Microstructural Characterization and Wear Behavior of Sintered Compacts Fabricated from Plasma-Nitrided Commercially Pure Titanium Powder

Abstract

The microstructure of commercially pure (CP) titanium having a bimodal nitrogen diffusion phase for biomedical applications, which was fabricated by sintering plasma-nitrided powders, was characterized, and its effect on the wear behaviors was examined. The maximum nitrogen concentration and hardness of CP titanium having a bimodal nitrogen diffusion phase depended on the powder plasma nitriding and sintering temperatures. The grain size of CP titanium made by sintering plasma-nitrided powders is smaller than that of the un-nitrided one. As results of ball-on-disk dry friction tests, CP titanium fabricated from powder plasma-nitrided at 873 K had lower wear resistance than compacts manufactured by sintering as-received CP titanium powder. In contrast, CP titanium fabricated from powder plasma-nitrided at 973 K having a continuous connected network nitrogen diffusion phase had high wear resistance due to the high hardness and differences in the wear mechanism. The wear resistance of CP titanium is dependent on the powder plasma nitriding and sintering temperatures.

Fig. 2 Nitrogen maps for sintered compacts analyzed by EPMA.

1. Introduction

Titanium-based materials, particularly commercially pure (CP) titanium, are used for bio-implants and in chemical plants because of their high corrosion resistance and superior biocompatibility owing to a passive film on the titanium surface.^1,2) Nevertheless, CP titanium has inferior tribological characteristics, such as high friction coefficient and poor abrasive wear resistance, and lower fatigue strength than ferrous materials and ceramics.^3,4) Modifying the surface microstructures of titanium-based materials is an effective approach for improving their characteristics^4–15) because the failure of bio-implants used in the human body induced by cyclic loading, friction, wear and corrosion occurs at the surface.

The friction and wear characteristics in titanium-based materials can be improved through the formation of a nitrided layer at the surface.^10–15) However, the titanium oxide layer on the surface with high corrosion resistance is lost during nitriding. Using the potentiodynamic polarization technique, Yilbas et al.¹⁶⁾ clarified that plasma nitriding reduces the corrosion resistance of Ti–6Al–4V alloy. Several concepts for designing a bimodal microstructure via powder metallurgy have been proposed to limit the deleterious effects of conventional nitriding on material properties.^17–24) Sueyoshi et al.^17–19) reported that TiN-contained composites can be manufactured via planetary ball milling of Si₃N₄ and CP titanium powders, followed by spark plasma sintering (SPS). The authors of the current study^4,20) developed a concept of manufacturing CP titanium using SPS that produced a passive film with high corrosion resistance and a high hardness phase.

Yamabe et al.²⁴⁾ reported that the strengths of titanium-based sintered compacts manufactured by adding light elements, including nitrogen, are improved compared to conventional titanium. To synthesize CP titanium having a nitrogen diffusion structure with sufficient performance for biomaterials, the present authors have devised a process for examining the mechanical properties and friction coefficients of sintered compacts. The process results in CP titanium²⁰⁾ with higher strengths and a lower friction coefficient than its homogeneous counterparts. Another important aspect of the microstructural design is that wear debris induced by contact influences cytotoxicity and leads to loosening of bio-implants in the human body. Thus, it is important to investigate the effects of a bimodal nitrogen diffusion phase on the wear resistance based on the existing microstructural characterization. The microstructure of CP titanium, which sinters powders having surface-modified layers,^20,25–27) can be controlled by changing the process parameters in our proposed method.

The purpose of this study is to characterize the microstructure of materials with a bimodal nitrogen diffusion phase fabricated by consolidating plasma-nitrided CP titanium powder, and to examine the wear mechanisms and behaviors based on microstructural characterization.

