Materials Processing Technology ELSEVIER mal of Materials Processing Technology 175(2006)364-375 Ceramic and metal matrix composites: Routes and properties Polytechnic of Turin, Department of Material Science and Chemical Engineering, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy Abstract The paper presents a brief state of the art of advanced ceramics, metal matrix and ceramic matrix composites. The attention is focused or rocess technologies involved, applications and future of these"potential"materials. Some experimental results are included. The future of advanced materials is related to systems solutions, economical manufacturing processing, diverse markets and new technolo- gies. The new materials will provide the opportunity for growth to a new and healthier balance, with vibrant commercial sector delivering improved quality of life and stronger technology base. 2005 Elsevier B. v. All rights Keywords: Ceramics; Metals; Composites; Processes; Applications 1. Ceramics rently many new classes of materials have been devised to satisfy various new applications. Advanced ceramics offer With technological progress, natural materials numerous enhancements in performance, durability, reliabil insufficient to meet increasing demands on product ity, hardness, high mechanical strength at high temperature, ities and functions stiffness, low density, optical conductivity, electrical insula- There are many combinations of metallic and non-metallic tion and conductivity, thermal insulation and conductivity, oms that can combine to form ceramic components, and radiation resistance, and so on Ceramic technologies have Iso several structural arrangements are usually possible for been widely used for aircraft and aerospace applications, each combination of atoms. This led scientists to invent many wear-resistant parts, bioceramics, cutting tools, advanced new ceramic materials to meet increasing requirements and optics, superconductivity, nuclear reactors, etc demands in various application areas. Advanced furnaces and Ceramics application could be categorised as struc heat engines played important roles in the success of the tural ceramics, electrical ceramics, ceramic composites, and ndustrial revolution, while ceramic materials were essen- ceramic coatings. These materials are emerging as the lead- tial for thermal insulation of various types of furnaces and ing class of materials needed to be improved to explore engines. Electrically insulating ceramic materials were devel- further potential applications. An advanced ceramics applica ped as electrical and electronic technologies matured. As tion tree, which classifies its current and potential application igher and higher frequencies and voltages were used, the areas and related advantageous properties, has been devel demand on ceramic dielectrics became more stringent. Also, oped and is shown in Fig. 1. Current and future advanced new specifications for the magnetic and optical properties of ceramic products derived from the application tree are indi- ceramics were developed as a part of the new electronic and cated in Table 1. Today, advanced ceramics have been widely electro-optical technology revolution used in wearing parts, seals, low weight components and fuel The technology of ceramics is a rapidly developing applied cells in transportation sectors, to reduce the weight of prod science in todays world. Technological advances result from uct, increase performance especially at high temperatures, unexpected material discoveries On the other hand, the new prolong the life cycle of a product and improve the effi echnology can drive the development of new ceramics Cur- ciency of combustion. As advances in ceramic technology offer potential and immediate opportunities, these materials Tel:+390115644664;fax:+390115644664. will translate into greater market shares in transportation sec- E-mail address: mario. rosso@ polito.it tors. On the other hand, future application is still very limited 0924-0136/s-see front matter 2005 Elsevier B. v. All rights reserved doi:10.1016/ 1-Imatprotec.2005.04038
Journal of Materials Processing Technology 175 (2006) 364–375 Ceramic and metal matrix composites: Routes and properties M. Rosso ∗ Polytechnic of Turin, Department of Material Science and Chemical Engineering, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy Abstract The paper presents a brief state of the art of advanced ceramics, metal matrix and ceramic matrix composites. The attention is focused on process technologies involved, applications and future of these “potential” materials. Some experimental results are included. The future of advanced materials is related to systems solutions, economical manufacturing processing, diverse markets and new technologies. The new materials will provide the opportunity for growth to a new and healthier balance, with vibrant commercial sector delivering an improved quality of life and stronger technology base. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Metals; Composites; Processes; Applications 1. Ceramics With technological progress, natural materials become insufficient to meet increasing demands on product capabilities and functions. There are many combinations of metallic and non-metallic atoms that can combine to form ceramic components, and also several structural arrangements are usually possible for each combination of atoms. This led scientists to invent many new ceramic materials to meet increasing requirements and demands in various application areas. Advanced furnaces and heat engines played important roles in the success of the industrial revolution, while ceramic materials were essential for thermal insulation of various types of furnaces and engines. Electrically insulating ceramic materials were developed as electrical and electronic technologies matured. As higher and higher frequencies and voltages were used, the demand on ceramic dielectrics became more stringent. Also, new specifications for the magnetic and optical properties of ceramics were developed as a part of the new electronic and electro-optical technology revolution. The technology of ceramics is a rapidly developing applied science in today’s world. Technological advances result from unexpected material discoveries. On the other hand, the new technology can drive the development of new ceramics. Cur- ∗ Tel.: +39 011 564 4664; fax: +39 011 564 4664. E-mail address: mario.rosso@polito.it. rently many new classes of materials have been devised to satisfy various new applications. Advanced ceramics offer numerous enhancements in performance, durability, reliability, hardness, high mechanical strength at high temperature, stiffness, low density, optical conductivity, electrical insulation and conductivity, thermal insulation and conductivity, radiation resistance, and so on. Ceramic technologies have been widely used for aircraft and aerospace applications, wear-resistant parts, bioceramics, cutting tools, advanced optics, superconductivity, nuclear reactors, etc. Ceramics application could be categorised as structural ceramics, electrical ceramics, ceramic composites, and ceramic coatings. These materials are emerging as the leading class of materials needed to be improved to explore further potential applications. An advanced ceramics application tree, which classifies its current and potential application areas and related advantageous properties, has been developed and is shown in Fig. 1. Current and future advanced ceramic products derived from the application tree are indicated in Table 1. Today, advanced ceramics have been widely used in wearing parts, seals, low weight components and fuel cells in transportation sectors, to reduce the weight of product, increase performance especially at high temperatures, prolong the life cycle of a product and improve the effi- ciency of combustion. As advances in ceramic technology offer potential and immediate opportunities, these materials will translate into greater market shares in transportation sectors. On the other hand, future application is still very limited 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.038
