SURFACE HARDENING FOR A DENTAL IMPLANT

Abstract

The present invention relates to a Group IV metal or alloy component having a protective oxide surface layer, the Group IV metal or alloy component having a core hardness, 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 or alloy, and a Group IV metal oxide layer at the surface of the component, the diffusion zone being between the Group IV metal oxide layer and the material core. In another aspect, the invention relates to a method of producing a protective oxide surface layer on a Group IV metal or alloy comprising: providing a workpiece of a Group IV metal or alloy, oxidising the Group IV metal or alloy in a first and a second oxidation step.

Claims

1. A Group IV metal or alloy component having a protective oxide surface layer, the Group IV metal or alloy component comprising a material core having a core hardness, 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 or alloy, and a Group IV metal oxide layer at the surface of the component, the diffusion zone being between the Group IV metal oxide layer and the material core, wherein the Group IV metal oxide layer has a thickness in the range of 5 μm to 100 μm, a carbon content and a cross-sectional hardness of at least 650 HV.sub.0.005.

2. The Group IV metal or alloy component according to claim 1, wherein the diffusion zone contains carbon in solid solution at a level above a minimum carbon content of the Group IV metal oxide layer.

3. The Group IV metal or alloy component according to claim 1, wherein the Group IV metal oxide layer contains nitrogen.

4. The Group IV metal or alloy component according to claim 1, wherein the component has a volumetric loss of up to 5% of the volumetric loss of a component not having the Group IV metal oxide layer when analysed after sliding wear testing under identical conditions.

5. The Group IV metal or alloy component according to claim 1, wherein the Group IV metal or alloy is selected from the list consisting of titanium, a titanium alloy, zirconium, and a zirconium alloy.

6. The Group IV metal or alloy component according to claim 1, wherein the Group IV metal or alloy does not comprise aluminium.

7. The Group IV metal or alloy component according to claim 1, wherein the component has a core of another material.

8. The Group IV metal or alloy according to claim 1, wherein the component is a titanium alloy or pure titanium and has a titanium oxide layer with a cross-sectional hardness of at least 800 HV.sub.0.005.

9. The Group IV metal or alloy according to claim 1, wherein the Group IV metal or alloy is a titanium/niobium alloy.

10. The Group IV metal or alloy according to claim 1, wherein the Group IV metal or alloy is a zirconium alloy or pure zirconium, and the component has a zirconium oxide layer with a cross-sectional hardness of at least 1000 HV.sub.0.005.

11. A method of producing a protective oxide surface layer on a Group IV metal or alloy, the method comprising the steps of: providing a workpiece of a Group IV metal or alloy, oxidising the Group IV metal or alloy in a first oxidation step at a temperature in the range of 500° C. to 900° C. using a carbon containing gaseous species having a first oxidising potential to provide an intermediary Group IV metal oxide, oxidising the intermediary Group IV metal oxide in a second oxidation step at a temperature in the range of 300° C. to 900° C. using a second gaseous species having a second oxidising potential to provide the protective oxide surface layer, which second oxidising potential is higher than the first oxidising potential.

12. The method according to claim 11, wherein the carbon containing gaseous species is CO.sub.2 or a mixture of CO.sub.2 and CO.

13. The method of claim 12, wherein the carbon containing gaseous species is a mixture of CO.sub.2 and CO, and wherein the ratio of CO.sub.2 to CO and CO.sub.2 is in the range of 0.4 to 0.9.

14. The method according to claim 11, wherein the second gaseous species is N.sub.2O or O.sub.2, or a combination of N.sub.2O and O.sub.2.

15. The method according to claim 11, wherein the first gaseous species and/or the second gaseous species do not comprise hydrogen containing species.

16. The method according to claim 11, wherein the first oxidation step is performed at a temperature in the range of 700° C. to 800° C.

17. The method according to claim 11, wherein the second oxidation step is performed at a temperature in the range of 600° C. to 700° C.

18. The method according to claim 11, wherein the oxidation step has a first reactive duration in the range of 0.2 hour to 24 hours.

19. The method according to claim 11, wherein the second oxidation step has a second reactive duration in the range of 1 hour to 24 hours.

