METHOD FOR MODIFYING SURFACE OF Co-Cr ALLOY, METHOD FOR MANUFACTURING HIGH FATIGUE STRENGTH Co-Cr ALLOY, AND HIGH FATIGUE STRENGTH Co-Cr ALLOY

Abstract

Provided is a method for modifying a surface of a Co-13 Cr alloy to obtain the Co-13 Cr alloy superior in fatigue strength. The method for modifying a surface of a Co-13 Cr alloy, comprising a step of shot peening of the Co-13 Cr alloy using a shot material including ZrO2.

Description

TECHNICAL FIELD

The present disclosure relates to a method for modifying a surface of a Co-13 Cr alloy, a method for manufacturing a high fatigue strength Co-13 Cr alloy, and a high fatigue strength Co-13 Cr alloy.

BACKGROUND

As an implant material, a biocompatible Co-13 Cr alloy is used. Co-13 Cr alloy has higher strength than Ti, and thus is used as materials for members undergoing high load such as pins for the spinal cord. A method for improving the strength by heat treatment of such a Co-13 Cr alloy is known (for example, Patent Literature 1).

CITATION LIST

Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Publication No. 2014-74227

SUMMARY

The implant material is required to be replaced in a timely manner because of deterioration over time and the like, and currently, the living body always undergoes a large load each time the replacement is performed. In order to reduce the frequency of the replacement, development of an implant material that can withstand longer use is required.

The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a method for modifying a surface of a Co-13 Cr alloy to obtain a Co-13 Cr alloy superior in fatigue strength. The other objects of the present disclosure are to provide a method for manufacturing such a high fatigue strength Co-13 Cr alloy and a high fatigue strength Co-13 Cr alloy obtained by the manufacturing method.

The present disclosure provides a method for modifying a surface of a Co-13 Cr alloy, the method comprising a step of shot peening of the Co-13 Cr alloy using a shot material including ZrO₂.

The present disclosure also provides a method for manufacturing a high fatigue strength Co-13 Cr alloy, the method comprising a step of shot peening of a Co-13 Cr alloy using a shot material including ZrO₂.

In one aspect of the surface modification method and the manufacturing method of the present disclosure, the average grain size of the shot material may be 0.05 to 1.0 mm.

The present disclosure also provides a high fatigue strength Co-13 Cr alloy in which the ε-hcp phase gradually increases and the γ-fcc phase gradually decreases from the inside to the surface.

In one aspect of the alloy of the present disclosure, the Vickers hardness (HV) of the surface may be 500 or more.

The present disclosure can provide a method for modifying a surface of a Co-13 Cr alloy to obtain the Co-13 Cr alloy superior in fatigue strength. The present disclosure also can provide a method for manufacturing such a high fatigue strength Co-13 Cr alloy and a high fatigue strength Co-13 Cr alloy obtained by the manufacturing method.

The present disclosure enables the production of an implant that can have high fatigue strength and a long life. The surface modification by shot peening does not change the component of the material and thus it is considered that the surface-modified material can easily meet the acceptance criteria as an implant material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing surface roughness Rz of the test material before and after shot peening.

FIG. 2 is a graph showing XRD diffraction results of the test material before and after shot peening by GB-K.

FIG. 3 is a graph showing the proportion of theε phase in the test material after shot peening.

FIG. 4 is a photograph showing EBSD observation results of the test material before and after shot peening.

FIG. 5 is a graph showing the Vickers hardness (HV) of surface and relationship of distance from surface and Vickers hardness of the test material before and after shot peening.

DETAILED. DESCRIPTION

The best mode for carrying out the present disclosure will be described below.

[Method for Modifying Surface of Co-13 Cr Alloy]

A method for modifying a surface of a Co-13 Cr alloy of the present embodiment comprises a step of shot peening of a Co-13 Cr alloy using a shot material including ZrO₂.

