SURFACE HARDENING OF GROUP IV METALS

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 H2 of up to 10−4 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.

Description

FIELD OF THE INVENTION

The present invention relates to hardening of Group IV metals or alloys. Specifically, a method of case hardening a Group IV metal or a Group IV metal alloy is provided as well as a hardened Group IV metal or Group IV metal alloy component. The method and the component are useful for implants, in particular dental implants.

BACKGROUND

Titanium is a light weight metal with a tensile strength comparable to stainless steel, which naturally reacts with oxygen to form a titanium oxide layer on the surface that provides corrosion resistance. These characteristics make titanium highly attractive in many fields, such as aerospace, military and for industrial processes, and moreover since titanium is biocompatible it is also relevant for medical uses, e.g. as implants. The naturally forming layer of titanium oxide is thin, e.g. in nanometre scale, and for certain applications it can be desirable to modify the surface-adjacent region of titanium and its alloys. It is well-known that titanium and other Group IV metals can be hardened by interstitial oxygen and other elements, and it is further well-known that hardening of titanium should be performed at a temperature as low as possible in order to avoid grain growth and general distortion of the material associated with phase transformations or creep.

Several examples of hardening titanium are known from the prior art. For example, EP 0885980 discloses a process for forming a superficial layer having high hardness and tribologic properties on parts of titanium or zirconium. The process may comprise raising the temperature to a temperature above 500° C. to obtain a homogeneous temperature of the parts; injecting a treatment gas containing ammonia, a hydrocarbon and/or an oxidizing gas onto the parts to be treated raised to a temperature greater than 500° C. and maintaining the pressure inside the furnace at a value of at least 100 mbar for at least a few minutes, depending on the depth of desired treatment. When ammonia is used to treat titanium, a yellow surface layer of Ti_xN_y is provided.

Preisser et al., 1991 (HTM Harterei-Technische Mitteilungen 46 (1991) Nov./Dez., No. 6, Munchen, DE) discloses high pressure nitriding of titanium workpieces where the workpieces are treated with ammonia at a pressure of 12 bar at nitriding temperatures of 700° C. or 900° C. The treated workpieces have an outer layer of TiN and a layer of Ti₂N is formed below the outer layer of TiN.

JPH0225559 discloses nitriding of titanium workpieces using ammonia at a temperature in the range of 400° C. to 850° C. for at least 1 hour, which is followed by treatment in an inert gas at at least 400° C. for at least 1 hour to remove hydrogen from the workpiece. The treatment provides a layer of TiN on the workpiece.

JPS5493700 discloses treating titanium in a nitriding reaction in an atmosphere of NH₃ gas or N₂—H₂ mixed gas followed by heating the titanium to above 600° C. in an inert atmosphere or a vacuum to thermally decompose hydride formed by the reaction.

In light of the prior art, there is still a need for an improved process of hardening titanium or other Group IV metals and their alloys, and it is an object of the invention to provide improved case hardening of Group IV metals and their alloys.

SUMMARY

The present invention relates to 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 NH₃ as a nitriding species at a first temperature in the range of 450° C. to 750° C. and a partial pressure of NH₃ in the range of 0.5 bar to 2 bar for a nitriding duration of at least 12 hours 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 H₂ (pH₂) of up to 10⁻⁴ mbar over a hydrogen removal duration of at least 4 hours to provide a hydrogen depleted diffusion zone.

The method includes a step of nitriding the workpiece. In the context of the present disclosure, this step may be referred to as the “nitriding step”.

The method comprises the step of removing hydrogen from the hydrogen containing diffusion zone. It is to be understood that by exposing the workpiece treated with NH₃ to a low pressure, especially a low partial pressure of H₂, at a high temperature, hydrogen will diffuse from the workpiece to the ambient atmosphere and thereby be removed from the workpiece. The step of removing hydrogen may also be referred to as “diffusing hydrogen” from the hydrogen containing diffusion zone, or as the “diffusive step”, and in the present context, the terms may be used interchangeably. Correspondingly, the hydrogen removal duration may also be referred to as a diffusive duration, and in the present context, the two terms may be used interchangeably.

A workpiece is provided in its final shape. Dissolution of nitrogen and hydrogen into the workpiece will generally increase the volume of the workpiece, but the workpiece will be returned to the shape it had before treatment in the method, i.e. the final shape. However, even though the workpiece is in its final shape after the treatment, it is possible to treat the workpiece further in steps that will not substantially affect the final shape, e.g. polishing and the like.

The treatment with NH₃, i.e. at the first temperature in the range of 450° C. to 750° C., provides a diffusion zone in the Group IV metal, which diffusion zone contains nitrogen in solid solution and hydrogen in solid solution and typically also hydrides of the Group IV metal. The subsequent diffusive treatment, i.e. at the second temperature of up to 750° C., provides a diffusion zone in the Group IV metal, which diffusion zone contains nitrogen in solid solution, but which is depleted in hydrogen. Thus, in the present context, the term “diffusion zone” may refer to either the diffusion zone after the nitriding treatment or after the diffusive treatment. The diffusion zone after the nitriding treatment but before the diffusive treatment is generally referred to as the “hydrogen containing diffusion zone”, and the diffusion zone after the diffusive treatment is generally referred to as the “hydrogen depleted diffusion zone”. However, the diffusion zone after removal of hydrogen may also be referred to as the “nitrogen diffusion zone”.