20. The method according to claim 11, wherein the carbon containing gaseous species is CO.sub.2 or a mixture of CO.sub.2 and CO, the first oxidation step temperature is in the range of 700° C. to 800° C., and the first reactive duration is in the range of 12 hours to 24 hours, and the second gaseous species is N.sub.2O in N.sub.2 at a ratio of 20% to 40% N.sub.2O to the total of N.sub.2O in N.sub.2, the second oxidation step temperature is in the range of 600° C. to 700° C., and the second reactive duration is in the range of 12 hours to 24 hours.

21. The method according to claim 11, wherein the first oxidation step is performed with the carbon containing gaseous species at ambient pressure and/or wherein the second oxidation step is performed with the second gaseous species at ambient pressure.

22. The method according to claim 11, wherein the second gaseous species and/or the carbon containing gaseous species is contained in an atmosphere comprising N.sub.2 or an inert gas.

23. The method according to claim 11, wherein the Group IV metal or alloy is selected from the list consisting of titanium, a titanium alloy, zirconium, and a zirconium alloy.

24. The method according to claim 23, wherein the Group IV metal or alloy is titanium grade 1 to 4, Zr702.sub.2, or a titanium/niobium alloy.

25. The method according to claim 11, wherein the Group IV alloy does not comprise aluminium.

26-27. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

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

[0049] FIG. 1 shows the hardness of titanium oxides as intermediary and surface oxide layers;

[0050] FIG. 2 shows the oxidising potential of a range of gaseous species;

[0051] FIG. 3 shows treatment of titanium in CO;

[0052] FIG. 4 shows titanium samples before and after treatment;

[0053] FIG. 5 shows a cross-section of titanium after the first oxidation step;

[0054] FIG. 6 shows a cross-section of titanium after the second oxidation step; step

[0055] FIG. 7 shows a cross-section of titanium after the first oxidation step;

[0056] FIG. 8 shows a cross-section of titanium after the second oxidation step;

[0057] FIG. 9 shows a hardness analysis in the cross-section of titanium hardened according to the invention;

[0058] FIG. 10 shows Glow Discharge Optical Emission Spectroscopy (GDOES) of untreated titanium;

[0059] FIG. 11 shows GDOES of titanium treated in the first oxidation step of the invention;

[0060] FIG. 12 shows GDOES of titanium hardened according to the invention;

[0061] FIG. 13 shows an enlarged section of the GDOES of FIG. 11;

[0062] FIG. 14 shows an enlarged section of the GDOES of FIG. 12;

[0063] FIG. 15 shows a spectrophotometric analysis of titanium hardened according to the invention;

[0064] FIG. 16 shows a cross-section of a titanium sample oxidised using O.sub.2;

[0065] FIG. 17 shows a cross-section of a zirconium component of the invention;

[0066] FIG. 18 shows the hardness of a zirconium component of the invention, the zirconium and the intermediary zirconium oxide;

[0067] FIG. 19 shows hardness analyses of a workpiece of Ti13Nb13Zr, and the workpiece after the first and second oxidation steps of the invention;

[0068] FIG. 20 shows cross-sections of a workpiece of Ti13Nb13Zr, and the workpiece after the first and second oxidation steps of the invention.

[0069] 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

[0070] The present invention relates to a method of producing a protective oxide surface layer on a Group IV metal or alloy component having a Group IV metal oxide layer at the surface of the component.

[0071] In the context of the invention “Group IV metal or alloy” 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.

[0072] 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 element in the alloy, or any constituent in the alloy. Titanium and zirconium alloys are well-known to the skilled person.

[0073] 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 or Grade 2 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. In particular, nitrogen and carbon contained in a Group IV metal or alloy 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”.

[0074] When a percentage is stated for a metal or an alloy the percentage is by weight of the weight of material, e.g. denoted wt %, 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).

[0075] The method of the invention employs a gaseous species. In particular, the method employs a gaseous species having an oxidising potential. A gaseous species having an oxidising potential may also be referred to as an oxidising species or an oxidising gaseous species. The method may also employ further gaseous molecules, e.g. inert gaseous molecules that are not oxidising. Unless noted otherwise, a “gaseous species” is an oxidising species. When an oxidising species takes part in a reaction with another species, the mixture of the oxidising species and the other species may be referred collectively to as the “gaseous species”, e.g. the carbon containing gaseous species may be a mixture of CO.sub.2 and CO.

[0076] In the context of the invention the hardness is generally the HV.sub.0.05 as measured according to the DIN EN ISO 6507 standard. If not otherwise mentioned the unit “HV” thus refers to this standard. The hardness is preferably recorded for a cross-section, e.g. of a treated Group IV metal or alloy, and it may be noted with respect to the depth from the surface 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”.

