Apparatus for forming metal oxide film and method for forming metal oxide film

Abstract

A triiron tetroxide film is efficiently formed using gas without the need for water vapor. [Solution] The present invention provides a method for forming a metal oxide film, comprising: a placement step of placing a metal workpiece in a heat treatment furnace; a replacement step of replacing the air in the heat treatment furnace with an inert gas; a heating step of raising the temperature of the heat treatment furnace after the replacement step; and a film formation step of forming at least a triiron tetroxide film and a diiron trioxide film by creating an oxygen atmosphere or a dry air atmosphere in the heat treatment furnace after the heating step.

Description

Technical Field

[0001] The present invention relates to an apparatus and a method for forming an oxide film on a metal such as steel.

Background Art

[0002] Conventionally, by oxidizing the surface layer of a metal such as steel, a hard and porous black film of magnetite (Fe3O4) has been formed. This magnetite film contributes to improvements in lubricity, wear resistance, corrosion resistance, anti-adhesive properties, etc. The target metals include steel, cast iron, sintered metal, copper, alloys, etc.

[0003] There are mainly three types of methods for forming a magnetite film on a metal surface. (1) A method of forming a film on a metal surface using an alkaline aqueous solution at around 150°C. (2) A method of forming a film on a metal surface using a nitrate-based salt bath at 300°C to 400°C. (3) A method of forming a film on a metal surface using a steam atmosphere at around 500°C.

[0004] In particular, the method (3) above is called the steam treatment method or the homo treatment method. Although there are drawbacks such as a decrease in the internal hardness of the target metal and the generation of heating strain due to the high treatment temperature, it is suitable for film treatment of mass-produced parts because a film can be formed at low cost.

[0005] In this steam treatment method, first, a workpiece is placed in a steam-compatible heating furnace (commonly called a retort furnace) and preheated to 300°C to 400°C. If steam is passed through the retort furnace without this preheating, it may condense on the workpiece surface to form water droplets, and there is a risk of red rust generation. When the preheating temperature is reached, steam is quickly guided into the retort furnace to expel the air in the retort furnace to the outside. For example, the air is discharged by replacement for about 30 minutes. Then, the retort furnace is heated up to a film formation temperature (treatment temperature) of around 500°C and this temperature is maintained for the required time (holding time). During this period, steam is continuously passed through the retort furnace, and the steam pressure inside the furnace is maintained at normal pressure of about 100 mmAq or more. After the holding time has elapsed, the workpiece is taken out of the retort furnace and the workpiece is air-cooled or liquid-cooled.

[0006] In the furnace, the reaction H2O = H2 + (1 / 2)O2 occurs at the processing temperature. During the coating process, the ratio of water (H2O) to hydrogen (H2) in the furnace is appropriately controlled. Figure 8 shows the equilibrium phase diagram of the iron oxidation reaction based on the volume ratio (%) of water (H2O) and hydrogen (H2) and the furnace temperature. By keeping the steam treatment temperature below 600°C and appropriately controlling the ratio of water (H2O) to hydrogen (H2), a triiron tetroxide film will be formed on the workpiece surface.

[0007] The processing temperature and holding time in steam treatment are set appropriately based on the type of workpiece and the thickness of the triiron tetroxide coating. Making the triiron tetroxide coating too thick is undesirable because it makes the coating prone to peeling. Also, if fine water droplets (condensation) are present in the steam, iron hydroxide (red rust) will form on the workpiece surface, and this iron hydroxide (red rust) will inhibit the formation of the triiron tetroxide coating. For this reason, steam treatment requires proper maintenance of the retort furnace, sufficient heating of the steam, and proper control of the preheating temperature and processing temperature. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 55-152182 [Overview of the project] [Problems that the invention aims to solve]

[0009] Steam treatment methods require steam generating equipment such as boilers, increasing the overall cost of the treatment system. Furthermore, because superheated steam directly contacts the insulation material placed inside the retort furnace, the insulation material is prone to deterioration. When the insulation efficiency decreases due to deterioration, condensation occurs, leading to the formation of iron hydroxide (red rust) on the workpiece. Additionally, the complex structure of the retort furnace increases repair costs, including the periodic replacement of insulation material and steam sealing materials.

[0010] In view of these circumstances, the present invention provides a method for forming a metal oxide film that can efficiently form an oxide film on an object (workpiece) using a gas without using water vapor. [Means for solving the problem]

[0011] To achieve the above objective, the present invention is a method for forming a metal oxide film, characterized by comprising: a placement step of placing a metal workpiece in a heat treatment furnace; a replacement step of replacing the air in the heat treatment furnace with an inert gas; a heating step of raising the temperature of the heat treatment furnace after the replacement step; and a film forming step of forming at least a triiron tetroxide film and a diiron trioxide film on the workpiece by creating an oxygen atmosphere or a dry air atmosphere in the heat treatment furnace after the heating step.

[0012] In relation to the above-described method for forming a metal oxide film, the film formation step may be characterized by circulating oxygen gas or dry air in the heat treatment furnace.

[0013] In relation to the above-described metal oxide film formation method, the film formation step is characterized by comprising: a degassing step of exhausting the inert gas to create a vacuum inside the heat treatment furnace; an oxygen gas purging step of supplying oxygen gas or dry air into the vacuum-state heat treatment furnace to restore pressure inside the heat treatment furnace to a specific pressure; and an oxygen gas supply step of supplying the oxygen gas or dry air into the heat treatment furnace while adjusting the flow rate so that the oxygen concentration inside the heat treatment furnace after restoration of pressure is maintained within a predetermined range.

[0014] In relation to the above-described metal oxide film formation method, the film formation step is characterized by comprising: a degassing step of exhausting the inert gas to create a vacuum inside the heat treatment furnace; an oxygen gas purging step of supplying oxygen gas or dry air into the vacuum-state heat treatment furnace to restore pressure inside the heat treatment furnace to a specific pressure; and an oxygen gas supply step of replenishing the oxygen gas or dry air into the heat treatment furnace each time the oxygen concentration inside the heat treatment furnace decreases after restoration of pressure reaches a lower threshold.

[0015] In relation to the above-described metal oxide film formation method, the film formation step may be characterized by maintaining the internal temperature of the heat treatment furnace at 600°C or below.

