Christopher DellaCorte, Malcolm K. Stanford, Fransua Thomas, and Brian J. Edmonds

Glenn Research Center, Cleveland, Ohio

The Effect of Composition on the Surface Finish 

of PS400: A New High Temperature 

Solid Lubricant Coating

NASA/TM—2010-216774

September 2010
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Christopher DellaCorte, Malcolm K. Stanford, Fransua Thomas, and Brian J. Edmonds

Glenn Research Center, Cleveland, Ohio

The Effect of Composition on the Surface Finish 

of PS400: A New High Temperature 

Solid Lubricant Coating

NASA/TM—2010-216774

September 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

65th Annual Meeting and Exhibition

sponsored by the Society of Tribologists and Lubrication Engineers

Las Vegas, Nevada, May 16–20, 2010
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Available electronically at http://gltrs.grc.nasa.gov

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

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2010-216774 1 

The Effect of Composition on the Surface Finish of PS400: 

A New High Temperature Solid Lubricant Coating 

Christopher DellaCorte, Malcolm K. Stanford, Fransua Thomas, and Brian J. Edmonds 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

A new composite, multi-constituent, solid lubricant coating, NASA PS400, developed for high 

temperature tribological applications, exhibits a smoother surface finish after grinding and polishing than 

its predecessors PS200 and PS300. In this paper, the baseline composition of PS400 is modified to 

investigate each individual constituent’s role on the achievable surface finish through a series of coating 

deposition, grinding, and polishing experiments. Furthermore, to explore the limits of compositional 

tailoring for improved tribological performance, several PS400 coatings were doped with additional solid 

lubricants (graphite, MoS2 and BN) and tribologically tested. 

The test results clearly showed that, compared to PS300 coatings, PS400 achieves a smoother surface 

finish via a reduced lubricant content. Coatings prepared with higher than the baseline level (10 wt%) of 

lubricants exhibited higher final surface roughness than the earlier generation PS300 coatings. Reducing 

or eliminating one or both lubricants (fluorides or silver) did not further improve the surface finish 

suggesting that the current composition of PS400 is near optimal with respect to surface finish. Lastly, 

attempts to improve the poor initial room temperature tribological behavior of PS400 via the addition of 

traditional solid lubricants were unsuccessful. Based upon this work and earlier results it is expected that 

future research will concentrate on developing methods to produce a lubricious glaze on the rubbing 

surface during “break in” to ensure that low friction and wear are rapidly achieved. 

Introduction 

NASA has recently developed a new solid lubricant coating, PS400, as an outgrowth of earlier work 

that resulted in the PS100, PS200 and PS300 families of plasma sprayed coatings (Ref. 1). These four 

distinct families of coatings were engineered over the last four decades to address specific tribological 

challenges encountered in various aerospace systems. 

The developer of the PS100 family of nickel-glass-solid lubricant-containing coatings pioneered the 

concept of combining the functions of individual constituents to produce a composite solid lubricant 

coating (Ref. 2). PS100 was originally formulated to lubricate lightly-loaded oscillatory bearings and 

bushings for re-entry flight vehicles, like the space shuttle. In this application, high temperature 

endurance and low friction were paramount. PS200 coatings, which utilized a hard nickel-cobalt-bonded 

chrome carbide matrix, were tailored for long life to lubricate piston ring-cylinder wall contacts found in 

Stirling and other heat engine applications (Ref. 4). The PS300 coating system was developed to fit 

between the earlier coatings in terms of hardness and was tailored specifically to meet the needs for shaft 

coatings operating against high temperature foil gas bearings (Ref. 4). The general compositions of these 

three coating families along with their general performance characteristics are given in Table 1. 

