The basic design principles are outlined in
The article also describes in passing how weak force fields could be realized. I’ll look at this more in future posts.
Militarising them is pretty obvious. Thicker and stronger materials for general shielding, reactive armour, faster engines, more maneuverability etc are all easy to engineer in. I’ll consider approaches to building reactive armor separately. For military use, comms, identification, encryption and so on are also essential but routine approaches to these would be fine.
Adding weaponry and surveillance capability is also easy, but different weapons and sensor capabilities deserve their own articles.
Adding invisibility to a hoverboard is not easy, and would be pretty pointless if the invisibility doesn’t also extend to the rider, but stealth for the car can certainly be optimized. The surface coating can act as a video display, so routine invisibility techniques would work – cameras on one side feeding display on the opposite side. This won’t make the car invisible, but it could make it less conspicuous. Similarly, straightforward camouflage based on surroundings could be implemented using surface-covering displays, without the need for extensive real-time camera links.
Mechanically, force fields would be far weaker than carbon materials, so they would only have military applications for disrupting electromagnetic attacks. They could have more use at high altitudes to help deflect radiation.