●" Science Direct O Solid state Materials science ELSEVIER Current Opinion in Solid State and Materials Science 9(2005)219-229 Interfaces in oxide-fibre-based composites S.T. Mileik Solid State Physics Institute of the Russian Academy of Sciences, Chernogolovka, Moscow 142432, Russia Received 8 May 2006: accepted 20 May 2006 Abstract The behaviour of the fibre/matrix interface makes a composite being a material different from just a mixture of two constituents. Some important features of the interface have been known from the very beginning of research in the composite field whereas some new fea- tures are becoming known when deeper insight into the behaviour of composites occurred to be vital. Oxide-fibre-based composites pro- vide clear illustrations of the primary importance of the interface in composite microstructures. Dealing with single crystalline oxide fibres in various matrices opens a number of new possibilities as well as yields new findings about a role of the interfaces in composites A review of both the possibilities and findings is a subject of the present paper o 2006 Elsevier Ltd. All rights reserved Keywords: Composites; Oxide fibres; Interface: Creep; Fracture; Crystallisation 1. Introduction ites) being fully or partly ceramic should be considered Most efforts in using advance materials in machines Single crystal oxides have always attracted attention as a for transportation and power generation have been potential base for heat-resistant materials because of a always directed towards enhancing energy efficiency of number of the reasons the machines, which means decreasing fuel consumption per unit of useful work. Enhancing the use temperature 1. They are inherently resistant to oxidation of structural materials for hot parts of machines has 2. Single crystals of oxides are inherently strong. been and will be always a most efficient way to reach 3. They have high melting points, high elastic moduli, and the goal low density Most heat-resistant materials for heavily loaded struc tural elements such as Ni-base superalloys are approaching ertainly, single crystals of oxides are most promising their physical limit set by their microstructural stability and substances since polycrystalline oxides, first, are not to be melting points. Perhaps, this limit will be around 1100 suficiently stable at high temperatures(say, at tempera and a dependence of the maximum use temperature on year tures higher than 1200 C for alumina) and, secondly, poly of the alloy development is asymptotically going to this crystals are known to reveal low creep resistance at high limit. Since no other metal can be a real base for heat-resis- temperatures because of the same reasons as those for tant alloys due to well-known reasons, the only alternative non-stability. Hence, single crystalline oxide fibres are is to exploit ceramics. Homogeneous ceramics cannot be wanted as a base for heat-resistant composites used in heavily loaded structures because of their inherent However, to realise the potential of single crystalline brittleness. Hence, non-homogeneous materials (compos- oxide fibres in either a metal-based or oxide matrix, it necessary to develop an appropriate design of a composite, and the main component of the composite to be properly E-mail address: mileiko @issp ac constructed is the interface or interphase, which has a 1359-0286S. see front matter 2006 Elsevier Ltd. All rights reserved doi:l0.1016 j. cossms.2006.05.004
Interfaces in oxide–fibre-based composites S.T. Mileiko Solid State Physics Institute of the Russian Academy of Sciences, Chernogolovka, Moscow 142432, Russia Received 8 May 2006; accepted 20 May 2006 Abstract The behaviour of the fibre/matrix interface makes a composite being a material different from just a mixture of two constituents. Some important features of the interface have been known from the very beginning of research in the composite field whereas some new features are becoming known when deeper insight into the behaviour of composites occurred to be vital. Oxide–fibre-based composites provide clear illustrations of the primary importance of the interface in composite microstructures. Dealing with single crystalline oxide fibres in various matrices opens a number of new possibilities as well as yields new findings about a role of the interfaces in composites. A review of both the possibilities and findings is a subject of the present paper. 2006 Elsevier Ltd. All rights reserved. Keywords: Composites; Oxide fibres; Interface; Creep; Fracture; Crystallisation 1. Introduction Most efforts in using advance materials in machines for transportation and power generation have been always directed towards enhancing energy efficiency of the machines, which means decreasing fuel consumption per unit of useful work. Enhancing the use temperature of structural materials for hot parts of machines has been and will be always a most efficient way to reach the goal. Most heat-resistant materials for heavily loaded structural elements such as Ni-base superalloys are approaching their physical limit set by their microstructural stability and melting points. Perhaps, this limit will be around 1100 C and a dependence of the maximum use temperature on year of the alloy development is asymptotically going to this limit. Since no other metal can be a real base for heat-resistant alloys due to well-known reasons, the only alternative is to exploit ceramics. Homogeneous ceramics cannot be used in heavily loaded structures because of their inherent brittleness. Hence, non-homogeneous materials (composites) being fully or partly ceramic should be considered as future heat-resistant materials. Single crystal oxides have always attracted attention as a potential base for heat-resistant materials because of a number of the reasons: 1. They are inherently resistant to oxidation. 