Metal infiltre edilmiş mikro poroz karbon kompozitlerin aşınma ve sürtünme davranışının karakterizasyonu
Başlık çevirisi mevcut değil.
- Tez No: 66407
- Danışmanlar: PROF. DR. ADNAN TEKİN
- Tez Türü: Doktora
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Metalurji Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 112
Özet
ÖZET Değişik oranlarda grafit içeren amorf karbon preformlar içersine (%100 amorf karbon, %80 amorf karbon-%20 grafit, %60 amorf karbon-%40 grafit) Cu-6Si-0.9Cr alaşımı basınçlı iufiltrasyonla emdirilerek metal matriksli seramik kompozit üretimi gerçekleştirilmiştir. Bu kompozitler metalürji sektöründe aşınma ve sürtünmenin önemli olduğu uygulamalarda, ısıl kontrol malzemelerinin üretiminde geniş kullanım alam bulmaktadır. Balar karbon asaslı metal matriksli kompozitlerde karbon daha çok grafit olarak kullanılmaktaydı. Diğer yandan amorf karbonun çok daha sert olması (8 Mohs) grafit-amorf karbon kombinasyonuyla üretilen yapıların aşınma ve sürtünme ömrünün daha iyi olacağı fikrini gündeme getirmiştir. Konvensiyonel olarak kullanılan karbon partiküllerinin yerine mikroyapıda homojenliği sağlamak amacıyla preformlar kullanılmıştır. Yapılan deneysel çalışmalar sıvı metal karbon preform arasındaki reaksiyonun karakterizasyonunu, üretilen kompozitlerin dökme demire ve kendisine olan aşınma ve sürtünme davranışının oda sıcaklığı, normal atmosfer şartlarında kuru kayma şartlan için belirlenmesini içermektedir. Sıvı metalin kalıba dolması esnasında preformun dış yüzeyinde gerçekleşen krom karbür oluşumu karbon ve bakır alaşımının birbirlerine karşı olan yüzey enerjilerini azaltıp ıslatmayı sağlamakta ve infiltrasyon prosesisine katkıda bulunmaktadır. Kompozitin iç bölgesinde ise bakır ve silisyumun homojen bir dağılım gösterdiği anlaşılmıştır. Kompozitin aşınma ve sürtünme davranışı mikroyapı bileşenlerinin (metalik faz ve karbon adacıkları) boyutlarından bilinci derece etkilenmektedir. Preforma grafit ilavesi yapıdaki por büyüklüğünü arttırmakta, dolayısıyla porlara giren metal miktarıda artmaktadır. Grafit içermeyen %100 amorf karbon esaslı preformdan üretilen I no lu kompozit için mİkro yapı bileşenlerinin grafit içeren kompozitlere oranla çok daha küçük olması dökme demire karşı aşınma esnasında sürtünen yüzeyler arasında bir karbon film oluşmasına neden olmaktadır. İki yüzey arasındaki sürtünme bu film üzerinde gerçekleşmektedir. Bütün yük koşullan içinde sürtünmenin genel karakteri abrasif aşınma şeklinde gözlenmiştir. Bu kompozitlerin aşınma direncinin ana alaşıma oranla on kat daha yüksek olduğu anlaşılmıştır, Diğer yandan grafit içeren kompozitler mikroyapı bileşenlerinin (metal ve karbon adacıklan) boyutlarının büyük olması nedeniyle farklı davranış göstermişlerdir. Düşük gerilme şartlarında amorf karbon içeren kompozitler gibi davranmaktadırlar. Fakat sürtünen yüzeyler arasında oluşan karbon film artan gerilme ile bozulduğu için yüzeyler arasında başlayan metal metal etkileşimini adhesif (yapışmah) aşınma şeklini sistemde hakim mekanizma haline getirmektedir. Kompozitin kendisi üzerindeki aşınma ve sürtünme davranışı da benzer sonuçlar gösterilmiştir. Grafitli ve grafitsiz kompozitlerden uygulanan normal gerilmeye bağlı olarak eşdeğer aşınma hızlan elde edilmiştir. Grafit ilavesi düşük gerilme değerlerinde aşınma ve sürtünme ömrü üzerinde bir fayda sağlamasına rağmen esas olan yüksek gerilme şartlan için bir fayda sağlamamaktadır. Kompozitin tribolojik davranışı mikroyapı bileşenlerinin boyutlarından birinci derecede etkilenmektedir. XI
Özet (Çeviri)
SUMMARY WEAR AND FRICTION BEHAVIOR OF METAL IMPREGNATED MICROPOROUS CARBON COMPOSITES Over the past decade and a half there has been strong expansion in the area of research, development and manufacturing of composite materials. The main emphasis has been to achieve high strength-to-weight ratio. The metal-matrix ceramic composite market has indicated a combined automotive and aerospace growth rate of 28% per year. This indicates a growth in dollar value from $37 million in 1988 to greater than $436 million in 1999. Since this healthy growth rate would be accompanied by an expansion of applications the interest in metal matrix composites has intensified. Composite materials are made of two or more dissimilar constituents, which are combined on macroscopic scale to form a new material, and bring out the best features of each constituent. Some properties which can thus be improved are: strength, strength to weight ratio, stiflhess, fatigue life, wear resistance, corrosion resistance, elevated temperature mechanical properties, and electrical and thermal conductivity. For use in tribological applications, the metal-matrix composites must be able to support a load without undue distortion, deformation or fracture during service life, and to maintain controlled friction and wear over long periods without seizure under working conditions. Composite materials consist of a bulk material known as the matrix phase, and a reinforcing phase, such as fibers, whiskers or particulates. The nomenclature of composites depends on the type of matrix material: plastic, metals or ceramics. If the bulk phase is made of metal the composite would be a Metal Matrix Composite. The reinforcing phase, usually harder than