Diamond-like carbon is an amorphous form of carbon, made of many small crystallites, mixing up the polytypes of diamond (most commonly the cubic lattice diamond, but also its lonsdaleite polytype with a hexagonal lattice and other forms). It is applied as a coating to lend some of the qualities of diamond to an object.
So if the goal is to give the object diamond-like qualities, why not just coat it with regular diamond instead of DLC?
You balance cost of manufacture and return of investment for the anticipated use.
If you read about the methods of production of diamond-like carbon (DLC), e.g., on Wikipedia, e.g.
«DLC is typically produced by processes in which high energy precursive carbons (e.g. in plasmas, in filtered cathodic arc deposition, in sputter deposition and in ion beam deposition) are rapidly cooled or quenched on relatively cold surfaces.»
credit to Wikipedia's article
you may infer that DLC may offer a smooth continuous coating of a surface at a layer thickness you may tune by adjusting the process parameters. Said Wikipedia article illustrates this with a cobblestone coverage:
On the other hand, how would you do with diamonds? You would need to find them or rely on synthetic diamonds and then to break and crash these into fine grains. Not all directions in a diamond crystal are of equal hardness (keyword cleavage planes), however diamonds are the most hard naturally occurring material. Even if you arrive this, crushing mineral yields to some dispersion of the grain sizes, so by sieves or floatation, you have to narrow this size distribution. But even then, the surface coating would be like icing a cake with shots (or sprinkles), potentially leaving some areas of the surface exposed without this coating. Instead, the sharp edges of the isolated diamond shots likely would be abrasive like a fine sanding paper.
Why do you think that would be preferable? A perfect crystal is rather fragile - diamonds are hard, but they also have perfect cleavage - one good hit and you have two diamonds (or a bunch of shards). We only use monocrystals when we have to - like for microchips. For material strength, they're usually really poor - you want many grain boundaries that halt the propagation of cracks, and you certainly don't want perfect cleavage.
It's also hard to imagine how you would coat anything with a single crystal of diamond in the first place. How would the atoms fit together? Using many grains allows you to nicely coat a surface, mostly without having to think too much about its shape - heck, if you look around yourself, we do that all the time with pretty much all the materials we use for strength - concrete and asphalt, stone walls and brick walls, metal/ceramic coatings...
Since DLC is pretty much a physical vapor deposition (PVD) thing, I will address this question from this perspective.
The simple answer to your question is: So far we cannot deposit diamonds in PVD processes (at least I have not heard about it). If you read into the literature of DLC, a measure of success for the coatings is the average hybridization, or in other words, how close the bonding structure gets to the sp3 bonds in proper diamonds. Reaching almost pure sp3 bonds in a DLC coating is a goal that many research groups try to reach. If we could deposit diamond properly, we would. Diamond would certainly be better than all alternatives for some problem, and would be used as such.
Chemical Vapor Deposited (CVD) diamonds are a thing, as a comment mentioned. I have seen a grown CVD diamond wth my own eyes for use as a window in a spectrometer. But CVD requires very high process temperatures that many substrates cannot endure. PVD on the other hand can be done at much lower temperatures.
Disregarding costs aspects, diamonds makes a good noble cathode, where I am assuming there is an intended electrochemical use for the coated object (else, this question is on aesthetics).
Generally, for the anode and cathode in the special case of an electrochemical cell, it is known that one can greatly accelerate galvanic based corrosion reaction, for example, other things held constant, by having a small area anode relative to a much larger area cathode.
Here is a supporting answer provide on Research Gate, to quote:
The influence of cathode/anode area ratio is a key factor to the rate of galvanic corrosion in seawater. It can be explained that the anode current is always equal to the cathode current when the galvanic corrosion occurs, and anode area is smaller, the anode current density is greater. That is, the corrosion rate of anode metal is greater. Increasing Sc/Sa accelerated galvanic corrosion rate of high potential difference coupling, the fitting curves of the galvanic current by experiments will show the parabola shape. When anode density is greater than corrosion rate will be higher. Galvanic corrosion rate linear growth with the Sc/Sa in seawater indicates, the galvanic potential shift and driving voltage decreases.
However, if we are talking about battery technology, we are in more complex arena of surface modifications. To quote a Science Direct source:
Electrode surface modifications generally prolong the lifetime and improve the safety of Li–ion batteries. Stabilization of electrode surfaces and the interphases reduces the extent of side reactions, thus suppressing impedance growth, capacity fade, and the release of significant heat and gases which cause safety risk of thermal runaway. Although significant progress has been achieved, particularly during the past decade, the field of electrochemical and chemical surface modification of battery electrodes is still wide open for further exploration. Novel synthetic approaches for electrode surface functionalization and preparation of thin-layer coatings are desired. However, the processing and manufacturing approach of electrode coatings must be facile and scalable at industrial level, as the main bottleneck for the large-scale implementation of Li–ion batteries today are the high cost of cathode materials and processing.4,6 Another practical issue is the stability of the surface modification, for example, the coating, which tends to degrade over time and cycling. Degradation is typically accelerated under harsh chemical and electrochemical operating conditions (e.g., high or low temperature, high voltage, and fast cycling rate). Further fundamental understandings of operational principles of the interphases, the synergetic interplay between their components, and how they may be modified for improved long-term cycling are necessary.
So, some knowledge of the intended purpose of the application for the diamond electrodes would appear to be important as to which is actually better as, for example, diamond-like carbon (an amorphous form of carbon) may not be suitable due to stability/degradation concerns for the targeted application.
