Description

METHOD OF IMPROVING SURFACE PROPERTIES OF THE METAL AND METAL WITH COATING LAYER PREPARED BY

THE SAME

Technical Field

[1] The present invention relates to a method of improving the surface properties of a metal and the metal with a coating layer prepared by the same and more particularly, it relates to a method of improving the surface properties of a metal, which does not cause damages such as heat distortation to a substrate during the formation of a coating layer, can form an intermetallic compound coating layer having superior properties with regard to surface properties such as wear resistance and fatigue crack property, enables heat treatment within a short time at a low temperature, thereby decreasing manufacturing costs, and can minimize the occurrence of residual stress between the substrate and coating layer according to heat treatment because the intermetallic compound is formed in the process of heat treatment, and the metal with a coating layer prepared by the same. Background Art

[2] A metal comprising a single metal or alloy is hardened by applying various hardening mechanism including precipitation hardening and dispersion reinforcement to the entire area inside the metal to improve its strength, hardness, wear resistance, etc.

[3] However, in the case of such property improvement applied to the entire area of substrates, the increase of excessive strength or hardness reduces the tenacity of materials and thus causes weak brittleness. Accordingly, in order to improve it, there are being developed various methods of improving only the surface of metals such that surface requirement properties such as high strength, high hardness, excellent wear resistance, etc. are satisfied at the surface and the substrates still secure high tenacity and thus overall mechanical properties depend on the substrates, i.e., for utilizing only the advantages of both sides.

[4] However, in such surface improvement methods, in case of deposition, it requires expensive equipments and manufacturing costs are high and thus, it is impossible to form a thick coating layer. In addition, in a method of arranging at the surface of a metal slurry or powder that reacts with the metal and performing heat treatment at a high temperature such that reactive materials having high strength are formed, as the heat treatment of high temperature based on equilibrium phase diagram is carried out for the reaction, damages such as heat distortation occur in the substrate during the

process of heat treatment, or the combination of the practically applicable substrate and powder is relatively low because the reaction cannot occur below the melting point of the powder or substrate when the equilibrium phase diagram is used as a referential point.

[5] Further, as in addition to heat treatment at a high temperature, heat treatment or reaction time over long time is required due to low driving force, manufacturing costs increase.

[6] Besides, there are methods of diffusion bonding or high-temperature spray to bind a coating layer with a substrate, but as they are carried out at high temperatures, manufacturing costs are high, heat treatment over long time for diffusion is required, and the strength of binding surfaces is decreased because residual stress between the substrate and coating layer occurs during the process of cooling materials after binding or spraying.

Disclosure of Invention Technical Problem

[7] In order to solve the problems of the prior arts, it is an object of the present invention to provide a method of improving the surface properties of a metal which is unlikely to cause damages by heat shock or thermal distortation to substrate and at the same time, can improve the surface properties of the metal, and the metal with a coating layer prepared by the same.

[8] Also, it is another object of the invention to provide a method of improving the surface properties of a metal capable of conducting surface improvement at a relatively low temperature with inexpensive costs and minimizing residual stress between the substrate and coating layer, and the metal with a coating layer prepared by the same.

[9] Furthermore, it is still another object of the invention to provide a method of improving the surface properties of a metal capable of forming surface improvement layers having a variety of combinations and types, of which the formation was impossible based on the equilibrium phase diagram which is a thermodynamic equilibrium state, and the metal with a coating layer prepared by the same. Technical Solution

[10] To achieve the aforementioned objects, the present invention provides a method of improving the surface properties of a metal comprising the steps of:

[11] providing a substrate comprising a single metal or alloy matrix at its surface;

[12] preparing a powder for coating comprising one or two more single metals or the powder of the alloy for forming an intermetallic compound, which forms an in- termetallic compound with the single metal or the metal element of the alloy;

[13] coating the prepared powder for coating onto the substrate by a cold spray method;

and

[14] forming the intermetallic compound by the heat treatment of the coated coating layer and the substrate.

[15] Also, the invention provides a metal with a coating layer prepared by the method of improving the surface properties of the metal as stated above. Advantageous Effects

[16] In accordance with the method of improving the surface properties of metal of the invention and the metal with coating layer prepared by the same, there is no likelihood of causing damage by heat shock or thermal distortation to a substrate because an intermetallic compound coating layer can be prepared at low temperatures in comparison with the prior art, mechanical properties such as high-temperature strength are enhanced because the growth of intermetallic compounds is suppressed, and resistance against crack generation due to the fatigue of the coating layer can be improved by preventing the accumulation of heat on the coating layer and inhibiting crack generation inside the coating layer or between the substrate and the coating layer.

