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SURFACE HARDENING OF GROUP IV METALS

20240133009 ยท 2024-04-25

    Inventors

    Cpc classification

    International classification

    Abstract

    The present invention relates to a method of case hardening a Group IV metal or a Group IV metal alloy and to components hardened in the method. The method comprising the steps of: providing a workpiece of a Group IV metal or a Group IV metal alloy, the workpiece being in its final shape; nitriding the workpiece in a nitriding atmosphere comprising NHs as a nitriding species at a first temperature in the range of 450? C. to 750? C. for a nitriding duration of at least 16 hours to provide a hydrogen containing diffusion zone; removing hydrogen from the hydrogen containing diffusion zone at a second temperature of up to 750? C. and a partial pressure of H.sub.2 of up to 10.sup.?4 mbar over a hydrogen removal duration of at least 4 hours to provide a hydrogen depleted diffusion zone. The method and the component are useful for implants, in particular dental implants.

    Claims

    1. A method of case hardening a Group IV metal or a Group IV metal alloy, the method comprising the steps of: providing a workpiece of a Group IV metal or a Group IV metal alloy, the workpiece being in its final shape, nitriding the workpiece in a nitriding atmosphere comprising NH.sub.3 as a nitriding species at a first temperature in the range of 450? C. to 750? C. and a partial pressure of NH.sub.3 in the range of 0.5 bar to 2 bar for a nitriding duration of at least 0.1 hour to provide a hydrogen containing diffusion zone, and removing hydrogen from the hydrogen containing diffusion zone at a second temperature in the range of 600? C. to 750? C. and a partial pressure of H.sub.2 (pH.sub.2) of up to 10.sup.?4 mbar over a hydrogen removal duration of at least 4 hours to provide a hydrogen depleted diffusion zone.

    2. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the nitriding atmosphere does not comprise oxidising species.

    3. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the first temperature is in the range of 580? C. to 700? C.

    4. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the total pressure in the step of removing hydrogen from the hydrogen containing diffusion zone is up to 10.sup.?4 mbar.

    5. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the Group IV metal is zirconium or a Group IV metal alloy containing at least 2 wt % zirconium.

    6. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the second temperature in the range of 600? C. to 700? C.

    7. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the Group IV metal is titanium or a titanium-based alloy not containing zirconium.

    8. A component of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium, the component having a core hardness and a nitrogen containing diffusion zone extending from a surface of the component to a depth from the surface, where the microhardness is equal to the core hardness plus 50 HV.sub.0.005, the component having a hardness in the range of 1000 HV.sub.0.005 to 1500 HV.sub.0.005 at a depth from the surface of 2.5 ?m, as measured according to the DIN EN ISO 6507 standard, which component does not comprise a nitride layer.

    9. A component of titanium or a titanium-based alloy not containing zirconium, the component having a surface hardness in the range of 700 HV.sub.0.005 to 2000 HV.sub.0.005, a core hardness and a nitrogen containing diffusion zone extending from a surface of the component to a depth from the surface, where the microhardness is equal to the core hardness plus 50 HV.sub.0.005, as measured according to the DIN EN ISO 6507 standard, the component having a surface nitride layer of Ti.sub.2N.

    10. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the surface does not comprise TiN.

    11. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the layer of Ti.sub.2N is identified using X-ray diffraction analysis.

    12. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the component has a microhardness in the range of 700 HV.sub.0.005 to 1200 HV.sub.0.005 at a depth from the surface of 2.5 ?m.

    13. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the absence of TiN is identified using X-ray diffraction analysis.

    14. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the surface of the component does not comprise an oxide layer, except from nanometre scale oxide layers formed naturally on the surface of Group IV metals in contact with air.

    15. The component of titanium or a titanium-based alloy not containing zirconium according to claim 9, wherein the component has a mirror polish appearance defined as a surface with an arithmetical mean deviation (Ra) roughness of <0.1 ?m in accordance with the ISO 1302:2002 standard.

    16. (canceled)

    17. The component of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium according to claim 8, wherein the absence of TiN is identified using X-ray diffraction analysis.

    18. The component of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium according to claim 8, wherein the surface of the component does not comprise an oxide layer, except from nanometre scale oxide layers formed naturally on the surface of Group IV metals in contact with air.

    19. The component of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium according to claim 8, wherein the component has a mirror polish appearance defined as a surface with an arithmetical mean deviation (Ra) roughness of <0.1 ?m in accordance with the ISO 1302:2002 standard.

    20. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 1, wherein the first temperature is in the range of 450? C. to 580? C. and the nitriding duration is in the range of 24 hours to 200 hours.

