Damage Behavior in Ceramic Plasma-Coated and Uncoated Glass with Steel-Ball Impact

A ceramic plasma-coating method that reduces damage from particle impact in a ceramic material is suggested. A steel-ball impact test was performed to investigate and compare the damage behavior of uncoated and ceramic-coated glass. Cone and lateral crack lengths were compared and damage volume analyzed and compared for different steel-ball diameters and types of coating materials, Al₂O₃-TiO₂, Al₂O₃, and Cr₂O_3. Damage resistance of the coating and Al₂O₃-TiO₂ was found to be best because of its mechanical properties and microstructure.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Measurement of the Fracture Toughness of Ceramic Materials Using a Miniaturized Disk-Bend Test

The fracture toughness of several ceramic materials has been measured using a miniaturized disk-bend test apparatus and methodology based on small disk-shaped samples 3 mm in diameter. The method involves the Vickers indentation of specimens ranging in thickness from 300 to 700 μm, and testing them in a ring-on-ring bending mode. New experiments on a glass-ceramic (GC) and Si₃N₄ have been performed to demonstrate the validity of the technique, supplementing the original work on ZnS. The fracture resistances of these materials increase with increasing crack length ( R -curve behavior). The data are analyzed using a specific model for the relationship between fracture resistance and crack length; this model enables the R -curve behavior to be treated analytically, and the fracture resistance at "infinite" crack length to be evaluated using a straightforward graphical procedure. The resulting values of the fracture toughness for ZnS, GC, and Si₃N₄ are 0.74 ± 0.02, 2.18 ± 0.09, and 4.97 ± 0.07 MPa-m^1/2, respectively, which are all in very good agreement with values obtained from conventional fracture toughness tests on large specimens. The results verify the utility of the miniaturized diskbend method for measuring the fracture toughness of brittle materials.     

γ-Y2Si2O7, a Machinable Silicate Ceramic: Mechanical Properties and Machinability

In this paper, the mechanical properties of bulk single-phase γ-Y₂Si₂O₇ ceramic are reported. γ-Y₂Si₂O₇ exhibits low shear modulus, excellent damage tolerance, and thus has a good machinability ready for metal working tools. To understand the underlying mechanism of machinability, drilling test, Hertzian contact test, and density functional theory (DFT) calculation are employed. Hertzian contact test demonstrates that γ-Y₂Si₂O₇ is a "quasi-plastic" ceramic and the intrinsically weak interfaces contribute to its machinability. Crystal structure characteristics and DFT calculations of γ-Y₂Si₂O₇ suggest that some weakly bonded planes, which involve Y–O bonds that can be easily broken, are the sources of the low shear deformation resistance and good machinability.     

Effects of Strain Induced by Pressing and Grinding BaTiO3 and SiO2

The simple ceramic processes of grinding and pelleting leave residual stresses in common materials such as SiO₂ and BaTiO₃. The magnitude of these stresses is easily detected by changes in the temperature of phase transitions; such processes are enough to completely eliminate the phase transitions.     

Thermoreversible Gelcasting: A Novel Ceramic Processing Technique

A new ceramic processing method, thermoreversible gelcasting (TRG), has been developed. The method uses a gelation process that can be reversed multiple times. Two Al₂O₃ systems have been explored with the use of a specific triblock copolymer to form a binding gel network. The mixture becomes a free-flowing liquid on heating above 60°C. However, on cooling below 60°C, the slurry instantaneously transforms to a physical gel. This process can be reversed easily (essentially with an infinite processing time window) while the solvent is present in the system, which is advantageous when trying to produce high-quality dense pieces if initial casting irregularities occur. Near-theoretically dense specimens have been produced with properties consistent with reported values of high-density Al₂O₃.     

Quantitative Studies of Thermal Shock in Ceramics Based on a Novel Test Technique

A thermal shock test was designed which permits the thermal fracture resistance and the mechanical strength of brittle materials to be quantitatively correlated. Thermal shock results for two materials, A1₂O₃ and SiC, were accurately predicted from biaxial strength measurements and a transient thermal stress analysis (performed using a finite element method). General implications for the prediction of thermal shock resistance, with special reference to ceramic components, are discussed.     

Effect of Silica and Processing Environment on the Toughness of Alumina Composites

Results are presented for the effect of the hot-pressing environment and the oxygen content of Sic whiskers on the fracture toughness of an A1₂0₃-matrix composite reinforced with 15 vol% Sic. For the composite with Sic whiskers having a 55 at.% oxygen layer, the fracture toughness (K_ic) decreases from 8.03 to 6.64 MPa-m^1/2 when the hot-pressing environment is changed from argon to a vacuum of ∼6.7 × 10⁻² Pa. The results indicate that the decrease in K_lc of the composite is related to the degradation of Sic whiskers by reaction with SiO₂, which is enhanced in a vacuum environment where the reaction products are constantly removed.     

