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wear detritus, where it is expected. Plastic deformation dominates this wear process. Diffusion (chemical reaction) dominates the wear rate. Thinking of wear in mass transport terms helps to clarify the process.

Diffusion

Compared to the three other wear processes, diffusion is relatively static, with short transport distances. This is not to say that diffusion does not play a significant role in wear processes; it does. A clean example of diffusive wear is corrosion or oxidation. Mass transport from one crystal lattice to another by diffusion is a common phenomenon. One ceramic example is the oxidation of the surface of a MoSi2 heating element. Oxygen diffuses in to form the silica film that protects the element from rapid deterioration. As the film forms, diffusion is slowed by the parabolic rate law where film thickness is inversely proportional to time. Eventually, the film spalls off by stresses caused by differential thermal expansion between the MoSi2 element and the silica film. Mass transport then is by fracture and kinetic displacement.

Viscous Flow

A ceramic example of viscous flow wear is in ferrous contact refractories, such as a teeming zirconia nozzle. It is not the molten steel that wears the nozzle, but the slag inclusions in the steel that wet and adhere to the zirconia surface. At these temperatures, the slag dissolves the zirconia and moves downstream by viscous flow. Solution is an active process that involves diffusion. However, this is not how the predominate mass transport occurs.

Fracture and Kinetic Displacement

While there are many good examples of the fracture and kinetic displacement wear process in ceramics, we will discuss just one example.

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A ceramic plant manufacturing BaO-TiO2 for capacitors developed a problem with alumina contamination that was serious as it negatively affects the dielectric properties. Running out of inventory of good product spurred some aggressive action. All of the raw materials were within specification.

Alumina had to be picked up in processing. One of these steps involved milling, using alumina milling media. This was the standard procedure. Small star cracks were observed on the surface of the mill balls. They then discovered that a new rubber lining with lifter bars had been installed in the mill. This caused the balls to be lifted higher than before, cracking them when they crashed down. Minute particles of alumina were fractured and displaced kinetically, contaminating the product.

Wear Tests

A common wear test is the pin on disc, where a rod is pressed against a rotating disc with a known force, at a known sliding speed, and with a lubricant. Figure 11.79 is a sketch of a pin on disc wear apparatus.

Interfacial temperatures can become very high at the points of the aspirates, making the ambient temperature deceptive as a measure of the processes at the interface. There are options on pin on disc test apparatus such as speed, force, temperature, and lubricant. Conditions are selected that most closely match those of the intended service.

A multitude of wear test machines and methods are available, ranging from sandblasting to reciprocating brushes.

While some wear tests are potentially useful for screening materials, there is no substitute for testing in the actual application. This requires a close relationship between the supplier of the wear parts and the potential customer. Liability can be a problem unless the user has a test facility for evaluating parts. There is an understandable reticence in placing experimental parts into commercial installations. This is especially true when failure of the part would result in a disaster, such as spilling a ladle of molten steel on the mill floor.

In one case, a well-conceived composition, carefully crafted, and thoroughly-lab-tested refractory never made it into the application.11 Later, a poorly conceived, shop-made, and scantily tested refractory made the installation.

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Figure 11.79: Pin and Disc Wear Test Configuration. A common wear test for evaluating materials.

Thermal Shock

Thermal shock is measured by heating the specimens to a temperature and then quenching them in water.12 Residual strength is measured as a function of the temperature differential between the specimens and the water. The typical response is shown in Figure 11.80.

Residual strength is unchanged up to a point and then suddenly plunges down to a lower value. It then flattens out for an interval and slowly drops off. The parameters of interest are the temperature differentials where the sudden drop occurs and how far the strength drops down. Curve "B" is better in thermal shock than curve "A". Constructing this diagram is a bit

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labor-intensive. For example, if there are four replicate test bars for each temperature and five temperatures, this results in 20 specimens that must be heated, shocked, and MOR-measured. It should be noted, however, that this procedure is derived from thermal shock theory and is valid. Other methods are empirical, which may be useful when simulating actual conditions in service. There are many of these, including an acetylene torch traverse, cyclic torch tests, placing the specimen over a burner, plunging into a molten metal, and inserting a specimen into a hot kiln, to name a few.

Figure 11.80: Thermal Shock, Strength Response. The two measurements of interest are shown in the figure.

Pycnometer

A pycnometer can be used for density measurements. The sample volume is obtained by the amount of gas displaced in a fixed volume. This, along with the sample weight, is used to measure density.

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Porosity Volume and Size Distribution

A porosimeter is used for measuring the pore volume and pore size distribution. Mercury is forced into the pores, with the pressure and displaced-pore volume measurements used to calculate the size distribution. This is a useful technique for looking at the early stages of sintering and porous materials. Since mercury does not wet the ceramic, higher pressures are required to infiltrate the smaller pores.

Mercury has a vapor pressure and is toxic over a period of time. Spills are inevitable and can be difficult to clean. Mercury can also be ingested by contaminating food or tobacco carelessly placed around the porosimeter. A mercury porosimeter should be isolated, with venting and a means to contain spills. Some mercury is left in the pores after the test, creating a sample disposal problem.

