most low friction coating for gears application

GEARS & TRANSMISSIONS Workshop paper X [ 203 ]

Faculdade de Engenharia da Universidade do Porto, Portugal, 5th June 2003

MoST low friction coating for gears application

R. I. Amaro a), R. C. Martins b), J. O. Seabra c) and A. T. Brito d)

a) INEGI, Instituto de Engenharia Mecânica e Gestão Industrial, Porto, Portugal

(presently at RENAULT - C.A.C.I.A., Aveiro, Portugal) [email protected]

b) INEGI, Instituto de Engenharia Mecânica e Gestão Industrial, Porto, Portugal [email protected]

c) FEUP, Faculdade de Engenharia da Universidade do Porto, Portugal, [email protected]

d) A. BRITO, Industria Portuguesa de Engrenagens, Lda, Porto, Portugal, [email protected]

ABSTRACT

The multi-layer composite surface coating made of MoS2/Titânium, produced by DC Magnetron Sputtering and known as MoST, exhibits excellent mechanical properties in several industrial applications.

Several tests, like Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear tests, were performed in order to evaluate the adhesion to the substrate and the tribological performance of this coating.

Twin-disc tests, performed at high contact pressure and high slide-to-roll ratios, confirmed the good adhesive and tribological properties of the MoST coating and left good indications about the applicability of the MoST coating in gears.

Gear tests were performed in the FZG machine in order to evaluate the anti-scuffing performance of this coating. Finally, the MoST coating was applied to the gearing in a gearbox and its influence on the gearbox efficiency was studied. 1. INTRODUCTION

Power transmission equipments employing gears dissipate significant amounts of power

and any improvement in there performance represents a significant reduction in energy

consumption. Beside this, the demands of modern mechanical transmissions require higher

operating torques, higher speeds, lower operating noise and lower weight.

In the last decades, surface coating technology was important to achieve increased

energetic performance, allowing lower friction coefficients, higher protection against surface

failures and higher load capacity. In applications with frequent start-stop operations such

protection against failures, mainly scuffing, is very important.

Another important objective to be accomplished by surface coatings in the near future is

the reduction / elimination of some toxic lubricant additives and consent the use of environment

friendly lubricants.

[204] paper X GEARS 2003

Faculdade de Engenharia da Universidade do Porto, Portugal, 5th June, 2003

There are several types of coatings that can be used in gears, for instance, MoS2

(Molybdenum Disulphide) [1], WC/C (Tungsten Carbide - Carbon) [2] or the B4C (Boron

Carbide – Carbon) [2, 3] among others.

The MoS2 coatings can be improved with the co-deposition of other metals such as Ti

(Titanium) [4] or Cr (Chromium) [5].

The tribological performance of a Molybdenum Disulphide – Titanium composite coating

is studied in this work. This coating, known as MoST [4], is harder, more resistant and less

sensitive to atmospheric water vapour than other common DLC coatings. It has already given

excellent results in a wide range of forming and cutting tools applications.

2. MOLYBDENUM DISULPHIDE – TITANIUM COMPOSITE COATING

2.1. Deposition procedure

The MoS2/Ti composite coatings [6, 7] is deposited by DC Magnetron Sputtering using a

standard CFUBMSIP [8] Teer Coatings PVD system, with four targets (one Ti and three MoS2

targets). The coating procedure starts with an ion cleaning, followed by a 70nm Ti layer, a

200nm MoS2/Ti multilayer, a 900 nm MoST (non multilayer) and a last step of 50 nm layer of

MoS2 for coloration. Further details about the coating deposition may be found in references [6,

7].

2.2. Physical properties and tribological performance of the MoST coating deposited

onto M42 polished 1200 SiC [6, 7]

This MoST coating was deposited on M42 polished 1200 SiC steel, and several

tribological tests were performed in order to evaluate this coating performance.

Ball Crater technique has been used to measure the coating thickness, as shown in figure

1. A thickness of about 1.2 µm was obtained.

Rockwell C indentation (Daimler Test) has been performed to assess qualitative coating

adhesion to the substrate, showing only a plastic deformation as can be observed in Figure 1.

Hardness, scratch, pin-on-disc (POD) and reciprocating wear (RWT) tests were

performed to assess the tribological properties of the deposited coating.



The hardness test indicated that the MoST coating had a plastic hardness

HP ≅ 1200kg/mm2 and the scratch test showed that no failure were found at loads up to 100N, as

shown in Figure 2.

Figure 1 - Ball crater and Rockwell indentation on MoSTTM coating.

Figure 2 - Scratch test on MoS2/titanium composite coating.

Pin-on-disc (POD) tests performed at 200mm/s and 50% r.h. (dry) have shown low wear

and low friction coefficient. For the highest load (80N) the friction coefficient is rather small

(0.04 – 0.045). Other results are given in Table 1.

Table 1 - Results of pin-on-disc tests of MoS2/titanium composite coating.

