Abstract
Niobium has been considered for applications in the aerospace sector, but its use at high temperatures is restricted, due to the great affinity of refractory metals with oxygen, which results in the formation of oxide layers and a decrease in their mechanical resistances. In the present work, Nb samples were submitted to High-Temperature Nitrogen Plasma-Based Ion Implantation (HT-NPBII). The process runs at a working pressure between 3 and 4 mbar and negative high voltage pulses of 7 kV/30 μs/300 Hz were applied to samples heated to 1000°C, at treatment times of 1 h, 4 h, and 8 h, respectively. Microstructural and mechanical characterizations of the treated samples revealed the formation of a layer of Nb2N, with 3.0 μm thickness and increase in the surface hardness from 225 HV for the untreated material up to about 2498 HV, for samples treated during 8 h. Creep tests were performed at 500°C and with loads varying from 25 to 40 MPa. Results indicated a decrease in the secondary creep rate for treated specimens when compared to the untreated ones. This behavior can be attributed to the formation of a nitride layer on the surface of Nb that acts as barrier to avoid the oxygen diffusion into the material under high temperature conditions.
Highlights
Niobium has been widely employed as an alloying element due to its outstanding properties such as good corrosion resistance, high melting point, high thermal conductivity, and high linear coefficient of thermal expansion [1] even though the pure metal presents low resistance to oxidation, especially at elevated temperatures, leading to its embrittlement. us, improving the surface properties of niobium is necessary for its application at high temperatures in an oxidative environment [2]. e effectiveness of HT-NPBII for the treatment of metal alloys and refractory metals was reported in [3,4,5]
Implantation of nitrogen into niobium at 1000°C and 1250°C via HT-NPBII led to the appearance of different phases of niobium nitrides (NbN, Nb2N, and Nb4N3), which resulted in significant increase in hardness and wear reduction, besides causing a retard in the oxidation of the metal by about 130°C [7,8,9,10]
Several studies have reported the influence of the creep response of the treated material under this condition [12,13,14,15,16]. e aim of this paper is to evaluate the creep behavior of niobium treated by HTNPBII
Summary
Niobium has been widely employed as an alloying element due to its outstanding properties such as good corrosion resistance, high melting point, high thermal conductivity, and high linear coefficient of thermal expansion [1] even though the pure metal presents low resistance to oxidation, especially at elevated temperatures, leading to its embrittlement. us, improving the surface properties of niobium is necessary for its application at high temperatures in an oxidative environment [2]. e effectiveness of HT-NPBII for the treatment of metal alloys and refractory metals was reported in [3,4,5]. E effectiveness of HT-NPBII for the treatment of metal alloys and refractory metals was reported in [3,4,5]. Us, improving the surface properties of niobium is necessary for its application at high temperatures in an oxidative environment [2]. Details about this process can be found in [6]. Microstructural changes caused by the surface treatment were evaluated, followed by mechanical tests performed to investigate the material creep properties. Microstructural changes caused by the surface treatment were evaluated, followed by mechanical tests performed to investigate the material creep properties. is is an important issue for the application of niobium in Advances in Materials Science and Engineering aerospace components submitted to harsh environments under high temperatures
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