Development of new ecofriendly detergent/dispersant/antioxidant/antiwear additives from L-histidine for biolubricant applications

Abstract

Two novel overbased Ca salts of histidine Schiff base esters Ca-HDS-L and Ca-HDS-M were synthesized following a three-step reaction sequence. The histidine Schiff base (HDS) was synthesized first by imine coupling of histidine with salicylaldehyde. Then its phenolic group was esterified using lauroyl chloride and myristoyl chloride to obtain HDS-L and HDS-M, respectively. Finally in the third step, their respective overbased salts Ca-HDS-L and Ca-HDS-M were synthesized by reaction with Ca(OH)₂. All the synthesized compounds were characterized using FT-IR, NMR, CHN and TG analysis. Panel coker federal test (FTM 3462), blotter spot test (ASTM D7899), universal oxidation test (IP-306) and four ball test (ASTM 4172A) were used for the evaluation of detergent, dispersant, antioxidant and antiwear activity, respectively, of the synthesized additives in polyol base oil. Both the additives are active but overall Ca-HDS-L is more effective as detergent and dispersant while Ca-HDS-M is more effective as antioxidant and antiwear multifunctional biolubricant additive.

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Despite its limitations related to environmental concerns and exhaust emission catalytic convertor poisoning, zinc dialkyldithiophosphates (ZDDP) remains an important and popular antiwear, antioxidant and anticorrosion multifunctional additive (MFA) since its discovery in the 1940s especially for automobile lubricant formulations.^1,2 Similarly, overbased metal sulfonate and mannich adducts are still in use as antioxidant, detergent and dispersant multifunctional additives.^3,4 Sulfonate and mannich bases have similar environmental problems. Although they work well in the emerging biolubricant base oils too,⁵ to formulate a complete ecofrienly biolube, the use of such additives is highly undesirable because it leads to toxicity in water bodies and land soils by decomposing and generating S- and P-containing toxic compounds.⁶ It is a challenge to replace such MFAs with environmentally friendly, biodegradable additives with comparable performance having no such toxic elements. Some efforts have been made to screen out the ecofriendly substitutes but most of them are restricted to additives having single property.^7,8

Further, it is desirable to develop these innovative MFAs from sustainable raw materials. Recently there have been some efforts to use sustainable materials to develop additives, but this was mostly with single additive character, e.g. natural garlic oil (NGO) has been evaluated as a high-performance, environmentally friendly, extreme pressure additive for lubricating oils.⁹ Cellulose fatty esters have been evaluated as the lubricity additive for biolubricant base oils.^10,11 Boron-containing soybean lecithin has been tribologically tested as environmentally friendly lubricant additive in synthetic base fluids.¹² The potential of dextrose, sucrose and cellulose dodecenylsuccinate esters have been investigated as lubricity additives.¹³ Acylated and isocyanate-functionalized chitin and chitosan have been reported to be used as thickening agents for the preparation of vegetable oil based grease.^14,15

Mixed esters of pentaerythritol monooleate with gallic acid and 3,5-di-tert-butyl-4-hydroxybenzoic acid have been evaluated for antioxidant and detergent dispersant along with lubricity properties using the rotary bomb oxidation test, blotter spot test, and four ball test.¹⁶ Homopolymers of sunflower oil and soybean oil have been evaluated as a pour point depressant and viscosity index improver or modifier for lube oil.¹⁷ Condensation product di(alkylphenyl)phosphorodithioic acid, derived from cashew nutshell liquid, with various amines have been evaluated as anti-oxidant, antiwear, friction-modifying, and extreme-pressure additives in lubricant compositions.¹⁸

Amino acids are abundant, natural, renewable and biodegradable resources, but still underutilized as lubricant additive feedstock. Few reports describe the use of amino acids, e.g. novel environmentally adapted lubricant additives were synthesized from cystine (Cys₂) by its carboxyl group derivatization to corresponding esters by reaction with long-chain alcohols. The Cys₂-derived additives exhibited comparable antiwear properties to the conventional additive ZDDP in synthetic hydrocarbons such as poly-alpha-olefin.¹⁹ Some ionic liquids derived from aspartic and glutamic acid were evaluated to be efficient antiwear and friction-reducing additives in mineral base oils.^20,21 Amino acids as such were also used sometimes, e.g. DL-valine was found to a good eco-friendly detergent/dispersant additives for vegetable-oil based lubricants when evaluated by blotters spot.⁷ So there exists ample scope to use them for the development of multifunctional lubricating oil additives.

