Effect of Thermal Degradation of Base Oil on the Performance of Friction Modifier Additive

Sophie Campen¹, Janet Wong¹

¹Department of Mechanical Engineering, Imperial College London, UK

INTRODUCTION: During an engine oil’s lifetime, thermal degradation or aging can occur. Thermal degradation may lead to changes in the oil’s viscosity.¹ Degradation also alters the oil’s chemical composition. Numerous different chemistries are generated during thermal oxidation of oils, and species may have molecular weights lower or higher than those present in the original oil.² A study of the thermal auto-oxidation of n-hexadecane found that the proportion of different oxidation species (hydroperoxides, alcohols, ketones, carboxylic acids and esters) varies with reaction time.³ Alkyl hydroperoxides were found to be the primary reaction products, however it was proposed that they rapidly decomposed and underwent secondary reactions.³ The oxidation reaction pathways are radical.^4,5 Furthermore, metals may act to catalyze these reactions.^6,7 The use of iron(II) and peroxide to generate radical initiator species, such as in Fenton’s reagent, has long been known.⁸ This is of significance, since steel surfaces are found in engines.

Previously it has been noted that the formation of oxidation products is beneficial, since they tend to be surface-active and act to lower the boundary friction of the oil.^9,10 However, in formulated oils these oxidation products may interact with and compete for surface sites with other additives present in the oil, potentially having an antagonistic effect.¹¹ In this paper we investigate the effects of temperature and the ensuing thermal degradation of the oil on the performance of organic friction modifier additive under boundary lubrication conditions.

METHODS:  Friction tests are carried out using a High Frequency Reciprocating Rig (HFRR) in which a fixed 6 mm diameter ball is slid against a flat. Both ball and flat are of AISI 52100 bearing steel hardened to 800 HV. During tests the stroke length is 1 mm, the frequency of the reciprocating motion is 50 Hz and the normal load is 4 N giving a mean Hertzian contact pressure of 0.69 GPa. Test lubricants include unadditivated base oil and friction modifier-containing base oil. Here, n-hexadecane is used as a model base oil and n-octadecanoic acid is used as a model organic friction modifier.

In these experiments the temperature is varied during the friction test and a single heating-cooling cycle is carried out. Initially the temperature is held constant at 100 °C for a period of 30 minutes. The temperature is then ramped to 150 °C at a constant rate of 1 °C per min. The temperature is held at 150 °C for a period of 30 minutes before decreasing to 100 °C at a rate of 1 °C per min. Finally, the temperature is held constant at 100 °C for a period of 30 minutes.

RESULTS:  The data in Figure 1 illustrate the effect of temperature on the coefficient of friction and the electrical contact resistance (ECR) during HFRR tests. The ECR, given as a percentage, is affected by the separation of the two opposing surfaces and provides a non-quantitative representation of the real film thickness. Although the real film thickness is not obtained, changes such as the formation or break-down of a boundary lubricating film are visible.

At a temperature of 100 °C, the mean coefficient of friction for the base oil (0.252) is larger than that observed with addition of organic friction modifier (0.097). Large fluctuations in friction with time suggest that wear processes are occurring in the unadditivated oil, while in the presence of organic friction modifier, the friction is stable with time. Previously, no measureable wear was observed by optical microscopy after 3 hour tests at 100 °C with the same

friction modifier. The ECR suggests that the organic friction modifier forms a boundary lubricating film which effectively separates the two opposing surfaces. On the other hand, little or no surface separation is achieved in the base oil alone.

Upon heating there is a sudden transition at 118 °C in the coefficient of friction of the base oil– the large fluctuations cease and the value drops to 0.176. With continued time spent at elevated temperature, the coefficient of friction continues to fall, before stabilizing upon cooling to 100 °C at a value of 0.129. During the test, the ECR increases, suggesting that a boundary lubricating film forms.

Transitions are also seen during the test with friction modifier, however they are less obvious in the graph plotted on this scale. With heating there is a gradual decrease in the coefficient of friction. A sharp decrease in friction occurs at a temperature of 125 °C, which is accompanied by a drop in the ECR, suggesting either changes in or disruption of the organic friction modifier boundary lubricating film. The coefficient of friction reaches a minimum value of 0.05 at a temperature of 150 °C, before subsequently increasing with sliding time. After the heating-cooling cycle, the coefficient of friction (0.110) is higher than its original value. Large fluctuations in the ECR hint at instability within the boundary lubricating film.

Figure 1 – High temperature HFRR friction experiments in base oil (BO) and base oil with organic friction modifier (BO + OFM). Graphs show coefficient of friction (COF, top), electrical contact resistance (ECR, middle) and temperature (T, bottom) vs. time.

DISCUSSION:  These experiments support that some degree of oxidation of unadditivated base oil is favorable since it facilitates boundary lubricating film formation. However, oxidation species can have a negative impact the performance of organic friction modifier additive. Further studies investigate base oil oxidation by spectroscopic techniques.

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