The effort involves two people with Ph.D. degrees, six M.S. candidates, and four undergraduate students. In addition, some assistance is being provided by three full-time engineers and two postdoctoral graduates who are part of an industrial development effort for composite materials that is being sponsored by the Taiwanese Government. Program Status The main thrust of the effort is to understand the influence of different mixtures of phenolic and furan, processing conditions, and resulting microstructure on the physical properties of C/Cs. They are trying to use their extensive knowledge of polymeric systems to see if there are unique combinations of precursors that may have been overlooked in the past by other investigators. They believe that it should be possible to reduce the total porosity of the C/Cs to less than 4%. Eventually these researchers will begin to study the pitch systems as a means of densifying the preforms. This research is just starting, so no results have been forthcoming nor are there any papers. Assessment Beijing Institute of Chemical • The Ministry of Aero Space Industry of China ⚫ Institute for Astronautics Information (Technology Dept.) ⚫ China Academy of Launch Vehicle Technology ⚫ Institute of Polymeric Materials My visits to the different organi- This is a relatively new research effort, so not much information is available at this time. But it is expected that an aggressive program will continue in the future because it is being funded by the Government so that C/Cs can be produced in this country for use in the military and eventually for industrial purposes. Prof. Ma would be interested Background in collaborative research with U.S. firms in the future. It is recommended that in a year or two a detailed review be conducted of the research progress that has occurred at this institute. This meeting, held in the Friendship Hall, Beijing, on 22 February 1991, was chaired by Prof. Jiang and attended by more than 55 persons from at least 10 different organizations that included: Shaanxi Nonmetallic Material and Beijing Institute of Chemical The purpose of this meeting was to discuss C/C research and related technology. The general question prior to this meeting was: Is there a technology for producing C/Cs and significant knowledge available in China for obtaining the desired thermal-mechanical properties of C/Cs based on R&D activities that have been conducted here since the early 1970s? In other words, has the research information that has been reported at past technical meetings been derived from only small-scale laboratory work that was performed in China? The discussion questions and information obtained from this meeting are presented according to the dif ferent topical areas listed below. Fibers. Programs are being conducted to determine the best methods for producing pitch- and PAN-based fibers on an industrial scale. Apparently, they are still undecided as to which type of fiber should be the primary industrial product. In general, the fiber diameters range from 5 to 10 microns in diameter. Pitch-based fibers with a 6-micron diameter have a maximum modulus of 700 GPa (102 Mpsi), which is 10% to 15% higher than the modulus for the PAN fibers. But they have tensile strengths of 3.2 GPa (464 kpsi) compared to 3.0 GPa (435 kpsi) for pitch fibers. Production lots of PAN fibers can have strain values as high as 1.8% compared to 1.2% for pitch fibers. The current emphasis is to develop processes for the domestic production of pitch fibers. Until recently, the precursors for the fibers as well as the PAN- and pitchbased fibers have been obtained from Russia. But China is developing methods for producing its own pitch precursors. No information was made available to me about domestically produced PAN precursors. Weaving and Architecture. There is general agreement among the participants of this meeting that the fiber/ yarn properties and architecture are very important ingredients in the determination of the properties of C/Cs. One of the questions was, "What type of yarn distribution will be best for maximum resistance to erosion and ablation?" This started a general discussion amongst the participants and their conclusion was that better resistance is attained by having the yarns/ fibers as evenly distributed throughout the preform rather than as discrete groups of yarns that are displaced from each other. The weaving capabilities are still being improved and they have the question whether 3D or 4D is a better architecture. Impregnation and Densification. Densification of preforms is by chemical vapor deposition (CVD) or liquid impregnation methods. The pitch that is used has characteristics that are similar to the U.S. 15V pitch. Billets are usually impregnated five times. It appears, although not specifically stated, that both organic and pitch types of impregnants are used, as it was stated that the last two impregnations are usually with coal tar pitch. Of the five impregnations, only four receive graphitization heat treatments (HT) as the last impregnation has a HT at <1,000 °C. This is to minimize the amount of open porosity that remains in the preform. The densities of the preforms are between 1.85 and 1.9 g/cc. The CVD impregnation method does have density gradients in thick samples of between 0.1 and 0.2 g/cc at a depth of 3 cm below the preform surface. Pyrolytic graphite from CVD or CVI is the preferred matrix for aircraft brakes. Processing Conditions. Processing temperatures between 400 and 700 °C are considered to be the carbonization range for mesophase-type pitches. Their research indicates that this temperature range is where the molecular order of the microstructure is