Diffusion-Hardened Medical Implant

Abstract

The present invention relates to a new composition and medical implant made therefrom, the composition comprising a thick diffusion hardened zone, and preferably further comprising a ceramic layer. The present invention relates to orthopedic implants comprising the new composition, methods of making the new composition, and methods of making orthopedic implants comprising the new composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 shows the hardness profile of Davidson-type oxidized zirconium composition. The thickness of the diffusion zone is 1.5 to 2 microns (Long et. al.)

FIGS. 2( a) and (b) are metallographic images of Zircadyne 702 and Zr-2.5Nb oxidized following the teachings of Kemp; (c) micro-hardness profile of the diffusion hardened zone

FIGS. 3( a) and (b) are metallographic images of Ti—Zr—Nb and Zr-2.5Nb oxidized by following teachings of Davidson; (c) Micro-hardness profile of diffusion hardened zone.

FIGS. 4( a) and (b) show samples of Ti-6Al-4V and Zr-2.5Nb oxidized at 850° C. for 0.3 hours respectively; (c) and (d) show samples of Ti-6Al-4V and Zr-2.5Nb diffusion hardened at 850° C. for 22 hours respectively.

FIGS. 5( a) and (b) show samples of Ti-6Al-4V and Zr-2.5Nb oxidized at 600° C. for 75 minutes respectively; (c) and (d) show samples of Ti-6Al-4V and Zr-2.5Nb diffusion hardened at 685° C. for 10 hours respectively, (e) shows the hardness profile of Ti-6Al-4V and Zr-2.5Nb after diffusion hardening.

FIG. 6 shows hardness profiles obtained on Zr-2.5Nb samples after vacuum diffusion process (685° C. for 10 hours). The starting oxide represents oxide thickness prior to vacuum diffusion treatment. The oxidation was carried out at 635° C. for different times to produce different starting oxide thickness.

FIG. 7 shows metallographic images of samples with hardness profile obtained in FIG. 3 were re-oxidized at 635° C. for 60 minutes.

FIG. 8 illustrates Rockwell indents showing the damage resistance of (a) and (b) Davidson-type oxidized zirconium composition and (c) and (d) composition disclosed in this invention with a total hardening depth of 20 to 25 microns.

FIG. 9 shows wear results of pin-on-disk testing of high carbon cast CoCr against itself and one of the oxidized zirconium compositions against itself (total hardened zone 20 to 25 microns) disclosed in this invention.

FIG. 10 shows the oxygen concentration profile of the diffusion zone. Analyses were carried out using a scanning auger microprobe with accelerating voltage of 10 kV; probe current of 18 nA and electron beam at 30° from sample normal. Oxide was retained on the sample after the vacuum treatment.

FIG. 11 illustrates the micro-hardness profile of Davidson-type oxidized zirconium composition and some of the compositions disclosed in this invention. Micro-hardness was carried out using a Knoop indenter at a load of 10 g.

FIG. 12 shows cross-sectional metallographic images; (a) Davidson-type oxidized zirconium composition, (b) oxidized at 635° C. for 75 minutes and diffusion hardened at 585° C. for 10 hours, (c) oxidized at 690° C. for 60 minutes and diffusion hardened at 685° C. for 20 hours, and (d) oxidized at 635° C. for 75 minutes and diffusion hardened at 750° C. for 20 hours. The dotted lines on the images show the demarcation of layers.

FIG. 13 shows XRD pattern of (a) Davidson-type oxidized zirconium and (b) one of the compositions of this invention. The M(−111) and M(111) are from −111 and 111 plane, T(111) is from tetragonal 111 plane. The T(111) peak for new composition is negligible indicating smaller tetragonal phase in the oxide compared to the oxide of Davidson-type oxidized zirconium. The monoclinic phase analysis was carried using ASTM F 1873.

