Overview of transient liquid phase and partial transient liquid phase bonding
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- Cook, G.O. & Sorensen, C.D. J Mater Sci (2011) 46: 5305. doi:10.1007/s10853-011-5561-1
Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented. Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side. This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.
Transient liquid phase (TLP) bonding
Transient liquid phase (TLP) bonding is a joining process that was developed to improve upon existing bonding technologies. Specifically, this process was patented by Paulonis et al. in 1971  to overcome deficiencies of then current bonding techniques in joining Ni-based superalloys [2, 3, 4, 5, 6]. TLP bonding’s main advantage is that resulting bonds have a higher melting point than the bonding temperature. This bonding process characteristically lies between diffusion bonding and brazing—for this reason, it is commonly called diffusion brazing. The process is also referred to by names such as transient insert liquid metal bonding  and is sometimes mistakenly referred to as diffusion bonding (which by definition relies solely on solid-state diffusion). See reference  for a detailed history of TLP bonding and its many names.
TLP bonding process
setting up the bond
heating to the specified bonding temperature to produce a liquid in the bond region
holding the assembly at the bonding temperature until the liquid has isothermally solidified due to diffusion
homogenizing the bond at a suitable heat-treating temperature.
thin foil (rolled sheet) [2, 3, 4, 7, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62]
evaporating an element out of the substrate material to create a “glazed” surface .
Fixturing pressures used during TLP bonding
The bonding process is usually confined in a vacuum [3, 4, 5, 7, 12, 14, 15, 16, 17, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 38, 39, 40, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 61, 62, 63, 65, 66, 68, 69, 70, 71, 72, 76, 77, 78, 79, 80, 81, 82, 83, 86, 87, 88, 93, 94, 95, 96, 97, 98, 99, 100, 103, 104, 105, 108, 110, 112, 113, 114, 115, 117, 119, 122, 123, 124, 125, 126, 127, 130, 132, 133, 134, 135, 136, 137, 138, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157], although an inert atmosphere, such as argon, can be used [6, 11, 14, 32, 33, 43, 45, 60, 67, 74, 75, 90, 111, 121, 134, 158, 159]. On rare occasions, TLP bonding is performed under a different atmosphere, such as nitrogen , hydrogen , nitrogen and hydrogen , or open air . The vacuum pressures used in the experiments referenced above are normally distributed about 0.1 μmHg (millitorr) with minimum and maximum values of 0.00015 and 34 μmHg, respectively.
TLP bonding kinetics
melting of the interlayer
dissolution of the substrate material
homogenization of the bond region.
Concentration profile 1 (CP1) in Fig. 1 shows the TLP bonding setup at room temperature. The interlayer element (i) is sandwiched between two pieces of the substrate material element (s). The thickness of the bond region in Fig. 1 has been exaggerated to display changes in the concentration profile. The interlayer can be composed of a single element, an alloy, or a multi-layer combination of elements and/or alloys.
Interlayer thicknesses for TLP bonding
Thickness range (μm)
Common thickness(es) (μm)
[5, 8, 10, 20, 21, 22, 23, 24, 25, 34, 35, 39, 44, 48, 54, 59, 60, 61, 62, 63, 66, 68, 74, 75, 76, 78, 79, 80, 82, 87, 89, 105, 107, 110, 117, 118, 120, 121, 122, 123, 126, 131, 134, 137, 141, 142, 143, 145, 146, 147, 150, 152, 155, 166, 167, 168, 169, 170, 171]
[2, 5, 20, 26, 27, 32, 33, 36, 37, 40, 42, 46, 51, 52, 55, 56, 57, 58, 59, 60, 64, 69, 70, 71, 77, 78, 81, 83, 84, 87, 88, 90, 101, 115, 119, 124, 125, 126, 127, 128, 135, 136, 145, 148, 149, 153, 155, 157, 166, 167, 172]
As the bond assembly is heated, the interlayer begins to diffuse into the substrate materials (CP2). The amount of diffusion that occurs is dependent upon the interdiffusion coefficient between the substrate and interlayer materials as well as the heating rate.
