Determining the hardest material to break is complex because material scientists use distinct metrics to define resistance. A substance that resists scratching might shatter easily under impact, while a material that bends rather than breaks might be easily dented. Materials science separates the concept of “breaking” into different mechanical properties, each describing a unique form of failure. Identifying the top contenders requires clarifying these definitions and examining materials that excel in each category.
Understanding Material Failure
The ability of a material to withstand damage is categorized by three metrics: hardness, strength, and toughness. Hardness defines a material’s resistance to localized surface deformation, such as indentation, scratching, or abrasion.
Strength is the material’s resistance to permanent deformation or yielding when a large load is applied. It measures the maximum stress a material can endure before it changes shape permanently or fails completely. For instance, high tensile strength allows a material to withstand being pulled apart without snapping.
Toughness describes the material’s ability to absorb energy and deform plastically before fracturing. This property is often considered the truest measure of “unbreakability,” as it measures resistance to catastrophic failure when a crack is introduced. Glass, which is hard and strong but brittle, contrasts with a rubber ball, which is soft but extremely tough because it absorbs impact energy by deforming.
Materials Built to Resist Scratching
Materials engineered for extreme hardness excel at resisting surface damage and indentation, a property quantified using scales like Vickers hardness (HV). Diamond, the benchmark for scratch resistance, is a carbon allotrope whose tightly bonded crystal lattice gives it a Vickers hardness ranging from 70 to 150 GigaPascals (GPa). Some synthetic materials, including certain forms of boron nitride, can surpass natural diamond.
Cubic boron nitride (\(\text{cBN}\)) is a synthetic compound analogous to diamond, exhibiting a Vickers hardness of approximately 50 GPa for a single crystal. While softer than diamond, \(\text{cBN}\) is often preferred for machining ferrous metals like steel. This is because diamond reacts chemically with the iron and carbon in steel at high temperatures, leading to rapid chemical wear.
Advanced engineering has led to materials that challenge diamond’s dominance. Nanocrystalline cubic boron nitride, engineered with nanometer-sized grains, has achieved Vickers hardness values near 108 GPa, comparable to synthetic diamond. Research into Wurtzite Boron Nitride (\(\text{wBN}\)), a rare form, suggests its unique atomic arrangement may give it a theoretical hardness advantage over diamond, though it is difficult to synthesize in bulk for testing.
Materials Built to Resist Shattering
Materials built to resist shattering are defined by high fracture toughness, which measures their ability to prevent cracks from spreading. High-performance alloys and ceramics utilize complex microstructures to dissipate impact energy rather than allowing it to concentrate at a crack tip. Zirconia Toughened Alumina (ZTA) is one example, a ceramic composite with a fracture toughness exceeding 12 \(\text{MPa} \cdot \text{m}^{1/2}\), significantly higher than commercial alumina’s \(\text{3 MPa} \cdot \text{m}^{1/2}\).
ZTA’s exceptional resistance comes from transformation toughening, involving the inclusion of tetragonal zirconia (\(\text{t-ZrO}_2\)) particles within the alumina matrix. When a crack approaches a particle, localized stress triggers a phase change from the tetragonal form to a monoclinic form (\(\text{m-ZrO}_2\)). This change is accompanied by a volumetric expansion of approximately 4 to 4.5%.
The resulting expansion creates a localized compressive stress field around the crack tip, forcing the crack closed and preventing further propagation. This mechanism allows the ceramic to absorb and dissipate fracture energy, a strategy different from the simple brittle failure seen in traditional, unreinforced ceramics. ZTA is reliable for applications like dental implants and armor plating, requiring both wear resistance and impact survival.
Maraging steels represent another class engineered for extreme toughness, balancing ultra-high strength with superior fracture resistance. These iron-nickel alloys contain very little carbon, deriving their strength instead from precipitation hardening. This process involves a specific heat treatment that forms a martensitic matrix followed by an aging step.
During the aging phase, tiny intermetallic precipitates (containing elements like molybdenum and titanium) form uniformly throughout the steel’s microstructure. These nanoscale particles act as physical obstacles, pinning the movement of dislocations—the defects that allow metals to deform plastically. This blockage significantly increases the material’s yield strength, while the low carbon content maintains the steel’s inherent ductility and toughness, resulting in a fracture toughness that can reach as high as \(175 \text{ MPa} \cdot \text{m}^{1/2}\).
The Search for the Ultimate Material
The current frontier involves creating substances that combine high hardness and high toughness, traditionally opposing properties. Bulk Metallic Glasses (BMGs) are promising because they lack the crystalline structure of traditional metals, possessing a random, amorphous atomic arrangement instead. This glassy structure provides BMGs with extremely high strength and elasticity, allowing them to deform significantly before yielding, resulting in impressive toughness.
Researchers are also looking to the natural world for design inspiration, particularly the structure of nacre, or mother-of-pearl. Nacre is a biocomposite composed of brittle aragonite (calcium carbonate) platelets (95% of its weight) cemented by a soft, ductile organic polymer matrix. This “brick-and-mortar” structure allows the material to achieve strength and toughness far greater than its individual components.
When stress is applied to nacre, the soft organic matrix stretches and the hard platelets slide against one another, dissipating energy over a large area and preventing catastrophic crack spread. Another area of research involves Q-carbon, a synthesized material created by melting and rapidly quenching amorphous carbon using a nanosecond laser pulse. This novel material is theorized to possess a hardness greater than diamond, while also exhibiting unique electronic and magnetic properties.