ARL-TR-7815 ● SEP 2016

US Army Research Laboratory

Evaluation for Loss of Lubrication Performance 

of Black Oxide, Superfinished, and As-Ground 

Surfaces for Use in Rotorcraft Transmissions

by Mark R Riggs, Nikhil K Murthy, and Stephen P Berkebile

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ARL-TR-7815 ● SEP 2016

US Army Research Laboratory

Evaluation for Loss of Lubrication Performance 

of Black Oxide, Superfinished, and As-Ground 

Surfaces for Use in Rotorcraft Transmissions

by Mark R Riggs and Stephen P Berkebile

Vehicle Technology Directorate, ARL

Nikhil K Murthy

Engility, Chantilly, Virginia

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1. REPORT DATE (DD-MM-YYYY)

September 2016

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Technical Report

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2 October 2015–26 April 2016

4. TITLE AND SUBTITLE

Evaluation for Loss of Lubrication Performance of Black Oxide, Superfinished,

and As-Ground Surfaces for Use in Rotorcraft Transmissions

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Mark R Riggs, Nikhil K Murthy, and Stephen P Berkebile

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US Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5066

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ARL-TR-7815

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

During a loss of lubrication event, gears and bearings no longer receive the proper amount of oil to prevent starved lubrication 

between contacting surfaces. This starved lubrication condition increases heat generation and decreases heat dissipation to 

facilitate a scuffing failure mode within the gear mesh. The time between the beginning of a loss of lubrication event and the 

initiation of scuffing is referred to as the time to failure. While a loss of lubrication condition can lead to catastrophic failure, 

the time to failure can be extended by varying the physical and chemical properties of the material and residual lubricant. 

This report compares the relative time to failure during loss of lubrication conditions for three 9310 gear steel surfaces: pristine 

as-ground, which serves as the baseline surface, as-ground with black oxide, and superfinished. Experiments are conducted 

using Mobil Jet II and AeroShell 555 rotorcraft lubricants with a ball-on-disc tribometer to simulate contact conditions relevant

to rotorcraft transmissions experiencing loss of lubrication. The ball-on-disc tribometer monitors the coefficient of friction 

within the contact, and scuffing initiation is indicated by a sharp rise in the coefficient of friction. The results from high-speed 

loss of lubrication evaluations showed that a superfinished surface was able to delay the initiation of scuffing by up 13 times 

longer than an as-ground surface, while the black oxide showed a modest 14% improvement.

15. SUBJECT TERMS

loss of lubrication, LoL, black oxide, ISF, as-ground, scuffing, gears, rotorcraft, superfinishing

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Mark R Riggs

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Contents

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Procedure 2

2.1 Test Apparatus 2

2.2 Materials 3

2.3 Experimental Protocols 4

2.3.1 Load Capacity Protocol 4

2.3.2 Loss-of-Lubrication Protocol 5

2.3.3 Friction Mapping Protocol 7

2.4 Test Matrices 9

3. Results and Discussion 10

3.1 Load Capacity 10

3.2 Loss of Lubrication 11

3.2.1 High Speed 12

3.2.2 Mobil Jet II 14

3.2.3 AeroShell 555 14

3.2.4 Low Speed 15

3.3 Friction Mapping 15

3.4 Microscope Analysis 21

4. Summary and Conclusions 25

5. References 27

Appendix. Low-Speed Loss of Lubrication Raw Data 29

List of Symbols, Abbreviations, and Acronyms 33

Distribution List 34
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List of Figures

Fig. 1 Ball-on-disc tribometer..........................................................................3

Fig. 2 Slip percentage and entrainment velocity for the mapping procedure ..8

Fig. 3 Typical time trace of traction coefficient vs. time during an HS LoL 

experiment............................................................................................11

Fig. 4 Example HS LoL raw data plot focused on the loss of lubrication 

portion of the experiment.....................................................................12

Fig. 5 HS LoL times to failure .......................................................................13

Fig. 6 HS LoL times to failure with a truncated Y-axis.................................13

Fig. 7 Example friction map, black oxide (first run)......................................16

Fig. 8 Contour map of friction data for black oxide (first run)......................17

Fig. 9 Final contour map for black oxide (final run)......................................18

Fig. 10 Final contour map for ISF....................................................................18

Fig. 11 Final contour map for radially as-ground ............................................19

Fig. 12 Subtraction for the first and last black oxide maps..............................20

Fig. 13 Subtraction of contour maps for the first and last ISF #2 maps...........20

Fig. 14 Subtraction for the first and last maps as-ground radial ......................21

Fig. 15 Color images of radially ground disc (left), black oxide disc (center), 

ISF #2 disc (right). The top of the image is the outside of the disc. 

Areas marked as invalid map were used for a mapping procedure but 

the protocol was not properly followed. ..............................................22

Fig. 16 Images of the radially ground (left), ISF #2 (center) and black oxide 

(right) discs after experimentation.......................................................23

Fig. 17 Roughness of mapping tracks..............................................................25

Fig. A-1 Black oxide low-speed loss of lubrication (LS LoL) experimental 

results...................................................................................................30

Fig. A-2 Isotropic superfinishing (ISF) LS LoL experimental results...............30

Fig. A-3 As-ground radial LS LoL experimental results ...................................31

Fig. A-4 As-ground circumferential LS LoL experimental results....................31
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v

List of Tables

Table 1 Surface materials process summary.......................................................4

Table 2 Load capacity protocol velocity conditions...........................................4

Table 3 Load capacity protocol load stages........................................................5

Table 4 HS LoL steady state conditions.............................................................6

Table 5 Original LS LoL loading progression during run-in..............................6

Table 6 Revised load stages for the LS LoL protocol.........................................7

Table 7 HS LoL experiment matrix ....................................................................9

Table 8 LS experiment matrix ............................................................................9

Table 9 Load capacity results............................................................................10

Table 10 Roughness of mapping tracks..............................................................24

Table A-1 LS LoL experimental results................................................................32
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vi

Acknowledgments

We would like to thank Brian Rich and his group at the Aviation Engineering 

Directorate/US Army Research, Development, and Engineering Command for his 

helpful suggestions and questions during review of this report. We would also like 

to thank Wedeven Associates, Inc., for their suggestions and comments in 

developing some of the protocol conditions.
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1. Introduction

A potential failure mode of interest in rotorcraft transmissions, “loss of 

lubrications” refers to the interruption of lubricant supply to one or more elements 

of the drive system, may result from one of many causes including ballistic impact. 

