Metal oxide film deposition apparatus and metal oxide film deposition method

Abstract

To efficiently deposit a triiron tetroxide film using gas without using water vapor.SOLUTION: A metal oxide film deposition method comprises: an arrangement step of arranging a metal workpiece in a heat treatment furnace; a substitution step of substituting air in the heat treatment furnace with inert gas or carbon dioxide; a heating step of heating the inside of the heat treatment furnace after the substitution step; and a film deposition step of depositing a triiron tetroxide film on the workpiece by changing the inside of the heat treatment furnace after the heating step into a carbon dioxide atmosphere.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an apparatus and method for forming an oxide coating on metals such as steel. [Background technology]

[0002] Traditionally, the surface layer of metals such as steel has been oxidized to form a hard and porous black coating of triiron tetroxide (Fe3O4). This triiron tetroxide coating contributes to improved lubricity, wear resistance, corrosion resistance, and adhesion resistance. Metals that can be treated include steel, cast iron, sintered metal, copper, and alloys.

[0003] There are three main methods for forming a triiron tetroxide coating on a metal surface: (1) A method for forming a coating on a metal surface using an alkaline aqueous solution at about 150°C. (2) A method for forming a coating on a metal surface using a peter-based salt bath at 300°C to 400°C. (3) A method for forming a coating on a metal surface using a water vapor atmosphere at about 500°C.

[0004] In particular, the above method (3) is known as the steam treatment method or homogeneous treatment method. Although it has drawbacks such as a decrease in the internal hardness of the target metal and the occurrence of thermal distortion due to the high treatment temperature, it is suitable for coating treatment of mass-produced parts because it can form a coating at low cost.

[0005] In this steam treatment method, first, the workpiece is placed in a steam-compatible heating furnace (commonly known as 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 surface of the workpiece and become water droplets, which may cause red rust. When the preheating temperature is reached, steam is quickly introduced 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. After that, the retort furnace is heated to a film formation temperature (treatment temperature) of about 500°C, and the temperature is maintained for the required time (holding time). During this time, steam continues to flow continuously into the retort furnace, and the steam pressure inside the furnace is maintained at normal pressure or higher, about 100 mmAq. After the holding time has elapsed, the workpiece is removed from the retort furnace and air-cooled or liquid cooled.

[0006] At the treatment temperature, the water vapor in the furnace undergoes the reaction H2O=H2+(1 / 2)O2. The water (H2O) and hydrogen (H2) content ratio in the furnace is appropriately controlled during the coating process. Figure 7 shows an equilibrium phase diagram of the iron oxidation reaction based on the volumetric content ratio (%) of water (H2O) and hydrogen (H2) and the furnace temperature. If the water vapor treatment temperature is kept below 600°C and the water (H2O) and hydrogen (H2) content ratio is appropriately controlled, a triiron tetroxide coating will be formed on the workpiece surface.

[0007] The treatment temperature and holding time in the steam treatment are appropriately set based on the type of workpiece and the thickness of the triiron tetroxide coating. If the thickness of the triiron tetroxide coating is too large, it is undesirable because it is prone to peeling. If the treatment temperature is 560°C or higher or the holding time is 90 minutes or longer, part of the triiron tetroxide coating will change to diiron trioxide (Fe2O3), which is undesirable. In addition, if fine water droplets (condensed water) are contained in the steam, diiron trioxide will be formed on the surface of the workpiece, and this diiron trioxide will inhibit the formation of the triiron tetroxide coating. For this reason, in the steam treatment method, it is required to properly maintain the retort furnace, to heat the steam sufficiently, and to properly control the preheating temperature and treatment temperature. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Publication No. 55-152182 Summary of the Invention [Problem to be solved by the invention]

[0009] The steam treatment method requires a steam generator such as a boiler, which increases the cost of the entire treatment device. In addition, the superheated steam comes into direct contact with the insulation material placed inside the retort furnace, which is prone to deterioration. If the insulation efficiency decreases due to deterioration of the insulation material, the steam condenses and a diiron trioxide film is formed on the workpiece. In addition, the structure of the retort furnace becomes complicated, which increases the repair cost, including the need to periodically replace the insulation material and sealing materials for the steam.

