WO2013141422A1

WO2013141422A1 - Method for forming hard layer on titanium, and talloy having hard layer formed by same

Info

Publication number: WO2013141422A1
Application number: PCT/KR2012/002146
Authority: WO
Inventors: 이동근; 이용태
Original assignee: 한국기계연구원
Priority date: 2012-03-23
Filing date: 2012-03-23
Publication date: 2013-09-26
Also published as: JP2015519470A; US20130248051A1

Abstract

The present invention relates to a method for forming a hard layer, having excellent and functionally gradient properties, on the surface of titanium at low cost. According to the present invention, the method for forming the hard layer comprises the steps of: (a) inserting the titanium in a heat treatment device, ventilating the same, and maintaining the air pressure to be 10-4 torr or less; (b) preprocessing the titanium by heating the same at 730 ~ 800℃ for 10 min.~ 5 hours and removing the oxide film formed on the surface of the titanium; (c) injecting one or more gases selected from nitrogen, oxygen, and carbon into the heat treatment device and heating the titanium at 740 ~ 950℃ for 30 min.~20 hours, thereby forming the hard layer, having a density nine times greater than the gas, on the surface of the talloy; and (d) cooling the titanium.

Description

Titanium alloy having a hard layer formed by the method and the hard layer formed by the titanium

The present invention relates to a method for forming a hard layer on the surface layer of titanium (pure titanium and titanium alloy), and more particularly, to a method for forming a hard layer having properties of continuously changing properties at low cost on the surface layer of titanium. It relates to titanium in which a hard layer is formed by the method.

Pure titanium and titanium alloys are relatively lighter than other structural materials and are widely used in the military and aerospace industries where weight reduction is essential because of their high specific strength in the temperature range from cryogenic temperatures to 400 to 500 ° C. In addition, since titanium has excellent corrosion resistance and biocompatibility, most biomaterials to be inserted into the human body are used. In addition, since titanium is capable of coloring through anodization, its use has recently been expanded in civilian products requiring various color expressions.

Despite such excellent properties of titanium, its application is limited because of its poor hardness and abrasion resistance, and recently, research on titanium surface modification treatments for improving it has attracted attention.

In this regard, as a method for improving the surface properties (particularly hardness and wear resistance) of the titanium alloy, a method of forming a hard film such as TiN on the surface of the titanium alloy has been proposed.

The TiN film, which has been actively studied among the coating materials for improving abrasion resistance, has excellent oxidation resistance, excellent surface roughness and ductility, and is beautiful in color due to its golden color, which is widely used not only for wear but also for corrosion resistance or decoration. to be.

However, since TiN film has very large volume expansion (volume expansion rate of about 64%) when TiO ₂ is formed through oxidation process, it forms a large compressive stress in the formed oxide layer and causes cracks in the film. Oxidation proceeds rapidly.

As a method of overcoming the disadvantages of the TiN film, an oxynitride material containing oxygen such as TiN _x O _y may be used as the surface curing film. This is because the oxynitride coating material based on the ternary composition also has excellent hardness, electrical properties, abrasion resistance, and corrosion resistance similar to the TiN coating due to the chemical bonding of atoms in the lattice and the electrical structure of the oxynitride.

On the other hand, as a method of forming a titanium oxynitride film having excellent properties such as TiN _x O _y on the surface of the titanium alloy, nitriding, carburizing, thermal spraying, physical vapor deposition (PVD) and chemical vapor deposition (CVD; Chemical Vapor Deposition) and the like.

Among them, PVD methods such as ion plating, cathode arc deposition, and reactive sputtering, and CVD methods using plasma are mainly considered.

The PVD process can be deposited at a lower temperature than the CVD process, minimizes the change of the structure at the interface between the titanium alloy surface and the coating layer, and can form a coating layer having excellent abrasion resistance, heat resistance, oxidation resistance, and corrosion resistance. However, there are disadvantages in that the adhesion between the coating layer and the titanium alloy base is weak, the coating equipment is expensive, and the formation of the coating layer takes a long time.

In addition, the CVD process has the advantage of easy control of the composition and coating thickness of the coating layer, but mainly due to the deposition process at a high temperature can cause a change in the structure of the coating layer and the titanium alloy in the mechanical properties and corrosion of the titanium alloy There are disadvantages that can have a bad effect.

