*3.1.1 Experiment*

The friction and wear life of journal bearings with bonded MoS2 film lubricant applied to some substrates were investigated in vacuum using the test equipment shown in **Figure 4** [8]. **Figure 5** illustrates the friction measurement part of the equipment set in the vacuum chamber. The shaft was oscillated by AC servo motor through reduction gear and feedthrough. The radial load was applied by an air cylinder outside the vacuum chamber through a bellows. The bore and width of the bearing were 10 mm and 7 mm, respectively. The lubricant was applied both to the bearing and the shaft. The frictional torque was measured by the load cell above the test bearing.

The commercially available bonded MoS2 film lubricant, including about 25 wt. % MoS2 and phenolic resin binder, was spray coated and heat cured at 150°C for 1 h. The film thickness was about 10 micro-meters and the diameter clearance between the bearing and the shaft with lubricant film was 10-20 micrometers.

**Table 1** [1] shows the substrate materials and pretreatment for the bearings and the shafts. **Table 2** [1] indicates the bearing and the shaft combinations. The circled numbers in the table correspond to those in **Table 1**. **Table 3** [1] shows the test conditions.

#### *3.1.2 Experimental results*

**Figure 6** [1] illustrated the typical measured friction evolution. The measured friction drew the rectangular wave due to the oscillation motion and half of the total

**Figure 4.** *In-vacuum journal bearing test equipment [7].*

#### *Wear Life of Bonded MoS2 Film Lubricant DOI: http://dx.doi.org/10.5772/intechopen.99802*


#### **Table 1.**

*Substrate materials and pretreatment for bearings and shafts [1].*


#### **Table 2.**

*Bearing and shaft combinations [1].*


#### **Table 3.**

*Test conditions [1].*

#### **Figure 6.**

*Typical measured friction evolution [1].*

amplitude was used as the frictional force. The friction gradually decreased with the number of oscillations and suddenly increased. This sudden increase point was used as the wear life of the lubricant.

**Figure 7** [1] shows the appearance of the tested shaft. A part of the film lubricant was removed and the metal substrate was exposed and scratched. **Figure 8** [1] shows

**Figure 7.** *Appearance of the tested shaft [1].*

**Figure 8.**

*Lubricant film profiles: (a) about 60% of wear life, (b) after the friction increase [1].*

the lubricant film profiles of Set No.3 specimen in **Table 2** at the points (a) and (b) in the wear life shown in the right diagram. Most of the film thickness remained near the end of the wear life, and the scratched part was observed at the end of the test.

**Figure 9** [1] indicated the wear life and friction coefficient of the tested combinations. There was no significant difference in the friction coefficients, but the wear life of the SUS304 was much longer than the others.

#### *3.1.3 Discussion*

It was observed that the film thickness of the lubricant gradually decreased, but it seems that the wear life suddenly came with most of the film thickness remaining. This form of wear has been observed in several previous studies [9, 10]. This means that specific wear rates cannot be used to predict wear life. In some studies, specific wear rates has been proposed for the wear life estimation of the bonded MoS2 film lubricant [9, 11]. It may be used for relatively large sliding speed and low load conditions, but cannot be used for small sliding speed and high load conditions like this case.

#### **Figure 9.**

*Wear life and friction coefficient of the tested combinations [1].*

The sudden decrease of the film is probably due to the de-bonding of the film from the substrate, possibly due to fatigue. The adhesion strength between the substrate and the film must affect the fatigue strength, that is, the wear life, and is strongly dependent on the anchoring effect by the surface morphology of the pretreated substrate.

The surface morphology of the pretreated substrate was investigated using "pretreated surface specimen" shown in **Figure 10** [1]. A portion of one side of a

**Figure 10.** *Pretreated surface specimen [1].*

#### *Tribology of Machine Elements - Fundamentals and Applications*

rectangular metal specimen was pretreated and the bonded MoS2 film lubricant was applied to half of the pretreated area. **Figure 11** [1] shows the surface profiles of the specimens. As shown in (a) and (c), the pretreated areas of the aluminum alloy substrate and the SUS304 stainless steel substrate had a roughness that went up and down across the original surface, while that of the SUS440C stainless steel substrate

**Figure 11.** *Surface profiles of the pretreated surface specimens, (a) aluminum alloy, (b) SUS440C, (c) SUS 304 [1].*

#### *Wear Life of Bonded MoS2 Film Lubricant DOI: http://dx.doi.org/10.5772/intechopen.99802*

was below the original surface. This means that ductile metal surfaces such as aluminum alloy and SUS304 stainless steel (austenitic stainless steel) were deformed plastically by the blasting and that of brittle metal such as SUS440C (martensitic stainless steel) seems to have had its surface layer taken away by the blasting. This resulted in a characteristic surface morphology.

**Figure 12** [1] shows the cross- sections of the pretreated surface specimens. The SUS304 specimen indicated the intricate surface morphology, while the SUS440C specimen had a monotonous wavy surface morphology. Probably these morphologies brought the strong adhesion between the substrate and the film, that is, the long wear life, to the SUS304 substrate and short wear life to the 440C substrate. The work hardening would also have contributed to the long wear life of the SUS304 substrate. The short wear life of aluminum alloy substrate could be attributable to the deformation of surface morphology by the load due to the lack of the hardness.

