I.L. Singer

Mechanical factors favoring release from fouling release coatings.

For some twenty years the marine coatings industry has been intrigued by polymer surfaces with low adhesion to other materials, especially to the biological glues used by marine organisms. Polymers with fouling release surfaces have been made from sundry materials, and their resistance to marine fouling in both static and dynamic tests has been evaluated in the worlds oceans. Although the polymer surface property most frequently correlated with bioadhesion is its critical surface tension (γ?), resistance to fouling is also influenced by other bulk and surface properties of the polymer. This paper reviews the types of bonding associated with polymeric materials used in fouling resistant coatings, describes the removal process in terms of fracture mechanics, and discusses the importance of surface energy, elastic modulus and coating thickness in the release of biofoulants.

Superlow friction behavior of diamond-like carbon coatings: Time and speed effects

The friction behavior of a diamond-like carbon coating was studied in reciprocating sliding contact at speeds from 0.01 to 5 mm/s, in dry nitrogen. “Superlow” friction coefficients of 0.003–0.008 were obtained in continuous sliding at the higher speeds (>1 mm/s). However, friction coefficients rose to values typical of diamond-like carbon in dry and ambient air (0.01–0.1) at lower speeds (<0.5 mm/s) as well as in time-delayed, higher speed tests. The rise of the friction coefficients in both speed and time-delay tests was in good quantitative agreement with gas adsorption kinetics predicted by the Elovich equation for adsorption onto carbon. More generally, superlow friction could be sustained, suppressed, and recovered as a function of exposure time, demonstrating that duty cycle cannot be ignored when predicting performance of superlow friction coatings in devices.

Hertzian stress contribution to low friction behavior of thin MoS2 coatings

Friction coefficients were measured for bearing materials slid in dry air against sputter‐deposited MoS2‐coated substrates. Ball versus flat tests were performed over a wide range of initial Hertzian pressures (200–1500 MPa) by varying loads (1–50 N), elastic moduli (70–615 GPa), and ball diameters (1.6–12.7 mm). The friction coefficient μ decreased as load L increased according to μ∝ L (−0.32), in agreement with the Hertzian contact model. Regression analysis of over 600 data points for friction coefficient versus Hertzian pressure (P H ), fitted to μ=(S 0/P H )+α, gave mean values of the shear strength S 0=24.8 MPa±0.5 and α= 0.001±0.001, with S 0 in good agreement with values in the literature.

Effect of roughness on the friction of diamond on cvd diamond coatings

Abstract Reciprocating sliding tests in air have been performed on polycrystalline diamond coatings with a range of surface roughnesses. Both the coatings and the single crystal diamonds used as sliders were abrasively worn during the tests. This wear reduced the roughnesses of the coatings, and was most severe at the beginning of the tests. The friction coefficient decreased as the coating surface was smoothed. Friction coefficients depended on both the roughness of the coating and the stylus: values ranged from 0.5 for rough styluses on rough coatings ( R a > 200 nm ), to 0.03 for smooth styluses on smooth coatings ( R a ≈ 1 nm ), the latter being comparable to the lowest value obtained with polished single crystal diamond. The higher initial friction coefficients could be eliminated by mechanical or chemical polishing. Roughness parameters relevant to loaded elastic solids are discussed, and related to a model for the dependence of friction on the angle of interlocking asperities. Wear behaviour and estimates of wear rates indicate that the wear resistance of the coatings is comparable to that of natural, single-crystal diamond, despite the polycrystalline nature of the coatings.

VISIBLE LIGHT EMISSION FROM SI NANOCRYSTALS GROWN BY ION IMPLANTATION AND SUBSEQUENT ANNEALING

Photoluminescence (PL), electron spin resonance (ESR), and high resolution transmission electron microscopy (HRTEM) were used to investigate the luminescence mechanism in Si nanocrystals. Si ions were implanted in SiO2 films at 190 keV to a dose of 3×1017/cm2.An intense photoluminescence (PL) band at 755 nm (1.65 eV) was observed when the implanted films were annealed above 800 °C in air or in nitrogen. HRTEM images showed Si nanocrystals of sizes between 1 and 6 nm from these annealed samples. ESR indicated Si dangling bonds. Upon annealing at 900 °C in air a few times, the particle sizes were reduced to less than 2 nm due to oxidation. The red PL band is attributed to emission from Si nanocrystals.

