Materials and Design 23 (2002) 377–383 0261-3069/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0261-3069 Ž 02 . 00002-X Effect of high-temperature surface hardening of metallic materials on their dimensional stability B.J. Fernandez*, J. de Damborenea, J. Ruiz ´ Centro Nacional de Investigaciones Metalurgicas (CENIMyCSIC) Avenida de Gregorio del Amo 8, Madrid, 28040, Spain ´ Received 2 October 2001; accepted 11 December 2001 Abstract Surface hardening thermo-chemical treatments provide metallic alloys with very hard corrosion and wear resistant coatings. As the layers obtained are so thin that dissuade from any machining operation after the treatment, it is useful to evaluate possible dimensional changes. This work studies the dimensional stability of alloy Ti6A14V after gas nitriding at 1100 8C and of superalloy MA956 after surface oxidation at the same temperature. For reference purposes, an analysis is also made of the deformations that appear on steel after conventional pack carburising treatment. In this study flat test specimens of 3 mm thickness were used, each of which had central drilled holes surrounded by six smaller outlying drilled holes. A calculation program allows the determination of changes in the drilled hole diameters and in the position of their centres as a consequence of the treatment.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Heat treatments; Surface treatments; Shrinkage 1. Introduction Both alloy Ti6A14V and ferric superalloy MA956 can be subjected to surface treatments that provide them with greater wear resistance and improve their chemical stability. The most outstanding advantages of alloy Ti6A14V, widely used for the manufacturing of osteoarticular prostheses, are its low density, good mechanical prop- erties, and its high corrosion resistance and biocompa- tibility w1x. However, it offers poor wear resistance due to the low hardness of titanium and its alloys. In order to improve the tribological properties of Ti6A14V, different surface modification techniques are being used to generate titanium nitride coatings which increase the hardness and abrasive wear resistance of the alloy, with the aim of reducing particle generation w2,3x. These techniques include ion implantation w4x, plasma spray w5x, laser nitriding w6,7x and gas nitriding in a nitrogen atmosphere w8x. The latter technique has *Corresponding author. Tel.: q34-1-553-8900, fax: q34-1-534- 7425. E-mail address: bfdez@cenim.csic.es (B.J. Fernandez). ´ the advantage of generating relatively thick titanium nitride layers, in the range of 20–50 mm, which offer greater guarantees against the risk of catastrophic break- down in service. Titanium nitride layers are achieved by long-duration treatments at temperatures of approximate- ly 1100 8C. Superalloy MA956 is being researched as a possible biomaterial, since surface oxidation treatment at this range of temperature leads to the formation of a dense, compact and hard surface alumina layer. The thickness of the protective coating obtained at 1100 8C is approx- imately 5 mm w9x. In both cases the surface characteristics of the mate- rials are improved thanks to the formation of thin but very hard coatings, whose low thickness makes it inad- visable to carry out any mechanical finishing operation after the treatment. Therefore, any deformations that might be caused by these treatments would have more serious consequences than in conventional hardening treatments after which slight final grinding is possible. The aim of this work is to study the dimensional alterations that appear as a consequence of high-temper- ature surface treatments of the materials under study.