CP Dofan, J.A Hawk/wear 203-204(1997)267-277 由9肠A0 g and (d Werestrucneral cha Random, intergrator Randoe imer and inmagraul 10, The matrix m amorphous phase is obsened at all homo- and heteropl a thin amorphous phase also wets all S C, -Si N, and SiCw. interfaces(including all whisker interfaces)in bothFig. I. General microruuetunl features oE (a) the Si,N,-A+SiCL eompositc; (b) dae Si,N,-B +Si composk: (c) the 99.11% Al&; aad (d) tbz AI,O, + SiC, compxite. Table I Microstructural characteristics of the ceramics and ceramic composites Material primary mabix phare Primwy grain boundary phase Si,N.-A .‘%Si,N. Si,N,-A + SiC, /3-SijNI S&N.-B p&N, S&N,-B + Sic, PW% 99.8% AlzO, a-AlzO, 99.5% Al,03 a!-ALO, AI,O, + Sic, a-AlrO, Ciyst;llline a-Y&% Crysclllii a-Y&O, Amor@ous Y.Al silicate Amqhous Y. Al silicate Graphite. (3+&O, Amorphous Ca. Al sdicate Amorphous Mg. Al silicate introducedintothematerialsduringprocessing,andtheirtotal thermal history. ;n the S&N,-A ceramic, this ,&n boundary phase is primarily crystalline a-Y&O,; however, a narrow amorphous phase is also observed at all home- and hetero￾phase boundaries, and is expected 10 be a continuous phase in these materials. The Si,N,-A +SiC, composite [Fig. 1 (a) ] is microstructurally identical except for the dis￾tribution of randomly oriented, intergranular SIC whiskers, which are. large relative to the matrix grains. Although not apparent at the magnification of the micrograph in Fig. 1 (a), a thin amorphous phase also wets all Sic, -S&N4 and SE,- Y&O, boundaries, and likely plays a key role in determin￾ing the fracture characteristics of the whisker-matrix interface in this composite. The matrix microstructure of the second series of silicon nitride ceramics, Si,N,-B, consists of a distrktion of SiaNa grains which are larger than those in the A-series (average matrix grain size is 0.75 Frn as opposed to 0.4 pm in S&N,- A), bonded by an amorphous yttrium atuminosilicate phase that is also believed to be continuous in these materials. This amorphous phase is obscrvcd at all home- and hetcrophase interfaces (Including all whisker-matrix interfaces) in both