Temperature-dependent tribological properties of low-energy N-implanted V5Ti alloys

Temperature-dependent tribological properties of low-energy N-implanted V5Ti alloys

Surface & Coatings Technology 188–189 (2004) 459 – 465 www.elsevier.com/locate/surfcoat Temperature-dependent tribological properties of low-energy N...

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Surface & Coatings Technology 188–189 (2004) 459 – 465 www.elsevier.com/locate/surfcoat

Temperature-dependent tribological properties of low-energy N-implanted V5Ti alloys J.A. Garcı´aa,*, G.G. Fuentesa, R. Martı´neza, R.J. Rodrı´gueza, G. Abrasonisb, J.P. Rivie`reb, J. Riusc a

Centro de Ingenierı´a Avanzada de Superficies AIN, E-31191 Cordovilla-Pamplona, Spain Laboratoire de Me´tallurgie Physique, Universite´ de Poitiers, 86960 Futuroscope, France c Institut de Cie`ncia de Materials de Barcelona, CSIC, 08193 Bellaterra, Catalunya Spain

b

Abstract In this contribution, we report on the structural and the tribological characterisation of low-energy nitrogen-ion-implanted Vanadium Titanium (V5Ti) alloys as a function of the annealing temperature during the bombardment process. The surface of the alloys was bombarded with N2+ ions accelerated at 1.2 keV in a vacuum chamber equipped with a Kaufman-type ion source. The temperature during the low-energy ion implantation were 480, 500, and 575 8C. In this temperature range, the penetration depth reached by the implanted ions is not only exclusively ballistic (i.e., kinetic energy dependent), but diffusion effects also play a decisive role. The structural changes can be drawn as the formation of a nitrogen diffusion layer containing a blend of pristine bcc V5Ti and a solid nitrogen solution having an expanded bcc phase. It has been found that the temperature favours the increase of both, the thickness of the diffusion layer and the relative nitrogen content in the solid solution. We have investigated the microhardness, friction and wear coefficients of the implanted V5Ti surfaces as a function of the annealing temperature. The reported results indicate that both hardness and wear resistance against mechanical abrasion increase as the temperature during the bombardment increases. This increment of the hardness and wear resistance could be directly correlated to the amount of the expanded phase exhibited by the V5Ti surfaces, as observed by X-ray diffraction. The hardness and wear resistance modification upon annealing temperature are discussed in terms of the plastic deformation undergone by the expanded bcc phase to release the accumulated lattice stresses. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Hardness; Wear; Friction; Tribology

1. Introduction Vanadium alloys, V–Cr–Ti have been recently investigated for its use in fusion reactors due to their excellent thermal stress factor, resistance to irradiation, high strength and superior ductility at elevated and low temperatures respectively [1,2]. Due to the fact that these materials are foreseen to operate under high aggressive conditions (i.e., high temperature, wear, corrosive or oxidant environments), it is evident that their surface

* Corresponding author. E-mail address: [email protected] (J.A. Garcı´a). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.053

properties such as corrosion and wear resistance, as well as its thermal fatigue behaviour should be optimised. A well-known surface treatment is the Ion Implantation at high energy, which is reported to bring significant benefits for the performance of steels [3–5], and alloys [6–11]. Alternatively, low-energy high-temperature implantation [12–14] has demonstrated some advantages such as easy and cheap experimental set up, reproducibility, and high performance when applied on steels or Ti alloys. Although both high-energy and low-energy high-temperature ion implantations stand on the same basis, the physical phenomena associated to them are indeed different. For instance, whereas implantation at high energies promotes

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penetration depths between 0.3 and 0.5 Am, implantation at low energy barely reaches few atomic layers. In order to achieve penetration depths usable for tribological application, low-energy implantation should be accompanied by thermally activated diffusion processes. This feature makes the sample temperature during low-energy implantation a critical parameter capable of controlling the mechanical properties of the treated surfaces. Williamson et al. [15] reported a linear increase of the surface hardening in stainless steel as a function of the temperature. Moreover, Wilbur et al. [16] found a critical temperature (i.e., around 450 8C) in which the surface hardening is maximal in Nimplanted high-speed steels (HSS), due to the influence of the tempering on the base material. In this work, we investigate the role of the temperature in the mechanical and tribological properties of low energy N ions implanted on V5Ti alloys. We discuss the results in terms of the modification of the lattice structure induced by the implantation–diffusion combined process as a function of the temperature. The observed results leads to the conclusion that this surface treatment could certainly be an alternative to standard high-energy ion implantation methods.

