This research investigated the relationship between the quantitative phase crystal structure and mechanical properties of ZrO₂ ceramic addition of Y₂O₃, MgO and BaCO₃ at 0.0, 2.0, 4.0 and 6.0 mol%. Ceramic samples were prepared using mixed oxide method under normal sintering at 1,600 ^oC with dwell time for 120 min . ZrO₂-Y₂O₃ and ZrO₂-MgO ceramics were obtained with bulk densities values between 5.324-5.722 g/cm³ while ZrO₂-BaCO₃ ceramic showed densification values about 4.412-4.827 g/cm³. It was found that ZrO₂-Y₂O₃ and ZrO₂-MgO ceramic showed higher fracture toughness values than ZrO₂-BaCO₃ ceramics. Refinement of lattice parameter using Rietveld analysis in ceramic samples revealed the percentage of fraction phase ratios of m-ZrO₂, t-ZrO₂ and c-ZrO₂. The refinement parameters result in sample ceramic which are Y₂O₃ addition between 4-6 mol% obtained a high ratio of t-ZrO₂ phase and the result supported optimal mechanical properties. ZrO₂-Y₂O₃ ceramic showed a higher lattice stain value compared with the additions of other ZrO₂ oxides and it was found that lattice strain increase with high ratio of t-ZrO₂ phase. Sample ceramics had crystallite size values between 56.65-82.30 nm. SEM micrographs revealed morphology and average grain sizes. All samples grains were spherical in shape combined with irregular shape and were gray in color and were obtained with an average grain size between 0.63 -2.18 µm. It was found that the ZrO₂-Y₂O₃ ceramic showed small crystallize size and size of grains. The optimal condition for addition of oxide were found in ceramics of ZrO₂-Y₂O₃ and ZrO₂-MgO and confirmed that good mechanical properties were obtained from a high ratio of t-ZrO₂ phase and fine grain size

Keywords: ZrO₂, Y₂O₃, MgO, BaCO₃, Rietveld refinement

ZrO₂ has attracted extensive attention for decades because it exhibit excellent characteristics such as low thermal conductivity and good mechanical properties under high temperature conditions. These properties have led to the use of zirconia-based components in many applications such as automobile engine part, medical devices and cutting tool. Zirconia is a polymorphic metastable material that exists in three crystallographic phases: monoclinic phase (up to 1,170 ^oC), tetragonal phase (1,170-2,370 ^oC), and cubic phase (2,370-2,680 ^oC) [1]. Zirconia is generally stable at room temperature in monoclinic phase. Zirconia in the tetragonal form at room temperature has the best mechanical properties among the forms [2]. The tetragonal (t-ZrO₂) to mono- clinic(m-ZrO₂) martensitic transformation occurring during cooling after sintering is detrimental to sintered zirconia integrity as this process is accompanied by a large increase in volume, leading to disintegration by crack formation and propagation [3]. The high temperature polymorphs of pure zirconia cannot be retained by quenching to room temperature [4]. Many reports are attempt to synthesize for a metastable tetragonal phase formation in the zirconia at low temperature [5-7].
Stabilizing oxides such as CaO, Co, Y₂O₃ and MgO are used to stabilize in the cubic and tetragonal zirconia form at room temperature. Previous works reported [8-10] effects of zirconia ceramic stabilizing oxides on the phase structure, microstructure and mechanical properties. The mechanism of t-ZrO₂ to m-ZrO₂ transition is typical of partially stabilized zirconia and commonly supported by many studies of the transformation strength process, which has been widely, discussed [11, 12]. However, the study of the relationship of quantitative analysis in crystal zirconia stabilizing oxides ceramic with XRD patterns on its characteristics is of interest to study.
Therefor the aim of this research is to investigate effects of oxides doping on characterization of zirconia ceramics such as physical properties, phase composition, crystalline structure, microstructure, mechanical properties. Rietveld refinement was used to calculate significant quantities in crystal to describe the relationship between crystal structure and ceramics properties.