2. Experimental Procedures

2.1 Material and specimen

The raw material in current study was CP titanium powder (25.6 µm particle diameter) with 0.122% O, 0.008% N, 0.005% H, 0.037% Fe, and 0.003% C (on a mass basis), with the remainder consisting of Ti. Plasma nitriding was conducted at temperatures of 873 K and 973 K, lower than that for conventional nitriding, in an atmosphere of H₂ and N₂ (13:5 ratio) for 18 ks to produce a nitrided layer at the powder particle surfaces; low temperature nitriding effectively suppresses grain-coarsening of titanium-based materials.^{12,13,15,25,28–36)} The plasma-nitrided powders were subsequently consolidated by SPS at 1073 K, 1273 K, or 1473 K for 1.8 ks under a vacuum of less than 15 Pa and a pressure of 50 MPa. CP titanium having a bimodal nitrogen diffusion structure (4 mm thick, 15 mm diameter) was produced using a 15 mm internal diameter graphite die. The as-received CP titanium powder was also consolidated at 1073 K for comparison (herein referred to as the un-nitrided series). The surfaces of CP titanium compacts were polished with emery papers and an SiO₂ suspension to obtain a mirror finish to examine the effects of plasma nitriding and SPS temperatures on the microstructure of CP titanium compacts.

2.2 Microstructural characterization

The surface hardness of CP titanium compacts (4 mm thick, 15 mm diameter) having a bimodal nitrogen diffusion phase and no titanium nitrides at the surface based on X-ray diffraction (XRD) results,²⁰⁾ was measured using a micro-Vickers hardness tester at a load of 10 mN. The microstructures of the sintered compacts were characterized using an electron probe micro analyzer (EPMA) at an accelerating voltage of 15 kV and electron backscatter diffraction (EBSD) at an accelerating voltage of 20 kV. The grain size was considered to be representative of the average value estimated by the area of the grains using the EBSD software; grain boundaries were defined as boundaries with misorientations greater than 15°. The microstructure of the un-nitrided specimen made by the as-received CP titanium powder was also analyzed using EBSD to examine the effect of plasma nitriding temperature on the microstructure of sintered compacts.

2.3 Testing

Ball-on-disk dry friction tests (TRIBOMETER, NANOTEC Co., Ltd.) were conducted using 6 mm SUJ2 steel ball bearings in air at room temperature to investigate the wear characteristics of sintered compacts (4 mm thick, 15 mm diameter) having a bimodal nitrogen diffusion phase. Movement rate was 100 mm/s, nominal applied force was 5 N, and the sliding distance was 300 m. The worn surfaces were observed using optical microscopy, and analyzed using EPMA and laser microscopy after testing. The counter material was also studied after testing, by optical microscopy to elucidate the wear mechanism of CP titanium compacts having a bimodal nitrogen diffusion phase; therefore, a SUJ2 steel ball bearing was used for each specimen.

3. Results and Discussion

3.1 Nitrogen analysis of CP titanium powder and sintered compacts having a bimodal nitrogen diffusion phase

Figure 1 shows SEM micrographs and nitrogen maps analyzed by EPMA for the as-received powder and plasma-nitrided CP titanium powders. The color in the EPMA maps indicates the composition of the nitrogen; red indicates higher nitrogen concentration. Every powder was composed of smooth spherical particles indicating that plasma nitriding did not change the size and shape of the particles. In addition, nitrogen concentration in CP titanium powders, which originally contained 0.008 mass% of nitrogen, tended to increase with increasing the plasma-nitriding temperature due to an increase in the nitrogen diffusion rate into CP titanium powder at higher temperatures. Furthermore, XRD analyses determined that a nitrogen diffused layer was observed on CP titanium powder at plasma nitriding temperatures above 873 K and that titanium nitride (Ti₂N) was formed on the powder plasma-nitrided at 973 K.²⁰⁾

Fig. 1

SEM micrographs and nitrogen maps for un-nitrided powder and plasma-nitrided powders analyzed by EPMA.

