ORIGINAL ARTICLES T h e tw o p ro p rie ta ry allo ys exh ibited d iffe re n c e s i n c o m p o s itio n an d m ic ro s tru c tu re an d in v a lu e s fo r m e c h a n ic a l p ro p e rtie s . O n e w a s s tro n g e r an d h a rd e r; th e o th e r y ie ld e d a llo y -p o rc e la in b o n d s tre n g th v a lu e s c o m p a ra b le to th o s e of p re c io u s a llo y -p o rc e la in c o m b in a tio n s .
Properties of two nickel-chromium crown-and-bridge alloys for porcelain veneering Eugene F. Huget, DDS, MS Nitin Dvivedi, M E Howard E. Cosner, Jr., W a s h in g to n , DC
The high cost of gold has stimulated interest in less expensive alloys for the casting of inlays, crowns, and bridges. Several castable alloys containing metals such as nickel and chromium rather than precious or noble constituents have become commercially available. Most of these materials are marketed primarily for use with fused-porcelain veneers. This report is based on data and observations on two nickel-chromium crown-and-bridge alloys suitable for veneering: Microbond-NP* and W iron-S.t Commercial materials and equipment are identified to explain the experimental procedure. (This does not im ply official recommendations or endorsement, or that the equipment and materials are neces sarily the best suited for the purpose.)
Materials and methods Constituents o f the as-received alloys, with the exception o f carbon, were determined quanti tatively by atomic absorption spectroscopy. $ Carbon was determined by combustion gravi metric procedures.
Patterns for tensile specimens were prepared by injecting wax into a split metal mold. The dimensions and design of the bars were within the tolerances of A D A Specification N o. 14 for dental chromium-cobalt casting alloys.1 Invest ments, burnout times, and burnout temperatures are given in Table 1. Both alloys were cast at 2,800 F on an automatic induction casting ma chine^ Hardness and metallographic specimens were cast disks 13x3 mm. The disks were mounted in plastic and polished with 240- to 600-grit abra sive papers. A diamond paste (6.0 /tm) was used with a cloth-covered wheefll to remove gross scratches. Final polishing** was accomplished with fine alumina abrasive (0.05 /am). Unetched mounted specimens were used for the measure ment of hardness. Specimens for metallographic exam inationtt were etched by immersion in a solution o f 55.2% HC1 (concentrated), 3% H 2 S O 4 (concentrated), 1.8% H N O 3 , and 40% water, by volume. Immersion time for specimens of both alloys was 25 minutes. Tensile properties were measured at a cross head speed of 0.02 inch per minute, t t Elonga-
Table 1 ■ Nickel-chrom ium alloys: investments and mold burnout procedure. Alloy
Investment
Wiron-S Microbond-NP
Aurovestf C eram igoldfi):
Burnout time minutes
Burnout temperature degrees F
30 60
1,700 1,500
JADA, Vol. 94, January 1977 ■ 87
Table 2 ■ Alloy compositions. Microbond-NP % 76.00 13.80 4.55 2.80 1.11 0.09 1.50 0.01 0.05
Element Nickel Chromium Molybdenum Aluminum Silicon Manganese Iron Cobalt Carbon
The ceramic portions of the test pieces were embedded in 9/16-inch cubes of a commercial tray acrylic. Each cube was mounted in a small vise for added support. The assembly was ori ented on a testing machine to allow loading o f the cast disk in a diametral direction. Bond strength was calculated on the basis of ultimate load at shear failure per unit area o f apparent porcelainto-metal contact.
