Contents

Influence of Temperature on the Tribochemical Reactions of Hexadecane

Czeslaw Kajdas
Warsaw University of Technology, Plock, Poland

Monika Makowska, and Marian Gradkowski
Institute for Terotechnology, Radom, Poland

Keywords : hexadecane, tribochemistry, thermochemical reactions, mechanochemistry, initiation of chemical reactions, reaction products
Published in: Lubrication Science 15-4, August 2003. (15) 329

Abstract

This paper investigates the tribochemical reactions of n-hexadecane proceeding in a tribosystem lubricated by n-hexadecane at ambient and elevated temperatures. It is hypothesised that, at ambient temperature, reactions are mostly initiated by the mechanical action of the system, and at elevated temperature (200°C) thermochemical reactions should be dominant. An experimental study was performed using a ball-on-disc machine with steel-on-steel mating elements. To analyse wear tracks, Fourier transform infrared microspectrophotometry (FTIRM) and electron spectroscopy for chemical analysis (ESCA/XPS) were used. To investigate chemical changes in the bulk lubricant, gas chromatography coupled with mass spectrometry (GC IMS) was applied. The results provide clear evidence for the hypothesis that two types of oxygenation processes of n-hexadecane under boundary lubrication conditions should be considered. The first, at ambient temperature, is controlled by the mechanical action and the second is clearly controlled by temperature. The analytical techniques applied gave evidence of the formation of some reaction products from hexadecane under boundary lubrication conditions. These products include compounds having Fe-0 bonding (salts and chelates), carbonyl compounds, and iron carbide.

INTRODUCTION

Early work on the chemistry of the boundary lubrication of steel by hydrocarbons, including hexadecane, demonstrated that the sliding behaviour of steel lubricated by hydrocarbons under boundary lubrication conditions could be related to chemical reactions at the sliding surfaces, involving metal, hydrocarbon, and oxygen [1]. The results suggested that the reactions occurred at sites where fresh metal surface was exposed by rubbing.
One of the basic questions in tribology is the mechanism by which antiwcar and extreme-pressure additives act to reduce wear and surface damage under boundary lubrication conditions. To study the tribochemistry of these additives, model base fluids have to be used. Hexadecane is widely used as the model fluid for such experiments. Major components of lubricating base oils, namely hydrocarbons, undergo chemical changes during operation of tribological systems. In this case, chemical reactions can be initiated by both temperature (a thermochemical process) and mechanical action of the system (a tribochemical process). The latter can be specifically traced to the disrupted surfaces caused by the sliding contact generating surface-active sites, so promoting reactions that otherwise might not occur.
Tribochemistry is generally referred to as the chemistry that occurs under rubbing conditions and producing molecular structures or interfacial layers that lubricate [2]. Reactions initiated by mechanical action are distinguished as triboreactions and they can be initiated by triboemission processes [3]. All these specific physicochemical processes lead to significant changes on the surface and in the bulk oil. The latter changes lead to lubricating oil deterioration.
To account for tribochemical reactions of hydrocarbons under boundary lubrication operating conditions, it is necessary to start such research using the most simple compounds, for example n-hexadecane, which is widely used as a low-viscosity model base oil [4-6]. etailed analysis of specific products formed at particular stages of the process may also allow a better understanding of the detailed chemical reactions taking place both in the bulk lubricant and on the contact surfaces.
This paper investigates the tribochemical reactions of n-hexadecane proceeding in a steel-steel tribosystem lubricated by n-hexadecane at ambient and elevated temperatures (200°C). It is hypothesised that at ambient temperature reactions are mostly initiated by the mechanical action of the tribological system, and at elevated temperatures thermochemical reactions should dramatically enhance the reaction pathways initiated mechanically.

Figure 1 Ball-on-discT- 11 tester

EXPERIMENTAL:

Lubricant

A sample of n-hexadecane (>99%; Merck-Schuchardt) was used as the lubricant (model mineral base oil) without any additional purification. Gas chromatographic analysis demonstrated that only one peak assigned to n-hexadecane was present.

