Christopher DellaCorte and Fransua Thomas

Glenn Research Center, Cleveland, Ohio

Olivia Ann Leak

North Carolina Agricultural and Technical State University, Greensboro, North Carolina

Tribological Evaluation of Candidate Gear 

Materials Operating Under Light Loads in

Highly Humid Conditions

NASA/TM—2015-218896

November 2015
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Christopher DellaCorte and Fransua Thomas

Glenn Research Center, Cleveland, Ohio

Olivia Ann Leak

North Carolina Agricultural and Technical State University, Greensboro, North Carolina

Tribological Evaluation of Candidate Gear 

Materials Operating Under Light Loads in

Highly Humid Conditions

NASA/TM—2015-218896

November 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

70th Annual Meeting and Exhibition

sponsored by the Society of Tribologists and Lubrication Engineers

Dallas, Texas, May 17–21, 2015
Acknowledgments

The authors wish to thank the ISS management (Penni Dalton, project manger) for their continuing support of this work. 

Thanks are also due to Mr. Walter A. Wozniak for his creative and capable specimen manufacture and to Karin Cline of 

Hohman Plating and Manufacturing Corporation for formulating and providing the DFL coatings.

Available from

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 

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subject to revision as analysis proceeds.

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NASA/TM—2015-218896 1 

Tribological Evaluation of Candidate Gear Materials Operating 

Under Light Loads in Highly Humid Conditions 

Christopher DellaCorte and Fransua Thomas 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Olivia Ann Leak 

North Carolina Agricultural and Technical State University 

Greensboro, North Carolina 27411 

Abstract 

A series of pin-on-disk sliding wear tests were undertaken to identify candidate materials for a pair of 

lightly loaded timing gears operating under highly humid conditions. The target application involves 

water purification and thus precludes the use of oil, grease and potentially toxic solid lubricants. The 

baseline sliding pair is austenitic stainless steel operating against a carbon filled polyimide. The test load 

and sliding speed (4.9 N, 2.7 m/s) were chosen to represent average contact conditions of the meshing 

gear teeth. In addition to the baseline materials, the hard superelastic NiTiNOL 60 (60NiTi) was slid 

against itself, against the baseline polyimide, and against 60NiTi onto which a commercially deposited 

dry film lubricant (DFL) was applied. The alternate materials were evaluated as potential replacements to 

achieve a longer wear life and improved dimensional stability for the timing gear application. An attempt 

was also made to provide solid lubrication to self-mated 60NiTi by rubbing the polyimide against the disk 

wear track outside the primary 60NiTi-60NiTi contact, a method named stick or transfer-film lubrication. 

The selected test conditions gave repeatable friction and wear data and smooth sliding surfaces for the 

baseline materials similar to those in the target application. Friction and wear for self-mated stainless steel 

were high and erratic. Self-mated 60NiTi gave acceptably low friction (~0.2) and modest wear but the 

sliding surfaces were rough and potentially unsuitable for the gear application. Tests in which 60NiTi pins 

were slid against DFL coated 60NiTi and DFL coated stainless steel gave low friction and long wear life. 

The use of stick lubrication via the secondary polyimide pin provided effective transfer film lubrication to 

self-mated 60NiTi tribological specimens. Using this approach, friction levels were equal or lower than 

the baseline polyimide-stainless combination and wear was higher but within data scatter observed in 

these preliminary tests. Based upon these results, self-mated 60NiTi gear teeth utilizing solid lubrication, 

is a reasonable approach for the target application. 

Introduction 

Polymer gear materials are often used for mechanisms that operate under light loads and low speeds 

because they are inexpensive to fabricate, are lightweight and do not rust. They are also self-lubricating 

and maintenance-free. Due to the intrinsically lower strength and wear resistance properties of polymers, 

compared to metals, they are rarely used for power transmission at high power levels. These attributes 

make them well suited to low cycle applications like office machines, automotive accessory applications 

(such as window lifts) and small appliances (Ref. 1). 

One application well suited to the use of polymer gears is a vacuum pump on board the International 

Space Station (ISS) inside the wastewater processing system (Refs. 2 to 4). In this application, two mating 

spur gears drive separate lobes of a roots type blower that is used to reduce the ambient pressure inside a 

rotary distillation unit. By design, the gear set is exposed directly to the moist, warm process stream. To 

avoid contamination of the process fluid, no oils, greases or other toxic lubricants can be used to mitigate 

friction and wear of the meshing gear teeth. Figure 1 shows a cross section of the rotary distillation 

assembly and the location of the gears. 
NASA/TM—2015-218896 2 

In this application, the gears ensure accurate timing of the pump lobes to prevent lobe-to-lobe contact. 