[0010] In view of the above circumstances, the present invention provides a method for forming a metal oxide film, which is capable of efficiently forming a triiron tetroxide film using a gas without using water vapor. [Means for solving the problem]

[0011] The present invention, which achieves the above-mentioned object, provides a metal oxide coating forming method comprising the steps of: placing a metal workpiece in a heat treatment furnace; replacing the air in the heat treatment furnace with an inert gas or carbon dioxide; heating the interior of the heat treatment furnace after the replacing step; and forming a coating on the workpiece by creating a carbon dioxide atmosphere in the heat treatment furnace after the heating step.

[0012] In the above-described method for forming a metal oxide film, the film forming step may be characterized in that the carbon dioxide is convected.

[0013] The above-mentioned method for forming a metal oxide film may be characterized in that at least oxygen is introduced into the heat treatment furnace in the film forming step.

[0014] The above-mentioned method for forming a metal oxide film may be characterized in that, in the film forming step, the temperature inside the heat treatment furnace is maintained at 650° C. or less.

[0015] The above-mentioned method for forming a metal oxide film may be characterized in that, in the film forming step, the temperature inside the heat treatment furnace is maintained at 450° C. or higher.

[0016] The above-mentioned method for forming a metal oxide film may be characterized in that in the film forming step, the carbon monoxide content of the heat treatment furnace is maintained at 40% or less.

[0017] The above-mentioned method for forming a metal oxide film may be characterized in that, in the film forming step, the carbon monoxide content of the heat treatment furnace is maintained at 20% or less.

[0018] The above-mentioned method for forming a metal oxide film may be characterized in that, before the replacing step, the atmosphere in the heat treatment furnace is evacuated to a pressure lower than atmospheric pressure.

[0019] In the above-mentioned method for forming a metal oxide film, the atmosphere in the heat treatment furnace may be evacuated to 10 kPa or less before the replacing step.

[0020] To achieve the above-mentioned object, the present invention provides a metal oxide film forming apparatus comprising a heat treatment furnace for accommodating a workpiece, a heating device for heating the inside of the heat treatment furnace, and a carbon dioxide gas supplying device for supplying carbon dioxide gas into the heat treatment furnace, and is characterized in that a triiron tetroxide coating is formed on the workpiece by creating a carbon dioxide atmosphere inside the heat treatment furnace after heating. Effect of the Invention

[0021] According to the present invention, a triiron tetroxide coating can be efficiently formed by gas without using water vapor. [Brief description of the drawings]

[0022] [Figure 1] 1 is an overall schematic view of a metal oxide film forming apparatus according to an embodiment of the present invention. [Diagram 2] 4 is a time chart of a method for forming a metal oxide film using the above-mentioned metal oxide film forming apparatus. [Diagram 3] FIG. 2 is an equilibrium diagram of oxidation states that are applied based on the content of carbon dioxide gas and carbon monoxide gas and the temperature in the furnace. [Figure 4]10 is a modified example of the above-mentioned time chart of the present metal oxide film forming method. [Diagram 5] FIG. 2A is an enlarged cross-sectional view of the surface of a workpiece according to Example 1, and FIG. 2B is a hardness distribution diagram of the surface of the same workpiece. [Figure 6] FIG. 1A is an enlarged cross-sectional view of a workpiece surface according to Example 2, and FIG. 1B is a hardness distribution diagram of the workpiece surface. [Figure 7] 1 is a diagram of oxidation state equilibrium relationships that are applied based on the water (H2O) and hydrogen (H2) content and furnace temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0024] 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 interior 10a to form a metal film on a workpiece 100 in the furnace interior 10a.

[0025] The metal oxide film forming apparatus 1 includes a heat treatment furnace 10, a heating device 12 for heating the workpiece 100 in the furnace interior 10a of the heat treatment furnace 10, a gas supplying device 20 for supplying various gases to the furnace interior 10a, a first stirring unit 16 provided in the furnace interior 10a, a negative pressure generating device 50 for creating a negative pressure state in the furnace interior 10a, a cooling device 80 for adjusting the temperature of the atmosphere in the furnace interior 10a, and a control device 30. The control device 30 controls the heating device 12, the gas supplying device 20, the stirring unit 16, the negative pressure generating device 50, and the cooling device 80 by a control program.