Load [N]

Test time [s]

Coefficient of Friction [/]

Coating Wear [%]

10 3600 0.09~0.1 10 40 3600 0.07~0.08 21 80 3600 0.04~0.045 24



Ball craters in the wear track have been performed to assess the coating wear as shown in

Figure 3.

The reciprocating wear test (RWT) was carried out using a sliding speed of 150mm/min

under 100N load at 50% r.h. (dry) and at room temperature. After 10.000 cycles, only 19% of

coating was worn away (see Figure 4) with a specific wear rate of 3.1x10-17m3/mN and a

friction coefficient of 0.04.

Figure 3 - Ball craters in wear tracks after POD test.

Figure 4 - Ball crater in wear tracks after RWT test.

This set of results confirms the excellent adhesion between the MoST coating and the

steel substrate, as well as, its interesting tribological properties.

3. TWIN-DISC TESTS

3.1. The twin-disc machine

The twin-disc machine [9,10] used in this study is shown in Figure 5. Two discs (1, 2)

roll against each other, in conditions of pure rolling or rolling and sliding, transmitting a

specified normal load, applied by a pneumatic cylinder (3). The discs are lubricated by oil

injection (4) and the lubricating oil is kept at the specified temperature in the tank (5).



1 2

3

4

5

6

7 9

98

12

12

1110

Figure 5 - Schematic view of the twin disc machine.

An electric AC motor (6), with frequency variation, drives an angle gearbox (7) by means

of a toothed belt (8) and two toothed pulleys (9). Two other toothed belts (10) connect the

toothed pulleys mounted on the disc shafts (11) to the toothed pulleys mounted on the angle

gearbox output shafts (12).

The specified rolling and sliding speeds are obtained selecting the electric motor rotating

speed and the adequate teeth numbers of pulleys (11) and (12).

3.2. Discs geometry, heat treatment and surface roughness

The discs have a diameter of 70 mm and a thickness of 7 mm. The upper disc, called

spherical disc, has a transversal curvature radius of 105 mm [9, 10]. The geometry of the discs is

indicated in Figure 6.

Figure 6 – Geometry of the contacting discs (dimensions in mm).

φ 70

φ 70

R105

7



The discs are manufactured in DIN 20MnCr5 steel. After machining they were heat

treated as follows:

1. Austenitisation at 900ºC;

2. Case hardened in controlled atmosphere during 5hours at 900 ºC (%Carbon = 1.05);

3. Diffusion during 2.5 hours while cooling till 860ºC;

4. Stabilisation for quenching during 0.5 hour

5. Quenching in oil at 60ºC;

6. Annealing during 1 hour at 220ºC.

After the heat treatment the surface hardness was measured in all discs and a mean value

of 59HRC was obtained. Afterwards, the discs were grinded in order to obtain the required

surface finishing. The surface roughness of each disc was measured in three different locations,

before and after the coating deposition, and the corresponding mean values are shown in Table 2.

The surface roughness is similar before and after coating. The roughness parameters of

the spherical discs show a slight decrease after coating while those of the cylindrical discs show

a slight increase.

The grinding operation is more difficult in the case of the discs with transverse curvature

(spherical discs), and consequently they show higher roughness values than the cylindrical discs.

However, the Rpk roughness parameter shows a very significant reduction after coating, for those

spherical discs, showing that the surface coating may have important effects on the roughness

peaks.

Table 2 - Surface roughness measurements of the discs before and after coating deposition.

After grinding After coating Roughness parameter Spherical

disc 1 Cylindrical

disc 2 Spherical

disc 1 Cylindrical

disc 2 RZ-DIN 2.550 2.280 2.443 2.570

Ra 0.500 0.310 0.480 0.332 Rq 0.620 0.400 0.600 0.426 Rpk 0.420 0.340 0.273 0.380

a1 a2R + R2

0.405 0.406

2 2q1 q2R + R 0.738 0.736



3.3. Twin-disc testing conditions

Four pairs of MoST coated discs were tested at constant maximum Hertzian pressure

(p0 = 1.5GPa) and constant rolling speed ( (U1 +U2)/2 = 11 m/s ). Each disc pair was tested at a

different slide-to-roll ratio, and the upper disc was always spherical and the faster one, while the

lower disc was always cylindrical and the slower one. Table 3 presents all the testing conditions.

Theoretically, the specific lubricant film thickness Λ is defined as

φΛ T 02 2q1 q2

h=R + R

where φT is lubricant film thickness correction parameter due to the contact inlet shear heating,

h0 is the centre film thickness according to Dowson and Hingginson and 2 2q1 q2R + R is the

composite surface roughness of the two contacting discs.

For each test, the inlet oil temperature was determined so that the specific lubricant film

thickness remains constant in all tests (Λ = 0.5). The values of Λ shown in Table 3 were

calculated considering the composite surface roughness 2 2q1 q2R + R before the test.

Table 3 – Twin-disc testing conditions.