As such, excessive wear of the gear mesh, as opposed to frictional power losses, is a primary 

consideration. Along with tooth wear, dimensional accuracy and precision is also a factor in maintaining 

proper timing of the pump lobes. Polymers tend to absorb moisture and have relatively high thermal 

expansion coefficients and these characteristics result in dimensional changes challenging timing 

accuracy. To minimize this effect while providing adequate self-lubrication, a mixed material, duplex 

gear approach that utilizes one stainless steel gear meshing with one polymer gear is employed. In this 

case, the stainless steel gear provides optimum dimensional stability while the mating polymer gear 

provides intrinsic self-lubrication to the meshing teeth. 

The use of the duplex gear mesh approach has been successful overall but leaves opportunities for 

improvement on several fronts. Net-shape injection molding is often the preferred approach to achieving 

precision polymer gears. With this fabrication technique a precision steel mold is filled with uncured 

polymer resin that then hardens into the exact shape and size required. Unfortunately, such an approach is 

only practical for applications in which large numbers of gears are to be made. The ISS application is not 

high volume and this leads to the use of conventional machining of the polymer gear. On a per gear basis, 

such a labor-intensive manufacturing approach is costly. Further, the polymer gear must be initially 

undersized so that it can “swell” under normal operating temperature and humidity to the desired 

dimensions. Since installation and assembly is done at room temperature and low humidity conditions 

while the gear is undersized, achieving accurate gear timing in use is difficult. It would be desirable for 

this application if more dimensionally stable materials, like stainless steel, could be used for both gears 

while maintaining proper solid lubrication to achieve long life. 

 
NASA/TM—2015-218896 3 

Aside from using the duplex gear approach, another means for solid lubrication is the use of thin solid 

lubricant coating on the surfaces of metal gear teeth to reduce friction and wear. For applications that 

have light loads, low sliding speeds and moderate wear life requirements, solid film lubricants are a 

reasonable approach (Ref. 5). Solid lubricant films for gear teeth applications can also be deposited and 

replenished by dynamically operating a metal gear against a sacrificial polymer gear through intentional 

transfer film (sometimes aptly named stick) lubrication (Ref. 6). Determining whether these approaches 

are valid normally requires a full-scale gear test. Such tests can be expensive and time consuming and are 

at times better preceded by a bench test experiment. 

In this paper, the friction and wear behavior of candidate gear and solid lubricant materials for the ISS 

application are evaluated using a pin-on-disk friction and wear test rig (tribometer). Candidate test 

specimens and configurations include austenitic stainless steel, the baseline polyimide, the superelastic 

60NiTi and a graphite filled PTFE solid lubricant coating. The tests are intended to identify new 

approaches to satisfy the application requirements and improve the overall cost and performance of the 

gear system. 

Materials and Procedures 

A pin-on-disk test is used to evaluate the candidate gear materials. Figure 2 shows the test rig. In this 

test, a hemispherical tipped pin is loaded in pure sliding against the face of a rotating disk. Friction is 

continuously monitored during the test and wear is measured by removing the specimens after testing is 

complete. The wear process leads to a flat, round wear scar on the pin and a shallow wear groove or track 

on the disk. Given the relatively modest loads and speeds for the current tests, disk wear is too small to 

accurately quantify so sliding friction and pin wear alone are used to gage material performance. 

 
NASA/TM—2015-218896 4 

Hemispherical tipped pins are fabricated from the baseline polyimide, austenitic stainless steel and 

the superelastic 60NiTi. Disk specimens are made from austenitic stainless steel and 60NiTi. A graphite 

and PTFE filled solid lubricant coating is applied to selected disks or a sacrificial polymer pin is rubbed 

against the wear track 90° ahead of the pin-disk contact to achieve solid lubrication. 

To maximize the relevance of this bench test to the full-scale gears, a series of calibration steps are 

taken to ensure that the bench test wear mechanism mimics the application. First an analysis of the gear 

contact is conducted to determine approximate load and sliding speeds. After these conditions are 

estimated, a series of calibration sliding tests are done to determine which test conditions result in wear 

surfaces and wear mechanisms that best resemble the gears taken from field service. 

Since meshing gear teeth experience a varying degree of rolling-sliding contact at ever changing 

loads, the use of a pure sliding test like the pin-on-disk requires care. The contact to be simulated by the 

pin-on-disk test is the meshing teeth of two spur gears, approximately 150 mm in diameter, transmitting 

tens of watts of power at speed up to a few thousand revolutions per minute. 

