Key Engineering Materials Vols. 434-435

Abstract: The Cf/SiC made from carbon fiber preforms infiltrated by phenol resin, pure carbon slurry and aqueous C/SiC slurry showed different binding strength between carbon fiber and SiC matrix, thus influenced the fracture behavior of the composite. The fracture toughness of the Cf/SiC composites with the value of 9.82MPa•m1/2,improved remarkably compared with reaction- bonded silicon carbide (RBSC). But the flexural strength was less than 100 MPa, because of the existence of considerable amount of pores in C/SiC composites.

Abstract: Si3N4/BN composite ceramics with 25vol% h-BN were prepared by pressure-less sintering process with Nd2O3/Al2O3/Y2O3 as sintering additives. The effects of these ternary additives on the densification behaviors and mechanical properties were investigated. XRD and FESEM were used to investigate the α-β phase transformation and microstructure. The XRD results showed that α-Si3N4 has transformed to β-Si3N4 completely in all the samples during the pressureless sintering process. The line shrinkage increased with the Nd2O3 contents increasing, and the highest line shrinkage (7.75%) was observed when 4wt% Nd2O3 was added, then decreased. The same trends were observed in flexural strength and fracture toughness testing. The ternary additives of Y2O3-Al2O3-Nd2O3 could improve the density, strength and fracture toughness of the material effectively.

Abstract: Stainless steel/TiC nanocomposite powder were prepared by high-energy ball-milling method using 316 stainless steel powder, carbon and titanium powder as raw materials. Microstructure of the nanocomposite powder was investigated with XRD and TEM techniques. The results showed that the stainless steel/TiC nanocomposite powder obtained when the ball-milling time was more than 40 hours. DSC analysis method was used to study the characteristics of oxidation resistance and the oxidation reaction kinetics of the nanocomposites powder. Results show that the oxidant resistance of nanocomposite powder was improved, the activation energy of oxidation reaction increased.

Abstract: The impact properties of TiC-Fe composite at different temperatures prepared by SHS / PHIP technique were investigated by impact test which was conducted in air from 20°C to 100020°C. Morphology of the impact fracture surface was observed by scanning electron microscope (SEM) and phase composition before and after the impact test was analyzed with X-Ray Diffraction. The results indicate that the impact toughness of TiC-Fe composite increases significantly with the temperature increasing above 700°C, representing a minimum value at 700°C. The impact fracture of TiC-Fe composite at room temperature, 700°C, 900°C and 1000°C is mainly brittle, showing certain plasticity with morphology of dimples as the temperature increases above 700°C. A small amount of Fe2O3 exists in the fracture surface, indicating TiC-Fe composite is oxidized partially during the impact test at elevated temperatures.

Abstract: The key problem about the application of TiC/Fe composite ceramics in high temperature is the oxidation resistance. With an aim to investigate oxidation properties, the high density TiC/Fe ceramics was produced by self-propagating high-temperature synthesis method combined with pseudo hot iso-static pressing (SHS/PHIP). The oxidation kinetics of TiC/Fe ceramics were investigated by means of conducting the isothermal oxidation experiments in air at temperature of 550°C  900°C for up to 150 hours, and measuring the variations of the weight gains of the composites with the oxidation times. The microstructure evolution of the composites during the oxidation processing was studied by using SEM/EDS and XRD. Experimental results show that TiC/Fe ceramics basically follows a parabolic rule at high temperature. A detailed characterization of the microstructure and distribution of the phases within the scale following oxidation studies has been undertaken to suggest the possible mechanism for the oxidation of TiC/Fe ceramics.

Abstract: To characterize wear behaviors of TiC ceramic particulate-reinforced metal matrix cermets in abominable condition, wear-resistance test was practiced in grain abrasive wear with the TiC-xNi cermets in-situ fabricated by self-propagating high-temperature synthesis method combined with pseudo hot iso-static pressing. Grain abrasion test was conducted with the MLS-225 type tester. Wear resistance was evaluated as weight loss obtained after each period of 10 minutes. Wear resistance of the cermets increased with the increase of TiC content in the composites. SEM was employed to analyze the wear process and wear mechanism. SEM examination of the wear tracks in the worn surface suggests that abrasive wear mechanisms of different binder content cermets are similar. The worn surface is characterized by long and deep parallel grooves. The fracturing of bigger carbide grains and carbide framework by a fatigue process under repeated abrasion is followed by weight loss of the material.

Abstract: The thermal conductivities of the LaPO4/Al2O3 composites that were fabricated by spark plasma sintering (SPS) were determined. The results revealed that their thermal conductivities displayed nearly a slow decrease with increasing temperature from 25oC to 800oC, having the classic 1/T dependence. In addition, the conductivities of the composites decrease monotonously with increasing the LaPO4 content because of the lower thermal conductivity of LaPO4. The calculated conductivities of the composites using Maxwell equation match well the experimental values at both the end members of LaPO4 and Al2O3 being the continuous phase, but showing a little deviation at intermediate composition.

Abstract: Hydroxyapatite (HA) has attracted many attentions because it has a similar structure to that of human bones and favorable biocompatibility. However, the antibacterial effect of HA must be improved. In this paper, HA nanoparticles were prepared by a simple method. Then, Ag-carrying HA powder (Ag-HA) was prepared by an ion-solution reaction and its antibacterial property was investigated. The results showed that Ag-HA powder had excellent ability in inhibiting the growth of two species of bacteria, namely E. coli and S. aureus. Ag ion was believed to play a key role in killing bacteria.

Abstract: Nd--sialons with the stoichiometric composition of Nd0.333Si10Al¬2ON15 were obtained by hot-press sintering at 1800°C for 1h. The thermal shock behavior of the Nd--sialons was examined by a water-quenching technique. The influence of the thermal shock temperature difference (T) and cycle times on the residual strength was evaluated. Equiaxed -sialon grains formed together with a small amount of intergranular phase M (Nd2Si3-xAl¬xO3+xN4-x) and -sialon phase. The residual strength after a thermal shock tended to decrease gradually with increasing T above 500°C. However, the specimens exhibited an improved residual strength (~94% of the room temperature strength) after a thermal shock of T=1100°C. The residual strength presented a gradual decrease with increasing the thermal shock cycle times at T=1100°C, and was still remained ~55% of the room temperature strength after 11-time cycle. It is contributed to the surface oxidation which may results in the healing of surface cracks and the generation of surface compressive stresses.

Abstract: The colored TaON and Ta3N5 were synthesized by ammonolysis of amorphous Ta(OH)5 in the temperature range of 800-1000°C. Ta(OH)5 were prepared by titration process from TaCl5. TaON were synthesized at 900°C for 2 hr, and Ta3N5 were synthesized at 1000°C for 5 hr. In general, Ta3N5 powders were easily oxidized at 500°C with changing colority from red to white. To minimize the oxidation problem of Ta3N5 powders, we proposed silica coated system using TEOS and APTES as a surfactant. Tonality of silica coated Ta3N5 has very little changing which means silica coating actually minimizes the oxidation of the Ta3N5 particles.