Ti metal matrix composite gradient coating on NiTi by laser cladding and nitriding

Surface & Coatings Technology 200 (2006) 4961 – 4966 www.elsevier.com/locate/surfcoat

In situ formation of a TiN/Ti metal matrix composite gradient coating on NiTi by laser cladding and nitriding H.C. Man a,*, S. Zhang a, F.T. Cheng b, X. Guo c a

Laser Processing Group, AMTRC, Department of Industrial and Systems Engineering, Hong Kong Polytechnic University, Hong Kong b Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong c Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong Received 16 November 2004; accepted in revised form 9 May 2005 Available online 21 June 2005

Abstract A TiN reinforced metal matrix composite (MMC) layer was fabricated in situ on a NiTi substrate aiming at improving the wear resistance and reducing the surface Ni content in view of potential medical applications. Ti powder was preplaced on a NiTi substrate and irradiated with a high-power CW Nd:YAG laser in N2 atmosphere for laser nitriding and alloying. SEM micrographs of the cross-section revealed a gradient coating with an almost compact TiN film of about 1 – 2 Am thickness as the outermost layer and an MMC layer beneath with the amount of TiN decreasing with depth. The hardness was increased from 250 HV in the substrate to 600 – 900 HV in the modified layer due to the presence of the hard TiN phase. The wear resistance against a diamond ball was correspondingly increased by a factor of two. EDS analysis also indicated a low Ni content in the surface layer. These results suggest that the laser modification technique employed in the present study is capable of enhancing the feasibility and biocompatibility of NiTi samples used as orthopedic implants. D 2005 Elsevier B.V. All rights reserved. Keywords: NiTi shape memory alloys; MMC coating; Laser surface alloying; Titanium nitride; Wear

1. Introduction Nickel Titanium shape memory alloy (NiTi) has been a popular functional and structural material for the last thirty years finding applications in many fields, especially in medicine and dentistry [1]. The attractiveness of NiTi mainly arises from its outstanding properties, which include in particular, its shape memory effect and superelasticity. Despite its attractive properties in relation to performance, the market of NiTi is incommensurate with its performance. For structural applications, NiTi would be rather expensive when used as a bulk material. In addition, its machinability is poor [2]. Thus NiTi has been proposed to be used as a cladding material rather than as the bulk [3,4]. For dental and medical applications, especially as implants, concern

* Corresponding author. E-mail address: [email protected] (H.C. Man). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.017

has been expressed with respect to its high Ni content [5]. This is particularly important when NiTi is used as orthopedic implant on which both wear and corrosion could be present, thus speeding up Ni ion release. In view of this, NiTi orthopedic applications are usually surface treated by some selected processes such as chemical etching using HF/ HNO3 followed by boiling in water [6], electropolishing [7] and autoclaving [8]. One inexpensive way of providing a reasonably thick oxide layer on NiTi is by thermal oxidation [8 –11] in which Ti in the surface layer is preferentially oxidized. Ceramics are in general more wear resistant than metallic materials due to their high hardness. Thus coating metallic substrates with a ceramic or a ceramic-reinforced MMC layer provides an effective method in prolonging the service life of metallic components in abrasive or erosive environments. Laser surface techniques are particularly suitable in fabricating such a layer because of the formation of strong fusion bonding between the modified layer and the

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substrate. Laser techniques have also been employed by our group to modify NiTi for better corrosion or erosion resistance [12,13] via laser surface melting or nitriding. The wear and corrosion resistance of the modified surface was improved by virtue of the chemical inertness and hardness of TiN present [14]. A similar degree of improvement was observed when a TiN/Ti metal matrix composite was fabricated by laser surface nitriding on commercial pure Ti metal [15]. As a further step in improving the surface properties of NiTi in applications as orthopedic implants for which a high wear resistance and a low surface Ni content are important, the present study aims at fabricating a TiN reinforced MMC on NiTi by laser treatment. Fig. 2. Optical micrograph showing the microstructure of the as-received NiTi substrate.