Tribological tests

To investigate the effects of temperature on tribochemical reactions proceeding under boundary lubrication conditions in a steel-on-steel rubbing system, a T-11 pin-on-disc tester was used. The tester (see Figure 1) was developed by the Institute for Terotechnology in Radom, Poland [7]. This apparatus allows one to run tests at elevated temperatures up to around 300°C. It also continuously records the coefficient of friction and linear displacement due to wear. Elements of the friction pair (balls and discs) were made from 52100 bearing steel, 60 HRC. Before each run, the balls and discs were cleaned ultrasonically in hexane for 15 min.
All tests were performed under the following operating conditions: load 9.81 N; sliding velocity, 0.25 m/s; sliding distance, 500 m; ambient temperature, 20 ± 2°C and 200 ± 4°C. After setting up the T-11 ball-on-disc tester and the test parameters, approximately 5 cm³ of hexadecane was placed in the disc-holding cup of the device. In the case of ambient temperature studies, the tests were performed immediately. In the case of 200°C tests, a software-controlled heating procedure was conducted prior to running the test. After the tests the discs were washed with hexane and stored above silica gel until further analysis.
The products of the tribochemical changes of n-hexadecane were investigated ex situ. To obtain detailed information concerning the tribochemical changes of hexadecane, wear tracks of the tested discs were examined using selected analytical techniques (Fourier transform infrared microspectrophotometry (FTIRM) and electron spectroscopy for chemical analysis (ESCA)), which allow one to investigate the chemistry of the products generated on the steel surface, and gas chromatography coupled with mass spectrometry (GC/MS) to determine the reaction products formed in the bulk hexadecane lubricant.

Analytical techniques

FTIRM To investigate the organic layer on the wear tracks an i-series PE Fourier transform infrared microspectrophotometer was used. Reflection spectra were recorded in the wavenumber range 4000-700 cm-1, with a resolution of 4 cm-1 (64 scans at each point). The region of analysis was totally within the disc wear track. The sample spot size was 35 µm x 35 µm. Before each sample analysis, the background spectrum (steel surface outside the wear track) was recorded and automatically subtracted from sample spectra. All sample spectra were corrected by removal of the spurious band originating from carbon dioxide [8], near 2350 cm-1, as well as by smoothing according to Savitsky-Golay's method and multipoint normalising of the baseline. The mathematical processing of the spectra showed no influence on their appearance.

ESCA In order to determine the types of chemical bonds present in the products layered on the disc steel surface, the wear track was subjected to ESCA/XPS* analysis. A Phi-5702 multifunctional X-ray photoelectron spectrometer was used with a Mg Ka radiation source and the binding energy of C 1s (284.6 eV) as the reference line. The energy resolution of high resolution spectra was ±0.2 eV. The Fe 2p, C 1s and O 1s profiles were recorded on a 0.8 mm diameter region at a constant pass energy of 93.9 eV. Ion sputtering of the disc surface was carried out with argon ions of 3 keV energy. It was done at a glancing angle of 45° and the depth profile was obtained during 7 min of sputtering.

XPS spectra were obtained in the LSL of the Institute of Chemical Physics, Lanzhou (China).

Figure 2 Result of GC/MS ion 60 (characteristic for monocarboxylic acids) selective analysis of bulk lubricant after friction in steel-steel system (200 °C)

GC/MS GC/MS was applied to identify the products of bulk lubricant chemical changes proceeding under boundary friction conditions. It was performed using an HP 5890 Series II gas chromatograph coupled with an HP 5972 Series mass spectrometer under the following conditions: injection, 1 µl; column, HP PONA (50 m, diameter 0.2 mm); oven programme, 60-260°C at 8°C/min; pressure programme, 100-200 kPa at 4 kPa/min; flow rate, 0.4 ml/min; carrier gas, He 6.0.

RESULTS AND DISCUSSION

On the basis of the results obtained for elevated temperature and previous results [9] relating to ambient temperature, it is clearly seen that the tribochemistry of n-hexadecane is significantly influenced by elevated temperature, which dramatically changes the oxidation process of this hydrocarbon. The chemically changed bulk lubricant at a temperature of 200°C is composed of various oxygen-containing compounds, especially carboxylic acids (see Figure 2). Other oxygenated products are
Figure 3 FTIR spectrum of triboreaction products formed during the friction process carried out at elevated temperature (200 C)

Figure 4 FTIR spectrum of triboreaction products formed during the friction process carried out at ambient temperature (20°C)

Figure 5 ESCA survey profile of the steel disc lubricated by n-hexadecane during friction

alcohols, aldehydes, and ketones. They are typical products of thermooxidative reactions proceeding under high-temperature conditions.

