MIATERIAL TENGE ENGMEERIM ELSEVIER Materials Science and Engineering A244(1998)91-9 Combustion chemical vapor deposition(CCvD)of LaPOa monazite and beta-alumina on alumina fibers for ceramic matrix composites T.J. Hwang*, M.R. Hendrick, H. Shao, H.G. Hornis, A.T. Hunt Micro Coating Technologies, 3901 Green Industrial Way, Chamblee, GA 30341, US.A Abstract This research used the low cost, open atmosphere combustion chemical vapor deposition(CCVDSM) method to efficiently deposit protective coatings onto alumina fibers(3M NextelTM 610) for use in ceramic matrix composites(CMCs). La-monazite (LaPO) and beta-alumina were the primary candidate debonding coating materials investigated. The coated fibers provide thermochemical stability, as well as desired debonding/sliding interface characteristics to the CMC. Dense and uniform La-phosphate coatings were obtained at deposition temperatures as low as 900-1000oC with minimal degradation of fibers However, all of the B-alumina phases required high deposition temperatures and, thus, could not be applied onto the Nextel M 610 alumina fibers. The fibers appeared to have complete and relatively uniform coatings around individual filaments when 420 and 1260 filament tows were coated via the CCvd process. Fibers up to 3 feet long were fed through the deposition flame in the laboratory of Micro Coating Technologies(MCT). TEM analyses performed at Wright-Patterson AFB on the CCVd coated fibers showed a 10-30 nm thick La-rich layer at the fiber/coating interface, and a layer of columnar monazite 0. 1-l um thick covered with sooty carbon of 50 nm thick on the outside. A single strength test on CCVd coated fibers performed by 3M showed that the strength value fell in the higher end of data from other CVD coated samples. o 1998 Elsevier Science S.A. All rights reserved Keywords: Combustion chemical vapor deposition: LaPO:: Beta-alumina; Ceramic matrix composites 1. Introduction erties, coating complexity, cost and manufacturability of that technique must be considered. Traditional CVD Current composite research(e.g. aerospace and en- methods have high fabrication costs and low deposition ergy field) has identified the need for ceramic fiber rates [3], while sol-gel and solution coatings can be systems to allow stronger, tougher composites for high non-uniform in thickness on complex substrates. This temperature applications [1, 2]. Alumina fibers possess research was directed toward use of the open-atmo- several advantages in these applications such as refrac- sphere combustion chemical vapor deposition tory properties, high strength, creep resistance and (CCVDSM)[I] method to deposit a compatible, debond- chemical stability. The loss of fiber strength, which ing layer on alumina fibers(in tows or in mats) at a results in less-than-expected composite strength, may very low cost and at a high efficiency. By operating in occur due to the reaction between the fiber and the the atmosphere with inexpensive precursors, this surrounding matrix. Therefore, fiber coatings are essen- process enables continuous feed of the fibers resulting sistance and a fiber/matrix reaction barrier, as wey te- in greater throughput and significant cost reduction debonding/sliding interface characteristics. Debondin mercial opportunity for CMCs to encompass gas tur- interfaces impede crack propagation and, therefore, bine conductors, high pressure heat exchangers and hot increase the fracture toughness of CMCs In the search for fiber coating methods, fiber/coating material prop- For a coating on alumina fibers to perform success fully in an oxide matrix composite at high temperature, Corresponding author. Tel :+1 770 4578400: fax: +1 770 the coating materials should have [5,6:(1)chemical 45784870. compatibility;(2)a high melting point;(3)appropriate 0921-5093/98/S19.00 0 1998 Elsevier Science S.A. All rights reserved PIS09215093097100831-9
Materials Science and Engineering A244 (1998) 91–96 Combustion chemical vapor deposition (CCVD) of LaPO4 monazite and beta-alumina on alumina fibers for ceramic matrix composites T.J. Hwang *, M.R. Hendrick, H. Shao, H.G. Hornis, A.T. Hunt MicroCoating Technologies, 3901 Green Industrial Way, Chamblee, GA 30341, USA Abstract This research used the low cost, open atmosphere combustion chemical vapor deposition (CCVDSM) method to efficiently deposit protective coatings onto alumina fibers (3M Nextel™ 610) for use in ceramic matrix composites (CMCs). La-monazite (LaPO4) and beta-alumina were the primary candidate debonding coating materials investigated. The coated fibers provide thermochemical stability, as well as desired debonding/sliding interface characteristics to the CMC. Dense and uniform La-phosphate coatings were obtained at deposition temperatures as low as 900–1000°C with minimal degradation of fibers. However, all of the b-alumina phases required high deposition temperatures and, thus, could not be applied onto the Nextel™ 610 alumina fibers. The fibers appeared to have complete and relatively uniform coatings around individual filaments when 420 and 1260 filament tows were coated via the CCVD process. Fibers up to 3 feet long were fed through the deposition flame in the laboratory of MicroCoating Technologies (MCT). TEM analyses performed at Wright-Patterson AFB on the CCVD coated fibers showed a 10–30 nm thick La-rich layer at the fiber/coating interface, and a layer of columnar monazite 0.1–1 mm thick covered with sooty carbon of B50 nm thick on the outside. A single strength test on CCVD coated fibers performed by 3M showed that the strength value fell in the higher end of data from other CVD coated samples. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Combustion chemical vapor deposition; LaPO4; Beta-alumina; Ceramic matrix composites 1. Introduction Current composite research (e.g. aerospace and energy field) has identified the need for ceramic fiber systems to allow stronger, tougher composites for high temperature applications [1,2]. Alumina fibers possess several advantages in these applications such as refractory properties, high strength, creep resistance and chemical stability. The loss of fiber strength, which results in less-than-expected composite strength, may occur due to the reaction between the fiber and the surrounding matrix. Therefore, fiber coatings are essential to provide thermochemical stability (oxidation resistance and a fiber/matrix reaction barrier), as well as debonding/sliding interface characteristics. Debonding interfaces impede crack propagation and, therefore, increase the fracture toughness of CMCs. In the search for fiber coating methods, fiber/coating material properties, coating complexity, cost and manufacturability of that technique must be considered. Traditional CVD methods have high fabrication costs and low deposition rates [3], while sol–gel and solution coatings can be non-uniform in thickness on complex substrates. This research was directed toward use of the open-atmosphere combustion chemical vapor deposition (CCVDSM) [1] method to deposit a compatible, debonding layer on alumina fibers (in tows or in mats) at a very low cost and at a high efficiency. By operating in the open atmosphere with inexpensive precursors, this process enables continuous feed of the fibers resulting in greater throughput and significant cost reduction. In turn, the use of this technique will broaden the commercial opportunity for CMCs to encompass gas turbine conductors, high pressure heat exchangers and hot gas filters. For a coating on alumina fibers to perform successfully in an oxide matrix composite at high temperature, the coating materials should have [5,6]: (1) chemical compatibility; (2) a high melting point; (3) appropriate * Corresponding author. Tel.: +1 770 4578400; fax: +1 770 45784870. 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(97)0083 1 -9
92 T.. Hwang et al./ Materials Science and Engineering 4244(1998)91-96 HPLC Pumps h-Line filter Marmal val Substrate Precursor Solutio Fig. 1. Schematic of the setup for thin film depositions via the CCVD process. All precursors are in liquid solution and pumped to a proprietary tomizer. Upon vaporization, the precursor solution is burned and the flame plasma is directed onto the sample substrate where the film is deposited Multi-layered films are deposited simply by altering the precursor feed material. coefficients of thermal expansion(CTE)in relation to environment in CCVD for the deposition of elemental both the fibers and the ceramic matrix; and (4)debond- constituents from solution, vapor, or gas sources. The ing/sliding characteristics. A high temperature, oxida- precursors are generally dissolved in a solvent that also tion-resistant, and alumina compatible material such as acts as the combustible fuel. This solution is submicron La-monazite (LaPO), which was delineated by P.E.D. atomized and then combusted in the flame Depositions Morgan et al. [5] as a candidate coating material, was can be performed at atmospheric pressure and tempera- deposited onto substrates using the CCVD technique. ture within an exhaust hood or outdoors. Depositions Monazite has been demonstrated to be a favorable of fully dense materials at a rate of I um min-I have coating for alumina fibers due to the nature of its been demonstrated. Today, over 40 different materials interface with alumina [5, 6]. Monazites are stable and have been deposited via CCVD at MCt. phase compatible with alumina at temperatures as high as 1750C in air [6]. The melting points of monazites are about 2000C, with no decomposition up to the 2. Experimental melting points [6,7]. La-monazite (LaPo4) has be shown to be sufficiently weak to allow for crack deflec- 2.1. Depositions tion and debonding at the fiber/matrix interface [5, 6] La-monazites have a cte of 9.6x10-6oc-l which is The thin films were deposited using the CCVD tech 25% higher than the CTE for alumina [6,7]. Since nique as represented in Fig. 1. Inexpensive, metalor ery thin films can deform without failure, the CTE ganic precursors were dissolved into organic solutions mismatch is not a problem if film thickness is main- A solution was nebulized into flowing oxygen or air tained below 0.5 um and then combusted in the flame. The heat from the flame provides the energy required for the precursors to I.I. Background on the CCVD process react and deposit onto the substrate. The substrate was positioned at the flame end in a location such that the The CCvd process is a recently invented open atmo- deposition took place at the desired temperature. All sphere CVD technique [4]. It holds considerable poten- depositions occurred at ambient conditions