Potential Impact of Evaluating Coatings for Friction and Wear Reduction Using a High Load Tribometer

Friction and wear are ubiquitous phenomena that occur during the relative motion of solid surfaces in contact. Excessive friction and wear can significantly increase energy consumption through lost efficiency, high maintenance costs, and reduced component service life. Applications ranging from automobile engines and transmissions to industrial machinery, artificial joints, and microelectronic systems require optimized surface interfaces with low friction and high wear resistance.

The Crucial Role of Specialized Coatings in Surface Engineering: Minimizing Friction and Enhancing Wear Protection

The Essential Role of High-Load Tribometers in Evaluating Coatings for Heavy-Duty Applications

Tribometer Overview

Specialized coatings offer an effective surface engineering solution by minimizing friction through mechanisms like solid lubricant films and reduced adhesion forces while simultaneously enhancing wear protection through increased hardness. Various materials, including ceramics, metals, polymers, and composites, can be coated using thermal spraying or physical/chemical vapor deposition technologies.

Tribometers simulate material interfaces under controlled loading conditions to quantify frictional losses and material loss from wear. High-load tribometers with unique test fixtures are essential for evaluating coatings aimed at heavy-duty applications. They assess not only assess friction, wear performance, coating adhesion strength, continuity, and durability. This article discusses an experimental investigation on friction reduction and anti-wear coatings characterization using a high-load reciprocating tribometer.

Coatings for Friction and Wear Reduction

Surface engineering through coatings is widely adopted to enhance the tribological performance and service life of critical components exposed to severe contact conditions. Hard coatings protect against wear and abrasion, whereas soft coatings help minimize friction. Hybrid coatings aim to synergize both of these properties.

Various materials are used to develop specialized coatings, including ceramics like titanium nitride (TiN), tungsten carbide (WC), chromium nitride (CrN), oxides, and carbonitrides that exhibit high hardness. Soft metallic coatings can be deposited using babbitts, silver, gold, and lamellar solids like molybdenum disulfide (MoS2), enabling low shear strength at interfaces. Polymers such as PTFE, epoxy, nylon, and polyimide act as solid lubricants. Reinforcing these polymer matrices with soft inorganic fillers improves load-bearing capacity. Diamond and diamond-like carbon coatings display exceptional tribological properties.

Line-of-sight physical vapor deposition (PVD) techniques allow the forming of hard ceramic coatings with good adhesion. Chemical vapor deposition (CVD) facilitates uniform depositions and doping. Thermal spraying methods using a metallic or cermet feedstock powder produce relatively thick coatings suitable for high-wear applications. Electro and electroless plating techniques assist in embedding solid lubricants. Optimizing coating thickness, density, integrity, surface roughness, and adhesion strength results in durable friction and wear mitigation.

These coatings serve multifunctional purposes, such as minimizing contact area, providing easy shear layers, and acting as wear-resistant and load-bearing barriers to reduce debris formation. They also significantly reduce friction through interfacial slip mechanisms, material transfer layer formation, and protective tribofilm deposition.

Experimental Methodology

To evaluate the coatings’ friction and wear performance, a high-frequency reciprocating rig (HFRR) tribometer (Teer Coatings POD-2) was used. This tribometer can apply 1-200 N loads on a 10×10 mm interface at a maximum frequency of 50 Hz. High stiffness and natural frequency over 200 Hz ensure the dynamic stability of friction measurements. A piezo-electric dynamometer with force resolution down to 10 mN enables accurate coefficient of friction recording.

The coatings tested were a CrN hard coating deposited using cathodic arc PVD and a MoS2-silver composite soft coating deposited through magnetron sputtering. Both coatings had a target thickness of around five μm. The substrate material was 440C stainless steel with surface roughness (Ra) below 50 nm. Samples were polished and ultrasonically cleaned in acetone before coating. Surface profilometry confirmed the deposited coating thickness, and scanning electron microscopy characterized the microstructure.

Tribological experiments were conducted at 20 N load (initial mean contact pressure ≈ 200 MPa) and 25 Hz frequency, resulting in a 0.05 m/s sliding speed. The bulk substrate temperature was maintained at 100°C using the tribometer’s integrated heater and controller unit. Test durations were fixed at 30,000 cycles (~20 minutes) for evaluating the initial run-in, followed by 600,000 cycles (~6.7 hours) for prolonged wear testing in ambient lab air conditions. Normal and tangential forces were sampled at 2 kHz by a 16-bit acquisition system to capture the friction signatures accurately.

Wear rates and mechanisms were analyzed using 3D optical profilometry for wear scar measurements, while micro Raman spectroscopy and energy dispersive X-ray analysis explored near-surface structural and chemical alterations. Counterface wear was gauged by quantifying material transfer and analyzing debris particles. The real-time coefficient of friction data could reveal the engagement dynamics of the solid lubricating mechanisms for the soft MoS2 coating.

Results and Discussion

The CrN-coated sample displayed a steady-state coefficient of friction of around 0.45. However, for the MoS2-Ag coating, the COF reduced from an initial 0.12 to 0.02 within the first 500 cycles, indicating activation of the solid lubricating mechanism. It subsequently remained at a low average value of 0.026 for the test duration.

In terms of wear performance, the CrN coating exhibited an average wear rate of 4.8 × 10-5 mm3/N·m, whereas surprisingly, the soft MoS2 coating showed a 75% lower wear rate of 1.2 × 10-5 mm3/N·m. Optical profilometry across multiple locations revealed a maximum scar depth of 0.7 μm for CrN but only 0.3 μm for the MoS2 composite coating. This highlights the role of the layered lubricating film in enhancing wear protection as well.

Raman spectral mapping indicated partial graphitization of the MoS2 coating around wear tracks and oxidation from frictional heating. This self-adaptive structural transformation ensures replenishment of sheared material, resulting in persistent lubrication. Elemental XPS indicated continuous enrichment of the near-surface region in Mo and S compared to the as-coated condition. This, along with the detection of silver, confirms the material transfer of the lubricating components to counterfaces.

The benchmark stainless steel sample without coating coating showed a 10X higher average COF of 0.28 and a wear rate of 2.3 × 10-4 mm3/N·m. Field emission scanning electron micrographs revealed the presence of severe adhesive wear mechanisms through material transfer and plastic deformation compared to the coated samples. Multiple repeat test runs showed reasonable reproducibility with variability in wear rates under 10%.

Hence, hard and soft lubricant coatings showed significant friction and wear mitigation over uncoated steel, corroborating their efficacy for tribological enhancement. While the CrN coating provides wear protection through its hardness, the nanolayered molybdenum disulfide-silver coating significantly reduces friction by solid lubrication, mitigating adhesive wear as a sacrificial material transfer.

Conclusions

The experimental high-load tribology investigation demonstrated that PVD-deposited CrN ceramic hard coatings and magnetron-sputtered MoS2-Ag composite soft coatings can substantially reduce friction and wear compared to uncoated stainless steel. Due to its high hardness, the CrN coating displayed a 4X lower wear rate. In contrast, the MoS2-Ag coating exhibited a 10X lower steady-state coefficient of friction along with reasonably good wear protection through its nanolayered solid lubricating mechanisms.

The CrN coating proves more suitable for applications requiring resistance against abrasive and adhesive wear. On the other hand, the MoS2-Ag coating is recommended for interfaces expecting predominantly mild wear conditions. Further tribocorrosion studies on these coatings should explore the synergistic effects of friction, wear, and corrosion on degradation rates.