Cesare Melandri

Tribological behaviour of Al2O3/ZrO2-ZrO2 laminated composites

Abstract Laminated composites were prepared in the system Al 2 O 3 -ZrO 2 . Starting from sheets produced by tape casting a symmetric structure was obtained by superimposing alternated layers of Al 2 O 3 /ZrO 2 composite and ZrO 2 . Due to the different thermal expansion coefficients and shrinkage of the two materials the alumina/zirconia external layers of the samples exhibit a compressive residual stress which leads to an improved apparent surface toughness. The value of this stress was determined by indentation technique. The tribological behaviour of the laminated composites and of a stress free material, with the same composition of the external layers, was assessed by Pin-on-Disc method at different loads and sliding speeds. The effect of differences in properties and microstructural features among the different materials on wear and friction were determined and discussed. Possible wear mechanisms are proposed. In spite of the increased surface toughness in the range of the experimental conditions considered, the presence of compressive residual stresses at the surface does not lead to an improvement of wear resistance in the laminated structure.

High temperature friction and wear testing of silicon nitride ceramics

This paper describes the results of unlubricated sliding wear tests carried out on ceramics and composites based on silicon nitride. These were silicon nitride, silicon nitride with titanium carbide, and an electrically conductive silicon nitride with titanium nitride. The tests were carried out from room temperature to 1200°C. For all the materials, there was a small increase in friction from room temperature to 300°C, followed by a small decrease as the temperature increased, until a sharp increase in friction coefficient at 1200°C. There was a maximum in wear as the temperature was varied for all materials, with almost no wear at room temperature and at 1200°C. The wear of the silicon nitride material was significantly higher than that of the composite materials at 900°C.

On data dispersion in pin-on-disk wear tests

Ten pin-on-disk sliding wear tests for each experimental condition were carried out with a commercial tungsten carbide (WC) pin on silicon carbide (SiC) disks in order to determine the wear and friction data dispersion. The tests were repeated using two sliding speeds (v), 0.1 and 1.0 m/s, and two applied loads (P), 5 and 50 N. The wear data showed a dispersion in the range of 28–47 and 32–56%, for disk and pin, respectively. For the disk, the dispersion decreased when increasing both sliding speed and applied load; for the pin, no clear relationship was found. The friction values spread in the range of 5–15%, with a lower dispersion at high applied load, independent of the sliding speed. From a statistical point of view, it was found that, in all the experimental conditions adopted, about 20% of the wear and friction values can be considered outliers.

Microstructural and mechanical characterization of bulk BSCCO (2223) superconductor

Abstract Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Ox bulk superconductor was prepared by hot pressing and pressureless sintering reaching relative density values in the 96–99 and 85–90% range, respectively. The hot-pressed specimens exhibited a textured microstructure with the “c”-axis preferentially aligned parallel to the direction of sintering pressure application, while the plastic flow into the particles, during hot pressing, favours the grain growth in the direction perpendicular to the pressure application. Both density and microstructure texturing results were greatly influenced by sintering temperature and pressure. More specifically for pressureless sintered samples, high cold uniaxial pressure was first applied to prepare highly dense green bodies characterized by unusually high degree of texture orientation. They were subsequently pressureless sintered to form links between grains by the activation of diffusion process and partial melting at the grain boundary. Mechanical characterizations, supported by microstructural evaluations, underline the anisotropic character of the material, its intrinsic brittleness and the low hardness.

Excimer laser-induced microstructural changes of alumina and silicon carbide

Surface treatments with a KrF excimer laser were applied on alumina and silicon carbide ceramic materials. Results on the surface modifications induced by laser were related to the processing parameters: laser fluence (1.8 and 7.5 J/cm2), number of laser pulses (1 to 500), frequency (1 to 120 Hz), pulse duration (25 ns), sample speed under the laser beam and working atmosphere. It was ascertained that alumina can be laser treated under air, while silicon carbide needs an inert atmosphere to avoid surface oxidation. Microstructural analyses of surface and cross section of the laser processed samples evidenced that at low fluence (1.8 J/cm2) the surface of both ceramics is covered by a scale due to melting/resolidification. At high fluence (7.5 J/cm2) there are no continuous scales on the surfaces; material is removed by decomposition/vaporisation and the depth of material removal is linearly dependent on the number of pulses. On alumina surface, a network of microcracks formed, while on silicon carbide different morphologies (flat and rugged areas, deposits of debris and discontinuous thin remelted scales) were detected. The evolution of surface morphology and roughness is discussed with reference to composition, microstructure and physical and optical properties of the two tested ceramics and to laser processing parameters.

Synthesis and mechanical behavior of β-tricalcium phosphate/titania composites addressed to regeneration of long bone segments

Bioactive tricalcium phosphate/titania ceramic composites were synthesized by pressureless air sintering of mixed hydroxyapatite and titania (TiO2) powders. The sintering process was optimized to achieve dense ceramic bodies consisting in a bioactive/bioresorbable matrix (β-tricalcium phosphate) reinforced with defined amounts of sub-micron sized titania particles. Extensive chemico-physical and mechanical characterization was carried out on the resulting composites, which displayed values of flexural strength, fracture toughness and elastic modulus in the range or above the typical ranges of values manifested by human cortical bone. It was shown that titania particles provided a toughening effect to the calcium-phosphate matrix and a reinforcement in fracture strength, in comparison with sintered hydroxyapatite bodies characterized by similar relative density. The characteristics of the resulting composites, i.e. bioactivity/bioresorbability and ability of manifesting biomimetic mechanical behavior, are features that can promote processes of bone regeneration in load-bearing sites. Hence, in the perspective of developing porous bone scaffolds with high bioactivity and improved biomechanical behavior, TCP/TiO2 composites with controlled composition can be considered as very promising biomaterials for application in a field of orthopedics where no acceptable clinical solutions still exist.

