JP2008195994A - Surface modification method for titanium product, and surface modified titanium product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a surface treatment method where the improvement of the fatigue strength and wear resistance of pure titanium or a titanium alloy can be simultaneously realized. <P>SOLUTION: With a product made of any of pure titanium, an α type titanium alloy, a β type titanium alloy or an α+β type titanium alloy as a treatment object, the treatment object is subjected to plasma nitrification at a temperature of 973 to 1,271 K for a treatment time of 0.5 to 12 hr, so as to form a hardened layer in which nitrogen is diffused, next, one or more kinds of particulates are collided against the treatment object, so as to remove compound layers of TiN, Ti<SB>2</SB>N or the like produced on the hardened layer, and, if required, residual stress is applied thereto. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention treats a product made of pure titanium, α-type titanium alloy, β-type titanium alloy, or α + β-type titanium alloy (referred to as “titanium product” in the present invention), and the surface of this treatment target. In particular, the present invention relates to a surface modification method for improving wear resistance and fatigue strength, and a titanium product surface-modified by the method.

  Titanium products made of pure titanium or titanium alloy described above have characteristics of high specific strength, that is, light and strong, and excellent corrosion resistance. Use as a component is effective in reducing the weight and efficiency of various machinery and equipment.

  On the other hand, titanium has a fatal defect that it is inferior in wear resistance, which makes it difficult to use titanium parts for sliding parts.

  The problem inherent to titanium, which is inferior in wear resistance, is not limited to not only pure titanium but also α-type, β-type titanium alloys, and α + β-type titanium alloys (eg, Ti-6Al-4V). To overcome such problems, various surface modification methods have been proposed so far.

  As one of the methods for improving the wear resistance, it has been proposed to perform surface treatment by gas nitriding on a titanium product, thereby forming a hardened layer on the surface of the titanium product to improve the wear resistance. (See Non-Patent Document 1).

According to this method, a compound layer made of TiN and Ti ₂ N having high hardness is formed on the surface of the titanium product to be treated, and a hardened layer hardened by diffusion of nitrogen inside the compound layer. It has been reported that the wear resistance is significantly improved.

  On the other hand, when such a gas nitriding is performed on a titanium product, it is reported that the fatigue strength is greatly reduced although the improvement in wear resistance is seen as described above (see Non-Patent Document 2). .

As a cause of such a decrease in fatigue strength,
(1) The structure of the base material (inner part of the compound layer) becomes coarse due to the thermal history that is received during surface modification.
(2) Since the compound layer on the surface has a higher Young's modulus than the base metal part, the compound layer has a higher Young's modulus than the base material part as long as the continuity of strain is maintained without peeling between the compound layer and the base material part. A large stress is applied,
(3) Since the compound layer formed on the outermost surface is fragile, its fracture occurs at a relatively low cyclic stress, and then the crack progresses inside without stopping.

  New attempts have been made to improve wear resistance while suppressing such a decrease in fatigue strength. After plasma carburizing a titanium product, steel-based hard fine particles are collided (Patent Document 1). (See Patent Document 2), in which oxygen is diffused and penetrated into the surface of a titanium alloy member in a solid solution state, and an air stream containing particles is projected onto the surface of the titanium alloy member.

Prior art document information of the present invention includes the following.
RW Hanzel, "Surface hardening processes for titanium and its alloys", Metal Progress, March, pp.89-96 (1954) T. Morita, M. Shimizu, K. Kawasaki and T. Chiba, "Fatigue property of Nitrided Ti-6Al-4V Alloy", Transactions of the Japan Society of Mechanical Engineers A, Vol.56, pp.1915-1919 (1990 ). JP 2002-371348 A JP-A-2005-68470

  The above-mentioned method in which fine particles collide with titanium products subjected to plasma carburizing and plasma oxidation not only improve the wear resistance but also can suppress a significant decrease in fatigue strength as in the case of gas nitriding described above. It has become a thing.

  However, with titanium products that have been surface-treated by the above method, although the surface hardness is increased, the hardness increase is sufficient to obtain the high level of wear resistance required for sliding parts. It has not been obtained yet.

  Further, in the case of the above-described method, even if the fatigue strength is brought close to the fatigue strength of the untreated titanium product before plasma carburizing, that is, the decrease in fatigue strength can be “suppressed”, this untreated titanium product The fatigue strength cannot be significantly improved beyond the fatigue strength.

  In particular, if an attempt is made to increase the surface hardness of the hardened layer, the fatigue strength will be lower than the fatigue strength of the untreated base metal, and the reduction in fatigue strength can be kept low compared to gas nitriding. Not too much.

  Accordingly, the present invention has been made to eliminate the above-mentioned disadvantages of the prior art, and wear resistance of titanium products made of pure titanium or titanium alloys, particularly α + β type titanium alloys, more preferably Ti-6Al-4V titanium alloys. An object of the present invention is to provide a titanium product surface modification method capable of improving the fatigue strength more than the fatigue strength of an untreated titanium product while improving the properties, and a titanium product surface-modified by the method. To do.

