Low temperature titanium hardening

Abstract

The invention relates to a method of oxygen hardening of a Group IV metal, comprising the steps of: providing a workpiece of a Group IV metal in its final shape; oxidizing the Group IV metal in an oxidizing atmosphere at a first temperature using a gaseous oxidizing species having an upper temperature limit of up to 800°C for an oxidation duration of at least 10 minutes to provide a non-delaminated Group IV metal oxide on the surface of the workpiece, wherein the first temperature is in the range of 500°C and the upper temperature limit of the gaseous oxidizing species; diffusing oxygen from the non-delaminated Group IV metal oxide into the Group IV metal in an inert atmosphere at a second temperature in the range of 500°C to 800°C and at a partial pressure of the gaseous oxidizing species of up to 10"4mbar for a diffusion duration of at least 0.1 hour to provide a surface diffusion zone comprising oxygen in solid solution. In another aspect, the invention relates to a Group IV metal component comprising a material core having a core hardness and a surface hardness of at least the core hardness + 200 HV 0.025 The component is obtainable in the method of the invention.

Description

TECHNICAL FIELD

[0001] The present invention relates to the low temperature hardening of titanium and other Group IV metals. In particular, a method of oxygen hardening of a Group IV metal is provided, as is a hardened Group IV metal component. The method and component are useful for any application where a hard metal can be used. The components of the present invention are particularly useful where the aesthetics of the metal are important, for example in watches, jewellery and spectacles. PRIOR ART

[0002] It is known that titanium and other Group IV metals can be hardened by interstitial oxygen and that there is a direct relationship between the oxygen content and the hardness, the higher the oxygen content the harder the metal. It is further known that the oxygen hardening of titanium should be carried out at as low a temperature as possible to avoid grain growth and general deformation of the material.

[0003] Accordingly, JPH 08134625 discloses a method of forming and surface hardening a titanium or titanium alloy sheet. The method employs a forming gas containing CO2 gas and by carrying out gas pressure forming, a titanium sheet can be formed into a predetermined shape and at the same time a hardened layer is formed. The method is carried out between 700°C and 1100°C, although preferably at a temperature below the beta transus point. Oxidation is excessive above 1100°C.

[0004] JPH 059703 discloses a surface hardening treatment method for titanium materials. The titanium material is heat treated in an atmosphere containing CO2 gas which is reduced and decomposed into oxygen and carbon, so the surface of the titanium material is hardened by solid solution strengthening with these as interstitial elements. A temperature of 700°C or higher is used and a curing time of preferably within 10 hours.

[0005] JPH 08104970 discloses a surface hardening treatment for titanium materials with the aim of easily forming a thick surface hardened layer without roughening the surface. The titanium surface is hardened by heating in an atmosphere composed of CO2 gas and an inert gas or nitrogen and then in an inert gas or vacuum to a predetermined heating temperature. In particular, it is found that by heating at a specific partial pressure of CO2 gas, oxygen and carbon atoms permeate to the surface of the titanium material and heat treatment in an inert gas or vacuum diffuses the oxygen and carbon into the interior, thereby forming a thick hardened layer with small surface roughness. In an example, a pure titanium is heated in a mixed gas of CO2 and argon at 800°C for 3 hours and then a diffusion heat treatment is carried out in a vacuum at 850°C for 3 hours.

[0006] JPH 1192911 aims to cure a titanium member at a temperature that does not cause surface roughness, but forms a Vickers hardness of at least 750 at a depth of 1 μm from the surface without forming a colored substance on the surface. In this process, the member is heated to 700°C to 800°C and treated in a reduced pressure atmosphere containing a nitrogen component and an oxygen component. Nitric oxide, nitrogen dioxide or nitrous oxide, or ammonia gas can be used as the nitrogen component, and water or oxygen can be used as the oxygen component. For example, the titanium member can be treated at 700°C-800°C under 0.3 torr of nitrous oxide for 1 to 10 hours.

[0007] US 6,221,173 discloses titanium or titanium alloys having a hard surface layer comprising a first hard layer containing nitrogen in solid solution in the range of 0.6 to 8.0 wt% and oxygen in solid solution in the range of 1.0 to 14.0 wt%, and a second hard layer beneath the first hard layer containing oxygen in solid solution in the range of 0.5 to 14.0 wt%. The hard surface layer can be provided by treating the alloy at 650°C to 830°C in a nitrogen atmosphere with a trace amount of oxygen, water, carbon dioxide or carbon monoxide at a total pressure of 1.33 to 1330 Pa. The length of the treatment time is from 1 to 10 hours. Thereby, nitrogen and oxygen diffuse into the interior of the titanium or titanium alloy without forming nitrides and oxides of titanium.

[0008] US 2003 / 041922 discloses a method of strengthening a titanium alloy to improve wear resistance. The method comprises heating the titanium alloy at 600°C to 900°C in a CO2 atmosphere to diffuse C and O atoms into the titanium alloy without forming titanium oxide. The preferred temperature range is 800°C to 850°C, and in particular at temperatures above 900°C, titanium oxide is formed. The reaction time can be 0.5 to 50 hours.

[0009] WO 97 / 14820 discloses a method of treating titanium parts at temperatures between 1450°F and 1850°F in a mixture of nitrogen, methanol and optionally natural gas or propane at atmospheric pressure. Thereby, a coating consisting of areas of titanium oxides and oxynitrides and sometimes some carbonitrides is formed on the surface of the alloy. The parts can then be treated at 1200°F to 1450°F in a vacuum furnace to reduce the hydrogen level in the parts to increase the yield strength.

[0010] WO 2008 / 154593 and WO 2007 / 078427 disclose a zirconium medical implant having a diffusion hardened zone and optionally also a ceramic zone having a layered structure, and a method of manufacturing a surface hardened medical implant. For example, a zirconium niobium alloy sample is oxidized at 635°C in air and then treated in a vacuum furnace.

[0011] WO 2017 / 207794 discloses a surface hardened component of a titanium alloy having a diffusion zone comprising oxygen and carbon in solid solution and having a distinct phase of a carbon oxide.

[0012] EP 0931848 discloses a decorative hardened titanium material, a method of processing a titanium material, and a method of processing a decorative titanium material. The hardened surface layer comprises nitrogen and oxygen and has a surface grain size in the range from 0.1 to 60 pm.

[0013] WO 99 / 04055 discloses a method of surface hardening an article formed of titanium, zirconium or an alloy of titanium and / or zirconium. The method involves heat treating the article in an oxidizing atmosphere to form an oxide layer and then heat treating the article in a vacuum to diffuse oxygen from the oxide layer into the article.

[0014] In view of the methods disclosed by the prior art, there is still a need for an improved method of hardening titanium or other Group IV metals and alloys thereof. SUMMARY

[0015] In a first aspect, the present invention relates to a method of oxygen hardening a Group IV metal, the method comprising the steps of:

[0016] - providing a workpiece of a Group IV metal in its final shape;

[0017] - oxidizing the Group IV metal in an oxidizing atmosphere at a first temperature using a gaseous oxidizing species selected from the group of CO2, N2O and a combination of CO2 and N2O for an oxidation duration of at least 10 minutes to provide a non-delaminated Group IV metal oxide on the surface of the workpiece, the gaseous oxidizing species having an upper temperature limit of up to 800 °C, wherein the first temperature is in the range of 500 °C and the upper temperature limit of the gaseous oxidizing species;

[0018] - diffusing oxygen from the non-delaminated Group IV metal oxide into the Group IV metal in an inert atmosphere at a second temperature in the range of 500 °C to 800 °C and at a partial pressure of the gaseous oxidizing species of up to 10 -4 millibar for a diffusion duration of at least 0.1 hour to provide a surface diffusion zone comprising oxygen in solid solution.

[0019] In a second aspect, the present invention relates to a Group IV metal component comprising a material core having a core hardness and a surface layer comprising a surface hardness of at least the core hardness + 200 HV 0.025surface hardness, a diffusion zone having a thickness in the range of 10 to 100 pm from the surface with oxygen in solid solution in a range providing a level of 120% of the hardness of the hardness of the material core to the saturation level of the Group IV metal, the diffusion zone further containing carbon and / or nitrogen in solid solution, the concentration of which shows a local maximum of carbon content and / or nitrogen content in the diffusion zone, as detectable by glow discharge optical emission spectroscopy (GDOES).

