A. Ramalho

Study of abrasive resistance of composites for dental restoration by ball-cratering

Abstract Two-body abrasion occurs in the mouth whenever there is tooth-to-tooth contact. This is what most dentists call attrition. Abrasive wear may also occur when there is an abrasive slurry interposed between two surfaces, such that the two solid surfaces are not actually in contact, this is called three-body abrasion, with food acting as the abrasive agent, and occurs in the mouth during mastication. Abrasion is the key physiological wear mechanism that is present in dental materials during normal masticatory function. The two main categories of restoration materials are dental amalgam and composite restorative material. Although amalgam has excellent mechanical properties, it also has certain limitations and disadvantages. The main negative factors for amalgam are: aesthetic factor, toxicity of mercury, weak adhesion to healthy dental tissue, duration of restoration, corrosion and feeble resistance to fracture. One of the main objectives in wear studies is to determine the nature of the dependency between the mechanical properties of materials and their tribological behaviour of the surfaces in contact. The aim of the present work is to study the effect of cure conditions on light curable composites on hardness and the respective abrasion resistance. Six commercially available composite restorative materials used for posterior restorations were selected for the study. In order to obtain comparative results one amalgam has been included in the study. The technique of ball-cratering is used to evaluate the resistance to abrasion.

Fretting Laboratory Tests: Analysis of the Mechanical Response of Test Rigs

The current state of the art of the experimental approach to fretting is discussed. The mechanical response of fretting equipment is analysed to establish the limits of the reliable use of fretting rigs. The effect of stiffness and dynamic response of fretting test rigs are extensively discussed, and a simple one freedom-degree model is proposed to describe, in a first approach, the response of the tangential force measuring systems.

High temperature fretting behaviour of plasma vapour deposition TiN coatings

Abstract Fretting tests (mode I) were performed on TiN coatings at test temperatures of 23–500 °C. The evolution of the coefficient of friction with the number of fretting cycles for tests performed in that temperature range was recorded. Differences in that evolution were analysed based on the frictional energy dissipated in the sliding contact during the fretting tests. That analysis demonstrated the role of dissipated frictional energy and thermal energy in initiating a structural modification of the debris. The structural modification of the debris coincides with the transition from high friction conditions taking place in the presence of amorphous debris to low friction conditions in the presence of nano-crystalline debris.

Fatigue and fretting fatigue of ion-nitrided 34CrNiMo6 steel

Abstract This work is concerned with the surface treatment (ion nitriding) of fretting fatigue and fatigue resistance of 34CrNiMo6. Tests are made on a servo-hydraulic machine under tension for both treated and non-treated specimens. The test parameters involve the applied displacements δ ±80–±170 μm; fretting pressure σ n =1000–1400 MPa; fatigue stress amplitude σ a =380–680 MPa and stress ratio R =−1. The ion nitriding process improves both fatigue and fretting fatigue lives. Subsurface crack initiation from internal discontinuities was found for ion-nitrided specimens.

Mixed Protein Carriers for Modulating DNA Release

Aqueous mixtures of oppositely charged polyelectrolytes undergo associative phase separation, resulting in coacervation, gelation, or precipitation. This phenomenon has been exploited in forming DNA gel particles by interfacial diffusion. We report here the formation of DNA gel particles by mixing solutions of double-stranded DNA with aqueous solutions containing two cationic proteins, lysozyme and protamine sulfate. The effect of the lysozyme/protamine ratio on the degree of DNA entrapment, surface morphology, swelling-deswelling behavior, and kinetics of DNA release has been investigated. By mixing the two proteins, we obtain particles that display higher loading efficiency and loading capacity values, in comparison to those obtained in single-protein systems. Examination of the release profiles has shown that in mixed protein particles, complex, dual-stage release kinetics is obtained. The overall release profile is dependent on the lysozyme/protamine ratio. The obtained profiles, or segments of them, are accuratelly fitted using the zero-order and first-order models, and the Weibull function. Fluorescence microscopy studies have suggested that the formation of these particles is associated with the conservation of the secondary structure of DNA. This study presents a new platform for controlled release of DNA from DNA gel particles formed by interfacial diffusion.