Wear of 3Y-TZP containing compressive residual stresses at the surface

Tetragonal zirconia polycrystals doped with 3 mol % yttria (3Y–TZP) was heat treated to stimulate the tetragonal to monoclinic phase transformation in order to induce compressive residual stresses at the surface. Pin-on-disc wear tests were performed at different sliding speeds to evaluate the wear behavior of treated and untreated materials. The friction coefficients were also measured. The conditions for mild and severe wear were defined. The material containing compressive residual stresses had lower wear resistance. A possible explanation for this behavior is discussed.

Indentation grid analysis of nanoindentation bulk and in situ properties of ceramic phases

Composites properties are often derived from the proper-ties of the constituent phases measured in bulk forms.However, in situ properties can be different from thosemeasured in bulk as a consequence of material processing[1–3]. In ceramic composites, for example, spurious phasescan form due to the chemical interaction of differentpowders. The knowledge of in situ properties would allowa better characterization and tailoring of composites per-formances. Many ceramic composites are particle-reinforced composites so that the evaluation of in situproperties involves measurements in very small volumes.For some mechanical properties, this can be accomplishedby nanoindentation tests. By nanoindentation, singlemicrostructural elements can be tested as grains in poly-crystals [4, 5] or single phases in composites [3, 6–8]. Inthis work, a comparison between nanoindentation bulk andin situ properties of some ceramic phases will be presented.Generally, in situ properties are evaluated by imaging theindentation marks, for example using a scanning electronmicroscope (SEM), to detect which phase was indented.Besides this traditional technique, which can be timeconsuming especially when indentations are tiny as ithappens in hard phases like advanced ceramics, in situproperties will be estimated applying a new type of anal-ysis to nanoindentation data [9–11]. According to this newanalysis, the mechanical properties of the constituentphases of a composite can be easily derived from a sta-tistical analysis of nanoindentation results without havingto image where indentation marks were placed. Basically,the analysis consists in fitting the experimental dataaccording to a proper number of statistical distributionswhose central values correspond to the specific propertiesof each phase. Constantinides et al. [10] called this analysisindentation grid (IG). In this work, IG will be applied toparticle-reinforced ceramic composites, some based onwell-known phases, such as MoSi

Effect of soft drinks on the physical and chemical features of nickel-titanium-based orthodontic wires

Abstract Objective. The purpose of this study was to evaluate the effect of three popular soft drinks on the Youngs modulus, hardness, surface topography and chemical composition of widely used nickel-titanium-based orthodontic wires. Materials and methods. Thirty-two specimens (20 mm in length) were cut from the straight portion of pre-formed 0.019 × 0.025 inch Nitinol Heat-Activated archwires and randomly divided into four groups of eight specimens each: Group A1 (Coca Cola® regular); Group A2 (Santal® orange juice); Group A3 (Gatorade®); Group B (distilled, deionized water; dH2O). Each specimen was immersed in 10 ml of one of the soft drinks or dH2O, control, for 60 min, at 37°C. At the end of the soaking time, the Youngs modulus and hardness were determined using a nanoindenter. Scanning Electron Microscope–Energy Dispersive Spectroscopy (SEM-EDS) was used to characterize the effects on the topography and chemical composition of the wires. Results. No statistically significant differences were found between the groups either in the Youngs modulus or in hardness after the selected soaking protocol. Besides some surface colour changes, the topography and the chemical composition of the wires were not affected by the immersion in any of the chosen soft drinks. Conclusions. These in-vitro results suggest that the consumption of soft drinks cannot be acknowledged as one possible reason for the degradation of the physical and chemical properties of heat activated nickel titanium orthodontic wires in patients undergoing fixed orthodontic treatment.

Densification, Microstructure Evolution and Mechanical Properties of Ultrafine SiC Particle-Dispersed ZrB2 Matrix Composites

The densification behavior along with the microstructure evolution and some mechanical properties of four ultrafine SiC particle-dispersed ZrB2 matrix composites were studied. The SiC–ZrB2 composites, with a SiC content of 5, 10, 15 and 20 vol%, were densified to near full density by vacuum hot pressing at 1,900°C under a maximum uniaxial pressure of 45 MPa. The presence of SiC greatly improved the sinterability of ZrB2. Grain growth of the diboride matrix was increasingly inhibited for larger amounts of SiC added. Elastic modulus, Poisson ratio, microhardness, flexural strength and fracture toughness were measured at room temperature. Unexpectedly, no obvious effect of the increasing amount of SiC on flexural strength and fracture toughness was found. The former property ranged from 650 to 715 MPa but was actually affected by the exaggerated size of several tenths of micrometers of sintered SiC clusters which acted as dominant critical defects. Also fracture toughness did not receive a marked contribution from the increase of the SiC content. As for the matrix, the prevailing fracture mode of the composites was intragranular, regardless of the SiC content.