JP7295511B2 - Plasma nitriding method - Google Patents

Description

The present invention relates to a plasma nitriding method for forming a nitrided layer on the surface of a metal workpiece using plasma.

One of the treatment methods for hardening the surface of metals such as steel is a nitriding process in which nitrogen is permeated near the surface to form a nitrided layer. Also, as one of nitriding process methods, a plasma nitriding process is known in which nitrogen or ammonia plasma is used to permeate nitrogen.
Conventionally, as a plasma nitriding process, DC plasma nitriding (sometimes called DCP nitriding) has been used in which a voltage is applied between a positive electrode in a processing container and a material to be processed to generate plasma. Various improvement techniques have also been proposed for DCP nitriding. For example, Patent Document 1 discloses removing residual gas and impurities on the surface of the material to be treated by applying a voltage to the material to be treated while injecting a mixed gas of argon and hydrogen into the furnace before starting the nitriding process. Techniques are disclosed for performing ablation sputtering.
On the other hand, DCP nitriding has disadvantages such as an edge effect in which the discharge concentrates on the edges of the material to be treated and the modified layer becomes non-uniform. Plasma nitridation (sometimes referred to as ASP nitridation) has also attracted attention. In ASP nitriding, a screen made of a conductive metal mesh or the like is installed so as to cover the material to be treated, a voltage is applied between the positive electrode and the screen to form plasma, and this plasma is used It is a method of nitriding process. Regarding ASP nitriding, Patent Document 2 discloses surface activation in which the internal atmosphere of the furnace is maintained with a mixed gas of argon and hydrogen, and residual gas and impurities on the surface of the material to be treated are removed by oxidizing gas and reducing gas. We are proposing a technique to do so. Further, Patent Document 3 discloses a technique for generating a uniform plasma atmosphere by micro-vibrating a material to be treated in ASP nitriding.

Japanese Patent No. 4644236 Japanese Patent No. 4378364 Japanese Patent Application Laid-Open No. 2008-115422

However, while the nitriding treatment improves the hardness of the surface, it sometimes causes the roughness of the surface. In order to secure the surface roughness, it is possible to consider polishing the surface after nitriding treatment, but such a method may scrape away the surface layer hardened by nitriding treatment, and requires very delicate processing technology. This will lead to another issue.
SUMMARY OF THE INVENTION In view of these problems, an object of the present invention is to provide a nitriding method capable of improving hardness while suppressing a decrease in surface roughness.

The present invention
A plasma nitriding method for forming a nitride layer on the surface of a metal material to be treated using plasma,
(a) placing the material to be treated in a closed treatment vessel;
(b) While supplying a process gas containing nitrogen or ammonia into the processing container, current is applied between the positive electrode and the negative electrode to generate plasma of the process gas, and the material to be processed is nitrided by the plasma. and
The energization in the step (b) is
Among the energization conditions for improving the hardness of the surface of the material to be treated in multiple stages, the plasma is performed under the energization conditions that can achieve the first stage improvement in hardness and do not lead to the second stage improvement. It can be configured with a nitriding method.

The inventors of the present invention found that there is a correlation between the hardness of the surface of the material to be treated and the surface roughness after the treatment, by changing the conditions of energization during plasma nitridation. It was also found that the hardness of the surface due to plasma nitridation does not change monotonously, but changes stepwise according to the conditions of energization. Then, the inventors found that the surface roughness is extremely reduced when the current is applied to the extent that the hardness of the surface is improved to the second stage. Therefore, in the present invention, by conducting the current under the above conditions, it is possible to suppress deterioration of the surface roughness while achieving the first level of improvement in surface hardness.

Conditions for energization include, for example, control of voltage, control of current, and control of both.
Various methods such as DCP nitridation, ASP nitridation, and nitridation using parallel plate electrodes can be applied to plasma nitridation.