    21. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 3, wherein the nitriding duration is in the range of 0.5 hours to 100 hours.

    22. The method of case hardening a Group IV metal or a Group IV metal alloy according to claim 6, wherein the hydrogen removal duration is in the range of 12 hours to 200 hours.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0048] In the following the invention will be explained in greater detail with the aid of an example and with reference to the schematic drawings, in which

    [0049] FIG. 1 shows pH.sub.2 as a function of the NH.sub.3:N.sub.2 ratio at different total pressures,

    [0050] FIG. 2 shows a hardness profile of Ti15Zr hardened using NH.sub.3;

    [0051] FIG. 3 shows a hardness profile of Ti15Zr hardened according to the present method;

    [0052] FIG. 4 shows a hardness profile of Ti15Zr hardened according to the present method;

    [0053] FIG. 5 shows an XRD analysis of Ti6Al4V hardened using NH.sub.3;

    [0054] FIG. 6 shows an XRD analysis of Ti6Al4V hardened according to the present method;

    [0055] FIG. 7 shows a hardness profile of titanium hardened according to the present method;

    [0056] FIG. 8 shows a microscopy image of a cross-section of titanium hardened using NH.sub.3;

    [0057] FIG. 9 shows a microscopy image of a cross-section of titanium hardened according to the present method.

    [0058] The invention is not limited to the embodiment/s illustrated in the drawings. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims.

    DETAILED DESCRIPTION

    [0059] The present invention relates to a method of case hardening a Group IV metal or a Group IV metal alloy, components of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium and components of titanium or a titanium-based alloy not containing zirconium. The components are obtainable in the method of the disclosure.

    [0060] In the context of the invention Group IV metal is any metal selected from the titanium group of the periodic table of the elements or an alloy comprising at least 50% of metals from the titanium group. Thus, a titanium alloy is any alloy containing at least 50% (a/a) titanium, and likewise a zirconium alloy is any alloy containing at least 50% (a/a) zirconium. It is contemplated that for the method of the invention and for the component of the invention any alloy containing a sum of titanium and zirconium of at least 50% (a/a) is appropriate. Likewise, the alloy may also comprise hafnium, which is a member of Group IV of the periodic table of the elements so that any alloy having a sum of titanium, zirconium, and hafnium of at least 50% (a/a) is appropriate for the invention.

    [0061] Alloys of relevance to the invention may contain any other appropriate element, and in the context of the invention an alloying element may refer to a metallic component or element in the alloy, or any constituent in the alloy. Titanium and zirconium alloys are well-known to the skilled person. Alloys of Group IV metals may also comprise metals from other groups of the periodic table of the elements, e.g. aluminium or niobium. An exemplary niobium containing alloy is Ti13Nb13Zr. Aluminium containing alloys are Ti6Al4V (Grade 5), which exists as an extra low interstitial (ELI) version, Ti6Al4V ELI that is commonly referred to as Grade 23. Further relevant alloys are Titanium 6Al-2Sn-4Zr-6Mo, Titanium 6Al-2Sn-4Zr-2Mo, Ti-3Al-8V-6Cr-4Mo-4Zr (TB9), Ti-5Al-2.5Sn (Grade 6), Ti-3Al-2.5V (Grade 9), Ti-15V-3Al-3Sn-3Cr, Ti-23Nb-0.7Ta-2Zr-1.2O (Gum metal), Ti-6Al-7Nb, Ti-15Zr-4Nb-4Ta, Ti-35Nb-7Zr-5Ta, Ti-29Nb-4.6Zr-13Ta, Ti-15Mo-5Zr-3Al, Ti-15Mo.

    [0062] Any grade of titanium containing at least about 99% (w/w) titanium is, in the context of the invention, considered to be pure titanium, e.g. Grade 1 titanium, Grade 2 or Grade 4 titanium; thus, the pure titanium may contain up to about 1% (w/w) trace elements, e.g. oxygen, carbon, nitrogen or other metals, such as iron. Pure titanium may also be referred to as commercially pure (CP). In particular, nitrogen and carbon contained in a Group IV metal in the context of the invention may represent unavoidable impurities. Elements present as unavoidable impurities are considered not to provide an effect for a workpiece treated according to the method of the invention or for the component of the invention. Likewise, any grade of zirconium containing at least about 99% (w/w) zirconium is, in the context of the invention, considered to be pure zirconium.

    [0063] When a percentage is stated for a metal or an alloy the percentage is by weight of the weight of material, e.g. denoted % (w/w), unless otherwise noted. When a percentage is stated for an atmosphere the percentage is by volume, e.g. denoted % (v/v), unless otherwise noted. Likewise, unless otherwise noted a composition of a mixture of gasses may be on an atomic basis and may then be provided as a percentage or in ppm (parts per million).

