Patent Publication Number: US-2013248051-A1

Title: Method of forming rigid layer on titanium and titanium alloy having rigid layer formed by the same

Description:
TECHNICAL FIELD 
     The present invention relates to a method of forming a continuously gradient-hardened rigid layer on the surface layer of titanium (pure titanium and titanium alloy), and more particularly, to a method of forming a rigid layer having characteristics in which physical properties are continuously varied at low costs on the surface layer of titanium and titanium on which the rigid layer is formed by the method. 
     BACKGROUND ART 
     Pure titanium and titanium alloy have widely been applied to military and aerospace industrial fields in which weight lightening is essential due to their relative light weight compared to other structural materials and high specific strength in a temperature range from extremely low temperatures to high temperatures of 400 to 500° C. Further, titanium has mostly been used as biomaterial inserted into the human body due to its superior corrosion resistance and biocompatibility. Additionally, titanium has recently been expanding its applications even in civilian articles requiring revelation of various shades since anodizing enables coloring of titanium. 
     Studies on titanium surface modification for improving titanium properties has recently been receiving attention since titanium limits its application fields due to its poor surface hardness and wear resistance in spite of such excellent properties of titanium. 
     In this connection, a method of forming a rigid film such as TiN on the surface of a titanium has been suggested as a method for improving surface properties, particularly, hardness and wear resistance of the titanium. 
     A TiN film among wear resistance-improving film materials, which has actively been studied, is a film material that has widely been used for corrosion resistance or decorating as well as wear resistance since the TiN film is excellent in oxidation resistance, has superior surface roughness and ductility, and is beautiful due to its yellow color. 
     However, when TiO 2  is formed through the oxidation process, the TiN film is subjected to very large volume expansion with a volume expansion ratio of about 64% to result in the formation of a large compressive stress in the formed oxide layer. Therefore, the TiN film has a drawback that cracks in the film are caused to rapidly proceed oxidation in a high temperature atmosphere of 500° C. or higher. 
     As a method that is capable of overcoming such a drawback of the TiN, an oxynitride material including oxygen such as TiN x O y  may be used as a film for surface hardening since an oxynitride film material based on a ternary system also has excellent hardness, electrical properties, wear resistance and corrosion resistance as in the TiN film by chemical bonds of atoms within lattices and electrical structures of oxynitrides. 
     On the other hand, examples of a method of forming a titanium oxynitride film having excellent properties such as TiN x O y  on the surface of a titanium alloy may include nitriding, carburizing, thermal spray process, physical vapor deposition (PVD), and chemical vapor deposition (CVD). 
     Among the examples of the method, the PDV method such as ion plating, cathode arc deposition and reactive sputtering, or the CVD method using plasma is mainly considered. 
     Although the PVD process has merits that deposition is performed at lower temperatures than the CVD process, structural changes are minimized in the interface between the coating layer and the surface of a titanium alloy, and a coating layer with excellent wear resistance, heat resistance, oxidation resistance and corrosion resistance can be formed, there are demerits in that there is weak adhesive strength between the coating layer and a titanium alloy matrix, a coating apparatus is expensive, and it takes a long time to form the coating layer. 
     Further, although the CVD process has a merit of facilitating the composition of the coating layer and control of coating thickness, there is a demerit in the CVD process that the deposition process mainly occurs at high temperatures such that the structural changes are caused in the interface between the coating layer and the titanium alloy to result in a bad effect exerted on mechanical properties and corrosion of the titanium alloy accordingly. 
     On the other hand, there are problems that separation and cracking of the film occur under the conditions of an external impact and multi-axial loading since a double layer is formed which is separated into a titanium alloy matrix layer with a relatively low hardness and a coating layer with a relatively high hardness when coating a hard film on the surface of a titanium alloy by all of the above-mentioned over-layer coating methods, wherein the matrix has metal characteristics while the coating layer has ceramic characteristics. 
     Further, when performing thermal spray coating, it is hard to expect physical properties such as sufficient oxidation resistance since lots of pores are contained in the coating layer, and it is also difficult to form a compact coating layer since the coating layer basically possesses such defects as pores although less pores are formed in a coating layer formed by the PVD or CVD process. Furthermore, since over-layer coating includes adding the coating layer to processed parts, there are problems that there is a growing need to perform the post-process to result in an increase in the manufacturing costs of the parts accordingly due to severe dimensional changes of the coating layer undergone between before the coating process and after the coating process in case of forming a thick coating layer. 
