Part B: engineering ELSEVIER Composites: Part B 30(1999)631-646 www.elsevier,com/locate/compositesb Characterization techniques for composites and other advanced materials A E. Pasto, D n. Braski, T.R. Watkins, W.D. Porter, E. Lara-Curzio,SB McSpadden High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6062, USA Abstract One of the key requirements in developing composites and other advanced materials is generation of a good understanding of the relationships between composition and structure on the one hand, and properties and behavior on the other. Another key requirement is application of this understanding to develop a material with the desired properties. a third key requirement is to understand the new material's failure mechanisms. All of these are wrapped in the term characterization, which is the subject of this paper. Application of umerous materials characterization techniques to the study of ceramic composites is described. o 1999 Elsevier Science Ltd. All rights Keywords: Materials characterization techniques 1. Introduction microstructural and microchemical analysis, equipment for One of the key requirements in developing composites ties of materials to elevated temperatures, X-ray amdp roper- measurement of the thermophysical and mechanical utron nd other advanced materials is generation of a good under- diffraction for structure and residual stress analysis, high- standing of the relationships between composition and speed grinding machines, and measurement of component structure on the one hand, and properties and behavior on shape, tolerances, surface finish and friction and wear prop- the other. Another key requirement is application of this erties. Users willing to publish the results of their work can understanding to develop a material with the desired proper perform no-cost materials characterization here, under the ties. A third key requirement is to understand the new sponsorship of OTT. materials failure mechanisms. All of these are wrapped in Over half of the work performed in the hTMl is the term "characterization, which is the subject of this sponsored by other R&d programs, from DOE and other paper. agencies. This research is also primarily characterization, The high temperature materials laboratory(HTML) is and it often involves composites. Composites work has art of the metals and ceramics division of Oak Ridge been sponsored by the continuous fiber ceramic composite National Laboratory (ORNL), where it serves the primary (CFCC) program, the ceramic technology for advanced heat purpose of providing equipment and staff to perform mate- engine program, now called the propulsion system materials rials characterization. It is a US Department of Energy program, and others (DOE)-designated National User Facility designed to assist The following sections will highlight some of the capabil- American industries, universities and governmental agen- ities resident at HTML, and illustrations of their application ies develop advanced materials, by providing a skilled to composite materials staff and numerous sophisticated, often one-of-a-kind pieces of materials characterization equipment. HTML is ponsored by DOE's office of transportation technologies 2. Microstructure/microchemistry characterization it is a 64, 500 sq. ft. building on the ORNL site, in which 2.1. Auger analysis of the interfaces in a composite eside six user centers", which are clusters of specialized equipments revolving around a specific type of properties The toughness offiber-reinforced ceramic-matrix compo- measurements. Available are electron microscopy for only controlled by the properties of the fiber and matrix materials, but the bonding forces between them Paradoxically, the strongest bonds do not increase tough Corresponding author. Tel: +1-423-574-5123; fax:+1-423-574-4913 E-imail address: pastoae @ornl. gov(AE Pasto) sliding of the fibers during fracture are more effective. A 1359-8368/99/- see front matter 1999 Elsevier Science Ltd. All rights reserved Pl:S13598368(99)00040-2
Characterization techniques for composites and other advanced materials A.E. Pasto*, D.N. Braski, T.R. Watkins, W.D. Porter, E. Lara-Curzio, S.B. McSpadden High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6062, USA Abstract One of the key requirements in developing composites and other advanced materials is generation of a good understanding of the relationships between composition and structure on the one hand, and properties and behavior on the other. Another key requirement is application of this understanding to develop a material with the desired properties. A third key requirement is to understand the new material’s failure mechanisms. All of these are wrapped in the term “characterization”, which is the subject of this paper. Application of numerous materials’ characterization techniques to the study of ceramic composites is described. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Materials characterization techniques 1. Introduction One of the key requirements in developing composites and other advanced materials is generation of a good understanding of the relationships between composition and structure on the one hand, and properties and behavior on the other. Another key requirement is application of this understanding to develop a material with the desired properties. A third key requirement is to understand the new material’s failure mechanisms. All of these are wrapped in the term “characterization”, which is the subject of this paper. The high temperature materials laboratory (HTML) is part of the metals and ceramics division of Oak Ridge National Laboratory (ORNL), where it serves the primary purpose of providing equipment and staff to perform materials characterization. It is a US Department of Energy (DOE)-designated National User Facility designed to assist American industries, universities and governmental agencies develop advanced materials, by providing a skilled staff and numerous sophisticated, often one-of-a-kind, pieces of materials characterization equipment. HTML is sponsored by DOE’s office of transportation technologies (OTT), energy efficiency and renewable energy. Physically, it is a 64,500 sq. ft. building on the ORNL site, in which reside six “user centers”, which are clusters of specialized equipments revolving around a specific type of properties measurements. Available are electron microscopy for microstructural and microchemical analysis, equipment for measurement of the thermophysical and mechanical properties of materials to elevated temperatures, X-ray and neutron diffraction for structure and residual stress analysis, highspeed grinding machines, and measurement of component shape, tolerances, surface finish and friction and wear properties. Users willing to publish the results of their work can perform no-cost materials characterization here, under the sponsorship of OTT. Over half of the work performed in the HTML is sponsored by other R&D programs, from DOE and other agencies. This research is also primarily characterization, and it often involves composites. Composites work has been sponsored by the continuous fiber ceramic composite (CFCC) program, the ceramic technology for advanced heat engine program, now called the propulsion system materials program, and others. The following sections will highlight some of the capabilities resident at HTML, and illustrations of their application to composite materials. 2. Microstructure/microchemistry characterization 2.1. Auger analysis of the interfaces in a fiber composite The toughness of fiber-reinforced ceramic-matrix composites is not only controlled by the properties of the fiber and matrix materials, but the bonding forces between them. Paradoxically, the strongest bonds do not increase toughness but, in fact, somewhat weaker bonds that promote some sliding of the fibers during fracture are more effective. A Composites: Part B 30 (1999) 631–646 1359-8368/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-8368(99)00040-2 * Corresponding author. Tel.: 11-423-574-5123; fax: 11-423-574-4913. E-mail address: pastoae@ornl.gov (A.E. Pasto) www.elsevier.com/locate/compositesb
L E Pasto et al./Composites: Part B 30(1999)631-646 using a profiling technique where material was intermit- tently ion-sputtered away and the fresh surface analyzed This produced composition depth profiles that extended from the fracture surface into the fibers or the matrix. It was found that the depth profiles of troughs (T in Fig. 1) djacent to particular fibers(f)were essentially the san those that mated directly above fibers. The profiles for the Fracture fibers were also similar to each other. Therefore. it was possible to combine the two types of depth profiles into Matrix one as shown in Fig. 2. Composition in at. is shown as a function of depth for a BN-coated SiC fiber/SiC matrix composite. Zero depth represents the fracture plane, which Fibe is also the weakest portion of the composite structure. The development engineer was able to pick out the location of the fracture plane and whether the coating had achieved the Fig. 1 Schematic diagram of fracture in a fiber-reinforced ceramic matrix desired effect in controlling fiber/matrix bond strength. It was also of interest to track the levels of oxygen from the fiber through the Bn coating and finally disappearing after convenient technique that has been used to control the fiber/ approximately 140 nm into the SiC matrix. The BN coating matrix bond strength involves coating the fibers such as Sic thickness was seen to be about 50 nm and appeared to have with an element such as C or BN before consolidation