Current Status of Research on Hybrid Ceramic Ball Bearings

Abstract

Rolling element bearings are essential components in modern mechanical equipment, providing crucial support for rotating parts. Hybrid ceramic ball bearings, consisting of steel rings and ceramic balls, have gained popularity in high-speed machinery to enhance performance. These bearings offer advantages such as longer fatigue life, improved performance, and higher speeds. Extensive research by scholars has been conducted to promote the wider adoption of hybrid ceramic ball bearings. This paper compiles relevant studies on hybrid ceramic bearings, organizing literature related to their lifetime, arranging literature pertaining to their performance analysis from the perspective of analytical methods, and collating literature on their lubrication techniques from the angle of lubrication methods. This paper covers research on lifetime modeling, fatigue spalling, wear, mechanical and tribological properties, dynamic performance, thermal analysis, temperature considerations, and lubrication techniques of hybrid ceramic ball bearings. The aim is to provide readers and researchers with a comprehensive overview of these innovative bearings.

1. Introduction

The demand for high-performance bearings has steadily increased over time to meet the needs of high-speed rotating machinery, particularly in industries such as aerospace and manufacturing. Conventional steel bearings often fall short in meeting all operational needs due to material properties and design limitations. This has sparked research interest in hybrid ceramic ball bearings, which incorporate ceramic rolling elements, as depicted in Figure 1.

Since the 1960s, foreign researchers have delved into the study of hybrid ceramic ball bearings. Talor et al. [1] examined bearings crafted from hot-pressed silicon carbide and hot-pressed alumina in high-temperature settings. Harris [2] explored the centrifugal force of silicon nitride rolling elements in hybrid ceramic ball bearings at elevated speeds in 1968. Scott et al. [3] conducted an initial comparison of hot-pressed silicon nitride and other ceramic bearing materials under various conditions including heavy loads, lubricated, unlubricated, and high-temperature rolling contact in 1973. Parker et al. [4] focused on investigating the fatigue life of hot-pressed silicon nitride ceramic ball bearings in 1975. Aramaki et al. [5] carried out thermal performance studies on both all-steel bearings and hybrid ceramic ball bearings in 1988. Subsequently, Rhoads et al. [6] delved into the impact resistance of hybrid ceramic ball bearings in 1994. Research on hybrid ceramic ball bearings in China began in the late 1980s. Due to the presence of inclusions, micro-porosity, and grain boundary glass phases, the brittleness of early ceramic materials has always been one of the most direct factors affecting the failure modes and impact resistance of long-life bearings. In recent years, the hot isostatic pressing sintering technology for ceramic blanks has developed. This technology can reduce impurities, pores, and grain boundary glass phases in ceramics, making the ceramic material structure more compact. Therefore, silicon nitride ceramics produced by hot isostatic pressing sintering technology have become the preferred material for manufacturing hybrid ceramic ball bearings. Currently, these bearings have Si₃N₄ rolling elements and steel inner and outer rings. The ceramic balls, being lighter due to the low density of ceramic materials, can achieve extremely high rotational speeds [7]. These bearings offer advantages such as wear resistance, high temperature resistance, corrosion resistance, non-magnetic properties, small thermal expansion coefficient, high Young’s modulus, and some self-lubricating properties. As a result, hybrid ceramic ball bearings are well-suited for use in high-speed electric spindles of machine tools, turbochargers, and gas turbine engines [8,9,10]. Moreover, in aerospace and military applications where extreme conditions like high maneuverability, heavy loads, rapid acceleration, extreme temperatures, and poor lubrication are common, challenges persist in bearing technology. The field of ceramic bearing technology is continuously evolving to address these challenges. Consequently, research on hybrid ceramic ball bearings holds significant importance [11]. In order to make it easier for readers to understand hybrid ceramic ball bearings and enable researchers to quickly obtain useful information, this paper organizes the research on hybrid ceramic ball bearings conducted in the past three decades.

2. The Study on the Lifetime of Hybrid Ceramic Ball Bearings

As a novel type of bearing, the service life of hybrid ceramic ball bearings stands out as a crucial property. The design and application of these bearings necessitate a thorough analysis of their service life, making research in this area fundamental. By delving deep into the service life of ceramic ball bearings, we can drive ongoing innovation and advancement in related technologies, thereby paving the way for the broader application of ceramic ball bearings.

2.1. Life Prediction

Lifetime is an important evaluation indicator of rolling bearing quality, as it directly relates to the reliability and stability of mechanical equipment. Even if bearings from the same batch with identical design, materials, and manufacturing processes operate under the same conditions, their lifetime data still exhibit significant variability.

Goepfert et al. [12] found through experimental research that compared to conventional bearings, hybrid ceramic ball bearings can reach higher rotational speeds while maintaining relatively stable life under the same temperature conditions. They not only have a longer fatigue life than all-steel bearings but also exhibit less variability in life compared to all-steel bearings.

Although hybrid ceramic ball bearings have a longer fatigue life compared to all-steel bearings, new life models are crucial for these innovative bearings. References [13,14,15] utilized the Weibull distribution statistical method to assess the fatigue life of hybrid ceramic ball bearings. Moreover, References [14,15] established a mathematical model correlating the rolling contact fatigue life of silicon nitride ceramic balls with contact stress. This study confirmed that the rolling contact fatigue failure of silicon nitride ceramic balls is primarily due to maximum tensile stress rather than maximum shear stress. The study suggests that the tensile stress life model is more appropriate than the Lundberg–Palmgren (L-P) shear stress life model for predicting the contact fatigue life of ceramic balls. Deutsch et al. [16] combined deep belief networks and particle filters to predict the remaining service life of hybrid ceramic bearings. Wang et al. [17] utilized the finite element method combined with ABAQUS submodel technology to accurately and efficiently calculate the contact deformation between the rolling elements and raceways of hybrid ceramic ball bearings as well as the contact subsurface stress of race, as illustrated in Figure 2. The calculated subsurface stress is then combined with the Ioannides–Harris (I-H) theory to predict the fatigue life of hybrid ceramic ball bearings.

Figure 2. The contact subsurface stress field of race [17].

Figure 2. The contact subsurface stress field of race [17].

Gupta et al. [18] conducted a study on the lifetime of hybrid ceramic ball bearings and found that the higher elastic modulus and lower Poisson’s ratio of silicon nitride would result in higher contact stress between the steel ring and the silicon nitride ball compared to full steel contact. Therefore, hybrid contact indeed reduces the contact life of the steel ring. Traditional fatigue life models are unable to accurately predict the life of hybrid ceramic ball bearings, so Gupta optimized the conventional fatigue life model. Morales-Espejel et al. [19,20,21] studied the life expectancy of hybrid ceramic ball bearings by separately considering the survival rates of the surface and subsurface of the ceramic balls. They found that when fatigue performance is mainly dominated by surface fatigue, hybrid ceramic ball bearings have advantages compared to all-steel bearings. However, in cases where subsurface fatigue dominates, all-steel bearings exhibit superior performance. Based on this research, they established the Generalized Bearing Life Model (GBLM). Reference [22] also found that the specific fatigue resistance of the ceramic-steel interface of hybrid bearings can compensate for the additional stress present in the subsurface contact area.

