(Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

Study on Tribology Performance of Different Lubricant Additives under Atmospheric Pressure and High Vacuum Conditions

Peng Qian; Huang Zhiyang; Yang He; Song Haiqing; Zhang Jianrong

(Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

By using PAO-10 as the base oil, the tribological behavior of 11 additives under high vacuum condition was evaluated. By adopting some surface analytical instruments, such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), the tribological mechanisms of these additives were studied. In air, O2can react with metal to form metal oxide that can protect the surfaces of rubbing pair during the tribological tests. According to the theory of the competitive adsorption, the function of some active elements is weakened. In a vacuum environment, the additives contributed more to the lubrication performance. The sulfur-containing additives could react with Fe to produce FeSxand “M—C” bonds (“M” represents metal). They both had contributions to the lubrication. As for the phosphorus-containing additives, they only generated the phosphates during the tests. When the sulfur and phosphorus-containing additives were applied, the generated phosphates and FeSx had the primary contribution to the lubrication performance during the tests.

anti-wear additive; high vacuum; lubrication mechanism; anti-wear properties; friction reducing properties

1 Introduction

Along with the rapid development of the aerospace industry, the need of increasing the lifetime of the spacecraft from 6—12 months at the early stage to the current 10—30 years, which has also raised the performance requirements for this space lubricant up to a new height. The vacuum liquid lubricant has become an essential part in the lubrication systems of spacecraft due to their advantages, viz.: ease in replenishment, ability to remove wear debris, low mechanical noise, no wear in the elastohydrodynamic regime, and insensitivity to environmental factors[1]. Currently, many types of space liquid lubricant base oils, which are used in the spacecraft, include: polyalpha-olefins (PAOs)[2-3], silahydrocarbons (SiHC)[4], alkylated cyclopentanes (MACs)[5-6], and perfuoropolyether (PFPE)[7]. However, there are few lubricant additives specialized for use in the spacecraft. Therefore, the development of high-effciency space liquid lubricants, specialized additives and the corresponding space lubrication formulation will play important roles to overcome the lubrication failures[8-11]of mechanical parts of spacecraft and to extend their lifetime. In comparison with other space base oils, PAO oils are widely used in the business. They not only have good lubricity, thermal and oxidation stability and shear stability, but also have good viscositytemperature properties. This is the reason why we chose PAO-10 oil as the base oil. Upon considering the specifcities of space liquid lubricants, three types of commercial lubricant additives were selected, namely: the oxidation corrosion resistant additives, the extreme pressure and anti-wear additives and the friction modifiers. The base oil and additives were mixed well at a certain proportion. The friction and wear performance of the mixtures was compared under atmospheric pressure and vacuum conditions. The anti-wear mechanism of the additives containing sulfur or phosphorus or sulfur and phosphorus species under vacuum conditions have been mainly discussed.

2 Experimental

2.1 Materials

On the basis of the PAO-10, which was produced by the ExxonMobil Co., 11 kinds of additives were selected,with their compositions listed in Table 1.

Table 1 Different types and kinds of the additives

2.2 Four-ball test condition

Following the standard test method GB/T 3142—1982“Lubricants: Determination of load carrying capacity (four balls method)”, the maximum non-seizure load (PB) of the oil PAO-10 was measured, and itsPBwas 490 N. Upon combining the load carrying capacity test results with its application of liquid space lubricants in space mechanical moving parts[12-15], the test load was selected at “400 N”. Due to the difficulty of heat-conduction in vacuum, the test speed should not be too large and the test time also should not be too long, and then eventually the speed was set at 100 r/min and the test time was 1.0 h. And according to the Stribeck curve[16], the lubrication process should belong to the boundary lubrication. The article[17]pointed out that the cabin pressure was at around 10-4Pa when the spacecraft was running outside the earth, and according to the operating instruction of the vacuum four-ball instrument, when the pressure of the cabin was less than 5.0×10-4Pa, the test results were reliable.

2.3 Experimental methods

One percent (mass fraction) of each of 11 kinds of additives was blended into the base oil (PAO-10). After being mixed homogeneously, the four-ball wear test was conducted at atmospheric pressure and under vacuum condition separately.

The test condition, which was implemented at room temperature, covered: a speed of 100 r/min, a load of 400 N, and a test time of 1 hour, with the diameter of steel ball (made of GCr15 steel) equating to 12.7 mm,. During the vacuum test, the experiment could start when the pressure of the chamber was not greater than 5.0×10-4Pa.

