Carbide Refractory Materials - From Synthesis to Finished Products

Carbide refers to a compound of carbon and metal or non-metal. The main refractory raw materials are refractory compounds formed by some transition metal elements and non-metal elements with carbon elements, including titanium carbide, chromium carbide, zirconium carbide, niobium carbide, molybdenum carbide, hafnium carbide, tantalum carbide, tungsten carbide, vanadium carbide and other metal carbides and silicon carbide, boron carbide and other non-metal carbides.

There are three methods for its synthesis:
(1) Direct combination of metal and carbon powder, the reaction temperature is about 1200~2000℃;
(2) Metal reacts with carbon-containing gas;
(3) Carbon and oxide reaction (carbon reduction method), that is, the mixture of oxide and carbon is melted and synthesized at high temperature, or in vacuum, hydrogen, inert gas or other reducing atmosphere, at a temperature below the melting point of the oxide, the oxide is reduced by carbon, and a solid phase reaction occurs to synthesize carbide. The synthesized carbide is used as the raw material and the powder metallurgy process is adopted to manufacture refractory products. Carbide refractory materials can be divided into pure carbide materials, such as SiC, B4C, Cr3C2 and TaC, etc. according to their chemical composition; composite carbide materials, such as TiC-WC-TaC, WC-Co, carbide-carbon system and "hypereutectic carbide" materials. 1. Performance of carbides Carbides are a group of materials with the highest melting point, and the melting point of many carbides is above 3000℃. Generally, carbides have the characteristics of high hardness, good chemical stability, high oxidation resistance, excellent melting point metals, and the electrical and thermal conductivity of metals.

Typical Carbide Material Properties
Name	Density/g·cm-3	Melting point/℃	Linear expansion coefficient  (20~1000℃)/C-1	Thermal conductivity  /W(m·K)-1	Specific resistance  /Π.cm	Microhardnes /MPa	Severe oxidation starting temperature/C°
SiC	3.21	2600	(5~7)x10-6	8.37	50	33400	1100~1400 1500
B4C	2.52	2450	4.5x10-6	(100'C)121.4	(20°C)0.44 (500'C)0.02	33400
TaC	14.3	3880	8.3x10-6	(700℃)62.8	42.1x10-6	16000
HfC	12.2	3890	5.6x10-6	22.19	45.0x10-6	29600
WC	15.7	2700	5.2x10-6	6.28	(20℃)53x10-6	24000
TiC	4.93	3107	7.74x10-6	24.28	52.5x10-6	30000

2. Synthesis of carbides
Usually, carbon (or its compounds) and metals (or their compounds) are interacted at the corresponding temperature to obtain, and protective gas is required when necessary. Preparation method of carbides

Method for preparing carbide
Method	Reaction	Main equipment	Temperature range/℃	Typical carbides
Smelting method	Me+C->MeC MeO+C->MeC+CO	Electric arc furnace	>2000	Boron carbide
Sintering method	Me+C->MeC MeO+C->MeC+CO	Carbon tube resistance furnace	1200~2200	Chromium carbide, tantalum carbide, tungsten carbide, hafnium carbide
Carbonization with carbon-containing gas	Me+CxHy->MeC+H2 Me+C0->MeC+CO	Special equipment	700~2900	Titanium carbide, molybdenum carbide, etc.
Vapor deposition method	Me+CxHy->MeC+H2 Me+H2+halogen+CmHm	Horizontal carbon tube furnace and special equipment	300~2500	Molybdenum carbide, chromium carbide, high-purity carbide