[0016] In relation to the above-described metal oxide film formation method, the film formation step may be characterized by maintaining the furnace temperature of the heat treatment furnace at 450°C or higher.

[0017] In relation to the above-described metal oxide film formation method, the method may be characterized by reducing the atmosphere inside the heat treatment furnace to below atmospheric pressure by suction before the substitution step.

[0018] In relation to the above-described method for forming a metal oxide film, the method may be characterized by reducing the atmosphere inside the heat treatment furnace to 10 kPa or less by suction before the substitution step.

[0019] In relation to the above-described method for forming a metal oxide film, the film thickness of the ferric oxide film may be characterized by being smaller than the film thickness of the triiron tetroxide film.

[0020] In relation to the above-described method for forming a metal oxide film, the film formation step may be characterized by forming at least an iron oxide film, the triiron tetroxide film, and the diiron trioxide film.

[0021] In relation to the above-described method for forming a metal oxide film, the iron oxide film, the triiron tetroxide film, and the diiron trioxide film may be stacked in this order from the inside to the outside of the surface of the workpiece, characterized in that the film thickness decreases in the order of iron oxide film, triiron tetroxide film, and diiron trioxide film.

[0022] To achieve the above objective, the present invention is a metal oxide film forming apparatus comprising a heat treatment furnace for housing a workpiece, a heating device for heating the inside of the heat treatment furnace, and a gas supply device for supplying oxygen gas or dry air into the heat treatment furnace, characterized in that at least a triiron tetroxide film and a diiron trioxide film are formed on the workpiece by creating an oxygen atmosphere or a dry air atmosphere inside the heat treatment furnace after heating.

Advantages of the Invention

[0023] According to the present invention, without using water vapor, a ferromagnetic oxide film can be efficiently formed by a gas.

Brief Description of the Drawings

[0024] [Figure 1] It is an overall schematic view of a metal oxide film forming apparatus according to an embodiment of the present invention. [Figure 2] It is a schematic view of a modification of a gas supply device of a metal oxide film forming apparatus according to an embodiment of the present invention. [Figure 3] It is a time chart of a metal oxide film forming method by the above metal oxide film forming apparatus. [Figure 4] It is a figure used to explain the relationship between the oxide film formed on the surface of the workpiece and the equilibrium oxygen partial pressure. [Figure 5] It is an Ellingham diagram related to the components of the oxide film formed on the surface of the workpiece. [Figure 6] It is a modification of the time chart of the metal oxide film forming method by the above metal oxide film forming apparatus. [Figure 7] (A) is an enlarged cross-sectional view of the workpiece surface according to Example 1, and (B) is a hardness distribution diagram of the same workpiece surface. [Figure 8] It is an equilibrium diagram of the oxidation state applied based on the content of water (H2O) and hydrogen (H2) and the furnace temperature.

Embodiments for Carrying Out the Invention

[0025] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification, the film represents a film formed on the surface of the workpiece, and may be re-expressed as a coating or simply a film as appropriate.

[0026] Fig. 1 shows the overall configuration of a metal oxide film forming apparatus 1. The metal oxide film forming apparatus 1 supplies gas into a furnace 10a to form a metal film on a workpiece 100 in the furnace 10a.

[0027] The metal oxide film forming apparatus 1 comprises a heat treatment furnace 10, a heating device 12 for heating the workpiece 100 inside the furnace 10a of the heat treatment furnace 10, a gas supply device 20 for supplying various gases to the inside of the furnace 10a, a stirring unit 16 provided inside the furnace 10a, a negative pressure generating device 50 for creating a negative pressure state inside the furnace 10a, a cooling device 80 for adjusting the temperature of the atmosphere inside the furnace 10a, and a control device 30. The control device 30 controls the heating device 12, the gas supply device 20, the stirring unit 16, the negative pressure generating device 50, and the cooling device 80 according to a control program.

[0028] (Heat treatment furnace) The heat treatment furnace 10 is capable of housing the workpiece 100, which is the material to be treated, inside the furnace 10a. The heat treatment furnace 10 employs a pressure-resistant airtight structure that can create a negative pressure (vacuum) state inside the furnace 10a. Multiple workpieces 100 are arranged in a vertical (first direction) and horizontal direction on a frame 11 installed inside the furnace 10a, for example. The workpieces 100 are metals such as low-carbon steel. An insulating material 10b is provided inside the furnace 10a to maintain the temperature inside the furnace 10a. Furthermore, a pressure adjustment valve 19 is connected to the heat treatment furnace 10 so that the inside of the furnace 10a is maintained at near normal pressure (approximately atmospheric pressure: about 101 [kPa]) during film formation. Preferably, the inside of the furnace 10a is set to or above normal pressure (atmospheric pressure), specifically to +100 mmAq or more relative to normal pressure (atmospheric pressure), for example, to atmospheric pressure + 160 mmAq. The pressure regulating valve 19 can also function as a relief valve to release the furnace interior 10a to the atmosphere in an emergency, or as an exhaust valve when replacing the atmospheric gas inside the furnace interior 10a.

[0029] Furthermore, the heat treatment furnace 10 is equipped with a thermometer 13 (e.g., a thermocouple) for measuring the temperature inside the furnace 10a, and an oxygen gas concentration meter 14 for the inside of the furnace 10a. For the oxygen gas concentration meter 14, it is preferable to use an infrared concentration analyzer from the viewpoint of heat resistance and pressure resistance, but other sensors may also be used. The thermometer 13 and the oxygen gas concentration meter 14 are installed at the top of the heat treatment furnace 10, but their installation positions are not particularly limited.

[0030] (heating device) The heating device 12 has heaters 12a for heating the inside of the furnace 10a. The heaters 12a are arranged in multiple pairs (6 pairs in this case) in the depth direction (vertical direction in Figure 1), with spacing between them in the horizontal direction (left-right direction in Figure 1) at the top of the inside of the furnace 10a. For example, radiant tube heaters can be used as heaters 12a, but are not limited to these.

