What a block-on-ring test tells you
The test ranks material pairs by their resistance to sliding wear under a line contact, and records how friction develops as the contact wears in. It is widely used to compare metals, polymers and coatings for parts that slide in dry or lubricated conditions, and to show how much a lubricant or surface treatment changes friction and wear.
How the test works
- The ring is mounted on the rotating spindle; the block is held in a fixture above it.
- A controlled normal load presses the block onto the outer surface of the ring, forming a line contact across the ring width.
- The ring rotates at the set speed (up to 5,000 rpm) for a set number of revolutions or sliding distance. Friction is measured continuously, with depth and acoustic emission if fitted.
- As the test proceeds, the line contact widens into a curved scar on the block whose shape follows the ring. After the test the scar is measured in 3D to obtain its volume.
Why line contact matters
A ball on a flat starts at a point, with very high initial contact pressure that falls quickly as a scar forms. A block on a ring starts along a line, with the load spread across the ring width. That makes the configuration representative of many real components: cam followers, gear flanks, journal and shaft surfaces, sliding guides. It also gives a large, easily measured wear scar, which is part of why block-on-ring testing has long been used to rank materials.
Dry versus lubricated
Lubrication can change block-on-ring results by orders of magnitude. In NANOVEA’s ASTM G77 study on an O1 tool-steel block against a hardened 4620 steel ring:
- Dry (72 rpm, 200 revolutions): COF ~0.8 after a short run-in, and a 9.45 mm³ wear scar. Oxidised metallic debris promoted severe three-body abrasion.
- Lubricated with heavy mineral oil (197 rpm, 500,000 revolutions): COF 0.09 throughout, and a scar of only 0.03 mm³.
The lubricated test ran 2,500 times more revolutions yet produced roughly 1/300 of the worn volume. Profilometry of the rings also showed the significant surface roughening caused by dry sliding.
Measuring the wear scar
The block’s scar is measured by chromatic confocal 3D non-contact profilometry. Volume comes directly from the 3D map rather than from a calculation based on scar width, so irregular scars are measured accurately. The specific wear rate follows from K = V / (F × s). The same profiler can measure the ring surface after the test, with its cylindrical form removed, to quantify roughening or material transfer.
