A METHOD OF PREPARING BORON CARBIDE/ALUMINUM CERMETS HAVING A CONTROLLED MICROSTRUCTURE

Abstract: Subject boron carbide to a heat treatment at a temperature within a range of 1250 C to less than 1800 C prior to infiltration with a molten metal such as aluminum. This method allows control of kinetics of metal infiltration and chemical reactions, size of reaction products and connectivity of B4C grains and results in cermets having desired mechanical properties.

Description: A Method of Preparing Boron Carbide/Aluminum Cermets Having a Controlled Microstructure Description Technical Field This invention relates generally to boron carbide/aluminum cermets and their preparation. This invention relates more particularly to boron carbide/aluminum cermets having a controlled microstructure and their preparation. Background Art U.S.-A 4,605,440 discloses a process for preparing boron carbide/aluminum composites that includes a step of heating a powdered admixture of aluminum (Al) and boron carbide (B 4 C) at a temperature of 1050°C to 1200°C. The process yields, however, a mixture of several ceramic phases that differ from the starting materials. These phases, which include AlB 2 , AI 4 BC, AIB 12 C 2 , AIB l2 and AI 4 C 3 , adversely affect some mechanical properties of the resultant composite. In addition, it is very difficult to produce composites having a density greater than 99% of theoretical by this process. This may be dueJn part, to reaction kinetics that lead to formation of the ceramic phases and interfere with the rearrangement needed to attain adequate shrinkage or densification. It may also be due, at least in part, to a lack of control over reactivity of molten Al. In fact, most of the Al is depleted due to formation of the reaction products. U.S.-A 4,702,770 discloses a method of making a B C AI composite. The method includes a preliminary step wherein paniculate B 4 C is heated in the presence of free carbon at temperatures ranging from 1800°Cto 2250°Cto reduce the reactivity of B 4 C with molten Al. The reduced reactivity minimizes the undesirable ceramic phases formed by the process disclosed in U.S.-A 4,605,440. During heat treatment, the B 4 C particles form a rigid network. The network, subsequent to infiltration by molten Al, substantially determines mechanical properties of the resultant composite. U.S.-A 4,718,941 discloses a method of making metal-ceramic composites from ceramic precursor starting constituents. The constituents are chemically pretreated, formed into a porous precursor and then infiltrated with molten reactive metal. The chemical pretreatment alters the surface chemistry of the starting constituents and enhances infiltration by the molten metal. Ceramic precursor grains, such as B 4 C particles, that are held together by multiphase reaction products formed during infiltration form a rigid network that substantially determines mechanical properties of the resultant composite. Disclosure of Invention One aspect of the present invention is a method for making a B 4 C/AI composite comprising sequential steps: a) heating a porous B 4 C preform to a temperature within a range of from greater than 1250°C to less than 1800°C for a period of time sufficient to reduce reactivity of the B 4 C with molten Al; and b) infiltrating molten Al into the heated B 4 C preform, thereby forming a B4C/AI composite that contains Al metal. The method allows control of three features of the resultant B4C/AI composites. The features are: amountof reaction phases; size of reaction phase grains or domains; and degree of connectivity between adjacent B4C grains. A second aspect of the present invention includes B4C/AI composites formed by the process of the first aspect. The B 4 C/AI composites are characterized by a combination of a compressive strength ≥ 3 GPa, a fracture toughness > 6 MPa mi, a flexure strength ≥ 250 MPa and a density < 2.65 grams per cubic centimeter (g/cc). The composites are suitable for use in applications requiring lightweight, high flexure strength and an ability to maintain structural integrity in a high compressive pressure environment. Automobile and aircraft brake pads are one such application. Other applications are readily determined without undue experimentation. Detailed Description Boron carbide, a ceramic material characterized by high hardness and superior wear resistance, is a preferred material for use in the process of the present invention. Aluminum, a metal used in