Tribological performance of nano-diamond composites-dispersed lubricants on commercial cylinder liner mating with CrN piston ring

2.1 Nanoparticles and lubricating oils

The base lubricant used in this study was commercially available synthetic oil (HVI-5 base oil). The ND powder was purchased from Nanjing XFNANO Materials Tech. The average size of the ND particle ranges from 5 to 10 nm. This study used polyaniline as the surface modifier, which contributes to the stabilization of ND dispersed in the base oil. The polyaniline (PANi)-modified ND particles were prepared with an in situ polymerization method, according to our previous work [39]. Figure 1(a) shows the SEM image of the polyaniline-modified ND particle. ND maintains uniform spherical particle morphology after the surface modification. From the XRD result (Figure 1(b)), the characteristic peaks of PANi at 2θ = 9.5, 15.9, 20.1, and 25.5° could all be observed, which reveals the successful synthesis of polyaniline-modified layer. On the other hand, the peak at 2θ = 43.6° is corresponding to the (111) plane of ND, which shows the good preservation of ND. The surface-modified nanoparticles were then predispersed by a combination of mechanical stirring and ultrasound irradiation in a reference oil. Then, this preconcentrate ND composite was diluted in the partially formulated base oil to obtain the nanolubricant. The concentration of nanoparticles was investigated included 0.5 wt%, 1 wt%, 2 wt%, and 3 wt%.

Figure 1

(a) SEM image and (b) XRD pattern of the polyaniline-modified ND particle.

2.2 Friction pairs

The developed lubricant with the ND additive was implanted in the typical friction pair of cylinder liner and piston ring, and the tribological performance of the pair directly indicates the effectiveness of the ND-based lubricant oil. The CrN-coated piston ring provided by CYPR ASIMCO Shuang Huan Piston Ring CO., Ltd was selected as the test specimen. The thickness of the CrN coating is about 30 µm, and the hardness is 1,066 HV_0.1. Cr liner treated by a platform honing process was selected as the counterpart materials, whose inner and outer diameters of the liner are 110 mm and 126 mm. The surface roughness of Cr liner is 0.8 µm (R _a), and the hardness is 701 HV_0.1. BP liner was applied as the other counterpart specimen, which consists of lamellar pearlite, graphite, and a small amount of phosphorus eutectic. The surface roughness of BP cylinder liner is 0.4 µm (R _a), and the hardness is 170 HV_0.1. The two liners were cut into 40 equal portions along the circumference (the central angle of the actual liner is about 9°, the arc length is 10 mm, and the length of cylinder liner specimen is 44 mm) [40].

2.3 Tribological test

The stepwise load tests and scuffing resistance tests were used by using the reciprocating sliding tribotester, which is purposely designed to simulate the working conditions of the actual top piston ring and cylinder liner [41]. Stepwise load tests adopt the way that the load increases with per 40 min, and the whole test was supplied with adequate lubricating oil at the speed of 0.1 ml/min. Before loading, the running-in stability under the previous load must be guaranteed. In other words, the friction coefficient must be kept at a stable value. The scuffing resistance tests were conducted with the reciprocating sliding tribotester as well, and it was divided into two stages: the running stage (included the light load period and higher load period, and the speed of lubricating oil is 0.1 ml/min) and the oil starvation stage. The lubricating oil supply was cut after a running stage, then the time interval between the oil cutoff and the onset of the scuffing (friction force increases sharply) was recorded to represent the scuffing resistance capacity [36]. At least five tests for each friction pair were conducted under the same conditions (Table 1).

The wear depth measured by the Olympus LEXT OLS4000 confocal laser scanning microscopy (CLSM) was used to characterize the wear loss of the liner and piston ring specimens. Before and after each stepwise load test, the surface morphologies and chemical composition of cylinder liner specimens and piston ring specimens were examined by ZEISS-SUPRA 55 SAPPHIRE scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS).

3.1 Effect of ND additives on the friction performance of CrN-coated ring mating with different cylinder liners

Figure 2 shows the friction force of Cr liner/CrN-coated piston ring changing with different ND concentrations in the stepwise load tests. It can be seen from the variation trend of friction force that the ND additive has a remarkable effect on the tribological performance of base oil between Cr liner and CrN coating piston ring. Besides raw base oil (black line), the friction force obtained from the tests lubricated with the ND composite added oil is relatively stable and lower (red, blue, green, and purple lines). Under the condition of the base oil without ND particles, the friction force begins to fluctuate, and scuffing occurs when the load increases to 30 MPa. In the meanwhile, when the load increases to 40 MPa, the friction force increases sharply, and the surface of the friction pair is severely scratched.

Figure 2

Variation of the friction force between Cr liner and CrN piston ring.