[0031] (Gas supply device) The gas supply device 20 includes an oxygen gas system 21 through which oxygen gas flows, a nitrogen gas system 23 through which nitrogen gas flows, and a supply system 24 that supplies the gases flowing through these systems 21 and 23 together to the furnace 10a. Furthermore, the gas supply device 20 includes an oxygen supply source 25 connected to the oxygen gas system 21 and a nitrogen supply source 28 connected to the nitrogen gas system 23.

[0032] (Oxygen supply source and oxygen gas system) The oxygen supply source 25 is, for example, a gas cylinder filled with oxygen. As shown in Figure 1, the oxygen gas system 21 has a supply pipe 21a that branches into two upstream and merges into one downstream. One branch of the supply pipe 21a, the first branch pipe section 21b which forms the first oxygen gas flow path, is equipped with an on-off valve 21c. The other branch of the supply pipe 21a, the second branch pipe section 21d which forms the second oxygen gas flow path, is equipped with an on-off valve 21e and a flow meter 21f. The first end of the supply pipe 21a is connected to the oxygen supply source 25 and the second end is connected to the connection part 40 of the supply system 24. Oxygen gas from the oxygen supply source 25 flows through the supply pipe 21a. The oxygen gas is supplied to the furnace 10a from the connection part 40 of the supply system 24 through the downstream pipe 42.

[0033] The on-off valves 21c and 21e are solenoid valves. The on-off valve 21c opens and closes the first oxygen gas flow path, and the on-off valve 21e opens and closes the second oxygen gas flow path. When the first oxygen gas flow path is open by the on-off valve 21c, the second oxygen gas flow path is closed by the on-off valve 21e. Conversely, when the first oxygen gas flow path is open by the on-off valve 21c, the second oxygen gas flow path is closed by the on-off valve 21e.

[0034] The flow meter 21f measures the flow rate of oxygen gas flowing through the second branch pipe section 21d (second oxygen gas flow path). Based on the measurement results from the flow meter 21f, the on / off valve 21e is opened or closed to control the flow rate of oxygen gas flowing through the second oxygen gas flow path.

[0035] Since the first oxygen gas channel does not have a flow meter 21f, the first oxygen gas channel is used to supply oxygen gas to the furnace 10a regardless of the flow rate. The first oxygen gas channel is also used as a dedicated purging route (a route with only opening and closing mechanisms, without a flow rate adjustment mechanism) for purging the furnace 10a with oxygen gas in the oxygen gas purging process described later. On the other hand, the second oxygen gas channel has a flow meter 21f, so the second oxygen gas channel is used to supply oxygen gas to the furnace 10a while controlling the flow rate. The second oxygen gas channel is also used as a dedicated route for supplying oxygen gas to the furnace 10a with a controlled flow rate in the oxygen gas supply process described later.

[0036] Furthermore, the on-off valve 21e only controls the opening and closing of the second oxygen gas flow path. In addition to the on-off valve 21e, a flow control valve (not shown) is provided to adjust the flow rate of oxygen gas flowing through the second branch pipe section 21d (second oxygen gas flow path), and the flow rate of oxygen gas may be manually adjusted while checking the value on the flow meter 21f.

[0037] Incidentally, as shown in Figure 2, the oxygen gas system 21 may be configured to have a supply pipe 21g that does not branch, and an on-off valve 21h and a flow meter 21i provided in the supply pipe 21g. Similar to the supply pipe 21a, the supply pipe 21g has its first end connected to the oxygen supply source 25 and its second end connected to the connection part 40 of the supply system 24. Oxygen gas from the oxygen supply source 25 flows through the supply pipe 21g.

[0038] The supply pipe 21g serves as both the first and second oxygen gas flow paths. When purging the furnace interior 10a with oxygen gas, the on-off valve 21h can be left open regardless of the measurement result of the flow meter 21f. In this way, the supply pipe 21g functions as the first oxygen gas flow path. On the other hand, when supplying oxygen gas to the furnace interior 10a with a controlled flow rate, the opening and closing of the on-off valve 21h is controlled based on the measurement result of the flow meter 21f. In this way, the supply pipe 21g functions as the second oxygen gas flow path.

[0039] The oxygen supply source 25 may be replaced with a dry air supply source that supplies dry air. In this case, the oxygen gas system 21 becomes a dry air system through which dry air flows and is connected to the dry air supply source. Dry air refers to air that has been treated to remove moisture. Dry air mainly contains nitrogen and oxygen.

[0040] A dry air supply source includes, for example, an air compression unit (compressor) that draws in and pressurizes atmospheric air, and a removal unit. The air drawn in and pressurized by the air compression unit is treated in the removal unit to remove moisture, resulting in dry air.

[0041] (Nitrogen supply sources and nitrogen gas systems) In this embodiment, the nitrogen supply source 28 has a gas separator 28a, which is a PSA-type nitrogen gas generator. The gas separator 28a separates nitrogen from the air by pressure swing adsorption (PSA) to produce nitrogen-containing gas. Therefore, the nitrogen-containing gas contains trace amounts of oxygen as an impurity other than nitrogen. The nitrogen gas system 23 has a supply pipe 23a that branches into two upstream and merges into one downstream. One branch of the supply pipe 23a, the first branch pipe section 23b which forms the first nitrogen gas flow path, is equipped with an on-off valve 23c. The other branch of the supply pipe 23a, the second branch pipe section 23d which forms the second nitrogen gas flow path, is equipped with an on-off valve 23e and a flow meter 23f. The first end of the supply pipe 23a is connected to the nitrogen supply source 28 and the second end is connected to the connection section 40 of the supply system 24. Nitrogen gas from the nitrogen supply source 28 flows through the supply pipe 23a. Nitrogen gas is supplied to the furnace 10a from the connection point 40 of the supply system 24 through the downstream piping 42.

[0042] The on-off valves 23c and 23e are solenoid valves. The on-off valve 23c controls the opening and closing of the first nitrogen gas passage, and the on-off valve 23e controls the opening and closing of the second nitrogen gas passage. When the first nitrogen gas passage is open by the on-off valve 23c, the second nitrogen gas passage is closed by the on-off valve 23e. Conversely, when the first nitrogen gas passage is open by the on-off valve 23c, the second nitrogen gas passage is closed by the on-off valve 23e.

