DLC wear resistance

Question

I read that a thickness of one micrometer of DLC coating (which, if I understood correctly, shows similar properties to diamond), increases the lifespan of an item plated with it from a week to more than a year, while a thickness of two micrometers can increase it up to 85 years. enter link description hereI find this difficult to imagine, as we all know cases of jewelry plated with a thin layer of gold, which loses its luster sometimes and after several months of use. Besides, it has occurred to me that every time any object is touched, it loses billions of atoms. If this is the case, are the atoms in diamond and DLC so tightly bound together by bonds that the material is actually so resistant to any major weight loss?

Answer 1

Background

Films protect in two primary ways, chemically and mechanically. Chemical protection is based on the film being inert in the given chemical environment, whether gaseous or liquid. Studies of this phenomena happen in the fields of materials oxidation (gaseous) or corrosion (liquids). We consider two aspects: the potential to react (thermodynamics) and the rate to react (chemical dynamics). Mechanical protection is based on the wear resistance of the film in sliding contact with another surface. Studies of this phenomena happen are in a field called tribology. In developing wear rate expressions, we can consider the influence of factors such as competing chemical bond strengths or mechanical interlocking from roughness.

The surface of perfect, pure diamond is chemically inert and has a high hardness or low wear factor in mechanical sliding. Both properties arise from the nature of the chemical bonding, the sp3 hybridization of the carbon atoms in a diamond cubic lattice structure. The perfect flatness of the diamond surface is also significant in setting its low wear rate.

The surface of diamond-like carbon (DLC) is not perfect diamond. Chemically, it has varying degrees of dangling (unfulfilled) sp2 bonds. This contributes to an increase in chemical reactivity, especially against oxidation. Physically, DLC has varying degrees of roughness. This, and the degree of weaker sp2 versus sp3 bonding, contribute to an increase in wear.

When two perfectly flat surfaces touch and release, the loss of material (atoms) from one surface to the other occurs solely because the atoms in one surface are chemically attracted to the other by higher chemical bonding forces. This is a discussion of adhesion (comparing the chemical binding forces between two dissimilar surfaces) versus the discussion of cohesion (contacting and separating two surfaces of the same material).

In Practice

Relate this now to some of the comments you make.

... as we all know cases of jewelry plated with a thin layer of gold, which loses its luster sometimes and after several months of use.

Two issues are involved. First, gold is softer than DLC, so gold will wear faster than DLC. Secondly, gold as a thin film coating on a metal can (and often will) allow the underlying metal to diffuse through it. By example, the best coatings of gold are often prefaced by an underlying layer of platinum, because the platinum is less prone to diffuse through the gold.

... it has occurred to me that every time any object is touched, it loses billions of atoms.

You would be hard pressed to prove a valid basis for promoting this concept, let alone stating a number, in any scientific manner for any material under anything but the most ideal of circumstances. Basically, you would have to reverse engineer the design of a system (this type of skin touching this object's surface for this amount of time) that will give you any loss of material from the surface rather than from your skin, let alone "billions of atoms" from the surface. So, consider such an idea simply as gossip and let it go.

I find this difficult to imagine, ...

The protective properties of DLC coatings arise because a) they are more inert than the underlying material that they cover and b) they are mechanically harder (have a lower wear rate) than the materials that they cover.

... a thickness of one micrometer of DLC coating ... increases the lifespan of an item plated with it from a week to more than a year, while a thickness of two micrometers can increase it up to 85 years

Thicker coatings improve the chemical inertness. This is primarily because one aspect of DLC is that it has grain boundaries. Thinner coatings allow the oxidizing or corrosive chemicals to diffuse through to the underlying substrate in a faster rate, decreasing the inertness. Thicker coatings can also have inherently higher film stiffness, thereby being less prone to mechanical spalling during sliding contact.

Summary

Dispense with the idea that touching a surface is a realistically indicative way to measure chemical inertness or mechanical wear resistance of the surface. When you touch a chemically inert and physically hard surface, you will leave behind skin cells, surface oils, and viruses or bacteria from yourself; you will not take away atoms from that surface. A DLC surface is protective because the surface mimics the high chemical inertness and low mechanical wear behavior of diamond surfaces. Thick DLC coatings have a higher protective lifespan because they are chemically less prone to allow grain-boundary diffusion and mechanically more stable against spalling.

Answer 2

If properly applied (which is not easy) diamond-like coatings can be as hard as diamond itself, in which the carbon atoms are extremely tightly-bound to one another and hence form an extremely hard, abrasion-resistant film.

But if the diamond-like coating is applied on top of something softer than diamond (which means any material at all), it's possible for the material under the DLC to deform under load, causing the DLC to crack and flake off. This limits the applications of DLC.

FYI in the early days of DLC technology it was known as "diamond-like carbon" and was so difficult to create that most batches wound up being a substance we called "carbon-like carbon". This is a materials science joke; you may now laugh.