Upon reaching the interlayer element’s melting point (CP3), the pure portion of the interlayer liquefies (L). Heating of the bond region continues until the bonding temperature has been reached. The bonding temperature is usually well above the interlayer’s melting point to ensure complete melting of the interlayer and to increase the rate of diffusion (see Optimal bonding temperature).
During heating past the melting point, the concentrations of the liquid region follow the solidus (c4,S) and liquidus (c4,L) lines of the phase diagram (CP4). This causes the liquid region to melt back, or dissolve, the substrate material to conserve mass. The movement of the solid–liquid interface continues until the bonding temperature has been reached (CP5a); at this point the liquid has attained its maximum width and has consumed some of the diffused solute. The amount of melt-back is dependent upon the solidus (c5,S) and liquidus (c5,L) compositions for the given material system at the bonding temperature (see Optimal bonding temperature). The main two effects that lower melt-back distance are (1) significant diffusion of the interlayer material into the substrate before melting (see Critical interlayer thickness) and (2) loss of liquid due to wetting of the substrate’s sides  or a high bonding pressure that squeezes liquid out [22, 44, 62, 134, 165].
Many materials that are joined by TLP bonding have carefully designed microstructures to achieve certain mechanical properties. Too much melt-back of the substrate by the liquid interlayer can have detrimental effects on the final bond in addition to lengthening the isothermal solidification time (see Critical interlayer thickness). And, in some systems melt-back can reach five to fifteen times the original interlayer thickness [43, 121, 146]. To prevent drastic melt-back that can adversely affect the microstructure, the interlayer should be thin [57, 62], of a eutectic composition , or of a composition similar to the substrate material .
After the liquid interlayer has reached its maximum width, the interlayer material diffuses into the substrates at a rate somewhere between the diffusivity of the liquid and solid [25, 43, 85, 163]. As this diffusion occurs isothermally, the liquid region contracts (CP5b) to conserve mass as the solidus and liquidus concentrations are now fixed. Isothermal solidification occurs until all of the liquid has disappeared (CP5c). At this point, the TLP bonding process can be stopped if desired . The bond already has an elevated remelting temperature (T5) compared to the melting temperature of the interlayer (T3).
In most cases TLP bonding is continued in order to homogenize the bond. This can be an extended time in the same heating apparatus or a post-bond heat treatment applied at some other time . Furthermore, if the substrate material’s microstructure is extremely sensitive, this stage can be conducted at a lower temperature [2, 9]. In either case, the bond undergoes homogenization for some predetermined time which causes smoothing of the solute peak (CP5d) that remained at the end of isothermal solidification (CP5c). The resulting remelting temperature of the bond in this case is TR,P. If the peak concentration (c5,P) is within the room-temperature solid-solubility limit of the binary system, the precipitation of strength-reducing intermetallic compounds upon cooling will be avoided [12, 110, 147, 150, 160].
If the bond is homogenized for a sufficient amount of time, there is no gradient in the concentration profile (CP5e) and the bond’s remelting temperature is even higher (TR,F). However, despite the increases in bond remelting temperature that can be achieved by complete homogenization, an adequate homogenization time is usually determined by a sufficiently high bond strength [2, 24, 26, 42, 83, 103, 122, 123, 146, 150] or economic considerations that limit furnace time [77, 125, 173]. Nonetheless, the bond’s remelting temperature is often hundreds of degrees (°C) above the melting point of the interlayer and can be about 1000 °C higher if refractory metals such as Ir, Mo, Nb, Os, Re, Ta, or W are used as the substrate or if low melting-point metals such as Al, Ga, In, Mg, Pb, Sb, Sn, or Zn are used as the interlayer.
Time frame of TLP bonding
Heating to the bonding temperature, CP1–5a: less than a minute to about an hour; dependent on the method of heating, the heating rate of the heating apparatus, and the substrate material’s thermal properties
Isothermal solidification, CP5a–5c: minutes [7, 25, 36, 37, 56, 74, 77, 95, 96, 105, 121, 125, 153] to hours [7, 11, 33, 36, 37, 43, 57, 58, 105, 146, 157, 175], although it can occur in less than a minute [73, 161] or take more than a day 
The general trend is that initial melting of the interlayer occurs an order of magnitude faster than melting back of the substrate, which occurs an order of magnitude faster than isothermal solidification, which occurs an order of magnitude faster than complete homogenization. Isothermal solidification ends up being the limiting, or controlling, time in producing a successful TLP bond [12, 36, 104, 125, 135, 138, 166, 167]. While the homogenization stage takes longer if carried to completion, it rarely is. As previously stated, homogenization can be performed during a subsequent heat treatment or skipped in some cases; it can even occur once the part is in service.
