Toughness Of Ceramics Research Articles

Effect of Artificial Saliva Storage on Microhardness and Fracture Toughness of a Hydrothermal Glass‐Ceramic

This study evaluated the effect of artificial saliva storage on the hardness, crack length, and fracture toughness of a glazed, polished, and bleached hydrothermal low-fusing glass-ceramic (Duceram LFC). Forty ceramic discs were constructed. The discs were assigned to four groups (n = 10) according to their surface finish: Gp1 -- Autoglaze, Gp2 -- Autoglaze/ground/diamond-polished, Gp3 -- Overglaze, Gp4 -- Overglaze/ground/diamond-polished. Each group was further divided into two subgroups forming eight total subgroups (n = 5). Subgroup A was unbleached; Subgroup B was bleached. Testing was performed before and after 21 days of artificial saliva storage. Data were presented as means and standard deviation (SD). ANOVA was used, along with Duncan's post hoc test for pairwise comparison between the means when ANOVA test was found significant (p< or = 0.05). Surface treatments such as glazing, polishing, and bleaching, saliva storage, and the interaction between these variables had a statistically significant effect on mean values of microhardness, crack length, and fracture toughness of the specimens. There was a statistically significant increase in microhardness and fracture toughness mean values, while crack length values decreased after saliva storage. Polished specimens recorded the smallest crack lengths and fracture toughness, and highest hardness values before and after saliva storage. No difference in fracture toughness values was evident between glazed and polished specimens. Mean crack lengths decreased after saliva storage in all the tested specimens. Hardness values increased after saliva storage. The autoglazed group showed significantly higher fracture toughness, lower crack length, and microhardness than the overglazed group. Surface finishing procedures and artificial saliva storage had a statistically significant effect on mean values of microhardness, crack length, and fracture toughness. This in vitro study suggests that fracture toughness of ceramics may be affected by different surface treatments such as glazing, polishing, bleaching, or a combination; however, in this study Duceram LFC proved its self-healing property after 3-week storage in artificial saliva.

Ceramic toughness assessment through edge chipping measurements—Influence of interfacial friction

In this paper, a comparative study between two techniques to determine material toughness through edge chipping measurements, is presented. The first technique relies on static Rockwell indentations, performed at well-defined distances from the edge with an increasing load until chipping occurs. The second technique keeps the applied load constant but drives the Rockwell indenter towards the edge at constant speed until a chip is formed (scratch testing). In most studied cases, our experimental results on ceramics are shown to conform with the predictions of previous models in the literature expressing the toughness K Ic as a function of chip size l c and critical chipping load P through K I c ≅ P / l c 3 / 2 . However, for silicon carbide, the toughness values obtained by both variants of the method are mutually incompatible. The discrepancy is shown to be related to the tangential loading occurring during scratch testing. A new equation for toughness assessment is derived that takes into account the influence of friction. This equation is demonstrated to match well the experimental results both for static and sliding indentation.

Universal Size Effect Law and Effect of Crack Depth on Quasi-Brittle Structure Strength

In cohesive fracture of quasi-brittle materials such as concrete, rock, fiber composites, tough ceramics, rigid foams, sea ice, and wood, one can distinguish six simple and easily modeled asymptotic cases: the asymptotic behaviors of very small and very large structures, structures failing at crack initiation from a smooth surface and those with a deep notch or preexisting deep crack, the purely statistical Weibull-type size effect, and the purely energetic (deterministic) size effect. Size effect laws governing the transition between some of these asymptotic cases have already been formulated. However, a general and smooth description of the complex transition between all of them has been lacking. Here, a smooth universal law bridging all of these asymptotic cases is derived and discussed. A special case of this law is a formula for the effect of notch or crack depth at fixed specimen size, which overcomes the limitations of a recently proposed empirical formula by Duan et al., 2003, 2004, 2006.

