Last Updated on June 16, 2021 by You Ling
In recent years, ultra-fine powders, especially nano-level ultra-fine powders, have received increasing attention due to their peculiar small size effects, surface and interface effects, and quantum size effects. At the same time, through experimental research, people have found that two or two types of The above powder particles can be surface coated or composite treated to obtain high-performance composite materials. In addition to the properties of a single powder, they also have composite synergistic multi-functions, changing the surface properties of a single particle, increasing two kinds of or The contact area of multiple components and so on.
Among them, the metal-ceramic composite powder refers to a composite ceramic powder formed by coating a layer of metal on the surface of ceramic particles. It has the properties of a metal coating layer and a ceramic core, and can achieve uniform mixing between individual particles. The sintered body or composite material made by it has the following characteristics: (1) Improve the interface bonding force between powder ceramic and metal, and improve the uniformity of ceramic and metal distribution in the sintered body; (2) It can realize multi-level and multi-mechanism composite strengthening (Fine grain reinforcement, phase change reinforcement, fiber reinforcement, etc.) to prepare metal composite ceramics with high strength and toughness; (3) Low-density functional powder materials (such as low-density conductive powder, magnetic powder, etc.) can be prepared ).
Commonly used metal-ceramic composite powder is composed of oxides (such as Al2O3, ZrO2, SiO2), carbides (such as WC, TiC, SiC), etc. and metals. Due to its excellent composite characteristics, it can meet the special requirements of many fields In recent years, it has become a hot spot in composite materials research.
Preparation of metal-composite ceramic powder
Metal coating technology is generally used in industry to prepare metal-ceramic composite powders. The preparation methods usually include the following: mechanical mixing, high-energy ball milling, self-propagating high-temperature synthesis, in-situ reaction, sol-gel, electroless plating, etc.
The mechanical mixing method is the earliest method used in the preparation of composite powders. This technology is easy to operate and simple in process. There are already complete sets of equipment for industrial production, with large output and low cost. Therefore, the preparation of some composite powders still uses machinery. Mixed method. However, due to the different particle size and surface characteristics of the powder, the doped components are easy to segregate, which easily leads to uneven composition and organization. The obtained powder has a large particle size, and it is difficult to obtain a uniform distribution of the reinforcement particles. It is suitable for the preparation of composite powders with high functional requirements.
The mechanical mixing method is widely used in the domestic preparation of ZnO pressure-sensitive composite porcelain powder.
High energy ball milling
High-energy ball milling is a new development in the research of mechanical alloy technology. It uses a mixture of two or more metal or non-metal powders to form an alloy or composite ceramic powder with a fine structure through high-energy ball milling. Compared with the traditional mechanical mixing method, it has the advantages of significantly reducing the activation energy of the reaction, improving the uniformity of particle distribution, and the interface bonding between the reinforcement and the matrix. However, the preparation of composite ceramic powder by high-energy ball milling is a complex material reaction and structure control process with many influencing factors and strict process requirements.
Self-propagating high-temperature synthesis
Self-propagating high-temperature synthesis technology (SHS) is to ignite the powder compact in a certain atmosphere to produce a chemical reaction. The heat of the reaction released makes the temperature rise suddenly and triggers a new chemical reaction of adjacent materials. The chemical reaction is in the form of a combustion wave. It spreads through the entire reactant, and the reactant turns into a product when the combustion wave advances. Self-propagating high-temperature synthesis technology has many advantages: simple production process, low investment, full energy utilization, and rapid response (0.1~15cm/s). The synthesis reaction temperature is generally very high, which can volatilize most impurities to obtain high-purity products.
The main disadvantage of the SHS method is that the reaction process and product performance cannot be strictly controlled, and it is difficult to obtain high-density products. In addition, the raw materials used in the SHS law are often flammable, explosive, or toxic substances, requiring special safety measures.
Sol-Gel technology (Sol-Gel) technology is a new process developed in the 1960s to prepare inorganic materials such as glass and ceramics. In recent years, many people have used this method to prepare nanomaterials. Its basic principle is to use metal alkoxides or inorganic salt water to hydrolyze to form a sol, then make it into a gel, and then dry and burn to make nanoparticles.
This method is more complicated and the raw materials are expensive. Some of the raw materials are organic, which is harmful to health. Secondly, the entire sol-gel process usually takes a long time, and there are a large number of micropores in the gel. A lot of gas and organic matter will escape, and shrinkage will occur, the loss will be large, and the preparation cost will be high.
