Research Progress and Trend of Plasma Metallurgy on Titanium Metallic Surface

By using vacuum plasma surface metallurgy technology, Chinese scien sts have carried out comprehensive research on improving the wear resistance, corrosion resistance and flame retardancy of tanium metal. In this paper, the latest research results of alloy layer forma on on tanium surface by plasma metallurgy technology and the development trend of plasma metallurgy technology on tanium surface are summarized. 1. Introduc on Titanium, although possesses some decent proper es of low density, high specific strength and good corrosion resistance, has some major drawbacks, like poor wear resistance, poor high temperature oxida on resistance, flammability and high manufacturing cost. Surface modifica on of tanium is adopted to overcome those drawbacks and improve their proper es. At present, surface modifica on technologies such as physical vapor deposi on, laser cladding, ion implanta on, ion nitriding, thermal spraying and micro-arc oxida on have been successfully applied to tanium alloys. Plasma metallurgy on tanium surface is a new kind of surface technology, upon which a lot of research work has been done by many Chinese scholars and researchers, and some important research results have been achieved [1-2]. 2. Double glow plasma surface metallurgy technology Double glow plasma surface metallurgy, a major breakthrough based on the “ion ntriding technology” invented by B. Berghaus in 1930, was invented by Professor Xu Zhong of Taiyuan University of Technology in 1980 to realize metalizing elements on the surface of metal materials. This technology has obtained patents form the United States, Great Britain, Canada and many other countries. It uses metal ions provided by plasma, which is generated by double glow discharge in vacuum chamber. Under the ac on of thermal diffusion and ion bombardment, a layer of coa ng is formed on the surface of workpiece. Its working principle and experimental setup are shown in Figure 1 [2]. MATEC Web of Conferences 321, 06007 (2020) https://doi.org/10.1051/matecconf/202032106007 The 14 World Conference on Titanium © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). Figure 1. Schema c diagram of Double glow plasma surface metallurgy experimental setup The characteris cs of double glow plasma surface metallurgy technology are: (1) can be applied to infiltrate one or several kinds of elements into the surface of metal material; (2) precise control of surface alloying process can be achieved by controlling discharge characteris cs; (3) plasma generated by double glow and hollow cathode can diffuse rapidly; (4) treatment, conducted in vacuum atmosphere, is environmental friendly. 3. Research progress of plasma metallurgy on tanium surface 3.1 Surface flame retardant tanium alloy There is a fatal problem of sustained combus on sensi vity in tanium alloy, which could lead to the failure of “ tanium fire”. Alloy C (Ti-35V-15Cr), Ti-45Nb, BTT-1 (Ti-13Cu-4Al-4Mo-2Zr) and BTT-3 (Ti-18Cu-2Al-2Mo) alloys are studied by Pra�Whitney, ATI Wah Chang, and a Russian company respec vely in order to improve the flame retardancy of tanium alloys [3]. 3.1.1 Plasma metallurgy Ti-Cu flame retardant alloy [4-6] Figure 2 shows the Ti-Cu alloy layer formed by plasma Cu diffusion on Ti6Al4V substrate. Cu element shows a gradient distribu on in surface alloy layer. The weight percentage of Cu in the outer surface is 14%. At the place 160μm from the surface, it remains at 12%, and the contents of other components are similar to those of Russian BTT-1 alloy. The content of Cu changes con nuously from the surface to the substrate, which gradually decreases along the depth with no muta on point, and the average thickness of the modified layer is over 200μ . Figure 2. Concentra on distribu on of Cu in Cu alloy layer on Ti6Al4V substrate The study found that the substrate is s ll α+β phase a�er plasma diffusion, the alloying layer consists of α+β phase and dispersively distributed Ti2Cu intermetallic compounds. The mel ng point of Ti2Cu is 990°C, means it will so�en or melt before combus on, which is helpful to the flame retardant property. Figure 3 shows the test results for Cu alloy layer formed on the surface of TC11 tanium alloy. Cu element content in the surface alloy layer maintains an approximate constant value of 20%, while that of external surface is rela vely low, about 1%. This result is not exactly the same as that of forming Ti-Cu alloy layer on Ti6Al4V substrate by plasma alloying. In order to make the alloy layer on the surface of TC11 has flame retardant proper es, Cu content of surface layer needs to reach 13% or more. Therefore, surface processing of TC11 surface a�er plasma Cu diffusion is considered. 2 MATEC Web of Conferences 321, 06007 (2020) https://doi.org/10.1051/matecconf/202032106007 The 14 World Conference on Titanium


Figure 1. Schema c diagram of Double glow plasma surface metallurgy experimental setup
The characteris cs of double glow plasma surface metallurgy technology are: (1) can be applied to infiltrate one or several kinds of elements into the surface of metal material; (2) precise control of surface alloying process can be achieved by controlling discharge characteris cs; (3) plasma generated by double glow and hollow cathode can diffuse rapidly; (4) treatment, conducted in vacuum atmosphere, is environmental friendly.