[0077] 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 H.sub.v0.5, 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.

[0078] 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. In the context of the invention a hardness measurement recorded in a cross-section at a depth of about 1 μm is considered to provide the actual hardness of the surface of the material. 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 or alloy, and likewise, the hardness will be maximal at the surface, e.g. as represented by measuring the hardness in a cross-section at a depth of about 1 μm.

EXAMPLES

Comparative Example 1

[0079] A cylindrical (Ø10 mm) grade 2 titanium samples was treated in a Netzsch 449 Thermal analyzer (furnace). In the experiment, the furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The sample was heated to 840° C. at a rate of 20° C./min in the same gas mixture and upon reaching the temperature held there for 16 hours. Cooling was carried out at 50° C./min in the flowing process gas. This treatment resulted in formation of a diffusion zone without visible a phase of titanium oxide as is evident from FIG. 3. Thus, without the presence of an oxidising species CO alone cannot provide an oxide layer.

Example 1

[0080] Cylindrical test samples with diameters of 15 mm and thicknesses of 2 mm were treated in oxidising atmospheres. The test samples were CP titanium grade 2. The oxidation was performed at 850° C. in atmospheres of either O.sub.2, Air, N.sub.2O, or CO.sub.2 for between 16 minutes and 900 minutes.

[0081] For oxidation in O.sub.2, the test sample was placed in a NETZCH STA449 F3 furnace. The furnace was evacuated and backfilled with O.sub.2. A continuous gas flow of 63 ml/min was used. N.sub.2 was used as a protective gas with a flow of 5 ml/min. The test sample was heated to 850° C. at a rate of 30 K/min. The furnace was kept at 850° C. for 16 minutes after which the furnace was allowed to cool down to room temperature at a rate of 20K/minutes. The experiment was repeated keeping the furnace at 850° C. for 36 minutes, 64 minutes, 400 minutes, and 900 minutes.

[0082] For oxidation in Air, the test sample was placed in a Nabertherm LE4/11 R6 furnace. The test sample was then heated to 850° C. at a rate of 30 K/minutes. The furnace was kept at 850° C. for 16 minutes after which the test sample was taken out of the furnace and allowed to cool down to room temperature. The atmosphere in the furnace was air. The experiment was repeated keeping the furnace at 850° C. and taking the samples out after 25 minutes, 36 minutes, 49 minutes, 64 minutes, 400 minutes, and 900 minutes.

[0083] For oxidation in N.sub.2O, the test sample was placed in a NETZCH STA449 F3 furnace. The furnace was then evacuated and backfilled with N.sub.2. When reaching ambient pressure, N.sub.2O was added at a continuous gas flow of 15 ml/minutes N.sub.2 was used as a protective gas with a flow of 30 ml/minutes The test sample was heated to 850° C. at a rate of 30 K/minutes The furnace was kept at 850° C. for 16 minutes after which the furnace was allowed to cool down to room temperature at a rate of 20K/minutes The experiment was repeated keeping the furnace at 850° C. for 25 minutes, 36 minutes, 49 minutes, 64 minutes, 225 minutes, 400 minutes, and 900 minutes.

[0084] For oxidation in CO.sub.2, the test sample was placed in a NETZCH STA449 C furnace. The furnace was then evacuated and backfilled with CO.sub.2. A continuous gas flow of 50 ml/minutes was used the test sample was heated to 850° C. at a rate of 30 K/minutes The furnace was kept at 850° C. for 16 minutes after which the furnace was allowed to cool down to room temperature at a rate of 20K/minutes. N.sub.2 was used as a protective gas with a flow of 5 ml/minutes The experiment was repeated keeping the furnace at 850° C. for 25 minutes, 36 minutes, 49 minutes, 64 minutes, 400 minutes, and 900 minutes.

[0085] The surface showed oxide layers on the surfaces of the CP titanium grade 2 test samples. The test samples were cut in half, embedded and polished. Using light optical microscopy, the thicknesses of the oxide layer were measured and plotted in FIG. 2. The thicknesses obtained for the tested gasses were considered to represent the oxidising potentials of the tested gasses, i.e. the thicker the oxide layer, the higher the oxidising potential. FIG. 2 thus shows a linear relationship in the oxide layer thickness with increasing treatment time and a clear difference in oxidising potential between the four different atmospheres used. Thus, the oxidising potentials of the tested gasses can be ordered as follows: CO.sub.2<air<O.sub.2<N.sub.2O.

