T h e e f f e c t of o x i d a t i o n h e a t t r e a t m e n t on p o r c e l a i n b o n d s t r e n g t h in s e l e c t e d b a s e m e t a l a l l o y s Y. Wu, B D S , M S , a J. B. M o s e r , P h D , b L. M. J a m e s o n , D D S , M S , c a n d W. F. P. M a l o n e , D D S , P h D d~
University of California, School of Dentistry, San Francisco, Calif., Northwestern Dental School, Chicago, Ill., and Washington University, School of Dental Medicine, St. Louis, Mo. B a s e metal alloys h a v e been w i d e l y used for fixed partial dentures in the past decade. The oxidation heat treatment (degassing) of t h e s e alloys is a c o n t r o v e r s i a l step to prepare the metal surface for bonding porcelain. This study e v a l u a t e d the effect of oxidation heat treatment on the porcelain bond strength of b a s e metal alloys and i n v e s t i g a t e d composition c h a n g e s that m a y h a v e occurred during this process. (J PROSTHET DENT 1991;66:439-44)
T h e popularity of the base metal alloys has dramatically increased in the past decade because of their advantageous mechanical properties and the high cost of gold. 14 The superior yield strength (resistance to permanent deformation) and modulus of elasticity (rigidity) of these alloys give them the potential advantage of thinner copings with less material and the required rigidity for long-span fixed partial dentures. Two types of base metal alloys, nickelLchromium and cobalt-chromium, have been commonly used in restorative dentistry. One disadvantage of the base metal alloys is the hard-to-control chromium oxide formation that results in lower bond strength between porcelain and metal. 5-7 Oxidation heat treatment (degassing, outgassing, and preoxidation) of the metal is used to remove the entrapped gas, eliminate surface contaminants, and form the metal oxide layer, s'l° This treatment may release stresses and cause distortion of the framework. 11,12 This study determined the effect of oxidation heat treatment on the porcelain bond strength of base metal alloys under varying time and atmospheric conditions and investigated ion diffusion at the interaction zone between porcelain and alloys. MATERIAL
AND
METHODS
Two nickel-chromium (Ni-Cr) alloys, NP2 and NP2 with beryllium (Be) (Austenal Dental, Chicago, Ill.), and two
Presented at the American Association for Dental Research meeting, Washington, D.C. From a thesis submitted in partial fulfillment of the requirement for the degree of Master of Science, Graduate Prosthodontics, Northwestern University Dental School. aClinical Assistant Professor, Department of Restorative Dentistry, University of California, School of Dentistry. bProfessor, Biological Material Division, Northwestern University, Dental School. CProfessorand Chairman, Section of Fixed Prosthodontics, Northwestern University, Dental School. dprofessor and Chairman, Department of Restorative Dentistry, Washington University, School of Dental Medicine. 10/1/29673 THE JOURNALOF PROSTHETICDENTISTRY
cobalt-chromium (Co-Cr) alloys, Vicomp (Austenal Dental) and Neobond II Special (Neoloy Products Inc., Chicago, Ill), were used in this study. The research was divided into two parts. Part 1 was a three-point bending test for the bond strength and part 2 consisted of the scanning electron microscope (SEM), energy-dispersive analysis of x-rays (EDAX), and a line scan for evaluation of boundary phase changes between metal and porcelain. Bending test Bending
test
Uniform wax test strips were made by lightly compressing two layers of occlusal indicator wax measuring 28 x 3.2 × 0.6 mm (Kerr Division of Sybron Corp., Emeryville, Calif.). The wax strips were sprued and invested with Hi-Temp phosphate investment (Whip-Mix Corp., Louisville, Ky.) under vacuum mixing with a ratio of 9 ml of liquid to 60 gm of powder. The invested rings were allowed to bench set before being placed in a furnace at 1700 ° F. Four alloys were cast with the induction casting machine (Nobilium Co., Chicago, Ill.) at approximately 2800 ° F. The cast strips were measured with calipers and a parallizer (Vickers Instruments, Inc., New York, N.Y.) and were adjusted to dimensions of 25 m m x 3 mm x 0.5 mm to ensure a flat surface. The standardized specimens were airabraded and ultrasonically cleaned and were divided into groups for various oxidation heat treatments (OHT) (Table I). Neobond II Special alloy (Table II) was subjected only to procedures recommended by the manufacturer. The specimens were degassed under vacuum conditions for 1 minute, bench-cooled, air-abraded with 50 gin aluminum oxide, and ultrasonically cleaned for 3 minutes. Microbond porcelain (Austenal Dental Inc.) was applied to the test strips. A thin layer of opaque porcelain was applied to an area 8 mm long located in the center of the metal strip. The metal strips with the opaque porcelain were vacuum fired with a rising temperature of 80 ° F/min. A 0.2 mm thick second coat of opaque porcelain was added and fired. Body porcelain was built up 1 mm using a custommade measuring jig on the opaque porcelain. The body porcelain was condensed by vibration using a Le Cron 4~9
WU ET AL
Table I. N u m b e r of specimens and oxidation heat t r e a t m e n t s for alloys "7
Alloys N u m b e r of Specimens Treatment
NP2
As Cast 5 Air, 0 min 1750 ° F 5 2000 ° F 5 Air, 10 min 1750 ° F 5 2000 ° F 5 Vacuum, 0 min i750 ° F 5 2000 ° F 5 Air, 0 rain + bonding agent 1750 ° F 5 2000 ° F 5 Air, 0 rain 2100 ° F 5 Total 170 50
N P 2 w i t h Be
6
6 6
6 6
6 6
6 6
6 6
6 6
6 6
6 6
6
6
60
60
Table II. N u m b e r of specimens and oxidation heat treatments scheduled for Neobond II Special alloy No.
As cast
5
1925 ° F v a c u u m , 1 m i n
5
1925 ° F v a c u u m , 10 m i n
5
1925 ° F air, 1 rain
5
1925 ° F, v a c u u m 1 rain a n d b o n d i n g a g e n t
5
1750 ° F v a c u u m , 1 m i n
5
2100 ° F v a c u u m , 1 m i n
5
Total
35
Vacuum, 27.5 inch Hg (700 mm Hg).
carver (Miltex, Germany) and was fired at 1775 ° F in a 700 m m Hg vacuum. The tensile bond strength of each sample was tested using a three-point bending device and was measured with an Instron testing machine (Instron Corp., Canton, Mass.). A 200 lb load cell and a crosshead speed of 0.01 in/min were used. A chart speed of 2 in/rain was used to record the load deflection curve. The porcelain-metal bond test subjected samples to a shear stress at the interface, and by measuring the compressive load on the free metal surface, the bond strength was determined. The first point of a sudden drop in the load curve is the bond fracture strength. The d a t a were subjected to the Bonferroni test and to one- or twoway analysis of variance (ANOVA) statistical analysis to determine the correlation between O H T and bond strength.
