AIM OF RESEARCH

• to present the tribological properties of fractions used to produce lowsulphur diesel fuels and to determine the influence of fatty acids on the tribological properties of these components
• to describe the influence of fatty acids on the wear of systems lubricated with fractions obtained by fractional separation of components
• to demonstrate how the chemical structure and viscosity of base fluids influence the antiwear and pro-wear behaviour of fatty acids.

  Table 3. Kinematic viscosity of base fluids
  Base fluid	Kinematic viscosity at 40°C, mm2/s 1-Methylnaphthalene	2.1
  Mixture of n-hexadecane and 1 -methylnaphthalene (volume ratio 1:1)	2.2
  n-Hexadecane	2.9
  PA04	16.6
  PAO 6	28.9
  Saturated hydrocarbon mixture from SN 400	50.4
  Mineral base oil SN 400	76.7

  EXPERIMENTAL

  Lubricant materials

  Base fluids Two major components of low-sulphur diesel fuel were tested: the HON component and the HK component. The HON component is the low-sulphur fraction from diesel fuel desulphurisation. The HK component is the ultralow-sulphur fraction from hydrocracking. The properties of low-sulphur diesel fuel components are given in Table 1.
  Additionally, tribological tests were carried out for distillation fractions obtained by fractional separation of the HON component (properties of fraction I-X are summarised in Table 2).
  Seven other base lubricants with a wide range of viscosities (Table 3) were used to find out the influence of fatty acids on the wear behaviour of some model saturated and aromatic hydrocarbons along with the typical mineral base oil. These fluids include:

  • two pure model hydrocarbons: n-hexadecane and 1 -methylnaphthalene (used as the reference paraffin and aromatic base fluid)
  • mixture of n-hexadecane and 1-methylnaphthalene (a low-viscosity hydrocarbon reference base fluid)
  • two synthetic hydrocarbon base oils: PAO 4 and PAO 6
  • saturated hydrocarbons separated from SN 400 base oil
  • mineral base oil SN 400.

  Table 4. Characteristics of fatty acids
  Fatty acid	Chemical structure	Molecular weight	Purity, %
  Caproic acid	CH3-(CH2)4-COOH	116.16	99
  Capric acid	CH3-(CH2)8-COOH	172.27	~99
  Lauric acid	CH3-(CH2)10-COOH	200.32	>99
  Palmitic acid	CH3-(CH2)14-COOH	256.43	>97
  Stearic acid	CH3-(CH2)16-COOH	284.49	≥90
  Behenic acid	CH3-(CH2)18-COOH	340.61	-

  Table 5. Solutions of fatty acids in different base fluids investigated
  Base fluid	Fatty acid	Fatty acid concentration, ppmt
  HON component	Caproic acid	50, 100, 500, 750, 1000
  	Capric acid	50, 100, 500, 750, 1000
  	Lauric acid	50, 100, 500, 750, 1000
  	Palmitic acid	50, 100, 500, 750, 1000
  	Stearic acid	50, 100,500,750, 1000
  	Behenic acid	500
  HK component	Stearic acid	100, 500, 1000
  Fraction I-X of HON component	Stearic acid	500
  1 -Methylnaphthalene	Palmitic acid	50, 100, 500, 750, 1000
  Mixture of n-hexadecane and 1-methylnaphthalene (volume ratio 1:1)	Palmitic acid	50, 100,500, 750, 1000
  n-Hexadecane	Palmitic acid	50, 100, 500, 750, 1000
  PA04	Palmitic acid	50, 100 ,500, 750, 1000
  PA06	Palmitic acid	50, 100, 500, 750, 1000
  Saturated hydrocarbon mixture from SN 400	Palmitic acid	50, 100, 500, 750, 1000
  Mineral base oil SN 400	Palmitic acid	50, 100,500,750, 1000

  Fatty acid solutions Table 4 presents the aliphatic fatty acids used as additives in the present work and the molecular weight and purity of these substances. All the compounds were applied as received, without further purification. Fatty acids were added to base fluids in the concentration range 50-1000) ppm. The solutions of fatty acids tested are listed in Table 5.