4. Rosso/Journal of Materials Processing Technology 175 (2006)364-375 High perform eigh rate defence High temperature strength High performance/weigh rate uclear Refractoriness insulation Heat collection thermal Radiation resistance hermal conductivity High temperature strength Hight strength oil industry mechanical Near resistance Lubrication Biologic compatibility Catalysis absorption Corrosion resistance Electrical insulation Optical condensing Piezoelectric Fluorescence Megnetic Translucence Dielectric Optical conductivity raw materials fibers ice g. 1. Advanced ceramic application tree [1]. if no breakthroughs are achieved in fundamental and applied After cold compaction, part of cylinders were pre-sintered research 1 while others underwent directly secondary mechanical oper- ations: drilling, internal and external trimming, milling and 1.I. A study for new application surface finishing This splitting was carried out in order to investigate the In the frame of a research project for the development influence of the pre-sintering cycle on the machinability of of components to be used in critical environments, studies compacted on the possibility of obtaining ceramic nuts and rods were Examples of "green"final components are shown in Fig. 2. carried out by the Centre for Study and Development of Met Mechanically worked samples then underwent pre- llurgy and Materials for mechanics at Politecnico di Torino sintering and final sintering this set of processes was per in alessandria formed to verify the previously calculated shrinkage, neces- The process of compacting and sintering powders, tradi- sary to keep the final desired dimensions of threads and of tionally used in the field of P/M, was investigated with the aim of evaluating the behaviour of alumina-based powder mixes Following the same scheme, the former pre-sintered sam- Different prototype cylinders were obtained through uni- es underwent mechanical operations of drilling, trimming, axial compaction, using a 50t Komage press. Since high etc In this case different pre-sintering cycles had to be inves- components were pressed in cold dies, particular care was tigated to mediate between workability and fragility of com- taken to avoid the presence of density gradients, detrimental ponents. for the possibility of inducing differential shrinkages durin At the end, the length of the pre-sintering cycle(for both sintering, thus leading to deformations in the final compo- the cases, i. e samples directly machined and machined after nents. Other important considered aspects were the choice the thermal treatment) was of 74 h with a peak temperature of the appropriate compacting speed, to minimise air trap- of 1000C Sintering was then carried out at 1600C for Ih ping, and all what related to the extraction of the sample, in order to avoid the slip and stick phenomena which leads to A general good response was obtained at the end of the whole process in terms of shapes and dimensions, in the sense Different compacting pressures were investigated, but at that all the obtained shrinkages(approximately 20%)fol- the end, chosen values were in the range of 200 and 300 MPa. lowed a predictable equation in every area of the specimen
M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 365 Fig. 1. Advanced ceramic application tree [1]. if no breakthroughs are achieved in fundamental and applied research [1]. 1.1. A study for new application In the frame of a research project for the development of components to be used in critical environments, studies on the possibility of obtaining ceramic nuts and rods were carried out by the Centre for Study and Development of Metallurgy and Materials for mechanics at Politecnico di Torino in Alessandria. The process of compacting and sintering powders, traditionally used in the field of P/M, was investigated with the aim of evaluating the behaviour of alumina-based powder mixes. Different prototype cylinders were obtained through uniaxial compaction, using a 50 t Komage press. Since high components were pressed in cold dies, particular care was taken to avoid the presence of density gradients, detrimental for the possibility of inducing differential shrinkages during sintering, thus leading to deformations in the final components. Other important considered aspects were the choice of the appropriate compacting speed, to minimise air trapping, and all what related to the extraction of the sample, in order to avoid the slip and stick phenomena which leads to the lamination of the component. Different compacting pressures were investigated, but at the end, chosen values were in the range of 200 and 300 MPa. After cold compaction, part of cylinders were pre-sintered while others underwent directly secondary mechanical operations: drilling, internal and external trimming, milling and surface finishing. This splitting was carried out in order to investigate the influence of the pre-sintering cycle on the machinability of compacted powders. Examples of “green” final components are shown in Fig. 2. Mechanically worked samples then underwent presintering and final sintering; this set of processes was performed to verify the previously calculated shrinkage, necessary to keep the final desired dimensions of threads and of the nuts. Following the same scheme, the former pre-sintered samples underwent mechanical operations of drilling, trimming, etc. In this case different pre-sintering cycles had to be investigated to mediate between workability and fragility of components. At the end, the length of the pre-sintering cycle (for both the cases, i.e. samples directly machined and machined after the thermal treatment) was of 74 h with a peak temperature of 1000 ◦C. Sintering was then carried out at 1600 ◦C for 1 h in air. A general good response was obtained at the end of the whole process in terms of shapes and dimensions, in the sense that all the obtained shrinkages (approximately 20%) followed a predictable equation in every area of the specimen
M. Rosso/Journal of Materials Processing Technology 175(2006)364-375 Table 1 Current and future products for advanced ceramics [1] Defense industry Cutting tools and dies m and valve Heat engin Tank po Abrasives Power units Catalytic converters ubmarine shaft seals Precise instruments parts ow weight componen Dri vitrain components Molten metal filter Fuel cells Turbines Turbine engine comp Thermal protec Fixed boundary recuperators Ground support vehicles Low weight components for rolary Turbine engine components Fuel injection components Wearing parts Combustors Turbocharger rotors Military aircraft(airframe and engine Bearings Low heat rejection diesels Seals Seals Solid lubricants Structures Biological, Chemical processing Electrical, Magnetic Engineering Nuclear industry engineenng Artificial teeth, bones and joints Nuclear fuel Catalysts and igniters ating elements Nuclear fuel aladdin Hearts valve Varistor senso Control materials Heat exchanger grated circuit substrate Moderating material multilayer capacitors Reactor mining Recuperators Advanced multilayer integrated Refractories Oil industry Electric power generation Thermal Engineerin Bearings bearings Laser diode Electrode materials Flow control valve Ceramic gas turbines Heat sink for electronic parts High-temperature industrial fur Refinery heater Fuel cells:(solid oxide) Blast sleeves impmmpmpmgnmqlll 99569956 Fig. 2.(a-c)Ceramic nuts and rods obtained through machining in the green state, work developed at Politecnico di Torino. As for the mechanical properties, hardness evaluations tures, lower coefficients of thermal expansion and better wear were carried out. as well as non-standard tensile tests in order resistance. Because of these attributes metal matrix com- to verify the properties of the threads posites(MMCs)are under consideration for a wide range of applications. However, on the debit side, their toughness is inferior to monolithic metals and they are more expen- 2. Metal matrix composites sive at present. In comparison with most polymer matrix composites, MMCs have certain superior mechanical prop Metal matrix composites have many advantages over ties, namely higher transverse strength and stiffness, greater monolithic metals including a higher specific modulus, shear and compressive strengths and better high temperature higher specific strength, betterproperties at elevated tempera- capabilities. There also advantages in some of the physical
