METHOD OF FORMING CERAMIC LAYER ON METALLIC BODY

Abstract: A method of forming a ceramic layer, which is compact and rich in adhesion to a metallic body, on the metallic body without adding binders even though said ceramics is hard to be sintered ceramics such as various kinds of non oxide ceramics. Said method comprises a step of placing ceramic powders and/or a mixture of ceramic powders and metallic powders or a mixture of metallic powders and non metallic powders on the metallic body and a step of forming the ceramic layer on the metallic body in a moment by a reaction heat of the Thermit<R> reaction under the pressurized condition. In addition, a metallic insert member can be disposed between said metallic body and various kinds of powder placed on said metallic body. Furthermore, the resulting ceramic/metal composite member is subjected to a hot hydrostatic pressing or a hot pressing under high temperatures and high pressures.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to a method of forming a compact ceramic layer on a metallic body, and more particularly to a method of forming a compact ceramic layer having an enhanced adhesion to the metallic body by utilizing a great deal of heat developed momentarily by Thermit reaction which occurs conveniently under a pressurized condition. BACKGROUND ART OF THE INVENTION As to the conventional methods of forming a ceramic layer on a metallic body, there are known CVD and PVD vapor deposition methods and plasma spray coating method, etc.. Further, a method for sintering a ceramic powder, a metal powder, or a mixture thereof, which utilizes the heat developed by Thermit reaction as the heat source, is disclosed in Japanese Laid-Open Patent Application No. SHO 61-186404 (which corresponds to U.S. Patent No. 4,761,262). The above-mentioned CVD und PVD vapor deposition methods, etc. quoted as prior art have so far involved problems or points at issue, in particular, on the adhesion of coating on metals, and are unable to provide excellent characteristics of ceramic layers themselves such as resistance to abrasion, resitance to oxidation, and resistance to corrosion, etc.. JP-A-62-238 303 discloses a method in which the heat of Thermit reaction is used for bonding a composite metallic body such as WC-Co to a metallic body, e.g. copper. However, this document does not disclose a method for bonding a ceramic layer to a metallic body. US-A-4 363 832 discloses a method for forming a ceramic lining layer on the inward surface of a hollow body such as a pipe. According to this method, a Thermit mixture is disposed inside of the hollow body, and the hollow body is rotated at a high speed so that the Thermit mixture is pressed against the wall of the hollow body. The Thermit composition is ignited, so that the molten metal generated in the Thermit reaction flows outwardly due to the centrifugal forces, and a ceramic lining at the internal surface of the metal body is formed by the oxide of the strongly reductive component of the Thermit composition. Thus, the ceramic layer can only be formed by materials which are part of the initial Thermit composition. OUTLINE OF THE INVENTION It is an object of the present invention to provide a method of forming a ceramic layer on a metallic body which overcomes the above-mentioned problems in the prior art. According to the invention, this object is achieved by the features indicated in the independent claims 1 and 3. Optional features of the invention are indicated in the dependent claims. Since the method of forming a ceramic layer on a metallic body according to the present invention is a process which utilizes the sudden generation of heat by Thermit reaction and the exothermic reaction of a ceramic compound which is induced effectively and efficiently by the Thermit reaction, various kinds of non-oxidizing ceramics, which were difficult to sinter and which could provide a compact ceramic layer only by using a binder, can be used to form a compact ceramic layer without having to use a binder, and also they can provide an enhanced adhesion to a metallic body. Further, the method of the present invention provides another advantage in that since the heating time is very short the ceramic crystal growth can be restrained markedly thus reducing thermal damage of the metallic body. Further, in addition to the characteristic feature of the method of forming a ceramic layer using Thermit reaction, the resistance to corrosion, resistance to heat and resistance to abrasion of the ceramic layer can be improved markedly by limiting the content of oxygen which is an impurity