K.C. Goretta et al /Materials Science and Engineering A 412(2005)146-152 Fig. 1. Schematic diagrams of(a)unidirectional FM and(b) four-ply laminate stacking. 22, 23]. A B2O3-based liquid is formed and rapid degradation and they develop a periodic distortion(pinching) along their of the FMensues For long-term use at elevated temperatures, in lengths(see the scanning electron microscopy(SEM) photomi many environments a highly effective environmental barrier will crographs in Fig. 2). The Si3N4/BN FMs that have been studied have to coat any FM that contains a BN-based cell boundary in detail have consisted of 80-85 vol. cells and 15-20 vol% Following the pioneering work by Halloran and co-workers cell boundary, and were generally >98%dense. An excellent [2-7, FMs are generally fabricated by a single processing review of their microstructural features can be found in Ref [7 sequence: blending of plastic masses of the cell and cell- boundary phases; coextruding a duplex filament consisting of a 3. Mechanical properties of Si3 N4/BN fibrous monoliths core and sheath, which form eventually the cell and cell bound- ary;optional bundling of duplex filaments and re-extruding to Mechanical properties that have been attained in Si3N4/BN form a filament with many smaller diameter cells surrounded FMs will be summarized. A series of papers by Halloran by a continuous cell boundary; assembly of the filaments into and co-workers examined, for various laminates, measurement a green body, removing the plastic constituents by heat treat- and modeling of thermal expansion, elastic properties, tensile ment; consolidating to a fibrous monolithic part by sintering or failure in various directions relative to lay-up, shear failure some type of hot pressing. Details on the processing sequence bending failure, and toughening mechanisms and the influ- and various plastic masses have been published [2-7, 12, 13]. ence of various materials properties on them. Ref [7] reviews Successful coextrusion requires matching of the rheological this body of work. Since that review, additional work has properties of the core and sheath plastic masses. Failure to match expanded greatly our knowledge of Si3 N4/BN FMs. In this them adequately can lead to pinching of cells or other structural section, we shall summarize recent studies of additional elas- irregularities, which lead to reduced properties tic properties and thermal expansion, in-plane fracture, and FMs can be unidirectional or cross-ply laminates(Fig. 1). Fil- residual and interfacial stresses. Because of likely exposure in ament placement can be by hand or, for example, by direct means service, we shall also summarize studies of high-temperature such as solid freeform fabrication[24, 26-28]. The resulting cells creep, resistance to solid-particle erosion, and dry and lubricated are typically 100-500 um wide. To date, high-quality Si3 N4/bn sliding wear FMs have been densified by hot pressing. The hot-pressing step The highly anisotropic thermal-expansion coefficient, a, of is the most costly single step in producing FMs Hot isostatic hexagonal BN(to 800C, axooc-l in the basal plane and pressing could possibly reduce costs significantly, and sintering aa 13-40 x 10-6oC-I perpendicular to the basal plane)con could reduce costs still further. The pressing steps induce sub- tributes to the weak bonding of cell boundary to the si3N4 stantial deformation of the cells In unidirectional laminates, the cells, and hence to the toughness of Si3 N4/BN FMs [7, 21].Ten- cells adopt a flattened-hexagonal cross-section; in cross-ply lam- sile strengths measured in flexure have exceeded 700 MPa, but inates, the cells are generally more rectangular in cross-section probably average closer to 450 MPa. Work-of-fracture value Fig. 2. SEM photomicrographs of cross-sections of (a)unidirectional and(b) cross-ply Si3Na/BN FMs; periodic distortions(arrows)along the lengths of cells evident in the cross-ply laminateK.C. Goretta et al. / Materials Science and Engineering A 412 (2005) 146–152 147 Fig. 1. Schematic diagrams of (a) unidirectional FM and (b) four-ply laminate stacking. [22,23].AB2O3-based liquid is formed and rapid degradation of the FM ensues. For long-term use at elevated temperatures, in many environments a highly effective environmental barrier will have to coat any FM that contains a BN-based cell boundary. Following the pioneering work by Halloran and co-workers [2–7], FMs are generally fabricated by a single processing sequence: blending of plastic masses of the cell and cell￾boundary phases; coextruding a duplex filament consisting of a core and sheath, which form eventually the cell and cell bound￾ary; optional bundling of duplex filaments and re-extruding to form a filament with many smaller diameter cells surrounded by a continuous cell boundary; assembly of the filaments into a green body; removing the plastic constituents by heat treat￾ment; consolidating to a fibrous monolithic part by sintering or some type of hot pressing. Details on the processing sequence and various plastic masses have been published [2–7,12,13]. Successful coextrusion requires matching of the rheological properties of the core and sheath plastic masses. Failure to match them adequately can lead to pinching of cells or other structural irregularities, which lead to reduced properties. FMs can be unidirectional or cross-ply laminates (Fig. 1). Fil￾ament placement can be by hand or, for example, by direct means such as solid freeform fabrication [24,26–28]. The resulting cells are typically 100–500
m wide. To date, high-quality Si3N4/BN FMs have been densified by hot pressing. The hot-pressing step is the most costly single step in producing FMs. Hot isostatic pressing could possibly reduce costs significantly, and sintering could reduce costs still further. The pressing steps induce sub￾stantial deformation of the cells. In unidirectional laminates, the cells adopt a flattened-hexagonal cross-section; in cross-ply lam￾inates, the cells are generally more rectangular in cross-section and they develop a periodic distortion (pinching) along their lengths (see the scanning electron microscopy (SEM) photomi￾crographs in Fig. 2). The Si3N4/BN FMs that have been studied in detail have consisted of 80–85 vol.% cells and 15–20 vol.% cell boundary, and were generally ≥98% dense. An excellent review of their microstructural features can be found in Ref. [7]. 3. Mechanical properties of Si3N4/BN fibrous monoliths Mechanical properties that have been attained in Si3N4/BN FMs will be summarized. A series of papers by Halloran and co-workers examined, for various laminates, measurement and modeling of thermal expansion, elastic properties, tensile failure in various directions relative to lay-up, shear failure, bending failure, and toughening mechanisms and the influ￾ence of various materials properties on them. Ref. [7] reviews this body of work. Since that review, additional work has expanded greatly our knowledge of Si3N4/BN FMs. In this section, we shall summarize recent studies of additional elas￾tic properties and thermal expansion, in-plane fracture, and residual and interfacial stresses. Because of likely exposure in service, we shall also summarize studies of high-temperature creep, resistance to solid-particle erosion, and dry and lubricated sliding wear. The highly anisotropic thermal-expansion coefficient, α, of hexagonal BN (to 800 ◦C, α ≈ 0 ◦C−1 in the basal plane and α ≈ 13–40 × 10−6 ◦C−1 perpendicular to the basal plane) con￾tributes to the weak bonding of cell boundary to the Si3N4 cells, and hence to the toughness of Si3N4/BN FMs [7,21]. Ten￾sile strengths measured in flexure have exceeded 700 MPa, but probably average closer to 450 MPa. Work-of-fracture values Fig. 2. SEM photomicrographs of cross-sections of (a) unidirectional and (b) cross-ply Si3N4/BN FMs; periodic distortions (arrows) along the lengths of cells are evident in the cross-ply laminate