Effect of Environmental Gas on Surface Initiated Rolling Contact Fatigue

Tribology Online, 8, 1 (2013) 90-96. ISSN 1881-2198 DOI 10.2474/trol.8.90

Article

Effect of Environmental Gas on Surface Initiated Rolling Contact Fatigue Hiroyoshi Tanaka1,2)*, Tatsuhiko Morofuji3), Kakeru Enami3), Masaaki Hashimoto1) and Joichi Sugimura1,2,4) 1)

Department of Mechanical Engineering, Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 2) International Institute for Carbon-Neutral Energy Research, Kyushu University 3) Graduate School of Engineering, Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 4) Hydrogenius, National Institute of Advanced Industrial Science and Technology 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan *Corresponding author: [email protected] ( Manuscript received 31 May 2012; accepted 18 December 2012; published 31 January 2013 ) ( Presented at Symposium S8: Hydrogen Tribology for Future Energy in the International Tribology Conference Hiroshima 2011 )

This paper describes an exploratory study on the effects of temperature on the formation of oxide film and rolling contact fatigue life in hydrogen, argon and air. Rolling contact fatigue tests were conducted at 333 K and 363 K by using a three-ball-on-disk type apparatus. The rolling contact fatigue life in hydrogen was shorter than that in argon, and life in air was the longest. Relationship was found between fatigue life and hydrogen concentration in steel. Cross sections of the specimens show that iron oxide grew to larger grain size in the subsurface in hydrogen environment, which may have resulted in shorter fatigue life. It was also found that fatigue failure occurs on ball surface in hydrogen at 363 K. Keywords: hydrogen, rolling contact fatigue, permeation, oxidation, AISI 52100 steel

1. Introduction Rolling element bearings are widely used because of its high load carrying capacity, high rotational speed capability, high reliability and stability. They work in many applications in normal air, and even in particular environment such as high pressure LNG and CO2 and in vacuum, under proper lubrication design. However, one exception is hydrogen. Life of rolling element bearings is significantly reduced by the hydrogen induced degradation of steel. Endo et al [1] and Imai et al [2] reported that hydrogen permeated into steel in ambient pressure hydrogen, resulting in reduction of life to one tenth of calculated life. They also suggested that the hydrogen embrittlement occurring in the subsurface of rolling elements promoted the crack propagation and reduced flaking life. Hydrogen embrittlement causes reduction of fatigue life not only in steels for structural materials but also in the hardened steel for rolling element bearings [1-7]. Ciruna and Szieleit [4] reported in 1960’s that hydrogen content in steel influenced flaking failure in bearings. Tamada and Tanaka [7] suggested that the hydrogen generated by decomposition of lubricant affected the Copyright © 2013 Japanese Society of Tribologists

formation of white structure which caused reduction in rolling contact fatigue life. More recently, Ohno et al. [8, 9] showed that fatigue cracks similar to those in hydrogen embrittlement were developed in a rolling contact fatigue test with two synthetic base oils and two greases for space applications. They demonstrated that a perfluoropolyethers base oil in atmospheric conditions reduced rolling contact fatigue life through permanent viscosity loss at high Hertzian pressure and hydrogen fluoride generation. When hydrogen embritlement occurs in rolling element bearings, hydrogen should diffuse into the material through its surface. Ciruna suggested that early flaking damage did occur only when the environment contained oxygen and did not in inert atmospheres [4]. Tanimoto et al. [10] demonstrated that the hydrogen permeated through fresh steel surfaces in ordinary polishing process in ambient air. They suggested that the formation of iron oxide on steel surface played an important role in the generation of hydrogen. The purpose of this study is to know the effect of environmental gases on rolling contact fatigue, especially in flaking failure in hydrogen. This study focuses on surface initiated fatigue cracks that are caused in relatively severe lubrication conditions. 90

