An evaluation of four variables affecting the bond strength of porcelain to nonprecious alloy

RESEARCH

AND

JOHN

Section

J. SHARRY,

editor

An evaluation strength Thomas George National

EDUCATION

of four

of purcs)ain

VW

s afkcting

to nonp~~us

the bad

aCky

A. Wight, D.D.S., i&S.,* John C. Bauman, B.,Pelleu, Jr., Ph.D.*** Naval Dental Center, Bethesda, Md.

D.D.S., MS.,**

and

lhe success and predictability of bonding porcelain to gold base alloys are well documented.‘-’ Unfortunately, the cost of using gold base alloys for bonding porcelain has increased to such an extent that the future use of the porcelain-gold system is severely jeopardized. Accordingly, interest has been renewed in the nonprecious metal alloys as a substitute for gold in all types of dental castings. Seed and McLean,’ in 1972, found that all base metal alloy systems break at the interface and do not break in the porcelain, as is typical for gold alloy systems. Other investigators, ‘)1 I” have shown that the porcelain/nonprecious alloy bond approaches equality with, or surpasses, the bond strength of porcelain to precious alloy. The authors believe, however, that an attempt should be made to strengthen the dimensions of clinical usefulness and predictability by using test specimens which more closely approximate actual tooth surfaces and that manipulation of the variables will produce more reliable results. We determined in preliminary investigations that the strength of the porcelain/nonprecious alloy bond is greatly affected by varying laboratory conditions and procedures. Four of the most promising variables which might affect porcelain/nonprecious alloy bond strength were selected for investigation. These were (1) direction of metal preparation, (2) degassing atmosphere, (3) deThe construed

opinions and assertions contained herein are those of the writers and as official or as reflecting the views of the Department of the Navy.

This article was submitted in partial fulfillment of Master of Science at The George Washington University, This project MR041.20.02-6052B3ID.

was

supported

by

*Commander

(DC)

USN:

second-year

**Commander ***Chairman,

570

(DC) Research

USN. Department.

Bureau

of Medicine graduate

student.

the requirements for Washington, D. C. and

Surgery

Research

are

not

the

degree Work

to he of Unit

“N”l~wl~r“5’

Variables

porcelain/nonprecious

in

”

Table

I. Summary

of procedural

gassing celain.

temperatures:

upper-limit

MATERIALS

AND

Levels 2

I

I. Metal preparation (directional) 2. Degassing atmosphere (mm. Hg vacuum) 3. Degassing upper-limit timing (minutes) firing

Experiment

timing,

and

bond

571

variations

Variables

4. Opaque

alloy

Unidirectional 0 (normal) 0 I, 1,760”

(4) firing

F. (960”

I

Bidirectional 25 4 C.); Experiment

temperature

3 Tridirectional 30 8

II, 1,840”

for the opaque

F. (1004.4”

C.).

layer of por-

METHODS

Table I summarizes the procedural variations followed in investigating the variables that might affect bond strength. The first variable, direction of metal preparation, was investigated by preparing the surface of the metal casting in three different directions: unidirectionally (or horizontally) , bidirectionally (or horizontally and vertically) , and tridirectionally (or horizontally, vertically, and diagonally) . Each application was made slowly and carefully with overlapping strokes. Preparation was accomplished with a PaaschC Air Eraser* and aluminum oxide abrasive. The second variable, the state of the atmosphere during degassing, was varied by changing the ambient atmospheric pressure in the furnace (0 mm. Hg) to a partial vacuum of 25 or 30 mm. Hg. During this cycle, the furnace temperature was raised 100’ F. per minute (38O C. per minute) from the low of 1,250° F. (676.5’ C.) to the high of 1,850’ F. (1,OlOO C.). The third variable evaluated, degassing upper-limit timing, was the time that the metal sample was heat-soaked at the upper limit of the degassing cycle. Times of 0 minute, when the metal was immediately removed from the furnace at the time the upper-limit temperature of 1,850° F. was reached, and 4 and 8 minutes at the upperlimit degassing temperature were used. Two experiments were conducted to compare the effects of changing the fourth variable, the firing temperature for the opaque layer of porcelain. Experiment I, which was performed at the manufacturer’s recommended firing temperature of 1,760’ F. (960° C.), included one sample for each of the 27 possible variations of the first three variables. Based on results of preliminary testing, a temperature of 1,840’ F. (1,000’ C.) was used in Experiment II, which included two samples for each of the 27 possible variations. A total of 162 Ticon test tabs+ were cast and remeasured. The test tab patterns were made from 0.060 inch methyl methacrylate sheets. A spruing procedure was devised using 0.050 inch methyl methacrylate sheets with ingate dimensions measuring precisely 0.5 by 3.00 mm. (Fig. 1) . It should be pointed out that this technique is not advocated for general clinical use, but was specifically designed for these tabs. *PaaschC tTicon,