[0020] The Group IV metal component of the second aspect is obtainable in the method of the first aspect. In particular, the Group IV metal component of the second aspect is obtainable when the oxidizing atmosphere comprises carbon-containing molecules or nitrogen-containing molecules, or both carbon-containing molecules and nitrogen-containing molecules.

[0021] Any Group IV metal is suitable for use in both aspects of the present application. In particular embodiments, the Group IV metal is selected from the list of titanium, titanium alloys, zirconium and zirconium alloys. In the context of the present application, the component can consist of the Group IV metal, e.g. a titanium alloy, or it can comprise other materials. For example, the component can have a core of another material, a polymer, a glass, a ceramic or another metal and an outer layer of titanium alloy or zirconium. Likewise, the workpiece treated in the method of the present application can also have a core of another material. The outer layer need not cover the outer surface of the component completely. The component can be prepared, e.g. by additive manufacturing or 3D printing, prior to being treated in the method of the present application.

[0022] Group IV metals and their alloys can be described in terms of their hardness. Group IV metals can be hardened by dissolving oxygen in the metal, but regardless of the oxygen hardening, the Group IV metal will have a core hardness. The core hardness thus corresponds to the hardness of the Group IV metal before hardening, e.g. the surface hardness. The core hardness is generally dependent on the specific Group IV metal, but after the Group IV metal has been treated in the method of the present application, the surface hardness will be at least 200 HV 0.025 units higher than the core hardness. Surface hardness is preferably analyzed using a load of up to 50 g, i.e. HV 0.05 , although any surface hardness value will be HV 0.025 unless otherwise stated. Surface hardness values obtained using a load of up to 50 g, e.g. HV 0.01 , HV 0.025 or HV 0.005 , are considered to be representative of HV 0.025 values as well. Grade 2 titanium will typically have a core hardness of about 200 HV 0.025 , so Grade 2 titanium hardened according to the present application will have a surface hardness of at least 400 HV 0.025 . Grade 5 titanium will typically have a core hardness of about 300 HV 0.025 , so Grade 5 titanium hardened according to the present application will have a surface hardness of at least 500 HV0.025 However, in general, higher surface hardness is obtained by the method of the present application. In particular embodiments, the surface hardness is at least 650 HV 0.025 For any Group IV metal, a surface hardness of at least 650 HV 0.025 is obtained. Preferably, the surface hardness is at least 700 HV 0.025 , for example at least 800 HV 0.025 .

[0023] The method of the present application allows the workpiece after treatment according to the method to regain its metallic luster, such that the component of the present application cannot be distinguished from the workpiece before treatment by visual inspection. Thus, when the workpiece has a mirror polished appearance, the mirror polished appearance will also be present on the component after treatment in the method. In the context of the present application, "mirror polished appearance" is defined as a surface having an arithmetic average deviation (Ra) roughness according to the ISO 1302:2002 standard of < 0.1 μιη. For example, the Ra value can be measured using a Taylor-Hubson Surtronic S25 measuring over a length of 1.25 mm. A mirror polished surface can also be referred to as an N3 surface, and the two terms can be used interchangeably. In preferred embodiments, the workpiece of the Group IV metal is polished before oxidizing the Group IV metal to provide a surface roughness according to the ISO 1302:2002 standard of < 0.1 μιη. A surface roughness of < 0.1 μιη will also be observed for the workpiece after the diffusion step. For Group IV metals, in particular titanium, a mirror polished appearance can generally be obtained, but when the workpiece contains aluminum, for example 5-grade titanium, the first temperature should not be higher than 700 °C, such that the method provides a mirror polished appearance. However, preferably, the Group IV metal does not contain aluminum when a mirror polished appearance is relevant. Furthermore, when a mirror polished appearance is relevant, the oxidizing atmosphere should not contain carbon-containing molecules other than CO2. When the gaseous oxidizing species is CO2and the oxidizing atmosphere is not supplemented with further carbon-containing molecules, commercial purity (CP) titanium, for example 2-grade or 4-grade, can be provided with a surface hardness of at least 1100 HV 0.025 while retaining the mirror polished appearance. Thus, the method of the present application provides a titanium component having a mirror polished appearance with a surface hardness of at least 1100 HV 0.025 . The presence of further carbon-containing molecules as unavoidable impurities does not affect the mirror polished appearance.

[0024] The inventors of the present invention have surprisingly found that if the Group IV metal comprises aluminum as alloying element, the appearance of the treated metal will be more dull than when a workpiece made of a Group IV metal not comprising aluminum is treated. Without being bound by theory, the inventors of the present invention believe that during the oxidation step, aluminum forms an oxide that is thermodynamically more stable than the Group IV metal, thereby preventing all oxygen from diffusing into the Group IV metal during the diffusion step. However, unavoidable aluminum impurities do not pose a problem for the formation of a metallic luster on the Group IV metal. The inventors of the present invention have further observed that a higher surface hardness can be obtained when aluminum is present than when no aluminum is present. In a preferred embodiment, the Group IV metal does not comprise aluminum as alloying element. In a particularly preferred embodiment, the Group IV metal is a CP Group IV metal, such as titanium of grade 2 and 4, or Zr702 zirconium. In another embodiment, the Group IV metal comprises aluminum, for example the Group IV metal can be titanium of grade 5 (also known as Ti6Al4V), or Ti6Al4VELI (also known as grade 23). When the Group IV metal is a CP grade, it is possible to obtain a mirror polished appearance after treatment in the method of the present invention. The treated workpiece of the present invention or the component of the present invention can comprise aluminum, for example as a separate part from the Group IV metal, while the Group IV metal does not comprise aluminum as alloying element. In an embodiment of the present invention, the component of the present invention, for example titanium of grade 2 or 4, has a mirror polished appearance. When the oxidation atmosphere contains other carbon-containing molecules than CO2, it can not be possible to obtain a mirror polished appearance, because the additional carbon, for example CO, tends to provide a darker non-delaminated oxide layer, which can be reflected on the surface of the final component. Therefore, a preferred embodiment is that the oxidation atmosphere does not contain CO. Likewise, using CO2 as oxidation atmosphere can result in a component of the present invention having a mirror polished appearance.

[0025] The method of the present invention provides a Group IV metal layer, for example titanium oxide or zirconium oxide, to the Group IV metal in the oxidation step, which acts as a reservoir for oxygen atoms, which diffuse into the Group IV metal in the diffusion step to form interstitially dissolved oxygen in the Group IV metal. The oxidation step can also be referred to as the first step. The inventors of the present invention have surprisingly found that it is possible to form an intermediate non-delaminated oxide layer, which is so robust and stably bound to the Group IV metal that oxygen atoms can diffuse into the Group IV metal to form a surface diffusion zone comprising oxygen in solid solution. The diffusion occurs in the diffusion step, which follows the oxidation step, and the diffusion step can also be referred to as the second step. Therefore, the non-delaminated oxide layer will effectively be removed from the surface, in its place a diffusion zone, and the Group IV metal will be restored to its metallic state on the surface, but with a greatly increased surface hardness, i.e. at least 200 HV 0.025 units, for example at least 650 HV0.025 or at least 800 HV 0.025 or at least 1000 HV 0.025 Even if the oxide layer is removed in the diffusion step, a natural oxide layer will inevitably be formed on the surface of the hardened Group IV metal. The naturally formed oxide layer will be in the nanometer range and will not change the metallic appearance of the hardened Group IV metal.

