The influence of surface roughness on the friction coefficient of silicon carbide bushings stems from the interplay between microscopic geometry and contact mechanics. As a typical hard ceramic material, silicon carbide's surface roughness significantly influences frictional behavior by altering the actual contact area, lubricating film distribution, and microscopic interaction patterns. This effect is not a simple linear relationship, but rather a complex coupling effect involving multiple scales and mechanisms.
Surface roughness primarily affects the friction coefficient by altering the actual contact area. When two objects come into contact, actual contact occurs only within the localized region of microscopic asperities. When the surface roughness of silicon carbide bushings is high, the number of asperities increases, and their heights vary significantly, resulting in a decrease in the actual contact area. This increases the load per unit contact area, causing local stresses to exceed the material's yield strength and induce plastic deformation. This deformation enhances the mechanical interlocking effect between the asperities, increasing sliding resistance and, consequently, the friction coefficient. Conversely, when the surface roughness is low, the asperity heights become more uniform, increasing the actual contact area, reducing the unit load, weakening the mechanical interlocking effect, and consequently decreasing the friction coefficient.
Surface roughness plays a key role in the distribution and stability of the lubricating film. Silicon carbide bushings often carry a lubricant during friction, and their surface roughness directly influences the formation and maintenance of the lubricating film. When the surface roughness is moderate, microscopic valleys act as reservoirs for lubricant, continuously replenishing lubricant to the contacting surfaces during sliding, forming a stable lubricating film. This lubricating film effectively isolates the two contacting surfaces, reducing direct contact and thereby lowering the coefficient of friction. However, if the surface roughness is excessive and the valleys are too deep, the lubricant cannot be evenly distributed, resulting in insufficient lubrication in localized areas and an increased coefficient of friction. If the surface is too smooth, the valleys are reduced, the lubricant storage capacity decreases, and the lubricating film is prone to rupture, similarly causing an increase in the coefficient of friction.
Surface roughness also regulates the friction coefficient by influencing the microscopic interaction patterns. During friction, the asperities on the surface of silicon carbide bushings interact with the mating material surface through mechanical engagement, adhesion, and plowing. When the surface roughness is high, the mechanical engagement is significant, and the asperities continuously cut into the mating material surface during sliding, forming plowing grooves and generating wear debris. This wear debris may adhere to the contacting surfaces, forming a transfer film that alters the contact state of the friction pair and, in turn, affects the coefficient of friction. Furthermore, high-roughness surfaces are prone to adhesion. High stress at microscopic contact points causes localized adhesion of the materials, which must be overcome during sliding, further increasing the coefficient of friction. Low-roughness surfaces, on the other hand, are primarily driven by intermolecular forces, resulting in a relatively low coefficient of friction.
The effect of surface roughness on the coefficient of friction is also closely related to material properties. Silicon carbide, as a hard ceramic material, has a high hardness and low toughness, making the effect of surface roughness more significant. Compared to metals, silicon carbide bushings are less susceptible to plastic deformation during friction, resulting in a lower wear rate on asperities and a more durable surface roughness. This means that the friction coefficient of silicon carbide bushings is more stable over time due to surface roughness, less susceptible to fluctuations caused by rapid changes in surface topography.
Dynamic changes in surface roughness can also affect the friction coefficient. During friction, the surface of silicon carbide bushings undergoes continuous wear and refining, and the surface roughness may change over time. During the initial wear phase, peaks are quickly smoothed, reducing surface roughness and the friction coefficient. During the stable wear phase, surface roughness stabilizes and the friction coefficient remains relatively constant. During the severe wear phase, fatigue cracks or spalling may occur, causing roughness to increase again and the friction coefficient to rise. This dynamic change causes the friction coefficient of silicon carbide bushings to exhibit nonlinear characteristics.
