Effects of pre-heat conditions on diffusion hardening of pure titanium by vacuum rapid nitriding

Effects of pre-heat conditions on diffusion hardening of pure titanium by vacuum rapid nitriding

SCT-21979; No of Pages 7 Surface & Coatings Technology xxx (2017) xxx–xxx Contents lists available at ScienceDirect Surface & Coatings Technology jo...

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SCT-21979; No of Pages 7 Surface & Coatings Technology xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Effects of pre-heat conditions on diffusion hardening of pure titanium by vacuum rapid nitriding Hyeon-Gyeong Jeong a, Yongtai Lee b, Dong-Geun Lee c,⁎ a b c

Gyeongnam Technopark Industrial Co. Ltd., Changwon 51395, Republic of Korea Department of Nano Materials Science and Engineering, Kyungnam University, 51767, Republic of Korea Materials Metallurgical Engineering, Sunchon National University, Suncheon 57922, Republic of Korea

a r t i c l e

i n f o

Article history: Received 30 August 2016 Revised 2 January 2017 Accepted in revised form 3 January 2017 Available online xxxx Keywords: Titanium Nitriding Pre-heat treatment Surface gradient hardening Inner-layer

a b s t r a c t The pre-heat treatment in vacuum rapid nitriding process was introduced to improve the surface hardening characteristics such as thick depth, high hardness and particularly to reduce the processing time. The pre-heat treatment was maintained for 30 min with an interval of 10 °C at the temperatures of 700– 800 °C under a high vacuum to allow the absorption of oxygen atoms on the titanium surface into the inner side. The following nitriding processes were equally performed at the same condition as 800 °C for 1 h. As a result, the depth of a hardened layer in the sample, which was treated at 800 °C for 1 h with the pre-heat treatment at 760 °C for 30 min, was similar to that of the treated sample at 800 °C for 3 h without pre-heat treatment and the depth was about 40 μm. The processing time can be largely reduced by introducing the pre-heat treatment process. The preheated specimens showed a longer wear lifetime by more than three times and higher friction resistance than that of PVD or non-nitrided specimens. The surface gradient-hardened layers exhibited excellent wear resistance and could prevent separation or debonding between matrix and the coated layer effectively. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Titanium and its alloys have excellent corrosion resistance because it exhibits not only a large effect of preventing corrosions penetrated inside a material due to its dense oxidized titanium film but also an immediate reproduction of damaged passive films within several micro seconds in the air [1–4]. In particular, titanium shows excellent corrosion resistance against chloride ions and can be used in marine structures and chemical process industries under severe environments. In addition, as titanium has no harmful reaction with human blood and superior bio-compatible characteristics to other metal materials, titanium and its alloys represent some advantages in bio-medical fields [5–8]. In spite of these excellent characteristics, titanium alloys show some disadvantages in their hardness and wear resistance. Thus, studies on the surface modification to improve such weaknesses have been attracted. Regarding the improvement in the surface characteristics of titanium, it was reported that the protective films which have excellent mechanical properties in low temperatures could be obtained by coating TiN onto the base material of titanium [9]. The TiN coating that has been most largely studied in wear resistive coating materials shows excellent oxidation resistance and surface roughness and ductility. Also, it exhibits an elegant gold color and has been largely used in not only wear ⁎ Corresponding author. E-mail address: [email protected] (D.-G. Lee).

resistance protective films but also corrosion resistive and ornament coating materials [10,11]. For improving the surface hardness and wear resistance of titanium using excellent coating materials, interstitial element hardening (using C, N, and O, etc.), plasma, Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) methods are used as general surface hardening methods [12–15]. Studies on applying the PVD method such as Ion Plating, Cathode Arc Deposition, Reactive Sputtering, and the CVD to coat the titanium surface, have been largely conducted. The CVD process is useful to vapor complex shapes because the CVD process has no dense coating microstructures, high adhesiveness, etc. Although it exhibits advantages of controlling the composition of coating materials and coating thickness, it may cause some changes in microstructures at an interface between a thin film coating layer and titanium matrix because vapor deposition processes are usually performed at high temperatures. Thus, it has been known that these changes significantly affect the mechanical and corrosion properties [16,17]. The PVD process has some advantages such as excellent wear resistance, heat resistance, oxidation resistance, and corrosion resistance. Whereas, it has some disadvantages such as a weak adhesive strength between the thin film coating layer and the matrix, high price equipments, a long processing time, separations in coated layers, and some cracks between layers, etc. In this study, a thermo-chemical treatment (TCT) processing is introduced in order to generate the hardened inner-layers with continuous

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hardness gradient. It enables some interstitial elements such as nitrogen, oxygen, and carbon to diffuse and penetrate into the matrix of titanium. By applying this method, it is expected that the wear resistance of the titanium surface is improved and the separation between the coating layer and the matrix as well as some cracks caused by external factors are restricted. Various pre-heat treatments are applied to implement the fast and effective absorptions of the TiO2 oxidized film which is existed on the titanium surface, under a high vacuum atmosphere. And the influences of some factors on the changes in surface hardness, surface gradient-hardened layers, and surface wear resistance are investigated.

