Papers by Keyword: Thermal Spray

Abstract: In the present work, a numerical investigation has been performed to study the thermal behavior of a flow through a pulsed detonation thermal spray device that employs a hydrogen-air mixture. In this device, a detonation wave travels inside a detonation tube and eventually exits the tube and impinges against a substrate. Two types of substrates have been investigated, namely, a sphere and a flat-plate. The present model is based on solving the two dimensional, axisymetric Euler reactive flow equations with temperature-dependent specific heats. A global single step finite rate reaction has been considered. The present results demonstrate the effects of two main parameters on the thermal performance of the device under consideration. These parameters are: the size and shape of the substrate, and the stand off distance.

Abstract: Thermal cycle resistance of Ni-20Cr, Ni-50Cr and CoNiCrAlY coatings produced by air plasma spraying was investigated according to Japanese Industrial Standard Testing method for thermal cycle resistance of oxidation resistant metallic coatings (JIS H 8452: 2008). The specimens were exposed to a cyclic heating and cooling regimen comprised of up to 100 cycles of 10 hours heating to 1000 °C or 1093 °C in air followed by cooling. The thermal cycle resistance of oxidation-resistant metallic coatings was found to depend strongly on testing temperature and on the chemical composition of the coating materials. In thermal cycle testing at 1000 °C, no remarkable failure was observed in any specimen. However, in thermal cycle testing at 1093 °C, spalling was observed over the entire surface of the Ni-20Cr coating, although the Ni-50Cr and the CoNiCrAlY coatings exhibited excellent thermal cycle resistance even upon exposure to 100 thermal cycles. The CoNiCrAlY coating showed mass gain with increasing number of thermal cycles due to preferential oxidation between thermal spray particle splats. Furthermore, the failure behavior of specimens was investigated in detail by SEM, XRD, EPMA, etc.

Abstract: The thermal spray coatings of stainless steel with nano-sized particles as reinforcement have been studied. Stainless steel powder mixed with 0, 2.5, 5 and 10 wt% WC-12wt%Co nano-sized powder was flame sprayed. The presence of WC-12wt%Co nano-particles in mixed powders as feedstock was confirmed. The microstructure of the coatings has been investigated using a wide range of characterization techniques: optical microscopy with image analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to understand the microstructure evolution. Chemical composition and microstructure of the coatings showed that the coatings contained stainless steel, WC, Co, and oxide layers. The addition of WC-12wt%Co increased cross-sectional hardness, reduced porosity and friction coefficient. The improved wear properties demonstrate a high potential for industrial applications of stainless steel/WC-12wt%Co coatings.

Abstract: In the paper, the indentation fracture toughness of the HVOF sprayed WC-12%Co coatings is determined by the Vickers indentation test in dependence on used load. The relations, proposed by Chicot, were used to calculate the value of KIc at condition of the Palmqvist, Intermediate and Radial-median cracking mode. The applicability of using different equations is discussed with respect to the size indentation effect of IFT.

Abstract: Electrodeposited hard chromium (EHC) coatings have been widely used for improving wear characteristics of engineering tools and components. In the electrodeposition processes, in general solutions containing hexavalent chromium have been used. Hexavalent chromium is classified as a carcinogen compound, which causes health risks if used in production, and environmental problems due to the toxic character of the wastes [1]. In the present study plasma sprayed coatings (aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide, chrome carbide) and electrolytic hard chrome coatings corrosion resistances have been compared through electrochemical polarization tests and immersion test in 0.1 N HCl aqueous solution.

Abstract: Hydroxyapatite (HA) coatings are widely used to improve the biocompatibility of titanium substrates. The main problem concerning these coatings is related with the bad adhesion characteristic of the ceramic/metal interface. A TiO2 interlayer can be used to improve the adhesion of the coating; however the TiO2 is a bioinert material. Then, a solution can be the use of a mixture of HA-TiO2 (60% wt HA- 40% wt TiO2) powder has been sprayed by using High Velocity Oxy-Fuel (DJH-2700) for different spraying distances. The adhesion of the coatings has been mechanically tested and the values have been compared with the values obtained for coatings that have been immersed in simulated body fluid solution (SBF) after 24 and 120 hours. The crystallinity degree of the HA plays an important role on it for the dissolution affecting the behaviour of the interface at the same time it is also important osteoclastic resorption and carbonate apatite precipitation.