ceramic-metal composites, or cermets, to impart toughness or ductility to the ceramic material is a second preferred material. The Al may either be substantially pure or be a metallic alloy having an Al content of greater than 80 percent by weight (wt-%), based upon alloy weight. The process aspect of the invention begins with heating a porous body preform or greenware article. The preform is prepared from B 4 C powder by conventional procedures. These procedures include slip casting a dispersion of the ceramic powder in a liquid or applying pressure to powder in the absence of heat. The powder desirably has a particle diameter within a range of 0.1 to 10 micrometers (μm). Ceramic materials in the form of platelets or whiskers may also be used. The porous preform is heated to a temperature within a range of from 1250°C to less than 1800°C. The preform is maintained at about that temperature for a period of time sufficient to reduce reactivity of the B 4 C with molten Al. The time is suitably within a range of from 15 minutes to 5 hours. The range is preferably from 30 minutes to 2 hours. As temperatures i n crease from 1250°C to less than 1800°C, the microstructure of the resultant cermet changes. At a temperature of from 1250°C to less than or equal to 1400°C, the microstructure undergoes rapid changes. In other words, temperatures of 1250°C to 1400°C constitute a transition zone. At one end, near 1250°C, the microstructures resemble the microstructure resulting from the use of untreated B 4 C. At the other end, near 1400°C, chemical reactions between B 4 C and Al are noticeably slower than at 1250°C. In general, the microstructure for a heat treatment within a temperature range of 1250°C to 1400°C is characterized by a continuous metal phase in an amount of > 0% by volume (vol-%) but < 10 vol-%, a discontinuous B 4 C phase and a reaction phase concentration of more than 10 vol-% . The volume percentages are based upon total chemical constituent volume Even though the microstructures of B 4 C/AI cermets that result from B 4 C preforms heat-treated at temperatures of 1250°C to 1400°C may resemble those resulting from the use of B 4 Cthat is chemically treated, molten Al penetrates into the former more rapidly than the latter. This promotes production of larger parts. Heat treatment at 1200°C or below provides no benefit. In fact, such a heat treatment leads to a reduced rate of infiltration and results in a cermet with increased porosity in comparison to that resulting from the 1250-1400°C heat treatment. At temperatures > 1400°C, but ≤ 1600°C, the microstructure is characterized by B 4 C grains that are isolated or weakly bonded to adjacent grains and surrounded byAl metal. The composite has a greater metal content than that of a composite prepared from an unheated, but substantially identical, porous precursor. The composite also has a reaction phase concentration of > 0 vol-%, but < 10 ol-%, based upon total chemical constituent volume. Temperatures near 1400°C typically yield the isolated grains whereas temperatures near 1600 D C usually result in weakly bonded B 4 C grains. Microstructures of cermets that result from heat-treatment within this temperature range are unique if the B 4 C has a size of < 10 μm. The unique microstructure leads to improvements in fracture toughness and flexure strength over cermets prepared from B 4 C that is heat treated below 1250°C At temperatures > 1600°C, but < 1800°C, the B 4 C has lower reactivity with molten Al than it does when given a heat treatment at temperatures < 1600°C. This results in lower hardness, but increased toughness and strength. Heat treatments change chemical reactivity between B 4 C and Al and affect the grain size of, or volume occupied by, reaction products or phases that result from reactions between B 4 C and Al. In the absence of a heat treatment or with a heat treatment at a temperature < 1250°C, comparatively large areas of AIB 2 and AI 4 BC form. Although B 4 C grains have an average size of 3 μm, an average area for AIB 2 or AI 4 BC may reach 50 to 100 μm. Large areas or grains of AI4BC are particularly detrimental because AI 4 BC is more brittle than B 4 C or Al. Large grains also affect fracture behavior and contribute to low strength (< 45 ksi (310 MPa)) and low toughness (K iC values < 5 MPa m ). Heat treatments at 1300°C for longer than one hour lead to reductions in AI 4 BC grain size to < 5 μm, frequently < 3 μm. Concurrent with the grain