  Table 6. Pin-on-disc experimental set-up and test conditions
  Material system	52100 steel-on-52100 steel
  Geometry	Sphere-on-flat (fixed ball on rotating disc)
  Specimens ball disc	Diameter: 3.18 mm; Ra = 0.3-0.35 µm; hardness: 63 HRC Diameter: 25.4 mm; Ra = 0.2 µm; hardness: 58-63 HRC
  Wear track radius, mm	8
  Applied load, N	9.81
  Sliding velocity, m/s	0.25
  Sliding distance, m	500
  Temperature, °C	25

  Apparatus and test conditions

  The wear tests were carried out using:

  • HFR2 apparatus (made by PCS Instruments, UK)
  • pin-on-disc contact machine (Tester T-01 M made at the Terotechnology Institute, Radom, Poland).

  HFR2 apparatus The HFR2 apparatus used an electromagnetic vibrator to oscillate a moving specimen (a ball) over small amplitude while pressing it against a fixed specimen (a disc). The lower, fixed specimen was held in a small bath, which contained the test lubricant. During the test three values were logged continuously: the friction coefficient, the fuel temperature (programmed temperature 60°C), and the electrical contact resistance.
  The test conditions were chosen according to CEC F-06-A-96 'Measurement of diesel fuel lubricity - approved test method. HFRR fuel lubricity test'.

  Pin-on-disc contact machine The specimens (a stationary ball and a rotating disc) were clamped in place with holders. The disc holder contained the test material, fully flooding the contact region. A dead-weight loading system applied normal force. The sliding velocity and temperature (programmed temperature 25°C) were recorded continuously during the test.

  The test conditions (summarised in Table 6) were designed to result in boundary lubrication at the sliding interface.

  Procedure

  Prior to use, the test specimens were ultrasonically cleaned in toluene for 20 min and in acetone for 20 min. The same cleaning procedure was applied to the holders before testing each lubricant. Tribological tests using the HFR2 apparatus were carried out under test conditions in accordance with CEC F-06-A-96: 17.0°C < temperature < 23.5°C; absolute humidity > 0.8 kPa, relative humidity <: 70%; 23.5°C < tem- perature ^ 28.0°C; and 0.8 kPa < absolute humidity < 2.0 kPa. The same con- ditions were selected for tests with the pin-on-disc contact machine. A minimum of two tests was done using the HFR2 apparatus and a min- imum of five tests was carried out by the pin-on-disc contact machine for a test material.

  Interpretation

  HFR2 apparatus test outputs The wear-scar diameter was measured in both the X and the Y directions using a photomicroscope to an accuracy of ±1 µm, and the mean wear-scar diameter (MWSD) was evaluated:

  MWSD =(X + Y)/2

  (1)

  In accordance with CEC F-06-A-96, the corrected wear-scar diameter was calculated in um as follows:

  WS1.4 = MWSD + HFC x (1.4 -AVR)

  (2)

  where HFC is the humidity correction factor in µm/kPa (for unknown fuel samples, ?HCF = 60 µm/kPa); and AVP is the mean vapour pressure during the test in kPa, given by

  AVP = (ACT1 + AVP2)/2

  (3)

  AVP₁ is the absolute vapour pressure at the start of the test in kPa, given by

  AVP2 = (%RH2 x 10n)/750

  (4)

  where %RH₁, is the relative humidity at the start of the test, and n is given by

  n = 8.017352- [1705.984/(231.8636 + T1)]

  (5)

  where T₁, is the air temperature at the start of the test in °C. AVP₂ is the absolute vapour pressure at the end of the test in kPa, given by

  AVP2 = (%RH2 x 10 n )/750

  (6)

  where %RH₂> is the relative humidity at the end of the test, and

  v = 8.017352 - [1705.984/(231.8636 + T2)]

  (7)

  where T₂ is the air temperature at the end of the test in °C.

  The volume of worn material from the ball (volume wear, VW) was calculated from the corrected wear-scar diameter as follows:

  (8)

  where D is the diameter of the test ball (D = 6.00 mm).