[0026] The method of the present invention uses a gaseous oxidizing species. Any gaseous oxidizing species that can be in gaseous form can be used in the method. The oxide layer provided in the oxidation step is non-layered, and the present inventors have found that the first temperature and also the oxidation duration can be chosen based on the oxidizing ability of the commonly used gaseous oxidizing species to provide a non-layered oxide layer. In the context of the present invention, a workpiece having a non-layered oxide layer can be referred to as an "intermediate workpiece", and the two terms can be used interchangeably. Layering of the oxide layer indicates that the oxide layer is not sufficiently stable to act as a reservoir for oxygen atoms diffusing into the Group IV metal. In general, the oxidizing ability of CO2, O2 and N2O can be ranked as follows: CO2 < O2 < N2O. Temperatures below 500 °C are too low to form an oxide layer, but for gaseous oxidizing species, an upper temperature limit can be determined. Water vapor, i.e. H2O, is also considered a gaseous oxidizing species. However, when hydrogen containing molecules are present in the oxidation atmosphere, or even in the inert atmosphere, hydrogen will dissolve in the Group IV metal, where the presence of interstitial hydrogen will lead to embrittlement of the Group IV metal. Therefore, it is preferred that neither the oxidation atmosphere nor the inert atmosphere contain hydrogen containing molecules, in particular H2O.

[0027] Regardless of which gaseous oxidizing species is chosen, it is preferred that the first temperature is in the range of 600 °C to 700 °C. At a first temperature in the range of 500 °C to 600 °C, the oxidation step will typically be undesirably slow, but if the first temperature is at least 600 °C, the workpiece will be oxidized at an acceptable rate. When the first temperature is in the range of 700 °C to 800 °C, i.e. as far as possible for the chosen gaseous oxidizing species, grain growth of the Group IV metal can be observed, and therefore the first temperature should be up to 700 °C, e.g. in the range of 500 °C to 700 °C, or in the range of 600 °C to 700 °C, when grain growth is not acceptable.

[0028] The oxidation step has an oxidation duration. The oxidation duration is determined by the time the workpiece is treated at the first temperature with the gaseous oxidizing species. The workpiece can be heated prior to exposure to the gaseous oxidizing species, for example from ambient temperature to the first temperature, or the workpiece, for example at ambient temperature, can be exposed to the gaseous oxidizing species at the first temperature. When the workpiece is heated prior to exposure to the gaseous oxidizing species, the workpiece can be heated in an inert atmosphere, in vacuum or in the gaseous oxidizing species, preferably minimizing the time the workpiece is left at a temperature from 500°C to the first temperature.

[0029] Generally, the thickness of the oxide layer is determined by the oxidation duration, and taking into account the oxidizing power of the gaseous oxidizing species employed. The longer the oxidation duration, the thicker the oxide layer, and generally, the dissolution of the element into the Group IV metal is considered to be parabolic, so that doubling the dissolution depth requires four times as long reaction duration. The composition of the oxide layer will generally be MeO2, where "Me" is the Group IV metal, for example TiO2or ZrO2, although traces of other elements, for example carbon and / or nitrogen or other metals from an alloy of the Group IV metal can also be present. The thickness of the diffusion zone will therefore depend proportionally on the thickness of the oxide layer. An oxide layer of only 1 pm thickness is considered sufficient to provide a diffusion zone of a thickness to provide a surface hardness of at least 200 HV 0.025 above, regardless of which Group IV metal is treated in the method. An oxide layer of at least 2 pm thickness is considered sufficient to provide a diffusion zone of a thickness to provide a surface hardness of at least 650 HV 0.025 However, it is preferred that the oxidation duration is sufficient to provide a thickness of the non-delaminated oxide layer in the range of 5 pm to 15 pm. The oxidation duration for providing a non-delaminated oxide layer having a thickness in the range of 5 pm to 15 pm can be determined by examining a cross-section of the intermediate workpiece.

[0030] Once the oxide layer is formed, the intermediate workpiece can be stored for a desired time before being treated in the diffusion step. The non-delaminated oxide layer is so stable that normal handling of the intermediate workpiece does not affect the non-delaminated oxide layer. The method of the present invention is therefore very flexible, as the oxidation step can be performed in a first furnace, for example a furnace without vacuum capacity, and transferred to another furnace with vacuum capacity. This advantageously allows for optimization in terms of scheduling coordination for producing the component of the present invention. Furthermore, the stability of the non-delaminated oxide layer and the intermediate workpiece allows for the conditions of the diffusion step to be chosen independently of the choice of the oxidizing gaseous species employed in the oxidation step.

[0031] CO2, as a gaseous oxidizing species, is the most benign gaseous oxidizing species, enabling the formation of a stable non-delaminated oxide layer at any temperature of 500 °C or higher, without considering the oxidation duration. However, to limit the deformation and grain growth of the Group IV metal, the upper temperature limit for CO2is 800 °C. For practical reasons and to limit the thickness of the oxide layer, the oxidation duration using CO2may be as long as 16 hours, although a stable non-delaminated oxide layer will also be formed at longer oxidation durations. For CO2, the oxidation duration is preferably in the range of 1 hour to 16 hours.

[0032] Among the commonly used gaseous oxidizing species, N2O is the most aggressive gaseous oxidizing species, and N2O has an upper temperature limit of 700 °C. Also, a non-delaminated oxide layer can be formed faster compared to CO2, and for N2O, the preferred oxidation duration is in the range of 10 minutes to 2 hours. In a certain embodiment, N2O is preferred as a gaseous oxidizing species because the first temperature can be lower, for example in the range of 500 °C to 650 °C, which further reduces the risk of deformation and grain growth of the Group IV metal.

[0033] It is also contemplated that O2 can be used as a gaseous oxidizing species. However, when O2 is used as a gaseous oxidizing species, the diffusion zone will not contain carbon and / or nitrogen in solid solution from the oxidation treatment. O2 has a higher oxidizing power compared to CO2, and the upper temperature limit is 750 °C. The higher oxidizing power of O2 also limits the oxidation duration compared to CO2. Therefore, if the gaseous oxidizing species is O2, the oxidation duration is limited to 3 hours to prevent the formation of a delaminated oxide layer. In the case of O2 as a gaseous oxidizing species, an oxidation duration of 30 minutes is considered to provide a non-delaminated oxide layer of sufficient thickness to provide a surface hardness of at least 1000 HV0.2after the diffusion step. The oxidation duration for O2 as a gaseous oxidizing species is preferably in the range of 30 minutes to 3 hours. 0.025

[0034] Figure 1 and Figure 2 Cross sections of oxide layers provided using CO2and N2O at different temperatures are shown. Specimens of CP 4-grade titanium were treated in CO2or N2O at ambient pressure, and the cross sections were analyzed with a microscope, and the results are shown in Figure 1 and Figure 2 Thus, treatment in CO2for all tested temperatures and for all tested times resulted in the formation of a strong and stable titanium oxide layer. Figure 2 Treatment of 4-grade titanium in N2O at 780 °C or higher resulted in the formation of a strongly delaminated oxide layer, while treatment at 680 °C resulted in the formation of a stable non-delaminated oxide layer. Figure 15 ​The boundary of when a layered titania layer is formed versus a non-layered titania layer is shown when N20 is used as the gaseous oxidizing species for CP 2 grade titanium, where any combination of first temperature and oxidation duration below the dashed line will provide a non-layered titania layer. Table 1 provides specific combinations of first temperature and oxidation duration for using N20 as the gaseous oxidizing species for treating titanium to provide a non-layered titania layer. The combinations of first temperature and oxidation duration in Table 1 provide a non-layered titania layer, and these combinations are believed to represent the limit, so increasing the first temperature beyond the values in Table 1 while keeping the oxidation duration constant will result in a layered titania layer. For CO2 and N20, preferred combinations of first temperature and oxidation duration when each is used individually as the gaseous oxidizing species are shown in Table 2. When CO2 and N20 are used in combination, the temperature and time for N20 will be used.

[0035] Table 1 Limiting combinations of first temperature and oxidation duration for using N20 as the gaseous oxidizing species for treating titanium.

[0036] Temperature [°C] 600 650 700 750 800 850 Time [hours] 100 25 8 3 1 0.5

[0037] Table 2 Preferred combinations of gaseous oxidizing species, first temperature and oxidation duration

[0038] Gaseous oxidizing substance First temperature Duration of oxidation CO2 500°C to 800°C 1 hour to 16 hours CO2 600°C to 750°C 1 hour to 8 hours CO2 620°C to 680°C 2 hours to 6 hours [N2O] 500°C to 700°C 10 minutes to 2 hours [N2O] 550°C to 600°C 30 minutes to 2 hours

[0039] When O2 is used as the oxidizing species, the first temperature can be in the range of 500 °C and 750 °C and the oxidation duration in the range of 30 minutes to 3 hours, or the first temperature can be in the range of 600 °C and 650 °C and the oxidation duration in the range of 30 minutes to 2 hours.

