METHOD OF PREPARING METAL MATRIX COMPOSITE AND COATING LAYER AND BULK PREPARED THEREBY

Technical Field

[1] This invention relates to a method of preparing a metal matrix composite, and a metal matrix composite coating layer and metal matrix composite bulk prepared by using the same. More particularly, the invention relates to a method of preparing a coating layer with wear resistance and excellent resistance against fatigue crack by securing high hardness by the dispersion of intermetallic compounds and ceramic particles without causing damages such as heat strain to a substrate in the process of preparing the coating layer and a method of providing a bulk which is prepared by separating the coating layer from the substrate, and a coating layer and bulk prepared thereby. Background Art

[2] Dispersion reinforcement methods are used to improve the strength, hardness, wear resistance of alloys or metals. The dispersion reinforcement methods form a structure of intermetallic compounds being dispersed within the matrix of alloys or metals and their representative examples are precipitation hardening or dispersion of high hardness particles. In particular, aluminum alloys are easy to dispersion reinforcement by the precipitation hardening of intermetallic compounds within the aluminum matrix and accordingly, it contributes to the improvement of their mechanical properties. The aluminum alloys have excellent strength, heat resistance and durability as well as light weight that is attributable to the aluminum itself and thus they have been widely used as a material for thermal, mechanical parts of aircrafts or engine parts of automobiles.

[3] The dispersion reinforcement methods will be described by example of aluminum.

[4] Prior dispersion strengthened aluminum alloys have been prepared by casting, powder metallurgy, thermal spraying and so on.

[5] In aluminum alloys prepared by the casting, precipitate phases where fine precipitates are precipitated are evenly distributed within aluminum matrix phases so that they exhibit excellent strength at room temperatures, but their strength is drastically decreased at temperatures above 200 °C due to sudden coarsening of the precipitate phases when exposed to high temperature. Accordingly, such dispersion reinforcements as precipitation hardening are inappropriate for heat resistant aluminum alloys.

[6] On the contrary, the powder metallurgy is a method of preparing aluminum alloys by forming aluminum and a dispersion (metal powder or ceramic powder) as an

additive into the form of powder and sintering them, and the thus prepared alloys are characterized in that fine dispersion phases are evenly dispersed within the alloys and in case of the ceramic powder, the coarsening of the dispersion phases does not occur at high temperatures and thus show excellent characteristics at high temperatures.

[7] Nevertheless, in case that the metal powder is contained as a dispersion, additional thermal treatment is needed to form intermetallic compounds. However, thin aluminum oxide (Al O ) is formed on the surface of the aluminum metal powder in air even at room temperatures and this surface aluminum oxide membrane interferes the reaction of aluminum and other metal elements and thus inhibits the formation of intermetallic compounds. Accordingly, as the powder metallurgy requires thermal treatment at a high temperature above the melting point of the alloys to produce intermetallic compounds, it proposes high costs resulting from high temperature operation and safety securing problems of facilities.

[8] Further, the powder metallurgy involves complex manufacturing process including the suitable control of sintering atmosphere in order to prevent the oxidation of aluminum at high temperature in the course of sintering and according to the powder metallurgy, the formation of intermetallic compounds with transition metals having high melting points such as Ti or Ni has been known to be very difficult. Besides, in the powder metallurgy, as a desired shape is prepared by a mold, it costs a lot to prepare the mold and it also has size limitation.

[9] The thermal spraying is a method of preparing dispersion strengthened aluminum alloys by spraying molten metals and cooling them. This method generates the same problems as in the casting. In particular, where aluminum-transition metal alloys are prepared by the thermal spraying, coarse secondary phases are formed within the aluminum matrix and they exhibit inferior alloy characteristics.

[10] Hence, according to the prior arts relating to dispersion strengthened aluminum alloys, especially, aluminum-transition metal alloys, it was difficult to obtain aluminum alloys where fine intermetallic compounds are evenly dispersed and only thermal treatment above the molten temperature of the alloys could form intermetallic compounds.

[11] As discussed by the example of aluminum, the prior dispersion reinforcement methods all require high temperature processing resulting in high costs and have difficulty in that they have to prevent high reactivity at high temperature in the course of thermal treatment and in case of the powder metallurgy, it has size limitation resulting from the preparation of molds and incurs high costs.

[12] Also, to elongate life of mechanical parts used in abrasive environment such as friction, fatigue, corrosion or erosion, methods of hardening the surfaces of the parts or coating them with wear-resistant materials have been used. As the coating materials to

improve wear-resistance, materials having high hardness, that is, ceramic materials such as oxides, for example, alumina, carbides, for example, SiC or TiC, and nitrides for example, Si N , TiN are mostly used.

[13] Particularly, Korean Patent Laid-Open No. 1997-0045010 discloses a method of forming a coating membrane instead of prior cast iron liners on the inner walls of cylinder bores and in this method, wear resistance is improved by forming coating powders comprising ceramics and their mixtures on the inner walls of bores by thermal spraying using plasma or arc as heat source.

