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DOI: 10.1016/j.jmrt.2018.07.017
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Effect of sintering temperature on the aging resistance and mechanical properties of monolithic zirconia
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Noor Faeizah Amata, Andanastuti Muchtara,
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muchtar@ukm.edu.my

Corresponding author.
, Muhammad Sufiyan Amrila, Mariyam Jameelah Ghazalia, Norziha Yahayab
a Centre for Materials Engineering and Smart Manufacturing, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
b Department of Prosthodontic, Faculty of Dentistry, Universiti Kebangsaan Malaysia, Jalan Raja Muda Abdul Aziz, 50300 Kuala Lumpur, Malaysia
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Received 22 March 2018, Accepted 13 July 2018
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Abstract

This study aimed to investigate the effect of final sintering temperature and grain size on the aging resistance and mechanical properties of zirconia fabricated through colloidal and cold isostatic pressing. Zirconia samples appearing as cylindrical discs were prepared and subjected to five different sintering temperatures ranging from 1400°C to 1600°C. Grain pullout and rough surfaces were detected on the micrographs of aged samples sintered at more than 1500°C, indicating surface degradation. The highest flexural strength of non-aged and aged samples was obtained at 1500°C, whereas excellent fracture toughness was demonstrated by the sample sintered at 1550°C under non-aged and aged conditions. The hardness properties of all of the samples sintered at various temperatures did not significantly differ. Under the aged conditions, their hardness properties slightly decreased. Overall, the mechanical performance of the aged zirconia slightly deteriorated but remained acceptable for use in an oral environment for 15–20 years. Mechanical performance evaluation after aging revealed that 1500°C was the appropriate sintering temperature. The mechanical properties and aging resistance of the zirconia samples were greatly dependent on sintering temperature during fabrication.

Keywords:
Monolithic zirconia
Aging
Biaxial flexural strength
Fracture toughness
Monoclinic transformation
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1Introduction

In comparison with other dental materials, monolithic zirconia has favorable mechanical properties and is clinically reported to be wear friendly on antagonist teeth [1]. Monolithic zirconia also possesses acceptable esthetic properties comparable to those of natural teeth. An advantage of using this monolithic zirconia restoration is its straightforward preparation because framework thickness adjustment is not required for veneer layering [2]. The fabrication of monolithic restorations is generally accompanied by high dimensional accuracy and minimal occlusal adjustment [3,4]. Monolithic zirconia restoration is also considered an economical procedure because it may not entail extensive treatment after few years of installation. However, zirconia used in dental applications is associated with some issues, including aging tendency or low-temperature degradation (LTD) when it is exposed to the oral environment. Monolithic zirconia restoration may be slightly deteriorated by LTD because the zirconia structure is absolutely exposed to the humid environment of saliva.

Few theories have described the aging mechanism. For instance, yttria, which is used as a dopant in zirconia to achieve phase stability, is exhausted through a reaction with water in the presence of moisture, resulting in tetragonal to monoclinic (t–m) transformation [5,6]. The zirconia bond of Zr–O is disturbed by water, thereby creating stress due to the diffusion of OH and forming lattice defects that cause the t–m transformation [7]. O2 derived from water likely fills the oxygen vacancies of the matrix and induces t–m phase transformation [8]. Nevertheless, the most accepted argument involves yttria-stabilized zirconia that tends to undergo t–m transformation when it is exposed to a humid environment. This transformation is associated with microcracks and grain pullout that are initially produced on the surfaces of zirconia. Water may penetrate cracks and promote surface degradation to expand further into the whole material [6].

LTD can be accelerated by a combination of mechanical stress and moisture in an environment [9,10]. However, LTD can also be associated with other factors, such as stabilizer type and amount [11], residual stress [12], microstructural and density properties [13], sintering temperature and fabrication method [14–16], surface finish processing [17], and different hydrothermal conditions. Aging usually accompanies grain pullout and increases the surface roughness, resulting in antagonist tooth abrasion, surface degradation, and microcrack formation [5,10,13,18]. Numerous studies have been conducted to improve aging resistance, particularly in relation to the grain size of zirconia, by altering the sintering parameter [13], stabilizer types and contents [8,11], and surface treatment [19].