Disc pair Parameter Design.

MoST-1 MoST-2 MoST-3 MoST-4

Maximum Hertzian pressure [GPa] p0 1.511 1.511 1.511 1.511

Rolling speed [m/s] 1 2U + U2

11.00 11.05 11.11 11.24

Sliding speed [m/s] U1-U2 0.75 2.17 3.16 4.60

Slide-to-roll ratio [%] ( )1 2

1 2

2 U - UU + U 6.8 19.6 28.4 40.9

Total nº cycles upper disc / 103 N1 931 993 1 038 1 108

Total nº cycles lower disc / 103 N2 870 816 780 731

Oil reference ISO VG 22, additive free

Oil temperature [ºC] T 93 89 87 83

Specific lubricant film thickness Λ 0.5 0.5 0.5 0.5

3.4. Twin-disc tests results



3.4.1. Contact track observation by video microscopy

During each test the contact tracks of both discs were periodically observed using a video

microscope, thus avoiding the dismounting of the disc pair. These systematic observations of the

contact track of both contacting discs showed that:

• The wear of the MoST coating was normal and progressive in both contact tracks in

all tests;

• Severe adhesive and/or abrasive wear of the contact tracks was not observed in any

test;

• Catastrophic coatings failures didn’t occurred in any test;

• Signs sustaining the need to stop any of the tests, due to coating failure were not

detected.

3.4.2. Contact track observation by optical microscopy

At the end of each test the contact tracks of the discs were observed by optical

microscopy and significant images were recorded. Figure 7 shows examples of these images, for

the upper fast disc (spherical) and the lower slow disc (cylindrical) corresponding to tests MoST-

2 (slide-to-roll ratio = 19.6%) and MoST-4 (slide-to-roll ratio = 40.9%).

These observations of the contact tracks of all discs by optical microscopy show that:

• Some small MoST coating particles were arrached fro the surface, originating small

“pits” in the contact track, although it couldn’t be verified if their depth was enough to

reach the steel substrate;

• In some locations the contact track has a blue colour, different from the colour of the

coating, being difficult to understand if the MoST coating has been completely worn

out or not;

• In some cases, the MoST coating shows transversal cracks in relation to the rolling

direction;

• The optical microscopy analysis was not always conclusive enough to identify the

zones of the contact tracks where the MoST coating has been completely worn out.



20 µm

20 µm

I. Test MoST-2, spherical disc. II. Test MoST-2, cylindrical disc.

III. Test MoST-4, spherical disc. IV. Test MoST-4, cylindrical disc.

Figure 7 – Images of the contact track from the spherical and cylindrical discs used in tests

MoST-2 and MoST-4 (rolling direction – left to right).

3.4.3. Surface observations by scanning electron microscopy

At the end of each test, both discs were observed by scanning electronic microscopy

(SEM) with reflected primary electrons, at 3 different locations along the contact track. In each

location 5 images were taken, in order to have a global view of the coating degradation across

the contact track.

Figure 8 and Figure 9 show, for test MoST-2 (slide-to-roll ratio = SRR = 19.6%), these 5

images across the contact track for the upper (spherical) and the lower (cylindrical) discs,

respectively.

With these SEM observations and analysis it was possible to identify that the dark areas

indicate the presence of the MoST coating while the white areas indicate zones where the coating

has been completely worn out.

The 5 images obtained per disc in each location were analyzed using Image Analysis

Software in order to measure the values of the coated and uncoated areas. The uncoated area was

divided by the total area under analysis (coated plus uncoated areas) defining the percentage of

coating wear (PCW).

20 µm 20 µm



Figure 8 - View of the contact track for the spherical disc MoST-2:

slide-to-roll ratio = 19.6%, uncoated area = 23.5%, total width = 3mm (50×).

Figure 9 - View of the contact track for the cylindrical disc MoST-2:

slide-to-roll ratio = 19.6%, uncoated area = 36.2%, total width = 3mm (50×).

In the case of the test MoST-2, shown in Figures 8 and 9, the percentage of coating wear

(PCW) at the end of the test was 23.5% for the upper spherical disc and 36.2% for the lower

cylindrical disc.

The average values of the percentage of coating wear are defined using the values

measured in each one of the 3 locations analysed for each test and are shown in Figure 10.

The percentage of coating wear (PCW) of the cylindrical discs is always smaller than

50% in all cases, and smaller than 35% for slide-to-roll ratios greater than 6.8%. However, the

most interesting feature concerning the cylindrical discs, is that the percentage of coating wear

decreases significantly when the slide-to-roll ratio increases, reaching the small value of PCW =

18% for a slide-to-roll ratio of 41%.

In the case of the spherical discs the percentage of coating wear (PCW) is smaller than

35% for the lower slide-to-roll ratios (SRR ≤ 19.6%), but there is a significant increase of the

percentage of coating wear (PCW ≥ 65%) for the higher slide-to-roll ratios (SRR ≥ 28.4%).

