J. Cent. South Univ. (2016) 23: 2771-2776
DOI: 10.1007/s11771-016-3339-y
Wear resistance of TC4 by deformation accelerated plasma nitriding at 400 °C
ZHU Xiao-shuo(朱小硕)1, FU Yu-dong(傅宇东)1, 2, LI Zi-feng(李子峰)1, LENG Ke(冷科)1
1. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China;
2. Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education,
Harbin Engineering University, Harbin 150001, China
Central South University Press and Springer-Verlag Berlin Heidelberg 2016
Abstract: An integrated low temperature nitriding process for TC4 (Ti6Al4V) has been developed and the effect on wear resistance has been investigated. Through the process of solid solution strengthening—cold deformation—nitriding at 400 °C, the TC4 alloy is nitrided on surface and dispersion strengthened in bulk at the same time. The white nitriding layer is formed after some time of nitriding. The nitriding speed increases with the deformation degree. The construction was investigated by XRD and the nitride is Ti3N2-X. The wear test was carried out and results exhibit that the nitrided samples have better wear resistance.
Key words: TC4 (Ti6Al4V); low temperature nitriding; cold deformation; wear resistance; tribology
1 Introduction
Titanium and its alloys are widely used today in aerospace, medical health, desalination, energy and other fields because of their excellent comprehensive performance such as high specific strength, excellent corrosion resistance, good creep strength and acceptable fatigue properties [1]. Among Ti alloys, TC4 (Ti6Al4V) occupies more than 50% market. However, poor tribological properties cause limitation of its applications in many industrial areas. A lot of work has been made to solve this problem [2-5]. Plasma nitriding (PN) has been proven a suitable technique to improve surface performance of TC4 [6-7]. Some processes which combine nitriding with other surface strengthening method are developed. MORITA et al [8] developed a combination treatment composed of plasma nitriding, short-time duplex heat-treatment, and fine-particle bombarding (FPB). The tensile strength was improved by 30% and the fatigue strength was improved by 59% by combination treatment compared with hybrid surface treatment, composed of plasma nitriding and FPB [8]. FAROKHZADEH et al [9] studied the impact of severe plastic deformation (SPD) to surface layer on plasma nitriding of TC4 alloy [9]. AMANOV et al [10] treated TC4 and Ti by using an ultrasonic nanocrystalline surface modification (UNSM) technique to improve the fretting wear and friction characteristics. The results showed that the fretting wear and friction coefficient characteristics of the UNSM-treated specimens were improved compared to those of the untreated specimens [10]. Nitride compositions of TC4 plasma nitriding were investigated [11-12]. DA SILVA et al [12] reported that composition of nitride of near face is influenced by treatment time, proportion of nitrogen, sample temperature and total pressure and got ε-Ti2N and δ-TiN at 500 °C in φ(N):φ(H)=3:2 atmosphere.
So far, the normal temperature of nitriding TC4 alloy is from 600 to 900 °C [13-15]. Nitriding at lower temperature is good for heat distortion and grain size growth, but decreases the diffusion speed [16]. High temperature improves the wear resistance, but hinders the bulk strength. FAROKHZADEH and EDRISY [17] reported that when being nitrided at 600 °C, the ductility and fatigue life of TC4 specimen are decreased by 43% and 23%, respectively. There is only a limited work done on low temperature plasma nitriding of TC4. RAJASEKARAN et al [18] carried out the PN treatment on TC4 at 500 °C and reported that plasma nitriding increases the plain and fretting fatigue lives due to improvement in surface hardness, surface residual stress, surface roughness and friction force. MUBARAK ALI et al [19] performed PN on TC4 at 520 °C. They investigated the influence of PN at low temperature on fretting wear in different environments and time periods.
In this work, we purposed a new integrated process to nitride TC4 alloy at low temperature. The process is solid solution strengthening—cold deformation— nitriding at 400 °C. The purpose of deformation is to increase the nitrogen diffusion and improve the nitriding layer formation [20]. The process is designed to have an integrated effect. At 400 °C, nitriding generates hard coating on the surface. At the same time, precipitation occurs in matensite phase and strengthens the substrate. So, the alloy gets strengthening effects on the surface and inside by a process.
2 Experimental
2.1 Substrate material and surface treatments
The as-received TC4 used in this work is commercial product with chemical compositions of Al 6.1%, V 4.0%, Fe 0.18%, C 0.03%, N 0.009%, H <0.0003%, O <0.16% and balance Ti (mass fraction). Samples of 6 mm thick were cut to dimensions of 50 mm × 60 mm. Microstructure was composed of annealing phases of α+β.
