Synthesis of titanium carbide and titanium diboride for metal processing and ceramics production

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 Introduction Titanium carbide and diboride relate to oxygen-free refractory compounds. They are characterized by high hardness and chemical inertness. Carbothermal reduction [1] is considered to be the most promising technique of titanium carbide preparation. However, carbon-diboride reduction is aimed at obtaining tita- nium diboride [2, 3]. Since tungsten is not readily available and very expensive, titanium carbide is used to manufacture tungsten-free hard alloys. The native industry has mastered the production of hard alloys based on titanium carbide 15Ni6Co0.1Nb-79TiC ( ТН 20) grade. The developed alloy corresponds in hardness, and in strength is close to tungsten-containing grades of the TiC-Co ( ТК ) and WC-Co ( ВК ) groups and has high wear re- sistance, reduced tendency to adhesion to the processed materials, resistant to oxidation in air at high tem- peratures and to aggressive media. This allows its wide application instead of a number of standard grades of hard alloys [4]: – for metal cutting by turning and milling of low-carbon, tool and rapid steels, non-ferrous metals, and some grades of cast iron in conditions where standard alloys 4Co-66WC-30TiC ( Т 30 К 4), 6Co-79WC- 15TiC ( Т 15 К 6), 8Co-78WC-14TiC ( Т 14 К 8), 94WC-6Co ( ВК 6) and 92WC-8Co ( ВК 8) are used; – in production of measuring tools, various wear-resistant parts and industrial equipment (spray nozzles, mud pump valves, mold and draw die blocks and etc.) instead of a standard alloy TiC-Co ( ВК ) grade; – to reinforce certain types of boring tools. It was reported [5] on ceramics production from titanium carbide by hot pressing of titanium/graphite powder mixture alloyed with nickel at 1,200 о С and 40 MPa for 30 minutes. Its relative density reached 98 %. The authors consider that the ceramics can be used as a cutting tool. When used as a cutting tool, TiB 2 -TiC ceramics composite was prepared by hot pressing of diboride/titanium carbide powder mixture in vacuum at 1,650 о С and 40 MPa [6]. The resulting ceramic with a mass ratio TiB 2 : TiC = 75 : 25, had a bending strength of 920 MPa and a microhardness of 22.6 GPa. Crack resistance was 7.6 MPa·m 1/2 . When used as a cutting tool, TiB 2 –TiC+Al 2 O 3 ceramics composite was prepared from TiB 2 , TiC and Al 2 O 3 pow- der mixture by hot pressing at 1,650 о С and 30 MPa [7]. The prepared samples had a bending strength of 1,100±62 MPa and hardness of 21.53±0.36 GPa. Crack resistance was 8.5±0.8 MPa·m 1/2 . Moreover, tita- nium carbide can be applied for wear-resistant coatings with high values of microhardness [8, 9]. Abrasive machining appears to be one of the most important steps of the machine building technology. Up to 60 % of the machinery is operational for its accomplishment in certain branches of industry. How- ever, conventional abrasives (corundum, silicon and boron carbides) do not comply with all the require- ments for these materials [10]. Titanium carbide is thought to be an advanced material. It is characterized by a combination of high hardness and certain plasticity along with chemical inertness. That is, it does not interreact with the iron-group metals being a base for most structural materials. Consequently, the usage of titanium carbide ensures highly ef fi cient abrasive machining. The technique of titanium carbide production by self-propagating high-temperature synthesis is presented in [11]. Chemical composition (wt. %) of tita- nium carbide powders prepared by this method is the following: С combined ranges from 19.3…19.7; С free and О don’t exceed 0.3; N – trace level; total impurities determined by spectral analysis can’t be more than 0.25. The abrasion resistance test has revealed that the titanium carbide powder prepared by self-propagating high-temperature synthesis is superior to the powder produced by carbothermal reduction of the titanium oxide in this respect. Consequently, it is a high-quality, technological and cost-effective tool material. The greatest technical and economic effect is brought by its use as a component of abrasive pastes for grinding and polishing. Carbidosteels relate to the materials which are composed of steel and carbides with the mass content ranging from 20 to 70 %. Titanium and tungsten compounds are used as carbides. They take intermediate position between rapid steels and hard alloys with regard to their properties and application area. Car- bidosteels combine properties of combines the properties of both components: a refractory solid base and a steel matrix. The refractory component gives hardness, strength, and wear-resistance to the alloy. Steel gives viscosity and plasticity. Carbidosteels are obtained by powder metallurgy methods [12]. Titanium car-