Powders with ZrO₂ addition of Y₂O₃, MgO and BaCO₃ at 0.0, 2.0, 4.0 and 6.0 mol% were prepared from ZrO₂, MgO, Y₂O₃ and BaCO₃ as precursors and isopropyl alcohol as solvent. All the ten different batches were then ball milled for 24 h. After ball-milling, drying in electronic furnaces and sieving with 120 mesh, the resulting powders were calcined at 1,200 ^oC, with dwell times for 120 min and heating/cooling rates of 10 ^oC/min. The powders were then pressed at 3 MPa into form pellets having 1.5 cm diameter using a hydraulic press and sintered in an alumina crucible at a temperature of 1,600 ^oC for 120 min with heating rate of 10 ^oC/min. The sintering experiments were carried out in an electrical furnace (Nabertherm, Germany). The bulk densities of sintered sample were calculated using Archimedes’ method and measured percentage of linear shrinkages. Phase identification was then performed using an X-ray diffractometer (XRD; Bruker model D8 advance). Next, Rietveld refinement of the XRD patterns of all samples was carried out by TOPAS software. Microstructural analysis was performed by using Scanning Electron Microscopy (SEM) (JEOL JSM-840A) of sintered samples on a polished surface. The micro hardness of the bulk ceramics was measured using a micro scan from Vickers and Knoops (FM- 700e type D, Future Tech., Japan).