The US Army requires rotorcraft transmissions and gearboxes to be capable of 

operating for 30 min from the time the lubricant low-level warning system is 

activated, according to the US Army Aeronautical Design Standard (ADS)-50-PRF 

5-4.3.5.

1 The operating requirement aims to allow improperly lubricated gearboxes 

to sustain flight for an emergency landing.

Meeting the ADS-50-PRF requirement is challenging because metal-to-metal 

asperity contact between previously lubricated surfaces rapidly leads to scuffing. 

While the mechanism behind scuffing is not fully understood, it is thought to be 

caused by high sliding velocities and contact stresses, which produce intense 

frictional heating and localized welding.

2 Additionally, losing the lubricant supply 

removes the mechanism responsible for dissipating frictional heat from the 

contacting surfaces through bulk lubricant flow. The combination of heat 

generation without the ability to dissipate the heat leads to a thermal runaway in the 

mechanical component. While there are several possible points of failure within the 

system due to a loss of lubrication event, this study chooses to focus on the effects 

within a gear mesh. Thermal runaway results in significant wear, through material 

loss and material transfer between gear teeth, and in plastic deformation. Plastic 

deformation results in a loss of the teeth’s profiles and the inability to transmit 

torque, which often leads to destruction of the component and loss of drive.3 One 

possible solution to increasing the time to scuffing is by varying the physical and 

chemical properties of the material and residual lubricant.

To evaluate this approach, 3 different types of American Iron and Steel Institute 

(AISI) 9310 gear steel surfaces, which is a common gear material for rotorcraft 

transmissions, were evaluated under a loss of lubrication condition. The surfaces 

are pristine as-ground, as-ground with black oxide, and superfinished. The pristine 

as-ground surface is a ground surface without additional treatment or wear. The 

black oxide and superfinishing processes are both surface treatments applied to 

pristine as-ground surfaces, which makes it a relevant baseline for comparison. 

Black oxide is commonly used in gearing systems as a surface treatment for 

corrosion resistance. Because of this, it is also a common surface treatment for 

qualification testing under the ADS-50-PRF by default. Superfinishing is evaluated 
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because it creates a smooth surface to reduce asperity interaction within the 

boundary lubrication condition resulting from residual oil films on the steel 

surfaces. 

2. Procedure

Gear contact conditions are simulated using a ball-on-disc tribometer to evaluate 

AISI 9310 aerospace gear steel samples with 3 evaluation protocols. Multiple 

protocols were conducted to give a well-rounded evaluation of the black oxide and 

superfinished surface treatments with respect to the pristine as-ground surface. The 

test apparatus, materials, and protocols are explained in Sections 2.1, 2.2, and 2.3, 

respectively.

2.1 Test Apparatus

The equipment used to evaluate the surfaces was a ball-on-disc tribometer.

4 A ballon-disc tribometer is capable of simulating gear contacts at high speed and high 

load with precise control over the operating conditions. Independent control of 

vertical load, oil flow, entrainment velocity, slip percentage, and geometric 

orientations enables the ball-on-disc tribometer to flexibly simulate gear and 

bearing contacts in a controlled and focused environment. The ball-on-disc 

tribometer specimen configuration is shown in Fig. 1, where the ball linear velocity 

(Ub) and disc linear velocity (Ud) are controlled with independent motors to create 

entrainment velocities (Ue) with controlled slip percentages (i.e., slide-to-roll 

ratios). The entrainment velocity is defined as the average of the ball and disc linear 

velocities and the slip percentage is the difference in ball and disc velocities divided 

by the average, as shown in Eq. 1. The ball thermocouple (Tb) and disc 

thermocouple (Td) give temperature readings of the specimen surface or the oil film, 

if one is present. When the ball and disc are rotating, the thermocouples remain 

stationary as they ride along the moving surfaces. An additional controlled 

parameter is the skew angle, which is the relative angle between the linear sliding 

velocities of the ball and disc. The orientation shown in Fig. 1 has a skew of 0° 

where the ball and disc linear velocities are parallel. Additional information 

regarding the ball-on-disc tribometer working envelope and geometric construction 

can be found in Riggs et al.

4
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Fig. 1 Ball-on-disc tribometer

SSSSSSSS PPPPPPPPPPPPPPPPPPPP (%) = UUbb−UUdd

1

2� (UUbb+UUdd) = UUbb−UUdd

UUee

∗ 100% . (1)

2.2 Materials

The base material for the balls and discs is AISI 9310 alloy steel lubricated with 

Mobil Jet II (MJII) or AeroShell 555 (A555). AISI 9310 was chosen because it is a 

common gear material in rotorcraft transmissions. The balls are typically 20.64 mm 

in diameter and the standard disc diameter is 109.7 mm. MJII is a MIL-PRF-236995

oil formulated as a gas turbine lubricant and A555 is a DOD-PRF-857346 oil that 

is formulated with a higher extreme pressure requirement for helicopter 

transmissions. While MJII is a gas turbine oil, it is used because of its historical use 

within the development of ball-on-disc protocols for high-speed conditions and its 

ability to distinguish between surfaces that otherwise would have all performed 

well with the extreme pressure additives of A555. The balls and discs are case 

carburized to a Rockwell C hardness of HRC 61 ± 2.5. There are 2 types of pristine 

as-ground discs that are used as a baseline, where pristine refers to the as-ground 

surface’s lack of treatment or wear. The 2 types are distinguished by the dissimilar 

grinding lay directions: radial and circumferential. The circumferentially ground 

disc results in a surface velocity parallel to its grinding lay when the ball and disc 

surface velocities are parallel. The radially ground disc results in a surface velocity 

perpendicular to its grinding lay when the ball and disc surface velocities are 

parallel. The different grinding lays allow us to consider the orientation of the lay 

with respect to entrainment velocity with similar average surface roughness, Sa, 

values of 148 nm and 175 nm. The black oxide treatment, processed according to 
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an original equipment manufacturer process, and isotropic superfinishing (ISF) 

treatment, processed by REM Surface Engineering, were applied to radially ground 

discs and a corresponding set of as-ground balls. The ISF specimens had an Sa

roughness value averaging to be 55 nm. A competitor process for improved surface 

finish, micro machining process (MMP) superfinished by MicroTek, was also 

compared. The MMP specimens had an average disc Sa roughness of 24 nm. 

