Patent Application
20120186723
The present invention relates to a Ti3SiC2 based material that exhibits excellent spark wear resistance and excellent oxidation resistance, an electrode, a spark plug, and methods of manufacturing the same.
An Ni alloy, a Pt alloy, or an Ir alloy has been used as a spark plug electrode material (see Patent Document 1). An Ni alloy is inexpensive, but exhibits poor spark wear resistance. Therefore, a spark plug made of an Ni alloy must be frequently replaced. A Pt alloy and an Ir alloy (noble metal alloy) provide a spark plug with a long lifetime due to excellent oxidation resistance and excellent spark wear resistance, but are expensive. An electrode obtained by welding a Pt alloy or an Ir alloy to the end of an Ni alloy electrode (i.e., only the end of the electrode is formed of a Pt alloy or an Ir alloy) has been proposed in order to reduce cost. However, separation of the Pt alloy or Ir alloy or the like may occur due to a difference in coefficient of linear expansion (see Patent Document 2). Specifically, a spark plug electrode material that is inexpensive and has a sufficient lifetime has not been proposed.
In recent years, a spark plug electrode has been exposed to a severe environment along with an improvement in engine fuel efficiency, performance, and the like. Therefore, an electrode material that exhibits more excellent resistance (e.g., oxidation resistance and spark wear resistance) has been increasingly desired. Ti3SiC2 that exhibits excellent spark wear resistance, oxidation resistance, and heat resistance, and is cheaper than an alloy material has been proposed as an alternative to an alloy material (see Patent Document 3).
Ti3SiC2 exhibits high thermal conductivity, electrical conductivity, thermal shock resistance, and workability (properties of a metal) and excellent heat resistance and oxidation resistance (properties of a ceramic), and is considered to be a promising spark plug electrode material. However, since it is difficult to synthesize single-phase Ti3SiC2, TiC and SiC may be present in the resulting Ti3SiC2 as heterophases. Moreover, since it is difficult to sufficiently density Ti3SiC2 under normal pressure, the resulting Ti3SiC2 may have an open porosity of several percent (see Patent Documents 4 to 6).
[Patent Document 1] JP-A-2002-235139
[Patent Document 2] Japanese Patent No, 3562532
[Patent Document 3] US2010/0052498A1
[Patent Document 4] Japanese Patent No. 4362582
[Patent Document 5] Japanese Patent No. 3951643
[Patent Document 6] JP-A-2005-89252
Ti3SiC2, exhibits excellent spark wear resistance, oxidation resistance, and heat resistance, but does not necessarily exhibit sufficient are resistance (spark wear resistance and oxidation resistance) for use as an electrode material (particularly a spark plug electrode material).
An object of the present invention is to provide a Ti3SiC2 based material that exhibits excellent arc resistance, an electrode, a spark plug, and methods of manufacturing the same.
The inventors of the present invention found a dense Ti3SiC2 sintered body in which almost no TiC phase remains. When an electrode material contains a TiC phase and pores, the TiC phase and the pores preferentially undergo oxidation or spark wear, so that the lifetime of the electrode material decreases. Therefore, a dense Ti3SiC2 sintered body in which almost no TiC phase remains is required as an electrode material (particularly a spark plug electrode material). The present invention provides the following Ti3SiC2 based material, electrode, spark plug, and methods of manufacturing the same.
The Ti3SiC2 based material according to the present invention exhibits improved arc resistance (spark wear resistance and oxidation resistance) as a result of significantly reducing the TiC (heterophase) content and the open porosity. The arc resistance of the Ti3SiC2 based material is further improved by dissolution of Al. An inexpensive spark plug electrode material that exhibits significantly improved arc resistance can thus be provided.
Exemplary embodiments of the present invention are described below with reference to the drawing. Note that the present invention is not limited to the following exemplary embodiments. Various modifications and improvements may be made of the following exemplary embodiments without departing from the scope of the present invention.
A Ti3SiC2 based material according to the present invention includes Ti3SiC2 as a main phase, the Ti3SiC2 based material having a TiC content of 0.5 mass % or less and an open porosity of 0.5% or less. Note that the term “main phase” used herein refers to a phase that accounts for 60 mass % or more of a material.