Effect of Environmental Gas on Surface Initiated Rolling Contact Fatigue

Rolling contact tests on ball-on-disk test rigs were conducted with synthetic base oil in hydrogen, argon and air. Temperature was carefully controlled so as to enable proper examination of the effects of the oil film thickness and oxide film formation on rolling tracks. Chemical analysis of specimen surfaces was made with Auger electron spectroscopy, and hydrogen concentration in the specimens were determined with thermal desorption spectroscopy. 2. Experimental 2.1. Test apparatus A ball-on-disk type apparatus shown in Fig.1 was used. The disk specimen was fixed at the bottom of the chamber, while the ball specimens were guided by a race of a thrust ball bearing, which was mounted on the upper rotating shaft. The balls of 6.35 mm diameter were used, and the diameter of the rolling track was 43 mm. Either three or six balls were used to change contact pressure. The chamber was pushed upward by a loading arm such that normal load was applied between the disk and the balls. With this configuration, the balls and the disk undergo almost pure rolling contact, although this is not literally pure rolling since the traction between the specimens and finite radius of the rolling track inevitably cause micro-slip at the contact. Lubricant of a certain amount was supplied in the chamber. Test gases were blown in the lower part of the chamber and vented out from the upper part of the chamber. Temperature within the chamber was controlled with a heater and a thermostat. Vibration sensor detected the occurrence of first flaking on the surfaces to stop the shaft. 2.2. Specimens The balls and disks made of JIS SUJ2 steel, which is equivalent to AISI 52100 steel, were used. Hardness of the specimens was 780 HV. The ball specimens were commercial bearing steel balls. The disk specimens were finished by lapping using with 3 µm diamond slurry.

Guide disc Gas out Ball specimens

2.3. Test conditions and procedure The test conditions are summarized in Table 1. Six balls were used in the present tests, and a total load of 2650 N was applied to the chamber. The normal load to each of the balls was 441 N, which produced a maximum Hertzian pressure of 4.8 GPa. The balls were guided by a retainer at even intervals. The speed of the rotating shaft was 1500 rpm. Two synthetic base oils PAO5 and PAO10, poly alpha olephine, with different viscosity were used. Their kinetic viscosities were 5.1 and 11.2 cSt at 313 K, respectively. Fresh oil of 220 ml was supplied in the chamber before tests such that both the disk and the ball specimens were immersed in the oil. The oil temperature was controlled by thermostat at 333 K and 363 K. Surfaces of the lower disk and ball were finished to 0.005 µmRq and 0.01 µmRq, respectively. With these configurations, the initial film parameter Λ, film thickness divided by composite roughness, was 2.8 and 1.7 at the oil temperature of 333 K and 363 K. Hydrogen, argon and dry air were used as test gases. Purity of hydrogen and argon were 99.97% and 99.9995% in the storage tanks, respectively. The test chamber was purged with the test gas for one hour before each test. The test gases were supplied to the chamber at a rate of 100 ml/min during the rolling contact tests. The tests were conducted for at least seven times for each test condition. The life data were plotted on Weibull charts, and the bearing life L10 and characteristic life η were obtained from their distribution. The disk specimens were cut into some pieces just after the tests. One of them was a block of about 8 mm wide, 1 mm long along the track and 4 mm thick. This was for the measurement of hydrogen concentration with TDS, thermal desorption spectroscopy. 3. Results 3.1. Fatigue life Figure 2 shows Weibull plots for rolling contact fatigue life. Solid marks indicate those lives caused by flaking on the disk surface and open marks indicate those on the ball surface. Table 2 summarizes the life data. Fatigue index in the table means the number of occurrence of flaking divided by the total number of tests. The data clearly demonstrates that life in hydrogen is shorter than that in other gases for each test condition.

