A METHOD OF FORMING A WEAR-RESISTANT LAYER

Abstract: A wear-resistant layer (2) is formed on the metallic surface of a worm gear (1), by coating a slurry mixture (formed by mixing tungsten (WC) powder or Mo₂B₅ powder with organic binder and organic solvent) onto the surface of the worm gear to form a covering layer, machining the covering layer to a thickness of 1mm, and dissolving a metallic binder (which serves as a matrix in the covering layer) into the covering layer in a vacuum furnance.

Description: This invention relates to the forming of a wear-resistant layer, on a surface for example of a metallic material. Such a layer may be particularly useful, for example on a machine part which is liable to wear, for instance on a machine used for forming plastics materials. Formation of wear-resistant materials on machine parts has previously been carried out by spray-coating, or sintering, after laminating alternately layers of wear-resistant powder (of metal) and slurry (of ceramic). It has been found, however, that layers formed by spray-coating often become detached from the base material. This is because the thickness of the layer is normally approximately 0.2mm, and this is too thin for the layer to withstand "Herz Stress", and satisfactory adhesion to the base material is thus difficult to obtain. Sintering suffers from the disadvantage that, after laminating alternately layers of wear-resistant metal powder and ceramic slurry, difficulties arise in forming a uniform layer. This is because the metal powder permeates into the ceramic slurry after sintering. Another disadvantage with this method is that a sintered workpiece requires a long machining time due to the roughness of the surface of the workpiece, which is caused by the organic binder reacting or melting at the boundary areas between the ceramic slurry, and the wear-resistant metal powder. US-A-4,017,480 discloses a process of forming a wear resistant layer comprising a high density of hard particles such as tungsten carbide bonded together in a softer matrix of a nickel alloy. The hard particle powder is filled with a brazing cement (a plastic binder in a volatile base) once it has been applied to a steel surface. The present invention therefore proposes the formation of a wear-resistant layer according to claim 1. Powders made of materials selected from carbides and borides, of metals belonging to groups IV and VI of the periodic table, are coated on the surface of the base material with organic binder to form a covering layer. The covering layer is then pressure moulded and/or machined and the organic binder in the covering layer is heated and eliminated. A metallic binder which serves as a matrix in the covering layer, is dissolved so as to permeate into the covering layer (this is done in a vacuum furnace), resulting in the combining or the joining of the metal binder with the base material. Embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which: Fig. 1 is a longitudinal-sectional view of tungsten (WC) slurry coated on the surface of a worm gear; Fig. 2 is a photomicrograph showing a section of a worm gear with a wear-resistant layer according to the present invention (magnification : 50) after sintering; Fig. 3 is a graph showing the results of an abrasion test on a wear-resistant layer according to the invention. Embodiment 1 In this first embodiment a slurry mixture, is formed by mixing the tungsten WC powder (which has a particle size preferably less than 50 microns), organic binder (which is referably an acrylic resin, but water may also be used) and an organic solvent (e.g. methylethylketone) in the ratio of 100:1:14 respectively. A part 1 of a worm gear shown in Fig. 1, forming a base material which is to receive the wear-resistant layer, is machined so as to reduce its size, in order to compensate for the eventual increase in size due to the application of the wear-resistant coating. The material of the worm gear may be e.g. that known as SCM 435. The slurry mixture is coated onto the surface of the worm gear 1 with a spraying device to create a covering layer 2, and the resulting covering layer 2, formed on the surface of the worm gear, is machined by a lathe to a thickness of approximately 1mm (as illustrated in Fig. 1). Fine particles of a nickel-based self-melting alloy (the preferred composition of which is shown in Table 1; a cobalt-based self-melting alloy may also be used) which are capable of being used as a matrix, are placed in a melting pot. The worm gear