[14] Korean Patent Laid-Open No. 1998-017171 discloses a method of forming wear- resistant coating layer on the bore side of aluminum cylinder blocks by plasma spray using silicone carbide particles.

[15] Korean Patent Laid-Open No. 2003-0095739 discloses a method of forming a coating membrane by spraying a powder composition for spray coating on the inner walls of stainless cylinder bores while fusing it with heat source of high temperature, and the powder composition for spray coating is a mixture alumina and zirconia.

[16] As described above, numerous attempts to form a wear-resistant coating on metal substrates with excellent wear-resistant ceramic materials have been conducted, but all of these methods are mainly based on plasma or electric arc spray. Such thermal spray methods provide substrates with powder particles by heating the powder particles to be coated nearly around or above fusion point thereby fusing at least a part thereof.

[17] Therefore, as the ceramic particles to be coated onto the substrates are heated to high temperature around 1000 °C, which is a fusion temperature of normal ceramic particles, and then provided to the substrates by contact, they cause damages by heat shock to the substrates during the coating process, induce residual stress in the process of cooling thereby decreasing adhesion and shorten the life of the parts.

[18] Also, due to the particle spray of high temperature, there is an increased risk with the handling of spray machine and complicated operation line is inevitable. In addition, the fused particles of high temperature might react with impurities on metal matrix or its surface and form another compounds, thereby adversely affecting the properties of the material.

Disclosure of Invention

Technical Problem

[19] In order to solve the problems of the prior arts, it is an object of the present invention to provide to a method of prepare a metal matrix composite coating layer or bulk which is not likely to cause heat strain or damages due to heat shock to a substrate and at the same time, has excellent wear resistance, and the coating layer and bulk prepared thereby.

[20] Also, it is another object of the invention to provide a method of preparing a metal matrix composite coating layer or bulk capable of obtaining dispersion reinforcement at a relatively low temperature with inexpensive costs and being applied to mass production, and the coating layer and bulk prepared thereby.

[21] Further, it is another object of the invention to provide a method of preparing a metal matrix composite coating layer or bulk having excellent resistance against crack generation resulting from fatigue of the coating layer by preventing the accumulation of heat on the coating layer and suppressing crack generation between substrates and the coating layers or within the coating layers, and the coating layer and bulk prepared thereby.

Technical Solution

[22] To achieve the aforementioned objects, the present invention provides a method of preparing a metal matrix composite, the method comprising the steps of:

[23] providing a substrate;

[24] preparing a mixed powder comprising i) a first metal powder comprising a metal, alloy or mixture particle thereof, ii) a second metal powder comprising an intermetallic compound forming metal particle which forms an intermetallic compound along with the metal or the alloy element of the alloy, and iii) a ceramic powder comprising a ceramic or mixture particle thereof;

[25] injecting the mixed powder prepared above into a spray nozzle for coating;

[26] coating the mixed powder on the surface of the substrate by accelerating the mixed powder in the state of non-fusion at a speed of 300 to 1,200 m/s by the flow of transportation gas flowing in the spray nozzle; and

[27] forming the intermetallic compound by the thermal treatment of the coated coating layer.

[28] Also, the invention provides a metal matrix composite coating layer characterized in that it is prepared by the metal matrix composite preparation method and a metal matrix composite bulk characterized in that it is prepared by the separation of the substrate from the substrate and coating layer prepared by the metal matrix composite preparation method. Advantageous Effects

[29] In accordance with the method of preparing metal matrix composite of the invention and the metal matrix composite coating layer and bulk prepared by using the same, the production of metal matrix composites where intermetallic compounds and ceramic powders are dispersed is possible at low temperatures in comparison with the prior art so that there is no likelihood of causing damages by thermal strain and thermal shock to the substrates, the growth of intermetallic compounds was suppressed

so that mechanical characteristics such as high temperature strength are improved, and the accumulation of heat on the coating layers is prevented and crack generation between the substrates and coating layers or inside the coating layers is suppressed so that resistance against crack generation due to the fatigue of the coating layers can be improved.

[30] In addition, the invention can be applied to prepare members with excellent mechanical strength and also, it can be used to dispersion reinforce the surface of the existing members. In particular, as it is carried out at low thermal treatment temperatures, there is a slight possibility of exercising bad influence on the properties of the members upon surface hardening.

[31] Besides, as the invention enables the processing under the environments of the thermal treatment temperature of relatively low temperature, low injection pressure of mixed powders and low transportation gas temperature, it can be manufactured with low costs and it is easy to mass production. Brief Description of the Drawings

[32] Fig. 1 is a schematic view of a low-temperature spray (cold spray) apparatus which is used to prepare metal matrix composites in this invention.

[33] Fig. 2 to Fig. 5 are phase diagrams illustrating intermetallic compounds formable with regard to Al matrix by a method of preparing metal matrix composites of the invention.