Friction and wear behavior of TiN in air: the chemistry of transfer films and debris formation*

Abstract Friction tests were performed on TiN-coated substrates at low speed (less than 0.1 m s −1 ) in air. Optical (Nomarski) and scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy (TEM) were used to characterize the transfer films and debris generated during sliding against steel and sapphire balls. Friction coefficients of steel against rougher ( R a ≈60–100 nm) TiN coatings started and remained relatively high (0.5–0.7) owing to wear and transfer of the steel. After the TiN coating was polished ( R a ≈4 nm), transfer was reduced and initial friction coefficients ranged from 0.15 to 0.2. Initial friction coefficients with sapphire balls sliding against polished TiN were even lower (0.05). However, friction coefficients with both balls increased as debris formed and transferred to the wear track. Auger analysis showed that steel balls accumulated metallic iron and/ or iron oxide as well as titanium oxide layers and debris, whereas sapphire balls acquired only titanium oxide transfer layers. TEM of debris stripped from contact areas of steel and sapphire balls identified the phases of debris as a rhombohedral ternary oxide (FeTiO 3 / α-Fe 2 O 3 ) and rutile (TiO 2 ) respectively; morphologies were either thin, uniform flakes — prevalent in low friction sliding — or spherical clusters. A thermochemical basis for oxide debris formation is given and the friction behavior is interpreted in terms of an oxide wear mechanism.

Factors That Influence Elastomeric Coating Performance: The Effect of Coating Thickness on Basal Plate Morphology, Growth and Critical Removal Stress of the Barnacle Balanus amphitrite

Abstract Silicone coatings are currently the most effective non-toxic fouling release surfaces. Understanding the mechanisms that contribute to the performance of silicone coatings is necessary to further improve their design. The objective of this study was to examine the effect of coating thickness on basal plate morphology, growth, and critical removal stress of the barnacle Balanus amphitrite. Barnacles were grown on silicone coatings of three thicknesses (0.2, 0.5 and 2 mm). Atypical (“cupped”) basal plate morphology was observed on all surfaces, although there was no relationship between coating thickness and i) the proportion of individuals with the atypical morphology, or ii) the growth rate of individuals. Critical removal stress was inversely proportional to coating thickness. Furthermore, individuals with atypical basal plate morphology had a significantly lower critical removal stress than individuals with the typical (“flat”) morphology. The data demonstrate that coating thickness is a fundamental factor governing removal of barnacles from silicone coatings.

Wear behavior of Pb–Mo–S solid lubricating coatings

Abstract Amorphous Pb–Mo–S coatings 200 to 510 nm thick were deposited by dual ion-beam deposition (IBD) onto steel and Si substrates. Coating wear studies were performed using ball-on-flat reciprocating sliding with steel ball counterfaces in dry air. Tests were run between 1 and 100,000 sliding cycles, and wear depths measured by interference microscopy. Morphology and chemistry of the as-deposited coatings and worn surfaces were investigated with optical microscopy, micro-Raman spectroscopy and cross-section high resolution transmission electron microscopy (HRTEM). Pb–Mo–S coatings were found to be quite wear resistant; no more than 25% of the coating thickness was removed by 10,000 sliding cycles. Two wear mechanisms were identified. At the nanometer scale, wear proceeded in a two-part process: transformation of the coating surface to MoS 2 , then layer-by-layer removal of MoS 2 . At the micrometer scale, wear occurred by plowing. The long endurance of Pb–Mo–S coatings was attributed to slow wear of the coatings, with lubricant redistribution processes playing a minor role.

Wear behavior of triode-sputtered MoS2 coatings in dry sliding contact with steel and ceramics

Abstract The endurance of MoS 2 sputter-coated steel balls was measured in continuous and stop-start sliding tests in a four-ball wear tester. Test variables were counterface materials, uncoated steel and two ceramics (Co-bonded tungsten carbide and sapphire), and gaseous atmospheres, dry Ar and dry air. In continuous tests, coating endurance increased from 14 to 80 min; combinations were ranked as follows: steel in air = ceramics in air

Low-friction, high-endurance, ion-beam-deposited Pb-Mo-S coatings

Abstract Thin solid lubricating coatings of PbMoS were deposited on steel substrates via ion-beam deposition. Coating endurance and friction coefficients under dry air sliding conditions were monitored with ball-on-disk tests; additional tribological testing was performed using a ball-on-flat reciprocating test rig to investigate intermediate sliding distances (100–32000 cycles). Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), scanning Auger microscopy and micro-Raman spectroscopy were used to examine the structure, composition and chemistry of the coatings. Worn surfaces were characterized by optical microscopy and micro-Raman spectroscopy. The average endurance (at 1.4 GPa stress) of ion-beam-deposited (IBD) PbMoS coatings (thickness, 160–830 nm) containing 4–26 at.% Pb was 160000 revolutions, more than twice that of MoS2 coatings obtained by ion-beam-assisted deposition. In addition, the IBD PbMoS coatings had friction coefficients between 0.005 and 0.02, similar to the MoS2 coatings obtained by ion-beam-assisted deposition. Friction coefficients were monitored as a function of the contact stress and found to obey the hertzian contact model; measured interfacial shear strengths (S0 ≈ 12 MPa) were similar to those observed for MoS2 coatings. Although XRD and micro-Raman spectroscopy indicated that the IBD PbMoS coatings were initially amorphous, micro-Raman spectroscopy showed that crystalline MoS2 was produced both in the wear tracks on coatings and in the transfer films on balls after as few as 100 sliding cycles. The wear resistance and low-friction properties of IBD PbMoS coatings are attributed to the combination of dense, adherent coatings and the formation of easily sheared, MoS2-containing sliding surfaces.