The oxygenated compounds formed react with the metal of the rubbing surfaces. Both the FTIRM and ESCA results suggest that, at ambient temperature, reactions between the iron and carboxylic acids lead to the formation of salts and/or chelates. Specific IR absorption bands [9] appear at around 1550, 1650, and 3300 cm-1. However, characteristic infrared absorption bands around 3500-3000, 1740, and 1600 cm occurring in the high-temperature spectrum presented in Figure 3 are different from those obtained at ambient temperature (see Figure 4). The signals observed are most probably related to carbonyl compounds, e.g., esters, aldehydes, and ketones. It appears that the thermal oxidation process proceeding in the bulk lubricant at elevated temperature is more enhanced and leads to the formation of different oxygen-containing compounds, and that the absence of bands assigned to —COOFe groups is caused by a relatively thick organic layer.
X-ray photoelectron spectroscopy is very sensitive for investigating not only the chemical composition but also the chemical environment of the atoms in a molecule. After running a survey profile of the disc (see Figure 5), the regions for high-resolution scans were selected.

Figure 6 Typical XPS spectra of

(a) Fe 2p,
(b) O 1s, and
(c)C 1s
photoelectrons recorded for the triboreaction product layers formed at elevated temperature (200°C)

Figure 7 Result of overlapping peak separation in C 1s photoelectron spectrum

Spectra of the iron, oxygen, and carbon regions revealed shifts in binding energy values due to chemical bonding. Figure 6 demonstrates that in spectra corresponding to external layers formed at elevated temperature there are no signals attributable to iron. For the other layers peaks appear at about 708 eV (Fe 2p), 532 eV (0 Is) and 284 eV (C Is).
In the carbon photoelectron region there are some overlapping signals at 288.28, 286.40 and 284.58 eV (see Figure 7). The results show that carbon might be in at least two forms: as Fe3C and as oxygen-containing organic compounds with C=O bonding.
Quantitative analyses of the films layered on the steel surface were done using the ESCA technique (see Table 1). All films contain considerable amounts (about 70%) of carbon.
Iron is the best candidate for electron donation, especially with increasing depth, indicating the formation of iron carbide, Fe3C, near to the steel. The existence of large amounts of carbon (determined using ESCA) and the detection of organic compounds with FTIRM analysis clearly show that a very specific tribochemical reaction layer was formed in the disc wear track. The layer consists of compounds including organic ligands and iron-oxygen bonding along with Fe-C bonding in the form of iron carbide (Fe3C).

Table 1 Atomic concentration of elements in the triboreaction product film

Ion sputtering time (min)Atomic concentration (%)
Fe 2pO 1sC 1s
0028.1171.89
1028.5571.45
220.321.8777.80
316.1710.3973.44
422.543.5373.93
523.282.5774.15
628.273.5668.17
728.102.0769.83

Figure 8 Typical XPS spectra of

(a) Fe 2p,
(b) 0 1s, and
(c) C 1s
photoelectrons recorded for the triboreaction product layers formed at ambient temperature (20°C)

Figure 8 shows the high-resolution spectra of iron, oxygen, and carbon photoelectrons corresponding to the products of n-hexadecane tribochemical changes layered on the steel surface at ambient temperature. The peak at about 711 eV is assigned to the formation of iron compounds (for elemental iron it is at 706.4 eV [10] ) and is typical for an Fe-O bond [11]. The binding energy of -530 eV, rather like -288.5 eV (C=0) and -285 eV (C-O), corresponds to oxygen in organic compounds, for example iron carboxylate [11].
The above results demonstrate that the reaction between iron and carboxylic acids leads to the formation of salts or chelates. It is worth noting again at this point that their characteristic infrared absorption bands are different from those obtained at elevated temperature (see Figure 3).

CONCLUSIONS

Using a ball-on-disc tribometer under steel-on-steel boundary lubrication conditions, a study has been made of the tribochemical changes of n-hexadecane initiated by friction and enhanced by elevated temperature. This is part of on-going research into the tribochemistry of lubricant base fluids. The use of FTIRM and ESCA analytical techniques to analyse the surface films and deposits in the contact region showed evidence of the formation of three reaction product types from hexadecane under boundary lubrication conditions. These products include compounds having Fe-O bonding (salts and chelates), carbonyl compounds, and iron carbide.
The results obtained also provide evidence for the hypothesis that two types of oxygenation of n-hexadecane under boundary lubrication conditions take place. The first, at ambient temperature, is controlled by the mechanical action, and the second is clearly controlled by temperature. More research is needed to clarify the reaction mechanisms, particularly the mechanisms of surface reactions.

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This is a revised version of a paper given at the 2nd World Tribology Congress, Vienna, Austria.