In this study feasibility of cold spraying to produce ceramic coatings is investigated. Ceramic coatings have high potential for various industrial applications. A main problem with these coatings is undesirable phase transformations caused by high temperatures, as in conventional thermal spraying methods. There have so far been few examples of successful cold spraying of ceramics. This work is to examine the problem further by synthesizing specially tailored porous powders, using the sol-gel method. The powders were sprayed on different substrates using a cold spray system. The deposits were characterized by optical and scanning electron microscopy. Based on the results, a discussion of the optimal powder characteristics for cold spraying of ceramics is presented.

In this work the suitability of two different liquid precursors of hydroxyapatite (HA) as feedstock for the air plasma spray (APS) coating technique was studied and compared. The precursors were organic (calcium nitrate tetrahydrate and triethyl phosphate) and inorganic (calcium nitrate tetrahydrate and ammonium dihydrogen phosphate) sol-gel solution precursors employed in a previous work to produce thin films by a dipping technique. We report on the formation of porous hydroxyapatite coatings on Ti6Al4V substrates. The microstructures found in of both types of coatings are characterized by features typically found in solution precursor plasma spray processes; a combination of melted and unmelted deposits and small hollow spheres. Transmission electron microscopy revealed submicron/nanocrystalline features forming those deposits. Hydroxyapatite was the main crystalline phase present in the coatings. Small amount of other crystalline phases were also found due to the high temperature of the substrates during the spraying.

Thermopile devices are useful in many applications such as thermoelectric generators, temperature sensors as well as heat sinks. This paper presents a rapid fabrication scheme for thermopiles by thermal spray technique. The scheme is based on direct deposition of two over lapping chromel/constantan layer separated by sprayed on copper connector on a tube. The insulating layer on the tube is obtained by curing a sprayed on sol-gel ceramic by the spray gun itself. Serially connected thermocouple junctions are created by machining a spiral track on the tube. Design and manufacturability of an exhaust waste heat recovery module has been discussed. The electrical and thermal characteristics of the thermopile module have been presented. These thermopiles are designed for high temperature and harsh environments.

Calcium phosphate (Ca-P) coatings have been used as surface coatings on porous metallic implants in dentistry and orthopaedics for the last twenty years. These Ca-P coatings, nominally hydroxyapatite (HA), have been shown to promote bone fixation and osteconductivity on Ti and Ti alloy substrates used for those purposes. Such coatings can be formed by different methods including plasma spray. In addition to the well known advantages of the plasma spray technique to deposit coatings, a new version of this technique, i.e. solution precursor plasma spray (SPPS), has been reported to produce submicron/nanocrystalline structured coatings. Nanocrystalline HA coatings may improve the resorption of the coating in the body, avoiding the irritant effect of large particles which may be seen in current thermal sprayed HA coatings. The main purpose of this work was to study the suitability of a sol-gel Ca-P solution precursor (calcium nitrate tetrahydrate and ammonium dihydrogen phosphate) as feedstock for the air plasma spray (APS) coating technique. We report on the formation and the characteristics of the coatings so formed on Ti6Al4V substrates. The presence of different Ca-P crystalline and amorphous phases was assessed by X-ray diffraction analysis. The X-ray photoelectron spectroscopy technique was used to characterize the surface chemical composition of the Ca-P coatings. The microstructural features of the coatings were characterized by scanning/transmission electron microscopy combined with image analysis in order to evidence the presence of submicron/nanocrystalline Ca-P features. Final results are discussed in terms of the spraying parameters. Abstract only; no full-text paper available.

This paper describes the manufacture of a new ceramic coating system based on Al 2 O 3 /SiC nanocomposite powder prepared by sol-gel processing followed by low pressure plasma spraying (LPPS) onto stainless steel substrates. In order to produce nanocomposite coatings of good adhesion and low porosity, the substrates were water-cooled to minimise thermal stresses associated with coefficient of thermal expansion mismatch and a CoNiCrAlY bond coat was used. The sol-gel powder feedstock and the as-sprayed coatings have been characterised by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), and Nuclear Magnetic Resonance (NMR). The coating characteristics were compared with a reference Al 2 O 3 coating prepared from commercial feedstock powder. The thermal exposure of the sol-gel powder during spraying caused phase-changes and phase-decomposition. Examination of the sprayed coatings showed that it was possible to maintain the 20-200 nm SiC particles in the final Al 2 O 3 /SiC nanocomposite coating. The coatings also contained both stable α-Al 2 O 3 and metastable γ-Al 2 O 3 . Some minority phases such as silica and aluminosilicate formed in the sol-gel feedstock powder were fully decomposed during LPPS. This preliminary study indicates that sol-gel and LPPS processing is a potential route for the manufacture of nanocomposite coatings, which may offer significant improvements in some aspects of coating properties.

Thermal barrier coatings have been extensively used in several industrial segments. The material used as an insulator in such systems has been partially stabilized zirconia (PSZ) plasma sprayed over a metallic bond coat layer. The ceramic layer is usually porous, thus improving insulation properties. The porosity also increases gas permeability and, therefore, reduces oxidation resistance of the coating. Post-treatments have been applied to reduce the open porosity and improve oxidation resistance. In this work thermal barrier coatings were applied on low carbon steel substrates using two sets of bond coat, i.e., metallic and metal-ceramic. The metallic bond coat was NiCrAlY. The metal-ceramic bond coat was a mixture of NiCrAlY and 8% yttria partially stabilized zirconia, which were applied by simultaneous feeding to the plasma torch from two powder feeders. A sol-gel method was employed to impregnate the porous ceramic top coat with alumina or zirconia. The samples in the as-sprayed and post-treated condition were characterized using mercury intrusion porosimetry (MIP), thermal conductivity. KEY WORDS: Thermal Conductivity, TBCs, Sol-Gel.

This paper explains how laser-beam remelting influences the quality of ceramic coatings on superalloy substrates. The coatings studied consist of Al2O3-Ni ceramic layers. Test results showed that a laser power density of 0.103 x 109 W/m2 is ideal for a beam rate of 1 to 2 m/min. It was also found that coating quality could be further improved by adding a diffusive Cr-Al layer prior to laser remelting. This added step reduces pore volume and layer thickness and increases adhesion without cracks, chips, or spalls. The examination also showed that, depending on cooling rate, it is possible to obtain layers that are similar in composition but different in structure or even amorphic.

Thermal sprayed coatings are frequently used in corrosive environments, even when their major purpose is to provide wear or thermal resistance, rather than corrosion resistance. This includes Thermal Barrier Coatings (TBC), where high porosity is a desired feature to give good thermal protection. However, as this proves to be a limiting factor in the corrosion protection, a trade off is involved. This is because the interconnected porosity in TBCs allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. This work addresses the problem of permeability of TBCs which can lead to premature delamination due to interfacial corrosion. The coatings studied were yttria-stabilized zirconia TBCs. A simple infiltration technique has been proposed using sol-gel ceramic precursors. The precursors studied include aluminum isopropoxide or pre-hydrolyzed ethyl silicate, which decomposed to alumina and silica respectively, at surface heat treatment temperatures as low as 550°C. In addition to sealing the surface, it is believed that some level of compressive stress is generated on the surface of TBCs on cooling from the processing temperature. Electrochemical tests in 3.0% NaCl have been carried out to study the effectiveness of the sealant. These potentiodynamic tests as well as permeability tests show a considerable decrease in interconnected porosity with sol-gel modifications of the coatings.