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The present work aims to provide fundamental insights into the grain size dependence and mechanical behavior of hot-pressed Al 2 O 3 -ZrO 2 ceramic at its eutectic composition, and further to explore the hardening effect of laser-induced surface nanoeutectic layer. The underlying correlations between densification behavior, grain size distribution and mechanical properties were elucidated. Sintering at 1550 °C promotes the densification without extensive grain growth, and in this case the sample exhibits a critical density of 99.3 %. The average grain size is tailored into a range of 0.6–0.9 μm, and the measured flexural strength and toughness reach 1100 MPa and 11 MPa·m1/2, respectively. The metastable t -ZrO 2 grains indeed play a pivotal role in energy dissipation at the crack tip through crack deflection and branching. In addition, the mechanical behavior is reasonably explained through constructing a multilevel toughening mechanism map associated with grain size distribution of ZrO 2. Particularly, surface nanocrystallized Al 2 O 3 -ZrO 2 eutectic layer with a thickness of 1000 μm free of pores and cracks is achieved by a rapid laser melting process. The outmost laser-modified nanoeutectic layer exhibits a fine cellular structure with an interphase spacing of only 105 nm and a hardness of as high as 26.1 GPa, which provides a promising potential in enhancing significantly the hardness and wear resistance for applications as sliding ceramic components. [ABSTRACT FROM AUTHOR]

This work concerns the microstructure-mechanical properties relationship in Al 2 O 3 /SmAlO 3 composites prepared by hot pressing at temperatures from 1450 °C to 1650 °C. Dense samples have been prepared using co-precipitated trigonal α -Al 2 O 3 and orthorhombic SmAlO 3 at the pseudo-binary eutectic composition. The overall microstructure coarsens with increasing processing temperature, but the nanoscale precipitates within α -Al 2 O 3 and SmAlO 3 grains contribute to coherency strains and distortions in Al 2 O 3 /SmAlO 3 prepared at 1650 °C. The intragranular SmAlO 3 precipitates in Al 2 O 3 grains exhibit semi-coherent interface and an orientation relationship of the intragranular phase SmAlO 3 and alumina matrix close to two different zone axes, namely [ 2 3 ¯ 1 ] SmAlO 3 and [ 4 ¯ 401 ] Al 2 O 3 . Consequently, significant strengthening effect is found from the measured hardness values on the sample sintered at 1650 °C. The measured flexural strength and single edged notched beam fracture toughness of the Al 2 O 3 /SmAlO 3 ceramic are comparable to traditional secondary phases reinforced alumina composites. The sample sintered at 1500 °C exhibits a flexural strength of 558 MPa and a fracture toughness of 5.59 MPa⋅m1/2. [ABSTRACT FROM AUTHOR]

In the present work, Al 2 O 3 -GdAlO 3 oxide nanopowders with eutectic composition of 77:23 (in molar ratio) were prepared via a chemical co-precipitation method. α-SiC particles were then incorporated with different volume fractions of 10, 15 and 20 vol% into the oxide powders. Al 2 O 3 -GdAlO 3 –SiC ceramics were hot-pressed at different sintering temperatures to obtain fully densified bulk ceramics. The as-sintered composites consist mainly of α-Al 2 O 3 , GdAlO 3 and α-SiC phases, and exhibit a dense and uniform microstructure. The GdAlO 3 grains are entangled with α-Al 2 O 3 grains as indicated by SEM observations. SiC nanoparticles are found within the interior of both α-Al 2 O 3 and GdAlO 3 grains or at the interfaces between entangled α-Al 2 O 3 and GdAlO 3 . With increasing the SiC content from 10 to 20 vol%, the high-temperature flexural strength of Al 2 O 3 -GdAlO 3 -SiC ceramics increases from 567 to 616 MPa, and the Vickers hardness increases from 17.5 to 19.5 GPa, while the fracture toughness of Al 2 O 3 -GdAlO 3 -SiC ceramics increases from 8.20 to 9.28 MPa m 1/2 , which is significantly improved with respect to that (7.5 MPa m 1/2 ) of unmodified Al 2 O 3 -GdAlO 3 ceramic. The toughening mechanisms of Al 2 O 3 -GdAlO 3 -SiC ceramics are attributed to crack deflection, pinning and bridging, and bifurcation due to the presence of SiC nanophases observed from the indentation. [ABSTRACT FROM AUTHOR]

Crack-healing behavior and strength recovery of ZrO 2 (Y 2 O 3 )–Al 2 O 3 –MoSi 2 (TZ3Y20A–MoSi 2 ) ceramics have been investigated. Heat treatment in air enhances distinctly the flexural strength of TZ3Y20A–MoSi 2 ceramics. Si–O oxide and ZrSiO 4 are found after heat treatment, in which the former heals the cracks and recovers the strength, however, the latter is one kind of brittle materials that reduces mechanical performance of ceramics. The TZ3Y20A–10 vol% MoSi 2 ceramic has a complete crack-healing effect and strength recovery by suitable heat treatment at 1373 K for 3 h in air, however, the strength does not recover completely under other heat treatment conditions. With further increasing the MoSi 2 contents to 20 vol%, the flexural strength of as heat-treated TZ3Y20A–MoSi 2 ceramics is higher than that of pre-cracked specimens, but remains lower than that of smooth specimens. Crack closure and rebonding of the crack walls are due to the formation of Si–O glassy phase by oxidation reaction. [ABSTRACT FROM AUTHOR]

