Ionic Liquids as Novel Lubricants and Additives - Department of Energy

Ionic Liquids as Novel Novel

Lubricants and Additives**

Jun Qu1, John J. Truhan2, Huimin Luo1,

Sheng Dai1, Peter J. Blau1

1Oak

Ridge National Laboratory

2University of Tennessee (currently at Caterpillar)

*Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy under Contract No. DE-AC05-00OR22725.

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY 1

Needs for Improved Lubricants

•

In transportation sector, 10~15% of the energy generated in automobile engines is lost to friction. In addition, production engine oils •

•

are one of the barriers for higher combustion temperatures to increase engine efficiency, limited by their low thermal stability above 250o C, and contribute hydrocarbon exhaust emissions due to oil blow by and burn-out.

A new class of more effective, environmentally-friendly

lubricants could lead to huge energy savings.


Introduction to Ionic Liquids

Ionic liquids (ILs) are composed of cations and anions, in stead of neutral molecules. {

Currently being used as green solvents in chemical synthesis, electrochemistry, catalysis, etc. Ionic liquid

Oil

Properties + { Inherent polarity + + { High thermal stability + - + { Negligible volatility Coulombic forces Van der Waals forces { Non-flammability { High flexibility of IL molecular design { Economical and environmentally friendly synthesis


Typical Molecular Structures of Ionic Liquids Common cations:

5

4 3

N

+

N

R1

1

2

R

1-alkyl-3-methylimidazolium

Common anions:

+ N R N-alkylpyridinium

water-insoluble [PF6][(CF3SO2)2N]- (or Tf2N-) [(C2F5SO2)2N]- (or BETI[BR1R2R3R4]- )

R2

+ N

R 4

R3

Tetraalkylammonium

R1 + P

R4

R2

R3

Tetraalkylphosphonium (R1,2,3,4 = alkyl) water-soluble

[BF4][CF3SO3]-

[CH3CO2][CF3CO2]-, [NO3]Br-, Cl-, I[Al2Cl7]-, [AlCl4]- (decomp.)

ORNL has been active in various areas of ionic liquids research since early 1990s, with a well equipped organic synthesis laboratory. OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY 4

Viscosity Strongly Correlates to Molecular Structure Examples With same cation, Cl-, Br-, or PF6- generate higher viscosities than Tf2N or BETI. With same anion, higher # of carbon in alkyl of cation leads to higher viscosity and lower density.

{

{

400

o

300 200 100

C[n]mim.PF6 - Viscosity

C[n]mim.Tf2N - Density

C[n]mim.PF6 - Density 1.5

700 1.4

600 500

1.3

400 1.2

300 200

1.1

100

I BE T

N 4m C

C

4m

im

im

Tf 2

PF 6

im

Cl C

4m

im

4m

C

4m

im

Br

0

C

C[n]mim.Tf2N - Viscosity 800 Viscosity @ 23 C (cP)

o

Viscosity @ 23 C (cP)

500

Density (g/cc)

0

1 2

4

6

8

10

12

Number of Carbon in Cation Alkyl, [n]


ILs have a Wide Range of Viscosities • •

Densities of ILs are in a narrow band, 1.03-1.46 g/cc @ 23 oC; Viscosities of ILs vary in a wide range, 50-1500 cP @ 23 °C. Lubricants

Hydrocarbon Mineral Oil oils 15W40 Oil C4mim.PF6 C6mim.Br Imidazolium C4mim.Tf2N ionic liquids C10mim.Tf2N C8mim.BETI [C6H13]3NH.Tf2N [C8H17]3NH.Tf2N Ammonium [C8H17]NH3.Tf2N ionic liquids [C2H5]3NH.BETI [C8H17]NH3.BETI OAK RIDGE NATIONAL LABORATORY

ρ (g/cc)

@ 23 °C 0.86 0.86 1.37 1.16 1.42 1.23 1.34 1.12 1.06 1.37 1.48 1.45

η (cP)

@ 23 °C 159 229 281 >1500 51 122 169 170 219 331 163 763

η(cP)