Disc specimen

Oil level

Table 1

Gas in

Test conditions

Test gases Maximum Hertzian pressure, GPa Rotational speed, rpm Temperature, K Load

Fig. 1 Schematic illustration of the test apparatus

Japanese Society of Tribologists (http://www.tribology.jp/)

Lubricant, Viscosity grade@313 K Initial film parameter Λ

Hydrogen, Argon, Air 4.8 1500 333 333 363 PAO 10

PAO 5

PAO 10

2.75

1.69

1.69

Tribology Online, Vol. 8, No. 1 (2013) / 91

Fatigue probability, %

Hiroyoshi Tanaka, Tatsuhiko Morofuji, Kakeru Enami, Masaaki Hashimoto and Joichi Sugimura

90 80 70 60 50 40 30

H2

a

Ar Air

20 10

10

100

1000

10000

100000


Fatigue life, x104cycles

90 80 70 60 50 40 30

H2

b

Ar Air

20 10

10

100

1000

10000

100000

L10 life in hydrogen is about half of that in argon. Also, the fatigue life is generally shorter under severer lubrication condition and higher temperature in all the gas environments. In the case in hydrogen at 333 K, the data can be fitted by two lines with different slopes. This means that two different mechanisms work, and they have different range of fatigue lives in hydrogen. At the higher temperature of 363 K, almost all failures have occurred on the ball surface, suggesting the second mechanism was dominant, and the data could be approximated by one single line. Figure 3 shows the concentration of hydrogen in the disk specimens averaged over seven tests. The concentration for each test was measured with TDS after the occurrence of flaking. The amount of hydrogen in steel slightly increased with increasing temperature of oil and/or reducing the film thickness, which could increase the probability of asperity contact. Figure 4 shows the relationship between L10 lives and the average concentrations of hydrogen shown in Fig. 3. The figure demonstrates a relationship between the rolling contact fatigue life and the concentration of hydrogen in steel. The life decreases with increasing the hydrogen

90 80 70 60 50 40 30

H2

c

Ar Air

20

Average concentration of hydrogen, ppm


Fatigue life, x104 cycles

0.3

0.2

0.1

10

0 H2

Ar

Air

PAO10 333K 10

100

1000

10000

100000

Fatigue life, x10 4cycles

Fig. 2 Weibull plots; (a) PAO10 at 333 K, (b) PAO5 at 333 K, (c) PAO10 at 363 K Table 2

H2

Ar

Air

PAO5 333K

H2

Ar

Air

PAO10 363K

Fig. 3 Average concentration of hydrogen permeated into disk specimens; error bars indicate the standard deviations

Results of Weibull analysis

Oil and Concentra Slope of Characteri Fatigue Environme stic temperatu tion of Weibull * ntal gas re hydrogen index distribution life,*10 4 L10 Ave. ppm β η 1.502 6002.7 1341.1 0.170 7/7 Air 0.724 24310.4 1084.8 0.170 6/7 Argon PAO10, 0.845 6383.4 444.7 0.186 5/7 333 K Hydrogen (5.455) (1271.5) (841.69) ** 3/3 1.279 30876.9 5315.0 0.159 6/6 Air 0.820 12944.3 832.2 0.221 6/6 Argon PAO5, 0.941 5511.4 503.6 0.188 7/7 333 K Hydrogen (16.98) (1441.2) (1262.2) ** 3/3 3.382 2233.7 1148.3 0.186 7/7 Air PAO10, 0.948 8824.1 821.5 0.211 6/7 Argon 363 K 1.657 1383.3 355.8 0.264 7/7 Hydrogen * **

Indicates number of failures out of total number of tests. Calculated by lower plots in Fig. 2(a)(b)



0.3

Average thickness of oxide layer, nm

Concentration of hydrogen, ppm


L10 H2 L10 Ar L10 Air 0.2

0.1 100

1000 Fatigue life, x104 cycles

250 200 150 100

H2 Ar

50

Air 0 100

10000

Fig. 4 Relationship between average concentration of hydrogen and L10 life; L10 life in hydrogen is calculated by using lower plots in Table 2

