Body made of a ceramic material

Abstract

A body made of a ceramic material based on zirconia, the body having a surface region extending from the surface of the body to a predetermined depth and a core region integrally formed with the surface region. The ceramic material in the surface region includes a crystalline phase A formed by zirconia in tetragonal phase. The ceramic material in the surface region further includes a crystalline phase B, the crystal structure of which including apart from zirconium and oxygen at least one further component X in a periodic arrangement, the crystalline phase B having a lower theoretical density than crystalline phase A.

Claims

1. A body made of a ceramic material based on yttria-stabilized zirconia, said body being an implant or an abutment for an implant, and comprising: a surface region extending from the surface of the body to a predetermined depth, the ceramic material in the surface region comprising: a crystalline phase A formed by zirconia in tetragonal phase, and a crystalline phase B having a crystal structure comprising, apart from zirconium and oxygen, at least one further component X in a periodic arrangement, the component X being calcium or calcium oxide, the crystalline phase B having a theoretical density that is lower than crystalline phase A and lower than 6.1 g/cm.sup.3; and a core region integrally formed with said surface region, wherein a proportion of yttrium in the ceramic material is higher in the surface region than in the core region.

2. The body according to claim 1, wherein the proportion of crystalline phase B in the surface region is higher than the proportion of crystalline phase B in the core region.

3. The body according to claim 1, wherein crystalline phase B is only present in the surface region.

4. The body according to claim 1, wherein separate areas of crystalline phase B are dispersed within the surface region.

5. The body according to claim 1, wherein the proportion of crystalline phase B in the ceramic material decreases continuously in a direction from the surface of the body towards the core region.

6. The body according to claim 1, wherein the surface region extends from the surface of the body to a depth of at least 5 nm.

7. The body according to claim 1, wherein the surface region extends from the surface of the body to a depth of 10 m at most.

8. The body according to claim 1, wherein the surface region extends from the surface of the body to a depth ranging from 5 nm to 10 m.

9. The body according to claim 1, wherein the proportion of component X in the ceramic material decreases continuously in a direction from the surface of the body towards the core region.

10. The body according to claim 1, wherein the proportion of yttrium in the ceramic material decreases continuously in a direction from the surface of the body towards the core region.

11. The body according to claim 1, wherein at least a part of the surface of the body has a roughened surface.

12. The body according to claim 1, wherein the proportion of crystalline phase B in the surface region is in total about 25% at most.

13. A process for preparing a body according to claim 1, said process comprising applying (a) component X and/or a precursor thereof, and (b) yttrium and/or yttria as a stabilizing agent onto the surface of a basic ceramic body made of zirconia; and thermally treating the basic ceramic body with the component X and/or the precursor, and the stabilizing agent applied thereon at a temperature of at least 500 C., whereby component X diffuses into the basic ceramic body in an amount sufficient to form crystalline phase B, and the stabilizing agent diffuses into the basic ceramic body by the thermal treatment.

14. The process according to claim 13, wherein component X and/or the precursor are applied onto the surface of the basic ceramic body in the form of calcium or a calcium containing substance selected from the group consisting of a calcium salt, calcium oxide, calcium hydroxide, metallic calcium, and a calcium containing gel.

15. The process according to claim 13, further comprising roughening at least a part of the surface of the basic body by a subtractive treatment before applying component X and/or the precursor.

16. The process according to claim 15, wherein the subtractive treatment comprises an etching step.

17. The body obtainable by the process according to claim 13.

18. A method comprising: utilizing the body of claim 1 as an implant or an abutment for a dental implant.

19. A method comprising: increasing the fracture toughness of a basic ceramic body, by the process according to claim 13.

20. A method comprising: increasing the flexural strength of a basic ceramic body, by the process according to claim 13.

21. The body according to claim 1, wherein the proportion of crystalline phase B in the surface region is in total about 20% at most.

22. The process of claim 13, wherein component X and/or the precursor are applied onto the surface of the basic ceramic body in the form of a calcium containing substance selected from the group consisting of CaO, CaCO.sub.3, Ca(HCO.sub.3).sub.2, Ca(NO.sub.3).sub.2, and mixtures thereof.

23. The process according to claim 15, wherein the subtractive treatment comprises a sand blasting step prior to the etching step.

