It is known that the hardened layers obtained using conventional boration, while having high hardness, are quite brittle. To eliminate this drawback, some of the specimens were subjected to complex boride hardening simultaneously with boration when, along with boron, atoms of other elements diffuse into the surface layers providing increased resistance to brittle fracture due to changes in the structural state. Especially effective in this regard is the use of boriding media, which contain components that provide, along with boron, the diffusion of silicon and other elements borosiliconizing and boroalumosiliconizing processes [1-12], as well as lanthanoboriding [5].
Powder mixtures were used for diffusion hardening, allowing boride hardening in an air environment without the use of additional protective equipment. At the same time, in the case of hardening of small-sized parts or test specimens, the powder mixture is poured into an unsealed container with saturated specimens and kept in an electric furnace with an air atmosphere at a given temperature followed by cooling in the air. In the case of powder mixture-based large-sized parts, after the introduction of water, a coating is prepared and applied only to the working parts of the specimen or part, followed by CVD, which can be combined with heating for hardening [2]. The change in the structural state of cyanated, lanthanoborated, borosilicited, and other layers as compared to borated ones has a very noticeable effect on their microbrittle properties.
Microbrittleness of diffusion layers was determined using the PMT-3 device. Microbrittleness was evaluated by the chipping stress (σ) of the diffusion-hardened surface (the lower the chipping stress, the higher the brittleness), which depends on l (the minimum distance from the center of the diamond pyramidal imprint to the edge of the specimen at load P) [6]:
where c is the diagonal length of the diamond pyramid print.
Figure 1 shows comparative microhardness diagrams for several phases of hardened layers of 45, 40Kh13 and 27KhGT kinds of steel (boration was carried out at 890 ºC for 3 hours). Individual phases: 1 - FeB; 2 - Fe2B; 3 - alphase (α).