The foregoing explanation of TLP bonding kinetics also applies to eutectic systems when the interlayer is a eutectic composition alloy. The kinetics are slightly different (and more importantly, TLP bonding takes much longer) for a eutectic system when pure elements are used. (See references [19, 44] for particulars of eutectic system kinetics). Tuah-Poku et al.  reported a drastic decrease in time to isothermally solidify by changing the interlayer: 200 h when using the pure element as compared to 8 h when using the eutectic composition. This occurs because (1) the interlayer has to undergo a certain amount of solid-state diffusion with the substrate at the bonding temperature before any liquid appears and (2) melt-back of the substrate is then greater.
Critical interlayer thickness
During initial heating, the interlayer element diffuses into the substrates. The magnitude of diffusion depends upon the specific material combination, but all solid-state diffusion rates increase as the temperature rises. Depending on the heating rate and the thickness of the interlayer, the amount of diffusion can significantly decrease the interlayer’s width. In fact, for a combination of high diffusion rate, slow heating, and/or thin interlayer, it is possible to diffuse all of the interlayer material into the substrate before reaching the interlayer melting point [114, 173], although this is a rare occurrence. Because TLP bonding requires the formation of a bulk liquid phase [9, 109, 114] to create a consolidated, void-free bond while also increasing diffusion rates, the interlayer must exceed a minimum, or critical, thickness [105, 114].
In addition to the parameters listed above, the critical interlayer thickness has been shown to depend on other variables such as applied clamping force, solid/liquid surface tension, surface roughness of the substrate, and intermetallic formation [18, 94, 114]. In short, experiments must be conducted for each material combination to empirically reveal its critical interlayer thickness.
On the other hand, analytical models of TLP bonding indicate that the isothermal solidification process time is roughly proportional to the square of the interlayer thickness [9, 18, 19, 25, 36, 44, 86, 104, 110, 114, 124, 138, 161, 173, 175, 176]; experimental data often corroborates this trend [3, 26, 43, 44, 71, 86, 88, 153]. Therefore, to minimize bonding time, an interlayer slightly thicker than the critical thickness is ideal.
Optimal bonding temperature
The bonding temperature is sometimes completely limited by the microstructural stability of the substrate material [7, 9, 87, 125]. If, however, the substrate material allows flexibility in selecting an optimized bonding temperature, a minimum isothermal solidification time (and therefore bonding time) can be achieved at a certain temperature.
If phase diagram and diffusion data are available for the material system in question, the isothermal solidification time can be characterized with respect to temperature. However, it is usually the case that experiments are the only way to discover this relationship. In general, the relationship is parabolic, yielding a minimum isothermal solidification time at a given intermediate temperature (between the melting points of the interlayer and substrate materials) [9, 18, 43, 50, 78, 79, 159, 162]. And yet, in some cases the variables of the system yield either (1) a monotonically increasing time, in which case the optimal bonding temperature is just above the interlayer’s melting point, [6, 9, 43, 114, 158, 162] or (2) a monotonically decreasing time, in which case the optimal bonding temperature is as high as the substrate material allows [4, 104, 114, 135, 141].
Systems a and b in Fig. 3 have the same convex-shaped liquidus line and therefore experience the same amount of melt-back (see the top concentration profiles). The same is true for the concave-shaped liquidus line in systems c and d. Systems a and c have the same partition coefficient (0.9). The same is true for systems b and d (0.4).
If the substrate material has a sensitive microstructure that could be damaged by significant melt-back, then phase diagrams such as systems c and d should be avoided. Systems b and d will take much longer to isothermally solidify, raising operating costs.
