Bearing Materials: 100Cr6 Steel, 440C Stainless, Si₃N₄ Ceramic & Polymer

Bearing material is the single most important factor determining a bearing's service life, load capacity, speed limit, temperature range, and corrosion resistance — because each material has distinct microstructure, hardness, and mechanical-physical-chemical properties suited to specific operating conditions.

Approximately 95% of industrial bearings worldwide use 100Cr6 bearing steel as the standard material. The remaining 5% — high-temperature M50 steel, stainless 440C and 316L, ceramic Si₃N₄ and ZrO₂, polymer PEEK and PTFE — serve applications where 100Cr6 falls short: jet engines above 300°C, food processing requiring absolute hygiene, CNC spindles above 40,000 rpm, or acid environments that destroy all metals. Understanding each material's characteristics is foundational engineering knowledge before selecting the right bearing construction. Material codes also appear as suffixes in bearing designations.

Standard Bearing Steel — 100Cr6

Chemical composition and international equivalents

100Cr6 bearing steel (EN ISO 683-17) is the standard material for over 95% of bearings manufactured worldwide — from the smallest deep groove ball bearing 608 to self-aligning spherical roller bearings 241/600 weighing several tons. International equivalents: AISI 52100 (USA), SUJ2 (Japan, JIS G4805), ShKh15 (Russia, GOST 801-78).

Nominal chemical composition of 100Cr6:

Oxygen content is the defining quality metric for bearing steel. Steel with oxygen ≤ 10 ppm (vacuum degassed) delivers contact fatigue life 3–5 times higher than steel at 20 ppm. SKF, FAG, NSK, NTN, and ZVL all specify oxygen ≤ 15 ppm per EN ISO 683-17:2023. SKF developed a proprietary "SKF Clean Steel" grade with oxygen below 8 ppm for the Explorer line.

Heat treatment and hardness

After heat treatment, 100Cr6 achieves a tempered martensite structure with uniformly distributed spheroidized carbides, reaching 58–64 HRC. Standard process:

1. Austenitizing: heating to 830–850°C, holding 20–30 minutes
2. Quenching: rapid cooling in oil at 60–80°C or molten salt at 180–220°C (martempering)
3. Tempering: holding at 150–180°C for 2–4 hours → 60–62 HRC standard; or 220–250°C for stabilized grade rated to 200°C (suffix S0/SN on SKF, S1 on FAG)
4. Deep cooling: –80°C to –196°C to convert retained austenite to martensite, improving dimensional stability

Through-hardening and case-hardening

Through-hardening is the standard method for 100Cr6: the entire cross-section achieves uniform hardness of 58–64 HRC. Suitable for bearings with outside diameters up to 300 mm — covering the vast majority of bearings produced worldwide.

Case-hardening (case-carburizing) uses 18CrNiMo7-6 or 20MnCr5 steels for large bearings: spherical roller bearings with outside diameters above 400 mm, large tapered roller bearings for steel rolling mills, and bearings subject to impact loads. The surface reaches 58–62 HRC through a carburized layer 2–5 mm deep; the core retains 30–40 HRC with high impact toughness. SKF uses the suffix "E" for some case-carburized spherical roller bearings.

Why 100Cr6 dominates

Three core reasons. First, optimal mechanical property balance: 60–62 HRC provides excellent contact fatigue resistance — precisely the hardness range where Hertzian stress distribution is most effective. Second, good machinability: spheroidized annealed at 180–210 HB, easy to turn, mill, and grind, keeping production costs low. Third, low-alloy steel with widely available raw materials and manufacturing processes refined over more than 100 years.

High-Temperature Steel — M50

M50 steel (AMS 6491; European equivalent 80MoCrV42-16) is the standard material for bearings operating above 150°C — particularly in jet engine mainshaft bearings, industrial gas turbines, and hot gas generators.

100Cr6 begins losing hardness significantly above 150°C as Cr₇C₃ carbides coarsen and martensite decomposes. At 250°C, 100Cr6 retains only 52–54 HRC — insufficient for bearing operation. M50 maintains 60–62 HRC up to 315°C thanks to thermally stable MC carbides, degrading significantly only above 400°C. According to NSK Technical Report CAT. No. E1266 (2021), M50 and its carburized variant M50NiL are the standard materials for mainshaft bearings in GE CF6, Pratt & Whitney PW4000, and Rolls-Royce Trent engines.

