ies of test metal ceramic
titanium
alloys
Koichi Akagi, DDS,a Yosbizo Okamoto, MEng, PhD,b Tomoji Matsuura, DDS, PhD,C and Takashi Horibe, PhDd Fukuoka Dental College, Fukuoka, Japan Four test alloys were prepared using a high frequency centrifugal casting machine and a ceramic crucible for the development of titanium bonding alloys that can be cast in the ordinary atmosphere. Of these alloys, 10.06% Ti, 78.79% Ni, 9.02% Pd, 1.77% Sn and 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir could be cast by the conventional high frequency centrifugal method; however, 89.18% Ti, 8.75% Ni, 1.03% Pd, 0.28% Sn and 89..81% Ti, 8.15% Ni, 1.01% Pd, 0.18% Sn, 0.67% Ir could be cast only by the argon arc melting method. The alloys 10.06% Ti, 78.95% Ni, 9.02% Pd, 1.77% Sn and 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir showed excellent physical and mechanical properties and bonding strengths, surpassing those of the commercial alloys TPW and Unimetal. Concerning the elution of component elements, the amounts of titanium eluted from these alloys were far smaller than those from pure titanium or a Ti-6AL4V alloy, and nickel elution, which has become an issue in relation to metal allergy, was almost nil in contrast to Unimetal (Ni-Cr alloy). The alloy 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir showed properties that indicated its favorable use as an alloy for the bonding of dental porcelain. (J PROSTHET DENT 1992;68:462-7.)
T.
itanium is used in a variety of fields because of its excellent properties, which include physical or mechanical characteristic& 2 such as a low specific gravity,3s 4 a high strength comparable to that of stainless steel,3 high heat resistance, and chemical properties2,4 such as corrosion resistance, which is derived from a thin passive layer (TiOs) formed on the surface at ordinary temperatures. Titanium has found acceptability as a biomateria14-6 in cardiac valves and pacemakers for surgery, in artificial joints for orthopedics, and in implants for dentistry. Being light, hard and firm, corrosion-resistant, and biocompatible, titanium fulfills the requirements of a dental material for potential use clinically in crowns, fixed partial dentures, and removable partial denture frameworks.7-g However, titanium has a high melting point and is active at a high temperature so that it may become fragile by reacting with atmospheric oxygen or may corrode the crucible (ceramics) during casting by conventional methods. For these reasons, it must be handled in an inactive gas atmosphere, which makes its casting and processing difficult.7, 8 Recently, casters and crucibles for argon arc melting have been developed,l* but they have not attained wide acceptance for practical use. In this study, four titanium test alloys were developed and their physical properties, bonding strength, and elution of component metal elements were
aInstructor, Department of Crown and Bridge Prosthodontics. Lecturer, Department of Dental Materials and Devices. CProfessor and Chairman, Department of Crown and Bridge Prosthodontics. dProfessor and Chairman, Department of Dental Materials and Devices. 1011137422
462
Table I. Percent composition
1OTi lOTi-Ir
90Ti SOTi-Ir
of test alloys (wt%)
Ti
Ni
Pd
Sll
Ir
10.06 9.91 89.18 89.81
78.95 78.F;6 8.75 8.15
9.02 9.07 1.03 1.01
1.17 1.86 0.28 0.18
0.65 0.67
compared with those of commercial metal ceramic alloys, pure titanium, and a Ti-6Al-4V alloy.