size reductions, the strength and toughness increase. The reduced grain size and increased strength and toughness can be maintained with heat treatment temperatures as high as 1400°C provided treatment times do not exceed five hours. As temperatures increase above 1400°C or treatment times at 1400°C exceed five hours, AI 4 BC tends to form elongated grains having an average diameter of 3-8 μm and a length of 10-25 μm. ->• The heat treatment does not require the presence of carbon. In fact, carbon is an undesirable component as it leads to an increase in AI4C3 when it is present. AI 4 C3 is believed to be an undesirable phase because it hydrolyzes readily in the presence of normal atmospheric humidity. Accordingly, the AI4C3 content is beneficially < 3 wt-%, based upon composite weight, preferably < 1 wt-%. Infiltration of a preform that is heated to a temperature of > 1250°C to < 1800°C occurs faster than in an unheated preform. In addition, the heat treated preform is easier to handle than the unheated preform and may even be machined prior to infiltration. The heat treatment temperatures suitable for use with porous preforms also provide beneficial results when loosely packed B C particles are heated to those temperatures. After heat treatment, the particles are suitably ground or crushed to break up agglomerates. The resulting powder may then be mixed with Al powder and converted to cermet structures or parts. The reduced reactivity of the heat treated B4C powder will minimize formation of the ceramic phases produced in accord with the teachings of U.S-A 4,605,440 at column 10. The ceramic phases include AI4C3, AIB24C 4 , AI4B 3C , AIB 12 C 2 , α-AlB 12 , AIB 2 and a phase X that contains boron, carbon and aluminum. It also maximizes retention of metallic Al. Infiltration of molten Al into heat-treated porous preforms is suitably accomplished by conventional procedures such as vacuum infiltration or pressure-assisted infiltration. Although vacuum infiltration is preferred, any technique that produces a dense cermet body may be used. Infiltration preferably occurs below 1200 α C as infiltration at or above 1200°C leads to formation of large quantities of AI4C3. A primary benefit of heat treatments at a temperature of from 1250°C to < 1800°C, is an ability to control the microstructure of resulting B 4 C/AI cermets. Factors contributing to control include variations in (a) amounts and sizes of resultant reaction products or phases, (b) connectivity between adjacent B 4 C grains, and (c) amount of unreacted Al. Control of the microstructure leads, in turn, to control of physical properties of the cermets. One can therefore produce near-net shape parts with improved mechanical properties without sintering B4C preforms at temperatures above 1800°C prior to infiltration. The production of near-net shapes below 1800°C eliminates problems such as warping of preforms at high temperatures and costly shaping operations subsequent to preparation of the cermets. Unique combinations of properties may also result, such as high compressive strength ( ≥ 3 GPa), high flexure strength ( > 250 MPa) and toughness ( ≥ 6 MPa mi) in conjunction with low theoretical density ( < 2.65 g/cc). The following examples further define, but are not intended to limit the scope of the invention. Unless otherwise stated, all parts and percentages are by weight. Example 1 B 4 C (ESK specification 1500, manufactured by Elektroschemeltzwerk Kempten of Munich, Germany, and having an average particulate size of 3 μm) powder was dispersed in distilled water to form a suspension. The suspension was ultrasonically agitated, then adjusted to a pH of 7 by addition of NH OH and aged for 180 minutes before being cast on a plaster of Paris mold to form a porous ceramic body (greenware) having a ceramic content of 69 vol-%. The B 4 C greenware was dried for 24 hours at 105°C. Several pieces of greenware were baked at temperatures of 1300°C to 1750°C for 30 minutes in a graphite element furnace. The baked greenware pieces were then infiltrated with molten Al (a specification 1 145 alloy, manufactured by Aluminum Company of America that is a commercial grade of Al, comprising < 0.55 % alloying elements such as Si, Fe, Cu and Mn) with a vacuum of 100 millitorr (13.3 Pa) at 1180°C for 105 minutes. Chemical analysis of the alloyed cermet body was completed using an MBX- CAMECA microprobe, available from Cameca Co., France. Crystalline phases were identified by X-ray