Densification of nanocrystalline hydroxyapatite powder via sintering: enhancing mechanical properties for biomedical applications

The effect of compaction load, sintering temperature and soaking time on the sinter-ability and densification of Nano crystalline hydroxyapatite (HA) was assessed. The compaction and sinter-ability of HA particles was done at three different compaction loads and temperatures ranging from 1 ton to 5 ton and 850 °C to 1250 °C, respectively. Compaction of the green pellets was best achieved at 5-ton compaction load and it’s percent green densification was up to 50% of the theoretical density of HA (3.16g cm−3). For sintered density, the best results were achieved at a temperature of 1250 °C and a compaction load of 1 ton which were 98% of the theoretical density. Soaking time at these sintering temperatures was varied between 1 and 3 h and was found that with the variation of soaking time from 3 h to 1 h, the sintered density decreased tremendously at 850 °C from 85% to 50% whereas at higher temperatures the decrease in density was only 4 to 6%. The maximum hardness of 625 (±28) HV1 was obtained for HA sintered at 1250 °C with a soaking time of 3 h. Phase analyses were carried out using an x-ray diffractometer. The HA phase was stable even at the highest sintering temperature of 1250 °C and did not decompose into α tri-calcium phosphate (TCP) and β TCP. The grain size was reduced by decreasing soaking time and lowering sintering temperature.


Introduction
The increasing need to revolutionize modern treatment and bio-implants has prompted a wave of research into the field of biomaterials. Bio-ceramics have become a popular choice for bone substitute materials in modern healthcare industries. This is because their compositional similarity to human bone, as well as their high wear resistance, low density, and chemical stability [1][2][3][4]. Biocompatibility and strength are the basic properties for a biomaterial to be used as in implant either temporary or permanent to interact with host environment. Although hydroxyapatite (HA) has gained attention for its bioactive properties among calcium phosphate materials, there are only a limited number of bio-material scientists who concentrate on its fabrication and enhancement [5,6].
The first officially reported medical application of calcium phosphate bio-ceramic in human body was in 1920, In 1975 the dental applications were also reported [3]. Biomaterials made from calcium phosphate are commonly utilized for repairing dental and musculoskeletal systems. Hydroxyapatite has been used as a coating for dental applications such as tooth implants [7]. For musculoskeletal systems, calcium phosphate has various significant roles from spinal fusions to the replacement of minor load bearing joints [8].
Hydroxyapatite material is widely used in clinical applications related to orthopedics and dentistry due to its exceptional bioactive and osteoconductive properties, which result from its chemical resemblance to the mineral component of hard tissues [1,[9][10][11]. In addition, because of its resemblance in chemical structure to hard tissues found in humans, HA often displays characteristics like bioactivity and biocompatibility [12,13]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
However, it has been found by researchers that in order for HA to be employed effectively in medical applications, it is necessary for the powders to have a well-defined particle morphology [14].
Due to its inferior mechanical properties, hydroxyapatite (HA) has not been widely used in load-bearing applications and has mostly been restricted to coatings. The low fracture toughness of HA, with sintered bodies exhibiting values below i.e., <1 MPam 1/2 , is a significant concern [15]. The selection of a suitable sintering method is a crucial factor that demands attention in the processing of hydroxyapatite, as it determines the final characteristics of the resulting HA body, such as high density and fine-grained microstructure [16]. As a result, researchers have investigated and developed several synthesis methods, including the sol-gel method [17], wet hydrothermal method [18], chemical method [19], and mechanochemical method [20]. As Sintered nanocrystalline hydroxyapatite holds great potential in biomedical engineering for bone tissue engineering, dental implants, and drug delivery systems. It offers improved osseointegration, biocompatibility, and controlled drug release. It can also be used as coatings for implants and in biosensors for early disease detection [21].
Zhou et al investigated that sintering of HA is hindered by two processes, namely dehydroxylation and decomposition of the HA phase, which occur at high temperatures. After the dehydroxylation process, HA transforms into two phases tetra-calcium phosphate and tricalcium phosphate, which further impedes sintering and causes a decrease in density. Thus, the HA decomposition on high temperatures in air is a complex process that affects the sintering of HA [22].
Significant research has focused on enhancing mechanical properties of synthetic HA. The improvement of the mechanical properties of HA is challenging due to lack of phase stability resulting from pressure and temperature variations. In this paper the combined effect of sintering temperature, pressure and sintering time has been investigated. The sintering of synthetic nanocrystalline HA under different compaction loads and sintering temperatures will be done to study the densification and other mechanical properties.

