GB2177116A

GB2177116A - High density sintered article of silicon carbide

Info

Publication number: GB2177116A
Application number: GB08516336A
Authority: GB
Inventors: Mitsuhiko Furukawa; Takashi Kitahira; Sakae Tanaka; Kiyohito Misumi
Original assignee: Nippon Tungsten Co Ltd
Current assignee: Nippon Tungsten Co Ltd
Priority date: 1984-04-27
Filing date: 1985-06-28
Publication date: 1987-01-14
Also published as: JPH044992B2; JPS60231466A; GB2177116B; GB8516336D0

Abstract

A sintered article of silicon carbide containing 1 to 12% by weight of erbuim oxide, not more than 2% by weight of aluminium oxide, existing in the form of a composite oxide exhibits remarkable characteristics in resisting to oxidation, thermal shock and corrosion, and shows increased strength at elevated temperatures due to their effect to compaction of resulting structure of the sintered article due to retaining fineness of crystal grains. Those meritorious effects can be enhanced by adding to the aforementioned composition 0.5 to 6.0% by weight of at least one element selected from among titanium, vanadium, chromium, manganese, magnesium, yttrium, zirconium, niobium, molybdenum, barium, lanthanum, cerium, gadolinium, hafnium, tantalum, tungsten, thorium, and cesium or a compound of this element.

Description

SPECIFICATION High-density sintered article of silicon carbide BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a sintered article of silicon carbide exhibiting resistance to oxidation, resistance to thermal shock, resistance to corrosion, and strength at elevated temperatures and, at the same time, possessing high density.

DESCRIPTION OF THE PRIOR ART In recent yea rs, sintered articles ofsilicon carbide, owing to theirfeature of excelling in resistance to oxidation, resistance to thermal shock, resistance to corrosion, and strength at elevated temperatures, have come to find growing utility in applications to various structural materials, check valves and sealing members designed to handle corrosive liquids, heat exchangermembersfor high-temperature furnaces, members expected to withstand heavy friction. Even the desirability of sintered articles which are substan tiallydevoid of pore and are stronger has come to find recognition.

As methods of producing such silicon carbide, (A) chemical vapor deposition (CVD), (B) reaction sintering, and (C) conventional sintering have been known.

The method of (A) is capable of producing homogeneousandcompactsiliconcarbidegenerally only in the form offilm and, therefore, is practically, barely suitableforthe purpose of coating various materials. The method of (B) which comprises sintering a compact of silicon carbide powderora mixed powder of silicon dioxide and silicon carbide is capable of producing articles of large dimensions but low density. Therefore, this method is now applied onlyto production of refractories and heat generators.

Forthe production ofsintered articles of large dimensions and high density, the method of (C) is considered as the optimum means.

Incidentally, silicon carbide, which is a compound of high covalent bond property and, therefore, is hard, tough, and stable at elevated temperatures, exhibits very poorsintering property and does not permit easy production of sintered articles when conventional sintering process is applied. Many studies have been being reported concerning adding various sintering aidstoimproveitssintering property of silicon carbide powder. Forexample, R.Alliegro etal.Journal of Ceramic Society, Vo. 39, pp.386-389 (1956), the specifications ofJapanese Patent Laid-open Publication 49-007311,49-099308, 50-078609,51-065000, 53-067711, and 53-084013 disclose the effect of use of Al, Fe, B, B4C, etc. as sintering aids permits production of sintered articles showing low pore contents and high strength.

Strengths of sintered articles is defected greatly by the factors of (A) porosity, (B) surface flaw, and (C) grain size. The problem of porosity of (A) can be remarkably solved by using varioussintering aids as mentioned above. Although, the sintered articles so produced by the incorporation of such sintering aids, contain extents of microscopic pores. Causing of said (B) surfaceflaw can be avoided by payment of careful attention to fabrication. The problem caused by the factor from grain size of (C) is most difficult, because of grain growth during the course of sintering, and difficulty of retaining the starting fine grains during the course of sintering. This inevitable growth of grains constitutes itself the cause forthe failure of sintered articles to acquire strength beyond a certain limit. This fact is reported by S.Procharzka etal.,Am.

Aram. Soc. Bull. 52,885-891(1973) purporting to concludethatowingtogrowth ofcrystal grains, the produced sintered articles fail to acquire any appreciable improvement in strength when using B as a sintering aid.