diffraction with a Phillips diffractometer using CuKα radiation and a scan rate of 2° per minute. The amount of Al metal present in the infiltrated greenware was determined by differential scanning calorimetry (DSC). The phase chemistry of infiltrated samples using greenware baked at 1300°C, 1600°C and 1750°C is shown in Table I. Composites or cermets prepared from unbaked greenware contain greater amounts of AIB 2 and AI 4 BC and lesser amounts of Al and B 4 Cthan those prepared from greenware baked at 1300°C. Table I - Phase Chemistry * - Chemical constituents normalized to 100 after void volume is removed. ** - Represents a mixture of B4C and AIB24C4 The flexure strengths were measured by the four-point bend test (ASTM C1 161) at ambient temperatures usi ng a specimen size of 3 x 4 x 45 mm. The upper and lower span dimensions were 20 and 40 mm, respectively. The specimens were broken using a crosshead speed of 0.5 mm/mi n. The broken pieces from the four-point bend test were used to measure density using an apparatus designated as an Autopycnometer 1320 (commercially available from Micromeritics Corp.). The bulk hardness was measured on surfaces polished successively with 45, 30, 15, 6 and 1 μm diamond pastes and then finished with a colloidal silica suspension using a LECO automatic polisher. Fracture toughness was measured using the Chevron notched bend beam technique with samples measuring 4 x 3 x 45 mm. The notch was produced with a 250 μm wide diamond blade. The notch depth to sample height ratio was 0.42. The notched specimens were fractured in 3-point bending using a displacement rate of 1 μm/minute. The results of physical property testing are shown in Table II. Table II also shows Al metal content and baking temperature. Table II The data presented in Tables I and II demonstrate several points. First, the temperature at which the greenware is baked has a marked influence upon the phase chemistry of the resultant B 4 C/AI cermets. The phase chemistry of a cermet formed from unbaked greenware or greenware baked at < 1250°C is believed to be similar to that of the cermet formed from greenware baked at 1300°C. Changes are, however, discernible. As the baking temperature increases above 1400°C, the amount of unreacted or retained Al metal is substantially greater than the amount in the cermet resulting from unbaked greenware or greenware baked at 1300°C. Similarly, the volume percentage of reaction products AIB 2 and AI 4 BC also goes down as the bake temperature increases. Second, the data demonstrate that one can now control both cermet microstructure and physical properties based upon the temperature at which the greenware is baked. Example 2 Ceramic greenware pieces were prepared by replicating the procedure of Example 1. The pieces were baked for varying lengths of time at different temperatures. Infiltration of the baked pieces occurred as in Example 1. The baking times and temperatures and the flexure strengths of resultant cermets are shown in Table III. The flexure strengths of cermets prepared from greenware baked at < 1250°C were lower than those of composites prepared from greenware baked at 1300°C. Table III The data presented in Table III show maxima in flexure strength with a baking temperature of 1400°C and baking times of one and two hours. Although not as high as the maxima, the other values in Table III are quite satisfactory. The flexure strength values shown in Table III are believed to exceed those of B4C/AI cermets prepared by other procedures. Samples prepared from cermets resulting from the heat treatment at 1300 C C were used to characterize fracture toughness (Kic). The fracture toughness values, in terms of MPa-m* were as follows: 5.6 at 0.5 hour; 5.8 at 1 hour; 6.4 at 2 hours and 6.9 at 5 hours. Fracture toughness, like flexure strength, tends to increase with baking time for a baking temperature of 1300°C. The variations in both fracture toughness and flexure strength between the sample baked for 0.5 hour at 1300°C in this Example and the sample baked for 0.5 hour at 1300°C in Example 1 indicate that temperatures of 1250°C to 1400°C constitute a transition zone. Within such a zone, small variations in temperature, baking time or both can produce marked differences in physical properties of resultant cermets. The cermets were subjected to analysis, as in Example 1 , to determine the average size of the AI4BC phase in μm. The data are shown in Table IV. Table IV The data in Table