The establishment of a life model and calculation method for hybrid ceramic ball bearings facilitates the estimation of their fatigue life. Currently, the GBLM model developed by SKF’s chief scientist Guillermo E Morales-Espejel is used as a general bearing life model by SKF to calculate bearing life, including the fatigue life of hybrid ceramic ball bearings, which is highly convincing in the industry.

2.2. Surface Cracks in Ceramic Balls

The service life of hybrid ceramic ball bearings is influenced by both their operating environment and the properties of the ceramic materials. Due to the uniqueness of ceramic materials, the difficulties in sintering and processing materials may lead to surface or internal crack defects in rolling elements [23], and it is difficult to detect surface cracks in mass production. The reliability requirements for hybrid ceramic ball bearings are extremely high, and cracks in the ceramic balls are absolutely unacceptable. Therefore, ceramic ball bearings require 100% non-destructive testing to ensure their quality. Among the ceramic non-destructive testing technologies, acoustic emission resonance technology and laser material evaluation technology show promising prospects for high-end applications. Tsai et al. [24] used the finite element method to simulate the damage form of ceramic balls with cracks. When ceramic balls with surface cracks are subject to vertical load and friction between the ball and the bearing race, tensile stress will be generated at the crack tip. The maximum tensile stress occurs at the crack tip. The stress distribution at the crack tip tends to open the crack, thus causing it to expand towards the free surface and leading to peeling. Therefore, surface cracks in ceramic balls pose a significant hazard to hybrid ceramic ball bearings.

The current method for detecting surface defects relies on fluorescent penetrant inspection under ultraviolet light. However, Sun et al. [25] researched a nondestructive evaluation (NDE) method based on ultrasonic Rayleigh waves generated and detected by phased array probes, which is used to detect defects on the surface and subsurface of ceramic balls. A vibration-based diagnostic system introduced in reference [26] is used to detect cracks in the balls of hybrid ceramic ball bearings. This diagnostic system is based on the Bayesian classifier and is capable of distinguishing healthy bearings from damaged balls and raceways.

Internal defects in ceramic balls can cause surface fatigue cracks. At this stage, there is a lack of large-scale nondestructive testing technology for ceramic ball cracks, and further research is needed in this area.

2.3. Fatigue and Wear of Hybrid Ceramic Ball Bearings

Common all-steel bearings exhibit various forms of damage, with the most prevalent being contact fatigue and wear. Hybrid ceramic ball bearings are no exception, but due to the superior wear resistance of ceramic materials, there are relatively few studies on the wear of hybrid ceramic ball bearings. Therefore, the typical failure mode of hybrid ceramic ball bearings is generally contact fatigue spalling.

2.3.1. Contact Fatigue Spalling of Hybrid Ceramic Ball Bearings

According to relevant studies, the ceramic balls of Si₃N₄ ceramic bearings generally do not undergo brittle failure under normal operating loads, but instead exhibit fatigue spalling similar to steel bearings [27]. The contact fatigue failure modes of hybrid ceramic ball bearings are shown in Figure 3 below, with the top two images representing the fatigue failure modes of the ceramic balls and the bottom two images representing the fatigue failure modes of the steel rings.

Hadfield et al. [29,30] investigated the impact of lubricant type on fatigue failure mode of hybrid ceramic ball bearings in rolling contact conditions. They conducted a comprehensive analysis of the surface and subsurface of the ceramic balls. In a similar study, Chiu et al. [31] examined the durability of hybrid ceramic ball bearings under high load conditions with proper lubrication. Additionally, they assessed the bearings’ performance after exposure to contaminated lubricants, resulting in raceway indentations. The study concluded that the durability of hybrid ceramic ball bearings surpassed that of all-steel bearings.

Gu et al. [32] analyzed the rolling contact fatigue behavior of steel rings and ceramic balls in liquid nitrogen. They found that in liquid nitrogen, fatigue spalling of the ceramic balls was the main cause of failure in hybrid ceramic ball bearings. They also discovered that most fatigue cracks originated from crack defects inside the ceramic balls.

Zhang et al. [33,34,35] conducted a comparative analysis on the surface damage of hybrid ceramic ball bearings and steel ball bearings, highlighting the distinctions in their surface fatigue mechanisms. The findings suggest that the surface fatigue behavior of hybrid ceramic balls deviates from the conventional rolling contact fatigue behavior. This is attributed to the ceramic ball’s significantly higher hardness compared to bearing steel, causing detached ceramic ball particles to be pressed into the raceway, resulting in the formation of numerous micro-pits. Moreover, the presence of cavities and micro-cracks within the raceway contributes to crack formation in the surface plastic deformation layer. The mechanisms responsible for crack formation in both the surface and plastic deformation layer exhibit a synergistic effect, ultimately leading to the development of a peeling band on the raceway.

The primary damage form of hybrid ceramic ball bearings is surface fatigue spalling. To improve the surface fatigue performance of hybrid ceramic ball bearings, it is necessary to address the issue of surface fatigue spalling in these bearings. Good lubrication can mitigate surface fatigue spalling.

2.3.2. Wear of Hybrid Ceramic Ball Bearings

Ghezzi et al. [36] analyzed the degradation scenarios and adaptation mechanisms of hybrid ceramic ball bearings, comparing them with steel bearings and studying the wear evolution of hybrid ceramic ball bearings. They found that hybrid ceramic ball bearings coated with lubricants exhibited a longer and more complex wear evolution. Stelmakh et al. [37] conducted a comparative evaluation of the wear resistance of different ceramic materials under various conditions. Based on the wear rate, they ranked the materials studied and improved the technology of hybrid ball bearing rolling elements. Pessolano et al. [38] tested the wear resistance of Si₃N₄ balls and found that sliding friction would increase wear. Wang et al. [39] operated a partially hybrid ceramic ball bearing with some rolling elements replaced by silicon nitride balls on a bearing tester under high-contamination lubrication conditions alongside a full steel bearing of the same model. The results indicated that the partially hybrid ceramic ball bearing exhibited less wear damage. Following the experiment, an inspection of the damaged surface of the bearing raceway revealed that the surface of the partially hybrid bearing was smoother than that of the full steel bearing. This suggests that ceramic balls have a smoothing effect on the protruding areas of bearing surfaces damaged by contaminants.

Hybrid ceramic ball bearings are recognized for their high wear resistance and corrosion resistance. However, inadequate lubrication or the ingress of external dust particles and contaminants can still lead to wear. Both fatigue spalling and wear have significant impacts on the normal operation of hybrid ceramic ball bearings. Therefore, it is essential to consider suitable surface treatments for hybrid ceramic balls or effective lubrication methods to mitigate surface fatigue or wear and prolong the life of hybrid ceramic ball bearings.

2.3.3. The Influence of Porosity in Ceramic Balls on Fatigue Failure and Wear in Hybrid Ceramic Bearings

Different manufacturers’ hybrid ceramic ball bearings made of Si₃N₄ ceramic balls have different properties, which are mainly due to the porosity of ceramic materials and the use of additives. Thoma et al. [40] tested the fatigue properties and wear of ceramic balls based on their porosity and found that a lower porosity will improve the tribological properties of hybrid ceramic ball bearings. When the porosity is less than 0.2 μm, the change in porosity will no longer affect wear and fatigue properties.

