| Description |
Ultra-thin multilayer diamond-like carbon (DLC) coatings are used in precision engineering applications (including hard disk drives (HDDs)); their high hardness, chemical stability, and low friction coefficient in a range of environments allow protecting delicate substrate materials from damage, wear, and corrosion. A critical challenge when designing ultra-thin DLC coatings is understanding how they deform and delaminate from the substrate as a function of operating and coating design parameters including coating layer thickness and composition. We use molecular dynamics simulations of the ultra-thin multilayer DLC coatings used in HDD recording heads, which consist of stacked layers of DLC and amorphous silicon (a-Si) on a Ni substrate, to quantify the effect of coating design parameters on the mechanical properties of the coating, plastic deformation of the substrate, and adhesion of the coating to the substrate. Based on the physical understanding gained from the simulations we derive design guidelines for ultra-thin multilayer DLC coatings. We find that the hardness and Young's modulus of the coating increase with increasing DLC layer thickness and decreasing a-Si layer thickness because DLC and a-Si are the hardest and softest materials in the coating, respectively. We observe that plastic deformation of the Ni substrate for a constant mechanical load increases with increasing coating hardness because plastic deformation is increasingly preferential to the substrate with increasing coating hardness, causing the DLC coating to bend like a plate into the plastically deformed substrate. We show that the presence of an intermediate a-Si layer is critical for improving adhesion of the DLC coating to the Ni substrate because bonding between Ni and DLC distorts the Ni lattice more than bonding between a-Si and Ni. Similarly, we observe that that an intermediate layer comprised of low sp3-fraction DLC improves adhesion of high sp3-fraction DLC to Si but not Ni substrates compared to coatings without an intermediate layer. For coatings with an intermediate a-Si layer, adhesion improves with decreasing a-Si layer thickness because less a-Si is present to plastically deform during loading, which displaces the coating failure region from the a-Si layer into the Ni substrate. |