E n e r g y - d i s p e r s i v e a n a l y s i s of x - r a y s T h e statistics showed t h a t two subgroups in the NP2 with Be and the Vicomp alloys had a significant difference 440
~ 2tO0*F ~2000°F
~ t750° F
Vicomp
6
Treatment
il
0 min
iO min
Vac
Bd Agent
No Rx
210O°F
Conditions
F i g . 1. Bond strength of NP2 alloy at oxidation heat treatments.
between the specimens "without O H T " and those with " O H T with bonding agent." These samples were selected for E D A X examination. Two specimens were e m b e d d e d in Stycast 1266 epoxy resin (Emerson and Cuming Inc., Canton, Mass.). After 2 days of curing, the e m b e d d e d specimens were longitudinally sectioned. The surfaces were smoothed and polished using successively finer grades of abrasive papers, from grit 240 to grit 600 (Buehler Ltd. Inc., Evanston, Ill.), followed by their polishing on a rotating wheel with slurries of 10, 5, and 0.5 ~m alumina oxide. The specimens were coated with a layer of graphite, with the exception of the metal and central p a r t of the porcelain at the metal border. The specimens were sputtered with carbon in a high-vacuum chamber (75 miUipore) for 5 to 15 minutes and were then transferred to the S E M (International Scientific Instruments Inc., Mountain View, Calif.) and E D A X (EDAX International Inc., Prairie View, Ill.) for line scan analysis. Four elements for each alloy were selected to compare the ion concentration profile at the interaction zone, i.e., A1, Si, Ni, and Cr ions in NP2 with Be alloy and A1, Si, Co, and Cr ions in Vicomp alloy. The ion concentration profiles were traced on the acetate paper and were superimposed on each other to compare the changes of ion diffusions.
RESULTS Ni-Cr alloys ( N P 2 ) In NP2 alloy, without Be, bond strengths for heat treatments at 1750 ° F ranged from 6.92 N for vacuum-treated specimens to 9.13 N for those t r e a t e d with the bonding agent. Again, for Be-free alloys for heat treatments at 2000 ° F, bond strengths ranged from 7.79 N for 10 minute air-fired specimens to 10.13 N for specimens with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 7.75 N, and O H T at 2100 ° F for 0 minutes in air, specimens had a bond strength of 8.66 N. No statistical difference was found at p < 0.05 for any of these values. Numerical values are shown in Table III and Fig. 1. W h e n Be was p a r t of the alloy, bond strengths were genOCTOBER 1991 VOLUME 66 NUMBER 4
PORCELAIN BOND STRENGTH TO BASE METAL
1No ~7
heat treatment 2000" F 1750=F ,
!~
mm 2 t 0 0 ' F
Z
ii~
mm 2000" F r~11750. F
m
tt
i5 i4
"~ co
t098755
g~
n 0 min
10 min
Vac
Bd Agent
No Rx
in
2~°OOF
Conditions F i g . 2. Bond strength of NP2 with Be alloy at oxidation heat treatments.
T a b l e III. Bond strength of NP2 alloy at oxidation heat treatments
1750 ° F 20000 F
0 min
8.25 _+1.60 8.41 _+2.30
210G=F
Fig. 3. Bond strength of Vicomp alloy at oxidation heat treatments.
T a b l e IV. Bond strength of NP2 with Be alloy at oxidation heat treatments Air
Air Temperature
Bd Agent No Rx
Conditions
10 m i n
Vacuum
7.02 _+1.92 7.79 _+1.32
Bonding agent
6.92 _+0.78 9.18 ± 1.24
9.13 _+1.63 10.13 _+1.41
No heat treatment, 7.75 ± 1.11 N. 2100 ° F air, 0 rain, 8.66 ± 2.33 N. 2000 ° F average bond strength, 8.875 ± 1.85 N. F value = 2.54, a = 0.05. Scheff~ critical difference = 3.59 N.
erally higher. For heat treatments at 1750 ° F, they ranged from 10.49 N (0 minutes in air) to 13.23 N for specimens with the bonding agent. For heat treatments at 2000 ° F, they ranged from 11.20 N (0 minutes in air) to 14.01 N for specimens with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 10.36 N and at O H T 2100 ° F with 0 minutes in air, specimens had a bond strength of 12.28 N. A significant difference was found a t p < 0.05 between no heat t r e a t m e n t and the mean of all heat treatments at 2000 ° F. Numerical values are shown in Table IV and Fig. 2.
Co-Cr Alloys (Vicomp and Neobond I I Special)
Vicomp. Bond strengths for heat treatments at 1750 ° F ranged from 9.93 N for vacuum-treated specimens to 10.93 N for specimens treated with the bonding agent. For heat t r e a t m e n t at 2000 ° F, bond strengths ranged from 10.14 N (0 minutes in air) to 13.18 N for specimens treated with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 9.93 N and at 2100 ° F in air at 0 minutes, specimens had a bond strength of 10.25 N. There was a significant difference at p < 0.05 between no heat t r e a t m e n t and THE J O U R N A L OF PROSTHETIC D E N T I S T R Y
Bonding agent
0 rain
10 rain
Vacuum
1750°F
10.49_+1.58
11.06_+1.31
11.43_+0.70
2000°F
11.20_+1.14 11.35_+1.23 12.72_+1.43 14.01_+1.68"
13.23_+1.78
No heat treatment, 10.36 ± 2.07 N.* 2100 ° F air, 0 min, 12.28 ± 1.90 N. 2 0 0 0 ° F average bond strength, 12.32 +_ 1.72 N . t F value = 2.45, a = 0.05. Scheff6 critical difference = 2.80 N. * and t, Mean significant difference between these two groups.
the mean of all heat t r e a t m e n t s at 2000 ° F. Numerical values are shown in Table V and Fig. 3. Neobond H Special. Bond strengths for heat treatments at 1925 ° F ranged from 13.17 N for 1 minute in vacuum to 14.75 N in air. Results for other temperatures and conditions ranged from 13.44 N to 14.95 N, as shown in Table VI and Fig. 4. No statistical difference at p < 0.05 was found for any of these values. Tables VII and VIII summarize statistical differences at p ~< 0.05 between NP2, NP2 with Be, and Vicomp alloys with heat t r e a t m e n t temperatures of 1750 ° F and 2000 ° F. No significant differences were found between any of the materials or t r e a t m e n t s at 1750 ° F; all significant differences occurred at a t e m p e r a t u r e of 2000 ° F. The Bonferroni multiple test was used to determine the effect of O H T on the metal by comparing the "without O H T " group to each subgroup "with OHT." The results are shown on Table VII. A significant difference was found between NP2 with Be without oxidation heat t r e a t m e n t and with oxidation heat t r e a t m e n t plus the bonding agent. Also, a significant difference was found between Vicomp alloy without O H T and with O H T plus the bonding agent. Scheff~ and two-way ANOVA tests were used to specify 441
WU ET AL
~ ~7 ~
g,
T a b l e VII. Statistical analysis of oxidation heat treatment
12tOO'F ~ 1750"F
1
mmNO
Temperature
Condition
NP2
NP2 with Be
Vicomp
1750° F
No OHT OHT-BA No OHT OHT-BA
NS NS NS NS
NS NS S* S*
NS NS St St
hllttreatment 2000 ° F
=
5
Bcl Agent No Rx 2tOO'F 1750°F
1 m;~n 10 min ~ . r
O H T , Oxidation heat treatment; O H T - B A , oxidation h e a t t r e a t m e n t plus bonding agent; NS, no significant difference; S, significant difference. *These two groups h a d significant differences. t T h e s e two groups h a d significant differences. p ,~ 0.05; critical value t(0.90~,1o)= 3.17.