2. Single crystals of oxides are inherently strong. 3. They have high melting points, high elastic moduli, and low density. Certainly, single crystals of oxides are most promising substances since polycrystalline oxides, first, are not to be sufficiently stable at high temperatures (say, at temperatures higher than 1200 C for alumina) and, secondly, polycrystals are known to reveal low creep resistance at high temperatures because of the same reasons as those for non-stability. Hence, single crystalline oxide fibres are wanted as a base for heat-resistant composites. However, to realise the potential of single crystalline oxide fibres in either a metal-based or oxide matrix, it is necessary to develop an appropriate design of a composite, and the main component of the composite to be properly constructed is the interface or interphase, which has a 1359-0286/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2006.05.004 E-mail address: mileiko@issp.ac.ru Current Opinion in Solid State and Materials Science 9 (2005) 219–229
S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 number of the purposes. In ductile-matrix composites the ever, the method does not allow an essential decrease in the interface is expected to be as strong as possible to allow fibre cost as compared to EFO transferring the load to the strong fibre; on the other hand Laser heated pedestal growth(LHPG)was applied for in brittle-matrix composites the interface(or interphase) is the first time to produce single-crystal Cr-doped Al2O expected to be sufficiently weak to trigger a mechanism of fibres. This was actually a floating-zone technique making the crack arrest and so providing sufficient fracture tough- use of small source rods locally melted at the end by a co ness to a composite structure. Hence, the microstructure laser [6]. There are some obvious advantages of the and properties of the oxide-fibre composites become pri- method: the absence of a crucible allows growing suffi- mary problems in the whole technology of future heat- ciently pure crystals, a small volume of the melts increa resistant compos thermal efficiency despite a low efficiency of the laser heat In the present paper, we start with a brief review of the ing and reduces mass exchange around the process zone fabrication technology schemes to produce oxide fibres that would be useful in growing such materials as mullite, focusing on the internal crystallisation method(ICM) which is characterised by a complicated phase diagram invented by the present author and his colleague, V I Kaz- But also the productivity rate of processes based on this min; then proceed with special features of some oxide fibres method can hardly be suitable to produce fibres to be used obtained by ICM, and continue with a brief outline of in structural material mechanical properties of ICM-fibres. a discussion of the properties of metal-matrix and oxide-matrix composites 2. 1. Internal crystallisation method (ICM) ith ICM-fibres focusing on the effect of the interface on mechanical behaviour of the composites is the main focus Internal crystallisation method invented in the authors of the paper laboratory was described in open literature for the first time nearly 15 years ago [7, 8]. Since then a main scheme of the 2. Single crystalline oxide fibres method as well as some variations of it have been published in a number of papers(see for instance Refs. [9-1l. Never- Certainly, the only way to produce either single crystal- theless, to make reading the present paper more convenient line oxides or those with typical eutectic microstructure is we are to describe here briefly the essence of the method to crystallise oxide melts. The following methods of crys- A schematic of the method is shown in Fig. 1. A molyb tallising oxide fibres are well known denum carcass with continuous channels in it. which is eas lly prepared by diffusion bonding or an assem blage of the 1. Edge Feeding Growth (EFG) wire and foil(step I in Fig. 1), is infiltrated with an oxide 2. Micro-pulling down(u-PD) melt(steps 2 and 3 )by the capillary force. The melt is then 3. Laser heated pedestal growth (LHPG) crystallised in the channels to form fibres in an oxide/ molybdenum block(step 4). This is a main scheme of the Strictly speaking all these methods are within a concept ICM, which can be varied to attain a particular goal. For of crystallising a melt by using a shaper, which was formu- example to ensure a homogeneous crystallographic orien- lated by Stepanov before the WWll [1]. Stepanov intro- tation of the fibres in a block, a seed is oriented in an duces a shaper to pre-determine a shape and size of the appropriate