In the plasma nitriding method of the present invention, for example,
It is preferable that the energization in the step (b) is performed under the condition that the Vickers hardness HV (0.5) of the surface of the material to be treated under a load of 0.5 kgf is less than 1250.

According to experiments, it was found that the surface Vickers hardness HV (0.5) was 1100 to 1200 in the first stage and 1250 or more in the second stage. Therefore, by setting the energization conditions such that the Vickers hardness HV (0.5) is less than 1250, it is possible to both improve the surface hardness and suppress the decrease in the surface roughness.

Further, in the plasma nitriding method of the present invention,
In the preceding step (b), in addition to the energization for generating the plasma, a bias current is energized between the positive electrode and the material to be treated;
The energization condition in the step (b) may be realized by controlling the bias current.

In the present invention, the principle by which the decrease in surface roughness can be suppressed has not been completely elucidated, but it is believed that one of the factors is the energy when the plasma acts on the material to be treated. be done. By applying a bias current, it is possible to control the potential of the material to be processed, so that it becomes easier to control the energy when the plasma acts on the material to be processed. Therefore, according to the above aspect, it is possible to effectively suppress a decrease in surface roughness.

Conditions for applying bias current include, for example,
When the process gas consists of nitrogen or ammonia and hydrogen,
Preferably, the bias current is <0.3A.
Further, it is more preferable to set the bias current <0.2A.

again,
When the process gas consists of nitrogen or ammonia, hydrogen, and argon,
The bias current is preferably 0.4 A or less.
Further, it is more preferable to set the bias current <0.35A.

Also, the bias current may be a pulse current.

A pulse current is a pulse-like current that flows in response to a pulse-like voltage applied from a power source. By doing so, there is an advantage that the pulse current can be easily controlled by adjusting the pulse width or pulse density.
In addition to the bias current, the current for generating plasma may also be a pulse current. By doing so, plasma can be generated stably, and plasma nitridation can be stably realized.

Regardless of whether the bias current is applied or not,
In the plasma nitriding method of the present invention,
The step (b) is
The plasma may be generated by energization between a positive electrode installed in the processing container and a conductive screen installed in the positive electrode.

That is, it is a mode by active screen plasma nitridation (ASP nitridation). It is known that ASP nitridation can avoid the edge effect, which means that it can avoid biased action of the plasma on the material to be treated. It is suitable because it can effectively prevent damage to the
Moreover, when ASP nitridation is used, the bias current can be controlled independently of the current for plasma generation, so there is an advantage that it is easy to suppress deterioration of the surface roughness. In order to realize independent voltage control, for example, circuits for applying the voltage of the screen and the bias voltage may be separately provided so that both can be controlled individually.

The present invention does not necessarily have all of the various features described above, and may be configured by omitting or combining some of them as appropriate.
Further, the present invention is not limited to the plasma nitriding method described above, and can be configured as a plasma nitriding system for performing a plasma nitriding process. In such a case, it is preferable to provide a control device for controlling the energization in the above-described step (b) so as to control the energization from the power source.

1 is an explanatory diagram showing the configuration of a plasma nitridation system; FIG. 4 is a flow chart of nitriding process control processing. FIG. 5 is an explanatory diagram showing an example of change in Vickers hardness with change in bias current; FIG. 5 is an explanatory diagram showing an example of change in surface roughness with change in bias current; FIG. 5 is an explanatory diagram showing an example of change in surface roughness when bias voltage is changed; It is an explanatory view showing the definition of surface roughness. FIG. 5 is an explanatory diagram showing an example of change in surface roughness accompanying change in bias current when argon gas is supplied; It is a micrograph of the surface after nitriding treatment.

Examples of the present invention will be described below.
A. Device configuration:
FIG. 1 is an explanatory diagram showing the configuration of a plasma nitridation system. This is an apparatus for performing a plasma nitriding process in which a voltage is applied to a process gas containing nitrogen or ammonia to form plasma, and nitrogen permeates the surface of a material to be treated such as steel to form a nitrided layer.
The plasma nitriding process includes a plasma nitriding device 10, a power supply device 20, a control device 40, and the like.