The average friction coeffcient and the average diameter of wear scars were measured, when the test terminated. The scanning electron microscopy (Quanta 200 FEG) was adopted to analyze the surface morphology of the wear scar, while the X-ray photoelectron spectroscopy (ESCA Lab 250) was used to study the changes on surface elements of wear scar.

3 Results and Discussion

3.1 Anti-wear and anti-friction properties

Figures 1 and 2 show the results for measuring the average friction coeffcient and the average diameter of wear scars.

Figure 1 WSD formed in relation to base oil containing different additives.

As shown in Figure 1, different additives had different ability of abrasion resistance under both atmospheric pressure and vacuum conditions. For the additives including T-1, T-2, T-3, T-6 and T-7, the diameters of their corresponding wear scars obtained under vacuum condition were greater than those formed under atmospheric pressure. On the contrary, those additives including T-4, T-5,T-10, T-8 and T-9 showed that the diameters of their corresponding wear scars obtained under vacuum condition were smaller than those obtained at atmospheric pressure. And as regards the additive T-11, the diameter of its WSD obtained under vacuum was almost the same as that obtained under the atmospheric pressure condition.

Figure 2 Friction coef fi cient measured in relation to base oil containing different additives

As shown in Figure 2, the friction coeffcient of all samples measured under the vacuum condition was greater than those measured under the atmospheric pressure condition.

3.2 Discussion on lubrication mechanism

According to the tribological properties of anti-wear additives and their compound types, we selected the additives T-4, T-10 and T-9 as the representatives. In order to study the lubrication mechanism of these additives, we analyzed the surfaces of their wear scars obtained under both the vacuum and the atmospheric pressure conditions by means of SEM, EDS and XPS.

3.2.1 Lubrication mechanism of base oil with sulfurcontaining additives

The sulfurized isobutylene (additive T-10) was used as the representative. The surface analytical methods were used and the mechanism for lubrication by base oil with sulfurcontaining anti-wear additives operating in vacuum and at atmospheric pressure was discussed.

(1) SEM and EDS analyses

Figures 3 and 4 show the surface topography of wear scar lubricated with base oil containing T-10 under the atmospheric pressure and vacuum conditions and measured by the SEM microscope.

Figure 3 SEM analysis of wear scar lubricated in air by T-10 doped base oil

Figure 4 SEM analysis of wear scar lubricated in vacuum by T-10 doped base oil

As shown in Figure 3, the grinding cracks on the surface of the wear scar were more obvious, deeper and worn severely, which were mainly caused by oxidation. While in Figure 4, the wear scars on the surface were relatively smooth. Some small cracks were observed on the wear scar surface. And some abrasive dust was accumulated along the edges of wear scars, which was mainly caused by adhesive wear.

EDS was used to investigate the elemental distribution on the surface of wear scar lubricated by oil containing the additive T-10 both in atmospheric pressure and vacuum environments. Figure 4b and Figure 5b show the analytical regional analysis and the point analysis of the surface wear scars, with the results depicted in Figures 5 and 6.

Upon comparing Figure 5 with Figure 6, it can be seen that the signal of the element sulfur on the wear scar surface that was lubricated by oil containing T-10 increased signifcantly under vacuum condition.

(2) XPS analysis

XPS was used to study the composition, structure and elemental valence of these compounds on the surface of thewear scars formed both in the atmospheric pressure and vacuum environments.

Figure 5 EDS elemental analysis of wear scar lubricated in air by base oil containing T-10

Figure 6 EDS elemental analysis of wear scar lubricated in vacuum by base oil containing T-10

The peak signal of carbon, oxygen, sulfur, iron and other elements in wear scars lubricated by T-10 doped oil had been analyzed by XPS under both in air and in vacuum. Figures 7 and 8 show the analytical results.

As shown in Figure 7a, under atmospheric pressure the element “C” whose peak located at 284.85 eV was associated with the “C—C” bond, while the peak at 288.10 eV was related with the “C—S” bond. Therefore, a certain amount of additive T-10 was still adsorbed on the surface of the wear scar after sputtering.

As shown in Figure 7b, under the atmospheric pressure condition, the element “O” the peak of which was located at 529.40 eV was associated with the “M—O” bond (in which “M” represented the metal), while the peak at around 530.83 eV might belong to the “S—O” bond or might also originate from the O2in the polluted air. Thus during the process of friction, O2molecules had been involved in the friction reaction, resulting in the formation of the compound FexOy.

