Prof. Volf LESHCHYNSKY, D.Sc.(Eng.), Hanna WISNIEWSKA-WEINERT, Ph.D. Metal Forming Institute, Poznan, Poland
Superlubricity and new superlubricant materials
made by powder spraying
Supersmarność oraz nowe materiały supersmarne
wytwarzane napylaniem proszkowym
Abstract
Superlubricity is the state in which two contacting surfaces exhibit no resistance to sliding. The work describes a ultra-low friction phenomenon due to application of MoS2 nanoparticle coatings results in ultra-low friction coefficients in ambient atmosphere.It is shown the concept of superlubricity of combined coatings can be effec-tively used to improve the friction under dry sliding conditions.
The additional application of solid lubricant films by powder spraying and MoS2 precipitation technologies results in low friction coefficients and a good tribological behaviour under dry sliding conditions. To clarify the tribological characteristics of MoS2 films, friction experiments were conducted at stick-slip regime at the high temperature. It was found that the friction coefficient value was very low and stable for prolonged periods.
Streszczenie
O supersmarności zazwyczaj mówimy wówczas, kiedy dwie stykające się powierzchnie wykazują brak oporu podczas tarcia ślizgowego. Artykuł opisuje zjawisko ultra-niskiego tarcia dzięki zastosowaniu powłok z nanoczą-stek MoS2, wynikiem którego jest otrzymanie ultra-niskiego współczynnika tarcia. Autorzy wskazują, Ŝe
zastoso-wanie wielokomponentowych supersmarnych powłok otrzymanych metodą napylania proszkowych materiałów zawierających dodatki smaru stałego MoS2 znacznie zmniejsza współczynnik tarcia w warunkach suchego tarcia ślizgowego.Przedstawiono charakterystykę tribologiczną powłok zawierających MoS2, przeprowadzono testy
tarciowo-zuŜyciowe w podwyŜszonej temperaturze. Określono wartość współczynnika tarcia w czasie.
Key words: solid lubricant, nanoparticles, friction coefficient Słowa kluczowe: smar stały, nanocząstki, współczynnik tarcia
INTRODUCTION
A critical need in tribology systems for engines and vehicles is an efficient, compact, and cost-effective lubrication to sharply fall friction losses. The efficiency, reliability and durability of engines depend on effective lubri-cation or friction and wear reduction in critical components such as bearings and seals. Con-ventional oil or grease lubrication of engine components is not desirable because such lu-bricants can contaminate and poison environ-ment. The objective of this paper is to develop and evaluate low-friction coatings and particu-late materials for critical components on the base of superlubricity approach. The work
is focused on short description of superlubricity effect and evaluation of materials and coatings for ultra-low friction process, as well as the development of generalized powder technology selection methodology for combined coatings with dry lubrication ability.
1. SUPERLUBRICITY
Superlubricity is the state in which two contacting surfaces exhibit no resistance to sliding. M. Hirano [1] have shown that su-perlubricity is related to the atomistic origin of friction and that the phenomenon appears when the sum of the force on each moving
atom against the entire system vanishes. In accordance with M. Hirano [1] the degree of freedom of atomic motion varies considerably. A motion of atoms at a contact surface is shown on Figure 1. The white circles show unstable areas in which atoms cannot stably exist and the shaded areas show stable places in which atoms they can stably exist. The past theories, that essentially analyze friction on the basis of one-dimensional models, (Fig. 1a) clearly show the degree of freedom in the mo-tion of an atom is low. This means that if the unstable areas appear, an atom will undergo non-adiabatic motion as it passes through those areas. These unstable area appears as a result of strong interaction between solids, the exis-tence of impurities, lattice defects, etc [1]. In two- and three-dimensional systems (Fig. 1b,c) the degree of freedom in the motion of an atom is high. As a result, even if unstable areas oc-cupy a lot, an atom can pass through the stable areas by moving around the unstable areas, as shown in Fig. 1b.
This case makes it easy for superlubricity to become apparent [1]. In this case the super-lubricity would be stable for a certain concen-tration of impurities and defects. An exceeding a certain value of impurities and defects will result in the rise of friction with friction forces increasing monotonically as that concentration increases (Figure 1c). As shown on Figure 1c, unstable areas will grow, and non-adiabatic motion will occur if stable areas become cut off. This occurrence of non-adiabatic motion results in a friction transition in which friction changes from zero to a finite value [1].
The described specific case of superlubri-city situation is frictional anisotropy [2] when in-commensurate contacting surfaces are sliding on each other. The superlubricity situation is believed to be realized for two contacting crystal lattices at a certain misfit angle [1]. The friction force is shown to be lowered by one order of magnitude [1] for the case of cleaved mica, when changing the lattice mis-fit angle. However, no frictional anisotropy could be seen in air atmosphere because of presence the surface contaminants.
