A spark plug insulator and a method of making the same

Abstract: A spark plug insulator has a sing a sintered body which has a nitride-based ceramic powder ranging from 60 wt% to 98 wt% and a sintering additive. On an entire surface of the sintered body, a pyrolytic boron nitride layer is uniformly deposited, a thickness of which ranges from 10 µm to 100 µm.

Description: This invention relates to a spark plug insulator and a method of making the same for use in an internal combustion engine. In a spark plug insulator for an internal combustion engine, a nitride-based sintered ceramic body has been employed since the sintered ceramic body has good thermal conductivity while maintaining good electric insulation. Taking Japanese Patent Publication No. 46634/1980 as one example of this type of insulator, an oxide of an element selected from group IIIA of the periodic table, silicate-based compounds and metallic oxides are sintered with aluminum nitride powder as a main component. The insulator thus sintered, however, suffers losses in electric insulation (i.e. to below 5 MΩ) when exposed to high ambient temperature so as to cause electrical leakage, and thus leading to misfire when high voltages are applied between a center electrode and an outer electrode. Therefore, it is an object of the invention to provide a spark plug insulator which is capable of maintaining an elevated insulation property at high ambient temperature with good thermal conductivity, thus preventing electrical leakage to protect against misfire, and contributing to an extended service life. EP-A-0 350 152 discloses a spark plug insulator including a sintered body which comprises between 85% and 99% by weight silicon nitride and a sintering agent consisting of magnesium oxide, aluminium oxide and yttrium oxide. According to a first aspect of the present invention there is provided a spark plug insulator including a sintered body which comprises: between 60 and 98 percent by weight of a nitride-based ceramic powder; and a sintering additive; characterised in that the insulator further comprises: a pyrolytic boron nitride layer provided on the surface of the sintered body, the thickness of the pyrolytic boron nitride layer being between 10 µm and 100 µm inclusive. According to a second aspect of the present invention there is provided a method of making a spark plug insulator constructed in accordance with the above-mentioned first aspect of the present invention. The method of making comprises the steps of: preparing and sintering a mixture of between 60 and 98 percent by weight of a nitride-based ceramic powder, and a sintering additive, to form a sintered boyd; and coating pyrolytic boron nitride on the surface of the sintered body to provide a pyrolytic boron nitride layer of a thickness of between 10 µm and 100 µm inclusive. The nitride-based ceramic powder is densely sintered using the sintering additive. Using less than 60% by weight of nitride-based ceramic powder deteriorates the resulting thermal conductivity thus reducing heat-dissipation. Conversely, using in excess of 98% by weight of the nitride-based ceramic powder hinders correct sintering. On the surface of the sintered body, the pyrolytic boron nitride layer is deposited. Preferably this has good electrical insulation properties (10⁵ - 1.5 x 10⁵/ mm MΩ at 700°C) with good thermal conductivity (80 W/ m.k at 700 °C). This makes it possible to prevent electrical insulation of the insulator surface from decreasing, and thus protects the insulator against electrical leakage so as to prevent misfire when high voltage is applied between a center electrode and an outer electrode. A pyrolytic boron nitride layer of less than 10 µm in thickness makes it difficult fully to cover a minute surface roughness in the sintered body, thus not improving its electrical insulation. On the other hand, a pyrolytic boron nitride layer of more than 100 µm in thickness tends to exfoliate from the surface of the sintered body owing to differential thermal expansion between the layer and the sintered body. With a thickness of the pyrolytic boron nitride layer between 10 µm to 100 µm, the layer may fully cover the surface of the sintered body while maintaining good electrical insulation and not exfoliating, with minimum use of pyrolytic boron nitride. The invention will further be understood from the following description, when taken together with the accompanying drawings, which are given by way of example only, and in which:- Fig. 1 is a schematic plan view showing a device for measuring the insulation resistance of test pieces at high temperature;and Fig. 2 is a graph showing how the insulation resistance of an insulator changes depending on the thickness of the pyrolytic boron nitride layer. Aluminum nitride (AlN) powder is prepared as a nitride-based ceramic powder according to the weight percentage listed in Table 1. Granular size of the aluminum nitride (AlN) powder measures 1.5 µm in average diameter (by sedimentation analysis) with an oxygen-laden rate of 0.8 percent by weight. Sintering additives employed herein are all 99.9% purity selected alone or combination from the group consisting of yttrium oxide (Y₂0₃), calcium oxide (CaO), barium oxide (BaO), calcium carbide (CaC₂), scandium oxide (Sc₂O₃) and neodymium oxide (Nd₂0₃). These sintering additives are added to the aluminum nitride (AlN) powder according to the weight percentage also listed in Table 1. Among test pieces prepared for a spark plug insulator, the test pieces (nos. 1 ∼ 22) are manufactured as follows: (1) A slurry mixture of the aluminum nitride powder, the sintering additive (sintering additives) and ethanol, wax-related binder are kneaded by means of a ball for 15 hours within a nylon pot. In this instance, a quantity of the sintering additive (sintering additives) is determined by taking the fact into consideration that the sintering additive disappear during a sintering process described hereinafter. (2) The slurry mixture is desiccated by means of a spray dryer. Then the mixture is pressed by a metallic die at the pressure of 1 ton/cm², and is formed into a compact plate which measures 50 mm in diameter and 1.5 mm in thickness. (3) The compact plate is degreased by primarily sintering (calcination) it in an atmospheric environment at the temperature of 500 ∼ 600 °C for 5 hours. A rate of the temperature rise is adapted to be 300 °C per hour. (4) Under the normal pressure, the compact plate is secondarily sintered at temperature of 1650 ∼ 1950 °C in nitrogen atmosphere for about 2 hours to form a sintered body. (5) The sintered body is placed in a carbon furnace in which boron chloride (BCl₃) and ammonia gas (NH₃) chemically react at the temperature of about 1900 °C under 10⁻² ∼ 10⁻³ Torr to form a pyrolytic boron nitride (referred to as PBN hereinafter). In the carbon furnace, the pyrolytic boron nitride is simultaneously deposited over the surface of the sintered body to provide a pyrolytic boron nitride layer, in a thickness which ranges from 10 µm to 100 µm inclusive. In this instance, the thickness of the PBN layer is controlled by the hours in which the boron chloride (BCl₃) and the ammonia gas (NH₃) react in the carbon furnace since it is known that the pyrolytic boron nitride deposits on the entire surface of the sintered body at the rate of 20 ∼ 30 µm per hour. Upon measuring the thickness of the PBN layer, the test pieces are sectioned and checked at their sectional area by means of an electronic microscope. The sintered body, thus conditioned, measures 40 mm in diameter and 1.0 mm in thickness. Among the test piece Nos. 1 ∼ 22 listed in Table 1, Nos. 1 ∼ 10 concerns to the subject invention, while Nos. 11 ∼ 17 concerns to counterpart insulators in which each thickness of PBN layer departs from the range of 10 µm to 100 µm. Nos. 18 ∼ 22 concerns to counterpart insulators in which PBN layer is not provided on a surface of the sintered body. A device shown in Fig. 1 is used to measure insulation resistance of the test piece Nos. 1 ∼ 22 at the temperature of 700 °C. The device has brass-made electrodes 100, 200, a heater 300 and a 500-volt digital resistance meter 400. The measurement result of the test piece Nos. 1 ∼ 22 is shown in Table 2 in which an insulation resistance of more than 50 MΩ at 700 °C is found substantially to reduce misfire caused from electrical leakage when high voltage is applied between a center electrode and an outer electrode of a spark plug as shown in Fig. 2. Fig. 2 indicates that the insulation resistance of more than 50 MΩ at 700 °C is presented when the thickness of the PBN layer ranges from 10 µm to 100 µm as designated by delta legends (Δ), while the insulation resistance of less than 50 appears when the thickness of the PBN layer is less than 10 µm as indicated by crosses (×). It is noted that the thickness of the PBN layer may be controlled by adjusting the amounts of both the boron chloride (BCl₃) and the ammonia gas (NH₃) chemically reacting in the carbon furnace. It is appreciated that the nitride-based ceramic power may include oxinite aluminum (Al₂O₃) and/or sialon. It is further appreciated that the sintering additive may be selected alone or combination from the group consisting of oxides of rare earth metals and oxides, fluorides, carbides, chlorides of alkali earth metals. While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by the skilled man without departing from the scope of the invention as defined in the appended claims.

