Experimental Study of the Service Performance of Full Ceramic Silicon Nitride Ball Bearings

Abstract

As the operating conditions of rolling bearings become increasingly demanding, traditional steel bearings can no longer fully meet the performance requirements of critical equipment. Silicon nitride full ceramic ball bearings, with intrinsic properties such as a low thermal expansion coefficient, low density, corrosion resistance, and wear resistance, offer significant advantages in extreme temperatures, high-speed operation, and harsh corrosive environments. As a result, they have become a key technical solution for the core transmission systems of high-end equipment. However, the dynamic evolution of their service performance under varying operating conditions—such as load and speed—remains insufficiently understood. This study systematically investigates the service performance evolution mechanism of silicon nitride full ceramic ball bearings under self-lubrication conditions. The key findings will provide a theoretical foundation for optimizing and regulating performance under extreme operating conditions.

1. Introduction

As the core transmission components in modern industrial equipment systems [1,2], bearing performance—encompassing friction torque, thermal characteristics, and vibration response—directly influences the operational efficiency of rotating machinery, including aerospace propulsion systems, precision instruments, and high-end CNC machine tools [3,4,5,6]. With increasingly demanding service environments, hybrid ceramic bearings, made of steel rings and ceramic rolling elements, outperform full steel bearings under challenging conditions such as insulation and boundary lubrication [7]. However, under extreme conditions such as vacuum, ultra-high/low temperatures, thermal shock, or lubrication failure, bearings must still maintain long-term reliability, presenting a significant challenge for both conventional metal bearings and hybrid ceramic bearings.

Thanks to their exceptional mechanical properties and chemical stability, ceramic materials are increasingly viewed as ideal bearing solutions for applications in aero-engines, high-speed machine tools, and next-generation energy drive systems [8,9,10,11,12]. Specifically, full ceramic ball bearings made from silicon nitride offer unique advantages, such as longer service life, outstanding wear and corrosion resistance, and a broad temperature adaptability range—attributes that are difficult to achieve with steel or hybrid bearings. Under self-lubrication conditions, where lubrication fails due to extreme temperatures, silicon nitride ceramic bearings demonstrate far superior performance compared to their metallic and hybrid counterparts [13].

It is noteworthy that, under extreme operating conditions, bearing failures account for approximately 30% of total failures in rotating machinery [14,15]. As demands for higher rotational speeds and precision increase, temperature rise becomes a critical factor that accelerates wear, causes accuracy loss, and triggers fatigue failure [16]. Additionally, bearing vibration signals, which directly indicate internal defects, are essential for fault diagnosis [17]. Given that most bearing heat generation results from internal frictional energy dissipation, and severe friction-induced wear leads to more than 30% of industrial productivity being spent on component replacement [18], the self-lubricating capabilities of silicon nitride ceramic bearings represent a significant engineering advantage. Therefore, studying their service performance under self-lubrication conditions is both theoretically and practically valuable.

Significant research has been conducted on the service performance of full ceramic bearings. For instance, Zhang et al. [19] developed a thermo-mechanical coupling model for ceramic motorized spindles to explore how thermal deformation affects bearing dynamics. They further analyzed the effects of temperature, preload, and rotational speed on vibration behavior. Based on kinematic and elastohydrodynamic lubrication theories, the Harris team [20] proposed a quasi-static mechanical analysis model for roller bearings and introduced a method for calculating the thermal generation rate of individual components. Sun et al. [21] established a vibration simulation model for full ceramic ball bearings, incorporating oil film stiffness, and investigated the effects of speed, load, and grease conditions on vibration characteristics. Under steady-state heat conduction assumptions, Cannell et al. [22] studied the temperature rise of rolling elements and developed a predictive model for contact surface temperature. Guo et al. [23] developed a dynamic model considering compressive stress on the inner ring of ceramic bearings, assessing how rotational speed and radial load direction affect the dynamic variation of bearing clearance. Vibration experiments on a steel shaft–ceramic bearing system showed that, at a constant load, higher rotational speeds shorten the time needed for the bearing–rotor system to reach thermal equilibrium, significantly reducing the RMS value of vibration acceleration. Conversely, with constant speed, an increased load leads to a more pronounced rise in vibration acceleration RMS values. Wu [24] proposed a dynamic model for silicon nitride full ceramic ball bearings with elastohydrodynamic lubrication effects, demonstrating that under constant load and speed, a reduction in lubricant viscosity due to elevated temperature results in increased vibration levels. Guo Jiancheng et al. [25] examined the lubrication characteristics of 6206-type silicon nitride bearings under varying loads. Their findings indicated that, under identical axial loads, the contact load between rolling elements and the outer ring exceeds that of the inner ring, and axial displacement increases linearly with axial load. Under self-lubrication conditions, bearing vibration initially decreases and then increases as axial load rises. Hanon [26,27] developed a correlation model linking internal bearing temperature to frictional heat generation, deriving quantitative relationships between thermal deformation and temperature gradients and proposing an improved frictional heat generation model for rolling bearings. Ionut Geonea [28] combined numerical simulations with experiments to investigate total friction torque in radial ball bearings, establishing a correlation with radial load. Niizeki [29] reviewed the development of ceramic bearings for extreme operating conditions—such as corrosive environments, vacuum, high temperature, absence of lubrication, and high failure risk—covering materials like silicon nitride, silicon carbide, and partially stabilized zirconia. Durability tests conducted in water, acidic solutions, high temperatures, and vacuum confirmed the superior performance of these bearings.