By comparing the friction coefficients between these five friction pairs lubricated by base oil with different concentrations of ND particles, the friction coefficients of the friction pairs with the ND lubrication oil were all smaller than that of the base oil regardless of the concentration of ND (see Figure 3). Meanwhile, the friction coefficients decrease first and then stabilize with the load increase under the ND lubrication oil. Although the friction coefficient of the tribopair under base oil decreases with the increase of load (from 10 to 20 MPa), it is still higher than the friction pairs lubricated by ND lubricating oil. With the respect of friction reduction, the best amount of ND added in the base oil is expected to be around 1 wt%.

Figure 3

Variation of the friction coefficient between the Cr liner and the CrN-coated piston ring.

The friction force of the BP liner and the CrN-coated piston ring changing with different ND contents in the stepwise load tests is illustrated in Figure 4. Contrary to the change of the friction force in the Cr liner/CrN-coated piston ring, ND lubrication seems to have no effect on improving the tribological properties. Furthermore, the friction force between tribopairs with the ND dispersed oil is all even higher than that obtained with the base oil and exhibits extreme instability. In addition, the ND-lubricated tribopairs needs a longer running-in time to reach a relatively stable state after each load increase, and the scuffing trends to occur at the high load, while the base oil is comparatively stable and the friction force is basically lower (see the black line).

Figure 4

Variation of friction force between the BP liner and the CrN-coated piston ring.

Figure 5 exhibits the friction coefficients of the BP liner/CrN-coated piston ring in stepwise load tests. As can be observed, the friction coefficient under the base oil is the lowest among all the tribopairs in different loads, and it first increases and then decreases with the increase of load (from 10 to 40 MPa). Meanwhile, the friction coefficients between other four tribopairs under ND lubrication exhibits unstable and are all higher than the friction coefficient under base oil. In addition, other four tribopairs under the condition of ND lubrication exhibit unstable friction force under higher load (20 or 30 MPa), implying the developed nanoadditive suppresses the lubrication of the base oil for the BP liner matching with the CrN-coated ring. It should be noted that the wear depths of the tribopair are not provided, because the wear behavior changes due to the onset of scuffing, and the material loss caused by the abnormal wear shows no indicative value.

Figure 5

Variation of the friction coefficient between the BP liner and the CrN-coated piston ring.

3.2 Effect of ND additives on the wear performance of CrN-coated ring mating with different cylinder liners

3.2.1 Effect of ND additives on the wear performance of Cr liner and CrN-coated ring pair

Figure 6 shows the wear depth of the cylinder liner and piston ring specimens with different concentrations of ND additive. In addition to the occurrence of scuffing between the tribopairs under the base oil condition, the wear depth between the other four tribopairs under ND lubrication can be effectively reduced. The wear depth of both the Cr liner and CrN-coated piston ring first drops and then rises with the increase of the ND content (from 0.5 wt% to 3 wt%). At the optimal ND concentration of 1 wt%, the lowest wear depth is obtained. The base oil-added ND additive exhibits the relatively lower wear depth, indicating the better wear resistance. It can thus be concluded that the ND modified with PANi as an additive in the base oil between Cr liner/CrN-coated piston ring friction pair has good antiwear and friction reduction properties, which are much better than that of the base oil without any additives under the same conditions, and the optimal additive concentration is around 1 wt%.

Figure 6

The wear loss of the friction pair: (a) wear depth of the cylinder specimen and (b) weight loss of the liner specimen.

The worn micrographs of the friction pair are shown in Figure 7. Severe plastic deformation and furrow could be found on the Cr liner surface lubricated with the base oil (Figure 7(a); ND% = 0%), whereas the plastic deformation and furrow on the worn surface lubricated with the base oil containing ND particles are significantly inhibited (Figure 7(a); ND% = 0.5–3%). When the concentration of ND is 1 wt%, the cross-hatch honing pattern is preserved very well. With the further increase in the ND composite content, the honing grooves gradually almost worn away (see Figure 7(a); ND% = 3%). Regarding the worn surface of the CrN-coated piston ring (Figure 7(b)), the degree of wear shows the similarity with that observed in the liner. With the ND composite lubricants, the wear grooves and scars are noticeably fewer and narrower than those found in the ring surface lubricated by the base oil. Moreover, the most slightly wear surface is observed in the 1 wt% lubrication. Consequently, adding the ND composite in the base oil can remarkably improve the wear resistance of Cr liner mating with the CrN-coated piston ring.

Figure 7

Worn micrographs of the Cr liner (a) and its counterpart CrN piston ring (b) lubricated with different ND content lubricants.

