Which material is generally harder?

Titanium is generally considered the hardest material among the options listed. This hardness is a result of its unique metallic bonding and crystalline structure, which contributes to its high strength-to-weight ratio. Titanium exhibits remarkable tensile strength and hardness, making it suitable for various demanding applications, including aerospace and medical implants, where durability and resistance to deformation are crucial.

In contrast, aluminum, though lightweight and easily machinable, is relatively softer and more ductile, which makes it less suitable for applications requiring high hardness. Copper, while it has good ductility and thermal conductivity, is also softer than titanium. Plastic, depending on the type, is usually much softer than metals and does not have the same level of hardness or structural integrity as titanium. Thus, titanium stands out as the hardest material in this context.