is also placed vertically in the melting pot. The melting pot and contents are then heated in a vacuum furnace (at a degree of vacuum of 1 x 10⁻²∼1 x 10⁻¹ Torr, 0.13 to 1,3 Pa), from 0 to 400°C at a rate of temperature rise of 30°C per hour, from 400 to 1100°C at a rate of temperature rise of 5°C per minute, and at 1100°C for 30 minutes. (% by weight) Ni B Si Fe C Others Balance 2.52 4.70 1.5 0.15 0.5 The heating results in the fine particles of the nickel-based self-melting alloy becoming fused and permeated into the mixture of slurry, and the materials of the worm gear and coating are then cooled to room temperature in the same vacuum furnace. Initially the tungsten (WC) powder adheres to the surface of the worm gear due to the acrylic resin, but when the resin is evaporated and/or fused in during heating, the tungsten (WC) powder depends on a certain amount of self-adhesion in order to adhere to the surface of the worm gear. Thus, in sintering, a wear-resistant material is formed, and is combined with the base material by permeating the nickel-based self-melting alloy into the covering layer; Fig. 2 is a microscopic photograph, showing a section through the surface of a worm gear 1, with the covering layer 3 thus formed. Embodiment 2 In a second embodiment of the present invention, the slurry is formed by the mixing of Mo₂B₅ powder (also preferably having a particle size of less than 50 microns). The ratio of quantities in the slurry mixture is the same as in the first embodiment. The mixture of slurry is coated onto the surface of the worm gear (which rotates at e.g. 5 rpm) by a spraying device (as may also occur in the first embodiment). It has been found, that the hardness of the covering layer is of the order of Hv800 1000, and the hardness of the base material is HRc10. Fig. 3 shows the specific amount of abrasion in wear-resistant layers of the type described in the present application. As can be seen in Fig. 3, the performance of wear-resistant layers prepared according to the present invention, is superior to those previously available (high speed steel SHK55, hardness HRc62) in terms of its wear-resisting properties. This data was obtained using a Dogoshi-type abrader, and the testing member of the abrasion test has the following properties: SKD11 (metal steel based on Japanese industrial standard) HRc58 (symbols showing the Rockwell hardness). The abrasion distance was 600m, the pressing load 18.9Kgf (189N), and the specific amount of abrasion is that amount of abrasion produced, at an abrasion distance of 1m and a pressing load of 1Kgf (10N). In this example of the present invention, when the slurry mixture is coated on the surface of the wear-resistant material, bubbles and irregular densities are sometimes found. However, when a CIP treatment is used for the slurry mixture, this is not the case. When the tungsten (WC) mould thickness required is 3mm, bubbles are not found if the pressure in the furnace is about 1000Kg/cm² (1 x 10⁸Pa) (when a CIP treatment is used). The present invention, enables the formation of a thin layer of wear-resistant material, without the presence of defects. Further, excessive machine-work is not required after sintering, and thus a wear-resistant layer may be formed at comparatively low cost.

Claims: A method of forming a wear-resistant layer on a base material, comprising the steps of: mixing organic binder with powder of carbides and/or borides of metals belonging to groups IV and/or VI of the periodic table; coating the mixed organic binder and powder onto the base material to form a covering layer on the base material; machining the covering layer; heating the covering layer in a sintering furnace containing metal binder, to eliminate the organic binder from the covering layer, permeate the metal binder into the covering layer and hence join the metal binder and the base material. A method according to claim 1, wherein the metal binder is a nickel-based self-melting alloy or a cobalt-based self-melting alloy. A method according to claim 1 or claim 2, wherein the organic binder is water or a volatile organic solvent of natural or synthetic high polymer. A method according to any one of the preceding claims, wherein the covering layer is moulded under compression prior to the machining of that layer.

Inventors: KUSANAGI, HIROAKI, MEGURO, TAKASHI, MIYAUCHI, MIKIYOSHI, SERIZAWA, HISAO
Applicants: TOSHIBA KIKAI KABUSHIKI KAISHA
Application year: 1987
Main classification: B22F7
IPC codes: B22F7, F16H55, C23C24