A reasonable explanation for the antiwear mechanism of ND particles is possible that the repairing effects (ND composites filled in the micro pits on the friction pair surfaces results in a smoother surface) [42,43,44]. The typical SEM morphology and EDX results of the wear surface of Cr liner when the concentration of ND composite additive is 1 wt% are illustrated in Figure 8. Only the Cr and O elements can be found on the plateaus of the Cr liner surface (Figure 8(a)). Cr is the dominating matrix element of Cr-plated coating, and O might originate from oxidation during sliding. However, more black sediments are deposited in the surface pit after the test. The elemental analysis of black sediments reveals that the main element of black sediments is C (Figure 8(b)). It is well known that C element can be resulted from the carbon deposition due to the combustion of furl or lubricating oil, but it has been reported that the occurrence of carbon deposition needs a temperature higher than 300℃ [45]. As the test was performed at the temperature of 150℃, it is considered that the observed black sediments mainly come from the ND composite. The ND composites are deposited in the pits during sliding, resulting in a smoother surface and contributing to sustain the oil film. Thus, the friction and wear of the friction pair can be reduced [46].

Figure 8

SEM micrographs and EDX spectra of Cr liner specimens after test (a) flat zone and (b) defect zone.

The surface roughness of Cr liner specimens before and after wear test is shown in Figure 9. It can be seen that the roughness in both the middle stroke and dead center decreases compared with the base oil lubrication surface, indicating the surface polishing effect of the ND composite. Furthermore, the roughness near the dead center is lower than that near the middle stroke, and this is because the friction force of the dead center is larger than that at the middle stroke in each reciprocation cycle [36]. Thus, the polish process is more effective. On the other hand, the lowest surface roughness is observed under the optimum concentration of ND composite (ND% = 1 wt%), and the smoother surface is more conducive to the formation of lubricating oil film. The ball-bearing effect of the ND particles may be activated as well [27]; therefore, the low friction and wear loss, as well as the slightly worn surface of the friction pair, was obtained. Noted that the amount of nanoparticles is not the more the better. The agglomeration will occur if the ND particle is over-added in the lubrication oil, thus, the abrasive dust can be formed between the friction pairs. As a result, the surface roughness increases again due to the abrasive wear (Figure 7; ND% = 2%, 3%).

Figure 9

Surface roughness of the Cr liner before and after the test.

3.2.2 Wear micrographs and elemental analysis of BP liner/CrN ring after stepwise load tests

Figure 10 presents the wear micrographs of the BP liner and the CrN-coated piston ring after stepwise load tests. Under the condition of the base oil, abrasion along sliding direction exists on the surface of the BP liner and piston ring after tests although the honing marks on the surface of liner are somewhat visible as shown in Figure 10(a) (ND% = 0–3%). Compared with the base oil, the scratches on the BP liner sliding against the CrN-coated piston ring are more apparent under the ND composite-added lubrication. Figure 10(a) (ND% = 2%, 3%) presents the wear damages on the BP liner surface. The results correspond to the high and unstable friction force. For the CrN surface, the severe wear can be observed as well.

Figure 10

Worn micrographs of the BP liner (a) and its counterpart CrN piston ring (b) lubricated with different ND content lubricants.

Figure 11 presents the wear surface of the BP liner under the ND composite-added lubrication. Similar to the phenomenon found in the pair of the Cr liner and CrN coating, many black sediments were deposited on the surface pits after the test. Through the EDS spectrum, black sediments are also dominated by the C element. As the hardness of the BP liner is very low, it is considered that the hard NDs are pressed into the soft surface of the BP liner during relative sliding of tribopairs, and the surface damage caused by ND particles is aggravated with the continuation of relative sliding [47]. Consequently, the ND composites deteriorate the friction and wear properties of the BP liner and CrN-coated friction pair. According to the result, it can be inferred that ND additives will aggravate the wear rather than protecting the friction pair surface if the friction coupling surfaces are too soft.

Figure 11

SEM micrographs and EDX spectra of cylinder liner specimens after test.

3.3 Effect of ND additives on the scuffing resistance performance of base oil

Since the developed ND composite has no beneficial effect on the tribology performance for the BP liner, the influence of ND particles on the antiscuffing properties of the base oil was only explored for the friction pair of the Cr liner and the CrN-coated piston ring. Figure 12 shows the typical friction forces of the whole scuffing resistance tests with different lubricants subjected to a load of 20 MPa. As the lubricating oil is cut off after the running-in stage, the oil film can be hardly uniformly dispersed on the contacting surface [48]. Under such oil starved condition, the additives and the oil retention capacity of the friction pair determine the anti-scuffing performance. All the friction force variations show the similarity in trend, but the scuffing time of the ND composites lubricated friction pairs is considerably longer than that of the base oil condition, and 1 wt% ND composite content still exhibits the best scuffing resistance.

Figure 12

The typical friction force variation of the Cr liner and the CrN-coated piston ring under 20 MPa in scuffing resistance tests.