Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Determination of residual stress on TIG-treated surface via nanoindentation technique in Co-Cr-Mo-C alloy M. Sahami-Nejad a , H.R. Lashgari b,∗ , Sh Zangeneh a,∗∗ , C. Kong c a Department of Materials and Textile Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran b School of Materials Science and Engineering, University of New South Wales, Sydney, NSW, 2052, Australia c Mark Wainwright Analytical Centre (MWAC), University of New South Wales, Sydney, NSW, 2052, Australia ARTICLE INFO Keywords: Co-Cr-Mo alloy TIG welding process Residual stress Martensite transformation Nano-indentation ABSTRACT The present study was undertaken to determine the amount of residual stress on the surface subjected to Tungsten Inert Gas (TIG) welding process in different protective gas environments (Ar-xN 2 , x = 20, 30 and 40 volume percent). It was shown that the increase of volume fraction of N 2 gas in the welding environment led to more residual stress mainly due to the higher heat input. Nanoindentation technique was employed to obtain the variations of residual stress when Ar-40N 2 protective gas was in use. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) method were used for microstructural study and phase identification. It was shown that applying higher heat input followed by quenching induced fully martensitic structure with very little amount of retained austenite. In addition, the magnitude of the residual stress in the TIG-treated area was measured to be ≈ 381 MPa using nanoindentation technique which was higher than the base metal. The type of residual stress was compressive which could be helpful for enhancing the surface properties and wear resistance of the alloy as a hip implant. 1. Introduction Cobalt-Chromium-Molybdenum alloys also known as ASTM F75 have increasingly been used as surgical implants for replacing hip and knee joints over recent years [1,2]. Co-Cr-Mo alloys possess acceptable mechanical properties, good wear resistance, excellent biological properties and also good corrosion resistance [3,4]. When a Co-Cr-Mo implant is placed in the human body, it releases ions (for example Cr 3+ , Cr 6+ and Co 2+ ) through pitting corrosion, crevice corrosion, and uni- form corrosion. Mechanical corrosion can also damage the protective oxide layer (Cr 2 O 3 , CoO, CrO 3 ) and releases more debris and particle into living tissues causing toxicity through the generation of reactive oxygen species (ROS) by Fenton reaction (FR) [5]. Therefore, it is quite crucial to improve the surface properties of Co-Cr-Mo alloys in order to increase the abrasion wear resistance and improve the biocompatibility and corrosion resistance [6]. The wear resistance of the Co-Cr-Mo alloys is attributed to the presence of high-volume fraction of M 23 C 6 and M 7 C 3 carbides (M = Cr, Mo) and ε-martensite formed during face-centered- cubic (fcc) to hexagonal close-packed (hcp) phase transformation [7,8]. The transformation of the fcc → hcp phase in pure cobalt is quite sluggish and takes place at T c = 417 °C. However, by the addition of hcp phase stabilizer such as Cr and Mo to Co-27Cr-5Mo-0.05 C alloy, the transformation temperature is increased to ≈ T c = 970°C. Solid- solution alloying (C, Ni, Nb, etc.) and precipitation of carbides are two main strengthening mechanisms in Co-Cr-Mo alloys. Solid-solution elements (such as Cr, Mo) tend to stabilize hcp phase and decrease the stacking fault energy whereby making the glide and climb of disloca- tions more difficult. Precipitation of carbides can also effectively pin the dislocations leading to higher hardness and mechanical strength. In Co- Cr-Mo alloys, the fcc (γ)→hcp (ε) transformation (so called martensitic transformation) is induced in three ways: (I) Strain-Induced Martensite Transformation (SIMT) resulting from plastic straining, (II) Isothermal transformation (holding at a constant temperature, T < T c , for a cer- tain time to develop hcp 1 and hcp 2 phases), (III) Athermal transfor- mation (quenching from high temperature T > T c ). The athermal ε- martensite is produced by regular overlapping of stacking faults and motion of Shockley partial dislocations [9]. Surface modification processes such as high energy laser beam, ion implantation and tungsten inert gas welding (TIG) can be employed to change the surface properties through composition change or phase transformation in order to enhance the surface hardness and wear re- sistance of the Co-27Cr-5Mo-0.05C alloy. Among these processes, TIG https://doi.org/10.1016/j.surfcoat.2019.125020 Received 26 June 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 ∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: h.lashgari@unsw.edu.au (H.R. Lashgari), Shzangeneh@razi.ac.ir (S. Zangeneh). Surface & Coatings Technology 380 (2019) 125020 Available online 20 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved. T