In monolithic zirconia application, translucency is the main characteristic that should be achieved when this material is used for the anterior region. For this reason, zirconia materials are sintered at high temperatures to meet this esthetic requirement. In our previous study, a colloidal-processing technique is prepared by using nano-zirconia powder for a monolithic zirconia block [20]. Preliminary results have revealed that this block shows great potential for further development and application as an alternative to currently available zirconia products [21–23]. The mechanical properties and aging resistance of new zirconia fabricated at different sintering temperatures should be evaluated in terms of LTD of zirconia. According to the International Organization for Standardization (ISO), standard for the use of zirconia in dental implants [24], the allowable monoclinic fraction must not exceed 25% after aging in an autoclave at 134°C and 2bar for 5h, but no guidelines for zirconia prostheses are available [25]. Inconsistent findings on LTD sensitivity have also been presented, although the type of zirconia used in previous studies is zirconia stabilized with 3mol% yttria and in accordance with the requirements of the ISO standard [26]. This observation confirms that the aging resistance of zirconia is related to the properties of primary zirconia powder and its processing method. Thus, the aging resistance of each zirconia product should be examined. The aging resistance of this material may depend on grain size. Therefore, this study aimed to investigate the effect of the final sintering temperature and grain size on the aging resistance and mechanical properties of zirconia.

2Experimental procedures2.1Sample preparation and sintering procedure

Zirconia powder stabilized with 3mol% yttria was used in this study and purchased from Inframat Advanced Materials, USA. Zirconia samples appearing as cylindrical discs were produced via colloidal slip casting and cold isostatic pressing [20]. All of the samples were placed in an alumina crucible and presintered at 1100°C with a heating rate of 3°C/min and 2h of soaking time. Sintering temperature between 1400°C and 1600°C is commonly used for zirconia. Thus, the presintered samples were then divided into five final sintering temperatures (1400°C, 1450°C, 1500°C, 1550°C, and 1600°C) with similar heating rates and soaking times as those during presintering. Pressureless sintering was performed in a furnace unit of VMK-1800 (Linn High, Germany).

2.2Aging procedure

The samples from each group were placed in a Petri dish and then aged using a steam autoclave unit (Hirayama/MV-50, Japan) at 134°C and 0.2MPa for 5h in accordance with the standard ISO 13356:2008 [24]. According to previous studies, aging for 1h in an autoclave is equivalent to 3–4 years of actual aging [6,27]. Therefore, in this study, autoclave aging was conducted for 5h, which is comparable to 15–20 years of actual aging.

2.3Field-emission scanning electron microscopy (FESEM)

For morphological analysis, the samples from the aged and non-aged groups of each sintering temperature were thermally etched at 900°C with a 20°C/min heating rate and a holding time of 2h. All of the etched samples were then characterized through FESEM analyses (Zeiss Merlin, Germany). The average grain size of each sample was examined using the FESEM micrographs in ImageJ of version 1.46r developed by National Institutes of Health, USA.

2.4Measurement of monoclinic transformation via X-ray diffraction (XRD) analysis

The phase contents of the aged and non-aged zirconia samples were identified with an X-ray diffractometer (XRD, D8 Advance, Bruker AXS, Germany). XRD data were acquired using Cu Kα radiation (40kV, 40mA) at a step size of 0.025° with an exposure time of 0.1s per step, and our detector was rotated between 10° and 80° (2θ).

A zirconia phase was determined using Diffrac.EVA (version V4.0, Bruker, Germany) and referred to the file data recorded in pdf file (PDF 2-2015) of International Center for Diffraction Data. Eq. (1) was used to quantify the monoclinic phase fraction (Xm) [13].

where Im is denoted as the monoclinic intensity at (111) and (−111) peaks, and It is the tetragonal intensity at (101). The volumetric monoclinic phase content (Vm) was then quantified with the equation described by Toraya et al. [28] by using the calculated monoclinic fraction Xm:

2.5Biaxial flexural strength (BFS) test

The mechanical strength of the samples before and after autoclaved aging was tested in terms of BFS by using a universal testing machine (Instron 5567, Singapore) in accordance with ISO 6872:2008E (Dentistry – Ceramics materials) [29]. Three hardened stainless steel balls with a diameter of 2.5mm were applied to support the disc samples [Fig. 1(a)] and placed on a jig with a distance of 120° from each other in a circular position with a diameter of 10.5mm. The hardened carbon steel flat pin with a diameter of 1.4mm was applied centrally on the sample with an increasing load at a crosshead speed of 1mm/min until a catastrophic fracture occurred. BFS was calculated by using Eq. (3) in accordance with ISO 6872:2008E (Dentistry – Ceramics materials) [29].

where σ is the maximum tensile stress (MPa), P is the total load causing fracture (N), and b is the sample thickness at the fracture origin (mm). X and Y were determined as follows:

Figure 1.
(0.28MB).

(a) Mounted load pin and jig on a universal testing machine and sample in position for biaxial flexural strength test and (b) fractured sample after the test.

where v is Poisson's ratio (0.25), r1 is the radius of the support circle (mm), r2 is the radius of the pin-loaded area (mm), and r3 is the radius of the sample (mm). The fracture sample yielded after the test is illustrated in Fig. 1(b).

2.6Fracture toughness (FT) test

The FT (KIC) of the sample was measured through an indentation method by using a Vickers hardness tester (HVS10 model, Mitaka, Japan). A Vickers hardness indenter was adopted to indent the samples with a load of 10kgf and a dwell time of 15s and thus produce cracks (Fig. 2). Two types of cracks, namely, Palmqvist crack and median crack (also known as half-penny crack), can form upon indentation [11]. Cracks are considered Palmqvist when c/a<2 and median when c/a2. Therefore, FT was calculated in accordance with Niihara formula through either Eq. (6) or (7) [30].

Figure 2.
(0.09MB).

Crack formation on an indented sample for fracture toughness measurement.

For Palmqvist crack (c/a<2):

For median crack (c/a2):

where Φ is a constant ≈ 3, Hv is Vickers hardness (GPa), and E is Young's modulus (GPa). In this study, 210GPa was used [31]. a is the half length of the indentation (μm), c is the length of the crack (μm), and l is the half length of the crack (μm).

2.7Vickers microhardness test

The hardness properties of the zirconia samples were examined using a Vickers microhardness tester (ZHV10, Zwick, Ulm, Germany) with a maximum load capacity of 2kgf. A microhardness test was conducted at a load of 0.2kgf and a dwell time of 15s. The indentation of Vickers hardness was carried out differently with Vickers hardness indentation for crack formation to measure FT because the latter involves a low load that is unable to produce crack on indented tips.

3Results and discussion3.1Morphological analysis

The FESEM micrograph of the non-aged and aged samples at various sintering temperatures is illustrated in Fig. 3. The grain size of the samples at 1400°C and 1450°C was initially homogeneous with a size in the range of 150–200nm. However, when temperature increased from 1500°C to 1600°C, the grain size appeared to be inhomogeneously distributed, where large (200–350nm) and small (90–120nm) grains were detected at certain areas. The grain pullout associated with roughened surfaces was notable at particular surfaces of the samples after autoclave aging. The increase in sintering temperature from 1550°C to the highest temperature of 1600°C promoted further obvious grain pullout and increased surface roughness upon autoclave aging.

Figure 3.
(1.1MB).

FESEM micrograph of non-aged and aged zirconia samples sintered at different temperatures.

FESEM micrographs reveal that autoclave aging conducted in this study promoted grain pullout in the microstructure of new fabricated zirconia, especially the samples that were sintered at high sintering temperatures. When the grain size grew because of increased sintering temperature, the density of zirconia also improved with fine grain boundary. The mechanical properties of zirconia were enhanced as the toughening mechanism of t–m transformation occurred because of microstructural changes. Excessive grain growth might occur at high sintering temperatures that led to crack and grain pullout. The presence of a humid environment from autoclave aging enlarged the t–m transformation zone and then promoted material degradation that initiated at the surface layer of zirconia and potentially grew into the bulk of the material [32]. Micrographs reveal that the sample sintered at 1500°C did not experience grain pullout and rough surface under aged or non-aged condition. Thus, the sintering temperature of 1500°C can be considered as the most suitable temperature for the production of zirconia samples with excellent LTD aging resistance.