within a tial for fast and inexpensive coatings on fibers in a fume hood. When continuous fiber tows were to be continuous manner. The process offers several advan- coated, the tows were continuously fed through the tages over other thin-film technologies, including tradi- flame. For even higher efficiency and for depositions tional CVD. The key advantage of the CCVd onto much longer fibers, multiple flames from at least technology comes from its ability to deposit films in the two directions can be used to ensure uniform coating open atmosphere without any costly furnace, vacuum, with minimal fiber handling or reaction chamber. As a result, the initial system The fiber used in this research was unsized NextelTM capitalization requirement is reduced at least ten times 610 Fiber from 3M. NextelTM 610 is composed of compared to a vacuum-based system. Instead of a >99% a-alumina with a filament size of 12 um and 420 pecialized environment, which is required by other filaments per tow. The reported degradation tempera technologies, a flame plasma provides the necessary ture of the fiber is above 1200@C. Prior to deposition, a
92 T.J. Hwang et al. / Materials Science and Engineering A244 (1998) 91–96 Fig. 1. Schematic of the setup for thin film depositions via the CCVD process. All precursors are in liquid solution and pumped to a proprietary atomizer. Upon vaporization, the precursor solution is burned and the flame plasma is directed onto the sample substrate where the film is deposited. Multi-layered films are deposited simply by altering the precursor feed material. coefficients of thermal expansion (CTE) in relation to both the fibers and the ceramic matrix; and (4) debonding/sliding characteristics. A high temperature, oxidation-resistant, and alumina compatible material such as La-monazite (LaPO4), which was delineated by P.E.D. Morgan et al. [5] as a candidate coating material, was deposited onto substrates using the CCVD technique. Monazite has been demonstrated to be a favorable coating for alumina fibers due to the nature of its interface with alumina [5,6]. Monazites are stable and phase compatible with alumina at temperatures as high as 1750°C in air [6]. The melting points of monazites are about 2000°C, with no decomposition up to the melting points [6,7]. La-monazite (LaPO4) has been shown to be sufficiently weak to allow for crack deflection and debonding at the fiber/matrix interface [5,6]. La-monazites have a CTE of 9.6×10−6 °C−1 , which is :25% higher than the CTE for alumina [6,7]. Since very thin films can deform without failure, the CTE mismatch is not a problem if film thickness is maintained below 0.5 mm. 1.1. Background on the CCVD process The CCVD process is a recently invented open atmosphere CVD technique [4]. It holds considerable potential for fast and inexpensive coatings on fibers in a continuous manner. The process offers several advantages over other thin-film technologies, including traditional CVD. The key advantage of the CCVD technology comes from its ability to deposit films in the open atmosphere without any costly furnace, vacuum, or reaction chamber. As a result, the initial system capitalization requirement is reduced at least ten times compared to a vacuum-based system. Instead of a specialized environment, which is required by other technologies, a flame plasma provides the necessary environment in CCVD for the deposition of elemental constituents from solution, vapor, or gas sources. The precursors are generally dissolved in a solvent that also acts as the combustible fuel. This solution is submicron atomized and then combusted in the flame. Depositions can be performed at atmospheric pressure and temperature within an exhaust hood or outdoors. Depositions of fully dense materials at a rate of 1 mm min−1 have been demonstrated. Today, over 40 different materials have been deposited via CCVD at MCT. 2. Experimental 2.1. Depositions The thin films were deposited using the CCVD technique as represented in Fig. 1. Inexpensive, metalorganic precursors were dissolved into organic solutions. A solution was nebulized into flowing oxygen or air and then combusted in the flame. The heat from the flame provides the energy required for the precursors to react and deposit onto the substrate. The substrate was positioned at the flame end in a location such that the deposition took place at the desired temperature. All depositions occurred at ambient conditions within a fume hood. When continuous fiber tows were to be coated, the tows were continuously fed through the flame. For even higher efficiency and for depositions onto much longer fibers, multiple flames from at least two directions can be used to ensure uniform coating with minimal fiber handling. The fiber used in this research was unsized Nextel™ 610 Fiber from 3M. Nextel™ 610 is composed of \99% a-alumina with a filament size of 12 mm and 420 filaments per tow. The reported degradation temperature of the fiber is above 1200°C. Prior to deposition, a