The main reason for the different behaviour between the spherical and cylindrical discs,

in terms of the PCW parameter, is probably related to their significant difference in surface



finishing. The surface roughness of the spherical discs is about 50% greater than that of the

cylindrical discs ( Sph.aR = 0.48µm , Cyl.

aR = 0.33µm ) and, probably, this has an unfavourable

effect on coating adhesion.

33,328,0

70,765,2

51,1

34,9 32,0

17,7

0102030405060708090

100

6,8 19,6 28,4 40,9Slide-to-roll ratio [%]

Perc

enta

ge o

f coa

ting

wea

r, P

CW

[%

] Spherical disc Cylindrical disc

Figure 10 - Percentage of uncoated area vs. slide-to-roll ratio in twin-disc tests with MoST

coated surfaces.

The fact that the spherical discs were always used as the fast discs, might be another

possible reason for their greater coating wear since they are submitted to a greater number of

cycles than the cylindrical discs. On the other hand the fast disc heats considerably less than the

slower one.

However, even taking into consideration the different number of cycles performed by

each disc in each test (see Table 3) the remarks made above are still valid as shown on Figure 11,

where the percentage of coating wear is related to the number of cycles.

3.5. BEHAVIOUR OF MOS2/TITANIUM COMPOSITE COATING

The most significant type of coating degradation is the progressive wear of the coating,

which happened in all tests. In some cases it was also possible to detect small “pits” due to the

extraction of small coating particles from the surface and the occurrence of transversal cracks of

the coating.



000E+0

10E-6

20E-6

30E-6

40E-6

50E-6

60E-6

70E-6

80E-6

6,8 19,6 28,4 40,9Slide-to-roll ratio [SRR, %]

Perc

enta

ge o

f coa

ting

wea

r /

Num

er o

f cyc

les [

PCW

/N] .

Spherical disc Cylindrical disc

Figure 11 - Percentage of coating wear per cycle vs. slide-to-roll ratio in twin-disc tests with

MoST coated surfaces.

The twin-disc tests show that the performance of the MoS2/Titanium composite coating

is in general quite good. The results obtained, both for the spherical and cylindrical discs,

confirm the excellent adhesion of this coating to the substrate, already shown by the Rockwell

indentations, ball crater, pin-on-disc and reciprocating wear tests mentioned before. They also

indicate that the MoST coating has high resistance to sliding at low specific film thickness and

high hertzian contact pressure (Λ = 0.5, po = 1.5GPa).

Thus, the MoST coating is able to support the severity of the tribological operating

conditions imposed by the twin-disc tests, similar to those found in gears in terms of contact

pressures, slide-to-roll ratios and specific lubricant film thickness, giving very good indications

about the successful application of these coatings in gear sets.

4. FZG GEAR TESTS

4.1. Test rig and FZG type C gears

The FZG machine, schematically represented in Figure 12, is well known and used to

perform several different types of gears and oils tests.



Figure 12 - Schematic representation of the FZG test rig.

The tests were performed using type FZG type C gears, whose characteristics are given in

Table 4. The steel (DIN 20MnCr5) and the heat treatment are the same used for discs used in the

twin-disc tests (see paragraph 3.2.).

Three different gears were tested:

1. N-CT - Uncoated gear with high surface roughness (Ra = 2.4 µm);

2. CT - MoST coated gear with high surface roughness (Ra = 2.4 µm);

3. CT* - MoST coated gear with very low surface roughness (Ra = 0.4 µm).

Table 4 – Characteristics of the FZG type C gears.

Parameters Designation Pinion Wheel Number of teeth [-] Z 16 24 Tooth Width [mm] b 14 Module [mm] m 4.5 Pressure angle [º] α 20 Profile shift factor [/] x 0.182 0.171 Tip circle [mm] da 82.45 118.35 Center distance [mm] aW 91.5 Working pressure angle [º] αW 22.5 Contact ratio [/] ε 1.47 Material / DIN 20MnCr5 Heat treatment / Case hardened, quenching and annealing Surface Hardness HRC 58 - 62 Surface roughness in teeth flanks (NC2.4 & C2.4) [µm] Ra 2.388 2.396

Surface roughness in teeth flanks (C0.4) [µm] Ra 0.405 0.420



4.2. Physical properties of the lubricant

All tests were performed using an ISO VG 100 mineral oil without extreme-pressure (EP) and anti-wear (AW) additives. The physical properties of the lubricant are presented in Table 5. During the tests the oil was kept at a constant temperature of 90 ºC.

Table 5 – Physical properties of the lubricant (ISO VG 100 mineral oil).

Parameter Designation Value Cinematic viscosity at 40ºC [mm2/s] ν0 100.0 Cinematic viscosity at 100ºC [mm2/s] ν1 11.1 Specific gravity [-] Sp Gr 0.891 Viscosity Index [-] VI 95 Additives / Additive free Oil operating temperature [ºC ] T 90 ± 2

4.3. Gear tests

Before each test the gears were run-in during 4 hours in FZG load stage 6 (KFZG = 6) at 1500 rpm (see figure 13), with the purpose of softening the surface roughness of the flanks of NC and C gears.

