Nanomechanical Characterisation of Diamond-Like Carbon Coatings for Tribological Performance

Diamond-like carbon (DLC) coatings are becoming increasingly popular in the automotive industry due to their high hardness, resistance to wear and low friction coefficient. Additionally, they have seen more recent use in the oil and gas industry as protective coatings for flow control devices such as gate and ball valves. They can suffer from poor adhesion at high loads and impact stresses. Well characterised coatings will enable the relationship between mechanical properties and tribological behaviour under different wear regimes to be studied.

To this end, three DLC variants have been produced; amorphous hydrogenated carbon, silicon-doped amorphous hydrogenated carbon and tungsten-doped amorphous hydrogenated carbon (a-C:H, Si:a-C:H and a-C:H:W) on two different substrates (316L stainless steel and hardened M2 tool steel) using the Hauzer Flexicoat 850 system located at the University of Leeds. Total thickness of the coating was varied from 1–5 µm. For the a-C:H coating, the substrate roughness was varied between 0.01 and 0.08 µm Ra. A Cr + WC/W-C:H interlayer is present in all coatings to aid adhesion to the substrate.

Mechanical characterisation has been performed using nanoindentation on the Micro Materials Nanotest Platform using a partial loading technique. Structural characterisation of the DLC was performed using Raman spectroscopy to measure graphitisation and disorder and electron energy loss spectroscopy (EELS) to determine sp2/sp3 ratio respectively. Throughout the testing, scanning electron microscopy (SEM) has been used to observe the deformation and failure mechanisms of the coatings.

Following this nano-scale fatigue resistance of the coatings was measured by comparing nano and micro-scale impact testing with solid particle erosion. Erosion testing was performed with a bespoke air powered flow system. Depth reached and relative depth increase with load during impact testing was compared with the amount of substrate visible (measured using optical pixel threshold method) after time-steps of erosion testing. For this application it was found that a lower H/E ratio and less severe cracking is beneficial as seen with a-C:H:W. Cross-section focused ion beam scanning electron microscopy was used to observe the coating-interlayer cracking phenomena.

Nano-scratch testing was performed to investigate the interfacial contribution to friction of various probe radii (4:5 µm, 8 µm, 72 µm and 170 µm) compared with one DLC coating, a-C:H on 316L stainless steel. All the DLC coatings were subsequently tested using the 4.5 µm probe to observe critical load failures of each coating architecture. Subcritical load scratch tests were also performed to investigate the number of passes to failure.

Fretting tests have been performed on two length scales. Larger length-scale utilised a 5 mm radius 52100 steel ball and a displacement amplitude of ±50 µm with 20 N and 40 N loads. Both dry and lubricated conditions were employed to assess the coating’s performance in the gross slip regime. Nano-wear fretting testing with a displacement amplitude of ±1 µm was performed to match the contact pressures of the larger scale fretting using the Nano-Fretting module of the Nanotest Platform.

Finally, DLC coatings previously studied, sharing similar architecture to the main DLCs produced for this study, have been tested using the NanoTriboTest module recently developed by Micro Materials Ltd. This module has allowed frictional tests to be performed on the nano-scale in the reciprocating sliding regime to compare with the fretting results. A 25 µm radius sphero-conical diamond probe was used giving larger contact pressures than typical tribological contacts (> 10GPa) to model accelerated wear.