《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_electrophoretic deposition-12.pdf_大学文库

c00.52es and manufacturing ELSEVIER Composites: Part A 32(2001)1007-1012 Fabrication of high-temperature resistant oxide ceramic matrix composites I.A. H. Al-Dawery". E. G. Butler Ceramic Matrix Composites Group, Interdisciplinary Research Centre(IRC) in Materials for High Performance Applications. The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Abstract The main objectives of this work are to develop and apply new ceramic matrix materials by which a relatively high level of densification of the final composite may be achieved by pressureless-sintering at temperatures close to 1200.C. A cost-effective processing method for the production of oxide/oxide CMCs materials and components has been achieved. A range of interphase materials is applied to Nextel 720 fibres. These include zirconia, neodymium and lanthanum phosphate. A combination of ZrO2 and the AKP50 alumina powder have been selected as the successful candidates for the interphase and the matrix material, respectively. Work has been conducted on the preparation of slurry systems consisting of these two materials. The prime target was to establish the possibility of achieving good coating around the bulk of the fibres together with a high level of impregnation with the matrix material. Furthermore, the pressure infiltration technique has been applied and as a result composite samples with fibre volume content of 40%0, and remaining porosity of around 20 vol% have been routinely produced. Composite room and high-temperature tensile strengths of around 200 MPa have been accomplished. o 2001 Published by elsevier Science Ltd. Keywords: A Ceramic matrix composities( CMCs); Nextel720 1. Introduction purity ultra fine alumina offers the advantage of being sintered readily at temperature above 800C [5]. Thus Continuous fibre reinforced ceramic matrix composites composite materials can be processed at temperatures that CMCs) with suitable interfaces can exhibit inelastic defor- mation behaviour which enable these materials to retain It is very essential during the fabrication stages that the trength in the presence of holes and notches. This damage intrinsic strength of the fibres is retained as far as possible tolerance, coupled with their inherent refractoriness, has and that the matrix material is able to translate efficiently the enabled CMCs to emerge as successful candidates for fibre properties into composite strength and stiffness many high-temperature thermostructural high performance Normally, the reinforcement material(Nextel720 fibres applications [1]. Careful microstructural design has been 3M, USA)is first coated with a low debond energy inter- defined as a key factor in the development of oxide/oxide phase(typically using ZrO2 sol). This would be followed by CMCs. The majority of these developments have been a second stage of incorporating the matrix material using based upon a selection of a weak interface concept, using pressure infiltration techniques. Eventually green com- stable oxide interphases with suitably low fracture tough- posites are densified by pressureless sintering at 1200.C ness[2,3]. Furthermore, sintering kinetics suggests adequate for 4 h. The method of matrix infiltration using aqueous microstructural stability for applications such as in the sols and slurries is an extremely demanding challenge gas turbine engine, where initial target wall tempera- Here there are two issues. The first one is that when fibre tures are in the range of 1100-1200"C. Here there is mats are coated with the Zro2 interphase a web-type solid a processing challenge. Most matrices require sintering structure, covering most voids within the fibre mats, is temperatures of or above 1200C to achieve sufficient formed. Such formation will act as a barrier towards incom- bonding between matrix particles. However, most ing sols and slurries aiming at filling the much bigger voids commercial oxide fibres are susceptible to micro- within fibre tows. As a result of this, incomplete infiltration structural degradation at these temperatures [4]. High with the matrix slurry cannot be avoided. It was therefore decided to conduct experiments through which the inter Corresponding author. Tel. +44-121-414-3449: fax: +44-121-414- phase sol and the matrix slurry were both applied in one simple and single stage. Composite samples were later made E-mail address: i aldawery ( bham ac uk (I.A.H. Al-Dawery ) using the above approach 9-835X/01/S- see front matter e 2001 Published by Elsevier Science Ltd S1359-835X(00)001676