FIG. 14( a) and (b) show a Davidson-type oxidized zirconium composition; (c) and (d) show one of the compositions of this invention. The sample shown in (c) and (d) was oxidized at 690° C. for 60 minutes and diffusion hardened at 685° C. for 20 hours. The oxide was retained on the surface. This is a longitudinal cross-section of the sample. The orientation of secondary phase is different in transverse section. A dotted line is drawn to show how far the secondary phase is present in the oxide. The samples are imaged using back scattered electron mode with accelerating voltage of 20 kV.

FIG. 15 illustrates (a) an oxide of Davidson-type oxidized zirconium composition, and (b) an oxide of the present invention. The bright white areas in image (b) are secondary phase.

FIG. 16 shows the ratio of atomic concentration of oxygen to atomic concentration of zirconium of Davidson-type oxidized zirconium composition and that disclosed in this invention. The depth profile analysis was carried out using x-ray photoelectron spectroscope (Al kα, take off angle 45°) and an ion gun for sputtering (Ar+, 3 keV, silica sputter rate of 48 angstroms/minute).

FIG. 17 illustrates an error function fit to the micro-hardness indents in the diffusion hardened zone to estimate the depth of hardening. The diffusivity values are in cm ²/s and are approximate. Time is in seconds and distance is in microns.

FIG. 18 illustrates the microstructure of (a) as received Zr-2.5Nb bar stock, (b) oxidized at 635° C. for 75 minutes and diffusion hardened at 585° C. for 10 hours, (c) oxidized at 690° C. for 60 minutes and diffusion hardened at 685° C. for 20 hours, and (d) oxidized at 635° C. for 75 minutes and diffusion hardened at 750° C. for 20 hours, and (e) oxidized at 850° C. for 20 minutes and diffusion hardened at 850° C. for 22 hours. The samples were polished using standard metallographic techniques and were heat tinted to reveal the grain size.

Claims

1. A medical implant comprising: a substrate comprising zirconium or zirconium alloy;a diffusion hardened zone in contact with said substrate, said diffusion hardened zone comprising zirconium or zirconium alloy and a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 2 microns; and,a substantially defect-free ceramic layer in contact with said diffusion hardened zone and comprising a surface of said medical implant, said ceramic layer ranging in thickness from 0.1 to 25 microns; and,wherein the total thickness of the ceramic layer and the diffusion hardened zone is 5 microns or greater.

2. The medical implant of claim 1, wherein the ceramic layer comprises a secondary phase; and,the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, the layered structure characterized by: a first layer directly below the ceramic layer, wherein the first layer is predominantly alpha phase zirconium;an interface between the first layer and the ceramic layer; and;a second layer directly below the first layer.

3. The medical implant of claim 2, wherein the substrate further comprises titanium, tantalum, hafnium, niobium, and any combination thereof.

4. The medical implant of claim 2, wherein said diffusion hardening species is selected from the group consisting of oxygen, nitrogen, boron, carbon, and any combination thereof.

5. The medical implant of claim 2, wherein said diffusion hardening species comprises oxygen.

6. The medical implant of claim 5, wherein the diffusion hardened zone has concentration of oxygen which decreases in the direction of the substrate, said decrease of oxygen concentration being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

7. The medical implant of claim 5, wherein the ceramic oxide has monoclinic content of greater than 93%.

8. The medical implant of claim 2, wherein said diffusion hardened zone has a hardness profile which is defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

9. The medical implant of claim 2, wherein said first layer has a thickness which is greater than or equal to the thickness of said second layer and of any subsequent layers if present.

10. The medical implant of claim 2, wherein said diffusion hardened zone has a thickness of 5 to 70 microns.

11. The medical implant of claim 2, wherein said diffusion hardened zone has a thickness of 10 to 50 microns.

12. The medical implant of claim 2, wherein said diffusion hardened zone has a thickness of 15 to 30 microns.

13. The medical implant of claim 2, wherein the hardness of the diffusion hardened zone is at least 10% greater than that of the substrate.

14. The medical implant of claim 2, wherein said medical implant is selected from the group consisting of a hip implant, a knee implant, and a spinal implant.

15. The medical implant of claim 2, wherein said substrate comprises an alloy of zirconium and niobium and has a niobium content of at least 1% (w/w).

