Continuous in situ functionally graded silicon nitride materials Manuel Belmonte * , Jesus Gonzalez-Julian, Pilar Miranzo, Maria Isabel Osendi Institute of Ceramics and Glass, CSIC, Campus de Cantoblanco, Kelsen 5, Madrid 28049, Spain Received 29 October 2008; received in revised form 8 January 2009; accepted 12 January 2009 Available online 25 March 2009 Abstract Functionally graded materials can enhance the performance of components under demanding operating conditions, although the development of residual stresses across the gradient and scaling up the manufacturing process to mass production present some limita- tions. To overcome these problems, we present a one-step approach for processing continuous functionally graded silicon nitride (Si 3 N 4 ) materials from a sole homogeneous powder composition, using spark plasma sintering as a densification technique. Through the control of the temperature profile within the compact, specimens with a continuous variation of a- and b-phase content, as well as grain size, were achieved. A continuous gradation of mechanical properties, in particular, hardness and toughness, were measured in these speci- mens. This approach offers unprecedented opportunities to design custom-made Si 3 N 4 components by taking advantage of the partic- ularities of field-assisted sintering methods. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Functionally graded materials; Ceramics; Spark plasma sintering; Hardness; Toughness 1. Introduction The concept of functionally graded materials (FGMs), first proposed in the mid-1980s in Japan [1], is based on the development of compositional and/or microstructural gradients within the material leading towards graded prop- erties in one, two or three dimensions [2,3]. Compared to monolithic and composite materials, these structures offer the possibility of improving performance of components under demanding technological applications, e.g. in fuse- lage surfaces of spacecraft, thermal and environmental bar- rier coatings for turbines and engines, super-hard cutting tools or artificial bones [2,4,5]. There are numerous meth- ods to process FGMs [3,4,6]; the main challenge is to reduce residual stress concentrations through the gradient in order to enhance the reliability of these materials. It is well known that silicon nitride (Si 3 N 4 ) materials possess good thermomechanical and tribological proper- ties, and hence these ceramics are suitable for use in wear-resistant technological applications, such as valves in diesel engines, bearings, sealing rings, and metal cutting and shaping tools [7]. One important advantage of Si 3 N 4 materials is the ability to tailor their microstructures and, consequently, properties, by controlling the a ? b phase transformation that occurs during sintering at high temper- ature of the mostly a-phase original Si 3 N 4 powders [8]. In this sense, the growth of large, elongated b-Si 3 N 4 grains of high aspect ratio produces an in situ toughening mecha- nism [9,10], although the material becomes less hard due to the decreasing a-phase content. The spatial control of this phase transformation leading to phase gradients in the specimens would open great technological possibilities, such as high toughness and hardness values in a single specimen. There is little information available regarding graded Si 3 N 4 materials [11,12], or their derivative SiAlON ceramics [13,14]. Some of these FGMs consist of layered systems which were step-wise fabricated by stacking either two a-Si 3 N 4 powders with different mean particle sizes [11] or tape casting a number of layers with gradual variations of the a/b SiAlON ratio [14]. The Si 3 N 4 bilayer system developed by Lee et al. [11] improved the contact damage resistance of Si 3 N 4 ceramics thanks to a hard coating of fine a-phase-rich grains on a soft substrate of coarse b-rich 1359-6454/$36.00 Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2009.01.043 * Corresponding author. Tel.: +34 917355863; fax: +34 917355843. E-mail address: mbelmonte@icv.csic.es (M. Belmonte). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 57 (2009) 2607–2612