Andreas A. Polycarpou

Contact of Rough Surfaces With Asymmetric Distribution of Asperity Heights

The Greenwood and Williamson (GW) statistical approach of characterizing rough sur-faces is extended to include asymmetric distribution of asperity heights using the Weibulldistribution. A key parameter that is used to characterize asymmetry is the skewness, andthe corresponding Weibull parameters are investigated for a range of practical skewnessvalues. The Weibull distribution is then adopted to model the asperity heights, and oncenormalized, is used to calculate the contact load, real area of contact and number ofcontacting asperities using the CEB elastic-plastic model of an equivalent rough surfacein contact with a smooth plane. The effect of skewness on different levels of surfaceroughness, ranging from very smooth surfaces encountered in microtribological applica-tions to rougher surfaces encountered in macrotribological applications is investigated,and also compared to the symmetric Gaussian case. Also, to allow for closed-form solu-tion of the contact equations, simpler exponential distributions are curved-fitted to thecontact side of the Weibull distribution, and the analytical results are favorably comparedwith the numerical results using the Weibull distribution. @DOI: 10.1115/1.1403458#

Analytical approximations in modeling contacting rough surfaces

A critical examination of the analytical solution presented in the classic paper of Greenwood and Williamson (1966), (GW) on the statistical modeling of nominally flat contacting rough surfaces is undertaken in this study. It is found that using the GW simple exponential distribution to approximate the usually Gaussian height distribution of the asperities is inadequate for most practical cases. Some other exponential type approximations are suggested, which approximate the Gaussian distribution more accurately, and still enable closed form solutions for the real area of contact, the contact load, and the number of contacting asperities. The best-modified exponential approximation is then used in the case of elastic-plastic contacts of Chang et al. (1987) ( CEB model), to obtain closed-form solutions, which favorably compare with the numerical results using the Gaussian distribution.

Detailed surface roughness characterization of engineering surfaces undergoing tribological testing leading to scuffing

Abstract Controlled tribological scuffing experiments were conducted on typical engineering surfaces (Al390-T6 discs and 52100 steel pins) under starved lubrication conditions (a mixture of R410A refrigerant combined with a polyolester (POE) lubricant) to simulate the contact conditions in an automotive air conditioning compressor. The time-to-scuff was established and was repeatable using a test protocol with increasing load increments. Test were then conducted for periods of 25, 50, and 75% of the time-to-scuff to investigate the progression of surface change preparatory to scuffing. Detailed studies of the surface topography were conducted using line and areal (usually referred as one dimensional (1D) and two dimensional (2D), respectively) analyses. Simple amplitude roughness parameters as well as more detailed spatial, hybrid, and functional parameters were calculated and used to track detailed roughness changes as the Al390-T6 samples undergo progressive wear until scuffing occurs. One-dimensional amplitude descriptors, such as the root-mean-square value, were not reliable in tracking surface topographic changes. However, 2D functional parameters, such as the surface bearing index and the fluid retention index, clearly showed progressive changes as the surfaces wear and reach scuffing.

Adhesive contact modeling for sub-5-nm ultralow flying magnetic storage head-disk interfaces including roughness effects

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 rou...

Reducing the effects of adhesion and friction in microelectromechanical systems (MEMSs) through surface roughening: Comparison between theory and experiments

Due to the large surface-to-volume ratios and the low loads encountered in microelectromechanical systems (MEMSs), the surface forces become important and may lead to permanent adhesion and high friction between near contacting and contacting surfaces. The effect of these forces can be reduced through surface texturing (roughening) at the contact interface. Moreover, modifying the distribution of the contacting surface asperities so that it becomes positively asymmetric (unbalance between the peak and valley heights) and as peaky as possible (making slender asperities) reduces these forces even further. In the current study, the effects of these parameters, i.e., roughness, asymmetry, and peakiness, on reducing the adhesion and friction in polycrystalline silicon (also referred as polysilicon) MEMS surfaces, were theoretically and experimentally investigated. Polysilicon films with different levels of roughness, asymmetry, and peakiness were fabricated. The roughness characteristics of these films were us...

Determination of hardness from nanoscratch experiments: Corrections for interfacial shear stress and elastic recovery

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.