Detailed surface roughness values before and after testing are given in the 

microscope analysis section. It should be noted that there were three 9310 discs that 

were all superfinished with an ISF treatment; ISF #1 and ISF #3 were treated with 

an aerospace process, and ISF #2 was treated with an automotive process. A 

summary of the discs used within this paper and their process differences are shown 

below in Table 1.

Table 1 Surface materials process summary

Disc Grinding lay Process notes Sa

(nm)

Black oxide Radial Boeing HP4-50 190

Superfinished Circumferential

ISF #1 (Aerospace) 52

ISF #2 (Automotive) 76

ISF #3 (Aerospace) 36

MMP 24

As-ground Radial None 148

Circumferential None 175

2.3 Experimental Protocols

The 3 evaluation protocols used to simulate relevant contact conditions are the load 

capacity (LC), loss-of-lubrication (LoL), and mapping protocols. 

2.3.1 Load Capacity Protocol

The first performance evaluation is the LC protocol. This experiment acts as a 

screening test to evaluate the ability of the lubricant and material to support the 

stress at which the LoL protocol is run to survive the run-in portion of the LoL 

experiment. The LC protocol is run with the velocity parameters listed in Table 2 

with increasing load stages, shown in Table 3, until scuffing initiation. If a scuff is 

not produced by stage 30, the contact is unloaded and its LC is determined to be 

greater than or equal to 2.38 GPa.

Table 2 Load capacity protocol velocity conditions

Description Entrainment velocity

(m/s)

Slip

(%)

Skew

(°)

LC 7 71.4 39.3
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Table 3 Load capacity protocol load stages

Load stage Load

(N)

Stressa

(GPa)

1 17.8 0.73

2 28.9 0.86

3 34.2 0.90

4 41.5 0.97

5 48.8 1.02

6 58.6 1.08

7 68.4 1.14

8 78.1 1.19

9 87.9 1.24

10 100.1 1.29

11 112.3 1.35

12 124.5 1.39

13 136.8 1.44

14 151.4 1.49

15 166.1 1.53

16 180.7 1.58

17 195.4 1.62

18 212.5 1.66

19 229.6 1.71

20 246.6 1.75

21 263.7 1.79

22 285.9 1.84

23 312.6 1.89

24 343.7 1.95

25 379.2 2.02

26 419.1 2.09

27 463.5 2.16

28 512.4 2.23

29 565.7 2.31

30 621.6 2.38 a

Calculated Hertzian contact stress.

2.3.2 Loss-of-Lubrication Protocol

The second evaluation method uses 2 LoL protocols. The first protocol simulates a 

loss of lubrication event experienced at the high sliding velocities within a spur gear 

mesh during tooth engagement and disengagement. This set of parameters ran at a 

relatively high entrainment velocity of 16 m/s and it is referred to as the high-speed 

(HS) LoL protocol. The second protocol utilizes steady state conditions similar to 

high sliding conditions of rotorcraft gear sets typical of spiral bevel gears. This set 

of parameters ran at a relatively low entrainment velocity of 7 m/s, and it is referred 

to as the low-speed (LS) LoL protocol. Both LoL protocols incorporate an initial 

run-in period under normal lubrication prior to removing the oil supply and 

measuring the time to failure. Failure is defined as the initiation of scuffing with a 

sharp rise in the traction coefficient. 
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2.3.2.1 High-Speed Loss-of-Lubrication Protocol

The HS LoL protocol begins with a run-in period followed by a period of operation 

without oil. The entrainment velocity, slip percentage, skew, and load are held 

constant throughout a 10-min lubricated run-in period according to the parameters 

in Table 4. Thermal parameters are also controlled to operate with a disc 

temperature of 126 °C and the lubricant is also independently heated to 126 °C to 

avoid cooling the disc. Once the run-in period is completed, the oil supply is 

removed and the operating conditions in Table 4 are held constant until a sharp rise 

in the traction coefficient indicates the initiation of a scuffing failure. The HS LoL

protocol is well developed and further discussion on this protocol can be found in 

Riggs et al.7

.

Table 4 HS LoL steady state conditions

Description

Entrainment 

velocity 

(m/s)

Slip 

(%)

Skew 

(°)

Load 

(N)

Contact stress 

(GPa)

LoL 16 –100 0 100 1.29

2.3.2.2 Low-Speed Loss-of-Lubrication Protocol

The LS LoL protocol presented several challenges to provide unbiased results and 

it continues to be in the development stage. As in the HS LoL protocol, the LS LoL

protocol also begins with a run-in period followed by a period of operation without 

oil, but a load schedule is adopted as shown in Table 5, rather than a single load. 

The velocity parameters are the same as the LC protocol, as shown in Table 2.

Table 5 Original LS LoL loading progression during run-in

Stage Elapsed time

(s)

Load

(N)

Stress

(GPa)

1 60 20 0.76

2 60 40 0.95

3 60 60 1.09

4 60 80 1.20

5 60 100 1.29

6 60 120 1.38

7 450 140 1.45

Thermal parameters were controlled in the same way as the HS LoL protocol, but 

additional oil mitigation was necessary. When the oil supply was removed during 

the LoL stage, unwanted excess oil from the center of the disc still entered the 

contact. To solve this issue, the following process was adopted. Before starting the 

LoL portion of the LS LoL protocol, the contact was unloaded and the lubricant 

supply was turned off. The ball and disc were allowed to spin for 30 s to remove 
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excess oil and then the motors were shutdown. Residual lubricant was physically 

wiped out of the oil slinger at the center of the disc and the test was resumed by 

bringing the motors back up to speed and loading the contact. The beginning of the 

LoL event was recorded as the moment 140 N was reached in the contact after 

reloading. This cleaning process resulted in additional load stages, which are 

presented in Table 6. The LS LoL protocol is under development because areas of 

concern developed for further oil mitigation requirements. In addition, a possible 

dependence on track diameter was observed that further highlighted the 

underdeveloped status of the LS LoL protocol. Therefore, these results are 

presented in the Appendix.