Ti3SiC2 exhibits excellent spark wear resistance, oxidation resistance, and heat resistance. When a Ti3SiC2 based material contains a TiC phase and pores, however, the TiC phase and the pores preferentially undergo oxidation or wear, so that the lifetime of the Ti3SiC2 based material decreases when used as an electrode material. The Ti3SiC2 based material according to the present invention is a dense Ti3SiC2 sintered body that contains only a small amount of TiC phase.
A Ti3SiC2 based material having a TiC content of 0.5 mass % or less and an open porosity of 0.5% or less may be synthesized by mixing a titanium source, a silicon source, an aluminum source, and a carbon source in a given mixing ratio (described later), and firing the mixture at a given firing temperature. The Ti3SiC2 based material may be used as an electrode material (particularly a spark plug electrode material). The Ti3SiC2 based material exhibits excellent arc resistance (spark wear resistance and oxidation resistance).
In the Ti3SiC2 based material according to the present invention, it is preferable that 0 to 30 mol % of Si contained in the main phase Ti3SiC2 be substituted with Al. Specifically, it is preferable that the Ti3SiC2 based material also include Ti3Si(1-x)AlxC2 (x=0 to 0.3). It is preferable that 0 to 20 mol %, and more preferably 0 to 10 mol %, of Si contained in the main phase Ti3SiC2 be substituted with Al. The arc resistance of the Ti3SiC2 based material is improved by substituting Si contained in Ti3SiC2 with Al.
It is preferable that the Ti3SiC2 based material according to the present invention have a Ti5Si3 content of 8 mass % or less. It is more preferable that the Ti3SiC2 based material have a Ti5Si3 content of 7 mass % or less, and more preferably 6 mass % or less. If the Ti3SiC2 based material has a Ti5Si3 content of 8 mass % or less, the Ti3SiC2 based material exhibits improved arc resistance.
It is preferable that the Ti3SiC2 based material according to the present invention have an SiC content of 5 mass % or less. It is more preferable that the Ti3SiC2 based material have an SiC content of 4.5 mass % or less, and more preferably 4 mass % or less. If the Ti3SiC2 based material has an SiC content of 5 mass % or less, the Ti3SiC2 based material exhibits improved arc resistance.
It is preferable that the Ti3 SiC2 based material according to the present invention have a TiSi2 content of 3 mass % or less. It is more preferable that the Ti3SiC2 based material have a TiSi2 content of 0 mass %. If the Ti3SiC2 based material has a TiSi2 content of 3 mass % or less, the Ti3SiC2 based material exhibits improved arc resistance.
It is preferable that the Ti3SiC2 based material according to the present invention have a thermal conductivity of 25 W/mK or more. It is more preferable that the Ti3SiC2 based material have a thermal conductivity of 30 W/mK or more. If the Ti3SiC2 based material has a thermal conductivity of 25 W/mK or more, the Ti3SiC2 based material exhibits an excellent heat dissipation capability when an increase in temperature has occurred due to ignition of fuel when using the Ti3SiC2 based material as a material for a spark plug, so that oxidation and wear can be suppressed.
It is preferable that the Ti3SiC2 based material according to the present invention have a bending strength of 200 MPa or more. It is more preferable that the Ti3SiC2 based material have a bending strength of 250 MPa or more, and more preferably 270 MPa or more. If the Ti3SiC2 based material has a bending strength of 200 MPa or more, the Ti3SiC2 based material may be used as a material for forming a member for which high strength is required.
It is preferable that the Ti3SiC2 based material according to the present invention have a coefficient of thermal expansion of 7 to 9 ppm/K. It is more preferable that the Ti3SiC2 based material have a coefficient of thermal expansion of 8 to 9 ppm/K. If the Ti3SiC2 based material has a coefficient of thermal expansion of 7 to 9 ppm/K, the Ti3SiC2 based material may be used as a material for forming a member that is used at a high temperature.
It is preferable that the Ti3SiC2 based material according to the present invention have a volume resistivity of 1×10−4 Ω·cm or less. It is more preferable that the Ti3SiC2 based material have a volume resistivity of 5×10−5 Ω·cm or less, and more preferably 3×10 −5 Ω·cm or less. If the Ti3SiC2 based material has a volume resistivity of 1×10−4 Ω·cm or less, the Ti3SiC2 based material may suitably be used as a material for forming a spark plug due to reduced energy loss.