Surface flame retardant tanium alloy
There is a fatal problem of sustained combus on sensi vity in tanium alloy, which could lead to the failure of " tanium fire". Alloy C (Ti-35V-15Cr), Ti-45Nb, BTT-1 (Ti-13Cu-4Al-4Mo-2Zr) and BTT-3 (Ti-18Cu-2Al-2Mo) alloys are studied by Pra�-Whitney, ATI Wah Chang, and a Russian company respec vely in order to improve the flame retardancy of tanium alloys [3] . [4][5][6] Figure 2 shows the Ti-Cu alloy layer formed by plasma Cu diffusion on Ti6Al4V substrate. Cu element shows a gradient distribu on in surface alloy layer. The weight percentage of Cu in the outer surface is 14%. At the place 160μm from the surface, it remains at 12%, and the contents of other components are similar to those of Russian BTT-1 alloy. The content of Cu changes con nuously from the surface to the substrate, which gradually decreases along the depth with no muta on point, and the average thickness of the modified layer is over 200μ . The study found that the substrate is s ll α+β phase a�er plasma diffusion, the alloying layer consists of α+β phase and dispersively distributed Ti2Cu intermetallic compounds. The mel ng point of Ti2Cu is 990℃, means it will so�en or melt before combus on, which is helpful to the flame retardant property. Figure 3 shows the test results for Cu alloy layer formed on the surface of TC11 tanium alloy. Cu element content in the surface alloy layer maintains an approximate constant value of 20%, while that of external surface is rela vely low, about 1%. This result is not exactly the same as that of forming Ti-Cu alloy layer on Ti6Al4V substrate by plasma alloying. In order to make the alloy layer on the surface of TC11 has flame retardant proper es, Cu content of surface layer needs to reach 13% or more. Therefore, surface processing of TC11 surface a�er plasma Cu diffusion is considered.  [7] The content distribu on of Ti-Cr alloy layer on TC4 substrate is shown in Figure 4. It can be seen that the content of Cr is 75% on the outmost surface. At a distance of 70μm from the surface, it remains above 20%. It fully meets the requirement. The composi on distribu on of diffusion layer on TC11 tanium alloy a�er double glow Cr diffusion is shown in Figure 5. The content of Cr is 100% on the outmost surface. The thickness of the layer is 45μm.   [8,9] The concentra on distribu on of alloying elements along the depth of the Nb modified layer in TC4 is shown in Figure 8. Nb element exhibits a gradient distribu on in alloy layer, and the weight percentage in the surface can reach more than 65%. Figure  9 shows the SEM photos of sec on of alloy layer on TC4. A high alloy layer (white bright layer) of several microns exists on the external surface of the layer. The diffusion layer is mainly composed of Nb solid solu on formed in α-Ti and β-Ti. The surface alloy layer of Ti-Mo also has flame retardant property. Molybdenum, with a the body centered cubic la ce, can form con nuous solid solu on with β-Ti who has the same crystal la ce, and limited solid solu on with α-Ti who has closepacking hexagonal la ce. Solubility of Mo in α-Ti is very low, no more than 0.8%. The forma on of Ti and Mo is subs tu onal solid solu on, with a small la ce distor on; therefore, the Ti-Mo subs tu onal solid solu on with high content of Mo not only has high strength, but also maintains high plas city. The composi on distribu on of diffusion layer on TC4 substrate is shown in Figure 10. The outmost surface is 3μm deposi on layer, with content of Mo for 100%. In the layer 3~30μm from surface, Mo element shows a gradient distribu on. The thickness of the layer is about 40μm. The composi on distribu on of diffusion layer on TC11 tanium alloy a�er surface plasma Mo diffusion is shown in Figure  11. Mo content at outmost surface is 1.5%, and remains above 0.8% at 30μm from the surface, with a low gradient concentra on. The thickness of the layer is about 100μm.

Other plasma metallurgy flame retardant alloys
A series of surface flame retardant alloy coa ngs including Ti-Cu, Ti-Mo, Ti-Cr, Ti-Nb, etc., have been successfully prepared on the surface of tanium alloy by adop ng the double glow plasma surface alloying technology, shows significant advantages compared with the flame retardant tanium alloy. [9,10] A typical structure of Ti-Pd layer on the tanium surface is shown in Figure 12. The thickness of Ti-Pd alloy layer is generally greater than 50μm, which is dense and forms a "canine" shaped interface with tanium substrate. TiPd2, TiPd3, Ti2Pd, and s ll α-Ti are formed in the Ti-Pd alloy layer. The content distribu on of Pd in alloy layers is shown in Figure 13. The content of Pd varies greatly in different layers. However, in each layer the content of Pd is linearly decreasing. Ti-Pd alloy layer on tanium surface has excellent corrosion resistance. The corrosion rate of Ti-Pd alloy layer on the tanium surface is only 0.682mm/a in the 25℃ water solu on of H 2 SO 4 , 1/6 of that of Ti-Pd alloy; in the 25℃ water solu on of 30% HCl, the corrosion rate of the Ti-Pd alloy layer is only 0.004mm/a, 1/8 of that of Ti-0.2Pd alloy. [11][12][13] From the X-ray diffrac on diagram of sample is shown in Figure 14 Figure 15 shows the indenta on morphology of two different cri cal loads. There is a small amount of redial cracks on the edge of the alloy layer, and there is no peeling off, which indicates that the combina on of the alloy layer and the substrate is firm, with no interface weakening phenomenon. The Pc value of the nitriding a�er molybdenizing layer is 180N, which is substan ally higher than 120N of the molybdenitriding layer.

