# Diamond-Like Carbon (DLC) Coatings: Classification, Properties, and Applications
## 3 Characterizations/Properties of DLC Coatings
### Thermal Stress: DLC Films  
Stress is a significant inherent parameter of DLC deposition, as stress leads to poor adhesion. Moreover, stress is a mechanical property that can be readily measured. As in bulk materials, the crack at the interference is resolved in large part by mechanical properties. Figure 3: Diagram representing the chemical test procedure [87]. The thermal stress at interference can be readily assumed [88, 89, 90]. Thermal stress is a big problem for DLC and diamond films due to the small coefficient of thermal expansion of diamond (see Table 2). However, if diamond films are deposited on a substrate at elevated temperature, after they cool at room temperature, the substrate will bond better than diamond over-layers. Later, the diamond film is bonded to the substrate, and the interface area will change to match that of the substrate's orientation, resulting in the development of compressive stress on films. The stress due to the thermal expansion mismatch between a substrate and over-layer can be determined utilizing Equation (1):  
\[\sigma=\frac{E(\alpha_{r}-\alpha_{s})\Delta T}{(1-V)} \tag{1}\]  
where \(\sigma\): stress; \(E\): modulus of elasticity; \(\alpha\)f and \(\alpha\)s: average coefficients of expansion of film and substrate, respectively; \(\Delta T\): change in temperature; and \(v\): Poisson's ratio. Accepting that the thermal coefficient of growth of the DLC film was equivalent to diamond and that Poisson's ratio is 0.2 [90], the pressure in a thin film was determined to be 1.4 x10* Pa compressive stress. The range of measured stress values for a film on Si may vary between \(5\times 10^{8}\) and \(4\times 10^{8}\) Pa (see Table 3). \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Materials**} & \multicolumn{2}{c}{**Linear Coefficient of**} & \multicolumn{2}{c}{**Modulus of**} & \multirow{2}{*}{**Density (g cm\({}^{-3}\))**} & \multirow{2}{*}{**Reference**} \\  & **Thermal Expansion** & & & \\  & **(at 20 °C) x10*0C-1** & & & \\ \hline Al & 23 & 0.706 & 2.702 & [91, 92, 93] \\ Al:O\({}_{3}\) & 5.3 & 4.0 & 3.965 & [91, 93, 94] \\ Chromium & 5.0 & 2.79 & 7.20 & [91, 92, 93] \\ Cu & 16.7 & 1.298 & 8.92 & [91, 92, 93] \\ Diamond & 1.0 & 9.1 & 3.51 & [91, 93, 94] \\ Germanium & 5.7 & ---- & 5.35 & [91, 92, 93] \\ Glass & 4–9 & 0.48–0.83 & ---- & [91, 95] \\ Graphite & 7.8 & ---- & 2.25 & [91, 92, 93] \\ Fe & 11.8 & 1.523–2.114 & 7.86 & [91, 92, 93] \\ Molybdenum & 5.0 & 3.248 & 10.2 & [91, 92, 93] \\ Ni & 12.8 & 1.995–2.192 & 8.9 & [91, 92, 93] \\ Platinum & 8.9 & 1.67 & 21.45 & [91, 92, 93] \\ Si & 2.5 & 6.6 & 2.33 & [91, 93, 96] \\ SiC & 3.7 & 4.8 & 3.217 & [91, 93, 94] \\ SiO\({}_{2}\) & 11.2 & ---- & 2.64 & [91, 92, 93] \\ NaCl & 42.3 & ---- & 2.165 & [91, 92, 93] \\ Stainless Steel & 15.9 & 2.153 & ---- & [91, 92, 93] \\ Ti & 8.6 & 4.7 & 4.5 & [91, 92, 93] \\ Tic & 6.2 & ---- & 4.93 & [91, 92, 93] \\ Tungsten & 4.5 & 4.11 & 19.35 & [91, 92, 93] \\ Uranium & 14.1 & ---- & 19.05 & [91, 92, 93] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical and thermal properties of various materials. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Film Stress (Pa)** & & & \\ **(C) = compressive** & **Film composition** & **Deposition process** & **Reference** \\ **(T) = tensile** & & \\ \hline
5. To \(7\times 10^{9}\) (C) & DLC on glass & Glow discharge & [97] \\ \(5\times 10^{8}\) (C) & DLC on glass & E-beam heated C rod & [98] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Quantitative stress measurements for DLC and diamond films. [PERSON] [109] and [PERSON] [110] have disclosed a quick survey on the issue of stress in DLC and diamond film coatings. As opposed to adhesion, the stress in films can be quantitatively measured. Generally, it can be performed by the beam deflection method, which was examined by [PERSON] [92]. Table 3 shows the investigations of quantitative measurements of the stress for films. [PERSON] and [PERSON] [100] analyzed the buckling of C-films on a glass substrate to infer internal stress.