Carbide layer coating on titanium by spark plasma sintering technique

Carbide layer coating on titanium by spark plasma sintering technique

Surface & Coatings Technology 353 (2018) 324–328 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsev...

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Surface & Coatings Technology 353 (2018) 324–328

Contents lists available at ScienceDirect

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

Carbide layer coating on titanium by spark plasma sintering technique a,⁎

Akio Nishimoto , Chihiro Nishi a b

T

b

Department of Chemistry and Materials Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan Graduate School of Science and Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Titanium Carburizing Surface modification Spark plasma sintering Carbide coating

Titanium materials are widely used in aerospace, automotive and biomaterial engineering fields due to high specific strength, superior fatigue and corrosion resistance as well as excellent biocompatibility. However, titanium exhibits low hardness and poor wear resistance. Therefore, the development of a suitable surface modification technology is necessary to expand the use of titanium materials. In order to improve hardness and wear resistance of materials, there is the method to form the hard ceramics layer on the matrix surface. In this study, carburizing method was applied. The carburizing method can form the carbide layer which is superior in adhesion with the matrix compared with PVD or CVD method. However, in conventional carburizing methods, the deterioration of the mechanical properties of the matrix as a result of long-term and high-temperature processing is problematic. Therefore, spark plasma sintering technique, which features short processing times, was applied to form a carbide layer in this study. The purpose of this research is to form a TiC layer on commercially pure Ti (CP-Ti) and evaluate its properties. CP-Ti was used as the substrate, and graphite powder was used as the carburizing source. XRD analyses indicated that a TiC layer was formed on the substrates. Corrosion tests indicated that the corrosion resistance of the carburized samples was remarkably improved compared to that of CP-Ti. Wear tests revealed that the carburized samples exhibited low friction coefficients and improved tribological properties.

1. Introduction Titanium materials are widely used in the aerospace, chemical, automotive, and biomaterial engineering fields because of their high specific strength, superior fatigue and corrosion resistance, and excellent biocompatibility. However, titanium exhibits poor wear resistance because of its high friction coefficient and low hardness. Therefore, a surface modification technology that maintains titanium's properties is needed to expand the use of titanium materials. Methods have been developed to form a hard-ceramic layer on the matrix surface of materials to improve their hardness and wear resistance [1–3]. In the case of titanium-based materials, a TiC layer is often formed via chemical vapor deposition (CVD) or physical vapor deposition (PVD) [3,4]. Titanium carbides exhibit high melting points, high hardness and good wear resistance. However, TiC coating layers formed by CVD or PVD do not exhibit good adhesion to their substrates; this poor adhesion has been reported to lead to exfoliation and fracture at the substrate-coating layer interface [5,6]. In contrast, the carburizing method is expected to form a carbide layer with matrix adhesion superior to that of carbide layers deposited via PVD or CVD. However, in conventional carburizing, the mechanical properties of the matrix ⁎

can deteriorate under extended high-temperature processing. Therefore, in the present study, the spark plasma sintering (SPS) technique, which can be used to treat materials with a short processing time [7], was applied to form ceramic layers. Recently, the utility of SPS has been demonstrated in ceramic/metal nano-materials, composites materials system functionally graded materials (FGMs), hard materials, electronic materials, thermoelectric conversion materials, and biomaterials [8–13]. In this technique, raw material powder is packed into a graphite die, to which a pulse current is subsequently applied while the die is maintained under uniaxial pressure. This process leads to Joule heating among powder particles, which promotes sintering. This method was also used for the joining of dissimilar ceramics and/or metals [14–19] and coating [20–28]. The fabrication of carbide [20], boride [21], silicide [22], ceramics [23,24], composites [25–27], and high entropy alloy coatings [28] on metal or ceramics substrate has also been achieved using SPS. The coatings fabricated using SPS demonstrated strong metallurgical bonding between the coating and substrate. In this study, commercially pure Ti (CP-Ti) was used as a substrate and graphite powder was used as the carburizing source. The advantages of the SPS technique include its moderate uniaxial pressure and shorter sintering time compared to

Corresponding author. E-mail address: [email protected] (A. Nishimoto).