Crack-healing ability and strength recovery of different ZrO 2 (Y 2 O 3 )–Al 2 O 3 –SiC (TZ3Y20A–SiC) ceramics have been investigated by tailoring heat treatment temperature, heat treatment time, SiC content and crack size. Heat treatment in air enhances distinctly the flexural strength of TZ3Y20A–SiC ceramics. A complete crack-healing of TZ3Y20A–10 vol%SiC ceramic can be realized by optional heat treatments at 1073 K for 30 h, 1273 K for 10 h or 1373 K for 5 h, respectively. With increasing the content of nanosize SiC particles from 10 to 20 vol%, the crack-healing process of TZ3Y20A–SiC ceramics speeds up at identical temperature levels. The pre-existing cracks with a width up to 0.45 μm are healed completely under the optimum heat treatment conditions, and the flexural strengths of crack-healed TZ3Y20A–SiC ceramics are completely recovered to the value of smooth specimens. Crack closure and rebonding of crack wall due to the formation of SiO 2 glassy phase by oxidation reaction between SiC and the oxidizing atmosphere are dominant healing mechanisms in TZ3Y20A–SiC ceramics. [ABSTRACT FROM AUTHOR]

Crack healing behavior of hot-pressed TZ3Y20A-SiC ceramics has been investigated by high-temperature oxidation. Semi-elliptical surface cracks with a length of 100μm have been set on the tensile side of each specimen using a Vickers hardness indenter. Based on flexural strength and observations of crack appearance, the effect of crack healing is finally judged. Cracks on ZrO2(Y2O3)–Al2O3 (TZ3Y20A) ceramics cannot be healed by heat treatment. However, for TZ3Y20A-SiC ceramics, complete crack-healing has been realized by heat treatment at 800°C, 1000°C and 1100°C for 30h, 10h and 5h, respectively. The crack-healing mechanism is attributed to the formation of SiO2 caused by high temperature oxidation during heat treatment. [ABSTRACT FROM AUTHOR]

Reaction process and phase formation mechanisms in reactive hot pressing (RHP) of titanium and boron nitride blend powders have been investigated in vacuum. In temperatures of up to 1300°C, chemical reactions in the Ti–BN blend powder proceed via a solid-state diffusion process. Intermediate phases of hexagonal α-TiN0.3, tetragonal Ti2N, nonstoichiometric cubic TiN0.9 and orthorhombic TiB are found before they finally transform to the stoichiometric cubic TiN and hexagonal TiB2. Both the TiN–TiB2 ceramics reactive hot-pressed at 1600 and 1800°C represent intragranular/intergranular distributions of TiB2 in TiN. In contrast with monolithic TiN and TiB2 ceramics, the reactive hot-pressed TiN–TiB2 exhibits a significant enhancement on flexural strength and fracture toughness. TiN–TiB2 ceramics reactive hot-pressed at 1800°C have a Vickers hardness of 17.8GPa, an elastic modulus of 475GPa, a flexural strength of 732MPa and a fracture toughness of 8.5MPa·m1/2. [ABSTRACT FROM AUTHOR]

In recent years, the increasing potential for the use of high temperature self-lubricating composites as sliding components at extreme environments have attracted great attention on these materials. In the present work, NiCr–BaCr2O4 self-lubricating composites have been prepared using the hot pressing method. The friction and wear properties of NiCr–BaCr2O4 composites were investigated by using a ball-on-disk friction and wear tester in sliding against alumina ball up to 800°C. At room temperature, the NiCr–20BaCr2O4 composite has a friction coefficient of 0.72, which is clearly higher than that (0.44) of NiCr alloy. However, the composites incorporated with BaCr2O4 exhibit distinct improvements in effectively reducing friction and wear in a temperature range of 400–800°C as contrasted with unmodified NiCr alloy. At 800°C, the NiCr–20BaCr2O4 composite exhibits a friction coefficient of 0.27 and a wear rate of 4.5×10–6 mm3/(Nm), while the unmodified NiCr alloy has a friction coefficient of 0.41 and a wear rate of 9.2×10–5 mm3/(Nm), respectively. At elevated temperatures, a dense and protective oxide glaze layer is generated on the worn surface of composites. The glaze layer was then investigated by means of scanning electron microscopy, laser Raman spectroscopy and nano-indentation. Tribo-oxidation reaction of BaCr2O4 during high temperature wear tests is responsible for the formation of the protective glaze layer. The newly formed BaCrO4 contains lubricious hexavalent-Cr6+, which can promote the densification of oxide glaze layer, and finally generate a dense lubricating film to effectively reduce friction and wear at elevated temperatures. [ABSTRACT FROM AUTHOR]

TiN–TiB2 composites incorporated with or without MoSi2 have been prepared by the reactive hot pressing method, using Ti, BN or MoSi2 powders as the starting materials. The influence of MoSi2 addition on tribological properties of TiN–TiB2 composites have been evaluated by using a ball-on-disk friction and wear tester in sliding against alumina ball from room temperature to 800°C. The wear performance depends mainly upon temperature as well as the applied load. At room temperature and 10N load, the TiN–TiB2 ceramic has a wear rate of 1.6×10–6 mm3 N–1 m–1, however, the TiN–TiB2–MoSi2 composite exhibits a wear rate of 1.5×10–4 mm3 N–1 m–1 under identical wear condition. At room temperature, the material removal of composites results from brittle fracture and abrasive wear during wear tests. The addition of MoSi2 can improve the oxidation resistance of TiN–TiB2 ceramic at high temperatures as it oxidizes to produce a protective SiO2 layer, which forms an effective barrier layer to restrain from further oxidation. At 800°C and 10N load, the wear rate of TiN–TiB2–MoSi2 composite is 1.7×10–6 mm3 N–1 m–1, which is only one fifth as much as the TiN–TiB2 ceramic. The low wear rate of TiN–TiB2–MoSi2 composite at 800°C is attributed to high oxidation resistance, the formation of dense SiO2 scale and self-healing of microcracks. [ABSTRACT FROM AUTHOR]