2. Experimental Low-energy ion implantation of nitrogen was carried out in a purpose designed preparation chamber with a base pressure of 10 5 Pa. Three different nitrogen implantations were performed on mirror polished V5Ti alloys using a Kaufman-type ion source at energy and current density of 1.2 keV and 1 mA/cm2, respectively. The treatments were carried out during 1 h keeping the substrates at 480, 500 and 575 8C. The ion beam was broad enough to assure a homogeneous irradiation of the alloys. Auger spectroscopy on the implanted samples revealed compositions of nitrogen around 5 at.%. X-ray diffraction patterns were recorded at a grazing incidence of 1.3 8 using a BRUKER-D5000 spectrometer with a Cu anode emitting at k=0.15408 nm. Plastic ultramicrohardness of the implanted surfaces were measured in a Fischercope H100VP microindenter using a Vickers indenter at three final loads of 2, 5, and 10 mN. In

addition, friction coefficient and wear test were monitored using a pin-on-disc tribometer FALEX 320 PC, with humidity controller unit. The test were performed against a 100 g/6 mm diameter WC-Co balls, at 50% of humidity and a linear speed of 0.04 m/s. A more quantitative insight of the wear produced after the ballon-disc tests were obtained by measuring the surface topography in an optical profilometer with an in-depth resolution better that 3 nm.

3. Results 3.1. Stoichiometry and structure Table 1 gathers the nitrogen atomic composition of the implanted alloys obtained by Auger spectroscopy in combination with in-depth profiling. The data revealed surface N concentrations below 5% in all cases studied, although the penetration ranges of the implanted nitrogen could not be accurately determined due to the inherent limitations of this spectroscopic method. Fig. 1 shows the diffraction peak (110) of the implanted V5Ti alloys measured at grazing angle as a function of the annealing temperature, and the corresponding for the pristine substrate. The nonimplanted substrate presents a sharp (110) feature at 2hc41.978 and 0.168 width, characteristic of polycrystalline bcc V5Ti. This feature becomes broader upon N implantation at 480 8C, as can be observed from the contribution emerging at the low angle slope of it. At 500 8C of implantation temperature, this emerging shoulder manifests now as a new contribution centered at 2hc41.368, i.e., shifted downwards by 0.618 with respect to this of the pristine V5Ti. In the case of the implantation at 575 8C, the low angle contribution further shifts downwards by 0.358, becoming the dominant contribution of the diffraction pattern, whereas the sharp peak characteristic of pristine V5Ti is barely observed and does not exhibits any significant shift. In all cases described previously, there is no evidence on new peaks arising from fcc VN or fcc TiN compounds, all that suggesting that new diffraction peak may stem from the appearance of an expanded bcc V5Ti phase as due to the ion implantation

Table 1 Elastic modulus (Er), universal hardness (HU) measured at 2 mN maximal load, and wear coefficient (K) after 5000 cycles (see text for details), of N implanted V5Ti alloys at different temperatures T (8C)

V5Ti 480 500 575

Unimplanted

Implanted

Er (N/mm2)

HU (N/mm2)

K (N m 2)1015

N (at.%)

Er (N/mm2)

HU (N/mm2)

K (N m 2)1015

110 112 114 115

1170 1900 1920 1800

383 – – –

– b5 b5 6

– 147 139 196

– 2730 5200 8600

– 153 22 b5

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Fig. 1. GIXRD diffraction patterns between 308b2hb468 of the vanadium alloys before and after the nitrogen implantation treatments as a function of the temperature, as indicated.

process. In addition, the observed results also indicate that the amount of such an expanded phase increases as the temperature during the implantation increases. 3.2. Microhardness Fig. 2 represents the load–unload indentation curves for the V5Ti alloys as a function of the temperature of implantation measured at a maximum load of 2 mN. The corresponding curve for the untreated alloy is also included. Fig. 2 exhibits how the V5Ti sample presents a large plastic deformation, with a very poor elastic recovery characteristic of these compounds. The implantation treatment at different temperatures modifies the elastic–plastic properties of the surface alloy as can be observed from the larger elastic recovery of the corresponding indentation experiments.

Fig. 2. Microindentation load–unload curves of the V5Ti alloys after the nitrogen implantation treatments as a function of temperature. The maximal load was set to 2 mN.