manner. Finally, the fibres are freed from the capillary column at the top of which the liquid/ solid inter- molybdenum carcass by dissolution of molybdenum in a face arises, although the authors were hardly aware about mixture of acids. It can be seen that the process of fibre Stepanov's ideas published in Russian. growth based on ICM is actually similar to growth of bulk EFG method was used for the first time to produce sap- single crystals; therefore, the fibre cost should be of the phire fibres by La Belle and Mlavsky [2), who lifted the same order of magnitudes as that of bulk crystals and they growth zone above the melt surface with a capillary tube can be used as reinforcements for structural composites in the crucible. the lower end of the tube is located near In the present context it is important to emphasize that the bottom of the crucible, and the growth zone is now the oxide/ molybdenum interface occurs to be ideal;an fixed relative to the heater independent of the level of the illustration is given in Fig. 2. It is also important to note melt surface which goes down with time. Both, a review that molybdenum foil is recrystallising in the process of of the corresponding techniques and discussion of the fibre melt infiltration and fibres are crystallizing in the channels growth parameters, structure and mechanical properties of Newly formed grains form steps on the surface, which are sapphire fibres are presented in Ref [3]. It appears that a approximately I um in height. The steps are replicated on stable growth takes place at rates no more than 0.5- the fibre surface as can be seen in Fig. 3. They can act as 1.0mm/ s, which makes productivity rate of the process stress concentrators, but perhaps, are not most dangerous. low, so that the cost of the fibres is too high to use them in structural applications 2.2. Fibres obtained by ICA Micro-pulling down method (u-PD)developed by Japanese researchers [4, 5] did actually turn up the eFg A family of single crystalline and eutectic oxide fibres scheme, which simplified slightly growth procedures. How- have been obtained by using ICM tion to sapphire
number of the purposes. In ductile–matrix composites the interface is expected to be as strong as possible to allow transferring the load to the strong fibre; on the other hand, in brittle–matrix composites the interface (or interphase) is expected to be sufficiently weak to trigger a mechanism of the crack arrest and so providing sufficient fracture toughness to a composite structure. Hence, the microstructure and properties of the oxide–fibre composites become primary problems in the whole technology of future heatresistant composites. In the present paper, we start with a brief review of the fabrication technology schemes to produce oxide fibres focusing on the internal crystallisation method (ICM) invented by the present author and his colleague, V.I. Kazmin; then proceed with special features of some oxide fibres obtained by ICM, and continue with a brief outline of mechanical properties of ICM-fibres. A discussion of the properties of metal–matrix and oxide–matrix composites with ICM-fibres focusing on the effect of the interface on mechanical behaviour of the composites is the main focus of the paper. 2. Single crystalline oxide fibres Certainly, the only way to produce either single crystalline oxides or those with typical eutectic microstructure is to crystallise oxide melts. The following methods of crystallising oxide fibres are well known: 1. Edge Feeding Growth (EFG) 2. Micro-pulling down (l-PD) 3. Laser heated pedestal growth (LHPG) Strictly speaking all these methods are within a concept of crystallising a melt by using a shaper, which was formulated by Stepanov before the WWII [1]. Stepanov introduces a shaper to pre-determine a shape and size of the capillary column at the top of which the liquid/solid interface arises, although the authors were hardly aware about Stepanov’s ideas published in Russian. EFG method was used for the first time to produce sapphire fibres by LaBelle and Mlavsky [2], who lifted the growth zone above the melt surface with a capillary tube in the crucible. The lower end of the tube is located near the bottom of the crucible, and the growth zone is now fixed relative to the heater independent of the level of the melt surface which goes down with time. Both, a review of the corresponding techniques and discussion of the fibre growth parameters, structure and mechanical properties of sapphire fibres are presented in Ref. [3]. It appears that a stable growth takes place at rates no more than 0.5– 1.0 mm/s, which makes productivity rate of the process low, so that the cost of the fibres is too high to use them in structural applications. Micro-pulling down method (l-PD) developed by Japanese researchers [4,5], did actually turn up the EFGscheme, which simplified slightly growth procedures. However, the method does not allow an essential decrease in the fibre cost as compared to EFG. Laser heated pedestal growth (LHPG) was applied for the first time to produce single-crystal Cr-doped Al2O3 fibres. This was actually a floating-zone technique making use of small source rods locally melted at the end by a CO2 laser [6]. There are some obvious advantages of the method: the absence of a crucible allows growing suffi- ciently pure crystals, a small volume of the melts increases thermal efficiency despite a low efficiency of the laser heating and reduces mass exchange around the process zone that would be useful in growing such materials as mullite, which is characterised by a complicated phase diagram. But also the productivity rate of processes based on this method can hardly be suitable to produce fibres to be used in structural materials. 