into a diffused to some extent into the SiC matrix. Such composi composite. Coating as well as consolidation are ofter tional details of the fiber/coating/matrix interface would accomplished using chemical vapor deposition methods. a have been extremely difficult, if not impossible, to analyze successful strategy that may be used to develop tougher by any other technique and were very helpful to the composites is to relate toughness to the fracture interface development and optimization of fiber-reinforced ceramic location and composition as an experimental measure of composites interface bonding. This type of experiment is ideally suited to a modern scanning Auger microprobe that has the ability to detect virtually all elements in areas as small as 0. 1 um. 3. Mechanical characterization Small, notched samples of the composite were loaded in special holders and placed in the HTML's PHI Model 660 The mechanical, physical and structural characterization Auger chamber and the sample was fractured in-situ in a of composites at the HTML has addressed different scales in special fracture stage under a vacuum of 10 Pa. Then these materials. In most cases, this characterization work areas on the fracture surface were observed at a high magni- has been driven by the need to understand property fication, where fiber surfaces(F) and other areas where performance relationships that may lead to the synthesi fibers had been pulled out(troughs or"T) were exposed, of better materials as illustrated in the schematic diagram in Fig. 1. Several The characterization of composites at the microscale has areas of each type were analyzed with the Auger microprobe been focused on the composite constituents, namely fibers 100 Coating 150 -100 Fig. 2. Compositional depth profiles of the interface region of a BN-coated SiC fiber/SiC matrix composite
convenient technique that has been used to control the fiber/ matrix bond strength involves coating the fibers such as SiC with an element such as C or BN before consolidation into a composite. Coating as well as consolidation are often accomplished using chemical vapor deposition methods. A successful strategy that may be used to develop tougher composites is to relate toughness to the fracture interface location and composition as an experimental measure of interface bonding. This type of experiment is ideally suited to a modern scanning Auger microprobe that has the ability to detect virtually all elements in areas as small as 0.1 mm. Small, notched samples of the composite were loaded in special holders and placed in the HTML’s PHI Model 660 Auger chamber and the sample was fractured in-situ in a special fracture stage under a vacuum of 1028 Pa. Then areas on the fracture surface were observed at a high magni- fication, where fiber surfaces (F) and other areas where fibers had been pulled out (troughs or “T”) were exposed, as illustrated in the schematic diagram in Fig. 1. Several areas of each type were analyzed with the Auger microprobe using a profiling technique where material was intermittently ion-sputtered away and the fresh surface analyzed. This produced composition depth profiles that extended from the fracture surface into the fibers or the matrix. It was found that the depth profiles of troughs (T in Fig. 1) adjacent to particular fibers (F) were essentially the same as those that mated directly above fibers. The profiles for the fibers were also similar to each other. Therefore, it was possible to combine the two types of depth profiles into one as shown in Fig. 2. Composition in at.% is shown as a function of depth for a BN-coated SiC fiber/SiC matrix composite. Zero depth represents the fracture plane, which is also the weakest portion of the composite structure. The development engineer was able to pick out the location of the fracture plane and whether the coating had achieved the desired effect in controlling fiber/matrix bond strength. It was also of interest to track the levels of oxygen from the fiber through the BN coating and finally disappearing after approximately 140 nm into the SiC matrix. The BN coating thickness was seen to be about 50 nm and appeared to have diffused to some extent into the SiC matrix. Such compositional details of the fiber/coating/matrix interface would have been extremely difficult, if not impossible, to analyze by any other technique and were very helpful to the development and optimization of fiber-reinforced ceramic composites. 3. Mechanical characterization The mechanical, physical and structural characterization of composites at the HTML has addressed different scales in these materials. In most cases, this characterization work has been driven by the need to understand property– performance relationships that may lead to the synthesis of better materials. The characterization of composites at the microscale has been focused on the composite constituents, namely fibers, 632 A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 Fig. 1. Schematic diagram of fracture in a fiber-reinforced ceramic matrix composite. Fig. 2. Compositional depth profiles of the interface region of a BN-coated SiC fiber/SiC matrix composite