Experimental procedure
The nanocrystalline HA powder was used as received (Amtech laboratories). The obtained powder was initially ground in mortar & pestle to get a uniform size. The micrograph, structure and size of the HA particles analyzed by SEM using JEOL Japan, JSM 6490A analytical scanning electron microscope, magnifications (10X-300,000X), powder and sintered pellet samples were initially coated with 200 Å thick layer of gold to make them conductive with the help of JFC 1500 ion sputtering device.
The ground HA powder was compacted under a uniaxial hydraulic press under three different compaction loads; 1 ton, 2 ton and 5 ton. 13 mm die was used for powder compaction.
Sintering of the green pellets was done in a muffle furnace KSL 1600X with different sintering temperatures and time as shown in table 1.
Heating rate was selected as 2°C min −1 on initial sintering till temperature of 700°C and then a fast heating rate of 5°C min −1 was selected. This was done to avoid the crack formation during sintering of the green pellets. Also, this change of heating rate shortened the overall sintering time for each sintering cycle. The sintering temperature was selected in a range of 850°C to 1250°C to study the sintering behavior of HA. Three temperatures were selected; 850°C, 1050°C and 1250°C. Soaking time at these sintering temperatures was varied from 1 to 3 h.
Density measurement of the sintered HA pellets were done by using Archimedes method. An average of three readings was taken for each sample. XRD analysis was carried out using STOE diffractometer, using Cu Kα monochromatic radiation at scan range of 20°to 80°with stay time at each step of 4 sec and step size of 0.4. SEM analysis was performed using JEOL Japan, JSM 6490A analytical scanning electron microscope for the grain size measurement. FTIR spectra (Perkin Elmer, USA) 400-4000 cm −1 range were collected; test samples prepared for this characterization were in pallet form. These pallets were made by mixing analytical grade Potassium Bromide (KBr) and synthesized powder in ratio 99:01 then pressing in hydraulic press at a pressure of 5 ton. For micro hardness, Vickers hardness test were considered under 1.0 kg load and 15 s dwell time and three reading were taken for each sample.

Results and discussions
The materials were characterized by different techniques to investigate the morphology, particle size, crystallite size, chemical groups attached, decomposition behavior, phase stability and phase purity of the as received HA powder. For the characterization of as received powder x-ray diffraction (XRD), Scanning electron Microscopy (SEM) and FTIR Spectroscopy were performed on the samples after the sintering process. Figure 1 displays the XRD pattern of the HA powder in its original state, where all diffraction peaks can be accurately matched to the hexagonal HA with its respective lattice constants a = 9.42Ǻ and c = 6.879Ǻ (JCPDS card No. 00-074-0565). Results evidently shows that the powder consists of hydroxyapatite, A phase that contains either no impurities or only negligible amounts of impurities. To measure the size of crystallite of the as received HA is calculated by the Scherer's Formula [1] as follows

XRD analysis
Where t represent crystallite size of the as received HA, λ represent the wavelength of Cu Kα radiation (1.54Ǻ), β represent peaks intensity at full width half maximum (FWHM) and while θ represent the Bragg's angle. The average crystallite size as calculated by this formula was 15 nm. No other phases such as α-TCP and β-TCP were observed confirming the phase purity of HA. The crystallite size increased with sintering temperature. Initially the crystallite size of un-sintered HA powder was 15 nm which then gradually increased to 54 nm, 73 nm and 95 nm respectively for 850°C, 1050°C and 1250°C. This grain size increases with increasing of soaking time as well as the grain size is get coarser with increasing in temperature [23,24]. The HA samples sintered at 850°C, 1050°C and 1250°C were then analyzed by XRD in order to check the phase stability of HA and the patterns obtained from XRD were matched with the standard pattern of HA. Figure 1 shows the indexed patterns of HA sintered as well as un-sintered HA. No new peaks were identified confirming the phase stability of HA. Even at 1250°C the HA phase was pure. As The phase stability provides controlled crystal structure affecting mechanical properties and biocompatibility. It enhances mechanical properties through densification and grain growth.
There was increase in crystallinity as the sintering temperature increased because increasing the sintering temperature enhances crystallinity due to improved atomic mobility, grain growth, impurity removal, and possible recrystallization [25]. Pang et al and Wang et al were also in agreement with the same results [26]. In addition, Yuan in agreement with the same phase purity observed for HA-ZnO composite [27].

Effect of different compaction loads on the green density of HA powder
It was noted that green density increased with the increase in compaction load. The percent densification value for 1 ton compaction load was 36% and it increased by 4% when compaction load was doubled. It further increased to 48% when compaction load was increased to 5 ton. It clearly shows that compaction load plays a significant role in the green densification of the HA pellets. There is a difference of 12% between the 1 ton and 5 ton compaction load values as shown in figure 3. An increase in compaction load leads to a higher green density due to improved particle packing, particle deformation, and enhanced interparticle bonding [23]. Majzoobi et al was also in agreement with the trend of green density against compaction of load [32].

