Microstructures and tribological properties of laser cladded Ti-based metallic glass composite coatings

Materials Characterization 120 (2016) 82–89

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Microstructures and tribological properties of laser cladded Ti-based metallic glass composite coatings Xiaodong Lan, Hong Wu ⁎, Yong Liu ⁎, Weidong Zhang, Ruidi Li, Shiqi Chen, Xiongfei Zai, Te Hu State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China

a r t i c l e

i n f o

Article history: Received 15 May 2016 Received in revised form 26 July 2016 Accepted 23 August 2016 Available online 24 August 2016 Keywords: Laser cladding Metallic glass Coating Tribological properties

a b s t r a c t Metallic glass composite coatings Ti45Cu41Ni9Zr5 and Ti45Cu41Ni6Zr5Sn3 (at.%) on a Ti-30Nb-5Ta-7Zr (wt.%) (TNTZ) alloy were prepared by laser cladding. The microstructures of the coatings were characterized by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray analyzer (EDXA), and transmission electron microscopy (TEM). Results indicated that the coatings have an amorphous structure embedded with a few nanocrystalline phases and dendrites. A partial substitution of Ni by Sn can improve the glass forming ability of Ti-base metallic glass system, and induce the formation of nano-sized Ni2SnTi phase during the cyclic laser heating. The tribological behavior of both the substrate and the coatings was investigated in detail. A significant improvement in both the hardness and the wear resistance of the coatings was achieved with the addition of Sn. The relationship between the wear resistance and the microstructures of the coatings was discussed. © 2016 Published by Elsevier Inc.

1. Introduction Titanium and its alloys have been widely used in aeronautical, biomedical and defense industries due to their low density, good mechanical properties, high strength-to-weight ratio, corrosion resistance and biocompatibility [1,2]. However, its poor wear resistance is a serious concern for applications. Protection against surface degradation is usually achieved through modification of microstructure and/or composition of the near-surface region (called surface engineering) without affecting the bulk [3]. Bulk metallic glasses (BMGs) have no longrange atomic order, and are deprived of the usual crystalline slip systems for plastic deformation, therefore, have high yield strength, low Young's modulus, high corrosion and fatigue resistance [4,5]. The unique combination of the good properties enables the BMGs to have potential to be used as emerging structural materials and biomaterials [6–10]. Among the BMGs systems, Ti-based BMGs are greatly attractive for structural and biomedical applications due to their high strength, high elastic limit, low Young's modulus, excellent corrosion resistance and good biocompatibility [11]. However, Ti-based BMGs possess quite low glass forming ability (GFA) when it comes to structural or ⁎ Corresponding authors at: State Key Laboratoryof Powder Metallurgy, Central South University, Changsha 410083, China. E-mail addresses: [email protected] (H. Wu), [email protected] (Y. Liu).

http://dx.doi.org/10.1016/j.matchar.2016.08.026 1044-5803/© 2016 Published by Elsevier Inc.

biomedical applications. Application on a large scale requires proper combination of biocompatibility, GFA and low cost. So it would be essential to further study Ti-based BMGs systems. To overcome the size limitations and expand the applications of metallic glasses, researchers have developed the process to deposit metallic glass coatings on structural or functional components [12–15]. Laser cladding has been proven to be capable of producing adherent, hard, wear corrosion fatigue and fracture resistant coatings on a diverse range of materials [16–19], which is widely used to overcome the surface-originated problems on metallic components [20]. It is expected that Ti-based metallic glass coatings can improve the wear resistance of titanium alloys. As a promising surface engineering technique, laser cladding also allows to obtain three dimensional implants [21–23]. With the crystallization characteristics controlled to eliminate the crystalline heat affected zone during laser cladding, the complete generation of a BMG implant without size limitation is also possible [24]. Sn is believed to be efficient to improve the corrosion resistance and biocompatibility of metallic materials [25]. In the present study, Tibased metallic glass composite coatings Ti45Cu41Ni9Zr5 and Ti45Cu41Ni6Zr5Sn3 (a substitution of Ni by 3% Sn) were deposited on biomedical titanium alloys by laser cladding. Both the microstructure characteristics and the tribological properties of the Ti-based metallic glass composite coatings were systematically studied. The underlying strengthening and wear mechanisms of the coatings were also

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discussed. The demonstration of the coating biocompatibility is out of the scope of the present work, and further biological characterization would be presented in our coming research paper concerning in biocompatibility.