Nitrogen analysis for the sintered compacts was also conducted. Figure 2 shows nitrogen maps analyzed by EPMA for CP titanium compacts made by consolidating plasma-nitrided powders. The map for the un-nitrided series manufactured by consolidating the as-received powder at 1073 K, is also shown in Fig. 2(a). The color in the CP titanium made by sintering powder plasma-nitrided at 973 K differed from the color of samples made by sintering as-received CP titanium powder and powder plasma-nitrided at 873 K. Nitrogen concentration in CP titanium compacts tended to increase with increasing the nitriding temperature, as was observed for the powders, as shown in Fig. 1. Figure 3 shows the nitrogen profiles analyzed by EPMA for sintered compacts manufactured by sintering the as-received powder and the plasma-nitrided CP titanium powders to investigate the effect of nitriding temperature on the nitrogen profiles. The intensity at the vertical axis in this study corresponds to the nitrogen concentration. Nitrogen concentration in CP titanium compacts varied as a function of position. In addition, the maximum nitrogen concentration of CP titanium sintered compacts having a bimodal nitrogen diffusion structure increased with increasing the plasma-nitriding temperature. A previous study²⁰⁾ demonstrated that Ti₂N formed in the CP titanium powder treated with plasma-nitriding was decomposed during the sintering process and nitrogen formed by the decomposition of Ti₂N on the powder diffused into the CP titanium compact during the sintering process. Thus, the unique nitrogen profile obtained in Fig. 2 corresponded to the nitrogen diffusion phase.

Fig. 2

Nitrogen maps for sintered compacts analyzed by EPMA.

Fig. 3

Nitrogen profiles for sintered compacts fabricated from un-nitrided powder and plasma-nitrided powders analyzed by EPMA (sintering temperature: 1073 K).

Figure 2 also shows that nitrogen diffusion behaviors in the CP titanium compacts depend on the temperature of sintering. In the images of the compacts consolidated at 1073 K for the plasma-nitrided powders, the high-nitrogen concentration region forms a continuous connected network texture that surrounds the substrate with low nitrogen concentration. This was because all of the nitrogen-diffused regions, which were formed at the surface of CP titanium powders containing nitrogen, became bonded during the SPS. An acicular nitrogen diffusion phase was formed in the low-nitrogen concentration region in the CP titanium fabricated by consolidating plasma-nitrided powders at 1473 K. Figure 4 shows the nitrogen profiles analyzed by EPMA for CP titanium made by sintering powders plasma-nitrided at 973 K to investigate the effect of sintering temperature on the nitrogen profiles. The maximum nitrogen concentration of sintered compacts having a bimodal nitrogen diffusion phase decreased with increasing the SPS temperature. Nitrogen contained in the powder surface was diffused into the core of the powder during the SPS; the trend observed in Fig. 4 became more pronounced due to an increase in the nitrogen diffusion rate.

Fig. 4

Nitrogen profiles for sintered compacts fabricated from powders plasma-nitrided at 973 K analyzed by EPMA.

Thus, nitrogen analyses by EPMA suggest that a profile of a nitrogen diffusion phase depends on both the plasma nitriding and sintering temperatures.

3.2 Microstructural characterization of CP titanium sintered compacts having a bimodal nitrogen diffusion phase

Figure 5 shows inverse pole figure (IPF) maps of the α-phase obtained by EBSD analysis for CP titanium compacts manufactured by consolidating plasma-nitrided powders. The map for the un-nitrided series, from consolidating the as-received powder at 1073 K, is also shown in Fig. 5(a). Equiaxed α-grains were observed in the un-nitrided series because the SPS temperature was low in comparison to the β-transus value (1155 K) for CP titanium. In contrast, the CP titanium compacts made by sintering plasma-nitrided powders appeared to contain different microstructures: a unique black structure and fine equiaxed grains. The formation of the unique black structure is influenced by the plasma nitriding process, because this structure was not detected in the un-nitrided series, as shown in Fig. 5(a). In addition, the β-phase was slightly observed²⁰⁾ inside the unique black structure of the compact sintered at 1273 K; therefore, the transformation mechanism would be changed during the SPS process. For compacts fabricated by sintering, the powder plasma-nitrided at 973 K had equiaxed grains, even though the sintering temperatures (1273 K and 1473 K) were higher than the β-transus value (1155 K). This result implies that the nitrogen diffusion phases suppress transformation to the β-phase during the SPS process because nitrogen stabilizes the α-phase. In contrast, acicular microstructures were formed in the CP titanium compacts manufactured by sintering powder plasma-nitrided at 873 K by sintering at 1473 K due to a lower nitrogen concentration than in compacts manufactured by sintering powder plasma-nitrided at 973 K.

Fig. 5

IPF maps obtained by EBSD of alpha-phase for sintered compacts.