1000 10000 Life, Nx104 cycles

Fig. 5 Changes in average thickness of oxide layer with fatigue life in the tests with PAO10 at 363 K; the curves are fitted by quadric approximation

O Carbide

In Air

Wustite

40µm

4µm

Fe

C In H2 40µm

4µm

(a)

(b)

1µm

Fig. 6 SEM and AES analysis; (a) SEM images of track surface tested in air and in hydrogen after 384 nm ion etching, (b) AES mapping for disk track surface tested in hydrogen; from the tests with PAO10 at 363 K concentration, which is in good agreement with many other studies. However, besides hydrogen concentration, other factors that may affect the rolling contact fatigue life should also be examined in detail. 3.2. Depth profiling In order to see the changes in properties of the rolling contact track, elementally analyses with AES, Auger electron spectroscopy, were performed using JEOL JAMP 9500F. Irradiation electron source at 10 kV with a beam current of 10 nA was employed. Depth profiles of elements were obtained by sputtering the track surfaces using a differentially pumped argon ion gun with an acceleration voltage of 2 kV, where the sputter rate was approximately 19.2 nm/min for the SiO2 control surface. From depth profiles, the thickness of oxide layer was determined by the intersection point depth of oxygen and iron element concentration. Figure 5 shows the relationship between the oxide film thickness and the number of cycles in the three gas environments. Oxide layer is hardly observed at an early Japanese Society of Tribologists (http://www.tribology.jp/)

stage during the series of tests in hydrogen and argon. However, the thickness is larger after larger number of cycles in hydrogen and argon. The oxide film grew in hydrogen ten times faster than in argon. There is a large scatter in the oxide layer thickness in the tests in air while the scatter in the life is relatively small. Stable oxide film might have been formed in the early stage of the test and grew in the latter cycles. Difference in rate of growth in oxide layer may suggest a chemical activity of the formed oxide in different environment, i.e. steel surface is strongly oxidized and stable oxide layer is produced in air. From the comparison between hydrogen and argon, hydrogen might have played a role in the oxidation of steel surfaces. 3.3. SEM observation and AES mapping of disk surface In order to identify the characteristics of oxide film, the secondary electron images of scanning electron microscope, SEM, and AES observations of track on disk surfaces were conducted after 384 nm argon ion etching. Figure 6(a) shows the typical differences in Tribology Online, Vol. 8, No. 1 (2013) / 93


Hydrogen Argon Air 1µm 0.5mm

0.1mm (a)

Fig. 7 Typical profiles of disk specimens tested in three gases

400µm

2µm

Origin of crack Contact track

(b)

100µm

Fig. 8 Cross section of disk specimen tested in argon; taken parallel to the traveling direction just below the center of the track, from the tests with PAO10 at 363 K surface films formed in air and hydrogen. Before ion etching, almost all contacting surface were covered with white rumpled layers regardless of the environment. After the ion etching, white layers were almost removed. Then innumerable, dark colored grains appeared in the case in hydrogen. They have diameters of roughly 1 µm and distributed. Similar grains were also observed in the disk surface with longer life tested in argon. AES mapping images in Fig. 6(b) shows that the dark grains in SEM images were iron oxide developed in hydrogen. These dark grains of oxide grew larger with increasing in cycles of fatigue test, so they might lead to increase in average thickness of oxide layer under hydrogen and argon shown in Fig. 5. The oxide layers were analyzed by narrow scan spectrum of AES by using Ishiguro’s method [11], which identified that the small rumpled white layer on the top surface formed in normal atmosphere was hematite (Fe2O3), whereas the rounded dark colored one was wustite (FeO), which was minute but dense spherical crystal as known in some literatures. When minute, spherical grains of FeO grew up on the surface, oxide layer on surface would have non-uniform thickness distribution. In contrast, the grains of hematite are too small to be identified even with SEM. They are accumulated beneath the rolling track at a depth of about 60 µm as shown in Fig. 5. 3.4. Roughness of the track on the disk surface The formation of different oxide might change the roughness of the surface. Surface roughness is important Japanese Society of Tribologists (http://www.tribology.jp/)