Check List, Physical Properties

• MOR

Bar size Surface finish

Four point bending Statistical analysis Examination of data

• Tensile tests

Dog bones are difficult, alignment Brittle ring test

Hydrostatic, true values

• Compressive

Square and parallel Alignment fixture Paper cushion Safety shield

• MOE

Direct strain measurement Machine compliance

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Sonic velocity Easy Useful

Amplitude attenuation Resonate Frequency

Easy

• Hardness

Polished surfaces Vicker's

Measure diagonals

• Fracture toughness Fine-grained materials

Methods

Vicker's, high load

Measure crack length and indent diagonal Calculations

Coarse-grained materials Other methods Consult an expert

•Wear resistance Wear maps

Mass transport mechanisms Variable conditions Interactions between variables Pin on disc

Use actual application

•Thermal shock

Quenching

MOR measurements

Curve critical parameters

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REFERENCES

1.Engineering Property Data on Selected Ceramics. Vol. III, Single Oxides. Battelle, July 1981.

2.Robert Samuel and Srinivasan Chandrasekar, "Effect of Residual Stresses on the Fracture of Ground Ceramics," J. Am. Ceram. Soc. 72[10] 1960-66 (1989).

3.A. G. King, "Tribology of Abrasive Machining," Thin Solid Films 108, 127-34 (1983).

4.Alan G. King and W. M. Wheildon, Ceramics in Machining Processes: New York: Academic Press, 1966.

5.A. H. Heuer, "Alloy Design in Partially Stabilized Zirconia," Science and Technology of Zirconia, Edited by A. H. Heuer and L. W. Hobbs. Westerville, Ohio: The American Ceramic Society, 1981, pp. 98-115.

6.S. A. Bortz and H. H. Lund,. "The Brittle Ring Test," Mechanical Properties of Engineering Ceramics, Edited by W. Wurth Kriegel and Hayne Palmour III. New York: Interscience Publishers, 1961, pp. 383-406.

7.Richard M. Anderson,. "Testing Advanced Ceramics," Advanced Materials and Processes (3/89).

8.Soo E. Lee, Stephen M. Hsu, and Ming C. Shen, "Ceramic Wear Maps: Zirconia," J. Am. Ceram. Soc. 76[8] 1937-47 (1993).

9.L. Coes Jr., "Chemistry of Abrasive Action," Ind. Eng. Chem. 47, 2493-4 (1955).

10.W. R. Brown, N. S. Eiss, H. T. McAdams, "Chemical Mechanisms Contributing to Wear of Single Crystal Sapphire on Steel," J. Am. Ceram. Soc. 47 157-62 (1964).

11.D. F. Beal, Personal Communication.

12.D. P. H. Hasselman, "Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics," J. Am. Ceram. Soc. 52[11], 600-04 (1969).

Tools 501

3.0 CRAFTSMAN HAND TOOLS

One commonly finds the following hand tools in a tool box: screw drivers, wrenches, hammers, pliers, cutters, and others. One must acquire a variety of tools and in different sizes. Additionally, one should check the quality of the manufacture, especially the quality of the steel. Cheap tools are made from soft steel. These are cheaper to make but more costly to use as they wear out quickly and do not work well. The round-ended screw driver is a common apparition.

Locked tool cabinets with the identity and location of each tool is a good way to store hand tools.

4.0 MEASURING HAND TOOLS

An initial question centers on the measuring system. Some people believe that the SI system is universal; however, there is a problem. Most laboratories have a collection of old and new equipment, with the old calibrated in English units and the new calibrated in either English or metric. SI units are not likely except where they coincide with metric. The bolts, screws, gears, slots, notches and shims are all in English units. The author has no problem with the SI or the metric systems as inherently more reasonable, based on decades of ten and standardization of fundamental units. However, at issue here is the everyday function of the laboratory that is predominantly English.

Geometrical

There are sophisticated measurement systems for almost everything. Of concern here are those hand-measuring tools kept in the tool cabinet or drawer.

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Linear

Micrometers often have carbide faces that grab and give false readings. Carbide faces wear at a slower rate than steel. Steel faces have a better feel and do not wear fast. Avoid vernier calipers that have tips ground down to a knife edge; these wear very quickly, especially with abrasive ceramics.

Angular

Angle measuring spirit levels should be rugged to sustain lab use. Accurate angular measurements are made in the shop with dial gages, sine bar, and gage blocks. Go to the shop for these measurements.

Mass

Digital scales have nearly displaced analog types, because they are so easy to use. The tare button is one of civilization's greatest inventions. Keep scales in calibration.

Temperature

When the indicator fluid separates, coalesce the fluid by chilling the thermometer to contain all the fluid in the bulb and then warm it again. Thermocouples or optical pyrometers are common for temperature measurements; these should be often calibrated against a traceable standard. As a backup, pyrometric cones are still useful as they also give a qualitative estimate of the total heat treatment.