Failure Mechanism of Bearings under Extremely Cryogenic Conditions

Bearing, as one of the most core components of rotating equipment, is prone to wear under cryogenic conditions, which seriously affects the efficiency of the whole system. Therefore, it is necessary to study the failure mechanism of bearing under cryogenic conditions. In this paper, friction and wear behavior of the bearings under cryogenic conditions is verified by using bearing simulation test platform. The results show that the bearing surface of raceway is prone to wear failure under cryogenic conditions, but the ball wear amount per unit time is small. This is because the cryogenic conditions seriously change the performance of the bearing material, the plasticity and strength of the material will be seriously reduced, and the brittleness will increase. Therefore, when the bearing ball and the bearing raceway surface contact friction, under the action of Hertz contact stress, the raceway surface is easy to produce pits and spalling, resulting in serious fatigue on the surface of the friction pair. The research provides a strongly theoretical and technical support for further improving the bearing performance and developing the tribological design of bearings.


Introduction
With the exploration of new fields and the progress of science and technology, the development of cryogenic tribology in new energy, aerospace and other fields has gradually attracted more and more scholars.No matter for new energy mining equipment (such as cryogenic pumps) or aerospace equipment (steering gear, rocker arm, etc.), bearing, as one of the most critical parts of rotating equipment, has gradually attracted the attention of more and more researchers in the field of cryogenic research [1][2][3][4][5][6].
According to studies, more than 40% of rotating machinery failures are caused by bearing failure [7][8][9].According to the current cryogenic conditions of the bearing, it is found that during the operation of the bearing, due to the extremely cryogenic conditions, the bearing will have serious wear and failure after running for a short time, which seriously affects the working efficiency of the entire equipment.Bearing failure to the normal operation of the whole system will pose a serious threat, frequent shutdown, maintenance and replacement will not only consume larger costs and time, but also may cause safety accidents.
At this stage, for cryogenic rotating equipment, the running status of its key components bearing directly affects the performance and safety of the entire equipment.Therefore, developed countries such as the United States and Japan will improve bearing performance and improve bearing life as a research focus [10][11][12][13].However, the key factors causing bearing failure and failure mechanism are still unclear.Moreover, it is difficult to carry out bearing tests under harsh and cryogenic working conditions, which brings huge challenges to the research work [14].Therefore, research on the key factors affecting bearing failure and failure mechanism will have important theoretical value and practical significance for further improving bearing performance, increasing bearing life, breaking foreign technology monopoly, and solving the problem of low bearing life "jam neck".
In summary, it is necessary to study the failure mechanism of bearings under cryogenic conditions.In this paper, the friction and wear performance of the bearing under cryogenic conditions is carried out by using the bearing simulation test platform to reveal the key factors and failure mechanism of the bearing.This study provides theoretical and technical guidance for further research on improving the cryogenic performance of the bearing.

Test Methods
To further study the failure mechanism leading to bearing failure under cryogenic conditions, bearing simulation test was carried out through the developed bearing simulation test bench, as shown in figure 1.The test platform is composed of magnetic drive wheel, cryogenic medium circulation system, AC motor, motor drive, speed control table, balance wheel, bearing and so on [15].The driving force of the entire device was provided by an AC motor.To prevent the leakage of the connection position between the transmission shaft and the motor due to insufficient sealing, a non-contact transmission structure is designed.The two strong magnetic drive wheels are connected to the motor shaft and the bearing shaft respectively to realize the non-contact transmission of the bearing.During the test, the entire test bearing needs to be completely immersed in a low temperature medium (liquid nitrogen).The load is applied by adjusting the balance wheel.The data acquisition system of bearing simulation test bench is composed of friction sensor, temperature sensor, data acquisition card, test control box, etc.The bearing adopts 6205 deep groove ball bearing.The bearing raceway material is G95Cr18 steel, the ball material is Si3N4 ceramic, the cage material is polyimide, and the bearing accuracy grade is P5.
To ensure that the bearing in the test box is heated evenly, it is necessary to ensure the circulation of cryogenic medium during the test.The specific test parameters are shown in table 1 Afterward, 3D surface tester (VK-X250K, KEYENCE, Japan) and scanning electron microscopy (SEM, Quanta 200F) were used to observe and analyze the worn surface at each wear stage.At the same time, FA1604 N electronic analytical balance was used to measure the wear of the bearing per unit time (10 h).Before and after the measurement, the raceway and ball which were restored to normal temperature were ultrasonic cleaned in ethanol solution for 30 min, and then the surface moisture was dried in a drying oven.

