What are the preparation methods and functional applications of diamond films?

Diamond possesses excellent physical and chemical properties: it is the hardest known material, with a low coefficient of friction, the highest elastic modulus, the highest thermal conductivity, good chemical stability, a wide bandgap, and a low dielectric constant. It holds great promise for applications in cutting tools, microelectromechanical systems, biomedicine, aerospace, nuclear energy, and other high‑tech fields. Diamond films inherit diamond’s outstanding characteristics, and their preparation and applications have attracted widespread attention from researchers. Let us now examine several other methods for fabricating diamond films and their functional applications.

1. Hot-Wire Chemical Vapor Deposition Method

Hot‑filament chemical vapor deposition is one of the earliest and most successful methods for fabricating diamond thin films. Compared with other techniques, this deposition process offers significant advantages, including simple equipment, rapid film growth, ease of operation, and low cost, making it the primary method currently used both domestically and internationally for depositing diamond coatings on cutting tools. However, it should be noted that because the hot filament is made of metallic materials—typically tungsten or tantalum—it inevitably contaminates the diamond film. Consequently, its use is often restricted in certain applications where stringent purity requirements are essential.

Depending on the specific application, diamond coatings prepared by the HFCVD method can be deposited on a variety of substrate materials. In the tool‑cutting sector, the primary substrate materials for diamond coatings include WC–Co carbide substrates; for electronic applications, Re‑based substrates and silicon wafers; and for drill bits, silicon carbide substrates as well as self‑supporting diamond coatings.

2. Microwave Plasma Chemical Vapor Deposition Method

Microwave plasma differs from other plasmas in that, under microwave irradiation, it can sustain electrodeless, stable discharges with minimal sample contamination. Moreover, the microwaves efficiently activate and rapidly excite the gas, leading to a high plasma density. Consequently, microwave‑assisted CVD is commonly employed to grow high‑quality diamond films. However, a notable drawback is its limited capability for large‑area diamond film deposition.

3. Chemical Vapor Deposition

A high‑temperature plasma generated by a DC arc discharge is used to dissociate gases in various deposition environments. During the preparation process, the plasma’s high energy density, coupled with the reactive chemical species it produces—such as atomic hydrogen, methyl radicals, and other activated atomic groups—results in an exceptionally high density of these species. Consequently, the rate of diamond film deposition via DC plasma‑jet CVD is very rapid, reaching tens to hundreds of micrometers per hour. Although this technique can deliver remarkably high growth rates, it suffers from substantial equipment costs, elevated overall expenses, and difficulties in achieving precise process control. Moreover, the deposited diamond films often exhibit small active areas, non‑uniform thicknesses, and significant thermal damage to the substrate.

4. Chemical Vapor Deposition by Combustion Flame Method

The principle of flame chemical vapor deposition involves pre-mixing a portion of oxygen into a hydrocarbon gas and then initiating diffusion combustion. The carbon source is acetylene, and the oxidizer is oxygen; the plasma generated by the combustion of acetylene and oxygen deposits a diamond film on the substrate surface. The resulting diamond films are of high quality and can be grown at a rapid rate (100–180 μm/h), making this method well suited for large‑area, complex surfaces. However, the films exhibit significant thermal stress and are prone to impurities.