366 M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 Table 1 Current and future products for advanced ceramics [1] Mechanical engineering Aerospace Automotive Defense industry Cutting tools and dies Fuel system and valve Heat engines Tank power trains Abrasives Power units Catalytic converters Submarine shaft seals Precise instruments parts Low weight components Dri vetrain components Improved armors Molten metal filter Fuel cells Turbines Propulsion system Turbine engine components Thermal protection systems Fixed boundary recuperators Ground support vehicles Low weight components for rolary equipment Turbine engine components Fuel injection components Military weapon system Wearing parts Combustors Turbocharger rotors Military aircraft (airframe and engine) Bearings Bearings Low heat rejection diesels Wear-resistant precision bearings Seals Seals Waterpump seals – Solid lubricants Structures – – Biological, Chemical processing engineering Electrical, Magnetic Engineering Nuclear industry – Artificial teeth, bones and joints Memory elements Nuclear fuel – Catalysts and igniters Resistance heating elements Nuclear fuel cladding – Hearts valves Varistor sensor Control materials – Heat exchanger Integrated circuit substrate Moderating materials – Reformers Multilayer capacitors Reactor mining – Recuperators Advanced multilayer integrated packages – – Refractories – – – Nozzles – – – Oil industry Electric power generation Optical Engineering Thermal Engineering Bearings Bearings Laser diode Electrode materials Flow control valves Ceramic gas turbines Optical communication cable Heat sink for electronic parts Pumps High temperature components Heat resistant translucent porcelain High-temperature industrial furnace lining Refinery heater Fuel cells; (solid oxide) Light emitting; diode – Blast sleeves Filters – – Fig. 2. (a–c) Ceramic nuts and rods obtained through machining in the green state, work developed at Politecnico di Torino. As for the mechanical properties, hardness evaluations were carried out, as well as non-standard tensile tests in order to verify the properties of the threads. 2. Metal matrix composites Metal matrix composites have many advantages over monolithic metals including a higher specific modulus, higher specific strength, better properties at elevated temperatures, lower coefficients of thermal expansion and better wear resistance. Because of these attributes metal matrix composites (MMCs) are under consideration for a wide range of applications. However, on the debit side, their toughness is inferior to monolithic metals and they are more expensive at present. In comparison with most polymer matrix composites, MMCs have certain superior mechanical properties, namely higher transverse strength and stiffness, greater shear and compressive strengths and better high temperature capabilities. There also advantages in some of the physical
4. Rosso/Journal of Materials Processing Technology 175 (2006)364-375 attributes of MMCs such as no significant moisture absorp- categories:(1) liquid-phase processes, (2)solid-liquid pro- tion properties, non-inflammability, high electrical and ther- cesses, (3)two-phase(solid-liquid) processes, (4)deposition mal conductivities, and resistance to most radiations 2 techniques and (5)in situ processes MMCs, in general consist of at least two components: one is the metal matrix and second is the reinforcement. In all 2.1 Liquid phase processes cases the matrix is defined as a metal, but pure metal is rarely used: it is generally an alloy. Some classes of MMCs, like In liquid phase processes, the ceramic particulates are cermets, diamond tools and hard metals, have different and incorporated into a molten metallic matrix using various pro- extensive applications and, even if they can be considered as prietary techniques. This is followed by mixing and eventual traditional materials, they are in continuous evolution [3-6]. casting of the resulting composite mixture into shaped com- Metal matrix composites have been extensively studied ponents or billets for further fabrication. The process involve since many years, the primary support has come from the a careful selection of the ceramic reinforcement depending on rospace industry for airframe and spacecraft components. the matrix alloy. In addition to compatibility with the matrix, More recently, automotive, electronic and recreation indus- the selection criteria for a ceramic reinforcement include the tries have been working diffusively with composites [7] following factors:(1)elastic modulus, (2)tensile strength, MMC reinforcements can be generally divided into five (3)density, (4)melting temperature, (5)thermal stability, (6) major categories: continuous fibres, discontinuous fibres, size and shape of the reinforcing particle and (7)cost. Since whiskers, wires and particulate(including platelets). With most ceramic materials are not wetted by molten alloys, intro- exception of wires, which are metals, reinforcements are gen- duction and retention of the particulates necessitate e eally ceramics. Typically these ceramics are oxides, carbides adding wetting agents to the melt or coating the ceramic nd nitrides which are used because of their excellent com- ticulates prior to mixing. It is possible to individuate four binations of specific strength and stiffness at both ambient methods temperature and elevated temperature The two most commonly used metal matrices are based liquid metal ceramic particulate mixing on aluminium and titanium. Both of these metals have com- melt infiltration: paratively low specific gravities and are available in a vari- melt oxidation processing: ety of alloy forms. Although magnesium is even lighter, its squeeze casting or pressure infiltration great affinity for oxygen promotes atmospheric corrosion and makes it less suitable for many applications. Beryllium is the 2.2. Solid-phase processes lightest of all structural metals and has a tensile modulus higher than that of steel. However, it suffers from extreme Solid phase processes involve the fabrication of brittleness, which is the reason for its exclusion as a potential particulate-reinforced MMCs from blended elemental pow- natrix material. Nickel-and cobalt-based super alloys have ders involves a number of steps prior to final consolidation also been used as matrices, but the alloying elements in these These processes are currently used for cemented carbides and materials tend to accentuate the oxidation of fibres at elevated for diamond tools, however they have good potentiality also for other systems, for examples Al-based MMC[9]. Methods Aluminium and its alloys have the most attention as matrix that fall in this category are SiC. MMC engine applications are produced and used high energy, high ate for automobile engine cylinder aluminium-Al2 O3 material. The titanium alloys that are most useful in MM 2.3. Two-phase processes B alloys and metastable B alloys. These titanium alloys have higher tensile strength-to-weight ratios as well as better Two-phase processes involve the mixing of ceramic and strength retentions at 400-500C than those of aluminium matrix in a region of the phase diagram where the matrix ontains both solid and liquid phases. Two-phase methods Titanium MMCs are used in applications where perfor- are mance is demanded without regard to cost-effectiveness. This is where one obtains high-temperature performance unattain-. ospray dep able with conventional materials [8 compocasting/rheocasting Over the years a spectrum of processing techniques have variable codeposition of multiphase materials evolved in an attempt to optimise the microstructure and mechanical properties of MMCs. The processing methods 2. 