On the other hand, if all of the above-described overlayer (over-layer) coating method to coat the cured coating on the titanium alloy surface as it is, a double layer separated into a relatively low hardness titanium alloy base layer and a high hardness coating layer (base metal properties, Coating layer has a ceramic characteristic) is formed, there is a problem that the film separation and cracking occurs under conditions such as external impact and multi-axial load.

In addition, in the case of spray coating, the coating layer contains a lot of pores, so it is difficult to expect sufficient physical properties such as oxidation resistance, and the coating layer formed by PVD and CVD has less pores than the spray coating, but basically It is difficult to form a dense coating layer because it has considerable defects. And since the over-layer coating is added to the processed part of the coating layer, when the thick coating layer is formed, the dimensional change before and after the coating is significant, the necessity of post-processing increases, which leads to an increase in the manufacturing cost of the component.

In addition, in the case of an inner-layer method such as carburization or sedimentation, it takes a long time to obtain predetermined physical properties, resulting in a change in the titanium alloy matrix structure, resulting in deterioration of the physical properties of the matrix structure or an increase in treatment cost. There was a problem.

The present invention has been researched and developed in order to solve the above-described problems of the prior art, through a thermo-chemical treatment (TCT) method of diffusion penetration of invasive elements such as oxygen, carbon and nitrogen from the surface of titanium. In order to solve the problem of the over-layer coating layer by forming inner-layer coating layer and at the same time, when forming inner-layer coating layer, do not perform separate process such as removing oxide film and suppressing oxide film formation on titanium surface. By performing the treatment for a short time, it is a problem to be solved to provide a hard layer formation method of titanium which can obtain a hard layer of excellent physical properties at low cost.

In addition, another object of the present invention is to provide a titanium in which a hard layer in which an invasive element such as oxygen, carbon or nitrogen obtained by the above method has a concentration gradient from the surface thereof is formed.

As a means for solving the above problems, the present invention provides a method for forming a hard layer on the surface layer of titanium, comprising the steps of: (a) injecting and evacuating titanium to a vacuum heat treatment apparatus to maintain an air pressure of 10 ^-4 torr or less; (b) a pretreatment step of heating the titanium at 730 to 800 ° C. for 10 minutes to 5 hours to remove an oxide film formed on the surface of the titanium; (c) injecting at least one gas selected from nitrogen, oxygen, and carbon into the vacuum heat treatment apparatus and heating the titanium at 740 to 950 ° C. for 30 minutes to 20 hours, thereby gradient of concentration of the gas on the surface of the titanium alloy. Curing step to form a hard layer having a; And (d) cooling the titanium.

In the method according to the invention, the air pressure of step (a) may be 5 × 10 ^-5 torr or less.

In addition, in the method according to the invention, step (b) may be performed at 740 ~ 780 ℃.

In addition, in the method according to the present invention, step (b) may be performed for 10 minutes to 1 hour.

In addition, in the method according to the invention, the temperature of the step (c) may be higher than the temperature of the step (b).

In addition, in the method according to the invention, the step (c) may be performed at 740 ~ 850 ℃.

In addition, in the method according to the present invention, step (c) may be performed for 30 minutes to 5 hours.

In the method according to the present invention, step (d) may be cooled by a step cooling method.

In addition, in the method according to the present invention, step (d) may include an aging step of maintaining for 30 minutes to 30 hours at 500 ~ 800 ℃.

In addition, in the method according to the present invention, after the step (d), by using an over-layer coating method such as CVD method or PVD method, additionally forming a coating layer for the purpose of rubbing, color expression, etc. on the surface of titanium. It may include the step.

In addition, in the method according to the present invention, the titanium may be pure titanium or a titanium alloy.

The present invention also provides titanium and its alloys with hard layers formed by the method described above and various components utilizing this technology.

According to the present invention, the following effects can be obtained.

First, according to the method according to the present invention, it is economical as compared to the conventional step of infiltrating gas can be obtained in a short time is economical, it is also advantageous to prevent the change of the structure of the base material due to gas penetration.

Second, through the step cooling method of an embodiment of the present invention, it is possible to obtain a control effect of a thick hard layer and the matrix structure on the titanium alloy surface to obtain a hard layer having very excellent physical properties.

