The answer to whether diamond is harder than steel is unequivocally yes. Diamond is the hardest known naturally occurring material, exhibiting a level of resistance to deformation that steel cannot match. This comparison highlights a significant difference in material science, where the term “hardness” is a specific, measurable property distinct from general strength or toughness.
Defining Scientific Hardness
Materials scientists define hardness as a measure of a material’s resistance to localized permanent deformation, which includes both scratching and indentation. This property is determined by how well a material’s bonds and structure resist being broken or rearranged when a force is applied to a small area. Since no single test captures all forms of resistance, engineers use different scales to quantify material hardness. One common method is the Mohs scale, which ranks materials from 1 to 10 based on their ability to scratch one another, measuring abrasive resistance. Indentation hardness, measured by scales like Vickers or Rockwell, applies a controlled force to a sharp indenter and measures the size or depth of the resulting permanent mark.
The Source of Diamond’s Extreme Hardness
Diamond’s unparalleled hardness is a direct result of its unique atomic arrangement and the strength of its chemical bonds. It is an allotrope of carbon where every atom is bonded covalently to four neighboring carbon atoms, forming a dense, rigid three-dimensional tetrahedral lattice, often called the diamond cubic crystal structure. The sp3 hybridization of the carbon atoms ensures that the bonds are strong and highly directional, locking the entire structure into place. Because of this structural perfection, diamond scores the maximum value of 10 on the Mohs scale, meaning only another diamond can scratch it. Its indentation hardness, measured using the Vickers scale, is also exceptionally high, ranging from approximately 70 to 150 gigapascals (GPa), depending on its purity and orientation.
Hardness in Metals and Alloys
Steel is an alloy primarily composed of iron and a small percentage of carbon, which dramatically increases its strength compared to pure iron. The atomic structure of steel is metallic, where atoms are held together by a “sea” of shared electrons, allowing layers of atoms to slide past one another under sufficient stress. This metallic bonding structure means that while steel is very strong, it is inherently less resistant to localized deformation than the rigidly-bonded diamond crystal. The hardness of steel varies widely based on its carbon content and, more importantly, its heat treatment, such as quenching and tempering. Rapid cooling, or quenching, locks the microstructure into a hard, brittle phase called martensite, significantly increasing its hardness.
Even the hardest tool steels, however, typically register Vickers hardness values in the range of 5 to 10 GPa, which is up to 30 times softer than diamond. Steel is valued for its high tensile strength, which is its ability to resist being pulled apart, and its high toughness, or its capacity to absorb energy without fracturing. A diamond is extremely hard but can shatter under a sharp impact due to its lower toughness, a property that steel excels at because its metallic structure can deform slightly to absorb shock. Therefore, steel is a superior choice for tools that require impact resistance, while diamond is unmatched for abrasive cutting and grinding applications.