first defined. The pressures that are used during impregnation can range from 300 to 700 psi and even, on special occasions, as high as 23,000 psi. The thermal and mechanical properties can be altered in coal tar pitch densified C/C preforms by varying the heat treatment between 2,400 and 2,500 °C, which means the amount of graphitic structure can be changed to fit the application requirements. Furthermore, for comparable amounts of graphitic matrix, the HT temperature is lower for pitch than it is for phenolics. Depending on the type of precursor, there is an optimum graphitization heat treatment for a particular desired maximum value of mechanical property. This optimum HT processing condition has also been found in Germany at Schunk. Another method for altering the properties of C/Cs is to treat the fiber surfaces of the yarns by electrolytic oxidation or by coating them with silicon-based oil sizing followed by a pyrolysis HT. Apparently these procedures will increase the degree of fiber-to-matrix bond strength. Microstructure and C/C ThermalMechanical Properties. It was indicated that the fiber-to-matrix interface bond is very important in determining the C/C's work of fracture, strength, and modulus properties. If the bond is very strong, the compressive modulus and strength will be high but the strain values will be low. But under tensile loads, it has been found that a reduction occurs of strength and strain values with an increase of the modulus. Their experience is that C/Cs with the highest work of fracture values and reasonably good strengths contain PAN fibers and graphitic matrices. Higher thermal expansion values are obtained with high fiber-matrix bond strengths and amorphous carbon matrices. The sheath effect has been observed in these studies where the graphitic a-b planes surround each of the fibers similar to rings in the trunk of a tree. This type of microstructure is considered to have an influence on the effective modulus and strength values of the fibers. One of the most interesting points was their discussion about the transverse orientation of the graphitic a-b planes between fibers. This means the normal of these planes is parallel to the fiber axis rather than perpendicular as it is for the previously mentioned sheath effect. Under proper processing conditions the microstructure is predominantly transverse, which results in a >25% increase of the transverse tensile strength and a 10% to 20% reduction in its longitudinal strength. Further studies are being undertaken to determine the optimum distribution of oriented microstructure, which will give the proper transverse and longitudinal strengths. Their descrip- Assessment R&D efforts that are related to the This meeting was very informative SHAANXI NONMETALLIC P.O. Box 72 Quality Assurance (QA) Techniques and Testing Methods. Mechanical testing of composites is being conducted between -196 and 2,600 °C and the data are used both for engineering design and as a gauge of the influence of different processing conditions that are used for developing advanced C/Cs. Thus, these physical property data, along with microstructural observations, are being used in an iterative manner to find the optimum processing procedures for improving these materials. Background At this time, no quantitative microstructural analysis research is being undertaken as they feel the problem is too complicated. Methods for conducting QA include x ray, ultrasonic, thermal and electrical resistance, and computer tomography. They believe that the main unresolved problem for obtaining reliable QA predictions is the inability to define the type, number, and distribution of defects that will cause the physical properties of the C/Cs to significantly degrade. Miscellaneous. Oxidation resistance work is taking place at the level of the shuttle technology, where silicon oxide is deposited on the C/C substrate and subsequently converted to silicon carbide. Only single layers of coating are being applied, but there are plans to investigate multiple layers. Xian, Shaanxi, China. This meeting, held at the Ceramic Society building, was composed of four persons from the institute and Prof. Jiang. The senior engineer of this group is Mr. Kang-li Wang, who stated the major purpose of the institute is the production and exportation of materials for the civilian market. One of their major goals is to become the major supplier of aerospace and aeronautical materials in Asia. This includes carbonaceous materials such as chopped carbon fiber composites that are densified by CVI for brakes; 2, 3, and 4D woven C/C nozzles and hot structures as well as high density graphite for electrical discharge machining (EDM) applications; low density felts for insulation; and metal-impregnated graphite for motor brushes. The members of this group are primarily involved in the The major portion of this discussion was concerned with the densification of woven carbon fiber preforms. Their method of densifying is primarily by using liquid pitch to impregnate the preforms. If their densities are required to be in the high density range of 1.8 to >1.9 g/cc, the impregnations are carried out at high pressures in a HIP facility. Then, the preforms are contained in a metal can that is placed inside an autoclave. The can is partially filled with pitch and the temperature and pressure of the system are raised at a predetermined rate to a maximum temperature of 700 °C. In general, the temperatures and pressures are increased to plateaus and held for predetermined times. These temperature plateaus are normally at 250, 400, and 700 °C and the increase of temperature is at a rate of about 15 °C per hour. The holding times