Airbrush Ticonium

Company, Company,

Chicago, Division

111. of CMP

Industries,

Inc.,

Albany,

N. Y.

572

Wight,

Bauman, k-5.72

.I. I’rorthet. Dent. Mav. 1977

and Pelleu

mm-l

-T

Fig. 1. Sprue Fig.

design

2. Schematic

for casting

nonprecious

configuration

alloy

(Ticon).

of test tabs.

The heat-rate settings on the Ticomatic casting machine* ranged from the No. I setting for 3 ingots to the No. 5 setting for 15 ingots. The heat-rate setting must be extremely low in order to produce sharply detailed, high-quality castings. If the slow, controlled heating procedure teas followed precisely, no difference was seen in the quality of castings, regardless of the size of the melt. If the metal Teas heated quickly during casting, gas inclusions appeared consistently in the porcelains at the third and fourth firings, especially in thin areas. A modified test version of the American Society for Testing Materials STM Designation D 2295-72 was devised to compare the complex combinations of tensile and shear strengths of bonds of porcelain to nonprecious alloy. Interfaces measuring 5.72 by 8.00 mm. (Fig. 1) were prepared on the tab surface by grinding with Dedeco No. 328 fine-cut abrasive wheels.? The test tabs were designed to closei) simulate the bond interface as it exists under complex clinical stress situations, but not to compare pure shear or pure tensile stress. Flat surfaces were used in this test to eliminate additional positive and negative stress components on curved surfaces resulting from different coefficients of thermal expansion. Consistency in construction of the samples, as well as in the testing method, \vas emphasized. Vita VMK-68$ porcelain was fused between the prepared interfaces to produce 81 test samples (Fig. 2). Bond strengths were tested in a combination tensilejshear stress test utilizing an Instron Universal testing machines with a crosshead speed of 0.01 inch per minute. A combination of shear and tensile strength results, because *Ticomatic casting machine, Albany, N. Y. tDedeco Dental Development $.Unitek

Corp.,

$Instron

Universal

Monrovia, testing

Ticonium

Company,

& Manufacturing

Division Corp.,

of

Brooklyn,

CMP N. Y.

Calif. machine,

Instron

Instrument

Corp.,

Quincy,

Mass.

Industries,

Inc.,

Volume Number

37 5

Fig. 3. Electron ment. (Original

Variables

micrograph magnification

of metal x750.)

surface

in porcelain/nonprecious

after

Fig. 4. Electron micrograph of sample interface retained porcelain is the dark area in the upper of the figure. (Original magnification x750.)

tridirectional

of partial left corner.

alloy

PaaschUaluminum

bond Bare

after metal

porcelain occupies

bond

oxide

573

treat-

fracture. The the remainder

the pull is made diagonally through the porcelain when the ends of the tabs are pulled. This testing system was intended to enable investigators to more closely simulate complex in vivotype stress vectors at the porcelain-metal bond interface. The breaking points of all samples were recorded.

RESULTS The appearance of the test tab surfaces after treatment with the PaaschC Air Eraser and aluminum oxide but before the application of opaque porcelain is shown in Fig. 3. Fig. 4 depicts the appearance of an interface at the same magnification after breaking (the specimen was fired at 1,840’ F.), with porcelain retained in the upper left portion and metal showing elsewhere. Note the similarity between the prepared metal surface in Fig. 3 and the area seen in Fig. 4 where the porcelain has been torn away. Note also that the metal is less well defined in Fig. 4 than in Fig. 3. In all instances, metallic oxide was found on the surface of the porcelain that corresponded to these bare metal sites, which indicates that the porcelain was originally

574

Wight,

Bauman,

.I. Prosthet. Dent.

and Pelleu

May.

l!J77

Experiment I Experiment II (17so0 F) W40“ Fl tNumbws within boxes are total samples.) Opaque Firing Temperatures

Fig. 5. The effect of opaque firing temperature on mean centered at the top level are the standard deviations.)

bond

strengths.