  The relative wear (RW) was calculated as the ratio of the volume of ball wear obtained for the solution of fatty acid additive in the base fluid (VW_fa) and the volume of ball wear for the base fluid (VW_bf):

  RW = (VWfa/VWfb) x 100%

  (9)

  The force transducer attached to the lower specimen housing measured the friction force throughout a test. An A/D converter captured the friction force signal from the force transducer and the centre-line average value of the signal was calculated. That value was divided by the applied load to give the friction coefficient. The limiting value of the friction coefficient is not defined by CEC F-06-A-96.
  The electrical contact resistance (a semi-quantitative indicator of the formation of a separating film in the contact) was monitored throughout a test. The contact resistance circuit applied a 15 mV potential across the specimen contact and a balance resistor in series. The contact and the balance resistor thus formed a potential divider circuit. The series resistance was set by the control software and was set to 10 W by default. The potential drop across the contact was thus a measure of the contact resistance, as compared to the balance resistor. A low or zero film reading meant that the potential dropped across the contact, and hence the contact resistance was very low, i.e., there was a significant metal-to-metal contact taking place between the test specimens. This is usually associated with a high friction force and with high wear. A high film reading meant that the metal surfaces were apart. The limiting value of the film is not defined in CEC F-06-A-96.

  Table 7. Tribological properties of tested diesel fuel components (HFR2 apparatus)
  Diesel fuel component	Tribological wear			Film %
  	Corrected wear-scar diameter, µm		Relative ball wear	Average value	Standard deviation
  	Average value	Standard deviation
  HON component	691	6	100.0	21	4
  HK component	703	3	100.0	13	2

  Pin-on-disc test outputs Wear-scar diameter was measured in both the X and Y directions using a photomicroscope to an accuracy of ±1 µm, and the MWSD was evaluated. The influence of the absolute humidity was not taken into account, and the corrected wear-scar diameter was not determined. VW was calculated from the MWSD as follows:

  (10)

  where D is the diameter of the test ball (D = 3.18 mm).

  The relative wear (RW) was defined in the same way as for the HFR2 test outputs.

  RESULTS AND DISCUSSION

  Test results

  Influence of fatty acids on the trihological properties of low-sulphur diesel fuel components The tribological properties of two low-sulphur diesel fuel components were compared. The results of tests carried out using the HFR2 apparatus show that the HK component, containing a lower amount of sulphur, has slightly worse antiwear behaviour than the HON component (Table 7).
  Tribological tests of 100, 500, and 1000 ppm stearic acid solutions in both components were performed using the HFR2 apparatus to determine the influence of small fatty acid additive amounts on the tribological properties of the components. The largest difference in wear value for these solvents appears at a concentration of 500 ppm. Furthermore, a 500 ppm solution of stearic acid has better antiwear characteristics in the HK component (Table 8).

  Table 8. Effect of stearic acid on tribological properties of HON component and HK component (HFR2 apparatus)
  Stearic acid concentration, ppm	Tribological wear			Film %
  	Corrected wear-scar diameter, µm		Relative ball wear	Average value	Standard deviation
  	Average value	Standard deviation
  HON component
  100	493	3	25.8	62	5
  500	409	2	12.2	87	2
  1000	319	8	4.5	90	3
  HK component
  100	470	15	19.7	44	4
  500	336	3	5.1	81	1
  1000	331	7	4.8	89	1

  The HON component was chosen as a base fluid for preparing C₆-C₁₆ acid solutions in the concentration range 50-1000 ppm. Tests carried out for those solutions using the pin-on-disc tester show that the wear does not decrease with increased solution concentration for each acid tested, and the wear does not decrease with increased molecular weight of the acid for each concentration (Table 9).
  The test results obtained from the HFR2 apparatus for 500 ppm solutions of C₆-C₂₂ acids (Table 10 ) confirm the untypical relationship between wear and the number of carbon atoms in the fatty acid molecule (molecular weight) (Figure 1). Additionally, it is found (Figure 2) that there is a correlation between wear and film thickness for capric, lauric, palmitic, stearic, and behenic acids (C₁₀-C₂₂ fatty acids). However, for caproic acid, in spite of a thin film, the wear-scar diameter is relatively small.