16. The medical implant of claim 15, wherein said substrate comprises an alloy of zirconium and niobium has a niobium content of at least 10% (w/w).

17. The medical implant of claim 2, further comprising an oxygen-containing zirconium alloy overlaying said ceramic oxide or nitride on the surface of said implant, said alloy being in the metallic state.

18. A medical implant comprising: a substrate comprising zirconium or zirconium alloy;a diffusion hardened zone in contact with said substrate, said diffusion hardened zone comprising zirconium or zirconium alloy and a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 5 microns; and,wherein the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, said layered structure characterized by: a first layer on a surface of the implant;a second layer directly below said first layer, wherein said first layer is predominantly alpha phase zirconium; and,said layered structure having a concentration of diffusion hardening species which decreases in the direction of the substrate, said decrease of concentration of diffusion hardening species being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

19. The medical implant of claim 18, wherein the substrate further comprises titanium, tantalum, hafnium, niobium, and any combination thereof.

20. The medical implant of claim 18, wherein said diffusion hardening species is selected from the group consisting of oxygen, nitrogen, boron, carbon, and any combination thereof.

21. The medical implant of claim 18, wherein said diffusion hardening species comprises oxygen.

22. The medical implant of claim 21, wherein the diffusion hardened zone has a concentration of oxygen which decreases in the direction of the substrate, said decrease of oxygen concentration being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

23. The medical implant of claim 18, wherein the diffusion hardened zone has a hardness profile which is defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function any sequential combination thereof.

24. The medical implant of claim 18, wherein said first layer has a thickness which is greater than the thickness of said second layer and of any subsequent layers if present.

25. The medical implant of claim 18, wherein said diffusion hardened zone has a thickness of 5 to 70 microns.

26. The medical implant of claim 18, wherein said diffusion hardened zone has a thickness of 10 to 50 microns.

27. The medical implant of claim 18, wherein said diffusion hardened zone has a thickness of 15 to 30 microns.

28. The medical implant of claim 18, wherein the hardness of the diffusion hardened zone is at least 10% greater than that of the substrate.

29. The medical implant of claim 18, wherein said medical implant is selected from the group consisting of a hip implant, a knee implant, and a spinal implant.

30. The medical implant of claim 18, wherein said substrate comprises an alloy of zirconium and niobium has a niobium content of at least 1% (w/w).

31. The medical implant of claim 30, wherein said substrate comprises an alloy of zirconium, titanium and niobium and has a niobium content of at least 10% (w/w).

32. A method of making a surface hardened medical implant comprising the steps of: forming said medical implant of zirconium or zirconium alloy; and,further treating said implant by any one of (a), (b), or (c), wherein (a), (b), and (c) are defined as follows: (a) treating said implant in the presence of ceramic-forming species at a temperature of less than 700° C. for greater than 5 minutes; and,thereafter treating said implant under vacuum or inert gas at a temperature of from 500° C. to 1000° C. for greater than 1 hour;(b) treating said implant in the presence of ceramic-forming species at a temperature of from 500° C. to 1000° C.; and,thereafter treating said implant under vacuum or inert gas at a temperature less than 700° C.;(c) treating said implant in the presence of ceramic-forming species at a temperature of less than 700° C.; and,thereafter treating said implant under vacuum or inert gas at a temperature less than 700° C.

33. The method of claim 32, further comprising the step of treating said implant in the presence of a ceramic-forming species at a temperature less than 700° C. for greater than 5 minutes after said step of thereafter treating said implant under vacuum or inert gas.

34. The method of claim 32, wherein said step of thereafter treating said implant under vacuum or inert gas is performed at a temperature of 600° C. to 700° C.

35. The method of claim 32, wherein said step of treating said implant in the presence of ceramic-forming species is performed for between 5 minutes to 12 hours.

36. The method of claim 32, wherein said step of thereafter treating said implant under vacuum or inert gas is performed for between 15 minutes to 30 hours.

