Comparative tensile strengths of nonnoble dental alloy solders

Comparative tensile strengths alloy solders W. G. Kaylakie,

D.M.D.,

of nonnoble

dental

M.S.,* and C. E. Brukl, Ph.D.**

Bergstrom Air Force Base,Austin, Tex., and The University of Texas Health ScienceCenter, Dental School,San Antonio, Tex

M

ost casting and soldering techniques currently used in dentistry were developed specifically for dental gold alloys. The newer base metal or nonnoble alloys have higher melting ranges and other characteristics that differ from those of gold alloys. Practical application and routine use of these alloys have drawn varied and often conflicting reports from dentists and laboratories concerning their abilities to be cast and soldered accurately. Cast crowns and fixed partial dentures are soldered either before or after the application of porcelain. These two operations are called preceramic soldering and postceramic soldering, respectively. The postsolder must melt below the fusing temperature of the porcelain. If a direct flame is placed on the porcelain, it can cause porcelain fracture. To avoid this damage, the operation is performed in an oven. In both techniques, it is of interest to determine the comparative tensile strengths of solder joints of commercially available nonnoble alloys and solders used according to manufacturers’ directions. Determination of the reliability and reproducibility of such solder joints is also of importance, as is the determination of the site of fracture (solder, parent metal, or interface of the two). In addition, technique recommendations to improve the strength and reliability of nonnoble solder joints are of value to the dental profession. Sloan et al.’ reported soldering various combinations of alloys with their recommended solders with noble, “semiprecious,” and nonnoble metals. When base alloys were involved, the resultant fracture was most often at the interface of the solder and the parent metal. To date,

Submitted as a Master’s Thesis to the Department of Graduate Prosthodontics, University of Texas Health Science Center, Dental School, San Antonio, Tex. Presented at the Annual Session of the American Association for Dental Research, Cincinnati, Ohio. Supported by the Education and Research Foundation of Prosthodontics, Seattle, Wash. *Major, USAF (DC), Bergstrom Air Force Base. **Assistant Professor, Department of Restorative Dentistry, Division of Biomaterials, University of Texas Health Science Center, Dental School. THE

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L

f

2.5mm

4.5mm

1

Tk-l2mm-------)/

Fig. 1. Half of cast tensile test dumbbell-shaped men (before soldering).

speci-

only a few experiments have actually tested the strength of dental solder joints.‘.’ Most of the studies were concerned with gold or noble solders. Sloan et al.,’ Stade et a1.,3 and Rasmussen et a1.4 each independently tested gold, semiprecious, and nonnoble solders. Their results were similar and reported ultimate tensile strengths from 206.9 to 600 MPa for the nonnoble alloys and their solders. Hendrickson et al.* reported a radiographic technique for the evaluation of dental solder joints to increase the clinical reliability of these restorations. Radiographic analyses of dental castings to disclose internal defects were also described by Mattila’ and Pasco and Wimmer.” These methods involved industrial radiographic equipment with high voltage ratings of 150 to 200 kilovolt (peak). Wise and Kaiser” described a technique for radiographic examination of nonnoble metal castings with the use of standard dental x-ray equipment. All the studies reported defects found radiographically that were not apparent on visual examination. The reported percent of base alloy dental castings that contained apparent internal defects ranged as high as 79%~~ The purposes of the present study were (1) to determine and compare the ultimate tensile strengths and examine the failure sites of six commercial, nonnoble dental alloy presolder and two postsolder combinations soldered according to manufacturers’ directions; (2) to observe soldering procedures and suggest technique improvements; and (3) to describe a radiographic method for the evaluation of soldered joints. 455

KAYLAKIE

AND

BRUKL

Fig. 2. Stainless steel holding fixture used for pairs of aligned and gapped tensile test specimen halves (before investing).