Figure 6 plots the grain size of the CP titanium compacts made by sintering plasma-nitrided powders as a function of the SPS temperature. The grain size of the CP titanium made by sintering the plasma-nitrided powders tended to increase with increasing the SPS temperature. Therefore, grain coarsening occurs during the SPS process. However, the grain size of the CP titanium compacts manufactured by sintering the plasma-nitrided powders was smaller than that of the sintered compacts made by the as-received powders, which suggests that designing a bimodal nitrogen diffusion structure via powder metallurgy limits the deleterious grain-coarsening effects. This is attributed to the solute drag effect of the additive nitrogen.²⁴⁾

Fig. 6

Average grain size of sintered compacts as a function of the sintering temperature.

Figure 5 also shows that the areal fraction of the α-phase increases with increasing the SPS temperature. The regions of fine α-grains in the compacts manufactured by sintering plasma-nitrided powders at 1073 K have formed continuous connected network textures, as observed in the nitrogen maps in Fig. 2. Thus, the α-phase of compacts made by sintering the plasma-nitrided CP titanium powders corresponds to the nitrogen diffusion structure because nitrogen stabilizes the α-phase of titanium. In addition, the areal fraction of the α-phase tended to increase with increasing the nitriding temperature. Thus, it was confirmed that the areal fraction of the nitrogen diffusion structure can be controlled by optimization of varying the plasma nitriding and SPS conditions.

The Vickers hardnesses of the CP titanium compacts were measured in order to investigate the effect of the nitrogen diffusion on the hardness distributions. To examine the effect of plasma nitriding temperature on hardness, Fig. 7 shows hardness profiles for un-nitrided compact and compacts made by sintering the powders plasma-nitrided at 873 K and 973 K. Hardness values in CP titanium compacts manufactured by consolidating plasma-nitrided powders varied as a function of position, and exhibited higher hardness values than the un-nitrided sintered compact. The maximum hardness of CP titanium compacts having a bimodal nitrogen diffusion structure increased with increasing the plasma nitriding temperature. The unique hardness variations correspond to the bimodal nitrogen diffusion phase, as observed for the nitrogen profiles in Fig. 3. Furthermore, the unique hardness variations are also influenced by the fine-grained structure, as observed for the IPF maps in Figs. 5(b) and (c). To examine the effect of the SPS temperature on hardness, Fig. 8 shows the hardness profiles for compacts fabricated by sintering powders plasma-nitrided at 973 K. The maximum hardness of CP titanium compacts having a bimodal nitrogen diffusion phase depended on the SPS temperature. The average hardness of the compact sintered at 1073 K was 801 HV, whereas CP titanium compacts sintered at 1273 K and 1473 K exhibited the almost the same average hardness values; 691 HV and 705 HV, respectively. The unique hardness variations correspond to the bimodal nitrogen diffusion phase, as observed for the nitrogen profiles in Fig. 4.

Fig. 7

Hardness distributions of sintered compacts fabricated from un-nitrided powder and plasma-nitrided powders (sintering temperature: 1073 K).

Fig. 8

Hardness distributions of sintered compacts fabricated from powders plasma-nitrided at 973 K.

Hardness variations of sintered compacts having a bimodal nitrogen diffusion phase are attributed to microstructural changes during the plasma nitriding and SPS processes.

3.3 Evaluation of wear behaviors of CP titanium sintered compacts having a bimodal nitrogen diffusion phase

Ball-on-disk dry friction tests were conducted to examine the wear behaviors of CP titanium having a bimodal nitrogen diffusion structure, and to elucidate the wear mechanisms. Figure 9 shows optical micrographs of a wear track formed on the compact surface after testing. The wear track for CP titanium compacts made by sintering powder plasma-nitrided at 973 K (Fig. 9(c)) is narrower than that for compacts fabricated by sintering the as-received powder (Fig. 9(a)) and powder plasma-nitrided at 873 K (Fig. 9(b)). In contrast, no effect of the sintering temperature on the macroscopic observations of wear tracks was observed. The worn surfaces were analyzed using laser microscopy to examine the contact areas in more detail. Figure 10 presents typical profiles of the wear tracks on the surfaces of the un-nitrided compact and the compacts manufactured by consolidating powders plasma-nitrided at 873 K and 973 K. The wear tracks on the CP titanium compacts made by sintering powder plasma-nitrided at 873 K (Fig. 10(b)) were wider and deeper than the wear tracks on the un-nitrided series (Fig. 10(a)). In contrast, the wear tracks on the CP titanium compacts made by sintering powders plasma-nitrided at 973 K (Fig. 10(c)) were narrower than the wear tracks on the un-nitrided series. Furthermore, the wear tracks for the CP titanium compacts made by sintering powder plasma-nitrided at 973 K were much shallower than the wear tracks in the un-nitrided sample and compacts manufactured by sintering powder plasma-nitrided at 873 K.