Table 6 Revised load stages for the LS LoL protocol

Stage Elapsed time 

(s)

Load 

(N)

Stress 

(GPa)

1 60 20 0.76

2 60 40 0.95

3 60 60 1.09

4 60 80 1.20

5 60 100 1.29

6 60 120 1.38

7 450 140 1.45

8 30 0 0

9 Until failure 140 1.45

2.3.3 Friction Mapping Protocol

Mapping experiments supplement the LoL data by showing a map of frictional 

behavior over a wide range of entrainment velocities and slide-to-roll ratios.8 This 

procedure provides data on friction across the gear tooth mesh, the operational 

speeds for different friction regimes (mixed, elastohydrodynamic), and the effects 

of running-in wear on the surface. The ball and disc velocity vectors are set to 0° 

(skew = 0°) with a 144-N load (1.46 GPa), and a disc temperature of 126 °C with 

an oil temperature of 80 °C. The slip percentage is ramped linearly from 0.02% to 

102% over 90 s at an entrainment velocity of 8 m/s, held for 10 s at 8 m/s and 102% 

slip, and ramped back to 0.02% at 8 m/s. Once this ramp cycle is complete, the 

entrainment velocity is decreased 1 step, held for 10 s, and the ramp is repeated. This 

procedure continues for all entrainment velocity values shown in Fig. 2. Once all of 

the entrainment velocity steps are complete, one map has been created. A standard 

mapping protocol runs 5 maps on the same track to compare the friction values across 

maps to determine the extent of running-in during operation.
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Fig. 2 Slip percentage and entrainment velocity for the mapping procedure

Time (Seconds)

500 1000 1500 2000 2500 3000 3500

Slip Percentage (%)

0

10

20

30

40

50

60

70

80

90

100

Time (Seconds)

0 500 1000 1500 2000 2500 3000 3500

Entrainment Velocity (m/s)

-1

0

1

2

3

4

5

6

7

8

9
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2.4 Test Matrices

Although 3 evaluation protocols are used, the main focus of the project is the LoL

performance. Five repetitions are typically devoted to LoL experiments for each set 

of surfaces. This is because of their tendency to have a relatively large variance 

among experimental groupings due to inherent variability. For the high-speed 

parameters, only LoL performance was evaluated. The high-speed test matrix, 

shown in Table 7, allows for a comparison within different subsets of experimental 

variables through 2 oils, grinding lays, and superfinishing techniques. The black 

oxide was tested 3 times due to a limited materials supply and MMP with A555 

was tested 7 times to gain additional confidence in the dramatic results. Three types 

of evaluations were done at low speed: 1) LC, 2) LoL, and 3) friction mapping as 

shown in Table 8. One repetition was devoted to each of the LC and friction 

mapping experiments as secondary evaluations and inherent repeatability. 

Table 7 HS LoL experiment matrix

Surface Lubricant Repetitions

Circumferential as-ground MJII 5 A555

Radial as-ground MJII 5 A555

Black oxide MJII 5

A555 3

ISF #1, 3 MJII 5 A555

MMP MJII 5

A555 7

Table 8 LS experiment matrix

Surface Test Repetitions

Radial as-ground

Load capacity 1

LS LoL 5

Friction mapping 1

Black oxide

Load capacity 1

LS LoL 5

Friction mapping 1

ISF #2

Load capacity 1

LS LoL 5

Friction mapping 1
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Color microscope images were taken of the low-speed experiments on each disc 

after evaluation using a Zeiss LSM700 confocal microscope. Surface profiles of 2

regions were taken: the unaffected disc area and an area within the wear tracks left 

by the mapping protocol. These images were taken using a 50× lens with a 

numerical aperture of 0.95. The surface roughness, Sa, was calculated according to 

International Organization for Standardization (ISO) 25178 and was taken as the 

average of 3 separate 128- × 128-μm surface profiles.9

3. Results and Discussion

The results for the test matrix are presented by test protocol. Discussion follows each 

set of results for the experimental protocols to compare among surface finishes. 

3.1 Load Capacity

The experimental results for the LC protocol are given as the load stage where 

failure is initiated. Failure is indicated by the traction coefficient increasing rapidly 

above 0.15 as scuffing initiates. If failure is not observed during the experiment, it 

is described as having “run out”. Each LC experiment was run with 9310 steel and 

MJII oil at the conditions given in Table 2.

Most LC experiments operated successfully through the maximum load stage of 

622 N (2.38 GPa), as shown in Table 9. The black oxide failed one stage before 

reaching the maximum load stage, 566 N (2.31 GPa), for the first repetition and it 

completed all load stages for the second repetition. This result is consistent with the 

authors’ previous experience, as it is not uncommon for the failures to be observed 

at bordering load stages as additional experiments are conducted. We can conclude 

that the as-ground, ISF, and black oxide surfaces are capable of a load carrying 

capacity of 140 N (1.45 GPa), which is used for the steady state condition in the LS 

LoL protocol. Additionally, much higher loads can be sustained for all surfaces 

under these velocity and temperature conditions with MJII up to at least 566 N 

(2.31 GPa).

Table 9 Load capacity results

Experiment no. Surface finish Failure load stage

(Stage #)

Failure load 

(N)

655 As-ground: radial Run-out N/A

712 As-ground: circumferential Run-out N/A

671 ISF Run-out N/A

675 Black oxide 29 566

707 Black oxide Run-out N/A
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3.2 Loss of Lubrication

The experimental results for the LoL protocol are given in the form of elapsed time 

from the oil supply shutoff until failure, called “time to failure”. Failure is defined 

by a rapid increase in the traction coefficient, which indicates scuffing initiation. 

An example HS LoL experiment, which is also applicable to LS LoL experiments, 

is shown in Fig. 3 with a zoomed-in view on the LoL portion of the experiment in 

Fig. 4. The sudden rise in the traction coefficient toward the beginning of the 

experiment corresponds with the ball entering contact with the disc and beginning 

the 10-min run-in period. The traction coefficient stabilizes a few seconds after 

contact and remains relatively constant throughout run-in. After the 10-min run-in 

period, the lubrication supply is shutoff and the time to failure begins. The traction 

coefficient steadily rises until a sharp increase indicates scuffing initiation. The 

experiment in Fig. 4 lasted 59.7 s before failure. 

