T. Aizawa, Effect of chlorine distribution profiles of tribological properties for chlorine-implanted titanium nitride films, Surf. Coat. Technol. 158–159 (2002) 694–698. [28] L.J. Liu, et al., Improvements in tribo-mechanical properties of commercial TiN coatings by argon ion implantation, in: M.A. Nastasi, et al. (Eds.), Beam–solid interactions, Materials Research Society, Pittsburgh, 1993, pp. 469–474. [29] F.H. Lu, H.Y. Chen, XPS analyses of TiN films on Cu substrates after annealing in the controlled atmosphere, Thin Solid Films 355–356 (1999) 374–379. [30] I.G. Polyakova, T. Hübert, Thermal stability of TiN thin films investigated by DTG/DTA, Surf. Coat. Technol. 14 (2001) 55–61. [31] R.S. Cowan, W.O. Winer, Frictional Heating Calculations, ASM Handbook, Friction, Lubrication, and Wear Technology, vol. 18, ASM International, Materials Park, OH, 1992, 39–44. [32] P. Waldner, G. Eriksson, Thermodynamic modeling of the system titanium–oxygen, Calphad 23 (2) (1999) 189–218.

T. Akhadejdamrong et al. / Wear 254 (2003) 668–679 [33] M. Ashino, T. Uchihashi, K. Yokoyama, Y. Sugawara, S. Morita, M. Ishikawa, Atomic-scale structures on a non-stoichiometric TiO2 (1 1 0) surface studied by noncontact AFM, Appl. Surf. Sci. 157 (2000) 212–217. [34] H. Nörenberg, R.E. Tanner, K.D. Schierbaum, S. Fischer, G.A.D. Briggs, Visualization of precipitation induced crystallographic shear planes as one-dimensional structure on surfaces: an STM and RHEED study on TiO2 (1 1 0), Surf. Sci. 396 (1998) 52–60. [35] A. Howard, C.E.J. Mitchell, D. Morris, R.G. Egdell, S.C. Parker, The surface structure of TiO2 (2 1 0) studied by atomically resolved STM and atomistic simulation, Surf. Sci. 448 (2000) 131–141. [36] M.N. Gardos, H.-S. Hong, W.O. Winer, The effect of anion vacancies on the tribological properties of rutile (TiO2−x ). Part II. Experimental evidence, Tribol. Trans. 22 (2) (1990) 209–220. [37] M. Woydt, A. Skopp, I. Dörfel, K. Witke, Wear engineering oxides/anti-wear oxides, Wear 218 (1998) 84–95. [38] M. Serratos, A. Bronson, The effect of oxygen partial pressure on the stability of Magnèli phases in high temperature corrosive wear, Wear 198 (1996) 267–270.

679

[39] F.S. Galasso, illustrated by W. Darby, International Series of Monographs in Solid State Physics, Structure and Properties of Inorganic Solids, vol. 7, Pergamon, New York, 1970, p. 264. [40] C. Grossiord, K. Varlot, J.-M. Martin, Th. Le Mogne, C. Esnouf, K. Inoue, MoS2 single sheet lubrication by molybdenum dithiocaramate, Tribol. Int. 31 (12) (1998) 737–743. [41] N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer, J. Hampshire, Performance of MoS2 /metal composite coatings used for dry machining and other industrial applications, Surf. Coat. Technol. 123 (2000) 84–91. [42] V. Derflinger, H. Brandle, H. Zimmermann, New hard/lubricant coating for dry machining, Surf. Coat. Technol. 113 (1999) 286– 292. [43] T.L. Huu, D. Paulmier, A. Grabchenko, M. Horvath, I. Meszaros, A.G. Mamalis, Autolubrication of diamond coatings at high sliding speed, Surf. Coat. Technol. 108–109 (1998) 431–436. [44] S.J. Shiao, T.Z. Wang, Dry self-lubricating composites, Composites 27B (1996) 459–465.