Sept. 2007 Journal of China University of Mining & Technology Vol.17 No.3

J China Univ Mining & Technol 2007, 17(3): 0363–0367

Load Dependence of Nanohardness in Nitrogen Ion Implanted Ti6Al4V Alloy

and Fractal Characterization LUO Yong1,2, GE Shi-rong1

1Institute of Tribology and Reliability Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China 2School of Mechanical and Electrical Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China

Abstract: Three different nitrogen ion doses were implanted into a Ti6Al4V alloy to improve its mechanical surface properties for the application of artificial joints. The titanium nitride phase and nitrogen element distribution profile were characterized with X-ray photoelectron spectroscopy (XPS). Nano-indentation tests were carried out on the surface of the Ti6Al4V alloy and implanted samples on a large scale of applied loads. The XPS analysis results indicate that ni-trogen diffuses into the titanium alloy and forms a hard TiN layer on the Ti6Al4V alloy. The nanohardness results reveal that nitrogen ion implantation effectively enhances the surface hardness of Ti6Al4V. In addition, the nanohardness clearly reveals load dependence over a large segment of the applied loads. Thus a concept of nanohardness fractal di-mension is first proposed and the dual fractal model can effectively describe nonlinear deformation in indentation areas on the Ti6Al4V surface. The fractal dimension shows a decreased trend in two regions of applied loads, indicating a de-crease of the self-similarity complexity in surface indentation owing to an increase in nanohardness after nitrogen ion implantation.Key words: nanohardness; nano-indentation; ion implantation CLC number: TB 302.3

1 Introduction

As load-bearing joint replacements, metals are still an alternative material because of their good elasticity and wear resistance. Stainless steel, cobalt-based al-loys and Ti-based alloys are the most commonly used metallic materials in orthopaedic implants[1–4]. How-ever, concern for loosening and the limitation of scratch and wear resistance has resulted in a search for enhancement of surface properties for Ti-based alloy application as heavy-duty implants. Surface coating technology is an effective way to modify mechanical surface properties and wear resistance of titanium alloys.

Nitrogen ion implantation can improve the surface properties and tribological behavior of metals and polymers[5–7]. Nitrogen ion implantation has also been applied to improve the surface hardness and wear resistance of Ti-based alloys. Kostov, et al applied a plasma-immersion ion implantation technique to modify the surface properties of the Ti6Al4V alloy,

AISI304 and H13 steel[8]. The hardness on the surface of an implanted Ti6Al4V alloy increased from 4.7 GPa to 7.5 GPa. Ueda, et al investigated the tri-bological properties of plasma immersion nitrogen implanted Ti6Al4V[9]. In their reports it was shown that a 50 nm surface layer with 40% concentration at a nitrogen peak was formed, which resulted in a re-duction of friction coefficients and improved wear resistance[9]. Fouquet, et al applied the plasma base ion implantation (PBII) to improve wear properties of Ti6Al4V material and analyzed nitrogen diffusion of the nitride growth mechanism[10].

Hardness is an important performance parameter for tribological coatings. Many important properties, such as wear resistance and adhesion, are related to this parameter. Hardness is commonly considered to be a convenient index of these mechanical properties. Traditionally, hardness is measured by a micro- hardness indentation test. However, since the film thickness of the implanted coating is in the submicron to nanometer regime, the micro-hardness test be-

Received 19 October 2006; accepted 06 January 2007 Projects 2007CB607605 supported by the Major State Basic Research and Development Program of China; 50535050, 50225519, 50405042 by the National Natural Science Foundation of China and 2005B032 by the Science Foundation of China University of Mining and Technology Corresponding author. Tel: +86-516-83591916; E-mail address: sulyflying@cumt.edu.cn

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comes inaccurate because of the substrate effects. Nano-indentation has been adopted as a new method to provide a more reliable thin film hardness. The measured results can provide further understanding of tribological behavior and surface properties[11].

In our study, we performed nitrogen ion implanta-tion on a Ti6Al4V alloy with three nitrogen ion doses of 1.3×1017 ions/cm2, 2.1×1017 ions/cm2 and 6.2×1017

ions/cm2. The surface structure was characterized by using X-ray photoelectron spectroscopy (XPS) analy-sis and nanohardness was measured on an in-situ nano-mechanical testing system. Subsequently, the load dependence of nanohardness of the Ti6Al4V alloy was investigated. Furthermore, fractal charac-terization was proposed to describe the variation of nanohardness to indentation loads.

2 Experiment

The Ti6Al4V samples tested were in a square shape of 15 mm × 15 mm, 5 mm in thickness. Before performing implantation, the Ti6Al4V samples were finished to Ra<0.05 m and ultrasonically cleaned in acetone for 15 min. The nitrogen ion implantation was made in a multi-functional ion implanting system, with a pressure of 10–5 Pa and an acceleration energy of 40 keV. Three different nitrogen doses, 1.3×1017

ions/cm2, 2.1×1017 ions/cm2, 6.2×1017 ions/cm2, were applied in the ion implantation.