[34] Fig. 6 to Fig. 9 illustrate embodiments of the nozzle used in a method of preparing coating layers of the invention.

[35] Fig. 10 is X-ray diffraction test results showing whether an intermetallic compound is generated and how much it is generated according to thermal treatment temperatures where the ratio of aluminum powder and nickel powder is 9: 1.

[36] Fig. 11 is X-ray diffraction test results showing whether an intermetallic compound is generated and how much it is generated according to thermal treatment temperatures where the ratio of aluminum powder and nickel powder is 75:25.

[37] Fig. 12 is EDX photographing results of each part where the ratio of aluminum powder and nickel powder is 75:25 and the thermal treatment temperature is 550 °C.

[38] Fig. 13 is EDX photographing results of each part where the ratio of aluminum powder and nickel powder is 75:25 and the thermal treatment temperature is 500 °C.

[39] Fig. 14 is X-ray diffraction test results showing whether an intermetallic compound is generated and how much it is generated according to thermal treatment temperatures where the ratio of aluminum powder and titanium powder is 9:1.

[40] Fig. 15 is X-ray diffraction test results showing whether an intermetallic compound is generated and how much it is generated according to thermal treatment temperatures

where the ratio of aluminum powder and titanium powder is 75:25.

[41] Fig. 16 is EDX photographing results of each part where the ratio of aluminum powder and titanium powder is 75:25 and the thermal treatment temperature is 630 °C.

[42] * Summary of Reference Numbers of Drawings *

[43] 2: convergence section 4: throat section

[44] 6: divergence/straight section 8: exit column

[45] 10: nozzle section 12: injection port

[46] 20: injection tube 22: base point

[47] 24: connection section 30: buffer chamber

[48] 110: gas compressor 120: gas heater

[49] 130: powder feeder 140: nozzle

Mode for the Invention

[50] The present invention is described in detail by way of the drawings and preferred embodiments.

[51] The present invention relates to a method of preparing a metal matrix composite, which comprises the steps of providing a substrate, preparing a mixed powder comprising i) a first metal powder comprising a metal, alloy or mixture particle thereof, ii) a second metal powder comprising an intermetallic compound forming metal particle which forms an intermetallic compound along with the metal or the alloy element of the alloy, and iii) a ceramic powder comprising a ceramic or mixture particle thereof, injecting the mixed powder prepared above into a spray nozzle for coating, coating the mixed powder on the surface of the substrate by accelerating the mixed powder in the state of non-fusion at a speed of 300 to 1,200 m/s by the flow of transportation gas flowing in the spray nozzle, and forming the intermetallic compound by the thermal treatment of the coated coating layer.

[52] That is, the invention focuses on the improvement of mechanical properties including fatigue characteristics, wear resistance, hardness of coating layers in the methods of preparing metal matrix composites on substrates by using cold spray (low-temperature spray) method and for the maximum improvement thereof, the invention is characterized in that in addition to mixing a metal matrix which is an existing metal matrix composite component with ceramic particles, it further comprises an intermetallic compound forming metal particle which forms an intermetallic compound along with the alloy element of an alloy or a metal that constitutes the metal matrix and such a mixed powder is sprayed and laminated via cold spray methods at relatively low temperatures in comparison with thermal spraying or sintering temperature.

[53] The cold spray method itself is already known and the schematic view of an

apparatus for such cold spray is shown in Fig. 1. That is, Fig. 1 shows a schematic view of a low-temperature spray (cold spray) apparatus (100) for preparing a coating layer on a substrate (S) in the invention.

[54] The spray apparatus (100) provides the substrate (S) with powders to form a coating layer by accelerating them at subsonic or supersonic speed. For this purpose, the spray apparatus (100) comprises a gas compressor (110), gas heater (120), powder feeder (130), and nozzle for spray (140).

[55] Compressed gas of about 5 to 20 kgf/cm provided by the gas of about 300 ~ 1200 m/s through the nozzle for spray (140). In order to generate the flow of such a subsonic or supersonic speed, a convergence-divergence nozzle (de Laval-Type) as shown in Fig. 1 is generally used as the nozzle for spray (140) and supersonic flow can be generated by such convergence and divergence process.

[56] A gas heater (120) on the route to feed the compressed gas in the apparatus (100) is an additional one for heating the compressed gas to increase its spray speed at the nozzle for spray by increasing kinetic energy thereof and it is not necessarily necessary. Also, as shown in Fig. 1, to enhance the powder supply to the nozzle of spray (140), a portion of the compressed gas in the gas compressor (110) can be supplied to the powder feeder (130).

[57] For the compressed gas in the apparatus, common gas, for example, helium, nitrogen, argon and air can be used and it can be suitably selected in consideration of spray speed at the nozzle for spray (140) and costs.

