Crystalline phase evolution and thermal behavior of zirconia-lanthanum aluminate ceramics produced by surface modification

Abstract

The purpose of the present work was to investigate the effect doping with lanthanum aluminate on the phase assemblage and thermal behavior of zirconia ceramics for biomedical applications. Four compositions were prepared by a surface modification route of either conventional tetragonal zirconia (3Y-TZP) or high translucency cubic-based zirconia (5Y-PSZ) to reach a nominal composition of either 0.5 wt. % (3Y-0.5LAO and 5Y-0.5LAO) or 5 wt. % of lanthanum monoaluminate (3Y-5LAO and 5Y-5LAO). Undoped powders were used as controls. DTA and XRD analyses revealed that lanthanum dizirconate crystallized in the 934– 936°C range, while lanthanum aluminate crystallized in the 1056–1063°C range in both types of zirconias doped at the 5% level. No second phase was found in compositions doped at the 0.5% level. The a lattice parameter and the amount of the cubic phase increased in both 3Y-5LAO and 5Y-5LAO. The microstructure of the compositions doped with 5% LAO was characterized by well distributed LAO twinned crystals and sparse needle-shaped lanthanum hexaaluminate crystals. A bimodal grain size distribution was observed in 5Y-doped compositions. This was attributed to abnormal grain growth of the cubic phase, and in line with aluminum segregation at grain boundaries and the presence of second-phase LAO crystals.

1. Introduction

Zirconia ceramics were introduced in dentistry in 2001 and have since continued to gain significant momentum with a span of dental applications ranging from single unit restorations to implant supported prostheses, abutments and dental implants^1–3. The recent introduction of translucent, albeit lower strength, compositions including substantial amounts of cubic zirconia⁴, as well as the technical evolution of CAD/CAM systems has further expanded the use of zirconia ceramics in dentistry to unprecedented levels for any dental ceramic system⁵. Meanwhile dental implants have become a standard of care for the treatment of simple and complex prosthodontic cases. Commercially pure titanium as well as titanium alloys have traditionally been used as implant materials and have demonstrated high levels of clinical success with excellent biocompatibility and osseointegration. However, there are some concerns regarding the use of metals as dental implant materials with a few reported cases of metal allergies⁶. As a result, the potential use of zirconia as an implant material has attracted considerable interest. Zirconia is considered bioinert and non-allergenic, while providing an aesthetic basis for anterior implant-supported restorations⁷. Conversely, zirconia has also been shown to be associated with lower levels of osseointegration than CP titanium due to difficulty of developing a zirconia surface treatment that compares to those of anodized or acid-etched titanium implants⁸. Furthermore, the susceptibility of zirconia (3Y-TZP) to low temperature aging may constitute a critical issue for dental implants with constant interaction with body fluids under cyclic loading conditions^9–11. To address this issue, recent work has demonstrated the efficacy of both lanthanum oxide and aluminum oxide in improving the low temperature aging resistance of zirconia^12,13. Lanthanum has also been shown to enhance the biocompatibility of ceramics for biomedical applications, particularly when combined with hydroxyapatite^14–17. Similarly, an enhancement of the osteogenic response has been demonstrated when calcium aluminate phases have been used in bone cements^18–20. Meanwhile, lanthanum monoaluminate phases have attracted considerable interest for their potential applications due to their dielectric properties²¹. Lanthanum aluminum oxide (LAO) belongs to the perovskite family²². LAO crystals are optically transparent and, similarly to 3Y-TZP, undergo ferro-elastic domain switching under stress, which can be advantageously exploited as a strengthening mechanism^23–25. LAO has been successfully used as a ferro-elastic second phase to strengthen lanthanum dizirconate ceramics²⁶. Development of dual-phase zirconia-LAO ceramics could therefore potentially lead to an increase in strength for translucent zirconia (5Y-PSZ) or an increase in biocompatibility and osseointegration capabilities for high strength zirconia (3Y-TZP).

