In the last article, we introduced the preparation of boron carbide ceramic powder to you. If you haven’t read this article, you can click: Preparation process of boron carbide ceramic powder, check it out.
After the powder is prepared, we will enter the next stage, which is the sintering of boron carbide ceramics, which is a very critical step. Ceramic sintering has been a very difficult process since ancient times.There are many uncertain factors in the sintering process, and these factors will directly affect the quality of ceramic products.
Let’s take a look at the current sintering processes of boron carbide ceramics. The main methods of sintering boron carbide ceramics are:There are four main types of boron carbide ceramic sintering: hot isostatic pressing, pressureless sintering, spark plasma sintering, and hot pressing.
Hot Isostatic Pressing Sintering
The biggest difference between HIP sintering and other sintering methods is that it can achieve densification without additives. Not only that, but this method also achieves a fine-grained microstructure and high flexural strength. The main steps are: 1. First, pressureless sintering is performed to obtain boron carbide without openings and communicating pores. 2. The remaining closed pores are eliminated by hot isostatic pressing, so as to achieve complete densification. In this process, the temperature is generally controlled at 1950-2050 °C.
Pressureless sintering of boron carbide ceramics is also a relatively common sintering method. It uses additives (additives include element additives such as C, Fe, Al, Ti, Ni, Cr, B, Cu, Mg, etc., and metal oxide additives include : Al2O3, TiO2, Y2O3, ZrO2, Fe2O3, carbide additives: SiC, Be2C, TiC, WC, etc., and boride additives: TiB2, CrB2, W2B3.) Remove the oxide layer on the surface of boron carbide to improve point defects or dislocation density to improve the activation of grain boundaries and bulk diffusion, resulting in higher densities (95-98%) at slightly lower temperatures (2100-2200 °C).
The advantage of pressureless sintering is that boron carbide ceramic products with complex shapes can be prepared. However, the boron carbide ceramics fired in this way often have the phenomenon of excessive grain growth, and also contain a porosity of 3-7 Vol.%, as well as the problems of low material strength and toughness.
Spark Plasma Sintering
Spark plasma sintering is a new sintering process that has only appeared in recent years, which enables fast and efficient sintering of materials at lower temperatures.In the sintering process, the discharge plasma generated instantaneously when the electrode is fed with a large pulsed DC current makes each particle in the sintering system evenly generate Joule heat to activate the surface of the particle, so as to achieve the goal of sintering and molding.
Spark plasma sintering utilizes the heat energy emitted by the ceramic powder itself to complete this process, so the energy consumption is low. At the same time, the powder is uniformly heated due to the dispersed distribution of the discharge points. Due to the effective discharge between the particles, a local high temperature is generated, which makes the oxide film on the surface of the particle fall off and the surface locally melts. This process can achieve the purpose of high densification and sintering of boron carbide ceramics without sintering aids. Products fired in this way tend to have a Pintejin structure and controllable chemical composition.
Hot Pressing Sintering
Finally, let’s take a look at the fourth sintering method – hot pressing sintering. This is a method of using high temperature and applying a certain pressure to make boron carbide powder particles rearrange and generate plastic flow, thereby causing grain boundary slip and strain-induced twinning, creep and volume diffusion. Under the combined action of these motions, high-density and high-strength boron carbide ceramics can be prepared.
The boron carbide ceramics prepared by hot pressing sintering have higher density and better mechanical properties, so the price of this material is also higher. The neutron absorption capacity of hot-pressed sintered boron carbide ceramics is stronger than that of pressureless sintered ones. Hot-pressed sintered boron carbide ceramics are ideal for absorbing neutrons without generating radioactive elements.
The above four types of sintering methods are currently several mainstream sintering methods for boron carbide ceramics. In the next article, we will focus on “boron carbide ceramic machining technology”, so stay tuned!
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