https://doi.org/10.1016/j.surfcoat.2018.08.092 Received 24 April 2018; Received in revised form 1 August 2018; Accepted 6 August 2018 Available online 05 September 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 353 (2018) 324–328

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diffractometer was equipped with a Cu-Kα radiation source operated at 40.0 kV and 300 mA, the samples were scanned at 40.0°/min. To investigate the corrosion resistance, we immersed the square plate samples in 2% HF–10% HNO3 solution and measured the decrease in weight after 10 min. The microstructure of the samples was characterized by scanning electron microscopy (SEM, JEOL, JSM-6060LV) and energy-dispersive X-ray spectrometry (EDX). Depth chemical profiles and element contents were obtained using glow-discharge optical emission spectroscopy (GD-OES, HORIBA, GD-Profiler2). The GD-OES conditions were a sputtering-mark diameter of 4 mm, a discharge pressure of 600 Pa, and a power of 35 W. The tribological performance of the sample was evaluated using a ball-on-disc tribometer (CSM Instruments, Tribometer, Switzerland). The tests were carried out under air atmosphere, with a sliding speed of 100 rpm, a wear-track radius of 5 mm, a sliding distance of 500 m, and a load of 2 N; Al2O3 balls with a diameter of 6.00 mm were used as the counter material. The surface hardness was measured using a Vickers microhardness tester (PMT-X7A, Matsuzawa, Akita, Japan) under a load of 0.25 N maintained for 10 s. The reported hardness values are the average results obtained for five measurements.

Vacuum chamber Pressure

Electrode

Power source

Upper punch Die (graphite) Graphite powder

Temperature analysis

Sample Lower punch

Controller • Atmosphere • Temperature • Pressure • Time

Electrode

Pressure Fig. 1. Schematic illustration of the setup for CP-Ti sample and graphite powder in a graphite die for SPS.

3. Results and discussion

those used in traditional methods, in addition, the applied pulse current may facilitate the diffusion of carbon atoms.

Fig. 3 shows XRD patterns of samples subjected to different carburizing temperatures. The current composites mainly consist of α-Ti phase and TiC phase, irrespective of the treatment conditions. The intensity of TiC phase increased with an increase of carburizing temperature. And impurities peaks associated with, for example, oxides were not detected. The lack of impurities is attributable to the carburizing treatment being conducted under vacuum, the carbon reduction effect stemming from the use of a graphite die, and the oxide film on the CP-Ti substrate being broken by the high-temperature plasma generated by the pulse current. Oxygen atoms have been reported to inhibit carbon diffusion into TiC layers, thus, to increase the thickness of the TiC layer, removal of oxygen is important [29]. In this study, the spark plasma sintering method effectively grew the TiC layer. In the XRD patterns, peaks associated with graphite were also intense. Fig. 4 shows the results of SEM-EDX analysis of the sample surface after carburizing treatment for 3.6 ks at 1343 K by SPS; particles similar to the raw powder are observed. We attributed the graphite peaks in the XRD pattern to graphite powder from the carburizing source. These results indicate that the SPS carburizing technique resulted in the formation of a TiC layer on the Ti substrate via treatment for 3.6 ks. Fig. 5 shows the test results for samples corroded using 2% HF–10% HNO3 solution. The weight loss per unit surface area is shown as a function of the immersion time. In this study, the weight loss is calculated for a surface area of 240 mm2. The samples carburized by SPS exhibited improved corrosion resistance compared to the substrate because of the formation of a TiC layer, which exhibits corrosion

2. Experimental details CP-Ti (purity 99.5%) was used as the substrate, and graphite powder (particle size 45 μm, purity 98.0%) was used as the carburizing source. The square plate sample (10 × 10 × 1 mm3) for corrosion test and disk sample (ϕ20 mm × 5 mm) were wet polished to #2000. Raw material powders were filled into a BN-coated cylindrical graphite die with an inner diameter of 20 mm. The polished substrate was first inserted into the powder, followed by the insertion of a graphite punch. The sample was coated using an SPS system (model SPS-1020, produced by Sumitomo Coal Mining Co., Japan) under a uniaxial pressure of 11 MPa and a vacuum lower than 10 Pa. Fig. 1 shows schematics of the SPS setup. The heating time was 0.6 ks, the coating time was 3.6 ks, and the coating temperature was 1143, 1243, and 1343 K. The sample temperature was measured using a thermocouple in the case of coating temperatures of 1143 K and 1243 K and using a radiation thermometer in the case of coating temperature of 1343 K. After coating, the sample was cooled to room temperature by furnace cooling. Fig. 2 shows the temperature program. After coating, to identify the phases, we analyzed the sample surface by θ–2θ X-ray diffraction (XRD; RINT-2550V, RIGAKU, Tokyo, Japan) at room temperature in the range 20° ≤ 2θ ≤ 90°. The X-ray