[0043] The flow meter 23f measures the flow rate of nitrogen gas flowing through the second branch pipe section 23d (second nitrogen gas flow path). Based on the measurement results from the flow meter 23f, the on / off valve 23e is opened or closed to control the flow rate of nitrogen gas flowing through the second nitrogen gas flow path.

[0044] Since the first nitrogen gas channel does not have a flow meter 23f, the first nitrogen gas channel is used to supply nitrogen gas to the furnace 10a regardless of the flow rate. The first nitrogen gas channel is then used as a dedicated purging route (a route with only opening and closing mechanisms, without a flow rate adjustment mechanism) for purging the furnace 10a with nitrogen gas in the first and second nitrogen gas purging processes described later. On the other hand, since the second nitrogen gas channel has a flow meter 23f, the second nitrogen gas channel is used to supply nitrogen gas to the furnace 10a while controlling the flow rate. The second nitrogen gas channel is then used as a dedicated route for supplying nitrogen gas to the furnace 10a with a controlled flow rate in the first and second nitrogen gas supply processes described later.

[0045] Furthermore, the on-off valve 23c only controls the opening and closing of the second nitrogen gas flow path, and a flow control valve (not shown) is provided separately from the on-off valve 23e to adjust the flow rate of nitrogen gas flowing through the second branch pipe section 23d (second nitrogen gas flow path). The flow rate of nitrogen gas may be manually adjusted while checking the value on the flow meter 23f. In addition, the nitrogen supply source 28 is not limited to a nitrogen gas generator; for example, a gas cylinder filled with nitrogen or liquid nitrogen can be used.

[0046] Incidentally, as shown in Figure 2, the nitrogen gas system 23 may be configured to have a non-branching supply pipe 23g, and an on-off valve 23h and a flow meter 23i provided on the supply pipe 23g. Similar to the supply pipe 23a, the first end of the supply pipe 23g is connected to the nitrogen supply source 28 and the second end is connected to the connection part 40 of the supply system 24. Nitrogen gas from the nitrogen supply source 28 flows through the supply pipe 23g.

[0047] The supply pipe 23g serves as both the first and second nitrogen gas flow paths. When purging the furnace interior 10a with nitrogen gas, the on-off valve 23h can be left open regardless of the measurement result of the flow meter 23f. In this way, the supply pipe 23g functions as the first nitrogen gas flow path. On the other hand, when supplying nitrogen gas with an adjusted flow rate to the furnace interior 10a, the opening and closing of the on-off valve 23h is controlled based on the measurement result of the flow meter 23f. In this way, the supply pipe 23g functions as the second nitrogen gas flow path.

[0048] The nitrogen gas system 23 and nitrogen supply source 28 do not necessarily have to be provided, but they allow the furnace interior 10a to be made even more inert before and during heating and / or cooling.

[0049] Note that nitrogen gas may be replaced with other inert gases. In this case, the nitrogen gas system 23 and nitrogen supply source 28 become the inert gas system 23 and inert gas supply source 28.

[0050] (supply system) The supply system 24 has a connection part 40 connected to the supply pipes 21a and 23a, and a downstream pipe 42 connected to the connection part 40. A gas supply nozzle 44 is provided at the end of the downstream pipe 42. The gas supply nozzle 44 is located at the bottom of the heat treatment furnace 10. In this embodiment, the gas supply nozzle 44 is located at the center of the width direction of the furnace interior 10a. This gas supply nozzle 44 has a supply port 44a that opens into the furnace interior 10a of the heat treatment furnace 10. Oxygen gas (or dry air) and nitrogen gas are ejected (or discharged) upward from the supply port 44a. In this embodiment, the case in which the supply port 44a is provided on the floor surface 10y is illustrated, but the present invention is not limited to this, and it may be provided on the side surface or ceiling surface of the furnace interior 10a.

[0051] The downstream piping 42 is equipped with an on-off valve 42a and a flow meter 42b. The on-off valve 42a is a solenoid valve that can adjust the flow rate of gas flowing through the downstream piping 42 based on the measurement results of the flow meter 42b.

[0052] (Agitation section) The stirring unit 16 is provided on the ceiling surface 10x of the furnace interior 10a and has, for example, a centrifugal fan for stirring the atmospheric gas inside the furnace interior 10a. In this embodiment, the stirring unit 16 is located in the upper part of the furnace interior 10a, in the horizontal center of the furnace interior 10a. The stirring unit 16 draws gas from the furnace interior 10a from the furnace core side (the horizontal center side of the furnace interior 10a) and diffuses the gas from the furnace interior 10a radially in the horizontal plane on the ceiling surface of the furnace interior 10a so that the gas flows downward along the plane direction of the inner circumferential wall of the furnace interior 10a. The installation position of the stirring unit 16 is not particularly limited; for example, it may be provided on the floor surface 10y of the furnace interior 10a and diffuse the gas radially in the plane direction of the floor surface 10y.

[0053] (Negative pressure generating device) The negative pressure generating device 50 includes a pressure gauge 51 and a vacuum pump (oil rotary vacuum pump) 52. The pressure gauge 51 detects the pressure inside the furnace 10a. The vacuum pump 52 is connected to the furnace 10a by an exhaust pipe 58. The exhaust pipe 58 is equipped with a main valve 58a, which switches the vacuuming of the furnace 10a by the vacuum pump 52 ON / OFF (open / closed).

[0054] To create a vacuum (negative pressure) state inside the furnace 10a, the main valve 58a is turned ON (open), and the vacuum pump 52 then pumps the air out of the furnace 10a. For example, the pressure inside the furnace 10a can be reduced to 1 kPa or less.

[0055] (cooling device) The cooling device 80 includes a circulation path 82, a heat exchanger 84, a blower 86, and an on / off valve 88. The circulation path 82 is connected to the furnace interior 10a at both ends and is a circulation path that draws gas from the furnace interior 10a out of the furnace at one end and returns that gas to the furnace interior 10a at the other end. The heat exchanger 84 is positioned in the middle of the circulation path 82 to cool the circulating gas. The blower 86 is positioned in the middle of the circulation path 82 to adjust the flow rate of the gas flowing through the circulation path 82. The on / off valve 88 is positioned in the middle of the circulation path 82 and determines whether or not to allow gas to flow through the circulation path 82 by switching it ON / OFF (open / closed). This cooling device 80 also creates convection in the furnace interior 10a, thereby enabling uniform cooling of the workpiece 100.

