Fracture Toughness of Advanced Ceramics at Room Temperature

Spark Plasma Sintering of SiAlON Ceramics Synthesized via Various Cations Charge Stabilizers and Their Effect on Thermal and Mechanical Characteristics

Oxygen-rich SiAlON ceramics doped with various nanosized metal oxide (MO) stabilizers were synthesized with a view to examine their effect on thermal and mechanical characteristics. The nanosized starting powder precursors comprising Si3N4, AlN, Al2O3, and SiO2 along with oxides of Ba, Y, Mg, La, Nd, Eu, Dy, Er, and Yb as the MO charge stabilizer were employed in developing different SiAlON samples. Ultrasonic probe sonication was utilized to develop a homogenous mixture of initial powder precursors followed by spark plasma sintering (SPS) of the samples at the low temperature of 1500 °C coupled with 30 min of isothermal treatment. Sample compositions (according to general formula of alpha SiAlON: Mm/vv+Si12−(m+n)Alm+nOnN16−n) selected in the present study are represented by m value of 1.1 and n value of 1.6. The synthesized samples were evaluated for their physical behavior, microstructural and crystal structure evolution, and thermal and mechanical characteristics. More specifically, the sintered ceramics were examined by X-ray diffraction and electron microscopy to comprehend and relate the structural characteristics with the densification, thermal conductivity, hardness, and fracture toughness. The high reactivity of the nanopowders and the localized heating provided by SPS resulted in densified ceramics with relative densities in the range of 92–96%. Vickers hardness values were found to be in the range of 12.4–17.0 GPa and were seen to be profoundly influenced by the grain size of the alpha SiAlON (primary) phase. The fracture toughness of the samples was measured to be in the range of 4.1–6.2 MPa·m1/2. SiAlON samples synthesized using Er and Yb charge stabilizers were found to have the highest fracture toughness of 5.7 and 6.2 MPa·m1/2, primarily due to the relatively higher content of the elongated beta phase. While there was no obvious relationship between the thermal conductivity and the alpha SiAlON metal charge stabilizers, the values were seen to be influenced by the grain size of alpha phase where Dy-SiAlON had the lowest thermal conductivity of 5.79 W/m⋅K and Er-SiAlON showed the highest value of thermal conductivity (6.91 W/m⋅K). It was concluded that scientifically selected metal oxide charge stabilizers are beneficial in developing SiAlON ceramics with properties tailored according to specific applications.

Toughening of Bioceramic Composites for Bone Regeneration

Bioceramics are widely considered as elective materials for the regeneration of bone tissue, due to their compositional mimicry with bone inorganic components. However, they are intrinsically brittle, which limits their capability to sustain multiple biomechanical loads, especially in the case of load-bearing bone districts. In the last decades, intense research has been dedicated to combining processes to enhance both the strength and toughness of bioceramics, leading to bioceramic composite scaffolds. This review summarizes the recent approaches to this purpose, particularly those addressed to limiting the propagation of cracks to prevent the sudden mechanical failure of bioceramic composites.

Investigation into the Structural, Chemical and High Mechanical Reforms in B4C with Graphene Composite Material Substitution for Potential Shielding Frame Applications

In this work, boron carbide and graphene nanoparticle composite material (B₄C–G) was investigated using an experimental approach. The composite material prepared with the two-step stir casting method showed significant hardness and high melting point attributes. Scanning electron microscopy (SEM), along with energy dispersive X-ray spectroscopy (EDS) analysis, indicated 83.65%, 17.32%, and 97.00% of boron carbide + 0% graphene nanoparticles chemical compositions for the C-atom, Al-atom, and B₄C in the compound studied, respectively. The physical properties of all samples’ B₄C–G like density and melting point were 2.4 g/cm³ density and 2450 °C, respectively, while the grain size of B₄C–G was in the range of 0.8 ± 0.2 µm. XRD, FTIR, and Raman spectroscopic analysis was also performed to investigate the chemical compositions of the B₄C–G composite. The molding press composite machine was a fabrication procedure that resulted in the formation of outstanding materials by utilizing the sintering process, including heating and pressing the materials. For mechanical properties, high fracture toughness and tensile strength of B₄C–G composites were analyzed according to ASTM standard designs. The detailed analysis has shown that with 6% graphene content in B₄C, the composite material portrays a high strength of 134 MPa and outstanding hardness properties. Based on these findings, it is suggested that the composite materials studied exhibit novel features suitable for use in the application of shielding frames.

Effect of Carbon Addition and Mixture Method on the Microstructure and Mechanical Properties of Silicon Carbide

High dense (>99% density) SiC ceramics were produced with addition of C and B₄C by spark plasma sintering method at 1950 °C under 50 MPa applied pressure for 5 min. To remove the oxygen from the SiC, it was essential to add C. Two different mixture method were used, dry mixing (specktromill) and wet mixing (ball milling). The effect of different levels of carbon additive and mixture method on density, microstructure, elastic modulus, polytype of SiC, Vickers hardness, and fracture toughness were examined. Precisely, 1.5 wt.% C addition was sufficient to remove oxide layer from SiC and improve the properties of dense SiC ceramics. The highest hardness and elastic modulus values were 27.96 and 450 GPa, respectively. Results showed that the 4H polytype caused large elongated grains, while the 6H polytype caused small coaxial grains. It has been observed that it was important to remove oxygen to achieve high density and improve properties of SiC. Other key factor was to include sufficient amount of carbon to remove oxide layer. The results showed that excess carbon prevented to achieve high density with high elastic modulus and hardness.