Fig. 2. The temperature program during SPS carburizing.

Fig. 3. XRD patterns of samples subjected to SPS carburizing treatment. 325

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SEI

C

Ti

20 µm

Fig. 4. SEM-EDX analysis of sample surface after carburizing treatment for 3.6 ks at 1343 K by SPS.

Fig. 7. GD-OES carbon profile of samples after carburizing treatment by SPS.

Fig. 5. Results of corrosion tests of carburized samples using 2% HF–10% HNO3 solution.

resistance superior to that of Ti. Fig. 6 shows SEM micrographs of the cross-sectional microstructures of the samples. These micrographs reveal that a TiC layer was formed on the substrate under all the investigated treatment conditions and that the layer thickness increased with increasing carburizing treatment temperature. The maximum thickness of the layer was approximately 5 μm at 1343 K. The TiC layer exhibited a porous structure. Fig. 7 shows the GD-OES carbon profile for the carburized samples. The carbon concentration is shown as a function of depth from the sample surface. The carburizing depth from the surface increased with increasing carburizing temperature. This is consistent with the SEM micrographs, as shown in Fig. 6. Figs. 8 and 9 show the tribological test results. In Fig. 8, the carburized samples exhibit low friction coefficients compared to the friction coefficient of the substrate. The friction coefficient decreased with increasing carburizing temperature, reaching a minimum of approximately 0.25. The value of μ = 0.25 is a little higher than the values of μ = 0.12–0.18 found in the literatures for TiC coatings deposited by plasma assisted CVD (PACVD) [30,31]. This difference may be related to the starting materials of gas for PACVD or raw elemental powders for SPS, resulting in the different roughness on the surface after carburizing. Fig. 9 shows the results of the ball-on-disk wear tests, including wear tracks and cross-sectional profiles of the samples, as well as ball surface wear. The untreated sample has poor wear resistance, as

Fig. 8. Friction coefficient of carburized samples by SPS and CP-Ti substrate.

indicated by the large wear loss area. This result indicates that the formation of the TiC layer on CP-Ti. Fig. 10 shows the results of surface hardness tests after carburizing treatments. As shown in Fig. 10, the surface hardness increased with carburizing temperature and a hardness of 1200 HV was obtained at 1343 K. This result indicates that substrate hardness increased considerably as a result of the formation of the TiC layer. However, the hardness of previously reported TiC layers exceeds approximately 2000 HV [30,32–34]. The lower hardness obtained for the formation TiC layers in this study may be a consequence of the formation of TiC layers with low density and numerous pores. The thickness of the TiC layer deposited by other processes such as PACVD [30], dual ion beam sputtering (DIBS) [32], pulsed laser deposition (PLD) [33], and reactive ion beam-assisted electron beam-PVD (RIBA EB-PVD) [34] was 1 μm or less. On the other hand, the thickness of the TiC layer carburized SPS was 5 μm in this study. These results indicate that SPS can be used to apply TiC coatings to titanium materials used in parts that require good wear resistance. 4. Conclusions In this study, the rapid formation of hard TiC layer on CP-Ti was carried out via the SPS technique. The results are summarized as follows: (1) XRD analyses indicated that a TiC layer was formed on the CP-Ti substrates at a carburizing time of 3.6 ks under all investigated carburization conditions. (2) Microstructure observations and GD-OES results revealed that the

Fig. 6. SEM micrographs of cross-sectional samples after carburizing treatments by SPS. 326

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Fig. 10. Surface hardness of samples after carburizing treatment by SPS.

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