MATERIALS
AND
METHODS
The metal materials used were four test alloys containing titanium (Table I), a commercial noble metal alloy for porcelain bonding, TPW (Tokuriki Honten Co., Ltd., Tokyo, Japan), a base metal alloy, Unimetal (Shofu Co., Ltd., Kyoto, Japan), pure titanium, and a 90% Ti, 6% Al, 4% V alloy (Ti-6Al-4V). Unibond vintage (Shofu Co., Ltd.) was used as the porcelain material for the extraction test. The test alloys were prepared as follows. A mother alloy was obtained by vacuum melting of nickel (88% ), palladium (lo%), and Tin (2%), and titanium was mixed at 10% (10Ti) or 90% (90Ti) with this mother alloy. Then another mother alloy was prepared by adding 1 gm of iridium to the first mother alloy, and titanium was mixed at 10 % (lOTi-Ir) or 90 % (SOTi-Ir). The 10Ti and lOTi-Ir were cast by the use of a high frequency centrifugal casting machine and a ceramic crucible in air, but 90Ti and SOTi-Ir were cast by the arc melting method. Samples of pure titanium and the Ti-6Al-4V alloy were prepared by machining.
SEPTEMBER1992
VOLUME68
NUMBER3
PROPERTIES
1C
OF TITANIUM
ALLOYS
(kcps!
F: IOTi-Nf-Pd-Sn-Ir E:SOTi-Ni-Pd-Sn-Ir
B:C?OTi-Ni-Pd-Sn A: Ni-Cr-@e Ati-Ft-Pd ii 90
100 (deal
Fig. 1. X-ray diffraction profiles of test alloys and commercial alloys. Ni-Cr-Be is Unimetal and Au-Pt-Pd is TPW.
X-ray diffraction analysis was made on test pieces prepared to a size of 20 x 20 x 1 mm. Their test surfaces were polished with emery paper to 1000 fineness under running water and were measured with an x-ray diffraction analyzer (XD-610, Shimadzu Co., Ltd., Kyoto, Japan). Scanning electron microscopy (SEM) was performed with a JSM-T20 instrument (JEOL Co., Ltd., Tokyo, Japan), and the Vicker’s hardness and melting point were determined with an MVK-E testing machine (Akashi Co., Ltd., Tokyo, Japan) and an ULVAC DT-7000 tester (Nihon Shinkuu Gijutu Co., Ltd., Kanagawa, Japan), respectively, on test pieces 5 x 5 x 1 mm. The test surfaces were mirror polished before the examination. The Vicker’s hardness was determined at 10 points with a load of 500 gf/lO seconds, and the melting point was determined at a te.mperature increase rate of 10’ C/min. The tensile strength was determined on rod-like test
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pieces 2 x 40 mm with an Autograph IS-5000 machine (Shimadzu Co., Ltd.) at a fulcrum distance of 20 mm and a cross-head speed of 1 mm/min. Heat expansion was studied on columnar test pieces 3 x 7 mm, which were polished with emery paper to 1000 fineness under running water, using a TMC-30 tester (Shimadzu Co. Ltd.) at a temperature increase rate of 20° Clmin. The amount of the oxide coating formed was determined on sample pieces 5 X 5 X 1 mm, which were polished with emery paper to 1000 fineness under running water, using a TGA-50 tester (Shimadzu Co., Ltd.) at a temperature increase rate of 35” C/min and an air flow of 30 ml/min. The bonding strength was measured on sample pieces 3.2 X 40 mm, which were sandblasted (aluminum oxide, 250 pm) and bonded to a porcelain material over an area of
463
AKAGI
I OTI
12
____--___---
90Ti I OTi-lr
I _..- - -
SOTi-lr
Ti _ . _ _ _ _ - Ti -6AI-4\!
_._._._. -.v TPw
“
ET AL
,’
08
04
400
600
Temperature
800
1000
200
("C)
400
600
Temperature
800
:( // !,
("Cl
Fig. 2. Heat expansion curves.
2 x 10 mm using an Autograph IS-5000 testing machine (Shimadzu Co., Ltd.) at a cross-head speed of 1 mm/ min. Elution tests of component elements were made on test pieces 5 x 5 x 1 mm polished with emery paper to 1000 fineness under running water. The test pieces were placed in an artificial saliva of 0.05 % HCl, 1% NaCl, and 1% lactic acid in a ba-th at 37O C for 3 days. The elements eluted were measured with an ICP-V1012P analyzer (Shimadzu Co., Ltd.).