The surface modification method of the present embodiment is to modify the alloy surface by utilizing phase transformation (stress-induced martensitic transformation from γ-fcc phase to ε-hcp phase) of a Co-13 Cr alloy. It is considered that using a zirconia material mainly composed of low thermal conductivity ZrO₂ as a shot material can efficiently provide the thermal energy of shot peening to the alloy side, and hence the treatment equivalent to heat treatment can be locally and instantaneously applied to the alloy. According to the surface modification method of the present embodiment, selective phase transformation in the vicinity of the surface of the Co-13 Cr alloy into the ε phase can realize a material in which the internal toughness is maintained while the vicinity of the surface is high in strength (gradient material). Such a material having strength against external load and also having flexibility is a material excellent in fatigue strength (high fatigue strength material).

As the Co—Cr alloy, a Co—Cr—Mo alloy can be used, and in particular, a Co—28Cr—6Mo alloy in accordance with ASTM F1537 can be used. This Co—28Cr—6Mo alloy includes Co as a main component, and the Cr content is 26.3 to 3 0.0% by mass, the Mo content is 5.0 to 7.0% by mass, and Ni, Mn, Si, C, Fe, N, and the like as other components are included in trace amounts. Such Co-13 Cr alloys are biocompatible and can be used in medical applications such as implants.

The shot material includes ZrO₂. Since the zirconia material has low thermal conductivity, it easily causes phase transformation in the vicinity of the alloy surface. From the viewpoint of suppressing the transfer of thermal energy generated in the shot peening step to the shot material side, the content of ZrO₂ in the shot material may be 30% by mass or more, 60% by mass or more, or 100% by mass (the shot material substantially consisting of ZrO₂). The shot material may include a small amount of a compound including Fe, Cr, Si, Al, Cu or the like as a constituent element (for example, Al₂O₃ or SiO₂) as a component other than ZrO₂.

The average grain size of the shot material may be 0.05 to 1.0 mm, 0.1 to 0.75 mm, 0.1 to 0.5 mm, 0.1 to 0.3 mm or 0.1 to 0.15 mm. An average grain size of less than 0.05 mm causes the shot material to be light and thus sufficiently modifying the Co-13 Cr alloy surface tends to be difficult, while an average grain size of more than 1.0 mm leads to a longer time for achieving the desired visual coverage (the ratio of the area occupied by the shot material dents by visual observation) and thus the treatment efficiency tends to deteriorate. The average grain size here is a value measured using a sieve.

The Vickers hardness (HV) of the shot material may be 500 to 1300, 600 to 1200, 700 to 1100, and 900 to 1100. The Vickers hardness of less than 500 is difficult to cause phase transformation to a sufficient depth, while the Vickers hardness of more than 1200 causes the surface roughness to be too large, tending to easily occur crack fracture.

The density of the shot material may be 1 to 10 g/cm³, 2.5 to 7.5 g/cm³, and 4 to 6 g/cm³. The density of the shot material of less than 1 g/cm³ leads to the weak intensity, tending to be difficult to cause phase transformation to a sufficient depth, while the density of the shot material of more than 10 g/cm³ leads to the strong intensity, causing the surface roughness to be larger to tend to easily occur crack fracture.

Examples of the method of the shot peening include a rotary projection method, an air suction method, and a pressure blast method. Among them, the air suction method can be used from the viewpoint that the construction of a complicated apparatus is unnecessary. The air suction method is also advantageous in that it is easy to suppress the destruction of the shot material itself including ZrO₂.

The injection pressure may be set appropriately, but can be, for example, about 0.1 to 0.5 MPa. The injection pressure of less than 0.1 MPa causes the shot material to tend to be difficult to eject, while the injection pressure of more than 0.5 MPa even causes the hardness after modification to tend to saturate.

The injection time can be set appropriately to achieve the desired coverage. The coverage may be at least 200% or more, 500% or more, 750% or more, or 900% or more. The upper limit of the coverage can be set to 1000% from the viewpoint of sufficient proceeding of the phase transformation in the vicinity of the surface of the Co-13 Cr alloy. The state where all regions are covered by the dents after striking the shot material on the surface is referred to as coverage of 100%. For example, coverage of 200% means that the shot is performed only for the time required to further reach coverage 0% to 100% from the state of coverage 100%.