Conditions F i g . 4. Bond strength of Neobond II Special alloy at ox-
idation heat treatments.
T a b l e VIII. Effect of oxidation t r e a t m e n t on oxidation heat t r e a t m e n t subgroups NP2
T a b l e V. Bond strength of Vicomp alloy at oxidation heat t r e a t m e n t s
Temperature
1750° F
Air 0 min
1 0 rain
Vacuum
Bonding agent
1750 ° F 10.20 _+ 1.70 9.99 + 2.56 9.93 _+ 1.97 10.93 _+ 2.06 2000 ° F 10.14 _+ L04 12.48 -+ 2.56 12.32 _+ 1.97 13.18 _+ 2.06 No h e a t t r e a t m e n t , 9.93 +. 0.85 N.* 2'100° F air, 0 min, 10.25 +- 1.18 N. 2000 Q F average bond strength, ]2.03 +- 1.58 N . t F Value = 2.45, ~ = 0.05. Scheff~ critical difference = 3,42 N, * and t, M e a n significant difference between these two groups.
T a b l e VI. Bond strength of Neobond II Special alloy at oxidation heat t r e a t m e n t s Vacuum 1 min
i0 rain
Air
Bonding agent
1925 ° F 13.17 +_ 1.33 14.35 _+ 0.88 14.75 +_ 0.92 13.56 _+ 0.62 No O H T , 14.52 _+ 1.88 N. 2100 ° F Vacuum, 1 min, 13.44 _+ 0.73 N. 1750 ° F Vacuum, 1 min, 14.50 +_ 1.78 N. 1925 ° F Vacuum, no air blast after O H T , 14.95 +_ 1.74 N.
the differences among O H T subgroups. The results are shown in Table VIII. A significant difference was found between NP2 with Be, heat t r e a t e d at 0 minutes in air and treated with the bonding agent. T h e ion diffusions, Ni, Cr, Col A1, and Si, were compared between specimens "without" and "with" the bonding agent in NP2 With Be alloy, as shown in Fig. 5, and in Vic0mp alloy, a s shown in Fig. 6. DISCUSSION Dent!stry has yet to establish a standardized bond test for porcelainLfused-to-metal because the in vitro bond tests
442
2000 ° F
Conditions
NP2
with Be
Vicomp
Air, 0 min Air, 10 rain Vacuum, 0 min Bonding agent Air, 0 rain Air, 10 min Vacuum, 0 rain Bonding agent
NS NS NS NS NS NS NS NS
NS NS NS NS S* NS NS S*
NS NS NS NS NS NS NS NS
NS, Not significant; S, significant difference. M e a n difference 2.81 N; critical value, 2.80 N, p < 0.05. *These two groups were significantly different.
do not correlate with clinical fractures. 13 The three-point bending test is recommended by the American Dental Association Council on Dental Materials and Devices. 14 The specimen is relatively easy to prepare and the d a t a can be compared with other published values using the same testing method. 15 O H T in base metal alloys is a controversial subject. In this study, the effect of O H T on the bond strength was determined by comparing the metal "without O H T " and the metal "with O H T " under different conditions: (1), temperature, such as 1750 ° F and 2000 ° F (within the manufacturer's suggested temperature) and 2100 ° F (overheating); (2), soaking time (0 minute and 10 minutes); and (3), pressure, (air and 700 mm Hg pressure). Only Neobond II Special alloy required different O H T procedures as recommended by the manufacturer. The Bonferroni multiple t test was used to compare the "without O H T group" with the " O H T group" and the "2100 ° F overheating group." Only two pairs of subgroups, '!without O H T " and "with O H T plus bonding agent," at 2000 ° F in NP2 with Be and Vicomp alloys had significant differences, with a small margin over the critical value (Table VII). B u t no difference was found in any group between "without O H T " and "with OHT." Two-way ANOVA and the Scheff@ test were also used to differentiate among those O H T subgroups. T h e results showed only the difference between the
OCTOBER 1991
VOLUME 66 NUMBER 4
P O R C E L A I N B O N D S T R E N G T H TO B A S E M E T A L
NP2 with Be
1orePS
~P2 with Be Cr C P ~
C~ | 25O
~i
250
500~ s
, |/
!
A1
.°°tli .°lli
8c
! I
200
m
E 10
ma
" 20 ~
0
DIS~
Diatance from Interface
L
M
CI~! ~ l e
10 u a
EEO~ IntezEa~a per saoaE~
,20 um
/li.
].
10
i0 ~ista~ce from interface 10
........ With bondln~ a~nt _ _ Without bo~Inq agent
A
.
4(
00
1Ù
'k0
;1'
II
100
LQ
6(
,:tl'°
150
0
1'0
Distance from interface CP5: Cycle per second.
....... with bonding agent B
- -
without bonding agent
Fig. 5. Ion diffusions on map of line scan in NP2 with Be alloy. A, Cr and Ni ion diffusions were suppressed after adding bonding agent. B, A1 and Si ions increased diffusion.
Ytcomp
Cr
vico~ CPS 25O
250C[S \:
c: IO0 A1
,
200 I
80
150
M
t'l
40O
"°ll
60
m
P
I00
a
i
. oolj.
• 10C
5O
N
40
P
"°Ii
5O
1. . . . . .
i0 I0 Distance from interface 20 um
10
0
~ ......
111
''
CP3, Cycle per second.
- -
without bonding agent
10
J 20 Ul
0
10
Distamce f r o m interface C~S, C y c l e
Per
sscon~
.........with bonding agent
......... with bonding agent A
io
Distance from interface
Distance from interface
B
~wlthout
b o n d i n g agent
Fig. 6. In Vicomp alloy, Cr and Co ion diffusions were suppressed in the bonding agent group (A). B, Si ions decreased and A1 ions increased diffusion.
subgroups " O H T with or without bonding agent" in NP2 with Be alloy (Table VIII). In the Neobond II Special alloy group, no significant difference was found among the subgroups after analysis by one-way ANOVA (Table VI). Based on these results, the variables of vacuum, temperatures, soaking time, and "without OHT" did not significantly affect the bond strength. However, the use of a bonding agent did affect bond strength. The probability of a type I error might occur in this statistical analysis because of the small value difference.