As shown in Figure 7c, peaks of element “S” at the positions 160.83 eV and 162.13 eV belonged to the spin splitting peaks of element “S”, the valence of which was negative two. This indicated that in the process of friction, S species in the additive T-10 reacted with Fe to form the Fe—S bonds[18].

As shown in Figure 7d, the peak at the location 709.80 eV might belong to the “Fe—S” bond or the “Fe—O” bond. But because of the overlaps of these two bonds, they could not be accurately distinguished.

It can be seen from the above analysis that under the atmospheric pressure condition both the oxygen from the air and the additive T-10 had participated in the friction reaction to generate the compounds FeSxand FexOywhich worked jointly as lubricant.

Compared with the results obtained at atmospheric pressure, the biggest difference of vacuum condition was the change in the element “C”. As shown in Figure 8a, there was a new peak produced at the position 282.10 eV, where the same peak did not appear in Figure 7a. According to the bond energy theory, the peak might belong to the “M—C” bond (in which the metal and “C” element were combined) or the “Si—C” bond. Figures 5 and 6 show thatthe element “Si” did not exist in the surface of the wear scar, since the elements “Fe” and “Cr” were dominant. Therefore, the peak herein would more likely belong to the “Fe—C” bond, which was produced by the reaction of the additive T-10 with the metal “Fe”. Figure 8b shows that under the vacuum condition the additive T-10 reacted with the metal and produced some compounds containing the “Fe—S” bond to function as the lubricant.

Figure 7 XPS analysis of element valence in wear scars lubricated in air by T-10 doped base oil

Figure 8 XPS analysis of element valence in wear scar lubricated in vacuum by T-10 doped oil

Due to the absence of O2in vacuum, the metal oxides were not produced during the friction process. But the diffculty of heat transfer in vacuum resulted in a highest temperature around the contact place between the friction pairs, which made the metal part of the steel balls react with the active “C” of the additive T-10 to generate some compounds with the “M—C” bond. It is supposed that due to the good anti-wear performance of the special compound the diameter of the wear scar under vacuum condition was smaller than that formed at atmospheric pressure.

3.2.2 Lubrication mechanism of the phosphoruscontaining additives

The extreme pressure antiwear additive of phosphate ester type (additive T-9) was used as a representative of the phosphorus-containing additives. The surface analytical methods were applied and the difference in the mechanism for lubrication by the additive-doped base oil in the vacuum and at atmospheric pressure was discussed.

(1) SEM analysis

The surface topography of the wear scar lubricated by the additive T-9 doped oil was analyzed by SEM, with the results presented in Figures 9 and 10.

Figure 9 Wear scar lubricated in air by T-9 doped oil (×1000)

Figure 10 Wear scar lubricated in vacuum by T-9 doped oil (×1000)

As shown in Figure 9, the grinding cracks on the surface of the wear scar were not obvious. There were a lot of abrasive dust on the wear scar with some fine cracks, which were mainly caused by oxidation and adhesive wear. In Figure 10 there were many pits on the surface of the wear scar, indicating to the severe corrosion. This phenomenon was mainly caused by the corrosive wear.

(2) XPS analysis

The surface of the wear scars lubricated by the additive T-9 doped base oil under atmospheric pressure and vacuum conditions was analyzed by XPS separately. The signals on peaks of carbon, oxygen, phosphorus, iron and other elements are shown in Figures 11 and 12.

As shown in Figure 11a, under atmospheric pressure the“C” elements the peaks of which were located at 282.53 eV, 284.80 eV, and 289.32 eV, belonged to “M—C” bond,“C—C” bond, and “C—O” bond, respectively.

As shown in Figure 11b, at atmospheric pressure the“O” elements the peaks of which were at 529.04 eV and 530.74 eV were associated with the “M—O” bond and the “P—O” bond or other pollutants.

As shown in Figure 11c, the peak of element “P” at the position of 133.70 eV was supposed to be the compound Fex(PO4)y.

As shown in Figure 11d, iron oxide existed on the surface of the wear scar.

It can be seen from the above analysis that at atmospheric pressure both the oxygen in the air and the additive T-9 had participated in the friction reaction, leading to the formation of compounds FeSxand FexOy, which worked jointly as lubricant.