Third, in the method according to the present invention, the oxide film removal of the titanium alloy surface, the formation of the inclined coating layer, etc. can be performed in one unit, so that the process is simplified and an excellent inclined hard layer can be obtained at low cost.

Fourth, the hard layer formed according to the present invention is because the concentration gradient is formed from the surface of the intrusion-type element from the surface of the base material is a continuous change of physical properties between the hard layer and the base material, the peeling phenomenon at the interface does not occur.

Fifth, since the hard layer formed according to the present invention is a penetration type element is introduced into the base material, there is almost no dimensional change, so that subsequent processing is not necessary after the hardening treatment and thus economical.

Sixth, since the hard layer formed according to the present invention is an intrusion type element is injected into the crystal lattice of the base material, the defects of the thermal spray coating, PVD or CVD coating layer does not occur.

1 is a schematic view of a cured layer forming process according to Example 1 of the present invention.

It is a schematic diagram of the hardened layer formation process which concerns on Example 2 of this invention.

Figure 3 is a photograph of the observation of the surface of the hard layer formed according to Examples 1, 2 and Comparative Examples of the present invention with an optical microscope.

Figure 4 is a photograph of the cross-section of the hard layer and the PVD coating layer formed according to Examples 1, 2 and Comparative Examples of the present invention with a scanning electron microscope.

Figure 5 shows the results of measuring the surface roughness of the hard layer and the PVD coating layer formed according to Examples 1, 2 and Comparative Examples of the present invention.

Figure 6 shows the results of measuring the friction characteristics of the hard layer formed according to Examples 1, 2 and Comparative Examples of the present invention.

Figure 7 shows the results of measuring the cross-sectional hardness of the hard layer formed according to Examples 1, 2 and Comparative Examples of the present invention.

Hereinafter, the present invention will be described in detail based on the preferred embodiments of the present invention, but the present invention is not limited to the following examples.

The inventors of the present invention found that when the surface of pure titanium and titanium alloys is coated with an over-layer coating method such as thermal spray coating, PVD, or CVD, a bilayer having a significant difference in physical properties at the interface between the titanium alloy base and the coating layer is essential. It is not only difficult to prevent the hard film from being peeled off due to external force, and the dimensional change due to the overlayer coating is inevitable, resulting in inevitable subsequent processing.Inner layers such as carburizing and sedimentation are noted. Attention was paid to the (inner-layer) coating method.

However, in order to quickly infiltrate invasive elements such as oxygen, nitrogen or carbon used in the inner layer coating method into the titanium or titanium alloy base, it is necessary to remove the stable layer of titanium oxide formed on the surface of the titanium or titanium alloy. Conventionally, a surface treatment is performed directly without performing a separate process for removing an oxide film, or a portion of nitrogen gas is ionized through a glow discharge in a low pressure nitrogen atmosphere such as plasma nitriding to impinge on the titanium surface. In this process, a method of removing the oxide film on the titanium surface and surface hardening is used. However, these methods have a high process temperature, and in particular, a long time surface treatment is required to obtain a cured layer having desired physical properties. In other words, the conventional method not only takes a long time but also can not be processed in a single process, and the cost of forming a hardened layer is considerably high.

In order to overcome this problem, the present inventors studied various inner layer coating methods, and as a result, several nanometer-thick oxide films formed on the surface of titanium when maintained at a high vacuum of 10 ^-4 torr or more in a specific temperature range in a vacuum chamber capable of heat treatment It can be confirmed that when the temperature range and gas pressure are maintained to facilitate the penetration of invasive elements such as nitrogen, oxygen, or carbon, in a short time, a hard layer of excellent physical properties having an inclination function can be obtained even in a short time. The present invention has been reached. The hard layer formed according to the present invention can prevent the separation between the coating layer and the matrix surface and the occurrence of cracking due to the external influence that may occur in the existing over-layer coating, and in particular, using a multi-step thermal diffusion surface hardening process technology. The surface hardening treatment can be performed continuously without separately performing processes such as removing the oxide film and suppressing the formation of the oxide film, thereby enabling economical advantageous surface hardening while having optimal physical properties.