T H E J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Wight et al.I¢ also reported that the vacuum had no effect on the bond strength but heat-soaked time significantly influenced the strength of bond. The shortest soaking time had the strongest bond strength. In that study, the opaque firing temperature was raised 80 ° F higher than the manufacturer recommended (representing overheating). The bonding agent significantly improved the bond strength in NP2 with Be and Vicomp alloys, but not significantly in NP2 and Neobon II Special alloys. The bonding agent gold tone catalyst liquid contained A1 9 %, Si 36 %,
443
WU ET AL
K 10%, Au 29.4%, and Zr 15.65% .* The gold-rich bonding agent reduced the interfacial stress by improving the compatibility between porcelain and metal. 17 Si and Zr can significantly improve the adherence of oxide to metal. 18 A1 and Zr are easily oxidized elements that can modify the oxide growth mechanism and suppress chromic oxide formation. 19 Chromium oxide grows via chromium ion diffusion through the oxide layer and leaves the metal atom lattice vacancies at the oxide-metal interface. The accumulation of vacancies and pore formation results in poor adherence and eventually separation of the oxide layer from the metal. 2°, 21 The oxide film thickness did not have a relationship to oxide adherence; the porcelain bonding could not be improved by minimizing the oxide thickness because the oxide adherence is related to the vacancy accumulation, s° The NP2 with Be alloy possessed a higher average bond strength than the NP2 alloy. The main difference in composition was 2 % beryllium in NP2 with Be alloy. The beryllium inhibits the excessive chromium oxide formation 22 and prevents the formation of brittle nickelchromium spinel. 23 Anusavice et al.24 stated that the presence of a bonding agent may broaden or suppress the width of the metal-ceramic interaction, depending upon the specific system. In this study, A1 ions, which mostly come from the bonding agent, diffused more across the interfacial zone and suppressed the diffusion of Cr, Ni, and Co ions (Figs. 5 and 6).25 Data from this study showed that a bonding agent (idem tiffed as a generic type) improved the bond strength by (1) increasing A1 ion diffusion and suppressing Cr, Ni, and Co ion diffusion; (2) preventing poor oxide adherence, from vacancy accumulation and pore formation at interfacial zone; and (3) improving the compatibility of thermal coefficients of porcelain and alloy thus reducing residual stress. SUMMARY
AND CONCLUSIONS
1. The oxidation heat treatment did not have a significant effect on porcelain bonding strength on selected base metal alloys. 2. A bonding agent significantly improved the bonding strength in NP2 with Be and Vicomp alloys but did not significantly improve the bonding strength of NP2 and Neobond II Special alloys. 3. The variables of temperatures, vacuum, and soakingtime in oxidation heat treatment did not have a significant effect on bond strength of porcelain to base metal alloys. 4. The ion concentration profiles showed that the A1 ion diffusion increased but Cr, Co, and Ni ion diffusion were suppressed in NP2 with Be and in Vicomp alloys. Si ion diffusion increased in NP2 with Be alloy but decreased in Vicomp alloy. 5. The gold-tone catalyst bonding agent and beryllium contained in the alloys significantly improved the bond strength of porcelain to metal. *Moser JB. Personal communication, 1985.
444
W e appreciate t h e invaluable guidance of Dr. E v a n H, G r e e n e r in this s t u d y a n d t h e assistance of Dr. D o n g - R u S h i e h in p r e p a r ing t h e article. REFERENCES
I. Kelly JR, Rose TC. Nonprecious alloys for use in fixed prosthodontics: A literature review. J PROSTHET DENT 1983;49:363-70. 2. Moffa JP, Jenkins WA. Status report on base-metal crown and bridge alloys. J A m Dent Assoc 1974;89:652-5. 3. Huget EF, Dvivedi N, Cosner HE Jr. Properties of two nickel-chromium crown and bridge alloys for porcelain veneering. J Am Dent Assoc 1977;94:87-90. 4. Philips RW. Skinner's science of dental materials. 8th ed. Philadelphia: WB Saunders Co, 1982:547-61. 5. Mclean JW, Sced IR. Bonding of dental porcelain to metal. II. The base-metal alloy/porcelain bond. Trans J Br Ceram Soc 1973;72:235-8. 6. Sced IR, Mclean JW. The strength of metal/ceramic bonds with base metals containing chromium. A preliminary report. Br Dent J 1972;132:232-4. 7. Mackert JR, Falrhurst CW. SEM and EDXS characterization of adherent and non-adherent oxides [Abstract]. J Dent Res 1983:62;255. 8. Mclean JW. The science and art of dental ceramics. Chicago: Quintessence Publishing Co Inc, 1980:242. 9. Brugger H, Rasmussen ST, Jeamsonne E. SEM of metallic oxidation of two alloys. Dental porcelain. The state of the art. Los Angeles: University of California Press, 1977:101-7. 10. Brooks MS. Metal preparating and conditioning for porcelain. Dental porcelain. The state of the art. Los Angeles: University of California Press, 1977:157-60. 11. Yashihiro T. Radiograph stress measurement of porcelain fused to metal. J PROSTHETDENT 1984;52:349-52. 12, Bryant RA, Nicholls JI. Measurement of distortions in fixed partial dentures; resulting from degassing. J PROSTHETDF.3qT1979;42:515-20. 13. Anusavice KJ, Dehoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13. 14. Council on Dental Materials, Instruments and Equipment. Porcelain metal alloy compatibility criteria and test method. J Am Dent Assoc 1981;101:71-2. 15. Schwickerath H, Mobel MA. Grundlagen zur prfifung des verbundes metal-ceramik. Dtsch Zahnarzil Z 1983;38:949-52. 16. Wight TA, Bauman JC, Pelleu GB Jr. An evaluation of four variables affecting the bond strength of porcelain to nonprecious alloy. J PROSTHET DENT 1977;37:570-7. 17. Hausselt JH. Improvement of porcelain-fused-to-metal compatibility by a new bonding agent [Abstract]. J Dent Res 1982;61:330. 18. Lustman B. The intermittent oxidation of some nickel-chromium base alloys. Trans AIME 1950;188;995. 19. Mackert J, Parry EE, Hashinger DT, Fairhurst CW. Measurement of oxide adherence to PFM alloys. J Dent Res 1984;63:1335-40. 20. Mackert JR, Ringle RD, Fairhurst CW. Oxide wrinkling and porcelain adherence on non-precious alloys [Abstract]. J Dent Res 1981;60:405. 21. Ringle RD, Fairhurst CW, Anusavice KJ. Microstructure in nonprecious alloys near the porcelain-metal interaction zone. J Dent Res 1979;58:1987-93. 22. Mclean JW. Dental ceramic. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co Inc, 1983:420. 23. Mclean JW. Dental ceramic. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co Inc, 1983:366. 24. Anusavice KJ, Ringle RD, Fairhurst CW. Adherence controlling elements in ceramic-metal systems. II. Nonprecious alloys. J Dent Res 1977;56:1053-61. 25. Anusaviee KJ, Ringle RD, Fairhurst CW. Bonding mechanism evidence in a ceramic-nonprecious alloy systems. J Biomed Mater Res 1977;11:701-9. Reprint requests to: DR. YN-LOWH. WU SCHOOLOF DENTISTRY UNIVERSITY OF CALIFORNIA,SAN FRANCISCO SAN FRANCISCO,CA 94143-0758