2. Materials and methods Ti-30Nb-5Ta-7Zr (wt.%) (TNTZ) sheets with a thickness of 10 mm were used as substrates without a special surface finishing after normal machining. Gas-atomized powder with the nominal composition of Ti45Cu41Ni9Zr5 and Ti45Cu41Ni6Zr5Sn3 (at. %) were prepared for laser cladding. The size of the particles was smaller than 75 μm. The laser cladding experiments were performed in a glove box containing an argon atmosphere with oxygen content less than 10 ppm to limit any oxidation using a 300 W Nd: YAG pulsed laser with a wavelength of 1.06 μm. The diameter of laser beam was 1 mm. The nominal single pulsed energy was 143 J with a pulse duration of 6 ms. The pulse frequency was 20 Hz. During the cladding, a powder layer with the thickness of 0.2 mm was firstly laid on the substrate or pre-deposited layer, then the powder layer was melted point by point along the pre-set trajectory by pulsed laser beam and resolidified to form a solid deposited layer. Thus, a metallic glass coating can be produced by this repeated laser deposition layer by layer. The laser scanning speed was 5 mm s−1, and the laser cladding tracks with an overlap ratio of approximately 50% was selected to obtain a continuous coating. The as-deposited coatings were cut by wire electrical discharge machining, and polished to mirror finish according to the standard procedures. The phases of the powders and the coatings were examined using a Rigaku D/max 2550-VB X-ray diffractometer (CuKα radiation) with 2θ diffraction angle ranging from 20° to 80°, at a scanning rate of 0.02°s−1. The degree of crystallinity of as-deposited coatings was quantitatively estimated with X-ray diffraction using the empirical method proposed by Ruland [26] and Vonk [27]. The degree of crystallinity is given by the ratio between the areas under the XRD peaks of crystals and the total area under the XRD curve. The equation for calculating the degree of crystallinity is as follows: Xc ¼ Ac =ðAc þ Aa Þ

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using a Buehler Micromet5104 microhardness tester at a load of 100 g. An average of 5 measurements for each position was reported. Wear tests were conducted under dry conditions using a ball-on-disk sliding apparatus (CERT UMT-3, USA) in air at room temperature. A Φ9.5 mm hardened chrome steel ball (HRC: 62) was used as a counterpart and a test load of 20 N was applied. The friction unit was rotated at a speed of 300 rpm for 30 min and the wear scar length was fixed at 7.5 mm. The friction coefficients of specimens were recorded during wear tests. The wear volumes (V) of specimens were determined as follows: V ¼ Mloss =ρ

ð2Þ

where Mloss was the weight loss of specimens after tests, ρ was the density of the material. The wear rates (ω) were thus calculated by: ω ¼ V=ðWLÞ

ð3Þ

where W was the contact load and L was the sliding distance. 3. Results 3.1. Microstructural characterization The SEM images of Ti45Cu41Ni9Zr5 and Ti45Cu41Ni6Zr5Sn3 powders are shown in Fig. 2, respectively. Both of the two Ti-based powders are of a spherical or nearly spherical shape. The XRD patterns of the as-

ð1Þ

Where Xc refers to the degree of crystallinity, Ac refers to the crystallized area on the X-ray diffractogram, and Aa refers to the amorphous area on the X-ray diffractogram. The coatings were observed by field emission scanning electron microscope (FESEM) combined with an EDXA. Transmission electron microscopy (TEM) specimens were prepared by using the focused ion beam (FIB) lift-out method (Fig. 1). TEM specimens extracted from the coatings were characterized using a JEOL-2100F high resolution transmission electron microscope (HRTEM). The Vickers microhardness of the coatings was measured

Fig. 1. SEM micrograph (secondary electron) showing the cross-sectional TEM sample prepared using the FIB lift-out method.