3.2XRD analysis

The XRD peak intensities of the tetragonal and monoclinic phases for the non-aged and aged samples formerly sintered at different temperatures are shown in Fig. 4. The XRD patterns suggest that autoclave aging slightly increased in the monoclinic phase contents as sintering temperature increased. For the sample sintered at 1400°C, a monoclinic peak only appeared at the −111 peak after autoclave aging. However, a low-intensity monoclinic peak was detected immediately after aging at peaks −111 and 111 when the sintering temperatures were increased from 1450°C to 1600°C. Before aging occurred, no monoclinic phase was detected in all of the samples except the sample sintered at 1600°C.

Figure 4.
(0.41MB).

XRD patterns of non-aged and aged zirconia samples sintered at different temperatures.

3.3Tetragonal to monoclinic transformation

Fig. 5 displays the amount of monoclinic phase for the non-aged and aged samples with respect to the sintering temperature conditions. In general, the amount of the monoclinic phase was found to increase in accordance with the increase in sintering temperature from 1400°C to 1500°C and almost plateaued when the temperature exceeded 1550°C for the aged and non-aged samples. The monoclinic fraction for non-aging was 2.5% for the sample sintered at 1400°C and increased to 3.2% when the sample was sintered at 1600°C. Autoclave aging only induced a monoclinic phase between 3.4% and 3.9%, which was slightly higher than that of non-aging, and the sample sintered at 1600°C gained the highest amount of monoclinic volume fraction upon autoclave aging.

Figure 5.
(0.07MB).

Volumetric monoclinic phase content of non-aged and aged zirconia samples sintered at different temperatures.

XRD analysis (Fig. 4) shows that the XRD pattern within 28° to 36° (2θ) was considered in the calculation of monoclinic fraction through Eq. (1) because this portion of the pattern covered the location of the highest peaks of the tetragonal (101) and monoclinic (111) and (−111) phases of the zirconia phase. The slight increase in the monoclinic phase at increased sintering temperature after aging can be related to the growing grain size (Fig. 3). When the grain size increased, initial t–m transformation might be induced because of the thermal stress of sintering, resulting in volume expansion that produced compressive stress on the affected zone. For example, a monoclinic phase was detected slightly higher at peak (111) at a sintering temperature of 1600°C before aging [Fig. 4(e)]. This t–m transformation is usually accompanied by grain pullout and microcracks created on the zirconia surfaces. Subjecting zirconia to autoclave aging promotes the transformation of the zirconia phase by causing water to penetrate the microcracks and enlarge the surface-degraded area [5].

XRD analysis also reveals that only a small amount of monoclinic transformation occurs on all of the samples after 5h of autoclave aging at 134°C in which the highest monoclinic fraction was only ≤ 3.9%. Egilmez et al. [33] identified up to 25.4% monoclinic phase at the surface of commercial zirconia under similar aging conditions as in the present study. This result concluded that zirconia fabricated through this new method considerably has adequate aging resistance when it is subjected to an oral environment that simulates the duration of 15–20 years. The correlation of sintering temperature and the presence of a monoclinic phase under aging conditions with the mechanical properties is explained further in the subsequent section to provide zirconia materials with good aging resistance and mechanical strength.

3.4Biaxial flexural strength (BFS)

Fig. 6 presents the BFS of the non-aged and aged samples sintered at various sintering temperatures. The BFS of the non-aged and aged samples similarly increased to a maximum value at 1500°C and gradually decreased as the sintering temperature of up to 1600°C further increased. The non-aged samples sintered at 1500°C yielded superior BFS of 1065.32MPa. The lowest BFS (410.06MPa) was achieved after autoclave aging was conducted for the samples sintered at 1400°C as they fractured rapidly.

Figure 6.
(0.08MB).

Biaxial flexural strength of non-aged and aged zirconia samples sintered at different temperatures.

