Diamond-like carbon
As the fifteenth most frequent element on Earth, carbon is widely used in engineering despite its resource-depleting and cost-effectiveness. Carbon-to-carbon bonds can transform into single, double, or triple bonds by several hybridizations, including sp1, sp2, and sp3 hybridizations. Altering periodic binding motifs in networks with these hybridizations enable the formation of multifarious allotropes, e.g., Diamond-like carbon (DLC), glassy carbon, diamond, lonsdaleite, graphite, graphene, fullerene, carbon nanotube (CNT), cyclocarbon, graphyne or graphdiyne. Carbon allotropes show strengths in mechanical properties and thermal conductivity, like in diamond with sp3 hybridization, or unusual electrical conductivity and lubricity, like in graphite with sp2 hybridization. In recent years, carbon has improved as an element for thin coatings like diamond, DLC, and various metal carbide coatings.
One of these carbonaceous compounds, DLC, is widely employed as tribologically functional coatings in large-scale, durable-critical parts because of its superior mechanical and tribological qualities. Among these, a variety of carbon-based coatings are attractive as tribological coating materials because of the controllable amount of carbon's sp2 and sp3 hybridization, which allows them to simultaneously possess the ability of both wear-resistant coatings and solid lubricants. DLC, one of the carbon-based materials, has a high hardness comparable to diamond and low shear strength like graphite.
DLC, which was first discovered in 1971 by S. Aisenberg, was synthesized by a CVD-based deposition process from a hydrocarbon gas source; it is a low-physical amorphous carbon film (a-C and a-C:H) used as a decorative coating and a simple anti-wear coating for gears. Afterward, hydrogen-free tetrahedral amorphous carbon with high physical properties comparable to diamond, first marked in 1996 by D.R. McKenzie, was developed by PVD including cathodic arc, pulsed laser ablation deposition, and has been used in hard disks, cutting tools, and engine sliding parts. Today, as industrial development requires surface treatments that include not only wear resistance but also various functions, DLC:X (X = alloying and dopant elements) incorporated with functional elements is currently being developed and is mainly applied as a durable coating endowed with functions such as electrical, optical, biocompatibility, and corrosion resistance. the development of environment-tailored DLC will be required due to extreme changes and diversification of the friction environment.