2.4. Analysis of Spalling Failures in Hybrid Ceramic Balls

Fatigue spalling is the primary cause of failure in hybrid ceramic ball bearings. When fatigue spalling occurs on the raceway, the acceleration vibration signals of the rolling elements exhibit different characteristics when entering and exiting the spalling area: upon entering the faulty area, a step response dominated by lower frequency components is generated; while exiting the spalling area, it causes a broader frequency band pulse response. The principle is illustrated in Figure 4. Effectively separating these two types of signal characteristics is of significant importance for measuring the length of the spalling area in hybrid ceramic ball bearings. Many scholars have conducted research on the analysis of vibration characteristics of spalling failures in hybrid ceramic ball bearings.

Yan et al. [41] studied the vibration characteristics of spalling failures in hybrid ceramic angular contact ball bearings. A finite element model of H7009C was established in ANSYS Workbench. By setting spalling failures on the outer ring raceway, inner ring raceway, and rolling elements, the force changes in various parts of the bearing under normal and spalling fault conditions were analyzed under specific working conditions. It was found that when a spalling failure occurs, the z-direction contact force of the rolling elements on the inner and outer rings tends to increase significantly compared to the normal bearing operation, manifesting as obvious impacts, severe vibrations, and distinct vibration characteristics.

Figure 4. Schematic diagram of double-impact phenomenon caused by spalling failure: (a) before entering the spalling area; (b) when entering the spalling area; (c) after leaving the spalling area [42].

Figure 4. Schematic diagram of double-impact phenomenon caused by spalling failure: (a) before entering the spalling area; (b) when entering the spalling area; (c) after leaving the spalling area [42].

Kong et al. [42] proposed a method for extracting double impact characteristics of spalling failures in hybrid ceramic ball bearings based on ensemble empirical mode decomposition (EEMD). Lu et al. [43] presented a method for extracting double impact characteristics of spalling failures in hybrid ceramic ball bearings based on dual-tree complex wavelet packet transform (DTCWPT). Both methods can effectively separate and extract the double impact characteristics of the vibration signals of hybrid ceramic ball bearings with spalling failures. The principle of double impact characteristic extraction is illustrated in Figure 5.

Kang et al. [44] introduced a method for estimating the width of the spalling area in rolling ball bearings based on vibration signal analysis. This method can effectively estimate the width of the spalling area in hybrid ceramic ball bearings based on the interval of double impact characteristics in the vibration signal and the rotational speed of the shaft. Guo et al. [45] studied the double pulse phenomenon generated by the outer ring spalling of hybrid ceramic ball bearings and compare it to metal bearings. It is found that the double pulse phenomenon generated by the outer ring spalling of hybrid ceramic ball bearings is sharper than that of metal bearings with the same outer ring spalling length. Shi et al. [46] proposed a novel method for locating outer ring spalling in hybrid ceramic ball bearings based on acoustic emission signal characteristics. This new method provides a new perspective for the detection and localization of spalling failures in hybrid ceramic ball bearings, helping to improve the reliability and operational efficiency of the equipment.

When fatigue spalling occurs on the surface of hybrid ceramic ball bearings, it will cause vibration impacts. Analyzing the vibration characteristics of spalling failures in hybrid ceramic ball bearings can determine the length and location of the spalling area during bearing operation, which is of great significance for studying bearing life issues.

3. Hybrid Ceramic Ball Bearing Analysis Method

Hybrid ceramic ball bearings exhibit superior properties, including excellent mechanical properties, tribological properties, and dynamic performance. The calculation of contact stress and deformation, which forms the basis of bearing performance analysis, is crucial for understanding mechanical performance. Dynamic characteristics are an extension of this analysis. These properties give ceramic ball bearings significant advantages in high temperature, high speed, and specialized environments. Therefore, in-depth research on the performance analysis of hybrid ceramic ball bearings can enhance our understanding and utilization of these advantages. The performance of hybrid ceramic ball bearings directly impacts their reliability during operation. Through theoretical analysis, software analysis, or experimental studies on the properties of ceramic ball bearings under various conditions, we can predict their performance in practical applications. This paper primarily consolidates relevant articles on the performance analysis of hybrid ceramic ball bearings.

However, since some references lack clear indication of the analysis methods for hybrid ceramic ball bearing performance, there may be inevitable errors in identifying them based on formulas, text, and other information.

3.1. Analysis of Hybrid Ceramic Ball Bearings Based on Finite Element Software

The finite element method (FEM) is a commonly used and efficient approach for solving mechanical and dynamic problems of hybrid ceramic ball bearings. It is feasible and can produce relatively accurate results. ANSYS and ABAQUS are powerful finite element analysis software that can simplify complex problems.

3.1.1. Analysis Based on Ansys Software

References [47,48,49,50] conducted modeling, meshing, and contact pair setting in ANSYS to obtain the internal and external contact stresses and deformations. The results were presented in the form of contour plots, which were very intuitive. Reference [48] also conducted experimental verification to validate the accuracy of the finite element analysis. Reference [50] compared the results with Hertzian contact and found little difference. It can be seen that using ANSYS to simulate and analyze the contact performance of hybrid ceramic ball bearings is feasible.

Li et al. [51] utilized ANSYS to conduct a finite element analysis of hybrid ceramic ball bearing of model 6026 based on Hertz theory. During modeling, the bearing cage was omitted to save computing resources and improve computational efficiency. Constraints were added to the sphere, and the influence of backlash was neglected to prevent calculation errors caused by rigid body displacement. Considering the symmetry of the bearing’s structure, loading, and constraints, it was simplified into a half bearing model for analysis, and a detailed mesh was performed. The meshing is shown in Figure 6.

The analysis yielded the distribution of contact stress between the rolling elements and the inner and outer rings, as shown in Figure 7, which aligns with the Hertz contact theory. This study also optimized the curvature radius of the inner and outer raceway grooves of the hybrid ceramic ball bearing, providing a theoretical basis for the structural design of silicon nitride hybrid ceramic ball bearings.

Zhao et al. [52] used ANSYS software to perform dynamic simulations on B7005C hybrid ceramic ball bearings and all-steel bearings. By comparing their cage slip rate, cage fluctuation range, and centroid eccentricity ratio, it was found that ceramic ball bearings operate more stably under heavy radial loads compared to steel bearings, and the use of ceramic balls reduces axial inner ring displacement and its fluctuation. While full ceramic bearings are slightly more stable than hybrid ceramic ball bearings, their impact resistance is significantly inferior. The use of hybrid ceramic ball bearings can effectively improve dynamic performance.

Chen et al. [53] used the LS-DYNA module in ANSYS simulation software to investigate the tribological and dynamic properties of hybrid ceramic bearings for aerospace engines. The simulation results showed that the maximum contact stress of the hybrid ceramic ball bearing is located on the surface of the ceramic ball, followed by the outer ring and inner ring raceways. The contact stress of the hybrid ceramic ball bearing was also verified and analyzed using Romax software. In addition, Chen also conducted wear experiments, with the wear test machine shown in Figure 8. After 2400 s of wear, the Si₃N₄-GCr15 friction pair gradually formed groove-like shapes on the disc surface, and the surface was relatively smooth. As seen in the microscopic image (Figure 9), the worn Si3N4 ball appeared almost unchanged, with only slight wear and white protrusions attached, indicating good overall wear surface conditions and a relatively smooth morphology. In contrast, the GCr15-GCr15 friction pair showed poorer overall wear conditions, with uneven surfaces, obvious peeling, and damage. This demonstrates that the Si₃N₄-GCr15 pair has better tribological performance.