The implanted samples were examined by X-ray photoelectron spectroscopy using a VG Thermo ES-CALAB 250 spectrometer fitted with monochro- matic Al-K X-ray radiation (150 W, h = 1486.6 eV). Analysis spectra were obtained using a pass energy of

550 eV in increments of 1 eV, but high-resolution spectra were obtained at a 20 eV pass energy in in-crements of 0.05 eV.

The nano-indentation on pure Ti6Al4V samples and nitrogen ion implanted samples was carried out on an in-situ nano-mechanical testing system (Tribo-Indenter, Hysitron), which can monitor and record the applied load and displacement of the indenter. A three-sided pyramid shaped diamond Berkovich in-denter, with a normal angle of 65.3°, was used in the nano-indentation tests. A three-stage mode of loading and unloading of the indent tip was designed for the nano-indentation tests of Ti6Al4V alloys. The applied load increased linearly in five seconds at the loading stage, then the peak load was maintained for five seconds. The applied load decreased linearly to zero in five seconds at the unloading stage. Load inde-pendence of nanohardness was investigated by vary-ing the peak load from 5 N to 6000 N for nano- indentation. The indentation images were in-situ ob-served through a SPM image system on the TriboIn-denter and the indentation profiles were measured for discussion of load dependent behavior of nanohard-ness of Ti6Al4V alloys.

3 Results and Discussion

3.1 XPS analysis

The XPS peak positions (electron binding energies (BE) for specific atomic levels) can be used for iden-tification of chemical states. Fig. 1 shows the typical X-ray photoelectron spectra of Ti6Al4V surfaces im-planted with a 2.1×1017 ions/cm2 nitrogen dose.

Fig. 1 XPS spectra of nitrogen implanted Ti6Al4V alloy at the dose of 2.1×1017 ions/cm2 on the surface and 1000 s sputtering depth

LUO Yong et al Load Dependence of Nanohardness in Nitrogen Ion Implanted Ti6Al4V Alloy … 365

Fig. 1a provides XPS surveys between binding energies 0 and 550 eV at the surface and 1000 s sput-tering depth, in which a series of N1s, Ti2p, O1s, Al2p and V2p spectra are observed. According to the NIST X-ray photoelectron spectroscopy database, the peak at a binding energy of 454.9 eV and 460.7 eV in Fig. 1b corresponds to the Ti2p3/2 and Ti2p1/2 compo-nents, which is attributed to the TiN phase. The N1s peak at 397.2 eV in Fig. 1d also corresponds to the formation of a TiN layer. The Ti3p peak at 35 eV and the Ti2p3/2 peak at 458 eV can be interpreted as a TiO2 component with evidence of a O1s peak at 531.3 eV in Fig. 1c, which results from the contami-nation on the sample surface during atmospheric ex-posure[12–14]. In addition the Ar2p peak at approxi-mate 120 eV, present in Fig. 1a, is due to the Ar+ ion sputter cleaning process. After 1000 s sputtering, the XPS spectra in Fig. 1 show an apparent reduction of the Ti2p3/2 peak at 458 eV and the O1s peak had be-come very weak, which suggests that the TiO2 phase disappears at a certain layer depth. However, the strong peak of N1s at 397.2 eV proved an abundant TiN formation.

3.2 Nano-indentation

The variation in nanohardness of implanted and unimplanted Ti6Al4V samples in response to differ-ent applied loads is shown in Fig. 2. Nanohardness is determined from load-displacement curves according to the methods by Oliver and Pharr. It is shown that the nanohardness values for nitrogen ion implanted Ti6Al4V samples in the three doses are higher than those of unimplanted samples. The Ti6Al4V samples implanted with 6.2×1017 ions/cm2 have the highest nanohardness value and samples implanted with 2.1×1017 ions/cm2 show the lowest nanohardness values. Compared to nanohardness of unimplanted samples, the maximum nanohardness value of im-planted Ti6Al4V samples is as high as 13.1 GPa, which is at least twice as large as that of the unim-planted samples. Such nanohardness enhancement can attribute to higher stiffness of the TiN hard layer on N+ implanted Ti6Al4V substrates.

Fig. 2 Nanohardness of nitrogen ion implanted and unim-planted Ti6Al4V as a function of increasing applied loads

Nanohardness of Ti6Al4V samples has strong load dependent behavior, which illustrates that the nano-hardness of Ti6Al4V samples varies with different applied loads. Fig. 2 shows two different types of variation in nanohardness. The nanohardness of Ti6Al4V samples increases sharply with increasing loads below 800 N. However, the nanohardness values decreases with loads increasing beyond 800

N. Because nanohardness is determined by the pro-jected contact area of indent tip on samples, the dif-ference in load dependent behavior of nanohardness of Ti6Al4V samples is due to the nonlinear behavior of indent deformation on Ti6Al4V substrates.