T.. Hwang et al./ Materials Science and Engineering 4244(1998)91-96 standard treatment was used to separate any clusters of fibers. Clusters were broken down(separated) with static force by lightly rubbing the entire sample to twice around a 7 mm diameter glass rod. Examinatin by SEM showed a great reduction in fiber cluster density within a fiber tow after the standard separation treatment The subsequent deposition of LaPO4 was performed on whole tows by moving the tows across the depo tion flame The force from the flame gasses effectuated more efficient separation of filaments and improved the coat ing uniformity. To ensure uniform coverage over the entire circumference of each fiber, the sample tow was coated from one side, rotated 180, and then coated or the other side. Ultimately, depositions could take place simultaneously from opposite sides of the fiber tow Deposition parameters such as solution composition and concentration, deposition temperature, and fiber/ substrate feeding mechanism were empirically deter- mined to optimize coating quality as well as deposition rate. No post deposition heat treatments were per formed Fig. 2. SEM micrograph of alumina fibers coated with LaPOa at 2. 2. characterization 900oC. The coating is dense, uniform and x400 nm thick. The phases of coating materials were determined by coatings were stoichiometric within equipment sensitiv X-ray diffractometry (XRD)using a Siemens GADDs ity X-ray Diffraction System. The quality and the compo The coatings are generally dense and uniform around sition of the coatings were assessed by scanning elec- individual filaments for both samples. No fiber degra SEM 25 keV Hitachi S-800 Field Emission SEM and a Kevex microanalyst 8000 with a Super Beryllium EDX detector A simple tensile strength test was performed or selected fiber tow samples at MCT. An electronic bal ance was used to measured the tensile force applied the fiber bundle. Samples with a similar number of fibers were attached to the balance and pulled in ten sion by hand. The results provided the relative strength of coated fibers and were used to optimize deposition conditions, especially temperature. TEM observations of coated fiber cross-section were done on a jeol 2000FX with EDS at the Wright Laboratory Materials Directorate. Tow bundle tensile strength tests of coated fibers were performed at 3M 3. Results and discussion 3. 1. Morphology of laPA coatings on fibers Two examples of coated NextelTM 610 alumina fibers are shown in Figs. 2 and 3. The deposition tempera 298 tures were 900 and 1000C, respectively. According to the EDX analysis performed by MCT and confirmed Fig. 3. SEM micrograph of alumina fibers coated with LaPO, at 3M(D. Wilson, personal communication), resulting 1000C. The coating is dense, uniform and <400-800 nm thick
T.J. Hwang et al. / Materials Science and Engineering A244 (1998) 91–96 93 standard treatment was used to separate any clusters of fibers. Clusters were broken down (separated) with static force by lightly rubbing the entire sample tow twice around a 7 mm diameter glass rod. Examination by SEM showed a great reduction in fiber cluster density within a fiber tow after the standard separation treatment. The subsequent deposition of LaPO4 was performed on whole tows by moving the tows across the deposition flame. The force from the flame gasses effectuated more efficient separation of filaments and improved the coating uniformity. To ensure uniform coverage over the entire circumference of each fiber, the sample tow was coated from one side, rotated 180°, and then coated on the other side. Ultimately, depositions could take place simultaneously from opposite sides of the fiber tow. Deposition parameters such as solution composition and concentration, deposition temperature, and fiber/ substrate feeding mechanism were empirically determined to optimize coating quality as well as deposition rate. No post deposition heat treatments were performed. 2.2. Characterization The phases of coating materials were determined by X-ray diffractometry (XRD) using a Siemens GADDS X-ray Diffraction System. The quality and the composition of the coatings were assessed by scanning electron microscopy (SEM) using a 25 keV Hitachi S-800 Field Emission SEM and a Kevex microanalyst 8000 with a SuperBeryllium EDX detector. A simple tensile strength test was performed on selected fiber tow samples at MCT. An electronic balance was used to measured the tensile force applied to the fiber bundle. Samples with a similar number of fibers were attached to the balance and pulled in tension by hand. The results provided the relative strength of coated fibers and were used to optimize deposition conditions, especially temperature. TEM observations of coated fiber cross-section were done on a JEOL- 2000FX with EDS at the Wright Laboratory Materials Directorate. Tow bundle tensile strength tests of coated fibers were performed at 3M. 3. Results and discussion 3.1. Morphology of LaPO4 coatings on fibers Two examples of coated Nextel™ 610 alumina fibers are shown in Figs. 2 and 3. The deposition temperatures were 900 and 1000°C, respectively. According to the EDX analysis performed by MCT and confirmed by 3M (D. Wilson, personal communication), resulting Fig. 2. SEM micrograph of alumina fibers coated with LaPO4 at 900°C. The coating is dense, uniform and 400 nm thick. coatings were stoichiometric within equipment sensitivity. The coatings are generally dense and uniform around individual filaments for both samples. No fiber degraFig. 3. SEM micrograph of alumina fibers coated with LaPO4 at 1000°C. The coating is dense, uniform and 400–800 nm thick.