Thermal Stress Analysis for Ceramics Stalk in the Low Pressure Die Casting Machine

Low pressure die casting (LPDC) is defined as a net shape casting technology in which the molten metal is injected at high speeds and pressure into a metallic die. The LPDC process is playing an increasingly important role in the foundry industry as a low-cost and high-efficiency precision forming technique. The LPDC process is that the permanent die and filling systems are placed over the furnace containing the molten alloy. The filling of the cavity is obtained by forcing the molten metal by means of a pressurized gas in order to rise into a ceramic tube, which connects the die to the furnace. The ceramics tube called stalk has high temperature resistance and high corrosion resistance. However, attention should be paid to the thermal stress when the stalk is dipped into the molten aluminum. It is important to develop the design of the stalk to reduce the risk of fracture because of low fracture toughness of ceramics. In this paper, therefore, the finite element method is applied to calculate the thermal stresses when the stalk is dipped into the crucible by varying the dipping speeds and dipping directions. It is found that the thermal stress can be reduced by dipping slowly if the stalk is dipped into the crucible vertically, while the thermal stress can be reduced by dipping fast if it is dipped horizontally.

Statistics of strength of ceramics: finite weakest-link model and necessity of zero threshold

It is argued that, in probabilistic estimates of quasibrittle structure strength, the strength threshold should be considered to be zero and the distribution to be transitional between Gaussian and Weibullian. The strength histograms recently measured on tough ceramics and other quasibrittle materials, which have been thought to imply a Weibull distribution with nonzero threshold, are shown to be fitted equally well or better by a new weakest-link model with a zero strength threshold and with a finite, rather than infinite, number of links in the chain, each link corresponding to one representative volume element (RVE) of a non-negligible size. The new model agrees with the measured mean size effect curves. It is justified by energy release rate dependence of the activation energy barriers for random crack length jumps through the atomic lattice, which shows that the tail of the failure probability distribution should be a power law with zero threshold. The scales from nano to macro are bridged by a hierarchical model with parallel and series couplings. This scale bridging indicates that the power-law tail with zero threshold is indestructible while its exponent gets increased on each passage to a higher scales. On the structural scale, the strength distribution except for its far left power-law tail, varies from Gaussian to Weibullian as the structure size increases. For the mean structural strength, the theory predicts a size effect which approaches the Weibull power law asymptotically for large sizes but deviates from it at small sizes. This deviation is the easiest way to calibrate the theory experimentally. The structure size is measured in terms of the number of RVEs. This number must be convoluted by an integral over the dimensionless stress field, which depends on structure geometry. The theory applies to the broad class of structure geometries for which failure occurs at macro-crack initiation from one RVE, but not to structure geometries for which stability is lost only after large macro-crack growth. Based on tolerable structural failure probability of <10−6, the change from nonzero to zero threshold may often require a major correction in safety factors.

Neodymium-doped laser yttrium oxide ceramics

We studied mechanical, optical, and lasing parameters of neodymium-doped yttrium oxide ceramics synthesised by using a new technology involving the laser synthesis of nanopowders and their magnetic pulsed compaction. The fracture toughness of ceramics to cracks and its microhardness were measured to be KIC = 0.9—1.4 MPa m1/2 and Hν = 11.8 GPa, respectively. Ceramic samples sintered in the temperature range from 1550 to 2050°C have the porosity (1—150)×10-4% and the optical loss coefficient α1.07 = 0.03—2.1 cm-1 at a wavelength of 1.07 μm. It is shown that such porosity does not affect the optical loss coefficient of light. Lasing at ≈1.079 μm with a slope efficiency of 15% was obtained in a 1.1-mm-thick sample pumped by laser diodes.

A Computational Approach to Design High Reliability Ceramic Laminates

A numerical design approach for ceramic multilayer laminates with predetermined stress profiles is here pro- posed and compared with a well established theoretical method. The design procedure allows for the optimization of layer composition, in order to obtain high reliability and at the same time control composition and stress state of each layer. The proposed approach is used to design alumina-mullite-zirconia laminates with high toughness, meeting at the same time the particular functional requirements of a pure alumina surface layer with residual compressive stresses with the aim to increase tribological properties. The main limitation to the use of ceramic materials in structural applications is owed to their low reliability, i.e. their high scatter in failure strength and low fracture tough- ness. This behavior is directly related to the presence of flaws originated during the production processes and the service. Although improvements in material quality, manu- facturing methods and component design have resulted in a drastic reduction of structure failures, the low fracture toughness of ceramics, coupled with their susceptibility to failure by slow crack growth under stresses well below their fracture strength, remains a concern for the reliability of ce- ramic bearings in many engineering applications (1). Many efforts have been made with the purpose to in- crease the reliability of ceramic materials: a promising ap- proach is the design and manufacturing of multilayered structures with a tailored residual stress field. Layers of ce- ramic composites with different composition are stacked, obtaining a graded ceramic, with the purpose to develop a specific residual stress profile as a result of the sintering process. The thermal stresses are due to the mismatch of the mean coefficient of thermal expansion between the layers and between the different grains in the same layer (2, 3). It was shown that in ceramic multilayer laminates with alumina surface layer, a compressive residual stress at the surface has beneficial effects on the wear and friction behav- ior, as a consequence of the reduction of crack nucleation and propagation and the formation of thinner and smoother tribolayers (4). Similarly, a compressive layer placed at a certain depth can significantly increase the toughness of the material (5).