The use of ANSYS software enables the analysis of both static and dynamic issues concerning hybrid ceramic ball bearings. The references mentioned above utilize ANSYS software to examine the contact stress, deformation, and dynamic behavior of hybrid ceramic ball bearings. From these analyses, it can be observed that the contact of hybrid ceramic ball bearings adheres to the distribution patterns predicted by the Hertz theory, and furthermore, they operate more stably compared to all-steel bearings.

3.1.2. Analysis Based on Abaqus Software

In reference [54], a finite element model of hybrid ceramic ball bearings was established in ABAQUS software, aiming at conducting precise static analysis on ceramic ball bearings under bearing loads. This model was used to analyze the effects of axial loads on the contact load, contact stress, contact angle, and axial elastic approach of hybrid ceramic angular contact ball bearings.

The use of ABAQUS software for analyzing hybrid ceramic ball bearings is not as common as ANSYS, which is favored by researchers due to its powerful capabilities and wider application range.

3.2. Methods for Analyzing the Performance of Hybrid Ceramic Ball Bearings

3.2.1. Static Analysis

Static analysis simplifies the kinematic relationship of the bearing by neglecting the influence of inertial forces and frictions within the bearing, treating the external loads as static. The primary computational method is still based on the Hertzian contact theory, but due to its limited number of considerations, the analysis results may deviate from reality to a certain extent.

Reference [55] uses MATLAB software to numerically solve the contact stress and deformation of hybrid ceramic ball bearings, laying a foundation for using MATLAB to solve problems related to hybrid ceramic balls. Wei et al. [56] used static equations based on the Hertzian contact theory to solve the load distribution and contact stress of bearings. Taking a deep groove ball bearing for a motor as an example, it calculates the load distribution and contact stress of all-steel bearings and ceramic ball bearings under different radial clearances. It is found that the influence of radial clearance on the load distribution of all-steel bearings and ceramic ball bearings is similar, and the optimal radial clearance for hybrid ceramic ball bearings is derived.

Although the results obtained from static analysis may not be entirely accurate, they still have a certain reference value as a basis for the optimized design of hybrid ceramic ball bearings.

3.2.2. Quasi-Static Analysis

The method of quasi-static analysis, compared to static analysis, takes into account inertial forces and can effectively predict the speed of revolution and rotation, fatigue life, bearing deformation, and stiffness of rolling elements. It also allows for the accurate determination of preload to prevent slipping of rolling elements. However, this method cannot analyze transient motion conditions of bearings.

Yuan et al. [57] established a quasi-dynamic model for high-speed ball bearings based on the ring control theory proposed by Jones [58]. By comparing the axial stiffness, radial stiffness, and spin-to-roll ratio between hybrid ceramic ball bearings and all-steel bearings using this model, it was found that under the same conditions, the stiffness of hybrid ceramic ball bearings is greater and that the spin-to-roll ratio is higher than that of all-steel bearings. Liu et al. [59] designed a silicon nitride hybrid ceramic ball bearing for satellite momentum wheels and optimized its contact stress, spin-to-roll ratio, axial stiffness, lubrication, and rated static load using quasi-static analysis. The results showed that the hybrid ceramic bearing has excellent impact load resistance, temperature adaptability, low power consumption, and long-term wear resistance. The friction torque of the hybrid ceramic ball bearing was also measured using a test machine, and it was found that under the same conditions, the friction torque of the hybrid ceramic ball bearing with the same parameters was slightly smaller than that of the all-steel bearing.

Xu et al. [60] analyzed the centrifugal force, gyroscopic moment, contact stress and deformation, axial and radial stiffness of hybrid ceramic ball bearings using quasi-static analysis based on the outer raceway control theory. It was found that hybrid ceramic ball bearings are more suitable for high-speed operation than all-steel bearings. He et al. [61] studied a hybrid ceramic ball bearing for the spindle of a CNC machine tool and established dynamic models for the hybrid ceramic ball bearing and all-steel bearing. Using the quasi-static analysis method with outer raceway control, the preload, maximum contact stress, heat, stiffness, and vibration of the hybrid ceramic ball bearing and all-steel bearing were investigated. Experiments were conducted on two identical bearings using a high-speed machine tool spindle test stand. It was found that within the speed range of 0 to 10⁵ rpm, the contact stress of the hybrid ceramic ball bearing was greater than that of the all-steel bearing at low speeds, but at high speeds, the contact stress of the hybrid ceramic ball bearing was smaller than that of the all-steel bearing. Applying a larger preload within this range helps control heat and vibration.

Tian et al. [62] established a computational model for ultra-high-speed angular contact hybrid ceramic ball bearings based on quasi-static analysis. Under the condition of outer raceway control, the influence of ball diameter on parameters such as contact stress, spin-to-roll ratio, and axial stiffness was analyzed at different speeds and axial loads. Lavrentyev [63] compared the contact stress between balls and raceways in hybrid bearings and steel bearings using Hertzian contact theory, considering the influence of centrifugal force. It was found that high speed and relatively low load are the most suitable operating conditions for hybrid ceramic ball bearings.

Gärtner et al. [64] conducted a study on the tribological properties of hybrid ceramic ball bearings, focusing on the 7008 model. A high-speed bearing test rig capable of applying radial loads was designed, as shown in Figure 10, for measuring the bearing friction torque of hybrid ceramic ball bearings under radial loads. During the testing, the hybrid ceramic ball bearing was subjected to a radial load of up to 3 KN and achieved a rotational speed of 24,000 r/min. A friction calculation model was established using the quasi-static method, which was controlled by the outer raceway. This model was then compared to the test results, verifying its correctness. The research in Reference [65] focuses on the performance analysis of a hybrid ceramic angular contact ball bearing that rotates at high speeds and experiences slipping. Based on the Hertzian contact theory, a quasi-static method is employed to analyze the performance of the micro-turbo hybrid ceramic ball bearing 7000 C. This analysis yields the optimal preload required for the hybrid ceramic ball bearing under different rotational speeds. When compared with all-steel bearings, the hybrid ceramic ball bearing requires a smaller axial preload to prevent slipping.

The quasi-static research methodology is used extensively in the aforementioned studies. References [61,65] analyze the optimal preload of the hybrid ceramic ball bearing, while References [57,59,60,64] examine the stiffness and spin-to-roll ratio of the hybrid ceramic ball bearing. Furthermore, References [57,60,61,62,64] analyze the dynamic performance of the hybrid ceramic ball bearing based on Jones’ raceway control theory and compare it with all-steel bearings. In summary, most quasi-static analyses of hybrid ceramic ball bearings are grounded in raceway control theory. The limiting speed of hybrid ceramic ball bearings far exceeds that of all-steel bearings, and their various performance metrics are also superior. Although the accuracy of the results obtained by the quasi-static analysis method is not as high as that of dynamics, it excels in simplicity. Compared to statics, it takes centrifugal force into account, making it more suitable for analyzing hybrid ceramic ball bearings.

3.2.3. Dynamic Analysis

The dynamic method considers the entire kinematic process from the start-up of the bearing, which not only provides dynamic performance parameters during steady-state operation but also effectively analyzes the working conditions when the load and speed vary over time. The results obtained by analyzing the dynamic performance of hybrid ceramic balls using this method are highly accurate, but due to the complexity of the method, it is difficult to solve.

