Tribo-mechanical features of nitride coatings produced on AISI 420 and HP13Cr stainless steels by cathodic cage technique

Full text: Martensitic (M) and supermartensitic (SM) stainless steels can be employed by the petrochemistry industry in the marine environment, where they are subjected to high temperatures and pressures in the presence of corrosive agents [1]. As bulk properties, surface features of oil pipelines and connective devices also demand attention under these circumstances, notably when intense abrasion came over by sand particles mixed in the product. In this study, AISI 420 (M) and HP13Cr (SM) stainless steels were plasma nitrided using the cathodic cage design [2]. Prior to nitriding, samples were martensitized by a quenching heat treatment, with a subsequent stress relief anneal. The nitriding consisted in positioning samples in the plasma chamber inside an AISI 1008 steel cage, in which sputtering took place. The processing time was 6 h, performed in different temperatures (350-450 deg C). Mechanical properties and scratch resistance were characterized at nanoscale by instrumented indentation and nanoscratch, and correlated with morphological and microstructural changes (AFM, SEM, XRD and profilometry). Nitride layers, consisting predominantly of Fe_2-3N phase, were deposited on both M and SM steels in all temperatures. The morphology was distinct between M and SM, despite the low average roughness (less than 60 nm). The thickest coating (~800 nm) was produced in the SM-450 deg C, even though the best performance in hardness (15 % increase) and critical load in scratch tests (120 mN) were achieved for M-400 deg C. An intensive cracking and layer detachment were disclosed around indentation imprints and scratch grooves. On the other hand, mechanical features of M and SM were significantly improved beneath these layers by the formation of expanded phases, presenting increase in hardness of 2.2 and 2.5 times, respectively. Reference: [1]. A. Turnbull, B. Nimmo, Corros. Eng. Sci. Technol. 40 (2005) 103. [2]. C. Alves Jr. et al., Surf. Coat. Technol. 201 (2006) 2450. (author)