Carbon member having a metal spray coating

Abstract: A carbon member having an excellent bonding property is provided at its surface with a metal spray coating layer, and comprises a carbon substrate and a spray coating layer made from at least one metal selected from Cr, Ti, V, W, Mo, Zr, Nb and Ta or an alloy thereof, in which the coating layer has a ratio of linear expansion coefficient to carbon of 0.73-1.44 and a large chemical affinity to carbon at its interface.

Description: This invention relates to a carbon member provided at its surface with a metal spray coating having an excellent adhesion property, and more particularly to carbon members suitable for use in various rolls made from sintered carbon, metal or vitreous crucibles, various cells, electrodes for electrolysis, structural members for flying objects, heating bodies and sport and leisure goods such as tennis racket, golf club, fishing rods and the like. The carbon products according to the prior art are light in the weight, excellent in the thermal stability, excellent in the thermal and electrical conductivities though they are non-metal, and have mechanical strengths under high temperature environment higher than those of iron steel in the fiber-shaped products, so that they are widely used in up-to-date industrial fields such as chemistry, fiber, high polymer, metal refining, ceramics and the like. However, the carbon products are poor in the wear resistance and low in the bonding force to metals, so that it is required to eliminate these drawbacks by combining with the other materials. As the conventional countermeasure for overcoming the above drawbacks produced in the carbon products, carbon has hitherto been combined with another different materials such as high polymers, metals or the like. Lately, there is noticed metal coating formed by electroplating process, chemical plating process, physical vapor deposition process (PVD) , chemical vapor deposition process (CVD) , spraying process or the like. Up to the present, however, the electroplating process, chemical plating process, PVD process and CVD process are critical in accordance with the shape and size of the product, and also the resulting metal coating layer is relatively thin in the thickness and can not sufficiently develop the functions as the metal coating layer. In the method of forming the metal coating layer by the spraying process, the restriction in accordance with the shape and size of the product is less and also the optional metal can freely be thickened as compared with the other methods for the formation of the metal coating layer. For instance, the method described in JP-A-60-221591 has taken notice of such advantages and proposed a method of forming a metal layer onto a contact surface of a current collecting member of a carbon electrode by the spraying process. In this conventional technique, at least one of Sn, Pb, Zn, Cu, Ag, Al, Ni, Fe, stainless steel, brass, bronze, monel metal and the like is used as a spraying material and spray coated by an electric arc type spraying process to form a carbon electrode. In this technique, however, when using plasma spraying process or detonation flame spraying process, the carbon product is undesirably oxidized or broken by a spraying heat source. In the method of spraying metal onto a surface of a carbon member through the electric arc spraying process described in JP-A-60-221591, there are problems that (1) the spraying material is restricted to a soft metal capable of working to a wire and hence a metal suitable for the joining to carbon may not be used, and (2) the joining strength of the spraying metal recommended by the conventional technique to carbon is weak, and (3) the coating work speed is slow and the operation efficiency is poor because of the electric arc spraying process. FR-A-1277578 discloses a carbon anode which has provided thereon a coating of Zr. Preferably, the surface of the carbon anode is roughened by sand blasting and the Zr is applied to the carbon anode by flame spraying. DE-A-2503374 describes a carbon boat for aluminum evaporation having provided on its surface a layer of refractory metals applied to the carbon boat by flame or plasma spraying, and the surface of the carbon boat has been roughened prior to the application of the refractory metals, for example, by sand blasting. Specific examples of the metals to be used are Ti, Zr and Ta. DE-A-1962036 discloses a carbon member having a protective metallic coating, said metallic coating being formed on the surface of the carbon member by using a spray coating process. Specifically disclosed is a Cr-Ni-alloy coating, preferably having a thickness of from 0.1 to 1 mm. The carbon member surfaces may have corrugations. It is an object of the invention to establish a technique effective for obtaining carbon products having a strong bonding strength between a metal spray coating layer and a carbon material as a substrate and developing excellent composite functions without causing the above problems. The inventors have made various studies in order to achieve the above object and found that the considerable effect of solving the above problems is obtained