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The Greenwood and Williamson (GW) statistical approach of characterizing rough surfaces is extended to include asymmetric distribution of asperity heights using the Weibull distribution. A key parameter that is used to characterize asymmetry is the skewness, and the corresponding Weibull parameters are investigated for a range of practical skewness values. The Weibull distribution is then adopted to model the asperity heights, and once normalized, is used to calculate the contact load, real area of contact and number of contacting asperities using the CEB elastic-plastic model of an equivalent rough surface in contact with a smooth plane. The effect of skewness on different levels of surface roughness, ranging from very smooth surfaces encountered in microtribological applications to rougher surfaces encountered in macrotribological applications is investigated, and also compared to the symmetric Gaussian case. Also, to allow for closed-form solution of the contact equations, simpler exponential distributions are curved-fitted to the contact side of the Weibull distribution, and the analytical results are favorably compared with the numerical results using the Weibull distribution.

Modeling of contact interfaces that inherently include roughness such as joints, clamping devices, and robotic contacts, is very important in many engineering applications. Accurate modeling of such devices requires knowledge of contact parameters such as contact stiffness and contact damping, which are not readily available. In this paper, an experimental method based on contact resonance is developed to extract the contact parameters of realistic rough surfaces under lightly loaded conditions. Both Hertzian spherical contacts and flat rough surfaces in contact under normal loads of up to 1000 mN were studied. Due to roughness, measured contact stiffness values are significantly lower than theoretical values predicted from smooth surfaces in contact. Also, the measured values favorably compare with theoretical values based on both Hertzian and rough contact surfaces. Contact damping ratio values were found to decrease with increasing contact load for both Hertzian and flat surfaces. Furthermore, Hertzian contacts have larger damping compared to rough flat surfaces, which also agrees with the literature. The presence of minute amount of lubricant and wear debris at the interface was also investigated. It was found that both lubricant and wear debris decrease the contact stiffness significantly though only the lubricant significantly increases the damping.

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A frequent application of the nanoscratch technique is to estimate hardness of ultrathin films when substrate effects are encountered with the nanoindentation technique. A model based on the work of Goddard and Wilman, which assumes a rigid-plastic behavior of the deformed surfaces, is commonly used for the determination of hardness from scratch tests, yet it overestimates the hardness of materials by as much as a factor of three at very shallow indentation depths on the order of 1-10 nm. The Goddard and Wilman model was extended in this paper to include the effects of the component of the shear stress tangential to the meridianal plane and the elastic recovery of the plastically deformed surfaces assuming elastic-perfectly-plastic material behavior. The proposed model was subsequently verified by performing nanoscratch experiments on fused quartz, which is homogeneous and isotropic with no known surface layers and with known hardness. The hardness was calculated using both the model based on the work of Goddard and Wilman and the extended model. The hardness calculated using the extended model was in very close agreement with the accepted value of bulk hardness of fused quartz over the range of scratch depths tested, showing the importance of the effects of elastic recovery and interfacial shear stress. The model was further verified for the case of a pure aluminum sample and the native thin film coating of alumina that forms on the surface upon air exposure.

As the slider flying height decreases to sub-5-nm to obtain extremely high-density magnetic recordings of the order of 1Tbit∕in.2, problems of adhesion can cause catastrophic behavior at the magnetic recording head-disk interface (HDI). In the earlier part of the paper, a number of interfacial adhesive models were implemented for simplified HDI configurations (i.e., two flat parallel surfaces and a sphere on a flat surface). With the use of realistic HDI properties, individual adhesive force models, such as van der Waals and electrostatic forces, can provide initial approximations to the adhesive forces present during sub-5-nm flying. In the second part of the paper, realistic roughness conditions applicable to actual HDI’s were modeled using an improved Derjaguin–Muller–Toporov-based elastic-plastic rough surface adhesion model. Specifically, the proposed adhesion model accounts for roughness, the presence of molecularly thin lubricant, and includes electrostatic forces. Using experimentally measured roughness values from ultralow flying HDI’s (root-mean-square roughness of 0.65–1.62 nm), it was found that while the contact force is negligible for an interface with low roughness, the adhesive force dominates such interface. Moreover, the effect of roughness promotes adhesion at higher separations than if a two flat parallel surface configuration is considered. Prior to the onset of contact, the total adhesive force for an interface with low roughness is comparable to a two flat parallel surface approximation. However, the simple flat parallel surface approximation fails to predict the realistic onset of contact due to the exclusion of roughness.