Surface & Coatings Technology 200 (2006) 4879 – 4884 www.elsevier.com/locate/surfcoat
Structure and wear properties of laser gas nitrided NiTi surface N.Q. Zhaoa,*, H.C. Manb, Z.D. Cuia, X.J. Yanga b
a School of Materials Science and Engineering, Tianjin University, China Laser Processing Group, Advanced Manufacturing Technology Research Center, Department of Industrial and Systems Engineering, Hong Kong Polytechnic University, Hong Kong
Received 24 March 2005; accepted in revised form 27 April 2005 Available online 3 June 2005
Abstract The laser gas nitriding process is an efficient technique for modification of materials. Composite coatings with a microstructure consisting of dendritic TiN/NiTi were fabricated on a substrate of NiTi by the laser gas nitriding process. Different nitrogen flow rates were supplied during the laser process in order to detect the influence of the TiN on the wear resistance. The wear resistance of the TiN/NiTi coatings was evaluated under sliding wear test conditions in 5% NaCl aqueous solution at room temperature. The results indicate that the TiN/NiTi coatings have excellent abrasive and adhesive wear resistance because of the high hardness and the proportion of the TiN. D 2005 Elsevier B.V. All rights reserved. Keywords: Laser gas nitriding; Wear resistance; NiTi shape memory alloy; TiN coating
1. Introduction Within the past 5 years, equiatomic NiTi alloy has become widely used in a variety of mainstream biomedical applications  because of its unique shape memory effect (SME) around the room temperature and superelasticity. From a product point of view, several candidate applications require good wear resistance. For example, the medical industry is very interested in the high compliance of NiTi alloys for use in joint replacement, where wear plays an important role. Recent studies have demonstrated that the equiatomic NiTi alloy exhibits a high wear resistance . The wear behaviour of NiTi alloy was expected to improve further if hard particles were embedded in the alloy . The hardphase particles may withstand the external load and the pseudoelastic matrix can absorb impact energy and accommodate a relatively large strain. Ye et al. 
* Corresponding author. Tel.: +86 22 87401601; fax: +86 22 27405874. E-mail address: [email protected]
(N.Q. Zhao). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.043
demonstrated a high wear resistant of TiNi-based TiC or TiN reinforced composites by using the vacuum sintering process. TiN as a modified layer to improve the corrosion resistance of the NiTi has been reported by Fu et al. , Endo et al. , Wu et al. and Lin et al. [8,9], Starosvetsky et al. , etc. The process for obtaining the TiN is concerned with the ion nitriding , arc ion plating , pulsed high-energy density plasma  and power immersed reaction assistant coating nitriding method . However, published results concerning the effects of TiN/NiTi on wear resistance, especially to the coating made by laser gas nitriding on NiTi shape memory alloys were sparse. The laser gas nitriding process (LGN) is an efficient technique for the modification of materials . Our previous work on the NiTi has shown that an alloyed layer with TiN on the NiTi substrate will be formed during the LGN treating . To our knowledge, no investigation of the wear behaviour of LGN NiTi has been reported yet. The present study obtained TiN/NiTi coatings by the laser gas nitriding process in different nitrogen flow rates. The purpose of this work is to investigate the effects of the
N.Q. Zhao et al. / Surface & Coatings Technology 200 (2006) 4879 – 4884 TiN
c b a
Fig. 1. Schematic of the block-on-wheel dry sliding wear tester.
laser nitriding process on the microstructure and the wear resistance of the TiN/NiTi coating.
2. Experimental materials and methods 2.1. Materials The material used for the experiment was a binary nickelrich NiTi alloy with a nominal composition of 50.8 at.% Ni.
Fig. 3. The XRD charts of LGN treated NiTi surface, a) 10 l/min N2 flow rate, b) 30 l/min N2 flow rate, c) 40 l/min N2 flow rate.
The hot rolled as-received NiTi alloy was as plate metal with a thickness of 4 mm. The specimens of 25 8 4 mm were cut for the experiment. A continuous wave 2-kW NdYAG laser was used to irradiate the specimen surface with a peak power intensity of 500 W. The laser beam was defocused to a 2-mm spot size. High purity nitrogen gas (99.9%) N2, was blown onto the specimen molten pool through a nozzle with f5 mm diameter. The nitrogen gas flow rate was changed from 10 to 40 l/min for investigating its influence on the NiTi microstructure. The scanning speed was kept as 5 mm/s. The overlap neighboring tracks was 0.5 mm (25% overlap). 2.2. Characterization methods Scanning electron microscopy (SEM) was performed using a Leica Stereo Scan 440 for investigating the microstructure of the LGN coatings. The microstructures of the composite layer were also studied by optical
10 l/min N2
20 l/minN2 30 l/minN2
600 500 400 300 200 100 0
Fig. 2. SEM images of the microstructure of the cross-section of LGN NiTi specimens nitrided by nitrogen flow rate of 10 l/min (a) and 40 l/min (b).