[58] For the detailed an apparatus, the first step is to provide a substrate. The substrate

(S) can be various kinds of known material that can be substrates of parts requiring wear-resistance where the improvement of wear resistance in parts requiring wear resistance is aimed and further, it can comprise any other materials. Particularly, the substrate can be aluminum, aluminum alloys, especially, Al-Si or Al-Mg aluminum alloys that are widely used as thermal, mechanical members, or iron alloys such as cast iron, or it can be semi-conductive materials such as silicone. Preferably, the substrate is aluminum or aluminum alloys that have poor wear resistance because it is remarkably improved according to the coating layer preparation of the invention. Also, unlike coating layers containing substrates, in case that a bulk form comprising metal matrix composite alone is prepared, as the separation of the coating layer from the substrate is needed, the substrate is preferably ceramic materials having low reactivity with metal powders or resin materials that can be destroyed and thus disappeared in thermal treatment step.

[59] For the metal, alloy or mixture particle thereof used in the first metal powder in the invention, various known metals, alloys or mixture particles thereof can be used and preferably, there are used iron, nickel, cupper, aluminum, molybdenum, titanium or

alloys thereof or mixture thereof. More particularly, in case of aluminum and titanium, aluminum, aluminum alloys, mixture of aluminum and aluminum alloys, mixture of aluminum and titanium, mixture of aluminum and titanium alloys, mixture of aluminum alloys and titanium alloys can be mentioned and especially, they can be aluminum alloys or titanium alloys that are often used as ordinary thermal, mechanical members. More preferably, the metal or alloy is aluminum or aluminum alloys because they are homogeneous with aluminum or aluminum alloy substrates that exhibit well- improved wear resistance according to the coating layer preparation of the invention.

[60] The intermetallic compound forming metal particle which forms an intermetallic compound along with the metal or the alloy element of the alloy used in the second metal powder is determined by the metal, alloy or mixture particle thereof of the first metal powder.

[61] That is, for example, where the first metal powder is aluminum or alloys thereof, a metal having higher melting point than aluminum among transition metals can be the second metal powder and as specific examples, it can be a metal selected from the group consisting of titanium, nickel, chromium, iron and combination thereof. As can be seen from the phase diagram of each system shown in Fig. 2 to 5, aluminum can form intermetallic compounds with regard to titanium, nickel, chromium, and iron, respectively.

[62] Below, there are described examples of transition metals capable of forming intermetallic compounds along with Al metal on the basis of phase diagrams. Fig. 2 to Fig. 5 are phase diagrams of two-element aluminum alloys as examples of aluminum alloys which are formable by the method of the invention.

[63] First, Fig. 2 is a phase diagram of Al-Ti type. With reference to Fig. 2, when Ti is added in several tens % by weight, Al phase where Ti is solid solubilized in a small amount in alloys and TiAl phase which is an intermetallic compound of Al-Ti exist as stable phases at temperatures lower than 664 °C (937 K). As the content of Ti increases (that is, it is added in an amount more than 38 % by weight), Al Tiphase and Al Ti phase exist as stable phases of the alloys. The relative weight ratio of Al, Al Ti and Al Tiphases present in the alloys according to the mixing ratio of the metal powders is determined by so called "the lever rule", which has been known by a person having ordinary knowledge in the art to which the invention pertains.

[64] Fig. 3 is a phase diagram of Al-Ni type. With reference to Fig. 3, at temperatures lower than 636 °C, intermetallic compounds of Al Ni, Al Ni , AlNi, AlNi and the like form the stable phases of alloys according to the amount of Ni. Fig. 4 is a phase diagram of Al-Cr type. With reference to Fig. 4, at temperatures lower than 663 °C (936 K), intermetallic compounds of CrAl form stable phases according to the addition of Cr. Meanwhile, Fig. 5 is a phase diagram of Al-Fe type and as shown in the

figure, even in case of Al-Fe type, intermetallic compounds of metastable phase such as FeAl can be formed at temperatures lower than 654 °C (927 K).

[65] As described with reference to the phase diagrams, as intermetallic compounds in

Al-Ti, Al-Ni, Al-Cr and Al-Fe two-element system exist as stable phases below certain temperature, the formation of intermetallic compounds within the alloys is possible by mixing Al metal powder with Ti, Ni, Cr or Fe metal powders.

[66] Besides, the second metal component can comprise alloy elements that can be obtained from existing Al alloys through precipitation hardening. That is, as precipitation hardening type aluminum alloys, various alloy systems such as Al-Cu, Al-Li and Al-Mg are possible, and in these cases, the alloys obtain dispersion reinforcement effects by precipitating precipitates that are intermetallic compounds and thus, the second metal components can comprise Cu, Lu or Mg where the metal matrix composite coating layer or bulk of the invention is applied to relatively low temperature.

[67] The ceramic or mixture thereof that is the ceramic powder in the invention can be various kinds of known ceramic and mixture thereof used in metal matrix composites for the improvement of known , alumina, nitrides such as TiN and Si 3 N 4 , and carbides such as TiC and SiC can be used, and alumina or SiC is preferable for the enhancement of wear resistance.