Fig. 9 Cross section of ball specimen tested in hydrogen; (a) taken parallel to the traveling direction, (b) taken transverse to the traveling direction; from the tests with PAO10 at 363 K with respect to the surface initiated fatigue crack, because greater roughness leads to shorter fatigue life by means of stress concentration. Surface roughness of the disk surfaces were measured with a stylus profilemeter. Figure 7 shows surface profiles of the tracks in the tests in different environments. Apart from the wavy shape of the track and side furrows, surface roughness of the track is larger for those tested in hydrogen and argon than that in air. This may have promoted the initiation and propagation of flaking cracks on both the ball and disk surfaces in hydrogen. On the other hand, the track tested in air was deeper and wider than those in hydrogen and argon. This may imply that larger amount of wear prevented the crack initiation and propagation, resulting in the longer life in air than in hydrogen and argon. 3.5. Observation of cross section of disk and ball specimens Flaking failure occurred on the ball surface for almost all the tests in hydrogen at 363 K, and on the disk surface tested in air and argon. This implies the fatigue failure in hydrogen occurred in different mechanisms from that in other gases. Crack initiation and/or propagation should depend on the mechanical properties of subsurface. In order to see the changes in damages of subsurface under rolling contact, cross sections of the specimens were etched with nital and observed. Figure 8 shows the cross section of the disk with cracks developed in Argon. The cracks look bi-directional crack [12], and the initiation point may be just below the surface indicated by arrow. On the other hand, cracks formed in hydrogen are quite different as shown in Fig. 9(a). It occurred on the ball surface. It Tribology Online, Vol. 8, No. 1 (2013) / 94


4. Discussion Rolling contact fatigue is affected by many factors such as initial film parameter Λ, contact pressure, lubricant and temperature. In this study, initial film parameter was set low such that surface or shallow subsurface initiated cracks would be formed under the mixed lubrication. In this situation, some features on rolling contact fatigue in hydrogen were found: Firstly, oxide layer formed in hydrogen differ from that in normal environment, reading to increase of the amount of permeated hydrogen in steel. Secondly, roughness on the rolling track on disk in hydrogen is larger than that in normal environment. Thirdly, initiation and/or propagation or crack in the ball subsurface is promoted in hydrogen. These factors may relate each other and cause the reduction of fatigue life in hydrogen. In this section, possible role of oxide products on rolling contact fatigue is discussed. Surface oxides on steel surfaces play an important role in the generation of hydrogen at the surfaces and the penetration of hydrogen through the surfaces. Formation of surface oxide film is affected by surrounding gas and lubricants. Endo et al. [1] suggested that phosphorous acid ester promoted the formation of oxide film which could prevent the permeation of hydrogen into steel. It elongated the fatigue life in the four-ball tests in hydrogen. On the other hand, surface oxidation is more likely to occur under mixed lubrication regime than in the conditions where the surfaces are completely separated by fluid film. It is expected that surface temperature under mixed lubrication condition locally