IV suggest that the size of AI 4 BC varies inversely with flexure strength. In other words, high flexure strength corresponds to small average size of the AI 4 BC phase. The data also suggest that by varying the baking temperature, one can control the size of reaction products in addition to kinetics of the reactions that form such products. Example 3 - Compressive Stress Testing Ceramic greenware pieces having a ceramic content of 70 volume percent were prepared by replicating the procedure of Example 1. The pieces were infiltrated with molten Al after heat treatment at 1300°C or 1750°C. The resultant cermets were subjected to uniaxial compressive strength testing. The uniaxial compressive strength was measured using the procedure described by C. A. Tracy in "A Compression Test for High Strength Ceramics", Journal of Testing and 0 Evaluation, vol. 15, no. 1, pages 14-18 (1987). A bell-shaped (shape "B") compressive strength specimen having a gauge length of 0.70 inch (1.8 cm) and a diameter at its narrowest cross section of 0.40 inch (1.0 cm) was placed between tungsten carbide load blocks that were attached to two loading platens. The platens were parallel to within less than 0.0004 inch (0.0010 cm). The specimens were loaded to failure using a crosshead speed of 0.02 in/min (0.05 5 cm/min). The compressive strength was calculated by dividing the peak load at failure by the cross-sectional area of the specimen. The compressive strengths of the cermets resulting from greenware baked at 1300°C and 1750°Cwere, respectively 3.40 GPa and 2.07 GPa. This example shows that compressive strength decreases as a result of heat- o treatment temperatures. The data demonstrate that temperatures between 1300°C and 1750°C constitute a transition zone for compressive strength. The data also suggest that an increased amount of metallic Al is present as temperatures increase within the transition zone. Example 4- Stepped-Stress Cyclic Fatigue Testing Ceramic greenware pieces having a ceramic content of 68 vol-% were prepared 5 by replicating the procedure of Example 1. The pieces were infiltrated with molten Al, as in Example 1, without prior heat treatment, after heat treatment at 1300°C or 1750°C or after sintering at 2200°C. The resultant cermets were subjected to stepped-stress cyclic fatigue testing. The stepped-stress cyclic fatigue test was used to evaluate the ability of the 0 materialsto resist cyclic load conditions. Specimens measuring 0.25 inch (0.64 cm) in diameter by 0.75 inch (1.90 cm) long were cycled at 0.2 Hertz between a minimum (σ m ) and a maximum (σ max ) compressive of 15 and 150 ksi (103.4 and 1034.2 MPa), respectively. If the specimen survived 200 cycles under this condition, o and σ max were increased to 20 and 200 ksi (137.9 and 1379.0 MPa), respectively, and the test was continued for an additional 200 cycles. If the 5 specimen survived 200 cycles underthis condition, σ m and σ ma χ were increased to 25 and 250 ksi (172.4 and 1723.7 MPa), respectively, and the test was continued for an additional 600 cycles or until the specimen broke. If the specimen broke during loading to 250 ksi (1723.7 MPa), the value at which it broke was reported to reflect passing the 200 ksi (1379.0 MPa) loading. If the specimen survived the additional 600 cycles, the test was stopped and the specimen was unloaded. The results of testing specimens prepared from the cermet pieces are shown in Table V. Table V The data in Table V demonstrate that resistance to cyclic fatigue decreases as baking or heat treatment temperatures increase. Baking at 1300°C does, however, improve resistance to cyclic fatigue over that of a cermet prepared from B 4 C having no prior heat treatment. Example 5 A porous greenware preform was prepared as in Example 1 and baked for 30 minutes at 1300°C. A bar measuring 6 mm by 13 mm by 220 mm was machined from the preform. The bar was placed in a carbon crucible having Al metal disposed on its bottom. The crucible was then heated to 1 160°C at a rate of 8.5°C per minute under a vacuum of 150 millitorr (20 Pa). The depth of metal penetration into the bar was measured at time intervals as shown in Table VI. Table VI Similar results are expected with baking or heat treatment temperatures > 1250°C, but < 1800°C. Metal infiltration occurs more slowly and to a lesser extent in unbaked greenware or greenware given a heat treatment at a temperature of < 1250°C. Heat treatment at temperatures > 1800°C does not produce