The diffusion zone extends from a surface of the Group IV metal or the Group IV metal alloy. In the context of this disclosure, the diffusion zone is considered to extend from the surface of the Group IV metal to a depth, where the microhardness is equal to the core hardness of the Group IV metal or the Group IV metal alloy plus 50 HV_0.005. The diffusion zone may also be defined to have a thickness, and the thickness if thus calculated from the depth from the surface, where the microhardness is equal to the core hardness plus 50 HV_0.005 to the surface of the Group IV metal. The treatment with NH₃, i.e. at the first temperature in the range of 450° C. to 750° C., also provides a nitride layer at the surface of the Group IV metal or the Group IV metal alloy. The Group IV metal may be titanium, zirconium or an alloy containing both titanium and zirconium, and in the context of the present disclosure, the nitride layer of the Group IV metal or the Group IV metal alloy may be collectively referred to as (Ti,Zr)N for an alloy containing both titanium and zirconium. Regardless of the presence of a nitride layer, the diffusion zone is considered to extend from the surface of the Group IV metal or the Group IV metal alloy, and the diffusion zone extends to a greater depth than the nitride layer. Thus, the diffusion zone can also be considered to be below the nitride layer, or the diffusion zone can also be considered to be between the nitride layer and the core of the Group IV metal or the Group IV metal alloy. The nitriding step can also be considered to provide a nitride layer of the Group IV metal or the Group IV metal alloy at the surface of the workpiece and a hydrogen containing diffusion zone between the nitride layer and a core of the Group IV metal or the Group IV metal alloy.

Ammonia (NH₃) is used as a nitriding species in the present method. When NH₃ is exposed to temperatures above 800° C., especially at even higher temperatures, e.g. above 1000° C., NH₃ will be dissociated into N₂ and H₂, and N₂ can generally also be used for nitriding metals. However, when NH₃ is employed as a nitriding species at temperatures up to 800° C., both of nitrogen and hydrogen will be dissolved simultaneously in the metal from NH₃. Specifically, the present inventors believe that NH₃ molecules on the metallic surface at the increased temperature will be split into N atoms and H atoms and that both the N atoms and the H atoms will diffuse into the metal. NH₃ can be considered to provide a much higher ‘virtual partial pressure’ of nitrogen than N₂ alone and also a much higher virtual partial pressure of hydrogen than H₂ alone so that NH₃ is a much more potent nitriding species than N₂ and at the same time a much more potent hydriding species than H₂. The virtual partial pressure of H₂ is illustrated in FIG. 1 as pH₂ as a function of the NH₃:N₂ ratio at different total pressures and a temperature of 690° C. However, interstitial hydrogen in Group IV metals is considered to embrittle the Group IV metal, and therefore hydrogen is normally not desired in hardening of Group IV metals, and moreover when a Group IV metal is treated with NH₃, more hydrogen is dissolved in the Group IV metal compared to when it is treated with H₂, so that the negative effect of interstitial hydrogen can be expected to be more pronounced when a Group IV metal is treated with NH₃ compared to when the Group IV metal is treated with N₂ and H₂. However, the present inventors have surprisingly found that by introducing hydrogen atoms from NH₃, i.e. at a temperature in the range of 450° C. to 750° C., the hydrogen atoms can subsequently be diffused out of the Group IV metal at a temperature up to 750° C., e.g. in the range of 400° C. to 700° C., in particular 600° C. to 700° C., thereby preventing problems with embrittlement despite the increased amount of hydrogen in the Group IV metal obtained in the nitriding step with NH₃. Moreover, the solution of hydrogen in the Group IV metal also causes formation of hydrides, and the removal of hydrogen from a Group IV metal containing Group IV metal hydrides, e.g. by treatment at high temperatures above 700° C., is known to provide grain refinement in the Group IV metal. Since treatment of the Group IV metal with NH₃ provides a higher concentration of hydrogen in the Group IV metal than treatment with H₂, and the present inventors believe that a more efficient grain refinement of the Group IV metal is thus obtained compared to grain refinement obtained using H₂. Thus, when a Group IV metal is treated with NH₃ at a temperature in the range of 450° to 750° C. followed by removal of hydrogen from the Group IV metal by diffusion at a temperature of up to 750° C., e.g. in the range of 400° C. to 700° C., a hardened and grain refined Group IV metal is obtained. Specifically, the hardened Group IV metal has a grain refined core zone defined by a core hardness of the Group IV metal and a hydrogen depleted diffusion zone with nitrogen in solid solution between the grain refined core zone and the surface of the Group IV metal.

The nitriding treatment employs a nitriding atmosphere comprising NH₃ as a nitriding species. The partial pressure of NH₃ may be chosen freely but should be sufficient to dissolve a desired amount of nitrogen and hydrogen in the Group IV metal. However, at a low partial pressure of NH₃ insufficient nitrogen is dissolved in the Group IV metal or the Group IV metal alloy, and in general, the partial pressure of NH₃ should thus be at least 1 mbar, although it is preferred that the partial pressure of NH₃ is in the range of 0.5 bar to 2 bar, e.g. at ambient pressure. When the partial pressure of NH₃ is below ambient pressure, the pressure may be reduced using any means as desired. For example, the partial pressure may equal the total pressure, i.e. the nitriding atmosphere is pure NH₃ with any unavoidable impurities, or the partial pressure may be reduced by including further gaseous species in the nitriding atmosphere. The further gaseous species may be an inert gas or a gaseous species providing a specific functionality. Since the nitriding is performed at a first temperature in the range of 450° C. to 750° C., nitrogen gas, i.e. N₂, is considered an inert gas that will not lead to dissolution of nitrogen. Further inert gasses are noble gasses, e.g. argon and helium. It is particularly preferred that the nitriding atmosphere does not comprise any oxidising species, such as CO₂, O₂ and N₂O. It is further preferred that the nitriding atmosphere does not comprise oxygen containing species. By avoiding oxidising species and oxygen containing species, it is ensured that the diffusion zone obtained in the method does not contain interstitial oxygen or oxygen in solid solution. However, components prepared according to the method may still have unavoidable oxide layers on the surface, since nanometre scale oxide layers form naturally on the surface of Group IV metals in contact with air. In the context of the disclosure, nanometre scale oxide layers are considered to not provide any negative effects, and a component having a naturally formed nanometre scale oxide layer is considered to be substantially free from oxide layers. Correspondingly, Group IV metals can contain unavoidable amounts of oxygen dissolved in the Group IV metal, and when the Group IV metal contains unavoidable amounts of oxygen dissolved in the Group IV metal, the Group IV metal is substantially free from interstitial oxygen.