increases by means of flash temperature. Formation of iron oxide depends on temperature and environment. Hydrogen reduces iron oxide from Fe2O3 to Fe3O4 at the temperature over 573 K and forms FeO and Fe over 627 K [14]. The changes in composition change the adsorption and permeation of hydrogen in steel. It is well known that minute FeO crystals prevent corrosion of iron. Figure 10 shows, however, thicker oxide layer in hydrogen could not always prevent the permeation of hydrogen. Minute FeO crystals grains were scattered and partially covered the disk surface. They could increase a probability of asperity contact to increase hydrogen generation but couldn’t prevent the permeation of hydrogen. In the contrast in air, Fig. 10 demonstrates that the thicker oxide layer leads to smaller amount of hydrogen permeated into the steel. This implies that the thick and uniform oxide layer prevents the hydrogen generation at the steel surface. In hydrogen gas environment, hydrogen molecules adsorb and decompose into atoms at the steel surface to enter into steel, resulting in a greater amount of permeated hydrogen than in other environments. Hydrogen gas environment also causes the increase in surface roughness, which may lead to increase in the probability of asperity contact and local temperature as well. The temperature increase may promote permeation and diffusion of hydrogen. Therefore, the severer contact condition causes larger amount of hydrogen permeation in hydrogen environment. In the mixed regime, asperity contact may intensify local stress and lead to initiation of cracks. The FeO grains which are relatively larger than Fe2O3’s can be a primary cause of surface damage in terms of asperity interference, which results in the reduction of fatigue life. Fujii et al. [12,15] suggested that fatigue damage induced by plastic deformation accumulates at the depth 0.4 Concentration of hydrogen, ppm

appears that the crack has propagated along many small black dots that were identified as ‘pit’ on etched surface, as shown in Fig. 9(b). The ‘pits’ were generated at around 200 µm from the surface, which was twice deeper than that of maximum shear stress. The ‘pits’ were also generated at shallow subsurface, i.e. about 10 µm below the surface. These pits may have helped the initiation and/or propagation of cracks, because several cracks grew passing through these pits in hydrogen. The pits however were also observed in the ball specimens tested in argon in which no flaking occurred. This may suggest that hydrogen diffused in subsurface somehow have accelerated the fatigue crack growth in the ball. This is supported by the long and relatively straight propagation, which is a characteristic feature of crack propagation in hydrogen in steels as reported by Fujita et al [13]. Fujii et al. [12] reported that a micro-crack initiated in a shallow subsurface area below a contact surface due to accumulating fatigue damage caused by indentation. The internal micro-crack extended almost parallel to the contact surface in the zone of maximum orthogonal shear stress, causing a large flaking failure with bi-directional crack. There is a similarity between Fujii’s results and the present study.

0.3

Hydrogen Argon

0.2 Air

0.1

H2 Ar Air

0 0 50 100 150 Average thickness of oxide layer, nm

Fig. 10 Relationship between concentrations of permeated hydrogen and average thickness of oxide layers on disk surfaces for the tests with PAO 10 at 363 K



of maximum equivalent stress, resulting in micro-crack initiation. The orthogonal shear stress is the attributed cause of micro-cracks that extend parallel to the contact surface in shallow subsurface. Recently, Uyama et al [16] reported that surface originated flaking life become shorter with hydrogen charged specimen than uncharged specimen. In present study, hydrogen environment changes the characteristics of surface, and also permeated hydrogen atom in steel induces micro-cracks formation in shallow subsurface, both may cause the reduction of rolling contact fatigue life. 5. Conclusions Rolling contact tests with JIS SUJ2 steel balls and disk were conducted in hydrogen, argon and air. Results are summarized as follows: 1. The rolling contact fatigue life in hydrogen was shorter than that in argon, and life in air was the longest, 2. A relationship was found between fatigue life and hydrogen concentration in the steel. 3. Iron oxide grew to larger grain size in the subsurface in hydrogen environment, which may have further shortened fatigue life of the steel. 4. Flaking failure on the ball surface occurred only in hydrogen. 5. Structural changes were found in the subsurface of the balls in hydrogen and argon.

[5]

[6]

[7]

[8]

[9]

[10]

[11]

Acknowledgment This work was conducted as a part of the “Fundamental Research Project on Advanced Hydrogen Science” administrated by New Energy and Industrial Technology Development Organization.