Plasma nitridation apparatus 10 has a vessel 11 for performing a plasma nitridation process. The container 11 is a closed container that can withstand vacuum.
Inside the container 11, a positive electrode 12 for applying a voltage for generating plasma is installed along the inner wall thereof.
Inside the positive electrode 12, a conductive mesh screen 13 is installed with a predetermined distance ds from the positive electrode. The distance ds between the positive electrode 12 and the screen 13 affects formation of the plasma P. Therefore, the interval between the two should be set analytically or experimentally so that the plasma P is well formed. The mesh shape of the screen may also be selected arbitrarily, but it is preferable that the shape of the opening is a rhombus with one diagonal length of 8 mm to 35 mm and the other diagonal length of 8 mm to 15 mm, more preferably: One diagonal length is 12 mm and the other diagonal length is 30 mm.
The inside of the screen 13 becomes a processing space for applying the nitriding process. A mounting table 16 for mounting the material W to be processed is installed inside the screen 13 .
A supply port 14 is provided at the top of the vessel 11 for supplying a series of process gases required for the nitriding process. A discharge port 15 for discharging the process gas is provided at the bottom of the container 11 .

In this embodiment, three types of gas tanks are connected to the supply port 14 . A nitrogen tank 31 for storing nitrogen, a hydrogen tank 32 for storing hydrogen, and an argon tank 33 for storing argon. A tank that stores ammonia instead of nitrogen may be used. The nitrogen tank 31, hydrogen tank 32, and argon tank 33 are connected to the supply port 14 through valves 34, 35, and 36, respectively. By controlling the opening and closing of these valves 34 , 35 and 36 , the amounts of nitrogen, hydrogen and argon supplied to the container 11 can be controlled. The amounts of gas supplied from the nitrogen tank 31, the hydrogen tank 32, and the argon tank 33 can be detected by flow sensors 37, 38, and 39, respectively.

During the nitriding process, the following sensors are attached to the vessel 11 in order to control the power source, the amount of process gas supplied, and the like. The arc sensor 17 is a sensor for detecting whether arc discharge is occurring within the container. For example, a sensor that detects light from arc discharge can be used as the arc sensor 17 . A temperature sensor 18 detects the temperature inside the container 11 . A pressure sensor 19 detects the pressure inside the container 11 .
In this embodiment, only one arc sensor 17 is provided, but a sensor for detecting arc discharge generated on the screen 13 and a sensor for detecting arc discharge generated on the mounting table 16 are separately provided. may

Next, the configuration of the power supply device 20 will be described. In this embodiment, a voltage for generating plasma is applied between the positive electrode 12 and the screen 13 from an AC power supply of 200V. This voltage is called the screen voltage. Also, a bias voltage is applied between the positive electrode 12 and the mounting table 16 . In this embodiment, the screen voltage can be output in the range of 0 to 600 V (50 A), and the bias voltage can be output in the range of 0 to 600 V (10 A).
The power supply device 20 includes a screen power supply circuit 22 for applying a screen voltage and a bias power supply circuit 23 for applying a bias voltage. The screen power supply circuit 22 and the bias power supply circuit 23 are circuits that output voltage pulses for applying a screen voltage and a bias voltage by turning on/off switching elements such as switching transistors and thyristors, respectively.
The on/off of the switching element is controlled by the power control device 21 . The power control device 21 modulates the time width and duty of the voltage pulse by controlling the ON/OFF of the switching element, and changes the voltage values of the screen voltage and the bias voltage. More specifically, a circuit that outputs a voltage pulse at a constant cycle is constructed, and part of the voltage pulse is paused according to the voltage. By doing so, the temporal pulse density, that is, the duty can be modulated, and the voltage can be controlled. In this example, a voltage pulse of 500 Hz was used.
Instead of such pulse width modulation control (PWM control), frequency modulation control (PFM control) that modulates the on/off frequency of pulses may be applied to voltage control.
In this embodiment, the power supply voltage is controlled according to the presence or absence of arc discharge inside the container 11 and the temperature. The presence or absence of arc discharge and temperature are detected by arc sensor 17 and temperature sensor 18 .

