Friction and adhesion hysteresis experiments were carried out on fluorocarbon surfactant monolayer-coated surfaces using the surface forces apparatus. Measurements were made as a function of temperature, load, sliding velocity, and relaxation time, and the resulting properties are contrasted with those of hydrocarbon monolayers and also with bulk fluorocarbon surfaces (e.g., Teflon, PTFE). The dynamic adhesion measurements show that the adhesion hysteresis and friction of fluorocarbon monolayer-coated surfaces are related:  large friction forces being associated with large adhesion hysteresis. The results also show that the overall tribological properties of fluorocarbon surfactants follow the same generic “friction phase diagram” behavior as do hydrocarbon surfactants. However, the friction phase diagram for fluorocarbon surfactant has at least two peaksone well above and the other well below room temperatureindicating that two different molecular relaxation processes are involved in friction and adhesion energy dissipation. Apparently, chain interdigitation, which is the most important molecular relaxation mechanism in the friction and adhesion hysteresis of hydrocarbon materials, does not play a major role with fluorocarbon surfaces. Instead, the surface topography and its change at the molecular and submolecular levels during shear is the most important factor determining the friction of these surfaces, but only so long as the monolayers remain molecularly smooth or “undamaged”. Reasons for the beneficial tribological properties of fluorocarbon surfaces are discussed.

A failure criterion for a laminated composite is presented. A salient nature of this criterion is the use of "in situ shear strength" measured in the form of cross-ply laminate. Then by statistical consideration, the model was further modified to incorporate probabilistic nature of composite fail ure. Rigorous mathematical development of probability was simply re placed by Monte Carlo simulation. As a result, the proposed model was proved simple and effective in predicting failure in both deterministic and probabilistic sense.

Friction measurements were carried out for molecularly thin films of a poly(dimethylsiloxane) (PDMS) melt (M w ≈ 80 000) as a function of the applied load (pressure) and sliding velocity using the surface forces apparatus. The PDMS films exhibit apparent layering transitions when the thicknesses of the films are decreased to the order of molecular dimensions. For four-layer and three-layer films, “solidlike” sliding is observed and the shear stresses are on the order of 105 Pa. Further compression and simultaneous lateral motion squeeze out the PDMS molecules to a final residual film two molecular layers in thickness, whose shear properties include “viscous” characters, and the shear stress increases abruptly by a factor of 6−8. This shear property change may arise from the different sliding mechanisms of “adsorbed” and “mobile” molecular layers. When thicknesses of the films are three layers and above, the first layers adjacent to mica substrates are strongly adsorbed onto substrate surfaces and immobile during sliding; shear is accomplished by the slipping of “mobile” middle layers (results in low friction). For the two-layer film (adsorbed layers in direct contact), sliding involves the deformation of adsorbed PDMS segments and wall slip, resulting in high friction and surface damage (wear). For PDMS films, a “fluidlike” response appears when molecules are squeezed out to a final residual thickness (two layers), which is very different from the typical behavior of most of the confined fluid systems (solidlike shift is commonly observed due to confinement). Effects of the substrate−molecule interaction strength on the layering structures and shear properties are also discussed.

Diffusion in a magnesium (Mg)-implanted homoepitaxial GaN layer during ultra-high-pressure annealing (UHPA, in ambient nitrogen, under 1 GPa) was investigated. Annealing at 1573 K resulted in Mg-segregation at the edge of the implanted region, which was suppressed using a higher temperature of 1673 K. Hydrogen (H) atoms were incorporated during the UHPA, resulting in the Mg and H developing the same diffusion profile in the deeper region. The diffusion coefficient of the Mg-implanted sample was 3.3 × 10 −12 cm 2 s −1 at 1673 K from the annealing duration dependence, 30 times larger than that of the epitaxial Mg-doped sample, originating from ion implantation-induced defects.