[0040] It is preferred that the pressure in the oxidizing atmosphere is ambient pressure. It is further preferred that the gaseous oxidizing species is at ambient pressure, i.e. the oxidizing atmosphere does not contain other molecules. However, it is also contemplated that the oxidizing atmosphere can be at ambient pressure and the partial pressure of the gaseous oxidizing species is reduced by the addition of an inert gas, such as argon or helium, or a noble gas. Operation at ambient pressure simplifies the process compared to operation at modified pressure, in particular reduced pressure. When the oxidizing atmosphere contains CO, it is preferred that the pressure of the oxidizing atmosphere is ambient pressure.

[0041] The oxidizing atmosphere can also contain further molecules which can provide atoms diffusing into the group IV metal and which can be incorporated into the non-delaminated oxide layer. For example, the oxidizing atmosphere can be ambient air, wherein O2 is the gaseous oxidizing species and thus present at a partial pressure of about 20% or about 0.2 atm and mixed with N2 at a partial pressure of about 80% or about 0.8 atm. The presence of N2 can increase the hardness of the treated workpiece, although the dissolution of nitrogen atoms in N2 can alter the appearance of the surface, so that a mirror polished appearance will not be obtained in case N2 is present in the oxidizing atmosphere. Therefore, it is preferred not to use atmospheric air and likewise, it is preferred not to include N2 in the oxidizing atmosphere. However, the interstitial nitrogen in the component of the invention or in the workpiece treated in the method of the invention provides an increased hardness, i.e. both the surface hardness as well as the cross-sectional hardness in the diffusion zone.

[0042] Oxidizing the group IV metal of the workpiece will increase the volume of the workpiece due to the inclusion of oxygen atoms in the oxide layer. Thus, the intermediate workpiece will have a larger volume than the untreated workpiece. However, the inventors have now surprisingly found that when the intermediate workpiece is treated in the diffusion step, the oxygen atoms will diffuse into the group IV metal and restore the component to its original dimensions in the untreated state prior to the first oxidation step. However, to ensure that the component of the invention has the same dimensions as the untreated workpiece, the thickness of the non-delaminated oxide layer should be limited to 50 pm, in particular 25 pm. Thus, the method of the invention enables hardening of the component of the group IV metal in its final shape without affecting its shape or dimensions. This is particularly relevant when grain growth is also undesirable and the first temperature is up to 700 °C. The component to be treated is in its final shape. When a mirror polished appearance is intended, the component can be polished to provide a mirror polished appearance, i.e. the surface has an Ra roughness of < 0.1 pm, prior to treatment in the method according to the invention. The method advantageously enables that the mirror polished appearance is observed also after treatment, i.e. the surface has an Ra roughness of < 0.1 pm after treatment.

[0043] In the diffusion step, the oxygen in the non-delaminated group IV metal oxide diffuses into the group IV metal and the diffusion step can be independent of the parameters employed in the oxidation step. However, to prevent further oxidation of the group IV metal, the partial pressure of the gaseous oxidizing species should be as low as possible. For example, the partial pressure of the gaseous oxidizing species can be up to 10 -4 millibar, but is preferably lower, for example up to 10 -5 millibar or up to 10 -6 millibar.

[0044] The diffusion step can be carried out in vacuum. In the context of the present invention, “vacuum” means a pressure of up to 10 -410 mbar, although the composition of the atmosphere is not limited. In another embodiment, the diffusion step is performed in an inert atmosphere. In the context of the present invention, an "inert atmosphere" is an atmosphere that is free of components that will interact with the Group IV metal, except for unavoidable impurities. A preferred inert atmosphere is a noble gas, such as argon. It is preferred that the diffusion step is performed in vacuum, as this will more easily ensure that the hardened part retains its metallic luster after being processed in the method of the present invention. The present inventors have surprisingly observed that when the pressure in the diffusion step is higher than 10 -4 mbar, contaminants present in the furnace, including gaseous residues, can prevent the reformation of a metallic luster on the part after being processed in the method of the present invention.

[0045] The diffusion step is performed at a second temperature in the range of 500 °C to 800 °C for a diffusion duration of at least 0.1 hour, such as at least 1 hour. The diffusion duration should be sufficient to remove the non-delaminated oxide layer of the intermediate workpiece by diffusing oxygen atoms into the Group IV metal. The diffusion duration is not limited. However, if the diffusion duration is prolonged too much, oxygen can eventually be distributed too uniformly in the Group IV metal to enable the Group IV metal to have a diffusion zone that provides sufficient hardness. Therefore, the diffusion duration should generally not be longer than 100 hours. The diffusion duration will depend on the thickness of the non-delaminated oxide layer, and the thicker the non-delaminated oxide layer, the longer the diffusion duration to remove the non-delaminated oxide layer. Therefore, the diffusion duration can be considered to be determined by the choice of gaseous oxidizing species, and also the first temperature and the oxidation duration. The diffusion duration is not related to the choice of Group IV metal.

[0046] In general, the higher the temperature, the faster the diffusion, and because the workpiece is not oxidized by a gaseous oxidizing species, the second temperature is less limited than the first temperature. Therefore, it is possible that the second temperature is higher than the first temperature. Therefore, when the second temperature is higher than the first temperature, the hardening can be completed faster. In an embodiment, the second temperature is higher than the first temperature. In a further specific embodiment, the second temperature is in the range of 600 °C to 750 °C, such as 650 °C to 700 °C. When grain growth is not desirable, a second temperature of up to 700 °C can be chosen, as a second temperature of up to 700 °C will prevent grain growth of the Group IV metal and also deformation. When the second temperature is in the range of 650 °C to 750 °C, the diffusion duration will typically be in the range of 2 hours to 40 hours.

[0047] In a particular embodiment, the gaseous oxidizing species is CO2, the first temperature is in the range of 600°C to 750°C, the oxidation duration is in the range of 1 hour to 8 hours, the second temperature is in the range of 650°C to 750°C, and the diffusion duration is in the range of 2 to 8 times the oxidation duration, for example in the range of 2 hours to 64 hours. In another embodiment, the gaseous oxidizing species is CO2, both the first temperature and the second temperature are in the range of 650°C to 700°C, wherein the second temperature is higher than the first temperature, the oxidation duration is in the range of 2 hours to 6 hours, and the diffusion duration is in the range of 3 to 6 times the oxidation duration, for example in the range of 6 hours to 36 hours.

[0048] In other embodiments, the gaseous oxidizing species is N2O and / or O2, the first temperature is in the range of 600°C to 650°C, the oxidation duration is in the range of 30 minutes to 2 hours, the second temperature is in the range of 650°C to 700°C, and the diffusion duration is in the range of 4 to 20 times the oxidation duration, for example in the range of 2 hours to 40 hours.

[0049] In particular embodiments, the oxidizing atmosphere further comprises CO. For example, the gaseous oxidizing species can be CO2 and the oxidizing atmosphere can contain both CO and CO2. The presence of carbon in the oxidizing atmosphere will dissolve carbon in the Group IV metal and it is believed that the carbon, even at trace levels, provides a more stable bonding of the oxide layer and the diffusion zone between the core of the Group IV metal and the oxide layer, thereby forming an extremely stable non-delaminated oxide layer. When the gaseous oxidizing species is CO2, without the addition of CO to the oxidizing atmosphere, a more stable non-delaminated oxide layer is formed compared to when the oxidizing is O2 or N2O. However, when O2 or N2O is the gaseous oxidizing species and the oxidizing atmosphere is supplemented with CO, carbon will also dissolve in the Group IV metal to provide a more stable non-delaminated oxide layer. Other carbon-containing molecules are also contemplated to dissolve carbon in the Group IV metal, although hydrogen-containing molecules (e.g., alkanes) should be avoided to prevent embrittlement of the Group IV metal.

