Study of colloidal behaviour and rheology of Al2O3-TiO2 nanosuspensions to obtain free-flowing spray-dried granules for atmospheric plasma spraying

This work deals with the dispersion and stabilisation of nanosized Al2O3 and TiO2 particles in aqueous medium. The dispersing conditions were studied as a function of pH, dispersant content, solid loading and ageing. Well-dispersed nanosuspensions of Al2O3 with solids contents up to 15 vol.% and TiO2 with solids contents up to 30 vol.% were obtained by dispersing each type of nanoparticles with 4 wt% of polyacrylic acid-based polyelectrolyte. In order to obtain a concentrated 87 wt% Al2O3–13 wt% TiO2 suspension, a 15 vol.% homogeneous nanosuspension, composed by alumina and titania nanopowders, was prepared and then reconstituted by spray-drying into free-flowing powders. This spray-dried powder showed an adequate granule size distribution, a very high flowability and a reasonable density for diverse purposes, such as to be used in atmospheric plasma spraying as a feedstock to obtain nanostructured coatings

Bioactive glass coatings by suspension plasma spraying from glycoletherbased solvent feedstock

Bioactive glasses are emerging as a substitute of hydroxyapatite in the development of bioactive coatings for biomedical applications. The deposition of these coatings is carried out by a wide range of methods, being atmospheric plasma spraying the most employed technique. However, the research on the deposition of these coatings from suspension feedstocks by thermal spraying is still incipient, therefore more research about this topic is needed. Thus, a bioactive glass suspension, composed of fine glass particles, was prepared and stabilised through rheological and sedimentation tests to be used as a feedstock in plasma spraying. The solvent used in the suspension preparation was dipropylene glycol methyl ether in order to develop a new type of bioactive suspension. Consequently, as a new type of solvent was used, its effect on the plasma torch properties was determined. On other hand, the rheological behaviour of the suspension feedstock was assessed by means of a simple viscosity model. This suspension was deposited onto metallic substrates by plasma spraying, employing several spraying distances. All coatings displayed a suitable adherence and similar thickness. However, the microstructure of the obtained coatings is highly affected by the spraying distance as it can be seen in coatings surface and cross-section field emission gun environmental scanning electron microscopy examination. Thus, a relation between the spraying distance and coatings microstructure was found. On the other hand, X-ray diffraction confirmed the amorphous nature of the obtained coatings

Activación alcalina de metacaolín. Efecto de la adición de silicato soluble y de la temperatura de curado

El objetivo del presente trabajo es determinar el efecto que la cantidad de sílice soluble presente en la disolución activadora (relación molar SiO2/Na2O entre 0 y 0.69) y la temperatura de curado (85ºC, 150ºC y 200ºC) ejercen sobre las propiedades físico-mecánicas y las características mineralógicas y microestructurales de los productos formados por activación alcalina de metacaolín. Para ello se determinaron los valores de resistencia mecánica a compresión de los materiales objeto de estudio y se realizó un estudio mineralógico y microestructural de los mismos por DRX, SEM-EDX, 29Si RMN-MAS y porosimetría de mercurio. Los resultados obtenidos muestran que la resistencia mecánica del material guarda una estrecha relación con la naturaleza, microestructura y composición química de los productos de reacción así como con las condiciones de curado. En todos los casos se genera, como principal producto de reacción, un polímero inorgánico alcalino con propiedades cementantes (gel N-A-S-H) responsable, en mayor medida, de las propiedades mecánicas del material. Como productos secundarios se forman algunas zeolitas cuya proporción y tipo (sodalita, zeolita A, faujasita, …) dependen tanto de la naturaleza del activador como de las condiciones de curado. La presencia de sílice soluble en la disolución activadora conduce a la formación de geles con una mayor relación Si/Al y a la ralentización de la zeolitización, factores, ambos, con un efecto positivo sobre la resistencia mecánica. Con respecto a la temperatura de curado, existe un valor umbral, en función de la composición del material, a partir del cual un incremento en dicha temperatura deja de tener efectos positivos sobre la evolución de la resistencia mecánicaThis study has been undertaken to determine the effect that the soluble silica content in the activating solution (molar ratio SiO2/Na2O between 0 and 0.69) and curing temperature (85ºC, 150ºC, and 200ºC) have on the physico-mechanical properties and mineralogical and microstructural characteristics of products formed by alkaline activation of metakaolin. The compression strength of the materials obtained was determined and a mineralogical and microstructural study of these materials was conducted by XRD, SEM-EDX, 29Si MAS-NMR, and mercury porosimetry. The results show that the mechanical strength of the material is closely related to the nature, microstructure, and chemical composition of the reaction products, as well as to the curing conditions. In all cases, the main reaction product that forms is an inorganic alkaline polymer with cementing properties (N-A-S-H gel), which is largely responsible for the mechanical properties of the material. Some zeolites form as by-products, whose quantity and type (sodalite, zeolite A, faujasite, etc.) depend on the nature of the activator and the curing conditions. The presence of soluble silica in the activating solution leads to the formation of gels with a larger Si/Al ratio and slower zeolitisation, these both being factors with a positive effect on mechanical strength. The curing temperature exhibits a threshold value that depends on the composition of the material, beyond which an increase in curing temperature ceases to positively affect the evolution of the material’s mechanical strengt

