Simultaneous Boronizing and Carburizing of Titanium via Spark Plasma Sintering
2019 年 60 巻 11 号 p. 2387-2391
詳細
2019 年 60 巻 11 号 p. 2387-2391
This study focused on a diffusion coating method for the formation of a hard layer with excellent adhesion through the formation of an interlayer and a gradient layer on titanium materials. A limitation of conventional diffusion coating methods is the deterioration of the mechanical properties of the matrix resulting from the long-term, high-temperature processing. Therefore, spark plasma sintering (SPS) was used to form a ceramic layer, as it allows the suppression of the growth of crystal grains via rapid heating and enables low temperature and short processing time. The purpose of this study is to simultaneously form borides and carbides on a titanium surface using the SPS method and evaluate their properties. Commercially pure titanium (CP-Ti) was used as the substrate, and B4C powder was utilized as both the boronizing and carburizing source. An analysis of the sample surface subjected to SPS processing indicated the formation of TiC, TiB2, and TiB. As a ceramic layer was formed on the titanium surface, a surface hardness of ∼1700 HV was obtained, and the wear resistance was improved compared with that of untreated CP-Ti.
Fig. 1 Cross-sectional SEM image and EPMA elemental mapping images of titanium, boron, and carbon for SPS-treated samples for 180 min at 1273 K.
Titanium (Ti) materials have been widely used in the aerospace, automotive, and biomaterial engineering industries owing to their high specific strength, superior fatigue, and excellent biocompatibility.1–4) However, further applications of such materials are limited owing to their low hardness, poor wear resistance, and reduced corrosion resistance to non-oxidizing acids. Therefore, the development of a surface modification method is necessary to expand the use of Ti materials.5,6)
Methods to coat the Ti alloy surface with a hard ceramic coating layer made of titanium compounds, such as TiN, TiC, and TiB2, have been proposed to improve their mechanical properties.7–9) Moreover, titanium composites composed of borides and carbides have attracted significant interest in recent years owing to their comparable properties, such as high hardness, good wear resistance, high electrical and thermal conductivities, and high fracture toughness, compared with single-phase ceramics.9–13) In general, coating methods such as physical vapor deposition (PVD) or chemical vapor deposition (CVD) are employed to produce such hard layers. A coating layer formed through these methods exhibits poor adhesion.14,15) In contrast, the diffusion coating method is expected to form a hard layer with matrix adhesion superior to that of hard layers deposited via PVD or CVD through the formation of an interlayer and a gradient layer. However, a limitation of conventional diffusion coating methods is the deterioration of the mechanical properties of the matrix resulting from long-term, high-temperature processing.6,16–19)
Therefore, spark plasma sintering (SPS) was used to form a ceramic layer, as it allows the suppression of the growth of crystal grain via rapid heating and enables low temperature and short processing times.20) This method involves generating high-temperature plasma at the powder contact portion by energizing a large current under pressure and promoting sintering by utilizing the high energy. Recently, the utility of SPS has been demonstrated in ceramic/metal nanomaterials, composite materials system functionally graded materials, and hard materials.21–23) This method was also used for coating metal substrates.24–27) The coatings fabricated using an SPS demonstrated strong metallurgical bonding between the coating and substrate, in addition, the applied pulse current may facilitate the diffusion of boron (B) and carbon (C) atoms. The poor tribological performance of Ti alloys restrict their wide applications in automotive and machinery fields. Simultaneous boronizing and carburizing can improve tribological properties, wear resistance, and corrosion resistance of Ti alloys. In this study, borides and carbides were simultaneously formed on a Ti surface using an SPS method. Effect of simultaneous boronizing and carburizing on surface properties such as microstructure, hardness and wear resistance has been investigated.
Commercially pure titanium (CP-Ti, purity 99.5%) was used as the substrate, and B4C powder (average particle diameter 0.5 µm, purity 99%) was utilized as both the boronizing and carburizing source. Each CP-Ti sample was 20 mm in diameter and 5 mm in thickness. Prior to boronizing and carburizing, the sample was polished to a surface finish of 600 grit, ultrasonically degreased in acetone, and dried in air.