Fig. 2. SEM micrographs (secondary electron) of metallic glass feedstock powder. a. Ti45Cu41Ni9Zr5, b. Ti45Cu41Ni6Zr5Sn3.

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The microstructure in the coating changes with the depth to the substrate. Figs. 4e–f show that a thin layer of the substrate is melted by the heat conduction during the laser cladding. In particular, the microstructures in the substrate reveal an epitaxial growth into the coating in the form of columnar grains indicating good fusion bonding. However, the sharp change of the composition and high cooling rate in the coating restrict the further growth of the columnar grains. In order to make further investigations on microstructures in the coatings, TEM observation was undertaken on the area near the surface region. The formation of amorphous phase in the coating was further confirmed by ring patterns in Fig. 5b. Some nanocrystalline phases are also found to be embedded in the amorphous matrix in Fig. 5a. In addition, diffraction patterns of nano-sized grains can also be seen in Fig. 5b. According to the selected area electron diffraction patterns (Fig. 5b), the nano-sized grains are identified as body centered cubic (bcc) phases with a lattice parameter of 0.267 nm, which is very close to that of CuNiTi2 phase (bcc, a = b = c = 0.3047 nm). The selected area electron diffraction pattern of a large grain in Fig. 5c is identified as Ti2Ni phase (a = b = c = 1.1319 nm) (Fig. 5d). The TEM results match well with the XRD patterns in Fig. 3a. As a result of the addition of 3% Sn, a substantial amount of nano-sized crystalline phases (gray phase in Fig. 5e) precipitated in the bcc matrix phase (CuNiTi2). The precipitates are characterized as Ni2SnTi (fcc, a = b = c = 0.609 nm). Peaks of Ni2SnTi phase are not observed in Fig. 3a because of its small amount. The diffraction results about Ti2Ni and Ni2SnTi are in accordance to those in Ref. [28]. Therefore, the nano-sized Ni2SnTi phases are evenly dispersed in the matrix of CuNiTi2 phases, and almost completely coherent with CuNiTi2 phases. 3.2. Microhardness

Fig. 3. XRD patterns of Ti-based metallic glass composite coatings (a) and powders (b) used in this study. (CuNiTi2 JCPDS card number 52–0860).

The microhardness for the two metallic glass composite coatings along the distance from the substrate is shown in Fig. 6. The Vickers hardness is around HV100260 ~ 270 in the substrate, and increases to HV100350 ~ 400 in the interfacial bonding zone, while in the coatings, the hardness increases significantly to about HV100565 ~ 582 (Ti45Cu41Ni9Zr5) and HV100615 ~ 665 (Ti45Cu41Ni6Zr5Sn3). The coating of Ti45Cu41Ni6Zr5Sn3 has a higher hardness, mostly due to its relative high volume fraction of amorphous phase. 3.1. Friction and wear behavior

deposited coatings are shown in Fig. 3a. The XRD patterns of the corresponding powders are included in Fig. 3b for comparison. The patterns in Fig. 3a exhibit a typical broad halo corresponding to amorphous phase at a diffraction angle of about 41°, combined with several sharp diffraction peaks corresponding to crystalline phases. No distinct oxidized material peaks are detectable, indicating low oxygen content in the coatings. As for the powder, the pattern of Ti45Cu41Ni9Zr5 powder shows typically brag peaks. With an addition of Sn, the Ti45Cu41Ni6Zr5Sn3 powder exhibits a quite prominent fraction of amorphous phase, compared to Ti45Cu41Ni9Zr5 powder. Crystallinity of the two deposited layers and the precursor powder were calculated from diffraction intensity data by using the Ruland-Vonk method. The order of the amorphous fractions in the coatings is Ti45Cu41Ni6Zr5Sn3 (60.52%) N Ti45Cu41Ni9Zr5 (49.46%). The results were given as the average value of three measurements with an error of ± 0.01. This means that the GFA of the Ti-based alloy can be improved by a minor addition of Sn. Fig. 4 shows SEM images of cross sections of the two coatings. It can be seen that both layers are dense and pore-free. The structures of coatings are quite homogeneous. Due to the high cooling rate, the solidified microstructures are mainly amorphous. There are also crystalline phase and some flower-like dendrites distributed in the coatings. However, the addition of Sn modified the microstructures. The Sn-containing coating has a higher content of amorphous phase than that without Sn, and the size of the dendrites is also smaller.