@ 40 °C 56 91 108 630 25 53 69 72 89 125 67 265

η (cP) @ 100 °C 6.3 11.3 13.3 n/m* 5.8 8.8 9.5 9.7 11.7 14.2 9.3 n/m*

Viscosity Index 78 128 110 n/m* 152 135 99 113 124 100 87 n/m*

*n/m - not measured

U. S. DEPARTMENT OF ENERGY 6

ILs Have High Thermal Stability Lubricant

Tonset (oC)

15W40 oil

236

C10mim.Tf2N

>400

[C8H17]3NH.Tf2N

357

TGA 100

Weight (%)

80 Mineral Oil 15W40 Oil [C8H17]NH3.NTf2

60 40

[C8H17]3NH.NTf2 [C8H17]NH3.BETI

20 0 0

100

200

300

400

500

Temperature (C)


Tribological Evaluation

Materials { Al 1100, Al 6061-T6, Al 319 { Counterface: AISI 52100 steel Lubricants: { 8 imidazolium ionic liquids { 5 ammonium ionic liquids { Mineral oil and 15W40 engine oil Testing temperature: RT and 100 oC Test configurations: { Ball-on-flat reciprocating sliding { Pin-on-disk unidirectional sliding


Friction Screening Tests Hydrocarbon oils

Imidazolium ionic liquids

Ammonium ionic liquids

Lubricants Mineral oil 15W40 diesel engine oil C6mim.Br C4mim.Cl C4mim.PF6 C6mim.PF6 C8mim.PF6 C10mim.Tf2N C8mim.BETI [C6H13]3NH.Tf2N [C8H17]3NH.Tf2N [C8H17]NH3.Tf2N [C2H5]3NH.BETI [C8H17]NH3.BETI

η @ 40 °C (cP) COF 56 91 630 136 108 153 245 53 69 72 89 125 67 265

0.10 0.09 0.09 0.21 0.18 0.20 0.07 0.16Æ0.10 0.21 0.11 0.06 0.07 0.20 0.08


Friction Reduction by ILs in All Lubrication Regimes

[C8H17]3NH.Tf2N produced 20-35% lower friction than 15W40 Engine Oil in all lubrication regimes. Suppress the transition from EHL to boundary lubrication (Stribeck curve shifted to the left); AISI 52100 steel against Al 6061-T6. Unidirectional sliding under 38.3 N load, 0.02-1.0 m/s.

Friction Coefficient

0.12

BL

Mixed

EHL

0.1 0.08 0.06 0.04 Mixed

EHL

15W40 Oil [C8H17]3NH.Tf2N

0.02 0 0

50

100

150

200

250

Viscosity x Sliding Speed (cP-m/s)


ILs Produce Low Friction

Ionic Liquid ([C8H17]3NH.Tf2N) vs. 15W40 Engine Oil • 20-35% friction reduction for Al alloys 0.16


0.14 0.12

15W40 Oil

[C8H17]3NH.Tf2N

0.102

0.1 0.078

0.08

0.096

0.087

0.079 0.057

0.06 0.04 0.02 0 Al 1100@RT

Al 6061@RT

Al 319@100C


ILs Produce Low Wear

Ionic Liquid ([C8H17]3NH.Tf2N) vs. 15W40 Engine Oil • 45-55% wear reduction for Al alloys • Virtually no wear on steel balls 4 3

Wear Volume (mm )

3.5

3.31

15W40 Oil [C8H17]3NH.Tf2N

3 2.5 2

1.88

1.5 1

0.59

0.5

0.06 0.03

0 Al 1100 @ RT

Al 6061 @ RT

0.26

Al 319 @ 100 C


Aluminum Had Less Material Transfer (Adhesive Wear) to the Steel Counterface in Ionic Liquid Lubrication Images of contact areas of the steel counterface

Fe

Fe

Fe

Al C 0

Fe

Fe

Cr 200

400

600

C 800

Fe Al

0

Mineral Oil OAK RIDGE NATIONAL LABORATORY

Fe

Cr 200

400

600

Mineral Oil + 10% [C8H17]3NH.Tf2N

C 800

0

Fe

Fe

Cr 200

400

600

800

[C8H17]3NH.Tf2N

U. S. DEPARTMENT OF ENERGY 13

Piston Ring-on-Flat Reciprocating Sliding Test (ASTM G 181-04)