Wiron-S % 70.60 15.70 4.50 3.80 1.54 3.20 0.23 0.19 0.12
tion was measured over a 1-inch gauge length with a breakaway extensometer.§§ Macrohard ness (Rockwell 30N) was determined with the use o f a testing m achine!! with a 120° diamond cone indenter. Response o f the alloys to heat treatment was determined in this manner. Tensile specimens and cast disks were subjected to a simulated por celain firing cycle. The cycle included four con secutive heat treatments: Alloy conditioning specimens were heated from 1,200 F to 1,950 F and kept at the latter temperature five minutes before removal from the furnace; opaque appli cation specimens were heated from 1,200 F to 1.825 F, removed from the furnace on reaching 1.825 F , and cooled to room temperature; body porcelain application specimens were heated from 1,200 F to 1,775 F and removed immed iately from the furnace; and glaze application specimens were heated from 1,200 F to 1,800 F and cooled to room temperature on reaching 1,800 F. Cooling after each step o f the treatment cycle was done in open air. Apparent alloy-porcelain bond strength was measured by the shear test method developed by Civjan and co-workers.2 Specimens were cast disks (!4 x 1/16 inch) to which porcelain cylind ers (V&X3/16 inch) were fused. The following variations were used in specimen preparation: porcelain*** was applied to coated tt and non coated disks; and alloy-porcelain combinations were subjected to either one firing or to three firings.
Results Compositions o f the alloys are given in Table 2. Nickel and chromium were the predominant components of both materials. Although the al loys exhibited qualitative similarities with re spect to minor elemental constituents, signifi cant quantitative compositional differences were detected. Mechanical properties o f the test materials are presented in Table 3. The reported values for tensile properties are based on a minimum of six observations. Values for hardness were ob tained from a minimum of 24 indentations on triplicate specimens. Strength and hardness val ues of Microbond-NP were much lower than those obtained for Wiron-S. Heat treatment of Microbond-NP by the simulated porcelain firing cycle elicited a reduction in strength and an in crease in elongation. The properties of Wiron-S were not significantly altered by heat treatment. As-cast microstructures showed extensive coring (illustration, A and C). Heat treatment did not produce a significant change in the metallographic features of Wiron-S (illustration, B). Exposure of Microbond-NP to the porcelain firing cycle yielded a slight improvement in grain homogeneity (illustration, D). Apparent alloy-porcelain bond strengths are given in Table 4. Difficulties were not exper ienced in the fusion o f porcelain to Microbond-
Table 3 ■ Mechanical properties of two nickei-chrom ium crown-and-bridge alloys. Properties
Alloy Microbond-NP as cast PFC* Wiron-S as cast PFC*
Tensile strength x 103 psif Av and SD
Yield strength 0.2% offset X103 psif Av and SD
Elastic limit X103 psif Av and SD
Young’s modulus X106 psif Av and SD
Elongation % Av and SD
Hardness Rockwell 30N Av and SD
67 58
4 2
65 46
3 4
56 37
3 4
27 29
2 3
2 7
1 1
33 30
1 1
105 107
3 5
100 97
3 1
84 81
9 9
29 29
2 1
1 2
1 1
54 53
1 1
* Heat treated by simulated porcelain firing cycle, t Pounds per square inch. 88 ■ JADA, Vol. 94, January 1977
Microstructures of nickel-chromium crown-and-bridge alloys (original magnification, x400). A, Wiron-S, as cast; B, Wiron-S, heat treated by porcelain firing cycle; C, Microbond-NP, as cast; D, Microbond-NP, heat treated by porcelain firing cycle.
N P . H ow ever, testable Wiron-S specim ens could not be made w ithout the use of a coating agent. All 15 test pieces using noncoated cast ings separated on cooling to room tem perature. A black scale rem ained attached to the contact surfaces of the porcelain cylinders. Inspection of 30 specim ens fabricated from noncoated disks o f M icrobond-N P revealed that shear failure, on testing, had occurred at the alloy-porcelain interface. Although use of the coating agent con siderably im proved the adhesion of porcelain to W iron-S, use of the agent with M icrobond-N P did not appear to be beneficial.