The surface roughness (Rz) of the Co-13 Cr alloy after the shot peening may be less than 10 μm or less than 5 μm. For example, when the use of a Co-13 Cr alloy in an artificial joint or the like is assumed, the polishing step is performed after the shot peening as described below for the purpose of suppressing the generation of wear powder on the sliding surface. When the surface roughness after the shot peening is 10 μm or more, the amount of polishing increases until the mirror surface is achieved. This is not only inefficient in the process but also leads to an excessive removal of the layer modified by the shot peening.

The surface roughness of the alloy and the amount of phase transformation in the vicinity of the alloy surface can be adjusted by changing various conditions of the shot peening (type of the shot material, injection pressure of the shot material, the amount of coverage, and the like). Since this can adjust the tensile strength, hardness, and the like of an alloy, a desired mechanical property can be given to the alloy.

The modification of the alloy surface by the shot peening (the introduction of a high strength phase on the surface) can be conveniently determined by measuring the Vickers hardness (HV) of the alloy surface.

[Method for Manufacturing High Fatigue Strength Co-13 Cr Alloy]

The method for manufacturing a high fatigue strength Co-13 Cr alloy according to the present embodiment comprises a step of shot peening of a Co-13 Cr alloy using a shot material including ZrO₂. Various types of materials, conditions, and the like regarding the step of shot peening conform to the contents of the method for modifying the surface of the Co-13 Cr alloy.

The manufacturing method of the present embodiment may further comprise a polishing step after the step of shot peening. That is, the shot-peened Co-13 Cr alloy surface may be further polished. The polishing step can be performed by, for example, mechanical polishing, electrolytic polishing, chemical polishing, or the like.

[High Fatigue Strength Co-13 Cr Alloy]

The high fatigue strength Co-13 Cr alloy of the present embodiment is an alloy in which the ε-hcp phase gradually increases and the γ-fcc phase gradually decreases from the inside to the surface. Such characteristic materials (gradient materials) can be introduced by the shot peening which can selectively cause phase transformation only in the vicinity of the surface. That is, the high fatigue strength Co-13 Cr alloy of the present embodiment can be a Co-13 Cr alloy which is subjected to shot peening (preferably using a shot material including ZrO₂). More macroscopically, the high fatigue strength Co-13 Cr alloy of the present embodiment can also be an alloy including an inner layer substantially consisting of a γ-fcc phase and an outer layer having an ε-hcp phase.

The Vickers hardness (HV) of the surface of the high fatigue strength Co-13 Cr alloy of the present embodiment may be 500 or more, 550 or more, or 600 or more. In view of the fact that the Vickers hardness of the surface of the Co-13 Cr alloy before the shot peening is about 400, the surface strength is significantly improved.

The effect of the shot peening reaches a certain depth from the alloy surface. Though not generally speaking because of the dependence on treatment conditions, for example, in the region of less than 400 μm, or less than 350 μm, or less than 300 μm from the surface, the Vickers hardness of the alloy is improved as compared to the untreated case. This is considered to be because the phase transformation from the γ-fcc phase (the phase having the existing toughness) to the ε-hcp phase (the phase having the high strength) occurs to such a depth. That is, in the high fatigue strength Co-13 Cr alloy of the present embodiment, the outer layer having the ε-hcp phase is formed with a thickness of less than 400 μm on the inner layer consisting of the γ-fcc phase.

The analysis by the XRD diffraction method can quantitatively calculate the proportion (volume ratio) of the formed ε-hcp phase. In addition, the analysis by the EBSD method can evaluate the proportion of the ε-hcp phase in consideration of the internal direction. In the high fatigue strength Co-13 Cr alloy, the proportion of the ε-hcp phase in the vicinity of the surface (a depth of about 15 μm from the surface) may be 30% or more, 35% or more, or 40% or more. Using a shot material including ZrO₂ easily allows for the proportion of the ε-hcp phase to be 40% or more. The remainder is substantially the γ-fcc phase. In view of the fact that the proportion of the ε-hcp phase before the shot peening is about 0%, a considerable amount of the ε-hcp phase is introduced. Increasing the coverage also can further increase the proportion of the ε-hcp phase.