Dimensional stability is the second critical factor. At high temperatures, retained austenite in 100Cr6 transforms to martensite, causing dimensional growth — altering internal clearance and potentially seizing the bearing. M50 receives specialized heat treatment (three tempering cycles × 2 hours at 540–560°C) to eliminate virtually all retained austenite, ensuring dimensional stability to 315°C.

M62 is a higher-carbon variant of M50 for extreme wear resistance. M50 costs 8–10 times more than 100Cr6 due to high alloy content and complex heat treatment; M62 costs 12–15 times more. Use only when actual operating temperature exceeds 150°C continuously and 100Cr6 stabilized S0/S1 is insufficient.

At a cement plant in Quang Ninh province, a 22324 EK bearing on a clinker grinding shaft operated at 130°C continuously, achieving 22,000 hours with standard 100Cr6 S0 stabilization. When capacity expansion pushed operating temperature to 180°C, M50 was considered but at 10× the cost. The practical solution: 100Cr6 with S1 stabilization (rated to 200°C, SKF designation 22324 E/C3/S1) combined with circulating oil through a cooler — keeping bearing temperature below 150°C and saving 85% versus the M50 option.

Stainless Steel — 440C and 316L

440C (X105CrMo17) — the most common choice

440C steel is the standard material for corrosion-resistant bearings. Its 16–18% chromium content forms a passive Cr₂O₃ oxide layer on the surface, protecting the bearing in moist, wet, steam, and mildly chemical environments.

The primary limitation of 440C: lower hardness than 100Cr6 means lower permissible loads and shorter fatigue life. At the same size, a 440C bearing handles only 70–80% of the load of a 100Cr6 bearing. Do not specify 440C unless there is a genuine corrosion requirement — it adds cost with no performance benefit in dry environments.

Per FAG/Schaeffler Technical Pocket Guide TPI 200 (2023), 440C fits these applications: food and beverage machinery, medical and pharmaceutical equipment, outdoor and marine environments, water pumps, and dilute chemical pumps.

X65Cr14 — food-grade steel

X65Cr14 (AISI 440A variant) with 0.60–0.70% carbon and 13–15% chromium meets FDA 21 CFR 175-178 and EU Regulation 1935/2004 for food-contact materials. Hardness 54–56 HRC — lower than 440C but with better corrosion resistance in food acids (citric acid, lactic acid, acetic acid).

Applications: bearings in bottle washing machines, food conveyor belts, packaging machinery, and dairy processing equipment — where bearings contact food directly or indirectly and must withstand CIP (Clean-In-Place) washing with 2–4% NaOH and 1–2% nitric acid.

At a seafood processing plant in Ca Mau province, standard 100Cr6 bearings 6205 on a shrimp washing conveyor corroded within 3 months despite adequate lubrication — the 3% saltwater environment combined with citric acid at pH 4.5 destroyed the raceway surface. Switching to SKF W 6205-2Z (440C stainless) extended service life to 14 months. Bearing cost increased 2.5× but downtime cost dropped 75%.

316L (X2CrNiMo17-12-2) — extreme corrosion resistance

316L is an austenitic stainless steel with 16–18% Cr, 10–14% Ni, and 2–3% Mo. Corrosion resistance far exceeds 440C — especially in dilute hydrochloric acid, seawater, and chloride environments. However, 316L cannot be hardened by quenching: it achieves only 25 HRC in cold-worked condition, versus 56–58 HRC for 440C.

The consequence: 316L bearings carry very low loads and suit only light-load, low-speed applications where corrosion resistance is the overriding requirement. Typical uses: chemical pump valves, wastewater treatment equipment, and measurement sensors in acid storage tanks.

Ceramic Si₃N₄ — High Speed and Electrical Isolation

Si₃N₄ material properties

Silicon nitride (Si₃N₄) is the advanced ceramic material that has redefined bearing performance limits over the past 30 years. Per CeramTec GmbH — Silicon Nitride for Rolling Bearings (2022):

Hybrid bearings and full ceramic bearings

Two configurations use Si₃N₄:

Hybrid bearings: Si₃N₄ rolling elements + steel inner and outer rings (100Cr6, M50, or 440C). This configuration accounts for over 95% of the ceramic bearing market — steel rings allow standard interference fit mounting, resist hoop stress better than ceramic rings, and cost significantly less.