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Interestingly, the elastic recovery increases as the temperature during implantation increases. In fact, the unloading curve for the V5Ti alloy implanted a 575 8C overlaps almost completely the loading curve indicating the large elastic behavior of the implanted surface. The relative elastic modulus (Er) and universal hardness (HU) of the treated alloys have been gathered in Table 1 as a function of the implantation temperature. The obtained HU and Er values can be only handled as a qualitative insight of the degree of hardening or softening of the treated samples with respect to the reference material. This is because the measured penetration depths at 2 mN load in this material (see Yaxis in Fig. 2) overcomes 1/10 of the layer thickness which has been hardened by the plasma treatment. This fact can be well seen in Fig. 3 where we have represented the universal hardness of the V5Ti samples as a function of the penetration depth (maximal load 750 mN) for different temperatures during the bombardment. Fig. 3 clearly shows that surface hardening decreases by 80% through the first 100 nm up to 200 nm for samples untreated and treated at 480, and 500, whereas in the case of T=575 8C, hardening decreases more progressive so that, at 0.5 Am, HU is half of the value on the top most surface layers. Taking this into account, Table 1 reveals that HU at 2 mN of the V5Ti alloys increases by a factor 2.3, 4.4 and 7 for implantation temperatures of 480, 500 and 575 8C, respectively, pointing out the enhancement of the surface mechanical hardness achieved upon these treatments. In addition, the elastic coefficients also revealed a significant increase after the treatments. Fig. 3 has also evidenced that the temperature during the bombardment process influence significantly the penetration range of the implanted ions into the surface of the samples and hence their in-depth mechanical properties. 3.3. Wear resistance and friction coefficient Fig. 4a–b exhibits the friction coefficient of the V5Ti alloys as a function of testing time for different implantation

Fig. 3. In-depth microhardness profiles of the V5Ti alloys as measured by microindentation at 750 mN of maximal load, as a function of the temperature during the implantation treatment.

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Fig. 4. Evolution of the friction coefficient against WC-Co of the treated alloys as a function of the temperature for applied loads (a) 0.5 N and (b) 1 N.

temperatures using 0.5 N (Fig. 4a) and 1 N (Fig. 4b) applied load. The test performed on pristine V5Ti at 0.5 N load (Fig. 4a) reveals that the friction coefficient against WC-Co shows two regions. During the first 450 cycles, it barely reaches 0.1, increasing abruptly up to 0.6 above 600 cycles of test. In the case of the implanted V5Ti alloys, the friction coefficients against WC-Co at 0.5 N load keeps below 0.15 along the experiment regardless the implantation temperature. Likewise, the measured friction coefficient at 1 N load for the pristine V5Ti alloys exhibit a similar qualitative behavior than that observed at 0.5 N (cf. Fig. 4b), but reaching a value of 0.75. The implanted alloy at 480 8C exhibits a first region up to 500 cycles where the friction coefficient ranges 0.15 and a second region characterized by

a rapid increase of up to 0.6. The implanted V5Ti samples maintain its friction coefficient below 0.2 up to 3000 cycles when annealed at 500 8C and more than 100.000 cycles in the case of 575 8C, all that suggesting the benefits of the implantation treatment on the tribological performance of the alloys. A more quantitative information on the wear of the alloys can be obtained by measuring the corresponding wear tracks caused by WC ball. Fig. 5a–d shows the wear tracks, after 5000 cycles at 0.5 N load, of the implanted alloys as a function of the temperature during bombardment, as indicated. The untreated alloy and this implanted at 480 8C exhibit wear tracks characteristic of a strong abrasive mechanism as indicate the presence of deep grooves and

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Fig. 5. Surface topographies of the wear tracks on the treated V5Ti alloys after 5000 cycles of pin-on-disc test using a WC ball and a load of 1 N for the (a) untreated sample, and these implanted at (b) 450 8C, (c) 500 8C and (d) 575 8C (vertical scale in Am for panels a, b and c and nm for panel d).

debris along the worn surface. Fig. 5 provides evidence of a radical improvement of the wear resistance of the V5Ti alloys upon N implantation at temperatures above 500 8C. The wear coefficient after 5000 cycles at 1 N load are also gathered in Table 1. In particular, whereas the wear coefficient of the alloy implanted a 480 8C (Fig. 5b) improved by a factor 2 with respect to that of the pristine V5Ti (Fig. 5a), this implanted at 500 8C (Fig. 5c) exhibited a decrease of the wear coefficient down to one order of magnitude. Moreover, the V5Ti alloy implanted at 575 8C did not show a wear track deep enough to accomplish an accurately measurement of the wear coefficient (Fig. 5d).