Silicon nitride full ceramic ball bearings, known for their high temperature resistance, corrosion resistance, and low density, show considerable promise in advanced equipment operating in high-speed, vacuum, and extreme environments. Although significant progress has been made in studying their service performance under oil and grease lubrication conditions, research on their behavior under self-lubrication remains limited. Comprehensive evaluations of their service performance under self-lubrication conditions have not yet been thoroughly conducted. Therefore, in-depth studies on their performance under self-lubrication are crucial for advancing their engineering applications and providing important theoretical support for enhancing bearing performance, extending service life, and improving related technologies.

2. Experimental Investigation on the Service Performance of Silicon Nitride Full Ceramic Ball Bearings

2.1. Experimental Setup

As shown in Figure 1, the GPM-30 bearing friction and wear testing machine, manufactured by Yihua Friction Testing Technology Co., Ltd. in Jinan, Shandong Province, China, was used in this experiment. The test rig primarily consists of a driven electric spindle, an axial load application system, a radial load application system, and a set of service performance measurement sensors. It enables the measurement of key service performance parameters, including the friction coefficient, frictional torque, temperature, and vibration characteristics of the bearing.

2.2. Experimental Bearings

In this study, P4-grade 7008 full ceramic ball bearings made of silicon nitride (Si₃N₄) were used to investigate their service performance under oil-free lubrication conditions. The structure of the test bearing is shown in Figure 2. The bearing was designed with a CN-class internal clearance, and its cage was outer-ring guided. Both the inner and outer rings, as well as the rolling elements, were manufactured from gas pressure sintered Si₃N₄ ceramic, while the cage was made of phenolic resin laminated fabric (commonly known as Bakelite).

The physical properties [6,7] of the Si₃N₄ ceramic are listed in

Table 1. Properties of silicon nitride ceramic materials.

Parameter	Taking Values
Density g·cm−3	3.20–3.30
Elastic modulus GPa	300–320
thermal expansion coefficient × 10−6 K	3.1–3.3
Poisson ratio	0.26
Hardness HV	1300–1800
bending strength MPa	800–1000
compressive strength MPa	2000–3500

, and the structural parameters of the test bearings are provided in

Table 2. Parameters of full ceramic silicon nitride bearing 7008.

Parameter	Taking Values
outside diameter D/mm	68
inside diameter d/mm	40
bearing width B/mm	15
Roller diameter Dw/mm	7.938
Inner channel diameter di/mm	47
Outer channel diameter De/mm	61
contact angle/°	15°
Inner raceway curvature coefficient fi	0.515
Curvature coefficient of outer raceway fo	0.525

.

2.3. Experimental Conditions

The experimental protocol is summarized in

Table 3. Test plan parameters table.

Radial Load Fr (N)	Rotational Speed ni (r/min)	Other
2000	6000/6500/7000/7500/8000	Axial load: 200 N
2000/2200/2400/2600/2800	6000

. During the tests, the bearing was subjected to a constant axial preload of 200 N, while the radial load and rotational speed were varied to investigate the influence of different operating conditions on the bearing’s service performance.

3. Experimental Results and Analysis

3.1. Overall Friction Coefficient of Silicon Nitride Full Ceramic Ball Bearings

In this study, the bearing was not lubricated with grease; instead, its lubrication relied entirely on the transfer film formed by the cage during service, achieving self-lubrication. The overall internal friction of silicon nitride full ceramic ball bearings mainly consists of differential sliding between the rolling elements and the raceway, normal spin friction of the rolling elements relative to the raceway, elastic hysteresis friction between the rolling elements and the raceway, and the friction between the rolling elements and the cage pockets. To measure the overall friction coefficient of the bearing, the torque method was employed. The calculation formula for the friction coefficient is as follows:

$$\mu ={\displaystyle \frac{2M}{d_m{F}_r}}$$

(1)

In the formula, M represents the overall frictional torque of the bearing, F_r denotes the radial load of the bearing, and d_m denotes the pitch diameter of the bearing.

