Doping the DLC with silicon for enhanced thermal stability

Some highlights in advance:

👉🏼 Adding Silicon into DLC coatings increases the thermal stability

👉🏼Undoped arc-deposited carbon collapses at 425°C, while addition of Silicon in DLC stabilizes the structure to further 125°C

👉🏼Addition of Silicon favors the sp3 hybridization and collapsing the compressive stress in the coatings

👉🏼Moreover Silicon incorporation reduces the friction and increases the wear resistance that enhances the machining properties of the coatings

Discussion in detail

DLC - Tribology of diamond like carbon films

Figure Curtsy: J. C. Sánchez-López & A. Fernández; Tribology of Diamond-Like Carbon Films pp 311–338

Diamond-like amorphous carbon (DLC) is an interesting material in the coating industries for various applications, such as

  • automobile

  • heavy machinery

  • tool industries and

  • weaving/textile manufacturing due to its impressive mechanical properties

  • luxury watch industries and enhanced tribological characteristics, predominantly as a solid lubricant.

Moreover, high hardness without a good wear resistance is always not a conducive mechanical property for enhancing the tribological behaviors of driving or machining parts because it can induce more wear, thus producing abrasive debris during machining process.

Also, the heat generated during the machining process jeopardize all beneficial applications if the DLC structure cannot maintain the stability at a working temperature of the machining process.

Therefore, low wear resistance H/E and the thermal degradation of DLC films are the major limiting factors for their use of DLC coated tools in high temperature applications such as high-speed cutting tools.

Wide possibilities exist for doping DLC with different elements such as non-metals Si, F, N, O and metals and combinations thereof to tune the nature and properties of DLC for improved machining functionalities while maintaining the amorphous phase of the coating intact.

Doping of DLC coatings alters the properties such as thermal stability, hardness and elastic modulus, intrinsic stress, tribological properties, thermal and electrical conductivity, surface energy and biocompatibility and hence these properties can be continuously tuned to the optimum value for specific applications.

DLC  dimensional wear coefficients of coatings

Dimensional wear coefficients of the coatings examined at 35 N, 0.75 GPa, 0.25 m/s, and 90 °C. Errors are with in 5% and error bars are omitted for better visualisation. The a-C samples show more severe wear than the Si-doped DLC (8 at% Si), with a candid evidence of polishing wear on the plate.

While doping of Nitrogen into Carbon matrix leads to increase in sp2 bonds, the doping of larger size elements damages the formation of sp3 hybridization. The ideal need is an element that is comparable to the size of carbon while encouraging the sp3 hybridization in the matrix for doping the DLC.

Silicon suits this criterion as it bounds with carbon COVALENTALY while favoring sp3 hybridization owing to the fact that the atomic radius of silicon (~0.111nm) is comparable to the atomic radius of carbon (~0.070 nm).

Moreover, when tested under water-assisted lubricating conditions, and Si-DLC can act as a solid lubricant under high humidity owing to the enhanced wear resistance.

When cathodic arc processes were performed along with the inlet of trimethyl-silane gas, an increased process pressure generates a stable plasma of carbon with the simultaneous deposition of fully and/or partially broken Si–CH3, –CH3, and Si–C components from the trimethyl-silane compound.

This could suppress the formation of macroparticles during the FCVA deposition and suppress the average roughness values. Alternatively, during the ball-on-disk tests, when the Si fraction was greater than ~ 7 at.% coefficient of friction initially increases however decreased abruptly after approximately 750 rotation cycles. Increasing Si content pushes the coefficient of friction below 0.1 and provides excellent machining properties and enhanced tool lifetime.