37. The method of claim 32, wherein said step of forming a medical implant of zirconium or zirconium alloy comprises forming said medical implant of zirconium alloy having an alloying element selected from the group consisting of titanium, tantalum, hafnium, niobium, and any combination thereof.

38. The method of claim 32, wherein said step of forming comprises forming said medical implant of an alloy of zirconium and niobium, said alloy having a niobium content of at least 1% (w/w).

39. The method of claim 38, wherein said step of forming comprises forming said medical implant of an alloy of zirconium and niobium, said alloy having a niobium content of at least 10% (w/w).

40. The method of claim 32, wherein said step of treating said implant in the presence of ceramic-forming species and said step of thereafter treating said implant under vacuum or inert gas comprise treating said implant with a diffusion hardening species selected from the group consisting of oxygen, nitrogen, boron, carbon, and any combination thereof.

41. A method of making surface hardened medical implant comprising steps of: forming said medical implant of zirconium or zirconium alloy;forming an oxide, carbide, nitride, boride or combination thereof, on a surface of said implant at a temperature of from 500° C. to 1000° C. for greater than 2 hours;removing the formed oxide, carbide, nitride, boride, or combination thereof, and,thereafter re-forming an oxide, carbide, nitride, boride, or combination thereof, on a surface of said implant at a temperature of from 500° C. to 1000° C. for greater than 5 minutes.

42. A method of making surface hardened medical implant comprising steps of: forming said medical implant of zirconium or zirconium alloy; diffusing oxygen or nitrogen into said implant at a partial pressure of oxygen or nitrogen of less than 0.05 bar and at a temperature ranging from 500° C. to 1000° C. for greater than 2 hours; and,thereafter oxidizing or nitriding the implant between 500° C. to 1000° C. for greater than 10 minutes.

43. A method of making a surface hardened medical implant comprising the steps of: forming said medical implant of zirconium or zirconium alloy;oxidizing or nitriding said implant at a temperature of from 500° C. to 700° C. to form at least a 2 micron thick oxide or nitride; and,thereafter treating said implant under vacuum or inert gas at a temperature less than 700° C. to retain at least 0.1 microns oxide, to form at least 0.005 microns metallic hardened layer, and to form a diffusion zone having a thickness of at least 2 microns.

44. The method of claim 43, wherein the substrate further comprises titanium, tantalum, niobium, hafnium, and any combination thereof

45. The method of claim 43, wherein the oxide or nitride thickness before said step of thereafter treating said implant under vacuum or inert gas is from 2 to 15 microns.

46. The method of claim 43, wherein the oxide or nitride thickness after said step of thereafter treating said implant under vacuum or inert gas is from 0.1 to 10 microns.

47. The method of claim 43, wherein the diffusion hardened zone is from 2 to 50 microns.

48. A medical implant produced by the process comprising the steps of: forming said medical implant of zirconium or zirconium alloy;further treating said implant by any one of (a), (b), or (c), wherein (a), (b), and (c) are defined as follows: (a) treating said implant in the presence of ceramic-forming species at a temperature of less than 700° C. for greater than 5 minutes; and,thereafter treating said implant under vacuum or inert gas at a temperature of from 500° C. to 1000° C. for greater than 1 hour;(b) treating said implant in the presence of ceramic-forming species at a temperature of from 500° C. to 1000° C.; and,thereafter treating said implant under vacuum or inert gas at a temperature less than 700° C.;(c) treating said implant in the presence of ceramic-forming species at a temperature of less than 700° C.; and,thereafter treating said implant under vacuum or inert gas at a temperature less than 700° C.

49. A medical implant, comprising: (a) a first implant portion comprising zirconium or zirconium alloy, said first implant portion having a bearing surface;(b) a second implant portion comprising zirconium or zirconium alloy, said second implant portion having bearing surface;(c) wherein the bearing surface of said first implant portion and the bearing surface of said second implant portion each have a size and shape to engage or cooperate with one another;(d) a diffusion hardened zone in contact with at least a portion of said zirconium or zirconium alloy, said diffusion hardened zone forming at least a part of the bearing surface of both of said first and second implant portions, said diffusion hardened zone comprising zirconium or zirconium alloy and a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 2 microns;and,(e) a substantially defect-free ceramic layer in contact with said diffusion hardened zone and comprising a surface of said medical implant, said ceramic layer ranging in thickness from 0.1 to 25 microns;wherein the total thickness of the ceramic layer and the diffusion hardened zone is 5 microns or greater.