Systems a and c are quite similar in rate of isothermal solidification. Because the solidus composition of system c is closer to the completely homogenized composition (shown as an x on the gray line), homogenization of the solute peak after isothermal solidification will likely proceed more rapidly than in system a. However, because the solidus line of system a has a convex shape, increases in bond remelting temperature due to homogenization will likely occur faster and be larger in this system.
Modeling of TLP bonding
Analytical models have been developed by many researchers for the four stages of TLP bonding to provide quick estimates or general trends, such as those illustrated in the previous section. Equations for and descriptions of TLP bonding analytical models are included in references [8, 19, 162, 178]. Assumptions made for these models are similar to those made in this article (see TLP bonding kinetics). In some cases these equations provide good results, but for many systems these simplified, binary-system approaches do not supply accurate estimates [5, 8, 38, 78, 79, 86]. This is due in part to the diffusion coefficients being assumed independent of composition.
Some complexities of TLP bonding are quite difficult to model. For example, grain boundaries can cause isothermal solidification to occur at a different rate than that predicted by analytical models using a bulk diffusion coefficient [25, 36, 37]. Indeed, grain boundary diffusion is faster than bulk diffusion in a certain temperature range (based on the alloy’s melting point) . Grain boundary diffusion rates also increase as the substrate material’s grain size decreases . Further, grain boundaries can be penetrated by the liquid to cause a non-planar solidification front, thereby increasing the area over which diffusion occurs [5, 11, 18, 138]. See references [180, 181] for more information on the effect of grain boundaries in TLP bonding.
Another interesting deviation is that isothermal solidification can occur in two different “regimes” [38, 50, 138]. The faster solute element of a multi-component interlayer controls the rate of solidification for the first regime. Then, a second solute element controls the rate of solidification during the second regime, resulting in complex concentration–time profiles.
Numerical models can account for some of the complexities of TLP bonding to accurately predict bonding kinetics [7, 8, 161, 162, 168, 173, 176, 179, 182, 183] and can even be extended to multi-component systems [174, 184]. Despite the complexities and extra time required in numerical modeling, especially for multi-component systems, the limiting factor is most often the lack of necessary diffusion data [7, 8, 178]. But, when the necessary data is available, modeling of TLP bonding can drastically reduce the number of experiments required to determine optimal bonding parameters [37, 162, 166].
Advantages and disadvantages of TLP bonding
The most distinctive advantage of TLP bonding is that the resulting bond can operate at the bonding temperature or higher temperatures. In other words, materials can be bonded at a temperature equal to or lower than what the assembled part will experience in service. This is especially important for temperature-sensitive materials whose microstructures can be damaged by too much thermal energy input  and therefore need to be joined at lower temperatures.
Another advantage is that the resulting TLP bonds often have microstructural, and therefore mechanical, properties similar to the properties of the base materials [7, 12, 13, 14, 24, 32, 42, 49, 61, 62, 75, 77, 81, 95, 105, 109, 115, 119, 123, 141, 149, 153, 166, 175]. In fact, in some cases the bond area becomes indistinguishable from other grain boundaries [18, 35, 37, 68, 108, 109, 130, 185] due to significant diffusion at high temperature. Such bonds are often as strong as the bulk substrate material [14, 164], or stronger, causing the joined assembly to fail in the substrate material rather than in the bond [14, 31, 71, 76].
the process is highly tolerant to the presence of a faying surface oxide layer [2, 6, 7, 11, 13, 21, 42, 47, 48, 56, 67, 80, 93, 136, 147, 148, 149, 186] and therefore requires less joint preparation and no fluxing agents [11, 18, 42, 173, 187]; in a few rare cases surface oxides are actually beneficial to the process 
For some material systems, bond properties and performance capabilities that are difficult or impractical to achieve using conventional joining methods are more accessible .
See reference  for examples of specific difficulties that occur in TLP bonding applications. Although many disadvantages of TLP bonding can be overcome by optimized bonding parameters, the optimization process often requires much experimentation.