     Further, there has been a problem that physical properties of the matrix structure deteriorate or treatment costs increase since changes in the titanium alloy matrix structure are caused by a long time consumed to obtain predetermined physical properties in case of an inner-layer coating method such as carburizing or nitriding. 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     The present invention is obtained through researches and development to solve the defects of the foregoing prior art, and provides a method of forming a rigid layer of titanium, which is capable of obtaining the rigid layer with excellent physical properties at low costs by performing a Thermo-Chemical Treatment (TCT) of diffusing and penetrating interstitial elements such as oxygen, carbon and nitrogen from the surface of titanium, thereby simultaneously forming an inner-layer coating layer to solve the defects of an over-layer coating layer and conducting a continuous treatment of one time for a short period of time without performing a separate process of removing the oxide film on the surface of titanium or suppressing the formation of an oxide film on the titanium surface when forming the inner-layer coating layer. 
     Furthermore, another aspect of the present invention is to provide titanium from the surface of which a rigid layer having a concentration gradient is formed using interstitial elements such as oxygen, carbon or nitrogen. The rigid layer is obtained by the above-mentioned method. 
     Technical Solution 
     In order to accomplish the foregoing objects, the present invention provides a method including the steps of (a) injecting titanium into a vacuum heat treatment apparatus and performing ventilation to maintain an atmospheric pressure of 10 −4  torr or less, (b) performing a pretreatment process 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 and its alloy, (c) performing a hardening process of injecting one or more gases 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 such that a continuously gradient-hardened rigid layer having a concentration gradient of the gases is formed on the titanium surface, and (d) cooling the titanium and its alloy. 
     In a method according to the present invention, the atmospheric pressure of the step (a) may be 5×10 −5  torr or less. 
     Further, in a method according to the present invention, the step (b) may be performed at 740 to 780° C. 
     Further, in a method according to the present invention, the step (b) may be performed for 10 minutes to 1 hour. 
     Further, in a method according to the present invention, the step (c) may have a temperature higher than that of the step (b). 
     Further, in a method according to the present invention, the step (c) may be performed at 740 to 850° C. 
     Further, in a method according to the present invention, the step (c) may be performed for 30 minutes to 5 hours. 
     Further, in a method according to the present invention, the step (d) may be performed by cooling titanium by a step cooling method. 
     Further, in a method according to the present invention, the step (d) may include an aging step of maintaining titanium at 500 to 800° C. for 30 minutes to 30 hours. 
     Further, a method according to the present invention may include, after the step (d), a step of additionally forming a coating layer for the purpose of fingerprinting resisting and color manifestation on the surface of titanium using over-layer coating methods such as a CVD method and a PVD method, etc. 
     Further, in a method according to the present invention, the titanium may be pure titanium or a titanium alloy. 
     Furthermore, the present invention provides titanium having a rigid layer formed by the above-mentioned method, and various parts utilizing its alloys and technologies thereof. 
     Advantageous Effects 
     The following effects may be obtained by the present invention. 
     First, a method according to the present invention is not only economical, but also advantageous in blocking structural changes of a matrix due to gas permeation since the method is capable of obtaining physical properties of the same level or higher within a short period of time compared to a convention process of permeating interstitial gases. 
     Second, a rigid layer having very excellent physical properties can be obtained by forming a thick rigid layer on the surface of a titanium alloy and obtaining the control effect of a matrix structure through the step cooling method that is an embodiment of the present invention. 
     Third, a method according to the present invention is capable of simplifying the process and obtaining an excellent inclined rigid layer at low cost by performing removal of an oxide film on the surface of the titanium alloy and formation of an inclined coating layer en bloc in an apparatus. 
     Fourth, a delamination phenomenon does not occur in the interface since a concentration gradient of interstitial elements is formed in the base metal direction from the surface of the rigid layer formed according to the present invention such that physical properties are continuously varied between the rigid layer and the base metal. 
     Fifth, the rigid layer formed according to the present invention is economical since the interstitial elements are injected into the base metal, and there are hardly any dimensional changes in the rigid layer accordingly such that a subsequent forming treatment is not required to be performed after performing a hardening treatment. 
