Biolubricants and Triboreactive Materials for Automotive Applications

Abstract The research institution TEKNIKER has coordinated the EUROPEAN Project EREBIO, were different biodegradable lubricants have been formulated by FUCHS and BAM for heavy duty engines (GUASCOR), and passenger cars (RENAULT). In the frame of this article, it has been summarised the results obtained when developing biodegradable passenger car lubricants in combination with triboreactive materials. Replacing hydrocarbon-based oils with biodegradable products is one of the ways to reduce adverse effects on the ecosystem caused by the use of lubricants. The application of low or no sulphur, ash and phosphorous (lowSAP) ester- or polyglycol-based oils, intended for passenger car engine lubricants as substitutes for hydrocarbon-based oils, required the preparation of a composition of lubricants with comparable tribological and functional properties. The study is focussed on passenger car motor oils (PCMO) with reduced metal-organic additives. This is necessary in order to reduce the ash build-up in the after treatment system and therefore improve its efficiency and lifetime. High fuel efficiency and long drain intervals are requested, as well. To follow a line in a consequent way, these oils have to be biodegradable and non-toxic to the aqueous environment according to the directive EC/1999/45, coherent with other international standard. In a modern diesel or gasoline engine, the engine oils has to fulfil quite a number of different functions, such as lubricating and cooling the system, wear protection, soot and particle handling with less deposit tendency and so on. In the paper a study of the biodegradability, toxicity and the tribological properties has been carried out for new developed prototype engine bio-oils. Also, some different plasma sprayed triboreactive coatings have been deposited on cast iron piston rings, being studied also their tribological properties. Finally, the behaviour of the new bio-oils selected and plasma sprayed triboreactive coatings on piston rings have been screened in a real engine.


Introduction
Replacing hydrocarbon-based oils with biodegradable products is one of the ways to reduce adverse effects on the ecosystem caused by the use of lubricants. The application of low or no sulphur, ash and phosphorous (lowSAP) ester-or polyglycol based oils are intended for passenger car engine lubricants as substitutes for hydrocarbon-based oils. The study is focused on passenger car motor oils (PCMO) with reduced metal-organic additives. This is necessary in order to reduce the ash build-up in the after treatment system and therefore improve its efficiency and lifetime. High fuel efficiency and long drain intervals are requested, as well.
In a modern diesel or Gasoline engine, the engine oils has to fulfill quite a number of different functions, such as lubricating and cooling the system, wear protection, soot and particle handling with less deposit tendency and so on.

Relevant aspects of influence
Generally speaking, engine oils consist of three major parts. The biggest part (per volume) is taken by the base fluid. It is responsible for the wear free hydrodynamic regime in lubrication and provides the basic lubricity. Besides, it serves as solvent for the additives. The additives can be subdivided into the group of surface active and lubricant active additives.
The first group consists of EP/AW-additives to reduce wear at heavily loaded tribo-contacts and so called friction modifiers (FM) that are used to reduce the coefficient of friction. These additives are polar and tend to adhere to the metallic surface of the engine components. Conventionally, they contain sulphur and phosphorous usually as metal-organic compounds. The other additive group is unpolar or less polar and consists of antioxidants (AO) to prolong the oil service life, dispersancy and detergency (D&D) to keep the engine clean and soot in dispersion and of corrosion inhibitors (Ci). Metal-organic compounds are widely used in that field, the most prominent being zinc-dialkyl-dithiophosphate (ZnDTP). These three components are not independent of each other; they all contribute to overall lubricant performance. The complex interplay of the components is shown schematically in figure 1 before. *The width of the arrows indicate the relative importance of the individual components