Zheng et al. [66] conducts design, performance analysis, and experimental verification of high-temperature, high-speed hybrid ceramic ball bearings. Based on the Adams dynamics simulation software, a rolling bearing dynamics model is established to analyze and compare the contact stress, spin-to-roll ratio, cage collision force, slippage rate, and centroid trajectory of steel bearings and hybrid ceramic ball bearings at different speeds (as shown in Figure 11 for the centroid trajectory of the hybrid ceramic ball bearing cage). An independently developed high-speed bearing performance test machine (as shown in Figure 12) was used. As the speed increases, the cage stability first increases and then decreases. When the speed is below 60,000 rpm, the cage stability of hybrid ceramic ball bearings is comparable to that of steel bearings. When the speed exceeds 80,000 rpm, the cage stability of hybrid ceramic bearings is better than that of steel bearings.

Apart from having higher contact stress than steel bearings, the hybrid ceramic ball bearings exhibit better or comparable performance in other aspects, and they are more suitable for high-temperature, high-speed operating conditions.

3.3. Other Analysis and Testing Methods

Since some references did not explicitly state the analytical methods used or only conducted experimental studies without theoretical analysis, they are summarized in this paper.

References [67,68] have investigated the performance of hybrid ceramic ball bearings under various operating conditions, including temperature, load, speed, corrosion, and the presence of lubrication. This research provides a basis for further experimental studies on the application of hybrid ceramic ball bearings in challenging environments.

References [69,70] analyzed the performance of hybrid ceramic ball bearings under liquid nitrogen conditions. Reference [69] conducted a two-ball compression test to simulate the pressure conditions of hybrid ceramic bearings and all-steel bearings in bearing applications. The experimental setup is shown in Figure 13 below. It measured the ultimate bearing capacity of Si₃N₄ balls and 9Cr18 steel balls under liquid nitrogen and room temperature conditions. The bearing capacity of Si₃N₄ balls at room temperature and cryogenic temperatures is higher than that of 9Cr18 steel balls. Therefore, when forming a hybrid ceramic ball bearing composed of Si₃N₄ balls and 9Cr18 steel rings, the surface of the 9Cr18 steel rings should be appropriately strengthened. Reference [70] compared the tribological properties of hybrid ceramic ball bearings composed of Si₃N₄ balls and 9Cr18 steel rings with all-steel bearings made of 9Cr18 steel through ball-on-disk friction and wear experiments conducted in air and liquid nitrogen without lubricants. It was found that the pairing of Si₃N₄ balls and 9Cr18 steel disks was stable and did not show severe adhesive wear, while the pairing of 9Cr18 steel balls and 9Cr18 steel disks showed poor friction properties and severe adhesive wear. Comprehensive performance tests were conducted on hybrid bearings and all-steel bearings on a high-speed bearing test machine under liquid nitrogen conditions. The life of the all-steel bearing was less than 8 min, while the running time of the hybrid ceramic ball bearing exceeded 120 min, indicating the superior overall performance of the hybrid ceramic ball bearing.

Grigorescu et al. [71] conducted experiments based on high-speed spindle testing to analyze the power loss and damage accumulation in full steel bearings and hybrid ceramic ball bearings. The power loss and temperature rise of hybrid ceramic ball bearings were analyzed and compared with the performance of full steel bearings. It was found that the physical and mechanical properties of ceramics used for rolling elements in hybrid bearings could reduce power loss by 25% compared to traditional bearings.

Fan et al. [72] conducted a comparative analysis of gyroscopic torque, spin sliding, revolution slippage at high speeds, hardness, adhesion resistance, resistivity, and surface roughness between hybrid ceramic angular contact ball bearings and full steel angular contact ball bearings. The hybrid ceramic angular contact ball bearings exhibit lower gyroscopic torque, lower friction coefficient, higher hardness, lighter weight, lower inertial force, slight adhesive wear, and insulating properties compared to full steel angular contact bearings. Therefore, hybrid ceramic angular contact ball bearings possess excellent lubrication performance, low friction temperature rise, and high limiting speed.

Wang et al. [73] studied hybrid ceramic ball bearings with Invar 36 alloy inner and outer rings under low-temperature conditions. Using an improved ball-on-disk friction tester with a low-temperature module, the tribological properties between Invar 36 alloy and Si₃N₄ ceramic balls were studied under extremely low temperature and dry conditions. Compared to G95Cr18 steel under the same conditions, it was proven that the tribological properties of Invar 36 alloy are superior to G95Cr18 steel.

Most of the above references conducted experiments on the performance of ceramic balls, and the results obtained indicate that hybrid ceramic ball bearings outperform full steel bearings in terms of performance, regardless of whether they are used under normal or cryogenic conditions.

4. Research on Temperature and Lubrication of Hybrid Ceramic Ball Bearings

During high-speed operation, bearings generate a significant amount of heat, with hybrid ceramic ball bearings being no exception. If the heat generated internally within the bearing and the heat transmitted from the high-temperature external environment cannot be effectively dissipated or cooled by the lubrication system, the bearing may suffer burns on the raceway and rolling element surfaces, ultimately leading to direct scrapping. Therefore, the temperature and lubrication of the bearing are interconnected and mutually influential. Delving into research on these two aspects will provide a deeper understanding of the bearing’s working conditions, ensuring its normal operation, enhancing its performance, and boosting its reliability.

4.1. Thermal Analysis of Hybrid Ceramic Ball Bearings

4.1.1. Research on Heat Generation in Hybrid Ceramic Ball Bearings

As early as 1988, Aramaki et al. [5] conducted research on the thermal performance of all-steel bearings and hybrid ceramic ball bearings. Slaney et al. [74] studied the heat generation of hybrid ceramic ball bearings and all-steel bearings, revealing that the heat generated by hybrid ceramic ball bearings was on average 40% less than that of standard all-steel bearings. Shoda et al. [75] conducted a temperature rise test on hybrid ceramic bearings with an inner diameter of 150 mm and M50 steel bearings. The study revealed that under a load of 19.6 KN, the heat generation of hybrid ceramic bearings was comparable to that of M50 steel bearings. However, when subjected to an axial load of 34.4 KN, the heat generation rate of hybrid ceramic bearings was lower than that of M50 steel bearings at low speeds. Nevertheless, at higher speeds, the heat generation of hybrid ceramic bearings increased significantly. This indicates that the heat generation rate of hybrid ceramic ball bearings under heavy loads is substantial. Through theoretical calculation, Cento et al. [76] compared the differences in heat generation and frictional torque between conventional all-steel bearings using M50 steel balls and hybrid ceramic ball bearings using silicon nitride (Si3N4) ceramic balls. Both types of bearings utilized M50 steel rings and were subjected to only radial loads. The findings showed that the friction torque of hybrid ceramic ball bearings was lower, resulting in 8–16% lower heat output compared to all-steel bearings.

Tian et al. [77] established a calculation model for the heat generation of high-speed hybrid ceramic ball bearings, analyzed the effects of rotational speed, load, initial contact angle, and kinematic viscosity of lubricant oil on the heat generation of hybrid ceramic ball bearings. As the rotational speed and load increase, the heat generation also increases. Measures such as reducing the diameter of rolling elements, using bearings with a smaller contact angle, reducing preload, and decreasing the viscosity of lubricant oil can be adopted to reduce the frictional heat generation of bearings.