24. The body according to claim 11, further comprising a surface topography defined by: an arithmetic mean height Sa in a range of from 0.1 m to 1.7 m; a skewness of a height distribution S.sub.sk in a range of from 0.6 to 0.6; and/or a developed surface area Sdr in a range of from 5% to 40%.

25. The body according to claim 11, further comprising a surface topography defined by: an arithmetic mean height Sa in a range of from 0.3 m to 0.9 m; a skewness of a height distribution S.sub.sk in a range of from 0.4 to 0.6; and/or a developed surface area Sdr in a range of from 10% to 30%.

26. The body according to claim 1, wherein the body is formed by co-diffusing yttria or yttrium with component X into the ceramic material.

27. The body according to claim 1, wherein the crystalline phase B is present in the surface region in a proportion in a range of from 4.1 to 25 vol. %.

Description

EXAMPLES

(1) Sample Preparation

(2) Sample 1

(3) Discs of yttria-stabilized zirconia (Y-TZP) having a machined surface, a thickness of about 1 mm and a diameter of about 5 mm were sand-blasted and subsequently treated with a HF etching solution comprising concentrated hydrofluoric acid (HF, 40%) at elevated temperature.

(4) The discs were then provided with a thin layer of yttria (about 20 nm thick) using PVD-sputtering.

(5) A 20 mM solution of Ca(HCO.sub.3).sub.2 was prepared by preparing a 20 mM Ca(OH).sub.2 solution (0.74 g/500 ml), sterile-filtering the latter and introducing CO.sub.2 into the solution, upon which the solution becomes turbid (CaCO.sub.3) and finally turns again into a clear solution.

(6) 20 l of the Ca(HCO.sub.3).sub.2 was pipetted on the surface of the disc with the layer of yttria applied thereon. Final sintering was then carried out at about 1100 C. for either 2, 12 or 48 hours. The samples were then cooled in air, rinsed using pure water and dried under a stream of argon.

(7) For the measurement of the depth distribution of calcium and calcium containing crystalline phase(s) by XPS (X-ray photoelectron spectroscopy), a sample 1.1 was prepared using 20 mM Ca(HCO.sub.3).sub.2 in analogy to the procedure above, whereby five times 3 l of the Ca(HCO.sub.3).sub.2 was pipetted on the surface of the disc, each time followed by drying in an oven at 60 C. Directly before the application of Ca(HCO.sub.3).sub.2, the disc was cleaned using oxygen plasma.

(8) A thermal treatment was then carried out at about 1100 C. for 48 hours.

(9) The sample was then cooled in air, rinsed using pure water (5 minutes in an ultrasonic bath, then two times for 5 minutes in a beaker glass) and dried under a stream of argon. For washing away any residuals, the samples were then washed using 20% HNO.sub.3, at 90 C., for 10 minutes and then rinsed using pure water (three times in an ultrasonic bath for 5 minutes).

(10) The samples were then cleaned using oxygen plasma and directly afterwards analysed by XPS.

(11) Sample 2

(12) The sample 2 discs were prepared in analogy to sample 1, but calcium was deposited by application of a calcium containing gel onto the surface before sintering.

(13) To this end, a calcium containing gel consisting of Ca(NO.sub.3).sub.2, PVA (polyvinyl alcohol, 20 kD molecular weight) and water was prepared. Specifically, solutions of 20 wt-% PVA and 20 wt-% Ca(NO.sub.3).sub.2.4H.sub.2O were prepared with water and mixed at a ratio of 1:1. After the oxygen plasma cleaning of the samples, the gel was applied to the discs in a thickness of about 2 mm.

(14) Evaluation Methods

(15) XPS (X-Ray Photoelectron Spectroscopy) Measurements

(16) The chemical composition of the surface composition (outermost 5-10 nm) of sample 1 and sample 2 discs (YCaZrO.sub.2) was determined by X-ray photoelectron spectroscopy (XPS). Furthermore depth-profiles measurements were carried out to determine the depth distribution of calcium within the top 10'000 nm (10 m) of the surface.

(17) XPS spectra were acquired on a PhI5000 VersaProbe spectrometer (ULVAC-PHI, INC.) equipped with a 180 spherical capacitor energy analyzer and a multi-channel detection system with 16 channels.

(18) Spectra were acquired at a base pressure of 5*10.sup.8 Pa using a focused scanning monochromatic AlKa source (1486.6 eV) with a spot size of 200 m. The instrument was run in the FAT analyzer mode with electrons emitted at 450 to the surface normal.

