Study On Process And Properties Of TiN Synthesized By Plasma Permeating Technique

As a new type of ceramic material, TiN is very suitable for surface coated material on cutting tools, moulds and bearings for its excellent properties, such as low density, high hardness, good conductivity, good wear resistance, oxidation resistance property, high corrosion resistance and so on . Therefore, the preparation of TiN hard coating has been getting more attention by researchers. Using double glow plasma surface alloying technique, titanizing was conducted on the surface of Q235 steel and tens of microns titanium solid solution diffusion layer formed. Later nitrogen gas was blown to synthesis TiN permeation layer directly. The microstructure, component concentration, hardness and phase analysis of TiN permeation layers were tested. The optimum technological parameters of titanizing and synthesis stage were obtained. The variation laws of phase structure, surface morphology, composition, appearance and micro-hardness of TiN permeation layer were achieved. The wear resistance tests for TiN permeation layer, untreated Q235 steel, quenched +tempered T10 steel and nitrided 3Cr13 steel were carried out under the same conditions. The electro-chemical corrosion tests for TiN permeation layer, 1Crl8Ni9Ti stainless steel and untreated Q235 steel in l mol/L H2SO4 solution,3.5% NaCl solution and 4% NaOH solution were carried out . The results showed that:(1) TiN permeation layer could be formed on the surface of Q235 steels by the double glow plasma surface alloying technology. The optimum technological parameters were as follows:The parameters of titanizing Stage included: Ultimate vacuum 1～3Pa, working gas Ar, working pressure 25～35 Pa, source voltage–1000V, sample voltage–600V, holding temperature 1050℃, holding time 2h. The parameters of synthesis Stage included: Working gas Ar and N2, working pressure 25～35 Pa, source voltage–1000V, sample voltage– 400V, holding temperature 1050℃, holding time 2 h, Ar/N2 gas flow ratio 8:1.(2) As TiN permeation layer thickness increase, the crystallographic preferential growth direction {100} converted into direction {111}. The direction {111} preferred orientation growth of TiN permeation layer helped to obtain TiN permeation layer of high rigidity and good wear resistance.(3) As TiN permeation layer thickness increasing, the surface cellular structure and the raised sections of the cellular structure became more obvious. When the uniform and compact cellular structure formed, TiN permeation layer could gain the higher hardness and the surface was smooth and no abscission.(4)The source voltage, sample voltages during titanizing stage and synthesis stage, holding time in synthesis stage had little effect on the surface Ti/N atomic ratio of TiN permeation layer. While working pressure, holding temperature and Ar/N2 gas flow ratio had a relatively high impact. As the working pressure increasing, the surface Ti/N atomic ratio of TiN permeation layer increased gradually. As the holding temperature increasing, the surface Ti/N atomic ratio of TiN permeation layer became more and more big. When Ar/N2 gas flow ratio was 8: 1, the surface Ti/N atomic ratio of TiN permeation layer was close to 1:1. When Ti/N atomic ratio was close to 1:1, the performance of TiN permeation layer was good.(5) When TiN permeation layer was thinner, surface color tends to light yellow. The color of TiN permeation layer changed along with the variations of technological parameters because of different contents of their phase essentially. The golden yellow TiN permeation layers contain the mostε-TiN phase, which have the best microhardness. When the optimum technological parameters was selected, TiN permeation layer showed golden yellow and no abscission.(6) TiN permeation layer thickness had great influence on the phase structure, surface morphology, composition, appearance and micro-hardness of TiN permeation layer. TiN permeation layer thickness which is too large or too small is unfavorable to obtain good TiN permeation layer.(7) The surface morphology of TiN permeation layer was cell shape. The golden yellow TiN grains were compact and uniform,the average surface microhardness was 3120 HV. TiN permeation layer was composed of external deposition layer and diffusion layer with evenly distributed TiN particles. The composition, hardness and structure of TiN permeation layer were in gradient distribution. The thickness of TiN permeation layer was 16μm. A metallurgical bonded layer was obtained between the permeation layer and the matrix. So the bonding strength was high.(8) Under the same conditions, TiN permeation layer had the least relative wear rate. Its wear resistance was 7.81 times higher than untreated Q235 steel, 5.625 times higher than quenched +tempered T10 steel and 7 times higher than nitrided 3Cr13 steel. With fineness of sand increasing, the relative wear rate of TiN permeation layer increased. As the load increasing, there were more wear mass loss and its relative wear rate was higher. Exfoliation and fragmentation were two main modes of wear failure for the permeation layer. Micro-cutting was slight nearly no plastic deformation. The degree of fragmentation and exfoliation deepened gradually with fineness of sand paper increasing. In this research, the wear mechanism of TiN permeation layer mainly was stress fatigue.(9) In l mol/L H2SO4 solution, the corrosion resistance of TiN permeation layer was increased 11.5 times compared with untreated Q235 steel and 2.65 times compared with 1Crl8Ni9Ti stainless steel. In 3.5% NaCl solution, its corrosion resistance was increased almost 11.3 times compared with untreated Q235 steel, but a little lower than 1Crl8Ni9Ti stainless steel. Compared with its corrosion resistance in salt solution, the corrosion resistance of TiN permeation layer was better in acidic solution. In 4% NaOH solution, the corrosion resistance of TiN permeation layer was increased 27.8 times compared with untreated Q235 steel and close to 1Cr18Ni9Ti stainless steel.