the matrix phase, are made of boron, silicon carbide, alumina, graphite, carbon and other refractory materials. Aluminum, copper, titanium, magnesium and other alloy matrices have been reinforced with particulates, whiskers or continues fibre of carbon, silicon carbide or alumina, boron, tungsten and various other ceramic. While solid state diffusion bonding techniques were commonly used earlier for fabrication the trend in composites processing in the eighties has been in casting techniques such as stir casting, compo casting and various form of infiltration techniques. Solid state and liquid state processing are used to produce metal-matrix composites. Both powder metallurgy and diffusion bonding process are in the framework of solid state method. Since the processing temperatures are relatively Xlllow for solid-state processing they are useful for reactive systems where exposure to high temperature can degrade the reinforcing phase. Liquid state methods include liquid metal infiltration (pressure, pressureless or vacuum), squeeze casting, compocasting, thixomat, centrifugal casting and stir casting. Liquid state processing are preferable from the standpoint of near net shape and fabrication cost, since conventional foundry techniques can be adopted. Several companies, such as, Toyota, Duralcan, Comalco are involved with large scale production of metal matrix composites. In the liquid production process Pressure Infiltration Casting is a very attractive route to produce metal matrix composites. Since it does not rely on the matrix wetting the reinforcement, almost any reinforcement- matrix alloy combination is possible. It utilizes pressurized inert gas to force liquid metal into a preform or reinforcement material. Preform and melt temperatures, and the environment can be controlled to minimize or increase the reaction between reinforcement and matrix. Infiltration pressure range from 10 atm to 70 atm depending on the volume fraction of the sample. The technique allows for inexpensive development of composite prototypes and net shape component production. This process is unlike other metal matrix composite production techniques because the complete operation is conducted within the controlled environment of a pressure vessel. It is possible to cast high volume fraction and complex structure in thin walled low strength molds. Controlled pressurization (dynamic pressure control) makes it possible for high infiltration pressures to be used while mamtaining a low differential pressure between the inside and outside of the mold. This reduces the mold sealing problem, lowers the required wall strength, minimizes the thermal mass of molds and lowers tooling cost. The economies of the process is strongly influenced by the viscosity of the molten matrix. Two critical steps, the melt infiltration and its subsequent solidification, control the soundness and microfracture of pressure infiltration cast composites. It is therefore important to control the preform temperature, the melt temperature, the metal infiltration rate and the pressure, the solidification front initiation point, solidification rate, and the pressure exerted on the mold. There are three basic modes of pressure infiltration casting, (1) bottom fill casting, (2) Top fill casting, and (3) top pour casting. All the modes of casting use dynamic pressure control and the choice of one mode over the other is governed by a particular type of component and alloy. The basic process of infiltration is a complex physicochemical hydrodynamic phenomenon that involves consideration of wettability and interfacial phenomena, chemical reactions, fluid flow through porous media, convective and diffusive mass transport and nucleation and growth of phases in finite- wit dh zone where the transport process such as diffusion and fluid flow are restricted by the dimensions of the pores and in the preform Superior mechanical properties of metal matrix composites have generated great interest, however, a potential application area of metal matrix composites in the X1Uautomotive and aircraft industry, as brake materials, has remained relatively unexplored. Selection of brake materials is based on their composition and design, the effect of temperature, rubbing speed and applied pressure on the friction and wear behavior. The wear and friction behavior are generally characterized by complex non steady state high temperature and high pressure process. Under actual operating conditions the contact surface can reach flash temperatures ranging from 1270 to 1370 K. in 1 ms. The subsequent cooling also occurs very rapidly as other areas came in contact. An important requirement of brake material is constant friction in order to prevent brake pulling and unexpected wheel lock up in the vehicles. A goal is to minimize the difference between static and dynamic coefficients of friction for avoiding the squeals or vibrations from brakes and clutches For automotives, the brake materials are attached to the rotor or stator, and for aircrafts they are attached to both. The braking system dynamics depend on brakes, wheel, tire, struts and hydraulics. Brake material composition for aircraft and automotive application are almost similar, the only difference