Claims: A spark plug insulator including a sintered body which comprises: between 60 and 98 percent by weight of a nitride-based ceramic powder; and a sintering additive; characterised in that the insulator further comprises: a pyrolytic boron nitride layer provided on the surface of the sintered body, the thickness of the pyrolytic boron nitride layer being between 10 µm and 100 µm inclusive. A spark plug insulator according to claim 1, wherein the pyrolytic boron nitride layer is substantially uniform. A spark plug insulator according to claim 1 or claim 2, wherein the pyrolytic boron nitride layer covers the entire surface of the sintered body. A spark plug insulator according to claim 1, 2 or 3, wherein the sintering additive comprises one or more of: yttrium oxide (Y₂0₃), calcium oxide (CaO), barium oxide (BaO), calcium carbide (CaC₂), neodymium oxide (Nd₂0₃) and scandium oxide (Sc₂0₃). A method of making a spark plug insulator according to claim 1 or claim 2, comprising the steps of: preparing and sintering a mixture of between 60 and 98 percent by weight of a nitride-based ceramic powder, and a sintering additive, to form a sintered boyd; and coating pyrolytic boron nitride on the surface of the sintered body to provide a pyrolytic boron nitride layer of a thickness of between 10 µm and 100 µm inclusive. A method according to claim 5, wherein substantially the entire surface of the sintered body is coated with pyrolytic boron nitride. A method according to claim 5 or 6, wherein the step of sintering includes: primarily sintering the mixture at a temperature of between 500°C and 600°C to provide a degreased compact body; and secondarily sintering the compact body at a temperature of between 1650 and 1950°C in a nitrogen atmosphere for about 2 hours to form a sintered body; and wherein the coating step includes: placing the sintered body in a carbon furnace in which boron chloride (BCl₃) and ammonia gas (NH₃) chemically react to form pyrolytic boron nitride (PBN). A method of making a spark plug insulator according to claim 5 or 6, wherein the sintering additive comprises one or more of: yttrium oxide (Y₂0₃), calcium oxide (CaO), barium oxide (BaO), calcium carbide (CaC₂), neodymium oxide (Nd₂0₃) and scandium oxide (Sc₂0₃. A spark plug including an insulator according to any one of claims 1 to 4. An internal combustion engine comprising a spark plug according to claim 9.

Inventors: MUSASA MAMORU C O NGK SPARK PL, SUGIMOTO MAKOTO C O NGK SPARK, TANABE HIROYUKI C O NGK SPARK, MUSASA, MAMORU, C/O NGK SPARK PLUG COMP. LIMITED, SUGIMOTO, MAKOTO, C/O NGK SPARK PLUG COMP. LIMITED, TANABE, HIROYUKI, C/O NGK SPARK PLUG COMP. LIMITED
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1991
Main classification: H01T13
IPC codes: C04B35, H01T13, C04B41