Flow and Fracture of Dental Alloys Determined by a Microbend Tester
The microstructure of an alloy is the fundamental variable on which the mechanical properties of an alloy depend. For testing the mechanical properties, however, common tests, such as tensile strength or hardness, are generally used. This may be due to the fact that the microstructure study is more complicated and time-consuming. The common method of microstructure study is to perform a series of tensile tests and then obtain the microstructure of the specimens after the tests are completed so as to correlate the structure with the performance data. The actual contribution of each observable microstructure to flow and fracture is rather difficult to detect. When the structure is examined after the fracture has taken place, some of the evidence is lost that would have been available if continuous observation of the structure had been possible. In 1955 Flinn and Trojan' developed an apparatus by means of which the microstructure of an alloy under stress can be studied. The stress can be increased while the microstructure is under observation. Thus any changes that may take place can be detected in any of the phases. When an alloy with a complicated microstructure is studied, the microbend test shows its main advantage, since it is rather difficult to predict how the different phases of a complicated structure will behave under different stresses. With this test the effect of such stress on various phases may be continuously watched under the microscope. The purpose of this study was to observe the behavior of different phases of cobalt-base and gold alloys used in dentistry for partial denture framework when they were subjected to bending stresses.
MATERIALS IN RESTORATIVE DENTISTRY
Two major areas of practice are recognized in dentistry. One is directed toward the prevention of oral disease by the reduction of dental caries and the control of periodontal disturbances and oral lesions of all types in the mouth, the patient thereby being assured of the best possible oral health services from such preventive practice. The other area is the practice of restorative dentistry, which includes the treatment of localized infections and the preparation of restorations to replace lost tissue. It is with regard to this second area that an understanding of materials is important. The construction of dental restorations and the practice of restorative dentistry are a necessity while people continue to suffer from involvements of the oral tissues. When teeth and adjacent tissues are lost by accident or disease, the lost tissues must be replaced by functionally and esthetically effective artificial substitutes. A major portion-variously estimated at SO-70% -of dental service and time is devoted to these biomechanical procedures. Unfortunately, this condition is likely to continue throughout the present generation, or until research provides methods to assure a reduction of prevailing dental disorders. One should recognize, therefore, that restorative dental practice is important to the patient, is an accepted fact of the profession, depends on a knowledge and choice of available materials, and will continue to be needed in the future as a part of our dental health service. In restorative procedures, two major activities must be recognized as fundamental: The proper treatment, preservation and care of the remaining oral tissues, with an evaluation of tissue tolerance, compatibility or tissue reaction to the various restorations; and the understanding of the fundamental nature, the physical and mechanical properties, and the functional and esthetic performance of the materials employed in the restoration. Dental materials science is concerned with all facets of these activities, and restorative dental practice involves the attempt to replace with appropriate substitutes either hard or soft tissues of the mouth which has been lost through accident or disease.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.