[0026] (Heat treatment furnace)

[0027] The heat treatment furnace 10 can accommodate the workpiece 100, which is the material to be treated, in the furnace interior 10a. The heat treatment furnace 10 employs an airtight structure having pressure resistance that can make the furnace interior 10a in a negative pressure (vacuum) state. For example, a plurality of workpieces 100 are arranged in a line in the vertical direction (first direction) and horizontal direction on a stand 11 installed in the furnace interior 10a. The workpieces 100 are metals such as low carbon steel. The furnace interior 10a is provided with a heat insulating material 10b to maintain the temperature of the furnace interior 10a. Furthermore, a pressure adjustment valve 19 is connected to the heat treatment furnace 10, so that the furnace interior 10a is maintained at a pressure close to normal pressure (approximately atmospheric pressure: about 101 [kPa]) during film formation. Preferably, the furnace interior 10a is set to normal pressure (atmospheric pressure) or higher, specifically +100 mmAq or higher based on normal pressure (atmospheric pressure), for example, about atmospheric pressure +160 mmAq. The pressure regulating valve 19 can also serve as a relief valve for opening the interior of the furnace 10a to the atmosphere in an emergency, and as an exhaust valve for replacing the atmospheric gas in the interior of the furnace 10a.

[0028] The heat treatment furnace 10 is also provided with a thermometer 13 (e.g., a thermocouple) for measuring the temperature inside the furnace 10a, and a carbon dioxide gas concentration meter 14 and / or a carbon monoxide gas concentration meter 15 for the furnace interior 10a. For example, it is preferable to use an infrared concentration analyzer as the carbon dioxide gas concentration meter 14 and / or the carbon monoxide gas concentration meter 15 from the standpoint of heat resistance and pressure resistance, but other sensors may also be used. The thermometer 13, the carbon dioxide gas concentration meter 14 and / or the carbon monoxide gas concentration meter 15 are provided at the top of the heat treatment furnace 10, but the installation positions thereof are not particularly limited.

[0029] (heating device)

[0030] The heating device 12 has heaters 12a for heating the furnace interior 10a. The heaters 12a are arranged in pairs (six pairs here) at intervals in the horizontal direction (left-right direction in FIG. 1) at the top of the furnace interior 10a, for example, in the depth direction (perpendicular to the paper surface of FIG. 1). For example, a radiant tube heater or the like is used as the heater 12a, but it is not limited thereto.

[0031] (Gas supply equipment)

[0032] The gas supply device 20 has a carbon dioxide gas system 21 through which carbon dioxide gas flows, an air system 22 through which air flows, a nitrogen gas system 23 through which nitrogen gas flows, and a supply system 24 that collectively supplies the gases flowing through these systems 21, 22, and 23 to the furnace interior 10a. The gas supply device 20 further has a carbon dioxide supply source 25 connected to the carbon dioxide gas system 21, an air supply source 26 connected to the air system 22, and a nitrogen supply source 28 connected to the nitrogen gas system 23.

[0033] (Carbon dioxide source and carbon dioxide gas system)

[0034] The carbon dioxide supply source 25 is, for example, a gas cylinder filled with carbon dioxide. As shown in FIG. 1, the carbon dioxide gas system 21 has a supply pipe 21a, and an on-off valve 21b and a flowmeter 21c provided in the supply pipe 21a. A first end of the supply pipe 21a is connected to the carbon dioxide supply source 25, and a second end of the supply pipe 21a is connected to a connection part 40 of the supply system 24. Carbon dioxide gas from the carbon dioxide supply source 25 flows through the supply pipe 21a. The carbon dioxide gas is supplied from the connection part 40 of the supply system 24 through a downstream pipe 42 to the furnace interior 10a. The on-off valve 21b is an electromagnetic valve and is provided in the middle of the supply pipe 21a. The on-off valve 21b opens and closes the flow path and adjusts the flow rate of the carbon dioxide gas. The flowmeter 21c is provided downstream of the on-off valve 21b and measures the flow rate of the carbon dioxide gas flowing through the supply pipe 21a. The on-off valve 21b only opens and closes the flow path, and a flow rate control valve (not shown) for adjusting the flow rate of the carbon dioxide gas may be provided separately from the on-off valve 21b, and the flow rate of the carbon dioxide gas may be adjusted manually while checking the value of the flowmeter 21c. Although not particularly shown here, the carbon dioxide gas system may be provided with a separate path dedicated to purging (a path that opens and closes only, omitting the flow rate control valve) for purging the furnace interior 10a.