Fig. 3 Typical time trace of traction coefficient vs. time during an HS LoL experiment
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Fig. 4 Example HS LoL raw data plot focused on the loss of lubrication portion of the 

experiment

3.2.1 High Speed

The HS LoL protocol experiments summarized in Table 7 were evaluated and the 

results are shown in Figs. 5 and 6 with standard error bars. While the results for 

MJII and A555 are given in the same figures, the discussions are separated to focus 

on the differences caused by the 9310 steel surfaces. The observations and analysis 

for the LoL results require data concerning the surface roughness, which are taken 

from the microscope analysis section. 
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Fig. 5 HS LoL times to failure

Fig. 6 HS LoL times to failure with a truncated Y-axis

0

200

400

600

800

1000

1200

Circumferential

As-Ground

Radial

As-Ground

Black Oxide

ISF

MMP

Circumferential

As-Ground

Radial

As-Ground

Black Oxide

ISF

MMP

MJII A555

Time to Failure (s)

Surface Finish and Lubricant

High Speed Loss of Lubrication with 9310

0

50

100

150

200

250

Circumferential

As-Ground

Radial

As-Ground

Black Oxide

ISF

MMP

Circumferential

As-Ground

Radial

As-Ground

Black Oxide

ISF

MMP

MJII A555

Time to Failure (s)

Surface Finish and Lubricant

High Speed Loss of Lubrication with 9310
Approved for public release; distribution is unlimited.

14

3.2.2 Mobil Jet II

The first major observation about the MJII data is the circumferentially as-ground 

surface could not reliably run-in without a scuffing failure. Because the LoL

protocol could not reach the end of the run-in period without a scuffing failure, the 

circumferentially as-ground surface does not have time-to-failure data. In addition, 

1 of the 5 tracks on the radially as-ground disc also did not survive run-in without 

scuffing. The second major observation is that the average time to failure for the 

superfinished sample was 3 times longer than the black oxide surface. This is 

because the smoother surface contributes substantially fewer asperity contacts to 

initiate scuffing within the residual fluid film lubrication contact. The third main 

observation is that the ISF #1 and MMP surfaces produced very similar times to 

failure despite the mixed lubrication regime for ISF #1 and the full film lubrication 

regime for MMP during run-in. The ISF #1 surface had an Sa roughness value of 

52 nm compared to the MMP value of 24 nm. The surface roughness is important 

when considering the contact lubrication regime, as suggested by the lambda ratio. 

The lambda ratio was calculated with the minimum film thickness for the HS LoL

parameters, using the Hamrock Dowson equation, divided by the composite surface 

roughness. Because the ball roughness was not measured, the composite surface 

roughness is assumed to be equal to the Sa values for each disc measurement on the 

pristine surface, 24 nm and 52 nm for the MMP and ISF #1 surfaces, respectively. 

The resulting lambda ratios are 4.5 and 2.1 for the MMP and ISF #1 surfaces, 

respectively. Typically, the mixed lubrication regime is estimated to start when the 

lambda ratio has a value below 3. A lambda value higher than 3 indicates a full film 

lubricated contact. The similar times to failure for MMP and ISF #1 indicate the 

difference in surface roughness between the 2 superfinished contacts does not have 

a large influence on the time to failure for MJII.

3.2.3 AeroShell 555

The first major observation about the A555 data in Figs. 5 and 6 is that the MMP 

surface lubricated with A555 has a significantly longer time to failure than all other 

combinations of surfaces and lubricants. This could be because of a combination of 

a smoother surface with an Sa roughness of 24 nm for the disc, compared to 36 nm 

for the ISF #3 disc, and additives used in the A555 lubricant formulation to pass 

the DOD-85734 specification. The lambda ratio was found to be 3.0 for the ISF #3 

contact and 4.5 for the MMP contact. Without further study, we can currently only 

speculate that the slightly higher lambda ratio, which lies securely within the full 

film lubrication regime during run-in, may have been enough to allow the A555 

extreme pressure additives to perform toward their full potential. The borderline 

mixed lubrication regime during run-in for the ISF #3 surface may have not allowed 
Approved for public release; distribution is unlimited.

15

the extreme pressure additives to form the chemically induced protective barrier on 

the surface needed to produce a relatively long time to failure. Alternatively,

perhaps there is a deeper chemical interaction between the steel surface and the 

lubricant that is being promoted by the MMP or hindered by the ISF process. These 

hypotheses are speculative and would need further experimental exploration to 

prove or disprove. The second major observation is a negligible difference between 

the circumferentially as-ground and radially as-ground times to failure. This is 

likely because of similar surface roughnesses that all cause a mixed lubrication 

regime during run-in. For example, a composite surface roughness, Sa, of 150 nm, 

similar to the radially ground disc, results in a lambda value of 0.73, which is well 

below the general guideline of 3 for a full film contact. The third observation is that 

black oxide lasts on average longer than pristine as-ground surfaces. This result is 

interesting in light of the increase in surface roughness during the black oxide 

process.

3.2.4 Low Speed

While the experimental results for the HS LoL experiments are presented within 

the body of this report, the raw data for the LS LoL experiments are included in the 

Appendix because of its underdeveloped protocol. The Appendix shows that the LS 

LoL protocol traction coefficient behavior varies between different surface 

treatments. The ISF traction coefficient generally increases to failure once 

lubrication has been removed, while the black oxide and as-ground surfaces 

increase and decrease in traction coefficient. This traction coefficient behavior may 

indicate the activation of lubricant chemistry when increasing asperity contact 

occurs as the residual oil film thickness diminishes over time. The LS LoL protocol 

is not fully developed and provided indications that the results may be biased in 

terms of track diameter. The HS LoL time-to-failure protocol is quite mature and 

has been compared favorably to more complex gear evaluations, leading us to have 

confidence in its ability to distinguish scuffing resistance performance in a loss of 

lubrication condition.