Emissions -Laboratory procedure
One focus was the development of lubricants which are compatible with latest exhaust gas after-treatment systems. This is the reason for the desired reduction in phosphorous, sulphur and metal-organic content (expressed as sulphated ash). These elements are suspected to contribute towards catalyst poisoning and deposit formation in particle traps. However, one has to keep in mind that there are two different ways how engine oil constituents can influence the performance of after-treatment systems. Figure 3 shows the result from the field test with a car equipped with a particle trap ('filtre à particule', FAP) and conventional engine oil. After the test the FAP was dismantled and the found deposits were analysed. Most residues were fuel related (FbA=fuel based ash), but the engine oil showed an influence, too. Interesting is the comparison between metal ions found as deposits and those present in the ACEA A3/B3 engine oil. Apparently, zinc is four times more critical towards after treatment deposits than calcium. Therefore, it developed engine oils without any Zinc; even so this is the most common metal organic compound in engine oils today.  . Analysis of residues found in particle filter (result of a field test). Zinc seems to give overproportional residues in the after-treatment system. Some engine oil is inevitably entering the fuel path, e.g. via the fuel pumps. The fuel/engine oil mixture is then burned in the combustion chamber and all chemical elements of the engine oil are likely to reach the after treatment system. Thus, phosphorous, sulphur and sulphated ash are appropriate parameters to characterise the oil compatibility with the exhaust gas systems.
The second way of contact between engine oil and after-treatment system arises from evaporation. Hot lubricated surfaces in the engine, e.g. cylinder walls, give rise to the evaporation of volatile substances. These are not necessarily present in the fresh lubricant but may be created by thermal cracking of higher molecular weight compounds at these elevated temperatures.
From the past, it is well known that the type of base oil has a significant influence on raw emissions. An example is the correlation between Noack evaporation loss and particle emissions (see figure 4). This correlation, together with concerns about the oil consumption led to the limitation of the Noack evaporation loss to 15% respectively 13% according to the ACEA regulation. Besides the base oil volatility, the volatility of some additives becomes interest. The content of chemical elements in the oil and their abundance in the exhaust gas need not to be proportional. For example, it is well-known that different phosphorous compounds with identical phosphorous content will show great differences in thermal stability and volatility.
In order to evaluate the element specific evaporation of fully formulated engine oils, we established a new test procedure. The new test procedure uses the Noack evaporation test equipment (DIN 51581, ASTM D5800) and the test condition 1h @ 250°C. However, not only the total mass loss but also the evaporation loss of additive elements is determined. A comparison of the two reference engine oils and the new candidate fluids in this test is shown in figure 5. To display hydrocarbon, sulphur and phosphorous emissions in one diagram, the test results have been normalised to a typical reference oil (=100% emissions).
Obviously, both developed engine oils have reduced evaporation losses compared to today's conventional oils. The hydrocarbon loss, dominated by the base oil, was cut to less than half of the original value. This should reduce the particle emission. On the additive side, sulphur and phosphorous volatility have been significantly reduced.
Under the described conditions, volatile metal-organic compounds in significant quantities have not been found in any of the tested oils. Figure 5. Element specific Noack evaporation losses (relative two reference engine oils).

Engine cleanliness & low ash
The main challenge in formulating engine oils with low ash and sulphur is the question of engine cleanliness. Metal soaps and similar components (commonly called detergents) are responsible for preventing deposits in the engine, predominantly at the piston rings and groves. Besides, their alkalinity is necessary to neutralise acidic combustion products. Today, the most prominent substance used is calcium sulphonate-accounting for approximately 80% of lubricant ash and sulphur. Current engine oils have typical values of 0.3% to 0.4% Calcium, corresponding to sulphated ash contents of about 1.3% to 1.5%. Thus, a reduction to 0.5% as expected by passenger car manufactures cannot be achieved by just reducing the concentration of well known additives, but needs a new additive technology.
To evaluate the capabilities of new D&D-additives, it made intense use of the so-called Wolf-test. This test is a modification of the well known panel-coker idea. The engine oil to be tested is dropping on a hot metal plate. This steel panel is mounted at a fixed angle to allow the oil to flow down the panel slowly. At the lower end it is captured in a glass vessel to be recirculated to the panel again. After a fixed period of time the test is stopped, the panels are washed and the deposit formation is observed. Weighting and a visual rating of the panels by an expert allows judging the deposit formation tendency. The results obtained in the WOLF test give a good indication for engine cleanliness in real engines. Figure 7 shows the correlation between the Wolf test and the VW TDI test according to CEC L-78-T-99 as established with new oils in the past. The CEC L-78 test belongs to the ACEA sequence of engine tests and is used to rate piston cleanliness. Results greater or equal to 65 points are supposed to be good in this engine test. Thus deposits in our Wolf screening test should be below approximately 8 to 10 mg. However, only a test in a real fired engine can give reliable information about the actual performance. As explained earlier, the major challenge in formulation the new oils is the reduction in crucial additive elements. Sulphur and phosphorous are the elements found in most antiwear agents, while the metal-organic compounds constitute important anti-oxidants and engine cleanliness additives. All three additive groups can have negative influence on the after-treatment system. Therefore so called lowSAP oils are required. The understanding of lowSAP is the reduction of sulphur ash (as a measure for metal-organic compounds) and phosphorous. The figure 8 demonstrates the comparison between the passenger motor oil reference lubricant and the new developed oils. The (sulphur) ash content is reduced to one third, phosphorous and sulphur to less than 50% of today's product. Despite this strong decrease in metal-organic compounds, the deposit formation tendency -as screened with the panel coker test according to Wolfremains as low as the reference oil (figure 9.b). This target was reached by using a new ash less EP/AW additive technology. However, the influence of the concentration of metalorganic compounds remains visible even with newest technologies.