Molybdenum and Tungsten disulphides are well known lamellar solid lubricant with hexa-gonal structure. The low friction of MoS2
coa-tings (friction coefficient f = 0.01-0.05) has been recognized in various cases in inert gas atmosphere or high vacuum [3,4]. J.M. Martin et al. [5] achieved the ultra-low friction coeffi-cients (in the range of f = 10-3) when tested in ultrahigh vacuum a sputtered MoS2 coatings
exempt of impurities such as carbon, oxygen, and water vapour. However, M. Chhowalla and G. Amaratunga [6] show an application of MoS2 nanoparticle coatings results in
ultra-low friction coefficients in ambient atmos-phere. The friction coefficient was obtained by a unidirectional `ball on disk' (pure sliding) test. The mating ball was also made from 440C stainless steel with a diameter of 0.7 cm and of comparable average roughness to the disk. The applied load was 10 N (using a dead-weight) on to the rotating disk (speed, 0.5 ms-1) which gave a maximum Hertzian pressure of 1.1 GPa and contact diameter of 100 mm. The mating ball was uncoated.
Fig. 1. Motion of atoms at a contact surface according M. Hirano [1]: (a) one-dimensional system; (b) and (c) two- and three-dimensional systems
Rys. 1. Ruch atomów na powierzchni kontaktowej według M.Hirano [1]: (a) układ jednowymiarowy; (b) oraz (c) układy dwu i trójwymiarowe
Discontinuous motion Discontinuous motion
The Figure 2 depicts the coefficient of fric-tion as a funcfric-tion of time for: sputtered MoS2
films in 45% relative humidity in ambient con-ditions (curve 1); sputtered MoS2 films in dry
nitrogen (curve 2); and nanoparticle MoS2
films in 45% humidity (curve 3).
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
5.0E+04 1.0E+05 1.5E+05 2.0E+05
Number of cycles F ic ti o n c o e ff ic ie n t 1 2 3
Fig. 2. The friction coefficient as a function of number of cycles according [6] for: sputtered MoS2 films in 45% relative humidity in ambient
conditions (curve 1); sputtered MoS2 films in dry
nitrogen (curve 2); and nanoparticle MoS2 films
in 45% humidity (curve 3)
Rys. 2. Współczynnik tarcia w funkcji ilości cyklów według [6] dla: rozpylonych warstw (filmów) MoS2
w 45% relatywnej wilgotności w warunkach otoczenia (wykres 1); rozpylonych warstw (filmów) MoS2 w suchym
azocie (wykres 2); oraz warstw (filmów) z nanocząstka-mi MoS2 w 45% wilgotności (wykres 3)
The results reveal there exist conditions of superlubricity effect in real macroscopic cases in spite of presence of contaminants at the interface. This behavior of solid lubricant coating at high contact loads for a long time seems to be result of :
(i) nanostructuring features of MoS2 films
that allow to achieve the effect of frictional anisotropy, and
(ii) high reliability of the sputtered MoS2
nanoparticle coating.
2. SUPERLUBRICANT MATERIALS
AND COATINGS
Fuel economy and reduction of harmless elements in lubricant are becoming crucial in the automotive industry. An approach to respond these requirements in engine
com-ponents is the potential use of low friction coat-ings exposed to specific boundary lubrication conditions. Superlubricity is a new research field in tribology, dealing with very low fric-tion values, typically below 0.01, and this even in dry or vacuum conditions. Superlow friction was already experimentally observed only in ultrahigh vacuum and inert gas environment, with pure molybdenite (MoS2) coatings [6,7]
and in presence of some hydrogenated DLC coatings. Under boundary lubrication, the au-thors [7] show that the coupling of hydrogen-free carbon coatings and selected organic lubri-cant additives permits to reach friction values approaching superlubricity nd also a wear-less behavior.
For very smooth surfaces, even film thick-ness less than the combined surface roughthick-ness can provide good lubrication [8] and superlu-bricity effects.. A development of the new technology combining very smooth surfaces with nanostructured solid lubricant powder coating seems to be important from the view-point of approaching superlubricity.
Due to the increasing importance of light-weight engineering and design driven by eco-logical and economical reasons, advanced composite coatings are needed to improve the surface properties of machine elements and system components made of light or soft metals and their alloys. Light metals in general exhibit very poor tribological properties in unlubricated condition resulting in severe seizing and wear. The application of thin solid lubricant coatings by PVD and CVD processes improves the tribological behavior signifi-cantly, but in many cases these coatings fail under high surface loading due to the low Young’s modulus of the light metal substrate and the mechanical incompatibility of thin solid film and substrate [9].
The strategy of new coating development is shown on Figure 3. These coatings are be-lieved to rise the dry lubrication ability both at ambient and high temperatures.
The wear resistant primary layer is applied by hardening or thermal spraying for all dif-ferent coating systems. The thermal spray tech-nology is well established and allows the appli-cation of a broad variety of materials in the form of spray powder on different substrate
materials. Most thermal spraying techniques use combustion gases and plasma as sources of the thermal and kinetic energy heating the powder particles and propelling them to the substrate to coat. Prior to the application of the thermally sprayed layer, the light metal substrates are degreased and grit blasted to in-crease the surface roughness, since the adhe-sion of the thermally sprayed layer is mainly caused by mechanical clamping. The atmos-pheric plasma spraying (APS) may be used to apply the wear resistant primary coatings as well [9].
