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The experts of the South-Ural State University, the Institute of Mechanics Problems named after A.Yu.Ishlinsky (IMP) of the Russian Science Academy (RSA), and the 25th State Science-Research Сhemmotology Institute of the Defense Ministry of the Russian Federation have held researches and comparative analysis of the solid-film coatings anti-friction properties that are intended, among the other purposes, to reduce friction, and to prevent scuffing forming in the piston skirt-and-cylinder liner pair of the high-powered diesel internal-combustion engines.
That work was fulfilled with support by the grant from the Department of Science and Higher Education of the Russian Federation as a part of the state task No. 9.7881.2017/БЧ, and partially on the theme of the state task for the tribology laboratory of the IMP of the RSA (State Registry No. АААА-А20-120011690132-4).
According to the tests results, it has been detected that use of the anti-friction solid-film coatings significantly reduces friction in the unit under study.
Some materials have demonstrated the low values that can be compared with the conditions of contacting the parts in present of a lubricating agent, and in absence of direct metallic contact between the elements.
Researches Relevancy
The overwhelming part (up to 66 %) of loads on the high-powered diesel engine are on piston skirts, piston rings, and bearings.
Directly in piston-to-cylinder, and piston ring-to-cylinder friction pairs, friction losses make 50% from the general friction losses in the engine.
Related to this, the role of the anti-friction coatings which are able to reduce friction in case of breaking friction hydrodynamic regime of the parts is very important.
Functions of the coatings in ICE:
- Reducing friction and wear coefficient
- Decreasing fuel consumption
- Preventing scores at motor oil lack: at cold starting, mechanical or heat piston deforming, exceeding acceptable service temperature, using non-quality fluid inclined to fast oxidation, etc
High-powered diesel engine operating has some features of its own. Firstly, these are heat deformations of the piston. Secondly, reinforced wear of the piston skirt as a result of mixed and boundary friction modes that occur.
That’s why to provide long service life for the parts, it is important both selecting the best coating on the service properties, and to provide its high adhesion to the base on the stage of preliminary preparation the surface of the part to be treated to applying the product.
The imported materials for piston skirts, as a rule, are made based on graphite, and other solid lubricating components, and for aluminum-to-aluminum friction pairs they are based on iron phosphates, and others. (Grafal/EvoGlide, Tin, Ferrostan/FerroTec, Ferroprint type wear-resistant netlike coating, and others).
Grafal/EvoGlide form a thin polymer matrix with the small graphite particles spread in it, and differ by a binder type used in their composition.
They can be applied onto the pistons of any constructive materials unlike, for example, Tin, and other compounds based on aluminum phosphate. They are used on aluminum elements exclusively.
To reduce friction on both aluminum, and steel pistons by 15 to 30%, anti-friction coatings based on iron phosphate are applied.
The coatings based on graphite or its composition with molybdenum disulfide, that is solid lubricating components of layered crystal structure are also used on piston skirts. Positive impact from using them to reduce wear, and to provide protection against scores is remarked in many science studies results.
Most coatings used in high-powered diesels, and internal-combustion engine units are produced abroad, at large.
Related to this, a relevant issue is replacing such materials with the Russian analogues which are designed and released by the only one company on the territory of the country – Modengy, LLC. It is situated in Bryansk town.
To hold researches, the company has provided a number of their coatings as samples.
About MODENGY coatings
MODENGY anti-friction solid-film coatings are liquid compounds containing superfine particles of solid lubricating material, binders, and solvents.
After being applied by method of spraying out, dipping with centrifuging, or screen printing onto the part surface prepared in a special way, and polymerizing at room temperature, or at heating in a kiln, they form a thin layer (up to 20 µm) which is a matrix of a binder with solid lubricants particles that are spread into its cells.
This enables to provide protection against scores during running-in of the rubbing parts of a new power unit as well as in the ‘oil starving’ moments.
MODENGY coatings based on graphite, or graphite and molybdenum composition took part in this experiment.
Tests conditions, and samples
The tests were held on the laboratory tribometer modelling operation of the given friction pair according to kinematic scheme of the cylinder liner fragment back-and-forth motion relative to the fixed piston skirt element.
The coatings from the Russian Modengy company (namely, MODENGY For the ICE Parts, MODENGY 1006, MODENGY 1066, MODENGY 1007) were applied onto the piston skirt segment surface.
The sample for comparison to detect efficiency of these coatings operation from the domestic producer was the imported coating based on graphite that is successfully used on steel pistons of the heavy-load vehicle diesel engines. It is marked as Sample No.1 on the graphs.
Fig.1. Coating No.1 material's photo (a) and spectrogram (b)
The basis of the given material is carbon without molybdenum added.
Test holding conditions were comparable with the diesel engine operational conditions: +110 °С temperature, standard loads within 50 to 450 N range, stable back-and-forth motion frequency of 20 Hz.
