Polycrystalline Diamond Hardness

Polycrystalline diamond (PCD) exhibits extreme hardness due to its unique microstructure and bonding characteristics, making it one of the most valuable materials for cutting, drilling, and wear-resistant applications. Understanding its hardness requires examining both the intrinsic properties of diamond and the collective behavior of the polycrystalline aggregate.

Atomic-Scale Origins of Hardness

Hardness in materials typically arises from resistance to plastic deformation, which in turn depends on the material's crystal structure and bonding. Diamond's exceptional hardness originates from its tetrahedral sp³ carbon-carbon bonding, which creates a highly symmetric, three-dimensional covalent network. This network resists dislocation motion, the primary mechanism for plastic deformation in most materials.

In monocrystalline diamond, the ideal lattice provides minimal sites for dislocation initiation or glide. However, when diamond is synthesized as a polycrystalline aggregate, the microstructure becomes more complex, introducing grain boundaries, residual stresses, and intergranular phases.

Structure of Polycrystalline Diamond

Polycrystalline diamond is composed of numerous diamond grains, typically ranging from nanometers to tens of microns in size. These grains are usually randomly oriented and densely packed. There are two dominant forms:

Conventional PCD: Manufactured by sintering micron-sized diamond grains under high-pressure high-temperature (HPHT) conditions with a metal catalyst (usually cobalt). This introduces a small volume fraction of metal at grain boundaries.

Binderless PCD (e.g., nano-polycrystalline diamond or NPD): Synthesized via direct conversion of graphite or amorphous carbon at ultrahigh pressures and temperatures, without any catalyst. These materials can exhibit isotropic, ultra-high hardness.

Grain Boundary Effects and Hall–Petch Scaling

In polycrystalline materials, grain boundaries act as barriers to dislocation motion. According to the Hall–Petch relationship, hardness increases as grain size decreases.

However, at extremely small grain sizes (<10 nm), inverse Hall–Petch behavior can emerge, where grain boundary-mediated mechanisms like grain sliding reduce hardness. Yet in practice, many nano-PCD materials remain extraordinarily hard due to constrained grain boundary mobility and strong intergranular bonding.

Measurement and Comparative Hardness

Vickers hardness tests are typically used, though interpretation is challenging due to the diamond indenter’s similarity in hardness to the sample. Reported hardness values include:

Monocrystalline diamond: ~90–115 GPa

Conventional PCD: ~60–90 GPa

Nano-polycrystalline diamond (e.g., NPD from Himeji group): >120 GPa, sometimes exceeding monocrystalline values

Such measurements are often approximate due to indentation cracking, anisotropy in the indenter/sample pair, and scale effects.

Anisotropy and Toughness Tradeoffs

While monocrystalline diamond is anisotropic in hardness (e.g. planes being hardest), polycrystalline forms are effectively isotropic due to random grain orientation. This makes PCD especially valuable in applications requiring uniform performance regardless of loading direction.

However, hardness is often inversely correlated with fracture toughness. Conventional PCD is tougher than single-crystal diamond due to crack deflection at grain boundaries and plasticity in metal binders. Binderless nano-PCD, despite its ultra-high hardness, tends to have lower fracture toughness, though advances in processing have partially mitigated this.

Applications and Performance Limits

The extreme hardness of PCD enables its use in:

• Cutting tools for hard metals and composites
• Drill bits for geological exploration
• Wear-resistant coatings and dies

Nonetheless, thermal stability is a limitation, especially for cobalt-sintered PCD, which degrades above ~700°C due to graphitization or cobalt-catalyzed diamond conversion. Binderless PCD improves thermal resistance, extending usable temperatures to 1000°C.

Conclusion:

Polycrystalline diamond represents a class of materials where grain boundary engineering, microstructure control, and synthesis methods converge to achieve mechanical properties surpassing even those of single crystals. Its hardness arises not just from atomic bonding, but from collective behavior at the nanoscale, with grain refinement and boundary chemistry playing decisive roles.