Fig. 9

Optical micrographs of the wear track formed on the sintered compacts.

Fig. 10

Profiles of wear tracks on the surfaces of sintered compacts fabricated from un-nitrided powder and plasma-nitrided powders (sintering temperature: 1073 K).

Figure 11 plots the specific wear rates of the CP titanium as a function of the SPS temperature. In the current study, the amount of wear loss was estimated based on cross-sectional profiles for each specimen, and specific wear rate was estimated by dividing the cross-sectional area of the wear track by nominal applied force (5 N). All of the compacts made by sintering powder plasma-nitrided at 973 K had lower specific wear rates than the un-nitrided compact, due to formation of a high hardness nitrogen diffusion phase. However, the specific wear rates of the CP titanium compacts manufactured by consolidating powder plasma-nitrided at 873 K were higher than those of the un-nitrided compact, even though the compact hardness was increased by nitriding, as shown in Fig. 7. The specific wear rate decreased with increasing the plasma nitriding temperature. The wear resistance tended to increase in conjunction with the hardness of the CP titanium made by sintering plasma-nitrided powder. Furthermore, the specific wear rate in both series increased with increasing the SPS temperature up to 1273 K, after which it decreased. A previous study⁴⁾ demonstrated the same trends; compacts produced by sintering CP titanium and zirconia powders by SPS, exhibited superior wear resistance during reciprocating wear tests with a low fraction of CP titanium or zirconia, but exhibited significant wear loss in the given volume fractions. The result was an increased specific wear rate due to the microstructural differences between the nitrogen diffusion phase and the substrate in the given volume fractions. Thus, the specific wear rate of the CP titanium made by plasma-nitrided powders was dependent on the sintering and plasma-nitriding temperatures.

Fig. 11

Specific wear rate of sintered compacts as a function of the sintering temperature.

To elucidate the wear mechanism of CP titanium compacts having a bimodal nitrogen diffusion phase, the counter material was observed with an optical microscope after testing. Figure 12 shows optical micrographs of the counter material used with the CP titanium compacts manufactured by sintering powders plasma-nitrided at 973 K. Local adhesion of the substrate was observed on the surfaces of the counter material for compacts sintered at 1273 K and 1473 K (Figs. 12(b) and 12(c)), indicating a relatively high specific wear rate in comparison with the compacts sintered at 1073 K, as shown in Fig. 11. The contact area in friction tests typically tended to decrease with increasing the hardness. However, the compacts sintered at 1073 K had the un-nitrided region with low nitrogen concentration and hardness due to the formation of a continuous connected network structure of a nitrogen diffusion phase. Thus, compared with the compacts sintered at 1273 K and 1473 K, the local temperature near the contact area did not relatively rise in the compacts sintered at 1073 K.

Fig. 12

Optical micrographs of the counter material tested for compacts sintered at (a) 1073 K, (b) 1273 K, and (c) 1473 K, for powders plasma-nitrided at 973 K.

