Anti-peeling Rolling Bearing With Manganese Phosphate Coating

17KOYO Engineering Journal English Edition No.156E (2000)

1. Introduction

As a result of environmental concerns in recent years, therehas been a trend among automobile manufactures to reducethe frequency of engine and transmission oil changes, andswitch to low-viscosity oils. As lubrication condition becomesincreasingly severe, various types of surface damage tend tooccur. This requires higher performance and high reliability ofmetal surfaces, but an increase in costs is severely restrained.

Peeling is a type of surface initiated failure, and is usually aconcentration of fine flakings of no more than 0.1 mm wideand 10 µm deep1). Peeling occurs on a component having asmooth surface when the mating component is rough and thelubricating film between the two surfaces is insufficient.Peeling is often seen, for example, on the surface in contactwith automobile engine roller follower bearing cams androllers of needle roller bearings.

This paper reports on a newly developed peeling-resistantbearing that is effective in preventing surface damage undersevere lubrication conditions and has low cost.

2. Concept of Development

Oil film thickness parameter Λ is commonly used as theparameter for expressing the severity of lubrication conditions.The value of Λ is usually defined as the ratio of minimum oilfilm thickness h0 and combined roughness of the two surfacesthat make contact with each other. The lower the value of Λ,the shorter the rolling contact fatigue life becomes2), and theorigin of damage shifts from subsurface to surface. Seizureand abnormal wear, which occur when the oil film is broken,are most likely connected with the value of Λ. Lubricationconditions must be improved in order to prevent damage,including peeling, from occurring on the surface. The value ofΛ is increased by using lubricants with higher kinematicviscosities or by reducing surface roughness, but this leads tohigher fuel consumption and cost.

Machine parts are generally subjected to running-inoperation under moderate conditions to improve contactsurfaces before actual use in the machine. Initial contactmicro-roughness is improved by running-in. According toF.Hirano, although the surface quality of machine parts mustbe improved, there is a limit to improving the precision of

machining and materials, but there is ample room forimprovement in the area of optimizing running-in operationtechnology3). Effective application of running-in as a means ofpreventing various types of surface damage must beinvestigated further.

Because of its initial running-in and oil retention properties,manganese phosphate coating is widely used for iron slidingsurfaces such as gears and cams. It used to be difficult to applymanganese phosphate to rolling bearings requiring highprecision due to problems such as an increase in roughnessafter treatment, coating unevenness, and coating falling off. Inrecent years, however, these problems have been minimizedby improvements in technology for controlling the grain sizeof manganese phosphate coating4), which is also relatively lowin cost because it is a wet process and mass quantities can beproduced at a time. In this paper we take a look at the running-in effectiveness of manganese phosphate coating and study itssuitability to rolling contact surfaces for the purpose ofpreventing peeling and other surface damage.

3. Peeling-Resistant Surfaces

3. 1 Application of Manganese Phosphate CoatingTo apply manganese phosphate coating to the contact

surface of rolling elements, the coating and surface of the basematerials must be optimized. In the case of manganesephosphate coating, various coatings and surface of basematerials are possible by combining chemicals (surfaceadjustment agent, phosphating solution) and processingconditions. The authors prepared various coated surfaces,evaluated them for peeling resistance under rolling contactconditions, and developed a new surface treated with a specialmanganese phosphate coating.

Fig. 1 shows the schematic diagram of the developed anti-peeling surface. The surface is coated with manganesephosphate. The coating thickness is 1~4 µm. The grains aresmall, so the roughness is less than that of conventionalmanganese phosphate coatings. The iron of the surface basemetal is dissolved during the initial manganese phosphatecoating process, forming pits in the surface of the base metal.These pits are arranged at a moderate size, depth and density.

Peeling occurs on smooth rolling bearing raceways in contact with rough mating surfaces under poor lubricationconditions. A relatively inexpensive manganese phosphate treatment was selected for preventing the contact surface frompeeling. We have confirmed by rolling fatigue test results that the manganese phosphate coating can prevent raceways frompeeling, and that its effect is a result of roughness truncation and the increase of oil film formation.

Anti-Peeling Rolling Bearing with Manganese Phosphate Coating

Y. ASAI* N. MOTOHASHI** Y. SAKAKI**

* Research & Development Center, Automotive Unitized Product Engineering Development Department**Automotive Engineering Center, Automotive Bearing Engineering Department

−Anti-Peeling Rolling Bearing with Manganese Phosphate Coating−

18 KOYO Engineering Journal English Edition No.156E (2000)

Coating

Base metal

Fig. 1 Schematic diagram of manganese phosphate coating

When actually applying manganese phosphate coating torolling bearings, we have taken into consideration dimensionalchanges during the process of coating and clearance changesproduced by coating fall-off during operation.

3. 2 Anti-Peeling PerformanceAnti-peeling properties for the newly developed surface

were evaluated with a two-ring type rolling fatigue tester for20 hours. Test conditions are given in Table 1, and thestructure of the tester is shown in Fig. 2. The results are shownin Figs. 3, 4 and 5.