When the teeth first contact they experience the most sliding but the lightest contact load. When fully 

engaged, they experience near perfect rolling (no sliding) but the highest load. During disengagement, 

both teeth experience diminishing load and an increasing proportion of sliding contact. Thus a reasonable 

approach for experimental simulation to identify a range of loads and sliding speeds that encompass those 

experienced by the gear teeth. Two sliding speeds, one low (1.35 m/s) and one high (2.7 m/s) are chosen 

based upon an established gear geometrical model of spur gears (Refs. 7 and 8). To establish a load range, 

the power transmitted (~100 W) is combined with the typical running speed (~2000 rpm) yielding the 

torque and the resulting tooth-to-tooth loading. Three test loads, one low (4.9 N), one medium (9.8 N) and 

one high (14.7 N) span the region of interest for the application. For estimating the maximum stress the 

load resulting from power transmission is divided by an estimated contact area. 

Using observations of the wear patterns of gears taken from service, the fully engaged tooth mesh 

area is approximately 32 mm2

. The product of these test conditions, also called the pressure-velocity (PV) 

value is a well-accepted parameter to judge the severity of operating conditions for polymer sliding. In 

our case, the P and V values yield a PV that ranges from ~3 to ~15 kg-m/cm2

-s. The product literature PV 

limits for the graphite filled polyimide tested in this study is over 100, thus the test conditions are well 

within the manufacturer’s load-speed design guidelines (Ref. 9). 

Based upon the estimates introduced above, an initial pin-on-disk test condition matrix of three loads 

(4.9, 9.8, and 14.7 N) and two sliding speeds (1.35 and 2.7 m/s) were selected for experimental evaluation. 

Preliminary sliding tests in which polymer pins sliding against austenitic stainless steel were conducted 

under each of these test conditions. Four repetitions were done for each test to assess consistency. The wear 

surfaces were examined to identify one or more pairs of conditions that gave sliding surfaces that appeared 

similar to the gently worn gears taken from service. The data is shown in Table I. 

The lowest load (4.9 N), yielded smooth wear surfaces and low friction at both the low and high 

sliding speeds. Inconsistent wear surfaces were observed for the tests at the medium load (9.8 N) and high 

friction and uniformly rough wear surfaces resulted from the highest test load (14.7 N). Based upon this 

data, the low load (4.9 N) and high-speed (2.7 m/s) test conditions were chosen for the remainder of the 

tribology tests. This load-speed pair rapidly produces measurable pin wear data while yielding 

consistently smooth running surfaces that mimic properly operating gear teeth. 

TABLE I.—TEST CONDITION EVALUATION-FRICTION AND WEAR SURFACE APPEARANCE 

[Air at 25 C, 50 to 60 percent relative humidity.]

Trial load, 

N 

Trial speed, 

m/s 

Friction range Surface appearance 

4.9 1.35 0.18 to 0.25 Smooth 

4.9 2.70 0.25 to 0.36 Smooth 

9.8 1.35 0.2 to 0.30 Rough to smooth 

9.8 2.70 0.5 to 0.6 Rough 

14.7 1.35 0.5 to 0.9 Rough 

14.7 2.70 0.3 to 0.5 Rough 
NASA/TM—2015-218896 5 

To conduct a materials evaluation, the selected pin and disk specimens are loaded into the tribometer; 

the motor is started and allowed to reach the test speed (1000 rpm, 2.7 m/s). The test load (0.50 kg, 4.9 N) 

is then applied. Four repeat tests of 1 hr each are run for each pin and disk specimen set. Pin wear after 

each one hour test is measured by removing the pin holder and measuring the flat, circular pin wear scar 

diameter that forms through the wear process on the pin tip using an optical microscope. The pin wear 

volume is calculated using simple geometrical relationships as described in the literature (Ref. 10). The 

surface appearance of the pin and the disk wear track are also noted after each test. The data is provided 

in the following results section. 

Two dry film lubricants (DFL), one PTFE-based and one graphite-based, were evaluated using a 

modified test protocol. In these DFL evaluations, the disk specimen is coated with the DFL and slid 

against a 60 NiTi pin. The DFL coatings are much softer than the pin thus pin wear is not expected. The 

tests are ended when the coating wears through to the substrate. Prior to the test, a flat, circular wear scar 

approximately 3 mm in diameter is created onto the tip of the pin by sliding it against an uncoated 60NiTi 

disk. This reduces the contact stress in the DFL to about 0.6 MPa (100 psi) and more closely simulates the 

lightly loaded gear application. Testing then proceeds with the pre-worn pin riding against the DFL 

coated disk until the coating is worn through to the metal substrate. In these DFL tests, sliding distance to 

coating wear-out, rather than the pin wear factor, is the experimental result. 