After running-in, all gear tests started on FZG load stage 7. The torque is progressively increased until FZG load stage 12 (or 13) or until any type of surface failure of the teeth flanks is detected. The duration of each load stage is 15 minutes, during which both the speed and the lubricant temperature are kept constant. At the end of each load stage the teeth flanks are visually inspected looking for surface failures, in particular scuffing and excessive wear.

For each type of gear (N-CT, CT and CT*) the test procedure was performed at two different speeds, 1500 rpm and 3000 rpm, respectively. The most significant operating conditions corresponding to each testing speed are detailed in Figures 13 and 14.

Figure 13 shows the torque applied to the pinion and wheel in each FZG load stage and the corresponding maximum Hertzian pressure at the wheel tip / pinion root contact point, of the gear meshing line. Figure 14 shows the power transmitted by the FZG gear in each FZG load stage, for the wheel speeds of 1500 rpm and 3000 rpm, and the corresponding lubricant film thickness at the wheel tip / pinion root contact point of the gear meshing line.

The tribological operating conditions of the contact between the teeth flanks are extremely severe, as the torque increases from load stages 7 to 13. At the wheel tip / pinion root contact point, the maximum Hertzian pressure increases from 1.13 GPa to 2.08 GPa, while the lubricant film thickness, although remaining almost constant, is very thin, 0.15 µm and 0.20 µm, at 1500 rpm and 3000 rpm, respectively.



275359

453

559

675

802

940

135

302373

450535

627

203 183239

0

100

200

300

400

500

600

700

800

900

1000

6 7 8 9 10 11 12 13FZG load stage [KFZG]

Tor

que

[Nm

]

0.0

0.5

1.0

1.5

2.0

2.5

Her

tzia

n pr

essu

re [G

Pa]

Wheel Pinion Hertzian pressure

Figure 13 - Wheel and pinion torque, and maximum Hertzian pressure (at point B of the gear

meshing line - wheel tip / pinion root) vs. the FZG load stage.

3256 71 88

106126

148

6486

113142

176212

252

43

295

0

50

100

150

200

250

300

350

400

450

500

6 7 8 9 10 11 12 13

FZG load stage [KFZG]

Inpu

t pow

er [k

W]

0.00

0.05

0.10

0.15

0.20

0.25

Lubr

ican

t film

thic

knes

s [µm

]

P 1500 P 3000 h 1500 h 3000

Figure 14 - Power transmitted and lubricant film thickness (at point B of the meshing line -

wheel tip / pinion root) vs. the FZG load stage, for the wheel speeds of 1500 rpm and

3000 rpm (ISO VG 100 mineral oil at 90 ºC).



4.4. Gear tests results

The influence of the surface coating on the gears performance is very significant, as shown on Figure 15.

At 1500 rpm the scuffing load stage of the uncoated gear is FZGN -CTK = 11 , while the

coated gears didn’t reach scuffing: FZGCTK > 12 and FZG

CT*K > 13 . At this speed, the improvement in scuffing resistance is at least of 2 FZG load stages for the gears coated with MoST.

Joachim et al [3] obtained a similar improvement in scuffing resistance (2 FZG load stages) in standard FZG scuffing tests A/8,3/90 lubricated with Mobil Jet Oil II, between uncoated and coated gears, using WC/C (tungsten carbon carbide) and B4C (boron carbide) coatings.

Weck et al [2] also obtained an improvement in scuffing resistance (1 FZG load stage) in standard FZG scuffing tests A/8,3/90 lubricated with COE 100 base oil, between uncoated and coated gears, using WC/C (tungsten carbon carbide) coating. They also noticed a considerable decrease in the wear of the teeth flanks.

At 3000 rpm the scuffing load stage of the uncoated gear is FZGN -CTK = 7 , the coated gear

with high surface roughness reached scuffing in load stage FZGCTK = 12 , while the coated gear

with very good surface finishing overcome load stage FZGCT*K > 12 . At 3000 rpm the MoST

coating improves the scuffing resistance by 5 FZG load stages, which is a very significant result. These scuffing results are directly related to the low friction coefficient of the MoST

coating that originates lower contact temperatures between the gear teeth and thus a higher scuffing load carrying capacity

The results obtained for the coated gears, CT and CT*, also indicate that the quality of the surface finishing of the teeth flanks has some influence on the scuffing resistance, improving it by at least 1 FZG load stage.

Joachim et al [3] obtained a similar improvement in scuffing resistance (1 FZG load stage) in standard FZG scuffing tests A/8,3/90 lubricated with Mobil Jet Oil II, between the usual and super-finished uncoated FZG gears. They noticed that the influence of the surface coating on scuffing resistance is more important than the influence of surface super-finishing.