T.. Hwang et al./ Materials Science and Engineering 4244(1998)91-96 degradation On the other hand, LaPO4 was detected in KRD analysis of coatings deposited at temperatures as low as 900C. Thus, the optimum deposition tempera ture should range from 900 to 1100C The effects of temperature were studied by pulling samples in tension to assess their relative breaking point. Sample tows were treated by the standard sepa ration method described above and then coated at 900 and 1000C. Another separation method that involved more handling(not a standard method) was used for another sample, which was coated with LaPOA at 1100C Samples were then tested by using the simple pull-test described above. An uncoated tow with sepa rated filaments failed at x 3. 4 kg. The sample de- posited at 900C broke at 1.8 kg, the sample deposited at 1000 C broke at 2.05 kg, while the sample deposited at 1100oC with more pre-deposition handling failed at 0.45 kg. This temperature study posed no statistical significance, but was used for adjusting deposition con ditions. The much lower failure strength of the 1100%C sample was attributed not only to the deposition tem 1100812KVK2.5K2,aum perature but also to the non-standard pre-deposition treatment 4. SEM micrograph of alumina fibers coated with LaPO4 at C. The coating is dense and uniform. The fibers has show 3.3. Analysis and preliminary test on LaPO4 coated dation(grain growth)was observed. More than 50% of the fibers coated at 900C have a coating thickness Two samples, as in Figs. 2 and 3, were sent to 3M for ranging from 300 to 500 nm. No uncoated fibers were characterization in order to direct further Ccvd depo- observed. The coating on the fibers appears to be Sitions bi-layered. Because the entire sample tow was coated The strand strengths, as assessed by 3M, for these and then rotated 180 for a coating on the other side, samples coated at 900 and 1000 C were 2.72 and 2.81 the manner in which the fibers were coated may have kg, respectively. This strength test was performed on contributed to the appearance of these layers the samples without prior thermal aging. A standard procedure for fiber strength test would include a ther The coating on more than 70% of the fibers coated at mal aging at 1200C for I h. The tensile strength 1000C was between 400 and 800 nm thick. Again, no uncoated fibers were noted. Observable bi-layering was the traditional cvd method generally ranges from 1. 82 fibers to 2.72 kg. These results indicate that CCVD coated fibers may have superior strength to other CVD coated 3. 2. Temperature study samples. A further test on another CCVD coated sam- ple after thermal aging confirmed this expectation. De During depositions at high temperatures, e.g. tails are described in the following A simple equipment configuration was set-up to al 1100oC, grain growth in fibers occurred, as noted low continuous deposition by the CCvd process onto Fig. 4, which resulted in a substantial decrease in three tows simultaneously up to 75 cm long. The fiber strength according to the test results obtained by 3M. length for deposition is currently limited only by the During preliminary studies, it became clear that p-alu- length of the fume hood at MCT. Two sets of samples as such as LaAl1O1g and BaMg2Al1sO27 req were sent to 3M and Wright-Patterson AFB for further higher deposition temperature(>1400oC)to form a analysis planar orientation than the NextelTM 610 fibers can Results obtained from 3M shot ed that the str withstand. It was not feasible to form the desired strength was 2.81 kg after thermal aging. This single B-alumina phase and stoichiometry on this particular test result fell in the upper end of data from all the grade of fibers. Therefore, no successful depositions of samples coated by conventional CVD at 3M and ATM B-alumina were performed on fibers without fiber (generally 1.82-2.72 kg)
94 T.J. Hwang et al. / Materials Science and Engineering A244 (1998) 91–96 Fig. 4. SEM micrograph of alumina fibers coated with LaPO4 at 1200°C. The coating is dense and uniform. The fibers has shown grain growth. degradation. On the other hand, LaPO4 was detected in XRD analysis of coatings deposited at temperatures as low as 900°C. Thus, the optimum deposition temperature should range from 900 to 1100°C. The effects of temperature were studied by pulling samples in tension to assess their relative breaking point. Sample tows were treated by the standard separation method described above and then coated at 900 and 1000°C. Another separation method that involved more handling (not a standard method) was used for another sample, which was coated with LaPO4 at 1100°C. Samples were then tested by using the simple pull-test described above. An uncoated tow with separated filaments failed at :3.4 kg. The sample deposited at 900°C broke at 1.8 kg, the sample deposited at 1000o C broke at 2.05 kg, while the sample deposited at 1100o C with more pre-deposition handling failed at 0.45 kg. This temperature study posed no statistical significance, but was used for adjusting deposition conditions. The much lower failure strength of the 1100°C sample was attributed not only to the deposition temperature but also to the non-standard pre-deposition treatment. 