Solid solution strengthening was carried out to obtain metastable martensite phase. Specimens were heated at temperature from 750 to 900 °C, and then quenched in water. The samples obtained the highest ductility and the lowest yield strength ratio at 820 °C, so the samples heated at 820 °C was applied in subsequent experiments.
After solid solution, the specimens were deformed on an industrial cold rolling mill with two rolls. The rolled specimens were ground through 400, 600, 800 grade sandpaper and ultrasonically washed before being placed inside the nitriding chamber. The plasma nitriding was performed at 400 °C in a pulsed plasma furnace. The environment was φ(N):φ(H)=1:8 and the voltage was 650 V. In order to study the effect of deformation degree and nitriding time on microstructure, factorial experiments with two facts of deformation degree and nitriding time were carried out. Deformation degree varied from 5% to 40%, and nitriding time varied from 0.5 to 7 h.
2.2 Characterization
Microstructure were observed under a OLYMPUS-PM3 microscope and a QUANTA200 SEM. Nital of microscope sample was mixture of HF and HNO3. The volume ratio was HF:HNO3:H2O=1:3:6. The etching time was 5 s.
Knoop microindentation hardness measurements were taken by means of a HVS-1000 hardness testing machine. The machine was set at a load of 0.25 N and a dwelling time of 15 s. Three measurements were taken on each test sample.
The phase constituents in nitrided coupons were studied by means of Panalytical X’Pert Pro XRD analyzer. The scan speed was 5 (°)/min and 2θ ranged from 20° to 80°.
Wear test was conducted on a Pin-On-Disk-1-Auto testing machine with the grinding ball material of GCr15. The setting was applied at load of 2 N, wear speed of 100 r/min and wear time of 25 min. The wear tracks were observed by SEM.
3 Results and discussion
3.1 Impact of deformation degree to nitriding speed
After 400 °C nitriding process, the nitriding layer can be observed. The surface of specimens turns into golden color and the color becomes dark golden with nitriding time increasing. This attributes to the golden color of TiN. The results by SEM and N energy spectrum analysis exhibit the nitriding layer. SEM image and N energy spectrum analysis results are shown in Fig. 1. The white layer in Fig. 1(a) is nitriding layer. In Fig. 1(b), it exhibits that the nitrogen concentration decreases sharply at distance around 5 μm. This could be attributed to low nitriding temperature. Nitrogen is not active enough at temperature below 400 °C, so higher concentration difference is needed to offer kinetics for nitrogen moving forward.
Fig. 1 SEM image (a) and N energy spectrum (b) of nitriding layer after nitriding at 400 °C
After studying the effects of deformation degree and nitriding time on the nitriding layer, as well as measuring the thickness of the nitriding layer, it has been found that when the nitriding time is shorter, the influence of the deformation degree is not obvious. But when the nitriding time is long enough, the influence of the deformation degree becomes more and more obvious. When the nitriding time is 2 h, there is only rarefied nitriding layer on un-deformation specimen; The nitriding layer of 30% deformed specimen starts to become different. The layer is constant and the thickness is 1 μm. The metallographs of nitriding layers after 2 h nitriding with different deformation degrees are shown in Fig. 2.
Fig. 2 Metallographs of nitriding layers after 2 h nitriding with different deformation degrees:
When the nitriding time is long enough, the thickness of the nitriding layer increases with the increase of deformation degree. When the nitriding time is 7 h, it is obvious that the thickness increases with the deformation degree increasing. Metallographs of 7 h nitriding specimens are shown in Fig. 3. In Fig. 3(a), the thickness of layer is 1 μm, and in Fig. 3(c), when the deformation degree is 30%, layer thickness is around 5 μm.
3.2 XRD test results
In Figs. 4 and 5, XRD patterns of 10% and 30% deformed specimen with different nitriding time are shown.
Fig. 3 Metallographs of nitriding layers after 7 h nitriding with different deformation degrees:
Fig. 4 XRD patterns of 10% deformed specimen with different nitriding time
As can be seen, the nitride with different deformation degree and nitriding time is Ti3N2-X steadily in experiments. The positions of the nitride peaks are the same under different conditions. Nitride peaks are Ti3N2-X (104), (015) and (0,1,14). It is noted that TiN and Ti2N are absent in composition of nitride. Composition of nitride is influenced by treatment time, proportion of nitrogen, sample temperature and total pressure [12]. In this experiment, temperature could be the main cause and proportion of nitrogen should be considered.
