LOW TEMPERATURE TITANIUM HARDENING

Abstract

The present invention relates to a method of oxygen hardening a Group IV metal, the method comprising the steps of: providing a workpiece of a Group IV metal in its final shape; oxidising the Group IV metal over an oxidising duration of at least 10 minutes in an oxidising atmosphere at a first temperature to provide a non-stratified Group IV metal oxide on the surface of the workpiece using a gaseous oxidising 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 oxidising species; diffusing oxygen from the non-stratified 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 oxidising species of up to 10-4 mbar over a diffusive duration of at least 0.1 hour to provide a superficial 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.sub.0.025. The component is obtainable in the method of the invention.

Claims

1. A method of oxygen hardening a Group IV metal, the method comprising the steps of: providing a workpiece of a Group IV metal in its final shape; oxidising the Group IV metal over an oxidising duration of at least 10 minutes in an oxidising atmosphere at a first temperature to provide a non-stratified Group IV metal oxide on the surface of the workpiece using a gaseous oxidising species selected from CO.sub.2, N.sub.2O and combinations of CO.sub.2 and N.sub.2O, the gaseous oxidising 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 oxidising species; diffusing oxygen from the non-stratified 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 oxidising species of up to 10.sup.−4 mbar over a diffusive duration of at least 0.1 hour to provide a superficial diffusion zone comprising oxygen in solid solution.

2. The method of oxygen hardening a Group IV metal according to claim 1, wherein the gaseous oxidising species is CO.sub.2, the upper temperature limit is 800° C., and the oxidising duration is in the range of 1 hour to 16 hours.

3. The method of oxygen hardening a Group IV metal according to claim 1, wherein the gaseous oxidising species is N.sub.2O, the upper temperature limit is 700° C., and the oxidising duration is in the range of 10 minutes to 2 hours.

4. The method of oxygen hardening a Group IV metal according to claim 1, wherein the pressure in the oxidising atmosphere is ambient pressure.

5. The method of oxygen hardening a Group IV metal according to claim 1, wherein the first temperature is in the range of 600° C. to 700° C.

6. The method of oxygen hardening a Group IV metal according to claim 1, wherein the total pressure in the inert atmosphere is up to 10.sup.−4 mbar.

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

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

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

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

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

13. The method of oxygen hardening a Group IV metal according to claim 1, wherein the gaseous oxidising species is CO.sub.2, the first temperature is in the range of 600° C. to 750° C., the oxidising 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 diffusive duration is in the range of 2 to 8 times the oxidising duration.

14. The method of oxygen hardening a Group IV metal according to claim 1, wherein the gaseous oxidising species is CO.sub.2, the first temperature and the second temperature are both in the range of 650° C. to 700° C. with the second temperature being higher than the first temperature, the oxidising duration is in the range of 2 hours to 6 hours, and the diffusive duration is in the range of 3 to 6 times the oxidising duration.

15. The method of oxygen hardening a Group IV metal according to claim 1, wherein the gaseous oxidising species is N.sub.2O, the first temperature is in the range of 600° C. to 650° C., the oxidising 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 diffusive duration is in the range of 4 to 20 times the oxidising duration.

16. The method of oxygen hardening a Group IV metal according to claim 1, wherein the oxidising atmosphere is a mixture of CO.sub.2 and CO with 40% to 90% CO.sub.2 compared to the total of CO.sub.2 and CO.

17. The method of oxygen hardening a Group IV metal according to claim 1, wherein the oxidising atmosphere is a mixture of N.sub.2O and CO with 40% to 60% N.sub.2O compared to the total of N.sub.2O and CO.

18. 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.sub.0.025, a diffusion zone having oxygen in solid solution in the range of a level providing a hardness of 120% of the hardness of the material core to the saturation level of the Group IV metal over a thickness from the surface in the range of 10 μm to 100 μm, the diffusion zone further containing carbon and/or nitrogen in solid solution at a concentration showing a local maximum in the diffusion zone of the carbon content and/or the nitrogen content as detectable with Glow Discharge Optical Emission Spectroscopy (GDOES).

19. The Group IV metal component according to claim 18, wherein surface hardness is at least 650 HV.sub.0.025.

20. The Group IV metal component according to claim 18, wherein the component has a surface roughness of <0.1 μm in accordance with the ISO 1302:2002 standard.

21. (canceled)

22. The Group IV metal component according to claim 18, wherein the diffusion zone has a thickness of at least 5 μm.

23. The Group IV metal component according to claim 18, wherein the Group IV metal is selected from the list consisting of titanium, a titanium alloy, zirconium, and a zirconium alloy.