2. Materials and methods The specimen for applying the surface hardening was a commercial product of pure titanium with a size of 10 × 12 × 10 mm3 (from TIMET) and its bulk chemical composition was presented in Table 1. The TCT processes were applied by the different processing conditions. For obtaining the surface gradient-hardened layers on the surface of titanium, a gas controlled vacuum furnace (GCVF) system was used, which is designed to control temperature, vacuum level, and gas flow, etc. Nitrogen was used as a reactive gas and the surface hardening process was implemented by decreasing the pressure to 5 × 10−5 Torr initially by the GCVF system, and then controlling the partial pressure in the chamber as 5 × 10−1 Torr through inserting nitrogen gas, followed as heating it up to 800 °C (Fig. 1). As titanium has a large affinity with nitrogen, it makes possible to easily form titanium nitride. The TCT process involves a pre-heat treatment process as shown in Fig. 1 in order to remove the passive oxide film existed on the titanium surface and effectively present such a nitriding process. The pre-heat treatment was maintained for 30 min in a specific temperature range in the initial heating stage and the nitriding process was carried out at 800 °C for 1 h. The pre-heat treatment was applied at four different temperatures in a range between 700 and 800 °C, such as 710 °C, 740 °C, 760 °C, and 790 °C. In addition, some TiN vapor deposited specimens using the PVD method were used for comparing them with the TCT specimens. For verifying the effects of the surface modification in these specimens, surface hardness, cross-sectional hardness, surface wear characteristics, and cross-section coating layers were analyzed as well as the morphologies of the hardened layers. The hardness characteristics of the cross-section were measured from the surface to the center of the specimens by a micro Vickers hardness tester (by FUTURE-TECH, Model FM-700). Wear characteristics were evaluated by using a Ball on Disk type wear tester (by J&L, Model JLTB-02 tribometer) in which a SUS304 stainless ball with a diameter of 1 mm was used as a counterface material. By generating frictional wear with a rotational radius of 3 mm between the ball and the specimen, the friction coefficients in each specimen were measured under a rotational speed of 100 rpm and a load of 1N, and the frictional behaviors were also investigated. Room-temperature X-ray diffraction analysis was carried out on a Philip X'PERT Diffractometer utilizing radiation at 30 kV and 40 mA, the scanning range being 2θ = 20°–80°. In addition, through observing the sections of the specimens by a scanning electron microscope (SEM, by JEOL, JSM-6610LV), the compound layers generated on the surface and the diffused layers of nitrogen gas and their boundaries were analyzed.

Fig. 1. Schematic diagram of the TCT process for nitriding titanium according to variable pre-heat treatment conditions.

3. Results and discussion 3.1. Surface gradient-hardened layers An XRD analysis was performed for verifying the compounds generated on the surface hardened layers (Fig. 2). It was possible to observe titanium matrix and TiN phases in the PVD specimens. In the case of the specimens in the TCT process, the generation of α-Ti, TiN0.26, and Ti2N phases was verified in all four specimens even though there were no generations and changes of other compounds according to preheat treatment temperatures. The peak of α-Ti phase in all TCT specimens was slightly moved from the original peak to a lower angle. This slightly shifts of α-Ti peak, which has a hexagonal close-packed (HCP) structure, is connected with the expansion or distortion of lattices due to the insertion of the interstitial element, nitrogen atoms [18]. For confirming the titanium nitrides analyzed by the XRD, the surface hardened layers were investigated by SEM analysis, as shown in Fig. 3. For the PVD process, TiN compound layer with a thickness of about 1.7 μm was only observed. For the TCT-ed specimens, it was verified that all specimens presented three layers. The results revealed that the compound generated at the most outside was Ti2N based on the studies performed by S. Gokul Lakshmi et al. [19], Erol Metin et al. [20], and S. Taktak et al. [21] and the hardened layer generated at the

Table 1 Chemical composition of commercially pure titanium material (wt.%). Components

C

Fe

H

N

O

Ti

wt.%

0.05

0.3

0.008

0.02

0.2

Bal.

Fig. 2. X-ray diffraction patterns of the nitrided surface compound layers according to the various pre-heat treatment temperatures; (a) as-received, (b) PVD (TiN), pre-heating at (c) 710 °C, (d) 740 °C, (e) 760 °C, and (f) 790 °C (the TCT treated at 800 °C, 1 h).

