Table of Content
DLC (Diamond-Like Carbon) is a thin-film coating that is primarily carbon-based and also possesses some of the properties of diamond. Its greatest value lies in its high hardness, low friction, wear resistance, and ability to be deposited at low temperatures. Therefore, it is widely used in automotive parts (piston rings, cams, fuel injectors), precision sliding parts, mold anti-adhesion, medical device surfaces, and 3C moving parts.
The structure and scientific principles of DLC: sp3/sp2, hydrogen content and triboelectric transfer layer
DLC is not a single material, but a collective term for a group of amorphous carbon thin films. The key factor affecting its properties is...The ratio of sp3 (diamond-like bonds) to sp2 (graphite-like bonds),as well asDoes it contain hydrogen (H) or doped elements (Si, W, F, etc.)? When the sp3 ratio is high...At that time, the bond is closer to that of a diamond.Increase in hardness and elastic modulusIt has better wear resistance, but at the same time, the membrane interior...Compressive residual stressIt will also increase, leading toAdhesion and thickness limits become more stringent.ta-C (tetrahedral amorphous carbon) is a representative example of high sp3 content. Conversely, when the sp2 content is higher, the structure is more like graphite, with slightly lower hardness but lower internal stress and better toughness.It is more friendly to thick films and complex shapes.
Hydrogenated DLC (aC:H) can form a low-shear carbon-based transfer layer under dry conditions through hydrogen passivation of surface bonds.The coefficient of friction can be reduced to 0.02–0.06; however, in humid air, water molecules interfere with this transfer layer, often increasing friction. Conversely,Silicon-containing DLC (Si-DLC) readily forms a silicon dioxide-like (Si–O–H) boundary layer in humid environments, but retains low friction under humidity.Very suitablePlastic Adhesion/Mold Demolding and Medical DevicesContact interface, etc.Metal-based DLC (Me-DLC, such as W-DLC) can improve toughness and impact resistance, reduce internal stress and the risk of brittle fracture, but its hardness and wear resistance will be lower than that of pure carbon-based DLC.There are also...Fluorinated DLC (F-DLC)By using extremely low surface energyAnti-stick and easy to clean.
DLC often produces [problems] when sliding.Friction-induced "transfer layer" or "graphite-like boundary layer"It acts like a solid lubricating film, determining the actual coefficient of friction. Ambient gases (dry nitrogen, air humidity), paired materials (steel, aluminum, stainless steel, ceramic), load, and speed all affect the formation and stability of this layer. This is the fundamental reason why DLC appears "sometimes very slippery, but sometimes normal" in different environments.
Want to learn about welding, dissimilar material joining, vacuum welding, and tack welding?
Further Reading :Bonding – The Best Choice for High-Precision and Dissimilar Material Joining
Common DLC Classifications and Key Parameters
The table below shows typical ranges commonly used in engineering practice (these ranges may vary depending on the process/brand; please refer to the supplier's data when selecting).
| type | sp3 content (%) | Hardness (GPa) | Elastic modulus (GPa) | Coefficient of friction (dry N2 / air) | Maximum operating air temperature (°C) | Typical film thickness (μm) | Volume resistivity (Ω·cm) | Features/Uses |
|---|---|---|---|---|---|---|---|---|
| aC:H (hydrogenation) | 30–60 | 10–20 | 80–150 | 0.02–0.06 / 0.08–0.20 | 200–300 | 0.2–3 | 10^6–10^9 | Low-temperature coating, ultra-low friction in dry environments, and sliding parts for automobiles and motorcycles. |
| ta-C (high SP3) | 60–85 | 30–60 | 200–400 | 0.02–0.08 / 0.10–0.20 | 350–500 | 0.2–2 | 10^9–10^13 | High hardness and wear resistance, small size or high contact pressure components, precision sliding |
| Si-DLC (containing silicon) | 30–60 | 8–18 | 60–130 | 0.08–0.15 / 0.05–0.10 | 300–400 | 0.5–3 | 10^7–10^10 | Low friction and anti-adhesion properties even under humidity, suitable for molds and medical devices. |
| Me-DLC (such as W-DLC) | 20–50 | 5–15 | 40–120 | 0.10–0.20 / 0.10–0.20 | 200–350 | 0.5–5 | 10^-3–10^2 | It has good toughness and impact resistance, but its hardness and wear resistance are relatively low. |
Replenish:Thermal ConductivityGenerally, DLC is about0.2–10 W/m·K(aC:H is low, ta-C can be higher), far lower than diamond (>1000 W/m·K), thereforeThermal conductivity and heat dissipation are not DLC's strong points.Regarding surface wettability, DLC contact angles are commonly...70–90°F-DLC can be used at temperatures above 100°C to enhance anti-sticking properties.