4. Discussion The averaged stoichiometries shown in Table 1 indicate that the retained doses measured at these conditions (~2–5 at.%) are rather low as compared to the implantation dose, i.e., 1019 ions/cm2. In stainless and tool steels, the amount of nitrogen accumulated into their surfaces reaches 20–30 at.% for implantation doses in the range of 108 ions/cm2, which are significantly larger than in the case of the V5Ti alloy, and temperatures in the range 450bTb550 8C. This difference could be tentatively associated to the larger diffusivity of nitrogen in fcc steels rather than in the bcc vanadium

alloys, due to the fact that the temperature needed for activate thermal diffusion in V5Ti is higher than in steels. The surface microstructure of the V5Ti alloy changes significantly upon low energy N implantation at high temperature as observed from the GIXRD patterns. The GIXRD results clearly indicate the appearance of a new (110) diffraction peak, which shifts to lower angles (larger interplanar distances) and becomes broader as the temperature increases. A closer examination of the diffraction patters did not evidenced the presence of nitrides species such as fcc TiN or VN, leading to the conclusion that the observed shifted reflection may arise from the formation an expanded bcc V5Ti phase, similar to the well-known solid solutions of nitrogen in steels or Ti alloys. In N-implanted steels it is well reported such shifted reflections arising from the presence of expanded austenitic or martensitic phases, which show up due to the trapping of nitrogen at interstitial sites. Moreover, the continuous shift of the diffraction feature associated to the expanded phase observed in Fig. 1 reflects also the fact that the V5Ti takes up more nitrogen as the temperature increases. The fact that the (110) diffraction of the pristine bcc V5Ti is still visible for temperatures up to 575 8C indicates that the treated surface is mainly constituted by a blend of solid nitrogen solution grains embedded in the original V5Ti matrix. In addition, the temperature during implantation

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seems conclusively to drive the shape and thickness of the formed N diffusion layer, and though no definite indications arose from the XRD data, these suggest that the total amount of the solid solution grains increases with respect to that of the pristine material as the temperature increases. The temperature not only determines the amount of nitrogen absorbed in the solid solution, but also the thickness of the diffusion layer. As observed in Fig. 3, the hardness of the V5Ti alloy implanted at 575 8C is significantly larger than these for the alloys treated at lower temperatures in a rather depth range. This may be an indication of the fact that, by increasing the temperature, the thickness of the diffusion layer also increases. It is also noteworthy to observe that the width of the peaks associated to the expanded phase is rather large. These effects can be correlated to the increase of the implanted surface hardness observed by microindentation at low load (cf. Fig. 2). Effectively, an excess of lattice expansion may promote the appearance of strong compressive stress that releases through plastic deformation mechanisms. These plastic deformation processes have reported to enhance the hardness of low energy implanted steels [17,18] and explain well the hardness enhancements observed experimentally. The structural changes of the V5Ti alloys due to the implantation treatment cause an important enhancement of its tribological performance when the applied temperature during the implantation is above 500 8C (cf. Fig. 4c–d). In the case of stainless steels, the implantation of nitrogen at low energies leads to a significant improvements of the tribological properties at temperatures in the range of 400– 450 8C. At these temperatures, no significant changes were observed in the case of the V5Ti (not shown here). This difference can be due to the fact that the diffusivity of nitrogen is larger in steels than in V5Ti. The amount of the expanded layer along with the relative nitrogen concentration of the solid solution is also observed to play an important role in the tribological performance on the diffusion layer of the treated surfaces. At 575 8C, the wear coefficient K is significantly lower than these measured at 480 and 500 8C. In fact, K could not be measured in the alloy implanted at 575 8C due to the lack of abrasion mechanisms in the corresponding wear track (cf. Fig. 5d). The plastic deformation mechanism responsible for the hardening of the alloys seems also to enhance the toughness and the wear resistance of the treated surfaces. Therefore, it can be concluded that the temperature during implantation is a critical parameter for the tribological properties of the alloys due to the role it plays in controlling the thickness of the nitrogen diffusion layer.

5. Conclusions We have investigated the structure and the tribological performance of low energy nitrogen implanted V5Ti

alloys as a function of the temperature during the treatment. The structural changes can be drawn as the formation of a nitrogen diffusion layer containing a blend of pristine bcc V5Ti and solid nitrogen solution having an expanded bcc phase. It has been found that the temperature favours the increase of both, the thickness of the diffusion layer and the relative nitrogen content in the solid solution. The microhardness, friction and wear coefficients of the implanted V5Ti surfaces exhibited significant improvements with respect to the untreated alloy, existing a clear correlation between tribological performance and temperature of treatment. According to the observed results, we can conclude that the relative amount of incorporated nitrogen in the solid solution and the thickness reached by the diffusion layer, both being growing parameters upon temperature, determine mechanical properties due probably to the presence of plastic deformations mechanism undergone by the excess of stress accumulated in the surfaces during the treatments.

Acknowledgements The authors want to express their gratefulness to the Regional Government of Navarra (Spain) for funding this research through the project PLASMANET under the program INTERREG IIIB.

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