Tribological Behavior of SiC-Whisker/Al2O3 Composites against Carburized 8620 Steel in Lubricated Sliding

Mineral oil lubricated sliding tests of Sic-whisker (SiC_w,)/A1₂0₃ composites and monolithic alumina against carburized 8620 steel were conducted on a cylinder-on-cylinder machine. The wear rate of the composites was one or two orders of magnitude less than that of pure alumina. Hot-pressed 25 wt% SiC_w,/A1₂0₃ composite had a lower wear rate than sintered and HIPed 7.5 wt% SiC/Al₂O₃ composite under the same conditions. The weight loss of the steel mating ring against the 25 wt% SiC_w, composite was a factor of four lower than against the 7.5 wt% SiC_w, composite, but the former was a factor of 50 to 60 less than that against pure alumina. The composites showed lower friction coefficients than alumina during the run-in stage. The friction coefficients decreased with initial wear. The steady-state friction coefficient decreased with increasing load up to 500 N for hot-pressed 25 wt% SiC,/Al₂0₃ composite. Further, SEM observation showed much less microfracture in composites than in alumina. EDAX analysis revealed less Fe transfer from the steel ring to the composites than to pure alumina. Wear by microfracture and by adhesion in composites was suggested to be suppressed by SIC whiskers. This in turn reduced wear of the steel because of the generation of fewer hard particles.     

Tribological Properties of Polycrystalline Ti3SiC2 and Al2O3-Reinforced Ti3SiC2 Composites

Tribological properties of Ti₃SiC₂ and Al₂O₃-reinforced Ti₃SiC₂ composites (10 and 20 vol% Al₂O₃) were investigated by using an AISI-52100 bearing steel ball dryly sliding on a linear reciprocating athletic specimen. The friction coefficients were found varying only in a range of 0.1 under the applied loads (2.5, 5, and 10 N), and the wear rates of the composites decreased with increasing Al₂O₃ content. The enhanced wear resistance is mainly attributed to the hard Al₂O₃ particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses.     

Studies in Lithium Oxide Systems: XII, Li2O-B2O3-Al2O3

Tentative phase relations in the binary system BnOa-A1₂O₃ are presented as a prerequisite to the understanding of the system Li₂O-B₂O₃-Al₂O₃. Two binary compounds, 2A1₂O₃.B₂O₃ and 9A1₂O₃.-2B₂O₃, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li₂O.A1₂O₃.B₂O₃ and 2Li₂O. 2AI₂O₃. 3B₂0₃. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Li_zO.-5Al₂O₃ and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.     

Effect of Trace A12O3 on Transformation of Quartz to Cristobalite

The effect of additions of 0.22, 0.44, 0.88, and 1.76% A1₂O₃ (Si⁴⁺/A1³⁺ ratio of 200:1, 100:1, 50:1, and 25:1) on the transformation of Brazilian quartz to cristobalite was studied at 1500°, 1530°, and 1570°C. The smaller percentages of A1₂O₃ (0.22 and 0.44%) catalyzed the transformation of quartz and the formation of cristobalite considerably. The rates of transformation of quartz with 0.88 and 1.76% A1₂O₃ were slower than with 0.22 or 0.44%, indicating a critical A1³⁺/Si⁴⁺ ratio where the catalytic effect was found to be maximum. This appeared to occur at about 0.5% A1₂O₃. The transformation rate of quartz indicated that the reaction was first order. Cristobalite, however, showed two different rates; the initial rapid growth was followed by a slower rate. The point of changeover was found to be at about 30 ± 5% cristobalite. The critical nature of the A1³⁻/Si⁴⁺ ratio at about 0.01 (or A1₂O₃/SiO₂± 0.005) may have some bearing on the properties of silica refractories with more or less than 0.5% A1₂O₃.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.     

Fracture Toughness of Al2O3 with an Unstabilized ZrO2Dispersed Phase

The fracture toughness of Al₂O₃ is considerably increased by the incorporation of fine monoclinic ZrO₂ particles. Hot-pressed composites containing 15 vol % ZrO₂ yield K_lcvalues of ∼ 10 MN/m^3/2, twice that of the A1₂O₃ matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO₂during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values.     

R-Curve Behavior of In-Situ-Toughened Al2O3:CaAl12O19 Ceramic Composites

The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al₂O₃:CaAl₁₂O_19,or A1₂O₃: CA₆) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA₆) phase possessed a platelike morphology showed more-pronounced R -curve behavior than a composite in which the CA₆ phase consisted of equiaxed grains. Toughness curves derived from the indentation-strength data exhibited a "crossover," such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA₆ resulted in enhanced toughening, compared to single-phase alumina of comparable grain size, thus demonstrating the viability of the in-situ -toughening approach. A simple grain-pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation-strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A1₂O₃:CA₆ composite were strongly bonded to the platelet A1₂O₃:CA₆ composite, demonstrated high strength over the range of tested flaw sizes.     