Rheological Property and Stress Development during Drying of Tape-Cast Ceramic Layers

Rheological property and stress development of tape-cast ceramic layers derived from nonaqueous alumina (A1₂O₃)-poly(vinyl butyral) (PVB) suspensions were observed during drying. Casting suspensions exhibited strong shear-thinning behavior, with a low shear Newtonian plateau apparent viscosity >10² Pa^.s. The apparent suspension viscosity displayed a power-law dependence on the A1₂O₃ volume fraction during the initial stage of drying (<30% solvent loss). Stress development, measured by a cantilever deflection method, and parallel weight loss measurements were performed during the drying of tape-cast layers and pure binder coatings. Maximum drying stresses (σ_max) of 1.37-0.77 MPa were observed for plasticized tapes cast at gap heights of 150-400 μm. In contrast, nonplasticized tapes of similar thickness displayed a more gradual stress increase, with σ_max values approximately an order of magnitude higher than their plasticized counterparts. The stress histories of the corresponding binder coatings were quite similar to the tape-cast layers, albeit slightly lower σ _max values were observed. Stresses decayed beyond σ_max with a logarithmic time dependence to an almost constant value of 0.2-0.4 MPa for the plasticized tapes. Based on these observations, process methodologies have been offered to minimize stress development and retention in tape-cast ceramic layers     

Influence of Test Methods on Fracture Toughness of a Dental Porcelain and a Soda Lime Glass

The aim of this study was to investigate the influence of the test method on fracture toughness of a dental porcelain and a soda lime glass. Three methods were used to determine fracture toughness: the indentation strength (IS) by bending, chevron-notched beam (CNB), and the single-edge-notched beam (SENB). In the IS method, the ratio of elastic modulus to hardness ( E / H ) in the formula was determined by two methods: individual measurement for E and H (IS_M) as well as direct estimation from Knoop's indentation method (IS_K). The tested materials were a dentin porcelain, a traditional feldspar-based leucite-reinforced glass ceramic (Carrara Vincent), and a soda lime glass. Carrara Vincent showed a higher toughness ( P <0.01) than glass with all three test methods. The toughness values manifested significant differences between the methods used ( P <0.01). The two-way analysis of variance suggested that the materials tested and the test methods used had interaction effects, which statistically means that differences in materials and methods influenced the comparability of the toughness result. In this study, a first step was made to compare different toughness test methods by testing the toughness of a traditional feldspar-based leucite-reinforced glass ceramic and a soda lime glass that has a homogeneous microstructure. An interaction effect of the method and the material used was shown. As a consequence, none of the methods tested is suitable as a universal fracture toughness test method. Further research is needed to investigate more extensively the influence of material composition on the fracture toughness test methods' comparability.     

Multilayer Construction with Various Ceramic Films for Electronic Devices Fabricated by Aerosol Deposition

Aerosol deposition (AD) is applicable as a fabrication technology for microstructures comprising different materials. We used this method for electronic devices that consist of ceramic films and metal electrodes. Various ceramic thick films (5–50 μm thickness), for example, Al₂O₃, 2MgO·SiO₂, and BaTiO₃, were deposited on substrates using room-temperature aerosol deposition. The dielectric constant of BaTiO₃ was 78 at 1 MHz. Multilayer constructions with ceramic films and copper electrodes were obtained using aerosol deposition and sputtering. During deposition, photoresist film masks were applied to produce patterns of ceramic films and connections between upper and lower electrodes through the ceramic films.     

Application of the J Concept to Alumina at High Temperatures

Conventional K _IC measurements exhibit a dependence of K _IC values on loading rate for material containing a second phase. An especially strong dependence had been observed at 900°C at which a maximum in K _IC also occurred. In an attempt to determine if this effect is caused by the measurement method and/or reflects a true material property associated with the second phase, an energy concept like the J integral was applied. J -calculation methods for a single load-displacement curve and for J -resistance curves ( J_R analyses) were applied to this alumina and the results compared to those for a pure alumina. The J_R analysis, in the form of the single specimen compliance method with partial and total unloading and the multiple specimen method, points out both the influence of the second phase and the problems of describing material constants for specific ceramic materials at high temperatures.     

Preparing Low-Loss Low-Temperature Cofired Ceramic Material without Glass Addition

A low-temperature cofired ceramic (LTCC) composition for radio-frequency purposes was accomplished without prior glass preparation. In this process, the formulation was made by mixing the glass-forming oxides (ZnO, SiO₂, and B₂O₃) with the commercial microwave ceramic MgTiO₃–CaTiO₃. The sintering, microstructure, and microwave properties were compared to a formulation with exactly the same composition, but a conventional production route, including glass preparation. The novel preparation route resulted in improved firing properties of the mixture. Also, the densities, porosities, and phases of the samples were almost the same as those of the conventional samples, but the phase fractions were different. Finally, this preparation route produced better dielectric values.     