[17] 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 heat treatment temperatures, there is a slight possibility of exercising bad influences on the properties of members upon surface hardening.

[18] Besides, as the invention enables the processing at the heat treatment temperature of relatively low temperatures, surface improvement layers having a variety of combinations and types of which the formation was impossible based on equilibrium phase diagram which is a thermodynamic equilibrium state can be formed, manufacturing costs are inexpensive and mass production is easy. Brief Description of the Drawings

[19] Fig. 1 is a schematic view of a cold spray apparatus used to form a metal matrix composite in the present invention.

[20] Fig. 2 to Fig. 5 are phase diagrams illustrating intermetallic compounds formable on

Al matrix by the method of improving the surface properties of a metal according to the invention.

[21] Fig. 6 shows EDX photographing results of each part when Example 1 according to the method of improving the surface properties of metal of the invention was performed.

[22] Fig. 7 shows EDX photographing results of each part when Example 2 according to the method of improving the surface properties of metal of the invention was performed.

[23] Fig. 8 shows EDX photographing results of each part when Example 3 according to the method of improving the surface properties of metal of the invention was performed.

[24] Fig. 9 shows EDX photographing results of each part when Example 4 according to the method of improving the surface properties of metal of the invention was performed.

[25] * Reference Number of Drawings*

[26] 110: Gas compressor 120: Gas heater

[27] 130: Powder feeder 140: Nozzle

Mode for the Invention

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

[29] The present invention relates to a method of improving the surface properties of a metal, which comprises the steps of providing a substrate comprising a single metal or alloy matrix at its surface, preparing a powder for coating comprising one or two more single metals or the powder of the alloy for forming an intermetallic compound, which forms an intermetallic compound with the single metal or the metal element of the alloy, coating the prepared powder for coating onto the substrate by a cold spray method, and forming the intermetallic compound by the heat treatment of the coated coating layer and the substrate.

[30] That is, the present invention is directed to a method of improving the surface properties of a metal wherein the metal herein is broadly used to encompass not only a metal of a single element but also alloys, and this invention is a method related to the improvement of improving the surface properties of such metals.

[31] For this, a substrate comprising a single metal or alloy matrix that is a target metal for surface improvement at its surface is provided wherein the single metal means a metal of a one element and the alloy means metal including two or more metals. The definition of the alloy encompasses alloys including precipitates or dispersion- strengthened materials and accordingly, the substrate can consist of the single metal or alloy as described above of which the entire surface can form an intermetallic compound or can consist of the single metal or alloy as described above of which the partial surface can form an intermetallic compound, and there can be mentioned various materials including composites or combinations having at their surface a certain single metal or alloy matrix forming an intermetallic compound with a metal or alloy to be coated by cold spray thereafter.

[32] The surface improvement of the invention is focused largely on the improvement of the mechanical properties including strength, hardness, wear resistance, fatigue

properties, etc. at the surface of substrates and intended to improve them at maximum, but it is not necessarily limited thereto. That is, it may aim at the improvement of various surface properties including plasma resistance. For this, one or two more single metals or the powder of the alloy for forming an intermetallic compound which form an intermetallic compound with the single metal or the metal element of the alloy exposed at the surface of the substrate is coated onto the surface of the substrate by a cold spray method.

[33] For this step, a powder for coating comprising one or two more single metals or the powder of the alloy for forming an intermetallic compound which forms an intermetallic compound with the single metal or the metal element of the alloy is prepared.

[34] The single metal or alloy of the substrate can be all single metals including a metal element capable of forming an intermetallic compound or alloys containing the metal element, and the single metal or alloy of the powder for coating can be all single metals including a metal element capable of forming an intermetallic compound with the single metal or alloy of the substrate or alloys containing the metal element. Also, the powder for coating can be one single metal powder, but two or more single metal powders can be used after being mixed in order to form multi-component, intermetallic compounds such as three-component system or four-component system. If necessary, in order to accelerate reaction or form three-component or four-component, intermetallic compounds or secure the mechanical properties of a residual layer after the formation of the intermetallic compound, for the powder for coating as described above, there can be used a variety of combinations such as one alloy powder, two or more alloy powders each of which is made respectively of two or more alloys, a mixture of a single metal powder and alloy powder, a mixture of one single metal powder and two or more alloy powders, a mixture of two or more single metal powders and one alloy powder, a mixture of two or more single metal powders and two or more alloy powders, etc.

