Diamond films, with their exceptional set of properties—ranging from extreme hardness and high thermal conductivity to electrical resistivity—have garnered significant attention in scientific research and technological advancements. [1] - [4] These extraordinary features make diamond films a material of choice for a plethora of industrial and scientific applications, such as cutting tools, thermal management systems, semiconductor devices, and even optoelectronic components [5] [6] . Among the various methodologies for synthesizing diamond films, Microwave Plasma Chemical Vapor Deposition (MPCVD) has emerged as a particularly effective and versatile technique, significantly propelling the field towards new frontiers. [7] - [9] The selection of an appropriate substrate material is a pivotal factor in the successful deposition of diamond films. While silicon substrates have been a traditional choice owing to their well-understood properties and compatibility with diamond, the focus has gradually shifted toward alloys. [10] - [12] Fe-based alloys such as 316, 1018, and 4140 are commonly used in structural applications. These alloys offer a combination of high strength, good corrosion resistance, and excellent mechanical properties. Fe-based amorphous/nanocrystalline alloys, for example, exhibit high saturation magnetic flux density (Bs), high permeability, low coercivity (Hc), and low magnetostriction, making them suitable for use in distribution transformers, reactors, and other devices [13] . Fe-based superelastic alloys are also widely used due to their low cost, easy processing, good plasticity and toughness, and wide applicable temperature range. They find applications in machinery, aerospace, transmission, and medicine [14] . Fe-based amorphous materials, on the other hand, offer opportunities for magnetic sensors, actuators, and magnetostrictive transducers due to their high saturation magnetostriction and low coercive field [15] . However, the intrinsic differences in physical and chemical properties between diamond and steel alloys often necessitate the use of an interlayer to enhance adhesion and compatibility [16] - [18] . Raman spectroscopy serves as an invaluable analytical tool in this context, offering insights into the quality, phase composition, and even the defect structure of the diamond films [19] [20] . Nonetheless, the interpretation of Raman spectra is not straightforward and can be affected by a myriad of factors, such as deposition pressure, gas composition, and plasma power during the MPCVD process, making it essential to carefully control and understand these variables [21] . The objective of this investigation is multifaceted. Primarily, it aims to meticulously analyze the Raman spectra of diamond films deposited on 316, 1018, and 4140 steel alloys via MPCVD. A special focus is accorded to the role of Vanadium Carbide (VC) interlayers, exploring how they influence the quality, adhesion, and compatibility of the diamond films with the steel substrates. This investigation extends beyond the boundaries of material science and engineering, positioning itself at the convergence of addressing global challenges and catalyzing advancements in emerging technologies. By harnessing Microwave Plasma Chemical Vapor Deposition (MPCVD) for the development of diamond-coated steel alloys, we illuminate pathways toward sustainable manufacturing practices, offering solutions that promise enhanced durability and thermal efficiency. Such innovations hold the potential to revolutionize sectors ranging from electronics—where the demand for heat management solutions grows alongside the miniaturization of devices—to renewable energy technologies, where efficiency and longevity are paramount. Moreover, the application of this research in biomedical engineering, through the development of biocompatible and durable materials, underscores its multifaceted impact. By articulating these connections, this work not only broadens its appeal to a diverse peer community but also underscores its contribution to a sustainable future, inviting interdisciplinary collaboration and exploration across scientific and technological domains.

The recipe for nanocrystalline diamond synthesis has been published previously elsewhere [22] . Nanocrystalline diamond films were synthesized using the DiamoTek 700-6 Microwave Plasma CVD system from Microwave Enterprises, Ltd. “1” diameter steel substrates of alloys 316SS, 4140 steel, and Low carbon steel (1018) were employed. Both 5 mm thick CeraTough-0401: Vanadium Carbonitride (VCN) and 0.5 mm thick Hot Thermo-diffused VC coated 316SS, 4140, and Low Carbon Steel (1018) were all purchased from IBC Coatings, Inc. and used as is.

Prior to deposition, the coated substrates were seeded with a diamond slurry in methanol containing 0.005 g of diamond nano-powder with an approximate size of 3.2 nm.

The film growth process was initiated by igniting a microwave plasma with a power of 6 kW at a frequency of 2.45 GHz. The plasma consisted of a mixture of H₂, CH₄, and Ar feeding gases. The chamber operated at a pressure range of 40 to 105 Torr. These variables—gas ratios, microwave power, and chamber pressure—served as the key parameters for controlling the grain size in the resulting films. Specifically, the nanocrystalline diamond was grown using a gas mixture of 2.45% CH₄, 90% Ar, and 7.55% H₂ at a chamber pressure and microwave power set at 105 Torr and 1330 W, respectively.

For characterization, Raman spectroscopy measurements were performed using a Renishaw InVia Raman microscope with a 633 nm light source. Additionally, scanning electron microscopy (SEM) images were captured using a Thermo Fischer Scientific Nova 600 NanoLab and the Carl Zeiss Merlin FE-SEM to confirm the diamond film thickness and compare film morphologies.