     Sixth, defects of a thermal spray coating layer, a PVD coating layer, or a CVD coating layer do not occur since the interstitial elements are injected into a crystal lattice of the base metal in the rigid layer formed according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a mimetic diagram of a hardening layer forming process according to Example 1 of the present invention. 
         FIG. 2  a mimetic diagram of a hardening layer forming process according to Example 2 of the present invention. 
         FIG. 3  illustrates photographs of an optical microscope for observing surfaces of rigid layers formed according to Examples 1 and 2 of the present invention and Comparative Example. 
         FIG. 4  illustrates photographs of a scanning electron microscope for observing cross-sectional views of rigid layers and PVD coating layers formed according to Examples 1 and 2 of the present invention and Comparative Example. 
         FIG. 5  illustrates measurement results of the surface roughness of rigid layers and PVD coating layers formed according to Examples 1 and 2 of the present invention and Comparative Example. 
         FIG. 6  illustrates measurement results of the friction characteristics of rigid layers formed according to Examples 1 and 2 of the present invention and Comparative Example. 
         FIG. 7  illustrates measurement results of the cross-sectional hardness of rigid layers formed according to Examples 1 and 2 of the present invention and Comparative Example. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, although the present invention is described in detail based on preferred examples of the present invention, the present invention is not limited to the following examples. 
     The present inventors have paid attention to the inner-layer coating method such as carburizing or nitriding after taking notice of problems that it is fundamentally difficult to prevent a rigid film from being delaminated due to an external force, and dimensional change due to over-layer coating generates inevitable subsequent processing since a double layer in which differences in physical properties are fundamentally remarkable in the interface between a titanium alloy matrix and a coating layer when coating is performed on the surface of pure titanium and a titanium alloy by over-layer coating methods such as thermal spray coating, PVD, and CVD, etc. 
     It is necessary to remove a stable titanium oxide layer formed on the surface of titanium or titanium alloy in order to promptly permeate an interstitial element such as oxygen, nitrogen or carbon used in the inner-layer coating method. In related arts, a method has been used that removes the oxide film on the titanium surface and carries out surface hardening by performing direct surface treatment without a separate process to remove the oxide film or by performing a plasma nitriding method of ionizing some of nitrogen gas through glow discharge in a low pressure nitrogen atmosphere, thereby colliding the ionized nitrogen gas with the titanium surface. However, these methods have limitations that process treatment temperatures are high, and surface treatment of a particularly long period of time should be carried out to obtain a hardening layer with desired physical properties. In another words, methods of the related art not only require a long period of time, but also would be completed in a single process to result in a considerably high amount of cost consumed in the formation of the hardening layer. 
     In order to overcome such problems, the present inventors have studies on diverse inner-layer coating methods. As a result of the study, the present invention has been accomplished by maintaining a gas pressure in a heat-treatable vacuum chamber to a high degree of vacuum of 10 −4  torr or high within a specific temperature range such that a several angstrom to nanometer-thick oxide film formed on the surface of titanium may be removed for a short period of time, and subsequently maintaining the temperature range and the gas pressure such that the interstitial element such as nitrogen, oxygen, or carbon is easily permeated into titanium to confirm that a rigid layer of excellent physical properties with an inclination function is obtained even in a short period of time. A rigid layer formed according to the present invention is capable of preventing separation or cracking between the coating layer and the matrix surface due to external effects, that is occurred in existing over-layer coating, and is capable of continuously performing surface hardening without separately performing processes of removing the oxide film and suppressing the formation of the oxide film particularly using a multistep heat diffusion surface hardening process technology such that surface hardening can be performed that not only has optimal physical properties, but also is economically beneficial. 
     A method of forming a titanium rigid layer according to the present invention includes the steps of: (a) injecting titanium into a vacuum heat treatment apparatus and performing ventilation to maintain an atmospheric pressure of 10 −4  torr or less, (b) performing a pretreatment process 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 one or more gases selected from nitrogen, oxygen, and carbon into the vacuum heat treatment apparatus after the removal of the oxide film and heating the titanium at 740 to 950° C. for 30 minutes to 20 hours such that a rigid layer is formed on the titanium surface, and (d) cooling the titanium. 
     In the present invention, “titanium” is used as a meaning including pure titanium and a titanium alloy. 