The method of forming a titanium hard layer according to the present invention comprises the steps of: (a) adding titanium to a vacuum heat treatment apparatus and evacuating to maintain an air pressure of 10 ⁻⁴ torr or less; (b) a pretreatment step of heating the titanium at 730 to 800 ° C. for 10 minutes to 5 hours to remove an oxide film formed on the surface of the titanium; (c) after removing the oxide film, injecting at least one gas selected from nitrogen, oxygen, and carbon into the heat treatment apparatus and heating the titanium at 740 to 950 ° C. for 30 minutes to 20 hours, thereby hardening the surface of the titanium alloy. Allowing a layer to be formed; And (d) cooling the titanium.

In the present invention, 'titanium' is used in the sense containing pure titanium and titanium alloy.

The degree of vacuum of the heat treatment apparatus should be lower than 1 × 10 ^-4 torr. If the pressure is 1 × 10 ^-4 torr or lower, the oxide film formed on the surface of titanium in step (b) cannot be removed cleanly. It is because the effect of the hardening process to do is not enough, It is more preferable to maintain so that the atmospheric pressure of a heat processing apparatus may be 5 * 10 <^-5> torr or less.

The oxide film removing step should be carried out at 730 ~ 800 ℃, if less than 730 ℃ the removal of the oxide film is not made sufficiently, the subsequent penetration process of intrusion type gas is long, if it exceeds 800 ℃ microhistological And a disadvantage in mechanical properties. More preferably, it is carried out at a temperature range of 740 ℃ to 780 ℃. In addition, the removal time in the oxide film removing step is more preferably 10 minutes to 1 hour, if less than 10 minutes is not sufficient oxide film removal, more than 1 hour is disadvantageous in terms of economic cost and mechanical properties Because.

The curing step may be carried out continuously with the removal of the oxide film, or may be performed by a two-stage heat treatment method performed by raising the temperature above the oxide film removal temperature, which is more preferable because the two-stage heat treatment method may shorten the curing time. . That is, the surface hardening treatment time can be considerably shortened by introducing a pretreatment process before performing the hardening heat treatment with the intruded gas element.

The temperature of the curing step should be carried out at 740 ℃ or more and can be carried out up to 850 ℃ depending on the depth and the required hardness of the cured layer. More preferably, it is performed at 780 ° C. or higher, which is higher than the oxide film removing step. In addition, the curing treatment time is preferably 30 minutes to 20 hours, but less than 30 minutes is not enough to penetrate invasive elements such as oxygen, nitrogen, carbon, etc., and if it exceeds 20 hours, the grain size grows microscopically. This is because it is disadvantageous in terms of declining mechanical properties and economic cost.

In addition, the gas used for the hardening treatment is an element that can easily penetrate into the titanium crystal lattice, carbon, nitrogen, oxygen gas or a mixture thereof may be used.

The cooling step may be a method of furnace cooling or air cooling in the heat treatment furnace, during the cooling or air cooling, at 500 ~ 800 ℃ to evenly form the structure of the hard layer to be formed and to form a thick hard layer on the surface of the hard layer It is also possible to use a step cooling method including an aging step for 30 minutes to 30 hours.

Example 1

The surface hardening specimen used in Example 1 of the present invention used commercially pure titanium (Gr. 2 material) having a width, length, and height of 30 mm × 25 mm × 2 mm, and the chemical composition of Gr.2 material suggested by the manufacturer. Is shown in Table 1 below.

Table 1

Chemical composition	C	Fe	H	N	O	Ti
Content (% by weight)	0.05	0.3	0.008	0.02	0.2	Bal.

The prepared titanium specimen was immersed in an acetone solution, ultrasonically cleaned and dried, and then surface hardened in the same manner as shown in FIG. 1.

Specifically, the specimen was charged in a chamber of a gas controlled vacuum furnace (GCVF), and then decompressed to 5 × 10 ⁻⁶ torr using a vacuum pump. Subsequently, the temperature of the heat treatment furnace was raised to 750 ° C. and maintained for 30 minutes, so that the titanium oxide film having a thickness of about 10 Å naturally formed on the surface of the titanium specimen could be removed by pyrolysis.

After the oxide film was removed in this manner, 100 ccm of a mixed gas of oxygen and nitrogen was injected, and the partial pressure in the chamber was adjusted so that the pressure in the chamber was maintained at 5 × 10 ⁻¹ torr. Then, the temperature of the heat treatment furnace was raised to 800 ° C. and maintained for 3 hours to allow the injected oxygen and nitrogen elements to penetrate into the interior from the surface of the titanium specimen. After the hardening treatment was completed as described above, the titanium specimen was cooled by air cooling or furnace cooling.

