Porous Si <sub> 3 </ Sub> N <sub> 4 </ Sub> Ceramics Reinforced Aluminum Matrix Composites Preparation

Two parts is included in this article. The first part is about the fabrication and properties of porous Si3N4 ceramic reinforcement; the second part focuses on the investigation of infiltration process and interface combination of porous Si3N4/ZL305 composite. The task's mainly target is to achieve a mature process of making metal-matrix interpenetrating phase composites, and in this work, appropriate materials and fabricating process about the task should be analyzed and conformed.β -Si3N4 was chosen as the skeletal material of the porous ceramic. Al powders was used as lower temperature sintering additive for Si3N4 ceramic and Al2O3 and ZrO2 powders as the higher temperature ones. Each addition of the three sintering additives was 5% based on the mass of Si3N4. Small quantity of calcium bentonite and carboxylic cellulose should also be added as rheopectic materials. All of the skeletal material, sintering additives and rheopectic materials were mixed up and ball milled to a homogeneous compound. Silica sol including 30wt.% SiO2 was used as agglomerant and dissolvent. The compound and silica sol were then mixed up uniformly.Impregnating process using polymeric sponges as templates was used to fabricate porous Si3N4 ceramic framework. The mixed slurry should be coat on the polymeric sponges equably during the flat extrusion and centrifugal swing.Twice immersing and sintering process was carried out orderly to ensure the high quality of porous ceramic framework. After the first sinter, ceramic framework was immersed into the lower thickness slurry for the second immersing process, and then, would be the second sinter. Some cavities which were left behind by polymeric sponges and other low melting volatile matter in the course of sinter could be filled up after the twice immersing and sintering procedure and ensured the high quality ofporous ceramic frameworkThe compressive strength of the porous ceramic increased with the decrease of porosity at the same sintering temperature. When the porosity was 78%, the value of compressive strength was 0.72MPa, while the porosity was 90% the corresponding value decreased to 0.1 MPa quickly. The testing results indicated that the best sintering temperature is 1400℃ at which the porous ceramic achieved the higher compressive strength and maintained its good open cells at the same time. When the sintering temperature exceeded to 1400℃, many disfigurements would come forth such as distortion, jam or collapse.The porous Si3N4 ceramic was composed of β -Si3N4 and some multiple Si-N-O-Al crystal or amorphous phases. These multiple phases which enclosed the β -Si3N4 was formed by fusion or reaction between low melting oxides and SiO2 at higher temperature.ZL305 alloy was chosen as the metal matrix in response to the structure characteristic and application of ceramic-metal interpenetrating phase composites. In the task, the molten ZL305 alloy completed the infiltration into the porous Si3N4 ceramic at the multiple traction of negative pressure and air pressure. The effect of infiltration changed with the variation of external conditions. Increasing the melting temperature of molten alloy and preheating temperature could have an advantaged work on the infiltration, the same effect as increasing the air pressure processed on the molten alloy. The casting results shown that, the alloy was difficult to wet with ceramic surface even at the higher temperature if there was not external pressure on the molten alloy. The oxidizing layer of molten alloy also hindered the bonding and reaction of the molten alloy with Si3N4 ceramic. In this task, the best effectiveness of infiltration was achieved at the condition of 8MPa of air pressure, 900 "C of the molten alloy and 800 °C of the infiltrating die. The interface of alloy with ceramichad a good bonding.Mg is an active element in the ZL305 alloy and inclined to congregate onto the surface of molten alloy. From the line scanning image of bonding micrographic, we can see that the Mg element had a higher content in the interface than that in the inner of the ZL305 alloy. The more content of Mg on the surface of molten alloy would thin the oxide surface and decrease the surface tension of molten alloy which assisted the bonding and reaction of interface.The reticulate Si3N4 reinforced ZL305 composite had much less wear rate than that of ZL305 alloy. The higher self-hardness of S13N4 and the special structure of reticulate ceramic decreased the contact area of ZL305 matrix with wear disk, and further more decreased wear rate of the composite.