2. Experimental details Hot rolled NiTi (Ti — 50.8 at.%) was spark cut into samples of dimensions 25 mm  50 mm  5 mm. The samples were polished to remove the oxide layer and then sandblasted to roughen the surface to enhance powder adhesion. The sandblasted samples were ultrasonically cleaned with acetone and deionized water. Commercial pure Titanium powder of particle size in the range 40 – 100 Am was mixed with a binder (4% polyvinyl alcohol, PVA) to form a slurry and then pasted on the samples. The pasted specimen was dried in an oven at 100 -C for two hours and then gently polished to ensure an even thickness of the pasted layer of 0.7 mm. The specimen was then put in a gas tight chamber with a glass window which the Nd-YAG beam can penetrate through and the specimen was irradiated inside the chamber. The chamber was constantly filled up and purged with a mixture of Nitrogen and Argon at a ratio of 2 : 1. By controlling the flow rates of Nitrogen and Argon at 20 and 10 l/min, respectively, the gases were mixed in a pre-chamber before they were delivered to the laser processing chamber.

Laser processing parameters such as laser power and scanning speed were optimized based on the criteria that the laser clad layer obtained should be free from porosity, crack and have relatively smooth surface. The range of laser power and speed studied were 1– 1.5 kW and 20 –30 mm/s. The parameters were varied systematically and the following set of parameters was optimized: CW Nd:YAG laser with a power of 1.5 kW at the workpiece, a laser spot of 1.5 mm and a scanning speed of 25 mm/s. The beam, delivered via optical fibre, was focused using a 100 mm focal length ZnSe lens. The laser tracks were overlapped at 50% track width to achieve surfacing. After laser treatment, the samples were sectioned, polished, and etched with an etchant (10 ml HF, 40 ml HNO3, 50 ml H2O) for scanning electron microscopy (SEM)/Energy Dispersive X-ray analysis (EDX) (JEOL JSM-6335F and Leica Stereo Scan440, respectively) analysis. The phases present were identified using X-ray diffractometer (XRD) (Bruker D8 Discover) with Ni filtered Cu Ka radiation generated at 40 mA and 40 kV, and a scanning rate of 1.5 -/min. The microhardness was NiTi(110)

P

40

50

60

70

NiTi(220)

NiTi(211)

NiTi(200)

TiO2(210)

Ti(101)

Counts

ω

80

2θ, deg Fig. 1. Schematic diagram showing the setup of the wear test.

Fig. 3. Diffractogram of the NiTi substrate.

90

100

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3. Results and discussion 3.1. Microstructure of the alloyed surface

Fig. 4. Macroscopic view of the surface of laser treated sample.

measured using a Shimadzu microhardness tester at a load of 300 g and a loading time of 15 s. The wear resistance was studied using a simple setup shown schematically in Fig. 1. The wear tester is not a standard one but nevertheless it serves the purpose of comparing the wear resistance of two different surfaces. The counter surface was a diamond ball of diameter 1.6 mm, with a load of 3.5 N and a rotating speed of 200 rpm. The sample was weighed at regular intervals. After the wear test, the morphology of the worn surface was studied by a surface profiler (Form Talysurf PG1240 Measurement System) and SEM.

Fig. 2 shows the microstructure of the as-received hotrolled NiTi alloy and Fig. 3 shows its XRD spectrum. The phases are identified based on the Powder Diffraction File [16]. The alloy is of single phase (B2) structure but contains particles of TiO2. The diffraction lines exhibit broadening due to the existence of non-homogenous residual stress in the alloy which has experienced thermomechanical processing. After laser surface processing, the specimen surface was fairly smooth, as shown in Fig. 4 and had a typical golden color TiN appearance. Cross-sectional views of a single track and of overlapping tracks shown in Fig. 5 revealed a modified layer of about 1000 Am thickness without cracks or pores. Compared with the as received NiTi substrate, (Fig. 3), the microstructure of the laser modified layer was characterized by the existence of various forms of dendrites embedded in a metal matrix, as shown in Fig. 6. When the specimen was irradiated by the Nd-YAG laser beam at optimum processing conditions, the Ti powder coating absorbs the laser energy effectively and melting of the Ti powder occurs immediately. As the molten metal is surrounded by nitrogen and because of the great affinity of Ti for nitrogen, TiN formed at the liquid/gas interface. At the same time, heat transfer to the NiTi substrate by

