Research On Properties Of The Nonhydrogen Carburizing Coating On Titanium Alloy By A Glow Plasma Method

As a novel structural and functional materials, titanium and titanium alloys possess highstrength and excellent corrosion resistance, and they have been applied in many fields, such asaerospace, ships, weapons, chemical engineering, medicine, marine. However, titanium andtitanium alloys have common defects including low hardness and poor wear-resistance, whichaffects the safety and the reliability of titanium alloy components, and limit their furtherapplications.TiC formed by titanium and carbon has high hardness and low friction coefficient andthe layer as a diffusion layer can meet the demand of heavy load. The traditional technique ofsolid carburizing is used to improve the surface hardness of steel materials, but it is notsuitable for processing titanium. In addition, for common methods, such as gas carburizing,vacuum carburizing, hydrogen can penetrate into the matrix of titanium alloy, and hydrogenembrittlement may occur.In this paper, using the double glow plasma surface metallurgy principle, a nonhydrogencarburized coating with certain thickness was prepared on the TC4alloy with pure graphite asthe target, and argon as the sputtering gas. The phases, microstructure, composition, hardnesswear resistance and mechanical properties of the carburized coating were investigated byXRD, SEM, EDS, etc. The conclusions are as follows:TiC phase appears in the coating, but no hybrids or H-containing phase are observed.The carburized coating is composed of a surface layer, a carburized layer and a diffusion layer.The surface layer containing Ti, Al and V elements inhibits the permeance of carbon. Thecarburized layer consists of dense layer I and equiaxed-crystal-shaped layer II. The diffusionlayer has a little carbon, so it is only a carburization-influenced zone. The HV microhardness of the carburized coating has a gradient change which reflects the change of carbon content.The wear loss is2%~7%of that before carburizing, while the friction coefficient is50%~60%of that before carburizing. With the ball-on-disc wear, TC4alloy is damagedseriously. However, the carburizing surface has only slight scratches without wear, indicatingthat carburizing can reduce the wear rate, and improve the wear resistance of alloy TC4alloy.Through1h,2h, and3h treating，the tensile strength and nonproportional extensionstrength are similar to those of the original sample. After the three heat treatment (withoutcarburized layer) elongation and reduction of area on the cross section are better than those ofthe original sample, but the sample with carburized layer are heavily decreased, and with thetime extension, the elongation and the reduction of area decline. In the tensile process, theouter surface layer, covered with ring-shaped cracks, flakes off gradually. The cracks areincreased from the root to the middle. There are small cracks on the root while the cracksbecome wider and larger in the middle. The cracks mainly concentrate near the fracture, and afew on the root. The fracture of the heat-treated TC4sample is plastic fracture, and thedislocation slip plays an important role in the fracture process. The layer close to the outersurface of the sample shows brittle fracture characteristics, the core is of plastic fracture, andthe fracture direction can be determined by the fracture morphology.