Surface treatment lapping

Surface treatment Lap shear (MPa) Ratio element to carbon ... 

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

The lap shear test involves measuring the adhesive shear strength between two surface fluorinated polyolefin sheet tokens that are adhesively secured with a reinforcement resin. The tokens are individually reinforced with steel backing plates to eliminate flexural distortion in the shear joint. Lap shear tests carried out with various reinforcing polyester-type resins, contrasting fluorination and oxyfluorination as surface treatment, are shown in Table 16.8. 

In order to understand the aim of surface treatments for composite bonding, we wiU concentrate on a real composite bonding problem for aeronautical purposes. Since classical surface treatments like peel ply can be limited by a cohesive failure occurring in the material, we wiU focus on a new kind of surface treatment (excimer laser) which can completely change surface parameters. The different aspects are presented in two steps the first consists in the surface characterization of the composite material and the second is related to results of destructive single lap shear tests of composite assemblies. Finally, both steps are Hnked in order to derive general mles on phenomena governing adhesion properties of polymer composites. 

Previous work performed on our composite materials [16] provided clear evidence of the influence of surface pollution prior to the use of any surface treatment on adhesion behavior. Several surface treatments can be performed to remove this surface pollution and increase single lap shear performances. As described in Section 20.1.2, peel ply treatment acts very specifically on composite materials. As several authors have already described [3, 5, 17] removing the peel ply before the application of the adhesive provides a rough (Fig. 20.2) and clean surface, free of pollution from the manufacturing process. 

The PPQ adhesive system made up of PPQ adhesive resin, dilute PPQ resin primer, and chromic-acid anodized surface treatment was selected based upon the data generated from this study, as well as from internal Boeing research programs. Lap shear data in Fig. 2 shows the phosphoric-acid anodized surface treatment superior to the chromic-acid anodized. However, Boeing data on both these surface treatments have shown that phosphoric-acid anodize is not stable in long-term (125 hr) elevated temperature (600 F) environments. Chromic-acid anodize exhibited good thermal... 

HR602 ws not selected because of its low lap shear properties with all metal surface treatments. Data generated on this adhesive for this program was very similar... 

It is strongly recommended that lap-joint samples are prepared and tested to control the bond quality (for test methods see Chapter 7). The samples shall have identical surface treatments to the actual bond surfaces and shall be cured identically to the actual bond. 

For thick adherend lap shear test specimens a surface treatment was performed. The aluminium adherends were 10 mm thick and 25 mm wide. After the specimens were cured and postcured, the adherends were cut to form a test area with an 8 mm lap length. 

Table 2 shows typical advancing contact angle measurements for a number of polymers, both before and after plasma treatment. Note that in all cases there is a significant improvement in wetting. Table 3 shows typical values for lap-shear bonding of polymers before and after plasma surface treatment. These data show the great improvement that can be obtained, and the reasons why plasma treatment is used on polymers. 

Fig. 2.17. Strengths of some single lap joints of aluminium bonded with an epoxy-polyamide adhesive on exposure to air at 43°C and 90% relative humidity for various times. Metal surface treatments are etching in chromic-sulphuric acid, O alkaline etching, solvent degrease, and phosphoric acid etching, (after Butt and Cotter ).

Dynamic mechanical thermal analysis (DMTA) has also been used by Brinson, et al.,t l to determine the suitability of the technique for evaluating damage in the adhesive bond from the viscoelastic properties of bonded beams and for evaluating the effects of various environmental conditions and various surface treatments. The authors considered that if the bond becomes damaged (either adhesive and/or interphase) due to excessive load, fatigue, moisture, or corrosion, it would seem likely that dissipation mechanisms or loss modulus and tan 5 would change. Therefore, they used DMTA to measure the viscoelastic properties of beams with simulated flaws and beams taken from lap specimens, which had been exposed to humidity and/or corrosion for extended periods. 

The single lap joint test is the most widely used test method for durability studies. Its popularity lies in its relative simplicity and perceived ease of interpretation. This test method is sensitive to changes in adhesives and surface treatments in joint combinations. Small differences in joint conditions can give rise to variability. 

Degradation rate in tensile butt joints is more sensitive to the bond area when compared with the lap shear test, therefore, smaller diameter joints allow a more rapid indication of durability. The absence of distortion and the general uniformity of fracture surface provide a good facility for analysis of the locus of failure. This makes this type of test particularly suitable for detailed analytical studies on specific surface treatments. 

In the case of bonded composite parts, the standard process inspections are incomplete, in the sense that only lap-shear coupons are mandated. There is rarely any use of a peel test to ensure that the adhesive is stuck properly. In the author s opinion, based on years of observation of manufacturing and in-service experience, there ought to be because there is no other way of discontinuing the use of surface treatments that have been found to be inadequate. This issue is discussed by Hart-Smith (2003). 

In a pilot study, it was discovered that an ultraviolet zone (UVO) based method, which has been developed for surface treating wool fibers, could be used to oxidatively modify polymer surfaces. Electron spectroscopy for chemical analysis (ESCA) and contact angle results indicated that the treatment was effective on PE and a polyetheretherketone (PEEK). It produced changes in surface oxygen chemistry and free energy, which increased polarity and improved wettability of the surface. Composite lap shear tests showed that the treatment gave a marked improvement in adhesion and that an optimum joint strength is achieved at low treatment times (<1 min). 

As stated earlier, adhesion is a major concern in electronic applications involving thin polyimide films either coated on hard substrates or laminated with metal ribbons. In these cases, neither lap-shear nor die-shear techniques allow the determination of the adhesion strength this can be done by using either the 90° peel test or the island blister test whose principles are sketched in Figs 12.26 and 12.27. The 90° peel test provides reliable data for the measurement of practical adhesion, especially useful for comparing the effect of surface treatments on the interfacial adhesion. The standard peel test procedure has been modified to determine the adhesive strength of thin polyimide films coated onto 10-cm silicon wafers. The equipment illustrated in Fig. 12.26 maintains a 90° peel effort during the test conducted at room temperature with a constant rate of crosshead displacement of 2 mm min . ... 

The mechanical surface treatments consist of grinding, lapping (smoothing) and polishing. For a detailed description of the processes involved the reader is refered to the literature [184, 284, 550] or Sect. 5.2.2. The brief description that follows is only related to surface treatments in general. 

Table 12.4 Effects of surface treatment versus lap shear strength for OOW and IW conditions...

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