EFFECTS OF PROCESSING TEMPERATURE AND FABRICATION METHOD ON MICROSTRUCTURE AND DENSITY OF MULTILAYER AL2O3-ZRO2 NANOCOMPOSITES

Relatively low strength and fracture toughness of ceramics have restricted their applications. Various solutions have been studied to overcome the abovementioned drawbacks. One of the most sophisticated and promising solution is production of multilayer ceramic composites. High hardness, heat and wear resistance, low electrical and thermal conductivity, and resistance to chemical attacks are the most important characteristics of Al 2 O 3. Therefore, it is a common candidate for various industrial applications. Multilayer alumina-zirconia composites were made using two methods of isostatic and hydraulic compaction of dry ceramic powders. The green articles were conventionally sintered at 1420, 1550 and 1650°C for 3h. The microstructure and density of composites were investigated. The results show that the pressing method and sintering temperature have a relatively moderate effect on the microstructure and density of the sintered samples.

Strength and fracture toughness of ceramics

Strength and fracture toughness of ceramics

Ab initio study of mechanical and thermo-acoustic properties of tough ceramics: applications toHfO2 in its cubic and orthorhombic phase

By means of the ab initio all-electron new full-potential linear-muffin-tinorbitals method, calculations were made for elastic constantsC11,C12 andC44 for Si,ZrO2 andHfO2 in their cubic phase,and constants C11, C22, C33, C12, C13, C23, C44, C55 and C66 for HfO2 in its orthorhombic phase. Using the Voigt and Reuss theory, estimations were made forpolycrystals of their bulk, shear and Young moduli, and Poisson coefficients. Thespeed of elastic wave propagations and Debye temperatures were estimated forpolycrystals built from Si and the above mentioned compounds. The semicore4f14 electrons should be included in the valence set of Hf atom in this all-electron approach ifaccurate results for elastic properties under pressures are looked for.

Maximum Stress for Shrink Fitting System Used for Ceramics Conveying Rollers

Steel conveying rollers used in hot rolling mills must be changed very frequently at great cost because hot conveyed strips induce wear on the roller surface in short periods. In this study new roller is considered where a ceramics sleeve is connected with two short shafts at both ends by shrink fitting. Here, the ceramics sleeve may provide longer life and reduces the cost for the maintenance. However, care should be taken for maximum tensile stress appearing between the sleeve and shaft because the fracture toughness of ceramics is extremely lower than the values of steel. In this study FEM analysis is applied to the new structure, and the maximum tensile stress has been investigated with varying the dimensions of the structure. It is found that the maximum tensile stress appearing at the end of sleeves takes a minimum value at a certain amount of shrink fitting ratio.

Heterocoagulation System Assisted Adsorption of Carbon Nanotubes on Alumina for Toughening Ceramics

A heterocoagulation process was adopted to disperse homogenously multi-walled carbon nanotubes (CNTs) into alumina powders via liquid medium and deposit uniform CNTs on the surfaces of alumina particles. Alumina composites with an addition of 2wt% CNTs were prepared by spark plasma sintering (SPS). The mechanical properties of the composites were characterized as a fracture toughness of 6.35 ± 0.15 MPa m0.5, 1.7 times that of pristine alumina ceramics, and also much improved compared to that of alumina ceramics added with pristine CNTs but without the heterocoagulation processing with surfactants. The fracture morphologies showed visually the bridging and pull out effects of the CNTs at interfaces between the alumina grains. The results indicated that the loads during the mechanical tests were efficiently transferred from the alumina matrix to the CNTs and dissipated, and then led to the improvement of mechanical properties.