(19) Charge neutralisation utilizing both a cool cathode electron flood source (1.2 eV) and very low energy Ark-ions (10 eV) was applied throughout the analysis.

(20) Depth-profiles were run using the ion gun (model 06-350) with a working distance of 50 mm at 450 to the sample normal. Each sample was ion etched on an area of 22 respectively 11 mm. Settings of 5 resp. 2 keV Ark-ions with an ion current of 5 resp. 2 A (measured inside a faraday cup) were used. Spectra were run in scanned mode with a pass energy of 58.7 eV.

(21) All samples were oxygen plasma treated for 2 minutes right prior to the XPS measurements.

(22) For determining the amount of calcium, the Ca LMM Auger peak was used.

(23) Crystal Structure Analysis by X-Ray Diffraction (XRD)

(24) In order to determine the crystal structure, the discs were further analysed by X-ray diffraction (XRD) using a diffractometer of the type Empyrean (PANalytical) in the /-constellation (radiation source: Cu (40 kV/40 mA); range of incidence angle: 20 to 80; step: 0.026 2; measuring time per measuring point: 300 s; continuous scan). The samples were measured in the Gonio-modus with an automatic slit. On the diffracted beam side a 0.04 mm soller slit was used as well as a Nickel filter. The analysis was performed with the software HighScore Plus (PANalytical, version 3.0.5) via Rietveld method.

(25) Contact Angle Measurements

(26) Contact angle measurements were performed in order to determine the degree of hydrophilicity or hydrophobicity. The contact angles were determined using a sessile drop test with ultrapure water (EasyDrop DSA20E, Krss GmbH). The water droplets were dosed using an automated unit and a droplet size of 0.1 l (microliter) was chosen for the samples. Contact angles were calculated by fitting a circular segment function to the contour of the droplet on the surface.

(27) Surface Roughness Measurements

(28) The microscopic roughness (conventionally also referred to as roughness strictu sensu) and macroscopic roughness (conventionally also referred to as waviness) of the sample surfaces was evaluated with confocal microscopy (surf explorer, NanoFocus AG). The same samples were used for the surface roughness measurements and for the contact angle measurements. For each sample, three probes were measured using a 20 objective with a lateral resolution of 1.56 m. The surface roughness values were determined on the entire surface image with a size of 798 m798 m.

(29) For the roughness measurements a Moving Average Gaussian filter with a cut-off wavelength of 30 m (x=31 m, y=30 m, 2019 pixels) was used. The determination of the roughness values was done by KFL analysis, limits of the amplitude density and a step width of 10 nm.

(30) The following 3D-roughness parameters were determined and analysed: arithmetic mean roughness (or arithmetic mean height) S.sub.a, topographic depth S.sub.t and skewness S.sub.sk.

(31) The following measurement parameters have been used:

(32) Piezo 0.59 m, algorithm=standard, search mode=maximized peak, threshold=4, brightness 80%, camera settings: exposure time 40 ms, gain 1.5 dB.

(33) Hydrothermal Aging Analysis

(34) Hydrothermal aging of the samples was simulated according to ISO 13356 procedure by autoclaving them at 135 C., respectively, using an autoclave of the type Systec DE-56. Assessment of the strength/hardness of the body

(35) Further experiments were carried out for assessing the strength or hardness of the body according to the present invention. To this end, discs of yttria-stabilized zirconia (Y-TZP) having a diameter of about 15 mm were polished on one side (sample 3) and subjected to the following treatment (in accordance to the treatments specifically described for sample 1 and sample 2, but without the step of providing a layer of yttrium):

(36) TABLE-US-00001 Calcium containing Sample No. component applied Thermal treatment 3.1 None (reference) 950 C., 2 h 3.2 Ca(HCO.sub.3).sub.2 950 C., 2 h 3.3 Ca(HCO.sub.3).sub.2 1150 C., 2 h 3.4 Calcium containing gel 1150 C., 2 h 3.5 Calcium containing gel 1100 C., 48 h

(37) Crystal structure analysis by XRD was carried out in analogy to the description above but with the incidence angle ranging between 20 to 80.

(38) Measurement of the surface fracture toughness was carried out by an instrumented indentation test (Zwick ZHU 2.5) using a Vickers diamond pyramid with an intersect of 136. A load of 147.2 N was used, the rate of application and relieve of the load being in each case 4.9 N/s. The holding time was set to 30 seconds.

