Sliding-Wear-Resistant Liquid-Phase-Sintered SiC Processed Using α-SiC Starting Powders

Low-cost α-silicon carbide (SiC) starting powder, instead of the more expensive β-SiC starting powder, has been used to process liquid-phase-sintered (LPS) SiC ceramics with different microstructures: (i) elongated SiC grains ( in situ toughened LPS SiC), (ii) fine equiaxed SiC grains, and (iii) coarse equiaxed SiC grains. The effects of microstructure on the sliding-wear properties of these LPS SiC ceramics have been studied. The sliding-wear resistance of the in situ toughened LPS SiC ceramic is found to be significantly better than that of two equiaxed-grain LPS SiC ceramics. This has been attributed to the existence of a hard, interlocking network of elongated SiC grains and the isolated nature of the yttrium aluminum garnet (YAG) second phase in the in situ toughened LPS SiC ceramic. This is in contrast to the equiaxed-grain LPS SiC ceramics, where the equiaxed grains are embedded within a continuous YAG phase matrix. The use of the α-SiC starting powder allows the processing of low-cost LPS SiC ceramics that are both sliding-wear resistant and tough.     

Microstructural design of sliding-wear-resistant liquid-phase-sintered SiC: An overview

We have reviewed the effect of microstructure – content of intergranular phase, grain size, and grain shape – on the lubricated, sliding-wear of pressureless liquid-phase-sintered (LPS) SiC ceramics. The sliding-wear resistance in LPS SiC decreases with an increase in the content of the intergranular phase or an increase in the equiaxed-grain coarsening. However, the sliding-wear resistance is dramatically improved with anisotropic-grain coarsening. Based on these results we suggest two strategies for the microstructural design of low-cost, sliding-wear resistant SiC-based ceramics: (1) grain refinement, and (2) grain elongation. The latter strategy allows the materials to be simultaneously in situ toughened, and we describe its optimization by judicious selection of the SiC starting powder.     

Effect of liquid-phase content on the contact-mechanical properties of liquid-phase-sintered α-SiC

Effect of the content of the intergranular phase (Y₃Al₅O₁₂ or yttrium aluminum garnet or YAG) on the room-temperature contact-mechanical properties of pressureless, liquid-phase-sintered (LPS) α-SiC ceramics has been studied. An increase in YAG vol.% is found to result in the expected degradation of the elastic modulus and the indentation yield strength in LPS SiC. However, with increasing YAG vol.% the degradation in the hardness and the sliding-wear resistance is found to be severe, while the indentation toughness first increases and then decreases. These results are analyzed, and discussed in the context of providing guidelines for the design and fabrication of low-cost LPS SiC ceramics with tailored contact-mechanical properties.     

Sliding-wear resistance of liquid-phase-sintered SiC containing graphite nanodispersoids

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.     

Effect of Microstructure on Sliding-Wear Properties of Liquid-Phase-Sintered α-SiC

We have studied, for the first time, the effect of the content of intergranular phase and grain size on the sliding-wear resistance of pressureless, liquid-phase-sintered (LPS) α-SiC (silicon carbide) ceramics. It was found that the sliding-wear behavior of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with well-defined wear transition. We have found that the increase in the content of intergranular phase and the grain coarsening leads to the degradation of the sliding-wear resistance in LPS α-SiC ceramics. A mechanistic model is used to rationalize these wear results, and to provide guidelines for the design and fabrication of low cost, highly wear-resistant SiC-based ceramics. This is likely to have important implications because SiC-based materials are being used increasingly in contact-mechanical and tribological applications.     

AlN-Re2O3液相烧结碳化硅

采用非氧化物AlN和Re2O3作为复合烧结助剂(Re2O3-La2O3与Y2O3)进行碳化硅液相烧结得到了致密的烧结体.烧结助剂占原料粉体总质量的20%,其中:AIN与(La0.5Y0.5)2O3的摩尔比为2:1,在30MPa压力下,1850℃保温0.5h热压烧结的碳化硅陶瓷,抗弯强度>800MPa,断裂韧性>8MPa·m1/2,明显高于同组分1 950℃无压烧结0.5h的碳化硅陶瓷的抗弯强度(433.7MPa)和断裂韧性(4.8MPam·m1/2.热压烧结的陶瓷晶粒呈单向生长,断裂模式为沿晶断裂.同组分无压烧结碳化硅陶瓷的显微结构可以观察到核壳结构.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Numerical Evaluation of Toughening by Crack-Face Grain Interlocking in Self-Reinforced Ceramics