[35] As preferred examples of the combinations of the single metal or alloy of the substrate and the powder for coating, the single metal or alloy of the substrate can be aluminum or alloy thereof and the powder for coating can be a single metal selected from the group consisting of titanium, nickel, chrome, iron and a mixture thereof, or the single metal or alloy of the substrate can be titanium or alloy thereof and the powder for coating can be a single metal selected from the group consisting of aluminum, nickel, and a mixture thereof, or the single metal or alloy of the substrate can be nickel or alloy thereof and the powder for coating can be aluminum. Thus, in the case of the alloys described above, since surface improvement such as wear resistance and hardness is required in many occasions and stable intermetallic

compounds can be produced, it is advisable to use such combinations. As specific examples for the formation of intermetallic compounds, examples between Al metal and transition metal element capable of forming an intermetallic compound with the Al metal are described on the basis of the phase diagrams. Fig. 2 to 5 are phase diagram of two-element aluminum alloys as examples of the formation of intermetallic compounds with aluminum which are formable by the method of the invention.

[36] First, Fig. 2 is a phase diagram of Al-Ti system. With reference to Fig. 2, when Ti is added in an amount of several to tens % by weight, Al phase where Ti is solid solubilized in a small amount within alloys and TiAl phase which is an intermetallic compound of Al-Ti exist as stable phases at 664 °C (937 K) or lower temperatures. As the amount of Ti increases (that is, it is added in an amount of 38 % or more by weight), Al Tiphase and Al Tiphase exist as stable phases of alloys.

[37] Fig. 3 is a phase diagram of Al-Ni type. With reference to Fig. 3, it can be seen that at 636 °C or lower temperatures, the intermetallic compounds of Al Ni, Al Ni , AlNi, AlNi and the like form the stable phases of alloys according to the amount of Ni.

[38] Fig. 4 is a phase diagram of Al-Cr type. With reference to Fig. 4, it can be seen that at 663 °C (936 K) or lower temperatures, the intermetallic compound of CrAl forms a stable phase according to the addition of Cr.

[39] Meanwhile, Fig. 5 is a phase diagram of Al-Fe type and as shown in the figure, in the case of Al-Fe type, the intermetallic compounds of metastable phase such as FeAl can be formed at 654 °C (927 K) or lower temperatures.

[40] With regard to the particle size of the powder of such elements for coating, particles having various sizes used in the known cold spray can be used and preferably, as the coating efficiency and reactivity depend on the type of powders to be used and thus optimal particle size can vary, it is necessary to select a suitable particle size in consideration of that and generally, ones having the size of 1 to 200 um are advantageous for dispersion and mixing. More preferably, as the powders for coating are changed into intermetallic compounds by subsequent heat treatment step, finer particles are advisable to obtain uniform and smooth reaction and thus it is preferable to have the size of 1 to 50 um. That is, if the size of the particles is too small, the weight of the particles is small and thus impulse becomes too small in spite of their fast speed when they collide with the coating layer and as a result, the accumulation of strain energy is small because strain owing to the collision is small and processed hardening such as shot peening is less generated. On the other hand, if the size of the particles is too big, although the impulse is big, collision frequency and area is small and thus the total strain is small, the accumulation of strain energy is small, processed hardening is small, and the formation of intermetallic compounds is not evenly made. Thus, there exist optimal medium size ranges to maximize the processed hardening and im-

provement effects through the formation of intermetallic compounds as described above.

[41] Upon the completion of such preparations, the coating step is carried out by coating the prepared powder for coating onto the substrate by a cold spray method. That is, the coating layer is formed by spraying the powder for coating prepared above via the cold spray method at relatively low temperatures in comparison with thermal spraying or sintering temperatures.

[42] The cold spray method itself is already known and preferably, the cold spray method can comprise the steps of injecting the powder for coating prepared above into a spray nozzle for coating, and coating the powder for coating onto the surface of the substrate by accelerating the powder for coating 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 the schematic view of an apparatus for such cold spray is as shown in Fig. 1.

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

[44] 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).

[45] Compressed gas of about 5 to 20 kgf/cm provided by the gas compressor (110) coats the powders fed from the powder feeder (130) by ejecting them at a speed 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.

[46] 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 the figure, 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).

[47] 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.

[48] For the detailed description about the operation and structure of the apparatus shown herein, see US Patent No. 5,302,414 by Anatoly P. Alkhimov et al.