    [0064] In the context of the invention the hardness is generally the HV.sub.0.005 or HV.sub.0.025 as measured according to the DIN EN ISO 6507 standard. If not otherwise mentioned the unit HV thus refers to this standard. The hardness may be recorded for a cross-section, e.g. of a treated Group IV metal, and it may be noted with respect to the depth of the measurement. The hardness measurement in the cross-section may also be referred to as microhardness, and the hardness measurement at the surface may also be referred to as macrohardness. When a hardness is measured in the surface, the hardness measurement may also be referred to as a top-down measurement.

    [0065] In general, microhardness measurements may be performed at a load of 5 g, i.e. HV.sub.0.005, 25 g, i.e. HV.sub.0.025, or 50 g, i.e. HV.sub.0.05. In contrast, the macrohardness may be performed from the surface with a much higher load, e.g. 0.50 kg, corresponding to HV.sub.0.5, so that the measurement represents an overall value of the hardness of the respective material and whatever surface layers it contains. In the context of the present disclosure, microhardness measurements obtained in cross-sections, e.g. of components prepared according to the method, are performed at a load of 5 g, i.e. HV.sub.0.005, and surface hardness values are obtained as top-down measurements using a load of 25 g, i.e. HV.sub.0.025.

    [0066] When the hardness is recorded at a cross-section the measurement is considered to represent a homogeneous sample with respect to the direction of the pressure applied. In contrast, when the hardness is obtained from measurements at the surface the measurement may represent an average of several different values of hardness, i.e. at different depths. Thus, when the surface hardness is measured at a high load, e.g. 0.50 kg, the value can be considered to provide an average value for both the surface and also depths below the surface. It is therefore preferred that surface hardness is measured with a load of 25 g or 50 g. When the surface hardness is measured with a load of 25 g, a value of 650 HV.sub.0.025 is considered to show that the material is scratch resistant. As an effect of the fact that nitrogen is dissolved from the surface the content of dissolved nitrogen will decrease from the surface towards the core of the Group IV metal, and likewise, the hardness will be maximal at the surface and decrease with depth.

    EXAMPLES

    Example 1

    [0067] Two samples of Ti15Zr alloy were provided and nitrided in NH.sub.3 in a Netzsch 449 Thermal analyzer (furnace). The samples were heated to 690? C. at a rate of 20? C./min and exposed to NH.sub.3 at ambient pressure for 20 hours. Both samples were cooled to ambient temperature, and one sample was not subjected to further steps. The other sample was subsequently treated in vacuum, i.e. in <10.sup.?4 mbar total pressure, provided with an Edwards 85 T-station turbo vacuum pump, at 690? C. for 4 hours.

    [0068] For both samples, the nitriding step provided a golden surface colour, but the golden colour was subsequently removed in the diffusive step. Thus, the nitride layer was removed and the nitrogen present in the nitride layer was dissolved into the diffusion zone.

    [0069] The treated samples were analysed for hardness (HV.sub.0.005), and the hardness profiles are shown in FIG. 2 and FIG. 3. In FIG. 2 and FIG. 3, the error bars represent 1 standard deviation from the mean. In both cases, the treatments provided a hardness >1100 HV.sub.0.005 at 2.5 ?m below the surface (FIG. 2, FIG. 3). A core hardness of <400 HV.sub.0.005 was first reached at a depth of about 50 ?m for the sample treated according to the disclosure (FIG. 2). For the sample not subjected to the diffusive step this hardness <400 HV.sub.0.005 was already reached at 30 ?m (FIG. 3).

    [0070] The hardness corresponds to a nitrogen uptake more than >10? larger in zirconium containing titanium alloys compared to conventional titanium alloys not containing zirconium.

    [0071] Furthermore, needle shaped hydrides formed within the grains and in the grain boundaries of the specimen in the nitriding step. After treatment in the diffusive step, formation of new alpha grains could be seen in the bulk as a result of the removal of hydrogen and the transformation of needle shaped hydrides in the diffusive step. Thus, the present method provides grain refinement of the treated metal.

    Example 2

    [0072] A sample of Ti13Zr13Nb was provided and nitrided in NH.sub.3 in a Netzsch 449 Thermal analyzer. The sample was heated to 690? C. at a rate of 20? C./min and exposed to NH.sub.3 at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10.sup.?4 mbar total pressure, at 690? C. for 4 hours. After the nitriding treatment, the sample had a golden colour, which was subsequently removed in the diffusive step. Thus, the nitride layer was removed. The hardness profile revealed a hardness of >1100 HV.sub.0.005 at 2.5 ?m below the surface and a core hardness of <500 HV.sub.0.005 was reached at a depth of about 35 ?m from the surface.