With a view to eliminating the drawback mentioned above, the inventors have already disclosed that a sintered article of silicon carbide containing erbium oxide and aluminum oxide as an independent composition shows features of high density and extremely fine size of crystal grains in the filed Japanese Patent Application 58-190361. Even the sintered article still contains pores measuring approximately um and is recognized as coarse as 8.5 pom. Thus, the need of producing sintered articles of silicon carbide possessing still higher density and containing crystal grains of still finer size still remains yet to be satisfied.

SUMMARY OF THE INVENTION An objectofthis invention isto provide a highdensity sintered article of silicon carbide containing finer pores and possessing more compact texture than the conventional sintered article of silicon carbide. Specifically, this invention aims to provide a high-density sintered article of silicon carbide containing pores measuring not more than 1.0 pm and crystal grains measuring not more than 5 cm.

The other and further objects and characteristics of this invention will becomeapparentfromthefurther disclosure ofthis invention to be made in the following detailed description of a preferred embodiment, with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The drawing attached hereto is an explanatory diagram illustrating a high-temperature fatigue test conducted in Example 6.

DETAILED DESCRIPTION OF THE INVENTION The sintered article of silicon carbide ofthe present invention has erbium oxide and aluminium oxide contained therein intheform of a composite oxide.

To cite a typical example of the composite oxide, they may be contained in the form of a garnetofthe following formulas.

Er3(AI, Er)2(A104)3 (1) (Er,Al)3AI2(AlO4)3 (2) As the garnet powder represented by the aforementioned formula (1) - (2), a polycrystlaline powder obtained by mixing aluminum oxide powder with erbiumoxidepowderandsubjectingthe produced mixed powder to a solid-phase reaction at elevated temperatures (generally about 1300"to 1600"C) can be used, for example.

Such solid-phase reaction described above does not always give rise to a product of the composition Er3Al2(AlO4)3. Often,thetwo powdered oxides under go mutual substitutive solid solution and give rise to garnets ofthe compositions indicated in (1) and (2) above. Otherwise, a garnet of the composition Er3AI2(AIO4)3 can be used. Besides such garnets, a composition represented by the formula ErxA x)o3 (whereinx < 1) can be used.

The amount of erbium oxide in the composite oxide must be at least 1 % by weight. Ifthis amountfalls short of the lower limit just mentioned, the density of the sintered article based on the theoretical value is insufficient and bending rupture and other properties become inferior. Ifthis amount is as large as 15% by weight, growth of crystal grains is observed to occur and, as the result, the magnitudes of bending rupture and impact value are lowered and most of the other properties are liable to be impaired. Thus, this amount is not allowed to exceed 12% by weight.

Asconcernstheamountofaluminum oxide,ifthe amount reaches 3% by weight, the properties such as breaking strength are observed to decline. If this amount is nil, the effect of the addition of a garnet formed jointly of terbium oxide and aluminium oxide is completely absent. Thus,the amount is desired not to exceed 2% by weight.

Although the mechanism of compaction ofthe structure by this composite oxide remains yet to be elucidated, it is considered that the low energy of activation is made the composite oxide to undergo solid solution in silicon carbide and consequently promote the sintering of silicon carbide.

This invention further embraces the addition of 0.5 to 6.0% by weight of at least one element selected from the group of components indicated below or its varying compound such as oxide, nitride, boride, or carbide, as an agent four promoting the compaction of sintered article. Concrete examples ofthe element possessing the aforementioned function includetitanium, vanadium, chromium, manganese, magnesium, yttrium, zirconium, niobium, molybdenum, barium, lanthanum, cerium, gadolinium, hafnium, tantalum,tungsten,thorium,and cesium.

The second additive element so used for promoting the sintering proves virtually effectless if the amount ofthis element added is 0.3% by weight, and is required to be at least 0.5% by weight. If this amount is as large as 6% by weight, the crystal grains are observed to grow in size and the properties of the sintered article are degraded. The effect ofthe added element upon the compaction of the sintered article has notyet been fully elucidated. It has been experimentally ascertained to the inventors that this added element synergistically cooperates with the aforementioned composite oxide to decrease micros copicporesto a great extent.

Optionally, part of silicon carbide may be substituted with Be, BeO, B, or B4C. When this substitution is effected, the addition of a proper amount of the composite oxide of erbium oxide and aluminum oxide enables the produced sintered article to acquire a compact structure formed of extremelyfine grains. If the amount of substitution is not more than 0.5% by weight, the effect of the added element is substantially nil.If the amount is as large as 3.0% by weight, however, the bending rupture and the hardness ofthe sintered silicon carbide are observed to decrease.

Thus, the amount of substitution must not exceed 2% by weight and is desired to fall in the range of 0.5 to 2% by weight.