Bioactive glass suspensions preparation for suspension plasma spraying

A bioactive glass was dry-milled and sieved in order to obtain powders with particle size finer than 63 μm. Two suspensions, with different particle size distribution (D50 = 8.3 μm and D50 = 2.2 μm), were subsequently obtained from milling those powders in an organic solvent (dipropylene glycol methyl ether) by using two different grinding steps. The obtained suspensions were characterised and stabilised to produce adequate feedstocks to be used in suspension plasma spraying technique. For that purpose, sedimentation tests as well as rheological characterisation were carried out on both suspensions. Only the suspension feedstock containing the finest particle size, managed to produce a coating with suitable thickness and adherence on the substrate when a TiO2 bond coat was used. Besides X-ray diffraction findings confirmed the amorphous nature of the obtained coatings. However coating microstructure displayed many round, closed pores and surface observation revealed the presence of abundant non-deformed splats.The authors of this study thank Universitat Jaume I, Castellón, Spain, for the support provided in funding the project RECUBIO (P1-1B2013-69)

Effect of TiO2 addition on the microstructure and nanomechanical properties of Al2O3 Suspension Plasma Sprayed coatings

Alumina–titania coatings are widely used in industry for wear, abrasion or corrosion protection components. Such layers are commonly deposited by atmospheric plasma spraying (APS) using powder as feedstock. In this study, both Al2O3 and Al2O3–13 wt% TiO2 coatings were deposited on austenitic stainless steel coupons by suspension plasma spraying (SPS). Two commercial suspensions of nanosized Al2O3 and TiO2 particles were used as starting materials. The coatings microstructure and phase composition were fully characterised using FEG-SEM and XRD techniques. Nanoindentation technique was used to determine the coatings hardness and elastic modulus properties. Results have shown that the addition of titania to alumina SPS coatings causes different crystalline phases and a higher powder melting rate is reached. The higher melted material achieved, when titania is added leads to higher hardness and elastic modulus when the same spraying parameters are use

Effect of particle size on processing of bioactive glass powder for atmospheric plasma spraying

The work addresses the effect of the particle size of a bioactive glass feedstock on the processing and microstructure of the resulting coatings obtained by atmospheric plasma spraying (APS). It was observed that the reduction of particle size negatively affects the flowability of the powder. In addition the thermal behaviour (weight losses, glass transitions, crystallisations, etc.) also depended on the particle size of the glass powder. No coating was obtained with the coarser fractions (higher than 200 μm) due to their low melting degree in the plasma. For the intermediate fractions (200–63 μm) coatings were obtained but insufficient particle melting was produced. On the contrary, the finest fraction (<63 μm) needed a fluidiser which enabled the samples to be sprayed.Authors wish to acknowledge the University Jaume I of Castellon for the support in the RECUBIO project (P1–1B2013–69) and Fritta S.L. for the support in feedstock synthesis

