4.1 Interface reaction between TiB2 coating and Al-adhesive transfer layer
Xue et al. [3] reported that there was a melting characteristic in the Al-hBN seal coating when rubbing against the Ti6Al4V blade tip at a linear speed of 150 m/s. Wang et al. [32] also found that the rubbing surface temperature between the AlSi-hBN seal coating and Ti6Al4V blade tip was higher than the melting point of Al (660 ℃) at 150 m/s. Thus, the rubbing temperature between the TiB2 deposited Ti6Al4V blade tip and the Al-hBN seal coating at 300 m/s could reach up to 660 ℃. With the accumulation of friction heat during the high-speed rubbing at 300 m/s, the rubbing surface of the Al-hBN seal coating melts and is smeared onto the TiB2 coating.
The thermodynamic calculation shows that the Gibbs free energy change(△G) of the TiB2/fused-Al interface reaction which forms free Ti (TiB2 + Al = Ti + AlB2, △G = 127.87 kJ 700 ℃)or TiAl3 ༈TiB2 + 4Al = TiAl3 + AlB2, △G=-15.89 kJ 700℃༉ is positive or close to zero. This interface reaction is difficult to occur directly. Similarly, Xi et al. [33] found that the TiB2 ceramic had an excellent chemical inertness at the static TiB2/fused-Al interface, and there was no obvious decomposition of TiB2 at 800℃.
However, it is different from the static corrosion by the fused Al, the tribology interface under high friction heat is exposed to the atmospheric environment during the high-speed rubbing. Kulpa et al. [34] reported that TiB2 coating exposed in the atmosphere would start to oxidize above 100 ℃. Therefore, the TiB2 coating would be oxidized with the increase of rubbing surface temperature during the high-speed rubbing of the TiB2 deposited Ti6Al4V blade tip against the Al-hBN sealing coating. Table 2 and Fig. 8 also show that there is a certain amount of oxides in the TiB2 coating after rubbing. Thus, under the influence of oxidation, the interface reaction between the TiB2 coating and the Al adhesive transfer layer could be described as follows [31, 35]:
\({\text{2Ti}}{{\text{B}}_{\text{2}}}{\text{+5}}{{\text{O}}_{\text{2}}}{\text{=2Ti}}{{\text{O}}_{\text{2}}}{\text{+2}}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}\) ,△G=-1565.73 kJ 700 ℃ (1)
\({\text{3T}}i{O_2}+{B_2}{O_3}+6Al={\text{3A}}{l_2}{{\text{O}}_3}{\text{+3Ti+2B}}\) ,△G=-653.61 kJ 700 ℃ (2)
The oxides of the TiB2 coating reacts with the Al-adhesive transfer layer, and the free Ti is released. the free Ti will dominate the following Ti-Al interface reaction [35]. It diffused into the fused Al and forms the TiAl3 phase firstly due to the low surface energy of TiAl3 compared with other Ti-Al intermetallic compounds [30]. The interface reaction between the free Ti and fused Al is as follows [36]:
\({\text{Ti+3Al=}}TiA{l_3}\) ,△G=-123.80 kJ 700 ℃ (3)
△G values of the above-mentioned reactions are all negative and at a low level, indicating their thermodynamic feasibility. Furthermore, the shear stress at the rubbing surface would also promote the tribology interface reaction from the kinetic respective [37]. After the free-Ti atoms near the interface are consumed, the TiB2/Al interface reaction will be inhibited [15]. In addition, the TiB2 coating deposited by magnetron sputtering had an over-stoichiometric ratio of B (B/Ti ratio = 2.61). The excess B exists as the B-rich phase at the grain boundary which will inhibit the further oxidation of TiB2 coating and the diffusion of Ti atoms to the TiB2/Al interface [38]. Therefore, the supply of free Ti at the TiB2/Al interface was limited, and there was no γ- TiAl phase was formed, only the TiAl3 interface reaction layer with a thickness of 0.5 ~ 0.8 µm could be observed as shown in Fig. 7b.