L-Histidine has attracted our attention as it shows anticorrosion activity when tested in hydrocarbon media and also its antioxidant property has been shown in pharmaceutical applications.^22,23 So in this paper, we have developed a new multifunctional additive having antioxidant, detergent, dispersant and antiwear properties from L-histidine. Novel histidine based salts (abbreviated as Ca-HDS-L and Ca-HDS-M) were synthesized and characterized. Their antioxidant, detergent, dispersant and antiwear additive performances were evaluated in polyol base oil.

Synthesis of Ca-HDS-L and Ca-HDS-M.

The synthesis of the Ca-HDS-L and Ca-HDS-M additives was completed using a three-step reaction route as shown in the Scheme 1.

Scheme 1 Reaction scheme for synthesizing Ca-HDS-L and Ca-HDS-M.

The L-histidine changes from white to yellow in the histidine Schiff base (HDS). This gives direct evidence of the successful imine bond formation as shown in the Fig. 1. This along with all other synthesized compounds were also characterized using the various analytical techniques like CHN analysis, FT-IR and NMR. The observed results of elemental analysis, given in Table 1, were found to be in good agreement with the calculated values for the given molecular structures in Scheme 1.

Fig. 1 Histidine Schiff base (HDS) with salicylaldehyde.

Table 1 The elemental analysis data of synthesized HDS, HDS-L and HDS-M^a

Sample	% Content
	C	H	N
a Values in parentheses are calculated.
HDS	59.17 (60.22)	4.96 (5.05)	16.42 (16.21)
HDS-L	68.25 (68.00)	8.17 (7.99)	9.12 (9.52)
HDS-M	69.14 (69.05)	8.78 (8.37)	9.15 (8.95)

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FT-IR spectroscopy

The successful synthesis of the compounds was also supported by FT-IR as shown in Fig. 2. The HDS spectrum shows a strong band at 3397.92 cm⁻¹ which indicates the N–H stretching of a secondary amine while the band at 3015.01 cm⁻¹ corresponds to aromatic C–H stretching. The inherent histidine CH₂ groups produce an asymmetric C–H stretching band at 2868.59 cm⁻¹. The most prominent evidence of imine bond formation between histidine amine group with salicylaldehyde is the appearance of a strong band at 1632.80 cm⁻¹ characteristic of C=N stretching. Although it contains an overlapping peak of histidine imidazole ring C=C stretching band. The peak at 1589.36 cm⁻¹ pertaining to C=C stretching of phenolic ring further confirms the Schiff bond formation. This may also contain an overlapping peak of C=O stretching vibrations of the histidine carboxylate group.

Fig. 2 FT-IR spectra of (a) histidine Schiff base, HDS; (b) histidine Schiff base ester, HSD-M; (c) calcium salt of histidine Schiff base ester, Ca-HSD-M.

The bands showing at 1457.45 cm⁻¹, 1413.13 cm⁻¹, 1271.66 cm⁻¹ and 1248.64 cm⁻¹ could be easily assigned to asymmetric C–H bending, symmetric C–H bending, C–O stretching (phenol) and C–N stretching (imidazole), respectively. Further, the bands at around 1145.9 cm⁻¹, 1112.26 cm⁻¹, 925.43 cm⁻¹ and 754.57 cm⁻¹ are attributed to C–N stretch (aliphatic amines), C–O stretching (acid), N–H wagging and CH₂ rocking respectively (Fig. 2a). In the FT-IR spectra of HDS-M (Fig. 2b), the appearance of a characteristic C=O stretching (ester) band at 1728.25 cm⁻¹ along with persistent sharp C=N stretching peaks at around 1640.18 cm⁻¹ determines the successful esterification. Further, the appearance of prominent peaks at around 2920.65 and 2850.43 cm⁻¹ are attributed to the asymmetric and symmetric C–H (CH₂) stretching band of the fatty myristyl chain. A similar IR spectrum was observed for the HDS-L. The FT-IR spectra of the Ca salts of histidine Schiff base esters Ca-HDS-L and Ca-HDS-M were also recorded showing all the characteristic peaks as in the HDS-L and HDS-M along with the appearance of an undesired OH stretching signal at 3643.77 cm⁻¹, which belongs to residual Ca(OH)₂ used during the third step of overbasing (Fig. 2c).

