Same word, different worlds
Spalling
Spalling turns up in 6 fields: tribology, thermal spray, refractories, gears, concrete, waterproofing. Here is what it means to the people in each one.
- 01TbTribology & LubricationSpalling in Tribology & Lubrication
Flaking of large chunks from a bearing race or gear tooth after fatigue cracks below the surface link up. Advanced rolling contact fatigue in which cracks starting just below the surface, near the depth of maximum shear stress, join up and release flakes of metal, leaving rough, deep cavities. In rolling bearings it is the classic end-of-life failure, often brought on early by contamination dents, misalignment or poor lubrication. It throws large fatigue particles into the oil, high PQ readings and clear bearing defect frequencies in vibration data.
On the job: Outer race is spalled across a third of the load zone, that bearing comes out this weekend.
- 04TsThermal SpraySpalling in Thermal Spray
Flakes or chunks of coating breaking away from the surface, typically under thermal cycling or impact. Failure in which pieces of coating detach, leaving patches of bare bond coat or substrate. In thermal barriers it is the end-of-life mode, driven by TGO growth, ceramic sintering and thermal cycling stresses. In wear coatings it follows contact fatigue, impact or excessive coating thickness. Spall location and the layer exposed are the first clues in a failure investigation.
On the job: TBC spalling on the leading edge down to bond coat, those blades come out at the next outage.
- 07RfRefractories & Furnace LiningsSpalling in Refractories & Furnace Linings
Cracking off of pieces or layers from the hot face of a refractory lining. Loss of refractory as fragments or slabs break away from the hot face. Thermal spalling comes from steep or sudden temperature changes; mechanical or pinch spalling from compressive stress when expanding brick has no room; structural spalling, or peeling, from a slag- or vapor-infiltrated layer that has different properties from the brick behind it. Reading the spall shape and depth usually tells you which cause it was.
On the job: Spalling on the hot face is coming off in clean slabs about an inch thick, that's structural, the slag's soaked in.
- 10GrGear EngineeringSpalling in Gear Engineering
Large, irregular flakes broken out of a tooth surface, usually where pits have grown and joined. Surface damage in which pieces of metal much larger than single pits break away, leaving shallow, irregular craters. It often grows from progressive pitting as pits link up, and on case-hardened gears it can start deeper, near the case-core boundary, when the case is too shallow for the load. Spalled teeth throw debris into the oil, which damages bearings and other gears.
On the job: Pits have joined up into spalling on the pinion, there's flakes the size of a fingernail on the magnet.
- 23CnConcrete & CementSpalling in Concrete & Cement
Chunks or flakes breaking away from a concrete surface or edge, often from corroding rebar beneath. When embedded steel rusts, the corrosion products occupy several times the volume of the original metal and push off the cover concrete in slabs, columns and balconies. Spalls also come from freeze-thaw damage, impact at joint edges, fire and trapped moisture. Repair means chipping back to sound concrete and behind the corroded bar, cleaning or replacing the steel and patching.
On the job: Garage level two has spalling at every column base, salt's been running down to that rebar for years.
- 26SwSealants & WaterproofingSpalling in Sealants & Waterproofing
Concrete, brick or stone breaking off in flakes or chunks at the surface. In concrete, spalls are usually caused by corroding reinforcing steel swelling as it rusts and pushing the cover off. In masonry, freeze-thaw cycling of saturated units, salts and hard mortar that will not give cause the face to pop off. Spalling is both a leak symptom and a falling hazard, so facade inspections map and tag every loose piece.
On the job: Balcony edge is spalling, rebar's showing, sound it with a hammer and tag everything loose before we drop.