(39) Based on the measurements, indentation hardness H.sub.IT and indentation elasticity E.sub.IT were determined. The indentation diagonal and the crack length were determined using a reflected-light microscope (Leica DM6000 M) at a 200-fold magnification. Determination of the fracture toughness was carried out using the assumption for the generation of Palmqvist-cracks according to Niihara (Niihara et al.; J. Mater. Sci. Letters 1 (1982), pp. 13-16). Specifically K.sub.1C, i.e. the critical stress intensity factor, was determined using the following equation:
K.sub.1C=0.018H.sub.ITa(E.sub.IT/H.sub.IT).sup.0.4(c/a1).sup.0.5, whereby a is half the diagonal of indentation and c=a+l, with l being the crack length.
Results
XPS Measurements

(40) The results of the XPS measurements of sample 1 and 2 discs (YCaZrO.sub.2) are shown in Table 1.

(41) TABLE-US-00002 TABLE 1 Zr Y O Si F Ca Y/2 [at [at [at [at [at [at (Zr + Y) %] %] %] %] %] %] [mol %] Sample 1 30.6 3.7 60.2 0.0 2.7 2.9 0.05 (Ca(HCO.sub.3).sub.2) 1150 C., 2 h Sample 1 33.3 2.4 60.7 0.0 1.7 1.6 0.03 1100 C., 12 h Sample 1 25.0 2.7 59.0 1.1 2.8 8.9 0.05 1100 C., 48 h Sample 2 22.8 1.0 58.0 0.8 3.9 13.1 0.02 (Ca-Gel) 1150 C., 2 h Sample 2 22.9 0.9 57.8 0.9 4.2 13.2 0.02 1100 C., 12 h Sample 2 21.9 0.9 57.9 0.9 3.8 13.7 0.02 1100 C., 48 h

(42) The results are further illustrated by way of FIG. 1 showing a graphical representation of the normalized atomic concentration of the three metals Zr, Y and Ca comprised in sample 1 and sample 2 discs for each different thermal treatment.

(43) As can be seen from Table 1, no carbon was detected on the sample 1 and 2 discs as they were measured directly after having been oxygen plasma cleaned.

(44) The sample 1 discs showed a significantly lower amount of calcium in the surface region (about 3-8 at. %) compared to sample 2 discs (about 13 at. %).

(45) Depth Profile Analysis

(46) The results of the depth profiles of sample 1 and sample 2 discs are summarized in the following figures of which: FIG. 2A shows a graphical representation of the atomic concentration of the material components in a sample 1 disc in relation to the depth of the disc body; FIG. 2B shows a graphical representation of the content of calcium in the material composition of a sample 1 disc in relation to the depth of the disc body; FIG. 3A shows a graphical representation of the atomic concentration of the material components in a sample 2 disc obtained by thermally treating the disc with the calcium containing gel applied thereon at 1100 C. for 12 hours in relation to the depth of the disc body; FIG. 3B shows a graphical representation of the content of calcium in the material composition of a sample 2 disc described for FIG. 3A in relation to the depth of the disc body; FIG. 4A shows a graphical representation of the normalized atomic concentration of Zr, Y, Ca and O in a sample 2 disc obtained by thermally treating the disc with the calcium containing gel applied thereon at 1100 C. for 48 hours in relation to the depth of the disc body, down to a depth of 10 m; and FIG. 4B shows a graphical representation of the content of calcium in the material composition of a sample 2 disc described for FIG. 4A in relation to the depth of the disc body, down to a depth of 10 m.

(47) The results of the depth profile XPS measurements presented in FIGS. 2A, 3A and 4A show that all samples contained Zr, Y and O as expected.

(48) As presented in FIGS. 2B, 3B and 4B, calcium diffused into the disc bodies and the calcium concentration decreased more or less linearly with increasing depth from the surface towards the core region.

(49) The diffusion depth was found to be dependent on the surface treatment: In case of a sample 1 disc treated at 1150 C. for 2 hours, the diffusion depth was about 1 m whereas sample 2 discs that were heated at 1100 C. for 48 hours, calcium diffused into the material down to a depth of 7 m (FIGS. 2B, 3B). It can therefore be concluded that temperature, time and sort of the calcium treatment influenced the diffusion depth of the calcium. Higher temperatures and longer heat treatment as well as calcium application as a calcium containing gel seemed to increase the diffusion depth of calcium.

(50) Another finding was that the presence of the yttrium-containing layer had a positive influence on the diffusion depth of the calcium. On samples prepared in analogy to sample 1 and sample 2 discs, but without an yttrium-containing layer, calcium was found to diffuse only down to a depth of 300 nm and 900 nm, respectively, compared to about 1000 nm and 2000 nm, respectively, measured for sample 1 and sample 2 discs treated for 2 hours at 1150 C.

(51) Without wanting to be bound by the theory, it is assumed that the additional yttrium present in the material lowers the diffusion barrier in the ZrO.sub.2 structure and facilitates the diffusion of calcium into the material.

(52) Crystal Structure Analysis

(53) XRD measurements were also performed to determine the different crystal phases in sample 1 and sample 2 discs. The results are shown in Table 2:

(54) TABLE-US-00003 TABLE 2 ZrO.sub.2 CaZrO.sub.3 tetrag- ZrO.sub.2 ZrO.sub.2 ortho- aging onal cubic monocl. rhromb. Sample 1 no 84.9 13.4 1.5 0.2 (Ca(HCO.sub.3)2) 1150 C., 2 h Sample 1 no 85.1 13.5 1.1 0.4 1100 C., 12 h Sample 1 no 77.8 16.3 1.4 4.5 1100 C., 48 h Sample 2 no 77.7 11.3 1.3 9.7 (Ca-gel) 1150 C., 2 h Sample 2 no 73.1 10.6 1.3 15.1 1100 C., 12 h Sample 2 no 65.7 9.5 1.4 23.3 1100 C., 48 h Sample 1 135 C. 78.6 11.9 9.2 0.2 1150 C., 2 h 5 h Sample 1 135 C. 82.2 13.2 4.4 0.2 1100 C., 12 h 5 h Sample 1 135 C. 77.6 16.0 1.6 4.8 1100 C., 48 h 5 h Sample 2 135 C. 77.8 11.3 1.3 9.7 1150 C., 2 h 5 h Sample 2 135 C. 74.2 9.6 1.3 14.9 1100 C., 12 h 5 h Sample 2 135 C. 65.4 9.9 1.7 23.0 1100 C., 48 h 5 h Sample 1 135 C. 51.4 14.0 34.5 0.1 1150 C., 2 h 20 h Sample 1 135 C. 59.9 13.3 26.3 0.5 1100 C., 12 h 20 h Sample 1 135 C. 74.2 17.0 4.7 4.1 1100 C., 48 h 20 h Sample 2 135 C. 74.8 11.8 5.1 8.1 1150 C., 2 h 20 h Sample 2 135 C. 72.3 9.6 5.0 13.1 1100 C., 12 h 20 h Sample 2 135 C. 62.3 5.6 3.8 28.3 1100 C., 48 h 20 h

(55) The results are further shown by way of FIG. 5 showing a graphical representation of the content of monoclinic phase as a function of the duration of a simulated aging treatment at 135 C. for sample 1 and sample 2 discs.

(56) As can be seen from FIG. 5, all sample 1 and sample 2 discs showed a content of monoclinic phase below 25% after an simulated accelerated hydrothermal aging at 135 C. for 5 hours. Even after 20 hours, only the sample 1 discs that were treated at 1150 C. for 2 hours and at 1100 C. for 12 hours showed a content of monoclinic phase that was above 25%.

(57) Sample 1 discs treated at 1100 C. for 48 hours and all sample 2 discs showed practically no hydrothermal aging, as shown in FIG. 5.

(58) XRD measurements revealed that new crystal phases developed by calcium diffusing into the material. All of these crystal phases had lower theoretical densities than the known crystal phases of zirconia (cubic, tetragonal, monoclinic).

(59) Contact Angle Measurements

(60) Table 3 gives the contact angles (CA) measured for the surfaces of the samples after having been subjected to different thermal treatments.