Takeuchi et al. [78] carried out research focused on the thermal conductivity of hybrid ceramic ball bearings within a vacuum environment. They identified the main factors impacting bearing temperature in such conditions as thermal conductivity and heat generation rate. This study mainly explored the lesser-known aspect of thermal conductivity. The findings demonstrated that steel bearings and hybrid ceramic ball bearings shared unexpected similarities in their thermal conductivity and behavior across various operating conditions and lubricant volumes. Therefore, it can be assumed that any temperature difference between hybrid ceramic ball bearings and steel bearings is determined by heat generation rather than bearing thermal conductivity. References [79,80,81] aimed to improve turbofan engines by using Si₃N₄ hybrid ceramic ball bearings in gas turbine engines. Lavrentyev et al. [79] tested hybrid ceramic ball bearings on the bearing test stand at the Central Institute of Aviation Motors. Based on the results of experimental data processing, empirical formulas were developed to determine the heat generation in high-speed hybrid ceramic ball bearings and the temperature values of the outer and inner rings. The correctness of the empirical formulas was verified using data from existing literature.

Although the temperatures of hybrid ceramic ball bearings and all-steel bearings may not differ significantly under heavy loads, the heat generation of hybrid ceramic ball bearings is far less than that of all-steel bearings under high-speed and light-load conditions. Furthermore, hybrid ceramic ball bearings exhibit superior performance and higher survivability under high-temperature conditions. Therefore, hybrid ceramic ball bearings possess unique advantages under high-speed and light-load working conditions.

4.1.2. Research on Temperature Distribution of Hybrid Ceramic Ball Bearings

Cao [82] utilizes the Abaqus finite element analysis software to study the temperature field of hybrid ceramic ball bearings. It argues that the majority of heat generation in the bearing comes from the self-rotation friction. The heat generation power of self-rotation friction is calculated using the local heat generation method and applied to the finite element analysis model as thermal loading. The finite element model is simplified into a binary model to simulate the temperature field of high-speed bearings. The resulting temperature distribution is shown in Figure 14, which concludes that the ceramic ball has the highest temperature, followed by the inner ring, and the outer ring has the lowest temperature. This lays a foundation for the analysis of the temperature field of hybrid ceramic ball bearings.

In [83], Wang et al. initially determined the heating rate and convective heat transfer coefficient of the hybrid ceramic ball bearing. These values were then utilized as boundary conditions in the simulation of the temperature field of the hybrid ceramic angular contact ball bearing using Ansys software. Subsequently, a temperature field distribution cloud map of the bearing at a rotational speed of 12,000 r/min was generated. Analysis of the cloud chart (refer to Figure 15) reveals that the ceramic ball exhibits the highest temperature, reaching 47.2 °C.

Cui et al. [84] introduce an innovative model that leverages the quasi-dynamic and moving heat source approaches to examine slip and overheating phenomena in ball bearings, incorporating dynamic-thermal coupling effects (illustrated in Figure 16). By developing specialized software, they determined the critical minimum load needed to prevent slip and the critical maximum load required to prevent overheating. Additionally, using Ansys finite element software, they calculated the temperature distribution of the hybrid ceramic ball bearing under critical overheating conditions (depicted in Figure 17), identifying a peak temperature of 150 °C. The model’s reliability was validated through experimental testing, with the experimental setup and principles shown in Figure 18.

Due to the complexity of calculating the temperature field of bearings, utilizing the finite element method to solve the temperature field of hybrid ceramic ball bearings involves theoretical calculations to determine parameters such as heat generation rate and convective heat transfer coefficient as boundary conditions. These are then used to simulate the heat generation of high-speed bearings, yielding an approximate solution. It is important to note that this approximate solution may differ from the actual temperature of the hybrid ceramic ball bearing under corresponding operating conditions.

4.2. Research on Lubrication of Hybrid Ceramic Ball Bearing

Lubrication plays a vital role in reducing friction and wear in bearings. While hybrid ceramic ball bearings may not always require lubrication at low speeds, they are commonly employed in high-speed machine tool spindles. Despite their inherent self-lubricating properties, appropriate lubrication is still necessary for these bearings. Optimal lubrication conditions can help minimize the negative effects of vibrations, decrease noise levels, and most importantly, prevent fatigue spalling in hybrid ceramic ball bearings, ultimately extending their lifespan. Therefore, there is a significant need for research on the lubrication of hybrid ceramic ball bearings. The lubrication of hybrid ceramic ball bearings has been studied as follows.

4.2.1. Oil Lubrication

There are various methods of oil lubrication, including oil bath lubrication, oil dripping lubrication, splash lubrication, circulating oil supply, jet lubrication, oil mist lubrication, oil–air lubrication, and under-race lubrication. Oil lubrication has good cooling effects and is the most common lubrication method for high-speed bearings. For high-speed bearings such as hybrid ceramic ball bearings, oil lubrication is undoubtedly a very suitable choice. Moreover, oil lubrication can carry away friction particles, effectively preventing abrasive wear of the hybrid ceramic balls. Currently, there are several types of research on oil lubrication for hybrid ceramic balls.

Under-race lubrication is a method where the lubricating oil first passes through the lower part of the bearing and then uses centrifugal force to splash the lubricating oil to the parts that need lubrication. Early scholars used under-race lubrication to lubricate hybrid ceramic ball bearings. In 1992, Schrader et al. [85] conducted a test on hybrid ceramic ball bearings of model 308, using diester oil and gas turbine fuel as lubricants. They tested both M50 steel bearings and hybrid ceramic ball bearings under different rotational speeds and load conditions using under-race lubrication. The bearings successfully underwent a 25-h durability test in JP-10 fuel, with an axial load of 7560 N and a rotational speed of 36,000 rpm. Shoda et al. [75] tested hybrid ceramic ball bearings and M50 steel bearings with under-race lubrication and conducted oil shut-off tests on both bearings. The results found that the hybrid ceramic ball bearings exhibited higher survivability in the oil shut-off tests due to their low thermal expansion and low sliding losses. Wang et al. [86] also conducted ball-on-disk experiments using the first-generation advanced aeroengine oil Mobil Jet II for hybrid ceramic bearings and M50 steel bearings under oil lubrication conditions and compared their performance.

Oil mist lubrication involves atomizing the lubricating oil through nozzles and spraying it onto the bearing surface under the drive of high-speed airflow, providing stable airflow and continuous lubricating oil for the ball bearings. Jeng et al. [87] tested hybrid ceramic ball bearings and steel ball bearings using oil mist lubrication under different conditions of preload, air flow rate, oil quantity, and rotational speed. It was found that as the rotational speed increased, the temperature of the bearing also increased, but the temperature rise of the hybrid ceramic ball bearing was always lower than that of the steel ball bearing. When the oil supply rate exceeded the appropriate range, the temperature rise of both the hybrid ceramic ball bearing and the steel ball bearing tended to be similar. Increasing the airflow rate was an effective method to effectively reduce the temperature rise of the hybrid ceramic ball bearing. Paleu et al. [88] has developed a new friction torque testing device and tested hybrid ceramic bearings and full steel bearings under the condition of oil mist lubrication. They found that hybrid ceramic ball bearings have a lower equilibrium temperature and can operate for a longer time under oil-cut conditions.