Crack bridging associated with the pull-out process of interlocking grains in self-reinforced ceramic materials is studied through a micromechanical simulation. The pullout of a single inclined grain is modeled via the numerical solution of a general contact problem. The bridging-force versus crack-opening-distance curve indicates a nonlinear, springlike response for the pullout of interlocking grains. The sliding friction along the debonded interface, induced by highly localized contact stresses, dominates the total bridging force. The bridging force increases with grain inclination until eventual bridge failure. The pullout of misaligned grains mainly affects short-crack toughening, with a rising R -curve, whereas aligned grains contribute to long-crack toughening. The residual stresses of the thermal expansion anisotropy play a minor role in the pull-out process of grain interlocking and the resultant toughening. The proposed mechanism is operative in both single-phase and composite ceramics in which pullout of elongated grains/reinforcements occurs.     

Mechanical and Thermal Properties of Pressureless Sintered Silicon Carbide Ceramics with Alumina–Yttria–Calcia

The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with Al₂O₃–Y₂O₃–CaO without applied pressure was investigated. SiC ceramics containing A₂O₃–Y₂O₃–CaO as sintering additives can be sintered to >97% theoretical density at temperatures between 1750°C and 1900°C without applied pressure. A toughened microstructure, consisting of relatively large elongated grains and relatively small equiaxed grains, has been obtained when sintered at temperatures as low as 1800°C for 2 h in an argon atmosphere without applied pressure. The achievement of toughened microstructures under such mild conditions is the result of the additive composition. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature because of the decrease in the lattice oxygen content of the SiC grains. Typical sintered density, flexural strength, fracture toughness, hardness, and thermal conductivity of the 1850°C‐sintered SiC, which consisted of 62.2% 4H, 35.7% 6H, and 2.1% 3C, were 99.0%, 628 MPa, 5.3 MPa·m^1/2, 29.1 GPa, and 80 W·(m·K)^?1, respectively.     

Dependence of the diamond coating adhesion on the microstructure of SiC-based substrates

Diamond thin films were deposited on the SiC–30TiC–10Cr₃C₂ substrates by using the microwave plasma CVD method and the effect of microstructural morphology of substrate was examined on the diamond–substrate adhesion strength. Two types of substrate were prepared by hot-pressing, one with small and equiaxed β-SiC grains and the other with large and elongated α-SiC grains. The SiC–30TiC–10Cr₃C₂ substrate surfaces were chemically etched to remove (Ti,Cr)C matrix phase by Murakami solution. Better diamond–substrate bonding was obtained on the substrate composed of large, elongated grains. Vickers indentation results indicated that mechanical interlocking between elongated and protruded grains on the etched surface and diamond thin film resulted in an increase in adhesion strength.     

Effect of intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered α-SiC

The effect was investigated of the intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered (PLPS) α-SiC densified with 10 vol.% 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, or Yb) additives. It was found that the sliding-wear behaviour of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Most importantly, the sliding-wear resistance of PLPS SiC is found to increase with decreasing size of the RE³⁺ cation in the rare-earth oxide additive, with a lower susceptibility to mild and severe wear and a delayed transition to severe wear. Underlying this effect is likely the hardening of the intergranular phase resulting from the increase in the field strength of the RE³⁺-O²⁻ bonds as the size of the RE³⁺ cation decreases. Tailoring the intergranular phase chemistry via the selection of RE₂O₃ sintering additives with cations as small as possible thus emerges as a potentially interesting approach to improving the sliding-wear resistance of PLPS SiC ceramics.     

Preparation of c-axis textured SiC ceramics by a strong magnetic field of 6 T assisted gel-casting process

C-axis textured SiC ceramics were prepared by a strong magnetic field of 6 T assisted gel-casting and subsequent pressureless sintering. The optimal suspension parameters for gel-casting were determined by analyzing the influences of pH value and dispersant content on the stability and dispersibility of suspensions. The effect of sintering conditions on the texture development and properties of SiC ceramics was discussed. It was found that the increasing sintering temperature or holding time promoted the densification process of SiC ceramics. The c-axis of SiC grain was aligned parallel to the magnetic field by applying a strong magnetic field of 6 T. The degree of texture of SiC ceramics showed a slightly increasing trend with the increase of sintering temperature or holding time. When the samples were sintered at 1950 °C for 4 h or 6 h, the large elongated grains were formed in the samples, leading to the extremely evident anisotropic microstructure on different planes. Textured SiC ceramics exhibited the anisotropic bending strength.     