LA.H. Al-Dawery, E.G. Butler/Composites: Part A 32(2001)1007-1012 size of 30 nm. Aqueous sols containing this powder were Nextel 720 Fibre Mat prepared. Sols with different solid loading were prepared and investigated. These include sols with 10, 15 and 20 wt% ZrO2 powder As an alternative, zirconia sol was also prepared in-house using hydrothermal synthesis. The process was carried out FibreSurface Conditioning in the medium size autoclave. 750 ml of zirconium acetate solution was placed in the autoclave, where a temperature of 220C was reached. After 2 h of reaction, taking place at this temperature, a white slurry(zirconium oxide and acetic Room Temperature Drying acid)was produced. The following step was to dry the slurry In and residual zirconium acetate was then obtained Calcina- on o tion was carried out at 600C for 4 h. The resultant zirco- and Matrix Mat nium powder was then ball milled in water. Initial coating experiments were carried out using nethod previously developed at the IRC/Birmingham University for coating Almaxwoven fabric [6]. The Cold/arm Pressure Infiltration process begins with conditioning of fibres surfaces by immersing the fibre mat in an ammonia based surfactant with a concentration of 1. 5% and a ph of 11.5. The fibre mat was left in surfactant over night and then it was dried in Room Temperature Drying air. To ensure removal of entrapped air, treated fibre mats were degassed under vacuum. They were then immersed in the interphase sol. This was followed by degassing in an ultrasonic bath. Afterwards, octanol was added on the top Pressureless Sintering surface of the sol. Fibre mats were then pulled out of the sol/ octanol. The octanol removes excess sol from the fibre tow to prevent bridging between fibres. Finally, interphase Re-Infiltration with Sol coated fibre mats were left to dry at room temperature in air, before being fired at 700C for 1 h At a later stage, an interphase coating process has been conducted using a simple dip coating technique, without Fibre-Reinforced ceramic pre-conditioning the fibre mats. After dipping in the sol the fibre mat is left under vacuum for 20 min to ensure the Fig. 1. Flow chart for processing oxide/oxide fibre-reinforced ceramic removal of the entrapped air within the fibre mat and hence matrix composite. increasing the chance of the sol material to fully surround the bulk of the fibres The coated fibre mat is then subjected 2.E to a room temperature drying over night. 2. Fibre conditioning 2.3. Vacuum-assisted impregnation of fibre tows with Experimental procedures applied for processing CMCs ceramic sols are illustrated in Fig. 1. Organic sizing on the Nextel720 Electrophoretic deposition(EPD) process was introduced fibre mats was removed by heating, in static air atmosphere, with the aim of ensuring the existence of sufficient matrix to a temperature of 600C for 2 h. During this process, the like material filling the voids within fibre tows [7]. At a later colour of the fibres changes from light green to dark and stage, comprehensive investigation on the whole process finally to a light cream-like colour. The above condition has shown that the process was ineffective when processi were found to be beneficial to allow even and sufficient large-sized samples and that densification should take place de-sizing over relatively large areas of fibre mats. by hot-pressing [8]. Thus, an alternative process has been developed here. Simple vacuum impregnation of fibre mats 2. 2. Processing and application of zirconia interphases with sols containing mullite composition and other suitable materials was found to be the best alternative approach to The first work on interphase coating begun by using a achieve full fibre-tow infiltration. The sols used here contai commercially available type VP zirconia powder(Degussa nano-metre sized particles. The dipping process was Ltd). This is a hydrophilic powder with an average particle assisted by considerably high vacuum levels for 30 min

