Effect of Diesel Soot on the Distribution, Composition and Mechanical Properties of ZDDP Tribofilm

Feng Weimin; Song Hui; Song Ruhong; Yang Bingxun; Hu Xianguo

(School of Mechanical Engineering, Hefei University of Technology, Hefei 230009)

Abstract: To investigate the effect of diesel soot on the distribution, composition and mechanical properties of ZDDP tribofilm, a HFRR tribometer was applied to study the tribological performance.Worn surfaces lubricated with ZDDP and soot were analyzed by laser microscopy, SEM/EDS, Raman spectroscopy, XPS, and a nano‐indentation equipment.Results show that soot scrapes off the ZDDP tribofilm and can be embedded into the worn surface, leading to the reduction of film thickness and non‐uniform distribution of tribofim.The phosphate structure in ZDDP tribofilm changes from short chain pyrophosphate to long chain metaphosphate due to the increased contact stress caused by the soot abrasive wear, which can promote the cross‐linking of ZDDP.The hardness (H) and elastic modulus (E) of the worn surfaces increase, while the ratio of hardness to elastic modulus, H/E, decreases, which indicates that the reduction of wear resistance is caused by the soot.

Key words: soot; ZDDP; tribofilm; chemical composition; mechanical properties

1 Introduction

The incomplete combustion of fuel in the engine will produce soot that contaminates the lubricating oil, which is predominantly severe in diesel engines where exhaust gas re‐circulation system is used.Soot will enter the key friction pairs with the flow of lubricating oil, promoting the wear of engine parts[1‐4].To solve this problem,the common way is to add functional additives to the lubricating oil to improve its lubricating ability[2‐3,5].In particular, ZDDP (zinc dialkyldithiophosphate) has always been favored for its excellent anti‐wear and friction reducing properties.

However, the enhancement of lubricating performance of engine oil by additives is affected by soot.In this regard, researchers have carried out many works[5‐9],and proposed the following main mechanisms.1) Soot will adsorb functional additives in lubricating oil and reduce the content of additives in the oil, so as to weaken the lubricating role of functional additives[10].2) Soot and functional additives form competitive adsorption on the surface of the friction pair, which hinders the additives from playing their protective role on the friction surface[8].3) Soot can agglomerate in the lubricating oil to form larger secondary particles, which would be accumulated around the friction pair and could block the flow of lubricating oil to reduce the effectiveness of additives[11].4) Soot can accelerate the decomposition of additives[12], and induce Fe3O4with anti-wear effect on the surface of friction pair to be transformed into FeO having an increased wear effect[13].5) The reason on why friction reducing and anti‐wear additives can play their role is that they can form a protective tribofilm on the surface, while soot will scrape off the tribofilm during the friction process, deteriorating the friction and wear performance[7,14].

In terms of the influence of soot on ZDDP, the most widely supported mechanism is that soot can abrade the tribofilm formed by ZDDP.The ZDDP tribofilm is composed of zinc phosphate, iron phosphate, ferrous sulfide and zinc sulfide.This film is softer than steel and thus should be much more susceptible to abrasion by soot.However, this mechanism still has some limitations due to the following two points.On the one hand, for the sake of easy access, some researchers use carbon black as a surrogate of engine soot in tribological tests and subsequent characterization analyses[12,15‐16].However,considering the differences in formation conditions,compositions, morphology and structures between soot and carbon black[17], the use of carbon black to simulate soot might lead to misunderstanding of the soot wear mechanism.On the other hand, the existing research conclusions are primarily based on the results obtained from surface chemical analysis equipment.For example,EDS was used to analyze the element composition of the surface[6,18], XPS/AES was used to analyze the valence state of the surface elements to determine the specific composition[4,15], Raman spectroscopy was used to investigate the graphitization degree of carbon on worn surfaces[12,19], etc.However, as it is known, the anti‐wear and friction reducing properties of ZDDP is ascribed to the tribofilm it forms to prevent contact of asperities.The mechanical properties of this tribofilm is critical to its role on tribological properties[17,20].As far as the authors know,there is no research on the effect of soot on the mechanical properties of tribofilm formed from ZDDP, which limits the understanding of the mechanism of soot wear.