[49] In the cold spray coating step, although the substrate can proceed at a room temperature or low temperature, it is preferable to proceed in heated state above a

certain temperature because the accumulation of strain energy resulting from the collision of the powder for coating and the deep collision of the powder for coating are induced. That is, although the powder is changed into an intermetallic compound in the subsequent heat treatment step, it is desirable that the powder for coating is deeply embedded in the substrate because the falling-off of particles can be prevented when in subsequent use of the substrate. More preferably, it is desirable that the heating temperature is 1/2 or less of the melting point of the substrate for the accumulation of strain energy and the deep embedment of the powder.

[50] Upon the completion of the coating step, the heat treatment step to form an intermetallic compound is carried out by the heat treatment of the coated coating layer and the substrate. The heat treatment can occur at appropriate temperatures on the basis of the equilibrium phase diagrams as shown in Fig. 2 to 5. In particular, in the present invention, as the collision particles and the substrates near them have high strain rate and undergo severe strain by the cold spray process and have high vacancy concentration due to the damage, they possess high driving forces and thus reactions where intermetallic compounds are formed occur at temperatures much lower than eutectic or peritectic temperatures exhibited in the equilibrium phase diagram. Accordingly, it is preferred that the heat treatment is carried out at the eutectic or peritectic temperatures of the intermetallic compounds or below for productivity and reduction in manufacturing costs.

[51] Fig. 2 to Fig. 5 are phase diagrams of equilibrium state showing examples of transition metal elements capable of forming intermetallic compounds with Al metal as described above.

[52] With reference to Fig. 2, when Ti is added in several to 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 664 °C (937 K) or lower temperature. Accordingly, although it is possible to raise the heat treatment temperature above 664 °C (937 K) and then cool it, the present invention enables the formation of the intermetallic compounds in the heat treatment at temperatures lower than that.

[53] Also, it can be seen from the phase diagram of Al-Ni type in Fig. 3 that at 636 °C or lower temperatures, the intermetallic compounds of Al Ni, Al Ni , AlNi, AlNi and the like form the stable phases of alloys according to the amount of Ni. Accordingly, in this case, although it is possible to raise the heat treatment temperature above 636 °C and then cool it, the present invention enables the formation of the intermetallic compounds in the heat treatment at temperatures lower than that. Also, in the phase diagrams of Al-Cr type in Fig. 4 and Al-Fe type in Fig. 5, it is possible to carry out heat treatment at temperatures not higher than 663 °C or 654 °C (927 K), respectively.

[54] More preferably, it is desirable that the heat treatment step is carried out at 500 °C or above for the easiness of heat treatment and the suitable formation time of in- termetallic compounds

[55] As described above, the intermetallic compounds are formed by solid-phase diffusion of solid-phase reaction in the heat treatment step. Accordingly, since liquid phase is not involved in the formation of the intermetallic compounds as in casting or thermal spraying, the substrate with fine intermetallic compounds distributed on the surface thereof can be obtained.

[56] Meanwhile, in the prior powder metallurgy, the formation of intermetallic compounds from aluminum and other metals at low temperatures of 900 °C or below, especially eutectic temperature or below 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.

[57] 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 powders sprayed in the invention are broken by collision energy upon collision with the substrate surface and thus substantial contact between Al and other metals becomes possible.

[58] 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 heat treatment step of the invention can be carried out not only in inert gas atmospheres such as nitrogen and argon but also in air.

[59] As described above, the reason why the thermal treatment in this invention is preferably carried out at eutectic temperature (including peritectic temperature) or below is that in principle, liquid phase is not involved in thermodynamic equilibrium state below this temperature and accordingly, it is suitable to obtain the intermetallic compounds of fine dispersion phase and it can be applied to circumstances where the involvement or production of liquid phase should be avoided. 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 intermetallic compounds can be ignored. Therefore, "eutectic Temperature or below" described in the appended claims is not intended to be literally interpreted to exclude the temperature ranges including such variation.

[60] The heat treatment step may have heat 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.

[61] The substrate after the heat treatment can be used as it is, or it can be used after the step of removing the powders for coating within the coating layer that are not reacted into intermetallic compounds.

[62] Further, in addition to the above methods, after the powders for coating are coated by the cold spray method, inert particles which are irrelevant to the formation of intermetallic compounds can be further cold sprayed onto the top thereof. The spraying of the inert particles can proceed such that the substrate is coated or it can proceed such that simple collision merely occurs and coating does not occur, and after the spraying process of the inert particles, the step of removing the inert particles can be further included. This processing can increase the improvement effects of surface properties as the penetration of the powder particles for coating can be evener and deeper. The inert particles are preferably ceramic particles or high-hardness ceramic particles. The high- hardness ceramic particles have the merit that they can contribute to the improvement of surface properties together with the intermetallic compounds where they remain on the surface of the substrate after the processing.