A E. Pasto et al./Composites: Part B 30(1999)631-646 . oad(grams) 88导8R Hi-NicalonTM 0.8 0.6 04 ceived hours 0.2 700°C ambient air 2.533.544.5 Ln(Load) Fig. 3. Effect of static exposure to ambient air at 700C for 500 h on the distribution of strengths of Hi-Nicalon fiber coatings, matrix and their interfaces. Characterization performance and the effects of temperature stress and envir- at the macroscale, through the evaluation of test coupons onment on service life. and components fabricated with these materials, has Although the original charter of the HTML addresses addressed issues such as the effect of specimen geometry "high temperature materials"(i.e ceramics, ceramic matrix and sample preparation on mechanical properties and composites), the infrastructure, expertise and tools available at the html have also been used for the characterization of composite systems with polymeric and metallic matrices 3.l. 3.1.. Fibers The major advances in the development of continuous fiber-reinforced ceramic composites(CFCCs)over the last 20 years have been possible, in part, thanks to the avail- ability of strong, small diameter ceramic fibers(ca 8- 20 um)[1, 2]. Most of the characterization of these ceramic aligning sphere flat glass slide fibers has been focused on their tensile properties, namely fiber fine dimensions, the direct determination of other physical and mechanical properties of these fibers has remained a challenge. However, current efforts at the HTml have been focused on the determination of their poisson's ratio and transverse coefficient of thermal expansion by means of laser diffraction techniques 3] ne ultimate in-plane tensile strength of 2D CFCI Fig 4 Schematic of the experimental setup used for lateral compression is controlled primarily by the strength of the reinforcing fibers. knowing how the strength of the fibers evolves
fiber coatings, matrix and their interfaces. Characterization at the macroscale, through the evaluation of test coupons and components fabricated with these materials, has addressed issues such as the effect of specimen geometry and sample preparation on mechanical properties and performance and the effects of temperature stress and environment on service life. Although the original charter of the HTML addresses “high temperature materials” (i.e. ceramics, ceramic matrix composites), the infrastructure, expertise and tools available at the HTML have also been used for the characterization of composite systems with polymeric and metallic matrices. 3.1. Constituents 3.1.1. Fibers The major advances in the development of continuous fiber-reinforced ceramic composites (CFCCs) over the last 20 years have been possible, in part, thanks to the availability of strong, small diameter ceramic fibers (ca 8– 20 mm) [1,2]. Most of the characterization of these ceramic fibers has been focused on their tensile properties, namely their elastic modulus and tensile strength. Because of their fine dimensions, the direct determination of other physical and mechanical properties of these fibers has remained a challenge. However, current efforts at the HTML have been focused on the determination of their Poisson’s ratio and transverse coefficient of thermal expansion by means of laser diffraction techniques [3]. Since the ultimate in-plane tensile strength of 2D CFCCs is controlled primarily by the strength of the reinforcing fibers, knowing how the strength of the fibers evolves A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 633 Fig. 3. Effect of static exposure to ambient air at 7008C for 500 h on the distribution of strengths of Hi-Nicalone. Fig. 4. Schematic of the experimental setup used for lateral compression experiments
TV CAMERA ⊙ -LOAD CELL INDENTOR 匚 Z-STAGE 匚 Y-STAGE DATA A X- STAGE当 OPTICAL BENCH Fig. 5. Schematic of the ITS which consists of a set of micropositioned XYZ stages, an optical microscope, a TV camera and a computer for user-friendly data acquisition and control
A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 634 Fig. 5. Schematic of the ITS which consists of a set of micropositioned XYZ stages, an optical microscope, a TV camera and a computer for user-friendly data acquisition and control
