An ideal kinetic armor — one that protects against physical attacks — needs several properties that often run in opposition to each other:
- It needs to be resistant to shear stress — fracture, slicing, puncture — in order to prevent penetrating attacks
- It needs to absorb and redistribute force laterally, so that a crushing impact towards a single point on the body is spread out across the surface of the body
- It needs to be light enough to allow (comparatively) unencumbered movement
- It either needs to remain within the elastic stress region where a material recovers its properties and can be immediately reused, or allow plastic deformation (with its greater potential for absorbing energy) and be constantly discarded and replaced
These are the advantages of both steel and kevlar. Both are highly resistant to shearing/penetration (kevlar more than steel); both are able to distribute the force of a blow over a large area (steel more than kevlar). Steel is more elastic, needing a fairly hefty blow to deform its shape; kevlar deforms easily, but absorbs more penetrative energy from plastic deformation, and so must be discarded after a few impacts. Of course, we have to consider the kind of attacks that are being made — kevlar is a wonder for stopping bullets, and would probably keep a battle axe from cutting deep, but would be of little use against a mace or a club — but in general there are always material improvements that can be made.
Gustavo's idea of transparent aluminum (aluminium oxynitride, a kind of ceramic) is an interesting choice, and would be a good protection for eyes where transparency is key, but I don't know its elasticity or tensile strength. Some transparent polycarbonates or transparent ceramics might also serve, though ceramics (as a rule) are brittle, which is not something one wants near the eyes. Ceramic plates would be an excellent substitute for steel generally — these are used in many modern armored vests — on the caveat that they absorb energy by crumbling (like crumple zones on a car) and have to be replaced fairly frequently. On the other hand there is titanium, which is as strong and shear resistant as steel, while having a weight equivalent to aluminum (maybe 4/10ths that of steel. Titanium is comparatively rare and difficult to extract, but that might not be an issue for magical compositions.
With respect to heat... Metals are (generally) good heat conductors — for good or bad — while ceramics tend to be heat insulators. However, body heat regulation is mostly a factor of airflow. For example, researchers have found that desert nomads do better with black robes the white: the heat absorbed by black robes creates convection cells in the air under the robe that tends to cool off skin through perspiration. The more layers packed under the armor, the more that air flow is inhibited, and the greater the tendency for overheating. Better would be a light, breathable material that allows airflow and keeps the armor from contacting the skin directly.