4. Deposition techniques utilised to manufacture MMCs can be grouped according to the temperature of the metallic matrix during process- Deposition techniques for MMCs fabrication involve coat ing. Accordingly, the processes can be classified into five ing individual fibres in a tow with the matrix material needed
M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 367 attributes of MMCs such as no significant moisture absorption properties, non-inflammability, high electrical and thermal conductivities, and resistance to most radiations [2]. MMCs, in general consist of at least two components: one is the metal matrix and second is the reinforcement. In all cases the matrix is defined as a metal, but pure metal is rarely used: it is generally an alloy. Some classes of MMCs, like cermets, diamond tools and hard metals, have different and extensive applications and, even if they can be considered as traditional materials, they are in continuous evolution [3–6]. Metal matrix composites have been extensively studied since many years, the primary support has come from the aerospace industry for airframe and spacecraft components. More recently, automotive, electronic and recreation industries have been working diffusively with composites [7]. MMC reinforcements can be generally divided into five major categories: continuous fibres, discontinuous fibres, whiskers, wires and particulate (including platelets). With exception of wires, which are metals, reinforcements are generally ceramics. Typically these ceramics are oxides, carbides and nitrides which are used because of their excellent combinations of specific strength and stiffness at both ambient temperature and elevated temperature. The two most commonly used metal matrices are based on aluminium and titanium. Both of these metals have comparatively low specific gravities and are available in a variety of alloy forms. Although magnesium is even lighter, its great affinity for oxygen promotes atmospheric corrosion and makes it less suitable for many applications. Beryllium is the lightest of all structural metals and has a tensile modulus higher than that of steel. However, it suffers from extreme brittleness, which is the reason for its exclusion as a potential matrix material. Nickel- and cobalt-based super alloys have also been used as matrices, but the alloying elements in these materials tend to accentuate the oxidation of fibres at elevated temperatures. Aluminium and its alloys have the most attention as matrix material for MMCs and the most common reinforcement is SiC. MMC engine applications are produced and used for automobile engine cylinders die-cast from carbon fibrealuminium–Al2O3 material. The titanium alloys that are most useful in MMCs are , alloys and metastable alloys. These titanium alloys have higher tensile strength-to-weight ratios as well as better strength retentions at 400–500 ◦C than those of aluminium alloys. Titanium MMCs are used in applications where performance is demanded without regard to cost-effectiveness. This is where one obtains high-temperature performance unattainable with conventional materials [8]. Over the years a spectrum of processing techniques have evolved in an attempt to optimise the microstructure and mechanical properties of MMCs. The processing methods utilised to manufacture MMCs can be grouped according to the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into five categories: (1) liquid-phase processes, (2) solid–liquid processes, (3) two-phase (solid–liquid) processes, (4) deposition techniques and (5) in situ processes. 2.1. Liquid phase processes In liquid phase processes, the ceramic particulates are incorporated into a molten metallic matrix using various proprietary techniques. This is followed by mixing and eventual casting of the resulting composite mixture into shaped components or billets for further fabrication. The process involves a careful selection of the ceramic reinforcement depending on the matrix alloy. In addition to compatibility with the matrix, the selection criteria for a ceramic reinforcement include the following factors: (1) elastic modulus, (2) tensile strength, (3) density, (4) melting temperature, (5) thermal stability, (6) size and shape of the reinforcing particle and (7) cost. Since most ceramic materials are not wetted by molten alloys, introduction and retention of the particulates necessitate either adding wetting agents to the melt or coating the ceramic particulates prior to mixing. It is possible to individuate four methods: • liquid metal ceramic particulate mixing; • melt infiltration; • melt oxidation processing; • squeeze casting or pressure infiltration. 2.2. Solid-phase processes Solid phase processes involve the fabrication of particulate-reinforced MMCs from blended elemental powders involves a number of steps prior to final consolidation. These processes are currently used for cemented carbides and for diamond tools, however they have good potentiality also for other systems, for examples Al-based MMC [9]. Methods that fall in this category are: • powder metallurgy; • high energy, high rate process; • diffusion bonding. 2.3. Two-phase processes Two-phase processes involve the mixing of ceramic and matrix in a region of the phase diagram where the matrix contains both solid and liquid phases. Two-phase methods are: • ospray deposition; • compocasting/rheocasting; • variable codeposition of multiphase materials. 2.4. Deposition techniques Deposition techniques for MMCs fabrication involve coating individual fibres in a tow with the matrix material needed