  Influence of fatty acids on tribological properties of separated fractions The HON component was studied in detail and separated into fractions (fraction I-X) by vacuum distillation. These fractions were tested using pin-on-disc and

  Table 9. Tribological wear in systems lubricated with 50-1000 ppm C6-C18 fatty acid solutions in HON component (pin-on-disc apparatus)
  Fatty acid	Fatty acid concentration, ppm	Wear-scar diameter, µm		Relative ball wear %
  		Average value	Standard deviation
  Caproic acid	50	299	27	15.4
  	100	240	24	6.4
  	500	272	26	10.6
  	750	237	35	6.0
  	1000	299	37	15.5
  Capric acid	50	305	17	16.8
  	100	183	25	2.2
  	500	300	23	15.6
  	750	234	39	5.8
  	1000	232	36	5.6
  Lauric acid	50	203	3	3.3
  	100	173	9	1.7
  	500	210	34	3.8
  	750	238	35	6.2
  	1000	197	32	2.9
  Palmitic acid	50	216	13	4.2
  	100	217	33	4.3
  	500	197	15	2.9
  	750	200	21	3.1
  	1000	207	28	3.6
  Stearic acid	50	191	15	2.6
  	100	206	21	3.5
  	500	221	37	4.6
  	750	196	22	2.8
  	1000	187	19	2.3

  Table 10. Tribological properties of 500 ppm C6-C18 fatty acid solutions in HON component (HFR2 apparatus)
  Fatty acid	Tribological wear			Film %
  	Corrected wear-scar diameter, µm		Relative ball wear	Average value	Standard deviation
  	Average value	Standard deviation
  Caproic acid	417	13	13.2	46	7
  Capric acid	446	3	17.3	60	4
  Lauric acid	438	1	16.1	63	3
  Palmitic acid	389	15	10.0	89	3
  Stearic acid	409	2	12.2	87	2
  Behenic acid	461	12	19.8	56	1

  Figure 1: Tribological wear in systems lubricated with 500 ppm solutions of fatty acids in the HON component under different test conditions

  Figure 2: Tribological wear and film thickness in systems lubricated with 500 ppm solutions of C6-C18 fatty acids in the HON component (HFR2 apparatus)

  Table 11. Tribological wear in systems lubricated with distillation fractions of HON component (pin-on-disc apparatus)
  Fraction	Tribological wear
  	Wear-scar diameter, µm		Relative ball wear %
  	Average value	Standard deviation
  I	522	33	144.2
  II	609	37	267.9
  III	582	36	224.4
  IV	619	72	286.3
  V	534	64	158.0
  VI	594	46	243.7
  VII	567	52	201.7
  VIII	547	71	174.2
  IX	527	55	150.4
  X	237	28	6.1

  Table 12 Tribological wear in systems lubricated with distillation fractions of HON component (HFR2 apparatus)
  Fraction	Tribological wear
  	Corrected wear-scar diameter, µm		Relative ball wear %
  	Average value	Standard deviation
  I	757	14	144.0
  II	780	24	162.6
  III	770	30	154.1
  IV	745	9	135.3
  V	743	16	133.6
  VI	723	22	119.7
  VII	699	25	104.8
  VIII	691	14	99.8
  IX	659	16	82.3
  X	533	15	35.3

  Figure 3: Influence of sulphur content on tribological wear in systems lubricated with distillation fractions of the HON component under different test conditions

  Figure 4: Influence of viscosity on tribological wear in systems lubricated with distillation fractions of the HON component under different test conditions

  Figure 5: Tribological wear in systems lubricated with distillation fractions of the HON component and 500 ppm solutions of stearic acid in these fractions (pin-on-disc apparatus)

  HFR2 apparatus. On the basis of the pin-on-disc results, clear evidence was found that only the last fraction (fraction X) gives lower wear than the HON component (Table 11). However, the HFR2 test results show that fraction IX also has antiwear properties (Table 12).