[0056] (Control device) The control device 30 is a computer equipped with a processor and the like, and controls the entire metal oxide film forming apparatus 1. Specifically, the control device 30 includes a CPU (central processing unit) on which various programs are executed, a memory for temporarily storing information required by the CPU, an information storage medium on which programs including the metal oxide film forming program and various data are stored, and a communication interface for communicating with various controlled devices.

[0057] The control device 30 controls the opening and closing of the on-off valves 21c, 21e, 21h, 23c, 23e, 23h, and 42a in the oxygen gas system 21, nitrogen gas system 23, and supply system 24, as well as various controls of the heating device 12, negative pressure generating device 50, and cooling device 80, based on the metal oxide film formation program. This enables control of the internal pressure of the furnace 10a, control of the supply (flow rate) of various gases supplied to the furnace 10a, and temperature control of the furnace 10a. The control device 30 is connected to a thermometer 13, an oxygen gas concentration meter 14, a pressure gauge 51, etc., and is capable of acquiring measurement results. These measurement results are output to external equipment and recorded as needed.

[0058] (Coating method) Next, referring to the timing chart in Figure 3, the procedure for forming a metal oxide film in this embodiment, which is executed by the control device 30 according to the metal oxide film formation program, will be described.

[0059] <Placement process (A)> First, the process of transporting and positioning the workpiece 100 into the furnace 10a is performed. Specifically, the workpiece 100 is transported into the furnace 10a by a transport device (not shown) located adjacent to the heat treatment furnace 10 and placed on the stand 11.

[0060] <First degassing process (B)> Once the workpiece 100 is placed inside the furnace 10a, a vacuum door for the entrance / exit (not shown) is closed to make the inside of the furnace 10a airtight. Then, the negative pressure generating device 50 is used to exhaust (suction) the gas (atmosphere) inside the furnace 10a to create a vacuum (negative pressure below atmospheric pressure). Specifically, the inside of the furnace is subjected to a negative pressure of 10 kPa or less, preferably 1 kPa or less.

[0061] <First nitrogen gas purging process (C)> After the furnace interior 10a reaches the desired vacuum state, nitrogen gas is supplied to the furnace interior 10a from the nitrogen supply source 28 to restore the pressure to approximately atmospheric pressure (preferably above atmospheric pressure). This procedure purges the furnace interior 10a with inert nitrogen gas. In the first nitrogen gas purging step, the first nitrogen gas flow path is opened by the on-off valve 23c, and the second nitrogen gas flow path is closed by the on-off valve 23e. As a result, in the first nitrogen gas purging step, nitrogen gas is supplied to the furnace interior 10a via the first branch pipe section 23b (first nitrogen gas flow path). The above <first degassing step (B)> and <first nitrogen gas purging step (C)> constitute a replacement step in which the air in the furnace interior 10a is replaced with an inert gas such as nitrogen gas.

[0062] <Heating process (D)> Subsequently, the heating device 12 raises the temperature of the furnace 10a to the coating treatment temperature. The coating treatment temperature is preferably in the range of 450°C to 650°C, and more preferably in the range of 500°C to 550°C. The temperature of the furnace 10a is then maintained at the coating treatment temperature by heating with the heating device 12 until the second nitrogen gas purging step (H) described later.

[0063] <First Nitrogen Gas Supply Process (E)> As the heating device 12 raises the temperature of the furnace 10a, nitrogen gas is supplied to the furnace 10a from the nitrogen supply source 28. In the first nitrogen gas supply process, the first nitrogen gas flow path is closed by the on-off valve 23c, and the second nitrogen gas flow path is opened by the on-off valve 23e. As a result, in the first nitrogen gas supply process, nitrogen gas is supplied to the furnace 10a via the second branch pipe section 23d (second nitrogen gas flow path). At this time, the pressure regulating valve 19 is kept open. As a result, when new nitrogen gas is supplied to the furnace 10a, the existing nitrogen gas in the furnace 10a is exhausted from the pressure regulating valve 19, and the nitrogen gas in the furnace 10a is replaced with new nitrogen gas. In addition, in the first nitrogen gas supply process, the flow rate of nitrogen gas is adjusted by the on-off valve 23e and the flow meter 23f so that the pressure in the furnace 10a is maintained at a desired value (for example, approximately atmospheric pressure).

[0064] When the furnace interior 10a is heated, the heat causes any deposits, oils, or other excess components adhering to the surface of the workpiece 100 to be separated from the workpiece 100 by vaporization or other means. Then, by continuously supplying nitrogen gas in the nitrogen gas supply process, the separated deposits and excess components are exhausted to the outside along with the nitrogen gas discharged from the pressure regulating valve 19. In this way, the deposits and excess components separated from the surface of the workpiece 100 are exhausted while the furnace interior 10a is filled with nitrogen gas.

[0065] <Film formation process (F)> After the heating is complete, the pressure adjustment valve 19 is closed, and the negative pressure generating device 50 is used to exhaust the nitrogen gas from the furnace 10a, creating a vacuum (negative pressure below atmospheric pressure) in the furnace 10a (second degassing step (F1)). Specifically, the furnace 10a is subjected to a negative pressure of 10 kPa or less, preferably 1 kPa or less. At this time, any remaining deposits (e.g., low-melting-point metallic substances) and excess components adhering to the surface of the workpiece 100 vaporize and are further exhausted along with the nitrogen gas.

[0066] After the furnace interior 10a reaches the desired vacuum state, oxygen gas is supplied to the furnace interior 10a from the oxygen supply source 25 to restore pressure to a specific pressure (e.g., approximately atmospheric pressure / preferably above atmospheric pressure) (oxygen gas purging process (F2)). In the oxygen gas purging process, the first oxygen gas channel is opened by the on-off valve 21c, and the second oxygen gas channel is closed by the on-off valve 21e. As a result, in the oxygen gas purging process, oxygen gas is supplied to the furnace interior 10a via the first oxygen gas channel. Through these procedures, the furnace interior 10a can be made into an oxygen atmosphere.