The FESEM micrograph in Fig. 3 shows that the grain of zirconia enlarged when sintering temperature increased, indicating that t–m transformation has occurred. The t–m transformation is a unique process that can cause volume expansion, thereby generating compressive stresses, consequently increasing zirconia toughness, and suppressing crack propagation. This notion was proven by the improvement of BFS of zirconia at increased sintering temperature from 1400°C to 1500°C. However, the BFS of the non-aged sample began to decrease from 1550°C to 1600°C possibly because of volume expansion that increased the compressive stress to the expanded grains; these grains are unable to hinder crack propagation, thereby failing [34]. This result is also consistent with previous findings [35]; that is, tetragonal zirconia is subjected to a sintering temperature exceeding 1550°C, and grain pullout combined microcracks are formed, thereby decreasing flexural strength properties.

The autoclave-aged zirconia samples were subjected to a BFS test to evaluate their flexural strength resistance in a simulated real oral environment. As expected, the BFS of the samples slightly depreciated upon autoclave aging, indicating that LTD occurred. In a humid intraoral environment, spontaneous t–m transformation occurred and led to microcrack formation on the surfaces that provided a good channel for water to travel further into the bulk of materials. Degradation intensified by creating grain pullout and roughening the surfaces. The lowest BFS obtained from this experiment was at a low sintering temperature of 1400°C for the aged samples. At this low sintering temperature, the grains were not fully densified with a coarse grain boundary, and the micropores possibly formed in the microstructure. The t–m transformation might not actively build up under this condition to generate a toughening process that hindered crack propagation. At the same time, aging condition introduced LTD and decreased the flexural strength.

The monoclinic phase was visible between 50 and 100h of autoclave aging when the increasing monoclinic phase fraction can significantly decrease in flexural strength [26]. This observation confirmed that the presence of the monoclinic phase can influence flexural strength when the duration of aging condition is prolonged [36]. Testing the aging sensitivity of up to 50–100h to simulate 250–500 years of application is impractical and irrelevant to a lifetime of application. Dental prostheses usually loosen, wear, dislodge, and debond over 7 years and thus require replacement [37]. Therefore, the assessment of aging resistance for zirconia becomes relevant in autoclave aging for 5h. In a previous study [38] on engineering guidelines for the use of zirconia-based materials in restorative application, the BFS of aged and non-aged zirconia requires at least 600MPa. In the present study, the sintering temperature of 1500°C is considered the proper temperature for the production of zirconia with acceptable flexural strength (1065.32 and 1019.47MPa for non-aged and aged samples, respectively) in restorative applications.

3.5Fracture toughness (FT)

FTs of the non-aged and aged samples in response to different sintering temperatures were measured (Fig. 7). The increase in sintering temperature increased the FT of the non-aged and aged samples, but FT eventually decreased at 1600°C and beyond. A slight difference in FT was noted between the non-aged and aged samples; in particular, autoclave aging lowered the FT of the samples. The highest FT (11.4MPam1/2) for the aged samples was achieved at a sintering temperature of 1550°C and slightly lower than that for the non-aged samples (12.6MPam1/2) at the same temperature.

Figure 7.
(0.08MB).

Fracture toughness of non-aged and aged zirconia samples sintered at different temperatures.

Sintering temperature influenced the size of zirconia grain and densification and thus affected the FT of the sample. At 1400°C, the grain size of the sample was initially about 150nm with a coarse grain boundary as illustrated in the FESEM image of Fig. 3(a). The increase in sintering temperature improved the grain size associated with fine grain boundary at the highest temperature of 1600°C. A fine grain boundary indicated the good packing density of a sample, possibly resulting in excellent mechanical properties. When fracture tests were conducted on high-packing-density samples, such as the samples sintered at 1500°C and 1550°C, the stresses generated increased and then induced t–m transformation around crack tips; such effects led to volume expansion and compressive stresses that hindered the crack from further propagating. Therefore, FT increases in a well-compacted body [39]. However, FT of the aged and non-aged samples decreased at 1600°C compared with that of BFS, which began to decline at 1550°C. This reduction could be due to the presence of a high amount of large grain sizes in the sample that cannot minimize crack propagation during fracture testing, thus allowing catastrophic failure and fracture.

