Journal of the European Ceramic Society 27 (2007) 3359–3364 Hardness degradation in liquid-phase-sintered SiC with prolonged sintering O. Borrero-L ´ opez a , A. Pajares b , A.L. Ortiz a , F. Guiberteau a,∗ a Departamento de Electr ´ onica e Ingenier´ıa Electromec´ anica, Escuela de Ingenier´ıas Industriales, Universidad de Extremadura, 06071 Badajoz, Spain b Departamento de F´ısica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain Available online 1 May 2007 Abstract The effect of the sintering time on the Vickers hardness of liquid-phase-sintered SiC with 10wt% YAG additives was studied for materials fabricated in Ar and N 2 atmospheres. The hardness of the Ar-sintered materials was found to decrease markedly with increasing sintering time, whereas, in contrast, the decrease was only marginal for the N 2 -sintered materials. Berkovich nanoindentation tests showed that the softening could not be attributed in either case to degradation of the SiC grains, thereby pointing to the intergranular YAG phase as the responsible. That the cause was degradation of the YAG phase was confirmed by X-ray energy-dispersive spectrometry and Hertzian indentation tests. The far more pronounced softening observed for the Ar-sintered materials reflects the more severe degradation that the YAG phase undergoes during sintering in this atmosphere. © 2007 Elsevier Ltd. All rights reserved. Keywords: Liquid-phase sintering; SiC; Hardness; Mechanical properties 1. Introduction Liquid-phase sintering is the most effective way of densify- ing silicon carbide (SiC) at a moderate processing temperature without the simultaneous application of external pressure, 1–8 and therefore at low cost. In this context, the most widely stud- ied additive system for SiC is Y 2 O 3 –Al 2 O 3 , especially when these components are chosen in the form Y 3 Al 5 O 12 . Typically, prolonged sintering in an Ar atmosphere is used to process such a liquid-phase-sintered (LPS) SiC to improve its long-crack toughness, 8–13 since the sintering duration promotes grain coars- ening, and the crack-bridging toughening mechanism in LPS SiC is more efficient with increasing grain size. 14 However, it has been shown that LPS SiC resulting from prolonged sintering, the so-called “in situ toughened SiC”, are much softer than those sintered for shorter times. 8,10,13,15 Whereas reduced hardness facilitates machining of LPS SiC parts, it can limit their utility in various structural applications. Although hardness degrada- tion with prolonged sintering is a well-documented effect, to the best of the authors’ knowledge no studies have explored the underlying causes of the phenomenon. ∗ Corresponding author. Tel.: +34 924 28 9530; fax: +34 924 289601. E-mail address: guiberto@unex.es (F. Guiberteau). Thus, the present study was designed with two objectives in mind. The first was to evaluate the influence of the sinter- ing atmosphere (Ar or N 2 -gas) on the hardness degradation in LPS SiC with prolonged sintering. For this propose, we used exclusively Vickers indentation tests. Our results showed that the Vickers hardness is much more stable when the sintering is performed in an N 2 atmosphere. The second objective was to investigate the origin of the softening in the two atmospheres. To this end, we combined mechanical tests of Berkovich nanoinden- tation, which allows the evolution of the SiC grain hardness to be followed, and Hertzian indentation, which allows the progres- sion of the interface weakness to be monitored through the yield stress values. The results point to intergranular phase degrada- tion as being the key to the softening in both cases, as was also confirmed by direct X-ray energy-dispersive spectrometry. 2. Experimental procedure 2.1. Processing -SiC powder (UF-15, H.C. Starck, Goslar, Germany) with 4.29 wt% Al 2 O 3 (AKP-30, Sumitomo Chemical Company, New York, NY, USA) and 5.71wt% Y 2 O 3 (Fine Grade, H.C. Starck, Goslar, Germany) as additives was ball-milled for 24 h in ethanol 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.02.189