[0067] Even after the repressurization (oxygen gas purging process), the oxygen concentration in the furnace interior 10a is maintained within a predetermined range by continuously supplying oxygen gas (oxygen gas supply process (F3)). In the oxygen gas supply process, the first oxygen gas flow path is closed by the on-off valve 21c, and the second oxygen gas flow path is opened by the on-off valve 21e. As a result, in the oxygen gas supply process, oxygen gas with an adjusted flow rate is supplied to the furnace interior 10a via the second branch pipe section 21d (second oxygen gas flow path). At this time, the pressure adjustment valve 19 is kept open. As a result, when supplying new oxygen gas to the furnace interior 10a, any excess oxygen gas in the furnace interior 10a is exhausted from the pressure adjustment valve 19, and the pressure in the furnace interior 10a can be kept constant. Although not specifically shown here, if dry air is supplied instead of oxygen gas, the oxygen is consumed in the furnace interior 10a and the nitrogen concentration increases, so the oxygen concentration in the furnace interior 10a can be maintained by continuously supplying dry air. Furthermore, in the oxygen gas supply process, the flow rate of oxygen gas is adjusted by the on-off valve 21e and the flow meter 21f so that the pressure inside the furnace 10a is maintained at a desired value (for example, approximately atmospheric pressure). In this process, it is preferable to use the stirring section 16 to create convection in the atmosphere inside the furnace 10a.

[0068] In the oxygen gas purging process and the oxygen gas supply process, oxygen gas is supplied to the furnace 10a, and an oxide film is formed on the workpiece 100. Incidentally, the following equation exists for the oxidation reaction of metal surfaces by oxygen, and on the surface of the workpiece 100, an iron oxide (FeO) film, a triiron tetroxide (Fe3O4) film, and a diiron trioxide (Fe2O3) film are formed as oxide films. Fe + 1 / 2O2 → FeO... Equation (1) 3FeO + 1 / 2O2 → Fe3O4 ... Equation (2) 2Fe3O4+1 / 2O2→3Fe2O3...Equation (3)

[0069] Generally, when an oxide film forms on a metal surface, the partial pressure of oxygen in the oxide film decreases from the atmosphere towards the metal surface. Furthermore, heating an oxide to a high temperature makes it easier for oxygen to dissociate (reduce) from the oxide. This means that the oxide becomes unstable under high-temperature conditions. Therefore, thermodynamically unstable oxides tend to form in environments with higher oxygen partial pressures.

[0070] Here, the equilibrium oxygen partial pressures Po2 in equations (1) to (3) are shown in the lower part of Figure 4. If the oxygen partial pressure in the atmosphere is higher than the equilibrium oxygen partial pressure Po2, the reaction proceeds to the right in equations (1) to (3), and oxidation proceeds. Conversely, if the oxygen partial pressure in the atmosphere is lower than the equilibrium oxygen partial pressure Po2, the reaction proceeds to the left in equations (1) to (3), and reduction proceeds. As is clear from the lower part of Figure 4, the equilibrium oxygen partial pressure Po2 increases in the order of the reaction equations (1), (2), and (3). Therefore, considering the magnitude of the oxygen partial pressure in the oxide film and the thermodynamically stable formation of the FeO film, Fe3O4 film, and Fe2O3 film, as shown in Figure 4, the FeO film of formula (1) is formed to be laminated on the metal base material surface, the Fe3O4 film of formula (2) is formed to be laminated on the outside of the FeO film, and the Fe2O3 film of formula (3) is formed to be laminated on the outside of the Fe3O4 film. In other words, the oxide film generated on the surface of workpiece 100 is formed in the order of FeO film, Fe3O4 film, and Fe2O3 film from the base material surface side to the atmosphere side of workpiece 100, according to the oxygen partial pressure in the oxide film. In addition, the film thickness decreases in the order of FeO film, Fe3O4 film, and Fe2O3 film. As a result, the surface of workpiece 100 has improved lubricity, wear resistance, corrosion resistance, and adhesion resistance.

[0071] The equilibrium oxygen partial pressures Po2 shown in the lower part of Figure 4 are for when the atmosphere is at a predetermined temperature, and the relationship between each equilibrium oxygen partial pressure Po2 and the processing temperature can be derived based on the Ellingham diagram shown in Figure 5. Each boundary line corresponding to each reaction equation shown in the Ellingham diagram represents the pressure (each equilibrium oxygen partial pressure Po2) at which each reaction equation reaches equilibrium. This means that reduction reactions proceed in oxygen partial pressure environments higher than the boundary line, and oxidation reactions proceed in oxygen partial pressure environments lower than the boundary line. Furthermore, oxides produced at boundary lines located on the lower side are thermodynamically stable and require a large amount of energy to reduce. On the other hand, oxides produced at boundary lines of reaction equations located on the upper side of the Ellingham diagram are thermodynamically unstable and are easily reduced. The inventors have optimized the equilibrium oxygen partial pressures of equations (1) to (3) by controlling the processing temperature within the range of 450°C to 600°C, thereby achieving efficient formation of FeO, Fe3O4, and Fe2O3 films. For example, if the processing temperature is controlled to below 450°C, the equilibrium oxygen partial pressure in equations (1) to (3) will decrease, making it difficult to secure a sufficient film thickness. Also, for example, if the processing temperature is controlled to above 600°C, the thickness of the FeO coating on the substrate side within the film will become too large, making delamination more likely.

[0072] <Third degassing process (G)> Once the required film thickness has been achieved on the surface of the workpiece 100 after a predetermined time has elapsed and the film formation process is complete, the supply of oxygen gas is stopped. Then, while closing the pressure adjustment valve 19, the gas containing oxygen gas in the furnace 10a is exhausted using the negative pressure generating device 50 to create a vacuum state (a negative pressure state below atmospheric pressure). Specifically, the furnace 10a is subjected to a negative pressure state of 10 kPa or less, preferably 1 kPa or less. This suppresses further oxidation of the surface of the workpiece 100 in the furnace 10a.

