Armor and Materials

Choosing materials is one of the biggest decisions in building a combat robot. Every part has to survive hits while staying within the weight limit, so builders pick the lightest material that can handle the forces on that particular part. This page covers the common materials for frames, armor, wedges and weapons, and the main ideas behind robot armor. For printed plastics, see 3D Printing; for getting parts made, see Fabrication Services.

Hardness vs. toughness

These two words are easy to confuse, and the difference matters a lot in robot combat:

Hardness is resistance to surface denting and penetration. Hard materials resist being cut or gouged by teeth and blades.
Toughness is the ability to absorb a sudden impact without cracking or shattering.

In most metals, making a material harder makes it less tough. A very hard plate may resist cuts but crack in half from one big hit, while a softer, tougher plate dents but holds together. Good weapon and armor materials balance the two.[1]

Hardness of steel is usually given on the Rockwell C scale (HRC) or the Brinell scale (HB or HBW).

Compare at equal weight

Because every robot has a weight limit, compare materials at the same weight, not the same thickness. A lighter material can be used much thicker. For example, UHMW plastic is about a third the density of aluminum, so a UHMW panel can be about three times thicker than an aluminum panel of the same weight.[2]

Plastics

Material	Typical uses	Notes
UHMW-PE (ultra-high-molecular-weight polyethylene)	Armor, chassis, especially in insect classes	Very impact-resistant: it flexes and stretches rather than breaking, and is easy to machine. Too soft for wedges, because sharp teeth dig into it.
HDPE (high-density polyethylene)	Chassis and armor in insect classes	Similar to UHMW but cheaper and slightly less tough. Many top beetleweights have been built mostly from HDPE.[3] Glues don't stick to it well.
Polycarbonate (Lexan, Makrolon)	Armor, covers, small-robot chassis	Absorbs big impacts by bending. Prone to cracking at stress points such as bolt holes, so let it flex rather than clamping it rigidly.
Acetal (Delrin, POM)	Machined parts such as hubs, spacers, gears	Much stiffer than UHMW, but breaks rather than stretching, so it's not great as armor.
Printed plastics (TPU, PLA+, nylon, carbon-fiber nylon and more)	Chassis, armor and parts in insect classes	See 3D Printing. TPU in particular has become very popular for small robots because it flexes and springs back.

Metals

Aluminum

6061-T6 is the common, affordable, easy-to-machine alloy, used widely for frames and structural parts.
7075-T6 is much stronger than 6061 (close to some stainless steels at about a third of the weight) but harder to work, and it cracks if bent. It is generally the best aluminum for armor.
5052 bends easily, which is useful for formed parts, but that same softness makes it a poor armor choice.
Hardware-store aluminum (often 3003) is too soft for robot armor.
Aluminum has a soft surface, so spinning weapons can dig into it. It makes good frames, but poor wedges.[4]

Titanium

Grade 5 (Ti-6Al-4V) is strong and very tough, and is a favorite for armor and wedges ("scoops") against spinners. It's expensive.
Grade 2 (commercially pure) titanium is much weaker – its yield strength is less than a third of Grade 5's – and isn't much better than a basic aluminum alloy for structure.
Titanium is about 1.6 times as dense as aluminum, so again compare at equal weight.[5]

Hardened steels

AR400 and AR500 are pre-hardened, abrasion-resistant steel plates (AR500 is roughly 470–530 HB, about 47–50 HRC). They are now widely used for armor and wedges in all weight classes. They're very hard to machine or weld, so they're usually laser, plasma or waterjet cut, and they crack if bent.[6]
Hardox is a brand of abrasion-resistant steel especially popular in the UK.[7]
Spring steel (for example 1075, 1095 or 5160) is resilient, but tends to break rather than dent.

Weapon steels

S7 is a shock-resistant tool steel, and the classic choice for spinner teeth and blades. Hardened to about 54 HRC, it's several times tougher than chromoly at a similar hardness.[8] It needs precise heat treatment, so most builders have it done professionally.
4130 / 4140 chromoly is cheap, widely available and easy to work, and is a common choice for home machinists. Experienced builders usually keep it at or below about 48 HRC (lower for highly stressed parts).
Some heavyweight robots use even harder steels such as AR600 for weapons.

Composites

Carbon fiber sheet is very stiff and light (about half the density of aluminum), and great for frames and plates that aren't hit directly. It doesn't handle sharp impacts well, can't be bent, and shouldn't be countersunk for flat-head screws. Cutting it creates harmful dust, so wear a mask and eye protection, or have it cut by a service.
G10 / FR4 (fiberglass laminate) is tough and stiff, and used for plates and electronics mounts.

Armor design ideas

Shock mounting: mounting armor or a whole chassis on rubber mounts spreads out the impact and protects the parts inside. Mounts should be snug without play. Shock-mounted single-piece armor has generally proven better than several layers bolted together.[9]
Spaced armor: if armor is meant to flex, leave enough space between it and anything it's protecting, so it can bend without hitting what's behind it.
Ablative armor: extra panels designed to be damaged or torn off, taking the brunt of a hit so the robot doesn't. Judges ignore damage to purely ablative armor under the SPARC Judging Guidelines.
Replaceable armor: many builders bolt on armor panels so damaged ones can be swapped quickly between matches. One BattleBots team, for example, went through UHMW, welded steel and welded aluminum before settling on bolted, bent aluminum panels designed to shear off and be replaced.[10]
Use the right material for each part: frames, armor, wedges and weapons all face different forces. Armor that's as thick as possible while still making weight is usually worth it.
Entanglement rules: armor or defenses that can entangle weapons or wheels (nets, tape, string, and some rubbery or soft plastic designs) are forbidden by the SPARC Robot Construction Specifications.

Weight class notes

150 g and 1 lb: mostly 3D printed plastics (especially TPU) and UHMW, often with thin titanium, aluminum or carbon fiber pieces.
3 lb: often mostly 3D printed plastics with metal armor added, frequently cut by an online service.[11]
12 lb and up: mainly aluminum or machined plastic frames, with steel or titanium armor and wedges.