ture alloys.3"5 T he properties o f M icrobond-N P, with the exception of m odulus of elasticity, fall within the ranges of properties exhibited by goldplatinum-palladium and gold-palladium-silverbased casting alloys.6,7 M odulus of elasticity values of both alloys approach 30 million pounds per square inch. T hese values are about two times higher than the m odulus o f elasticity val ues of high-fusing golds.6'7 T he nickel-chrom i um alloys, how ever, exhibit relatively low elong ation (ductility) as do the high-fusing precious metal alloys. High modulus of elasticity (rigidity) and rela tively high yield strength (resistance to perm a nent deformation) suggest the potential useful ness of the nickel-chrom ium alloys for the cast ing of thin copings and retainers, as well as for the construction of long-span fixed partial den tures. T hese properties, how ever, when coupled with low ductility and high hardness, impede finishing and adaptation o f margins. High hard ness also com plicates the adjustm ent of prox
D is c u s s io n
W iron-S and M icrobond-N P have properties that w arrant careful consideration in the selec tion of m aterials for fixed dental prostheses. T he values for mechanical properties of Wiron-S are com parable with those o f base-m etal partial den
T a b le 4 ■ Alloy-porcelain bond strength. Alloy
Porcelain
Bond strength Noncoated specimens
Wiron-S Microbond-NP
Ceramco’ ” Ceramco***
Coated specimens
Single firing psi*
Multiple firing psi*
Av and SD
Av and SD
7,000
900
7,500
1,300
Single firing psi’ Av and SD 6,800 7,500
1,200 900
Multiple firing psi* Av and SD 7,400 8,300
1,500 400
'Pounds per square inch. Huget— Dvivedi— Cosner; NICKEL-CHROMIUM ALLOYS ■ 89
imal contact areas and occlusal equilibration. Alterations in the properties of Microbond-NP that follow heat treatment by the porcelain firing cycle appear to be related to microstructural changes. Improved ductility and concurrent reduction o f yield strength and elastic limit are attributed to improvement in grain homogeneity. The application of dental porcelain to nickelchromium crown-and-bridge alloys is a sensitive technique. Success of the procedure depends on meticulous surface preparation of the substrate alloys. Bond strengths obtainable with Microbond-NP and Ceramco porcelain are comparable with those obtained with many precious alloy porcelains. If a coating agent is not used, the for mation o f strong and reliable Wiron-S-porcelain bonds apparently is precluded by excessive ox ides that form on the metal during the fusion pro cess. The most perplexing problems associated with the clinical use o f available nickel-chromium crown-and-bridge alloys are difficulties in tech niques in the casting of these materials using techniques, equipment, and materials designed for the centrifugal casting of gold alloys. Thin sections o f castings fabricated from base metals often are incomplete. Fine detail of margins is prone to obliteration by rounding. Extracoronally retained castings made for preparations with relatively parallel walls (
Properties of two nickel-chromium crown-and-bridge alloys for porcelain veneering Eugene F. Huget, DDS, MS Nitin Dvivedi, M E Howard E. Cosner, Jr., W a s h in g to n , DC
The high cost of gold has stimulated interest in less expensive alloys for the casting of inlays, crowns, and bridges. Several castable alloys containing metals such as nickel and chromium rather than precious or noble constituents have become commercially available. Most of these materials are marketed primarily for use with fused-porcelain veneers. This report is based on data and observations on two nickel-chromium crown-and-bridge alloys suitable for veneering: Microbond-NP* and W iron-S.t Commercial materials and equipment are identified to explain the experimental procedure. (This does not im ply official recommendations or endorsement, or that the equipment and materials are neces sarily the best suited for the purpose.)
Materials and methods Constituents o f the as-received alloys, with the exception o f carbon, were determined quanti tatively by atomic absorption spectroscopy. $ Carbon was determined by combustion gravi metric procedures.