being their configuration to suit the operating conditions. Choices of friction materials determines the coefficient of friction and the brake design. For example the iron base matrix tends to have a lower coefficient compared to the copper based matrix. The thermal and strength properties are influenced by the chemistry, particle shape and particle size of the material. Generally it is desirable to design a friction pair such that the heat is conducted away from the interface quickly. An aircraft brake is expected to operate over a vide range of energy input. These varying energies control the temperature at the friction interface. During low energy, that is low interface temperature, the wear is characterized by abrasive wear. When the brake energies are higher, the interface temperature is also higher, and adhesive type of wear takes place. Adhesive wear occurs when surface aspirates bond together under localized high temperatures and pressures, and subsequently shear apart during sliding of the surfaces. This cycle repeats. The wear debris become trapped between the mating surfaces and further contribute to the wear. Current brake lining materials are based mostly on the following constituents; organic materials, metallic materials, and carbon based materials Organic Brake Linings Currently used organic friction linings fall into following three subclasses, asbestos based materials, nonasbestos based materials (NAO), and semimetallic materials. Because of the environmental concerns, the asbestos based materials are not currently used. Semunet linings have unique friction and wear characteristic. However, compared to NAO linings they are limited in composition range. NAO brake linings are rapidly evolving due to the numerous combinations of fiber, binders and inorganic materials. Inorganic constituents are added to improve the structural strength, thermal expansion, heat absorption, friction coefficient, wear and other properties. XIVSintered Metallic Brake Linings These linings are extensively used in high performance car, high speed railways and aircrafts. Metallic friction materials are mainly a mixture of various metallic and non metallic powders processed at high temperatures, and sometimes under pressure. The metallic matrix phase can be an alloy of copper or iron and the uonmetallic phase consists of natural or synthetic graphite, and silicon. These materials are sintered at high temperatures and sometimes under ressure to form a metal matrix composite. The processing conditions greatly influence the wear life of the brake lining. Large commercial aircrafts get around 1000 landings on one brake stack before replacement. The aircraft brake industry is constantly trying to improve wear characterictics of metallic friction materials to achieve increased number of landings for a given brake system. Carbon Carbon Brake Linings Carbon friction materials show broad range of friction over wide operating conditions. They consist of highly dense carbon fibers embedded in a carbon matrix. The carbon fibers can be woven in the form of a textile fabric or any other desired configuration to obtain specific properties. Carbon-carbon composites have an advantage in brake application because they serve as a friction surface, heat sink, and structural member of the brake, in addition they are light in weight. The most common method of producing carbon carbon composites is chemical vapor infiltration (CVI) or chemical vapor deposition (CVD). These processes are earned out in reactors under high vacuum, high temperatures and with hydrocarbon gas flowing through the reactor. Second method is to mold the carbon fibers with a resin and form the carbon matrix by charring. Metal-matrix composites, reinforced by non metallic fibers, whiskers and particulates, are being seriously examined for structural, thermal management, and wear applications. Copper matrix-carbon composites are specially suited for thermal management applications, because of their low coefficient of thermal expansion, high thermal conductivity, and low density. They are also attractive for wear applications, such as, sliding electrical contacts, bearings and bushings. Solid-state consolidation techniques, such as, sintering and hot pressing, and casting processes have been used to prepare these composites. However, the casting techniques, such as, stir casting, compocasting, and various forms of milt infiltration, art more attractive because of their net shape capability and low processing cost. Mostly, graphitic carbon, has been used for copper matrix-carbon composites in wear applications because its addition (graphite particle volume fraction>0.2) reduces the coefficient of friction and increases the wear resistance, as compared with the matrix. Easy glide of the basal planes under ambient conditions is responsible for the lubricity and antiseizure characteristics of graphite. Glassy (amorphous) carbon, on the other hand, is much harder, about 8 Mohs. One would, therefore, expect a higher strength and wear resistance from a copper alloy composite containing glassy carbon. There is only one study on copper-amorphous carbon composite, where copper wires (30-50 mm in diameter) were introduced into an organic material, which after polymerization and XVpyrolysis yielded a glassy carbon matrix. This composite showed low wear rates and friction coefficients, comparable to those typically observed in graphite and copper matrix-graphite composites. Graphite sliding against itself yields a low coefficient of friction, about 0.1, independent of the sliding distance, but the frictiou coefficient between two amorphous carbon surfaces increases from 0. 