Key Engineering Materials Vols. 434-435

Abstract: Machinability of ceramics is a comprehensive behavior resulting from the interactions of several factors. In this paper, the fuzzy comprehensive evaluation system was applied to establish a model for machinabilities of several dental ceramics, including zirconia, empress feldspar, empress toughening leucite and lithium disilicate glass. The hardness (HV), fracture toughness (KIC) and elastic modulus (E) of ceramic samples were used in evaluation model with different weighted indexes. The ceramics could be thus divided into 4 groups of “easiest”, “easy”, “difficult” and “most difficult”, based on evaluated results. Lithium disilicate glass belongs to “easiest”, empress feldspar and empress toughening leucite are at the “easy” level, and zirconia is “difficult” to be machined compared with other counterparts. The evaluations by established model are well supported by the practical machining experiments while these dental ceramics being undergone cutting. This implies that the derived evaluation model is an easy and simple way to estimate machinability of dental ceramics.

Abstract: Two groups of silicon carbide ceramics were prepared by pressureless sintering processing with a small amount of boron and carbon as sintering additives. The flexural strength, Vickers hardness, fracture toughness and compressive strength of the samples were measured and compared. Sample A achieved much higher flexural strength (389.4MPa) than that of sample B (250.3MPa) due to the difference of additive dosage. However, the two groups of samples attained comparable values for the other three mechanical properties. SEM observation revealed that the stoichiometric ratio of the Si/C atom in the sintered body is an important factor to affect the mechanical performance of the silicon carbide. The ballistic performance of the samples was also evaluated in this study. The results showed that sample B exhibited more excellent ballistic property, which is probably related to the high flexural strength. The detailed influencing factors need to be studied further.

Abstract: Ti(C,N) cermets were prepared by SPS method. The effects of nickel salt coating as well as ball milling times on the microstructure and mechanical properties of the cermets were investigated. SEM images shows that the nickel was coated homogeneously on the surface of Ti(C,N) powders by mixing Ni salt and Ti(C,N) powder. The grains of Ti(C,N)-based cermets became finer with increasing ball milling time. The flexural strength increased when the balling milling was below 48h, and then decreased with increasing of ball milling time, which was due to the decrease of the flake structure in cermets.

Abstract: Porous SiO2 ceramics had been successfully prepared by gelcasting. The open porosity of obtained samples was between 0.40 and 0.46, which reached its maximum at 1100°C. The change of line shrinkage rate was opposite to the open porosity. The microstructure was investigated though X-ray diffraction (XRD) and scanning electronic microscope (SEM).

Abstract: Porous mullite was prepared from a reactive mixture of Al metal powder and the fly ash produced by power plant. The sintering ceramics was characterized for the pore structure and phase identification studies. It was found that the sintered specimen expands at first (1300-1400°C) and then shrinks which was correspondence to the changing of open porosity. The expansion was attributed to the fact that the alumina produced from the oxidation reaction of Al powder made the volume larger than the starting mixtures. At 1550°C, it was inferred that the alkaline element in the fly ash made the mullite decomposed then destroyed the mullite structure. The SEM photographs suggested us that the pore diameter of the sintered ceramics might be adjusted by adding Al powder with different grain sizes. Moreover, because of the rod-like mullite produced in the pores, Mg metal addition could develop the further modification of the pore structure with similar porosity (47% at 1400°C/4h) compared with that (53%) with 0wt % Mg metal powders.

Abstract: Porous mullite were prepared by coating Al(OH)3 on the surface of the particles of waste fly ash. The effects of content of urea on formation of mullite and sintering temperature on microstructure of mullite were investigated by XRD and SEM analysis. It was showed that two kind mullite were obtained, one was growing from original Needle-like mullite existing in original ashfly, and the other was formed through by the reaction between glassy phase and Al2O3 coated on spherical waste fly ash.

Abstract: Two domestic silicon carbide powders with different particle size distribution and one petroleum coke powder were blended in proportion and then dispersed in aqueous medium. Green bodies were solidified from these suspensions via conventional slip casting. The effects of pH, solid loading, and the amount of dispersant on the formability and packing ability were evaluated. The results showed that the pH within 9.5-10.5, solid loading of 42vol%, aging time more than 24h, and 0.3wt% of dispersant were optimal. Complicated green bodies with height of 300mm and thickness of 3.5mm were obtained. The corresponding suspension viscosity was 1200mPa∙s and the relative packing density was 64.8%. The density is 3.01g∙cm-3 and the flexural strength is 305±15 MPa after reaction sintering. These results may be attributed to accurately using of dispersant and in-depth study of processing parameter.

Abstract: A novel evaluation method of pozzolanic activity of metakaoline is studied. The pozzolanic activity can be evaluated by the measurement of the content of AlO4 tetrahedra. Base on this method, the influence of calcination time on pozzolanic activity of metakaoline is studied. The results indicate that the pozzolanic activity increases with the calcination time when calcination time is below 4 hours, and it decreases with the calcinations time increasing when the calcination time is 4-8 hours.

Abstract: Carbon fibre reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) are new type of high performance brake materials, and possess a series of outstanding advantages. In the present study, the C/C-SiC brake composites were fabricated by the combination of chemical vapor infiltration with liquid silicon infiltration. The fabric preforms were prepared by three dimension needling method. Results show the SiC form inside the short-cut fabric, around the needing fibre and among the fibre bundles in the non-woven web of the C/C-SiC composites. The average static friction coefficient of the C/C-SiC materials was 0.61. The average dynamic friction coefficient were between 0.41 and 0.54 with the increase of brake speed, and the wear rates are not sensitive to the brake speed and maintained about 0.02 cm3•MJ-1 all along.

Abstract: Al4SiC4 is probably used as non-oxide raw material for high-temperature ceramics. Al4SiC4 was synthesized by using starting materials of metal-carbon, metal-oxide- carbon or carbides. In this paper, Al4SiC4 has been synthesized by using oxides (Al2O3 and SiO2) and carbon as starting materials through carbothermal reduction process. The oxidation properties in air and O2 atmosphere by TG method and the stability of the synthesized Al4SiC4 heated in moisture were investigated. Pure Al4SiC4 phase was synthesized when heating the mixture powder of Al2O3, SiO2 and C (C : Al2O3 : SiO2 = 8 : 2 : 0.8, in molar ratio) at 1700°C for 8h in flowing Ar atmosphere. The synthesized Al4SiC4 powder consists of platelet shape grains with size of 68μm length, 35μm width and 1μm thickness. Al4SiC4 obviously oxidized from 800°C when heated in flowing air or O2 atmosphere. When keeping Al4SiC4 powder in moisture, it was not observed to be reacted with water by XRD and SEM analysis.