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LA H. Al-Dawery, E.G. Butler /Composites: Part A 32(2001)1007-1012 2. 4. Processing of CMCs using pressure infiltration matrix materials. The prime target was to establish the ity of achieving good coating around the bulk of Initial work involved fabrication of fat discs with a the fibres together with a high level of impregnation with diameter of 70 mm. Progressively, the process has been the matrix material. Nextel720 fibre reinforced composite adapted so that flat plates of pe can be processed The amounts of reinforcement material and matrix were samples were produced using combinations of interphase and matrix materials as the slurries and the pressure infiltra- carefully calculated so that the final thickness of the sintered tion process was applied to consolidate green samples.The composites is around 3.0 mm. total solid loading of the final slurry was kept to the 40 vol% Slurries for pressure infiltration were initially prepared constant level from alumina powders mixed with a silica sol so that the Efforts have been made to investigate the best startin final slurry would have the composition of mullite materials for the matrix slurries that lead to a successful (Al2O3: 2SiO2). Later on, mullite powder(Keith Ceramic fabrication of crack-free composite samples after both the Materials Ltd), with a mean particle size less than 10 um drying and sintering stages. The best solution for the above was dispersed in water and used as the matrix material. challenge was found through the usage of a combination of a Different concentrations were used. This involved prepara and a small particle sized mullite powders. This tion of slurries with solid loading of 40 vol%. Investigation combination system is known as a bi-modal system. It has on decreasing matrix cracking over drying has led to the been reported, for some composites, that to achieve maxi introduction of bi-modal systems which contain two differ ent particle sizes of mullite powders. For this, a combination composite must be designed to have a reinforcement mate- of mullite KCM 73 powder(Keith Ceramic Materials, rial with a volume fraction of typically 29-55%[10]. In this O um) and mullite SASM powder (Baikoski Chimie, work, composites were fabricated with a 40%o as the fibre France) with a mean particle size of 0. 70 um has been volume fraction. This figure is applied to any given thick- used as the source of the matrix material ness of the final composite. Early pressure infiltrated and Pressure infiltration experiments were conducted using a sintered composite samples were found to have a fibre cylindrical die. Fibre preforms were prepared by dipping volume fraction of only 30%, leading to samples with 70 mm diameter Nextel "720 fabric into a matrix slurry poor properties. and laminating them within the die. A filter paper and One of the challenges in processing CMCs is that the porous stainless steel filtering plate were placed on the sintering temperature is limited to a maximum level of upper surface of the preform. The die assembly is place 1200.C. This is due to the fact that Nextel720 fibres within a instron load frame model 1195 with a maximum start to loose their strength at temperatures above that pability of 100 kN. The pressure level was varied from 5 limit. Unfortunately, at 1200.C matrix materials consistin to 13 MPa(20-50 kN). The cross head speed was 0.5 mm/ mainly of a mullite composition are difficult to sinter min. Once the pressure reaches its required level it under- without application of pressure. No matter what precau goes a cycling process. During cycling (+5% of applied tions one has taken and no matter how well packed the d)excess liquid within the preform is removed through matrix material is, if the matrix is not sintering well, the porous filter plate. Upon completion of the filtration appreciable properties of the final composite cannot be process, samples were removed from the die and then they achieved unless materials with relatively low sintering were subjected to a room temperature drying. More details emperature are used on the pressure infiltration process have been given in Ref. [9]. 2.6. Densification process Densification of CMCs samples was carried out via 2.5. Combined application of interphase and matrix pressureless sintering in static air atmosphere. Sintering was performed at 1200.C for 4 h, with a heating rate of For the application of the interphase and the matrix mat 300 C/h and a cooling rate of 600C/h Samples were placed fat inside the furnace chamber. These conditions were rials in a single stage, several systems have been assessed order to establish the best parameters through which the carried out for our standard sintering process. In the case hole process can be put under control. For this purpose. where reinfiltration cycles were applied, similar conditions high purity (99.99)and fine(. 1-0.3 um) alpha alumina for heat treatment were used powder type AKP50(Mandoval Ltd, UK) has been investi- 2.7. Sample microstructural characterisation gated. This type of alumina is manufactured by hydrolysis of aluminium alkoxide. Since the ZrO2 system and the This mainly involves scanning electron microscop AKP50 alumina powder have been selected as the success (SEM) analysis of the prepared samples SEM characteris ful candidates for the interphase and the matrix material, tion was carried out using JEOL WINSEM type JSM-6300 respectively, work has been conducted on the preparation SEM. Thus, as received fibre mats, interphase coated fibre of slurries containing a mixture of both interphase and mats, vacuum impregnated fibre mats, and composite samples

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LAH. Al-Dawery, E.G. Butler/Composites: Part A 32(2001)1007-101 were prepared for this analysis by vacuum impregnation of 3. 1200.C as the sintering temperature of the final com- the sample with epoxy resin. This was followed by typical polishing used for any ceramic material. Fractured surfaces of composite samples were also characterised using SEM After the initial heat treatment at 1200C for 4 h, samples analysis. were subjected to a total of five vacuum-assisted rein- filtration cycles. Sols of alumina AKP50, Condea Disperal 2.8. Composite mechanical characterisation boehmite solution and Zro,, with 20 wt% solid loading. Bar-shaped samples were machined for a three-point flex- were used as the reinfiltrating materials. After each cycle, ural test. The length of the samples was 60 mm. The test was samples were heat treated under the conditions mentioned bove. As for the last stage, samples were subjected to a conducted using an Instron type 4467 computer controlled final sintering at 1200'C for 4h machine. Furthermore, samples were also machined for Results have indicted that the best and the highest levels room and high-temperature tensile strength analysis. The of room temperature tensile strength for Nextel"720 fibre length of these samples was 120-200 mm with a width of reinforced composites was 194 MPa. This figure was 8-10 mm. The thickness of these samples was 2.5-3.0 mm. Tests were conducted with a strain rate of 0.5 mm/min, and achieved for those samples processed with the application of the zirconia interphase and alumina matrix materials the maximum testing temperature was 1150C. Metal end- together in a single step. Furthermore, high-temperature abs were introduced to both ends of the bar-shaped samples. The test was conducted using a Zwick type 1484 tensile strength tests conducted at 1150%C revealed a computer controlled tensile machine. Tensile strengths of strength level of 180 MPa. These values were obtained for samples were later calculated. composite samples processed at a temperature of 1200C 3. Results and discussion It is important to mention that the manufacturing sequence has been shown to strongly affect the properties of the final composite. Not to forget the essential role of th densification process. The whole of the sintering process, particular the sintering temperatures for green composite samples and those for reinfiltrated components, was reviewed. Mullite emerged as an attractive matrix material owing to its excellent creep resistance, low modulus and an dequate microstructural stability in the temperature range 1000-1200C. However, the sluggish sintering kinetics presents a challenge in processing. That is, temperatures 00000220KU x20 15mm bove 1300C are required to achieve the required bondin between the matrix particles, yet the currently used Nextel720 fibres are susceptible to microstructural degra- dation at these temperatures. Ideally, neck formation with minimal shrinkage should be promoted by operating in egimes dominated by surface and vapour transport mechanisms. However, this approach cannot provide the requisite strengths. Liquid precursor impregnation and pyro- lysis provide another avenue to build the inter-particle necks. but the initial matrix must have sufficient strength to withstand handling In this work, the main element of the matrix is the AKP50 type alumina powder, and the impreg- nation sources(secondary elements of the matrix)are diluted sols of alumina. boehmite and zirconia. The sintering regime was selected as in the following: 1. 1200C as the temperature for the initial heat treatment of filtrated green samples. This is a matrix bonding treat- ment 2. 1200C as the pyrolysis temperature for heat treating Fig. 2. SEM micrographs of:(a) low; and(b)high magnification images of a reinfiltrated samples polished surface of a composite showing a high level of matrix infiltration

