Special preparation technology of silicon carbide ceramics

Silicon carbide ceramic materials have excellent properties such as high high-temperature strength, strong high-temperature oxidation resistance, good wear resistance, thermal stability, small thermal expansion coefficient, high thermal conductivity, high hardness, thermal shock resistance and chemical corrosion resistance. It is increasingly widely used in automobiles, mechanical and chemical industry, environmental protection, space technology, information electronics, energy and other fields. It has become an irreplaceable structural ceramic with excellent performance in many industrial fields.

The excellent properties of SiC ceramics are closely related to their unique structure. SiC is a compound with strong covalent bonds, and the ionic nature of Si-C bonds in SiC is only about 12%. Therefore, SiC has high strength, high elastic modulus, and excellent wear resistance. Pure SiC is not attacked by acid solutions such as HCl, HNO3, H2SO4 and HF, as well as alkaline solutions such as NaOH. Oxidation is easy to occur when heated in air, but the SiO2 formed on the surface during oxidation inhibits the further diffusion of oxygen, so the oxidation rate is not high. In terms of electrical properties, SiC is semiconductor, and the introduction of a small amount of impurities will exhibit good conductivity. In addition, SiC has excellent thermal conductivity.

SiC has two crystal forms: α and β. The crystal structure of β-SiC is a cubic crystal system, and Si and C form a face-centered cubic lattice respectively, and there are more than 100 polytypes of α-SiC, such as 4H, 15R and 6H, among which 6H polytype is the most common one in industrial applications. There is a certain thermal stability relationship between the various types of SiC. At temperatures below 1600°C, SiC exists in the form of β-SiC. When above 1600°C, β-SiC slowly transforms into various polytypes of α-SiC. 4H-SiC is easy to generate at about 2000°C, 15R and 6H polytypes need to be generated at high temperatures above 2100°C, and 6H-SiC is very stable even if the temperature exceeds 2200°C. The free energy difference between the various polytypes in SiC is very small, so the solid solution of trace impurities will also cause changes in the thermal stability relationship between polytypes.

The production process of SiC ceramics is briefly described as follows: The preparation technology of silicon carbide powder is divided into solid-phase synthesis method and liquid-phase synthesis method according to its original raw material state.

Solid-phase synthesis method

The solid-phase method mainly includes carbon-thermal reduction method and silicon-carbon direct reaction method. The carbon-thermal reduction method includes the Acheson method, the vertical furnace method and the high-temperature converter method. The Acheson method was first invented by Acheson, which is in the Acheson electric furnace, the silica in the quartz sand is reduced by carbon to produce SiC, which is essentially an electrochemical reaction under the action of high temperature and strong electric field, and has a history of large-scale industrial production for hundreds of years, and the SiC particles obtained by this process are coarse. In addition, the process consumes a lot of electricity, which is used for production, which is heat loss.

The method developed in the 70s of the 20th century improved the classical Acheson method, and in the 80s, new equipment for synthesizing β-SiC powder such as vertical furnaces and high-temperature converters appeared, and this method was further developed in the 90s. Ohsakis et al. used the SiO gas released by the mixed powder of SiO2 and Si powder when heated to react with activated carbon to prepare Riyi, and the specific surface area of the SiO gas released decreased with the increase of temperature and the extension of holding time.

The silicon-carbon direct reaction method is an application of the self-propagating high-temperature synthesis method, which ignites the reactant body from an external heating source and uses the chemical reaction heat released by the material during the synthesis process to maintain the synthesis process by itself. In addition to ignition, there is no need for an external heat source, which has the advantages of low energy consumption, simple equipment and high productivity, but its disadvantage is that the eye reaction is difficult to control. In addition, the reaction between silicon and carbon is a weak exothermic reaction, which is difficult to ignite and maintain at room temperature.

Liquid-phase synthesis method

The liquid-phase method mainly includes the sol-gel method and the polymer decomposition method. Ewell et al. first proposed the sol-gel method, and the real use of ceramic preparation began around 1952. The method uses an alkoxide precursor prepared by liquid chemical reagents, dissolves it in a solvent at low temperature to form a uniform solution, adds an appropriate coagulant to hydrolyze the alkoxide, polymerizes the reaction to generate a uniform and stable sol system, and then after a long time of placement or drying, it is concentrated into a mixture or polymer of Si and C at the molecular level, and continues to heat to form a two-phase mixture of Si and C with uniform mixing and fine particle size, and a carbon reduction reaction occurs at about 1460-1600 °C to finally prepare SiC fine powder. The main parameters to control the gelatinization of sol are the pH value of the solution, the concentration of the solution, the reaction temperature and time, etc. This method is easy to realize the addition of various trace components in the process of operation, and the mixing uniformity is good, but the hydroxyl group and organic solvent are often left in the process products, which are harmful to human health, and the high cost of raw materials and the large shrinkage in the treatment process are its shortcomings.