RESULTS Fig. 1 shows x-ray diffraction profiles of the four titanium test alloys, commercial metal/ceramic alloys TPW and Unimetal, pure titanium, and Ti-6Al-4V. Table II shows the properties of the test alloys and the commercial alloys. The Vicker’s hardness values of the test alloys were three to four times the value for TPW (103.7), comparable with or greater than that of Unimetal(394.6); three to four times the value for pure titanium (119.5), and higher than that of Ti-6Al-4V (312.9). The melting point of the test alloys increased slightly after the addition of iridium, but high frequency melting was possible in these temperature ranges. The addition of iridium increased the tensile strength of the 1OTi test alloy but slightly decreased that of the 90Ti alloy. However, as homogeneous samples of 90Ti were difficult to obtain even by arc melting in an argon atmosphere, 464
a material prepared in a wire form was used for measurement. The tensile strengths of 1OTi and lOTi-Ir were about five times and about 1.5 times greater than those of TPW and Unimetal, respectively. Their elongation percentages were two to three times greater than those of TPW and Unimetal. The values for the coefficient of heat expansion of 1OTi and lOTi-Ir were comparable with those of TPW and Unimetal. They were also close to the value for Unibond Vintage used in the test of bonding strength (15.9 x 10e6/” C). As shown in Fig. 2, changes in the percent expansion with temperature changes to 890° C were similar among lOTi, lOTi-Ir, TPW, and Unimetal, but transition points were observed near 750“ C in 90Ti and SOTi-Ir. The amount of oxide coating produced (Fig. 3) increased rapidly at about 800’ C and above in 90Ti and SOTi-Ir when compared with 1OTi and lOTi-Ir. The bonding strength (Table II) for 1OTi and lOTi-Ir was far higher than values for TPW and Unimetal, but the values of 90Ti and SOTi-Ir were considerably lower. After extraction, the oxide coating of the lOTi-Ir alloy was found on the broken surface on the porcelain side, and the porcelain material was found on the broken surface of the alloy side. The oxide coating was also thin in this material, showing features of aggregation breakage. The results of the elution test of component metal elements are shown in Tables III and IV. Elution of each metal component was of an extremely small quantity, including that of nickel, SEPTEMBER
1992
VOLUME
68
NUMBER
3
PROPERTIES OF TITANIUM ALLOYS
Table
II.
Properties of test alloys and commercial alloys Melting point HMV
1OTi
433.4
(“C)
1223-1250
90Ti
SOTi-Ir TPW
Unimetal
1165-1423
27.4 (1.7) 25.0*
1382-1429
895.7
15.0
437.0 (5.5) 402.0 (13.4) 322.8 (10.1) 103.7 (12.7)
1268-1294
394.6
1148-1250
119.5
Ti-6Al-4V
(3.3) 312.9 (9.7)
24.8
Coefficient of expansion (X lo-s/%)**
Amount of oxide coating produced (xlO-%ng/mm2)
Bonding strength (MW
14.84
10.7
28.39
15.12
23.5
(2.5) 31.61
10.54
45.2
(3.1) 8.68
10.06
50.5
(0.8)
(2.1) 1.96
(1.2) 1100-1250
(1.3)
Ti
Elongation (W)
1127.0 (41.4) 1239.2 (37.6) 1136.8*
(6.8) LOTi-Ir
Tensile strength (IMPa)
1668 1650
263.4 (27.4) 826.8 (65.7) 461.2 (23.6) 730.1 (28.9)
9.2 (1.9) 12.2 (1.9) 14.2 (3.4) 11.0
15.48
22.1
23.54 (1.5) 14.38
15.68
-29.9
10.37
109.4
1.91
56.3
(0.3) 2.97
(1.6) 12.43
(0.2)
Standard deviations in parentheses. H.~v, Vicker’s hardness value. *Measured in machined samples. **Coefficient of heat expansion in a range of 200° to 700° C.