PCMO -EP/AW properties
The second concern while using lowSAP oils is wear protection. To check this performance, screening tests have been performed. Next figure 10 displays the results obtained with the SRV test rig. On the left hand side, wear scars according to DIN 51834-2 are displayed. This is a 2 hour test at constant load. Here, both new engine oils show slightly smaller wear scars. On the right hand side, results according to ASTM 5706 are displayed. In this SRV test, the load is increased in fixed intervals until seizure occurs. Higher load corresponds to a better protection at highest load spikes. The reference oil and both new candidate oils easily supersede the values typically found in engines.

Fuel efficiency test
The fuel efficiency potential of epc 50 was tested in an M111-FE engine test bench which is part of different ACEA specification. The matter of this test is to evaluate the fuel consumption under defined conditions. The fuel saving compared to the 15W40 CEC reference oil RL 191 is calculated in %. The formidable result of 3,9% fuel benefit demonstrate the potential of epc 50.

Passenger car motor oil candidates
At least two candidate oils named epc-48 and epc-49 passes the laboratory optimization process. A comparison of the passenger car specifications and the candidate oil data is given in next table 1.
Epc 48 and epc 49 are based on the same base fluid composition but differ in additivation. Due to their high viscosity index, the desired high temperature, high shear viscosity (HTHSV) at 150°C of 2.9 mPas is reached at significantly lower viscosities at 100°C, 40°C and at -25°C. This offers the potential for improved fuel efficiency.
To meet the passenger car oil specifications, the additives and also their treat rates have to be balanced well.  One important aspect of the engine oil specification is to meet all viscosity properties according to SAE J300. During the study a change in the in SAE J300 requirements occurred.

PCMO-Specifications
The changes relates to the low temperature cranking viscosity (CCS viscosity) and low temperature pumping viscosity (MRV). The defined measuring temperature switched to a lower value whereas the limit value was adjusted to the lower temperature.
Due to this epc 48 and epc 49 was developed in accordance to the old SAE J300 viscosity specification. Nevertheless epc 48 and 49 fulfil also the new SAE J300 low temperature requirements.
To follow a line in a consequent way, these oils have to be biodegradable and non-toxic to the aqueous environment according to the directive EC/1999/45, coherent with other international standards.

Aquatic toxicity
The following basic level test are proposed and included in the EU hazard assessment of substances: Daphnia acute immobilization test (OECD 202; Table 4).

Lubricant *EC50 (mg/l) Classification
EPC-48 EL50>100 "Not harmful" to aquatic organisms EPC-49 EL50>100 "Not harmful" to aquatic organisms *EC50: Effective Concentration. The EC50 value means the mean effective concentration of a substance that produces a particular, previously defined behaviour in 50% of the organisms of a test series.

Plasma powder characterization
Three types of triboreactive powders were used for the production of ring prototypes. Piston rings were produced with the reference coating, CIE-TARABUSI-PL72, and with the new coatings E1, E2 and E3 with successful characteristics in terms of hardness, porosity, adherence and machine ability. In all cases an intermediate layer is applied to obtain good adherence with the substrate.
Production of plasma powder with Magnéli phases and deposition of plasma sprayed coatings containing Magnéli phases or triboactive suboxides.
Plasma powder and coating were characterized with:  Metallography, optical microscopy and image analysis  Profilometry  Mastersizer for particle size distribution  SEM + EDX  X-ray diffraction  Raman spectroscopy    TinO2n-1 powder (n = 4 -6) is not stable against thermal stress.  Atmospheric plasma spraying as well as laser radiation cause oxidation with increasing formation of rutile. With APS process Magnéli phases cannot be fully transferred from the TinO2n-1 spray powder to the APS coatings.  Magnéli phases are stabilized against thermal stresses, laser radiation and oxidation, if chromium oxide was added to TinO2n-1.