Per SKF Rolling Bearings Catalogue PUB BU/P1 10000/2 EN (2018), hybrid bearings versus all-steel:

• Limiting speed increased 30–40%: 60% lighter balls reduce centrifugal force substantially
• Service life increased 2–5 times: no cold welding, reduced adhesive wear, better Hertzian stress distribution
• Complete electrical insulation: prevents shaft current damage — critical for VFD-driven motors
• Lower starting torque and heat generation at high speed
• Extended grease service life due to lower operating temperature

Full ceramic bearings: all rolling elements, inner ring, and outer ring made from Si₃N₄. Advantages: complete corrosion resistance, absolute electrical insulation, operation in vacuum and at extreme temperatures. Disadvantages: brittle (fracture toughness K_IC ≈ 6 MPa√m, versus 15–20 for steel), cannot withstand impact loads, and difficult to mount because ceramic does not deform elastically like steel.

Cost and economically justified applications

Hybrid Si₃N₄ bearings cost approximately 3–5 times more than 100Cr6 steel bearings of the same size. Full ceramic Si₃N₄ costs 10–20 times more.

Economically justified applications for hybrid Si₃N₄:

• High-speed CNC spindles (ndm > 1,000,000): SKF S7014 CE/HCP4A, FAG HCB71914-C-T-P4S (suffix HC = Hybrid Ceramic)
• VFD-driven electric motors preventing shaft current erosion: SKF 6205/HC5, FAG HCS7005-C-T-P4S-UL
• Compressors and turbines: reduced lubrication requirements, high-temperature capability
• Vacuum pumps: no lubrication required, zero contamination

At a precision machining shop in Hanoi, a 6308 hybrid bearing (Si₃N₄ balls + 100Cr6 rings) installed in a 30,000 rpm CNC spindle achieved 3.5× longer service life than standard 6308 C3 steel bearings, while reducing bearing operating temperature by 15–20°C at maximum speed.

Ceramic ZrO₂ — Extreme Chemical Resistance

Zirconia (ZrO₂) — specifically Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) — is the second ceramic material used in bearings, with distinctly different characteristics and applications from Si₃N₄.

The core advantage of ZrO₂ over Si₃N₄: higher fracture toughness — less susceptible to sudden cracking under impact loads. The transformation toughening mechanism (tetragonal-to-monoclinic phase transformation at crack tips creates compressive stress that arrests crack propagation) is unique to ZrO₂.

ZrO₂ bearings are used almost exclusively as full ceramic in: extreme corrosive environments (HCl, HF, concentrated H₂SO₄, NaOH, KOH) where even 316L stainless steel corrodes; medical equipment including MRI (non-magnetic, biocompatible); food and pharmaceutical processing (no metal contamination, CIP-compatible); and marine seawater pumps and offshore drilling equipment. Because ZrO₂ is nearly as heavy as steel (6.0 versus 7.8 g/cm³), it offers no speed advantage — do not specify it for high-speed applications.

Polymer PEEK and PTFE — Self-Lubricating, Electrically Insulating

PEEK (Polyether Ether Ketone)

PEEK is the highest-performance polymer used for bearings:

• Continuous temperature rating to 250°C (short-term peak 300°C)
• Resists virtually all acids, alkalis, and organic solvents — attacked only by hot concentrated sulfuric acid and select strong oxidizing acids
• Coefficient of friction 0.35–0.45 dry, unlubricated — lower than steel-on-steel (0.6–0.8)
• Electrically insulating and non-magnetic: suited for MRI, electronics, and strong magnetic field environments
• Density 1.3 g/cm³ — one-sixth that of steel

PEEK bearings are typically reinforced with carbon fiber (CF-PEEK, 30% carbon fiber) or glass fiber to increase stiffness and load capacity. Primary limitation: permissible surface pressure below 5 MPa at room temperature (dropping to 2–3 MPa at 200°C), and limiting speed 50–70% lower than metal bearings.

PTFE (Polytetrafluoroethylene)

PTFE (Teflon) has the lowest coefficient of friction of any polymer: 0.04–0.10 — approaching oil-lubricated bearing levels. Near-absolute chemical resistance — resists all acids, alkalis, and solvents (attacked only by molten alkali metals and fluorine). Continuous temperature rating to 260°C.

Disadvantage: PTFE is very soft and creeps under sustained load, so pure PTFE bearings carry very low loads. PTFE is typically used as a coating or composite (PTFE + glass fiber + graphite) to improve mechanical properties.

Phenolic resin and PA66

Cotton-fabric reinforced phenolic resin (Textolite, Bakelite) is the traditional material for self-lubricating bearings in water pumps, hydroelectric turbines, and industrial washing machines. Good water resistance, self-lubricating when wet, low cost. Limitations: maximum temperature 120°C, cannot withstand organic solvents.