OCTOBER 1991
VOLUME 66
NUMBER 4
University of California, School of Dentistry, San Francisco, Calif., Northwestern Dental School, Chicago, Ill., and Washington University, School of Dental Medicine, St. Louis, Mo. B a s e metal alloys h a v e been w i d e l y used for fixed partial dentures in the past decade. The oxidation heat treatment (degassing) of t h e s e alloys is a c o n t r o v e r s i a l step to prepare the metal surface for bonding porcelain. This study e v a l u a t e d the effect of oxidation heat treatment on the porcelain bond strength of b a s e metal alloys and i n v e s t i g a t e d composition c h a n g e s that m a y h a v e occurred during this process. (J PROSTHET DENT 1991;66:439-44)
T h e popularity of the base metal alloys has dramatically increased in the past decade because of their advantageous mechanical properties and the high cost of gold. 14 The superior yield strength (resistance to permanent deformation) and modulus of elasticity (rigidity) of these alloys give them the potential advantage of thinner copings with less material and the required rigidity for long-span fixed partial dentures. Two types of base metal alloys, nickelLchromium and cobalt-chromium, have been commonly used in restorative dentistry. One disadvantage of the base metal alloys is the hard-to-control chromium oxide formation that results in lower bond strength between porcelain and metal. 5-7 Oxidation heat treatment (degassing, outgassing, and preoxidation) of the metal is used to remove the entrapped gas, eliminate surface contaminants, and form the metal oxide layer, s'l° This treatment may release stresses and cause distortion of the framework. 11,12 This study determined the effect of oxidation heat treatment on the porcelain bond strength of base metal alloys under varying time and atmospheric conditions and investigated ion diffusion at the interaction zone between porcelain and alloys. MATERIAL
AND
METHODS
Two nickel-chromium (Ni-Cr) alloys, NP2 and NP2 with beryllium (Be) (Austenal Dental, Chicago, Ill.), and two
Presented at the American Association for Dental Research meeting, Washington, D.C. From a thesis submitted in partial fulfillment of the requirement for the degree of Master of Science, Graduate Prosthodontics, Northwestern University Dental School. aClinical Assistant Professor, Department of Restorative Dentistry, University of California, School of Dentistry. bProfessor, Biological Material Division, Northwestern University, Dental School. CProfessorand Chairman, Section of Fixed Prosthodontics, Northwestern University, Dental School. dprofessor and Chairman, Department of Restorative Dentistry, Washington University, School of Dental Medicine. 10/1/29673 THE JOURNALOF PROSTHETICDENTISTRY
cobalt-chromium (Co-Cr) alloys, Vicomp (Austenal Dental) and Neobond II Special (Neoloy Products Inc., Chicago, Ill), were used in this study. The research was divided into two parts. Part 1 was a three-point bending test for the bond strength and part 2 consisted of the scanning electron microscope (SEM), energy-dispersive analysis of x-rays (EDAX), and a line scan for evaluation of boundary phase changes between metal and porcelain. Bending test Bending
test
Uniform wax test strips were made by lightly compressing two layers of occlusal indicator wax measuring 28 x 3.2 × 0.6 mm (Kerr Division of Sybron Corp., Emeryville, Calif.). The wax strips were sprued and invested with Hi-Temp phosphate investment (Whip-Mix Corp., Louisville, Ky.) under vacuum mixing with a ratio of 9 ml of liquid to 60 gm of powder. The invested rings were allowed to bench set before being placed in a furnace at 1700 ° F. Four alloys were cast with the induction casting machine (Nobilium Co., Chicago, Ill.) at approximately 2800 ° F. The cast strips were measured with calipers and a parallizer (Vickers Instruments, Inc., New York, N.Y.) and were adjusted to dimensions of 25 m m x 3 mm x 0.5 mm to ensure a flat surface. The standardized specimens were airabraded and ultrasonically cleaned and were divided into groups for various oxidation heat treatments (OHT) (Table I). Neobond II Special alloy (Table II) was subjected only to procedures recommended by the manufacturer. The specimens were degassed under vacuum conditions for 1 minute, bench-cooled, air-abraded with 50 gin aluminum oxide, and ultrasonically cleaned for 3 minutes. Microbond porcelain (Austenal Dental Inc.) was applied to the test strips. A thin layer of opaque porcelain was applied to an area 8 mm long located in the center of the metal strip. The metal strips with the opaque porcelain were vacuum fired with a rising temperature of 80 ° F/min. A 0.2 mm thick second coat of opaque porcelain was added and fired. Body porcelain was built up 1 mm using a custommade measuring jig on the opaque porcelain. The body porcelain was condensed by vibration using a Le Cron 4~9
WU ET AL
Table I. N u m b e r of specimens and oxidation heat t r e a t m e n t s for alloys "7
Alloys N u m b e r of Specimens Treatment
NP2
As Cast 5 Air, 0 min 1750 ° F 5 2000 ° F 5 Air, 10 min 1750 ° F 5 2000 ° F 5 Vacuum, 0 min i750 ° F 5 2000 ° F 5 Air, 0 rain + bonding agent 1750 ° F 5 2000 ° F 5 Air, 0 rain 2100 ° F 5 Total 170 50
N P 2 w i t h Be
6
6 6
6 6
6 6
6 6
6 6
6 6
6 6
6 6
6
6
60
60
Table II. N u m b e r of specimens and oxidation heat treatments scheduled for Neobond II Special alloy No.
As cast
5
1925 ° F v a c u u m , 1 m i n
5
1925 ° F v a c u u m , 10 m i n
5
1925 ° F air, 1 rain
5
1925 ° F, v a c u u m 1 rain a n d b o n d i n g a g e n t
5
1750 ° F v a c u u m , 1 m i n
5
2100 ° F v a c u u m , 1 m i n
5
Total
35
Vacuum, 27.5 inch Hg (700 mm Hg).
carver (Miltex, Germany) and was fired at 1775 ° F in a 700 m m Hg vacuum. The tensile bond strength of each sample was tested using a three-point bending device and was measured with an Instron testing machine (Instron Corp., Canton, Mass.). A 200 lb load cell and a crosshead speed of 0.01 in/min were used. A chart speed of 2 in/rain was used to record the load deflection curve. The porcelain-metal bond test subjected samples to a shear stress at the interface, and by measuring the compressive load on the free metal surface, the bond strength was determined. The first point of a sudden drop in the load curve is the bond fracture strength. The d a t a were subjected to the Bonferroni test and to one- or twoway analysis of variance (ANOVA) statistical analysis to determine the correlation between O H T and bond strength.