The nitriding atmosphere may further comprise a gaseous species providing a specific functionality. The nitriding atmosphere may for example comprise a carbon containing gaseous species. In general, carbon may be dissolved in the Group IV metal, but the carbon containing gaseous species should be present at a concentration too low for the formation of carbide, and also carbonitrides, in the Group IV metal. In general, carbide formation will be avoided when the first temperature does not exceed 700° C. Exemplary carbon containing gaseous species are alkanes, e.g. methane, alkenes, e.g. ethylene, and alkynes, e.g. acetylene. It is also contemplated that carbon containing gaseous species may contain nitrogen, however a carbon containing gaseous species should not contain oxygen.

It is preferred that the nitriding atmosphere does not comprise oxygen containing species. However, for certain species containing oxygen, the oxygen will not be available for dissolving significant amounts of oxygen in the Group IV metal. For example, CO may be included to provide a high carbon activity but without dissolving oxygen in the Group IV metal. It is furthermore contemplated that urea (H₂NCONH₂) may be used as a nitriding species or to provide NH₃. For example, urea may be heated to provide a mixture of NH₃, CO and other species, and the mixture may be used as the nitriding atmosphere.

The hydrogen containing diffusion zone is formed in the nitriding step at a first temperature in the range of 450° C. to 750° C. The nitriding duration will generally depend on the first temperature and at a first temperature in the range of 450° C. to 580° C., the nitriding step will generally be undesirably slow, but if the first temperature is at least 580° C., e.g. at least 581° C. or at least 585° C., the workpiece will be nitrided at a more acceptable rate. When the first temperature is in the range of 700° C. to 750° C., undesirable grain growth may be observed for the Group IV metal. Thus, when grain growth is unacceptable, the first temperature should be up to 700° C., e.g. in the range of 450° C. to 700° C., or in the range of 580° C. to 700° C., e.g. 582° C. to 700° C. However, the increased amount of hydrogen in the hydrogen containing diffusion zone allows that the grain refinement provided in the diffusive treatment can be sufficient to avoid negative effects of grain growth above 700° C. The present disclosure thus provides a method allowing hardening of a Group IV metal at a temperature above 700° C. with a decreased risk of undesirable grain growth. Preferred combinations of the first temperatures and nitriding durations are shown in Table 1.

TABLE 1
Preferred combinations of first temperatures and nitriding durations
First temperature  Nitriding duration
450° C. to 580° C. 2 hours to 200 hours
580° C. to 700° C. 0.5 hours to 100 hours
700° C. to 750° C. 0.1 hour to 50 hours

Particularly preferred combinations of the first temperature and the nitriding duration are 450° C. to 580° C. and at least 24 hours, e.g. up to 200 hours, respectively; 580° C. to 700° C. and at least 12 hours, e.g. at least 16 hours or at least 20 hours, and up to 100 hours, respectively; and 700° C. to 750° C. and at least 1 hour, and up to 50 hours, respectively. A most preferred combination of the first temperature and the nitriding duration is 600° C. to 700° C. and at least 12 hours, e.g. at least 16 hours, respectively.

It is preferred that the nitriding atmosphere does not comprise oxidising species. However, in an example, the workpiece is treated to dissolve oxygen in the Group IV metal prior to treatment in the nitriding step or after treatment in the diffusive step. For example, the Group IV, e.g. the workpiece of a Group IV metal or a Group IV metal alloy in its final shape or the workpiece of the Group IV metal or the Group IV metal alloy after treatment in the nitriding step and the diffusive step, may be treated to have an oxygen diffusion zone with interstitial oxygen, the oxygen diffusion zone having a thickness from the surface in the range of 10 μm to 100 μm. The oxygen diffusion zone may for example extend from the surface of the Group IV metal to a depth from the surface where the microhardness is equal to the core hardness of the Group IV metal plus 50 HV_0.005. Near the surface, e.g. at a depth of 5 μm, the oxygen diffusion zone may have a microhardness in the range of 600 HV_0.005 to 800 HV_0.005. The oxygen diffusion zone may for example be provided by initially oxidising the Group IV metal to provide an oxide layer on the Group IV metal and subsequently treating the Group IV metal with the oxide layer in a vacuum treatment to dissolve the oxygen from the oxide layer in the Group IV metal. Alternatively, the workpiece of the Group IV metal or the Group IV metal alloy having been treated in the nitriding step and the diffusive step may be treated in an oxidising atmosphere to dissolve oxygen in the Group IV metal or the Group IV metal alloy or to form an oxide layer on the surface of the Group IV metal or the Group IV metal alloy followed by treatment at a low partial pressure of the oxidising species in the oxidising atmosphere, e.g. in vacuum. The conditions, e.g. the temperature, duration and partial pressures of active species, the oxidising species, for the oxidising step and the dissolution step, may be as for the present nitriding step and the present diffusive step with respect to the oxidising species. When the Group IV metal is provided with an oxygen diffusion zone, the nitriding step followed by the diffusive step, the hydrogen depleted diffusion zone will contain both oxygen and nitrogen.

Group IV metals, and their alloys, can be described in terms of their hardness. Group IV metals can be hardened by dissolution of nitrogen, oxygen and other elements in the metal, but regardless of any case hardening, the Group IV metal will have a core hardness. Thus, the core hardness generally corresponds to the hardness, e.g. the surface hardness, of the Group IV metal before case hardening. The hardness is generally measured according to the DIN EN ISO 6507 standard. The core hardness generally depends on the specific Group IV metal, but when the Group IV metal has been treated in the method of the invention, the surface hardness will be at least 200 HV_0.025 units higher than the core hardness. It is preferred that the surface hardness is analysed using a load of up to 50 g, i.e. HV_0.05, although any value for surface hardness will be HV_0.025 unless noted otherwise. Surface hardness values obtained using loads of up to 50 g, e.g. HV_0.01, HV_0.025 or HV_0.005, are considered to be representative also for the HV_0.025-value. Grade 2 titanium will typically have a core hardness of about 200 HV_0.025, and Grade 5 titanium will typically have a core hardness of about 300 HV_0.025. In the nitriding step, a diffusion zone is obtained, which extends from the surface of the Group IV metal to a depth, where the microhardness is equal to the core hardness plus 50 HV_0.005, so that the diffusion zone has a thickness calculated from the depth, where the microhardness is equal to the core hardness plus 50 HV_0.005 to the surface of the Group IV metal.