In order to achieve the above object, the method for modifying the surface of a titanium product according to the present invention uses a titanium product made of pure titanium, α-type titanium alloy, β-type titanium alloy, or α + β-type titanium alloy as a processing target. Plasma nitriding treatment that forms a hardened layer by diffusing nitrogen ions on the surface of the object to be treated, and fine particle collision that causes one or more kinds of fine particles to collide with the object to be treated after the plasma nitriding treatment And a compound layer such as TiN or Ti ₂ N existing on the surface of the hardened layer is removed by the fine particle collision treatment (claim 1).

  The plasma nitriding is preferably performed at a temperature of 973 to 1271 K and a processing time of 0.5 to 12 hours. (Claim 2) The fine particles used in the fine particle collision have a maximum particle diameter of 20 μm to 200 μm. (Claim 3).

  Further, in the fine particle collision treatment, it is preferable to apply a residual stress to the treatment target in addition to the removal of the compound layer (Claim 4), and the surface of the treatment target may be made smooth ( Claim 5).

In the fine particle collision treatment, for the purpose of removing the compound layer, the compound layer can be removed so that the compound layer has a certain degree of hardness, and a crack is generated in the compound layer at the time of removal so as not to induce cracking of the hardened layer. Fine particles that have a low specific gravity and do not deteriorate the corrosion resistance are preferable, and include a treatment for colliding non-metallic fine particles such as silicon carbide (SiC), alumina (Al ₂ O ₃ ), and zirconia (ZrO ₂ ) (Claim 6). Can do. Preferably, those having a Mohs hardness of 9 or more and a specific gravity of 6 or less can be applied.

  For the purpose of imparting residual stress, it may include a treatment for colliding metal steel fine particles such as steel such as high-speed steel, titanium, tin, and zinc (claim 7).

  From the hardness and specific gravity of the titanium product, for example, fine particles having a Vickers hardness of 300 or more and a specific gravity of 4 or more are preferable.

In addition, in the fine particle collision treatment, a non-metallic system such as alumina or zirconia (ZrO ₂ ) is used for removing the components of the metallic fine particles remaining on the surface to be treated after the collision of the metallic fine particles and for smoothing the surface. A process of further colliding the fine particles can be included (claim 8).

  In addition, the collision of each of these fine particles is performed by injecting the solid-gas two-phase mixed fluid of the fine particles and the high-pressure gas at an injection pressure of 0.3 MPa or more or an injection speed of 80 m / sec or more. It is preferable to carry out such that the area is 100% or more (claim 9).

  Further, the surface-modified titanium product of the present invention is obtained by performing surface modification by the above-described method, and is formed by plasma nitriding, and the surface compound is formed by collision of one kind or two or more kinds of fine particles. The hardened layer from which the layer has been removed is formed on the surface of a titanium product made of pure titanium, α-type titanium alloy, β-type titanium alloy, or α + β-type titanium alloy (claim 10).

  The hardened layer can be formed by plasma nitriding at a temperature of 973 to 1271 K and a processing time of 0.5 to 12 hours (claim 11). In removing the compound layer, the maximum particle size of 20 μm to 200 μm is obtained. Fine particles can be used (claim 12).

  Further, it is preferable that residual stress is applied to the hardened layer by collision with the fine particles.

  With the configuration of the present invention described above, the following remarkable effects can be obtained according to the surface modification method of the present invention.

 (1) While improving the wear resistance of the titanium products to be treated, the fatigue strength could be improved more than the fatigue strength of untreated titanium products.

 (2) In particular, by applying residual stress by collision of fine particles, the surface hardness can be further improved and fatigue strength can be improved as compared with the case where only the surface compound layer is removed. Furthermore, by smoothing the surface by collision of fine particles, it was possible to smooth the surface without impairing the effect thus obtained.

(3) In the case of collision of fine particles such as silicon carbide (SiC), alumina, zirconia (ZrO ₂ ) in the fine particle collision treatment, TiN, Ti ₂ N formed on the surface to be treated It was possible to remove the compound layer such as.

 (4) In the case of collision of fine particles, steel such as high-speed steel, metal fine particles such as titanium, tin, zinc, etc. could be applied with the above-mentioned residual stress. In addition, when not only the application of residual stress but also the removal of the compound layer can be performed by the collision of the metal-based fine particles, the process of causing the non-metallic fine particles to collide to remove the compound layer can be omitted. is there.

(5) In addition, when collisions with non-metallic fine particles such as alumina and zirconia (ZrO ₂ ) are performed after the metal fine particles have collided, the wear resistance and fatigue strength imparted to the object to be treated It was possible to remove the metal-based fine particle component adhering to the surface to be treated at the time of the collision of the metal-based fine particles and to flatten the surface without impairing the effect of improving the quality.

  Next, embodiments of the present invention will be described below with reference to the accompanying drawings.

1. Overall Structure The surface modification method for a titanium product according to the present invention is a plasma nitriding treatment in which a hardened layer is formed near the surface of a base material portion of a titanium product to be treated by plasma nitriding as shown in FIG. )) And fine particles collide with the titanium product surface after the plasma nitriding process is completed, and the compound layer such as TiN or Ti ₂ N generated on the surface of the hardened layer during the nitriding is removed. Accordingly, it is constituted by a fine particle collision process (FIG. 1B) that applies compressive residual stress to the base material portion.

