Vol.:(0123456789) 1 3 Journal of the Brazilian Society of Mechanical Sciences and Engineering (2022) 44:199 https://doi.org/10.1007/s40430-022-03506-x TECHNICAL PAPER Controlled addition of air in the gas mixture of plasma nitriding: an analysis of nitrided layer microstructure and microhardness of carbon steels Francisco Cavilha Neto 1  · Tatiana Bendo 1  · Bruno Borges Ramos 1  · Walter Dal’Maz Silva 1  · Cristiano Binder 1  · Aloisio Nelmo Klein 1 Received: 15 September 2021 / Accepted: 30 March 2022 © The Author(s), under exclusive licence to The Brazilian Society of Mechanical Sciences and Engineering 2022 Abstract In this study, the efect of the controlled injection of air as a source of contamination, corresponding to 10%, 20% and 30% of total renewal fow, on the microstructural characteristics and microhardness of plasma-nitrided plain carbon steels was investigated. Samples were submitted to microstructural, mechanical and chemical characterization by optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray difraction and microhardness measurements. It was found that all percentages of air addition promoted a reduction in surface hardness of all samples treated due to the formation of a thinner compound layer, with a more dramatic drop observed at 30% of air addition. The formation of Fe 3 O 4 iron oxide in the surface was detected in the X-ray difraction analysis of SAE 1005 samples processed under this condition. Also, SEM and EDS analysis showed that a thin layer of iron oxide was formed in the treatment with the same atmosphere. Keywords Plasma nitriding · Contamination · Oxygen · Iron nitrides · Carbon steel · Air leak 1 Introduction Plasma-assisted nitriding is a thermochemical surface treat- ment used to modify tribological systems [1, 2] and the mechanical properties [3] of metallic materials. It is known to improve the wear [4, 5], fatigue [6] and corrosion [4, 7] resistance of steels. This process can provide a controlled- nitrogen environment through the use of a (generally abnor- mal) glow discharge, allowing for the difusion of nitrogen into the treated parts. In solid state, nitrogen produces a series of phase changes in a reaction–difusion pathway over the afected depth [8, 9]. The fnal surface layer structure is not only dependent on the temperature but also on the pres- ence of alloying elements and treatment duration. Nitriding can be carried at temperatures as low as 180 ºC [10, 11] up to around 900 ºC [12]. The process can be controlled through the N 2 /H 2 ratio [13, 14] and discharge characteristics [15, 16]. Oxygen contamination of N 2 -H 2 gas mixture discharge is a potential problem depending on the material grade and desired properties [17]. It is relatively common in reac- tors that do not operate under ultra-high vacuum (UHV), a regime characterized by pressures lower than 100 nano- pascals. This is mostly due to leaks, low-purity gas sources and gas adsorption onto the system walls [18] and can result in a polluted flm or layer, depending on the afnity of the material components to oxygen and other elements, although residual amounts of impurities can be tolerated. The infu- ence of oxygen contamination on the formation of a nitrided layer over sintered Fe-1.5%wt Si alloy during ion nitriding was studied by Maliska et al. [19] and Moskalioviene and Galdikas [20]. These authors reported a practically undis- turbed compound layer with regard to the reference plasma nitriding in a N 2 -H 2 mixture, demonstrating through mass spectrometry studies that hydrogen neutralizes the harmful efects of oxygen. It is often accepted that the presence of hydrogen in plasma nitriding increases the mass transfer of nitrogen from the gas to solid state because, besides help- ing to inhibit oxygen, OH radicals are formed, reducing the potential for the formation of oxide barriers. It also increases Technical Editor: Izabel Fernanda Machado. * Francisco Cavilha Neto franciscocavilha@gmail.com 1 Mechanical Engineering Department, Federal University of Santa Catarina, Rua Doutor Abel Capela, 863, Florianópolis, SC CEP 88080-251, Brazil