Therefore, in view of the above shortcomings that limit the understanding of the soot wear mechanism,an improved work was carried out.Diesel soot was prepared according to reference[21]and was used in tribological tests at different concentration added to liquid paraffin containing ZDDP.The worn surfaces were analyzed by EDS, XPS and Raman spectroscopy that could study the chemical state of the tribofilm formed thereby.Particularly, the mechanical properties of the worn surfaces were analyzed by nano‐indentation equipment, which for the first time had demonstrated the effect of soot on the hardness and elastic modulus of ZDDP tribofilm.These results combined together could give a comprehensive understanding of the soot wear mechanism from both the chemical and the mechanical aspects.

2 Experimental

2.1 Preparation of lubricating oil

The base oil used for preparing lubricating oil was a liquid paraffin (LP, provided by the Sinopharm Chemical Reagent Co., Ltd.), the kinematic viscosity of which was 42.5 mm2/s at 40 ℃ and 6.4 mm2/s at 100 ℃.The ZDDP(provided by the Jinzhou Shengda Chemical Co., Ltd.)was a primary alkyl type compound, with its specific elemental composition shown in Table 1.The diesel soot was prepared according to reference[21].The ZDDP was added to the liquid paraffin at a dosage of 1.0%,and the soot at a mass fraction of 0, 1%, 2%, 3%, 4%,and 5%, respectively, was added into the mixture.Then each lubricating oil sample was magnetically stirred and ultrasonically treated at 60 ℃ successively for 8 h to throroughly disperse the soot.

Table 1 Elemental composition of ZDDP

2.2 Tribological tests

The tribological tests were conducted using a high frequency reciprocating rig (HFRR, MGW‐001, Jinan Yihua Tribology Testing Technology Co., Ltd.).The schematic diagram of the tribometer is shown in Figure 1.The ball is made of GCr15 steel with a diameter of 6 mm,a hardness of HV 700‒760, and a roughness ofRa0.016 μm.The disc is made of GCr15 steel with a diameter of 10 mm, a hardness of HV 190‒210, and a roughness ofRa0.014 μm.The tribological tests were conducted under a load of 2 N and a sliding velocity of 0.1 mm/s for 60 min at 100 ℃.A laser microscope (VK‐X100, KEYENCE)was used to measure the wear scar diameter of the ball for anti‐wear performance analysis.

Figure 1 Schematic diagram of HFRR

2.3 Surface characterisation

2.3.1 Distribution characterisation

The distribution of ZDDP tribofilm formed on the worn surfaces was analyzed by an optical microscope and an energy dispersive spectrometer (EDS, Inca X‐MAX 80, Oxford Instruments) attached to a scanning electronmicroscope (SEM, LSM‐6490LV, JEOL).The secondary electron image was used in SEM, operating at an acceleration voltage of 20 kV.The EDS analysis was conducted at the mapping mode to detect the elemental distribution of oxygen, phosphorus and sulfur from ZDDP tribofilm and carbon from soot.

2.3.2 Surface chemical characterisation

The confocal laser Raman spectroscopy (Raman,LabRAM HR Evolution, HORIBA Jobin Yvon) and X‐ray photoelectron spectroscopy (XPS, ESCALAB250Xi,Thermo) were used to study the chemical properties of the worn surfaces.Coupled with the Raman spectroscopy,the laser length was 532 nm and the laser intensity was set to be 5%.The excitation source used for XPS analysis was Al‐Kα and the penetration energy was 20 eV.The binding energy of C1s at 284.8 eV was used to correct the XPS spectrum.

2.3.3 Surface mechanical characterisation

A nano‐indentation equipment (Nano‐indenter G200,Agilent) was used to measure the mechanical properties of the worn surfaces and the measured parameters included the hardness and the elastic modulus[22].The indentation tip used was a diamond Berkovich tip with a tip radius of 20 nm.The maximum indentation depth was set to be 500 nm and at least 9 different locations were selected on each worn surface for repeatability test.The Poisson’s ratio used in calculating the elastic modulus and hardness was set to be 0.3, which was the real value of GCr 15 steel.The continuous stiffness measurement(CSM) method was used to perform the nano‐indentation test, which could continuously measure the change of hardness and elastic modulus with the indentation depth.