Example 2

[0086] Samples of pure titanium (Grade 2) with diameters of 15 mm and thicknesses of 2 mm were treated according to the invention by using CO.sub.2 as the first reactive species at 650° C., 700° C. or 750° C. for 16 hours, and this was followed by oxidation with N.sub.20 as the second reactive species at 750° C. for 64 minutes or 400 minutes. Both oxidations were performed at ambient pressure. Specifically, the first oxidation step was performed in a MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with CO.sub.2 at a continuous gas flow of 50 ml/min before heating the test sample to the tested temperatures at a rate of 12 K/min. The second oxidation step was performed in a NETZCH STA449 F3 furnace. After placing the samples in the furnace, the furnace was evacuated and backfilled with N.sub.2. A continuous gas flow of 15 ml/min N.sub.2O was used. N.sub.2 was used as a protective gas with a flow of 30 ml/min. The furnace was kept at 750° C., 700° C., or 650° C. for 400 min or 64 min after which the furnace was allowed to cool down to room temperature at 20 K/min.

[0087] Following the two oxidations, the samples were analysed for surface hardness (HV.sub.0.010). For comparison, a sample not exposed to the second oxidation step was also analysed (this sample was treated in CO.sub.2 at 750° C.) for surface hardness. The hardness measurements are shown in FIG. 1, where the “boxes” show the measurements with standard deviations, and the “bars” show the confidence intervals (α=0.05). Thus, the intermediary titanium oxide and the protective oxide surface layers were considerably harder than the core hardness (of about 300 HV). The second oxidation step treatment did not provide a significantly higher hardness to the outer titanium oxide layer on the surface than the first oxidising step. The intermediary titanium oxide was dense and robust, but the protective oxide surface layer had a higher resistance to wear than the intermediary titanium oxide layer.

Example 3

[0088] Samples of pure titanium were hardened according to the invention or in a treatment using only N.sub.2O. Specifically, in the first oxidation step the samples were placed in a MTI OFT-1200 glass tube furnace before evacuating and backfilling with the appropriate gas. A continuous gas flow of 200 ml/min was used, and the test samples were heated at a rate of 12 K/min. After this first step, the furnace was allowed to cool down to room temperature unaided. The treatment in the second oxidation step was done separately from the first oxidation step although the same furnace was used. After placing the intermediary workpiece in the furnace, the furnace was evacuated and backfilled with the appropriate gas at continuous gas flow of 300 ml/min, and the intermediary workpiece was heated at a rate of 12 K/min. After the second oxidation step, the component the furnace was allowed to cool down to room temperature unaided.

[0089] Thus, a sample was treated in CO.sub.2 at ambient pressure and 780° C. for 16 hours followed by treatment in N.sub.2O at 680° C. for 3 hours, also at ambient pressure. Treatment in N.sub.2O alone without hardening with CO.sub.2 was performed at 880° C. for 16 hours. The treated samples and an untreated titanium sample were analysed for wear resistance by performing a ball on disc tribology testing in Ringers solution. Specifically, the wear counterpart was a 6 mm diameter Al.sub.2O.sub.3 ball loaded with a normal force of 5N on the rotating sample disc for total 50 meter with a speed of 0.5 cm/s. The test solution was Ringers solution containing 0.12 g/l CaCl.sub.2, 0.105 g/l KCl, 0.05 g/l NaHCO.sub.3 and 2.25 g/l NaCl. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Wear resistance test and comparison Sample Volume loss [mm.sup.3] Untreated reference 0.3170 N.sub.2O 880° C. 16 h 1.4290 CO.sub.2 780° C. 16 h + N.sub.2O 680° C. 3 h 0.0011

[0090] As seen from Table 1, hardening according to the invention significantly improved the wear resistance compared to the untreated sample. In contrast, treatment in N.sub.2O provided a worse performance than the untreated sample.

[0091] Specifically, the treatment in N.sub.2O at 880° C. resulted in a stratified titanium oxide layer that was highly susceptible to spallation. In contrast, the component of the invention had a volumetric loss of 0.35% of the volumetric loss of the untreated workpiece.