In addition, in order to lower the roughness of the titanium surface and to realize a uniform color, a TiN coating layer was formed by PVD. The PVD coating layer was formed using a Ti target in a nitrogen gas atmosphere at 150 ° C. for 10 minutes, and the thickness of the PVD coating layer formed was 2.1 μm.

Example 2

Example 2 of the present invention, after the specimen pretreatment and curing treatment under the same conditions using the same specimen as in Example 1 of the present invention, as shown in Figure 2, in the cooling step, homogenization of the microstructure and In order to maximize the strength and increase the thickness of the surface hard layer, an aging treatment held at 700 ° C. for 1 hour is performed, and then the hard layer is removed by air cooling (ie, step cooling). Formed. Here, as in Example 1, the TiN coating layer was formed by PVD in order to lower the roughness of the titanium surface and to realize uniform color. The PVD coating layer was formed using a Ti target in a nitrogen gas atmosphere at 150 ° C. for 10 minutes as in Example 1, and the thickness of the formed PVD coating layer was 2.1 μm.

[Comparative Example]

Comparative Example is a TiN coating layer formed on the surface of the titanium specimen prepared in the same manner as Example 1 of the present invention by using a PVD method, the TiN coating layer is formed by using a Ti target in a nitrogen gas atmosphere at 150 ℃ for 10 minutes At this time, the thickness of the TiN coating layer formed was 3.4㎛.

The surface shape, cross-sectional shape, cross-sectional hardness, surface roughness, surface wear characteristics, and the like of the surface hard layer formed as described above were analyzed.

Specifically, the shape of the surface was observed through an optical microscope, and the shape of the cross section was observed by a scanning electron microscope. In addition, the surface roughness was measured by fixing the scan length to 9000㎛ using a surface profiler (Model TENCOR P-11). In addition, wear characteristics were measured using a ball on disk wear tester (J &L; Model JLTB-02 tribometer). At this time, as a counterpart, a stainless steel ball having a diameter of 1 mm was used. The friction coefficient of each specimen was measured under friction conditions of rotation radius 3mm, rotation speed 100rpm and load 1N, and the friction behavior was observed. In addition, in order to measure the cross section of a thin plate-shaped specimen effectively, the cross-sectional hardness measured the cross-sectional hardness of the hard layer after cutting and grinding the specimen to the inclined surface. At this time, the hardness was measured from the surface of the specimen toward the matrix center while maintaining the load at 100 g for 10 seconds using a micro-Vickers hardness tester (FUTURE-TECH; Model FM-700).

Surface and cross-sectional organization

3 is a photograph of the surface of the titanium specimen in which the hard layer was formed according to Examples 1, 2 and Comparative Examples of the present invention by an optical microscope.

3 is a result of observing the surface shape of each specimen 50 times and 200 times, respectively, using an optical microscope, the apparent structure of each specimen showed a surface structure having an α phase of equiaxed crystal shape.

In addition, the cross-sectional shape of the three surface-hardened specimens was observed using a scanning electron microscope. In order to observe the formed thin film layer, the boundary between the thin film layer and the base material, and the diffusion layer in a wider range, the cross section was polished to an inclined surface and the diffusion hard layer was observed in FIG. 4. As shown in Figure 4, all three specimens can confirm the coating layer interface.

Specifically, in the case of Figure 4a (comparative example), it can be seen that the surface hard layer of about 3.4㎛ by PVD deposition. In addition, only PVD (Physical Deposition) is performed on the surface of pure titanium without any pretreatment. Accordingly, the surface friction characteristics are also the lowest (Fig. 6a). This is due to the low adhesion between the base material and the coating layer, resulting in thin film separation in harsh environments such as wear tests.

In the case of FIG. 4B (Example 1), the entire surface hard layer is formed to have a thick surface hard layer of about 84 μm, including a TiN thin film layer of about 2.1 μm formed by PVD.