E n e r g y - d i s p e r s i v e a n a l y s i s of x - r a y s T h e statistics showed t h a t two subgroups in the NP2 with Be and the Vicomp alloys had a significant difference 440
~ 2tO0*F ~2000°F
~ t750° F
Vicomp
6
Treatment
il
0 min
iO min
Vac
Bd Agent
No Rx
210O°F
Conditions
F i g . 1. Bond strength of NP2 alloy at oxidation heat treatments.
between the specimens "without O H T " and those with " O H T with bonding agent." These samples were selected for E D A X examination. Two specimens were e m b e d d e d in Stycast 1266 epoxy resin (Emerson and Cuming Inc., Canton, Mass.). After 2 days of curing, the e m b e d d e d specimens were longitudinally sectioned. The surfaces were smoothed and polished using successively finer grades of abrasive papers, from grit 240 to grit 600 (Buehler Ltd. Inc., Evanston, Ill.), followed by their polishing on a rotating wheel with slurries of 10, 5, and 0.5 ~m alumina oxide. The specimens were coated with a layer of graphite, with the exception of the metal and central p a r t of the porcelain at the metal border. The specimens were sputtered with carbon in a high-vacuum chamber (75 miUipore) for 5 to 15 minutes and were then transferred to the S E M (International Scientific Instruments Inc., Mountain View, Calif.) and E D A X (EDAX International Inc., Prairie View, Ill.) for line scan analysis. Four elements for each alloy were selected to compare the ion concentration profile at the interaction zone, i.e., A1, Si, Ni, and Cr ions in NP2 with Be alloy and A1, Si, Co, and Cr ions in Vicomp alloy. The ion concentration profiles were traced on the acetate paper and were superimposed on each other to compare the changes of ion diffusions.
RESULTS Ni-Cr alloys ( N P 2 ) In NP2 alloy, without Be, bond strengths for heat treatments at 1750 ° F ranged from 6.92 N for vacuum-treated specimens to 9.13 N for those t r e a t e d with the bonding agent. Again, for Be-free alloys for heat treatments at 2000 ° F, bond strengths ranged from 7.79 N for 10 minute air-fired specimens to 10.13 N for specimens with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 7.75 N, and O H T at 2100 ° F for 0 minutes in air, specimens had a bond strength of 8.66 N. No statistical difference was found at p < 0.05 for any of these values. Numerical values are shown in Table III and Fig. 1. W h e n Be was p a r t of the alloy, bond strengths were genOCTOBER 1991 VOLUME 66 NUMBER 4
PORCELAIN BOND STRENGTH TO BASE METAL
1No ~7
heat treatment 2000" F 1750=F ,
!~
mm 2 t 0 0 ' F
Z
ii~
mm 2000" F r~11750. F
m
tt
i5 i4
"~ co
t098755
g~
n 0 min
10 min
Vac
Bd Agent
No Rx
in
2~°OOF
Conditions F i g . 2. Bond strength of NP2 with Be alloy at oxidation heat treatments.
T a b l e III. Bond strength of NP2 alloy at oxidation heat treatments
1750 ° F 20000 F
0 min
8.25 _+1.60 8.41 _+2.30
210G=F
Fig. 3. Bond strength of Vicomp alloy at oxidation heat treatments.
T a b l e IV. Bond strength of NP2 with Be alloy at oxidation heat treatments Air
Air Temperature
Bd Agent No Rx
Conditions
10 m i n
Vacuum
7.02 _+1.92 7.79 _+1.32
Bonding agent
6.92 _+0.78 9.18 ± 1.24
9.13 _+1.63 10.13 _+1.41
No heat treatment, 7.75 ± 1.11 N. 2100 ° F air, 0 rain, 8.66 ± 2.33 N. 2000 ° F average bond strength, 8.875 ± 1.85 N. F value = 2.54, a = 0.05. Scheff~ critical difference = 3.59 N.
erally higher. For heat treatments at 1750 ° F, they ranged from 10.49 N (0 minutes in air) to 13.23 N for specimens with the bonding agent. For heat treatments at 2000 ° F, they ranged from 11.20 N (0 minutes in air) to 14.01 N for specimens with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 10.36 N and at O H T 2100 ° F with 0 minutes in air, specimens had a bond strength of 12.28 N. A significant difference was found a t p < 0.05 between no heat t r e a t m e n t and the mean of all heat treatments at 2000 ° F. Numerical values are shown in Table IV and Fig. 2.
Co-Cr Alloys (Vicomp and Neobond I I Special)
Vicomp. Bond strengths for heat treatments at 1750 ° F ranged from 9.93 N for vacuum-treated specimens to 10.93 N for specimens treated with the bonding agent. For heat t r e a t m e n t at 2000 ° F, bond strengths ranged from 10.14 N (0 minutes in air) to 13.18 N for specimens treated with the bonding agent. No heat t r e a t m e n t yielded a bond strength of 9.93 N and at 2100 ° F in air at 0 minutes, specimens had a bond strength of 10.25 N. There was a significant difference at p < 0.05 between no heat t r e a t m e n t and THE J O U R N A L OF PROSTHETIC D E N T I S T R Y
Bonding agent
0 rain
10 rain
Vacuum
1750°F
10.49_+1.58
11.06_+1.31
11.43_+0.70
2000°F
11.20_+1.14 11.35_+1.23 12.72_+1.43 14.01_+1.68"
13.23_+1.78
No heat treatment, 10.36 ± 2.07 N.* 2100 ° F air, 0 min, 12.28 ± 1.90 N. 2 0 0 0 ° F average bond strength, 12.32 +_ 1.72 N . t F value = 2.45, a = 0.05. Scheff6 critical difference = 2.80 N. * and t, Mean significant difference between these two groups.
the mean of all heat t r e a t m e n t s at 2000 ° F. Numerical values are shown in Table V and Fig. 3. Neobond H Special. Bond strengths for heat treatments at 1925 ° F ranged from 13.17 N for 1 minute in vacuum to 14.75 N in air. Results for other temperatures and conditions ranged from 13.44 N to 14.95 N, as shown in Table VI and Fig. 4. No statistical difference at p < 0.05 was found for any of these values. Tables VII and VIII summarize statistical differences at p ~< 0.05 between NP2, NP2 with Be, and Vicomp alloys with heat t r e a t m e n t temperatures of 1750 ° F and 2000 ° F. No significant differences were found between any of the materials or t r e a t m e n t s at 1750 ° F; all significant differences occurred at a t e m p e r a t u r e of 2000 ° F. The Bonferroni multiple test was used to determine the effect of O H T on the metal by comparing the "without O H T " group to each subgroup "with OHT." The results are shown on Table VII. A significant difference was found between NP2 with Be without oxidation heat t r e a t m e n t and with oxidation heat t r e a t m e n t plus the bonding agent. Also, a significant difference was found between Vicomp alloy without O H T and with O H T plus the bonding agent. Scheff~ and two-way ANOVA tests were used to specify 441
WU ET AL
~ ~7 ~
g,
T a b l e VII. Statistical analysis of oxidation heat treatment
12tOO'F ~ 1750"F
1
mmNO
Temperature
Condition
NP2
NP2 with Be
Vicomp
1750° F
No OHT OHT-BA No OHT OHT-BA
NS NS NS NS
NS NS S* S*
NS NS St St
hllttreatment 2000 ° F
=
5
Bcl Agent No Rx 2tOO'F 1750°F
1 m;~n 10 min ~ . r
O H T , Oxidation heat treatment; O H T - B A , oxidation h e a t t r e a t m e n t plus bonding agent; NS, no significant difference; S, significant difference. *These two groups h a d significant differences. t T h e s e two groups h a d significant differences. p ,~ 0.05; critical value t(0.90~,1o)= 3.17.