#### **3.2 Effects of atmosphere**

#### *3.2.1 Experiment and results*

Friction and wear life characteristics were investigated under various loads, sliding speeds in air and vacuum atmospheres using the test equipment used in Section 3.1. Test materials and test conditions are shown in **Table 4** [8] and **Table 5** [8], respectively. SUS630 is a precipitation hardened stainless steel with high strength, and was chosen for the shaft specimen in consideration of actual applications.

**Figure 13** [8] shows a typical change in the friction coefficient with the number of oscillations in air and in vacuum. The friction coefficient was several times larger and the wear life was several ten times shorter in air than in vacuum. The relationship between the friction coefficient and the load is shown in **Figure 14** [8]. Friction coefficient used was in the steady state as shown in **Figure 13**. Friction coefficient was about 0.2 in air and about 0.05 in vacuum regardless of test conditions.

**Figure 15** [8] shows the relationship between the bearing pressure and the wear life. The wear life refers to the number of oscillations when friction increased sharply. There are two groups, one in vacuum and one in air, with differences in wear life of hundreds of thousands of oscillations. It seems that there is no relationship between the groups.

#### *3.2.2 Discussion*

**Figure 16** [8] shows the lubricant film profile at about 70% wear life in air, obtained under the same test conditions as No. 5 in **Table 5**. Most of the film thickness remained, as in the in-vacuum test shown in **Figure 8**. This suggests that the wear life in air, as well as in vacuum, is due to the de-bonding of the film from the substrate, and that the fatigue strength of the film-substrate interface may determine the wear life.

Fatigue strength of some metals are known to be affected by atmosphere and be larger in vacuum than in air (e.g. [12]). However, it has been shown that the propagation rate of fatigue cracks in epoxy resins is almost the same in both


#### *Tribology of Machine Elements - Fundamentals and Applications*


#### **Table 5.** *Test conditions [7].*

**Figure 13.** *Typical change in friction coefficient in air and in vacuum [7].*

**Figure 14.** *Relationship between the friction coefficient and load [7]. The numbers correspond to those in Table 5.*

**Figure 15.** *Relationship between bearing pressure and wear life [7]. The numbers correspond to those in Table 5.*

**Figure 16.**

*Lubricant film profile at about 70% wear life in air [7].*

vacuum and air [13], and in general, the fatigue of resins, which are the binders of the films, is considered to be less affected by the atmosphere. Therefore, the short wear life due to fatigue of film lubricants in air is not considered to be due to their reaction to the environmental atmosphere. The factor that differs between vacuum and air and is considered to affect fatigue is the friction coefficient.

Stress analysis was performed to investigate the effects of the friction coefficient on the stress in the film [8]. The analysis was performed as a plane strain perfect elasticity problem. Young's modulus of the film was measured in dry air to be about 10 GPa. Calculation conditions are shown in **Table 6** [8].

**Figure 17** [8] shows examples of the calculated stresses in the film. Since the film thickness is small compared to the contact length, the stress is almost constant in the depth direction of the film.

**Figure 18** [8] shows the relationship between the maximum shear stress at the interface between the film and the substrate in the direction parallel to the interface and wear life. All points are on a straight line, whether they are in vacuum

#### *Tribology of Machine Elements - Fundamentals and Applications*


#### **Table 6.**

*Calculation conditions [7].*

#### **Figure 17.** *Examples of calculated stresses in the film [7].*

or in air. This is a typical S-N curve for fatigue phenomenon. Therefore, the wear life of the bonded MoS2 film lubricant can be attributed to fatigue due to shear stress at the interface. Since the contact width is much larger than the film thickness, the shear stress at the interface is almost the same as the shear stress at the film surface, i.e., the product of the friction coefficient and the contact pressure,

**Figure 18.** *Relationship between the maximum shear stress at the interface between the film and the substrate and wear life [7].*

**Figure 19.**

*Repeated vertical loading machine [13].*

as shown in **Figure 17(b)** and **(d)**. Since the maximum von Mises stress, which contains a large component of vertical loading, did not show the same relationship as the shear stress, damage inside the film is not considered to be the cause of wear life in this case. Thus, the difference in wear life between vacuum and air is due to the friction coefficient between vacuum and air.

The effect of repeated vertical loading was investigated separately. **Figure 19** shows the "repeated vertical loading machine" [14], in which the bonded MoS2 film lubricant on the flat surface was subjected to the repeated vertical pressure by a steel ball with 5/16 in. (~7.9 mm) diameter. A sinusoidal load of 0.98 N to 4.4 N was applied at a frequency of 1000 cpm.

Only dents with a few micrometer depth were observed on the tested lubricant films after more than 107 times loading, as shown in **Figure 20** [14], and no debonding was observed. Hence, the repeated shear stress, i.e. the friction, rather than the repeated vertical load causes the de-bonding of the film lubricant.