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Fig. 3. SEM micrographs of the cross-sectional hardened layers after (a) PVD and (b) ~ (e) various pre-heat treatments (the TCT treated at 800 °C, 1 h).

lowest section near the matrix was the diffused layer, in which nitrogen atoms are located at the interstitial sites of HCP structure. The generation of nitrides was initiated from the reaction between titanium and nitrogen and the thickness of the Ti2N compound was increased to about 0.9 μm, 1.1 μm, 2.0 μm, and 2.0 μm according to the increase of the pre-heat treatment temperature as 710 °C, 740 °C, 760 °C, and 790 °C, respectively. In the case of the temperature more than 750 °C, the thickness of the compound was increased by twice. The reason is that the surface is activated by the enough absorption of the protective film, in order to allow more effectively the penetration of nitrogen atoms.

temperature. That is, it was possible to form the continuous surface gradient-hardened layers according to the following chemical reactions. TiOx → Ti + [O]: oxide film dissolution

3.2. Hardness distribution For observing the effects of the pre-heat treatment temperatures at more extended ranges, nitriding processes were performed after applying pre-heat treatment processes. Although titanium has a very large affinity with nitrogen and it makes possible to easily form titanium nitride, it requires a long processing time for implementing the nitriding process due to the dense passive oxide film on the titanium surface. Thus, the TCT process can be used to present a short processing time and effectively implement the gradient-hardened depths by removing the oxide film under a high vacuum atmosphere and a proper

Fig. 4. Hardness variations in the various pre-heat treatment conditions (the TCT treated at 800 °C, 1 h).

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Fig. 5. Surface hardness and hardened layer depth profiles of the various pre-heat treatment processes (the TCT treated at 800 °C, 1 h).

Ti + N2 → TiNx + Ti[N]: nitride film and diffused layer formation Fig. 4 showed the variation of the cross-sectional hardness of each specimen after applying the TCT process with a pre-heat treatment process. All specimens by the nitriding process formed the hardened layers with a thickness of more than 10 μm and the surface hardness and depth of hardened layers were varied according to pre-heat treatment temperatures even though the same nitriding conditions were applied. The deep hardened layers with a thickness of more than 30 μm were formed at the temperature above 740 °C and there were similar

Fig. 7. Relationship between the friction coefficient and the cycles for the nitrided titanium according to the pre-heat treatment.

hardening behaviors without any specific increases in the temperature at the pre-heat treatment process more than 750 °C and the deep hardened layers with a thickness of more than 40 μm were presented. It showed similar hardening depths to that of the TCT nitriding process at 800 °C for 3 h without applying a pre-heat treatment process and the surface hardening processing time can then be largely reduced by introducing the pre-heat treatment process. In addition, as mentioned above, it was verified that the effects of the oxide film can be significantly reduced

Fig. 6. Depth profiles of the various pre-heat treatment processes (the TCT treated at 800 °C, 1 h).

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Fig. 8. Relationship between the friction coefficient and the cycles for the as-received pure titanium.

in the nitriding process by absorbing effectively the oxygen atoms on the surface into the inner side through maintaining the temperature more than 750 °C under a high vacuum atmosphere [22–24].

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The changes in the surface hardness and depth of the hardened layers for the TCT-ed specimens with the pre-heat treatment temperatures of 710 °C, 740 °C, 760 °C, and 790 °C, respectively were compared one another with respect to the TiN vapor deposited specimens using the PVD method, as shown in Fig. 5 and Fig. 6. Regarding the surface hardness, the specimen of the PVD process showed 315Hv and the TCT-ed specimens exhibited 445Hv, 489Hv, 549Hv, and 562Hv according to pre-heat treatment temperatures of 710 °C, 740 °C, 760 °C, and 790 °C, respectively. The specimens of the TCT process represented higher hardness values than that of the PVD process by more than 100Hv. In addition, in the case of applying the pre-heat treatment processes with different temperatures for the four specimens, it revealed that the surface hardness was gradually increased according to the increase of the pre-heat treatment temperature. The depths of the TCT-ed specimens were measured as 16 μm, 35 μm, 45 μm, and 42 μm for each pre-heat treatment temperature, and in particular, the deep hardened layers with a thickness of more than 40 μm were formed at the temperature above 750 °C. However, in the case of increasing the temperature up to 790 °C, there were no increases in the hardness and were similar results to the surface hardness and the depth of hardened layers of the specimen with the pre-heat treatment at 760 °C. Thus, the oxide film was not fully removed at the pre-heat treatment temperature below 750 °C, whereas the oxide film was effectively removed by increasing the temperature more than

Fig. 9. Wear surface obtained by applying a tribology test after 5000 cycles (SEM).