(a)

Single track

NiTi substrate

(b)

200μ μm

Bakelite

Modified layer, overlapped tracks

NiTi substrate

300μm

Fig. 5. Optical micrographs showing the cross-section of (a) single laser track, (b) overlapping tracks.

30

40

50

60

70

80

TiN(400)

Ti2Ni(771)

Ti2Ni(931)

TiN(311) Ti2Ni(842)

Ti2Ni(733) Ti(103)

Ti2N(002) TiN(220)

Ti2N(520)

TiO2(210) Ti2Ni(440)

Laser modified layer

Ti2Ni(311) TiN(111) Ti(002) Ti N(111) Ti(101) 2 Ti2N(511)

Bakelite

Counts

(a)

TiN(200)

H.C. Man et al. / Surface & Coatings Technology 200 (2006) 4961 – 4966

Ti2Ni(442)

4964

90

100

2θ, deg

(b)

(c)

Fig. 6. SEM micrographs showing cross-sectional views of TiN/Ti MMC layer formed by in situ reaction in laser treatment: (a) surface layer, (b) near the middle, and (c) near the layer/substrate interface.

convection of the molten metal and conduction lead to melting of the substrate. Because of the Maragoni convective fluid flow of the molten metal due to the thermal gradient and the surface tension [17], mixing of the molten metal (Ti and NiTi) occurred. Also TiN particles formed at the outmost surface were transported and mixed within the molten metal pool. These particles act as dendritic nuclei and were dispersed in the molten pool. Vigorous convection in the melt pool also facilitated the transport of N2 to various parts of the melt pool. TiN dendrites grow in the molten pool owing to constitutional

Fig. 7. Diffractogram of the laser modified laser.

supercooling during solidification. As determined by the concentration of N2 and the liquid phase lifetime at different locations in the melt pool, the amount of TiN formed varied in a gradient manner (Fig. 6) along the depth of the modified layer. The longer the time the molten Ti stays in the liquid state, the more TiN dendrites can be formed. At the outmost surface, because the molten metal there solidifies last, TiN formed at the outmost surface is a continuous and compact layer of thickness around 2 Am [18,19]. At the substrate interface, solidification occurred first and the solid/liquid front moved towards the surface of the pool. Hence the amount of TiN is minimal at the interface. Due to maximum heat flow at the substrate interface, the epitaxial grains at the interface are of columnar shape and mainly consist of NiTi and Ni Ti2 owing to the extra amount of Ti added into the layer. At the centre of the molten pool, there is no preferred heat flow direction and hence the dendritic growth is multidirectional and coral like dendrites resulted. Beneath the compact TiN surface layer is a metal matrix composite layer composed of TiN dendrites, NiTi2 dendrites and Ti matrix, as indicated in Fig. 7 which is the XRD pattern of the composite layer taken after the top 10 Am of the surface were removed by grinding. According to EDXS analysis, as shown in Table 1, the laser modified layer had a much lower surface Ni content (about 7 wt.%) than that in the substrate (about 54 wt.%). The surface composition data of the modified layer was collected after about 10 Am from the outmost surface was ground off. The release of Ni ions from a NiTi implant is always a concern. The dramatic decrease