Parameters affecting sintered density
Various parameters were investigated to check the resultant effect on the sintered density. The following three parameters were studied: Effect of compaction load on sintered density The compaction loads were varied from 1 ton to 5 ton and their effect on the resultant sintered densities were studied. These results were studied for three various temperatures 850°C, 1050°C and 1250°C as shown in figure 4.
At 850°C it was observed that the sintered density increased with the increase in compaction load. The sintered density increased from 85% at 1 ton load to 90% at 5 ton compaction load. There is a minor change in the sintered values of 1 ton and 2 ton compaction load.
At 1050°C it was observed again that the sintered values were highest for the 5 ton compaction load and the overall sintered values increased from the previous values sintered at 850°C. The sintered value difference between the three compaction loads also decreased. The percent sintered densification for 5 ton compaction load was only 2% more than the 1 ton compaction load whereas previously at 850°C it was 5%. This shows that effect of load at higher sintering temperature plays very little role in densification of HA.
At 1250°C no difference was observed in the sintered values between the two compaction loads at the extreme. The sintered values at higher temperatures are almost independent of compaction load, because at 1250°C, there is no significant difference in sintered density between the two compaction loads, indicating that the compaction load has limited impact on density at that temperature [23].
Effect of sintering temperature on sintered density The sintering temperature was varied from 850°C to 1050°C and 1250°C at different compaction loads to study the effect on the resultant sintered densities of hydroxyapatite. It was observed from figure 5 that the sintered density increased with the increase in the sintering temperature. It can be noticed that sintered densities increased linearly with increase in sintering temperature as discussed in [33]. The maximum sintered density was achieved for 1250°C which is 98.7% of the theoretical density. The lowest sintered density was observed of the pellet which was sintered at 850°C and it was found to be 85% of the theoretical density of hydroxyapatite. In agreement with theoretical density of 98.7% Muhmmad ul Hassan et al [34] extracted 98.8% sintered bulk density for Hap at pressure 500 MPa, temperature 200°C and holding time of 20 min. The following plot shows that there is a linear trend in the increase of sintered density with the increase of sintering temperature of the HA pellets. The sintered density of the pellets compacted at 5 ton and 2 ton was greater than pellets compacted at 1 ton except for the highest sintering temperature.

Effect of sintering time on sintered density
The sintering time at these sintering temperatures was varied from 1 h to 3 h to investigate the effects on sintered density with sintering time.
It was observed that with the decrease in sintering time at 850°C from 3 h to 1 h, the resultant sintered density decreased tremendously from 85% to 50% of the theoretical density of HA as shown in figure 6. This occurred due to the incomplete sintering of HA at this temperature. It shows that the sintering just started at this temperature which would complete only by increasing the sintering time to 3 h or sintering HA higher temperatures.
At 1050°C, the sintered density decreased to 83% (at 1 h sintering time) from 93% sintered densification which was achieved by sintering for 3 h. This investigation showed us that at 1050°C the sintering was 83% complete and it further can be increased by giving the proper time to get sintered densification more than 90%.
The effect of varying sintering time at 1250°C from 3 h to 1 h showed a little decrease in sintered densification of only 6% and 4% sintered densification difference by varying sintering time from 3 h to 2 h. It showed that the sintering is in the final stage at 1250°C and very little effect is observed in the final densification of HA as shown in figure 7. In agreement with thermal stability Syazwan et al also investigated that HA powder shows the crystalline structure at 800°C temperature [35].

Parameters affecting hardness
The resultant effect on the Vickers Hardness values of the following three main parameters was studied: Effect of compaction load on vickers hardness value The first parameter which was studied to observe the Vickers Hardness of HA was the variation of compaction load. Three different compaction loads of 1 ton, 2 ton and 5 ton were applied on the HA pellets prior to sintering. Then the sintering was done at three different sintering temperatures of 850°C, 1050°C and 1250°C. The sintering time was selected for 3 h.
It was observed that as the compaction load is increased the hardness values are also increased. The results clearly show that at 850°C the hardness value for 5 ton compaction load is much higher than 1 ton and 2 ton loads. The reason for increase in hardness value at higher compaction load is due to the initial higher values of green density which affects the sintering rate, thus increasing hardness values. At 1050°C there is no difference in the hardness values between 1 ton and 2 ton whereas the Vickers hardness value of the 5 ton load is 522HV1.
At 1250°C, the greatest value of hardness was achieved for 1 ton compaction load which had also the maximum sintered densification as well. The plot of hardness versus compaction load clearly shows the increasing trend of hardness values by the increase of compaction loads at lower sintering temperatures whereas at 1250°C, where sintering is almost complete the highest value is achieved by the pellet compacted at 1 ton compaction load as shown in figure 8. At the atomic scale, applying a compaction load to a HA causes compression and rearrangement of its atomic structure. This increased density and altered bonding make the HA more resistant to indentation, resulting in a higher Vickers Hardness value [36]. Niakan et al were also in agreement with the same results for sintering temperatures versus Vickers hardness [37].