Figure 11 also shows that the compacts fabricated by sintering at 1073 K for powder plasma-nitrided at 973 K exhibited the lowest specific wear rate. These results suggest that the formation of a continuous network texture of a nitrogen diffusion phase with high hardness, as shown in Fig. 2(c), effectively increases the wear resistance of CP titanium. The worn surfaces of the CP titanium compacts were analyzed by EPMA to examine the wear mechanism in more detail. Figure 13 shows iron and nitrogen elemental maps obtained by EPMA and SEM micrographs, with the lowest specific wear rate corresponding to the analysis area of the compacts consolidated at 1073 K for the powder plasma-nitrided at 973 K. Iron from the counter material was clearly evident on the CP titanium compacts made by sintering powder plasma-nitrided at 973 K (Fig. 13(b)). The nitrogen concentration fluctuated, which suggests that a bimodal nitrogen diffusion structure suppresses the local adhesion of the CP titanium to the surfaces of the counter material. On the other hand, local adhesion of the substrate was not observed on the surfaces of the CP titanium compacts manufactured by consolidating powder plasma-nitrided at 873 K.²⁰⁾ The local temperature near the contact area rose, resulting in adhesion friction due to the hardness gap during processing of the compacts made by sintering powder plasma-nitrided at 973 K. The results indicate that the wear behaviors in CP titanium having a bimodal nitrogen diffusion structure depends on the plasma nitriding temperature. In particular, abrasive wear occurred in the CP titanium compacts manufactured by consolidating powder plasma-nitrided at 873 K, whereas adhesion wear was dominant for the compacts made by sintering powder plasma-nitrided at 973 K. Thus, these differences are related to the plasma nitriding temperature, which produces differences in the wear resistance of CP titanium.

Fig. 13

(a) SEM micrograph, (b) elemental intensity determined by EPMA analysis for the worn surface of a compact sintered at 1073 K for powder plasma-nitrided at 973 K.

It appears that CP titanium with superior wear properties can be fabricated by optimizing the plasma nitriding and SPS conditions. Thus, the formation of a continuous connected network nitrogen diffusion phase is an effective approach for increasing the wear resistance of CP titanium.

4. Conclusion

The microstructure of CP titanium having a bimodal nitrogen diffusion phase manufactured by consolidating plasma-nitrided CP titanium powders was characterized, and its wear characteristics were examined. The main conclusions of the present work are as follows:

1. (1)    The maximum nitrogen concentration and hardness of the nitrogen diffusion phase increase with increasing nitriding temperature and decrease with increasing sintering temperature.
2. (2)    The grain size of CP titanium made by sintering plasma-nitrided powders increases with increasing the sintering and plasma nitriding temperatures, and is smaller than the grain size of the un-nitrided compact.
3. (3)    The wear mechanism of CP titanium depends on the powder plasma-nitriding temperature.
4. (4)    The wear resistance of CP titanium manufactured by sintering plasma-nitrided powders depends on the sintering and plasma nitriding temperatures. CP titanium having a continuous connected network nitrogen diffusion phase has higher wear resistance than compacts manufactured by sintering un-nitrided CP titanium powder and powders plasma-nitrided at lower temperature, due to differences in the wear mechanism.

Acknowledgements

The authors would like to thank the Light Metal Educational Foundation, Inc. and Nippon Sheet Glass Foundation for Materials Science and Engineering for financial support.