In addition, in FIG. 4C (Example 2), it can be seen that the thickest surface hard layer is formed to have a thickness of about 99 μm, including both the TiN thin film layer formed by PVD and the hard layer formed by the TCT process. . In addition, by performing the aging treatment, the diffusion of the invasive element is promoted, and the boundary of the diffusion layer is changed obliquely, thereby preventing the thin film separation phenomenon during the abrasion test. For this reason, as shown below, the specimen according to Example 2 of the present invention exhibits the best surface friction characteristics (FIG. 6C).

Surface Roughness and Surface Friction Characteristics

5 is an arithmetic mean roughness value by measuring the surface roughness to investigate the influence of the hard layer formed according to Examples 1, 2 and Comparative Examples of the present invention and the effect of the respective process conditions on the surface and the orientation of the hard layer It is the result shown by (Ra).

As confirmed in FIG. 5, the hard layer formed according to the comparative example had a Ra = 0.13 μm value, the hard layer formed according to Example 1 had a Ra = 0.13 μm value, and the hard layer formed according to Example 2 Ra has a value of 0.14 µm.

That is, when the PVD coating layer is formed on the surface of the hard layer formed according to Examples 1 and 2 of the present invention, it can be seen that the surface roughness is similar without significant change compared to the case where the surface hardening treatment is not performed. It can be seen that is almost similar to the comparative example in which PVD was formed directly on the titanium base material. That is, it can be seen that even if the titanium base material, or its hardening treatment or hardening treatment, the aging treatment does not significantly affect the surface roughness of the final PVD coating layer.

Surface friction characteristics were measured by using SUS ball as a counterpart under no lubricating conditions in the air, and the wear test was measured up to 20,000 reciprocating times considering initial wear.

Figure 6 shows the surface friction coefficient measured by the wear test, Figure 6a is a comparative example, Figure 6b is a wear test result of the specimen formed with a cured layer by Example 1, Figure 6c is Example 2.

As shown in FIG. 6A, in the comparative example, the average friction coefficient μ = 0.53 indicates the highest friction coefficient value. In contrast, the average friction coefficient μ = 0.42 for the specimen according to Example 1 of the present invention and the average friction coefficient μ = 0.44 for the specimen according to Example 2 were significantly lower than those of the comparative example.

This result is due to the strengthening of the base material by the hardening process by the thermal diffusion method according to the embodiment of the present invention, the high hardness of the formed hard layer itself, and the excellent adhesion between the base material and the thin film, compared to the coating layer formed only by PVD deposition. Is evaluated.

In particular, the friction coefficient values of the three specimens showed a significant difference in the initial change within about 3,000 cycles, and the specimen according to the comparative example reached the average friction coefficient value before 50 revolutions, unlike the specimens according to Examples 1 and 2. Shows a sharp increase.

In contrast, the specimen according to Example 1 reached the average friction coefficient value only after reaching 500 rotations, and exhibited excellent wear resistance compared to the comparative example. Particularly, the specimen according to Example 2 had up to about 2,500 rotations. It has the best wear resistance with low coefficient of friction.

This is because the PVD TiN layer formed by the proportional example causes the TiN layer to easily separate and fall off at the beginning of the abrasion test due to the distinct characteristic difference (i.e. hardness difference) formed between the base material and the TiN layer. It is.

In contrast, in the case of the specimens according to Examples 1 and 2 of the present invention, thin film separation does not occur since the TCT process is strengthened to a slope functional type in which no strong boundary line exists between the base material and the hard layer. Meanwhile, in the case of Example 2, since the toughness and strength of the base metal are increased through the recovery process due to the aging treatment, the average friction coefficient value seems to be reached most stably and slowly.

On the other hand, the average coefficient of friction of CP Ti (Gr. 2), the base material of the three specimens used to analyze the wear characteristics, was about 0.7, and all three specimens showed relatively lower coefficients of friction than the untreated specimens. It is judged that the coating layer is not completely removed even up to 20,000 times due to the small load of 1N, and it is possible to obtain a certain effect of increasing the surface hardness only by physical vapor deposition. It can be seen that the wear characteristics can be obtained.

