Núria Llorca-Isern

High-pressure plasma spraying of hydroxyapatite powders

Thermal spray processes with their wide array of operating parameters are flexible manufacturing tools in the production of bioactive hydroxyapatite (HA) coatings. In this study, two spherical HA powders, namely, spray-dried hydroxyapatite and spheroidized hydroxyapatite powders were sprayed with suitable parameters in a controlled atmosphere plasma spraying (CAPS) system. This unique system was operated in three distinct modes: High-pressure plasma spraying (HPPS), atmospheric plasma spraying (APS) and inert plasma spraying. The HPPS mode has three different pressure settings up to 250 kPa. The APS mode is operated at normal atmospheric pressure of 100 kPa (sea level), and, the other modes utilised argon gas to provide the inert atmosphere during plasma spray. These were applied in order to assess the influence of chamber pressure and chamber atmosphere on the deposition of HA coatings. The microstructures and phase compositions of the plasma sprayed HA coatings are evaluated using standard X-ray diffraction (XRD) and electron microscopy techniques. These established the influence of the plasma spray parameters in the CAPS chamber. HPPS led to enhanced heating of the powder and dense HA coatings with a high content of the amorphous calcium phosphate phase. Small amounts of other calcium phosphates, tetracalcium phosphate and tricalcium phosphate were detected. Calcium oxide was not detected. Such coatings are useful for subsequent investigation of biological and mechanical properties where phase composition and porosity are deciding factors. It is found that the degree of melting of the HA powder can be controlled with CAPS system. This has the advantage of tailoring the final coating microstructure.

Effect of heat treatment on high pressure plasma sprayed hydroxyapatite coatings

Abstract This paper reports the characteristics and properties of two different sets of hydroxyapatite (HA) coatings obtained by thermal spraying two different HA powders, spray dried HA (SDHA) and spheroidisided HA (SHA), at various chamber pressures. The evaluation of the coatings following post-spray treatment at 800°C is also included. Comparisons to the results from the as sprayed coatings were made to show the effects of post-spray heat treatment. The median particle sizes for SDHA and SHA were 35 and 47 μm, respectively. The main difference between the powders is their density and morphology. The SHA powder provides higher density and spherical morphology, and a more porous and rough surface is present in the SDHA powders. These two powders were sprayed using a controlled atmosphere plasma spraying (CAPS) system in the inert plasma spray (IPS) and high pressure plasma spray (HPPS) modes. Four different pressures of 100, 150, 200, and 250 kPa were applied in an argon atmosphere. Post-spray heat treatment was performed to increase the crystallinity of the coatings, to homogenise their structure, and to convert other calcium phosphate phases to HA. Different techniques were used to characterise these coatings. Scanning electron microscopy (SEM) was used to observe surface and cross-section morphology of both powder and sprayed coatings. The phase and chemical characteristics of the samples were evaluated by X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR). Finally, in order to have a qualitative measure of the integrity of the coatings, Vickers hardness testing was performed on polished cross-sections of the coatings.

Grain Refinement of Pure Copper by ECAP

Pure commercial Cu of 99,98 wt % purity was processed at room temperature by Equal- Channel Angular Pressing (ECAP) following route Bc. Heavy deformation was introduced in the samples after a considerable number of ECAP passes, namely 1, 4, 8, 12 and 16. A significant grain refinement was observed by transmission electron microscopy (TEM). Tensile and microhardness tests were also carried out on the deformed material in order to correlate microstructure and mechanical properties. Microhardness measurements displayed a quite homogeneous strain distribution. The most significative microstructural and mechanical changes were introduced in the first ECAP pass although a gradual increment in strength and a slight further grain refinement was noticed in the consecutive ECAP passes.

Metallographic structure of bainitic compacted graphite cast irons

Abstract The microstructural characteristics of austempered unalloyed compacted graphite cast irons have been studied. The material was subjected to a variety of bainitic treatments based on four different combinations of austenitizing and austempering temperatures, plus various periods of time at the temper temperature. Scanning electron microscopy was used to reveal changes in the microstructure with the different heat treatments. Austenitizing at 1000°C followed by austempering at 400°C produced microstructures consisting of carbide-free ferrite, together with untransformed austenite and martensite for tempering times out to 75 min. Tempering at 300°C showed incipient bainitic ferrite at short times, with carbide precipitation occurring after 16 min. Austenitizing at 900°C followed by austempering at 400°C produced little or no microstructural change until 16 min had elapsed, then carbide precipitation could be seen after 20 min. Austempering at 300°C gave rise to a diffuse microstructure that was visible after the shortest times at this temperature.