The differences occurred in the scuffing load stages also represent very significant differences in the power transmitted by the FZG type C gear, as shown in Figure 16.

At 1500 rpm the coated gear CT transmits 40% more power than the uncoated gear N-CT. At 3000 rpm the coated gear with very good surface finishing CT* transmits, at least, 17% more power than the coated gear CT, and this one transmits 77% more power than the non coated gear N-CT.



N-CT

N-CTCT

MoSTCT

MoSTCT*

MoST

CT*MoST

0123456789

1011121314

1500 3000Speed of gear wheel [rpm]

FZG

scuf

fing

load

stag

e [K

FZG]

N-CT CT MoST CT* MoST

Figure 15 - Scuffing (or maximum) load stage at 1500 rpm and 3000 rpm gear tests (FZG type C

gears lubricated with dditive free ISO VG 100 mineral oil at 90 ºC).

N-CT

N-CT

CT MoST

CT MoST

CT* MoST

CT* MoST

0 25 50 75 100 125 150 175 200 225 250 275 300

1500

3000

Spee

d of

gea

r w

heel

[rpm

]

Input power at scuffing [KW]

CT* MoST

CT MoST

N-CT

Figure 16 - Power transmitted by the FZG type C gear at the scuffing load stage or at the end of

the test (Additive free ISO VG 100 mineral oil at 90 ºC).



The teeth flanks of the pinion and wheel of each gear were examined at the end of the tests in order to assess the scuffing severity (if occurred), the surface wear and the existence of other types of surface failures. Images of those flank surfaces were made and are shown in Figure 17 for the tests performed at 3000 rpm.

Gear reference Pinion Wheel

Gear N-CT

uncoated

Ra = 2.4 µm

FZGN -CTK = 7

at 3000 rpm

Gear CT

MoST Coated

Ra = 2.4 µm

FZGCTK = 12

at 3000 rpm

Gear CT*

MoST Coated

Ra = 0.4 µm

FZGCT*K > 12

at 3000 rpm

Figure 17 - Images of the teeth flanks at the end of the tests performed at 3000 rpm.



The scuffing failure always occurred at the wheel tip / pinion root contact point on the

meshing line of the FZG type C gear. This was case for the uncoated gear N-CT at 1500 rpm and

at 3000 rpm and for the coated gear CT at 3000rpm, as shown on Figure 17. Besides the scuffing

marks, the gear flanks also presented severe wear all over the surface.

It’s normal that scuffing always occurred at the wheel tip / pinion root contact point of

the gear meshing line, since at that point the product µ . p .v reaches its highest value, as shown

on figure 18 ( µ . p .v = friction coefficient x Hertzian pressure x sliding velocity).

0,0

0,5

1,0

1,5

2,0

2,5

0 5 10 15 20 25Meshing line [mm]

Max

imum

Her

tzia

n Pr

essu

re

[GPa

]Sl

ide-

to-r

oll r

atio

[/]

0,0

0,1

0,2

0,3

0,4

0,5

Lub

rica

nt fi

lm th

ickn

ess [

mm

]

p0 SRr h0T

Figure 18 - Variation of the maximum Hertzian pressure, slide-to-roll ratio and lubricant film

thickness along the gear meshing line. (KFZG = 12, n = 1500 rpm, □ ○ contact point

between wheel tip / pinion root).

The MoST coated gears CT* show a very good performance in all situations. Scuffing

never occurred ( FZGCT*K > 13 at 1500 rpm and FZG

CT*K > 12 at 3000 rpm) and the teeth flanks

showed a normal and reduced amount of wear, as can be observed in Figure 17. The surface

roughness measurements made after test confirmed this observation, showing a very smooth

surface (RZDIN ≈ 1.9 µm) in both flanks of the pinion and of the wheel.



5. TESTS WITH A TRANSFER GEARBOX

5.1. Gearbox test rig

The transfer gearbox tests were performed in a back-to-back test rig with re-circulating

power. Thus, the driving electrical motor only supplies the power needed to overcome the

frictional and inertial losses and to reach and maintain the desired operating speed. Figure 19

shows a schematic view of the test rig.

Figure 19 - Schematic view of the gearbox test rig.

This test rig was developed to allow the testing of different types of gearboxes. The test

and the slave gearboxes are attached to adjustable platforms, and connected by the output torque

and speed sensor (Sen.2), which is mounted on a mobile platform allowing the adjustment of its

height and depth, as shown in Figure 19.

In order to close the loop and allow the re-circulation of power two gear sets are needed

at each end connected by a long rear shaft.

The torque is applied with a hydraulic cylinder connected to a helical gear. When the

hydraulic cylinder moves forward the helix angle of the helical gear twists the connecting gears

and applies the desired torque.