DISCUSSION Titanium has many physical and mechanical properties suitable for a dental material but, at the same time, has a high melting point and is highly active and readily oxidized at high temperatures. Both pure titanium and the Ti-6Al-4V alloy, which are used in dentistry at present, have satisfactory physical properties and biocompatibilityj1m6but their casting is difficult as it requires nonceramic (e.g., copper, calcium) crucibles and arc melting in an inactive gas atmosphere. 7,8 In an attempt to develop titanium alloys for porcelain bonding, we tentatively prepared alloys consisting o.f titanium, nickel, palladium, and tin. Titanium was selected as the base or as an additive of these alloys because it is light (specific gravity, 4.5), hard (comparable to stainless steel), firm, and corrosion-resistant,l, 2,6 cosmetically acceptable, and biocompatible,“-6 fulfilling the conditions required of dental metals today. However, titanium-based alloys and alloys containing titanium are prone to gap corrosion and discoloration in the oral cavity. Therefore titanium was electrochemically inactivated by the addition of a small percentage of a metal of the platinum group to improve the anticorrosion and antidiscoloration properties of the alloys by inducing a firm passive coating. Palladiuml’~ I2 was chosen among the platinum group metals, as it prevents corrosion of titanium by the addition of only a small amount (0.15%). A problem encountered here was that it is difficult to obtain homogeneous alloys of titanium and palladium because of the wide difference in the specific gravity (4.5 for titanium and 12.0 for palladium) and the shortness of the interatomic distance of the two metals, the former showing a bodycentered cubic lattice and the latter showing a faceTHE JOURNAL OF PROSTHETIC DENTISTRY
centered cubic lattice at high temperatures. Therefore nickel, with a high affinity for platinum group metals, and tin, which improves the castability and manipulability of the alloys, were added. Iridium was added to some alloys to further increase the hardness and firmness. Although it was difficult to homogeneously mix a platinum group metal with titanium, titanium-based alloys and alloys containing titanium could be prepared by adding titanium to a mother alloy of nickel, palladium, and tin. Test alloys were prepared by adding titanium to vacuum-melted mother alloys of Ni-Pd-Sn and Ni-Pd-Sn-Ir. Of the four test alloys, 1OTi and lOTi-lr had melting points of 1223“ to 1250° C and 1268’ to 1294OC, respectively, and could be cast with a high frequency centrifugal casting machine presently employed in dentistry; 90Ti and SOTi-Ir, however, could not be cast by this method. Concerning the physical and mechanical properties of these alloys, 10Ti and lOTi-Ir had a specific gravity of 8.3, which was smaller than the 18.7 of TPW and comparable with the 7.9 value for Unimetal, which are commercial alloys for bonding to porcelain. The Vicker’s hardness of 1OTi and lOTi-Ir, which reflects the hardness and firmness of the materials, was about four times that of TPW and comparable with or greater than that of Unimetal. The tensile strength of 1OTi and lOTi-Ir was about five times that of TPW and 1.5 times that of Unimetal. The elongation percentage was about three times that of TPW and about two times that of Unimetal. These physical and mechanical properties of 1OTi and IOTi-Ir were satisfactory as materials for dental prostheses. The coefficient of heat expansion is a most important factor in bonding of an alloy to porcelain. The difference in 465
AKAGI ET AL
400
I 600
800
1000 CC)
r
I
1
200
400
600
800
1000 (‘c )
0
200
200
400
600
800
1000 i-c 1
0
200
T-- 200
400
600
800
1000
-0 0
200
0---X0
-I
0
0
-I
-0
0
Fig. 3. Amount of oxide coating produced commercial materials. III.