Due to several drawbacks of PS300, namely the need to undergo a heat treatment for dimensional 

stabilization and poor initial surface finish, PS400 was invented. PS400 is similar to its sibling coatings in 

that fluoride and silver lubricants are combined with a hardened metal matrix. PS400, however, differs in 

that it relies on the matrix material, nickel-aluminum-molybdenum, to enhance oxidative and dimensional 

stability. Also, the solid lubricant content was lowered from 20 to 10 wt% to lower material costs. Greater 

detail of PS400’s formulation and baseline tribological performance has been previously reported 

(Ref. 1). 
NASA/TM—2010-216774 2 

TABLE 1.—COMPARISON OF THE NASA PLASMA SPRAY (PS) COATING 

Coating 

designation 

Binder 

matrix 

Hardener Solid 

lubricants 

General 

attributes 

PS100 NiCr Glass Ag+ Fluorides Soft-high wear

PS200 Ni-Co Chrome 

Carbide 

Ag+ Fluorides Hard-low wear, (abrasive to counter face dimensionally stable)

PS300 NiCr Chrome 

Oxide 

Ag+ Fluorides Moderate hardness, mildly abrasive to counter face, poor 

dimensional stability-requires heat treatment

PS400 NiMoAl Chrome 

Oxide 

Ag+ Fluorides Excellent dimensional stability and surface finish, poor initial 

low temperature tribology 

While researching PS400 it was observed that, in addition to its superior dimensional stability, it 

achieved a better surface finish compared to PS300. The reasons for this improvement were not known. 

Earlier research on PS300 aimed specifically at improving the initial surface finish had largely been 

unsuccessful (Refs. 5 and 6). Only through repeated, high-temperature start-stop sliding could PS300 

surfaces become smooth (Ref. 7). The first part of this investigation seeks to explore and understand the 

reason or reasons for PS400’s superior surface finish. A series of PS400 coatings are prepared in which 

the major constituent phase composition and proportions are varied and the effect on measured surface 

finish is examined. 

Previous tests with PS300 consistently showed that its friction and wear performance was deficient at 

room temperature when compared to conventional solid lubricants like graphite, molybdenum disulfide 

and similar materials. Further, if not previously operated (in sliding) at elevated temperatures during 

which time a lubricating surface glaze was formed, even markedly higher wear and friction at room 

temperature was observed. Such behavior was also noted in the first tests with PS400 (Ref. 1). One 

solution adopted for PS300 was to overlay a thin sacrificial film of solid lubricants onto the coating 

surface prior to first use (Ref. 5). Such sacrificial “break in” layers are commonly used in machinery 

systems. 

The later part of this investigation seeks to improve the room temperature performance of PS400. To 

enhance the initial as-installed tribological performance of PS400, several compositional variations were 

prepared that contain conventional solid lubricants (BN, graphite and MoS2) at varying doping levels. In 

addition, several tests are done using thin overlay coatings of MoS2 to determine if their presence can 

reduce initial friction and wear. In the present work, surface roughness measurements were made using 

stylus and optical profilometry on coated stainless steel coupons. For the tribological evaluations coated 

disks are rubbed against superalloy pins in a pin-on-disk tribometer. 

Materials 

PS400 is a plasma sprayed, composite, solid lubricant coating that employs a nickel-molybdenumaluminum binder to which chrome oxide (Cr2O3) is added as a hardening agent and silver and barium 

fluoride-calcium fluoride (BaF2/CaF2) eutectic are added as low and high-temperature solid lubricants, 

respectively. By weight, the baseline PS400 coating contains 70 percent binder, 20 percent hardening 

phase, and 5 percent each of the solid lubricant phases. The binder phase, by weight percent, contains 

90 percent nickel and 5 percent each of Mo and Al. It is available as a commercial plasma spray feedstock 

and is often used by itself as a wear resistant, high temperature protective coating. 

To elucidate the role of the various constituents and their relative proportions on the final achievable 

surface roughness, several model-coating compositions were formulated. These coatings are not intended 

or expected to be viable tribological coatings and are not tribologically tested. Rather, they are prepared 

as small (25 mm diameter, 6 mm thick) coupons coated by plasma spraying the following: a coating 

designated PS304-NiMoAl in which the NiCr binder normally used for PS300 is replaced by the binder 

used in PS400; PS400-NiCr in which NiCr replaces the NiMoAl binder normally used in PS400; 

PS400-A in which the fluoride content is removed; PS400-B in which the silver content is removed; 
NASA/TM—2010-216774 3 

PS400-C in which the silver lubricant content is increased; and PS400-D in which the fluoride lubricant 

content is increased. In addition, three PS400 solid lubricant doped coatings were formulated and 

tribologically tested by adding between 2 and 4 wt% of boron nitride (BN), graphite (C) or molybdenum 

disulfide (MoS2) to reduce initial room temperature friction. These doped coatings were deposited onto 

50 mm diameter, 12.5 mm thick superalloy test disks. Table 2 gives the composition of PS400 along with 

the variations formulated to determine their effects on surface roughness. 