In general, the depth will depend on the first temperature and the nitriding duration, but also on the partial pressure of NH₃. The higher the first temperature and the longer the nitriding duration, the greater the depth. In general, a case hardening is obtained already when the depth is about 1 μm, but it is preferred that the depth is up to about 50 μm. The depth will typically be in the range of 10 μm to 30 μm. Thus, the case hardened Group IV metal or Group IV metal alloy will have a diffusion zone with nitrogen in solid solution to a depth of up to 50 μm, e.g. in the range of 10 μm to 30 μm. The microhardness in the diffusion zone will increase from a microhardness equal to the core hardness plus 50 HV_0.005 to the hardness value observed at the surface, e.g. from about 300 HV_0.005 to a value of up to 1000 HV_0.005 or higher. In particular, when the surface has a (Ti,Zr)N layer, the hardness can be in the range of 1000 HV_0.005 to 1500 HV_0.005 at a depth from the surface of 2.5 μm. In general, workpieces treated in the method of the invention, e.g. components of the invention, have a very high surface hardness, but the depth from the surface at which the very high hardness is obtained does not need to be particularly high, and therefore the components are described in terms of a microhardness, HV_0.005, obtained at a depth from the surface of 2.5 μm.

In the diffusive step, hydrogen is diffused out of the hydrogen containing diffusion zone, i.e. removed from the hydrogen containing diffusion zone. Hydrogen will generally diffuse out of the hydrogen containing diffusion zone when the partial pressure of any hydrogen containing species is low, and in the present method the diffusive step is defined i.a. in terms of the partial pressure of H₂ (pH₂). The diffusive step may be performed at a second temperature of up to 750° C., but for practical reasons the second temperature will normally be at least 200° C. For example, the second temperature may be in the range of 300° C. to 750° C., in particular in the range of 600° C. to 700° C. Hydrogen containing gaseous species, e.g. NH₃, can dissociate into H₂ and other gasses as relevant from other elements of the gaseous species and thereby pH₂ can be defined even though H₂ is not included in the diffusive step, so that pH₂ is appropriate to define the diffusive step. For example, FIG. 1 illustrates pH₂ as a function of the NH₃:N₂ ratio at different total pressures. In particular, pH₂ should be as low as possible, e.g. pH₂ may be up to 10⁻⁵ mbar, or pH₂ may be up to 10⁻⁶ mbar, and the diffusion step may be performed in a vacuum. In the context of the disclosure, “vacuum” means that the total pressure is up to 10⁻⁴ mbar although the composition of the atmosphere is not limited, except in order to ensure that pH₂ is up to 10⁻⁴ mbar. The conditions in the nitriding step and the diffusive step can be selected independently. The partial pressure of H₂ may be controlled as desired. For example, pH₂ may equal the total pressure so that the total pressure is up to 10⁻⁴ mbar, or pH₂ may be reduced by supplementing the nitriding atmosphere with an inert gas and optionally also reducing the total pressure.

The nitriding step and the diffusive step may be performed in the same furnace. For example, the nitriding atmosphere may be evacuated directly to provide pH₂ of up to 10⁻⁴ mbar, or the nitriding atmosphere may be replaced with an inert atmosphere, e.g. N₂ or argon, before evacuating the furnace. When the nitriding atmosphere is replaced with an inert atmosphere it is easier to ensure that a sufficiently low pH₂ is obtained, since NH₃ is removed from the furnace. It is also possible to include further steps between the nitriding step and the diffusive step. For example, the workpiece may be removed from the furnace where the nitriding step is performed and cooled to ambient temperature before conducting the diffusive step in the same or another furnace. In addition to ensuring removal of hydrogen from the hydrogen containing diffusion zone, the diffusive step also redistributes interstitial nitrogen and nitrogen in the nitride layer. In particular, nitrogen will migrate to a greater depth and thereby push the interface between the core and the diffusion zone to a greater depth. For example, when a Ti15Zr alloy was treated in the nitriding step, the Ti15Zr alloy had a hardness of <400 HV_0.005 at a depth of 30 μm, but when the Ti15Zr alloy had been subjected to the diffusive step, the hardness of <400 HV_0.005 was observed at a depth of 50 μm (see FIG. 2 and FIG. 3).

For the diffusive step, even small amounts of contaminants may provide undesirable colourations and other unwanted effects on the surface of the workpiece, e.g. contaminants may prevent that the Group IV metal retains its metallic lustre, and it is therefore preferred that the partial pressure of NH₃, and other unwanted species, e.g. CO₂, O₂ and N₂O, is controlled by reducing the total pressure in the diffusive step to be up to 10⁻⁴ mbar and/or by only including inert gaseous species resulting in partial pressures of the respective species, in particular pH₂, of up to 10⁻⁴ mbar. It is especially preferred that the partial pressure of NH₃, and also of any oxidising species, such as CO₂, O₂ and N₂O, is lower, e.g. up to 10⁻⁵ mbar or up to 10⁻⁶ mbar. In general, inert gasses, such as argon and N₂, contain sufficient amounts of contaminants, especially O₂, wherefore it is preferred that only very pure forms of the inert gasses are included in the diffusive step when it is intended for the Group IV metal to retain its metallic lustre.