The pyrolysis of organic polymers is an effective technology for the preparation of silicon carbide : one is the decomposition reaction of the heating gel polysiloxane to release small monomers, and finally form SiO2 and C, and then prepare SiC powder by carbon reduction reaction. The other type is to heat polysilane or polycarbonatesilane to release small monomers and then form a skeleton, which eventually forms SiC powder.

At present, the sol-gel technology is used to make SiO2 into a sol/gel material based on SiO2-based hydroxide derivatives, which ensures that the sintering additives and toughening additives are evenly distributed in the gel to form a high-performance one

Pressureless sintering

Pressureless sintering is considered to be the most promising sintering method for SiC sintering. According to the different sintering mechanisms, pressureless sintering can be divided into solid phase sintering and liquid phase sintering. S.Proehazka adds appropriate amounts of B and C to ultra-fine β-SiC powder (oxygen content less than 2%) at the same time, and sinters it at normal pressure at 2020°C to form a SiC sintered body with a density higher than 98%. A. Mulla et al. used Al2O3 and Y2O3 as additives to sinter 0.5 μm β-SiC (the particle surface contains a small amount of SiO2) at 1850-1950°C. The relative density of the SiC ceramic obtained was greater than 95% of the theoretical density, and the grains were fine and the average size was is 1.5μm.

Hot press sintering

Nadeau pointed out that without adding any sintering aids, pure SiC can only be sintered densely at extremely high temperatures, so many people implement hot pressing sintering processes for SiC. There have been many reports on hot-pressing sintering of SiC by adding sintering aids. Alliegro et al. studied the effects of metal additives such as B, Al, Ni, Fe, and Cr on the densification of SiC, and found that Al and Fe are the most effective additives in promoting hot-press sintering of SiC. F.F. Lange studied the effects of adding different amounts of Al2O3 on the performance of hot-press sintering SiC, and believed that hot-press sintering densification relies on the dissolution-reprecipitation mechanism. However, the hot-pressing sintering process can only produce SiC parts with simple shapes, and the number of products produced in one hot-pressing sintering process is very small, so it is not conducive to industrial production.

reaction sintering

Reactive sintering SiC, also known as self-bonded SiC, is a process in which a porous blank undergoes a chemical reaction with the gas phase or liquid phase to increase the mass of the blank, reduce the pores, and sinter it into a finished product with certain strength and dimensional accuracy. α-SiC powder and graphite are mixed into a green body in a certain proportion and heated to about 1650°C. At the same time, Si is infiltrated or infiltrated into the green body through gas phase Si, causing it to react with graphite to generate β-SiC. The existing α-SiC particles are combined. If the Si infiltration is complete, a completely dense reaction sintered body without dimensional shrinkage can be obtained. Compared with other sintering processes, reaction sintering has small dimensional changes during the densification process and can produce products with precise dimensions. However, the presence of a considerable amount of SiC in the sintered body makes the high-temperature performance of reaction-sintered SiC ceramics poor.

SiC ceramics using pressureless sintering, hot press sintering, hot isostatic pressing sintering and reaction sintering have different performance characteristics. For example, in terms of sintering density and flexural strength, hot pressing sintering and hot isostatic pressing sintered SiC ceramics are relatively more, while reaction sintering SiC is relatively low. On the other hand, the mechanical properties of SiC ceramics also vary with different sintering additives. Pressureless sintering, hot press sintering and reaction sintering SiC ceramics have good resistance to strong acids and strong alkali, but reaction sintering SiC ceramics have poor corrosion resistance to super strong acids such as HF. In terms of high temperature resistance performance comparison, when the temperature is lower than 900°C, the strength of almost all SiC ceramics increases; when the temperature exceeds 1400°C, the flexural strength of reaction-sintered SiC ceramics decreases sharply. (This is because the sintered body contains a certain amount of free Si, and the flexural strength drops sharply when it exceeds a certain temperature.) For pressureless sintering and hot isostatic pressing sintered SiC ceramics, the high temperature resistance is mainly affected by the type of additives.