Metallic adhesion strength

The curve Da(Gz 1) is limiting, because it corresponds to process parameters, which provide the minimum necessary level of adhesive strength. The area below this curve (at a definite mold temperature Tm) corresponds to the Da and Gzn Numbers, for which the degree of conversion is less than the critical level these parameters do not provide the maximum output. Points above this curve, represent inadmissible process parameters, which do not provide the required level of polymer-metal adhesive strength. 

BeO substrates are available with thin-film (TiW/Ni/Au) and refractory (MoMn) metallizations. Metallization-adhesion strengths are generally lower with refractory metallization 134 MPa (tape-cast) and 115 MPa (dry-pressed) vs. 160 MPa (tape) and 150 MPa (pressed) observed with thin-film metallization [42]. BeO can be brazed to a variety of heat sink, lead, and seal ring materials, including both metals and ceramics. 

Conductive Adhesives. Electrically conductive adhesives are used today for specialized applications such as connections to LCD displays and attachment of small resistors and capacitors. These materials consist of conductive particles, usually silver flakes or carbon, suspended in a polymer matrix, most commonly epoxy.The electrical resistance of the contact to the PCB tends to be unstable over time, so these materials are not suitable for applications requiring a constant, low-resistance contact. The primary failure mechanism is moisture migration through the epoxy to the interface, resulting in oxidation of the contact metal. Adhesion strength is also a reliability concern. New materials suitable for a broader range of applications are under development. Further information can be found in Ref 39. 

Waterborne contact adhesives contain an elastomer in latex form, usually an acryflc or neoprene-based latex, and a heat-reactive, cross-linkable phenohc resin in the form of an aqueous dispersion. The phenoHc resin improves metal adhesion, green strength, and peel strength at elevated temperature. A typical formulation contains three parts latex and one part phenohc dispersion (dry weight bases). Although metal oxides may be added, reaction of the oxide with the phenohc resin does not occur readily. 

Adhesive strength is evaluated at room temperature as well as at the extreme temperatures of —65°F and 180°F. Aircraft structure can reach —65°F at cruise altitudes and 180°F on the ground in a hot, sunny location. The types of toughened epoxies commonly used for metal bond adhesives have glass transition temperatures not much greater than 200°F, so properties fall off drastically at higher temperatures. 

Since the m,m -DABP polyimide is known to be an outstanding adhesive, lap shear strength tests employing titanium-titanium adherends and metal ion filled polyimides were conducted. Tests were performed at room temperature, 250°C and 275°C employing either DMAC or DMAC/Diglyme as the solvent. At room temperature regardless of the metal ion employed adhesive strength is de-... 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

This review will first present the metal/epoxy resin adhesive system and discuss how the presence of metal oxides influences adhesion secondly, present the reasons why these systems exhibit very good adhesion strength under dry conditions and why this adhesion strength is greatly reduced in the presence of water thirdly, examine possible methods of increasing their durability in wet environments and finally, discuss some of the spectroscopic techniques that are currently being used to aid in the advancement of metal/polymer adhesion technology. 

High initial adhesion strength between epoxy resins and metals is readily obtainable as long as surface contamination and weak oxide layers have been removed from the... 

All adhesion scientists will agree that water is a very destructive environment for metal/polymer adhesion systems (see Fig. 5). Since water is one of the most commoi environments encountered, the effectiveness of metal/polymer coating and structural bonding systems has been severely limited by this great loss of adhesion strength in the presence of water. 

Water can reduce adhesion strength by reducing the strength of the metal oxide layer via hydration52,81 . Hydration of the oxide layer is detrimental because the resulting aluminum-, iron-, or other metal-hydrates generally exhibit very poor adhesion to their base metals 52 Therefore, the produced layer of hydrates will effectively act as a weak boundary layer in the system and decrease adhesion strength. Since the hydration reaction has been most heavily studied on aluminum oxides, the authors have chosen to base the discussion of the hydration mechanism on this case. 

Locus of failure studies 75 80) on metal/epoxy joints that had been exposed to water indicate that corrosion of the metal substrate does not occur until after interfacial failure has occurred. This suggests that corrosion itself does not play a primary role in the loss of adhesion strength mechanism of metal/epoxy joints, but rather is a post-failure phenomenon. However, for the case of metal/epoxy protective coating systems, Leidheiser and coworkers 88-91 -92) and Dickie and coworkers 5 87-89-90> have proposed that localized corrosion processes are part of an important delamination mechanism. 

In the past, the detrimental effect of internal stresses on adhesion strength has often been ignored. Conversely, in a few cases, the presence of internal stresses has been proposed as the primary reason for adhesive failure96 "-104-108) authors believe that internal stresses can play an important role in reducing the adhesion strength of metal/epoxy systems and that the theories developed by Croll 96 102) and Shimbo et al.99) should lead to an increased awareness of this fact. In addition, further work must be completed in this area so that the usefulness of these theories and the effect of internal stresses on metal/polymer adhesion systems can be successfully analyzed. 

In Section 4.2, the strength loss mechanisms of metal/polymer adhesion systems in the presence of water were discussed. From this discussion it is evident that high initial adhesion strength is not the only important property of these systems. Actually, if a metal/polymer adhesion system is exposed to humid environments, it is more important for the system to exhibit good durability. 

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