The tribological behavior of the Ti-based metallic glass composite coatings was assessed by dry sliding wear tests. Fig. 7 presents the variation of the friction coefficient as a function of sliding time for the substrate and the metallic glass composite coatings. The friction coefficient of Ti45Cu41Ni9Zr5 metallic glass composite coatings exhibited an apparent local fluctuation, with a mean value reaching 0.4796. Compared to the fluctuating friction coefficient of TNTZ substrate and Ti45Cu41Ni9Zr5 composite coatings, the Ti45Cu41Ni6Zr5Sn3 composite coatings exhibited a stable friction coefficient in the range of 0.35–0.40 (a mean value of 0.3946). To some degree, the relatively steady wear behavior shows a self-lubricating ability, which mainly attributes to the formation of Sncontaining phase Ni2SnTi. It could be explained based on the reports [29,30] that during dry sliding wear experiments Ni2SnTi acts as an extreme pressure lubricant and minimizes direct contact between the surfaces in relative motion, thus providing anti-friction characteristics. The friction coefficient of the Ti45Cu41Ni9Zr5 metallic glass composite coating shows large local fluctuations in the steady state regime, even more severe than that of TNTZ substrate. This may attribute to the uneven distribution of crystalline phases in the coatings. The wear rate and average friction coefficient of different materials are shown in Fig. 8. The wear rate of materials is 1.46 × 10− 4 mm3 N−1 m−1 for Ti45Cu41Ni9Zr5, and 2.09 × 10−5 mm3 N−1 m−1 for Ti45Cu41Ni6Zr5Sn3 coatings, while it is 6.24 × 10−4 mm3 N−1 m−1 for TNTZ substrate. The wear rate of both

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Fig. 4. SEM micrographs (backscattered electron) showing the cross-section microstructures of the laser-cladded Ti45Cu41Ni9Zr5 (a、b) and Ti45Cu41Ni6Zr5Sn3 (c, d) metallic glass composite coating: a, c low magnification, b, d high magnification; Microstructures of bonding interface between the coatings and the substrate. e Ti45Cu41Ni9Zr5, f Ti45Cu41Ni6Zr5Sn3.

metallic glass composite coatings are significantly lower than that of the substrate, indicating that both metallic glass composite coatings have better wear resistance. The wear resistance of metallic glass coating is about 4.3 and 30 times higher than that of TNTZ substrate, respectively. Further, the wear rate of Ti45Cu41Ni6Zr5Sn3 metallic glass composite coating is much lower, only one seventh of that of Ti45Cu41Ni9Zr5 coating. Detailed SEM studies of the corresponding worn surfaces are shown in Fig. 9 and the underlying mechanisms for the formation of various topographical features were proposed to be as follows. The worn surface of the substrate (Fig. 9a) is very rough, primarily consisting of adhesive zones and parallel, deep grooves representing severe abrasion wear. The presence of loose fragments at the edges of grooves revealed the local

severe deformation and plowing of the surface during sliding. And this is consistent with the result of the wear rate in Fig. 8. Compared to the substrate, the worn surfaces of the metallic glass composite coatings in Fig. 9b and Fig. 9c show much shallower grooves, thereby reducing the mean friction coefficient and attendant wear rate to 1.46 × 10−4 and 2.09 × 10−5 mm3 N−1 m−1, respectively (Fig. 8). For the coating of the Ti45Cu41Ni9Zr5, there are some large plastic deformation zones, indicating an adhesive wear. While for the Ti45Cu41Ni6Zr5Sn3 coating, though the worn surface shows an adhesive wear, the plastic deformation zones are much less and finer. It can be concluded that within a certain range, the higher the GFA of the coating, the better the wear resistance, because the wear resistance of the coatings can be improved by a proper increase of the amorphous phase [31,32].