[63] Also, the invention provides a metal with a coating layer of which the surface is improved through the method of improving the surface properties of metal as described above. The metal refers to the substrate and it is broadly used to include not only the metal of a single element but also an alloy and further to include a substrate with a single metal or alloy matrix on the surface thereof. That is, it can include a variety of materials such as a single metal of one element metal, an alloy which is a metal containing two or more metals, an alloy which contains a precipitate or dispersion strengthened material in addition to the alloy, a composite or combination with a certain single metal or alloy matrix on the surface thereof and so on.

[64] The coating layer can be formed into a uniform layer according to the formation of intermetallic compounds or it can be constituted in the form of intermetallic compound particles being dispersed.

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

[66]

[67] EXAMPLES

[68] Example 1

[69] After Al powder having an average particle diameter of 77 um was prepared, it was injected into a nozzle having an aperture of 4 X 6 mm and a throat gap of 1 mm as a standard laval type nozzle, using air as a compression gas at 7 atmospheres with the

flow of transportation gas of 330 °C whereby a coating layer was formed on a Ni substrate. The formed coating was subject to heat treatment at about 450 °C for 4 hours. The heat treatment was carried out under nitrogen atmosphere.

[70] The EDX (back scattered electron microscope) photographing results about the formation of intermetallic compounds between Al powder and Ni matrix are as shown in Fig. 6. That is, it was observed that Al Ni intermetallic compound was formed between the Al powder coating layer and Ni matrix.

[71]

[72] Example 2

[73] With the exception that the heat treatment was carried out under air atmosphere, the same procedures as used in Example 1 were carried out to form a coating layer. The EDX photographing results about the formation of intermetallic compounds between Al powder and Ni matrix are as shown in Fig. 7. That is, it was observed that Al Ni intermetallic compound was formed between the Al powder coating layer and Ni matrix.

[74]

[75] Example 3

[76] After Ti powder having an average particle diameter of 77 um was prepared, it was injected into a nozzle having an aperture of 4 X 6 mm and a throat gap of 1 mm as a standard laval type nozzle, using air as a compression gas at 7 atmospheres with the flow of transportation gas of 330 °C whereby a coating layer was formed on an Al substrate. The formed coating was subject to heat treatment at about 450 °C for 4 hours. The heat treatment was carried out under nitrogen atmosphere.

[77] The EDX photographing results about the formation of intermetallic compounds between Ti powder and Al matrix are as shown in Fig. 8. That is, it was observed that Al Ni intermetallic compound was formed between the Ti powder coating layer and Al matrix.

[78]

[79] Example 4

[80] With the exception that the heat treatment was carried out under air atmosphere, the same procedures as used in Example 3 were carried out to form a coating layer. The EDX photographing results about the formation of intermetallic compounds between Ti powder and Al matrix are as shown in Fig. 9. That is, it was observed that Al Ni intermetallic compound was formed between the Ti powder coating layer and Al matrix.

Industrial Applicability

[81] In accordance with the method of improving the surface properties of metal of the invention and the metal with coating layer prepared by the same, there is no likelihood of causing damage by heat shock or thermal distortation to a substrate because an in-

termetallic compound coating layer can be prepared at low temperatures in comparison with the prior art, mechanical properties such as high-temperature strength are enhanced because the growth of intermetallic compounds is suppressed, and resistance against crack generation due to the fatigue of the coating layer can be improved by preventing the accumulation of heat on the coating layer and inhibiting crack generation inside the coating layer or between the substrate and the coating layer.

[82] 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 heat treatment temperatures, there is a slight possibility of exercising bad influences on the properties of members upon surface hardening.

[83] Besides, as the invention enables the processing at the heat treatment temperature of relatively low temperatures, surface improvement layers having a variety of combinations and types of which the formation was impossible based on equilibrium phase diagram which is a thermodynamic equilibrium state can be formed, manufacturing costs are inexpensive and mass production is easy.

[84] The invention is not restricted by the detailed description of the invention, examples and the drawings and there is no doubt that various changes and modifications made by those skilled in the art within the range of not departing from the spirit and scope of the invention defined in the appended claims are included within the scope of the invention.