Table 1 Results of the EDX analysis Specimen

NiTi alloy Laser modified layer

Element (wt.%) Ni

Ti

54.1 7.3

45.9 92.7

H.C. Man et al. / Surface & Coatings Technology 200 (2006) 4961 – 4966

The hardness profile along the depth of the layer is shown in Fig. 8 and exhibits a gradual decrease. The outermost TiN compact layer which has a thickness of about 2 Am, has a hardness of 1200 Hv as measured by a nano-indentation tester (NANO INDENTATOR II with a Berkovich indentator, loading time 10 s, 200 mN). This hardness value is not shown in Fig. 8 as the outermost layer is only 2 Am thick. Beneath this compact layer is the composite layer and the hardness value decreased from about 900 to about 600 HV at the interface, and then rapidly dropped to about 250 HV in the substrate. The large increase in hardness was obviously due to the presence of TiN, and the change in hardness was consistent with the graded distribution of the hard TiN phase in the matrix. The cumulative weight loss in the wear test as a function of time is shown in Fig. 9. At the end of the 5 h test, the laser modified layer exhibited an improvement in wear resistance by a factor of 2 over the as received NiTi. Crosssectional profiles of the worn surface shown in Fig. 10 indicate different wear mechanisms for the substrate and the TiN/Ti MMC layer. In the laser modified layer, the wear scar exhibited a rugged profile near the bottom, indicating the role played by the hard phase in resisting stress and abrasive wear [20]. In the NiTi substrate, material was removed by plastic deformation and ploughing, as evidenced by material pile-up and a smoother profile. Evidence of the difference in wear mode was also found in the SEM micrographs of the worn surface in Fig. 11. A seriously worn surface with troughs and cracks was visible in the untreated NiTi substrate, while in the laser modified

Weight loss (x10-2mg)

3.2. Hardness and wear resistance

200 as recevied NiTi alloy laser modification layer

160

120

80

40

0

0

1

2

3

4

5

6

Wear time (h) Fig. 9. Weight loss against time in wear test for NiTi and laser modified samples.

sample, the surface was much smoother with the TiN particles still visible. This indicates that the TiN particles formed in situ were tightly bound to the metal matrix and remained effective in resisting wear. Thus the higher wear resistance of the MMC layer could be attributed to the

+8.0

(a) Wear scar depth /μ μm

of Ni content in the surface layer of the specimen obtained in this work is significant in biomedical application as the rate of Ni ion release into the human body can be greatly reduced.

4965

+4.0 +0.0 -4.0 -8.0 -12.0 +0.0

+0.3

+0.6

+0.9

+1.2

+1.5

Wear scan width /mm

1000 +4.0 900

Wear scar depth /μm

(b)

800

HV0.3

700 600 500 400 300

+0.0 -2.0 -4.0 -6.0 +0.0

200 0

+2.0

200

400

600

800

1000

1200

1400

+0.3

+0.6

+0.9

+1.2

+1.5

Wear scan width /mm

Distance from surface (μm) Fig. 8. Hardness profile along the depth of the laser modified layer.

Fig. 10. Depth profiles of the wear scar for (a) NiTi and (b) laser modified sample.

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(a)

3. The Ni content at the surface of the laser modified layer was significantly reduced. Thus the results of the present study indicate that the laser-fabricated TiN/Ti MMC layer improves the applicability of NiTi as orthopedic implants.

Acknowledgements

(b)

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5190/02E). Support from the infrastructure of the Hong Kong Polytechnic University is also acknowledged.

References

Fig. 11. SEM micrographs showing the worn surface of (a) NiTi and (b) laser modified sample.

complementary effects of the hard TiN phase and the tough metal matrix.

4. Conclusions 1. In situ fabrication of a TiN reinforced metal matrix composite layer on NiTi has been achieved by laser cladding pre-placed Ti powder in a nitrogen atmosphere. The pre-placed Ti powder reacted with N2 to form an MMC layer containing a gradient distribution of TiN and NiTi2 dendrites in a Ti rich matrix. 2. The wear resistance of the MMC layer was increased by a factor of two relative to the substrate owing the presence of the hard TiN phase.

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