Research on the Dependence of the Microhardness of Modified Surfaces of Titanium Alloys on the Depth of Nitrogen Saturation During Vacuum Ion Plasma Nitriding in the Thermocyclic Mode
Authors:   National Academy of sciences of Ukraine G.S. Pisarenko institute for problems of strength, Rutkovskіy Anatoly , Markovych Sergiy ,   Central Ukrainian National Technical University, Magopec Sergiy ,   Central Ukrainian National Technical University, Markovych Viktor ,   Central Ukrainian National Technical University
Published: December 2023

A study of titanium alloys with a strengthened nitrided layer by the method of vacuum ion plasma nitriding in thermocyclic mode was carried out to determine the regularity of the effect of diffusion saturation parameters on microhardness. At the same time, the effect of anomalous mass transfer of nitrogen in the surface of the part being processed was used, by creating a field of thermal stresses in the surface layer due to the cyclic inclusion and exclusion of the glow discharge and cyclic temperature changes. Microhardness studies of strengthened surface layers were carried out on metallographic slides using a PMT-3M microhardness meter, the thickness of the nitride layer was controlled using microstructural analysis using a MIM-10 microscope, and the phase composition of the surface layer was monitored using a DRON-3M device. Vacuum ionic nitriding in the thermocyclic regime of titanium alloys allows changing the physical and mechanical characteristics within wide limits (diffusion layer depth up to 500 μm, microhardness up to 9600 MPa, phase composition of nitrided surfaces, etc.), obtaining surface layers with different phase composition (α , γ' and ε - phases) with and without the nitride zone, depending on the temperature, pressure, composition of the saturating medium, and the size of the temperature cycles. Nitriding in a glow discharge achieves a high hardness of the surface of titanium alloys – up to 10,000 MPa, but at the same time the plasticity of the nitrided layers is sharply reduced and the tensile strength of the material is reduced by 30%. Conclusions. 1. The amount of microhardness depends on the phase composition of the surface. Three phases TiN, Ti2N and Ti(N) are formed on the surface of the VT1-0 alloy after nitriding. The hardness of the surface layer of nitrided titanium depends on the ratio of these phases and is higher, the greater the amount of the TiN phase. The TiN phase has the highest hardness, the Ti2N phase has the lowest hardness. The hardness of the internal nitriding zone (Ti(N)) varies depending on the concentration of nitrogen in it. 2. By changing the parameters of the vacuum ion nitriding process in the pulse mode (temperature, pressure, composition of the saturating medium and nitriding time), it is possible to change the physical and mechanical characteristics (diffusion layer depth up to 300 μm, microhardness up to 9600 MPa, different hardness gradient along the depth , phase composition of nitrided surfaces, etc.), obtain surface layers with different phase composition (α, γ' and ε - phases) with and without a nitride zone, depending on temperature, pressure, composition of the saturating medium, and size temperature cycles, which allows to optimize the properties of the surface layer in specific operating conditions. 3. Increasing the nitriding time of titanium alloys contributes to increasing the thickness of the nitrided layer to 300 microns. Addition of inert helium and argon gases to the saturating medium helps to increase the plasticity and thickness of the nitrided layer.