Study of erosion behaviour of conventional and nanostructured WC-12Co coatings sprayed by atmospheric plasma

Thermal sprayed WC-Co coatings are used extensively to enhance the wear resistance of a wide range of engineering components. In this paper, erosive resistance of plasma atmospheric sprayed WC-12Co coatings has been evaluated. Solid particle erosion tests were conducted on these coatings at different angles of impact with silica and alumina abrasives of size 250 µm. Coatings have been deposited by using micrometric and nanometric agglomerated powders, employing H2 and He as plasmogen gas. In order to determine the erosion regime (ductile or brittle), the influence of impact angle on the erosion rate has been studied. Optical microscope and FESEM have been used to analyze the eroded surface. The influence of the plasmogen gas and the powder employed on the erosive behaviour of the coating has been evaluated. An attempt to connect the erosive behaviour with mechanical properties and microstructure has been made. Hardness has been determined by means of several measurements of Vickers microhardness; fracture toughness has been estimated through indentation method. Identification of phases has been made by means of X Ray diffractio

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO2 powders with an average primary size of ∼20–40 nm and surface area of ∼50 m2 g−1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800–1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively

Effect of the initial particle size distribution on the properties of suspension plasma sprayed Al2O3-TiO2 coatings

Al2O3–TiO2 coatings have been deposited by atmospheric plasma spraying from agglomerated, nanostructured powders showing better properties than those of their conventional (microstructured) counterparts. These nanostructured coatings can be also obtained by suspension plasma spraying however the research on suspension plasma sprayed Al2O3–TiO2 is still scarce. Consequently, it is crucial to study the effect of the suspension characteristics on the coating properties and to optimize the deposition process. In this work, Al2O3–13 wt.% TiO2 tribological coatings were successfully deposited by suspension plasma spraying from three different feedstocks: a nanometric suspension and two bimodal suspensions with different solid contents made up of titania nanoparticles and alumina submicron-sized particles. The coating microstructure and phase composition were characterized using scanning electron microscopy and X-ray diffraction analysis. Moreover, nanoindentation technique was used to determine the nanomechanical properties of coatings. The influence of the feed suspension characteristics on the final coating quality was analyzed. Findings showed that similar microstructures and phases were developed after depositing the different feedstocks. In addition suspension feedstock made up of nanoparticles resulted in a coating with better mechanical properties. However the use of submicron-sized particles in the suspension feedstocks gives rise to some technical and economic advantages in the process which should be taken into account when a suspension plasma spraying process is to be set up

Large scale synthesis of nanostructured zirconia-based compounds from freeze-dried precursors

Nanocrystalline zirconia powders have been obtained at the multigram scale by thermal decomposition of precursors resulting from the freeze-drying of aqueous acetic solutions. This technique has equally made possible to synthesize a variety of nanostructured yttria or scandia doped zirconia compositions. SEM images, as well as the analysis of the XRD patterns, show the nanoparticulated character of those solids obtained at low temperature, with typical particle size in the 10–15 nm range when prepared at 673 K. The presence of the monoclinic, the tetragonal or both phases depends on the temperature of the thermal treatment, the doping concentration and the nature of the dopant. In addition, Rietveld refinement of the XRD profiles of selected samples allows detecting the coexistence of the tetragonal and the cubic phases for high doping concentration and high thermal treatment temperatures. Raman experiments suggest the presence of both phases also at relatively low treatment temperatures.This study has been supported by the Spanish Ministry of Science and Technology and EU FEDER Program (MAT2009-14144-C03-01, MAT2009-14144-C03-03, MAT2012-38364-C03-01, MAT2012-38364-C03-02). The SCSIE of the Universitat de València is gratefully thanked for the use of the X-ray diffraction, electron microscopy, and analytical facilities. R. Moreno and T. Molina, from Instituto de Cerámica y Vidrio, CSIC, Madrid, are acknowledged by the realization of the agglomerate size measurements