[68] The ceramic particle to be mixed into the mixed powder in the invention can be provided in the form of an agglomerated powder. The agglomerated powder is easy to be pulverized into fine particles and thus become fine particles when the powder particles collide with substrates in the coating process. Accordingly, it is advantageous in that a coating layer where fine ceramic particles are uniformly dispersed can be formed.

[69] For the particle size of the first metal powder, the second metal powder and the ceramic powder to be mixed into the mixed powder, particles having various sizes used in the known cold spray can be used and preferably, ones having size of 1 to 100 um are advantageous in respect to dispersion and mixing. More preferably, as the second metal powder is changed into intermetallic compounds by subsequent thermal treatment step, finer particles are advisable to obtain uniform, strong dispersion reinforcement effects and preferably it has smaller particle size than the first metal powder. Particularly, it is preferred that in case of mixing aluminum powder and Ni powder, the aluminum powder is 50 to 100 um and the nickel powder is 1 to 100 um, preferably, 1 to 50 um, and in case of mixing aluminum powder and Ti powder, the aluminum powder is 50 to 100 um, and the Ti powder is 1 to 100 um, preferably, 1 to 50 um. For the ceramic powder to be mixed together, powders having various sizes used in the preparation of known metal matrix composites can be employed and preferably, ones

having size of 1 to 100 um are advantageous in respect to dispersion and mixing. In case that the aluminum powder is used as the first metal powder, the ceramic powder is preferably SiC or alumina because they are advantageous in respect to reactivity and dispersion effects and in connection with their sizes, where the aluminum powder is 50 to 100 um, the ceramic powder is preferably 1 to 50 um. That is, in case of the first metal powder and the ceramic powder, if the size of the particles is too small, the weight of the particles is less and thus impulse becomes too less in spite of their fast speed when they collide with coating layers and as a result, processed hardening such as shot peening is less generated. On the other hand, if the size of the particles is too big, dispersion reinforcement effects is decreased. Thus, optimal medium size ranges as described above exist to maximize the processed hardening and dispersion reinforcement.

[70] For the mixing ratio of the first metal powder, the second metal powder and the ceramic powder, various mixing ratios can be chosen and in case of the second metal powder, as it is almost changed into intermetallic compounds in subsequent thermal treatment step, it is mixed in a ratio corresponding to the amount of dispersion required when metal matrix composite is designed and in case of the ceramic powder, as it functions as a dispersion by itself without additional reactions, it is mixed in a ratio corresponding to the amount of dispersion required when metal matrix composite is designed. In particular, the mixing ratio of the first metal powder and the ceramic powder is preferably 1:1 to 3:1 of metal: ceramic by volume to maximize micro Vickers hardness value that is a relative index of wear resistance.

[71] The mixed powders of the first metal powder, the second metal powder and the ceramic powder can be prepared by ordinary methods. As simple methods, these powders can be dry mixed by v-mill. The dry mixed powders themselves can be used in the powder feeder without additional treatment. Although the mixing ratio of each powder of the mixed powders can be suitably controlled according to their use, they are mixed within appropriate ranges according to their designed values for the optimization of mechanical properties such as wear resistance. When the volume ratio of the ceramic particles exceeds 50 %, there is a problem that coating layers may not increase beyond certain thickness and accordingly, it is mixed within the above ranges.

[72] Generally, a convergence-divergence nozzle is used for the nozzle in the invention, and in case of having a common structure, a compressed gas of about 5 - 20 kgf/cm is supplied to the mixture powder. For the compressed gas, helium, nitrogen, argon or air can be used. The gas is supplied, while being compressed to about 5 - 20 kgf/cm by a gas compressor. If necessary, the compressed gas can be supplied in the state of being heated to the temperature of about 200 - 500 °C by heating means such as the gas heater (120) in Fig. 1.

[73] There are lots of control parameters such as compression pressure against powders, flow rate of transportation gas and temperature of transportation gas in the cold spray process but for the improvement of wear resistance, it is preferable that about 50 % of the powders sprayed from the nozzle participate in substantial coating process and the others fall apart after collision to contribute to processed hardening such as shot peening on the coating surface instead of all of the sprayed powders being used for coating in respect to the improvement of hardness according to the processed hardening of coating layers and the increase of wear resistance. More preferably, the range of the coating efficiency is 10 to 20 % for the improvement of hardness and the increase of wear resistance.

[74] Accordingly, if the above coating efficiency is maintained, it is preferable to keep the speed of the mixed powders relatively low when they are collided. As the speed approximately changes in proportion to the square root of the temperature of the transportation gas, the temperature of the transportation gas supplied to the nozzle may be maintained relatively low when the mixed powders are coated through the nozzle. Preferably, the temperature of the transportation gas is 280 + 5 °C. More preferably, the temperature of the transportation gas is advisable because it shows appropriate coating efficiency in case that aluminum is used as the first metal powder.