Table I. Nonnoble Alloys

solders

alloys, solders, and fluxes used _--fluxes

Biobond Biobond solder Biobond soldering flux Microbond NPZ Microbond NP strip solder Microbond NP post solder Microbond presoldering flux Microbond postsoldering flux Ceramalloy II Ceramalloy NP solder Ceramalloy soldering flux Unibond Unibond nonnoble solder Cospan Cospan solder Cospan soldering flux Litecast B Super solder

MATERIAL

AND

Batch

114anufacturer Dentsply York,

h-l 0-CB-Strip 762335 06-058

international Pa.

Howmedica, Chicago,

042180 060037

Inc Ill.

040044

Ceramco, Long

00001 OE002

C. Vaupel Houston, 4350198 23773M

METHODS

Inc. Island

Unitek Corp. Monrovia,

31280-7

Six commercially available nonnoble alloys and their recommended presolders, as well as two of the six alloys with their recommended postsolders, were used in this study (Table I). A master stainless steel die (half of a finished tensile test dumbbell-shaped bar) was machined according to American Society for Testing Methods tensile testing specifications (Fig. 1).12Molds of the die were made in a silicone impression material (Vescote, batch No. 801001-02, Teledyne Dental Products, Elk Grove Park, Ill.). The molds were injection molded with inlay wax (Inlay Wax, batch No. C2403T, Maves Company, Cleveland, Ohio) to form casting patterns. The casting patterns were invested five at a time and cast according to the manufacturers’ specifications with a phosphatebonded investment material (High Temp, Lot No. 0176101, Whip-Mix Corp., Louisville, KY.). For specimen soldering, a stainless steel holding fixture was machined to accommodate five pairs of coaxially aligned specimens with a variable gap distance (Fig. 2). A gap distance of 0.33 mm was chosen because 456

.-__--No.

City,

N.Y.

Calif.

Dental, Tex

Inc.

Williams Gold Refining Buffalo, N.Y. __ ------

Co. Inc.

it was within the range recommended by all of the manufacturers, and previous studies had shown it to be a favorable separation for dental soldering.3,4, I3 The five specimens were invested in soldering investment (HiHeat, lot No. 098001, Whip-Mix Corp.) according to the manufacturer’s directions and then sawed into individual sections for soldering (Table II). Other factors such as oxygen pressure to the torch, solder form, and the time the torch flame remained in contact with the joint after the solder flowed are also given in Table II. All presoldering was accomplished with a natural gas/ oxygen torch (Perkeo No. 65 Multiplex Torch and Perkeo P-2-4 tip, Perkeo Mfg. Co., West Germany). In the postsoldering experiments, a porcelain oven (Jelenko Jelcraft, model H.T., Jelrus Technical Products Corp., New Hyde Park, N.Y.), adaptable for air or vacuum firing, was used according to the alloy manufacturers’ directions. Table III shows the procedural details of the postsoldering operations for the two alloy-solder combinations investigated. To simulate possible heatconditioning effects on the alloys, postsolder specimens were subjected to cyclic firing in the porcelain oven. This APRIL

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Table II. Technique

SOLDERS

variations

among proprietary

nonnoble

alloy presolders

tested

Torch oxygen Solder form

Alloy Biobond Ceramalloy Cospan Litecast B Microbond Unibond

Strip Rod Rod Strip Strip Paste

II

NP,

Table III. Technique Alloy

Investment drying temp. (degrees F) Cold furnace to 1400 1200-1500 1100 1200 Heat only enough to dry investment At mouth of furnace (900”)

variations

between

proprietary

Solder form

Furnace atmosphere

Biobond

Strip

Vacuum

Microbond

Strip

Air

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5 3 6 5 specified

3

Flame contact after solder flows (seconds) None None 60 5 None None

alloy post solders tested Soldering

steps

1. Apply flux to solder joint area 2. Place % inch solder strip in solder joint area 3. Dry investment and flux for 15 minutes at open oven door with temperature held at 900” F 4. Insert carbon purge block and sample into oven 5. Apply vacuum; raise temperature to 1550” F and hold for 1 minute 6. Release vacuum, remove and bench cool sample 1. Apply flux to solder joint area 2. Dry investment and flux for 15 minutes at open oven door with temperature held at 900” F 3. Place specimens in oven and raise temperature to 1650” F; hold for 1 minute 4. Dry flux coated solder strip in bunsen burner 5. Open oven door, touch solder strip to solder joint; feed solder until joint fills 6. Remove and bench cool specimens

