Selection of the optimal composition of plasma coating of the Ni‒B‒Si system by the CALPHAD methods

Copper and alloys are widely used in parts of metallurgical equipment. Due to high heat capacity and reflectivity of IR radiation, copper parts have found application in water‒cooled blast furnace elements, such as tuyeres, which are subject to active gas‒abrasive, erosive and other types of wear and gas corrosion. Copper and its alloys have low resistance to wear and corrosion. To increase the resistance of copper parts, thermal barrier coatings of the Ni–B–Si, Ni–Cr–Al–Y and ZrO₂ systems are offered. However, the first layers of the coating have low adhesion, and consequently, low strength of the first and subsequent layers. Laser remelting solves the problem of adhesion of the first layer to copper and the remaining layers to the fused layer. Using the CALPHAD methods in the TermoCalc software package (software version number 2024.1.132110‒55), the effect of reflow on the properties of the protective coating of the Ni‒B‒Si system was simulated. The following composition was chosen as the base: Ni – 86.97 at.%, B – 6.93 at.%, Si – 6.1 at.%. When laser radiation is applied to a coating applied by the gas‒thermal method, active interaction of the coating components with copper is observed, forming a continuous coating containing new phases and chemical elements. The appearance of some of these phases occasionally leads to cracking due to the formation of a stable compound of copper‒nickel alloy (monel metal), which has relatively low plasticity. Using X‒ray phase analysis data, it was confirmed that during the melting process, active mixing of the coating components (Ni–B–Si) with the substrate components (Cu) occurs, forming a stable compound of Cu with Ni. In this regard, using mathematical modeling, the density changes were predicted and the crystallization rates were determined using the Sheil method, as well as the phases formed during cooling in the coating, namely: Ni_86.97‒B_6.93‒Si_6.1, Ni_84.47‒Cu_2.5‒B_6.93‒Si_6.1, Ni_81.97Cu₅B_6.93Si_6.1, Ni_76.97Cu₁₀B_6.93Si_6.1, Ni_71.97Cu₁₅B_6.93Si_6.1, Ni_66.97Cu₂₀B_6.93Si_6.1. Using calculation methods, based on the provisions of thermodynamics, the process of laser melting is described during heating from 1750 K to 3000 K and subsequent cooling from 1750 K to 500 K. When studying the melting process, for all compositions it was determined that a copper content in the coating of about 15‒20 at.% is favorable for the formation of a good quality coating, since at these concentrations the most complete release of copper atoms from the grain boundaries occurs, their transition to the surface layers of the coating and their binding with nickel into stable compounds of the monel‒metal type.