  In order to study the factors influencing the tribological properties of fractions, the sulphur content and viscosity of fractions I-X were determined (Table 2). The relative ball wear obtained from tests carried out using the HFR2 apparatus decreases with increasing viscosity and correlates with sulphur content for all the fractions except fraction VII (Figures 3 and 4). The pin-on-disc test results indicate that for lighter fractions wear does not depend on sulphur content and viscosity; the difference in the wear values for fractions I-VI may be caused by the tribological properties of particular groups of hydrocarbons in individual fractions under these test conditions (Figures 3 and 4).
  Figure 5 shows wear in the systems lubricated with fractions I-X, and wear in the systems lubricated with 500 ppm solutions of stearic acid in these fractions plotted against the initial boiling point of the fractions. The addition of 500 ppm of stearic acid into fractions I-IX produces marked wear reduction but does not influence the tribological properties of fraction X (Table 13).

  Influence of fatty acids on the tribological properties of different hydrocarbon base fluids Figure 6 (see p. 312) shows the relative change in wear for n-hexadecane, 1-methylnaphthalene, and their mixture containing palmitic acid

  Table 13 Tribological wear in systems lubricated with 500 ppm stearic acid solutions in distillation fractions of HON component (pin-on-disc apparatus)
  Fraction	Tribological wear
  	Wear-scar diameter, µm		Relative ball wear %
  	Average value	Standard deviation
  I	236	67	4.1
  II	212	64	1.5
  III	155	15	0.5
  IV	188	36	0.8
  V	184	32	1.4
  VI	225	35	2.0
  VII	200	47	1.5
  VIII	166	17	0.8
  IX	189	13	1.7
  X	249	21	122.4

  
  *The ratio of the volume of wear of the ball obtained for a fraction containing 500 ppm stearic acid additive and the volume of wear of the ball obtained for the fraction, multiplied by 100%.

  at different concentrations. In considering the influence of palmitic acid concentration on both 1-methylnaphthalene and its 1:1 ratio by volume mixture with n-hexadecane, three effects can be seen:

  • a 100 ppm concentration of palmitic acid reduces the ball wear for both lubricants very significantly
  • for the mixture of aromatic and paraffin hydrocarbons at a 750 ppm concentration of palmitic acid, a maximum value of wear appears, as in the case of n-hexadecane containing the same amount of acid (note that in Figure 6, this latter value is multiplied by 10 ^-1)
  • no wear maximum appeared for 1-methylnaphthalene.
  The effect of viscosity on the tribological behaviour of fatty acids was studied using two polyalphaolefin base oils. Figure 7 shows that an

  Figure 6: Relative ball wear in systems lubricated with 50-1000 ppm solutions of palmitic acid in n-hexadecane, 1-methylnaphthalene, and equivolume mixture of n-hexadecane and 1-methylnaphthalene (pin-on-disc apparatus)

  increase in the viscosity of the base oils decreases the wear reduction at all palmitic acid concentrations in the range 100-1000 ppm. Figure 7 demonstrates also that a 100 ppm addition of acid to synthetic base fluids PAO 4 and PAO 6 reduces the wear significantly, and at this acid concentration a minimum wear value appears.

  Interestingly, for high-viscosity base fluids with 100 ppm addition of acid, a clear maximum wear value was found (Table 14). Additionally, a 50 ppm concentration of palmitic acid in these base fluids also increased the wear. These trends are true for the saturated hydrocarbons (paraffin and naphthenic) separated from mineral base oil SN 400 and for the mineral base oil SN 400.

  Discussion

  Oxygen-containing impurities (e.g., naphthenic acids, phenols) are usually present in diesel fuels. Over the past few years a range of hydrogenation treatments of varying severity and hydrocracking processes have become

  Figure 7: Relative ball wear in systems lubricated with 50-1000 ppm solutions of palmitic acid PAO 4 and PAO 6 (pin-on-disc apparatus)

  common. These are especially effective at reducing the concentration of polar compounds and are potentially responsible for a large decrease in fuel lubricity.