existent unavoidably in powdery raw materials to a particular range and by limiting the particle diameter of the powdery raw material in the same way so that various kinds of very excellent ceramic layers can be provided. Thus, the method of the present invention can be widely used to form excellent lens forming molds, provision of which has come to be required with the recent improvement in the performance of glass lenses, various kinds of materials for chemical industry for use in severe environmental conditions, and various kinds of mechanical parts which require high resistance to abrasion, and can therefore contribute to industrial development. The above-mentioned and other objects, aspects and advantages of the present invention will become apparent to those skilled in the art by making reference to the following description and the accompanying drawings in which a preferred embodiment incorporating the principles of the present invention is shown by way of example only. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a sectional view showing one embodiment of the pressing apparatus adapted for use in the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail below by way of example only with reference to the accompanying drawing. Fig. 1 shows a manufacturing apparatus which is used to form a ceramic layer on a metallic body according to the present invention. In the drawing, reference numeral 1 denotes a cylinder, 2 a pressing punch, 3 a platform made of a metal or ceramic, 4 a cylinder made of a ceramic, 4a a plate, 5 an electric wire made of a ceramic, 6 a Si-Thermit composition, 7 an Aℓ-Thermit composition, 8 a hexagonal boron nitride molded article, 9 a ceramic powder or a mixture of a metallic powder with a ceramic powder for providing a compact ceramic layer, and 10 a metallic body for forming a ceramic layer thereon. Out of the above-mentioned component parts, the cylinder 1, the pressing punch 2 and the metal platform 3 form a pressure generating vessel, whilst the ceramic cylinder 4 serves as a thermal insulation material and a pressure seal. The electric wire 5 made of a ceramic constitutes an internal heater to ignite the Si-Thermit composition 6 to thereby ignite the Aℓ-Thermit composition 7 with a chain-reaction. The hexagonal system boron nitride molded article 8 functions to prevent reaction from occurring between the ceramic layer 9 to be formed on the metallic body and the Thermit compositions 6 and 7 and serves to achieve excellent condition of the heat generated by Thermit reaction. The operation of the above-mentioned pressing apparatus will be described below. The metallic body 10 on which a ceramic powder or a mixture 9 of a ceramic powder with a metallic powder is placed is disposed on the metal platform 3 forming the bottom wall of the cylinder 1. Further, the hexagonal system boron nitride molded article 8 is placed on the ceramic powder or the mixture, and then the Aℓ-Thermit composition 7 around which the Si-Thermit composition 6 is placed is disposed on the molded article 8. Further, the ceramic plate 4a is placed on the Si-Thermit composition. Thereafter, a load is applied by means of the pressing punch 2 so as to apply more than 50 MPa (500 atmospheres) of pressure onto the ceramic powder or mixture 9 of a ceramic powder with a metal powder. By turning on electricity to the ceramic electric wire 5 under the above-mentioned pressurized condition, the Si-Thermit composition 6 disposed around the Aℓ-Thermit composition 7 is ignited thereby causing the ignition of the Aℓ-Thermit composition with a chain-reaction thus generating a great deal of heat by the Thermit reaction. The time series chemical reaction commencement of the Thermit compositions is represented by the following formula. Si + 2/3Fe₂O₃ → SiO₂ + 4/3Fe + 360 KJ (86 Kcal) 2Aℓ + Fe₂O₃ → Aℓ₂O₃ + 2Fe + 854 KJ (204 Kcal) A fine ceramic layer is formed on the metallic body by the heat developed by the ignition of the Thermit composition. After that, the loading is released, and the sample is recovered. EXAMPLE 1 0.02 g of TiB₂ powder having an oxygen content of 0.6 % and a mean particle diameter of 1.0 µm to be used as the ceramic powder 9 was placed on a 4.0% Co-WC disc having a diameter of 6.0 mm and a thickness of 2.5 mm. Whilst, as to the Thermit composition, 42 gr of a Thermit composition prepared by mixing Aℓ powder and Fe₂O₃ powder at a molar ratio of 2 : 1 was cold molded on a disc having a diameter of 30 mm. At the same time, 6 gr of a Si-Thermit composition was placed in the above-mentioned manufacturing apparatus to form a compact ceramic