D B. Marshall, J B. Davis /Current Opinion in Solid State and Materials Science 5(2001)28. all the proposed compounds, has attracted the most atten- 39% Nextel 720 fiber Further studies of debonding and sliding at the LaPo Al, O3 interface have confirmed the weak bondi 0, * 31], but have also indicated a possibly important role of plastic deformation in the monazite itself to accommo- date interfacial roughness [31. Both La-monazite and CaWO,(scheelite) have been shown to deform relatively 48% Nextel 720 fiber easily at room temperature by twinning and dislocation motion[ 32]. Connections may exist between unique resistance of monazite to amorphizaton from o00 hour exposure radiation damage, which makes it the prime candidate for long-term storage of actinide wastes 33] 1100 The effectiveness of monazite interface coatings in Heat Treatment Temperature (C) providing a phase-compatible debond layer has now been us matrix composites after high temperature confirmed from tensile strength testing in a variety aging in air(from Ref.[#16 specimen configurations(using uncoated control specimens or comparisons): coatings and coating/matrix combina- tions on individual sapphire fibers [31, 341: individual tows of nextel 610 alumina fibers coated with monazite Detailed measurements are now available for the room and infiltrated with alumina matrix 35]; and unidirectional mperature in-plane tensile properties of a 2-D composi composites of Nextel fibers(610, 650 and 720)with consisting of Nextel 610 fibers(stacked layers of woven monazite coatings and alumina matrix [**36]. In the last fabric)in a porous mullite-alumina matrix(both 0/90 and case the matrix, although porous, was sufficiently strong 45)[*16, 25]. Measurements of notch sensitivity using that very little fragmentation of the matrix took place center-hole tension tests indicate some loss of net -sectio during fracture. Clear evidence was found on the fracture strength in the presence of notches, but much less than for surfaces for the role of the weak interface rather than a a fully notch-sensitive material. Effects of hole size on weak matrix in allowing extensive debonding and pullout strength were rationalized by comparing with stress dis- of fibers. After heat treatment at 1100 and 1200.C, the ributions predicted using non-linear continuum and shear strengths of composites with monazite coatings were all band models: a size-scale dependence in the failur higher than those of the corresponding control specimens, condition is implied. The trends are similar to those the largest differences being seen after 1200.C treatment observed in conventional weak-interface composites (e.g, strength of 200 MPa for coated Nextel 610 composite Systematic studies have now been done to vary the after 5 h at 1200'C compared with 45 MPa for uncrate strength and density of the matrix of the mullite-based control composite composite and measure the effect on the mechanical Although an effective method has been developed for properties [17, *26]. These involved further processing producing thin coatings of monazite on fiber tows(see with multiple cycles of reinfiltration and heat treatment Section 6), a method for coating fibers in a woven fabric using an alumina precursor(Al,(OH)S CI). Bonding of the has not been demonstrated. An approach that circumvents particle network by evaporation-condensation in reactive the need for a separate coating step has been developed on atmosphere has also been used to strengthen the matrix the basis of observations that monazite deposits pref- without decreasing the porosity [23]. As the matrix was erentially between adjacent fibers in tows or fabrics after strengthened and densified there was a trend to increase the infiltration with solution precursors and heat treatment tensile strength in the +45 direction, decrease the strength [ 37]. Infiltration of a slurry consisting of alumina powder in the 0/90 direction, and decrease the degree of uncorre- in a solution precursor for monazite results in a two-phase lated fiber fracture and pullout lengths on the fracture (porous) matrix of alumina and monazite, with a continu- surface ous layer of monazite around all of the fibers. Composites formed by infiltrating fabrics of Nextel 610 and 720 in this manner, followed by stacking, pressing and sintering, have 4. Weak interface developments shown damage-tolerant behavior and absence of notch sensitivity when tested at room temperature and at 1000C Since the discovery that rare-earth phosphates (most (Fig. 2)[37-39]. Although this behavior may be partly notably La-monazite)bond sufficiently weakly with other due to the porous nature of the mati significant oxides to allow debonding of fibers [27], other mixed difference in notch sensitivity is seen between this and the oxide compounds have been proposed and tested for the porous matrix composites described in the previous sec same purpose(tungstates, vanad dates, niobates [9, 28, 29]). tion, thus implying an additional effect of the Nevertheless, La-monazite, which is the most refractory of interfaceD.