International Journal of Scientific and Technological Research www.iiste.org ISSN 2422-8702 (Online), DOI: 10.7176/JSTR/5-10-12 Vol.5, No.10, 2019 88 | Page www.iiste.org The Effect of Aluminizing Nb Refractory Metal Surface on Increasing Oxidation Resistance Pinar Koymen Cagar (Corresponding author) Dokuz Eylul University, The Graduate School of Natural and Applied Science, Tinaztepe Campus, Buca, Izmir, PO box 35160, Turkey E-mail: pinarkoymen@hotmail.com Ali Bulent Onay Dokuz Eylul University, The Graduate School of Natural and Applied Science, Tinaztepe Campus, Buca, Izmir, PO box 35160, Turkey E-mail:bulent.onay@deu.edu.tr bulentonay@gmail.com Abstract In order to increase oxidation resistance of Nb refractory metal, Aluminizing and Aluminizing + Siliconizing with co-deposition were applied to its surface through the method of Halide-activated pack cementation (HAPC). The characterization of diffusion coating was conducted via the analyses of SEM, XRD, and EDS. After that, oxidation tests were conducted, and the effect of coating on oxidation resistance was examined. As a result, according to XRD analysis, it was found that there were Nb5Si 3 phase structures in both coatings. In addition to this, Nb5Si 3 and Nb 3 Si phases emerged in co-deposition coating. Protective NbSi 2 phase was not observed because an activator with fluoride was used but an activator with chloride was not. Both methods increased the oxidation resistance of Nb. However, it was observed that the target value which was “metal loss of 25μm at 1300°C 100 hours after oxidation” was not obtained only through cementation aluminizing process. We are indebted to TUBITAK for their support. Keywords: Refractory metals, oxidation, pack cementation, co-deposition, aluminizing DOI: 10.7176/JSTR/5-10-12 1. Introduction As of the 1970s, Ni-based metal alloys have started to replacing steel. As a result of mixing these alloys with various elements and heat treatment, new generation metallic engineering materials which are defined as “superalloys” have emerged. Through these alloys, the service temperatures of heat-energy conversion systems could be increased to 900 o C (Stringer 1975). These systems can be exemplified by steam and gas turbines used to produce electricity, motors of aircraft and spacecraft, and rockets developed for defense industry. Although superalloys are still used for the abovementioned purposes, productivity growth in energy conversion systems since 1980s has been possible with the development of the methods to cool the parts made of these superalloys and oxidation-resistant coating techniques (Perepezko 2009). As a result of these developments, the operating temperature of metallic materials has gone above 1000 o C. In order to sustain productivity growth, it is necessary to introduce both creep-resistant and oxidation-resistant materials at temperatures above 1200 o C, though. Today, Nickel (Ni) and Cobalt (Co) based superalloys with high strength and oxidation resistance used in electric power generation systems, aircraft, and space vehicles approach the limits of use (~1200 o C); therefore, there is a need for engineering materials with improved mechanical properties and oxidation resistance which can operate at higher temperatures than now. The new materials with these properties are refractory metals with melting temperatures higher than 2000 o C. The studies on refractory metals that started in the 1960s in the USA are still being conducted in lots of countries, yet there are no materials at the technically and commercially desired level. The studies based on Niobium (Nb) have become prominent since this metal has the lowest density in this group (Briant 2000).