[0050] When the oxidizing atmosphere contains CO2 and CO, CO2 and CO will participate in Reaction 1 and Reaction 2, determined below.

[0051] Reaction 1 CO(g) + ½ O2(g) = CO2(g)

[0052] Reaction 2 2CO(g) = CO2(g) + C

[0053] In general, CO is believed to be a carbon-providing molecule without oxidizing ability, while CO2 has oxidizing potential but limited carbon activity. The partial pressure of O2 (pO2) and carbon activity (aC) present in Reactions 1 and 2 are determined by the partial pressure of the gaseous oxidizing species and the partial pressure of CO, if present. c) can be determined from Equation 1 and Equation 2. Thus, the partial pressure of O2 is:

[0054] Equation 1

[0055] and the carbon activity is:

[0056]

[0057] where AG1= -282.200 + 86.7T (J), and AG2= -170.550 + 174.3T (J).

[0058] By using a mixture of CO2 and CO, the amount of carbon dissolved into the Group IV metal can be controlled, and thus the oxidation atmosphere can be tailored to ultimately also tailor the diffusion zone of the component of the present invention. Carbon will typically increase the microhardness of the diffusion zone compared to a diffusion zone comprising oxygen but no carbon. Furthermore, CO is believed to provide a thicker diffusion zone and a thinner oxide layer. The combination of a thinner oxide layer and a thicker diffusion zone is believed to be advantageous in the subsequent step of diffusing oxygen from the non-delaminated Group IV metal oxide into the Group IV metal, as the oxygen atoms in the Group IV metal oxide will more effectively diffuse into the Group IV metal. Furthermore, without being bound by theory, the present inventors believe that carbon in the Group IV metal oxide will further stabilize the non-delaminated oxide layer, resulting in an improved process compared to using only CO2, O2 or N2O as gaseous oxidizing species. In a specific embodiment, the oxidation atmosphere thus has a mixture of CO2 and CO, wherein CO2 comprises 40% to 90% compared to the total amount of CO2 and CO. In another embodiment, the oxidation atmosphere has a mixture of CO2 and CO, wherein CO2 comprises 40% to 60% compared to the total amount of CO2 and CO. Similar ratios apply for other gaseous oxidizing species, such as O2 or N2O. For example, the oxidation atmosphere can have a mixture of O2 and CO, wherein O2 comprises 40% to 60% compared to the total amount of O2 and CO. The oxidation atmosphere can also have a mixture of N2O and CO, wherein N2O comprises 40% to 60% compared to the total amount of N2O and CO.

[0059] Treating the Group IV metal in an oxidizing atmosphere will dissolve oxygen into the Group IV metal, thereby forming a diffusion zone between the core of the Group IV metal and the non-delaminated oxide layer. In the context of the present invention, a "diffusion zone" is any zone, e.g. determined from the surface of the Group IV metal to a certain depth in the Group IV metal, in which oxygen is dissolved in the Group IV metal. The interstitial oxygen will harden the Group IV metal, and thus the diffusion zone can be determined by measuring the hardness of a cross-section of the Group IV metal. After the diffusion step, the oxygen atoms in the non-delaminated oxide layer have diffused into the Group IV metal, and thus only an unavoidable nanometer-thick oxide layer is present on the Group IV metal. The concentration of interstitial oxygen is highest at the surface of the hardened Group IV metal, and decreases with increasing depth. In general, the diffusion zone is considered to extend from the surface of the Group IV metal to a depth at which the cross-sectional hardness is 120% of the hardness of the core of the Group IV metal.

[0060] Preferably, the concentration of interstitial oxygen near the surface of the Group IV metal is close to saturation. Even if the oxygen content near the surface is below the saturation level for oxygen, a surface hardness of 1000 HV 0.025 can be obtained for any Group IV metal. A surface hardness of 650 HV 0.025 is considered to be sufficient to provide scratch resistance to the hardened Group IV metal. A diffusion zone thickness of about 5 μιη will result in a surface hardness of 1000 HV 0.025 . Thus, in a preferred embodiment, the diffusion zone has a thickness of at least 5 μιη. The thickness of the diffusion zone is not limited, but no effect was observed for increasing thickness when the thickness exceeds 50 μιη. Thus, in embodiments, the diffusion zone has a thickness in the range of 5 μιη to 50 μιη, i.e. a hardness of 120% of the core hardness of the Group IV metal can be recorded at a depth of 50 μιη from the surface. Figure 3 A cross-section of a hardened Grade 4 titanium and the corresponding hardness profile are shown; the core hardness is about 230 HV, and a hardness of about 280 HV is observed at a depth of 0.06 mm from the surface, thus the diffusion zone has a thickness of 60 μιη.

[0061] The removal of the non-delaminated oxide layer is easily detected by visual inspection, by analysis of a cross-section, or by GDOES analysis. Visual inspection of the surface of the component will show whether the component has a metallic shine or whether an oxide layer is present on the surface. Figure 4 and Figure 6 Exemplary components of Grade 2 titanium and Grade 23 titanium, respectively, are shown. Figure 4 The corresponding cross-sections are shown in Figure 5 In Figure 4 to Figure 6 (a) shows the untreated workpiece, (b) shows the intermediate workpiece, and (c) shows the component of the present invention.

[0062] Figure 9 and Figure 10 show exemplary GDOES analysis of a 2-grade titanium, respectively, and Figure 12 and Figure 13 show exemplary GDOES analysis of a 5-grade titanium, respectively, with and without the oxide layer removed. The intermediate workpiece has a stable oxygen content at a certain depth from the surface, which is indicative of the presence of an oxide layer. In contrast, the final component will show a decrease in the amount of oxygen starting from the surface, thus exhibiting a diffusion zone.

[0063] When carbon is present in the oxidizing atmosphere, for example when the gaseous oxidizing species is CO2, or when the oxidizing atmosphere is supplemented with CO, carbon will be present in the non-layered oxide layer. Likewise, when nitrogen is present, for example as N2or N2O, nitrogen will be present in the non-layered oxide layer. As will be apparent from Figure 9 and Figure 12 The carbon content and / or the nitrogen content is detectable using GDOES. When oxygen diffuses into the Group IV metal in the diffusion step, carbon will also diffuse into the Group IV metal. However, the distribution of carbon atoms will be different from the distribution of oxygen atoms, and thus, as it is detectable by GDOES analysis, the intensity of carbon will show a peak, i.e. a local maximum of intensity, in the diffusion zone. Likewise, the intensity of nitrogen can show a peak, i.e. a local maximum of intensity, in the diffusion zone. Without being bound by theory, the present inventors believe that the local maximum of carbon intensity represents a peak in the concentration of interstitial carbon in the diffusion zone, which in turn increases the hardness of the diffusion zone at the respective location. Further without being bound by theory, the present inventors believe that this peak in carbon concentration is also reflected in the surface hardness of the component of the present application. The same considerations apply to the nitrogen peak in the diffusion zone, when present. Thus, hardening using CO2as gaseous oxidizing species or when the oxidizing atmosphere is supplemented with CO provides a higher surface hardness than obtainable using a gaseous oxidizing species that does not contain carbon. For example, using CO2as gaseous oxidizing species, a surface hardness of about 1300 HV 0.025 was obtained for a 5-grade titanium, compared to a surface hardness of about 360 HV 0.025 of the untreated metal. For a 2-grade titanium, using CO2as gaseous oxidizing species, a surface hardness of about 1100 HV 0.025 was obtained, compared to a surface hardness of about 360 HV 0.025 of the untreated metal. In a particular embodiment, the component is a 5-grade titanium and has a surface hardness of at least 1300 HV 0.025 . In another embodiment, the component is a 2-grade titanium and has a surface hardness of at least 1100 HV 0.025 .

[0064] A local maximum in carbon intensity is also relevant when the oxidizing atmosphere contains CO, and in this case a higher carbon activity will provide an even harder diffusion zone and also a harder surface. Thus, a surface hardness of at least 800 HV 0.025 for grade 5 titanium and a surface hardness of at least 800 HV 0.025 for grade 2 titanium can be obtained when the oxidizing atmosphere has a mixture of CO2 and CO, where CO2 comprises 40 to 60 % compared to the total amount of CO2 and CO, when the oxidizing atmosphere has a mixture of O2 and CO, where O2 comprises 40 to 60 % compared to the total amount of O2 and CO, or when the oxidizing atmosphere has a mixture of N2O and CO, where N2O comprises 40 to 60 % compared to the total amount of N2O and CO.