DNA gel nanoparticles: preparation and controlling the size

A simple and novel method was used for the preparation of nano-/micro-sized DNA gel particles by nebulisation of a solution of DNA (single- or double-stranded) into an oppositely charged surfactant (cetyltrimethylammonium bromide) or protein (lysozyme, protamine sulfate) solution. The size and size distribution of the particle populations were investigated by means of fluorescence microscopy (FM), photon correlation spectroscopy (PCS) and scanning electron microscopy (SEM). Particles measuring from 0.1 to 10 µm were obtained. FM studies suggest that the formation of the particles was carried out with conservation of the secondary structure of the nucleic acid molecules. SEM on freeze-dried and Au-shadowed samples showed a distribution of virtually spherical particles. It was found that, in addition to the size of the initial DNA droplets, the cationic agent is a controlling parameter of the particle size.

Identification of fatigue crack propagation modes by means of roughness measurements

This paper is a study of the applicability of fracture surface roughness measurements to identify the crack propagation mode in nickel base superalloys (transgranular, intergranular or mixed). The results obtained indicated that this technique can be used as an alternative or as a complement to standard fractography. The best roughness parameters for identifying the crack propagation mode are average roughness, mean roughness depth and mean height profile peak (a, Rz and Rpm). Analysis of roughness spectra in terms of the frequency range showed that the amplitude values of the profiles with wavelengths identical to the grain size are significant when propagation is intergranular. The roughness measurements were used to study the influence of loading frequency and stress state on the fatigue crack propagation mode occurring in Inconel 718 tested at 600°C.

Micro-Scale Abrasive Wear Test of Thin Coated Cylindrical Surfaces

This paper is concerned with whether the micro-scale abrasive test is suitable for studying non-flat surfaces. A new micro-scale abrasion test has been developed to do studies on cylindrical shapes. A complete procedure to determine the intrinsic wear resistance of coatings is presented and discussed. The new method allows the direct characterization of small cylindrical parts, such as pins, bolts, shafts, etc. The method developed is used to study the abrasion resistance of W–Si–N sputtered coatings and electroplated chromium films.

Fretting behaviour of glass-fibre-reinforced polypropylene composite against 2024 Al alloy

Abstract Composite materials, mainly fibre type ones, are used to respond to crucial demands in engineering applications. Various limitations mean that it is usually impossible to produce structures without mechanical joints. Fretting is an important failure mode for such joints, especially for dynamic loads. This paper sets out to assess the influence of this failure mode—fretting—in association with the effect of displacement, surface treatment with aluminium (anodisation) and the effect of environment, temperature and relative humidity. A series of experiments was carried out, changing each of the variables. To analyse the influence of each parameter, tangential force and displacement were used to establish the fretting cycles for every condition tested. Variations in the shape of the cycles revealed three regimes typical of fretting: stick, slip and partial slip, but the most effective way to characterize the transition between regimes was based on energy dissipation by friction. Surface treatment by anodisation leads to lower wear values, for small amplitude displacements, while for higher displacement amplitudes the wear volume was larger, for the case of room temperature and humidity. Increased temperature resulted in a rise in wear volumes, especially for non-anodised aluminium. Variation in humidity did not greatly influence the behaviour of the specimens studied.

Failure modes observed on worn surfaces of W-C-Co sputtered coatings

Abstract During scratch testing, the indenter gives rise to a distribution of stresses similar to that observed in tribocontacts. In this work, r.f.- sputtered W-C-Co coatings deposited from sintered WC + Co (6, 10 and 15wt.%Co) at various substrate biases were scratched and tested tribologically and the morphology of the damaged surfaces was analysed. The cobalt content of the coatings is the main factor determining their tribological characteristics. The failure modes observed on the worn pin-on-disc tested surfaces are explained and compared with those obtained by scratch testing. In spite of it not being possible to establish quantitative results for the wear resistance of W-C-Co coatings from scratch testing, an estimation cen be performed based on the observation of the failure modes in the scratch track. Thus scratch testing can be used to predict the tribological behaviour of coated surfaces. This possibility can reduce the number and cost of tribological tests.