1 to a steady-state value of about 0.8. One would therefore, intuitively, expect that metal-matrix composites containing amorphous carbon would show similar high friction coefficients. However, the observed behavior is contrary. The copper fiber-reinforced amoiphous carbon yielded a low and constant coefficient of friction, about 0.16, independent of sliding distance. The role of graphitic versus amorphous carbon in the metal matrix- composites in determining their friction and wear behavior is not understood. The purpose of this research was to develop copper matrix-carbon composites by melt infiltration into microporous carbon preforms, containing different volume fractions of amorphous and graphitic carbon, and study their wear and tribological behavior against cast iron and composite's own plates under ambient conditions for brake application Microporous carbon preforms were selected instead of the conventionally used carbon particulates, because their melt infiltration was expected to result in a fine and uniform distribution of the two phases, metallic matrix and carbon. Such a fine distribution would be expected to yield improved mechanical properties and wear resistance. The Cu-6Si-0.9Cr alloy was selected as the matrix because of the high thermal conductivity and heat capacity of copper, improved wettability between the copper melt and carbon due to the addition of chromium, increased fluidity due to the addition of silicon, and the expected improvement in the wear resistance due to the presence of silicon and chromium carbide particulates. The microporous carbon preforms were made from a mixture of furfuryl alcohol resin, diethylene and triethylene glycol, and p-toluene sulphonic acid. The mixture was polymerized to form a porous solid polymer which is heated up to 973 K to yield porous amorphous carbon preform. Graphite particulates (-440 mesh) were mixed into the above liquid mixture to yield varying proportions of amorphous versus graphitic carbon in the porous preforms. Mercury porosimetry was used to characterize the pore size distribution of the preform. The Cu-6Si-0.9Cr (by weight percent) alloy ingots were prepared by induction melting the charge in an alumina crucible under a flowing UHP argon atmosphere. This alloy has a liquidus of 1 129 K. The alloy was remelted in a pressure infiltration casting facility and was pushed upwards into cylindrical microporous preforms (0.6 cm diameter, 10 cm long) kept in an alumina tube by the help of 500 psi argon pressure. The pressure infiltration casting facility provided independent control of the melt and preform temperatures in order to achieve successful infiltration. Wear and friction experiments were carried out in a pin on plate reciprocating wear tester under ambient conditions at room temperature. Composite pins (2.2 cm long and 0,6 cm in diameter) were tested against cast iron and composite plates (wear track length=4 cm). The load, ranging from 4 to 35 Kg was applied on the composite pin, rubbing against the reciprocating plate (linear speed 20 cm s“1). The friction force XVIand pin displacements, measured by transducers, were recorded by the help of a Hewlett Packard data acquisition unit. Because of the reciprocating motion involved, the plate and the pin came to rest at the end of each cycle, and the rod impacting on the friction force transducer vibrated vigorously. Therefore, a triggering arrangement was used to stop the data acquisition near the end of the cycle, to prevent collecting these erratic data. The time for each run, 90 minutes, and all other variables were kept constant. A careful control of mould and melt temperatures is crucial for successful infiltration of molten metal into the microporous preforms. The process of pressurization causes a cooling of the two graphite susceptors, the bottom one, heating the melt, and the top one, heating the mould (containing the preform.The alloy has a liquidus temperature of about 1 129 K. The melt temperature in the alumina crucible, below, and the preform temperature in the mould, above, were carefully controlled during pressurization of the microporous carbon preforms. A typical plot of pressure and temperature data recorded during bottom fill pressure infiltration casting of the carbon preform. Before the vessel was pressurized the melt and mold temperatures were 1480 K and 1320 K, respectively. The superheats for the melt and mold were 300 K, and 147 K respectively. At 1563 seconds the vessel was pressurized at the rate 0.1 MPa/sec for 41 seconds. Both the melt and mould temperatures dropped during the first seven seconds of pressurization as the argon gas contacted the melt, and the mould. For the next 10 seconds the mould temperature increased as the melt flowed through the mould, whereas the melt temperature