TABLE 2—COATING COMPOSITION SUMMARY 

Coating 

designation 

Binder 

(wt%) 

Hardener 

(wt%) 

Low temp 

lubricant, 

(wt%) 

High temp 

lubricant, 

(wt%) 

Additional 

solid lubricant, 

(wt%) 

Purpose 

PS304 NiCr /60% Cr2O3/20% Ag/10% Fluorides/10% ------ Baseline 

PS400 NiMoAl / 70% Cr2O3/20% Ag/5% Fluorides/5% ------ Baseline 

PS304-NiMoAl NiMoAl /60% Cr2O3/20% Ag/10% Fluorides/10% ------ Binder substitution 

PS400-NiCr NiCr /70% Cr2O3/20% Ag/5% Fluorides/5% ------ Binder substitution 

PS400-A NiCr /68% Cr2O3/17% Ag/5% Fluorides/10% ------ Reduced silver 

PS400-B NiMoAl /68% Cr2O3/17% Ag/10% Fluorides/5% ------ Reduced fluoride 

PS400-C NiCr /68% Cr2O3/22% Ag/0% Fluorides/10% ------ Additional fluorides 

PS400-D NiMoAl /68% Cr2O3/22% Ag/10% Fluorides/0% ------ Additional silver 

PS400-BN NiMoAl /68.6% Cr2O3/19.6% Ag/4.9% Fluorides/4.9% BN/2% Added BN 

PS400-G NiMoAl /67.9% Cr2O3/19.4% Ag/4.85% Fluorides/4.85% C/3% Added graphite 

PS400-M NiMoAl/68.95% Cr2O3/19.7% Ag/4.93% Fluorides/4.93% M0S2/1.5% Added M0S2

The following general steps are taken to prepare the PS400 and its variant coatings. First, powders of 

the individual components, as listed in Table 2, are weighed and mixed in a suitable container. The 

powders in the container are first hand mixed then poured into the rotary drum of a commercial plasma 

spray powder feeder. In this device, the rotating motion of the drum continuously mixes the powder blend 

before and during the spray deposition process. To deposit a coating, the powders are injected into an 

argon gas carrier stream and fed into a plasma spray gun. Inside the gun, an electric field ionizes inert gas 

to provide the high temperatures (~10,000 K) needed to melt the particles. The molten particle-inert gas 

stream is projected onto the substrate where they form splats that solidify and adhere to form a coating. 

Many passes are made to build up a thick, dense coating approximately 300 μm thick. Table 3 gives the 

nominal plasma spray parameters used in this process. 

TABLE 3.—PLASMA SPRAY PARAMETERS 

Parameter Value 

Current ..................................................................................... 600 A 

Voltage ............................................................................. 30 to 32 V 

Stand-off distance ....................................................... 80 to 100 mm 

Argon arc gas flow ............................................... ~35 standard l/min 

Powder flow ........................................................................ ~1 Kg/hr 

Powder gas flow ............................................................... ~0.4 m3/hr 

The plasma spray process results in a rough surface that must be ground smooth to the desired coating 

thickness and dimensions. 

Typical acceptable coating thicknesses range from about 200 to 400 μm. Thicker coatings can be 

made but often suffer from poor strength and residual stresses. Coatings thinner than 125 μm often lack 
NASA/TM—2010-216774 4 

adequate bond strength. Figure 1 shows a typical cross-section sample of a PS400 coating in which the 

splat-type morphology from the plasma spray process can be readily seen. 