Zrc ceramics incorporated with different-sized SiC particles

ABSTRACT

Three different SiC powders with average particle sizes of 0.45, 3.5 and 10?µm were used to prepare ZrC-20vol.-% SiC ceramics by hot pressing. The effects of SiC particle size on the densification, microstructure, mechanical properties and thermal properties of ZrC–SiC ceramics were studied. Ceramics prepared from SiC with finer particle sizes exert higher bending strength, hardness and lower thermal conductivity. The ZrC–SiC ceramics with a starting SiC particle size of 3.5?µm has relative high fracture toughness than others. Analysis indicates that SiC grain size and the grain boundaries control the thermal conductivity ZrC–SiC ceramics. Ceramics prepared from SiC with the particle size of 10?µm exhibits the highest thermal conductivity due to the larger grains and less grain boundaries.     

Heat-Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide

Fully dense SiC ceramics with high strength at high temperature were obtained by hot-pressing and subsequent annealing under pressure, with AlN and Er₂O₃ as sintering additives. The ceramics had a self-reinforced microstructure consisting of elongated SiC grains and a grain-boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m^1/2 at room temperature. The beneficial effect of the new additive composition on high-temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain-boundary glassy phase.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

A route for the pressureless liquid-phase sintering of SiC with low additive content for improved sliding-wear resistance

A colloidal processing route has been developed for the pressureless sintering of dense SiC with a low content of sintering additives. In this route, a sol-gel solution precursor of the sintering additives is deposited onto the surface of the SiC particles, achieving a uniform distribution of the sintering additives in the green compact. This in turn promotes complete densification at short sintering times, which is not otherwise achievable when the batch is prepared by the standard method of mechanically mixing powders. It is also shown that the resulting ceramic has improved sliding-wear resistance compared to its counterpart prepared by the classical method, with essentially the same rate of mild and severe wear but a notably delayed transition from the mild to the severe wear regimes. This improvement is attributed to the reduction in the microstructural defect size achieved by the colloidal processing. Implications for the fabrication of low-cost SiC ceramics for wear-resistance applications are discussed.     

Reaction forming of silicon carbide ceramic using phenolic resin derived porous carbon preform

SiC ceramics were prepared with porous carbon preforms derived from phenolic resin by a reaction-forming method. The effects of the structure of the preform pores and the infiltration process on the properties of SiC ceramics were investigated, and components with complex shapes were fabricated by combining this process with stereolithography (SLA). Dense SiC ceramics were obtained from carbon preforms with high apparent porosities, but SiC ceramics with many macrodefects resulted from a carbon preform with an apparent porosity of 39%. The infiltration of molten silicon into the preform pore channel was accelerated under vacuum pressure, resulting in an increase in the depth of the Si infiltration. The growth of SiC was predominantly controlled by carbon diffusion at the last stage of the reaction. The extended grain growth caused the SiC grains to coalesce and some free Si was enveloped in the SiC grains. SiC components with complex geometries were fabricated by combining reaction forming with SLA. The geometry was controlled by SLA.     

Effect of microstructure on the mechanical properties of liquid-phase-sintered silicon carbide at pre-creep temperatures

The effect of the microstructure on the mechanical properties of pressureless, liquid-phase-sintered (LPS) α-SiC ceramics above room-temperature was studied. LPS-SiC ceramics were fabricated with different microstructural features (grain size and morphology, and content of the intergranular phase), and their mechanical behaviour under contact stresses was evaluated by high temperature Hertzian testing (HTHT) from room temperature up to the creep temperature (1000 °C). The amount of intergranular phase was found to control the elasto-plastic properties of LPS-SiC at intermediate temperatures. Grain size and morphology had a significant influence only on toughness, since the crack bridging mechanism was enhanced by elongated grains, the more so the larger their size. Implications of these results for the design and fabrication of LPS-SiC ceramics with tailored contact-mechanical properties are discussed.     

Effects of Elemental Additives on Electrical Resistivity of Silicon Carbide Ceramics

Effects of various elemental additives on the electrical resistivity of hot-pressed SiC ceramics were studied. The electrical resistivity at room temperature of dense SiC ceramics varied greatly depending on the additives used. SiC ceramics with added Be had an extremely high electrical resistivity of 3 × 10¹²°.cm. On the other hand, SiC ceramics with added B and Al had electrical resistivities of 2 × 10 and 0.8 °cm, respectively. The differences in the electrical resistivity of the dense SiC ceramics were considered to be due to different solubilities of the additives in SiC grains. SiC ceramics with added Be had a low level of impurities in the SiC grains as a result of the low solubility of Be in these grains.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.