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Biomedical subjects

K Asgar

Publications and source records attributed to K Asgar.

At least 19 recordsLinked to original sources
PubMed1993

Effect of interfacial variables on metal-porcelain bonding.

While ceramic-to-metal bonding has been used in many applications, the actual chemical and physical factors leading to optimum bond strength are not well understood. In this work, several variables affecting the bonding between dental porcelain and a palladium alloy (85% Pd, 10% Cu, and 5% Ga) were investigated: 1) precoating the metal by sputtering various oxides before porcelaining; 2) peroxidation of the metal base before porcelaining; 3) porcelaining under reducing atmosphere; and 4) surface roughening at controlled levels before porcelaining. Using a modification of the push doughnut shear bond strength test to measure bond strength the following results were obtained. 1) Compared with standard "control" samples, the aluminum oxide precoated specimens showed a bond strength improvement of 46%, while the copper, manganese, and tin oxide precoatings exhibited smaller effects. 2) Preoxidation of the metal base led to pronounced bond strengthening (152%) by surface roughening as well as oxide formation. 3) Porcelaining under a reducing atmosphere severely reduced bond strength (88% lower than the controls) indicating the role of oxidation during the standard firing cycle. 4) Mechanical roughening of the surface by controlled amounts gave pronounced improvements with greater notch depth. Coarse roughening produced the highest bond strength improvements (486%).

Copper↗
PubMed1988

In vivo wear. Part I: The Michigan computer-graphic measuring system.

Three-dimensional coordinate measuring machines for examining the quality of industrial castings were reviewed. The concept was modified and successfully converted to examine the minute geometric configurations of the surfaces of dental materials. The wear of composites has undoubtedly precipitated this perceptive, thorough study.

Computer Systems↗
PubMed1987

Effects of brackets and ties on stiffness of an arch wire.

Beam theory was used to evaluate the stiffness of a simulated orthodontic model as affected by the type of bracket, interbracket distance, type of ligature tie, and size of segment. For a given deflection, the model generated greater force (increased stiffness) as the beam constant (N) increased. N increased as interbracket distance increased. Metal ties were as rigid or more rigid than o-rings. Four bracket segments were more rigid than two-bracket segments when tied with o-rings but not metal ligatures. Values of N of Lewis and narrow twin brackets with metal ties were similar and greater than the N of wide twin and medium single brackets. Wide twin brackets were more rigid than others with o-rings.

Elasticity↗
PubMed1986

Tensile strength comparison of presoldered and postsoldered joints.

Twenty half-dumbbell shaped rods were cast in a silverless gold-palladium alloy that contained a minimum of 42% fresh alloy and the rest once-melted alloy. Each pair of half-dumbbells was assembled and either presoldered or postsoldered using a vacuum-oven and a torch-soldering technique. There was no significant difference in the ultimate tensile strength values between the presoldered and postsoldered joints within the same technique group, but the torch-soldered joints for the same solder were significantly stronger (p less than .01). It was believed that slow heating and cooling has deleterious effects on the ultimate tensile strength joints (UTS) possibly because of the microstructural changes and the highly oxidizable content of the postsolder used. Another 15 samples cast using once, twice, and three-times remelted parent alloy were presoldered and postsoldered with the torch only. In each of these groups, specimens joined by presolder showed significantly higher strength than those joined by postsolder (p less than .02). Remelting the parent alloy affected the strength differences between the presoldered and postsoldered joints. It was assumed that on remelting, the nature of the parent alloy is changed because some original elements are volatilized and newly formed oxides dissolved into it. Because it contains copper, the postsolder is more affected by this contamination than presolder. The UTS of the parent alloy, as well as that of the presolder and postsolder, were also determined. As expected, the parent alloy possessed the highest strength and the postsolder, the lowest.

Dental Porcelain↗
PubMed1985

Castability of crown and bridge alloys.

A special saucer-shaped wax pattern was designed to evaluate the castability of five casting machines and four casting alloys. The casting machines used included one broken arm unit, one induction unit, one resistance unit, and two vacuum air pressure units. The alloys included one base metal alloy, two high-fusing noble metal alloys, and one type III gold alloy. Results of analysis of variance showed that at the 95% confidence level there was a significant difference among casting machines and alloys, the casting machines had a stronger effect on castability. No attempt was made to evaluate the fit quality of the castings with regard to surface roughness or porosity.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed1983

Measurement of resistance of amalgam mixes to condensation.

A method was designed to evaluate the resistance of amalgam mixes to condensation forces. For the purpose of classification with respect to the plasticity of amalgam, the optimum test condition was to apply 40 or 50 lb of static load at 30 sec after trituration.

Chemical Phenomena↗