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Fig. 9 (continued).

750 °C and that led to present similar results even though the different pre-heat treatments were applied. The TiN layers by PVD showed a thickness of 1– 2 μm and the surface hardness was measured as about 300Hv because it was affected by the base material as the Vickers hardness is measured by the load of 100 g. Because the surface hardened layers were not deeply formed compared to that of the TCT-ed specimens and showed no surface gradient-hardened characteristic, it could be difficult to restrict the separation of the coated layers under frictional and wear environments. 3.3. Surface friction characteristics For verifying the friction and wear characteristics of the surface hardened specimens, the friction coefficients were measured as shown in Fig. 7. For the PVD specimen, a low friction coefficient of 0.1– 0.2 was maintained up to about 500 cycles and increased rapidly by 0.45 after increasing more than 500 cycles. The reason was that the deposited TiN layer was separated from the base material after above 500 cycles. Also, it showed that the same value continued to proceed from 500 cycles up to 5000 cycles and was similar to the friction coefficient measured in the as-received pure titanium, which was carried out under the same wear condition as presented in Fig. 8. It revealed that

the value was the friction coefficient of the base material based on the separated coating layers after 500 cycles. For the TCT-ed specimens, they maintained a low friction coefficient for a longer time than that of the PVD process and showed a gentle increase in the friction coefficient due to the gradient-hardened layers. In the case of the TCT process, the number of cycles that maintained the low friction coefficients was varied according to these pre-heat treatment conditions. The cycles were significantly increased up to about 2500 cycles for the samples pre-heated at the temperature above 750 °C. It showed the same tendency to the results of the crosssectional hardness and it is due to the fact that the hardened layer with high hardness is more deeply formed as the pre-heat treatment temperature at the temperature above 750 °C. However, the diverged friction coefficient after 2500 rotations was about 0.7 that was a higher value than that of base pure titanium. It was due to the friction caused by a relatively weak counterface material. The surface hardened layers formed by the TCT process were not the easily separated layers like that of the PVD process but the deeply gradient-hardened layers with a high hardness level and a thickness of more than 40 μm. The results of the wear surface observation by SEM were shown in Fig. 9. The wear surfaces showed different aspects according to the process conditions in the PVD and TCT processes. The width of wear tracks

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of the PVD specimen was averagely about 904 μm and that was the largest wear surface among these specimens. In the case of the surface wear layers for the PVD specimen, it showed typical abrasive wears that form deep and thick wear tracks along the rotation direction and lots of grooves. In addition, there was much debris due to the separated coating layers generated in the wear test. However, in the results of the wear tests for the TCT-ed specimens with the pre-heat treatment at 710 °C, 740 °C, 760 °C, and 790 °C, they showed relatively narrow wear tracks as 633 μm, 457 μm, 390 μm, and 387 μm, respectively, and typical adhesive wear with vertical stripes against the rotation direction. It was created by the adhesion of the counterface material due to the friction in the wear test and there was almost no debris caused by the wear. The width of the wear tracks was reduced by half at the preheat treatment temperature above 750 °C and the wear surface became to be gradually clean. Also, the adhesion phenomenon was reduced. A proper surface condition for applying a nitriding process was formed at the temperature more than 750 °C. It revealed that the formation of the deep gradient-hardened layers with a high hardness represents excellent wear resistance. 4. Conclusions The pre-heat treatment and vacuum rapid nitriding technology were investigated to improve the surface characteristics of titanium by the Thermo-Chemical Treatment process. The results of this study can be summarized as follows; 1. It was possible to successfully form the gradient-hardened layers with a gradient inner hardness distribution by applying vacuum rapid nitriding process to pure titanium. And the surface hardening processing time can be largely reduced by introducing the pre-heat treatment process in TCT process. 2. The TCT nitriding after applying a pre-heat treatment process to dissolve the passive oxide film from titanium surface, could improve the surface hardness more than 2– 3 times and form the surface gradient-hardened layers with a thickness more than 40 μm. 3. By applying a pre-heat treatment above 750 °C for 30 min, it was possible to obtain a longer wear lifetime by more than three times and higher friction resistance than that of the PVD or the non-nitrided specimens. This tendency agreed with the hardness results of the surface gradient-hardened layers and the depth of the hardened layers. It showed excellent wear resistance characteristics that represent no debonding between the base titanium and the surface hardened layers. Acknowledgements

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This study was supported by the Korean government MSIP (the Ministry of Science, ICT and Future Planning), MOTIE (the Ministry of Trade,

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