50. The medical implant of claim 49, wherein the ceramic layer comprises a secondary phase; and,the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, the layered structure characterized by: a first layer directly below the ceramic layer, wherein the first layer is predominantly alpha phase zirconium;an interface between the first layer and the ceramic layer; and;a second layer directly below the first layer.

51. The medical implant of claim 50, wherein the substrate further comprises titanium, tantalum, hafnium, niobium, and any combination thereof.

52. The medical implant of claim 50, wherein said diffusion hardening species is selected from the group consisting of oxygen, nitrogen, boron, carbon, and any combination thereof.

53. The medical implant of claim 50, wherein said diffusion hardening species comprises oxygen.

54. The medical implant of claim 53, wherein the diffusion hardened zone has a concentration of oxygen which decreases in the direction of the substrate, said decrease of oxygen concentration being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

55. The medical implant of claim 53, wherein the ceramic oxide has monoclinic content of greater than 93%.

56. The medical implant of claim 50, wherein the diffusion hardened zone has a hardness profile which is defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function and any sequential combination thereof.

57. The medical implant of claim 50, wherein said first layer has a thickness which is greater than or equal to the thickness of said second layer and of any subsequent layers if present.

58. The medical implant of claim 50, wherein said diffusion hardened zone has a thickness of 5 to 70 microns.

59. The medical implant of claim 50, wherein said diffusion hardened zone has a thickness of 10 to 50 microns.

60. The medical implant of claim 50, wherein said diffusion hardened zone has a thickness of 15 to 30 microns.

61. The medical implant of claim 50, wherein the hardness of the diffusion hardened zone is at least 10% greater than that of the substrate.

62. The medical implant of claim 50, wherein said medical implant said medical implant is selected from the group consisting of a hip implant, a knee implant, and a spinal implant.

63. The medical implant of claim 50, wherein said substrate comprises an alloy of zirconium and niobium and has a niobium content of at least 1% (w/w).

64. The medical implant of claim 63, wherein said substrate comprises an alloy of zirconium and niobium has a niobium content of at least 10% (w/w).

65. The medical implant of claim 50, further comprising an oxygen-containing zirconium alloy overlaying said ceramic oxide or nitride on the surface of said implant, said alloy being in the metallic state.

66. A medical implant, comprising: (a) a first implant portion comprising zirconium or zirconium alloy, said first implant portion having a bearing surface;(b) a second implant portion comprising zirconium or zirconium alloy, said second implant portion having bearing surface;(c) wherein the bearing surface of said first implant portion and the bearing surface of said second implant portion each have a size and shape to engage or cooperate with one another;(d) a diffusion hardened zone in contact with at least a portion of said zirconium or zirconium alloy, said diffusion hardened zone forming at least a part of the bearing surface of both of said first and second implant portions, said diffusion hardened zone comprising zirconium or zirconium alloy and a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 5 microns;wherein the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, said layered structure characterized by: a first layer on a surface of the implant;a second layer directly below said first layer, wherein said first layer is predominantly alpha phase zirconium; and,said diffusion hardened zone having a concentration of diffusion hardening species which decreases in the direction of the substrate, said decrease of concentration of diffusion hardening species being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

67. The medical implant of claim 66, wherein the substrate further comprises titanium, tantalum, hafnium, niobium, and any combination thereof.

68. The medical implant of claim 66, wherein said diffusion hardening species is selected from the group consisting of oxygen, nitrogen, boron, carbon, and any combination thereof.

