Wear-Resistant Coating Evaluation by High-Load Tribometer

A valve is used to regulate, direct or control the flow of a fluid such as gases, liquids, fluidized solids, or slurries. Valves are widely applied in virtually every industrial sector including water and sewage processing, upstream oil and gas, downstream and chemical processing, mining and minerals processing, nuclear power, hydro power and countless others. Valves are often exposed to high stress levels at the contact and extreme application environments combining wear, corrosion, high temperature, etc. Therefore, protective coatings are applied to extend the lifetime of the valves and to cut the cost and time of repair and replacement.

A number of techniques have been applied to deposit hard valve coatings, such as high velocity oxygen fuel spraying (HVOF), plasma transferred arc welding (PTA), laser cladding, physical vapor deposition (PVD) and others. These coatings possess superior wear resistance under conditions of high pressure and stress concentration. Reliable tribological evaluation under a high load is hence critical to simulate and compare the wear behaviors of different valve coatings in a controlled and monitored manner and to select the best candidate for the targeted industry.

Conventional pin-on-disc tribometers usually have a load capacity below 100 N, which makes it difficult to simulate severe service conditions under high pressure. Moreover, it may take days or even weeks to test one sample using a low load tribometer due to the superior wear resistance of the test sample.

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The coefficient of friction, COF, and the wear resistance of two wear-resistant valve coatings and the bare metal substrate were evaluated by the NANOVEA Macro Tribometer. An Al₂O₃ ball (10 mm diameter) was used as the counter material. The wear track and the Al₂O₃ ball were examined using the NANOVEA 3D non-contact profilometer after the tests. The test parameters are summarized in Table 1.

Please note that the Al₂O₃ ball as a counter material was used as an example in this study. Any solid material with different shapes can be applied using a custom fixture to simulate the performance of different material couplings under actual application conditions.

The wear rate, K, was evaluated using the formula K = V/(F × s) = A/(F × n), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolutions. Wear track profiles and wear scars of the balls were evaluated by the NANOVEA Optical Profilometer, and the wear track morphology was examined using an optical microscope.

The NANOVEA Macro Tribometer was employed to measure the COF and wear rate of the valve coatings using a high load of 100 N and a large contact area in order to better simulate the application conditions of the valves. Wear rate is a vital factor for determining the service lifetime of the valves, while a low friction is desirable to improve equipment performance and efficiency and to avoid damaging issues caused by excessive friction such as galling, deposit formation, or over-tightened packing. Fig. 2 compares the evolution of COF during the tests. Due to the substantial difference in wear resistance between the test coatings and substrate, the substrate was tested for 3 min, compared to 30 min for the two coatings. The two valve coatings show a comparable COF of ~0.6 throughout the wear test. The substrate exhibits a slightly higher average COF of ~0.65 after the run-in period of the first 30 sec.

Fig. 3 compares the wear track profiles after the tests measured by the NANOVEA non-contact optical profilometer, and Table 2 summarizes the results of the wear track analysis. Fig. 4 shows the wear tracks of the test samples under the optical microscope. It can be observed that Coating 2 exhibits a much larger wear track compared to Coating 1 after the 30 min wear test. The wear rate of Coating 1 is 1.2E-04 mm³/N m, which is about nine times lower compared to Coating 2. The wear track depths of Coating 1 and Coating 2 are 29 and 160 µm, respectively. Since the coatings have a thickness of 300 µm, the wear rate calculated here reflects the wear resistance of the coatings. The substrate has a wide and rough wear track after the three min wear test. The formation of such a peculiar wear track on the substrate is further analyzed by 3D scan of the wear scar on the counter material, the Al₂O₃ ball.

Fig. 5 displays the 3D wear scar morphology on the Al₂O₃ balls against the two coatings and the substrate after the wear tests. The volume loss and maximum height loss of the balls are summarized in Table 3 as calculated using the NANOVEA analysis software. Coating 1, which possesses the best wear resistance among the tested samples, creates a wear scar with a volume loss of 0.073 mm³ on the Al₂O₃ ball, compared to 0.149 mm³ for Coating 2. In comparison, the Al₂O₃ ball against the substrate after only a three min wear test exhibits a big flattened wear scar with a volume loss of 1.510 mm³. This is attributed to the formation of a large amount of abrasive debris removed from the substrate during the wear test, which in turn substantially accelerated the wear rate of both the substrate and the Al₂O₃ ball. As a consequence, a wide and rough wear track was quickly generated on the substrate in three min.

This finding demonstrates the significant enhancement of wear resistance by the valve coatings compared to the metal substrate. The formation of abrasive debris plays an important role in the wear behavior of both the test sample and the counter material. Other materials with different shapes besides the Al₂O₃ ball can also be applied as counter materials to simulate and compare the wear performance of different material couplings under actual application conditions, e.g. high temperature, corrosion, lubrication, etc.

The wear resistance of valves under high pressure and concentrated stress plays a vital role in their service performance. Valve coatings can significantly extend the lifetime of the valves under high stress conditions and reduce the time and cost of valve repair or replacement.

The NANOVEA Macro Tribometer is designed with high torque capability for loads up to 500 N and a precisely controlled motor for rotational speeds from 0.01 to 2000 rpm. It offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear and lubrication modules available in one pre-integrated system. This range allows users to simulate different severe work environments of the valves, including high stress, wear and high temperature, and select the best candidate for the targeted industrial applications. It also provides an ideal tool to quantitatively assess the tribological behaviors of superior wear resistant materials under high loads.

The NANOVEA 3D non-contact profilometer enables precise wear volume measurement and offers a tool to analyze the detailed morphology of the wear tracks, providing more insight into the fundamental understanding of wear mechanisms.