The additive T-9 reacted with the iron during the friction process, and generated the soft phosphate of the iron. So there was a lot of abrasive dust on the surface of the wear scar. In addition, O2species, which were also involved in the friction process, could react with iron to produce the compound FexOy. They worked together with the iron phosphate as the lubricants.

Under the vacuum condition, only the peaks of element“C” were quite different. As shown in Figure 12, the “C”element peaks of which were located at 284.90 eV, 287.33 eV, and 291.50 eV belonged to the “C—C” bond, the “C—O” bond and the “C—OR” bond, respectively.

Due to the absence of O2in vacuum, complete oxidation cannot happen in the process of friction. Meanwhile, the chemically reacting films were composed mainly of the metal phosphates, which were produced by the reaction of the additive T-9 and the metal. The phosphates were soft[19]and the metal was worn seriously, so the diameterof the wear scar under vacuum conditions was much bigger than that identifed in air.

Figure 11 XPS analysis of element valence in wear scar lubricated in air by T-9 doped base oil

Figure 12 XPS analysis of element C in wear scar lubricated in vacuum by T-9 doped base oil

3.2.3 Lubrication mechanism of the sulfur and phosphorus-containing additives

The nitrogen-containing derivatives of dithiophosphoric acid (the additive T-4) were used as a representative of the sulfur-phosphorus type additives. The surface analytical technique was used and the difference of the lubrication mechanism in a vacuum environment and at atmospheric pressure was discussed.

(1) SEM analysis

The surface topography of the wear scar lubricated by base oil containing the additive T-4 was studied by SEM, with the results shown in Figures 13 and 14.

Figure 13 Wear scar lubricated in air by T-4 doped oil (×1000)

Figure 14 Wear scar lubricated in vacuum by T-4 doped oil (×1000)

After conducting the friction test of the additive T-4-containing base oil at atmospheric pressure, the surfaces of the wear scar were observed by SEM, with the results shown in Figure 13. There were fne grooves on the surface of the wear scars along with a certain amount of abrasive dust, which was formed due to the oxidation and adhesive wear.

As shown in Figure 14, in a vacuum environment the surface of wear scars was rougher. Fine and small abrasion, serious grinding cracks and lots of abrasive dust appeared along with many pits, which were mainly caused by the interaction of adhesive wear and abrasion.

(2) EDS analysis

The elemental distribution of the wear scar lubricated by base oil containing the additive T-4 in an atmospheric pressure environment and under vacuum was analyzed by EDS, respectively. We selected the analytical area and points studied in Figure 13 and Figure 14, respectively, with the results presented in Figure 15 and Figure 16, respectively.

As shown in Figure 15a, the signal intensity of elements“S” and “P” was very weak, indicating that the content of elements “S” and “P” was small on the surface of the wear scar lubricated in air by oil containing the additive T-4. In regard to the point analysis results, the signal intensity of element “P” was stronger than that of element“S”. It might occur because the element “P” was more actively prone to react with the metal than the element “S”during the process of friction.

Figure 15 EDS spectra of wear scar lubricated in air by T-4 doped base oil

Figure 16 EDS spectra of wear scar lubricated in vacuum by T-4 doped oil

As shown in Figure 16, under the vacuum condition thesignal intensity of the element “P” was stronger than that measured under atmospheric pressure, indicating that the additive T-4 was involved more actively in the friction reaction in vacuum than in air. But the results of element point-scanning analysis by EDS would be affected by the analytical locations selected.

(3) XPS analysis

The surface of the wear scars that were lubricated in the air and in the vacuum environment, respectively, by base oil containing the additive T-4, was studied by XPS. The signal of peaks of carbon, oxygen, phosphorus, iron and other elements was analyzed. Because of the similarity of the peak shapes identifed in the air or in the vacuum environment, only the results obtained under atmospheric pressure are given in Figure 17.

As shown in Figure 17a, under atmospheric pressure the element “C” peaks of which were located at 283.11 eV, 284.90 eV, and 289.50 eV belonged to the “M—C” bond, the “C—C” bond, and the “C—O” bond, respectively.

As shown in Figure 17b, under atmospheric pressure the element “S”, the peak of which was at 162.03 eV and 163.14 eV, belonged to the spin splitting peaks of element“S”, with its valence equating to negative two. The peak at 168.00 eV belonged to thespecies. And we can conclude that the element “S” of the additive T-4 reacted with iron to generate the compound FeSx.