Equal Channel Angular Pressing of Cu-Al Bimetallic Rod

Coextrusion and corolling are the major processes to produce bimetallic rods, tubes and wires, the objective being to perform clad metals, bimetallic joints or seals. The aim of the present work is to produce bimetallic rods showing an ultrafine grained microstructure with enhanced properties. Bimetallic Cu-Al rods were deformed by equal channel angular pressing (ECAP) in order to study their microstructure. ECAP is an interesting process for producing bulk materials with refined microstructure and, consequently, changes in physical, chemical and mechanical properties can be observed. Higher shear strength and dimensional stability are among the advantages of this process. A comparative experimental study of pure commercial copper with cylindrical inner aluminium rods of different diameters processed by one-pass equal channel angular pressing has been carried out. The ECAP die used in this research was a 90º 2-channels intersecting angle. Electron backscattered (EBSD) and X-ray diffraction techniques were used for microstructure characterization (deformation, grain fragmentation and microstrain evaluation) at the interfaces and away from them. It was found that the microstructure in the ECAP deformed Cu-Al bimetallic rods was influenced by the dimensions of the aluminium inner rod. In fact, the microstructure appeared to be much more elongated and refined in the samples containing smaller diameter aluminium rods.

Severe Plastic Deformation of a Commercial Aluminium-Lithium Alloy (AA8090) by Equal Channel Angular Pressing

The enhanced mechanical properties of crystalline materials are linked to very small grain sizes. The AA8090 is a commercial aluminium lithium alloy is referred in the metallurgical literature by its superplastic behavior linked as well to nano-grains as obtained by ECAP. On this research-work in progress, the 8090 aluminium alloy is studied after being processed in a 400 kN ECAE press up to nine passes through a 90º angle-die at 150 °C following Route A (constant path) at a processing speed of 10 mm/min. The room temperature mechanical properties of the extruded alloy are investigated through hardness measurements and tensile tests. Elastic measurements have been carried out by means of nanoindentation and ultrasonic testing, showing that both texture and nanostructuration of the processed alloy influence on the elastic constant of the processed material. In order to understand the mechanisms involved in the SPD-ECAP process, microstructural characterisation of the alloy has been carried out using different high resolution techniques such as Transmission Electron and Atomic Force Microscopy. TEM results have also been used in order to evaluate grain size evolution. From this analysis it is possible to see that after 4 passages the initial grains have already a high degree of dislocations. Among the characterisation techniques, AFM in the surface potential mode has been used to evaluate discontinuities in the matrix and /or decohesion between matrix and precipitates. After nine ECAP passages, no signs of decohesion at the interface have been observed neither using FEG-SEM nor using AFM.

Relationships between microstructure and properties of unalloyed compacted graphite cast irons

Abstract Austempered cast irons have been the subject of much attention in recent years because of their excellent mechanical properties. The hardness, ultimate tensile strength and dynamic elastic modulus are presented for a commercially available unalloyed compacted iron (C.E. 4.31) and correlated with different matrix microstructures (as-cast, ferritized, normalized and austempered). For this study, two isothermal temperatures for the austempering treatment were chosen: 400°C and 300°C. The influence of a ferritizing treatment prior to normalizing and austempering has been evaluated, the results indicating that no advantages are obtained with this additional treatment. The influence of microstructure on properties and on the resulting fracture surfaces in tensile tests are discussed.

Microscopic Characterization of Different Shrinkage Defects in Ductile Irons and their Relation with Composition and Inoculation Process

AbstractShrinkage becomes one of the most important defects that negatively affects the production of ductile cast iron parts. Regular inspections made on the affected parts show that different morphologies of shrinkage can be obtained according to the melt composition, to the layout and to a number of other processing variables that have been reported in the literature. However, minimization of these defects demands a more detailed understanding of their internal features and the relationship with the processing variables causing the greatest effect. In the present study, carbon equivalent content, inoculation and thermal characterization of melts have been studied as variables for producing eight test parts which have been designed for promoting the formation of shrinkage. The different defects obtained in each case have been analyzed by means of metallographic techniques and FE-SEM and their characteristics and size correlated with the selected variables. It has been found that carbon equivalent shows the strongest effect on shrinkage incidence. A discussion about the features found in the internal surfaces of the different closed defects is done, and possible explanations for each case are also described.

Hafnium-silicon precipitate structure determination in a new heat-resistant ferritic alloy by precession electron diffraction techniques.

The structure determination of an HfSi4 precipitate has been carried out by a combination of two precession electron diffraction techniques: high precession angle, 2.2°, single pattern collection at eight different zone axes and low precession angle, 0.5°, serial collection of patterns obtained by increasing tilts of 1°. A three-dimensional reconstruction of the associated reciprocal space shows an orthorhombic unit cell with parameters a = 11.4 Å, b = 11.8 Å, c = 14.6 Å, and an extinction condition of (hkl) h + k odd. The merged intensities from the high angle precession patterns have been symmetry tested for possible space groups (SG) fulfilling this condition and a best symmetrization residual found at 18% for SG 65 Cmmm. Use of the SIR2011 direct methods program allowed solving the structure with a structure residual of 18%. The precipitate objects of this study were reproducibly found in a newly implemented alloy, designed according to molecular orbital theory.

Processing Dense Hetero-Nanostructured Metallic Materials for Improved Strength/Ductility Balance through High Strain Deformation and Electrical Current Assisted Sintering (ECAS)

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by ECAE) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) for producing multi-scale microstructures when a milled powder is used is also demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of ECAS), altering the nature of the milled powder - by Y2O3 addition during the milling stage - is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.