24. The Group IV metal component according to claim 23, wherein the titanium is titanium of grade 2, 4, or 5 or wherein the zirconium is Zr702 zirconium.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0058] 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

[0059] FIG. 1 shows oxide layers from treatment with CO.sub.2;

[0060] FIG. 2 shows oxide layers from treatment with N.sub.2O and CO.sub.2;

[0061] FIG. 3 shows a hardness profile and the cross-section of a titanium component of the invention;

[0062] FIG. 4 shows an untreated workpiece, an intermediary component and hardened component;

[0063] FIG. 5 shows cross-sections of an untreated workpiece, an intermediary component and hardened component;

[0064] FIG. 6 shows an untreated workpiece, an intermediary component and hardened component;

[0065] FIG. 7 shows a hardness profile a titanium component of the invention;

[0066] FIG. 8 shows a Glow Discharge Optical Emission Spectroscopy (GDOES) curve for an untreated titanium workpiece;

[0067] FIG. 9 shows a GDOES curve for an intermediary titanium component;

[0068] FIG. 10 shows a GDOES curve for a titanium component of the invention;

[0069] FIG. 11 shows a GDOES curve for an untreated titanium workpiece;

[0070] FIG. 12 shows a GDOES curve for an intermediary titanium component;

[0071] FIG. 13 shows a GDOES curve for a titanium component of the invention;

[0072] FIG. 14 shows a hardness profile and the cross-section of a titanium component of the invention;

[0073] FIG. 15 shows a plot of time vs. temperature for the provision of non-stratified titanium oxide layers;

[0074] FIG. 16 shows a roughness measurement of mirror polished component of the invention;

[0075] FIG. 17 shows a photographic representation of a mirror polished component of the invention.

[0076] Reference to the figures serves to explain the invention and should not be construed as limiting the features to the specific embodiments as depicted.

DETAILED DESCRIPTION OF THE INVENTION

[0077] 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.sub.0.025 higher than the core hardness.

[0078] 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.

[0079] 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.

[0080] 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”.

[0081] 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).

[0082] In the context of the invention the hardness is generally the HV.sub.0.025 as measured according to the DIN EN ISO 6507 standard. If not otherwise mentioned the unit “HV” thus refers to this standard. The hardness may be recorded for a cross-section, e.g. of a treated Group IV metal, and it may be noted with respect to the depth of the measurement. The hardness measurement in the cross-section may also be referred to as “microhardness”, and the hardness measurement at the surface may also be referred to as “macrohardness”.

[0083] The microhardness measurement is generally independent of the testing conditions, since the measurement is performed at microscale in the cross-section. Microhardness measurements are typically performed at a load of 25 g, i.e. HV.sub.0.025, or 50 g, i.e. HV.sub.0.05. In contrast, the macrohardness may be performed from the surface with a much higher load, e.g. 0.50 kg, corresponding to HV.sub.0.5, so that the measurement represents an overall value of the hardness of the respective material and whatever surface layers it contains. Microhardness measurements at loads of 25 g or 50 g typically provide the same value, “HV”, but measurement at 25 g is preferred since the measurement requires less space in the cross-section.

[0084] When the hardness is recorded at a cross-section the measurement is considered to represent a homogeneous sample with respect to the direction of the pressure applied. In contrast, when the hardness is obtained from measurements at the surface the measurement may represent an average of several different values of hardness, i.e. at different depths. Thus, when the surface hardness is measured at a high load, e.g. 0.50 kg, the value can be considered to provide an “average” value for both the surface and also depths below the surface. It is therefore preferred that surface hardness is measured with a load of 25 g or 50 g. When the surface hardness is measured with a load of 25 g, a value of 650 HV.sub.0.025 is considered to show that the material is scratch resistant. As an effect of the fact that oxygen is dissolved from the surface the content of dissolved oxygen will decrease from the surface towards the core of the Group IV metal, and likewise, the hardness will be maximal at the surface.

EXAMPLES

Example 1

[0085] Specimens of CP Grade 4 titanium were provided and treated in a Netzsch STA449 C (furnace) using CO.sub.2 or a Netzsch STA449 F3 (furnace) using N.sub.2O as the gaseous oxidising species at ambient pressure at different first temperatures and oxidising durations. Following the oxidising treatments, the cross-sections of the treated samples were analysed microscopically. Thus, FIG. 1 shows how CO.sub.2 as the gaseous oxidising species provides stable non-stratified oxide layers at any temperature and duration tested where the temperature and duration are indicated in FIG. 1. As expected the thickness increased with increasing oxidising duration. FIG. 1 thus shows that CO.sub.2 as the gaseous oxidising species provides a robust process allowing formation of the non-stratified oxide layer at the lowest temperature tested, 730° C.