Fig. 5 XRD patterns of 30% deformed specimen with different nitriding time
By comparing Figs. 4 and 5, the height of the nitride peaks is different, so deformation affects the nitriding process. This is consistent with former results.
3.3 Microstructure analysis
As samples are nitrided on the surface, the substrate is aged in the nitriding process simultaneously. When aging time is short (0.5 h), the impact of deformation degree is not significant. But when aging time is enough long, it comes to be obvious. The morphologies of pieces after 3 h nitriding are shown in Fig. 6. Compared to the undeformed samples, β phase in the sample with 30% deformation degree is much smaller and more aging dispersed particles exist in 30% deformed specimen. It exhibits that there is a positive correlation between deformation degree and aging rate.
Microstructure of 30% deformed specimen after aging for 7 h is shown in Fig. 7. When aging at 400 °C for 7 h, α′and metastable β phase transform into α and β phase, so microstructure becomes clear and is composed of α+β.
3.4 Wear resistance analysis
3.4.1 Microhardness
Surface microhardness of specimen of 30% deformed specimen is shown in Fig. 8. For specimens of un-nitriding and 0.5 h nitriding, the hardness change is small. And when the nitriding time reaches 2 h, the hardness increase is significant. The highest hardness exhibits at the sample nitrided for 6 h and is HV 300 more than that of un-nitrided sample.
Fig. 6 Microstructures of different deformed specimens after aging for 3 h:
Fig. 7 Microstructure of 30% deformed specimens after aging for 7 h
Surface microhardness of specimen after 6 h nitriding with different deformation degrees is shown in Fig. 9. It exhibits that the impact of deformation degree to nitriding is significant. As can be seen, the hardness climbs sharply when deformation degree increases. It shows a double-peak profile. At 30% deformation degree, the microhardness gets the highest peak which is almost HV 200 higher than that of non-deformation specimen. At 15% deformation degree, the hardness comes down and is as same as un-deformed one. This could be attributed to deformation texture which makes nitrides oriented.
Fig. 8 Surface microhardness of 30% deformed specimen with different nitriding time
Fig. 9 Surface microhardness of specimen after 6 h nitriding with different deformation degrees
3.4.2 Wear
The wear rate of specimens with different deformation degrees after nitriding for 6 and 7 h are illustrated in Fig. 10. As can be seen, no matter the nitriding time is 6 or 7 h, the wear rate of 30% deformed specimen is lower than that of the undeformed specimen. Compared with the wear rate of undeformed specimen, the wear rate of 30% deformed specimen after nitriding for 6 h decreases by 21%. The decreasing ratio becomes 19% when the nitriding time is 7 h. When the deformation degree is the same, the wear rate of 7 h nitriding sample is always lower than that of 6 h nitriding one.
Friction coefficient analysis result also supports the above conclusions. Friction coefficient analysis result is illustrated in Fig. 11. When the nitriding time is the same, the stable stage of 30% deformed specimen is always longer than that of un-deformed specimen. And no matter deformation degree is 0% or 30%, 7 h nitriding specimen has a longer stable stage than 6 h nitriding one. Wear scar morphology analysis indicates that the wear scar is shallow and has less adhesive wear when the deformation degree is 30%.
Fig. 10 Wear rates of specimens with different deformation degrees after nitriding for 6 and 7 h
Fig. 11 Friction coefficients of specimens with different deformation degrees after nitriding for 6 and 7 h
4 Conclusions
1) TC4 alloy was deformed and then nitrided at 400 °C. The nitriding rate increases with the increase of the deformation. The nitride is Ti3N2-x.
2) The aging rate has positive correlation to deformation degree. Since the normal aging temperature of TC4 in industry is 500 °C, the experiment with higher temperature is promising.
3) The microhardness has positive correlation to deformation degree. The wear rate has negative correlation to deformation degree.
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(Edited by YANG Bing)
Foundation item: Projects(51275105, 51375106) supported by the National Natural Science Foundation of China
Received date: 2016-08-03; Accepted date: 2016-10-08
Corresponding author: FU Yu-dong, Professor, PhD; Tel: +86-15204692272; E-mail: fuyudong@hrbeu.edu.cn