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Fig. 5. TEM micrographs of the microstructures and selected area electron diffraction patterns of Ti45Cu41Ni6Zr5Sn3 composite coating.

4. Discussion 4.1. Glass forming ability and crystallization The atomic radius and mixing enthalpy play important roles in the glass forming ability [33]. The Sn has the largest atom radius compared with the other elements in the Ti-base alloy system. In the present work, with the addition of Sn, the GFA is improved. Some reasons should be considered. First, The addition of Sn could increase the atomic size

mismatches, and subsequently retard the long range rearrangement of atoms required for the crystallization, thus improving the GFA of the BMG [34,35]. Also, the addition of Sn increases the complexities in element species. The majority of BMG compositions are associated with a complex multicomponent chemistry. The large complexities in element species and atomic sizes are believed to significantly frustrate the crystallization tendency of the undercooled liquid and hence to favor glass formation [36]. Secondly, the Sn-Ti, Sn-Zr and Sn-Ni pairs exhibit negative mixing enthalpies (−21, −43 and −4 kJ/mol, respectively [34]). In

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Coating

1.0

6.24 Wear rate Friction coefficient

Wear rate (×10-4 mm3N-1m-1)

6

Bonding zone

500

Ti45Cu41Ni9Zr5

400

Ti45Cu41Ni6Zr5Sn3

Substrate

Microhardness (HV)

600

300

0.8

5

0.6209 0.6

4

0.4796 0.3946

3 2

0.4

Friction coefficient

700

87

1.46 0.2

1

0.209 200 -0.4

0

-0.2

0.0

0.2

0.4

0.6

0.8

0.0

Ti45Cu41Ni9Zr5

Ti45Cu41Ni6Zr5Sn3

TNTZ

Distance from interface (mm) Fig. 8. Comparison of wear rate and average friction coefficient of different materials. Fig. 6. Hardness variation across the coatings on TNTZ substrate.

a thermodynamic point of view, the substantially negative heat of mixing would result in a deeply depressed free energy curve of the undercooled liquid and, hence in small free energy driving forces for crystallization [37]. Hence, the addition of Sn is effective for increase in the degree of the satisfaction of the empirical rules for obtaining high GFA proposed by Inoue et al. [33]. The temperature interval of the supercooled liquid region defined by the difference between crystallization temperature (Tx) and glass transition temperature (Tg), ΔTx (= Tx − Tg) increases significantly with a minor addition of Sn [38]. The high thermal stability of the supercooled liquid leads to a higher GFA [38,39]. Therefore, an addition of Sn element in the Ti-Cu-Ni-Zr metallic glass system can increase the fraction of amorphous phase in the composite coatings. However, due to the positive mixing enthalpies of Sn-Cu pairs (7 kJ/mol [34]), the addition of Sn should facilitate the nucleation of crystalline phase resulted from the crystallization in the heat affected zone in the pre-deposited layer during laser depositing [24]. The formation of nano-sized CuNiTi2 and Ni2SnTi particles in the composite structure may be a result of the high cooling rate of laser shock. 4.2. Strengthening mechanism and wear resistance According to the Archard's law, the wear loss of the material is inversely proportional to the hardness of the material [40]. A good balance between the crystalline and amorphous phases can effectively increase both the hardness and the toughness of metallic glass, and consequently 1.0 0.9

