The core performance advantages of silicon nitride ceramics in high-end bearings

Silicon nitride (Si₃N₄) ceramic, with its unique physical and chemical properties, has become an ideal choice for high-end bearings operating under high speeds, high precision, and extreme operating conditions.

Core Application Advantages of Silicon Nitride Bearings

Excellent Physical Properties

Low Density, High Hardness: Silicon nitride (Si₃N₄) ceramic has a density of only approximately 3.2 g/cm³, only about 40% of GCr15 bearing steel. With the same design volume, it significantly reduces centrifugal forces during high-speed rotation and suppresses vibration harmonics. Its Vickers hardness reaches 1500–1700 HV, 2.1–2.4 times that of GCr15, endowing rolling elements with excellent wear and fatigue resistance. Under oil lubrication conditions (paraffin oil with a kinematic viscosity of 68 mm²/s, oil temperature of 40°C, and linear speed ≤1 m/s) and a contact load of 100 MPa, the wear rate of silicon nitride rolling elements on steel/ceramic bearings is as low as 0.02 mm³/N·m (ASTM G99), an 80% reduction compared to steel bearings under the same operating conditions. Its self-lubricating properties even maintain a wear rate of less than 0.08 mm³/N·m even in oil-depleted conditions, providing dual lubrication support for high-speed, high-precision, and extreme operating conditions.

Vibration harmonics refer to vibration components in a vibration system that exist at frequencies that are integer multiples of the fundamental frequency, in addition to the fundamental frequency. In other words, if an object vibrates at a certain frequency (the fundamental frequency), its vibrations may also contain vibrations at frequencies that are double, triple, or even higher multiples of the fundamental frequency. These multiples are called harmonics.

High elastic modulus (typically 320 GPa): This provides excellent resistance to elastic deformation—the deformation under the same load is significantly lower than that of conventional bearing steel (approximately 200 GPa). This property directly supports the motion stability of precision machinery. For example, in high-speed ceramic bearings, the micron-level deformation control of silicon nitride rolling elements reduces rotational vibration and improves axis positioning accuracy (radial runout ≤ 0.1 μm).

Low thermal expansion coefficient: Dimensionally stable at high temperatures, it avoids the "shaft seizure" problem caused by thermal expansion, and is applicable over a temperature range of -100°C to 1200°C.

Silicon nitride has a coefficient of thermal expansion (CTE) of 2.8–3.2×10⁻⁶/°C (room temperature to 1000°C), significantly lower than that of metals (e.g., stainless steel, 17×10⁻⁶/°C) and alumina ceramics (8×10⁻⁶/°C).126 This property is the core foundation of its high-temperature dimensional stability.

Silicon nitride has a thermal expansion of 0.38 ± 0.02% (CTE = 3.2 × 10⁻⁶/°C, a temperature gradient of 1175°C) over a temperature range of 25°C to 1200°C. Even after 500 cycles, its dimensional fluctuation remains less than 0.3%, ensuring millimeter-level precision for high-temperature systems (e.g., a 100 mm component experiences a total expansion of ≈ 0.38 mm, with a cyclic deviation of less than 0.3 mm).

Shaft seizure: This refers to an abnormal condition between a shaft and a bearing, typically manifested as severe friction or seizure, resulting in the shaft becoming unable to rotate properly. This condition can severely impact the operation of mechanical equipment and may even cause damage.

Excellent Tribological Properties: Low Friction Coefficient: Under dry or high-temperature lubrication conditions, the friction coefficient of silicon nitride (typically 0.1-0.3) is significantly lower than that of bearing steel (approximately 0.5-0.8 under dry friction), effectively reducing energy loss and temperature rise. For example, under the same load (100N) and speed (3000r/min), frictional heat generation is approximately 80% of that of a steel bearing, making it suitable for applications sensitive to energy efficiency and temperature rise.

Self-lubricating: It maintains stable operation even when oil is scarce or lubrication is degraded. Under specific operating conditions (e.g., a load of 500N and a speed of 3000r/min), it can safely operate without noticeable wear for up to 5.5 minutes without oil (a typical reduction of less than 30 seconds for steel bearings), making it suitable for extreme scenarios where lubrication system failure is a possibility.