Patterns for tensile specimens were prepared by injecting wax into a split metal mold. The dimensions and design of the bars were within the tolerances of A D A Specification N o. 14 for dental chromium-cobalt casting alloys.1 Invest ments, burnout times, and burnout temperatures are given in Table 1. Both alloys were cast at 2,800 F on an automatic induction casting ma chine^ Hardness and metallographic specimens were cast disks 13x3 mm. The disks were mounted in plastic and polished with 240- to 600-grit abra sive papers. A diamond paste (6.0 /tm) was used with a cloth-covered wheefll to remove gross scratches. Final polishing** was accomplished with fine alumina abrasive (0.05 /am). Unetched mounted specimens were used for the measure ment of hardness. Specimens for metallographic exam inationtt were etched by immersion in a solution o f 55.2% HC1 (concentrated), 3% H 2 S O 4 (concentrated), 1.8% H N O 3 , and 40% water, by volume. Immersion time for specimens of both alloys was 25 minutes. Tensile properties were measured at a cross head speed of 0.02 inch per minute, t t Elonga-
Table 1 ■ Nickel-chrom ium alloys: investments and mold burnout procedure. Alloy
Investment
Wiron-S Microbond-NP
Aurovestf C eram igoldfi):
Burnout time minutes
Burnout temperature degrees F
30 60
1,700 1,500
JADA, Vol. 94, January 1977 ■ 87
Table 2 ■ Alloy compositions. Microbond-NP % 76.00 13.80 4.55 2.80 1.11 0.09 1.50 0.01 0.05
Element Nickel Chromium Molybdenum Aluminum Silicon Manganese Iron Cobalt Carbon
The ceramic portions of the test pieces were embedded in 9/16-inch cubes of a commercial tray acrylic. Each cube was mounted in a small vise for added support. The assembly was ori ented on a testing machine to allow loading o f the cast disk in a diametral direction. Bond strength was calculated on the basis of ultimate load at shear failure per unit area o f apparent porcelainto-metal contact.
Wiron-S % 70.60 15.70 4.50 3.80 1.54 3.20 0.23 0.19 0.12
tion was measured over a 1-inch gauge length with a breakaway extensometer.§§ Macrohard ness (Rockwell 30N) was determined with the use o f a testing m achine!! with a 120° diamond cone indenter. Response o f the alloys to heat treatment was determined in this manner. Tensile specimens and cast disks were subjected to a simulated por celain firing cycle. The cycle included four con secutive heat treatments: Alloy conditioning specimens were heated from 1,200 F to 1,950 F and kept at the latter temperature five minutes before removal from the furnace; opaque appli cation specimens were heated from 1,200 F to 1.825 F, removed from the furnace on reaching 1.825 F , and cooled to room temperature; body porcelain application specimens were heated from 1,200 F to 1,775 F and removed immed iately from the furnace; and glaze application specimens were heated from 1,200 F to 1,800 F and cooled to room temperature on reaching 1,800 F. Cooling after each step o f the treatment cycle was done in open air. Apparent alloy-porcelain bond strength was measured by the shear test method developed by Civjan and co-workers.2 Specimens were cast disks (!4 x 1/16 inch) to which porcelain cylind ers (V&X3/16 inch) were fused. The following variations were used in specimen preparation: porcelain*** was applied to coated tt and non coated disks; and alloy-porcelain combinations were subjected to either one firing or to three firings.
Results Compositions o f the alloys are given in Table 2. Nickel and chromium were the predominant components of both materials. Although the al loys exhibited qualitative similarities with re spect to minor elemental constituents, signifi cant quantitative compositional differences were detected. Mechanical properties o f the test materials are presented in Table 3. The reported values for tensile properties are based on a minimum of six observations. Values for hardness were ob tained from a minimum of 24 indentations on triplicate specimens. Strength and hardness val ues of Microbond-NP were much lower than those obtained for Wiron-S. Heat treatment of Microbond-NP by the simulated porcelain firing cycle elicited a reduction in strength and an in crease in elongation. The properties of Wiron-S were not significantly altered by heat treatment. As-cast microstructures showed extensive coring (illustration, A and C). Heat treatment did not produce a significant change in the metallographic features of Wiron-S (illustration, B). Exposure of Microbond-NP to the porcelain firing cycle yielded a slight improvement in grain homogeneity (illustration, D). Apparent alloy-porcelain bond strengths are given in Table 4. Difficulties were not exper ienced in the fusion o f porcelain to Microbond-
Table 3 ■ Mechanical properties of two nickei-chrom ium crown-and-bridge alloys. Properties
Alloy Microbond-NP as cast PFC* Wiron-S as cast PFC*
Tensile strength x 103 psif Av and SD
Yield strength 0.2% offset X103 psif Av and SD
Elastic limit X103 psif Av and SD
Young’s modulus X106 psif Av and SD
Elongation % Av and SD
Hardness Rockwell 30N Av and SD
67 58
4 2
65 46
3 4
56 37
3 4
27 29
2 3
2 7
1 1
33 30
1 1
105 107
3 5
100 97
3 1
84 81
9 9
29 29
2 1
1 2
1 1
54 53
1 1
* Heat treated by simulated porcelain firing cycle, t Pounds per square inch. 88 ■ JADA, Vol. 94, January 1977
Microstructures of nickel-chromium crown-and-bridge alloys (original magnification, x400). A, Wiron-S, as cast; B, Wiron-S, heat treated by porcelain firing cycle; C, Microbond-NP, as cast; D, Microbond-NP, heat treated by porcelain firing cycle.
