A note here about quality, and that is while quality affects how well armor is at intercepting an attack (so the attack is affected by the armor at all), material determines what, if any, effect the armor has on the blow. So while a artifact helm may be able to intercept several times more attacks then a normal helm, if the artifact is made of something like bone then it will be practically worthless since even the attacks that it intercepts will simply punch right through it due to being a better material. In general with armor, weapons, and shields then as long as:
material(your equipment) > material(their equipment)
then
quality>material. Elsewise material>quality.
So against a goblin wearing iron armor a steel weapon will always outperform the same weapon made of copper. This is because while the steel weapon will penetrate the iron armor, the copper one will "glance away" and not hurt the goblin. Now take the same goblin wearing copper armor. If you attack it with a steel sword and an iron sword, both of them will penetrate the armor, and at this point quality becomes much more important then material as it helps the weapon hit more easily and deal more damage when it hits.
There are two small exceptions to the rule above though.
1)In shields/bucklers quality always beats material because a shield block is equivalent to a dodge. As such a wooden shield will block dragon fire just as well as an adamantine one will.
2)Adamantine bends the quality>material rule just slightly for edged weapons due to having [MAX_EDGE:100000] instead of [MAX_EDGE:10000] this lets it be somehow "sharper" then swords or edged weapons made of a lesser material, and as such a sword of adamantine
will perform better then a sword of steel of the same quality against copper armor. This is different then the iron/steel example, where quality is a much more important factor.
Maybe someone who knows more about how coverage works in armor can explain more on how little is understood about it
Coverage works as a sort of "block fail chance" that you can build into armor. Basically when a body part is hit the game looks at quality of the armor to help determine whether or not the armor intercepts the blow (though the chance of armor failing to intercept a blow is very low, even with normal quality armor). If the armor does intercept the blow then the game looks at the [COVERAGE:%] value of the armor and rolls an RNG. Because the default [COVERAGE:%] value is 100, for most armor this roll does absolutely nothing and the game simply goes on to determining how well the armor blocked the blow. If the piece of armor had only [COVERAGE:50] though, then that would mean that 50% of the time that the armor would normally intercept a blow, it fails to do so. You can sort of imagine it as the, well, 'coverage' of a piece of armor. So while a t-shirt might have [COVERAGE:100], meaning that every blow to your torso is intercepted and padded (excluding the critical hits where they manage to stab you up the arm whole or something), a swimsuit might only have [COVERAGE:15], meaning that only 15% of the hits to your torso are intercepted and padded. As such [COVERAGE:%] tends to be the main factor in most pieces of armor at how well it is at intercepting attacks, with the quality more dealing with how often your enemy gets a critical hit that ignores the armor completely.
Two mini walls of text, but those are the best explanations that I've got.