[0073] <Second nitrogen gas purging process (H)> After the furnace interior 10a reaches the desired vacuum state, nitrogen gas is supplied to the furnace interior 10a from the nitrogen supply source 28 to restore the pressure to approximately atmospheric pressure (preferably above atmospheric pressure). This procedure purges the furnace interior 10a with inert nitrogen gas. In the second nitrogen gas purging step, the first nitrogen gas channel is opened by the on-off valve 23c, and the second nitrogen gas channel is closed by the on-off valve 23e. As a result, in the first nitrogen gas purging step, nitrogen gas is supplied to the furnace interior 10a via the first branch pipe section 23b (first nitrogen gas channel).

[0074] <Cooling process (I)> After the furnace interior 10a is vacuum-purged with inert nitrogen gas, nitrogen gas is further supplied to the furnace interior 10a from the nitrogen supply source 28 to replace the nitrogen gas in the furnace interior 10a (second nitrogen gas supply process (J)), while the temperature of the furnace interior 10a is lowered by the cooling device 80. In the second nitrogen gas supply process, the first nitrogen gas flow path is closed by the on-off valve 23c, and the second nitrogen gas flow path is opened by the on-off valve 23e. As a result, in the second nitrogen gas supply process, nitrogen gas is supplied to the furnace interior 10a via the second branch pipe section 23d (second nitrogen gas flow path). At this time, the pressure regulating valve 19 is kept open. As a result, when new nitrogen gas is supplied to the furnace interior 10a, the existing nitrogen gas in the furnace interior 10a is exhausted from the pressure regulating valve 19, and the nitrogen gas in the furnace interior 10a is replaced with new nitrogen gas. Furthermore, in the second nitrogen gas supply process, the flow rate of nitrogen gas is adjusted by the on-off valve 23e and the flow meter 23f so that the pressure inside the furnace 10a is maintained at a desired value (for example, approximately atmospheric pressure).

[0075] <Export process (K)> Once the workpiece 100 has cooled sufficiently, the vacuum door for the entrance / exit (not shown) is opened, and the workpiece 100 is removed from the furnace 10a to the outside. The removal of the workpiece 100 is expected to be done by a removal device (not shown), but it may also be removed manually.

[0076] Furthermore, the present invention also includes cases in which the oxygen gas supplied in the above <film formation process (F)> is replaced with dry air. When oxygen gas is replaced with dry air, the furnace interior 10a becomes a dry air atmosphere.

[0077] (modified version) Next, with reference to Figure 6, a timing chart of a modified version of the metal oxide film formation method of this embodiment is shown. In the timing chart of this modified version, the <film formation step (F)> differs from that of the above embodiment. Essentially, in the <film formation step (F)>, it is sufficient for an amount of oxygen gas to be present in the furnace 10a to form an oxide film (FeO film, Fe3O4 film, Fe2O3 film) of the desired thickness on the surface of the workpiece 100. For example, if an oxide film of the desired thickness can be obtained on the surface of the workpiece 100 with only the oxygen gas supplied to the furnace 10a in the oxygen gas purging step of the <film formation step (F)> in the above embodiment, the subsequent oxygen gas supply step may be omitted. In this case, the pressure regulating valve 19 is kept closed so that the oxygen gas is not exhausted to the outside through the pressure regulating valve 19.

[0078] On the other hand, in forming an oxide film on the surface of the workpiece 100, if the oxygen gas supplied to the furnace 10a in the oxygen gas purging step is insufficient, instead of continuously supplying oxygen gas to the furnace 10a as in the oxygen gas supply step of the <film formation step (F)> in the above embodiment, the oxygen gas supply step may be configured to supply only the amount of oxygen gas necessary for forming an oxide film of the desired thickness to the furnace 10a.

[0079] Whether or not further oxygen gas supply is necessary is determined by the control device 30 based on the measurement results from the oxygen gas concentration meter 14 (see Figure 1) installed in the furnace interior 10a. In Figure 1, the oxygen concentration in the furnace interior 10a is measured by the oxygen gas concentration meter 14 installed in the furnace interior 10a, but this is not the only method; the atmosphere inside the furnace interior 10a may be taken outside the furnace and measured outside the furnace.

[0080] Focusing on the oxygen concentration in furnace 10a during the <film formation process (F)> in the timing chart of Figure 6, the supply of oxygen gas during the <film formation process (F)> will be explained below. Assume that the supply of oxygen gas in the oxygen gas purging process begins at time T0. Subsequently, the oxygen concentration in furnace 10a increases over time. At time T1, the supply of oxygen gas in the oxygen gas purging process ends. After that, since oxygen gas is used to form an oxide film on the surface of the workpiece 100, the oxygen concentration in furnace 10a decreases over time.

[0081] For example, at time T2, when the oxygen concentration in the furnace 10a reaches the lower threshold M0, the oxygen gas supply process is started and oxygen gas is supplied to the furnace 10a. In the oxygen gas supply process, for example, at time T3, when the oxygen concentration in the furnace 10a reaches the upper threshold M1, the supply of oxygen gas to the furnace 10a is stopped. The oxygen concentration in the furnace 10a is measured by the oxygen gas concentration meter 14. Once the supply of oxygen gas to the furnace 10a is stopped, the oxygen gas is used to form an oxide film on the surface of the workpiece 100, so the oxygen concentration in the furnace 10a decreases over time. Similarly thereafter, each time the oxygen concentration in the furnace 10a reaches the lower threshold M0 (time T4 in Figure 6), oxygen gas is supplied to the furnace 10a until the oxygen concentration reaches the upper threshold M1 (time T5 in Figure 6). In other words, in this modified example, the supply of oxygen gas to the furnace chamber 10a in the oxygen gas supply process is performed intermittently according to the oxygen concentration in the furnace chamber 10a.

[0082] In Figure 6, oxygen gas is supplied to the furnace interior 10a twice during the oxygen gas supply process, but this is not limited to this; it may be supplied once or three or more times. Also, in Figure 6, the oxygen concentration in the furnace interior 10a at the end of the oxygen gas purging process and the oxygen concentration in the furnace interior 10a at the end of the oxygen gas supply process are set to the same value (upper threshold M1), but this is just an example, and the oxygen concentrations at the two points may be different.