NMR spectroscopy

In addition to FT-IR analysis, NMR also presents strong evidence in favour of successful synthesis of HDS-L and HDS-M. Fig. 3 shows the ¹³C NMR of the additive HDS-L in d⁶-DMSO at 25 °C. Lauroyl fatty chain carbons (C1–C9) are observed in the range 0–40 ppm where the signal at 15 ppm corresponds to the terminal methyl group carbon and other signals between 20 to 40 ppm are attributed to the other methylene carbons of the long alkyl chain. The histidine imidazole ring carbons (C11–C13) are observed between 105–120 ppm. The aromatic carbons (C16–C20) are observed between 130–140 ppm. The existence of the C15 downfield signal at 158 ppm is strong evidence of imine coupling of histidine. The appearance of the signal of C21 ([double bond splayed left]C=O) at 172.5 ppm along with the C22 signal at 175 ppm is strong evidence of esterification. HDS-M also shows all the characteristic NMR signals too (ESI Fig. S1†).

Fig. 3 ¹³C NMR of HDS-L in d⁶-DMSO.

Thermogravimetry

TG curves were recorded for determining the thermal stability of the synthesized additives. Fig. 4 shows the TG profiles of the HDS, HDS-L, HDS-M, Ca-HDS-L and Ca-HDS-M. It is clear from the graph that esterification lowers the thermal stability of the histidine Schiff base. HDS starts to degrade at around 270 °C while the degradation temperature for HDS-L and HDS-M is near to 200 °C. Finally the Ca salt formation provides stability as the degradation temperature for Ca-HDS-L and Ca-HDS-M was found to be 379.75 °C and 372.67 °C, respectively. So both compounds Ca-HDS-L and Ca-HDS-M show good thermal stability to be used as lubricant additives.

Fig. 4 TG curves of histidine Schiff base, histidine Schiff base esters and their Ca salts.

Detergency test

After characterization of the synthesized additives and determining their working temperature range, samples with different concentrations of 1000, 2000 and 3000 ppm were prepared by dispersing the additives Ca-HDS-L and Ca-HDS-M in the poly base oil by sonication with heating. The first additive property tested was detergency, since both molecules have a hydrophilic polar end (carboxylate) and a lipophilic fatty chain. The panel coker federal test method (FTM 3462)²⁴ determines the tendency of oil to form solid deposits on a metal panel surface at elevated temperatures and thereby evaluates the detergency. The “coking value” determined by the panel coker apparatus is indicative of a high temperature detergency. A lower coking value means a higher detergency. It is evident that the polyol has quite a high coking value, i.e. 0.043 g (σ; 0.0017). The test results indicate that both the additives, Ca-HDS-L and Ca-HDS-M, show antioxidant behaviour even at higher temperature. The behaviour increases as the concentration increases. The additive Ca-HDS-L decreases the coking value at 1000, 2000 and 3000 ppm to a value of 0.041 g (σ; 0.0010), 0.039 g (σ; 0.0017) and 0.035 g (σ; 0.0016), respectively. A similar trend was observed with Ca-HDS-M but it was less effective than Ca-HDS-L (Fig. 5). The reason may be the slightly higher thermal stability of Ca-HDS-L. Comparison was also made with a commercially available detergent, i.e. calcium alkyl salicyalate (TBN no. ≥150). Its detergency observed with the panel coker test is high (coking value 0.027 g (σ; 0.0015)) compared to both the synthesized additives and this may be due to its higher TBN number. In spite of this low activity compared to the commercial additive, the work is a significant breakthrough in the direction of developing environmentally benign additives from a sustainable resource.

Fig. 5 Panel coker test specimens, (a) specimen before test; (b) specimen after test with polyol; (c) specimen after test with 3000 ppm Ca-HDS-L in polyol.