N P . H ow ever, testable Wiron-S specim ens could not be made w ithout the use of a coating agent. All 15 test pieces using noncoated cast ings separated on cooling to room tem perature. A black scale rem ained attached to the contact surfaces of the porcelain cylinders. Inspection of 30 specim ens fabricated from noncoated disks o f M icrobond-N P revealed that shear failure, on testing, had occurred at the alloy-porcelain interface. Although use of the coating agent con siderably im proved the adhesion of porcelain to W iron-S, use of the agent with M icrobond-N P did not appear to be beneficial.
ture alloys.3"5 T he properties o f M icrobond-N P, with the exception of m odulus of elasticity, fall within the ranges of properties exhibited by goldplatinum-palladium and gold-palladium-silverbased casting alloys.6,7 M odulus of elasticity values of both alloys approach 30 million pounds per square inch. T hese values are about two times higher than the m odulus o f elasticity val ues of high-fusing golds.6'7 T he nickel-chrom i um alloys, how ever, exhibit relatively low elong ation (ductility) as do the high-fusing precious metal alloys. High modulus of elasticity (rigidity) and rela tively high yield strength (resistance to perm a nent deformation) suggest the potential useful ness of the nickel-chrom ium alloys for the cast ing of thin copings and retainers, as well as for the construction of long-span fixed partial den tures. T hese properties, how ever, when coupled with low ductility and high hardness, impede finishing and adaptation o f margins. High hard ness also com plicates the adjustm ent of prox
D is c u s s io n
W iron-S and M icrobond-N P have properties that w arrant careful consideration in the selec tion of m aterials for fixed dental prostheses. T he values for mechanical properties of Wiron-S are com parable with those o f base-m etal partial den
T a b le 4 ■ Alloy-porcelain bond strength. Alloy
Porcelain
Bond strength Noncoated specimens
Wiron-S Microbond-NP
Ceramco’ ” Ceramco***
Coated specimens
Single firing psi*
Multiple firing psi*
Av and SD
Av and SD
7,000
900
7,500
1,300
Single firing psi’ Av and SD 6,800 7,500
1,200 900
Multiple firing psi* Av and SD 7,400 8,300
1,500 400
'Pounds per square inch. Huget— Dvivedi— Cosner; NICKEL-CHROMIUM ALLOYS ■ 89
imal contact areas and occlusal equilibration. Alterations in the properties of Microbond-NP that follow heat treatment by the porcelain firing cycle appear to be related to microstructural changes. Improved ductility and concurrent reduction o f yield strength and elastic limit are attributed to improvement in grain homogeneity. The application of dental porcelain to nickelchromium crown-and-bridge alloys is a sensitive technique. Success of the procedure depends on meticulous surface preparation of the substrate alloys. Bond strengths obtainable with Microbond-NP and Ceramco porcelain are comparable with those obtained with many precious alloy porcelains. If a coating agent is not used, the for mation o f strong and reliable Wiron-S-porcelain bonds apparently is precluded by excessive ox ides that form on the metal during the fusion pro cess. The most perplexing problems associated with the clinical use o f available nickel-chromium crown-and-bridge alloys are difficulties in tech niques in the casting of these materials using techniques, equipment, and materials designed for the centrifugal casting of gold alloys. Thin sections o f castings fabricated from base metals often are incomplete. Fine detail of margins is prone to obliteration by rounding. Extracoronally retained castings made for preparations with relatively parallel walls (
