Structure and Tribological Characteristics of Steels after Liquid Boriding with Additions of Rare Earth Oxides

Abstract

The effect of boriding on the structure and tribological characteristics of VKS-5, Kh12MF, and 40Kh steels was studied. The samples were processed in a melt based on sodium tetraborate and boron carbide with the addition of lanthanum, yttrium, and scandium oxides in an amount of 5%. It was shown that this type of chemical-thermal treatment (CTT) is an effective means of increasing the tribological efficiency of steel joints for operating conditions without a lubricant. When testing the friction pair of VKS-5 steel–SS440 steel, the maximum decrease in the friction coefficient is observed if the steel is borided in the melt with the addition of Y₂O₃, the smallest, with the addition of La₂O₃. The values of the friction coefficient in the steady-state mode are: ≈0.45 for the original (not subjected to CTT) surface, steel after boriding in a standard melt, and in a melt with the addition of La₂O₃; ≈0.20 for steel after boriding with the addition of Sc₂O₃; and ≈0.14 for steel borided in a melt with the addition of Y₂O₃. The wear of VKS-5 steel decreases by ≈6.7 times after treatment in a standard melt, by 5.0, 4.2 and 3.6 times, respectively, in a melt with the addition of La₂O₃, Sc₂O₃, and Y₂O₃. Boriding of Kh12MF steel in a standard melt reduces the friction coefficient by 1.7 times, and wear by 1.8 times compared to steel without CTT. The average value of the friction coefficient decreases by 2.9, 2.7, and 2 times in melts with the addition of La₂O₃, Y₂O₃, and Sc₂O₃, respectively; the wear intensity in the melt with La₂O₃ does not increase significantly, with Y₂O₃, it decreases by 1.6 times, and with the addition of Sc₂O₃, it decreases by 1.3 times. Boriding forms wear-resistant diffusion layers on the surface of 40Kh steel. The best results in the combination of two tribotechnical characteristics (wear intensity and friction coefficient) for the friction pair 40Kh steel—SS440 steel were obtained under conditions when the steel was borided in a melt of standard composition.