Oil–air lubrication utilizes high-speed airflow to deliver low-speed lubricating oil to the corresponding lubrication positions. The lubricating oil and air have significantly different speeds, and the lubricating oil is not atomized. Additionally, the high-speed airflow also plays a role in lubrication. In their study, Jiang et al. [89] conducted experiments to analyze the temperature rise of hybrid ceramic bearings and all-steel bearings under oil-air lubrication conditions. They examined the impact of various factors such as the length of the oil-air supply pipe, preload, lubrication interval, lubricating oil viscosity, lubricating oil type, and nozzle on the temperature rise of the bearings. The research highlighted that lubricating oil viscosity and the specific nozzle design were crucial factors influencing the bearing temperature rise. Interestingly, the temperature rise of hybrid ceramic ball bearings was consistently lower than that of steel ball bearings under similar operating conditions. Additionally, Wang [90] investigated oil–air lubrication for hybrid ceramic ball bearings and found that the oil supply needed for hybrid ceramic bearings was approximately 1/2 to 2/3 less than that required for steel bearings.

Dotsenko et al. [91] conducted comparative tests on all-metal bearings and Si₃N₄ hybrid ceramic ball bearings using both jet lubrication and oil-air mixed lubrication. It was found that without external heating, slightly increasing the bearing temperature and using oil- air mixed lubrication could reduce bearing losses by more than 2 times, while the bearing temperature increased slightly. Hadfield et al. [29] conducted high-speed performance tests on spindle bearings lubricated with grease or oil-air mixtures. Additionally, it discussed the latest material developments, bearing temperatures under high speed, and reliability considerations.

Zhu [92] analyzed thermally mixed lubrication for hybrid ceramic ball bearings. Mechanical equipment often generates a significant amount of heat due to contact friction, causing an increase in temperature in the contact area. Considering the impact of temperature on the properties of lubricating oil is referred to as thermally mixed lubrication. Taking into account oil supply conditions, the study investigated the lubrication state of hybrid ceramic ball angular contact bearings under oil-starved conditions. It was found that the thermally mixed lubrication performance under oil-starved conditions was poorer compared to adequate oil supply.

Currently, there is minimal research on oil-starved hybrid ceramic ball bearings. However, in practice, failures in the oil supply system or unreasonable design of the lubrication system can lead to oil starvation in bearings, increasing friction and wear and potentially causing bearing failures. For hybrid ceramic ball bearings, their unique material properties may have more specific requirements for performance under oil-starved conditions. Scholars should conduct research on hybrid ceramic ball bearings under oil-starved conditions, focusing on:

(1)

Friction and wear performance of hybrid ceramic ball bearings under oil-starved conditions: Through experiments and simulation analysis, study the friction coefficient, wear rate, and other performance parameters of hybrid ceramic ball bearings under oil-starved conditions to understand the impact of lubrication status on performance.

(2)

Temperature rise and heat dissipation performance of hybrid ceramic ball bearings under oil-starved conditions: Study the temperature rise pattern and heat dissipation performance of bearings under oil-starved conditions to understand the impact of insufficient lubrication on the thermal stability of hybrid ceramic ball bearings.

(3)

Fatigue life of hybrid ceramic ball bearings under oil-starved conditions: Through accelerated life tests and other methods, study the fatigue life and failure mechanisms of bearings under oil-starved conditions to assess their reliability in practical applications.

(4)

Lubrication system design for hybrid ceramic ball bearings under oil-starved conditions: Optimize the design and operating parameters of the lubrication system for oil-starved conditions to ensure that the bearings maintain stable performance even when oil starved.

Current research on oil lubrication for hybrid ceramic ball bearings includes under-ring lubrication, oil mist lubrication, oil-air lubrication, and jet lubrication. Overall, hybrid ceramic ball bearings require less lubricating oil than all-steel bearings and exhibit better lubrication performance.

4.2.2. Grease Lubrication

The method of grease lubrication includes brushing lubrication, filling and sealing lubrication, dripping lubrication, and centralized lubrication, which are widely used. However, grease lubrication may not be suitable for all situations as grease can attract and retain dust and pollutants, and cannot remove abrasive particles, potentially causing increased friction and accelerated wear.

Lugt et al. [93] conducted a study comparing the grease life of hybrid ceramic deep groove ball bearings and all-steel deep groove ball bearings. Their findings demonstrated that the grease life of hybrid ceramic ball bearings consistently exceeded that of equivalent all-steel bearings. The extent of this difference varied based on the type of grease utilized, ranging from two to nine times. In a separate study, Zheng et al. [94] designed a hybrid ceramic ball bearing with excellent performance under high temperature and high speed for grease lubrication. If the contact area between the ceramic ball and the steel raceway lacks grease, it will lead to thermal instability and friction failure of the contact surface, manifesting as a combination of fatigue and adhesion. Wei et al. [95] compared the rolling friction and wear characteristics of Si₃N₄ ceramic bearing balls under no lubrication and grease lubrication conditions based on a ball-on-disk friction and wear tester. Grease lubrication significantly reduced the rolling friction coefficient, especially alleviating the fatigue damage of Si₃N₄ ceramic bearing balls. Under no lubrication conditions, the rolling friction damage mechanism of Si₃N₄ ceramic bearing balls was mainly fatigue damage and adhesive wear. Under grease lubrication conditions, the rolling friction damage mechanism of Si₃N₄ ceramic bearing balls was mainly abrasive wear and micro brittle fracture. Li et al. [96] conducted early failure tests on hybrid ceramic ball high-speed spindle bearings under different working conditions based on the working characteristics of grease-lubricated hybrid ceramic ball high-speed spindle bearings. It was found that the main reasons for early failure of hybrid ceramic ball spindle bearings under high-speed grease lubrication conditions were severe impacts, friction, and high operating temperatures caused by high-speed operation and rapid and frequent start-stops, leading to fractures in weak parts of the cage, and their operating time was far from reaching the expected fatigue life.

Wu et al. [97] utilized polyalphaolefin 40 as the base oil in the synthesis of three types of greases: lithium-based grease, polyurea-based grease, and calcium sulfonate complex grease. Their study focused on the impact of grease thickener morphology on the tribological properties of the contact surface between Si₃N₄ and GCr15 under varying sliding speeds. Findings indicated that at lower speeds, the thickener could create a physical deposit film and friction film to safeguard the worn surface. The continuous shearing of the lithium-based grease by the ceramic ball allowed for stable release of the base oil, resulting in the formation of a thick oil film with the broken thickener at the center of the raceway, maintaining the bearing in a state of moderate lubrication. Consequently, lithium grease shows promise as a lubricant for silicon nitride hybrid ceramic ball bearings.

The above studies mainly focused on the performance and failure modes of ceramic bearings under grease lubrication conditions. It was found that hybrid ceramic ball bearings have a long grease life. Under the condition of lack of lubrication, hybrid ceramic ball bearings are prone to fatigue damage and adhesive wear. Under the condition of grease lubrication, due to the inability to discharge abrasive particles, dust, pollutants, and other impurities, their failure modes are generally abrasive wear or brittle fracture of the cage.