For most combined polymer-solid lubricant coatings the typical surface roughness and to-pography of a substrate is beneficial since the ‘valleys’ act as a reservoir for the lubricant-polymer varnish which is constantly released during operation. In the case the surface rough-ness is too high after pre-processing steps, an optional mechanical processing might be added to remove the most protruding asper-ities by mechanical grinding and polishing
and to create a more homogeneous load bearing area and to facilitate approaching superlubricity regime. Different technologies can be used to apply the thick lubricating varnish, like dip-ping, brushing and rolling. A special MoS2
precipitation technology was developed by INOP to deposit the nanoparticle lubricant coating with thickness of 0.3-0.5 mm (Figure 3a).
Surface fragmentation is the new nanos-tructuring technology of a coarse-grained solid lubricant material which is developed as the main step of technology route shown on Figure 3b. It provides a new approach to the improvement of its properties without changing its chemical composition. With the increasing evidences of unique properties for nanostructured materials, it is reasonably expected to achieve surface modification by generation of a nanostructured surface layer, i.e. surface fragmentation of solid lubricant particles.
Fig. 3. Fabrication routes for combined coatings with dry lubrication ability: a - polymer-solid lubricant coating route; b – surface nanostructuring – solid lubricant deposition by deformation techniques
Rys. 3. Sposób wytwarzania wielokomponentowych powłok stosowanych w warunkach suchego tarcia ślizgowego: a – powłoka polimer-smar stały; b – nanostrukturyzacja powierzchni – nałoŜenie smaru stałego z zastosowaniem
technik odkształcania Pre-processing metal substrate (degreasing, grit-blasting) Hardening ( Nitriding, Chemical Vapor Deposition)
Optional mechanical proc-essing (grinding, polishing)
Degreasing with solvents (e.g. acetone)
Precipitation technology of solid lubricant powder
layers
Post-processing powder coatings
Pre-processing metal substrate (degreasing,
grit-blasting) Coating with metal/ceramic by thermal or cold spraying Optional mechanical processing (grinding, polishing)
Degreasing with solvents (e.g. acetone)
Solid lricant particle deposition by surface deformation techniques
Post-processing powder coatings
a) b) Fig. 4. SEM of MoS2 (a) and WS2 nanoparticles obtained by surface fragmentation
Rys. 4. Obraz SEM nanocząstek MoS2 (a) i WS2 otrzymanych poprzez fragmentację powierzchni
3. TRIBOLOGICAL TEST RESULTS
The ring-shaft tribology tests were made to evaluate the potential of nanostructured powder coatings in the harsh temperature conditions. The ring samples were tested at T = 300 oC in stick-slip friction regime (cy-cle of sleeve pendulum motion is 90° and 100 cycles/min). The first results of tribology tests are shown on Figure 5 (samples designation is shown on the Figure 5). The data reveal the friction coefficient after running stage is about 0.04-0.1. The friction coefficient falls up to 0.028 at the temperature of 300 oC. Thus, only solid lubricant is responsible for a such ultra-low friction in spite of high oxidation impact of air atmosphere. The run-in period is different for each sample which depends on surface hardness of sleeve. The example of long run-in period is presented on the dia-gram for sample 4.
0 0.05 0.1 0.15 0.2 0.25 0.3 10000 20000 30000 40000 50000 60000 Number of cycles F ri c ti o n c o e ff ic ie n t 1 2 3 4 1- SleeveNiCr-021+ (MoS2+NC111) -20oC 2- SleeveNiCr-021+ (MoS2+NC111) -300oC 3- SleeveNiCr-021+ (MoS2+NC111) -300oC 4- SleeveNiCr-021+ (MoS2+NC111) -300oC
Fig. 5. High temperature friction test results
Rys. 5. Wyniki testów zuŜyciowych w podwyŜszonej temepraturze
The dry friction coefficient at the 20 oC is about 0.03-0.04 which confirms our
sugges-tion about possibility of approaching ultra-low friction regime.
4. CONCLUSION
The concept of superlubricity of combined coatings can be effectively used to improve the friction under dry sliding conditions.
The additional application of solid lubri-cant films by surface fragmentation and MoS2
precipitation technologies results in low fric-tion coefficients and a good tribological beha-viour under dry sliding conditions.
To clarify the tribological characteristics of MoS2 films, friction experiments were
con-ducted at stick-slip regime at the high tempera-ture. It was found that the friction coefficient value was very low and stable for prolonged periods.
A wide range of applications of these coa-ting combinations in sliding components is expected.
Materiały prezentowane były na Seminarium pt. „New materials for advanced applica-tions”, 18-19.09.2006 r. Poznań-Wąsowo.
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