Dispersancy test

Both the additives, Ca-HDS-L and Ca-HDS-M, were evaluated for their dispersant capabilities as per modified ASTM D7899 method which is popularly known as the blotter spot test.²⁴ Although the dispersing capability of the additives can be evaluated in terms of several parameters such as size of centre black spot, colour of centre spot, size of diffusion zone, colour of diffusion zone (black, gray), density (translucent or opaque), we have only measured the size of the spot in this study (Fig. 6) as described by the N. S. Ahmed.²⁵ The results indicate that both the additives have dispersing property. The dispersion is found to be increasing with increasing Ca-HDS-L concentration from 1000–2000 ppm (spot size increased from 0.50 cm to 0.65 cm). Ca-HDS-L (spot size 0.65 cm) is dispersing soot in base oil little bit better than Ca-HDS-M (spot size 0.60 cm) at 2000 ppm with respect to polyol blank (spot size 0.45 cm). At 3000 ppm concentration the dispersion is found to be decreased as compared to 2000 ppm. So the optimum concentration is 2000 ppm.

Fig. 6 Blotter spot test of additives Ca-HDS-L and Ca-HDS-M.

Antioxidant performance

Then the antioxidant property was also evaluated as L-histidine is reported in the literature as having antioxidant character albeit for pharmaceutical applications.²⁶ The histidine Schiff base moiety may provide metal chelating abilities to the additives too. The overbasing in the third step of the synthesis of the additives also gives it acid neutralization capabilities. The total base number (TBN) was determined by ASTM standard D974 (ref. 24) is 57.5 mg KOH g⁻¹ and 61.32 mg KOH g⁻¹ for Ca-HDS-L and Ca-HDS-M, respectively. These entire factors are supposed to make Ca-HDS-L and Ca-HDS-M good antioxidants and this was realized too through the experiments being carried out on the universal oxidation test apparatus following IP-306 (Fig. 7).²⁴ The antioxidant potential of the additives was estimated in terms of volatile acidity, soluble acidity, total sludge (S%) and total oxidation products (TOP%). The results tabulated in Table 2 reveal that both the additives show antioxidant properties at concentrations higher than 1000 ppm. At 1000 ppm the volatile acidity is higher than the blank polyol. Otherwise, the antioxidant property increases with increasing concentration of both the additives from 2000 ppm to 3000 ppm in terms of all determined parameters. Ca-HDS-M shows slightly better results than Ca-HDS-L at 3000 ppm concentration. The values of volatile acidity, soluble acidity, total sludge (S%) and total oxidation products (TOP%) for polyol base oil are quite high as 4.095, 2.019, 32.454 and 34.416, respectively. 3000 ppm Ca-HDS-M reduces these value to 2.165, 1.458, 0.021 and 1.183 while 3000 ppm Ca-HDS-L reduces these values to 3.478, 1.402, 0.013 and 1.579, respectively. The higher activity in the case of Ca-HDS-M could be explained on the basis of its little bit higher solubility in comparison to Ca-HDS-L due to the longer fatty chain and also the TBN value.

Fig. 7 Arrangement of universal oxidation test (IP 306).

Table 2 Additive effect on oxidative characteristics at various concentrations of additives in base oil in universal oxidation test (IP 306)

Samples	Total acid number (mg KOH g^−1)		Total sludge (S%)	Total oxidation products (TOP%)
	Volatile acidity	Soluble acidity
Blank-polyol	4.095	2.019	32.454	34.416
1000 ppm Ca-HDS-L	7.854	1.598	0.497	3.530
2000 ppm Ca-HDS-L	2.468	1.514	0.022	1.300
3000 ppm Ca-HDS-L	3.478	1.402	0.013	1.579
1000 ppm Ca-HDS-M	6.874	0.476	0.065	2.423
2000 ppm Ca-HDS-M	2.244	1.514	0.076	1.282
3000 ppm Ca-HDS-M	2.165	1.458	0.021	1.183

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Anti-wear property

It is a well reported fact that in the surface-complex film forming process, organic Schiff compounds hindered the metal–metal contact leading to anti-wear properties.²⁷ As our designed additives have an imine bond along with a polar end and imidazole ring which may provide the film forming tendency, we have tested Ca-HDS-L and Ca-HDS-M as antiwear additives too. The antiwear potential is estimated in terms of the WSD (wear scar diameter) using a four ball test machine following standard test conditions (ASTM D4172).²⁴ Both the additives are found to have the antiwear property as a value of WSD for the base oil i.e. 896 μm reduces to a value of 690 μm and 614 μm at 1000 ppm concentration of Ca-HDS-L and Ca-HDS-M, respectively (Table 3). Ca-HDS-M is comparatively more effective may be due to the little higher solubility. At higher additive concentrations also, the WSD is lower than the blank but higher than what we observed at 1000 ppm concentration. So the optimum concentration is 1000 ppm (Fig. 8).