Novel dual-phase ceramics have recently been introduced using an innovative processing route^27,28. Briefly, the surface of the ceramic powder is coated with inorganic precursors of a second phase. Thermal treatment then prompts crystallization of the second phase on the surface of the parent material. Adequate mixing of between parent phase and inorganic precursors ensures an homogeneous distribution of the second phase after crystallization^28,29.

The aim of the present work was to screen new zirconia compositions for potential dental applications and investigate the synthesis, crystallization behavior and microstructure of dual phase LAO-zirconia ceramics prepared via surface coating. We hypothesized that homogeneously distributed dual phase zirconia/LAO ceramics could be produced by using the grafting technique and that microstructure could be controlled by subsequent heat treatment.

2. Materials and methods

2.1. Dual phase ceramic powder preparation

An alumina-free commercial zirconia powder stabilized with 3 mol. % yttria (TZ-3YS, Tosoh, Japan) and a high translucency commercial zirconia powder stabilized with 5.2 mol.% yttria³⁰ (ZPex Smile, Tosoh, Japan) were used as raw materials for the preparation of dual phase ceramic powders containing either 0.5 wt. % LAO (referred to as 3Y-0.5LAO and 5Y-0.5LAO, respectively) or 5 wt. % LAO (referred to as 3Y-5LAO and 5Y-5LAO, respectively). The nominal composition of the starting zirconia powders and prepared zirconia/LAO mixes is given in Table 1.

Table 1.

Nominal compositions of the various powders prepared.

Wt. %	3Y(TZ-3YS*)	3Y-0.5LAO	3Y-5LAO	5Y (ZPex Smile*)	5Y-0.5LAO	5Y-5LAO
ZrO2	94.8	94.3	90.1	90.65	90.2	86.2
Y2O3	5.2	5.2	4.9	9.30	9.3	8.8
Al2O3	0.005	<0.005	<0.005	0.050	<0.05	<0.05
LAO	0	0.5	5	0	0.5	5

^*From manufacturer’s data.

Aluminum nitrate nonahydrate Al(NO₃)₃.9H₂O (>98% purity, Sigma-Aldrich, USA) and lanthanum nitrate hexahydrate La(NO₃)₃.6H₂O (>99.0% purity, Sigma-Aldrich) were used as aluminum and lanthanum precursors, respectively. The powders were prepared according to a technique adapted from Palmero et al.²⁹. Briefly, the zirconia powder was dispersed in ultrapure water at a solids loading of 49 wt. %. The pH of the dispersion was adjusted to 3 by adding diluted hydrochloric acid. The dispersion was mixed for 24 h in a shaker-mixer (Turbula, GlenMills, USA) with the aid of 2 mm-diameter zirconia balls as milling media. Aluminum and lanthanum nitrates were then dissolved in ultrapure water at an Al/La molar ratio of 1. This aqueous solution was added dropwise to the zirconia slurry and stirred for 2 h at room temperature. The slurries were freeze-sprayed in liquid nitrogen using an ultrasonic probe, transferred to a freeze dryer (Advantage, Virtis, USA) and dried for 48 h. Recovered powders were placed in polyurethane molds and cold isostatically pressed into cylindrical blanks (10 mm in diameter, 25 mm in length) at 300 MPa (Flow Autoclave CIP, Autoclave Engineers, USA).

2.2. Crystallization behavior

Differential thermal analyses (DTA) were performed on the freeze-dried powders under nitrogen gas flow to investigate the thermal behavior (Q600 DTA/TGA, TA Instruments, USA). Analyses were performed at a heating rate of 40 °C/min. up to 1100°C. Aluminum oxide (99.99%) powder served as reference standard.