REFERENCES

• 1)   Y.  Kurashina,  A.  Ezura,  R.  Murakami,  M.  Mizutani and  J.  Komotori: J. Mater. Sci. Mater. Med. 30 (2019) 57.
• 2)   M.  Long and  H.J.  Rack: Biomaterials 19 (1998) 1621–1639.
• 3)   K.G.  Budinski: Wear 151 (1991) 203–217.
• 4)   S.  Kikuchi,  K.  Katahira and  J.  Komotori: J. Jpn. Inst. Met. Mater. 82 (2018) 341–348.
• 5)   H.  Dong and  T.  Bell: Wear 238 (2000) 131–137.
• 6)   C.  Lee,  A.  Sanders,  N.  Tikekar and  K.S.R.  Chandran: Wear 265 (2008) 375–386.
• 7)   S.  Kikuchi and  J.  Komotori: J. Jpn. Inst. Met. Mater. 80 (2016) 114–120.
• 8)   J.  Umeda,  T.  Mimoto,  K.  Kondoh and  B.  Fugetsu: Trans. JWRI 41 (2012) 49–52.
• 9)   S.  Kikuchi,  S.  Yoshida,  Y.  Nakamura,  K.  Nambu and  T.  Akahori: Surf. Coat. Tech. 288 (2016) 196–202.
• 10)   D.  Nolan,  S.W.  Huang,  V.  Leskovsek and  S.  Braun: Surf. Coat. Tech. 200 (2006) 5698–5705.
• 11)   K.  Katahira,  Y.  Tanida,  S.  Takesue and  J.  Komotori: CIRP Ann. 67 (2018) 563–566.
• 12)   S.  Takesue,  S.  Kikuchi,  H.  Akebono,  Y.  Misaka and  J.  Komotori: Surf. Coat. Tech. 359 (2019) 476–484.
• 13)   S.  Kikuchi,  A.  Ueno and  H.  Akebono: Int. J. Fatigue 139 (2020) 105772.
• 14)   K.  Farokhzadeh,  A.  Edrisy,  G.  Pigott and  P.  Lidster: Wear 302 (2013) 845–853.
• 15)   T.  Morita,  H.  Takahashi,  M.  Shimizu and  K.  Kawasaki: Fatigue Fract. Eng. Mater. Struct. 20 (1997) 85–92.
• 16)   B.S.  Yilbas,  A.Z.  Sahin,  Z.  Ahmad and  B.J.  Abdul Aleem: Corros. Sci. 37 (1995) 1627–1636.
• 17)   N.  Ahmad,  K.  Obara,  S.  Sameshima and  H.  Sueyoshi: Trans. Mater. Res. Soc. Jpn. 34 (2009) 793–797.
• 18)   N.  Ahmad and  H.  Sueyoshi: Ceram. Int. 36 (2010) 491–496.
• 19)   N.  Ahmad and  H.  Sueyoshi: Mater. Res. Bull. 46 (2011) 460–463.
• 20)   S.  Kikuchi,  H.  Akebono,  A.  Ueno and  K.  Ameyama: Powder Technol. 330 (2018) 349–356.
• 21)   Y.S.  Zhang,  Y.H.  Zhao,  W.  Zhang,  J.W.  Lu,  J.J.  Hu,  W.T.  Huo and  P.X.  Zhang: Sci. Rep. 7 (2017) 40039.
• 22)   O.E.  Falodun,  B.A.  Obadele,  S.R.  Oke,  O.O.  Ige and  P.A.  Olubambi: Particul. Sci. Technol. 38 (2020) 156–165.
• 23)   A.  Lisiecki: Metals 5 (2015) 54–69.
• 24)   Y.  Yamabe,  J.  Umeda,  H.  Imai and  K.  Kondoh: Mater. Trans. 59 (2018) 61–65.
• 25)   S.  Kikuchi,  Y.  Nakamura,  A.  Ueno and  K.  Ameyama: Mater. Trans. 56 (2015) 1807–1813.
• 26)   K.  Osaki,  S.  Kikuchi,  Y.  Nakai,  M.O.  Kawabata and  K.  Ameyama: Mater. Sci. Eng. A 773 (2020) 138892.
• 27)   S.  Kikuchi,  T.  Mori,  H.  Kubozono,  Y.  Nakai,  M.O.  Kawabata and  K.  Ameyama: Eng. Fract. Mech. 181 (2017) 77–86.
• 28)   K.  Farokhzadeh and  A.  Edrisy: Mater. Sci. Eng. A 620 (2015) 435–444.
• 29)   S.  Kikuchi,  S.  Heinz,  D.  Eifler,  Y.  Nakamura and  A.  Ueno: Key Eng. Mater. 664 (2015) 118–127.
• 30)   S.  Kikuchi,  Y.  Nakamura,  A.  Ueno and  K.  Ameyama: Adv. Mater. Res. 891–892 (2014) 656–661.
• 31)   T.  Morita,  N.  Uehigashi and  C.  Kagaya: Mater. Trans. 54 (2013) 22–27.
• 32)   M.M.  Ali and  S.G.S.  Raman: Tribol. Lett. 28 (2010) 291–299.
• 33)   A.  Raveh: Mater. Sci. Eng. A 167 (1993) 155–164.
• 34)   J.  Sun,  W.P.  Tong,  L.  Zuo and  Z.B.  Wang: Mater. Des. 47 (2013) 408–415.
• 35)   S.  Kikuchi,  S.  Yoshida and  A.  Ueno: Int. J. Fatigue 120 (2019) 134–140.
• 36)   S.  Takesue,  S.  Kikuchi,  H.  Akebono,  T.  Morita and  J.  Komotori: Results Mater. 5 (2020) 100071.