[0065] Generally, all variations and features of any aspect and embodiment of the present application can be freely combined. Thus, features described above in relation to the method apply equally to the components of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0066] In the following, the present application will be explained in more detail by means of examples and with reference to the schematic drawings, in which:

[0067] Figure 1 shows an oxide layer after treatment with CO2;

[0068] Figure 2 shows an oxide layer after treatment with N2O and CO2;

[0069] Figure 3 shows the hardness profile and cross-section of a titanium component of the present application;

[0070] Figure 4 shows an untreated workpiece, an intermediate component and a hardened component;

[0071] Figure 5 shows a cross-section of an untreated workpiece, an intermediate component and a hardened component;

[0072] Figure 6 shows an untreated workpiece, an intermediate component and a hardened component;

[0073] Figure 7 shows the hardness profile of a titanium component of the present application;

[0074] Figure 8 shows the glow discharge optical emission spectroscopy (GDOES) profile of an untreated titanium workpiece;

[0075] Figure 9 shows the GDOES profile of an intermediate titanium component;

[0076] Figure 10 GDOES profile of a titanium component of the invention is shown;

[0077] Figure 11 GDOES profile of an untreated titanium workpiece is shown;

[0078] Figure 12 GDOES profile of an intermediate titanium component is shown;

[0079] Figure 13 GDOES profile of a titanium component of the invention is shown;

[0080] Figure 14 Hardness profile and cross-section of a titanium component of the invention is shown;

[0081] Figure 15 Graph showing time versus temperature to provide a non-layered titanium oxide layer is shown;

[0082] Figure 16 Roughness measurement of a mirror polished component of the invention is shown;

[0083] Figure 17 Photographic illustration of a mirror polished component of the invention is shown.

[0084] Reference to the accompanying drawings is made in the description of the invention and should not be construed as limiting the features to the particular embodiments depicted. DETAILED DESCRIPTION

[0085] The present invention relates to a method of oxygen hardening a Group IV metal and to a Group IV metal component having a surface hardness of at least 200 HV 0.025 higher than the core hardness.

[0086] In the context of the present invention, a "Group IV metal" is any metal selected from the titanium group of the periodic table or an alloy comprising at least 50% of a metal of the titanium group. Thus, a "titanium alloy" is any alloy containing at least 50% (atom / atom) titanium and likewise a "zirconium alloy" is any alloy containing at least 50% (atom / atom) zirconium. It is contemplated that any alloy containing at least 50% (atom / atom) of the sum of titanium and zirconium is suitable for the method of the invention and the component of the invention. Likewise, the alloy can also comprise hafnium, which is a member of Group IV of the periodic table, and thus any alloy having at least 50% (atom / atom) of the sum of titanium, zirconium and hafnium is suitable for the present invention.

[0087] Alloys relevant to the present invention can contain any other suitable elements and in the context of the present invention, "alloying elements" can refer to the metallic constituents or elements in the alloy or any component in the alloy. Titanium and zirconium alloys are well known to the person skilled in the art. Alloys of Group IV metals can also comprise metals of other groups in the periodic table, for example aluminium or niobium. An exemplary niobium-containing alloy is Ti13Nb13Zr. Aluminium-containing alloys are Ti6Al4V (Grade 5), which is available in "extra-low interstitial" (ELI) form, Ti6Al4V ELI, commonly referred to as Grade 23.

[0088] In the context of the present invention, any grade of titanium containing at least about 99% (w / w) titanium is considered to be "pure titanium", for example Grade 1 titanium, Grade 2 titanium or Grade 4 titanium; thus, pure titanium can contain up to about 1% (w / w) of trace elements, for example oxygen, carbon, nitrogen or other metals, such as iron. Pure titanium can also be referred to as "commercially pure" (CP). In particular, in the context of the present invention, nitrogen and carbon contained in Group IV metals can represent unavoidable impurities. Elements present as "unavoidable impurities" are considered not to have an impact on the workpiece treated according to the method of the present invention or the component of the present invention. Likewise, in the context of the present invention, any grade of zirconium containing at least about 99% (w / w) zirconium is considered to be "pure zirconium".

[0089] When stating a percentage of a metal or alloy, the percentage is by weight of the material weight, for example expressed as % (w / w), unless otherwise specified. When stating a percentage of an atmosphere, the percentage is by volume, for example expressed as % (v / v), unless otherwise specified. Likewise, the composition of a gas mixture can be on an atomic basis and then can be provided in the form of a percentage or ppm (parts per million), unless otherwise specified.

[0090] In the context of the present invention, hardness is generally HV 0.025 . The unit "HV" refers to this standard, if not otherwise specified. The hardness of a cross-section of, for example, a treated Group IV metal can be recorded and can be recorded relative to the depth of measurement. Hardness measurement of a cross-section can also be referred to as "microhardness" and hardness measurement at the surface can also be referred to as "macrohardness".

[0091] Microhardness measurement is generally independent of the testing conditions, as the measurement is made on the microscale of the cross-section. Microhardness measurement is typically made at a load of 25 g (i.e. HV 0.025 ) or 50 g (i.e. HV 0.05 ). In contrast, macrohardness can be measured from the surface at a much higher load (e.g. 0.50 kg, corresponding to HV 0.5) are performed so that the measured values represent the total hardness value of the respective material and any surface layer contained therein. Microhardness measurements at a load of 25 g or 50 g typically provide the same value "HV", but the measurements are preferably performed at 25 g because this measurement requires less cross-sectional space.

[0092] When hardness is recorded on a cross-section, the measured value is considered to represent a homogenous sample with respect to the direction of the applied pressure. In contrast, when hardness is obtained from a measurement at the surface, the measured value can represent an average of several different hardness values (i.e. at different depths). Thus, when surface hardness is measured at a high load (e.g. 0.50 kg), the value can be considered to provide an "average" value of both the surface and also depths below the surface. Therefore, surface hardness is preferably measured at a load of 25 g or 50 g. When surface hardness is measured at a load of 25 g, a value of 650 HV 0.025 is considered to indicate that the material is scratch resistant. Due to the effect of dissolution of oxygen from the surface, the content of dissolved oxygen will decrease from the surface of the Group IV metal towards the core, and likewise, the hardness will reach a maximum value at the surface.

[0093] Examples

[0094] Example 1

[0095] Specimens of CP 4 grade titanium were provided and treated in a Netzsch STA449 C (furnace) using CO2 or in a Netzsch STA449 F3 (furnace) using N2O as gaseous oxidizing species at ambient pressure at different first temperatures and oxidation durations. After the oxidation treatment, cross-sections of the treated samples were analyzed with a microscope. Thus, Figure 1 it is shown how CO2 as gaseous oxidizing species provides a stable non-delaminated oxide layer at any temperature and duration tested, wherein Figure 1 the temperature and duration are indicated. As expected, the thickness increases with increasing oxidation duration. Thus, Figure 1 it is shown that CO2 as gaseous oxidizing species provides a robust method so that a non-delaminated oxide layer is formed at the lowest temperature tested (730 °C).

[0096] In Figure 2 , cross-sections of oxide layers provided using CO2 or N2O as gaseous oxidizing species are compared. The oxidation duration is 16 hours. Figure 2It is shown that at a temperature of 880°C, CO2provides a thick and stable non-delaminated oxide layer, which illustrates the robustness of using CO2as gaseous oxidizing species. In contrast, treatment with N2O as gaseous oxidizing species results in the formation of delaminated oxide layers at 780°C and higher temperatures. These delaminated oxide layers can even be easily removed using a fingernail and are not suitable for the diffusion of oxygen into titanium. However, at an oxidation temperature of 680°C, both N2O and CO2provide non-delaminated oxide layers, e.g. of about 5 pm thickness, suitable for treatment in the diffusion step.

