JOURNAL OF MATERIALS SCIENCE 31 (1997) 3733 — 3738 Dislocations studies in -silicon nitride X. MILHET, H. GAREM, J. L. DEMENET, J. RABIER Laboratoire de Me ´ tallurgie Physique, URA 131 CNRS, Universite ´ de Poitiers, UFR Sciences-SP2MI, Bd. 3 Te ´ le ´ port 2 BP 179, 86960 Futuroscope cedex, France T. ROUXEL Laboratoire des Mate ´ riaux Ce ´ ramiques et Traitements de Surface, ENSCI, URA 320 CNRS, 47 Av. A. Thomas, 87065 Limoges, France Some preliminary results of dislocation analysis and associated glide systems in as-grown aswell asin superplastically deformed silicon nitridearepresented.Transmission electron microscopy observationsusing the weak-beam technique, are reported.[0001] 101 0 and 1/312 10101 1 glide systems have been characterized. In the basal plane, a superposition of two hexagonal networks built with screw dipoles has been observed. Both a sequence extended node—constricted node and partial dislocations have been identified in these networks which clearly evinces dislocation dissociation in -silicon nitride following the reaction 1/3 21 1 0 P 1/3 101 0#1/3 11 00. 1. Introduction Silicon nitride, Si N , is a covalently bonded thermo- mechanical ceramic. The high-temperature mechan- ical properties of this compound are of great interest for industrial applications. However, its brittleness, as well as the lack of cheap and reliable shaping tech- niques, still prevent use of this material on a large scale. This has motivated research for new processing and shaping routes and recently, a superplastic Si N was successfully synthesized [1—4]. Such deformation behaviour results mainly from grain-boundary sliding; however, it seems important, for a complete under- standing of silicon nitride deformation mechanisms, to perform detailed analysis of dislocation configurations as well as slip systems characterization. This paper presents some preliminary results obtained by trans- mission electron microscopy (TEM) of dislocation configurations and slip systems in silicon nitride. 2. Structure of silicon nitride Silicon nitride exists with two different crystallo- graphic modifications: the phase, the metastable one, crystallizes above 1300 °C, and the phase, the stable one, above 1700 °C. The reaction P has been found to occur, but not the reverse one [5]. - and -Si N have a hexagonal structure with, respectively, P and P symmetry group [5 —7]. The structure of the -phase can be described by the piling up of four planes A, B, C, D, spaced out by c /4, each of them containing a planar arrangement of three silicon and four nitrogen atoms (Fig. 1). The lattice parameters are a "0.7753 nm, c "0.5618 nm. The structure of the -phase is a piling up of two planes A, B, spaced out by c /2, with an atomic arrangement similar to the -phase (Fig. 2). The lattice parameters are a "0.7606 nm, c "0.2909 nm. Owing to the similarity between the two structures, c could be expected to be twice as large as c . However, the volume of the interstitial sites is smaller in the struc- ture; this results from atomic rearrangements and ex- plains the variance of ratio c /c from 1/2 [6]. 3. Previous dislocation observations Some papers have been already published on disloca- tion observations in - and -Si N . Most of the studies have been performed on non-deformed silicon nitride. 3.1. Dislocations in -Si 3 N 4 In the structure, c [0 0 0 1] is the most probable Burgers vector: dislocations with such a Burgers vec- tor have been observed by several authors [8, 9]. Moreover, evidence of dislocation dissociation follow- ing the reaction [0 0 0 1]P1/2[0 0 0 1]#1/2 [0 0 0 1] has been provided [9]. In addition, dislocations with a/3 112 0 Burgers vector have been observed sev- eral times by TEM in as-sintered materials [8—10] and also after deformation [11]. Associated with this Bur- gers vector, 11 01 was found to be the primary glide plane [11]. 1/3112 3 has also been reported as a possible Burgers vector [8, 9, 12], with the 1 1 21 habit plane [12]. Recently, Zhou and Mitchell [9] have characterized 101 0 planes to be the glide planes of these three types of dislocations in chemical vapour deposited (CVD) material. Dislocations with 1/9 112 3 Burgers vector have been also observed by Suematsu et al. [13]. 3.2. Dislocations in -Si 3 N 4 A theoretical analysis on Burgers vectors and different glide planes in -silicon nitide has been performed by 0022—2461 1997 Chapman & Hall 3733