The densification and shrinkage of ceramic samples which were doped with the three oxides are shown in Fig. 1 and 2. ZrO₂ ceramics with addition of Y₂O₃ at 0.0, 2.0, 4.0 and 6.0 mol% showed a density of between 4.93 and 5.51 g/cm² and percentage of linear shrinkage values between 10.54 and 14.52. ZrO₂ ceramics with addition of MgO at 0.0, 2.0, 4.0 and 6.0 mol% showed a density of between 4.93 and 5.75 g/cm² and percentage of shrinkage values between 10.54 and 16.01. ZrO₂ ceramics with addition of BaCO₃ at 0.0, 2.0, 4.0 and 6.0 mol% were showed a density of between 4.54 and 4.93 g/cm², percentage of shrinkage values between 9.60 and 10.54. Densities value of ZrO₂ ceramics with addition of Y₂O₃ and MgO tended to increase with increasing added oxides while density values of ZrO₂ ceramics with addition of BaCO₃ tended to decrease with increasing BaCO₃ content. This result may be due to the different ionic radius of Ba and Zr, therefor making it difficult for Ba²⁺ substitution of Zr⁴⁺. The XRD patterns of ZrO₂ ceramics sample which were doped with the three oxides are shown in Fig. 3, 4 and 5. The three figures reveal the characteristic peaks of XRD patterns of ZrO₂ ceramics with different oxides contents. Fig. 3 shows XRD patterns of ZrO₂ ceramics with added 0.0-6.0 mol%Y₂O₃, which were identified using the main refraction of m-ZrO₂ (011) at 24.4^o, (-111) at 28.2^o, (111) at 31.45^o, (020) at 34.70^o and t-ZrO₂ (101) at 30.41^o, (002) at 34.81^o, (110) at 35.10^o and c-ZrO₂ (010) at 30.10^o. Fig. 4 shows XRD patterns of ZrO₂ ceramics with added 0.0-6.0 wt% MgO, which were identified as the main refraction of m-ZrO₂ (011) at 24.4^o, (111) at 31.45^o, (020) at 34.70^o and t-ZrO₂ (101) at 35.10^o. Fig. 5 shows XRD patterns of ZrO₂ ceramics with added 0.0-6.0 mol% BaCO₃, which are identified using the main refraction of m-ZrO₂ (011) at 24.4^o, (-111) at 28.2^o, (111) at 31.45^o, (020) at 34.70^o and t-ZrO₂ (110) at 35.10^o. The results indicated the appearance of ZrO₂ polymorphous crystalline phase consistent with previous reports [8, 13, 14]. The intensity of ZrO₂ peaks in three polymorphous are the result of different oxides addition at varies proportions of dopants. It was found that XRD patterns in Fig. 4 and Fig. 5 are nearly similar patterns. Full pattern matching refinement of XRD patterns was performed using the TOPAS program based on the Rietveld method to obtain more detailed information on crystallographic spectra of ZrO₂ ceramics with three oxides addition (Y₂O_{3 ,} MgO, BaCO₃ ) using 0.0, 2.0, 4.0 and 6.0 mol% and selected sample with added 4.0 mol%Y₂O₃, shown as the XRD refinement pattern in Fig. 6. The fitted patterns of ZrO₂ ceramics with added three oxides are in good agreement with the respective experiment data, denoted by R_p, R_wp and GOF factors listed below Table 1. ZrO₂ ceramics with added 4.0 mol%Y₂O₃ had a larger lattice parameter compared to ZrO₂ ceramics with added MgO and BaCO_{3 ,}which may be attributed to substitution to Zr⁴⁺site due to the addition of substances with similar ion sizes in agreement with the studies of M.T. Vinas et al. [9], M. Borik et al. [10]. This is clearly seen from the result of the higher tetragonality (c/a) and strain lattice in ZrO₂ ceramics with added Y₂O₃. Moreover, when considering the percentage ratio of tetragonal phase, it was obviously related to crystallize size and average grain size of ZrO₂ ceramics with oxides addition. The high tetragonal fraction of ZrO₂ ceramics with small crystallize size and fine grains was found in ZrO₂ ceramics with added Y₂O₃, supporting previous works [15-17]. The micro- structure of ZrO₂ ceramics is shown in Fig. 7, where Fig. 7(a) is a micrograph of pure ZrO₂ ceramic and Fig. 7(b-d) are micrographs of ZrO₂ ceramics with three different oxides addition using a ratio of 4.0 mol%. Microstructural evaluation was performed, i.e., uniformly sized grains with well-packed, continuous grain structure and spherical shape combined with irregular shape in gray color. By applying the linear intercept method [18] to these SEM images, grain sizes were estimated for these samples as given in Table 1. It can be seen that ZrO₂ ceramics with no added oxide exhibited large grains with average grain sizes in the range of 2.65-2.91 mm. While, ZrO₂ ceramics with addition of the three oxides showed average grain sizes between 0.63-2.18 mm. Comparing the grain sizes of pure ZrO₂ ceramics and ZrO₂ ceramics added oxides, it is found that the grain of ZrO₂ ceramics with addition of oxides had smaller sizes than grain sizes of pure ZrO₂ ceramics. Thus, the optimal contents and type of stabilizing oxides is an important parameter for development of ceramic microstructures. The mechanical properties of ZrO₂ ceramics were investigated by measuring micro- hardness by Knoop and Vickers techniques and then calculating the fracture toughness values. The fracture toughness of ZrO₂ ceramics as a function of different type and concentration of oxide additions is shown in Fig. 8. The toughness values of ZrO₂ ceramics with additions of Y₂O₃ and MgO were in the range 3.51-5.64 MPa m^1/2. Pure ZrO₂ ceramics and ZrO₂ ceramics with additions of BaCO₃ had toughness values that were not much different and in the range 1.52-1.58 MPa m^1/2. This result indicated that the mechanical properties are related to grain size and phase trans- formation, which increase as grain size decrease and high fraction of tetragonal phase, in agreement with previous reports by J. Vleugels et al. [19] and Chun-Feng Hu et al. [20]. The highest of fracture toughness value was found in ZrO₂ ceramics with 4 mol% Y₂O₃ added, which corresponds to high ratio of tetragonal phase by XRD refinements and optimal microstructure.

In the present work, ZrO₂ ceramics with addition of three oxides (Y₂O₃, MgO, BaCO₃ ) at 0.0, 2.0, 4.0 and 6.0 mol% were prepared by solid state reaction method. The effects of addition of different oxides and concen- trations on the properties of ZrO₂ ceramics were studied. This samples had phase compositions of polymorphous combine with t-, c- and m- ZrO₂ phases. Quantitative analysis from XRD pattern using Rietveld technique was performed to explain the crystal structure. The relationship between the tetragonal phase ratio, densifi- cation, grain size and mechanical properties is discussed.

This research project was financially supported by Mahasarakham University, Thailand.