     The heat treatment apparatus should maintain an atmospheric pressure of 1×10 4  torr or less as a degree of vacuum since the oxide film formed on the surface of the titanium cannot be removed clearly in the step (b) in case of the atmospheric pressure of 1×10 −4  torr or less to result in an insufficient effect of the subsequent hardening process. Therefore, it is more preferable to maintain the atmospheric pressure of the heat treatment apparatus to an atmospheric pressure of 5×10 −5  torr or less. 
     The step of removing the oxide film should be conducted at a temperature of 730 to 800° C. since the oxide film is not sufficiently removed to result in an increase of the subsequent permeation process of the interstitial gas when the temperature is less than 730° C., and there are disadvantages in terms of microstructural and mechanical properties when the temperature is more than 800° C. More preferably, the step of removing the oxide film should be conducted at a temperature range of 740 to 780° C. Further, it is more preferable that the step of removing the oxide film should be conducted when an oxide film-removing time is from 10 minutes to 1 hour since removal of the oxide film is not completely accomplished when the oxide film-removing time is less than 10 minutes, and there are disadvantages in terms of an economical aspect and mechanical properties when the oxide film-removing time is more than 1 hour. 
     The hardening step may be continuously conducted together with removal of the oxide film, or may be carried out in a second-step heat treatment method by increasing the temperature to higher than the oxide film-removing temperature, wherein the second-step heat treatment method is more preferable since hardening time is shortened. Namely, surface hardening time may considerably shortened by employing the pre-treatment process before conducting hardening heat treatment by the interstitial gas element. 
     The hardening step may be conducted at a temperature of 740° C. or higher, and may be conducted at a temperature up to 850° C. according to depth and required hardness of the hardening layer. More preferably, the hardening step is conducted at a temperature of not less than 780° C. which is higher than the oxide film-removing temperature. Further, the hardening time is preferably from 30 minutes to 20 hours since the permeation amount of the interstitial element such as oxygen, nitrogen or carbon is insufficient in case of the hardening time of less than 30 minutes, and there are disadvantages in terms of an economical cost aspect and a drop in mechanical properties according to microstructural growth of grain size in case of the hardening time of more than 20 hours. 
     Further, the gas used in the hardening step may include carbon, nitrogen, oxygen, and mixture gases thereof elements that may easily permeate into lattices of a titanium crystal. 
     The cooling step may be performed by a furnace cooling method or an air cooling method in a heat treatment furnace, and a step cooling method including an aging step of maintaining titanium at 500 to 800° C. for 30 minutes to 30 hours to uniformize structure of a rigid layer formed and form a thick rigid layer on a surface part of the rigid layer may be used during furnace cooling or fast cooling. 
     Examples 1 
     A commercial pure titanium (Gr. 2 material) having a size of 30 mm (length)×25 mm (width)×2 mm (height) as a surface hardening sample used in the Example 1 of the present invention was used, and a chemical composition of a Gr. 2 material suggested by the manufacturer is represented in the following Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Chemical components 
                 C 
                 Fe 
                 H 
                 N 
                 O 
                 Ti 
               
               
                   
               
             
            
               
                 Content (weight %) 
                 0.05 
                 0.3 
                 0.008 
                 0.02 
                 0.2 
                 Balance 
               
               
                   
               
            
           
         
       
     
     After immersing a prepared titanium sample into an acetone solution to subject the titanium sample to ultrasonic cleaning and drying the ultrasonic-cleaned titanium sample, the dried ultrasonic-cleaned titanium sample was subjected to surface hardening by a method such as a mimetic diagram as illustrated in  FIG. 1 . 
     Specifically, after charging the sample into the chamber of a Gas Controlled Vacuum Furnace (GCVF, the chamber was decompressed to a pressure of 5×10 −6  torr using a vacuum pump. Subsequently, after increasing temperature of a heat treatment furnace to 750° C., the heat treatment furnace with the increased temperature was maintained for 30 minutes such that a titanium oxide film with a thickness of about 10 Å naturally formed on the surface of the titanium sample was removed by thermal decomposition. 