[0035] (Air supply source and air system)

[0036] The air supply source 26 is, for example, a compressor 26a that draws in and pressurizes the atmosphere. As shown in FIG. 1, the air system 22 has a supply pipe 22a, an on-off valve 22b and a flow meter 22c provided on the supply pipe 22a. A first end of the supply pipe 22a is connected to the air supply source 26, and a second end of the supply pipe 22a is connected to a connection part 40 of the supply system 24. Pressurized air from the air supply source 26 flows through the supply pipe 22a. Air is supplied from the connection part 40 of the supply system 24 through a downstream pipe 42 to the furnace interior 10a. The on-off valve 22b is an electromagnetic valve and is provided in the middle of the supply pipe 22a. The on-off valve 22b opens and closes the flow path and adjusts the flow rate of air. The flow meter 22c is provided downstream of the on-off valve 22b and measures the flow rate of air flowing through the supply pipe 22a. In addition, the on-off valve 22b only opens and closes the flow path, and a flow rate adjustment valve (not shown) that adjusts the air flow rate is provided separately from the on-off valve 22b, and the air flow rate can be adjusted manually while checking the value of the flow meter 22c.

[0037] (Nitrogen source and nitrogen gas system)

[0038] In this embodiment, the nitrogen supply source 28 has a gas separation device 28a, which is a PSA type nitrogen gas generator. The gas separation device 28a separates nitrogen from air by pressure swing adsorption (PSA) to generate a nitrogen-containing gas. Therefore, the nitrogen-containing gas contains a small amount of oxygen as an impurity other than nitrogen in addition to nitrogen. The nitrogen gas system 23 has a supply pipe 23a, an on-off valve 23b and a flow meter 23c provided on the supply pipe 23a. A first end of the supply pipe 23a is connected to the nitrogen supply source 28, and a second end of the supply pipe 23a is connected to a connection part 40 of the supply system 24. Nitrogen gas from the nitrogen supply source 28 flows through the supply pipe 23a. The nitrogen gas is supplied from the connection part 40 of the supply system 24 through a downstream pipe 42 to the furnace interior 10a. The on-off valve 23b is an electromagnetic valve and is provided in the middle of the supply pipe 23a. The flow rate of the nitrogen gas is adjusted by the on-off valve 23b. The flowmeter 23c is provided on the supply pipe 23a downstream of the on-off valve 23b to measure the flow rate of the nitrogen gas flowing through the supply pipe 23a. The on-off valve 23b only opens and closes the flow path, and a flow rate control valve (not shown) for adjusting the flow rate of the nitrogen gas may be provided separately from the on-off valve 23b, and the flow rate of the nitrogen gas may be manually adjusted while checking the value of the flowmeter 23c. Although not particularly shown here, the nitrogen gas system may be provided with a separate path dedicated to purging (a path for opening and closing only, omitting the flow rate control valve) for purging the furnace interior 10a. The nitrogen supply source 28 is not limited to a nitrogen gas generator, and may be, for example, a gas cylinder filled with nitrogen or liquid nitrogen.

[0039] The nitrogen gas system 23 and the nitrogen supply source 28 are not necessarily provided, but they make it possible to make the interior of the furnace 10a in a more inert state before and during the heating and / or cooling of the interior of the furnace 10a.

[0040] (supply system)

[0041] The supply system 24 has a connection part 40 connected to the supply pipes 21a, 22a, and 23a, and a downstream pipe 42 connected to the connection part 40. A mixer 49 is provided in the middle of the connection part 40 or the downstream pipe 42. The mixer 49 has a function of mixing various gases flowing from upstream. A gas supply nozzle 44 is provided at the end of the downstream pipe 42. The gas supply nozzle 44 is provided at the lower part of the heat treatment furnace 10. In this embodiment, the gas supply nozzle 44 is provided at the center position in 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 10a. Carbon dioxide gas, air, and nitrogen gas are ejected (or flowed out) upward from the supply port 44a. In this embodiment, the supply port 44a is provided on the floor surface 10y as an example, but the present invention is not limited to this, and may be provided on the side surface or ceiling surface of the furnace interior 10a.

[0042] An on-off valve 42a and a flow meter 42b are provided in the downstream pipe 42. The on-off valve 42a is an electromagnetic valve, and is capable of adjusting the flow rate of the gas flowing through the downstream pipe 42.