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LA H. Al-Dawery, E.G. Butler /Composites: Part A 32(2001)1007-1012 pressureless-sintering, as can be seen in Fig 3. This beha- viour supports the claim of the existence of a zirconia inter ace that separates the fibres from the matrix material, leading to high level of fibres debonding. Composites processed with the zirconia-alumina system, as mentioned in this paper, have been found to exhibit damage tolerant mechanical properties. This is a very essential phenomenon for the development of composite components for high temperature high performance applications Two important factors affecting properties of the final composite have been identified. These are processing temperature and matrix. The ultimate performance of 1 00Nm 20KV composites processed using Nextel720 fibres will depend on the processing temperature on a combination of both. The temperature effect is the 3. SEM micrograph of a fractured surface of a standard composite. a degradation of fibres strength as the temperature increases high level of fibre pull-out is seen. The matrix effect, on the other hand, is possible chemical reactions between fibres and matrix materials. To investi This is a significant achievement in the sense that gate both parameters, two sets of experimental work were Nextel M720 reinforced composites can designed. Details of these sets and corresponding results are considerable level of tensile strength at such a high shown in table 1. results showed that there is no significant temperature. It is very important to mention that the above effect of matrix materials on the strength of composite findings can only be achieved when using zirconia as the samples processed at 1200.C. Whereas strength of the interphase material On the other hand preliminary studies resin composite made with fibres preheated to 1200.C was considerable damage to the 206 MPa, a room temperature tensile strength of 194 MPa fibres strength after undergoing a heat treatment at 1200C, was recorded for a ceramic composite processed at the same with an average of 30 MPa for the tensile strength [11]. This temperature. These results clearly indicate that: (1)there is degradation in the tensile properties of the fibres was attrib- no negative effect of the matrix materials on the final uted to a possible chemical reaction between the fibres strength of the composite; and (2)only processing tempera- mullite composition and the deposited NdPOa due to the ture have a significant effect on the mechanical strength acidic nature of the precursor solutions [ll]. Thus, based on these results, it was decided that ZrOz sols are the only 4. conclusions and most suitable and stable interphase material for proces composite samples reinforced with Nextel720 fibres With respect to our work on the combined application of The above findings confirm that the combination of both the zirconia interphase and the alumina matrix in a single the main constituent of the matrix has been an excellent step, it was found that although near to full fibre-tow infil tration was achieved (see Fig 2), it was difficult to establish materials choice, since there was no chemical degradation of the final composite strength comparing to that of the re- a visible coating around the bulk of the fibres. This is due to enforcement materials, i.e. Nextel 720 fibres. It is there- the fact that it is possible that the thickness of the deposite fore decided that this combination should be used as a stan- coating is so small, in a nanometer scale, that it cannot be detected by SEM. However, in one way or another the exis- dard composition for the matrix material in processing of tence of a zirconia solution in the alumina matrix seems to high-temperature high performance CMCs. have played a role, since high strengths were only obtained for those samples fabricated with this combination. Indeed, Acknowledgements le test of these composites has resulted in samples fail- in a composite behaviour with a high level of fibre The author would like to acknowledge gratefully financial Mechanical properties of ceramic and resin matrix composites. Effect of matrix and processing temperatures Type of matri Ceramic Ceramic Resin Resin Resin 000° 1200° 200°C. 1000°C, 1200C, conditions Tensile strengt (MPa)

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_electrophoretic deposition-12

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_electrophoretic deposition-12