M. Rosso/ Journal of Materials Processing Technology 175(2006)364-375 to form the composite, followed by diffusion bonding to form The MMCs fabrication procedures with matrix a consolidated composite plate or structural shape. Since the reinforcement best associations are shown in Table 3, composite is composed of identical units, the microstructure with distinction between continuous and discontinuous is more homogeneous than that of cast composites. Several reinforcement, while the process routes for the production deposition techniques are available of continuous fibre-reinforced MMCs are shown in fig 3. The wide choice between long and short fibers as wel Immersion platin as particulate reinforcement, Fig. 4, offers the possibility to · electroplating; esign the composite with the best properties as a function ●CvD of the application requirements. An example of microstruc ture of spray/wind Al-12Si reinforced composites is in Fig. 5 ·PVD; [10] and Fig. 6 [8] describes the main process routes for the spray forming techniques. production of discontinuous fibre, whiskers and particulate reinforced composites 2.5. In situ processes The microstructure and the properties of MMCs with con- tinuous reinforcement are quite homogeneous, in the case of In these techniques the reinforced phase is formed in situ. discontinuous reinforcement the homogeneity of reinforcing The composite material is produced in one step from an particle distribution within the matrix must be evaluated in appropriate starting alloy order to improve the mechanical properties. Moreover, for In Table 2 there is a comparison of different MMCs tech- applications demanding high toughness, a proper choice of niques. The route, the related cost, the pos and most metal matrix, both in terms of chemical composition and per- suitable applications, together with some comments, are here centage must be considered. In particular, a higher fracture presented. In particular the techniques are related to diffu- toughness can be obtained owing to a continuous and regu- sion processes, to powder metallurgy, to casting techniques lar ceramic network throughout the microstructure Contrary, as well as to spray processes distributing the same volume of ceramic particles in a metal Table 2 omparison of MMCs techniques [81 Diffusion bonding Used to make sheets, blades. vane shafts. structural Handles foils or sheets of matrix and filaments Powder metallurgy Mainly used to produce small objects(especially Both matrix and reinforcements used in pow- round),bolts, pistons, valves, high-strength and heat- der form; best for particulate reinforcement; since no melting is involved, no reaction zone develops, showing high-strength composite Liquid-metal infiltration Low/medium Used to produce structural shapes such as rods, tubes, Filaments of reinforcement used beams with maximum properties in a uniaxial direc- Widely used in automotive industry for producing dif- Generally applicable to an of rein. ferent components such as pistons, connecting rods, forcement and may be used for large scale rocker arms, cylinder heads; suitable for making com- ufacturing ex Spray casting Used to produce friction materials, electrical brushes Particulate reinforcement used: full-density and contacts, cutting and grinding tools material e produced Compocasting/rheocasting Low Widely used in automotive, aerospace, industrial Suitable for discontinuous fibres, especially equipment and sporting goods industries: used to particulate reinforcement anufacture bearing materials Table 3 MMCs fabrication procedures [8] Processing route Continuous reinforcement Discontinuous reinforcement Monofilament Multifilament Staple fibre Whiskers articulate Squeeze infiltrate preform √ Stir mixing and casting Powder premix and extrude Slurry coat and hot pressin Interleave and diffusion bonding x: not practicable; (V): not common; V: current practice
368 M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 to form the composite, followed by diffusion bonding to form a consolidated composite plate or structural shape. Since the composite is composed of identical units, the microstructure is more homogeneous than that of cast composites. Several deposition techniques are available: • immersion plating; • electroplating; • spray deposition; • CVD; • PVD; • spray forming techniques. 2.5. In situ processes In these techniques the reinforced phase is formed in situ. The composite material is produced in one step from an appropriate starting alloy. In Table 2 there is a comparison of different MMCs techniques. The route, the related cost, the possible and most suitable applications, together with some comments, are here presented. In particular the techniques are related to diffusion processes, to powder metallurgy, to casting techniques, as well as to spray processes. The MMCs fabrication procedures with matrixreinforcement best associations are shown in Table 3, with distinction between continuous and discontinuous reinforcement, while the process routes for the production of continuous fibre-reinforced MMCs are shown in Fig. 3. The wide choice between long and short fibers, as well as particulate reinforcement, Fig. 4, offers the possibility to design the composite with the best properties as a function of the application requirements. An example of microstructure of spray/wind Al-12Si reinforced composites is in Fig. 5 [10] and Fig. 6 [8] describes the main process routes for the production of discontinuous fibre, whiskers and particulatereinforced composites. The microstructure and the properties of MMCs with continuous reinforcement are quite homogeneous, in the case of discontinuous reinforcement the homogeneity of reinforcing particle distribution within the matrix must be evaluated in order to improve the mechanical properties. Moreover, for applications demanding high toughness, a proper choice of metal matrix, both in terms of chemical composition and percentage must be considered. In particular, a higher fracture toughness can be obtained owing to a continuous and regular ceramic network throughout the microstructure. Contrary, distributing the same volume of ceramic particles in a metal Table 2 Comparison of MMCs techniques [8] Route Cost Application Comments Diffusion bonding High Used to make sheets, blades, vane shafts, structural components Handles foils or sheets of matrix and filaments of reinforcing element Powder metallurgy Medium Mainly used to produce small objects (especially round), bolts, pistons, valves, high-strength and heatresistant materials Both matrix and reinforcements used in powder form; best for particulate reinforcement; since no melting is involved, no reaction zone develops, showing high-strength composite Liquid–metal infiltration Low/medium Used to produce structural shapes such as rods, tubes, beams with maximum properties in a uniaxial direction Filaments of reinforcement used Squeeze casting Medium Widely used in automotive industry for producing different components such as pistons, connecting rods, rocker arms, cylinder heads; suitable for making complex objects Generally applicable to any type of reinforcement and may be used for large scale manufacturing Spray casting Medium Used to produce friction materials, electrical brushes and contacts, cutting and grinding tools Particulate reinforcement used; full-density materials can be produced Compocasting/rheocasting Low Widely used in automotive, aerospace, industrial equipment and sporting goods industries; used to manufacture bearing materials Suitable for discontinuous fibres, especially particulate reinforcement Table 3 MMCs fabrication procedures [8] Processing route Continuous reinforcement Discontinuous reinforcement Monofilament Multifilament Staple fibre Whiskers Particulate Squeeze infiltrate preform (√) √ √√ ( √) Spray coat or codeposit √ √ × × √ Stir mixing and casting × × ( √) (√) √ Powder premix and extrude × × √ √√ Slurry coat and hot pressing (√) √ × ×× Intercleave and diffusion bonding √ × × ×× ×: not practicable; (√): not common; √: current practice