further improvements in infiltration. Infiltration is believed to occur faster in a preform baked at temperatures of 1250°C to < 1800°Cthan in a preform prepared from B 4 Cthat is chemically pretreated by, for example, washing with ethanol. Example 6 Boron carbide greenware materials were prepared as in Example 1 and baked at different temperatures and different lengths of time. After baking, the materials were infiltrated with Al metal as in Example 1 save for reducing the temperature to 1 160°C and the infiltration time to 30 minutes. Bulk hardness of the infiltrated materials, measured as in Example I Js shown in Table VII together with baking time and temperature. Table VII The data shown in Table VII demonstrate that hardness values tend to decrease with increased temperature, increased baking temperature or both. The data at 1400°C and 1600°C are quite similar. This suggests the existence of a transition zone between 1250°C and 1400°C wherein small changes in time, temperature or both may cause large changes in chemistry as reflected by variations in physical properties such as hardness. The data presented in Examples 1-6 demonstrate that heat treatment prior to infi Itration at temperatures withi n the range of 1250°C to < 1800°C provides at least two benefits. First, it enhances the speed and completeness of infiltration. SecondJt allows selection and tailoring of physical properties. The changes in physical properties are believed to be a reflection of changes in microstructure.

Claims: CLAIMS : 1. A method for making a boron carbide/aluminum composite comprising sequential steps: a) heating a porous boron carbide preform to a temperature within a range of from 1250°C to less than 1800°C for a period of time sufficient to reduce reactivity of the boron carbide with molten aluminum; and b) infiltrating molten aluminum into the heated boron carbide preform, thereby forming a boron carbide/aluminum composite that contains aluminum metal. 2. A method as claimed in Claim 1 wherein the temperature is ≥ 1250°C, but < 1400°C, and the heated preform is subjected to shaping operations prior to step b). 3. A method as claimed in Claim 2 wherein the composite has a microstructure characterized by a continuous metal phase in an amount of > 0% by volume, but < 10% by volume, a discontinuous boron carbide phase and a reaction phase concentration of more than about 10% by volume, the volume percentages being based upon total chemical constituent volume. 4. A method as claimed in Claim 1 wherein the temperature is from 1400°C o less than 1600°C, the composite has a microstructure characterized by boron carbide grains that are isolated or weakly bonded and surrounded by aluminum metal, and the composite has a greater metal content than that of a composite prepared from an unheated, but substantially identical, porous precursor and a reaction phase concentration of greater than 0% by volume but less than 10% by volume, based upon total chemical constituent volume. 5. A method as claimed in Claim 1 wherein the temperature is from 1600°C to less than 1800°C, the composite has a microstructure characterized by a nearly continuous, low surface area boron carbide skeleton with uniformly distributed aluminum metal and discrete concentrations of AIB 2 and AI4BC reaction products. 6. A method as claimed in Claim 1 wherein the composite has a concentration of Al 4 3 of less than 1 % by weight, based upon total composite weight. 7. A method as claimed in Claim 1 wherein the period of time and temperature are from 2 hours or more at 1300°C to from 0.5 hour to 2 hours at 1400°C and the composite has a microstructure characterized by AI4BC grains having an average diameter of less than 5 μm. 8. A boron carbide/aluminum composite characterized by a combination of a compressive strength greater than or equal to 3 GPa, a fracture toughness greater than or equal to 6 MPa mi, a flexure strength greater than or equal to 250 MPa and a density of less than or equal to 2.65 grams per cubic centimeter. 9. The composite of Claim 8 wherein the flexure strength is within a range of from 300 to 675 MPa and the fracture toughness is within a range of from 6 to 7.5 MPa mi.

Inventors: CARROLL DANIEL F, OTT JACK J, PRUNIER ARTHUR R JR, PYZIK ALEKSANDER J, CARROLL, DANIEL, F., OTT, JACK, J., PRUNIER, ARTHUR, R., JR., PYZIK, ALEKSANDER, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1993
Main classification: C22C1
IPC codes: C22C1, C22C21, C22C29