Application Note | Humidity Tribology
Humidity Effect on DLC Coating Tribology
How Relative Humidity Changes the Friction, Transfer Film, and Wear of Diamond-Like Carbon
Research & Experimental Testing
NANOVEA Laboratory
Visual Design & Editorial
Andrew Shore
Introduction
Diamond-like carbon (DLC) coatings possess enhanced tribological properties, namely excellent wear resistance and a very low coefficient of friction (COF). DLC coatings impart diamond characteristics when deposited on different materials. Favorable tribo-mechanical properties make DLC coatings preferable in various industrial applications, such as aerospace parts, razor blades, metal cutting tools, bearings, motorcycle engines, and medical implants.
Why Wear Evaluation of DLC in Humidity Matters
DLC coatings exhibit very low COF (below 0.1) against steel balls under high vacuum and dry conditions1, 2. However, DLC coatings are sensitive to environmental condition changes, particularly relative humidity (RH)3. Environments with high humidity and oxygen concentration may lead to significant increase in COF4. Reliable wear evaluation in controlled humidity simulates realistic environmental conditions of DLC coatings for tribological applications. Users select the best DLC coatings for target applications with proper comparison of DLC wear behaviors exposed to different humidity.
ℹ️ Learn more about humidity-controlled friction and wear testing lab services.
Measurement Objective
This study showcases the NANOVEA Tribometer equipped with a humidity controller is the ideal tool for investigating wear behavior of DLC coatings at various relative humidity.
NANOVEA T200 Compact
Pneumatic Tribometer
Test Procedure
Friction and wear resistance of DLC coatings were evaluated by the NANOVEA Tribometer. Test parameters are summarized in Table 1. A humidity controller attached to the tribo-chamber precisely controlled the relative humidity (RH) with an accuracy of ±1%. Wear tracks on DLC coatings and wear scars on SiN balls were examined using an optical microscope after tests.
Note: Any solid ball material can be applied to simulate the performance of different material coupling under environmental conditions such as in lubricant or high temperature.| Parameter | Value |
|---|---|
| Coupon sample material | Stainless steel SS304 |
| Ball material | SiN |
| Ball diameter | 6 mm |
| Normal force | 2 N |
| Rotational speed | 200 rpm |
| Duration of test | 15 min |
| Wear track radius | 3 mm |
| Lubricant | None |
| Atmosphere | Air |
| Temperature | 24°C (room) |
| Humidity | 10%, 30%, 50%, 70% and 90% |
Table 1: Test parameters of the wear measurements.
Results and Discussion
DLC coatings are great for tribological applications due to their low friction and superior wear resistance. The DLC coating friction exhibits humidity dependent behavior shown in Fig. 2. The DLC coating shows a very low COF of ~0.05 throughout the wear test in relatively dry conditions (10% RH). The DLC coating exhibits a constant COF of ~0.1 during the test as RH increases to 30%. The initial run-in stage of COF is observed in the first 2000 revolutions when RH rises above 50%. The DLC coating shows a maximum COF of ~0.20, ~0.26 and ~0.33 in RH of 50, 70 and 90%, respectively. Following the run-in period, the DLC coating COF stays constant at ~0.11, 0.13 and 0.20 in RH of 50, 70 and 90%, respectively.
| Relative humidity | Maximum COF (run-in) | Steady COF |
|---|---|---|
| 10% | – | ~0.05 |
| 30% | – | ~0.1 |
| 50% | ~0.20 | ~0.11 |
| 70% | ~0.26 | ~0.13 |
| 90% | ~0.33 | ~0.20 |
Table 2: COF of the DLC coating at different relative humidity.
Fig. 3 compares SiN ball wear scars and Fig. 4 compares DLC coating wear tracks after the wear tests. The diameter of the wear scar was smaller when the DLC coating was exposed to an environment with low humidity. Transfer DLC layer accumulates on the SiN ball surface during the repetitive sliding process at the contact surface. At this stage, the DLC coating slides against its own transfer layer which acts as an efficient lubricant to facilitate the relative motion and restrain further mass loss caused by shear deformation. A transfer film is observed in the wear scar of the SiN ball in low RH environments (e.g. 10% and 30%), resulting in a decelerated wear process on the ball. This wear process reflects on the DLC coating’s wear track morphology as shown in Fig. 4. The DLC coating exhibits a smaller wear track in dry environments, due to the formation of a stable DLC transfer film at the contact interface which significantly reduces friction and wear rate.
Conclusion
Humidity plays a vital role in the tribological performance of DLC coatings. The DLC coating possesses significantly enhanced wear resistance and superior low friction in dry conditions due to the formation of a stable graphitic layer transferred onto the sliding counterpart (a SiN ball in this study). The DLC coating slides against its own transfer layer, which acts as an efficient lubricant to facilitate the relative motion and restrain further mass loss caused by shear deformation. A film is not observed on the SiN ball with increasing relative humidity, leading to an increased wear rate on the SiN ball and the DLC coating.
The NANOVEA Tribometer offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional humidity modules available in one pre-integrated system. It allows users to simulate the work environment at different humidity, providing users an ideal tool to quantitatively assess the tribological behaviors of materials under different work conditions.
References
1. C. Donnet, Surf. Coat. Technol. 100–101 (1998) 180.
2. K. Miyoshi, B. Pohlchuck, K.W. Street, J.S. Zabinski, J.H. Sanders, A.A. Voevodin, R.L.C. Wu, Wear 225–229 (1999) 65.
3. R. Gilmore, R. Hauert, Surf. Coat. Technol. 133–134 (2000) 437.
4. R. Memming, H.J. Tolle, P.E. Wierenga, Thin Solid Coatings 143 (1986) 31.
Frequently Asked Questions About DLC Coating Tribology in Humidity
Why does humidity matter for DLC coatings?
DLC coatings show very low friction (below 0.1) against steel under high vacuum and dry conditions, but they are sensitive to environmental changes, particularly relative humidity; high humidity and oxygen concentration can significantly increase friction.
How is DLC wear tested in controlled humidity?
A tribometer with a humidity controller attached to the tribo-chamber holds relative humidity within ±1%. Here a 6 mm SiN ball ran on the DLC coating at 2 N and 200 rpm on a 3 mm radius for 15 minutes at 10%, 30%, 50%, 70% and 90% RH.
How did friction change with humidity?
The COF was ~0.05 at 10% RH and ~0.1 at 30% RH. Above 50% RH a run-in stage appeared in the first 2000 revolutions, peaking at ~0.20, ~0.26 and ~0.33 at 50%, 70% and 90% RH before settling at ~0.11, 0.13 and 0.20.
Why is DLC friction lower in dry conditions?
A DLC transfer layer accumulates on the counter ball during sliding, so the coating slides against its own transfer film, which acts as an efficient lubricant and restrains mass loss. The film forms at low humidity (10% and 30% RH) but is not observed as humidity increases.
How did humidity affect wear?
The SiN ball wear scars were smaller at low humidity, and the DLC coating showed a smaller wear track in dry environments thanks to the stable transfer film; wear of both ball and coating increased with relative humidity.
Where are DLC coatings used?
In aerospace parts, razor blades, metal cutting tools, bearings, motorcycle engines and medical implants, thanks to their excellent wear resistance and very low coefficient of friction.