[0043] (Mixing section)

[0044] The stirring unit 16 is provided on the ceiling surface 10x of the furnace interior 10, and has, for example, a centrifugal fan or the like for stirring the atmospheric gas in the furnace interior 10a. In this embodiment, the stirring unit 16 is disposed in the upper part of the furnace interior 10a, in the horizontal center of the furnace interior 10a. The stirring unit 16 sucks in the gas in the furnace interior 10a from the core side (the horizontal center of the furnace interior 10a), and diffuses the gas in the furnace interior 10a radially in the horizontal surface direction on the ceiling surface of the furnace interior 10a so that the gas in the furnace interior 10a flows downward along the surface direction of the inner peripheral wall of the furnace interior 10a. The installation position of the stirring unit 16 is not particularly limited, and may be provided on the floor surface 10y of the furnace interior 10a, for example, to diffuse the gas radially in the surface direction of the floor surface 10y.

[0045] (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 interior 10a. The vacuum pump 52 is connected to the furnace interior 10a by an exhaust pipe 58. A main valve 58a is provided on the exhaust pipe 58, and switches ON / OFF (open / close) the evacuation of the furnace interior 10a by the vacuum pump 52.

[0046] When the interior 10a of the furnace is put into a vacuum (negative pressure) state, the main valve 58a is turned ON (opened) and the interior 10a of the furnace is fully evacuated by the vacuum pump 52. For example, the pressure in the interior 10a of the furnace can be reduced to 1 kPa or less.

[0047] (cooling device) The cooling device 80 has a circulation path 82, a heat exchanger 84, a blower 86, and an on-off valve 88. The circulation path 82 is connected at both ends to the furnace interior 10a, and serves as a circulation path that draws gas from the furnace interior 10a out of the furnace from one end and returns the gas to the furnace interior 10a from the other end. The heat exchanger 84 is disposed in the middle of the circulation path 82 to cool the circulating gas. The blower 86 is disposed in the middle of the circulation path 82 to adjust the flow rate of gas flowing through the circulation path 82. The on-off valve 88 is disposed in the middle of the circulation path 82 and determines whether or not gas is allowed to flow through the circulation path 82 by switching ON / OFF (open / close). This cooling device 80 also forms convection in the furnace interior 10a, which allows the workpiece 100 to be uniformly cooled.

[0048] (Control device)

[0049] The control device 30 is a computer having a processor and the like. Based on the metal oxide film formation program, the control device 30 controls the on-off valves 21b, 22b, 23b, and 42a provided in the carbon dioxide gas system 21, the air system 22, the nitrogen gas system 23, and the supply system 24, and various controls the heating device 12, the negative pressure generating device 50, and the cooling device 80. This allows the control of the internal pressure of the furnace 10a, the control of the supply (flow rate) of various gases to be supplied to the furnace 10a, and the temperature of the furnace 10a to be controlled. The control device 30 is connected to a thermometer 13, a carbon oxide gas concentration meter 14 and / or a carbon monoxide gas concentration meter 15, a pressure gauge 51, and the like, so that the measurement results can be acquired. These measurement results are output to and recorded in an external device as necessary.

[0050] (Coating method)

[0051] Next, the procedure of the metal oxide film forming method of this embodiment executed by the control device 30 according to the metal oxide film forming program will be described with reference to the timing chart of FIG.

[0052] <Placement process> First, a step of carrying and arranging the workpiece 100 in the furnace interior 10a is performed. Specifically, the workpiece 100 is transported to the furnace interior 10a by a transport device (not shown) provided adjacent to the heat treatment furnace 10, and is arranged on the stand 11.

[0053] <Degassing process> When the workpiece 100 is placed in the furnace 10a, a vacuum door for entrance and exit (not shown) is closed to make the furnace 10a airtight. Then, the 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 is made into a negative pressure state of 10 kPa or less, preferably 1 kPa or less.

[0054] <Purge process> After the interior of the furnace 10a has reached a desired vacuum state, nitrogen gas is supplied from the nitrogen supply source 28 into the interior of the furnace 10a to restore the pressure to approximately atmospheric pressure. Through this procedure, the interior of the furnace 10a is vacuum purged with inert nitrogen gas.

[0055] <Heating process> Thereafter, the temperature inside the furnace 10a is raised to a film treatment temperature by the heating device 12. Note that this film treatment temperature will be described later.