     After removing the oxide film, 100 ccm of a mixture gas of oxygen and nitrogen was injected into the chamber, and partial pressure within the chamber was adjusted such that a pressure within the chamber was maintained to 5×10 −1  torr. After increasing temperature of the heat treatment furnace to 800° C., and the heat treatment furnace with the increased temperature was maintained for 3 hours such that the injected oxygen and nitrogen elements could permeate into the titanium sample from the surface of the titanium sample. After completing hardening, the titanium sample was cooled by an fast cooling method or a furnace cooling method. 
     Further, the titanium sample had a TiN coating layer formed thereon by the PVD method in order to lower surface roughness of the titanium sample and realize uniform colors of the titanium sample. A PVD coating layer was formed at 150° C. for 10 minutes in a nitrogen gas atmosphere by using a Ti target, and the formed PVD coating layer had a thickness of 2.1 μm. 
     Example 2 
     A rigid layer was formed in the Example 2 of the present invention by a method, i.e., a step cooling method including conducting pre-treatment and hardening of the same sample as that in the Example 1 of the present invention under the same conditions as those in the Example 1 of the present invention, performing an aging process of maintaining the sample in a heat treatment furnace at 700° C. for 1 hour in order to homogenize microstructure of the sample, maximize strength of the sample, and increasing thickness of the surface rigid layer of the sample in the cooling step, and then taking out the sample from the heat treatment furnace to cool the sample by air cooling as illustrated  FIG. 2 . The titanium sample had a TiN coating layer formed thereon by the PVD method in order to lower surface roughness of the titanium sample and realize uniform colors of the titanium sample in the same method as that in the Example 1. A PVD coating layer was formed at 150° C. for 10 minutes in a nitrogen gas atmosphere by using a Ti target in the same method as that in the Example 1, and the formed PVD coating layer had a thickness of 2.1 μm. 
     Comparative Example 
     In the Comparative Example, a titanium sample prepared in the same method as in the Example 1 had a TiN coating layer formed on the surface thereof by a PVD method. The TiN coating layer was formed at 150° C. for 10 minutes in a nitrogen gas atmosphere by using a Ti target, and the formed TiN coating layer had a thickness of 3.4 μm. 
     Surface shape, cross-sectional shape, cross-sectional hardness, surface roughness, surface wear characteristics, and the like of the surface rigid layer formed as described above were analyzed. 
     Specifically, surface shape was observed through an optical microscope, and the cross-sectional shape was observed through a scanning electron microscope. Further, the surface roughness was measured in the state that scan length was fixed to 9,000 μm by using a surface profiler (Model TENCOR P-11). Further, wear properties were measured by a ball-on-disk type abrasion tester (Model JLTB-02 tribometer manufactured by J&amp;L Corporation), wherein stainless steel balls with a diameter of 1 mm were used as counter material, and balls and samples were subjected to abrasion friction under conditions of a radius of rotation of 3 mm, a rotation velocity of 100 rpm, and a load of 1 N to measure friction coefficients of the respective samples and observe friction behaviors of the samples. Further, cross-sectional hardness of a rigid layer was measured after cutting the samples into the inclined plane and polishing the cut samples in order to effectively measure cross-sectional hardness of thin plate-like samples. Hardness values of the samples were measured from the surface of the samples to the central part of matrix while maintaining a load of 100 g for 10 seconds using a Micro-Vickers Hardness Tester (Model FM-700 manufactured by Future-Tech Corporation). 
     Surface and Cross-Sectional Structure 
       FIG. 3  illustrates photographs for observing the surface of a titanium sample on which a rigid layer is formed by an optical microscope according to the Examples 1 and 2 of the present invention and Comparative Example. 
       FIG. 3  illustrates results in which surface shapes of the samples were magnified to 50 times and 200 times respectively using an optical microscope, wherein apparent structures of the respective samples all showed surface structures having an equiaxed shaped a phase. 
     Further, cross-sectional shapes of the three surface hardened samples were observed using a scanning electron microscope. The observed diffusion rigid layers were illustrated in  FIG. 4  by observing diffusion rigid layers of the samples after polishing cross-sections of the samples into an inclined plane in order to observe a formed thin film layer, a boundary between the thin film layer and matrix, and a diffusion layer to a wider range. As illustrated in  FIG. 4 , coating layer boundary surfaces were all confirmed from the three samples. 
