Spalling Failure in Bearings and Gears: Mechanisms, Causes, and Prevention

Spalling is the dominant terminal failure mode of rolling contact fatigue (RCF) in bearings and gears, appearing as flakes of material separating from a loaded raceway or tooth flank along a subsurface crack. This guide covers the Hertzian stress mechanics behind spall nucleation, the metallurgical role of inclusions and case depth, and the practical levers available to engineers to extend contact fatigue life.

What Is Spalling?

Spalling is the fracture-driven separation of a discrete flake of material, typically 0.1 mm to several millimetres thick, from a rolling or meshing contact surface. It is the macroscopic, load-bearing-capacity-ending stage of rolling contact fatigue (RCF), and in bearings and gears it is functionally distinct from the shallower, more gradual damage modes of micropitting and pitting, although one can transition into the other. Spalling is addressed alongside related crack-growth phenomena discussed in our guide to fatigue S-N behaviour and the fracture mechanics framework in our Paris Law fatigue crack growth article.

Mechanisms: Subsurface-Initiated vs. Surface-Initiated Spalling

Two competing but related crack-initiation pathways drive spalling. Distinguishing between them is central to root-cause failure analysis.

Subsurface-Initiated Spalling

Under Hertzian rolling contact, the maximum orthogonal (reversing) shear stress occurs not at the surface but at a depth of roughly 0.4 to 0.5 times the contact semi-width, b. In clean, well-lubricated, well-aligned bearings this subsurface plane is historically regarded as the classical origin of RCF spalling. Cracks nucleate at non-metallic inclusions, principally oxides and manganese sulphides, that sit at or near this depth. Cyclic shear around the inclusion produces localized plastic strain that transforms the surrounding martensite into a characteristic butterfly-shaped, white-etching microstructure visible in metallographic sections. The crack then grows along the shear plane and eventually turns toward the surface, releasing a spall when it breaks through.

Surface-Initiated Spalling

In practice, the majority of highly loaded bearing failures are not purely subsurface. Surface stress risers, including debris dents, machining marks, insufficient elastohydrodynamic (EHL) film thickness, and asperity-level contact, raise local stress at or just beneath the surface and nucleate micropits. These coalesce into macropits and, under continued cycling, propagate into full spalls. Because the crack path in this mode is shallower and driven by the interaction between asperity contact pressure and near-surface residual stress, spall depth and morphology differ visibly from the subsurface case, and the two are considered competing but interacting mechanisms rather than entirely separate phenomena.

Hertzian Contact Stress and Rolling Contact Fatigue

Spalling life is governed by the magnitude and cyclic reversal of stress under a rolling or partially sliding Hertzian contact. For two parallel cylinders (a simplified roller-raceway or gear-tooth-flank model), the maximum contact pressure and half-width are:

p_max = sqrt( (F' * E*) / (pi * R) )

b = sqrt( (4 * F' * R) / (pi * E*) )

1/E* = (1 - v1^2)/E1 + (1 - v2^2)/E2
1/R  = 1/R1 + 1/R2

where F’ is load per unit length, R is the effective (relative) radius of curvature, E* is the effective (reduced) modulus, and v is Poisson’s ratio for each body. The maximum orthogonal shear stress beneath the surface is approximately 0.30 to 0.33 times p_max, occurring near z ≈ 0.4b to 0.5b depending on the friction coefficient and whether pure rolling or rolling-with-slip is present. Every pass of the mating surface produces a full reversal of this shear stress at that depth, giving classical high-cycle fatigue loading with millions to billions of stress cycles accumulated over service life, conceptually related to the mechanics in our S-N fatigue guide.

Spalling vs. Pitting vs. Micropitting vs. Scuffing

Root Causes of Spalling

Steel Cleanliness and Inclusion Control

Rolling contact fatigue life correlates strongly with the size, density, and composition of non-metallic inclusions within the stressed volume rather than with bulk alloy chemistry alone. Vacuum degassing and electroslag remelting of bearing steels such as 52100 (100Cr6) target very low oxygen content, since larger oxide inclusions act as stronger stress concentrators and shorten L10 life disproportionately. Extreme-value statistics on the maximum expected inclusion size within the Hertzian-stressed volume are used industrially to predict scatter in fatigue life.

Lubrication and the Film Thickness Ratio

The specific film thickness ratio, lambda, compares the calculated EHL minimum film thickness to the composite RMS surface roughness of the two contacting bodies. When lambda exceeds roughly 2 to 3, full-film separation dominates and failure trends toward the slow, subsurface path. When lambda falls below about 1, asperities interact directly, raising local traction and near-surface stress and promoting rapid surface-initiated micropitting, pitting, and eventual spalling.