(The

vertical

lines

bonded to these sites. It was observed in Experiment II (1,840’ F.) that in 36 of the 54 test samples, complete breaks occurred entirely in the porcelain with no metal visible. Some porcelain was retained in all but two samples. In Experiment I ( 1,760° F.), all of the samples broke at the metal-porcelain interface. The mean breaking strengths for the opaque firing temperatures in the two experiments are shown in Fig. 5. In Experiment I, when the opaque layer was fired at 1,760’ F., the mean bond strength was 29.20 & 6.55 Kg. per square centimeter. In Experiment II, when the opaque layer was fired at 1,840° F., the mean bond strength was significantly* increased to 65.48 _+ 13.70 Kg. per square centimeter. The effect on the bond strength of different times at the upper limit of the degassing cycle in Experiment II is shown in Fig. 6. At 0 minute, the mean bond strength was 75.04 + 15.51 Kg. per square centimeter. Increasing the time to 4 and 8 minutes resulted in mean bond strengths of 64.65 t 10.06 and 56.75 + 8.08 Kg. per square centimeter, respectively. Mean bond strength differences were significant* when comparing 0 and 8 minutes, and 4 and 8 minutes, but the difference was not significantt when comparing 0 and 4 minutes. The other variables examined, the direction of metal preparation using the PaaschC Air Eraser and aluminum oxide abrasive and the use of a partial vacuum during degassing, had no effect on the bond strength in either Experiment I or II. Mean bond strengths for all of the combinations of these variables were not significantly different.$ *P

< 0.01

(Student’s

t test).

+P > 0.01

(Student’s

t test).

$P >

(Student’s

t test).

0.05

Variables

in porcelain/nonprecious

alloy

bond

575

t

0 minutes (Numbers

4 minutes within Upper

Fig. 6. The effect of upper-limit cal lines centered at the top level

boxes

are total

Limit

Timing

timing on mean are the standard

8 minutes sampled

bond strengths deviations.)

in Experiment

II.

(The

verti-

DISCUSSION Cost effectiveness is one of the most important considerations concerning the future use of non-noble metals in dentistry. Ticon was selected for this study because of its low cost and the high prevalence of its use in the Navy system. Other nonprecious alloys also must be evaluated, however, in order to build a profile of comparative bond strengths. Slight variations in laboratory procedures can have significant impact on bond strength, as seen in this study. For this reason, detailed experimentation is essential if nonprecious alloys are to be used effectively. Our previous experiences and experimentation have suggested that specific laboratory procedures were required to ensure high-quality test samples. Casting patterns and sprues were made from methyl methacrylate sheets, because the material is strong and quickly and easily manipulated. By using this material, we were able to make accurate clinical castings of various shapes and dimensions in a predictably consistent manner. Only one porceIain was used in this investigation, Vita VMK-68. It was chosen because of its small multiple-grain size which, in our experience, results in greater condensation at a faster rate, yielding greater strength. Although some investigators have reported that bonds are not possible between porcelain and nonprecious alloys, this investigation has shown that such bonds are possible. Firing of the opaque layer of porcelain to 1,840° F., rather than to the recommended temperature of 1,760° F., significantly increased the strength of the bond. The increase in bond strength appeared to be directly related to the binding of the porcelain, since the samples fired at the higher temperature broke totally or primarily in the porcelain, while the samples fired at the lower temperature broke exclusively at the interface.

576

Wight,

Bauman,

and Pelleu

1. 1’1oQlret. r)cnt. May.