Nickel and zirconia toughened alumina prepared by hydrothermal processing

Ceramics are attractive candidates for structural systems due to their strength at high temperatures. However, the major limitation to their application is their low fracture toughness. A major research objective for ceramic communities has, therefore, been to improve the fracture toughness of ceramics. The past two decades have seen the emergence of a number of studies concerned with toughening [1–3]. There are two types of mechanisms to improve the resistance to crack propagation, that is, by increasing the inherent toughness (energy dissipation) of the material and reducing the local crack-tip driving force [4], such as residual stress effects, phase transition toughening, crack deflection, nano-composites toughing, and bridging by ductile particles, fibers, and whiskers [5]. The addition of a dispersed second-phase inclusion, which limits the propagation of cracks is one of the most commonly used approaches. With the addition of either zirconia particles [6] or silicon carbide whiskers [7], the fracture toughness and strength were increased [8, 9]. Alternatively, continuous metal and ceramic phases can improve fracture toughness and creep resistance, respectively. A tri-phase structure is of benefit for the suppression of grain growth [10]. In this paper, two toughening agents, nickel and zirconia particles, were added to an alumina matrix to enhance its toughness. Although the simultaneous use of nickel and zirconia particles is not new, few studies have been reported on triphase composite powders obtained by combining hydrothermal processing with co-precipitation. In the present communication, two-phase and tri-phase composites were investigated. Aluminum hydroxide was prepared by precipitation by slowly adding ammonia liquor into a 1 M Al(NO3)3 9H2O solution while vigorously stirring to attain the desired pH value of the solution in contact with the precipitated solid. In general, the pH value was controlled to be in the range of 10–10.5 [11]. The as-obtained aluminum hydroxide was washed by centrifugal filtration with deionized water and methanol for several times and then washed in methanol accompanied by ultrasonic mixing. The addition of Y2O3 to ZrO2 is known to increase the stability of the t-ZrO2 phase in composite powders [3]. Zirconium oxychloride octahydrate was dissolved in deionized water and yttria was dissolved in a hydrochloric acid solution. The yttrium ionic solution was added into the zirconium oxychloride solution. ZrO2 was doped with yttrium to stabilize its final structure. Colloidal zirconium hydroxide was precipitated by drop-wise addition of 2 M ammonia solution into the mixture under vigorous stirring until precipitation reaction was completed. Colloid was washed by centrifugal filtration with deionized water and methanol for several times and then washed in methanol accompanied by ultrasonic mixing. Nickel chloride hexahydrate was dissolved in absolute ethyl alcohol, as also described by Gibson and Kathy [12]. The nickel was reduced by the drop-wise addition of 2 M NaOH containing hydrazine hydrate. The activation of Ni ions by coordinating and surface absorption may overcome H. Jia X. Liu T. Li H. Yan X. Liu B. Xu (&) Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China e-mail: xubs@public.ty.sx.cn

Nanocomposites – Toughened Ceramics

In order to improve fracture toughness of ceramics, an intrinsically small frontal process zone (FPZ) size must be expanded. An intra-type nano-structure, where nano-particles are embedded within matrix grains, yields dislocations around the dispersed particles due to residual stresses. These dislocations become sessile dislocations at room temperature, operate as origins of small stress concentration in the matrix, and create nano-cracks in the FPZ. To produce the intra-type nano-structure, we developed a soaking method. This method makes it possible to produce nano-sized metallic particles dispersed within ceramic powders. In this research, alumina-nickel nanocomposite powder was obtained using the soaking method. The powder mixed with α-alumina as a seed was sintered using a pulse electric current sintering technique. The sintered nanocomposites are then annealed to disperse dislocations around the nanoparticles into alumina grains. Results showed that the maximum fracture toughness was 7.6 MPam1/2, which was two times higher than that of alumina.

Novel Estimation of Critical Frontal Process Zone of Ceramics by a Single-Edge V-Notched-Beam Technique

A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.