when the surface of the carbon product is preliminarily subjected to a blast treatment with alumina and then sprayed with a metal having a linear expansion coefficient near to that of carbon and a strong chemical affinity to carbon or an alloy thereof or a mixture with another metal, alloy, ceramic or the like, and as a result the invention has been accomplished. According to one aspect of the invention, there is provided a carbon member comprising a carbon substrate which has been subjected to a blast treatment and a first metal spray coating layer formed on the surface thereof and made from at least one metal selected from V, W, Mo and Nb or an alloy thereof, said coating having a ratio of linear expansion coefficient to carbon of 0.73-1.44 and a large chemical affinity to carbon at its interface. According to another aspect of the invention, there is provided a carbon member comprising a carbon substrate which has been subjected to a blast treatment and a first metal spray coating layer formed on the surface thereof and made from a mixture or an alloy of (a) at least one metal selected from V, W, Mo, Nb or an alloy thereof and (b) at least one member selected from Ni, Al, Cu, Co or an iron alloy, said coating having a ratio of linear expansion coefficient to carbon of not more than 1.85 and a large chemical affinity to carbon at its interface. In a preferred embodiment of the invention, the first metal spray coating layer is further provided at its surface with a second spray coating layer of a metal having a ratio of linear expansion coefficient of more than 1.44 or a non-metallic compound having a ratio of linear expansion coefficient of less than 0.73. The invention will be described with respect to the accompanying drawing. Fig. 1 is a sectional view showing a method for measuring a bonding strength of a metal spray coating layer formed on a sintered carbon substrate. According to the invention, the reason why a material of spraying metal to be applied onto a surface of a carbon substrate is restricted to V, W, Mo and Nb or an alloy thereof is due to the fact that the linear expansion coefficient of these metals or alloys is within the range of 4.5 to 8.9 x 10 -6 /°C (room temperature to 100°C) and is near to that of the carbon substrate (4.2 to 6.5 x 10 -6 /°C: room temperature to 100°C). In general, when different materials are joined to each other, if there is a difference in the linear expansion coefficient between both the materials, shearing stress is caused at the joint face in accordance with the temperature change or the like in use, and if such a stress exceeds the joining strength, both the materials are peeled off from each other. As the spraying metal used in the invention, at least one metal or alloy having a linear expansion coefficient of 4.5 x 10 -6 /°C to 8.9 x 10 -6 /°C is used. These metals and alloys are characterized by having a small difference to the linear expansion coefficient of the carbon substrate. The ratio of the linear expansion coefficient of the metal to the linear expansion coefficient of the carbon substrate (6.2 x 10 -6 /°C) , i.e the linear expansion coefficient ratio (metal/carbon), is within a range of 0.73-1.44. (x10 -6 /°C, room temperature to 100°C) Metal V W Mo Nb Linear expansion coefficient 8.3 4.5 5.1 7.2 Linear expansion coefficient ratio 1.34 0.73 0.82 1.16 If two or more of these metals are mixed or alloyed for use in the spraying materials, the ratio of linear expansion coefficient to carbon of the mixture or alloy has to be within the range of 0.73-1.44. Although a metal or alloy having a linear expansion coefficient ratio of more than 1.59 such as e.g. Ni, Al, Cu or Fe alloy (stainless steel) is not a spraying material used according to the invention, it may be mixed with the above spraying material for the spray coating. In the latter case, the ratio of the linear expansion coefficient of the metal or alloy mixture to the linear expansion coefficient of the carbon substrate is not more than 1.85. The peeling phenomena at the joint portion due to the thermal expansion difference can be prevented as long as the above linear expansion coefficient ratio is maintained. The spraying metal or alloy used according to the invention is required to have an excellent chemical affinity to carbon. That is, fine particles of the metal or alloy fly toward the surface to be coated at a molten state during the spraying and collide with the surface of the carbon substrate to form a coating layer. In this case, it has been found that the metal or alloy takes a lamination structure physically bonded to the substrate and also is bonded thereto through strong chemical affinity. In general, it is known that the intensity of the chemical affinity between metal and carbon has the following order, from which it is apparent that all of the metals used according to the invention have strong affinity. Nb> V> W> Mo> Cr> Mn> Fe> Ni> Co> Si Moreover, the metals and alloys used according to the invention have a side that they are easily oxidized by oxygen under a high temperature spraying environment. However, the resulting oxides are low in the sublimation temperature (for example, sublimation