The main reason catalyst metal impurities cause diamond graphitization is that they not only trigger destructive thermal stress inside the crystal but also directly act as "catalysts" to promote the heterogeneous nucleation of graphite.

Specifically, catalyst metal impurities (such as Ni, Fe, Mn, etc.) lead to graphitization through the following two mechanisms:

1.Promoting the heterogeneous nucleation of graphite (Core chemical/physical mechanism) During the growth of synthetic diamonds, bubble inclusions are extremely prone to form inside the crystal, and catalyst metal impurities like Ni, Fe, and Mn are usually attached around these bubbles. The presence of these catalyst metal impurities makes graphite highly susceptible to heterogeneous nucleation. Whether during the cooling stage in the later period of synthesis or in the subsequent high-temperature heating process, as long as specific temperature conditions are met, these impurities will cause the diamond matrix around the inclusions to begin transforming into graphite first.

2.Huge internal stress caused by mismatched physical properties (Mechanical mechanism) After impurities such as catalyst metals enter the diamond crystal, due to the difference in atomic radius from carbon atoms, most of them exist in an interstitial state, meaning they are incoherent with the diamond matrix. More importantly, the thermal expansion coefficients of the catalyst metal inclusions and the diamond matrix are different.

When heating the diamond, due to their inconsistent expansion, the inclusions will exert an increasing internal pressure and shear stress on the surrounding diamond matrix.

As the temperature rises, the yield stress of the diamond matrix will decrease. When the pressure generated by the internal inclusions equals or exceeds the yield limit of the diamond matrix at high temperatures, the diamond will undergo plastic deformation.

Once plastic deformation occurs, the diamond matrix around the inclusions will begin to graphitize. At this point, plastic deformation and graphitization coexist, eventually causing the entire diamond crystal to be destroyed.