layer. The amount of heat generated by Thermit reaction in this example was 183 KJ (43.8 Kcal.) It was possible to scratch the 4.0% Co-WC hard metal by means of a ceramic edge composed solely of the TiB₂ prepared in this example, which exhibited that a compact ceramic layer was obtained. When the ceramic layer obtained in this example was subjected to grinding by means of a diamond grinding wheel, the ground surface had silver luster. Even when the TiB₂/WC-4% Co pellets obtained in this example were subjected repeatedly to heating and cooling treatments in atmosphere and at a temperature of 700°C, peeling of the ceramic layer from the metallic body did not occur, and so it was confirmed that the TiB₂ ceramic layer had an extremely excellent compactness to the WC-4% Co pellet. To investigate the adhesive interface of the TiB₂/WC-4% Co pellet in details, the hard metal pellet obtained in this example was cut by means of a diamond cutter, and then ground. As a result of observation of the cut portion by SEM, it revealed that the adhesive interface between the ceramic layer and the WC-4% Co disc exhibited an extremely excellent adhesion. As to the metallic body on which a ceramic layer is to be formed, tests were performed using samples made of high speed steel, stainless steel and cast iron, etc., respectively. As a result, it was found out that they provided a extremely excellent compact ceramic layer and extremely excellent adhesion to the metallic body, as in the case of the above-mentioned 4% Co-WC hard metal. EXAMPLE 2 A ceramic layer (stating exactly, cermet layer) was formed under the same conditions as in EXAMPLE 1, except that 0.1 gr of a mixture of TiB₂ and Ni (percent of Ni by volume: 3%) was used as the ceramic powder and metal powder 9 used in EXAMPLE 1, the oxygen contents of TiB₂ and Ni were set at 0.6% and 0.4%, respectivley, and the mean particle diameters of TiB₂ and Ni were set at 1.0 µm and 3.0 µm, the heat value by Thermit reaction was set at 146 KJ (35 KCAL) and carbon steel was used as the material of the metallic body 10. As a result of observation by SEM on the pellet after cutting and grinding thereof, it revealed that the ceramic layer obtained in this example exhibited extremely excellent compactness and adhesion to the carbon steel, and extremely uniform distribution of Ni, and that an extremely thin Ni layer was bonded TiB₂ particles. It was possible to scratch carbon steel easily by means of a ceramic layer edge. EXAMPLE 3 A ceramic layer was formed under the same conditions as in EXAMPLE 1, except that 0.02 - 0.04 gr of each of TiC (oxygen content: 0.5%, mean particle diameter: 1.2 µm), TiN (oxygen content: 0.7%, mean particle diameter: 1.0 µm) , and TiC 0.5 N 0.5 (oxygen content: 0.8%, mean particle diameter: 1.3 µm) were used as the ceramic powder 9 in EXAMPLE 1, the heat value by Thermit reaction was set at 167 KJ (40 KCAL), and stainless steel was used as the material of the metallic body. The ceramic layer obtained in this example exhibited extremely excellent performance as in the case of EXAMPLE 1. EXAMPLE 4 A ceramic layer was formed under the same conditions as in EXAMPLE 1, except that, as the ceramic powder 9, 0.01 gr of B₄C (oxygen content: 0.8%, mean particle diameter: 2 µm) was placed on a 4% Co-WC disc, and further 0.2 gr of a mixture of Ti powder and B powder in a molar ratio of 1 : 2 was placed in layer, and the heat value by Thermit reaction was set at 146 KJ (35 KCAL). The ceramic layer resulted from the mixture of TiB₂, B₄C and TiC in this example provided an extremely compact hard layer having an excellent adhesion to the metallic body 10. Further, in this example, by placing a mixture of Ti/C in a molar ratio of 1 : 1 or a mixture of TiO₂, C and Aℓ in a molar ratio of 3 : 3 : 4, as a substitute to Ti/B mixture which was placed just above the B₄C powder 9 in the previous case, through a hexagonal boron nitride layer on the B₄C powder, and by keeping the heat value by Thermit reaction at the same value as in the previous case, a compact B₄C ceramic layer was formed on the 4% Co-WC disc, which layer exhibited an excellent adhesion to the metallic body 10. EXAMPLE 5 A ceramic layer was formed under the same conditions as in EXAMPLE 1, except that 0.03 gr of a mixture of Ti powder (mean particle diameter: 5 µm, oxygen content: 0.5%) and B powder (mean particle diameter: 0.5 µm, oxygen content: 0.8%) in a molar ratio of 1 : 2, and 0.02 gr of a mixture of Si powder (mean particle diameter: 2 µm, oxygen content: 0.6%) and C powder (mean particle diameter: 3 µm, oxygen content: 0.5%) in a molar of 1 : 1 were