Grinding, using silicon carbide or diamond abrasive wheels, is usually the preferred finish process for 

PS400. Using standard grinding practices, surface root-mean-square (rms) finishes of about 0.25 μm are 

typical for PS400. Following machine grinding, selected coupons were further finished using varying 

grades of silicon carbide abrasive paper and water. Figure 2 shows a photograph of a turbine engine shaft 

that has been prepared for service with a ground and polished PS400 coating. 
NASA/TM—2010-216774 5 

Two types of coating specimens are fabricated for use in the present investigation; small round wafer 

coupons made from stainless steel and large superalloy test disks. These stainless steel wafers are 25 mm 

in diameter and 6 mm thick and are coated on one face with PS400 or one of the variants. These are 

utilized for the surface roughness measurements. The test disks are 63 mm in diameter and 12.5 mm thick 

and are made from Inconel X-750. These disks are coated on one face with PS400 which is ground to a 

smooth surface and a final coating thickness of about 250 μm. Several standard PS400-coated disk 

specimens were overlayed with a thin layer (~5 μm thick) of MoS2 after grinding to reduce initial friction 

using the procedure outlined in Reference 5. For the pin-on-disk testing, hemispherical-tipped pins are 

made Inconel X-750. Figure 3 shows a sketch of the test rig and specimen configuration. 

Test Apparatus and Procedure 

Surface Roughness Measurements 

The surface roughness coupons were evaluated primarily using stylus surface profilometry 

techniques. The wafer coupons were first plasma spray coated and then ground with SiC grinding wheels 

(400 grit) and polished using SiC abrasive paper and water (400 and 600 grit). The polished surface was 

subjected to repeated profilometry scans, 10 mm in length, at varying locations and directions across the 

entire coupon surface. Fifteen scans were obtained for each coated sample to get a representative 

roughness value. At least five coupons were prepared for each coating composition. The primary 

roughness parameter measured and recorded was the arithmetic mean, Ra, value. The scan speed was 

0.5 mm/s giving a total scan time of 20 ces. This speed was found to be sufficient to minimize 
NASA/TM—2010-216774 6 

measurement time while still preventing dynamic measurement problems such as stylus skipping. Typical 

variation in the roughness measurements of repeated samples was about 20 percent. 

Optical profilometry was also used on selected samples but was found to give widely variable 

roughness results. When pits were part of the analyzed area, the roughness was inordinately skewed 

yielding much higher values not truly representative of the surface. With stylus profilometry, artifacts 

such as pits cause less effect on roughness because the relatively large probe tip radius (2 μm) glides over 

the voids effectively filtering them from the measurement. Only when the selected analysis area was free 

of deep pits (exposed porosity) did the optical roughness measurements correlate well with those from 

stylus profilometry. For these reasons, only stylus profilometry results are presented and used to compare 

coating roughness. 

Pin-On-Disk Tribotesting 

Figure 4 shows the high temperature pin-on-disk test rig is used to evaluate the friction and wear 

properties of PS400 and its variants. In this rig, coated disks were rotated against stationary metal pins to 

evaluate the effect of solid lubricant additives to the baseline PS400 coating. A hemispherical tipped 

superalloy pin is loaded against a coated disk under a 4.9 N dead weight load. Three disk-pin specimen 

sets are tested to assess repeatability. Each specimen set is run for a total of nine 30-min tests; three at 

25 C followed by three at 500 C and then three at 650 C in ambient air. Selected specimen sets were 

additionally tested at 25 C following the standard sequence of nine tests to assess the possible formation 

of lubricious surface glazes during high-temperature sliding. An infrared pyrometer is used to measure 

disk surface temperature just ahead of the sliding contact and heating is accomplished using a lowfrequency induction-heating coil located around the disk specimen. Sliding velocity is 3 m/s and the pin 

generates a 51 mm diameter wear track on the disk surface. Friction is measured using a load cell and 

wear is measured after each 30 min test using optical microscopy (pin wear) and stylus profilometry 

(coating wear) methods as described in the literature (Refs. 3 and 4). 
NASA/TM—2010-216774 7 

Results and Discussion 

Surface Roughness Measurements 

Table 4 shows the roughness measurement results from the stylus profilometry tests. Figure 4 shows a 

representative stylus roughness profile for PS400 and Figure 5 shows an area profile derived from optical 

profilometry measurements. Both methods reveal a fairly uniform surface texture with occasional deep 

pits typical for thermal spray coatings of this type. The roughness values show significant data scatter and 

overlap, yet deeper examination reveals some interesting trends and patterns. 