Table 1 Test conditions

Maximum contact pressure, GPa 2.25

Rotational speed, min−1 1 000

Running time, h 20

Oil film thickness parameter (Λ) 0.55

Driven roller

Drive roller

Motor

Fig. 2 Test equipment

(a) Untreated (b) Treated80µm

Fig. 3 Appearance of surface after testing

Fig. 4 EPMA analysis of treated surface

(a) Surface appearance (b) P analysis (c) Mn analysis10µm



The load was applied by a coil spring. As a lubricant,3 ml/min of no-additive turbine oil was dripped from the topof the tester. The outer diameters of both the drive roller anddriven roller were 60 mm. The drive roller had an R30 crown-radius and the driven roller was flat. Both rollers were made ofhigh-carbon chromium steel (JIS/SUJ2) and hardened by heattreatment to 700~750 HV. The roughness of the drive rollerwas increased by grinding, and the roughness of the drivenroller was reduced by lapping. Sample driven rollers with andwithout manganese phosphate-treated anti-peeling surfaceswere tested.

Fig. 3 shows the appearance of the surface after testing.Peeling accompanied by two surface cracks5) occurred in theuntreated surface. In the case of the coated surface, the coatingcame off and some pits were observed in the surface of thebase metal, but they were of undefined shape, unlike finepeeling. EPMA analysis of Mn and P confirmed thatmanganese phosphate remained in the pits, as shown in Fig. 4,and that the pits were not minute peeling.

Fig. 5 shows the peeling damage of the surface after the testas observed using a laser microscope. There was no peeling onthe anti-peeling surface. The peeling damage of the untreatedsurface ranged from 13.5% to 36%.

Running time, h0

0

0.10

0.1

20

30minutes

Treated

Untreated

Fig. 6 Change of voltage by separation of two surfaces

Con

tact

vol

tage

, V

4. 2 Change in Roughness of Drive Roller duringTesting Time

The authors turned their attention to the roughness of thedrive roller to investigate the reason why separation betweenthe two surfaces by oil film is large and peeling is suppressed.

With the two-ring type rolling fatigue tester, the authorscompared changes in initial operating period roughness of thecompanion drive roller both when the driven roller had apeeling-resistant surface and when it had an untreated surface.Test conditions are shown in Table 2.

36

0

13.5

0 0 00

5

10

15

20

25

30

35

40

Untreated Treated

Pee

ling

dam

age

(%, d

amag

ed a

rea

ratio

)

Fig. 5 Peeling damage of treated and untreated surfaces

Fig. 7 shows a typical example of roughness change for thesurface of companion drive roller. Where the driven roller wasuntreated, roughness amplitude was observed to be less thaninitial period roughness both three minutes and one hour afterstarting the test. In the case of the treated surface, in additionto a decrease in roughness amplitude, the asperity becamerounded.

Concerning the roughness curve of the drive roller, theradius of curvature R for the asperity was determined usingthe McCool method6) (following equation).

Here, m4 represents the 4th spectral moment.

a

Change in roughness Rq and the radius of asperitycurvature (hereinafter referred to as "radius of peaking") fromthe start of running until one hour is shown in Fig. 8. Thisshows that the roughness amplitude decreased earlier in thecase of the treated surface than in the case of the untreatedsurface, and the radius of asperity increased. This is due to thetruncation accompanying the running-in. The test proved thatcoating with manganese phosphate helps decrease ofroughness of the companion surface and smoothes asperitiesto enhance response to running-in.

4. Peeling Suppression Effect

4. 1 Oil Film Formation in Peeling-Resistant SurfaceCeramic ball bearings were used for support bearings in the

previously described tester. Both roller shafts were electricallyseparated in order to measure progressive change in contactvoltage between the two rollers over time. Contact voltagemeasured during the test is given in Fig. 6. Separation voltagewas measured using the electrical resistance method, applying0.1 volts of electricity. The figure shows perfect contact at0 volts and perfect separation at 0.1 volts. Separation voltageshifts almost to contact during the test in the case of theuntreated surface, whereas in the case of the peeling-resistantsurface, while making perfect contact during the initialoperation period, after 30 minutes the separation voltage risesand shifts to the separation side. This indicates that the oil filmtends to be formed on the peeling-resistant surface more thanon the untreated surface, thus suppressing peeling.

Table 2 Test Conditions



Drive roller initial roughness Rq, µm 0.84


=R 0.375(π ⁄ ) m41 ⁄ 2



4. 3 Affect of Roughness Radius of AsperityThe representative index for severity of asperity contact,

plastic index ψ, has been defined by Greenwood7) et al. as inthe following equation. Plastic index ψ is inverselyproportional to the square root of the roughness radius ofasperity R.

s

Here E* is equivalent elastic modulus, H is metal hardness,and σ is RMS roughness. Radius of asperity is thought toaffect bearing life and cause sharing8) of total pressure over theasperities in mixed lubrication condition9).

The authors tested the effect of change in radius of asperitydue to running-in confirmed in the previous section on thedegree of peeling damage.