Figure 3 shows the variation of the overall friction coefficient of silicon nitride full ceramic ball bearings under different radial loads, where the error bars in Figure 3b represent the standard deviation. As shown in Figure 3a, the overall friction coefficient exhibits a clear decreasing trend with increasing radial load. When the load reaches 2600 N, the friction coefficient tends to stabilize, with little further reduction observed.

The comparison of average overall friction coefficients in Figure 3b further confirms this trend. At a radial load of 2000 N, the average friction coefficient is 0.00615. When the load increases to 2200 N, the value drops to 0.00312, representing a 49.2% reduction. As the load continues to increase to 2400 N, 2600 N, and 2800 N, the average friction coefficients decrease to 0.00261, 0.000619, and 0.000676, respectively—corresponding to reductions of 57.5%, 89.9%, and 89.1% relative to the 2000 N case.

A comprehensive analysis indicates that when the radial load of the bearing increases from 2000 N to 2800 N, the friction coefficient decreases significantly by nearly one order of magnitude. This phenomenon can be attributed to the combined effects of an enlarged contact area, a reduced proportion of sliding, a diminished contribution from cage-related friction, and the surface self-stabilizing effect of ceramic materials under higher radial loads. This effect refers to the ability of ceramic rolling elements and raceways, under elevated contact stresses, to achieve a more uniform and stable contact state through microscopic elastic deformation and local surface rearrangement, thereby alleviating localized stress concentrations and microslip. Such a mechanism effectively mitigates the growth of frictional force with increasing load, ultimately leading to a sharp reduction in the friction coefficient and confirming the existence of the observed “tenfold decrease.”

Figure 4 shows the variation in the overall friction coefficient of the silicon nitride full ceramic ball bearing under different rotational speeds. As shown in Figure 4a, the overall friction coefficient increases first and then stabilizes with the increase in rotational speed, with only a slight overall variation. The rotational speed has little effect on the overall friction coefficient of the bearing.

The comparison of average overall friction coefficients in Figure 4b further supports this observation. When the spindle speed is 6000 r/min, the average friction coefficient is 0.00615. As the speed increases to 6500 r/min, the coefficient rises to 0.00749, reflecting a 21.7% increase. With further increases in speed to 7000 r/min, 7500 r/min, and 8000 r/min, the average friction coefficients become 0.00737, 0.00730, and 0.00700, corresponding to increases of 19.8%, 18.6%, and 13.8% compared to the baseline at 6000 r/min.

3.2. Service Temperature Range of Silicon Nitride Full Ceramic Ball Bearings

Figure 5 illustrates the evolution of the steady-state temperature of the silicon nitride full ceramic ball bearing under various operating conditions during service. As shown in Figure 5a, the bearing’s steady-state temperature increases significantly with increasing radial load. At a spindle speed of 6000 r/min, the temperature rises by 15.1% as the radial load increases from 2000 N to 2800 N. At spindle speeds of 6500 r/min, 7000 r/min, 7500 r/min, and 8000 r/min, the corresponding temperature increases over the same load range are 14.7%, 19.1%, 13.8%, and 18.9%, respectively.

Figure 5b further shows that temperature increases considerably with rotational speed. At a radial load of 2000 N, increasing the speed from 6000 r/min to 8000 r/min leads to a 25.2% temperature rise. When the radial load is increased to 2200 N, 2400 N, 2600 N, and 2800 N, the corresponding temperature rises over the same speed range are 25.8%, 29.1%, 30.3%, and 29.4%, respectively.

It is noteworthy that the fluctuation range of temperature rise under different radial loads is limited (25.2–30.3%), indicating that the effect of load variation on the temperature rise is relatively small. The initial ambient temperature in this experiment was 20 °C. Even under the most severe operating condition (2800 N, 8000 r/min), the steady-state bearing temperature reached 44.5 °C, which means that the maximum steady-state temperature rise of the bearing was only 24.5 °C. Specifically, under a radial load of 2000 N and a spindle speed of 8000 r/min, the temperature rise of the silicon nitride full ceramic bearing was 16.5 °C. Compared with the temperature rise of 18.5 °C reported by Sun et al. [30] for hybrid bearings under the same conditions, this indicates that silicon nitride full ceramic bearings, like hybrid bearings, also exhibit low-temperature-rise service performance.