Friction and wear performance: the effects of environment, mating materials and load
- Atmosphere
- Dry nitrogen/vacuum: aC:H, ta-C usually appearExtremely low friction (0.02–0.06).
- In normal air (medium to high humidity): the aC:H friction causes the air to rise;Si-DLC, on the contrary, is stable with low friction..
- High-temperature air: DLC will graduallyOxidation/GraphitizationHardness and wear resistance decrease (>300–500°C depending on the type).
- Matching materials
- Steel vs DLCCommon and stable; ta-C can support higher contact pressure.
- Aluminum/Brass for DLCIt can reduce adhesive wear (anti-gluing); Si-DLC or F-DLC performs better.
- Ceramic for DLCIt can achieve very low friction, but attention should be paid to the local stress caused by surface roughness and edges.
- Loads and geometry
- DLC isHard and thinA brittle film; ifThe substrate is too softOr contact geometry leads toHertz contact pressure too highIt is prone to yielding of the substrate or stress concentration at the edges.Membrane cracking or peeling.
- Practical advice:First, determine the hardness and surface roughness of the substrate.(Common Ra≤0.02–0.05 μm); Polish until there are no sharp burrs to avoid "scraping" the cutter/hole.
Process and Design: Substrate, Pretreatment, Thickness, Interface Layer and Application Examples
Process technologyPVD magnetron sputtering, cathode arc (CA)FCVA (Filtered Cathode Vacuum Arc, high sp3 but high internal stress)PECVD/HiPIMS, etc. DLC can be performed at temperatures below 200°C.The deposition process is complete and compatible with heat-sensitive substrates (spring steel, tempered tool steel, stainless steel, aluminum alloy, and medical-grade stainless steel/titanium alloy). Interface layer: Commonly used.Cr, Ti, Si等Adhesive/gradient layer** (100–300 nm) to reduce internal stress gradient and improve adhesion.Thickness selection：
- Precision component low-load sliding:0.5–1 μmIt's sufficient, and the size has little impact.
- General wear resistance:1–2 μm.
- High wear or high contact pressure:2–4 μm(Internal stress and adhesion need to be assessed.) PreprocessingVacuum baking for degassing, plasma cleaning and activation;Surface Ra controlIt is crucial; do it when necessary.Rounded corners/bevelsReduce edge peeling.
Common Examples
- Automobiles and motorcyclesCam/taper, piston ring, fuel injector needle valve — reduce wear and improve efficiency.
- MoldingPlastic mold runners, gates, ejector pins—anti-sticking/easy demolding (Si-DLC or F-DLC recommended).
- knivesAluminum alloy machining tools—inhibit tool sticking and BUE; ta-C is an option for small tools.
- Medical suppliesFor joints/catheters requiring low friction and biocompatibility, use aC:H/Si-DLC (in accordance with regulations and sterilization conditions).
Want to learn about Vf, thermoforming, carbon fiber, and high-pressure RTM?
Further Reading :How to Mass Produce Carbon Fiber Composites? 5 Key Differences and Application Guidelines Between Hot Press Molding and High-Pressure RTM
Quick Selection Table
| Use Cases | Recommended coating | Recommended interface layer | Typical thickness | Remark |
|---|---|---|---|---|
| Ultra-low friction in dry environments (precision sliding) | aC:H or ta-C | Cr/Ti | 0.5–1 μm | aC:H offers lower friction, while ta-C provides greater pressure resistance. |
| Low friction and non-stick properties are still required in humidity/air. | Si-DLC | Si/Cr | 1–2 μm | Mold demolding, commonly used medical materials |
| High contact pressure, intermittent impact | Me-DLC (W-DLC) or ta-C | Cr/Ti | 2–4 μm | Good toughness but low hardness |
| Aluminum alloy cutting anti-sticking tool | ta-C or F-DLC | Cr/Ti | 1–2 μm | Low surface energy is beneficial for chip removal |
| Low-temperature substrate (not suitable for high-temperature treatment) | aC:H/Si-DLC | Choose according to material | 0.5–2 μm | <200°C sedimentation compatibility |
Conclusion
DLC isn't a "cure-all," but as long as...Choose the right type + do a good job of interface and surface engineeringIt canLow friction, wear resistance, low temperature depositionThree key aspects offer extremely high cost-performance. In practice, first clarify the usage environment (dry/wet/temperature), the matching materials and contact geometry, then select aC:H/ta-C/Si-DLC/Me-DLC according to the tables in this article; simultaneously...Substrate hardness, surface roughness, chamfering and interface layerDoing this properly can significantly reduce the risks of early stripping and performance instability. In short,The key to DLC's success lies in the systematic integration of "materials, interface, environment, and geometry".Instead of just looking at a single hardness value.