placed on a stainless steel disc, in place of the ceramic powder 9, and the heat value by Thermit reaction was set at 126 KJ (30 KCAL). In both cases of Ti/B powder mixture and Si/C powder mixture, as a result of X-ray diffraction, only single phase of ceramics of TiB₂ and SiC, respectively, were detected on the stainless steel disc. However, in view of the increase in the oxygen content of Si/C powder mixture, it is preferable to keep the ratio of Si/C less than 1. EXAMPLE 6 A ceramic layer was formed under the same conditions as in the case of EXAMPLE 1, except that, in place of the ceramic powder 9, 0.06 gr of a mixture of B₄C powder (whose oxygen content and mean particle diameter were the same as those in EXAMPLE 4) and Ti powder (whose oxygen content and mean particle diameter were the same as those in EXAMPLE 5) in a molar ratio of 4 : 1 was placed on a 4% Co-WC hard metal disc, and the heat value by Thermit reaction was set at 146 KJ (35 KCAL). The resultant ceramic layer was comprised of B₄C, TiB₂ and TiC and exhibited a satisfactory compactness and an excellent adhesion to the metallic body. EXAMPLE 7 A ceramic layer was formed under the same conditions as in the case of EXAMPLE 1, except that a thin Ta plate, 0.05 mm thick, was previously placed on the metallic body 10, 0.02 gr of ZrN powder (mean particle diameter: 1 µm, oxygen content: 1%) was placed, as the ceramic powder, on the thin plate, and the heat value by Thermit reaction was set at 146 KJ (35 KCAL). The Ta thin plate used in this example was effective for relaxing the thermal stress on the ceramic layer and the metallic body and forming an extremely excellent crack-free ceramic layer. The same satisfactory effect was obtained in case a thin plate of Mo, Ti-Cr alloy, Ta-Zr alloy or Cu-Ti alloy, etc. was used in place of the Ta thin plate used in this example, or in case a powder was placed in place of the above-mentioned Ta thin plate. EXAMPLE 8 A ceramic layer was formed under the same conditions as in the case of EXAMPLE 1, except that a mixture of TiB₂ and Ni₄B₃ (in a volume ratio of 6 : 4) was used as the ceramic powder 9 in EXAMPLE 1, a Ni-Ti thin plate, 0.05 mm thick, was previously placed on the metallic body, and the heat value by Thermit reaction was set at 126 KJ (30 KAL). The ceramic/WC-Co composite body obtained by this ceramic layer forming method was subjected to HIP (hot hydrostatic pressure pressing) treatment in an Argon atmosphere kept at a temperature of 600°C and a 100 MPa (1,000 atmospheres) of pressure, for 30 minutes. As a result of thermal shock tests on the resultant TiB₂ ceramic composite layer on the metallic body by repeatedly immersing the ceramic layer kept at 600°C into water, it revealed that the ceramic composite layer maintained more stable adhesion to the metallic body than that in case it was not subjected to HIP treatment. HIP and HP (hot pressing) treatments are indispensable, in particular, for ceramic/metal composite materials for use under severe thermal conditions. Further, the above-mentioned ceramic layer was subjected to HP treatment (700°C, 20 MPa (200 atmospheres) of pressure), as a similar treatment, and as a result, a similar improvement in the resistance to thermal shock was achieved. No difference in the result can be recognized between those subjected to HP and HIP treatments, respectively, and those which were not subjected to such treatments, at a temperature of 500°C and at a pressure of 20 MPa (200 atmospheres) or under. Further, at a temperature of 1,200°C or above, significant changes occur in the shape and structure of the metallic body, whilst pressing treatments by using a pressure of more than 200 MPa (2,000 atmospheres) involve increases in cost, and little industrial effect can be expected. It is of course possible to select any one of N₂, H₂, Ar (or O₂ in a special case), or a mixture of them, as the atmosphere in which HIP or HP treatment is to be made, depending on the purpose of use. COMPARATIVE EXAMPLE 1 Examples of the effect of oxygen, which is an unavoidably existent impurity, on the compactness of the ceramic layer and on the adhesion thereof to the metallic body, and examples of the effect of particle diameter of metallic powder and non-metallic powder on the characteristics of the ceramic layer are shown in EXAMPLES 1 to 7. To make these results more clearly, a ceramic layer was formed using TiB₂ powder having an oxygen content of 1.8%, according to a similar procedure to that in EXAMPLE 1. The resultant ceramic layer was less compact than that in the case of using the powder with a low oxygen content