Effects of AGG on fracture toughness of tungsten carbide

The cobalt-bonded tungsten carbide has been regarded as a specific success in improving the fracture toughness of ceramics. However, the metal binder phase lowers the hardness and corrosion resistance of the composite and limits its applications. In this paper, the possibility of in situ-toughening tungsten carbide, making use of abnormal grain growth of WC grains to reinforce WC–Co composites, is investigated. The volume fraction of abnormally large grains in the samples is controlled by tailoring compositions and sintering conditions. The fracture toughness of the samples is evaluated using the indentation technique. The experiment results show that the fracture toughness of the sample could be improved by introducing a certain volume fraction of abnormal grains, but at some expense of the hardness. The highest value of fracture toughness, 7.34 MPa m 0.5, is achieved for WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 °C for 4 h. The main mechanisms for the improved toughness are cutting elongated grains, crack bridging and crack deflection, which could resist crack growth in the sample. Therefore, it is possible to in situ-toughen tungsten carbide using the abnormal grain growth of WC.

Influence of t-ZrO 2 addition on mechanical property and electrical conductivity of YSZ electrolyte

8YSZ material that has high electrical conductivity is widely used as electrolytes for solid oxide fuel cells (SOFCs). But its low strength and low fracture toughness hampered the development of SOFCs. In order to find a best method to improve the capability of YSZ electrolyte, the effects of 3Y-TZP additive on the density, strength, conductivity and microstructure were studied by means of X-ray diffraction and Vicker's hardness apparatus. The strength and conductivity of YSZ electrolyte doped with different amounts of 3Y-TZP were determined. It is shown that the samples sintered at 1450 °C for 2 h are the best in properties. When 3Y-TZP powders are added to the YSZ system, the results demonstrate that strength of the electrolyte increases remarkably, and the fracture toughness is improved. The electrical conductivity is lowered only slightly. The results display that the flexural strength and the fracture toughness of ceramics with 30wt. % TZP reach 300 MPa and 3.7 MPa·m 1/2, respectively, and the conductivity at 1000 °C reaches 0.11 S·cm −1.

Prediction of Fracture Toughness in Fibrous Si&lt;sub&gt;3&lt;/sub&gt; N&lt;sub&gt;4&lt;/sub&gt; Monolithic Ceramics

A fracture toughness model of fibrous monolithic ceramics revealed that the major factor that contributed to the fracture toughness in ceramics was the actual energy absorbed by crack propagation rather than the total work of fracture. The load-displacement curve and the crack propagation path were predicted using the derived model mirror image with that of experimental data.

A novel test method to measure the fracture toughness of ceramic balls used in bearings

ABSTRACTA novel mechanical test has been developed to measure the fracture toughness of the silicon‐nitride (Si3N4) balls used in modern hybrid bearings. The ball is compressed diametrally between two hemispherical conforming dies, which causes the ball's equator to bulge, generating a tensile hoop stress. Under applied load, a precrack placed at the equator grows. To calculate the ball's fracture toughness at crack instability, finite‐element calculations of the applied stress field and an analytical solution for the stress intensity factor are used in the ‘two point plus semiellipse’ method. Tests of 16 Si3N4 balls and three soda‐lime glass balls gave fracture toughness measurements in good agreement with accepted published values. The new technique appears to be more accurate than the indentation technique used to measure the toughness of ceramics. As future work, the test can be extended to measure fatigue and stress corrosion properties for Si3N4 balls.

R-curve behavior of Ti 3SiC 2

One of the characteristic material properties for many of the high toughness ceramics (ZrO 2, Sialon, etc.) is the enhanced resistance to crack growth during crack extension, a phenomenon known as ‘R-curve behavior’. Herein, we report, the crack growth resistance/R-curve behavior of Ti 3SiC 2. The crack growth resistance property of Ti 3SiC 2 is experimentally measured using single-edge-precrack-notched beam (SEPB) method. At room temperature, the flexural tests of hot pressed Ti 3SiC 2 bars are performed under 4-point bending mode and the precrack length is varied in our experiments. Additionally, the fracture toughness/crack growth resistance behavior as well as fracture mechanisms of Ti 3SiC 2 are explained on the basis of microstructure and bonding in Ti 3SiC 2. SEM investigation reveals significant crack wake bridging, which is a characteristic feature of many high toughness damage-tolerant ceramic.