5.2. Transfer gearbox

Figure 20 shows a cross section of the two speeds gearbox used in this study. It is usually

used as a transfer gearbox, mounted after the conventional gearbox of the vehicle, allowing the

vehicle to have 2 drive axles (4 wheel drive) and an auxiliary power output.

Fixed platform

Test Gearbox

A

Slave Gearbox

B

Gears Gears Adjustable platforms

Mobile platform



This transfer gearbox uses 5 gears mounted in three shafts employing gibs. The gears

mounted on the input and output shafts are supported by needle roller bearings. The gears are

manufactured with DIN 15CrNi6 steel. After machining, the gears are case hardened, quenched

in oil and annealed. The geometric characteristics of the gears are given in Table 6.

Figure 20 – Cross section of the two speeds transfer gearbox.

Table 6 - Geometric characteristics of the transfer gearbox gears.

Gear wheel nº Parameter [units] Design.

1 3 2 4 6

Module [mm] m 4 4 3.5 3.5 3.5

Number of teeth [/] Z 17 28 27 23 32

Profile shift factor [/] x 0.051 -0.24 0.161 0.415 0.381

Width [mm] b 35 33.5 35 35 35

Pressure angle [º] α 20 20 20 20 20

Helix angle [º] β 20 20 20 20 20

Max. addendum diameter [mm] damax 80.7 125.2 108.4 95.3 128.6

The gearbox is lubricated with mineral base industrial gear oil, with a viscosity grade ISO

VG 150 and containing EP and AW additives. The transfer gearbox is filled up with 2.85 littera

of lubricant oil, as recommended by the gearbox manufacturer. The most significant properties

of the lubricant are given in Table 7.

21

43

6



Table 7 – Transfer gearbox lubricant properties.

Parameter Designation Value Type of base oil / Mineral Viscosity at 40ºC [mm2/s] ν 150 Viscosity at 100ºC [mm2/s] ν 14.5 Viscosity Index [-] VI 95 Specific gravity at 15ºC [kg/dm3] sp. gr. 0.896 Viscosity grade ISO VG 150 FZG Rating KFZG 13>

5.3. Transfer gearbox testing

Before each test the transfer gearbox is submitted to a running-in period of more than 530

000 cycles, during which the input power is softly increased, as shown in Table 8. In the end of

the running-in period the lubricating oil is changed.

Table 8 - Running-in operating conditions of the transfer gearbox.

Stage T [N.m] n [rpm] Pin [kW] N cycles t [minutes] 1 125 573 7.5 68 760 120

750 300 23.6 6 000 10 480 410 20.6 12 300 15 250 790 20.7 31 600 20 155 1265 20.5 63 250 25

2

130 1571 21.4 141 390 45

3 170 2000 35.6 120 000 60

4 130 3000 40.8 90 000 30 Total = 533 300 325

The transfer gearbox was tested in the lower speed range, when it is used as a torque

multiplier, considering realistic operating conditions in terms of input speed and torque. The

testing programme is defined in Table 9 and the operating conditions are related to the

characteristics of the vehicle using this transfer gearbox (power and torque of the diesel engine

and gear ratios of the main gearbox). The test time is 120 minutes.

The oil temperature when the test starts is 40ºC. The oil temperature is measured during

the test and depends on the operating conditions in each stage. The test is stopped if the lubricant

temperature exceeds 120 ºC.



Table 9 - Operating conditions of the transfer gearbox with MoST coated gears.

Parameter Design. Stages Input speed to transfer gearbox [rpm] n 573 1376 2178 2981 3784

Vehicle speed [km/h] V 5.7 13.6 21.5 29.4 37.4 P ≅ 36 kW Tin 600 250 160 120 90

Input torque [N.m] P ≅ 72 kW Tin 1200 500 320 240 180

5.4. Efficiency of the transfer gearbox

The gearbox tested has MoST coated gears and its efficiency is calculated for the input

conditions of torque and speed mentioned in Table 9.

The efficiency results presented in Figures 21 and 22 are compared with previous data

obtained with the same gearbox using uncoated gears. In both cases the operating conditions are

very similar and the oil temperature is in the range between 60ºC and 70ºC.

For an input power of 36 KW the maximum efficiency is obtained between 1400 rpm and

2400 rpm, for both gearboxes, coated and uncoated. However, in all the speed range considered

(700 rpm to 2300 rpm) the efficiency of the gearbox with coated gears is always better in about

0.5%, resulting from the reduction of the friction losses between the gear teeth due to the action

of the MoST coating.

Figure 21 - Efficiency of the transfer gearbox at constant input power (36 KW). Influence of the

MoST surface coating.



For an input power of 72 KW both gearboxes, with coated and uncoated gears, show the

same efficiency between 2300 rpm and 3800 rpm. For these high speeds and low torques the

lubricant film thickness between the gear teeth is higher and the corresponding contact pressure

and friction smaller. On the other hand, the churning losses become much more important than

the friction losses and thus the efficiency of both gearboxes is very similar.