2. Process Target Titanium products to be processed by the method of the present invention include all products made of pure titanium or titanium alloy, and these titanium alloys include α type, β type, and α + β type.

  As an example, it is widely used mainly in the aerospace industry, and accounts for a high percentage of total titanium consumption (for example, in the US, 56% of total titanium consumption is consumed as Ti-6Al-4V alloy) .) Ti-6Al-4V alloy, which is a typical α + β type titanium alloy, is the object of treatment by the method of the present invention, and the application as a sliding member can be further expanded by applying the surface modification treatment of the present invention. is there.

3. Plasma nitriding treatment The titanium product to be treated is first subjected to plasma nitriding treatment, as described above, whereby a hardened layer is formed on the surface of the base material portion of the titanium product. The formation of this hardened layer increases the surface hardness of the titanium product and improves the wear resistance.

  Here, as a method of forming a hardened layer on the surface of the titanium product by nitriding, there is a method such as gas nitriding in addition to the plasma nitriding employed in the method of the present invention, but as already described in the section of the prior art. In addition, when a hardened surface layer is formed by gas nitriding, a change in the microscopic structure of the base material due to the thermal history experienced during the surface modification is one of the causes for reducing the fatigue strength.

  Therefore, by forming the surface hardened layer at a relatively low temperature by plasma nitriding, the surface hardened layer having a high hardness is formed while minimizing the microstructure growth of the base material part due to the heat treatment. It was. Specifically, ionized nitrogen (+) is attracted to the material to be treated (titanium) as a negative electrode and collides at high speed, resulting in the formation of a thick hardened layer under relatively low temperature and short time conditions.

  Thus, nitriding at a relatively low temperature is effective for minimizing the microscopic structure growth of the base material, while the hardened layer formed becomes more resistant to thickening. This is effective in improving the wear resistance, and the thickness of the surface hardened layer increases as the nitriding temperature increases.

  Therefore, in the present invention, the thickness of the hardened layer can be ensured in order to make the hardened layer as thick as possible while suppressing the microscopic structure growth of the base material portion, and the β transformation point or less. In this range, plasma nitridation is performed at a temperature of 973 K to 1271 K, preferably 973 to 1123 K, and more preferably 1023 K.

  In addition, in the case where the titanium product to be processed is made of Ti-6Al-4V as the base material part, it undergoes transformation when the processing temperature exceeds 1271K and becomes a single phase of β, so the upper limit of the nitriding temperature is 1271K. To do.

  The plasma nitriding treatment time depends on the material to be treated and other treatment conditions, but in order to ensure the necessary thickness of the hardened layer that is not destroyed by the fine particle collision, as an example, 1.8ks to 43.2ks (0 .5 to 12 hours), and when Ti-6Al-4V described above is a processing target, it is preferably 14.4ks (4 hours) to 32.4ks (9 hours).

4). Particulate collision treatment Titanium products that have been subjected to plasma nitriding as described above are then subjected to particulate collision treatment for colliding the particulates.

  In this fine particle collision treatment, the compound layer formed on the surface of the titanium product (the surface of the hardened layer) is removed during the plasma nitriding described above, and residual stress is applied to the hardened layer as necessary. A finishing process such as surface flattening is performed as necessary.

(1) Removal of compound layer As described in the description of the prior art, when a hardened surface layer is formed by gas nitriding, the cause of the decrease in fatigue strength is that the compound layer formed by nitriding, the base material part, It was mentioned that there was a shift in Young's modulus between the two, and that the fracture occurred at a relatively low cyclic stress because the compound layer was fragile.

In this step, the compound layer such as TiN or Ti ₂ N that is also generated by plasma nitriding is removed to eliminate the decrease in fatigue strength caused by the compound layer as described above.

Thus, as an example of the fine particles used to remove the compound layer, non-metallic fine particles such as silicon carbide (SiC), alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ) can be used. .

  As the non-metallic fine particles to be ejected here, particles having a maximum particle diameter of 20 μm to 200 μm are used, and the ceramic fine particles are caused to collide with the surface of the object to be treated by, for example, being projected by a known blast or shot peening apparatus. .

  As the blasting or shot peening device to be used, various known devices such as those that project particles by centrifugal force, those that project particles by collision with a rotating impeller, and those that inject particles with compressed gas are used. Although it is possible, it is preferable to use a blasting or shot peening device of a type that ejects particles together with compressed gas, which is relatively easy to handle the projection material which is fine particles and the adjustment of the projection conditions is relatively easy. .

  As an example, the injection condition of the nonmetallic fine particles may be an injection pressure of 0.3 MPa or more, or an injection speed of 80 m / sec or more, preferably 100 to 150 m / sec, and projected so that the treatment rate is 100% or more. preferable.

(2) Application of residual stress As described above, steel such as high-speed steel, or metal such as titanium, tin, or zinc is further applied to the surface of the titanium product from which the compound layer is removed and the hardened layer is exposed. Residual compressive stress accompanying plastic deformation of the hardened layer is applied by colliding the system fine particles.