[75] Also, in case that the first metal powder is aluminum or aluminum alloys, the in- termetallic compound forming metal particle in the second metal powder is a metal selected from the group consisting of titanium, nickel, chromium, iron, combination thereof, if the speed of the powder to be coated on the substrate is maintained at 300 to 500 m/s regardless of the types of the ceramic particles, the processed hardening effects of the coating layer as described above can be obtained and accordingly, wear resistance can be maximized.

[76] For the nozzle of the cold spray apparatus, besides ordinary convergence- divergence nozzles of de Laval-Type as described above, there can be used convergence-straight nozzles or convergence-divergence nozzles with throat as depicted in Fig. 6 to 9. The injection of the mixed powders can be carried out at the divergence or straight section of the nozzle via an injection tube located through a throat. As the injection of the mixed powders is carried in the divergence or straight section at a relatively low pressure, the pressure for the injection of the mixed powder can be maintained low and it is thus possible to design a cold spray apparatus with low costs and further, as the powders are injected in the divergence or straight section, it can prevent the powders from being coated inside the nozzle, especially, throat and accordingly, long time operation is possible.

[77] Accordingly, in the case that the nozzle and injection tube as described above are used, it is preferred that the pressure when the mixed powders are injected into the

nozzle is as low as 90 to 120 psi, which is much lower than the ordinary pressure.

[78] More preferably, in the case that the nozzle and injection tube of the above forms are used, it is preferred that the pressure when the mixed powders are injected into the nozzle is 90 to 120 psi and the temperature of the transportation gas is 280 + 5 °C for the formation of coating layers with excellent wear resistance, especially when the first metal powder is aluminum and the ceramic is SiC.

[79] Furthermore, in the coating step, the mixing ratio of the ceramic powder or the mixing ratio of the second metal powder to the first metal powder may have a concentration gradient as it goes to outer surface from the substrate surface and the particle size of the ceramic powder or the particle size of the second metal powder may have a constant gradient with regard to particle sizes as it goes to outer surface from the substrate surface.

[80] That is, the mixing ratio of the second metal powder to the first metal powder can be designed to have various concentration gradients such as i) increase from the substrate surface to outer surface, ii) decrease from the substrate surface to outer surface, iii) peak in the middle and decrease to the substrate surface and outer surface, iv) be lowest in the middle and increase to the substrate surface and outer surface, etc. Such mixing ratio concentration gradient can be equally applied to the ceramic powder and the concentrations of the ceramic powder and the second metal powder can be adjusted altogether, and the concentration gradient direction of the ceramic powder and the second metal powder can be designed differently or conversely.

[81] Also, together with such concentration gradient or separately therefrom, particle sizes can have gradient and in case of the ceramic powder, its particle size can i) increase as it goes to outer surface from the substrate surface, ii) decrease as it goes to outer surface from the substrate surface, iii) peak in the middle and decrease to the substrate surface and outer surface, iv) be lowest in the middle and increase to the substrate surface and outer surface, etc. Such particle size gradient can be equally applied to the second metal powder and the particle size of the ceramic powder and the second metal powder can be adjusted altogether, and the particle size gradient direction of the ceramic powder and the second metal powder can be designed differently or conversely.

[82] Such gradient can minimize the heat stress which is generated by difference in thermal expansion coefficients between the substrate and the coating layers and minimize peeling, residual stress, etc. which might be generated by heat cycling by activating heat transfer.

[83] The formation of such additional intermediate layers is preferable when the first metal powder is aluminum and the ceramic is SiC.

[84] The mixed powder sprayed at high speed forms a coating layer with high density

upon collision with the substrate. After the coating step is carried out until the coating layer with a desired thickness is obtained, the coated coating layer is subject to thermal treatment where intermetallic compounds are formed, which is intended in the preparation step of the mixed powder. The thermal treatment step in this invention is characterized in that it is carried out at low temperatures. While in the prior casting and thermal spray, the metal mixed powder was heat-treated at a high temperature of 900 -1200 °C or so, the thermal treatment in the method of the invention is carried out at temperatures not higher than 900 °C. More particularly, the thermal treatment is preferably carried out below the lowest liquid phase formation temperature which the mixed combination of different first metal powder and second metal powder can attain, that is, below eutectic temperature. In the present invention, the term "eutectic temparature" encompasses peritectic temperature. For example, in case of the mixed powder where the first metal powder is Al and the second metal powder is Ti, the thermal treatment of the invention is preferably carried out at temperatures not higher than 664 °C as shown in Fig. 2. Also, when the mixed powder of the first metal powder and the second metal powder is Al-Ni, Al-Cr or Al-Fe, the thermal treatment step is preferably carried out at temperatures not higher than 636 °C, 663 °C or 654 °C (927 K), respectively. More preferably, the thermal treatment step is carried out above about 500 °C because of the easiness of thermal treatment and the appropriate maintenance of time for intermetallic compound formation.