A.E. Pasto et al. /Composites: Part B 30(1999)631-646 Fig. 6. Fracture surface of a 2D CFCC with time under service conditions has been the focus of recent research work. For example, Fig 3 shows the effect of static exposure to ambient air at 700C for 500 h on the distribution of strengths of Hi-Nicalon"[3]. The avail- ability of these data has aided the formulation of models to predict the mechanical behavior and service life of CFCCs at elevated temperatures in oxidizing environments for example [4-7 A major milestone in the development of fiber and polymer technology occurred in the late 60s and early 70s Fig 8. Fracture surface of pulled-out fibers when new high-modulus organic fibers having strengths and moduli five times larger than then existent nylon fibers were compressive properties consists in subjecting these fibers to first synthesized [8]. The new fibers were composed of stiff, lateral compression. Through the of the hTml's highly aligned aromatic molecules, and although these interfacial test system(ITS), it was possible to characterize fibers exhibit outstanding tensile properties, their compres- the lateral compressive behavior of thermally cross-linkable sive properties are poor as a result from their large structural poly(p-1, 2-dihydrocyclobutaphenylene terephthalamide) anisotropy. Whereas, the axial tensile properties are domi-(PPXTA)fibers to assess the role of intermolecular cross- nated by covalent bonds within the polymer backbone, the links on the elastic and plastic transverse properties of these axial compressive properties depend more on the weaker fibers, and hence on their axial compressive behavior secondary intermolecular bonds. One way of characterizing Fig 4 shows a schematic of the experimental setup used for the secondary intermolecular bonding, and hence their axial the lateral compression experiments which was integrated to the html 's its. The html's its is a universal micro- mechanical testing machine that has been used for a large variety of tests including lateral compression tests on single fibers and single-fiber indentation tests as described below Fig. 5 is a schematic of the Its which consists of a set of micropositioned XYZ stages, an optical microscope, a TV camera and a computer for user-friendly data acquisition 200 Fig. 9. The tensile load versus cross-head displacement response of a Fig. 7. Scanning electron micrograph showing the fracture surface of CC Nicalon/CVI SiC minicomposite with a 1.0 um thick carbon fiber minicomposite after tensile testing
with time under service conditions has been the focus of recent research work. For example, Fig. 3 shows the effect of static exposure to ambient air at 7008C for 500 h on the distribution of strengths of Hi-Nicalone [3]. The availability of these data has aided the formulation of models to predict the mechanical behavior and service life of CFCCs at elevated temperatures in oxidizing environments for example [4–7]. A major milestone in the development of fiber and polymer technology occurred in the late 60s and early 70s when new high-modulus organic fibers having strengths and moduli five times larger than then existent nylon fibers were first synthesized [8]. The new fibers were composed of stiff, highly aligned aromatic molecules, and although these fibers exhibit outstanding tensile properties, their compressive properties are poor as a result from their large structural anisotropy. Whereas, the axial tensile properties are dominated by covalent bonds within the polymer backbone, the axial compressive properties depend more on the weaker secondary intermolecular bonds. One way of characterizing the secondary intermolecular bonding, and hence their axial compressive properties consists in subjecting these fibers to lateral compression. Through the use of the HTML’s interfacial test system (ITS), it was possible to characterize the lateral compressive behavior of thermally cross-linkable poly(p-l,2-dihydrocyclobutaphenylene terephthalamide) (PPXTA) fibers to assess the role of intermolecular crosslinks on the elastic and plastic transverse properties of these fibers, and hence on their axial compressive behavior [9]. Fig. 4 shows a schematic of the experimental setup used for the lateral compression experiments which was integrated to the HTML’s ITS. The HTML’s ITS is a universal micromechanical testing machine that has been used for a large variety of tests including lateral compression tests on single fibers and single-fiber indentation tests as described below. Fig. 5 is a schematic of the ITS which consists of a set of micropositioned XYZ stages, an optical microscope, a TV camera and a computer for user-friendly data acquisition A.E. Pasto et al. / Composites: Part B 30 (1999) 631–646 635 Fig. 6. Fracture surface of a 2D CFCC. Fig. 8. Fracture surface of pulled-out fibers. Fig. 7. Scanning electron micrograph showing the fracture surface of a minicomposite after tensile testing. Fig. 9. The tensile load versus cross-head displacement response of a CC Nicalone/CVI SiC minicomposite with a 1.0 mm thick carbon fiber coating