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 285 all the proposed compounds, has attracted the most atten￾tion. Further studies of debonding and sliding at the LaPO –4 Al O interface have confirmed the weak bonding 2 3 [30,*31], but have also indicated a possibly important role of plastic deformation in the monazite itself to accommo￾date interfacial roughness [*31]. Both La-monazite and CaWO (scheelite) have been shown to deform relatively 4 easily at room temperature by twinning and dislocation motion [*32]. Connections may exist between this and the unique resistance of monazite to amorphizaton from radiation damage, which makes it the prime candidate for long-term storage of actinide wastes [33]. The effectiveness of monazite interface coatings in providing a phase-compatible debond layer has now been confirmed from tensile strength testing in a variety of Fig. 1. Strengths of porous matrix composites after high temperature aging in air (from Ref. [**16]). specimen configurations (using uncoated control specimens for comparisons): coatings and coating/matrix combina￾tions on individual sapphire fibers [*31,34]; individual tows of Nextel 610 alumina fibers coated with monazite Detailed measurements are now available for the room and infiltrated with alumina matrix [35]; and unidirectional temperature in-plane tensile properties of a 2-D composite composites of NextelE fibers (610, 650 and 720) with consisting of Nextel 610 fibers (stacked layers of woven monazite coatings and alumina matrix [**36]. In the last fabric) in a porous mullite-alumina matrix (both 0/90 and case the matrix, although porous, was sufficiently strong 458) [**16,*25]. Measurements of notch sensitivity using that very little fragmentation of the matrix took place center-hole tension tests indicate some loss of net-section during fracture. Clear evidence was found on the fracture strength in the presence of notches, but much less than for surfaces for the role of the weak interface rather than a a fully notch-sensitive material. Effects of hole size on weak matrix in allowing extensive debonding and pullout strength were rationalized by comparing with stress dis- of fibers. After heat treatment at 1100 and 12008C, the tributions predicted using non-linear continuum and shear strengths of composites with monazite coatings were all band models: a size-scale dependence in the failure higher than those of the corresponding control specimens, condition is implied. The trends are similar to those the largest differences being seen after 12008C treatment observed in conventional weak-interface composites. (e.g., strength of 200 MPa for coated Nextel 610 composite Systematic studies have now been done to vary the after 5 h at 12008C compared with 45 MPa for uncoated strength and density of the matrix of the mullite-based control composite). composite and measure the effect on the mechanical Although an effective method has been developed for properties [*17,*26]. These involved further processing producing thin coatings of monazite on fiber tows (see with multiple cycles of reinfiltration and heat treatment Section 6), a method for coating fibers in a woven fabric using an alumina precursor (Al (OH) Cl). Bonding of the has not been demonstrated. An approach that circumvents 2 5 particle network by evaporation–condensation in reactive the need for a separate coating step has been developed on atmosphere has also been used to strengthen the matrix the basis of observations that monazite deposits pref￾without decreasing the porosity [23]. As the matrix was erentially between adjacent fibers in tows or fabrics after strengthened and densified there was a trend to increase the infiltration with solution precursors and heat treatment tensile strength in the 6458 direction, decrease the strength [*37]. Infiltration of a slurry consisting of alumina powder in the 0/908 direction, and decrease the degree of uncorre- in a solution precursor for monazite results in a two-phase lated fiber fracture and pullout lengths on the fracture (porous) matrix of alumina and monazite, with a continu￾surface. ous layer of monazite around all of the fibers. Composites formed by infiltrating fabrics of Nextel 610 and 720 in this manner, followed by stacking, pressing and sintering, have 4. Weak interface developments shown damage-tolerant behavior and absence of notch sensitivity when tested at room temperature and at 10008C Since the discovery that rare-earth phosphates (most (Fig. 2) [*37–39]. Although this behavior may be partly notably La-monazite) bond sufficiently weakly with other due to the porous nature of the matrix, a significant oxides to allow debonding of fibers [27], other mixed difference in notch sensitivity is seen between this and the oxide compounds have been proposed and tested for the porous matrix composites described in the previous sec￾same purpose (tungstates, vanadates, niobates [9,28,29]). tion, thus implying an additional effect of the weak Nevertheless, La-monazite, which is the most refractory of interface