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60Driv

e ro

ller

roug

hnes

s R

q, µ

m

UntreatedTreated

01234567

0 10 20 30 40 50 60

Running time, h

Rad

ius

of a

sper

ity R

, µm

Fig. 8 Change of roughness Rq and asperity curvature radius

The results are given in Fig. 9. A correlation betweenroughness radius of asperity of the drive roller and peelingarea was noted, with peeling damage decreasing as radius ofasperity increases.

EH

φ =∗ σ

R




Running time, h 20


0

5

10

15

20

25

30

35

0 1 2 3 4 5 6

Radius of asperity, µm

Fig. 9 Relationship between asperity curvature radius and

peeling damage

Pee

ling

dam

age,

%

Test conditions are given in Table 3. Equipment used forthe test was the previously mentioned two-ring type rollingfatigue tester. The drive rollers were prepared by running-inoperation. Drive rollers of the same degree of roughness (Rq)were selected from among various radiuses of asperities andtested. All driven rollers were lapped and were untreated. Theauthors evaluated peeling damage by measuring peeling area.

Treated

Untreated

New After 3 min. After 60 min.

Fig. 7 Change of roughness on drive roller



This suggests that the manganese phosphate coatingsuppresses peeling by running in the companion surface andsmoothing the asperities. It is surmised that the coating itselfrelaxes asperity interference in the initial period of rotationand contributes to the lubricating effect by maintaining an oilfilm due to the manganese phosphate remaining in the pitseven after the coating drops off.

5. Application to Roller Bearings

The authors applied the developed manganese phosphatecoating to engine cam roller follower bearings, which aresubject to peeling, and evaluated them.

The tester is shown schematically in Fig. 10, and the testconditions are given in Table 4. Actual cams were used as thecompanion parts. Needle roller type roller follower bearingNAP0817 (outer diameter φ17, width 9, basic dynamic loadrating 11.1 kN) was used as the specimen. Bearing loadproduced by cam lift was adjusted by spring force to be thesame as that of an actual engine. SAE 10-W30 engine oil wasused for lubrication, and the oil temperature was regulated at130; . We applied the coating to the outer ring of thespecimen bearings making contact with the cam and comparedthem with untreated outer rings. The untreated outer ringswere super-finished.



Lubricant amount, ml/min 60

Oil temperature, ; 130

Running time, h 200


Lubricant SAE10-W30 engine oil

Coil spring

Specimen

Cam

Fig. 10 Cam follower test equipment

0

50

100

150

200

Untreated Peeling-resistant bearing

Run

ning

tim

e, h

Fig. 11 Test results

Fig. 12 Appearance of specimen after running

(a)Untreated part (b)Peeling-resistant bearing

The results are shown in Fig. 11. The appearance aftertesting is shown in Fig. 12. Visual inspection revealed lightsingle-layer peeling in untreated outer rings after 50 hours ofrunning. The surface of the peeling-resistant bearing becameglossy after 200 hours of running, but no peeling wasobserved. It was confirmed that the treated cam roller follower

bearings showed sufficient resistance to peeling. There was noabnormal wear of the companion cam surface.

By measuring the surface profile of the glossy surface, theauthors confirmed that this was not caused by wear but wasrather the result of coating fall-off. Even though the coatingfall off, it was thin so there was no problem with precision.Application of manganese phosphate coating to sliding partsfor engines been proven sufficiently through actual usage, sofallen-off manganese phosphate will not negatively affectother parts.

6. Conclusion

1) The authors developed a peeling-resistant coating surfacewith optimized surface of base metal by treating withmanganese phosphate coating.

2) The probable reasons the manganese phosphate coatingsuppresses peeling are as follows:(1) Manganese phosphate coating runs-in the companion

surface and increases surface separation of rollercontact surfaces.

(2) Radius of asperity affects degree of peeling damage andis increased by running-in.

3) Peeling-resistant bearings with manganese phosphatecoating showed sufficient resistance to peeling in endurancetesting on a bench machine. Manganese phosphate coatingwas observed to have no detrimental effect on actual parts.Finally, the authors would like to express their appreciation

to Nihon Parkerizing Co., Ltd., for its cooperation in preparingtest samples, etc.



References

1) Y. Akamatsu: Journal of Japanese Society of Tribologists,

37, 7 (1992) 533.

2) J. C. Skurka: Trans. ASME, J. Lub. Tech. (1971) 28.

3) F. Hirano: Journal of Japan Society of Lubrication

Engineers, 30, 5 (1985) 377.

4) J. Kawaguchi: The Journal of the Surface Finishing Society

of Japan, 49, 8 (1998) 377.

5) H. Muro and T. Yamamoto: Proceeding of JAST Tribology

Conference, 785 (1992-10, Morioka).

6) J. I. McCool: Trans. ASME, JOT. 109 (1987) 264.

7) J. A. Greenwood and J. B. P. Williamson: Proc.Roy.Soc.

London, A295 (1966) 300.

8) K. L. Jhonson, J. A. Greenwood, and S. Y. Poon: Wear, 19(1972) 91.

9) J. Y. Li., J. J. Kauzlarich and W. E. Jamison: Trans. ASME,

J. Lub. Tech. (1976) 530.

Anti-peeling Rolling Bearing With Manganese Phosphate Coating

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