TABLE 4.—STYLUS SURFACE ROUGHNESS RESULTS SUMMARY 

[10 mm long traces, 0.5 mm/s scan velocity] 

Coating 

designation 

Hard phase 

content 

(wt%) 

Soft phase 

content 

(wt%) 

Surface roughness 

Ra 

(m) 

PS304 80 20 0.61 ± 0.12 

PS400 90 10 0.43 ± 0.08 

PS304-NiMoAl 80 20 0.92 ± 0.18 

PS400-NiCr 90 10 0.37 ± 0.07 

PS400-A 85 15(5 Ag + 10 FL) 0.32 ±0.06 

PS400-B 85 15(5 FL + 10Ag) 0.37 ± 0.07 

PS400-C 90 10 (Fl only) 0.30 ± 0.06 

PS400-D 90 10 (Ag only) 0.24 ± 0.05 
NASA/TM—2010-216774 8 

For instance, the roughness measurements of the PS304 and PS400 coatings in which the matrix 

compositions have been reversed demonstrate that the matrix has little effect on the roughness. The 

roughness of PS304 and PS304-NiMoAl are high and similar, about 0.7 μm. Roughness of PS400 and its 

variant PS400-NiCr are also similar but lower, about 0.4 μm. Clearly the improved surface finish of 

PS400 compared to PS304 is not solely driven by the change in binder from NiCr to NiMoAl. When 

examining these four compositions (PS304, PS304-NiMoAl, PS400 and PS400-NiCr), however, one can 

observe a different correlation. 

The two coatings with 20 percent soft phases (solid lubricant content) and 80 percent hard phases 

(binder and hardener content) have higher roughness than the coatings with 90 percent hard phases and 

only 10 percent soft phases. This suggests that the softer phases are preferentially removed during 

grinding and polishing leaving low spots, pits and depressions that contribute to a higher measured 

surface roughness. Such roughness can degrade performance in selected applications like gas bearings 

(Ref. 7). Furthermore, since the soft phases are solid lubricants, there is a loss in lubrication performance. 

Thus their absence or depletion leads to higher friction and wear during sliding until the wear process can 

remove the surrounding hard phases revealing fresh lubricants to the surface (Ref. 8). 

The science and art of metallographic polishing is faced with preferential material removal challenges 

on a regular basis (Ref. 9). Many techniques are available that may improve the surface finish of the PS 

series of coatings such as oil-based lapping compounds, reduced polishing rates and the use of 

structurally-rigid lapping plates. These techniques, however, are not necessarily practical or possible for a 

manufactured coating for which contamination and costs must be minimized and complex shapes are 

often involved. To explore the effects of soft-phase concentration on surface finish, four alternative 

compositions were prepared PS400-A, PS400-B, PS400-C, and PS400-D. 

PS400-A and PS400-B both contain 85 percent hard phases and 15 percent soft phases. The primary 

difference is that PS400-A has more fluoride than silver and PS400-B has more silver than fluoride. In 

terms of roughness, PS400-A achieved a slightly smoother finish than PS400-B though the data for the 

two coatings overlap considerably. This is not surprising since the fluorides and silver are soft and are 

susceptible to removal during finishing to a similar extent. The smoothest surfaces were achieved for the 

remaining two model coatings, PS400-C and PS400-D, which contained only fluoride or silver lubricants, 

respectively. The silver containing coating, PS400-D, achieved the lowest roughness of all, perhaps due to 

silver’s high ductility allowing it to resist micro-fracture type wear during polishing. However, it may 

also simply be experimental scatter since there is considerable data overlap. 