Table 3 Four ball test results

Compounds	Concentration (ppm)	WSD (μm)	Std. dev. σ
Blank	—	896	13.8684
Ca-HDS-L	1000	690	6.0277
Ca-HDS-L	2000	736	7.5498
Ca-HDS-L	3000	744	8.5049
Ca-HDS-M	1000	614	9.0737
Ca-HDS-M	2000	680	14.1067
Ca-HDS-M	3000	727	15.7162

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Fig. 8 Reduction in WSD with increasing concentration of Ca-HDS-M in polyol base.

Also the feedstock of both the additives, being a natural amino acid, and considering the results of ASTM D5864 (ref. 24) may be considered as environmental friendly to a significant extent.

Conclusions

Two new histidine based additives Ca-HDS-L and Ca-HDS-M were synthesized and evaluated as detergent, dispersant, antioxidant and anti-wear additives in polyol, which was taken as biolube reference base fluid. Both the additives were found to have the tested activities but Ca-HDS-L is comparatively more effective as detergent and dispersant additive while Ca-HDS-M is more effective as antioxidant and antiwear additive. A 1000 ppm concentration of Ca-HDS-M reduces the wear of the polyol base oil to 31.47%.

We kindly acknowledge the Director of CSIR-IIP for his kind permission to publish these results. The Analytical science division of the Institute is kindly acknowledged for providing support in analysis of samples.

Experimental

Materials

L-Histidine, N,N′-dimethylacetamide and calcium hydroxide were purchased from Merck Millipore. Salicyldehyde, lauroyl chloride, myristoyl chloride, 4-(dimethylampino)pyridine and methanol were purchased from Sigma-Aldrich.

Synthesis of histidine Schiff base (HDS)

3.1 g (∼20 mmol) L-histidine and 2.5 g (∼20 mmol) salicyldehyde were placed in a 250 mL round bottomed flask equipped with a water condenser. Methanol was used as solvent. The mixture was refluxed with stirring for 18 hours. The yellow coloured compound was obtained by filtration followed by drying in an oven. The yield obtained was 5.2 g.

Synthesis of histidine Schiff base esters (HDS-L and HDS-M)

2.6 g (∼10 mmol) of the synthesized HDS was reacted with 4.37 g (∼10 mmol) lauroyl chloride in a round bottomed flask using 1.22 g (∼10 mmol) 4-(dimethylamino)pyridine (DMAP) as HCl scavenger and N,N′-dimethylacetamide (DMAc) as solvent. The stirring was carried out at 130 °C for around 15 hours. The content was poured in cold water and the precipitate was filtered and washed several times with methanol and water. The dark brown compound (HDS-L) was obtained after drying in a vaccum oven overnight at 60 °C. The yield of HDS-L obtained was 3.98 g. Similarly, HDS-M was synthesized by the same reaction protocols using 4.92 g (∼20 mmol) myristoyl chloride. The yield of HDS-M obtained was 4.32 g.

Synthesis of Ca salt of histidine Schiff base esters (Ca-HDS-L and Ca-HDS-M)

An excess amount (twice the equimolar concentration) of calcium hydroxide was moistened with few drops of water and then chloroform was added. The solution was sonicated for 30 min for proper dispersing. HDS-L or HDS-M was added slowly to this solution and vigorous stirring was continued for 8–10 hours with CO₂ being purged in to it. The precipitate was washed several time with water to remove the excess calcium hydroxide. The product was dried vacuum oven overnight.

Techniques used

The synthesized additives were characterized by CHN analysis, FT-IR, NMR and TG. Fourier transform infrared spectra were recorded on a Thermo-Nicolet 8700 Research spectrophotometer with a 4 cm⁻¹ resolution using a potassium bromide window. NMR measurements were carried out on a Bruker Avance 500 spectrometer in the proton noise-decoupling mode with a standard 5 mm probe. Thermo gravimetric analyses (TG) of samples were carried out using Perkin Elmer EXSTAR TG/DTA 6300 using aluminum pans. Analysis was carried out in the temperature range of 30 to 900 °C under nitrogen flow (200 mL min⁻¹) with heating rate of 10 °C min⁻¹.