Results and Discussion 

Table II gives the friction and wear summary for the selected tribomaterials candidates. An inspection 

of the data reveals some interesting observations. 

The friction coefficient levels for the polymer pins sliding against stainless steel and 60NiTi disks 

were fairly consistent and relatively high (~0.3) considering that the pins are made from solid lubricant 

(graphite) containing self-lubricating plastic. Generally, one expects friction levels to be below 0.2 for 

tribomaterials marketed as solid lubricants (Ref. 9). However, the polyimides are well known to exhibit 

increased friction in the presence of atmospheric moisture. Both the application and these tests are under 

relatively humid conditions. In this context, the observed friction levels are not surprising. Further, 

because the loading between gear teeth is low, such friction coefficients are not a hindrance to good gear 

operation. The more important consideration is the running surface appearance after testing. For all of the 

polymer pin-metal disk tests, the wear surfaces are smooth. This behavior rightly indicates that the 

polymer gear-metal gear design approach is valid. 

Another interesting and perhaps unexpected observation is the low friction coefficient observed for 

the case of self-mated 60NiTi. In this instance the friction was only 0.18. Literature friction values for dry 

sliding of 60NiTi disks against itself, alumina (sapphire) and 440C stainless give much higher friction 

coefficients ranging from 0.4 to over 1.0 (Refs. 11 to 13). Delving into this data more deeply, however, 

reveals that these high friction levels are normally associated with low sliding speeds, on the order of 

mm/s whereas our present tests are conducted at speeds greater by an order of magnitude or more. 

TABLE II.—FRICTION AND PIN WEAR DATA SUMMARY 

[Test conditions: 4.9 N load, 2.7 m/s sliding speed, air at 25 C.] 

Pin material Disk material/surface coating Friction 

coefficient 

Pin wear factor, 

mm3/N-m 

Surface 

appearance 

SP21 Polyimide 316L SS 0.290.07 1.90.710–6 Smooth 

SP21 Polyimide 304 SS 0.340.08 0.70.210–6 Smooth

SP21 Polyimide 60NiTi 0.280.04 2.11.510–6 Smooth

60NiTi 60NiTi 0.180.03 8.33.210–6 Rough 

60NiTi + SP21 60NiTi 0.150.03 3.11.910–6 Smooth a

60NiTi PTFE DFL 0.150.02 b184 to 348 km Smooth a

60NiTi Graphite DFL 0.170.02 b24 to 135 km Smooth a

Tests initiated with pre-worn pin (~3 mm dia. Wear scar). bTests terminated when DFL wore through to substrate. No additional pin wear was observed.
NASA/TM—2015-218896 6 

To verify a speed effect, several short duration tests were run for self-mated 60NiTi from 0.27 m/s 

(100 rpm) to 5.4 m/s (2000 rpm). The friction coefficient dropped continuously from 0.45 to 0.20, 

respectively. At the highest test speeds vibration of the pin was observed. It is possible that simple rig 

dynamic loading-unloading effects caused by the rough surfaces resulted in lower average friction, in a 

skip-slip fashion. It is also possible, but not confirmed, that surface friction heating effects play a role. 

60NiTi has a relatively low thermal conductivity, around 10 W/m-°K. It is only about half that of 

austenitic stainless steel and silicon nitride. For this reason, it is possible that during sliding the pin and 

disk wear surfaces are heating significantly resulting in the formation of lubricious surface oxides. Others 

have suggested that such oxides, especially of titanium, can have a beneficial friction reduction effect 

(Ref. 14). Notwithstanding the observed low friction, the self-mated and lubricated 60NiTi tests resulted 

in rough wear surfaces and relatively high pin wear. Based upon this alone, using a set of 60NiTi gears 

without adequate lubrication to mitigate wear is not a sound approach. 

When solid lubrication of self-mated 60NiTi specimens is introduced via transfer film (stick) 

lubrication, the observed friction coefficient is reduced to 0.15 and wear is comparable to the baseline 

polymer-stainless tribopair. In this case, a second sliding contact consisting of a polymer pin pressed 

against the disk just ahead of the 60NiTi-60NiTi sliding contact effectively reduced pin wear, resulted in 

smooth surfaces and low friction. In effect, wear of the polymer pin provided a means to replenish solid 

lubricant to the 60NiTi disk surface. This approach has been demonstrated for metal gears destined for 

space use (Ref. 6). 