It has also been ascertained experimentallythatthe results of the present invention are not affected at all even when silicon carbide contains 0.5 to 2% by weight of free carbon.

In the manufacture ofthe sintered article according to this invention, the composite oxide, the agent four promoting the sintering, etc. are required to be uniformly dispersed in silicon carbide.

For the manufacture ofthe sintered article according to this invention, the conventional sintering method such as hot press method or HIP method can be advantageously utilized. In order to obtain compact and strong sintered articles, the hot pressure temperature is required to exceed 1900"C. lfthistemperature is as high as 2100 C, however, growth ofgrainsoccurs heavily and excessive growth of grains sets in before the compaction of structure proceeds sufficiently and, as the result, pores persist in the produced sintered article. Forthe purpose of the hot press method, the pressure has only to exceed 100 kg/cm2to be sufficient. No upper limit is specifically fixed for this pressure.The sintering can be carried out effectively in a vacuum or in an atmosphere of inactive gas. In the case of the HIP method, the sintering is desired to be carried out in an atmosphere of inactive gas. Even by the normal sintering method, the sintered article can be produced in substantiallythesame quality. When the sintering is performed in the atmosphere of inactive gas without application of pressure,the temperature fails in the range of 2050 to 23000C. In the atmosphere of compressed gas under 10 atm, the temperaturefalls in the range of 2000"to 22500C.

EXAMPLES Example 1: First, aluminum oxide powder of purity of 99.9% and average particle diameter of 0.4 pm and erbium oxide powder of purity of 99.9% and average particle diameter of 0.8 pm were mixed in a varying ratio indicated in Table 1. The mixed powder was heated at 1300 to 1 6000C for three to ten hours to synthesize a garnet. The garnetwas finelygroundto an average grain size of 0.5 m The garnet powder was mixed with silicon carbide powder of purity of 98.5% and average grain size of 0.5 pm and magnesium oxide powder of purity of 99.9% and average grain size of 1 pm in a varying ratio indicated in Table 1. The resuitantcompositionwaswet-pulverized in a ball mill mixerfor 15 hours and then dried thoroughly to prepare a raw material forsintering. Agraphite mold the square of50 mm in area and 60 mm in height was packed with the raw material and inserted in a high-frequency coil. The raw material was held at 1 9500C under 200 kg/cm2 of pressure for 60 minutes and then relieved of pressure and left cooling off. As the result, a sintered article 50 x 50 x 5.5 mm in size was obtained. The sintered article so obtained was cut and ground with a diamond cutting tool to be 10 test pieces 3 x4x 36 mm. These test pieces were tested for various properties. The results are shown in Table 1.

The test pieces were visually examined to test for structure. Coarse pores measuring about 2 pm are indicated by the markX and fine pores measuring not morethan 1 pom bythe mark G.

Table 1-1 (Example)

Run No. 1 2 3 4 5 6 7 8 9 SiC 97 95 94 93 90 86 84 83 81 MgO 3 3 3 3 3 3 3 3 3 3 Al2O3 - 1 1 2 2 1 1 2 1 Mixing ratio Garnet (% by weight) Kr2O3 - 1 2 2 5 10 12 12 15 Al2O3 - - - - - - - - Kr2O3 - - - - - - - - Relative density (%) 74.9 87.2 98.8 98.9 99.1 99.1 99.3 99.1 95.3 Grain size (um) 1.0 3.5 3.5 4.0 4.0 4.5 4.0 4.5 8.5 Bending rupture strength (1w/~2) RI BR 84 88 88 @8 85 83 63 Charpy impact value (kg.m/cm) 0.06 0.11 0.24 0.25 0.25 0.25 0.25 0.25 0.22 Hardness (Mg 3ON) - 86.0 93.9 94.0 95.5 95.1 95.3 95.0 93.8 Pore size (G, X) X G G G G G G G X Table 1-2 (Comparative experiment)

Run No. | 10 11 12 SiC 94 90 86 MgO 3 3 3 Al2O2 - - Mixing ratio Garnet (% by weight) Kr2O3 - - A1203 1 2 1 Kr2O3 2 5 10 Relative density (%) 97.7 97.9 98.8 Grain size (;;-) 3.5 7.0 7.5 | Bending rupture strength (kg/mm) 81 82 79 Charpy impact value (kg.m/cm) 0.20 0.23 0.23 Hardness (HR 3ON) 93.5 93.8 93.7 Pore size (G, X ) Example 2: Same test pieces in Example 1 were out with a diamond cutting tool to be a plate 10x 10 x 5 mm in size. Those plates were given to surface polishing with &num; 200 grit diamond.And the polished surface, 10 x 10 mm, of the plate was blasted with abrasive grits (METCOLITE C, No.40) blown at a distance of 50 mm underairpressure of 5 kg/cm2 bya sand blasting machine provided with a nozzle 8 mm in inside diameter, to test for weight loss. The results are shown in Table 2.