The results of the surface finish experiments point to the existence and extent of soft phases as the 

cause for high finished surface roughness. Since the lubricants are needed to mitigate friction and wear 

their presence cannot be avoided. Efforts to develop more effective polishing techniques would be 

beneficial as would the infusion or overlaying of the finished surface with solid lubricants. Another 

approach is to add small quantities of highly effective solid lubricants to retain good tribological 

properties in the event that some soft phase depletion occurs during finishing. These approaches are 

addressed by the tribology experiments discussed below. 

Tribology Results 

Five coatings are evaluated, the baseline PS400, PS400 doped with several weight percent of BN, 

graphite or MoS2 and one PS400 coating onto which a thin layer of pure MoS2 was applied after finish 

grinding and polishing was completed. The intent of the pin-on-disk testing was to determine if roomtemperature friction and wear behavior of the baseline PS400 coating could be enhanced through the 

addition of traditional solid lubricants. Table 5 displays the tribological results obtained in this research. 

The results clearly show that this approach is not successful. 
NASA/TM—2010-216774 9 

TABLE 5.—FRICTION * AND WEAR ** SUMMARY 

Coating 

designation 

Test temperature, 

°C 

PS400 Friction +

Kpin +

Kdisk 

25 °C 500 °C 650 °C 25 °C-Retest 

0.80 

NM 

NM 

0.16 

0.21×10–6

6.3×10–6

0.21 

0.89×10–6

7.6×10–6

0.31 

14.1×10–6 mm3

/N-m 

118×10–6 mm3

/N-m 

PS400-BN 

Friction 

Kpin 

Kdisk 

1.30 

230 

1500 

0.35 

7.8 

1220 

0.36 

1.7 

2510 

0.40 

135×10–6 mm3/N-m 

NM 

PS400-G 

Friction 

Kpin 

Kdisk 

0.064 

172×10–6 

143 

0.45 

40.8 

3140 

0.32 

–9.5 

4070 

0.60 

9.9×10–6 mm3

/N-m 

NM 

PS400-M 

Friction 

Kpin 

Kdisk 

0.76 

14.6 

85 

0.40 

0.92 

1703 

0.22 

1.52×10–6 

2570×10–6

0.40 

2.5×10–6 mm3

/N-m 

NM 

PS400-MoS2

Overlay 

Friction 

Kpin 

Kdisk 

0.20*** 

2.71 

NM 

 

Notes: 

*Data scatter for friction typically േ 20 percent nominal 

**Data scatter for wear (Kpin, Kdisk) typically േ 50 percent nominal; all valves are ×10–6 mm3

/N-m 

***Friction coefficient for PS400 – MoS2 overlay represents first 10 min of sliding only + 

Wear factors (Kpin, Kdisk) are all ×10–6 mm3/N-m 

NM-not measured 

The baseline PS400 coating exhibits low friction coefficients (0.16 to 0.21), low pin wear 

coefficients (~10–6 mm3

/Nm) and low coating wear coefficients (~710–6 mm3/Nm) at elevated 

temperatures. If room temperature testing commences before previous sliding at high temperatures, 

friction and wear are excessive. This has been attributed to the absence of lubricious glazing on the 

tribological surfaces that only form during high-temperature sliding. For this reason, the PS400 data at 

room temperature was collected after the high temperature tests were complete. That data shows a low 

friction coefficient (~0.3), low pin wear coefficient (~1410–6 mm3

/Nm) and a moderate coating wear 

coefficient (11810–6mm3

/Nm). 

The goal of the alternate composition studies discussed below was to improve the room temperature 

performance and provide low friction coating without the need to first run at high temperature. The 

addition of BN to PS400 caused degradation in performance in every respect. The friction coefficient at 

room temperature exceeded 1.0. Pin wear and coating wear increased by at least an order-of-magnitude 

and the wear couple could be described as abrasive. When the coating was tested following high 

temperature operation, friction was high (0.4), pin wear was excessive and disk wear could not be 

characterized due to the excessively rough nature of the wear track. In these tests, the addition of BN had 

deleterious effects on the tribological performance of PS400. 