3 Results and Discussion

3.1 Tribological test results

Figure 2 shows the average friction coefficient and average wear scar diameter (ASWD) as a variation of soot concentration in the lubricating oil from three tribological tests.It can be seen that with more soot being added,the average friction coefficient and AWSD increased.Interestingly, the friction coefficient decreases with the addition of 1.0% of soot.This indicates that lower content of soot can help to reduce friction, while higher content of soot increases the friction and wear.The reason may be that lower content of soot is easy to be dispersed in the lubricating oil, leading to the formation of small‐sized secondary soot particles, which can play the role of rolling‐bearing in the interface.However, higher content of soot is prone to agglomerate around the inlet of the friction pair, thus blocking the flow of lubricating oil and causing oil starvation during lubrication[6].

Figure 2 Variations of average friction coefficient and average wear scar diameter (AWSD) with soot content

3.2 Distribution of tribofilm

Figure 3 shows the optical photos of worn surfaces.It can be seen from the photos that only slight scratch is identified on the worn surfaces lubricated by the lubricating oil without soot.After adding the soot, there are obvious furrows on the worn surfaces.This indicates that soot causes abrasive wear[9,23].More importantly,the worn surfaces lubricated without soot are dark black,which suggests the existence of ZDDP tribofilm.With the soot being added, the color of the worn surfaces become lighter, and the color of the ball is even lighter than the disc.This implies that the soot scrapes off the ZDDP tribofilm.Because the ball is always in contact with the disc during the tribological test, the stripping rate of ZDDP tribofilm on the ball is faster than the disc.

To further study the influence of soot on the distribution of ZDDP tribofilm, the distribution of oxygen,phosphorus, and sulfur from ZDDP tribofilm and carbon from soot were analyzed by the SEM/EDS techniques.The selected analysis areas are shown as the black boxes in Figure 3, with the results presented in Figure 4.The signals of oxygen, phosphorus and sulfur are intense andthese elements are evenly distributed on the worn surface lubricated without soot.This fact indicates that the ZDDP tribofilm is relatively thick and is uniformly distributed on the worn surface.With the addition of soot, the signals of oxygen, phosphorus and sulfur on the worn surfaces are significantly weakened and the distribution of these elements are uneven.This suggests that the thickness of the ZDDP tribofilm is reduced and its distribution is not uniform, which is probably attributed to the scraping role of soot.Moreover, it is also noticed that the carbon is also enriched on local positions of worn surfaces lubricated with soot.This implies that soot may be embedded into the worn surface.

Figure 3 Optical photos of worn surfaces

Figure 4 SEM images and EDS mapping results of worn surfaces

3.3 Composition of tribofilm

Raman spectroscopic analyses were conducted on the worn surfaces and the soot, with the results shown in Figure 5.The soot shows typical D‐band and G‐band of carbon at around 1360 cm‐1and 1590 cm‐1, respectively.ItsID/IGvalue is 2.88, which is similar to that reported in the reference[21].For the worn surfaces lubricated without soot, the D‐band and G‐band are relatively weak and theID/IGvalue is 6.65.With soot being added, the D‐band and G-band of carbon are intense and theID/IGvalue is around 2.6, which is similar to that of soot.This indicates that soot is indeed embedded into the worn surface during the tribological process.Therefore, upon combinig Figure 3 and Figure 4, the reduction of the thickness and the nonuniform distribution of ZDDP tribofilm caused by soot are not only related to its abrasive wear, but are also partly related with the fact showing that the embedded soot hinders the tribo‐reaction between ZDDP and iron substrate from forming the tribofilm.

Figure 5 Raman spectra of soot and worn surfaces

Figure 6 shows the XPS spectra of the worn surfaces and the corresponding percentage of the elements.Under the lubrication without soot, after combining the valence state of phosphorus, oxygen, zinc and sulfur, the worn surface is primarily composed of zinc phosphate and sulfides.These elements are just expected to be the composition of the ZDDP tribofilm[24].With soot being added, the zinc phosphate on the worn surfaces decreases, and its existing form has changed from short chain zinc pyrophosphate to long chain zinc metaphosphate.It is also noticed that the percentage of sulfur increases with the soot content.Upon consideringthat sulfur is usually used as the extreme‐pressure additive[5], the increase of sulfur indicates that soot causes the increase of local stress on the worn surface.This is probably attributed to the fact showing that the abrasive wear behavior of soot leads to the decrease of the actual contact area between the counter faces and thus increases the normal stress and shear stress.Furthermore, this may also be the reason for the structural change of zinc phosphate in ZDDP tribofilm as the increased stress can promote the cross-linking of ZDDP to form long chain phosphate[25].In addition, the carbide on the worn surfaces has increased according to the valence state of carbon.This is in accordance with Figure 5, which indicates the embedding of soot again.