2.1. Internal crystallisation method (ICM) Internal crystallisation method invented in the author’s laboratory was described in open literature for the first time nearly 15 years ago [7,8]. Since then a main scheme of the method as well as some variations of it have been published in a number of papers (see for instance Refs. [9–11]). Nevertheless, to make reading the present paper more convenient we are to describe here briefly the essence of the method. A schematic of the method is shown in Fig. 1. A molybdenum carcass with continuous channels in it, which is easily prepared by diffusion bonding of an assemblage of the wire and foil (step 1 in Fig. 1), is infiltrated with an oxide melt (steps 2 and 3) by the capillary force. The melt is then crystallised in the channels to form fibres in an oxide/ molybdenum block (step 4). This is a main scheme of the ICM, which can be varied to attain a particular goal. For example, to ensure a homogeneous crystallographic orientation of the fibres in a block, a seed is oriented in an appropriate manner. Finally, the fibres are freed from the molybdenum carcass by dissolution of molybdenum in a mixture of acids. It can be seen that the process of fibre growth based on ICM is actually similar to growth of bulk single crystals; therefore, the fibre cost should be of the same order of magnitudes as that of bulk crystals and they can be used as reinforcements for structural composites. In the present context it is important to emphasize that the oxide/molybdenum interface occurs to be ideal; an illustration is given in Fig. 2. It is also important to note that molybdenum foil is recrystallising in the process of melt infiltration and fibres are crystallizing in the channels. Newly formed grains form steps on the surface, which are approximately 1 lm in height. The steps are replicated on the fibre surface as can be seen in Fig. 3. They can act as stress concentrators, but perhaps, are not most dangerous. 2.2. Fibres obtained by ICM A family of single crystalline and eutectic oxide fibres have been obtained by using ICM. In addition to sapphire 220 S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229
S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 [7-9] and alumina-YAG-eutectics [7, 8, 10]. YAG with the 2. Coating fibres with a thin layer of various materials (100) orientation of the fibre axis [12] and single crystalline yields healing of surface defects and an essential mullite [13, 14]fibres have been produced. Crystallization of nhancement of the fibre strength. A CVd process was sapphire fibres [9] was conducted by using a seed to get used to coat the fibres to ensure an intimate contact fibres with the c-axis coinciding with the fibre axis: YAG on the interrace and mullite fibres have been crystallised without using seed, so their axis coincides with the (00 1) and c-directions, The second feature is of an obvious importance in the respectively(see Fig. 4) present context. There is a clear dependence of the fibre strength on the coating thickness presented in Fig. 6. How- 2.2.1. Strength of ICM-fibres ever, at the present time it is not clear as to the extent of the Room temperature strength of the fibres was measured maximum fibre strength that can be obtained. A maximum by bending a fibre over rigid cylinders of decreasing diam- can be expected as the coating layer also contains defects eters and counting an average distance between fibre gh temperature strength of the fibres has been mea breaks on each step of the experiment [15]. This yields a sured by testing oxide/molybdenum composites obtained dependence of ultimate strain on the fibre length provided by ICM in either tension [7, 8] or bending. Some results the Youngs modulus of the fibre material is also known. are shown in Fig. 7 The room temperature strength of ICM-fibres(see an example in Fig. 5)are characterised by two features [9, 10]: 2. 2. High temperature creep characteristics of ICM-fibres Creep tests of ICM-fibres are normally performed by 1. The strength/scale dependence is very strong, so the bending oxide-fibre/Mo-matrix specimens. The analysis Weibull exponent varies usually between 3 and 5. A rea- of the experimental data allows obtaining tensile creep son for it is certainly an existence of sufficiently rough characteristics of the fibres. The analysis [16] is based on defects, which are located mainly on the fibre surface. the following assumptions: Molybdenum carcass Oxide/molybdenum block oxide melt the carcass Fig. 1. Schematic of the internal crystallization method (ICM)