The present method allows that the workpiece after treatment according to the method regains its metallic lustre so that a component of the invention cannot be differentiated from the workpiece before treatment by visual inspection, although when a surface layer of (Ti,Zr)N is intended, a metallic lustre is not available. Thus, when the workpiece has a mirror polish appearance the mirror polish appearance will also be found on the component after treatment in the method. In the context of the invention, a “mirror polish appearance” is defined as a surface with an arithmetical mean deviation (Ra) roughness of <0.1 μm in accordance with the ISO 1302:2002 standard. For example, the Ra value can be measured using a Taylor-Hubson Surtronic S25 measuring over a length of 1.25 mm. A mirror polish surface may also be referred to as an N3 surface, and the two terms can be used interchangeably. In a preferred embodiment, the workpiece of a Group IV metal is polished prior to nitriding the Group IV metal to provide a surface roughness of <0.1 μm in accordance with the ISO 1302:2002 standard. The surface roughness of <0.1 μm will also be observed for the workpiece after the diffusion step. Furthermore, the nitriding atmosphere should not contain carbon containing molecules when a mirror polish appearance is relevant. When the nitriding atmosphere is not supplemented with further carbon containing molecules a commercially pure (CP) titanium, e.g. Grade 2 or Grade 4, can be provided with a surface hardness of at least 1100 HV_0.005 while retaining the mirror polish appearance. Thus, the method of the disclosure provides a titanium component having a mirror polish appearance with a surface hardness of at least 1100 HV_0.005.

In general, the higher the temperature in the diffusive step, i.e. the higher the second temperature, the faster the diffusion. It is therefore preferred that the second temperature is as high as possible while still preventing undesirable grain growth. It is therefore preferred that the second temperature of up to 700° C., e.g. the second temperature may be in the range of 600° C. to 700° C. Exemplary combinations of second temperatures and hydrogen removal durations, which cover both removal and retainment of the nitride layer, are shown in Table 2. However, in order to ensure removal of the nitride layer, the second temperature should be at least 600° C., and the hydrogen removal durations should be at least 4 hours.

TABLE 2
Exemplary combinations of second temperatures
and hydrogen removal durations
Second temperature Hydrogen removal duration
450° C. to 600° C. 1 hour to 500 hours
600° C. to 650° C. 0.5 hours to 200 hours
650° C. to 700° C. 0.5 hours to 100 hours
700° C. to 750° C. 0.1 hour to 50 hours

In general, the nitriding duration and the hydrogen removal duration can be considered in combination, and the present inventors have surprisingly found that when the nitriding duration is at least 12 hours, e.g. at least 16 hours, in particular at a first temperature in the range of 650° C. to 700° C., a sufficient amount of nitrogen is dissolved into the workpiece of the Group IV metal or the Group IV metal alloy that the nitride layer can be removed, i.e. by using a hydrogen removal duration of at least 4 hours at a second temperature of at least 600° C., while retaining a nitrogen diffusion zone providing a hardened component not having a nitride layer, in particular a visible nitride layer, on its surface. In particular, the method of the invention provides that when a component of titanium or a titanium-based alloy not containing zirconium is treated in the method, the component obtains a surface hardness in the range of 700 HV_0.005 to 2000 HV_0.005 but without a layer of TiN. Correspondingly, when a component of zirconium or a Group IV metal alloy containing at least 2 wt % zirconium is treated in the method, the component the component obtains a hardness in the range of 1000 HV_0.005 to 1500 HV_0.005 at a depth from the surface of 2.5 μm but without having a nitride layer so that the hardness is obtained with requiring a nitride layer.

The present inventors have furthermore surprisingly found that when a component of titanium or a titanium-based alloy not containing zirconium is treated in the method, the treated component retains a surface nitride layer of Ti₂N but without TiN. In the present context, removal of nitride from a component therefore does not include removal of Ti₂N from a component of titanium or a titanium-based alloy not containing zirconium. Ti₂N cannot visually be differentiated from the titanium or titanium alloy, whereas TiN has a golden or yellow colour. Likewise, nitrides of zirconium or zirconium alloys also have a golden or yellow colour. By removing the golden or yellow nitride layer, the component regains the metallic lustre it had before the treatment while also being hardened by having a nitrogen diffusion zone. Thus, the invention provides a hardened component of a Group IV metal, e.g. titanium or a titanium-based alloy not containing zirconium, zirconium or a zirconium alloy, which has a metallic lustre. Moreover, the component does not have a golden or yellow colour associated with a nitride of a Group IV metal.

In an example, the nitriding step and the diffusive step are repeated for the same workpiece. The nitriding step and the diffusive step may be repeated any number of times as desired. In general, the first repetition will increase the surface hardness compared to the surface hardness of a workpiece treated only a single time in the nitriding step and the diffusive step.

Any Group IV metal or Group IV metal alloy is appropriate for the present method. In specific embodiments the Group IV metal is selected from the list of titanium, titanium alloys, zirconium and zirconium alloys. In the context of the invention the component may consist of a Group IV metal or Group IV metal alloy, e.g. a titanium alloy or a titanium-zirconium alloy, or it may comprise other materials. For example, the component may have a part being of another material, a polymer, glass, ceramic or another metal, and an outer layer of the titanium alloy or zirconium. Likewise, a workpiece treated in the method of the invention may also have a centre of another material. The outer layer need not completely cover the outer surface of the component. The component may for example be prepared from additive manufacturing or 3D printing prior to be treated according to the method of the disclosure.