3.3. Analysis and preliminary test on LaPO4 coated fibers Two samples, as in Figs. 2 and 3, were sent to 3M for characterization in order to direct further CCVD depositions. The strand strengths, as assessed by 3M, for these samples coated at 900 and 1000°C were 2.72 and 2.81 kg, respectively. This strength test was performed on the samples without prior thermal aging. A standard procedure for fiber strength test would include a thermal aging at 1200°C for 1 h. The tensile strength measured from samples coated by 3M and ATMI by the traditional CVD method generally ranges from 1.82 to 2.72 kg. These results indicate that CCVD coated fibers may have superior strength to other CVD coated samples. A further test on another CCVD coated sample after thermal aging confirmed this expectation. Details are described in the following. A simple equipment configuration was set-up to allow continuous deposition by the CCVD process onto three tows simultaneously up to 75 cm long. The fiber length for deposition is currently limited only by the length of the fume hood at MCT. Two sets of samples were sent to 3M and Wright-Patterson AFB for further analysis. Results obtained from 3M showed that the strand strength was 2.81 kg after thermal aging. This single test result fell in the upper end of data from all the samples coated by conventional CVD at 3M and ATMI (generally 1.82–2.72 kg). dation (grain growth) was observed. More than 50% of the fibers coated at 900°C have a coating thickness ranging from 300 to 500 nm. No uncoated fibers were observed. The coating on the fibers appears to be bi-layered. Because the entire sample tow was coated and then rotated 180° for a coating on the other side, the manner in which the fibers were coated may have contributed to the appearance of these layers. The coating on more than 70% of the fibers coated at 1000o C was between 400 and 800 nm thick. Again, no uncoated fibers were noted. Observable bi-layering was minimized at 1000°C but was still observed on some fibers. 3.2. Temperature study During depositions at high temperatures, e.g. \ 1100°C, grain growth in fibers occurred, as noted in Fig. 4, which resulted in a substantial decrease in strength according to the test results obtained by 3M. During preliminary studies, it became clear that b-aluminas such as LaAl11O19 and BaMg2Al15O27 require a higher deposition temperature (\1400°C) to form a planar orientation than the Nextel™ 610 fibers can withstand. It was not feasible to form the desired b-alumina phase and stoichiometry on this particular grade of fibers. Therefore, no successful depositions of b-alumina were performed on fibers without fiber
T.J. Hwang et al./ Materials Science and Engineering 4244(1998)91-96 The TEM analysis shows a uniform CCVD coating between 0. I and I um thick completely covering the fibers, Fig. 5(a). According to the EDS analysis, the coating consists of a 10-30 nm thick La-rich layer at the coating/fiber interface and a layer of columnar grained monazite covered with sooty carbon of <50 nm thick on the outside, Fig. 5(b). This layer sequence was present in all filaments. The La-rich inner layer may either be La2O3 or LaAlO3 Spot size limitations of the tem did not allow unambiguous determination of the phase of this layer. The thickness of this inner layer scaled with the total coating thickness. The coating microstructure was uniform from filament to filament The composition, although slightly off stoichiometry ecause of this La-rich inner layer, did not vary from filament to filament, as sometimes observed for other LaPOa coatings deposited by CVD(R. Hay, personal communication). At present, it is not clear whether the observed thickness, composition, and microstructure uniformity are intrinsic to the CCVd process. Observed uniformity may also be dependent on the method used to hold and/or spread the tows during deposition. 3. 