in EXAMPLE 1 and exhibited an inferior adhesion to the 4% Co-WC hard metal disc. Further, the particle diameter of the crystal of TiB₂ forming the ceramic layer grew partially abnormally, and in a heating and cooling cycle at a temperature ranging from a room temperature in the atmosphere to 700°C, a portion of the ceramic layer broke down. Regarding the other examples 2 to 6, oxygen, which is an unavoidably existent impurity, lowered the compactness of the ceramic layer and the adhesion to the metallic body. COMPARATIVE EXAMPLE 2 To clarify the effect of the particle diameter of each of powdery raw materials forming the ceramic layer, a ceramic layer was formed using TiC powder having a mean particle diameter of 15 µm, according to a similar procedure to that in EXAMPLE 3. The compactness of the resultant ceramic layer deteriorated, and in most cases molten metal from the substrate metallic body intruded into the ceramic layer. Similar troubles occurred in EXAMPLE 1 to 6. Further, in particular, in cases of ceramics having strong thermal anisotropy in the crystal structure thereof, a great many intergranular cracks occur with the increase in the crystalline particle diameter thus increasing the frequency of occurrence of break-down. In addition thereto, the ceramic compound layer resulted from a starting material such metallic elements/non-metallic elements as shown in EXAMPLE 5 will contain non-reacted metallic elements and non-metallic elements. It is necessary to keep the mean particle diameter of powdery raw materials 10 µm or under.

Claims: A method of forming a ceramic layer on a metallic body (10), comprising the steps of: disposing a ceramic powder (9) on the surface of the metallic body (10), disposing a Thermit composition (6,7) above said ceramic powder (9), and igniting said Thermit composition while exerting a pressure of at least 50 MPa on the layered structure comprising said metallic body (10), said powder (9) and said Thermit composition (6,7), so that the powder (9) is sintered by the heat generated in the Thermit reaction. A method as claimed in claim 1, wherein a layer of a mixture of a metallic powder with a non-metallic powder or a mixture of a ceramic powder with a metallic powder is superposed directly on said layer of ceramic powder. A method of forming a ceramic layer on a metallic body (10), comprising the steps of: disposing a powder (9) on the surface of the metallic body (10), said powder (9) being a mixture of a metallic powder with a non-metallic powder or a mixture of a metallic compound powder with a non-metallic compound powder, the molar ratios of metal elements and non-metal elements in said mixture being such that these elements can react to form a ceramic layer, disposing a Thermit composition (6,7) above said powder (9), and igniting said Thermit composition (6,7) while exerting a pressure of at least 50 MPa on the layered structure comprising said metallic body (10), said powder (9) and said Thermit composition (6,7), so that the powder (9) is sintered by the heat generated in the Thermit reaction and reacts to form said ceramic layer. A method as claimed in claim 1 or 3, wherein at least one kind of metal selected from among the groups Ib, IIb, IVa, Va or VIa of the periodic table or an alloy of these metals is inserted between said metallic body (10) and said powder (9). A method as claimed in anyone of the preceding claims, wherein the resultant ceramic/metal composite member is subjected to hot hydrostatic pressure pressing treatment or hot pressing treatment at a temperature ranging from 500 to 1,200°C and at a pressure ranging from 20 to 200 MPa. A method as claimed in anyone of the preceding claims, wherein said metallic body (10) is a 4 % Co-WC hard metal body. A method as claimed in claim 1, wherein said ceramic powder comprises at least one compound selected from among TiB₂, TiC, TiN, B₄C, Ni₄B₃. A method as claimed in claim 3, wherein said metallic powder is a powder selected from a group consisting of Ti powder, Ni powder and Si powder, and said non-metallic powder is selected from a group consisting of carbon powder and boron powder. A method as claimed in anyone of the preceding claims, wherein the content of oxygen contained as in impurity in said powder (9) is not more than 1 %. A method as claimed in anyone of the preceding claims, wherein the mean particle size of the powder (9) is not more than 10 µm.

Inventors: TAKEDA SHYUICHI, TAKEDA, SHYUICHI
Applicants: KOMATSU MFG CO LTD, KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1989
Main classification: C23C24
IPC codes: C23C24, B22F3