temperature of each oxide MoO 3 : 795°C, WO 3 : 1000°C, Nb 2 O 5 : not lower than 1370°C), and evaporate very easily in the spraying heat source such as plasma, combustion gas or the like. Therefore, when they collide with the carbon substrate, the oxide film formed on the surface of the metal particle becomes fine and hence the bonding strength of the coating layer is improved without obstructing the chemical affinity to carbon. Moreover, a slight amount of oxide film retained without evaporation is microscopically removed by reduction reaction. In the carbon member according to the invention, after the first spray coating layer of the above metal or alloy is formed, a second layer having a ratio of linear expansion coefficient of more than 1.85 formed from e.g. Ni, Al, Cu or Ca may be sprayed thereon to form a multilayer-like coating layer. Alternatively, ceramics such as e.g. Al 2 O 3 , ZrO 2 , TiO 2 , Cr 2 O 3 , WC, TiC or Cr 3 C 2 or cermet material thereof, which have a ratio of linear expansion coefficient smaller than 0.73, may be sprayed to form a multi-layer like coating layer. According to the invention, many kinds of e.g. the metal, alloy or ceramics may be coated onto the surface of the carbon substrate. In this meaning, the invention is expected to be applied to various industrial fields and is considerably large in the industrial merit. In the invention, the thickness of the metal spray coating is desirable to be 0.05-5 mm on the surface of the carbon substrate. When the thickness is less than 0.05 mm, the effect of the metal coating layer can not sufficiently be developed, while when it exceeds 5 mm, the operation takes a long time and the amount of metal consumed becomes large and disadvantageous in the economical reasons. If the material having a linear expansion coefficient larger than the above defined range or the material having a linear expansion coefficient smaller than the above defined range is sprayed to form a multi-layer like coatings, the thickness of the metal spray coating as an under layer may be thinned. In the production of the metal spray coating layer, obtained by spraying the metal or alloy used according to the invention, e.g. plasma, combustion gas flame or explosion energy may be used as a spraying heat source. Further, the spraying may be conducted in atmosphere or an argon gas environment under a low pressure as a spraying atmosphere. According to such a spraying method, the coating metal or alloy may be rendered into a linear or powdery form. In the invention, the metal spraying material of powdery state may be used, so that different metals may be mixed at an optional mixing ratio to form a metal spray coating layer for various applications. Moreover, when such spraying powdery materials are passed through a high temperature plasma flame even at the mixed state, the powdery materials are rendered into a molten state containing a large amount of metal completely alloyed through metallurgical reaction and adhere to the surface of the carbon substrate, which largely contributes to improve the bonding strength of the coating layer. As mentioned above, the metal spray coating layer having an excellent bonding property is formed on the surface of the carbon substrate. The following example is given in illustration of the invention. Example A high density carbon sintered body of a discontinuous layer structure (width 50 x length 100 x thickness 10 mm) was used as a test mother material. It was first subjected to a blasting treatment with Al 2 O 3 (# 60) and further at the surface thereof to an electric arc spraying process, a powdery spraying process using oxygen·acetylene combustion flame as a heat source (flame spraying), an atmospheric plasma spraying process mainly using argon gas (atmospheric plasma spraying), or a spraying process in which plasma as a heat source is used in an atmosphere adjusted to 1.10 4 to 2.10 4 Pa (100-200 mbar) with argon gas after the removal of air (low-pressure plasma spraying), whereby various spraying materials shown in table 2 were spray-coated at a thickness of 150 µm. Thereafter, a circular specimen of 25 mm in diameter was cut out from a carbon sintered body provided with the resulting metal spray coating and subjected to a method of measuring the bonding strength as shown in Fig. 1 to measure the bonding property between the metal spray coating layer and the carbon substrate. In Fig. 1, numerals 1, 2 are fixing jigs of carbon steel, numeral 3 a sintered carbon body, numeral 4 a metal spray coating layer, and numeral 5 a joint portion with a synthetic resin adhesive. The measurement of the bonding property was carried out by providing two carbon steel (SS400) jigs of 25 mm in diameter and 80 mm in length, applying a thermosetting epoxy resin to each end portion of the jigs, pushing the carbon specimen provided with the metal spray coating layer onto the resin surfaces between the jigs, and baking the resin through heating at 150°C for 90 minutes. The measured results of the bonding strength of the metal spray coating layer in accordance with the spraying material are shown in Table 2. No. Coating metal Ratio of linear expansion coefficient