4. Rosso/Journal of Materials Processing Technology 175 (2006)364-375 Monofilament fibres alloy sheets with binder Infiltration by liquid metal Sheet Multifilament fibres filtration by powder meta Diffusion bonding Fibre preform with binder deposition as liquid (arc vapour(CVD, PVD) uperplastic forming Braze bonding Near net Fig 3. Continuous fibre-reinforced MMCs process routes [8] matrix rather frequently promote the formation of reinforce- coatings applied on cast iron discs, as well as on Al-based ment clusters. These are weak zones that usually constitute MMCs [15,16] fracture initiation sites l1 con rying to reduce this drawback, interpenetrating phase 2.6. Study of different ceramic coatings on cast iron and posites or co-continuous ceramic composites, called C4, MMCs disk brakes were developed [12, 13]. The production of this materials con- cerns with infiltration process of silica perform with molten This work was an estimation of the possible automotive aluminium alloy without the application of external pressure. employment of a braking system with brake disks ceramic The low production cost and the near net shape capability of coated coupled with metal/ceramic-based friction materials C4 materials are further advantages of co-continuous com- The aim was to improve braking system performance, main- posites. The mechanical properties of these materials are very taining low production costs atisfying and their combination with thermal and electrical In literature and already on the market there are brakin conductivity characteristics and with good tribological prop- disks entirely made of ceramic materials(excellent perfor- erties allows C4 composites very promising for applications mances)but their prices are only competitive for special such as automotive disc brake rotors and callipers. To develop production and not for mass production. Therefore the idea this application a project is under way in the frame of the was to start from the same base concept(the employing of bilateral cooperation between Italy and Slovenia[14] ceramic materials)but obtain a good performant, costs com- There is a strong demand for the development of disc brake petitive solution. with improved resistance and properties and good result have The tested brake disks come from line production com- been recently obtained at Politecnico di Torino using ceramic ponents and are employed on medium-high car sector and ig. 4. Long fibers, short fibers and paniculate reinforcement
M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 369 Fig. 3. Continuous fibre-reinforced MMCs process routes [8]. matrix rather frequently promote the formation of reinforcement clusters. These are weak zones that usually constitute fracture initiation sites [11]. Trying to reduce this drawback, interpenetrating phase composites or co-continuous ceramic composites, called C4, were developed [12,13]. The production of this materials concerns with infiltration process of silica perform with molten aluminium alloy without the application of external pressure. The low production cost and the near net shape capability of C4 materials are further advantages of co-continuous composites. The mechanical properties of these materials are very satisfying and their combination with thermal and electrical conductivity characteristics and with good tribological properties allows C4 composites very promising for applications such as automotive disc brake rotors and callipers. To develop this application a project is under way in the frame of the bilateral cooperation between Italy and Slovenia [14]. There is a strong demand for the development of disc brake with improved resistance and properties and good result have been recently obtained at Politecnico di Torino using ceramic coatings applied on cast iron discs, as well as on Al-based MMCs [15,16]. 2.6. Study of different ceramic coatings on cast iron and MMCs disk brakes This work was an estimation of the possible automotive employment of a braking system with brake disks ceramic coated coupled with metal/ceramic-based friction materials. The aim was to improve braking system performance, maintaining low production costs. In literature and already on the market there are braking disks entirely made of ceramic materials (excellent performances) but their prices are only competitive for special car production and not for mass production. Therefore the idea was to start from the same base concept (the employing of ceramic materials) but obtain a good performant, costs competitive solution. The tested brake disks come from line production components and are employed on medium-high car sector and Fig. 4. Long fibers, short fibers and paticulate reinforcement
M. Rosso/ Journal of Materials Processing Technology 175(2006)364-375 Fig. 5. Spray/wind Al-12Si composites reinforced with(a) stainless wire and(c) Sigma 1140+SiC.(b and d) Interface region of SS/Al-Si and SiC/Al-Si respectively [101 port cars. In Fig. 7 are shown the main aspects attained high melting point materials such as the previously indicated ceramIc systems. The disks were plasma sprayed with different types Ceramic coatings were tested with a thermal fatigue lachine to simulate brakes work condit ZrO2/Y2O3. This coating technique is the best one with coating adhesion Coated samples were also tested with the ulate or whiskers Short fibres and/or whiskers Squeeze casting Infiltration by molten metal Preform with binder o-spraying Molten metal Particulate or whiskers Infiltration by molten metal
370 M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 Fig. 5. Spray/wind Al-12Si composites reinforced with (a) stainless wire and (c) Sigma 1140 + SiC. (b and d) Interface region of SS/Al-Si and SiC/Al-Si, respectively [10]. from sport cars. In Fig. 7 are shown the main aspects attained during the research. The disks were plasma sprayed with different types of materials: Al2O3/TiO2 of two granulometry range, ZrO2/Y2O3. This coating technique is the best one with high melting point materials such as the previously indicated ceramic systems. Ceramic coatings were tested with a thermal fatigue machine to simulate brakes work conditions and evaluate the coating adhesion. Coated samples were also tested with the Fig. 6. Discontinuous fibre, whiskers and particulate-reinforced composites process routes [8]
4. Rosso/Journal of Materials Processing Technology 175 (2006)364-375 friction assessment and screening test(FAST) machine to ner, which contrast with the failure of the continuous fibre evaluate the friction coefficient. These tests took about one composite where a substantial load carrying capacity is main- hundred of minutes and the results were plotted as friction tained after failure has commenced. therefore not only has the coefficient versus time. Finally, coated discs were tested on continuous fibre composite a better toughness but the failure the bench test apparatus and tested against a suitable pads, mode is more desirable. However, fibres are a more expensive to check their behaviour, resistance and life. The test results reinforcement than particles and the processing is more com- very satisfying, the photo A and B in Fig. 7 show very impor- plex, therefore the improvement in toughness is associated tant wear differences between the MMC coated disc (A)and with an extra cost burden the uncoated one(B). Both the coating materials given good Ceramic matrix composite (CMC) devel results without significant differences about performances lagged behind other composites for two main and they appear to be very profitable also for the MMC discs. more of the processing routes for CMCs involve high tem- Particulate-reinforced 3. Ceramic matrix composites Monolithic ceramics have reasonably high strength stiffness but are brittle. Thus one of the main objectives Fibre-reinforced in producing ceramic matrix composites is to increase the toughness. Naturally it is also hoped, and indeed often found that there is a concomitant in strength and stiffness. Fig 8 compares typical stress-strain curves for composites with that for a monolithic ceramic. the area under the stress-strain Monolithic is the energy of fracture of the sample and is a mea- sure of the toughness. It is clear from this figure that the reinforcement with particulates and continuous fibres has lead to an increase in toughness but that the increase is more significant for the latter. Both the monolithic and the Fig 8 Schematic force-displacement curves for a monolithic ceramic and particulate-reinforced composite fail in a catastrophic man- CMCs illustrating the greater energy of fracture of the CMCs [21
