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Vojnotehnički glasnik
2010, vol. 58, iss. 4, pp. 5-16
article language: Serbian
document type: Scientific Paper
Effect of the powder deposition rate on the mechanical properties and the structure of the APS-NiCr/Al coating
IMTEL Institute, Belgrade



The paper presents the results of the examination of atmospheric plasma spraying of APS-NiCr/Al coatings. Composite NiCr/Al powders, composed of NiCr particles coated with fine Al particles, enable an exothermic reaction during the deposition process. The exothermic reaction of these metals results in the formation of selfbonding NiCrAl coatings with a unique combination of properties. For the purpose of obtaining excellent structural and mechanical properties, the deposition parameters were optimized. In this study, three groups of NiCrAl coating samples with different powder deposition rate were made to get deposits of greater strength and toughness. In the first group of samples, the deposition rate was 50g/min, in the second group the powder deposition rate was 40 g/ min. while in the third group it was 30 g/min. The assessment of the properties of deposits was made by the HV0.3 microhardness testing method and the bond strength was examined by tensile strength testing. The metallographic evaluation of the structure was done by optical microscopy The verified coatings were applied for the protection and revitalization of parts exposed to oxidation at elevated temperatures. Introduction Plasma spraying is widely used in many industries to improve performance of components. This process has found wide applications in many industries such as aviation, petrol chemistry or automotive industry. Plasma spraying is one of the most efficient thermal processes. A jet of plasma particles has a wide range of both temperature and rate. A typical plasma temperature range is from 8700 to 12000°C and the rate range is from 80 to 400ms-1. Composite NiCr/Al powder consists of NiCr alloy particles (nuclei) coated with fine Al particles. Deposited coatings are better than NiCr and Ni/Al-type coatings. They are resistant to oxidation, hot corrosion, sudden changes in temperature, abrasion, erosion, and they react well to mechanical damage. Mechanical strength of bonding remains adequate up to a working temperature of 980°C. For designing NiCrAl coatings with the best structural mechanical properties, three groups of samples with different powder deposition rates were made. The aim of this work was to improve the repair of the sections of turbo jet engines rejected due to wear-based oxidation, hot corrosion and other damage mechanisms. Experiment details and materials The coated AMDRY 960 powder of the Sulzer METCO company was used in the experiment. The NiCr/Al powder was developed for the production of coatings which are often used separately as a protection of the base metal from oxidation and hot corrosion at high temperatures or in a combination with ceramic coatings as a middle layer. Powder deposition was performed at atmospheric pressure with a mixture of Ar-He plasma gases and the power supply of 40KW. The study examined three groups of samples to get deposits of high toughness. The powder deposition rate in the first group of samples was 50g/min with a carrier gas flow of 7 l/min, the second group of samples had a powder deposition rate of 40 g min. with a carrier gas flow of 6 l/min. while the third group of samples had 30 g/min. of the deposition rate with a carrier gas flow of 5 l/min. Testing microhardness, bond strength and microstructure The testing and characterization of the coating layers were made in accordance with the TURBOMECA standard. Microhardness was measured along the lamellar layers with the Vickers test with 300 g load (HV 0.3). The microhardness test samples and the samples for microstructure analysis were made of Č.4171 (X15Cr13 EN10027) steel in the thermally raw state with the dimensions of 70x20x1.5 m. Testing the coating/supstrate interface strength was done by the tensile test. The specimens for tensile bond strength testing were made in accordance with the TURBOMEC standard, out of Č.4171 (X15Cr13 EN10027) steel in the raw thermal state with 25x50 mm in diameter. The microstructure of the layers and the quality of the coating/supstrate interface was analyzed using an optical microscope with a magnification of 400X. Results and discussion The obtained results show that the microhardness values for layers in all three groups of specimens were over 170HV0.3.The highest microhardness values (from 346 to 357 HV0.3) occurred in the layers deposited with the powder deposition rate of 30 g/min. The values of the bond strength confirm that there is good adhesion between the coating and the substrate; the cohesion strength of the lamellae in coating layers is good as well. The destruction in all coatings was of an adhesive type along the coating/supstrate interface. The coating qualitative analysis showed that the uniformity of layers on the substrate was obtained without the segment separation of layers from the surface of the substrate. The coating quantitative analysis showed that the structure of the substrate is a solid solution of chromium and aluminum in nickel light gray color. On the light gray lamelar boundaries, there are dark gray oxide (NiO and Cr2O3) lamellae and black α-Al2O3 lamellae as a result of air penetration into the plasma jet and micro pores. The existing pores are black and account for less than 8%. Conclusion Atmospheric plasma spraying (APS) was used for three NiCrAl coatings with three different powder deposition rates (30.40 and 50 g/min). Their mechanical properties were investigated and the coating microstructure was analyzed, resulting in satisfactory mechanical properties. The following conclusions were drawn: The highest values of micro hardness of 346-357HV0.3 were found in the layers deposited with the powder deposition rate of 30 g/min. With the increase of the powder deposition rate, the microhardness of layers decreases. The highest values of the bond strength of 72MP were found in the layers deposited with the powder deposition rate of 30 g/min. The coating properties were confirmed in the exploitation of functional parts.



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