Example 4

[0092] Samples of commercially pure (CP) titanium (Grades 2 and 4) with diameters of 15 mm and thicknesses of 2 mm were hardened according to the invention. In the first oxidation step, the samples were 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 test sample was heated to 750° C. at a rate of 12 K/min. The furnace was kept at 750° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided.

[0093] In the second oxidation step, test sample was again placed in the MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with 30% N.sub.2O and 70% N.sub.2. A continuous gas flow of 300 ml/min total was used. The test sample was heated to 650° C. at a rate of 12 K/m in. The furnace was kept at 650° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided.

[0094] Photographs of the untreated sample, the intermediary sample and the final sample (Grade 4 titanium) are shown in FIG. 4. The intermediary test sample had a dense robust grey oxide layer, and the final component had a surface showing a white, dense, hard, and robust oxide layer.

[0095] For both Grade 2 and Grade 4 titanium the cross-sections of the samples were analysed after the first and the second oxidation steps. The results are depicted FIG. 5 to FIG. 8, where FIG. 5 shows the Grade 4 titanium sample after the first oxidation step, FIG. 6 shows the Grade 4 titanium sample after the second oxidation step, FIG. 7 shows the Grade 2 titanium sample after the first oxidation step, and FIG. 8 shows the Grade 2 titanium sample after the second oxidation step. It is seen that for both Grades the intermediary oxide layer and the final oxide layer, i.e. the protective oxide surface layer, have the same approximate thickness, whereas the oxide layers are thicker for Grade 4 titanium compared to Grade 2 titanium. Without being bound by theory, the present inventors believe that the higher thickness, about 60 μm, for the Grade 4 titanium compared to the thickness of about 20 μm for Grade 2 titanium can be attributed to a higher content of iron in Grade 4 titanium compared to Grade 2 titanium: Specifically, the inventors believe that iron increases the speed of the reaction so that a thicker oxide layer will form on

[0096] Grade 4 titanium compared to Grade 2 titanium. However, for both Grades of titanium the intermediary oxide layers were dense and allowed formation of sufficiently thick protective oxide surface layers.

Example 5

[0097] The Grade 4 sample of Example 4 was selected for further analysis, and the cross-section was thus analysed for hardness. Hardness measurements were performed using a 5 g load, and the hardness measurements are shown in FIG. 9, which also shows the actual cross-section after analysis. The protective oxide surface layer had a hardness >800 HV.sub.0.005, and at a depth of about 60 μm the interface between the oxide layer and the diffusion zone is visible. The hardness is seen to drop from a level near the interface suggesting titanium saturated with oxygen, i.e. >800 HV.sub.0.005 to a level of 120% of the hardness of the core metal at a depth of about 100 μm.

[0098] The untreated Grade 4 titanium sample, the Grade 4 titanium sample with the intermediary oxide layer, and the hardened Grade 4 titanium sample were also exposed to Glow Discharge Optical Emission Spectroscopy (GDOES) analysis. 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 all three layers, the GDOES analysis appropriately provides a comparison of the compositions of the protective oxide surface layer (in the left side of the plot) with the diffusion layer (middle section of the plot) and the core of the metal (right section of the plot). Thus, FIG. 10 shows a GDOES analysis of untreated titanium, FIG. 11 shows a GDOES analysis of titanium after the first oxidation, and FIG. 12 shows a GDOES analysis of titanium after the second oxidation. FIG. 13 and FIG. 14 show enlarged sections of FIG. 11 and FIG. 12, respectively. FIG. 10 initially, i.e. in the first 50 seconds, shows apparent sharp decreases in contents of non-metallic elements, which are considered to represent inevitable contents of these elements in titanium. The initial sharp decreases are also observed after the first and second oxidation steps (FIG. 11, FIG. 12, FIG. 13 and FIG. 14). FIG. 11 and FIG. 12 show a content of carbon over the thickness of the intermediary titanium oxide layer, and FIG. 13 and FIG. 14 shows how the carbon content appears to have migrated towards the surface of the component of the invention. The drop in oxygen signal and the increase in titanium signal is considered to represent the diffusion zone, and the signals after about 1300 seconds are considered to represent the metal core. Enlarged sections of the plots in FIG. 11 and FIG. 12 are shown in FIG. 13 and FIG. 14, respectively, where FIG. 11 and FIG. 12 show 8 intensity units on the Y-axis and FIG. 14 show 1.5 intensity units on the Y-axis. FIG. 12 shows that the protective oxide surface layer has a composition of mainly titanium and oxygen, and at the interface between the protective oxide surface layer and the diffusion zone the oxygen content drops towards the metal core. The enlarged section in FIG. 14 highlights that the protective oxide surface layer has a detectable content of carbon over the thickness of the protective oxide surface layer, although the carbon content is higher near the surface of the component. The titanium sample was treated with CO.sub.2 in the first oxidation step to that the carbon content seen for the protective oxide surface layer was also present in the intermediary oxide layer. Without being bound by theory the present inventors believe that this detectable content of carbon stabilises the intermediary oxide layer and allows that the intermediary oxide layer is oxidised further in the second oxidation step to provide a protective oxide surface layer that is as stable as the intermediary oxide layer but harder than the intermediary oxide layer. The increased hardness of the protective oxide surface layer compared to the intermediary oxide layer provides a component with wear resistance beyond what has been available in methods of the prior art.