Conditions F i g . 4. Bond strength of Neobond II Special alloy at ox-
idation heat treatments.
T a b l e VIII. Effect of oxidation t r e a t m e n t on oxidation heat t r e a t m e n t subgroups NP2
T a b l e V. Bond strength of Vicomp alloy at oxidation heat t r e a t m e n t s
Temperature
1750° F
Air 0 min
1 0 rain
Vacuum
Bonding agent
1750 ° F 10.20 _+ 1.70 9.99 + 2.56 9.93 _+ 1.97 10.93 _+ 2.06 2000 ° F 10.14 _+ L04 12.48 -+ 2.56 12.32 _+ 1.97 13.18 _+ 2.06 No h e a t t r e a t m e n t , 9.93 +. 0.85 N.* 2'100° F air, 0 min, 10.25 +- 1.18 N. 2000 Q F average bond strength, ]2.03 +- 1.58 N . t F Value = 2.45, ~ = 0.05. Scheff~ critical difference = 3,42 N, * and t, M e a n significant difference between these two groups.
T a b l e VI. Bond strength of Neobond II Special alloy at oxidation heat t r e a t m e n t s Vacuum 1 min
i0 rain
Air
Bonding agent
1925 ° F 13.17 +_ 1.33 14.35 _+ 0.88 14.75 +_ 0.92 13.56 _+ 0.62 No O H T , 14.52 _+ 1.88 N. 2100 ° F Vacuum, 1 min, 13.44 _+ 0.73 N. 1750 ° F Vacuum, 1 min, 14.50 +_ 1.78 N. 1925 ° F Vacuum, no air blast after O H T , 14.95 +_ 1.74 N.
the differences among O H T subgroups. The results are shown in Table VIII. A significant difference was found between NP2 with Be, heat t r e a t e d at 0 minutes in air and treated with the bonding agent. T h e ion diffusions, Ni, Cr, Col A1, and Si, were compared between specimens "without" and "with" the bonding agent in NP2 With Be alloy, as shown in Fig. 5, and in Vic0mp alloy, a s shown in Fig. 6. DISCUSSION Dent!stry has yet to establish a standardized bond test for porcelainLfused-to-metal because the in vitro bond tests
442
2000 ° F
Conditions
NP2
with Be
Vicomp
Air, 0 min Air, 10 rain Vacuum, 0 min Bonding agent Air, 0 rain Air, 10 min Vacuum, 0 rain Bonding agent
NS NS NS NS NS NS NS NS
NS NS NS NS S* NS NS S*
NS NS NS NS NS NS NS NS
NS, Not significant; S, significant difference. M e a n difference 2.81 N; critical value, 2.80 N, p < 0.05. *These two groups were significantly different.
do not correlate with clinical fractures. 13 The three-point bending test is recommended by the American Dental Association Council on Dental Materials and Devices. 14 The specimen is relatively easy to prepare and the d a t a can be compared with other published values using the same testing method. 15 O H T in base metal alloys is a controversial subject. In this study, the effect of O H T on the bond strength was determined by comparing the metal "without O H T " and the metal "with O H T " under different conditions: (1), temperature, such as 1750 ° F and 2000 ° F (within the manufacturer's suggested temperature) and 2100 ° F (overheating); (2), soaking time (0 minute and 10 minutes); and (3), pressure, (air and 700 mm Hg pressure). Only Neobond II Special alloy required different O H T procedures as recommended by the manufacturer. The Bonferroni multiple t test was used to compare the "without O H T group" with the " O H T group" and the "2100 ° F overheating group." Only two pairs of subgroups, '!without O H T " and "with O H T plus bonding agent," at 2000 ° F in NP2 with Be and Vicomp alloys had significant differences, with a small margin over the critical value (Table VII). B u t no difference was found in any group between "without O H T " and "with OHT." Two-way ANOVA and the Scheff@ test were also used to differentiate among those O H T subgroups. T h e results showed only the difference between the
OCTOBER 1991
VOLUME 66 NUMBER 4
P O R C E L A I N B O N D S T R E N G T H TO B A S E M E T A L
NP2 with Be
1orePS
~P2 with Be Cr C P ~
C~ | 25O
~i
250
500~ s
, |/
!
A1
.°°tli .°lli
8c
! I
200
m
E 10
ma
" 20 ~
0
DIS~
Diatance from Interface
L
M
CI~! ~ l e
10 u a
EEO~ IntezEa~a per saoaE~
,20 um
/li.
].
10
i0 ~ista~ce from interface 10
........ With bondln~ a~nt _ _ Without bo~Inq agent
A
.
4(
00
1Ù
'k0
;1'
II
100
LQ
6(
,:tl'°
150
0
1'0
Distance from interface CP5: Cycle per second.
....... with bonding agent B
- -
without bonding agent
Fig. 5. Ion diffusions on map of line scan in NP2 with Be alloy. A, Cr and Ni ion diffusions were suppressed after adding bonding agent. B, A1 and Si ions increased diffusion.
Ytcomp
Cr
vico~ CPS 25O
250C[S \:
c: IO0 A1
,
200 I
80
150
M
t'l
40O
"°ll
60
m
P
I00
a
i
. oolj.
• 10C
5O
N
40
P
"°Ii
5O
1. . . . . .
i0 I0 Distance from interface 20 um
10
0
~ ......
111
''
CP3, Cycle per second.
- -
without bonding agent
10
J 20 Ul
0
10
Distamce f r o m interface C~S, C y c l e
Per
sscon~
.........with bonding agent
......... with bonding agent A
io
Distance from interface
Distance from interface
B
~wlthout
b o n d i n g agent
Fig. 6. In Vicomp alloy, Cr and Co ion diffusions were suppressed in the bonding agent group (A). B, Si ions decreased and A1 ions increased diffusion.
subgroups " O H T with or without bonding agent" in NP2 with Be alloy (Table VIII). In the Neobond II Special alloy group, no significant difference was found among the subgroups after analysis by one-way ANOVA (Table VI). Based on these results, the variables of vacuum, temperatures, soaking time, and "without OHT" did not significantly affect the bond strength. However, the use of a bonding agent did affect bond strength. The probability of a type I error might occur in this statistical analysis because of the small value difference.