[85] The coating layer formed on the substrate by the thermal treatment step forms Al matrix composite where intermetallic compounds and ceramic powders are dispersed inside. In case that thermal treatment is carried out below eutectic temperature as in this invention, intermetallic compounds are formed by solid-phase diffusion by solid- phase reaction. Accordingly, as liquid phase is not involved in the formation of intermetallic compounds as in the casting or thermal spraying, it is possible to obtain Al matrix composites where fine intermetallic compounds are dispersed within Al matrix phase.

[86] Meanwhile, in the prior powder metallurgy, the formation of intermetallic compounds from aluminum and other metals at low temperatures not higher than 900 °C, especially below eutectic temperature has been known to be very difficult. This seems because oxides formed on the surface of aluminum powder inhibit the reaction of aluminum with other metals. Accordingly, in the prior powder metallurgy, the formation of intermetallic compounds by the reaction of Al and other metals hardly occurs unless liquid phase is formed in an amount enough to break surface membranes.

[87] However, in accordance with the invention, the reaction of Al and other metals can occur at lower temperatures. This is considered to result from the fact that the surface membranes of the aluminum powders sprayed in the invention are broken by collision

energy upon collision with the substrate surface and thus substantial contact between Al powders and other metal powders becomes possible.

[88] Also, the coating layer formed by the method of the invention has a very high density. Accordingly, although it is exposed to oxygen included in air or atmosphere gas in the process of thermal treatment, the possibility of forming oxidation membrane on the surface of individual Al powder particle is decreased. For such a reason, the thermal treatment step of the invention can be carried out not only in inert gas atmospheres such as nitrogen and argon but also in air.

[89] As described above, the reason why the thermal treatment in this invention is preferably carried out below eutectic temperature (including peritectic temperature) is that liquid phase is not involved in thermodynamic equilibrium state below this temperature and thus the intermetallic compounds of fine dispersion phase are suitably obtained. In actual system, however, as the involvement of liquid phase is slight at temperatures somewhat exceeding the eutectic temperature (including the peritectic temperature), in fact, the role of the liquid phase affecting the formation of the intermetallic compounds can be ignored. Therefore, "below eutectic temperature" described in the appended claims is not intended to be literally interpreted to exclude the temperature ranges including such variation.

[90] The thermal treatment step may have thermal treatment effects for the improvement of the adhesion of the coating layer or mechanical processing for surface illumination control as well as the formation of intermetallic compounds.

[91] Furthermore, the method of preparing metal matrix composites in the invention may further comprise the step of separating the coating layer which is formed in the coating step from the substrate and thus there can be provided a metal matrix composite bulk comprising the metal matrix composite alone.

[92] Also, the invention provides a metal matrix composite coating layer characterized in that it is prepared by the method of preparing metal matrix composite described above. The thickness of the coating layer is preferably 10 um to 1 mm If it is too thin, wear resistance is decreased and if it is thick, it will be expensive to prepare a coating layer, and peeling or heat stress by thermal expansion may be generated.

[93] Further, the invention provides a metal matrix composite bulk characterized in that it is prepared from the metal matrix composite coating layer prepared by the method of preparing metal matrix composite described above by further comprising the step of separating the coating layer formed in the coating step from the substrate.

[94] The wear-resistant metal matrix composite coating layer or bulk obtained from the method of the invention improves mechanical properties of the substrate, coating or bulk.

[95] First, the wear resistance of the members can be improved by containing in-

termetallic compounds and ceramic particles with high hardness in the coating layer or bulk.

[96] Second, the coating layer or bulk prepared by the invention enhances the fatigue properties of the coated parts. Thus, strong binding between the coating layer and the substrate inhibits crack from being generated and as the coating layer possesses the characteristics of metal matrix composite, its fine structure reduces the generation of crack and its propagation rate and therefore, fatigue properties are enhanced. In addition, it helps the parts have high resistance against thermal fatigue destruction. One of the main causes for the generation and propagation of crack in parts used in heat resistant engines such as gas turbines is heat stress due to local temperature difference. In engine blocks, a portion close to the cylinder has high temperature and a portion far from the cylinder has low temperature due to combustion of the engine. Such temperature difference generates heat stress, which causes crack on the engine block surfaces. In particular, where periodical combustion and cooling occur, for example, in engines, it is very important to control heat fatigue destruction properties by periodical heat stress. In the present invention, the thermal conductivity properties of the member can be enhanced by preparing the coating layer using particles having high thermal conductivity such as aluminum or aluminum alloys as a metal and SiC as a ceramic. The improvement of the thermal conductivity properties reduces temperature difference present in the parts thereby resulting in improvement in the heat fatigue destruction properties of the parts. Further, as the formation of composite can reduce difference in thermal expansion coefficient from the substrate, heat stress occurring during heating can be reduced and the peeling and crack generation of the coating layers can be minimized.