[0099] FIG. 14 further shows that the diffusion zone, especially near the interface with the protective oxide surface layer has an increased amount of carbon compared to both the content in the metal core and also the protective oxide surface layer. Without being bound by theory, the present inventors believe that this carbon, i.e. present in solid solution in the diffusion zone, further stabilises the protective oxide surface layer on the surface of the hardened component thereby contributing to the stability of the protective oxide surface layer on the component of the invention.

[0100] The second oxidation step was performed in an atmosphere of N.sub.2O in N.sub.2. The presence of N.sub.2 in the second oxidation step has provided the protective oxide surface layer with a stable content of nitrogen, as shown by the GDOES analysis in FIG. 14. The level of the nitrogen content can be controlled by controlling the amount of N.sub.2 in the second oxidation step, but regardless of the N.sub.2 in the second oxidation step, the amount of nitrogen in the protective oxide surface layer will be stable over its thickness. The present inventors have now surprisingly found that nitrogen in the protective oxide surface layer will change its colour from white towards a more yellow tinge, and furthermore that modification of the content of N.sub.2 in the second oxidation step can be used to adjust the colour of the protective oxide surface layer from white to a broken white; the higher the content of N.sub.2 in the second oxidation step, the more yellow the colour of the protective oxide surface layer.

Example 6

[0101] The surface of the Grade 4 sample of Example 4 was analysed spectrophotometrically and compared to a standard white, RAL9003 (“signal white”), and black, RAL9004 (“signal black”) together with an untreated sample and a sample after the first oxidation step. The component of the invention is seen to have a white and reflective surface as indicated by the high reflectance (>60%) and the little variation in reflectance over the visible range of wavelengths.

Example 7

[0102] A sample of Ti6Al4V alloy with diameters of 15 mm and thicknesses of 2 mm was treated according to the invention in two separate oxidation steps. In the first oxidation step, the 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 test sample was then heated to 750° C. at a rate of 12 K/m in. The furnace was kept at 750° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided. The intermediate test sample at this point had a dense robust grey oxide layer.

[0103] In the second oxidation step, test sample was again placed in the MTI OFT-1200 glass tube furnace. The furnace was evacuated and backfilled with 30% N.sub.2O and 70% N.sub.2. A continuous gas flow of 300 ml/min total was used. The test sample was heated to 750° C. at a rate of 12 K/m in. The furnace was kept at 650° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided.

[0104] The surface of the sample after the final oxidation step displayed a grey dense, hard, and robust oxide layer. The oxide layer has a thickness of 10 to 15 μm. The oxide does not turn white when applying this thermochemical treatment due to the presence of N.sub.2.

Example 8

[0105] In a variant of the invention, ambient air was used as the second gaseous species for the treatment of CP titanium of grade 4. A samples with a diameter of 15 mm and a thickness of 2 mm 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 test sample was then heated to 750° C. at a rate of 12 K/m in. The furnace was kept at 750° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided. The test sample has at this point a dense robust grey oxide layer.

[0106] In the second oxidation step, test sample was placed in a Nabertherm N 7/H furnace. The test sample was heated to 650° C. at a rate of 12 K/min. The furnace was then kept at 650° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided. The atmosphere in the furnace was ambient air at ambient pressure. No circulation of the air was used.

[0107] The thus provided component had a white, dense, hard, and robust oxide layer on the surface, which was similar in appearance to the surface seen using N.sub.2O in second step.