1OTi
lOTi-Ir
Concentrations
of component
1% NaCl 0.05% HCI
SOTi-Ir
600
800
400
6000
I
‘;Wy
r
400
600
800
1000 (‘C)
800
titanium
alloys
Koichi Akagi, DDS,a Yosbizo Okamoto, MEng, PhD,b Tomoji Matsuura, DDS, PhD,C and Takashi Horibe, PhDd Fukuoka Dental College, Fukuoka, Japan Four test alloys were prepared using a high frequency centrifugal casting machine and a ceramic crucible for the development of titanium bonding alloys that can be cast in the ordinary atmosphere. Of these alloys, 10.06% Ti, 78.79% Ni, 9.02% Pd, 1.77% Sn and 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir could be cast by the conventional high frequency centrifugal method; however, 89.18% Ti, 8.75% Ni, 1.03% Pd, 0.28% Sn and 89..81% Ti, 8.15% Ni, 1.01% Pd, 0.18% Sn, 0.67% Ir could be cast only by the argon arc melting method. The alloys 10.06% Ti, 78.95% Ni, 9.02% Pd, 1.77% Sn and 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir showed excellent physical and mechanical properties and bonding strengths, surpassing those of the commercial alloys TPW and Unimetal. Concerning the elution of component elements, the amounts of titanium eluted from these alloys were far smaller than those from pure titanium or a Ti-6AL4V alloy, and nickel elution, which has become an issue in relation to metal allergy, was almost nil in contrast to Unimetal (Ni-Cr alloy). The alloy 9.91% Ti, 78.56% Ni, 9.07% Pd, 1.86% Sn, 0.65% Ir showed properties that indicated its favorable use as an alloy for the bonding of dental porcelain. (J PROSTHET DENT 1992;68:462-7.)
T.
itanium is used in a variety of fields because of its excellent properties, which include physical or mechanical characteristic& 2 such as a low specific gravity,3s 4 a high strength comparable to that of stainless steel,3 high heat resistance, and chemical properties2,4 such as corrosion resistance, which is derived from a thin passive layer (TiOs) formed on the surface at ordinary temperatures. Titanium has found acceptability as a biomateria14-6 in cardiac valves and pacemakers for surgery, in artificial joints for orthopedics, and in implants for dentistry. Being light, hard and firm, corrosion-resistant, and biocompatible, titanium fulfills the requirements of a dental material for potential use clinically in crowns, fixed partial dentures, and removable partial denture frameworks.7-g However, titanium has a high melting point and is active at a high temperature so that it may become fragile by reacting with atmospheric oxygen or may corrode the crucible (ceramics) during casting by conventional methods. For these reasons, it must be handled in an inactive gas atmosphere, which makes its casting and processing difficult.7, 8 Recently, casters and crucibles for argon arc melting have been developed,l* but they have not attained wide acceptance for practical use. In this study, four titanium test alloys were developed and their physical properties, bonding strength, and elution of component metal elements were
aInstructor, Department of Crown and Bridge Prosthodontics. Lecturer, Department of Dental Materials and Devices. CProfessor and Chairman, Department of Crown and Bridge Prosthodontics. dProfessor and Chairman, Department of Dental Materials and Devices. 1011137422
462
Table I. Percent composition
1OTi lOTi-Ir
90Ti SOTi-Ir
of test alloys (wt%)
Ti
Ni
Pd
Sll
Ir
10.06 9.91 89.18 89.81
78.95 78.F;6 8.75 8.15
9.02 9.07 1.03 1.01
1.17 1.86 0.28 0.18
0.65 0.67
compared with those of commercial metal ceramic alloys, pure titanium, and a Ti-6Al-4V alloy.