Operation of the plasma nitridation system is controlled by controller 40 . The control device 40 is a computer internally equipped with a CPU, RAM, and ROM. The control device 40 is provided with illustrated functional blocks. These functional blocks can be configured by installing control software, or can be configured in hardware by dedicated circuits that implement control functions.
The process condition input unit 43 inputs the conditions of the nitriding process according to the operator's operation. Conditions include the temperature, pressure, bias current, screen current, etc. of the nitriding process.
The process main control unit 41 performs processing of the entire plasma nitriding system based on the conditions of the nitriding process. For the power supply device 20, the conditions required for power supply control, such as the set temperature inside the container 11, the bias current, and the screen current, are specified. The composition ratio of the process gas and other conditions are specified for the process gas control unit 42 . As will be described later, in this embodiment, as a pretreatment for the nitriding process, a temperature raising process for raising the temperature of the material to be treated is performed. In this process, a process gas containing argon is used. Use a process gas that does not The process main control unit 41 instructs the process gas control unit 42 about the compounding ratio of the process gas used in each of these processes.
The process gas controller 42 controls the opening and closing of the valves 34 , 35 and 36 to control the supply of nitrogen, hydrogen and argon. In this embodiment, it is controlled according to the pressure inside the container and the supply amount of each gas.

B. Nitriding process:
The contents of the nitriding process will be described below.
FIG. 2 is a flow chart of the nitriding process control process. This process is a process executed by the control device 40 .
When the process starts, the controller 40 inputs process conditions (step S10). The process conditions can be specified by an operator operating a keyboard or other input device provided in the control device 40 . The process conditions include the process temperature, pressure, etc. when performing the nitriding process.

The control device 40 sets conditions for the power supply control device 21 according to the input process conditions (step S11). In this embodiment, the process temperature, the holding time of the nitriding process, the bias current, the screen current, etc. are indicated.
After setting the conditions, the control device 40 starts supplying the argon-containing process gas and instructs the power control device 21 to start power control (step S12). The argon-containing process gas is supplied by opening the valves 34, 35 and 36 and supplying gases from the nitrogen tank 31, the hydrogen tank 32 and the argon tank 33, respectively. In this embodiment, the supply amount is controlled so that the mixing ratio of nitrogen, hydrogen and argon is 2:2:1.

Controller 40 senses the temperature of vessel 11 and continues to supply argon-containing process gas (step S13) until it reaches the process temperature (step S14). By applying a voltage while supplying the argon-containing process gas, plasma is generated inside the container 11 and the temperature of the material to be treated is raised.

When the temperature inside vessel 11 reaches the process temperature (step S14), controller 40 supplies an argon-free process gas (step S15). In this embodiment, the valve 36 connected to the argon tank 33 should be closed. At the same time, the blending amounts of nitrogen and hydrogen are controlled so as to achieve the ratio set for nitriding treatment. In this embodiment, the ratio of nitrogen:hydrogen was set to 1:1. Other values may be used for the blending ratio of nitrogen and hydrogen.
By applying a voltage while supplying an argon-free process gas, a plasma is generated inside the vessel 11 and a nitridation process is performed.
The controller 40 continues to supply the argon-free process gas (step S15) until the set holding time has passed (step S16).
When the holding time is reached, the control device 40 instructs the power control device 21 to stop power control (step S17), and terminates the nitriding process control process.