As shown in Figure 17c, the peak of element “P” was supposed to be thespecies.

As shown in Figure 17d, at least one “Fe—O” bond and one “Fe—S” bond existed on the surface of the wear scar. It can be seen from the above analysis that under atmospheric pressure both the element “S” and the element“P” of the additive T-4 had participated in the friction reaction to generate the compounds FeSxand Fex(PO4)y, which could decrease the wear of tribopairs. Meanwhile, the oxygen from the air could react with iron to generate the compound FexOy, which could work as a lubricant to a certain extent. At atmospheric pressure under the lubrication by the T-4 doped base oil the compounds FexOy, FeSxand Fex(PO4)ywere identifed on the surface of wear scar. These three compounds formed a composite flm, which could work jointly in the lubrication process.

Figure 17 XPS analysis of element valence in wear scar lubricated in air by T-4 doped base oil

Compared with the case of lubrication under the atmospheric pressure condition, the shape of each element inthe wear scar was similar to that obtained in the vacuum environment. But due to the absence of O2species, the chemical film, which was mainly composed of phosphates and FeSx, could be formed in the friction process. However, under atmospheric pressure, the composite flm, which was mainly composed of oxides, phosphates and sulfdes, could serve as a supplement to the lubricant. In comparison with the element “S”, the element “P” always played a dominant role both in the vacuum and in the atmospheric pressure environment.

The anti-wear property of the composite flm formed under the vacuum condition might be better than that formed at atmospheric pressure, and therefore the additive T-4 could behave better in vacuum.

4 Lubrication Mechanism under Atmospheric Pressure and Vacuum Conditions

It can be seen from the above analysis that with regard to the sulfur-containing type additive (T-10) during the friction process under the atmospheric pressure condition, the chemical flm mainly contained FexOyand FeSx, while under the vacuum condition the chemical film mainly contained FeSxand the “M—C“ bonds. The anti-wear properties of the chemical flm under the vacuum condition could be superior to those measured under the atmospheric pressure condition. Therefore, the additive T-10 behaved better under the vacuum condition.

As for the phosphorus-containing type additive (T-9) in the friction process under the atmospheric pressure condition the chemical film mainly consisted of FexOyand the phosphate compounds. While in vacuum, the chemical flm mainly consisted of the phosphate compounds. Possibly due to the good anti-wear properties of the chemical flm formed under the vacuum condition, the anti-wear properties of the additive T-9 were worse in the vacuum environment.

As regards the sulfur and phosphorus-containing type additive (T-4), in the process of friction under the atmospheric pressure condition, FexOy, FeSxand the phosphate compound constituted the composite film. While in the vacuum environment, the phosphate compound and FeSxconstituted the primary composition of the composite flm. Possibly because of the good anti-wear performance of the composite flm under the vacuum condition, the additive T-4 had better anti-wear performance in the vacuum environment.

5 Conclusions

The friction and wear properties of different kinds of additives were evaluated, and their mechanisms of lubrication have been drawn up.

(1) Different anti-wear additives behaved differently both under the atmosphere pressure and the vacuum conditions, but their friction reducing properties turned out to be worse in the vacuum environment.

(2) As for the sulfur-containing additive T-10 both under the atmospheric pressure and the vacuum conditions, the adsorbed flm and the chemical reaction flm formed thereby could work jointly as lubricant. But in the vacuum environment, the element “S” participated further in the friction reactions, thus generating FeSxand the “M—C” compounds. These two species then constituted a protective flm for lubrication of tribopairs. Therefore, the anti-wear properties of the additive T-10 were better in the vacuum environment.

(3) As regards the phosphorus-containing type additive T-9 both under the atmospheric pressure and the vacuum conditions, phosphates were formed to lubricate the tribopairs during the friction process. But FexOyspecies were not generated in vacuum, which resulted in poor lubrication performance.

(4) As for the sulfur and phosphorus-containing type additive T-4, as compared to the atmospheric pressure condition, the elements “S” and “P” could participate further in the reactions in vacuum. During the friction process, the phosphate compounds were mainly generated and they comprised the primary components of the chemical flm, which played a protective effect of lubrication.

Acknowledgement: Financial support from the SINOPEC Research Program (No. ST13164-19]) is gratefully acknowledged.

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Received date: 2016-06-08; Accepted date: 2016-08-24.

Ms Peng Qian, E-mail: pengqian050924119@163.com.