  Wei and Spikes [4] considered that the significant wear reduction was produced by oxygen compounds with phenolic-type or carboxylic acid groups and occurred at low concentrations (Table 15). Our studies confirm that fatty acids added to low-sulphur diesel fuel components in a concentration range of 50-1000 ppm produced excellent antiwear properties (Tables 8-10).
  Fatty acids of higher molecular weight, starting from lauric acid (C₁₂), appear to be superior friction-reducing agents. A wide variety of early systematic studies [11, 13, 14] indicate that friction is reduced by the existence of a thin layer of a fatty acid material different from that of the contacting surfaces and which adheres strongly to the solid surface. Depending on the substrate surface, physical adsorption or chemisorption or both play an important part in friction-reducing film formation. At this point a question arises: what is the relation between friction and wear? According to Bowden and Tabor,13 although friction shows a strong correlation with wear, there is no simple general relation between the two. The fact that low friction does not necessarily mean low wear

  Table 14 Wear data for model and real base fluids containing palmitic acid in the concentration range 0-1000 ppm (pin-on-disc apparatus)
  Base fluid	Palmitic acid concentration, ppm	Wear-scar diameter, µm		Relative ball wear %
  		Average value	Standard deviation
  1 -Methylnaphthalene	0	385	13	100.0
  	50	279	22	27.4
  	100	208	37	8.4
  	500	215	16	9.6
  	750	192	7	6.1
  	1000	174	9	4.2
  Mixture of 1-methylnaphthalene and n-hexadecane	0	346	21	100.0
  	50	268	47	36.0
  	100	178	11	7.0
  	500	178	7	7.0
  	750	255	42	29.5
  	1000	241	43	23.4
  PA04	0	281	35	100.0
  	50	252	37	64.9
  	100	181	9	17.0
  	500	199	41	25.2
  	750	205	59	28.1
  	1000	192	56	21.7
  PA06	0	213	16	100.0
  	50	194	39	68.1
  	100	163	21	34.0
  	500	172	22	42.0
  	750	175	8	45.9
  	1000	174	7	43.9
  Saturated hydrocarbon mixture from SN 400	0	212	23	100.0
  	50	232	17	145.2
  	100	238	14	159.3
  	500	205	16	88.3
  	750	190	26	65.4
  	1000	156	10	29.6
  Mineral base oil SN 400	0	222	18	100.0
  	50	264	10	202.3
  	100	270	18	219.0
  	500	215	13	88.7
  	750	225	14	107.0
  	1000	167	15	32.4

  Table 15 Influence of oxygen-containing compounds on tribological properties of hydrotreated diesel fuel [4]
  Fluid	Oxygen from added compound, ppm	Wear-scar diameter, µm
  Hydrotreated diesel fuel	0	350
  Solution of 8-hydroxyquinoline in hydrotreated diesel fuel	10	320
  Solution of 8-hydroxyquinoline in hydrotreated diesel fuel	100	180
  Solution of 1,4-hydroquinoline in hydrotreated diesel fuel	100	220
  Solution of caproic acid in hydrotreated diesel fuel	100	230
  Solution of palmitic acid in hydrotreated diesel fuel	1	330
  Solution of palmitic acid in hydrotreated diesel fuel	10	220
  Solution of palmitic acid in hydrotreated diesel fuel	100	180
  Solution of palmitic acid in hydrotreated diesel fuel	300	170

  was emphasised over fifty years ago [19]. However, this fact is often overlooked and it has also contributed somehow to the lack of systematic and detailed studies on the influence of fatty acids on the wear of tribological mating elements. It has been shown, however, that although there is no simple, universally applicable relation between friction and wear, for specific systems, where effects such as ploughing and corrosion are negligible, a relation can be established [20]. It has been shown (Figure 2) that there is a correlation between wear and film thickness for capric, lauric, palmitic, stearic, and behenic acids (C₁₀—C₂₂) fatty acids). For caproic acid, in spite of a thinner film, the corrected wear-scar diameter is relatively small. For all fatty acids tested, the film thickness is greater than for neat diesel fuel components. Figure 8 shows wear and friction coefficient against number of carbon atoms and it can be seen that there is no correlation between wear and friction coefficient for C₆—C₂₂ fatty acids.

  In our opinion, two points seem to be of importance:

  • a kind of molecular interaction exists between the fatty acid additive and the chemistry of the base fluids
  • the viscosity of the base fluids seems to be a factor in the boundary lubrication process.

