## 2 Materials and Methods  
Table 1 shows the different parameters used to grow PCD films on silicon (Si) substrates using the HFCVD process described in the literature [5; 17]. All substrate surfaces were seeded with 5-10 nm diamond particles from Blueseeds (Adams Nano, Raleigh, NC\(<\) USA 27617) using the standard ultrasonication process described in several publications [1; 2; 5; 6; 9]. For the first 4 films, the film growth time was kept constant at 2 h, the filament-substrate surface distance was kept at 20 mm, the filament temperature was retained at 2300 \({}^{\circ}\)C, and the total pressure during film growth was 10 Torr. The remaining 3 PCD films were grown using the same gas mixture but varying the growth time. The temperature of the substrate surface during film growth was determined using a calibration curve based on thermocouple measurements between a thermocouple in the substrate holder and thermocouples pasted on the surface of a Si substrate. To be able to vary the grain size, two procedures were used: (1) PCD films grown with constant total flow of 102 sccm, varying the H\({}_{2}\):Ar flow rate from 10:90 to 75:25, and (2). PCD films grown with the same H\({}_{2}\):CH\({}_{4}\) flow (no Ar flow) but with increased film growth \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Sample Name**} & **H\({}_{2}\)-CH\({}_{4}\)-Ar** & **Deposition** & **Filament-Substrate** & **Temperature** \\  & **(sccm)** & **Time (h)** & **Distance (mm)** & **(\({}^{\circ}\)C)** \\ \hline A & 10-2-90 & 2 & 20 & 550 \\ B & 25-2-75 & 2 & 20 & 575 \\ C & 50-2-50 & 2 & 20 & 600 \\ D & 75-2-25 & 2 & 20 & 675 \\ E & 200-3-0 & 2 & 10 & 725 \\ F & 200-3-0 & 4 & 10 & 725 \\ G & 200-3-0 & 8 & 10 & 725 \\ \hline \end{tabular}
\end{table} 
Table 1: Process parameters used for the growth of different PDC films on silicon (Si) substrates. Column 1 shows the cross-referenced name use for each condition. time. The CH\({}_{4}\) gas flow was kept constant at 2 sccm, while changing the H\({}_{2}\):Ar flow ratios (10:90, 25:75, 50:50, and 75:25 sccm, respectively). The PCD films grown under the conditions described above are marked in this paper as A, B, C, and D, respectively. In addition, 3 other films were grown using a high H\({}_{2}\):CH\({}_{4}\):Ar flow ratio (200 H\({}_{2}\)/3 CH\({}_{4}\)/0 Ar sccm), varying the film growth times to 2, 4, and 8 h. These samples are marked as E, F, and G, respectively. The parameters described in Table 1 were used in order to increase the grain size from UNCD (2-5 nm) to MCD (\(\geq\)1 \(\upmu\)m) based on prior research [18; 19]. The films were cooled in a hydrogen atmosphere via flowing H\({}_{2}\) gas into the chamber. No other surface treatment was performed on these samples. The type of carbon chemical bonding in the films was analyzed via visible Raman spectroscopy using a Thermo DXR Raman spectrometer (Thermo Fisher Scientific, Waltham, MA, USA 02451) with a 532 nm wavelength laser beam. The deconvolution of the peaks was done using the program Fityk ([[https://fityk.nieto.pl/](https://fityk.nieto.pl/)]([https://fityk.nieto.pl/](https://fityk.nieto.pl/)) (accessed on 14 March 2021)). The morphology, grain size, and thickness of the PCD films were characterized using scanning electron microscopy (SEM, ZEISS SUPRA-40, Zeiss Corporation, Oberkochen, Germany). The grain size was determined using Digital Micrograph software from the Gatan Microscopy Suite (GMS-3, Pleasanton, CA, USA 94588). After uploading the high-resolution SEM image and adjusting the contrast to determine the grain boundaries, the software processed the image to determine the mean size and standard deviation of the grain in the specified area. XRD patterns were obtained with a Rigaku Ultima III X-ray diffractometer (Rigaku, Tokyo, Japan) using the CuK\(\alpha\) radiation (\(\lambda\) = 1.5406 A) and an angular region of 2\(\theta\) = 10-100\({}^{\circ}\) with step-scanning of 0.05\({}^{\circ}\). The grain sizes for samples A and B were calculated from the XRD analysis using the Scherrer equation.