The load dependence of nanohardness of Ti6Al4V samples is intensified after nitrogen ion implantation, as shown in Fig. 2. Although the load dependent be-havior of implanted samples and unimplanted sam-ples shows similar trends, the variation in the gradient of nanohardness in response to applied loads for im-planted samples is higher than that of unimplanted samples, which is caused by the hard TiN layer formed on the Ti6Al4V substrate.

3.3 Fractal characterization

Fractal theory, introduced by Mandelbrot in 1977, is non-Euclidean geometry, allowing the study of ir-regular shapes and chaotic phenomena present in na-ture and can describe, in a very concise manner, ob-jects characterized by the properties of self-similarity or scale effect. Conventional hardness is character-ized by a determined applied load. However, many experiments prove that the hardness values vary with the applied loads, which can be called load depend-ence of hardness. The load dependence of nanohard-ness has strong fractal behavior.

As shown in Fig. 2, there are two different regions, both revealing totally different load dependent be-havior, which can be described by using fractal theory. In order to find the fractal characteristic of the load dependence of nanohardness in the applied low load region in Fig. 2, twenty indentations were performed with applied loads between 5 N and 200 N. The double logarithmic plotting of the nanohardness and applied loads is shown in Fig. 3. The results show that the logarithm of nanohardness, as a function of the logarithm of applied loads, is approximately lin-ear in two parts of the load dependence. The elas-tic-plastic deformation appears in the first region.The linear relationship of the double logarithm between measure (the nanohardness) and scale (the applied load) shows indentation self-similarity, both in in-dentation shape and in the results of hardness. We can conclude this to be fractal behavior. The load de-pendence of nanohardness shows dual fractal behav-iour given the two different regions of nanohardness. The fractal dimension in the first region can be de-picted as the following equation.

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Fig. 3 Double logarithmic coordinates for the nanohardness and applied load

11lg lg( ) DH bP

(1)

where 1D is the fractal dimension in the first region, P the scale (the applied load), and H the measure (the nanohardness).

According to equation (1), the slopes of the regres-sion lines, shown in Fig. 4, which can be called the nanohardness fractal dimensions for the unimplanted Ti6Al4V sample, the 1.3×1017 ions/cm2, 2.1×1017

ions/cm2 and 6.2×1017 ions/cm2 nitrogen ion im-planted samples, are 0.99, 1.00, 0.96 and 0.95 in the first region of the load dependence of nanohardness, respectively. The fractal dimension shows a decreas-ing trend, which indicates that the increase of the nanohardness reduces the elastic-plastic deformation, therefore decreasing the self-similarity complexity after nitrogen ion implantation.

(a) First region with applied load less than 200 N (b) Second region with applied load more than 1000 N

Fig. 4 Double logarithmic coordinates for the nanohardness and applied load

The nanohardness fractal dimension in the second region can be described by the following equation.

211lg lg( ) DH bP

(2)

where D2 is the fractal dimension in the second re-gion, P the scale and H the measure.

According to equation (2) and Fig. 4, the nano-hardness fractal dimensions for the unimplanted Ti6Al4V sample, the 1.3×1017 ions/cm2, 2.1×1017

ions/cm2 and 6.2×1017 ions/cm2 nitrogen ion im-planted samples are 0.76, 0.56, 0.57 and 0.59, respec-tively. The fractal dimension also shows a decreasing trend in the second region, suggesting a decrease of the self-similarity complexity owing to the increase of nanohardness after nitrogen ion implantation. Compared to the nanohardness fractal dimensions of the two regions, the variation of the deformation segment can be distinctly described by using the nanohardness fractal dimensions of the materials in nano-indentation. The nanohardness fractal dimen-sion in the two regions both maintain a decreasing trend. Fractal theory provides an effective method to estimate the material state under practical load bear-ing conditions. All of the evidence suggest that nanohardness fractal dimensions can be used to de-scribe the load dependence of nanohardness of mate-

rials. There may be essential properties of nanohard-ness of distinctly different kinds of materials, which will be investigated in our further studies.

4 Conclusions

In summary, the mechanical surface properties of the implanted Ti6Al4V alloys with three different nitrogen ion doses were investigated with nano-in-dentation technology. The XPS results indicate that nitrogen diffuses into the titanium alloy and formed a TiN layer on the Ti6Al4V alloy. The nano-indentation testing results reveal that nitrogen ion implantation effectively enhances the surface hardness of Ti6Al4V. As well, the nanohardness over a large segment of the applied loads clearly reveals load dependence, which also shows that the nanohardness is characterized by dual fractal behavior. Thus the concept of the nano-hardness fractal dimension is first proposed. The dual fractal model can effectively describe nonlinear de-formation in indent areas on the Ti6Al4V surface. The fractal dimension shows a decreasing trend in both regions, indicating a decrease of self-similarity complexity in surface indentation owing to the in-crease of nanohardness after nitrogen ion implanta-tion.

LUO Yong et al Load Dependence of Nanohardness in Nitrogen Ion Implanted Ti6Al4V Alloy … 367

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