Strength interfacial

A reasonably close match of thermal expansion of the coating and substrate over a wide temperature range to limit failure caused by residual stresses is desired for coatings. Because temperature gradients cause stress even in a weU-matched system, the mechanical properties, strength, and ductUity of the coating as well as the interfacial strength must be considered. 

LI. I. The meaning of adhesion, adhesive failure and interfacial strength... 

It must also be recognized that adhesive interfaces are not static entities, but may deteriorate or even strengthen over time, and often it is the time course of interfacial strength or durability under different conditions and in different environments that is of greatest concern [4]. As important as durability issues are, they too will not be a direct concern of this chapter. 

In a mechanical test, interfacial strength may be quantified in terms of either the minimum load required for interface disruption or the total integral energy or work expended. In many situations, due to non-uniformity of chemical or morphological conditions over the area of the interface or to non-uniformity of the applied stress in a given test [7], the two criteria are different. The investigator must thus strive to minimize or deal with both of the above complications, i.e. the interfaces studied should be chemically and morphologically uniform, and the stresses applied in the test should be uniform or distributed in way which is quantitatively describable. 

It is only in the context of the systematic variation of the properties of the adhesive and/or the adherend surface in a set of otherwise identical specimens subjected to a given mechanical testing procedure that it is reasonable to think of predicting relative interfacial strength. 

Most of the various strategies which have been proposed to predict relative adhesive interfacial strength are based on thermodynamics. One may define, without ambiguity, as shown in Fig. 3, a thermodynamic work of adhesion , Wa,... 

The two issues that are dominant in determining the interfacial strength in the case of contact adhesion are (1) the completeness and intimacy of contact between the adhesive and adherend at the interface and (2) the strength of the intermolecular interactions across the interface. Methods for predicting both of these factors are discussed below. 

Recent unpublished results from the author s laboratory, shown in Fig. 26, confirm the good correlation obtainable between mechanically measured interfacial strengths, as detailed above, and UNIFAC calculations of ( —AG )o5 for... 

Fig. 25. Relationship between the measured interfacial strength and the (negative) Gibbs free energy of mixing, (-AG )o5, for glass beads treated with various silane coupling agents embedded in a PVB matrix. Error bars correspond to 95% mean confidence intervals. Redrawn from ref. [165].

To prepare an interpenetrating polymer network (IPN) structure, PU networks having ACPA units were immersed with MMA and polymerized. PU-PMMA semi-lPN thus formed was given improved interfacial strength between PU and PMMA phases and showed flexibility with enforced tear strength [65,66]. 

In the single fiber pull out test (SFPO), a small portion of the fiber is embedded in the bulky matrix and the interfacial strength is calculated from the peak load when the fiber is pulled out of the composite. 

It is suggested that TOR having lower viscosity locates at the boundary of NR/EPDM layers, thereby increasing their interfacial strength. This was confirmed by measuring the adhesive strength (Ga) between NR and EPDM sheets with and without TOR as shown in Table 11.13. It is speculated that TOR is co-vulcanized with component elastomers, thereby increasing compatibility. 

Liu Y-N, Li M, Gu Y, ZhangX, Zhao J, Li Q, et al. The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites. Carbon. 2013 Feb 52(0) 550-8. 

Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

Broutman L.J. (1969). Measurement of the fiber-polymer matrix interfacial strength. In Interfaces in Composites, ASTM STP 452, ASTM, Philadelphia, PA, pp. 27-41. 

Scherf, J., Cohen, Y. and Wagner, H.D. (1992). Interfacial strength measurements in poly (p-phenylene benzobisthiazole)/epoxy composites. Intern. J. Adhesion Adhesive, 12, 251-256. 

Wang, S.W., Kahn, A, Sands, R. and Vasudevan, A.K. (1992). A novel nanoindenter technique for measuring fiber-matrix interfacial strength in composites. J. Mater. Sci. Lett. 11, 739-741. 

Koenig, J.L., Emadipour, H. (1985). Mechanical characterization of the interfacial strength of glass reinforced composites. Polym. Composites 6, 142 150. 

Fig. 7.1. Fibers containing deliberately produced regions of low interfacial strength are shown in the...

Thus, Kic can be used as an accurate descriptor of the interfacial strength in a film/substrate system. 

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