Effect of sintered density on hardness value
The effect of sintered density on hardness values was studied and observed that sintered density has direct relationship with the hardness values. The hardness values were found maximum for the pellets having the highest sintered densification and vice versa. It is clear from the figure 9 that 625HV1 was achieved for the pellet having 98% sintered densification. Also the pellet having minimum sintered densification of 85% had the lowest hardness value of 200HV1.

Effect of sintering temperature on vickers hardness value
The sintering temperature has also the same effect as the sintered density. It was observedthat at 850°C the hardness values remained in the range of 200 to 366 HV1 and it increased with the increase in sintering temperature. At 1050°C the hardness value were in the range of 422 to 525 HV1.The hardness values reached a maximum value of 625 HV1 at a sintering temperature of 1250°C which had also maximum sintered densification of 98% as shown in figure 10. At the atomic scale, applying a compaction load to hydroxyapatite compresses its crystal structure, increasing atomic density and enhancing atomic bonding. This leads to improved resistance to indentation and higher Vickers hardness [36].

Effect of sintering time on vickers hardness value
The effect of sintering time on the resultant hardness values was also studied. The results clearly show that when the sintering time at the sintering temperature is decreased from 3 h to 1 h, the hardness values at lower temperatures are very much affected. At higher sintering temperature 1250°C, the effect of decreasing sintering time on the hardness values was very little. This is because of the different sintering stages at these temperatures. At lower temperature 850°C, the sintering is at its initial stage. So it needs much more time to achieve better results.
The data below shows that the hardness values at 850°C decreased from 200 HV1 to 45 HV1 when the sintering time is decreased from 3 h to 1 h. The hardness values at 1050°C are decreased from 422 HV1 to 361 HV1 when the sintering time is decreased from 3 h to 1 h. This time there is little difference in the two values as sintering is in the intermediate stage and the effect of time is also decreased as compared to 850°C values.
At the final stage of sintering at 1250°C, the hardness values were studied by varying the time from 3 h to 2 h and 1 h. It was found that the hardness decreased from 625 HV1 to 570HV1 and 512HV1 respectively when the sintering time was decreased from 3 h to 2 h and 1 h, The details are shown in table 2

Effect of sintering variables on grain growth
The effect of sintering time and sintering temperature on the resultant grain growth was studied by using SEM. Effect of sintering temperature on grain growth HA was sintered at three temperatures 850°C, 1050°C and 1250°C. As the temperature increased, sintering started, and it was observed in the SEM that with increasing sintering temperature the grain size grows as shown in figures (11)(12)(13). While the SEM of the as-received HA is shown in figure 14. The larger grains grow at the expense of smaller grains. The effect of sintering temperature on grain growth shows that higher temperatures promote larger grain sizes, while lower temperatures result in smaller grains [23]. The grain size was calculated   by Lineal Intercept Method and averages of 200 grains were measured for each sample. Different areas were selected during SEM so that actual size can be measured. The growth of grain size was also examined in SEM results by Wang and Shaw [38].

Effect of sintering time on grain growth
The sintering time/soaking time was also varied to study the behavior of grain growth and it was investigated that the grain size was decreased to 1.

Conclusion
Nano crystalline HA powders have been compacted under three different types of load ranging from 1 to 5 ton in a uniaxial hydro press and then sintered them in a temperature range from 850°C to 1250°C with a variation of soaking time from 1 to 3 h. The effect of various parameters such as compaction load, sintering temperature and soaking time on the resultant sintered density, micro hardness and flexural strength was thoroughly investigated. The green density was maximum 50% for 5 ton compaction load and pellets were sintered to a relative density of 98% theoretical density at a temperature of 1250°C and compaction load 1 ton which is greater than previous work reported under pressure less sintering. A maximum average Vickers Hardness was recorded to a value of 625 HV1 and maximum flexural strength was achieved of 40MPa for the pellet having maximum sintered density. Results of XRD analyses showed no significant phase changes even at a temperature  of 1250°C. It was concluded that at a higher compaction load of 5 ton, we can achieve a relative sintered density of 95% even at 1050°C. Also, by using Nano crystalline HA, we can achieve a relative sintered density of 90% even at very low sintering temperature of 850°C at 5 ton compaction load. This study optimizes the sintering of hydroxyapatite powder to enhance its mechanical properties for biomedical applications, by controlling sintering conditions, it improves densification, hardness, and grain growth. This contributes to more durable materials for bone implants and tissue regeneration. However, it's challenging for the densified HA upon compacted structure, in addition, how to handle the sintered pallets with 3D printer, cold spray techniques to target the complex shapes/geometries.

Data availability statement
All data that support the findings of this study are included within the article.

Conflicts of interest
The authors declare that they have no conflict of interest.