The powders were heat treated at various temperatures ranging from 750 to 950°C in 100°C increments, and from 950 to 1500°C in 50°C increments, at a heating rate of 10 °C per minute. Disc-shaped specimens were sectioned from the pressed cylindrical blanks with a low speed diamond saw and sintered at 1500°C for 2 h. Crystalline phases present in powders and sintered specimens were characterized by x-ray diffraction (XRD). Scans were performed in the two-theta range 20–90°, at 40 kV and 44 mA in Bragg-Brentano configuration (Smartlab, Rigaku Americas, USA.). Peak positions were determined using PDXL-2 analysis software (PDXL-2, Rigaku Americas, USA) after calibration using silicon powder standard (NIST, 640d). Lattice parameters were determined by Rietveld refinement³¹ and compared to published values and powder diffraction files to assess relative amounts of crystalline phases. The amount of cubic phase was determined by performing additional scans in the 72–76° range (2θ) at a scanning speed of 0.2 deg.min⁻¹, and a step size of 0.04° (λCuKα=1.5406Å). Scans were analyzed with Rigaku PDXL-2 software. Background subtraction was first performed, followed by Kα₂ elimination and profile fitting. This angular region focuses on 004 and 400 reflections of the t-phase and the 400 reflection of the cubic phase. The amounts of the various phases were determined from integrated intensities using equation (1)³²:

$$Vc=\frac{I{c}_{\left(400\right)}}{I{c}_{\left(400\right)}+I{t}_{\left(400\right)}+I{t}_{\left(400\right)}}$$ (1)

Where Vc is the volume fraction of the cubic phase and I represents the integrated intensity for each listed reflection. When the cubic phase was replaced by the cubic-derived t’ phase, summed integrated intensities of the 004 and 400 reflections of the t’ phase were used in replacement of the integrated intensity of the 400 reflection of the cubic phase in equation (1).

2.3. Microstructural characterization –

The microstructure was characterized by scanning electron microscopy (Hitachi S-4800 field emission SEM equipped with XRF target x-ray tube for microanalysis, Hitachi, Japan) on sintered specimens. Routine SEM preparation techniques including fine polishing to a 1 μm-finish, thermal etching and gold coating were used to analyze the microstructure. X-ray fluorescence microanalyses were performed on compositions 3Y-5LAO and 5Y-5LAO. The mean grain size was determined by image analysis on digital micrographs (n=4 per group) using NIH Image J software³³, averaging 400 grains per group. A correction of 1.56 was used to calculate the real grain size, assuming equiaxed grains³⁴. This technique was selected preferentially to the lineal intercept technique³⁵ due to the bimodal distribution observed for some of the experimental groups. Atomic force micrographs were also acquired on thermally etched but uncoated specimens in contact mode to complete microstructural characterization (Model: INNOVA AFM, Bruker, USA).

2.4. Statistical analyses

Results were analyzed by ANOVA using the Tukey adjustment for multiple comparisons in conjunction with an overall 0.05 level of Type I error. A P-value of less than 0.05 was considered statistically significant. Grain size distributions were analyzed using JMP statistical software (JMP®, Version 15.0, SAS Institute Inc., Cary, NC, 1989–2019). Fits for a mixture of two normal distributions, a single normal distribution, and a lognormal distribution were compared for each experimental group using the Schwarz Bayesian Information Criterion (BIC). The model with the lowest BIC value is considered to have the best fit.

3. Results

3.1. Crystallization behavior

Results from differential thermal analyses for 3Y-5LAO and 5Y-5LAO powders are displayed in Figure 1. Both compositions exhibited two exothermic peaks located at 934°C and 1058°C for 3Y-5LAO, and 936°C and 1063°C for 5Y-5LAO, respectively. All other compositions were devoid of exotherms or endotherms (plots not shown).

Figure 1.

Differential thermal analysis plots for 3Y-5LAO and 5Y-5LAO compositions.

