Hard-material Adhesion: Which Scales of Roughness Matter?

Background Surface topography strongly modifies adhesion of hard-material contacts, yet roughness of real surfaces typically exists over many length scales, and it is not clear which of these scales has the strongest effect. Objective: This investigation aims to determine which scales of topography have the strongest effect on macroscopic adhesion. Methods Adhesion measurements were performed on technology-relevant diamond coatings of varying roughness using spherical ruby probes that are large enough (0.5-mm-diameter) to sample all length scales of topography. For each material, more than 2000 measurements of pull-off force were performed in order to investigate the magnitude and statistical distribution of adhesion. Using sphere-contact models, the roughness-dependent effective values of work of adhesion were measured, ranging from 0.08 to 7.15 mJ/m2 across the four surfaces. The data was more accurately fit using numerical analysis, where an interaction potential was integrated over the AFM-measured topography of all contacting surfaces. Results These calculations revealed that consideration of nanometer-scale plasticity in the materials was crucial for a good quantitative fit of the measurements, and the presence of such plasticity was confirmed with AFM measurements of the probe after testing. This analysis enabled the extraction of geometry-independent material parameters; the intrinsic work of adhesion between ruby and diamond was determined to be 46.3 mJ/m2. The range of adhesion was 5.6 nm, which is longer than is typically assumed for atomic interactions, but is in agreement with other recent investigations. Finally, the numerical analysis was repeated for the same surfaces but this time with different length-scales of roughness included or filtered out. Conclusions The results demonstrate a critical band of length-scales—between 43 nm and 1.8 µm in lateral size—that has the strongest effect on the total adhesive force for these hard, rough contacts.

Effect of the load pad size on the output of the column-type force transducer

The size of the load pad had a significant effect on the output of the column-type force transducer. It was important to eliminate the influence when the transducer was loaded in a force transducer build-up system which worked as the primary force transfer standard. A common model of the output about the column-type force transducer affected by the shape of load pad was analysed. Seven load pads with various dimensions were designed and fixed on a 300-kN force transducer to investigate the influence of the dimension on the indication error of the transducer. The results showed that the load pad of the sphere–plane contact design would cause the smallest indication error but the largest contact stress. It was better to use the load pad of the sphere–sphere contact design with the spherical radius ratio of ball to cup equalled to 0.2. The spherical radius of the ball should be small under the permission of material strength. In addition, it was important to enhance the stiffness of the build-up system to eliminate the effect of the load pad size on the indication error of the transducer.

Numerical Simulation of Convective Heat Transfer in Structured Packed Beds of Particles With Different Particle Contact Treatment Methods

The treatment of sphere contact is a crucial issue on the numerical analysis of the flow and heat transfer in packed beds of particles. In this work, the effects of three different methods including introducing a gap, area contacting and cylindrically bridging on the flow and heat transfer performances of structured packed beds of particles are investigated with CFD methods. The simulation results have shown that the influence of different treatment methods on the contact point on flow and heat transfer gets greater as the porosity decreases and the Reynolds number increases. There are no large differences among these three methods on the pressure drop and heat transfer under laminar flow regime. Compared with the experimental results, cylindrically bridging method is proved to be reasonable to simplify the sphere–sphere contact points. Therefore, cylindrically bridging treatment method is encouraged to be used instead of point contact method between particles with CFD analysis in structured packed beds of particles.