4.2 Mechanism of anti-Al adhesive transfer of TiB2 deposited blade tip
The frictional flash temperature of the rubbing surface is extremely high due to the cutting and rubbing between the blade tip and seal coating with a linear speed as high as hundreds of meters per second[39]. The high-speed rubbing is an intermittent interaction process, during which the Al-adhesive blade tip surfers both the rubbing heating and idling cooling cycles [7]. When the temperature changes, the difference in thermal expansion coefficient and elastic modulus between the Al-adhesive transfer layer, interface reaction interlayer and TiB2 coating or Ti6Al4V blade tip will produce thermal stress at their interfaces. The existence of high thermal stress will seriously deteriorate the adhesion strength of the interface [40].
The calculation equation of the thermal stress at an interface in the elastic range is [41]:
$${\sigma _i}= - {\sigma _j}$$
4
\({\text{=}}\frac{{{E_i}{E_j}}}{{{E_i}+{E_j}}}\left( {{\alpha _i} - {\alpha _j}} \right)\Delta T\)
Where E is the elastic modulus, α is the thermal expansion coefficient, △T is the temperature cooling range, and i and j represent different materials on each side of the interface respectively [41]. As Xue [3] and Wang [32] reported that Al-hBN seal coating at the rubbing surface was melted at the linear speed as high as 300 m/s. Accordingly, it is assumed that the cooling range of △T temperature is from aluminum melting point (660 ℃) to room temperature (25 ℃), to facilitate the calculation of thermal stress. The thermal stress distribution between the reaction interlayers was calculated with the elastic modulus and thermal expansion coefficient given in Table 3.
It should be noticed that the real interface reaction interlayer is a composition gradient transition microstructure. There is no obvious interlayer interface in the interface reaction interlayer. But for the convenience of calculation and comparison, the thermal stress was calculated layer by layer and the influence of their oxidization was neglected.
Table 3
Elastic modulus and coefficient of thermal expansion of Ti, Al, Ti-Al intermetallic and TiB2 [42]
Alloy/Metal | Elastic modulus E | Coefficient of thermal expansion α |
GPa | 10− 6×K− 1 |
TiAl | 173 | 9.6 |
TiAl3 | 216 | 12 |
TiB2 | 575 | 6.7 |
Ti | 115 | 8.6 |
Al | 84 | 20.2 |
Figure 9 shows the high-speed rubbing interface reaction and thermal stress distribution model of Al-adhesive blade tips. The TiAl3 interface reaction interlayer with a thickness of less than 0.8 is formed at the interface between the Al-adhesive transfer layer and TiB2 coating. It is difficult for the thin TiAl3 interface reaction interlayer to fully release the thermal stress up to 645 MPa at the TiB2/TiAl3 interface [43, 44]. The cracks tend to initiate and propagate in the area under higher thermal stress [45]. Under such high thermal stress, the TiAl3 interface reaction interlayer together with the Al-adhesive transfer layer is apt to fall off, and the TiB2 coating will be re-exposed. Therefore, TiB2 deposited Ti6Al4V blade tip can effectively inhibit the adhesive transfer of Al-hBN seal coating during the high-speed rubbing.
As the results in the previous research [7], the Ti-Al interface reaction on the Al-adhesive Ti6Al4V bare blade tip, was carried out in two steps. First, the TiAl3 phase is formed. Second, TiAl3 reacts with Al to form γ- TiAl phase and γ- TiAl locates closer to the Ti6Al4V side as shown in Fig. 9 (b). Their thermal stress between layers gradually increases and can be effectively released [30, 31]. Therefore, the Al-adhesive transfer layer and the interface reaction interlayer are closely bound to the Ti6Al4V tip surface and the Al-adhesive transfer layer can reach a thickness about several hundred microns [7].
|
Figure 9 Model of interface reaction and thermal stress distribution (a) TiB2 deposited Ti6Al4V blade tip (b) Bare blade tip |