Both, Tin-2Cr2O2n-1 powder and thermal sprayed coating contain high amount of stable H-Magnéli phases. Tin-2Cr2O2n-1 can be sprayed with APS to produce triboactive coatings.  Vacuum plasma spraying (VPS) in reducing gas atmospheres using hydrogen addition to plasma gas allows to produce coatings with Magnéli phases. With VPS process Magnéli phases of TinO2n-1 (n = 4 -6; IKTS) powder can be transferred to the coatings. Additionally, other Magnéli phases were detected with XRD.  The substoichiometry and electric conductivity of TiO2-x can be effected by the amount of hydrogen in plasma gas. Substoichiometry and electric conductivity increase with the amount of hydrogen in plasma gas as reducing medium.  Magnéli phases were found with LRS and XRD.  Owing to the stability of the Tin-2Cr2O2n-1 powder, thermal sprayed coatings with stable Magnéli phases can be produced with APS process -using Ar/He process gas mixture with Ar = 35 SLPM and H2 = 8 SLPM.  Thermal sprayed (Ti,Mo)(C,N)+Ni+Mo coatings have been produced with HVOF and plasma spraying process.  Thermal sprayed coatings (Ti-and TiCr-suboxides with crystallographic planar defect structure (Magnéli phases)) on flat disks have been produced for prescreening tribotests and evaluation of interactions of different lubricant / coating combinations.

Reference Components:
These components were designed and manufactured by CIE-TARABUSI with own knowledge and technology.
Piston: Design for Turbo Diesel application with oil cooling gallery and steel struts for expansion control, in aluminum alloy AT12 with 12% silicon content and iron insert for the top groove improved wear resistance. The piston head is coated by hard anodizing to avoid thermal cracks. The piston skirt has a graphite coating to improve lubrication during runningin. 2nd ring: Chromium plated taper ring with conical periphery and positive torsion internal step, manufactured in STD material AT110, non heat treated gray cast iron, ISO 6621/3 MC 11 type, of diameter 96 mm and 2 mm height 3rd ring: Chromium plated helicoidal spring loaded oil control ring with symmetric bevels, produced in STD material AT110 non heat treated gray cast iron, ISO 6621/3 MC 11 type. The helicoidal spring is made of oil hardened spring steel, of 96 mm of diameter and 3 mm of height.
Pin: DIN 17Cr3 carburized, hardened and tempered steel pin, with 30 mm of outer diameter, 15 mm of inner diameter and 80 mm length.

E3: Top ring coated with titanium oxide
Top ring: As the reference ring but coated by atmospheric plasma with E1 powder developed. The composition of the powder is given below: E3: TinO2n-1 (Ti4O7, Ti5O9, Ti6O11) sintered and agglomerated, 20-63µm:

Piston ring cylinder liner simulation
To study these new oils have been tested different piston rings coated with triboreactive powders deposited by Plasma spray against cast iron cylinder liner, using the PCMO oil as reference. In the next figure 21, the configuration to carry out the simulation tests developed by TEKNIKER is shown.   0.13 0.6 E3:TinO2n-1 0.14 0.2 E1:Tin-2Cr2O2n-1 0.14 1.5 *The roughness of E2 77(Ti,Mo) (C,N)-20Ni-3Mo are very high, so these piston rings will not be tested in laboratory simulation.

Specific wear energy
Specific wear energy is a criterion, which takes both into account: it is the ratio of the friction work spent in the interface divided by the mass loss due to the wear.
The specific wear energy Ew is the amount of energy needed to wear a certain mass of matter. Consequently, the higher the value of the Ew is, the more difficult it is to wear the material.
The Figures 22 and 23 show the cylinder liner wear specific energy.

Engine tests. Pass/No pass results for biodegradable fluids in combination with triboreactive materials. Evaluation of friction, wear, life, oil consume
The piston ring component engine tests have been performed by CIE TARABUSI in a 2.7 L turbo-charged 4-stroke diesel engine, using as reference the Mo-based plasma coating PL72 (reference used by CIE-Tarabusi). The test conditions and parameters were fixed for three different type of tests: (a) Scuff test to determine the scuffing resistance of the coating at high temperature and pressure with minimum component clearance, (b) Hot test, long run at high power and high speed to set the reference values for wear and (c) Cycle test between low and high thermal conditions to determine the thermal fatigue and adherence resistance of the developed coatings. The components were measured before and after the test.