Mo-N-Ti surface wear resistant alloy
With the running-in distance of 200m, compared with the original Ti6Al4V sample, the wear rate of Mo-N modified sample decreased by 405 mes, and the wear rate of N a�er Mo modified sample decreased by 1081 mes.

Hydrogen embri�lement of tanium materials
Carburizing technology is a tradi onal surface chemical heat treatment technology, and has been widely adopted in iron and steel industry. The exis ng carburizing technologies includes solid carburizing, gas carburizing, ion carburizing and so on. It is well known that hydrogen is involved in those carburizing technologies. Titanium is an ac ve metal with strong affinity to hydrogen, especially at high temperature, it will absorb hydrogen, so the hydrides are generated. When the accumula on of hydrides reaches a certain level, the impact toughness and extensibility of tanium reduce rapidly, hydrogen embri�lement phenomenon occurs [13] . [14] Figure 16 shows the surface X-ray diffrac on pa�ern of industrial pure tanium a�er double glow plasma non-hydrogen carburizing. High hardness phase TiC is formed in the diffusion layer, as well as some free-state carbon elements. The forma on of TiC phase will undoubtly increases the hardness of the material, reduces fric on coefficient and improves the wear resistance of the material. The free-state carbon elements will mainly exist on the surface of the sample, which will play a role in reducing the fric on, as well as improving wear resistance. The surface layer of the carburized layer on tanium alloy formed by non-hydrogen carburizing is composed of TiC and a small amount of non-carburized C. Shown in Figure17, in the carburizing layer, the grain is fine due to the infiltra on of C. From the surface to the matrix direc on, the grain size increases with the decrease of C element, and there is fine and uniform needlelike TiC in the layer. Figure 18 shows the rela onship between the thickness of the carburizing layer and the heat-retaining me at the workpiece temperature. The thickness of the carburizing layer is increasing with the prolonging of the heat-retaining me.  Figure 19 shows the Ti and C element distribu on of the carburizing layer on the industrial pure tanium. Ti and C showed a gradient distribu on from the outside to the inside of the layer. Ti showed a gradient ascent, while C a gradient descent, especially in the range of 0~2μm, Ti increased rapidly, while C decreased rapidly. C decreased from 95.71% to 6.583%, while Ti 10 MATEC Web of Conferences 321, 06007 (2020) https://doi.org/10.1051/matecconf/202032106007 The 14 th World Conference on Titanium increased from 3.218% to 93.35%. Then the decrease became slower a�erward, un l 16.577μm the C content is s ll 0.204%, higher than that of substrate. Figure 19. The composi on of carburizing layer on pure tanium a�er 960℃×3h non-hydrogen carburizing [15][16][17][18] Figure 20 shows the hardness curve of non-hydrogen carburized pure tanium. The hardness is a parabolic-shaped curve, with the maximum value at the surface. The hardness decreases rapidly within the 25μm from the surface, and a�erward, there is a stable decreasing zone. It is because TiC prevents the diffusion of C to a certain degree. The forma on of TiC on the surface will block the further inward diffusion of C, which decreases the surface hardness. Industrial pure tanium, TC4, TC6 and TC11 all exhibit the above hardness distribu on rules, only with different the absolute hardness of the non-hydrogen carburized layer due to differences of element contents in substrate alloys. Studies show that, a�er a brief running-in stage, when carburized TC4 grinding with GCr15 under dry fric on condi on entered the normal stable wear stage, the fric on coefficient of carburized TC4 tended stable, at around 0.23, which is about 30% of that of the untreated sample. The wear rates of carburized samples are about 2%-7% of that of the untreated one, which shows that wear rate can be reduced by carburizing.

The proper es of non-hydrogen carburized layer on tanium surface
The carburized samples and pure tanium samples were soaked for a week (168h) under different concentra ons of sulfuric acid and hydrochloric acid at room temperature. The corrosion rates are shown in Figure 21 and 22. When the concentra on is higher than 10%, the corrosion rates of the carburized samples are substan ally lower than those of the untreated samples. The corrosion rate is at a low level, about 0.0599mm/a. In the 10%~80% H 2 SO 4 solu on, the carburized samples also show decent corrosion resistance, the corrosion rates maintain at 0.002-0.0820mm/a. The improvement of corrosion resistance lays a theore cal founda on for the applica on of tanium in chemical engineering. HCl(wt%)

Conclusion and Prospect
1. The study of engineering applica on technology of double glow plasma surface metallurgy wear resistant TiC tanium alloy needs to go deeper and further; solve the rela onship between the structure size precision of the parts and the wear resistance of the alloying layer in the project; produce the tanium alloy parts with high performance, high precision, to meet the more extensive engineering and technical requirements.