Upper temperature limit of operating all coatings under study doesn’t exceed +250 ºС.
The samples to hold the tests were cut out of the high-powered diesel engine parts.
The coatings were by turn applied onto the fragment in the shape of a pin gained of the piston skirt made of steel.
Prior applying the first coating's sample, and after each stage of testing, the pin was subjected to sandblasting. This enabled to get rid of the residues of already tested materials, to activate the surface, and to remove a shielding film.
The Samples 1, 3, 4, 5, and 6 (see Table 1) after sandblasting were also treated with manganese phosphate what enable additionally to improve cohesion between the coating, and the steel pin.
A plate made of the cylinder liner of alloyed cast iron (ρ=7000 kg/m3) from the same power unit served as a counter sample for the steel pin covered with the coating.
Tests holding methods
The tests were held on the laboratory tribometer with a heat chamber.
The kinematic scheme intended back-and-forth motion of the plate relative to the pin fixed and pressed to it.
Fig. 2. Photos of the friction pair installed in the tribometer holder where 1 – pin-sample, 2 – plate, 3 – heat chamber
Prior starting the experiment, 5-minute long washing of the samples in the Galosha solvent in the ultrasonic bath, and next in ethyl alcohol to remove all contaminations was held.
No fitting-in of the friction pair to be studied was held.
The tests recreated one of the characteristic real-life operational conditions: boundary friction appearance for a short while at extreme operational modes of the engine.
While testing, the pin sample, and the plate were put into the holders of the tribotechnical plant, the heat chamber was closed with a lid. After that heating up to +105 to +115 °С was done with exposure during 55 to 60 minutes to imitate operational conditions of the materials to be studied.
The back-and-forth motion frequency of the plate sample was stable - 20 Hz, and movement amplitude made 6 mm.
Standard load at testing to detect friction coefficient grew stepwise within 50 to 450 N range by a 100 N step.
At maximum loading, it was possible to achieve extreme values of the diesel specific load.
At each load level, the samples were testing during 5 minutes. The complete test for each coating made 25 minutes.
To make the experiment whole, the tests were also held on the samples treated by the sandblasting method as well as on the elements with Castrol 5W40 motor oil in volume of 2 to 4 ml applied onto their surface.
To gain the perceivable and reliable results, all the experiments were recreated 3 times.
Research Results
On Fig.3, typical dependences of friction coefficient μ from the standard load P are presented.
It is necessary to remark that at motor oil presence in the contact, friction coefficient value had no differences for the samples both with and without the coating, that’s why on Fig.3 the results for one coating variant only (Coating No.1) with use of the lubricating material are presented.
Fig. 3. Friction coefficient µ dependence from Test time t where the samples numbers represented with the figures
All the experiments were held by series, each following experiment was done on the newly prepared samples.
The diagrams gained according to the experiment results were processed by using the statistical analysis method for the experiments. The calculated friction coefficient average values, and their changing range for all tested samples are presented in Table 1, and on Fig.4.
Table 1
|
Product name and composition |
Sample number |
Friction coefficient at standard load P, N |
||||
|
50 |
150 |
250 |
350 |
450 |
||
|
Coating No.1, special binder, graphite |
1* |
0,42±0,09 |
0,39±0,07 |
- |
- |
- |
|
Modengy 1007 Polyamidimide, graphite |
2 |
0,26±0,03 |
0,16±0,04 |
0,14±0,05 |
0,14±0,03 |
0,18±0,04 |
|
3* |
0,22±0,07 |
0,17±0,05 |
0,17±0,03 |
0,13 |
0,12±0,03 |
|
|
MODENGY For the ICE Parts, special binder, graphite, and MoS2 |
4* |
0,16±0,03 |
0,11±0,01 |
0,11±0,01 |
0,10±0,02 |
0,11±0,02 |
|
Modengy 1066 Modengy 1006 Polyamidimide, graphite, and MoS2 (different concentrations) |
5* |
0,13±0,05 |
0,12±0,01 |
0,11±0,03 |
0,12±0,02 |
0,11±0,01 |
|
6* |
0,12±0,04 |
0,10±0,02 |
0,10±0,01 |
0,10±0,01 |
0,11±0,02 |
|
|
Coating No.1 (with motor oil) |
7 |
0,14 |
0,13 |
0,13±0,01 |
0,13 |
0,12 |
|
No coating |
8 |
0,69±0,08 |
0,58±0,06 |
- |
- |
- |
Fig. 4. Friction coefficient µ dependence from Standard load P where the samples numbers represented with the figures
For all the coatings subjected to the tests, reducing friction coefficient with increasing standard load was observed what is typical for the coatings based on solid lubricating material of layed structure.
Using manganese phosphating enables to improve wear-resistance of the coating. At high loads (350 to 450 N) on the samples without phosphate coating fast destruction of the anti-friction coating on the base surface was observed.