Ti45Cu41Ni9Zr5

Coefficient of friction

0.8

TNTZ

0.7 0.6 0.5

improve the wear resistance of metallic glass composites [32,41–43]. The cyclic heating and cooling process caused by a pulsed laser irradiation will lead to a heat shock to the adjacent amorphous layer. During a heat shock, the zone in the amorphous layer whose temperature was higher than Tg can pass through supercooled liquid region, but the relaxation and crystallization will occur. In the early stages of crystallization, the nanocrystalline particles impede the crack propagation, but the ductile dendrites have an overall toughing effect [43]. This means converting the plastic deformation mechanism from the formation of multiple shear bands to active promotion of slips or twins, and finally the plastic deformation mechanisms of dendrites [44]. Therefore, the contributions of the microstructures to the hardness and wear resistance can be divided into two aspects, i.e., crystalline phase and amorphous phase [45]. The two Ti-based metallic glass composite coatings both have ductile dendrites distributed in the amorphous matrix. Elastic-plastic deformation can occur sufficiently by slips or twins inside the dendrites, toughening the coatings, and thus, increase the wear resistance. The Sn-containing coating has nano-sized CuNiTi2 particles that can effectively strengthen the amorphous matrix. The nano-sized Ni2SnTi particles evenly dispersed in the CuNiTi2 dendrites can further strengthen the dendrites. Kim et al. [46] also pointed out that the nanocrystalline grains are too small to contain defects (such as dislocations, stacking faults, microtwins) and the ultra-high strength would explain the higher resistance to deformation than the amorphous phase. In amorphous alloys, the deformation mechanism is inhomogeneous, and involves shear on highly localized shear bands which also can be effectively suppressed by interfaces (induced by nanocrystalline particles) or by a possible interaction between shear bands and a nanocrystalline phase [47]. Therefore, it can be expected that the nanocrystalline particles dispersion strengthen the amorphous alloys, increase the fracture stress, and decrease the crack growth [48]. In summary, the amorphous structure improves the wear resistance of the coating, while the nanocrystalline particles and dendrites in the amorphous matrix have a much significant strengthening and toughing effect. 5. Conclusions

0.4

Ti45Cu41Ni9Zr5 and Ti45Cu41Ni6Zr5Sn3 metallic glass composite coatings were successfully synthesized by laser cladding. The following conclusions are drawn:

0.3 0.2

Ti45Cu41Ni6Zr5Sn3

0.1 0.0 0

200

400

600

800

1000

1200

1400

1600

1800

Time/sec Fig. 7. Friction coefficients versus time on substrate TNTZ, Ti45Cu41Ni9Zr5 composite coating and Ti45Cu41Ni6Zr5Sn3 composite coating.

1) The partial substitution of Ni by Sn increases GFA of the coating, and induces the precipitation of nano-sized Ni2SnTi phase during the cyclic laser heating (annealing). 2) The microhardness of the coatings is significantly enhanced due to the formation of crystalline phase and amorphous phase composite structures.

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Fig. 9. SEM micrographs (secondary electron) of worn surfaces for the substrate and the composite coatings. a substrate, b Ti45Cu41Ni9Zr5 composite coating, c Ti45Cu41Ni6Zr5Sn3 composite coating.

3) A significant improvement in the wear resistance can be achieved by laser cladding of metallic glass composite coatings. The Sn-containing coating has an even better wear resistance than that of Ti45Cu41Ni9Zr5. A good balance between the crystalline and amorphous phases can effectively improve the wear resistance of metallic glass composite coatings. 4) The TNTZ substrate shows a typical abrasive wear, while adhesive wear is the main mechanism in the metallic glass composite coatings during sliding. Acknowledgment Financial support from the National Natural Science Foundation of China (51301205 and 51571214), Innovation-driven Plan in Central South University (2016CX003), Specialized Research Fund for the Doctoral Program of Higher Education of China (20130162120001) and Changsha Municipal Major Science and Technology Program (K1502003-11) is gratefully acknowledged. References [1] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Ti based biomaterials, the ultimate choice for orthopaedic implants – a review, Prog. Mater. Sci. 54 (2009) 397–425. [2] M. Peters, J. Kumpfert, C.H. Ward, C. Leyens, Titanium alloys for aerospace applications, Adv. Eng. Mater. 5 (2003) 419–427. [3] W.L. Johnson, Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials, Prog. Mater. Sci. 30 (1986) 81–134. [4] C. Suryanarayana, A. Inoue, Bulk metallic glasses, CRC Press, 2010. [5] J. Schroers, Processing of bulk metallic glass, Adv. Mater. 22 (2010) 1566–1597.

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