Silicon nitride full ceramic ball bearings exhibit lower temperature rise compared with hybrid ceramic bearings, mainly owing to the advantages of their all-ceramic structure in terms of material compatibility and tribological properties. On the one hand, ceramic–ceramic contact avoids the thermal expansion mismatch between ceramic balls and steel rings found in hybrid bearings, resulting in a more uniform distribution of contact stress. On the other hand, silicon nitride possesses a low friction coefficient characteristic, which significantly reduces frictional power loss and local heat concentration. Consequently, the overall temperature rise remains at a lower level, highlighting the superior low-temperature-rise performance of full ceramic bearings under high-speed self-lubrication conditions [31].

3.3. Service Vibration Range of Silicon Nitride Full Ceramic Ball Bearings

Figure 6 presents the variation in the vibration velocity RMS value of the silicon nitride full ceramic ball bearing under different operating conditions. As shown in Figure 6a, the RMS value of vibration velocity increases significantly with increasing radial load. At a spindle speed of 6000 r/min, increasing the radial load from 2000 N to 2800 N results in a 139.2% increase in vibration velocity RMS. At spindle speeds of 6500 r/min, 7000 r/min, 7500 r/min, and 8000 r/min, the corresponding increases over the same load range are 160.7%, 63.6%, 81.8%, and 89.8%, respectively.

As shown in Figure 6b, the vibration velocity RMS value also increases markedly with rotational speed. At a radial load of 2000 N, increasing the speed from 6000 r/min to 8000 r/min leads to a 182.1% increase. When the radial load is increased to 2200 N, 2400 N, 2600 N, and 2800 N, the vibration increases over the same speed range are 158.3%, 124.4%, 124.1%, and 123.8%, respectively.

It is worth noting that, when the radial load exceeds 2400 N, the increase in vibration RMS tends to stabilize, ranging narrowly between 123.8% and 124.4%. In contrast, within the lower load range (2000–2200 N), a more pronounced load-dependent attenuation trend is observed, with the vibration increase decreasing from 182.1% to 158.3%.

The phenomenon that the vibration velocity RMS value of silicon nitride full ceramic ball bearings increases significantly with radial load and rotational speed is mainly attributed to the combined effects of friction-induced excitation and dynamic responses. As the radial load increases, the contact stiffness is enhanced and the friction force grows, thereby amplifying the vibration caused by micro-slip and stick-slip. It is noteworthy that, when the radial load exceeds 2400 N, the increase in vibration velocity RMS tends to stabilize (approximately 123.8–124.4%), whereas, in the lower load range (2000–2200 N), a pronounced load-dependent attenuation is observed (from 182.1% down to 158.3%). This phenomenon indicates that, in the high-load range, the contact state tends to become relatively stable, and the sensitivity of vibration growth to load variation decreases.

4. Conclusions

(1) The friction coefficient of the silicon nitride full ceramic ball bearing decreases significantly with increasing radial load and tends to stabilize when the load reaches 2600 N. Specifically, when the radial load increases from 2000 N to 2200 N, 2400 N, 2600 N, and 2800 N, the corresponding reductions in friction coefficient relative to the 2000 N baseline are 47.8%, 57.5%, 89.9%, and 89.1%, respectively. This suggests that 2600 N serves as a critical threshold for friction behavior stabilization.

(2) The friction coefficient exhibits a non-monotonic variation with increasing rotational speed, showing a trend of initial increase followed by a slight decrease. However, the overall fluctuation is limited (maximum variation < 21.7%), confirming that rotational speed has a relatively minor effect on the bearing’s frictional characteristics.

(3) The steady-state service temperature of the bearing increases significantly with the rise in radial load and rotational speed. It is noteworthy that, under the most severe operating condition (2000 N, 8000 r/min), the steady-state temperature rise is markedly lower than that of hybrid bearings, fully verifying the inherent low-temperature-rise service advantage of silicon nitride ceramic materials.

(4) The vibration velocity RMS value also increases sharply with rising radial load and rotational speed. When the load increases from 2000 N to 2800 N, the vibration rise ranges from 63.6% to 160.7%. Similarly, when the speed increases from 6000 r/min to 8000 r/min, the rise reaches 123.8% to 182.1%, indicating that dynamic operating conditions exert a strong excitation effect on the bearing’s vibration response.

Future work will further combine numerical simulations with long-term service tests to systematically reveal the full-life service mechanisms of silicon nitride full ceramic ball bearings under extreme operating conditions, thereby providing theoretical and technical support for their engineering applications and service life prediction.