The addition of graphite to the PS400 coating also did not improve the performance. While not as 

damaging to the behavior of the baseline PS400 as BN, graphite was unsuccessful in significantly 

lowering friction and wear during initial room-temperature sliding. Friction was high (0.64) and pin and 

coating wear coefficients were both in the 10–4 mm3

/Nm range. Friction and wear at elevated temperature 

suffered also, especially coating wear which ranged from 3×10–3 to 4×10–3 mm3

/Nm at 500 and 650 C, 

respectively. Pin wear at the highest test temperature, 650 C, was negative indicating gross material 

transfer from the disk. Clearly, adding graphite to the PS400 coating was not an improvement. 

MoS2, considered among the best solid lubricants, was added to the PS400 coating in two ways. As a 

constituent, MoS2 was not able to reduce room-temperature friction significantly from the baseline case 

(0.76 versus 0.80). Both pin wear and coating wear were in the moderate 10–5 mm3

/Nm ranges. At 500 C, 

friction was twice as high as the baseline coating and pin wear was about 4 times higher. Coating wear 

was excessive (10–3 mm3

/Nm). At 650 C, friction was comparable to the baseline case as was pin wear 
NASA/TM—2010-216774 10 

but coating wear remained high (10–3 mm3/Nm). During the room temperature re-test, following hightemperature sliding, friction was moderate, and pin wear remained low (10–6mm3

/Nm), but the coating 

wear track became roughened and was not measurable (NM). These results show that adding MoS2 as a 

coating constituent is not an acceptable path to improved tribological performance. 

As found previously with PS304 coatings for foil gas bearings, thin overlay coatings of MoS2

deposited onto PS400 provided a temporary reduction in friction and wear. Testing was conducted only at 

room temperature. The friction coefficient was low, about 0.20, and was comparable to that obtained for 

the baseline coating at elevated temperatures. After 10 min of sliding, the friction gradually increased 

eventually reaching about 0.4 by the end of the 30-min test period. Pin wear was low, 10–6 mm3

/Nm, 

and coating wear was too low to reliably measure for such a short test and is estimated to be below 

10–6 mm3

/Nm. Subsequent tests at elevated temperatures yield performance comparable to baseline PS400 

coatings indicating that the MoS2 overlay is a benign, short-term solution that can function as a break-in 

lubricant. This behavior corroborates experience with PS304 coatings and gas foil bearings in which the 

overlay coatings are employed as a “one-time” use coating to prevent machine damage during initial 

running (Ref. 7). 

Summary Remarks 

The goals of this research were to elucidate the factors affecting the achievable surface roughness 

levels of the newly developed PS400 coating and to investigate methods to improve its friction and wear 

performance especially during initial sliding contact at room temperatures. A series of coatings were 

prepared at the coupon level specifically to determine surface roughness effects. Modified PS400 

coatings, doped with different solid lubricants, were prepared on tribological test disks to evaluate the 

potential for improved friction and wear performance. Based upon the results in this effort the following 

conclusions are drawn: 

 The improved final surface roughness of PS400, compared to its predecessor PS304, is largely 

due to the reduction in soft phase content, the solid lubricants silver and fluorides. Altering the 

coating matrix has no significant effect on surface finish. 

 Standard grinding and polishing techniques appear to preferentially remove the softer lubricant 

phases at the finished surface resulting in high surface roughness and possibly high initial friction 

and wear. Enhanced techniques for finishing are needed to reduce roughness and improve initial 

tribological performance. 

 The addition of solid lubricants graphite, BN and MoS2 did not improve any of the tribological 

properties of PS400. In general, observed friction and wear increased dramatically when these 

solid lubricants were added to the coating. 

 Overlay coatings of MoS2 sprayed over the PS400 surface provided low friction and wear for a 

limited duration. This behavior corroborates earlier experience with such overlay coatings used 

on foil bearing shafts. Such an approach is likely an engineering solution to using PS400 in 

engines and other machinery. 

References 

1. DellaCorte, C., and Edmonds, B.J.: “NASA PS400: A New High Temperature Solid Lubricant 

Coating for High Temperature Wear Applications,” NASA/TM—2009-215678. 