It is preferable that an oxide film formed when allowing the Ti3SiC2 based material according to the present invention to stand at 1000° C. for 5 hours under atmospheric pressure have a thickness of 40 μm or less. It is more preferable that the oxide film have a thickness of 35 μm or less, and more preferably 30 μm or less. If the oxide film has a thickness of 40 μm or less, the Ti3SiC2 based material may be used as a conductive material that is used at a high temperature due to high oxidation resistance.
The Ti3SiC2 based material according to the present invention may be used as an electrode material. More specifically, a spark plug electrode may be formed using the Ti3SiC2 based material.
As shown in
The ceramic insulator 2 has an axial hole that is formed through the ceramic insulator 2 in the axial direction, and holds the center electrode 1 within the axial hole. The metal shell 3 surrounds the ceramic insulator 2 to hold the ceramic insulator 2. The metal shell 3 secures the spark plug 10 on an internal combustion engine. The ground electrode 4 is bonded to the metal shell 3 at one end, and faces the end of the center electrode 1 at the other end.
A method of manufacturing the Ti3SiC2 based material according to the present invention is described below. Ti3(Si,Al)C2 having a low TiC content may be obtained by mixing a titanium source, a silicon source, an aluminum source, and a carbon source in a given mixing ratio (described later), and firing the mixture at a given firing temperature. The amounts of the silicon source, the aluminum source, and the carbon source may be adjusted so that a single-phase material is obtained. When mixing the titanium source, the silicon source, the aluminum source, and the carbon source for a short time (several to several tens of minutes) using a mortar or the like, it is preferable to adjust the amount of the silicon source (about 1.2-fold molar excess with respect to the desired composition). When mixing the titanium source, the silicon source, the aluminum source, and the carbon source for a long time (several hours) using a pot mill or the like, the raw materials tend to be oxidized. In this case, it is preferable to adjust the amount of the carbon source (about 1.1-fold molar excess with respect to the desired composition) in addition to the amount of the silicon source. Metallic aluminum may be vaporized during synthesis due to a low inciting point, so that the desired composition may not be obtained. Dense Ti3(Si,Al)C2 having almost the desired composition may be obtained by firing the mixture at 600 to 1400° C. over 0.5 to 20 hours to suppress vaporization of unreacted Al.
The method of manufacturing the Ti3SiC2 based material according to the present invention includes mixing a titanium source, a silicon source, and a carbon source in a mass ratio of 68.0 to 73.5/14.0 to 19.0/11.0 to 14.0 (mass %) to obtain a raw material powder mixture, forming the raw material powder mixture to obtain a formed body, and firing the formed body. Note that the term “mass ratio” used herein refers to the ratio of the elements (Ti, Si, Al, and C) included in the raw materials. For example, when using a silicon carbide powder as the silicon source, the mass ratio of the silicon source is referred to as 14.0 mass % when the mass ratio of Si contained in the silicon carbide powder to the total raw materials is 14.0 mass %. It is preferable to fire the formed body obtained by forming the raw material powder mixture in two stages that differ in firing temperature. More specifically, it is preferable to subject the formed body to a first firing process at 600 to 1400° C. for 0.5 to 20 hours under vacuum or in an Ar atmosphere, and then subject the formed body to a second firing process for 0.5 to 20 hours at a temperature within a range of 1000 to 1750° C. that is higher than that employed during the first firing process, Note that the formed body subjected to the first firing process is heated to the temperature employed for the second firing process without cooling the formed body. The first firing process and the second firing process arc more preferably performed for 0.5 to 10 hours, and still more preferably 0.5 to 5 hours.
A metallic titanium powder, TiH2, or the like may be used as the titanium source. A metallic silicon powder or the like may be used as the silicon source. A carbon powder or the like may be used as the carbon source. A phenol resin that is pyrolyzed into carbon may also be used as the carbon source.
A metallic aluminum powder may be used as the aluminum source. A compound of each element (e.g., silicon carbide powder) may also be used. A metallic titanium powder, a silicon carbide powder, and a carbon powder are relatively inexpensive raw materials.