In addition, the power supply control device 21 controls the application of the screen power supply voltage by the screen power supply circuit 22 and the application of the bias power supply voltage by the bias power supply circuit 23 according to the instruction from the control device 40 . In this embodiment, the control is performed so that the specified bias current value is realized instead of the voltage value.
The power supply control device 21 temporarily stops applying voltage to the screen power supply circuit 22 and the bias power supply circuit 23 until the process temperature is reached, when an arc discharge is detected. The stop time can range from 0.5 to 2.0 seconds. After stopping for a predetermined time, energization is started again. Also, after reaching the process temperature, control is performed to maintain the specified bias current while maintaining the process temperature.

The supply of the process gas and the adjustment of the power supply may be manually performed by the operator without relying on the control device 40 .
Also, the supply of argon gas may be omitted in the process until the process temperature is reached. Conversely, the supply of argon gas may be continued even after reaching the process.

C. Effects and process conditions:
FIG. 3 is an explanatory diagram showing an example of change in Vickers hardness with change in bias current. A process gas containing nitrogen gas and hydrogen gas at a ratio of 1:1 was used, and the results were obtained when the process was performed for 5 hours under the conditions of a pressure of 350 Pa and a process temperature of 525°C. The material to be treated is a member made of SKD61 (steel material) having a square planar shape of 12 mm×12 mm and a thickness of 5 mm.

FIG. 3(a) shows the results when the bias current is varied in the range of 0 to 0.35A. Hardness HV (0.5) is Vickers hardness with a 0.5 kgf load. Appearance is the result of visual evaluation of surface roughness due to nitriding treatment, which will be described later.
FIG. 3(b) is a graph showing the relationship between the bias current and hardness HV (0.5) shown in FIG. 3(a). As shown in the figure, it can be seen from the hardness of the untreated state (broken line in the figure) that the hardness is improved by the nitriding treatment. Further, it can be seen that the hardness does not monotonically improve with an increase in bias current, but increases stepwise as indicated by straight lines L1 and L2. The hardness of the first stage (straight line L1) ranges from 1,100 to 1,200. The hardness of the second stage (straight line L2) is 1258 or more.

FIG. 4 is an explanatory diagram showing an example of changes in surface roughness accompanying changes in bias current. FIG. 4(a) shows the surface of the material to be treated in an untreated state. After mirror-finishing the surface of the material to be treated in this manner, characters were written on the material, and it was confirmed how this would be affected by the nitriding treatment. 4(b) to 4(h) are the results at bias currents of 0.05 A, 0.1 A, 0.15 A, 0.2 A, 0.25 A, 0.3 A and 0.35 A, respectively. As can be seen, as the bias current increases, the lettering on the surface becomes illegible. This means that the surface, which was mirror-finished before the treatment, is roughened by the nitriding treatment. FIG. 3(a) shows the results of evaluating the appearance after mirror finishing according to the degree to which the characters are legible. When the bias current is 0.15 A or less (FIGS. 4(b) to 4(d)), the characters can be read well both at the center and at the edges (indicated by ◯ in FIG. 3(a)). ). When the bias current is 0.2 A and 0.25 A (FIGS. 4(e) and 4(f)), the edges are difficult to read, but the characters in the center can be read well.
When the bias current is 0.3 A (FIG. 4(g)), the characters in the center are somewhat readable, but the edges are almost unreadable. When the bias current is 0.35 A (FIG. 4(h)), both the central portion and the edges are illegible. In other words, it can be said that the surface roughness is so low that it cannot be overlooked when the bias current is in the range of 03 A or more.

According to the above results, in order to improve the hardness of the surface by nitriding treatment and to suppress the decrease in surface roughness, the bias current is preferably set to a range not reaching 0.3 A, and is set to 0.2 A. It can be seen that it is more preferable to make it a range that does not reach. Further, as shown in FIG. 3(b), the Vickers hardness is improved in a stepwise manner, with the first stage in the range of the bias current <0.3 A and the second stage in the range of the bias current of 0.3 A or more. there is Therefore, in order to simultaneously suppress the decrease in surface hardness and surface roughness, the value of the bias current is set within a range in which the Vickers hardness can be improved in the first stage but does not reach the second stage. It can be said that it is preferable to In the case of a process gas composed of nitrogen and hydrogen, it is preferable to set the value of the bias current so that the Vickers hardness is less than 1250, for example.