X-ray diffraction patterns of 3Y-5LAO and 5Y-5LAO powders heat treated at various temperatures ranging from 600 to 1500°C are shown in Figure 2A and 2B, respectively. After heat treatment at 600°C, monoclinic zirconia (29 vol.%) was present in 3Y-5LAO powder, in addition to the tetragonal phase. A smaller amount of monoclinic phase (3 vol.%) was present in 5Y-5LAO powder. However, none of the nitrate precursors was found in any of the powders. Crystalline phase evolution followed the same pattern for both compositions. After heat treatment at 1100°C for 1h, lanthanum dizirconate (La₂(Zr₂O₇); LZ) was present, together with lanthanum monoaluminate (LaAlO₃; LAO). The amount of LZ decreased significantly after heat treatment at 1250°C for 1h, while the amount of LAO increased. There was no LZ remaining after heat treatment at 1300°C for 1h. Heat treatment at 1500°C for 1h led to the crystallization of a significant amount of cubic phase (25±8%) in 3Y-5LAO powder. The amount of cubic phase and cubic-derived tetragonal prime phase present in each composition after full sintering heat treatment at 1500°C for 2h, and lattice parameters are summarized in Table 2. Representative x-ray diffraction patterns of bulk sintered specimens are displayed in Figure 3A and 3B. Traces of lanthanum hexaaluminate (LaAl₁₁O₁₈; LH) were found, in addition to LAO in 3Y-5LAO and 5Y-5LAO compositions. The amount of cubic or cubic-derived t’-phase increased with the lanthanum doping level, for both ceramic materials. Only tetragonal or cubic zirconia was found in the 3Y and 5Y undoped compositions and in 3Y-0.5LAO or 5Y-0.5LAO compositions. Lattice parameters results from Rietveld refinement analyses are listed in Table 2. The a lattice parameter for the cubic phase present in 3Y-5LAO and 5Y-5LAO was 5.1395 and 5.1416 Å, respectively. There was no t’-phase in these compositions.

Figure 2.

X-ray diffraction patterns after heat treatment at various temperatures. A: 3Y-5LAO powder, B: 5Y-5LAO powder.

Table 2.

Amount of cubic (c) and cubic-derived (t’) phase, lattice parameters and mean real grain size for the various compositions. Identical letters denote no statistically significant difference (P>0.05).

Composition	Amount of cubic (c) or t’-phase (t’) % (SD)		Lattice parameters t-phase (Å)	Lattice parameters t’-phase (Å)	Lattice parameter c-phase (Å)	Mean grain size μm (SD)
3Y	10 (2)	t’	a = 3.6061 c = 5.1802	a = 3.6246 c = 5.1548	N/A	0.50 (0.16)a
3Y-0.5LAO	21 (2)	t’	a = 3.6055 c = 5.1820	a = 3.6266 c = 5.1572	N/A	0.61 (0.22)ab
3Y-5LAO	25 (2)	c	a = 3.6044 c = 5.1847	N/A	a = 5.1416	0.65 (0.26)b
5Y	71 (5)	t’	a = 3.6067 c = 5.1796	a = 3.6247 c = 5.1554	N/A	1.04 (0.48)c
5Y-0.5LAO	80 (1)	t’	a = 3.6056 c = 5.1839	a = 3.6253 c = 5.1561	N/A	1.11 (0.36)c 3.48 (1.48)d
5Y-5LAO	90 (8)	c	a = 3.6045 c = 5.1848	N/A	a = 5.1395	0.80 (0.23)b 2.30 (0.93)d

Figure 3.

X-ray diffraction patterns after full sintering at 1500°C for 2h. A: 3Y series; B: 5Y series.

3.2. Microstructure and elemental analyses

Characteristic micrographs for each ceramic composition after sintering at 1500°C for 2h are displayed in in Figure 4 (A through F). Mean grain size distributions are shown in Figure 5. Distributions followed a log normal fit, except for 5Y-0.5LAO and 5Y-5LAO, which were best described by a bimodal distribution fit (mixture of two normal distributions) based on the Bayesian Information Criterion (BIC). Note that the difference in the BICs of the bimodal fit versus each of the other two models exceeds ten for both 5Y-0.5LAO and 5Y-5LAO, indicating very strong support for superior fit of the mixture model.³⁶ The overall grain size increased significantly with doping level for 5Y compositions, while this increase was more modest for 3Y compositions. Both 5Y-5LAO and 3Y-5LAO ceramics exhibited submicrometer polyhedral twinned crystals and needle-shaped submicrometer crystals appearing in darker contrast (Figure 4C and 4F). Polyhedral twinned crystals were present at the surface of larger grains and between grains for 5Y-5LAO. They were present only in intergranular positions in 3Y-LAO. Needle-shaped crystals were located mostly along grain boundaries in both materials. Lower magnification atomic force micrographs illustrating crystal distributions are shown in Figure 6 (A through F). The trend towards the development of a bimodal distribution can be seen from Figure 6A through 6C. Larger cubic grains appear to grow while medium and smaller sized-grains remain unchanged. Semi-quantitative x-ray fluorescence elemental analyses were performed to help identify twinned polyhedral crystals and needle-shaped crystals in both materials. Elemental maps are shown in Figure 7 A and B for 3Y-5LAO and 5Y-5LAO, respectively. In both ceramic materials, needle-shaped crystals appeared rich in aluminum while polyhedral twinned crystals were rich in both aluminum and lanthanum.