Panel coker federal test (FTM 3462)

The panel coker federal test method (FTM 3462) performance test²⁴ was used to determine the detergency property of the synthesized additives in a polyol lube base oil in terms of the coking value. The “coking value”, i.e. solid deposits on metal panel surface, at elevated temperatures was determined by the panel coker apparatus is indicative of the detergency. A panel coker test apparatus from Tribotech Technologies Pvt. Ltd., New Delhi was used. The tarred, polished steel test panel is placed in the coking splashed apparatus in a position such that the polished surface thereof comes in contact with oil thrown on it by means of the splashing brush immersed in the sample. The test sample was prepared and placed in a coker bath. The test panel is heated to a temperature of 300 ± 10 °C and the oil was heated to a temperature of 120 ± 10 °C and maintained at this temperature while the splashing is done for a period of 30 min, after which the panel is removed, cooled, washed with petroleum ether, dried and weighted. The difference in weight before and after the test was taken as the “coking value”.

Blotter spot test (ASTM D7899)

A blotter spot test was performed following the modified ASTM D7899 (ref. 24) for analyzing the dispersing efficiency of the synthesized additives. At first, a reference blank stock was prepared by dispersing 5.0 g lamp black in 1000 mL base oil (polyol ester) taken in a beaker. Now the three different samples were prepared by adding the 1000, 2000 and 3000 ppm concentration of the additives to this stock solution. Samples were homogenized by using a sonicator and used for the blotter spot test in which a sheet of whatman filter paper was clamped in aluminiumium frame having circular holes of equal area. The boundaries and centres of these circles were marked with a pencil. Oil samples were drawn with the help of capillary tube and one drop of the sample was placed on the centre of each circle. The oil spread on the filter paper as it was kept for 10–12 hours. The dispersancy of samples is evaluated in terms of the size of the black carbon spot.

Universal oxidation test (IP-306)

The potential of synthesized compounds as antioxidant additives was analyzed by a universal oxidation test performed as per IP 306.²⁴ Samples with different concentrations of 1000, 2000 and 3000 ppm were prepared in polyol base oil. 25 g of the each sample was taken in am oxidation tube and connected with the absorption tube with a connecting plastic tube. The absorption tube was also filled with 25 mL water. The apparatus was run at 120 °C for 48 hours. The flow of the oxygen was maintained at a rate of 1 litre per hour. After the test the oil sample was recovered using 150 mL heptane. For some samples, which were difficult to remove, chloroform was added. The samples were kept in the dark for 48 hours and were then filtered by vaccum filtration using a filtration crucible and the solution was made up to 500 mL by adding heptane. Titration of 10 mL aliquots was done using a 0.1 M alcoholic KOH solution using phenolphthalein as an indicator and calculate soluble acidity as per the given formula:
where A is the volume of KOH used to neutralised n-heptane/oil solution. M is the molarity of KOH solution used. S.A. is the soluble acidity.

The water from the absorption tube was used to calculate the volatile acidity. 5 mL water was titrated against 0.1 M KOH solution. The volatile acidity was calculated according to the formula:

where A is the volume of KOH used to neutralise the n-heptane/oil solution. M is the molarity of the KOH solution used. V.A. is the volatile acidity.

The sludge formed was measured after filtration of the oil/heptane solution through crucibles.

Total sludge (S%) = a × 4

where a is the weight of the sludge.

For analysing the additive effect on oxidative characteristics of additives in base oil, TOP (total oxidation product) was calculated according to the formula:

Four ball test (ASTM 4172A)

The anti-wear performance evaluation of synthesized additives as blends with polyol base oil was carried out on a four-ball rolling contact fatigue tribotester (Ducom India) as per ASTM D4172A standard test method.²⁴ The anti-wear characteristics were estimated in terms of average wear scar diameter (WSD) of the four balls in contact in a tetrahedral geometry in which the top ball is fixed into the spindle rotating at a predefined speed while the bottom three balls were kept in a ball pot filled with the blended sample making three point contact with the top ball. The 12.7 mm test ball specimens are made up of AISI standard steel no. E-52100 and have Rockwell C64–66 hardness. Tests were performed at a rotating speed of 1200 rpm; load, 198 N; temperature, 75 °C and time, 60 min. Each of the additive blends was tested thrice and the average results were reported.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra03113c

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