Bonelike Coatings onto Ceramics by Reactive Magnetron Sputtering

Bonelike coating of partially carbonated, calcium hydroxyapatite onto strong ceramics and metals is one of the current objectives in biomaterials science for the development of bioactive implant materials. We recently accomplished bonelike apatite coating onto ceramic alumina by the newly developed reactive rf-magnetron sputtering method, which was undertaken in a mixed CO₂/Ar gas. The key of this method is the use of calcium phosphate glass targets with much lower Ca/P ratios than the stoichiometric value of hydroxyapatite. Surface analyses by XRD, EDX, and FT-IR identified the formation of bonelike carbonated calcium hydroxyapatite, in which CO^2-₃ ions occupied the lattice positions of PO^3-₄. The bioactivity of the coated ceramic materials was confimed by in vitro experiments on the bonelike crystal growth on their surfaces in a simulated body fluid, and the growth rate on the coated layers was experimentally proved to be much faster than that on uncarbonated hydroxyapatite     

Synthesis of Ultrahigh-Temperature Si–B–C–N Ceramic from Polymeric Waste Gas

A "green" route to ultrahigh-temperature Si–B–C–N ceramic from vacuum-degassing waste gas of polyborosilazane {B[C₂H₄Si(CH₃)NH]₃} _n (T2-1) has been developed. After gas-to-gel transformation, an amorphous ceramic Si_5.3B_1.0C₁₉N_3.7 was derived from the gel by dehydrocoupling and polymer-to-ceramic transformation. The ceramic started to form a nanostructure at 1700°C and resisted thermal degradation up to 2200°C in argon. This suggests that vacuum-degassing waste gases of polymer precursors may be perfect raw materials for various advanced ceramics.     

Theoretical Studies of the Electronic Properties of Ceramic Materials

The first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method for electronic structure studies has been applied to a variety of complex inorganic crystals. The theory and the practice of the OLCAO method in the local density approximation are discussed in detail. Recent progress in the study of electronic and optical properties of a large list of ceramic systems are summarized. Eight selected topics on different ceramic crystals focusing on specific points of interest are presented as examples. The materials discussed are AIN, Cu₂O, β-Si₃N₄, Y₂O₃, LiB₃O₅, ferroelectric crystals, Fe-B compounds, and the YBa₂Cu₃O₇ superconductor. The results include the band structure, density of states, charge density distribution, spin density distribution, effective charges, total energy and totalenergy-derived results, optical absorption, positron annihilation spectra, and more. Extension of the band theoretical approach to the study of other areas of fundamental ceramic science is also discussed.     

Physical and Mechanical Properties of Bulk Ta4AlC3 Ceramic Prepared by an In Situ Reaction Synthesis/Hot-Pressing Method

Bulk Ta₄AlC₃ ceramic was prepared by an in situ reaction synthesis/hot-pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X-ray diffraction data were obtained. The physical and mechanical properties of Ta₄AlC₃ ceramic were investigated. Ta₄AlC₃ is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa·m^1/2, respectively. Typically, plate-like layered grains contribute to the damage tolerance of Ta₄AlC₃. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta₄AlC₃ was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200°C in air, Ta₄AlC₃ still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

Thick Nb-Doped Bismuth Titanate Film with Controllable Grain Orientation

In the current work, we reported a potential approach to obtain thick ceramic films with controllable grain orientation based on magnetic alignment and polymerization techniques. The slurry containing 40 vol% Bi₄Ti_2.96Nb_0.04O₁₂ (BINT) ceramic powder, monomer, initiator, and catalyst was drop coated on a Pt substrate and then moved into a vertical 10 T magnetic field. In 1–2 min, the ceramic particles were aligned by a strong magnetic force in slurry and then in situ locked by polymerization on the substrate. After sintering at 1000°C, a BINT ceramic film (50–80 μm in thickness) with a highly a / b plane orientation was obtained. Theoretically, the grain orientation in the films can be easily controlled by adjusting the magnetic field direction. This approach is readily applicable to other materials with a non-cubic structure and is expected to facilitate the fast preparation of grain-oriented thick films.     

Estimation of Hamaker Constants of Ceramic Materials from Optical Data Using Lifshitz Theory

The Hamaker constants of eight different ceramic materials, 6H-SiC, tetragonal, partially stabilized ZrO₂ (3% Y₂O₃), β-Si₃N₄, α-Al₂O₃, Y₂O₃, sapphire (single-crystal α-Al₂O₃), MgO, MgAl₂O₄, and fused silica, across air, water, and n -dodecane at room temperature and across silica at 2000 K have been calculated from optical data using the Lifshitz theory. Spectroscopic ellipsometry was used to measure the photon energy dependence of the refractive index, n , and the extinction coefficient, k , in the visible and near-UV range on several important ceramic materials. This relatively simple, nondestructive technique has proved to yield reliable optical data on sintered, polycrystalline materials such as Si₃N₄, SiC, ZrO₂, Al₂O₃, and ZnO. For the other materials, Y₂O₃, sapphire, MgO, MgAl₂O₄, and fused silica, optical data from the literature were used to calculate the Hamaker constants. The calculated Hamaker constants were estimated to be accurate within ±10%.