Crystal grains in the vicinity of the alloy surface are refined by the shot peening. In the high fatigue strength Co-13 Cr alloy, the average size of the crystal grains may be less than 3 μm, less than 2 μm, less than 1.5 μm, or less than 1 μm. In view of the fact that the average size of the crystal grains before the shot peening is about 6 μm, the crystal grains are significantly refined.

The existence mode of the γ-fcc phase and the ε-hcp phase in the Co-13 Cr alloy and the existence mode of the refined crystal grains can be observed by, for example, EBSD (Electron Back Scatter Diffraction Patterns) method.

The high fatigue strength Co-13 Cr alloy of this embodiment is suitable as an implant material, and the surface roughness and hardness can be adjusted, so that it can be widely used for sliding parts, pins for the spinal cord, and the like.

EXAMPLES

Hereinafter, the present disclosure will be more specifically described referring to Examples. However, the present invention is not limited to these Examples.

(Experiment 1: Comparison of Material of Shot Material)

As a test material, a medical Co—28Cr—6Mo alloy (ASTM F1537) was used. Table 1 shows the alloy composition (unit: % by mass). In the shot peening, the surface to be treated was polished in advance with Al₂O₃ powder and used.

TABLE 1
Co	Cr	Mo	Ni	Mn	Si	Fe	N	C
Bal.	26.3-	5.0-	≤1.0	≤1.0	≤1.0	≤0.75	≤0.25	≤0.14
	30.0	7.0

Under the shot peening conditions shown in Table 2, the test material was subjected to the shot peening with a nozzle size of 6 mm, an injection distance of 100 mm, and a coverage of 300%. The apparatus of the air suction method was used for the shot peening.

The surface roughness Rz of the test material before and after the shot peening was measured using a contact-type surface roughness measuring device. The measurement results are shown in FIG. 1. In the figure, NP is data of an untreated test material (only subjected to Al₂O₃ polishing) without the shot peening.

The test material before and after the shot peening was analyzed by XRD diffraction using an X-ray diffractometer. The XRD diffraction results of the test material before and after the shot peening are shown in FIG. 2. From the main peak in the XRD chart, the proportion of ε phase was calculated using the following formula. In the following formula, V represents a volume fraction and I represents an integrated intensity of a peak.

$\begin{array}{cc}{V}_{\varepsilon }=\frac{I_{\varepsilon }\left(10\stackrel{\_}{1}0\right)+I_{\varepsilon }\left(10\stackrel{\_}{1}1\right)}{I_{\varepsilon }\left(10\stackrel{\_}{1}0\right)+I_{\varepsilon }\left(10\stackrel{\_}{1}1\right)+I_{\gamma }\left(111\right)}& \left[\mathrm{Formula}1\right]\end{array}$

The proportion (volume ratio) of the ε phase in the test material after the shot peening calculated above is shown in FIG. 3. From the figure, it is found that the shot material having the largest transformation ratio to the ε phase is GB-K. From this, it is considered that the shot material having a small grain size had a high frequency of collision with the projection surface, and as a result, a large strain was formed. Comparison of SBM44T with ZB120 finds that the proportion of ε phase in ZB120 with high hardness is larger. From this, it is considered that the collision energy is larger as the hardness of the shot material is higher. As described above, it is considered that a shot material having a small grain size and high hardness is suitable to promote the stress-induced martensitic transformation.

