Over the years newer and tougher composites have been developed, giving about five times the protection value of the original pure ceramics, the best of which were again about five times as effective as a steel plate of equal weight. These are often a mixture of several ceramic materials, or metal matrix composites which combine ceramic compounds within a metal matrix. The latest developments involve the use of carbon nanotubes to improve toughness even further. Commercially produced or researched ceramics for such type of armour include boron carbide, silicon carbide, aluminium oxide (sapphire or "alumina"), aluminium nitride, titanium boride and Syndite, a synthetic diamond composite. Of these boron carbide is the hardest and lightest, but also the most expensive and brittle. Boron carbide composites are today favoured for ceramic plates protecting against smaller projectiles, such as used in body armour and armoured helicopters; this was in fact in the early sixties the first general application of ceramic armour. Silicon carbide is better suited to protect against larger projectiles than boron carbide as the latter material suffers a phase collapse when impacted by a projectile travelling at a speed over 850 m/s. Silicon carbide was at that time only used in some prototype land vehicles, such as the MBT-70. The ceramics can be created by pressureless sintering or hot pressing. A high density is required, so residual porosity must be minimised in the final part.