process simulated the thermal treatment the alloys would receive in zin actual porcelainizing operation. After the different presoldering and postsoldering operations, all specimens were handled in an identical manner in the radiography, machining, and tensile testing procedures. Prior to lathe truing, all specimens were sandblasted to remove residual investment and flux. To assure concentricity and accurate diameters for all specimens, the samples were lathe machined to uniform diameters of 2.05 + 0.05 mm. After the samples were machined, the final diameters of the reduced sections of the tensile test specimens were measured to 0.001 mm with a vernier caliper. Radiographic examination was used to detect internal void defects in the solder, parent metal, and solder-metal interface. A standard dental radiographic unit (model 1000, General Electric Corp., Dental Systems Operation, Milwaukee, Wis.) was used. In numeric order, the samples were secured on sheets of pink baseplate wax. Each film was labeled with radiopaque symbols for identification and orientation. The samples were placed on top of a standard intraoral 2% X 3 inch occlusal film (Kodak Mfg. Co., Rochester, N.Y.); the end of the x-ray cone was placed 3 inches from the specimens. Settings of 30 impulses, 15mA and 90 kV were used for each THE

nonnoble

pressure

half-second exposure. All radiographs were developed in an automatic processor (model 810, Philips Medical Systems, Inc., Stamford, Conn.) with a digital control monitor module. Samples with apparent surface defects or internal radiographic defects were recorded for elimination from the final sample group evaluation. All samples were tensile tested to confirm radiographic findings. Tensile testing was performed on an Instron Universal testing machine (model 1125, Instron Corp., Canton, Mass.). Special slotted holding devices were machined from heat-hardened block steel and used to engage and firmly hold the test specimens during testing. The upper holding device was connected to the load cell by a universal joint to avoid torsional forces,on the specimens during loading (Fig. 3). Each specimen was loaded to fracture with a crosshead speed of 1 mm/min. The data for each sample was automatically recorded on a strip chart recorder. The ultimate tensile strength for each specimen was calculated by means of the sample’s previously recorded exact cross-sectional diameter. The ultimate tensile strength data were analyzed statistically at the 95% confidence level with one- and two-way analyses of variance and the Tukey honestly significant difference (HSD) multiple comparison test. 457

KAYLAKE

Fig.

3. Tensile

test specimen

in slotted

holding

device

60,000 Ultimate Tensile Strength in 40,000 I? S.I.

attached

to Instron

AND

BRUR!

load frame.

i-400

7

T

Ultimate L 3oo Tensile Strength in MPa

Pre- Soldered Alloys Fig. 4. Solder joint statistical groupings

ultimate tensile strengths and standard deviations).

All fractured specimens were viewed under a metallographic microscope (Metalograph, Carl Zeiss Co., Oberkochen, West, Germany) at magnifications of x50 to x1000 to study the morphology of the fracture surface and to discern whether the fracture had occurred in the solder, the parent metal, or the interface. Typical fracture surfaces and morphologies were photographed through the metallographic microscope.

RESULTS There was considerable difficulty in obtaining consistent, reliable specimens from both the presoldering and

postsoldering opertitions, although manufacturers’ soldering directions were followed precisely by an experienced operator. This difficulty appears to be related to both material and technique. A total of 81 pairs of specimens were soldered with six proprietary presolders and two postsolders. Before tensile testing, 17 solder 458

of presoldered

alloys

(Tukey

HSD

joints fractured during the truing phase, which indicates extremely weak, defective solder joints. Eleven samples were rejected because of visible surface or radiographic defects. Of the remaining 53 sampies, 42 had ultimate tensile strengths that exceeded the 310.3 MPa reported for conventional gold solder.‘-5 The solder joint strength values for the presoldered specimens are depicted in Fig. 4. The mean ultimate tensile strength values (N = 5) and standard deviations in MPa for these alloys were (1) U&bond, 525.4 rt 56.5; (2) Micrabond, 468.4 + 32.8; (3) Biobond, 463.5 + 103.0; (4) Ceramalloy II, 427.6 +- 52.3; (5) Cospan, 254.8 f 102.3; and (6) Litecast B, 189.5 -t 100.3. A one-way analysis of variance was applied to the tiresolder values. The interaction that was indicated was further described by a multiple comparison Tukey HSD test, which showed that the strengths of Cospan and APRIL

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600

Ultimate Tensile Strength in es. I.