Distance from surface, μm
Fig. 4. Profile of the microhardness of the LGN NiTi samples.
N.Q. Zhao et al. / Surface & Coatings Technology 200 (2006) 4879 – 4884 9
equipment were 40kV and 40mA, respectively using Cu – K alpha radiation. The microhardness of the crosssection of LGN treatment specimen were measured with Shimadzu micro hardness tester with the load of 3 N. Every specimen was measured for 5 times for obtaining the average microhardness.
Weight loss, mg
6 5 4
2.3. Wear test
3 2 1 0 0
Nitrogen flow rate, l/min
Fig. 5. Weight loss via nitrogen flow rates on NiTi specimens. a) 10 l/min b) 20 l/min, c) 30 l/min d) 40 l/min.
microscopy (OM) using a Leica digital camera system microscope. The volume fraction of the TiN was estimated with the image analysis software in Leica digital camera system. A Bruker D8 Discover equipment was used for the XRD study and the voltage – current settings of the
Room-temperature wear resistance of the laser nitrided TiN/NiTi coatings was evaluated on a MM-200 block-onwheel sliding wear tester in 5% NaCl water solution, as schematically shown in Fig. 1, where a surface of the TiN/ NiTi coating on the block specimen (10 7 3 mm in size) is pressed under the applied test load of 70 N against the outer periphery surface of a stainless steel wheel (HV560) rotating at 200 rpm, resulting in a relative sliding speed of 0.489 m/s. The wear test cycle lasted for 60 min and the total wear sliding distance is 1764 m. The wear mass loss was measured using an analysis balance with an accuracy of 0.01 mg and was utilized to rate the relative wear resistant properties of the coatings in comparison to the test materials.
Fig. 6. Worn surface morphologies of LGN NiTi with different N2 flow rate a) 10 l/min b) 20 l/min c) 30 l/min d) 40 l/min.
N.Q. Zhao et al. / Surface & Coatings Technology 200 (2006) 4879 – 4884
3. Results and discussion 3.1. Microstructure and hardness The cross-section microstructures of the LGN NiTi specimens by different nitrogen flow rate are shown in Fig. 2. The features of the microstructure can be described as follows: 1) A continuous thin surface layer was formed on the outmost surface of the specimen with the different thickness from 1 to 2 Am, which increased with the flow rate of nitrogen. 2) A dendritic phase exists in the NiTi matrix in the melted zone of the laser nitriding. The typical dendritic structure existed in all the specimens made by different N2 flow rates in the present work. 3) The thickness of the melted zone with TiN is about 200 – 300 Am measured by OM. 4) The size of the dendrites and its proportion increased with the flow rate of nitrogen. The XRD result of the LGN treated surface proves that the surface is composed of the y-TiN with face-centered cubic close-packed structure , while the diffraction peaks of the NiTi phase were hardly detected in the XRD spectrum, as shown in Fig. 3. This demonstrates that the surface of the NiTi specimen is primarily covered by the TiN. It indicates that laser nitriding in a pure nitrogen environment produced dendritic structures. No Ti2N phase was detected in the specimen treated with low nitrogen flow rate. This result is different with that of in Ti alloys. We have demonstrated the formation of acicular Ti2N during cooling in zones where the N atom concentration was not high enough to form TiN at room temperature in Ti alloy . The hardness profile along the depth direction of the TiN/NiTi coating of the cross-section is shown in Fig. 4. Because of the rapidly solidified fine microstructure and the high volume proportion of TiN, the laser gas nitrided TiN/ NiTi coating exhibits a high hardness property. The hardnesses drop considerably as the distance from the surface increases. When the N2 flow rate changes, the hardness shows the evident difference due to the change of the nitride titanium in proportion and type. The hardness curves are well matched with the observation of the microstructures (Fig. 2a and b).
istics of NiTi specimens can be effectively improved by laser nitriding because TiN/NiTi compound layers provide an important contribution to the improvement of wear resistance. In other words, the laser nitrided NiTi shape memory alloy, being hardened by TiN, can exhibit excellent wear resistance and a low friction coefficient. Fig. 6 shows the surface morphologies of worn tracks after sliding wear on the NiTi LGN specimens treated by different nitrogen flow rates. A comparison between the tracks on different N2 flow rate specimens reveals remarkable differences. In Fig. 6a, the compound layer prepared with 10 l/min N2 flow rate has been almost worn out and few TiN dendritic phase can be seen; and a typical worn morphology of NiTi matrix can be observed. The NiTi matrix (HV = 200) is much softer than the against-wear stainless steel (HV = 560); hence, the adhesive and abrasive wears occur. The adhesive wear may cause the fragments of NiTi matrix to be pulled off and adhere to the surface of the against-wear stainless steel. The abrasive wear introduces the ploughing grooves, which originate from the interaction of micro-cutting and plastic deformation . Similar worn morphologies were observed in the specimen nitrided with the nitrogen flow rate of 20 l/min, as shown in Fig. 6b. But the ploughing grooves in Fig. 6b are shallower than that shown in Fig. 6a. The TiN phase in the specimen prepared
3.2. Wear resistance and wear morphology In comparison to the raw material, the laser nitrided coating exhibited a quite outstanding wear resistant capability at room temperature under sliding wear test conditions, coupling with the stainless steel wheel as the mating counterpart, as indicated in Fig. 5. It can be seen that the wear mass losses of the TiN/NiTi coatings are much lower than that of non-LGN treated specimen. These results come from the fact that the wear interfaces are TiN/NiTi composite layers and stainless steel, and hence the friction coefficients and wear rates maintain the low values due to their high hardness. This indicates that the wear character-
Fig. 7. TiN morphology of the coating by LGN treating with 40 l/min N2 flow rate. a) The cross section of coating b) the inner coating.