Materials { {

Lubricants { {

Slider: Cr-plated piston ring Flat: Grey cast iron with simulated honing marks 15W40 diesel engine oil IL1 ([C8H17]3NH.Tf2N)

Temperature: 100 oC Normal loads: 240 N Sliding speed: 0.2 m/s (ave.) { 10 Hz, 10 mm stroke Test duration: 6 hours


Latest Results of Piston Ring-on-Flat Reciprocating Sliding Test (ASTM G 181-04) Compared with 15W40 oil, [C8H17]3NH.Tf2N (IL1) Reduced the initial COF by 25% and the final COF by 55% at the end of the six-hour wear test. Reduced the total wear rate (flat+ring) by 15%. 5.E-08

0.2 15W40 oil

IL1

0.14

0.12

0.11

0.12 0.1

3

0.16

0.09

0.08 0.05

0.06

Wear Rate (mm /N-m)


0.18

0.04

4.E-08 3.E-08

3.7E-08

15W40 oil

IL1

2.9E-08

2.E-08 1.1E-08 1.0E-08

1.E-08

0.02

0.E+00

0 Ini-COF

Final-COF

GCI Liner

Cr-Ring


Summary

A group of ammonium ionic liquids have been developed with promising lubricating performance and benchmarked with 15W40 engine oil. {

{

{

20-35% friction reduction and 45-55% wear reduction in lubricating steel-aluminum contacts. 25-55% friction reduction and 15% wear reduction in lubricating Cr-plated piston rings against cast iron. A surface boundary film was detected and is believed to be responsible for the firition/wear reductions.

A U.S. Patent was filed on September 19, 2006 (Application# 11533098)

Hint: No single ionic liquid can work for all materials, but with the uncountable species available, one would expect to design appropriate ionic lubricants for specific applications. OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY 16

Ionic Liquids Offer Significant Potential •

• •

• •

• • •

Reduce parasitic energy loss by friction reduction and allowing higher engine combustion temperatures. Extend service life and maintenance cycle by wear reduction. Expand the usage of lubricants to higher temperatures with higher thermal stability. Reduce air emissions due to ultra-low vapor pressure. Require fewer expensive lubricant additives with better intrinsic properties, e.g. boundary film formability and solvent nature. Serve as ashless additives for oil- and water-based lubricants. An effective replacement for catalyst-poisoning ZDDP. Safer transportation and storage because of non-flammability.


- Backup Slides -


Corrosion Behavior Electrochemical measurement Potentiodynamic polarization curves – both steel and aluminum showed active-passive corrosion behavior in [CH3(CH2)7]3NH.Tf2N. Steel

Aluminum


Friction Results for 3 Al Alloys

[C8H17]3NH.Tf2N produced lower COF by 20-35% than the 15W40 engine oil for different Al alloys. 0.14


0.12

15W40 Oil

C10mim.Tf2N

[C8H17]3NH.Tf2N

0.1 0.08 0.06 0.04 0.02 0 Al 1100@RT

Al 6061@RT

Al 319@100C


Wear Results for 3 Al Alloys

[C8H17]3NH.Tf2N produced lower wear by 45-55% than that the 15W40 engine oil for different Al alloys.

C10mim.Tf2N produced unexpectedly high wear for Al 6061. 6 5.69 3

Wear Volume (mm )

5 4

15W40 Oil C10mim.Tf2N [C8H17]3NH.Tf2N

Why? React with Mg?

3.31

3 2.17 1.88

2 1

0.59 0.44 0.06

0 Al 1100 @ RT

0.03

0.26

Al 6061 @ RT Al 319 @ 100 C


Surface Chemistry - [C8H17]3NH.Tf2N

Boundary films are detected on aluminum surfaces. {

Inherent polarity and tribo-chemical reactions.

XPS analysis of Al 6061 worn surface lubricated by [C8H17]3NH.Tf2N OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY 22

Surface Chemistry - [C8H17]3NH.Tf2N

Possible composition of the surface boundary film: AlF3, Al2O3, Al2S3, Al metallic phase, and organic compounds. Al 2p 2000

Al2S3

C/S

1800 1600

AlF3

Al

1400

Al2O3

1200 1000 72

73

74

75

76

77

Binding Energy (eV)