     Specifically, it could be known in case of  FIG. 4   a  (Comparative Example) that a surface rigid layer with a thickness of about 3.4 μm was formed by PVD. Further, it is illustrated that the boundary between matrix and thin film layer is the clearest by performing PVD only on the surface of pure titanium without pretreatment. Accordingly, surface friction properties are also dropped most as illustrated in  FIG. 6A . This comes from that a thin film separation phenomenon is represented in a test of a severe environment such as abrasion test according as adhesive strength between the matrix and coating layer is low. 
     In case of  FIG. 4B  (Example 1), there is a thick surface rigid layer having a total surface rigid layer of about 84 μm including a TiN thin film layer with a thickness of about 2.1 μm formed by PVD. 
     Furthermore, in case of  FIG. 4C  (Example 2), it can be seen that there is the thickest surface rigid layer with a thickness of about 99 μm including a TiN thin film layer with a thickness of about 2.1 μm formed by PVD and a rigid layer formed by the TCT process. Further, aging is additionally conducted to prevent a thin film separation phenomenon during abrasion test by promoting diffusion of interstitial elements, thereby inclinedly changing the boundary of the diffusion layer. Due to such a reason, the sample according to Example of the present invention illustrates the best surface friction properties as described below referring to  FIG. 6C . 
     Surface Roughness and Surface Friction Properties 
       FIG. 5  shows results represented as arithmetic mean roughness values (Ra) by measuring surface roughness values to investigate the forming state of a rigid layer formed according to Examples 1 and 2 of the present invention and Comparative Example, effect of the respective process conditions on the surface, and directional properties of the rigid layer. 
     As confirmed in  FIG. 5 , the rigid layer formed according to Comparative Example has an arithmetic mean roughness value (Ra) of 0.13 μm, the rigid layer formed according to Example 1 has an arithmetic mean roughness value (Ra) of 0.13 μm, and the rigid layer formed according to Example 2 has an arithmetic mean roughness value (Ra) of 0.14 μm. 
     That is, it can be seen that surface roughness values in case of forming a PVD coating layer on the surface of rigid layers formed according to Examples 1 and 2 of the present invention are similar to those in case of performing no surface hardening without large changes, and it can be seen that results of the surface roughness values are almost similar to those in Comparative Example in which PVD is directly formed on the titanium matrix. That is, it may be seen that no great effect on the surface roughness of a final PVD coating layer is obtainable although hardening or aging after hardening of the titanium matrix is conducted. 
     Surface friction properties were obtained by conducting a wear test using stainless steel balls as counter material under non-lubricant in the atmosphere, and abrasion test was measured up to a number of alternating motions of 20,000 considering initial abrasion. 
       FIG. 6  illustrates surface friction coefficients measured by abrasion test, wherein  FIG. 6A  is an abrasion test result of a sample on which hardening layer is formed by Comparative Example,  FIG. 6B  is an abrasion test result of a sample on which hardening layer is formed by Example 1, and  FIG. 6C  is an abrasion test result of a sample on which hardening layer is formed by Example 2. 
     As confirmed in  FIG. 6A , Comparative Example shows the highest friction coefficient value by representing an average friction coefficient μ of 0.53. Contrary to this, a sample according to Example 1 of the present invention represents an average friction coefficient μ of 0.42, and a sample according to Example 2 of the present invention represents an average friction coefficient μ of 0.44. Therefore, the average friction coefficients of the samples according to Examples 1 and 2 were remarkably lower than the average friction coefficient of the sample according to Comparative Example. 
     Such a result is evaluated to be attributed to matrix reinforcement by hardening, high hardness of a formed rigid layer itself, and superior adhesive strength between a matrix and a thin film due to a heat diffusion method according to Examples of the present invention compared to a coating layer formed only by PVD. 
     Particularly, friction coefficient values of the three samples show clear differences in the initial variations of turnover number of no more than about 3,000 cycles, wherein the sample according to Comparative Example represents a rapidly increasing pattern in which the friction coefficient value of the sample according to Comparative Example reaches the average friction coefficient value before the turnover number of 50 cycles differently from the samples according to Examples 1 and 2. 
     In comparison, the sample according to Example 1 represents superior wear resistance properties than the sample according to Comparative Example by reaching the average friction coefficient value at the turnover number of about 500 cycles, and particularly the sample according to Example 2 represented the most excellent wear resistance properties by maintaining the lowest friction coefficient value to the turnover number of about 2,500 cycles. 