[0083] At time T6, an oxide film of the desired thickness is obtained on the surface of workpiece 100, and the process moves to the second degassing step (G). [Examples]

[0084] Steel (specifically SPHC) was used as the base material for workpiece 100. A nitride layer of 13-15 μm was formed on workpiece 100 by gas soft nitriding at 580°C. Then, a metal oxide film was formed on the surface of workpiece 100 according to the timing chart in Figure 3. The film formation temperature was 520°C, and oxygen gas was supplied at 20 Nl / min during both the heating and film formation processes. The film formation process took 60 minutes. Nitrogen gas was supplied at 2 Nm / min during the cooling process. 3 The supply was provided at / hr. Figure 7(A) shows a 500x magnified cross-section of the surface layer of workpiece 100, and Figure 7(B) shows the hardness distribution of the surface.

[0085] As shown in Figure 7(A), a 4 μm metal oxide film was formed on the surface of workpiece 100. As shown in Figure 7(B), the hardness of the outermost surface exceeded 400 HV, specifically reaching 420 HV. The hardness of the nitrided layer averaged 150 HV. As a result, a hardness increase of approximately 270 HV was achieved by the metal oxide film.

[0086] As described above, according to the metal oxide film formation method of this embodiment, after placing the metal workpiece 100 in the heat treatment furnace 10, the air inside the heat treatment furnace 10 is replaced with an inert gas (nitrogen gas in this embodiment), the temperature is raised to the treatment temperature, and the inside of the heat treatment furnace 10 after the temperature is raised is made into an oxygen atmosphere, thereby efficiently forming an oxide film (FeO film, Fe3O4 film, Fe2O3 film) on the workpiece 100. In particular, since the air inside the heat treatment furnace 10 is vacuum purged with an inert gas before the temperature is raised, the formation of unintended films on the surface of the workpiece 100 during the temperature rise can be suppressed. Furthermore, since oxygen is convection is used in the film formation process, a film is formed densely even on the fine parts of the workpiece 100. Moreover, in the film formation process of this embodiment, the internal temperature of the heat treatment furnace 10 is controlled within the range of 450°C to 600°C, so that the oxide film formation can be optimized.

[0087] Furthermore, the metal oxide film formation method and metal oxide film formation apparatus of the present invention are not limited to the embodiments described above, and it goes without saying that additions, omissions, substitutions, and various modifications can be made to the configuration without departing from the spirit of the present invention. Naturally, all metal oxide film formation methods and metal oxide film formation apparatuses constructed by extracting and appropriately combining each component of the above embodiments, including modified versions, are also included in the scope of the present invention. [Explanation of Symbols]

[0088] 1. Apparatus for forming metal oxide films 10 Heat treatment furnace 12 Heating device 16. Stirring section 20 Gas supply equipment 30 Control device 80 Cooling device 21. Oxygen gas system 22 Air System 23 Nitrogen gas system 24 Supply system 25. Oxygen supply sources 26 Air supply source 28 Nitrogen sources 30 Control device 40 Connection part 42 Downstream Piping 44 Gas supply nozzles 44a Supply port 50 Negative pressure generating device 100 Work

Claims

1. The process involves placing metal workpieces inside a heat treatment furnace, A replacement step in which the air in the heat treatment furnace is replaced with an inert gas, A heating step for raising the temperature inside the heat treatment furnace after the substitution step, A film formation step is performed by creating an oxygen atmosphere or a dry air atmosphere inside the heat treatment furnace after the heating step, thereby forming at least a triiron tetroxide film and a diiron trioxide film on the workpiece. A feature comprising: A method for forming a metal oxide film.

2. The film formation step is characterized by circulating oxygen gas or dry air inside the heat treatment furnace. The method for forming a metal oxide film according to claim 1.

3. The aforementioned film formation step is, A degassing step is performed by exhausting the inert gas to create a vacuum inside the heat treatment furnace. An oxygen gas purging step in which oxygen gas or dry air is supplied into the heat treatment furnace in a vacuum state to restore the pressure inside the heat treatment furnace to a specific pressure, An oxygen gas supply step in which oxygen gas or dry air is supplied into the heat treatment furnace while adjusting the flow rate so that the oxygen concentration inside the heat treatment furnace after repressurization is maintained within a predetermined range, A feature having The method for forming a metal oxide film according to claim 1.

4. The aforementioned film formation step is, A degassing step is performed by exhausting the inert gas to create a vacuum inside the heat treatment furnace. An oxygen gas purging step in which oxygen gas or dry air is supplied into the heat treatment furnace in a vacuum state to restore the pressure inside the heat treatment furnace to a specific pressure, An oxygen gas supply step is performed to replenish the oxygen gas or dry air into the heat treatment furnace each time the oxygen concentration in the heat treatment furnace decreases after repressurization and reaches a lower threshold, A feature having The method for forming a metal oxide film according to claim 1.

5. The film formation process is characterized by maintaining the internal temperature of the heat treatment furnace at 600°C or below. The method for forming a metal oxide film according to claim 1.

6. The coating formation step is characterized by maintaining the internal temperature of the heat treatment furnace at 450°C or higher. The method for forming a metal oxide film according to claim 5.

7. The method is characterized by removing the atmosphere inside the heat treatment furnace before the substitution step to bring it below atmospheric pressure. The method for forming a metal oxide film according to claim 1.

8. The method is characterized by removing the atmosphere inside the heat treatment furnace by suction before the substitution step to reduce it to 10 kPa or less. The method for forming a metal oxide film according to claim 1.

9. The thickness of the ferric oxide film is smaller than the thickness of the triiron tetroxide film. The method for forming a metal oxide film according to claim 1.

10. The film formation step is characterized by forming at least an iron oxide film, a triiron tetroxide film, and a diiron trioxide film. The method for forming a metal oxide film according to claim 1.

11. The iron oxide film, the triiron tetroxide film, and the diiron trioxide film are layered in this order from the inside outward on the surface of the workpiece. The film thickness decreases in the order of the iron oxide film, the triiron tetroxide film, and the diiron trioxide film. The method for forming a metal oxide film according to claim 10.

12. A heat treatment furnace for housing the workpiece, A heating device for heating the inside of the heat treatment furnace, A gas supply device that supplies oxygen gas or dry air into the heat treatment furnace, Equipped with, The heat treatment furnace, after heating, is subjected to an oxygen atmosphere or a dry air atmosphere to form at least a triiron tetroxide film and a diiron trioxide film on the workpiece. Metal oxide film forming apparatus.

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