# Diamond-Like Carbon (DLC) Coatings: Classification, Properties, and Applications
## 2 Classification of DLC Coatings  
DLC is a type of thin-film material of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C: H), which has a high binding rate of the metastable sp\({}^{3}\)C bond. These particles provide an ascent for the sp\({}^{3}\) bond, which is in stark contrast to the sp\({}^{2}\) bond unless balanced by the C-H bond. It has been suggested that cyclic particles are different from plasma polymerization. In this reaction, with the development of subatomic H, the accumulation of C-H groups will form C sp\({}^{3}\) bonds. The DLC can be linked with different compositions (such as Si and F). This section discusses unalloyed materials first, rather than specifying alloyed materials. Specifically, it has four bases: (1) processing, (2) structure, (3) properties, and (4) performance, and improves the bond between them\(-\) each edge of the tetrahedron is connected at the edge to each other corner [21].  
However, academicians and industrial researchers have turned their attention toward the development of DLC coatings due to excellent optimization among tribological, mechanical, and chemical properties. DLC coatings are deployed in automotive engines because of superior friction behavior, increased life cycle, and compatibility with chemicals because of their inert nature. However, such kinds of properties will be influenced by mixing nonmetals and metals (see Figure 1).  
In this classification section, the sp\({}^{3}\) ratio, H content, and other properties of several types of C-films were studied to categorize the amorphous C-films, including DLC. The results demonstrated that amorphous C exists in a wide range in the ternary diagrams of sp\({}^{2}\), sp\({}^{3}\), and H. Subsequently, amorphous C was categorized in four groups of the DLC region and one group of the PLC region. Herein, the percentage contribution factors of categorized groups were observed as ta-C content >50% of sp\({}^{3}\) ratio and <5% of H. At the same time, a-C content was <50% of sp\({}^{3}\) ratio and <5% of H. As they categorized four DLC regions, it was more reasonable to classify these DLC groups as polymer-like C-films rather than DLC films. The maximum level of sp\({}^{3}\) ratio was expected to suppress film corrosion, due to mellite reactions for corrosion resistance; therefore, this was almost distinct from the sp\({}^{3}\) ratio. Typically, it has been introduced to internationally standardize C-films, including DLC, without disclosing data, and reports have been published as ISO20523:2017 [22]. The major findings of this study are expected to serve as a basis for deriving the sp\({}^{3}\) ratio and H content at high precision using XPS and glow-discharge optical emission spectroscopy. DLC films can be further classified as a biocompatible material [23] in a refraction index plane and extinction coefficient [24], which can help in large  
Figure 1: The general classification of DLC coatings.  
scale development and industrial applications. However, as per ISO 20523:2017 disclosed of the standard specifies classification, and designations, and predominant constituent on DLC films. Amorphous C-based films are also known as DLC, as well as graphite, polymer-like films, and CVD diamond films. This applies to mass-production on an industrial scale, which is appropriate for the C-based films. However, it does not apply to the entire coating, and it can be made up of a main functional layer and additional layers on the top and bottom. Over the development of its thickness, a material's property may be changed. These kinds of layers are known as gradient layers. C-based films can include other elements like H, metal, and other elements. Metal carbide can be used as a metal constituent with supplementary elements, and are only covered if C is the predominant constituent part. However, a detailed informative fact was found regarding the DLC films classification based on the experimental results, and their characteristics and applications are explained in different subsections.