2.2 Boronizing and carburizing via spark plasma sinteringBoronizing and carburizing experiments were conducted with a SPS unit (SPS-1020, produced by Sumitomo Coal Mining Co., Japan). Each sample was embedded in a closed die (inner diameter of 20 mm) and punch (graphite) containing B4C powder. After they were placed, the chamber was evacuated to a pressure of no more than 10 Pa. Subsequently, a large pulsed current was applied. The treatment was performed for 30–180 min at 1073–1273 K. The applied pressure of SPS was 11 MPa and the heating rate was 100 K min−1. Once the treatment was completed, each sample was cooled to room temperature inside the furnace. The treatment temperature was continuously monitored using a thermocouple directed at the sample.
2.3 Surface characterizationThe phase structure of each treated sample surface was determined by analyzing the entire area of the surface using an X-ray diffractometer (XRD; RINT-2550V, RIGAKU, Japan). The X-ray diffractometer was equipped with a Cu-Kα radiation source operated at 40.0 kV and 300 mA, and the samples were scanned at 40.0°/min. The microstructural and compositional analyses of the treated samples were performed using a scanning electron microscopy (SEM; JSM-6060LV, JEOL, Japan), an electron probe micro-analysis (EPMA; JXA-8800, JEOL, Japan), and a glow discharge-optical emission spectroscopy (GD-OES; GD-profiler2, Horiba, Japan). The GD-OES conditions were a sputtering-mark diameter of 4 mm, a discharge pressure of 600 Pa, and a power of 35 W. Accordingly, cross-sections of each sample were cut using a low-speed saw and thereafter polished. The hardness values of the cross-sections of the treated samples were measured using a Vickers microhardness tester (PMT-X7A, Matsuzawa, Japan) under a load of 0.1 N. Five indentations were made on each sample, and a three-point average value (excluding both the maximum and minimum values) was reported as the hardness. Wear testing was performed at room temperature using a ball-on-disk tribometer (CSM Instruments, Tribometer, Switzerland). The conditions for wear testing were as follows: running distance of up to 500 m, wear load of 2 N, rotation speed of 100 rpm, wear radius of 5 mm, and diameter of 6.35 mm for the Al2O3 ball used for the counter material. After the wear test, the wear volume was calculated using the depth and width of the wear track.
In the treated sample, the modified layer was observed under all the treatment conditions. The external appearance of the treated samples was visually examined. Appearance of sample surface treated by SPS has uniformly and the surface gloss was lost. The microstructures of the cross-sections and the EPMA elemental mapping images of Ti, B, and C for the SPS-treated sample for 180 min at 1273 K are shown in Fig. 1. Cracks and peeling were not observed at the interface between the modified layer and the substrate, indicating good adhesiveness. The diffusion of B and C into the substrate was observed. The concentrated region of B corresponds to the modified layer. The concentrated region of C corresponds to the outermost surface, is dispersed in the upper half of the modified layer. Figure 2 shows the GD-OES profiles of the SPS-treated sample for 30–180 min at 1073–1273 K, obtained for Ti, B, and C. The obtained profiles indicate a surface region composed of a B and C-rich layer. Moreover, B and C were diffused deeper with the increase in the treatment temperature and time. The concentration of B is shown as a function of depth from the sample surface irrespective of the treatment conditions. This B concentration region is a compound layer, and beneath this layer, the sloped region is a diffusion layer. Such an elemental distribution is consistent with the EPMA results, as shown in Fig. 1, which is reported to occur by reaction behavior of Ti–B4C system.28,29) Figure 3 shows the XRD patterns of the SPS-treated sample for 30–180 min at 1273 K. On the sample surface, relatively weak peaks of TiB2 were observed and the intensity of TiC and TiB increased with the increase in treatment temperature and time.30,31) No impurities such as oxides and ternary phase of Ti–B–C were observed.32) Thus, these results demonstrate that borides and carbides were simultaneously formed on the Ti surface using the SPS method.