     Due to a clear property difference (i.e., hardness difference) between a matrix and a PVD TiN layer formed by Comparative Example, the TiN layer is easily separated and come away from the matrix in the early stage of abrasion test to result in an initial rapid increase in the friction coefficient. 
     In comparison, thin films are not separated since the thin films have been reinforced with functionally graded material in which a clear boundary line does not exist between the matrix and the rigid layer through the TCT process in case of the samples according to Examples 1 and 2 of the present invention. On the other hand, the sample according to Example 2 seems to reach the average friction coefficient value most stably and slowly since toughness of the matrix is increased, and strength of the matrix is improved through the recovery process due to aging. 
     On the other hand, a matrix CP Ti (Gr. 2) for three samples used for analyzing wear properties had an average friction coefficient value of about 0.7, all of the three samples representing relatively low friction coefficient values compared to a non-treated specimen. It is judged that the coating layer is not completely removed up to the turnover number of 20,000 cycles due to a small load of 1 N, and a certain effect of increasing surface hardness through physical deposition only can be obtained. However, it can be seen that further improved surface wear resistance properties can be obtained by the TCT process of single or multiple treatment of aging. 
     Cross-Sectional Hardness 
     Surface hardness values of three samples were measured to study if a nitride was formed on the surface of a pure titanium matrix and effects of surface treatment conditions of respective samples on the formation of the nitride. Hardness values of the three samples according to Comparative Example and Examples 1 and 2 were measured as 373 Hv, 441 Hv, and 489 Hv respectively while Vicker&#39;s hardness of pure titanium CP Ti (Gr. 2) on which surface treatment had not been conducted were measured as about 167 Hv. Namely, it can be seen that surface hardness values of the samples increased considerably by the formation of rigid films regardless of a surface hardening method. 
     However, it can be seen that hardness values of the samples according to Examples 1 and 2 in which a matrix is reinforced by the TCT process to conduct thin film coating on the reinforced matrix are more improved than hardness value of a sample on which a simple PVD TiN layer is formed by directly coating a TiN thin film on a CP Ti matrix. This shows that the heat treatment processes according to Examples 1 and 2 are effective in obtaining stronger surface strength. 
       FIG. 7  illustrates measurement results of hardness changes along the cross-sectional depths from surfaces of the samples according to Comparative Example and Examples 1 and 2 using Vicker&#39;s hardness tester. 
     The cross-sectional hardness is a result of measuring hardness from the surface of a sample to the central part of a matrix. The outermost surface parts of the samples represented the cross-sectional hardness values that are at least three times higher than that of the matrix by showing that cross-sectional hardness values of the samples according to Comparative Example and Examples 1 and 2 were about 340 Hv, about 450 Hv, and about 500 Hv or more respectively. 
     Further, it is observed that a rigid layer of the sample according to Comparative Example has a thickness of within several micrometers and has its hardness dropped discontinuously and rapidly to about 160 Hv of the standard hardness value of CP Ti formed after the surface rigid layer. 
     Comparably, hardness values of the samples according to Examples 1 and 2 of the present invention were continuously decreased according to increases in depths from surfaces of the sample, wherein the sample according to Example 1 maintained a hardness value higher than that of the matrix up to a depth of about 80 μm, and the sample according to Example 2 maintained a hardness value higher than that of the matrix up to a depth of about 100 μm. Such results mean that interstitial elements such as nitrogen and oxygen are diffused into the matrix up to respective converged predetermined depths to form rigid layers, and these results correspond with observation results of cross-sectional structures of the samples by a scanning electron microscope. 
     Further, hardness distribution results of such functionally graded rigid layers correspond with trends that, compared with friction properties previously evaluated and analyzed as illustrated in  FIG. 6 , a friction coefficient value of the sample according to Example 2 is most slowly risen in spite of an initial increase in friction cycles due to a high internal hardness value and a more thickly formed functionally graded rigid layer such that it reaches an average friction coefficient for other parts except the rigid layer and is converged at last when the friction cycles reach 2,500 cycles or more. 
     It can be seen from above-mentioned results of the friction properties and hardness properties that a method of forming a rigid layer of the present invention enables a rigid layer having excellent physical properties to be formed within a short time compared to a conventional method of forming a rigid layer.