Douglas T. Smith

Effects of different whiskers on the reinforcement of dental resin composites.

OBJECTIVE Whiskers were recently used to reinforce dental composites to extend their use to large stress-bearing restorations. The aim of this study was to investigate the effects of different types of whiskers on composite properties. METHODS Silicon nitride and silicon carbide whiskers were each mixed with silica particles at whisker/silica mass ratios of 0:1, 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0, and thermally treated. The composite was heat-cured at 140 degrees C. Strength and fracture toughness were measured in flexure, while elastic modulus and hardness were measured with nano-indentation. RESULTS Both whisker type and whisker/silica ratio had significant effects on composite properties (two-way ANOVA; p<0.001). Silicon nitride whiskers increased the composite strength and toughness more than did silicon carbide. Silicon carbide whiskers increased the modulus and hardness more than silicon nitride did. The silicon nitride whisker composite reached a strength (mean+/-SD; n=6) of 246+/-33 MPa at whisker/silica of 1:1, while the silicon carbide whisker composite reached 210+/-14 MPa at 5:1. Both were significantly higher than 114+/-18 MPa of a prosthetic control and 109+/-23 MPa of an inlay/onlay control (Tukeys multiple comparison test; family confidence coefficient=0.95). Fracture toughness and work-of-fracture were also increased by a factor of two. Higher whisker/silica ratio reduced the composite brittleness to 1/3 that of the inlay/onlay control. SIGNIFICANCE Whisker type and whisker/silica ratio are key microstructural parameters that determine the composite properties. Reinforcement with silica-fused whiskers results in novel dental composites that possess substantially higher strength and fracture toughness, and lower brittleness than the non-whisker control composites.

Review of SI traceable force metrology for instrumented indentation and atomic force microscopy

This paper reviews the current status of small force metrology for quantitative instrumented indentation and atomic force microscopy (AFM), and in particular focuses on new electrical and deadweight standards of force developed at the National Institute of Standards and Technology (NIST). These standards provide metrological infrastructure so that users of instrumented indentation and AFM can achieve quantitative nanomechanical testing of materials, engineered surfaces and micro and nanoscale devices in terms of forces that are expressed in internationally accepted units of measure with quantified uncertainty.

Dental resin composites containing silica-fused whiskers—effects of whisker-to-silica ratio on fracture toughness and indentation properties

Dental resin composites need to be strengthened in order to improve their performance in large stress-bearing applications such as crowns and multiple-unit restorations. Recently, silica-fused ceramic whiskers were used to reinforce dental composites, and the whisker-to-silica ratio was found to be a key microstructural parameter that determined the composite strength. The aim of this study was to further investigate the effects of whisker-to-silica ratio on the fracture toughness, elastic modulus, hardness and brittleness of the composite. Silica particles and silicon carbide whiskers were mixed at whisker:silica mass ratios of 0:1, 1:5. 1:2, 1:1, 2:1, 5:1, and 1:0. Each mixture was thermally fused, silanized and combined with a dental resin at a filler mass percentage of 60%. Fracture toughness was measured with a single-edge notched beam method. Elastic modulus and hardness were measured with a nano-indentation system. Whisker:silica ratio had significant effects on composite properties. The composite toughness (mean+/-SD; n = 9) at whisker:silica = 2:1 was (2.47+/-0.28) MPa m(1/2), significantly higher than (1.02+/-0.23) at whisker:silica = 0:1, (1.13+/-0.19) of a prosthetic composite control, and (0.95+/-0.11) of an inlay/onlay composite control (Tukeys at family confidence coefficient = 0.95). Elastic modulus increased monotonically and hardness plateaued with increasing the whisker:silica ratio. Increasing the whisker:silica ratio also decreased the composite brittleness, which became about 1/3 of that of the inlay:onlay control. Electron microscopy revealed relatively flat fracture surfaces for the controls, but much rougher ones for the whisker composites, with fracture steps and whisker pullout contributing to toughness. The whiskers appeared to be well-bonded with the matrix, probably due to the fused silica producing rough whisker surfaces. Reinforcement with silica-fused whiskers resulted in novel dental composites that possessed fracture toughness two times higher than, and brittleness less than half of current dental composites.

Indentation modulus and hardness of whisker-reinforced heat-cured dental resin composites.

OBJECTIVES Recent studies showed that ceramic whisker reinforcement imparted a two-fold increase in the strength of dental composites. The aim of this study was to investigate the indentation response and measure the elastic modulus, hardness, and brittleness of whisker-reinforced heat-cured resin composites as a function of filler level, heat-cure temperature, and heat-cure duration. METHODS Silica particles were fused onto silicon nitride whiskers to facilitate silanization and to roughen the whiskers for improved retention in matrix. Whisker filler mass fractions of 0, 20, 40, 60, 70, 74 and 79% were tested. Heat-cure temperature ranged from 100 to 180 degrees C, and duration from 10 min to 24 h. A nano-indentation system enabled the measurement of elastic modulus. Fracture toughness was measured and composite brittleness index was calculated. An inlay/onlay composite and a prosthetic composite were tested as controls. RESULTS Whisker filler level and heat-cure duration had significant effects on composite properties, while heat-cure temperature had non-significant effects. The whisker composite with 79% filler level had a modulus in GPa (mean (SD); n = 6) of 26.9 (1.0), significantly higher than 15.1 (0.2) of an inlay/onlay control, and 16.1 (0.3) of a prosthetic control (Tukeys multiple comparison test; family confidence coefficient = 0.95). The fracture toughness in MPa.m1/2 was 2.22 (0.26) for the whisker composite, higher than 0.95 (0.11) for inlay/onlay control, and (1.13 +/- 0.19) for prosthetic control. The brittleness index was (0.49 +/- 0.07) for whisker composite, lower than (1.02 +/- 0.12) for inlay/onlay control and (0.63 +/- 0.13) for prosthetic control. SIGNIFICANCE Whisker filler level had a profound influence, heat-cure duration had significant effects, while temperature did not have significant effects, on the properties of whisker composite. The whisker composite had significantly higher elastic modulus and fracture toughness, and lower brittleness than the inlay/onlay and prosthetic controls.