Adaptability to Extreme Environments

High-temperature resistance: High-purity silicon nitride ceramics exhibit excellent high-temperature stability under typical loads. Mechanical property degradation at 900°C is typically kept to less than 10%, making it considered essentially stable. Even at the more challenging 1000°C environment, the material maintains excellent structural integrity. While its core mechanical properties (such as strength) degrade (approximately 10%-20% below room temperature), they remain at a high level. This makes high-purity silicon nitride ceramics ideal for medium- and high-temperature applications. However, it should be noted that long-term performance may be affected by creep and oxidation at temperatures approaching or reaching 1000°C.

Corrosion Resistance: Silicon nitride ceramics exhibit a corrosion rate of less than 0.01 mm/year in dilute acid and alkaline solutions with a pH of 2–11 and temperatures ≤80°C (ASTM G31 standard). They are resistant to chemical media (such as 30% H₂SO₄) and marine salt spray environments (with a salt spray corrosion rate of only 0.002 mm/year, equivalent to 1/24 that of 316L stainless steel), making them particularly suitable for applications such as chemical reactors and offshore platform bearings. However, grain boundary dissolution may occur in high-temperature concentrated alkaline solutions (corrosion rate of 0.85 mm/year in 60% NaOH solution at 140°C) or strong oxidizing acids (such as concentrated H₃PO₄ at 150°C), requiring the following protection measures:

Surface Modification: CVD deposition of a 2–5 μm pyrolytic carbon coating (corrosion rate reduced to 0.02 mm/year)

Grain Boundary Engineering: Adding TiN/Al₂O₃ sintering aids improves grain boundary stability.

Operating Condition Monitoring: Alarms are triggered when the medium pH value deviation exceeds ±1.5 or the temperature exceeds the limit.

Electrical Insulation: Excellent insulation performance with a room-temperature volume resistivity of 10⁴-10⁶ Ω・cm and a power-frequency breakdown field strength ≥20 kV/mm. Under the 800V platform of new energy vehicles (shaft voltage ≤ 200 V) and conventional wind power operating conditions, it can block the shaft current path (leakage current <1 μA), completely preventing electrolytic corrosion damage to metal components. However, the following should be noted:

High-Temperature Attenuation: Resistivity drops to 10⁸-10¹⁰ at 150°C. Ω・cm (high-power motor operating conditions require verification)

Transient High Voltage: 10,000-volt transients, such as those caused by lightning strikes, require independent grounding protection.

Impurity Risk: Conductive contaminants (such as metal debris) may reduce surface insulation.

Long Life and High Reliability

Fatigue Life: Under high-speed, light-load conditions (DN > 1×10⁶ mm·r/min, P/C < 0.1), fatigue life (L₁₀) can reach 8–10 times that of a steel bearing of the same size. Fatigue damage manifests as progressive spalling originating from the subsurface (steel bearings often experience sudden fractures caused by surface defects). Monitoring vibration accelerations > 8 m/s² (4–8 kHz frequency band) or acoustic emission count rates > 1000 hits/s can provide 50–100 hours of advance warning, significantly improving system safety. However, it should be noted that under extreme overload conditions, there is still a <0.01% probability of brittle fracture, requiring a load threshold (contact stress ≤ 4 GPa).

High-speed, low-load operating conditions: This refers to the situation where mechanical equipment operates at high speeds but with low loads. This is common in components such as bearings in high-speed trains and has a significant impact on equipment performance, reliability, and maintenance. In these operating conditions, special attention should be paid to specific issues arising from high speed and low loads, such as bearing slip, lubrication, and vibration.

Progressive spalling: This refers to the gradual onset of surface or near-surface damage in materials under cyclic loading or mechanical contact, typically manifesting as the gradual peeling or shedding of material layers. This phenomenon is particularly common in rolling contact (e.g., bearings and gears) and sliding friction (e.g., brake discs). Progressive spalling is a multi-stage process involving plastic deformation, microcrack initiation and propagation, and ultimately material shedding.