continued to drop, but not below its melting point. During the next 24 seconds the melt temperature increased and the mould temperature decreased due to the convectfve effects of the argon gas. The carbon struts and the interconnected continuous porosity can be clearly seen in from scanning electron micrographs. Addition of graphite particles has resulted in significant increase in the pore sizes, for otherwise identical processing conditions. The pore size distributions, obtained by mercury porosimetry, showed preform preform type I which has a median pore diameter (volume) of 1.68 fim, average pore diameter of.10 urn, fraction porosity.48, bulk density of.77 gem”3, skeletal density 1.48 gem“3. For the ”Type 1“ preform, the skeletal density measured by mercury porosimetry, 1.48 gem”3, is in close agreement with the density of amorphous carbon, 1.5 gem“3, indicating the continuity of pores (in the presence of isolated pores the skeletal density would appear to be less than the true density of the amorphous carbon). Addition of graphite has resulted in increased pore sizes. Typical microstructures of the three infiltration cast composites show that the light regions are the copper alloy matrix and the gray regions are the amorphous carbon. Both, the uninfiltrated pores and graphite particles appear dark. Both constituents in the microstructure, the carbon and the metallic phase, are much finer in the ”Type 1“ as compared with the ”Type 2“ and ”Type 3“ composites. Increasing volume fraction of graphite particles also resulted in more uninfiltrated regions in the vicinity of the graphite particles. Agglomeration of the graphite particles was not XVUobserved. However, amorphous carbon appears to nucleate on the preexisting graphite particles, and produce isolated pores in their vicinity which do not get infiltrated by the copper alloy melt. The infiltration, occurs both from the bottom and the side of the preforms. The composite interior consists of finely distributed two phases, the bright looking copper alloy matrix and the dark looking carbon struts. Wear and friction between two metals is believed to initiate by deformation and cutting of the softer asperities by the harder phase, resulting in an increased contact area. This is followed by adhesion between the two surfaces, plastic deformation and crack nucleation just below the contact surface, and the subsequent delamination, The increased adhesion and delamination ”ploughing“ causes an increase in the wear and friction, and also the thickness of the wear debris increases. Finally, an equilibrium is reached between the rate of debris formation (by adhesion and delamination) and the rate of debris removal, which results in a steady-state wear and a constant coefficient of friction. The processes of adhesion and delamination, respectively, result in high and low shear stresses, and, therefore, cause fluctuations in the coefficient of friction. Severity of the adhesion and delamination increases with increasing normal stress, resulting in higher coefficient of friction and also its larger fluctuation. The experimental observations for the softer copper alloy (168 VHN) pin wearing against harder cast iron ( 23 1 VHN) plate, such as, increasing coefficient of friction during the initial transient, severe fluctuations in the coefficient of friction and increase in the coefficient of friction with increasing normal stress on the pin are all in agreement with the above expected behavior. In the presence of a third body, such as, graphite or amorphous carbon particles in the metal-matrix composites, the wear and friction process would become more complex. As pointed out by Rohatagi et.al., the nature of the wear debris and its interaction with the two mating surfaces would be determined by factors, such as, size, shape, distribution and volume fraction of the particles; the inteifacial strength between the matrix and the particles; the strength, ductility and deformation behavior of the particles and the matrix; and the presence of defects (porosity). For the ”Type 1“ composite, which has finely distributed amorphous carbon in a copper alloy matrix, two processes would compete during the initial transient in the coefficient of friction versus sliding distance: (1) cutting and plastic deformation of the softer copper alloy matrix asperities by the harder cast iron, which would increase the coefficient of friction, and, (2) ”bleed out“ of the carbon particles on to the surface, which can create a carbon coating on the exposed metallic surfaces, decrease the likelihood of metallic adhesion, and, result in decreased coefficient of friction. The wear debris would consist of carbon and the metallic matrix phase. However, the friction induced hot surface temperature would tend to oxidize the matrix phase. The thickness of the debris layer, containing amorphous carbon and copper oxide slowly build up until a steady-state is reached. With smaller applied normal stress on the pin, it would take longer to reach the steady-state, as compared to the larger stresses (about 500 m at 8 MPa, versus only 20 m at 43 MPa), It appears that at low stress the carbon readily bleeds out on the surface before any significant cutting of the matrix. This