3.3 Friction Mapping

Friction maps provide a measurement of friction that can be expected across the 

gear mesh for different operational speeds throughout the mixed to 

elastohydrodynamic lubrication regimes. Differences in subsequent friction maps 

can be used to characterize the running-in process. The friction map data have been 

recorded for both increasing and decreasing slip values. We will be using the 

portion of the data when the slip percentage is increasing to be consistent and avoid 

possible hysteresis caused by additional run-in. This is why each plot is labeled 
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16

with the test number followed by “Increase.” The first black oxide map is shown as 

an example in Fig. 7, with the traction coefficient plotted against the entrainment 

velocity and slip percentage. To view traction values for specific parameters and 

decipher small changes in the traction coefficient, a contour map is used, such as 

Fig. 8. In general, entrainment velocity correlates with rotational velocity of a gear 

pair, while slip percentage correlates with position in the gear mesh along the 

profile of the gear tooth. The pitch point in a gear contact correlates to a slip of 0%. 

These maps correspond to spur gear contacts since the relative surface velocities 

are all parallel.

8

Fig. 7 Example friction map, black oxide (first run)

100

80

60

Slip (%)

40

Test #0682 Increase

20

0

Entrainment velocity (m/s)

8

6

4

2

0.04

0.1

0.08

0.06

0.02

0

0

Traction coefficient
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17

Fig. 8 Contour map of friction data for black oxide (first run)

The traction coefficient contour plots of the final, fifth, map for each surface are 

shown in Figs. 9–11. The traction coefficient in all 3 cases displays typical features: 

a rapid increase at low slip across all entrainment velocities, a large relatively flat 

area throughout most entrainment velocities indicating elastohydrodynamic 

lubrication, and an increase at low entrainment velocities across nearly all slip 

values, indicating the mixed lubrication regime where asperity contact occurs. 

These absolute traction coefficients show the ISF #2 surface has the lowest traction 

coefficient at most entrainment velocities in the elastohydrodynamic lubrication 

regime. However, the black oxide surface has the lowest traction coefficient at low 

entrainment velocity in the mixed lubrication regime. The final traction coefficients 

at low entrainment velocities seem to correlate to the roughness measured on the 

final experiment tracks, as shown in the microscope analysis section, since the 

black oxide is smoothest after running-in. This is because, at lower entrainment 

velocities, the roughness has a larger effect on the traction coefficient due to 

operation firmly in the mixed lubrication regime. The smaller differences in traction 

coefficient at higher entrainment velocities among the different surfaces is 

presumably due to reduction of asperity contact during operation in or closer to the 

elastohydrodynamic lubrication regime in which the lubricant film more fully 

separates the surfaces.

0.01 0.020.030.04 0.06 0.03 0.04 0.05 0.02 0.02 0.03 0.03 0.04 0.05

0.06 0.06

0.06

0.06

0.06

0.07

0.07

0.07

0.08

0.08

Test #0682 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%)

10

20

30

40

50

60

70

80

90
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18

Fig. 9 Final contour map for black oxide (final run)

Fig. 10 Final contour map for ISF

0.01

0.02 0.02 0.03 0.03 0.04 0.05 0.01 0.02 0.03 0.04

0.05

0.05

0.05

0.06

0.06

0.06

0.07

Test #0686 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%) 10

20

30

40

50

60

70

80

90

0.01 0.02 0.01 0.03

0.02

0.03

0.03

0.04

0.04

0.04

0.05

0.05

0.06 0.06

0.07 0.07

Test #0706 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%) 10

20

30

40

50

60

70

80

90
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19

Fig. 11 Final contour map for radially as-ground

The analysis of the running-in process using the traction coefficient is best 

evaluated by subtracting the starting contour map from the final contour map. The 

difference in the first and fifth contour maps is shown in Fig. 12. The subtraction 

of contour maps gives a good perspective of how much change in traction 

coefficient occurs during a running-in process. Figures 12–14 are the subtraction of 

the first and last contour maps for the black oxide, ISF #2, and as-ground surfaces,

respectively. The black oxide and as-ground maps are very similar with large 

amounts of wear occurring during the first few cycles of the mapping procedure at 

the initial contact condition of Ue = 8 m/s and where mixed lubrication is more 

prominent at low entrainment velocities. These maps show the black oxide and asground surfaces change significantly during running-in at about the same set of 

entrainment velocities and slip percentages, although the as-ground sample has a 

greater change at low entrainment velocities. This greater change can indicate either 

a greater change overall for the as-ground specimen or a more rapid and earlier 

change in friction that occurred during the first mapping procedure for the black 

oxide specimen before the low entrainment velocities were initially measured. For 

the ISF treatment, Fig. 13 shows there is little change in the traction coefficient 

from running-in, unsurprisingly as the surface was polished by ISF before the first 

test. 

0.01 0.04 0.02 0.03 0.02 0.03 0.04

0.04

0.04

0.05

0.05

0.05

0.05

0.06

0.06

0.06

0.07 0.07

0.08 0.08

0.09

0.08

Test #0696 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%) 10

20

30

40

50

60

70

80

90
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20

Fig. 12 Subtraction for the first and last black oxide maps

Fig. 13 Subtraction of contour maps for the first and last ISF #2 maps

0 0.002 0.004 -0.002 -0.004 0.0020.004 0 0.004

0.004

0.004

0.006 0.006 0.006

0.006

0.006

0.006

0.006

0.006

0.006

0.006

0.006

0.008

0.008

0.008

0.008

0.008

0.008 0.008

0.008

0.008

0.01

0.01

0.01 0.01

0.01

0.012 0.012

0.012

0.012

0.014

0.014

0.014

0.016

0.016 0.018 0.02

0.022 0.024

Test #0682 - Test#0686 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%)

10

20

30

40

50

60

70

80

90

0 -0.002

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0.002

0.002

0.002

0.002

0.004

0.004

0.006

Test #0702 - Test#0706 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%)

10

20

30

40

50

60

70

80

90
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21

Fig. 14 Subtraction for the first and last maps as-ground radial

3.4 Microscope Analysis

The mapping disc tracks were analyzed with a Zeiss LSM700 confocal microscope, 

as described in the procedure section. Color images of the disc samples, showing 

example test tracks, can be seen in Figs. 15 and 16. The tracks in Fig. 15 are labeled 

with a protocol abbreviation for reference (LC=load capacity, OO=Oil Out/LowSpeed Loss of Lubrication). The black oxide disc differs from the radially ground 

disc in appearance by the dark purplish hue of the oxide layer. Figure 15 shows that 

the coating is somewhat uneven with a varying hue across the surface of the disc. 