Applications of TLP bonding
A spectrum of materials joined by TLP bonding
NB 30, NB 150, BNi-3, MBF-60, MBF-80, DF-3
Ni–B, Ni–Cr–Si–Fe–B, MBF-80
F20, F24, F25, F26, F27, MBF-80
F20, F24, F25, F26, F27, MBF-80
Ni–Ge, Ni–Mn, Ni–Mn–Si, D-15
Ni–15Cr–11.5Al–3 W–0.2Hf–0.1Si–0.1Mn (γ/γ′/β type)
MBF-80, Ni–Cr–B–Ce (various combinations)
Ag, Al–Si, BAg-8
Ni–Cr, 304L SSc, BNi-2
Cu, Fe–B–Si, Ni–Si–B, MBF-20, MBF-30, MBF-35, MBF-50, MBF-80
Ni–B–Cr–Si (various combinations)
Fe–B–Si, BNi-1a, BNi-3
Low carbon steel
ODSc steel (Fe–Cr–W–Y2O3–Ti)
Fe–B–Si, Fe–Ni–Cr–Si–B, BNi-2
Cu–Ti, Ni–Ti, Ti–Cu–Zr
Co alloy (unspecified)
Cu | Sn | Cu
Cu | Sn | Cu
Sn–Bi, Bi–Sn (various combinations)
IC 6 (with and without B)
PWA 1483 (Ni–Cr–Co–Ta–Ti–W–Al–Mo)
Ni (glaze), BNi-3
Ti | Cu, Ti | Ni, Ti | Fe
Ti–45Al–2Nb–2Mn (a) + 0.8 vol.% TiB2
Ti–Cu–Ni, Cu–Ni | Ti | Cu–Ni
γ-TiAl [Ti–47Al–2Cr–2Nb (a)]
Cu, Cu & Ti–Al–Cr–Nb, Cu & TiAl
Gamma Met PX
Cu & Gamma Met
Ag, Cu, Ga, Al–Cu, Al–Si–Cu
Au–Sn, Sn, In, Ti | In
Ag, Sn, Ag–Cu, BiIn, BiIn2, BiSn, InSn, NB 51
Sb, Fe–P, Fe–B
Ti, Zr, V
B, Cu, Hf, BNi-3, BNi-6, MBF-60, MBF-80
Metal matrix compositesd
Ag, Cu, Al–Cu, Cu–Ti
Haynes 230 doped with B
Al, Al & SiO2, B2O3
Mar-M247 (directionally solidified)
Cu–Cr–Zr and Cu (ODSc)
Cu | Sn | Cu
Inconel 738 and 939
BNi-3, Niflex-110, Niflex-115
M963 (Ni–W–Co single crystal)
NiAl-Hf (single crystal)
Cu, NiAl & Cu, Ni3Al & Cu
Low carbon steel
TS7 (Ti alloy)
5VMTs (Nb alloy) with W, Mo, & Zr; and TV10 (Ta alloy) with W
Ti–45Al–2Nb–2Mn (a) + 0.8 vol.% TiB2
Metals to Metal matrix compositesd
Metals to Ceramics
Ni–Si | Mo
ODSc Fe alloy (Fe–Cr–Al–Y2O3)
W18Cr4 V tool steel
Ti(C,N) (50%TiC & 50%TiN)
Metal matrix compositesdto ceramics
Variants of TLP bonding
Wide-gap TLP bonding: gaps of 100–500 μm can be bonded or repaired by the use of a melting and a non-melting constituent (multiple layers or mixed powders) [7, 16, 57, 92, 94, 95, 96, 100, 101, 136, 149, 173, 195]. This technique can also be used in conventional TLP bonding to accelerate isothermal solidification [13, 99, 140]
Active TLP bonding: a ceramic and metal can be joined by a multi-component interlayer; at least one constituent reacts with the ceramic while another diffuses into the metal to cause isothermal solidification [28, 42, 52, 54, 116, 132, 196]
Partial TLP bonding (see next section).
Bonds made using temperature gradient, wide-gap, and active TLP bonding have been included in Table 3.
Partial transient liquid phase (PTLP) bonding
Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding mainly used to join ceramics. PTLP bonding overlaps both wide-gap and active TLP bonding, although articles defining PTLP bonding predate the other two techniques by a few years. Many advantages of conventional TLP bonding carry over to PTLP bonding . The ensuing sections focus on how PTLP bonding differs from TLP bonding.