[0056] <Film formation process> After the temperature increase is complete, the nitrogen gas in the furnace interior 10a is evacuated using the negative pressure generating device 50 to create a vacuum state (negative pressure state below atmospheric pressure). After the desired vacuum state is achieved in the furnace interior 10a, carbon dioxide is supplied from the carbon dioxide supply source 25 to the furnace interior 10a to restore the pressure to approximately atmospheric pressure. By carrying out these steps, the furnace interior 10a is made into a carbon dioxide atmosphere, and a triiron tetroxide coating of Fe3O4 is formed. By continuing to supply carbon dioxide even after the pressure is restored, the carbon monoxide concentration in the atmosphere in the furnace interior 10a is maintained within a predetermined range. In this process, it is preferable to convect the atmosphere in the furnace interior 10a using the stirring unit 16.

[0057] Specifically, there are three types of oxidation reactions of metal surfaces by carbon dioxide: Fe + CO2 → FeO + CO Equation (1) 3FeO+CO2→Fe3O4+CO...Formula (2) 3Fe+4CO2→Fe3O4+4CO...Formula (3)

[0058] FIG. 3 shows the equilibrium relationship of the oxidation state of the above formulas (1) to (3) that is applied based on the volume content (%) of carbon dioxide gas and carbon monoxide gas in the furnace 10a and the temperature in the furnace 10a. The oxidation reaction of the FeO film in formula (1) occurs in the range surrounded by the Tb line and the Tc line. In other words, the condition is that the carbon monoxide content (or carbon dioxide content) in the furnace 10a is within a predetermined range and exceeds the temperature T (570 degrees). The oxidation reaction of the Fe3O4 film in formulas (2) and (3) occurs in the range below the aTc line. Note that no oxidation reaction occurs in the range above the aTb line. For reference, in the range above the aTb line, the range surrounded by the def line is the range in which the ferrite phase is formed, and the other ranges are the ranges in which the austenite phase is formed.

[0059] The oxidation reaction of formula (2) occurs when the inside of the furnace 10a is in a range that satisfies formula (1) and an FeO film is formed, or when an FeO film is formed on the surface of the workpiece 100.

[0060] The jJ line indicates the boundary of the carbon deposition reaction of the following formula (4). If the reaction is above (to the left of) this line, soot will be generated. 2CO→C+CO2...Equation (4) In other words, as the amount of carbon monoxide in the atmosphere inside the furnace 10a increases, the oxidation reactions of formulas (1) to (3) become weaker, and the generation of soot by formula (4) takes precedence.

[0061] Therefore, in order to suppress the formation of the FeO film according to formula (1) and to actively cause the oxidation reaction of formula (3) while suppressing the generation of soot according to formula (4), it is desirable to keep the carbon monoxide content in the atmosphere inside the furnace 10a low and to maintain the furnace 10a at a low temperature. In other words, it is preferable to maintain the furnace 10a in a state within the range of region X in FIG. 3. Therefore, the carbon monoxide content in the furnace 10a is preferably set to be near point g in FIG. 3 or less, that is, 40% or less (carbon dioxide content over 60%), and more preferably, near point s where the jg line intersects at the same temperature as the temperature at point T, that is, 20% or less (carbon dioxide content over 80%). More preferably, it is set to be near point c or less, that is, 17% or less (carbon dioxide content over 83%). In addition, the film treatment temperature in the furnace 10a is preferably set to be near point g in FIG. 3 or less, that is, 650° C. or less, and more preferably, near point T or less, that is, 600° C. or less. More strictly, it is desirable to set it below point T (point s), i.e., below 570°C. On the other hand, if the coating temperature is too low, the jg line is exceeded, which makes it easy for soot of formula (4) to be generated, so it is preferable to set it above 450°C, and more preferably above 480°C, taking into consideration the margin of response speed during temperature control and carbon monoxide content control. In this embodiment, the temperature is adjusted to within the range of 500°C to 550°C.