[0086] In FIG. 2 the cross-sections of oxide layers provided using CO.sub.2 or N.sub.2O as the gaseous oxidising species are compared. The oxidising durations were 16 hours. FIG. 2 shows that at a temperature of 880° C., CO.sub.2 provided a thick and stable non-stratified oxide layer, which illustrates the robustness of using CO.sub.2 as the gaseous oxidising species. In contrast, treatment with N.sub.2O as the gaseous oxidising species resulted in formation of stratified oxide layers at 780° C. and higher. These stratified oxide layers can be remove easily using even a finger nail and are not appropriate for diffusing oxygen into the titanium. However, at 680° C. as the oxidising temperature, both N.sub.2O and CO.sub.2 provided non-stratified oxide layers, e.g. of about 5 μm thickness, appropriate for treatment in the diffusion step.

Example 2

[0087] A sample of Grade 4 titanium was treated with N.sub.2O for 64 minutes at ambient pressure at 600° C. This oxidising step was followed by the diffusion step conducted in vacuum at about 10.sup.−6 mbar at 750° C. for 4 hours. The treatment provided the sample with an unaffected surface finish so that the sample regained its metallic lustre. The cross-section of the hardened sample was analysed microscopically, and the hardness profile measured. The results are shown in FIG. 3. The Grade 4 titanium had a core hardness of about 230 HV.sub.0.025, and a hardness of about 280 HV.sub.0.025 was observed at a depth from the surface of 0.06 mm, so that the diffusion zone has a thickness of 60 μm.

Example 3

[0088] Workpieces with diameters of 15 mm and thicknesses of 2 mm of CP titanium (Grade 2) and Ti6Al4V ELI were treated in two separate steps.

[0089] In the first step, the oxidising step, a sample was placed in a MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with CO.sub.2. A continuous gas flow of 200 ml/min was used. The workpiece was heated to 650° C. at a rate of 12 K/m in. The furnace was kept at 650° C. for 4 hours after which the furnace was allowed to cool down to room temperature unaided.

[0090] In the second step, the diffusion step, the cooled workpiece from the first step was placed in a tube furnace with an Edwards 85 T-station turbo vacuum pump. The vacuum pump was allowed to evacuate the chamber to <10.sup.−3 mbar before the furnace was turned on. The furnace heats at a rate of approx. 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 approx. 25 K/min. The end pressure in the furnace chamber was <10.sup.−4 mbar.

[0091] Photos of the components are shown in FIG. 4 (Grade 2) and FIG. 6 (Ti6Al4V ELI), where (a) shows the untreated workpiece, (b) shows the intermediary workpiece, and (c) shows the component of the invention. The oxided surface is clearly visible for the intermediary workpieces (b), whereas the components (c) have regained the metallic lustre.

[0092] The cross-sections of (a), (b) and (c) of FIG. 4 (Grade 2) are shown in FIG. 5, where the black bars correspond to 5 μm. Thus, the non-stratified oxide layer had a thickness of about 2 μm, which provided a diffusion zone with a thickness in excess of 5 μm. The cross-sectional hardness of the component of Ti6Al4V ELI is shown in FIG. 7, which shows that the diffusion zone had a thickness of about 15 μm. The untreated Grade 2 had a surface hardness of about 361 HV.sub.0.025, and the untreated Ti6Al4V ELI had a surface hardness of about 450 HV.sub.0.025. After the hardening, the surface hardnesses were 1152 HV.sub.0.025 and 1382 HV.sub.0.025, respectively.

Example 4

[0093] The samples of Example 3 were further analysed using Glow Discharge Optical Emission Spectroscopy (GDOES) analysis, and the results are shown in FIG. 8 to FIG. 13. Thus, FIG. 8 shows the GDOES analysis for the untreated Grade 2 titanium workpiece, FIG. 9 shows the GDOES analysis for the intermediary Grade 2 titanium workpiece, and FIG. 10 shows the GDOES analysis for the Grade 2 titanium component of the invention. Likewise, FIG. 11 shows the GDOES analysis for the untreated Ti6Al4V ELI workpiece, FIG. 12 shows the GDOES analysis for the intermediary Ti6Al4V ELI workpiece, and FIG. 13 shows the GDOES analysis for the Ti6Al4V ELI component of the invention.