To give a perspective of the disc tracks, Fig. 16 shows a picture of all 3 discs after 

experimentation. The coloration of the discs did not change over the course of 

experimentation, with the exception of a scar produced from the point contact at 

each track diameter. The 3 discs used for microscope analysis were the discs used 

for the mapping experiments because these mapping tracks were not marred by a 

scuffing failure and they were able to be measured for a meaningful roughness 

value. 

0.002 0 -0.0020 0.002

0.002

0.002

0.004

0.0040.004

0.004

0.004

0.004

0.004

0.004

0.004

0.0060.006 0.006

0.006

0.006

0.006

0.006

0.006

0.006

0.006

0.008 0.008

0.008

0.008

0.008

0.01

0.008

0.01

0.01

0.01

0.012

0.01

0.012

0.012

0.012

0.012

0.012

0.014

0.014

0.014

0.014

0.014

0.014

0.016 0.016

0.016

0.016

0.016

0.018 0.018

0.018

0.018

0.018

0.02

0.02

0.02 0.02

Test #0692 - Test#0696 Increase

Entrainment velocity (m/s)

1 2 3 4 5 6 7

Slip (%)

10

20

30

40

50

60

70

80

90
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22

 

Fig. 15 Color images of radially ground disc (left), black oxide disc (center), ISF #2 disc 

(right). The top of the image is the outside of the disc. Areas marked as invalid map were used 

for a mapping procedure but the protocol was not properly followed.
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23

Fig. 16 Images of the radially ground (left), ISF #2 (center) and black oxide (right) discs 

after experimentation

One group of observations that can be made from the optical images are related to 

the surface topography. The untested portions of the discs contain features 

characteristic to the individual finishing processes. The radially ground discs, with 

and without black oxide, have large grind marks aligned along the radius, whereas 

the ISF #2 disc has nearly isotropic surface features. These surface topographies 

are worn away within the track formed by the point contact during experiments.

The experiment tracks can be seen as bands running perpendicular to the radius of 

the disc with varying affects to the topography. The tracks can be divided into 2 

categories: tests that ended due to scuffing and tests that did not. The LS LoL 

experiments ended due to scuffing along with one of the black oxide LC

experiments. The remaining LC experiments and all of the mapping experiments 

did not initiate a scuffing failure. The scuffed tracks are all very dark with large 

scratches and crater-like features due to the localized tearing. The original surface 

features are not recognizable on the scuffed tracks. The tracks without scuffing tend 

to have a slight browning of the surface. The surface also tends to have a much 

brighter and more polished appearance for the radially ground discs, with and 

without black oxide. There is a very subtle difference between the track and 

unaffected surface for the nonscuffed ISF #2 experiments. Features of the original 

finish can be seen within the ISF #2 track along with wear created during the 

experiment. The direction of scuffing can be seen at an angle for the oil out and LC

tests due to the skew parameter. Scratches form circumferentially on the mapping 

tracks because the experiments were run with parallel surface velocities. Scuff 

marks were produced on the ISF #2 ball and disc similar to a dotted line along the 

tracks with a uniform spacing because of skipping between the ball and disc likely 

due to small vibrations in the tribometer. These are generally only observed on the 

smoothest of samples. Finally, the LC experiments produced wider tracks due to 

larger applied loads. 
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24

An important characteristic of the surface features is the roughness before and after 

experimentation. Table 10 and Fig. 17 show the arithmetic average of 3-D 

roughness, Sa, of the disc on the unaffected surface and the final surface roughness 

after the mapping experiment from the center of the tracks. In addition, the 

roughness values for each of the 3 ISF discs, as explained in the materials section, 

are given for reference across the ISF superfinished surfaces. The roughness of the 

unaffected ISF #2 surface is roughly half of the pristine radially ground and black 

oxide discs. The black oxide disc has a larger surface roughness than the radially 

ground disc due to the processing during coating. After the mapping experiments, 

the radially ground disc shows very little change in roughness, only decreasing 9% 

from Sa = 129 nm to Sa =117 nm. The black oxide disc had a much larger decrease 

in roughness, 62%, resulting in a final roughness lower than both the radially 

ground disc and the ISF #2 disc. The ISF #2 disc roughness actually increased to a 

value greater than the final black oxide roughness during the mapping procedure, 

although traction coefficient decreased.

Table 10 Roughness of mapping tracks

Disc Grinding lay Process notes Test Sa

(nm)

Error 

(nm)

Sa

(μin)

Error 

(μin)

Black oxide Radial Boeing 

HP4-50

None/pristine 190 7 4.83 0.18

Mapping 88 7 2.24 0.18

Superfinished Circumferential

ISF #1 

(Aerospace) None/pristine 52 6 1.32 0.15

ISF #2 

(Automotive)

None/pristine 76 10 1.93 0.25

Mapping 94 6 2.39 0.15

ISF #3 

(Aerospace) None/pristine 36 2 0.91 0.05

MMP None/pristine 24 N/A 0.61 N/A

As-ground

Radial None None/pristine 148 10 3.76 0.25

Mapping 115 9 2.92 0.23

Circumferential None None/pristine 175 16 6.89 0.63
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25

Fig. 17 Roughness of mapping tracks

4. Summary and Conclusions

Black oxide, ISF, and as-ground surfaces were evaluated with LC, LoL, and 

mapping protocols. 

• The ISF and as-ground surfaces passed the LC test at low-speed conditions 

during their single experiment. The black oxide surface failed at stage 29 of 

30 and was run a second time, which passed through all 30-load stages. This 

indicates that the 3 surfaces all have a LC of 2.3 GPa or higher under these 

contact speeds. 

• The HS LoL experiments showed superfinishing techniques, such as ISF 

and MMP applied to 9310 gear steel, improve the time to failure across both 

MJII and A555 oils. The superfinished surfaces had very similar times to 

failure for MJII and very different times to failure for A555, although both 

were substantially improved over the as-ground and black oxide surfaces. It 

was also found that black oxide might last a bit longer in loss of lubrication

than the pristine rough as-ground surfaces. This result is interesting in light 

of the slight increase in surface roughness during the black oxide process. 