The present inventors have surprisingly found that the presence of zirconium will affect the available case hardening when NH₃ is employed to harden a Group IV metal or a Group IV metal alloy. Regardless of the Group IV metal, e.g. titanium or zirconium, NH₃ will diffuse atomic N and atomic H into the Group IV metal at the low temperature and form the hydrogen, and nitrogen, containing diffusion zone. The present inventors have particularly found that when zirconium is present at a content of at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium, the zirconium containing Group IV metal alloy, especially a zirconium containing titanium-based alloy, allows a nitrogen uptake at least 5 times, e.g. about 10 times, larger than a Group IV metal alloy not containing zirconium, e.g. titanium of Grade 2, Grade 4 or Grade 5, and they have further observed that such a high nitrogen uptake is not available when a zirconium containing titanium-based alloy is nitrided using N₂, e.g. at a temperature of at least 800° C. Thus, nitriding zirconium or a titanium-based alloy containing at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium using NH₃ as a nitriding species at a first temperature in the range of 450° C. to 750° C. provides a nitrogen uptake at least 5 times larger than for nitriding the alloy using N₂ as a nitriding species at a temperature of at least 800° C. The present disclosure provides a solution to the problem of how to increase the nitrogen uptake of zirconium or a titanium-based alloy containing at least 2 wt % zirconium. In addition to the increased content of nitrogen dissolved in the hydrogen containing diffusion zone, nitrides of the zirconium containing titanium-based alloy will be present on the surface of the titanium-based alloy after the nitriding step, and the specific nitrides may be dissolved or retained in modified form in the diffusive step.

In a specific example, the Group IV metal alloy is zirconium or a Group IV metal alloy containing at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium, e.g. with other optional metals, e.g. the titanium-based alloy contains zirconium in the range of 10 wt % to 20 wt %. For example, the Group IV metal may be a titanium-based alloy with at least 5 wt % zirconium or a zirconium-based alloy, e.g. pure zirconium. Exemplary zirconium containing Group IV metals and Group IV metal alloys are Zr702 zirconium, titanium/niobium alloys, e.g. Ti13Nb13Zr, and Ti15Zr (alpha alloy). When the Group IV metal alloy, e.g. a titanium-based alloy, contains at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium the nitriding step will provide a much greater content of nitrogen in the diffusion zone and also a nitride layer of zirconium nitride (ZrN) and titanium nitride (TiN), e.g. (Ti,Zr)N, when the alloy is a titanium-based alloy or contains titanium at the surface of the zirconium containing Group IV metal alloy. This nitride layer can be retained or removed in the diffusive step. In particular, when the zirconium content is in the range of 10 wt % to 20 wt %, a hardness of at least 1000 HV_0.05 at a depth of 2.5 μm is obtained.

The nitride layer formed on the surface of a zirconium containing Group IV metal in the nitriding step can be retained or removed in the diffusive step, although when the component is of titanium or a titanium-based alloy not containing zirconium the component will retain a surface nitride layer of Ti₂N but without TiN regardless of the second temperature. In general, the nitride layer can be retained by performing the diffusive step at a low second temperature. For example, the nitride layer may be retained by performing the diffusive step at a significantly lower second temperature than the first temperature, e.g. the first temperature is in the range of 650° C. to 700° C., and the second temperature is in the range of 400° C. to 600° C. Thus, when removal of the nitride layer is intended the second temperature should be at least 600° C., and when retainment of the nitride layer is intended the second temperature should be up to 600° C. When removal of the nitride layer is intended, the hydrogen removal duration should be sufficient to ensure removal of the nitride layer, but when retainment of the nitride layer is intended, the hydrogen removal duration should be limited to prevent removal of the nitride layer. In general, it is possible to stop the diffusive step, e.g. by lowering the temperature, and inspect the progress of the removal or retainment of the nitride layer and subsequently restart the diffusive step.

It is possible to remove the nitride layer at a temperature lower than 600° C., although this will generally involve performing the hydrogen removal step for an extended period of time and monitoring the status of the removal of the nitride layer. Thus, in a specific example, the step of removing hydrogen from the hydrogen containing diffusion zone comprises selecting a second temperature in the range of 400° C. to 600° C. and removing hydrogen from the hydrogen containing diffusion zone at the selected second temperature at a partial pressure of H₂ (pH₂) of up to 10⁻⁴ mbar for a hydrogen removal duration sufficient to ensure removal of the nitride layer, except for Ti₂N when the Group IV metal is titanium or a titanium alloy not comprising zirconium. The removal of the nitride layer may be confirmed using any method as desired, e.g. X-ray diffraction (XRD) analysis. In general, the nitride layer can be removed with a hydrogen removal duration of at least 48 hours when the second temperature in the range of 500° C. to 550° C., or with a hydrogen removal duration of at least 24 hours when the second temperature in the range of 550° C. to 600° C. The step of removing hydrogen from the hydrogen containing diffusion zone may also comprise monitoring the presence of the nitride layer, e.g. using XRD analysis.

Particularly preferred combinations of the second temperature and the hydrogen removal duration are 600° C. to 650° C. and at least 4 hours, e.g. up to 200 hours, respectively; 650° C. to 700° C. and at least 4 hours, e.g. up to 100 hours, respectively; 700° C. to 750° C. and at least 2 hours, e.g. up to 50 hours, respectively.

The most preferred combination of conditions in the nitriding step and the hydrogen removal step are: 580° C. to 700° C. and at least 12 hours, e.g. at least 16 hours or at least 20 hours, and up to 100 hours, for the first temperature and the nitriding duration, respectively, in the nitriding step, and 650° C. to 700° C. and at least 4 hours, e.g. up to 100 hours, for the second temperature and the hydrogen removal duration, respectively. When the first temperature is in the range 580° C. to 700° C. and the nitriding duration is in the range of 16 hours to 100 hours, a sufficient amount of nitrogen is dissolved in the Group IV metal, and when second temperature is in the range of 650° C. to 700° C., and the hydrogen removal duration is in the range of 4 hours to 100 hours, the nitride layer, except for Ti₂N when the Group IV metal is titanium or a titanium alloy not comprising zirconium, is removed while retaining the hardening obtained from the nitrogen in solid solution in the nitrogen diffusion zone.

In other aspects, the disclosure relates to components obtainable in the methods of the disclosure. Thus, in another aspect the disclosure relates to a component of zirconium or a Group IV metal alloy containing at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium, the component having a core hardness and a nitrogen containing diffusion zone extending from a surface of the component to the depth from the surface, where the microhardness is equal to the core hardness plus 50 HV_0.005, the component having a hardness in the range of 1000 HV_0.005 to 1500 HV_0.005 at a depth from the surface of 2.5 μm. The component does not comprise a nitride layer, although it is also contemplated that the nitride layer can be retained, e.g. when the removal of hydrogen is performed at a temperature of up to 600° C.

