Substrate surface treatment

Since slower-curing epoxy adhesives systems flow over and wet high-energy surfaces very well, there is little chance for air to become trapped at the interface. As a result, mechanical abrasion is often recommended as a substrate surface treatment prior to application of the epoxy adhesive. The added surface area and the mechanical bonding provided by the additional peaks and valleys on the surface will enhance adhesive strength. If the adhesive does not wet the substrate surface well, such as in the case of epoxy resin on polyethylene, mechanical abrasion is not recommended since it will only encourage the probability of gas voids being trapped at the interface. 

Unlike substrate surface treatments, primers always add a new organic layer to the surface and two new interfaces to the joint structure. Most primers are developed for specific adhesives, and many are developed for specific adhesive/substrate combinations. 

Various substrate surface treatments suggested for use with a common epoxy-substrate joint and service environment combinations are discussed in this chapter. Surface preparation processes for a range of specific substrates and detailed process specifications are provided in App. F. The reader is also directed to several excellent texts that provide prebond surface treatment recipes and discuss the basics of surface preparation, the importance of contamination or weak boundary layers, and specific processes for adhesive systems other than epoxy.1,2,3... 

Morphology of the Metal-Organic Semiconductor Contacts The Role of Substrate Surface Treatment... 

Material Substrate Surface treatment Load at failure (lb)... 

Fig. 1.17 For some display devices it is preferable to have a substrate surface treatment which gives a tilted alignment. This drawing shows a smectic A phase with a small tilt, above a glass substrate coated with a conducting layer (usually indium and tin oxide) and an alignment layer giving a small tilt angle...

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

Surface Modification. Plasma surface modification can include surface cleaning, surface activation, heat treatments, and plasma polymerization. Surface cleaning and surface activation are usually performed for enhanced joining of materials (see Metal SURFACE TREATMENTS). Plasma heat treatments are not, however, limited to high temperature equiUbrium plasmas on metals. Heat treatments of organic materials are also possible. Plasma polymerization crosses the boundaries between surface modification and materials production by producing materials often not available by any other method. In many cases these new materials can be appHed directly to a substrate, thus modifying the substrate in a novel way. 

Electroplating. Chromium is electroplated onto various substrates in order to realize a more decorative and corrosion- or wear-resistant surface (24—32). About 80% of the chromium employed in metal treatment is used for chromium plating over 50% is for decorative chromium plating (see Metal surface treatments). Hard chromium plating differs from decorative plating mostiy in terms of thickness. Hard chromium plate may be 10 to several 100 p.m thick, whereas the chromium layers in a decorative plate may be as thin as 0.25 p.m, which corresponds to about two grams Cr per square meter of surface. 

Although there is considerable evidence that chemical surface treatments improve the substrate bondability of stainless steels, there is no general agreement on which is the best. One etchant commonly used with stainless steels is an HNO3-HF mixture [128-131] others are chromic acid and ferric chloride/hydrochloric... 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

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