[97] The invention is further described in detail by illustration of preferred embodiments of the invention.

[98]

[99] EXAMPLES

[100] Example 1

[101] Mixed metal powders where a weight ratio of Al powder having an average particle diameter of 77 um and Ni powder having an average particle diameter of 3 um is 90: 10 (Al-10%Ni) and 75:25 (Al-25%Ni), respectively were prepared and SiC powder having an average particle diameter of 35 um was mixed therewith in an amount of 5 parts by weight of 100 parts by weight of the mixed metal powder thereby to prepare the final mixed powder. The mixed powder was inserted into a nozzle having an aperture of 4 x 6 mm and a throat gap of 1 mm as a standard laval type nozzle in the conditions of air as a compression gas, 7 atm and flow of transportation gas of 330 °C whereby a coating layer was prepared. The prepared coating was subject to thermal

treatment for four hours at about 450 °C, 500 °C and 550 °C. The thermal treatment was carried out under nitrogen atmosphere. With the surface of the thermally treated substrate, its X-ray diffraction patterns were determined and the results were depicted in Fig. 10 (Al-10%Ni) and Fig. 11 (Al-25%Ni). According to the X-ray diffraction results, as the content of Ni increases and as the thermal treatment temperature increases, Al Ni intermetallic compounds and Al Ni intermetallic compounds were formed a lot, but even though the thermal treatment temperature is low, the intermetallic compounds were absolutely produced.

[102] EDX photographing results for the formation of intermetallic compounds from Ni powder and Al matrix adjacent thereto are as shown in Fig. 12. That is, Al Ni intermetallic compounds were formed in the vicinity of Al matrix where the concentration of Ni is low and Al Ni intermetallic compounds were formed at the inside of Ni powder particles where the concentration of Ni is high.

[103] Fig. 13 showed that where such reactions did not totally occur, residual Ni remained inside the Ni powder.

[104]

[105] Example 2

[106] Mixed metal powders where a weight ratio of Al powder having an average particle diameter of 43 um and Ti powder having an average particle diameter of 43 um is 90:10 (Al-10%Ti) and 75:25 (Al-25%Ti), respectively were prepared and SiC powder having an average particle diameter of 35 um was mixed therewith in an amount of 5 parts by weight of 100 parts by weight of the mixed metal powder thereby to prepare the final mixed powder. The mixed powder was inserted into a nozzle having an aperture of 4 x 6 mm and a throat gap of 1 mm as a standard laval type nozzle in the conditions of air as a compression gas, 7 atm and flow of transportation gas of 330 °C whereby a coating layer was prepared. The prepared coating was subject to thermal treatment for four hours at about 450 °C, 500 °C, 550 °C and 630 °C. The thermal treatment was carried out under nitrogen atmosphere. With the surface of the thermally treated substrate, its X-ray diffraction patterns were determined and the results were depicted in Fig. 14 (Al-10%Ti) and Fig. 15 (Al-25%Ti). According to the X-ray diffraction results, as the content of Ni increases and as the thermal treatment temperature increases, Al Ti intermetallic compounds were formed a lot, but even though the thermal treatment temperature is low, the intermetallic compounds were absolutely produced.

[107] In case that the thermal treatment temperature was 630 °C, EDX photographing results for the formation of intermetallic compounds from Ti powder and Al matrix adjacent thereto are as shown in Fig. 16. That is, it was observed that Al Ti intermetallic compounds were formed in the boundary region of the powder through

mutual diffusion of Al atoms and Ti atoms. In case of Ti, due to its relatively low diffusion rate in comparison with Ni, intermetallic compounds were formed in the in- terfacial regions, and residual Ti which was not totally involved in any reactions remained inside the Ti powder.

[108] The invention is not restricted by the detailed description of the invention and the drawings and it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. Industrial Applicability

[109] In accordance with the method of preparing metal matrix composite of the invention and the metal matrix composite coating layer and bulk prepared by using the same, the production of metal matrix composites where intermetallic compounds and ceramic powders are dispersed is possible at low temperatures in comparison with the prior art so that there is no likelihood of causing damages by thermal strain and thermal shock to the substrates, the growth of intermetallic compounds was suppressed so that mechanical characteristics such as high temperature strength are improved, and the accumulation of heat on the coating layers is prevented and crack generation between the substrates and coating layers or inside the coating layers is suppressed so that resistance against crack generation due to the fatigue of the coating layers can be improved.

[110] In addition, the invention can be applied to prepare members with excellent mechanical strength and also, it can be used to dispersion reinforce the surface of the existing members. In particular, as it is carried out at low thermal treatment temperatures, there is a slight possibility of exercising bad influence on the properties of the members upon surface hardening.

[Ill] Besides, as the invention enables the processing under the environments of the thermal treatment temperature of relatively low temperature, low injection pressure of mixed powders and low transportation gas temperature, it can be manufactured with low costs and it is easy to mass production.