PTLP bonding process
A spectrum of materials joined by PTLP bonding
Cr | Cu | Ni | Cu | Cr, Cu | Nb | Cu, Cu | Ni | Cu, Cu | Ni–Cr | Cu, Cu | Pt | Cu, In | Ag ABAc | In, In | Cusil ABAc | In, In | Incusil ABAc | In, Ni | Nb | Ni, Ti | Al | Ti
Au | InBi | Au
Al | Ti | Al, Au | Ni–Cr | Au, Cu–Au | Ni | Cu–Au, Co | Nb | Co, Co | Ta | Co, Co | Ti | Co, Co | V | Co, Cu–Au–Ti | Ni | Cu–Au–Ti, Cu–Ti | Pd | Cu–Ti, Ni | Ti | Ni | Ti | Ni, Ni | V | Ni, Ti | Au | Cu | Au | Ni | Au | Cu | Au | Ti, Ti | Cu | Ti, Ti | Cu | Ni | Cu | Ti, Ti | Ni | Ti, Ti | Ni | 304SSc | Ni | Ti, Ti | Ni | Kovarc | Ni | Ti, V | Co | V
C | Si | C, Cu–Au–Ti | Ni | Cu–Au–Ti, Ni–Si | Mo | Ni–Si, Ti | Au | Cu | Au | Ni | Au | Cu | Au | Ti
Zn | Pd | Zn
Al | Ni | Al, Ni | Nb | Ni
Ni | Nb | Ni
Al / SiC
Cu | Ni | Cu
Si3N4 / TiC
Ti | Ni | Ti
Al 6061 / Al2O3
Cu | Ni | Cu
C / C
Ti | Ni | Ti
Metals to ceramics
FA-129 (Fe3Al alloy, Fe–Al–Cr–Nb)
Cu–Ti (ABAc) | Cu | Cu–Ti (ABAc), Cu–Ti | Cu | Ni | Al
Ni | Ti | Ni
Ti | Ni | Ti
Ti | Cu | Sn | Au | Cu
The refractory core tends to be a foil that is 20–30 μm [143, 207, 208, 209, 210, 211, 212, 213, 214] or 100–127 μm thick [116, 188, 189, 201, 207, 211, 212, 213, 215, 216, 217, 218, 219, 220, 221, 222], although it can be in the 200–1000 μm range [202, 203, 204, 205, 208, 223, 224, 225]. The refractory core element is often Ni [144, 189, 196, 201, 202, 203, 204, 207, 208, 209, 210, 218, 224, 226, 227]; other elements (and an alloy) that have been used include Au, Co, Cu, Nb, Ni–Cr, Pd, Pt, Si, Ta, Ti, and V [117, 188, 189, 200, 201, 202, 205, 206, 207, 208, 214, 215, 216, 217, 218, 222, 227, 228, 229]. The thin layers can be most of the formats used for TLP bonding interlayers (see TLP bonding process) and are often in the 1–10 μm thick range. The ratio of the thin layer thickness over the refractory core thickness is usually 1–5% [143, 188, 189, 201, 202, 203, 204, 207, 208, 211, 212, 213, 215, 216, 217, 218, 220, 222, 223, 224, 225, 230, 231], although it can be 6–20% [207, 210, 211, 212, 213, 219, 221, 223], and some PTLP bond experiments have utilized a ratio of 50% or higher [116, 144, 208, 214, 226].
PTLP bonding kinetics
The second and fourth assumptions highlight the major differences between TLP and PTLP bonding. First, the multi-layer interlayer used during PTLP bonding has been termed “self-contained”  because the liquid phases must diffuse into the rc, rather than the much larger substrate materials, to induce isothermal solidification. Second, the liquid phases must wet the ceramic substrates to create a strong bond. This tends to be difficult due to the chemical inertness of ceramics [117, 180, 196, 216] and usually requires the use of active elements such as Al, Cr, Hf, Nb, Ni, Sc, Ta, Ti, V, or Zr [65, 117, 187, 189, 198, 200, 201, 202, 206, 215, 216, 218, 222, 228, 232, 233]. Also, when analyzing the critical interlayer thickness of the thin layers, a portion of the liquid that forms from those thin layers will react with the ceramic substrate and add to the critical thickness.