Tests results
The pistons and rings are inspected visually after the scuff, life tests and cyclic tests. The appearance observations of the tested components are presented comparatively to the reference components. We summarized the aspect of the prototypes as follows.

Reference components
The piston has good appearance for the time of running and type of tests run. The wear pattern of the piston skirt is smooth and wide. The rings have normal periphery and side wear. The pin also has a typical appearance with normal signs of friction effects.

Thrust Side
Anti-thrust Side  The engine test results are as follows: 1) Reference engine test: The studied tribological pair is the cylinder liner of 96mm internal diameter and made of uncoated pearlitic cast iron AT182 and top piston rings of 96mm outside diameter, height 2mm and made of nodular graphite cast iron AT126 coated with PL72 standard atmospheric plasma layer. The results are the reference for wear, scuff resistance and adherence. Engine oil consumption and blow-by values are also considered for comparison.
These tests were performed with the reference materials and using the oil SAE 5W30. The scuffing test (2h30m) was done with 100% load and speed engine conditions. Water and oil temperatures were measured. Neither piston nor piston rings show scuff marks after testing. Concerning hot test (100h), the engine conditions were 105% load, 100% speed, water temperature 110ºC and oil temperature 130ºC. Results were an oil consumption of 48,3 g/h and blow by 55,7 l/min. The cold/warm test consists of (100h) with cycle time of 14 min.
2) Engine testing of piston rings with new coating E2 (1st batch) and reference oil SAE 5W30. After 14 hour of hot test running the engine is stopped due to high blow-by. In the engine disassembly it is observed that the E2 coated rings show scuffing and the liners are severely deteriorated.
3) Engine testing of piston rings with new coating E3 and reference oil SAE 5W30. After an initial test failure due to reasons not caused by the new rings, the tested ring set goes successfully under the hot test running (100 h) and the cold/warm cycles (100 h). The E3 coated rings show no sign of scuffing and a periphery wear 30% more than the reference coating. On the contrary the induced liner wear is lower more than 50%.
4) Engine testing of piston rings with new coating E1 and reference oil SAE 5W30 was successfully concluded after hot test running (100h) and cold/warm cycles (100h). The ring periphery wear of E1 is equal to the reference coating and the induced wear in the cylinder is importantly reduced in around 70%. Unfortunately it is observed after the engine disassembly that the top ring of piston no. 4 shows peripheral erosion and breakage.
5) Engine testing of piston rings with new coating E2 (2nd batch) and reference oil SAE 5W30 was successfully concluded after hot test running (100h) and cold/warm cycles (100h). The ring periphery wear of E2 is 80% higher than the reference coating and the induced wear in the cylinder is reduced in around 40%.
6) Reference engine test with EPC-49 oil: The studied tribological pair is the cylinder liner of 96mm internal diameter and made of uncoated pearlitic cast iron AT182 and top piston rings of 96mm outside diameter, height 2mm and made of nodular graphite cast iron AT126 coated with PL72 standard atmospheric plasma layer. The results are the reference for wear, scuff resistance and adherence with special triboreactive epc-49 oil.
These tests were performed with the reference materials and using the oil EPC-49. The scuffing test (2h30m) was done with 100% load and speed engine conditions. Neither piston nor piston rings show scuff marks after testing. Concerning hot test (100h), the engine conditions were 105% load, 100% speed, water temperature 110ºC and oil temperature 130ºC.
7) Engine testing of piston rings with new coating E3 and EPC-49 oil. The tested ring set goes successfully under scuff test (2.5 h), the hot test running (100 h) and the cold/warm cycles (100 h). The E3 coated rings show no sign of scuffing and a periphery wear and cylinder bore induced wear performance similar to the result obtained with the reference components. In figure 27 the difference between measurements are presented. These values are a combination of the wear and deformation that the components have suffered during the test. Figure 27. Testing of piston rings in a turbo diesel engine testing arrangement.

Conclusions
The main conclusions of the tests are summarized as follows: -Good wear and anti-scuffing properties of Titanium Oxide followed by the (Ti, Mo)(C,N)+23NiMo coatings. -Wear resistance of TinO2n_1 coating is similar to standard Mo-based coating for both standard and triboreactive oil.
The best extreme pressure properties (in tribological tests) were found for the Titanium Chromium Oxide coating but it was detached from the iron substrate during the engine tests.
Wear results with triboreactive oil EPC-49 are similar to the standard oil, and oil consumption results were reduced by 45%.