FIG. 5 is an explanatory diagram showing an example of changes in surface roughness when the bias voltage is changed. FIGS. 5(a) to 5(c) are photographs of the surface after treatment with bias voltages of 280 V, 290 V and 300 V, respectively. In both FIGS. 5(a) and 5(b), characters can be clearly read at the center and edges. In FIG. 5(c), although the edges are difficult to read, the characters in the center can be read well. Vickers hardness HV (0.5) is in a range smaller than 1250.

FIG. 6 is an explanatory diagram showing the definition of surface roughness. The surface roughness Ra in FIG. 5 was calculated by the method shown in FIG. Specifically, as shown in FIG. 6, the surface roughness Ra is obtained by measuring the height Z of each point with a stylus roughness meter and calculating the average value.
As shown in FIGS. 5(a) to 5(c), the surface roughness Ra is 0.007 micrometers, 0.024 micrometers and 0.033 micrometers. Therefore, if the surface roughness is within this range, it can be said that both surface hardness and surface roughness can be achieved.
As shown in FIGS. 5 and 6, both surface hardness and surface roughness can be achieved by controlling the bias voltage.

FIG. 7 is an explanatory diagram showing an example of changes in surface roughness accompanying changes in bias current when argon gas is supplied. The results are shown when nitrogen, hydrogen, and argon are mixed at a ratio of 1:1:1 and supplied as the process gas. In the nitriding process control process (FIG. 2), an argon-free process gas is supplied (step S15), but in the case of FIG. 7, argon was continuously supplied at the above rate.
In FIG. 7B, Δ indicates the results when nitrogen, hydrogen, and argon were used as the process gas. The results obtained when nitrogen and hydrogen were used as the process gas (the same as in FIG. 3) are also indicated by ●. As shown, the treatment increases the hardness of the surface. However, even when the bias current reaches 0.4 A, the improvement in surface hardness has not reached the second stage. The Vickers hardness HV (0.5) of the surface is 967, 1140, 1184, 1223, ie less than 1250 at bias currents of 0.05 A, 0.2 A, 0.35 A and 0.4 A, respectively. there is
FIG. 7A also shows the evaluation results of the surface roughness. In the range where the bias current was 0.2 or less, the characters were clearly legible both at the center and at the edges. When the bias current was 0.35 A and 0.4 A, the edges were difficult to read, but the center was clearly readable. In both cases, it is evaluated that the surface roughness has not decreased to the extent that it cannot be overlooked.

As shown in the above examples, it is possible to achieve both surface hardness and surface roughness by controlling the bias current conditions during the nitriding treatment. The reasons why the bias current affects both the surface hardness and the surface roughness have not been clarified. It is thought that the sedimentary layer has an influence.
The reasons why the surface is hardened by nitriding are that nitrogen atoms enter the crystals of the treated material and harden the crystal structure, and that a compound layer of nitrogen and iron is formed on the surface of the treated material. can be considered. Although these phenomena are thought to occur in parallel, first, the phenomenon of nitrogen atoms penetrating into the crystals of the material to be treated and hardening the crystal structure mainly occurs, resulting in the first stage of improvement in surface hardness. it is conceivable that. After that, a phenomenon occurs in which a compound layer of nitrogen and iron is formed on the surface of the material to be treated. Then, it is considered that the surface becomes rough due to the formation of a compound layer on the surface. In order to confirm this phenomenon, the results of observing the surface of the material to be treated are shown below.