Example 9

[0108] The effect of using O.sub.2 as the first oxidising species in the treatment of CP titanium of grade 2 was analysed. A sample with a diameter of 15 mm and a thickness of 2 mm was placed in a NETZCH STA449 F3 furnace. The furnace was evacuated and backfilled with O.sub.2. A continuous gas flow of O.sub.2 of 63 ml/min was used together with a flow of N.sub.2 of 5 ml/min. N.sub.2 was contemplated as a protective gas considering the strong oxidising potential of O.sub.2. The test sample was heated to 850° C. at a rate of 30 K/min. The furnace was kept at 850° C. for 400 min after which the furnace was allowed to cool down to room temperature at a rate of 20 K/min.

[0109] The treatment with O.sub.2 provided the surface with a white oxide layer with a thickness of 50 to 55 μm as seen on the light optical images. The white oxide layer had a hardness of 350-700 HV.sub.0.025, but the oxide layer showed a stratified structure indicating a low resistance to spallation. The cross-section of the treated sample is shown in FIG. 16. The oxide layer was thus observed to easily detach from the surface when handled.

Example 10

[0110] The effect of using ambient air as the first oxidising species in the treatment of CP titanium of grade 2 was analysed. The procedure of Example 9 was repeated but with ambient air in place of the mixture of O.sub.2 with N.sub.2. Since ambient air was used, no flow through the furnace was employed.

[0111] This treatment provided a white oxide layer having a thickness of 25 to 30 μm and an apparent hardness of 700 to 1000 HV.sub.0.025. However, as for Example 9, the oxide layer showed a stratified structure indicating a low resistance to spallation. The oxide layer was thus observed to easily detach from the surface when handled.

Example 11

[0112] The effect of using N.sub.2O as the first oxidising species in the treatment of CP titanium of grade 2 was analysed. A sample with a diameter of 15 mm and a thickness of 2 mm was placed in a NETZCH STA449 F3 furnace. The furnace was evacuated and backfilled with N.sub.2O. A continuous gas flow of 15 ml/min was used with a flow of N.sub.2 of 30 ml/min. The test sample was heated to 750° C. at a rate of 30 K/min, and the furnace was kept at 750° C. for 400 minutes after which the furnace was allowed to cool down to room temperature at a rate of 20 K/min.

[0113] The procedure was repeated for a new, untreated sample with a treatment temperature of 1000° C. and a duration of 64 minutes.

[0114] Both treatments provided white oxide layers having a thickness of 15 to 20 μm and 60 to 65 μm, respectively, and apparent hardnesses of 700 to 1000 HV.sub.0.025. However, as for Examples 9 and 10, the oxide layers showed a stratified structure indicating a low resistance to spallation. The oxide layer was thus observed to easily detach from the surface when handled.

Example 12

[0115] A square test sample of Zr702 with a side length of 15 mm and a thickness of 1.5 mm was treated in two separate oxidation steps according to the invention. In the first oxidation 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 test sample was then heated to 750° C. at a rate of 12 K/m in. The furnace was kept at 750° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided. The intermediary test sample at this point had a dense robust grey oxide layer.

[0116] In the second oxidation step, the sample was again placed in the same furnace that was evacuated and backfilled with 30% N.sub.2O and 70% N.sub.2. A continuous total gas flow of 300 ml/min was used. The test sample was heated to 750° C. at a rate of 12 K/m in. The furnace was kept at 650° C. for 16 hours after which the furnace was allowed to cool down to room temperature unaided.

[0117] The provided component had a grey dense, hard, and robust oxide layer with a thickness of 8 to 12 μm. The cross-section of the zirconium component is shown in FIG. 17.

[0118] The hardened component, the workpiece with the intermediary zirconium oxide and the workpiece prior to treating in the method of the invention were analysed for surface hardness, and the results are depicted in FIG. 18. The untreated zirconium had a hardness of about 200 HV.sub.0.05, whereas the treated zirconium component had a surface hardness of about 1150 HV.sub.0.05.

Example 13

[0119] A sample of Ti13Nb13Zr was treated as outlined in Example 4. Cross-sections of the workpiece before treatment, the workpiece after the first oxidation step and the final component are shown in FIG. 20 in the top panel, the middle panel and the bottom panel, respectively. Thus, the workpiece did not have a visible oxide layer, whereas the first oxidation step provided a dense intermediary oxide layer. The hardness values found in FIG. 19 show that the hardness increased upon oxidation. The component had a more stable oxide layer than the workpiece after the first oxidising step.