[0062] However, when the oxidation reaction of formula (3) is actively caused to occur, carbon monoxide is generated, and the carbon monoxide content in the furnace interior 10a naturally increases. Therefore, in this embodiment, in the film formation process, air (at least oxygen) is supplied from the air supply source 26 to the furnace interior 10a, causing the reaction of formula (5) below to occur, thereby maintaining the carbon monoxide content in the furnace interior 10a at a low level (maintaining the carbon dioxide concentration at a high level). 2CO+O2→2CO2...Equation (5)

[0063] <Cooling process> In the film formation process, after a predetermined time has elapsed until the required film thickness is obtained, nitrogen gas is supplied from the nitrogen supply source 28 to the furnace interior 10a to replace the atmosphere in the furnace interior 10a with nitrogen gas, while the temperature in the furnace interior 10a is lowered by the cooling device 80. Once the workpiece 100 has been sufficiently cooled, the vacuum door is opened and the workpiece 100 is carried out of the furnace interior 10a. Through the above processes, a triiron tetroxide film is formed on the surface of the workpiece 100.

[0064] (Modification)

[0065] 4, a timing chart of a modified example of the method for forming a metal oxide film according to the present embodiment is shown. In this timing chart, in the degassing and purging steps, carbon dioxide gas is supplied to the furnace interior 10a to restore the pressure to approximately atmospheric pressure (vacuum purging with carbon dioxide gas). In this case, there is an advantage that the step of evacuating the furnace interior 10a and replacing it with carbon dioxide gas can be omitted in the film forming step.

[0066] In the above embodiment, the air in the furnace 10a is replaced with nitrogen gas or carbon dioxide gas, but the present invention is not limited thereto. The air may be replaced with an inert gas other than nitrogen gas (e.g., argon gas, etc.). In addition, the air in the furnace 10a is degassed by a degassing process before the purge process, but the present invention is not limited thereto. For example, the air in the furnace 10a may be replaced by pushing it out to the outside by supplying an inert gas or carbon dioxide gas to the furnace 10a while opening the pressure regulating valve 19. In this case, it is not necessary to make the heat treatment furnace 10 vacuum-specified (pressure-resistant), and the negative pressure generating device can be omitted.

[0067] Furthermore, in the film formation process, the carbon monoxide content in the furnace interior 10a is maintained at or below a desired value by supplying air to the furnace interior 10a, but the present invention is not limited to this. For example, pure oxygen may be supplied to the furnace interior 10a. Also, if the carbon monoxide content in the furnace interior 10a can be maintained low by continuously supplying carbon dioxide gas, the supply of oxygen or air may be omitted. EXAMPLES

[0068] Carbon tool steel (specifically, SK5) was used as the base material of the workpiece 100, and a compound layer of carbonitride was formed to a thickness of 13 to 15 μm on this base material by gas soft nitriding at 570°C. Then, a metal oxide coating was formed on the surface of the workpiece 100 based on the timing chart of FIG. 4. The coating treatment temperature in the coating formation process was 520°C, and carbon dioxide gas was supplied at 20 Nl / min during the heating process and coating formation process, with the supply of air (oxygen) omitted. The duration of the coating formation process was 60 minutes. In the cooling process, nitrogen gas was supplied at 2 Nm 3 5A shows a cross section of the surface of the workpiece 100 enlarged 500 times, and FIG. 5B shows the hardness distribution of the surface.

[0069] As shown in FIG. 5(A), a 3-4 μm Fe3O4 metal oxide coating was formed on the surface of the workpiece 100. As shown in FIG. 5(B), the hardness of the outermost surface exceeded 350 HV, specifically 388 HV. The hardness of the compound layer was an average of 208 HV. As a result, the metal oxide coating achieved an increase in hardness of about 150 HV or more. EXAMPLES

[0070] Carbon tool steel (specifically, SK5) was used as the base material of the workpiece 100, and a compound layer of carbonitride was formed to a thickness of 13 to 15 μm on this base material by gas soft nitriding at 570°C. Then, a metal oxide coating was formed on the surface of the workpiece 100 based on the timing chart of FIG. 4. The coating treatment temperature in the coating formation step was 520°C, and in the heating step and coating formation step, carbon dioxide gas was supplied at 20 Nl / min and air was supplied at 0.5 Nl / min. The duration of the coating formation step was 60 minutes. In the cooling step, nitrogen gas was supplied at 2 Nm 3 6A shows a cross section of the surface of the workpiece 100 enlarged 500 times, and FIG. 6B shows the hardness distribution of the surface.

