Effect of cooling rate on properties of plasma nitrided AISI 1010 steel C. Alves Jr. a, ⁎ , J.A. Lima a , V. Hajek a , J.B. Marimon da Cunha b , C.A. dos Santos b,c a LabPlasma, Departamento de Engenharia Mecânica, Universidade Federal do Rio Grande do Norte (UFRN), Campus Universitário-Lagoa Nova, 59072-970 Natal, RN, Brazil b Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), C.P. 15051, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil c Núcleo de Educação a Distância, Universidade Estadual do Rio Grande do Sul (UERGS), Rua Gal. João Manoel, 50, 90010-030 Porto Alegre, RS, Brazil Received 28 December 2005; accepted in revised form 15 February 2007 Available online 1 March 2007 Abstract In this work, AISI 1010 steel samples were plasma nitrided into 20% N 2 plus 80% H 2 atmosphere at a pressure of 500 Pa (partial pressure of 100 Pa and 400 Pa for N 2 and H 2 , respectively), temperatures of 500 and 580 °C, during 2 h. Three different procedures for cooling were accomplished after nitriding. In the first procedure the cooling occurred naturally, that is, the sample was kept on substrate holder. In the second one the sample was pulled off and cooling in a cold surface. Finally, in the third cooling process the sample was pulled off the substrate holder down into special reservoir filled with oil held at ambient temperature. The properties of the AISI 1010 steel samples were characterized by optical and electron microscopy, X-ray diffraction, Mössbauer spectroscopy and microhardness tests. Thermal gradient inside the sample kept on substrate holder during cooling process was measured by three inserted thermocouples at different depths. When samples were cooled rapidly the transformation of ϵ-Fe 2 - 3 N to γ′-Fe 4 N was inhibited. Such effect is indicated by the high concentration of ϵ-Fe 2 - 3 N phase observed in the compound zone. To get solid state solution of nitrogen in the diffusion zone, instead of precipitates of nitride phases, the cooling rate should be higher than a critical value of about 0.95 °C/s. When this value is reached at any depth of the diffusion zone, two distinct diffusion zones will appear. Temperature gradients were measured inside the samples as a consequence of the plasma treatment. It's suggested the need for standardization of the term “treatment temperature” for plasma treatment because different nitrided layer properties could be reported for the same “treatment temperature”. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma nitriding; Plasma nitrided AISI 1010; Cooling rate effect on plasma nitriding; Thermal gradient on plasma nitriding 1. Introduction Advanced surface modification processes using plasma have undergone substantial industrial development over the past sev- eral decades [1,2]. One of the fields where these processes are widely used is machining. To increase productivity and efficiency, high speed and high load operating machines were developed. Components of such machines are exposed to severe conditions. They have to be tougher, more resistant to wear at high operating temperatures (when working without cooling emulsion) and be efficient during a long lifetime. Modification of the surface of such mechanical components leading to increased hardness and wear resistance comes to address those needs. One of the most studied and industrially adopted processes is plasma nitriding. This process is used to introduce nitrogen to the surface of mechanical components. While the component is treated by plasma at elevated temperature nitrogen diffuses inside and forms various surface and subsurface nitrides [3–6]. The plasma nitriding has been intensively studied over the several decades and the basic principles of the process are already well known, so that the final results can be reproduced [7–10]. One of the less studied process parameters, which can signifi- cantly influence the resulting properties of plasma nitrided sam- ples, is their cooling rate after the treatment. Depending on the cooling rate different phases precipitate which means that different mechanical properties can be reached [3]. The cooling rate can be controlled in various ways, for example by decreasing the discharge voltage (till plasma turns off), the flux of the cooling gas, by introducing cooling liquid inside the substrate holder. One Surface & Coatings Technology 201 (2007) 7566 – 7573 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +55 84 215 3800x210; fax: +55 84 215 3791. E-mail address: alvesjr@dfte.ufrn.br (C. Alves). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.02.036