Table 2

Run No. 1 2 4 5 8 9 Loss of weight, g/(cm .hr) 2.03 0.98 0.65 0.60 0.60 1.11 Example 3: Same test pieces in Example 1 were cut with a diamond cutting tool to obtain a plate 10x 10x5 mm in size. All the surfaces of this plate were wrapped with &num; 200 grit diamond. The test piece thus prepared was left standing at 1300 C for 20 hours in air, to test for weight increase per unit area. The results are shown in Table 3.

Table 3

Run No. 1 2 4 5 8 9 Weight increase x 10-7 g/mm 15.0 5.2 0.4 0.3 0.3 6.7 Example 4: Same test pieces in Example 1 were cut with a diamond paste to obtain a rod3 x 4 x 36 mm in size.

All the surfaces of this rod were wrapped with a diamond paste. The test pieces so prepared were subjected to Charpy impact test at 950 C in the atmosphere.

The results are shown in Table 4.

Table 4

Run No. 0 11 2 4 5 Impact strength at elevated temperatures, 0.08 0.22 0.42 0.42 0.43 0.21 kg.m/cm Example 5: Same test pieces in Example 1 were directly subjected to high-temperature fatigue test. Specifically, with a flex tester, the samples are held in the position by the single point loading method with the span distance of 20 mm under the atmospheric pressure at 1000 C in air stress cycles 1325 CTM.The repeated stress were applied in a pattern as illustrated in the accompanying drawing, under the conditions as to satisfy #max = 15 kg/cm2 and i = 0.73 wherein #max denotes the upper limit of repeating stress, #min denotes the lower limit of repeating stress, am denotes the average stress, #a denotes the amplitude of stress, and i denotes the ratio of #a/#m.

The results are shown in Table 5.

Table 5

Run No. 1 2 4 5 8 9 Flexible fatigue number 9.8 0.8 8.2 4.7 5.6 0.6 of cycle x x x x x x 102 104 104 105 105 104 Example 6: First, 10% by weight of aluminum oxide powder of purity of 99.9% and average grain size of 0.4 m; and 90% by weight of erbium oxide powder of purity of 99.9% and average grain size of 0.8 pm were mixed.

The mixed powder was heated at 1400 C for five hours to synthetise a garnet. The garnet so obtained was finely ground to average grain size of 0,5 m. The resultant fine powder used in an amount of 10% by weight. a varying second additive element for promo tion of sintering used in a varying amount indicated in Table 6, and the balane to make up 100% by weight of silicon carbide of purity of 98.5% and average grain size of m were wet pulverized in a ball mill mixer for 15 hours. From the resultant composition, a sintered articles were produced by same procedure in Example 1. The sintered article was tested forvarious properties.The results are shown in Table 6. Table 6

Second additive element for Relative Grain Bending Charpy Hardness promoting density size rupture impact (HR 3ON) sintering, (%) ( m) strength value compound/amount (kg/mm) (kg.m/cm) (% by weight) Ti2O(0.3 98.0 4.0 81 0.23 94.7 Cr2O3/0.5 98.5 4.0 83 0.24 95.1 MnO2/3 98.9 4.5 83 0.25 95.0 MgO/0.3 98.1 4.5 81 0.23 94.6 MgO/0.5 98.5 4.5 84 0.24 95.0 MgO/3.0 99.0 4.5 88 0.25 95.0 MgO/6.0 99.1 5.0 84 0.25 94.6 MgO/7.0 98.8 6.0 71 0.23 92.2 Y2O3/0.3 98.0 4.5 79 0.22 94.5 Y2O3/0.5 98.6 4.5 83 0.25 95.3 Y2O3/3.0 98.9 4.5 87 0.24 94.9 Y2O3/6.0 99.0 5.0 94 1 0.25 ! 95.0 Y2O3/7.0 98.8 6.5 72 0.22 92.3 ZrSiO4/3 99.2 4.5 85 0.25 95.0 Nb2O3/3 99.4 4.5 83 0.24 95.2 Mo/1.BaO/1 99.0 4.5 86 0.25 94.9 La2O3/0.5 CeO2/2 99.4 4.5 83 0.24 95.2 W/0.5 Sm2O3/0.5 99.2 5.0 87 0.26 94.8 Example 7: A par tof silicon carbide powder used for sintering was substituted with Be, BeO, B, or B4C.

