A new article published in the journal Polymers studies the tribology of carbon fiber reinforced polymer composites (CFRPS), an emerging material solution to meet the demands of several industries. The work was carried out by scientists from the Technical University of Cluj-Napoca in Romania.
Study: Experimental study of the tribological behavior of carbon fiber reinforced polymer composites under boundary lubrication. Image Credit: GO BANANAS DESIGN STUDIO/Shutterstock.com
Carbon fibers were first produced in the middle of the 19th century, with considerable progress in their development and industrial applications in the second half of the 20th century. Defined as fibers containing at least 92% carbon by weight, they have excellent mechanical, physicochemical, chemical and thermal properties.
Beneficial properties of carbon fibers include their superior tensile and flexural properties, low density and high chemical and thermal stability in the absence of oxidizing agents, superior creep resistance, and high electrical and thermal conductivity. Fibers containing more than 99% carbon are called graphite fibers.
Several industries are exploiting the properties of carbon fibers, with the carbon fiber industry developing rapidly over the past decades to meet their demands. They are used in the aerospace industry, automotive manufacturing, military applications, and structural and non-structural elements of construction. Carbon fibers are used in medical items, pressure valves, cars, sporting goods and drilling components.
SEM images of the layers that make up the composite material tested. Image Credit: Birleanu, C et al., Polymers
Carbon fiber reinforced composites
Recent research over the past decades has seen the development of several advanced and innovative carbon fiber reinforced composite materials. In the automotive industry, for example, these composites offer the advantages of low weight and improved aesthetic appeal, and they are used in elements such as doors, bumpers and hoods.
The microstructures of carbon fibers are the main reason why fibers and their composites have found wide commercial viability in several industries. In addition, their length, orientation and concentration in composites are characteristics that govern their effectiveness in a wide range of industrial applications.
The superior damping performance of carbon fiber reinforced composites makes them incredibly useful materials for applications such as high speed drive shafts, robotic arms and machine tool shafts. Thanks to this damping effect, composites can effectively dissipate vibrations, improving the durability of these technologies.
Carbon fiber reinforced composites are typically prepared by carbonizing a liquid precursor impregnated fiber platform with gaseous carbon-containing precursors such as methane. The resulting materials possess a number of beneficial properties, such as high specific strength, low coefficients of thermal expansion, toughness, stiffness, and self-lubricating capabilities. In addition, they have improved refractory properties.
Some studies have been conducted recently on the tribological properties of carbon fiber reinforced composites. Stick-slip phenomena are undesirable in many engineering applications, causing wear, vibration, noise, energy loss and component damage. Wear and friction in composites depends on several parameters such as sliding speed, normal load, friction surface roughness and lubrication.
Layer 1 SEM images and EDS analysis. Image credit: Birleanu, C et al., Polymers
Currently, there is a lack of knowledge about the tribological properties of CFRPs, although pioneering studies have revealed important factors that govern wear and friction behavior.
The current paper in Polymers studied wear and friction in unidirectionally oriented CFRPs. The tribological behavior with respect to the sliding direction of composites under dry lubrication and boundary conditions has been studied extensively in research.
Tribological experiments were performed at a range of normal loads from 20 to 80 N at room temperature. Speeds from 0.4 to 2.4 ms-1 were used in the tests. Experiments were carried out over different time periods to evaluate the tribological behavior of the material samples.
The wear and friction mechanisms of the tribosystem were analyzed and interpreted, as well as the morphological characteristics induced by wear and the resulting wear residues. In addition, the influence of friction time on the friction coefficients of composite materials was examined under stationary and moving conditions (using a rotating disk).
Experimental setup of roughness measurements with the INSIZE ISR-C300 device. Image Credit: Birleanu, C et al., Polymers
It has been revealed that the sliding speed and the applied load contribute significantly to the control of wear and friction. In addition, the samples all had a mostly worn surface due to abrasive wear. In the disk experiment, intermittent loading caused wear effects. Additionally, the authors concluded that analysis of the resulting wear debris is difficult. In the rotating disc experiment, comparatively higher wear values were observed compared to the stationary spindle experiment.
Carbon fiber reinforcement has a noticeable effect on wear and friction characteristics, with experiments demonstrating the lowest wear limit and coefficient of friction observed. In addition, the hardness and elastic modulus of the samples are significantly improved.
There appear to be two factors that contribute to reduced polymer wear. First, the fibers support some of the applied load when exposed to sliding surfaces. Second, the fiber reinforcements reduce the stresses between the contacts of the roughness peaks by smoothing the surface of the reverse face.
In summary, the research revealed several important findings on the tribological behavior of CFRPs, which provide crucial information for future studies of these industrially important materials.
Birleanu, C et al. (2022) Experimental study of tribological behaviors of carbon fiber reinforced polymer composites under boundary lubrication Polymers 14(18) 3716 [online] mdpi.com. Available at: https://www.mdpi.com/2073-4360/14/18/3716