Friction process on the samples with the coating can be characterized as stable as they have sound radiation by 70% lower than the samples without the coating have.
Sample No.1 has demonstrated increased friction coefficient. To explain this phenomenon, microscopic studies of the samples surfaces were held after passing the tests by using Jeol JSM-7001F scanning electronic microscope in the magnification range of 50 ... 5000 times.
Fig. 5. Coating No.1 SEM photos gained on BSE (а) EDS (b) detectors
The X-ray fluorescence analysis of the coating was performed by using Oxford INCA X-max 80 energy dispersive spectrometer installed on the microscope. The spectrometer enabled to analyze the elements with atomic numbers from 5 (В) to 92 (U). The system is automated, and has sensibility up to 0.1 mass.%, required to hold this analysis.
Microtexture analysis of the butt-end microscopic sections of the samples with Coating No.1 enabled to make a number of conclusions: prior applying the coating, manganese phosphating was held, the layer thickness is 3 µm; coating thickness after the tests taking into account the phosphate layer is about 10 µm.
It can be supposed that increased friction coefficient is related with nature of the binder retaining graphite.
Conclusion
These experiments with the coatings were performed without applying any liquid lubricating material onto them to study anti-friction and anti-scuffing properties of the solid lubricating materials that are used to protect high-powered diesel piston skirts which short-time modes of both boundary and dry friction are typical for.
According to the research results, it has been proved that use of anti-friction coatings enables to reduce friction significantly, and to increase seizure load.
MODENGY For the ICE Parts, and MODENGY 1006 enable to reduce friction losses by 5 times, and to extend service lifetime of the high-powered engine.
Further on, for the purpose of studying the cylinder-to-piston tribological conjunction, it is necessary to hold a number of experiments oriented to research tribological characteristics, and to assessment of wear intensity of the parts friction surfaces.
Besides, it is necessary to study operational conditions of the samples covered with the coatings at presence in the contact motor oils with various rheological properties.
Bibliography:
1. Holmberg K., Andersson P., Nylund N.O., Makela K., Erdemir A. Global energy consumption due to friction in trucks and buses // Tribology International. — 2014, V. 78, 94 –114.
2.Bartz W.J. Fuel economy improvement in engine and gear oils. In: Proceedings of the
24th Leeds–Lyon symposium on tribology, tribology for energy conservation. — 1998, 13—24.
3. Taylor R.I., Coy R.C. Improved fuel efficiency by lubricant design: a review //
Proceedings of the Institution of Mechanical Engineers. —2000, 1–15.
4. Ye Z., Zhang C., Wang Y., Cheng H.S., Tung S., Wang Q.J., He X. An experimental
investigation of piston skirt scuffing: a piston scuffing apparatus, experiments, and scuffing mechanism analyses // Wear. —2004, Vol. 257, № 1–2, 8–31.
5 I.G.Goryacheva, A.V.Morozov, Yu.V.Rozhdestvensky, K.V.Gavrilov, A.A.Doykin. Developing a method for experiment-calculated assessing diesel piston-to-cylinder conjugation tribological parameters // Friction and Wear. — 2013, Vol. 34, No.5,
446–457.
6. Milojević S., Pešić R., Davinić A., Taranović D. Coated al piston as technological
solution to lowering of friction losses inside IC engine // 12. International Conference on Accomplishments in Electrical and Mechanical Engineering and Information Technology. —
2015, 741—746.
7. Zhang J., Li H. Influence of manganese phosphating on wear resistance of steel piston
material under boundary lubrication condition // Surface & Coatings Technology. — 2016,
Vol.34, 530 —536.
8. Buyukkaya E. Thermal analysis of functionally graded coating AlSi alloy and steel
pistons // Surface and Coatings Technology. — 2008, V.202, 3856–3865.
9. MAHLE GmbH (Ed.), Pistons and engine testing, DOI 10.1007/978-3-8348-8662-0
10. Shaw A., Qu J., Wang C., England R. Tribological study of diesel piston skirt coatings
in CJ-4 and PC-11 engine oils // Wear. — 2017, Vol.376-377, 1673—1681.
11. Kumar V., Kumar S.S., Kumar A.A. Wear evaluation of engine piston rings coated
with dual layer hard and soft coatings // Journal of Tribology, —2019, V. 141, 10.
12. Westerfield Z., Totaro P., Kim D. and Tian T. An Experimental Study of Piston Skirt
Roughness and Profiles on Piston Friction Using the Floating Liner Engine // SAE Technical Paper. — 2016.
13. MAHLE GmbH (2012) Piston materials. In: MAHLE GmbH (eds) Pistons and engine testing. ATZ/MTZ-Fachbuch. Vieweg+TeubnerVerlag, Wiesbaden.
14. L.N.Sentyukhina, E.M.Oparina. Solid lubricants of molybdenum disulfide. М.:
Chemistry, 1966. – 152.