The decrease in FT upon autoclave aging was expected because of the LTD occurrence that slightly reduced the FT of the material. XRD analysis shows that the content of the monoclinic fraction was only 3.4–3.9%, a low amount and hence not mainly responsible for diminishing FT and BFS. By contrast, the changes in grain size and the densification of the sample are caused by the sintering parameter, and these phenomena are the probable reasons. Nevertheless, the aging condition accelerated the LTD phenomenon and affected the mechanical properties. Given these results, the sintering temperature of 1550°C provided zirconia with excellent FT even under the influence of aging.

3.6Vickers hardness

The hardness measurement of each aged and non-aged sample fabricated at various sintering temperatures is presented in Fig. 8. The hardness values of the non-aged samples did not differ among the various studied temperatures. Thus, this finding implies that the temperature range did not significantly influence the hardness properties of the zirconia samples. Upon autoclave aging, hardness was downgraded relative to that of the non-aged sample, and hardness showed no significant effect when different sintering temperatures were applied. The hardness ranges for the aged and non-aged samples were 14.7–15.5GPa and 15.7–15.9GPa, respectively, under the different test sintering temperatures.

Figure 8.
(0.07MB).

Vickers microhardness of non-aged and aged zirconia samples sintered at different temperatures.

The undetected difference in hardness of the samples in response to the different sintering temperatures might be attributed to the high densification generated upon sintering that improved the materials’ hardness properties. There was only a slight dip in hardness for those samples sintered at 1400°C followed by aging. This might be caused by the samples not having achieved full densification at this sintering temperature which resulted in residual micropores which in turn, were prone to surface degradation during autoclave aging, hence the reduced hardness resistance. Nonetheless, the hardness properties of these new formulated zirconia samples were considered excellent because their hardness remained intact even when they were subjected to aging condition.

4Conclusions

In the present study, the effect of sintering temperature on the aging resistance and mechanical properties of the newly formulated zirconia materials was successfully studied. Based on microstructural study, phase analysis, and mechanical property examination, the main conclusions can be summarized as follows:

  • 1.

    FESEM micrographs showed that increasing the sintering temperature allowed the existence of noticeable grain pullout and rough surfaces after the samples sintered to 1550°C and above were subjected to autoclave aging. This finding indicated surface degradation. Minimum surface roughening could be observed in the samples sintered below 1550°C.

  • 2.

    Only a small volume fraction of the monoclinic phase (from 3.4% to 3.9%) was detected after autoclave aging. This result indicated that the performance of the fabricated material was better than that of commercially available zirconia.

  • 3.

    The BFS of the zirconia samples sintered at 1500°C regardless of aging was relatively higher than that of the samples sintered at other temperatures.

  • 4.

    The zirconia sample sintered at 1550°C exhibited excellent FT under non-aged and aged conditions.

  • 5.

    Slight differences in hardness were noted between the non-aged and aged samples, whereas no significant difference in hardness was observed among the samples at various sintering temperatures. These findings confirmed that zirconia prepared in this study has good hardness properties.

Sintering temperature affects the microstructure of the zirconia samples. Uneven grain size distributions occur specifically at 1550°C and 1600°C, thereby permitting an enhanced monoclinic transformation. Sintering temperature, not autoclave aging, appeared as the sole factor affecting the mechanical performance of the zirconia sample. On the basis of the other mechanical test results, we concluded that the sintering temperature of 1500°C could be considered an appropriate temperature for sintering zirconia restorations because acceptable flexural strength, FT, and hardness properties could be achieved through the process when zirconia was exposed to a simulated oral environment corresponding to 15–20 years of actual oral exposure. Moreover, zirconia materials sintered at 1500°C satisfied the requirements of ISO 13356:2008 (Implants for surgery-Ceramic materials based on yttria-stabilized tetragonal zirconia [Y-TZP]) in terms of grain size (≤ 0.4μm) and aging sensitivity (≤25% of the monoclinic fraction after 5h of autoclaving at 134°C).

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflicts of interest

The authors declare no conflicts of interest

Acknowledgments

This work was supported by Universiti Kebangsaan Malaysia (UKM) via the research sponsorship of DIP-2016-001. The authors gratefully acknowledge the Center for Research and Instrumentation Management (CRIM), UKM for providing excellent testing equipments.

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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