T H E J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Wight et al.I¢ also reported that the vacuum had no effect on the bond strength but heat-soaked time significantly influenced the strength of bond. The shortest soaking time had the strongest bond strength. In that study, the opaque firing temperature was raised 80 ° F higher than the manufacturer recommended (representing overheating). The bonding agent significantly improved the bond strength in NP2 with Be and Vicomp alloys, but not significantly in NP2 and Neobon II Special alloys. The bonding agent gold tone catalyst liquid contained A1 9 %, Si 36 %,
443
WU ET AL
K 10%, Au 29.4%, and Zr 15.65% .* The gold-rich bonding agent reduced the interfacial stress by improving the compatibility between porcelain and metal. 17 Si and Zr can significantly improve the adherence of oxide to metal. 18 A1 and Zr are easily oxidized elements that can modify the oxide growth mechanism and suppress chromic oxide formation. 19 Chromium oxide grows via chromium ion diffusion through the oxide layer and leaves the metal atom lattice vacancies at the oxide-metal interface. The accumulation of vacancies and pore formation results in poor adherence and eventually separation of the oxide layer from the metal. 2°, 21 The oxide film thickness did not have a relationship to oxide adherence; the porcelain bonding could not be improved by minimizing the oxide thickness because the oxide adherence is related to the vacancy accumulation, s° The NP2 with Be alloy possessed a higher average bond strength than the NP2 alloy. The main difference in composition was 2 % beryllium in NP2 with Be alloy. The beryllium inhibits the excessive chromium oxide formation 22 and prevents the formation of brittle nickelchromium spinel. 23 Anusavice et al.24 stated that the presence of a bonding agent may broaden or suppress the width of the metal-ceramic interaction, depending upon the specific system. In this study, A1 ions, which mostly come from the bonding agent, diffused more across the interfacial zone and suppressed the diffusion of Cr, Ni, and Co ions (Figs. 5 and 6).25 Data from this study showed that a bonding agent (idem tiffed as a generic type) improved the bond strength by (1) increasing A1 ion diffusion and suppressing Cr, Ni, and Co ion diffusion; (2) preventing poor oxide adherence, from vacancy accumulation and pore formation at interfacial zone; and (3) improving the compatibility of thermal coefficients of porcelain and alloy thus reducing residual stress. SUMMARY
AND CONCLUSIONS
1. The oxidation heat treatment did not have a significant effect on porcelain bonding strength on selected base metal alloys. 2. A bonding agent significantly improved the bonding strength in NP2 with Be and Vicomp alloys but did not significantly improve the bonding strength of NP2 and Neobond II Special alloys. 3. The variables of temperatures, vacuum, and soakingtime in oxidation heat treatment did not have a significant effect on bond strength of porcelain to base metal alloys. 4. The ion concentration profiles showed that the A1 ion diffusion increased but Cr, Co, and Ni ion diffusion were suppressed in NP2 with Be and in Vicomp alloys. Si ion diffusion increased in NP2 with Be alloy but decreased in Vicomp alloy. 5. The gold-tone catalyst bonding agent and beryllium contained in the alloys significantly improved the bond strength of porcelain to metal. *Moser JB. Personal communication, 1985.
444
W e appreciate t h e invaluable guidance of Dr. E v a n H, G r e e n e r in this s t u d y a n d t h e assistance of Dr. D o n g - R u S h i e h in p r e p a r ing t h e article. REFERENCES
I. Kelly JR, Rose TC. Nonprecious alloys for use in fixed prosthodontics: A literature review. J PROSTHET DENT 1983;49:363-70. 2. Moffa JP, Jenkins WA. Status report on base-metal crown and bridge alloys. J A m Dent Assoc 1974;89:652-5. 3. Huget EF, Dvivedi N, Cosner HE Jr. Properties of two nickel-chromium crown and bridge alloys for porcelain veneering. J Am Dent Assoc 1977;94:87-90. 4. Philips RW. Skinner's science of dental materials. 8th ed. Philadelphia: WB Saunders Co, 1982:547-61. 5. Mclean JW, Sced IR. Bonding of dental porcelain to metal. II. The base-metal alloy/porcelain bond. Trans J Br Ceram Soc 1973;72:235-8. 6. Sced IR, Mclean JW. The strength of metal/ceramic bonds with base metals containing chromium. A preliminary report. Br Dent J 1972;132:232-4. 7. Mackert JR, Falrhurst CW. SEM and EDXS characterization of adherent and non-adherent oxides [Abstract]. J Dent Res 1983:62;255. 8. Mclean JW. The science and art of dental ceramics. Chicago: Quintessence Publishing Co Inc, 1980:242. 9. Brugger H, Rasmussen ST, Jeamsonne E. SEM of metallic oxidation of two alloys. Dental porcelain. The state of the art. Los Angeles: University of California Press, 1977:101-7. 10. Brooks MS. Metal preparating and conditioning for porcelain. Dental porcelain. The state of the art. Los Angeles: University of California Press, 1977:157-60. 11. Yashihiro T. Radiograph stress measurement of porcelain fused to metal. J PROSTHETDENT 1984;52:349-52. 12, Bryant RA, Nicholls JI. Measurement of distortions in fixed partial dentures; resulting from degassing. J PROSTHETDF.3qT1979;42:515-20. 13. Anusavice KJ, Dehoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13. 14. Council on Dental Materials, Instruments and Equipment. Porcelain metal alloy compatibility criteria and test method. J Am Dent Assoc 1981;101:71-2. 15. Schwickerath H, Mobel MA. Grundlagen zur prfifung des verbundes metal-ceramik. Dtsch Zahnarzil Z 1983;38:949-52. 16. Wight TA, Bauman JC, Pelleu GB Jr. An evaluation of four variables affecting the bond strength of porcelain to nonprecious alloy. J PROSTHET DENT 1977;37:570-7. 17. Hausselt JH. Improvement of porcelain-fused-to-metal compatibility by a new bonding agent [Abstract]. J Dent Res 1982;61:330. 18. Lustman B. The intermittent oxidation of some nickel-chromium base alloys. Trans AIME 1950;188;995. 19. Mackert J, Parry EE, Hashinger DT, Fairhurst CW. Measurement of oxide adherence to PFM alloys. J Dent Res 1984;63:1335-40. 20. Mackert JR, Ringle RD, Fairhurst CW. Oxide wrinkling and porcelain adherence on non-precious alloys [Abstract]. J Dent Res 1981;60:405. 21. Ringle RD, Fairhurst CW, Anusavice KJ. Microstructure in nonprecious alloys near the porcelain-metal interaction zone. J Dent Res 1979;58:1987-93. 22. Mclean JW. Dental ceramic. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co Inc, 1983:420. 23. Mclean JW. Dental ceramic. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co Inc, 1983:366. 24. Anusavice KJ, Ringle RD, Fairhurst CW. Adherence controlling elements in ceramic-metal systems. II. Nonprecious alloys. J Dent Res 1977;56:1053-61. 25. Anusaviee KJ, Ringle RD, Fairhurst CW. Bonding mechanism evidence in a ceramic-nonprecious alloy systems. J Biomed Mater Res 1977;11:701-9. Reprint requests to: DR. YN-LOWH. WU SCHOOLOF DENTISTRY UNIVERSITY OF CALIFORNIA,SAN FRANCISCO SAN FRANCISCO,CA 94143-0758
OCTOBER 1991
VOLUME 66
NUMBER 4
