A New Method for Fabrication of Functionally Gradient Materials

Abstract

Functionally gradient materials (FGM) are a new kind of composite material, which were introduced as the thermal barrier materials of spacecraft, and have been found to be of great use as medical materials, electronic materials, nuclear energy materials, etc. Design and techniques of fabrication are the foundation of FGM and its applications. Up to now, a great many means have been developed to obtain functionally gradient materials [1, 2]: chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spraying (PS), self-propagating high temperature synthesis (SHS), and powder metallurgy (PM). In these processes, the powder metallurgy has been researched by many investigators and is considered to be one of the promising methods for fabricating large size FGM samples. In current powder metallurgy processes, however, the gradient of composition and=or structure is formed by the stage ®lling. According to the composition gradient designed, mix the two powders used to prepare the sample of functionally gradient material into several compositions, and tier the powders in a compacting tool set, and then compact and sinter it to obtain the gradient material. It is clear that the distribution of composition in the samples prepared by the stage ®lling process is not continuous, and there are some interfaces of step change in composition in it. Therefore, the powder metallurgy method by stage ®lling is not only dif®cult for operation but also inadequate for thermal stress relief. This paper reports a new process, co-sedimentating method for fabrication of functionally gradient materials, by which the continuous distribution of composition can be formed, and the gradient layer of free size and free thickness can be prepared. It is well known that the sedimentating behavior of powder in a liquid can be discussed by the Stokes equation. The sedimentating velocity of a powder particle in a liquid medium is mainly dependent on the size and density of the particle. When a kind of powder sedimentates, the particles of larger size have greater velocities. When two (or more than two) kinds of powders sedimentate together, the particles of larger size or=and larger density sedimentate more quickly. Accordingly, a deposit layer with a continuous change in composition can be obtained by the co-sedimentation of two kinds of powders through conditioning the parameters of sedimentation (the density and viscidity of the liquid medium used for sedimentating, the sedimentating height, etc.), the size, and the size distribution of the powders. In this paper, a functionally gradient material of metal=ceramics is fabricated by the co-sedimentation of metal and ceramics powders in ethanol. The ceramics powder is the 3 mol % yttria partially stabilized ZrO2(3Y-PSZ) powder of particle size 0.3 im, which has been granulated into the particle size 30 im. The stainless steel powder is Japanese SUS 316 powder, whose particle size is 8 im. The concentrations of the 3Y-PSZ and SUS 316 in the ethanol are 45 3 10y3 g=ml and 90 3 10y3 g=ml, respectively. The sedimentation was carried out in a glass pipe of internal diameter 40 mm, and length 200 mm. After drying and compacting (150 MPa), the deposit layers were sintered at 1300 8C for 1 h in argon atmosphere to obtain the samples of SUS 316=3Y-PSZ gradient material. The optical microscope and the graphical analysis instrument were used to observe the microstructure and measure the distribution of composition. The samples of SUS 316=3Y-PSZ functionally gradient material prepared by the process described in this paper are disks of diameter 33 mm and thickness 2±5 mm. Fig. 1 shows the microstructure of the cross-section in a sample of thickness 3.5 mm (black PSZ, white stainless steel). The distribution curve of composition is shown in Fig. 2, which is measured by a graphical analysis instrument. It can be seen from Fig. 1 that a compactive structure and good interface binding between stainless steel and ceramics have been obtained after sintering. Both the microstructure and the distribution curve of composition show a continuous distribution of composition in the sample. It is important and necessary for thermal stress relief and property design that the composition distribution (gradient) in functionally gradient materials can be designed and conditioned. In the cosedimentating method, the composition distribution (gradient) can be controlled by the properties (particle sizes and distributions) of the two powders and the conditions for sedimentating (the density and viscidity of the liquid used as the sedimentating medium, the sedimentating height, and the contents of two powders, etc.). If the sedimentating behaviors of the two powders do not interfere together and a