When mixing the raw materials, the silicon source may be used in about 1.2-fold molar excess with respect to the desired composition. The carbon source may be used in about 1.1-fold molar excess with respect to the desired composition. A Ti3SiC2 based material having a low heterophase content can be obtained by setting the mixing ratio of the raw materials and the firing temperature and the firing time employed for the first firing process and the second firing process, within the above ranges.
A method of manufacturing the Ti3SiC2, based material according to the present invention in which 0 to 30 mol % of Si contained in the main phase Ti3SiC2 is substituted with Al, includes mixing a titanium source, a silicon source, an aluminum source, and a carbon source in a mass ratio of 68.0 to 73.5/9.0 to 19.0/0 to 5.0/11.0 to 14.0 (mass %) to obtain a raw material powder mixture, forming the raw material powder mixture to obtain a formed body, and firing the formed body. It is preferable to fire the formed body obtained by forming the raw material powder mixture in two stages that differ in firing temperature. More specifically, it is preferable to subject the formed body to a first firing process at 600 to 1400° C. for 0.5 to 20 hours under vacuum or in an Ar atmosphere, and then subject the formed body to a second firing process for 0.5 to 20 hours at a temperature within a range of 1000 to 1750° C. that is higher than that employed during the first firing process. Note that the formed body subjected to the first firing process is heated to the temperature employed for the second firing process without cooling the formed body. The first firing process and the second firing process are more preferably performed for 0.5 to 10 hours, and still more preferably 0.5 to 5 hours.
When mixing the raw materials, the silicon source may be used in about 1.2-fold molar excess with respect to the desired composition. The aluminum source may be used in about 1.1-fold molar excess with respect to the desired composition. The carbon source may be used in about 1.1-fold molar excess with respect to the desired composition. A Ti3SiC2 based material- having a low heterophase content can be obtained by setting the mixing ratio of the raw materials and the firing temperature and the firing time employed for the first firing process and the second firing process within the above ranges.
The first firing process and the second firing process may be implemented by hot pressing regardless of whether or not Si contained in the main phase Ti3SiC2 is substituted with Al. Hot pressing is preferably performed at a pressure of 50 to 450 kg/cm2. Hot pressing is more preferably performed at a pressure of 100 to 350 kg/cm2, and still more preferably 200 to 250 kg/cm2. A Ti3SiC2 based material having the desired composition and a low heterophase content can be obtained by performing the first firing process and the second firing process by hot pressing at a pressure within the above range.
A method of manufacturing the spark plug 10 is described below. The center electrode 1 and the ground electrode 4 of the spark plug 10 may be formed by cutting (machining) the Ti3Si C2 based material according to the present invention.
The ceramic insulator 2 is obtained as described below. For example, a raw material powder including alumina, a binder, and the like is press-formed to obtain a tubular formed body. The formed body is ground to obtain a ceramic insulator formed body. The electrode (center electrode 1) is embedded in the ceramic insulator formed body, and the electrode and the ceramic insulator formed body are fired to convert the ceramic insulator formed body into the ceramic insulator 2 and bond the ceramic insulator 2 to the electrode. Since the ceramic insulator can be bonded to the electrode by firing the ceramic insulator formed body, the production cost can be reduced.
The metal shell 3 is formed using a tubular metal material, and the ground electrode 4 is bonded to the metal shell 3. The ceramic insulator 2 that is bonded to the center electrode 1 is then secured on the metal shell 3 to which the ground electrode 4 is bonded. The spark plug 10 is thus obtained.
The present invention is further described below by way of examples. Note that the present invention is not limited to the following examples.
A metallic titanium powder was used as a titanium source. A silicon carbide powder was used as a silicon source. The silicon carbide powder and a carbon powder were used as a carbon source. A metallic aluminum powder was used as an aluminum source. The raw materials were mixed in the mixing ratio (mass %) shown in Table 1. The ratio of each element contained in the raw material mixture is shown in Table 1 (see “Element ratio”). The raw materials were mixed for 10 minutes using a pestle and a mortar. About 20 g of the mixed powder was weighed, and press-formed to a diameter of 35 mm and a thickness of 10 mm.