Practical and theoretical considerations on the fracture toughness testing of dental restorative materials

BACKGROUND

An important tool in materials research, development and characterization regarding mechanical performance is the testing of fracture toughness. A high level of accuracy in executing this sort of test is necessary, with strict requirements given in extensive testing standard documents. Proficiency in quality specimen fabrication and test requires practice and a solid theoretical background, oftentimes overlooked in the dental community.

AIMS

In this review we go through some fundamentals of the fracture mechanics concepts that are relevant to the understanding of fracture toughness testing, and draw attention to critical aspects of practical nature that must be fulfilled for validity and accuracy in results. We describe our experience with some testing methodologies for CAD/CAM materials and discuss advantages and shortcomings of different tests in terms of errors in testing the applicability of the concept of fracture toughness as a single-value material-specific property.

In vitro fracture toughness of commercial Y-TZP ceramics: a systematic review

PURPOSE

The aim of this review was to assess research methods used to determine the fracture toughness of Y-TZP ceramics in order to systematically evaluate the accuracy of each method with regard to potential influencing factors.

MATERIALS AND METHODS

Six databases were searched for studies up to April 2013. The terms "tough*," "critical stress intensity factor," "zirconi*," "yttri*," "dent*," "zirconia," "zirconium," and "stress" were searched. Titles and abstracts were screened, and literature that fulfilled the inclusion criteria was selected for a full-text reading. Test conditions with potential influence on fracture toughness were extracted from each study.

RESULTS

Ten laboratory studies met the inclusion criteria. There was a significant variation in relation to test method, ambient conditions, applied/indentation load, number of specimens, and geometry and dimension of the specimen. The results were incomparable due to high variability and missing information. Therefore, 10 parameters were listed to be followed to standardize future studies.

CONCLUSIONS

A wide variation in research methods affected the fracture toughness reported for Y-TZP ceramics among the selected studies; single-edge-precracked beam and chevron-notched-beam seem to be the most recommended methods to determine Y-TZP fracture toughness; the indentation methods have several limitations.

CLINICAL SIGNIFICANCE

The accurate calculation of toughness values is fundamental because overestimating toughness data in a clinical situation can negatively affect the lifetime of the restoration.

Fracture toughness determination of two dental porcelains with the indentation strength in bending method

OBJECTIVE

This study was to investigate the influence of the bending test configurations and the crosshead displacement speeds on the fracture toughness (K(Ic)) of dental porcelains obtained with the indentation strength in bending (ISB) method.

METHODS

The strength of the dental veneering porcelains Duceram and Sintagon Zx, Vickers' indented at a load of 2 kg was measured at a crosshead speed of 0.5mm/min with three test configurations, which were 3-point, 4-point, and biaxial bending. Two more groups of Sintagon Zx were tested the same way, but at speeds of 0.1, and 0.05 mm/min, respectively. Both porcelains, the three crosshead speeds, and the three test configurations were compared statistically.

RESULTS

Duceram had a higher toughness than Sintagon Zx with all three test configurations and there was no significant difference between three test configurations with either porcelain. Within the crosshead speed groups of Sintagon Zx, a significant difference was found only in the 0.5mm/min group between the 3-point, and 4-point configurations. Within the configuration groups, significant differences were found between all speeds with the 3-point configuration and only between the highest and lowest speed with the 4-point and the biaxial tests.

CONCLUSION

The crosshead displacement speed can cause statistically different results of fracture toughness obtained with the ISB method.

Load-bearing behavior of a simulated craniofacial structure fabricated from a hydroxyapatite cement and bioresorbable fiber-mesh

Calcium phosphate cements (CPC) have proven successful in the repair of small, non-stress bearing skeletal defects. These cements do not have sufficient tensile strength or fracture toughness to allow their use in stress-bearing applications. It was hypothesized that a bioresorbable fiber mesh would improve the load-bearing behavior of shell structures fabricated of CPC. This study used a biaxial flexure fixture to compare the work-to-fracture values of discs made of: (1) CPC; (2) CPC reinforced with a bioresorbable two-dimensionally oriented poly(glactin) fiber-mesh; and (3) poly(methyl methacrylate) (PMMA) that were immersed in a serum-like solution for 0-28 days. CPC-mesh and PMMA discs were indistinguishable at 0, 1 and 7 days, based on work-to-fracture data. CPC and CPC-mesh discs were indistinguishable at day 28, because of fiber hydrolysis. The knitted fiber-mesh was effective in improving load-bearing behavior of a calcium phosphate cement for potential structural repair of bone defects.

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic

OBJECTIVES

Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test.

METHODS

The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05).

RESULTS

For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values.

SIGNIFICANCE

The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.