4. LaPO4 coatings on alumina mats Several pieces of alumina fiber fabric were also coated. Unlike the fiber tows, the fabric had sizing on it which had to be burned off prior to coating. A special design using forced vapor infiltration was set up for deposition onto the fiber cloth. As shown in the SEM micrograph(Fig. 6) the LaPOa coating is not as dense and uniform on the fabric as on the fiber tows due to the forced vapor infiltration needed for this tightly woven material. The region of the coated cloth shown in Fig. 6 represents the thickest coating area. However, some filaments of the alumina cloth had little or no coating at all. Because the sample is densely woven, sputtering of a conductive coating onto the fiber mat or sem analysis did not cover all areas of interest Therefore, high magnification SEM observation of the non-sputter-coated area was not possible due to a severe charging problem The cloth was weighed after sizing burn off, and again after coating. The weight change indicated a deposition efficiency of at least 29%, after a correction for the oxygen mass in LaPO4 4. Conclusions (1) The combustion chemical vapor deposition pro cess was used to deposit LaPO4 coatings, from inexpen Fig. 5. TEM micrographs of alumina fiber coated with LaPO4 at sive precursors, onto NextelTM 610 alumina fibers 1000C.(a) Uniform coatings of 450-750 nm thick completely cove Continuous fiber coating was made possible by ing the fibers.(b)A 10-30 nm thick La rich layer at the coating/fiber interface and a layer of columnar grained monazite covered with CCVD's unique open-atmosphere processing. of sooty carbon on the outside
T.J. Hwang et al. / Materials Science and Engineering A244 (1998) 91–96 95 The TEM analysis shows a uniform CCVD coating between 0.1 and 1 mm thick completely covering the fibers, Fig. 5(a). According to the EDS analysis, the coating consists of a 10–30 nm thick La-rich layer at the coating/fiber interface and a layer of columnar grained monazite covered with sooty carbon of B50 nm thick on the outside, Fig. 5 (b). This layer sequence was present in all filaments. The La-rich inner layer may either be La2O3 or LaAlO3. Spot size limitations of the TEM did not allow unambiguous determination of the phase of this layer. The thickness of this inner layer scaled with the total coating thickness. The coating microstructure was uniform from filament to filament. The composition, although slightly off stoichiometry because of this La-rich inner layer, did not vary from filament to filament, as sometimes observed for other LaPO4 coatings deposited by CVD (R. Hay, personal communication). At present, it is not clear whether the observed thickness, composition, and microstructure uniformity are intrinsic to the CCVD process. Observed uniformity may also be dependent on the method used to hold and/or spread the tows during deposition. 3.4. LaPO4 coatings on alumina mats Several pieces of alumina fiber fabric were also coated. Unlike the fiber tows, the fabric had sizing on it which had to be burned off prior to coating. A special design using forced vapor infiltration was set up for deposition onto the fiber cloth. As shown in the SEM micrograph (Fig. 6) the LaPO4 coating is not as dense and uniform on the fabric as on the fiber tows due to the forced vapor infiltration needed for this tightly woven material. The region of the coated cloth shown in Fig. 6 represents the thickest coating area. However, some filaments of the alumina cloth had little or no coating at all. Because the sample is densely woven, sputtering of a conductive coating onto the fiber mat for SEM analysis did not cover all areas of interest. Therefore, high magnification SEM observation of the non-sputter-coated area was not possible due to a severe charging problem. The cloth was weighed after sizing burn off, and again after coating. The weight change indicated a deposition efficiency of at least 29%, after a correction for the oxygen mass in LaPO4. 4. Conclusions (1) The combustion chemical vapor deposition process was used to deposit LaPO4 coatings, from inexpensive precursors, onto Nextel™ 610 alumina fibers. Continuous fiber coating was made possible by CCVD’s unique open-atmosphere processing. Fig. 5. TEM micrographs of alumina fiber coated with LaPO4 at 1000°C. (a) Uniform coatings of 450–750 nm thick completely covering the fibers. (b) A 10–30 nm thick La rich layer at the coating/fiber interface and a layer of columnar grained monazite covered with B50 nm of sooty carbon on the outside