  If the compound layer can also be removed by the collision of the metal fine particles without removing the compound layer using the non-metallic fine particles, the collision of the non-metallic fine particles is omitted directly. You may perform the process which makes a metal type fine particle collide.

  In order to surely remove the compound layer, it is preferable to perform the collision of the nonmetallic fine particles as described above prior to the collision of the metallic fine particles.

  As the metal-based fine particles used here, those having a maximum particle diameter of 20 μm to 200 μm are used. As the projection method, as in the case of the removal of the compound layer described above, various known devices can be used. As an example, the collision of the metal fine particles is converted into a compressed air flow by a direct pressure type blast or shot peening device. In the case where it is placed, injection can be performed under the same conditions as when the compound layer is removed.

(3) Finishing As described above, after colliding with metallic fine particles, if necessary, further colliding non-metallic fine particles and remaining metallic fine particle components such as high-speed steel remaining on the hardened layer of titanium products. Is removed and the hardened layer is flattened.

  Such non-metallic fine particles for finishing must be made of a material in which the particle component does not easily remain on the surface to be processed, and can exhibit grinding and polishing properties that can flatten the surface to be processed. Any fine particles such as alumina, zirconium, and glass can be used as an example.

  The particle diameter of the non-metallic fine particles for finishing is, for example, 20 μm to 200 μm, and various known devices can be used as the projection method as in the case of removing the compound layer described above. As an example, in the case of projecting with a direct pressure blast device, injection can be performed under the same conditions as in the case of removing the compound layer.

  Next, a test example in which a surface modification treatment is performed on a titanium product made of a Ti-6Al-4V alloy which is a typical α + β type titanium alloy by the method of the present invention will be described below.

  Table 1 shows the chemical composition of the Ti-6Al-4V alloy used in this test.

1. Test conditions and test method (1) Test piece The Ti-6Al-4V alloy rolled round bar having a chemical composition shown in Table 1 and having a diameter of 14 mm is formed into three types of test piece shapes shown in FIGS. Machined into test specimens for use in each test. In addition, the numerical value described in FIG. 2 is the size of each part, and a unit is mm.

  Among these, the button-type test piece (FIG. 2A) was finished in a mirror-like shape by emery polishing (# 100 to # 2000) and buffing (alumina powder, average particle size 0.03 μm) on one of the flat portions.

  Moreover, about the tensile test piece (FIG.2 (b)) and the fatigue test piece (FIG.2 (c)), the test part was finished in mirror shape by emery polishing and electrolytic polishing.

(2) Sample
(2-1) Plasma nitriding sample (one that does not collide with fine particles)
In order to accumulate basic data on plasma nitriding materials, the processing time is 14.4 ks (4h), the processing temperatures are 973K (700 ° C), 1023K (750 ° C), 1073K (800 ° C), and Four types of samples subjected to plasma nitriding treatment at 1123K (850 ° C.) were prepared.

  For comparison, an untreated sample and two types of plasma carburized materials that were subjected to plasma carburizing treatment at 1073 K (800 ° C.) and 1123 K (850 ° C.) were prepared as samples.

(2-2) Particulate collision sample Sample obtained by performing the particle collision treatment shown as abbreviated symbol FPB1 or FPB2 in Table 2 for a sample obtained with a plasma nitriding temperature of 1023K and a sample obtained with a plasma nitriding temperature of 1123K ( A total of four types of Examples 1 to 4) were produced.

  For comparison, either FPB1 or FPB2 particle collision was performed on an untreated sample that was not subjected to plasma nitridation or particle collision (Comparative Example 1), or an untreated sample that was not subjected to plasma nitridation. Samples (Comparative Examples 2 and 3), a sample (Comparative Example 4) subjected to only plasma nitriding at 1023K (750 ° C.), and a sample (Comparative Example 5) subjected to only nitriding at 1123K (850 ° C.) were prepared.

  In addition, as samples for plasma carburization, a sample subjected to plasma carburization at 1023K (no particle collision; Comparative Example 6), a sample subjected to particle collision of FPB1 or FPB2 (Comparative Examples 7 and 8), and 1123K A sample subjected to plasma carburization (no particle collision; Comparative Example 9) and a sample subjected to particle collision of FPB1 or FPB2 (Comparative Examples 10 and 11) were prepared.

  Table 2 summarizes the contents of each sample and the processing performed on each of these materials.

  The abbreviation FPB1 in Table 2 above indicates the collision treatment of SiC fine particles for the purpose of removing the compound layer, and FPB2 gives the residual stress by plastic deformation by colliding high-speed steel fine particles after the collision of SiC fine particles. A three-stage process of final finishing by colliding alumina fine particles is shown (see Table 3).

  The numerical values shown in parentheses in Table 3 are the maximum diameters of the fine particles used in this test example.

(3) Test method
(3-1) Measurement of surface hardness Surface hardness was measured on the surface of a button-type test piece (Fig. 2 (a)). At that time, the hardness was measured at 5 points per material using an ultra micro Vickers hardness meter (load 0.03N), and the average value was measured.