Cross-sectional SEM image and EPMA elemental mapping images of titanium, boron, and carbon for SPS-treated samples for 180 min at 1273 K.
GD-OES profiles of titanium, boron, and carbon concentration for the SPS-treated sample for 30–180 min at 1073–1273 K.
XRD patterns of the SPS-treated samples for 30–180 min at 1273 K.
The modified layer as a compound layer on the top surface formed via SPS exists as a composite layer of TiC, TiB2, and TiB with no oxide. The reason for the absence of oxide is that the SPS method was performed in a vacuum atmosphere and high-temperature plasma generated via pressurization and energization destroyed the oxide film on the Ti surface.25,26) In addition, it has been reported that oxygen atoms block the diffusion of atoms in compounds,33) and the disposal of oxygen is important for good growth of the layer. In this study, the SPS method effectively grew the modified layer.
Figure 4 shows the relationship between the square of thickness of the compound layer and treatment time. The square of the layer thickness linearly increases with the treatment time. Assuming that the growth of the compound layer is rate-controlled by diffusion and it occurs perpendicular to the substrate, on the basis of the classical kinetic theory, the square of thickness of the compound layer as a function of treatment time is expressed by eq. (1).34)
| \begin{equation} x^{2} = kt \end{equation} | (1) |
| \begin{equation} k = k_{0}\exp\left(-\frac{Q}{RT}\right) \end{equation} | (2) |
Relationship between the square of thickness of the compound layer including borides and carbides, and the treatment time.
Arrhenius plots for the rate constant of the growth of the compound layer.
Figure 6 shows the cross-sectional hardness distributions of the SPS-treated samples. All the treated samples are observed to be harder than the untreated sample; the hardness increased considerably at the surface to approximately 1700 HV and decreased toward the core of the substrate. This increase in surface hardness can be explained by the formation of the TiC (3200 HV), TiB2 (3400 HV), and TiB (900 HV) phase and solid solution strengthening in the diffusion layer owing to C and B diffusion via carburizing and boronizing.41–43) Figure 7 shows the results of the ball-on-disk wear tests, including wear tracks and cross-sectional profiles of the samples, in addition to ball surface wear. Figure 8 shows the effect of the treatment temperature on wear volumes of the sample and ball. The untreated sample has poor wear resistance, as indicated by the large wear loss area. The wear resistance of each treated sample is higher than that of the untreated sample. The depth of wear decreased with the increase in the treatment temperature. The wear track of the untreated Ti sample and treated samples at 1073 K and 1173 K after wear tests exhibited a rough surface appearance due to the heavy plowing action of the counter material of Al2O3 ball, indicating adhesive wear.44) Indications of severe plastic deformation along with grooves parallel to the sliding direction were detected on the worn surface of these samples. The boride layer treated at 1273 K was slightly worn without any evidence of plastic deformation, however shallow scratches were detected. No cracking, delamination or spalling were observed in the wear track. In addition, wear volumes of both the sample and ball decreased considerably with increasing treatment temperature, as shown in Fig. 8. Considerable reduction in wear volume through boronizing and carburizing can be associated with the superior hardness of the boride layer, as shown in Figs. 6 and 8. During wear testing, the boride layer exhibited limited plastic deformation and the contact between the boride layer and the Al2O3 ball tended to be elastic. Therefore, the boronized and carburized sample at 1273 K exhibited superior tribological performance than the untreated sample and treated samples at 1073 K and 1173 K.
Cross-sectional hardness profile of SPS-treated samples for 60 min.
Wear track and its profile of SPS-treated samples for 60 min and wear track of counter material Al2O3 ball.
Effect of treatment temperature on wear volumes of the SPS-treated sample (a) and the ball (b).
In this study, borides and carbides were simultaneously formed on a pure Ti surface via SPS. The microstructure and properties of the treated samples were investigated using various experimental methods. The following findings were obtained:
