ORIGINAL RESEARCH |
https://doi.org/10.5005/jp-journals-10015-2501 |
Characterization of Aluminum Oxide–Hydroxyapatite Ceramic Composites Sintered with Hydroxyapatite at Various Concentrations and Post-sinter Degradation By-products Analysis: An In Vitro Spectroscopic Study
1–3Department of Prosthodontics and Crown and Bridge, Vivekanandha Dental College for Women, Namakkal, Tamil Nadu, India
Corresponding Author: Ranganathan Ajay, Department of Prosthodontics and Crown and Bridge, Vivekanandha Dental College for Women, Namakkal, Tamil Nadu, India, Phone: +91 8754120490, e-mail: jrangclassiq@gmail.com
Received: 02 October 2024; Accepted: 12 November 2024; Published on: 13 March 2025
ABSTRACT
Aim: To characterize newly formulated aluminum oxide–hydroxyapatite (AHA) ceramic composites sintered with hydroxyapatite (HA) incorporated at four different concentrations and to ascertain the absence of post-sinter degradation by-products in the ceramic composite.
Materials and methods: Control group A contains plain sintered aluminum oxide (Al2O3). Four experimental groups were formulated and sintered by admixing HA with Al2O3 at 2.5 wt% (group B), 5.0 wt% (group C), 7.5 wt% (group D), and 10 wt% (group E) concentrations. Field emission scanning electron microscopy (FESEM) combined with energy dispersive X-ray (EDX) spectroscopy were employed for surface topographical and elemental composition analysis, respectively. To detect post-sinter degradation by-products, X-ray diffraction (XRD) spectroscopy of ground specimens was performed.
Results: Hydroxyapatite hexagonal crystallites were absent in the control group and were observed in all the experimental groups. There were significant differences in the EDX spectra between the control and the experimental groups. EDX spectra showed peaks of calcium (Ca), phosphorus (P), and calcium aluminates (CA) in all the experimental groups, thereby ascertaining the presence of undecomposed HA. An exclusive peak corresponding to krotite appeared only in groups D and E.
Conclusion: The newly formulated AHA ceramic composites sintered with incorporated HA in all four concentrations do not contain post-sinter degradation by-products, and the HA was not decomposed when sintered with Al2O3.
Clinical significance: Neither with any prior surface treatment nor with the use of primers, this novel attempt would be beneficial in chemically bonding alumina-based ceramics with 10-methacryloxydecyl dihydrogen phosphate (10-MDP)-containing resin cements, thereby increasing the clinical serviceability of the prostheses.
Keywords: Aluminum oxide, Calcium aluminate, Ceramic composite, Chemical characterization, Hydroxyapatite
How to cite this article: Balu K, Yasmin-Fathima J, Ajay R. Characterization of Aluminum Oxide–Hydroxyapatite Ceramic Composites Sintered with Hydroxyapatite at Various Concentrations and Post-sinter Degradation By-products Analysis: An In Vitro Spectroscopic Study. World J Dent 2025;16(1):8–13.
Source of support: Nil
Conflict of interest: None
INTRODUCTION
The growing interest in esthetic dentistry has focused numerous studies on dental ceramics due to their unique mechanical, chemical, esthetic, and biological properties.1 Depending on the chemical composition, ceramics can be classified as oxide ceramics [aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂)-based] and glass ceramics [silica (SiO₂)-based]. The stable chemical structure of oxide ceramics has led to significant improvements in their mechanical properties, referred to as high-strength core ceramics.2 Although superior in terms of performance, the bonding of oxide ceramics to luting agents remains a challenge due to their nonpolar properties.3 Al₂O₃ ceramics need surface alterations to chemically bond with 10-methacryloxydecyl dihydrogen phosphate (10-MDP)-based cement. Moreover, previous studies cannot provide direct evidence of the chemical bonding between 10-MDP and ZrO₂ ceramics.4 Numerous methods that aim to alter the surface characteristics have been explored to increase the bondability of oxide ceramics, including air abrasion, acid etching, SiO2 coating, laser irradiation, etc.5 Because of the absence of a silicon oxide phase, acid etching and silanization are unreliable treatments for oxide ceramics.6 Furthermore, there is controversy regarding the effects of air abrasion on bonding because it may lead to crack development.7 Although the above approaches have been partly successful, there is no unanimity on the best surface treatment yet.8
Hydroxyapatite (HA) has gained greater significance as a biomaterial in prosthetic and restorative dentistry due to its biocompatibility, radiopacity, abrasion resistance, hardness, interfacial surface tension, and wettability.9 The antibacterial effect10 and tensile strength11 of HA added to dental ceramics are well documented in previous studies. The chemical bonding between 10-MDP and HA has been explained by Fukegawa et al.12 For many years, numerous techniques have been introduced for the deposition of HA to modify the surface of prosthetic materials such as dental implants and dental ceramics, including thermal coating, plasma spraying, sol–gel deposition, aerosol deposition, etc. However, it is important to note that each method could change the mechanical properties of the parent material, and the coating was susceptible to contamination, delamination, and disintegration, which would greatly influence the bond strength.13 Hence, it would be a breakthrough to blend the dental ceramics and HA into a composite ceramic material with a chemical bonding ability facilitated by 10-MDP at the adhesive resin-ceramic interface.
Previous attempts have been made to improve the properties of ZrO₂ by incorporating HA.14,15 Despite the superior mechanical properties, HA decomposition was observed during high-temperature sintering of ZrO₂, resulting in the formation of decomposed by-products such as alpha-tricalcium phosphate (α-TCP), beta-tricalcium phosphate (β-TCP), calcium oxide (CaO), and tetracalcium phosphate (TTCP). Such decomposition inadvertently affected the properties of ZrO₂ through poor densification and degradation.16 The incorporation of HA in Al₂O₃ and characterization of the resultant Al₂O₃–HA (AHA) composite ceramic has hardly been reported in the dental literature. Due to the lower sintering temperature of Al₂O₃ compared to ZrO₂, the incorporation of HA in the former shall be experimented with. At low sintering temperatures, the presence of post-sinter degradable by-products was presumed to be negligible. Such incorporation would be hypothetically beneficial in promoting chemical adhesion between dental aluminous ceramic prostheses and 10-MDP-containing self-adhesive resin cement. Hence, the present research aims to chemically characterize the surface topography of the novel AHA ceramic composite and to confirm the absence of post-sinter decomposition by-products.
MATERIALS AND METHODS
The present in vitro study was conducted at Vivekanandha Dental College for Women, Tiruchengode, for 6 months (November 2023–May 2024) after obtaining institutional approval. The research comprised five groups: a control group (group A) with Al2O3 without HA, and four experimental groups (groups B, C, D, and E) with Al2O3 containing HA at 2.5, 5.0, 7.5, and 10 wt% concentrations.
Preparation of Aluminum Oxide–Hydroxyapatite Ceramic Composite Powder
Pure HA powder at four concentrations (2.5, 5.0, 7.5, and 10 wt%) [<200 nm particle size, Lot No.: MKCQ2685, Sigma-Aldrich] was mixed with concentrated nitric acid to form a clear, turbid-free solution. Al2O3 powder [≤10 µm particle size, Lot No.: MKCQ4209, Sigma-Aldrich] was then incorporated into the mixture. The resulting solution was placed in an ultrasonic bath for 3 minutes to prevent particle agglomeration and ensure the uniform distribution of HA in Al2O3. The mixture was then gradually added to the ammonia solution while continuously stirring with a magnetic stirrer for 2 hours. The solution’s pH was maintained at 10. Finally, the mixture was allowed to settle overnight, after which decantation and rinsing with demineralized water were performed. After dehydrating the precipitate for 1 day in a hot air oven at 80°C for 3 hours, it was fired at 850°C for 4 hour and then ground to powder using a mortar.17 The powder was then passed through a 10 µm sieve (TIMC test sieves, Testing Instruments Manufacturing Company Pvt., Ltd., Kolkata, India) to obtain a uniform particle size. This powder was labeled as AHA ceramic composite core powder.
Preparation of Specimens
The corresponding ceramic powders of each group were mixed with ceramic modeling liquid (VITA modeling fluid, VITA Zahnfabrik, Germany) to form a slurry. The resultant slurry was poured into a cuboidal stainless-steel mold (5 × 5 × 3 mm3) using the vibration-condensation method and was uniaxially pressed. It was then dried in an oven preheated to 480°C for 8 minutes until the compacted specimen reached the green stage. The final sintering of the specimen was done at 1200°C for 1 hour (MIHM-VOGT HT-S, Germany). Three sintered specimens (n = 3) were fabricated for each group.
Field Emission Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy and X-ray Diffraction Analysis
The microstructure and surface topographies of the sintered block specimens of each group were obtained using field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray (EDX) spectroscopy (Carl Zeiss, SUPRA 55 VP). One random sintered specimen (n = 1) from each group was selected for scanning and imaging. Images were obtained at 25000× magnification. For X-ray diffraction (XRD) analysis (PW3040/60 X’pert Pro PANalytical diffractometer), all three sintered block specimens of each group were ground to powder and scanned continuously for 1 hour using Cu–Kα X-ray radiation at 1.54 Å, 40 kV, 30 mA, with a diffraction angle ranging from 10° ≤ 2θ ≤ 90°, 0.02° step size, and 20 seconds per step. The crystalline structures were determined in comparison to the known standards provided by the Joint Committee on Powder Diffraction Standards (JCPDS) and the International Centre for Diffraction Data (ICDD). The peak intensities were assessed using the Origin data analysis software (OriginLab Corporation, Massachusetts, USA).
RESULTS
The FESEM image of the group A specimen (Fig. 1) showed well-crystallized (≤10 µm) structures that appeared porous with agglomerates. The free-grain surface and boundary were irregular. Groups B to E (Fig. 2) revealed the presence of hexagonal rod-shaped HA crystallites scattered in the Al2O3 matrix. With the incorporation of HA, the free-grain surface and boundaries of Al2O3 were regular and smooth. Nevertheless, HA incorporation did not affect the pore shrinkage. An increase in the appearance of hexagonal crystallites of nano-HA was observed as the wt% of HA nano-powder increased from the experimental groups B to E. Quantitative (Table 1) and qualitative data (Fig. 3) from the EDX spectrum of group A showed visible peaks of oxygen (O) and aluminum (Al), whereas the spectra of groups B to E showed visible peaks of calcium (Ca) and phosphorus (P) additionally. This ascertains the presence of HA in the AHA ceramic composites.
| Groups | Wt% of elements | |||
|---|---|---|---|---|
| O | Al | P | Ca | |
| A | 53.38 | 46.62 | – | – |
| B | 54.68 | 43.18 | 0.63 | 1.51 |
| C | 53.99 | 41.79 | 1.64 | 2.58 |
| D | 54.56 | 38.66 | 2.14 | 4.64 |
| E | 54.36 | 37.96 | 2.56 | 5.13 |
Fig. 1: FESEM image of the control group (group A); 25000× magnification
Fig. 2: FESEM images of the experimental groups—groups B, C, D, and E at 25000× magnification
Fig. 3: EDX spectra of the control (group A) and an experimental group (group C)
The peak intensities of α-Al2O3 (JCPDS card 42-1468 and ICDD card 00-010-0173) in crystallographic planes or Miller indices were observed in the XRD spectrum of group A (Fig. 4). There were significant spectral differences between the control and experimental groups. Peaks attributable to Al2O3, HA (JCPDS card number 09-0432 and ICDD card 01-073-0293), and a range of calcium aluminate (CA) molecules such as calcium dialuminate/grossite (CaAl4O7), dodecacalcium hepta-aluminate/mayenite (Ca12Al14O33), pseudo-mayenite (Ca5Al6O14), tricalcium aluminate (Ca3Al2O6), and calcium hexa-aluminate [CaO(Al2O3)6], were observed in the XRD spectra of groups B to E (Fig. 5). Furthermore, an exclusive phase peak attributed to monocalcium aluminate/krotite (CaAl2O4) was evident in groups D and E, which was absent in groups B and C. This signified the addition of HA in the Al2O3 and the solid-state interaction between Al2O3 and HA, yielding various CAs. Moreover, there was viable HA in the ceramics without decomposition. Hence, sintering the Al2O3 ceramic matrix with a maximum of 10 wt% HA did not cause its decomposition without degradation by-products.
Fig. 4: XRD spectrum of the control group (group A)
Fig. 5: XRD spectra of the experimental groups—groups B, C, D, and E
DISCUSSION
In the present research, HA nanoparticles were utilized as the medium for bonding the Al2O3 ceramic core with the tooth by 10-MDP primers. Hence, the availability of HA nanoparticles after sintering without decomposition is crucial. The topographical evaluation of the AHA ceramic composite specimen by FESEM showed the presence of hexagonal agglomerates of HA, and different molecules of CA were seen surrounded by Al2O3 particles. The hexagonal structure consisted of a network of phosphate (PO4) tetrahedra interconnected by Ca ions interspersed among them, which is similar to the study conducted by Ma and Liu.18 EDX stoichiometric analysis of AHA ceramic composite specimens revealed the elemental composition of groups A to E. The predominant elements were Al and O, followed by Ca and P, which were also detected in groups B to E since they were included in the precursor. The diffraction peaks from the XRD spectra were found to be a good match to the standards provided by the JCPDS and ICDD for Al2O3 and HA. Besides the HA peaks in the XRD spectra of groups B to E, the appearance of various molecules of CA was found as a secondary phase due to the solid-state reaction between Al2O3 and HA.
The formation and kinetics of these CAs have been well documented in previous literature.19 Solid-state reactions for the formation of CA take place by the diffusion of Al from the Al2O3 into the HA or Ca from the HA into the Al2O3. Ca is known to be the diffusing species in the CaO–Al2O3 system.20 Since the majority of CA intermediate compounds are Al2O3-rich, it follows that the reaction proceeds by Ca diffusion from the HA to the Al2O3 at the interface, resulting in Al-rich CAs. Ji and Marquis characterized HA reinforced with Al2O3 at 1200–1400°C sintering temperatures. The XRD analysis revealed the presence of α-TCP at 1300°C. At elevated temperatures (>1300°C), further decomposition of HA was noted with no evidence of Al2O3 being traced. This indicated that Al2O3 was completely consumed by solid-state reaction with the formation of various CA molecules evident in the XRD spectra.21 Viswanath and Ravishankar found no peaks of Al2O3 in the XRD spectra when HA was reinforced with Al2O3 at 1000°C sintering temperature. This absence of peaks was attributed to the appearance of CA at temperatures starting from 1000°C, and additional peaks were observed as the wt% of Al2O3 in the ceramic composite increased. In the composites containing 20 and 30 wt% Al2O3, HA started to decompose into TCP [3Ca3(PO4)2] even at 1000°C as a result of its reactivity with Al2O3.22 The abovementioned studies focused on HA being the major ingredient with Al2O3 as a reinforcer.
On the contrary, in the present research, Al2O3 served as the major ingredient with HA as the incorporated compound. In contrast to the abovementioned studies, peaks of Al2O3 and HA were traced in the XRD spectra of the experimental groups in addition to CA peaks. The nondisappearance of the peaks of Al2O3 indicated that it was partially consumed by solid-state reaction with the formation of different formulations of CA. Likewise, the nondecomposition of HA when sintered with Al2O3 at 1200°C was attributed to the difference in Al2O3–HA ratio with a maximum of 10 wt% of HA incorporated in Al2O3.
Muralithran and Ramesh characterized the effect of sintering temperature (1000–1450°C) on the commercially available HA and stated that the XRD traces of specimens sintered below 1350°C showed no evidence of any phase except for HA. However, the presence of α-TCP and β-TCP was revealed at sintering temperatures of 1400 and 1450°C, respectively. This was due to the calcium–phosphate (Ca–P) proportion being identical to the stoichiometric value. If there had been any variation from the stoichiometric value, β-TCP would have formed in the specimens that were sintered at temperatures <1350°C.23 The result of this present research was found to be congruent with the result of the earlier studies conducted, with the absence of post-sinter degradation products at 1200°C and the Ca–P proportion identical to the stoichiometric value. Sintering of HA at temperatures >1200°C tends to eliminate the hydroxyl ion (OH–) (dehydroxylation) by losing the water molecule (H2O) in the HA matrix to form oxyapatite [Ca10(PO4)6(OH)2] with a noncharged vacancy.24,25 There will not be a reverse phase transformation, and this oxyapatite phase is stable.26 Nevertheless, additional heating to 1450°C may cause HA to break down into α-TCP, β-TCP, TTCP [Ca4P2O9], and CaO.27
Numerous studies reported the reaction of HA with ZrO2. Irrespective of the wt% of ZrO2 and HA being added, HA started to decompose at 1250°C.16 Microstructural analysis revealed that there was no significant reaction between ZrO2 and HA at 1250°C, and the reaction was only significant at temperatures above 1250°C.16 Lim et al. noted the release of water vapor during the decomposition, which was said to contribute to the hike in the porosity and reduction in the density of the composite.14 On the contrary, in this present research, the reaction between Al2O3 and HA was evident at 1200°C with no water vapor release during the reaction and accompanied by the formation of intermediate compounds of CAs.
The primary focus of this research was to characterize the AHA ceramic composite and to ascertain the availability of HA without decomposition when sintered with Al2O3 to aid in chemical bonding with 10-MDP-based resin cements. Fukegawa et al. outlined two chemical binding models of HA with 10-MDP. In the ionic binding model, 10-MDP interacts with the Ca²+ ions of HA by either having one of its P-OH groups release an H+ ion to create a P–O− group, or by both P–OH groups releasing H+ ions to generate two P–O− groups during the electrostatic interactivity with HA. In the covalent binding model, 10-MDP reacts with the PO₄³ ions of HA to produce pyrophosphates.12 Therefore, with the availability of HA in the AHA ceramics, there would be a chemical or covalent bonding at the adhesive-Al2O3 ceramic prostheses interface.
The limitation of the current research is that it neither focuses on the densification of the composite nor the mechanical properties of the AHA ceramic composites when sintered at 1200°C. Moreover, the influence of CA formation on the mechanical properties of the novel AHA ceramic composite needs to be investigated in future research.
CONCLUSION
Within the limitations, it can be concluded that a quantitative and qualitative increase in the appearance of HA was observed as the concentration of the HA increased from the experimental groups B to E. HA did not decompose at 1200°C when sintered with Al2O3, with no traces of post-sinter degradation by-products detected in all the experimental groups. In addition, various molecules of CA were found to appear due to the diffusion of Ca from the HA into the Al2O3 matrix.
DATA AVAILABILITY STATEMENTS
Data is available on request.
ORCID
Kandasamy Balu https://orcid.org/0009-0004-7886-0155
Jailaudeen Yasmin-Fathima https://orcid.org/0009-0001-5079-5177
Ranganathan Ajay https://orcid.org/0000-0001-9315-8912
REFERENCES
1. Gonzaga CC, Okada CY, Cesar PF, et al. Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics. Dent Mater 2009;25(11):1293–1301. DOI: 10.1016/j.dental.2009.03.013
2. Rizkalla AS, Jones DW. Mechanical properties of commercial high strength ceramic core materials. Dent Mater 2004;20(2):207–212. DOI: 10.1016/s0109-5641(03)00093-9
3. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89(3):268–274. DOI: 10.1067/mpr.2003.50
4. Nagaoka N, Yoshihara K, Feitosa VP, et al. Chemical interaction mechanism of 10-MDP with zirconia. Sci Rep 2017;7:45563. DOI: 10.1038/srep45563
5. Atsu SS, Kilicarslan MA, Kucukesmen HC, et al. Effect of zirconium-oxide ceramic surface treatments on the bond strength to adhesive resin. J Prosthet Dent 2006;95(6):430–436. DOI: 10.1016/j.prosdent.2006.03.016
6. Amaral R, Ozcan M, Bottino MA, et al. Microtensile bond strength of a resin cement to glass infiltrated zirconia-reinforced ceramic: the effect of surface conditioning. Dent Mater 2006;22(3):283–290. DOI: 10.1016/j.dental.2005.04.021
7. Zhang Y, Lawn BR, Rekow ED, et al. Effect of sandblasting on the long-term performance of dental ceramics. J Biomed Mater Res B Appl Biomater 2004;71(2):381–386. DOI: 10.1002/jbm.b.30097
8. Azari A, Nikzad Jamnani S, Yazdani A, et al. Deposition of crystalline hydroxyapatite nanoparticles on Y-TZP ceramic: a potential solution to enhance bonding characteristics of Y-TZP ceramics. J Dent (Tehran) 2017;14(2):62–68. PMID: 29104596.
9. Pepla E, Besharat LK, Palaia G, et al. Nano-hydroxyapatite and its applications in preventive, restorative and regenerative dentistry: a review of literature. Ann Stomatol (Roma) 2014;5(3):108–114. DOI: 10.11138/ads/2014.5.3.108
10. Mohsen CA, Abu-Eittah MR, Hashem RMM. The antibacterial and antifungal effect of silver nanoparticles and silver hydroxyapatite nanoparticles on dental ceramic. Rep opinion 2015;7(7):83–88.
11. Katamish HA, Karaksy AO, el-Mahallawy OS. The effect of the dissolution process of hydroxyapatite added to conventional dental porcelain on its mechanical strength. Egypt Dent J 1995;41(2):1085–1094. PMID: 9497644.
12. Fukegawa D, Hayakawa S, Yoshida Y, et al. Chemical interaction of phosphoric acid ester with hydroxyapatite. J Dent Res 2006;85(10):941–944. DOI: 10.1177/154405910608501014
13. Ong JL, Chan DC. Hydroxyapatite and their use as coatings in dental implants: a review. Crit Rev Biomed Eng 2000;28(5–6):667–707. DOI: 10.1615/critrevbiomedeng.v28.i56.10
14. Lim KF, Muchtar A, Mustaffa R, et al. Synthesis and characterization of hydroxyapatite-zirconia composites for dental applications. Asian J Sci Res 2014;7(4):609–615. DOI: 10.3923/ajsr.2014.609.615
15. Towler MR, Gibson IR. The effect of low levels of zirconia addition on the mechanical properties of hydroxyapatite. J Mater Sci Lett 2001;20(18):1719–1722. DOI: 10.1023/A:1012435124012
16. Kutty MG, Bhaduri S, Bhaduri SB. The effects of zirconia additions on the sintering behaviour and phase tability of hydroxyapatite ceramics. Ceram Eng Sci Proc 2001;22(4):601–606. DOI: 10.1002/9780470294703.ch68
17. Singh A, Purohit KM. Chemical synthesis, characterization and bioactivity evaluation of hydroxyapatite prepared from garden snail (Helix aspersa). J Bioprocess Biotechniq 2011;1:1–5. DOI: 10.4172/2155-9821.1000104
18. Ma G, Liu XY. Hydroxyapatite: hexagonal or monoclinic? Cryst Growth Des 2009;9(7):2991–2994. DOI: 10.1021/cg900156w
19. Singh VK, Ali MM, Mandal UK. Formation kinetics of calcium aluminates. J Am Ceram Soc 1990;73(4):872–876. DOI: 10.1111/j.1151-2916.1990.tb05128.x
20. Kohatsu I, Brindley GW. Solid state reaction between CaO and α-Al2O3. Z Phys Chem 1968;60:79–89. DOI: 10.1524/zpch.1968.60.1-6.079
21. Ji H, Marquis PM. Preparation and characterization of Al2O3 reinforced hydroxyapatite. Biomaterials 1992;13(11):744–748. DOI: 10.1016/0142-9612(92)90011-c
22. Viswanath B, Ravishankar N. Interfacial reactions in hydroxyapatite/alumina nanocomposites. Scr Mater 2006;55(10):863–866. DOI: 10.1016/j.scriptamat.2006.07.049
23. Muralithran G, Ramesh S. The effect of sintering temperature on the properties of hydroxyapatite. Ceram Int 2000;26(2):221–230. DOI: 10.1016/S0272-8842(99)00046-2
24. Juang HY, Hon MH. Fabrication and mechanical properties of hydroxyapatite-alumina composites. Mater Sci Eng C 1994;2(2):77–81. DOI: 10.1016/0928-4931(94)90033-7
25. Kijima T, Tsutsumi M. Preparation and thermal properties of dense polycrystalline oxyhydroxyapatite. J Am Ceram Soc 1979;62(9–10):455–460. DOI: 10.1111/j.1151-2916.1979.tb19104.x
26. Zhou J, Zhang X, Chen J, et al. High temperature characteristics of synthetic hydroxyapatite. J Mater Sci: Mater Med 1993;4:83–85. DOI: 10.1007/BF00122983
27. Wu JM, Yeh TS. Sintering of hydroxylapatite-zirconia composite materials. J Mater Sci 1998;23:3771–3777. DOI: 10.1007/BF00540526
________________________
© The Author(s). 2025 Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and non-commercial reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.