Figure 4.

Representative scanning electron micrographs of each experimental ceramic. A: 5Y; B: 5Y-0.5LAO; C and C1: 5Y-5LAO; D: 3Y; E: 3Y-0.5LAO; F and F1: 3Y-5LAO. Arrows point at lanthanum monoaluminate twinned crystals.

Figure 5.

Grain size distributions for the various experimental groups. A: 3Y; B: 3Y-0.5LAO; C: 3Y-5LAO; D: 5Y; E: 5Y-0.5LAO; F: 5Y-5LAO

Figure 6.

Representative atomic force micrographs for each experimental ceramic. A: 5Y; B: 5Y-0.5LAO; C: 5Y-5LAO; D: 3Y; E: 3Y-0.5LAO; F: 3Y-5LAO

Figure 7.

XRF elemental maps for A:3Y-5LAO; B: 5Y-5LAO.

4. Discussion

Results from DTA analyses revealed a similar behavior for 3Y-5LAO and 5Y-5LAO compositions, with the presence of two exotherms occurring in the temperature range (934–1063°C). XRD analyses confirmed a similar behavior between the two compositions with crystallization of lanthanum zirconate (LZ) first, followed by crystallization of lanthanum monoaluminate (LAO), in that temperature range. These two exotherms are therefore tentatively attributed to LZ (934, 936°C) and LAO (1058, 1063°C).

XRD analyses shoed that LZ was present in 3Y-5LAO and 5Y-5LAO from 1000°C to 1300°C. This temperature range is in agreement with the study by Miura et al.³⁷ in Ce-TZP, which showed that LZ formed at 800°C and remained in the temperature range 800–1500°C, while LAO crystallized in the 1200–1400°C range. In the present work, LAO was already present at 1000°C and up to 1500°C. This discrepancy could be due to the processing route. Miura et al.³⁷ used a mixture of alumina, lanthanum oxalate and zirconia, while the present study explored a novel processing route^29,38, based on surface coating and crystallization of the second phase on the zirconia grains to ensure homogeneous phase distribution. This technique could have favored reaction and crystallization of lanthanum monoaluminate at lower temperature, as described by Kuo et al.³⁹, who successfully synthesized LAO using a co-precipitation process from nitrate precursors, with a crystallization exotherm as low as 810°C.

The increase in the amount of LAO at the expense of LZ as the heat treatment temperature increased can be described by the following reactions^37,40:

$$L{a}_2{O}_3+2Zr{O}_2\to L{a}_2\left(Z{r}_2{O}_7\right)\left(\text{lanthanum}\text{dizirconate;}\text{Lz}\right)$$

$$L{a}_2\left(Z{r}_2{O}_7\right)+A{l}_2{O}_3\to 2{\mathit{\text{LaAlO}}}_3+2Zr{O}_2$$

These reactions could explain the increase in the amount of cubic phase in 3Y-5LAO after sintering at 1500°C, with some availability of lanthanum as additional stabilizer. Sintering at 1500°C for 2h also led to the crystallization of lanthanum hexaaluminate (LaAl₁₁O₁₈, LH), traces of which were found after sintering at 1500°C, in both 3Y-5LAO and 5Y-5LAO. According to previous literature^40,41, the reaction kinetics for lanthanum hexaaluminate are very slow. Crystallization was reported to begin above 1500°C and complete formation was achieved at 1600°C for 1h, according to the following reaction³⁷:

$${\mathit{\text{LaAlO}}}_3+5A{l}_2{O}_3\to LaA{l}_{11}{O}_{18}$$

The presence of LH in 3Y-5LAO and 5Y-5LAO compositions could also be due to the processing route used, permitting in situ reaction at the surface and between the zirconia grains at lower temperatures.