• The mapping protocols show a large change in traction coefficient from the 

first map to the final mapping repetition for the as-ground surfaces and the 

black oxide–treated surfaces in excess of 0.02 for some parameters. The ISF 

surface did not have a large change in traction coefficient during the 

mapping tests due to their prepolished surfaces. The final mapping contour 

plot shows the absolute value of the traction coefficient, which appears to 

0

50

100

150

200

250

None /

Pristine

Mapping None /

Pristine

Mapping None/

Pristine

None /

Pristine

Mapping None/

Pristine

None/

Pristine

Radial Black Oxide ISF #1 ISF #2 ISF #3 MMP

Roughness, Sa (nm)

9310 Surface Treatment and Experiment Track 
Approved for public release; distribution is unlimited.

26

show a correlation at low entrainment velocity between the traction 

coefficient and the surface roughness such that the traction coefficient is 

lower for surfaces that have a lower surface roughness after run-in.

Overall, the measurements demonstrate that superfinished specimens have the 

longest times to failure until the onset of scuffing at contact conditions representing 

the most critical high sliding portion of a gear mesh during a loss of lubrication

event. The black oxide showed a slight improvement in loss of lubrication

performance over its base radially ground counterpart, despite creating a rougher 

surface. This indicates the black oxide treatment may have a chemical effect that 

could also improve the survivability of a superfinished surface. We recommend 

evaluating a superfinished surface that has also been treated with black oxide to see 

if a cumulative effect can be achieved.
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27

5. References

1. ADS-50-PRF. Rotorcraft propulsion performance and qualification 

requirements and guidelines. St. Louis (MO): Army Aviation and Troop 

Command (US), 1996 Apr 15.

2. Davis J, editor. Gear materials, properties, and manufacture. Materials Park 

(OH): ASM International; 2005. Chapter 2, Gear tribology and lubrication; p. 

19–38.

3. Transportation Safety Board of Canada. Main gearbox malfunction/collision 

with water: Cougar Helicopters Inc., Sikorsky S-92A, C-GZCH, St. John’s, 

Newfoundland and Labrador, 35 nm E, 12 March 2009. Gatineau (QC); 2010 

Dec 29. Report No.: A09A0016.

4. Riggs M, Berkebile S, Hood A, Dykas B. Simulating army-relevant spur gear 

contacts with a ball-on-disc tribometer. Aberdeen Proving Ground (MD): 

Army Research Laboratory (US); 2015 Sep. Report No.: ARL-TR-7492. 

5. MIL-PRF-23699G. Lubricating oil, aircraft turbine engine, synthetic base, 

NATO code numbers: O-152, O-154, O-156, and O-167. Lakehurst (NJ): 

Naval Air Warfare Center Aircraft Division (US); 2014 Mar 13.

6. DOD-PRF-85734A. Lubricating oil, helicopter transmission system, synthetic 

base. Patuxent River (MD): Naval Air Systems Command (US); 2004 June 29. 

7. Riggs M, Berkebile S, Murthy N. Ball-on-disc tribometer's protocol 

development: loss of lubrication evaluation. Aberdeen Proving Ground (MD): 

Army Research Laboratory (US); 2016 Feb. Report No.: ARL-TR-7588. 

8. Berkebile S, Murthy N. Friction mapping as a tool for measuring the 

elastohydrodynamic contact running-in process. Aberdeen Proving Ground 

(MD): Army Research Laboratory (US); 2015 Oct. Report No.: ARL-TR7501. 

9. ISO 25178 general product specification (GPS) – surface texture: areal – part 

70: material measures. Geneva (Switzerland): International Organization for 

Standardization; 2014. 
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28

INTENTIONALLY LEFT BLANK.
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29

Appendix. Low-Speed Loss of Lubrication Raw Data
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30

Fig. A-1 Black oxide low-speed loss of lubrication (LS LoL) experimental results

Fig. A-2 Isotropic superfinishing (ISF) LS LoL experimental results

Time (sec)

1000 1200 1400 1600 1800 2000 2200

Traction Coefficient

0.035

0.04

0.045

0.05

0.055

0.06

0.065

0.07

0.075

data # =0676

data # =0678

data # =0679

data # =0680

data # =0681

Time (sec)

1000 1100 1200 1300 1400 1500

Traction Coefficient

0.03

0.035

0.04

0.045

0.05

0.055

0.06

0.065

0.07

data # =0666

data # =0667

data # =0668

data # =0669

data # =0670
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31

Fig. A-3 As-ground radial LS LoL experimental results

Fig. A-4 As-ground circumferential LS LoL experimental results

Time (sec)

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Traction Coefficient

0.04

0.045

0.05

0.055

0.06

0.065

0.07

data # =0661

data # =0708

data # =0709

data # =0710

data # =0711

Time (sec)

1000 1100 1200 1300 1400 1500 1600

Traction Coefficient

0.04

0.045

0.05

0.055

0.06

0.065

0.07

0.075

0.08

data # =0662

data # =0663

data # =0665

data # =0713

data # =0714
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32

Table A-1 LS LoL experimental results

Experiment no. Surface finish Time to failure 

(s)

Average time 

to failure 

(s)

Error 

(±)

661

As-ground- radial

934

731 164

708 618

709 932

710 598

711 576

662

As-groundcircumferential

440

400 136

663 174

665 356

713 435

714 594

666

ISF

563

480 67

667 523

668 487

669 461

670 365

676

Black oxide

1158

853 214

678 1019

679 827

680 659

681 599
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33

List of Symbols, Abbreviations, and Acronyms

3-D 3-dimensional

A555 AeroShell 555

ADS Aeronautical Design Standard

AISI American Iron and Steel Institute

HS high speed

ISF isotropic superfinishing

ISO International Organization for Standardization

LC load capacity

LoL loss of lubrication

LS low speed

MJII Mobil Jet II

MMP micro machining process

Sa average surface roughness

Tb ball thermocouple

Td disc thermocouple

Ub ball linear velocity

Ud disc linear velocity

Ue entrainment velocity
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34

1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL CIO L

IMAL HRA MAIL & RECORDS

MGMT

1 GOVT PRINTG OFC

(PDF) A MALHOTRA

3 DIR USARL

(PDF) RDRL VTP

M RIGGS

N MURTHY

S BERKEBILE