(3-2) Measurement of hardness distribution Hardness distribution is measured by cutting a button-type test piece that has been subjected to the necessary treatment, finishing the longitudinal section into a mirror surface by emery polishing and buffing, and then micro-Vickers hardness. Using a meter (load 0.25N), the hardness at each position was measured at five points, and the average value was taken as the measured value at that position.

(3-3) Observation of surface condition Observation of the surface condition was performed using an optical microscope on the longitudinal section of the button-type specimen used for the hardness distribution measurement. After that, it was corroded with Kroll's solution and the base material structure was observed, and the α particle size was obtained by the line segment method.

(3-4) Tensile test The tensile test was performed by measuring the strain while increasing the load stepwise using a hydraulically controlled universal testing machine. Three specimens per material (JIS Z 2201, No. 14A specimen) were used for the tensile test, and the average value of each value obtained from them was taken as the measured value.

(3-5) X-ray analysis X-ray diffraction (Cu Kα-ray) was performed on the surface of the button-type test piece in the range of 2θ = 33 to 43 ° with a scanning speed of 0.05 ° / s and a step of 0.01 °. .

(3-6) X-ray residual stress measurement In addition, X-ray residual stress measurement (Co Kα ray) is performed using a fatigue test piece at a diffraction angle of 2θ = 154.3 ° in the direction of the specimen axis, and the stress constant is K = -170.77. The residual stress value was determined as MPa / deg.

(3-7) Plane bending fatigue test The plane bending fatigue test was performed under the conditions of a stress ratio R = -1 and a repetition rate of 33 Hz. The specimen used for the fatigue test was in the shape of the No. 2 specimen specified in JIS Z 2274.

2. Test results (1) Confirmation of changes in various characteristics of sample with changes in nitriding temperature Untreated samples and samples subjected to plasma nitriding as 973K, 1023K, 1073K, 1123K by changing nitriding temperature in 50K increments Were measured for Young's modulus, yield stress, tensile strength, elongation, drawing, α particle size, thickness of the hardened layer, and surface hardness. The results are shown in Table 4, a graph showing the relationship between the distance from the surface and hardness is shown in FIG. 3, and the cross-sectional state of the base material is shown in FIG.

  From the above results, as the plasma nitriding temperature increases, the thickness of the hardened layer and the surface hardness increase, which contribute to the improvement of wear resistance, are observed, while the growth of α grains (structure) (Table 4) That is, it was confirmed that the base metal structure was coarsened, which caused a decrease in fatigue strength (FIG. 4).

  Therefore, plasma nitriding at a relatively high temperature is effective in improving wear resistance, but if it is too high, the fatigue strength will be reduced. Although it is effective in suppressing the coarsening and preventing the fatigue strength from being reduced, if it is too low, it will not be possible to obtain an increase in the thickness of the hardened layer and the increase in surface hardness that contributes to the improvement in wear resistance. It was confirmed that the selection of the nitriding temperature is important to improve the wear resistance without sacrificing the strength.

  As α grains (structures) grow with increasing nitriding temperature, the tensile strength tends to decrease slightly, and the stretch index (elongation / squeezing) decreases as the thickness of the hardened surface layer increases. It was done.

  In addition, it was confirmed from FIG. 3 that the hardness decreased smoothly toward the inside.

(2) Comparison of hardness distribution in plasma nitriding and plasma carburizing In order to confirm the superiority of forming a hardened layer by plasma nitriding, the samples subjected to plasma nitriding treatment at 973K and 1023K, the processing temperature The hardness distribution from the surface was compared for each of the samples that were subjected to plasma carburizing treatment at 1073K and 1123K and untreated samples (see FIG. 5).

  As is clear from FIG. 5, in plasma nitriding, a hardened layer with high hardness and thickness is obtained at a lower temperature than plasma carburizing, and plasma nitriding improves wear resistance compared to plasma carburizing. It turns out that it is superior in doing.

  In addition, when the same increase in hardness is obtained, the plasma nitriding can lower the processing temperature compared to plasma carburizing, so that the coarsening of the base metal structure that causes a decrease in fatigue strength can be suppressed, and the fatigue strength can be improved. However, it was confirmed that it is superior to plasma carburizing.

(3) Comparison of hardness distribution before and after collision with fine particles For the samples subjected to plasma nitriding (Examples 1 to 4 and Comparative Examples 4 and 5 in Table 2), the results of measuring the hardness distribution are shown in FIG. Shown in (b).

  Compared to the sample (FIG. 6 (a)) subjected to the plasma nitriding treatment at the temperature condition of 1023K, the sample (FIG. 6 (b)) subjected to the plasma nitriding at 1123K has a greater depth from the surface. It can be confirmed that the hardness has increased to a deep position and the thickness of the hardened layer contributing to the improvement of wear resistance is larger.

