The Quest for Highest Toughness: Beyond Diamond and Steel

Toughness is a multifaceted property that plays a crucial role in determining the durability of materials under various stress conditions. While hardness and strength are often considered the gold standard in materials science, recent findings have introduced wurtzite boron nitride and metals as contenders for the highest toughness. This article delves into the definitions, applications, and key attributes of materials that exhibit exceptional toughness.

Understanding Toughness

The term toughness is defined differently depending on the context in which it is used. For metallurgists, the definition includes:

Tensile Testing: the capacity of a material to absorb energy and exhibit plastic deformation without fracturing. Impact Testing: the ability of a material to absorb energy per unit volume before breaking. Fracture Toughness: the capacity of a material to resist fracture under specific conditions of flaws and applied loads. Stress Resistance: the inherent ability of a material to resist fracture under stress.

Many people mistakenly assume that materials with high strength and hardness must also be the toughest. However, the relationship between these properties is not always straightforward. Hard materials may resist crushing but may not necessarily absorb energy and deform plastically before breaking.

Testing and Measurement of Toughness

Measuring toughness can be challenging, especially for small-diameter fibers like those used in aramid and carbon fibers. These fibers have strength-to-weight ratios that often surpass those of traditional materials like steel and have applications in lightweight composites. However, published research on the toughness of these fibers is limited due to the intricate experimental challenges.

Natural vs. Man-Made Materials

Nature offers some extraordinary examples of tough materials, such as spider silk. Spiders produce threads with tensile strengths of about 1.3 GPa, which might be slightly less than steel's 1.65 GPa. However, the density of spider silk allows it to be five times stronger than an equal weight of steel. This makes spider silk an ideal candidate for high-performance applications in lightweight structures.

Man-Made Materials in the Spotlight

Among man-made materials, metals stand out for their high fracture toughness values. They excel in resisting crack propagation due to their inherent hard and durable nature, which results in larger plastic flow areas in stress-strain curves. Examples such as aramid, carbon, and polyethylene fibers have grown in popularity for use in composites due to their excellent strength and stiffness-to-weight ratio.

Experimental Challenges

While aramid and carbon fibers are known for their exceptional strength, the measurement of their toughness remains a research challenge. This is because testing small-diameter fibers involves significant experimental difficulties. The toughness of these fibers, combined with the fiber/matrix interface, plays a vital role in comprehending the ultimate failure stress and energy dissipation mechanisms in brittle unidirectional composites.

Materials scientists continue to explore these challenges, aiming to further enhance the toughness of these advanced materials, making them even more suitable for a wide range of applications in aerospace, automotive, and construction industries.

In conclusion, while diamonds may adorn the top spot in hardness, metals, wurtzite boron nitride, and the natural marvels like spider silk highlight the diverse landscape of toughness in materials science. The quest to find the material with the highest toughness is ongoing, driven by both natural and man-made developments.