VCN failed to show any result after diamond deposition. Therefore, this analysis is for Long Diamond Microwave Plasma Chemical Vapor Deposition on Vanadium Carbide interlayer was tested directly on 316 L, 1018, and 4140 substrates since the vanadium carbide interlayer thickness was ~.5 mm the distance between the steel alloy substrate is increased to prevent the initiation of a ~20 um thick and sp3-bonded tetrahedral amorphous carbon (ta-C) coating Diamond and rapid carbon diffusion into the substrate and the preferential formation of an intermediate graphite layer on steel surface catalyzed by iron and nickel in the substrate.

Figure (A1, SI) Shows coated VCN (top) and VC (bottom) coated steel surface before deposition. In Figure A2 , a clear deposition of VCN-coated samples delaminated after long MPCVD diamond deposition. Raman spectra will be discussed in Figure 2 . In Figure A2 and Figure A3 , 4140 VC coated steel shows that during the growth process, a diamond film occurs on the top of the graphite, which has separated the diamond from the steel substrate.

As depicted in the SEM comparison, Figure 1 , we examine the surface transformations of SS316L and Silicon substrates layered with VC, following the CVD process. On the VC-laden SS316 substrate, the diamond structures in nature after extended CVD processing (27 hours). This implies a robust barrier effect of the 500-micron VC interlayer against carbon-metal interdiffusion. The thickness of the resultant film is ~ 20 um. Figure 1 right, shows the silicon substrates distinct formation of diamond nanocrystallites, indicating a re-nucleation pathway of diamond film formation. From the difference in the images, it can be determined on silicon, the clear diamond nanocrystallites ( Figure 1 , right) that can coalesce quickly to form microcrystalline diamond thin films, as seen in the 316L SS diamond-coated image ( Figure 1 , left).

Figure A4 illustrates the delamination phenomena observed in diamond samples post-MPCVD on VCN-coated steel. An intriguing aspect of these images is the observed partial detachment of the VCN interlayer alongside the diamond film, despite expectations of a 32-micron diamond layer from linear deposition models.

The D band and G Band shifts and FWHM can be found in the supplementary data ( Figure A5 and Figure A6 ). The Raman spectrum analysis in Figure 2 shows an overlay of short-term ~2 hours inhomogeneously broadened control small nanocrystalllite coated diamond silicon sample and nanocrystal growth coated steel alloys post a long-term MPCVD process. The silicon sample is broadened due to the many microstates of the smaller nanocrystallites, revealing the disorder evident in the D and G bands. The ratio of D/G gives the relative disorder of the diamond coating [23] . Particularly striking are the distinct diamond peaks at 1332 cm⁻¹ across 1018 and 4140 spectra and a unique, blue-shifted peak at ~1395 cm⁻¹ in the 316-steel diamond-coated spectra, likely indicating diverse bonding environments [24] . The peak height ratio in the 1018 sample points to suppressed graphitization and low disorder. In the 4140 coated steel samples, the Peak to Peak height ratio shows a large amount of graphitization and induced diamond disorder. The main blue shifted portion of the 316L SS spectra which is around 63.15 cm⁻¹ from the expected 1332 cm⁻¹. In order to determine the strain in the 316L SS diamond-coated

sample, we use the following equation:

${\epsilon }_c=-\frac{\Delta \omega }{\left({\omega }_0\times \gamma \right)}$ (1)

where:

We used this formula to calculate the strain for 316L SS, using 1332 cm⁻¹ for ω₀ and 1.2 as γ, as has been reported for the Gruneisen parameter for diamonds [25] [26] . The calculated value for the strain using Equation (1) is −0.04% or −4%. This value corresponds to the Δω = 63.15 cm⁻¹. The 316 SS spectra correspond to an extreme compressive strain, which is essentially tightening of the diamond matrix to a sp3-bonded tetrahedral amorphous carbon with an average calculated bond length of 1.479 Å. the introduction of strain in the first order Raman peak, is evident in the blueshift and splitting of the peak at ~1395 cm⁻¹ due to the transverse optical (TO) and longitudinal optical (LO) phonon modes (BZ) [27] . The extremely strained structural distortion induces relaxation of the q ≈ 0 BZ zone and allows detection of the phonon modes away from the first Brillouin zone [27] . Note the first order BZ peak as the small broad shoulder at the bottom of the low wavenumber side of the 316L SS Raman spectrum at 1332 cm⁻¹. The majority of diamonds have been grown to a high concentration due to re-nucleation and have compressively strained due to bonding with and the intrinsic nature of the softness of the thick VC interlayer top surface. The surface topology is a soft impression of the supporting structural bonded bottom V-C layer and the steel surface. The ta-C is high quality by SEM and completely covers the diamond-like FCC 316L SS surface ( Figure 1 ) with a sp3-bonded tetrahedral amorphous carbon coating (ta-C) of extreme strain. The extremely strong directional sp3 bonds lead to many extraordinary properties and applications of ta-C, such as the highest known hardness, very low friction and adhesion, unmatched thermal conductivity, electronic mobility, the highest electron dispersion, high dielectric breakdown, radiation hardness, biocompatibility, and chemical inertness. [28] [29] The graphitic peaks are at the background level and almost absent in the 316L SS coated Raman Spectrum, indicating that the formation of graphite has been prevented from being formed. This sample has been completely coated with a continuous and thick diamond cladding. This is advantageous, and it has surpassed the quality of the nanocrystalline standard sample. In the 1018 and 4140 spectra, Figure 2 , the presence of sharp graphitic peaks at ~1620 cm⁻¹ point towards a complex interplay of sp₃ and sp₂ carbon structures [24] . The difference in diamond coating morphology is confirmed by the SEM image of the 4140 sample (A4, SI). Figure 3 shows the second-order phonon mode of graphite [30] . Graphitization and Disorder are the highest in the 4140-diamond-coated sample. The 4140 This spectral analysis provides insightful clues into the nature of the diamond films and the underlying carbide layer, especially in terms of graphite formation and its potential impact on film uniformity and adhesion.