M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 371 Fig. 7. Brake components and coated discs; view of a ceramic coating layer. The surface appearance the coated disc (A) and of the MMC disc after the wear test at the bench. friction assessment and screening test (FAST) machine to evaluate the friction coefficient. These tests took about one hundred of minutes and the results were plotted as friction coefficient versus time. Finally, coated discs were tested on the bench test apparatus and tested against a suitable pads, to check their behaviour, resistance and life. The test results very satisfying, the photo A and B in Fig. 7 show very important wear differences between the MMC coated disc (A) and the uncoated one (B). Both the coating materials given good results without significant differences about performances and they appear to be very profitable also for the MMC discs. 3. Ceramic matrix composites Monolithic ceramics have reasonably high strength and stiffness but are brittle. Thus one of the main objectives in producing ceramic matrix composites is to increase the toughness. Naturally it is also hoped, and indeed often found, that there is a concomitant in strength and stiffness. Fig. 8 compares typical stress–strain curves for composites with that for a monolithic ceramic; the area under the stress–strain curve is the energy of fracture of the sample and is a measure of the toughness. It is clear from this figure that the reinforcement with particulates and continuous fibres has lead to an increase in toughness but that the increase is more significant for the latter. Both the monolithic and the particulate-reinforced composite fail in a catastrophic manner, which contrast with the failure of the continuous fibre composite where a substantial load carrying capacity is maintained after failure has commenced. therefore not only has the continuous fibre composite a better toughness but the failure mode is more desirable. However, fibres are a more expensive reinforcement than particles and the processing is more complex, therefore the improvement in toughness is associated with an extra cost burden. Ceramic matrix composite (CMC) development has lagged behind other composites for two main reasons. First more of the processing routes for CMCs involve high temFig. 8. Schematic force–displacement curves for a monolithic ceramic and CMCs illustrating the greater energy of fracture of the CMCs [2]
M. Rosso/ Journal of Materials Processing Technology 175(2006)364-375 peratures and can only be employed with high temperature Table 4 reinforcements. It follows that it was not until fibres and for CMCs [3 whiskers of high temperature ceramics, such as silicon car- bide, were readily available was there much interest in CMCs. Chemical va infiltration Carbides. nitride carbon. oxides. borides The high temperature properties of the reinforcement are also Viscous phase hot pressing of importance during service. A major attribute of monolithic (2D performs) ceramics is that they maintain their properties to high temper (2D,3D Oxides atures and this characteristic is only retained in CMCs if the reinforcements also have good high temperature properties -eye Polymer precursor route (3D SiC, SiNy, Si, CyNz Hence, there is only limited interest in toughening ceram- Liquid metal infiltration Si→SiC ics by incorporation of reinforcements of materials, such as Gas-metal reaction Oxide(Al, nitrides [Al, Zn, TiD ductile metals, that lose their strength and stiffness at inter- Solid-state hot pressing SiC, Si3 Na Prepreg curing and pyrolysis SiC, Si3N4 mediate temperatures Hot pressing(2D preforms) Oxides The second factor that has hindered the progress of CMCs is also concerned with the high temperatures usu- ly employed for production. Differences in coefficients of thermal expansion, a, between the matrix and the reinforce- Ceramic fibres such as SiC and Si3N4 use polysilane ment lead to thermal stresses on cooling from the process base material. CMCs, in which ceramic or glass matrices are ing temperature. However, whereas the thermal stresses can reinforced with continuous fibres, chopped fibres, whiskers, generally be relieved in metal matrix composites by plastic platelets or particulates, are emerging as a class of advanced deformation of the matrix, this is not possible for CMCs and engineering structural materials. They currently have limited cracking of the matrix can result. The nature of the cracking high-temperature applications but a large potential for much depends on the whether the reinforcement contracts more or wider use in military, aerospace and commercial applications less than the matrix on cooling as their determines the char- such as energy-efficient systems and transportation acter(tensile or compressive)of the local thermal stresses. If There are also other specialty CMCs such as nanocom- aR for a particulate reinforcement is great than that for the posites(made from reactive powders)and electroceramics matrix aM then the circumferential cracks may be produced CMCs are unique in that they combine low density with high in the matrix, and for aR aM the axial tensile ceramics)and strength retention at high temperatures. Many stresses induced in the fibres produce an overall net residual have good corrosion and erosion characteristics for high tem- compressive stresses in the matrix and, as the fibres contract, perature applications. CMcs have been used in jet fighter there is a tendency for them to pull away from the matrix. The Industrial uses of CMCs include furnace materials, energy stress situation is reversed when aR <aM and cracking of the conversion systems, gas turbines and heat engines matrix due to the axial tensile stresses may occur. Clearly rocessing methods can be broken down into two broad there has to be some matching of the coefficients of therm groups: powder consolidation and chemically based method expansion in order to limit these problems The latter class consists of wmm如mm Fig 9. Photomicrographs of glass/spinel composites [17]
372 M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 peratures and can only be employed with high temperature reinforcements. It follows that it was not until fibres and whiskers of high temperature ceramics, such as silicon carbide, were readily available was there much interest in CMCs. The high temperature properties of the reinforcement are also of importance during service. A major attribute of monolithic ceramics is that they maintain their properties to high temperatures and this characteristic is only retained in CMCs if the reinforcements also have good high temperature properties. Hence, there is only limited interest in toughening ceramics by incorporation of reinforcements of materials, such as ductile metals, that lose their strength and stiffness at intermediate temperatures. The second factor that has hindered the progress of CMCs is also concerned with the high temperatures usually employed for production. Differences in coefficients of thermal expansion, , between the matrix and the reinforcement lead to thermal stresses on cooling from the processing temperature. However, whereas the thermal stresses can generally be relieved in metal matrix composites by plastic deformation of the matrix, this is not possible for CMCs and cracking of the matrix can result. The nature of the cracking depends on the whether the reinforcement contracts more or less than the matrix on cooling as