3. Production process of carbide products
(I) Selection According to different synthesis methods, artificial selection, water selection and pre-burning in vacuum or H2 atmosphere can be used to remove excess carbon and other impurities.
(ii) Crushing: Use carbide balls as grinding bodies, add grinding aids, and use ball milling to make the raw material fineness reach several microns to tens of microns.
(iii) Molding
(1) Machine pressing: Generally, gasoline rubber solution, phenolic resin, etc. are used as binders, and the molding pressure is 50~80MPa;
(2) Film rolling: Use polyvinyl alcohol solution as a binder and repeatedly roll it on a double-roll film rolling machine to form a film with a thickness of 0.5~1mm. This method is suitable for preparing carbide materials for plasma spraying.
(3) Isostatic pressing: Put the powder into a rubber mold and press it on an isostatic press. Generally, no binder is added, or a small amount of polyvinyl alcohol is added when the raw material is finely ground. The molding pressure is 250~350MPa.
(4) Melt casting: This method is suitable for preparing "hypereutectic carbide" products. The carbide powder is mixed with graphite powder in a certain proportion, melted in a vacuum arc furnace, and cast by gravity to obtain the desired shape.
(5) Hot pressing: This method can produce dense high-performance carbide products. The specific process is as follows: the powder is loaded into a graphite mold and heated in a protective atmosphere in a carbon tube hot pressing furnace. When the temperature rises to a certain predetermined temperature, the pressure is slowly increased, and the temperature and pressure are simultaneously raised to the sintering temperature and the predetermined pressure. The heat preservation is started and the pressure is maintained for a certain period of time (until the entire hot pressing forming and sintering process is completed). This method combines forming and sintering together and is a more commonly used forming method for carbide refractory materials. The volume density of the products made by this method can be close to the theoretical density. The general hot pressing temperature is 1500~2600℃ and the pressure is 25~35MPa. For example, the density of titanium carbide products reaches 4.90~4.93g/cm³.

(IV) After the products formed by sintering machine pressing, rolling film and isostatic pressing are dried, they are sintered in a horizontal carbon tube furnace under a protective atmosphere. The sintering temperature should be higher than the hot pressing temperature, generally at 1600~2700℃. Sintering aids must be added to prepare dense carbide materials.

IV. Titanium carbide products are industrially made by reacting titanium dioxide and carbon black at a high temperature of 1700~2100℃ for a short time to produce titanium carbide. This can be done in an inert atmosphere, preferably in a vacuum. Titanium carbide has a high melting point (3140℃) and high hardness (Mohs hardness 9). It is mainly used to manufacture titanium carbide-based cermets, heat-resistant alloys and cemented carbides.

Titanium carbide products are mostly made by hot pressing, and the product density is close to the theoretical density of 4.90∼4.93g/cm³. In special occasions, the method of vapor deposition of titanium carbide can be used, such as depositing titanium carbide on cemented carbide or mold steel to improve hardness and wear resistance (the friction coefficient between steel and titanium carbide is 0.14, while the friction coefficient between steel and steel is 0.7).

V. Boron carbide products Boron carbide is black, very hard, with a Mohs hardness of 9.3. It is an excellent grinding material and wear-resistant material. The melting point of boron carbide is 2350℃ (decomposition), the linear expansion coefficient is 4.5×10-6/℃-1 (20~1000℃), the product compressive strength is 2250MPa, it is resistant to acid and alkali corrosion, does not wet most metals, and has a fairly high chemical stability.

At present, the domestic mass production of boron carbide is to use boric acid, petroleum coke, and artificial graphite, calculate the ratio according to the reaction formula, and synthesize it by arc melting. The products of this method contain a large amount of free carbon. A better method is to prepare it by heating a mixture of boric anhydride and carbon in a resistance furnace at a temperature below the decomposition temperature of boron carbide. This method contains very little free carbon and sometimes free boron.

The reaction formula is: The production process of boron carbide products is immature, and products are usually made by hot pressing. It is usually carried out in a hot pressing furnace with carbon tube heating under argon protection. The hot pressing temperature is 2050~2150℃, the pressure is 30MPa, and the insulation time is 30min. The product density is 2.46~2.51g/cm³, and the porosity is 0.46%~0.62%.