The component can also be obtained, so that the component further comprises a nitride layer at the surface of the component. The nitride layer comprises ZrN and also TiN when titanium is present in the alloy. In particular, ZrN and TiN are mixable and isomorphic so that the nitride layer can also be described as (Ti,Zr)N. When a nitride layer is retained on the component, the surface hardness of the component is generally higher than when the nitride layer is removed, although the diffusion zone near the surface will have the highest nitrogen content and a hardness of at least 1000 HV_0.05. In general, the component with the (Ti,Zr)N layer has a surface hardness in the range of 1000 HV_0.005 to 2000 HV_0.005.

In examples of both of these components, the diffusion zone does not comprise interstitial oxygen or dissolved oxygen beyond the naturally unavoidable amounts of oxygen, and in particular, the hardness of the diffusion zone is attributable to the dissolved nitrogen content. In particular, the component may be substantially free from interstitial oxygen or dissolved oxygen. Without being bound by theory, the present inventors believe that there is a direct correlation between the content of nitrogen in the diffusion zone and the hardness of the diffusion zone so that the hardness will increase over the diffusion zone from the core of the zirconium or the Group IV metal alloy containing at least 2 wt %, e.g. at least 3 wt % or at least 5 wt % zirconium to the surface of the component. The component furthermore preferably 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, so that it is substantially free from any oxide layers. However, in other examples, the diffusion zones of the components also comprise interstitial oxygen. Interstitial oxygen may be provided by treating a workpiece to dissolve oxygen in the zirconium or the Group IV metal alloy containing at least 2 wt % zirconium prior to treatment in the nitriding step.

It is preferred that the component is from a titanium-based alloy containing 10 wt % to 20 wt % zirconium, although the titanium-based alloy may also contain further elements. Exemplary Group IV metals and alloys include Zr702 zirconium, Ti13Nb13Zr, and Ti15Zr.

In a further specific example, the Group IV metal alloy is a titanium-based alloy not containing zirconium or pure titanium. Exemplary Group IV metals and Group IV metal alloys are commercially pure (CP) titanium, e.g. Grade 2 or Grade 4, Grade 5 titanium, which is also known as Ti6Al4V, or Ti6Al4V ELI also known as Grade 23. When a titanium-based alloy not containing zirconium or pure titanium is treated in the nitriding step, the nitriding step provides the alloy or the titanium with a golden colour that is representative of TiN, and upon treatment in the diffusive step, the diffusive step removes the golden colour and returns the treated workpiece of the titanium-based alloy not containing zirconium or the pure titanium to its original appearance, including the metallic lustre of the workpiece prior to the nitriding treatment. The present inventors have surprisingly found that the golden coloured nitride layer contains both TiN and Ti₂N, but that the diffusive step removes TiN without removing Ti₂N, so that the method provides a surface nitride layer of Ti₂N (see e.g. FIG. 5 and FIG. 6), which has a high hardness, e.g. the component has a surface hardness in the range of 700 HV_0.005 to 2000 HV_0.005, e.g. 1000 HV_0.005 to 1800 HV_0.005, but which cannot visually be differentiated from the titanium-based alloy not containing zirconium or the pure titanium before treatment in the nitriding step. The component will typically have a microhardness in the range of 700 HV_0.005 to 1200 HV_0.005 at a depth from the surface of 2.5 μm. The present inventors have in particular found that XRD analysis of the treated workpiece revealed a layer of Ti₂N on the surface of the workpiece. An exemplary XRD plot for a treated workpiece is shown in FIG. 6. The XRD plot can be compared to an XRD plot obtained after the nitriding step but without the diffusive step, e.g. in FIG. 5. Comparison of FIG. 6 with FIG. 5 shows that Ti₂N is present after both steps, but after the diffusive step, the XRD plot no longer shows TiN peaks. The Ti₂N layer is sufficiently hard, e.g. a surface hardness of at least 1000 HV_0.005, for the workpiece to be scratch resistant. Thus, the present disclosure provides a scratch resistant workpiece of titanium or a titanium alloy having a metallic lustre.

In another aspect the disclosure relates to a component of titanium or a titanium-based alloy not containing zirconium, the component having a core hardness and a nitrogen containing diffusion zone extending from a surface of the component to the depth from the surface, where the microhardness is equal to the core hardness plus 50 HV_0.005, the component having a surface with a layer of Ti₂N, in particular a surface nitride layer of Ti₂N. The component has a surface hardness in the range of 700 HV_0.005 to 2000 HV_0.005. The layer of Ti₂N is obtainable by nitriding a workpiece with NH₃ in the nitriding step at a first temperature in the range of 450° C. to 750° C. for a nitriding duration of at least 12 hours, e.g. at least 16 hours, followed by a diffusive step at a second temperature in the range of 600° C. to 750° C. and a partial pressure of H₂ of up to 10⁻⁴ mbar over a hydrogen removal duration of at least 4 hours. It is preferred that the surface does not comprise TiN. In particular, both Ti₂N and TiN can be identified using XRD, and in a specific example, the component comprises Ti₂N as identifiable by XRD. In a further specific example, the component does not comprise TiN as identifiable by XRD. It is most preferred that the component comprises Ti₂N and does not comprise TiN as identifiable by XRD.

It is further preferred that the component does not have a golden colour, and also that it has a metallic lustre. For example, the component may have a mirror polish appearance defined as a surface with an arithmetical mean deviation (Ra) roughness of <0.1 μm in accordance with the ISO 1302:2002 standard.