[7–9] and alumina–YAG-eutectics [7,8,10], YAG with the h100i orientation of the fibre axis [12] and single crystalline mullite [13,14] fibres have been produced. Crystallization of sapphire fibres [9] was conducted by using a seed to get fibres with the c-axis coinciding with the fibre axis; YAG and mullite fibres have been crystallised without using seed, so their axis coincides with the h001i and c-directions, respectively (see Fig. 4). 2.2.1. Strength of ICM-fibres Room temperature strength of the fibres was measured by bending a fibre over rigid cylinders of decreasing diameters and counting an average distance between fibre breaks on each step of the experiment [15]. This yields a dependence of ultimate strain on the fibre length provided the Young’s modulus of the fibre material is also known. The room temperature strength of ICM-fibres (see an example in Fig. 5) are characterised by two features [9,10]: 1. The strength/scale dependence is very strong, so the Weibull exponent varies usually between 3 and 5. A reason for it is certainly an existence of sufficiently rough defects, which are located mainly on the fibre surface. 2. Coating fibres with a thin layer of various materials yields healing of surface defects and an essential enhancement of the fibre strength. A CVD process was used to coat the fibres to ensure an intimate contact on the interface. The second feature is of an obvious importance in the present context. There is a clear dependence of the fibre strength on the coating thickness presented in Fig. 6. However, at the present time it is not clear as to the extent of the maximum fibre strength that can be obtained. A maximum can be expected as the coating layer also contains defects. High temperature strength of the fibres has been measured by testing oxide/molybdenum composites obtained by ICM in either tension [7,8] or bending. Some results are shown in Fig. 7. 2.2.2. High temperature creep characteristics of ICM-fibres Creep tests of ICM-fibres are normally performed by bending oxide–fibre/Mo–matrix specimens. The analysis of the experimental data allows obtaining tensile creep characteristics of the fibres. The analysis [16] is based on the following assumptions: Fig. 1. Schematic of the internal crystallization method (ICM). S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229 221
S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 Interface where nm, On, and n are constants. The value of n can be chosen arbitrary. In what follows, mn=10h, which means that on is the stress to cause 1% creep strain for 100 h. We call this value as creep resistance of a material on 100-h time base Matrix A solution of a creep problem for a beam under bending yields a dependence of the deflection rate, f, of the beam at its centre on applied load 0. For a beam of rectangula cross-section of height 2h, width b we have Fibre Q (2) in case of 3. 0.5 um b7|1q in case of 4-point bending. Here L and Li are the distances Fibre in=a, Mn=nanbh- and M is the bending moment Matrix The solution was obtained by neglecting a contribution of shear deformations to the displacement that can be essential in case of 3-point bending Creep characteristics of ICM-fibres tested up to now Fig.2.TEM images of the sapphire-fibre/molybdenum-matrix interface fibres are presented in Fig. 8. Here alo stalline mullite in an oxide/ molybdenum block [32]. for Nextel 720 fibre (a-Al2O3 mullite) evaluated from experimental data presented in Ref [17] is shown. A num- ber of important conclusions can be now drawn; here we emphasize just three points 1. In temperature interval from 1100 to 1600C, values of the creep resistance of single crystalline YAG and mull- ite as well as that of alumina-YAG-eutectic fibres obtained by using ICM are nearly the YAG fibre looks slightly better than the others. Still, their creep resistance can be certainly enhanced by crystallising them in the(111) direction 2. Surprisingly enough, single crystalline mullite fibres pro- duced by ICM do not seem to be superior to, say, YAG fibres. Their creep resistance differs essentially from the experimental value(Dokko et al. [18] 3. Polycrystalline oxide fibres, available at th Fig. 3. A view of the flat surface of a sapphire fibre; a replica of the time, obviously lose their creep resistance below a tem- molybdenum foil can be seen with a grain size of 10 um. perature of 1200C certainly due to an intrinsic behav lour of 1. A contribution of the matrix, which is fully recrystallised molybdenum, to the creep resistance of oxide/molybde t hum composites at temperatures above 1000.C is negli-3.Oxide-fibre/metal-matrix composites A composite is characterised by identical creep behav- Metal-matrix composites reinforced with ICM-fibres our under tension and compression are obtained via liquid phase route [19, 20]. Hence, the 3. The creep law of the material is fibre/ matrix interface strength depends on wettability of an oxide with a metal melt; the issue is analysed in details (1) in a review paper[21].Ti- and Ni-based alloys as matrices are of an immediate practical interest. They represent also