their determines the character (tensile or compressive) of the local thermal stresses. If R for a particulate reinforcement is great than that for the matrix M then the circumferential cracks may be produced in the matrix, and for R M the axial tensile stresses induced in the fibres produce an overall net residual compressive stresses in the matrix and, as the fibres contract, there is a tendency for them to pull away from the matrix. The stress situation is reversed when R < M and cracking of the matrix due to the axial tensile stresses may occur. Clearly there has to be some matching of the coefficients of thermal expansion in order to limit these problems. Table 4 Main processing routes for CMCs [3] Processing route Matrices Chemical vapour infiltration Carbides, nitride carbon, oxides, borides Viscous phase hot pressing (2D performs) Glasses, ceramic-glasses Sol–gel route (2D, 3D performs) Oxides Polymer precursor route (3D performs) SiC, SixNy, SixCyNz Liquid metal infiltration Si→SiC Gas–metal reaction Oxide (Al, nitrides [Al, Zn, Ti]) Solid-state hot pressing SiC, Si3N4 Prepreg curing and pyrolysis SiC, Si3N4 Hot pressing (2D preforms) Oxides Ceramic fibres such as SiC and Si3N4 use polysilane as the base material. CMCs, in which ceramic or glass matrices are reinforced with continuous fibres, chopped fibres, whiskers, platelets or particulates, are emerging as a class of advanced engineering structural materials. They currently have limited high-temperature applications but a large potential for much wider use in military, aerospace and commercial applications such as energy-efficient systems and transportation. There are also other specialty CMCs such as nanocomposites (made from reactive powders) and electroceramics. CMCs are unique in that they combine low density with high modulus, strength and toughness (contrasted with monolithic ceramics) and strength retention at high temperatures. Many have good corrosion and erosion characteristics for high temperature applications. CMCs have been used in jet fighters. Industrial uses of CMCs include furnace materials, energy conversion systems, gas turbines and heat engines. Processing methods can be broken down into two broad groups: powder consolidation and chemically based methods. The latter class consists of: Fig. 9. Photomicrographs of glass/spinel composites [17]
4. Rosso/Journal of Materials Processing Technology 175(2006)364-375 373 Table 5 Some processes for continuous fibre-reinforced CMCs [8 metl Disadvantages Fibre range(°Om I Slurry infiltration (a)Glass ceramic matrix Commercially developed Limited max temperature due to Glass-ceramic 800-1000 Good mechanical properties Needs to be hot pressed, expensive Nicalon Glass-ceramic 800-1000 ormations of complex shapes difficult (b) Ceramic matrix LS Potentially inexpensive Shrinkage during sintering cracks Alumina 800-1400 matrIx Could produce complex shapes Temperature limit due to glassy phase Sic 800-1600 SiaN 8001500 2. " Cement bonded Inexpensive Relatively poor properties to date 400-1400 Ability to produce large complex Low temperature processing New"fibres 3. Reaction bonded Good mechanical properties Has required hotpressing of Si SiaN 800-1500 matr powder in silicon nitride system prior to reaction bonding Pressureless densification New fibres sic 800-1600 ll Sol-gel and polymer processing Good matrix composition control Low yields Nonoxide 800-1200 Easy to infiltrate fibres Very large shrinkage Alumina 800-1400 Lower densification temperature Woul require multiple Silicates infiltration/densification steps promising results reported IIL Melt infiltration Potentially inex High melting temperatures would Graphite Alumina 800-1100 age fibres (a) Ceramic melt Should be easy to infiltrate fibres Nicalon Oxides 800-1100 Lower shrinkage on solidification "New " fibres (b) Metal melt, follo Potentially inexpensive Difficult to control chemistry and Graphite 800-1200 roduce all ceramic system Cermet type material Difticult to envision in use for large. Nicalon Alumina 800-1200 complex parts for aerospace IV Chemical vapour infiltration v"fibres BC Has been commercially Slow and expensive developed Best mechanical properties Requires iterative process Nextel 800-1800 (a) General approach Considerable flexibility in fibres Never achieved full density and matrices High quality matrix, very pure Capital intensive Nitrides Little fibre damage Oxides In situ fibre surface treatment Borides (b)Lanxide Slow reaction and growth kinetics Graphite Alumina 800-1200 shapes Properties dominated by cerami rocessing time& high temp. Nicalon 800-1200 hemistry Very pore grain boundaries d reaction are limitations 800-1200 Systems include: AIN/AL, TINTi, 800-1200 ArNEr [1] Temperature limit depends on fibre. Currently all systems are limited to A 1200 C available fibres
M. Rosso / Journal of Materials Processing Technology 175 (2006) 364–375 373 Table 5 Some processes for continuous fibre-reinforced CMCs [8] Processing method Advantages Disadvantages Fibre Matrix Temperature range (◦C) [1] I. Slurry infiltration (a) Glass ceramic matrix Commercially developed Limited max. temperature due to matrix Graphite Glass-ceramic 800–1000 Good mechanical properties Needs to be hot pressed, expensive Nicalon Glass-ceramic 800–1000 Formations of complex shapes is difficult (b) Ceramic matrix 1. Sintered matrix Potentially inexpensive Shrinkage during sintering cracks matrix Alumina 800–1400 Could produce complex shapes Temperature limit due to glassy phase SiC 800–1600 Si3N4 800–1500 2. “Cement bonded matrix” Inexpensive Relatively poor properties to date Graphite Cements 400–1400 Ability to produce large complex shapes Nicalon Low temperature processing “New” fibres 3. Reaction bonded matrix Good mechanical properties Has required hotpressing of Si powder in silicon nitride system prior to reaction bonding Nicalon Si3N4 800–1500 Pressureless densification Simple shapes only “New” fibres SiC 800–1600 II. Sol–gel and polymer processing Good matrix composition control Low yields Nicalon Nonoxide 800–1200 Easy to infiltrate fibres Very large shrinkage – Alumina 800–1400 Lower densification temperature Woul require multiple infiltration/densification steps – Silicates No promising results reported III. Melt infiltration Potentially inexpensive High melting temperatures would damage fibres Graphite Alumina 800–1100 (a) Ceramic melt Should be easy to infiltrate fibres Nicalon Oxides 800–1100 Lower shrinkage on solidification “New” fibres (b) Metal melt, followed by oxidation Potentially inexpensive Difficult to control chemistry and produce all ceramic system Graphite 800–1200 Cermet type material Difficult to envision in use for large, complex parts for aerospace applications Nicalon Alumina 800–1200 IV. Chemical vapour infiltration – – – “New” fibres B4C 800–1200 – Has been commercially developed Slow and expensive Nicalon SiC 800–1600 – Best mechanical properties Requires iterative process Nextels SiC 800–1800 (a) General approach Considerable flexibility in fibres and matrices Never achieved full density – HfC – – High quality matrix, very pure Capital intensive – Nitrides – – Little fibre damage – – Oxides – – In situ fibre surface treatment – – Borides – – Ability to fill small pores – – – – (b) Lanxide Ability to produce complex shapes Slow reaction and growth kinetics Graphite Alumina 800–1200 – Properties dominated by ceramic Long processing time & high temp. limits chemistry Nicalon AlN 800–1200 – Very pore grain boundaries Wetting and reaction are limitations – TiN 800–1200 – Systems include: AlN/Al, TiN/Ti, ZrN/Zr – – ZrN 800–1200 [1] Temperature limit depends on fibre. Currently all systems are limited to ≈ 1200 ◦C available fibres