In this study, we unravel the intricate interplay between diamond deposition variables and the resultant film qualities on steel alloys, underpinned by comprehensive Raman spectral analysis. Our findings illuminate the pivotal role of D and G band shifts in demarcating the crystallinity and disorder within carbon materials, a phenomenon that is intricately tied to the electronic, mechanical, and thermal properties of the resultant films. The nuanced understanding of D and G band behaviors, garnered through meticulous spectral analysis, provides a robust framework for tailoring material properties to specific application needs. This research not only advances the frontier of material science by offering a deeper insight into the structural nuances of diamond-coated steel alloys but also opens new avenues for innovation in fields as diverse as electronics, energy storage, and nanotechnology. By marrying the unique properties of diamond with the versatility of steel, we set the stage for next-generation materials that promise to revolutionize industrial applications, from enhancing the durability of mechanical parts to enabling more efficient thermal management solutions. Thus, our work stands at the confluence of fundamental science and practical technology, heralding a new era of materials engineering that leverages the full potential of composite materials for sustainable and high-performance applications.

The successful suppression of graphite formation in ta-C diamond films grown on 316L stainless steel, as compared to the results on diamond-coated 1018 and 4140 steel alloys, can be attributed to several key factors inherent to the composition and microstructure of 316L stainless steel. Notably, 316L is a low-carbon variant of the 316 stainless steel, typically containing less than 0.03% carbon. This low carbon content is crucial in reducing the formation of chromium carbide (Cr₇C₃ or Cr₂₃C₆) at grain boundaries, especially under the high-temperature conditions of Microwave Plasma Chemical Vapor Deposition (MPCVD). The minimized carbide formation at the interface between the steel and the vanadium carbide (VC) coating significantly reduces the likelihood of graphite nucleation, thereby promoting the growth of high-quality diamond films.

In addition to its low carbon content, 316L stainless steel contains high levels of chromium (16% - 18%), along with nickel (10% - 14%) and molybdenum (2% - 3%). The presence of chromium enhances the oxidation resistance of the alloy, which is vital for maintaining a stable interface with the VC coating during the MPCVD process. This stability is critical in suppressing graphite formation and facilitating diamond growth. The other alloying elements, such as nickel and molybdenum, contribute to the overall structural stability of the alloy, particularly under the extreme conditions of diamond film deposition.

Furthermore, the austenitic microstructure of 316L stainless steel, characterized by its face-centered cubic (FCC) crystal structure, provides a compatible and uniform lattice for the VC coating. This compatibility ensures a stable and conducive growth surface for ta-C diamond deposition. The thermal properties of 316L, including its conductivity and coefficient of thermal expansion, also play a significant role in determining the stress states at the interface during the MPCVD process, further influencing the diamond film’s quality.

In contrast, 1018 and 4140 steels, with their higher carbon contents and lack of substantial chromium and nickel, offer favorable conditions for diamond growth. These differences in composition and microstructure lead to a higher tendency for diamond growth.

Overall, the unique combination of low carbon content, specific alloying elements, and austenitic microstructure in 316L stainless steel creates an optimal substrate for high-quality diamond film growth using MPCVD, effectively minimizing the risk of graphite formation and enhancing the diamond film’s quality compared to 1018 and 4140 steels.

Clyde Varner II was involved in Conceptualization, Investigation, Data Curation, Visualization, Writing and Editing of the Manuscript. Leslie Wilson was involved in Investigation, Methodology and Data Curation, Writing and Editing the Manuscript. Angela Davis was involved in Editing of the Manuscript. Padmaja Guggilla was involved in Editing of the Manuscript.

MPCVD, SEM, and Raman Spectroscopic Analysis research were conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.

This work was supported by the National Science Foundation under Grant No. #2331969. Any opinions, findings, conclusions, or recommendations expressed in this work are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Data will be made available upon request.