1977

It is recommended that at least two procedural considerations be applied in bonding procedures involving this porcelain/nonprecious alloy system, but further investigation is needed to determine optimal conditions. Our findings suggest firing the opaque layer of porcelain to 1,840’ F. at 75O F. (about 24’ C.) per minute, instead of using the manufacturer’s suggested temperature of 1,760° F., and removing the metal immediately when the upper limit of the degassing cycle (1,850’ F.‘: is reached. This phase of the study is being extended, first, to establish the optimal firing temperature for the opaque layer and, later, to compare the characteristics of bonding other porcelains to nonprecious alloys. SUMMARY Four variables that could possibly affect the bond strength of the porcelain to nonprecious alloy \vere investigated. The variables were directional variations of metal preparation using the PaaschC Air Eraser with aluminum oxide fast-cut abrasive, atmosphere variations in the furnace from low to high temperature limits of the degassing cycle, time variations at normal atmosphere of 1,850’ F. (degassing upper-limit timing), and firing of the opaque layer of porcelain at different temperatures. A total of 162 Ticon alloy interfaces were prepared, from which 81 paired test samples were constructed. Porcelain was fused to the samples and tested under specified conditions of preparation utilizing an Instron Universal testing machine. It was determined that firing the opaque layer at 1,840’ F. at a rate of 75’ F. per minute more than doubled the mean bond strength of all samples. The time at the upper limit of the degassing cycle also had a significant effect on the bond. As the time increased, the bond strength decreased. Complete bonds between porcelain and nonprecious metals were demonstrated when the fracture occurred in the porcelain and not at the interface. It is recommended that the opaque firing and degassing be done in accordance with the foregoing findings. The authors are deeply indebted to Nelson W. Rupp, D.D.S., for examining the test specimens by electron microscopy and for making testing instruments available and also to the staff of the American Dental Association Research Unit, National Bureau of Standards, for technical services. The authors are grateful to Dr. Thomas Burnett, Associate Professor of Operation Rcsearch, Management Science Department, Naval Academy, for the use of the computer and for writing the computer program and analyzing the data.

References 1. 2. 3. 4. 5.

Kelly, M., Asgar, K., and O’Brien, W. J.: Tensile Strength Determination of the Interface Between Porcelain Fused to Gold, J. Biomed. Mater. Res. 3: 403-408, 1969. Knapp, F. J., and Ryge, G.: Study of Bond Strength of Dental Porcelain Fused to Metal, J. Dent. Res. 45: 1047-1051, 1966. McLean, J. W., and Seed, I. R.: The Gold Alloy/Porcelain Bond, Trans. Br. Ceramic Sot. 5: 229, 1973. Nielsen, J. P., and Tuccillo, J. J.: Calculation of Interface Stress in Dental Porcelain Bonded to Gold Substrate, J. Dent. Res. 51: 1043-1047, 1972. Nishiyama, Y., Hisashi, H., and Noguchi, H.: Various Factors Affecting the Bonding Strength of Porcelain Fused to Gold Alloys, Trans. Bull. Tokyo Dent. Coil. 12: 99-116, 1971.

Volume

Number 6. 7. 8. 9.

10.

37

Variables

5

in porcelain/nonprecious

alloy

bond

577

Shell, J. S., and Nielsen, J. P.: Study of the Bond Between Gold Alloys and Porcelain, J. Dent. Res. 41: 1424-1437, 1962. Tuccillo, J. J., and Nielsen, J. P.: Shear Stress Measurements at a Dental Porcelain-Gold Bond Interface, J. Dent. Res. 51: 626-633, 1971. Seed, I. R., and McLean, J. W.: The Strength of Metal/Ceramic Bonds With Base Metals Containing Chromium, Br. Dent. J. 132: 232-234, 1972. Moffa, J. P., Lugassy, A. A., Guckes, A. D., and Gettleman, L.: An Evaluation of Nonprecious Alloys for Use With Porcelain Veneers. Part I. Physical Properties, J. PROSTHET. DENT. 30:424-441, 1973. Civjan, S., Huget, E. F., De Simon, L. B., and Risinger, R. J.: Determination of Apparent Bond Strength of Alloy-Porcelain Systems, J. Dent. Res. 53: 240, 1974. (Abst.) Reprint requests to: DR. PELLEU NATIONAL NAVALDENTAL BETHESDA, M~.20014

ARTICLES Vinyl

chloride

Oscar

N. Guerra,

CENTER

TO APPEAR concentrations M.S.P.H.,

F. Guichet,

Preparation tion James

Charles Jerome

B. Horton, J. Rudolph,

Facial

prosthesis

Clinical

prosthetics

J. Kronoveter,

of muscles

laboratories

MS.

that

move

the

mandible.

treated

teeth to receive

a post-core

restora-

D.D.S.

of commercial D.D.S., D.D.S.,

Helen M.S.D.

following D.D.S.,

management L. Miller,

maxillofacial

and Kenneth

functions

of endodontically

An evaluation

Ernest

ISSUES

D.D.S.

L. Gutmann,

R. C. McClelland,

in

D.D.S.,

Biologic laws governing Parts I, II, Ill, and IV Niles

IN FUTURE

D.D.S.,

M.S.,

pastes M.

radical

Paulus,

D.M.D.,

maxillofacial

composite George

B. Pelleu,

surgery

F.R.C.D.

of denture-induced M.S.

for finishing

infiammations

resin Jr.,

surfaces Ph.D.,

and