1. A contribution of the matrix, which is fully recrystallised molybdenum, to the creep resistance of oxide/molybdenum composites at temperatures above 1000 C is negligible, less than 10 MPa [7]. 2. A composite is characterised by identical creep behaviour under tension and compression. 3. The creep law of the material is e_ ¼ gn r rn n ð1Þ where gn, rn, and n are constants. The value of gn can be chosen arbitrary. In what follows, gn = 104 h1 , which means that rn is the stress to cause 1% creep strain for 100 h. We call this value as creep resistance of a material on 100-h time base. A solution of a creep problem for a beam under bending yields a dependence of the deflection rate, _ f , of the beam at its centre on applied load Q. For a beam of rectangular cross-section of height 2h, width b we have: _ f ¼ gn 1 23nþ2 nnðn þ 2Þ Q rnh2 n L b n L h L ð2Þ in case of 3-point bending and _ f ¼ vn M Mn n L2 1 8 ð3Þ in case of 4-point bending. Here L and L1 are the distances between periphery and internal supports, respectively, vn ¼ gn h , Mn ¼ 2n 2nþ1 rnbh2 and M is the bending moment. The solution was obtained by neglecting a contribution of shear deformations to the displacement that can be essential in case of 3-point bending. Creep characteristics of ICM-fibres tested up to now including preliminary data for single crystalline mullite fibres are presented in Fig. 8. Here also creep resistance for Nextel 720 fibre (a-Al2O3 + mullite) evaluated from experimental data presented in Ref. [17] is shown. A number of important conclusions can be now drawn; here we emphasize just three points: 1. In temperature interval from 1100 to 1600 C, values of the creep resistance of single crystalline YAG and mullite as well as that of alumina–YAG-eutectic fibres obtained by using ICM are nearly the same. YAG fibre looks slightly better than the others. Still, their creep resistance can be certainly enhanced by crystallising them in the h111i direction. 2. Surprisingly enough, single crystalline mullite fibres produced by ICM do not seem to be superior to, say, YAG fibres. Their creep resistance differs essentially from the experimental value (Dokko et al. [18]). 3. Polycrystalline oxide fibres, available at the present time, obviously lose their creep resistance below a temperature of 1200 C certainly due to an intrinsic behaviour of grain boundaries. 3. Oxide–fibre/metal–matrix composites Metal–matrix composites reinforced with ICM-fibres are obtained via liquid phase route [19,20]. Hence, the fibre/matrix interface strength depends on wettability of an oxide with a metal melt; the issue is analysed in details in a review paper [21]. Ti- and Ni-based alloys as matrices are of an immediate practical interest. They represent also Fig. 2. TEM images of the sapphire–fibre/molybdenum–matrix interface in an oxide/molybdenum block [32]. Fig. 3. A view of the flat surface of a sapphire fibre; a replica of the molybdenum foil can be seen with a grain size of 10 lm. 222 S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229
S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 Fig 4. Single crystalline YAG(a)and mullite fibres(b)obtained by using ICM 8000 吵∴! Coated 2zum∽gz 8。8 FIBRE LENGTH/ mm Fig. 5. Room temperature bending strength of sapphire fibres as extracted Temperature/12001400-1600 and coated with silicon carbide Fig. 7. The temperature dependence of the bending and tensile strength of sapphire and some eutectic fibres 5000 two cases of the oxide/ metal composites from the point of view of the interface strength. Results of the study of these two cases will be presented below, but we start with a brief outline of micromechanical creep models, which are neces- sary to interpret test results in an appropriate way 3.1. Creep models We consider composites with creeping matrix and ini- tially continuous fibres. A continuous fibre means that its length is much larger than a critical fibre length. Obviously, there can be observed at least four creep regimes of such composite [22]: Coating thickness/micron 1. E: fibres are elastic and non-breaking 2. Br-NCr: fibres are elastic and brittle thickness. Solid points stand for fibres of batch vo453 with pyrocarbon 3. Cr: fibres are creeping and non-breaking bating: open points are for fibres of batch vo86 with SiC-coating 4. Br-Cr: fibres are creeping and brittle
two cases of the oxide/metal composites from the point of view of the interface strength. Results of the study of these two cases will be presented below, but we start with a brief outline of micromechanical creep models, which are necessary to interpret test results in an appropriate way. 3.1. Creep models We consider composites with creeping matrix and initially continuous fibres. A continuous fibre means that its length is much larger than a critical fibre length. Obviously, there can be observed at least four creep regimes of such composite [22]: 1. E: fibres are elastic and non-breaking. 2. Br–NCr: fibres are elastic and brittle. 3. Cr: fibres are creeping and non-breaking. 4. Br–Cr: fibres are creeping and brittle. Fig. 5. Room temperature bending strength of sapphire fibres as extracted and coated with silicon carbide. Fig. 6. Average sapphire fibre strength on a length of 1 mm versus coating thickness. Solid points stand for fibres of batch V0453 with pyrocarbon coating; open points are for fibres of batch V086 with SiC-coating. Fig. 7. The temperature dependence of the bending and tensile strength of sapphire and some eutectic fibres. Fig. 4. Single crystalline YAG (a) and mullite fibres (b) obtained by using ICM. S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229 223