N.Q. Zhao et al. / Surface & Coatings Technology 200 (2006) 4879 – 4884
with 20 l/min nitrogen flow rate can be still clearly seen. The dominating wear mechanism is micro-cutting. In contrast, the influence of the high N2 flow rate is remarkable. In comparing Fig. 6c and d, the composite layers of the nitrided specimen could sustain a higher wear load than those of the low N2 flow rate specimen. As shown in Fig. 6c, main adhesive wear morphology occurs. However, the fragments of stainless steel have been adhesively transferred to the TiN/NiTi composite layers, instead of the NiTi alloy being transferred to the steel, due to their significant difference of hardness. It is interesting, as shown in Fig. 6d, that the worn surface of the coating after a sliding wear test cycle of 60 min is very similar to a wellpolish and etched metallographic section on which even the fine microstructural features of the TiN can be clearly resolved. No any characteristic features of metallic adhesion and abrasive wear on the worn surfaces of the coating, whereas noticeable grooves and adhesion and deformation features are visible on the worn surface of the comparison test materials, as shown in Fig. 6a and b. Fig. 7a) and b) show the morphologies of the TiN on the coating surface and coating inside of the sample by LGN with 40 l/min N2 flow rate, respectively. The plump dendritic TiN can be seen clearly, implying the important role to the good wear resistance. The effect of the reinforcement in the matrix on the wear resistance depends on its type, amount and size as well as its distribution within the matrix. We can use a parameter A to describe the effect of the reinforcement on the wear resistance: A ¼ Volume fraction ð%Þ of reinforcement Hardness of reinforcement Size of reinforcement: The value A is directly affected by the TiN/NiTi composite layer hardness, which increases with the volume fraction and the size of TiN within the matrix. The hardness of the coating reveals the ability of resisting the abrasive to enter into the surface. The larger the A, the lower wear rate the material. According to our microstructure analysis, the results is in good agreement with the rule, as shown in Table 1 in which the data was measured on the specimen section from the LGN surface about 30 Am. Thus, the wear mechanism changes from adhesive and abrasive wear for the specimen nitrided by the low N2 flow rate to main adhesive wear to those by high N2 flow rate. The disagreement of the mechanism between the specimens nitrided by low and high N2 flow rate results from the difference of the specimens’ hardness. The work of Wu et al.  on the ion-nitrided NiTi specimens being hardened by the TiN/Ti2Ni compound layers, exhibited excellent wear resistance and a low friction coefficient. Their result demonstrated that only main adhesive wear morphology occurs for the ion-nitrided
Table 1 The hardness and TiN volume fraction of the composite coating nitrided by different nitrogenflow rates Nitrogen flow rate (l/min)
TiN volume fraction (%)
10 20 30 40
752 783 806 857
25 32 38 43
specimen instead of adhesive and abrasive wears for the non-ion-nitrided specimen in dry sliding contact against SKS-95 steel (HV-700). The present work on the laser gas nitrided NiTi has a good agreement but smoother worn morphology compared with their work on the ion-nitrided NiTi specimens, which may result from the different composition of the composite layer and the wear test condition.
4. Conclusion 1. TiN/NiTi composite has been produced by laser nitriding on NiTi alloy. The volume fraction of TiN increased with increasing the N2 flow rate, resulting in the enhancement of the surface hardness of NiTi alloys. The amount of dendritic TiN can be controlled by varying the nitrogen flow rate during laser processing. 2. The wear characteristics of the NiTi shape memory alloy can be effectively improved by laser gas nitriding. The laser nitrided specimens, being hardened by TiN, can exhibit the excellent wear resistance. The wear resistance increases with the TiN amount in the TiN/ NiTi composite. 3. The wear mechanism for the TiN/NiTi composite layer against the stainless steel wheel changes from adhesive and abrasive wear to main adhesive wear with the TiN volume fraction increases, resulting from the different N2 flow rate during laser gas nitriding.
Acknowledgements The work described in this paper has been supported by the Research Grants Council of Hong Kong, China (Project No. PolyU 5157/00E). Support from the Tianjin Univerisyt and Hong Kong Polytechnic University is also acknowledged.
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