    Example 3

    [0073] A sample of Ti15Zr was provided and nitrided in NH.sub.3 in a Netzsch 449 Thermal analyzer. The sample was heated to 690? C. at a rate of 20? C./min and exposed to NH.sub.3 at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10.sup.?4 mbar total pressure, at 550? C. for 24 hours. After the nitriding treatment, the sample had a golden colour, which was subsequently retained in the diffusive step. Thus, the nitride layer was retained. The hardness profile is shown in FIG. 4, and reveals a surface hardness >1200 HV.sub.0.025, and a hardness of >1100 HV.sub.0.005 at 2.5 ?m for the surface. In FIG. 4, the error bars represent 1 standard deviation from the mean. The hardness profile shows that the treated sample had a very high surface hardness.

    Example 4

    [0074] A sample of Ti13Zr13Nb was provided and nitrided in NH.sub.3 in a Netzsch 449 Thermal analyzer. The sample was heated to 690? C. at a rate of 20? C./min and exposed to NH.sub.3 at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10.sup.?4 mbar total pressure, at 550? C. for 24 hours, thus retaining the nitride layer in the diffusive step. Correspondingly, the treated sample obtained a golden colour in the nitriding step, which golden colour was also visible after the diffusive step. The treated sample had a surface hardness >1000 HV.sub.0.025, and a hardness of >900 HV.sub.0.005 at 2.5 ?m for the surface. The core hardness of <400 HV.sub.0.005 was reached at about 40 ?m below the surface.

    Example 5

    [0075] Two samples of Ti6Al4V (alpha-beta alloy) were nitrided in NH.sub.3 at ambient pressure at 700? C. for 16 hours in a Netzsch 449 Thermal analyzer. The treatment provided a nitride layer having a surface hardness of >2000 HV.sub.0.005. The nitride layer had a golden colour and was subjected to X-ray diffraction (XRD) analysis. The XRD plot is shown in FIG. 5, which confirms the presence of Ti.sub.2N and TiN.

    [0076] One of the nitrided samples was subjected to the diffusive step. Specifically, the sample was treated in vacuum, i.e. in <10.sup.?4 mbar total pressure, provided with an Edwards 85 T-station turbo vacuum pump, at 680? C. for 16 hours. The nitride layer lost its golden colour in the diffusive step, and the sample was again subjected to XRD analysis, which is shown in FIG. 6.

    [0077] Comparison of FIG. 6 with FIG. 5 documents that TiN disappeared while Ti.sub.2N was still detected.

    [0078] The sample treated according to the present disclosure had a reduced surface hardness of >1300 HV.sub.0.005 compared to the sample not exposed to the diffusive step (i.e. >2000 HV.sub.0.005), albeit more than sufficient to provide scratch resistance in spite of the reduced hardness. Moreover, the removal of the golden colour from the surface provided a much more attractive metallic appearance, identical to the initial condition. During the redistribution of nitrogen in the diffusion step, hydrogen was removed, thereby strongly reducing the risk for hydrogen embrittlement.

    Example 6

    [0079] Two samples of commercially pure (CP) titanium were provided and nitrided in NH.sub.3 in a Netzsch 449 Thermal analyzer. The samples were heated to 690? C. at a rate of 20? C./min and exposed to NH.sub.3 at ambient pressure for 20 hours before treating one sample in vacuum, i.e. in <10.sup.?4 mbar total pressure, at 690? C. for 4 hours and not subjecting the other sample to further treatment.

    [0080] The nitriding step provided a golden surface colour that disappeared in the subsequent diffusive step thus documenting that no TiN was present after treatment according to the present two-step method.

    [0081] The hardness profile of the sample treated according to the present method is shown in FIG. 7, and reveals a surface hardness >1200 HV.sub.0.005, and a hardness of >800 HV.sub.0.005 at 2.5 ?m below the surface, thereby providing the sample with scratch resistance. A hardness of <300 HV.sub.0.005 was reached at about 20 ?m below the surface. In FIG. 7, the error bars represent 1 standard deviation from the mean

    [0082] Both samples were cut to reveal the cross-sections that were analysed microscopically, as shown in FIG. 8 and FIG. 9. FIG. 8 clearly shows that needle shaped hydrides are visible after treatment in the nitriding step and FIG. 9 clearly shows that the needle shaped hydrides have disappeared in the diffusive treatment, thereby documenting removal of hydrogen in the diffusive step.