  Figure 8: Tribological wear and friction coefficient in systems lubricated with 500 ppm solutions of C6-C22 fatty acids in the HON component (HFR2 apparatus)

  Our previous research [21], in which n-hexadecane was used as the base fluid, indicated a new phenomenon concerning a very specific prowear and antiwear behaviour of fatty acids in the 50-1000 ppm concentration range. For this specific behaviour of fatty acids, two kinds of effects could be distinguished.

  • a clear corrosive effect at a concentration of 750 ppm of myristic acid (C₁₄), palmitic acid (C₁₆), and stearic acid (C₁₈)
  • a distinct antiwear effect at a concentration of 500 ppm of caproic acid (C₆).
  Results concerning 1-methylnaphthalene and its mixture with n-hexadecane (1:1 by volume) confirm that at a 100 ppm concentration of palmitic acid, a significant wear reduction of the steel-on-steel system can be achieved. For the mixture, the low-wear region extends to 500 ppm. At a 750 ppm concentration a maximum wear value appears (Figure 6); however, the maximum is located in the low-wear region (relative wear around 30%). In the case of 1-methylnaphthalene the increasing acid concentration slightly decreases the wear. Figure 7 demonstrates that for the synthetic base fluids, a clear minimum wear value exists at a 100 ppm concentration of palmitic acid; however, at a 750 ppm concentration of the acid, only a slight maximum is observed. In comparison with the wear data for n-hexadecane-based lubricants, both PAO 4- and PAO 6-based lubricants have these maxima at a relative wear reduction below 50%. Surprisingly, it was found that the addition of 50 and 100 ppm of palmitic acid to SN 400-saturated hydrocarbons increases wear by around 50% (Table 14). The same is true for the mineral base oil SN 400. However, in this case the corrosive effect is even larger; the wear is increased by a factor of two. When the concentration of palmitic acid was increased to 1000 ppm, the antiwear behaviour of both oils was improved (relative wear around 30%). The viscosity can influence the wear behaviour of fatty acids used at the lowest concentrations. For polyalphaolefin base oils, an increase in viscosity decreases wear reduction at all palmitic acid concentrations of 100-1000 ppm (Figure 7).
  All the above-discussed findings are challenging from the viewpoint of their interpretation as well as from their relation to the lubricating oil formulation. The results obtained evidently confirm the complexity of the boundary lubrication approach.

  CONCLUSIONS

  The results of the influence of 50-1000 ppm C₆-C₂₂ fatty acid solutions in diesel fuel components, their fractions, and other real and model solvents obtained from tribological tests carried out using a pin-on-disc tester and HFR2 apparatus have been described and discussed.
  One of the most interesting conclusions is that fatty acids added to low-sulphur diesel fuel components in the concentration range 50-1000 ppm produce excellent antiwear properties.
  Interestingly, tribological wear does not decrease with increased concentration of any fatty acid in a diesel fuel component. It was also found that the fatty acid chain length was not a controlling factor in the wear process for either the pin-on-disc apparatus or the HFR2 apparatus.
  Palmitic acid solutions in real and model solvents show that the antiwear behaviour of fatty acids is the result of complex interactions between molecules of the acid and particular types of hydrocarbons. This has been confirmed by results obtained for palmitic acid solutions ofn-hexadecane, 1-methylnaphthalene, and mixtures of these hydrocarbons.
  The antiwear behaviour of fatty acids under boundary lubrication conditions is influenced by the rate of acid molecule diffusion to the friction surface. This conclusion is drawn from results of experiments obtained for systems lubricated with palmitic acid solutions in synthetic oils (polyalphaolefins) of various viscosities. It has also been found that in some cases the structure of the solvent molecule has a predominant influence on wear.

  References

  1. Spikes, H.A., and Wei, D.-P., 'Fuel lubricity - fundamentals and review', Proc. 1st International Colloquium on Fuels, Technische Akademie Esslingen, Germany, 1997, pp. 249-58.
  2. Spikes, H.A., and Cameron, A., 'A comparison of adsorption and boundary lubricant failure', Proc. Roy. Soc. London Ser. A, 336 (1974) 407-19.
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  A revised version of a paper first presented at the 2nd International Colloquium on Fuels, Technische Akademie Esslingen, Germany.