Production of honeycomb structure-extruding dies

Abstract: A process for producing a honeycomb structure-extruding die, includes the steps of etching a surface of a base material for a honeycomb structure-extruding die with a gas, and coating the etched base material by CVD, before an oxidized film is formed after the etching.

Description: The present invention relates to a process for producing a honeycomb structure-extruding die. The aim is to provide a die having excellent wear resistance by coating a surface of a base material for the die through a CVD technique, or particularly through a CVD technique at medium temperature. It is known from, for example, Japanese patent application Laid-open No. 60-145,805 (extruding die and production method thereof, Corning Glass Works) that wear resistance of a honeycomb structure-extruding die is improved by treating a surface of a base material for the die through a CVD process. It is also disclosed, for example, in Japanese patent application Laid-open No. 61-69,968 (corresponding to DE-A-3 431 892, steel matrix, producing process thereof, Berna AG Olten) to apply a CVD process to the surface of the honeycomb structure-extruding die base material made of stainless steel. However, when the die base material is to be coated by the CVD process, it is indispensable to remove contaminant, a processed layer, an oxidized film, etc. from the base material to be coated, by fully washing the base material before the coating. If this step is not satisfactorily effected, no coating film is formed or the coating formed is peeled during cooling. Thus, even if the coating film is formed, such a film has a practically short life only. Particularly, if the base material for the honeycomb structure-extruding die is stainless steel, it is likely that an oxidized film is formed on the surface, and particular care needs to be made to remove the oxidative film. An acidic solution is usually used to remove such an oxidized film. However, this process suffers a problem that since the resulting die is rinsed with water to remove the acidic solution remaining on the surface after being washed with the acidic solution, and the die is dried in an oven or the like, so that oxidized film is formed again during the drying, no excellent coating film can be obtained by the CVD process. The above peeling problem can be solved by ensuring a relatively long cooling time after the coating by the CVD process. However, if the cooling time is made longer, the total coating time by the CVD process increases, resulting in cost increase in producing the die. On the other hand, if a material, which is required to be hardened, is coated by CVD, the coated material needs be quenched after the coating, so that it is substantially impossible to prolong the cooling time. It is an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a process for producing a honeycomb structure-extruding die in which a coating film having good adhesion and excellent wear resistance is formed on a base material of the die by a CVD process. The present invention is set out in claim 1. In the present invention, the step of etching the die base material with a gas, for example, with hydrogen chloride gas generated by reacting titanium tetrachloride with hydrogen is introduced before the CVD coating. By so doing, the oxidized film on the surface of the die base material is removed, and good CVD coating film is formed. However, the gas etching is not limited to this specific method, and etching may be effected by using hydrogen chloride gas, phosphoric acid gas or hydrogen fluoride gas. When hydrogen chloride gas itself is used as the etching gas, a corrosion-resistive feed pipe line needs to be employed, resulting in cost increase of the equipment. On the other hand, it is preferable to use hydrogen chloride gas generated by reacting titanium tetrachloride with hydrogen in an etching chamber, because such a corrosion-resistive feed pipe line is unnecessary. The gas etching is preferably effected in a temperature range from 700-900°C. This temperature range is preferred from the standpoint of preventing deformation of the base material for the die and effecting sufficient etching with the gas. The gas etching is preferably effected for a period of time from 0.5 to 3.5 hours. If the etching time is shorter than 0.5 hour, no sufficient etching effect may be obtained, or the coating film becomes ununiform, or the coating film is likely to peel during the cooling step. On the other hand, if the etching time is longer than 3.5 hours, the die base material may be corroded to form a brittle layer on its surface so that the coating film having wear resistance cannot be obtained. The base material for the honeycomb structure-extruding die may be a stainless steel. Since stainless steel to be used for the base material for the honeycomb-extruding die is very sensitive to oxygen and water, it is preferable to effect the etching with the gas in reducing atmosphere. If oxygen or water is contained in the atmosphere, it is feared that the sufficient etching effect cannot be exhibited so that the coating film becomes non-uniform or peels during the cooling step. Further, the gas etching is preferably effected inside a CVD coating apparatus. The gas etching according to the present invention is effected immediately before the coating by the CVD process starts, and the etching is preferably effected as one step in a CVD coating schedule. That is, after the die base material is set inside the CVD coating apparatus, a reducing gas is passed through the CVD coating device, and the temperature is raised to a given level. Then, the substrate is etched with gas, and the CVD coating is consecutively effected. When the gas etching and the CVD coating are effected continuously in the closed state like this, it is possible to completely prevent re-attachment of contaminant to the surface of the die base material and re-formation of an oxidized film. Thus, a uniform coating film having excellent adhesion can be obtained. These and other optional features and advantages of the invention will be appreciated upon reading of the description of the preferred embodiment of the invention when taken in conjunction with the attached drawings. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a sectional view of a CVD coating apparatus used in Examples of the present invention; Fig. 2 is a schematic view illustrating a treating process in Example 1; Fig. 3 is an enlarged view of a honeycomb structure-extruding die obtained by a process in Example 1; Fig. 4 is an enlarged view of a honeycomb structure-extruding die obtained by a process in Example 2; and Fig. 5 is an enlarged view of a honeycomb structure-extruding die obtained by a process in Example 3. The present invention will be explained in more detail by description of preferred embodiments of the present invention with reference to the attached drawings. Example 1 A die base material was prepared from PSL (Commercial name) (martensitic precipitation hardened stainless steel manufactured by Hitachi Metal, Co., Ltd). The base material had an elliptical shape with a major axis of 215 mm, a minor axis of 130 mm and a thickness of 21 mm. In this base material, holes having a diameter of 1.5 mm were bored from one side surface by drilling or electrolytic working, and then shaping channels having a width of about 200 µm were formed by discharge from the other side opposite to the side from which the above holes were bored. The resulting base material was dewaxed with an alkali, washed with water, and further supersonically washed by using an acidic solution, followed by washing with water again. Then, four such die base materials 2 were set inside a standard type CVD coating apparatus manufactured by Bernex Div. having an outer diameter of 275 mm and a height of 750 mm as schematically shown in Fig. 1. After the chamber was closed, the interior of the chamber was evacuated to vacuum. While hydrogen gas was passed through a gas feed nozzle 3 at the center, the temperature was raised up to 770°C under reduced pressure. Then, titanium tetrachloride gas was led into the chamber through the gas feed nozzle 3, and the base material was etched, for 3 hours, with a hydrogen chloride gas which was being generated by reacting hydrogen gas with the titanium tetrachloride gas. Thereafter, hydrogen, titanium tetrachloride and acetonitrile gas were passed into the CVD coating apparatus through the gas feed nozzle 3, and CVD coating was effected with Ti(C, N) (titanium carbonitride) at 770°C for 10 hours. At that time, the die base material 2 was so arranged that the shaping channels in the base material might be directed toward the gas feed nozzle, and the flow rate of the total material gas was 27.0 liters/min. and the pressure was 60 mbar. The above process is schematically shown in Fig. 2. Results were checked by a microscope at a magnification of 20-30 times. In this Example, as shown in Fig. 3, honeycomb structure-extruding dies having a (C,N) coating Ti film 4 on the surface of the die base material 2 at a thickness of 15 µm were obtained, the coating film being uniform and free from peeling. Example 2 A die base material having the same material and the same shape as those in Example 1 was prepared, washed and then placed in the CVD coating apparatus 1 in the same manner. The conditions in Example 2 were the same as in Example 1 except that the etching time period was changed to 0.5 hour. As a result, it was revealed that a Ti(C,N) coating film 4 was formed on the surface of the die base material 2 with a thickness of 15 µm. However, as shown in Fig. 4, the coating film 4 was partially peeled, and adhesion of the coating film 4 was slightly inferior to that of the coating film in Example 1. The resulting die having undergone such a partial peeling could be used without any practical problem. Comparative Example A die base material having the same material and the same shape as those in Example 1 was prepared, and washed and placed in the CVD coating apparatus 1 in the same manner, except that a CVD coating was effected immediately after raising the temperature, without effecting the gas etching in this Comparative Example. The conditions other than this were all the same as those in Example 1. As a result, it was revealed that a Ti(C,N) coating film 4 was formed on the surface of the die base material 2 with a thickness of 15 µm. However, as shown in Fig. 5, the coating film 4 was peeled over its entirety, and the coating film 4 having good adhesion could not be obtained. As mentioned above, according to the present invention, since the die base material is etched with the gas, preferably inside the CVD apparatus, before the base material is coated by the CVD, the honeycomb structure-extruding die having, on its surface, the coating film possessing excellent adhesion and free from peeling can be obtained. Since the peeling of the coating film can be prevented like this by etching the base material with the gas, the die can be quenched to shorten the time required for the coating, and the production cost of the die can be reduced. Therefore, the present invention solves the problems possessed by the prior art, and greatly contributes to the industrial development as the process for producing the honeycomb structure-extruding die.

Claims: A process for producing a honeycomb structure-extruding die, including coating a surface of a stainless steel base material for the honeycomb structure-extruding die by CVD, characterized in that prior to the step of coating by CVD, a gas etching step using a gas etchant is performed on said surface in a reducing atmosphere so as to remove an oxidized film on the surface, and in that the step of CVD coating is performed immediately after the etching step so that a further oxidized film is substantially not formed on the etched surface. The producing process according to claim 1, wherein the gas etching is effected for a period of time of from 0.5 to 3.5 hours. The producing process according to claim 1 or 2, wherein the gas etching is effected at 700-900°C. The producing process according to any one of claims 1 to 3 wherein the gas etching is effected inside a CVD coating apparatus. The producing process according to any one of claims 1 to 4, wherein the etching is effected with hydrogen chloride gas generated by reacting hydrogen with titanium tetrachloride.

Inventors: ARAI KENJI, MATSUZAKI MASAYUKI, SUZUKI KAZUO, ARAI, KENJI, MATSUZAKI, MASAYUKI, SUZUKI, KAZUO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1992
Main classification: C23C16
IPC codes: C23F1, C23C16, B28B3