Whether bone is harder than glass is an interesting scientific question. At first glance, glass may seem very hard and bone more flexible. But looking closer reveals that bone has some remarkable properties that allow it to withstand more stress than glass. In this article, we’ll compare the hardness, strength, toughness, and microstructures of bone and glass to see which material is truly harder.
The Hardness of Bone and Glass
Hardness refers to a material’s resistance to deformation, particularly permanent deformation that causes surface damage. The traditional measure of hardness is the Mohs scale, which ranks materials on a scale from 1 (talc) to 10 (diamond). On this scale:
- Glass has a Mohs hardness around 5.5.
- Bone ranges from 3 to 4 in hardness.
Based solely on the Mohs scale, glass appears much harder than bone. However, the Mohs scale has limitations. It measures scratch resistance, which relates more to surface hardness. Mohs hardness is a qualitative test, not a precise measure.
Quantitative hardness tests better capture microscopic properties. For example, Knoop hardness uses a pyramidal diamond indenter to make micro-indentations. The indentation depth indicates hardness. In Knoop tests:
- Glass scores between 400 and 700 KHN (Knoop hardness number).
- Bone measures around 50 KHN.
Again, glass seems far harder by this metric. But hardness alone doesn’t tell the whole story of how materials perform mechanically.
Strength Differences
The strength of a material indicates how much force it can withstand before failing. Substances can be hard yet brittle and prone to cracking under stress. Here are typical strength values for glass and bone:
- Glass has compressive strength around 1000 MPa but weak tensile strength below 100 MPa.
- Bone has a compressive strength from 131 to 224 MPa and tensile strength between 104 to 121 MPa.
Although much harder than bone, glass is far more brittle. Bone has more balanced compressive and tensile strength. This greater isotropy allows bone to handle multi-axial stresses. Pound for pound, bone is stronger than standard glass.
Measuring Toughness
Hardness and strength measure how much force a material can resist under static loading. Toughness indicates a material’s resistance to fracture when subjected to sudden impacts or dynamic loads. Toughness depends on both material strength and ductility (ability to deform before fracturing).
There are several ways to quantify toughness:
- Fracture toughness – Measures resistance to crack propagation in units of MPa·m^1/2.
- Impact energy – Absorbed energy before fracturing in Joules/m2.
- R-curve – Plots fracture toughness vs. crack growth to show rising resistance.
By all measures, bone exhibits much greater toughness than glass:
| Material | Fracture Toughness | Impact Energy |
|---|---|---|
| Glass | 0.7 – 0.95 MPa·m^1/2 | 0.01 – 0.1 J/m2 |
| Bone | 2 – 12 MPa·m^1/2 | Up to 50 J/m2 |
Bone’s higher toughness allows it to resist impacts and dissipate stress without shattering. This helps explain why bone rarely fractures under sudden blows in the body, unlike glass.
Microscopic Structure
The remarkable toughness of bone originates from its complex microscopic structure. Here are key structural features that enhance bone’s mechanical performance:
Composite Material
Bone consists mainly of a stiff mineral phase of hydroxyapatite crystals (65% by weight) bound within a tough protein matrix (25% by weight) of mainly collagen fibers. This reinforces bone, much like fiberglass. The remaining 10% includes water and cells.
Hierarchy
Bone has a hierarchical structure spanning different size levels. The smallest motif is mineralized collagen fibrils (~50-300 nm wide). These assemble into fibers, fiber bundles, lamellae, and osteons at increasing length scales. This hierarchy dissipates stresses and resists cracks.
Laminates
At the micrometer scale, layers of bone (lamellae) have alternating orientations, like plywood. This prevents cracks from spreading across lamellar boundaries, increasing fracture resistance.
Tubular Design
Bone is structured into hollow tubular units called osteons (Haversian systems). This geometry evenly distributes stresses in bone and avoids stress concentrations.
In contrast, glass has an isotropic, amorphous structure without internal interfaces. Cracks easily propagate through the uniform glassy matrix under stress. Glass lacks the microstructures that provide bone’s unique combination of strength and toughness.
Conclusion
Based on microscopic structure and mechanical properties, bone is indeed harder than glass in the most meaningful sense. Although glass has high surface hardness, bone has superior strength and especially toughness. The complex composite architecture of bone at multiple size scales enables it to resist fracture from static and dynamic loads. In fact, ounce for ounce bone is stronger than steel. So while glass seems hard, it cannot withstand real-world stresses as well as the intricately structured biological material called bone.