T.. Hwang et al./ Materials Science and Engineering 4244(1998)91-96 (6)Except for a very thin La-rich inner layer and some sooty carbon outer layer, CCVD coatings were phase pure columnar grained monazite. Coating thick ness varied from 0.1 to I um, but the microstructure and compositional layering were uniform from filament to filament ( Depositions onto fiber fabric mats yielded a de- position efficiency of 29%, but coating density and uniformity suffered due to the difficulty in infiltrating the tightly woven material Acknowle Financial support was provided by the National Sci- nce Foundation SBIR/DMII program, contract #DMI9561712. The authors would like to thank David wilson at 3M and Dr Randy Hay at Wright-Pat terson AFB for coating evaluations and technical ad- vise References Fig 6 SEM micrograph of alumina fiber cloth coated with LaPoa at 950C. The image was taken at a thicker coating area. The coating is [J.A. Dicarlo, Fibers for ceramic c need and opportu- not as dense and uniform as that on fiber tows nities Flight Vechicle Mater. Struct. D Assess Future dir 2(1994)285-307. (2)The LaPO4 coatings deposited by the CCVD [2J. Porter, Properties of advanced fibers for reinforcing metal and method are dense and uniform with complete coverage ceramic matrix composites, Proc. Int. Conf. Adv. Compos. Mater.(1993)785-790 around individual filaments 3] C.. Griffin, R.R. Leschke, CVD processing of fiber coating for (3)The LaPO4 coatings were applied to fibers at CMCs, Cera. Eng. Sci. Proc. 164, July-August(1995) emperatures as low as 900-1000oC with minimal fiber 4 A.T. Hunt, W.B. Carter, J K. Cochran, Combustion degradation apor deposition: a thin novel thin film deposition Appl.Phys.Lett63(2)(1993)266-268. (4)Beta-aluminas, such as LaAlnO19 and [5] P.E. Morgan, D B. Marshall, Functional interfaces for oxide/ox- BaMg2Al1sO27, required high deposition temperatures de composites, Mater. Sci. Eng. Al62(1993)15-25 and, thus, did not yield the desired stoichiometry and 6 P.E. Morgan, D B. Marshall, Ceramic composites of monazite phases on NextelTM fibers without degradation and alumina, J. Am. Ceram. Soc. 78(6)(1995)1553-1563 (5)The strand strength for CCVD coated fibers [7 Y. Hikichi, T. Nomura, Y. Tanimura, S. Suzuki, Sintering and properties of monazite-type CePO4, J. Am. Ceram Soc. 73(12) about 2.72-2.81 kg with or without thermal aging (1990)3594-3596
96 T.J. Hwang et al. / Materials Science and Engineering A244 (1998) 91–96 Fig. 6. SEM micrograph of alumina fiber cloth coated with LaPO4 at 950°C. The image was taken at a thicker coating area. The coating is not as dense and uniform as that on fiber tows. (6) Except for a very thin La-rich inner layer and some sooty carbon outer layer, CCVD coatings were phase pure columnar grained monazite. Coating thickness varied from 0.1 to 1 mm, but the microstructure and compositional layering were uniform from filament to filament. (7) Depositions onto fiber fabric mats yielded a deposition efficiency of 29%, but coating density and uniformity suffered due to the difficulty in infiltrating the tightly woven material. Acknowledgements Financial support was provided by the National Science Foundation SBIR/DMII program, contract cDMI9561712. The authors would like to thank David Wilson at 3M and Dr Randy Hay at Wright-Patterson AFB for coating evaluations and technical advise. References [1] J.A. Dicarlo, Fibers for ceramic composites: need and opportunities, Flight Vechicle Mater. Struct. Dyn.—Assess. Future Dir. 2 (1994) 285–307. [2] J. Porter, Properties of advanced fibers for reinforcing metal and ceramic matrix composites, Proc. Int. Conf. Adv. Compos. Mater. (1993) 785–790. [3] C.J. Griffin, R.R. Kieschke, CVD processing of fiber coating for CMCs, Cera. Eng. Sci. Proc. 164, July–August (1995) 425–433. [4] A.T. Hunt, W.B. Carter, J.K. Cochran, Combustion chemical vapor deposition: a thin novel thin film deposition technique, Appl. Phys. Lett. 63 (2) (1993) 266–268. [5] P.E. Morgan, D.B. Marshall, Functional interfaces for oxide/oxide composites, Mater. Sci. Eng. A162 (1993) 15–25. [6] P.E. Morgan, D.B. Marshall, Ceramic composites of monazite and alumina, J. Am. Ceram. Soc. 78 (6) (1995) 1553–1563. [7] Y. Hikichi, T. Normura, Y. Tanimura, S. Suzuki, Sintering and properties of monazite-type CePO4, J. Am. Ceram. Soc. 73 (12) (1990) 3594–3596. (2) The LaPO4 coatings deposited by the CCVD method are dense and uniform with complete coverage around individual filaments. (3) The LaPO4 coatings were applied to fibers at temperatures as low as 900–1000°C with minimal fiber degradation. (4) Beta-aluminas, such as LaAl11O19 and BaMg2Al15O27, required high deposition temperatures and, thus, did not yield the desired stoichiometry and phases on Nextel™ fibers without degradation. (5) The strand strength for CCVD coated fibers is about 2.72–2.81 kg with or without thermal aging.