The test material before and after the shot peening was then analyzed by the EBSD method using a field emission scanning electron microscope. FIG. 4 shows EBSD observation results of the test material before and after the shot peening. The upper drawing in FIG. 4 shows the existence modes of the γ phase and the ε phase in the vicinity of the surface of the test material, and the lower drawing shows the appearance of crystal grains in the vicinity of the surface of the test material. The ε phase proportion in the upper drawing was obtained from the observation image by analysis software. GS in the lower drawing means an average grain size. After taking out the band contrast from the observation image by the analysis software, the grain boundary line was drawn, the part surrounded by the line was made into one grain boundary, and the average grain size of the obtained crystal was calculated. As a result of observation by EBSD, it was found that the shot peening introduced the ε phase and that the crystal grains in the vicinity of the surface were refined. For GB-K (refer to FIG. 3) in which the amount of transformation to the ε phase was the largest, phase transformation to the ε phase mainly occurred in the vicinity of the surface, and many untransformed γ phases were present inside. From this, it is considered that the collision energy is larger as the hardness of the shot material is higher, and as a result, the stress-induced martensitic transformation occurred to the inside.

FIG. 5 shows the Vickers hardness (HV) of the test material before and after the shot peening.

	TABLE 2
	Shot material
	GB-K	GB-D	SBM100C	SBM44T	RCW06PS	ST600	ZB120
	Material
	Glass	Iron-based	WC	ZrO2
Grain size (mm)	0.09	0.3	0.15	0.1	0.6	0.12
Hardness (HV)	550	830	750	1450	700
Injection pressure	0.3	0.2	0.3
(MPa)
Density (g/cm3)	2.3	7.9	15.6	5.7
ε phase proportion	33.4	27.5	15.0	5.5	40.5	41.6	46.0
(%)

(Experiment 2: Comparison of Type of Zirconia Shot Material)

The shot peening was performed on the test material in the same manner as in Experiment 1 except that the shot peening conditions were changed as shown in Table 3. All shot materials A to E include ZrO₂ as a main component. In the table, shot materials A, C, D, and E are manufactured by Saint-Gobain company, and shot material B is manufactured by Tosoh Corporation.

	TABLE 3
	Shot material
	A	B	C	D	E
	B120	TZ-B90	B60	B30	B205
Material	Ceramic (main component ZrO2)
Grain size (mm)	0.1	0.12	0.2	0.6	0.05
Hardness (HV)	700	1085	700
Injection pressure (MPa)	0.3
Density (g/cm3)	3.8	6.05	3.8	3.8	3.8
Coverage (%)	300 or 1000

In the same manner as Experiment 1, the proportion of the ε phase after the shot peening under each condition was calculated from the observation results of EBSD. The results are shown in Table 4.

	TABLE 4
	Shot material	Coverage	ε phase proportion
	NP	—	1.0%
	A	300%	42.1%
		1000%	64.4%
	B	300%	37.7%
		1000%	74.9%
	C	300%	34.5%
		1000%	64.8%
	D	300%	44.7%
		1000%	63.4%
	E	300%	32.9%
		1000%	35.8%

According to Examples, the gradient material was formed by selective phase transformation (stress-induced martensitic transformation) in the vicinity of the surface of the Co-13 Cr alloy into the ε phase, and thereby it was possible to obtain a material in which the internal toughness was maintained while the vicinity of the surface was high in strength. That is, the material excellent in fatigue strength (high fatigue strength material) having strength against external load and also having flexibility was able to be obtained.

Claims

1. A method for modifying a surface of a Co-13 Cr alloy, comprising a step of shot peening of the Co-13 Cr alloy using a shot material including ZrO2.

2. The method for modifying a surface of a Co-13 Cr alloy according to claim 1, wherein an average grain size of the shot material is 0.05 to 1.0 mm.

3. A method for manufacturing a high fatigue strength Co-13 Cr alloy, comprising a step of shot peening of a Co-13 Cr alloy using a shot material including ZrO2.

4. The method for manufacturing a high fatigue strength Co-13 Cr alloy according to claim 3, wherein an average grain size of the shot material is 0.05 to 1.0 mm.

5. A high fatigue strength Co-13 Cr alloy, wherein an ε-hcp phase gradually increases and a γ-fcc phase gradually decreases from inside to surface.

6. The Co-13 Cr alloy according to claim 5, wherein a Vickers hardness of the surface is 500 or more.