XRF elemental analyses for these compositions revealed that submicrometer polyhedral twinned crystals were rich in aluminum and lanthanum and could therefore correspond to the LAO phase, which appeared homogeneously distributed throughout the ceramics. Needle-shaped crystals appearing in darker contrast were rich in aluminum with a few lanthanum counts, and could be identified as lanthanum hexaaluminate crystals with a similar morphology to that described in the literature^37,42,43

The a lattice parameter of the cubic phase present in 3Y-5LAO and 5Y-5LAO was higher than expected for 3Y or 5Y zirconia powders⁴⁴. This could indicate enrichment in the amount of yttrium stabilizer, with lanthanum potentially acting as co-stabilizer. The a and c lattice parameters for the tetragonal phase did not change significantly with the amount of doping for either series, and were within the range expected for 3Y-TZP.

The evolution of the grain size as a function of doping amount in the 5Y series confirmed the significant effect of both aluminum and lanthanum on grain growth in partially stabilized zirconia. According to previous studies^45,46, alumina additions enhance the sinterability of 3Y-TZP as alumina ions tend to segregate at grain boundaries and promote grain boundary diffusion because the ionic radius of Al³⁺ ions (0.068 nm) is smaller than that of Y³⁺ ions (0.104 nm). It is expected that at 1500°C and above, grain boundary diffusion, favored by Al³⁺, dominates the grain growth behavior over the solute-drag effect of Y³⁺ ions. Furthermore, Al³⁺ ions segregated at grain boundaries also accelerate the formation of cubic phase. Conversely, numerous studies have demonstrated that co-doping 3Y-TZP with lanthanum caused an inhibition of both grain growth and densification as a result of decrease in interfacial energy as lanthanum segregated to both surfaces and grain boundaries^47–49. Zhang et al.^13,50 showed that co-doping 3Y-TZP with both aluminum and lanthanum led to segregation of both dopants at grain boundaries and successfully increased the resistance to low temperature degradation. In the present work, it is postulated that although no phases other than zirconia were found in 5Y-0.5LAO composition, aluminum ions segregated at grain boundaries and dominated the grain growth mechanism, leading to a significant increase in grain size of the cubic phase. Meanwhile, yttrium and lanthanum segregation at grain boundaries in 3Y-0.5 LAO led to a solute drag effect that surpassed that of aluminum and this kept grain growth to a minimum. The situation was somewhat different for 5Y-5LAO, it is hypothesized that the crystallization of LAO and LH depleted the amount of lanthanum and aluminum ions available and that grain boundary segregation was less prevalent, permitting ample grain growth of the cubic grains, as illustrated by a bimodal grain size distribution. Furthermore, the presence of secondary phases at grain boundaries likely increased grain boundary energy and favored abnormal grain growth⁵¹. The observed increase in grain size was modest for 3Y-5LAO but significant for 5Y-5LAO. This is consistent with previous literature showing that grain growth is slower in tetragonal zirconia compared to cubic zirconia^52,53.

5. Conclusions

Dual-phase zirconia-lanthanum aluminate ceramics were successfully produced by a surface coating technique using only inorganic precursors and aqueous media and ensuring a homogeneous distribution of second-phase crystals in both 3Y and 5Y-PSZ.

Significant grain growth occurred for the 5Y-LAO doped compositions regardless of doping level. This result is in line with an established faster grain growth rate for the cubic phase, compared to the tetragonal phase, and the prominent role of aluminum ions on grain growth.