The same raw materials as those used in Example 1 were mixed in the mixing ratio (mass %) shown in Table 1. The raw materials were mixed as described below. Specifically, nylon cobblestones (with an iron core) were put in a pot mill, and the raw materials were mixed for 4 hours in the pot mill using IPA as a solvent. The mixed powder was dried using a evaporator, and sieved (#30 mesh). About 20 g of the mixed powder was weighed, and press-formed to a diameter of 35 mm and a thickness of 10 mm.
The formed body obtained by press forming was hot-pressed at 1600° C. and 210 kg/cm2 for 4 hours in an Ar atmosphere to obtain a sintered body (sample).
The formed body obtained by press forming was hot-pressed at 1000° C. and 210 kg/cm2 for 4 hours in an Ar atmosphere, and then hot-pressed at 1600° C. for 4 hours to obtain a sintered body (sample).
The formed body obtained by press forming was hot-pressed at 1000° C. and 210 kg/cm2 for 4 hours in an Ar atmosphere, and then hot-pressed at 1675° C. for 4 hours to obtain a sintered body (sample).
A commercially available spark plug center electrode Ni alloy was used.
The X-ray diffraction pattern of the sintered body (sample) was measured by a 0-20 method. The X-ray diffraction pattern was measured using an X-ray diffractometer (manufactured by Rigaku Corporation) (X-ray source: Cu-Ku). The amount (mass %) of each crystal phase was calculated from the intensity ratio of the main peak attributed to Ti3SiC2 and the peak attributed to each crystal phase other than Ti3SiC2 in the X-ray diffraction pattern.
The open porosity of the sintered body (sample) was calculated by the Archimedes method.
The bending strength of the sintered body (sample) was measured by performing a four-point bending test (JIS R 1601).
The thermal conductivity of the sintered body (sample) was calculated from the specific heat measured by differential scanning calorimetry (DSC), the thermal diffusion coefficient measured by a laser flash method, and the density measured by the Archimedes method (JIS R 1611).
The volume resistivity of the sintered body (sample) was measured by a four-terminal method (JIS R 1650-2).
About 0.5 to 1.5 g (surface area: 1.6 to 3.2 cm2) of the sintered body (sample) was placed in a rectangular alumina box, and allowed to stand at 1000° C. for 5 hours under atmospheric pressure. The thickness of an oxide film formed by this operation was observed and measured using an SEM.
The sintered body (sample) (diameter: 0.6 mm, length: 15 to 20 mm) was used as an anode, and SUS 304 was used as a cathode. The anode (sintered body) was disposed at an angle of 45° relative to the cathode (SUS 304). The distance between the cathode and the anode was set to 5 mm. An arc discharge was generated at room temperature under atmospheric pressure using a 100 kV power supply. The arc discharge was generated for 5 minutes (current value of power supply: 0.15 A), and the arc discharge area was observed using an optical microscope to determine the presence or absence of wear. When wear due to the arc discharge was not observed, an arc discharge was generated for 5 minutes in a state in which the primary-side current value was increased by 0.05 A, and the arc discharge area was observed using an optical microscope. This cycle (arc discharge and optical microscope observation) was repeated until wear was observed. The primary-side current value at which wear was initially observed was taken as the arc wear start current value. An arc discharge was also generated for 5 minutes in a state in which the current value of the power supply was set to 0.45 A, and a decrease in diameter due to the arc discharge was taken as the amount of wear. The arc wear rate was calculated by dividing the amount of wear by the discharge time.
A case where the arc wear rate was 20 μm/min or less was evaluated as “Very Good”, a case where the are wear rate was 21 to 30 μm/min was evaluated as “Good”, and a case where the arc wear rate was 31 μm/min or more was evaluated as “Bad” (see Table 2).
The arc wear rate obtained in each example and the arc wear rate obtained in each comparative example are compared below. The sintered bodies of Examples 1 to 12, in which the TiC content was 0.5 mass % or less and the open porosity was 0.5%, had a low arc wear rate (i.e., exhibited excellent arc resistance) as compared with the sintered bodies of Comparative Examples 1 to 4. The wear state of the sintered body of Example 1 observed using an SEM was compared with the wear state of the sintered body of Comparative Example 1 observed using an SEM. In the sintered body of Comparative Example 1 in which the TiC content was high, TiC was oxidized and scattered. In the sintered body of Example 1 in which the TiC content was 0.5 mass % or less, an area in which TiC was scattered was rarely observed.