[0071] As shown in FIG. 6(A), a 3-4 μm Fe3O4 metal oxide coating was formed on the surface of the workpiece 100. As shown in FIG. 6(B), the hardness of the outermost surface exceeded 350 HV, specifically 381 HV. The hardness of the compound layer was an average of 209 HV. As a result, the metal oxide coating achieved an increase in hardness of about 150 HV or more.

[0072] As described above, according to the method for forming a metal oxide film of this embodiment, the metal workpiece 100 is placed in the heat treatment furnace 10, the air in the heat treatment furnace 10 is replaced with an inert gas or carbon dioxide, and the temperature is raised to the treatment temperature, and the heat treatment furnace 10 after the temperature is raised is filled with a carbon dioxide atmosphere, so that a triiron tetroxide film can be efficiently formed on the workpiece 100. In particular, since the air in the heat treatment furnace 10 is vacuum purged with an inert gas or carbon dioxide before the temperature is raised, it is possible to prevent the formation of an unintended film on the surface of the workpiece 100 during the temperature rise. Furthermore, since carbon dioxide is convected in the film forming process, it is possible to form a dense film even on the fine parts of the workpiece 100.

[0073] Furthermore, according to the metal oxide film forming method of this embodiment, at least oxygen is introduced into the heat treatment furnace 10 in the film forming step, so that the content of carbon monoxide that adversely affects the formation of the triiron tetroxide film can be reduced, and film quality can be improved. Also, in the film forming step, the temperature inside the heat treatment furnace 10 is maintained at 650°C or lower, so that the formation of the FeO film can be suppressed. Also, the temperature inside the heat treatment furnace 10 is maintained at 480°C or higher, so that a slight increase in the carbon monoxide content can be tolerated. As a result, it is possible to suppress the adverse effects of carbon monoxide that occurs when the triiron tetroxide film is formed.

[0074] According to the method for forming a metal oxide film of the present embodiment, the carbon monoxide content in the heat treatment furnace is maintained at 40% or less (preferably 20% or less), thereby improving the film quality. [Explanation of symbols]

[0075] 1 Metal oxide film forming equipment 10 Heat treatment furnace 12 Heating device 16 Mixing section 20 Gas supply equipment 30 Control device 80 Cooling device 21 Carbon dioxide gas system 22 Air System 23 Nitrogen gas system 24 Supply system 25 Carbon Dioxide Sources 26 Air supply source 28 Nitrogen Sources 30 Control device 40 Connection 42 Downstream piping 44 Gas supply nozzle 44a Supply port 49 Mixer 50 Negative pressure generating device 100 Work

Claims

1. 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 or carbon dioxide; a heating step of heating the inside of the heat treatment furnace after the replacement step; a coating formation process in which a carbon dioxide atmosphere is created in the heat treatment furnace after the temperature increase process to form a triiron tetroxide coating on the workpiece; A method for forming a metal oxide film, comprising:

2. In the film forming process, the carbon dioxide is convected. The method for forming a metal oxide film according to claim 1 .

3. In the film forming step, at least oxygen is introduced into the heat treatment furnace. The method for forming a metal oxide film according to claim 1 .

4. In the coating process, the temperature inside the heat treatment furnace is maintained at 650° C. or less. The method for forming a metal oxide film according to claim 1 .

5. In the coating process, the temperature inside the heat treatment furnace is maintained at 450° C. or higher. The method for forming a metal oxide film according to claim 4.

6. In the coating process, the carbon monoxide content of the heat treatment furnace is maintained at 40% or less. The method for forming a metal oxide film according to claim 1 .

7. In the coating process, the carbon monoxide content of the heat treatment furnace is maintained at 20% or less. The method for forming a metal oxide film according to claim 6.

8. Before the replacement step, the atmosphere in the heat treatment furnace is evacuated to a pressure lower than atmospheric pressure. The method for forming a metal oxide film according to claim 1 .

9. Before the replacement step, the atmosphere in the heat treatment furnace is evacuated to 10 kPa or less. The method for forming a metal oxide film according to claim 1 .

10. A heat treatment furnace that accommodates the workpiece; A heating device for heating the inside of the heat treatment furnace; a carbon dioxide gas supplying device that supplies carbon dioxide gas into the heat treatment furnace; The inside of the heat treatment furnace after the temperature is increased is made into a carbon dioxide atmosphere, thereby forming a triiron tetroxide coating on the workpiece. Metal oxide film forming equipment.

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