  In addition, there is a large difference in hardness distribution between the samples that have been subjected to plasma nitriding but have not collided with fine particles thereafter (Comparative Examples 4 and 5) and the samples that have undergone fine particle collisions (Examples 1 to 4). Therefore, it can be confirmed that the particle collision does not affect the hardness distribution caused by plasma nitriding, and the effect of improving wear resistance by the diffusion layer formed by plasma nitriding is maintained. However, it was confirmed that the compound layer which is one of the causes of the fatigue strength reduction can be removed and the residual stress can be applied by the collision of the high hardness fine particles.

(4) Comparison of mechanical properties Table 5 shows the results of measuring the mechanical properties of the samples of Examples 1 to 4 and Comparative Examples 1 to 11 shown in Table 2 above. Table 6, the results of measuring the ratio of hardness, surface hardness, compressive residual stress, fatigue strength and fatigue strength / tensile strength are shown in FIG. 7, and cross-sectional micrographs of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in FIG. .

  In Tables 5 and 6, the abbreviations for FPB1 and FPB2 are as shown in Table 3, respectively.

  From the results shown in Table 5, samples that were not plasma-nitrided (Comparative Examples 1 to 3), samples that were plasma-nitrided at 1023K (Comparative Example 4 and Examples 1 and 2), and samples that were plasma-nitrided at 1123K In any of (Comparative Example 5 and Examples 3 and 4), there is no significant change in the mechanical characteristics before and after the particle collision, and the collision of the particles does not significantly affect the mechanical characteristics. confirmed. The same applies to the samples that have been plasma carburized.

  Further, from the results of Table 6, it was confirmed that the residual stress was applied with the fine particle collision treatment with the high-speed steel fine particles, and the fatigue strength was improved with the application of the residual stress.

  Further examination of the fatigue strength reveals that the fatigue strength (630 MPa) of the sample (Comparative Example 4) in which the nitriding temperature is 1023K and the fine particles are not collided is the untreated sample (Comparative Example) in which neither the nitridation nor the fine particles are collided. It was confirmed that the fatigue strength (570 MPa) of 1) was slightly increased, and further improved by carrying out fine particle collision (increased to 740 MPa in Example 1 and 840 MPa in Example 2).

  In particular, it was confirmed that the fatigue strength reached an extremely high level (840 MPa) in the sample (Example 2) in which the removal of the compound layer by fine particle collision and the application of compressive residual stress were both performed. . The improvement in fatigue strength achieved here is at a level that cannot be achieved by any existing technology. In particular, the fact that the fatigue strength / tensile strength ratio of 91% is achieved is that the untreated material is 55 to 55%. This is a dramatic improvement from 60% (59% in Table 6).

  The fatigue strength (470 MPa) of the sample with nitriding temperature 1123 K (no collision of fine particles) decreased to a value lower than the fatigue strength (570 MPa) of the untreated material (Comparative Example 1). This is considered to be because the microscopic tissue grew and a relatively thick compound layer was formed.

  Therefore, even when the nitriding temperature is 1123 K, as shown in Example 3 and Example 4, collision of high-hardness fine particles is performed thereafter to remove the compound layer, thereby significantly increasing fatigue strength. The removal of the compound layer is extremely effective in improving the fatigue strength, and the fatigue strength of the untreated material (570 MPa) exceeds the fatigue strength (660 MPa in both Examples 3 and 4). Was confirmed.

  Note that in any of the samples nitrided at 1023K and 1123K, there is no change in the thickness of the hardened layer after the fine particle collision, and it can be said that the wear resistance is not reduced as the thickness of the hardened layer is reduced.

  Moreover, when fine particle collision is performed on the samples nitrided at 1023K and 1123K, the compound layer having high hardness and weakness is removed as shown in FIG. 7, and as a result, the surface hardness is somewhat increased. In particular, in the sample subjected to plasma nitriding at 1123K where the thickness of the compound layer is thick, the amount of decrease in the surface hardness is slightly large.

  However, the surface hardness of all samples is still at a high level even after the collision with fine particles, and the decrease in wear resistance caused by the decrease in surface hardness is such that there is no practical problem.

  On the other hand, the fatigue strength (590 MPa) of the sample (Comparative Example 6) that was subjected to plasma carburization at a temperature of 1023 K and not collided with fine particles was approximately the same as the fatigue strength (570 MPa) of the untreated material, Fatigue strength is hardly improved. Also, the degree of improvement in surface hardness remains low compared to the case where plasma nitriding is performed.

  Further, even in the sample (Comparative Example 7) in which SiC fine particles collided after the plasma carburization, the fatigue strength did not change at 590 MPa, and the fatigue strength was not improved.

  Further, in the sample (Comparative Example 8) subjected to the collision of the high-speed steel fine particles and the alumina fine particles (Comparative Example 8), although the fatigue strength is slightly improved (740 MPa), it is similar to the plasma nitride shown in Example 2. In addition, a dramatic improvement in fatigue strength could not be obtained.