SO,OOO-

400

Ultimate 3oo Tensile Strength in 200 MPa

40,00020,000-

100

Soldered Alloys Fig. 5. Solder HSD statistical

joint ultimate tensile strengths of post and presoldered groupings and standard deviations).

alloys

(Tukey

Fig. 6. Photomicrograph of fractured presoldered Unibond solder joint. Sample shows irregular surface, numerous voids, and small oxidelike inclusions. Ultimate tensile strength: 490.4 MPa. (Original magnification x480.)

Litecast B solders were significantly different at the 95% confidence level from those of Microbond, Biobond, Unibond, and Ceramalloy II solders. The multiple comparison groupings are also shown in Fig. 4. The ultimate tensile strength values (N = 5) for the two alloys soldered with the postsoldering technique are shown in Fig. 5 with their presolder tensile values for comparison. The postsolder values in MPa were Biobond, 550.0 + 46.1, and Microbond, 275.6 -+-70.6. A two-way analysis of variance was applied to the postsolder and presolder tensile strength values. The interaction indicated was defined by a subsequent Tukey test, which showed a significant difference @ < .Ol) THE

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between the mean values for Biobond and Microbond postsolders. The mean value obtained from Microbond presolder differed significantly (ic,< .Ol) from the mean for Microbond postsolder. Significant differences were not found between the Biobond presolder and postsolder values (Fig. 5). Microbond postsolder tensile tested specimens, which were oven soldered in an air environment, were the only specimens that fractured at the parent

metal-solder

interface;

all other

tested samples

fractured in the solder. In contrast, Sloan et all’ reported that most fractures

of nonnoble

solder joints

occurred

at

the interface of the solder and the parent metal. The fracture surfaces of all soldered specimens exam459

KA%LAKIE

AND

BRlJKi

Fig. 7 Photomicrograph of fractured presoldered Unibond solder joint. Sample shows few if any voids and no inclusions. Ultimate tensile strength: 624.4 MPa. (Original magnification X480.)

ined with the optical metallography microscope showed various details. In most tests, the number and size of the voids, as well as the amount of impurity inclusions at the fracture surface, correlated well with the measured ultimate tensile strength of the specimen: the greater the porosity and/or inclusions, the lower the strength. In several tests some of the specimens that broke prematurely before the tensile test were found to have significant quantities of black-gray oxidelike inclusions at the fracture surfaces either in the solder or at the parent metal-solder interface. Fig. 6 shows a photomicrograph of a presoldered Unibond sample; this solder joint has an irregular surface with numerous voids. A few small inclusions, which may be flux particles or oxide contamination, are seen. This sample had an ultimate tensile strength of 490.4 MPa. In contrast, Fig. 7 shows the fracture surface of another presoldered Unibond sample; this surface has few if any voids and no inclusions. The ultimate tensile strength of this sample was 624.4 MPa. Because oxygen is below the detection limits of the electron microscopy equipment available for this study, the identity of probable oxide contaminants on the fracture surfaces of the solder joints could not be ascertained. In radiography of the soldered specimens, the relatively short exposure (30 impulses, ‘/z second) successfully penetrated the nonnoble alloys. It did not penetrate the denser strip’ solders. These solders, however, could be clearly seen at the interface of the cast specimens. The nonnoble rod or paste form solders appeared to have the

same amount of radiopacity as the cast specimens. Voids and partially joined interfaces can be seen clearly on the radiographs (Figs. 8 and 9), although no evidence of the small quantity of oxidelike contaminants observed after fracture could be discerned. The more the solders appeared to present a radiographically homogeneous appearance with the cast parent metal, the stronger were the resultant tensile strengths for those joints. Joints with visible voids or continuity breaks at the interface were rejected from the sample group evaluation.