4.2.3. Self-Lubrication

Self-lubrication is a lubrication method that does not require external lubrication oil or grease, commonly used in places where oil supply is difficult or to avoid the pollution of lubricating oil, such as the aerospace field. There are two methods for self-lubrication. One is an oil-containing cage made of porous pure polyimide, which utilizes its porous structure to store lubricating oil. When the bearing starts to operate, the lubricating oil is released to achieve a lubricating effect. The other method uses composite materials to make the cage, which provides a lubricating transfer film to the cage and rolling elements when they come into contact and collide. The following references introduce the application of self-lubrication on hybrid ceramic ball bearings.

According to Reference [59], the hybrid ceramic ball bearing of the designed momentum wheel utilizes an oil-containing cage made of porous pure polyimide, which has good lubricating effects.

Since traditional lubrication methods cannot be used for aviation bearings, these bearings need to rotate at very high speeds, withstand radial and thrust loads, and require high wear resistance. As early as 2001, Gibson et al. [98] proposed using composite materials to make cages for hybrid ceramic ball bearings. When the cage comes into contact and collides with the rolling elements, it provides a lubricating transfer film to the rolling elements and raceways. The combination of hybrid ceramic ball bearings and composite materials of the cage can achieve better self-lubricating effects.

References [99,100,101] have studied the self-lubricating properties of hybrid ceramic ball bearings in a liquid hydrogen environment. The hybrid ceramic ball bearings were tested at ultra-high speeds of up to 120,000 r/min. Due to the reducing ability of liquid hydrogen, the formed CaF₂ or FeF₂ films exhibited excellent self-lubricating properties, and the hybrid ceramic ball bearings maintained a good wear state. These research results are invaluable for the aerospace field.

4.2.4. Special Lubricants

Liu et al. [102] introduces a high-speed and high-precision spindle system with hybrid ceramic ball bearings, which utilizes porous restrictors for lubrication. A green and viscosity-controllable water-based fluid is designed as the lubricant for the spindle. By using a lubricant with controllable viscosity, the matching between the bearing and the spindle system can be optimized. ANSYS software is employed for thermal-elastic fluid-dynamic simulation to test the change in lubricant viscosity with temperature. References [103,104] studied the performance of hybrid ceramic ball bearings under water lubrication conditions. Reference [104] primarily focuses on the impact of water supply pressure on the power loss, friction coefficient, and temperature of hybrid ceramic ball bearings. It can be concluded that with the increase in water supply pressure, the power loss and friction coefficient increase significantly, while the temperature rise decreases as the water supply pressure increases.

Wu et al. [105] conducted a study on the effect of six different choline chloride (ChCl)/diol deep eutectic solvents (DESs) with varying alkyl chain lengths and hydroxyl positions as lubricants for hybrid ceramic ball bearings. The research found that choline chloride (ChCl)/ethylene glycol (EG) exhibits superior lubrication performance, making it a promising candidate as a high-quality lubricant for Si3N4 hybrid ceramic ball bearings.

4.3. Outer Ring Direct Cooling Technology

Under conditions of high speed and heavy loads, rolling bearings used in aeroengines can generate significant heat. Additionally, the lubricating oil flowing to the bearings is already preheated, considerably diminishing its cooling effectiveness. Studies referenced in [106,107] explored a technique to actively cool the outer ring of bearings and confirmed its reliability when applied to hybrid ceramic ball bearings. This outer ring cooling concept is inspired by the heat sinks used in cooling electronic devices. In this approach, a spiral groove is etched into the bearing’s outer ring material, functioning as a heat sink body with oil employed as the cooling medium. The findings indicated a reduction in the outer ring’s temperature by more than 20 °C. The bearing showcased its ultra-high-speed capability, achieving a rotational rate of 24,000 rpm while maintaining the bearing temperature below 200 °C.

The outer ring cooling technology not only improves the cooling efficiency of the bearing but also reduces the consumption of lubricating oil, having a positive impact on the performance and reliability of aeroengines.

5. Discussion

Although hybrid ceramic ball bearings offer superior performance compared to all-steel bearings, they have not yet been able to completely replace all-steel bearings due to several challenges that need to be addressed.

(1)

The firing and processing of ceramic balls are extremely difficult, which easily leads to surface crack defects. Surface cracks are difficult to detect in large-scale production processes [23]. Therefore, rapid and reliable nondestructive testing is an important technology for manufacturing high-reliability hybrid ceramic ball bearings.

(2)

The brittleness of early ceramic materials has always been one of the most direct factors affecting the failure modes and impact resistance of hybrid ceramic ball bearings. Although the comprehensive performance of ceramic bearings has been greatly improved through hot isostatic pressing sintering technology, their low toughness, high hardness, and moderate flexural strength remain fatal weaknesses of ceramic materials [27]. However, it is not always the case that the less impurities there are, the better. Silicon nitride ceramics with low impurity content have better porosity and higher hardness, while those with high impurity content have better toughness. Controlling the impurity content of ceramic materials is the key to producing high-quality silicon nitride ceramic balls [108]. In the future, more high-quality ceramic blank materials should be obtained through process optimization.

(3)

The high elastic modulus and hardness of silicon nitride in hybrid ceramic ball bearings lead to increased stress on the raceway due to the relatively small contact area between the ball and the raceway. This can result in a sharp rise in heat generation under heavy loads and high rotation speeds. It is essential to focus on developing effective lubrication methods to prevent fatigue spalling and ensure the longevity of hybrid ceramic ball bearings.

(4)

The surface fatigue spalling mechanism in hybrid ceramic ball bearings differs from traditional steel bearings, with ceramic particles generated by spalling being pressed into the raceway, causing irreversible damage in the form of micro-pits [33]. Therefore, it is necessary to reduce the hardness of the ceramic balls and apply surface treatments to the steel raceways to minimize the hardness difference between them. In addition, understanding the failure mechanisms of hybrid ceramic balls is also very important.

(5)

The development of hybrid ceramic ball bearings is severely restricted due to their high costs in both R&D and production, the limited market demand, and the high requirement for reliability.

6. Conclusions

(1)

Currently, the longevity of hybrid ceramic ball bearings is primarily determined using SKF’s GBLM general bearing life model. While these bearings typically last two times longer than standard all-steel bearings, issues such as double-impact vibration from surface cracks and fatigue spalling prevent hybrid ceramic ball bearings from reaching their full potential lifetime.

(2)

Various studies have delved into the mechanical and kinematic performance of hybrid ceramic ball bearings, highlighting their ability to achieve much higher limit speeds compared to all-steel bearings and showcasing superior stability.

(3)

Research on the temperature field of hybrid ceramic balls is currently limited, with a focus on thermal analysis. Findings suggest that hybrid ceramic ball bearings are best suited for high-speed and light-load applications, showing improved performance and greater resilience under high-temperature conditions.

(4)

For hybrid ceramic ball bearings, in high-speed conditions, oil lubrication offers superior performance with reduced resistance and provides a certain degree of prevention against abrasive wear. At medium to low speeds, grease lubrication can effectively mitigate fatigue failure. However, irrespective of the lubrication method used, the primary goal is to lower bearing temperature and mitigate surface fatigue spalling in hybrid ceramic ball bearings. Scholars are encouraged to concentrate on lubrication studies for these bearings, conducting thorough research into lubrication technology. This research is crucial for enhancing bearing performance and longevity, ultimately contributing to advancements in high-end equipment manufacturing and offering vital support to aerospace, manufacturing equipment, and related industries.