Sectional hardness

The surface hardness of three specimens was measured to investigate the effect of nitride on the surface of pure titanium matrix and the effect of surface treatment conditions on the specimens. In the case of pure titanium without surface treatment, CP Ti (Gr. 2), the Vickers hardness was about 167 Hv, whereas for the three surface-hardened specimens, Comparative Example 373 Hv, Example 1 441 Hv, Example 2 was measured at 489 Hv, respectively. That is, it can be seen that the surface hardness increases considerably due to the formation of the hard film regardless of the method of surface hardening treatment.

However, when the TiN thin film was directly coated on the CP Ti base material, the hardness of the specimens according to Examples 1 and 2 in which the base material was first strengthened by the TCT process and the thin film was coated thereon was compared with that of the PVD TiN layer. It can be seen that it can be further improved, which shows that the heat treatment process according to Examples 1 and 2 is effective in obtaining a stronger surface strength.

Figure 7 shows the results of measuring the hardness change according to the cross-sectional depth from the surface of the specimens according to Comparative Example, Example 1 and Example 2 using Vickers hardness measurement.

The cross-sectional hardness is measured from the surface of the specimen toward the center of the matrix. The cross-sectional hardness of the outermost surface portion is about 340 Hv of the specimen according to the comparative example, while the specimen according to Example 1 is 450 Hv, and the specimen according to Example 2 is 500 Hv or more, three times higher than known. The value is shown.

In addition, the hard layer of the specimen according to the comparative example has a thickness within a few micrometers, and after the surface hard layer, a sudden sudden decrease in hardness up to about 160 Hv, which is the standard hardness value of CP Ti, is observed.

In contrast, in the case of the specimens according to Examples 1 and 2 of the present invention, the hardness decreases continuously as the depth increases from the surface. In Example 1, the hardness value is higher than the known hardness value even at a depth of about 80 μm. In the case of Example 2, a hardness value higher than the known hardness value was maintained up to about 100 μm. These results indicate that the penetration element consisting of nitrogen and oxygen was diffused to form a hard layer by a predetermined depth converged to the base metal, which is consistent with the observation of the cross-sectional structure of the scanning electron microscope.

In addition, the hardness distribution results of the inclined functional hard layer is initially due to the higher internal hardness and thicker inclined functional hard layer formed in the specimen according to Example 2 when compared with the friction characteristics (Fig. 6). In spite of the increase in the number of frictions, the coefficient of friction rises the slowest and reaches more than 2,500 times, which coincides with the convergence tendency to reach the average coefficient of friction other than the hard layer.

From the results of friction and hardness characteristics as described above, it can be seen that the hard layer forming method according to the present invention enables formation of a hard layer having excellent physical properties in a short time as compared with the conventional hard layer forming method.

Claims

By forming a hard layer on the surface layer of titanium,

(a) injecting titanium into the heat treatment apparatus and evacuating to maintain the atmospheric pressure below 10 -4 torr;

(b) a pretreatment step of heating the titanium at 730 to 800 ° C. for 10 minutes to 5 hours to remove an oxide film formed on the surface of the titanium;

(c) injecting at least one gas selected from nitrogen, oxygen, and carbon into the heat treatment apparatus and heating the titanium at 740 to 950 ° C. for 30 minutes to 20 hours to obtain a concentration gradient of the gas on the surface of the titanium alloy. Allowing a hard layer to be formed; And

(d) cooling the titanium.
The method of claim 1,

The pressure in step (a) is characterized in that less than 5 × 10 -5 torr.
The method of claim 1,

The step (b) is characterized in that it is carried out for 10 minutes to 1 hour.
The method of claim 1,

The process of step (c) may be higher than the temperature of step (b).
The method of claim 1,

Step (c) is characterized in that it is carried out at 740 ~ 850 ℃.
The method of claim 1,

Step (d) comprises the step of cooling by a step cooling method.
The method of claim 1,

The step (d) is characterized in that it comprises an aging step of maintaining for 30 minutes to 30 hours at 500 ~ 800 ℃.
The method of claim 1,

After the step (d), further characterized in that to form one or more coating layer by using an over-layer coating method such as CVD method or PVD method.
The method of claim 1,

The gas of step (c) is characterized in that the oxygen and nitrogen, or a mixture of carbon and nitrogen gas.
The method according to any one of claims 1 to 9,

The titanium is pure titanium or a titanium alloy.
Titanium provided with the hard layer formed by the method of any one of Claims 1-9.
The titanium component provided with the hard layer formed by the method of any one of Claims 1-9.