E≈S ournal of the European Ceramic Society 20(2000)589-597 Effect of coating deposition temperature on monazite coated fiber R.S. Haya, E Boakye, M.D. Petry lir Force Research Laboratory, Materials Directorate, Wright Patterson Air Force Base, OH 45433, USA 4401 Davi lia Road, Dayton, OH 45432 USA Accepted 13 August 1999 Abstract Monazite (apoA) was continuously coated on 3M Nextel 720 fiber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement Coating deposition temperatures were varied from 900 to 1300.C. Fibers coated at 900C were heat-treated up to 100 h at 1200C. Coated fibers were characterized by analytical TEM, and tensile strengths were measured by structure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and ther subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200 C there was significant roughening of the coating-fiber interface, with facetting of alumina grains in the fiber and some lanthanum segregation to these facetted boundaries Spheroidization of thin coatings was also observed. Tensile strength of coated fiber decreased with increasing deposition tempera- ture and with time at temperature after deposition. Possible reasons for the strength decrease are discussed lished by elsevier Science ltd Keywords: Aluminosilicate fibers; Coating: Grain growth; Interfaces; LaPO4: Mechanical properties 1. ntroduction itu between the fiber and matrix during processing. Sol and solution precursors have been used to coat fiber Improvement of intermediate and high temperature tows with monazite, as has continuous chemical vapor properties of ceramic matrix composites (CMCs) deposition(CVD). 10,15,16 Limited experiments demon- requires an oxidation resistant alternative to carbon or strate crack deflection and fiber pullout in tensile tests of boron nitride fiber-matrix interfaces. -6 One possible composites with CVD monazite coated fiber tows. 10 For alternative is monazite(LaPO4). Monazite is refractory liquid-phase precursors, crack deflection and debonding (mp 2072C)and thermochemically stable with many have only been demonstrated on composites with low other common refractory oxides such as alumina and volume fractions of dip coated, large diameter(100 um) mullite. Crack deflection and fiber pushout experi- single crystal alumina(Saphikon )monofilaments. The ments,as well as some limited mechanical tests on thickness uniformity of sol and solution derived coat composites suggest that monazite bonds weakly with ings on filaments in fiber tows is often poor, although other oxides 8-10 Some similar results were found for stoichiometry is easily controlled. 6 In contrast, limited xenotime(YPO4) and scheelite(CaWO4). 2 Monazite data suggest that CVD coatings have good thickness containing ceramics were demonstrated to be machine- uniformity, but are sometimes off stoichiometry. 15, 16 able, 3 which implies significant plasticity from some Filament tensile strength is often degraded during combination of cleavage microcracking, twinning, or fiber coating, 6. 17-19 or during exposure to various envir dislocation glide. 4 onments.20), 2I Strength increases after coating or envir Use of monazite as a fiber-matrix interface in MCs onmental exposure are also known 22.23 Large requires that it either be coated on fibers or formed in differences in tensile strength have been observed between fibers coated with different monazite pre- 4 Corresponding author Tel: 1-937 255 9825: fax: 1 937 6564296. strength dependence on coating temperature were also E- mail address: hayrs(@ ml wl wpafb af. mil(RS. Hay) found. Excessive degradation in fiber tensile strength 0955-2219/00/S- see front matter. Published by Elsevier Science Ltd PII:S0955-2219(99)00257-5E€ect of coating deposition temperature on monazite coated ®ber R.S. Haya,*, E. Boakyeb, M.D. Petryb a Air Force Research Laboratory, Materials Directorate, Wright Patterson Air Force Base, OH 45433, USA bUES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432, USA Accepted 13 August 1999 Abstract Monazite (LaPO4) was continuously coated on 3M NextelTM 720 ®ber tows with an ethanolic precursor using hexadecane for immiscible liquid displacement. Coating deposition temperatures were varied from 900 to 1300C. Fibers coated at 900C were heat-treated up to 100 h at 1200C. Coated ®bers were characterized by analytical TEM, and tensile strengths were measured by single ®lament tensile tests. The monazite precursor was characterized by X-ray, DTA/TGA, and mass spectrometry. Micro￾structure evolution was complex and may have involved recrystallization of large defective grains into smaller grains and then subsequent growth of these grains, along with coarsening of porosity. After 100 h at 1200C there was signi®cant roughening of the coating±®ber interface, with facetting of alumina grains in the ®ber and some lanthanum segregation to these facetted boundaries. Spheroidization of thin coatings was also observed. Tensile strength of coated ®ber decreased with increasing deposition tempera￾ture and with time at temperature after deposition. Possible reasons for the strength decrease are discussed. Published by Elsevier Science Ltd. Keywords: Aluminosilicate ®bers; Coating; Grain growth; Interfaces; LaPO4; Mechanical properties 1. Introduction Improvement of intermediate and high temperature properties of ceramic matrix composites (CMCs) requires an oxidation resistant alternative to carbon or boron nitride ®ber-matrix interfaces.1±6 One possible alternative is monazite (LaPO4). Monazite is refractory (mp 2072C) and thermochemically stable with many other common refractory oxides such as alumina and mullite.7±9 Crack de¯ection and ®ber pushout experi￾ments, as well as some limited mechanical tests on composites suggest that monazite bonds weakly with other oxides.8±10 Some similar results were found for xenotime (YPO4) 11 and scheelite (CaWO4).12 Monazite containing ceramics were demonstrated to be machine￾able,13 which implies signi®cant plasticity from some combination of cleavage microcracking, twinning, or dislocation glide.14 Use of monazite as a ®ber-matrix interface in CMCs requires that it either be coated on ®bers or formed in situ between the ®ber and matrix during processing. Sol and solution precursors have been used to coat ®ber tows with monazite,6 as has continuous chemical vapor deposition (CVD).10,15,16 Limited experiments demon￾strate crack de¯ection and ®ber pullout in tensile tests of composites with CVD monazite coated ®ber tows.10 For liquid-phase precursors, crack de¯ection and debonding have only been demonstrated on composites with low volume fractions of dip coated, large diameter (100 mm) single crystal alumina (Saphikon1) mono®laments.8 The thickness uniformity of sol and solution derived coat￾ings on ®laments in ®ber tows is often poor, although stoichiometry is easily controlled.6 In contrast, limited data suggest that CVD coatings have good thickness uniformity, but are sometimes o€ stoichiometry.15,16 Filament tensile strength is often degraded during ®ber coating,6,17±19 or during exposure to various envir￾onments.20,21 Strength increases after coating or envir￾onmental exposure are also known.22,23 Large di€erences in tensile strength have been observed between ®bers coated with di€erent monazite pre￾cursors.6 Preliminary indications of a strong tensile strength dependence on coating temperature were also found. Excessive degradation in ®ber tensile strength 0955-2219/00/$ - see front matter. Published by Elsevier Science Ltd. PII: S0955-2219(99)00257-5 Journal of the European Ceramic Society 20 (2000) 589±597 * Corresponding author Tel.:1-937 255 9825; fax: 1 937 656 4296. E-mail address: hayrs@ml.wl.wpafb.af.mil (R.S. Hay)