He, Y.J.; Winnubst, L.; Burggraaf, A.J.; Verweij, H.; van der Varst, P.G.T.; de With, G.

The thermodynamic concept used to quantify adhesion on a fundamental molecular level is the work of adhesion. However, most of the experimental techniques give no, or very limited information about its magnitude. In this paper, a way to estimate the work of adhesion for copper-(acrylonitrile-butadienestyrene) (ABS) interface using molecular dynamics simulations is presented. The work of adhesion is calculated from the interactions between single molecules constituting the ABS polymer (poly(styreneco-acrylonitrile) and polybutadiene molecules) and copper (oxide) surface, using their van der Waals contact area. The calculated work of adhesion seems to be independent of the number of polymer molecules present on the copper surface, monomer residue unit sequence within the polymer molecule, and the type of copper surface. Introduction of oxygen atoms to the metallic surface and the polymer molecules significantly increases the work of adhesion. The highest work of adhesion was found between the oxidized copper surface and high oxygen content copolymer poly(styrene-alt-maleic anhydride). Results are shown to qualitatively correspond to previously reported experimental observations.

Evaluation of long-term mechanical behavior of new types of restorations in clinical trials is time-consuming. A partial alternative can be found in experimental fatigue-testing, which simulates accelerated mechanical deterioration. The aim of this study was to determine the feasibility of using fatigue-testing of a complex dental restoration and to evaluate the mechanical fatigue behavior of premolar teeth restored with a titanium alloy post and an amalgam or composite core. Eighty-seven human upper premolar teeth were decoronated, embedded, and restored with a prefabricated post of 1 mm diameter. The teeth were randomly assigned to one of two groups corresponding with a core build-up of amalgam or chemically-cured core composite, respectively. Five to 21 days after restoration, the specimens were subjected to cyclic loading (frequency, 5 Hz), at an angle of 45 degrees to the long axis of the tooth. The boundary technique was used for determination of the mean fatigue strengths of the restorations at 10(4), 10(5), and 10(6) cycles, simulating up to 1-3 years of clinical functioning. Mean fatigue strength was expressed in percentage of initial strength: For 10(4), 10(5), and 10(6) cycles, the results were 66%, 58%, and 52%, respectively, for the amalgam and 62%, 62%, and 53% for the composite group. It is concluded that fatigue-testing of more complex systems is possible, if a suitable testing method is selected. The restorations showed a comparable strength reduction after 10(6) cycles of about 50% of their initial strength. The composite core build-up showed a behavior less predictable than that of the amalgam, which might be attributed to handling parameters.

Sliding wear properties of ultra-fine-grained (180 nm)very slow grain growth at 1150ЊC in a large range of dwell times. 9 (2) The stress-induced phase transformation from yttria-doped tetragonal zirconia (Y-TZP) ceramics were examined with porosities from 1.5% to 7%. On a pin-ontetragonal to monoclinic is almost completely suppressed by the ultra-fine grain size. 6,10 A tetragonal-to-monoclinic phase plate tribometer under dry-N 2 conditions, e.g., wear rates of the material increase by a factor of 5 by increasing porosity transformation has a significant effect on the tribological properties of TZP ceramics during sliding wear, 8,11 resulting in a with a factor of 5 (from 1.5 to 7.0 vol%). In all cases no (irreversible) phase transformation to monoclinic zirconia more complex wear process. took place during wear tests. The results for the relatively dense nanostructured materials showed significant evidence II. Experimental Procedure of plastic deformation and less microcrack formation. The morphology of the wear tracks in these ceramics indicate Nanocrystalline and weakly agglomerated zirconia powders (Y-TZP) are prepared by a gel-precipitation method using that the degree and amount of microcracks on the contact surface increased with porosity. A change in wear mechametal chlorides as precursor chemicals. More details about the preparation process have been reported elsewhere. 12 The nism is observed.specimens are uniaxially pressed in a die at 80 MPa to a rectangular form and subsequently isostatically pressed at 400 MPa. I. IntroductionThe ultra-fine-grained Y-TZP specimens with grain size 180 nm are densified at 1150ЊC for varying times by pressureless sin-P OLYCRYSTALLINE tetragonal zirconia (TZP) ceramics are tering or by sinter forging 13 to obtain 1.5%-7.0% porosities. promising in tribological applications due to their high Porosity was measured using the Archimedes technique in Hg flexural strength, hardness, and fracture toughness at room and grain size by SEM pictures using the linear intercept techtemperature or elevated temperature.1,2 The presence of porosity nique. 14 The surface of TZP specimens was subsequently generally has a negative influence on mechanical properties, polished with 0.05 m alumina powder. Subsequently the specsince pores can cause stress concentration, resulting in low imens were ultrasonically cleaned for 45 min in ethanol and strength.3 The presence of pores, distributed at grain boundthen annealed at 950ЊC for 10 min (heating and cooling rates aries, weakens the grain boundaries and leads to a reduction of 2ЊC/min) to remove the residual stresses produced by machinfracture energy. It is found that for TZP ceramics the variation ing and polishing. The pin materials used in all tests are comof fracture energy (␥), Young's modulus (E) and fracture toughmercially available sintered SiC spheres (diameter 4 mm) with ness (k IC ) with residual porosity follows an exponential relaa mirror-polished surface (Gimex Technische Keramiek B.V., tion. [4][5][6] It is expected that the...

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