In a research program from the middle of the last century, engineers were faced with providing a 

means to replenish solid lubrication to gears and bearings operating in a space vacuum environment 

where contamination by oils and greases were a key concern. In the test set-up described by Bowen, an 

idler gear made from polymer composite (PTFE and WSe2) loaded against the metal gears in need of 

lubrication. The arrangement is shown in Figure 3. 

 
NASA/TM—2015-218896 7 

No other lubrication, such as the use of a solid lubricant coating, on the gear teeth was used. Their 

results showed adequate lubrication at light to moderate loads but gear seizure at the highest test loads. 

Since the intended gear application currently under consideration is lightly loaded it is reasonable to view 

the idler gear approach for lubrication in a favorable light. Another approach for lubrication is the use of a 

purposefully formulated and applied solid lubricant coating. 

The use of dry film lubricants (DFL) for gears and space mechanisms is well established (Refs. 5 and 

15). Soft metal films like lead and silver are often used for limited life applications and low loads. MoS2

and graphite based coatings can be effective for highly stressed applications and are particularly well 

suited for vacuum and cryogenic operation because of their stability. For many dry running applications, 

DFL’s are formulated from mixtures of solid lubricant pigments and tough, wear resistant binders. In a 

sense, many DFL’s resemble thin layers of the graphite filled polymer currently used in the ISS 

application for one of the gear set. In this work, the DFLs tested were carefully formulated to avoid the 

use of toxic constituents and to be insensitive to the presence of moisture at temperatures between 25 and 

100 C. One DFL is a PFTE-based coating and the other is graphite-based. Both are intended to be air 

sprayed onto the gear teeth surfaces and heat cured in an oven prior to use. Based upon literature 

experimental results of similar DFL’s under conforming contact sliding similar to the current pin-on-disk 

test, wear life was expected to be on the order of 105

 to 106 cycles (Refs. 16 and 17). 

Examination of the DFL test results shown in Table II confirms the effectiveness of solid film 

lubrication for 60NiTi. Friction coefficients between 0.13 and 0.19 were observed and wear lives ranged 

from 105

 to over 106 sliding cycles. Furthermore, the use of DFLs resulted in smooth wear surfaces and 

steady friction coefficient levels with little scatter. Such characteristics are important to achieving quiet 

running gears. 

Summary 

The study described in this paper was undertaken to identify a solid lubrication method for a set of 

high-speed lightly loaded spur gears used to drive the compressor of the Urine Processor Distillation 

Assembly within the wastewater treatment system on the ISS. The solution currently employed utilizes 

one 316L stainless steel gear that meshes with a solid polymer gear. Dimensional stability and wear of the 

polymer gear are the impetus for seeking a more robust approach. 

A series of pin-on-disk tribology tests were developed and run to provide a reasonable simulation of 

the gear tooth wear contact. Wear pins made from the gear polymer material were slid against disks made 

from the stainless steel gear material as a baseline. Polymer pins were slid against the 60NiTi candidate 

gear material. Self-mated 60NiTi tests were run with and without additional lubrication provided by 

rubbing a polymer pin against the disk just ahead of the primary wear contact. Lastly, dry film lubricant 

(DFL) coatings made from a polymer binder with graphite and PTFE (Teflon) solid lubricant additives 

were deposited on both stainless steel and 60NiTi test disks for evaluation. 

The pin-on-disk test results illustrate that the baseline polymer-stainless steel wear couple exhibits 

generally low friction and modest wear. Tests of the polymer (pin) against 60NiTi (disk) give results 

comparable to the polymer-stainless steel baseline. Surprisingly low friction was observed for self-mated 

60NiTi wear specimens but the wear was high. 

The use of a polymer wear pin to deposit lubricant as a transfer film onto 60NiTi was effective at 

reducing friction and wear. By far, the best lubrication approach was the use of a DFL coating deposited 

onto the 60NiTi surface. Friction was smooth, steady and low and metal wear was too low to be 

measureable in these limited duration tests. 

Though the quantitative predictability of the pin-on-disk test with regards to gear life is yet to be 

proven, the use of a DFL deposited onto 60NiTi gear teeth appears to be a reasonable approach. Should 

full-scale gear life tests show unacceptable wear, lubricant replenishment could be achieved through the 

use of a third idler gear made entirely out of polymer. 
NASA/TM—2015-218896 8 

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