However, for the low input speeds and higher torques, the importance of the friction

losses rise again and the efficiency of the gearbox with coated gears is again better than that

obtained with uncoated gears in about 0.5%.

These results clearly show that at low speed and high torque, when the contact pressure

and friction between the gear teeth are important and the lubricant film thickness reduced, the

friction losses are much more important than the churning losses and the influence of the MoST

coating becomes fundamental improving the efficiency of the gearbox.

Figure 22 - Efficiency of the transfer gearbox at constant input power (72 KW). Influence of the

MoST surface coating.

Figure 23 shows the active flanks of wheel nº 4 (see Figure 20) of both gearboxes, coated

and uncoated. The analysis of those surfaces shows that they are in perfect state exhibiting small

normal wear.



Uncoated

MoST coated

Figure 23 - Teeth flanks after test.

6. CONCLUSIONS

The MoST coating shows a very good tribological performance, confirmed by the results

of all tests made, such as, pin-on-disc, twin disc machine, FZG scuffing tests and transfer

gearbox efficiency tests. The MoST coating shows the following characteristics:

► High hardness and low friction coefficient.

► Very thin coating film almost not affecting the surface roughness of the substrate.

► Excellent adhesion to the steel substrate at high contact pressure, high slide-to-roll

ratio and low specific lubricant film thickness.

► Very significant increase of the load carrying capacity of FZG type C gears against

scuffing - 2 FZG load stages at 1500 rpm and 5 FZG stages at 3000 rpm.

► Improvement of gearbox efficiency at high torque and low speed when the friction

losses between the gear teeth are most sifnificant.

These characteristics shown that the MoST coating is of great interest at least in two

particular cases:

➼ Severe applications involving high contact pressures and high sliding, frequent start-

ups, or deficient lubrication.

➼ Acting as tribo-reactive materials and substituting non-biodegradable and toxic

additives in environmental friendly lubricants.



REFERENCES

[1] K. Holmberg, A. Mathew sand H. Ronkainem, “Friction and wear mechanisms of coated surfaces”, Finnish Journal of Tribology, Nº 3-4, Volume 17, 1998

[2] M. Weck, O. Hurasky-Schonwerth and Ch. Bugiel, “Service behaviour of PVD-Coated gearing lubricated with biodegradable synthetic Ester oils”, VDI-Berichte, Nº 1665, 2002

[3] F. Joachim, N. Kurz and B. Glatthaar, “Influence of coatings and surface improvements on the lifetime of gears”, VDI-Berichte, Nº 1665, 2002

[4] D. Teer, J. Hampshire and V. Bellido. EU patent Nº 96924987.9

[5] Y. Su, W. Kao, “Tribological behaviour and wear mechanism of MoS2 – Cr coatings sling against various counterbody”, Tribology International, Nº 36, 2003

[6] N. Renevier, N. Lobiondo, V. Fox, D. Teer and J. Hampshire, “Performance of MoS2/metal composite coatings used for dry machining and other applications”, Surface and coatings technology 123, 2000

[7] N. Renevier, V. Fox, D. Teer and J. Hampshire, Surface and coatings technology 127, 2000

[8] N. Renevier, V. Fox, J. Hampshire, D. Teer, “Performance of low friction MoS2/titanium composite coatings used in forming applications”, Materials and Design 21, 2000

[9] D. Teer, U. K. Patent Nº GB 2 258 343, EU Patent Nº 0 521 045, U.S.A. Patent Nº 5 554 518

[10] L. Magalhães, J. Seabra, “Reduction of the film lubricant use in heavy loaded motion transmission throght the aplication of self-lubricated coatings”, Technical Report T01, CRAFT Project BE-S2-5389, 1999

[11] R. Amaro, J. Seabra, “Reduction of the film lubricant use in heavy loaded motion transmission through the application of self-lubricated coatings”, Technical Report T02, CRAFT Project BE-S2-5389, 1999

[12] J. Silva, R. Martins, “Relatório Técnico T01 - Dimensionamento e analise experimental de uma caixa transfer de 2 velocidades”, FEUP, 1999 (in portuguese)

[13] R. Amaro, “Comportamento tribologico de revestimentos auto-lubrificantes para engrenagens, Master of Science Thesis, DEMEGI, FEUP, 2000 (in portuguese)

[14] R. Martins, “Avaliação experimental do desempenho energético de uma caixa transfer para veículos de tracção integral”, Master of Science Thesis, DEMEGI, FEUP, 2002 (in portuguese)

ACKNOWLEDGEMENTS

The authors would like to thank the European Comission for the finantial support given

to this study throught the project “Reduction of fluid lubricant use in heavily loaded motion

transmission systems through the application of sel-lubricating coatings”.

Project nº BE-S2-5389, Contract nº BRST-CT97-5363, 1998 – 2000.

The authors would like to thank Mr. Dennis Teer, from Teer Coatings L.td in UK, thar

kindly coated the gears used in this work.

most low friction coating for gears application

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