PA66 (nylon 6,6) is the most common material for bearing cages — lightweight, elastic, low cost. PA66+GF25 (25% glass fiber reinforced) is the standard for deep groove ball bearing cages in small-to-medium sizes.

Cage Materials and Surface Treatments

Cage (retainer) materials

The cage maintains equal spacing between rolling elements in the bearing structure, prevents ball-to-ball contact, and distributes lubricant. Cage material directly affects limiting speed and lubrication service life.

Pressed steel — suffix J (SKF), C (FAG): low-carbon steel DC01/DC04, lowest cost, rated to 300°C. Heavy — high centrifugal force at elevated speeds.

Machined brass — suffix M (SKF), MA (FAG): CuZn39Pb3 or CuSn8 alloy, CNC-machined. Rated to 250°C, mildly self-lubricating, suited for high-speed and heavy-load applications. Standard for large bearings such as 22220 EK/C3 (d=100, D=180, B=46). Limitation: incompatible with sulfur-containing EP lubricant additives.

PA66+GF25 — suffix TN9 (SKF), TVP (FAG): lightest of all common cage types, low friction, good grease retention. Standard for small-to-medium deep groove ball bearings. Maximum temperature 120°C; absorbs moisture causing slight dimensional change.

PEEK — suffix P (FAG): used for super-precision CNC spindle bearings. Rated to 250°C, non-hygroscopic, compatible with all lubricant types. Highest cost.

Phenolic resin — suffix T (SKF): cotton fabric impregnated with phenolic resin, lathe-machined. Self-lubricating, lightweight, good impact resistance. Used for high-speed cylindrical roller bearings (NU, NJ series). Maximum temperature 120°C.

Surface coatings and treatments

Beyond core heat treatment, surface coatings extend application limits without changing the base material:

Black oxide: Fe₃O₄ layer 1–2 µm thick created by chemical reaction at 140–150°C. Improves oil retention, reduces running-in friction, and provides mild corrosion protection. SKF applies it broadly on standard deep groove ball bearings.

DLC coating (Diamond-Like Carbon): 1–4 µm carbon coating deposited by PVD/PECVD. Hardness 2,000–5,000 HV, coefficient of friction 0.05–0.15, extreme wear resistance. SKF NoWear and FAG Durotect CK are DLC products for automotive gearbox and wind turbine gearbox bearings where mixed lubrication causes surface wear.

SKF INSOCOAT (Al₂O₃ plasma spray): 100–300 µm alumina ceramic layer plasma-sprayed on the outer or inner ring. Provides complete electrical insulation (resistance > 50 megaohms at 1,000 VDC), preventing shaft current erosion in VFD-driven motors. Costs approximately 50% less than hybrid ceramic bearings for this application.

Selecting Bearing Material for Specific Applications

Material selection is not a search for the "best" material — it is matching material properties to actual operating conditions. Four deciding questions:

1. Operating temperature?

• < 120°C → standard 100Cr6
• 120–200°C → 100Cr6 stabilized (S0/S1)
• 200–315°C → M50

• 315°C → full ceramic

2. Corrosive environment?

• None → 100Cr6
• Water or moisture → 440C
• Mild acid or alkali → 316L
• Strong acid or alkali → full ceramic ZrO₂ or polymer

3. Required speed?

• ndm < 500,000 → 100Cr6 + standard grease
• ndm 500,000–1,000,000 → 100Cr6 P5 + oil
• ndm > 1,000,000 → hybrid Si₃N₄ P4 + oil-air

For evaluating material impact on surface fatigue life, refer to bearing life calculation.

4. Shaft current present?

• No → standard steel bearing
• Yes → hybrid Si₃N₄ or INSOCOAT ceramic-coated bearing

Decision matrix

Browse matching products at ball bearings, spherical roller bearings, and tapered roller bearings.

ZVL in the materials context

ZVL uses 100Cr6 steel per EN ISO 683-17 — the same composition and quality standard as SKF, FAG, NSK, and NTN. Oxygen content of 15 ppm or less, microstructure meeting SEP 1520. At the same size and precision class, ZVL bearings have contact fatigue life equivalent to other tier-1 bearings of the same type.

The differences between ZVL and premium-tier manufacturers lie in proprietary heat treatment processes, finish machining tolerances (surface roughness Ra, waviness, roundness), and internal quality control acceptance limits — not in steel chemistry. For standard industrial applications — electric motors, pumps, fans, conveyors, gearboxes — ZVL's 100Cr6 material fully meets technical and service life requirements at pricing significantly more competitive than SKF and FAG for the same bearing designation.