2. Sliney, H.E.: “Wide Temperature Spectrum Self-Lubricating Coatings Prepared by Plasma Spraying,” 

Thin Solid Films, Vol. 64, 1979, pp. 211–217. 

3. DellaCorte, Christopher: “Tribological Composition Optimization of Cr3C2 Coatings for Foil Bearings of 

Use to 650 °C,” NASA CR–179049, Surface Coatings and Technology, AVS, 36, (1988) pp. 87–97, 

Oct. 1988. 
NASA/TM—2010-216774 11 

4. DellaCorte, Christopher and Fellenstein, J.F.: “The Effect of Compositional Tailoring on the Thermal 

Expansion and Tribological Expansion and Tribological Properties of PS300: A Solid Lubricant 

Composite Coating,” NASA TM–107332, Prepared for the 1997 STLE Annual Meeting. Tribology 

Transactions, Vol. 40, pp. 639–642, Oct. 1997. 

5. DellaCorte, Christopher; Zaldana, Antonio; Radil, Kevin: “A Systems Approach to the Solid Lubrication 

of Foil Air Bearings for Oil-Free Turbomachinery,” ASME Journal of Tribology, Vol. 126, pp 200-207, 

Jan. 2004. Also NASA/TM—2002-211482, ARL–TR–2867, Oct. 2002. 

6. Blanchet, Thierry A.; Kim, John-Hwan; Calabrese, Salvadore J.; DellaCorte, Christopher: “ThrustWasher Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding 

Contact,” Tribology Transactions, Vol. 45, 4, pp. 491-498, 2002. 

7. Radil, Kevin C. and DellaCorte, Christopher: “The Effect of Journal Roughness and Foil Coatings on the 

Performance of Heavily Loaded Foil Air Bearings,” Tribology Transactions, Vol. 45, No. 2, Apr. 2002, 

pp. 199–204. Also NASA/TM—2001-210941, June 2001. 

8. DellaCorte, Christopher and Sliney, H.E.: “Composition Optimization of Self-Lubricating ChromiumCarbide-Based Composite Coatings for Use to 760 °C,” ASLE Trans., Vol. 30, No. 1, Jan .1987, 

pp. 77–83. 

9. Gray, R.J., and Vander Voort, G.F.: “Metallography” in Metals Handbook-Desk Edition, Ch. 35, 

American Society of Materials (ASM), 1985. 
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The Effect of Composition on the Surface Finish of PS400: A New High Temperature Solid 

Lubricant Coating 

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6. AUTHOR(S)

DellaCorte, Christopher; Stanford, Malcolm, K.; Thomas, Fransua; Edmonds, Brian, J. 

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WBS 877868.02.07.03.01.01.04 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

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14. ABSTRACT

A new composite, multi-constituent, solid lubricant coating, NASA PS400, developed for high temperature tribological applications, 

exhibits a smoother surface finish after grinding and polishing than its predecessors PS200 and PS300. In this paper, the baseline 

composition of PS400 is modified to investigate each individual constituent’s role on the achievable surface finish through a series of 

coating deposition, grinding, and polishing experiments. Furthermore, to explore the limits of compositional tailoring for improved 

tribological performance, several PS400 coatings were doped with additional solid lubricants (graphite, MoS2 and BN) and tribologically 

tested. The test results clearly showed that, compared to PS300 coatings, PS400 achieves a smoother surface finish via a reduced lubricant 

content. Coatings prepared with higher than the baseline level (10 wt%) of lubricants exhibited higher final surface roughness than the 

earlier generation PS300 coatings. Reducing or eliminating the one or both lubricants (fluorides or silver) did not further improve the surface 

finish suggesting that the current composition of PS400 is near optimal with respect to surface finish. Lastly, attempts to improve the poor 

initial room temperature tribological behavior of PS400 via the addition of traditional solid lubricants were unsuccessful. Based upon this 

work and earlier results it is expected that future research will concentrate on developing methods to produce a lubricious glaze on the 

rubbing surface during “break in” to ensure that low friction and wear are rapidly achieved. 

15. SUBJECT TERMS

Solid lubricants; Friction; Wear; High temperature; Turbines; Bearings 

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