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 sapphirenumber 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 tough￾ness to a composite structure. Hence, the microstructure and properties of the oxide–fibre composites become pri￾mary problems in the whole technology of future heat￾resistant 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. Kaz￾min; 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 crystal￾line oxides or those with typical eutectic microstructure is to crystallise oxide melts. The following methods of crys￾tallising 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 formu￾lated by Stepanov before the WWII [1]. Stepanov intro￾duces a shaper to pre-determine a shape and size of the capillary column at the top of which the liquid/solid inter￾face arises, although the authors were hardly aware about Stepanov’s ideas published in Russian. EFG method was used for the first time to produce sap￾phire 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 EFG￾scheme, which simplified slightly growth procedures. How￾ever, 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 heat￾ing 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]). Never￾theless, 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 molyb￾denum carcass with continuous channels in it, which is eas￾ily 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 orien￾tation 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