  In addition, the fatigue strength (490 MPa) of the sample (Comparative Example 9) that was subjected to plasma carburization at a temperature of 1123 K and that did not collide with fine particles was lower than the fatigue strength (570 MPa) of the untreated material. Is the same as when plasma nitriding is performed at a temperature of 1123 K (see Comparative Example 5), but in the case of plasma nitriding, the fatigue strength is increased to a value higher than that of the untreated material by collision of fine particles thereafter. In contrast to the improvement, in the sample (Comparative Example 10) in which the SiC fine particles collided after plasma carburizing at a temperature of 1123 K, the fatigue strength remained unchanged at 490 MPa.

  Further, after that, in the sample (Comparative Example 11) in which high-speed steel fine particle collision and alumina fine particle collision were performed, although the fatigue strength was slightly improved (increased to 560 MPa), the fatigue of the untreated material (Comparative Example 1) It was not possible to recover the strength (570 MPa).

  Based on the above results, the combination of the formation of a hardened surface layer by plasma nitriding and the removal of the compound layer by collision of fine particles is extremely effective for obtaining improved wear resistance while maintaining and improving fatigue strength. Furthermore, it was confirmed that fatigue strength can be improved more effectively by applying residual stress due to fine particle collision.

(5) Results of X-ray analysis test An untreated sample (Comparative Example 1) that was not nitrided and not subjected to particle collision was subjected to different particle collisions (FPB1, FPB2) on the untreated sample. Fig. 8 (a) shows the result of X-ray analysis of the sample (Comparative Examples 2 and 3). The sample subjected to nitriding at 1023K (Comparative Example 4) and different fine particle collisions (FPB1 and FPB2) are shown. The X-ray analysis results of the samples (Examples 1 and 2) are shown in FIG. 8 (b), the sample subjected to nitriding treatment at 1123K (Comparative Example 5), and the different fine particle collisions (FPB1, FPB2). FIG. 8C shows the X-ray analysis results of the samples subjected to the above (Examples 3 and 4).

  From the analysis results shown in FIG. 8 (a), titanium peaks (α phase, β phase) are recognized, but the peak height decreases and the peak width (half-value width) increases with the particle collision treatment. ing. From this, it is expected that the microstructure of the extreme surface is nanocrystallized in the sample subjected to particle collision.

From the analysis result shown in FIG. 8B, it can be confirmed that a compound (Ti ₂ N) layer is formed on the surface of the sample due to plasma nitridation at 1023K. With the fine particle collision treatment, the peak indicating the presence of the compound disappears, and it can be seen that the compound layer was removed by the fine particle collision. In addition, with this fine particle collision treatment, the peak height of titanium decreases, the peak width (half-value width) increases, and it is expected that the structure on the extreme surface of the sample is nanocrystallized.

From the analysis result shown in FIG. 8 (c), it was confirmed that the compounds (TiN and Ti ₂ N) were formed with the plasma nitriding at 1123K, and the peak indicating the presence of both compounds with the fine particle collision treatment was confirmed. Since it has disappeared, it can be confirmed that the compound layer has been removed. In addition, as the particle collides, the peak height of titanium decreases and the peak width (half-value width) increases, so it is predicted that the microstructure of the extreme surface of the sample is nanocrystallized. The

  Here, miniaturization of the extreme surface (nanocrystallization) has the effect of suppressing the occurrence of fatigue cracks, which is a surface phenomenon. For titanium products that have been surface-treated by the present method, this nanocrystal The improvement in fatigue strength and wear resistance can be expected even by making it easier.

(6) Fatigue test results Untreated sample (Comparative Example 1) that was not nitrided and not subjected to fine particle collision, and different types of fine particle collisions (FPB1, FPB2) were performed on the untreated sample. The fatigue test results of the samples (Comparative Examples 2 and 3) are shown in FIG. 9 (a). The sample subjected to nitriding treatment at 1023K (Comparative Example 4) and different types of fine particle collisions (FPB1 and FPB2) were performed. Fig. 9 (b) shows the fatigue test results of the samples (Examples 1 and 2). Fig. 9 (b) shows a sample subjected to nitriding treatment at 1123K (Comparative Example 5) and different types of fine particle collisions (FPB1, FPB2). The fatigue test results of the samples (Examples 3 and 4) performed are shown in FIG. 9 (c), respectively.

  In FIG. 9 (a), it is confirmed that the fatigue life and fatigue strength of the untreated material (Comparative Example 1) are greatly improved by increasing the surface hardness and applying the residual stress accompanying the fine particle collision treatment. It was.

  In FIG. 9B, it was confirmed that the fatigue strength slightly increased with plasma nitriding at a temperature of 1023 K, but the fatigue life was significantly shortened. In contrast, in the samples (Examples 1 and 2) in which the compound layer was removed by the fine particle collision treatment and a slight residual stress was applied, the fatigue life and fatigue strength were greatly improved. It was also clarified in the results of this fatigue test that a dramatic improvement in fatigue strength was obtained in Example 2 where the test was performed.

  In FIG. 9 (c), the fatigue strength significantly decreases with plasma nitriding at a temperature of 1123K, but with the fine particle collision treatment, the compound layer that causes the fatigue strength to decrease is removed, and the fatigue life and fatigue strength are reduced. Was significantly improved, and it was confirmed that it reached a level higher than that of untreated materials.