[12]

References

[13]

[1]

[2]

[3]

[4]

Endo, T., Dong, D., Imai, Y. and Yamamoto, Y., “Study on Rolling Contact Fatigue in Hydrogen Atmosphere – Improvement of Rolling Contact Fatigue Life by Formation of Surface Film – in Life Cycle Tribology,” Tribology and Interface Engineering Series, 48, 2005, 343-350. Imai, Y., Endo, T., Dong, D. and Yamamoto, Y., “Study on Rolling Contact Fatigue in Hydrogen Environment at a Contact Pressure below Basic Static Load Capacity,” Tribology Transactions, 53, 2010, 764-770. Grunberg, L., Jamieson, D. T. and Scott, D., “Hydrogen Penetration in Water-Accelerated Fatigue of Rolling Surfaces,” Philosophical Magazine, 8, 93, 1963, 1553-1568. Ciruna, J. A. and Szieleit, H. J., “The Effect of Hydrogen on the Rolling Contact Fatigue Life of AISI 52100 and 440C Steel Balls,” Wear, 24, 1973, 107-118.


[14]

[15]

[16]

Murakami, T., Naka, M., Iwamoto, A. and Chatell, G., “Long Life Bearings for Automotive Alternator Applications,” SAE Technical Paper, 1995, 950944. Shibata, M., Goto, M., Oguma, N. and Mikami, T., “A New Type of Micro-Structural Change Due to Rolling Contact Fatigue on Bearing for the Engine Auxiliary Device,” Proc. Int. Trib. Conf., Yokohama 1995, 1996, 1351-1356. Tamada, K. and Tanaka, H., “Occurrence of Brittle Flaking on Bearing Used for Automotive Electrical Instruments and Auxiliary Devices,” Wear, 199, 1996, 245-252. Ohno, N., Komiya, H., Mia, S., Morita, S., Satoh, N. and Obara, S., “Bearing Fatigue Life Tests in Advanced Base Oil and Grease for Space Applications”, Tribology Transactions, 52, 2009, 114-120. Ohno, N., Rahman, M. Z., Yamada, S. and Komiya, H. “Effect of Perfluoropolyether Fluids on Life of Thrust Ball Bearings,” Proc. 2004 ASME/STLE IJTC, Long Beach, TRIB 2004-64018, 1-7. Tanimoto, H., Tanaka, H. and Sugimura, J., “Observation of Hydrogen Permeation into Fresh Bearing Steel Surface by Thermal Desorption Spectroscopy,” Tribology Online, 6, 7, 2011, 291-296. Ishiguro, K. and Homma, T., “The Chemical Effect of Auger Spectrum in the Transition Metal Oxides and its Influence on the Relative Sensitivity Factor,” J. Japan Inst. Metals, 45, 4, 1981, 360-376. Fujii, Y. and Maeda, K., “Flaking Failure in Rolling Contact Fatigue Caused by Indentations on Mating Surface (III) Mechanism of Crack Growth in the Direction Opposite to the Load-Movement,” Wear, 252, 2002, 811-823. Fujita, S., Matsuoka, S., Murakami, Y. and Marquis, G., “Effect of Hydrogen on Mode II Fatigue Crack Behavior of Tempered Bearing Steel and Microstructural Changes,” International Journal of Fatigue, 32, 2010, 943-951. Jozwiak, W. K., Kaczmarek, E., Maniecki, T. P., Ignaczak, W. and Maniukiewicz, W., “Reduction Behavior of Iron Oxides in Hydrogen and Carbon Monoxide Atmospheres,” Applied Catalysis A: General 326, 2007, 17-27. Fujii, Y. and Maeda, K., “Flaking Failure in Rolling Contact Fatigue Caused by Indentations on Mating Surface (II) Formation Process of Flaking Failure Accompanied by Cracks Extending Bi-Directionally Relative to the Load-Movement,” Wear, 252, 2002, 799-810. Uyama, H., Yamada, H., Hidaka, H. and Mitamura, N., “The Effects of Hydrogen on Microstructural Change and Surface Originated Flaking in Rolling Contact Fatigue,” Tribology Online, 6, 2, 2011, 123-132.