at each plateau vary, too, from as short as just reaching thermal equilibrium to holding >10 hours at about 400°C. In this case the long time is used to advance the pitch toward the carbonaceous state at a slow enough rate to allow the gaseous products from it to diffuse out of the preform without blowing the pitch out of it. They believe the major difficulty in the densification of C/Cs is getting the pitch into the preform and that keeping the pitch in the voids is no problem during the pyrolysis steps that Occur between about 350 and 500 °C. This finding is not consistent with the interpretation of what is occurring according to experiments that some of us have conducted in the United States. Perhaps there are other considerations that the Chinese were not willing to discuss. The can, into which the preform is placed, is either vented or sealed, with the appropriate amount of void space above the pitch in order to minimize the pressures that are developed due to the gaseous products that are evolved from the pitch as it is heated. The purpose for these measures is to prevent the can from rupturing and to minimize the pitch-derived gas pressures, which will counteract the autoclave's high pressures outside the can that are being used to force the pitch into the preforms. Apparently, both vented and unvented cans have been tried. Currently, they are using unvented cans where the volume above the pitch must be carefully selected so as not to cause the can to rupture during the pyrolysis stage. The gas pressures are first applied at 250°C and increased to a maximum value at about 400 °C. It is held at this value until the matrix has been solidified. They have found that the rate of depressurization is very important in terms of the subsequent performance of the C/Cs. The autoclave wall is cold and the maximum pressure that can be attained is about 15 kpsi. The working space for the sample is 120 mm outside diameter and about 300 mm long. This R&D effort started more than 6 years ago and the goal is to find the optimum combinations of pressure, temperature, and time conditions to obtain the maximum density of the C/Cs with the least number of impregnations steps. Interestingly and most important, these processing conditions are also being determined with the intent of obtaining a specific type of matrix microstructure that is desired for particular engineering applications of C/C. This means for a high modulus and high strength C/C the matrix should be carbonaceous. Whereas if the C/C is to have more toughness, the matrix should be more graphitic. The topic of discussion then changed toward applications and the types of preforms and manufacturing techniques. They are making thin (35 mil), wall long (3 to 4 m) tubes by winding cloth on mandrels much as is being done in Prof. Jiang indicated that in the U.S.S.R. the group size doing R&D at the Institute of Astronautics and Space is >6,000 people, with half of these involved in production. He did not specify the overall size of a similar effort in China, although my impression is that it is in the hundreds if not several thousands of people. the United States. There is interest and plans are just being formulated to develop techniques for making nuts and bolts that are cut out of either 2D or 3D C/Cs. This latter form of C/C is being used in the U.S.S.R. The subject of porosity was also discussed and its importance to the performance characteristics of C/Cs. Prof. Jiang made the point that a certain amount of porosity is necessary to accommodate Assessment the differential expansion problems that occur between the fibers and the different types of matrices. They, too, have found that billets can crack during processing if the densities are >1.9 g/cc. But a reduced porosity with the proper size distributions is desired for C/Cs that most resist ablative environments. So for them there is a balance of porosity that must be maintained. Thermal shock characteristics are another aspect that they are trying to enhance and the direction of development appears to be towards increasing the thermal conductivity and work of fracture of the C/Cs. The general impression is that this organization has significant background and experience in the areas of fabrication and the processing to fabricate C/Cs that are of a quality that meets their engineering requirements. They appear to be continually developing advanced techniques to improve these materials without the help of advisors from aboard, such as from the U.S.S.R. Aside from the usual language problems, there was a good exchange of technical detail and no apparent withholding of information. BEIJING RESEARCH No. 1 Nan Da Hong Men Road Beijing 100076, China Quality assurance methods are another area of interest. There was no clear definition, according to them, of which properties to monitor. Currently it appears that x ray, microstructure, porosity, and mechanical properties are being used. Everyone agreed that this system is too costly and time consuming, but an alternative procedure has not been devised as of yet. One possibility that was briefly discussed is the use of pulsed laser heating on different areas and measuring the changes with time of temperature rise and expan- Background sion. These values are then compared on a relative basis in order to obtain the quality of the piece of C/C that is being processed. In concept this approach is a good integrator of the microstructure that is being evaluated in each of the areas that is being measured. So the quality of an entire piece can be evaluated by taking a sufficient number of measurements. The visit was conducted by Prof. Jiang, who worked here for over 20 years, and