(61) TABLE-US-00004 TABLE 3 disc 1 disc 2 disc 3 treatment CA [] CA [] CA [] Sample 1 1150 C., 2 h 0 0 0 (Ca(HCO.sub.3).sub.2) Sample 1 1100 C., 12 h 0 0 0 (Ca(HCO.sub.3).sub.2) Sample 1 1100 C., 48 h 0 0 0 (Ca(HCO.sub.3).sub.2) Sample 2 1150 C., 2 h 0 0 0 (Ca-Gel) Sample 2 1100 C., 12 h 0 0 0 (Ca-gel) Sample 2 1100 C., 48 h 0 0 0 (Ca-gel)

(62) As can be seen from Table 3, all samples were ultrahydrophilic independent of the time of thermal treatment.

(63) Surface Roughness Measurements

(64) The microscopic and macroscopic roughness values of the surface topographies on sample 1 and sample 2 discs, treated according to table 3, were determined. The mean values obtained for the respective samples are given in Table 4.

(65) TABLE-US-00005 TABLE 4 microscopic roughness S.sub.a Std S.sub.a S.sub.t Std S.sub.t S.sub.sk Std S.sub.sk [m] [m] [m] [m] [m] [m] Sample 1 0.465 0.128 2.85 0.76 0.123 0.032 (Ca(HCO.sub.3).sub.2) Sample 2 0.505 0.700 3.10 0.58 0.140 0.038 (Ca-Gel)

(66) TABLE-US-00006 TABLE 5 macroscopic roughness S.sub.a Std S.sub.a S.sub.t Std S.sub.t S.sub.sk Std S.sub.sk [m] [m] [m] [m] [m] [m] Sample 1 0.457 0.133 2.88 0.80 0.403 0.087 (Ca(HCO.sub.3).sub.2) Sample 2 0.516 0.120 3.30 0.73 0.378 0.092 (Ca-Gel)

(67) The results are further illustrated by way of FIG. 6A showing a graphical representation of the measured microscopic roughness values on sample 1 and sample 2 discs; and FIG. 6B showing a graphical representation of the measured macroscopic roughness values on sample 1 and sample 2 discs.

(68) The results show that also when performing the process according to the present invention after the surface roughening treatment, a surface with desirable surface parameters in view of a good osteointegration can be achieved.

(69) Still further to the depth profile analysis mentioned above, the content of calcium in the material composition of a sample 1.1 disc was measured and the proportion of calcium zirconate phase formed was calculated under the assumption that all calcium is present in a calcium zirconate phase.

(70) The results are given in FIG. 7 showing a graphical representation of the molecular proportion (circles), the mass proportion (squares) and volume proportion (diamonds) of the calcium zirconate phase as well as the measured content of calcium (crosses) and the calculated content of calcium (triangles) in a sample 1.1 disc in relation to the depth of the disc body.

(71) As is obvious from the decrease in the molecular proportion of the calcium zirconate phase with increasing depth in comparison to the respective decrease in the volume proportion, the formation of the calcium zirconate phase results in building up a compressive stress within the surface region.

(72) The results of the crystal structure analysis mentioned above is shown in Table 6, showing that calcium containing crystalline phases, i.a. a calcium zirconate phase, is formed and that the formation of the calcium zirconate phase is favoured with increasing temperature.

(73) TABLE-US-00007 TABLE 6 Sample ZrO.sub.2 ZrO.sub.2 ZrO.sub.2 No. tet. mon. cub. CaZrO.sub.2 CaZrO.sub.3 3.1 82.4% 2.0% 15.6% 0.0% 0.0% 3.2 85.4% 0.6% 10.5% 3.4% 0.1% 3.3 73.9% 0.6% 8.4% 4.0% 13.0% 3.4 66.3% 1.4% 10.0% 3.4% 18.9% 3.5 74.4% 1.0% 12.0% 3.4% 9.2%

(74) The results of the determination of the E.sub.IT-, H.sub.IT- and K.sub.1C-values is given in Table 7, showing a clear trend for an increased indentation hardness and an increased fracture toughness for the samples comprising an increased amount of calcium containing crystalline phases.

(75) TABLE-US-00008 TABLE 7 Sample No. E.sub.IT [GPa] H.sub.IT [MPa] K.sub.1C [MPa .Math. m] 3.1 135.15 10727.0 5.17 3.2 159.85 11716.5 5.70 3.3 176.45 14234.5 6.29 3.4 177.40 14901.0 6.43 3.5 172.70 15826.5 6.21

(76) The results obtained show a clear indication for the amount of calcium containing phases formed correlating with the fracture toughness of the body and a saturation for the achievable fracture toughness to be obtained at a proportion of crystalline phases of about 25%.