Adhesion forces for Sub-10 nm Flying-Height Magnetic Storage Head Disk Interfaces

A quasi-dynamic adhesion model is used to calculate the intermolecular adhesion forcespresent in ultra low flying Head Disk Interfaces (HDI’s). The model is a continuum-basedmicromechanics model that accounts for realistic surfaces with roughness, molecularlythin lubricants, and is valid under both static and dynamic sliding conditions. Severaldifferent levels of surface roughness are investigated ranging from extremely smoothsurfaces having a standard deviation of surface heights s52 A to rougher interfaces withseveral nanometer roughness. It is found that when the flying-height is greater than 5 nm,there are no significant adhesive forces, whereas for flying-heights less than 5 nm, adhe-sion forces increase sharply, which can be catastrophic to the reliability of low flyingHDI’s. In addition to roughness, the apparent area of contact between the flying recordingslider and the magnetic disk is also found to significantly affect the magnitude of theadhesion forces. The adhesion model is validated by direct comparisons with adhesion‘‘pull-off’’ force measurements performed using an Atomic Force Microscope with con-trolled probe tip areas and magnetic disks having different lubricant thickness.@DOI: 10.1115/1.1645299#

Dynamic Head-Disk Interface Instabilities With Friction for Light Contact (Surfing) Recording

Recent advances in hard-disk drive technology involve the use of a thermal fly-height control (TFC) pole tip protrusion to bring the read/write recording elements of the slider closer to the disk surface and thus achieve terabit per square inch recording densities. A dynamic, contact mechanics-based friction model of the head-disk interface (HDI) that includes roughness and accounts for the TFC geometry and its influence on the HDI dynamics is presented. The model is based on physical parameters and does not include any empirical coefficients. Experimental flyability/touchdown measurements were performed and used to examine in detail the HDI contact criterion in the presence of surface roughness and dynamic microwaviness. Using the model, a procedure is outlined that identifies the optimal clearance and light contact conditions, i.e., the amount of thermal actuation that minimizes, both, the clearance, as well as the flying height modulation. Through calculation of the time varying interfacial forces, mean pressure and shear stress at the HDI can be predicted and used to characterize the contact regime. Based on our results, a light contact regime with reduced bouncing vibrations and low stresses (thus, low wear) that would enable surfing recording is identified.

Combining and Contacting of Two Rough Surfaces with Asymmetric Distribution of Asperity Heights

The statistical approach of describing rough surfaces is extended to include the contact oftwo rough surfaces in which their distribution of asperity heights can either be symmetricor asymmetric, and the asymmetry is modeled using the normalized Weibull distribution.In considering the contact between two rough surfaces, as in most practical applications,the contact can be approximated by an equivalent rough surface in contact with a smoothplane. The roughness parameters of the equivalent surface are obtained using the spectralmoment method, and its validity is verified using realistic surface roughness measure-ments. This paper presents a method to obtain the equivalent rough surface with a Weibulldistribution of asperity heights, in which the standard deviation and skewness parametersof asperity heights of the actual contacting surfaces are preserved. The advantages of thismethod are demonstrated via direct comparisons with a previously proposed method aswell as with exact numerical simulation of the contact parameters of several differentactual surfaces from magnetic storage and MEMS applications. For practical engineeringapplications, where the roughness parameters of each individual surface are known,contour plots for the skewness value of the equivalent rough surface are provided forpractical ranges of combinations of standard deviation ratios and skewness values. Asexpected when the roughness of one of the contacting surfaces dominates, the skewness issolely determined by the rougher surface. @DOI: 10.1115/1.1614822#

Microtribodynamics of pseudo-contacting head-disk interfaces intended for 1 Tbit/in/sup 2/

A nonlinear dynamic model that includes realistic roughness, adhesion and friction, as well as the dynamics of a flying and contacting HDI was developed to characterize a pseudo-contacting HDI, intended for 1 Tbit/in/sup 2/. A pseudo-contacting recording system is designed to fly at few nanometers using an air-bearing and at the same time some features of the air-bearing surface are designed to contact with the rotating disk during operation. The model was favorably compared with flyability measurements, and then applied to a pseudo-contacting interface to investigate adhesion, friction, and contact forces as well as bouncing vibration. Contrasting earlier studies adopting a simple Coulomb friction, the friction model used in this work calculates the friction force at the interface, accounting for roughness and adhesion. It was found that unlike a fully flying HDI, adhesion plays a positive role in attaining pseudo-contact recording by reducing bouncing vibrations.