  The surface treatment method of the present invention and the titanium product surface-treated by the method of the present invention can be applied to all known uses of the titanium product, particularly for applications requiring wear resistance and fatigue strength. It can be applied to titanium products used in various applications such as railways, ships, automobiles, various machine tools, building materials, as well as applications in the aerospace industry such as jet engine parts, which are one of the main applications. is there.

It is explanatory drawing of the surface treatment method of this invention, (a) is a plasma nitriding process, (b) is a fine particle collision process, It is explanatory drawing of the test piece used in the Example, (a) is for structure observation, surface hardness, hardness distribution measurement, and X-ray diffraction, (b) is a tensile test piece (JIS Z 2201, No. 14A test) Pieces), (c) are fatigue test pieces (JIS Z 2274, No. 2 test piece), The graph which shows the change of hardness distribution accompanying the change of nitriding temperature (no fine particle collision). It is a cross-sectional micrograph of an untreated material and a plasma nitride material, (a) is an untreated material, (b) is a plasma nitride material at a temperature of 973K, (c) is a plasma nitride material at a temperature of 1023K, and (d) is a temperature of 1073K. (E) is a plasma nitride material at a temperature of 1123K, Graph showing hardness distribution in comparison between plasma nitriding material and plasma carburizing material, It is a graph which shows the hardness distribution from the surface of each sample, (a) is a plasma nitride material at the temperature of 1023K (Examples 1 and 2 and Comparative Example 4), (b) is a plasma nitride material at the temperature of 1123K (Example) 3, 4 and Comparative Example 5), It is a cross-sectional micrograph of an untreated material and a plasma nitride material, (a) Comparative example 1, (b) is Comparative Example 2, (c) is Comparative Example 3, (d) is Comparative Example 4, and (e) is an implementation. Example 1, (f) is Example 2, (g) is Comparative Example 5, (h) is Example 3, (i) is each sample of Example 4, It is a graph which shows a X-ray-analysis result, (a) is Comparative Examples 1-3, (b) is Comparative Example 4 and Examples 1 and 2, (c) is each sample of Comparative Example 5 and Examples 3 and 4. Is the analysis result of It is a graph which shows a fatigue test result, (a) is Comparative Examples 1-3, (b) is Comparative Example 4 and Examples 1 and 2, (c) is each of Comparative Example 5 and Examples 3 and 4. The test results are shown.

Claims (13)

  A plasma nitridation treatment in which a titanium product made of pure titanium, an α-type titanium alloy, a β-type titanium alloy, or an α + β-type titanium alloy is subjected to plasma nitridation, and a hardened layer is formed on the surface of the treatment target; A fine particle collision treatment is performed on the object to be treated after the plasma nitriding treatment so that one or more fine particles collide with each other, and the compound layer existing on the surface of the hardened layer is removed by the fine particle collision treatment. A surface modification method for titanium products.   The method for modifying the surface of a titanium product according to claim 1, wherein the plasma nitriding is performed at a temperature of 973 to 1271K and a treatment time of 0.5 to 12 hours.   The method for modifying the surface of a titanium product according to claim 1 or 2, wherein the fine particles have a maximum particle size of 20 µm to 200 µm.   The surface modification method for a titanium product according to any one of claims 1 to 3, wherein a residual stress is applied to the object to be treated in the fine particle collision treatment.   The surface modification method for a titanium product according to any one of claims 1 to 4, wherein in the fine particle collision treatment, a surface of the treatment target is smoothed.   The method for modifying the surface of a titanium product according to any one of claims 1 to 5, wherein the fine particle collision treatment includes a treatment of causing nonmetallic fine particles to collide for the purpose of removing the compound layer.   The method for modifying the surface of a titanium product according to any one of claims 1 to 6, wherein the fine particle collision treatment includes a treatment of causing metal fine particles to collide for the purpose of imparting residual stress.   The fine particle collision treatment further includes a process of removing a component of the metallic fine particles remaining on the surface of the processing target after the metallic fine particles collide and a treatment of causing the nonmetallic fine particles to collide for surface smoothing. The method for modifying the surface of a titanium product according to claim 7.   The solid-gas two-phase mixed fluid of fine particles and high-pressure gas is injected from a nozzle at an injection pressure of 0.3 MPa or more, or an injection speed of 80 m / sec or more, and collides so that the treatment rate becomes 100% or more. The method for modifying the surface of a titanium product according to any one of claims 1 to 8.   A hardened layer formed by plasma nitriding and having a surface compound layer removed by collision of one or more fine particles is made of pure titanium, α-type titanium alloy, β-type titanium alloy, or α + β-type titanium alloy. A surface-modified titanium product characterized by being formed on the surface of any titanium product.   The surface-modified titanium product according to claim 10, wherein the hardened layer is formed by plasma nitriding at a temperature of 973-1271K and a processing time of 0.5-12 hours.   The surface-modified titanium product according to claim 10 or 11, wherein the removal of the compound layer is performed with the fine particles having a maximum particle size of 20 µm to 200 µm.   The surface-modified titanium product according to any one of claims 10 to 12, wherein a residual stress is applied to the hardened layer by collision of the fine particles.