Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests

Significance 

Hard coatings are commonly used to protect the surfaces of moving parts from wear by reducing friction. It uses nitrides and carbide of transition metals or carbon derivatives as a protective layer. More efficient friction-reducing strategies have been developed with emphasis on friction properties of two-dimensional (2D) structures. Despite their high mechanical properties, their low oxidation resistance at elevated temperatures or due to friction-generated heat remains a big challenge. Thus, self-lubrication by introducing the metal oxide supplements is an approaches that can reduce friction and increase the oxidation resistance of the contact surfaces.. For applications characterized with elevated temperatures, this approach becomes highly susceptible to the high-temperature induced effects detrimental to the wear mechanism.

Recently, metal oxides with crystallographic planes in their lattice have been identified as promising candidates for reducing friction. This has been followed by a significant amount of research on oxygen deficient metal oxides of different systems. Although W – O systems show relatively stable lubricious oxide phases, there are limited studies on their tribological benefit, and none of the reported systems contain easy shear oxides of WnO3n-2. Additionally, it is well-known that ceramic composites fabricated using pulsed electric current sintering (PECS) benefit from its advantages like rapid densification. Unfortunately, the tribological and mechanical behavior of WO2.9 have not been fully explored despite its potential application in boundary lubrication to reduce friction.

Herein, researchers at Aalto University: Dr. Erkin Cura, Dr. Michal Trebala, Dr. Yanling Ge, and led by Professor Simo-Pekka Hannula together with Dr. Piotr Klimczyk from Krakow Technology Institute studied the behavior of mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites. Monolithic WO2.9 and self-lubricating ZrO2 + WO2.9 composites were both synthesized via PECS and their friction and mechanical properties were studied by nanoindentation and reciprocating wear tests, respectively. The research team aimed at establishing the possibility of utilizing oxygen deficient tungsten oxide phase as a solid lubricant in a ceramic matrix composite at room temperature. Their work is currently published in the research journal, Wear.

The authors showed that at room temperature (25°C), monolithic WO2.9 recorded a hardness of approximately 11GPa and a decrease in the elastic modulus to about 150 GPa. The monolithic also exhibited significant dependence on normal load and ploughing against the diamond tip geometries, and both ploughing and measured CoF were lower for the Berkovich tip. Additionally, the presence of WO2.9 and applied normal load in test conditions were characterized by a significant decrease in the CoF at room temperature. ZrO2 + 10 vol% WO2.9 composite had the lowest CoF of 0.108 ± 0.030 and a wear rate of mm3/Nm lower than other test materials under 10N applied load because WO2.9 acted as a solid lubricant.

In a nutshell, the study investigated the nano-mechanical and frictional properties of the W20O58CS oxide for potential application as a solid lubricant. The Raman spectroscopy studies of the W20O58CS oxide agreed well with the previous reports with few minor variations in structure which was attributed to the differences in their structural evolution owing to the synthesis techniques. Besides the remarkable frictional properties at room temperature, the authors noted the possibility of enhancing the wear and mechanical properties of the composite by either using a modified matrix or optimizing the sintering process. In a statement to Advances in Engineering, Dr. Erkin Cura, first author explained the study findings contribute to the development of high-performance ceramic matrix composite based solid lubrication for reducing friction between contacting surfaces.

Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests - Advances in Engineering
WO2.9 whiskers as synthesized
Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests - Advances in Engineering
Force/displacement curve of bulk WO2.9 with visible periodic pop-in effect referring to easy shear planes
Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests - Advances in Engineering
CoF of ZrO2 + 10 vol % WO2.9 composites under different loads
Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear tests - Advances in Engineering
Wear scar for ZrO2 + 10 vol % WO2.9 when lowest CoF was achieved

Reference

Cura, M.E., Trebala, M., Ge, Y., Klimczyk, P., & Hannula, S. (2021). Mechanical and tribological properties of WO2.9 and ZrO2 + WO2.9 composites studied by nanoindentation and reciprocating wear testsWear, 478-479, 203920.

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