First, aluminium oxide powder of purity of 99.9% and average grain size of 0.4 pm and erbium oxide powder of purity of 99.9% and average grain size of 0.8 m were mixed in a varying ratio indicated in Table 7. The powder mixtures were heated at 1300 C to 1600 C for three to ten hours to synthesize a garnet.

Then, the garnet so obtained was finely ground to average grain size of 0.5 pm, The finely ground garnet powderwas mixed with silicon carbide powder of purity of 98.5% and average grain size of m and magnesium oxide powder of purity of 99.9% and average grain size of 1 pm in a varying ratio indicated in Table 7. The composition was wet pulverized in a ball mill mixerfor 15 hours. Then byfollowing the procedure of Example 1, the raw material so prepared was subjected to hot press sintering at 1950 C. The sintered article consequently produced was tested for various properties.The results are shown in Table 7. Table 7

Mixing ratio (% by weight) Relative Grain Bending Charpy Hardness density size rupture impact Garnet (%) ( m) strength value (HR 3ON) MgO Additive SiC (kg/mm) (kg.m/cm) Al2O3 Br2O3 2 3 3 - bal. 97.5 6.0 79 0.22 93.6 2 3 3 0.5 B bal. 98.5 4.0 84 0.24 95.6 2 3 3 3.0 bal. 98.3 5.0 75 0.21 94.8 2 5 3 0.5 B4C bal. 98.3 5.0 86 0.24 95.3 2 9 3 3.0 B4C hal. 98.6 5.5 78 0.23 94.5 2 5 3 1.5 Be bal. 98.6 4w0 | 85 0.25 95.3 2 5 3 3.0 Be bal. 97.3 5.5 57 0.22 95.0 2 10 3 0.5 BeO bal. 98.6 4.5 85 0.25 95.4 2 10 3 3.0 BeO bal. 97.0 6.0 73 0.22 95.4 2 10 3 1.0 BeO bal. 98.0 5.0 85 0.24 96.0 1.0 B Sintered articles of silicon carbide, according to this invention, as described in above examples, shows increased density by the addition ofthe composite oxide of aluminum oxide and erbium oxide and an element capable of promoting the sintering, and toughness by making the crystal grain size below 5 m to be very fine, and decreasing the size of pores belowl pm.

In contrast, in the cases of using mixed powder consisting of aluminum oxide and erbium oxide as shown in above comparative experiments in Table 1-2, the pores contained in the sintered article are increased being as 2 m.

MERITORIOUS EFFECTS OF INVENTION Thus, the sintered article of this invention, is preferably applicable to structural materials and abrasive materials which are expected to offer high resistance to oxidation, thermal shock, and corrosion and retain high strength at elevated temperatures.

Since the sintered article contemplated by the present invention can be manufactured by the hot press method orthe HIP method, it can be obtained easily in a large size. Even when the sintered article of this invention is manufactured by the normal sintering method, it acquires substantially the same quality as when it is manufactured by the hot press sintering method.

Claims

1. A high-density sintered article of silicon carbide, composed of 1 to 12% by weight of erbium oxide, not more than 2% by weight of aluminum oxide, and the balance to make up 100% by weight of silicon carbide and characterized by said erbium oxide and aluminum oxide wholly existing in the form of a composite oxide.

2. A high-density sintered article of silicon carbide as claimed in claim 1,wherein component of erbium oxideis2to 12% byweight.

3. A high-density sintered article of silicon carbide according to Claim 1,wherein part of silicon carbide powder is substituted with Be, BeO, B, or B4C.

4. A high-density sintered article or silicon carbide, composed of 1 to 12% by weight of erbium oxide, not more than 2% by weight of aluminium oxide, 0.5 to 6.0% by weight of at least one element selected from the group of components, A, indicated beloworacompound of said element, and the balanceto make up 100% by weight of silicon carbide and characterized by said erbium oxide and aluminum oxide wholly existing in the form of a composite.

ComponentsA: titanium,vanadium,chromium, manganese, magnesium, yttrium, zirconium, niobium, molybdenum, barium, lanthanum, cerium, gadolinium, hafnium, tantalum,tungsten, thorium, and cesium.

5. A high-density sintered article of silicon car bide as claimed in claim4,wherein component of erbium oxide is 2 to 12% by weight.

6. A high-density sintered article of silicon carbide according to Claim 3,wherein part of silicon carbide powder is substituted with Be, BeO, B, or B4C.