Misalignment, Edge Loading, and Crowning

Shaft misalignment or an undersized crown radius on rollers concentrates load at the ends of the line contact rather than distributing it evenly, producing a localized pressure spike well above the nominal Hertzian value. This edge-loading condition is a common practical driver of premature spalling in both cylindrical and tapered roller bearings, and is a recurring theme in the metallurgical failure analysis workflow applied across rotating equipment.

Surface Finish and Residual Stress

Grinding marks, honing lay direction, and near-surface residual stress state all influence whether a surface defect nucleates a fatigue crack. Compressive residual stress from shot peening or hard turning retards crack initiation and closure, while tensile residual stress from grinding burn or improper heat treatment accelerates it.

Case Depth in Carburized and Nitrided Gears

In case-hardened gears, if the effective case depth is too shallow relative to the applied contact stress, the subsurface shear stress maximum falls below the hard case into the softer core. Because the core lacks the strength to sustain the cyclic shear, a large subcase spall (case crushing) can separate the case layer from the core in a single catastrophic event rather than developing as gradual surface pitting. Correct case depth specification against the calculated shear stress profile is therefore a core design check, closely related to the hardenability principles in our Jominy end-quench hardenability guide.

Metallurgy of Bearing and Gear Steels in Spalling Resistance

52100 (equivalent to 100Cr6 / SUJ2), a through-hardening 1 percent C, 1.5 percent Cr steel, remains the dominant bearing steel because its high carbon content supports a fully martensitic, high-hardness (58-64 HRC) microstructure after quenching and low-temperature tempering. For gears, case-carburizing or carbonitriding grades such as 8620, 4320, or 20MnCr5 build a hard, compressively stressed surface case over a tough, lower-carbon core, combining wear and contact-fatigue resistance at the surface with bending fatigue and impact resistance in the core, discussed further in our case hardening and precipitation strengthening content.

Retained austenite plays a nuanced role. A moderate fraction, typically 5-15 percent in through-hardened bearing steel and higher in some carbonitrided cases, can relieve local stress concentration around inclusions through strain-induced martensitic transformation, extending fatigue life. Excess retained austenite, however, reduces hardness and promotes dimensional instability under service temperature and stress cycling, so it is controlled tightly through austenitizing temperature, quench severity, and tempering practice, related to the phase behaviour covered in our martensite formation article.

Lundberg-Palmgren basic life relation (simplified form)

L10  is proportional to  ( C / P )^p

where:
  L10 = basic rating life (10% failure probability), millions of revolutions
  C   = basic dynamic load rating (bearing-specific)
  P   = equivalent dynamic bearing load
  p   = 3 for ball bearings, 10/3 for roller bearings

Industrial Significance

Spalling failures are a leading cause of unplanned downtime in gearboxes, wind turbine main bearings, rolling mill stands, and automotive transmissions. Because a spall generates debris and vibration, it frequently triggers secondary damage, cage fracture, or cascading tooth failure if not detected early through vibration monitoring or oil debris analysis. Correct steel selection, EHL lubrication design, and case-depth specification against calculated contact stress remain the primary engineering levers for extending component life between overhauls.

Diagnosis and Failure Analysis Approach

• Visual and stereomicroscopic examination of spall morphology, depth, and orientation relative to the rolling direction.
• Sectioning through the spall and adjacent unaffected material for metallographic examination, looking for butterfly formations, inclusion chemistry (via SEM-EDS), and case depth profile.
• Hardness traverse from surface to core to confirm case depth and rule out soft spots or decarburization.
• Calculation of the theoretical Hertzian contact pressure and subsurface shear stress profile for the actual operating load, compared against the observed crack depth.
• Review of lubrication history, filtration practice, and alignment records to establish whether the surface- or subsurface-initiated pathway is more likely.

Prevention and Mitigation Strategy

• Specify clean, low-oxygen bearing and gear steel with documented inclusion rating (per ASTM E45 or ISO 4967) appropriate to the load and life target.
• Design crowning and verify shaft alignment to avoid edge loading in line-contact elements.
• Maintain lubricant film thickness ratio lambda above roughly 1.5-2 through correct viscosity grade, operating temperature, and surface finish.
• Filter lubricant to control particle contamination below the critical size that causes denting-induced surface spalling.
• Specify effective case depth against the calculated subsurface shear stress profile, not by rule-of-thumb alone, for carburized and nitrided gears.
• Consider superfinishing, shot peening, or coatings where marginal lubrication or high specific loads are unavoidable.

Frequently Asked Questions

Recommended Reference Reading

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