Core Application Industries and Scenarios

New Energy Field

Wind Turbine Equipment: Silicon nitride ceramic bearings (hybrid) have been successfully applied in pitch control systems and generator bearings and are gradually being adopted in medium-load main shaft bearings (contact stress ≤ 2.5 GPa). Their resistance to salt spray corrosion (corrosion rate 0.002 mm/year) and high-cycle alternating stress (fatigue life 3-6 times that of steel bearings), combined with automatic lubrication systems, can support the wind turbine's 20-year design life. The global wind turbine ceramic ball market (primarily silicon nitride) is estimated to be worth approximately 1.12 billion yuan per year in 2024 and is expected to exceed 2 billion yuan by 2030.

New Energy Vehicles: Motor bearings must withstand high speeds (>16,000 rpm), and the alternating electromagnetic fields generated by motor operation can easily induce shaft currents. Silicon nitride ceramic bearings, with their excellent electrical insulation properties, can block shaft currents, effectively preventing electrical corrosion. Currently, hybrid ceramic bearings are used in drive motors of vehicles such as Tesla and Audi.

High-end Manufacturing and Precision Machinery

High-speed Machine Tools/Electric Spindle: Hybrid ceramic bearings featuring silicon nitride rolling elements and bearing steel rings support maximum speeds of DN values ≤ 1.5 × 10⁶ mm·r/min (e.g., 75,000 r/min for a φ20 mm bearing). This is achieved through:

60% reduction in centrifugal force → Temperature rise reduced from ΔT = 120°C to 60°C

30% increase in radial stiffness → Feed rate increased from 8 m/min to 16 m/min

20 dB reduction in vibration amplitude → Surface roughness Ra improved from 0.8 μm to 0.1 μm

When combined with ultra-fine oil-air lubrication (oil droplets ≤ 0.5 μm) and diamond tools, aluminum alloy finishing efficiency is increased by 5–10 times (actual measurement: Makino iQ500 machining time for an aircraft part was reduced from 45 minutes to 4.5 minutes).

Aerospace and Defense

Aircraft Engines / Turbocharger: Silicon nitride ceramics offer excellent high-temperature and high-pressure resistance:

Extreme Environment Resistance: Long-term resistance to temperatures of 1200°C (air) / 1400°C (inert gas), with thermal shock resistance up to 1000°C.

Fail-Safe Design: Self-lubrication ensures safe operation for >5 minutes after oil loss (depending on lubrication film regeneration temperature range of -200-1200°C).

Improved Power Efficiency: Density of 3.2 g/cm³ (41% of steel) reduces moment of inertia, resulting in a 40% increase in turbine response, a 5.5-8.2% increase in efficiency, and a 35-40% reduction in bearing system temperature rise.

Application Limitations: Impact loads >4.5 MPa must be avoided (to prevent brittle fracture), and the cost of complex components is 3-5 times that of alloys.

Medical and Specialty Equipment

Dental drill bearings: As core components of high-speed dental handpieces, they support ultra-high-speed operation at 400,000–450,000 rpm and offer a clinical service life of ≥6 months (up to 8 months with some optimized designs). Their low vibration characteristics (noise ≤ 65 dB) significantly reduce patient pain during treatment. These bearings utilize a hybrid ceramic structure: 440C stainless steel rings, silicon nitride (Si₃N₄) ceramic balls, and a polyimide retainer. The ceramic balls' lightweight design (density of 3.2 g/cm³, 60% lower than steel balls) and ultra-high wear resistance (Vickers hardness of 1500–2000 HV, five times the wear resistance of steel) synergistically enhance device performance.

Chemical and Energy

Nuclear Power Equipment: Silicon nitride ceramics boast excellent radiation stability, maintaining stable material structure and performance in nuclear radiation environments. Their covalently bonded crystal structure resists lattice distortion caused by neutron irradiation (volume expansion <0.2% at a dose of 10⁷ Gy). Furthermore, the SiO₂ oxide layer formed at high temperatures dynamically repairs microcracks, effectively inhibiting corrosion and creep. This property makes them suitable for critical components such as bearings in nuclear power cooling pumps. They can withstand temperatures exceeding 300°C, pressures of 15 MPa, and long-term corrosion from coolants such as boron-containing water and liquid sodium. Furthermore, their self-lubricating properties (friction coefficient <0.1) enable long-life, lubrication-free operation (over 12 years of trouble-free operation), significantly improving the reliability and safety of nuclear power equipment.