DISCUSSION All six commercial alloys and solders could be soldered successfully. However, there were inconsistencies among the various alloys in obtaining predictable, defect-free solder unions. The desired number of visibly and radiographically -defect-free specimens for tensile strength testing (five for each alloy) were obtained. However, an initial rejection of 14% of the specimens because of visible and radiographic defects and a 21% specimen loss because of fracture during the machiningtruing operation are not indicative of an aceeptabIe success rate for dental laboratory procedures. In this study, 35% of the prepared specimens that were eliminated from final strength evaluations because of defects or fracture during machining broke at the parent metalsolder interface. Only 6% of the remaining tensile-tested specimens fractured in the same mode. Ninety-four percent of the tensile-tested specimens showed a fracture through the solder itself. In other less exacting studies, APRIL

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Fig. 8. Radiographs of three samples with solder-joint voids. Left to right, Litecast B, ultimate tensile strength (UTS): 71.6 MPa; Unibond, UTS: 142.3 MPa; and Ceramalloy II, IJTS: 104.6 MPa.

Fig. 9. Radiographs of three samples with sound solder joints. Left to right, Litecast B, UTS: 204.9 MPa; Unibond, UTS: 518.8 MPa; and Ceramalloy II, UTS: 455.6 MPa.

materials may be thought to be at fault for poor mechanical properties or characteristics when in reality the culprit is faulty technique. The data from this study indicate that when soldered specimens that have technique-related flaws are eliminated from evaluation, the interfacial bond strength is not the weak link and a material’s true strength is evaluated. The ultimate tensile strength differences shown among the six alloysolder combinations are therefore differences caused by the manufacturers’ solder-alloy compositional variations. Technique-related problems appear to be the major contributor to the low reproducibility of integral, highstrength solder joints. The most important consideration with the nonnoble alloys is the control of surface oxide formation. This problem has been encountered for years with similar alloys in the aviation industry. Most industries today successfully combat the soldering and welding problem with the use of vacuum or protective atmosphere ovens, while dental technology appears still to be relegated in a large part to the protection and

cleaning of solder joints with fluxes. A better directed approach with vacuum and/or protective gas environments for dental pre- and postsoldering would be beneficial. The results of this study fully emphasize the importance of protection against oxidation and contamination. Indeed, a recent publication describes a technique for argon-protected atmospheric soldering of nonnoble alloys.‘4 The natural gas used with the torch may contain some trace contaminants as well as a small amount of moisture. One manufacturer (Williams Gold Refining Co.) recommended the use of a propane torch rather than natural gas. None of the torch-soldered samples yielded as high mean ultimate tensile strength values as the postsoldered Biobond specimens oven soldered under vacuum (Figs. 4 and 5). The Microbond postsolder samples were oven soldered also but without vacuum. These samples yielded significantly lower ultimate tensile strength values (Fig. 5). This may be due in part to the availability of oxygen to the hot specimens because the open oven door technique is used. It is also likely that the oven and specimen temperatures fluctu-

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ated as the cool room air mixed with the hot oven air. The probable surface oxide formation may also explain why the Microbond postsolder specimens were the only macrodefect-free samples that exhibited fractures at the solder-parent metal interface. One presolder combination, Unibond, was supplied with the solder in a paste form mixed with flux (Table II). The specimens soldered with this material had the highest presolder tensile strength values. Although these specimens were open-air torch soldered, the solder is flux-coated until it melts. This material preparation reduces the chances for oxidation and contamination. However, to further improve this alloy-solder combination as well as the other combinations supplied with rod and strip solder, oven soldering with either vacuum or protective atmosphere is recommended to assure more predictable, uniform, and stronger solder bonds. The pretensile testing radiographic screening technique proved to be a valuable aid in eliminating specimens with gross structural defects in the solder joints. Without this elimination, considerably lower tensile test values would have been falsely included in the final evaluation to lead to an erroneous solder and solder joint strength judgment influenced by technique-created imperfections. Although the radiographic technique used in this study is not capable of detecting small but detrimental quantities of oxide contaminants, voids are caused in part by a decrease in wettability of the parent metal by the molten solder in the presence of contaminant oxides. The real value of the technique lies in its ability to detect internal voids in solder joints. On this basis, routine radiography for all soldered nonnoble dental prostheses should be given serious consideration.