[0094] The GDOES analyses the content of specified elements shown as an intensity (in the unit V) over time (in second). Thus, the intensity reflects the relative amount of the element and the time reflects the depth from the surface. By analysing the sample for a sufficient time to reflect the composition of layers relevant for the workpiece or component, the GDOES analysis appropriately provides a comparison of the compositions of the untreated workpiece, the intermediary workpiece having the non-stratified oxide layer and the diffusion zone between the non-stratified oxide layer and the material core, and the core of the Group IV metal.

[0095] Thus, FIG. 8 and FIG. 11 illustrate how the composition of the metal is generally stable over the thickness. FIG. 9 and FIG. 12 show an approximately stable amount of oxygen, which represents the non-stratified oxide layer, which at higher values of time changes to a gradually increasing titanium signal with a correspondingly decreasing oxygen signal, which together represent the diffusion zone. At higher values for time the oxygen signal stabilises thus representing the core of the Group IV metal. Since CO.sub.2 was used as the gaseous oxidising species carbon is also present in both the non-stratified Group IV metal oxide and also in the diffusion zone below the non-stratified metal oxide layer. FIG. 10 and FIG. 13 show the final components of Grade 2 titanium and Ti6Al4V ELI, respectively. The carbon signal shows that the carbon intensity increases from the surface, which is visible as a local maximum in the intensity curve for carbon. This local maximum is considered to represent a peak in the concentration of interstitial carbon in the diffusion zone after removal of the non-stratified metal oxide layer, and it is further considered to increase the hardness beyond the hardness available had the carbon not been present.

Example 5

[0096] A workpiece of Ti6Al4V (Grade 5) was treated to provide a component of the invention. Specifically, the workpiece was created by 3D printing as a cylindrical workpiece with a diameter of 12 mm and a height of 15 mm, and the workpiece was subsequently treated in two separate steps.

[0097] In the first step, the sample was placed in a MTI OFT-1200 glass tube furnace, which was evacuated and backfilled with CO.sub.2. 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 unaided.

[0098] In the second step, the diffusion step, the cooled workpiece from the first step was placed in a tube furnace with an Edwards 85 T-station turbo vacuum pump. The vacuum pump was allowed to evacuate the chamber to <10.sup.−3 mbar before the furnace was turned on. The furnace heats at a rate of approx. 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 approx. 25 K/min. The end pressure in the furnace chamber was <10.sup.−4 mbar. A photo and the hardness profile of the component are illustrated in FIG. 14. The component showed an increased hardness at a depth of up to 20 μm. The hardness profile is similar to the one seen on a non-3D printed Ti6Al4V component.

Example 6

[0099] Workpieces with a diameter of 15 mm and a thickness of 2 mm were treated in two separate steps in a variant where CO was included in the oxidising atmosphere. The sample were of CP titanium of Grade 4 and Ti6Al4V (Grade 5).

[0100] In the first step, a sample was placed in a MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with CO.sub.2/C0 at a 50/50 ratio using a continuous gas flow of 200 ml/min. 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 unaided.

[0101] In the second step, the cooled workpiece from the first step was placed in a tube furnace with an Edwards 85 T-station turbo vacuum pump. The vacuum pump was allowed to evacuate the chamber to <10.sup.−3 mbar before the furnace was turned on. The furnace heats at a rate of approx. 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 approx. 25 K/m in. The end pressure in the furnace chamber was <10.sup.−4 mbar.

[0102] The provided component showed a similar surface to the one seen on the component oxidised only in CO.sub.2, and the components thus provided regained their metallic lustre after the diffusion step.

Example 7

[0103] Samples of CP titanium of Grade 4 and Ti6Al4V (Grade 5) with a diameter of 15 mm and a thickness of 2 mm were treated in a variant where ambient air at ambient pressure was used as the oxidising atmosphere.

[0104] In the first step, the sample was placed in a Nabertherm LE4/11 R6 furnace, and the sample was then heated to 650° C. at a rate of 12 K/m in. The furnace was kept at 650° C. for 4 hours after which the furnace was allowed to cool down to room temperature unaided.

[0105] In the second step, the cooled sample from the first step was placed in a tube furnace with an Edwards 85 T-station turbo vacuum pump. The vacuum pump was allowed to evacuate the chamber to <10.sup.−3 mbar before the furnace was turned on. The furnace heats at a rate of approx. 50 K/m in (>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 approx. 25 K/m in. The end pressure in the furnace chamber was <10.sup.−4 mbar.

