Abrasive Sampling

FIGURE 4.45 Left A silicon carbide disk with an adhesive back. Center A metal post that fits into a DRIFTS accessory. Right A silicon carbide disk adhered to the metal post. 

FIGURE 4.46 The abrasive sampling spectrum of white paint from a light fixture. 

Comparatively easy sample preparation Spectra may be noisy 

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Hoult, R.A. Spragg, R.A. Abrasive sampling for diffuse reflectance using reflective substrates, Fourier Transform Spectroscopy, 12th International Conference Itoh, K., Tasumi, M., Eds. Waseda University Press Tokyo, 1999, 506-507. 

The ease with which samples in gaseous, liquid and solid phases can be studied is one factor that accounts for the wide use of IR spectroscopy. Glass and silica are opaque in the relevant IR region, so other materials must be used as windows for sample cells. Salts such as NaCl and KBr are adequate down to 600 and 350 cm, respectively, and Csl, softer and a lot more expensive, down to 200 cm. Other options include zinc selenide (which is transparent down to 500 cm ), and offers the considerable advantage that it is not water soluble. These windows are easily scratched, so they are best used for non-abrasive samples. Durability... 

In addition to obtaining the spectra of powdered samples, DRIFTS accessories can be used to take the spectra of large intractable solids using a technique known as abrasive sampling. In this technique, abrasive paper is used to collect particles from the sample of interest silicon carbide is conunonly used. A convenient way of obtaining abrasive sampling spectra is to pnrchase (from an FTIR accessory company) pieces of precut abrasive paper with an adhesive on the back, one of which is seen on the left in Figure 4.45. 

The beauty of the abrasive sampling technique is that it is fast, easy, and in theory can work on many solid samples. The drawback to the technique is that the spectra are frequently noisy, as can be seen in Figure 4.46. The problem is the infrared beam is scattered and reflected in many directions by the sample particles, reducing the amount of signal measured and causing unwanted noise. Frequently, more scans are needed to obtain usable spectra via abrasive sampling spectra than for other sample preparation techniques. 

Powders KBr pellets, mulls, DRIFTS, abrasive sampling, ATR... 

In general, polycarbonate resins have fair chemical resistance to aqueous solutions of acids or bases, as well as to fats and oils. Chemical attack by amines or ammonium hydroxide occurs, however, and aUphatic and aromatic hydrocarbons promote crazing of stressed molded samples. Eor these reasons, care must be exercised in the choice of solvents for painting and coating operations. Eor sheet appHcations, polycarbonate is commonly coated with a sihcone—sihcate hardcoat which provides abrasion resistance as well as increased solvent resistance. Coated films are also available. 

Eor the free swelling index which is also known as the cmcible swelling number (ASTM D720), a coal sample is rapidly heated to 820°C in a covered cmcible. Then the profile of the resulting char is compared to a series of standard numbered profiles (18). Eor the Roga index weighed amounts of coal and standard anthracite are mixed and carbonized, and the product coke is tested in a Roga dmm for its resistance to abrasion (89). 

The drop shatter test indicates the resistance of a coal or coke to breakage on impact (see D440). A sample is dropped in a standard way a number of times from a specified height. Eor the tumbler test or abrasion index (ASTM D441), the coal or coke is rotated in a dmm to determine the resistance to breakage by abrasion (18). 

Most ceramics (as we have seen) contain flaws holes and cracks left by processing, cracks caused by thermal stress, corrosion or abrasion. Even if there are no cracks to start with, differences in elastic moduli between phases will nucleate cracks on loading. And most of these flaws have a size which is roughly that of the powder particles from which the ceramic was made. If the flaw size can be reduced, or if samples containing abnormally large flaws can be detected and rejected, the mean strength of the ceramic component is increased. 

Below about 0.75 T, polymers are brittle (Fig. 23.9). Unless special care is taken to avoid it, a polymer sample has small surface cracks (depth c) left by machining or abrasion, or caused by environmental attack. Then a tensile stress tr will cause brittle failure if... 

Diesel fuel may contain ash-forming materials in the form of abrasive solids or soluble metallic soaps. These solids cause wear of injection equipment, pistons, piston rings, and liners as well as increasing engine deposits. Ash content is expressed as a percentage of the weight of the original test sample of the fuel when burned to completion m an open container. 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Methods are available for the determination of relative abrasion resistance using either a mixed jet of air and abrasive, as recommended in the appropriate British Standard or an abrasive wheel or disc. Owing to variations in the quality of the abrasive, and the performance of individual jets, a standard comparison sample is included in each batch. 

The abrasive in use should not be ignored by the inspector. He should have a standard reference sample in order to be able to determine when the grade is wrong or the grit is dirty. 

The material s loss in weight or the change in optical transmission usually measures abrasion resistance and reflectance after a sample has been exposed to an abrasive surface. This is usually done under load, for a predetermined number of cycles or a time period specified by ASTM methods. 

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