Phase Equilibrium in a Portion of the Ternary System Ba OA-I2,O3,-SiO2,

Phase-equilibrium relations on the liquidus surface in the system Ba0-A1₂O₃-SiO₂ have been investigated by the quenching method. The compositions investigated within the ternary area were those containing less than 30%, A1₂O₃ and more than 20% SiO₂ by weight. Petrographic and X-ray techniques were employed in the determination of the crystalline phases.
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO₂,), barium metasilicate (BaO 2SO₂,), solid solutions, sanbornite (BaO 2SiO₂), tridymite and cristobalite (SO₂), mullite (3A1₂O₃ 2SiO₂), and celsian (BaO A1₂O3_.2SiO₂). Diagrams show the isotherms and indices of refraction of the glasses.
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Development of Nano-Composite Microstructures in ZrO2-Al2O3 via the Solution Precursor Method

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr_{1- x} Al _x O_{2− x /2} ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO₂+γ-Al₂O₃ with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al₂O₃ phase transformation in the t -ZrO₂ matrix are reported for compositions containing 10, 20, and 40 mol% A1₂O₃. During this phase transformation, the α-Al₂O₃ grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO₂ inclusions and are separated by ZrO₂ grains. The volume fraction of A1₂O₃ and the heat treatment conditions influence the final microstructure. At lower volume fractions of A1₂O₃, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO₂. Interpenetrating microstructures produced at high volume fractions of A1₂O₃ exhibit very little grain growth for periods up to 24 h at 1400°C.     

Reactions Between Aluminum Oxide and Carbon The Al2O3—Al4C3 Phase Diagram

A phase diagram of the system A1₂O₃-A1₄C₃ is proposed. Two intermediate oxycarbides, A1₄O₄C and A1₂OC, were established. Eutectic melting between alumina and A1₄O₄C occurred at 1840° C. No other low melting was observed. The alumina phase was not corundum but was similar to delta-alumina. Because of the high reactivity of aluminum carbide and all the intermediate compounds with moisture and oxygen, use of refractories based on the system A1₂O₃-A1₄C₃ must be limited to applications where these agents are excluded. The behavior of high-alumina refractories in the presence of carbon is explained.     

Preparation of Composite Particles by Pulsed Nd-YAG Laser Decomposition of [(CH3)2N]4Ti to TiN-Coat TiO2, Al2O3, or Si3N4 Powders

Irradiation of Ti[N(CH₃)₂]₄ by the 1.064-μm line of a pulsed Nd: YAG laser in the presence of TiO₂, Al₂O₃, or Si₃N₄ particles has been found to form amorphous deposits on the oxide particles. The resulting materials can be processed into TiN/TiO₂, TiN/Al₂O₃, or TiN/Si₃N₄ composites with the TiN component on the surface of the particles. The powders have been characterized by Raman spectroscopy and X-ray powder diffraction studies. The surface analysis of the composites by X-ray photoelectron spectroscopy and high-resolution electron microscopy is presented.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Tribological Properties of Ti3SiC2

The present contribution reports the unlubricated friction and wear properties of Ti₃SiC₂ against steel. The fretting experiments were performed under varying load (1–10 N) and the detailed wear mechanism is studied using SEM-EDS, Raman spectroscopy, and atomic force microscopy. Under the selected fretting conditions, Ti₃SiC₂/steel tribocouple exhibits a transition in friction as well as wear behavior with coefficient of friction varying between 0.5 and 0.6 and wear rate in the order of 10⁻⁵ mm³·(N·m)⁻¹. Raman analysis reveals that the fretting wear is accompanied by the triboxidation with the formation of TiO₂, SiO₂, and Fe₂O₃. A plausible explanation for the transition in friction and wear with load is proposed.     

Fabrication and Microstructure of Al2O3–SiC–YAG Hybrid Composites Prepared by Particulate Dispersion

Stable Al₂O₃–SiC–YAG hybrid composites were successfully fabricated by reaction of Al₂O₃ and Y₂O₃ and incorporation of SiC. The hot-pressed bodies consisted of uniformly dispersed grains of microsized YAG particulates and nanosized SiC particulates in an Al₂O₃ matrix. Although the grain size of monolithic A1₂O₃ increases markedly with increased temperature, the grain size of the Al₂O₃–SiC–YAG hybrid composites was effectively restrained due to grain-boundary pinning by the particulates.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

Phase Relations in the System Bi2O3-WO3

Phase relations in the system Bi₂O₃-WO₃ were studied from 500° to 1100°C. Four intermediate phases, 7Bi₂O₃· WO₃, 7Bi₂O₃· 2WO₃, Bi₂O₃· WO₃, and Bi₂O₃· 2WO₃, were found. The 7B₂O · WO₃ phase is tetragonal with a ₀= 5.52 Å and c ₀= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi₂O₃· 2WO₃ has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi₂O₃· WO₃ and Bi₂O₃· 2WO₃ are orthorhombic with (in Å) a ₀= 5.45, b ₀=5.46, c ₀= 16.42 and a ₀= 5.42, b ₀= 5.41, c ₀= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO₃, 907°± 3°C and 70 mol% WO₃, and 965°± 5°C and 10 mol% WO₃.     

Reaction Sintering of ZnO-AI2O3

The reaction sintering of equimolar mixtures of ZnO and A1₂O₃ powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A1₂O₃ powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the A1₂O₃ particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single-phase powder of zinc aluminate, ZnAl₂O₄. The chemical reaction between ZnO and A1₂O₃ occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl₂O₄ powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl₂O₄ powder. The difference in sintering between the ball-milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.