[0106] The components included interstitial nitrogen, which was reflected as a higher surface hardness. The Grade 4 titanium component had a surface similar to the one seen on the component oxidised only in CO.sub.2, whereas the Grade 5 titanium component was less aesthetically pleasing.

Example 8

[0107] Samples of zirconium (Zr702) and a niobium containing alloy (Ti13Nb13Zr) were treated using CO.sub.2 as gaseous oxidising species at 650° C. for 4 hours followed by the diffusion step in vacuum for 16 hours at 680° C. as described in Example 3. The Ti13Nb13Zr workpiece had a diameter of 10 mm and a thickness of 1 mm, whereas the Zr702 workpiece was a square with a side length of 15 mm and a thickness of 1.5 mm.

[0108] The treatment resulted in surface hardnesses of the components of about 860 HV.sub.0.025 and about 1218 HV.sub.0.025, respectively for the Ti13Nb13Zr and the Zr702, compared to surface hardnesses of the workpieces of 264 HV.sub.0.025 and 185 HV.sub.0.025, respectively, before treatment.

Example 9

[0109] A workpiece with a diameter of 15 mm and a thickness of 2 mm was treated in the two steps outlined in Example 3 followed by an additional anodisation step. The workpiece was of CP titanium grade 4.

[0110] In the third step, the anodisation step, the component was cleaned in first distilled water, followed by ethanol and then turpentine. The component was lowered 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 regained its metallic lustre, but the anodisation provide a visible oxide layer typical of anodised titanium that has not been treated in the method of the invention.

Example 10

[0111] Experiment were conducted to define the limits for formation of stratified titanium oxide layers vs. non-stratified titanium oxide layers when using N.sub.2O as the gaseous oxidising species. Specifically, specimens of CP Grade 2 titanium were provided and treated as done in Example 1.

[0112] Following the oxidising treatments, the cross-sections of the treated samples were analysed microscopically, and the results are plotted in FIG. 15. Thus, FIG. 15 is a plot of time vs. temperature, and the markers show the limits, so that the “area” below the dotted line is the region where a non-stratified titanium oxide layer will form when N.sub.2O is used as the gaseous oxidising species. For example, for N.sub.2O as the gaseous oxidising species treatment at a temperature in the range of 500° C. to 700° C. with an oxidising duration in the range of 10 minutes to 2 hours, or a temperature in the range of 550° C. to 600° C. with an oxidising duration in the range of 30 minutes to 2 hours will yield a non-stratified oxide layer of a sufficient thickness to subsequently harden the titanium in the diffusion step.

Example 11

[0113] A CP Titanium 030 mm disc with a thickness of 15 mm was polished to a mirror like surface finish. Prior to treatment the CP titanium specimen was polished to a mirror polished surface finish, i.e. having an arithmetical mean deviation (Ra) roughness of <0.1 μm (thus being in accordance with the ISO 1302:2002 standard). The Ra value was measured using a Taylor-Hubson Surtronic S25 measuring over a length of 1.25 mm. The measurement was repeated 12 times on the surface of the specimen. The mean Ra value was measured to <0.1 μm, and the surface of the specimen was approved as mirror polished.

[0114] The specimen was placed in a Nabertherm 3-Zone furnace. The furnace was evacuated and backfilled using CO.sub.2 twice. The furnace was then heated to 650° C. for 4 hours using a continuous flow of CO.sub.2 at 500 ml/min. The furnace was allowed to cool down using furnace cooling. The specimen was, for a 2.sup.nd treatment, placed in a furnace capable of achieving a pressure <10.sup.−4 mbar. The Furnace was heated to 680° C. and held there for 16 hours. The furnace was cooled down using furnace cooling.

[0115] The Ra value of the surface on the specimen was measured using the same procedure as prior to the thermochemical treatment (described above). The specimen still displays a surface roughness with a Ra value of <0.1 μm. The surface roughness is shown in FIG. 16. In FIG. 16 a graphical representation of the surface roughness after treatment, as measured by the Taylor-Hubson Surtronic S25, is shown. FIG. 17 shows a photo of the mirror polished surface after applying the thermochemical low temperature hardening. FIG. 17 shows how the shapes in the plot next to the specimen are reflected in the surface of the specimen, and in particular, the reflections are shown without distortion and the colours of the plot are also reflected in the surface.