defects in nonnoble alloy solder joints; it is recommended that this technique be used for soldered nonnoble prosthodontic appliances. To reduce or eliminate the inconsistencies and voids in nonnoble alloy solder joints that are currently made with air furnace or gas/oxygen torch techniques; and to improve solder bond strengths, the soldering procedures should be performed in closed vacuum furnaces or protective atmospheres.

CONCLUSIONS

11

Six commercially available nonnoble alloys and their recommended presolders yielded solder joints with mean ultimate tensile strengths between 198.5 and 525.4 MPa when macrodefect-free samples were made by gas/ oxygen torch soldering according to manufacturers’ directions. Strengths of the solder joints decreased in the following order: Unibond, Microbond, Biobond, Ceramalloy II, Cospan, and Litecast B. Biobond vacuumpostsoldered joints were significantly stronger (550 MPa) than Microbond air-postsoldered joints (275.6 MPa), and Microbond torch-presoldered joints are significantly stronger (468.4 MPa) than Microbond airpostsoldered joints (275.6 MPa). Radiography is a valuable tool for detecting internal

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REFERENCES 1

2

3 4

Sloan, Xl., Reisbick, M. H., and Preston, J. D.: Post ceramic soldering of various alloys. J Dent Res 59(Special issur .4):472, 1980. Coleman, R. L.: Physical properties of dental materials Research paper No. 32. J Res Nat1 Bureau Standards 1:814. 1928. Stade, E. H., Reisbick, M. H., and Preston, J. D.: Preceramic and postceramic solder joints. J PrtosrrrEr DENT 34~527, 1975. Rasmussen, E. J., Goodkind, R. J., and Gerberich, W. W.: An investigation of tensile strength of dental solder joints. J PROSTHLT

5

6

7

8

9

IO

12. 13. 14.

DENT

41:418,

1979.

Kava, S., Lautenschlager, E. P., Higgins, M. R.. and Bruen, E Optimal gap for gold soldering. J Dent Res 59(Special issue A):475, 1980. Bogan, R. L.: An evaluation of the comparative strengths of some high fusing solders used for porcelain restorations. Thesis. Indiana University, School of Dentistry, 1967. Wictorin, L.. Bjelkhagen, H., and Abramson, N.: Holographic investigation of the elastic deformation of defective gold solder joints. Acta Odontol Stand 30:659, 1972. Hendrickson, C. O., Wictorin, L.. and Osterberg, J.: Radiographic detection of defects in solder .joints of dental gold alloys Odont Revy 24~161, 1973. Mattila, K.: A roentgenographic study of internal defects in chrome cobalt implants and partial dentures. Acta Odontol Stand 22:215, 1964. Pascoe, D. F., and Wimmer, J.: A radiographic technique for the detection of internal defects in dental castings. Quintessence Dent Techol 9:45, 1978. Wise, H. B.. and Kaiser, D. I\.: A radiographic technique lot examination of internal defects in metal frameworks. .J PROP. T,,ET DEN.,- 42:594, 1979. ,4ZSTM E8-60: Tension Testing of Metallic Materials. Philadelphia, 1969, American Society for Testing Materials. Ryge, G.: Dental soldering procedures. Dent Clin North :\m November 1958, p 747. Prasad, A., Day, G., and Best, R.: The USP of argon during the brazing of nonprecious alloys. Jenerir Times. Laboratorv Techniques. May 1982.

Ke,hrml

requesl.\

ill

DR. CH.~RLES E. BRLK UNIVERSITY OF TEX.G HEALTH

SCIENCE CENTEK

DIVISION OF BIOMATERIALS DENTAL SCHOOL SAN ANT~NIO,

TX

78284

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