causes an xvininitial decrease in the coefficient of friction. With increasing wear the metal-to metal contact surfaces area increases and hence the coefficient of friction begins to increase. Ultimately a steady-state condition is reached where the existing wear debris allows the pin surface to be continually covered by a thin coating of carbon, resulting in an abrasive wear and a constant coefficient of friction, about 0.16. At high stress, the process of plastic deformation and cutting of the matrix asperities overpowers the extent of the carbon film formation in the beginning and results in an initial increase in the coefficient of friction. With increasing amount of carbon in the wear debris, the extent of carbon film coverage on the tribo-surface increases and the coefficient of friction begins to decrease. Ultimately a steady-state is reached where the pin surface is continually covered by the thin carbon layer resulting in a constant coefficient of friction, about 0.21. Unlike, the alloy-matrix versus cast iron wear, where the friction coefficient increases with increasing applied stress, the steady-state value of the friction coefficient is independent of the applied stress for the ”Type 1“ composite pins. This is because the pin surface gets covered by the same carbon film for all the normal stresses examined in this study, only the variation in the film thickness across the specimen surface is more at the higher stresses. This variation is produced because higher normal stress leads to the formation of larger film thickness, and layers within these films tend to shear off from each other. The integrity of the carbon film during sliding wear of the composite is expected to depend on several factors, such as, the size and extent of porosity, and, size of the metallic and carbon constituents. Porosities in the composite, when exposed to the wearing surface, would make the process of ploughing and delamination easier and tend to brake the carbon film. The integrity of the carbon film is easier to maintain in the presence of a very fine size and uniform distribution of carbon and metallic phases, because as the metallic constituent deforms and flows (exposing a fresh metallic surface which can cause adhesion) the carbon is right there to create the next thin carbon layer. This is not likely for a composite where the metallic constituents are large and the spacings between the carbon particles are large. Such a micro structure would, therefore, cause more adhesive wear, yielding higher coefficients of friction and their larger fluctuations, especially at the higher normal stresses where plastic deformation and fracture of the matrix phase is more likely. The ”Type 2“ and ”Type 3“ composites, therefore, despite the presence of significant amount of soft graphitic carbon, show higher coefficients of friction at large stress (47 MPa) as compared to ”Type 1“ because they have significant porosity (9 percent, about 57 mm average size) and their metallic constituents are much larger (83 mm and 50 mm versus 12 mm for the ”Type 1“ composite), causing adhesive wear. The adhesive wear is also reflected by the large fluctuations in their coefficient of friction and their high wear rates at the high loads. At low stress, 17 MPa, the ”Type 2“ and ”Type 3“ composites yield coefficient of friction values similar to the abrasive wear of ”Type 1“ composite, because of the presence of the thin carbon film on their tribo- surface, The transition from the abrasive to the adhesive wear, observed in the graphite containing ”Type 2“ and ”Type 3“ composites, was not seen in the ”Type 1" composite, because the uniform and fine distribution of the two phase, copper alloy matrix and amorphous carbon, promoted the formation of the thin carbon film and did not allow adhesion to occur XIXAs a result Cu-6Si-.9Cr alloy have been succesfully infiltrated into the inicroporous carbon preforms containing varying quantity of graphite. Cr addition decreased the wetting angle between carbon preform and alloy in which caused formation of chromium carbide at the outer part of composite. Most of the chromium was simply concentrated in this reaction zone, only little quantity of it distributed in the inner part of composite. Since this formation decreased the surface energies in between two constituents, metal infiltration has been achieved. On the other hand inner part of composite showed homogeneous distribution of copper and silicon in the matrix. Occurrence of some precipated particules indicates that Silicon tends to enhance formation of silicon carbide particules. Load dependence of the wear and friction behavior of the composite pius under ambient conditions against cast iron plates and composite plates, using a pin on plate reciprocating wear tester showed that the wear resistance of the composite is significantly improved, as compared with the base alloy. Contrary to the normally expected behavior, the addition of graphite to the amorphous carbon does not reduce the friction coefficient, especially at high loads. The wear and friction behavior of the composites is very sensitive to the size and distribution of the microstructural constituents XX
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