The PTLP bonding setup at room temperature is shown in Figs. 5 and 6 as concentration profiles 1A and 1B, CP1A and CP1B, respectively. Both binary systems exhibit complete solid solubility. As the temperature of the bond is raised to the melting points of each thin layer (T4 for tlA and T3 for tlB), both thin layers diffuse into the rc (see CP2A and CP3A as well as CP2B). Despite the small amount of liquid that initially forms from tlB due to its high diffusivity (CP3B), the liquid drastically melts back the rc on further heating (CP4B) due to the concave shape of the liquidus. This melt-back continues until the assembly is heated to the bonding temperature (T5) shown in CP5aB. On the other hand, the liquid formed from tlA (CP4A) widens slightly to be about the same width as the original thin layer (CP5aA) due to that system’s convex liquidus.
At this point, isothermal solidification occurs on both sides of the multi-layer interlayer. It proceeds much faster for tlB due to its high partition coefficient and diffusivity. In fact, isothermal solidification is complete for tlB (CP5bB) when the other liquid region has only solidified about halfway (CP5bA), despite the considerable melt-back of the rc.
The liquid formed from tlA eventually solidifies isothermally (CP5cA). On the other side of the bond, the solute peak has been smoothed due to homogenization (CP5cB), and the remelting temperature on that side has increased to TR,P.
Further homogenization causes the remaining gradient in the tlB element to disappear (CP5dB), thereby raising the remelting temperature of the bond next to substrate B to its final value, TR,F. A similar melting temperature increase (to TR,P1) simultaneously occurs on the other side of the bond due to smoothing of its solute peak (CP5dA).
Prolonging the homogenization process continues to raise the remelting temperature of the left side of the bond. However, once its remelting temperature has reached TR,P2 (CP5eA), which is higher than TR,F for the right side of the bond, further homogenization will have little effect on raising the bond’s remelting temperature. From an optimization standpoint, homogenization should be stopped at this time. However, real-world considerations usually determine the homogenization time, which can be less than—or greater than—the optimized time due to various factors, such as cost, microstructural considerations, or resulting bond strength.
The time frame of PTLP bonding is very similar to that of TLP bonding. Isothermal solidification and homogenization times for TLP bonding depend on high-diffusivity elements diffusing into “infinite” substrate materials. In PTLP bonding, the elements tend to have lower diffusivities, but the maximum diffusion path is on the order of 100 μm, resulting in similar bonding times.
Advantages and disadvantages of PTLP bonding
because diffusion occurs on a smaller scale (on the order of 100 μm), bonding using slow-diffusing elements occurs in a reasonable amount of time.
matching the thermal expansion coefficients of the ceramic substrates and metallic interlayer elements is sometimes necessary to prevent thermally induced stresses and cracking [117, 143, 188, 196, 203, 215]
However, most disadvantages of PTLP bonding can be overcome by proper design. In the end, the limiting factor is wettability on the specific ceramic material.
Applications of PTLP bonding
TLP bonding is a relatively new bonding process that results in a bond with a higher melting temperature than that used to join the materials. Specific details of this process, including experimental details, process kinetics, and optimal bonding temperature, have been outlined in this article. Also, the broad range of materials that have been joined by TLP bonding was presented.
PTLP bonding, a more recent variant of TLP bonding used to bond hard-to-join materials, was also outlined. PTLP bonding has been successful in joining a smaller range of materials, most notably, ceramics.
Both TLP and PTLP bonding are specialized joining processes that require more resources to implement compared to typical bonding processes. However, in some cases these bonding processes are the best—or only—way to join materials for specialized applications.
A numerical model was developed to calculate one-dimensional solid-state diffusion in conjunction with a liquid region that expands or contracts, assuming infinite diffusivity for the liquid region. The model also accounts for a heating period and diffusivity data as a function of concentration and temperature. Diffusivity data along with solidus and liquidus profiles for a hypothetical binary system were used to output concentration profiles that were the basis for the concentration profiles in Figs. 1, 3, 5, and 6.
This study was funded by the Office of Naval Research under grant number N00014-07-1-0872, Dr. William Mullins, Program Officer.