Electroless plating is an advanced method for preparing metal-ceramic composite powders. This method can obtain a uniform metal coating on the surface of various powder materials such as glass, ceramics, plastics or metal surfaces. The reaction mechanism is based on the controlled autocatalytic oxidation-reduction reaction does not need to provide current, and there is no restriction on the shape of the substrate. Therefore, it has attracted extensive attention as a new method for preparing metal composite powders. Studies have found that the metal-ceramic composites prepared by electroless plating have higher toughness, better dispersion, and a more uniform surface coating.
Application of metal-composite ceramic powder
At present, some domestic enterprises can mass-produce metal-composite ceramic powders. The products are mainly metal-ceramic composite powders such as Al2O3, TiC, SnO2, SiO2, CeO2 coated with metal Ni, Co, Cu, Ag, etc. The metal content ranges from 5 to 90%. Product application areas include ceramic cutting tools, electrical contacts, conductive pastes, rubber fillers, and materials for the manufacture of auto parts.
Foreign companies with similar products mainly produce Ag coated SiO2 powder for stealth and electrostatic shielding coatings, Cu and Fe coated SiC, graphite, etc. for particle-reinforced metal matrix composite materials, Au, Ni coated polymer, and ceramic powders The body is used in conductive paste, etc. Most manufacturers use electroless plating technology.
The metal-composite ceramic powder has great potential, and its applications are far beyond the existing products on the market and are widely used in military, aviation, aerospace, chemical, pharmaceutical, and other fields.
(1) Metal toughened ceramic materials
Metal-ceramic composite powder is sintered (including pressureless sintering, hot pressing sintering, hot isostatic pressing sintering, etc.) to prepare high-performance cermets. Compared with single ceramic powder sintering, it has the following characteristics: the sintering temperature is greatly reduced, melting or The metal phase in the semi-molten state is uniformly distributed between the ceramic particles, which inhibits the growth of ceramic grains and prevents the formation of gas-phase or glass phase. The metal phase in the sintered body is continuously distributed, and the ceramic particles are interlaced to improve the bonding state of the ceramic phase, increase the bonding strength of the interface, and can give full play to the plasticity and toughness of the metal, and improve the stress state of the sintered body under load, thereby effectively The strength and fracture toughness of the cermet sintered body are improved.
(2) Ceramic particle reinforced metal matrix composites
Combine high-hardness wear-resistant ceramic particles with metal materials, combine the high hardness and high wear resistance of ceramic particles with the toughness of the metal matrix material, and form a ceramic-metal composite layer with a certain thickness on the working surface of the wear-resistant part. The composite layer bears abrasion, and the metal matrix plays a bearing role. This local compounding method can not only improve the wear resistance of the wear-resistant parts but also ensure their overall toughness.
Common particles used to prepare particle-reinforced metal matrix composites are WC, TiC, Al2O3, ZTA (zirconia toughened alumina) ceramic particles. Among them, ZTA ceramic particles have great advantages in hardness, toughness and cost, and are now widely used in ceramic-metal composite square hammers, plate hammers, throwing hammers, grinding rollers and other products.
(3) Thermal spray powder
Thermal spraying materials are mainly used for corrosion, oxidation and wear protection of high-temperature components. However, a single ceramic coating has more pores, poor fracture toughness, and has a large difference in thermal expansion coefficient from metal matrix materials, so its application is greatly restricted. Therefore, in recent years, the use of metal-ceramic composite powder as a spraying material for thermal spraying technology has received widespread attention.
(4) Special functional materials
The surface modification of SiC and hollow glass microspheres by electroless nickel plating can realize the micro-layer composite of the material, improve the absorbing ability of SiC itself, and make the hollow glass microspheres have good absorbing performance. Precious metals are plated on sub-micron and nano-scale inorganic particles. Due to the nano-sized structure of the noble metal particles adsorbed on the surface of the matrix, the entire composite powder has special optical and electrical properties.
At present, the most widely used is to prepare cermets. For example, in the aerospace field, many parts of the aircraft need to use high-temperature, wear-resistant, and high-strength materials, which have been gradually replaced with cermets and cutting made of cermets. Cutting tools are also extremely popular in the field of processing and manufacturing. For example, compared with ordinary refractory materials, cermets have higher thermal shock properties and can be used for high-temperature equipment components and so on.