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Humidity Effect on DLC Coating Tribology

Importance of Wear Evaluation on DLC in Humidity

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. DLC coatings exhibit very low COF (below 0.1) against steel balls under high vacuum and dry conditions12. 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.


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.

 

 


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.




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 Figure 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.

 



Figure 3 compares SiN ball wear scars and Figure 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 Figure 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.

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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

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3D Surface Analysis of a Penny with Non-contact Profilometry

Importance of Non-contact Profilometry for Coins

Currency is highly valued in modern society because it is traded for goods and services. Coin and paper bill currency circulates around the hands of many people. Constant transfer of physical currency creates surface deformation. Nanovea’s 3D Profilometer scans the topography of coins minted in different years to investigate surface differences. Coin features are easily recognizable to the general public since they are common objects. A penny is ideal for introducing the strength of Nanovea’s Advanced Surface Analysis Software: Mountains 3D. Surface data collected with our 3D Profilometer allows for high level analyses on complex geometry with surface subtraction and 2D contour extraction. Surface subtraction with a controlled mask, stamp, or mold compares the quality of manufacturing processes while contour extraction identifies tolerances with dimensional analysis. Nanovea’s 3D Profilometer and Mountains 3D software investigates the submicron topography of seemingly simple objects, like pennies.


Measurement Objective

The full upper surface of five pennies were scanned using Nanovea’s High-Speed Line Sensor. The inner and outer radius of each penny was measured using Mountains Advanced Analysis Software. An extraction from each penny surface at an area of interest with direct surface subtraction quantified surface deformation.
 


Results and Discussion

3D Surface
The Nanovea HS2000 profilometer took only 24 seconds to scan 4 million points in a 20mm x 20mm area with a 10um x 10um step size to acquire the surface of a penny. Below is a height map and 3D visualization of the scan. The 3D view shows the High-Speed sensor’s ability to pick up small details unperceivable to the eye. Many small scratches are visible across the surface of the penny. Texture and roughness of the coin seen in the 3D view are investigated.
 







Dimensional Analysis
The contours of the penny were extracted and dimensional analysis obtained inner and outer diameters of the edge feature. The outer radius averaged 9.500 mm ± 0.024 while the inner radius averaged 8.960 mm ± 0.032. Additional dimensional analyses Mountains 3D can do on 2D and 3D data sources are distance measurements, step height, planarity, and angle calculations.






Surface Subtraction
Figure 5 shows the area of interest for the surface subtraction analysis. The 2007 penny was used as the reference surface for the four older pennies. Surface subtraction from the 2007 penny surface shows differences between pennies with holes/peaks. Total surface volume difference is obtained from adding volumes of the holes/peaks. The RMS error refers to how closely penny surfaces agree with each other.

 






Conclusion




Nanovea’s High-Speed HS2000L scanned five pennies minted in different years. Mountains 3D software compared surfaces of each coin using contour extraction, dimensional analysis, and surface subtraction. The analysis clearly defines the inner and outer radius between the pennies while directly comparing surface feature differences. With Nanovea’s 3D profilometer’s ability to measure any surfaces with nanometer-level resolution, combined with Mountains 3D analysis capabilities, the possible Research and Quality Control applications are endless.  

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