69. The medical implant of claim 66, wherein said diffusion hardening species comprises oxygen.

70. The medical implant of claim 69, wherein diffusion hardened zone has a concentration of oxygen which decreases in the direction of the substrate, said decrease of oxygen concentration being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

71. The medical implant of claim 66, wherein the diffusion hardened zone has a hardness profile which is defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function and any sequential combination thereof.

72. The medical implant of claim 66, wherein said first layer has a thickness which is greater than the thickness of said second layer and of any subsequent layers if present.

73. The medical implant of claim 66, wherein said diffusion hardened zone has a thickness of 5 to 70 microns.

74. The medical implant of claim 66, wherein said diffusion hardened zone has a thickness of 10 to 50 microns.

75. The medical implant of claim 66, wherein said diffusion hardened zone has a thickness of 15 to 30 microns.

76. The medical implant of claim 66, wherein the hardness of the diffusion hardened zone is at least 10% greater than that of the substrate.

77. The medical implant of claim 66, wherein said medical implant is selected from the group consisting of a hip implant, a knee implant, and a spinal implant.

78. The medical implant of claim 66, wherein said substrate comprises an alloy of zirconium and niobium has a niobium content of at least 1% (w/w).

79. The medical implant of claim 78, wherein said substrate comprises an alloy of zirconium, titanium and niobium and has a niobium content of at least 10% (w/w).

80. A medical implant comprising: (a) a first implant portion, said first implant portion having a bearing surface;(b) a second implant portion, said second implant portion having a bearing surface;(c) wherein the bearing surface of said first implant portion and the bearing surface of said second implant portion each have a size and shape to engage or cooperate with one another;(d) wherein one or both of the two portions of the medical implant comprises a biocompatible alloy having an elastic modulus less than 200 GPa; and,(e) wherein the difference in radius of the mating portions is greater than about 50 microns.

81. The medical implant of claim 80, wherein one or both of said first implant portion and said second implant portion further comprises: a substrate;a diffusion hardened zone in contact with said substrate, said diffusion hardened zone comprising a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 2 microns; and,a substantially defect-free ceramic layer in contact with said diffusion hardened zone and comprising a surface of said medical implant, said ceramic layer ranging in thickness from 0.1 to 25 microns; and,wherein the total thickness of the ceramic layer and the diffusion hardened zone is 5 microns or greater.

82. The medical implant of claim 81, wherein one or both of said first implant portion and said second implant portion further comprises: the ceramic layer comprises a secondary phase; and,the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, the layered structure characterized by: a first layer directly below the ceramic layer;an interface between the first layer and the ceramic layer; and;a second layer directly below the first layer.

83. The medical implant of claim 80, wherein one or both of said first implant portion and said second implant portion further comprises: a substrate;a diffusion hardened zone in contact with said substrate, said diffusion hardened zone comprising a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 5 microns; and,wherein the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, said layered structure characterized by: a first layer on a surface of the implant;a second layer directly below said first layer; and,said diffusion hardened zone having a concentration of diffusion hardening species which decreases in the direction of the substrate, said decrease of concentration of diffusion hardening species being defined by a function selected from the group consisting of an error function, an exponential function, a near uniform distribution function, and any sequential combination thereof.

84. The medical implant of claim 80, wherein one or both of said first implant portion and said second implant portion further comprises: a substrate;a diffusion hardened zone in contact with said substrate, said diffusion hardened zone comprising a diffusion hardening species, said diffusion hardened zone having a thickness of greater than 2 microns; and,a substantially defect-free ceramic layer in contact with said diffusion hardened zone and comprising a surface of said medical implant, said ceramic layer ranging in thickness from 0.1 to 25 microns; and,wherein the total thickness of the ceramic layer and the diffusion hardened zone is 5 microns or greater.

85. The medical implant of claim 80, wherein one or both of said first implant portion and said second implant portion further comprises: the ceramic layer comprises a secondary phase; and,the diffusion hardened zone has a layered structure comprising at least two distinct layers under metallographic analysis, the layered structure characterized by: a first layer directly below the ceramic layer;an interface between the first layer and the ceramic layer; and;a second layer directly below the first layer.