The sintered bodies of Example 1 to 5 and 9 to 11, in which 30 mol % or less of Si contained in Ti3SiC2 was substituted with Al, had a low arc wear rate (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 12.
The sintered bodies of Example 1 to 5, in which the Ti5Si3 content was 8 mass % or less, had a low arc wear rate (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 6. The sintered bodies of Example 1 to 5, in which the SiC content was 5 mass % or less, had a low arc wear rate (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 7. The sintered bodies of Example 1 to 5, in which the TiSi2 content was 3 mass % or less, had a low arc wear rate (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 8.
The arc wear start current value obtained in each example and the arc wear start current value obtained in each comparative example are compared below. The sintered bodies of Example 1 to 5 and 9 to 11, in which 30 mol % or less of Si contained in Ti3SiC2 was substituted with Al, had a large arc wear start current value (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 12. The sintered bodies of Example 1 to 5, in which the Ti5Si3 content was 8 mass % or less, had a large arc wear start current value (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 6. The sintered bodies of Example 1 to 5, in which the SiC content was 5 mass % or less, had a large arc wear start current value (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 7. The sintered bodies of Example 1 to 5, in which the TiSi2 content was 3 mass % or less, had a large arc wear start current value (i.e., exhibited excellent arc resistance) as compared with the sintered body of Example 8.
The sintered bodies of Example 1 to 12 that exhibited excellent arc resistance had a thermal conductivity of 25 W/mK or more, a bending strength of 200 MPa or more, a coefficient of thermal expansion of about 7 to 9 ppm/K, and a volume resistivity of 1×10−4 Ω·cm or less.
When the sintered bodies of Examples 1, 3, 4, 6, and 9 to 11 were allowed to stand at 1000° C. for 5 hours under atmospheric pressure, separation of an oxide film from the matrix was rarely observed. When the sintered body of Example 12 was allowed to stand at 1000° C. for 5 hours under atmospheric pressure, an oxide film having a large thickness was formed due to low oxidation resistance. The arc wear rate due to high thermal load was low (i.e., excellent arc resistance was obtained) when the Al content was 30 mol % or less.
The main component (alumina) of a ceramic insulator and the dense Ti3SiC2 body were fired together to determine whether or not the dense Ti3SiC2 body could be bonded to alumina. The main component (alumina) of a ceramic insulator and the Ni alloy were fired together to determine whether or not the Ni alloy could be bonded to alumina. A case where the material could be bonded to alumina was evaluated as “Good”, and a case where the material could not be bonded to alumina was evaluated as “Bad” (see Table 2).
As shown in Table 2, the dense Ti3SiC2 body of Example 1 having a coefficient of thermal expansion relatively close to that (about 8 ppm/K) of the spark plug ceramic insulator could be bonded to the ceramic insulator by firing the dense Ti3SiC2 body together with the ceramic insulator. The dense Ti3SiC2 bodies of Examples 2 and 10 could also be bonded to the ceramic insulator. However, the Ni alloy used in Comparative Example 5 could not be bonded to the ceramic insulator. Therefore, it is expected that the spark plug production process can be simplified (i.e., the production cost can be reduced) by utilizing the dense Ti3SiC2 body of Example 1, 2, or 10 that can be bonded to the ceramic insulator by firing.
As described above, the Ti3SiC2 based material according to the present invention exhibited improved arc resistance as a result of significantly reducing the TiC (heterophase) content and the open porosity. An inexpensive spark plug electrode material that has a long lifetime and exhibits significantly improved arc resistance can thus be provided.
The Ti3SiC2 based material according to the present invention may be used as an electrode material (particularly a spark plug electrode material). An electrode and a spark plug formed using the Ti3SiC2 based material according to the present invention are inexpensive, and exhibit excellent arc resistance.
| Number | Date | Country | |
|---|---|---|---|
| 61436262 | Jan 2011 | US |