MATERIALS
AND
METHODS
The metal materials used were four test alloys containing titanium (Table I), a commercial noble metal alloy for porcelain bonding, TPW (Tokuriki Honten Co., Ltd., Tokyo, Japan), a base metal alloy, Unimetal (Shofu Co., Ltd., Kyoto, Japan), pure titanium, and a 90% Ti, 6% Al, 4% V alloy (Ti-6Al-4V). Unibond vintage (Shofu Co., Ltd.) was used as the porcelain material for the extraction test. The test alloys were prepared as follows. A mother alloy was obtained by vacuum melting of nickel (88% ), palladium (lo%), and Tin (2%), and titanium was mixed at 10% (10Ti) or 90% (90Ti) with this mother alloy. Then another mother alloy was prepared by adding 1 gm of iridium to the first mother alloy, and titanium was mixed at 10 % (lOTi-Ir) or 90 % (SOTi-Ir). The 10Ti and lOTi-Ir were cast by the use of a high frequency centrifugal casting machine and a ceramic crucible in air, but 90Ti and SOTi-Ir were cast by the arc melting method. Samples of pure titanium and the Ti-6Al-4V alloy were prepared by machining.
SEPTEMBER1992
VOLUME68
NUMBER3
PROPERTIES
1C
OF TITANIUM
ALLOYS
(kcps!
F: IOTi-Nf-Pd-Sn-Ir E:SOTi-Ni-Pd-Sn-Ir
B:C?OTi-Ni-Pd-Sn A: Ni-Cr-@e Ati-Ft-Pd ii 90
100 (deal
Fig. 1. X-ray diffraction profiles of test alloys and commercial alloys. Ni-Cr-Be is Unimetal and Au-Pt-Pd is TPW.
X-ray diffraction analysis was made on test pieces prepared to a size of 20 x 20 x 1 mm. Their test surfaces were polished with emery paper to 1000 fineness under running water and were measured with an x-ray diffraction analyzer (XD-610, Shimadzu Co., Ltd., Kyoto, Japan). Scanning electron microscopy (SEM) was performed with a JSM-T20 instrument (JEOL Co., Ltd., Tokyo, Japan), and the Vicker’s hardness and melting point were determined with an MVK-E testing machine (Akashi Co., Ltd., Tokyo, Japan) and an ULVAC DT-7000 tester (Nihon Shinkuu Gijutu Co., Ltd., Kanagawa, Japan), respectively, on test pieces 5 x 5 x 1 mm. The test surfaces were mirror polished before the examination. The Vicker’s hardness was determined at 10 points with a load of 500 gf/lO seconds, and the melting point was determined at a te.mperature increase rate of 10’ C/min. The tensile strength was determined on rod-like test
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pieces 2 x 40 mm with an Autograph IS-5000 machine (Shimadzu Co., Ltd.) at a fulcrum distance of 20 mm and a cross-head speed of 1 mm/min. Heat expansion was studied on columnar test pieces 3 x 7 mm, which were polished with emery paper to 1000 fineness under running water, using a TMC-30 tester (Shimadzu Co. Ltd.) at a temperature increase rate of 20° Clmin. The amount of the oxide coating formed was determined on sample pieces 5 X 5 X 1 mm, which were polished with emery paper to 1000 fineness under running water, using a TGA-50 tester (Shimadzu Co., Ltd.) at a temperature increase rate of 35” C/min and an air flow of 30 ml/min. The bonding strength was measured on sample pieces 3.2 X 40 mm, which were sandblasted (aluminum oxide, 250 pm) and bonded to a porcelain material over an area of
463
AKAGI
I OTI
12
____--___---
90Ti I OTi-lr
I _..- - -
SOTi-lr
Ti _ . _ _ _ _ - Ti -6AI-4\!
_._._._. -.v TPw
“
ET AL
,’
08
04
400
600
Temperature
800
1000
200
("C)
400
600
Temperature
800
:( // !,
("Cl
Fig. 2. Heat expansion curves.
2 x 10 mm using an Autograph IS-5000 testing machine (Shimadzu Co., Ltd.) at a cross-head speed of 1 mm/ min. Elution tests of component elements were made on test pieces 5 x 5 x 1 mm polished with emery paper to 1000 fineness under running water. The test pieces were placed in an artificial saliva of 0.05 % HCl, 1% NaCl, and 1% lactic acid in a ba-th at 37O C for 3 days. The elements eluted were measured with an ICP-V1012P analyzer (Shimadzu Co., Ltd.).