Tribological Experimental Results
Through the bearing friction and wear test under cryogenic conditions, the friction coefficient curve with time and the corresponding temperature rise curve of the bearing friction pair surface are obtained, as shown in figure 2. The results show that when the bearing just starts, that is, the initial stage.Because the speed is low and the friction resistance is large, the friction coefficient at this stage is large.Then, the friction coefficient decreases instantaneously and gradually becomes stable.When the wear of the bearing enters the T1 stage, the fluctuation of the friction coefficient curve is stable, and the fluctuation amplitude is small, about 0.01, indicating that the bearing is in a stable running state at this stage.The change of bearing surface temperature rise at T1 stage was analyzed, and it was found that with the running of time, the surface temperature rose slowly to about -148 ℃ at a small rate, and the temperature increased by about 8.6%, indicating that the surface temperature rise at this stage was small.
When the bearing runs for about 50 h, the bearing wear enters the T2 stage.At this stage, the fluctuation amplitude of the friction coefficient curve increased by about 0.02, and the curve gradually increased with time.The bearing surface temperature rise corresponding to the T2 stage was analyzed, and it was found that the surface temperature rose suddenly increased when the stage was just entered, increasing from -148 ℃ to about -140 ℃ in only 10 hours.By observing the macroscopic images of the bearing raceway and ball surface at this stage, it was found that there were many furrows and pitting pits on the raceway surface.It indicates that the surface wear of the bearing is intensified at this stage, and the abrasive wear occurs on the surface of the friction pair.
When the bearing runs to about 76 h, the bearing wear enters the T3 stage.At this stage, the fluctuation amplitude of the friction coefficient curve increases instantaneously, with an increase of about 0.035.And the corresponding bearing surface temperature rise change at this stage also increased suddenly, indicating that the wear degree of the bearing at this stage is more serious, and the bearing has been in a state of instability.It can be shown that in the cryogenic medium environment, the bearing will suffer serious wear and failure after running for a short time.

Wear Mechanism
To further explore the wear mechanism of cryogenic bearings, the bearing samples after the test were sliced and sampled, and the wear surfaces were observed and analyzed by SEM. Figure 3 shows the surface wear of the test bearing raceway at various stages.As shown in figure 3 (a), in the bearing wear T1 stage, there are few cracks on the raceway surface, and the cracks gradually expand from the surface of the contact surface to the subsurface.In addition, there are a small amount of wear debris and shallow scratches, indicating that the bearing ball and the raceway surface have slight wear particles at this stage.As shown in figure 3 (b), with the bearing wear entering the T2 stage, the surface wear intensifies.There are more abrasive debris, pits and larger scratches on the surface of the wear marks, indicating that more serious abrasive wear occurs in the bearing at this time.With the further intensification of wear, the bearing enters the T3 stage, as shown in figure 3 (c).In this stage, a large area of peeling off, cracks and wear debris on the wear surface, indicating that more serious fatigue wear, abrasive wear and plastic deformation occur on the bearing surface at this stage.Figure 4 shows the wear morphology of the ball surface at each wear stage.It can be found from the figure that in the T1 wear stage, the wear of the ball surface is relatively slight, and only a small number of micro-pits exist.When entering the T2 wear stage, the number of micro-pits on the ground surface increased and a few furrows appeared.When the wear entered the T3 stage, the size and number of and scratches increased, and some areas appeared spalling, indicating that the bearing was running in the cryogenic medium.Due to the impact between the ball surface of the bearing and the raceway surface, the ball surface is broken and more grinding chips are produced, which further leads to the abrasive wear on the surface of the friction pair.Figure 5 shows the change curve of wear amount of bearing raceway and ball under unit time (10 h).It can be found from the figure that between 10 and 80 h, the wear amount of the bearing raceway maintains a relatively stable process, and the wear amount per unit time is about 15 mg.After 80 hours, there was a sudden increase in wear.Moreover, the amplitude of unit time increase is about 20 mg and 30 mg respectively, indicating that the bearing has wear failure at this time.However, compared with the bearing raceway, the wear amount of the ball in unit time is small, and the wear amount is maintained in a relatively stable state throughout the wear stage.Even when the raceway wear is more serious, the corresponding ball wear is only about 8 mg, indicating that the bearing in the entire wear stage, the ball wear is relatively slight.

Conclusion
In this paper, bearing simulation test platform was used to verify the bearing friction and wear behavior under cryogenic conditions.According to the above experimental observation and detailed analysis, following important conclusions can be drawn: (1) Bearing simulation test shows that under cryogenic conditions, the bearing outer ring fails due to wear after running for a short time, but the wear amount of ball per unit time is small.
(2) The cryogenic environment seriously changes the performance of the bearing material, the plasticity and strength of the material will be seriously reduced, and the brittleness will be increased.Therefore, when the contact friction occurs between the bearing ball and the bearing raceway surface, the raceway surface is easy to produce pits and spalling under the action of Hertz contact stress, resulting in serious fatigue on the surface of the friction pair.
(3) In future research, the bearing raceway material with good cryogenic performance can be selected or prepared to improve the bearing's anti-friction and anti-wear performance under cryogenic conditions, so as to reduce the wear.

Figure 2 .
Figure 2. Friction coefficient and local temperature curve with time.

Figure 3 .
Figure 3. SEM images of surface wear of bearings at different stages.

Figure 4 .
Figure 4. Surface wear morphology of rolling body at different stages.

Figure 5 .
Figure 5. Wear amount of bearing raceway and rolling body in unit time (10 h).

Table 1 .
. Specific test parameters of friction and wear test.