[0097] Example 2

[0098] A sample of 4-grade titanium was treated with N2O at 600°C for 64 minutes at ambient pressure. This oxidation step was followed by a diffusion step of 4 hours at 750°C in a vacuum of about 10 -6 millibar. This treatment provided the sample with an unaffected surface finish, restoring the metallic luster of the sample. The cross-section of the hardened sample was analyzed with a microscope and the hardness profile was measured. The results are shown in Figure 3 0.025 The 4-grade titanium had a core hardness of about 230 HV 0.025 and a hardness of about 280 HV -3 was observed at a depth of 0.06 mm from the surface, so the diffusion zone had a thickness of 60 pm.

[0099] Example 3

[0100] Workpieces of CP titanium (grade 2) and Ti6Al4V ELI with a diameter of 15 mm and a thickness of 2 mm were treated in two separate steps.

[0101] In the first step (oxidation step), the samples were placed in an MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with CO2. A continuous gas flow of 200 ml / min was used. The workpieces were heated to 650°C at a rate of 12 K / min. The furnace was kept at 650°C for 4 hours, after which the furnace was allowed to cool down to room temperature without assistance.

[0102] In the second step (diffusion step), the cooled workpieces from the first step were placed in a tube furnace with an Edwards 85T-station turbo vacuum pump. The vacuum pump evacuated the furnace chamber to <10 -3 millibar before the furnace was opened. The furnace was heated at a rate of about 50 K / min (>250°C). The furnace was kept at 680°C for 16 hours, after which the furnace was cooled to room temperature at a rate of about 25 K / min. The final pressure in the furnace chamber was <10 -4 millibar.

[0103] Photos of the components are shown inFigure 4 (Level 2) and Figure 6 The diagram (Ti6Al4V ELI) shows an untreated workpiece, an intermediate workpiece, and a component of the present invention. The oxidized surface is clearly visible on the intermediate workpiece (b), while the component (c) has been restored to a metallic luster.

[0104] Figure 4 (Level 2) sections (a), (b) and (c) are in Figure 5 As shown, the black bars correspond to 5 μm. Therefore, the non-layered oxide layer has a thickness of approximately 2 μm, which provides a diffusion region exceeding 5 μm in thickness. The cross-sectional hardness of the Ti6Al4V ELI component is... Figure 7 As shown, this indicates that the diffusion region has a thickness of approximately 15 μm. The untreated Stage 2 exhibits a voltage of approximately 361 HV. 0.025 The surface hardness of untreated Ti6Al4VELI is approximately 450 HV. 0.025 The surface hardness is 1152 HV after hardening. 0.025 and 1382HV 0.025 .

[0105] Example 4

[0106] The sample from Example 3 was further analyzed using glow discharge photoemission spectroscopy (GDOES), and the results were as follows: Figure 8 to Figure 13 As shown in the diagram. Therefore, Figure 8 GDOES analysis of an untreated Grade 2 titanium workpiece is shown. Figure 9 The GDOES analysis of the intermediate level 2 titanium workpiece is shown, and Figure 10 GDOES analysis of the grade 2 titanium component of the present invention is shown. Similarly, Figure 11 GDOES analysis of an untreated Ti6Al4VELI workpiece is shown. Figure 12 The GDOES analysis of the intermediate Ti6Al4V ELI workpiece is shown, and Figure 13 GDOES analysis of the Ti6Al4V ELI component of the present invention is shown.

[0107] GDOES analysis measures the content of a specified element as intensity (in V) over time (in seconds). Therefore, intensity reflects the relative amount of the element, and time reflects the depth from the surface. By analyzing samples for a sufficient time to reflect the composition of the layers associated with the workpiece or component, GDOES analysis appropriately provides a comparison of the composition of untreated workpieces, intermediate workpieces with non-layered oxide layers and diffusion zones between the non-layered oxide layers and the material core, and the core of Group IV metals.

[0108] Thus, Figure 8 and Figure 11 It is shown that the composition of the metal is generally stable over the thickness. Figure 9 and Figure 12 It is shown that the amount of oxygen is generally stable, which represents a non- layered oxide layer, which at higher time values becomes a gradually increasing titanium signal with a corresponding decreasing oxygen signal, which together represent the diffusion zone. At higher time values, the oxygen signal is stable, thus representing the core of the group IV metal. Because CO2is used as gaseous oxidizing species, carbon is also present in the non-layered group IV metal oxide as well as in the diffusion zone below the non-layered metal oxide layer. Figure 10 and Figure 13 The final composition of the 2-grade titanium and Ti6Al4V ELI, respectively, is shown. The carbon signal shows that the carbon intensity increases from the surface, which is shown as a local maximum in the carbon intensity curve. This local maximum is considered to represent the peak of the interstitial carbon concentration in the diffusion zone after removal of the non-layered metal oxide layer, and it is further considered to increase the hardness beyond the hardness that can be obtained in the absence of carbon.

[0109] Example 5

[0110] A workpiece of Ti6Al4V (grade 5) was processed to provide a component of the present application. In particular, the workpiece was produced by 3D printing in the form of a cylindrical workpiece with a diameter of 12 mm and a height of 15 mm, and the workpiece was subsequently processed in two separate steps.

[0111] In the first step, the sample was placed in an MTI OFT-1200 glass tube furnace, the furnace was evacuated and backfilled with CO2. A continuous gas flow of 200 ml / min was used. The workpiece was heated to 650 °C at a rate of 12 K / min. The furnace was kept at 650 °C for 4 hours, after which the furnace was allowed to cool down to room temperature without assistance.

[0112] In the second step (diffusion step), the cooled workpiece of the first step was placed in a tube furnace with an Edwards 85T-station turbo vacuum pump. Before the furnace was opened, the vacuum pump evacuated the furnace chamber to <10 -3 mbar. The furnace was heated at a rate of approximately 50 K / min (>250 °C). The furnace was kept at 680 °C for 16 hours, after which the furnace was cooled down to room temperature at a rate of approximately 25 K / min. The final pressure in the furnace chamber was <10 -4 mbar. The photo and hardness profile of the component are shown in Figure 14 The component showed an increased hardness up to a depth of 20 pm. The hardness profile was similar to the hardness profile observed on a non-3D printed Ti6Al4V component.

[0113] Example 6

[0114] Workpieces with a diameter of 15 mm and a thickness of 2 mm were treated in two separate steps in a variant in which the oxidizing atmosphere comprised CO. The samples were CP titanium of grade 4 and Ti6AI4V (grade 5).

[0115] In the first step, the samples were placed in an MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with C02 / CO in a 50 / 50 ratio using a continuous gas flow of 200 ml / min. The workpieces were heated to 650 °C at a rate of 12 K / min. The furnace was kept at 650 °C for 4 hours, after which the furnace was allowed to cool down to room temperature without assistance.

[0116] In the second step, the cooled workpieces from the first step were placed in a tube furnace with an Edwards 85T-station turbo vacuum pump. Before the furnace was opened, the vacuum pump evacuated the furnace chamber to <10 -3 mbar. The furnace was heated at a rate of approximately 50 K / min (>250 °C). The furnace was kept at 680 °C for 16 hours, after which the furnace was cooled to room temperature at a rate of approximately 25 K / min. The final pressure in the furnace chamber was <10 -4 mbar.

[0117] The provided components showed a surface similar to the surface observed on components oxidized in C02 only, and the provided components recovered their metallic shine after the diffusion step.

[0118] Example 7

[0119] Samples of CP titanium of grade 4 and Ti6AI4V (grade 5) with a diameter of 15 mm and a thickness of 2 mm were treated in a variant in which ambient air at ambient pressure was used as oxidizing atmosphere.

[0120] In the first step, the samples were placed in a Nabertherm LE4 / 11R6 furnace and then heated to 650 °C at a rate of 12 K / min. The furnace was kept at 650 °C for 4 hours, after which the furnace was allowed to cool down to room temperature without assistance.

[0121] In the second step, the cooled samples from the first step were placed in a tube furnace with an Edwards 85T-station turbo vacuum pump. Before the furnace was opened, the vacuum pump evacuated the furnace chamber to <10 -3 mbar. The furnace was heated at a rate of approximately 50 K / min (>250 °C). The furnace was kept at 680 °C for 16 hours, after which the furnace was cooled to room temperature at a rate of approximately 25 K / min. The final pressure in the furnace chamber was <10 -4 mbar.

