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I'm a watch collector with a lot of interest in material science. Most of the watches I own are frequently worn and wear marks over time.

These "marks" can be categorized into 2: 1. Scratches - look like very thin wear lines. 2. "Dings" - look like miniature bites, usually occur on sharp edges as a result of impact.

Modern watches come in a wide variety of materials: Gold, Platinum, 316L Stainless Steel, Ceramics, Titanium and other exotics... And I know that a harder material will do better against the first kind of damage, but about the second kind ("dings") ?

At first I thought that this type of failure is due to a material being brittle and I know that the harder the material - generally the more brittle it is... But I was surprised to find out that Ceramic (which is harder and therefore more brittle than 316L SS) performed better against Scratches AS WELL AS "Dings".

Can you explain this ? What material property helps avoid the second type of damage ?

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The reason is that both of those types of damage that you mention are in the same category: both are surface indentation.

Surface indentation is preventable with hardness. Ceramics in general are much harder. They are also much more brittle, yes. This is a different property, though: while hardness is the resistance against deformation, brittleness (the opposite of ductility) is the amount of deformation the material can take before failure (breaking).

  • Ceramics, being harder as well as more brittle, can take more beating but will fail (splinter) immediately, when the beating is too much. Like glass.
  • A metal on the other hand, being less hard and less brittle, can take less beating before giving in (deforming), but can accept such deformation for much longer.

In fact, the laboratory method for hardness measurements is to measure the indentation depth (size) caused by a pin/mandrill at a fixed pressure for a fixed duration. Look up the Vickers hardness test for example.

Geometry and thickness of materials may cause some variation in where you see damages. At edges, the pressures are relatively much larger and possibly the material weaker due to being thinner.

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I would suspect it is compressive strength, and ceramics typically have very high compressive strength. However, when it is exceeded, ceramics can shatter.

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