Mr. Jiliang Wu, who is the vice director of this institute. The other senior engineers and members included Mr. Zi-chun Wang, who is in charge of the carbon R&D effort and who worked as Jiang's assistant when he was here, Prof. Jiaxiang Zhao, Prof. Lian Cheng Hu, the vice general engineer, and Mr. Zheng Guo. While driving or walking to the different locations within this institute, my impression was that this is a very restricted access area. The security was so tight even Prof. Jiang could not get me entrance into the area until a particular person arrived for work who could grant the permission. Another impression was that this facility is very large and contains many buildings and people, many of whom live on this site. This arrangement is similar to what was observed at the graphite institute in Moscow. Apparently, the research, development, preproduction, and production activities are all conducted at this location. In general, the appearance of the buildings gave the same impression as those at the Wright Development Center in Dayton. Program Status The central theme of these discussions was concern with the research and related technology of C/Cs. The general objective at this institute is to develop advanced C/Cs through knowledge of what the processing conditions do to the physical properties for these types of materials. About 700 persons are involved in the R&D and production of C/Cs. The approximate breakdown of this figure was given as 16 professors, 200 senior engineers, and 500 engineers and technicians for R&D work. No further information was conveyed concerning the number of professionals and support persons. I was told that all the analytical and experimental facilities required for this work are available at this site. Their efforts are divided into two general areas: Division 1 - metallic materials Clearly, this meeting was being sponsored by Dept. 2. The major thrust of these R&D efforts is to advance the technology and processing capabilities of composite materials, such as C/Cs, for governmental use. In general, the development of the materials that go into these composites is not being performed at this facility. For example, a major effort is being conducted at the Anshan Research Institute of ThermoEnergy in Anshan Liaoning concerning the development of pitch that is used for the impregnation of C/Cs and for the production of fibers. I met and had a discussion with Dr. Jianfeng Wang, who is a senior engineer, at a 117 Committee meeting on 30 November 1990 in Japan. He was on a 6-month leave doing research with Prof. Otani. Prof. Jiang, my host, knows Dr. Wang, as they were in graduate school together in Moscow. An area that received much attention during the morning discussion was quantitative microstructural analysis and especially possible methods of evaluating the degree of bonding that exists between the fiber and the matrix. Apparently, they are knowledgeable about the open literature on this subject, including my work in this area, for the questions were both general and very specific. They have a keen interest in this whole subject of microstructure in this whole subject of microstructure because they know from experience the area is complex. Experimental and analytical work is ongoing in how to define defects and how they influence the physical properties of C/Cs. The laboratory tour involved going to a number of buildings that contained special types of equipment that is of particular relevance to the processing and fabrication of large pieces of both organic and carbon composites. One of the most impressive facilities is their HIP facility for pitch-impregnating preforms that are as large as 15 inches in diameter and over 40 inches long. Prior to being placed in the autoclave, the preforms are rigidized through the infiltration of pyrolytic graphite (PG) into the preforms. Then the preforms are placed in a sealed metal container that contains the pitch. The temperature and the autoclave pressures are increased to make the can and autoclave pressures approximately equal so the can does not rupture. The maximum conditions that can be obtained in the autoclave are 1,750 °C and >23,000 psi. This autoclave was bought in Sweden about 8 years ago; its overall dimensions are about 3 meters in diameter by 10 meters long. Although I asked, it was not clear to me why these extreme conditions were selected. One possibility is that high pressures are known to produce fine-grained microstructures. The autoclave's conditions are controlled manually based on temperatures and pressures that are continually monitored and recorded by the operator. I'm told that this is the largest autoclave of this type in Asia. A second autoclave was for the processing of organic matrix composites. It is about 5 meters in diameter and 14 meters long and can be operated up to 250 °C and at a pressure of 90 psi using computer-controlled programs. This facility is about 5 years old and again it is reported to be the largest one in Asia. The proper thermal cycles for different materials are determined by the "Edisonian" method where test samples, 500 mm in diameter by 100 mm thick, are cured at different heating cycles and then characterized for their properties. The tour also included a visit to see some of the characterization equipment such as SEMS, TEMS, electron spectroscopy for chemical analysis (ESCA), scanning ion microscopes (SIMs), and other equipment that is normally used in this type of a laboratory. Qualitative determination is the main thrust of their micrographic studies. Optical micrography is used to determine void and fiber content and distributions. SEM is used for evaluating the fracture behavior of these |