In an example of the component of titanium or the titanium-based alloy not containing zirconium, the diffusion zone does not comprise interstitial oxygen or dissolved oxygen, and in particular, the component may be substantially free from interstitial oxygen or dissolved oxygen, and the hardness of the diffusion zone is attributable to the nitrogen content. Without being bound by theory, the present inventors believe that there is a direct correlation between the content of nitrogen in the diffusion zone and the hardness of the diffusion zone so that the hardness will increase over the diffusion zone from the core of the titanium or a titanium-based alloy not containing zirconium to the surface of the component. However, in another example, the diffusion zone of the component also comprises interstitial oxygen. Interstitial oxygen may be provided by treating a workpiece to dissolve oxygen in the titanium or the titanium-based alloy not containing zirconium prior to treatment in the nitriding step or after treatment in the diffusive step.

The component has a surface layer of Ti₂N, which provides the surface with scratch resistance. In general, scratch resistance is considered present when the surface has a hardness of at least 1000 HV_0.025. The component may, for example, have a surface hardness in the range of 700 HV_0.005 to 2000 HV_0.005, e.g. in the range of 1000 HV_0.005 to 1800 HV_0.005. It is especially preferred that the component has a surface hardness in the range of 1000 HV_0.005 to 1800 HV_0.005 and a mirror polish in accordance with the ISO 1302:2002 standard.

Any embodiment of the invention may be used in any aspect of the invention, and any advantage for a specific embodiment applies equally when an embodiment is used in a specific aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 shows pH₂ as a function of the NH₃:N₂ ratio at different total pressures,

FIG. 2 shows a hardness profile of Ti15Zr hardened using NH₃;

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

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

FIG. 5 shows an XRD analysis of Ti6Al4V hardened using NH₃;

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

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

FIG. 8 shows a microscopy image of a cross-section of titanium hardened using NH₃;

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

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

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.

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.

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.

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”.

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).

In the context of the invention the hardness is generally the HV_0.005 or HV_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.

In general, microhardness measurements may be performed at a load of 5 g, i.e. HV_0.005, 25 g, i.e. HV_0.025, or 50 g, i.e. HV_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_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_0.005, and surface hardness values are obtained as top-down measurements using a load of 25 g, i.e. HV_0.025.

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_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

Two samples of Ti15Zr alloy were provided and nitrided in NH₃ in a Netzsch 449 Thermal analyzer (furnace). The samples were heated to 690° C. at a rate of 20° C./min and exposed to NH₃ 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 <10⁻⁴ mbar total pressure, provided with an Edwards 85 T-station turbo vacuum pump, at 690° C. for 4 hours.

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.

The treated samples were analysed for hardness (HV_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_0.005 at 2.5 μm below the surface (FIG. 2, FIG. 3). A core hardness of <400 HV_0.005 was first reached at a depth of about 50 μm for the sample treated according to the disclosure (FIG. 2). For the sample not subjected to the diffusive step this hardness <400 HV_0.005 was already reached at 30 μm (FIG. 3).

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

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

A sample of Ti13Zr13Nb was provided and nitrided in NH₃ in a Netzsch 449 Thermal analyzer. The sample was heated to 690° C. at a rate of 20° C./min and exposed to NH₃ at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10⁻⁴ 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_0.005 at 2.5 μm below the surface and a core hardness of <500 HV_0.005 was reached at a depth of about 35 μm from the surface.

Example 3

A sample of Ti15Zr was provided and nitrided in NH₃ in a Netzsch 449 Thermal analyzer. The sample was heated to 690° C. at a rate of 20° C./min and exposed to NH₃ at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10⁻⁴ 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_0.025, and a hardness of >1100 HV_0.005 at 2.5 μm 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

A sample of Ti13Zr13Nb was provided and nitrided in NH₃ in a Netzsch 449 Thermal analyzer. The sample was heated to 690° C. at a rate of 20° C./min and exposed to NH₃ at ambient pressure for 20 hours before treating the sample in vacuum, i.e. in <10⁻⁴ 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_0.025, and a hardness of >900 HV_0.005 at 2.5 μm for the surface. The core hardness of <400 HV_0.005 was reached at about 40 μm below the surface.

Example 5

Two samples of Ti6Al4V (alpha-beta alloy) were nitrided in NH₃ 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_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 Ti₂N and TiN.

One of the nitrided samples was subjected to the diffusive step. Specifically, the sample was treated in vacuum, i.e. in <10⁻⁴ 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.

Comparison of FIG. 6 with FIG. 5 documents that TiN disappeared while Ti₂N was still detected.

The sample treated according to the present disclosure had a reduced surface hardness of >1300 HV_0.005 compared to the sample not exposed to the diffusive step (i.e. >2000 HV_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

Two samples of commercially pure (CP) titanium were provided and nitrided in NH₃ in a Netzsch 449 Thermal analyzer. The samples were heated to 690° C. at a rate of 20° C./min and exposed to NH₃ at ambient pressure for 20 hours before treating one sample in vacuum, i.e. in <10⁻⁴ mbar total pressure, at 690° C. for 4 hours and not subjecting the other sample to further treatment.

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.

The hardness profile of the sample treated according to the present method is shown in FIG. 7, and reveals a surface hardness >1200 HV_0.005, and a hardness of >800 HV_0.005 at 2.5 μm below the surface, thereby providing the sample with scratch resistance. A hardness of <300 HV_0.005 was reached at about 20 μm below the surface. In FIG. 7, the error bars represent 1 standard deviation from the mean

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.

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 NH3 as a nitriding species at a first temperature in the range of 450° C. to 750° C. and a partial pressure of NH3 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 H2 (pH2) of up to 10−4 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 10−4 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 HV0.005, the component having a hardness in the range of 1000 HV0.005 to 1500 HV0.005 at a depth from the surface of 2.5 μm, 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 HV0.005 to 2000 HV0.005, 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 HV0.005, as measured according to the DIN EN ISO 6507 standard, the component having a surface nitride layer of Ti2N.

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 Ti2N 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 HV0.005 to 1200 HV0.005 at a depth from the surface of 2.5 μm.

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 μm 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 μm 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.