3.4 Mechanical properties of tribofilm

Figure 7 shows the nano‐indentation test results on the worn surfaces.It can be seen from Figure 7 (a) that the repeatability of load‐indentation depth curves on the worn surface lubricated without soot is relatively high.With soot being added, the test repeatability is significantly reduced.However, the test repeatability of worn surface lubricated with 5% of soot is better than that obtained with 1% of soot.The improvement in the stability causedby addition of more soot may not be owing to the fact that the soot itself can alleviate the worsening mechanical properties of the worn surfaces, but is ascribed to the fact that the scraping of ZDDP tribofilm by soot leads to the exposure of the substrate and makes the indentation test affected by the friction pair material.

Figure 6 XPS spectra of worn surfaces lubricated with different content of soot

With the increase of indentation depth, the hardness of the worn surfaces shown in Figure 7 (b) decreases, while the elastic modulus shown in Figure 7 (c) increases.This is attributed to the fact showing that the influence of substrate on the mechanical properties of ZDDP tribofilm will be enhanced with the indentation depth.

Within 100 nm of indentation depth, the hardness and elastic modulus measured at each location on the worn surface lubricated without soot is around 9 GPa and 150 GPa, respectively, which is typical of ZDDP tribofilm[19].With soot being added, the hardness and elastic modulus of the worn surfaces increase.The highest hardness is found on the worn surface lubricated with 5% of soot,which is 15 GPa.The highest elastic modulus is identified on the worn surface lubricated with 1% of soot, which exceeds 300 GPa.This is quite surprising since the ZDDP tribofilm has been scraped by the soot, as shown in Figure 3 and 4, and the hardness and elastic modulus of the wornsurface should be the value of the substrate, but actually they do not meet the expectation.A possible explanation for this outcome is that the abrasive wear caused by soot leads to the reduction of contact area and thus the increase of contact stress, which enhances the friction hardening effect[26].

Figure 7 Nano-indentation results of worn surfaces, (a) Load-depth, (b) Hardness-depth, (c) Elastic modulus-depth,and (d) H/E-depth

However, it is also worth noting that the hardness of some test points on the worn surfaces lubricated with soot are significantly reduced.For example, on the worn surface lubricated with 5% of soot, the hardness is even as low as 0.4 GPa.Upon combining the results presented in Figure 4 and Figure 5, it is likely that these points with very low hardness are located in the position where the soot is embedded.This means that the data measured at these points are the mechanical properties of soot.

The ratio of hardness and elastic modulus,H/E, is usually introduced as a main parameter to estimate the relative wear resistance of materials.It can be seen from Figure 7 (d) that theH/Evalue of the worn surface lubricated without soot is around 0.07 within the 100 nm indentation depth.With soot being added, theH/Evalue decreases, which is 0.05 and 0.04 for the worn surfaces lubricated with 1% of soot and 5% of soot, respectively.The reduction ofH/Evalue is primarily attributed to the fact that the soot after having scrapped off the ZDDP tribofilmm can lead to the reduction of film thickness and inhomogeneity of film distribution, which can enhance the effect of substrate on the indentation test.In general,these results imply that soot reduces the wear resistance properties of the worn surfaces, which is in accordance with the tribological test results.

4 Conclusions

(1) Diesel soot may cause the thickness reduction and the nonuniform distribution of ZDDP tribofilm, which might be attributed to the abrasive role of soot, and the hindrance function of the tribo‐reaction between ZDDP and the substrate.

(2) The phosphate structure in ZDDP tribofilm can change from short chain pyrophosphate to long chain metaphosphate due to the increase of contact stress in the presence of soot.

(3) Soot causes the instability of mechanical properties and the decrease in the ratio of hardness (H) to elastic modulus (E) of ZDDP tribofilm, resulting in the reduction of wear resistance.

Acknowledgment:This research was supported by the National Natural Science Foundation of China (Grant No.52075141).