Sputtered amorphous carbon nitride films

The recent announcement of the synthesis of C 3 N 4 has increased interest in this unique material. Carbon nitride may have several useful applications as wear and corrosion resistant coatings, electrical insulators, and optical coatings. We have produced amorphous carbon nitride coatings containing up to 40% nitrogen using planar magnetron RF sputtering with and without an ion beam in a nitrogen atmosphere. Both wavelength dispersive x-ray spectrometry (WDX) and x-ray photoelectron spectroscopy (XPS) indicate this composition. Coatings up to 2 μm thick were produced on alumina, silicon, SiO 2 , and glass substrates using a graphite target. Films with transparency greater than 95% in the visible wavelengths and harder than silicon have been produced. The properties of these films are correlated with composition, fabrication, conditions, and subsequent heat treatments. A scanning tunneling microscope (STM) and transmission electron microscopy (TEM) were used to characterize the morphology of the films. XPS studies confirm the stability of a carbon nitrogen phase up to 600 °C. Compositional variations were determined with secondary ion mass spectrometry (SIMS) depth profiling, and the Raman spectra are compared with those of carbon and carbon nitride films prepared by other methods.

Measuring surface forces to explore surface chemistry: Mica, sapphire and silica

In this paper measurements of the forces acting between two solid surfaces separated by a thin liquid film are discussed. By investigating these forces in a range of different liquids and solutions, it is possible to acquire an understanding of the surface properties of the solid material. The surface of mica has been studied extensively in this way, and the results obtained are reviewed to illustrate how the surface force measurements can give surface chemical information. Recent measurements on two other materials, sapphire and silica, which are of greater practical interest are also discussed.

Analytic solution for the three-layer multiple beam interferometer.

We present a simple analytic solution for the condition of constructive interference for light transmitted through an interferometer incorporating three ideally transparent layers of arbitrary thickness and refractive index. We also consider the effect of adding two metallic coatings to the outer surfaces of the interferometer and give empirical expressions for the associated phase changes for silver coatings on silica, sapphire, and mica substrates. A particular application to fringes of equal chromatic order can be utilized to obtain precise measurements of the thickness of extremely thin films sandwiched between two substrates.

Effect of thermal cycling on whisker-reinforced dental resin composites

The mechanical properties of dental resin composites need to be improved in order to extend their use to high stress-bearing applications such as crown and bridge restorations. Recent studies used single crystal ceramic whiskers to reinforce dental composites. The aim of this study was to investigate the effects of thermal cycling on whisker-reinforced composites. It was hypothesized that the whisker composites would not show a reduction in mechanical properties or the breakdown of whisker–resin interface after thermal cycling. Silicon carbide whiskers were mixed with silica particles, thermally fused, then silanized and incorporated into resin to make flexural specimens. The filler mass fraction ranged from 0% to 70%. The specimens were thermal cycled in 5 °C and 60 °C water baths, and then fractured in three-point bending to measure strength. Nano-indentation was used to measure modulus and hardness. No significant loss in composite strength, modulus and hardness was found after 105 thermal cycles (family confidence coefficient=0.95; Tukeys multiple comparison test). The strength of whisker composite increased with filler level up to 60%, then plateaued when filler level was further increased to 70%; the modulus and hardness increased monotonically with filler level. The strength and modulus of whisker composite at 70% filler level were significantly higher than the non-whisker controls both before and after thermal cycling. SEM revealed no separation at the whisker–matrix interfaces, and observed resin remnants on the pulled-out whiskers, indicating strong whisker–resin bonding even after 105 thermal cycles. In conclusion, novel dental resin composites containing silica-fused whiskers possessed superior strength and modulus compared to non-whisker composites both before and after thermal cycling. The whisker–resin bonding appeared to be resistant to thermal cycling in water, so that no loss in composite strength or stiffness occurred after prolonged thermal cycling.

Nanoindentation study of sputtered nanocrystalline iron thin films

In this report, the mechanical behavior of nanocrystalline iron thin films prepared by sputtering is presented. The hardness of a series of as-deposited films with grain sizes between 10.3 and 21.0 nm displays a positive Hall-Petch slope although there is a great deal of scatter. A much better fit is obtained from hardness measurements made on a film with an as-deposited grain size of 11.4 nm that was annealed to increase its grain size up to 31.5 nm. These results are analyzed in terms of an Orowan bowing model that involves a grain size dependent term for the hardness proportional to d{sup {minus}1}.

Measuring contact charge transfer at interfaces: a new experimental technique

Abstract A new experimental technique has been developed for measuring the amount of electric charge transferred when two dissimilar surfaces are brought into contact and separated without lateral motion in a controlled environment. The primary surfaces studied, silica and mica, are prepared with very low surface roughness (average roughness less than 0.5 nm); the macroscopic contact area, which can be measured directly, is therefore a better measure of microscopic contact area than is typically the case in contact electrification experiments, and the surface charge density can be more accurately determined. Observations reported include (1) the transfer of charge densities as high as 1 × 10−2 C/m2 after only a few contacts of mica and silica surface in dry air or N2 gas at room temperature and (2) a reduction in transferred charge density in the silica-mica system as the relative humidity of the air in the environmental chamber is increased from 0% to 98%. The technique also permits the study of the rate at which the surface charge decays under various environmental conditions.