Spraying process peeling arc spraying flame spraying atmosphere plasma spraying low-pressure plasma spraying Acceptable Example 1 V 1.34 - 320 350 450 ○ 2 W 0.73 - - 405 430 ○ 3 Mo 0.82 310 - 380 420 ○ 4 Nb 1.16 - - 390 420 ○ Comparative Example 5 Al 3.80 55 35 18 20 × 6 Co 2.02 65 40 58 60 × 7 Fe 1.95 140 130 145 180 × 8 Ni 2.15 35 38 42 51 × 9 Cu 2.74 28 25 25 20 × 10 Zn 5.00 35 41 20 20 × 11 Stainless steel 2.58 30 41 28 22 × [Note] - : not conducted ○ : no peeling × : peeling occurred As seen from the results of Table 2, the bonding strength in all of the acceptable examples exceeded 300 kgf/cm 2 , while the bonding strength in the metal spray coating layer as a comparative example was very low and the peeling was caused at a bonding strength of less than 200 kgf/cm 2 . Furthermore, when the peeled portion was observed by means of an optical microscope, the peeling was almost caused at an interface between the metal spray coating layer and the carbon substrate. In the metal spray coating layers having a very small bonding strength (Nos. 5, 8, 9, 11), when the section of the spray coated portion in these layers before the tensile test or immediately after the spraying was observed by means of an optical microscope, the metal spray coating layer was at a state just before the peeling from the carbon substrate, or the interface between the metal spray coating layer and the carbon substrate was completely peeled though the abnormal appearance was not observed. On the contrary, all of the acceptable metal spray coating layers (Nos. 1-4) showed a bonding strength of not less than 300 kgf/cm 2 . Furthermore, the breaking was observed in the vicinity of the central portion of the carbon substrate, but the interface between the metal spray coating layer and the carbon substrate showed a good bonding property. Among the above spraying processes, a linear metal was used in the arc spraying process, but a metal wire of e.g. V, W or Nb was not commercially available (which could not be produced technically or economically), so that each of these metals was used in form of powder, from which the metal spray coating layer was formed by the other spraying process. As the spraying process, it has been found that the low-pressure spraying process shows the best bonding property, but the excellent bonding strength was provided even in the atmosphere spraying process and other spraying processes. That is, as seen from the above test, the bonding property is largely influenced by the kind of the spraying metal rather than the kind of the spraying process. As mentioned above, the metal spray coating layer made from the metal or alloy having a linear expansion coefficient ratio to carbon of 0.73-1.44, or from a mixture (with another material) having a linear expansion coefficient ratio of not more than 1.85 has a strong bonding strength to the sintered carbon substrate. Therefore, in the carbon member according to the invention, the joining property of the carbon substrate to the abrasion-resistant metallic member, which has not been attained in the conventional technique, can be improved while maintaining the properties of the carbon substrate. Furthermore, it is possible to produce carbon members coated with various ceramics, and the formation of the ceramic insulation coating is easy together with the improvement of the appearance, so that the invention develops large effects on the improvement of the properties as a carbon product and the enlargement of applications.

Claims: A carbon member comprising a carbon substrate which has been subjected to a blast treatment and a first metal spray coating layer formed on the surface thereof and made from at least one metal selected from V, W, Mo and Nb or an alloy thereof, said coating having a ratio of linear expansion coefficient to carbon of 0.73-1.44 and a large chemical affinity to carbon at its interface. A carbon member comprising a carbon substrate which has been subjected to a blast treatment and a first metal spray coating layer formed on the surface thereof and made from a mixture or an alloy of (a) at least one metal selected from V, W, Mo, Nb or an alloy thereof and (b) at least one member selected from Ni, Al, Cu, Co or an iron alloy, said coating having a ratio of linear expansion coefficient to carbon of not more than 1.85 and a large chemical affinity to carbon at its interface. The carbon member of claim 1 or 2, wherein a second spray coating layer of a metal having a ratio of linear expansion coefficient of more than 1.44 or a non-metallic compound having a ratio of linear expansion coefficient of less than 0.73 is provided on the surface of said metal spray coating layer. The carbon member of claim 1 or 2, wherein said first metal spray coating layer formed on said carbon substrate has a thickness of 0.05-5 mm.

Inventors: HARADA YOSHIO, HIRAIWA JIRO, KURODA KOJI, OKUNISHI SHIGEKI, TAKEUCHI JUNICHI, TOJO TETSURO, HARADA, YOSHIO, HIRAIWA, JIRO, KURODA, KOJI, OKUNISHI, SHIGEKI, TAKEUCHI, JUNICHI, TOJO, TETSURO
Applicants: TOCALO CO LTD, TOYO TANSO CO, TOCALO CO. LTD., TOYO TANSO CO., LTD.
Application year: 1992
Main classification: C23C4
IPC codes: C04B41, C01B31, C23C4