[0122] The components included interstitial nitrogen, which was reflected in the higher surface hardness. The Grade 4 titanium components had a surface similar to that observed on components oxidized in CO2 only, while the aesthetics of the Grade 5 titanium components were less satisfactory.

[0123] Example 8

[0124] Samples of zirconium (Zr702) and a niobium-containing alloy (Ti13Nb13Zr) were treated at 650°C for 4 hours using CO2 as the gaseous oxidizing species, followed by a diffusion step in vacuum at 680°C for 16 hours, as described in Example 3. The Ti13Nb13Zr workpiece had a diameter of 10 mm and a thickness of 1 mm, while the Zr702 workpiece was a square with a side length of 15 mm and a thickness of 1.5 mm.

[0125] For Ti13Nb13Zr and Zr702, respectively, the treatment resulted in a component surface hardness of about 860 HV 0.025 and about 1218 HV 0.025 , respectively, as compared to a workpiece surface hardness of 264 HV 0.025 and 185 HV 0.025 , respectively, prior to treatment.

[0126] Example 9

[0127] A workpiece having a diameter of 15 mm and a thickness of 2 mm was treated in two steps as outlined in Example 3, followed by an additional anodization step. The workpiece was CP titanium, Grade 4.

[0128] In the third step (anodization step), the component was first cleaned in distilled water, then ethanol, and then turpentine. The component was placed into a solution containing 15% phosphoric acid. A voltage of 70 V was applied for 5 to 10 seconds. After the diffusion step, the component had regained its metallic luster, but the anodization provided a visible oxide layer, which is typical of anodized titanium that has not been treated in the method of the present application.

[0129] Example 10

[0130] An experiment was conducted to determine the boundary between formation of a layered and non-layered titanium oxide layer when N2O was used as the gaseous oxidizing species. Specifically, a sample of CP Grade 2 titanium was provided and treated as done in Example 1.

[0131] After the oxidation treatment, the cross-section of the treated sample was analyzed with a microscope, and the results are plotted in Figure 15 Thus, Figure 15is a plot of time versus temperature, and the markers show the boundaries, so the "region" below the dashed line is the region where a non-delaminated titania layer will form when N20 is used as the gaseous oxidizing species. For example, for N20 as the gaseous oxidizing species, a treatment using an oxidation duration in the range of 10 minutes to 2 hours at a temperature in the range of 500°C to 700°C or a treatment using an oxidation duration in the range of 30 minutes to 2 hours at a temperature in the range of 550°C to 600°C will produce a non-delaminated oxide layer of sufficient thickness so as to subsequently harden the titanium in a diffusion step.

[0132] Example 11

[0133] A CP titanium sample having a thickness of 15 mm was polished to a mirror-like surface finish. Prior to treatment, the CP titanium sample was polished to a mirror-polished surface finish, i.e. having an arithmetic average deviation (Ra) roughness of <0.1 pm (this is in accordance with the ISO 1302:2002 standard). The Ra value was measured using a Taylor Hobson Surtronic S25 measuring over a length of 1.25 mm. The measurement was repeated 12 times on the surface of the sample. The average Ra value was measured to be <0.1 pm and the surface of the sample was accepted as mirror-polished.

[0134] The sample was placed in a Nabertherm 3-zone furnace. The furnace was evacuated and backfilled twice using CO2. The furnace was then heated to 650°C using a continuous flow of 500 ml / min of CO2for 4 hours. The furnace was cooled using furnace cooling. For the second treatment, the sample was placed in a furnace capable of reaching a pressure of <10 -4 millibar. The furnace was heated to 680°C and held at this temperature for 16 hours. The furnace was cooled using furnace cooling.

[0135] The Ra value of the surface of the sample was measured using the same procedure as before the thermo-chemical treatment (described above). The sample still showed a surface roughness with a Ra value of <0.1 pm. The surface roughness is shown in Figure 16 . In Figure 16 , a graphical representation of the surface roughness after treatment as measured by a Taylor Hobson Surtronic S25 is shown. Figure 17 A photograph of the mirror-polished surface after application of the thermo-chemical low temperature hardening is shown. Figure 17 It is shown how the shapes in the picture next to the sample are reflected on the surface of the sample and in particular, the reflection is shown without distortion and the colours in the picture are also reflected on the surface.

Claims

1. A method of oxygen hardening of a Group IV metal, the method comprising the steps of: - providing a workpiece of a Group IV metal in its final shape; - oxidizing the Group IV metal in an oxidizing atmosphere at a first temperature using a gaseous oxidizing species selected from the group of CO2, N2O and a combination of CO2 and N2O for an oxidation duration of at least 10 minutes to provide a non-delaminating Group IV metal oxide on the surface of the workpiece, wherein the first temperature is in the range of 500°C - 680°C when the gaseous oxidizing species is N2O or a combination of CO2 and N2O, or in the range of 500°C - 800°C when the gaseous oxidizing species is CO2; - diffusing oxygen from the non-layered Group IV metal oxide into the Group IV metal at a second temperature in the range of 500°C to 800°C and at a partial pressure of the gaseous oxidizing species up to 10 -4 millibars for a diffusion duration of at least 1 hour to provide a surface diffusion zone comprising oxygen in solid solution.

2. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the gaseous oxidizing species is CO2 and the oxidation duration is in the range of 1 hour to 16 hours.

3. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the gaseous oxidizing species is N2O and the oxidation duration is in the range of 10 minutes to 2 hours.

4. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the pressure in the oxidizing atmosphere is ambient pressure and the oxidizing atmosphere consists of the oxidizing species or consists of the oxidizing species and an inert gas.

5. The method of oxygen hardening of a Group IV metal according to claim 1, wherein neither the oxidizing atmosphere nor the inert atmosphere contains hydrogen containing molecules.

6. The method for oxygen stiffening of Group IV metals according to claim 1, wherein the total pressure in the inert atmosphere is up to 10 -4 mbar.

7. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the inert atmosphere is a noble gas.

8. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the second temperature is in the range of 650°C to 750°C.

9. The method of oxygen hardening of a Group IV metal according to claim 8, wherein the diffusion duration is in the range of 2 hours to 40 hours.

10. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the Group IV metal contains aluminum as alloying element and the first temperature is in the range of 500°C to 700°C.

11. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the oxidizing atmosphere further contains CO.

12. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the workpiece of the Group IV metal is polished prior to oxidizing the Group IV metal to provide a surface roughness according to the ISO 1302:2002 standard of < 0.1 pm.

13. The method of oxygen hardening of a Group IV metal according to claim 1, wherein the gaseous oxidizing species is CO2, the first temperature is in the range of 600°C to 750°C, the oxidation duration is in the range of 1 hour to 8 hours, the second temperature is in the range of 650°C to 750°C and the diffusion duration is in the range of 2 to 8 times the oxidation duration.

14. The method of oxygen hardening of Group IV metals of claim 1, wherein the gaseous oxidizing species is CO2, both the first temperature and the second temperature are in the range of 650°C to 700°C, wherein the second temperature is higher than the first temperature, the oxidation duration is in the range of 2 hours to 6 hours, and the diffusion duration is in the range of 3 to 6 times the oxidation duration.

15. The method of oxygen hardening of Group IV metals of claim 1, wherein the gaseous oxidizing species is N2O, the first temperature is in the range of 600°C to 650°C, the oxidation duration is in the range of 30 minutes to 2 hours, the second temperature is in the range of 650°C to 700°C, and the diffusion duration is in the range of 4 to 20 times the oxidation duration.

16. The method of oxygen hardening of Group IV metals of claim 1, wherein the oxidizing atmosphere is a mixture of CO2 and CO, wherein CO2 is in the range of 40% to 90% compared to the total amount of CO2 and CO.

17. The method of oxygen hardening of Group IV metals of claim 1, wherein the oxidizing atmosphere is a mixture of N2O and CO, wherein N2O is in the range of 40% to 60% compared to the total amount of N2O and CO.

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