Abstract: Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Abstract: Thermal spray coatings provide good tribological and corrosion-resistant properties. Coatings with carbides or diborides improve resistance to oxidation. However, depending on the thermal spray conditions some carbide and diboride oxidation takes place. The aim of this study is to analyze the oxidation of TiC or TiC+TiB2 powders embedded into a stainless steel matrix. The starting powders were obtained by Self-propagating High-temperature Synthesis (SHS). Oxidation was studied at two temperatures, 700°C and 800°C. The results aid our understanding of the powders’ behaviour during coating and use. An open electric furnace was used, and the samples were analyzed using a SEM coupled with an EDS. Thermal treatment time ranged from 2 minutes to 9 hours. A continuous oxidized layer was observed for the TiC particles. This was not the case for TiB2. The decomposition/oxidation of TiB2 begins before the oxidation of TiC. An oxidized layer of around 4 m is formed at a temperature of 800°C after 690 s, when using a powder of 45 m mean size.

Abstract: Today, in the period of one year, in Brazil and throughout world, hundreds of thousands of surgical implants, both permanent and temporary, are carried out with a high cost for the patients, especially those of SUS – Sistema Único de Saúde. In addition, the SUS costs for treatment of infections that appear as a result of introduction of an implant into the human body create expenses about three times greater than the use of implants coated with biomaterials. Hydroxyapatite (HA) constitutes the base of one important group of biomaterials with excellent implant characteristics. Hydroxyapatite has been extensively studied, but the adherence relationship at the coating/substrate interface is not yet well understood. The coatings in this study were produced using substrates of AISI 316L stainless steel shaped as external fixation pins, with HA applied by a high energy plasma spray gun (HEP). The coating thicknesses were between 15µm and 98µm. The mechanical evaluation of coating adhesion was conducted by scratch testing with progressive loads. Results show an increase of failure load with an increase in thickness. The clinical evaluation was carried out in the Trauma and Orthopedics Sector of the Baleia Hospital, and shows a significant decrease in inflammatory response in the human body.

Abstract: The machinery used in coal thermoelectrical plants usually is submitted to erosive wear. The erosive wear occurs mainly in the metallic pipe set of heat exchangers due the flow of hot gases carrying erosive particles. Jorge Lacerda’s thermoelectrical complex at Capivari de Baixo city holds seven power units, where two units use approximately 20 000 ASTM A178 heat pipes. The set is submitted to a semester maintenance schedule (preventive and corrective) where the damaged pipes are changed. So, in this work a set of erosive wear accelerated tests according ASTM G76 were performed in order to develop and specify materials and methods to diminish the erosive action caused by the combustion gases over the heat pipes. Specimens were coated with WC12Co and Cr3C2-25NiCr alloys using the HVOF technique and the coated specimens were tested at 450°C, the heat pipes working temperature. Silica was used as abrasive material at 30° and 45° impact angles, simulating a harder erosive condition than the real condition. The best performance coating at laboratory scale was later used in field condition. The results showed the coated specimen performance is better than the ASTM A178 alloy. The erosion resistance of the Cr3C2-25NiCr and WC12Co coatings is eight times higher than the uncoated alloy, and the coatings also presented a better corrosion resistance. This feature is important, because despite the erosive action the circulating gases also present a large amount of sulfur in their composition. Sulfur at lower temperatures forms H2SO4, causing intense corrosion of the pipes located at the heat exchangers colder parts. Based on the results and considering the coating costs the Cr3C2-25NiCr alloy was selected to coat a set of pipes mounted at the region of the heat exchanger with the most intense erosive wear. At the moment these coated tubes are in field operation and under observation regarding their performance in comparison with the uncoated pipes located at the same heat exchanger. The real operation conditions of the coated pipes will be estimated from the field life cycle analysis, and after all the cost-benefit of the studied coating.