RESULTS Fig. 1 shows x-ray diffraction profiles of the four titanium test alloys, commercial metal/ceramic alloys TPW and Unimetal, pure titanium, and Ti-6Al-4V. Table II shows the properties of the test alloys and the commercial alloys. The Vicker’s hardness values of the test alloys were three to four times the value for TPW (103.7), comparable with or greater than that of Unimetal(394.6); three to four times the value for pure titanium (119.5), and higher than that of Ti-6Al-4V (312.9). The melting point of the test alloys increased slightly after the addition of iridium, but high frequency melting was possible in these temperature ranges. The addition of iridium increased the tensile strength of the 1OTi test alloy but slightly decreased that of the 90Ti alloy. However, as homogeneous samples of 90Ti were difficult to obtain even by arc melting in an argon atmosphere, 464
a material prepared in a wire form was used for measurement. The tensile strengths of 1OTi and lOTi-Ir were about five times and about 1.5 times greater than those of TPW and Unimetal, respectively. Their elongation percentages were two to three times greater than those of TPW and Unimetal. The values for the coefficient of heat expansion of 1OTi and lOTi-Ir were comparable with those of TPW and Unimetal. They were also close to the value for Unibond Vintage used in the test of bonding strength (15.9 x 10e6/” C). As shown in Fig. 2, changes in the percent expansion with temperature changes to 890° C were similar among lOTi, lOTi-Ir, TPW, and Unimetal, but transition points were observed near 750“ C in 90Ti and SOTi-Ir. The amount of oxide coating produced (Fig. 3) increased rapidly at about 800’ C and above in 90Ti and SOTi-Ir when compared with 1OTi and lOTi-Ir. The bonding strength (Table II) for 1OTi and lOTi-Ir was far higher than values for TPW and Unimetal, but the values of 90Ti and SOTi-Ir were considerably lower. After extraction, the oxide coating of the lOTi-Ir alloy was found on the broken surface on the porcelain side, and the porcelain material was found on the broken surface of the alloy side. The oxide coating was also thin in this material, showing features of aggregation breakage. The results of the elution test of component metal elements are shown in Tables III and IV. Elution of each metal component was of an extremely small quantity, including that of nickel, SEPTEMBER
1992
VOLUME
68
NUMBER
3
PROPERTIES OF TITANIUM ALLOYS
Table
II.
Properties of test alloys and commercial alloys Melting point HMV
1OTi
433.4
(“C)
1223-1250
90Ti
SOTi-Ir TPW
Unimetal
1165-1423
27.4 (1.7) 25.0*
1382-1429
895.7
15.0
437.0 (5.5) 402.0 (13.4) 322.8 (10.1) 103.7 (12.7)
1268-1294
394.6
1148-1250
119.5
Ti-6Al-4V
(3.3) 312.9 (9.7)
24.8
Coefficient of expansion (X lo-s/%)**
Amount of oxide coating produced (xlO-%ng/mm2)
Bonding strength (MW
14.84
10.7
28.39
15.12
23.5
(2.5) 31.61
10.54
45.2
(3.1) 8.68
10.06
50.5
(0.8)
(2.1) 1.96
(1.2) 1100-1250
(1.3)
Ti
Elongation (W)
1127.0 (41.4) 1239.2 (37.6) 1136.8*
(6.8) LOTi-Ir
Tensile strength (IMPa)
1668 1650
263.4 (27.4) 826.8 (65.7) 461.2 (23.6) 730.1 (28.9)
9.2 (1.9) 12.2 (1.9) 14.2 (3.4) 11.0
15.48
22.1
23.54 (1.5) 14.38
15.68
-29.9
10.37
109.4
1.91
56.3
(0.3) 2.97
(1.6) 12.43
(0.2)
Standard deviations in parentheses. H.~v, Vicker’s hardness value. *Measured in machined samples. **Coefficient of heat expansion in a range of 200° to 700° C.