FIG. 8 is a micrograph of the surface after nitriding. FIG. 8(a) shows the state before the nitriding treatment. Only polishing scratches during mirror finishing are confirmed.
FIG. 8(b) shows the results when the bias current is 0.3 A and the surface roughness is 0.021 micrometers. Microscopic crystals can be confirmed when compared with the untreated state. Measurements confirmed that crystals of about 100 nanometers were attached to the surface. The appearance of the surface is also shown in the upper right of FIG. 8(b). As shown in the figure, the characters on the surface are legible, indicating that the mirror surface is maintained.
FIG. 8(c) shows the results when the bias current is 1.2 A and the surface roughness is 0.091 micrometers. Clearer crystals can be confirmed. According to measurements, it was confirmed that crystals of about 200 nanometers were attached to the surface. The appearance of the surface is also shown in the upper right of FIG. 8(c). As shown in the figure, the characters on the surface are almost unrecognizable, and the surface roughness is so low that it cannot be overlooked.
As a result of FIG. 8, it can be seen that as the bias current increases, the crystal on the surface thickens and the surface roughness decreases. Therefore, by setting the bias current to such an extent that such crystals cannot be formed, nitrogen atoms penetrate into the crystals of the material to be treated and the crystal structure is hardened while suppressing the surface crystal formation and suppressing the deterioration of the surface roughness. You can get the effect.

According to the nitriding method of the embodiment described above, it is possible to achieve both improvement in hardness of the surface of the material to be treated and suppression of decrease in surface roughness.
The various features described in the above embodiments do not necessarily have all, and the present invention can be practiced by omitting or combining some of them as appropriate. Further, the present invention can realize various modifications.
In the embodiment, ASP nitridation is exemplified, but nitridation using parallel plate electrodes may be performed.

INDUSTRIAL APPLICABILITY The present invention can be used for a nitriding treatment that achieves both improvement in surface hardness and suppression of a decrease in surface roughness in an ASP nitriding process.

10: Plasma nitriding device 11: Container 12: Positive electrode 13: Screen 14: Supply port 15: Discharge port 16: Mounting table 17: Arc sensor 18: Temperature sensor 19: Pressure sensor 20: Power supply device 21: Power control device 22: Screen Power supply circuit 23 : Bias power supply circuit 31 : Nitrogen tank 32 : Hydrogen tank 33 : Argon tank 34 : Valve 35 : Valve 36 : Valve 37 : Flow rate sensor 38 : Flow rate sensor 39 : Flow rate sensor 40 : Controller 41 : Process main controller 42: Process gas control unit 43: Process condition input unit

Claims (3)

A plasma nitriding method for forming a nitrided layer on the surface of a steel material to be treated using plasma,
(a) placing the material to be treated in a closed treatment vessel;
(b) While supplying a process gas composed of nitrogen or ammonia and hydrogen or a process gas composed of nitrogen or ammonia, hydrogen, and argon into the processing container, current is applied between the positive electrode and the negative electrode to generate plasma of the process gas. and performing a nitriding process on the material to be treated by the plasma,
In step (b) of the preceding paragraph, a bias current is applied between the positive electrode and the material to be treated in addition to the energization for generating the plasma , so that the surface of the material to be treated is loaded with a load of 0.5 kgf. Vickers hardness of 1100 to 1200 and surface roughness of 0.007 to 0.033 micrometers, which is represented by the height of each point on the surface of the material to be treated, measured with a stylus type roughness meter. is a process to realize
The energization condition of the step (b) is realized by controlling the bias current,
if the process gas consists of nitrogen or ammonia and hydrogen, the bias current <0.3 A;
A method of plasma nitridation, wherein said bias current <0.4A when said process gas consists of nitrogen or ammonia, hydrogen and argon. The plasma nitridation method of claim 1 , comprising:
The plasma nitridation method, wherein the bias current is a pulse current. The plasma nitridation method according to claim 1 or 2 ,
The step (b) is
A plasma nitriding method in which the plasma is generated by energization between a positive electrode installed in the processing container and a conductive screen installed in the positive electrode.

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