DISCUSSION Titanium has many physical and mechanical properties suitable for a dental material but, at the same time, has a high melting point and is highly active and readily oxidized at high temperatures. Both pure titanium and the Ti-6Al-4V alloy, which are used in dentistry at present, have satisfactory physical properties and biocompatibilityj1m6but their casting is difficult as it requires nonceramic (e.g., copper, calcium) crucibles and arc melting in an inactive gas atmosphere. 7,8 In an attempt to develop titanium alloys for porcelain bonding, we tentatively prepared alloys consisting o.f titanium, nickel, palladium, and tin. Titanium was selected as the base or as an additive of these alloys because it is light (specific gravity, 4.5), hard (comparable to stainless steel), firm, and corrosion-resistant,l, 2,6 cosmetically acceptable, and biocompatible,“-6 fulfilling the conditions required of dental metals today. However, titanium-based alloys and alloys containing titanium are prone to gap corrosion and discoloration in the oral cavity. Therefore titanium was electrochemically inactivated by the addition of a small percentage of a metal of the platinum group to improve the anticorrosion and antidiscoloration properties of the alloys by inducing a firm passive coating. Palladiuml’~ I2 was chosen among the platinum group metals, as it prevents corrosion of titanium by the addition of only a small amount (0.15%). A problem encountered here was that it is difficult to obtain homogeneous alloys of titanium and palladium because of the wide difference in the specific gravity (4.5 for titanium and 12.0 for palladium) and the shortness of the interatomic distance of the two metals, the former showing a bodycentered cubic lattice and the latter showing a faceTHE JOURNAL OF PROSTHETIC DENTISTRY
centered cubic lattice at high temperatures. Therefore nickel, with a high affinity for platinum group metals, and tin, which improves the castability and manipulability of the alloys, were added. Iridium was added to some alloys to further increase the hardness and firmness. Although it was difficult to homogeneously mix a platinum group metal with titanium, titanium-based alloys and alloys containing titanium could be prepared by adding titanium to a mother alloy of nickel, palladium, and tin. Test alloys were prepared by adding titanium to vacuum-melted mother alloys of Ni-Pd-Sn and Ni-Pd-Sn-Ir. Of the four test alloys, 1OTi and lOTi-lr had melting points of 1223“ to 1250° C and 1268’ to 1294OC, respectively, and could be cast with a high frequency centrifugal casting machine presently employed in dentistry; 90Ti and SOTi-Ir, however, could not be cast by this method. Concerning the physical and mechanical properties of these alloys, 10Ti and lOTi-Ir had a specific gravity of 8.3, which was smaller than the 18.7 of TPW and comparable with the 7.9 value for Unimetal, which are commercial alloys for bonding to porcelain. The Vicker’s hardness of 1OTi and lOTi-Ir, which reflects the hardness and firmness of the materials, was about four times that of TPW and comparable with or greater than that of Unimetal. The tensile strength of 1OTi and lOTi-Ir was about five times that of TPW and 1.5 times that of Unimetal. The elongation percentage was about three times that of TPW and about two times that of Unimetal. These physical and mechanical properties of 1OTi and IOTi-Ir were satisfactory as materials for dental prostheses. The coefficient of heat expansion is a most important factor in bonding of an alloy to porcelain. The difference in 465
AKAGI ET AL
400
I 600
800
1000 CC)
r
I
1
200
400
600
800
1000 (‘c )
0
200
200
400
600
800
1000 i-c 1
0
200
T-- 200
400
600
800
1000
-0 0
200
0---X0
-I
0
0
-I
-0
0
Fig. 3. Amount of oxide coating produced commercial materials. III.
1OTi
lOTi-Ir
Concentrations
of component
1% NaCl 0.05% HCI
SOTi-Ir
600
800
400
6000
I
‘;Wy
r
400
600
800
1000 (‘C)
800
